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 μm 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 ∼1 μm for this FSP alloy.