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Development of Niobium-Boron Grain

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Development of Niobium-Boron Grain DEVELOPMENT OF NIOBIUM-BORON GRAIN REFINER FOR ALUMINIUM-SILICON ALLOYS A thesis submitted for the degree of Doctor of Philosophy (PhD) by Magdalena Nowak Brunel Centre for Advanced Solidification Technology (BCAST) Brunel University September 2011 To my mother and my late father... II Abstract Aluminium castings with a large grain structure have poor mechanical properties which are primarily due to casting defects as opposed to fine grain structure. The grain refinement practice using chemical addition is well established for wrought alloys, however in the case of casting alloys, the practice of adding grain refiners and the impact on castability is not well established. The addition of well known Al-5Ti-B grain refiner to casting alloys with silicon (Si) content above 3 wt.% is not effective. This is believed to be due to the chemical reaction between Ti and Si. The current research aim is to find an alternative, but effective, chemical phase which can refine Al-Si alloy grains. Based on a crystallographic database search and intermetallic phases found in Aluminium–Niobium-Boron, there exists several iso-structural phases similar to those of Al3Ti and TiB2. We have selected a phase which exhibits chemical phase stability with Si (below 900 oC) and developed a potential novel grain refiner Nb-B for Al-Si cast alloys. Various Al-Si binary alloys and a commercial sourced LM6 (Al-10Si-Mg) cast alloys were cast after novel grain refiner addition to the melt. It is the first time that such fine grain structures were achieved for Al-Si alloys when Si >4wt.%. It is believed that Nb-B grain refiner enhances the heterogeneous nuclei in the melt. The effectiveness of this grain refiner under various cooling rate conditions is investigated to simulate various practical casting conditions. Due to increased heterogeneous nuclei density, a fine grain structure is also obtained at low cooling rates and the grain size is less sensitive to the cooling rate. The processing of high Si containing alloys for complex shaped castings with reduced defects, fine grain structure and improved mechanical properties are now possible. III Acknowledgment It is a pleasure to thank those who made this thesis possible. I am thankful to my supervisor, Dr Hari Babu Nadendla, whose encouragement, guidance and support from the initial to the final level enabled me to develop an understanding of the subject for this thesis. He has been everything that one could want in an advisor. I would like to thanks Isambard Research Scholarships for the financial support of my three years research and I would like to thank Prof. Z.Fan for providing casting facilities. I which to thank Mr. A.R. Dennis from the Bulk Superconductivity Group at Cambridge University for measurements of the magnetic moment, Dr W. Kong from University of Sydney, for the TEM; also I would like to thank Dr K. Sundaram from Oxford University for SEM. I am grateful to all my family specially my mother Bozena Nowak for her unending love and support. I am also very grateful to my brother Damian Nowak for encouragement. I would like to acknowledge my late father Slawomir Nowak for whom this PhD was completed. I would like, also, to thank my boyfriend Adam Coventry for support over this last three years and help in reviewing this thesis. I would like, also, to thank my friends, inside and outside of the studies who have been a constant source of intellectual generosity, personal support and leisurely relief. I thank the members of BCAST for providing an intellectually and socially stimulating and well-organized environment. IV Contents Abstract......................................................................................................................................III Acknowledgment.......................................................................................................................IV Contents......................................................................................................................................V Abbreviations............................................................................................................................IX List of symbols...........................................................................................................................X List of Figures..........................................................................................................................XII List of Tables.........................................................................................................................XXV Chapter 1. Introduction................................................................................................................1 Chapter 2. Literature Review ...................................................................................................... 5 2.1 Introduction .......................................................................................................................5 2.2 Solidification of Aluminium Alloys .................................................................................5 2.2.1 Classical Nucleation Theory ......................................................................................5 2.2.2 Growth .....................................................................................................................11 2.3 Lattice Mismatch .............................................................................................................15 2.3.1. Crystal Structure......................................................................................................15 2.4 Grain Refinement ............................................................................................................17 2.4.1 Introduction ..............................................................................................................17 2.4.2 Grain Refinement by Al-Ti-B System .....................................................................17 2.4.3 The Grain Refinement Theories ...............................................................................18 2.5 Alloys Classification .......................................................................................................21 2.5.1 Wrought Alloys ........................................................................................................21 2.5.2 Casting Alloys ..........................................................................................................26 2.6 Casting Processes ............................................................................................................30 V 2.7 Defects in Castings ..........................................................................................................32 2.7.1 Gas Porosity .............................................................................................................32 2.7.2 Solidification Shrinkage ...........................................................................................33 2.8 Novel Grain Refiners...........................................................................................................33 Chapter 3 Experimental Procedures .......................................................................................... 34 3.1. Materials .........................................................................................................................34 3.1.1. Commercial Aluminium and Wrought Alloys ........................................................34 3.1.2. Aluminium-Silicon Alloys ......................................................................................34 3.1.3. Metal Powders and Plates .......................................................................................35 3.2. Casting Processes ...........................................................................................................36 3.2.1 Test Procedure-1 (TP-1 test) Mould ........................................................................36 3.2.2 Cylindrical Moulds and Wedge-shape Moulds ........................................................37 3.2.3 The High Pressure Die-Casting (HPDC) Process ....................................................39 3.3 Microstructural Characterization ....................................................................................41 3.3.1 Metallographic Sample Preparation .........................................................................41 3.3.2 Optical Microscopy (OM) ........................................................................................42 3.3.3 Scanning Electron Microscopy (SEM) ....................................................................43 3.3.4 Transmission Electron Microscope (TEM) ..............................................................44 3.3.5 Quantitative Metallographiy ....................................................................................45 3.4. Mechanical Properties Testing .......................................................................................45 3.4.1 Tensile Testing .........................................................................................................45 3.4.2 Hardness Testing ......................................................................................................46 3.5.Thermal Analysis.............................................................................................................47 3.6.Summary of Experiments and Alloys Studied................................................................48
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