Friction stir processing (FSP), developed based on the basic principles of friction stir welding – a solid-state joining process originally developed for aluminum alloys, is a relatively new metal-working technique that can provide precise control of microstructures in metallic materials. FSP causes intense plastic deformation, material mixing, and thermal exposure, resulting in the generation of fine and equiaxed recrystallized grains with a size of 0.1-20 μm and high ratio of high-angle grain boundaries. Such a microstructure in the FSP alloys is amenable to superplastic deformation. In the past few years, enhanced superplasticity was achieved in a number of fine-grained alloys prepared by FSP, such as aluminum alloys 7075, 2024, 5083, A356, Al-Mg-Zr, Al-Zn-Mg-Sc, Al-Mg-Sc and magnesium alloys Mg-Zn-Y-Zr, AZ91, ZK60. Especially, high strain rate/low temperature superplasticity was observed in several aluminum alloys. In this article, the current state of understanding and development of the FSP superplasticity are addressed. Particular attention is paid to the microstructural characteristics of the FSP fine-grained alloys and superplastic deformation mechanism.
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