Redistribution Mechanisms and Quantification of Homogeneity in Friction Stir Welding and Processing of an Aluminum Silicon Alloy
Woertz, Jeffrey C.
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The uniformity (homogeneity) of nondeforming particle distributions in a cast alloys microstructure is highly important in controlling the materials strength, ductility, and fatigue resistance. Friction stir processing (FSP) is an effective post-casting technique for redistributing constituent material into a more homogeneous state, although the mechanism of particle transport remains unclear and no concise method exists for quantifying and comparing homogeneity. Advective and diffusive redistribution processes are investigated in Al-7 per cent Si and AA356 to determine the magnitude and rate of particle transport within a high-temperature deforming metallic matrix. High temperature deformation experiments were conducted via Equal Channel Angular Processing (ECAP), hot rolling, and FSP. Processed material was then examined using optical / scanning electron microscopy. The experimentally observed particle transport was compared against modeled and analytically predicted transport, while microstructural homogeneity was measured and compared using digital image analysis and a six-parameter variance model. FSP redistribution is proposed to be the result of a matrix shear / layer sliding advective mechanism, the probable result of fine layers of material (ranging from 5 to 15 m in thickness) that are sheared and rotated by the tools pin face. Diffusive processes have only a small role in FSP redistribution. FSP is shown to increase homogeneity by a factor of ~5 (compared to as-cast), increasing linearly with tool RPM over the measured range.
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