Laser Material Processing Springer-Verlag London Ltd. William M. Steen Laser Material Processing Third Edition With 257 Figures , Springer William M. Steen, PhD, MA, CEng British Library Cataloguing in Publication Data Steen, W.M. Laser material processing. -3rd ed. 1. Lasers -Industrial applications I. Tide 621.3'66 Library of Congress Cataloging-in-Publieation Data A catalog record for this book is available from the Library of Congress. Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographie reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency. Enquiries concerning reproduction outside those terms should be sent to the publishers. ISBN 978-1-85233-698-1 ISBN 978-1-4471-3752-8 (eBook) DOI 10.1007/978-1-4471-3752-8 http://www.springer.co. uk © Springer-Verlag London 2003 Originally published by Springer-Verlag London Limited in 2003. 1s t edition published in 1991 2nd edition published in 1998 The use of registered names, trademarks etc. in this publieation does not imply, even in the absence of a specific statement, that such names are exempt from the relevant laws and regulations and therefore free for general use. The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or liability for anyerrors or omissions that may be made. Typesetting: Sunrise Setting Ud, Torquay, Devon, UK 69/3830-543210 Printed on acid-free paper SPIN 10894150 To Margaret, Pip, Melanie, Donald, Jane, William and Jonathan The stimuli Jor this emission Acknowledgements The author would like to acknowledge the support he has gained from the enthusiasm of the many students who have passed through the research schools he has led at Imperial College and Liverpool University. Many of these now run their own laser businesses, teach the subject or have found other ways of making money from laser material processing. This is one of the greatest pleasures an academic can have. He is particularly grateful to Professor Ken Watkins, who now leads the Liverpool laser group, for writing Chapter 9 on laser cleaningj Dr Bill O'Neill, who now runs his own laser group at Cambridge University, for his help in editing the original vers ions of Chapters 1 and 2 and some of the material for Chapter 7j to Professor Lin Li, who now runs his own laser group at UMIST, for his help in editing the original version of Chapter 10j to Dr Rehan Akhter, who works with lasers in Pakistan, for his help in editing and assembling the illustrations for the original version of Chapter 4j to Professors Dennis Hall and Julian Jones of Heriot-Watt University for guidance on brightness and aspects of focussingj and last but not least to his wife, Margaret, who created an ambience in which to live and complete this third edition. On the principle of a picture being worth a thousand words and a spoonful of sugar helping the medicine go down, the author is also most grateful to the artistry of the two cartoonists Patrick Wright and Noel Ford. The cover picture of a Nd-YAG laser drilling was taken for the opening of the new Laser Engineering Centre in Birkenhead in 1997. The photograph is courtesy of Bernard Rose Photography. Contents Prologue ................. . 1 1 Background and General Applications . 11 1.1 How the Laser Works . . . . . . . . 11 1.1.1 Construction........ 11 1.1.2 Stimulated Emission Phenomenon 14 1.2 Types ofIndustrialLasers . . . . 22 1.2.1 Carbon Dioxide Lasers . 22 1.2.2 Carbon Monoxide Lasers 28 1.2.3 Solid State Lasers 28 1.2.4 Diode Lasers . . . . 32 1.2.5 Excimer Lasers 33 1.3 Comparison Between Lasers 34 1.4 Applications of Lasers . 34 1.4.1 Powerful Light 36 1.4.2 Alignment 36 1.4.3 Measurement of Length . 38 1.4.4 Pollution Detection 40 1.4.5 Velo city Measurement 41 1.4.6 Holography . 45 1.4.7 Speckle Interferometry 46 1.4.8 Inspection 47 1.4.9 Analytic Technique 48 1.4.10 Recording. 49 1.4.11 Communications 51 1.4.12 Heat Source . 52 1.4.13 Medical . 52 1.4.14 Printing ..... 56 1.4.15 Isotope Separation 56 1.4.16 Atomic Fusion .. 56 1.5 Market for Laser Applications . 58 2 Basic Laser Optics . . . . . . . . 61 2.1 The Nature ofElectromagnetic Radiation. 61 2.2 Interaction of Electromagnetic Radiation with Matter 64 2.2.1 Nonlinear Effects 67 2.3 Reflection or Absorption . . . 69 ix x Contents 2.3.1 Effect of Wavelength 71 2.3.2 Effect of Temperature . 71 2.3.3 Effect of Surface Films 72 2.3.4 Effect of Angle of 1ncidence . 72 2.3.5 Effect of Materials and Surface Roughness 74 2.4 Refraction..... 74 2.4.1 Scattering. 76 2.5 1nterference.... 77 2.6 Diffraction.... 78 2.7 Laser Beam Characteristics 78 2.7.1 Wavelength ... . 78 2.7.2 Coherence ... . 79 2.7.3 Mode and Beam Diameter 79 2.7.4 Polarisation ..... 81 2.8 Focusing with a Single Lens . 83 2.8.1 Final Spot Size 84 2.8.2 Depth of Focus 94 2.9 Optical Components 95 2.9.1 Lens Doublets . 95 2.9.2 Depolarisers 95 2.9.3 Collimators .. 96 2.9.4 MetalOptics 97 2.9.5 Diffractive Optical Elements - Holographie Lenses 98 2.9.6 Laser Scanning Systems . 100 2.9.7 Fibre Delivery Systems 100 3 Laser Cutting. . . . . . . . . . 107 3.1 1ntroduction........ 107 3.2 The Process - How It 1s done 108 3.3 Methods of Cutting . . . . . 111 3.3.1 Vaporisation Cutting/Drilling . 111 3.3.2 Fusion Cutting - Melt and Blow . 114 3.3.3 Reactive Fusion Cutting . 121 3.3.4 Controlled Fracture . 122 3.3.5 Scribing......... 123 3.3.6 Cold Cutting .... . . 123 3.3.7 Laser Assisted Oxygen Cutting - The LASOX Process . 124 3.4 Theoretical Models of Cutting . 125 3.5 Practical Performance 126 3.5.1 Beam Properties .. 126 3.5.2 Transport Properties 131 3.5.3 Gas Properties .. 133 3.5.4 Material Properties 138 3.5.5 Practical Tips . . . 139 3.6 Examples of Applications 140 3.6.1 Die Board Cutting . 141 Contents xi 3.6.2 Cutting of Quartz Tubes . 141 3.6.3 Profile Cutting ... 141 3.6.4 Cloth Cutting . . . . 142 3.6.5 Aerospace Materials . 142 3.6.6 Cutting Fibre Glass 142 3.6.7 Cutting Kevlar ... 142 3.6.8 Prototype Car Production . 142 3.6.9 Cutting Alumina and Dielectric Boards 143 3.6.10 Furniture Industry . . . . 143 3.6.11 Perforated Irrigation Pipes 144 3.6.12 Perforating Cigarette Paper 144 3.6.13 Flexographic Print Rolls 144 3.6.14 Cutting Radioactive Materials . 144 3.6.15 Electronics Applications 145 3.6.16 Hole Drilling . . 145 3.6.17 Scrap Recovery 147 3.6.18 Laser Machining 148 3.6.19 Ship Building . . 148 3.6.20 The Laser Punch Press 148 3.6.21 Manufacture ofBikes and Tubular Structures 149 3.6.22 Cutting and Welding of Railcars . 149 3.7 Costed Example ........... . 149 3.8 Process Variations ......... . 149 3.8.1 Are Augmented Laser Cutting . 149 3.8.2 Hot Machining . . . . 150 3.9 Future Developments . . . . . . . 151 3.9.1 Higher Powered Lasers .. 151 3.9.2 Additional Energy Sources 151 3.9.3 Improved Coupling 151 3.9.4 Smaller Spot Size 151 3.9.5 Increased Drag .. 152 3.9.6 Increased Fluidity . 152 3.10 Worked Example ofPower Requirement 152 4 Laser Welding . . . . . 157 4.1 Introduction.... 157 4.2 Process Arrangement 160 4.3 Process Mechanisms - Keyholes and Plasmas . 162 4.4 Operating Characteristics . 166 4.4.1 Power....... 166 4.4.2 Spot Size and Mode 170 4.4.3 Polarisation. 172 4.4.4 Wavelength.. 172 4.4.5 Speed..... 174 4.4.6 Focal Position . 175 4.4.7 Joint Geometries 176 xii Contents 4.4.8 Gas Shroud and Gas Pressure . . . . . . . . . . . . . . . 182 4.4.9 Effeet of Gas Pressure - Due to Velocity and Environment 185 4.4.10 Effeet of Material Properties 186 4.4.11 Gravity . . . . . . . . . . . . 187 4.5 Proeess Variations .......... 188 4.5.1 Are Augmented Laser Welding 188 4.5.2 Twin Beam Laser Welding. . 190 4.5.3 Walking and Spinning Beams 191 4.6 Applieations ..... 191 4.6.1 Tailored Blanks 193 4.7 Costed Example . 195 5 Heat Flow Theory . 201 5.1 Introduetion.. 201 5.2 Analytie Models in One-dimensional Reat Flow 205 5.3 Analytie Models for a Stationary Point Souree 207 5.3.1 The Instantaneous Point Souree. . 207 5.3.2 The Continuous Point Souree . . . 209 5.3.3 Sourees Other than Point Sourees. 210 5.4 Analytic Models for a Moving Point Souree 210 5.5 Alternative Surfaee Heating Models. . . . 212 5.5.1 The Ashby-Shercliffe Model: The Moving Hypersurfaee Line Souree ....................... 212 5.5.2 The Davis et al. Model: The Moving Gaussian Souree 212 5.6 Analytic Keyhole Models - Line Souree Solution 213 5.7 Analytic Moving Point-Line Souree Solution 216 5.8 Finite-differenee Models . 217 5.9 Semi-quantitative Models 219 5.10 Flow Models 222 5.11 Stress Models . 223 5.12 Conclusions.. 223 5.13 List of Symbols 223 6 Laser Surfaee Treatment 227 6.1 Introduetion.... 227 6.2 Laser Heat Treatment 229 6.2.1 Reat Flow .. 232 6.2.2 Mass Flow by Diffusion 237 6.2.3 Meehanism of Transformation Proeess 237 6.2.4 Properties of Transformed Steels 239 6.3 Laser Surfaee Melting . . . . . . . 242 6.3.1 Solidifieation Meehanisms 246 6.4 Laser Surfaee Alloying .. 251 6.4.1 Proeess Variations 251 6.4.2 Applieations 252 6.5 Laser Cladding . . . . . . 253