DOCTORAL T H E SIS Department of Engineering Sciences and Mathematics Sundqvist Jesper Division of Product and Production Development Aspects of Heat Flow in ISSN 1402-1544 Laser Processing of Metals ISBN 978-91-7790-051-1 (print) ISBN 978-91-7790-052-8 (pdf) Aspects of Heat Flow in Laser Processing of Metals in Laser Processing Aspects of Heat Flow Luleå University of Technology 2018 Jesper Sundqvist Manufacturing Systems Engineering Doctoral Thesis Aspects of heat flow in laser materials processing Jesper Sundqvist Luleå University of Technology Department of Engineering Sciences and Mathematics Division of Product and Production Development 971 87 Luleå Sweden Luleå, March 2018 Printed by Luleå University of Technology, Graphic Production 2018 ISSN 1402-1544 ISBN 978-91-7790-051-1 (print) ISBN 978-91-7790-052-8 (pdf) Luleå 2018 www.ltu.se I. Preface The writing of this Ph. D thesis and the research behind it has been conducted at the research subject ('chair') of Manufacturing Systems Engineering at Luleå University of Technology. I want to express my gratitude to my supervisors Prof. Dr. Alexander Kaplan and Adj. Prof. Dr. John Powell for their support and guidance. Without them this work would not have been possible. I would like to thank all the colleagues at the research team for help given during my research and also all colleagues at the division for all the interesting conversations during coffee breaks. Particular thanks to Stephanie Robertson for proof reading of parts of the thesis. I also want to thank Christina Lundebring and Marie-Louise Palmblad for all the help with administrative tasks. I am very grateful to Prof. Bang Hee-Seon and Prof. Bang Han-Sur for giving me the opportunity to do research during four months at Chosun University, Gwangju, South Korea. It was a great experience which I will never forget. I met many new friends there. During the time leading up to this thesis I have worked mainly in the projects PROLAS, TailorWeld, NorFaSt-HT and SPAcEMAN. I am grateful for the corresponding funding from EU-ERDF-Interreg IVA Nord (project no. 304-58- 11), EC-FP7-SME-2013 (no. 606046), EU-ERDF-Interreg Nord 2014-2020 (project no. 304-15588-2015) and EC EIT-KIC Raw Materials (project, no. 17070) respectively, as well as from the STINT Korea-Sweden Research Cooperation Programme (no. KO2012-5164). I want to thank all project partners and co-authors as well. Last but not least, I want to thank my family and friends for all the support during this time as a PhD student. Jesper Sundqvist Luleå, March 2018 II. Abstract Since the laser was invented in 1960, its use in manufacturing industry has been growing rapidly. Laser processing of metals is based on the flow of heat that is generated by the absorbed laser beam. One outstanding aspect of laser beams is high precision along with high controllability of energy transfer, which includes creative techniques of shaping the beam and in turn the process. This thesis presents six Papers A-F on different metal processing techniques, namely welding, hardening and cutting, the latter combined with additive manufacturing. For each respective technique it was studied how desired properties can be optimized by controlled use of the laser beam and in turn of the temperature field. Addressing their different complexity of the heat transfer, various theoretical and experimental analysis methods were applied. Laser beam welding is usually conducted with standard beam shapes, i.e. Gaussian or top-hat like, which is not always optimal for the process. Identification of an optimised weld pool shape or temperature cycle through beam shape or beam path tailoring could increase the quality of welded products or even enable new applications. Papers A and B aim to increase the knowledge for non-standard beam shapes, particularly for single-pulse conduction mode welding. Paper A presents an investigation on an industrial application where a C-shaped weld joint is desired. The sensitivity to and optimization of different C-shaped beam irradiation profiles is discussed. The analysis is mainly carried out by applying Finite Element Analysis, FEA, to calculate the heat conduction contributions, showing unexpected sensitivity in certain regimes. Paper B presents a semi-analytical model for fast calculation of the temperature field from different beam profiles. Examples include multi-spots or the misalignment sensitivity of Diffractive Optical Elements. In Paper C, for laser hardening of 11% Cr ferritic stainless steel the temperature field was studied to enable hardening. It was shown that single-track hardening without sensitisation could be achieved but overlapping tracks had a continuous network of ditched grain boundaries and is thereby at risk for sensitisation; the sensitised area is caused by a reheating cycle. The same mechanism for the same material was studied in Paper D when applying a recently developed drop-deposition technique, where additive manufacturing is fed by laser cutting. The same reheating isotherm could be critical, but here sensitisation tests show a discontinuous network of ditched grain boundaries in the added material. The never melted heat-affected zone on the other hand has a continuous network of ditched grain boundaries, which implies a sensitisation risk. The continuous network is however not in contact with the surface. The tested parameters is thus not at risk for intergranular corrosion through sensitisation. For friction stir welding of dissimilar metals, Ti-6Al-4V with AISI 304L stainless steel, Paper E, the influence of a laser-induced preheating temperature field on the tool forces was investigated through numerical simulation. By suitable application of laser preheating, the forces acting on the tool can be substantially lowered, in a robust manner. The temperature field from seam welding induces a residual stress field. In Paper F, for continuous wave laser keyhole welding of high strength steel butt joints, a method is presented to identify the residual stress behaviour of laser welded sheets by measurement of the fatigue crack growth rate during testing, by deriving the crack acceleration. The analysis was confirmed by hole drilling tests and by FEA. The knowledge and methods of the above different experimental and theoretical studies complement each other. They contribute to further optimize certain aspects through laser-induced temperature fields, for different manufacturing techniques. III. List of publications and author contribution The core of the thesis is composed of the following appended publications: Paper A: Numerical optimization approaches of single-pulse conduction laser welding by beam shape tailoring J. Sundqvist, A. F. H. Kaplan, L. Shachaf, A. Brodsky, C. Kong, J. Blackburn, E. Assuncao, L. Quintino Optics and Lasers in Engineering, 79 (2016) 48-54. Paper B: Analytical heat conduction modelling for shaped laser beams J. Sundqvist, A. F. H. Kaplan, L. Shachaf Journal of Materials Processing Technology, 247 (2017) 48-54. Paper C: Laser surface hardening of 11% Cr ferritic stainless steel and its sensitisation behaviour J. Sundqvist, T. Manninen, H.-P. Heikkinen, S. Anttila, A. F. H. Kaplan Surface & Coatings Technology, editing of minor revisions from review Paper D: Sensitisation behaviour of drop-deposited 11% Cr ferritic stainless steel J. Sundqvist, A.F.H. Kaplan Submitted for publication Paper E: Numerical simulation of laser preheating of friction stir welding of dissimilar metals J. Sundqvist, K. H. Kim, H. S. Bang, H. S. Bang, A. F. H. Kaplan Science and Technology of Welding and Joining, published online, no volume allocated yet Paper F: Identifying residual stresses in laser welds by fatigue crack growth acceleration measurement J. Sundqvist, A. F. H. Kaplan, J. Granström, K. G. Sundin, M. Keskitalo, K. Mäntyjärvi, X. B. Ren Journal of Laser Applications, 27(4) (2015) 042002 (8 p). Author contribution Paper A: The author carried out methodological planning, heat flow modelling, analysis and editing, with input from the other authors. Experiments were carried out by other co-authors, with the main author present. Calculations of the optics were conducted by co-authors. Paper B: The author carried out methodological planning, heat flow modelling, analysis and editing, with input from the other authors. Experiments were carried out by co-authors, with the main author present. Optical calculations were carried out by co-authors. Paper C: The author carried out methodological planning, experiments, analysis, modelling of temperature and editing, with input from the other authors. Equilibrium diagram, material knowledge and material composition was provided by co-authors. Paper D: The author carried out methodological planning, experiments, analysis and editing, with input from the co-author. Paper E: The author carried out methodological planning, modelling, analysis and editing, with input from the other authors. The co-authors performed the experiments. Paper F: The author carried out methodological planning, laser experiments, hole drilling, analysis and editing with input from the other authors. The co-authors performed the fatigue testing and numerical modelling. Abbreviations AHSS – Advanced High Strength Steel AM – Additive Manufacturing BEM – Boundary Element Method BPP – Beam Parameter Product CAD – Computer Aided Design CFD – Computational Fluid Dynamics CVM – Control Volume Method CYCLAM – Recycling by Laser to feed Additive Manufacturing DED - Direct Energy Deposition DMD – Direct Metal Deposition DOD –