2008:08 DOCTORAL T H E SIS The Absorption of Laser Light by Rough Metal Surfaces David Bergström Luleå University of Technology Department of Applied Physics and Mechanical Engineering Division of Manufacturing Systems Engineering 2008:08|: 02-544|: - -- 08⁄08 -- Doctoral Thesis The Absorption of Laser Light by Rough Metal Surfaces David Bergström Division of Manufacturing Systems Engineering Department of Applied Physics and Mechanical Engineering Luleå University of Technology Luleå, Sweden in cooperation with Department of Engineering, Physics and Mathematics Mid Sweden University Östersund, Sweden Luleå, February 2008 Dedicated to the Curious Mind. Cover figure: A laser profilometer image of the topography of a hot rolled stainless steel surface (middle), with a photograph of an experimental setup for absorptance measurements using an integrating sphere (left) and a computer-generated Gaussian random rough surface illustrating the principles of ray-tracing (right). Acknowledgements The work presented in this Doctoral thesis is devoted to the theoretical and experimental study of the light absorption mechanisms in laser processing of rough metals and metallic alloys. The Doctoral thesis consists of a short introduction and a series of six papers, all dealing with different aspects of this topic. This work has been carried out at the Division of Manufacturing Systems Engineering at Luleå University of Technology (LTU) in Luleå, Sweden, and at the Department of Engineering, Physics and Mathematics at Mid Sweden University (MIUN) in Östersund, Sweden, from 2003 to 2008. I would like to express my gratitude to my supervisors Prof. Alexander Kaplan (LTU) and Prof. Torbjörn Carlberg (MIUN) for their advice and support during these past years. My deepest thanks go to Prof. John Powell (Laser Expertise Ltd., Nottingham, UK) for always keeping me focused on the essentials. I’d like to thank him for all the time he has devoted in the discussions of different thoughts and ideas and for proof-reading all my written work. It’s been a pleasure working with you and Alexander and I wish and hope for further collaboration in the future. Thanks are also due to M.Sc. Rickard Olsson and his colleagues at Lasernova AB in Östersund, for once upon a time introducing me to this exciting subject and for their support and friendship. I’m also indebted to Patrik Frihlén of Azpect Photonics AB and M.Sc. Ingemar Eriksson for helping me setting up the equipment for the laser lab in Östersund. Further thanks go to Doc. Mats Tinnsten and Dr. Leon Dahlén for providing the financial circumstances in making this work possible. I would also like to thank my colleagues at my university department in Östersund, especially my fellow PhD student Marianne Olsson for sharing countless cups of coffee over conversations about the trials and tribulations of postgraduate studies as well as of topics beyond. Finally I would like to thank my family for their support and encouragement and my girlfriend Susanne for her love, patience and understanding. Östersund, February 2008 David Bergström i Abstract In Laser Material Processing of metals, an understanding of the fundamental absorption mechanisms plays a vital role in determining the optimum processing parameters and conditions. The absorptance, which is the fraction of the incident laser light which is absorbed, depends on a number of different parameters. These include laser parameters such as intensity, wavelength, polarisation and angle of incidence and material properties such as composition, temperature, surface roughness, oxide layers and contamination. The vast theoretical and experimental knowledge of the absorptance of pure elements with smooth, contamination-free surfaces contrasts with the relatively sparse information on the engineering materials found in real processing applications. In this thesis a thorough investigation of the absorption mechanisms in engineering grade materials has been conducted, both experimentally and theoretically. Integrating sphere reflectometry has been employed to study the impact of surface conditions on Nd:YAG and Nd:YLF laser absorptance of some of the most common ferrous and non-ferrous metallic alloys found in Laser Material Processing. Mathematical modelling and simulations using ray-tracing methods from scattering theory have been used to analyze the influence of surface topography on light absorption. The Doctoral thesis consists of six papers: Paper 1 is a short review of some of the most important mathematical models used in describing the interaction between laser light and a metal surface. Paper 2 is a review of experimental methods available for measuring the absorptance of an opaque solid such as a metal. Papers 3 and 4 are experimental investigations of the absorptance of some of the most frequently found metallic alloys used in Laser Material Processing today. Paper 5 presents results from 2D ray-tracing simulations of random rough metal surfaces in an attempt to investigate the influence of surface roughness on laser scattering and absorption. Paper 6 is a full 3D ray-tracing investigation of the interaction of laser light with a rough metallic surface, where some comparisons also are made to the previous 2D model. ii List of Papers Paper 1 Bergström, D.; Kaplan, A.; Powell, J.: ”Mathematical modelling of laser absorption mechanisms in metals: A review”. Presented at the M4PL16 workshop, Igls, Austria, January 20-24, 2003, Ed: A. Kaplan, published by Luleå TU, Sweden. Paper 2 Bergström, D.; Kaplan, A.; Powell, J.: ”Laser absorption measurements in opaque solids”. Presented at the 10th Nordic Laser Materials Processing (NOLAMP) Conference, Piteå, Sweden, August 17-19, 2005, Ed: A. Kaplan, published by Luleå TU, Sweden. Paper 3 Bergström, D.; Powell, J.; Kaplan, A.: ”The absorptance of steels to Nd:YAG and Nd:YLF laser light at room temperature”. Paper published in Applied Surface Science, Volume 253, Issue 11, pp. 5017-5028, 2007. Paper 4 Bergström, D.; Powell, J.; Kaplan, A.: ”The absorptance of non-ferrous alloys to Nd:YAG and Nd:YLF laser light at room temperature”. Paper published in Applied Optics, Volume 48, Issue 8, pp. 1290-1301, 2007. Paper 5 Bergström, D.; Powell, J.; Kaplan, A.: ”A ray-tracing analysis of the absorption of light by smooth and rough metal surfaces”. Paper published in Journal of Applied Physics, Volume 101, pp. 113504/1-11, 2007. Paper 6 Bergström, D.; Powell, J.; Kaplan, A.: ”The absorption of light by rough metal surfaces - A three-dimensional ray-tracing analysis”. Submitted to Journal of Applied Physics. Shorted version presented at the 26th International Congress on Applications of Lasers and Electro-optics (ICALEO), Orlando, Florida, USA, October 28-November 1, 2007. iii Table of Contents Acknowledgements i Abstract ii List of Papers iii Table of Contents iv Introduction 1 1. Motivation of the thesis 1 2. Methodological approach 1 3. Laser absorption in metals – a short introduction 3 4. Conclusions of the thesis 6 5. Future outlook 7 6. Appendix 9 6.1 Further comparison between 2D and 3D ray-tracing models 9 6.2 AFM characterization and ray-tracing of real engineering grade 13 metal surfaces References 17 Paper 1: Mathematical Modelling of Laser Absorption Mechanisms in 19 Metals: A Review Paper 2: Laser Absorption Measurements in Opaque Solids 49 Paper 3: The Absorptance of Steels to Nd:YLF and Nd:YAG Laser Light 89 Paper 4: The Absorptance of Non-Ferrous Alloys to Nd:YLF and Nd:YAG 117 Laser Light Paper 5: A Ray-Tracing Analysis of the Absorption of Light by Smooth and 143 Rough Metal Surfaces Paper 6: The Absorption of Light by Rough Metal Surfaces - A Three- 179 Dimensional Ray-Tracing Analysis iv Introduction Introduction 1. Motivation of the thesis Metal processing with lasers has reached a high level of maturity and acceptance in industry. It is used for cutting, drilling, welding, forming, engraving, marking, hardening and various forms of surface treatment of metals in a broad spectrum of modern industries, including the automotive and aerospace industries, the shipbuilding industry, the microelectronics industry and the medical instrument industry to name a few. Common to all these applications, is that the laser light is transformed into heat. This is the fundamental process of light absorption and can often be a determining factor in whether a process will be successful for a particular material under certain circumstances or not. In the field of material optics the studies of light interaction with metals have usually been concentrated on pure metallic elements with surfaces as smooth and clean as possible. This is understandable from the point of view of finding correlation with existing theories from solid state and condensed matter physics. If any handbook of chemistry or physics is picked up from a library, this is usually the experimental data that will be found. However, the engineering grade metals and metallic alloys found in real life processing with lasers have various forms of texture and roughness on their surfaces and also have layers of oxides on top of them. The sparse amount of experimental data and mathematical models for these kinds of non-perfect but more realistic surfaces is a limitation to the modellers of laser metal processing who need accurate heat input data for their heat conduction problems. The main motivation for this research was to provide more realistic information about the absorption of laser light by engineering metal surfaces. 2. Methodological approach The methodological approach of this Doctoral thesis can be understood by reference to the structure and order of the papers included. The work began by making an overview of the existing mathematical models of laser absorption in 1 Introduction metals (see Paper 1 for details) and this lead to a realisation of the lack of theory and experimental data for the engineering grade metals and metallic alloys found in real life processing applications. A study and review of the available experimental methods for measuring laser absorption in metals was then initiated and a potential candidate was found in the method of measuring reflectance using an integrating sphere.