CO2 Laser Cutting

256 Pages·1993·3.665 MB·English

by  Dr John Powell DSc, DIC, CEng, CPhys (auth.)

CO Laser Cutting
2
John Powell
CO Laser Cutting
2
With 103 Figures
Springer-Verlag
London Berlin Heidelberg New York
Paris Tokyo Hong Kong
Barcelona Budapest
Dr John Powell, DSc, DIC, CEng, CPhys
Technical Director, Laser Expertise Limited, Unit H, Acorn
Park Industrial Estate, Harrimans Lane, Nottingham
NG72TR, UK
Visiting Professor, Dept. of Materials Processing, Lulea
University of Technology, S-951 87 Lulea, Sweden
Academician, Engineering Academy of St. Petersburg, Russia
Cover illustration: Ch. 5, Fig. 7b. Nozzle with a movable lens.
ISBN-13: 978-1-4471-3386-5 e-ISBN-13: 978-1-4471-3384-1
DOl: 10.1007/978-1-4471-3384-1
British Library Cataloguing in Publication Data
Powell, John
CO Laser Cutting
2
I. Title
671.53028
Library of Congress Cataloging-in-Publication Data
Powell, John, 1955-
CO laser cutting/John Powell.
2
p. cm.
Includes index.
(Springer-Verlag Berlin Heidelberg New York:
acid-free paper). - (Springer-Verlag New York
Berlin Heidelberg: acid-free paper): $85.00 (approx.)
1. Laser beam cutting. 2. Carbon dioxide lasers - Industrial
applications. I. Title.
TJ1191.P68 1993 92-36190
671.5'3 - dc20 CIP
Apart from any fair dealing for the purposes of research or private study, or
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© Springer-Verlag London Limited 1993
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Typeset by Best-set Typesetter Ltd., Hong Kong
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Contents
Foreword .......................................... xi
Introductiou . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii
Acknowledgements .................................. xv
Notes on the Units Used .............................. xvii
1. The Basic Principles. . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 The Cutting Process. . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.1 Cutting Parameters . . . . . . . . . . . . . . . . . . . . . . 5
1.2 How Materials Respond to Laser Light . . . . . . . . . . . 6
1.3 How CO Lasers Work. . . . . . . . . . . . .. . . .. . . . . .. . 7
2
1.3.1 The Storage and Release of Energy from a
CO Molecule .......................... 8
2
1.3.2 Lasing................................. 9
1.3.3 The Role of Nitrogen. . . . . . . . . . . . . . . . . . . . 10
1.3.4 The Role of Helium. . . . . . . . . . . . . . . . . . . . . 12
1.3.5 Gas Mixture Cooling. . . . . . . . . . . . . . . . . . . . . 13
1.3.6 Gas Recirculation. . . . . . . . . . . . . . . . . . . . . . . 14
1.3.7 A Summary of the Basic Energy Exchanges
in a CO Laser. . . . . . . . . . . . . . . . . . . . . . . . . . 14
2
1.4 Laser Modes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.4.1 Mode Types. . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.4.2 A Quick Guide to TEM Nomenclature. . . . . 18
1.4.3 The Effect of Laser Design on Mode and the
Implications for Materials Processing. . . . . . . 18
2. Laser Cutting Steels. . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.1 Mild steels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.1.1 Introduction............................ 23
2.1.2 The Cutting Mechanism for Mild Steels. . . . . 24
2.1.3 Cutting Speeds and Cut Quality. . . . . . . . . . . 26
2.1.4 Cut Initiation and Termination. . . . . . . . . . . . 31
2.1.5 Techniques to Avoid Unwanted Material
Burning................................ 35
2.1.6 The Effect of Sheet Surface Finish on the
Cutting Process. . . . . . . . . . . . . . . . . . . . . . . . . 40
vi Contents
2.1.7 The Importance of Axial Symmetry of the
Energy Input to the Cutting Zone ......... 44
2.1.8 Pulsed Laser Cutting of Mild Steels. . . . . . . . 50
2.2 Alloy Steels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
2.2.1 Introduction............................ 56
2.2.2 Stainless Steels. . . . . . . . . . . . . . . . . . . . . . . . . . 57
2.2.3 Low Alloy Steels . . . . . . . . . . . . . . . . . . . . . . . . 65
2.2.4 Silicon Iron. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
2.2.5 Free Cutting Steels . . . . . . . . . . . . . . . . . . . . . . 67
2.2.6 Carbon Steels. . . . . . . . . . . . . . . . . . . . . . . . . . . 67
3. Cutting Non-ferrous Metals ..................... 71
3.1 Introduction.................................. 71
3.2 Titanium Alloys. . . . . . . . . . . . . . .. .. . .. . . . . . . . . . . 74
3.2.1 Laser-Oxidation Cutting. . . . . . . . . . . . . . . . . 75
3.2.2 Laser-Inert Gas Cutting . . . . . . . . . . . . . . . . . 76
3.3 Aluminium Alloys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
3.3.1 Introduction............................ 82
3.3.2 The Effect of Surface Condition on Cutting
(Anodising etc.). . . . .. . . . . . . . ... . . . . . . . . . 84
3.3.3 Pulsed Laser Cutting. . . . . . . . . . . . . . . . . . . . . 86
3.3.4 Piercing a High Reflectivity Metal. . . . . . . .. 86
3.4 Nickel Alloys. . . . . . . . . . . . . . . .. . .. . . . . . . . . . . ... 87
3.5 Copper Alloys ................................ 88
3.6 General Comments... ......................... 89
4. Cutting Non-metals ............................ 91
4.1 Introduction.................................. 91
4.1.1 Melt Shearing. . . . . . . . . . . . . . . . . . . . . . . . . . . 91
4.1.2 Vaporisation............................ 91
4.1.3 Chemical Degradation . . . . . . . . . . . . . . . . . . . 92
4.2 Polymers..................................... 93
4.2.1 Cutting by Melt Shearing. . . . . . . . . . . . . .. . . 93
4.2.2 Cutting by Vaporisation. . . . . . . . . . . . . . . . . . 96
4.2.3 Cutting by Chemical Degradation ......... 98
4.2.4 Cutting Speeds for Polymers . . . . . . . . . . . . . . 101
4.2.5 Cut Speed Forecasting for Polymers Using a
Standard Processing Curve ............... 104
4.3 Wood Based Products. . . . . . . . . . . . . . . . . . . . . . . . . . 108
4.4 Ceramics and Glasses. . . . . . . . .. . .. . . . . . . . . . . . .. 111
4.4.1 Ceramics............................... 111
4.4.2 Glasses................................ 113
4.5 Composites and Miscellaneous Materials. . . . . . . . . . 114
4.5.1 Composite Materials. . . . . . . . . . . . . . . . . ... . 114
4.5.2 Miscellaneous Materials. . . . . . . . . . . . . . . . . . 115
5. Setting Up for Cutting. . . .. . . . . . . . . . . . . . . . . . .. . . 117
5.1 Introduction.................................. 117
5.2 Beam Alignment....... . ...... ................ 117
Contents vii
5.2.1 Introduction............................ 117
5.2.2 Taking a Beam Print. . . . . . . . . . . . . . . . . . . . . 118
5.2.3 Alignment by the Beam Print Method...... 120
5.2.4 Alignment by use of a ReNe Laser... ..... 125
5.3 Finding the Focus Position. . . . . . . . . . .. . . . . . . . . .. 128
5.3.1 Introduction............................ 128
5.3.2 The Blue Flash Test ..................... 129
5.3.3 The Drilling Test. . . . . . . . . . . . .. . . . . . . . . . . 130
5.4 Nozzles and Nozzle Alignment. . . . . . . . . . . . . .. . . . 131
5.4.1 General Comments. . . . . . . . . . . . . . . . . . . . . . 131
5.4.2 Practical Considerations. . . . . . . . . . . . . . . . . . 132
5.4.3 Nozzle Alignment. . . . . . . . . . . . . . . . . . . .. . . 134
5.4.4 The Gas Dynamics of Nozzles. .. . . . . . . . .. . 138
5.5 Jigging....................................... 144
5.5.1 Methods of Supporting the Workpiece. . . . . 144
5.5.2 Laser Cut Supports. . . . . . . . .. . . . .. .. .. . . . 145
5.5.3 Chutes................................. 146
5.5.4 Overlay Jigging for Flimsy Materials. . . . . . . 146
5.5.5 Clamps and Vices ....................... 146
5.6 Tuning the Laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
5.6.1 Fine Tuning. . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
5.6.2 Mode Prints. . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
5.6.3 Use of a Laser Beam Analyser. . . . . . . . . . . . 150
5.6.4 Laser Alignment. . . . . . . . . . . .. . . . . . . . . . . . 150
6. Troubleshooting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
6.1 Introduction and Checklist. . . .. . . . . . . . . . . . . . . . . . 153
6.2 Notes on Checklist. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
6.2.1 Nozzle Contamination .................. 153
6.2.2 Laser Power Level and Type. . . . . . . . . . . . . 154
6.2.3 Cutting Speed ......................... 155
6.2.4 Cutting Gas Type, Pressure and/or Flow
Rate.................................. 155
6.2.5 Nozzle-Material Standoff . . . . . . . . . . . . . . . 156
6.2.6 Nozzle Type, Condition and Alignment ... 156
6.2.7 Material Specification. . . . . . . . . . . . . . . . . . . 159
6.2.8 Lens Type, Condition and Alignment ..... 160
6.2.9 Laser Mode Quality............... ..... 162
6.2.10 External Mirror Condition and Alignment 163
7. Safety GuideUnes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
7.1 Introduction.................................. 165
7.2 Beam Exposure.... .. ... . . ... .. ...... . ........ 166
7.2.1 Skin Damage. . . . . . . . . . . . . . . . . . . . . . . . . . . 166
7.2.2 Eye Safety ............................. 167
7.3 Fumes....................................... 168
7.3.1 Metals................................. 168
7.3.2 Non-metals............................. 170
7.4 Electrocution................................. 171
7.5 Fires......................................... 172
viii Contents
8. Alternative Cutting Methods. . . . . . . . . . . . . . . . . . . . . 173
8.1 Nd: Y AG Laser Cutting ........................ 173
8.1.1 The Principle of Operation of Nd: YAG
Lasers. . . . . . . . ... . . . . . . . . . . . . . . . . . . ... . 175
8.2 Plasma Arc Cutting . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176
8.3 Abrasive Water Jet Cutting ..... . . . . . . . . . . . . . . . . 179
8.3.1 Abrasive-Free Fluid Jets ................. 184
8.4 Oxygen-Flame Cutting........................ 184
8.5 A Summary of the Strengths and Weaknesses of
Each Process Compared with CO Laser Cutting. . . 186
2
9. The Physics and Design of CO2"L asers ............ 189
9.1 Introduction.................................. 189
9.2 The Physics of CO Lasers. . . . . . . . . . . . . . . . . . . . . . 190
2
9.2.1 The CO Energy Spectrum............... 190
2
9.2.2 Interactions in a Pure CO Laser. . . . . . . . . . 191
2
9.2.3 The Role of Nitrogen. . . . . . . . . . . . . . . . . . . . 193
9.2.4 The Role of Helium ..................... 194
9.2.5 The Energy Cycle of Mixed Gas CO
2
Lasers. . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . .. 195
9.2.6 Population Inversion.. . . . . . .. . . . . . . . . . . . . 197
9.2.7 Laser Modes and TEM Nomenclature...... 197
9.3 Aspects of the Design of CO Lasers . . . . . . . . . . . . . 200
2
9.3.1 Introduction............................ 200
9.3.2 Methods of Exciting the Lasing Gas
Mixture. . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . 201
9.3.3 Methods of Cooling the Lasing Gas
Mixture. . . . . . . . . . . . . ... . . . . .. . . .. . . . . . . 201
9.3.4 Designs of Optical Cavity. . .. . .. . . . . . . . . . . 202
10. Some Aspects of the Physics and Chemistry of Laser
Cutting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . 205
10.1 Introduction.................................. 205
10.2 The Energy Balance in the Cut Zone and Its
Relationship to the Efficiency of the Process. . . . . . . 206
10.2.1 The Effect of Decreasing the Material
Thickness on Cutting Speeds. . . . . . . . . . . . . 209
10.2.2 The Limits on Material Thickness ........ 209
10.2.3 The Implications of the Energy Balance
Argument to Changes in Laser Power. . . . . 210
10.3 The Role of Oxidation when Cutting Steels ....... 211
10.3.1 Mild Steel. . . . . . .. . . . . . . . . . . . . . .. . . . . . . 211
10.3.2 Stainless Steel ......................... 213
10.4 Conductive Losses Experienced when Cutting
Steels........................................ 215
10.5 Notes on Reflected, Transmitted, Radiated and
Convective Losses from the Cut Zone . . . . . . . . . . . . 217
10.5.1 Reflected and Transmitted Losses. . . . . . . . 218
10.5.2 Radiation and Convective Losses. . . . . . . . . 222
Contents ix
10.6 Notes on the Focusing Characteristics of CO
2
Lasers....................................... 225
10.6.1 Theoretical Focused Spot Size and Depth
of Focus .............................. 225
10.6.2 The Discrepancy Between Theory and
Practice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226
10.6.3 Concluding Comments. . . . . . . . . . . . . . . . . . 228
11. Bibliography and Further Reading. . . .. . . . . . . . ... 231
11.1 Conference Proceedings and Journals ............ 231
11.1.1 Conference Proceedings... . . .. . . . . .. . ... 231
11.1.2 Journals............................... 232
11.2 Books........................................ 233
11.3 Specific Papers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234
11.3.1 General Reviews of Laser Processing and
Cutting ............................... 234
11.3.2 Cutting Metals. . . . . . . . . . . . . . . . . . . . . . . . . 235
11.3.3 Cutting Non-metals. . . . . . . . . . . . . . . . . . . . . 236
11.3.4 Theoretical Analyses of the Cutting
Process ............................... 236
11.3.5 CO Lasers. . . . . . . . . . . . . . . . . . . . . . . . . . . . 237
2
11.3.6 Nozzle Design and Gas Dynamics........ 237
11.3.7 Laser Beam Analysis. . . . . . .. . . . . .. . . . . . 237
11.3.8 Alternative Cutting Methods.. . ... .. . . . . . 237
11.4 Miscellaneous References. . . . . . . . . . . . . . . . . . . . . . . 238
Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239
Foreword
The laser has given manufacturing industry a new tool. When
the laser beam is focused it can generate one of the world's
most intense energy sources, more intense than flames and arcs,
though similar to an electron beam. In fact the intensity is such
that it can vaporise most known materials. The laser material
processing industry has been growing swiftly as the quality, speed
and new manufacturing possibilities become better understood.
In the fore of these new technologies is the process of laser
cutting. Laser cutting leads because it is a direct process substitu
tion and the laser can usually do the job with greater flexibility,
speed and quality than its competitors.
However, to achieve these high speeds with high quality con
siderable know how and experience is required. This information
is usually carefully guarded by the businesses concerned and has
to be gained by hard experience and technical understanding.
Yet in this book John Powell explains in lucid and almost non
technical language many of these process wrinkles concerning
alignment, cornering, pulsing, water jets, material properties,
cutting speeds as well as tricks with surface coating and much
much more. It is a book which managers and technicians in laser
job shops and laser processing facilities would be foolish not to
read.
John Powell has been working with lasers since the 1970s. He
is one of the most experienced specialists in the subject of laser
cutting today. He was awarded one of the early doctorates in the
subject of laser material processing while working with me at
Imperial College. He is now the Technical Director of one of the
more successful laser job shops in the UK. Perhaps reading the
book will tell the reader why his business is successful. He is also
a visiting Professor to the University of Lulea in Sweden, an
Academician of the Engineering Academy of St. Petersburg and
a well known character at many laser conferences. Certainly the
book helps the whole subject to take a step forward and not
become constipated in secrecy. There is much more to learn
about laser material processing for it is the application of a new
energy source with the new horizons that must open for us. This
book is a step on this long and exciting journey that has just
xii Foreword
begun. But apart from all that this book is a pleasure to read - so
learn, enjoy and relax in John Powell's down to earth style!
W.M. Steen
James Bibby Professor of Engineering Manufacture,
Mechanical Engineering Department,
The University of Liverpool,
Liverpool L69 3BX