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Weldability of High Strength Aluminium Alloys

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Weldability of High Strength Aluminium Alloys Muyiwa Olabode WELDABILITY OF HIGH STRENGTH ALUMINIUM ALLOYS Thesis for the degree of Doctor of Science (Technology) to be presented with due permission for public examination and criticism in lecture hall 1382 at Lappeenranta University of Technology, Lappeenranta, Finland on the 1st of December, 2015, at noon. Acta Universitatis Lappeenrantaensis 666 Supervisors Professor Jukka Martikainen Laboratory of Welding Technology LUT School of Energy Systems Lappeenranta University of Technology Finland Associate Professor Paul Kah Laboratory of Welding Technology LUT School of Energy Systems Lappeenranta University of Technology Finland Reviewers Professor Leif Karlsson Department of Engineering Science University West Sweden Professor Thomas Boellinghaus Department of Component Safety Federal Institute of Material Research and Testing Germany Opponent Professor Leif Karlsson Department of Engineering Science University West Sweden ISBN 978-952-265-865-4 ISBN 978-952-265-866-1 (PDF) ISSN-L 1456-4491 ISSN 1456-4491 Lappeenrannan teknillinen yliopisto Yliopistopaino 2015 Abstract Muyiwa Olabode Weldability of high strength aluminium alloys Lappeenranta 2015 59 pages Acta Universitatis Lappeenrantaensis 666 Diss. Lappeenranta University of Technology ISBN 978-952-265-865-4, ISBN 978-952-265-866-1 (PDF), ISSN-L 1456-4491, ISSN 1456-4491 The need for reduced intrinsic weight of structures and vehicles in the transportation industry has made aluminium research of interest. Aluminium has properties that are favourable for structural engineering, including good strength-to-weight ratio, corrosion resistance and machinability. It can be easily recycled saving energy used in smelting as compared to steel. Its alloys can have ultimate tensile strength of up to 750 MPa, which is comparable to steel. Aluminium alloys are generally weldable, however welding of high strength alloys like the 7xxx series pose considerable challenges. This paper presents research on the weldability of high strength aluminium alloys, principally the 7xxx series. The weldability with various weld processes including MIG, TIG, and FSW, is discussed in addition to consideration of joint types, weld defects and recommendations for minimizing or preventing weld defects. Experimental research was carried out on 7025-T6 and AW-7020 alloys. Samples were welded, and weld cross sections utilized in weld metallurgy studies. Mechanical tests were carried out including hardness tests and tensile tests. In addition, testing was done for the presence of Al2O3 on exposed aluminium alloy. It was observed that at constant weld heat input using a pulsed MIG system, the welding speed had little or no effect on the weld hardness. However, the grain size increased as the filler wire feed rate, welding current and welding speed increased. High heat input resulted in lower hardness of the weld profile. Weld preheating was detrimental to AW- 7020 welds; however, artificial aging was beneficial. Acceptable welds were attained with pulsed MIG without the removal of the Al2O3 layer prior to welding. The Al2O3 oxide layer was found to have different compositions in different aluminium alloys. These findings contribute useful additional information to the knowledge base of aluminium welding. The application of the findings of this study in welding will help reduce weld cost and improve high strength aluminium structure productivity by removing the need for pre-weld cleaning. Better understanding of aluminium weld metallurgy equips weld engineers with information for better aluminium weld design. Keywords: Aluminium alloys, aluminium welding processes, high strength aluminium, anodising, Al2O3, 7025-T6, AW-7020 Acknowledgements I would like to express my appreciation to the many people that have in one way or the other helped me in the completion of this thesis. I gratefully acknowledge the efforts of Dr. Paul Kah for his input in the form of research methodology, article corrections, availability and readiness to guide. I thankfully acknowledge the efforts of Esa Hiltunen, Antti Heikkinen and Antti Kähkönen in carrying out laboratory weld experiments. I would also like to thank Dr. Liisa Puro and Toni Väkiparta for their assistance in carrying out O2 composition experiments on the Al2O3 layer. I would like to thank Peter Jones for the valuable input on the academic presentation of this thesis. I wish also to extend my thanks to Martin Kesse for all his support. I would like to thank the pre-examiners of this work, Professor Leif Karlsson and Professor Thomas Boellinghaus for their valuable comments and suggestions that helped in improving the quality of this work. I wish to acknowledge the encouragement of friends and families. My special thanks are extended to my families, the Olabodes, the Pöllänens and the Olamilehins for their encouragement and support. Additionally, my appreciation goes to Bidemi Orebiyi, Samuel Okunoye, Kevin Eyiowuawi, Edith Emenike, and others that have in one way or the other supported my journey. My sincere appreciation is expressed to my immediate family, especially my wife, Olaitan Olabode, for all the encouragement, forbearance and understanding exercised during the course of this research. I feel blessed, thank you all. Muyiwa Olabode October 2015 Lappeenranta, Finland Dedication Dedicated to almighty God, The one who was, is, and is to come; allowing the acquisition of knowledge and giving the wisdom to know when, where and how to apply the acquired knowledge. Contents List of publications 11 Author's contribution 11 Nomenclature 13 1 Introduction 15 1.1 Research problem and research questions ............................................... 16 1.2 Scope and limitations of the study .......................................................... 17 1.3 Contribution of the work ......................................................................... 18 1.4 Social and environmental impact ............................................................ 18 1.5 Thesis outline .......................................................................................... 18 2 State of the art of Al welding 19 2.1 Alloy designation .................................................................................... 19 2.2 HSA ......................................................................................................... 19 2.3 Weldability of HSA ................................................................................. 22 2.3.1 Joint types and process limitations .............................................. 22 2.3.2 Work preparation ........................................................................ 26 2.3.3 Welding defects in HSA ............................................................. 30 2.4 Hybrid laser beam welding (HLBW) of HSA ......................................... 32 2.4.1 HLBW focusing head .................................................................. 33 2.4.2 Challenges of HLBW of Al ........................................................ 36 3 Experimental work 37 3.1 Welding metallurgy of HSA (7025-T6) .................................................. 37 3.2 Investigation of the Al2O3 layer in Al alloys ........................................... 39 3.3 Effect of Al2O3 layer on HSA (AW-7020) weld metallurgy ................... 39 4 Results 41 4.1 Findings on the welding metallurgy of HSA (7025-T6) ......................... 42 4.2 Findings on the Al2O3 layer of Al alloys ................................................. 45 4.3 Findings on the effect of Al2O3 on HSA (AW-7020) weld metallurgy .. 45 5 Discussion 49 5.1 Welding metallurgy of HSA (7025-T6) .................................................. 49 5.2 Effect of Al2O3 on HSA (AW-7020) weld metallurgy ............................ 50 6 Conclusions 53 7 Future work 55 References 56 11 List of publications This thesis is based on the following papers. The rights have been granted by publishers to include the papers in the dissertation. I. Olabode, M., Kah P., and Martikainen J. (2012). Experimental review on the welding metallurgy of HSA (7025-T6) alloy. The Paton Welding Journal, 4, pp.88-96. II. Olabode, M., Kah, P., and Martikainen, J. (2013). Aluminium alloys welding processes: Challenges, joint types and process selection. Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture, 227(8), 1129-1137. III. Olabode, M., Kah, P., and Martikainen, J. (2015). Effect of Al2O3 film on the mechanical properties of a welded high-strength (AW-7020) aluminium alloy. Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture. DOI: 10.1177/0954405415600678 IV. Olabode, M., Kah, P., and Salminen, A. (2015). Overview of laser systems and optics applicable to hybrid laser welding of aluminium alloys. Rev. Adv. Mater. Sci, 42, (2015) 6-19. Author's contribution The candidate was the main author of all the publications attached to the doctoral thesis. The candidate generated the ideas and the conclusions presented in the publications. Revision was carried out together with the co-authors and reviewers as a joint effort. The contribution of the author to the publications was as summarized below: I. Made the research design and experimental design, carried out the experiment, literature review and analysis, drew inferences and wrote the paper. II. Made the research design, carried out the literature review and analysis, drew inferences and wrote the paper. III. Made the research design and experimental design, carried
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