FRICTION STIR PROCESSING OF ALUMINIUM-SILICON ALLOYS A THESIS SUBMITTED TO THE UNIVERSITY OF MANCHESTER FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN THE FACULTY OF ENGINEERING AND PHYSICAL SCIENCES 2011 CHUN YIP CHAN SCHOOL OF MATERIALS Contents Abstract .......................................................................................................................... 21 Declaration ..................................................................................................................... 23 Copyright Statement ..................................................................................................... 24 Acknowledgements ........................................................................................................ 25 1 Introduction ............................................................................................................... 26 2 Literature Review...................................................................................................... 29 2.1 Aluminium Alloys............................................................................................................. 29 2.1.1 Strengthening Mechanism......................................................................................... 33 2.2 Automotive Pistons ........................................................................................................... 35 2.2.1 Introduction ............................................................................................................... 35 2.2.2 Alloy Additions in Al-Si Casting Alloys .................................................................. 36 2.2.3 Microstructure Development .................................................................................... 39 2.2.4 Second Phase Particles in Al-Si Piston Alloys.......................................................... 43 2.2.5 Fracture and Failure Behaviour of Al-Si Poston Alloys ........................................... 47 2.3 Friction Stir Welding (FSW) ............................................................................................. 50 2.3.1 Microstructure Development .................................................................................... 51 2.3.2 The Effects of FSW on Heat Treatable Alloys ......................................................... 52 2.3.3 The Effects of FSW on Non-Heat Treatable Alloys ................................................ 56 2.4 Friction Stir Processing (FSP) ........................................................................................... 57 2.4.1 Tool Design ............................................................................................................... 58 2.4.2 Tilt and Plunge Depth ............................................................................................... 60 2.4.3 Transverse and Rotational Speed .............................................................................. 61 2.5 Heat Generation ................................................................................................................. 62 2.6 Material Flow .................................................................................................................... 66 2.6.1 Flow Visualisation .................................................................................................... 67 2.6.2 Material Flow Modelling .......................................................................................... 70 2.7 FSP of Different Classes of Materials ............................................................................... 74 2 2.7.1 FSP of Al-Si Alloys .................................................................................................. 79 2.8 Post-Weld/Process Heat Treatment .................................................................................. 82 2.9 Summary ............................................................................................................................ 87 3 Experimental Procedures ......................................................................................... 90 3.1 Material ............................................................................................................................. 90 3.2 Phase Identification ........................................................................................................... 91 3.2.1 JMatPro ..................................................................................................................... 91 3.2.2 X-Ray Diffraction (XRD) ......................................................................................... 92 3.2.3 Scanning Electron Microscopy and Energy Dispersive X-Ray Spectroscopy .......... 92 3.3 Friction Stir Experiments .................................................................................................. 93 3.3.1 Friction Stir Processing ............................................................................................. 93 3.3.2 Stop-Action Experiment ........................................................................................... 95 3.4 Post-Process Heat Treatment ............................................................................................ 96 3.5 Mechanical Testing ........................................................................................................... 97 3.5.1 Vickers Hardness Testing ......................................................................................... 97 3.5.2 Uni-Axial Tensile Testing ......................................................................................... 97 3.5.3 Uni-Axial Compression Testing ............................................................................... 98 3.6 Characterisation of Second Phase Particles ...................................................................... 99 3.6.1 Metallographic Preparation ..................................................................................... 100 3.6.2 Particle Statistic Measurements .............................................................................. 100 3.6.3 Particle Distribution Analysis ................................................................................. 101 3.7 Grain Structure Characterisation ..................................................................................... 104 3.7.1 Optical ..................................................................................................................... 104 3.7.2 Electron Back Scattering Diffraction (EBSD) ........................................................ 105 3.8 Computer Models ............................................................................................................ 106 3.8.1 ABAQUS Thermal Finite Element Model .............................................................. 106 3.8.2 Computational Fluid Dynamics (CFD) Model ....................................................... 111 4 Microstructures Prior to FSP ................................................................................ 116 4.1 Introduction ..................................................................................................................... 116 4.2 Grain Structure Analysis ................................................................................................. 116 4.3 Second Phase Particle Analysis ...................................................................................... 118 4.4 Phase Identification ......................................................................................................... 120 4.4.1 Morphology of Individual Phases ........................................................................... 123 4.5 Summary ......................................................................................................................... 130 3 5 Particle Refinement in FSP .................................................................................... 132 5.1 Introduction ..................................................................................................................... 132 5.2 Overview ......................................................................................................................... 132 5.3 Effect of Processing Parameters on Particle Refinement by the Tool Shoulder ............. 139 5.4 Effect of Processing Parameters on Particle Refinement by the Tool Pin and Shoulder 142 5.5 Particle Size Distribution ................................................................................................ 145 5.5.1 Effect of Rotation Rate ........................................................................................... 149 5.5.2 Effect of Transverse Speed ..................................................................................... 155 5.6 Particle Spatial Distribution ............................................................................................ 158 5.6.1 Effect of Rotation Rate ........................................................................................... 163 5.6.2 Effect of Transverse Speed ..................................................................................... 166 5.7 Effect of FSP on the Hypereutectic Alloys ..................................................................... 168 5.7.1 Al-Si Hypereutectic Alloy (Alloy-2) ...................................................................... 169 5.7.2 Al-Si-Ni Hypereutectic Alloy (Alloy-3) ................................................................. 171 5.7.3 Effect