Feedstock Technology for Reactive Metal Injection Molding: Process, Design, and Application [1 ed.] 012817501X, 9780128175019

Table of contents :
Cover
Feedstock Technology for Reactive Metal Injection Molding: Process, Design, and Application
Copyright
Contents
1 Reactive powder metal injection molding
1.1 Metal injection molding—a standout manufacturing technology?
1.2 Overview of metal injection molding
1.2.1 Metal injection molding processes
1.2.2 Design consideration
1.2.3 Powders for metal injection molding
1.2.4 Binder selection
1.2.5 Feedstock preparation
1.2.6 Molding operation
1.2.7 Debinding
1.2.8 Sintering
1.3 Evolution of metal injection molding technology
1.3.1 Materials development
1.3.2 Technological advancements
1.3.3 Current status
1.4 Opportunities for metal injection molding of reactive metals
1.4.1 Increasing demand for miniaturization
1.4.2 Advantages over conventional manufacturing techniques
1.4.3 Demand from the medical sector
1.4.4 Materials that are hard to process
1.4.5 Market statistics and research direction
1.4.6 Applications
1.5 Constraints on the reactive powders metal injection molding
References
2 Design strategy of binder systems and feedstock chemistry
2.1 The role of binder
2.2 Basics of binder
2.2.1 Binder chemistry
2.2.2 Classifications of binder system
2.2.2.1 Wax-based binder system
2.2.2.2 Polyoxymethylene-based binder system
2.2.2.3 Water-based binder system
2.3 Feedstock chemistry and properties
2.3.1 Feedstock flow: powder characteristics and optimal solids loading
2.3.2 Shear sensitivity
2.3.3 Temperature sensitivity
2.3.4 Thermal conductivity and heat capacity
2.3.5 Strength model
2.4 Summary
References
3 Binder system interactions and their effects
3.1 Interactions between binder components
3.1.1 Polymer blends
3.1.2 Thermodynamics of polymer blends
3.1.2.1 Flory–Huggins theory
3.1.2.2 Solubility parameter approach
3.1.3 Experimental methods
3.1.3.1 Determination of interaction parameters for binary systems
3.1.3.2 Glass transition temperature (Tg) measurements
3.1.3.3 Infrared spectroscopy
3.1.3.4 Microscopy
3.1.4 Common binder blends
3.1.5 Further remarks for binder blends
3.1.5.1 Case study: complex interactions and their effects on reactive powders-MIM
3.2 Interactions between powder and binder
3.2.1 Role of surfactant
3.2.2 Basic chemistry of surfactant
3.2.3 Case study: surfactants other than stearic acid for reactive powders-MIM
3.3 Summary
References
4 Impurity management in reactive metals injection molding
4.1 The importance of impurity control
4.2 Methods of controlling impurities
4.2.1 Selection of primary component
4.2.2 Control of impurities and thermal debinding mechanisms
4.2.3 Sintering and impurity control
4.3 Points to consider for other reactive powders metal injection molding
4.3.1 Pure Al-metal injection molding
4.3.2 Metal injection molding of aluminum alloy 6061 with tin
4.3.3 Metal injection molding of Mg and its alloys
4.4 Process control
4.5 Summary
References
5 Potential feedstock compositions for metal injection molding of reactive metals
5.1 Polymers that thermally degrade by depolymerization
5.2 Minimizing oxidation in Al or Mg-MIM
5.2.1 Attempts to address Al-MIM
5.3 Commercial feedstocks and their properties
5.4 Ti-MIM success stories
5.4.1 Medical implants by Ti-MIM
5.4.2 Smart-glasses titanium arm by MIM
5.5 Summary
References
6 Outlook of reactive metal injection molding
6.1 Future trends
6.1.1 Market opportunities
6.1.1.1 Electronics
6.1.1.2 Consumer decorative products
6.1.1.3 Medical applications
6.1.2 Patents highlighting the success of reactive powders metal injection molding
6.2 Summary
References
Index
Back Cover

Citation preview

Feedstock Technology for Reactive Metal Injection Molding

Feedstock Technology for Reactive Metal Injection Molding Process, Design, and Application

PENG CAO MUHAMMAD DILAWER HAYAT Department of Chemical and Materials Engineering, The University of Auckland, Auckland, New Zealand

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2020 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN: 978-0-12-817501-9 For Information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisitions Editor: Christina Gifford Editorial Project Manager: Ana Claudia A. Garcia Production Project Manager: Sojan P. Pazhayattil Cover Designer: Mark Rogers Typeset by MPS Limited, Chennai, India

Contents 1. Reactive powder metal injection molding 1.1 Metal injection molding—a standout manufacturing technology? 1.2 Overview of metal injection molding 1.3 Evolution of metal injection molding technology 1.4 Opportunities for metal injection molding of reactive metals 1.5 Constraints on the reactive powders metal injection molding References

2. Design strategy of binder systems and feedstock chemistry 2.1 The role of binder 2.2 Basics of binder 2.3 Feedstock chemistry and properties 2.4 Summary References

3. Binder system interactions and their effects 3.1 Interactions between binder components 3.2 Interactions between powder and binder 3.3 Summary References

4. Impurity management in reactive metals injection molding 4.1 The importance of impurity control 4.2 Methods of controlling impurities 4.3 Points to consider for other reactive powders metal injection molding 4.4 Process control 4.5 Summary References

5. Potential feedstock compositions for metal injection molding of reactive metals 5.1 Polymers that thermally degrade by depolymerization 5.2 Minimizing oxidation in Al or Mg-MIM 5.3 Commercial feedstocks and their properties

1 1 1 13 23 35 39

43 43 45 64 81 81

87 87 127 136 138

145 145 147 170 183 185 186

191 191 195 207

v

vi

Contents

5.4 Ti-MIM success stories 5.5 Summary References

6. Outlook of reactive metal injection molding 6.1 Future trends 6.2 Summary References Index

219 233 233

237 237 248 251 255

CHAPTER 1

Reactive powder metal injection molding 1.1 Metal injection molding—a standout manufacturing technology? The market of metal injection molding (MIM) has expanded significantly over the last decades to include a broad array of applications such as consumer electronics, automotive, medical, and firearms. MIM fabrication results from the application of plastic injection molding technology to powder metallurgy. It is a low-cost forming method best suited for the metals and alloys that are difficult to machine or cast. The process is used to make small-to-medium and complex-shaped parts from metal or alloy powders and relies on shaping metal particles and subsequently sintering those particles. Hence, the process is capable of producing parts having higher strength compared with die casting, improved tolerances compared with investment or sand casting, and more shape complexity compared with most other forming routes. The competitive advantage comes from the ability of MIM to manufacture complex parts at high production rates and near-net-shaping capability, which results in high yield savings. Conventional production processes are being increasingly replaced by MIM—machining, casting, and press-sintering are some of the processes that have been replaced by MIM.

1.2 Overview of metal injection molding 1.2.1 Metal injection molding processes Although powder injection molding (PIM) was first demonstrated during the 1930s when the ceramic spark plug bodies were produced, MIM did not achieve commercial status until the 1970s. The delay was due to the lack of sophistication in the process equipment. With the advent of microprocessor-controlled processing equipment, such as molders and sintering furnaces, which enabled repeatable and defect-free cycles with tight

Feedstock Technology for Reactive Metal Injection Molding DOI: https://doi.org/10.1016/B978-0-12-817501-9.00001-6

© 2020 Elsevier Inc. All rights reserved.

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Feedstock Technology for Reactive Metal Injection Molding

tolerances, the manufacturing infrastructure of the MIM strategy improved dramatically.1 MIM attracted major attention in 1979 when two design awards were won. One award was for a screw seal used on a Boeing jetliner. The second award was for a niobium-alloy thrust chamber and injection for a liquid-propellant rocket engine.2 These awards provided the necessary springboard for extensive research in this field. Several patents emerged, with one of the most useful being issued in 1980 to Ray Wiech. From this beginning, a host of other patents, applications, and firms strictly dealing with MIM rose. By the middle 1980s, the MIM technology had developed a firm base in the manufacturing sector. Since the mid-1990s, the MIM technology has expanded to include an array of different material families and new and innovative product designs that were not possible with conventional processing. The key steps of MIM include (1) selecting and tailoring a powder for the process; (2) mixing metallic powder and a binder system to form a homogenous feedstock; (3) molding the feedstock to achieve the required shape and geometry (green parts); (4) removal of the binder while keeping the geometry (debinding); (5) sintering the debound parts (brown parts) to achieve the desired mechanical properties; and (6) post sintering treatments to further improve properties if required. The steps involved in the MIM are illustrated in Fig. 1.1. MIM can process any metal if the metal is produced in a suitable powder form. Some common material families used in MIM are stainless steel, low alloy steel, tool steel, titanium, copper, tungsten, and hard metals. MIM products' mechanical properties are superior to cast products in most cases and slightly inferior to wrought products. Cast and MIM components both have some microstructural voids as a result of the processing methods. The cast voids are usually large due to the cooling of liquid to solid while the MIM voids are typically fine and well distributed across the microstructure after sintering. The large voids of the cast components result in the inferior properties than MIM components. Even full densification (microstructure without voids) can be attained in MIM by post sintering techniques such as hot isostatic pressing. The dimensional variability of the MIM process is associated with the amount of shrinkage that the component experiences during the debinding and sintering. Components shrink about 1% during the debinding operation and about 15% 25% after sintering.

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Figure 1.1 Steps involved in a MIM operation.3

1.2.2 Design consideration Before selecting this technology, capital investment, production costs, production rate as well as performance and quality of the part are the factors that need to be taken into consideration. As a general rule of thumb, components that are produced by press and sinter technology (generally less than 100 g) can be easily manufactured by MIM. Typically, an average size of 15 g is common for a MIM component; however, components in the range around 0.030 g are also possible.4 Table 1.1 presents the lower and upper specifications of the MIM process. MIM process generally produces good surface finish. Typically, the surface finish of 0.8 μm can be achieved easily. However, surface finish as smooth as 0.3 μm is also possible. The surface finish generally depends on the chemistry of powders used and the sintering conditions.

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Feedstock Technology for Reactive Metal Injection Molding

Table 1.1 Typical attributes produced by the MIM process. Attributes

Minimum

Typical

Maximum

Component mass (g) Max. dimension (mm) Min. wall thickness (mm) Tolerance (%) Density (%) Production quantity Surface finish (μm)

0.030 2 0.025 0.2 93 1000 0.3

10 15 25 5 0.5 98 100,000 0.8

300 150 15 1 100 100,000,000 1

As MIM involves postmolding steps of debinding and sintering, there are some design recommendations that should be considered to get highquality MIM product, as listed: 1. Avoid components over 12.5 mm thick. In cases where thick sections are desired, special modifications to the binder system should be made to debind thick sections. 2. Avoid components over 100 g in mass. 3. Avoid holes smaller than 0.1 mm in diameter. 4. Avoid wall thinner than 0.1 mm. 5. Maintain uniform wall thickness in order to attain smooth flow during molding. 6. Avoid sharp corners. The desired radius is greater than 0.05 mm. 7. Avoid internal undercuts. 8. Design with a flat surface to aid in sintering.

1.2.3 Powders for metal injection molding Powders shape, size, and its distribution play a decisive role in determining the overall quality of the MIM product. Metals or alloy powders that can be manufactured to a sufficiently small size (,45 μm), sinterable, and do not possess a melting point lower than the decomposition temperature of the binders can be utilized for MIM. The low-melting temperature and strong surface oxides and their interference with sintering make aluminum and magnesium less desirable for MIM. However, both have been successfully processed by MIM with limited commercial success.5 More details on Al and Mg-MIM can be found in Chapter 4, Impurity Management in Reactive Metals Injection Molding, and Chapter 5, Potential Feedstock Compositions for MIM of Reactive Metals, of this book. Common MIM metals and alloys include stainless steel, low-alloy

Reactive powder metal injection molding

5

steels, tool steels, copper and its alloys, titanium and its alloys, soft magnetic alloys, refractory metals, and cemented carbides. The ideal MIM powder characteristics are as follows6 8: 1. Powder particle size (D90) of ,22 μm for most of the metals and alloys for good sinterability and surface finish since finer powders sinter more readily than coarser powders. However, for reactive powder such as titanium, this limit can be set at # 45 μm as the risk of impurity pick-up during sintering also increases with a higher surface area of the fine particles. In addition, for reactive powders (titanium, aluminum, magnesium, and zirconium), the probability of explosion increases simultaneously with decreasing particle size. 2. It is often sought to incorporate a high proportion of metal powder in the feedstock. In other words, powders having a high packing density are desirable. The spherical or near-spherical shape should, therefore, be preferred (see Chapter 2: Design Strategy of Binder Systems and Feedstock Chemistry for more details). However, irregular shape powders in the case of titanium and its alloys have also been widely studied as they offer much lower cost (B45 $/kg) compared to spherical powders (B250 $/kg). 3. The powder particles should have high surface purity to maintain uniform interaction with the binder system. 4. The powder particles should be void-free. Table 1.2 compares the different powder production methods with respect to price, shape, size, and materials that can be processed. High-purity argon atomization is the principal technique used to produce reactive metals powders. However, aluminum powders are also produced via air atomization. Other fabrication techniques, such as plasma atomization, are also sometimes used for reactive metals powder production. Table 1.2 Common techniques for powders production in MIM. Technique

Size (μm) approx.

Shape

Materials

Cost

Gas atomization Water atomization

5 45 5 45 0.2 20

Metals, alloys Metals, alloys except reactive Metals only

High Moderate

Thermal decomposition Chemical reduction

Spherical Semispherical Spherical to spiky Angular, spherical

Metals only

Moderate to high

0.1 10

Moderate

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Feedstock Technology for Reactive Metal Injection Molding

1.2.4 Binder selection The binder system is an integral part of the MIM process. It controls the shaping stage of the MIM process and then holds the powder particles until the initial stage of sintering. By achieving this, a binder system usually has three components: a low molecular weight component that provides the necessary flowability during molding, a backbone polymer that provides the green strength, and a surfactant which acts as a bridge between the binder and powder particles (see Chapter 2: Design Strategy of Binder Systems and Feedstock Chemistry for more details). Some common binder systems are listed in Table 1.3.9,10 Thermoplastic and thermosetting are two common types of polymers. Thermoplastic polymers are formed by repeating small monomer groups along the chain length without cross-linking. On the other hand, in thermosetting polymers, monomers undergo cross-linking, which results in the formation of a three-dimensional rigid structure. The cross-linking of thermosetting polymers upon heating can be helpful during the molding process since it may provide the necessary green strength. However, due to their complicated decomposition processes, thermosetting polymers are rarely used in MIM. The composition of the binder plays a significant role in determining the binder viscosity and flow behavior, especially for mixtures with large differences in viscosities among the components. Viscosity increases as the molecular weight is increased, so a proper selection of the molecular weight of each binder component is necessary. For titanium MIM, careful Table 1.3 Common binder systems for metal injection molding. Binder type

Main ingredients

Polymer backbone

Additives

Thermoplastic

Waxes (paraffin/ microcrystalline/ carnauba/natural waxes), oils (vegetable/peanut oil), acetanilide, naphthalene, PEG Polyoxymethylene Epoxy resin Water

PE, PP, PS, PA, PVB, HDPE, LDPE, PBMA, CAB, EVA, PMMA

Stearic/oleic acid, oils

Polyacetal Thermosetting Gelation

Waxes Methyl cellulose, agar

Proprietary Stearic acid Glycerine, boric acid

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selection of binder components is extremely important (discussed more in detail in Chapter 2: Design Strategy of Binder Systems and Feedstock Chemistry) because titanium is extremely reactive to elements such as O, N, and C.

1.2.5 Feedstock preparation In the next step, the metal or alloy powders are mixed with preselected binders to produce a homogeneous mixture—also known as a feedstock. The mixing operation should be thorough enough to ensure that each particle is coated with the selected binder. Also, homogeneous mixing of feedstock is crucial to the final product quality, as any inhomogeneities such as bubbles, binder pockets, and powder segregation will subsequently be carried over to the injection molding stage. To ensure this, different types of shear mixers are available in the market nowadays. These include twin-screw extruder, double planetary, single screw extruder, plunger extruder, twin-cam extruder, shear roll compounder, and most common sigma or z-blade mixers. Some of the common mixers are shown in Fig. 1.2.

Figure 1.2 Some common mixers/kneaders used in feedstock preparation: (A) a zblade sigma mixer,11 (B) a shear roller,12 and (C) torque mixer.13 (B) Courtesy Mr. Frank Langer, Bellaform GmbH, Germany; (C) courtesy HAAKE ThermoFisher Scientific.

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Feedstock Technology for Reactive Metal Injection Molding

With the recent advancements in MIM technology and a better understanding of the process, some companies such as ThermoFisher Scientific offer a wide range of feedstock preparation equipment from laboratory-scale simple mixing to industrial-scale compounding and pelletizing, Fig. 1.3.

1.2.6 Molding operation The design of injection molding machines irrespective of the suppliers has certain general design features in common, which are necessary to carry out and control the injection molding process. The most important components of the injection molding machine are the injection unit, the clamping unit, and the tooling attached to the clamping unit (Fig. 1.4). These units are generally placed horizontally. In fact, the maximum clamping force is the main characteristic by which the power and size of an injection molding machine are defined. Molding parameters such as injection pressure, injection speed, mold quality, and mold temperature are very important for preparing parts without defects and minimum porosity.

Figure 1.3 Classic strand pelletizing.14 Courtesy ThermoFisher Scientific.

Figure 1.4 A schematic overview of injection molding machine.15

Reactive powder metal injection molding

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Figure 1.5 HAAKE MiniJet pro piston injection molding system by ThermoFisher Scientific. Such small injection molders provide an excellent opportunity to test-run feedstocks on a laboratory scale.

Barrel temperature is very important; if the temperature is too low, the feedstock may freeze before the mold cavity is completely filled. If the temperature is too high, it will lead to very low viscosity that causes problems such as molten feedstock dripping out of the nozzle, flashing, and prolonged cooling times. It should also be considered that some heat is generated in the barrel by the frictional forces between the screw and the feedstock. Typical melt temperatures for common wax-polymers systems are between 150°C and 190°C, and the mold temperature is 25°C 55° C. Typical melt temperatures for catalytic systems are 200°C 260°C, and the mold temperature is 100°C 150°C. Owing to the higher thermal conductivity of the feedstock, MIM injection speeds are typically higher than pure polymers. The injection speed is typically set at the minimum injection speed required to completely fill the component's cavity without any defects. Too low an injection speed will result in surface imperfections such as flow lines and incomplete fill. Too high an injection speed will result in a flash due to powder/binder separation and can result in an explosion in the case of reactive powders. These days even small bench-top injection molders are also available in the market, Fig. 1.5.

1.2.7 Debinding After the injection molding step, the binder system becomes a disposable component and hence, must be removed from the samples. During

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Feedstock Technology for Reactive Metal Injection Molding

debinding operation, special attention is paid to the shape retention of the molded samples. The method of binder removal has a considerable effect on the molded sample shape retention, uniformity of shrinkage, and final product mechanical properties. Therefore proper selection of the debinding method is important for good quality control, particularly for reactive materials such as titanium, as explained in Chapter 2, Design Strategy of Binder Systems and Feedstock Chemistry. The most commonly applied debinding techniques include thermal debinding, vacuum debinding, catalytic debinding, and solvent-thermal debinding. The solvent debinding, combined with thermal debinding, is the most widely used technique in the MIM industry. It involves two steps: (1) removal of primary binder usually by solvent dissolution, (2) removal of backbone polymer, and surfactant by a thermal process without cracking or degrading the molded part. Some of the important points that must be considered during solvent-thermal debinding are summarized below: 1. The dissolution rate of the primary binder in the solvent increases as the primary binder is liquefied. This implies the temperature of the solvent must be chosen with great care. 2. The reaction of the backbone polymer with the solvent should not cause any part distortion. 3. The solvent should not have high vapor pressure at debinding temperatures. Care must be taken if any of the fire hazard solvents are used. When the primary binders are removed, no diffusion bonding between powder particles takes place, as the first-step debinding is usually performed at low temperatures. It is the backbone binders and interparticle friction that hold the powder particles together and maintain the shape after the solvent debinding. The secondary binders are removed thermally at moderate-to-high temperatures and called thermal debinding. The thermal debinding is achieved by heating the parts slowly to the temperature where the secondary binder evaporates. At those temperatures, interparticle diffusion is enough to hold the parts together.16 Table 1.4 presents multiple binder systems with their debinding method, debinding temperature, and approximate debinding rates for regular MIM parts.17 Another attractive and innovative method for extracting waxes from the molded samples is to use supercritical solvent debinding. In this process,18 the waxes are removed using liquid CO2 as the solvent at about 50°C 70°C. What makes this process attractive is the fast dissolution of the wax (in some cases, complete removal of paraffin wax was achieved

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Table 1.4 Examples of debinding rates of the primary component for different binder systems. Primary binder

Secondary binder

Primary debinding method

Debinding temperature (°C)

Debind rate (mm/h)

Paraffin wax Synthetic wax Polyethylene glycol Polyethylene glycol Polyacetal

Polypropylene Polypropylene

Heptane Perchloroethylene

50 70

1.5 2

Polyacetal

Water

60

0.5

Polymethy methacrylate Polyethylene

Water

50

0.2

Nitric acid catalyst

120

1.5



It is worth mentioning here that these debinding rates are just an estimate. Actual debinding rates may greatly vary depending on the powder particle size, debinding temperature, molecular weight of the primary component, and its interactions with metal powder and the other components of the binder system.

in 2 h19). After the soaking process, the liquid CO2 is removed from the chamber, the waxes are left behind in the solid form and can be easily removed. As a high pressure (as high as 35 MPa) is required to keep CO2 liquid at these temperatures, special high-pressure vessels are required for this purpose. Therefore this process has not seen commercial success.17

1.2.8 Sintering Any left-over binder residue is subsequently removed during the sintering step of MIM. During sintering, the component shrinks to form a dense shape and attain a high density approx. 96% 99% of that of wrought material. During sintering densification, several material transport mechanisms may be active, such as surface diffusion, vapor transport, boundary diffusion, and lattice diffusion, all of which lead to the reduction in surface energy. Surface diffusion and vapor transport are surface transport mechanisms and mainly related to particle coarsening. They promote the neck growth among the particles without increasing the density of the compact, as only particle surfaces are involved. Density is increased by the bulk transport mechanisms, which include lattice (volume) diffusion, boundary diffusion, plastic flow, and viscous flow. Depending on the type of material being sintered, one or more of these mechanisms may be active during the sintering.

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Feedstock Technology for Reactive Metal Injection Molding

The debound parts are sintered in either a continuous or a batch-type high-temperature furnace. The sintering parameters such as temperature, atmosphere, heating and cooling rate, and holding time are selected based on material type, composition, and size of the part being sintered. For example, the preferred sintering conditions for tool steel H11 are a sintering temperature of 1200°C 1275°C and under N2 atmosphere. For reactive metallic powders, it is not possible to avoid oxygen pickup entirely during sintering. However, the oxygen pickup can be kept to a minimum by using a high vacuum, generally .1022 Pa. High purity argon can also be used as a sintering atmosphere, but it leads to lower sintered density due to trapping of gas in the pores. The selection of sintering parameters for reactive powders is a tough task and is always a compromise between low contents of impurity, low residual porosity, and small grains. Generally speaking, higher sintering temperatures and longer holding times lead to a higher density and result in higher strength. On the contrary, a higher sintering temperature also leads to more oxygen contamination and hence increased brittleness. Moreover, it also leads to grain coarsening, which is detrimental to strength. Therefore the optimal sintering cycle for reactive powders depends on the starting powder characteristics, desired density, alloying approach, microstructure, and final impurity level requirements. As sintering temperatures are usually close to the melting range of the component’s material and, in some cases, also involve a liquid phase, the selection of charge carriers (also known as racking materials) is extremely important. To prevent any reaction between the component and the charge carriers, a layer of inert ceramic is used as a barrier between the two. The charge carrying shelves in a graphite furnace are usually also graphite. Direct contact of these graphite charge carriers would result in the formation of a low melting eutectic in several material systems. In the case of refractory metal furnaces, molybdenum (Mo) or a molybdenum alloy is used as shelving material. However, Mo reacts with ferrous materials to form a liquid phase. Hence, a nonreactive barrier is a must to prevent the unwanted reaction irrespective of the furnace type. Alumina is a widely used charge carrier for most of the materials except for titanium alloys because titanium reacts with alumina. For titanium, the most suitable and successful carrier materials are zirconia (ZrO2) and yttria (Y2O3). Zirconia stabilized yttria (YSZ) is also among the best. High-purity yttria plates have also been used, but these are even more

Reactive powder metal injection molding

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expensive and difficult to source than the YSZ plates. The only metal that can be used as sintering support is molybdenum.

1.3 Evolution of metal injection molding technology MIM technology has significant advantages over conventional machining and wrought technologies such as die casting and investment casting when it comes to producing small and complex components in mediumto-high volumes (Fig. 1.6). The cost advantage depends on the component material, shape and size, and production volumes. Some of the key benefits are listed: It is near-net-shape manufacturing for complex geometries It minimizes material wastage and reduces finishing requirements It can combine multiple components into one, thus reducing part and assembly costs. It reduces costs due to lower raw material consumption.

1.3.1 Materials development From the past 2 decades, research studies are being carried out to manufacture and test new materials systems via MIM. These research efforts have led to setting up of many industrial collaborations and companies that can supply commercial feedstocks and can create proprietary recipes for the clients.21,22 For instance, the internationally active family company OBE Ohnmacht & Baumgaertner GmbH & Co KG22 for large-scale manufacturing of precision mechanical metal parts has set up a MIMplus

Figure 1.6 Diagram showing where MIM is most appropriately used in comparison with other fabrication processes.20

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research and development department. This department is focused on developing innovative and new MIM materials that are not available so far. This includes permanent magnetic alloys (e.g., NdFeB) or complex composite materials. The attractive features of the MIM process can be applied advantageously to the fabrication of metal matrix composite (MMC), particularly the particulate reinforced MMCs. Although many MMCs offer unique properties that cannot be achieved by monolithic metals or alloys, the high cost of manufacturing often restricts their commercial uses. By successfully applying the MIM strategy, the cost for commercial use of composite materials can be significantly reduced. In recent years, comprehensive work has been carried out to explore the potential of MIM for the fabrication of MMC components.23 Many companies have been working toward identifying the commercial capacity of the MIM technology for the fabrication of composites.22,24,25 The widely studied and research on MMCs include stainless steels, refractory metals, intermetallic compounds, and titanium alloys composites. Although MMCs can be either continuously or discontinuously reinforced, the flow and mold filling requirements of the MIM process ascertain that it is most applicable for processing the feedstock containing particles or short fibers. Thereof, all of the MMCs fabricated by MIM to date are strengthened by discontinuous reinforcements. In the early 2000s, Thian et al.26 28 led the research in exploring the fabrication of titanium-hydroxyapatite (HA) composites by MIM, focusing applications in the biomedical field. The use of Ti-HA composites for biomedical applications has increased due to the resulting advantageous combination of HA bioactivity and favorable mechanical properties of Ti. HA (Ca10(PO4)6(OH)2) is a bioceramic material frequently used for implants of human hard tissue because its chemical and crystallographic structure is similar to that of bone minerals. Moreover, HA is nontoxic, bioactive, and biocompatible, which results in better osseointegration between implants and bones. However, a major limitation of using HA is its poor mechanical properties. On the contrary, Ti has superior mechanical properties and is considered a biocompatible metal—although its biocompatibility is not as good as that of HA. Consequently, to create high-efficiency biomaterials for medical implants, Ti-HA composites have been considered an encouraging group of materials. Since the early research on HA/Ti composites, new studies have been carried out to study these composites made by

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MIM.29,30 During the high-temperature sintering of HA/Ti composites, a key challenge is to prevent HA from decomposing. The degree of HA decomposition increases with sintering temperature. On the contrary, to achieve a high degree of densification, a relatively high sintering temperature is required. This dilemma is one of the main reasons that is hindering the commercial success of this composite system made by MIM. Another major challenge for using Ti-MIM to manufacture orthopedic devices is that conventional Ti-MIM components do not have adequate fatigue strength for most load-bearing applications. Typical fatigue strengths, when measured by rotating-beam fatigue testing are around 480 MPa at 10 million cycles. The commonly accepted minimum for load-bearing applications is, however, around 620 MPa at 10 million cycles. Focus is laid now on the optimization of fatigue properties. Recently, Praxis Powder Technology has developed a processing route— branded “TiRx”—to improve the final microstructure of the sintered titanium. This process results in fatigue strength over 620 MPa and achieves this performance while still meeting the chemical and mechanical requirement of ASTM F2885. At present, aluminum matrix composites are highly demanding material in aerospace and automobile industries due to their outstanding properties such as high strength, high specific modulus, good wear resistance, and low thermal expansion coefficient. Recently, Abdoos et al.31 have studied nanoalumina-reinforced aluminum matrix composites fabricated by MIM. Alumina particles are favorable as a reinforcement because of their low cost, superior high-temperature mechanical properties, and excellent oxidation resistance. Their study showed that by incorporating nanoalumina, mechanical properties of the Al matrix composites can be further enhanced. In addition to general progression on materials development, different legislations are also fueling the research to develop new materials for MIM. For instance, in the currently used medical austenitic stainless steel 316L and 17-4PH, nickel is an important alloying element. The main role of nickel is to form a stable austenitic structure, thereby improving the corrosion resistance, weldability, and toughness of the steel. However, nickel can cause allergic reactions in humans.32 EU has now posted restrictions on the use of nickel for medical implants. The alternative is to use high-nitrogen, nickel-free, chromium manganese molybdenum steel, which can be processed easily by MIM technology.

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1.3.2 Technological advancements Since its introduction to mainstream applications, MIM has gone through significant technological advancements. Perhaps, the most significant among these advancements is the development of micrometal injection molding (microMIM or μMIM). There are some fields, for example, chemistry, biology, and telecommunications that require highly resistant microcomponents made of metals. It is, therefore, necessary to enhance manufacturing methods for products using metal-based components in microdimensions. MIM is expected to become increasingly popular for the precision manufacturing of complex microparts.33 A comprehensive survey exploring market trends of microMIM can be found in the references.34,35 For a successful microMIM process, a careful selection of feedstock components and compounding techniques is mandatory. Macroscopic MIM is commonly performed with particles sized up to 20 μm or even larger. However, microMIM requires particle size in the micro or even in submicrometer range to meet the specific requirements with regard to true-to-detail design, surface roughness, and the mechanical properties of the sintered part.36 One plausible reason for using submicron-sized powders in microMIM is so that the grain sizes are kept to a minimum after sintering to maintain a polycrystalline structure of the component.37 One option to ensure this outcome is to start the process with the smallest possible powder particles. Some of the common metal powders currently used or are being trialed for microMIM are presented in Table 1.5. MicroMIM needs feedstocks of very low viscosity as compared to macroMIM for fast, effective filling of the mold. The binder system, therefore, plays a decisive role in the success of microMIM. As microcomponents are subject to high demolding forces due to relatively large surface-to-volume ratios and small load-bearing cross-sections, the microMIM requires a binder system that can provide high green strength. However, high green strength requires backbone polymers with a high molecular weight. The usage of high molecular weight polymers unfortunately leads to the higher viscosity of the feedstock. There are very few binder systems that can successfully meet both conditions. From the authors’ personal experience, water-based binder systems with high contents of surfactants are a good choice. Custom microinjection molding machines have been developed by companies like Arburg GmbH 1 Co KG (Germany), MicroMIM Taisei

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Table 1.5 Typical powder sizes for metal powders currently applied for microMIM. Material

Mean particle size (μm)

Typical aspect ratio (AR)

Stainless steel 316L Stainless steel 17-4PH Carbonyl iron Titanium alloys Copper Hard metal (WC-xCo) Tungsten-copper alloy

1.5 5 3 5 1.5 5 $ 20 0.5 2 0.5 4 1.5 3

1 5 1 5 Up to 15 / Up to 100 Up to 10 /

Figure 1.7 An appropriate example of a microinjection molding machine developed and distributed by Wittmann Battenfeld GmbH, Austria. This machine is equipped with a special plunger system for the replication of microcomponents. As the plunger diameters can be much smaller than those of the screws, even the lowest shot volumes can be measured out and injected precisely.37,38

Kogyo (Japan), and Sodick Co. Ltd. (Japan). One such example is shown in Fig. 1.7. It is beyond the scope of this chapter to describe the microMIM process in full detail. Interested readers are advised to refer to the reference37 and the cited literature thereof. Examples of industrial microMIM products already on the market are given in Fig. 1.8. Two-material MIM or more commonly known as two-color or twocomponent MIM (2C-MIM) is another extension of the conventional MIM process that has seen significant interest in recent times. The early investigations into 2C-MIM were carried out by Pischang et al.40 Since

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Figure 1.8 A selection of various microMIM samples: (A) the world’s smallest check valve, less than 0.2 mm in thickness; (B) microgears; (C) soft-magnetic material; (1) plug-in element for glass fiber connectors, 7%NiFe; (2, 3) components for dental electro engines, 17-4PH; (4) clamping jaw for medical instruments, 17-4PH; (5) arresting bolt for ammunition, 316L; (6) gearwheel carrier for microelectroengines, AlSi4140. Pictures (A C) courtesy Taisei Kogyo Co. Ltd. https://www.taisei-kogyo.com/en/facility/ enmicromimtech.php., while pictures (1 6) are taken from Piotter, V., 15 Micro Metal Injection Molding (MicroMIM). In Handbook of Metal Injection Molding, 2nd ed., Heaney, D. F., Ed.; Woodhead Publishing, 2019, pp 333 360.

then, the concept and evaluation of 2C-MIM have been applied to many material systems with a variety of potential applications.41 Like in MIM, 2C-MIM starts with the preparation and rheological characterization of feedstock, followed by injection molding to form a green part. Molding of the parts can be accomplished either via overmolding or coinjection molding. The overmolding is typically a manual

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Figure 1.9 Cross-sectional difference between the two 2C-MIM variants.42

process. In this variant, the first one component is molded; once the component is cooled down to room temperature, it is then transferred to another mold where overmolding is carried out to achieve the desired shape. The resulting molded part is composed of two different materials. After thermal debinding and sintering, a single, integrated component results. This process is best achieved using a twin-barrel injection molding units. In coinjection molding, a functionally graded structure is produced using distinct flow behavior of the materials via the same runner system. The resulting component has a core and skin of two different materials. Fig. 1.9 schematically illustrates the cross-sectional difference between overmolding and coinjection molding. Successful prototypes made using 2C-MIM technology are shown in Fig. 1.10. Companies such as Taisei Kogyo Co. Ltd. are leading the way in research and development of MIM technology. Apart from microMIM, the company has successfully been able to produce hollow, complex components or complex parts with undercuts. This method is illustrated in Fig. 1.11.

1.3.3 Current status According to Matthew Bulger, MIM is now recognized as one of the top ten advanced manufacturing technologies, ranking only second to additive manufacturing.20 MIM has continued to gain interest in recent years. In particular, over the past 7 years, it has achieved exceptional success. This is especially attributed to the significant growth in the electronics industry in China20 (Fig. 1.12). According to a leading MIM industry consultant Dr. Y.H. Chiou, widely known as Dr. Q, the ability of the MIM industry to manufacture high volumes of small, light parts coincides with the consumer’s desire to carry and wear ever more electronic devices. The competitiveness of MIM keeps pace with the demands of the smartphone industry, and the technology continues to thrive. When we look at each generation of

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Figure 1.10 A few pictures and schematic diagrams of a few prototypes explored via 2C-MIM, (A) copper-based heat sink with porous channels acting as heat pipes, (B) and (C) automotive sensor holders, (D) housing for a hermetically sealed electronic package with an embedded heat sink. These pictures and their description are taken from Ref. [41] For readers seeking detailed information regarding 2C-MIM, please see Ref. [38] and the cited literature thereof.

mobile networks and telecommunication technology upgrades, a corresponding proportional growth can be seen in MIM’s global gross product for this sector43 (Fig. 1.13). The advent of new high-speed communication technology in the 5G era means that MIM is also facing an important era of “hidden” needs in the wider 5G ecosystem. Physically, it is difficult to solve the challenges of 5G simply using conventional technology, because of the demand for heat dissipation, electromagnetic compatibility, current conduction, and

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21

Figure 1.11 (A) A new method developed by Taisei Kogyo Co. Ltd. for the production of complex parts. With this technique, hollow-shaped complex parts such as closed impeller, as shown in (B) can be made.

Figure 1.12 Trends in global MIM markets. Since 2013, MIM part sales have grown rapidly in China/Taiwan, largely thanks to advancements in the electronics sector. The MIM sales for the electronics sector in China make up to 66% of total China’s MIM sales.

manufacturing tolerances of conventional manufacturing. These problems are being solved through materials science and technology, resulting in a sharp increase in the demand for MIM parts made from a wider range of

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Figure 1.13 Growth in global MIM gross output coincides with a corresponding proportional upgrade of mobile phone networks and communication technology.43

materials for specific applications in 5G communication signal receiving boxes, local servers, and terminal devices. In addition to the use of conventional stainless steel as structural parts, the heat dissipation and electromagnetic isolation functions of copperbased alloys and the nonmagnetic properties of cobalt-based alloys are a few prerequisites of 5G technology. Such requirements are constantly bringing forth new and disruptive innovations to replace existing designs in MIM as well. Hence, there is an increase in the use of the sinter joining of “green” MIM parts to make hollow, highly complex, or long structures. According to Dr. Q,43 “MIM will be the best partner for product design in 5G communications. With more than two hundred MIM factories in China, the technology is able to support the move to the 5G era.” Although the electronics sector dominates MIM applications in China, across the globe, in North America, firearms and medical applications are prevalent, and automotive along with armament MIM applications in Europe (Fig. 1.14). In North America, companies such as Smith Metal Products44 are leading the way in research and development of MIM for medical applications. This trend is likely to continue as the demand for intricate, small, and lightweight medical components is ever increasing. One of the great benefits of using MIM for biocompatible medical implants is its cleanliness compared to conventional manufacturing technologies. The only possible toxic substances are from the binder residues. Therefore it is suggested to use nontoxic, biocompatible binder components exclusively if medical applications are sought for.

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Figure 1.14 Breakdown of MIM sales by market: (top) North America; (bottom) Europe.

Although MIM is now regarded as an established manufacturing process, MIM technology still has a promising and bright future in areas such as automotive, medical, aerospace, and even energy (fuel cell plates, Fig. 1.15). The technology’s prospect could be further improved by developing sophisticated methods to fabricate high-quality powders at a lower cost.

1.4 Opportunities for metal injection molding of reactive metals 1.4.1 Increasing demand for miniaturization The prevailing trend toward product miniaturization these days is one of the major factors propelling the growth of the global MIM market. In recent years, the trend has been about product miniaturization across various industries, that is, manufacturing of even smaller electronic, optical, mechanical, and medical devices and products (Fig. 1.16). The initial push for small parts and components arose from the aerospace sector with the demand for electronic components in rockets and control to be lighter and more compact to reduce fuel costs. The medical sector also demanded

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Figure 1.15 MIM bipolar plates for proton exchange membrane (PEM) fuel cells: (A) made from pure copper; (B) made from 316L stainless steel.45 Courtesy Joseph W. Newkirk, Missouri S&T, United States.

Figure 1.16 An example of miniature parts production by MIM: (left) series production of “green” ear implant; (right) sintered part. Investigations of 100 green parts via microbalance showed a mean weight of 9.38 mg for each part with a deviation of 6 0.29%.46

reliable, small components that use minimal power for placement inside the body. Additionally, this trend has been further bolstered by the demand from the consumer products sector for portable, compact and smarter devices. Thus, many companies have realized the benefits of miniaturization in terms of efficiency, logistics, and material reduction. Owing to this, it is expected that this trend will continue to be a major paradigm shift in the advanced manufacturing sector.47 The rising trend toward product miniaturization coupled with the design engineer’s quest to develop newer components with better performance at low costs is offering lucrative growth opportunities to the growth of MIM technology. As mentioned earlier, MIM offers significant advantages when manufacturing small parts with high density and complex geometry (Fig. 1.6). MIM is capable of producing small parts with comparable properties of wrought or machined metal. Intricate parts with tight tolerances can be produced by means of this process at a lesser cost than machining

Reactive powder metal injection molding

25

or investment casting. These benefits combined with widespread miniaturization of products are fueling the growth of MIM globally.

1.4.2 Advantages over conventional manufacturing techniques MIM has several advantages over conventional manufacturing techniques such as machining, casting, and investment casting, and in some cases over additive manufacturing as well. Machining is a well-established manufacturing technique. Many metals can be machined. However, it is capital and labor-intensive. Furthermore, the lag time associated with the machining operation is a drag on productivity. On the other hand, investment casting can precision manufacture complicated shapes; it is an expensive and laborious process. Press and sinter operations of conventional powder metallurgy are useful in producing simpler shapes and generally result in lower densities and inferior mechanical properties, as compared to MIM. MIM is capable of producing parts that are as dense as machinedwrought metal parts. It provides net- or near-net-shape metal components with complex geometries, high precision, superior mechanical properties, in medium or large production quantities at a very competitive cost (Fig. 1.6). Small parts with tight tolerances can be produced at a lower cost than machining or investment casting. One may argue that additive manufacturing can be an excellent alternative for MIM. However, the current status of additive manufacturing dictates that it is best situated for low production volumes only. Moreover, some alloys are yet to be successfully developed by a viable additive manufacturing process. Nonetheless, MIM is cost-effective in large production volumes only. If the limited volume or customer-specific products are required, then additive manufacturing or investment casting should be preferred.

1.4.3 Demand from the medical sector The ability of MIM to manufacture large volumes of small precision components has resulted in its increased demand in the medical sector in recent decades. From the early use of MIM for the production of endoscopy instruments to a new generation of implants in recent times, the demand for MIM has risen considerably in the medical sector. This increase in demand is due to the flexibility of the MIM process.

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Figure 1.17 Examples of MIM fabricated parts for the medical sector: (A) parts from an orthodontic tooth positioning system manufactured by Flomet LLC, USA; (B) 17-4 PH stainless steel articulation gear manufactured by Parmatech Inc. USA and used in a surgical stapling unit. The part has a density of more than 7.65 g/cm3, an ultimate tensile strength of 900 MPa, yield strength of 730 MPa, and a 25 HRC hardness. This part won an MPIF Grand Prize in 2008. (A) Courtesy MPIF, United States; (B) courtesy MPIF, United States.

Innovations in this process have made it possible to fabricate porous components with desired porosities. Even components having density variations within them are also realizable. Therefore MIM fabrication has evolved as an essential manufacturing technology for the medical sector and is constantly expanding as more innovative research is being carried out in this field. Some of the MIM fabricated components used in medical sector are shown in Fig. 1.17.48

1.4.4 Materials that are hard to process Owing to its high material utilization and near-net-shape manufacturing, MIM is ideal for metals and alloys that are otherwise hard to convert into useful products using conventional fabrication techniques. Titanium is a good example of this kind. The spring-back characteristics of titanium and relatively low thermal conductivity make it difficult to machine. The high processing costs are related to the very high material wastage associated with conventional machined and wrought products, for example, many aerospace applications involve removal of over 90% of the starting material during the machining process. A fly-to-buy ratio describes the relationship of material required to manufacture the finished product. In 2010, the average fly-to-buy ratio for commercial aerospace titanium components was between 1:10 and 1:20.49 In other words, one needs to buy 20 kg of titanium to make a 1 kg flight component (Fig. 1.18). It also means 98% of titanium is wasted during machining. Moreover, titanium scraps are difficult to recycle.50,51 To mitigate these problems, MIM can be effectively applied to produce components of titanium and its alloys.

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Figure 1.18 A comparison of fly-to-buy ratios of conventional machining technology and Ti-MIM. MIM does not only reduce the material wastage, it also allows a reduction of energy usage and therefore reduces the overall CO2 footprint of the finished product.49

Although MIM is increasingly being used to produce metals such as stainless steel parts with complex geometries, MIM of reactive powders such as titanium, aluminum, and magnesium is still not well established. Among these reactive powders, only titanium MIM has some presence in the global market of MIM—approx. 4% 5% of total global sales. On the other hand, aluminum and magnesium MIM are still going through the development process. So far, no standardized process has been adopted in the industry. Hence, there is very little information available regarding the commercial products of Al/Mg-MIM.

1.4.5 Market statistics and research direction In 2018, the market for metal injection molding fabrication was around $3.2 billion and is expected to reach around $4.5 billion in 2023 with a compound annual growth rate (CAGR) of 7.5% during this period—a steady growth (Figs. 1.19 and 1.21). The growth of MIM market is expected to rise owing to the rising demand during the forecast period for small and complex metal parts for industries such as automotive, consumer products, electronics, medical, and defense. The MIM process is now accepted by several sophisticated customers, such as Bosh, Chrysler, Siemens, Honeywell, Mercedes Benz, Volkswagen, BMW, Chanel, Pratt and Whitney, Apple Computer, Samsung, Texas Instruments, General Electric, Nokia, Motorola, Rolls Royce, Continental, Stryker, LG, Sony, Philips, Seagate, Toshiba, Ford, General Motors, IBM, Hewlett-Packard, Seiko, Citizen, Swatch, and similar firms. Most of the

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Figure 1.19 Global market share of different MIM materials in 2018 and projected for 2023. Data were taken from the BCC research report.52

common engineering metals are available in MIM; Fig. 1.20 illustrates the global MIM sales based on different materials.2 The global material sales values are as follows: 53% stainless steels, 27% steels, 10% tungsten alloys, 7% iron nickel alloys (mostly magnetic alloys), 4% titanium alloys, 3% copper, 3% cobalt chromium, 2% tool steels, 2% nickel alloys (superalloys), and 1% electronic alloys (Kovar and Invar). TiMIM sales contribute to only 4% of the total global sales, even though it enjoys a reasonable total MIM market share of B14%. Given the much higher cost of titanium compared to steel, the percentage of titanium MIM in the total global MIM market share would inevitably be high. In reality, the overall share of Ti-MIM in global MIM sales is certainly quite small; the Al and Mg-MIM even do not have any known presence yet. The respective CAGR% during this period (2018 23) for some of these common material systems and titanium are shown in Fig. 1.21, respectively.

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Figure 1.20 Global MIM sales by the main material category. Taken from German, R. M., 1 Metal Powder Injection Molding (MIM): Key Trends and Markets. In Handbook of Metal Injection Molding, 2nd ed.; Heaney, D. F., Ed.; Woodhead Publishing, 2019, pp 1 21.

Figure 1.21 Projected compound annual growth rate (CAGR) of different MIM materials for 5 years between 2018 and 2023.52

MIM of ferrous material has become well established and advanced since the recent developments dating back to the 1990s and hence contributes the most to the global sales of MIM followed by tungsten. On the other hand, Ti-MIM shares a relatively small portion of the global market; its projected CAGR% (8.4) is the highest among all metals MIM. This indicates the growing interest of industry in Ti-MIM, as research in this field is bringing up new and innovative solutions to some of the critical problems hindering success.

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Table 1.6 Ti-MIM market by end-use industry (in millions $). End-use industry

2017

2018

2023

CAGR% 2018 23

Automotive Consumer products Medical and orthodontics Manufacturing Electrical and electronics Firearms and defense Others Total

100.9 80.6 59.7 53.2 45.1 12.2 10.9 362.6

11.3 89.0 65.7 58.9 50.1 13.7 12.0 400.7

164.9 132.6 96.4 89.0 76.5 22.5 18.1 600.0

8.2 8.3 8.0 8.6 8.8 10.4 8.6 8.4

The higher CAGR% in Ti-MIM shows that the interest of industry in this field is growing, largely thanks to continuous research efforts. Two ASTM standards for Ti-MIM53,54 surgical implants have been published, and several Ti-MIM implant components have received approval across the globe, including from the Food and Drug Administration of USA. Based on this, various established medical technology companies have started to develop the next generation of implants using metal injection molding as the preferred fabrication method. In addition, the aerospace automotive industries have also started to recognize the advantages associated with Ti-MIM. Table 1.6 presents the global market for Ti-MIM with respect to end-use industry. The automotive sector was the major end-user industry segment for Ti-MIM in 2017. It was valued at $100.9 million in 2017 and is expected to reach $164.9 million in 2023, growing at a CAGR of 8.2% during the forecast period from 2018 to 2023.52 The other important segment is consumer products, which was valued at $80.6 million in 2017 and is expected to reach $132.6 million by 2023. Apart from these industries, aerospace is considered to be one of the few industries that have recognized the advantages of Ti-MIM. According to a report produced by Industry Arc,55 “There are many metal injection molded titanium parts that are already being used in production quantities in aircraft.” According to the fact sheet released by Boeing, on average, there are 600,000 parts in a Next-Generation 737 airplane.56 Traditionally, aircraft manufacturers have used fabrication processes, including precision machining, plastic molding, and powder metallurgy, to produce these parts, but a comparison unveils several advantages associated with MIM. This method provides an alternative and better processing method for the high production of latches, fasteners, screws, seatbelt components, hinges, bushings, and

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many more components. The parts produced by Ti-MIM are proved to be the imperative possibilities for cost savings, lightweight, enhanced durability, and improved cosmetic appearance. Titanium MIM, which is efficient to produce intricate parts through an unseamed process, is thus gaining attention in the aerospace industry. Moreover, the increasing number of airplanes across the world will foster the growth of the Ti metal injection molding market. According to the International Air Transport Association, the number of air passengers is estimated to reach 8.2 billion by 2037.57 Moreover, to cater to this meteoric rise in aviation activities, the industry requires more planes and aircraft. As per Commercial Market Outlook (CMO) unveiled by Boeing on June 17, 2019, the airline industry will need more than 44,000 new commercial airplanes by 2038.58 Therefore the high surge in demand for new airplanes is considered to be a significant factor in creating growth opportunities to the Ti metal injection molding market. Another sector that is particularly lucrative for Ti-MIM is medical. The medical technology companies are exploiting the Ti-MIM method to manufacture intricate products such as orthodontic brackets, tissue ablation electrodes, drug delivery devices, small surgical instruments, vascular access ports, and various implant parts. In recent years, there has been an increasing trend for smaller, intricate devices for minimally invasive surgeries. Against this background, Ti-MIM has been realized as the optimal technology to produce such parts in large production quantities with minimal cost. The electronics industry is another sector where MIM is being extensively studied for and gradually employed. The miniaturization of electronic devices and the increasing desire for lightweight assemblies call for even smaller parts with better performance at a lower cost. This is where Ti-MIM fits well. MIM is used for the production of thin-wall fiber optic parts, mainframe body, and connectors. With respect to the global market of Ti-MIM by region, Asia-pacific is the largest market and is expected to remain the market leader in the coming years. The titanium material market in Asia-Pacific was valued at $114.1 million in 2017 and is expected to reach $192.7 million by 2023, growing at a CAGR of 8.8% from 2018 to 2023. Table 1.7 presents the Ti-MIM global market structure by region. The dominance of Asia-Pacific is expected to continue. According to a press release by Market Watch,56 “Rapid industrialization in emerging economies such as India, China, and Japan is presumed to fuel market

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Feedstock Technology for Reactive Metal Injection Molding

Table 1.7 Global market of Ti-MIM by region (in millions $). Region

2017

2018

2023

CAGR% 2018 23

Asia-Pacific North America Europe Rest of the world Total

114.1 94.4 97.3 56.8 362.6

126.5 104.1 107.4 62.7 400.7

192.7 154.0 160.2 93.1 600.0

8.8 8.1 8.3 8.2 8.4

development in this region. The growing trend for miniaturization of metal parts across the end-use industries is expected to be the significant factor driving the market growth. The automotive industry in China and India has emerged as a major consumer of titanium injection molded parts. Owing to the fact that high strength and high complexity parts are used in engines, gearboxes, turbochargers, locking mechanisms, steering systems, and electronic systems, there is hefty demand from the automotive industry for precision auto parts with high performance and even higher reliability at a competitive cost, which can be met by MIM. Thus, the burgeoning automotive industry is expected to fuel the growth of the regional market. The demand in APAC automotive industry can be easily fathomed from the bulk production of automobiles in China and India, which were estimated to produce around 33 million motor vehicles cumulatively, in 2018, according to the International Organization of Motor Vehicle Manufacturers (OICA).”

1.4.6 Applications As Al and Mg-MIM are relatively new techniques and still in the design process, particularly Mg-MIM, there are very limited commercial products belonging to Al-MIM to the best of our knowledge. It is fair to assume that most of the Al-MIM commercial products are for heat-sink applications. As mentioned earlier, consumer demand is driving products and technologies toward further miniaturization. Simultaneously, consumers want more capabilities in these smaller, yet more powerful platforms. As the chip technology is becoming increasingly smaller yet powerful, the amount of heat generation has also gone up concurrently. Current processors generate power densities in the order of 10 40 W/cm2. The power density is expected to increase to 20 60 W/cm2 in the near

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future. To remove this heat away from the circuits, passive heat sinks are used. Heat sinks use a mass of thermally conductive material to move heat away from the device and the design typically includes fins or other protrusions to increase the surface area. Common materials to manufacture heat sinks include tungsten-copper, aluminum, and copper composites. However, the current trend of miniaturized products and thereof, miniaturized heat sinks with profound heat dissipation characteristics are putting strain on the manufacturing technologies. New heat-sink materials and designs are needed to cope with these increased demands, and injection molding is potentially a key element of the solution. Recently, Singapore-based Advanced Materials Technologies (AMT)59 has commercialized an Al-MIM process to produce novel heat sink. AMT has manufactured Al heat sinks with tapered fins, a design that aerodynamically maximizes surface area to improve thermal management,60 Fig. 1.22. In another recent example, TU Wien has realized a working strategy for Al-MIM (see Chapter 4: Impurity Management in Reactive Metals Injection Molding and Chapter 5: Potential Feedstock Compositions for MIM of Reactive Metals for more details). They have asserted that by precise control of temperature and atmosphere during the sintering process, dense Al-MIM parts can be manufactured, offering a significant weight reduction (Fig. 1.23).

Figure 1.22 AMT Al-MIM heat sink. This design offers two significant advantages: (A) it can provide significantly improved thermal conductivity compared to aluminum parts manufactured by conventional means such as casting, extrusion, and machining; (B) the thermal properties of Al-MIM parts are isotropic hence, uniform expansion in all directions. Courtesy Lye King Tan, Advanced Materials Technologies Pte Ltd.

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Figure 1.23 An example showing that Al-MIM can be successfully employed for achieving significant weight reductions. Courtesy Dr. Christian Gierl-Mayer, TU Wien.

Figure 1.24 Decorative parts made of Ti-MIM. (A) An eyewear frame part made of pure Ti having a weight of 10 g and tolerance of 6 0.5% or less. The maximum length of the part is B30 mm.39 (B) luxury watch chain parts made by Ti-MIM. (A) Courtesy Taisei Kogyo; (B) Courtesy Prof. Xin Lu, University of Science and Technology Beijing, China.

The application of Ti-MIM is strongly influenced by the contents of impurities in the final products. Currently, depending on powder and process quality, the applications can be classified into three general segments: 1. Decorative, where mechanical and other properties are not demanding, such as in eyewear frame, watch cases, or SIM cardholders in mobile phones. A few examples of Ti-MIM decorative parts are shown in Fig. 1.24. 2. Structural components, where mechanical and corrosion properties are of prime importance (Fig. 1.25).

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Figure 1.25 Ti-MIM structural components. (A) Propeller for radio-controlled air balloon made of Ti 6Al 4V. The thickness of the wing is only 0.5 mm.39 (B) A Ti-MIM tripod base.61 (C) Left: airplane fastener; right: a lever.62 (D) Part of a sleep apnea device made by MIM from Grade 4 Ti feedstock.63 (A) Courtesy Taisei Kogyo.

3. Biomedical applications, where titanium is used to produce biomedical implants/components, Fig. 1.26. MIM for fabricating titanium medical implants is also interesting because of the possibility of generating porous components. Such porous components are beneficial to tissue ingrowth. An example of a Ti-MIM device with the purposely designed porosity is shown in Fig. 1.27.

1.5 Constraints on the reactive powders metal injection molding The small share of titanium in the overall MIM industry is due to many factors. The most important feature that represents the greatest challenge to the success of Ti-MIM is the high affinity to interstitial elements like oxygen, nitrogen, carbon, and hydrogen.66 The fact that titanium is used as a getter material for purification of the atmosphere from oxygen clearly shows the problem. Titanium can absorb 13 wt.% oxygen on interstitial lattice sites at elevated temperatures as per the Ti O phase diagram. The great affinity to interstitial elements is combined with a strong influence on the mechanical properties even at low contents. To better illustrate

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Figure 1.26 Examples of medical implants parts manufactured using the Ti-MIM process: (A) by Element 22 GmbH, Kiel, Germany,64 (B) a titanium heart valve, and (C) a dental implant. Pictures (B) and (C) are taken from Miura, H.; Osada, T.; Itoh, Y. Metal Injection Molding (MIM) Processing. In Advances in Metallic Biomaterials: Processing and Applications; Niinomi, M., Narushima, T., Nakai, M., Eds.; Springer: Berlin, Heidelberg, 2015, pp 27 56.65

Figure 1.27 An example of a Ti-MIM dental implant with a porous region for bone ingrowth.2 Photograph courtesy Eric Baril, National Research Council Canada.

this, Table 1.8 shows the maximum contents of oxygen, nitrogen, carbon, and hydrogen for powder metallurgy titanium and its alloys structural components and their influence on tensile properties according to the ASTM standard B988.67

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Reactive powder metal injection molding

Table 1.8 Maximum levels of interstitial elements and tensile properties according to ASTM B988-18 for a few selected Ti grades.

Ti Grade 1 Ti Grade 2 Ti Grade 3 Ti Grade 4 Ti 6Al 4V Grade 5 Ti 3Al 2.5V Grade 9 Ti 6Al 6V 2Sn

Max. O content (wt.%)

Max. C content (wt.%)

Max. N content (wt.%)

Max. H content (wt.%)

Min. YS (MPa)

Min. UTS (MPa)

Min. εf (%)

0.18 0.25 0.35 0.40 0.30

0.08 0.08 0.08 0.08 0.08

0.03 0.03 0.05 0.05 0.05

0.015 0.015 0.015 0.015 0.015

138 275 380 483 828

240 345 450 550 895

24 20 20 18 10

0.30

0.08

0.03

0.015

483

620

15

0.30

0.1

0.04

0.015

883

958

13

Among the interstitials, oxygen is the most important element because it is picked up preferentially. From Table 1.5, it can be estimated that in pure titanium, an increase of just 0.22 wt.% oxygen doubles the tensile strength. At the same time, ductility is reduced significantly. The ductility reduction is essentially the reason why the pick-up of interstitial elements (particularly oxygen) during MIM processing has to be minimized. It is rather easy to obtain a high strength in a Ti-MIM component, but the provision of good ductility is a serious challenge. The high affinity of titanium to interstitial elements put an immense strain on titanium powder production. The available powders can be divided into two categories: (1) high purity and high cost; (2) low purity and low cost. In this context, the word “purity” relates to the oxygen and carbon content mainly, but also to residuals from the specific production process, for example, Cl, Mg, and Na. In addition, the high purity and expensive powders also differ in geometry: the expensive ones are more spherical, the low-cost powders are typically irregular shaped. Current techniques for the production of spherical, pure powders are inert gas atomization,68 plasma atomization,69 and plasma rotating electrode processing.70 Low-purity and low-cost powders are made by milling of scrap, sponge, or ingots, for example, as done in the hydride dehydride (HDH) process. Choosing the right powder for MIM requires taking into account the desired properties. If the requirements of strength and ductility are not that high, HDH powder may be sufficient. On the contrary, if mechanical

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Feedstock Technology for Reactive Metal Injection Molding

Figure 1.28 Cost of spherical titanium powder over the past 15 years based on our personal observation. The current surge of additive manufacturing technologies has led to many new titanium powder manufacturing companies and new powder processes under development.

strength is the priority, then one should choose spherical powder. It is also possible to mix both spherical and HDH powders to reduce the overall cost while attaining sufficient mechanical properties. Fortunately, the growing interest and applications of additive manufacturing technologies have led to better availability of spherical powders along with decreasing prices (Fig. 1.28). Another reason for the lack of industrial applications of Ti-MIM is the unavailability of a viable binder system. The early choices for Ti-MIM were to use already developed binder systems for other metals PIM, such as stainless steel. Therefore the initial development of a binder for TiMIM was based on the knowledge of binder systems developed for other powders. However, the solubility of impurities such as oxygen, carbon, hydrogen, and nitrogen in titanium at higher temperatures was given little attention. This improper selection of binder system was one of the main reasons that led to poor mechanical properties of initial Ti-MIM practice. Over time, the importance of the proper selection of the binder system for successful Ti-MIM has been realized and has received much attention. The control of impurity contents arising from the binder system is, therefore, considered the most challenging part of Ti-MIM and is one of the biggest hurdles in convincing product designers to use this technology. Nevertheless, there is a large number of possible binder systems that have been used successfully for Ti-MIM (see Chapter 2: Design Strategy of Binder Systems and Feedstock Chemistry for details), and there are also commercial feedstock materials available in the market. As many

Reactive powder metal injection molding

39

practitioners do not adjust the binder recipe for Ti, nor employ MIM equipment specific to Ti, the material properties of the manufactured products do not meet industry expectations. The major challenge in the processing of aluminum or magnesium powders for MIM is the steady surface oxide layer coating with a thickness of B4 10 nm on the powder particles. This oxide layer needs to be reduced to attain good interparticle contact during sintering. Al-MIM has been around for quite a long time, and the most likely application is thermal management devices. Mg-MIM, on the other hand, is relatively new, and very little research has been carried out in this regard. Only a few research groups in the world are actively carrying out research for creating a viable Mg-MIM process strategy. The rapidly growing electronics market has led to an increased demand for advanced heat-sink materials and an innovative way of producing them. Aluminum alloys are the most common heat-sink material because the price of aluminum is about one-third of that of copper. The global trend of miniaturization, as well as potential for higher thermal conductivity and greater flexibility in design over die casting, has led to the exploration of Al-MIM. Nevertheless, Al-MIM has not gained widespread popularity due to resulting lower mechanical properties, difficulty in sintering, and the lack of availability of commercial feedstocks that can be processed easily. According to our personal experience, only one commercial Al feedstock is available in the market by Ryer. Inc,71 despite several patents72 75 published the advantages of Al-MIM parts along with their test results.

References 1. German, R. M. Powder Injection Molding: Design and Applications. Innovative Material Solutions; State College, PA, 2003260. 2. German, R. M. 1 Metal Powder Injection Molding (MIM): Key Trends and Markets. In Handbook of Metal Injection Molding; Heaney, D. F., Ed.; . 2nd ed. Woodhead Publishing, 2019; pp 1 21. 3. Wen, G.; Cao, P.; Gabbitas, B.; Zhang, D.; Edmonds, N. Development and Design of Binder Systems for Titanium Metal Injection Molding: An Overview. Metal. Mater. Trans. A 2012, 44 (3), 1530 1547. 4. Heaney, D. F. 2 Designing for Metal Injection Molding (MIM). In Handbook of Metal Injection Molding; Heaney, D. F., Ed.; . 2nd ed. Woodhead Publishing, 2019; pp 25 43. 5. Tan, L.-K. Characterisation of Powder Injection Molded Aluminum (aluMIM). Adv. Powder Metall Part Mater. 2003, 8, 282 288. 6. Heaney, D. F. 3 Powders for Metal Injection Molding (MIM). In Handbook of Metal Injection Molding; Heaney, D. F., Ed.; . 2nd ed. Woodhead Publishing, 2019; pp 45 56.

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7. German, R. M. Powder Metallurgy and Particulate Materials Processing: The Processes, Materials, Products, Properties, and Applications; Metal Powder Industries Federation: Princeton, 2005. 8. German, R. M. Particle Packing Characteristics, 1989. 9. Nyborg, L.; Carlström, E.; Warren, A.; Bertilsson, H. Guide to Injection Moulding of Ceramics and Hardmetals: Special Consideration of Fine Powder. Powder Metal. 1998, 41 (1), 41 45. 10. German, R. M. Powder Injection Molding; Metal Powder Industries Federation: Princeton, NJ, 1990p xii, 521 p. 11. González-Gutiérrez, J.; Stringari, G. B.; Emri, I. Powder Injection Molding of Metal and Ceramic Parts; INTECH Open Access Publisher, 2012. 12. https://bellaform.com/en/shear-roll-mixing-systems-for-cim-and-mim/lab-shear-rollerbsw-100.html. 13. https://www.thermofisher.com/order/catalog/product/10134964#/10134964. 14. https://www.thermofisher.com/order/catalog/product/554-2110#/554-2110. 15. https://techminy.com/common-injection-molded-plastics/reciprocating-screw-injection-molding-machines/. 16. Hayat, M. Development of PEG Based Binders for Metal Injection Moulding with Special Focus on Titanium; ResearchSpace@ Auckland, 2015. 17. Banerjee, S.; Joens, C. J. 7 Debinding and Sintering of Metal Injection Molding (MIM) components. In Handbook of Metal Injection Molding; Heaney, D. F., Ed.; . 2nd ed. Woodhead Publishing, 2019; pp 129 171. 18. http://www.appliedseparations.com/. 19. Kim, Y.-H.; Lee, Y.-W.; Park, J.-K.; Lee, C.-H.; Lim, J. S. Supercritical Carbon Dioxide Debinding in Metal Injection Molding (MIM) Process. Korean J. Chem. Eng. 2002, 19 (6), 986 991. 20. Williams, B. WORLDPM2018 Congress: Global MIM Markets Show Healthy Growth PIM International, 2018; pp 67 75. 21. https://www.parmaco.com/en/engineering-and-design/material-development.html. 22. https://www.mimplus.com/advantages-of-mim. 23. Ye, H.; Liu, X. Y.; Hong, H. Fabrication of Metal Matrix Composites by Metal Injection Molding—A Review. J. Mater. Process. Technol. 2008, 200 (1), 12 24. 24. Decker, R. Net Shape Metals and MMCs Produced by ThixomoldingTM. Mater. Process. Report 1989, 4 (9), 1 2. 25. New Generation Powders Target High Alloy PM Parts. Metal Powder Report 2006, 61 (2), 17 18. 26. Thian, E. S.; Loh, N. H.; Khor, K. A.; Tor, S. B. Effects of Debinding Parameters on Powder Injection Molded Ti-6Al-4V/HA Composite Parts. Adv. Powder Technol. 2001, 12 (3), 361 370. 27. Thian, E. S.; Loh, N. H.; Khor, K. A.; Tor, S. B. Ti-6A1-4V/HA Composite Feedstock for Injection Molding. Mater. Lett. 2002, 56 (4), 522 532. 28. Thian, E. S.; Loh, N. H.; Khor, K. A.; Tor, S. B. In Vitro Behavior of Sintered Powder Injection Molded Ti-6Al-4V/HA. J. Biomed. Mater. Res. 2002, 63 (2), 79 87. 29. Salleh, F. M.; Sulong, A. B.; Muhamad, N.; Mohamed, I. F.; Mas’ood, N. N.; Ukwueze, B. E. Co-Powder Injection Moulding (Co-PIM) Processing of Titanium Alloy (Ti-6Al-4V) and Hydroxyapatite (HA). Procedia Eng. 2017, 184, 334 343. 30. Egorov, A. A.; Smirnov, V. V.; Shvorneva, L. I.; Kutsev, S. V.; Barinov, S. M. HighTemperature Hydroxyapatite-Titanium Interaction. Inorg. Mater. 2010, 46 (2), 168 171. 31. Abdoos, H.; Khorsand, H.; Yousefi, A. Nano-particles in Powder Injection Molding of an Aluminum Matrix Composite: Rheological Behavior, Production and Properties. Int. J. Mater. Res. 2017, 108.

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32. Sezer, N.; Evis, Z.; Kayhan, S. M.; Tahmasebifar, A.; Koç, M. Review of Magnesium-Based Biomaterials and Their Applications. J. Magnes. Alloy 2018, 6 (1), 23 43. 33. Piotter, V. 13 Micro Metal Injection Molding (MicroMIM). In Handbook of Metal Injection Molding; Heaney, D. F., Ed.; Woodhead Publishing, 2012; pp 307 337. 34. Petzoldt, F. Micro Powder Injection Moulding-Challenges and Opportunities. Powder Inject. Mould. Int. 2008, 2 (1), 37 42. 35. German, R. M. Materials For MicrominiaTure Powder Injection Molded Medical and Dental Devices. Int. J. Powder Metal. 2010, 46 (2).. 36. Choi, J.-P.; Park, J.-S.; Hong, E.-J.; Lee, W.-S.; Lee, J.-S. Analysis of the Rheological Behavior of Fe Trimodal Micro-Nano Powder Feedstock in Micro Powder Injection Molding. Powder Technol. 2017, 319, 253 260. 37. Piotter, V. 15 Micro Metal Injection Molding (MicroMIM). In Handbook of Metal Injection Molding; Heaney, D. F., Ed.; . 2nd ed. Woodhead Publishing, 2019; pp 333 360. 38. https://www.wittmann-group.com/injection-molding/micro-injection-molding.html. 39. https://www.taisei-kogyo.com/en/facility/enmicromimtech.php. 40. Pischang, K.; Birth, U.; Gutjahr, M. In Investigation on P/M-composite Materials for Metal Injection Molding, International conference and exhibition on powder metallurgy and particulate materials, Toronto, ON, Canada; Lall, C., Ed.; Metal Powder Industries Federation: Toronto, ON, Canada, 1994; pp 273 284. 41. Suri, P. 16 Two-material/Two-color Powder Metal Injection Molding (2C-PIM). In Handbook of Metal Injection Molding; Heaney, D. F., Ed.; . 2nd ed. Woodhead Publishing, 2019; pp 361 370. 42. Heaney, D. F.; Suri, P.; German, R. M. Defect-free Sintering of Two Material Powder Injection Molded Components Part I Experimental Investigations. J. Mater. Sci. 2003, 38 (24), 4869 4874. 43. https://www.pim-international.com/metal-injection-moulding-in-the-5g-era-opportunities-for-growth/. 44. http://www.smithmetals.com/MIM-Medical_The-Future-Is-Now.html. 45. Williams, N. MIM2011 Conference: Industry Remains Confident as Markets and Regions Continue to Evolve. PIM Int. 2011, 44 45. 46. Micro MIM Approaches Mass Production. Metal Powder Report 2005, 60 (12), 16 20. 47. Global Powder Injection Molding Market Outlook 2019 2024 Rising Demand from the Automotive Industry and Growing Inclination Towards Miniaturized Complex Components. GlobeNewswire 2019/04/26/, 2019. 48. https://www.pim-international.com/metal-injection-molding/applications-for-mimi-medical-and-orthodontic/. 49. Scharvogel, M. Titanium Metal Injection Molding A Commercial Overview. Key Eng. Mater. 2016, 704, 107 112. 50. Froes, F. H.; Imam, M. A. Cost Affordable Developments in Titanium Technology and Applications. Key Eng. Mater. 2010, 436, 1 11. 51. Whittaker, D. Developments in the Powder Injection Moulding of Titanium PIM International 2007, 27 32. 52. Sengupta, A. Metal Injection Molding Fabrication: Global Markets to 2023; BCC Research LLC, 2019. 53. ASTM, Standard Specification for Metal Injection Molded Unalloyed Titanium Components for Surgical Implant Applications. In ASTM F2989-13; ASTM International: West Conshohocken, PA, 2013. 54. ASTM, Standard Specification for Metal Injection Molded Titanium-6Aluminum4Vanadium Components for Surgical Implant Applications. In ASTM F2885-17; ASTM International: West Conshohocken, PA, 2017.

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55. Titanium Metal Injection Molding Market Forecast (2019 2024), 2019. 56. https://www.marketwatch.com/press-release/titanium-metal-injection-molding-market-estimated-to-grow-at-a-cagr-of-74-through-to-2025-2019-09-06, Titanium Metal Injection Molding Market estimated to grow at a CAGR of 7.4% through to 2025, 2019. 57. https://www.iata.org/en/pressroom/pr/2018-10-24-02/. 58. https://www.cnbc.com/2019/06/17/boeing-raises-outlook-for-airplane-demandwhile-airbus-rolls-out-new-model.html. 59. http://www.amt-mat.com/guides/. 60. http://exclusive.multibriefs.com/content/advanced-mim-electronic-materials-andprocessing/engineering. 61. German, R. M. 20 Powder-Processing Linkages to Properties for Complex Titanium Shapes by Injection Molding. In Titanium Powder Metallurgy; Qian, M., Froes, F. H., Eds.; Butterworth-Heinemann: Boston, 2015; pp 361 382. 62. Ebel, T.; Friederici, V.; Imgrund, P.; Hartwig, T. 19 Metal Injection Molding of Titanium. In Titanium Powder Metallurgy; Qian, M., Froes, F. H., Eds.; ButterworthHeinemann: Boston, 2015; pp 337 360. 63. Whittaker, D.; Froes, F. H. 30 Future Prospects for Titanium Powder Metallurgy Markets. In Titanium Powder Metallurgy; Qian, M., Froes, F. H., Eds.; ButterworthHeinemann: Boston, 2015; pp 579 600. 64. http://www.element22.de/. 65. Miura, H.; Osada, T.; Itoh, Y. Metal Injection Molding (MIM) Processing. In Advances in Metallic Biomaterials: Processing and Applications; Niinomi, M., Narushima, T., Nakai, M., Eds.; Springer Berlin Heidelberg: Berlin, Heidelberg, 2015; pp 27 56. 66. Ebel, T. 19 Metal injection molding (MIM) of Titanium and Titanium Alloys. In Handbook of Metal Injection Molding; Heaney, D. F., Ed.; . 2nd ed. Woodhead Publishing, 2019; pp 431 460. 67. ASTM, Standard Specification for Powder Metallurgy (PM) Titanium and Titanium Alloy Structural Components, Vol. B988-18; ASTM: US, 2018; p 4. 68. Hohmann, M.; Jönsson, S. Modern Systems for Production of High Quality Metal Alloy Powder. Vacuum 1990, 41 (7), 2173 2176. 69. Entezarian, M.; Allaire, F.; Tsantrizos, P.; Drew, R. A. L. Plasma Atomization: A New Process for the Production of Fine, Spherical Powders. JOM 1996, 48 (6), 53 55. 70. Aller, A. J.; Losada, A. Rotating Atomization Processes of Reactive and Refractory Alloys. Metal Powder Report 1990, 45 (1), 51 55. 71. http://www.ryerinc.com/SolvMIM.html. 72. Liu, Z.; Sercombe, T. B.; Schaffer, G. B. Metal Injection Moulding Method, US20100183471A1, 2007. 73. Liu, Z.; Sercombe, T. B.; Schaffer, G. B. Metal Injection Moulding Method, 2010. 74. Danninger, H.; Gierl, C.; Zlatkov, B.; Ter, M. J. Method for Producing Moldings from Aluminum Alloys, 2010. 75. JP2013524006A Method for Producing Molded Product of Aluminum Alloy, 2010.

CHAPTER 2

Design strategy of binder systems and feedstock chemistry 2.1 The role of binder The binder is a temporary vehicle that helps powder particles to flow into the mold and then holds the powder particles until the early stage of sintering. The properties of binders thus have a significant effect on the overall performance of the feedstocks including metal particle distribution, shaping process, shape retention of the molded products, and the final properties of the sintered products. Consequently, the binder or binder system usually has three components: a low molecular weight component that provides the necessary flowability during molding, a backbone polymer that provides the green strength, and a surfactant that acts as a bridge between the binder and powder particles. The important characteristics required in binders are summarized in Table 2.1. Some of these requirements are even more stringent for reactive powders metal injection molding (MIM). Perhaps, the most sought-after characteristics of the binder system in the current world scenario is their environment-friendly nature followed by cost and availability. Good mixing and homogeneity of feedstocks depend on the adhesion of the binder system with metal powders. For this reason, the binder system should have a low contact angle with metal particles. The low contact angle results in better spreading of binder on powder particles assisting mixing and molding processes. The binder system and the metal powder particles should be inert with respect to each other. In other words, at mixing or injection temperatures any component of the binder system should not react with particles and simultaneously, powder particles should not act as a catalyst for polymerization or degradation of the binder system. To achieve defect-free molding operation, the feedstock should possess certain rheological traits. The viscosity of the feedstock should be in an ideal range. Too low viscosity can cause issues such as flashing, binderpowder separation during the molding process. On the other hand, too high viscosity will impair the mixing and molding operation. Similarly, Feedstock Technology for Reactive Metal Injection Molding DOI: https://doi.org/10.1016/B978-0-12-817501-9.00002-8

© 2020 Elsevier Inc. All rights reserved.

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Feedstock Technology for Reactive Metal Injection Molding

Table 2.1 Basic requirements for binders.

Commercial point of view

Interactions with metal powders

Flow characteristics

Material’s properties

Debinding characteristics

• • • • • • • • • • • • • • • • • • • • • •

Low cost Easily available Easy to manufacture Long shelf life Safe and environmentally acceptable Good wetting with powder (low contact angle) Good adhesion with powder Chemically passive with respect to powder Capillary attraction of powder particles Low viscosity and high flowability at molding temperatures Less variations in viscosity with temperature change Shear sensitive flow behavior (pseudoplastic behavior) High strength and stiffness Low thermal expansion coefficient High thermal conductivity Not degraded due to cyclic heating Isotropic properties Soluble in common solvents Degradation temperature should be above molding and mixing temperatures Progressive decomposition temperatures Noncorrosive and nontoxic decomposed products Low residual char after burnout

the flow behavior of feedstock is critical to a successful molding operation. The feedstock should possess pseudoplastic behavior (i.e., a decreased viscosity with increasing shear rates), which ensures a uniform filling of the mold cavity and shape retention of the product. Ideally, the binder should be easy to remove at an adequate rate without creating defects in injection molded parts. The green part is most susceptible to the formation of defects during the debinding stage. Poor backbone polymer strength leads to poor shape retention during the initial stage of debinding. On the other hand, low interconnected open porosity during thermal debinding results in the formation of defects such as cracking and blistering—further highlighting the importance of a multicomponent binder system. Similarly, the binder should completely burn out during the debinding stage without leaving any residues. The decomposed products (usually small molecules) should be noncorrosive to the equipment and nontoxic for the environment.

Design strategy of binder systems and feedstock chemistry

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Finally, the binder system must be cost-effective, easily available, and should have good recyclability. This is because the recycling of feedstock is commonly practiced in the industry. It is impractical for a single binder to fulfill all the above characteristics to achieve an excellent feedstock. Therefore a typical binder system for a metal injection molding feedstock contains 3
4 components, each specifically chosen for a specialized job. The contamination issue for reactive powders is the most challenging for reactive powders MIM and is always the greatest hurdle in convincing the product designers to use this technology.

2.2 Basics of binder 2.2.1 Binder chemistry Before going into binder chemistry, some basic polymer chemistry must be understood. The two common types of polymers are thermoplastic and thermosetting. The thermoplastic polymer is a long molecule consisting of many small monomers, joined end to end (Fig. 2.1A). Polyethylene (PE), polypropylene (PP), polymethyl methacrylate (PMMA), and waxes are some common examples of thermoplastic polymers. Upon heating, thermoplastic polymers flow in the manner of a highly viscous liquid and do so reversibly repeatedly on subsequent heating and cooling. Two molecular factors govern the mechanical properties of a thermoplastic polymer: (1) molecular weight, which depends on the length of the molecule, and (2) shape of the molecule, that is, linear or branched. The number of side branches may be varied by changing the polymerization conditions. For example, the first polymerization of polyethylene produced molecules with small side branches (Fig. 2.1A). Even small variations in the number of side branches can be of technological significance and cause substantial changes in elastic modulus, creep resistance and toughness. Although different in properties, the linear and side-branched polymers are identical in one respect; they may be reversibly heated to melt and then cooled to crystallize repeatedly. Thermoplastics can be amorphous, crystalline, or a combination of both. The amorphous and crystalline polymers show different behaviors on exposure to high temperatures. The crystalline polymers have a distinct melting point Tm, for example, polyethylene glycol (PEG) can have a melting point anywhere between 0°C and 60°C depending on molecular weight. When a crystalline polymer melt is cooled below Tm, crystallization is initiated at nuclei at different points in the melts. From these

46

Feedstock Technology for Reactive Metal Injection Molding

Figure 2.1 (A) Side-branched polyethylene—an example of thermoplastics. Thermoplastic polymers can either have linear or branched chains. (B) Molecular structure of cross-linked polyethylene in the liquid state. The spaces between the cross-linked net are filled with other parts of the network.

nuclei, crystallization proceeds by the growth of the spherulites (a complex ordered aggregation of the submicroscopic crystals). This stage of crystallization is called primary crystallization and is completed when the spherulites completely effectively fill the space. This is accompanied by a sharp decrease in specific volume (Fig. 2.2). For a crystalline polymer, the modulus, strength, and most other mechanical properties critically depend on the degree of crystallinity. The melting point of a crystalline polymer strongly depends on the molecular weight. Generally speaking, for molecular weight below 103, there is only one melting point that decreases with decreasing chain length. Above the molecular weight of 103, the sharp melting point broadens into a melting range. However, in reality, most engineering polymers are either amorphous or are semicrystalline

Design strategy of binder systems and feedstock chemistry

47

Figure 2.2 Change in specific volume with respect to temperature is shown for polymeric materials. For polymers with lower molecular weight, Tg marks the transition from glass to liquid; for higher molecular weight polymers it marks the transition from glass to rubber.

with the degree of crystallinity anywhere between B10% and 80%. Some low molecular weight polymers though can be fully crystalline (with a degree of crystallinity above 90%) for example, PEG, polyoxyethylene, and polyoxymethylene (POM). Compared to crystalline polymers the amorphous polymers have longer chains and are thus more ductile in nature. The amorphous polymers do not crystallize even when cooled from the melt extremely slowly. Hence, amorphous polymers do not possess a melting point. Instead, the main effect of cooling on the melt is the reduction in thermal agitation of the molecular segments. If the cooling is continued a temperature is reached at which the rate of movement is extremely sluggish and then on further cooling stops altogether. At this stage, the polymer consists of long molecules tangled in a liquid-like manner, but with a complete absence of the rapid molecular motion. This state is called the glassy state and the temperature at which this transition occurs is called glass transition temperature Tg. The Tg is usually obtained from a volume
temperature observation during cooling. At Tg, modulus drops significantly (in the order of 103). For this reason, Tg is sometimes vaguely (incorrectly) referred to as the softening point. In the case of a semicrystalline polymer,

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Feedstock Technology for Reactive Metal Injection Molding

the amorphous fraction exhibits a glass transition. The ratio of the glass transition temperature to the melting point is recognized as1 Tg 5 0:6 Tm

(2.1)

With respect to binder systems for MIM and thereof for reactive powders MIM, the two critical properties of polymers are molecular weight and crystallinity. Polymers with a high degree of crystallinity undergo rapid volume changes upon cooling (Fig. 2.2). This can cause shrinkage related defects during the cooling sequence of the injection molding process. Various properties such as glass transition temperature, melting point, viscosity and tensile strength of the polymers depend on the molecular weight (Fig. 2.3). For the primary component of the binder systems, generally small chain polymers are preferred to provide sufficient flowability. On the contrary, medium-to-high chain polymers (depending on commercial availability) are favored for achieving necessary green strength during the injection molding process. Thermosets are those polymers whose precursors, when heated to an appropriate temperature for a short time, undergo chemical cross-linking and form an infusible mass. The precursors can be of low molecular

Figure 2.3 Properties dependence on molecular weight for a typical thermoplastic polymer.

Design strategy of binder systems and feedstock chemistry

49

weight. The chains once cross-linked form one giant molecular network (Fig. 2.1B), which cannot be reversed on reheating. In the liquid state, the cross-linking inhibits the flow, causing polymers to remain stable and possess properties typical of rubbers. Once a polymer specimen is cross-linked its shape is fixed. It can be forced into a different shape under heating, which can be then frozen in by cooling to a sufficiently low temperature. However, it will revert to the original shape it had when it was cross-linked as soon as it is reheated. There are many ways of preparing a cross-linked network. For example, for polyethylene, the simplest way is to irradiate the molten polymer with ionizing radiation, such as β or γ-rays. However, many cross-linked networks are produced by chemical reactions triggered by heating. After heating, the network/sample shape is set, thus the origin of the term “thermoset.” The thermoset polymer directly vaporizes on heating without melting or softening. As the thermoset polymers do not undergo softening/melting like thermoplastics, the shape loss during debinding cycle thus can be avoided using a thermoset polymer. Thermoset polymers usually result in higher green strength of injection molded samples as compared to thermoplastics. However, it is extremely difficult to completely burn out thermosetting polymers. Therefore the use of thermoset polymers in MIM is very rare2 and is not used for reactive powders MIM.

2.2.2 Classifications of binder system Logically, the development of a binder for reactive powders MIM draws on the knowledge about the binders developed for other common materials, for example, stainless steel. The early choices for reactive powders MIM were to use the existing binder systems. Since then, a wide range of binders is tailored/developed for reactive powders MIM. Nevertheless, every binder system contains multiple components as mentioned earlier. The binder systems for reactive powders MIM can be classified into three major groups based on their primary components and their subsequent depending behavior. 2.2.2.1 Wax-based binder system Wax-based binder systems are perhaps the most commonly used binder system in MIM and hence, are instinctively adopted for reactive powders MIM. Waxes are a class of chemicals which are plastic near ambient temperatures. Some of the common waxes include paraffin wax (PW),

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Feedstock Technology for Reactive Metal Injection Molding

beeswax, carnauba wax (CW), and several wax-like short oligomers or low molecular-weight analogs of polymers such as PE or PP. However, PE and PP waxes are rarely used in the MIM industry due to their complicated production process. On the contrary, PW is a widely accepted choice. The PWs are hydrocarbonaceous mixtures of alkanes (straight carbon chains), having chain length ranging from C20 to C40 and melting point ranging from 52°C to 74°C. They have consistent chemical structures and thus consistent rheological properties, which are important for consistency and reproducibility of the product quality.3 In addition, PWs are cost-effective compared to other waxes. In a wax-based binder system, the popular backbone polymers are industry-grade PE, PP, ethylene-vinyl acetate (EVA) and PMMA. Early attempts on reactive powder MIM utilized this abundant literature on wax-based binder systems. In the first published study by Kaneko et al.,4 wax-PBMA [poly(butyl methacrylate)]-EVA-DBP (dibutyl phthalate) was used as the binder for titanium MIM (Ti-MIM). Although the initial trials did not produce the desired results (Fig. 2.4), it provided the necessary platform for future research in wax-based binder systems for reactive powders MIM. The early research on the wax-based binder system for Ti-MIM was focused on simple one-step thermal pyrolysis only, which explains the initial poor performance of titanium parts due to incomplete burnout resulting in severe contamination. Soon the problem was identified and a two-step debinding process—solvent extraction of wax component followed by thermolysis—was designed. This significantly reduces contamination after sintering. The goal of solvent extraction is to create interconnected pores throughout the molded body so that the backbone component can be removed through these pores without distorting the shape when the second debinding is applied. The solvent extraction of wax component is based on its high solubility in organic solvents; the most common solvents for wax extraction are hexane and heptane. The process is driven by capillary action and concentration gradient across the solvent bath. Solvent debinding can be divided into three phases: Phase I: The process starts from the surface of the specimen. During this phase, the debinding rate is quite high as a binder is directly in contact with the solvent. Phase II: The rate of debinding decelerates. The solvent penetrates the molded body by capillary action. It extracts and dissolves the wax molecules, which are then transported into the surrounding solvent

Design strategy of binder systems and feedstock chemistry

51

Figure 2.4 Compression test results of early Ti-MIM study carried out by Kaneko et al. in Japan. The sintered specimen showed higher compression strength but exhibited brittle fracture. The reduced ductility was attributed to the high impurity levels in the samples. Taken from Wen, G.; Cao, P.; Gabbitas, B.; Zhang, D.; Edmonds, N. Development and Design of Binder Systems for Titanium Metal Injection Molding: An Overview. Metall. Mater. Trans. A 2012, 44 (3), 1530
1547, with permission.

bath due to the concentration difference between the molded part and the solvent bath. Phase III: The debinding rate gradually levels out as concentration equilibrium is achieved. The three phases are pictorially illustrated in Fig. 2.5. The debinding rate depends on several processing factors. The time t for debinding depends on the section thickness H and absolute temperature T6:
   Vb 2 t 5 H =β ln exp Q=kT (2.2) 12[ where ð1 2 [Þ is the starting binder volume, Vb is the fraction of binder to be removed, β depends on the binder solubility in the solvent, Q is the activation energy associated with binder
solvent interactions and k is the Boltzmann’s constant. Temperature plays a critical role in solvent

52

Feedstock Technology for Reactive Metal Injection Molding

Figure 2.5 Different stages of solvent debinding illustrated. Redrawn from Dandang, N. A. N.; Harun, W. S. W.; Khalil, N. Z.; Ahmad, A. H.; Romlay, F. R. M.; Johari, N. A. Paraffin Wax Removal From Metal Injection Moulded CoCrMo Alloy Compact by Solvent Bebinding Process. IOP Conf. Series: Mater. Sci. Eng. 2017, 257 (1), 012020 [5].

debinding. If the temperature of solvent debinding exceeds the melting/ softening point of the soluble binder component, MIM compacts will start to collapse. Similarly, too quick solvent debinding can lead to cracks and blisters. In addition, complete removal of the soluble binder component is not possible. However, above 90% removal rate is generally accepted. Moreover, since the main component is extracted at room temperature to low temperature, no to little contamination results from solvent debinding. During the solvent debinding, the backbone polymer (secondary component) provides the necessary strength. Hence, while designing the solvent debinding process, the chemistry of the backbone polymer should also be considered. Solvent debinding is followed by thermolysis of backbone polymer and exploits thermal degradation of thermoplastics, which is based on the successive dissociation of polymers to produce light molecules that are later carried out of the surface of the molded part either by a sweeping inert gas such as Ar or by a vacuum pump. As the thermal degradation process is different for different polymers, the thermal debinding parameters are set accordingly. During thermal debinding, low heating rates are applied to give ample time for the vaporized product to dissipate. Increasing temperature too fast may produce an excessive increase of vapor pressure inside molded

Design strategy of binder systems and feedstock chemistry

53

parts leading to defects. The low heating rates generally result in a long debinding cycle (commonly between 10 and 60 h). 2.2.2.2 Polyoxymethylene-based binder system POM-based binder system was first disclosed by a US company Celanese Corp is 1984 and are practically developed and distributed by BASF. As the name suggests, POM is the major component in the binder system, while PE, PP, and PMMA are generally added as backbone components. POM is characterized by its high strength, hardness, and rigidity. Another important characteristic of POM is its strong sensitivity to acid hydrolysis by acidic agents. This allows POM in the feedstock to be removed by treating the green parts in a gaseous acid-containing atmosphere such as nitric acid or oxalic acid at a temperature well below its softening temperature (Fig. 2.6). As the debinding can be carried out in the solid state, the cracks and bloating caused by the boiling of the binder are avoidable. The gaseous catalyst does not penetrate the polymer; instead, the decomposition proceeds only at the gas/binder interface (Fig. 2.7). The planar debinding interface advances inward at a linear speed depending on the processing conditions. The small formaldehyde gas molecules can escape easily through the already porous outer zone of the part without disrupting the powder particles packing structure. This allows a faster debinding rate compared to other debinding techniques and permits easy and uniform debinding of thick sections. In addition, due to faster debinding rates, it is claimed that this binder system may lead to lower contaminations in the final sintered part. Nevertheless, due to the health concerns of formaldehyde, there are some limitations with regard to the use of catalysts and their contents. This problem can be averted by using a two-step burning process during

Figure 2.6 Catalytic debinding of POM in the presence of nitric acid produces toxic bi-products: nitrogen dioxide and formaldehyde, which must be treated before releasing into the atmosphere. Taken from Gonzalez-Gutierrez, J.; Stringari, G.; Emri, I. Powder Injection Molding of Metal and Ceramic Parts, 2012 [7], with permission.

54

Feedstock Technology for Reactive Metal Injection Molding

Figure 2.7 Illustration of catalytic debinding mechanism.

the debinding stage, which requires significant investment. This put additional pressure on already expensive reactive powders MIM. POM-based binder usually contains a secondary thermoplastic backbone component. Therefore a thermal debinding step is still required before sintering. 2.2.2.3 Water-based binder system The involvement of organic solvents such as heptane and hexane for the removal of the wax component makes wax-based binder systems less attractive due to toxicologic and environmental concerns. This has led to the development of new environmentally-friendly binders that can be extracted by an environmentally friendly solvent such as ethanol and water. Water-soluble binders have received increasing interest in recent years. Water-soluble binders can be divided into two subgroups: gelationbased and nongelation-based, depending on the interaction of the binder component and water. The gelation-based systems are principally the same as wax-based binder systems in terms of feedstocks formulations and debinding approach with the only difference that water is used as the medium for extracting the soluble components in the binder system. In this category, PEG is the most commonly used binder component because of its commercial availability and environment-friendly nature. PEG is

Design strategy of binder systems and feedstock chemistry

55

soluble in water even at low temperatures, nontoxic, and has a simple structure—(CH2CH2O)n. Depending on the molecular weight, PEG can be either liquid or low-melting-point solid at room temperature. This signifies that PEGs are very similar to wax. In PEG-based binders, currently employed backbone polymers are PMMA, PVB, EVA, and HDPE. Among these, the PEG/PMMA system has found widespread recognition and numerous reports have been published in the literature on this system. The main reason for this increased interest is mainly due to the thermal decomposition behavior of PMMA. PMMA thermal decomposition can produce more than 90% of pure gaseous methyl methacrylate monomer depending on molecular weight and leaves very little residue behind in both vacuum and inert atmospheres. Although PEG-based binders can be a good choice for reactive powders MIM, they are not free of problems mainly due to PEG crystallinity. PEG-based binders undergo considerable volumetric shrinkage upon cooling. This can lead to defects such as voids and dimensional variations (Fig. 2.8). The shrinkage defects caused by PEG crystallinity can be minimized or even eliminated using a crystallization inhibitor such as polyvinylpyrrolidone (PVP) during feedstock formulations.9 However, this adds additional costs at the expense of productivity. On the contrary, the gelation-based binders involve solvents when formulating the feedstock: water being the most common one. The binder interacts with water molecules to form a gel, which can occur either before or during feedstock formulation. Occasionally, feedstock compounding and injection molding are combined in one step. After injection molding, the green parts are dried to remove the water. Subsequently, the remaining polymers and other additives are removed by thermal debinding. Most common gelation-based binders are natural polymers including polysaccharides (cellulose, starch, and agar) and polyamino acids (proteins). Among these, agar and its derivatives have received much attention for reactive powders MIM. Agar is a polymer made up of subunits of the sugar galactose; it is a component of the algae’s cell walls. When dissolved in hot water and cooled, agar becomes gelatinous. Given the gel strength is critical for the feedstock preparation, the optimum molecular weight of agar ranges from 30,000 to 150,000 g/mol. However, agar with a too high molecular weight is difficult to decompose completely by heating, resulting in higher levels of impurities (Fig. 2.9) in the sintered reactive powders MIM parts.

56

Feedstock Technology for Reactive Metal Injection Molding

Figure 2.8 PEG-based binders can lead to defects such as voids in injection molded parts due to uncontrolled volumetric shrinkage upon cooling. Taken from Hayat, M. D.; Li, T.; Wen, G.; Cao, P. Suitability of PEG/PMMA-Based Metal Injection Moulding Feedstock: An Experimental Study. Int. J. Adv. Manuf. Technol. 2015, 80 (9), 1665
1671 [8], with permission.

The increased amount of left-over residue in the case of gelationbased binder systems is the main reason for the lack of industrial confidence in adopting this system for reactive powders MIM. Another disadvantage associated with this binder system is the lack of dimensional control of the final parts.3 As water accounts for a substantial portion in this system, evaporation of water during mixing and molding can significantly change the composition of the feedstock and green parts. There is a high risk of part distortion and contaminations when only thermal debinding is used, as was the case in early attempts of Ti-MIM.3 However, when used in conjunction with either solvent or catalytic debinding, the risk of distortion is greatly reduced since open pores created during initial debinding act as a conduit to degraded products. Table 2.2 summarizes the three types of debinding techniques. Arguably, the best option for reactive powder MIM is to employ a two-stage debinding strategy—solvent/catalytic/wicking removal of primary component followed by thermal debinding to remove the remaining binder. Apart from these conventional binder systems, other systems have also been tested and tried for reactive powders MIM. In recent

Design strategy of binder systems and feedstock chemistry

57

Figure 2.9 Relationship between the average molecular weight of agar and carbon residue in titanium parts. Redrawn from Suzuki, K.; Fukushima, T. Binder for Injection Molding of Metal Powder or Ceramic Powder and Molding Composition and Molding Method Wherein the Same Is Used. US6171360B1, 2001 [10].

times, an aromatic-based binder system has been used for Ti-MIM.11 The aromatic compound can be used both as binder and solvent, and therefore, only a small fraction of traditional binder materials is required as additive. One such binder system consists of naphthalene, 1 vol.% stearic acid, and 3
12 vol.% EVA. Naphthalene then can be subsequently removed via sublimation at 80°C under a vacuum of 2.67 Pa. Since sublimation involves low surface energies in the vaporization process, the specimen volume remains constant throughout the debinding process. This means that common debinding problems such as part distortion and cracking can be avoided. Additionally, the use of lower volume percent of backbone polymer reduces or mitigates the contamination issue for reactive powders. However, the health and environmental concerns associated with the toxic aromatic compounds are a big hurdle in adopting this binder system on an industrial scale. Table 2.3 summarizes some commercial and lab-scale binder formulations that has been utilized/studied for reactive powders MIM.

58

Feedstock Technology for Reactive Metal Injection Molding

Table 2.2 Comparison of three primary debinding techniques. Debinding technique

Key features

Advantages

Disadvantages

Thermal

Slow heating of molded parts in either sweeping inert gas or high-vacuum to melting/ degradation temperature of binder system

One-step process, no need to handle product between debinding and sintering. Applicable to wide range of binders

Solvent 1 thermal

Molded parts are placed in a solvent to extract main binder component via dissolution followed by thermolysis for complete removal

Lower contents of impurities, as the major component is removed at lower temperature. Lower defects and distortions of parts

Catalytic 1 thermal

Heating of green part in atmosphere containing catalyst to depolymerize main binder component and sweeping away monomers followed by thermal debinding to remove backbone component

Rapid process that works well on thick and thin sections with excellent shape retention. Lower contents of impurities

High levels of impurities, poor dimensional control, requires extensive study and control of processing parameters over a wide temperature range for multicomponent system, relatively slow process Solvent hazard, chemical handling, and environmental concern (unless water soluble binder is used). Drying before thermal debinding is usually required. Still a lengthy process—can take up to 40 h Highly toxic decomposition products. Requires sophisticated equipment for byproducts treatment before release

Table 2.3 Case study: compilation of the binders that are used in reactive powders MIM. Base metal

Binder

Solids loading

Debinding

Sintering

Impurity (wt.%)

Density (%)

Sintered property UTS/YS (MPa); El (%)

References

Al alloy 6061 and 2 wt. %Sn

Palm oil wax: 52 wt. % HDPE: 45 wt.% SA: 3 wt.% PW SA PP
copolymer
PE PW SA PP

62 vol.%

Solvent 1 thermal

2 h at 630°C under high vacuum ,5.0 1022 torr in the presence of Mg blocks




.97

UTS: 157 6 11 El: 9.5 6 3.8

[12]

64 vol.%

Solvent 1 Thermal

64 h at 635°C furnace temperature in high-purity Ar







[13]


̋

̋







UTS: 142 YS: 67 El: 8 UTS: 161 YS: 123 El: 3.4 UTS: 240 YS: 118 El: 5.1


UTS: 550
710 EL:0.25
0.41 YS: 485 Modulus: 101 GPa EL: 18 UTS: 640 YS: 680 El: 3 UTS: 605 YS: 500 El: 5 UTS: 389
419 El: 2.0
4.0

[16]

Mg
0.9Ca

Mg
2.6Nd
1.3Gd
0.5Z
0.3Zn Mg
8Al
1Zn

̋


̋

̋







c.p. Ti

PW: 6.8 wt.% PP: 3.4 wt.% Acrylate resin: 3.4 wt.% DBP: 1.4 wt.% Wax-based

85.0 wt.%

Thermal debinding in air or argon

1300°C, 1 h, 1023 torr







Thermal debinding in argon

1250°C, 3 h













23

1025°C, 10

Wax 1 resin

56.1 vol.%

Wax 1 resin

66.7 vol.%

PW: 60
65 wt.% PEG20,000: 10
15 wt.% LDPE: 12
16 wt.% PP: 8
12 wt.% SA: 1
5 wt.%

67 vol.%

Pa

Thermolysis: 374°C vacuum B100 Pa flowing Ar Thermolysis

925°C
1075°C/2 h B1022 Pa

C: 0.06 O: 0.45

925°C
1075°C/2 h B1022 Pa

C: 0.05 O: 0.28

Heptanes 1 ethanol, 6h Thermolysis: 350°C/ 1h 420°C/1 h; 600°C/ 1h at 1023Pa

1150°C, 2
4 h under vacuum 1025 Pa

C: 0.06 O: 0.3

97

[14]

[14]

[15]

[17]

[18]

[18]

[19]

(Continued)

Table 2.3 (Continued) Base metal

Ti
6Al
4V

Binder

Solids loading

Debinding

Sintering

Impurity (wt.%)

Density (%)

Sintered property UTS/YS (MPa); El (%)

References

Naphthalene: 93 vol. % SA: 1 vol.% EVA: 6 vol.%

65 vol.%

1100°C/4 h 1026 torr Cooled under argon flowing




94




[20,21]

PEG4000 PMMA Lubricants (20 wt.%) of total binder PEG1500 PMMA PVP SA BASF Catamold Ti

58 vol.%

Thermolysis: 80°C/48 h at 2 3 1022 torr 375°C/3 h, Ar 1 2.75%H2; 750°C, 3 h vacuum Solvent 1 thermal debinding

1300°C/2 h 1022 Pa







UTS: 635 El: B2%

[22]

67 vol.%

Solvent 1 thermal debinding

1300°C/2 h 1022 Pa

O: 0.28 C: 0.05 N: 0.01

98

UTS: 504 El: 9.5%

[9]




Catalytic 1 thermal debinding

Sintering in vacuum or pure argon

$ 93

1200°C
1260°C 2
4 h vacuum

Solvent: Heptane 50°C, 5 h Thermolysis: 750°C Vacuum Heptane, 250°C, 5 h 600°C, 1 h, 1023 Pa

1260°C 1
10 h 1023Pa

C: 0.13
0.19 O: 0.35
0.72

96.9

UTS $ 550 YS $ 480 El $ 5% UTS: 1020
1060 YS: 880
960 El: 8.0
9.5 YS: 850

[23]

Solvent 1 thermal debinding

O: # 0.4 C: # 0.2 N: # 0.1


1100 21300°C, 2
8 h, 1023 Pa

O: 0.26 C: 0.08

96.1

UTS: 890 EL: 10

[26]

59 vol.%

Solvent: 1,1,2,2tetrachloroethane, 70°C, 5 h

1050°C, 5
72 h vacuum




96

UTS: 950

[27]

68 vol.%

Heptanes, 40°C, 20 h Thermal debinding is combined in sintering

1250°C, 2 h, 1025 mbar

O: 0.19 C: 0.015




UTS: 800 YS: 700 EL: 15

[28]

PW 1 PEG 1 LDPE 1 PE 1 SA PW: 63 wt.%, PEG: 12 wt.%, PE: 24 wt.% SA: 1 wt.% PW: 69 wt.% CW: 10 wt.% PP: 10 wt.% EVA: 10 wt.% DBP: 1 wt.% PW: 8.6 wt.% CW: 1.2 wt.% PP: 2.5 wt.% SA: 0.1 wt.% PW 1 PE 1 SA

71 vol.%

65 vol.%

[24]

[25]

Palm stearin: 60 wt. % PE: 40 wt.% PW: 63 wt.% PP: 10 wt.% LDPE: 14 wt.% PEG20,000: 12 wt.% SA: 1 wt.%

65 vol.%

Solvent: Heptane, 60°C, 6 h

1150°C, 4 h vacuum

C: 0.33

99

UTS: 830 El: 1.74

[29]

70 vol.%

1230
1260, 2
4 h, vacuum




99

UTS: 1060 YS: 960 EL: 9.5

[30]

PW: 63 wt.% PP: 10 wt.% LDPE: 14 wt.% PEG20,000: 12 wt.% SA: 1 wt.%

70 vol.%

1230°C, 3 h 1023 Pa

O: 0.26 C: 0.095




UTS: 1030 El: 12

[31]

Naphthalene: 93 vol. % SA: 1 vol.% EVA: 6 vol.%

65 vol.%

1100°C, 4 h 1026 torr Cooled under argon flowing

C: 0.019

97




[11]

PW: 60 wt.% PE: 35 wt.% SA: 5 wt.%

69 vol.%

450°C 1250°C
1400°C, 1024 mbar, Ar flow







UTS: 861 YS: 757 EL: 14.3

[32]

PEG: 87 wt.% PMMA: 11 wt.% Stearic acid: 2%

69 vol.%

1300°C, 3 h, in Ar

O: 0.19




UTS: 876 El: 15.5

[33]

Agar 1
3 wt.% Water 45
55% CaBO3 0.2
0.5 wt. % polyMIM Ti6Al4V

61 vol.%

Heptane 1 ethanol, 6 h, dried 1 h Thermal: 350°C, 1 h 420°C, 1 h; 600°C, 1h at 1023 Pa Heptane 1 ethanol, 6h Thermolysis: 350°C, 1h 420°C, 1 h; 600°C, 1h at 1023 Pa Thermolysis: 80°C, 48 h at 2 3 1022 torr 375°C, 3 h, Ar 1 2.75%H2; 750°C, 3 h vacuum Heptane, 40°C, 20 h Thermal debinding is combined into sintering Water: 55°C, 6 h Thermolysis: 350°C, 1h 440°C, 1 h in Ar


1150°C Ar/vacuum

C: 0.33 O: 0.30




UTS: 784 El: 10

[34]

Solvent 1 thermal debinding

Sintering in vacuum or pure argon at 1360°C for 1 h

O: # 0.25 C: # 0.08 N: # 0.05

$ 96

UTS $ 850 YS $ 750 El $ 10

[35]




(Continued)

Table 2.3 (Continued) Base metal

Binder

Solids loading

Debinding

Sintering

Impurity (wt.%)

Density (%)

Sintered property UTS/YS (MPa); El (%)

References

Ti
6Al
4V
0.5B

PW: 60 wt.% EVA: 35 wt.% SA: 5 wt.% PW: 29 wt.% PP: 30 wt.% PMMA: 40 wt.% SA: 1 wt.% Wax-based

65 vol.%

Heptane, 40°C, 20 h No separate thermolysis Hexane, 70°C, 6 h No separate thermolysis

1250°C
1350°C, 1025 mbar, 2 h







[36]

1023 2 1022 Pa, 1200°C
1300°C, 8 h, 1024 2 1023 Pa

O: 0.29 C: 0.048

99.5

UTS: 900 YS: 790 EL: 12 UTS: 970 EL: 13.8

Solvent 60°C/5 h Thermolysis: up to 450°C under N2 partial Heptane, 40°C, 10 h 300°C, 1 h then 450°C, 1 h Trichloroethylene, 40°C, 12 h, dried for 6 h 30°C
600°C, 1023Pa Hexane, 40°C Thermal: 250° C
400°C, vacuum Solvent: Hexane, 40° C, ? h Thermolysis: 1023 mbar 250°C
400°C Solvent 1 thermal debinding Solvent debinding. thermal debinding is combined into sintering

Stepwise heating to 1370°C, 6.7 3 1023 Pa

C: 0.03
0.16 O: 0.67
1.48

97.7
98.7

UTS: 260
454 YS: 266
435 EL: 0.32
1.88

[38]

1000°C, 3 h, 1025 torr then 3°C/min to 1350°C, then hold 30 h 23 10 Pa, 1400°C
1500°C, 2 h




98.8




[39]

O: 0.18

96.2

UTS: 382 6 3 EL: 0.46 6 0.03

[40]

1360°C, 3.5 h, 1025 mbar

O: 0.135 C: 0.035 N: 0.009

95
96

UTS: B350 YS: 324
328 EL: 0.59

[41]

1250°C, 2 h 1025 mbar

O: 0.25 N: 0.0149 C: 0.037

UTS: 846 YS: 726 El: 13.7

[42,43]

1350°C, 4 h 1022 Pa 600°C then 1150°C
1350°C, 2
6 h, 1023 2 1022 Pa




UTS: .800 EL: .10 UTS: 900 EL: 15

[44]

Ti6Al
4V
4Mo

65 vol.%

32.6
35.8

PW 1 EVA 1 SA

PW: 63 wt.% LDPE: 20 wt.% SA: 5 wt.%

PW 1 PE 1 SA

68 vol.%

PW:? wt.% PE? wt.% SA:? wt.%

68 vol.%


PW 1 CW 1 PP 1 EVA 1 DBP

65 vol.%

98 96

[37]

[45]

PW: 29 wt.% PP: 30 wt.% PMMA: 40 wt.% SA: 1 wt.% PW: 69 wt.% PP: 20 wt.% CW:10 wt.% SA: 1 wt.%

65 vol.%

NiTi

Wax/EVA







Ti
4%Fe

Polyamide wax: 60 vol.% PE: 40 vol.% Wax/EVA

Ti
6Al
2Sn
4Zr
2Mo Ti
5Al
2.5Fe

Ti
4.3Fe
7.1Cr

PW: 29 wt.% PP: 30 wt.% PMMA: 40 wt.% SA: 1 wt.%

59




65 vol.%

Hexane, 70°C, 6 h Thermal debinding is combined in sintering 1,1,2,2 tetrachlore thylene Thermolysis: 300°C/ 2 h 1 450°C/2 h 1024 Pa 150°C/Ar
H2 400°C/ vacuum Wicking (180°C) 1 thermal debinding (480°C) 250°C
320°C in air

1023 2 1022 Pa, 1050°C
1350°C, 3
8 h, then cooled to r.t. at 1024 2 1023 Pa

O: 0.24 C: 0.038

98.2

UTS: 1010 EL: 14.7

[46]

1000°C
1100°C 5
24 h 1024 Pa

O: 0.27
0.30

94
97

UTS: 970 EL: 3
6

[47]

900°C
1150°C 2
20 h Vacuum 1270°C In vacuum




60
67

UTS: 115 EL: 1.5

[48]

O: 0.30 C: 0.087




[49
54]

1250°C under vacuum

C: 0.20
3 O: 1.55
1.61

99

Hexane, 70°C, 6 h Thermal debinding is combined in sintering

1023 2 1022 Pa, 1100°C
1250° C, 3
8 h, then cooled to r.t. at 1024 2 1023 Pa

O: 0.22 C: 0.07

96.9

Max. strain at failure of 5% EL: 3.8 UTS: 152
230 EL: 0.08
0.23 HRC: 44
49 UTS: 1160 EL: 3

[55,56]

[57]

64

Feedstock Technology for Reactive Metal Injection Molding

2.3 Feedstock chemistry and properties 2.3.1 Feedstock flow: powder characteristics and optimal solids loading In MIM, successful mold filling depends on the rheological properties of the feedstock. The two most important properties are flow behavior and viscosity in particular. Both properties depend on starting powder characteristics and solids loading employed for feedstock formulation. The same claim can be made for reactive powders MIM. For reactive powders MIM, specific powder characteristics to consider are listed: • Powder particle size distribution • Particle size • Interstitial level (generally quantified by oxygen and carbon contents). The above three characteristics are linked with each other via powder cost, which determines the powder final size, shape, and impurity. Table 2.4 summarizes the typical characteristics of titanium powders used for metal injection molding. Fig. 2.10 shows some typical reactive powders used for metal injection molding. Because of difficulty in controlling impurities during reactive powders MIM, it is necessary to start with impurity levels as low as possible. Generally speaking, reactive powders MIM processing can cause an additional pickup of interstitials from 0.02% to 0.1%. If the processes are not well controlled, the interstitial uptakes may be even worse. Hence, the powder purity is one of the critical factors determining the end-product Table 2.4 Commonly used titanium powders in MIM and their characteristics. Type

Sponge Hydride
dehydride (HDH) Titanium hydride Reactive Gas atomized Plasma atomized Rotating electrode

Mean size (μm)

Typical shape

38 38

Irregular Irregular

35 30 32 60 130

Irregular Irregular Spherical Spherical Spherical

Tap density (%)

Interstitials (wt.%) Oxygen

Carbon

48 38

0.35 0.25

0.05 0.04

40 47 60 62 72

0.20 0.30 0.15 0.15 0.15

0.02 0.10 0.03 0.04 0.02

Design strategy of binder systems and feedstock chemistry

65

Figure 2.10 (A) Commercially pure gas atomized Mg powder58; (B) HDH
Ti powder with d50 of 47 μm; (C) PREP NiTi powder. Taken from Chen, G.; Zhao, S.-y.; Tan, P.; Yin, J.-o.; Zhou, Q.; Ge, Y.; Li, Z.-f.; Wang, J.; Tang, H.-p.; Cao, P. Shape Memory TiNi Powders Produced by Plasma Rotating Electrode Process for Additive Manufacturing. Trans. Nonferrous Met. Soc. China 2017, 27 (12), 2647
2655 [59], with permission.

properties and potential application. However, purity comes with the price: a gas atomized titanium powder with oxygen less than 0.10 wt.% and carbon less than 0.02 wt.% can cost as much as US$300/kg while an HDH titanium powder having oxygen above 0.3 wt.% will cost around US$50/kg. Although HDH powders are comparatively cheap, their flow properties are significantly inferior. In fact, any particle shape other than spherical results in poor rheological properties because of the lower packing density and higher interparticle friction. To accommodate this, solids loading is kept relatively low to achieve a good molding operation. The fact that

66

Feedstock Technology for Reactive Metal Injection Molding

surface roughness and particle shape are two different aspects must also be kept under consideration. Surface roughness is related to the texture of the surface and greatly affects the wetting and spreading of the binder; however, it has a relatively small influence on the packing density and viscosity. Particle shape has a more profound effect on the particle packing density and inevitably, on the feedstock viscosity (Fig. 2.11). Fine powders sinter more readily than coarser powders and result in better surface finish. Therefore they are the best choice for MIM. However, in the case of reactive powders MIM, powder selection favors a large particle size with less surface area to limit impurities uptake. A typical compromise for reactive powders is to use 2325 mesh (below 45 μm), spherical or rounded powder to give a high packing density. The recommended starting oxygen level should be below 0.20 wt.%. Homogeneous mixing of powder with binder system during feedstock formulation and optimal solids loading are the two factors that have a strong influence over the sintered product properties. The relative viscosity—viscosity of the feedstock divided by the viscosity of pure binder— increases with the addition of solid particles (Fig. 2.11). The addition of solid particles eventually reaches a critical point at which the relative viscosity becomes infinite and mixture no longer remains viscous, that is, the feedstock becomes stiff and does not flow. The amount of solid particles in the feedstock at this point/limit is called critical solids loading. The critical solids loading limit depends on parameters such as particle size distribution, particle shape, binder viscosity and wettability, surfactant, etc. In general, solids loading 3%
5% less than the critical value is considered as an optimal value.

Figure 2.11 Effect of different shapes of glass particles on relative viscosity. Redrawn from German, R. M. Powder Injection Molding. Metal Powder Industries Federation: Princeton, NJ, 1990; p xii, 521 p. [60].

Design strategy of binder systems and feedstock chemistry

67

The effect of solids loading on the viscosity of feedstock can be realized using theoretical models. Einstein provided a basic direction of the solids loading effect on viscosity for monosized spherical particles dispersed at random in a fluid61: ηr 5 1 1 2:5ɸ

(2.3)

where ηr is the relative viscosity, and ɸ is solids loading. Eq. (2.3) assumes that the particles are monosized, nondeforming spheres that do not experience Brownian motion. Thus this equation is applicable for mixtures having solids loading less than approximately 15%. Several modifications of the Einstein relation have been presented. Some models are listed61: ηr 5 Að12Φr Þ2n ηr 5 A exp ð2:5 ηr 5

Φ Þ 1 2 kΦ

Aɸ 1 2 Bɸ

h i 1 1 ηr 5 Aɸ3r = 1 2 ɸ3r ηr 5 1 1 Aɸr 1 Bɸ2r where ηr is the relative viscosity, ɸ represents solids loading, ɸr relative solids loading, that is, solids loading divided by the maximum solids loading and A, B, k, and n are system-dependent constants. Based on experimental studies, the most widely used model to estimate the change in relative viscosity with relative solids loading is: ηr 5 A ð12Φr Þ2n

(2.4)

Generally, the exponent n is found to be 2.0. The coefficient A is a constant and depends on factors such as shear rate sensitivity and particle size effects and is usually near 1. For experimental determination of solids loading, torque rheometers are commonly used and are therefore applied for reactive powders MIM as well. Fig. 2.12 shows one such example. The mixing torques varies as solid particles are added into the binder, but at lower solids loadings the

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Feedstock Technology for Reactive Metal Injection Molding

Figure 2.12 Determination of solids loading by torque rheometer for Ti-MIM feedstocks with different powder shapes: (A) plot of torque versus time during mixing; (B) torque versus solids loading. Taken from Park, S.-J.; Wu, Y.; Heaney, D. F.; Zou, X.; Gai, G.; German, R. M. Rheological and Thermal Debinding Behaviors in Titanium Powder Injection Molding. Metall. Mater. Trans. A 2009, 40 (1), 215
222 [62], with permission.

variation in torque soon levels out (i.e., stabilized). The subsequent addition of solid particles gradually increases the overall mixing torque and the time taken to achieve a stable value until critical solids loading. At this point, the torque values are significantly high with no stable value even after mixing for hours. By setting up a torque value criterion, for instance, 1 N m in this case, an optimal solids loading can be determined.

Design strategy of binder systems and feedstock chemistry

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Although torque measurements during mixing are generally used to determine the optimum solids loading, this technique may produce erroneous results, if low shear mixers are used. On the other hand, the capillary rheometer produces reliable and accurate results because of more realistic shear rates and conditions to injection molding. It is widely accepted that for excellent molding the viscosity of the feedstock should be below 1000 Pa s at molding temperature in the shear rate range of 102
105 s21.63,64 In the context of injection molding, viscosity at low-to-medium shear rates (102
104 s21) is generally more important, as most of the injection molding machines work in this shear rate range with the highest shear rates occurring at the gates. By selecting 1000 Pa s viscosity as the limiting value, optimal solids loadings can be realized (Fig. 2.13). When reactive powder feedstocks are formulated, additional provisions of protective atmosphere cover should be made in either vacuum or inert gas.

2.3.2 Shear sensitivity For low molecular weight liquids like water, the viscosity is independent of shear rates at any given temperature and pressure. Such liquids are termed as Newtonian fluids. The Newtonian flow concept does not apply to MIM feedstocks. Most polymers exhibit complex behavior, in which

Figure 2.13 Optimal solids loading determination by capillary rheometer for titanium-based feedstocks having different powder morphologies. The contact points between 1000 Pa s limiting line and solid loadings curves give optimal solid loading. Taken from Hayat, M. D.; Cao, P. A New Lubricant Based Binder System for Feedstock Formulation From HDH-Ti Powder. Adv. Powder Technol. 2016, 27 (1), 255
261, with permission.

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Feedstock Technology for Reactive Metal Injection Molding

viscosity is dependent on the shear rates as well as on pressure and temperature. Such fluids are termed as non-Newtonian fluids. In such type of fluids, either viscosity increases (dilatant behavior) or decreases (pseudoplastic behavior) with increasing shear rates; whereas, some fluids exhibit a yield stress point that must be exceeded to initiate the flow. Such systems are termed as Bingham (Fig. 2.14). MIM feedstock, irrespective of the powder type and shape, possesses a yield shear stress at low temperatures and zero shear rate (i.e., Bingham behavior), but exhibits a pseudoplastic behavior over the shear rates that are encountered during injection molding. Such fluids are termed as Herschel
Bulkley fluids (Fig. 2.14) and can be expressed as τ 5 τ y 1 K γ_ n

(2.5)

where K is a constant, τ y yield shear stress, γ_ shear rate, and n the exponent used to characterize the fluid. For Newtonian fluids n 5 1, for pseudoplastic fluids n , 1, and for dilatant fluids n . 1, as yield stress can be considered as a minimum force required for the flow, the yield stress can be calculated by the Bingham model: τ 5 τ y 1 μp γ_

Figure 2.14 Common flow behaviors.

(2.6)

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where μp is the plastic viscosity determined by the slope of the shear stress versus shear rate plot above the yield point. It represents the viscosity of the feedstock when extrapolated to infinite shear rate based on the Bingham model. By plotting τ versus γ_, yield stress and plastic viscosity can be calculated. However, the yield stress evaluation only gives an estimate of fluidity, pressure required to initiate the flow, and an approximated working temperature of feedstocks. In most cases, the applied pressure in injection molding is much higher than the yield point of the feedstock. Therefore viscosity dependence on shear rates is more desired due to the wide shearing range of injection molders. Thus the initial yield stress can be neglected in Eq. (2.5): τ 5 K γ_ n

(2.7)

where K is a constant and n has the same meaning as in Eq. (2.5). This is known as the power-law model. Although it is widely used to characterize fluids, there are certain limitations. For instance, the viscosity becomes infinite at low shear rates and vice versa. However, if used within the measured shear rates, this model can produce reasonably accurate results. Eq. (2.7) is often used to describe shear rate dependence of viscosity and is given by η 5 K γ_ n21

(2.8)

where K is a constant and, η is viscosity. The exponent (n 2 1) value shows the viscosity dependence on the shear rate (also termed shear sensitivity) and can be calculated from a double logarithm plot (logη vs logγ_). Generally, a high (n 2 1) value or a high negative slope is desired for MIM. However, too high (n 2 1) values or too high shear sensitivity may also cause problems such as flashing during molding. By far capillary rheometer is the most useful testing technique for characterizing feedstocks. However, since capillary rheometers are designed to calculate viscosity based on Newtonian fluids and given non-Newtonian flow behavior of MIM feedstocks; certain corrections are required to get the true viscosity. The two most important are Bagley end correction and Rabinowitsch correction. The Bagley end correction: In reality, the MIM feedstock is not an incompressible fluid and its behavior might be disturbed at the entrance of the capillary, which might lead to instability if the velocity gradient is too high. It entails that the pressure at the entrance due to the force applied to extrude the feedstock is different from the pressure at the exit of the capillary. Thus some pressure losses have important consequences regarding the shear stress calculations (Fig. 2.15).

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Feedstock Technology for Reactive Metal Injection Molding

For a Newtonian fluid, the ΔP versus capillaries of different L/R ratio will always be a straight line with zero intercepts, the slope of which is equal to twice the shear stress. However, for non-Newtonian fluid, the pressure drop will increase as die length will increase. Bagley end correction simply considers that pressure drop through a die length of L will be equivalent to the die length of L 1 e, as shown in Fig. 2.16. The procedure to calculate Bagley’s end correction is quite simple. The pressure is plotted against different L/R ratio dies at a constant shear rate to obtain the e value. Thus the true shear stress can be calculated as P  τ ðtrueÞ 5 l 2 r 1e

(2.9)

Figure 2.15 Entrance pressure loss in case of non-Newtonian fluid (right) compared with a Newtonian fluid (left).

Figure 2.16 Bagley end correction. The y-intercepts are the entrance pressure loss.

Design strategy of binder systems and feedstock chemistry

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Rabinowitsch correction: The wall shear rate is generally greater for nonNewtonian fluid and the velocity profile is not parabolic, as shown in Fig. 2.17. A special step is used to determine this wall shear rate for non-Newtonian fluid and this is called as Rabinowitsch correction. Moreover, it is generally given by ð3n 1 1Þ 4n

(2.10)

where n is the slope of the log
log graph of apparent shear rate and true shear stress. The true shear rates are thus calculated as γ_ ðtrueÞ 5

ð3n 1 1Þ γ_ ðapparent Þ 4n

(2.11)

The above corrections can be overlooked if the capillary die with L/D ratio of more than 20 is used. Additionally, most of the latest versions of capillary rheometers have build-in software protocols for such calculations. Nevertheless, experiments at different L/D ratio dies are still need to be carried out. Typical viscosity versus shear rate curves for different titanium feedstocks as determined by a capillary rheometer are shown in Fig. 2.18. Feedstock C&D would be ideal for injection molding since they demonstrate a more consistent dependence of viscosity on shear rates (shear sensitivity) over the entire temperature range. One important thing that should be monitored during shear sensitivity testing of reactive powders MIM is feedstock separation (Fig. 2.19). Every feedstock has a flowtransition shear rate (usually quite high) above that it starts showing dilatant flow behavior instead of pseudoplastic. This occurs because an extremely low viscosity of binders at high shear rates disables them to

Figure 2.17 Velocity profile for a Newtonian and non-Newtonian flow.

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Feedstock Technology for Reactive Metal Injection Molding

(A)

(B)

1.7

80ºC 90ºC 100ºC 110ºC 120ºC 130ºC

1.6

1.3 1.2

+

1.5 1.4 1.3 1.2 1.1

+

+

1.4

1.7

+

+

1.0

++

0.9

1.1

+

+

0.8 0.7

1.0

+

0.6 0.5

0.9 2.6

2.8

3.0

3.2

3.4 -

3.6

3.8

4.0

2.5

4.2

3.0

3.5

–1

-

logγ (s )

4.0

4.5

–1

logγ (s )

(D)

(C)

3.4

+

2.2

3.0

+

2.1 2.0

+

1.9

2.8

+

+

2.3

+ +

2.6

+ + +

2.4 2.2

Pseduoplastic flow

1.8

1.6 1.5 1.4 2.0

2.5

3.0

3.5 -

–1

logγ (s )

++

2.0

++

++ + +

1.7

+ +

2.4

+ 4.0

4.5

80ºC 90ºC 100ºC 110ºC 120ºC 130ºC 140ºC

+

2.5

Feedstock D

3.2

+

2.6

80ºC 90ºC 100ºC 110ºC 120ºC 130ºC 140ºC

logη (Pa s)

Feedstock C

2.7

logη (Pa s)

80ºC 90ºC 100ºC 110ºC 120ºC 130ºC

+

1.5

1.8

logη (Pa s)

1.6

logη (Pa s)

Feedstock B

1.9

Feedstock A

1.8

++

1.6

Transition point 1.5

2.0

2.5

3.0 -

3.5

4.0

4.5

–1

logγ (s )

Figure 2.18 Plot of the double logarithm of viscosity versus shear rate for different titanium feedstocks in the temperature range of 80°C
140°C. The feedstocks had the same solids loading but different binder compositions. Taken from Hayat, M. D.; Wen, G.; Zulkifli, M. F.; Cao, P. Effect of PEG Molecular Weight on Rheological Properties of Ti-MIM Feedstocks and Water Debinding Behaviour. Powder Technol. 2015, 270, 296
301 [65].

hold solid particles together. For irregular HDH powders, this problem can even occur at low shear rates as well because of the inherently high interparticle friction and poor flowability of irregular particles. For nonreactive powders, feedstock separation generally does not present a threat apart from molding difficulties. However, for reactive powders, it can have serious consequences. Fig. 2.20 offers one such insight. Although titanium powders do not react with oxygen during short duration exposures at room temperature, higher frictional forces and presence of air molecules inside the mold cavity can spark the naked titanium powders separated from the feedstock. Once titanium powders are ignited, the reaction propagates with nearexplosive speed reaching up to 100 MPa/s pressure rise. This can potentially result in life-threatening injury and serious damage to equipment.

Design strategy of binder systems and feedstock chemistry

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Figure 2.19 Feedstock separation for a titanium feedstock as observed during testing of shear sensitivity via capillary rheometer. Note the binder accumulation on the die surface leaving behind a solid compact with no flowability.

Figure 2.20 Damaged mold due to explosion caused by feedstock separation during injection molding of HDH Ti feedstock.

Therefore reactive powders feedstocks must have a high value of the flow-transition shear rate.

2.3.3 Temperature sensitivity Another important factor that must be considered during feedstock design is temperature sensitivity. In general, a good reactive powder MIM feedstock should have a low-temperature sensitivity of its viscosity. The high

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Feedstock Technology for Reactive Metal Injection Molding

sensitivity arises from a large difference in the coefficient of thermal expansion between binder polymer and metal powders. Additionally, temperature decreases sharply from nozzle to cavity during injection molding. If a feedstock viscosity is too sensitive to temperature change then a sudden decrease in viscosity can affect the quality of the product by inducing cracks, stresses, and distortion in the final part. The temperature dependence of viscosity can be explained by the Arrhenius equation:
 E η 5 η0 exp (2.12) RT where η0 is the viscosity at a reference temperature, R is the universal gas constant, E is the activation energy for viscous flow, and T is the absolute temperature. A higher value of E indicates a high sensitivity of viscosity to temperature change. The activation energy can be calculated from the plot of lnη BT1 , the slope of which is equal to E/R. Fig. 2.21 shows typical lnη versus T curves for different titanium feedstocks obtained from Fig. 2.18 at a constant shear rate. A highly temperature-sensitive feedstock has a steep slope and requires more accurate temperature control of the mold. Otherwise, the temperature gradient across injection mold will induce residue stresses that can lead to distortion in the final sintered part (Fig. 2.22).

2.3.4 Thermal conductivity and heat capacity During the injection process, the feedstock first fills the mold cavity under pressure and is then subsequently cooled under pressure as well. The pressure is removed once the compact has cooled completely. As mentioned in the previous section, the feedstock should have suitable temperature sensitivity—a large increase in viscosity on cooling to avoid distortion of the compacts during cooling and subsequent handling. Similarly, the feedstock should possess high thermal conductivity to minimize the formation of shrinkage cracks. The thermal conductivity values of the feedstocks lie in between pure binder and the metallic particles. The thermal conductivity of the feedstock (kf Þ can be estimated as kf 5 kb ð1 1 AΦÞ=ð1 2 BΦÞ

(2.13)

where kb is the thermal conductivity of binder, A and B are constants while Φ is solids loading. The temperature sensitivity of feedstock and thermal conductivity are important with regards to defect formation and product quality. The heat

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77

Figure 2.21 Temperature dependence of viscosity for different titanium feedstocks in the temperature range of 80°C
140°C. Values were obtained from shear sensitivity experiments by capillary rheometry.65

capacity of the feedstock, on the other hand, influences productivity. Generally, the cooling rate of any substance is the measure of the rate of heat loss by the substance and ultimately decrease in the temperature. The rate of heat loss can be described by the sensible heat equation: _ 5 mC Q _ p ΔT

(2.14)

_ is the rate of heat change ( J/s), m where Q _ is the mass flow rate, Cp is the specific heat capacity, and ΔT is the temperature difference. As the mass of sample products and the injection temperature during MIM are kept constant, the assumption of constant mass flow rate and the temperature difference can be deduced. Hence, Eq. (2.14) can be rearranged as Q1 Q2 5 5 constant Cp 1 Cp 2

(2.15)

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Feedstock Technology for Reactive Metal Injection Molding

Figure 2.22 Temperature-induced distortion of sintered titanium samples. All the samples were made from the same feedstock using the same injection parameters only mold temperature was varied.

Eq. (2.15) indicates that heat removal is directly proportional to the Cp. In other words, more heat will need to be removed to decrease the temperature of a particular substance as Cp increases. This will have serious ramifications during the metal injection molding process as a feedstock with higher Cp value would require either a longer cooling cycle or a too low molding temperature. Both long cycle and too low molding temperature are detrimental to the overall quality and efficiency of the MIM process. For comparison, we compared the Cp value of a commercial PolyMIM Ti feedstock with our own lab Ti feedstock. The polyMIM feedstock has a Cp value of 0:746 g℃J at 22°C, while our own feedstock formulation has a Cp value of 1:073 g℃J at the same temperature. Because of this higher Cp, the lab Ti feedstock took nearly 10 s more during the cooling sequence of injection molding before ejection for a given sample shape and size. This difference may not be a big issue for lab or smallscale production. However, for large volume production, this can adversely affect production efficiency. The rule of mixtures provides a

Design strategy of binder systems and feedstock chemistry

79

very good estimate of the heat capacity of the feedstock. For accurate measurements, differential scanning calorimetry (DSC) is the classic approach. Consider a typical DSC measurement consisting of an empty pan, reference sample, and test sample. The specific heat capacity of the sample (at a given temperature) can be calculated (Fig. 2.23) by the comparison of the heat flow rates into the sample (Δhsample ) and into the reference sample (Δhref ) according to     Cpsample Δhsample mreference 5 3 (2.16) Cpreference msample Δhreference However, there are uncertainties in this simple model. The main sources of uncertainty are the masses of sample msample and reference sample mreference , the heat flow rates Δh of the three experiments and the specific heat capacity of the reference material. Nevertheless, by carefully calibrating the instrument and controlling the working parameters, an accurate heat capacity value can be obtained.

Figure 2.23 Example of heat capacity determination using DSC.

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Feedstock Technology for Reactive Metal Injection Molding

2.3.5 Strength model During the debinding stage, the injection-molded sample experiences stress or pressure due to various activities particularly in the thermal debinding process. Defects occur in the molded samples when these stresses exceed the in situ strength of the compact. A model based on the in situ strength of the component can be used to predict defect formation during thermal debinding. The in situ strength of the sample with localized bonding due to the polymer can be estimated by the Rumpf equation66,67: σc 5 1:1

12εH ε d2

(2.17)

where H is the bonding force, ε is the porosity in the debound sample, and d is the diameter of the particle. The bonding force H is estimated from61
2 πx H 5 σo (2.18) 4 where x is the average of the contact diameter between the particles, and σo is the inherent strength of the metal powder. Taking the stress concentration factor into consideration, the strength of the debound sample is given as σc 5 1:1

12εH Kε d2

where K is the stress concentration factor and is obtained from x h  x i2 lnðK Þ 5 0:457 1 0:175ln 1 0:095 ln 8d 8d

(2.19)

(2.20)

However, this model does not include the fact that the in situ strength of the compact during the thermal debinding process is also dependent on the viscosity and thermal degradation characteristics of the polymer. Therefore the final equation for the in situ strength of the compact during the thermal debinding process should also include backbone polymer parameters such as molecular weight, viscosity, and thermal degradation behavior. Further research studies are required to fine-tune this model. Nevertheless, Ying et al.68,69 estimated the value of green strength for carbonyl iron powder using the observations made by Suri et al.67 for powder injection molding feedstock of tungsten alloy powder mixed with

Design strategy of binder systems and feedstock chemistry

81

paraffin wax
polypropylene binder. The observations suggested that the average diameter of the contact between the particles (x) is 0.3 times the diameter of the particles (x 5 3d). Ying et al. then used the obtained green strength value to estimate stresses developed in the compact during the thermal debinding process. The in situ strength of the carbonyl iron compact was estimated as 10.6 MPa for ε value of 0.4, which decreased to 0.79 MPa with an increase in ε to 0.9. The stresses in the compact simulated by Ying et al. were in the order of 10 MPa due to polymer content change and 0.1 MPa due to gas pressure and temperature change. The in situ strength of the compact as predicted by the model is in the same range as the stress estimated by Ying et al. The analysis shows that modeling with the approach of in situ strength variation during thermal debinding is a promising way to develop heating cycles to minimize the formation of defects.

2.4 Summary To put it succinctly, the binder systems for reactive powders such as Al, Ti, and Mg MIM require special attention and must fulfill additional requirements (when compared to MIM of stainless steel for example) due to the sensitivity to impurities. Nevertheless, most of the binder systems used for reactive powders MIM are either directly adopted from common powders MIM or being employed after some modifications. For feedstock formulation, it is recommended to use the spherical powder to obtain high solids loading. However, the powder should be handled completely under a protective argon atmosphere. Appropriate selection of the solids loading in the case of reactive powders MIM is another parameter that should be given special focus. If a solids loading is chosen above the optimal value, high shear rates during the injection process can cause feedstock separation that may lead to explosive ignition of reactive powders.

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41. Gerling, R.; Aust, E.; Limberg, W.; Pfuff, M.; Schimansky, F. P. Metal Injection Moulding of Gamma Titanium Aluminide Alloy Powder. Mater. Sci. Eng. A 2006, 423 (1
2), 262
268. 42. Aust, E.; Limberg, W.; Gerling, R.; Oger, B.; Ebel, T. Advanced Ti6Al7Nb Bone Screw Implant Fabricated by Metal Injection Moulding. Adv. Eng. Mater. 2006, 8, 365
370. 43. Limberg, W.; Aust, E.; Ebel, T.; Gerling, R.; Oger, B. In Metal Injection Moulding of an Advanced Bone Screw 7Nb Alloy Powder, Euro PM2004: Powder Metallurgy World Congress & Exhibition, Vienna, Austria, Vienna, Austria, 2004; pp 457
462. 44. Itoh, Y.; Miura, H.; Sato, K.; Niinomi, M. Fabrication of Ti
6Al
7Nb Alloys by Metal Injection Molding. Mater. Sci. Forum 2007, 534-536, 357
360. 45. Osada, T.; Miura, H.; Itoh, Y.; Fujita, M.; Arimoto, N. Optimization of MIM Process for Ti
6Al
7Nb. J. Jpn. Soc. Powder Powder Metall. 2008, 55 (10), 726
731. 46. Itoh, Y.; Uematsu, T.; Sato, K.; Miura, H.; Niinomi, M. Fabrication of High Strength α 1 β Titanium Alloy Compacts by Metal Injection Molding. J. Jpn. Soc. Powder Powder Metall. 2008, 55, 720
725. 47. Xu, Y.; Nomura, H.; Takita, M.; Toda, H. Characteristics of Metal-Injection Processed Ti
5Al
2.5Fe Alloy for Implant Material. J. Jpn. Soc. Powder Powder Metall. 2001, 48 (4), 316
321. 48. Kyogoku, H.; Kumatsu, S. Fabrication of NiTi Shape Memory Alloy by Powder Injection Molding. J. Jpn. Soc. Powder Powder Metall. 1999, 46, 1103
1107. 49. Bram, M.; Ahmad-Khanlou, A.; Heckmann, A.; Fuchs, B.; Buchkremer, H. P.; Stöver, D. Powder Metallurgical Fabrication Processes for NiTi Shape Memory Alloy Parts. Mater. Sci. Eng. A 2002, 337 (1
2), 254
263. 50. Köhl, M.; Habijan, T.; Bram, M.; Buchkremer, H. P.; Stöver, D.; Köller, M. Powder Metallurgical Near-Net-Shape Fabrication of Porous NiTi Shape Memory Alloys for Use as Long-Term Implants by the Combination of the Metal Injection Molding Process With the Space-Holder Technique. Adv. Eng. Mater. 2009, 11 (12), 959
968. 51. Krone, L.; Mentz, J.; Bram, M.; Buchkremer, H. P.; Stöver, D.; Wagner, M.; Eggeler, G.; Christ, D.; Reese, S.; Bogdanski, D.; Köller, M.; Esenwein, S. A.; Muhr, G.; Prymak, O.; Epple, M. The Potential of Powder Metallurgy for the Fabrication of Biomaterials on the Basis of Nickel
Titanium: A Case Study With a Staple Showing Shape Memory Behaviour. Adv. Eng. Mater. 2005, 7 (7), 613
619. 52. Krone, L.; Schüller, E.; Bram, M.; Hamed, O.; Buchkremer, H. P.; Stöver, D. Mechanical Behaviour of NiTi Parts Prepared by Powder Metallurgical Methods. Materials Science and Engineering: A 2004, 378 (1
2), 185
190. 53. Mentz, J.; Bram, M.; Buchkremer, H. P.; Stöver, D. Improvement of Mechanical Properties of Powder Metallurgical NiTi Shape Memory Alloys. Adv. Eng. Mater. 2006, 8 (4), 247
252. 54. Schöller, E.; Krone, L.; Bram, M.; Buchkremer, H. P.; Ståaver, D. Metal Injection Molding of Shape Memory Alloys Using Prealloyed NiTi Powders. J. Mater. Sci. 2005, 40 (16), 4231
4238. 55. Kyogoku, H.; Komatsu, S.; Shinohara, K.; Jinushi, H.; Toda, T. Microstructures and Mechanical Properties of Sintered Ti
4% Fe Alloy Compacts by Injection Moldings. J. Jpn. Soc. Powder Powder Metall. 1994, 41, 1075
1079. 56. Kyogoku, H.; Komatsu, S.; Tsuchitori, I.; Toda, T. Tensile Properties of Sintered Ti4%Fe Alloy Compacts by Injection Moldings. J. Jpn. Soc. Powder Powder Metall. 1995, 42 (9), 1052
1056. 57. Itoh, Y.; Uematsu, T.; Sato, K.; Miura, H.; Niinomi, M.; Ikeda, M. Sintering Behavior and Mechanical Properties of Injection Molded Ti
4.3Fe
7.1Cr Alloys. J. Jpn. Soc. Powder Powder Metall. 2006, 53, 821
826.

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58. Neikov, O. D.; Gopienko, V. G. Chapter 17
Production of Magnesium and Magnesium Alloy Powders. In Handbook of Non-Ferrous Metal Powders; Neikov, O. D., Naboychenko, S. S., Yefimov, N. A., Eds.; . 2nd ed. Elsevier: Oxford, 2019; pp 533
547. 59. Chen, G.; Zhao, S.-y; Tan, P.; Yin, J.-o; Zhou, Q.; Ge, Y.; Li, Z.-f; Wang, J.; Tang, H.-p; Cao, P. Shape Memory TiNi Powders Produced by Plasma Rotating Electrode Process for Additive Manufacturing. Trans. Nonferrous Met. Soc. China 2017, 27 (12), 2647
2655. 60. German, R. M. Powder Injection Molding; Metal Powder Industries Federation: Princeton, NJ, 1990p xii, 521 p. 61. Enneti, R. K.; Onbattuvelli, V. P.; Atre, S. V. 4
Powder Binder Formulation and Compound Manufacture in Metal Injection Molding (MIM). In Handbook of Metal Injection Molding; Heaney, D. F., Ed.; Woodhead Publishing, 2012; pp 64
92. 62. Park, S.-J.; Wu, Y.; Heaney, D. F.; Zou, X.; Gai, G.; German, R. M. Rheological and Thermal Debinding Behaviors in Titanium Powder Injection Molding. Metall. Mater. Trans. A 2009, 40 (1), 215
222. 63. Smallman, R. E.; Bishop, R. J. Modern Physical Metallurgy and Materials Engineering; Elsevier, 1999. 64. German, R. M.; Cornwall, R. G. Summary Report on the Worldwide Market and Technology for Injection Molding of Metals and Ceramics. Adv. Powder Metall. Part. Mater. 1997, 3 18-13. 65. Hayat, M. D.; Wen, G.; Zulkifli, M. F.; Cao, P. Effect of PEG Molecular Weight on Rheological Properties of Ti-MIM Feedstocks and Water Debinding Behaviour. Powder Technol. 2015, 270, 296
301. 66. Rumpf, H.; Knepper, W. A., Eds. Agglomeration; Interscience Pub: New York, 1962. 67. Suri, P.; Atre, S. V.; German, R. M.; de Souza, J. P. Effect of Mixing on the Rheology and Particle Characteristics of Tungsten-Based Powder Injection Molding Feedstock. Mater. Sci. Eng. A 2003, 356 (1), 337
344. 68. Shengjie, Y.; Lam, Y. C.; Yu, S. C. M.; Tam, K. C. Thermal Debinding Modeling of Mass Transport and Deformation in Powder-Injection Molding Compact. Metall. Mater. Trans. B 2002, 33 (3), 477
488. 69. Shengjie, Y.; Lam, Y. C.; Yu, S.; Tam, K. C. Thermo-mechanical Simulation of PIM Thermal Debinding. Int. J. Powder Metall. 2002, 38 (8), 41
55.

CHAPTER 3

Binder system interactions and their effects As mentioned in the previous chapters, the same binder systems are often used for different powders in metal injection molding (MIM). A poor binder may still lead to acceptable properties for less reactive metals/alloys, but for reactive powders-MIM, it can be detrimental to the final mechanical properties. At present, the binder’s properties for MIM are assessed by rheological measurements and thermal decomposition temperature of the backbone polymer. Although blend studies of common polymers are available in the literature, little attention has been paid to study specific interactions among the particular binder system components and the effects of such interaction on reactive powders-MIM feedstocks are largely missing. This chapter aims to give readers some basic knowledge about polymer blends, their thermodynamics, and the possible interactions among individual components of common binder systems. In the last section, interactions between a binder system and metallic powder are also explained. After going through this chapter, the readers should be able to identify the importance of such interactions and know-how these interactions can be used to improve the properties of a feedstock.

3.1 Interactions between binder components As described in Chapter 2, Design Strategy of Binder Systems and Feedstock Chemistry, a binder system for MIM usually involves components of plasticizer (wax), polymer, and surfactant, to possess some specific characteristics vital to the success of each step involved in MIM. The success of a particular binder system depends on how different binder components interact with each other. The two basic requirements for feedstocks—homogeneity and green strength—are primarily dependent on compatibility and interactions between binder components and metal powders. Fig. 3.1 presents one such example of incompatibility between binder components. Feedstock Technology for Reactive Metal Injection Molding DOI: https://doi.org/10.1016/B978-0-12-817501-9.00003-X

© 2020 Elsevier Inc. All rights reserved.

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(A)

(C)

(B)

(D)

Figure 3.1 (A) a binder system resulting from poor miscibility and compatibility between binder components and (B) the resultant Ti-feedstock as prepared using mechanical mixer, (C) a typical example of a compatible binder system and (D) the resultant Ti-feedstock.

The incompatibility between binder components usually leads to phase separation and inhomogeneous aggregates in the resulting feedstocks. On the other hand, feedstock made of a compatible binder appears and behaves as a well-prepared pizza dough. To understand the interactions between binder components, we first need to understand the fundamentals of polymeric blends.

3.1.1 Polymer blends The simplest definition of a polymer blend is given by Utracki et al.1 and is stated as a “mixture of at least two macromolecular substances, polymers or copolymers, in which the ingredient contents are above 2 wt.%.” Polymer blends can be either miscible or immiscible and most of the polymer blends are immiscible. The miscibility of polymer blends depends

Binder system interactions and their effects

89

on factors such as chemical structure, molecular weight distribution and the molecular architecture of individual polymers. Table 3.1 defines some basic terms related to polymer blend miscibility. Fig. 3.2 presents how the properties of a polymer blend change as the component concentration changes. In the case of polymers that are miscible in all proportions, it is fair to assume that the resultant blend is an average of the physical properties of each component and depends on the proportion of each polymer present. For example, the Tg of a miscible blend will vary linearly from that of the lower Tg polymer to that of the higher Tg polymer as the higher Tg polymer increases in proportion in the blend (Fig. 3.3). The miscibility of two polymers depends on the specific interactions between polymer chains. This can be explained by the free energy of mixing: ΔGM 5 ΔHM 2 T ΔSM

(3.1)

where ΔGM is the change in the Gibbs free energy of mixing, ΔHM and ΔSM represent the excess enthalpy and the mixing entropy, and T is the absolute temperature.

Table 3.1 Terminology of polymer blends.

Miscible polymer blend: polymer blends that show single phase over certain ranges of temperature, pressure, and composition, homogeneous down to the molecular level; associated with a negative value of the free energy of mixing, ΔGm DΔHm # 0, where ΔGM is the change in the Gibbs free energy of mixing and ΔHM is the enthalpy of mixing. Immiscible blends: polymer blends whose free energy increase upon mixing, that is, ΔGm DΔHm . 0; distinctive phases Compatible polymer blend (polymer alloy): immiscible polymer blend that exhibits macroscopically uniform physical properties. The macroscopically uniform properties are usually caused by sufficient interactions between the polymeric components, resulting in enhanced performance compared to constituent polymers. Occasionally, two polymers can inherently form immiscible yet a compatible blend. However, in most cases, a compatibilizer is added to modify the interfaces between the components. Compatibilization: a process to modify the interfaces in immiscible polymer blends, resulting in a reduction of the interfacial energy, development, and stabilization of the desired morphology, leading to the creation of a compatible polymer blend with enhanced performance.

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Figure 3.2 Idealized expected properties arising from blending two polymers.2

Figure 3.3 The ideal miscible blend has a single Tg that varies linearly. However, realistically the variation may look like the ones shown in dotted lines depending on how well the two polymers bind together. If the two polymers bind more strongly to each other than to themselves, the Tg will be higher than expected and vice versa.

For a homogeneous miscible blend, the Gibbs free energy of mixing requires a negative value. The free energy of mixing can only be negative if the heat of mixing is negative given that for most of the systems the entropy of mixing is small. This means that the mixing must be exothermic, which usually requires specific interactions between the blend components. These interactions may range from strongly ionic to weak and nonbonding, including hydrogen bonding, ion
dipole, dipole
dipole, and donor
acceptor interactions. Fig. 3.4 presents one such example.

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91

Figure 3.4 (Left) Schematic representation of interactions in the poly(N-vinylpyrrolidone) (PVP) and polystyrene terminated with sulfonic acid (PSS). PVP acts as a proton acceptor and PSS as a nonpolar polymer having a proton donor at one end. (Right) Optical micrograph of the PVP-PSS blend. No phase separation can be seen. It is considered that the hydrogen bonding between the terminal SO3H group of PSS and the repeating unit of PVP carbonyl group causes the homogenization of the PVP-PSS blend.3

As miscibility requires a specific set of conditions to be followed, immiscibility dominates. Most polymers form immiscible blends that require compatibilization. When two immiscible polymers are blended without compatibilization, generally one obtains a mixture with physical properties worse than those of the individual polymers. Usually, such a blend has poor structural integrity and poor heat stability. When two immiscible polymers are blended with compatibilization, a synergistic combination of properties derived from each polymer can be achieved. The compatibilization process must accomplish three tasks: (1) reducing the interfacial tension; (2) stabilizing the morphology against shear and thermal effects during the processing steps; and (3) providing interfacial adhesion in the solid state. The compatibilization can be achieved via the following: 1. addition of a small quantity of cosolvent, which is miscible with both phases; 2. addition of a copolymer, one part of which is miscible with one polymer and another part is miscible with other polymers in the blend; 3. addition of a large amount of a core
shell copolymer; 4. reactive compounding that leads to modification of at least one polymer and results in the development of local miscibility regions

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5. addition of a small fraction of nanoparticles, which influence blend structure similarly to particle-stabilized oil emulsions. One may use a single compatibilizer or a combination of various agents, each playing different roles. One example is shown in Fig. 3.5, where the two phenolic terminals of bisphenol A (BPA) provide hydrogen bonds with both poly(vinyl acetate) (PVAc) and poly(N-vinylpyrrolidone) (PVP).

3.1.2 Thermodynamics of polymer blends The performance of polymer blends depends on the properties of individual polymeric components, as well as how they are arranged in space. The spatial arrangement is controlled by thermodynamics and flow-imposed morphology; the latter is more related to the processing of polymer blends and hence not discussed here. The word “thermodynamics” invariably brings to mind “miscibility.” However, it should be noted that thermodynamics has a broader use for the practitioners of polymer science and technology than predicting miscibility. Nevertheless, in the context of this book, we will discuss thermodynamics concerning miscibility only. Determination of such properties for polymeric blends is in principle difficult due to the high viscosity of macromolecular species, slow diffusion, heat generation when mixing and thermal degradation at processing relevant temperatures. For these reasons, there is a tendency to use values obtained from low molecular analogs or solutions. In the subsequent section, we will introduce two common theoretical approaches, followed by a few typical experimental techniques. 3.1.2.1 Flory
Huggins theory The first attempts to calculate the entropy change in polymer solutions with nonideal thermodynamic behavior were due to Flory5 and Huggins,6 commonly termed the Flory
Huggins model. The Flory
Huggins model uses a simple representation for the binary systems and calculates the total number of ways the lattice can be occupied by small molecules and by connected polymer segments. For binary systems that contain two components (traditionally, for polymer solutions the subscript 1 indicates solvent and 2 the polymer) the Flory
Huggins relation can be expressed as7
 ΔGm [1 [2 χ12 0 0 (3.2) ln[1 1 ln[2 1 χ12 [1 [2 withχ12  5 RT V1 V2 Vref

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Figure 3.5 (A) Hydrogen bonding interaction of bisphenol A (BPA) with poly(vinyl acetate) (PVAc) and PVP. BPA acts as a compatibilizer and forms hydrogen bonds with both PVP and PVAc enhancing the miscibility of PVAc/PVP immiscible binary blend; (B) DSC thermograms of BPA/PVAc/PVP ternary blends. The miscibility of PVAc/PVP blend increases with the increasing content of BPA and ultimately, resulting in a complete miscible blend exhibiting a single value of Tg.4

where G: the Gibbs free energy and the subscript m represents a change of the state corresponding to the formation of mixture, R: the universal gas constant, 8.314 J/K/mol,

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T: the temperature in Kelvin, [: the volume fraction of the individual components, V : the molar volume of individual components, χ12 : the interaction parameter, Vref: the reference volume is usually taken as Vref 5 100 mL/mol (i.e., the liquid density is 1), χ012 : the Flory
Huggins interaction parameter; this parameter is the polymer-solvent/polymer interaction energy per mole of solvent/mixture divided by RT, which itself has the dimensions of energy. In Eq. (3.2), the first two logarithmic terms give the combinatorial entropy of mixing, which are by definition of [ negative and always promote mixing, while the third term is the enthalpy of mixing. For polymer blends, V of individual components is large; thus the combinatorial entropy becomes vanishingly small. Therefore the higher the molecular weights of the components ΔGm is less negative and the mixture is less stable. In fact, mixtures of highmolecular-weight polymers are indicated to be always incompatible unless χ12 # 0. This situation will occur only when the enthalpy of mixing is less than or equal to zero, that is, when there are some specific interactions (not van der Waal type) between the components of the mixture.8 Using [1 1 [2 5 1 and the monomeric volume as a reference volume, the free energy of mixing per monomer basis can be written as follows: ΔGm [ 12[ ln[ 1 lnð1 2 [Þ 1 χ012 [ð1 2 [Þ 5 N1 N2 kT

(3.3)

where Ni is the degree of polymerization of individual components and k is the Boltzmann constant. To determine miscibility, it does not matter if one uses the change in free energy of mixing expressed per unit volume, per mole of lattice sites, or per monomeric volume. Due to the assumption in the original Flory
Huggins model, it can predict the upper critical solution temperature (UCST) only (Fig. 3.6). According to Eq. (3.3), the critical point conditions (the critical point is located on the spinodal, thus, @2 ΔGm @2 [ 5 0, and is the extremum of the spinodal curve, thus, @3 ΔGm @3 [ 5 0Þ and treating χ12 as composition independent, the critical conditions for phase separation can be expressed as follows: !2 1 1 0 χ12;cr 5 1=2 1 (3.4) ON1 ON2 Eq. (3.4) gives the miscibility condition for polymer blends as follows:

Binder system interactions and their effects

95

Figure 3.6 An example of a phase diagram predicted by the FH model for various ratios of molar sizes. Solid lines are binodals and dashed lines are spinodals; for each, a generic χ12 5 2 0:6 1 300=T was assumed. All spinodals, UCST, and the bimodal of the symmetric N1 5 N2 mixture are analytically calculated from the derivatives of Eq. 3.3. The rest of the binodals is from numerical solutions.7

Polymer blends (N1  N2 c1Þ are miscible when χ12 , 0 or χ12 , χ12;cr and from Eq. (3.4), χ12;cr 5 2=N  0 (for N 5 N1 5 N2 ) Originally, the parameter χ12 was assumed to be a function of the nature of the components only in a binary mixture. However, it was soon found that even for polymer solutions χ12 is a complex function of many independent variables such as concentration, temperature, pressure, molecular weight, and molecular weight distribution. The Flory
Huggins theory has certain limitations even when all of its restrictive assumptions such as weak interactions and entropy-independent enthalpy are satisfied. Nevertheless, a large number of Flory
Huggins theory extensions and modifications are available in the literature. These extensions aim to address the type and geometry of monomer, stiffness of the backbone, existence of unsaturated carbons, etc. The readers are encouraged to read a review by Freed et al.9 for details. In most cases, such extensions retain the Flory
Huggins equation for the free energy of mixing and redefine the χ0 parameter as an appropriate function, rather than a system-specific constant: 0

χ ð[; T Þ 5 a 1 ðb 1 c[Þ=T

(3.5)

where a, b, and c are corrections due to monomer geometry, packing and other considerations (please refer to Ref. [9]). The details of such corrections go beyond the scope of this chapter.

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Feedstock Technology for Reactive Metal Injection Molding

3.1.2.2 Solubility parameter approach The concept of the solubility parameter originates from Hildebrand’s work on the enthalpy of regular solutions.10,11 The solubility parameter, δ, is defined as the square root of the cohesive energy density: pffiffiffiffiffiffiffiffiffiffi δ  E=V (3.6) where E is cohesive energy and V denotes molecular volume. Additionally, this approach assumes that the molecular interactions are nonspecific, that is, they do not form associations or orientation, hence not polar nor hydrogen-bonding interactions. Another fundamental assumption is that the intermolecular interactions 1
2 are the geometric mean of the intramolecular interactions 1
1 and 2
2, where “1” and “2” indicates individual polymeric components 1 and 2. If the contact energies (interaction energies) are assumed to be temperature independent, the change of enthalpy on mixing is ΔHm  ΔGmnon comb 5 [1 [2 Vm ðδ1 2δ2 Þ2 $ 0 2sffiffiffiffiffiffiffi sffiffiffiffiffiffiffi32
V  V V V V E E E 2E E 1 2 1 12 2 5 5 2 (3.7) 2 pffiffiffiffiffiffiffiffiffiffiffi 1 ΔHm  [1 [2 Vm 4 V1 V2 V1 V2 V1 V2 where EV is the molar energies of vaporization of individual substances (1 and 2), ΔGmnon comb is noncombinatorial free energy of mixing, Vm is the volume of the mixture, while δ represents solubility parameters of individual substances. Comparing Eqs. (3.2) and (3.7), the binary interaction parameter χ012 can be written as   (3.8) χ012  Vref =RT ðδ1 2δ2 Þ2 where the reference volume is usually taken as Vref 5 100 mL/mol. It is important to note that the above interaction parameter is limited to the enthalpic part of binary interaction parameter, that is,   χ012 5 χS 1 χH 5 χS 1 Vref =RT ðδ1 2δ2 Þ2 (3.9) The entropic term in Eq. (3.9), χS , originates from local configurational effects, as well as combinatorial entropy contributions. When Eq. (3.9) is used, χS must be accounted for via other means. For molecules without polar groups, the solubility parameter δ can be determined or approximated from its definition:

97

Binder system interactions and their effects

δ2i 5 EiV =V 5 ðΔHiV 2 PV Þ=V

(3.10)

Given that polymer evaporation experiments are impossible, the solubility parameter of a polymer is usually determined by measurements of its oligomeric liquids or by indirect measurements of its behavior in a solvent of known solubility parameters. The solvent approach allows for the polymer to be cross-linked (the degree of swelling, Ds is measured) or simply dissolved in the solvent (the intrinsic viscosity, η, is usually measured).7 The value of δpolymer then can be determined by plotting either Ds or η versus δsolvent as the peak location.12 A major drawback of the solubility parameter approach is the omission of entropic and specific interactions’ effects. Furthermore, the fundamental dependencies do not take into account either the structural orientation or the neighboring group effects.13,14 However, this approach can provide a guide for miscibility. Since miscibility only occurs if ΔGm # 0, and given ΔSm is always positive, the component of a mixture is assumed to be compatible only if ΔHm # TΔSm (zero or a small value of ΔHm). As this theory allows only positive heats of mixing (Eq. 3.6), the mixture may be miscible if the absolute value of the ðδ1 2δ2 Þ2 difference is zero or small. In the simplest approach, the solubility parameter of a polymer can be calculated by a summation of group contributions.13,14 The quality of this method is it assume pairwise additivity for the interaction of submonomeric building blocks “groups,” which can be added to form the monomer units. For example, a simple hydrocarbon such as n-octane is assumed to consist of six
CH2
and two
CH3 groups. Coleman et al.13 then estimated molar attraction constants for
CH2
and
CH3 using the energy of vaporization. Additionally, by including branched hydrocarbons, and molecules containing other functional groups, Coleman et al. obtained a table of constants. Readers are encouraged to read the Miscible Polymer Blends book by Coleman et al. for details.13) These constants can be used to calculate the solubility parameter for various polymers, as per the relationship: P g F δ 5 P i ig (3.11) i Vi g

g

where Fi is molar attraction with units of (cal cm3)1/2/mol, and Vi is molar volume (cm3/mol). Refs. [13,14] contain additional groups and important guidelines of how, and when, meaningful solubility parameters for polymers can be obtained.

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The Hildebrand solubility parameter concept was developed for nonpolar, low molecular weight liquids at room temperature only. For polar molecules, the method does not provide consistent information. To cope with polar molecules, Hansen15 postulated that all types of intermolecular forces can be combined and grouped into three types of interactions: dispersive, polar, hydrogen bonding, respectively. These intermolecular forces include London dispersion forces between nonpolar molecules, Coulombic ion/ion interactions, dipole/dipole interactions between the permanent dipoles, repulsive forces between nonpolar molecules, induced dipole/ion interactions, hydrogen bonding, permanent dipole/ion interactions, and charge-transfer forces. When the intermolecular forces are categorized into these three groups, a substance’s total solubility parameter can be written as δ2i 5 δ2id 1 δ2ip 1 δ2ih

(3.12)

where the subscripts d, p, and h represent the dispersive, polar, and hydrogen-bonding interactions, respectively. Accordingly, two substances would be miscible only when their solubility parameter place them within the critical radius of a spheroid as defined by Hansen15,16:  2 2 crit 5 Θðδ1d 2δ2d Þ2 1 δ1p 2δ2p 1 ðδ1h 2δ2h Þ2 5 χ12 $ 0 (3.13) R12 

Crit R1;2

2

 2  2  2 5 Θ δ1;d 2δ2;d 1 δ1;p 2δ2;p 1 δ1;h 2δ2;h 5 χ1;2 $ 0

where Θ is the semiempirical fudge parameter with the values of 4
5 and accounts for the dominant role that the dispersion forces play in binary solubility. Fig. 3.7 illustrates the Hansen concept. Examples of the numerical value of the Hansen’s parameters are reported in the literature,15
18 whereas a comprehensive collection of values has been compiled in a handbook.19 Like the Coleman approach, the values of Hansen’s partial solubility parameters (δ1d , δ1p , δ1h ) can also be calculated from the molecular structure of a polymer by using additive group contributions.20 The details of which are beyond the scope of this book. Readers are encouraged to refer to cited references for detailed information about theoretical approaches to the thermodynamics of polymer blends.

3.1.3 Experimental methods Several experimental methods are used to characterize (mainly miscibility) of polymer blends. These methods can be divided into four groups, Table 3.2.

Binder system interactions and their effects

99

Figure 3.7 Schematic representation of Hansen’s miscibility sphere.7,16

Some of these techniques are discussed in subsequent sections. 3.1.3.1 Determination of interaction parameters for binary systems All types of radiation scattering techniques, such as light, X-ray, and neutron, have been used to measure the interaction parameters and study the phase equilibria in polymer blends and solutions. As the great majority of polymer blends have domain sizes in the range of 50 nm
5 μm, both light scattering and SAXS techniques have limited use for studies of phase morphology (Table 3.3). Small-angle neutron scattering (SANS) has been widely used to study macromolecular morphology, size, and conformation, in a single or multicomponent system (in molten or solid state). As the phase difference is based on the mass number, it is very useful to replace the hydrogen atoms in one polymer, or parts of a polymer, by deuterium (deuterium is a stable isotope of hydrogen, consisting of 1 proton, 1 neutron, and 1 electron). The deuterated polymer is mixed with its hydrogenated homolog at a selected low concentration, usually, B0.1%, providing a means to control contrast. The mixture can be then used as one of the blend’s components. However, deuteration changes the conformation of

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Feedstock Technology for Reactive Metal Injection Molding

Table 3.2 Characterization techniques for polymer blends. Indirect methods of miscibility

Direct methods of miscibility

Studies of phase equilibria

Determination of “χ” interaction parameter

Opacity (light scattering

Microscopy: • Phase Contrast • Electron

Melting point depression

Glass transition: • DSC • Dynamic Mechanical • Dielectric Spectroscopy: • NMR • Infrared ODT through rheology

Turbidity measurements

Scattering methods: • PICS • SAXS • SANS • Turbidity Fluorescence techniques

Combinational approaches

Ultrasonic measurements

Inverse phase gas chromatography







SAXS, SANS

Vapor sorption

N.B.: DSC, Differential scanning calorimetry; NMR, nuclear magnetic resonance spectroscopy; ODT, order to disorder transition; PICS, pulse-induced critical scattering; SAXS, small-angle X-ray scattering; SANS, small-angle neutron scattering.

Table 3.3 Approximate ranges of the dimensions of scattering phases for the light, neutron, and X-ray techniques. Method

Origin of contrast

Scattering domain size (μm)

Light scattering (LS) Small-angle neutron scattering (SANS) Small-angle X-ray scattering (SAXS) Wide-angle X-ray scattering (WAXS)

Refractive index Mass number

1
100 0.01
3

Electron density

1
100

Electron density

0.1
1

macromolecules and also their miscibility/solubility, especially for high molecular weight (Mw) polymers.21 SANS has also been used extensively to determine χ12 of polymer blends by fitting SANS profiles measured from blends to random phase

Binder system interactions and their effects

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approximation (RPA).22
25 This method uses RPA theory to relate χ12 to the composition fluctuations of a polymer blend melt.26 These composition fluctuations can be measured using SANS and χ12 can be extracted from the scattering data. Example: SANS is an effective characterization method to investigate nanoscale structures. It is based at neutron scattering facilities around the world and has experienced steady growth over the past 40 years. It examines structures with sizes from the near-atomic to the near micrometer scale and has had a significant impact in many research fields including materials science and biology. The partial deuteration method gives the SANS technique unique advantage.25 The SANS instrument uses the following basic steps: monochromation, collimation, scattering, and detection as shown in Fig. 3.8. Like other scattering methods, SANS yields measurements in the reciprocal (Fourier transform) space and depends therefore on data interpretation using models. The most common model for polymer blends studies is RPA, the details of which are beyond the scope of this book and can be found in online resources.29

Figure 3.8 Schematic of a SANS instrument. Monochromation consists of producing a monochromatic neutron beam from the Maxwellian neutron source spectrum and is performed using a velocity selector. Collimation is performed using a source aperture and a sample aperture to define an incident neutron beam with a very small divergence. Scattering from samples of various forms and in various environments is measured in special cells. Detection of the scattered neutrons is performed using a 2D area sensitive detector. For readers interested in detailed SANS theory of operation, please refer to Refs. [27,28] and the references within. From Ref. [25], courtesy American Chemical Society, 2010.

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Kobayashi et al.30 studied the effects of ring topology on the miscibility of polymer blends via SANS. For this purpose, highly purified hydrogenous ring poly(4-trimethylsilylstyrene) (h-PT) and deuterated ring polyisoprene (d-PI) samples, as well as their linear counterparts were prepared. PT and PI are known to form a miscible polymer blend with the lower critical solution temperature (LCST) type phase diagram. The authors evaluated the miscibility of three kinds of polymer blends, that is, linear
linear, ring
linear, and ring
ring, denoted as L
L, R
L, and R
R, respectively, with all 50/50 vol.% in the composition. The sample preparation details can be found in their publication.30 The respective SANS measurements were conducted on the 40 m SANS at high-flux advanced neutron application reactor (HANARO) at Korea Atomic Energy Research Institute (KAERI), Korea. The neutron wavelength, λ, was 7 Å, the sample-to-detector distance was 3 m, and a pinhole with 3 mm in diameter was set just upstream of the samples. This equipment condition covered the q range of 0.01 # q/Å21 # 0.2, where q is the scattering vector given by q 5 (4π/λ)sin(θ/2), and θ is the scattering angle respectively. The intensities were counted on the 2D position-sensitive detector (for complete details regarding SANS measurements please refer to Ref. [30]). To complement the SANS results, the authors also carried out optical microscopy (OM) measurements. However, for OM, a hydrogenous PI (h-PI) sample was used instead of d-PI for the blend. Fig. 3.9 presents the overview of the SANS profiles (double-log plots of I (q) vs q) for L
L, R
L, and R
R blends, respectively. It is clear from Fig. 3.9D that the intensities of the R
R blend are higher than the other two L
L and R
L blends covering all q range, indicating that the R
R blend exhibits much larger concentration fluctuation than the others. As other characteristics of the blends such as the molecular weight of individual polymers and blend compositions were the same, this result points to the chain topology effects on blend miscibility. Recent experimental and theoretical studies31,32 suggest that ring chains possess more compact and isolated conformation, which is associated with stronger intermolecular repulsive forces. This could be the reason for the lower miscibility of R
R blend compared to L
L or R
L blends in this study. Comparing the L
L and R
L blends, it is evident that L
L exhibits higher intensities at low q (#0.03 Å21), while exhibiting lower intensities at high q ($0.03 Å21) than R
L. This crossover in I(q) reflects the difference in the intrinsic component chain architecture as well as the

Figure 3.9 (A
C) Double-logarithmic plots of the SANS profiles (I(q) vs q) for (A) L
L, (B) R
L, and (C) R
R blends at various temperatures; (D) Comparison of the SANS profiles for L
L, R
L, and R
R blends at 170°C only. From Ref. [30], courtesy American Chemical Society, 2018.

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concentration fluctuation. However, at low q the contribution of concentration fluctuation is dominant. This suggests that the R
L blend has better miscibility than the L
L blend as it exhibits smaller concentration fluctuation at low q. This result was also confirmed with optical microscopic observations (Fig. 3.10). The authors observed that at the lower T side, the blend exhibits the homogeneous phase as shown in Fig. 3.10A, while on the higher T side, it demonstrates micrometer-ordered domain structures as evidenced in Fig. 3.10B. From the observation of this blend, the authors confirmed that phase separation began to occur at 187°C. In this way, the authors examined the phase behavior for the hL
hL and hR
hL blends at various compositions by observing the phase separation induced via change of temperature. The results are summarized in Fig. 3.11. It is clear from Fig. 3.11 that the hR
hL blend exhibits higher phaseseparation temperatures, that is, better miscibility, than the hL
hL blend, at all ΦPT ranges examined. This result is also in good accordance with the SANS results. Based on the results the authors concluded that since the miscibility of R
R blend is considerably lower than the other two blends, it is a clear manifestation of the topological effect on the phase behavior of the studied blend. Nishi et al.33,34 used melting point Tm depression to determine χ12 . It should be noted that Tm depends on two factors: (1) the unit cell geometry and dimensions of the crystallites, (2) the interactions between the crystalline polymer and other ingredients. To determine χ12 from Tm, there must be no chemical reactions between blend components and all specimens must have identical thermal history. However, it is important to point out that the incorporation of other ingredients changes

Figure 3.10 Optical images of the hR
hL blend of ΦPT 5 0.47 as observed by authors at (A) 180°C and (B) 190°C. The scale bars in the images indicate 50 μm. It is worth noting that the temperature was continuously increased at a rate of 1°C/min. From Ref. [30], courtesy American Chemical Society, 2018.

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Figure 3.11 Phase diagram, lower critical solution temperature (LCST) of the hL
hL and hR
hL blends. All plots have a margin of error (at most 6 3°C). The difference between the two blends is pronounced at lower ΦPT. From Ref. [30], courtesy American Chemical Society, 2018.

crystallinity only through thermodynamic interactions. Blending a crystalline polymer with an amorphous one can affect crystallinity in diverse ways, due to the effects of added components on nucleation and growth rates (just like solute atoms affect solidification of a pure metal). The presence of a miscible amorphous polymer is that the blend can slow down (or even prevents) crystallization of the semicrystalline polymer. Hence, despite its simplicity, the values of χ12 obtained through this method must be confirmed by other techniques. Example: Miscibility and specific interactions in blends of poly(L-lactide) (PLLA) with poly(vinylphenol) (PVPh) were studied by Emilio et al.35 They used the melting point depression method to calculate the polymer
polymer interaction parameter. Differential scanning calorimetry (DSC) was used to carry out the subsequent thermal analysis. For melting point depression studies, samples were allowed to crystalize isothermally until crystallization was complete. The samples (approximately 5 2 10 mg of each blend) were then heated with a scan rate of 10°C/min to obtain the melting point values. The melting point of a crystalline polymer in the presence of an amorphous polymer can be used to extract a value for the Flory
Huggins interaction parameter, χ12 . This information can be used to estimate the

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free energy of mixing of the two components.36 The depression of the melting point is caused by morphological effects and thermodynamic reasons. Thermodynamic considerations predict that the chemical potential of a polymer will be decreased by the addition of a miscible component. This decrease in chemical potential will result in a decreased equilibrium melting point in the case of a crystallizable polymer. Strong exothermic mixing (miscibility) will produce a considerable melting point depression while weakly exothermic, athermal, and endothermic mixing, will give consecutively less melting point depression meaning immiscible blend. The equilibrium melting temperature can be analyzed by the Flory 2 Huggins equation:


 1 1 2 R V2u ln[2 1 1 2 2 0 5 1 2 ð1 2 [2 Þ 1 χ12 ð12[2 Þ Tm0 Tmb ΔH2u V1u x2 x2 x1 (3.14) where the subscripts 1 and 2 refer to the amorphous and the crystallizable 0 polymer, Tm0 and Tmb represents the equilibrium melting points of the pure crystallizable component and the blend, Vu is the molar volumes of the repeating unit of the polymers, R is the universal gas constant, ΔHu is the heat of fusion of the pure perfectly crystallizable polymer, x is the degree of polymerization, [ is the volume fraction of the crystallizable component in the blend, and χ12 is the polymer
polymer interaction parameter, respectively. When both x1 and x2 are large, these terms can be neglected, which gives

1 1 ΔH2u V1u 2 2 0 5 χ12 [21 (3.15) Tm0 Tmb R V2u χ12 is independent of composition, a plot of If we assume  2 2u V1u 2 T10 2 T10 ΔH R V2u versus [1 will produce a straight line passing m mb through the origin. As χ12 is dependant on Vref. (Eq. (3.8)), this makes χ12 inadequate to compare different systems given monomer volumes of polymers are usually significantly different from each other. The interaction energy density, B, can be used in such cases and is related to χ12 as B5

RT χ12 Vref

Substituting Eq. (3.16) into Eq. (3.15) yields

(3.16)

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0 Tm0 2 Tmb 5 Tm0

BV2u 2 [ ΔH2u 1

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(3.17)

Eq. (3.17) is known as the Nishi Wang equation.33 The equilibrium melting temperatures for pure PLLA and blends of given compositions can be determined via DSC. To relieve the observed results from any secondary effects, the method of Hoffman and Weeks37 can be used. The equilibrium melting points obtained by the authors in this study are listed in Table 3.4. The decrease in the equilibrium melting point of PLLA upon the addition of PVPh suggests the miscibility of the system. However, the higher addition of PVPh causes a low incremental decrease of the melting point, which suggests a relatively low value of the interaction parameter. Once the equilibrium melting temperatures are determined, Eqs. (3.16) and (3.17) can then be used to determine χ12 and B values (Fig. 3.12). It can be seen from Fig. 3.12 that the experimental data line does not pass through the origin. This behavior is attributed to a residual entropy effect. The values obtained for χ12 and B in this study were, 20.42 and 28.8 J/cm3, respectively. The negative value of the interaction parameter (a value near 1 means increasing miscibility and vice versa) confirms a thermodynamically miscible blend. 3.1.3.2 Glass transition temperature (Tg) measurements Perhaps the most common use of Tg in the determination of polymer/ polymer miscibility is based on the presumption that a single Tg indicates a uniform blend. However, it is important to remember that a single Tg is not a measure of miscibility, but rather an indication of the state of dispersion. One approach proposes the following relation for Tg of binary blends38: Table 3.4 Equilibrium melting temperature (Tm0 ) for pure PLLA and PLLA/PVPh blends. PLLA (wt.%)

Tm0 (°C)

100 90 85 80

211.6 203.6 203.0 201.7

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Figure 3.12 Flory
Huggins equation plot for the equilibrium melting temperatures obtained for studied PLLA/PVPh blends. The parameters used in the calculations were V1u 5 100 cm3, V2u 5 53.3 cm3, and ΔH2u 5 93 J/g. From Ref. [35], courtesy American Chemical Society, 2005.

    w1 ln Tg =Tg1 1 kw2 ln Tg =Tg2 5 0

(3.18)

where w1 and w2 are the weight fractions of polymers, Tg is the glass transition temperature of the blend while Tg1 and Tg2 are glass transition temperatures of the individual polymers in the blend. For a miscible blend, the parameter k is equal to k 5 ΔCp1 =ΔCp2 where ΔCp is a difference of the isobaric heat capacity in the liquid and glass states of the polymer, and is assumed independent of temperature. The dependence should be symmetrical, that is, it must be valid when the indices are exchanged. Thus miscibility requires that k 5 1/k 5 1. The larger the difference between k and 1/k, the larger is the immiscibility of the system.7 However, this relation should not be used for strongly associating polymer blends where the blend Tg may reach values higher than those observed for pure components. Such miscible systems are described by a single parameter relation21:

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h i2=3 3=2 3=2 Tg 5 ð1 1 K  w1 w2 Þ w1 Tg1 1w2 Tg2

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(3.19)

where K  is a material parameter with a value that increases with stronger polymer/polymer association. The value of the width of the glass transition temperature is more reliable in assessing the degree of -miscibility than Tg. Moreover, there are equipment available, for example, DTA and DSC, which can measure the width of the glass transition with ease. By measuring Tg and the width of the glass transition temperature for samples annealed at different temperatures and then quenched, one may be able to determine the level of miscibility. One shortcoming of this approach is the limitation of resolution. Even for immiscible blends, it is difficult to detect two glass transition temperatures for compositions containing less than 20 wt.% of the dispersed phase. Therefore this approach can only be used for blends having the difference in Tg . 20°C between the two polymer constituents. Example: Hong et al.39 investigated the phase behavior of a ternary polymer blend of PMMA, poly(ethylene oxide) (PEO) and poly(hydroxy ether of bisphenol-A) (phenoxy) via Tg studies. To study the corresponding thermal properties, they used DSC. For the measurement of the glass transition temperature, blend samples were first heated to 170°C at a heating rate of 20°C min21, maintained at 170°C for 5 min to ensure complete melting of PEO crystals, and then quenched to 290°C. They were then reheated to 170°C at a heating rate of 20°C min21. Importantly, all the scans were carried out under a nitrogen atmosphere to minimize oxidative degradation. Fig. 3.13 shows the composition dependence of the Tg of the three respective binary blends. It can be seen from Fig. 3.13 that all three binary blends exhibit single, composition-dependent Tg, indicating that the three binary blends are miscible. The subsequent DSC thermograms of ternary blends are presented in Fig. 3.14. Although all the three binary blends were miscible, the ternary blend shows two Tgs at any given composition (Fig. 3.14) indicating phase separation or immiscibility. This implies that there exists a closed immiscibility loop in the phase diagram of this particular ternary blend. The authors advocated that this closed immiscibility loop may result from the asymmetry in the interaction energy of binary systems, the so-called “Δχ effect.” Generally, the ternary phase behavior of polymer blend system is primarily

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governed by differences in the physical interaction among the components—expressed by the magnitude of the binary interaction parameters— in the absence of strong specific interactions between the components. If one of the binary interaction parameters is significantly larger than a critical value, a large portion of the ternary phase diagram will likely be heterogeneous. If there are significant differences in the solubility parameter values (χab
χac6¼0), a strong driving force toward phase separation exists. This is called the Δχ effect. For readers wanting to broaden their knowledge on ternary blends, it is advised to visit Refs. [40,41]. 3.1.3.3 Infrared spectroscopy Polymer blends have been extensively characterized using infrared spectroscopy.21,42,43 The applicability, fundamental aspects, and principles of experimentation using an infrared dispersive double-beam spectrophotometer (IR) or computerized Fourier transform interferometers (FTIR) are well described in the literature.44

Figure 3.13 Composition dependence of the glass transition temperature of three binary blends: (A) PMMA/PEO; (B) PEO/phenoxy; (C) phenoxy/PMMA, as studied by Hong et al.39

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Figure 3.14 DSC thermograms for ternary blends of PMMA/PEO/phenoxy at various compositions. The arrow indicates the position of the Tg.39

FTIR is predominantly used to study hydrogen bonding in polymer blends. The hydrogen bonding affects the
OH absorption region (3500
3600 cm21), the 5 CO stretching (1737 cm21), the
CH2 symmetric stretching (2886 cm21), as well as the fingerprint frequency region (1300
650 cm21). Guo et al.45 studied the hydrogen-bonding interactions in biodegradable polymer blends of poly(3-hydroxybutyrate) (PHB) and PVPh via FTIR. PVPh is completely amorphous, while PHB is semicrystalline. The selection of PVPh was based on the hydroxyl group. Because of the existence of C 5 O in PHB, the blend is possible to form hydrogen bonding (Fig. 3.15). Fig. 3.15 presents the FTIR spectra obtained at room temperature for the as-prepared PHB/PVPh blends in the C 5 O stretching vibration region. With increasing WPVPh%, the breadth of the band at 1724 cm21 increases. In addition, new band appears at the lower wavenumber side around 1713 cm21. Interestingly, this peak exists neither in the pure PHB spectrum nor in the pure PVPh spectrum. Hence, this new band is inherent to PHB/PVPh blend system and is attributed to hydrogen bonding between the two components. 3.1.3.4 Microscopy Microscopy methods provide direct morphology imaging and can be divided into several categories: OM, scanning electron microscopy (SEM),

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Figure 3.15 An example of normalized FTIR spectra recorded at room temperature in the C 5 O stretching region for the as-prepared poly(3-hydroxybutyrate) (PHB)/ poly(4-vinylphenol) (PVPh) blends. For the spectrum with wPVPh 5 90 wt.%, the peak around 1700 cm21 (shown by the arrow) was due to the preparation method employed in the study. From Ref. [45], courtesy American Chemical Society, 2010.

transmission electron microscopy, atomic force microscopy (AFM), and several modifications of these techniques. For instance, the scanning transmission electron microscopy and low-voltage scanning electron microscopy (LVSEM, at 0.1
2 kV accelerating voltage) are particularly useful for polymer blends.46 In particular, LVSEM provides an approximately tenfold increased image contrast compared to the conventional SEM with almost no charging problem.47 Moreover, due to the low energy of the secondary electrons, the conductive coating is not required. To study the polymer blends via microscopy, some techniques of sample preparation has to be used after the formation of a blend (e.g., staining, swelling, fracturing, or etching). Although microscopy has been extensively used to characterize the morphology of immiscible blends, it has severe shortcomings in miscible or partially miscible blends. Even at the highest resolution, it is difficult to obtain sufficient confidence to declare that a blend is thermodynamically miscible. Therefore in most cases, microscopy is generally considered as a necessary secondary method

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to characterize polymer blends and validate the results obtained from other techniques such as spectroscopy, Tg, or other measurements. Meredith et al.48,49 developed a combinatorial method involving optical microscopy to determine the entire phase diagram for the polymer blends. The method involves the formation of samples with a gradient of blend composition in one direction (x-axis) and a linear change in temperature in the normal direction (y-axis). After sufficient annealing of the samples, the LCST phase diagram can be directly observed with optical microscopy (Fig. 3.16).

3.1.4 Common binder blends Although studies on the blending of common polymers are available in the literature, surprisingly, little attention has been paid to study specific interactions among the components of a particular binder system. Here are some examples of common polymer blends that are used as binders for reactive powders-MIM as well. These examples do not depict the exact nature of the interactions among binder components. Instead, they provide a reference to readers for future studies. 1. Paraffin wax/ethylene vinyl acetate blend Standring et al.50 recently investigated paraffin wax (PW) and ethylene vinyl acetate (EVA-28) blends for their use as a binder for ceramic injection molding applications. They stated that PW and EVA-28, in most circumstances, combine to form stable homogeneous blends, which experience relatively small changes in the melting and solidification phase transition behavior (Fig. 3.17). The heating profiles of PW and EVA show strikingly different thermal behaviors as can be seen in Fig. 3.17, which is a direct result of the different levels of crystallinity in their structures. Due to the normalized nature of the DSC thermograms, the EVA trace appears as only a very marginal deviation from the baseline. Increasing the EVA content in PW/EVA blends has a significant effect on the melting profile. A general feature of both shallower and broader peaks (as EVA wt.% increases) is observed, which is a direct indication of interactions between PW and EVA. The authors also verified the interactions by studying the mechanical and rheological properties of the blends. For instance, they reported that the melt flow behavior of the blends at shear rates of 100 s21 varied from 0.01 Pa s for PW to 10 Pa s for the blend containing 50% PW and 50% EVA by weight. Similarly,

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Figure 3.16 Optical microscopy micrograph of a polystyrene (PS) and poly(vinyl methyl ether) (PVME) combinatorial sample after 16-h of annealing, showing the lower critical solution temperature cloud-point curve. From Ref. [48], courtesy American Chemical Society, 2000.

increasing the EVA content in a blend caused the initiation of shear thinning at progressively lower shear rates and formed a blend with an increasing elastic character at typical injection temperatures. However, they did not study the nature of specific interactions between PW and EVA, which has been investigated to a limited degree in the literature. 2. PEG/PMMA blend PEG or PEO/PMMA blends are among the most studied polymer systems51
55 (it is worth mentioning here that materials with Mw , 100,000 are usually called PEGs, while higher molecular weight polymers are classified as PEOs). It is reported in the literature that linear PMMA can form stable blends with linear PEO due to van der Waals type bonding between the PMMA chains and the planar PEO segments.51 The miscible domain starts around 10% of PEO by weight and goes up to 30%; the blend is presumed immiscible with PEO . 30 wt.% and crystalline aggregates of PEO can be observed.51 Since the weight fraction of PEG in the PEG/PMMA binder system for MIM is generally higher than 50 wt.%, it is fair to assume that the blends of the PEG/PMMA binder system typically used for MIM

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Figure 3.17 DSC thermograms of different PW and EVA blends (data from the heating cycle, 220°C to 600°C at a rate of 10°C/min). The DSC thermograms illustrate predominantly crystalline features of the blends.50

are immiscible. Sari et al.56 studied PEG/PMMA blends as novel form-stable phase-change materials for thermal energy storage. The studied blend composition was 70/30 w/w% PEG/PMMA. They revealed that the blend showed a nonuniform single-phase appearance (Fig. 3.18). They claimed that the single phase on the surface of the blend did not mean miscibility of PEG with PMMA on a microscopic scale. However, PEG was well distributed in the PMMA matrix owning to its excellent compatibility. To investigate the possible interaction between PEG and PMMA, the authors carried out FTIR spectroscopy (Fig. 3.19). Stretching of CH peaks at nearly the same wavenumbers was observed. The carbonyl peak position in the blend was almost the same as that in PMMA, but it had a narrow shape. Finally, they concluded that the broadening of OH stretching peaks at about 3450 cm21 was caused by the possible carbonyl
hydroxyl interaction. Silvestre et al.52 reported that the PMMA tacticity strongly influences the miscibility of PEO/PMMA blends. They reported that for a

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Figure 3.18 Optical microscopy image of a form-stable PEG/PMMA blend.56

given temperature and composition, in the melt phase, the syndiotactic PMMA is more miscible with the PEO than the isotactic PMMA. In addition, they explained that for PEO to interact with PMMA, PEO has to act as a proton acceptor through the negative oxygen, whereas PMMA acts as a proton donor through the positive carbonyl atom. Possible hydrogen bonding is shown in Fig. 3.20. It should be noted that in any case, there cannot be a strong interaction between PEO and PMMA in PEO/PMMA blends, as the attractive forces between negatively charged oxygen atoms of PEO and positively charged carbonyl carbon atoms of PMMA are weakened by the repulsive forces due to the two negatively charged oxygen atoms of PMMA. 3. Paraffin wax/different polyethylene blends As with PEG/PMMA blend, wax and polyethylene (PE)-based blends have been studied as form-stable phase change materials (PCMs). One can use the results of those studies as guidance during the evaluation of binder systems for MIM. Krupa et al.58 studied blends based on low-density polyethylene (LDPE) and paraffin waxes. Paraffin waxes are classified into two groups, depending on the molecular weight and melting point. The soft paraffin has a low melting point so it is soft at room temperatures. In the context of MIM, hard paraffin wax is primarily used. These authors reported that the hard paraffin wax was much more miscible with LDPE because of the hard paraffin wax co-crystallize well with LDPE, as compared to the soft

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Figure 3.19 FTIR spectra of PEG, PMMA, and the PEG/PMMA blend.56

Figure 3.20 Possible hydrogen bonding between PEG and PMMA.57

paraffin wax. LDPE blended with hard paraffin wax decomposes in just one step, while blends containing soft paraffin wax decomposes in two distinguishable steps (Fig. 3.21). The blends miscibility was also confirmed with SEM, which showed contrasting morphologies for the blends based on different waxes (Fig. 3.22).

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Figure 3.21 (Left) Thermogravimetric analysis (TGA) curves of LDPE, hard wax (Wax FT), and LDPE/hard wax blends (Right) TGA curves of LDPE, soft wax, and LDPE/soft wax (Wax S) blends.58

Figure 3.22 SEM micrographs of (A) 50/50 w/w LDPE/hard wax blend; (B) 50/50 w/w LDPE/soft wax blend.58

The SEM micrographs clearly show the differences in the morphologies of the two types of blends. The LDPE/hard wax blend shows a homogeneous surface with only a slight indication of wax separation, indicating excellent compatibility between the two components. On the other hand, clear phase separation is visible in the case of LDPE/soft wax blend. Nonetheless, the authors claimed that LDPE/soft wax blend showed partial miscibility in the molten state, as the DSC result showed a shift of the melting peak of LDPE in the blend to lower temperatures and that of soft wax to higher temperatures, as compared to the pure components. The authors further

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claimed that the probable reason for contrasting behaviors of the two blends is the different molecular weights of waxes since the crystalline structure is identical for both types of waxes. Soft wax due to its lower molecular weight and resulting lower viscosity can separate from the blends much easier than hard wax. In addition, PE chain structure and morphology have a substantial influence on its miscibility with paraffin wax, as discussed in the literature.59,60 Chen et al.61 studied the binary PE/paraffin wax blends at different ratios of high-density polyethylene (HDPE), LDPE and linear lowdensity polyethylene (LLDPE) using DSC and AFM. They reported that for blends containing 30% paraffin wax, two distinct phases were found for each sample, with an intermediate phase depending on the type of PE (Fig. 3.23). Fig. 3.23 shows the paraffin wax is well dispersed in the PE, though the blends are not uniform, as can be seen by the shape and size of the paraffin domains. The clear phase separation supports the notion that PE and paraffin are not miscible, with miscibility increases in the order of LLDPE . LDPE . HDPE. Nevertheless, paraffin wax showed good compatibility with PE in every case. 4. Paraffin wax/polypropylene Another widely used binder system for reactive powders MIM is based on paraffin wax and polypropylene (PP). Similar to wax/PE blends, wax/PP blends have also been studied as potential PCMs, albeit in less detail.62,63 Krupa et al.62 studied isotactic-PP (the isotactic-PP offers more crystallinity and has a higher melting point than atactic or syndiotactic-PP. Most of the commercial PP is isotactic and has an intermediate level of crystallinity, blended with soft and hard paraffin wax respectively, for this purpose. They used DSC, dynamic mechanical analysis, thermogravimetric analysis (TGA), and SEM to determine the structure and properties of the blends. Strong phase separation was observed in both cases, which was more pronounced in the case of soft paraffin wax (Fig. 3.24). The SEM micrographs indicate the complete immiscibility of both waxes with PP. Both micrographs clearly show separate crystal fractions for PP and wax and there is no indication of miscibility or co-crystallization on micrometer level. The authors confirmed the SEM results with TGA analysis as well. The TGA curves are shown in Fig. 3.25. The results indicated that the thermal stability of the blends decreased with an increase in wax

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Figure 3.23 AFM phase images of different PE/paraffin wax blends. The light color corresponds to paraffin wax, the dark color corresponds to PE while green color shows intermediate phase.61

content because of the lower thermal stability of the wax. The blends consisting of hard wax had significantly higher thermal stability than those containing soft wax at the same paraffin wax content. Moreover, blends consisting of soft wax and PP decompose in two distinguishable steps (Fig. 3.25A), whereas blends containing hard wax decompose in only one step (Fig. 3.25B). This fact also indicates a higher level of compatibility of hard paraffin wax with PP in comparison to soft paraffin wax. Therefore if soft wax in conjunction with PP is to be used as the binder system for reactive powders-MIM, it is recommended to use a

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Figure 3.24 SEM micrographs of 50/50 w/w: (A) hard wax/PP and (B) soft wax/PP blends.62

Figure 3.25 TGA curves for (A) soft wax/PP blend and (B) hard wax/PP blend. In both cases, the measurements show no char yield at temperatures higher than 500° C.62

compatibilizer or a secondary polymeric component to achieve good homogeneity of the resulting feedstock.

3.1.5 Further remarks for binder blends Based on the earlier discussion, one can see it is not easy to select a proper binder system. One cannot simply mix two polymers and use them as binders. During the selection of binder components particularly the primary and secondary components, it is critical to check important parameters such as molecular structure, molecular weight, and polarity that can affect compatibility and miscibility between the two components. In

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some cases, the primary component may act as a compatibilizer. For example, Itoh et al.64 fabricated high strength Ti alloy compacts by MIM using a binder system containing PP, PMMA, PW, and stearic acid in a weight ratio of 30:40:29:1. Interestingly, PMMA is antagonistically immiscible with PP.65 As both polymers are immiscible, compatibilization is required, which is generally achieved by adding a block or graft copolymer of the two polymer components in the blend or by forming such a copolymer through covalent or ionic bonds in situ during blending. Without any compatibilizer, the binder system would simply collapse in this case. As the authors did not provide any information regarding compatibilizer, it begs the question of how a homogeneous mixing and feedstock was achieved. The answer may lie with the third component PW. Shi et al.66 showed that PW can form stable blends with PMMA. Similarly, PP can form compatible blends with PW as was shown in the previous section. It is worth noting that PW may act as a compatibilizer agent for PP/PPMA/PW binder system. Moreover, the authors used molybdenum (Mo) powder with a mean particle size of 1.59 μm, which may also help in stabilizing blend. Similarly, Hayat et al.67,68 recently incorporated PVP into PEG/PMMA binder system. The resulting feedstock had excellent homogeneity, rheological properties. The overall performance of the feedstock was better than the feedstock made of the unaltered PEG/PMMA binder system. This is because the addition of PVP acts as a compatibility enhancer in the PEG/PMMA blend. PVP is miscible with both PMMA69 due to dipole
dipole interactions between them and PEG (up to a molecular weight of 1500 g/mol)70 due to hydrogen bonding between them. As the authors used a PEG molecular weight of 1500 g/mol, the addition of PVP inevitably made the immiscible (yet compatible) PEG/PMMA blend into a miscible blend leading to improved rheological properties. 3.1.5.1 Case study: complex interactions and their effects on reactive powders-MIM As explained in the previous section, one can further improve the homogeneity of a binder system by incorporating a compatibilizer to enhance the compatibility/miscibility between the components of the binder system. However, it does not always lead to excellent results particularly in the case of reactive powders MIM. In general, the more the binder components, the more will be the complex interactions between them that can leave unwanted char inside the thermally debound samples. In one

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such example, Hayat et al.71 recently studied PEG/poly(propylene carbonate) (PPC) binder system for titanium metal injection molding. PPC is a biodegradable, clean polymer that leaves no residue behind. Follow up trials on the PEG/PPC binder system showed promising results in terms of feedstock homogeneity, thermal degradation behavior and formation of injection molded parts. However, at the subsequent water debinding step for Ti-MIM, the injection-molded samples were completely disintegrated due to insufficient green strength. It was postulated that the insufficient shape retention was due to insufficient interaction/binding between PEG and PPC even though the PEG/PPC blend was homogenous after the mixing process. To improve the green strength, PMMA was incorporated in the PEG/PPC binder blend. The addition of PMMA affects this interaction between PEG and PPC because it is more compatible with PEG and can also form a stable blend with PPC.72 An illustration of potential hydrogen bonding among the components is shown in Fig. 3.26A. The PMMA addition drastically improved the green strength while maintaining excellent rheology (Fig. 3.26B). However, increased interactions in the case of PEG/PMMA/PPC blend also mean a different thermal degradation behavior (Fig. 3.27). The PEG/PMMA binder system starts decomposition at approximately 300°C and ends at 430°C.73 Moreover, it follows a gradual weight loss over most of the decomposition range (Fig. 3.27). On the other hand, the thermal degradation behavior of the PEG/PPC binder system has two distinctive patterns; the degradation of the less stable PPC starts at around 200°C. The initial weight loss of 17% at 200°C implies that PPC was almost completely decomposed, followed by the decomposition of PEG. This binder decomposes completely around 320°C
340°C. The addition of PMMA in the PEG/PPC blend system shifts the onset decomposition temperature from 230°C to a higher temperature of 300° C. The two distinctive thermal degradation patterns remain identical, albeit the addition of PMMA further broadens the temperature range over which the second weight loss takes place. Furthermore, the clean nature of PEG/PMMA and PEG/PPC binder blends is evident with no char formation. The addition of PMMA in the PEG/PPC system, however, leads to the formation of some unwanted thermally stable complex carbonaceous compounds, which does not decompose completely until B600°C. This also provides evidence for strong interactions between the polymeric components present in the binder (Fig. 3.28).

Figure 3.26 (A) Schematic presentation of possible H-bonding interactions between the polymers in the binder system. Development of this chemical interaction in PEG/PPC/PMMA blend would contribute towards enhancing mechanical and thermal stability; (B) Incorporation of PMMA in PEG/PPC blend significantly improved the green strength of injection-molded samples and kept the samples in shape during subsequent water debinding.71

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Figure 3.27 Thermal degradation behavior of different binder blends. Identical conditions were applied in each case.71

Complete wrapping of Ti powder with the binder can be seen in Fig. 3.28A, further confirming the excellent homogeneity of the feedstock. Backscatter mode in SEM, which gives the atomic number contrast, can also be used to detect the impurity content. Some dark spots among Ti powder particles can be seen in Fig. 3.28B. The energydispersive X-ray spectroscopy over different selected areas confirmed the presence of carbon and oxygen in the thermal debound samples. The impurity contents in the samples after thermal debinding were measured by the LECO impurity analyzer and listed in Table 3.5. Generally, thermal debinding under argon gas leads to a higher content of oxygen, compared to thermal debinding in a high vacuum.74 Carbon and nitrogen do not adversely affect the sintering process unless the binder removal is not properly carried out. However, the oxygen pick-up during sintering cannot be avoided. Nonetheless, it can be kept minimum by using a clean, high vacuum sintering furnace (vacuum 1022 Pa or better). According to Ref. [75], the increase in oxygen content can be kept to 0.05 wt.% using a high-quality furnace. Assuming an increase of 0.1 wt.% in oxygen content during sintering under a high vacuum, the overall impurity content should remain below the maximum

Figure 3.28 Backscattered images of (A) Ti Feedstock composed of PEG/PPC/PMMA binder system, (B) thermal debound sample (in argon). The maximum temperature attained during thermal debinding was 650°C. Residue after binder burn-out is clearly visible.

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Table 3.5 Impurity content of thermally debound samples under high vacuum and pure argon gas, respectively. Element

Oxygen Carbon Nitrogen

Thermally debound samples In high vacuum (wt.%)

Under purging argon gas (wt.%)

0.17 0.02 0.01

0.25 0.03 0.02

Sintered samples (wt.%)

0.5 0.2 0.04

Content of sintered samples are also presented for comparison purpose.

limit of 0.30 wt.% (ASTM standard Grade 3)76 in this case. However, as can be seen from Table 3.4, this was not the case. It is fair to assume that the residue left during thermal debinding could be in the form of some complex cross-linked carbonaceous compounds (which are difficult to detect by the LECO impurity analyzer for inorganic materials). At the next stage, when a higher temperature is applied during the sintering process, the carbonaceous residues would finally degrade into C and O2. Some of these residues may not be able to escape from the interstitial spaces, ultimately resulting in higher C and O2 impurities. Concurrently, titanium powders may have additional catalytic effects on the decomposition of binders in mixed feedstocks as well.77 It should also be noted that the maximum temperature employed during the thermal debinding process generally ranges from 650°C to 700°C and at these temperatures presintering process has already begun, further complicating the decomposition process of left-over binder residue. This example shows even a small amount of a third component/compatibilizer can have a significant effect on final properties. Therefore it is recommended whenever a change in binder composition is sought, a thorough evaluation of the binder system should be carried out in the case of reactive powders-MIM.

3.2 Interactions between powder and binder 3.2.1 Role of surfactant Considering the role of the binder in MIM processes, it can be argued now that the binder system should have at least partially miscible polymer components (ideally, immiscible yet highly compatible). However, incompatible immiscible will result in phase separation and agglomeration

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during feedstock preparation. Also, it is important to have a good interaction between the metal powder and binder system to uniformly distribute metal particles and produce defect-free green parts. Surfactants are generally added in binder systems to enhance these interactions between metal powders and binder components by improving properties such as surface wetting, spreading, adsorption and binder strengthening. In other words, the surfactant can improve the compatibility of binder and powders by increasing adhesion between binder and powder particles, thereby leading to a better molding operation. Therefore the surfactant acts as a bridge between the powder and the binder. The surfactant is thus critical to the binder system and MIM process.78 Furthermore, it can also reduce the friction between the powder particles and die wall by acting as a lubricant and enhance compatibility between binder components.

3.2.2 Basic chemistry of surfactant Surfactants are amphiphilic compounds having a lyophilic, in particular, hydrophilic part (polar group) and a lyophobic, in particular, hydrophobic part, which is often a hydrocarbon chain.79 This structure of surfactants is responsible for their tendency to concentrate at interfaces and to aggregate in solutions into various supramolecular structures. According to the nature of the polar group, surfactants can be classified as nonionic and ionic. The ionic surfactants may be of anionic, cationic, amphoteric, or zwitterionic in nature (Fig. 3.29). Anionic surfactants make up 65% of all surfactants manufactured and it is not surprising that the bulk of literature on surfactants deals with the analysis of these compounds. Some common types of anionic surfactants are alkyl carboxylates, alkylarylsulfonates, and alkyl sulfates. The two significant characteristics of a surfactant are the adsorption at interfaces and the aggregation in the bulk solution. In the MIM regime, the most important consideration is adsorption on metal powder surfaces. The adsorption of surfactants at the solid
liquid interface is governed by several factors such as the following79,80: 1. Nature of the structural groups on the solid surface—morphology, chemical composition, etc., whether the surface contains highly charged sites or nonpolar groupings. 2. Molecular structure and the nature of the surfactant—whether it is ionic or nonionic, and whether the hydrophobic group is long or short, straight-chain or branched, aliphatic or aromatic.

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Figure 3.29 Typical molecular structures of the four classes of surfactants: (A) anionic surfactant, (B) amphoteric surfactant, (C) cationic surfactant, (D) nonionic surfactant.

3. The environment of the aqueous phase—its composition, pH, and temperature. The above-mentioned factors determine the mechanism by which adsorption occurs and the effectiveness and efficiency of adsorption. In general, numerous mechanisms govern the adsorption of surfaceactive solutes onto solid particles from an aqueous solution. However, the adsorption of surfactants involves single ions.81 A few mechanisms are as follows: 1. Ion exchange, which involves the replacement of counter-ions adsorbed onto the solid surface from the solution by similarly charged surfactant ions82 (Fig. 3.30A). 2. Ion pairing, which involves adsorption of surfactant ions from solution onto oppositely charged sites82 (Fig. 3.30B). 3. Acid
base interaction. This occurs via either hydrogen bonding between particle and adsorbate83 (Fig. 3.30C) or Lewis acid
Lewis base reaction (Fig. 3.30D). 4. Ion
dipole, dipole
dipole and induced dipole
dipole interactions. 5. Adsorption by universal dispersion forces. This occurs via London
van der Waals dispersion forces acting between adsorbent and adsorbate molecules. Adsorption by this mechanism generally increases with an increase in the molecular weight of the surfactant.

Figure 3.30 Illustrations of different mechanisms of surfactant’s adsorption onto a solid surface: (A) ion exchange; (B) ion pairing; (C) hydrogen bonding; (D) via Lewis acid
Lewis base interaction. It should be noted that the rigid arrangement of the surfactant molecule (hydrophobic group) shown is only for simplicity. In reality, the hydrophobic groups may assume all conformations including the interweaving of the hydrophobic chains of adjacent surfactant molecules.

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It is also an important supplementary mechanism in all other types. For example, it accounts in part for the definite ability of surfactant ions to displace equally charged simple inorganic ions from solid surfaces by an ion-exchange mechanism.84 6. Hydrophobic bonding. This is a typical mechanism for aqueous surfactant solutions. It involves the minimization of the contact between hydrocarbon chains and the surrounding water by adsorbing onto the solid surface by aggregating their chains.85 Critical analyses of the adsorption of ionic surfactants from an aqueous solution onto a polar solid showed that the first layer of the ionic surfactant adsorbs on the solid particle with its hydrophilic group such that a hemimicelle forms with its hydrophobic group pointing toward the aqueous phase. The additional layer of surfactant adsorbs onto the adsorbed initial layer of hemimicelle. The adsorption is in such a way that the hydrophobic group is oriented towards the adsorbed hemimicelle and its hydrophilic group is oriented towards the aqueous phase.86 In aqueous systems, the formation of the structures depends on the interaction of the surfactant molecules with the solid surface in such a manner to minimize exposure of the hydrophobic groups to the aqueous phase. Adsorption of a surfactant molecule to a powder surface is usually through hydrogen bonding,87 although covalent bonding is also possible using silanes88 or titanates.89 Hydrogen bonding involves the reaction between an electron acceptor and an electron donor. For interaction among organic materials, the strength of this reaction is determined by the “shift” of the functional groups using IR spectroscopy. Similarly, the interaction occurring between a powder particle surface and a surfactant can be recognized by observing the shifting of the IR absorption peak of the key functional groups. Yi-min et al.90 presented an adsorbing model to estimate the least amount of stearic acid (SA) for a stainless steel feedstock using the same reasoning mentioned earlier (Fig. 3.31). According to this model, the polar group of the surfactant is dispersed within the binder, while the hydrophobic group latches onto the particle surface. Assuming monolayer adsorption of surfactant and uniform dispersion of mono-sized particles, the minimum content of the surfactant in the binder system that forms a single adsorption layer on the powder particle surface can be calculated:

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Figure 3.31 An illustration of the adsorbing model of surfactant on the powder particle surface.

Ws 24M ϕ 5 πNA d1 ð1 2 ϕÞρB d22 WB

(3.20)

where WB is the mass of the binder, Ws mass of the surfactant that forms a single molecular layer on the particle surface, M is the molar mass of the surfactant, ϕ is solid loading, NA is the Avogadro’s number (6.02 1023), d1 is the diameter of the powder particles, ρB is the density of the binder and d2 is the diameter of the polar head of the surfactant, respectively.

3.2.3 Case study: surfactants other than stearic acid for reactive powders-MIM It can be said with fair confidence that in almost 90% of the binder systems for reactive powders-MIM, SA has been used as the surfactant (see Table 2.3). The reports on feedstocks comprising of surfactants other than SA in reactive powders-MIM are few and far between. Palm stearin (which can also act as the primary component) and dibutyl phthalate and castor oil are a few examples. Ibrahim et al.91 evaluated binder system based on palm oil derivative (palm stearin) for Ti6Al4V as a possible alternative binder system. The reason for using palm stearin as a binder system was due to its contents that could be advantageous during the debinding process. The binder system comprised 40% of polyethylene and 60% of palm stearin (percentage by weight). The powder was mixed with the binder system at 130° C
160°C for 2 h using a z-blade mixer. Solids loading between 65 and 67 vol.% were used. After injection molding, the green molded samples were immersed into heptane for 6 h at 60°C to remove two-third of the binder system. The specimen was then continuously heated at 10°C min21 up to 1150°C under a vacuum atmosphere with a holding time of

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Table 3.6 Physical and mechanical properties of sintered titanium alloy. Properties 23

Density (g cm ) Hardness (Hv) Porosity (%) Shrinkage (%) Strength (MPa) Elongation (%)

Sintered Ti6Al4V

MPIF 35

4.39 381.20 2.47 11.86 541.53 0.90

.96% 300
400 ,5 12
15 .700 10
15

5 h. However, the achieved mechanical properties were well below par due to the formation of titanium carbide (TiC), Table 3.6. The authors did not provide any information about the formation of TiC. It is highly likely the formation of TiC was caused by either an unsatisfactory binder system and debinding process or the lack of clean processing equipment. The lack of available literature on this binder system suggests the former case. In an interesting study, Hayat et al.78 carried out a series of experiments to study the effects of different types of surfactants on the properties of Ti-MIM feedstock. The binder system contained 83 wt.% PEG as the primary component and 15 wt.% PMMA (Mw B60,000 g/mol) as the backbone polymer and 2 wt.% surfactant. Three different surfactants were chosen: stearic acid (SA, density 5 0.87 g/cm3), liquid peanut oil (density 5 0.92 g/cm3), and castor oil (density 5 0.96 g/cm3) for feedstock formulation and designated as feedstock A, B, and C, respectively. The solid loading was fixed as 60 vol.% for this purpose. Although the feedstock made of SA had better shear sensitivity, lower viscosities were achieved when SA was replaced with liquid surfactants (Fig. 3.32). In this case, almost 5% reduction in viscosity was achieved using liquid surfactants. The authors attributed this low viscosity to higher interaction between metal powder and binder system, which increases the powder dispersion and thereby a higher solid loading can be employed. It should also be noted that there is a nominal viscosity difference between feedstocks containing castor and peanut oils, respectively, suggesting that the oil type has little effect over the shear sensitivity properties of feedstocks. The authors claimed that the good rheological properties of feedstock containing castor oil arise from the higher interaction between the binder system and Ti powders (Fig. 3.33).

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Figure 3.32 Double logarithm of viscosity versus shear rate plot of feedstocks containing different surfactants at 120°C.

For feedstock A, it can be seen that the fracture surface is of a rounded nature (dimples), arising from the particle pull-out during fracture (Fig. 3.33, left). This particle pull-out occurs due to poor adhesion between metal particles and the binder system. On the contrary, the fracture surface of feedstock C is smooth and very few dimples are present (Fig. 3.33, right), which suggests good adhesion between the binder system and titanium particles. This strong adhesion improved the powder dispersion in the binder during mixing, which in turn reduced the flow viscosity of the feedstock, resulting in better rheological properties as well as increased green strength of the molded samples in the case of feedstock C (Fig. 3.34). In feedstock formulation, the primary and backbone polymeric constituents control the green strength of the samples, which is dependent on their respective contents. Nevertheless, the surfactants can also increase the green strength by effectively increasing the dipole
dipole attractive forces. The authors suggested that the better performance in the case of feedstock containing castor oil as a surfactant lies in the chemistry of castor oil (Fig. 3.35).

Figure 3.33 Fracture surface morphologies of feedstock A (left) and feedstock C (right). To study the fracture surface, green parts were first submerged in liquid nitrogen and were subsequently fractured.

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Figure 3.34 Green strength comparison of two feedstocks containing SA (feedstock A) and castor oil (feedstock B) as surfactants, respectively. The mean flexural strength value for feedstock A is 7.3 MPa and for feedstock C it is 8.1 MPa almost 10% higher than feedstock A.

As can be seen in Fig. 3.35, in contrast to SA, the castor oil chain has at least three polar ester functional groups in addition to a hydroxyl group, which not only enhances the dipole
dipole attractive forces but also provides hydrogen bonding via hydroxyl group. Consequently, in the case of castor oil feedstock, a better powder dispersion and higher powder binder interaction can be achieved, which ultimately leads to lower viscosity and better rheological properties.

3.3 Summary This chapter discusses specific interactions among individual components of the binder system with special emphasis on the thermodynamics of polymer blends. After a brief introduction on polymer blends, selected common theoretical approaches concerning the miscibility of polymer blends are presented: specifically, Flory
Huggins (FH) lattice theory and those evolving from it such as solubility parameter model are discussed. The largest data pool on thermodynamics of polymer blends is based on

Figure 3.35 Comparison of chemical formula of stearic acid (left) and castor oil (right). Castor oil is a combination of different fatty acids. However, ricinoleic, oleic and linoleic acids are the three main ingredients.92

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Flory
Huggins lattice theory given it is the oldest and well-known theory. To use the theory, one must know the pressure, temperature and concentration dependence of the enthalpic and entropic contributions to the binary interaction parameter. The solubility approach divides all thermodynamic influences into three groups: the van der Waals interactions, the configurational entropy, and the specific interactions. Both of these approaches suffer from the fundamental drawbacks of the FH theory that is, inability to take into account the fine structure of polymeric chains, orientation, and nonrandomness. In the second half, a few selected experimental methods to determine the miscibility and interaction parameter of polymer blends are presented along with some classic examples. Some examples of common compatible polymer blends that are used as binders for reactive powders-MIM are also presented in the subsequent section. In the last section, the importance of interaction between metal powder and binder system is discussed. It is important to have a good interaction between the metal powder and binder system to uniformly distribute metal particles and produce defect-free green parts. Surfactants are employed to enhance or create this interaction by improving properties such as surface wetting, spreading, adsorption and binder strengthening. Several factors govern the adsorption of surfactants at the solid
liquid interface including the nature of the structural groups on the solid surface and surfactant chemistry. Lastly, a case study is presented on surfactants other than SA for Ti-MIM; demonstrating that a better dispersion can be achieved by using a surfactant having more functional groups.

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907. 63. Krupa, I.; Luyt, A. S. Thermal Properties of Polypropylene/Wax Blends. Thermochim. Acta 2001, 372 (1), 137
141. 64. Itoh, Y.; Miura, H. Fabrication of High Strength Ti Alloy Compacts by Metal Injection Molding. 2016, 63 (7), 438-444. 65. Song, J.; Thurber, C. M.; Kobayashi, S.; Baker, A. M.; Macosko, C. W.; Silvis, H. C. Blends of Polyolefin/PMMA for Improved Scratch Resistance, Adhesion and Compatibility. Polymer 2012, 53 (16), 3636
3641. 66. Tong, X. M.; Zhang, M.; Song, L.; Ma, P. Paraffin/Poly(methyl methacrylate) Blends as Form-Stable Phase Change Materials for Thermal Energy Storage. Adv. Mater. Res. 2011, 183-185, 1573
1576. 67. Hayat, M. D.; Li, T.; Cao, P. Incorporation of PVP into PEG/PMMA-Based Binder System to Minimize Void Nucleation. Mater. Des. 2015, 87, 932
938. 68. Hayat, M. D.; Goswami, A.; Matthews, S.; Li, T.; Yuan, X.; Cao, P. Modification of PEG/PMMA Binder by PVP for Titanium Metal Injection Moulding. Powder Technol. 2017, 315, 243
249. 69. Chen, W.-C.; Kuo, S.-W.; Jeng, U. S.; Chang, F.-C. Self-Assembly Through Competitive Interactions of Miscible Diblock Copolymer/Homopolymer Blends: Poly(vinylphenol-b-methyl methacrylate)/Poly(vinylpyrrolidone) Blend. Macromolecules 2008, 41 (4), 1401
1410. 70. Bairamov, D. F.; Chalykh, A. E.; Feldstein, M. M.; Siegel, R. A. Impact of Molecular Weight on Miscibility and Interdiffusion Between Poly(N-vinyl pyrrolidone) and Poly(ethylene glycol). Macromol. Chem. Phys. 2002, 203 (18), 2674
2685.

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71. Hayat, M. D.; Jadhav, P. P.; Zhang, H.; Ray, S.; Cao, P. Improving Titanium Injection Moulding Feedstock Based on PEG/PPC Based Binder System. Powder Technol. 2018, 330, 304
309. 72. Yang, G.; Hu, X.; Su, J.; Geng, C.; Yao, W.; Zhang, Q.; Fu, Q. Significant Reinforcement of Poly(propylene carbonate): Nanostructured Polymer Composites of Poly(propylene carbonate)/Poly(methyl methacrylate) via a Supercritical Carbon dioxide Route. J. Supercrit. Fluids 2013, 82, 200
205. 73. Chen, G.; Wen, G.; Cao, P.; Edmonds, N.; Li, Y. Processing and Characterisation of Porous NiTi alloy Produced by Metal Injection Moulding. Powder Injection Moulding Int. 2012, 6, 83
88. 74. Shibo, G.; Xuanhui, Q.; Xinbo, H.; Ting, Z.; Bohua, D. Powder Injection Molding of Ti
6Al
4V Alloy. J. Mater. Process. Technol. 2006, 173 (3), 310
314. 75. Ebel, T. 17
Metal Injection Molding (MIM) of Titanium and Titanium Alloys. In Handbook of Metal Injection Molding; Heaney, D. F., Ed.; Woodhead Publishing, 2012; pp 415
445. 76. ASTM, Standard Specification for Metal Injection Molded Unalloyed Titanium Components for Surgical Implant Applications. In ASTM F2989-13, ASTM International: West Conshohocken, PA, 2013. 77. Lin, D.; Sanetrnik, D.; Cho, H.; Chung, S. T.; Kwon, Y. S.; Kate, K. H.; Hausnerova, B.; Atre, S. V.; Park, S. J. Rheological and Thermal Debinding Properties of Blended Elemental Ti
6Al
4V Powder Injection Molding Feedstock. Powder Technol. 2017, 311, 357
363. 78. Hayat, M. D.; Wen, G.; Li, T.; Cao, P. Compatibility Improvement of Ti-MIM Feedstock Using Liquid Surfactant. J. Mater. Process. Technol. 2015, 224, 33
39. 79. Pletnev, M. Y. 1. Chemistry of Surfactants. In In Studies in Interface Science; Fainerman, V. B., Möbius, D., Miller, R., Eds.; Vol. 13; Elsevier, 2001; pp 1
97. 80. Rosen, M. J.; Kunjappu, J. T. Surfactants and Interfacial Phenomena, Vol. 82. Wiley Online Library, 2004. 81. Griffith, J. C.; Alexander, A. E. Equilibrium Adsorption isotherms for Wool/ Detergent Systems: I. The Adsorption of Sodium Dodecyl Sulfate by Wool. J. Colloid Interf. Sci. 1967, 25 (3), 311
316. 82. Rupprecht, H.; Liebl, H. Einfluß von Tensiden auf das kolloidchemische Verhalten hochdisperser Kieselsäuren in polaren und unpolaren Lösungsmitteln. Kolloid-Z. und Zeitschrift für Polymere 1972, 250 (7), 719
723. 83. Snyder, L. R. Interactions Responsible for the Selective Adsorption of Nonionic Organic Compounds on Alumina. Comparisons adsorptisilica. J. Phys. Chem. 1968, 72 (2), 489
494. 84. Law, J.; Kunze, G. Reactions of Surfactants with Montmorillonite: Adsorption Mechanisms 1. Soil. Sci. Soc. Am. J. 1966, 30 (3), 321
327. 85. Gao, Y.; Du, J.; Gu, T. Hemimicelle Formation of Cationic Surfactants at the Silica Gel
Water Interface. J. Chem. Society, Faraday Trans. 1: Phys. Chem. Condens. Phases 1987, 83 (8), 2671
2679. 86. Kunjappu, J. T. Comments On Structure Of Sodium 4-(L’-Heptylnonyl) Benzenesulfonate Adsorbed On Alumina Using H-2 NMR. J. Colloid Interf. Sci. 1994, 162 (1). 261-261. 87. Fowkes, F. Ceramic Powder Science. Advances in Ceramics; Vol. 21, p 411. 88. Zhang, J. G.; Edirisinghe, M. J.; Evans, J. R. G. The Use of Silane Coupling Agents in Ceramic Injection Moulding. J. Mater. Sci. 1988, 23 (6), 2115
2120. 89. Lindqvist, K.; Carlström, E.; Persson, M.; Carlsson, R. Organic Silanes and Titanates as Processing Additives for Injection Molding of Ceramics. J. Am. Ceram. Soc. 1989, 72 (1), 99
103.

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90. Li, Y.-m; Liu, X.-q; Luo, F.-h; Yue, J.-l Effects of Surfactant on Properties of MIM Feedstock. Trans. Nonferrous Met. Soc. China 2007, 17 (1), 1
8. 91. Ibrahim, R.; Azmirruddin, M.; Jabir, M.; Ismail, M.; Muhamad, M.; Awang, R.; Muhamad, S. Injection Molding of Titanium Alloy Implant for Biomedical Application Using Novel Binder System Based on Palm Oil Derivatives. Am. J. Appl. Sci. 2010, 7 (6), 811. 92. Bantchev, G. B.; Biresaw, G. Elastohydrodynamics of Farm-Based Blends Comprising Amphiphilic Oils. In Surfactants in Tribology; Biresaw, G., Mittal, K. L., Eds.; 2013; Vol. 3, pp 266
296.

CHAPTER 4

Impurity management in reactive metals injection molding 4.1 The importance of impurity control The most important feature of reactive powders metal injection molding (MIM)—also a lingering challenge—is their high affinity to impurity elements representative of oxygen. The fact that some of the reactive powders such as titanium and magnesium are used in the vacuum technique as getter materials for purification of the atmosphere highlights the problem that arises when the reactive fine powders have to be handled. For example, according to the binary phase diagram of Ti O, titanium can absorb 13 wt.% oxygen on interstitial lattice sites. The great affinity of titanium to oxygen is combined with a strong influence on its mechanical properties, even at low contents in the range of 0.1 0.4 wt.%.1 For illustration, Fig. 4.1 gives an example of the effects of oxygen on the mechanical properties of Ti 6Al 4V, while Table 4.1 shows the values for the maximum content of oxygen according to the ASTM standard F2989-13 for metal injection molded unalloyed titanium components for surgical implant applications and its influence on tensile properties.

Figure 4.1 Effect of oxygen on the ductility, yield, and ultimate tensile strength of Ti 6Al 4V specimens produced by MIM.2 Feedstock Technology for Reactive Metal Injection Molding DOI: https://doi.org/10.1016/B978-0-12-817501-9.00004-1

© 2020 Elsevier Inc. All rights reserved.

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Table 4.1 Maximum oxygen level and mechanical requirements according to ASTM F2989.3

Grade MIM 1 Grade MIM 2 Grade MIM 3

Oxygen %, (mass/ mass)

UTS, MPa, min

YS (0.2%), MPa, min

Elongation, %, min

Reduction of area, %, min

0.18 0.25 0.30

370 420 495

315 360 390

23 17 10

25 20 15

It is worth noting from these numbers that for pure titanium, an increase of only 0.12 wt.% in oxygen content can halve the ductility of titanium. This signifies why the pick-up of interstitial elements during reactive powders-MIM processing has to be minimized. Due to the wellknown solid solution strengthening, high strength in a Ti-MIM component is readily obtained, simply by increasing the oxygen content. However, the provision of good ductility is a challenge. In the case of titanium alloys like Ti 6Al 4V, the oxygen limits are even smaller. There is also a standard for metal injection molded Ti 6Al 4V components for surgical implant applications (ASTM F28854) allowing a maximum oxygen content of 0.2 wt.% only. From the viewpoint of solution hardening in titanium, oxygen, nitrogen, and carbon have the same effect, albeit with different potencies. Therefore it is prudent to combine these elements in the form of an oxygen equivalent, Oeq: Oeq 5 CO 1 2CN 1 0:75CC where CO , CN , and CC represents the concentrations of oxygen, nitrogen, and carbon, respectively. In the literature, the coefficient of CC differs somewhat between 0.5 and 0.75.5 7 Although the effect of nitrogen is twofold that of oxygen, in practice the latter remains the most important interstitial element. This is mainly because oxygen has high solubility and fast diffusivity in titanium. Standard specifications for reactive powders other than titanium such as aluminum or magnesium are not available in the literature, primarily due to the lack of research and applications. The lack of research stems from the inferior sinterability of aluminum and magnesium powders. In contrast to titanium, Mg and Al metals do not show any solubility for oxygen solutes. The oxygen pick-up during processing leads to the

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Table 4.2 Influence of different processing conditions on the oxygen and carbon contents, and relative density of the sintered Al-MIM compacts. Avg. particle size (μm)

Debinding atmosphere

Degree of vacuum in sintering (Pa)

Oxygen content (mass %)

Carbon content (mass %)

Relative density (%)

20 20 10 10 3 3

Air Ar gas Air Ar gas Air Ar gas

1023 1023 1023 1023 1023 1023

2.31 0.75 2.95 0.92 1.99 1.47

1.04 0.21 1.03 0.23 0.87 0.17

66.5 86.5 67.2 89.8 85.9 96.3

Source: Extracted from Katou, K.; Matsumoto, A. Application of Metal Injection Molding to Al Powder. J. Jpn. Soc. Powder Powder Metall. 2016, 63 (7), 468 472 [5].

formation of a thermodynamically stable oxide layer on the surfaces of powder particles, which do not dissolve during sintering. Accordingly, these oxide layers drastically inhibit diffusion processes that are essential for sintering and densification. The relative density tends to be high when the oxygen content is low (Table 4.2). The limited research in Al or Mg MIM is, therefore, targeted at improving the sinterability for achieving a high relative density.6 8 For the above reasons, it is not surprising that the main parameter for reactive powders MIM is the control of impurity uptake, in particular, oxygen. In the following sections, we highlight the main sources of impurities and pinpoint approaches that can be employed to keep impurity uptake in check for achieving a successful MIM process. It should be noted that the next section is presented for Ti-MIM only. For other reactive powders such as Al, Mg where there is a little solid solution, a separate section is devoted.

4.2 Methods of controlling impurities 4.2.1 Selection of primary component Baril et al. published a detailed study of the sources of oxygen during MIM processing. As demonstrated in Fig. 4.2, there are several sources, such as the starting powder and the sintering support material that is commonly made of ceramics like ZrO2. The contributions from these sources slightly change with the powder particle size; nevertheless, the overall pick-up remains very similar. Among these sources, the initial oxygen content of starting powder is the greatest contributor, followed by the

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Figure 4.2 Contributions of main MIM variables to the final oxygen content in Ti components. Redrawn from Baril, E.; Lefebvre, L. P.; Thomas, Y. Interstitial Elements in Titanium Powder Metallurgy: Sources and Control. Powder Metall. 2011, 54 (3), 183 186 [9].

sintering atmosphere and debinding atmosphere. Given the fact that highquality powders with oxygen content as low as 0.09 wt.% are available in the market and coupled with a high-quality sintering furnace, the resulting oxygen content from these two sources can be kept to a minimum. On the other hand, oxygen content arising from the binder system and its subsequent debinding is a lingering threat to the successful reactive powders-MIM operation. The primary component may make up for a major portion of a typical binder system for reactive powder MIM. This means the volume fraction of the primary component can be anywhere from 30 to 80 vol.% depending on applications. The high volume fraction dictates the need for the primary component that can be removed cleanly and at relatively low temperatures in the case of reactive powders MIM. The primary component is usually removed via either solvent extraction or catalytic debinding—the former being more common. As mentioned in Chapter 2, Design Strategy of Binder Systems and Feedstock Chemistry, the solvent extraction of the primary component (also known as solvent debinding) is driven by capillary action and concentration gradient across the solvent bath. It must be kept in mind that the time required to remove the primary component is a function of the debinding temperature and set-up, the size of the part, as well as the particle size of the powders. Some of the important aspects that must be considered are listed:

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Table 4.3 Some important debinding parameters of different binders. Primary binder

Wax-based Paraffin wax Synthetic wax Water-soluble PEG 200 PEG Catamold Polyacetal

Secondary binder

Primary debind method

Debind temperature

Debind rate (mm/h)

Polypropylene Polypropylene

Solvent Heptane Perchloroethylene

50 70

1.5 2

Polypropylene Polyacetal

Water Water

40 60

0.3 0.5

Polyethylene

Nitric acid catalyst

120

1.5

1. The dissolution rate of the primary binder component in the solvent increases as the primary binder is liquefied. This implies the temperature of the solvent must be chosen with great care. 2. If there are any reactions of the secondary binder component with solvent, they should not cause any part distortion. 3. The solvent should not have high vapor pressure at debinding temperatures. Table 4.3 presents some common binder systems used in Ti-MIM, their respective primary debinding method, temperatures used for debinding, and approximate debinding rates achieved for standard MIM parts.10 Fig. 4.3 presents the effects of sample thickness and temperature on the debinding rate of PEG for a Ti-MIM feedstock. It is clear that PEG removal largely depends on sample thickness. In the first few hours of debinding, the removal rate is generally rapid; afterward, it levels off. The debinding time can be significantly reduced if the debinding temperature is increased to 60°C from 40°C. This is because a higher debinding temperature increases the molecular diffusivity and mobility. However, a further increase in temperature is not recommended because too high binder removal rate will result in loss of integrity, the formation of cracks and blisters, distortion, or even collapse of the compacts. From the author’s experience, the debinding temperature should be selected as 0.90% of the melting point of the primary component. In industrial practices, up to B80% removal of the primary component from the MIM component via solvent debinding is considered sufficient, as necessary open porosity is already created at this point for

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Figure 4.3 PEG removal percentage versus debinding time: (A) for various sample thicknesses at 60°C debinding temperature; (B) at three different temperatures for the 6 mm-thick sample. The samples were produced from Ti 6Al 4V feedstock with the PEG/PMMA binder system (particle size: ,74 μm, solid loading: 69.5%).11

subsequent thermal debinding.12 This constitutes to debinding time of anywhere between B1 h and more than 24 h, depending on the molecular weight of the primary component, debinding temperature, thickness of the samples, volume of the solvent, and stirring frequency. However, our own experience suggests that more than 90% removal of the primary component should be sought after in the case of Ti-MIM. To highlight the importance of solvent debinding, PEG as a primary binder component is presented here. PEG is a crystalline polymer (chemical structure: HO [CH2 CH2 O]n H) with an open helical structure. The molecular chain of PEG with an average molecular weight of 4000 g/mol approximately contains carbon 54.41 wt.%, oxygen 36.5 wt.%, and hydrogen 9 wt.%, respectively. PEG-4000 thermal decomposition behavior is shown in Fig. 4.4. If we neglect impurity pick-up below 400°C (Fig. 4.4), the increase in the levels of impurity due to improper water debinding for a typical 67 vol.% Ti MPIF standard injection molded part containing 1.3 g of PEG is estimated in Table 4.4. From Table 4.4, it is clear that for Ti-MIM, 90% of the primary component should be removed to keep impurity content in-check. However, this may take the debinding time anywhere between 6 h and more than 24 h, which may affect production efficiency. The invention of the Catamold system by BASF, based on polyoxymethylene (POM), has revolutionized the MIM industry and provided the much-needed impetus for the development of a reliable MIM manufacturing

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Figure 4.4 Pure PEG (molecular weight 4000 g/mol) thermal decomposition behavior under the argon atmosphere.

process for volume production. Binder removal is done in a gaseous acidic environment, that is, highly concentrated nitric or oxalic acid, at a temperature of approximately 120°C, which is below the softening temperature of the binder. The acid attacks the side OH groups and creates molecules of formaldehyde. Formaldehyde has a very low dew point (221°C), and it directly vaporizes from solid to vapor at the typical debinding temperature. The molecules are small; therefore, the velocity of the diffusion process is high and the debinding process is fast. The process yields parts with an interconnected porosity in approximately 3 h. Owing to faster debinding rates, it can be argued that this binder system may lead to lower contamination in the final sintered part. Interestingly, this is not the case. The final contents of impurities are not modest as shown in Table 2.3. In 2006, Weil et al.14 developed a binder formulation for a Ti-MIM process, in which an aromatic compound was used as the binder and solvent, and therefore, only a small fraction of the traditional binder materials were required as minor additives. The main advantage of aromatics such as naphthalene, anthracene, and pyrene is that they melt at relatively low temperatures and can be completely removed from the green parts by sublimation under reduced pressure at temperatures well below their

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Feedstock Technology for Reactive Metal Injection Molding

Table 4.4 Increase in impurity contents due to the improper first stage of debinding.13 PEG (wt.%) removed after water debinding

0 50 75 90

Initial Impurity contents (wt.%) (GA-Powder)

Approx. increase in impurity contents after thermal debinding (considering PEG thermal decomposition after 400°C only) (wt.%)

Carbon

Oxygen

Carbon

Oxygen

B0.003

B0.13

B0.186 (6100%) B0.0949 (3000%) B0.048 (1500%) B0.0159 (430%)

B0.253 (94%) B0.1916 (47%) B0.1608 (23%) B0.140 (8%)

melting points. Their binder formulation consisted of naphthalene, 1 vol. % stearic acid, and 3% 12% ethylene-vinyl acetate (EVA). Their debinding cycle involved sublimation for 20-h at 80°C. They demonstrated that complete removal of naphthalene can be achieved if the debinding is done under a vacuum of 2.67 Pa (Fig. 4.5). In the subsequent debinding step, EVA was thermally removed in the temperature range of 360°C 450°C. The main benefit of the aromatic binder is, as sublimation involves low surface energies in the vaporization process, the specimen volume remains constant throughout the debinding process. This means that common debinding problems such as part distortion and cracking can be avoided by using such types of binder systems. Moreover, they claimed that the sintered titanium samples showed little impurity pick-up (maximum wt.% of carbon B0.02). Although this binder system provides an excellent solution to different solvent debinding issues such as swelling and cracking, along with potentially low impurity contents, there are no further reports on this binder system. The main reason for this may be due to health and environmental concerns associated with toxic aromatic compounds. Interactions between components of the binder system can also affect the solvent debinding behavior. In one of our recent studies, we incorporated poly(vinylpyrrolidone) (PVP) into a PEG/PMMA binder system to reduce the void nucleation.15,16 The problem of void

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Figure 4.5 Weight loss as a function of time and temperature for feedstock containing 65 vol.% Ti 6,4/6 vol.% EVA/1 vol.% stearic acid/balance naphthalene. The prepared samples were heated under 2 3 1022 Torr vacuum. Taken from Scott Weil, K.; Nyberg, E.; Simmons, K. A New Binder for Powder Injection Molding Titanium and Other Reactive Metals. J. Mater. Process. Technol. 2006, 176 (1), 205 209 [14].

nucleation stems from the molecular interaction between PEG and PMMA, which hinders the volumetric shrinkage during PEG recrystallization (solidification) and hence leads to void formation. These solidification voids cannot be removed during the subsequent hightemperature sintering process and consequently result in poor mechanical properties. To overcome this issue, the use of PVP as a crystallization inhibitor was proposed. Our hypothesis was that the PVP would act as a bridge between PEG and PMMA as PVP is compatible with both PEG and PMMA; it also reduces the direct interaction between PEG and PMMA. In addition, PVP is also water-soluble and can be extracted along with PEG.17,18 It has been shown that PEG, PVP, and water molecules interact with each other to form hydrated complexes, and thus the debinding behavior does not change in the PVP-modified PEG/PMMA binder system, Fig. 4.6. It can be seen from Fig. 4.6 that approximately 98% of the PEG/PVP mixture was removed after 30 h of water debinding. Feedstock D (no PVP substitution) initially leached faster and resulted in a greater sample mass loss as compared to the other feedstocks, suggesting lower molecular mobility in the case of the PVP 1 PEG mixture (the diffusion of PEG PVP water hydrated complexes depends on molecular mobility that, in turn, is generally a function of temperature, molecular weight, and powder particle size. The higher the molecular weight and the smaller the particle size, the lower is the molecular mobility). However, the binder removal rate significantly slowed down after 6 h of leaching and all

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Feedstock Technology for Reactive Metal Injection Molding

Figure 4.6 Water debinding profile for feedstocks containing different amounts of PVP; Feedstock A, 15 wt.% PVP was substituted for PEG while for Feedstocks B and C 20 wt.% PEG and 25 wt.% PEG were replaced with PVP, respectively. Feedstock D was prepared with an unaltered PEG/PMMA binder.

feedstocks followed the same trend with a little higher removal rate in the case of feedstock made of unaltered PEG/PMMA binder system. Nevertheless, feedstocks containing PVP in binder systems led to higher contents of impurity in the sintered Ti parts for the same debinding conditions (Table 4.5). It is apparent that Feedstock C had the highest oxygen content and was well above the maximum permitted concentration that is, 0.30% according to the ASTM standard.3 It is therefore suggested that the amount of PVP replacing PEG should not exceed 20 wt.%. It may be concluded that any primary component can be selected for Ti-MIM as long as it can be removed at low temperatures efficiently. The selected primary component should not interact with the other binder components to form any hard-to-remove compounds. In other words, the selection of the primary component must ensure that it is not completely miscible with any other binder components. By all means, the selection of binder components must comply with the current environmental protection protocols. The debound waste should not be harmful or toxic.

4.2.2 Control of impurities and thermal debinding mechanisms As more than 90% of the primary component of the binder systems can be removed either via solvent/catalytic debinding or by sublimation at

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Impurity management in reactive metals injection molding

Table 4.5 Average mechanical properties and chemical analysis of the sintered Ti samples. Sample

Feedstock A Feedstock B Feedstock C Feedstock D

Mechanical properties

Relative density (%)

0.2% yield stress (MPa)

0.2% yield stress (MPa)

Elongation (%)

400

444

7.1

410

504

496.7 386.7

Impurity content (%) O

C

N

97

0.21

0.03

0.01

9.5

98

0.28

0.05

0.01

617

5

96

0.35

0.07

0.04

390 (break)

3

94

0.19

0.02

0.02

relatively low temperatures, they pose little threat to the success of reactive powders-MIM unless incomplete debinding is carried out. However, contamination during thermal debinding, which arises from thermal decomposition of backbone polymers, remains a big concern for product designers. For instance, in the case of Ti-MIM, it is suggested to use backbone polymers that leave minimum residues upon decomposition and have low-to-moderate decomposition temperatures, preferably below 400°C as titanium readily reacts with oxygen above 400°C, Fig. 4.7. However, it must also be noted that the backbone polymer provides the necessary green strength until the initial stage of sintering; too early decomposition may lead to shape distortion. In addition, most of the typical thermoplastic polymers (employed in MIM) have decomposition temperatures well above 400°C. Hence, a comprehensive understanding of the thermal decomposition behavior of the backbone polymer is crucial to achieving desired mechanical properties. There is an extensive body of literature devoted to polymer degradation studies. However, one noteworthy deficiency in most of these decomposition studies is the lack of quantitative information about nonvolatile carbon residue yields, as they are primarily concerned with understanding initial polymer degradation mechanisms and the compositions of volatile species. A few studies have also been carried out to investigate the kinetics of binder decomposition concerning MIM. Mohsin et al.20 studied the thermokinetics of thermal debinding using Fourier transform

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Feedstock Technology for Reactive Metal Injection Molding

Figure 4.7 Oxygen intake with respect to debinding temperature. Redrawn from Lefebvre, L.-P.; Baril, E. Effect of Oxygen Concentration and Distribution on the Compression Properties on Titanium Foams. Adv. Eng. Mater. 2008, 10 (9), 868 876 [19].

infrared and mass spectrometry, respectively. These techniques are effective for determining the thermal stability and product evolution as a function of temperature. Keeping in mind the thermal decomposition behavior of common polymers, it can be assumed that polymeric binder may decompose into hydrocarbons ranging from C1 to C6. These molecules may then be reactants themselves and break down further into small hydrocarbon chains by radical processes. The process may consist of thermolytic bond cleavage, recombination reactions, and volatilization of low molecular-weight products. Polymers can degrade in several ways, including chain depolymerization, random scission, and side-group elimination.21,22 The thermal degradation mechanisms of different types of polymers lead to the production of different kinds and amounts of residues. Without knowing the amounts of each species and the decomposed gases, it is very challenging to predict which reactions are occurring but it is possible to give a generic description.23 For example, the decomposed products of a depolymerization type polymer such as polyacetal (POM) or polybutyl methacrylate (PBMA) are completely different from the residues produced by polyolefins such as polypropylene (PP). Kankawa24 studied the thermal degradation mechanisms of several polymers for the MIM process. Their results are shown in Fig. 4.8. The thermal degradation speed of depolymerization type polymer (POM, PBMA, PMMA, etc.) is faster than that of the random type polymers (PP, PE, EVA, etc.) (Fig. 4.8). Moreover, the former type of polymers produces no carbon residue both in the air as well as in the inert

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Figure 4.8 TGA curves of different polymers in nitrogen atmosphere.

Figure 4.9 Degradation mechanisms of (A) PP and (B) POM.

atmosphere—an important consideration for reactive powders-MIM. The thermal degradation mechanisms of POM and PP are shown in Fig. 4.9. POM decomposes into the monomer from the side of the main chain, which is easily evaporated at a low temperature, as illustrated in Fig. 4.9A. Monomers are sequentially lost at the chain’s end. On the other hand, polyolefins decompose to low molecular weight compounds at high temperature and thus the decomposition requires a longer time. The degradation, in this case, can be described as the random scission of the chain (Fig. 4.9B). This scission process could produce some carbon double bonds, which require even higher decomposition temperatures and generally yields higher carbon residues. All other degradation possibilities lie in between these two ends (depolymerization and random scission) of the degradation spectrum. Therefore the kinetic scrutiny of such behaviors can become quite complicated. Side group elimination produces a cluster of low molecular weight species as a result of scission of attached pendant groups along the

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Feedstock Technology for Reactive Metal Injection Molding

backbone of the polymeric chain. Polyvinyl butyral (PVB), polyvinyl alcohol, and polyvinyl acetate are the few common backbone polymers used in MIM that initially degrade by side group elimination. Fig. 4.10 presents a degradation sequence for PVB. The initial PVB degradation involves the loss of water, butanal, and acetic acid leaving behind long organic chains containing appreciable unsaturation and conjugation. The remaining chains subsequently degrade via random scission, with concurrent cyclization and cross-linking reactions yielding significant amounts of carbon residue. Side reactions such as cyclization and cross-linking are considered particularly harmful as they lead to the formation of non-volatile carbonaceous residue. The residual carbon resulting from these processes will have one of two forms: pseudographitic carbon, which is composed of multiple polycyclic groups (similar to graphite) resulting from cyclization, or highly branched carbon, which is a nonaromatic carbon residue forms as a result of cross-linking reactions (Fig. 4.11). It is conceivable that carbon residue containing a mixture of these two forms may be produced in the case of polymer blends.

Figure 4.10 Thermal degradation sequence of PVB.

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Figure 4.11 Schematics of the chemical structure of different types of carbonaceous residue: (A) pseudographitic; (B) cross-linked.

The elimination of nonvolatile residue species in the final stage of binder burn-out requires the presence of an oxidative atmosphere, which for obvious reasons, cannot be applied in the case of reactive powders. Therefore it is often recommended to use components that volatize efficiently in inert atmospheres instead of designing an elaborate binder removal process to minimize carbon retention. Higgins25 has extensively studied the formation of carbon residue during thermal decomposition of additives widely used in ceramics processing. These findings were very valuable for MIM processing as well. In his work, the specimens were heated to 1000°C at 5°C/min in gettered argon with ,1027 ppm O2(g) and then cooled to ambient conditions in the same atmosphere. Table 4.6 lists the yield of residue for the studied additives. According to this study, polymers that initially decompose by side group elimination and surfactants containing polar functionalities that degrade by random scission processes produce considerable amounts of nonvolatile carbon residue. On the other hand, the polymers that decompose via depolymerization and hydrocarbon polymers that degrade by random scission into volatile alkanes and alkenes appear to be among the cleanest additives. On the contrary, cellulosic polymers, which inherently contain cyclic backbone structures, are among the least clean additives, and thus should be avoided for reactive powders-MIM at all costs. Despite the clean nature, the use of depolymerization type of polymers as a single backbone component of the binder could lead to the formation of cracks and blisters during thermal debinding because of the faster decomposition rate. Hence, the heating rate during thermal debinding of binder systems containing this type of polymers requires robust control. PMMA has excellent compatibility with PEG as described in Chapter 3, Binder System Interactions and Their Effects. In addition to

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Table 4.6 Carbon residue yields for different organic additives as studied by Higgins.25 Polymers

Carbon residue yield (wt.%)

Polyethylene Polyisobutylene Polypropylene carbonate Polymethyl methacrylate Low mol. wt. Medium mol. wt. High mol. wt. Polybutyl methacrylate Polyethylene oxide 3400 g/mol 20,000 g/mol 100,000 g/mol Polyvinyl butyral Butvar B-76 Butvar B-98 Menhaden fish oil Ethyl cellulose

,0.1 ,0.1 ,0.1 0.2 6 0.1 0.2 6 0.1 0.2 6 0.1 0.2 6 0.1 0.3 6 0.1 2.5 6 0.1 3.1 6 0.2 1.8 6 0.1 1.3 6 0.1 3.5 6 0.2 6.0 6 0.3

the clean nature of PMMA thermal decomposition, PEG/PMMA binder system is widely regarded as a good binder system for reactive powdersMIM. However, we found that this binder system is not free of problems. Many irregular-shape voids appeared as the feedstock reached room temperature after compounding at a high temperature (Fig. 4.12).15 Initially, it was thought that these voids were formed due to gas entrapment and may be rectified during the injection molding process. However, the voids were persistent even in injection-molded samples, regardless of the injection parameters. In most cases, the voids were found in the middle of the samples (an example is presented in Fig. 2.8). Fig. 4.13 shows a typical morphology of one void. There are no particle pull-out impressions within the surface of the void. Moreover, the irregular internal surface and a wormhole type appearance imply a typical solidification defect, whereas voids due to air bubbles are round in shape and have smooth internal surfaces. The void formation is an intrinsic result of PEG/PMMA binder and not related to the injection molding parameters. This has been verified by hot mixing the same binder system. Upon cooling, the similar voids

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Figure 4.12 Comparison between feedstock at a high temperature of 130°C (left) and feedstock at room temperature (right). Despite the voids, the feedstocks showed good homogeneity, which was verified by the pycnometer density results.15

Figure 4.13 Morphology of a void occurring during solidification of a conventional MIM sample.15

appeared with rough surface morphology. In addition, the PMMA molecular weight did not influence void formation. Based on the experimental results, we proposed that these solidification defects are a result of the difference between the crystallization temperature of PEG and the glass transition temperature of PMMA. During solidification, as temperature drops, PMMA molecules start to segregate at interlamellar regions (similar to the solidification of metal castings, where

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impurities segregate in the center) and form a network, while PEG spherulite starts to nucleate and grow around PMMA molecular chains with decreasing temperature. When the temperature drops below the glass transition temperature of PMMA, its molecular chains become rigid again. As temperature further falls and PEG crystallization temperature is reached, the rigid PMMA network impedes the volumetric shrinkage upon crystallization. This hindrance to volumetric shrinkage along with the interaction between PEG and PMMA molecules thus results in void formation. The tendency of PMMA segregation increases with PMMA content up to 40% and results in more voids nucleation. In fact, when PMMA content is increased to 15%, void nucleation became more frequent. Based on our hypothesis, it can be argued that polymers that have a glass transition temperature below the crystallization temperature of PEG will not produce shrinkage voids. It was indeed confirmed by preparing feedstocks with different backbone polymers such as polypropylene (PP, Tg B 210°C) and poly(butyl methacrylate) (PBMA, Tg B 15°C). Both polymers have Tg well below the crystallization temperature of PEG (approximately 40°C for pure PEG26). Particularly, in the case of PP as backbone polymer, no voids were visible at macroscopic or microscopic levels, as shown in Fig. 4.14. Although the use of PP as a backbone polymer can eliminate void formation and produce homogeneous feedstocks, the high content of PP ( . 35 wt.%) is required to maintain the shape during subsequent water debinding. Such high content may not pose a significant threat for less reactive powders, but for reactive materials such as Ti, such a high content will lead to increased impurity contents during thermal debinding.

Figure 4.14 SEM morphology of Ti feedstocks comprising binder containing PP as a backbone polymer.

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PBMA can be an excellent alternative to PMMA, considering that it has a very similar structure to PMMA; the only difference is that the methyl group is replaced with butyl (Fig. 4.15). The use of PBMA was reported previously in the literature.24 Although PBMA seems a good choice, there are some compatibility issues between PEG and PBMA during feedstock formulation. The SEM analysis of injection molded samples revealed different phases within the sample (Fig. 4.16). In addition, these samples showed poor shape retention during water debinding due to phase segregation, which can be seen in Fig. 4.16. The shape retention during water debinding can be improved by incorporating a very small amount of PMMA (5 6 wt.% of binder). Injection-molded Ti-samples made of the binder compositions (PEG 73 wt.%, PBMA 15 wt.%, PMMA 6 wt.%, SA 6 wt.%) showed good shape retention but had mediocre mechanical properties (e.g., ultimate tensile strength, UTS: B620 MPa, elongation: B4%). However, low solids loading ,55 vol.% due to less compatibility between PEG and PBMA remains a concern.

Figure 4.15 Comparison of the molecular formula of PBMA and PMMA.

Figure 4.16 Fracture surface analysis of Ti-feedstock comprising of PBMA as a backbone polymer. Two phases can be easily seen. The presence of phase segregation clearly suggests poor compatibility between the two components of the binder system.

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Figure 4.17 Weight loss of PPC in argon atmosphere compared to PMMA. Decomposition starts around 190°C and ends at 340°C approximately. On the other hand, PMMA decomposition starts at 340°C and ends at 435°C.27

Apart from PMMA, polypropylene carbonate (PPC) decomposes cleanly via an unzipping mechanism even in an inert atmosphere. In addition, the decomposition temperature is below 400°C—almost 100°C less than PMMA (Fig. 4.17). However, our follow-up trials have shown that the binder system comprising of PPC has poor shape retention, as described in Chapter 3, Binder System Interactions and Their Effects. The one final and important aspect for binder decomposition to be considered is the presence of metallic powder. Metallic powders can have a catalytic effect on the decomposition behavior of the binder. It can alter the mechanism or the rate of binder decomposition.28,29 For example, Aggarwal et al.28 showed that for a particular binder weight percent, a lower temperature (higher work of decomposition, Θ) is required for the feedstocks than just the binder without metal powder (Fig. 4.18). They concluded that the plausible reason for this is that the metallic powder enhances the debinding process as a consequence of the catalytic effect or faster heat transfer due to higher thermal conductivity of the metal powder (54 W/m K for Nb, 12 W/m K for 316L, vs 0.1 W/m K for typical polymer). In another study, Machaka et al.29 acknowledged the catalytic effects of Ti 6Al 4V powder on the thermal decomposition of the used binder system (Table 4.7). In a nutshell, the debinding cycle must be tailored according to the binder. A model proposed by Lin30 to predict the residual carbon content

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Figure 4.18 Catalytic effect of metal powders on the decomposition behavior of solvent debound samples as illustrated by Aggarwal et al. where binder system consisted of PW, PP, PE, and SA. Nb and 316L refer to metallic powders. Both feedstocks had the same solids loading (57 vol.%) and the same debinding conditions were applied in each case.

Table 4.7 Ti 6Al 4V powder effect on the thermal decomposition of the used binder system. Material

Melting range (°C)

Decomposition range (°C)

Pure binder system Ti 6Al 4V feedstock

37 108 46 112

154 557 113 506

after debinding under N2 H2 atmosphere suggests that the isothermal holding time at the maximum debinding temperature is one of the most important parameters. Hence, the debinding process should not be extended to any unnecessary length. In addition, the debinding atmosphere also plays an important role. A detailed study carried out by Guo et al.31 on the Ti6Al4V system demonstrated that although an argon atmosphere results in lower residual carbon content, a vacuum atmosphere aids the reduction of the oxygen content (see Fig. 4.17, left). In the same study, measurements of carbon and oxygen content after different debinding times and temperatures under vacuum showed that while the carbon content was reduced to 0.095 wt.% after 1 h at 600° C, the oxygen content increased continuously during debinding (Fig. 4.19, right). Therefore a balance between debinding temperature and time with the correct selection of atmosphere is vital to the success of the MIM process in the case of reactive metals or for alloys where carbon content has to be kept in check.

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Figure 4.19 (Left) Correlation between carbon and oxygen contents of the specimens with different debinding atmospheres. (Right) Correlation of maximum debinding temperature and carbon and oxygen content of the specimens during thermal vacuum debinding of Ti 6Al 4V. The holding time was 1-h and the starting levels of carbon and oxygen were 0.056 wt.% and 0.192 wt.%, respectively.

4.2.3 Sintering and impurity control As presented in Fig. 4.2, sintering is the most critical production step concerning contamination by oxygen. Also, the resulting microstructure is defined by this process and so are the mechanical properties. The key considerations for Ti-MIM sintering cycle are as follows: rack material, peak temperature, holding time, and most importantly sintering atmosphere.1 It is not possible to entirely avoid oxygen pick-up. However, it can be kept to a minimum by using a high vacuum ( . 1022 Pa) during sintering. High purity argon can also be used as a sintering atmosphere, but it leads to lower sintered density due to trapping of gas in the pores. In addition, powder with low oxygen content, for example, the extra low interstitials variant of the Ti 6Al 4V alloy, should be used. From the author’s personal experience, if a high-quality sintering furnace is used and the impurity pick-up during debinding is kept in check, the difference in oxygen between powder and sintered part can be limited to about 0.07 wt.%. However, in practice, the oxygen pick-up up to 0.35 wt.% is possible, depending on initial oxygen content, the type of the binder used, debinding efficiency, quality of furnaces, and so on. Proper selection of sintering parameters is a tough task and is usually a compromise between porosity and grain size. Obasi et al.32 studied the influence of different processing parameters on the mechanical properties of MIM fabricated Ti 6Al 4V alloy. According to them, the heating rate from ambient temperature to debinding temperature does not

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Table 4.8 Variation of heating rate from ambient temperature to debinding temperature dTdebind and its effects on tensile properties for Ti 6Al 4V. Heating rate dTdebind (° C/min)

Tensile properties

Microstructural features

Yield stress (MPa)

UTS (MPa)

Elongation (%)

Average colony size (μm)

Densification (%)

Carbon content (μg/g)

2 5 10

728 6 5 734 6 3 728 6 3

832 6 2 835 6 2 828 6 4

13.4 6 0.5 14.0 6 0.4 13.5 6 1

91 6 2 92 6 3 92 6 2

97.1 97.1 97.1

400 6 34 395 6 14 403 6 47

Sintering temperature was 1350°C, while the heating rate after thermal debinding up to the sintering temperature was 5°C/min in each case.

Source: Data were taken from Obasi, G. C.; Ferri, O. M.; Ebel, T.; Bormann, R. Influence of Processing Parameters on Mechanical Properties of Ti 6Al 4V Alloy Fabricated by MIM. Mater. Sci. Eng. A 2010, 527 (16), 3929 3935 [32].

influence the final mechanical properties nor induce any surface or internal defects (Table 4.8). This result is also in agreement with the previous investigation of Yimin et al.,33 who reported that increasing the heating rate did not promote defects in compacts with thicknesses smaller than 10 mm. However, it should be kept in mind that the optimal heating rate depends on the geometry of the particular component. Nonetheless, it is fair to assume that there is a rather wide scope within which this parameter can be varied without influencing the properties of the sintered bodies. Contrary to the heating rate in the debinding stage, the authors found that the subsequent heating rate up to the sintering temperature is the most crucial and influences final density and grain size. They concluded that for Ti 6Al 4V powder sintering, the heating rate should not exceed 10°C/min. Generally, longer holding time and higher sintering temperatures lead to higher density resulting in higher strength and ductility. On the contrary, a higher sintering temperature also leads to more oxygen contamination and hence reduced ductility. In addition, high temperature and long sinter time lead to grain coarsening that is detrimental to strength.34 Pure titanium and typical alloys like Ti 6Al 4V are sintered above the β-transus temperature in the single-phase β region, which boosts grain growth. It should be kept in mind that the reduction in the grain size of titanium-based materials is not possible without a mechanical treatment. Therefore the optimal sintering cycle depends on the starting powder characteristics, desired density, alloying approach,

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microstructure, and final impurity level requirements. Zhang et al.35 showed that sintering at a low temperature for a long time is a better approach than sintering at a high temperature but for a short time. Typical sintering parameters include the sintering temperature at around 1300°C and a holding time of 2 h. Lower temperature and shorter sintering time are favored if hot isostatic pressing is used to reach full density, as described by Chen.36 Smaller powder particle size can lower the sintering temperature, but usually result in increased oxygen pick-up. Similar studies were performed for TiNi and TiNiAl alloys by Liu et al.37 and Ti 6Al 4V by Nor et al.38 For optimizing the sintering parameters, statistical tools can be useful. For example, Sidambe et al.39 used the Taguchi method to find the best combination of sintering time, temperature, heating rate, and atmosphere for MIM of pure titanium and Ti 6Al 4V, for a given binder system. As titanium reacts with most materials at high sintering temperatures, it limits the materials that can be suitable for sintering support, and only less than five materials are suitable for racking. Any deviation from these choices would result in a higher content of impurity or a reaction between the sample and the racking material. The most suitable and successful racking materials are zirconia (ZrO2) and yttria (Y2O3). Uematsu et al.40 studied the effects of different substrates (ZrO2 and Y2O3) for sintering on the mechanical properties of injection-molded Ti 6Al 4V. They established that both zirconia and yttria substrates require a prebake at 1250°C for 2 h in vacuum to remove impurities before first use. They concluded that to achieve the best results, prebaked zirconia should be used (Fig. 4.20). Similarly, Y2O3 was found as the most promising lining material for handling molten metals.41,42 However, these materials are expensive and to reduce cost, some firms rely on plasma sprayed coatings on alumina setters. The only metal that can be used as sintering support is molybdenum. However, it is susceptible to grain coarsening at high temperatures and therefore, its service time is relatively limited. In addition, oxygen scavengers can also be employed to further reduce oxygen pick-up during sintering. Compounds containing rare earth elements offer combined possibilities for oxygen scavenging and microstructure refinement.2,43,44 Limberg et al.43 added elemental yttrium to Ti 6Al 4V. The authors claimed that the addition of yttrium serves two purposes: (1) the in situ formation of Y2O3 allows the usage of cheap titanium powder with a high initial oxygen content like HDH-powder (the Ellingham diagram

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Figure 4.20 Highlighting decisive results from Uematsu et al. study. Effects of various types of substrates on (A) tensile strength; (B) elongation; (C) carbon content; (D) oxygen content, of sintered Ti 6Al 4V alloy compacts. Taken from Uematsu, T.; Itoh, Y.; Sato, K.; Miura, H. Effects of Substrate for Sintering on the Mechanical Properties of Injection Molded Ti 6Al 4V Alloy. J. Jpn. Soc. Powder Powder Metall. 2006, 53 (9), 755 759 [40].

shows that the energy of formation for TiO is higher than for Y2O3, so yttrium should be able to scavenge the oxygen out of the titanium matrix by forming of Y2O3 during sintering); (2) the formed oxides lead to a colony refinement, which enhances the mechanical properties. The results were, however, not in accordance with the proposed rationale— the addition of yttrium led to increased porosity with a negative impact on tensile strength and little impact on the contents of oxygen (Table 4.9).43 A common practice is to use a mixture of loose titanium and alumina powders (in 50/50 weight ratio) in an alumina crucible as an oxygen getter. It is recommended that the sintering furnace should be maintained as clean as possible. Any cross-contamination should be avoided.

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Table 4.9 Mechanical properties of different Ti 6Al 4V MIM samples sintered at 1300°C.

Ti 6Al 4V Ti 6Al 4V 1 0.1Y mesh 120 Ti 6Al 4V 1 0.2Y mesh 120 Ti 6Al 4V 1 0.2Y mesh 400

YS (MPa)

UTS (MPa)

εf (%)

Porosity (%)

Oxygen content (μg/g)

769 733

876 832

15.2 8.4

3.4 4.0

1850 6 33 1935 6 56

712

803

6.1

4.3

1840 6 91

728

823

12.9

4.2

1828 6 18

4.3 Points to consider for other reactive powders metal injection molding As mentioned at the start of this chapter, MIM of reactive powders other than titanium, such as aluminum or magnesium is not common and is, in fact, difficult to achieve economically. The reason for this being none other than their extreme reactivity. However, unlike titanium, Al or Mg do not form a solid solution with oxygen.45,46 Instead, a thermodynamically stable oxide layer (usually a few nanometers thick) is formed. This oxide layer inhibits atomic diffusion required for densification, rendering a high density (at least .99% of theoretical value) unattainable. This put MIM at an economic disadvantage over other metal forming processes such as die-casting in the business world. Therefore overcoming the oxide layer and getting the powder sell sintered is the most challenging technical hurdle for these powders. Nevertheless, there have been a few research studies performed in the past years. The most common way of achieving a successful Al or Mg-MIM process is to employ alloy powders to keep the oxide formation to a minimum. The following section presents some case studies involving Al or Mg powder systems-MIM.

4.3.1 Pure Al-metal injection molding Al-based materials are still largely manufactured via ingot metallurgy.47 49 These products are obtained by hot working, either by extrusion, rolling, or forging. Since the early 1960s, aluminum P/M processing includes aluminum powder production by gas (air) atomization and powder consolidation into billets by extruding, forging, or rolling into plate or sheet. As explained earlier, a primary problem with the consolidation of the Al

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powder during sintering is due to the obtrusive and intrinsic oxide film of the particle surfaces. The breakthrough in Al P/M was made in 1969 when suitable internal lubricants were identified, the processing difficulties were addressed and commercially viable sintering processes were developed.50 The compositions of current Al P/M alloys are primarily based on wrought or cast alloys, whose P/M products, by and large, have less satisfactory mechanical properties. The poor mechanical properties limit the use of Al PM alloys to low-stress applications (for readers seeking advanced knowledge on aluminum metallurgy, refer to Ref. [48] for details). Given the limited work and insufficient mechanical properties of Al PM alloys, it is not a surprise that only limited work has been carried out in the case of MIM of aluminum and its alloys, and hence, it is not a standardized process yet. Nevertheless, several research studies and patents51 53 have been published in recent years detailing the advantages of MIM aluminum parts along with their test results. In one of the studies, Katou et al.5 applied the MIM process to Al powder. First, they investigated the influence of different processing conditions on the properties of sintered pure-Al compacts. In the second part, the addition of silicon powder to Al powder to improve the sinterability of Al powder was investigated. For the first part, Al powders of different particle size 20, 10, and 3 μm were used. The binder system was based on paraffin wax and PMMA. For debinding in air, 325°C was selected as the maximum debinding temperature while for debinding in argon 380°C was used. All compacts were sintered at 650°C for 2 h under a high vacuum of 1023 Pa. The influence of different processing and materials parameters on the properties of sintered pure Al compacts as studied by Katou et al. are summarized in Table 4.10. The following results can be concluded based on these studies: (1) the density of sintered compacts increases with decreasing powder particle size (as expected for any metallic powder MIM) for the same processing conditions; (2) degree of vacuum during sintering affects the content of oxygen of sintered compacts; (3) naturally, the sintered compacts debound in air contain more oxygen than those debound in Ar gas; (4) the relative density increases as the particle size decreases; (5) similarly, the tensile properties improve with decreasing Al powder particle size. Apart from the above observation, the relative density of Al sintered compacts increases with increasing temperature (Fig. 4.21). From the above observations, it can be concluded that reasonable mechanical properties can be achieved in pure Al by employing a small

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Table 4.10 Influence of the process condition on the relative density, oxygen and carbon contents of the sintered compacts. Average particle size (μm)

20 10 3 20 3 10

Debinding atmosphere

Air Ar gas Air Ar gas Air Ar gas Ar gas Ar gas

Vacuum degree in sintering (Pa)

1023 1023 1023 1023 1023 1023 10 102

Oxygen content (wt.%)

Carbon content (wt.%)

Relative density (%)

0.57 0.81 0.99 2.31 0.75 1.99 1.47 2.95 0.92 1.14 2.40

0.01 0.01 0.01 1.04 0.21 0.87 0.17 1.03 0.23 0.20 0.32

66.5 86.5 85.9 96.3 67.2 89.8 83.8 64.8

UTS (MPa)

ε (%)

B60

B7

B120

B19

B75

B12

Figure 4.21 Relationship between sintering temperature and relative density of sintered Al compacts.

powder particle size, using a clean polymer (PMMA being one of the cleanest polymers) and carrying out sintering under high vacuum. It is well known from the literature54 56 that the addition of elemental powders such as Cu, Si to Al powder promotes densification. In the second part of their study, Katou et al. tested this theory to the MIM process. Silicon was selected as an additive. As shown in the Al Si phase diagram, there is a eutectic point at 12.2 at.% Si at 577°C (see Fig. 4.22). Consequently, it can be expected that this eutectic reaction will promote densification of Al powder by liquid phase formation. The initial

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Figure 4.22 Binary phase diagram of Al Si.57

Figure 4.23 Relationship between sintering temperature and relative densities of sintered Al 1Si and pure-Al MIM compacts.5

trial suggested that silicon weight percent of more than 1 wt.% did not promote the densification of Al compacts effectively. The average particle sizes of Al powder used for this study were 35, 20, and 11 μm. The particle size of Si powder was 2 μm. The solids loading for this study was 62 vol.%. After injection molding, all the green samples were debound in Ar at 380°C and then sintered at various temperatures under high vacuum 1023 Pa for 2 h. Fig. 4.23 compares sintered densities of pure-Al and Al 1Si compacts at various temperatures. From this result, it is clear that the addition of Si powder effectively improves the densification of pure Al compacts prepared by MIM regardless of Al powder size. The increase in density also leads to improved tensile properties (Table 4.11).

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Table 4.11 Comparison between tensile properties at room temperature of sintered pure Al and Al 1Si compacts prepared by MIM and commercial wrought Al. Composition

Pure-Al Al 1Si

110058

Particle size (μm)

Sintering temperature (° C)

Relative density (%)

Tensile properties Y.S (MPa)

UTS (MPa)

ε (%)

11 35 35 20 20 11 11

650 625 635 625 635 625 635

83.3 85.3 87.7 91.8 93.0 95.7 96.9

41.7 26.0 25.8 29.5 31.7 42.7 41.7 35.0

74.6 63.6 69.7 77.0 79.2 104.6 107.8 90.0

5.2 9.3 10.2 9.1 8.5 20.7 24.6 35.0

4.3.2 Metal injection molding of aluminum alloy 6061 with tin The previous studies have shown that the atmosphere plays an important role in the sintering of aluminum. Inert gas atmospheres or a high vacuum should be employed to limit oxidation. Intriguingly, dry nitrogen has proven to be a more effective sintering atmosphere than either argon or high vacuum.59 61 It is believed that nitrogen plays an active role in the sintering of aluminum—the key feature of which is the formation of aluminum nitride (AlN).62 The formation of AlN facilitates sintering by enhancing the liquid filling of pores through the reduction of internal pores pressure during the late stages of sintering. However, excessive formation of AlN on the liquid hinders wetting and negatively impacts upon sintering. Concurrently, effective solid-state sintering of Al powders under nitrogen is also attributed to the formation of AIN, which disrupts the oxide layer on the surfaces of the Al powder particles, thereby, promoting diffusion. Under favorable conditions, including environmental and compositional control, the oxide layer can be overcome and Al can be sintered to a near full density. Similar to Si, other elemental powder or powder systems can also be used to enhance the sinterability of Al. Tin is such an elemental powder for the enhanced liquid phase sintering of Al63,64 (see Fig. 4.24 for Al Sn phase diagram). The Al Sn phase diagram shows many of the features of an ideal liquid phase sintering system. The melting point of tin (232°C) is

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Figure 4.24 Al Sn phase diagram.65

significantly lower than that of Al (660°C) and there are no intermetallic phases. Additionally, Sn solubility in solid Al is moderate with maximum solubility being ,0.15%. Al is completely soluble in liquid Sn, and no immiscible liquids are formed. Moreover, the surface tension of liquid Sn is considerably less than that of Al, which, in turn, can improve the wetting characteristics and sintering behavior of Al when Sn is present in trace amounts. Sn also affects both the mechanism and rate by which AlN grows on Al powder when it is processed in an oxygen-free nitrogencontaining environment at temperatures greater than 540°C. In the absence of Sn, the AlN on the powder surface grows rapidly with growth occurring both into and out from the surface of the particles. On the contrary, in the presence of Sn, slow and controlled nitridation occurs. The initial tin concentration determines the point at which the growth rate changes. Sercombe et al.66 studied the role of Sn in the nitridation of Al powder in detail, it is advised to interested readers seeking more knowledge on this phenomenon to follow their work and the cited literature thereof. Liu et al.67 developed an Al-MIM system based on AA6061 alloy mixed with Sn powder. For this system, AA6061 powder with a D50 of 13.4 μm and Sn powder with a particle size of ,43 μm were used as the starting materials. In the MIM feedstock formulation, they used a powder loading of 62 vol.% with the binder system consisting of 3% stearic acid, 52% palm oil wax and 45% high-density polyethylene. To remove more than 90% of wax and stearic acid, they employed solvent debinding (conducted in hexane at 40°C for 8 h). The thermal debinding of the remaining binder and

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Figure 4.25 Sintered densities of AA6061 with different additions of Sn at various sintering temperatures: (A) in argon; (B) in nitrogen. The holding time was kept constant at 2-h for each case. From above comparison, it is evident that 2 wt.% addition of Sn results in higher sintering density at any given temperature for AA6061 alloy.67

sintering were combined in one furnace cycle. The alloy discs were surrounded by alumina blocks to control the gas flow pathway and to provide greater temperature homogeneity. To keep the oxygen pick-up to a minimum level, magnesium sacrificial blocks were used as oxygen getters. For thermal debinding, the samples were heated to 450°C over 16.5 h and then sintered to various temperatures at 0.8°C/min and held at the respective temperatures for 2 h under the gas (either nitrogen or argon) flowrate of 0.5 L/ min. The samples were then subsequently cooled at 1°C/min to 550°C and then at 10°C/min to room temperature. The authors also carried out heat treatment of sintered samples according to different standard conditions68 (T4 and T6) to further improve mechanical properties. The achieved sinter density results with and without the addition of Sn and in the absence/presence of nitrogen atmosphere are shown in Fig. 4.25. Evidently, the sintering response of AA6061 alloy with Sn addition is much better than that of without Sn. Sn significantly increases sinter density and expands the sintering window: higher sintered density can be obtained at lower temperatures. Surprisingly, AA6061 can be sintered to a density of 95% at 630°C in argon without Sn (Fig. 4.25A). However, the narrow temperature range means poor process control and such a sharp increase in density can lead to cracking/distortion of the samples. With Sn, the density increases gradually with temperature. The negative effect of excessive AlN formation is also evident from Fig. 4.25B. In the case of pure AA6061 alloy (without Sn addition) sintered in a nitrogen atmosphere, the maximum obtainable density remains below 95% of the theoretical value. However, in conjunction with Sn, the nitrogen atmosphere increases the sinterability of Al, as shown in the microstructure of different compacts (Fig. 4.26).

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Figure 4.26 Optical micrographs of AA6061 1 2 wt.% Sn compacts sintered at 620°C for 1 h: (A) in argon; (B) in nitrogen. The original powder size was B13 μm.67

For the sample sintered in argon, the grain size increases from 13 to 100 μm. Contrary to this, the grain size of the sample sintered in nitrogen remains approximately the same. In addition, there is no liquid phase network visible on the grain boundaries. Instead, the particle boundaries are decorated by AIN. This formation of AIN is not uncommon when the atmosphere is carefully controlled.69 The formation of AIN inadvertently minimizes grain growth and limit distortion upon shrinkage. By comparing sinter density at different sintering conditions (temperature, time, and atmosphere) and corelating them to tensile properties for AA6061 1 2 wt.% Sn, the authors found that comparable UTS can be achieved when sintering is carried out in nitrogen atmosphere for 2 h at 630°C. However, elongation results remained poor. Table 4.12 compares the tensile properties of studied MIM AA6061 1 2 wt.% Sn alloy with wrought AA6061 and other commercial powder metallurgy alloys. Based on the combined density/tensile results, the author also obtained the optimum thermal profile for debinding and sintering of MIM AA6061 alloy (Table 4.13). A demonstration part (see Fig. 4.27) with a high aspect ratio (wall thickness 1 mm and height 33 mm) was also processed using this scheme. The resulting sintered part (shrinkage was B9%) was free of common MIM defects such as blisters, cracks and warpage, and had a very good surface finish. In another study, Liu et al.62 developed a MIM process for producing fine AlN particle reinforced AA6061 matrix composites. The polymer binder they used was composed of palm oil wax, stearic acid, and highdensity polyethylene. The powder loading was 61 vol.%. Thermal debinding and sintering were conducted in a single heating sequence. Before initial heating, a vacuum level below 5.0 3 1022 Torr (B6 Pa) was achieved. The samples were first heated to 450°C over 16.5 h and

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Table 4.12 Comparison between mechanical properties of MIM AA6061 1 2 wt.% Sn alloy and selected commercial wrought and other Al alloys produced by PM. Test condition

YS (MPa)

UTS (MPa)

ε (%)

As sintered T4 T6 aluMIM (as sintered) 601AB (as sintered)70 AA6061 (T6)71

75 6 7 78 6 3 279 6 13 50 94 276

157 6 11 209 6 4 303 6 19 100 145 310

9.5 6 3.8 10.4 6 0.7 1.5 6 0.8 18 6 17

Table 4.13 Optimal processing parameters for MIM of AA6061 alloy as proposed by Liu et al. Step

1

2

3

4

5

6

7

Heating rate (°C/min) Set temperature (°C) Holding time (min)

3 150 0

0.5 250 120

0.5 375 120

0.5 450 60

0.8 630 120

1.0 550 0

10 25 End

Figure 4.27 Thin-walled complex-shaped MIM demonstration part.67

then heated to the sintering temperature at 0.8°C/min and held for 2 h. The temperatures evaluated for sintering of the tensile test bars were 630, 635, 640, and 645°C.62 At 120°C, the backfilling of nitrogen was started at a gas flow rate of 1 l/min. To further keep oxygen level to a minimum, several Mg blocks were placed beside the samples. The authors reported that with the addition of AlN reinforcement, the hardness, 0.2% yield strength, and tensile strength of the AA6061 matrix increase in both as-

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Table 4.14 Selected mechanical properties of MIM AA6061 1 2Sn 1 10AlN composite. The MIM AA6061 1 2Sn alloy properties are presented as a comparison. Process conditions

Density (%)

Hardness (HRH)

YS (MPa)

UTS (MPa)

εf (%)

Sintered at 630° C Sintered at 635° C Sintered at 640° C Sintered at 645° C Sintered at 640° C and T4 treated AA6061 1 2Sn as sintered AA6061 1 2Sn T4 condition

94.7 6 0.6

75.3 6 1.7

80.8 6 3.5

134.6 6 23.6

1.66 6 0.88

96.4 6 0.4

80 6 0.9

82.0 6 3.6

149.7 6 28.3

2.33 6 1.88

97.1 6 0.1

80.2 6 0.9

84.0 6 3.6

184.7 6 9.2

6.63 6 3.26

96.9 6 0.1

80.0 6 1.4

79.5 6 2.4

168.3 6 16.7

4.65 6 2.48

97.1 6 0.1

92.0 6 0.9

118.7 6 3.2

263.3 6 5.8

8.1 6 0.52

97.5 6 0.4

68.2 6 2.4

75.0 6 7.2

157.0 6 11.3

9.47 6 3.77

97.5 6 0.4

81.8 6 2.8

78.2 6 3.4

208.8 6 4.2

10.41 6 0.69

Source: Taken from Liu, Z. Y.; Kent, D.; Schaffer, G. B. Powder Injection Moulding of an Al AlN Metal Matrix Composite. Mater. Sci. Eng. A 2009, 513 514, 352 356 [62].

sintered and T4 conditions. On the other hand, ductility is reduced at the expense of these property gains (Table 4.14). Although the addition of AIN into the AA6061 matrix reduces the ductility, the high thermal conductivity and low thermal expansion coefficient of such systems can lead to potential applications in microelectronics. The authors, however, did not provide results of thermal conductivity testing. Nevertheless, this example shows that even Al matrix composites can also be made by MIM.

4.3.3 Metal injection molding of Mg and its alloys Powder metallurgy of Mg and its alloys offers three distinct advantages in comparison to conventional ingot metallurgy and metal forming techniques: 1. P/M processing facilitates the homogeneous distribution of elements. 2. P/M processing allows control over grain size for Mg and its alloys. Using PM metal forming techniques, for example, extrusion, the grain size can be kept to the submicrometer scale.

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3. The difficulties occurring during metal forming due to the hexagonal lattice structure of Mg can be avoided by using P/M—near-net shaping techniques. Since the recognition of Mg and its alloys as being highly suitable biodegradable orthopedic implant materials for biomedical applications,72 75 MIM of Mg expectedly seems to be a very attractive fabrication route for complex-shaped implants. The new Mg-based biodegradable and biocompatible materials degrade completely, consists of nontoxic elements, and show mechanical properties matching those of cortical bone tissue. The corrosion behavior of Mg enables well-directed biodegradation and the corrosion products generated during biodegradation are identified as noncytotoxic. However, similarly like Al, sintering of pure Mg is reported to be unfeasible due to the formation of a thermodynamically stable MgO layer on the surface of powder particles, which acts as a diffusion barrier for the essential diffusion process during sintering. This is the main reason why the MIM of Mg is still not a standardized process. Hence, a big portion of research on powder metallurgy of Mg has been focused on understanding the sintering performance of Mg and to develop a successful sintering process.6,76,77 The main challenges of MIMMg and its alloys are (1) the prevention of any additional oxygen pick-up, and (2) the prevention of any reaction between Mg and degradation products during thermal debinding. MIM of pure Mg is out of the question (remember Mg is often used as oxygen getter during sintering of Ti or Al!). Therefore MIM-Mg is not possible without a sintering aid. Small amounts of Ca can be employed as the sintering aid in this case. Akin to Sn in Al, it forms a liquid phase and weakens the oxide layer on the Mg particles; promoting the diffusion processes necessary for sintering. Wolff et al.78 recently have shown that 0.9 wt.% of Ca yield the optimal results in terms of mechanical properties. Considerable work in developing a design strategy for a successful Mg-MIM process has been done by the same author and his team.77 81 In one such study, they have evaluated the impact of several polymeric components such as polybutylene (PB), polypropylene (PP), polypropylene copolymer polybutene (PPcoPB), and polypropylene copolymer polyethylene (PPcoPE) on the mold filling quality during injection molding and on the sinterability of Mg. To carry out this research, they blended pure spherical and commercially available gas atomized Mg-powder with spherical gas atomized master alloy powder Mg 10Ca to prepare Mg 0.9Ca powder blends. For the primary component of the binder system, paraffin wax was used and stearic acid as the

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Figure 4.28 Thermal debinding and sintering profile for MIM Mg 0.9Ca parts.82

surfactant in each case. Polymer content was varied between 5 and 35 m. % of the binder system while the powder loading was 64 vol.% for all feedstock batches. To avoid any oxygen uptake of the magnesium powder components, the materials were handled in a protective argon atmosphere. Paraffin wax and stearic acid were subsequently removed via solvent debinding in a hexane bath at 45°C for 10 15 h. Thermal debinding and sintering were then carried out in a combined debinding and sintering hot-wall tube furnace with integrated binder precipitation zone (see Fig. 4.28 for debinding profile). The thermal debinding took place under a reactive atmosphere (Ar 1 5% H2) using alternating pressure between 1 and 800 mbar at 1 L/min gas flow to reduce reactive monomers that may occur. It should be noted that Mg has the highest vapor pressure among all technical metals83 (231.5 Pa at 627°C84), so the sintering of Mg under a high degree of vacuum is not possible. Any such “adventure” would result in evaporation of material and consequent deposition in the relative colder areas. Hence, the sintering of Mg is generally carried out in an argon atmosphere as was in this case, which was conducted for 64 h at 635°C in a high-purity argon atmosphere. The prolonged sintering time seems unreasonable. However, according to the authors, it is necessary to destabilize the oxide layer encasing each Mg particle. In addition to the above sintering profile, the authors also employed a unique labyrinth-like crucible configuration that contained Mg getter material (particle size 0.06 0.3 mm with purity .98.5%). This set-up is shown in Fig. 4.29. Expectedly, the backbone polymer component has a considerable effect on the sintering performance of the Mg powder compact. Initially, the authors used a traditional binder system based on polyethylene copolymer

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Feedstock Technology for Reactive Metal Injection Molding

Figure 4.29 Labyrinth like crucible set-up as employed by Wolff et al. for the sintering of MIM Mg alloy parts.82

Figure 4.30 SEM micrographs of sintered Mg MIM specimens after 64 h sintering time showing residual porosity Px, shrinkage sf, UTS, and elongation as a function of used backbone polymer: (A) PE-VA copolymer; (B) PPcoPE copolymer.82

vinylacetate (PE-VA), which had a satisfactory rheological behavior and a reasonable sintering performance for MIM of titanium.85 Therefore it is logically correct to assume that the binder system that works for Ti-MIM will also work for Mg-MIM. However, as the authors found out this was not the case (see Fig. 4.30A). The use of PE-VA resulted in inhibition of the sintering of Mg alloys giving UTS of 6 MPa at 26% of residual porosity. Other polymers such as PP, PB or PPcoPB although resulted in good sintering performance, but had poor rheological properties. The main difference between the performance of PE-VA and other polymers in the case of MIM of Mg can be traced back to their thermal degradation behavior (see Section 4.2.2). PE-VA generally leads to a higher amount of char due to the chain scission reactions during its thermal degradation. On the contrary, PB decomposes by depolymerization and produce volatile degradation products. PP although also degrades via chain scission mechanism, the presence

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of PB can influence its degradation behavior. In addition, PE-VA degrades at a higher temperature compared to PP and PB. For a low melting point metal, this can have serious consequences as the degradation temperature can coincide with the initial stages of sintering. The binder residue may be entrapped inside the pore, resulting in the increased porosity and in the case of oxygen-containing polymers such as PEcoEV. It may lead to oxygen pick-up and consequently to the formation of MgO on particle surfaces. The PPcoPB nevertheless resulted in poor rheological properties of the feedstocks. Although paraffin wax and PP are compatible to some extent (see Section 3.1.3), the PB presence may have further decreased this compatibility, which, in turn, resulted in poor rheological properties. The authors claimed that these process challenges were overcome using a novel polymer backbone component, polypropylene copolymer polyethylene (PPcoPE). This unified the good rheological behavior of PE with the satisfying sintering performance of PP. The use of 35 wt.% PPcoPE in the binder system resulted in low residual porosity (Fig. 4.30B). Moreover, increased UTS and YS could be achieved by carefully controlling the content of PPcoPE in the binder system. Table 4.15 provides an overview of the relation between the used polymer and the resulting tensile properties. The authors further demonstrated that complex parts could also be manufactured using the novel binder system, Fig. 4.31.

4.4 Process control When we talk about the MIM processing of reactive powders, there is nothing really special with regard to the equipment. The main consideration, however, is the contamination problem during the whole production chain. The powder handling should be strictly carried out under a protective atmosphere although it does not pose a significant contamination risk in the case of titanium. Nevertheless, titanium powder dust can Table 4.15 Relation between PPcoPE content in the binder system and mechanical properties of resulting Mg 0.9Ca alloy. Polymer

UTS (MPa)

YTS (MPa)

εf (%)

35% PE-VA 35% PPcoPE 25% PPcoPE

461 136 6 6 142 6 5

68 6 2 67 6 1

762 861

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Feedstock Technology for Reactive Metal Injection Molding

Figure 4.31 Different demonstration parts made by MIM of Mg 0.9Ca feedstock at Helmholtz-Zentrum Geesthacht (HZG), (A) bookmark demonstrator parts. The parts on the left are in the as sintered condition; the parts on the right are the corresponding green parts. The shrinkage of the sintered parts is very clearly visible in comparison to the size of the green parts; (B) suture anchor screws. Lower screw shows a sintered part while upper screw was made from PLDLA (poly L-lactide/DL-lactide copolymer) using the same mold as the MIM Mg 0.9Ca screw. Courtesy Conmed Linvatec Wolff, M.; Schaper, J.; Suckert, M.; Dahms, M.; Feyerabend, F.; Ebel, T.; Willumeit-Römer, R.; Klassen, T. Metal Injection Molding (MIM) of Magnesium and Its Alloys. Metals 2016, 6 (5), 118 [82].

explode in the air (It is presently considered to be 20 30 g/m3—an extremely small amount. Titanium dust will ignite with as little as 9% oxygen, with the balance helium or 3% oxygen with the remainder carbon dioxide.86) This limit is more or less the same for Al and Mg powders as well. As a protection gas, argon is the most suitable inert gas. Any device suitable for MIM of stainless steel can be used for MIM of reactive powders. During kneading, binder and powder are heated to certain temperatures depending on the type of binder system used, but usually ranging from 120°C to 180°C. To guarantee no additional oxygen pick-up, kneading should be done preferably under a protective atmosphere. As in some cases, low molecular weight additives in polymers (sometimes additives are added in minute quantities in commercial polymers to make them more processable) can degrade at a much lower temperature in the presence of oxygen. It is extremely important to make sure that no reactive powders feedstocks are contaminated by other materials. For example, in the case of titanium, steel powder impurity can lead to brittle or low melting phases in the microstructure—degrading the mechanical properties significantly. Thus the cleanliness of the machine is critically important. It is necessary

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to carefully clean all parts of the injection molder in contact with the feedstock whenever the feedstock material is changed.

4.5 Summary The control over impurity content is one of the critical factors that determine the success of metal injection molding of reactive powders. However, it is easier said than done. This chapter highlights this issue. After a brief introduction to the importance of impurity control, this chapter is divided into two sections depending on the interaction between oxygen and the powders. In the first part, methods to control impurities for titanium and its alloys have been discussed. The most important parameter to keep impurity content in-check is complete and removal of the binder system during the debinding stage. Any leftover binder will subsequently be picked up during the early stages of sintering. The thermal degradation of the backbone polymer may follow chain depolymerization, random scission, and side-group elimination mechanisms depending on the type of the polymers. The polymers that decompose via depolymerization degradation mechanism generally lead to little to no residue. However, their fast degradation rate demands careful control of processing parameters during the thermal debinding stage. On the contrary, polymers that degrade by random scission processes produce considerable amounts of nonvolatile carbon residue. In particular, cellulosic polymers, which inherently contain cyclic backbone structures, are among the least clean additives, and thus should be avoided for reactive powdersMIM at all costs. In the subsequent section, the impact of sintering parameters on impurity control is discussed. The most suitable and successful racking materials are zirconia (ZrO2) and yttria (Y2O3). In the second part, MIM of reactive powders that do not form a solid solution with oxygen such as Al or Mg is discussed. Unlike titanium, Al and Mg form a stable oxide layer on the powder surface, which inhibits the diffusion processes required to achieve acceptable densification. Some case studies involving Al or Mg powder systems-MIM are presented. The different case studies show that by controlling the sintering atmosphere and adding small quantities of additives into pure Al or Mg powder, a successful MIM strategy for such powders can be accomplished. The growing research interest in Al/Mg powders-MIM is encouraging and may soon lead to standardization of a MIM process for such powders.

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62. Liu, Z. Y.; Kent, D.; Schaffer, G. B. Powder Injection Moulding of an Al AlN Metal Matrix Composite. Mater. Sci. Eng. A 2009, 513 514, 352 356. 63. Sercombe, T. B.; Schaffer, G. B. The Effect of Trace Elements on the Sintering of Al Cu Alloys. Acta Mater. 1999, 47 (2), 689 697. 64. Sercombe, T. B.; Schaffer, G. B.; Calvert, P. Freeform Fabrication of Functional Aluminium Prototypes Using Powder Metallurgy. J. Mater. Sci. 1999, 34 (17), 4245 4251. 65. Saito, Y.; Todoroki, H.; Kobayashi, Y.; Shiga, N.; Tanaka, S.-I. Hot-Cracking Mechanism in Al Sn Alloys From a Viewpoint of Measured Residual Stress Distributions. Mater. Trans. 2018, 59, 908 916. 66. Sercombe, T. B.; Schaffer, G. B. On the Role of Tin in the Nitridation of Aluminium Powder. Scripta Mater. 2006, 55 (4), 323 326. 67. Liu, Z. Y.; Sercombe, T. B.; Schaffer, G. B. Metal Injection Moulding of Aluminium Alloy 6061 with Tin. Powder Metall. 2008, 51 (1), 78 83. 68. Benedyk, J. International Temper Designation Systems for Wrought Aluminum Alloys: Part II Thermally Treated (T Temper) Aluminum Alloys 2010, 68, 16 22. 69. Sercombe, T. B.; Schaffer, G. B. On the Role of Magnesium and Nitrogen in the Infiltration of Aluminium by Aluminium for Rapid Prototyping Applications. Acta Mater. 2004, 52 (10), 3019 3025. 70. Committee, A. S. M. I. H. ASM Handbook, Volume 07 Powder Metal Technologies and Applications. ASM International. 71. Committee, A. I. H. ASM Specialty Handbook: Aluminum and Aluminum Alloys. Materials Park; ASM International, 1993. 72. Li, Z.; Gu, X.; Lou, S.; Zheng, Y. The Development of Binary Mg Ca Alloys for Use as Biodegradable Materials Within Bone. Biomaterials 2008, 29 (10), 1329 1344. 73. Staiger, M. P.; Pietak, A. M.; Huadmai, J.; Dias, G. Magnesium and Its Alloys as Orthopedic Biomaterials: A Review. Biomaterials 2006, 27 (9), 1728 1734. 74. Parai, R.; Bandyopadhyay-Ghosh, S. Engineered Bio-Nanocomposite Magnesium Scaffold for Bone Tissue Regeneration. J. Mech. Behav. Biomed. Mater. 2019, 96, 45 52. 75. Ali, M.; Hussein, M. A.; Al-Aqeeli, N. Magnesium-Based Composites and Alloys for Medical Applications: A Review of Mechanical and Corrosion Properties. J. Alloys Compd. 2019, 792, 1162 1190. 76. Hort, N.; Dieringa, H.; Thakur, S.; Kainer, K. Magnesium Matrix Composites; Springer-Verlag: Berlin, 2006315 334. 77. Wolff, M.; Deussing, J.; Dahms, M.; Ebel, T.; Kainer, K.; Klassen, T. Advances in the Metal Injection Molding of Mg Ca Alloys for Biomedical Applications. PIM Int. 2012, 6 (4), 59 63. 78. Wolff, M.; Schaper, J.; Dahms, M.; Ebel, T.; Kainer, K.; Klassen, T. Magnesium Powder Injection Moulding for Biomedical Application. Powder Metall. 2014, 57 (5), 331 340. 79. Wolff, M.; Schaper, J. G.; Suckert, M. R.; Dahms, M.; Ebel, T.; Willumeit-Römer, R.; Klassen, T. Magnesium Powder Injection Molding (MIM) of Orthopedic Implants for Biomedical Applications. JOM 2016, 68 (4), 1191 1197. 80. Wolff, M.; Deussing, J.; Dahms, M.; Ebel, T.; Klassen, T. In Production of Biodegradable Mg 0.9Ca Implants by Powder Injection Moulding (PIM), 9th International Conference on Magnesium and Their Applications, Vancouver, BC, Canada, Poole, W. J., Kainer, K. U., Eds. Vancouver, BC, Canada, 8 12 July 2012. 81. Wolff, M.; Wiese, B.; Dahms, M.; Ebel, T. Binder Development for Magnesium Powder Injection Moulding, 2011; Vol. 2.

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82. Wolff, M.; Schaper, J.; Suckert, M.; Dahms, M.; Feyerabend, F.; Ebel, T.; Willumeit-Römer, R.; Klassen, T. Metal Injection Molding (MIM) of Magnesium and Its Alloys. Metals 2016, 6 (5), 118. 83. Zhao, H.; Debroy, T. Weld Metal Composition Change During Conduction Mode Laser Welding of Aluminum Alloy 5182 2001, 32, 163 172. 84. Kammer, C. Eigenschaften von reinem Magnesium. Magnesium Taschenbuch, 2000; pp 77 95. 85. Ebel, T.; Milagres Ferri, O.; Limberg, W.; Oehring, M.; Pyczak, F.; Schimansky, F. P. Metal Injection Moulding of Titanium and Titanium-Aluminides. Key Engineering Materials; Trans Tech Publ, 2012153 160. 86. Poulsen, E. Safety-Related Problems in the Titanium Industry in the Last 50 Years. JOM 2000, 52 (5), 13 17.

CHAPTER 5

Potential feedstock compositions for metal injection molding of reactive metals As discussed in the previous chapters, many binder systems commercially available do not result in the desired compositional requirements and mechanical properties when they are used in the metal injection molding (MIM) of reactive powders. This chapter introduces a few feedstocks that can be successfully employed for reactive powders MIM.

5.1 Polymers that thermally degrade by depolymerization It is generally agreed that polymers, which thermally degrade by depolymerization, are better in keeping the impurity pick-up to a minimum. In particular, we are interested in the polymers that degrade into volatile monomers. In this respect, polymethyl methacrylate (PMMA) seems to be an excellent option. In the PMMA-based systems, polyethylene glycol (PEG) is widely used as the primary component due to the excellent compatibility between PEG and PMMA. In one such report, Sidambe et al.1 carried out MIM of CP-Ti components for biomedical applications using a binder system that consisted of a major fraction of PEG and a minor fraction of PMMA. The molecular weights of PEG and PMMA were 1500 and 106 g/mol, respectively. The weight ratios of the constituent binder materials were 87:11:2 of PEG, PMMA, and SA, respectively, while the powder loading was 69 vol.%. The handling of the materials was carried out in an inert argon atmosphere. The injection molded parts were then subjected to solvent debinding for 6 h at 55°C to remove PEG. PMMA was then removed via thermal degradation in an argon atmosphere using a heating profile: 2.5°C/min to 350°C; holding for 1 h; heating at 2°C/min to 440°C; and again holding there for 1 h. Sintering was also carried out in the argon atmosphere at 1300°C for 3 h. The resulting sintered samples had excellent mechanical properties owing to Feedstock Technology for Reactive Metal Injection Molding DOI: https://doi.org/10.1016/B978-0-12-817501-9.00005-3

© 2020 Elsevier Inc. All rights reserved.

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Figure 5.1 Photograph illustration of excellent shape retention of CP-Ti MIM dogbone samples. The linear shrinkage was 11 6 1%.1

their low impurity contents and excellent shape retention (see Fig. 5.1 and Table 5.1). Undoubtedly, the excellent properties were the result of clean and complete removal of PMMA in the argon atmosphere (Fig. 5.2). The excellence of this binder system was not limited to CP-Ti only. The same binder system and processing parameters were applied to Ti 6Al 4V powder as well, and the resulting sintered samples had again demonstrated excellent mechanical properties (Table 5.2). However, from the authors’ personal experience, the PEG/PMMA system leads to void formation in the feedstocks, as discussed in the previous chapters. The MIM industry is still wary of the adoption of this binder system. Nevertheless, it is one of the cleanest binder systems and, perhaps, the ideal choice for reactive powders MIM. Therefore efforts have been made to rectify the problems associated with the PEG/PMMA binder systems. In recent years, we have attempted to improve the properties of this binder system while maintaining the clean nature of it.3 5 This includes the addition of a “third component” such as PVP into the PEG/PMMA binder systems.3 The addition of PVP does help improve the overall properties of the PEG/PMMA-based feedstocks; however, it has a negative impact on efficiency. The PVP modified PEG/PMMA binder system leads to longer cooling times (more than 60 s) during the injection molding process. For small molding samples, where cooling rates are inherently high, it may not pose a significant threat. A longer cooling time could, however, be a limiting factor for large samples or for processes where a high production rate is required. The increase in cooling time is related to interactions between the three components.

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193

Table 5.1 Mechanical and chemical composition results of CP-Ti MIM samples.1 Sample

UTS (MPa)

ε (%)

Oxygen (wt.%)

Porosity (%)

1 2 3 4 5 6

483 460 467 475 466 451

21 24 21.5 20 27 24.5

0.21 0.21 0.19 0.20 0.19 0.17

3.5 4.2 3.0 5.2 4.7 3.8

Figure 5.2 TGA analysis showing the weight loss curves for PMMA for Ti/binder mix in argon and in the air before and after thermal pyrolysis. The heating rate was 5°C/min.1

Table 5.2 Mechanical and chemical results of MIM Ti6Al4V components.2 Sample

UTS (MPa)

ε (%)

Oxygen (wt.%)

1 2 3 4 5

880 876 852 876 873

14 16 8.5 15.5 9.5

0.21 0.20 0.19 0.19 0.21

The lowest PEG melting energy is observed when the PVP content is 32 wt/wt.% of PEG. It is interesting to note that when the PVP content increases to above 30 wt/wt.% of PEG, there is a sudden reduction in crystallization temperature. In the range of 10% and 30% of PVP, the

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Feedstock Technology for Reactive Metal Injection Molding

crystallization temperature of PEG does not change much (around 17°C). However, the crystallization temperature drops to 6.7°C when 32% of PVP is used (Fig. 5.3). This decrease in crystallization temperature means that for the cooling sequence of the injection molding process it would take longer for the molded samples to solidify at an appropriate mold temperature (generally around 30°C 40°C) unless a significantly lower mold temperature is applied. However, high moisture condensation may occur at the mold surface if the mold temperature is below 20°C, thus affecting the final product quality. In addition, the sharp cooling gradient can induce thermal stresses inside the molded parts (Fig. 5.4). To realize a working PEG/PMMA binder system, the author’s group has worked out an alternative polymer—polyvinyl acetate (PVAc)—to PVP. The initial trials have shown that the studied polymer is not only effective in reducing the formation of the voids but also does not lead to excessive cooling times (Fig. 5.5 and Table 5.3). By increasing the PVAc content, the crystallization energy drops from 164.2 to 124.6 J/g. Meanwhile, the crystallization temperature decreases from 40.0°C (pure PEG) to 38.9°C (PEG: PVAc, 78:5), and then remains stable at B38.5°C, irrespective of the PVAc content. These crystallization temperatures are significantly higher than those observed in the case of PEG/PVP blends (Fig. 5.3) and hence will not require a too low mold temperature for the molded body to solidify. The resulting sintered tensile

Figure 5.3 Impact of PVP contents on PEG crystallization—reduction in crystallization energy was observed with increasing PVP content.5

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Figure 5.4 The effect of mold temperature on the shape retention of the sintered molded samples for the PEG/PMMA binder system. The same injection molding processing parameters apart from the mold temperature were applied. The mold temperature for the middle sample was 35°C while the mold temperatures for left and right samples were 5°C and 10°C, respectively. The shape distortion is because of the thermal stresses induced during the cooling sequence due to the sharp temperature gradient.

specimen had a UTS of B600 MPa and elongation of B13%, satisfying the requirements for Grade 3, as specified in the ASTM F2989-13 standard.6

5.2 Minimizing oxidation in Al or Mg-MIM As stated in the previous chapter (Chapter 4: Impurity Management in Reactive Metals Injection Molding), the MIM of Al or Mg is not efficient due to the poor sinterability of Mg and Al powders. However, by using an alloy composition or an additive that enhances sinterability in the case of these powders, a successful Al/Mg-MIM process can be realized.

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Feedstock Technology for Reactive Metal Injection Molding

Figure 5.5 DSC thermograms of different ratio PEG and PVAc blends. Table 5.3 Heat absorption and crystallization energy of different ratio PEG and PVAc blends. PEG: PVAc

Heat absorption (J/g)

Heat absorption/g PEG (J/g)

Crystallization energy (J/g)

crystallization energy/g PEG (J/g)

100:0 78:5 73:10 63:20

2181.22 2153.89 2141.18 2120.27

2181.22 2163.755 2160.52 2158.451

164.2 148.1 138.5 113.8

164.2 154.3 144.4 124.6

Jiaqi et al.7 fabricated hypereutectic Al Si (20 wt.%) alloy parts by MIM. The hypereutectic Al Si alloy composites are attracting much attention for heat dissipation and electronic packaging applications due to their low density, low thermal expansion coefficient, high wear resistance, and excellent thermal conductivity. As modern electronic packaging is continuously moving toward smaller sizes, higher integration, and more complex geometries, conventional ingot metallurgy cannot meet such requirements. MIM provides an excellent opportunity to counter this. Jiaqi et al.7 developed an injection molding process for hypereutectic

Potential feedstock compositions for metal injection molding of reactive metals

197

Al Si alloy using a binder system that consisted of 35 wt.% high-density polyethylene (HDPE), 62 wt.% carnauba wax (CW), and 3 wt.% stearic acid (SA). The feedstock was prepared with a very high powder loading of 83 wt.%. The binder was eliminated in a two-step process: solvent debinding to extract CW and SA, and thermal debinding to remove HDPE. The thermal debinding and sintering both were carried out under a nitrogen atmosphere. Fig. 5.6 presents the DTA curve of the studied Al Si (20 wt.%) alloy powder. An endothermic peak can be seen around 590°C, which corresponds to the eutectic melting of the alloy powder. The authors suggest that the sintering temperature should be between 520°C and 600°C. The effects of sintering temperature on different properties are shown in Fig. 5.7. The above results show that the relative sintered density increases rapidly from 55.6% to 90.7% when the sintering temperature increases slightly from 520°C to 540°C. Afterward, the densification slows down from 540°C to 600°C. The reason for this, as pointed out by authors, was the excessive increase of Si particles, which hindered the contact of grains. The increase in Si particle size actively influenced the mechanical properties of the sintered part. Similarly, sintering time plays an important role in the properties as mentioned earlier (Fig. 5.8). These authors believe that the poor mechanical performance associated with longer sintering times is related to the size of the prior Si phase, which increased from less than 5 μm to greater than 25 μm. Below 3 h of sintering, the reaction does not progress significantly. Beyond 3 h, the Si

Figure 5.6 DTA curve of Al 20 wt.% Si alloy powder. A heating rate of 10°C/min in nitrogen was applied.7

Figure 5.7 Effect of sintering temperature on (A) relative density, (B) hardness, and (C) tensile strength. The sintering hold time was 1 h.7

Figure 5.8 Effect of sintering time on (A) relative density, (B) hardness, and (C) tensile strength. The sintering temperature was 550°C. The ideal sintering time can be marked as 3 h.7

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Feedstock Technology for Reactive Metal Injection Molding

phase grows considerably around the grain boundaries; it will eventually lead to the formation of defects, and reduced density (Fig. 5.9). Although the mechanical properties, in this case, seem reasonable, the authors did not provide information on the thermal properties of the studied material. The intended applications of this system manufactured by MIM are yet to be seen. However, it provides a general overview that MIM can be successfully applied to manufacture the Al alloys components for electronic packaging. So far, a series of biopolymers,8 bioceramics,9 and biometals10 have been developed for bone repair application. Among them, Mg and its alloys, owing to their high specific strength with appropriate Young’s modulus, natural degradability, good osteopromotive property, and biocompatibility, have been regarded as revolutionary biometals. Mg and its alloys exhibit the similar Young’s modulus (about 45 GPa) to that of human bone, which is between 15 and 30 GPa.11 This can significantly reduce stress shielding during load-bearing at the bone implant interface. Its density is B1.79 g/cm3, which is very close to that of human bone (B1.75 g/cm3). Apart from the favorable mechanical properties, Mg and its alloys can completely degrade in vivo (the recommended daily allowance of Mg is 250 350 mg for a healthy adult).12 Hypothetically, magnesium-based implants can be produced by MIM. However, MgMIM is not as easy as it seems and there is still no standard MIM process for Mg and its alloys. Wolff et al.13 16 have made significant strides in improving the full processing chain of MIM of Mg from feedstock and specimen shaping, sintering, and furnace conditions. The result of their rigorous scheme is satisfactorily consolidated sinter parts with improved material conditions that are sufficient for biomedical applications. For instance, by adding calcium into Mg, an improved sintering density was achieved by the formation of a transient liquid phase. Further, it was found that Mg Ca master alloy powder with calcium content between 5 and 10 wt.% mixed with pure Mg powder in the appropriate ratio can produce the Mg Ca alloy having the required amount of Ca.17 Fig. 5.10 gives an illustration of the material processing chain from Mg base materials MIM as developed by Wolff et al.13 The backbone polymer used in this study was poly(propylene-co-1butene) (PPco1PB) based on earlier research.18 The backbone polymer content was adjusted between 15 and 35 wt.% in accordance with rheology and green strength. The optimal minimum powder loading was

Figure 5.9 Optical microstructures of parts sintered at 550°C for different times: (A) 1 h; (B) 3 h; (C) 5 h; (D) 8 h. The localization of the Si phase around grain boundaries can be clearly seen.7

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Feedstock Technology for Reactive Metal Injection Molding

Figure 5.10 Material processing chain for Mg-MIM, starting from Mg base material (1) and Mg 5Ca MAP (2) to Mg 0.9Ca feedstock (6) and lastly, biodegradable Mg 0.9Ca implant prototypes. The green part and debound part are present for comparison purposes only.13

found to be 64 vol.%. The wax was then removed by solvent debinding as usual. The thermal debinding and sintering were carried out using the profile, as shown in Fig. 5.11. The addition of master alloy powder (MAP) improves the sinterability of the Mg alloys via liquid phase sintering.19 This route enables the fractional melting of the MAP below the sintering temperature. The Ca-rich liquid phase helps destabilize the Mg oxide layer and initiates the sintering process. Fig. 5.12 schematically illustrates how the liquid phase sintering in the powder compound occurs. Sintering takes place at a temperature slightly above the solidus temperature of the alloy composition. Hence, a permanent liquid Ca-rich phase occurs during sintering and ensures a better consolidation of the compound. To further improve sintering performance, the authors employed a labyrinth-like crucible set-up (see Section 4.3.3). The set-up was filled with pure irregular magnesium powder as an oxygen getter, which was used to protect thermally debound brown parts from picking up

Figure 5.11 Thermal debinding and sintering profiles for Mg-MIM parts as detailed by Wolff et al.13

Figure 5.12 The MAP route for better sinterability of Mg. (A) Details of the Mg Ca phase diagram. The arrow shows the diffusion path of MAP into Mg; (B) and (C) demonstrate how molten MAP coats Mg particles and infiltrates the Mg matrix.13

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205

Table 5.4 Mechanical properties of sintered MIM Mg 0.9Ca and in comparison with cast Mg 0.9Ca, polylactide acid (PLA), and cortical bone tissue.13 Property

MIM Mg 0.9Ca

Cast Mg 0.9Ca

PLA

Cortical bone

UTS (MPa) YS (MPa) Elongation, ε (%) Young’s modulus (GPa) Density (g/cm3) Average grain size (μm)

131 6 2.9 64 6 1.8 6.8 6 1.8 19.4 46 1.6 1.7 23.3 6 1.3

98.8 6 7.8 81.3 6 2.3 4.1 38 46 1.73 350.4 6 129.3

49 78

165 6 24 140 6 20 2.1 6 0.3 22.2 6 8.3 1.8 2.1

3 5 3.5 1.2

additional oxygen during sintering. The getter bed generates a highly reducing gaseous Mg vapor atmosphere due to inherently high Mg vapor pressure. This helps keep the oxygen level to the minimum. Using the mentioned thermal debinding and sintering profiles and the sintering set-up, the authors showed that a relative density between 94% and 98% is achievable. The obtained results during this study are presented in Table 5.4. Table 5.4 reveals that the material properties of the sintered MIM Mg 0.9Ca are significantly higher than those of the as-cast Mg 0.9Ca and PLA despite the residual porosity between 2% and 6%. These surprisingly higher material properties in MIM Mg 0.9Ca is associated with the grain size. The as-cast alloy reveals an average grain size 14 times higher than its counterpart in the as-sintered condition. This increased grain size to decreased strength in the as-cast material. Moreover, by carefully monitoring binder and sintering parameters, porosity can be controlled. Although the overall properties of the MIM Mg alloy may still not be optimal in this study, it provides hope for the future development in MIM of Mg for biomedical applications.

5.2.1 Attempts to address Al-MIM The sintering process for Al-MIM can be achieved in two main ways. The first possibility is the inclusion of Mg with high chemical activity. It can be an elemental Mg or AlMg master alloy. Due to its inherently high vapor pressure as well as the high free energy of its oxide, Mg can partially reduce the protective surface Al2O3 and form spinel (MgAl2O4). The MgAl2O4 oxide is much more benign than Al2O3 to atomic diffusion, which leads to the formation of sintering contacts between the Al particles. The second possibility is sintering under a nitrogen atmosphere.

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In combination with Mg, nitrogen leads to the formation of nitrides, causing cracks in the oxide layer subsequently. The nitridation is an exothermic reaction, which increases the temperature locally and leads to the formation of a sinter-promoting liquid phase. Without any of these two effects, it will be impossible to sinter Al-MIM parts. There is every possibility that the thermal debinding process to remove the backbone polymer may overlap with the sintering process in the case of Al powder. The overlapping of the temperature range for binder removal and sintering means that residues of the backbone polymer are incorporated into the sintered workpiece. However, researchers at the Technical University Vienna (TU Wien)20,21 have succeeded in developing a MIM process for aluminum that can be used to manufacture complex-shaped, weight-saving components with high utilization of material. The trick to their successful Al-MIM process lies in the controlled thermal debinding process. As generally agreed upon, a low oxygen environment is used to prevent the oxidation of Al powder. However, the TU Wien researchers found that an oxygen-rich atmosphere is more beneficial for Al powders. The formation of oxide inhibits the diffusion of any other secondary impurity. At the same time, oxygen aids the combustion of the backbone polymer. The subsequent sintering is carried out under a nitrogen atmosphere (see Fig. 5.13). There is a clear correlation between the residual carbon content and the densification of Al-MIM parts (Fig. 5.14). In this study, the researchers used BASF modified Catamold binder. As described in Chapter 2, Design Strategy of Binder Systems and Feedstock Chemistry, and Chapter 3, Binder System Interactions and Their Effects, Catamold is designed to be catalytically debound in flowing nitrogen using evaporative nitric acid HNO3, which is a well-established procedure for feedstocks based on polyacetals. However, the researchers found that for Al-MIM, nitric acid is not a suitable choice, as it resulted in blistering of the surface (Fig. 5.15) and layered cracking of the bodies. On the other hand, oxalic acid ((COOH)2) as a catalyst resulted in excellent defect-free brown parts. Consequently, they have achieved reasonably good mechanical properties, as shown in Table 5.5. The mechanical strength of the Al-MIM parts lies in the expected range for equivalent wrought alloys of the 5xxx-type, for example, AA 5052. However, the elongation values are well below those of the comparable alloys. This is well expected for the sintered Al-parts considering

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207

Figure 5.13 Effects of different debinding atmospheres on the interstitial contents of Al 3%Mg MIM samples. Two different powder loadings were used for complete analysis. Each sample was sintered under N2 atmosphere. The oxygen atmosphere results in the lowest amounts of impurities in the final parts.21 Courtesy Herbert Danninger, TU Wien.

the residual surface oxides and porosity. Nevertheless, this example shows that although Al-MIM is a tricky technique, it is manageable for the production of net-shape complex parts (Fig. 5.16) if proper procedures are applied.

5.3 Commercial feedstocks and their properties Although most of the feedstocks based on reactive powders are custom made and hence proprietary protected. Nevertheless, a few commercial feedstocks are available for general purpose applications (Table 5.6). Catamold is a ready-to-use feedstock based on the polyacetal-based plastic binder in conventional injection molding machines designed and distributed by BASF. Its revolutionary principles of catalytic debinding have transformed the MIM industry since its invention. The polyacetal—the main component of the Catamold feedstock—is rapidly hydrolyzed into its monomers by acid catalysis. Depending on particle sizes of the metal powder, debinding rates of 1—4 mm/h can be

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Figure 5.14 Correlation of the interstitial element content and sintering density of Al 3%Mg-samples. The poorly sintered specimens show inconsistent C, O, and N-contents while the specimens sintered to a high density contain low levels of C as well as relatively low contents of N and O; signifying the importance of the interstitial elements for the successful sintering of Al-MIM.21 Courtesy Herbert Danninger, TU Wien.

Figure 5.15 (Top) Al 3% Mg bar catalytically debound with HNO3, blistering is easily seen. (Bottom) Injection-molded part catalytically debound with (COOH)2, no defects can be seen. The downside of using (COOH)2 was the insufficient green strength of the brown part.21 Courtesy Herbert Danninger, TU Wien.

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209

Table 5.5 Mechanical and chemical properties of MIM samples Al 3%Mg.21 Density (g/cm3)

Hardness (Hv5)

Tensile strength (MPa)

Elongation (%)

Oxygen (wt.%)

Carbon (wt.%)

Nitrogen (wt.%)

2.57

60

180

4.5

0.27

0.07

0.12

achieved. The most suitable catalyst is nitric acid.27 One downside to this process is the need for specially designed debinding furnaces (Fig. 5.17). Polyacetal depolymerization is achieved at approximately 110°C—well below the softening temperature of the polymer. In other words, during debinding, the polymer transits directly from the solid to the gaseous form, hence, bypassing the softening phase normally associated with solvent extraction or pyrolysis of the main component. Catamold Ti is a ready-to-mold feedstock for the production of sintered Ti components. According to Catamold Ti brochure, the sintered components should exhibit the following properties.22 Density (g/cm3)

YS0.2 (MPa)

UTS (MPa)

ε (%)

Hardness (HV1)

Carbon (wt.%)

Nitrogen (wt.%)

Oxygen (wt.%)

$ 4.2

$ 480

$ 550

$ 5%

160 240

# 0.2

# 0.1

# 0.4

Catamold Ti is perhaps the most commonly used commercial Ti feedstock readily available on the market. It has been used for research purposes and is being trialed for production in some countries including China. For instance, Demangel et al.28 used Catamold to make Ti-MIM parts and studied their cytocompatibility with various anodic oxidation posttreatments. It is well known that surface roughness has a direct impact on cellular response.29 Since the MIM is considered a near net-shape technique, special attention should be paid to the final product surface properties. The authors used bone explant cultures to compare cytocompatibility between the Ti components made by conventional machining and MIM. This was carried out by comparing cell behavior, that is, cell migration, colonization, and adhesion. MIM samples were made using Catamold Ti feedstock following the procedure as prescribed by BASF.30 Sintering was carried out at 1300°C under argon partial pressure (65 mbar) for 200 min. During sintering, the samples were placed on zirconia plates. The initial analysis of the feedstock revealed that the particle

Figure 5.16 Al-MIM demonstration parts. By precise temperature and especially atmosphere during the whole sintering process control dense, metallic Al-MIM parts can be produced. Courtesy Christian Gierl-Mayer, TU Wien.

Table 5.6 Commercial feedstocks based on reactive powders and their properties. Feedstock

Supplier

Primary debinding method

Achievable UTS (MPa)

Achievable ε (%)

Catamold Ti22

BASF

$ 550

$5

PolyMIM Ti Grade 223

POLYMIM

PolyMIM Ti6Al4V23

POLYMIM

$ 850

$6

Metamold Ti24 Metamold Ti alloy24 Ti6Al4V25 Ti6Al4V Grade 526 2024 aluminum26 6061 aluminum26 7075 aluminum26

NPR Co. Ltd. NPR Co. Ltd. RYER, INC. RYER, INC. RYER, INC. RYER, INC. RYER, INC.

Catalytic debinding according to the BASF system Water debinding (also available as catalytically debound feedstock) Water debinding (also available as catalytically debound feedstock) Thermal debind Thermal debind Water debind Solvent debind Solvent debind Solvent debind Solvent debind

555 780

15 5

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Feedstock Technology for Reactive Metal Injection Molding

Figure 5.17 Schematics of a debinding furnace for Catamold.27

diameter ranged from 2 to 35 μm while the initial carbon and oxygen contents were 0.01% and 0.12%, respectively. The achieved sintering density was 4.32 g/cm3. This density corresponded to porosity of 4% with localized pores on the surface of the material and denser core. The carbon content after sintering was in the range of 0.09 0.11 wt.% while oxygen content was 0.23 wt.%. The carbon content was significant after sintering. Although the Catamold Ti feedstocks provide a fast processing route, as compared to other commercial/noncommercial feedstocks, they may lead to high interstitial contents. Commercial Ti feedstocks other than Catamold are few and far between.30 Only a handful of companies provide feedstocks that are based on solvent debinding despite the fact Catamold requires specialty furnaces. PolyMIM GmbH is one of the companies that supply commercial PEG-based titanium feedstocks. According to the PolyMIM Ti6Al4V datasheet,23 the sintered component should possess the following properties:

Potential feedstock compositions for metal injection molding of reactive metals

213

Density (g/ cm3)

YS0.2 (MPa)

UTS (MPa)

ε (%)

Carbon (wt.%)

Nitrogen (wt.%)

Oxygen (wt.%)

$ 4.2

$ 750

$ 850

$6

# 0.08

# 0.05

# 0.25

The injection-molded samples made from PolyMIM feedstocks must undergo solvent debinding afterward to remove the primary component—PEG. The solvent debinding temperature should be between 40°C and 60°C while the solvent medium is water/distilled water. The total debinding time depends on the part thickness but may take more than 12 h depending on the temperature. The morphology of commercially available PolyMIM feedstock is shown in Fig. 5.18. It is clear from the micrographs, PolyMIM is a mono-sized spherical powder feedstock with particle size less than 47 μm. Small particles generally have good packing density, but the viscosity is also higher than large particles due to inherently large surface areas. However, the high packing density is required for excellent rheological properties. When pressure is applied to a high packing density feedstock, all the particles contribute to the flow. On the other hand, binder-particle separation may occur in coarse particle feedstocks. The biggest advantage of PolyMIM is its rheological properties. In our opinion, PolyMIM has the best rheological properties for any given Ti feedstock. The rheological data of PolyMIM and our in-house built feedstock based on binder formulation (PEG-PMMA-PPC, hereafter

Figure 5.18 Morphology of PolyMIM.31

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designated as Feedstock D) was compared, and viscosity dependence on the shear rate for the two feedstocks is presented in Fig. 5.19. It is clear from Fig. 5.19 that the commercial feedstock possesses the best rheological properties at the molding temperature of 160°C. In comparison, Feedstock D is more shear sensitive; its viscosity varies rapidly over the tested shear rate range (as verified by activation energy calculations, see Table 5.7). This can create defects during the injection molding process. MIM feedstocks viscosity increases as temperature decreases. The temperature dependence of viscosity can be explained by the Arrhenius equation:

Figure 5.19 Viscosity dependence of shear rate for commercial feedstock and Feedstock D.32

Table 5.7 Comparison between activation energies of in-house made feedstock and commercial feedstock. Shear rates (S21)

800 1200

Activation energy (E) KJ/mol PolyMIM

Feedstock D

34 19

90 99

Potential feedstock compositions for metal injection molding of reactive metals



E η 5 η0 exp RT

215



where η0 is the viscosity at a reference temperature, R is the gas constant, and E is the activation energy for viscous flow. A high value of E indicates a high sensitivity of viscosity to temperature change. The temperature dependence of viscosity should be as small as possible to avoid sharp changes in viscosity as temperature may vary during injection molding. High sensitivity to temperature also causes stress concentrations, cracking, and distortion in molded parts. As expected, the reference feedstock (PolyMIM) has the lowest activation energy value at both shear rates (Table 5.6). Its viscosity varies gradually with temperature. Thus it retains better shape during molding with less residual stresses. Mohammed et al.31 used PolyMIM Grade 2 Ti feedstock as a benchmark to compare their own feedstock made of coarse powder. Their feedstock was composed of PEG/PMMA binder system. PEG molecular weight was 1500 g/mol, and it constituted 70 vol.% of the total binder. The other binder components included 25 vol.% PMMA and 5 vol.% stearic acid as a lubricant. A relatively coarse spherical Ti powder Grade 2 with a mean particle size of roughly 75 μm was used and the respective solids loading was 58 vol.%. The results of the DSC analyses for both the commercial polyMIM and developed feedstocks are presented in Fig. 5.20. The results reveal that the PolyMIM feedstock has a higher peak melting temperature of approximately 64°C compared with that of PEG 1500 (approximately 45°C) used in this study. Nevertheless, this temperature corresponds to high-molecular-weight PEG. The second part of the binder for PolyMIM has a lower peak melting temperature of about 126°C. One might suspect that this could be high-density polyethylene, as the peak melting temperature lies in the peak melting temperature range for high-density polyethylene, which is about (125.8°C 138° C).33 In addition, a small third peak is also noticeable in PolyMIM, which can be attributed to the peak melting temperature of the isotactic polypropylene (ranging from 160°C to 166°C).34 Needless to say, there could be other polymeric components present in the PolyMIM feedstock as well. The sintering was carried out at 1320°C for 2 h under the flow of argon. The contents of impurities after sintering are summarized in Table 5.8.31

Figure 5.20 DSC thermographs for the (A) PolyMIM feedstock and (B) as-studied PEG/PMMA feedstock.31

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Table 5.8 Comparison of impurity contents between two feedstocks. Interstitial elements

PolyMIM grade 2 (wt.%)

Sintered samples of studied feedstock (wt.%)

Carbon Oxygen Nitrogen

0.044 0.273 0.03

0.079 0.277 0.023

It is clear from Table 5.8 that the contents of oxygen and nitrogen are almost the same in these two feedstocks. However, the carbon content for parts made of PolyMIM is much lower than the in-house made one— even lower than those of Catamold feedstock. This confirms the nature of clean and thorough debinding of PolyMIM feedstock that leaves little-tono carbonaceous residue behind. Comparing the studies on PolyMIM Ti feedstock and Catamold Ti feedstock, it is clear that PolyMIM will result in better properties since it leads to lower interstitial contents. However, Catamold Ti feedstock offers a faster processing route. The selection of a commercial feedstock hence depends on the final product quality and production requirements. Considering even only limited options available for commercial Ti feedstocks, it is not a surprise that no such liberty is available for Al-MIM and in particular Mg-MIM. There has been not much interest in the industry to develop Al-MIM processes as MIM is hardly in a position to compete with conventional manufacturing technologies such as die casting. There is no commercial feedstocks available for Mg-MIM either, as it is still in the early stages of development. For Al-MIM, Ryer Inc.35 provides two different Al alloys (6061 and 7075)-MIM feedstocks. The feedstocks are debound following the solvent debinding method. However, no reports are available in the literature suggesting the use of these commercial feedstocks. In the early 2000s, Tan and Ma made significant progress in commercializing Al-MIM process. They claimed that their process, patented and trademarked as aluMIM36 by Advanced Materials Technologies Pte Ltd., offers many advantages Al. They reported the following properties of Al and Al-Cu alloy36:

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Property of Al and Al Cu alloy

aluMIM process

Density (g/cm3) UTS (MPa) ε (%) Thermal conductivity (W/mK) RFI/EMI shielding (dB) Dimensional tolerance

2.5 2.6 100 150 20 30 180 90 6 0.5%

A comparison of parts using the aluMIM and die-casting process also showed a distinct advantage of aluMIM in the mechanical properties (Fig. 5.21). Another important characteristic is the improvement in the thermal conductivity of the aluMIM parts compared with die-casted parts. The improvement was found to be 30% 80% higher than die-castings. The thermal performance of the round fin heat-sink produced by aluMIM process is compared with the machined square pin (6061) in Fig. 5.22. Thermal conductivity (W/mK) ASTM E146137 Heat dissipation capacity ΔT (°C)

aluMIM

Commercial Al6061

180 30

120 150 16

The authors also showed a couple of demonstration parts (Fig. 5.23), which can be made with ease using aluMIM technology. However, to the best of our knowledge, no commercial products are being made using this technology.

Figure 5.21 Comparison between stress strain curves of aluMIM and die-casting parts. aluMIM offers better mechanical properties on top of dimensional tolerances similar to those in metal injection molding.36 Picture courtesy Lye King Tan, Advanced Materials Technologies Pte Ltd.

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Figure 5.22 Comparison in thermal performance of the two heat-sink parts. At any given wind speed, the thermal resistance of aluMIM part is lower than the machined part indicating its higher efficiency.38 Courtesy Lye King Tan, Advanced Materials Technologies Pte Ltd.

5.4 Ti-MIM success stories 5.4.1 Medical implants by Ti-MIM There have been considerable efforts toward the commercialization of medically implantable MIM components focusing on titanium and cobalt chromium materials since the mid-2000s. MIM can be leveraged to take advantage of the low-volume/high-variety (given the customized nature of most orthopedic implants) mix on these complex, tighttolerance geometry parts. Most of the large orthopedic implants require functional surfaces as ingrowth mediums. Fig. 5.24 presents one such example. Titanium materials are preferred due to their superior biocompatibility over cobalt chromium materials for long-term implants. To penetrate the orthopedic market, a Ti-MIM process needs to robust enough to demonstrate that the product can consistently meet the chemical and mechanical performance requirements of the design specifications. Because of the high reactivity of titanium powder, few MIM companies have made successful entries into the Ti-MIM implant market. Praxis Powder Technology39 is one such company with FDA registration and ISO-13485 certification. Praxis specializes in porous titanium and Ti-MIM and has developed the only known qualified Ti-MIM process in the world. They have perfected their proprietary process to provide highperformance titanium parts for a variety of demanding applications while

Figure 5.23 (Left) An aluMIM demonstration part of the wall thickness of 1 mm. (Right) A stainless steel and aluMIM demonstration parts.36 Picture courtesy Lye King Tan, Advanced Materials Technologies Pte Ltd.

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Figure 5.24 Orthopedic MIM Ti 6Al 4V implant with porous ingrowth features. Courtesy Praxis Technology, New York, United States. Table 5.9 Summarized results of some of Praxis Ti 6Al 4V-MIM process capability studies.40 Property

ASTM F2885 limit

Average

Std. Dev

Cpk or Ppk

UTS (MPa) Yield strength (MPa) Elongation (%) Oxygen (ppm) Carbon (ppm)

900 830 10 2000 800

964 860 19.8 1720 361

3.77 6.68 1.19 6.5 5.6

5.70 1.47 2.74 1.42 2.61

satisfying stringent regulatory requirements. Table 5.8 presents a summary of capability data for Praxis’ Ti 6Al 4V-MIM process. In this table, Cpk represents the process capability index while Ppk stands for a process performance index. Many industries use a benchmark value of 1.33. Table 5.9 shows that the Praxis process is capable and meets the requirement. Moreover, data for mechanical properties were collected at the upper and lower boundaries of the sintering window (the lesser of these values is presented in Table 5.9). In addition, data for the interstitial

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Feedstock Technology for Reactive Metal Injection Molding

Figure 5.25 Schematic of a typical porous ingrowth segment. Courtesy Praxis Technology, New York, United States.

content was collected across three sequential furnace runs. The capability objective was met for all of the evaluated properties. As mentioned before, most orthopedic implants are manufactured with some type of integration surface. This surface may be a simple rough surface or a more complex porous ingrowth surface. Ingrowth surfaces are becoming a standard practice for many orthopedic devices. A typical porous ingrowth segment is shown in Fig. 5.25 and may include one or more of the following regions: fixation texture, ingrowth medium and substrate interface. A fixation texture region provides for the initial fixation of the device during surgery. The ingrowth region refers to the portion of the implant that is intended to promote the growth of tissue into the device for long-term fixation. The last portion is a substrate interface region that is the transitional area between the ingrowth medium and the dense portion of the implant. Commonly, the porous ingrowth segments are added to an orthopedic implant in a separate step that is independent of the main dense portion fabrication. However, from a cost perspective, it is always desirable to reduce the number of manufacturing steps and MIM is playing a vital role in achieving this. There are several approaches to form a porous segment on a MIM article. These include the following: 1. Coforming with porous feedstock In this approach, two different feedstocks are molded together. One feedstock contains a space holder that can be successfully removed after injection molding. The porous feedstock is used to form the porous section, and feedstock without space holder is used to create the solid portion of the article. In a typical approach, the conventional feedstock is the first injection molded and then used as an insert for the subsequent porous feedstock injection molding.

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However, there are several challenges associated with this technique. The most problematic aspect of this approach is the conflicting need to create substantially interconnected porosity via contact between the space holder particles and to provide a highly loaded MIM feedstock with a usable viscosity. In addition, it generally results in a very irregular distribution of porosity with reduced porosity at the surface. 2. Coforming with a compacted preform In this approach, first, an insert comprising a metal powder and a space holder is fabricated using a non-MIM method. This insert is then placed in the mold and the MIM feedstock is molded around it, creating a component with both porous and solid sections. This method has been successfully tested and trialed, using porous sections formed by cold isostatically pressing a space holder and titanium powder. After pressing, the compact is then machined into the desired shape as required, inserted in the mold cavity, and the MIM feedstock is injected into and around the insert. This process overcomes several of the challenges of porous feedstock injection molding. As the blend of a space holder and metal powder is compacted, the space holder may be slightly deformed, ensuring interconnectivity. The issue of irregular porosity distribution can be overcome by incorporating some homogenizing aids into the blend of metal powder and space holders. In addition, due to the absence of the binder system, strong porous structures are formed. The problem of reduced surface porosity is, however, still present. Moreover, the need to machine a precision insert adds cost to the process. Finally, engineering the shrinkages between the two independently formed powder articles is very challenging, particularly, for large parts. 3. Coforming with a sacrificial insert To overcome the issues of the two methods mentioned earlier, Praxis has successfully developed a technology called 3DT, which utilizes a sacrificial insert.41 This enables coforming of complex porous ingrowth segments during the injection molding process. The insert typically comprises a negative of the desired porous ingrowth segment and a section to secure it in the mold. The feedstock is injected directly against the insert to form different aspects of the porous segment. The insert is then subsequently removed during the first stage of debinding, and the product is processed similarly to a conventional metal injection molding process. The use of sacrificial inserts with MIM provides control over three critical aspects of the ingrowth segment on the implanted devices.

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These are (1) control of the fixation texture, (2) control of the ingrowth region, and (3) control of the interface between the ingrowth region and the solid substrate. This is a significant advantage over using a porous preform or coforming with porous feedstock processing routes. In addition

Figure 5.26 3D porous ingrowth surface manufactured using 3DT technology. Courtesy Praxis Technology, New York, United States.

Figure 5.27 Close-up of highly interconnected ingrowth medium and fixation surface. As can be seen, the pores are highly interconnected and the struts are wellformed. The ingrowth region is 70% porous and the major pores have an average diameter of 500 μm. Tensile testing performed on these surfaces yielded an average UTS of 64 MPa. Picture courtesy Praxis Technology, New York, United States.

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Figure 5.28 Praxis TiRx leads to better fatigue performance—a critical parameter for orthopedic implants. Courtesy Praxis Technology, New York, United States.

to unparalleled control over the nature of the various regions of the porous segment, this approach also enables the addition of these surfaces to geometries that are challenging to manufacture via conventional approaches. Due to the complex nature of these inserts, additive manufacturing is used to manufacture them. Thus this technology combines the benefits of both additive manufacturing and MIM processing routes to overcome the shortcomings of individual processes. The versatility of additive manufacturing allows unprecedented control of the molded surface and can provide for many different types of surfaces useful for orthopedic applications. The primary surfaces that have been successfully manufactured using this approach are ongrowth, ingrowth, and polymer anchoring. Ingrowth surfaces: ingrowth surfaces demonstrate the fullest potential of 3DT technology. Fixation texture, ingrowth medium, and the substrate interface are all defined precisely, repeatable and independently by different sections of the sacrificial insert. Fig. 5.26 is a photo of the crosssection of the porous ingrowth segment manufactured using a sacrificial insert. Fig. 5.27 presents a close-up of a porous ingrowth surface made by this technology.

Figure 5.29 The arm of augmented reality smart glasses. Courtesy Dr. Peng Yu ElementPlus, Shenzhen, China.

Figure 5.30 (A) Rheology of one of the CP-Ti feedstocks designed by ElementPlus. It can be seen that the feedstock is pseudoplastic in nature and has viscosity well below 1000 Pa s in the shear rate range of 102 105 s21. (B) The cross-sectional SEM micrograph of the developed feedstock. A homogeneous distribution of binder-coated Ti particles can be seen.42

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Feedstock Technology for Reactive Metal Injection Molding

Table 5.10 Impurity content, density, and mechanical properties of the sintered tensile specimen.43 Specimen

O (wt. %)

C (wt. %)

Density (%)

UTS (MPa)

Elongation (%)

GA-Ti, 1200°C vacuum GA-Ti, 1200°C Ar Ti 6Al 4V, 1200°C Ar

0.215

0.126

94.0

507

14.5

0.229 0.224

0.097 0.089

95.9 95.3

545 710

15.4 7.8

The 3DT technology has been demonstrated to enable the conformation of porous ingrowth segments on large orthopedic devices. One such example is titanium tibial tray with ingrowth surface in Fig. 5.24. Another major challenge for using Ti-MIM to manufacture orthopedic devices is that conventional Ti-MIM components do not have adequate fatigue strength for most load-bearing applications. When measured by rotating-beam fatigue testing, typical fatigue strengths are around 480 MPa at 10 million cycles. The commonly accepted minimum for load-bearing applications is, however, about 620 MPa at 10 million cycles. Efforts are being made to improve the fatigue properties. Recently, Praxis Powder Technology has developed a processing route—branded “TiRx”—to improve the final microstructure of the sintered titanium. This process results in fatigue strength in excess of 620 MPa while still meeting the chemical and mechanical requirements specified in the ASTM F2885 standard. Fig. 5.28 compares the fatigue performance of Ti made of conventional Ti-MIM and TiRx material.

5.4.2 Smart-glasses titanium arm by MIM Fig. 5.29 presents an example of augmented reality smart glasses fabricated for a Chinese company of wearable devices. The part is 170 mm long and has a thickness of B500 μm, featuring a complex curved surface. Conventional machining, in this case, is not productive due to the risk of deformities during machining of the thin-wall structure. Even MIM of such parts would require careful control of processing parameters, as defects such as black lines can be easily developed during injection molding. Also, such parts are prone to warpage during sintering. ElementPlus successfully solved this issue by employing a

Figure 5.31 Microstructure of sintered CP-Ti specimen. Pores can be seen. However, the pores can be eliminated later through hot isostatic pressing. The HIPed samples were pore-free and resulted in improvement in elongation from 15% to 30% while UTS was increased to 635 MPa.43

Figure 5.32 SEM micrographs revealing delicate structures of the MIMed Ti glasses arm.43

Potential feedstock compositions for metal injection molding of reactive metals

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POM-based custom-made feedstock that has high powder loading and good viscosity (Fig. 5.30). Before the real parts were molded, the tensile bar was first fabricated to test the material properties. DSC and TGA were used to optimize the sintering parameters. The Ti tensile samples were thermally debound and sintered at 1200°C under vacuum or flowing Ar. The sintered samples exhibited low impurity contents and good mechanical properties. The sintered density varied between 94% and 96% of the theoretical density (Table 5.10). The sintered part also showed a typical microstructure of CP-Ti (Fig. 5.31). The resulting sintered smart-glass arms achieved a tolerance within 6 0.5% consistently with good corrosion resistance. Even the delicate structures at the end of the parts could be precisely duplicated (Fig. 5.32). A Germany-based company Element 22 has optimized the MIM process for the production of Ti 6Al 4V parts resulting in outstanding properties. Their selective bead-sintered (SBS) MIM Ti 6Al 4V possesses outstanding strength and ductility, high fatigue and corrosion resistance, excellent biocompatibility and weldability with potential applications in structural components, medical devices (FDA approved, 2011), and aerospace parts (Fig. 5.33). Fig. 5.34 presents a typical microstructure of their SBS Ti 6Al 4V MIM.

Figure 5.33 Element 22 SBS Ti-MIM process44 results in excellent mechanical properties without the need for secondary hot isotactic pressing (HIP) process.45 Courtesy Matthias Scharvogel, Element 22, Kiel, Germany.

Figure 5.34 Resulting microstruture of Ti 6Al 4V processed via different techniques: (A) conventionally sintered, (B) conventionally sintered and isostatic pressed, (C) selective bead sintered. Compared to conventionally sintered, SBS Ti 6Al 4V has a fine microstructure with isotropic material behavior. Globular alpha grains are surrounded by a small fraction of β phase.45 Courtesy Matthias Scharvogel, Element 22, Kiel, Germany.

Potential feedstock compositions for metal injection molding of reactive metals

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5.5 Summary This chapter presents some potential feedstock compositions that can be employed for reactive metals MIM and also highlights the possible mechanisms behind their success. The chapter starts with discussing the importance of polymers that thermally degrade by depolymerization. An example of one such system—polyethylene glycol/polymethyl methacrylate binder system—is presented for Ti-MIM. Problems related to this binder system are also highlighted and the latest research that has been carried out to rectify the problems is also presented. In the subsequent section, procedures for a successful MIM process in case of reactive metals that have a very stable oxide layer such as Al or Mg are presented. It is postulated that to achieve full density in the case of these metals MIM, it is imperative to use sintering aids in the form of alloying elements. The success of the MIM process in the case of such powders is highly dependent on the starting powder composition, sintering aid in the form of alloying element, binder system, and sintering atmosphere—sintering aid and atmosphere being the key factors. In the second half, few available commercial feedstocks and their properties are presented. Among the reactive metal commercial feedstocks, Catamold Ti (a ready-to-use feedstock based on polyacetal-based plastic binder) is the most common and readily available. Despite the fact, Catamold requires specialty furnaces, commercial Ti feedstocks other than Catamold are few and far between. To the best of the author’s knowledge, for Al-MIM, only one company Ryer Inc. is providing a commercial feedstock while for Mg-MIM no commercial feedstock is available to date. In the last section, success stories of Ti-MIM are discussed with a couple of industrial case studies. Importantly, implantable grade Ti-MIM has moved from an academic undertaking to a production capable process. Recently, the Ti-MIM technology has been augmented to provide fatigue strength suitable for load-bearing implants as well.

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20. Kuzmanovic, J.; Gierl-Mayer, C.; Danninger, H.; Nenning, A.; Avakemian, A. In Interstitial Effects During Sintering of Injection Moulded Al Base Alloys, Euro PM2015 Advanced Materials and Applications 2, Reims, France, EPMA: Reims, France, 2015. 21. Gierl, C.; Danninger, H.; Avakemian, A.; Synek, J.; Sattler, J.; Zlatkov, B.; Maat, J. t; Arzl, A.; Neubing, H.-C. Carbon Removal as a Crucial Parameter in the Powder Injection Moulding of Aluminium Alloys; Powder Injection Moulding International, 2012. 22. BASF, Catamolds Ti. Aktiengesellschaft, B., Ed. Ludwigshafen, Germany, 2006. 23. https://www.polymim.com/. 24. https://www.npr.co.jp/english/products/products03.html#point03. 25. http://www.ryerinc.com/AquaMIM.html. 26. http://www.ryerinc.com/SolvMIM.html. 27. Bloemacher, M.; Weinand, D. CatamoldTM-A New Direction for Powder Injection Molding. J. Mater. Process. Technol. 1997, 63 (1), 918 922. 28. Demangel, C.; Auzène, D.; Vayssade, M.; Duval, J.-L.; Vigneron, P.; Nagel, M.-D.; Puippe, J.-C. Cytocompatibility of Titanium metal Injection Molding With Various Anodic Oxidation Post-Treatments. Mater. Sci. Eng. C 2012, 32 (7), 1919 1925. 29. Le Guéhennec, L.; Soueidan, A.; Layrolle, P.; Amouriq, Y. Surface Treatments of Titanium Dental Implants for Rapid Osseointegration. Dental Mater. 2007, 23 (7), 844 854. 30. http://www.ryerinc.com/AquaMIM.html. 31. Menhal Shbeh, M.; Yerokhin, A.; Goodall, R. Microporous Titanium Through Metal Injection Moulding of Coarse Powder and Surface Modification by Plasma Oxidation. Appl. Sci. 2017, 7 (1), 105. 32. Zhang, H. Z.; Hayat, M. D.; Qu, X. H.; Jadhav, P. P.; Wang, X. G.; Cao, P. Study of a Binder System for Ti-MIM: A Potential Low Temperature Backbone Polymer. Key Eng. Mater. 2018, 770, 206 213. 33. Peacock, A. J. The Chemistry of Polyethylene. J. Macromol. Sci. Part C: Polym. Rev. 2001, 41 (4), 285 323. 34. Maier, C.; Calafut, T. Polypropylene: The Definitive User’s Guide and Databook. William Andrew, 1998. 35. http://www.ryerinc.com/. 36. Yeo, C. T.; Tan, L. K.; Ma, J. Metal Injection Molding of Aluminum. Technologies, A. M., Ed., 2002. 37. Standard Test Method for Thermal Diffusivity by the Flash Method. 38. https://www.electronics-cooling.com/2004/11/metal-injection-molding-of-heatsinks/. 39. https://praxisti.com/. 40. Piemme, J. Metal Injection Molding for Implantable Device Applications. Int. J. Powder Metall. 2018, 54, 21 24. 41. Grohowski, J. A.; Piemme, J. C. In Titanium MIM: Technology for Orthopedic Devices, 2014. 42. Ye, S.; Mo, W.; Lv, Y.; Wang, Z.; Kwok, C. T.; Yu, P. The Technological Design of Geometrically Complex Ti 6Al 4V Parts by Metal Injection Molding. Appl. Sci. 2019, 9 (7), 1339. 43. Ye, S.; Mo, W.; Lv, Y.; Li, X.; Kwok, C. T.; Yu, P. Metal Injection Molding of Thin-Walled Titanium Glasses Arms: A Case Study. JOM 2018, 70 (5), 616 620. 44. Viehöfer, U.; Winkelmüller, W.; Lang, M.; Scharvogel, M. EP3231536B1, Method for Producing Components From Titanium or Titanium Alloys With Powder Metallurgy, 2016. 45. http://www.element22.de/.

CHAPTER 6

Outlook of reactive metal injection molding 6.1 Future trends Metal injection molding (MIM) is projected to be one of the most important manufacturing technologies for the future, second only to additive manufacturing. The industry has enjoyed close to double-digit average growth for the last two decades and is expected to continue its unabated growth in all geographical regions. In particular, after decades of research and development, Ti-MIM is finally gaining attention for applications where the use of titanium can be fully justified. These include biomedical implants, military and firearms, automotive, aerospace, chemical devices, and recently electronics. With the development of nontoxic, water-soluble binder systems, this potential has further increased. The future of Ti-MIM fabrication seems lucrative as major technological advancements are paving the way for its market growth. One of the prominent trends in this market is miniaturization. The growing consumer’s demand for lightweight, miniaturized, yet high-performance products are set to offer potential growth opportunities in the Ti-MIM fabrication market. In the case of Al or Mg-MIM, the progress is quite slow. For these two metals, the biggest hurdle is in achieving full densification of the components. Although Al-MIM has been studied since the early 2000s, no commercial products highlight the significance of Al-MIM. Nevertheless, there is a renowned interest in Al-MIM recently due to the current global push towards environmentally friendly, less energyintensive schemes. TU Wien, along with industry partners such as BASF SE, has set up a consortium “INDALMIM” to bring Al-MIM into the mainstream manufacturing industry.1 The mission of INDALMIM is “to develop the required process stages for the industrial application of aluminum metal injection molding and to refine alloying techniques to the point where implementation by the automotive industry appears realistic. This involved finding solutions to the sintering challenge as well as the Feedstock Technology for Reactive Metal Injection Molding DOI: https://doi.org/10.1016/B978-0-12-817501-9.00006-5

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recurring problem of surface staining.” According to the report released by INDALMIM, full consolidation of Al was achieved by sintering in a nitrogen atmosphere with the debound body containing low oxygen and carbon concentrations. A solution to the unattractive surface layers formed during the cooling phase was also found. In the process of sintering aluminum alloys, particular care must, therefore, be taken to control the temperature and the partially reactive atmospheres. It is claimed that using this knowledge, continuous production of components through industrial injection molding is possible. In addition, it has been claimed that the new technology developed by INDALMIM uses relatively coarse, offthe-shelf—and therefore cheap—powders to even allow the production of large components. It can immediately be integrated into existing production lines and saves material by up to 50%. For the energy-intensive metal aluminum, this means a significant reduction in energy consumption. Mg-MIM, on the other hand, is quite young yet full of potential. Recently, magnesium alloys have been highlighted as the potential biodegradable materials for future orthopedic applications due to their elastic moduli and strength matching those of bone tissues. In addition, the corrosion products of magnesium generated during biodegradation support osteoconductivity2,3 (Fig. 6.1). A porous Mg implant would support not

Figure 6.1 The dynamic absorption and excretion equilibrium of magnesium in the human body.3

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only osteoconductivity but also osseointegration (ingrowth of bone cells into the degrading implant) as well. With the MIM processing route, the generation of such parts having a porous skeleton on a dense platform is possible. However, magnesium suffers from the same problem as aluminum—a stable oxide layer that prevents densification. The fact that magnesium is used as an oxygen getter during aluminum and titanium sintering further highlights this problem. Recently, efforts have been made to develop streamline processing of magnesium and its alloys for medical implants applications. Helmholtz-Zentrum Geesthacht’s Magnesium Innovation Centre (MagIC) in Germany is one of such examples.4 The main focus of the research in MagIC is the development of magnesiumbased materials for diverse applications with special emphasis on alloy development and on the optimization of existing and new processing technologies. The Centre is currently organized into three research groups: magnesium processing, wrought magnesium alloys, and corrosion and surface technology. MagIC is leading the research in Mg-MIM. It has been shown that using their novel binder system and sintering strategy, Mg-MIM components can be realized.

6.1.1 Market opportunities Listed below are some potential market opportunities for MIM of the reactive powders. 6.1.1.1 Electronics Cell phone and computer components continue to grow and potentially present the most lucrative market for titanium and aluminum MIM. The growing demand of consumers for lightweight, miniature, and power electronics devices are paving the way for industries to adopt Ti and AlMIM. Ti-MIM can potentially replace stainless-steel structural parts such as mainframe body, SIM card holders, phone buttons. One example of the Ti-MIM products is shown in Fig. 6.2. Al-MIM can be utilized for heat-sink applications. Table 6.1 summarizes the properties of metal injection molded aluminum (aluMIM), as compared to those of 6061 (a common extrusion alloy) and A380 (a common casting alloy). Although Al-MIM has a lower density, its thermal conductivity is higher. This is because the die-cast and extruded aluminum heat sinks often contain alloying elements for uncomplicated processing. However, these alloying elements (and other impurities picked up during processing) are detrimental to the thermal properties. On the contrary, the higher

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Figure 6.2 Frame shell of a smartwatch made by Ti-MIM weighs only 6 g.5 Table 6.1 The properties of metal injection molded aluminum (aluMIM) as compared to those of 6061 (a common extrusion alloy) and A380 (a common casting alloy).

3

Density (g/cm ) Thermal conductivity (W/mK) Net-shape capability

MIM

Extruded 6061-T6

Die-Cast A380

2.5 170 180

2.7 120 150 Directional

2.7 80 100

Excellent

2D only

Good

purity of metal injection molded aluminum results in higher thermal conductivity. This is one example where Al-MIM can be effectively utilized.6 6.1.1.2 Consumer decorative products The consumer products industry is a major user of titanium metal injection molded parts. Early Ti-MIM reached production status for decorative applications as early as 1991. The most notable applications were some watch cases and running shoe spikes. The demand for precision, compact, and high-volume components has ensured the growing demand for MIM fabrication in the consumer products sector. The potential for growth of Ti-MIM in the consumer products sector of the market is very high. The consumer products market is very lucrative and is projected to exhibit high growth rates owing to miniaturization, innovations, and new product developments. In 2017, the consumer products of Ti-MIM were 80.6 million US dollars, and it is expected to grow to US$132.6 million in 2023. The exclusive capability of metal injection molding technology

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to produce small parts is extremely advantageous, and thus it has gained widespread popularity in the consumer products end-user application. 6.1.1.3 Medical applications The drive for less invasive surgical techniques has created a global demand for intricate and highly complex part geometries. Such components are typically manufactured using a wrought metal bar via conventional costintensive techniques such as multiaxis milling and turning and electrical discharge machining. Currently, the MIM process is posing a challenge to these traditional processes by offering a more cost-effective and viable alternative. One of the major driving forces for this industry is the growth of the aging population across the globe coupled with the desire for miniaturization. Many medical devices are produced from difficult-to-machine materials such as stainless-steel, cobalt-chromium alloy, titanium and nickel-titanium, and other titanium alloys. Titanium and its alloys have always been considered the star material for medical applications, but owing to its high cost and limited availability of high-quality powder its use has been constrained. However, there has been tremendous research and advancements and its industrial applications now encompass medical implants and devices. Some common products for Ti-MIM in this category now include body implants, dental braces, and endo-surgery equipment. The recent successful application of MIM to produce biodegradable magnesium implant forecasts a prosperous future in medical applications for Mg-MIM as well. Fig. 6.3 presents some magnesium bone fixation devices that may potentially be manufactured by MIM in the near future. Automotive applications: The potential for growth of titanium and aluminum alloys MIM in the automotive sector of the market is very high. The global automotive industry remains especially interested in near-net-shape manufacturing methods, and the global drive of weight reduction to cut fuel cost is expected to boost research and growth of titanium and aluminum alloys MIM in this industry. Some of the automotive parts where titanium and aluminum alloys MIM can be effectively applied are shown in Fig. 6.4.

6.1.2 Patents highlighting the success of reactive powders metal injection molding With a gradual increase in reactive powders-MIM, novel techniques and new materials are being developed for this process, which is evident from the patents filed over the past two decades (Table 6.2).

Figure 6.3 (A D) Several typical Mg bone fixation devices that present an excellent market opportunity for Mg-MIM and (E) shows one case of their bone repair application. The traditional bone fixation device is made of 316L stainless-steel or titanium alloy. However, these alloys possess much greater modulus than that of natural bone resulting in osteoporosis and other symptoms due to insufficient load-bearing of the bone. Mg bone fixation device, on the other hand, ensures proper mechanical support at the early stage. Over time, it undergoes a dynamic degradation with its load-bearing support gradually decreases, which is conducive to promoting the healing of new bone tissue. For more details on magnesium implants, interested users are encouraged to read Refs. [3,7 10].

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Figure 6.4 Potential applications of titanium and aluminum alloys MIM in the automotive sector: (A) seat belt components, (B) gears and wheels, (C) oil pump parts, and (D) different valves. Courtesy Dr. Peng Yu ElementPlus, Shenzhen, China.

Some important highlights from the selected patents are briefly discussed here. Patent US20030170137A1 (Forming complex-shaped aluminum components) is perhaps the first patent that described the successful application of MIM for the production of complex-shaped aluminum components. To solve the issue of the tough oxide layer that grows on aluminum particles preventing metal metal bonding between the particles, the inventors added a small amount of material that formed a eutectic mixture with aluminum oxide, thus reducing the oxide layer and aiding sintering by allowing intimate contact between aluminum surfaces. This was achieved by mixing a composition of elemental powders into a feedstock that included aluminum in the amount of at least 95% by weight, the rest being silicon carbide or a metallic fluoride in an amount sufficient for the required density and strength. According to the inventors, the desired alloy comprises approximately of 97 wt.% of aluminum, and the rest 3 wt.% could be either silicon carbide or metallic fluorides with a sintering temperature between 600°C and 650°C and a sintering time of approximately 60 min in a vacuum atmosphere of ,10 Pa. Patent WO2009029993A1 is another patent disclosing a method for producing Al components by MIM. To achieve successful sintering, the inventors suggest a sintering aid in the form of a metal having a melting point lower than that of Al and is preferably insoluble in solid Al. Some suggested examples of suitable sintering aids include tin, lead, indium, bismuth, and antimony. In particular, tin has been found especially suitable in assisting in the sintering of aluminum and aluminum alloys.

Table 6.2 A summary of patents filed during recent decades exclusively for reactive powders MIM. Patent number

Based on Al-MIM

US20030170137A1

11

Patent title

Filing date

Patent claim

Forming complex-shaped aluminum components

03/2002 by Advanced Materials Technologies Pte Ltd 09/2007 by Seiko Epson Corp.

A process to manufacture an aluminum alloy object having a complex shape A method to safely, easily, and inexpensively produce a metalsintered body having a low content of metal oxide and excellent dimensional accuracy is provided A method for forming an article by metal injection molding of aluminum or an aluminum alloy A process for the production of moldings based on aluminum alloys by metal powder injection molding. An aluminum powder metallurgy injection molding process

US7811512B212



WO2009029993A113

Metal injection moulding method

09/2008 by The University of Queensland

EP2552630A114

Method for producing shaped bodies from aluminium alloys

03/2011 by Rupert Fertinger GmbH, TU Wien, BASF SE

CN104227002A15

Metallurgical injection molding process of aluminum powder A kind of injection molding method of high-strength aluminum alloy turbine wheel

06/2013

CN107775005B16

Method for producing sintered body (translated from Korean)

11/2017

The invention belongs to aero-turbine preparation technical fields, and in particular to a kind of high-strength aluminum alloy turbine wheel Injection molding method

Based on Mg-MIM

US20100274292A117

Based on Ti-MIM

JP2005281736A18

Process for producing components consisting of magnesium or magnesium alloy by sintering Method for producing titanium alloy sintered compact by metal powder injection molding method

04/2010 by GkssForschungszentrum Geesthacht GmbH

US20050196312A119

Feedstock composition and method of using same for powder metallurgy forming of reactive metals

03/2004 by Battelle Memorial Institute Inc.

KR100929135B120

Method for producing a titanium material of precision parts by powder injection molding Powder injection molding product, titanium coating product, sprayer for

11/2005

KR100749395B121

03/2004 by Shizuoka Prefecture, Teiboo KK

01/2006

The invention relates to a process for producing components consisting of magnesium or magnesium alloy by sintering To provide a method capable of producing a titanium alloy sintered compact combining high strength and ductility at a low cost by jointly using low-cost hydrogenated titanium powder with hydrogenated-dehydrogenated titanium powder A composition comprising an aromatic binder system and a metal powder, wherein said aromatic binder system and said metal powder are mixed to form a feedstock for powder metallurgy forming techniques To manufacture Ti components using TiH2 powder

Manufacturing of titanium product with improved sintered performance (Continued)

Table 6.2 (Continued) Patent number

KR20070106079A22

EP2292806B123

US20100178194A124 US9334550B225 DE102010028432A126

CN102242288A27

Patent title

titanium coating and paste for titanium coating Manufacturing method of spectacles frame using a titanium powder injection molding Method for producing components from titanium or titanium alloy using MIM technology Powder extrusion of shaped sections Method of controlling the carbon or oxygen content of a powder injection Manufacturing a spatial implant structure from pure titanium, comprises producing a green compact using metal powder injection molding, transforming the green compact into a brown body, and holding the brown body on solid implant base body Preparation method of porous titanium

Filing date

Patent claim

04/2006

A glass frame manufacturing method using titanium powder injection molding

08/2009 by HZG GmbH

Manufacturing of boron added Ti/Ti alloy components using MIM

01/2009 by Accellent Inc.

Manufacturing of Ti and other profiles using MIM A method for controlling the content of at least one of carbon and oxygen A method for producing a spatial structure implant having an openmesh, three-dimensional spatial network structure that at least partially covers its surface

10/2010 by Anglo Platinum Marketing Limited 04/2010 by S and G IMPLANTS GmbH

06/2011

Method for preparing a porous titanium

CN103042219A28

Titanium glasses frame molding method Manufacture method of titanium nail clippers

12/2012

CN104148644A30

Manufacturing method for titanium alloy products

08/2014

CN104690271A31

Powder injection molding process by utilizing lowcost hydrogenateddehydrogenated titanium powder A kind of preparation method of medical bone fixation means Precision titanium part manufacturing method

02/2015

Titanium mobile phone chassis and methods of making and using same

05/2016 by Essential Products Inc.

KR20130110423A29

CN10536906332 CN105382261A33

US20170251085A134

03/2012

A method of forming titanium spectacle frames A method for manufacturing a titanium nail clipper by injection molding A method to manufacture Ti components using MIM at high injection pressure Addition of rare earth boride or hydride to HDH titanium powders and ultrasonic-assisted injection molding method

08/2015

Development of a new biomedical Ti alloy

11/2015

A method to achieve economical scale preparation of high dimensional accuracy and high-performance titanium complex shape parts To manufacture titanium or titanium alloy (e.g., titanium/copper alloy) mobile phone chassis, and methods for making and using same

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Feedstock Technology for Reactive Metal Injection Molding

Patent US20100274292A1 describes a process, for the first time, for producing components consisting of magnesium or magnesium alloy by sintering. Materials of this type can be used as biocompatible endosseous implant materials. A decisive factor for the successful sintering of the magnesium or magnesium alloys is the use of getter material. The preferred getter material is magnesium powder. In addition, the set-up of the samples, along with the getter material, is very important for achieving successful sintering. According to the inventors, it is possible to select a socalled “labyrinth sintering crucible arrangement,” in which the potential impurities such as oxygen first have to pass completely through the bed of getter material before they reach the main component. Patent JP2005281736 discloses a new solution for low-cost manufacturing of Ti-MIM alloy components with high mechanical properties and low oxygen content. The inventors mixed different fractions of TiH2, HDH titanium, and 60Al-40V prealloyed powders to manufacture Ti 6Al 4V components. Optimum mechanical properties of YS 5 910 MPa, UTS 5 950 MPa, and El 5 14% were obtained by mixing 25 wt.% TiH2 and 75 wt.% HDH powders. Contrary to general perception, they observed that increasing the TiH2 fraction in the powder mixture increases the oxygen content, resulting in a decrease in elongation. Patent CN103266319 describes a new technique based on the insert molding and Ti-MIM processes to cover the surface of Ti implants by a thin and porous layer to improve the biocompatibility and reduce the Young modulus of the implants. The inventors created a porous-coated surface with an oxygen content of 0.32 wt.%, the porosity of 60%, and the bonding strength of 420 MPa.

6.2 Summary Among the metal injection molding (MIM) of different reactive powders such as aluminum (Al), magnesium (Mg), and titanium (Ti), only Ti-MIM has established itself to some extent. The applications of MIM to manufacture titanium components have been considered for many years now, but the progress has been still slow. The two main reasons for this slow progress are the high cost of starting powder and the contamination during the entire processing routes. Owing to the high melting point and high reactivity of titanium, the production of high-quality spherical powder by inert gas atomization requires significant investment. The price of titanium powder has come down in recent years due to significant interest and investments

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by many countries and international companies in additive manufacturing (AM). The success of AM highly depends on the quality of starting powder, which must be high-quality spherical powders for better flowability. Despite the decreasing prices in recent years, the cost of fine (#45 µm), low oxygen, and spherical titanium powder remains excessive for industrial applications. Additionally, the high affinity of titanium for oxygen, carbon, and nitrogen requires extraordinary considerations during MIM processing, including the requirement for a specialized binder system. These are the principal reasons for the lack of Ti-MIM business today compared to the established MIM of the stainless-steel business. As a result, from an economic perspective, Ti-MIM components are not yet competitive enough when compared to similar components made by machining or casting. Due to the high cost of spherical titanium powder, efforts have been made to produce titanium components using low-cost hydride dehydride (HDH) Ti powder. However, there are no commercial titanium feedstocks available on the market made of HDH Ti powder, suggesting the use of such powders is confined to laboratory-scale research only. The commonly used binder systems for MIM involve components of a plasticizer (waxes), polymers, and surfactants to possess specific characteristics vital to the success of the MIM operation. Each binder component affects the overall properties of the system. Therefore a binder system must meet certain criteria. Some of these conditions are even more stringent for titanium and its alloys metal injection molding, given that Ti is a universal solvent for many common interstitials such as H, O, C, and N. The pick-up of these elements affect the overall mechanical properties of the final component, increasing strength at the cost of decreasing elongation. The most important characteristics of a binder system for Ti-MIM include the following: • good adhesion to titanium particles; • safe and environmentally acceptable; • high strength and stiffness with low thermal expansion coefficient; • not chemically reactive with titanium; • most importantly, complete decomposition at low temperatures (,300°C) without any residue after thermal debinding. It is worth mentioning here that titanium readily reacts with oxygen above 400°C. Experimental studies have shown that the pick-up of nitrogen during Ti-MIM processing is usually not a problem. Hydrogen can also be

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Feedstock Technology for Reactive Metal Injection Molding

ignored because the sintering of titanium is done under a high vacuum at high temperatures. Under these conditions, the hydrogen contents are drawn out from the material. On the other hand, carbon and oxygen are the two most important elements to consider. This is because carbon is present in all polymeric binders, and oxygen has high solubility and high affinity for titanium above 400°C. The primary component of binder systems (plasticizers) is usually removed completely by solvent extraction at low temperatures. Therefore it does not increase the overall impurity level unless improper extraction is done. Contaminations during sintering is also a major contributor to the overall impurity levels in the final products. In this regard, sintering should be undertaken in a high-quality vacuum furnace. It is often noted that contaminations during thermal debinding, which arise from the thermal decomposition of backbone polymers (as most of the typical backbone polymers employed in MIM have decomposition temperature above 400°C), remain a big concern for product designers. Therefore the proper selection of binder components is key to success in the case of the Ti-MIM process. As mentioned in Chapter 3, Binder System Interactions and Their Effects, of this book, we simply cannot mix two polymers and use them as binders. During the selection of binder components, particularly the primary and secondary components, it is critical to check important parameters such as molecular structure, molecular weight, and polarity, which can affect compatibility and miscibility between the two components. In Chapter 3, Binder System Interactions and Their Effects, we describe a case study highlighting the impact of interactions of the binder components on the overall performance of a Ti-MIM feedstock. Ideally, a binder system for Ti-MIM should fully decompose below 300°C. However, no commercially available polymer can decompose below 300°C, while providing good green strength. The commercially available feedstocks are generally distinguished by the primary debinding methods they employ and thence fall into three categories: water debind feedstock, catalytic debind feedstock, and solvent debind feedstock. Among these, catalytic feedstocks are widely used in industry due to faster debinding rates and excellent shape retention properties, as compared to other types of feedstock. The most common catalytic Ti-MIM feedstocks are provided by BASF. Chapter 2, Design Strategy of Binder Systems and Feedstock Chemistry, lists some of the commercially available reactive powders-MIM feedstocks and the resulting properties that can be achieved.

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Unlike Ti, Al or Mg do not form a solid solution with oxygen. Instead, a thermodynamically stable oxide layer (4 6 nm thick) is formed. This oxide layer inhibits atomic diffusion required for densification and presents a technical challenge for Al- and Mg-MIM. This problem has been solved successfully by adding small amounts of sintering aids—in the form of a material that forms a eutectic mixture and reduces the oxide layer. Some commercial feedstocks for Al-MIM are now available on the market. However, the industrial applications of Al-MIM remain limited, and diecasting remains the first-hand choice fabrication technique in the industry. So far, limited research has been conducted in the case of Mg-MIM. Nevertheless, the results are promising. The biggest competitor of MgMIM is Thixomolding. If it has to be successful, its cost will have to be lower than those of thixomolding or die-casting components. Chapter 4, Impurity Management in Reactive Metals Injection Molding, and Chapter 5, Potential Feedstock Compositions for MIM of Reactive Metals, present some case studies of Al and Mg-MIM. Lastly, special care must be taken when dealing with titanium, aluminum, and magnesium powders. Ideally, all the processing and handling should be carried out under a protective gas atmosphere. These powders may explode when dispersed in the air at concentrations greater than 0.03 kg/m3. Even during the injection molding, an explosion may occur, in particular, when the feedstock is recycled.

References 1. ,https://produktionderzukunft.at/en/projects/indalmim-aluminium-injection-molding-for-industry.php.. 2. Kainer, K. U.; Ebel, T.; Ferri, O. M.; Limberg, W.; Pyczak, F.; Schimansky, F. P.; Wolff, M. From Titanium to Magnesium: Processing by Advanced Metal Injection Moulding. Powder Metall. 2012, 55 (4), 315 321. 3. Yang, Y.; He, C.; Dianyu, E.; Yang, W.; Qi, F.; Xie, D.; Shen, L.; Peng, S.; Shuai, C. Mg Bone Implant: Features, Developments and Perspectives. Mater. Des. 2020, 185, 108259. 4. ,https://www.hzg.de/institutes_platforms/materials_research/magnesium_technology/index.php.en.. 5. Ye, S.; Mo, W.; Lv, Y.; Wang, Z.; Kwok, C. T.; Yu, P. The Technological Design of Geometrically Complex Ti 6Al 4V Parts by Metal Injection Molding. Appl. Sci. 2019, 9 (7), 1339. 6. ,https://www.electronics-cooling.com/2004/11/metal-injection-molding-of-heatsinks/.. 7. Zhao, D.; Witte, F.; Lu, F.; Wang, J.; Li, J.; Qin, L. Current Status on Clinical Applications of Magnesium-Based Orthopaedic Implants: A Review From Clinical Translational Perspective. Biomaterials 2017, 112, 287 302.

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8. Chaya, A.; Yoshizawa, S.; Verdelis, K.; Myers, N.; Costello, B. J.; Chou, D.-T.; Pal, S.; Maiti, S.; Kumta, P. N.; Sfeir, C. In Vivo Study of Magnesium Plate and Screw Degradation and Bone Fracture Healing. Acta Biomater. 2015, 18, 262 269. 9. Waizy, H.; Seitz, J.-M.; Reifenrath, J.; Weizbauer, A.; Bach, F.-W.; MeyerLindenberg, A.; Denkena, B.; Windhagen, H. Biodegradable Magnesium Implants for Orthopedic Applications. J. Mater. Sci. 2013, 48 (1), 39 50. 10. Windhagen, H.; Radtke, K.; Weizbauer, A.; Diekmann, J.; Noll, Y.; Kreimeyer, U.; Schavan, R.; Stukenborg-Colsman, C.; Waizy, H. Biodegradable Magnesium-Based Screw Clinically Equivalent to Titanium Screw in Hallux Valgus Surgery: Short Term Results of the First Prospective, Randomized, Controlled Clinical Pilot Study. Biomed. Eng. Online 2013, 12 (1), 62. 11. Yeo, C.-T.; Tan, L.-K. Forming Complex-Shaped Aluminum Components. US20030170137A1, 2002. 12. Sakata, M.; Hayashi, J. Method for Producing Sintered Body and Sintered Body. US7811512B2, 2007. 13. Liu, Z.; Schaffer, G. B. Metal Injection Moulding Method. WO2009029993A1, 2008. 14. Danninger, H.; Gierl, C.; Zlatkov, B.; Maat, J. T. Method for Producing Shaped Bodies from Aluminium Alloys. EP2552630A1, 2011. 15. Shu, T. Metallurgical Injection Molding Process of Aluminum Powder. CN104227002A, 2013. 16. Xia, J.; Chang, F. A Kind of Injection Molding Method of High-Strength Aluminum Alloy Turbine Wheel. CN107775005B, 2017. 17. Wolff, M.; Ebel, T.; Hort, N. Process for Producing Components Consisting of Magnesium or Magnesium Alloy by Sintering. US20100274292A1, 2010. 18. Hariyuki, T.; Ito, Y.; Komatsu, T.; Sato, K. Method for Producing Titanium Alloy Sintered Compact by Metal Powder Injection Molding Method. JP2005281736A, 2004. 19. Nyberg, E.; Weil, K.; Simmons, K. Feedstock Composition and Method of Using Same for Powder Metallurgy Forming of Reactive Metals. US20050196312A1, 2004. 20. Lee, J. Method for Producing a Titanium Material of Precision Parts by Powder Injection Molding. KR100929135B1, 2005. 21. Park, Y. Powder Injection Molding Product, Titanium Coating Product, Sprayer for Titanium Coating and Paste for Titanium Coating. KR100749395B1, 2006. 22. Yun, J.-H.; Hwang, K.-C. Manufacturing Method of Spectacles Frame Using a Titanium Powder Injection Molding. KR20070106079A, 2006. 23. Ferri, O. M.; Ebel, T. Method for Producing Components from Titanium or Titanium Alloy Using MIM Technology. EP2292806B1, 2009. 24. Broadley, M. W.; Eckert, J.; Rilling, R.; White, R. J.; Farina, J. M.; Sago, J. A. Powder Extrusion of Shaped Sections. US20100178194A1, 2009. 25. Hamilton, H. G. C. Method of Controlling the Carbon or Oxygen Content of a Powder Injection. US9334550B2, 2009. 26. Hans, D.-I.G. Manufacturing a Spatial Implant Structure From Pure Titanium, Comprises Producing a Green Compact Using Metal Powder Injection Molding, Transforming the Green Compact into a Brown Body, and Holding the Brown Body on Solid Implant Base Body. DE102010028432A1, 2010. 27. Liu, H.; Yi, D.; Wang, J.; Hu, H. Preparation Method of Porous Titanium. CN102242288A, 2011. 28. Gu, Y.; Jiang, K.; Shili, Z.; Fei, W. Titanium Glasses Frame Molding Method. CN103042219A, 2012. 29. Ja-Sil, G.; Sung-Hoon, J. Manufacture Method of Titanium Nail Clippers. KR20130110423A, 2012.

Outlook of reactive metal injection molding

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Index Note: Page numbers followed by “f” and “t” refer to figures and tables, respectively.

A Additive manufacturing (AM), 25, 237, 248 249 Advanced Materials Technologies (AMT), 33 Agar, 55 AA6061 alloy, 176 177, 176f Al Si alloy composites, 196 197 Al Sn phase diagram, 174 175, 175f Aluminum alloy 6061 with tin, 174 179 Aluminum-MIM (Al-MIM), 195 207 industrial applications of, 251 mechanical strength of, 206 207 sintering process for, 205 206 thermal debinding process, 206 Aluminum nitride (AlN), 174 Amorphous polymers, 45 47 Arrhenius equation, 214 215 Atomic force microscopy (AFM), 111 112, 120f

B Bagley end correction, 71 Binary phase diagram, 145, 173f Binder systems, 6 7, 6t basic requirements for, 44t binder chemistry, 45 49 common binder blends different polyethylene blends, 116 119 ethylene vinyl acetate, 113 114 PEG/PMMA blend, 114 116 polypropylene, 119 121 complex interactions and their effects on reactive powders-MIM, 122 127 experimental methods determination of interaction parameters, 99 107 glass transition temperature (Tg) measurements, 107 110 infrared spectroscopy, 110 111

microscopy, 111 113 feedstock chemistry and properties heat capacity, 76 79 powder characteristics and optimal solids loading, 64 69 remarks for binder blends, 121 127 shear sensitivity, 69 75 strength model, 79 81 temperature sensitivity, 75 76 thermal conductivity, 76 79 H-bonding interactions, 124f metal injection molding, 87 polymer blends compatibilization, 89t compatible, 89t definition of, 88 89 immiscible, 89t miscible, 89t thermodynamics of, 92 98 polyoxymethylene-based binder system, 53 54 role of, 43 45 surfactant basic chemistry of, 128 132 role of, 127 128 stearic acid for reactive powdersMIM, 132 136 water-based binder system, 54 63 wax-based binder systems, 49 53 Bioceramics, 200 Biomedical implants, 237 Biometals, 200 Biopolymers, 200 Bisphenol A (BPA), 92, 93f Brownian motion, 67

C Carnauba wax (CW), 49 50, 196 197 Catamold, 150 151, 207, 212f Cellulosic polymers, 185

255

256

Index

Commercial feedstocks and properties, 207 218 Common binder blends different polyethylene blends, 116 119 ethylene vinyl acetate, 113 114 PEG/PMMA blend, 114 116 polypropylene, 119 121 Compound annual growth rate (CAGR), 27 31, 29f Consumer decorative products, 240 241 Crystalline polymers, 45 47

D

Δχ effect, 109 110 Debinding techniques, 9 11, 11t catalytic, 56 comparison of, 58t solvent, 51 52 thermal, 52 53 Demand from the medical sector, 25 26 Densification of Al, 206 Depolymerization, 191 195 Deuterated ring polyisoprene (d-PI), 102 Differential scanning calorimetry (DSC), 78 79, 105, 109, 111f, 196f, 216f, 231 heat capacity determination, 79f

E Electronics, 239 240 Ellingham diagram, 168 169 Ethylene-vinyl acetate (EVA), 49 50, 113 114, 151 152, 156 157 Experimental methods determination of interaction parameters, 99 107 glass transition temperature (Tg) measurements, 107 110 infrared spectroscopy, 110 111 microscopy, 111 113

F Feedstock chemistry and properties, 7 8, 7f heat capacity, 76 79 powder characteristics and optimal solids loading, 64 69 shear sensitivity, 69 75

strength model, 79 81 temperature sensitivity, 75 76 thermal conductivity, 76 79 water debinding profile, 154f Fixation texture, 222, 225 Flory Huggins (FH) lattice theory, 136 138 Food and Drug Administration (FDA), 30 Formaldehyde, 150 151 Fourier transform interferometers (FTIR), 110 111, 112f, 114 116, 117f Future trends, 237 248

G Gibbs free energy of mixing, 90 Global MIM sales, 29f

H Hansen’s parameters, values of, 98 Heat capacity, 76 79 Helmholtz-Zentrum Geesthacht (HZG), 184f Herschel Bulkley fluids, 70 71 High-density polyethylene (HDPE), 6t, 54 55, 116 119, 196 197 High-flux advanced neutron application reactor (HANARO), 102 Hot isotactic pressing (HIP) process, 231f Hydride dehydride (HDH) process, 37

I Impurity control importance of, 145 147 selection of primary component, 147 154 sintering, 166 169 and thermal debinding mechanisms, 154 165 INDALMIM, 237 238 Infrared spectroscopy, 110 111 Injection molding process, 192 International Organization of Motor Vehicle Manufacturers (OICA), 31 32

L Light scattering (LS), 100t

Index

Linear low-density polyethylene (LLDPE), 116 119, 118f Low-density polyethylene (LDPE), 116 119 Lower critical solution temperature (LCST), 102, 105f Low voltage scanning electron microscopy (LVSEM), 111 112

M Magnesium Innovation Centre (MagIC), 238 239 Market opportunities consumer decorative products, 240 241 electronics, 239 240 medical applications, 241 Market statistics and research direction, 27 32 Master alloy powder (MAP), 202, 204f Medical applications, 241 Metal injection molding (MIM), 87, 145 additive manufacturing, 237 advantages over conventional manufacturing techniques, 25 applications of, 32 35, 248 249 attempts to address Al-MIM, 205 207 binder system for, 6 7, 6t, 87 commercial feedstocks and properties, 207 218 constraints on the reactive powders, 35 39 current status, 19 23 debinding, 9 11, 11t demand from the medical sector, 25 26 depolymerization, 191 195 design consideration, 3 4 feedstock preparation, 7 8, 7f fixation texture, 223 225 HAAKE MiniJet pro piston, 9f increasing demand for miniaturization, 23 25 ingrowth region, 223 225 manufacturing technology, 1 market opportunities consumer decorative products, 240 241 electronics, 239 240 medical applications, 241

257

market statistics and research direction, 27 32 materials development, 13 15 materials that are hard to process, 26 27 mechanical and chemical properties of, 209t molding operation, 8 9, 8f optimal processing parameters, 178t powders for metal injection molding, 4 5 powders production in, 5t process control, 1 2, 4t, 183 185 reactive powders, 241 248 of aluminum alloy 6061 with tin, 174 179 of Mg and its alloys, 179 183 pure Al-metal injection molding, 170 173 sintering, 11 13 smart-glasses titanium arm by, 228 232 technological advancements, 16 19 thermal debinding and sintering profile, 181f thin-walled complex-shaped, 178f Ti-MIM, 219 232 Metal matrix composite (MMC), 14 Mg-MIM, 195 207 MicroMIM process, 16 17, 17t, 18f industrial products, 17 submicron-sized powders in, 16 Microscopy, 111 113 MIMplus research, 13 14 Miniaturization, increasing demand for, 23 25 Molybdenum (Mo), 121 122 Monomers, 157

N Naphthalene, 56 57 Nishi Wang equation, 107

O Optical microscopy (OM), 102, 114f Orthopedic implants, 222

P Paraffin wax (PW), 49 50, 113 114, 121 122

258

Index

Patent CN103266319, 248 Patent JP2005281736, 248 Patent US20030170137A1, 243 Patent US20100274292A1, 248 Patent WO2009029993A1, 243 Phase change materials (PCMs), 116 119 Poly(3-hydroxybutyrate) (PHB), 111 Poly(4-trimethylsilylstyrene) (h-PT), 102 Polyacetal (POM), 156 depolymerization, 209 Polybutylene (PB), 180 181 Polybutyl methacrylate (PBMA), 156, 162 fracture surface analysis of, 163f molecular formula of, 163f Polyethylene (PE), 45, 116 119 Polyethylene copolymer vinylacetate (PEVA), 181 182 Polyethylene glycol (PEG), 45 47, 56f, 191 192 crystalline polymer, 149 150 crystallization temperature of, 161 162, 193 194 debinding rate of, 149 FTIR spectra of, 117f heat absorption and crystallization energy of, 196t hydrogen bonding, 117f molecular chain of, 149 150 thermal decomposition, 151f thermal degradation behavior of, 123 void formation, 160 161 Poly(ethylene oxide) (PEO), 109 Poly(hydroxy ether of bisphenol-A) (phenoxy), 109 Polylactide acid (PLA), 205t Poly(L-lactide) (PLLA), 105 equilibrium melting point of, 107, 107t Polymer blends, 156 characterization techniques for, 100t compatibilization, 89t compatible, 89t definition of, 88 89 immiscible, 89t miscible, 89t poly(3-hydroxybutyrate), 111 TGA curves of, 157f thermodynamics of, 92 98

Flory Huggins theory, 92 95 solubility parameter approach, 96 98 thermoplastic, 6 thermosetting, 6 Polymethyl methacrylate (PMMA), 45, 49 50, 54 55, 191 192 carbonyl carbon atoms of, 114 116 FTIR spectra of, 117f glass transition temperature of, 161 162 hydrogen bonding, 117f molecular formula of, 163f optical microscopy image of, 116f oxygen atoms of, 114 116 PolyMIM advantage of, 213 214 DSC analyses, 215 mono-sized spherical powder feedstock, 213 morphology of, 213f rheological data of, 213 214 Ti6Al4V datasheet, 212 213 Poly(N-vinylpyrrolidone) (PVP), 92, 93f Polyoxymethylene (POM), 45 47, 53 54, 150 151 catalytic debinding of, 53f degradation mechanisms of, 157f thermal degradation mechanisms of, 156 157 Polypropylene (PP), 45, 119 122, 156, 180 181 degradation mechanisms of, 157f Polypropylene carbonate (PPC), 122 123, 124f, 164 weight loss of, 164f Polypropylene copolymer polybutene (PPcoPB), 180 181 Polypropylene copolymer polyethylene (PPcoPE), 180 181, 183 Polystyrene (PS), 114f Polyvinyl acetate (PVAc), 92, 93f, 157 158, 194 heat absorption and crystallization energy of, 196t Polyvinyl alcohol, 157 158 Polyvinyl butyral (PVB), 157 158 thermal degradation sequence of, 158f Poly(vinyl methyl ether) (PVME), 114f

Index

Poly(vinylphenol) (PVPh), 105 Polyvinylpyrrolidone (PVP), 55, 152 153 Powder characteristics and optimal solids loading, 64 69 Powder injection molding (PIM), 1 2 Powders for metal injection molding, 4 5 Praxis Powder Technology, 15, 219 222, 228 Primary crystallization, 45 47 Process control, 183 185 Proton exchange membrane (PEM), 24f Pure Al-metal injection molding, 170 173

R Rabinowitsch correction, 73 Racking materials, 12 Random phase approximation (RPA), 100 101 Reactive powders, 241 248 of aluminum alloy 6061 with tin, 174 179 commercial feedstocks, 211t of Mg and its alloys, 179 183 pure Al-metal injection molding, 170 173 Rheology, 227f Rumpf equation, 79 80

S Scanning electron microscopy (SEM), 111 112 in backscatter mode, 125 Scanning transmission electron microscopy (STEM), 111 112 Selection of primary component, 147 154 Selective bead-sintered (SBS), 231 Shear sensitivity, 69 75 Sintering, 11 13, 166 169, 177 179, 202 Small-angle neutron scattering (SANS), 99 100, 100t Small-angle X-ray scattering (SAXS), 100t Smart-glasses titanium arm, 228 232 Solvent debinding, 50 52, 148 149 Stearic acid (SA), 131, 177 179, 196 197 chemical formula of, 137f

259

Stearic acid for reactive powders-MIM, 132 136 Strength model, 79 81 Surfactant basic chemistry of, 128 132 for reactive powders-MIM, 132 136 role of, 127 128

T Technological advancements, 16 19 3DT technology, 223, 224f, 225, 228 Temperature sensitivity, 75 76 Thermal conductivity, 76 79 Thermal debinding mechanisms, 154 165, 177 179, 203f Thermodynamics, 92 98 Thermogravimetric analysis (TGA), 118f, 119 121, 121f, 193f, 231 Thermoplastic polymer, 45 amorphous, 45 47 crystalline, 45 47 Thermoplastic polymers, 6 Thermoset polymer, 48 49 Thermosetting polymers, 6 Ti 6Al 4V alloy, 166 167, 167t Titanium carbide (TiC), 132 133 Titanium-hydroxyapatite (Ti-HA), 14 15 biomedical applications, 14 15 mechanical properties of, 14 15 orthopedic devices, 15 Titanium MIM (Ti-MIM), 219 232 aromatic-based binder system, 56 57 binder system, 38 39 global market of, 31, 32t industrial applications of, 38 39 medical implants, 219 228 potential growth opportunities, 237 structural components, 35f wax-based binder system for, 50 Transmission electron microscopy (TEM), 111 112 Two-component MIM (2C-MIM), 17 18

U Upper critical solution temperature (UCST), 94

260

Index

V

Y

Vaporization process, 152

Yttria (Y2O3), 168, 185

W

Z

Water-based binder system, 54 63 Wax-based binder systems, 49 53 Wide-angle X-ray scattering (WAXS), 100t

Zirconia (ZrO2), 168, 185 Zirconia stabilized yttria (YSZ), 12 13