Materials and Manufacturing Processes [1st ed.] 978-3-030-21065-6;978-3-030-21066-3

Table of contents :
Front Matter ....Pages i-xiv
Front Matter ....Pages 1-1
Introduction to Materials (Kaushik Kumar, Hridayjit Kalita, Divya Zindani, J. Paulo Davim)....Pages 3-20
Mechanical Behaviour of Materials (Kaushik Kumar, Hridayjit Kalita, Divya Zindani, J. Paulo Davim)....Pages 21-34
Front Matter ....Pages 35-35
Casting (Kaushik Kumar, Hridayjit Kalita, Divya Zindani, J. Paulo Davim)....Pages 37-52
Forming (Kaushik Kumar, Hridayjit Kalita, Divya Zindani, J. Paulo Davim)....Pages 53-63
Welding (Kaushik Kumar, Hridayjit Kalita, Divya Zindani, J. Paulo Davim)....Pages 65-81
Front Matter ....Pages 83-83
Machining Process (Kaushik Kumar, Hridayjit Kalita, Divya Zindani, J. Paulo Davim)....Pages 85-100
Back Matter ....Pages 101-104

Citation preview

Materials Forming, Machining and Tribology

Kaushik Kumar Hridayjit Kalita Divya Zindani J. Paulo Davim

Materials and Manufacturing Processes

Materials Forming, Machining and Tribology Series Editor J. Paulo Davim, Department of Mechanical Engineering, University of Aveiro, Aveiro, Portugal

This series fosters information exchange and discussion on all aspects of materials forming, machining and tribology. This series focuses on materials forming and machining processes, namely, metal casting, rolling, forging, extrusion, drawing, sheet metal forming, microforming, hydroforming, thermoforming, incremental forming, joining, powder metallurgy and ceramics processing, shaping processes for plastics/composites, traditional machining (turning, drilling, miling, broaching, etc.), non-traditional machining (EDM, ECM, USM, LAM, etc.), grinding and others abrasive processes, hard part machining, high speed machining, high efficiency machining, micro and nanomachining, among others. The formability and machinability of all materials will be considered, including metals, polymers, ceramics, composites, biomaterials, nanomaterials, special materials, etc. The series covers the full range of tribological aspects such as surface integrity, friction and wear, lubrication and multiscale tribology including biomedical systems and manufacturing processes. It also covers modelling and optimization techniques applied in materials forming, machining and tribology. Contributions to this book series are welcome on all subjects of “green” materials forming, machining and tribology. To submit a proposal or request further information, please contact Dr. Mayra Castro, Publishing Editor Applied Sciences, via [email protected] or Professor J. Paulo Davim, Book Series Editor, via [email protected]

More information about this series at http://www.springer.com/series/11181

Kaushik Kumar Hridayjit Kalita Divya Zindani J. Paulo Davim •

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Materials and Manufacturing Processes

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Dr. Kaushik Kumar Department of Mechanical Engineering Birla Institute of Technology Ranchi, Jharkhand, India

Dr. Hridayjit Kalita Department of Mechanical Engineering Birla Institute of Technology Ranchi, Jharkhand, India

Divya Zindani Department of Mechanical Engineering National Institute of Technology Silchar Silchar, Assam, India

Prof. J. Paulo Davim Department of Mechanical Engineering University of Aveiro Aveiro, Portugal

ISSN 2195-0911 ISSN 2195-092X (electronic) Materials Forming, Machining and Tribology ISBN 978-3-030-21065-6 ISBN 978-3-030-21066-3 (eBook) https://doi.org/10.1007/978-3-030-21066-3 © Springer Nature Switzerland AG 2019 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

The authors are pleased to present the book Materials and Manufacturing Processes under Book Series Materials Forming, Machining and Tribology. This book would serve as an initial part of the book Advanced Machining and Manufacturing Processes (ISBN 978-3-319-76074-2), under the same series and also published by Springer International Publishing AG. These books together would provide readers a complete picture of manufacturing technology starting from materials to virtual manufacturing covering the basic and advanced aspects. This book, the preamble of the two, has been written with a view to project the basic understanding of the materials and traditional processes involved in a manufacturing industry. The properties and application of different engineering materials have been discussed and failure tests performed on the work materials of both destructible and non-destructible in nature have been explained in detail. The processes and the design associated with manufacturing processes like casting, forming, welding, machining, etc. have been covered in a simple and lucid language. The main emphasis, hence, is directed towards industrial engineering outlook. The target audience is academics students, researchers and industry practitioners, engineers, research scientists/academicians working in this vast field. This book is divided into three parts ranging from engineering materials to various basic manufacturing and machining processes that may be employed by manufacturing industries for better understanding of the intricacies of the system to improve their output towards a better socio-economic development. As the sections are independent of each other, in each part, chapters are independently numbered. Part I deals with Engineering Materials. The part contains two chapters. Chapter 1 deals with Introduction to Materials. There has been a major development in the material research and applications in industrial and commercial components or products. With the increase in demand of customers for various customized products there is need to be durable, functionally reliable and low cost at the same time. These requirements motivate designers and manufacturers to tackle the challenging task of optimizing the control factors that determine the quality of the final product. In this chapter, the properties, compositions and characteristics of different materials have been described in detail under the heading of all engineering materials used presently v

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Preface

in industries and structural works. The production procedure of these materials, processes for heat treatments for altering properties of the material and applications of these materials are described in detail in this chapter. Chapter 2 explores Mechanical Behaviour of Materials. Material is the most important aspect of research in all engineering domains which gives an understanding of the real physical structure of various products and objects that are available all around. The behaviour of these various products in their service life is very important to analyse as it might not be desirable to have a product fail at the most unexpected time. In order to quantify these behaviors of material, various properties, as described, in the earlier chapter are determined by running material tests. The procedure for carrying out these tests, feasibility of these test to be able to run for different materials and the behaviour of the part to be tested are some of the aspects which have been explained in detail in this chapter. Material tests have been detailed in two major branches namely, destructible and non destructible tests. Some other fracture surface behaviour in ductile and brittle material upon application of loads higher than their fracture strength and responses for plastic deformation in the form of heat generation and build up of residual stresses are also described in this chapter. This ends Part I. Part II containing three chapters covers Conventional Manufacturing. Chapter 3 of this section elaborates Casting, the most widely used manufacturing process. Here the basic understanding of the casting process, operations and design involved in the sand mould casting process, various metal mould casting processes and few other commonly used casting processes are described in detail. Chapter 4 of this section deals with Forming Process. Right from the advent of industrial revolution, this has been the most important and versatile operation in any manufacturing and metal working industries. Metals are needed to be deformed for a variety of functional requirements in the product or for enhancing the strength and hardness. This deformation produces a complex flow of material which basically is dependent on the amount of force application, ductility of the material, temperature of the material and modulus of elasticity. Deformation can be in the form of bulk deformation under compressive loads or bending, stretching, shrinking or shearing in the sheet metal operations based on which different tools (rollers in rolling, dies and punch) have to be designed accordingly for operations such as blanking, extrusion, wire drawing, rolling, etc. The temperature of the metal has to be maintained such that sufficient plasticity is induced and can be operated with the given capacity of the machine. Few positive aspects of forming processes include insignificant wastage of material, high output material strength, dimensional accuracy and simpler mechanism. Chapter 5, the last chapter of the section, discusses another very important manufacturing process i.e. Welding. It is a process which enables large and complex shaped products to be divided into small and simple parts to be manufactured independently and later can be rigidly joined by fusion of the material at the interface between two metal pieces, stiff enough to operate as a single piece. Welding process is mainly characterized by the melting of the interface material between the two metals, fusion or mixing of the material and then solidifying to

Preface

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produce a rigid and high strength joint. In some cases or applications, even an axial or shear pressure may accompany the fusion phenomenon to trigger a plastic deformation of material at the interface for tight bonding. In this chapter, the fundamental concept behind the various welding processes and how are these classified based on different heat source utilization and techniques of utilization are explained in detail. The last section of the book Part III focuses on Conventional Machining and contains a single chapter, Chap. 6. All metal parts, after being casted or formed has to be shaped into its final dimensions so that it could meet the required functionality, durability and aesthetic characteristics of a product. This chapter elaborates the shaping process which requires metal removal involving various complex phenomenon and factors and a set of mechanisms that can be controlled by considering various tool geometry, optimized cutting conditions and better tool–work material combination. In this chapter processes like turning, milling, boring, drilling, reaming, planning and shaping, sawing, filing, broaching, etc have been discussed in detail. Augmentation of this introductory book along with Advanced Machining and Manufacturing Processes (ISBN 978-3-319-76074-2), published by Springer International Publishing AG would provide, all readers, complete domain of manufacturing world. First and foremost we would like to thank God for providing the power of thinking and means of expression. We would like to thank all of our colleagues, friends in different part of the world for sharing of ideas in shaping our thoughts. We owe a huge thanks to all of our Technical reviewers, Editorial Advisory Board Members, Book Development Editor and the team of Publisher Springer Nature for their availability to work on this huge project. All of their efforts helped to make this book complete and we could not have done it without them. Throughout the process of writing this book, many individuals, from different walks of life, have taken time out to help us out. Last, but definitely not least, we would like to thank them all, our well wishers, for providing us encouragement. We would have probably given up without their support. Ranchi, India Ranchi, India Assam, India Aveiro, Portugal

Kaushik Kumar Hridayjit Kalita Divya Zindani J. Paulo Davim

Contents

Part I

Engineering Materials

1 Introduction to Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Highlights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Ferrous Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.1 Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.2 Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.3 Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.4 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Non Ferrous Metal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.1 Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.2 Properties and Applications . . . . . . . . . . . . . . . . 1.4.3 Heat Treatment Processes in Non Ferrous Metals . 1.5 Ceramic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6 Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.7 Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.8 Graphite and Diamond . . . . . . . . . . . . . . . . . . . . . . . . . . 1.9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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2 Mechanical Behaviour of Materials 2.1 Highlights . . . . . . . . . . . . . . . . 2.2 Introduction . . . . . . . . . . . . . . . 2.3 Destructible Tests . . . . . . . . . . . 2.3.1 Tensile Test . . . . . . . . . 2.3.2 Compression Testing . . 2.3.3 Hardness Test . . . . . . . 2.3.4 Fatigue Testing . . . . . .

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2.3.5 Creep Test . . . . . . . . . . . . . . . 2.3.6 Impact Test . . . . . . . . . . . . . . 2.4 Non Destructible Tests . . . . . . . . . . . . 2.4.1 Visual Inspection . . . . . . . . . . 2.4.2 Radiography . . . . . . . . . . . . . 2.4.3 Ultrasonic Inspection . . . . . . . 2.4.4 Magnetic Particle Test . . . . . . 2.4.5 Eddy Current Testing . . . . . . . 2.4.6 Acoustic Method . . . . . . . . . . 2.4.7 Liquid Penetration Method . . . 2.5 Failure Characteristics in Materials . . . 2.5.1 Fracture Types . . . . . . . . . . . . 2.5.2 Plastic Deformation Responses 2.6 Summary . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . .

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4 Forming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Part II

Conventional Manufacturing

3 Casting . . . . . . . . . . . . . . . . . . . . . . . 3.1 Highlights . . . . . . . . . . . . . . . . . 3.2 Introduction . . . . . . . . . . . . . . . . 3.3 Fundamentals of Casting . . . . . . . 3.3.1 Solidification of Metal . . 3.3.2 Fluidity of Molten Metal 3.3.3 Pouring Temperature . . . 3.4 Sand Mould Casting Process . . . . 3.4.1 Pattern Making . . . . . . . 3.4.2 Mould Making . . . . . . . . 3.4.3 Core Making . . . . . . . . . 3.4.4 Casting . . . . . . . . . . . . . 3.5 Metal Mould Casting Process . . . 3.5.1 Slush Casting . . . . . . . . . 3.5.2 Die Casting . . . . . . . . . . 3.5.3 Centrifugal Casting . . . . 3.5.4 Continuous Casting . . . . 3.6 Investment Mould Casting . . . . . 3.7 Plaster Mould Casting . . . . . . . . . 3.8 Ceramic Mould Casting . . . . . . . 3.9 Summary . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . .

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4.3 Hot Working and Cold Working 4.4 Types of Forming . . . . . . . . . . . 4.4.1 Bulk Forming . . . . . . . 4.4.2 Sheet Metal Forming . . 4.5 Summary . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . .

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5 Welding . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Highlights . . . . . . . . . . . . . . . . . . . . 5.2 Introduction . . . . . . . . . . . . . . . . . . . 5.3 Welding Processes . . . . . . . . . . . . . . 5.3.1 Gas Welding . . . . . . . . . . . . 5.3.2 Arc Welding . . . . . . . . . . . . 5.3.3 Resistance Welding . . . . . . . 5.3.4 Laser Beam Welding (LBW) 5.3.5 Friction Welding . . . . . . . . . 5.3.6 Cold Welding . . . . . . . . . . . 5.3.7 Braze Welding . . . . . . . . . . . 5.3.8 Soldering . . . . . . . . . . . . . . . 5.3.9 Mechanical Fasteners . . . . . . 5.4 Summary . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . .

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Part III

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Conventional Machining

6 Machining Process . . . . . . . . . . . . 6.1 Highlights . . . . . . . . . . . . . . . 6.2 Introduction . . . . . . . . . . . . . . 6.3 Mechanics of Metal Cutting . . 6.4 Machines Involved . . . . . . . . . 6.4.1 Lathe Machine . . . . . . 6.4.2 Milling Machines . . . . 6.4.3 Drilling Machines . . . 6.5 Traditional Machining Process . 6.5.1 Turning Process . . . . . 6.5.2 Milling Process . . . . . 6.5.3 Boring Process . . . . . . 6.5.4 Drilling Process . . . . . 6.5.5 Reaming Process . . . . 6.5.6 Planning . . . . . . . . . . 6.5.7 Shaping . . . . . . . . . . . 6.5.8 Sawing . . . . . . . . . . .

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6.5.9 Filing . . . 6.5.10 Broaching 6.6 Summary . . . . . . . References . . . . . . . . . .

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. 98 . 99 . 99 . 100

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

About the Authors

Kaushik Kumar B.Tech. (Mechanical Engineering, REC (Now NIT), Warangal), MBA (Marketing, IGNOU) and Ph.D. (Engineering, Jadavpur University), is presently an Associate Professor in the Department of Mechanical Engineering, Birla Institute of Technology, Mesra, Ranchi, India. He has 16 years of teaching and research and over 11 years of industrial experience in a manufacturing unit of Global repute. His areas of teaching and research interest are Conventional and Non-conventional Quality Management Systems, Optimization, Non-conventional machining, CAD/CAM, Rapid Prototyping and Composites. He has 9 Patents, 28 books, 15 Edited Book Volume, 43 Book Chapters, 137 international Journal, 21 International and 1 National Conference publications to his credit. He is on the editorial board and review panel of 7 International and 1 National Journals of repute. He has been felicitated with many awards and honours. Hridayjit Kalita (BE, Mechanical Engineering, SRM University, Kattankulathur), presently pursuing ME (Design of Mechanical Equipment, BIT Mesra). His areas of interests are Conventional and advanced machining and manufacturing, Optimization, 3D printing. He has 2 Books, 4 Book Chapters, 2 Scopus Indexed international journal and 4 International Conference publications to his credit. Divya Zindani (BE, Mechanical Engineering, Rajasthan Technical University, Kota), M.E. (Design of Mechanical Equipment, BIT Mesra), presently pursuing Ph. D. (National Institute of Technology, Silchar). He has over 2 years of industrial experience. His areas of interests are Optimization, Product and Process Design, CAD/CAM/CAE, Rapid prototyping and Material Selection. He has 1 Patent, 4 Books, 6 Edited Books, 18 Book Chapters, 2 SCI journal, 7 Scopus Indexed international journal and 4 International Conference publications to his credit. J. Paulo Davim received his Ph.D. degree in Mechanical Engineering in 1997, M.Sc. degree in Mechanical Engineering (materials and manufacturing processes) in 1991, Mechanical Engineering degree (5 years) in 1986, from the University of Porto (FEUP), the Aggregate title (Full Habilitation) from the University of Coimbra in 2005 xiii

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About the Authors

and the D.Sc. from London Metropolitan University in 2013. He is Senior Chartered Engineer by the Portuguese Institution of Engineers with an MBA and Specialist title in Engineering and Industrial Management. He is also Eur Ing by FEANI-Brussels and Fellow (FIET) by IET-London. Currently, he is Professor at the Department of Mechanical Engineering of the University of Aveiro, Portugal. He has more than 30 years of teaching and research experience in Manufacturing, Materials, Mechanical and Industrial Engineering, with special emphasis in Machining & Tribology. He has also interest in Management, Engineering Education and Higher Education for Sustainability. He has guided large numbers of postdoc, Ph.D. and master’s students as well as has coordinated and participated in several financed research projects. He has received several scientific awards. He has worked as evaluator of projects for ERC European Research Council and other international research agencies as well as examiner of Ph.D. thesis for many universities in different countries. He is the Editor in Chief of several international journals, Guest Editor of journals, books Editor, book Series Editor and Scientific Advisory for many international journals and conferences. Presently, he is an Editorial Board member of 30 international journals and acts as reviewer for more than 100 prestigious Web of Science journals. In addition, he has also published as editor (and co-editor) more than 100 books and as author (and co-author) more than 10 books, 80 book chapters and 400 articles in journals and conferences (more than 250 articles in journals indexed in Web of Science core collection/h-index 50+/7050+ citations, SCOPUS/h-index 56+/10500+ citations, Google Scholar/h-index 71+/16500+).

Part I

Engineering Materials

Chapter 1

Introduction to Materials

Abstract There has been a major development in the material research and applications in industrial and commercial components or products. With the increase in demand of customers for various customized products that need to be durable, functionally reliable and low cost at the same time. These requirements motivate designers and manufacturers to tackle for the challenging task of optimizing the control factors that determines the quality of the final product. Material is one of the control factors that has been extensively analysed since the industrial revolution began using various material tests as described in Chap. 2. In this chapter, the properties, compositions and characteristics of different materials have been described in detail under the heading of all engineering materials used presently in industries and structural works. The production procedure of these materials, processes for heat treatments for altering properties of the material and applications of these materials are described in detail in this chapter.

1.1 Highlights In this chapter, the major industrial materials have been classified and discussed in details. The structure, properties, production procedures, heat treatment processes and application of ferrous metals, non ferrous metals, ceramics, polymers, composites, graphite and diamond have been explained in detail.

1.2 Introduction The products used in our day to day life are all made of one or more than one materials. The properties and behavior of a material (or metal) are mainly influenced by the structure and the arrangement of the atoms in the metal. Variations in the sizes and positions of the atoms in the unit cell, presence of interstitial sites, presence of impurity atoms, grain size, grain boundary, composition of the material and surface characteristics are some of the factors which characterizes the suitability of © Springer Nature Switzerland AG 2019 K. Kumar et al., Materials and Manufacturing Processes, Materials Forming, Machining and Tribology, https://doi.org/10.1007/978-3-030-21066-3_1

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a material for a product. These micro-structural characteristics of materials can be well modified in case of ferrous metals and few non ferrous metals to synchronize with the desirable properties for better product functioning and durability. These modifications are carried out by adopting a specific production method, heat treatment process and desired composition. With improvement in the knowledge of the material structures and development in the material processing, synthetic materials such as polymers, composites, ceramics have also come to light and are extensively employed in industries today. The most commonly used industrial materials are ferrous and non ferrous metals. Ferrous metal tools were used by primitives from the early period of 4000–3000 B.C. [1]. And are still considered today as the most important technological development in the history of mankind. Modern system of production of ferrous metal in larger quantity began only in the year 1340 AD with the invention of the blast furnace [1]. Ferrous metals are generally classified into cast iron, wrought iron, steel and alloy steel which find applications in a variety of products due to their better mechanical property, lower cost, easy availability and ease of production. Non ferrous metals such as aluminium, titanium, nickel, magnesium, copper, etc. are more expensive than ferrous metals but finds application in industries due to their refractoriness, corrosion resistance, low density and ease of fabrication.

1.3 Ferrous Metals All metals and alloys can be mainly classified into two main groups based on the composition of the material. The metal alloys which consist of iron as the main ingredient in the solid mixture are called the ferrous metals. These materials are the most widely used ingredient in any construction and manufacturing jobs and excel in their wide variations in utility and property. They are cheap, have high strength and are abundantly available. The main varieties of the iron based metal are pig iron, wrought iron, cast iron, steels and alloy steel.

1.3.1 Structure As already known, all metals or non metals are ultimately build up of atoms which consist of protons and neutrons at the nucleus with electrons revolving around it. The factor that determines the characteristic of the material whether it is metallic, non-metallic or semi-metallic, is the number of protons present in the atom. With insufficiency in the total number of electrons in the outer shells and inability to replenish electrons from other electron deficient adjacent atoms, the positive charge in the nuclei dominates, and the remaining electrons are equally shared among all the atomic nuclei. This strong attractive force between the atomic nuclei and electron cloud binds the metal atoms together and distinguishes metallic bonds from all other types of bonds [2].

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Ferrous metals obey the same arrangement of atoms in their body as discussed above. With repeated pattern of arrangements of atoms throughout the body, the metal attains a crystalline structure consisting of a combination of multiple crystal lattices. Each crystal lattice is made up of a repeated configuration of atoms known as unit cells. There are basically three types of arrangement of atoms in a crystal lattice, which are body-centred cubic (BCC) lattice, face-centred cubic (FCC) lattice and hexagonal close packed (HCP) crystal lattice [2]. Among the three, the HCP exhibits the highest density and packing factor followed by FCC and BCC. Alpha and gamma iron which forms the basic composition of ferrous metals exhibit BCC and FCC lattice structure respectively. For pure metal, the above arrangements hold and all the atoms on all unit cells are the same. These pure metals have limited range of properties, which are much inferior to most of the present industrial materials. Alloying is a common practice which involves combination of two elements, one of which must be a metal. They remain in two forms which are solid solution and inter-metallic compounds. Ferrous alloy basically has iron as its main ingredient with other elements for alloying such as carbon in steel.

1.3.2 Properties Some of the ferrous metals and their properties are described below: (a) Cast iron: Cast iron is generally obtained from re-melting of pig iron (detailed in the next section) along with coke, limestone and steel scraps in a cupola furnace. It is an alloy of iron and carbon and is brittle and hard. Since it is brittle, it is weaker in tension and cannot be employed in applications where tensile load is subjected. It also cannot be employed where shock loading is dominant in their service life. Some of the advantageous characteristics of cast iron are high compressive strength, high wear resistance and good machinability. The compressive strength of cast iron ranges from 400 to 1000 MPa which is much higher than their tensile strength which ranges from 100 to 200 MPa [3]. The carbon content in cast iron is either in free form or in combined form. Cast iron can be classified into grey cast iron, white cast iron, alloy cast iron, ductile cast iron, malleable cast iron, nodular cast iron, mottled cast iron and meehanite cast iron [3]. i. In grey cast iron, the carbon remains in graphite form which gives a grey colour to the material. Some of the properties of grey cast iron are good machinability (better than steel), high vibration damping capacity, good resistance to wear, high fluidity (which enables easy casting of complex parts and thin sections), low ductility and low impact strength. ii. In white cast iron, the carbon content is in the form of combined state (iron carbide) which is basically known as cementite. The name is derived from the fact that bright white colour appears on its broken surface, while it is

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fractured. It is produced by refining of the pig iron in the cupola furnace. Some of the characteristic properties of white cast iron are its hardness, excellent wear resistance, high tensile and compressive strength and low machinability. iii. Alloy cast iron includes alloying elements such as nickel, copper, manganese, molybdenum and chromium as ingredients in higher amount as compared to the above mentioned types of cast irons. These inclusions of alloying elements enhances their strength, corrosion resistance, heat resistance and wear resistance. (b) Wrought iron: Wrought iron is the purest form of iron having 99.5% of pure iron in it. It can be produced from pig iron by re-melting it in the furnace. Since the content of carbon in the wrought iron is negligible, it generally cannot be hardened or remains soft having high tensile and compressive strength. Wrought iron is easily formable using hammer and press. Some of the other properties are high ductility and plasticity, corrosion resistance, impact strength or shock loading and electrical conductivity. (c) Stainless steel: The composition of stainless steel includes iron (0.1% carbon) as the main ingredient with some major alloying elements such as chromium and nickel and few minor elements such as molybdenum, manganese, etc. The amount of chromium ranges in 12–18% with minimum resistance to corrosion to maximum. Addition of nickel also builds resistance to corrosion in the material and enhances toughness and strength. Some of the other properties of stainless steel are high resistance to creep, good hot/cold workability, good machinability and weldability, high thermal conductivity, good surface finish and high resistance to scaling and oxidation at elevated temperature. (d) Mild steel: Mild steel comes under plain carbon steel having 0.2–0.3% of carbon content and are structurally fibrous. It possess high tensile strength and toughness, can be easily formed and worked upon, having higher elasticity than the wrought iron and can sustain shock loading. Some of the drawbacks of mild steel are they get rust easily and lack hardness resulting in poor surface finish and polish.

1.3.3 Processes The processes involved in manufacturing of fully functional and cost effective ferrous metal stocks which includes steel, cast iron etc. can be detailed as below.

1.3.3.1

Production

Pig iron is the initial material for production of any kind of steel, cast iron or other ferrous metals. The raw materials are iron ore, limestone and coke which is smelted

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or chemically reduced in a cupola furnace to obtain a high concentration of iron. The molten metal is then sand cast to form a solid mass having the shape of a reclining pig from which the pig iron get its name. The iron ore such as taconite, hematite and limonite are first crushed into particles and impurities are removed out of it by various mechanisms such as magnetic separation. The ore having higher concentration of iron in it can be directly used as charge in the blast furnace while the other crushed iron ore particles are formed into pallets (round balls or solid mass) by mixing with various binders or water. These pellets or high concentration iron ore are charged into the blast furnace along with coke or carbon as reducing agent and limestone acting as a flux. When hot air is blast into the furnace, the coke burns at high temperature producing carbon-monoxide (CO). This evolved CO eliminates the oxygen from the iron oxide and converts it to pure iron at a temperature of around 1650 °C. The impurities in the ore react with limestone to decrease in its melting temperature and weight. The light content moves to the surface of the molten metal as slag and then eliminated out of the furnace. Thus limestone acts like a flux in the iron making process. The molten metal is then extracted out of the furnace into the ladles to be poured over the mould. At this stage the hot metal is called the pig iron. The refinement of the pig iron is performed in the cupola furnace itself which yields a number of grades of cast iron. Wrought iron is produced by puddling process from pig iron. Steel is produced from pig iron by the processes such as open hearth, bessemer, etc.

1.3.3.2

Heat Treatment

Thermal treatment of iron carbon alloy steel is necessary for better functionality of the metallic products. The strength, toughness, hardness, corrosion and surface properties, impact strength can be altered by adopting appropriate heat treatment processes such as annealing, quenching, tempering, hardening, etc. Few factors which determines the after effect of any heat treatment processes include composition of the material, heating and cooling rates and the amount of prior work on the material. All ferrous metal (cast iron or steel) are composed of binary elements which are iron and carbon. Iron in its purest form or 0% carbon content suffers a crystal structural change from a BCC lattice in its alpha form or ferrite to a FCC lattice in its gamma form or austenite when temperature is raised above 912 °C. Further raising the temperature above 1394 °C the FCC structure is re-transformed into BCC structure which is known as delta ferrite. This region does not have much significance in engineering works due to its high temperature stability [2]. In the usual iron carbon phase diagram, the right vertical line at a carbon concentration of 6.67% represents the iron carbide (Fe3 C) or pure cementite while the left vertical line represents pure ferrite. When austenite steel is cooled very slowly through the eutectoid point at 0.77% carbon content, micro-structural changes occurs from gamma grain structure to pearlitic structure which is composed of alternate

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layers (lamellae) of pearlite and cementite. Since, at 727 °C, the solid solubility of carbon in ferrite is only 0.022%, therefore the remaining carbon forms the cementite part. When the austenite having carbon content lesser than 0.77% is cooled, the microstructures formed are in pearlite and ferrite phase, the ferrite structure in this case being named as proeutectoid ferrite. On the other hand, when austenite having carbon content greater than 0.77% is cooled below the temperature 727 °C, the microstructures formed is a combination of pearlite and cementite phase, the cementite content being in this case named as proeutectoid cementite. The micro-structural changes during the transformation from austenite to ferrite and cementite depends on factors like the carbon content in the specimen, the method of heat treatment applied and the amount of plastic work or deformation subjected. The rate of cooling at the reaction site of the conversion of the austenitic phase into ferrite and cementite phase, has a direct effect on the changes in micro-structures during the transformation. These changes can be characterized by variations in the shape of the cementite laminae and by transformation of FCC lattice (austenite) into BCC ferrite lattice. Some of the microstructural changes that are commonly observed with changes in cooling rates at the reaction site are: • Pearlite: During the transformation of austenite to pearlite, if the rate of cooling at the site of reaction or at the eutectoid temperature is sufficiently high (air quenced), the microstructure of the pearlite presents an alternating thin layers of cementite and ferrite which is termed as fine pearlite. On the other hand, when the cooling rates are slow at the eutectoid temperature (furnace cooled), the microstructure exhibits thicker layers of cementite widely separated by ferrite layers. This is called the coarse pearlite (Fig. 1.1). • Spheroidite: When the pearlite is taken to a temperature of 700 °C, just below the eutectoid temperature and kept for a longer period of about 1 day time, the lamillae shape of the cementite is transformed to round or spherical shape. Since, the lamillae shape of the cementite contributes more to the stress concentration

Fig. 1.1 Pearlite. Source Michelshock, (2009), McGill University [5]

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with negligible in case of round structures, the sphroidites enable the metal to enhance in its strength, ductility and toughness. Due to ductility, the spheroidites exhibits a very high resistance to crack initiation and propagation in metal. • Bainite: Bainite is a microstructure formed at higher rate of cooling as compared to pearlite formation, in steels with alloying elements. It has the composition of ferrite and cementite as in the case of pearlite but having different morphology. Bainite is more ductile and strong as compared to the pearlite at same hardness level. • Martensite: Martensite is formed during rapid cooling (water quenched) of austenite through the eutectoid point or at the reaction site for transformation of FCC structure of austenite to body centered tetragonal structure of martensite. These are hard and brittle due to the lack of many slip systems as compared to the BCC structures and presence of hard carbon at the interstitial positions. Martensite increases in volume during phase transformation from austenite by about 4% (Fig. 1.2). • Tempered martensite: as the martensite remains hard and brittle after rapid cooling of austenite, it needs to be softened and increase in its toughness. This is done by the process of tempering where the body centered tetragonal structure of martensite decomposes into two phase system consisting of alpha ferrite and cementite at an intermediate temperature of about 150–650 °C. The cementite particles coalesce as the time and temperature of tempering is increased and are spaced at a higher distances from each other in the ductile ferrite matrix. It is due to this phenomenon, the material gets more ductile and tough. All the above described microstructures are obtained by employing few heat treatment techniques, the principle of which is mainly to alter the rate of cooling of the heated iron-carbon specimen. These are described below: • Annealing: This heat treatment process is employed to obtain coarse pearlite structure by reducing the rate of cooling at the eutectoid temperature or the reaction

Fig. 1.2 Martensite. Source Martensite rinvenuta, (retrieved on 11th feb, 2019), Wikipedia [6]

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site of the heated iron specimen. The specimen is heated in the furnace itself, maintaining a long period of time for each stage in temperature drop. The final output obtained is a ferrous metal having high ductility and toughness, reduced hardness and internal stresses, improved machinablity, etc. • Normalizing: Normalizing is a heat treatment process which is mainly employed to release residual stresses, refine grain size, improve weld joints and soften ferrous alloys. The cooling takes place at a much higher rate than in the annealing process and is achieved by exposing the heated ferrous alloy (at a temperature of about 40–50 °C above the upper critical limit of steel) to still air. The high rate of cooling enables desirable micro-structural transformation with uniformly fine grained pearlite and equally dispersed cementite. This transformation enhances the tensile strength and hardness of the alloy. • Tempering: As already described above, martensite which is very hard and brittle as a result of high rate of cooling from an austenitic temperature to room temperature, cannot be employed in applications where impact resistance is required. These martensite alloys are reheated to required tempering temperature in the range of 200–400 °C and kept at that temperature for a specified amount of time. It is followed by air cooling till the temperature reduces to room temperature. The resulting product is called “tempered martensite” and possess microstructures in the form of dispersed cementite in ferrite matrix as already described in the previous section. With higher tempering temperature, the final hardness is normally observed to be decreasing in nature.

1.3.4 Applications Steel is extensively used commercially worldwide in many industrial applications from car and space industries to medium and regular service industries. Few of the ferrous metals and alloys with their applications are given below. Grey cast irons have some of their applications in building of machine tool frames, columns and bed, rolling mills, cylinder heads, piston rings and blocks in engines, underground water or gas pipes, manhole covers, etc. White cast irons have some of their applications in building car wheel rims due to its hardness and abrasion resistance, moulds for casting of malleable iron and railway brake blocks. Wrought iron has wide range of applications in building chains, crane hooks, railway couplings, electrical conduits, rivets, bridge railings, condenser tubes, sludge tanks, fittings in welding, drainage lines, weir plates and most of the structural goods. Mild steel is used for building gears, valves, connecting rod, crankshafts, railway axles, rods, tubes and forged components. Stainless steels are used in the production of ball bearings, knife blades, springs, valves, dental and surgical instruments, utensils, etc.

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1.4 Non Ferrous Metal Non ferrous metals are also extensively used for their high corrosion resistance, low density, high electrical and thermal conductivity and ease of fabrication. These are mainly composed of metals like aluminium, titanium, copper and magnesium as the major ingredient (instead of iron in case of ferrous metals) and other alloying elements. Non ferrous metals are generally more expensive than ferrous metals.

1.4.1 Types The various types of non ferrous metals and alloys include aluminium metal and alloys, magnesium metals and alloys, copper and copper alloys, nickel and their alloys, titanium and titanium alloys, superalloys, shape memory alloys, amorphous alloys and refractory metals and alloys. The properties and applications of some of the non ferrous metals are described in the next topic.

1.4.2 Properties and Applications Few of the properties and applications of non ferrous metals and alloys are detailed below [1]. Aluminium and their alloys: Aluminium is the most important and widely used among all other non ferrous metals due to their excellent properties such as high electrical and thermal conductivity, high corrosion resistance, high strength to weight ratio, low density, low melting point, non toxicity, non magnetic, high reflectivity and ease of forming and machining. Aluminium has a high affinity to atmospheric oxygen and when it is exposed, the outer skin of the metal forms an oxide layer which remains strongly bonded to the parent metal, thus eliminating further oxidation and preventing corrosion. Some of the major applications of aluminium and their alloys are the production of packaging and containers, aircraft and aerospace applications, automobiles and buses, radiator fins, light reflectors, electrical conductors, cooking utensils, furniture and a variety of other structural components. Aluminium alloys can be easily rolled, extruded, drawn and forged in forming and forging processes. Magnesium and their alloys: Magnesium is used where the primary concern of the product or component is weight. It is light weight, fatigue resistant and ease of production. It has excellent machining characteristic utilizing 15% of the power used in case of machining ferrous metals. The major drawback of magnesium is its poor strength in pure form due to which it is generally alloyed with other elements. Magnesium has the disadvantage of its rapidly oxidising property which creates risk

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of any fire ignition. Therefore, care must be taken while carrying out machining, forming and casting operation on magnesium. Some of the major applications of magnesium alloy include its use in aircraft and space industries, missile components, bicycle, luggage, power tools, automotive industries and printing and textile industries. Copper and their alloys: Some of the characteristic properties of pure copper are its excellent electrical and thermal characteristics, corrosion resistance and ease of joining by brazing. Presence of a small amount of impurities in copper reduces its electrical conductivity significantly. Though pure copper has poor mechanical properties as it is weaker and softer, it can be alloyed with zinc, tin, phosphorus and lead to enhance in its strength, resistance to wear and thermal stability. Commonly used brass and bronze are alloys of copper with zinc and tin respectively. Applications of pure copper include its use as lubricant in hot metal forming process. Some other applications of copper alloys are in the production of utensils, decorative objects, coins, springs, bearings, heat exchangers and electrical and electronic products. Titanium and their alloys: Titanium is a white silvery metal which has excellent strength to weight ratio and resistance to corrosion even at high temperatures. Pure titanium is extremely corrosive resistant with low strength, so it is normally employed where strength consideration is the lowest. To enhance the strength, hardenability and formability of the metal, titanium is generally alloyed with various other elements such as vanadium, manganese, aluminium, molybdenum, etc. Titanium alloys are generally designed to sustain for long period of working hour at a temperature limit of 550 °C and for shorter period of working hour at a temperature limit of 750 °C. Embrittlement of titanium is a great inconvenience in the production of titanium products which reduces toughness and ductility and is triggered with surface contamination by coming into contact with gases like hydrogen, oxygen and nitrogen during the production phase. Prevention must be taken to restrict contact of these gases with the metal. Alloying materials in the titanium matrix must be precisely controlled to avoid any sensitive changes in the required properties. Titanium aluminide inter-metallic (TiAl or Ti3 Al) is a titanium alloy which is better than all other conventional alloys in providing higher stiffness at lesser density. Titanium was first commercially produced in the 1950s and finds application in a variety of industrial component manufacturing today in spite of it being expensive. Some of its applications include building of aircraft and jet engines, racing cars, armor plates, orthopaedic implants and other medical instruments, submarine hulls, marine and petrochemical components, etc. Nickel and its alloys: Nickel is used as an alloying element which enhances the strength, toughness, corrosion and wears resistances. Nickel alloys have magnetic properties due to which these are employed for electromagnetic solenoid building. Nickel alloys can maintain its strength and corrosion resistance even at elevated temperature. Nickel based alloys (also called super alloys) are generally used where high temperatures are faced in the service phase. Thus they are extensively used in the pro-

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duction of resistors, heating rods, jet engines, rockets, nuclear plant components, marine applications and food handling and processing equipments. Refractory metals and alloys: Refractory metals generally includes molybdenum, tungsten, tantalum and niobium which exhibits excellent strength even at elevated temperature, have high melting point, ductility and toughness and few of them also possessing good electrical and thermal conductivity (molybdenum). Refractory metals and alloys are extensively used in aircraft and aerospace industries in building rockets, jet engines and in tool and die making industries. Some other products are gas turbines, electronic, nuclear and chemical industry components.

1.4.3 Heat Treatment Processes in Non Ferrous Metals The general heat treatment processes for ferrous metals cannot be applied to non ferrous metals as they do not involve phase transformation like in the case of ferrous metals. However few of the non ferrous metals such as aluminium, copper, stainless steels can be heat treated over the entire volume of the metal. The heat treatment employed for these metals is generally the precipitation hardening process. Precipitation hardening process mainly involves second phase particles uniformly dispersed into a single phase matrix in a solid solution which enhances the strength and hardness. The second phase particle is an inter-metallic compound and is termed as precipitates. The increase in strength is due to restriction in the movement of dislocation due to formation of precipitates. The process is mainly accomplished by two methods which are solution treatment and precipitation hardening. In the solution treatment method, the alloy is heated to its single phase temperature and then cooled rapidly usually by immersing it into water resulting in a soft and ductile metal having moderate strength. In the precipitation hardening method, the obtained single phase metal alloy is reheated to a temperature known as precipitation temperature for a specified duration of time which is called precipitation time. After the specified duration of time is completed, the optimized size and shape of the precipitates is obtained which improves the hardness and strength of the alloy. When the precipitation process is carried out at room temperature, it is called age hardening. Sometimes, when the aged or precipitation hardened alloy is reheated in their service life (or after the precipitation phase is over), the microstructure displays oversized and lightly dispersed bigger inter-metallic particles which degrade the strength and hardness of the alloy. Case hardening: In some cases, metal alloys are required to be hard from its outer surfaces to resist any abrasion or wear and at the same time maintain toughness in the material to resist any impact load. In that condition, case hardening is the eligible operation to be performed. Case hardening is mainly employed for low carbon steel and alloy steels. Low carbon steel is generally very insensitive to all the above mentioned heat treatment processes due to lack of presence of carbon in their

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structure. The processes involved in case hardening operation includes carburizing, cyaniding, nitriding, flame hardening and induction hardening [4]. a) Carburizing: Carburizing is performed by exposing the metal to a gaseous environment containing natural gas, propane or methane in gas carburizing or packed with charcoal in pack carburizing, heated to about 870–950 °C and then kept at that temperature for a specific period of time. The process involves generation of CO at high temperature by reacting with limited oxygen. When this CO comes into contact with the surface of the heated steel, oxygen is released and the carbon is left behind, further penetrating inside with time. The released oxygen is used up again to produce CO from charcoal or the carbonaceous gas. Sometimes an “after carburizing” procedure has to be undergone due to change in grain size by prolonged heat exposure. The procedures involve quenching and tempering. Applications include making of gears, shafts, sprockets, cams, bearings and clutch plates. b) Cyaniding: In cyaniding, the alloy steel or low carbon steel is heated by immersing it in a molten bath of cyanide solution which is at a temperature of 760–845 °C along with salts to enhance the fluidity. After about 30–60 min the heated steel is lifted out and quenched in oil or water bath according to the desired quality of the resulting metal. The resultant steel is high in hardness with penetration of about 0.025–0.25 mm. The cyanide solution consists either of sodium cyanide, potassium cyanide and potassium ferrocyanide and salts used are sodium chloride and sodium carbonate. Applications include small gears, nuts, bolts and screws. c) Nitriding: Nitriding is mainly employed for alloy steels where the alloying elements remain very prone to react with nitrogen at an elevated temperature. This process does not require any after heat treatment process and a complete case hardened alloy steel can be obtained. The steel is inserted in a sealed container with ammonia gas filled inside and then heated to about 500–600 °C for specific period of time. The alloying elements such as aluminium, chromium, molybdenum and vanadium forms their nitride on contact with nitrogen which are hard and enhances strength. Case depth of about 0.1–0.6 mm is obtained in this process. Applications include gears, sprockets, fuel injection pump, cutters, valves and boring bars. d) Flame hardening: Flame hardening is employed for steels having carbon content in the range of 0.3–0.6% i.e. medium carbon steel and cast iron. The alloy is heated with an oxy-acetylene flame for a short interval of time and then water is sprayed to quench it for required hardness. Due to short interval of time, the heat is only able to penetrate at a small depth from the surface to form martensite or bainite structure after rapid quenching. Case depth ranging from 0.7–6 mm is obtained in this operation and is followed by reheating to about 200 °C for release of stress. Applications include making of crankshafts, sprocket teeth, gears, lathe bed and axles.

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e) Induction hardening: Induction hardening is similar to flame hardening process, the difference being only that the heat in this case is derived from electromagnetic induction. The workpiece is first enclosed inside an induction copper coil through which high frequency electricity is passed to heat up the surface of it for a short interval of time and then quenched by spraying water onto it. Case depth within the range of 0.25–1.5 mm and a hardness of 50–60 Rc of the outer surface is obtained by this operation. Applications involved are as same as in flame hardening operation.

1.5 Ceramic Ceramics can be considered as the most complex material composed of both metallic and non-metallic elements which results in their outstanding properties as compared to other materials. The term “Ceramic” is derived from a Greek word named “keramos” which means “potter’s clay”. Ceramic has been used since before 2000 B.C. in the making of pottery items and bricks and now hardware, electrical, automotive and space industries have found its application in a variety of their specialized products. An important combination of physical properties which distinguishes it from the other materials include its high temperature stability, high electrical resistance, good elastic modulus, softness, high chemical stability, high compressive strength and high creep resistance. Structure and properties: The unusual combination of properties of ceramics are a result of its complex structure. The structure of most of the ceramic material is in the form of crystals consisting of atoms that varies in sizes and remains strongly packed. The strength of its structure is the result of strong ionic and covalent bonds between their atoms. Due to inefficiency in their packing of the ionic and covalent structures, it generally possesses low density. Due to non availability of free electrons, it exhibits high electrical resistance. Ceramic can exist in more than one structural arrangement or forms as also can be seen in metals, the property which is called polymorphism. The silica (SiO2 ) exist in three forms which are quartz, crystobalite and tridymite. This mainly depends on the conditions of temperature and pressure. Ceramic remain in both single and poly crystalline structure and their grain size determines its strength and properties needed for a specific purpose. Finer the grain size, higher is the strength and toughness. The condition for the non crystalline structure of ceramic is termed the “glassy state” and the material is termed “glass”. Composition: The raw materials for ceramics includes clay, feldspar and flint. A common ‘white clay’ known as kaolinite is composed of structures consisting of alternate layers of silicon and aluminium ions bonded weakly. These layers on contact with the water, adsorb it on its surface resulting in slipping of the layers which makes it soft. Feldspar is composed of a variety of minerals including aluminium silicate, sodium, potassium and calcium. Flint consists of rocks of fine grained silica or AlO2 .

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Oxide ceramics can be obtained in the form of alumina and zirconia. Alumina is obtained in nature but to have control over the various ingredients in the ceramic for alteration in properties or quality, ceramics now are generally synthetically obtained by electric furnace heating and fusion of aluminium ore (bauxite), iron fillings and coke. Zirconia or Zirconium oxide is obtained by doping zirconia with oxides of calcium, magnesium and yttrium. There are also a few major ceramics in the form of carbides (of titanium, tungsten and silicon) and nitrides (of silicon nitride, titanium nitride and cubic boron nidride). Applications: Ceramics are employed in a number of electrical components as in thermistors, rectifiers, resistors or heating elements, capacitors, transducers and high voltage insulators. Structural ceramics are mainly used for its high melting temperature and high compressive strength in many mechanical applications such as spark plugs, cylinder liners, exhaust port liners, coated pistons, for materials in tools and dies, etc.

1.6 Polymers Polymers are composed of large molecules of carbon, hydrogen and other elements which are synthetically produced and can also be obtained in nature such as cellulose. Phenol formaldehyde is the earliest synthetically produced polymer which is also called “Bakelite” and is a thermoset. Other synthetic polymers include ethylene, polypropylene, poly carbonate, polyvinyl chloride and polymethylmetacrylate which are extensively used in various products today and are extracted from petroleum and coal. Structure and production processes involved: Polymers are long chains of molecules of mainly carbon and hydrogen atoms bounded by covalent bonding and are a combination of smaller structural units called mers derived from a greek word ‘meros’ i.e., ‘part’. Many ‘mer’ units combine in different orientation to give rise to a variety of polymer structures namely linear, branched, cross-linked and network polymers. These molecules differ in their property, weight and chemical stability. The weight increases with the addition of monomer units to form larger polymer molecules. Monomer units are accumulated to form polymer molecules by a reaction process called “polymerization”. Polymerization process can be classified into addition polymerization and condensation polymerization. In addition polymerization, there is no by-product and the reaction is faster than in case of condensation polymerization. Initiators are added with the monomer units to initialize the reaction which then opens up the monomer double bonds for adding carbon atoms. In case of condensation polymerization, two different monomer units are reacted to form bonds between the atoms and the reaction proceeds until all the reacting mers gets exhausted. An important characteristic of this polymerization is the formation of water as a by product which is condensed out.

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Characteristics and properties of polymer: Some of the major desirable characteristics and properties of polymers include manufacturing ease, relatively low cost, low density, chemically stable, environmentally stable (resistance to corrosion), good electrical and thermal insulators, high strength to weight ratio, reduction of noise and high thermal expansion. The undesirable properties include lack of strength and stiffness which in some applications can also be considered as desirable. Applications: Polymers have extensively replaced the use of metals in many applications in military, automotive and household equipments. Some of the applications include production of packaging, foams, television and monitor housings, lenses, gears, paints, beverage containers, medical equipments, textiles, electrical and electronic components and automotive components.

1.7 Composites Composites are a material system of a combination of two or more different phases (insoluble and chemically stable) with distinctive interfaces between them. This combination of different phase materials enhances the strength, stiffness, toughness and creep resistance of the matrix composite as compared to individual phases or materials being incorporated. Common examples of composites can be taken as the reinforced concrete (which is strengthened by the incorporation of metallic bars into the concrete matrix) and steel reinforced vehicle tyres. Types: Composites can be mainly classified based on the matrix material used. These are polymer matrix composites, metal matrix composites and ceramic matrix composites. Polymer matrix composites or fibre reinforced plastics consists of fibres in the range of about 10–60% in volume of the total volume of the polymer matrix. This percentage of volume is generally determined by the desired average separation between the fibres in the matrix and the purpose of use. The incorporation of fibres enhances its specific strength and stiffness, creep resistance, fatigue resistance and toughness. The major drawbacks associated with fibre reinforced polymer are its stiffness and strength only in the longitudinal direction of loading but fails to exhibit these characteristics in the transverse direction of loading. The reinforcing fibres commonly employed includes carbon, ceramic, glass, aramid and boron. Metal matrix composites are superior in strength, toughness, ductility and heat resistivity as compared to fibre reinforced polymers. The metals mostly used as matrix are non ferrous metals such as magnesium, copper, aluminium, titanium, super alloys and lead. The materials for fibres include graphite alumina, boron, silicon carbide, tungsten and molybdenum. In ceramic matrix composites, matrix material mainly include all the refractory materials which can sustain a temperature of up to 1700 °C such as silicon carbide, aluminium oxide and silicon nitride, while the fibre material includes carbon and aluminium oxide. A common ceramic matrix composites, namely carbon-carbon

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composites can sustain a temperature of up to 2500 °C, though it is not immune to oxidation at high temperature. Applications: Polymer matrix composites are mainly employed in the construction of military aircrafts, helicopters, commercial aircrafts, helicopter blades, rocket components, automobile bodies, helmets, leaf springs, pressure vessels, boat hulls, etc. the advantage is its low weight and stiffness. Metal matrix composites are employed in the production of satellites, missiles, helicopter components, compressor blades, engine and transmission components, antenna, etc. Ceramic matrix composites finds applications in the production of automotive components, jets, deep sea mining equipments, tools and dies, pressure vessels, etc.

1.8 Graphite and Diamond Graphite is the crystalline form of carbon which is composed of thin layers of sheets of closely packed carbon. Due to its layered structure, graphite is generally a very good solid lubricant that has numerous applications. Some of its major properties are: Mechanical property: Graphite is available in many sizes which can be square block or round shaped. The mechanical property of graphite is generally influenced by the size factor of it which is usually graded as industrial grains, fine grains and micro grains. Finer or more micro the grain is, higher is the improvement in the mechanical property of graphite. The strength of the graphite also increases with increase in temperature. Lubrication property: Graphite is a good solid lubricant but works only in a wet or air filled interfaces. The frictional resistance of a graphite increases with decrease in interface air, or in vacuum thus behaving as abrasives. Thermal and electrical property: Graphite provide a very good electrical conductivity and resistant to thermal shocks and high temperatures. It though starts degrading at temperature above 500 °C. Chemical property: Graphite has a very low affinity to reactions with chemicals. It is also employed in nuclear applications due to its low absorption cross sectional area and high scattering area for neutrons. Some of the applications of graphite are: Lampblack or black soot is an amorphous graphite which is used for pigmentation. It is used as a filter for corrosive liquid. It is used for production of electrodes in electrical discharge machining (EDM) process by impregnating it with copper. As a fibre in composites or reinforced plastics. Diamonds are covalently bonded forms of carbon which is the hardest substance known. Diamonds possess a hardness value of 7000–8000 HK and can sustain a temperature of 700 °C without degradation in contact with oxygen, though it can sustain

1.8 Graphite and Diamond

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a higher temperature in a non corrosive environment. The production technique mainly involves compression of graphite mass, under a pressure of up to 14 MPa at a temperature of 3000 °C. This process yields synthetic diamonds which are much more advanced in their mechanical characteristics and remain free of impurities. The effectiveness in grinding operation can be improved by coating diamond particles with nickel, titanium and copper. Diamonds can be applied in cutting tool applications, as abrasives in grinding wheel, in dressing of grinding wheels, as windows for high power laser path, as dies in wire drawing of wires lesser than 0.06 mm in diameter and in heat sinks for computers in tele-communication and electronics industries.

1.9 Summary From the above discussion, it is clear that all materials in the engineering application have variety of uses. Ferrous metals are the most widely used materials in industrial applications which can be characterized by their ability to exhibit variations in the properties, cost effectiveness and ease of manufacturing. Commonly available ferrous metals are cast iron, wrought iron, mild steel, stainless steel, etc. which are produced from the same initial material called ‘pig iron’ prepared by fusion of iron ore, limestone and coke in a blast furnace. Non ferrous metals on the other hand, are employed where weight of the structural material is of prime concern and at the same time which is strong, easy to work upon, corrosion resistant and other desired specific properties like refractoriness and chemical stability. Aluminium, magnesium, copper, titanium, nickel and their alloys are included under non ferrous metals which presents a number of applications as already detailed in the above sections. Ferrous metals can be suitably heat treated due to availability of reaction sites (eutectoid point) for phase transformation. The heat treatment processes include annealing, normalizing and tempering. Non ferrous metals does not exhibit phase transformation and particles of different phase called precipitates has to be incorporated into the matrix from outside for regulating strength, hardness and toughness. However, this process can only be applied in selective non metals such as aluminium, stainless steel and copper as they exhibit volumetric grain alteration upon heating unlike the others. If the metal is desired to maintain toughness through the entire volume and only the outer surface is needed to be maintaining its hardness for better wear resistance and surface finish, then case hardening is the preferred heat treatment process. Some other materials such as ceramics, polymer, composites, graphite and diamond are also described in details which also forms a part of the engineering materials in industrial applications.

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References 1. Kalpakjian, S., & Schimid, S. R. (2009). Manufacturing, engineering and technology (p. 6). Prentice Hall. 2. DeGarmo, E. P., Black, J. T., & Kohser, R. A. (2008). Materials and processes in manufacturing (p. 8). USA: Prentice Hall. 3. Singh, R. (2006). Introduction to basic manufacturing processes and workshop technology. Ltd.: New Age International Pvt. 4. Rao, P. N. (2013). Manufacturing technology (Vol. 1, No. 4). McGraw Hill Education Pvt. Ltd. 5. https://upload.wikimedia.org/wikipedia/commons/7/77/Pearlite.jpg. 6. https://upload.wikimedia.org/wikipedia/commons/9/9f/20180308_Martensite_rinvenuta.jpg.

Chapter 2

Mechanical Behaviour of Materials

Abstract Material is the most important aspect of research in all engineering domains which give us an understanding of the real physical structure of various products and objects that we come across in our daily life. The behaviour of these various products in their service life is very important to analyse as it might not be desirable to have a product fail at the most unexpected time. In order to quantify these behaviour of material, various properties have been described in detail in Chap. 1 which is determined by running few material tests. The procedure for carrying out these tests, feasibility of these test to be able to run for different materials and the behaviour of the part to be tested are some of the aspects which have been explained in detail in this chapter. Material tests have been detailed in two major branches namely, destructible and non destructible tests. Few other fracture surface behaviour in ductile and brittle material upon application of loads higher than their fracture strength and responses for plastic deformation in the form of heat generation and build up of residual stresses are described in this chapter.

2.1 Highlights The chapter includes the evaluation of mechanical properties and characteristics using destructible tests (such as tensile test, compression test, fatigue test, creep test, hardness test and impact test) and non destructible testing (such as visual inspection, radiography, ultrasonic inspection, magnetic particles, eddy current inspection and liquid penetrant method). Few other behavioural aspects of ductile and brittle material upon application of load and responses in terms of residual stress, heat and temperature during plastic deformation are discussed.

2.2 Introduction In order to obtain better functionality and durability of a product, so that customers can avail the best benefit out of it, the manufacturer needs to know the suitability of a material, to be worked upon for the product. The suitability of a material is © Springer Nature Switzerland AG 2019 K. Kumar et al., Materials and Manufacturing Processes, Materials Forming, Machining and Tribology, https://doi.org/10.1007/978-3-030-21066-3_2

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determined by its properties and behavior in the form of responses to any external forces and pressure. The functional (mechanical or thermal) and economical aspects of the product must be kept in mind as the chief parameters in deciding the suitability of the material. The properties and applications of various materials such as ferrous, non ferrous metals and few non metals such as ceramics, composites, etc. have been described in detail in the previous chapter. These properties of materials can be quantifiably represented by examining few behavioral aspects of materials, which is done by running few simple tests. These tests can be destructible or non destructible in nature. In the destructible one, the material is subjected to external forces, moments and torsions to analyse the response and quantify the behavioral parameters such as ultimate tensile strength, ultimate compressive strength, maximum shear strength, bending strength, toughness, fatigue strength, hardness, brittleness and ductility. In the non destructible testing, the material is examined by imaging the interior faults and grain discontinuity, without the job being tampered. The behavioral aspects of fracture in ductile and brittle material upon application of forces having different orientation and nature has been discussed which enables designers and manufacturers to decide for the best possible optimized product in the market.

2.3 Destructible Tests The major destructible tests for quantifying various physical and mechanical properties of materials includes tension test, compression test, hardness test, fatigue test, creep test and impact test. These are described below.

2.3.1 Tensile Test Tensile test is performed to evaluate for the static material properties such as strength, resilience, toughness, ductility, elastic modulus, maximum strain and strain hardening characteristics [1]. It quantifies these properties to compare among different materials for the most suitable material which would get synchronised with the service loading conditions of the product to be made out of the concerned material (Fig. 2.1). The method begins with the preparation of a standard specimen of rectangular or circular cross sections. The gage length of the specimen is generally taken as 50 mm and cross section diameter as 12.5 mm [1]. The operation is carried out using universal testing machine (UTM) machine having a pair of vertical jaws and extensometer (an optical, mechanical or electrical device to measure elongation). The upper jaw assembly is generally movable having a single degree of freedom movement along the axis of the bottom static jaw. The specimen is first roughened on the surfaces to be held by the jaws and then fixed on one end by the bottom jaw.

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Fig. 2.1 Universal testing machine. Source UTM ich, (retrieved on 11th Feb, 2019) Wikipedia [4]

The movable jaw is moved towards the other end of the specimen and fixed. Load is applied gradually at a pre defined elongation rate and the stress strain curve is plotted which quantifies the above mentioned properties. Accessories and controls are installed in some cases for evaluating deformation at an elevated temperature of the specimen.

2.3.2 Compression Testing Compression testing is required for the evaluation of compressive strength property of a material which is needed by designers in determining the factors in manufacturing processes like rolling, extrusion, wire drawing and all other forming processes (as the material is subjected to compressive load during these operations). A cylindrical specimen is prepared with the ‘length to diameter’ ratio lesser than 3:1 so that the specimen does not get buckled due to slenderness. The specimen is placed between two flat dies which holds the specimen from both its circular flat ends, giving an initial load. During the compression, the cylindrical stock is bulged out from its central region, the effect which is known as barrelling. The barrelling

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effect can be characterized by the difference in cross sections at the central region and both the ends of the cylindrical specimen. The end section is unable to increase in its cross section due to friction (between the flat dies and specimen ends) holding the specimen. The barrelling effect creates difficulty for design engineers to evaluate for the relation between the true stress and true strain which is often seen tackled by applying lubricants at both the interfaces between the specimen end surfaces and the flat dies. Application of lubricant enables free expansion of the specimen ends, thus maintaining a constant cross sectional area throughout the specimen length. Brittle material generally exhibits higher compressive strength as compared to its tensile strength. Ductile material, on the other hand show similar strength both in tensile and compression loading which coincides perfectly on the true stress-true strain curve.

2.3.3 Hardness Test Hardness test is a commonly employed test in industries for better products having better surface finish, wear resistance, longetivity and crack resistant. Hardness tests are simpler, non destructive, cheaper and rapid in nature which makes it a feasible operation to be carried out in all industries for characterizing materials. Hardness can be defined as the ability to resist impression by hard tools on a job surface which cannot be considered as a fundamental property of material due to a number of factors including size factor, variations in hardness value with depth and its dependency on the shape of the indenter and load application. Hardness can be tested using different machines based on the material used for the indenter and the shape of it. These are explained below. Brinell hardness test: Brinell hardness test is commonly employed for measuring the hardness values in iron and steel metals. It consists of an indenter in the form of steel or tungsten carbide balls of diameter 10 mm which presses against any smooth surfaces of the specimen with a load in the range of 500–3000 kg for a time period of about 10–15 s [2]. The diameter of the impression formed is measured using microscope and with the data for load application and indenter diameter, the brinell hardness number can be evaluated. Two types of indentation shapes are generally seen, one of them of which is round in shape on annealed metal and the other having sharp features in cold condition. The formula for brinell hardness value is evaluated using the formula given below: BHN =

2P  √ π D D − D2 − d 2 

(2.1)

where, BHN = Brinell hardness number, P = load applied (kg), D = diameter of the indenter (mm), d = diameter of the impression made by the indenter (mm).

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Hardness test cannot be considered accurate due to elastic deformations on steel balls under high load, though it can be avoided to some extent by using tungsten carbide balls having higher values of elastic modulus and higher resistance to distortion [1]. Rockwell hardness test: Rockwell hardness test was developed by S. P. Rockwell in 1992 and it differs from the brinell hardness test based on the physical dimension of the impression considered for measurement of hardness. While in the brinell hardness test the diameter is considered, in rockwell hardness test the depth of the impression is considered [1]. In rockwell hardness test, the indenter is made of a conical shaped diamond with an angle of 120° between the opposite surfaces and having a diameter of 0.2 mm. At first, a minor load (around 10 kg) is applied on the specimen surface to eliminate the effects of any surface defects and rust. The penetration depth of the indenter at this position is measured. Major load is then applied for a time period of about 15 s which gives the final penetration depth. The difference in the penetration depth between the major and minor loads gives the measure of hardness. Softer materials are generally indented using steel balls of diameter 1.5 mm [2]. Rockwell hardness test has been incorporated in a number of industries dealing with metals and the feasibility of the test can be realised considering their accuracy, simplicity, rapidity and smaller impression produced. Vicker’s hardness test: Vicker’s hardness test or diamond pyramid hardness test is a technique to measure a wide range of hardness values of hard and small cross section materials including heated steel using a square diamond shaped indenter having an angle of 136° between opposite faces. The load applied is in the range of 5–120 kg on polished surface of the specimen which mainly depends on the thickness and hardness of it. Vicker’s hardness test is similar to brinell hardness test in that both the techniques consider the area of the indentation and the load applied in evaluating for the hardness value, thus having the same unit for both the hardness scale. The hardness value from both the scales does in fact coincide to values up to 400. The vicker’s hardness value or diamond pyramid hardness (DPH) is evaluated as given below: D P H = 1.8544

P d2

where, P = load applied (kg), d = diagonal of the square shaped impression made by the square pyramid (mm) [2].

2.3.4 Fatigue Testing Fatigue testing is mainly employed for machine elements or components in products that are subjected to cyclic loads such as dies, tools, springs, shafts, gears, cams, etc. These cyclic loads have the effect on the failure characteristics of the components which is much lesser in stress limit or amplitude stress at which the component can

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operate for unlimited numbers of cycles, as compared to the yield strength in static tensile testing. The stress amplitude for an infinite number of cycles is termed as endurance limit which generally ranges in 0.4–0.5 of the tensile strength in carbon steels. The testing is accomplished considering three states of stresses which are tensile stress, bending stress and compression stress. Stresses are being applied on cylindrical metallic bar which are completely reverse in nature, from the limit of maximum tensile stress to maximum compressive stress and so on. In some cases, bending of the cylindrical bar placed horizontally and rotating about its axis at the same time, is utilized to generate alternate tensile and compressive loads on each fibre in the bar. In many occasion, the specimen is tested with both static and fluctuating load subjection (Fig. 2.2). Whatever might be the nature of testing of the specimen for fatigue strength evaluation, the end objective is to plot the S-N curve with logarithmic values of stress amplitude as abscissa and logarithmic value of the number of cycles as ordinate. It is observed in most of the materials, that the number of cycles sustained by a specimen decreases with increase in stress amplitude (or fluctuating load) but below a certain limit of stress amplitude, the number of cycles sustained by the specimen exceeds a limit which may be considered equivalent to infinite based on its desired service life. This stress amplitude is called endurance limit of the material. Aluminium on the other hand, does not have a distinct endurance limit and follows a downward trend in S-N curve until fracture condition is reached.

Fig. 2.2 Diagram showing the S-N curves of aluminium and steel. Source S-N curves, (retrieved on 11th Feb, 2019) Wikipedia [5]

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2.3.5 Creep Test Creep may be considered a high temperature phenomenon where metals subjected to temperature of about 40% of their melting point elongates under static load after a period of time. The creep gets even more significant and important to consider at temperatures higher than 40%. The rate of creep and the temperature at which creep occurs, varies with materials as such lead is observed to get elongated under its own weight at room temperature whereas aluminium alloys tends to get elongated at a temperature of around 200 °C, while refractory materials at 1500 °C. Creep test is performed by subjecting a constant tensile load on a specimen exposed to a higher temperature and a graph is plotted which is called creep curve with increments in elongation or strain on its ordinate axis and time period on the abscissa axis. The time period may extend to a few hours or few days or even few years. The creep curve generally can be divided into four regions namely, instantaneous elongation, primary creep, secondary creep and tertiary creep (Fig. 2.3). The instantaneous elongation is the initial elongation in the specimen when loaded with a tensile stress which is supposed to be constant throughout the test. Primary creep is the region of decreasing slope of the curve which generally denotes a gradual reduction of work hardening rate. The curve exhibits a constant slope in the secondary region which can extend to a longer period of time. During the tertiary phase, the rate of elongation increases significantly which leads to necking or reduction of cross sectional area of the specimen, until it fractures.

2.3.6 Impact Test Impact test is usually carried out for components that are expected to get subjected to dynamic or impact loads in their service life. It is generally employed to test impact

Fig. 2.3 Creep curve showing the three stages of time period during elongation of material. Source Stage creep, (2009) Wikimedia common [6]

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toughness in notched specimen by a hammering effect of the pendulum so that its fracture strength can be evaluated for component design purposes. Operations like drop forging and bolt heading involves components that are subjected to impulsive force on a regular basis. On the basis of beam loading i.e. simply supported and cantilever, impact tests can be divided into charpy test and izod test respectively. In charpy test, the specimen by both its ends is fixed and the notched region remains in the centre. The presence of notch facilitates accurate identification of the breaking point and the amount of energy absorbed by the specimen. A pendulum with its bob weight is pulled to some height and released to impact the specimen just behind the notch. After the specimen breaks, the pendulum rises on the other side by some height which is noted. The potential energy that was available at the initial height subtracted by the inertia energy of the pendulum while rising to some height on the other side gives the impact strength of the specimen. The working principle remains the same in the izod test, difference being only that the arrangement of the specimen is in the form of cantilever beam with notched region touching the edge of the fixture grip. Impact tests are generally useful for determining the ductile-brittle transition temperature of materials. Materials with high impact strength are generally tough and ductile.

2.4 Non Destructible Tests Non destructible testing (NDT) is employed to detect flaws or cracks in the component without getting it damaged in the process like in any other destructible testing methods. These techniques require excellent operator skills and experience to identify faults which remains undetected to an unskilled personnel as the result is quite subjective and interpretation is difficult. Some of the NDT techniques are described below [2].

2.4.1 Visual Inspection Visual inspection is carried out to detect surface defects or faults that are visible to unaided eyes or through a magnifying glass or microscope. It is the preliminary common step in almost all testing operations and requires a specialized set of skills to identify faults such as weld joint flaws, composite structural flaws, poor fits, missing of screws and bolts, cavities, dents, dimension errors and poor surface finishes.

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2.4.2 Radiography Radiography is a technique of irradiating small wavelength of electromagnetic waves such as x-ray and gamma rays. In X-ray radiography, the specimen is bombarded with X-ray radiations which travels through the interior structure of the specimen and then gets impinged on the photo sensitive film. Higher the density of the specimen, lesser the radiation through it resulting in darker shade on the film. This property of the x-ray enables it to detect material flaws, voids and inclusions. Inclusions generally display a darker shade on the photo sensitive film due to less amount of x-rays passing through it while on the other hand, voids allow excess rays to pass through it to produce a light shade on the film. X-ray radiography is generally large and bulky and can detect voids having dimension of up to 0.5 mm but are unable to detect voids lesser than that. It is employed in laboratory tests as it produces sharper pictures and facilitates higher control over the intensity. Gamma rays have relatively shorter wavelength and can be used to detect flaws in thicker sections.

2.4.3 Ultrasonic Inspection Ultrasonic inspection generally employs piezoelectric effect in some crystals like quartz, ceramics or lithium sulfate which produces a high frequency sound wave in the range of 1–25 MHz. This high frequency sound wave travels through a medium of water, glycerin, grease or oil into the specimen to be reflected back by interior material discontinuities and then received by the crystal probe itself in the form of sharp peaks on the cathode ray oscilloscope. These peaks in energy and the time taken to get reflected back to the receiver, gives an indication of the size and location of the interior flaws. Ultrasonic inspection facilitates smaller voids, cracks or other minute flaws to be detected in thicker sections with ease and requires operator skills and experience to perfectly interpret the results.

2.4.4 Magnetic Particle Test In a magnetic particle test, the surface or near surface flaws of a ferromagnetic material such as iron, cobalt, nickel or their alloys are detected by placing an electromagnet near to it. Iron fillings are spread over the surface of the specimen to be detected and when electromagnet is bought near to it, these particles align themselves based on the magnetic field lines. In presence of any faults or discontinuity in the material, the particles on the region gets distorted in uniformity, outlining the edges of the fault which is much more visible than the original micro crack (invisible to visual

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inspection). It is the most commonly employed test in industries due to its low cost and good sensitivity. The magnetic lines of force are generally aligned perpendicular to the defect for higher sensitivity.

2.4.5 Eddy Current Testing Eddy current testing involves the principle of electromagnetic induction, where an alternating current is passed through a conductor and a fluctuating magnetic field is generated which induces a current on a second conductor placed close to the first conductor. Presence of faults on the second conductor surface causes a distortion in the flow of current which in turn affect the current on the first conductor, thus detecting discontinuities. The apparatus consists of a probe which acts as the first conductor and is placed close to the conducting surface to be analysed. The signal is picked up by the same probe, evaluating the distance of different regions on the conducting surface from the probe and identifying defects such as cracks, voids and grain size alteration. Some other important detectible characteristics in eddy current testing are thickness measurements, heat damage and conductivity of the specimen. Limitations of eddy current testing includes requirement of an accessible surface to be analysed, interference due to surface roughness and cracks, requirement of a conductive specimen, requirement of skilled personnel and limited penetrating depth.

2.4.6 Acoustic Method Acoustic method generally involves stressing of the specimen elastically or tapping the specimen which releases acoustic waves to be received by the piezoelectric sensors. Two methods are generally involved in this technique, which are acoustic emission and acoustic impact technique. In acoustic emission, the specimen is elastically deformed as in bending of the beam, pressurizing of vessels and torque application in shafts. Few sources of acoustic emission include crack initiation and propagation, cavitation due to formation of bubbles, plastic deformation, phase transformation and grain re-orientation. Acoustic impact technique employs the operation of tapping the specimen to listen to the noise emitted by different region on the specimen surface. The specimen is generally standardized to eliminate the effect of mass and dimension of the specimen and the noise received is analysed for any flaws and discontinuities.

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2.4.7 Liquid Penetration Method In this method, surface is prepared first (cleaned thoroughly but delicately) and made free of any dirt, coatings, grease, paint, etc. The specimen surface is first washed with alcohol and kept still for 5 min to dry. Next, the liquid penetrant is applied onto the surface of the specimen and brushed delicately to enhance the penetration. The liquid penetrant infiltrates into the cracks and voids on the surface (or into a limited depth of the specimen) and is left to dry for about 20 min. The surface is then cleaned with textiles and made free of extra residue using remover spray. After waiting for some minutes, developer spray is then applied from a distance of about 30 mm from the specimen surface which absorb the solidified penetrant, forming a negative of cracks and faults on the surface.

2.5 Failure Characteristics in Materials Failure can be considered the end of product life cycle or any structural build, signifying total damage of the material by either breaking into two (or more) or buckle due to longitudinal loads (in case of slender columns). On the basis of the type of damage of the specimen, failures can be classified into buckling and fracture. Buckling is caused by exerting a load longitudinally on a slender column which tends to bend the column generating a couple which further intensifies with increase in the deflection, until it finally fails. Fracture is caused by stress applications on a body which separates it into two or more than two pieces.

2.5.1 Fracture Types Fracture can again be divided into two, namely, ductile failure and brittle failure [1] (Fig. 2.4).

2.5.1.1

Ductile Failure

Ductile failure is mainly caused by stressing a body beyond its plastic deformation state, resulting in coalescence of micro voids into larger void which develops necking on the body, until it fractures completely. The material fracture for a ductile material is generally along the maximum shear plane which presents a unique form of fracture surfaces on application of tensile and shear stresses. Since, ductile material is weaker in shear, on application of tensile stress, the fracture surface exhibits a cup and

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Fig. 2.4 a Brittle fracture upon tensile load b semi-ductile fracture upon tensile load c ductile fracture upon tensile load. Source Ductility, (retrieved on 11th of Feb, 2019), Wikipedia [7]

cone feature (denoting fracture planes along 45° to the load application) while on application of torsion, the material fails along the plane perpendicular to the axis of the twist.

2.5.1.2

Brittle Failure

Brittle fracture has the tendency to propagate along certain crystallographic planes called cleavage planes with little or without the plastic deformation stage, upon application of stresses. Since brittle material is weak in tension, upon application of tensile stress, brittle material fractures along the plane perpendicular to the stress direction, exhibiting a granular form of fracture surface. On the other hand, upon application of torsion, brittle material tends to fail along the plane at an angle of 45° to the torsional plane. Brittle fracture is generally observed in body centred cubic structure and hexagonal packed structure lattice materials and it further intensifies with decrease in temperature and increase in deformation rate.

2.5.2 Plastic Deformation Responses When the material is plastically deformed, some responses are felt in terms of material discontinuity and temperature effect. These are described below.

2.5 Failure Characteristics in Materials

2.5.2.1

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Residual Stresses

Residual stresses are generally caused by non uniform plastic deformation of a body upon application of an external load and the effect is observed after the load is released. The common example can be taken of a bending beam where the beam is plastically deformed by exerting a moment. After the moment is released, the beam regains its form by elastically recovering to its desired state, with residual stresses being developed inside of the material. These residual stresses tend to balance itself to reach an equilibrium state of condition and when there is unbalance in the compressive and tensile residual forces in the specimen (due to removal of material by grinding or machining), the beam gets warped. Residual stresses can also be developed due to non uniformity in temperature gradient caused by the heat treatment process and then rapidly cooling the specimen which triggers expansion and compression in the grains of the material. Tensile residue stresses on specimen surfaces are generally not desirable in any structural or mechanical parts as it loses the capacity to withstand higher tensile stress. Whereas, on the other hand, compressive residual stresses are desirable as it enhances the fatigue strength of the material.

2.5.2.2

Heat and Temperature Responses

Plastic deformation is directly related to the generation of heat and rise in temperature of the specimen material. Almost all of the mechanical work in stressing the material is converted into heat energy during plastic deformation phenomenon with few percentage of about 5–10% of the total energy (30% in case of few alloys), stored in the material as elastic energy. The heat generated during the plastic deformation phase is lost to the outer environment through coolants, tools and dies. The heat loss can be minimized by increasing the rate of deformation due to which the material does not get enough time to transfer its generated heat into the surrounding. While on the other hand, heat loss is maximum when the deformation rate is slow. The rise in temperature (T ) of the material during frictionless plastic deformation can be calculated as given below, assuming total conversion of the work to heat. T =

u ρc

(2.1)

where, u = specific energy (work per unit volume), ρ = density of material and c = specific heat.

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2.6 Summary From the above discussion, it has been observed that the properties and behaviour of materials which are identified by employing specific destructible and non destructible tests and enables designers to make the suitable material choice for its products. Tension tests are performed to evaluate for the properties like elastic modulus, ultimate tensile strength, yield strength, ductile, brittle, etc. while fatigue tests are performed to evaluate for the behaviour of material in cyclic loadings where machine elements or components are constantly being subjected to repeated loads and can cause failure at a much lower stress amplitude than in the case of static loading. Endurance limit is generally determined using the fatigue test which ensures safety of the component for infinite number of repeating cycles of load. Hardness tests have been divided into brinell, rockwell and vicker’s hardness test which employs the same principle of making impressions on hard surfaces using indenters that has been pressed against the specimen and evaluating for the hardness. Non-destructible tests have their own set of advantages over the destructible ones as it does not require the tool to have direct contact with the specimen and the evaluation of properties can be accomplished without even damaging the specimen. Micro cracks, flaws, discontinuities in the grain structure and shape and presence of voids on specimen surface or to a limited depth from the surface can be evaluated using the non destructible tests which employs energy sources such as ultrasonic sound waves, x-rays, gamma rays, magnetic particles and chemicals. Some important ductile and brittle material behaviour upon application of different forms of loads such as shear, tensile or torsion are discussed which determines the fracture surface characteristics of the materials. Responses in plastic deformation loading in the form of residual stresses, heat and temperature are discussed which provide us a solution to incorporate these responses in further design and material research.

References 1. Kalpakjian, S., & Schimid, S. R. (2009). Manufacturing, engineering and technology (p. 6). Prentice Hall. 2. Hazra Choudhury, S. K., & Hajra Choudhury, A. K. (1986). Elements of workshop technology. In Manufacturing processes (Vol. 1, No. 10). Media Promoters and Publishers Pvt. Ltd. 3. DeGarmo, E. P., Black, J. T., & Kohser, R. A. (2008). Materials and processes in manufacturing (p. 8). USA: Prentice Hall. 4. https://upload.wikimedia.org/wikipedia/commons/4/46/UTM_ich.jpg. 5. https://upload.wikimedia.org/wikipedia/commons/3/30/S-N_curves.PNG. 6. https://upload.wikimedia.org/wikipedia/commons/thumb/4/4d/3StageCreep.svg/2000px3StageCreep.svg.png. 7. https://upload.wikimedia.org/wikipedia/commons/thumb/b/be/Ductility.svg/1065px-Ductility. svg.png.

Part II

Conventional Manufacturing

Chapter 3

Casting

Abstract Casting is the most widely used manufacturing process which involves melting of metal ores, scrap metals, carbonaceous material, alloying elements, fluxes and other additives in hot furnaces from where the liquid metal is collected by the ladle to be poured over the pouring basin, delivering the molten metal into the mould cavity. After pouring, the metal solidifies slowly inside the mould cavity. The solidification starts with the material phenomenon called nucleation which triggers growth of solid particles (transformation of liquid metal into solid crystals with release of energy) into fine/coarse grains possessing individual boundaries. During this solidification phase, all metals tend to shrink in their volume but which vary with different metals and alloys. This solidification phenomenon governs the design of various components of casting such as riser and gating system and also influences the dimensions of the mould cavity required for a particular metal cast. Allowances are provided on the patterns itself which are copies of the final cast. Moulding sand is prepared, the properties of which affects the quality of the final casting and that is dependent on different proportion of combinations of ingredients such as clay content, moisture content, carbon additives, etc. In this chapter, the basic understanding of the casting process, operations and design involved in the sand mould casting process, various metal mould casting processes and few other commonly used casting processes are described in detail.

3.1 Highlights The chapter begins with the basic fundamentals of casting; various operations involved in pattern, mould and core making under the heading of sand moulding; designs of gates and risers; permanent mould casting operations and few other commonly used processes like investment casting, plaster mould casting and ceramic mould casting.

© Springer Nature Switzerland AG 2019 K. Kumar et al., Materials and Manufacturing Processes, Materials Forming, Machining and Tribology, https://doi.org/10.1007/978-3-030-21066-3_3

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3.2 Introduction Casting is the initial process in any material processing procedures where the scrap metals and raw metals along with various additives are melted in a furnace which is then poured into the mould for solidification and obtaining the desired shape. Parts having complex sections can be easily moulded using this process and are directly used as metal stocks for other operations such as machining and forming. Mechanical working of metal is important for obtaining a product, enhanced in strength and hardness. All casting operations that will be discussed in detail in the chapter can be employed for producing all types of complex parts. But the main concern of the designer lies in the economic and financial considerations of the operation where selection of a proper casting process influences the optimality condition of the entire manufacturing unit. The chapter describes in detail the various types of casting processes such as sand casting, metal mould casting and few other processes that are commonly used in industries. Solidification factors, fluidity, pouring temperature and properties of the sand mould are some of the physical-thermal factors which determine the criteria for design of gates and risers and in adopting various mould and pattern construction operation.

3.3 Fundamentals of Casting After pouring the molten metal into the mould cavity, solidification process initiates, the quality of which depends on factors like ratio of the volume to surface area of the casting, casting process employed, flow temperature of the melt inside the mould cavity, mould and gating system design, composition of the metal and pouring methods. These factors can alter the micro-structural characteristics of the final casting from the desired properties which results in poor product or component quality. So, a proper knowledge on various aspects of casting is needed by all designers to produce a defect free casting and that can easily be worked upon for proper functionality and durability of the final product. Some of the major factors in casting can be described as below.

3.3.1 Solidification of Metal Solidification of metal generally involves two micro-structural phenomenon that holds the trigger to generate different combination of properties in them. These can be termed as nucleation and growth of grains in the material. Nucleation is the phenomenon where micro solid particles build up from their molten state due to release of heat, indicating more solid stability and marks the start of the solidification phase. The phenomenon involves generation of surfaces to mark the territory of each grain

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in the metal. The surface generation at the interfaces of the molten metal and the solid particles requires energy which is derived from the surrounding superheated melt. It is due to this reason, that nucleation starts at a temperature below the equilibrium melting temperature [1]. Nucleation triggers the formation of grains and crystals, the sizes of which differs with the rate of temperature drop. Higher the temperature drop, smaller is the grain size leading to fine grain that enhances strength in the material. Lower rate of temperature drop leads to coarse grain formation, reducing the strength of the material. In order to obtain finer grain, the molten metal is generally mixed with solid particle impurities that enhance the uniformity in the distribution of nucleation sites. Nucleation is followed by grain growth, the direction and type of growth of which can be controlled by the above explained cooling rates. Directional solidification is an important phenomenon during solidification that needs to be taken care of while designing moulds, cores and risers. In order to compensate for the solidification shrinkage during solidification, it is desired to initiate grain growth from the extreme end of the mould away from any open surface or heat reservoir such as riser which ensures continuous feeding of the melt from the riser into the mould cavity. Solidification process of metals can be well represented with the help of cooling curves which is evaluated by recording the temperature of the molten metal during solidification and plotting it with time. The superheated metal is poured into the mould cavity which flows through every section and corner, gradually cooling in the process until the melting temperature is reached. At this temperature, the nucleation begins to occur resulting in formation of grains and at the same time releasing heat in the form of latent heat. The temperature remains constant for sometimes and when the solidification is complete, the temperature drops to room temperature. The time taken from the start of the pouring process of the molten metal to the complete solidification of the material can be termed the total solidification time while from the start of the nucleation to the complete solidification, it is termed as local solidification time. The solidification time mainly depends on the geometry of the casting which determines the amount of heat that is expelled out of the solidified metal. The parameter that influences this rate of solidification can be given by Chvorinov’s rule which suggests the solidification time (ts) as directly proportional to the square of the ratio of volume to surface area (V/A) of the casting. ts = B where ‘n’ holds the value from 1.5 to 2.

 n V A

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3.3.2 Fluidity of Molten Metal Fluidity is the ability of the molten metal to flow through all the sections in the mould cavity with ease. The design of the mould cavity must be such that the molten metal is able to fill in all the regions in the mould cavity and then freeze to solidification gradually from the extreme corners away from the riser. Defects like misrun and coldshut are associated with the reverse of the above mentioned sequence where the molten metal freezes before completely filling the cavity and results in lack of sufficient molten metal for compensation of the solidification shrinkages and non compatibility of the fluid at different temperatures to fuse, arriving at a common location inside the mould cavity from two different direction. Since, fluidity has not been accepted to be measured by a single method, standard mould remains as the vital method for measuring fluidity of various metals.

3.3.3 Pouring Temperature The cooling curves for solidification of metals suggest a superheated pouring temperature leading to increase of the time period of the freezing range and fluidity in melt. The melt is able to fill in all sections and corners of the mould before it finally freezes for solidification. However, pouring temperature must be controlled and should not exceed a certain limit, as it accelerates metal-mould reaction inside the cavity and also enhances fluidity which may penetrate deep into voids and pores in the mould sand. The result is the embedded small sand particle on the metal surface.

3.4 Sand Mould Casting Process Sand casting process is the earliest used technique of casting metal parts by pouring molten metal into sand moulds that are designed considering various factors as described above to make it free from all defects and selecting proper pattern-mould design and material. Sand casting process is divided into 4 processes. These are described in detail below.

3.4.1 Pattern Making Pattern can be considered as a duplicate copy of the final cast to be produced. The mould cavity is given its final shape by ramming the mould sand over the designed pattern placed inside of a flask. All the allowances in the cast such as machining allowances, solidification shrinkage allowance, draft allowances, etc are to be

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41

accommodated in the pattern itself so that the mould cavity can be build for the final desired cast size. Pattern making mainly involves material selection for the pattern and types of pattern to be used. These are described below: 1. Material selection: Material selection for patterns mainly depends on factors like number of moulds to be produced, type of material to be cast and processes adopted in mould making. Wood is the most commonly used pattern material which is preferred due to its low cost and ease of producing [2]. The drawbacks of wood pattern can be realised by its tendency to warp and wear on repeated use of it and on using it for large castings. The condition of wood worsens if the storage is maintained under humid air which induces warping on re drying. Metal patterns are more suitable in this case which has no tendency to warp and remain dimensionally stable even after repeated use, but it is costly. Hard plastic (eg: urethane) is another option for building patterns, but in this case the mould is made of organically bonded sand particles which stick to the pattern material. Expanded polystyrene is used in full mould process and wax for investment casting [1]. 2. Types of patterns: Patterns can be classified based on the their number of parting interfaces and which is dependent on factors like number of duplicate casts to be made and complexities in section design of the cast. Few of the major patterns are described below. Single piece pattern is employed where the dimensions of the cast is not complex, does not include any intricate sections and number of duplicate casts to be produced is small. These patterns are not attached to any frame or plate so these are also called loose patterns. Single piece patterns are used where large casting is to be made with simpler shape as these are much cheaper than other patterns. Single piece patterns with one of their sides flat can be placed over the follow board directly which acts as a parting plane and later on removed easily. Patterns having no flat sides are a bit difficult to part for which the moulder has to estimate the parting plane to be selected for cut and make a free hand cut. Moulder needs to cut their own gates, runners and risers in the mould. Split patterns are used when the required cast consists of intricate sections, which cannot be used as a single pattern and therefore needs to be split into two. These patterns can be split into two (symmetrical as possible) equal halves, both of which represents the cope and the drag portion of the cast. The procedure begins with placing the drag pattern on top of the follow board and surrounding it on all sides with the drag portion of the flask. Moulding sand is poured over the pattern and rammed. The flask is then inverted and pattern taken out. Similar steps are also followed with the cope pattern. The cope and drag portions of the flask are then fastened with pins and holes, after the parting surfaces have been properly sprinkled with parting sand. Match plate patterns are generally used when small metal parts in large number are required to be produced by simultaneously loading liquid metal into all mould cavities. The process can be automated and is cheap if the production requirement is high. It basically involves split patterns mounted on a plate. The plate is generally made of metal or wood while the patterns are always made of metal. Each of the two

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halves of the pattern is mounted on each side of the plate and all other components such as runner and gates are attached to the drag (lower) side of the plate. After the match plates are taken out of the mould, it drags out the patterns along with it and both the cope and drag portions of the mould are matched perfectly. Cope and drag patterns are used where there is a need for the production of large castings. A dual match plates are taken onto which cope and drag parts of the patterns are mounted. This enables independent production of cope and drag portion of the mould and simultaneously can be produced by workers, enhancing productivity. Fillets are an important feature on pointed corners of the pattern so that the cast metal does not crack due to accumulation of residual stresses on account of solidification shrinkages. The designer must consider fillets, the most commonly used radius of which is taken as 1/8 and 1/4.

3.4.2 Mould Making Mould making generally involves two major processes, namely sand conditioning (and testing) and mould construction. 1. Sand conditioning and testing: Sand conditioning is the process of mixing the ingredients of sand (sand, clay and water) in desired proportions for the best quality of the moulding sand and to exhibit the required properties needed for obtaining a sound casting. Silica (SiO2 ), zircon and olivine are few sands that are mixed with various other ingredients and additives. The properties that mainly affect the quality of moulding sand are refractoriness, permeability, cohesiveness and collapsibility. Refractoriness is a structural property of sand which enables sand to resist high temperature and remain intact during pouring of molten metal. Permeability depends on the size of the grains in the moulding sand. It basically represents the amount of air that can pass through the mould sand particles for a specific time period if the test is carried out of a standard specimen. Finer the grain size of the sand particles, lower is the permeability whereas coarser the grain size, higher is the permeability. Cohesiveness is determined by the amount of clay content and water, acting as a bonding agent between different moulding sand particles. It is the main property which binds the sand particles together to attain its desired solid shape. Clays can be in the form of bentonite, illite or kaolinite. Collapsibility is the property of the moulding sand to remain in shape and does not promote inclusion of sand particles onto surfaces of the casting during solidification shrinkage. Collapsibility is improved by adding organic material such as cellulose. These carbonaceous material on the inside surfaces of the mould burns during pouring of molten metal to release carbon dioxide leaving few vacant pores and voids within the mould surface material which makes it weak and solidification shrinkage can then be easily achieved without any sand inclusion. A good moulding sand must consider the above mentioned four properties while processing the mixture of sand, clay and water. The size of the sand grains, the amount

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of clay acting as a bonding agent and the moisture content for forming thin layers around the clay particles which remain adhered and enhances cohesiveness between the particles, are few of the control parameters that must be regulated while mixing the various ingredients of the moulding sand. But moisture content in excess must be avoided which can lead to reduction in collapsibility. Moulding sand generally are reused again and again for production of casts, though a small volume of this sand is burned during pouring of the melt as to improve collapsibility which is discarded and new sand is incorporated to fill the vacant volume [2]. Various sand testing procedures are available to assess the different characteristic of moulding sand such as moisture content, sand grain size, clay content, permeability, compressive strength and compactability. Grain sizes are usually determined by pouring and letting the sand particle to pass through a series of different sized mesh. These standard sized meshes gives the grain fineness number based on the weight of the remaining particles on each sieves. Moisture content is evaluated by drying off a 50 g sample of moulding sand and measuring the weight after the application of heat (temperature of 110 °C) for sufficient duration of time. Clay content is measured by washing a 50 g sample of moulding sand with water containing alkaline solution of sodium hydroxide. The sand is washed again and again until all of the clay gets removed out. The final remaining sand is weighed again to evaluate the amount of clay that was present in the original sample. Permeability test is conducted to evaluate for the amount of air that can be passed through the voids and pores within the moulding sand with sufficient ease. It is an important property which ensures the proper release of the gases from the mould cavity during pouring of the molten metal into it. A sample of rammed moulding sand is taken inside of a tube across which a pressure of 10 g/cm2 is maintained. By noting the time for complete transfer of air from one side to the other side of the compacted sample, the flow rate can be determined which gives the permeability number. Compression test is performed on compacted moulding sand under a load of 10–30 psi until the specimen breaks. Compactability of moulding sand is determined by pressing a fixed volume of sand under a load of a standard weight striking 3 times with it. The change in height of the sample specimen inside of the tube to the original height multiplied by 100 gives the percentage of compactability. 2. Mould construction: Mould construction involves mould making processes which can be broadly classified into hand moulding and machine moulding [2]. When there is a requirement of casts that are small and for a small lot production, hand moulding is generally preferred. Moulding sand is hand rammed slowly which is first poured over the pattern enclosed by a flask on all sides. Small foundry shops still employ this traditional approach of hand moulding that turns out to be quite affordable. Moulding machines are used where the production lot size is big and repeated casts are needed to be produced. It consumes less time, can produce accurate castings and requires no special skills. The procedure for mould construction begins with placing the pattern at the bottom of the flask generally made of aluminium or magnesium, which is then filled with moulding sand. The sand is levelled from the top of the

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moulding flask and then placed on machines. These machines can be classified based on the way of packing the moulding sand into jolt and squeezer machine. In the jolt machine, the pattern loaded moulding flask filled with sand is taken to some height and dropped repeatedly so that the sand particles get enough flexibility in arranging themselves to correct packing density. The sand gets rammed, filling in every void space inside of the mould. The major drawback of this process is that while the sand particles around the pattern or the lower part of the mould remain closely packed, the upper part of the mould sand is less dense and loosely packed. In the squeezer machine, the moulding sand inside of the flask is compressed between the squeezing head and the table on which the flask is placed. The sand layers close to the squeezing head is compressed to high density while the sand particles close to the pattern, remains loose which results in uneven distribution of density of moulding sand inside of the flask. Another type of machine called jolt-squeezer machine, eliminates the drawbacks of both the above described machines to obtain a high quality and uniform density mould. A flask filled in moulding sand is first taken through the jolting action which packs the bottom sand in correct density and then is taken for squeezing action. The squeezing action packs the sand from the top as well to form a uniformly rammed and hard mould.

3.4.3 Core Making Cores are an added feature on moulds which facilitates incorporation of holes in the final cast product. It is an economical process which eliminates the need of the machining process in making holes on cast parts. Few holes that have a tapered shape from both the cope and drag sides can be obtained by selecting a hollow pattern design which is buried under green mould sand to obtain a mould cavity with green sand cores. But as the hole becomes narrow and straight, the risk of mould break upon withdrawing the pattern from the mould, gets more severe which is a costly affair. Green sand cores have relatively less strength and can collapse if the core is less tapered or straight. Dry sand cores are generally made separately from the remaining mould construction in small core boxes where the sand mixed with binders is packed, baked and then coated for refractoriness and smoothness such as graphite, silica or mica. These dry cores are then mounted inside of the mould cavity with the help of the core prints. Baking is required to cure the binders to form cross linked polymers which strongly bind the sand grains. The cores are subjected to a temperature of about 230 °C in the hot box method, where catalyst in contact with the heated surface activates and cures the binder within 10–30 s. Baking ovens can be classified into batch type core ovens, continuous type core ovens and dielectric bakers. Batch type core ovens employ the use of drawers and racks which store the cores for baking in batches. Continuous type core ovens are generally used for high rate production where the racks holding the cores are moved

3.4 Sand Mould Casting Process

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through the heated oven for a specified amount of time. Dielectric bakers enable fast production of cores which is achieved by passing high frequency current through the dielectric cores in between two cement bonded asbestos plates acting as electrodes.

3.4.4 Casting Casting is the final step, where molten metal is poured into the prepared mould with embedded cores for obtaining the final cast. Apart from mould cavity, runners and gates, few other components such as sprue, pouring basin and risers are also cut from the mould which facilitates pouring of the molten metal, avoiding air inclusions while moving through the tapered sprue, maintaining a smooth laminar flow throughout the mould and replenishing during solidification shrinkage. Some design considerations have to be considered while designing for these components such as riser design and gating design which affects the flow pattern of the melt and maintains a smooth delivery of molten metal into the mould without turbulence and inclusions. These are described below. Design of casting: The major design of the casting includes gating system and riser system. Gating system mainly includes pouring basin, sprue, sprue well, runners and gates. These components are shown below (Fig. 3.1). The basic functions of the gating system are to reduce turbulence, maintain fluidity of the molten metal during casting and avoid inclusions of sand and air. Turbulence in the metal flow can be the main factor for air inclusion and entrapment of mould material in the casting which is caused by rapidly filling the metal into the mould, facilitating absorption of gases to produce cavitation, promoting oxidation and eroding away mould material from the surface of the sprue, runner and mould cavity which ultimately gets deposited in the cast. To avoid turbulence, short sprues

Fig. 3.1 Gating system [4]. Source Casting gating system (2009) Wikimedia commons

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are adopted which minimizes the distance of fall of the molten metal from the pouring basin to the choke area of the sprue. Sprues are made tapered as the velocity of flow increases with depth and larger choke area would accelerate entrapment of gases and air from the surrounding resulting in blow holes on the surfaces of the final casts. The pouring basin is made rectangular to avoid vortex generation of flow while metal flows through the sprue, that leads to air absorption. The velocity of flow at the choke area is the maximum which cannot be directly fed to the mould due to risk of erosion. Runners are installed to avoid this by increasing the time of flow gradually slowing down for a smooth flow into the mould through gates. Fluidity of molten metal can be hampered by slow filling of the molten metal into mould cavities. Slow movement of the flow also promotes temperature drop by expelling heat from the walls of the runner and sprue and defects like misrun and cold shuts can occur. Trapping of sand particles and dross is another concern for designers which is tackled by employing long flat rectangular runners which gives sufficient time for the low density material to rise above to the top of the molten metal and installing runner extension and well to trap oxides of metals and sand particles eroded during the initial pouring of the molten metal. The gates can also be installed with ceramic filters and screens to trap foreign particles. Molten metal can enter into the mould cavity by three different ways of gate designs, each of them of which has its own advantages, disadvantages and compatibility issue with the desired cast to be produced. Top gate system facilitates quick delivery of molten metal into large moulds maintaining an excellent fluidity until the mould is completely filled. In some cases, top gates can also replace risers both of which are supposed to protrude out from the mould cavity. As the hottest molten metal remains at the top, proper directional solidification towards the riser can be achieved and at the same time oxide layers build up at the top, which prevents the contact of the atmospheric air with the molten metal layers underneath. Top gates generally are installed with ceramic strainers at the bottom of the pouring basin which controls the flow of molten metal and collects impurities. Top gates are generally located wholly in the cope part of the mould cavity. Bottom gates are generally employed to reduce the turbulence of the molten metal during entering into the mould cavity. The molten metal is fed from the bottom of the mould which rises gently inside the mould cavity, releasing heat to the mould wall at the same time. This hampers the directional solidification towards the riser, which is located far away from the gate (where the metal is the hottest). Due to this reason, bottom gates are not generally employed for large castings and only find applications in small castings. Parting gates are located in between the cope and the drag portions of the mould or at the interface between the two portions which facilitates easy capture of impurities and sand particles from the molten metal after leaving the sprue and before entering the gates. In some cases, metal can also be directly poured into the riser which promotes directional solidification towards the riser. Parting gates facilitates easy maintenance of the mould and the cleaning process is much easy.

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Riser design is another design consideration that must be kept in mind while going for an efficient and effective casting. The main objective of providing riser in the casting is to act as a reservoir of molten metal for solidification shrinkage and to promote directional solidification. Metals have a unique property to lose its volume during transformation from liquid to solid state. This loss of volume is called the solidification shrinkage. After the liquid metal has been completely filled into mould cavities and riser, the metal solidifies which triggers solidification shrinkage. It is always desirable for the cast metal to solidify first followed by the liquid metal in the riser. In that way, the riser can supply the adequate liquid metal to replenish the casting during solidification shrinkage. In order to obtain slow rate of cooling of the liquid metal in the riser and to enhance the yield of the casting operation, the riser needs to be as small as possible and at the same time must possess a minimum surface area to volume ratio, according to Chvorinov’s rule. Though spherical shapes of riser would yield the best design in this case, there are difficulties in their production and does not prove to be a feasible one where mould needs to be prepared multiple times. Cylindrical shape comes next to a spherical shape in terms of having minimum surface area, the riser of which can also be constructed with minimum time and ease. Cylindrical risers can be adjusted in its height to diameter ratio depending on the nature of the alloy (their solidification shrinkage), flask size requirement and location of the riser. Based on the location on the mould, risers can be classified into top riser and side riser. Top risers are generally located on the top of the mould cavity in the cope region of the mould flask and are cylindrical in shape. Top riser facilitates maximum space utilization within the mould, requiring less space. It has shorter feeding distance and are acted upon by two forces, namely atmospheric pressure force and metallostatic (weight) force. Side risers are generally located near to the adjacent side of the mould cavity (occupying more spaces) and are made spherical which remains completely covered with the moulding sand as a result of which only the metallostatic force of the liquid metal comes into play during supply of the melt for solidification shrinkages.

3.5 Metal Mould Casting Process Also known as permanent mould casting, these casting processes consists of metal moulds that are re-usable, provides high dimensional accuracy and are fast. Few of the processes in this category includes [3].

3.5.1 Slush Casting After the molten metal is poured into a mould, the solidification of liquid metal starts from the surfaces of the mould which forms a thin layer around. Slush casting utilizes

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this characteristic of metal-mould interaction to build hollow metallic casts with lesser importance given to the quality. These mainly include decorative, ornamental products and toys. Liquid metal is poured into the permanent moulds and after holding for few seconds, it is poured out. A thin layer is formed on the inside wall of the mould which hardens and is taken out by opening the mould halves.

3.5.2 Die Casting Die casting is a permanent mould casting where molten metal is poured into a shot chamber and pressurized by pistons or plunger to flow through the gooseneck channel and nozzle into the permanent die moulds. This process is used for casting high quality products, having high strength and that is devoid of any defects. The process is less time consuming, does not require much human labour and are generally automated or semi-automated. Metals having low to medium melting temperatures such as magnesium, lead, tin, zinc and aluminium are generally cast using this method. Based on the mounting of the furnace whether or not attached along with the feeding unit, pressure die casting can be classified into hot chamber die casting and cold chamber die casting. Hot chamber die casting is generally used to cast products made out of low melting temperature metals and alloys such as magnesium, lead, tin, zinc. Aluminium is not cast using this process due to its high melting temperature. The hot chamber die casting involves a heating flask (furnace) into which a shot chamber (vertically) and gooseneck channel is submerged below the surface of the molten metal that is being heated constantly. The liquid metal enters into the shot chamber and when it is time for the operation, the plunger moves down to pressurize the liquid metal (at about average value of 15 MPa) into the die moulds. The plunger is then held at the innermost position for sometimes until the metal in the mould solidifies completely and no solidification shrinkage volume is left unfilled. The cast is then ejected out by ejector pins after opening the mould halves. The plunger reverts back and repeats the process multiple times at the rate of about 200–300 shots per hour. The final cast is of high surface finish, less porosity, compact and defect free. Cold chamber die casting is generally employed for casting products made of higher melting temperature metals and alloys such as aluminium, magnesium and copper based alloys. The principle of injecting the liquid metal into the die moulds remains the same as in the case of hot chamber die casting, difference being only that the furnace is not equipped along with the feeding unit and aluminium metal has to be externally heated, melted and poured into the horizontally inclined shot chamber. The plunger pressure ranges in 20–70 MPa delivering a pressurized liquid metal into the mould.

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3.5.3 Centrifugal Casting Centrifugal casting basically employs centrifugal force to feed liquid metal into the moulds or to shape cylindrical products (by direct pouring of the molten metal into the mould). The final product obtained is of high quality with uniform density and impurity free. Centrifugal casting can be of three types based on the type of the cast to be produced, amount of centrifugal force required, mould design and purpose. These are described below. True centrifugal casting is generally employed to build cylindrical parts such as pipes, barrels, bushings, lamp posts, etc which consists of a cylindrical horizontal mould made out of steel iron and graphite with refractory layers being coated on the inside wall of the mould. The molten metal is poured directly into the rotating cylindrical mould and the quantity required is determined by the thickness of the cast to be produced for a fixed length. The molten metal is held to the inside wall of the cylindrical mould under a pressure of about 150 G and high density metal accumulates far from the centre of the cylindrical mould, while the low density and light impurities accumulates towards the centre. Cylindrical parts with thicknesses 6–125 mm and diameter of 13 mm–3 m, having length upto 16 m can be produced using this technique. Semi centrifugal casting involves a pouring basin, top gate and a general mould with cope and drag arrangement, which is symmetrical along a vertical plane. The entire arrangement is rotated (vertically) at an RPM very much lower than in true centrifugal casting while the liquid metal is poured over the pouring basin that is centrally located. The centrifugal force feeds the metal into the mould and a good strength casting is obtained. Centrifuge castings are generally employed for moulds having intricate sections where the liquid metal cannot be delivered by the normal operation of pouring and requires a pressurized system to do it. The moulds can be of any shape that are placed at some specified distance from the centre of rotation. The moulds are connected through runner channels to the centrally located sprue which delivers the molten metal into the mould by centrifugal action. The properties of castings in all the processes mentioned above vary with the distance from the centre of rotation.

3.5.4 Continuous Casting Continuous casting is mainly employed for mass production of metal stocks by an uninterrupted casting process, feeding the liquid metal into the mould and withdrawing the semi solidified cast with the help of series rollers, at the same time. The mould is generally of a rectangular, square or circular cross section and is made of copper which is kept cool by installed cooling channels around the mould wall. Complex cross section parts with large length can be obtained which later can be cut to individual pieces. Parts produced from this operation are generally taken directly

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for forming processes or machining and the desired standard cross section of the stock can be controlled by altering the mould design.

3.6 Investment Mould Casting Investment mould casting process is also called the lost wax casting due to involvement of wax patterns which are later melted out after the mould has been constructed around it. Wax patterns are first prepared in mould blocks, onto which a slurry coating is applied and then a coating of silica sand mixed with binders and other additives to form a layer of refractory mould around the wax pattern. Fine sand is coated at the starting face of the pattern so that a good quality cast with better surface finish is obtained. The mould thickness can be increased rapidly by repeatedly coating it with relatively larger size sand particles. After the coating is dried in air, the mould is heated to about 90–175 °C by placing it inverted so that the melted wax can be easily removed. The mould is then further taken to a temperature of about 650–1050 °C where the mould is dried off completely, eliminating the water produced from chemical interaction and any remaining waxes. This multi-cavity mould is then ready to be poured with molten metal. After the cast is cooled completely, the mould is broken [3].

3.7 Plaster Mould Casting Plaster mould casting is mainly employed for making locks, gears, valves, tools, ornaments, etc, or products having lesser weight than 10 kg. It involves the use of plaster of paris (gypsum) which is mixed with silica flour and talc in desired proportion to influence the time of set and desired strength. The prepared ingredients are mixed with water to form a slurry, which is then poured over the pattern made of aluminium, plastic, etc to take the shape of it and dried at a temperature of about 120–260 °C. The mould halves are joined and preheated for pouring in liquid metal. The major disadvantage of this process is the lack of permeability which hinders the removal of gases produced inside the mould cavity from the pouring molten metal. Therefore pressure casting is generally used to cast parts using plaster mould casting. Molten metal delivery under vacuum condition or using foamed plaster with air cavities to enhance permeability can also be done for better quality, dimensional accuracy and surface finish. Plaster moulds have a limited refractoriness and burns if the temperature exceeds above 1200 °C. Therefore, these moulds are mainly used for casting metals having medium melting temperatures such as aluminium, magnesium and copper alloys. It has a poor conductivity as a result of which the metal gets cooled very slowly and the grain growth in the material is uniform.

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3.8 Ceramic Mould Casting Ceramic mould casting involves refractory materials such as fine grains of zircon, silica, aluminium oxide mixed with binders which is poured over the pattern for complete mould preparation. Due to the use of these refractory materials, this process can be employed for casting higher melting temperature metals and alloys such as ferrous metals, stainless steel, etc. After the mould is prepared, it is heated to high temperature which bakes the mould, drying off the volatile impurities in it. The mould is then poured with molten metal which on cooling down produces a high quality cast with high dimensional accuracy and surface finish. Parts like impeller, moulding dies and machining tools are manufactured using this process.

3.9 Summary From the above discussion, it is clear that various factors that comes into play during the casting process such as fluidity of molten metal, pouring temperature of the metal, directional solidification, etc are all related to the same contraction behaviour of metals during solidification from liquid to solid state. Various design alteration in riser and gates are all the outcomes of this phenomenon of solidification in metals including material consideration which has the effect on the dimensions of the mould on account of varying solidification shrinkage volumes with different metals. While considering a production system, various other economic and financial factors also has to be considered while designing for the mould, patterns and cores that form the basic components in a sand casting operation. A different variety of casts having complex shapes and dimensions can be produced and is most commonly preferred casting methods in various industries along with other casting methods involving metallic moulds or permanent moulds. Sand casting procedure begins with the pattern design which involves selection of different materials for pattern construction and selection of the types of patterns such as single piece, split, match plate, etc. Metal, plastic and wax are the most commonly used materials for pattern making which provide a good dimensional accuracy, smoothness in the cast surface and surface strength to resist any deformation during sand ramming. Mould making involves preparation of moulding sand and ramming the sand over the pattern which is placed at the bottom of a drag flask, flasks having two halves for production of cope and drag parts of the mould separately and integrating into one at the end. Various mould making machines such as jolt machine, squeezer machine, etc are implemented in various industries for rapid and ease of production. Mould making is followed by core making which involves material selection and processing techniques, considering the high exposure of the core material to the extreme heat of the molten metal.

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Various other casting processes involving metal moulds are discussed in detail such as die casting, centrifugal casting, continuous casting, etc and few other processes employing ceramic and plaster moulds such as ceramic mould castings, investment castings and plaster mould casting.

References 1. DeGarmo, E. P., Black, J. T., & Kohser, R. A. (2008). Materials and processes in manufacturing (p. 8). USA: Prentice Hall. 2. Hazra Choudhury, S. K., Hajra Choudhury, & A. K. (1986). Elements of workshop technology. In Manufacturing processes (Vol. 1, No. 10). Media Promoters and Publishers Pvt. Ltd. 3. Kalpakjian, S., & Schimid, S. R. (2009). Manufacturing, engineering and technology (p. 6). Prentice Hall. 4. https://upload.wikimedia.org/wikipedia/commons/thumb/a/ac/Casting_gating_system.svg/ 2000px-Casting_gating_system.svg.png.

Chapter 4

Forming

Abstract Right from the advent of industrial revolution, forming process has been the most important and versatile operation in any manufacturing and metal working industries. Metals are needed to be deformed for a variety of functional requirements in the product or for enhancing the strength and hardness. This deformation produces a complex flow of material which basically is dependent on the amount of force application, ductility of the material, temperature of the material and modulus of elasticity. Deformation can be in the form of bulk deformation under compressive loads or bending, stretching, shrinking or shearing in the sheet metal operations based on which different tools (rollers in rolling, dies and punch) have to be designed accordingly for operations such as blanking, extrusion, wire drawing, rolling, etc. The temperature of the metal has to be maintained such that sufficient plasticity is induced and can be operated with the given capacity of the machine. Few positive aspects of forming processes include insignificant wastage of material, high output material strength, dimensional accuracy and simpler mechanism.

4.1 Scope The chapter mainly deals in the mechanism behind the different forming operations in the two different sections of the types of forming processes namely, bulk forming and sheet metal forming. The hot and cold working conditions of metals in the deformation process are described in detail.

4.2 Introduction The records for the forming processes such as wire drawing and rolling has been found to be extensively used in the middle ages and even further. The basic principle of material deformation has remained same throughout history, but in recent times

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with the advent of the industrial revolution, a lot of developments in determining the nature of the deformations, controlling the material flow and integration of machines and even computer operated automated machines, has been introduced for obtaining the optimized quality of the product and to synchronize with the other available manufacturing processes in the industry [1]. Forming process involves material deformation under extreme compressive load to transform the material to its final shape. The deformation mechanism utilizes the plasticity characteristics of the material which is easy to attain if the material is very ductile and for other high strength metal and alloys, it can be achieved by heating the metal above the re-crystallization temperature. Compared to the other manufacturing operation as in casting and machining, forming process does not produce any wastage of materials, the technique is simple and easy to operate as there is no requirement of handling any molten metal and parts with high dimensional accuracy and strength can be obtained with relative ease. Forming process lags behind in their expensive use of machineries, requirement of large forces and non economical in mass production of parts.

4.3 Hot Working and Cold Working Hot working of metal is generally conducted prior to the cold working so that sufficient improvements in grain structure or uniformity is achieved before the cold working process is implemented on the metals with the aid of taking the metal to a temperature which is above the re-crystallization temperature. Hot working temperature generally differs with material to material based on the composition of the metal, pre worked conditions and impurities contained. The re-crystallization temperature generally determines the range of working temperature that a metal can be worked upon. Hot working enables high deformation in the material flow without significant effect of strain hardening upon application of external pressure. If not hot worked, the strain hardening will persist in the plastic deformation region, resulting in a brittle material. Hot working condition reduces the forces required for deforming the metal as compared to the cold working condition but fails to maintain the dimensional tolerance and surface finish in the metal [2]. After gaining uniformity in the grain structure following the hot working of the metal, the metal is cold worked for dimensional accuracy, good surface finish and increased strength. Cold working is carried out at room temperature or at a temperature below the re-crystallization temperature ensuring a plastic deformation with no recovery effect. Forces required for cold working are higher than in the hot working process.

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4.4 Types of Forming Forming can be mainly classified into bulk forming, which exhibits the volume constancy condition to shape the metal volume into desired shape, and sheet metal forming where bending and shearing are the dominant features. These are described below.

4.4.1 Bulk Forming 4.4.1.1

Rolling

Rolling is the process by which initial wrought stocks are given their finished shapes by making it pass through a pair of power driven rotating cylindrical rollers of designed capacity. The finished shapes can be in the form of blooms, billets and slabs based on the gap between the rollers [1]. The process begins with the hot rolling, ensuring uniform grain size transformation, followed by cold rolling or machining into finished products. Blooms generally are larger in size and can be of square or rectangular cross section with the width not exceeding twice the thickness and the thickness greater than six inches. Billets can be of square, rectangular and circular cross sections which are smaller in size than the blooms. Slabs are having rectangular cross section with width greater than twice the thickness and can be further cold rolled into sheets, foils and plates. Billets and blooms are rolled into bars, rods and other structural components or can be directly shaped into its finished product such as pipes, wires, tubes and rails (Fig. 4.1). The mechanism behind the rolling process involve frictional force that drives the initial work metal between the rollers, compressing and lengthening the work to maintain volume constancy in the work material and finally delivering after the required thickness of the metal is achieved, to the feed rollers. To analyze the rolling mechanism analytically, a number of assumptions are considered which facilitates an approximate determination of force and power requirements during the operation [3].

Fig. 4.1 Rolling operation [5]. Source Rolling, (2007) Wikipedia

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The assumptions taken are the width of the stock which generally is taken to be constant throughout the length (or reduction in the thickness of the sheet is replenished by the increase in length of the stock), rolls are rigid and straight, uniform co-efficient of friction (and low) across the whole contact interface and constant yield strength of the job which is mainly the average of the yield strength of the work material at the start and end of the rolling.

4.4.1.2

Extrusion

Extrusion is the process where a solid billet is forced to move through die openings which can be of different shapes based on the requirement of the dimensions of the final product. During the process, the billet surface is subjected to compressive stress from the die surface (that remains rigid and hard) to deform the metal plastically. Metals having a relatively higher ductility are often extruded at room temperature and differ from few other metals which need to be extruded at an elevated temperature to achieve ductility. Based on this, extrusion can be divided into hot extrusion and cold extrusion. Commonly used metals in extrusion are copper, aluminium, magnesium, lead, steel, etc. (Fig. 4.2).

Fig. 4.2 Extrusion process showing the blank, die opening and the final product [6]. Source Extrusion, (2006) Wikipedia

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Based on the relative motion between the chamber and billet surface, extrusion can be classified into direct and indirect extrusion. In direct extrusion or forward extrusion, a hydraulically powered ram is employed to force the billet pass through the die opening and frictional forces act between the surfaces of the chamber wall and the billet. A dummy block is used to protect the tip surface of the ram from the heat of the billet in case of hot extrusion by placing it between the ram and the billet. In indirect extrusion or backward extrusion, the die end is pushed against the billet to extrude out the metal through the die openings. There is no relative motion between the billet and chamber surface and no frictional forces are generated. Few high strength steels are extruded using this technique. Hydrostatic extrusion is another technique where the billet is smaller in size than the chamber and the space between them is filled with incompressible fluid which exerts hydrostatic pressure on the surface of the billet upon application of ram pressure, forming it to required shape. The force (F) required to extrude out a metal (or extrusion force) to its final shape can be approximately calculated using the expression as given below.   A0 F = A0 kln Af where, A0 and Af are the initial and final cross-sectional areas of the billet respectively, k determines the strength of the work material and the frictional characteristics between the billet and the chamber surface. Few other factors that affect the extrusion forces are the temperature of the billet and the extrusion rate.

4.4.1.3

Drawing

Drawing is the process where the plastic deformation of the metal takes place which are initially in the form of sheets and plates and are deformed between the punch and the die to give it the desired 3D shape having certain depth. The procedure is accomplished by carrying out the drawing operation in multiple steps of reduction in diameter and increase in depth. The depth of the cup is generally calculated approximately as the half of the difference between the diameters of the blank and the punch and is affected by defects like wrinkles on the surface of the finished cup and tearing of the blank in the region between the punch and die diameters at the bend area. The procedure begins with the placing of a thick heated blank for a hot working process on top of a female die. A male punch is then pressed against the blank which draws the plate, lengthening and thinning the metal as the punch goes down into the female die. This process is repeated with successive die punch combinations having a reduced diameter of punch and die but with greater depth until the desired thickness and depth is obtained. For thinner blanks, cold working is generally employed to shape the metal into its desired dimension. The thickness is generally reduced a little or not at all in case of cold drawing.

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Fig. 4.3 Wire drawing [7]. Source Wire drawing, Wikipedia

In some other drawing process, the continuous thick wire is made to pass through die opening and is pulled under tensile load into smaller diameter wires and fibres. Unlike extrusion, this process is a continuous process with thick wire continuously being fed through the die. The material is plastically deformed and due to this act of pulling the metal through the die opening under cold working condition, the process is called wire drawing. The force required in wire drawing process can be calculated as similar to the extrusion process with reduction of cross-sectional area, tool-work interface frictional characteristics, work material, drawing speed and die angle as process variables (Fig. 4.3).

4.4.1.4

Forging

Forging is the process by which metals are plastically deformed to give their desired shape by hammering the specimen with various tools under hot or cold working condition depending on the ductility of the metal whether sufficient for working at room or elevated temperature. Cold forging generally yields product with high dimensional accuracy, good surface finish and good strength. Hot worked products fail to maintain a good dimensional tolerance and surface finish, but force required is lowered as compared to the cold working condition due to enhancement in ductility at elevated temperature. Forging is extensively used in today’s industries in the making of large rotors for turbines, shafts and gears, hand tools, bolts, rivets, railroad components, structural components, etc. and the advantage lies with the ability to change the grain structure as suited functionally and plastically deforming to any shape enhancing its strength, durability and toughness. Forging can be followed by some finishing operations depending on the requirement of the grain structure and dimension, such as heat treatment and machining. Forging can be carried out in two ways. Namely open die forging and closed die forging. In open die forging, metal bars and cylinders are placed in between two flat dies which exerts a compressive load altering the height or thickness of the specimen.

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Along with the decrease in height, the cylinder tends to form a barrelling effect on its cylindrical surface which occurs due to the frictional force generated at the top and the bottom of the specimen. This top and bottom surfaces of the specimen which are under high compression, are unable to move in the radial direction while the central part expands freely, causing a barrelling effect. In an ideal condition with frictionless surface between the job and the dies, the material freely flow radial at the top and bottom surfaces of the specimen causing a uniform increase in diameter throughout the length with corresponding decrease in height. Effective lubricants can be used at the interfaces to achieve the frictionless conditions. Barrelling effect can also be seen during open die forging of hot specimen with cold dies. The top and bottom surfaces of the job cool down rapidly reducing the ductility of the material and increasing the resistance to movement, causing the material at the central region to bulge out. In a closed die forging, the material is compressed between a pair of shaped dies which deforms the material to fill the mould cavity entirely. The job is basically heated to its plastic state so that forces required is reduced and ease of material deformation is enhanced. Due to extreme pressure, the extra work material that exceeds the volume of the mould cavity tends to flow outward through gaps between the dies which are generally called flash. As the volume of the flash increases with pressure being exerted in the mould cavity, the frictional forces generated between the flash material and the die gap facilitates building up of more pressure to force the material into the entire cavity. In a perfectly closed die forging, there is no gap between the dies during compression and so the formation of flash is avoided and the material completely fills the mould cavity. For this, precise volume of the work material is to be decided beforehand so that there is shortage or excess of material and required dimensions of the product is obtained. Forces required for forging in a closed die types is high and can be given by the expression as given below [4]: F = kY f A where ‘k’ is a multiplying factor obtained from experiments. ‘Yf ’ is the flow stress of the work material at the specified temperature and ‘A’ is the total interface area between the work material and die.

4.4.2 Sheet Metal Forming 4.4.2.1

Shearing

The initial stock for the sheet metal forming operation is obtained from the rolling process which are then are taken in between punch and dies where blanking and punching operations are carried out by shearing the metal. Shearing of the metal starts from the crack generation at the top and bottom of the sheared region on the blank over the clearance gap between the die and punch. The cracks propagate from

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both directions resulting in complete separation of the blank from the sheet with shiny edge formation caused due to the sliding of the blank edges against the die surface. The clearance between the punch and the die serves the prime factor in determining the quality of the edges of the output blank. With larger clearance gap, the bending of the sheet metal occurs which deforms the material along the gap and a precise cut is not obtained. The edges become rough and need arises to carry out secondary operations to remove the excess material. The edge roughness can be reduced by adopting a specified clearance gap and increasing the punch speed which can be as high as 10–12 m/s. Due to high punch speed, the strain rate increases and the strain becomes sufficiently high for work hardening to occur which tends to make the cut region brittle and thus break off with the minimum deformation. It is commonly observed that the shiny burnished region does not extend throughout the entire thickness and depends mainly on the blank material ductility, punch speed, thickness of the blank and clearance. With increase in punch speed, the sheared area is reduced and lesser deformation zone is obtained leading to smoother surfaces of the blank edges devoid of any burr. Most commonly used shearing operations include punching and blanking. In punching operation, the sheet with the punched hole is the required product as output and the blank is left out as wastage or as materials for other forming processes whereas, in blanking operation, the blank is the required out product while the sheared job is discarded.

4.4.2.2

Bending of Sheets

Ductile sheets can be bend in many ways to achieve different requirements of a product. Sheets can be bent to provide stiffness (seams, beads, etc.) in the product, to provide structural strength, for a cost effective production and for securing the edges in cans, cylinders, etc. Bending of sheets mainly involves elastic deformation of the fibres on the outer and inner layers of the sheet or blank, the outer fibres being at tensile loading and inner one at compressive loading, under a limit of bend radius below which the material remain in elastic condition and no crack is initiated. Due to elastic stretching, the poisson’s ratio comes into effect and the fibres on the outer bend surface tend to contract in its width, while the inner fibres tend to expand. This creates a non uniformity in the cross sectional area along the bend profile. The neutral axis along the central layer of an ideal bend sheet does not deform and the length is assumed to remain fixed which is called bend allowance. Bend allowances facilitates estimation of appropriate blank size for the bending operation. The expression for bend allowance can be given as below [4]: L b = α(R + kt)

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where α is the bend angle, R is the bend radius, t is the thickness and k is a constant ranging from 0.33 to 0.5. k equals to 0.5 is the most ideal condition where the neutral axis is completely aligned at the centre. The bendability of a metal sheet mainly depends on the thickness of the metal sheet and on the tensile reduction of area in the fibres (achieved under high working temperature or pressure). With increase in tensile reduction of area, the minimum bend radius also reduces to zero, i.e. the sheets that can be bend over itself. With increase in thickness of the sheet, the minimum bend radius also increases proportionally i.e., the bendability decreases. Bendability of a metal sheet is reduced with factors that induce brittleness such as edge roughness, inclusions of hard particles, edge hardness from the shearing operation and any prior cold working procedures. Springback is a common elastic phenomenon in bending operation of metal sheets and plates which tend to decrease the bend angle of the plates as a recovery after being plastically deformed to a specified bend angle. Springback effect mainly depends on the modulus of elasticity of the metal, higher the elastic modulus lower will be the springback effect. Other factors which influence the springback effect are the bend radius to thickness (R/T) ratio and the yield stress of the material. Upon increasing these parameters, the springback effect is found to be larger. The solution to the springback effect is to overbend the metallic sheet so that after the springback effect, the material attains the desired bend angle.

4.4.2.3

Spinning of Metal Blanks

Spinning is a process where axis-symmetric products that are made out of metallic blanks by mounting the blank onto a mandrel (having the profile of the final product), rotating the mandrel and at the same time deforming the blank to the profile of the mandrel using a relatively simple tool made of tool steel. The process is economical and time saving and is commonly used in industries in the making of missile nose and rocket engine components. The operation can be carried out in cold condition with the exception of the thick plates which are difficult to deform at room temperature due to less ductility and are generally hot worked. Parts as large as 6 m in diameter are generally shaped using this process. Spinnability is a measure of the ability of the material to get reduced in thickness during a spinning operation without any crack formation. Spinnability is generally dependent on the tensile reduction of area as in the case of bending. As the tensile reduction of the area or ductility is reduced (or in case of high strength steel or alloys), the metal blanks are first heated to elevated temperature to build ductility in the material and are then spinned. For tube spinning, the hollow cylindrical blank is fitted over a solid round mandrel and rotated. Spinning tool is used to reduce the thickness of the cylindrical blank and shape it into desired length and profile.

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Deep Drawing

Deep drawing has already been described in the “drawing” section which is basically deforming a metal blank in between the die and the punch and where the depth of the cup formed is larger than the diameter of the punch or die. Springback effect is a major disadvantage in a deep drawn cup which tends to reform the blank back to its original position. This is tackled by considering straight dies and punches for drawing the whole length of the cup in a single operation instead of drawing in steps of reduction of the die and punch diameters. But in such cases, difficulty arises in carrying out the forming operation and therefore, a relief angle of at least 3° is provided on the wall of the die to maintain a simpler and easy deep drawing operation. The formed cups are gone through further ironing operation to eliminate the springback effect.

4.4.2.5

Embossing

Embossing is a shallow sheet metal forming process which generally is used to form impressions of letters or different shapes on the surface of the blank or sheet. The thickness of the sheet remains intact and the depth of the impression ranges in only two to three times the thickness.

4.5 Summary From the above discussion it is clear that ductility of material plays an important role in defining the complex material flow in the forming process. The materials lacking in ductility are generally formed at an elevated temperature so that necessary plasticity can be achieved in the material for ease of deformation and reduced force requirement. Various mechanisms of deformation in the forming operations are described in the chapter in two different sections which are bulk forming and sheet metal forming. Bulk forming includes rolling, forging, bulk drawing, extrusion, etc. which deal with the entire volume of the work part and compressive forces are applied to deform the material. Sheet metal forming includes shearing, deep drawing, spinning, bending and embossing which deal with 2D circular or rectangular metal sheets or blanks. Shear, bending and tensile force mechanisms are the dominating characteristics in sheet metal forming. Various defects in the sheet metal drawing and bending are wrinkle formation and springback effects which are tackled employing different mechanisms as explained in the chapter.

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References 1. DeGarmo, E. P., Black, J. T., & Kohser, R. A. (2008). Materials and processes in manufacturing (p. 8). USA: Prentice Hall. 2. Hazra Choudhury, S. K., & Hajra Choudhury, A. K. (1986). Elements of workshop technology. In Manufacturing processes (Vol. 1, No. 10). Media Promoters and Publishers Pvt. Ltd. 3. Ghosh, A., & Malik, A. K. (2005). Manufacturing science. New Delhi: East-West Press Pvt. Ltd. 4. Kalpakjian, S., & Schimid, S. R. (2009). Manufacturing, engineering and technology (p. 6). Prentice Hall. 5. https://upload.wikimedia.org/wikipedia/commons/thumb/b/ba/Rolling.png/1280px-Rolling. png. 6. https://upload.wikimedia.org/wikipedia/en/c/c0/Extrusion.JPG. 7. https://upload.wikimedia.org/wikipedia/en/thumb/f/fd/Wiredrawing.svg/1280px-Wiredrawing. svg.png.

Chapter 5

Welding

Abstract Welding is an important process in manufacturing which enables large and complex shaped products to be divided into small and simple parts to be manufactured independently and later can be rigidly joined by fusion of the material at the interface between two metal pieces, stiff enough to operate as a single piece. Welding process is mainly characterized by the melting of the interface material between the two metals, fusion or mixing of the material and then solidifying to produce a rigid and high strength joint. In some cases or applications, even an axial or shear pressure may accompany the fusion phenomenon to trigger a plastic deformation of material at the interface for tight bonding. Some other important materials used in welding are filler metals and fluxes. Filler metals are employed to provide the same material (as that of the base metals) at the interface gap between them in molten condition. Fluxes are employed to remove oxide formations in the weld pool and other impurities and form a layer of slag above the molten pool, protecting it from further oxidation. In this chapter, the fundamental concept behind the various welding processes and how are these classified based on different heat source utilization and techniques of utilization are explained in detail.

5.1 Highlights This chapter basically deals in the fundamental concepts of various techniques and processes employed in welding, the application of these processes and their classification based on different techniques of heat generation.

5.2 Introduction There is a need of large size, complex and intricate shaped components that are difficult to manufacture as a single piece. To avoid such problems in material handling, the components are generally manufactured in parts which are much simpler to produce individually and can be later assembled by joining into the final products. Welding mainly employs the phenomenon of metal fusion at the interface between two fitted © Springer Nature Switzerland AG 2019 K. Kumar et al., Materials and Manufacturing Processes, Materials Forming, Machining and Tribology, https://doi.org/10.1007/978-3-030-21066-3_5

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workpieces which upon solidification forms a rigid and strong joint. In many case, pressure is being applied along with the fusion process to produce a defect free and tight joint. Other joining process apart from welding, includes soldering, brazing, adhesives and mechanical fasteners. Soldering and brazing are employed when there is a need of a strong joint between two dissimilar metal without the fusion of the base metal and by only melting the filler metal to fill the gaps between the work pieces. Welding processes like any other manufacturing process employ numerous techniques and types, each of which is unique in their utilization of the heat source, weld joint strength, ease of manufacture, welding speed and job materials. These processes can be mainly classified into 3 categories namely, fusion welding, solid state welding and soldering-brazing. Fusion welding basically utilizes the phenomenon of melting of metal surfaces at high temperature for joining and includes gas welding, arc welding, resistance welding, etc. On the other hand, in solid state welding the material is not melted by any heating source and the joining is done by applying external pressure or forces on the either side of the metal-metal interfaces. The extreme pressure deforms the interface material plastically to fuse together into one. Common examples of solid state welding are cold welding and friction welding. In the 3rd category, soldering and brazing employs the phenomenon of adhesion and diffusion between the molten filler metal and the faying surfaces of the fitted metal interface.

5.3 Welding Processes Based on the characteristics of the heat source, welding processes can be classified into gas welding, arc welding, resistance welding, thermit welding, friction welding and laser beam welding. These are described in detail below.

5.3.1 Gas Welding In Gas welding, dual gases are generally used which ignites to melt the metal around the joint forming an autogenous or homogenous weld joint. One of the gases is generally a fuel gas while the other gas is oxygen. Based on the combination of the type of these gases, gas welding can be distinguished into oxy-acetylene gas welding, air-acetylene gas welding and oxy-hydrogen gas welding. These are described below.

5.3.1.1

Oxy-acetylene Gas Welding

In oxy-acetylene gas welding, the acetylene is used as the fuel which is mixed with oxygen in desired proportion and released through the nozzle to produce different types of flames that are useful for joining different metals and alloys. Various ferrous

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Fig. 5.1 Oxy-acetylene welding [5]. Source Oxy acetylene welding (2008) Wikimedia commons

and non ferrous metals are joined using this process and the equipment mainly consists of two cylinders (each of acetylene and oxygen), long hoses, welding torch, pressure gages and regulators. The weld torch releases the gases in desired proportion through the nozzle which ignites the flame. The flame is directed sharply over a narrow weld area to melt the metal around the joint and creating a fusion zone. The gases are delivered to the torch from the acetylene and oxygen cylinders via hoses and are connected to the pressure gages and regulators at the top of the cylinder to control the gas flow (Fig. 5.1). The flames produced in the oxy-acetylene gas welding are of different types based on the amount of heat required to melt the joining metals and oxidizing properties of different metals. The reaction basically involves combustion of acetylene gas in presence of oxygen gas to produce heat, carbon-dioxide and water vapour as the final product. These are shown below [1]: yields 1 C2 H2 + O2 −−−→ 2CO + H2 + rd of total heat 3 yields 2 2CO + H2 +1.5O2 −−−→ 2CO2 + H2 O + rd of total heat 3 In the first reaction, the acetylene dissociates into carbon monoxide (CO) and hydrogen (H2 ), generating heat which is 1/3rd of the total heat produced and forming the inner core of the flame. In the second reaction, the carbon monoxide and hydrogen produced from the first reaction reacts again with the surrounding atmo-

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spheric oxygen for the final output products generating heat which is 2/3rd of the total heat. The temperature generally reaches to about 3300 °C in this process which is way above the melting temperature of most of the metals including the ferrous metals. As can be observed from the above equations, the ratio of the volumes of acetylene and oxygen is 1:1 which when supplied accordingly, there does not remain any extra oxygen and the concerned flame is termed the neutral flame. Neutral flames are generally used for welding majority of the ferrous and non ferrous metals including steel. When oxygen gas is released in excess, the metals to be joined especially steel tends to get oxidized around the weld region due to high temperature which is an undesirable outcome as it weaken the joint by forming oxides. This type of flame is called the oxidizing flame. Oxidizing flames are limited in its use and are generally used to join copper metals and alloys, the oxidized molten metal of which forms a protective layer on top of the weld pool, called slag [2]. The third type of flame is the reducing flame, which burns at a limited supply of oxygen producing low heat and high amount of soot. This flame is generally employed for low heat application such as soldering, brazing and flame hardening process. An important material that is used which acts as a filler metal along the joints and are generally in the form of rods, sticks and wires, made of the same metal as that of the base metal and which is coated with flux. The forward end of the filler stick is placed on top of the joint, above which the flame is directed, melting the filler end to accumulate inside the weld pool, forming stiff joints upon solidification. The flux in the weld pool reacts with the surrounding oxides and other impurities which gets lighter and lifted up to the top of the weld pool surface preventing further oxidation. The flux also produces a gaseous envelope around the arc and weld region for shielding the molten metal from atmospheric oxygen.

5.3.1.2

Air-Acetylene Gas Welding

In air-acetylene gas welding, air is drawn into the torch, mixed with the fuel gas such as acetylene, propane, butane and natural gas, deriving the necessary oxygen from the air for combustion and igniting to produce flame. Since the heat produced is low, this welding process is employed for low melting temperature metals and alloys such as lead and for joining processes like brazing and soldering [3].

5.3.1.3

Oxy-hydrogen Gas Welding

In oxy-hydrogen gas welding, the setup is similar to oxy-acetylene gas welding with the only difference of installing a special regulator in the hydrogen metering unit and storing hydrogen in cylinder instead of acetylene. The heat produced in this case is low and therefore is employed for low melting temperature metals such as aluminium, magnesium and lead. The flame produced by hydrogen is not visible and so is difficult to adjust for the desired flame [3].

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5.3.2 Arc Welding With the development of the commercial electricity, people have started recognizing the potential of an arc between two electrodes produced by high voltage supply, to produce enough heat, raising the temperature of the medium between them to about 3900 °C. The earliest use of the welding process involved a carbon electrode and the workpiece served as the other electrode [4]. The arc between them produces heat which melts the workpieces to form rigid joints. The temperature of the arc depends on factors like current supplied, electricity supply type, voltage applied and the polarity of the current. The carbon electrodes were soon replaced by filler metal rods or wires (consumed in the process and having lower melting temperature than the arc temperature), whose properties are similar to the base metal with fluxes being provided in the wire for removal of impurities from the weld pool and protection from the atmospheric oxygen by formation of slag which remain floated above the fused region. This type of filler wires with a coating of flux and other elements are also called shielded metal electrodes which stabilizes the arc and protects the molten metal from oxidation. As the wire gets consumed gradually, proper feed mechanism is installed to maintain a constant stabilized arc length. The temperature produced in the weld region by the arc can be controlled considering the polarity of the current in a DC supply. When the positive terminal of the DC supply is connected to the workpiece and negative terminal to the electrode, the temperature produced near the workpiece is maximum, facilitating quick melting of the metal joints. This is due to higher concentration of collision of electrons on the metal surface released through the arc from the negative electrode and this method is called straight polarity direct current (SPDC). Another alteration in characteristics of the welding process is observed when the positive terminal of the DC supply is connected to the electrode and negative to the work piece. This condition is called reverse polarity direct current (RPDC) which produces minimum heat near to the workpiece and vice versa near to the electrode end of the arc. Low melting temperature metals and alloys are generally welded using this method. Since, AC supply produces rapid fluctuation in the polarity of the current at high frequency, it distributes the heat generation equally towards both the ends of the arc. The above described general welding process involves consumable electrodes, the materials of which get deposited in the weld pool by the heat of the arc. Another type of welding processes utilises tungsten electrodes which does not melt during the welding process but vaporizes slowly. These electrodes are called non consumable electrodes [4].

5.3.2.1

Arc Welding Processes with Consumable Electrodes

Below mentioned welding processes utilizes electrodes which are consumed by the heat of the arc and fills the weld pool producing a defect free and quality joints.

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Metal Arc welding Metal arc welding involves a bare metal (serving as a filler metal) electrode with special coating in case of a shielded metal arc welding and creating a shielding gas atmosphere in case of Gas metal arc welding [4]. In shielded metal arc welding (SMAW), a special coating around the metal electrode wire serves the purposes of providing various desirable functions such as arc stabilizing by vaporizing the coating material into shielding gas, removal of impurities from the molten weld pool (acting as a flux), preventing the molten pool from further oxidation by forming slag above it, reducing weld spatter, adding alloying elements, increasing efficiency of deposition of filler metal and enhancing penetration of heat for deeper welds. The special coatings can be of cellulose and titania which are composed of mainly SiO2 , TiO2 with addition of small quantity of FeO, MgO and Na2 O, and volatile materials. The volatile material releases hydrogen which might sometimes get dissolved into the weld pool causing embrittlements and cracks. Low hydrogen evolving coatings are available which eliminates this drawback and maintains a required level of shielding atmosphere around the arc and the weld pool. Electrodes are generally baked before using it for welding in order to remove the moisture content for limiting the evolving hydrogen during the reaction. The operation of shielded metal arc welding involves touching of the electrode to the metal joint region to initiate the spark and then moving it back to maintain a stable arc length. The process is versatile and is economically efficient with low equipment cost. Stainless steel, carbon steel, alloy steel and cast steels are generally welded using this process and a welding voltage of about 15–45 V, welding current ranging between 10 and 500 A is utilized to produce an arc temperature of about 5000 °C. The filler metal wire has a diameter of about 1/16–1/4 in. and a length of about 9–14 in. (Fig. 5.2).

Fig. 5.2 Gas metal arc welding [6]. Source Equipment gas metal arc welding (2014) Wikipedia

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Metal Inert Gas Welding (GMAW) In Gas metal arc welding (GMAW) or metal inert gas (MIG) welding, the bare metal wire used as electrode is similar to that in case of the shielded metal arc welding, the difference being in the fact that no coating has been provided in this welding process. Instead of the special coating in SMAW, all the various functions of arc stabilizing, penetration of the heat and protecting of the molten weld pool from the atmospheric oxidation are provided by gas shielding. The gases mainly used are inert and consist of a mixture of argon and helium (used for welding non ferrous metals) with addition of small amount of O2 and CO2 in case of ferrous metals. The GMAW process is relatively fast and economical and there is no requirement of frequent alteration of electrodes as in the case of SMAW. No fluxes are required and hence no slag formation is triggered and concentrated heat quickly penetrates deep within the joints. It is due to this penetration capability, GMAW is generally used in reverse polarity direct current mode. Due to its smooth and clean weld formation and operational simplicity, the GMAW can be automated and robotic implementation is possible.

Submerged Arc Welding (SAW) In submerged arc welding process, a bare metal electrode end is submerged inside the granular layer of fluxes (composed of lime, magnesium oxide, silica, calcium fluoride and other compounds) and electricity is switched on. The arc produced melts the electrode metal wire and a portion of the surrounding fluxes to get deposited into the weld pool. The deposited molten flux combines with the impurities in the weld pool to get lighter in weight which rises to the surface as slag. The remaining flux (including the molten flux layer on top of the weld pool and the un-melted portion of the flux granules) provides extra shielding from the atmospheric oxygen, preventing weld spatter, blocking ultraviolet radiations and fumes emitted in the process and lowers the rate of cooling of the metal in the joint to give a soft, ductile metal joint. The molten flux over the weld pool upon solidification becomes brittle and gets easily separated from the weld surface. A vacuum suction is used to remove the remaining unmelted flux granules which are then re-utilized for later processes. Since the fluxes and molten weld are held in its position under gravity, the process involves horizontal working (Fig. 5.3). The suitable joints formed by submerged arc welding are butt joint and fillet joint. Multiple electrodes can also be used in this process when a higher deposition rate of welding is desired. Other significant characteristics of SAW are high welding speed, deeper penetration and clean operation. Electricity source used can be both DC and AC with currents ranging from 600 to 2000 A. Submerged arc welding is mainly employed for welding low carbon steels but can also be used for medium carbon steel, cast irons, alloy steel, stainless steel, copper alloys and nickel alloys, with some pre and post welding measures.

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Fig. 5.3 Submerged arc welding [7]. Source Submerged arc welding schematic (2010) Wikimedia commons

Welding of high carbon steel, tool steel, aluminium, magnesium, lead, zinc and titanium are not recommended using this process due to unavailability of desired quality fluxes, high reactivity at higher temperature and low sublimation temperatures. Requirement of excess handling of fluxes and large volume of slag, need for horizontal placement of the workpieces, large grain structured joints obtained due to high temperature working and absorption of moisture by the flux materials (leading to porosity) are few other drawbacks of this process.

Electro Gas Welding Electro gas welding is mainly employed for joining vertically placed pieces by their edges or in a butt joint technique using a special equipment consisting of a pair of water cooled shoes, one of which is fixed while the other moves vertically upward along the joint to be made (placed between the shoes) and at the same time depositing metal from a single flux cored electrode or single solid electrode (or multiple electrodes) into the weld pool by passing electric arc. The flux melted by the heat of the arc gets deposited in the weld pool and accumulates the impurities in the form of slag on top of the molten pool which is prevented from flowing out by the pair of shoes placed on either side of the joint. The slag provides protection to the molten metal from the atmospheric oxygen and the shielding is done by the gases evolved such as CO2 , inert gases like argon and helium, etc. either from the cored flux or from external source.

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Electro gas welding is extensively used in the construction of bridges, ships, thick and large diameter pipes, pressure vessels and storage tanks. Metals mostly welded using this process are structural steels, aluminium and titanium alloys. Apart from vertical edges, this process can also be employed for welding circumferential joints in large pipes by rotating it. The current for a flux cored electrode can reach up to 750 A while for a solid electrode it can reach up to 400 A with a power consumption of about 20 kW [1].

5.3.2.2

Arc Welding Processes with Non Consumable Electrodes

The arc welding processes that utilize electrodes which are not consumed by the heat of the arc produced and thus can be used for an extended period of time. These processes are described below.

Gas Tungsten Arc Welding (GTAW) In GTAW, tungsten electrodes are used which is basically not consumed during the arc generation as in the case of any consumable electrodes and helps maintain a constant arc length throughout the operation. The shielding is done (without application of any flux material) by releasing inert gases like argon and helium around the weld region which prevents oxidation of the molten metal by contacting with the atmospheric air. The electricity supply used can be DC or AC depending on the nature of the metal to be joined. In case of DC supply, the current is kept at about 200 A while for AC supply, the current is kept at 500 A. The power supply ranges between 8 and 20 kW (Fig. 5.4). For welding aluminium and magnesium, GTAW is commonly used with AC supply, the frequency of which has an effect on the cleaning of the weld surfaces by

Fig. 5.4 Gas tungsten arc welding [8]. Source GTAW setup (2005) Wikimedia commons

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removing the oxides of these metals and impurities. This gives a good quality weld, defect free and smooth surface finish. Titanium and refractory metals are other materials which can be welded using GTAW. A couple of major limitations of the GTAW are the contamination of the tungsten electrode during contact with the molten metal in the weld pool. This contamination effects the proper arc generation and can cause discontinuities in the weld bead. Another drawback is the cost of the inert gases is too high making it an expensive process. Some of the advantages of this process includes good surface characteristics of the weld surface or smooth surface finish is obtained, ability to weld a range of workpiece thicknesses and equipment portability.

Carbon Arc Welding As already mentioned at the starting of the “arc welding” section, carbon (graphite) electrodes were the earliest used electrodes which generates an arc between the electrode and the workpiece to melt the workpiece edges, forming rigid joints. With the development of tungsten electrodes and application of these in welding processes like GTAW, PAW and AHW (atomic hydrogen welding), the use of carbon electrodes have almost declined which can otherwise be employed in brazing operations, repairing iron castings and depositing wear resistant materials on the surfaces.

Plasma Arc Welding (PAW) Plasma arc welding involves an inert gas like argon which is passed through an orifice (between the tungsten electrode and the plasma gun or plasma nozzle) to produce a concentrated plasma gas on passing electric arc. This plasma beam can reach a temperature of up to 33,000 °C which can easily melt the edges of two metal plates fitted at a close proximity. The process begins with the generation of arc between the tungsten electrode and the plasma gun at the orifice which blocks the path of argon gas being forced to move through the orifice under a certain pressure. The argon gas absorbs the heat of the arc to ionize completely and in the process generates tremendous amount of energy in the form of plasma gas which moves down, transferring the energy to the workpiece for melting. Another opening around the plasma gas nozzle, releases shielding gas (argon or helium or mixture between them) which protects the molten weld pool from coming into contact with the atmospheric oxidation. The heat supplied to the workpiece depends on the pressure at which the plasma gases get released. With lower pressure of the plasma gas, the metal just gets melted and proper fusion and solidification of the weld region takes place. With increase in pressure, a depression begins to form on the surface of the weld pool leading to a keyhole through the joining metal plates just below the plasma orifice. With constant movement of the plasma gun along the metal joint, the keyhole formed is filled with the surrounding molten metal pool and solidifying upon cooling to form strong joint. As the pressure is increased to its maximum, the molten metal under the

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Fig. 5.5 Non-transferred Plasma arc welding [9]. Source Non-transferred DC plasma torch (retrieved on 9th of April, 2019) Wikipedia

plasma orifice is thrown out from beneath (a keyhole) which is basically a plasma cutting operation (Fig. 5.5). The above described plasma arc welding is basically a non-transferred plasma arc welding method which does not include the workpiece in its electrical circuitry and involves an arc produced between the tungsten electrode and the plasma gun nozzle with the heat being transferred to the workpiece by hot plasma gas. Another method called transferred plasma arc welding produces arc between the tungsten electrode and the workpiece, transferring heat directly. Plasma arc welding finds its application in a variety of metals with part thicknesses not exceeding 6 mm and due to its advantageous characteristics like arc stabilizing, low distortion, concentrated heat supplied, deeper and narrow weld, the process is widely used for both butt and lap joints. Skilled personnel are required while carrying out operation in PAW.

5.3.3 Resistance Welding In resistance welding, the heat energy required to melt the metal surfaces to be fused together for rigid joints, is obtained from the highly concentrated electrical resistance at various locations on the interface between the surfaces of the metal parts on passing electricity. Lap joints are mainly welded using resistance welding and electricity is passed between a pair of electrodes pressed tightly on top and bottom of the lap joint. With high resistance offered by interfaces between electrodes and metal surfaces,

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metal to metal surfaces and by the metal parts itself, the concentrated building of resistance between the electrodes heats up the metal-metal interface surfaces (at a small region) which melts to fuse together upon solidification. Since no contact with the atmospheric oxygen is made, the molten pool remains protected without the use of shielding gas and fluxes, provided the surface of the metals are cleaned and made free of any impurities (oils, paints, films, etc.) and oxide layers prior to the welding. There is also no requirement of filler metals as in the case of shielded arc welding processes or any other conventional welding processes. Thermal conductivity and specific heat of the metals to be joined plays a vital role in deciding the temperature rise at the interface between them. Since in resistance welding, heat is concentrated around a small region or point, aluminium and copper metals are welded using resistance welding as they have higher thermal conductivities. Based on the type of electrodes used and methods implemented, resistance welding can be divided into resistance spot and seam welding. In resistance spot welding, a pair of electrodes are used (tapered at the front) which are placed on either side of a lap joint (between two metal plates) and applies pressure during the time of current flow through the metal interface and remains pressed (forming a slight depression on the metal outer surfaces) until the molten bead solidifies at the interface. Then the pressure is released and moved to another position on the metal-metal interface for welding. The pressure may be applied by mechanical means (rocker arm type) or pneumatic means (press type) which are used for small and large workpieces respectively. Multiple pairs of electrodes can also be used simultaneously to perform spot welds and current provided through each pair ranges in (3000–40,000) A to obtain a weld nugget of 6–10 mm in diameter. The spot welding process is economical and easy to perform and the major application of this technology can be frequently seen in automobile industries and repairing shops. Resistance seam welding is an extension of spot welding where the electrodes used are in the form of rotating and conducting wheels (and rollers) and continuous spot welds are being formed along the movement of the electrodes on the metal surfaces. AC current is generally used which on higher frequency (or slow rotational speed of the electrode wheels) produces continuous beads of spot welds, closely placed in the form of a seam. Resistance seam welding is mainly used for producing seam welds on cans, gasoline tanks and other household containers.

5.3.4 Laser Beam Welding (LBW) Laser beam welding utilizes the heat from a high power concentrated laser beam to melt thin or thick metal interfaces for high strength joint. Due to high energy density of the beam, it has an excellent penetration characteristic and is generally used for producing narrow and deep joints of depth to width ratio ranging between 4 and 10. The joint produced is ductile, free from impurities, shrinkages or distortions and porosity, and the welding speed can vary from 2.5 m/min for thick metal joints to about 80 m/min for thin joints. For thin metal joints, power supplied by high energy dense

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laser beam ranges up to 100 KW with pulsed energy input in milliseconds. For thick metal joints, a multi KW continuous system of energy input is provided by the laser beam. LBW can be automated due to its processing nature, therefore minimum skill is required of the operator and can be performed in remote inaccessible locations. In automotive industries, LBW is mainly employed for welding of transmission components with high precision. Laser beam welding has the advantage over electron beam welding in that it does not require vacuum atmosphere for operation, the beam can be optically focussed just like any other light beams so they are economical and can be robotized, weld quality is excellent with no porosity and complete fusion of metal joints takes place with no X-ray emission.

5.3.5 Friction Welding Friction welding can be distinguished from all other welding processes by the fact that it does not require an external energy source for melting of the interface material in the welding procedure and can be directly joined, utilizing the frictional characteristics between two metal surfaces upon relative motion. The axis-symmetry piece of a pair of metal components is held on a rotating chuck while the other piece of metal (axissymmetry or non axis-symmetry) remains stationary. The rotating piece is axially pressurized against the surface of the stationary piece of metal at a relative surface velocity of up to 15 m/s at the interface and when sufficient contact is made at the joint interfaces the rotation is suddenly bought to halt while the axial pressure compresses the plastic state of the material at the interface to form a properly fused strong joint. During the rotation of the workpiece at the interface the oxide layers gets eroded and thrown out radial by centrifugal action including all other impurities. A flash of material generally emerges out of the interface in radial direction due to plastic compression of the material and can be later machined out or grounded. The extent of the weld effected region in this process mainly depends on factors like conductivity of the metals to be joined, heat generated during frictional contact and mechanical properties of the metals (Fig. 5.6). Solid bars of diameter up to 100 mm, hollow cylinders having outer diameter of up to 250 mm and all other pairs of components consisting of at least a single piece of axis-symmetry work can be affordably and easily welded using friction welding.

5.3.6 Cold Welding Cold welding is mainly employed by application of pressure at the interface between two dissimilar or similar (more preferable) metals without employing any heating source or frictional heat, but using pairs of rolls and dies. The process is completely based on the principle of plastic deformation and therefore it is desirable for at least

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Fig. 5.6 Rotary friction welding [10]. Source Rotary friction weld (2009) Wikipedia

a single metal piece to be ductile if not both of it. Since this process involves just the interface contact which merges into one on application of roll or die pressure due to plastic deformation and fusion, interface surfaces needs to be made free of oxide layers, contaminants, oil, grease and other particles, prior to the operation which ensures greater strength of the joint. Joining of aluminium and steel by this process involves formation of intermetallic brittle compounds at the interface which makes the joint brittle and low in strength. Dissimilar metals that are mutually soluble into each other can produce weak joints leading to failure and due to which similar metal jobs are most preferred for this operation. Roll bonding or roll welding is a common cold welding process which employs rolls (on either side of a sandwiched layers of two dissimilar or similar metals) to apply pressure on the surfaces to get bonded at desired locations on the interface that has been previously thoroughly cleaned. Common examples of roll bonding are bimetallic strips in thermos heaters and thermostats and roll welding of a layer of stainless steel on either surfaces of a mild steel [1]. Apart from the welding processes some other joining processes for metals and other materials are brazing, soldering and mechanical fastener.

5.3.7 Braze Welding In brazing operation, a filler metal known as braze metal is used to fill the gaps between the two faying surfaces of the metal edges in molten condition without the melting of the base metals. The filler metal has a melting point of about 450 °C which is lower than the base metal (ensuring no melting of the base metal) and is melted by various external heat sources thus, penetrating into the gaps between the faying surfaces and later solidifying to produce high strength joints. Fluxes are used to remove oxides and impurities from the joint gap and are fed along with the filler

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molten metal into it. The filler metal used is generally made of brass and fluxes used are borax, fluorides, chlorides, boric acid and borates. Braze welding is mainly employed for repairing works, joining intricate and lightweight shapes and for joining metals such as cast steel and iron. Thus with this process, joining between two dissimilar metals can be effectively performed having a good joint strength.

5.3.8 Soldering Soldering is similar to brazing operation where filler metal in the form of solder (made of an alloy of tin and lead) is melted (using soldering torches and irons) along with the flux and then diffused into the gaps between two tightly fitted faying metal surfaces by capillary action. The flux reacts with the impurities and forms slag on the surface preventing oxidation of the joined metal surfaces. The filler metal upon solidification, thus make a high strength and defect free joint. One specific significant property of solder metal is its low surface tension or its wetting action which ensures proper filling of the gap between the metals (Fig. 5.7). The melting point of solder alloy depends mainly on the composition and its eutectic point. Various other combination in alloys such as tin-zinc, zinc-aluminium, lead-silver and cadmium-silver are also commonly used as solder metal. As there is concern regarding the hazardous emission from lead compounds, solder alloys are generally nowadays lead free. Fluxes used can be inorganic acids and salts (zincammonium-chloride solution), or resin based fluxes (used in electronic applications). When soldering is performed with inorganic acids and salts, the joints need to be made free of any flux residue from the surface to avoid corrosion.

Fig. 5.7 Soldering [11]. Source Soldering (2018) Pixabay

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5.3.9 Mechanical Fasteners Mechanical fasteners include nut-bolts, nails, rivets, screws car bumper clips, mechanical seams, staples, stitches, crimping and any other specific mechanical fastening methods that involve formation of holes through both the mating metal pieces for a permanent or temporary joint.

5.4 Summary From the above discussion, it is clear that simple to complex parts can be joined using traditional and advanced welding processes. Welding processes can be mainly classified into fusion welding, solid state welding and other joining processes like soldering and brazing. Fusion welding processes includes gas welding, arc welding and resistance welding which involves melting of the metal interfaces to fuse together forming a uniform strength joint but which varies in their heat sources. Gas welding employs thermal heat of the gases (oxygen and fuel gas) to melt the metal interfaces for fusion welding, arc welding employs heat from an electric arc to melt the metal at the interface using different modes of polarity (straight polarity direct current and reverse polarity direct current) and resistance welding employs resistance heat offered by the material of the metal pieces that has been lap jointed, between the two electrodes. Solid state welding includes friction welding and cold welding which does not require any heating source and can be joined while in a solid state. Friction welding employs heat generated by frictional stress subjected between the surfaces of the two joining metals and by plastic deformation. While on the other hand, cold welding involves application of pressure on top of a sandwiched layer of two metal parts. Soldering and brazing operations involve use of filler metals which melts and diffuses into the gap between tightly fitted metal plates. Mechanical fasteners are another mechanism of joining parts employing mechanical means.

References 1. Kalpakjian, S., & Schimid, S. R. (2009). Manufacturing, engineering and technology (p. 6). Prentice Hall. 2. Beddoes, J., & Bibby, M. J. (2003). Principle of metal manufacturing processes. Elsevier Butterworth Heinemann. 3. Hazra Choudhury, S. K., & Hajra Choudhury, A. K. (1986). Elements of workshop technology. In Manufacturing Processes (Vol. 1, No. 10). Media Promoters and Publishers Pvt. Ltd. 4. DeGarmo, E. P., Black, J. T., & Kohser, R. A. (2008). Materials and processes in manufacturing (p. 8). USA: Prentice Hall. 5. https://upload.wikimedia.org/wikipedia/commons/1/15/Oxy_acetylene_welding_rig.jpg. 6. https://upload.wikimedia.org/wikipedia/commons/e/e8/Equipment-gas-metal-arc-welding. png.

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7. https://upload.wikimedia.org/wikipedia/commons/8/82/Submerged_arc_welding_schematiccz.png. 8. https://upload.wikimedia.org/wikipedia/commons/5/5e/GTAW_setup.png. 9. https://upload.wikimedia.org/wikipedia/commons/b/bc/Nontransferred_DC_plasma_torch. png. 10. https://upload.wikimedia.org/wikipedia/en/d/dc/Rotary_friction_weld.jpg. 11. https://cdn.pixabay.com/photo/2018/04/01/06/13/soldering-3280085_960_720.jpg.

Part III

Conventional Machining

Chapter 6

Machining Process

Abstract All metal parts, after being casted or formed has to be shaped into its final dimensions so that it could meet the required functionality, durability and aesthetic characteristics of a product. This shaping process requires metal removal which involves various complex phenomenon and factors, governed by a set of mechanisms that has been experimentally and analytically discovered and can be controlled by considering various tool geometry, optimized cutting conditions and better tool-work material combination. Merchant’s force model of the machining process recognizes the influence of the shear plane in determining all controlling factors such as the cutting conditions and inputs for optimized force and power utilization, good surface finish and longer tool life. In this chapter, these factors and conditions that govern the basic mechanics of metal cutting including the various type of chips and their desirability issues, crater wear and flank wear (that determines the limit of usage of any cutting tool) has been explained in detail. Turning is a fundamental operation (carried out on lathe machines) in any machining research due to its involvement of a single point cutting tool and the ability to reduce the complex problem (representing complex phenomenon) into 2 dimensional simple problems. Other operations involve either a single point or multi point cutting tool and includes boring, milling, drilling, reaming, etc.

6.1 Highlights This chapter mainly includes the fundamentals of basic machining process, machines involved and the various operations in the machining process.

6.2 Introduction Machining is an important process which shapes the material to its final dimensions by employing cutting tools which can be a single point cutting tool in case of turning and boring and multipoint cutting tool in case of milling, broaching, sawing, etc. © Springer Nature Switzerland AG 2019 K. Kumar et al., Materials and Manufacturing Processes, Materials Forming, Machining and Tribology, https://doi.org/10.1007/978-3-030-21066-3_6

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Machining is mainly followed after the casting operation and sometimes even in between different operations to scrap away any extra material. This process utilizes the maximum power (as compared to other manufacturing processes) and involves complex phenomenon of plastic deformation, material separation and temperature effects. The tools that are basically studied for determining the factors behind these phenomena are the single point cutting tools which are analysed considering orthogonal cutting rather than oblique cutting. Orthogonal cutting reduces the complex geometry of the metal cutting mechanism into 2 dimensional problems, assuming a plane strain condition. Oblique cutting is generally avoided for analysis purpose as it considers an obliquely aligned cutting edge rather than a straight edge. Machining process is generally influenced by factors like the depth of cut, feed, rake angle, nose radius and cutting speed of the cutting tool, work and tool material, frictional condition at the tool-chip interface, cutting fluids used and the strength of the tool. Ferrous metals are generally classified into cast iron, wrought iron, steel and alloy steel which find applications in a variety of products due to their better mechanical property, lower cost, easy availability and ease of production. Non ferrous metals such as aluminium, titanium, nickel, magnesium, copper, etc. are more expensive than ferrous metals but finds application in industries due to their refractoriness, corrosion resistance, low density and ease of fabrication.

6.3 Mechanics of Metal Cutting The interaction between the tool and the work piece during the process of metal cutting generates a complex phenomenon where a number of factors comes into play, that are already been discussed in the introduction part. Ernst was the first person to detect the presence of a shear plane at the chip removal zone during machining. He urged the shear plane having a significant influence in determining the complex behaviour in material separation event. Merchant who worked under Ernst gave the first analytical force model for the machining process involving significant contribution of shear plane in determining all the parameters and computing for the best possible optimized cutting conditions. The Merchant’s force model was a representation of orthogonal cutting which takes a 2-dimensional tool-workpiece cross section and the forces remain perpendicular to each other. Below given is the expression for shear plane angle (∅), tan∅ =

r cosα 1 − rsinα

where α is the rake angle of the tool, along which the chip slides after separation from the workpiece. The cutting ratio “r” or the chip thickness ratio is defined as the ratio of the depth of cut (feed in case of turning operation) to the chip thickness. The chip thickness is always greater than the depth of cut which implies that the value of ‘r’ is always lesser than unity. The reciprocal of the cutting ratio is called the chip

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compression ratio which gives the measure of the amount of thickness increased in the chip compared to the depth of cut [1]. The cutting ratio plays a significant role in analysing the force and power utilization in machining operation by optimizing the cutting conditions that can be controlled by the user such as depth of cut, feed, rake angle and cutting velocity. The above equation was derived from the geometrical consideration of the merchant’s given model. Earlier research works on determination of the inclination of the shear plane angle with the cutting direction suggests that during metal cutting, shear plane adjusts itself along the maximum shear plane where the force and energy requirement in the process will be the minimum. Based on this assumption, shear plane angle can be related with other cutting factors such as rake angle (α) and friction angle (β). ∅ = 45◦ +

α β − 2 2

Friction angle represents the frictional characteristics at the interface between the tool rake face and the chip. It can be denoted as = tan −1 μ, where ‘mu’ is the co-efficient of friction and can be defined as the ratio of the frictional shear stress at the tool-chip interface to the normal contact stress on the rake face. Research experiments conducted to determine the frictional characteristics of the tool-chip interface suggests a μ value ranging from 0.5 to 2 along the rake face which can be reasoned with the extreme contact pressure at the sharp edge of the tool. Strain In the metal cutting operation, the material above the shear plane is completely separated from the work piece while the material below is completely attached to the work. This phenomenon indicates the existence of a higher shear stress along the shear plane which generates deformation and strain on the material and that which takes place in a fraction of a second. The strain in the material can be calculated by considering the mechanics of the chip formation, i.e. resembling a deck of cards inclined at an angle and sliding one above the other. This mechanism has been observed experimentally, which enabled researcher to find out the expression for shear strain (γ ) along the maximum shear plane in terms of the shear angle and the rake angle. This is given as below: γ = cot∅ + tan(∅ − α) Shear strain tends to increase with decrease in shear angle. This is caused due to the decrease in rake angle (reducing the shear angle) which causes the cutting force to elevate. Material strain in the machining operation has been observed to be greater than 5 which is higher than all other operations in manufacturing processes. In some metals such as cast iron exhibits the presence of a shear zone rather than shear plane at low speed. In high speed however the shear zone reduces to a line.

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Velocity determination Velocity determination is necessary in a machining operation, to predict the power consumption during interactions between the tool-chip, tool-workpiece and chipworkpiece interfaces. Chip velocity (Vc ) can be evaluated based on the mass continuity principle and assuming width of the cut to remain constant. The final expression can be given in terms of cutting velocity (V) and chip thickness ratio (r) or in terms of shear angle and rake angle by substituting the expression for ‘r’ as under: Vc = V r Vc =

V sin∅ cos(∅ − α)

A velocity diagram can be constructed out of which the relationships between different velocities associated with machining can be evaluated as given below: Vs Vc V = = , cos(∅ − α) cosα sin∅ where Vs denotes the shear velocity along the maximum shear plane. Based on the morphological characteristics, chips can be classified into continuous chips, discontinuous chip and chip with build up edge. These are described below: Continuous chip is generally produced in ductile material where the machining is carried out at high cutting speed and higher rake angle. With decrease in rake angle, continuous chip forms a shear zone bounded by curved lines, one which is curved outward towards the chip and the other towards the work material. The upper curved line is due to the curl or curvature of the chip after leaving the rake surface. The lower curved line can be considered the main reason behind the distortion lines on the machined surface at lower cutting speed. Though continuous chip formation is desirable in a number of cases as it provides a good surface finish and reduced cutting force, still it is not considered desirable where automation is applied as the chip tends to curl around in the regions of fixtures, tool holders and on the work piece which causes inconvenience in the CNC machine operation. This problem can be tackled by employing chip breaker which provides a curl effect to the chip by increasing the curvature and breaks off when the curl goes above its limit. Discontinuous chips are generally not desirable due to its effect on the cutting force during machining. The cutting force fluctuates and if the base fixtures and tool holders are not rigid and firm, it might lead to damaged work surface, roughness and tool failure. Discontinuous chips are observed in cutting of brittle material due to its lower achievable strains and in materials where inclusions and impurities are provided. Some other causes are lower rake angle, lower speed, large depth of cut and cutting fluid shortage. Build up edge chip generally occurs in materials which have been annealed before the machining process is being conducted. These are layers of materials being deposited one above the other on the rake surface of the tool tip. With time, the

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build up edge gradually grows in size until it becomes unstable to breakdown into fragments. Few fragments are carried over the tool-chip interface, while the others are deposited randomly onto the machined surface, thus creating surface roughness and wear. In some cases, the layers deposited on the rake surface are compressed to its work hardened state and strongly adheres to form an extra layer on the rake surface, changing the geometry of the tool tip. Build up edge effect during machining can be reduced by increasing the cutting speed, applying sufficient cutting fluids, using sharp or high rake angle tool and reduced depth of cut. Crater wear and flank wear are common phenomenon of wear on the surface of the tool in a machining operation which occurs due to the abrasive and adhesive interactions at the tool-chip and tool-work piece interfaces respectively. These are mainly accelerated by the intensity of heat generated at these interfaces and the amount of diffusion of atoms from one surface to the other. Crater wear interestingly occurs at the region on the rake surface where the temperature is observed to be the highest just slightly ahead of the cutting edge. The amount of wear on the tool surfaces can be set as a standard figure in measuring the tool life. The tool life can be empirically related to the cutting velocity as given by Taylor as under, VTn = C where V is the cutting velocity, T is the tool life, C is a constant and n is an exponent that depend on the tool and work piece material and cutting conditions. To take into account the influence of the depth of cut (d) and feed (f), the Taylor’s relation can be further expanded as: V T ndx f y = C Here, x and y are generally determined experimentally.

6.4 Machines Involved Major metal cutting machines are employed in shaping different forms of metal based on single point cutting tool in lathes, multi point cutting tool in milling and drill teeth in drilling. These machines are described below [2].

6.4.1 Lathe Machine Lathe machine is mainly employed for operations like turning, facing and boring and due to its versatility in carrying out different operations it is the most commonly

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used metal cutting machine in all industries. It has a single point cutting tool which is interacted with the rotating workpiece mounted on a spindle, to strip away materials from its surface in the form of chips. Lathe machine mainly consists of four main components namely, the headstock, the tailstock, the carriage and the bed (Fig. 6.1). The bed is generally made out of grey cast iron which provides a rigid, heavy and damp proof base support to all the components mounted on it. Two sets of ways namely, inner and outer ways are embedded on top of the bed which guides the headstock and the tailstock in its longitudinal movement. The ways are precision components that are not to be tampered with as it can affect the accuracy of the entire machine. These ways are generally surface hardened and possess a high wear resistance and hardness. The headstock remains on one end of the inner ways which consists of the transmission gear mechanism and spindle mechanism in it. The transmission gear mechanism is similar to any conventional diesel engine which enables the spindle to rotate at different rpm’s, deriving its power from an engine ranging from 5 to 25 hp via a V-belt and through the gear system. Lathe machine generally comes with variations in rpm of the spindle ranging in 8–18 rpm. Spindles are generally hollow in shape, the diameter of which determines the size of the job stock that can be fitted into it. The spindle can also hold other work holding devices such as chuck, face plate and collets. The tailstock assembly of the lathe includes three components namely, the lower casting fit, the upper casting fit and the tailstock quill having relative motions between the components and is used to hold cutting tools and dead centres and maintaining a proper alignment with the work part mounted on the headstock. Lower casting fits are generally mounted on the bed ways which enables it to translate in the longitudinal directions. On top of the lower casting fit, sits the upper casting fits which can be adjusted to align itself to the headstock by translating in proper keyways. The upper casting fit includes a hollow barrel known as tailstock quill which can be moved longitudinally in and out of the upper casting to adjust the lathe centres or cutting tools in alignment to the headstock work part, by employing a hand wheel and a screw mechanism through the other end of the barrel. The carriage assembly mainly consists of the tool post, carriage, cross slide, compound rest, all controls (hand wheel, dial gauges and screws) and the apron. The

Fig. 6.1 Lathe machine. Source Fay automatic lathe (1921) A.S.M.E. (11) [4]

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base of the assembly can be taken as the carriage which can translate longitudinally mounted on the outer guide ways. On top of the carriage, the cross slide is mounted which can move transversely to feed the tool into the work part using a screw controlled by a hand wheel. The tool post generally sits on top of the compound rest which is mounted on the cross slide and can rotate relative to it to provide angular cuts or cutting the job in different angular positions. The rotation can be controlled by a hand wheel attached to the compound rest and can be fixed at the desired orientation. In automatic systems, all the movements in the carriage including the rotation of the compound rest, transverse movement of the cross slide and the longitudinal movement of the carriage can be controlled by the powered mechanisms provided in the apron that remains mounted towards the front part of the carriage.

6.4.2 Milling Machines Milling machines are the most versatile machines in cutting complex metal parts and can be found in a variety of types, the most common type of all being the knee column type general purpose milling machine. These machines are available in two types namely, horizontal and vertical types which are classified based on the inclination of the spindle axis, whether vertical or horizontal (Fig. 6.2). The most basic type of milling machine consists of a base and a column. The column is mounted on the rigid and heavy base, supporting all other components in

Fig. 6.2 Universal milling machine. Source Universal milling machine (2014) Wikimedia commons [5]

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the machine including the main drive mechanism of the spindle. In the horizontal type milling machine, an adjustable over-arm protrudes out longitudinally from the column and supports the arbor by the bearings attached to it, while the other end of the arbor is fitted to the spindle, rotating along with it and carrying the cutter at its central region. The knee remains above the base which can be moved up and down on guide ways embedded in the front of the column. On top of the knee, sits the work table which can be moved in the X-Y direction along the plane of it and which holds the work part firmly and rigidly to be aligned perfectly with the cutter. The motion of the knee and the work table in all three mutually perpendicular directions give collectively enough flexibility between the work-tool interactions. These movements can be hand or power driven and can be automated as in the case of CNC machining centres. Horizontal milling type machines are mainly employed for operations like slab, straddle and side milling. In the vertical type milling machine, the arrangement of the work table and the knee remains similar to that as in the case of horizontal type, difference being only that the spindle axis is perpendicular to the work table and remains attached to the head that is mounted on the column. Vertical milling machines can be employed for operations like facing, end milling, drilling and boring.

6.4.3 Drilling Machines Drilling machines are usually called drill presses which are mainly employed for operations like drilling and boring. The construction mainly comprises of a base, a column, power head, spindle head and a work table. The base is generally mounted on a workbench in smaller drill presses whereas it rests on the floor in case of larger drill presses. The power head remains attached to the column which consists of an electric motor to drive the spindle mechanism. The power is transmitted between the engine and the spindle by belt transmission in smaller drills, but a gear transmission mechanism is used to transmit power in case of larger drill presses. The spindle is generally hollow (Morse tapered) from the bottom which can accommodate tapered shank drill or drill chucks and that is supported by taper roller bearing. The spindle can be operated by a capstan wheel in smaller drill presses to feed the drill into the work piece that is firmly supported on a work table, while in larger drill presses, feeding mechanism is generally power controlled. The spindle is the major component in the drill press which can affect the accuracy of the drilling operation, if not rigidly fitted. The work table is mounted on the column which can move up and down and can also be rotated (about round columns) to clear away from the path of the drills that is being fed into the work parts (Fig. 6.3).

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Fig. 6.3 Drilling machine. Source Drilling machine (2015) Pixabay [6]

6.5 Traditional Machining Process Few traditional metal cutting processes that are executed employing the above described metal cutting processes are [1, 3].

6.5.1 Turning Process Turning process is a metal removal process where a single point cutting tool is fed along the longitudinal axis of a rotating work piece, maintaining a fixed depth of cut. The operation initiates with a roughing cut at a high material removal rate with a high depth of cut. The dimensional accuracy and tolerance are not considered during this operation and the main objective is to remove material as fast as possible. The machined surface of the metal after this procedure is rough and scaled. The roughing cut is followed by a finishing cut where the metal is turned at a slower cutting speed, lower feed and depth of cut. The resulting surface is well surface finished and dimensionally accurate. Forces are the main factors to be considered while designing for a turning operation. There are mainly 3 types of forces in the turning process namely, cutting force, thrust force and radial force. All the forces are mutually perpendicular to each other,

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the characteristics of which depend on factors like materials used for the tool and work piece, tool geometry, build up edge formation, use of cutting fluid, type of chips formed, cutting conditions and rigidity of the tool holder and the machine. Cutting force is the main force that can be measured and provided during the operation, acting tangentially to the work surface and opposite to the cutting direction, towards the tool. The cutting force multiplied by the radius of the cylindrical work piece gives the torque resisted by the spindle and this torque multiplied by the rotational speed of the spindle gives the power requirement in the operation. The feed force acts along the longitudinal axis of the work piece towards the tool which is very difficult to measure and is influenced by few undesirable factors like machine vibration and chatter, type of chips formed and rigidity of the tool holder and machine. Another force which acts in the radial direction of the cylindrical work exerted by the work on the tool is called the radial force. As in the case of feed force, radial forces are also very difficult to measure. These forces though can be experimentally found out during the time of need. Material removal rate in the turning process can be measured by considering the area of cut i.e., the product of feed (f) and depth of cut (d) multiplied by the average diameter of the cut (Davg. ). The expression for material removal rate can be given as below. M R R = π Davg d f N , where ‘N’ is the rotational speed of the work piece and D +D Davg = 0 2 f . D0 and Df are the diameters of the work piece before and after the cut.

6.5.2 Milling Process Milling process is a metal cutting operation where the material is removed by employing a circular cutter which rotates over the workpiece surface to produce multiple chips in one revolution based on the number of teeth being provided on the cutter circumference. Each tooth acts as a single point cutting edge with its respective rake and flank surfaces. The cutter rotation on the work surface to chip out material can be done in two ways, which is either by clockwise or anticlockwise rotation relative to the workpiece feed direction. The anticlockwise rotation of the cutter or the motion of the teeth in the opposite direction to the work piece feed motion is called the up milling (conventional milling) whereas, the clockwise rotation of the cutter or the motion of the teeth along the work piece feed direction, is called the down milling (climb milling). Up milling is more desirable as it provides a smooth surface finish to the machined part, non dependency on surface wear and scales of the work part surface, gradual increase in the chip thickness and low cutter vibration and chatter resulting in enhanced life. Down milling is relatively less desirable as it is more sensitive to work surface characteristics (roughness and scale formations), higher tendency to vibration and chatter formation, larger initial chip thickness and lower tool life. Apart from the above mentioned undesirable effects, down milling proves

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to be useful in applying a downward force on the work part which eliminates the tendency of the work part to move away from the cutter as can be observed in case of up milling condition. Based on the orientation of the cutter axis to the work surface and their applications, milling can be classified into three types which are peripheral milling, face milling and end milling. In peripheral milling, the cutter made of high speed steel has its teeth circumferentially distributed along the periphery and each one of it acts like a single point cutting tool. In peripheral milling, the cutter axis remains parallel to the work surface, the length of the cutter is usually lesser than the diameter and the teeth can be either helical or straight. Helical teeth are more desirable as they engage gradually with the material of the workpiece, resulting in lower force subjection and smooth operation with no chatter or vibration. When the length of the cutter exceeds the diameter, the milling is usually called slab milling. In face milling, the cutter axis remains perpendicular to the surface of the work part and the cutter is mounted with carbide inserts around its circumference, each acting a single point cutting tool. The cutter rotates and at the same time translates along the direction parallel to the surface, removing material in the process. Face milling generally leaves feed marks on the machined surface and is very much similar to the marks in the turning process. The surface characteristic of the machined surface is completely dependent on the material, geometry and nose radius of the carbide inserts and the feed/per tooth. The ratio of the diameter to the width of the cutter is generally kept greater than 3:2. In end milling, the cutter (or the end mill) is larger in length as compared to its diameter and consists of teeth on its cylindrical surface made out of high speed steel or carbide inserts as in the case of face milling. The cutter axis generally remains perpendicular to the work surface and can be tilted to cut curves and fillets at the edge. The teeth provided on its cylindrical surface enables it to cut complex profiles and shapes (curved, pocketed, stepped, etc.) on the work part and produce a smooth surface finish devoid of any scales. The cutter is generally mounted into the spindle with tapered or straight shank, tapered being for larger cutter size and straight for smaller cutter size. Hollow end mills are also available with internal teeth which are used to machine the outer surfaces of solid cylindrical shafts.

6.5.3 Boring Process Boring process basically involves a boring bar with a single point cutting insert attached to its front end and which is fed parallel to the rotational axis of the work piece. This process can also be called the internal turning of the hollow work pieces which aims to enlarge the already made holes by drilling or in cored castings. Misalignment in the holes and non uniformity in the cross section of the circular holes can be easily perfected to its concentricity about the axis of rotation of the work piece. Boring bars are generally designed to withstand extreme deflection and vibration on account of cutting force and should be stiff enough to resist bending. These

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bars are therefore made out of tungsten carbide having high elasticity modulus and damp resistance. Boring operation can be performed on lathes by mounting the work piece on the spindle chuck or plate and feeding the tool parallel to the axis of the rotation of the work piece. Apart from the lathe, boring operation on large work pieces are performed on boring machines or boring mills which are available in vertical and horizontal types and are able to carry out operations like turning, facing, chamfering and grooving as well.

6.5.4 Drilling Process Drilling process mainly involves a long metallic bar or drill (made out of high speed steel) which is used to make deep and through in holes for various assembly applications (such as holes for bolt, rivets and screws) and for design purposes (such as for ventilation and appearance). The drills are generally very tough and are flexible enough to absorb torsion and bending deflections. The flute region of the drill ensures smooth removal of the unwanted chips from deep inside the drilled hole during the operation. The most commonly used drills are the twist drills that have a pair of chisel shaped cutting edges running longitudinally in helix along the length of the drill with a tapered tip. The normal rake angle of the cutting edge and the velocity varies with the distance from the axis of the drill. The internal surface of the hole produced by drilling consists of circumferential marks on it which are caused by the rotational friction between the surface of the hole and the outer diameter of the tool. Finishing operations are thus carried out after the drilling operation such as reaming and honing. The bottom surface of the hole develops small burrs or sharp edges on it after a drilling operation. So a deburring operation is required post drilling. The drilled hole diameter is generally larger in size than the diameter of the drill tool itself which can be realised while lifting out the drill from the hole and which does seem to be very easy. Some materials with high thermal expansion also tend to expand by the frictional heat produced due to the interaction between the tool and the hole internal surface. In such cases, the final hole becomes lesser in diameter to the drill diameter. The drill bars are often seen subjected to bending stresses which is caused mainly due to the thrust force applied onto the bar by the inner wall of the hole during drilling. This thrust force is very difficult to measure and it depends on a number of factors such as work material, feed, drill geometry, drill diameter and cutting fluids. Thrust forces can vary from few newtons to 100 kN depending on the size and strength of the work material and the drill respectively. Material removal rate (MRR) in drilling is generally given by the expression. 2 M R R = π 4D f N , where ‘D’ is the diameter of the drill, ‘f’ is the feed and ‘N’ is the rotational speed.

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6.5.5 Reaming Process Reaming process involves a multiple tooth cutting tool (also known as reamers) made out of high speed steel or solid carbide, which is used to enhance the accuracy of the hole that has already been made previously by the drilling and boring operations. It smoothen the inner wall of the hole to perfect surface finish. Reamers can have straight or helically fluted edges that remove material from the surfaces in very small amount of layer thickness of about 0.2 mm on the diameter in a ductile metal whereas a layer thickness of about 0.13 mm in case of harder metal. Fluted reamers generally have a rake angle of 5° and are employed for light cuts. Another type of machine reamer named rose reamer is used prior to the fluted reamer and is employed to remove materials in high amount for perfect true up of the hole. Reaming does not require a separate machine for the operation and the same drilling machine can be employed by just mounting the reamer tool into the spindle. Guide blocks can be clamped above and below the work piece to perfectly align the reamer into the hole.

6.5.6 Planning Planning is the operation where large work pieces of dimensions ranging in (25 * 15) m2 can be grooved or edge cut by moving the work piece mounted on a table, forth and back in straight lines below the cutting tool which is generally made out of high speed steel and carbides. Cutting takes place during the forward movement of the work piece and remain idle during the return stroke. To avoid surface wear out of the tool during the return stroke, cutting tools are generally lifted up above the work piece mechanically or hydraulically. Cutting tools in the planning process are generally mounted onto the heads that can be moved on horizontal rails and vertically up and down along the ways made onto columns. The drawbacks of the planning process lies in its unproductive time spent during the return stroke. Since the material used is long and cuts are made along the length, a lot of time is consumed by reverting back the work piece after a full cut and then starting a new cut. This makes the process less economic and efficient and can be tackled by cutting the work even during the return stroke. Cutting tools are sometimes equipped with chip breaker which prevents interference of the long chips with the cutting operation. Cutting speed varies from 3 to 6 m/min in case of cast irons and stainless steel and up to 90 m/min in case of aluminium and magnesium. Power ranges up to 110 kW and feed ranges from 0.5 to 3 mm/stroke.

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6.5.7 Shaping Shaping is similar to the planning process, the difference being only that it uses smaller work pieces of size ranging in 1 * 2 m2 and the tool reciprocates to make cuts and grooves on the work piece rather than the movement of the work piece as in the case of planning. The cutting tool is generally mounted on a ram that reciprocates to and fro to give the tool its cutting and return stroke.

6.5.8 Sawing Sawing is the process where material is removed by the reciprocating motion of a thin metallic sheet with teeth being cut on its edges. The teeth are generally made of carbide or high speed steel which individually removes a bit of a material in each stroke. Materials belonging to both metallic and non metallic in type can be cut efficiently without wastage of much material. This reduction of wastage can be related to the kerf of the cut made by the sawing operation which is very narrow. Three sets of teeth are generally used in sawing blades based on the pattern or orientation of the individual tooth namely, straight tooth, raker tooth and wave tooth. As these teeth are arranged in different orientation, the blades are widened at the cutting edge which enables the blade to remain cool by not heating up due to frictional resistance between the blade surface and the walls of the cut in the work piece. Cutting fluids are generally used during cutting to obtain a smooth and accurate cut and to increase the life of the teeth. Hand hacksaws and power hacksaws are commonly employed sawing equipments which are manual and power operated respectively. These are having straight blades with teeth being cut on its edges which are used for cutting rods, tubes sheets, bars, etc. Some other types of saws are circular saw and band saw.

6.5.9 Filing Filing is a process by which material is removed in very small quantity from surfaces, edges, holes, corners, etc. employing long files of various cross sections and shapes. Cross sections of files are generally available in round, half round, flat and triangular shapes. Few files are tapered towards the front while others are straight throughout and are basically made of hardened steel. Files are available with different orientations of its teeth on both of its sides or only on a single side. The teeth can be finer or coarser depending on the grades of teeth on the file and are decided based on the operation and the level of surface finish needed. Commonly used files are band files and rotary files.

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6.5.10 Broaching Broaching is a process of material removal by involving a long cutting tool (or broach) with teeth being cut on its surface and reciprocating back and forth longitudinally to remove the material from the surface of the work piece either in the forward or backward stroke. The teeth on the surface of the broach are gradually increasing in nature of its heights and therefore with every set of circumferentially arranged teeth interacting with the work material, the broach goes deeper into the work material. The total depth of cut in a broach operation in one stroke is thus the addition of the depth of cuts of individual sets of circumferentially arranged teeth. Broaching is mainly employed to shape the surfaces of long solid or hollow bars, deep holes with circular, square and triangular cross sections, splines and outer surfaces as well. The completed part is much enhanced in its surface finish and dimensional accuracy. The series of teeth being cut on the broach tool has a rake angle ranging from 0 to 20° and clearance angle ranging from 1 to 4°. In most of the broach tool, chip breakers are installed which prevents long chips to interfere with the cutting operation.

6.6 Summary From the above discussion, it is clear that there is prevalence of complex phenomenon in metal cutting operation where a number of factors collectively influence the economy, accuracy, predictability, power consumption and tool durability of a machining operation. Various mathematical expressions for metal deformation and flow associated with machining are described in detail in the chapter which arises from an important assumption of the involvement of a shear plane in chip removal site in the machining process. This shear plane inclination with the cutting direction depends on the cutting conditions and the material of the work piece. Three types of chip formation namely, continuous chip, discontinuous chip and chip with build up edge formation have been discussed, among which the continuous chip is considered the most desirable one as it ensures a lower cutting force, lower power consumption, higher tool life, predictability and better surface finish whereas, chip with build up edge formation is the least desirable. As continuous chips are considered an undesirable factor in automation as in CNC machine due to formation of long chips which interfere with the cutting operation, the occurence can be prevented by employing chip breakers on tools which curls and break the chip into small fragments that can easily be separated out from the machine. Various machines are involved for metal cutting processes, the major one being the lathe machine, milling machine and drilling machine. Other operations carried out in these machines can be classified based on the type and amount of the material being removed and its purpose in the production run. These are turning, planning and shaping, reaming, boring, sawing, filing and broaching.

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References 1. Kalpakjian, S., & Schimid, S. R. (2009). Manufacturing, engineering and technology (p. 6). Prentice Hall. 2. DeGarmo, E. P., Black, J. T., & Kohser, R. A. (2008). Materials and processes in manufacturing (p. 8). USA: Prentice Hall. 3. Beddoes, J., & Bibby, M. J. (2003). Principle of metal manufacturing processes. Elsevier Butterworth Heinemann. 4. https://upload.wikimedia.org/wikipedia/commons/b/b3/Fay_automatic_lathe.jpg. 5. https://upload.wikimedia.org/wikipedia/commons/a/af/Universal_Milling_Machine_% 28193%29.jpg. 6. https://cdn.pixabay.com/photo/2015/02/23/00/03/drilling-machine-645617_960_720.jpg.

Index

A Acetylene, 66–68 Acoustic emission, 30 Air-acetylene gas welding, 68 Allowances, 37, 40, 60 Alloy cast iron, 5, 6 Alloying, 5, 6, 9, 11, 12, 14, 37, 70 Alloy steel, 4, 7, 13, 14, 70, 71 Alpha iron, 5, 7 Alumina, 15, 17 Aluminium, 4, 11–15, 17, 19, 26, 27, 43, 48, 50, 51, 56, 68, 72, 73, 76, 78, 79, 86, 97 Annealing, 7, 9, 10, 19 Arc welding, 66, 69–76, 80 B Bainite, 9, 14 Bakelite, 16 Baking, 44 Barrelling, 23, 59 Barrelling effect, 23, 59 Batch type core ovens, 44 Bed, 10, 14, 90 Bendability, 61 Bend allowance, 60 Bending of sheets, 60 Billets, 55 Blanking, 53, 59, 60 Blast furnace, 4, 7, 19 Blooms, 55 Body Centred Cubic (BCC), 5, 7–9, 32 Boring bars, 14, 95 Boring process, 95 Bottom gates, 46 Braze welding, 78, 79

Brazing operation, 74, 78–80 Brinell hardness test, 24, 25 Brittle failure, 31, 32 Broaching, 85, 99 Buckling, 31 Build up edge chip, 88 Bulk forming, 53, 55, 62 C Carbon arc welding, 74 Carriage assembly, 91 Casting, 5, 10, 12, 37–43, 45–52, 54, 74, 86, 90, 95 Cast iron, 4–7, 10, 14, 19, 71, 86, 87, 90, 97 Cavitation, 30, 45 Cementite, 5, 7–10 Centrifugal casting, 49, 52 Ceramic, 3, 4, 15–19 Ceramic matrix composite, 17, 18 Ceramic mould casting, 51 Ceramics, 3, 4, 15, 16, 19, 22, 29 Charpy test, 28 Chip breaker, 88, 97, 99 Chvorinov’s rule, 39, 47 Closed die forging, 59 Cohesiveness, 42, 43 Coke, 5, 6, 15, 19 Cold chamber die casting, 48 Cold shut, 46 Cold working, 53, 54, 57, 58, 61 Collapsibility, 42, 43 Compactability, 43 Composites, 4, 17–19, 22, 28 Compression test, 21–23, 43 Continuous casting, 49, 52

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102 Continuous chip, 88, 99 Cope and drag patterns, 42 Copper, 4, 6, 11–14, 17–19, 48–50, 56, 68, 71, 76, 86 Corrosion resistance, 4, 6, 11, 12, 86 Crater wear, 85, 89 Creep test, 21, 22, 27 Crystal lattices, 5 Crystobalite, 15 Cupola furnace, 6, 7 Cutting ratio, 86, 87 Cyaniding, 13, 14 D Deep drawing, 62 Destructible tests, 21, 22, 28 Diamond, 3, 18, 19, 25 Die casting, 48, 52 Dielectric bakers, 44, 45 Direct extrusion, 57 Directional solidification, 39, 46, 47, 51 Discontinuous chips, 88, 99 Down milling (climb milling), 94 Draft allowance, 40 Drawing, 19, 23, 44, 53, 57, 58, 62 Drilling machines, 92 Drilling process, 96 Drill press, 92 Drills, 92, 96 Ductile failure, 31 Ductility, 5, 6, 9, 10, 12, 17, 22, 32, 53, 56, 58–62 E Eddy current testing, 30 Electro gas welding, 72, 73 Embossing, 62 Embrittlement, 12, 70 End milling, 92, 95 Expanded polystyrene, 41 Extrusion, 23, 53, 56–58, 62 F Face-centred cubic, 5 Face milling, 95 Fatigue testing, 25 Feldspar, 15 Ferrous metals, 3–7, 10, 11, 13, 19, 51, 67, 68, 71, 86 Filing, 98, 99 Filler metal, 65, 66, 68–70, 76, 78–80 Fillets, 42, 71, 95

Index Flank wear, 85, 89 Flash, 59, 77 Flint, 15 Fluidity, 5, 14, 38, 40, 45, 46, 51 Fluted reamers, 97 Flux, 7, 37, 65, 68–73, 76, 78, 79 Forging, 11, 28, 58, 59, 62 Forming process, 23, 50, 53, 54, 60, 62 Friction welding, 66, 77, 78, 80 Fusion welding, 66, 80 G Gamma iron, 5 Gas Metal Arc Welding (GMAW), 70, 71 Gas Tungsten Arc Welding (GTAW), 73 Gas welding, 66–68, 71–73, 80 Gates, 27, 37, 38, 41, 42, 45, 46, 49, 51, 59 Glassy state, 15 Graphite, 3, 5, 17–19, 44, 49, 74 Grey cast iron, 5, 10, 90 H Hardening, 7, 13–15, 19, 27, 54, 60, 68 Hardness test, 21, 22, 24, 25, 34 Headstock, 90 Helical teeth, 95 Hematite, 7 Horizontal type milling machine, 96 Hot chamber die casting, 48 Hydrostatic extrusion, 57 I Impact test, 21, 22, 27, 28 Indirect extrusion, 57 Induction hardening, 13, 15 Interstitial sites, 3 Investment mould casting, 50 Iron fillings, 15, 29 Izod test, 28 J Jolt machine, 44, 51 K Kaolinite, 15, 42 Keramos, 15 L Laser Beam Welding (LBW), 66, 76, 77 Lathe machine, 85, 89, 90, 99 Limestone, 5–7, 19 Limonite, 7

Index Liquid penetration, 31 Lower casting fits, 90 M Machinability, 5, 6 Machining, 11, 12, 18, 21, 33, 38, 44, 50, 51, 54, 55, 58, 85–89, 92, 93, 99 Magnesium, 4, 11, 12, 16, 17, 19, 43, 48, 50, 56, 68, 71–73, 86, 97 Magnetic particle test, 29 Magnetic separation, 7 Martensite, 9, 10, 14 Match plate patterns, 41 Material removal rate, 93, 94, 96 Mechanical fasteners, 66, 78, 80 Merchant’s force model, 85 Metal arc welding, 70, 71 Metal matrix composites, 17, 18 Mild steel, 6, 10, 19, 78 Milling machines, 91, 92, 99 Misrun, 40, 46 Moulding sand, 37, 41–44, 47, 51 N Neutral flames, 68 Neutrons, 4, 18 Nickel, 4, 6, 11, 12, 18, 19, 29, 71, 86 Nickel alloys, 12, 71 Nitriding, 13, 14 Non Dstructible Tests (NDT), 21, 28, 34 Nucleation, 37, 39 Nuclei, 4 O Oblique cutting, 86 Open die forging, 58, 59 Orthogonal cutting, 86 Oxidizing flames, 68 Oxy-acetylene flame, 14 Oxy-acetylene gas welding, 66–68 Oxy-hydrogen gas welding, 68 P Parting gates, 46 Pattern, 5, 37, 38, 40–45, 50, 51, 98 Pearlite, 8–10 Pellets, 7 Peripheral milling, 95 Permeability, 42, 43, 50 Phenol formaldehyde, 16 Pig iron, 4–7, 19 Planning, 97–99 Plasma Arc Welding (PAW), 74, 75 Plaster mould casting, 37, 50, 52

103 Plastic deformation, 21, 30–34, 54, 57, 65, 77, 78, 80, 86 Plasticity, 6, 53, 54, 62 Polymer matrix composites, 17 Polymers, 3, 4, 16, 17, 19, 44 Polymorphism, 15 Pouring basin, 37, 45, 46, 49 Pouring temperature, 38, 40, 51 Precipitation hardening, 13 Protons, 4 Punching, 59, 60 Q Quenching, 7, 14 R Radiography, 21, 29 Reaming process, 97 Re-crystallization temperature, 54 Refractoriness, 4, 19, 42, 44, 50, 86 Residual stresses, 10, 21, 33, 34, 42 Resistance seam welding, 76 Resistance spot welding, 76 Resistance welding, 66, 75, 76, 80 Reverse Polarity Direct Current (RPDC), 69, 71, 80 Riser design, 45, 47 Risers, 37–41, 45–47 Rockwell hardness test, 51 Roll bonding, 78 Rolling, 10, 23, 53, 55, 56, 59, 62 Runners, 42, 45, 46, 49 S Sand casting, 38, 40, 51 Sand conditioning and testing, 42 Sawing, 85, 98, 99 Semi centrifugal casting, 49 Shaping, 85, 89, 98, 99 Shearing, 53, 55, 59–62 Shielded Metal Arc Welding (SMAW), 70, 71 Shock loading, 5, 6 Side risers, 47 Single piece pattern, 41 Slab milling, 95 Slag, 7, 65, 68–72, 79 Slurry, 50 Slush casting, 47 Soldering, 66, 68, 78–80 Solidification, 37–40, 42, 45–48, 51, 66, 68, 71, 74, 76, 79 Solidification shrinkage, 40, 42, 45, 47, 48, 51 Solution treatment method, 13 Spheroidite, 8, 9

104 Spinning, 61, 62 Split patterns, 41 Springback, 61, 62 Sprue, 45, 46, 49 Squeezer machine, 44, 51 Stainless steel, 6, 10, 13, 19, 51, 70, 71, 78, 97 Straight Polarity Direct Current (SPDC), 69, 80 Submerged Arc Welding (SAW), 71, 72, 98 T Taconite, 7 Tailstock, 90 Tailstock quill, 90 Tempered martensite, 10 Tempering, 7, 9, 10, 14, 19 Tensile strength, 5, 6, 10, 22, 24, 25, 34 Titanium, 4, 11, 12, 16–19, 72–74, 86 Titanium aluminide, 12 Top gate system, 46 Top risers, 47 Transmission gear mechanism, 90 Tridymite, 15 True centrifugal casting, 49 Tungsten carbide balls, 24

Index Turning process, 93–95 U Ultrasonic inspection, 21, 29 Unit cells, 3, 5 Universal testing machine, 22, 23 Up milling (conventional milling), 94, 95 Upper casting fit, 90 Urethane, 41 V Vertical type milling machine, 92 Vicker hardness test, 25, 34 W White cast iron, 5, 6, 10 Wire drawing, 19, 23, 53, 58 Work hardening rate, 27 Wrought iron, 4, 6, 7, 10, 19, 86 Z Zirconia, 15