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Diffusion Bonding Aluminium Alloys and Composites: New Approaches and Modelling

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Diffusion Bonding Aluminium Alloys and Composites: New Approaches and Modelling Diffusion Bonding Aluminium Alloys and Composites: New Approaches and Modelling Amir A. Shirzadi King’s College Cambridge A thesis submitted for the degree of Doctor of Philosophy at the University of Cambridge December 1997 Declaration This thesis describes work undertaken in the Department of Materials Science and Metallurgy at the University of Cambridge between April 1994 and December 1997. The work is entirely my own and includes nothing which is the outcome of work done in collaboration, except where reference is explicitly made to the work of others. The work has not been submitted, either in whole or in part, for any degree or qualification at any other university and does not exceed 60,000 words. This work was financially supported by the Ministry of Higher Education of Iran, the Committee of Vice-Chancellors and Principals of the Universities of the United Kingdom (ORS Award Scheme), and King’s College, the University of Cambridge. Amir A. Shirzadi Cambridge December 1997 Acknowledgement I am wholeheartedly grateful to my supervisor, Dr Rob Wallach, for his guidance, encouragement and invaluable friendship during my research period in Cambridge. It is a pleasure for me to acknowledge Dr Sue Jackson for her advice and ever friendly support. Also, I would like to thank Dr Ian Bucklow for the useful discussions I had with him on my work. I sincerely thank Dr Hamid Assadi for his advice and help throughout my research, and also Fataneh who provided an enjoyable atmosphere for Hamid and me to carry on with our discussions. I am also grateful to Mohammed Balamdi for “tidying” my room frequently and Kaveh Chamandar who kept correcting my English accent so patiently. Thanks are also due to my friends at King’s College: Clea for her kind friendship and food-wise logistic; Liz, Ken, Kai and Paul for improving my knowledge to the single currency issue during our outings. I am grateful to Professor Alan Windle, the Head of the Department, for providing laboratory facilities, and I express my gratitude to my research colleagues, academic and technical staff in the Department who helped me so sincerely in the course of my research. Finally, thanks to the staff in my college for their help and many others who made my time in Cambridge so enjoyable. A.A. Shirzadi University of Cambridge Ph.D. Thesis Abstract Development of a suitable joining technique for advanced aluminium alloys and composites will enable them to be more widely used. The aim of this Ph.D. research was to develop new joining methods for these materials for which conventional welding methods have been unsuccessful. The research led to six new bonding methods and also to an analytical model which may be applicable to all transient liquid phase (TLP) bonding processes. In the early stage of the research, two new methods for TLP diffusion bonding of aluminium-based composites (aluminium alloys with silicon carbide particles as reinforcement) were developed. The methods were based on applying isostatic pressure (rather than conventional uniaxial compression), and bonds were fabricated with shear strengths as high as 242 MPa which is 92% of the shear strength of the parent material. This value is far greater than the highest bond strength reported to date for these aluminium-based composites. Based on simple finite element analysis modelling, a third method was developed which allows the joining of superplastic alloys/composites with minimal deformation. This method is based on a combination of conventional TLP diffusion bonding and hot isostatic pressing without encapsulation. It allows the fabrication of intricate parts with virtually no deformation during the bonding process so that dimensional tolerances are preserved. A fourth method, which is based on a new approach to TLP diffusion bonding by introducing a temperature gradient, is capable of producing reliable bonds with shear strengths as high as those of the parent alloys. The use of this method led to the formation of non-planar interfaces, compared to planar interfaces associated with i A.A. Shirzadi University of Cambridge Ph.D. Thesis conventional diffusion bonding methods. Therefore the strength and reliability of the bonds, made using this method, were improved considerably. This fourth method has already been patented in the United Kingdom (UK 9709167.2). A comprehensive analytical model was developed to predict the bonding time and the microstructure of the bond line when using temperature gradient TLP diffusion bonding. The model has been experimentally verified. The model also may be applicable to all TLP diffusion bonding approaches. The fifth method, developed in the current research, is capable of fabricating reliable bonds in air with shear strengths as high as 90% of those of the parent material. This is the highest bond strength, reported to date, for diffusion-bonded aluminium joints made in air. The approach overcomes the limitations associated with bonding aluminium- based materials in vacuum; this has been a major restriction in exploitation of the process. Patent protection has been sought (UK 9811860.7). Based on a combination of the fourth and fifth methods, a sixth method is proposed to improve the reliability of bonds made in air (temperature gradient TLP diffusion bonding in air). Preliminary results are very promising and some suggestions for further work on this method are proposed. ii A.A. Shirzadi University of Cambridge Ph.D. Thesis Table of Contents 1. Introduction..........................................................................................1 Summary of the work in this dissertation ..........................................................................2 2. Literature survey..................................................................................4 2.1 Theoretical aspects of solid-state diffusion bonding .......................7 2.2 Theoretical aspects of transient liquid phase (TLP) diffusion bonding ........................................................................10 Modelling of TLP diffusion bonding................................................................................12 Modelling of TLP diffusion bonding of composites........................................................16 2.3 Solid-state diffusion bonding of aluminium alloys .......................18 Imposing macroscopic plastic deformation ....................................................................18 Enhancing microplastic deformation of the surface asperities .....................................18 Use of interlayers and the effect of alloying elements....................................................20 Non-conventional bonding and testing methods.............................................................21 2.4 TLP diffusion bonding of aluminium alloys.................................24 Zinc based interlayers ......................................................................................................24 Silver interlayers...............................................................................................................25 Copper interlayers ............................................................................................................25 2.5 Joining aluminium metal matrix composites ................................27 2.6 Diffusion bonding of aluminium-based materials in air................31 2.7 Summary......................................................................................33 Appendix.....................................................................................................35 iii A.A. Shirzadi University of Cambridge Ph.D. Thesis 3. Experimental procedure ....................................................................37 3.1 Parent materials and interlayers....................................................37 3.2 Bond fabrication...........................................................................39 3.2.1 Pre-bonding procedure.......................................................................39 3.2.2 Bonding procedure.............................................................................40 Method I: Isostatic TLP diffusion bonding....................................................................42 Method II: Low pressure conventional TLP diffusion bonding followed by isostatic solid-state diffusion bonding in air .........................................42 Method III: Low pressure conventional TLP diffusion bonding followed by hot isostatic pressing (HIP) without encapsulation ..............................44 Method IV: Temperature gradient TLP diffusion bonding ..........................................44 Method V: High heating rate TLP diffusion bonding in air.........................................44 Method VI: Temperature gradient TLP diffusion bonding in air ................................47 3.2.3 Post bond heat treatment ....................................................................49 3.3 Bond testing and evaluation .........................................................51 3.3.1 Mechanical testing .............................................................................51 3.3.2 Metallographic examination...............................................................52 4. Methods I, II and III: New methods for TLP diffusion bonding aluminium-based composites .............................................55 4.1 Introduction..................................................................................55 4.2 The aim of using isostatic pressure for TLP diffusion bonding
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