High strain rate superplasticity of a fine-grained AZ91 magnesium alloy prepared by submerged friction stir processing

The as-cast AZ91 plate was subjected to normal friction stir processing (processed in air) and submerged friction stir processing (processed in water, SFSP), and microstructure and superplastic tensile behavior of the experimental alloys were investigated. SFSP results in remarkable grain refinement due to the enhanced cooling rate compared with normal FSP, with an average grain size of 1.2 μm and 7.8 μm. The SFSP AZ91 specimen exhibits considerably enhanced superplastic ductility with reduced flow stress and higher optimum strain rate, as compared to the normal FSP specimen. The optimum superplastic deformation temperature is found to be 623 K for both the normal FSP and SFSP AZ91 specimens. An elongation of 990% is obtained at 2×10−2 s−1 and 623 K for the SFSP specimen, indicating that excellent high strain rate superplasticity could be achieved. By comparison, maximum ductility of the normal FSP specimen strained at high strain rate is 158%. Grain boundary sliding is the main mechanism for the superplastic deformation of the normal FSP and SFSP specimens. The excellent high strain rate superplasticity of the SFSP alloy is attributed to its finer grain structure and higher fraction of grain boundary.