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Selective Laser Melting of Nickel Superalloys: Solidification, Microstructure and Material Response

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Selective Laser Melting of Nickel Superalloys: Solidification, Microstructure and Material Response Selective Laser Melting of Nickel Superalloys: solidification, microstructure and material response By: Neil J Harrison A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy The University of Sheffield Faculty of Engineering School (or Department) of Mechanical Engineering Submission Date: 28th of October 2016 Abstract The Selective Laser Melting (SLM) process generates large thermal gradients during rapid melting, and during solidification certain nickel superalloys suffer from thermally induced micro-cracking which cannot be eliminated by process optimisation. This investigation sought to investigate and understand the root cause of micro-cracking in nickel superalloys when processed by SLM, with the aim of ultimately being able to predict the crack susceptibility of an alloy from composition alone. Microstructural analysis as well as implementation of Rapid Solidification Processing (RSP) theory and solute redistribution theory was used to establish SLM as a rapid solidification process. As a consequence, secondary dendrite arm formation and solute redistribution is largely inhibited, resulting in a bulk material which is near to a super saturated solid solution. The establishment of SLM as an RSP along with morphological and chemical analysis of micro-cracks support Elevated Temperature Solid State (ETSS) cracking as the primary cracking mechanism in SLM processed nickel superalloys. The crack susceptibility of a nickel superalloy, χ, was defined as the ratio between the solid solution strengthening contribution from alloying elements and apparent thermal stress generated by the process. Minor increases in the wt% of solid solution strengthening elements in Hastelloy X, a high crack susceptibility alloy, resulted in average reductions of crack density of 65%. Thereby supporting solid solution strength as a key factor in the crack susceptibility of a nickel superalloy. The addition of the apparent thermal stress component, further supported 1 the crack susceptibility model, with the modified Hastelloy X being predicted to have a lower crack susceptibility. Additional validation of the crack susceptibility predictor was determined by taking compositions and material properties from published SLM investigations and calculating the crack susceptibility of the respective alloy. The results were found to be in good agreement with the reported observations. Acknowledgments I would like to thank everyone who has made success in this project possible. As with anything of this scale, there are countless people involved at various points, all of whom have had an effect on the process and outcome of the work. However, I feel it necessary to thank the following people specifically for their significant contributions and support: I would like to thank my primary academic supervisor, Dr Kamran Mumtaz, for his support and guidance throughout this project. His encouragement and knowledge of this field ensured a successful conclusion to this project, along with publication of part of this work in a high ranking journal. I would also like to thank my secondary supervisor, Professor Iain Todd, whose extensive knowledge of metallurgy and enthusiasm for the project was invaluable. I can attribute much of my learning regarding the fundamental aspects of this work to his guidance. I thank LPW Technology Ltd and EPSRC for their financial support in this project. I also wish to thank my industrial supervisor, Dr Robert Deffley, whose work provided the platform for this project and whose knowledge of the process and nickel alloys 2 specifically was also invaluable. In addition, I personally thank Dr Phil Carroll (CEO of LPW Technology) for his technical support and helping me integrate with the company. I would like to thank my colleague Omar Lopez, whose temperature field model featured in this work, in addition to his general support and imparting of knowledge in this area. Other colleagues without whose assistance I would not have be able to complete this work in its entirety are Dr James Hunt, Dr Fatos Derguti, Ian Woods, Kyle Arnold, Pratik Vora and Dr Cheryl Shaw. I would also like to thank the Departments of Mechanical Engineering and Materials Science and Engineering at the University of Sheffield for administrative support, use of equipment, and provision of courses and literature for personal development. The guys in D14 Garden Street also deserve significant praise for breaking up the long days with occasional games of office golf and various internet base games, but punctuated with actual work. Without them, I doubt I would have had the drive required to put in a full day every day. So thank you Tom, Ryan, Ste and Luke. Perhaps the greatest thanks goes to my family, without the support, encouragement and love from them, I would have struggled to get through the difficult periods of the last 4 years. Mum, Dad, Mark, Sue, Alastair, Grandma, I love you and thank you. Finally my beautiful wife, Catherine, thank yous will never be enough to balance the love, support and encouragement you have given me. Without you I would never have considered this avenue in my career, and therefore I would have never have found myself in the position that I am in now. From long before this project began, throughout its duration and every day after, you have been my constant, my rock, and my best friend. I could not have done this without you. I love you more than anything. 3 Table of Contents Abstract ................................................................................................................................................................................ 1 Acknowledgments ............................................................................................................................................................ 2 Table of Contents .............................................................................................................................................................. 4 Publications ........................................................................................................................................................................ 8 List of Tables ....................................................................................................................................................................... 8 List of Figures ..................................................................................................................................................................... 9 List of Abbreviations .................................................................................................................................................... 15 Nomenclature .................................................................................................................................................................. 17 1 Introduction ........................................................................................................................................................... 21 1.1 Problem statement .................................................................................................................................... 21 1.2 Aims and Objectives of Research ......................................................................................................... 22 1.3 Thesis structure .......................................................................................................................................... 22 2 Background and Literature review .............................................................................................................. 25 2.1 Selective Laser Melting: An Additive Manufacturing technique ............................................. 25 2.1.1 The process ......................................................................................................................................... 25 2.1.2 Applications ........................................................................................................................................ 27 2.2 Problem characteristics in SLM ............................................................................................................ 29 2.2.1 High thermal and residual stresses ........................................................................................... 29 2.2.2 Porosity ................................................................................................................................................. 37 2.2.3 Surface roughness ............................................................................................................................ 39 2.2.4 Process induced cracking .............................................................................................................. 42 2.3 Failure by fracture and critical crack length ................................................................................... 44 2.4 Solidification and microstructure theory ......................................................................................... 47 2.4.1 Solidification fundamentals.......................................................................................................... 47 2.4.2 Rapid solidification .......................................................................................................................... 55 2.4.3 In SLM ...................................................................................................................................................
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