Friction stir processing (FSP) and superplasticity
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FSP is an allied technology of friction stir welding (FSW). The development and applications of these technologies are reviewed and additional considerations such as processing pattern and step over distance between successive passes in FSP are discussed. The deformation field for a rotating and advancing tool is not symmetric about the axis of tool motion. Thus, various spiral patterns with step over distances less than the tool pin diameter result in replacement of advancing interfaces (tool tangential velocity and traversing speed add) retreating interfaces (tool tangential velocity and traversing speed subtract) and greater homogeneity of the resulting stir zone microstructures. Microstructures formed as a result FSP often exhibit refined and uniform distributions of non-deforming constituents as well as highly refined grain structures, and superplastic response following FSP of wrought 7XXX and 5XXX Al alloys has been achieved. The application of multi-pass FSP to cast metals, including continuously cast (CC) AA5083 and AA356 as well as a Na modified Al-7Si will be summarized and the conversion of as-cast microstructures to a wrought condition in the absence of external shape change will be shown. Grain refinement is the result of recrystallization during the rapid thermomechanical cycle of FSP. The mechanisms involved in homogenization of constituent particle distributions remains to be determined. The FSP-induced superplastic response of the AA5083 will be documented. In contrast, grain refinement in the Na modified Al-7Si alloy was insufficient to support superplasticity.
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