The origin of non-uniform microstructure and its effects on the mechanical properties of a friction stir processed Al-Mg alloy

The microstructure of the stirred zone (SZ) resulting from friction stir processing or welding (FSP/FSW) has usually been assumed to be uniform when discussing the mechanical properties. However, numerous works have indicated that the fine-grained microstructures in the SZ were non-uniform, with precipitate, texture and grain size gradients caused by the severe plastic deformation and heat distribution. In this work commercial aluminum alloy 5083-H112 was subjected to FSP and fine-grained microstructures with an average grain sizes of 2.7-13.4 mu m were obtained by controlling the FSP conditions. The stress-strain curves exhibited stepped yield point elongation, which was suggested to be associated with these characteristic non-uniform microstructures. Tensile tests indicated that the Hall-Petch relationship held in this FSP alloy when taking account of the average grain size. Toughness analysis indicated that the optimum toughness was anticipated to be obtained around a grain size of similar to 1 mu m for this FSP alloy. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.