AWS C7.2 - Recommended Practices for Laser Beam Welding, Cutting, and Allied Processes [2 ed.] 9780871717771, 0871715627

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AWS C7.2M:2010 An American National Standard

Recommended Practices for Laser Beam Welding, Cutting, and Allied Processes

AWS C7.2M:2010 An American National Standard Approved by the American National Standards Institute June 24, 2010

Recommended Practices for Laser Beam Welding, Cutting, and Allied Processes 2nd Edition

Supersedes ANSI/AWS C7.2:1998

Prepared by the American Welding Society (AWS) C7 Committee on High-Energy Beam Welding and Cutting Under the Direction of the AWS Technical Activities Committee Approved by the AWS Board of Directors

Abstract This document presents recommended practices for laser beam welding, cutting, drilling, and transformation hardening. It is intended to cover common applications of the process. Processes definitions, safe practices, general process requirements and inspection criteria are provided.

AWS C7.2M:2010

International Standard Book Number: 978-0-87171-777-1 American Welding Society 550 N.W. LeJeune Road, Miami, FL 33126 © 2010 by American Welding Society All rights reserved Printed in the United States of America Photocopy Rights. No portion of this standard may be reproduced, stored in a retrieval system, or transmitted in any form, including mechanical, photocopying, recording, or otherwise, without the prior written permission of the copyright owner. Authorization to photocopy items for internal, personal, or educational classroom use only or the internal, personal, or educational classroom use only of specific clients is granted by the American Welding Society provided that the appropriate fee is paid to the Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923, tel: (978) 750-8400; Internet: .

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Statement on the Use of American Welding Society Standards All standards (codes, specifications, recommended practices, methods, classifications, and guides) of the American Welding Society (AWS) are voluntary consensus standards that have been developed in accordance with the rules of the American National Standards Institute (ANSI). When AWS American National Standards are either incorporated in, or made part of, documents that are included in federal or state laws and regulations, or the regulations of other governmental bodies, their provisions carry the full legal authority of the statute. In such cases, any changes in those AWS standards must be approved by the governmental body having statutory jurisdiction before they can become a part of those laws and regulations. In all cases, these standards carry the full legal authority of the contract or other document that invokes the AWS standards. Where this contractual relationship exists, changes in or deviations from requirements of an AWS standard must be by agreement between the contracting parties. AWS American National Standards are developed through a consensus standards development process that brings together volunteers representing varied viewpoints and interests to achieve consensus. While the AWS administers the process and establishes rules to promote fairness in the development of consensus, it does not independently test, evaluate, or verify the accuracy of any information or the soundness of any judgments contained in its standards. AWS disclaims liability for any injury to persons or to property, or other damages of any nature whatsoever, whether special, indirect, consequential, or compensatory, directly or indirectly resulting from the publication, use of, or reliance on this standard. AWS also makes no guarantee or warranty as to the accuracy or completeness of any information published herein. In issuing and making this standard available, AWS is neither undertaking to render professional or other services for or on behalf of any person or entity, nor is AWS undertaking to perform any duty owed by any person or entity to someone else. Anyone using these documents should rely on his or her own independent judgment or, as appropriate, seek the advice of a competent professional in determining the exercise of reasonable care in any given circumstances. It is assumed that the use of this standard and its provisions are entrusted to appropriately qualified and competent personnel. This standard may be superseded by the issuance of new editions. Users should ensure that they have the latest edition. Publication of this standard does not authorize infringement of any patent or trade name. Users of this standard accept any and all liabilities for infringement of any patent or trade name items. AWS disclaims liability for the infringement of any patent or product trade name resulting from the use of this standard. Finally, the AWS does not monitor, police, or enforce compliance with this standard, nor does it have the power to do so. On occasion, text, tables, or figures are printed incorrectly, constituting errata. Such errata, when discovered, are posted on the AWS web page (www.aws.org). Official interpretations of any of the technical requirements of this standard may only be obtained by sending a request, in writing, to the appropriate technical committee. Such requests should be addressed to the American Welding Society, Attention: Managing Director, Technical Services Division, 550 N.W. LeJeune Road, Miami, FL 33126 (see Annex B). With regard to technical inquiries made concerning AWS standards, oral opinions on AWS standards may be rendered. These opinions are offered solely as a convenience to users of this standard, and they do not constitute professional advice. Such opinions represent only the personal opinions of the particular individuals giving them. These individuals do not speak on behalf of AWS, nor do these oral opinions constitute official or unofficial opinions or interpretations of AWS. In addition, oral opinions are informal and should not be used as a substitute for an official interpretation. This standard is subject to revision at any time by the AWS C7 Committee on High-Energy Beam Welding and Cutting. It must be reviewed every five years, and if not revised, it must be either reaffirmed or withdrawn. Comments (recommendations, additions, or deletions) and any pertinent data that may be of use in improving this standard are required and should be addressed to AWS Headquarters. Such comments will receive careful consideration by the AWS C7 Committee on High-Energy Beam Welding and Cutting and the author of the comments will be informed of the Committee’s response to the comments. Guests are invited to attend all meetings of the AWS C7 Committee on HighEnergy Beam Welding and Cutting to express their comments verbally. Procedures for appeal of an adverse decision concerning all such comments are provided in the Rules of Operation of the Technical Activities Committee. A copy of these Rules can be obtained from the American Welding Society, 550 N.W. LeJeune Road, Miami, FL 33126.

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Personnel AWS C7 Committee on High-Energy Beam Welding and Cutting P. W. Hochanadel, Chair T. A. Palmer, 1st Vice-Chair K. W. Lachenberg, 2nd Vice-Chair M. Rubin, Secretary P. Blomquist D. D. Kautz G. R. LaFlamme E. D. Levert

Los Alamos National Laboratory Lawrence Livermore National Laboratory Sciaky, Incorporated American Welding Society Applied Thermal Sciences, Incorporated Los Alamos National Laboratory PTR Precision Technologies Inc. Lockheed Martin Missiles and Fire Control

Advisors to the AWS C7 Committee on High-Energy Beam Welding and Cutting P. E. Denney R. D. Dixon P. W. Fuerschbach R. W. Messler Jr. J. O. Milewski T. M. Mustaleski D. E. Powers R. C. Salo

Connecticut Center for Advanced Technology Retired Sandia National Laboratory Rensselaer Polytechnic Institute Los Alamos National Laboratory BWXT Y-12 LLC PTR – Precision Technologies, Incorporated Sciaky, Incorporated

AWS C7C Subcommittee on Laser Beam Welding and Cutting P. Blomquist, Chair P. E. Denney, Vice-Chair M. Rubin, Secretary R. D. Bucurel S. L. Engel D. F. Farson A. P. Hoult J. P. Hurley D. D. Kautz R. P. Martukanitz V. Merchant L. R. Migliore T.Palmer G. C. Schmid

Applied Thermal Sciences, Incorporated Connecticut Center for Advanced Technology American Welding Society WEC Welding & Machining HDE Technologies, Incorporated The Ohio State University IPG Photonics Cosma Power Laser Los Alamos Natcional Laboratory Applied Research Laboratory, Pennsylvania State University Electrical and Optical Solutions Coherent, Incorporated. Applied Research Laboratory, Pennsylvania State University Bechtel Bettis, Incorporated

Advisors to the AWS C7C Subcommittee on Laser Beam Welding and Cutting R. D. Dixon P. Hochanadel, Ex-Officio S. Jensen T. A. Jones J. O. Milewski D. E. Powers

Retired Los Alamos National Laboratory Visotek, Incorporated Ford Motor Company Los Alamos National Laboratory PTR – Precision Technologies, Incorporated

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Foreword This foreword is not part of AWS C7.2M:2010, Recommended Practices for Laser Beam Welding, Cutting, and Allied Processes, but is included for informational purposes only.

The first practical demonstration of a laser occurred in 1960. As laser output power increased through maturing technology, the use of lasers for material processing became widespread by the early 1970s. Currently, lasers are an accepted industrial tool for traditional and nontraditional materials processing operations that benefit from the laser’s unique characteristics. While the materials processing applications of lasers in industry are diverse, this document focuses on welding, cutting, drilling, and transformation hardening. The data contained in this Recommended Practices has been compiled and reviewed by the C7C Laser Beam Welding and Cutting Subcommittee of the American Welding Society, which included representatives from manufacturers and users of laser beam welding, cutting, and drilling equipment. As industrial lasers gain wider acceptance, there is a greater need for skilled process engineers and technicians. The American Welding Society is contributing in this regard, by offering these Recommended Practices for Laser Beam Welding, Cutting, and Allied Processes. The intended users of these recommended practices are engineers and technicians involved, or planning to become involved, in laser materials processing. It should be noted that the operating and processing parameters given in this Standard may not be the only parameter combinations that can be employed for successfully processing the materials and thicknesses shown. Changes in material chemistry, dimensional tolerances, laser beam characteristics, machine calibration, and other factors can produce different results. Therefore, the procedures presented here are simply meant to provide a set-up and design guide to help users to organize and learn the process of developing and refining a particular application. Comments and suggestions for the improvement of this standard are welcome. They should be sent to the Secretary, AWS C7 Committee on High-Energy Beam Welding and Cutting, American Welding Society, 550 N.W. LeJeune Road, Miami, FL 33126.

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Table of Contents Page No. Personnel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii List of Tables. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii List of Figures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii 1.

Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2.

Normative References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

3.

Terms and Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

4.

Safety Considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 4.2 General Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 4.3 Laser Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 4.4 Electrical Hazards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 4.5 Laser Radiation Hazards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 4.6 Visible Radiation Hazards. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 4.7 Hazards from Fumes and Gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5.

Equipment Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 5.1 Description of Process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 5.2 CO2 Lasers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 5.3 Nd: YAG Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 5.4 Output Characteristics of Nd: YAG Lasers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 5.5 Other Types of Material Processing Lasers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 5.6 Laser Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 5.7 Fixturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

6.

Process Control and Monitoring. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 6.2 Laser Beam Quality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 6.3 Power Density Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 6.4 Laser Power Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 6.5 Laser Beam Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

7.

Laser Beam Welding (LBW) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 7.1 Process Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 7.2 Advantages and Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 7.3 Types of Welding Lasers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 7.4 LBW Process Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 7.5 Laser–Material Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 7.6 Designing for Laser Welding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 7.7 CNC for Laser Welding. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 7.8 Thick-Section LBW with High-Power CO2 Lasers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 7.9 Thin-Section LBW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

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8.

Laser Welding Metallurgy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 8.2 Heat-Affected Zone (HAZ) Effects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 8.3 Fusion Zone Effects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 8.4 Metallurgical Considerations for Welding Various Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 8.5 Dissimilar Metal Joining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

9.

Laser Beam Cutting (LBC) and Laser Beam Drilling (LBD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 9.1 Process Descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 9.2 Advantages and Limitations of LBC and LBD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 9.3 Types of Cutting and Drilling Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 9.4 Laser Beam Cutting Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 9.5 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 9.6 Characteristics of Cuts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 9.7 CNC for Laser Cutting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 9.8 Parameter Selection for LBC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 9.9 Parameter Selection for LBD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

10.

Laser Transformation Hardening (LTH) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 10.2 The Laser Transformation Hardening Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 10.3 Equipment and Processing Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 10.4 Materials Consideration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 10.5 Process Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 10.6 LTH Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

11.

Record Keeping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 11.2 Laser Parameter Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

12.

Inspection and Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 12.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 12.2 Weld Inspection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 12.3 Inspection Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 12.4 Inspecting Laser-Cut Parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 12.5 Acceptability Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

13.

Specifications for Laser Beam Welding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 13.1 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 13.2 Alternatives to Use of Published Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

14.

Equipment Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 14.1 Maintenance Schedule and Records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 14.2 Troubleshooting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 14.3 Specific Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 14.4 Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

15.

Personnel Training . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 15.1 Safety. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 15.2 Laser System Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 15.3 Laser System Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 15.4 Other Training . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 15.5 Sources of Training . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

Annex A (Informative)—Informative References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 Annex B (Informative)—Guidelines for the Preparation of Technical Inquires . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

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List of Tables Table 5.1 5.2 9.1 9.2 9.3 9.4 9.5 11.1

Page No. Operation Comparison of CO2 and Nd:YAG Material Processing Laser . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Spatter Masking Materials for Laser Processing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Assist Gas Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 Typical Gas Pressure for CO2 Laser Beam Cutting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 Typical Power Densities for Drilling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 Typical Pulsed Laser Drilling Parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 Hole Diameters for Specific Drilling Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 Laser Equipment Set-up Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

List of Figures Figure 4.1 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 5.11 5.12 5.13 5.14 5.15 5.16 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 7.1 7.2 7.3 7.4 7.5

Page No. Sample Precautionary Sign for a Class 4 Laser System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Four Major Components of a Laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Stable Laser Resonator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Confocal Unstable Laser Resonator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Examples of laser Temporal Pulse Shapes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Spatial Intensity Distribution for a Near Gaussian Mode Laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Schematic Diagram of a Slow Axial Flow Laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Schematic Diagram of a Transverse Flow Laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Schematic Diagram of a Fast Axial Flow Laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Schematic Diagram of a Solid-State Nd:YAG Rod Laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Schematic Diagram of a Total Internal Reflection-Face Pumped(TIR-FP) Slab Laser . . . . . . . . . . . . . . . 19 Schematic of the Basic Components of a Fiber Laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Principle of Thin-Disk Laser Cavity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Laser Beam Welding Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Focusing Head. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Transmission of Laser Beam Through a Fiber Optic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Schematic of Fiber-Optic Focusing System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 The Equations for Propagation of Multimode Laser Beam Through a Focusing Lens . . . . . . . . . . . . . . . 29 Schematic Showing Focusing of a Beam in the Far Field of a Laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 An Example of the Effect of Average Power of a Pulsed Nd:YAG Focused Spot Diameter. . . . . . . . . . . 31 Comparison of Predicted and Measured Spot Size for a CO2 Laser at 300 W . . . . . . . . . . . . . . . . . . . . . . 31 Proper Lens Type Orientation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Principle for Establishing Focus Position Using an Acrylic Wedge Block . . . . . . . . . . . . . . . . . . . . . . . . 33 Example of Technique for Finding Focal Plane Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Calorimeter Type Power Meter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 Photomicrograph of Conduction Mode Weld . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Schematic View of Keyhole (Deep Penetration)Welding. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Room Temperature Reflectivity as a Function of Wavelength for Several Metals. . . . . . . . . . . . . . . . . . . 42 Absorption at 250 °C as a Function of Roughness RZ (Mean Value of Maximum Height) between Peaks and Valleys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Transition in Laser Welding Energy Transfer at the Grit Blasted Region on Aluminum for Aluminum for the Pulsed Nd:YAG Laser Beam Welding Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

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Figure 7.6 7.7 7.8 7.9 7.10 7.11 7.12 7.13 7.14 7.15 7.16 7.17 7.18 7.19 7.20 7.21 7.22 7.23 7.24 7.25 7.26 8.1 8.2 8.3 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9 9.10 9.11 9.12 9.13 9.14 9.15 9.16 9.17 9.18 10.1 10.2 10.3 10.4 10.5

Page No. Ray diagrams Showing the Refection of Light from the Workpiece for Different Lens Positions . . . . . . 44 Microprobe Profile Across 5456 Aluminum CW Nd:YAG Laser Weld . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Microhardness Traverse of 5456 Aluminum CW Nd:YAG Laser Weld . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Laser Welds in Butt Joint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 Laser Welds in Corner Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Laser Welds in T-Joints. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 Laser Welds in Lap Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Laser Welds in Edge Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 Self-Aligning Joints. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 Schematic Plasma Control Concepts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 Schematic Relationship between Power (P), Travel Speed (S), and Depth of Penetration . . . . . . . . . . . . 55 Welding Parameter to Fabricate through Thickness Weldments in ASTM A36 Plate of Different Thicknesses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Heat Input for Incipient Complete Joint Penetration vs. Laser Power for 304 Stainless Steel . . . . . . . . . . 56 Comparison of (a) Autogenous Laser Weld and (b) Narrow-Gap Laser Weld. . . . . . . . . . . . . . . . . . . . . . 57 Schematic Representation of Interaction Region between the Gas Management System, Wire Delivery System, and Laser Beam Delivery System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 Metallographic Determination of Optimum Lens Position Relative to Workpiece for Pulsed Nd: YAG Laser Beam Weld . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 Effect of CO2 Laser Power on Melting Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Comparison of Laser Beam Welding Melting Efficiency with Conventional Arc Welding Processes . . . 61 Bead-on-Plate Welding Speeds for High-Power CW Nd:YAG Laser Beam Welding of Mild Steel. . . . . 62 Welding Speeds for 1.7 kW and 2.5 kW CW CO2 Laser Beam Welding of Carbon and Stainless Steel . . . 62 Dependence of Penetration in Three Metals on the Operating Conditions for Pulsed Lasers of Three Power Ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 Conduction Mode Pulsed Nd:YAG Spot Weld in Type 304L Stainless Steel . . . . . . . . . . . . . . . . . . . . . . 66 Continuous Wave CO2 Welding Edge Joint in 17-4 PH Stainless Steel. . . . . . . . . . . . . . . . . . . . . . . . . . . 66 Subsolidus HAZ Cracks in Nitronic 60 Pulsed CO2 Weld . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 Mechanism of Laser Cutting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 Geometry of an Electromagnetic Wave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 Geometry of Reflection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 P-Polarized Light. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 S-Polarized Light. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 Reflectivity of a Dielectric Surface for P-and S-Polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 Reflectivity of a Metal for P-and S-Polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 The Effect of Polarization on Laser Cutting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 Programming Sharp Corners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 Typical Cutting Rates for CW CO2 Laser Beam Cutting of Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 Typical Cutting Rates for CW CO2 Laser Beam Cutting of Stainless Steel . . . . . . . . . . . . . . . . . . . . . . . . 88 Typical Cutting Rates for CW CO2 Laser Beam Cutting of Aluminum Alloys . . . . . . . . . . . . . . . . . . . . . 89 Cutting Rates for High-Power CW Nd:YAG Laser Beam Cutting of 1010 Steel . . . . . . . . . . . . . . . . . . . 89 Cutting Rates for Low-Power Pulsed Nd:YAG Laser Beam Cutting of Stainless Steel . . . . . . . . . . . . . . 90 Examples of Taper Effect from Relationship of Short Lens Focal Length to Material Thickness in Percussion Drilling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 Examples of Taper Effect from Relationship of Long Lens Focal Length to Thin and Thicker Materials in Percussion Drilling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 Focusing Technique for Material Processing with an Excimer Laser. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 Temporal Pulse Shaping for Drilling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 Isothermal Transformation Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 Laser Hardened Groove in AISI 4140 Steel, for Ball Bearings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 Tempering of Steel Due to Reheating by Side-by-Side Passes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 Beam Integrator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 Beam Scanner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

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Figure 10.6 11.1 11.2 12.1 14.1 14.2 14.3 14.4

Page No. Kaleidoscope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 Laser Cutting and Drilling Process Parameter Schedule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 Laser Welding Process Parameter Schedule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 Radiographic Windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 Typical Maintenance Schedule (Daily). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 Typical Maintenance Schedule (Weekly/Monthly) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 Typical Maintenance Schedule (Six Month/Yearly). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 Typical Maintenance Schedule (Periodic) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

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Recommended Practices for Laser Beam Welding, Cutting, and Allied Processes

1. Scope These recommended practices present a description of laser beam equipment and procedures that can be used for welding, cutting, drilling, and transformation hardening of various materials. These recommended practices stress the process basics, parameters, and applications. Practical information has been included in the form of figures, tables, and graphs which should prove useful in determining capabilities and limitations in the processing of various materials. Readers who desire additional information about lasers and laser materials processing should consult the Reference Documents shown in Annex A, as well as the various references given throughout this document. Any specific manufacturer product depicted in any sketch, figure, table, or product description in the document, shall not be construed as an endorsement of that particular manufacturer or product by AWS. This standard makes sole use of the International System of Units (SI). Safety and health issues may not be fully addressed by this standard. Users of this standard should consult ANSI Z49.1, Safety in Welding, Cutting, and Allied Processes, applicable federal, state, and local regulations, and other relevant documents concerning safety and health issues not addressed herein. ANSI Z136.1, Safe Use of Lasers, is another important source for safe operation of laser equipment. Please consult Clause 4 for more information.

2. Normative References The following standard contains provisions which, through reference in this text, constitute provisions of this AWS standard. For undated references, the latest edition of the referenced standard shall apply. For dated references, subsequent amendments to, or revisions of, any of these publications do not apply. AWS Documents:1 AWS A3.0, Standard Welding Terms and Definitions

3. Terms and Definitions The following terms and definitions are specific to laser beam material processing. Terms that are not part of common welding vocabulary are defined and the definitions of some standard welding terms are changed slightly to more accurately describe those terms as they apply to the laser beam process. Common terms pertaining to all welding processes that are not listed in this compilation shall be found in AWS A3.0, Standard Welding Terms and Definitions. Some of the Terms and Definitions have been extracted from The Photonic Dictionary. For the purposes of this document, the following definitions apply: absorption. The transfer of electromagnetic energy to an atom or a group of atoms that may be in a gas, gaseous plasma, liquid, or solid form. This energy may be released as heat (dissipative absorption) or re-emitted as electromagnetic energy. Most laser material processing involves the transfer of energy by dissipative absorption. 1

AWS Documents are published by the American Welding Society, 550 N.W. LeJeune Road, Miami, FL 33126.

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absorption coefficient. In the passage of a beam through an absorptive material, the loss of intensity of the beam per unit length of the material, usually expressed in cm-1. absorptivity. A dimensionless measure of absorption which has a value of 1 for a perfect absorber. For an opaque material, absorptivity has a value of 1 – (reflectance). For a transparent material, absorptivity has a value of 1 – (reflectance) – (transmittance). active medium. A substance in a laser generator in which stimulated emission of radiation, rather than absorption or spontaneous emission, will take place after pumping from an energy source has produced a population inversion. aerodynamic window. A device used to transmit the beam from inside of the resonator to the beam delivery optics in some high-power gas lasers (>10 kW). It uses a rapid flow of gas, usually air, to create a pressure gradient between laser cavity pressure and atmospheric pressure. This pressure gradient isolates the gases inside the laser cavity from the atmosphere, and prevents contamination of the laser gas. amplitude. The magnitude of the electric field in a radiation field. The intensity of the radiation in W/cm2, is proportional to the square of the amplitude. amplification. The growth of the radiation field in the laser resonator cavity. As the light wave bounces back and forth between the cavity mirrors, it is amplified by stimulated emission on each pass through the active medium. angle of incidence. In laser materials processing, the angle between the optical axis of the incident laser beam and the normal or perpendicular to the workpiece or an optical component. A perpendicular angle is most often used. aperture. The diameter of an opening through which light or laser radiation passes. Also, a device containing a hole through which some percentage of laser radiation passes to alter or measure beam characteristics. aspheric lens. A lens element in which at least one face is shaped to a nonspherical surface of revolution about the lens axis. assist gas. A gas used to blow molten metal away to form the kerf in laser beam inert gas cutting, or to blow vaporized metal away from the beam path in laser beam evaporative cutting. In some cases, the assist gas may chemically react with the laser-heated workpiece, producing additional heat which aids the laser-cutting process. attenuate. To reduce the intensity of the laser beam. This can be accomplished by passing the beam through a partially transmitting medium or through an optical element. autofocus. A method or device to automatically set the focusing head so that the focusing optic is maintained at a constant distance above the workpiece. average power. The pulsed laser beam power that is the summation of all the laser pulses averaged over time. Average power (in watts or kilowatts) can be determined by multiplying pulse energy (in joules or kilojoules) times pulse frequency (in hertz). beam alignment. Adjustment of the optical elements so that the axis of the laser beam coincides with the optical axis of the lenses, mirrors, beam tubes, etc. beam delivery system. The use of optics, such as mirrors, lenses, tubes, and fibers arranged in such a way that a laser beam can be precisely directed to a specific location in a safe manner. beam diameter. The diameter of a circular aperture which, if placed in the beam path, would transmit 86.5% (1e2 value) of the total power in the beam. beam expander. An optical system used to increase the diameter of an optical beam. In a laser, this device also serves to collimate the beam. beam quality. The focusability of a laser beam, commonly specified by the product of the radius of the resonator beam waist and the far field divergence angle. See M2. beam splitter. An optical device used to split an optical beam into two or more optical beams at predetermined power levels. beam waist. The narrowed region of a laser beam as produced by an optical resonator or at the focal plane of a focusing optic. brightness. A term often used to describe laser sources, typically by describing the power delivered into the beam focus area expressed in watts per square centimeter. The term can have several meanings, however.

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CAD. An acronym for computer-aided design describing a drawing created using computer drafting software, often used with drafting software linked to computer-aided engineering (CAE) software for calculating stress or thermal distributions of objects under load. CAD/CAM. An acronym for the process of computer-aided design and computer-aided manufacturing, where computers are used both in the design and manufacture of a part. calorimeter. A thermally sensitive device used to measure the amount of laser output power or pulsed energy. carbon dioxide laser (CO2). A gas laser which uses carbon dioxide as the lasing medium and which radiates at a wavelength of 10.6 micrometers (µm). Nitrogen and helium are also used within the laser medium for better excitation and efficiency. cavity. See resonator. circularly polarized light. A light beam in which the electrical vector is rotating around the axis of propagation at the frequency of the beam. A circularly polarized beam is often used to optimize performance in contour cutting. classification of lasers. The American National Standards Institute (ANSI) has introduced a classification scheme for lasers or laser systems, based on the laser’s potential to inflict injury to personnel. The system is based upon the intensity of emitted radiation from the laser or laser systems. The higher the classification number, from one to four, the greater the potential hazard. CNC. An abbreviation for computer numerical control, a device that controls a machine tool by reading numerical data and transforming it into movement of machine axes. coherence. A property of radiation in which the phase between any two points in the radiation field has a constant difference, or is exactly the same throughout the duration and extent of the radiation. Laser light is coherent radiation. collimated beam. Radiation in which a ray in a given point in the cross-section of a beam is nearly parallel to every other ray, with the degree of parallelism limited by the laws of diffraction. collimator. An optical system which creates a collimated beam. conduction mode welding. A melting and fusing operation in which the incident beam energy is transferred to the root of the weld solely by conductive and convective heat flow in the molten metal. See keyhole mode welding. continuous power. See continuous wave. continuous wave (CW). A laser beam which is produced continuously rather than as a series of pulses. contour cutting. Laser cutting involving motion of the beam relative to the workpiece either by moving the beam or by the workpiece in two or more axes. cosmetic pass. A partial penetration weld pass made primarily to enhance surface quality and appearance. damage threshold. The power density at which optical components such as windows, mirrors, and rods are damaged by laser radiation. depth of focus. The distance along the direction of propagation of a beam for which the power density is greater than 90% of its peak value in the focal plane. diffraction. The tendency of light to spread out as it passes through space due to wave nature of light. As laser light passes by an opaque edge or through an aperture, secondary weaker wavefronts are generated, apparently originating at that edge. These secondary wave fronts will interfere with the primary wave front as well as with each other to form various diffraction patterns. diffraction limited. The property of a high quality laser beam, whereby only the effects of diffraction determine the spot size produced by an optical system. diode laser. A type of laser in which the active medium is a semiconductor with polished end facets forming mirror surfaces. Also called semiconductor laser. Diode lasers used for alignment emit in the visible; other diode lasers emitting in the infrared are used for material processing.

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disk laser. Characterized by a heat sink and laser output that are realized on opposite sides of a thin layer of active gain medium. Despite their name, disk lasers do not have to be circular; other shapes have also been tried. Disk lasers should not be confused with Laserdiscs, which are a disk-shaped optical storage medium. divergence. The expansion angle (measured in milliradians) of an optical beam in the far field. In specifying the beam divergence, one should note whether it is the full-angle or half-angle divergence. divergence, full-angle. The expansion angle of an optical beam measure on both sides of the optic axis. divergence, half-angle. The expansion angle of an optical beam measured from the optic axis to the extremities of the beam (as specified by the beam diameter). doping. The process of adding a small concentration of a lasing ion or atom to a host material. The host material supports the dopants, influences the emission of energy from the dopants, and conducts away excess heat. dross. The melted and resolidified metal or metal oxide produced during cutting or drilling that adheres to the top or bottom edge of a cut or drilled surface. duty cycle. The fraction of time that a repetitively pulsed laser is producing pulsed output. effective spot diameter. See melted spot diameter. energy density. Laser output energy per unit area, expressed, for example, as J/cm2. energy transfer efficiency. The ratio of the heat absorbed by the workpiece to the incident laser beam energy. enhanced pulse. A laser pulse output containing an initial short-duration burst of power when the discharge is first energized, which subsequently decays to a lower level of output. See leading edge spike. excimer laser. A laser that emits ultraviolet energy from molecules called excimers that are created in pulsed electrical discharges and exist only for a very short period of time, on the order of nanoseconds. Excimer lasers use combinations of a gas such as argon, xenon, or krypton with halogens such as fluorine. Wavelengths from 0.193 to 0.348 µm have been generated. excitation. Process of transferring energy from the energy source to the active medium. far field. Far from the laser source; a term used in describing how a laser beam propagates. Mathematically, far field occurs at a distance D such that π w2 (Equation 1) D > ––––o λ where, λ = laser wavelength wo= minimum radius of the output laser beam fast axial flow (FAF). A laser design in which the active medium (gas) is transported at a high speed along the optical axis of the laser resonator so that it can be cooled by external heat exchangers. In industrial CO2 lasers, where this design is common, either a Roots pump or a centrifugal compressor is used to move the laser gas through the discharge region and heat exchangers. fiber laser. A solid-state laser design in which the active medium is a doped optical fiber. Ytterbium is the most common active element doped into the fiber. fiber optic. A small diameter solid fiber made of a transparent material such as fused quartz (or fused silica) and coated to achieve total internal reflection of a laser beam along its path. A fiber optic beam delivery system transmits the output power of the laser to the workpiece and can be bent at a shallow radius, eliminating the use of mirrors. flashlamp. A device that converts electrical energy into light by means of a sudden electrical discharge. Flashlamps are a source of excitation in a pulsed solid-state laser. fluence. See energy density. fluence threshold. The energy density level (measured in joules/cm2) at the surface of a material, at which there is sufficient energy to cause molecular bonds to break or some interaction to take place.

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F-number. The ratio of the focal length of a lens or focusing mirror to the diameter of the beam incident on the lens or mirror. Note the definition is somewhat different than that for photographic systems. focal length. In a lens or lens system, the distance from the principal plane, the surface at which the projections of an entering and exiting ray intersect, to the focal point. In a thick lens or system of lenses, the principal plane is often inside the lens itself; for setup purposes, operators often use the back focal length, which is the distance from the front surface of a focusing lens or mirror system to the focal point. focal plane. A plane through the focal point at right angles to the direction of propagation of the beam where the minimum spot size occurs. focal point. A nonstandard term for focal spot. focal spot. A location at which the beam has the most concentrated energy and the smallest cross-sectional area. focusing head. An assembly located near the end of the beam path that is used to converge an unfocused laser beam for processing materials. The assembly consists of one or more focusing lenses or mirrors, and may include assist- and purge-gas delivery system, a focal point adjustment mechanism, and optic protection devices. following error. In CNC equipment, the deviation of the actual path traced by the focused beam or by a machine tool from the programmed motion. This error is most important when making abrupt turns at high speed and can be reduced by slowing the speed. fused quartz. A glass formed by heating crystalline quartz to white heat, then cooling. It has a lower index of refraction than crystal quartz. fused silica. Glass consisting of almost pure silicon dioxide. It is purer than fused quartz. gallium arsenide (GaAs). A synthetically grown crystal used, among other things, in laser optics like lenses and mirrors. gas laser. A laser in which the lasing medium is a gas. This type of laser is subdivided by medium into atomic (such as helium–neon laser), molecular (such as carbon dioxide laser), and ionic (such as argon, krypton, xenon, or helium–cadmium laser, for example). Excimer lasers are also gas lasers. gas recirculation. A system to circulate the laser gas through the active laser region and the heat exchangers in a closed loop. The gases are cooled and re-excited continually. Commonly used in high-power lasers, a means to circulate the laser gas through the active laser region and heat exchangers in a closed loop. Gaussian distribution. A symmetric two-dimensional equation which approximately describes the spatial power distribution of many laser beams. The equation of the Gaussian distribution is: 1

–2r –––

P(r) = Poe w 2

(Equation 2)

where, P(r) = power density Po = maximum power density r = radial distance out from the center of distribution W = radius at which power density is 0.135 times maximum (see beam diameter) Gaussian mode. The fundamental transverse mode of a stable laser resonator (called TEM00 mode). A laser operating in this mode produces a beam having a Gaussian distribution. glass laser. A laser in which the active medium is an optical glass doped with a small concentration of a lasing material, typically neodymium. Glass is easier to produce than YAG, especially in large sizes, but its thermal properties are not as good as the YAG crystal. See YAG. helium-neon laser. A laser in which the active medium is a mixture of helium and neon, and which commonly has wavelengths in the visible range (400 to 700 µm). They are used widely with materials processing lasers for alignment. hermetic. The rating of a seal that prevents the exchange of fluids (gases or liquids) between a contained volume and an exterior environment.

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irradiance. See power density. infrared (IR). Electromagnetic radiation possessing wavelengths between those of visible light and microwaves, i.e., approximately 0.8 µm to 1 mm. joule. An SI unit of energy equivalent to 1 W of power lasting 1 second. Used as a unit of measure of the output of pulsed lasers. kaleidoscope. Optical device used to integrate the transverse mode of the laser beam to be evenly distributed across the beam. Both transmission and reflective types are available. kerf. The width of a cut produced during a cutting process. keyhole mode welding. A technique that employs a concentrated heat source with sufficient intensity to vaporize some workpiece material. This results in the formation of a vapor hole (“keyhole”), surrounded by molten material that penetrates deeply into or through the work piece. As the concentrated heat source is advanced, molten metal flows around the walls of this vapor hole, fills in the trailing edge, and solidifies to produce a continuous weld. kissing weld. A weld on a corner, T-, or lap joint in which the laser beam is incident at a small angle with respect to one of the surfaces being joined, and with respect to the joint to be welded. laser. A device that produces a concentrated coherent light beam by stimulated electronic or molecular transitions to lower energy levels. Laser is an acronym for “Light Amplification by Stimulated Emission of Radiation.” laser beam. The output radiation of a laser that can be steered and focused by mirrors, lenses, or both, to do useful work. laser system. A machine or fabrication tool consisting of a laser, a workpiece holder, and a means of delivering the beam to the workpiece and deflecting it over the workpiece in a controlled manner. The most common type of controller is a computer numerical controller, or CNC. leading edge spike. The initial phase of a pulse from some pulsed lasers, in which the instantaneous peak power is considerably greater than the average power during the pulse. The leading edge spike is useful in rapidly heating a workpiece to temperatures at which the absorption is greater than that at room temperature. The trailing edge of the laser pulse is used to maintain the high temperature and process the material. lens. A transparent optical element with curved surfaces, used to converge or diverge light rays via refraction. M. The magnification ratio of an unstable resonator (i.e., the ratio of the outer diameter to the inner diameter in an annular-shaped beam). M2. A measure of beam quality that compares a laser beam with a theoretically ideal beam that is limited solely by diffraction (i.e., when M 2 = 1). melted spot diameter. The diameter of a melted spot on the workpiece (also known as effective spot diameter). meniscus lens. A lens having one surface convex and the other concave. micrometer (micron). A distance of one millionth (10–6) of a meter. This term is commonly used in specifying the wavelength of light and is abbreviated as µm. mirror. An optical element which reflects nearly 100% of the laser beam. mode. A stable condition of oscillation in a laser. There are transverse modes and longitudinal modes. Longitudinal modes occur at slightly different frequencies; most lasers operate on a large number of longitudinal modes simultaneously. Transverse modes describe the distribution of energy in a plane perpendicular to the direction of propagation. A laser can operate in one transverse mode (single mode) or in many modes simultaneously (called multimode or mixed mode operation). monochromatic. Light which consists of single wavelength. Most lasers emit their light in a very narrow band of wavelengths around a central wavelength. multimode. See mode. near field. A term used to describe the propagation of a laser beam, referring to the beam characteristics at positions close to the laser.

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Nd:glass. Neodymium-doped glass which is utilized as the lasing material in some solid-state lasers. The neodymium atoms are the active medium. Nd:YAG. Neodymium-doped yttrium aluminum garnet (YAG) crystal which is utilized as the lasing material in a common type of solid-state laser. The neodymium atoms form the active medium. See YAG. nozzle. A device projecting from the focusing head with an outlet to direct gas into the laser beam processing area. optical axis. An imaginary line through an optical system along which the center of a laser beam is expected to follow; the line usually passes through the center point of all optical elements such as lenses, mirrors, apertures, and gain media. optical coating. Dielectric or metallic coating on an optic with one or more thin surface layers to control surface reflection or transmission as a function of wavelength. output coupler. A partially transmitting mirror at one end of the laser resonator which allows the extraction of laser energy from the laser. peak power. The peak power is often approximated by dividing the energy of the pulse, or the energy contained in a leading edge spike, in joules, by the pulse duration in seconds. percussion drilling. A process by which repeated pulses remove material and increase the depth of a hole until the workpiece is perforated. Subsequent pulses enlarge the hole and remove recast material. photodiode. A semiconductor device in which light is directly converted to a flow of electricity which can be displayed in a time resolved fashion, for example with an oscilloscope. photon. The quantum, or small discrete amount, of electromagnetic energy that moves at the speed of light. plane polarized light. Light in which the oscillation of the electric field is only in a single plane (also called linearly polarized light). There are two possible plane polarizations, each one perpendicular (orthogonal) to the other. plano convex. An optical element with a flat (planar) face on one side and a rounded outward (convex) face on the opposite side, which converges the light incident on the convex side. plasma plume. See plume. plume. The vapor cloud that forms above the laser material interaction zone. It can consist of excited and partially ionized assist gases and atoms of the material vaporized by the intense laser beam and can have a temperature equivalent to tens of thousands of degrees. Under certain conditions, the plume can absorb or scatter the incoming laser beam, reducing transmission of the beam, the energy absorbed by the workpiece, and weld penetration. polarization. Limitation of the vibration direction of the propagated electromagnetic field (laser beam) to a single plane. Various forms of polarization can be found; these include random, linear, elliptical, and circular. polarizer. An optic which transmits light of only a single polarization, or otherwise alters the polarization of radiation. population inversion. A state in which more atoms or molecules of a lasing medium are at a higher energy level than at some lower energy level. This is necessary to promote the release of photons so that the lasing process can occur. power. With respect to lasers, power is the rate of energy flow from the radiation source. Power is usually measured in watts (W), an energy flow of a joule per second, or kilowatts (kW). power density. Intensity, or the power per unit area, of the output of a laser; expressed as, for example, W/cm2. power meter. A device used to measure laser beam power. See also calorimeter. power ramping. A controlled change in the power level of a laser beam. Power ramping is useful for smooth completion of circular welds and for preventing depressions or fractures at the end of the weld. pulsed laser. A laser which emits light as a pulse or a series of pulses rather than continuously. pulse duration. The duration measured between the half-peak-power points at the front and trailing edges of a pulse.

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pulse energy. The total energy contained in a laser pulse, or a series of laser pulses from a repetitively pulsed laser, usually measured in joules. It is calculated by dividing the measured average power by the pulse repetition rate. pulse enhancement. See enhanced pulse. pulse frequency. The output pulse repetition rate, in pulses per second (Hz), for a pulsed laser. pulse length. See pulse duration. pulse overlap. In spot welding with a repetitively pulsed laser, the amount (usually expressed as a percentage of the diameter of the melted zone) by which the diameter of the melted area created by one pulse overlaps that created by the preceding pulse. pulse shape. The variation in time of the amplitude of a single laser pulse during the pulse duration. pulse width. See pulse duration. pump. An energy source used to excite the active medium and create a population inversion. pumping. See excitation. pyroelectric. A crystal which is a source of electrical current when radiation is incident on it; the current is proportional to the time rate of change of the crystal’s temperature. The term also refers to laser detectors containing these crystals, which may be used to measure either the laser energy or laser power depending on crystal properties and associated circuitry. Q-switch. A device used to produce very short duration and intense pulses from ruby or pulsed YAG lasers. A large amount of energy is stored during the off time and a burst of energy is released during the on time. quarter-wave plate. A polarization retarder which causes light of one linear polarization to be retarded by a quarter wavelength (90 degrees) relative to the other polarization. The quarter wave plate is utilized to convert linear polarization to circular polarization, primarily for cutting applications. recast. A layer of metal which melts and then solidifies along the walls of a hole or cut edge of a laser processed metal. reflectance. The ratio of reflected power to incident power. repetition rate. See pulse frequency. resonator, optical. An optical system formed by two or more mirrors that reflect most light emitted by the medium back into the medium allowing light amplification to take place. resonator, stable. A resonator in which the radiation reflected between the cavity mirrors converges on the longitudinal axis. A stable resonator can produce a beam with a Gaussian mode. resonator, unstable. A laser resonator in which a light ray reflected between the cavity mirrors will move away from the cavity axis. The resonator is “unstable” for light propagation rather than mechanically unstable. Usually, laser energy is extracted by a mirror at a 45 degree angle to the optical axis which contains a hole which is aligned with optical axis. RF excitation. A method of laser excitation using radio frequency waves. This allows excitation though the glass or quartz envelope of the tube, so that the electrodes are isolated from the lasing gas. rod, laser. A cylinder of transparent lasing material with polished end faces used as the active medium in a laser. ruby laser. A laser in which the lasing material is a single crystal of aluminum oxide doped with chromium ions. The cylindrical ruby rod is pumped by a pulsed xenon flash lamp, producing a laser output in the red region of the spectrum, at a wavelength of 0.694 µm. Ruby is the material in which laser operation was first demonstrated in 1960. shutter. A mechanical or optical device used to block or control the transmission of laser beams. slab laser. A solid-state laser in which the host medium is a rectangular or trapezoidal block of material, usually YAG. Formation of the light beam occurs more uniformly in the material than in a rod laser reducing the thermal gradients in the material.

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slow flow. A gas laser in which the flow of gas is along the axis of the laser beam, but is relatively slow so that the extraction of waste heat from the gas is done primarily by thermal conduction through the walls of a tube to a cooling medium. spherical aberration. The failure of a spherical surface of a lens or mirror to form a perfect image; light rays incident on the surface at different radii come to focus at a different distance from the surface along the optical axis. spot size. The diameter of a focused laser beam. See beam diameter. stability. The ability of a laser system to maintain a beam with constant output characteristics, such as power level or mode. standoff. The distance between the focus head’s beam exit aperture and the work surface. The standoff distance influences the dynamics of the assist gas interaction with the material being processed. stimulated emission. The emission of a photon from an atom or a molecule in an excited state in response to the presence of another photon of approximately the same wavelength. The resulting photon will have the same wavelength, polarization, phase, and spatial coherence as the original one. TEA laser. An acronym for “transversely excited, atmospheric pressure laser.” The TEA laser is a gas laser for which excitation of the active medium is provided by an electric discharge transverse to the optical axis of the medium at pressures of one atmosphere or greater. This type of laser is used primarily for marking applications. TEM00. Laser radiation of the fundamental transverse electric mode. The spatial intensity has a Gaussian distribution. See transverse mode. thermal diffusivity. The controlling transport property for the transient diffusion of heat into the base metal during laser materials processing. κ (Equation 3) α = ––– ρcp where, κ = thermal conductivity ρ = density Cp = specific heat thermocouple. A type of sensor consisting of two electrically conducting circuit elements of different thermoelectric characteristics joined at a junction. thermopile. An arrangement of thermocouples in series so that the voltages from individual thermocouples add. In a thermopile, the series of thermocouples are arranged such that the alternate junctions of the thermocouple are at the measuring temperature and the reference temperature. Transverse Electromagnetic Mode (TEM). See transverse mode. transverse flow. Refers to the direction of the gas lasing medium in a gas laser. Transverse flow lasers have the gases flowing at right angles to the direction of laser beam propagation. transverse mode. The distribution of energy in a plane perpendicular to the direction of propagation. See mode. transmittance. The ratio of transmitted power to incident power. trepan. A type of drilling where a circular hole is made either by rotating the laser optics within the focusing head, or by programming a circular motion of the focusing head. up-collimator. See beam expander. wavelength (λ). The distance between two points in a periodic wave which have the same phase. YAG (Yttrium Aluminum Garnet). A synthetic crystal used as a laser rod host material in Nd:YAG laser.

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4. Safety Considerations 4.1 Introduction. The scope of this clause is to introduce users and potential users of laser beam materials processing to important safety considerations. Some of the governing bodies for these areas will be identified and a list of minimum guidelines for safe use of lasers for materials processing will be provided. It should be noted, however, that this is not an all-encompassing document by itself and that all aspects of safety should be considered in developing or implementing a laser application. Personnel involved with lasers and laser materials processing should always read, understand, and follow all of the safety guidelines provided by the laser and machine tool manufacturer, as well as all other pertinent safety standards. 4.2 General Safety. There are a wide variety of potential hazards that can be discussed under the category of general laser safety. These hazards relate to typical minimum safety requirements for any manufacturing or development facility. The American National Standards Institute (ANSI) in association with The American Welding Society (AWS) has issued a standard which covers the potential hazards associated with welding and cutting processes, ANSI Z49.1, Safety in Welding and Cutting. Also, the Occupational Safety and Health Administration (OSHA) has established a range of regulations designed to protect personnel working on or around industrial equipment. As these bodies have established the general safety requirements for welding and cutting, and for industrial environments, no further detail will be provided. OSHA has published Instruction Pub. 8-1.7 on Guidelines for Laser Safety and Hazard Assessment. This document was developed to provide guidelines to Federal OSHA and Plant Site Officers, consultants, and employees for the assessment of laser safety. In general, this document cites the standards of the appropriate governing body (AWS, Laser Institute of America (LIA), ANSI) as the minimum acceptable safety criteria. Also, many foreign regulatory agencies have safety standards, regulations, or recommended practices that are either based on ANSI standards or are similar to them. There are two main bodies that provide specifications on laser safety. The Food and Drug Administrations’ Center for Devices and Radiological Health (part of the U.S. Department of Health and Human Services) is concerned primarily with the performance characteristics as they apply to the manufacturer and puts forth its regulations in the Code of Federal Regulations, Title 21, Subchapter J, Part 1040.10 on “Performance Standards for Laser Products.” These specifications apply to all laser installations, whether they are new installations or modifications to existing facilities. Of primary concern to the laser user, ANSI describes the minimum safety requirements for laser users in their specification ANSI Z136.1, Safe Use of Lasers. 4.3 Laser Safety 4.3.1 Classification of Lasers. ANSI Z136.1 divides lasers into four broad categories based on their potential hazards. The following laser classes are derived from the ANSI definitions: CLASS 1 LASER PRODUCT: Denotes exempt lasers or laser systems that cannot, under normal operating conditions, produce a hazard. CLASS 2a LASER PRODUCT: Denotes lower power visible laser systems that are not intended for prolonged viewing and, under normal operating conditions, will not produce a hazard if viewed directly for periods not exceeding 1000 seconds. CLASS 2b LASER PRODUCT: Denotes low-power visible lasers or laser systems which, because of the normal human aversion responses, do not normally present a hazard, but may present some potential for hazard if viewed directly for extended periods of time (like many conventional light sources). CLASS 3 LASER PRODUCT: Class 3a denotes lasers or laser systems that normally would not produce a hazard if viewed for only momentary periods with the unaided eye. They may present a hazard if viewed using collecting optics. Class 3b denotes lasers or laser systems that can produce a hazard if viewed directly. This includes intrabeam viewing of specular reflections. Except for the high-power Class 3b lasers, this class will not produce a hazardous diffuse reflection (radiation reflected over a wide angular range). CLASS 4 LASER PRODUCT: Denotes lasers or laser systems that can produce a hazard not only from direct or specular reflections, but also from a diffuse reflection. In addition, such lasers may produce fire and skin hazards. Of these four classifications, only Class 4 lasers are incorporated into systems used for materials processing. A Class 4 laser may be part of a system designed in such a manner as to be considered a Class 1 laser system. Such a system cannot, under normal operating conditions, produce a hazard. This can be achieved using engineering controls such as enclosures, interlocks, and other mechanisms.

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4.3.2 Laser Safety Officer. All facilities with materials-processing laser installations shall designate a Laser Safety Officer (LSO) per ANSI Z136.1. The ANSI standard describes the position and duties of the LSO. These include ensuring that the correct engineering controls are in place and functional. These engineering controls may include, but are not limited to, safety interlocks, signs, signals, and light-tight beam enclosures. Other duties include the implementation of the proper administrative and procedural controls, such as the posting of safety signs and equipment, and ensuring standard operating procedures are posted and followed. All laser installations are required to have a standard format sign posted in the laser operation area, which lists the type of laser, its wavelength, power level, and classification type. Figure 4.1 shows an example of a sign for a Class 4 laser system. The proper format is listed in ANSI Z136.1. The Laser Safety Officer is also required to determine and inform the pertinent personnel of the maximum permissible exposure (MPE) for the laser system installed in the facility. The MPE is calculated using the formulas and tables in ANSI Z136.1 and limits are given for intrabeam viewing, extended source viewing, and skin exposure. It should be noted that a high-power collimated or unfocused laser beam is more dangerous over large distances than a focused beam, which diverges rapidly. Some laser materials processing operations may be extremely noisy, especially if used in enclosed areas. Proper controls should be implemented to ensure that adequate hearing protection or noise reduction techniques are used. 4.4 Electrical Hazards. The voltages used in lasers are sufficient to cause fatal injuries to personnel and account for most laser-related fatalities. All electrical equipment associated with laser beam materials processing should be installed in conformance to ANSI/NFPA 70, National Electric Code and ANSI/NFPA 79, Industrial Machinery. All doors and access panels should be properly secured, either electrically or mechanically, to prevent access by unauthorized personnel to electrical components, especially those operating at the laser excitation potential. All personnel working on or around high-voltage components should be trained in the proper safety techniques for electrical systems, as well as in the technique of removing a victim from an electrical circuit and administering cardiopulmonary resuscitation (CPR). Personnel should be aware of and adhere to any additional electrical safety requirements of the laser system installed in their facility. Wherever access to high voltage is possible, the area should be properly posted in conformance to ANSI Z535.1, Safety Color Code, and ANSI Z535.5, Accident Prevention Tags. 4.5 Laser Radiation Hazards. There are two potential dangers associated with laser radiation: eye and skin hazards. Depending on wavelength, damage to either the cornea, the retina, or both, of the eye is possible. Exposure to radiation from a CO2 laser (10.6 µm) typically results in corneal damage. The radiation from an Nd:YAG laser, at 1.06 µm, is much closer to the visible spectrum (0.4 to 0.7 µm) and can be transmitted by the cornea and lens. The lens will focus the laser light on the retina, causing severe and permanent damage to the retina and other intraocular material. This focusing, by the lens of the eye, can cause even low-power diffuse laser light to be focused to a sufficient power density to cause retinal damage. Low-power helium–neon (He–Ne) lasers (0.633 µm), often used for alignment purposes, may also present a hazard. Requirements for eye protection are described in ANSI Z87.1, Practice for Occupational and Educational Eye and Face Protection, and the Laser Institute of America’s Guide for the Selection of Laser Eye Protection. It is important that the selected eye protection be clearly marked to ensure it is only used for the laser wavelength and power levels for which it is intended. ANSI Z136.1, and the LIA’s Laser Safety Guide describe the proper warning signs and labels that should be employed wherever beam exposure is possible.

Figure 4.1—Sample Precautionary Sign for a Class 4 Laser System 11

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Skin damage is restricted primarily to burns. It should be noted, however, for high-powered lasers these burns can be deep and cause severe and permanent damage. As stated above, ANSI Z136.1 defines the MPE for all laser wavelengths, including the typical industrial lasers and He–Ne alignment lasers. 4.6 Visible Radiation Hazards. Viewing of the visible radiation emitted during laser materials processing can also be harmful to eyesight. During welding, a bright plume, similar in appearance to a welding arc, is generated from the interaction between the laser beam and material being processed. The size and intensity of this plume is a function of the material being processed, the power level, and the shielding gas used. Consequently, no exact guidelines can be given. However, the radiation emitted is broadband, from the ultraviolet, which can cause skin erythema (sunburn) and photokeratitus (arc eye) through the visible to the infrared, which can contribute to the formation of cataracts. As the plume is generally too bright for direct viewing, adequate filtering, such as welding shades, should be employed for eye protection. As a general guideline, the filter used should be of sufficient optical density to ensure the viewer’s comfort at the highest level of light intensity encountered and there should be no evidence of eye irritation after exposure. The optical viewing system should provide filtering in conformance to ANSI Z87.1 and should include provisions for filtering the visible and ultraviolet radiation from the plume as well as the laser radiation. All persons involved with laser beam materials processing should be instructed in the use of proper optical filtering and should be required to use such protection. 4.7 Hazards from Fumes and Gases. Welding, cutting, drilling, and surface modification with lasers may result in the generation of fumes, dust, and gases that can be hazardous to personnel. These airborne contaminants may include metal particles and oxides, ozone, and other toxic gases. The hazards associated with welding and cutting of metals have been documented in a variety of American Welding Society publications, including Fumes and Gases in the Welding Environment. It should be noted that some organic materials, such as plastics, can generate fumes that are hazardous. Care should be taken to avoid the excessive buildup of laser discharge gases, shielding gases, and assist gases, especially in enclosed spaces where oxygen can be displaced, as described in ANSI Z117.1, Safety Requirements for Confined Spaces. All necessary environmental engineering measures for fume and gas control (external venting, filtering, etc.) should be taken to prevent the accidental inhalation of harmful concentrations of fumes and gases by personnel working on or around laser materials processing equipment. Exhaust of these fumes may violate local or federal standards, and implications should be considered before using equipment. The possible toxicity of the workpiece and consumables (wire, powder, etc.) should be determined before laser beam materials processing begins. Adequate protection to personnel should be provided in conformance to ANSI Z49.1. Also, for all materials, the Material Safety Data Sheet (MSDS), available from the material supplier, should be consulted to determine what hazards exist.

5. Equipment Description 5.1 Description of Process. Laser is an acronym of Light Amplification by Stimulated Emission of Radiation, a process first demonstrated in 1960. The laser generates a coherent optical beam that has an essentially constant wavelength; the phase and amplitude at any particular time has a known relationship to that at another time. Laser output has been demonstrated at thousands of different wavelengths, but only a few have industrial application. The laser consists of several major components as shown in Figure 5.1. These components include: (1) a lasing gain medium, (2) a means to excite the gain medium, (3) an optical resonator, and (4) a heat sink. The gain medium may be a solid, liquid, or gas. The optical cavity is used to provide a feedback system for the system to achieve enough amplification to overcome the losses within the system. There are many different types of lasers available for material processing, which have significantly different kinds of outputs. Wavelengths vary from less than 0.2 to over 10 micrometers (µm); pulse durations range from below 1 nanosecond (ns) to continuous wave; peak powers can be measured in megawatts (MW); and average powers go up to several kilowatts (kW). The laser beam is sent either through the atmosphere or through fiber optics to a work station. For most production applications, the carbon dioxide (CO2) and the neodymium doped yttrium-aluminum-garnet (Nd:YAG) lasers are used (see Table 5.1). A brief discussion of the operation of these lasers follows. 5.2 CO2 Lasers. The carbon dioxide (CO2) laser is the most powerful type of industrial laser presently available. It is in general use for contour cutting and deep penetration welding. The long wavelength of CO2 light, 10.6 µm, is absorbed by most solids. This allows CO2 lasers to process a wide variety of materials.

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Figure 5.1—Four Major Components of a Laser

Table 5.1 Operational Comparison of Co2 and Nd: YAG Material Processing Laser CO2

Nd:YAG

Active Medium Excitation Wavelength (microns) Average Power (kW) Peak Power (kW) Pulse Frequency (kHz) Efficiency (%) Beam Quality (M2)

CO2, N2 He Gases Electrical Discharge 10.6 0.1–45.0 0.1–50 CW—100 5–15 1–3

Consumables Transmissive Optics Reflective Optics Fiber Delivery Safety Shield Capital Cost (S/Watt)

CO2, N2 He Gases ZnSe, GaAs Metal Not Available Acrylic, Glass 50 200

Nd:YAG Crystal Lamp 1.06 0.1–5.5 0.1–100 CW—50 1–4 1–2 special applications 10–100 typical Lamps Quartz Metal or Dielectric Quartz Filters 100–400

5.2.1 Principle of Operation. The CO2 laser is a gas discharge device; it operates by sending an electric current through a gas. In industrial lasers, high efficiency is obtained by using a mixture of helium, nitrogen, and carbon dioxide. Other gases such as oxygen or carbon monoxide may be added to alter the chemical equilibrium in the laser gas. Electrical energy is coupled to the gas by establishing a glow discharge in the nitrogen. The nitrogen transmits this energy to the CO2 molecules by resonant transfer, which puts a large percentage of the molecules in an elevated energy state. The carbon dioxide provides the population inversion required for operation as a laser. Laser emission at 10.6 µm in the infrared is produced when these molecules drop to an intermediate state. Collisions between carbon dioxide molecules and helium atoms return the CO2 to the ground level, where the process can begin again. The helium also provides discharge stability. The gas is typically passed through a heat exchanger where it is cooled, allowing it to be recycled. Large chillers are required for high-power CO2 lasers. CO2 Laser efficiencies, which are high for lasers, are still only 6% to 10%, and all the rest of the incoming power is converted to heat. 5.2.2 CO2 Laser Resonators. Two groups of resonator configurations, stable and unstable, are used in industrial carbon dioxide lasers. 5.2.2.1 Stable Resonator. To sustain laser oscillation, optical feedback must be provided. This is usually accomplished with an optical resonator consisting of a pair of spherical mirrors facing each other (see Figure 5.2).

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Figure 5.2—Stable Laser Resonator

Figure 5.3—Confocal Unstable Laser Resonator A resonator is stable if rays of light initially near the axis of the mirrors are kept confined between them. This condition is met if, for mirrors of radius R1 and R2 spaced a distance L apart:

(

)(

)

L L 0 < 1 – –– 1 – –– < 1 R1 R2

(Equation 4)

The radii, R1 and R2 are defined as positive for concave mirrors. Most lasers up to the 6 kW level have stable resonators. Output mirrors for this optical cavity are generally transmissive materials, with antireflective (AR) and reflective thinfilm coatings. Stable resonators generally use pairs of concave mirrors, or a flat mirror and a concave mirror. Such structures can sustain oscillation of electromagnetic waves at well-defined axial resonance frequencies; these frequencies are referred to as longitudinal modes. The particular amplitude (or intensity) patterns of the waves in a plane transverse to the direction of propagation are referred to as the transverse modes. 5.2.2.2 Unstable Resonator. A laser beam can only get out of a stable resonator by transmission through one of the resonator mirrors. This requires that the mirror be partially transmitting to the wavelength of light generated in the laser, and that it be mounted so that the beam can go through it. At very high powers, absorption in the mirror can cause heating, optical distortion, and damage. To avoid these consequences, high-power systems are often designed with unstable resonators in which energy is not confined within the cavity and can be extracted using reflective optics. Instead of being transmitted through a partially transparent mirror, the transmitted beam goes around the edge of an otherwise totally reflecting mirror. Unstable resonators have modes analogous to those of stable resonators, although their mathematical form is not as simple, and they require involved numerical calculations to define them. A confocal unstable resonator, as shown in Figure 5.3, produces an output with an annular power distribution. For CO2 lasers with output powers greater than 6 kW, a transparent front mirror cannot be used and the excitation gases are kept inside the resonator by the use of an aerodynamic window through which the laser beam exits. The magnification ratio, M, of unstable resonators is the ratio between the outer and inner diameters of the annular power distribution and is important in establishing a balance between power and beam quality. For a particular resonator configuration, the power that can be extracted is zero at M = 1, increases to a maximum near M = 2, and then drops off slowly

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as M increases further. When an unstable resonator beam is focused, there is a central power peak surrounded by diffraction rings. At low magnifications near M = 2, most of the power is in the diffraction rings, so the focusability and power density are low. As M increases, more of the power goes into the central spot, allowing high-power densities to be reached. For CO2 lasers, magnification ratios of about 4, which reduce power by about 25% from the maximum, produce beams that are nearly as focusable as those from low-order stable resonators. Note that the magnification, M, of an unstable resonator is quite different from M2 used to indicate beam quality. 5.2.3 Output Characteristics of CO2 Lasers 5.2.3.1 Temporal. CO2 lasers can operate in continuous wave (CW) or in a variety of pulsed modes controlled by the power supply driving the discharge (see Figure 5.4). The pulse frequency may be as high as 100 kHz, although complete modulation of the output is seldom achieved above 2 kHz. The most common modes of pulsing are termed gated and enhanced. In a gated mode, the laser operates at a peak power level that is within its normal CW range. The output is modulated to generate a reduced duty cycle. Gated pulses can be any length that is compatible with the chosen repetition rate. Lasers that can produce enhanced pulses have peak powers that are several times their CW rating. 5.2.3.2 Spatial. The low density and high thermal diffusivity of a gaseous laser medium reduce the tendency to distort the light that goes through it. This allows even high-power CO2 lasers to have good beam quality. Beams from many lasers with outputs of up to 1.5 kW are close approximations to the fundamental Gaussian mode TEM00. These beams produce a spatial intensity distribution in a plane normal to their propagation direction that is as shown in Figure 5.5. Such beams may be focused to the limit set by the diffraction of light through an aperture (diffraction limited). A spot size of 0.1 mm is easily achieved by normal focusing lenses for CO2 lasers with a TEM00 beam. Another property of TEM00 beams is low divergence, which allows great flexibility in laser system design since the laser does not have to be near the focusing lens. Divergence is simply the angle at which the laser beam spreads as it propagates. The typical values are in the range of 1 milliradian. CO2 lasers with maximum continuous powers over 1.5 kW usually have higher order or multimode outputs. The effect of high order or multimode outputs in material processing is that such beams do not focus to as small a spot as the beam from a TEM00 laser although the spatial intensity distribution may still be bell-shaped and quite similar to a Gaussian distribution in appearance. These beams can be focused to a spot with a diameter M2 times the diffraction limit, where M2 is a measure of beam quality associated with the mode (see 5.2). It is generally not possible for higher power lasers to generate a TEM00 beam because output mirrors cannot handle high central power densities, and their resonators are intentionally configured to produce higher order modes. Most CO2 lasers exceeding 6 kW in power use an unstable resonator, which produces a beam with good focusing properties without having to send high-power densities through transmissive optics.

Figure 5.4—Examples of Laser Temporal Pulse Shapes 15

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Figure 5.5—Spatial Intensity Distribution for a Near Gaussian Mode Laser

Figure 5.6—Schematic Diagram of a Slow Axial Flow Laser

5.2.4 Types in Use. The earliest industrial CO2 lasers consisted of glass tubes with mirrors on both ends. The laser gas flowed down the tube while electricity was applied near each mirror. These devices are very simple and reliable, but are limited to about 50 W per meter of discharge length because there is no means to cool the gas. These are called slow-flow lasers (see Figure 5.6). The size of slow-flow lasers becomes impractical if more than one or two kilowatts is required. They are in use today because they can produce stable high quality outputs and because the large volume of active medium allows for massive pulse enhancement. The transverse flow laser was developed to produce high power in a relatively small package. It does this by circulating the laser gas perpendicular through the discharge region at high speed and then cooling it with a heat exchanger so that it can be reused (see Figure 5.7). Transverse flow lasers tend to have asymmetrical modes because the gain characteristics of the discharge vary across the beam. In addition, they are hard to pulse because they operate at high discharge currents. Despite these limitations, transverse flow lasers have been extensively implemented into laser welding systems.

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Figure 5.7—Schematic Diagram of a Transverse Flow Laser Fast axial-flow lasers are a modification of the slow-flow laser using either a Roots or turbine pump to circulate the gas (see Figure 5.8). The high-speed gas movement has resulted in laser designs with up to 1 kW power output per meter of resonator length. Fast axial lasers are small, powerful, inexpensive to build, and can have excellent beam quality. Thus, greater power in a smaller equipment package is achieved. 5.3 Nd:YAG Lasers. A solid-state laser uses a transparent solid substance as the active medium. The most common solid-state laser in industrial applications is the neodymium doped yttrium-aluminum-garnet laser, commonly referred to as the Nd:YAG laser. Nd:YAG is used as the host crystal because it has relatively high thermal conductivity, high mechanical strength, good optical quality, and can be grown in large sizes. Because the 1.06 µm light from the Nd:YAG is transmitted easily through flexible quartz fibers, system design can be considerably simpler than with CO2 lasers. In addition, the Nd:YAG wavelength is absorbed more readily by metals than CO2 laser radiation, further improving process efficiency. 5.3.1 Principle of Operation. An industrial Nd:YAG laser contains a crystalline rod surrounded by xenon or krypton lamps, or by light-emitting diodes. Energy is transferred from the lamps or diodes to the host crystal by reflectors parallel to the crystal. Light from these sources excites the neodymium atoms, which emit light at a wavelength of 1.06 µm (see Figure 5.9). In order to get higher powers, several water-cooled rods may be placed in series. Nd:YAG lasers have electrical efficiencies of about 1% to 4%, and need large chillers to maintain their optical elements at stable temperatures. The majority of Nd:YAG lasers currently installed are pulsed; however, continuous power units are available for highrate cutting and welding applications. Pulse durations may be as short as 0.1 milliseconds, pulse rates as high as 2000 Hz, and peak power levels are up to 100 kW. The mode structure of the majority of these lasers is multimode. Multimode Nd:YAG lasers are focusable down to a spot size of 75 µm. Resonator mirrors for this laser are fused quartz with AR and reflective thin-film coatings.

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Figure 5.8—Schematic Diagram of Fast Axial Flow Laser

Figure 5.9—Schematic Diagram of a Solid-State Nd:YAG Rod Laser

5.4. Output Characteristics of Nd:YAG Lasers 5.4.1 Temporal. Nd:YAG lasers may be operated in either pulsed or continuous wave (CW) modes. Pulsed Nd:YAG lasers have flashlamps or diodes, while CW Nd:YAG lasers use continuous arc lamps. Nd:YAG lasers cannot be switched from pulsed to CW operation because the excitation equipment (lamps and power supplies) is distinct and significantly different for each type. Pulse repetition rates are generally below 200 Hz. Control of the power going into the lamps allows tailoring of the laser-pulse shape and duration. The solid laser medium has a high concentration of light-emitting atoms, so the peak power can be very high. Material removal is usually accomplished with short, high-power pulses, while joining is done with longer pulses of lower peak power or with a continuous beam. 5.4.2 Spatial. Laser rods accumulate heat in the center as they are cooled along their outside surface. Therefore, there will be a temperature gradient across the rod’s diameter whenever substantial power is produced. This gradient induces changes in the rod’s refractive index, which degrades the optical performance of the laser. High-power Nd:YAG lasers have multimode outputs with high divergence. This limits the ability of optics to focus the beam to a small spot, and requires the resonator to be near the work area. The total internal reflection face pumped laser (TIR-FPL), also called the “slab” laser, reduces this problem by bouncing the beam from side to side within a rectangular crystal, compensating for thermal gradients in the material

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Figure 5.10—Schematic Diagram of a Total Internal Reflection-Face Pumped (TIR-FP) Slab Laser

(see Figure 5.10). The thermal stresses in the YAG medium, which distort the laser light, are canceled-out as the light crisscrosses the long axis between slab-surface reflections. 5.5 Other Types of Material Processing Lasers 5.5.1 Nd:glass. Nd:glass lasers are very similar to Nd:YAG lasers. The laser rod is made of neodymium-doped glass rather than garnet. When glass rather than YAG is used as a matrix, a higher concentration of neodymium atoms can be incorporated in the laser rod. This allows glass lasers to produce stronger pulses than Nd:YAG lasers, which makes them more appropriate for deep drilling. Because the poor thermal conductivity of glass limits the pulse frequency to about one pulse per second, this type of laser is not used for contour cutting. 5.5.2 Ruby. Ruby was the first material in which laser emission was observed. The ruby laser is a flashlamp-pumped solid-state device like the Nd:YAG and Nd:glass lasers, but emits visible light. Although largely replaced by other types, it is still suitable for drilling, with characteristics similar to Nd:glass lasers. 5.5.3 Excimer. The excimer laser is an electrically excited gas laser. The gases used vary, but are always combinations of inert gases (argon, xenon, and krypton) with halogens such as fluorine. In the discharge, excited dimmers such as KrF, and trimers such as KrF2, can be formed. These are unstable molecular forms that emit ultraviolet light when they dissociate. The name excimer is a contraction of excited dimmer. Wavelengths from 0.193 to 0.348 µm have been generated. Because of the short lifetimes of the excited states, excimers operate only in a pulsed mode. Pulse duration is very short, in the range of 0.01 to 0.5 µsec, while peak power can be as high as 100 kW. The UV output and high peak power of the excimer laser are very attractive for material processing. Drilling, marking, material removal, and cleaning have been done which exploit these unique characteristics. 5.5.4 Fiber Lasers. Fiber lasers and fiber laser amplifiers were originally developed for the telecommunications industry. As the power output capability increased, fiber lasers were packaged for industrial applications and are finding extensive use in cutting and welding, applications. Just as neodymium ions can be doped into YAG rods and slabs for applications in industrial Nd:YAG lasers, these and other ions can be doped into silica or other substrates that can be drawn into long thin fibers. The surface layer of the fiber, called cladding, is doped to give it a higher index of refraction so that it acts like a mirror; the laser radiation is reflected off the walls of the fiber but can be transmitted down the core of the fiber in a nearly lossless fashion. The core of the fiber is typically 2.54

Percussion Drilling Trepanned Drilling Contoured drilling

9.9.3.1 Percussion-Drilled Holes. Percussion-drilled holes are produced by repeatedly firing laser pulses into the material, with the laser beam stationary on the material. To obtain the best surface finish and the greatest accuracy, it is a good practice to drill the hole with a large number of pulses, as opposed to one high-energy pulse. Each one of the small energy pulses removes only a small amount of material. Typical pulse rates are 10 to 100 pulses per second. Pulse durations are 0.1 to 1.5 msec. The range of hole diameters that may be drilled with this method is from approximately 0.03 to 0.5 mm. Note—It is not considered a good practice to change the hole diameters by defocusing the laser beam. When the laser beam is defocused, the power density at the focal point of the lens is lowered and the material may no longer strictly vaporize; it may melt and then vaporize. When the latter occurs, the quality of the hole becomes worse, there will be excessive slag around the holes, and the HAZ would increase significantly. The surface finish inside percussion drilled holes may be as good as 0.2 µm; typically it is 0.8 to 1.6 µm. The accuracy of these holes is typically within ±0.005 and 0.025 mm. The roundness of these holes is also within that range. The depth to diameter (aspect) ratio of percussion drilled holes may be as high as 100:1. For example, a 0.25-mm diameter hole can be drilled through 25-mm thick stainless steel. 9.9.3.2 Trepanned Hole. Trepanned holes are produced by moving the laser beam in a circular pattern, typically by rotating the focusing lens or the focusing mirror. The material remains stationary during the drilling cycle. A pulsed laser is operated at a high pulse rate, between 30 and 200 pulses per second. The holes are usually produced in only one rotation. The range of hole diameters that may be drilled with this method is from 0.5 to 2.5 mm. The mode of the laser beam is usually not a factor in the sizing of the holes; the hole diameter determined by the off-set of the focusing lens or mirror. Both Nd:YAG and CO2 lasers perform equally well in the drilling of trepanned holes. The surface finish inside these holes is typically 0.8 to 3.2 µm, depending on the pulse rate of the laser and the rotational speed of the optic. The accuracy of the diameter of trepanned holes is typically ±0.025 mm, or better. The aspect ratio of trepanned holes is limited by the thickness of the material that may be cut with the laser. This method of laser drilling can handle only about half of the thickness of material that can be percussion drilled. Today’s Nd:YAG and CO2 lasers can trepan drill up to 12-mm thick stainless and carbon steels, and 8-mm aluminum and refractory metals. 9.9.3.3 Contoured Hole. Contour-drilled holes are produced by moving either the workpiece or the focusing head in a programmed circular pattern, similar to laser beam cutting. The laser is operated at a high pulse rate, between 30 and 200 pulses per second. The holes are usually produced with only one pass. The range of hole diameters that may be drilled with this method is from approximately 0.5 mm to any size within the mechanical capability of the equipment. The attributes of contour drilled holes are similar to those of trepanned holes. 9.9.4 Optimizing Drilled Holes 9.9.4.1 Minimizing Taper. In percussion-drilled holes, minimizing the taper is achieved by some of the following methods: (1) By focusing the laser into the material to a depth where the differences between the entrance, central, and exit diameters are minimized. (2) By using longer focal length lenses, while still maintaining the correct power density at the focal point to vaporize the material. (3) By “firing” an additional number of laser pulses into the material, removing more material from the hole. (A side-effect of this technique may be excessive HAZ.)

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(4) By using very high gas pressures in the nozzle, to assist the clearing of all molten material. (To employ this technique, the focusing lens must be of the correct thickness to withstand the high gas pressures.) In trepanned holes, minimizing the taper is achieved by the above listed methods and the correct choice of focal length lens, consistent with the diameter and the depth of the hole. 9.9.4.2 Improving Roundness. The need to improve the roundness of the hole appears mostly in the case of percussion-drilled holes. Should the energy distribution across the laser beam not be symmetrical, the cross-section of the hole will likely not be symmetrical either. The solution to this problem is to use a trepanning focusing head with zero offset radius. Rotation of the trepanning head rotates the laser beam making it symmetrical and results in the drilled hole being round. The disadvantages of this method are that a longer drilling cycle is required and a greater HAZ is produced in the material around the hole. Drilling cycles may increase from 4 to 10 sec per hole (as in the case of 0.5-mm diameter holes through 3-mm Hastalloy-X). The gain in quality may be worth the extra time. 9.9.4.3 Drilling at Shallow Angles. Drilling at shallow angles with CO2 lasers is difficult, and is not usually done. Nd:YAG lasers may be used to drill holes at angles as small as 12 degrees to the surface. No special fixtures or tooling are needed to accomplish this task, hence the use of these lasers is very economical in these applications. Careful attention must be paid to the power density at the focal point of the final focusing lens, since the spot size will be elongated over a larger area. If the power density is too low, the laser beam will not couple into the material and will reflect off the surface. Nozzles of special geometry are needed, so that the gas assist may be applied accurately at the focal point of the lens. The following are typical settings for drilling at shallow angles: (1) 150 × 106 W/cm2, or greater (2) oxygen gas assist, to help absorption (3) low pulse rates (4 pulses per second, or less) 9.9.4.4 Aerospace Engine Component Drilling. Drilling of many aerospace products is specified by total flow rate of air to be achieved, and not by hole size, because the holes are not perfectly round and the surface roughness of the hole sidewalls plays a large part in flow-rate results. This allows some hole-to-hole variation in diameter. If very stringent flow rates are specified, it is best to trepan the hole, rather than to use percussion-drilling. This takes more time, but the holes will be more consistent. Percussion-drilled holes will result in flow rates within 10% of the target value, while trepanned holes will achieve flow rates within approximately 2% of nominal. Some assemblies are drilled by either method, flow-tested, and then a calculated number of extra holes are drilled to bring the flow to within a tight specification. In critical aerospace applications, the primary concerns are with metallurgical requirements, such as recast thickness and microcrack levels in the recast and parent metal. Of secondary concern is the diameter, taper, drilling time, and roundness of the holes. Once parameter windows are established to meet the metallurgical requirements, the parameters are adjusted within the window to meet flow and hole diameter requirements on a day-to-day basis. Most laser drilled superalloys have a minimal HAZ. However, there is a recast layer on the sides of the hole, surrounded by parent metal. Typical thickness of this recast layer is less than 100 µm. Usually there is some microcracking in the recast layer, and there is a specification for the maximum distance these microcracks can propagate into the parent material. 9.9.5 Gas Assist for LBD. It is necessary to apply a gas assist during the laser drilling process. Typically, the gas is applied coaxially with the laser beam through a nozzle. One of the functions of the gas assist is to protect the optical components. Hence, there are various sensors and interlocks that detect the presence of gas and gas pressure in the nozzle and prevent the operation of the system unless all interlocks are satisfied. The cost of the gas may be a significant factor in the total cost of laser drilling. 9.9.5.1 Gas Nozzle Operation. The design of (coaxial) nozzles used for laser drilling is similar to the design of nozzles used for laser cutting. The general configuration of a nozzle was shown in Figure 9.1. Nozzle robustness is especially critical for laser drilling. During most of the laser drilling cycle, the vaporized and molten material is violently ejected upwards, directly into the incident laser beam and will adhere to the surface of the lens and the nozzle. The material of the tip is often made of copper or even ceramic, since it has to withstand the temperature of the superheated by-products of the laser drilling operation. The geometrical configuration of the tip of the nozzle will carry greater importance, since the geometry of the laser-drilled hole is greatly affected by the flow profile of the compressed gas assist, as it exits through the nozzle. Highly

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turbulent gas flow will tend to increase the taper of the laser-drilled hole. Nozzle standoff distance is mainly dictated by accessibility. A standoff distance between the tip of the nozzle and the material is usually about 3 mm. A standoff distance of 8 to 10 mm is typically required to allow low-incident angle drilling of complex part shapes. 9.9.5.2 Function of Gas Assist. The focusing optics are located within several centimeters of the surface of the metal being drilled. As the drilling commences, all the vaporized and molten material is ejected out of the surface directly toward the focusing optics. The optics, therefore, are encased in a gas nozzle having a small orifice near the focal point A high gas pressure in the nozzle results in a high gas flow which minimizes the deposition of vaporized and molten metal that would otherwise deposit on the lens. The gas pressure must be present before commencing the drilling operation. Once the laser beam pierces through the material, the high gas pressure helps to blow the by-products of the drilling process through the hole and out of the way. The high volume of gas flow also acts to cool the workpiece. This helps to minimize the HAZ in the material adjacent to the drilled hole. 9.9.5.3 Assist Gas Selection. Assist gas selection is application-specific and requires empirical development of parameters. By the proper selection of the assist gas, one may achieve favorable cost savings, special effects, or control the quality of the laser-drilled holes and their effect on the parent material. Compressed shop air that is clean and dry is the most commonly used assist gas in percussion drilling with Nd:YAG lasers, because of its economy and its tendency to be slightly oxidizing. In the case of materials that are exothermic, using compressed air or oxygen will assist the laser absorption process and result in faster (or deeper) drilling. Typical gain is 20% to 40% in drilling speed, which often results in favorable economics to offset the cost of the oxygen. Holes drilled with oxygen assist may exhibit microcracks in the recast layer or in the HAZ surrounding the hole. Such microcracks will lower the fatigue strength of the material. Oxygen is typically not used in percussion drilling, because it increases recast thickness and does not significantly increase drilling capability, except at thicknesses approaching 12 mm in superalloys. It can, however, reduce the amount of dross that adheres to the back side of a drilled component. Oxygen assist gas can increase drilling capability when used with a trepan drilling technique. With CO2 lasers, oxygen is used almost exclusively for trepanning in metals, but air is used for processing all other materials. Shop air is used for many automotive drilling applications, because oxygen is far too reactive with ferrous-based metals. Because of the metallurgical characteristics of high-nickel alloys, microcracks will develop in the recast layer and the HAZ, and the fatigue strength of the metal will be lowered significantly. Therefore, it is recommended that the assist gas used in drilling these metals be an inert gas such as argon. Argon is used when drilling reactive or refractory metals, such as Mg, Zr, and Ti. For reasons of economy, nitrogen is also a candidate for nonoxidizing, assist gas applications, but it is reactive with materials such as Ti and Zr. When drilling stainless steels or superalloys, the use of argon or nitrogen will produce holes with no oxides on the sidewalls. This is important for some applications in the nuclear industry and in the production of medical components. Drilling can be accomplished in a vacuum with very satisfactory results and the technique is used in research and nuclear applications. The peak power in pulsed Nd:YAG lasers is more than sufficient to remove debris in a vacuum. Special attention, however, must be paid to protecting the optics from spatter, a task for which an assist gas is used at ambient pressures. Windows used to view vacuum drilling should be sufficiently thick and correctly mounted to withstand the forces developed on them by outside atmospheric pressure. It has been found that if helium is used to drill acrylic plastics and quartz, the surface finish inside the hole is very smooth and appears to have been fire polished. 9.9.5.4 Assist Gas Pressure Selection. Assist gas pressure should be high enough to prevent spatter contamination of the focusing optic. Typical gas pressures are 1.5 to 7 bar. Typical gas usage is 275 to 1400 liters per hour, depending on the pressure, through a 1.5 mm diameter gas jet. Some experimental work is being conducted on high-pressure (greater than 10 bar) drilling. 9.9.6 Hole Geometry Inspection. Laser-drilled holes look different from holes drilled by conventional machining methods or by electro-discharge machining (EDM). Therefore, it is important to review the geometry of laser-drilled holes and some of the common and proper methods of inspecting these holes. The call-out of laser-drilled holes in engineering drawings should incorporate the description of the laser-drilled holes and the proper methods to inspect them. Laser-drilled holes in metals are evaluated by recast thickness, microcrack length, diameter, taper, and roundness. Recast may be specified as a maximum thickness, average thickness, or both. Laser-drilled holes typically have an hourglass

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shape, i.e., the entrance diameter of the hole is larger than the center portion, then the exit widens again. Typically, the diameter of the hole is that dimension which is the smallest of the three diameters and is typically located inside the material. The entrance diameter is usually 10% to 20% larger than the smallest diameter, while the exit diameter is 5% to 10% larger. The geometry of laser-drilled holes is easily reproduced and maintained by a laser that operates steadily. The hole diameter may be maintained within ±0.01 to 0.08 mm, depending on the diameter and the depth of the hole. The recast layer is usually only about 0.01 to 0.08 mm thick, depending on the material and the specific laser parameters used. The HAZ is typically 1% to 5% of the thickness of the material; again, depending on the material and the specific laser parameters used. The shape of the HAZ closely follows the shape of the hole, though it tends to get wider at the exit side, due to the secondary heating effects caused by the reattachment of some of the expelled metal. By cross-sectioning the laser-drilled holes many important pieces of information may be obtained. Cross-sectioning will display the following: (1) Shape of the hole (2) Extent of the recast layer (3) Extent of the HAZ (4) Presence of microcracks and metallurgical changes (5) Extent of the reattached material (burrs) To inspect finished products for proper hole characteristics, without damaging the part, the following techniques can be used: (1) Gage pins can be used to check the diameter of the hole. (2) Gage pins can be used to measure the angular orientation of the axis. (3) Optical (inspection) projector can be used to check the diameter and the cylindricity of the hole. (4) Optical microscope can be used. (5) Air (flow) gages can be used to check the flow characteristics of the holes. (6) Fluid (flow) gages can be used. (7) Wire gages can be used to check the depth of the holes. Several of these methods may have to be applied to completely characterize the holes.23

10. Laser Transformation Hardening (LTH) 10.1 Introduction 10.1.1 Definition of Laser Transformation Hardening. Laser Transformation Hardening (LTH) is the use of laser radiation absorbed in a metal to change the microstructure of the metal, producing a hard surface with beneficial properties. LTH is a solid-state reaction which is performed under processing conditions that do not result in surface melting. It is performed in steels which transform to a hard metallurgical state designated martensite. Transformation hardening can also occur with surface melting. The metallurgical processes resulting in laser transformation hardening are the same processes that result in hardening in the heat-affected zone in welding of hardenable materials. However, transformation hardening in the context of this clause is a process applied to increase the hardness and other characteristics of steel components without surface melting. In the early laser-processing literature, the transformation hardening process was often called “laser heat treating.” However, there are many different heat treatment processes: not only transformation hardening, but also stress-relieving, normalizing, annealing, tempering, martempering, and austempering. Of these processes, only transformation hardening is normally done with a laser, and hence the term “laser transformation hardening” is preferred to “laser heat treatment.” Laser Surface Melting can also result in profound metallurgical changes in a workpiece, which may be beneficial; this process is not covered in this clause. 23

For additional information on “Small Hole Drilling,” see Terrell, Norman E., Laser Precision Small Hole Drilling, SME Paper, MR 80–849.

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10.1.2 Advantages of LTH. The primary advantage of LTH is its controllability. The intensity of the light and the duration of heating may be controlled with great accuracy and may be precisely shaped to cover any specific area. Using a turning mirror, LTH may be applied to interior bores. Fiber optics allow some wavelengths of laser energy to be delivered to otherwise inaccessible areas. Some subsidiary advantages to LTH are that it is easy to achieve high heating and cooling rates and it may be performed in vacuum or in an inert gas atmosphere. With a change in focusing optics, LTH can be done with the same laser generator and workstation as used for other applications such as welding. 10.1.3 Comparison of Laser Transformation Hardening with Other Processes. LTH is a process for generating localized hardening and may be compared with other such processes. There are two classes of localized hardening processes: diffusion methods and selective hardening methods. Diffusion methods such as carburizing and nitriding change the surface composition of a steel so that its hardness after quenching is greater than that of the bulk material. Selective hardening methods such as flame, induction and laser apply heat to a localized area so that it is austenitized and hardens upon cooling. 10.1.3.1 Comparison of LTH with Carburizing. Articles to be carburized are made of low-carbon steel. Carbon diffuses into the surface while the workpiece is held between 800 and 1000°C. The carbon is supplied from gas, solid, or liquid. Areas not to be carburized must be masked to prevent diffusion of carbon into these regions, and the masking material subsequently removed. The case depth is controlled by time and temperature. Hardening is achieved by quenching the entire part; the high-carbon regions reach a much higher hardness than the interior or any masked surfaces. From this description, it is clear that carburizing is preferable to laser hardening if the entire surface of a part is to be hardened. LTH is better for hardening small areas of a workpiece since the time-consuming process of masking is unnecessary in the laser process. 10.1.3.2 Comparison of LTH with Nitriding. Like carburizing, nitriding changes the surface composition of steel so that it is harder than the bulk material. Nitriding is typically performed on steels alloyed with aluminum or chromium (stainless steels may be nitrided, although their corrosion resistance is reduced) and generates an extremely hard case without requiring quenching. The depth of the case is quite shallow unless parts are kept in the nitriding furnace for long periods of time. A 24-hour soak time produces a case depth around 300 µm. As a diffusion process, nitriding requires masking to allow selective hardening. It is a good way to harden stainless steel, and nitrided surfaces have excellent wear resistance. Compared to laser transformation hardening, nitriding is slow and cannot generate deep cases in reasonable times. 10.1.3.3 Comparison of LTH with Flame Hardening. Flame hardening is very similar to laser hardening except that lasers can apply more power per unit area with lower total heat input and a much faster thermal cycle. In both processes, heating occurs at the surface of the steel, with the depth of the case being controlled by the steel’s thermal diffusivity. Torches are drastically less expensive than lasers but have less control of heat input and the area being heated. If the power and positioning of the flame produces sufficiently reproducible hardening for an application, it is not costeffective to use a laser. 10.1.3.4 Comparison of LTH with Induction Hardening. Induction hardening is a very powerful selective hardening method. A coil is placed close to the location to be hardened; when the coil is energized with a high-frequency alternating current, electric current is induced in the workpiece in a manner similar to the way in which electric current is induced in the secondary windings of a transformer. Heat is generated within a workpiece; the depth of heating may be controlled by varying the frequency of the source. Either shallow or deep cases may be produced by the induction hardening process. Lasers are superior to induction where the part geometry is not suitable to act as a transformer secondary. Such geometries are the insides of bores, external corners, and narrow stripes on surfaces. 10.1.4 Surface Preparation. Another disadvantage of LTH with respect to other hardening processes is the need to paint the surface to absorb the energy of CO2 lasers, the most common high-power industrial laser in use today. As described below, metals are excellent reflectors at this wavelength, so coatings are mandatory for effective power absorption. High-power Nd:YAG lasers operating at 1.06 µm are available. Steel absorbs their light much better, so they can be used for hardening without coating. The lasers, though, are costly and inefficient, so they have not been widely adopted. High-power diode lasers are extremely efficient and have a wavelength that is better absorbed by steel; use of these lasers may reduce or eliminate the need for painting.

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10.2 The Laser Transformation Hardening Process 10.2.1 Metallurgy of Transformation Hardening. Laser Transformation Hardening is often carried out in steel alloys possessing a pearlite structure as well as many grades of cast iron. LTH can also be applied to material with a tempered martensite structure. Pearlite is formed when steel is slowly cooled. Immediately after solidifying, steel is composed of a high-temperature crystalline phase called “austenite.” This phase contains all the carbon in the steel but is unstable below 750°C. Below this temperature, it decomposes to the low-carbon phase ferrite and the high-carbon compound cementite (Fe3C). In this decomposition, carbon diffuses through the austenite matrix to a cementite crystal, allowing the cementite to grow. The region around the crystal becomes depleted in carbon, so a layer of ferrite that contains no carbon forms. As ferrite grows, carbon atoms are rejected from the crystal structure, and the concentration builds up in the matrix around the crystal, until it reaches such a concentration that ferrite can no longer be formed. Then cementite forms again, removing the excess carbon from the matrix, and the process repeats itself. The result of this process is the pearlite structure consisting of alternate layers of cementite and ferrite. Thickness of the layers, or laths, in the grain depends on the cooling rate. In a relatively low-carbon steel, the pearlite grains can be isolated specks in a low-carbon matrix. In a relatively highcarbon alloy, the pearlite grains can be interspersed with relatively massive carbide grains. When this structure is subjected to a heat source and is heated above approximately 800°C, the crystal structure converts to the high-temperature phase austenite. The carbon diffuses away from the regions of the former carbide laths in which it was concentrated. If the temperature is maintained above the phase-transformation temperature for a sufficiently long period of time, a uniform carbon concentration is achieved in the heated region. The length of time that it takes for this to happen depends on the coarseness of the original pearlite microstructure. If this austenite structure is allowed to cool slowly, allowing time for the diffusion of carbon, the pearlite structure will be reformed and no hardening will take place. If the cooling takes place very rapidly, there will be no time for the diffusion of carbon, and a crystal structure called “martensite” will form. Martensite is a very hard crystal structure and consequently has beneficial wear properties. Transformation hardening is the process of converting the microstructure of the steel into martensite. This is a solid-state reaction and doesn’t require melting of the steel. A degree of control of the temperature cycling of the steel is required. If the heating doesn’t take the material over the phase-transition temperature, the microstructure won’t be changed. If the cooling rate after the phase transition is too slow, then the pearlite structure will be reformed, perhaps with a different degree of coarseness of the original structure. Fast cooling is required to achieve martensite. At intermediate cooling rates, a microstructure called “bainite” can be formed. Sometimes, a mixture of some or all of pearlite, bainite, and martensite is formed; individual regions of the different microstructures can be seen upon microscopic examination of polished cross-sections. Bainite structure is harder than pearlite but not as strong as hard as martensite, and is usually not the desired microstructure. These concepts are illustrated in Figure 10.1, which is a sketch of what metallurgists call an Isothermal Transformation Diagram. The specific shape of the diagram is different for every steel alloy. Curve A shows a case where the heating cycle is insufficient to form austenite, and no phase transition takes place. Curve B shows the desirable thermal cycle in which the austenite phase-transition temperature is exceeded, and cooling is sufficiently rapid that martensite is formed. Curve C shows a thermal cycle in which the cooling is so slow that pearlite is reformed. A thermal cycle between B and C is one in which bainite or a mixture of the microstructures would form. The Isothermal Transformation Diagrams have a pronounced vertical rise, designated the nose of the curve. To transform the structure to martensite, it is important that the cooling rate is sufficiently rapid that the cooling line is to the left of the nose of the curve. The thermal cycles shown in Figure 10.1 show the material is maintained above the austenite phase-transition temperature for a length of time. This is required for the carbon constituent of the alloy, which was originally resident in the cementite, to diffuse into the austenite, forming a homogeneous mix. If sufficient time for diffusion is not allowed, there will be carbon rich and carbon depleted areas in the crystal structure, and full hardness will not occur upon cooling. Figure 10.1 also shows two temperatures, Ms and Mf. Ms is the martensite start temperature at which the transformation from austenite to martensite starts, and Mf is the martensite finish temperature at which the transformation is completed. These temperatures occur at lower values for heavily alloyed materials and in many cases can be below room temperature. If this is the case, the microstructure resulting from transformation hardening would be a mixture of martensite and retained austenite. This is usually indicated by the hardness readings; the detection of retained austenite is fairly difficult and requires techniques such as X-ray diffraction. In these alloys, the conversion to martensite can be completed and full hardness achieved by soaking the part in a dry-ice slurry or in liquid nitrogen. This is not practical for large components. 10.2.2 Transformation Hardening with a Laser. The laser is only one of many heat sources that can be used for transformation hardening. The essence of the laser process is the scanning of a partly focused beam over a metal surface,

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END OF LASER BEAM START OF CONTINUOUS COOLING CURVE FOR STEEL AUSTENITE

730°C

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Figure 10.1—Isothermal Transformation Diagram

having taken steps to ensure the beam is absorbed by the material. Methods of producing a partly focused beam and of assuring beam absorption are discussed in later sections. The component being hardened is heated above the austenite phase transition temperature, but without melting the surface. The laser parameters (power, scanning rate, beam size) are chosen so there is a sufficient time at the elevated temperature for carbon to diffuse and form a homogeneous austenite. Under the right conditions, this can be achieved without heating the whole of the structure on which surface hardening is desired. The bulk of the material remains at or near room temperature. When the heat source is removed, for example by the laser scanning to a different part of the structure, there is rapid heat transfer from the heated surface layers to the cool region underneath. As described above, the high cooling rate results from the self-quenching results in a hard martensite microstructure with beneficial properties. In steels similar to AISI 1040 and 4140, a hardened case layer of about a millimeter in depth can readily be achieved, as shown in Figure 10.2. Within the case depth, the hardness is relatively constant, and is close to the maximum of which the steel is capable. A careful choice of laser processing conditions is required to produce significantly deeper case depths while avoiding surface melting.

Figure 10.2—Laser Hardened Groove in AISI 4140 Steel, for Ball Bearings 103

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10.2.3 Side-by-Side Passes. To surface harden a significant surface area, it is often necessary to use many side-by-side passes of the partly focused laser beam. This can lead to a condition known as tempering, sometimes called “back-tempering.” Tempering occurs when a steel with a martensitic microstructure is raised to an elevated temperature, but below the austenitic phase transformation temperature. At the elevated temperature, some of the carbon diffuses out of the ironcarbon alloy and forms cementite inclusions. Because the carbon content in the lattice is lower, the material is somewhat softer than it had been before the tempering process, and thought to be less wear resistant. The tempered martensite microstructure, although softer than untempered martensite, is strengthened by the carbide inclusions and more ductile. A single pass of the partly focused beam produces hard untempered martensite. A second later pass, slightly adjacent to the first, will reheat some of the material in the first pass, tempering and slightly softening the microstructure. A scan of microhardness measurement across two adjacent passes of the partly focussed beam is shown in Figure 10.3. The tempered region is clearly visible as a dip in the microhardness values, although even the depressed values are significantly harder than the untreated base metal. The softening due to adjacent passes is an unavoidable feature of laser transformation hardening, although special optics can be envisioned to minimize this phenomenon, as discussed later. 10.2.4 Surface Melting. It has been emphasized above that laser transformation processing is a solid-state process that occurs without surface melting. The process will also take place when surface melting occurs; indeed, this is the only way to dissolve large carbide inclusions found in many cast irons. After surface melting, the part will solidify forming austenite, and if the rate of cooling is sufficiently fast as described above, the hard martensite crystal structure will form. But when surface melting occurs, the surface of the workpiece is left in a roughened condition, and in most cases a subsequent grinding operation will be required to produce a smooth surface. The secondary operation detracts from the cost effectiveness of the process. 10.3 Equipment and Processing Considerations 10.3.1 Lasers. Both CO2 and Nd:YAG lasers described in clause 5 have been used to perform laser transformation hardening. Average power levels in excess of a kilowatt are usually used, and power levels of three kilowatts and higher most often used. Continuously operating lasers are usually used, except where the region to be hardened is small enough that it can be treated with a single-laser pulse. Laser diodes producing a line focus can also be used for LTH; at the time of writing there is no case of a laser diode being used in a production environment for LTH.

VICKERS HARDNESS NO. (3009)

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DISTANCE (MM) Tempering of steel due to reheating by side-by-side passes. The beam from a 5 kW laser was concentrated with a beam integrator on a rotating shaft of AISI4140 steel to produce a spiral path of hardened area.

Figure 10.3—Tempering of Steel Due to Reheating by Side-by-Side Passes 104

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10.3.2 Interactions of Laser Beams with Surfaces. For normal incidence, only 5 percent of a CO2 laser beam or 30 percent of a Nd:YAG laser beam is absorbed by the polished surface. Some means is usually undertaken to increase the fraction of the laser energy used for transformation hardening. Absorption of CO2 laser light can be increased from 5 to 12 percent by simply roughening the surface with 100 grit sandpaper. For some specialized workpieces that cannot be treated with a normally incident beam, one can take advantage of the fact that the absorption increases, at least for one polarization of incident light, when the beam is at a sharp angle to the surface. This phenomenon has been used in hardening small areas inside a cylindrical bore. The disadvantage of this treatment is that the beam becomes spread out when incident on a surface at a sharp angle; a circular beam has an elliptical contact with the surface and a rectangular beam has a trapezoidal contact area. The most usual method of increasing the absorption of the incident beam is by using a surface treatment, as discussed in the next section. 10.3.3 Surface Treatment to Enhance Laser Absorption. A variety of different surface treatments have been used to enhance the absorption of laser radiation in the workpiece. Approximately 80 percent of CO2 laser radiation is absorbed into a phosphatized surface; both zinc phosphate and manganese phosphate have been used. About 88 percent of the laser energy is absorbed into a surface coated with a colloidal graphite; black spray paint seems to perform somewhat better. Further improvements in the case depth from transformation hardening have been noted when the spray treatments is applied after the surface is shot-blasted or grit-blasted, but this is not desirable on some workpieces where a smooth finished surface is required. The phosphate treatment is preferred by many workers because it is readily available and can be applied uniformly by a simple dip treatment. A spray treatment requires a specially vented work area and is difficult to apply uniformly on some workpieces. Surface treatment is a requirement for transformation hardening with a CO2 laser. Transformation hardening can be performed with a Nd:YAG or a diode laser without surface treatment, but absorption and subsequent case depth is improved when a surface treatment is used. There are disadvantages to surface treatment. The main disadvantage is that it requires at least one extra processing step. Full automation of the surface treatment operation can be very difficult, particularly if areas of the workpiece are masked off so that they’re not treated. If paint is used, charred residues of the coating may remain after the operation, requiring a cleaning step for their removal. Steps must be taken to prevent fumes from the laser interaction with the painted surface from contaminating the optics or work area. Finally, since emissivity of the coatings may not be well known, the presence of a coating may impede the use of a pyrometer for process control. In spite of these difficulties, taking the extra step makes more effective use of the laser energy and hence is worthwhile. 10.3.4 Systems and Beam Delivery Optics. In general, the spatial energy profile of a laser beam is not well suited for uniformly heating a surface. The energy is often concentrated in the center of the partly focused beam. In many highpower lasers, the distribution is sensitive to mirror alignment and may drift with time as the system warms up. For this reason, most laser-heat-treating systems incorporate optics described below to alter the laser beam so that it delivers power where it is needed. 10.3.4.1 Integrators, Reflective, and Transmissive. If a laser beam is sent into a faceted mirror which is designed so as to superimpose the light striking each facet at one location in space, the superimposed elements will serve to integrate the beam, transforming a laser beam into a uniform illumination source. This is illustrated in Figure 10.4. Transmissive optics can also be designed to perform the same function. These are more useful for Nd:YAG lasers than for CO2 lasers because of the limited durability of optics that transmit high-power 10.6-µm light. Integrators are useful in production because of their stability; they will produce the same pattern despite highly variable input beams. This makes them less useful in development work where it is often necessary to change the illumination pattern. 10.3.4.2 Scanners, One- and Two-Dimensional. Since laser surface heating without melting has a time constant on the order of 100 msec or more for significant case depths in steel, a rapidly scanned spot has the same effect as a uniform line of power. Scanners, such as that shown in Figure 10.5, are able to vary line width and scan length over a considerable range. If two orthogonal scanners are combined, it is possible to generate arbitrary patterns of power on the surface of the workpiece, in two dimensions. 10.3.4.3 Kaleidoscope. A simpler device than the scanner, the kaleidoscope as shown in Figure 10.6 is simply a tube with reflective walls. The shape of the tube may be square, rectangular, or more complex. If a highly divergent beam (typically obtained by setting the kaleidoscope beyond the focal point of a lens) is sent into the tube, the light will bounce several times off the walls, emerging with a relatively uniform intensity distribution. The output is only useful very near 105

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Figure 10.4—Beam Integrator

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Figure 10.5—Beam Scanner

the opening of the tube unless it is re-imaged by another lens. Kaleidoscopes are useful in development work because they are easily made and relatively inexpensive. Because of the multiple reflection and absorption events, they are quite inefficient, losing 30% to 50% of the incoming power, unless internally gold-coated. 10.3.4.4 Partly Focused Beam, or Direct Fiber Illumination. The simplest beam conditioning is none at all. Some lasers, particularly those with high-order modes, do not have central hot spots in their beams. A laser operating in TEM01 produces a “doughnut” mode that has no power in the center. Such beams can be used to heat treat surfaces if the surface is not brought too near the melting point of the steel. Fiber outputs from Nd:YAG or diode lasers are quite uniform and may be used for heat treating, although circular beams are not ideally shaped for uniform case depth.

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INCIDENT LASER BEAM

WAVE GUIDE (REFLECTIVE METAL INTERIOR CHANNEL) FOCUSING LENS

INTEGRATED BEAM PATTERN (DIVERGES AT ANGLE CREATED BY FOCUSING LENS)

Figure 10.6—Kaleidoscope 10.3.4.5 Special Optics, e.g. To Treat Both Sides of a Gear Simultaneously, Avoiding Tempering, Ring of Fire Optics. The optics as described above are suited for general transformation hardening work. In certain cases, more specialized optics are needed. Gear teeth, for example, cannot tolerate the distortion that occurs if one side is hardened before the other. To reduce this distortion, the incoming laser beam must be split so as to heat both sides of the tooth simultaneously. Splitting the beam also controls the back-tempering caused by successive passes. If tempering is not allowable on a cylindrical part, the entire circumference must be heated at once. For inner diameters, the beam must impinge on a conical mirror that sends the light out in a circular pattern. The mirror is then moved along the axis of the bore to harden it. For outer diameters, the beam is first transformed into a ring. A second optic directs this ring inward to hit the surface. Again, axial motion is used to perform hardening. 10.4 Materials Considerations 10.4.1 Materials for Laser Transformation Hardening. Laser Transformation Hardening is performed on steels or cast irons with carbon contents ranging from 0.3% to over 1%. Either straight carbon steel (10xx series) or alloy steels are suitable, and free machining grades may be used. The higher hardenability of alloy steels such as 4140 or 8650 allows case depths up to 2 mm to be generated, although most LTH applications require case depths of 1 mm or less. 10.4.2 Effect of Carbon Content. The hardness of martensite is almost exclusively controlled by its carbon content. Carbon also slows down austenite decomposition. Steels with less than 0.3 percent of carbon are considered unhardenable; even when cooled quickly enough to form martensite, the martensite is soft. 10.4.3 Unsuitable Materials. Steels for diffusion hardening such as 8620 are unsuitable because their intrinsic carbon content is too low to allow them to transform to martensite. Austenitic stainless steels do not respond to laser hardening since their carbon contents are extremely low, and they don’t transform to martensite on cooling. 10.4.4 Effect of Initial Microstructure. Laser Transformation Hardening is generally a very fast process, with austenitization times on the order of 1 sec. Since the austenite picks up carbon from dissolution of carbides at the elevated temperatures required for austenitizing, the carbide must be finely dispersed to allow uniform carbon diffusion. Rolled steel often exhibits banding, where the composition varies from point to point. Hot-rolled steel is often decarburized on the surface. This is clearly detrimental to surface hardening. A uniform pearlitic structure responds quite well to rapid heating because of the short diffusion distance. A quenched and tempered structure is even better because the furnace treatment homogenizes the steel and a proper furnace atmosphere can eliminate any surface decarburization that might have occurred. 10.5 Process Control 10.5.1 Control Factors. The parameters that influence the quality of the laser transformation process are as follows:

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Laser power at the workpiece: This has to be carefully controlled to bring the metal above the A3 temperature (the temperature where it transforms to austenite) but not to exceed the melting point. Travel speed: Since the workpiece temperature is a function of laser power and travel speed, this parameter must be held closely to achieve consistent case depths. Laser mode: A beam integrator, mentioned above, consists of a number of flat segments arranged on a curved surface so that the energy reflected from, or transmitted through, each segment overlaps at a particular point along the transmission path. At the position of overlap, an approximately uniform distribution of laser energy is produced, if a sufficient number of flat elements of the beam integrator are illuminated by the incoming beam. It is important that the laser mode largely fill the integrator so that many segments are utilized. If only a few segments are used, the uniformity and distribution of the concentrated beam may critically depend on the position of the beam on the integrator. Paint thickness and consistency: If paint becomes too thick, insufficient heat is conducted to the metal surface. If the paint is too thin, insufficient heat is absorbed by the metal. Metal microstructure: Variations in material of the same nominal composition can effect the hardening process. Processing conditions used to harden a fine-grained pearlite will not harden a coarse-grained pearlite. If a material has been decarburized at the surface due to a prior treatment such as hot forging, it won’t harden to the extent as expected for that composition of material. Some vendors using LTH put strict controls on the metallurgy of as-received materials. Initial temperature of the workpiece: Variations in the initial temperature of the workpiece will affect the cooling rate and hence the hardness achieved. 10.5.2 Quality Control during Transformation Hardening. The best all-round method of quality assurance is random sampling of production parts, with each sample part being sectioned, etched to show the depth of the case, and its hardness measured with a microhardness tester. This will show if there is any change in the distribution of the heat-affected region or in the hardness achieved due to any of the factors described in the previous section. This is also the most expensive method of testing, unless the sampling can be done on parts that are scrapped for other, independent reasons. Simulation parts, rather than fully machined production parts, may be used to reduce costs. It is important that the simulation parts be made from the same batch of material as the production parts. Witness plates may also be used; they provide a quick and permanent record of the process quality. A witness plate would be a relatively thin piece of metal, cut to the same shape as the workpiece being treated and subjected to the same treatment. For example, in hardening a gear, a 6 mm thick piece of flat stock of the same material, cut with the same gear teeth but without any hub or other structure, may be placed immediately adjacent to and in contact with the gear as it is treated. Sandblasting will show the depth and shape of the hardened region, as the sand will roughen the base metal next to the hardened area but not the hardened area itself. This will not give actual hardness values, however. On-line instrumentation such as eddy current monitor has been investigated as on-line instrumentation for microstructure achieved, but to the present time have not been implemented in practice. In some cases, infrared detectors monitoring the temperature of the laser-heated surface have been employed in a job shop environment. 10.6 LTH Applications. Examples of products that have been laser transformation hardened are diesel cylinder liners, housings for power steering mechanisms, piston rings, splines of drive shafts, grooved rails in automatic teller machines, grooves in high-pressure swivel joints, parts of gas turbine engines, and the material surrounding a guide for a sliding joint in a oil tool. Diesel cylinder liners are treated with 5 kW laser beams concentrated by a beam integrator on the internal surface; the liner is rotated and moved vertically to produce a spiral hardened region. The grooved rails in the automatic teller machines used a 2.5 kW CO2 laser partly focused to produce a 3-mm spot to harden the grooves to HRC60 to 65. The power-steering gear housings used a TEM01* mode from a 1 kW CO2 laser focused to produce a 2-mm spot size. Only part of the inside of the liner was hardened; in this case the laser process satisfied the engineering requirements for part lifetime with less-hardened surface than possible with other techniques.

11. Record Keeping 11.1 Introduction. The accurate documentation of the laser process parameters is vital for reproducing previous results of the laser process. These parameters will sometimes vary between different models and manufacturers of laser systems. This section identifies many of the factors that may need to be recorded.

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The results from a procedure developed on one laser system and transferred to another system will usually vary. A procedure developed and used on only one system can vary over time due to optics or flash lamp degradation. The end result of the process should always be the governing factor for final process parameter selection or modification. Statistical Process Control (SPC) methods can be employed. Many of the following parameters can be measured and calibrated for accuracy. Individual users or organizations should decide which factors must be routinely calibrated, as well as the accuracy and frequency of calibration necessary for their application. As a guideline, existing ISO and AWS Technical Standards (written for this industry) may be consulted. Please see more information on this subject in Clause 13.2 and Annex A. Some factors will vary over time, for example, mirror alignment, flash lamp performance for Nd:YAG lasers, and gas mixture variations in CO2 lasers. These and other parameters will affect beam quality which will, in turn, affect processing capabilities. Proper record keeping should include recording these parameter variations, but more importantly, quality of the processed material should be verified through product inspection techniques, such as nondestructive testing, periodic destructive testing, and metallography. Table 11.1

Table 11.1 Laser Equipment Set-up Factors System Factors Laser Resonating Medium Laser gas flow rate Laser gas mixturea Laser tube pressure Number of cavity mirrors Upcollimator ratioa Modea Beam path length Number of beams (split) Focus lens coating Lens shield typea Assist gas orifice typea

Medium Excitation Source Tube purge rate Laser gas purity Lamp time in service Resonator Mirror Type and Coating Collimated Beam Diametera Polarization Number of Tarning Mirrors (type and cooling) Focus Lens Typea Focus Lens Diameter Assist Gas Nozzle Typea Beam Quality (M1)

Welding Set-up Factors

Cutting and Drilling Set-up Factors

a

Pulsed or CWa Pulse ratea Pulse durationa Pulse shape Aperture diametera Travel Speeda Lens Focal Length Focal Point Locationa Nozzle Tip Sizea Nozzle tip distance from workpiecea Assist Gas Type Assist Gas Pressurea Shutter Delaya Kerf Allowance (cutter-path compensation) Resonatur current Resonator Voltage PFN Voltage Capacitor Voltage Measured Powera Pulse Energya Energy density Toolinga

Pulsed of CW Pulse ratea Pulse durationa Pulse overlapa Pulse shape Aperture diametera Travel Speeda Filler Metal (Type, Size, Speed, Angle) Lens Focal Length Focal Point Locationa Shield Gas Typea Shield Gas Flow Rate and Pressurea Shield Gas Prepurge Timea Shielding Gas Postpurge Timea Nozzle distance from workpiecea Plume Suppression Gas Flow Rate Plume Suppression Direction Shutter Delay Timea Resonator Current or Voltage PFN Voltage Capacitor Voltage Measured Powera Pulse Energya Energy Density Toolinga a

Strongly recommended as reference data.

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identifies many of the system and set-up parameters for laser materials processing. The need to record these factors, and their classification, will depend on the type of laser, its construction, and the application. System parameters are those which are not necessarily recorded on a routine basis but are, however, necessary to assist in transfer of procedures from one laser to another, and to ensure long-term reproducibly for a specific laser system. Setup parameters are established for a specific application and are required in order to reproduce previous results. The asterisk items are strongly recommended to ensure a reference point for requalification and verification of parameters. Figures 11.1 and 11.2 show examples of laser documentation.

PART NAME

SAMPLE NUMBER

DATE

DESCRIPTION OF PROCESS (WELDING/CUTTING/DRILLING/SURFACING/ETC. MATERIAL TYPE

THICKNESS (mm)

LASER TYPE

LASER MANUFACTURER AND MODEL NUMBER

MICROPROCESSOR (MAKE AND MODEL)

PROGRAM NAME/REVISION

AXES CONTROLLED

BEAM MODE

LASER SETTING (LAMP OR DISCHARGE SETTING)

APERTURE DIAMETER AND POSITION

LASER PULSE LENGTH (mS)

LASER PULSE RATE (Hx)

LASER POWER (WATTS/OR ENERGY (JOULES/PULSE)

MIRROR/LENS FOCAL LENGTH (STATE WHICH)

POSITION OF FOCAL POINT (CONVERGENT/ DIVERGENT)

MIRROR/LENS TO PART DISTANCE (mm)

ASSIST GAS TYPE (NOZZLE)

ASSIST GAS FLOW AND PRESSURE (NOZZLE)

NOZZLE ORIFICE SIZE

NOZZLE STANDOFF

AUXILIARY SHIELDING GAS TYPE AND FLCW/PRESSURE

BEAM ANGLE TO PART (PITCH/ROLL/YAW)

TRAVEL SPEED

NUMBER OF PASSES

FILLER METAL SPECIFICATION

WIRE DIAMETER/POWER MESH SIZE

FILLER FEED RATE (mm/MIN)

FILLER NOZZLE ANGLE AND SETBACK

DEPTH OF PENETRATION OR BEAM SIZE/PASS

WELD WIDTHKERF WIDTH

NOZZLE SHAPE/MODEL

TIME BETWEEN PASSES

DIAMETER OF HOLE

FILLER VETAL AIM POSITION ADDITIONAL DATA/COMMENTS:

SKETCH

Figure 11.1—Laser Cutting and Drilling Process Parameter Schedule

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PART NAME

DATE

DESCRIPTION OF PROCESS (WELDING/CUTTING/DRILLING/SURFACING/ETC.) MATERIAL TYPE

THICKNESS

LASER TYPE

LASER MANUFACTURER AND MODEL NUMBER

MICROPROCESSOR (MAKE AND MODEL)

PROGRAM NAME REVISION

AXES CONTROLLED

BEAM MODE

APERTURE DIAMETER AND POSITION

MIRROR/LENS FOCAL LENGTH (STATE WHICH)

ASSIST GAS TYPE (NOZZLE)

ASSIST GAS FLOW AND PRESSURE (NOZZLE)

NOZZLE ORIFICE SIZE

NOZZLE STANDOFF

AUXILIARY SHIELDING GAS TYPE AND FLOW/PRESSURE

BEAM ANGLE TO PART (PITCH/ROLL/YAW)

FILLER METAL SPECIFICATION

WIRE DIAMETER/POWDER MESH SIZE

FILLER FEED RATE (in/MIN OR mm/MIN)

FILLER NOZZLE ANGLE AND SETBACK

ADDITIONAL DATA/COMMENTS:

SKETCH:

Sample No.

Power Level

Pulse Energy (J/P)

Avg. Power (W)

Pulse Freq. (Hz)

Pulse Length (mS)

Focus Pos. (mm)

NOZZLE SHAPE/MODEL

Travel Speed (mm/sec)

NUMBER OF PASSES

Penetration Kert Width/ Holo Size (mm)

Comments

Figure 11.2—Laser Welding Process Parameter Schedule 11.2 Laser Parameter Description 11.2.1 Average Power and Energy Measurement. It is recommended that the specific instrument used for power or energy measurement be recorded, calibrated at intervals recommended by the manufacturer, and the method used is consistent and repeatable between laser operators. 11.2.2 Aperture Identification. The diameter and type of aperture used with an Nd:YAG laser and the resulting output beam size shall be documented. If the aperture size is not commercially available, the aperture should be machined to a sharp or knife edge. 11.2.3 Laser Beam Quality. Laser beam quality should be documented along with laser power as a method of ensuring a consistent energy density is maintained at the work surface. Clause 6, Process Control and Monitoring, describes a number of methods by which beam quality is typically measured that range from relatively simple mode burns in acrylic (or photo detector plates) to detailed analysis of energy distribution and beam propagation. The method of measurement and the results of the measurements should be included as part of the record keeping.

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11.2.4 Carbon Dioxide Gas Mixture. CO2 laser gas mixtures shall be identified by the percentages used for each gas. If a gas-mixing system is used in place of premixed bottles, the operating instructions should be available for the operator, and the instrument settings should be documented. Gas regulator pressure and flow rates shall be documented. Gas purity should also be recorded to ensure consistent results. 11.2.5 Focusing Lens or Mirror. The optic diameter, focal length, and special optical coating shall be documented. Other features of the optic (e.g., plano-convex, aspheric, or meniscus) should be identified in the documentation. Identification numbers can be engraved into the mount and recorded to ensure direct reference to the focusing optic used for the task. 11.2.6 Focal Point Location to Material Surface. The focal point location in respect to the material surface is critical to maintain specific kerf width for cutting, and power density for welding operations. The effective focal point can be identified several ways. The procedure for focal-point adjustment for solid-state and gas lasers can vary, so it is critical that the procedure used be standardized and documented within each company. Refer to Clause 6, Process Control and Monitoring, for a detailed discussion on how to accurately locate the laser focus. 11.2.7 Gas Assist Methods. The type of gases used to assist process operations (i.e., cutting, welding) shall be identified. The flow rate or pressure of the gas, or both, should be documented for each type of gas used. Ensure that the proper gas regulator/flowmeter is used for each specific gas. If a multi-gas flowmeter is used for all gases, then the specific flowmeter should be documented. The purity of the gas used should also be recorded. The type of gas assist devices (e.g., gas nozzle, plume suppression) should be identified. If the gas assist device is unique, then a supplementary diagram or photograph should be provided with the gas documentation. The nozzle orifice diameter or a detailed description of the gas delivery system, including the method for centering the laser beam in the nozzle, should be provided in the documentation. The nozzle standoff distance, gas delivery tube pointing attitude/orientation, or both, are important and should be provided in the documentation. 11.2.8 Resonator. The laser system model, manufacturer, and detailed information about any system modifications should be documented. For solid-state lasers, the laser medium, the mode, type, and divergence, if adjustable, should be documented. Also, resonator current, resonator voltage, PFN voltage, and capacitor voltage as applicable to the system should be identified as reference. For gas lasers, the following should be documented for reference: (1) laser gas mixture, (2) laser gas flow rate, (3) laser tube pressure, (4) tube purge rate, (5) resonator mirror type, (6) mirror coating time in service, (7) number of cavity mirrors, (8) polarizing optics, (9) resonator current, and (10) resonator voltage. 11.2.9 Beam Delivery. The beam delivery system should be clearly identified and documented to ensure a repeatable set-up. If the beam delivery is unique or difficult to set up, a diagram or photo of the beam delivery system may be included in the documentation. Beam delivery information to be recorded should include the beam path length, the number of turning mirrors, the number of beams, and the upcollimator ratio. Beam alignment procedures should also be included if the beam delivery is unique or complex. 11.2.10 Process Tooling. All tooling associated with the process shall be identified and any specific instructions required for proper tool use should be included with the documentation. Depending on the complexity of the tooling, a diagram or photograph may be necessary to ensure repeatable results.

12. Inspection and Testing 12.1 Introduction. The inspection and testing of laser-processed materials is primarily regulated by engineering requirements. In most instances, an inspection process is selected based on the general purpose of the end article. This section identifies the most commonly used inspection and testing practices for laser welding and cutting. A general overview of appropriate inspection techniques for welding is found in AWS B1.10, Guide for the Nondestructive Inspection of Welds. More specific guidance may be found in AWS C7.4/C7.4M2008, Process Specification and Operator Qualification for Laser Beam Welding and other information may be found in published ISO standards (see Annex A). Excessive inspection of a weld joint or cut area adds unnecessary costs to the final product, whereas insufficient inspection could result in the failure to detect weld discontinuities that may affect the fitness for service of the weldment. Nonetheless, consideration of the inspection methods and acceptability limits is a vital part of establishing a laser materials processing procedure.

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12.2 Weld Inspection. In order to provide an overview of the various inspection and testing methods, it is appropriate to first define the weld characteristics that affect the service of a laser-welded joint. These characteristics can be categorized as geometric features or as weld discontinuities. Features of laser welds that are geometric in nature include: weld reinforcement or undercut, melt-through, mismatch, weld width at the face and root, and weld aspect ratio. These features affect the load-carrying capability of the joint as well as fatigue strength and the amount of distortion. The types of weld discontinuities are similar to those found in most other fusion welds. Clause 8, Laser Welding Metallurgy, describes the metallurgical causes of discontinuities in laser welds. Included among the types of weld discontinuities are: linear indications or cracks (both surface and sub-surface), porosity, inclusions, undercut, incomplete fusion, and incomplete joint penetration. In applying the laser welding process and selecting inspection methods, it is necessary to specify limits for the discontinuities that affect the weld’s fitness for service, resulting in some discontinuities being classified as defects. It is also important to verify that the sensitivity of the inspection processes and techniques selected are adequate to detect the size of discontinuities deemed unacceptable. 12.3 Inspection Techniques. Inspection techniques for welds are classified as either destructive or nondestructive. Destructive testing methods are often used to verify the accuracy of the nondestructive testing techniques. 12.3.1 Destructive Testing 12.3.1.1 Metallographic Examination. Metallographic examination of weld cross-sections is used to measure weld fusion zone geometry and characterize the weld contour, size of reinforcement, depth of penetration, seam tracking efficiency, defects relative to microstructures (e.g., cracks, transgranular, or intergranular) and weld HAZ microstructure. 12.3.1.2 Mechanical Testing. Mechanical testing is used to determine the structural integrity of a weldment. Examples of mechanical testing include: tension testing, fatigue testing, and bend testing. In many manufacturing facilities, welded assemblies are destructively tested on a sampling schedule. 12.3.2 Nondestructive Examination 12.3.2.1 Visual Testing (VT). Visual testing or inspection is the most common inspection method used. Visual inspection is normally used to measure undercut, weld width, and mismatch. It is also used to detect surface defects such as lack of fusion, lack of penetration, and some linear indications. Visual inspection is used on nearly every laser weld, but is only used as a primary inspection method for noncritical weldments. Visual inspection is often aided with a handheld magnifier or portable microscope. Full penetration is a common criteria for lap welds. The presence of weld metal on the bottom is a indication of penetration to the joint position. Sometimes the presence of backside oxidation (heat-bands) is a requirement on lap welds. For butt welds, presence of weld metal on the root side is an indication of full penetration. Another criteria for visual inspection of laser welds is specifying a minimum weld width on the basis of the joint thickness and the weld width achieved during parameter development. The minimum weld width is then used in conjunction with a maximum allowable joint run-out to establish a level of confidence that the joint is fully consumed. A technique used to assist the visual inspection of laser welds is that of witness features. Witness features are often used on concealed joints or weld joints where it is difficult to verify joint consumption by the weld. The most common type of witness feature is that of witness lines. Witness lines are scribed or machined lines located on both sides of the joint to be welded. The lines can be located on the face side or root side of the joint, depending on the joint design and the accessibility to the weld joint. Witness lines can be categorized as consumable or nonconsumable. With consumable witness lines, the centerline location of the weld relative to the joint is verified by the complete consumption of scribe lines on both sides of the joint by the weld. Likewise, nonconsumable witness lines are used by verifying the centrality of the weld between the two scribed lines on both sides of the joint. Other witness features include the use of grooves or lips adjacent to the weld joint. With the distance from the feature to the weld joint being established, the centering of the weld is verified by making measurements from the weld centerline to the witness feature and comparing the measurement to the established distance. 12.3.2.2 Penetrant Testing (PT). Penetrant testing or inspection is capable of detecting surface discontinuities. The PT method is relatively economical and the sensitivity can vary depending on the system used. Since PT will not detect subsurface defects, the method is not used in critical applications. It is also used as an in-process inspection method prior to, and after, heat treatment of weldments.

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12.3.2.3 Radiographic Testing (RT). Radiographic testing or inspection is used to detect both surface and subsurface discontinuities, but is used primarily for the detection of subsurface discontinuities. Radiography is used for the inspection of moderately to highly stressed joints or for inspection of cracks, porosity, incomplete fusion, and incomplete joint penetration in the weldment. A variation of the technique is black-line X-ray. Application of high-energy laser-welding process requires weld joints with very tight fit-ups. The tight fit-up assists welding but can also make it difficult to interpret radiographs of weld joints that might contain a lack of fusion discontinuity. Thus, in order to provide a comparison standard for the inspection of the radiograph, a laser joint that has been tack-welded together is radiographed. This also verifies that the radiographic setup has the X-ray source aligned with the joint so that the tight joint can be detected. This black-line X-ray is then used by the inspector to interpret radiographs of welds that might contain a lack of fusion discontinuity. Another variation of radiographic testing, applicable to thick-section welding, is the use of radiographic windows. This entails placing a machined groove or “window” at a certain level of weld penetration. The weld is acceptable if the groove is shown by radiography to be filled during welding. This technique would require some development and process validation before implementation and should be coordinated with engineering requirements. Figure 12.1 shows designs of radiographic windows. 12.3.2.4 Ultrasonic Testing (UT). Ultrasonic testing or inspection is a testing method capable of detecting both surface and subsurface discontinuities. As a result of the cost and the high level of operator skill required, ultrasonic inspection is usually reserved for critical or highly stressed joints and thick section welds. The ultrasonic inspection process has an advantage over radiographic inspection in that it is more capable of detecting the approximate size and depth of a subsurface discontinuity. 12.3.2.5 Magnetic-Particle Testing (MT). Magnetic-particle testing or inspection is another process capable of detecting surface and local subsurface discontinuities. This process is only used on ferromagnetic materials. The magnetic-particle testing process is usually used to test moderately stressed laser welds and is often limited by the size of the welded assembly. 12.3.2.6 Leak Testing (LT). A technique commonly used in the inspection of hermetically sealed assemblies and pressure vessels is that of leak testing (LT). There are a variety of testing methods used to inspect laser welds for cracks, voids, or other defects that affect the integrity of a hermetic seal. Among these are leak testing by pressurizing welded

Figure 12.1—Radiographic Windows 114

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assemblies with fluids, bubble testing, flow testing, and testing with specific-gas detectors. One common specific-gas detector type is helium leak testing. 12.3.2.7 Witness Plates. A dummy workpiece, of similar dimensions, attached to the workpiece during the processing operation, is an in-process test of the quality of the processing job without performing destructive testing of the production part. 12.4 Inspecting Laser-Cut Parts. In many instances, the high-quality attainable through laser cutting is insured by specifying critical characteristics of the cut part. 12.4.1 Physical Dimensions. The most universal requirements are the physical dimensions of the part. Laser-cut parts typically have to meet dimensional requirements that are measured by calipers, micrometers, vision systems, templates, go/no-go gauges, or other standard measuring equipment. Inspection schedules depend on the criticality of the part, the allowable tolerances, and the applicable specifications. Statistical process control (SPC) techniques may be used to determine if the laser cutting process is under control when long production runs are made. In the usual case of multiple shortrun jobs, more sophisticated techniques such as normalizing the variations of a series of parts can serve to establish process control. Edge taper may also need to be controlled, and the amount will generally be specified separately from the part dimensional tolerance. 12.4.2 Surface Roughness. The surface roughness of laser-cut edges may be characterized per AMSE B46.1, Surface Texture Roughness, Waviness and Lay. The average roughness, Ra, is commonly specified. Since the surface texture of laser-cut edges varies from the top to the bottom of most cuts, Ra values for top, bottom, and center may be specified. 12.4.3 Dross. In some cases, recast material adheres to the bottom of a laser-cut edge. This recast is commonly referred to as dross, slag, or burrs. If a burr is critical to the function of a part, its maximum height should be specified in the engineering requirements. 12.4.4 Chemistry. Surface chemistry of the cut edge may need to be controlled. Common examples are oxides on stainless steel or depth of alpha case on titanium alloys. Visual examination is generally adequate in the former situation, while metallographic sectioning is often required to determine compliance to the latter. 12.4.5 Edge Quality. Metallographic techniques are also required for inspection of HAZ, micro-cracking, and recast layer. The HAZ may be specified in terms of observable changes in microstructure or may be determined by microhardness measurements. A requirement of “no HAZ” is typically unattainable if determined by micro-hardness. Recast is generally determined by observation of microstructure. 12.4.6 Fatigue Life. In some instances, depending on engineering design requirements, the laser-cutting process must be qualified by fatigue testing of laser-cut parts. 12.5 Acceptability Limits. An important aspect of incorporating testing methods into the fabrication of a product is specifying acceptability limits. These limits are determined by the designer of the product based upon the nature of the service that the product will experience. However, establishing practical limits of acceptability for discontinuities requires a knowledge of the discontinuities inherent in a given combination of process and material. For example, HAZ microfissures are common in certain precipitation hardenable nickel-base alloys. Also, a certain level of porosity is inherent when welding materials with high levels of interstitial elements. Thus, it is important to specify practical limits for these types of discontinuities. Acceptability limits are required for weld discontinuities such as linear indication, voids, undercut, under fill, lack of penetration, and lack of fusion. These limits are specified on an engineering drawing of the finished product, in a process specification, or in a legal agreement between the designer and the manufacturer. Certificates of conformance are the responsibility of the manufacturer upon completion of the welding and inspection of the product

13. Specifications for Laser Beam Welding 13.1 Purpose. The purpose of a welding specification is to ensure that a manufacturer or subcontractor will produce a welded assembly that will meet all of the design requirements and the process will be repeatable for each and every

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assembly. The specification will usually include inspection requirements and criteria for acceptance and rejection of welded parts. 13.2 Alternatives to Use of Published Specifications 13.3.1 Drawing Notes. Another method is to write an extensive set of drawing notes that are specific to the part being welded. The notes may include many of the requirements stated above. Depending on the extent of the drawing notes, the production of the laser weld can be adequately controlled. 13.2.2 In-House Specifications. The most common method is the writing of a laser welding specification by the design or production organization. The specification can be written specifically for a particular part or for a group of parts with similar design or welding requirements. These specifications usually resemble the format of other welding specifications, but are very specific towards laser welding variables and laser process control.

14. Equipment Maintenance 14.1 Maintenance Schedule and Records 14.1.1 Preventive Maintenance Schedule. Each manufacturer of a laser, laser system, or other equipment should specify detailed preventive maintenance procedures and schedules. A facility manager should have the various schedules coordinated to produce a daily checklist and lists of maintenance procedures that should be carried out at weekly, monthly, yearly, or at other intervals. 14.1.2 Maintenance Sign-Off Sheets. Maintenance procedures should be specified on a sign-off sheet, that is signed by the individual performing the maintenance at the time the maintenance is performed. These form part of a permanent record of the maintenance of a machine. Figure 14.1 is an example of a maintenance schedule.

DAILY MAINTENANCE ITEM

COMMENTS

LENS

REMOVE, CLEAN, AND STORE THE FOCUSING LENS AT THE END OF EACH WORK DAY. IF MORE THAN ONE SHIFT, CLEAN LENS BETWEEN SHIFTS.

CUTTING HEAD ASSEMBLY

INSPECT CUTTING HEAD FOR CLEANLINESS AND DAMAGE, PARTICULARLY THE NOZZLES. CLEAN AND REPLACE AS REQUIRED.

CUTTING RING/PIPE

CLEAN OFF ALL DEBRIS

TABLE/CHUTES

CLEAN TABLE/CHUTES OF ALL WATER AND DEBRIS; LUBRICATE (OIL) THE FREE TRAVEL BEARINGS.

SCRAP BOX

REMOVE AND EMPTY.

VENTILATION SYSTEM

REMOVE AND CLEAN ALL FILTER DEVICES.

LUBRICATION

LUBRICATE ALL POINTS INDICATED ON THE LUBRICATION CHART LOCATED ON THE SIDE OF THE MACHINE.

GAS CYLINDERS

CHECK THE LASER AND ASSIST GAS CYLINDERS FOR INTERNAL GAS PRESSURE. (CYLINDERS SHOULD NOT BE ALLOWED TO DROP BELOW 1379 kPa)

Figure 14.1—Typical Maintenance Schedule (Daily) 116

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14.1.3 Log Book. Operators should be provided a log book at each laser system to record problems or other notes to the maintenance personnel. The problems noted in the log book should not be a reason for immediate shut down of the system, but draw attention to items requiring the attention of maintenance personnel routine during preventive maintenance. 14.1.4 Parts List and Stock. A spare parts list complete with purchasing sources should be required and available. A stock list should be kept for all components that are expendable, for example, oil and gas filters, optics, nozzles, flash lamps, pump oil, etc. Stocked items should also include cleaning supplies. 14.1.5 Maintenance Intervals. Maintenance should be performed on a timely basis, but the intervals may be dependent on the amount of laser time accumulated and on the environment. A laser system operated three shifts per day will require a different maintenance schedule than one used intermittently. A laser system in a stamping plant in which the surrounding environment is comprised of oil vapors and dirt particles will have shorter maintenance periods than one in a research laboratory. All major pieces of equipment should have hour-meters installed. The hour-meter readings should be recorded on the maintenance sign-off sheets. Figure 14.2 contains typical maintenance items and intervals. 14.1.6 Laser Efficiency. A daily record of the laser power at the workpiece for a particular programmed setting provides a history of the system’s overall operating condition. Deviations from a standard (established when the laser is new) indicate degradation of the system. How efficiency is indicated depends on individual laser systems and whether the laser is pulsed or continuous. If the laser controls do not indicate power input directly, gas discharge current or flash-lamp current may be a good measure of the relative power input. This requirement means that a suitable power meter should be used at regular intervals to measure the power. Many laser users automate this measurement by using an electronic power meter and then record the data in a computer.

WEEKLY MAINTENANCE ITEM

COMMENTS

AIR PRESSURE

CONFIRM MAIN REGULATOR SET TO 5.7 KG.

LUBRICATION

LUBRICATE ALL POINTS INDICATED ON THE LUBRICATION CHART LOCATED ON THE SIDE OF THE SYSTEM.

MONTHLY MAINTENANCE ITEM

COMMENTS

VENTILATION SYSTEM

REMOVE AND REPLACE CHARCOAL FILTER (IF APPLICABLE).

AIR FILTER

CHECK, CLEAN, AND/OR REPLACE IN-LINE AIR FILTER.

CHILLER

CHECK WATER TANK FOR PROPER LEVEL, AND REFILL IF NECESSARY, CHECK AND/OR REPLACE INLET WATER FILTER. CLEAN AIR INTAKE FILTER.

OPERATION BUTTONS

CHECK ALL PUSHBUTTONS AND LAMPS FOR DEFECTS.

LUBRICATION

LUBRICATE ALL POINTS INDICATED ON THE LUBRICATION CHART LOCATED ON THE SIDE OF THE MACHINE.

MODULATOR

CHECK OIL LEVEL. REFER TO PRC AM 2000 MANUAL SECTION 7, PARAGRAPH IV FOR DETAILS, WARNING: HIGH VOLTAGES MAY BE PRESENT. FAILURE TO FOLLOW THE INSTRUCTIONS MAY RESULT IN LETHAL ELECTROCUTION.

LENS COOLER

CHECK AND REFILL WATER LEVEL.

Figure 14.2—Typical Maintenance Schedule (Weekly/Monthly) 117

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14.2 Troubleshooting. In spite of the best intentioned maintenance procedures, unscheduled system shutdowns may occur. In these cases, it is usually essential that the system be returned to operational as soon as possible and records made that indicate the cause of failure and the steps taken to return the system to operation. The manufacturers of laser equipment specify troubleshooting procedures. System modifications or changes in the routine maintenance procedures can be made with the approval of the process engineer in charge of the equipment to minimize the future occurrence of this downtime. 14.3 Specific Components 14.3.1 Gas Supply Consideration. Gas supplies are used with gas lasers, such as the CO2 laser, and for processing. The condition of gas bottles should be routinely observed, and replaced if necessary. Upon replacing a bottle, the line should be flushed out to remove the air contaminants introduced by the changing operation. It is a good practice to not completely discharge gas cylinders, but to leave a slight overpressure to prevent entrapment of debris. Note that most gas lasers have interlocks that prevent the laser from operating if a gas bottle becomes empty during operation. Thus, the laser itself cannot be damaged; however, a workpiece may have to be scrapped if the laser turns off in the middle of a job. If there is a danger of this occurring, gas bottles may have to be replaced when they are low. An alternate suggestion is to install a manifold system with a low-pressure alarm to warn of impending shutdown, allowing completion of a workpiece before automatic laser shutdown. Gas lines may contain in-line filters which should be cleaned or replaced on a regular basis. Compressed air supplies may need routine maintenance (grease, oil); in addition, an in-line oil or water trap may need systematic attention. 14.3.2 Vacuum Pumps. Gas lasers use vacuum pumps, which require periodic changing of the oils; the oil level should be systematically checked. Overall performance of the pumps can be checked by measuring the lowest pressure achieved when the pumping port is blanked off. 14.3.3 Circulation Blowers. High-power gas lasers use circulation blowers for circulation of the gases. Roots-type blowers will require routine checking of the oil level and periodic changing of the oil. Other types of blowers may require periodic inspection and possible replacement of the bearings, particularly if the bearings are located inside the vacuum enclosure. 14.3.4 Material Handling Consideration. Most moving parts, such as translating stages that move optics or parts, may require periodic lubrication. They should be periodically cleaned as an accumulation of grit may cause excessive wearing and a loss of accuracy. Clamps and rollers for manipulation of parts should be kept clean and lubricated. 14.3.5 Air Quality Equipment. Fume removal devices contain filters or electrostatic precipitators which should be routinely cleaned or replaced. 14.3.6 Controls. Control equipment is cooled by forced circulation; the filters in this equipment should be routinely cleaned (see Figure 14.2). The calibration of meters in the control equipment should be periodically confirmed and adjusted, if necessary. Data accumulated should be routinely backed-up. Much of the high-voltage equipment is encapsulated, but some laser suppliers may have the equipment in a restricted access cabinet. This should be checked for a buildup of dust which could cause an arc-over. The integrity of high-voltage connections should be occasionally checked by qualified personnel. 14.3.7 Cooling Water. Because lasers are extremely inefficient in converting electrical energy to light output, excess heat must be removed through proper cooling. Different systems may be used for cooling: water-to-water coolers, waterto-air heat exchangers, closed-cycle refrigeration units, or cooling towers. The performance of these systems in assuring temperature stability in the laser system can directly affect the laser output. Consequently, assuring the cooling system is performing properly is extremely important. Most cooling water systems contain an in-line filter, which should be periodically checked or replaced. Closed-cycle cooling systems for high-voltage components may contain deionizers; the cartridges should be checked. Other closedcycle cooling supplies may contain anticorrosive additives or glycol for cold weather operation; the content should be periodically checked. For large systems, temperatures, pressures, and flow rates should be periodically checked. Compressors used in a cooling system should be maintained by an air-conditioning specialist. Some laser systems have fail-safe circuits for control or monitoring of system-operating parameters. These circuits should be checked on receipt and periodically for reliability.

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14.3.8 Flashlamps. Nd:YAG and some other laser types are pumped by the light from flashlamps. The flashlamps degrade with time, but seldom fail catastrophically (see Figure 14.3). Some laser manufacturers recommend changing the lamps after a specified number of shots, which is counted in the laser control circuitry. If this is not done, welding trials should be done in which failure to reach a pre-established depth of penetration indicates the need to replace flashlamps. 14.3.9 Laser Rods. Laser rods have theoretically no life-limiting factors. Failures attributable to malfunction of the cooling system may occur. Periodic inspection of the end-coatings should be made. Glass or quartz tubes around the rod of a Nd:YAG laser which confine the flow of coolant require periodic inspection and possible cleaning of deposits. 14.3.10 Optics Cleanliness. The condition of laser optics is an important factor in the condition of the laser system. The optics should be inspected regularly for cleanliness, and cleaned if necessary (see Figure 14.4). Laser operation with dirty optics will lead to the dust being burned into the mirrors and lenses, and could lead to catastrophic failure of the optics. Because of the expense of the optics and the risk of damage during the cleaning process, it is recommended that the optics not be cleaned on a routine basis, but only cleaned when necessary. Note that internal laser optics may require cleaning even if they are not visibly dirty; this can be observed by a degradation in the efficiency of the laser. A recommended cleaning procedure for reflective optics is as follows: (1) Blow away all dust and debris from the optical surface with clean dry air. Do not use shop air lines because they usually contain significant amounts of oil and water. These contaminants can form detrimental absorbing films on optical surfaces; (2) Dampen a cotton swab or a cotton ball with acetone or methanol. Gently wipe the surface with the damp cotton. Drag the cotton across the surface just fast enough so that the liquid evaporates right behind the cotton. This should leave no streaks; (3) For severely contaminated and dirty parts, an optical polishing compound may be needed. Gingerly apply the polishing solution with a cotton ball (do not use swabs) or a special polishing pad. Use very light pressure and as few strokes as possible. Wipe with a clean cotton ball dampened with water and then repeat the previous step; and (4) Examine the part carefully. If the color of the surface has changed or appears to be abraded, then it needs to be reworked or replaced. Extreme caution should be maintained while cleaning and handling the optics. Insure the optic is free of dust or fingerprints prior to use. Also make sure that all systems in place for protecting the optics (see 5.5.3.4) are operating correctly.

SIX MONTH MAINTENANCE ITEM

COMMENTS

RESONATOR DUST TRAPS

CHECK AND CLEAN THE GLASS DUST TRAPS AND ALUMINUM DUST TRAPS.

HIGH VOLTAGE POWER SUPPLY

CHECK INTERNAL OIL LEVEL.

PRC DIAGNOSTICS FAULT PANEL

TEST FOR CORRECT OPERATION.

YEARLY MAINTENANCE ITEM VACUUM PUMP

COMMENTS REPLACE DEMISTING FILTER AT LEAST ONCE A YEAR.

Figure 14.3—Typical Maintenance Schedule (Six Month/Yearly) 119

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200–250 HOUR MAINTENANCE ITEM BEND MIRROR

COMMENTS INSPECT AND CLEAN BEND MIRROR.

500 HOUR MAINTENANCE ITEM

COMMENTS

VACUUM PUMP

CHANGE THE OIL.

OPTICS

OPTICS CLEANING SHOULD BE PERFORMED BY A TRAINED AND AUTHORIZED TECHNICIAN.

600 HOUR MAINTENANCE ITEM CHILLER

COMMENTS DRAIN AND REPLACE WATER.

Figure 14.4—Typical Maintenance Schedule (Periodic)

14.3.11 Nozzles and Shields. A cutting nozzle should be periodically checked for damage since a high gas pressure is required for cutting, and a worn or partially melted nozzle will not result in high-quality cuts. Shields providing cover gas for welding should be cleaned of spatter or replaced. If an inspection finds a damaged nozzle, this may mean the beam is not aligned with the center of the nozzle opening and it will be necessary to realign the beam. 14.4 Alignment. Internal laser optics should be aligned whenever a mode check indicates a degradation in the mode or spatial distribution of the laser output, or when maintenance of the laser cavity or delivery system may have disturbed the optical alignment. External optics should be aligned so that the beam is centered in the focusing lens and in the nozzle used to provide cutting assist gas or weld shielding gas. The beam should remain centered as the optics are moved over their full range of motion. This alignment operation may involve manipulation of several optical elements. The manufacturer of a laser system should provide detailed instructions on performing laser alignment. 14.4.1 Resonator Alignment. Proper alignment of the resonator cavity is critical for optimum laser performance. Alignment of the resonator is accomplished with one or more of several techniques, depending on the system design. Systems with transparent mirrors can be roughly aligned with an autocollimator. This instrument is used to determine small relative angles of reflecting surfaces with arc-second accuracy. The device projects a beam of visible light to the mirrors and collects the reflected light through a lens, which is then transmitted through a beam splitter for reflection to an eyepiece. The person performing the alignment views the projected spots through the eyepiece and aligns the spots to coincide with each other. Proper alignment of the resonator cavity should be verified with mode checks. See 6.5 for a complete description of mode checks. The laser manufacturer should be consulted for the proper alignment procedures for each specific laser. 14.4.2 Beam Delivery Alignment. The laser beam alignment is critical to the repeatability of laser processing. It is relatively easy to align a fixed-beam system. As long as the beam does not clip (hit an opaque support structure such as the side of a mirror housing) and goes through the middle of the focusing lens and gas nozzle, the system is aligned. The stationary elements of a fixed-beam delivery tend to stay aligned because they aren’t shaken or accelerated. A moving-beam system is aligned when there is no change in the beam location when the axes are run through their full range of motion. This is usually done by checking the alignment of each axis at both extremes of its travel and adjusting mirrors until the beam stays in place. Moving-beam systems have a tendency to become misaligned because they have a lot of mirrors, long beam paths, and moving parts.

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There are a number of laser-beam alignment techniques that are used to establish proper beam propagation throughout the beam delivery path, which will ensure that all the available laser power reaches the workpiece with the desired mode and diameter. Beam delivery alignment techniques that are available range from relatively simple, static methods through complex, automated dynamic alignment methods. The cross-wire alignment technique places a cross-wire or grid target into the beam path perpendicular to the direction of propagation. If the cross-wire is exposed to either the Nd:YAG or CO2 laser for a relatively short duration at low power, an image can be displayed on a thermal imaging plate. A permanent image can be obtained by burning into wood, Transite, or photosensitive paper. Correct beam alignment is indicated by the illuminated or burned pattern being centered on and symmetric about the cross-wire image. Alignment is corrected by adjusting the mirrors and retesting. The cross-wire is not a permanent part of the laser-beam delivery system and is removed after the alignment. Some systems employ a visible diode or helium neon (He–Ne) laser to assist with mirror alignment. It is critical that the visible beam be aligned to coincide with the CO2 or Nd:YAG beam. The system manufacturer’s recommendations should be followed for alignment of the delivery optics with a He–Ne beam. Commercial quadrant detectors are available that can be used to determine beam alignment. These devices measure the intensity of the laser beam relative to the center of the detector. The spatial arrays described in clause 6.4 can also be used for beam delivery alignment. In automated systems, the delivery mirrors can be servo-mechanically repositioned to center the beam based on the signal received from the spatial array. 14.4.3 Nozzle Alignment. Alignment of the laser beam through an assist gas nozzle is critical, particularly in cutting operations. Poor alignment will adversely affect kerf width, dross, and process speed, as well as damage beam delivery components. Some of the methods described above for beam delivery alignment can be used for nozzle alignment. Several other methods exist for determining nozzle alignment. Adhesive tape, such as masking, cellophane or Kapton® can be placed over the nozzle tip and exposed to a short, low power burst of laser energy. The resultant hole should be centered in the tape’s impression of the nozzle orifice edge. Photosensitive paper can be placed below the nozzle to determine beam alignment. A beam that is properly aligned will present a well-defined, circular image. Energy reflected by the beam impinging on the nozzle will result in a scattered radiation pattern on the photosensitive paper. Proper alignment can also be determined by piercing a hole in thin (0.8 mm thick) steel while observing the material to see the direction that metal is ejected. The lens or nozzle is then adjusted to make the ejected metal form a uniform star burst around the nozzle. This will occur when the beam and nozzle are concentric to within 0.5 mm, which is the order of accuracy needed. A coating of oil on the surface of the workpiece and impingement of a short burst of laser power on the oil can also be used to determine nozzle alignment. Proper alignment is indicated by the spot centered in the oil spray. 14.4.4 Vision System Alignment. Many laser systems, in particular Nd:YAG systems, have some type of vision system to assist with positioning parts for processing. This vision capability is extremely important in aligning weld joints with respect to the focused beam. The most common system employs a video camera with an electronically generated cross-hair superimposed on the video screen. The video camera can be either off-axis (viewing from the side of the laser beam material interaction point) or on-axis (viewing directly through the final focusing lens or though a mirror placed in front of the final focusing lens). Another type of viewing system is direct visual viewing though the final focusing lens, or a mirror placed in front of the lens, with a binocular or monocular lens. This system also employs a cross-hair to assist in alignment. Regardless of the system used, it is necessary to verify proper alignment of the components. For ease of processing, the vision system should be adjusted so its focus position, and the focus position of the laser beam being used for the materials processing application, is the same. It is also necessary to align the cross-hair with the beam interaction point. This can be accomplished by creating a weld spot or drilled hole, on a fixed piece of material at the proper focus position, and aligning the cross-hair with this spot. 14.4.5 Fiber Optic Alignment. The critical part of fiber optic alignment is getting the beam from the laser into the fiber. The beam should be focused to about 80% of the fiber diameter and be coaxial with the fiber to minimize exit-side divergence. The input optics are generally attached directly to the laser, so the alignment is quite stable. Once the beam is coupled to the fiber, the fiber and output housing may be moved freely without affecting alignment.

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15. Personnel Training Personnel operating laser systems must be comfortable and confident with the control of the machine. They must be aware of potential hazards and be able to perform day to day preventive maintenance to ensure the machine performs satisfactorily. Areas in which personnel may require training include the following: (1) Safety (2) Laser system operation (3) Laser system maintenance (4) CNC programming (5) Metallurgy (6) Inspection 15.1 Safety. At a minimum, the operator must have a general understanding of laser safety. Hazards associated with laser processing systems include the following: (1) General safety (2) Electrical shock (3) Laser safety (4) Welding and cutting safety (5) Fumes and gases (6) Compressed gases Personnel performing maintenance procedures on the laser power supply or resonator must be familiar with general safety and electrical shock hazards. Personnel performing maintenance on the laser beam delivery and operating the laser to perform material processing must be familiar with general safety, direct and reflected laser beam, visible radiation, and fumes and gas hazards. For further information on safety considerations, refer to Clause 4, Safety Considerations. 15.2 Laser System Operation. Laser and laser system manufacturers will provide detailed operation and maintenance information for their products. Operation and maintenance training should be included in the system purchase contract. The operator must be proficient with startup and shutdown procedures for the particular laser. Required procedures vary significantly depending on the type of laser and particular installation. The owner’s manual should be referred to or the manufacturer consulted for details. The operator must be familiar with both manual and automatic operation of the laser. Manual operation includes “beam on/beam off” and “single shot” for power calibration, mode analysis, and other diagnostic evaluation procedures. Automatic operation includes setting power levels, pulsing parameters, and power cycles. The operator must be able to focus the laser beam on the workpiece with accuracy and repeatability. The operator must be able to assess the quality of the results achieved and to adjust parameters as required to achieve desired results. 15.3 Laser System Maintenance. Daily maintenance of the laser processing system must be performed by the operator (or other maintenance personnel as appropriate) to ensure optimum performance. CO2 lasers require the correct laser gas purity, mixture ratio, and operating pressure. The owner’s manual should be referred to for specifications on the particular laser. Nd:YAG lasers require maintenance and replacement of the excitation flashlamps as well as the correct cleaning and alignment of the laser rod. The correct operating temperature of both CO2 and Nd:YAG lasers must be maintained to achieve the designed power output and mode. The owner’s manual should be referred to or the manufacturer consulted with for details. Beam delivery components must be cleaned and aligned properly to maintain optimum performance of a laser processing system. The operator must be able to remove, clean, and reinstall optics to their original condition. Internal optics (rear mirror and output coupler) must be clean and properly aligned to provide the best possible beam mode. External optics must be clean and properly aligned to deliver the beam in the best possible mode condition to the precise point needed. Extreme care must be taken to not damage optical components when cleaning. For detailed recommended procedures on cleaning and aligning optics, refer to the optics manufacturer’s recommendations and Clause 14, Equipment Maintenance.

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15.4 Other Training. Additional areas of training that need to be considered are listed in the following sections. 15.4.1 CNC and CIM Programming. Positioning system control includes both manual (jogging and positioning motion for set-up) and automatic (CNC) operation. These controls are dependent on the type and complexity of the positioning system. There are many different types of positioning systems ranging from manufacturing line automation to point-to-point contouring. The operator must understand how to safely control the particular system. This includes the use of hard-wired overrides and emergency stops. Training in CNC programming is recommended for all personnel involved with laser materials processing. A machine operator should have, as a minimum, the ability to make minor program changes to compensate for changes in the process. Advances in technology have led to an increase in the use of computer-aided design (CAD) and computer-integrated manufacturing (CIM) for rapid manufacture of contour cut parts. Using the proper computer software, CIM allows the generation of CNC part programs directly from CAD designs. Personnel involved in the manufacture of contour cut parts should evaluate the benefits from this technology and obtain the necessary training. 15.4.2 Metallurgy. It is recommended that personnel involved in designing laser-welded components, and those responsible for the development of welding procedures, have a basic understanding of the metallurgy of laser welding. Without this understanding, often unweldable alloys are specified for laser weldments and incorrect parameters are chosen to weld parts. 15.4.3 Inspection. Operators of laser systems should be capable of some level of inspection of the parts they process. The ability to perform the required dimensional inspection of laser-cut parts is necessary, especially when statistical process control is used for the process. Operators of laser welding systems should understand, and be able to perform, the visual inspection required for a welded assembly. The operator can play a key role in maintaining product quality by stopping the process before a significant number of discrepant parts are welded. 15.5 Sources of Training. The best source of laser personnel training is the manufacturer (final system integrator or the laser manufacturer). In addition, training is available through formal education programs (colleges and vocational schools), consultants, job shops, and seminars such as those sponsored by University extension programs.

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Annex A (Informative) Informative References This annex is not part of AWS C7.2M:2010, Recommended Practices for Laser Beam Welding, Cutting and Allied Processes, but is included for informational purposes only. The following codes, standards, specifications, books, and reports have either been cited in these recommendations or contain information that is useful. These references are included as a source of helpful information.

ANSI B46.1–2002, Surface Texture, New York: American Society of Mechanical Engineers (ASME). Arata, Y., 1986, Plasma, Electron, and Laser Beam Technology, Materials Park, Ohio: ASM International. AWS A1.1:2001, Metric Practice Guide for the Welding Industry, Miami: American Welding Society. AWS B1.10:1999, Guide for the Nondestructive Inspection of Welds, Miami: American Welding Society. C7.4/C7.4M:2008, Process Specification and Operator Qualification for Laser Beam Welding, Miami: American Welding Society. Charschan, S. S., ed., 1972, Lasers in Industry, Orlando: Laser Institute of America. Code of Federal Regulations, Title 29-Labor, Part 1910, “Occupational Safety and Health Standards,” 2005, Washington D.C.:U.S. Department of Labor. Dawes, C., 1992, Laser Welding, New York: McGraw Hill. Duley, W. W., 1976, CO2 Lasers: Effects and Applications, New York: Academic Press. Effects of Welding on Health, Vol. 11, 2004, Miami: American Welding Society. Fumes and Gases in the Welding Environment, 1979, Miami: American Welding Society. Gregson, V.G., 1983. Laser Heat Treating, Laser Materials Processing, ed. Bass, M., 201–234, New York: North-Holland Publishing Company. ISO 13919–1, Welding – Electron and laser-beam welded joints – Guidance on quality levels for imperfections – Part 1: Steel, 1, ch. de la Voie-Creuse, Case postale 56 CH-1211 Geneva 20, Switzerland: International Organization for Standardization (ISO). ISO 15609–4, Specification and qualification of welding procedures for metallic materials – Welding procedure specification – Part 4: Laser beam welding, 1, ch. de la Voie-Creuse, Case postale 56 CH-1211 Geneva 20, Switzerland: International Organization for Standardization (ISO). ISO 15616–1, Acceptance tests for CO2-laser beam machines for high quality welding and cutting — Part 1: General principles, acceptance conditions, 1, ch. de la Voie-Creuse, Case postale 56 CH-1211 Geneva 20, Switzerland: International Organization for Standardization (ISO). ISO 15616–2, Acceptance tests for CO2-laser beam machines for high quality welding and cutting — Part 2: Measurement of static and dynamic accuracy, 1, ch. de la Voie-Creuse, Case postale 56 CH-1211 Geneva 20, Switzerland: International Organization for Standardization (ISO). ISO 22826, Destructive tests on welds in metallic materials — Hardness testing of narrow joints welded by laser and electron beam (Vickers and Knoop hardness tests), 1, ch. de la Voie-Creuse, Case postale 56 CH-1211 Geneva 20, Switzerland: International Organization for Standardization (ISO). ISO 22827–1, Acceptance tests for Nd:YAG laser beam welding machines – Machines with optical fibre delivery – Part 1: Laser assembly, 1, ch. de la Voie-Creuse, Case postale 56 CH-1211 Geneva 20, Switzerland: International Organization for Standardization (ISO). Laser Safety Training Manual, 6th Edition, Cincinnati: Rockwell Associates, Inc.

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Migliore, L., 1996, Heat Treatment, Laser Materials Processing, ed. Migliore, L., 209–238, New York: Marcel Dekker Publishing. National Fire Protection Association (NFPA), 2002, Electrical Standard Industrial Machinery, NFPA 79, Quincy, MA: National Fire Protection Association. National Fire Protection Association (NFPA), 2003, Standard for Fire Prevention During Welding, Cutting, and Other Hot Work, NFPA 51B, Quincy, MA: National Fire Protection Association. National Fire Protection Association (NFPA), 2005, National Electric Code, NFPA 70, Quincy, MA: National Fire Protection Association. Nonhof, C. J., 1988, Material Processing with Nd Lasers, Ayr, Scotland: Electrochemical Publications Limited. Powell, J., Egstrom, H, and C. Magnusson, 1994. Laser Surface Treatment, Mechanisms and Techniques, The Fabricator, May 1994: 80–83 and June 1994: 24–27. Ready, J. F., 1978, Industrial Applications of Lasers, New York: Academic Press. Sandven, O., 1991, Laser Surface Transformation Hardening, Volume 4 of ASM Handbook, 286–296, Materials Park, Ohio: ASM International. Seaman, F.D., 1986. Laser Heat Treating, The Industrial Laser Annual Handbook, eds. Belforte, D., and Levitt, M., Tulsa: PennWell Publishing Company. Siegman, A. E., 1986, Lasers, Mill Valley, California: University Science Books. Sliney, D.H. and M.L. Wolbarsht, 1980, Safety with Lasers and Other Optical Sources, New York: Plenum Publishing.

Additional standards for lasers, laser equipment, laser optics, and laser processing are published by the International Organization for Standardization, 1, ch. de la Voie-Creuse, Case postale 56 CH-1211 Geneva 20, Switzerland.

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Annex B (Informative) Guidelines for the Preparation of Technical Inquiries This annex is not part of AWS C7.2M:2010, Recommended Practices for Laser Beam Welding, Cutting, and Allied Processes, but is included for informational purposes only.

B1. Introduction The American Welding Society (AWS) Board of Directors has adopted a policy whereby all official interpretations of AWS standards are handled in a formal manner. Under this policy, all interpretations are made by the committee that is responsible for the standard. Official communication concerning an interpretation is directed through the AWS staff member who works with that committee. The policy requires that all requests for an interpretation be submitted in writing. Such requests will be handled as expeditiously as possible, but due to the complexity of the work and the procedures that must be followed, some interpretations may require considerable time.

B2. Procedure All inquiries shall be directed to: Managing Director Technical Services Division American Welding Society 550 N.W. LeJeune Road Miami, FL 33126 All inquiries shall contain the name, address, and affiliation of the inquirer, and they shall provide enough information for the committee to understand the point of concern in the inquiry. When the point is not clearly defined, the inquiry will be returned for clarification. For efficient handling, all inquiries should be typewritten and in the format specified below. B2.1 Scope. Each inquiry shall address one single provision of the standard unless the point of the inquiry involves two or more interrelated provisions. The provision(s) shall be identified in the scope of the inquiry along with the edition of the standard that contains the provision(s) the inquirer is addressing. B2.2 Purpose of the Inquiry. The purpose of the inquiry shall be stated in this portion of the inquiry. The purpose can be to obtain an interpretation of a standard’s requirement or to request the revision of a particular provision in the standard. B2.3 Content of the Inquiry. The inquiry should be concise, yet complete, to enable the committee to understand the point of the inquiry. Sketches should be used whenever appropriate, and all paragraphs, figures, and tables (or annexes) that bear on the inquiry shall be cited. If the point of the inquiry is to obtain a revision of the standard, the inquiry shall provide technical justification for that revision. B2.4 Proposed Reply. The inquirer should, as a proposed reply, state an interpretation of the provision that is the point of the inquiry or provide the wording for a proposed revision, if this is what the inquirer seeks.

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B3. Interpretation of Provisions of the Standard Interpretations of provisions of the standard are made by the relevant AWS technical committee. The secretary of the committee refers all inquiries to the chair of the particular subcommittee that has jurisdiction over the portion of the standard addressed by the inquiry. The subcommittee reviews the inquiry and the proposed reply to determine what the response to the inquiry should be. Following the subcommittee’s development of the response, the inquiry and the response are presented to the entire committee for review and approval. Upon approval by the committee, the interpretation is an official interpretation of the Society, and the secretary transmits the response to the inquirer and to the Welding Journal for publication.

B4. Publication of Interpretations All official interpretations will appear in the Welding Journal and will be posted on the AWS web site.

B5. Telephone Inquiries Telephone inquiries to AWS Headquarters concerning AWS standards should be limited to questions of a general nature or to matters directly related to the use of the standard. The AWS Board Policy Manual requires that all AWS staff members respond to a telephone request for an official interpretation of any AWS standard with the information that such an interpretation can be obtained only through a written request. Headquarters staff cannot provide consulting services. However, the staff can refer a caller to any of those consultants whose names are on file at AWS Headquarters.

B6. AWS Technical Committees The activities of AWS technical committees regarding interpretations are limited strictly to the interpretation of provisions of standards prepared by the committees or to consideration of revisions to existing provisions on the basis of new data or technology. Neither AWS staff nor the committees are in a position to offer interpretive or consulting services on (1) specific engineering problems, (2) requirements of standards applied to fabrications outside the scope of the document, or (3) points not specifically covered by the standard. In such cases, the inquirer should seek assistance from a competent engineer experienced in the particular field of interest.

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