Development of Hypereutectic Al-Si based P/M Alloys By Shei Sia Su A thesis submitted to the University of Birmingham for the degree of Doctor of Philosophy School of Metallurgy & Materials College of Engineering and Physical Sciences The University of Birmingham Birmingham B15 2TT England April 2012 University of Birmingham Research Archive e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder. Synopsis The compaction, sintering and heat treatment processing conditions for the commercial powder, Ecka Alumix 231 with the nominal composition of Al-15Si- 2.5Cu-0.5Mg(wt%) have been optimized in this study. It has been found that densification of Ecka Alumix 231 depended largely on the sintering atmosphere and the amount of liquid phase presented. Sintering in nitrogen atmosphere was found to be beneficial due to the formation of AIN which later induced pore filling effect. Presence of hydrogen in the sintering atmosphere, however, was not desirable. Decomposition of MgH2 at higher temperature was suggested to increase the pore pressure and caused detrimental effect on compact densification. The effects of alloy additions (i.e. Cu and/or Ni) on sintering and heat treatment response were also investigated in this study. Addition of copper was found to enhance the sintering response by a significant increase in the sintered density, reduced the peak ageing temperatures and time. Nickel addition, however, was detrimental to both sintering and heat treatment response since it reduced the amount of copper content dissolved in the α-aluminum due to the formation of Al3(Ni,Cu)2 phase. The precipitation sequence for Al-Si-Cu-Mg-(Ni) was suggested to be: Supersaturated solid solution (SSS)→GP zones → θ” → θ’ → θ. Preface The work described in this thesis was carried out by the author in the School of Metallurgy and Materials, The University of Birmingham from September 2008 to February 2012, under the supervision of Dr. I T H Chang To the best of my knowledge, the material presented in this thesis is original, except otherwise noted within the text. None of this research has been submitted for any degree at this or any other university. Part of the present work was presented in international conference and published as follows: [1] Su.S.S. and I.T.H. Chang, “ Effect of Processing Conditions on Hypereutectic Aluminum-Silicon P/M Alloys”, World PM2010 Proceedings, Vol. 4, pp 49-57 (2010) [2] Su.S.S. and I.T.H. Chang, “ Development of Hypereutectic Al-Si based alloys”, World PM2010 Proceedings, Vol.4, pp 58-62 (2010) Acknowledgements This dissertation would not have been completed without the support of many wonderful individuals. In particular, I would like to acknowledge the enthusiasm and dedication of my supervisor, Dr I T H Chang. I am deeply grateful for his valuable guidance, support and constant encouragement throughout my PhD. His patience, wisdom and excellent supervision have made this PhD a very rewarding journey. My sincere gratitude also goes to Dr Winson Kuo from Oxford Instruments who has always offered insightful suggestions and gave significant contribution to boost my knowledge in electron microscopy and enhance my analyzing skills. I would also like to thank Mr. Hang Wang in offering lots of valuable inputs on thermodynamics and is always willing to lend an attentive ear and a critical eye. I would also like to express my sincere gratitude to many other friends and colleagues who have contributed in countless ways to my dissertation. My sincere thanks go to Dr Martin Strangwood for offering training in optical microscope and Thermocalc software, Dr Yulung Chiu and Dr Rengen Ding for providing valuable advices on TEM sample preparations, Dr Ming Chu for offering training in TEMs, Mr. Paul Stanley for offering training in SEMs/EDS, Mr. David Price for assistance in tensile test, Mr. Frank Biddelstone for assistance in DSC, Mr. Jeff Sutton for assistance in XRD and to Mr. Jas Singh for assistance in the grinding and polishing. I am also grateful to all technical staff of Met & Mat; particularly Tony Burbery, Geoff Dolman, Mick Cunningham, Avril Rogers, Paul Osborne and Anne Cabezas, for their support and help. Special thanks also go to my group mates: Nick Harrison, Andy Poole, Andy Ward and for providing a pleasant working environment, to Yina Guo, Jiayun Jiang, Chao Yang and all my bible study members for their friendship and unfaltering encouragement over the years of my academic pursuit. I would like to take this opportunity to also thank Overseas Research Scholarship Fund and Professor Paul Bowen, School of Metallurgy and Materials for providing financial support. Last but not least, I would like to express my gratitude to all of my family. Their support, patience and encouragement have significantly contributed to the completion of my PhD. Content Chapter 1 Introduction ......................................................................................... 1 Chapter 2 Literature Review 2.1 Introduction ....................................................................................................... 4 2.2 Powder Production Methods ............................................................................. 7 2.2.1 Gas Atomization .................................................................................. 7 2.2.2 Water Atomization ............................................................................... 9 2.2.3 Centrifugal Atomization ..................................................................... 11 2.2.4 Mechanical Milling ............................................................................. 12 2.2.5 Electrolysis ........................................................................................ 13 2.2.6 Thermal decomposition ..................................................................... 14 2.3 Compaction ..................................................................................................... 15 2.3.1 Uniaxial Compaction .......................................................................... 17 2.4 Sintering .......................................................................................................... 20 2.4.1 Driving Force for Sintering ................................................................. 21 2.4.2 Wetting Characteristic ....................................................................... 25 2.4.3 Solid State Sintering .......................................................................... 28 2.4.4 Liquid Phase Sintering ....................................................................... 30 2.4.4.1 Particles Rearrangement ................................................................ 34 2.4.4.2 Solution-Reprecipitation ................................................................. 35 2.4.4.3 Pores Filling .................................................................................... 38 2.4.4.4 Particle Coarsening ........................................................................ 40 2.4.5 Transient Liquid Phase Sintering ....................................................... 42 2.4.6 Supersolidus liquid phase sintering ................................................... 44 2.5 Effects of Sintering Variables .......................................................................... 46 2.5.1 Particle Size ....................................................................................... 46 2.5.2 Particle Shape ................................................................................... 48 2.5.3 Sintering Atmosphere ........................................................................ 50 2.5.4 Sintering Temperature ....................................................................... 54 2.5.5 Sintering Time ................................................................................... 55 2.6 Aluminum Alloys Powder Metallurgy ............................................................... 56 2.6.1 Al-Si P/M alloys ................................................................................. 59 2.6.2 Effect of Alloy Additions ..................................................................... 61 2.6.3 Heat Treatment of Aluminum Alloys .................................................. 63 2.6.4 Major Precipitation Hardening Alloy Systems .................................... 70 2.6.4.1 Al-Cu System ................................................................................. 73 2.6.4.2 Al-Cu-Mg System .......................................................................... 74 2.6.4.2 Al-Mg-Si System ............................................................................ 75 2.6.4.3 Al-Cu-Mg-Si System ...................................................................... 76 2.6.4.4 Al-Zn-Mg-Cu System ..................................................................... 76 2.7 References ...................................................................................................... 78 Chapter 3 Experimental Methods 3.1 Introduction ....................................................................................................