New Approaches to Casting Hypereutectic Al-Si Alloys to Achieve Simultaneous Refinement of Primary Silicon and Modification of Eutectic Silicon A thesis submitted for the degree of Doctor of Philosophy by Kawther W. A. Al-Helal Brunel Centre for Advanced Solidification Technology (BCAST), Brunel University October 2013 This thesis is dedicated to my family for their love, support and encouragement i َ بِ ْس ِمِِّللاِِال َر ْح َم ِنِِال َر ِحي ِمِِ آ ُتو ِني ُز َب َر ا ْل َح ِدي ِد ۖ َح َّت ى إِ َذا َسا َو ى َب ْي َن ال َّص َد َف ْي ِن َقا َل انفُ ُخوا ۖ َح َّت ى إِ َذا َج َع َل ُه َنا ًرا ُ ْ َ ْ َقا َل آ ُتو ِني أ ْف ِر ْغ َع َل ْي ِه ِقط ًرا َف َما ا ْس َطا ُعوا أن َيظ َه ُروه ُ َو َما ا ْس َت َطا ُعوا َل ُه َن ْق ًبا. صدقِّللاِالعظيمِ الكهف:ِمنِاﻵية 95 In the name of Allah, the Entirely Merciful, the Especially Merciful ‘’ Bring me blocks of iron, when he had filled up the space between the two steep mountain-sides, He said, "Blow (with your bellows)" Then, when he had made it (red) as fire, he said: "Bring me molten Lead, that I may pour over it. So Gog and Magog were unable to pass over it, nor were they able (to effect) in it any penetration’’. Surat Al-Kahf: 95 Holy Quran ii Abstract Hypereutectic Al-Si alloys are of increasing interest for applications that require a combination of light weight and high wear resistance, such as pistons, liner-less engine blocks and pumps. The wear resistance of this class of alloys is due to the presence of hard primary Si particles formed during casting. The objective of this work was to develop one or more methods of refining primary silicon in cast hypereutectic Al-Si alloys to compete with the conventional process of adding phosphorous and to achieve the simultaneous modification of silicon in the Al-Si eutectic. A robust sampling/casting technique was developed to minimise macro-segregation of primary silicon during solidification of hypereutectic Al-Si alloys by using water cooled steel mould with cooling rate in excess of 15 K/s. The morphology of silicon phases was found to change with increasing melt temperature and cooling rate. The high cooling rate and superheat temperature produces a good distribution of polyhedral primary silicon particles in a refined lamellar eutectic matrix in solidification of commercial purity Al-Si alloys. Removing Ca by fluxing with K2SiF6 prior to casting can improve the refinement and modification effect of Mg and Sb respectively. Effects of various inoculants were studied. Microstructural analysis showed that Mg and ZnS refined primary Si whereas MgO, CaO and Na2S coarsened the primary Si together with a modification effect on the eutectic Si. Adding Zn had no effect on morphology of Si phases. Refinement of both primary and eutectic silicon phases was observed for the Al-15Si alloy with Mg content ≤ 0.3 wt%. P-doped γ-Al2O3 was found to be a potent substrate to nucleate primary silicon whilst good modification of the eutectic matrix is retained during solidification of hypereutectic Al-Si alloys. On using P-doped γ-Al2O3 could be a perfect and clean source of P without additional impurities. A new solid-liquid duplex casting process was devised to achieve simultaneous refinement and modification of Si phases in hypereutectic Al-Si alloys with improvement in mechanical properties. The static mechanical properties of Al-Si produced by the solid-liquid duplex casting process are significantly better than conventionally cast untreated Al-Si and slightly better than conventionally cast Al-Si treated with P and/or Sr. A novel Al-ZnS master alloy was developed by in situ reaction of Zn and Na2S in the Al melt. The results from this study leave little doubt that this novel Al-ZnS master alloy is a promising refiner in solidification of hypereutectic Al-Si alloys. It refines primary silicon to the same extent as that achieved by adding P via Cu-P following the same refinement mechanism. iii Preface This thesis is a description of work that I performed in the Brunel Centre for Advanced Solidification Technology, Brunel University, West London from 22nd October 2009 to 30th October 2013. To the best of my knowledge, this work is original, except where suitable references are made to previous work. Neither this, nor any substantially similar dissertation has been submitted to any other institution nor is it the result of my own work. The content of the dissertation does not exceed 60,000 words. iv Related Publications International Journal Papers Kawther W. Al-Helal, Ian C. Stone and Zhongyun Fan, ‘’Simultaneous Primary Si Refinement and Eutectic Modification in Hypereutectic Al-Si Alloys”, Trans. Indian Inst. Metals. (December 2012) 65(6):663–667. Kawther W. Ahmed Al-Helal, Yun Wang, Ian C. Stone and Zhongyun Fan, ’Effect of Ca Level on the Formation of Silicon Phases During Solidification of Hypereutectic Al-Si Alloys’’, Mat. Sci. Forum Vol. 765 (2013) pp 117-122. Kawther Al-Helal, Ian C. Stone and Z. Fan, ‘’Morphology of Silicon Phases in Solidification of High Purity Hypereutectic Al-15Si Alloy’’, 2013. The paper is in preparation. Kawther Al-Helal, Ian C. Stone and Z. Fan, ‘’Novel Al-ZnS Master Alloy for Refinement of Primary Silicon Crystals in Solidification of Hypereutectic Al-Si Alloys’’, 2013. The paper is in preparation. Kawther Al-Helal, Ian C. Stone and Z. Fan, “Effect of Solidification Rate on Macro Segregation and Morphologies of Silicon phases in Solidification of Hypereutectic Al-15Si Alloy”, 2013. The paper is in preparation. Conferences The John Hunt International Symposium, Brunel University, UK, December 12 -14, 2011. National Student Conference in Metallic Materials, Manchester Conference Centre, UK, June 25-26, 2012. Fifth International Conference on Solidification Science and Processing, Bhubaneswar, India, November 19-22, 2012 / Poster. Sixth International Light Metals Technology Conference (LMT2013), Windsore, UK, July 24-26, 2013. v Acknowledgements I would like to express my sincere gratitude and grateful admiration to my supervisors Prof. Zhongyun Fan and Dr. Ian Stone for their encouragement, continuous guidance, utmost effort and interest through the work that contributed in its completeness. I am grateful to the EPSRC, UK for funding this project and my special thanks and sincere gratitude to Prof. Paul Harris / Wolfson Materials Processing Centre, Prof. Andy Feest and Dr. Kumar Sundaram / Oxford University for their help and useful discussion throughout my PhD programme. My thanks to Aura Metals Ltd for supplying the CuP shot that I used during my research. I am in great debt to Prof. Dmitry Eskin and Dr. Hari-babu Nadendla for their obliging and fruitful support on all topics and through all stages of this work. I have received technical help/suggestions from many people at BCAST during the course of this work. I want to specially mention Dr. Yun Wang, Dr. Jayesh Patel, Dr. Yubo Zuo, Dr. Magdalena Nowak, Dr. H-T Li, Dr. Anil Midathada and Dr. Prasadd Ayyagari for their scientifically helpful and discussions. My special thanks to Mr. Stephen Cook, Mr. Peter Lloyd and Mr. Carmelo Nunez for their technical support. I wish to express my thanks to my colleagues at BCAST for their support and letting me fell part of the team whenever we worked together. Finally, I wish to express my deepest respect and sincere appreciation to my family for their encouragement, patience and great understanding. vi Table of Contents Abstract ........................................................................................................................ iii Preface ........................................................................................................................ iv Related Publications ..................................................................................................... v Conferences ................................................................................................................. v Acknowledgements ...................................................................................................... vi Table of Contents ........................................................................................................ vii List of Figures .............................................................................................................. xii List of Tables ............................................................................................................. xxii Nomenclature ........................................................................................................... xxiv Chapter 1 Introduction ........................................................................................... 1 1.1 Background ........................................................................................................... 1 1.2 Objectives and Scope ............................................................................................. 3 1.3 Outline of Thesis …...……………………………………………………………………..4 Chapter 2 Literature Review ................................................................................. 5 2.1 Introduction to Al-Si Alloys ...................................................................................... 5 2.2 The Al-Si Alloy System and Properties.................................................................... 6 2.3 Hypereutectic Al-Si Alloys ....................................................................................... 9 2.4 Inclusions and Oxides ..........................................................................................