Microstructure and superplasticity of AZ31 sheet fabricated by differential speed rolling

Microstructure evolution and superplasticity in AZ31 alloy by differential speed rolling (DSR) with a high-speed ratio (=3) between the upper and lower rolls was examined. With increase in thickness reduction by DSR at 473 K, degree of grain refinement and microstructure homogeneity increased. The microstructure obtained by a single rolling pass for a 70% reduction in thickness consisted of recrystallized grains with a mean size of ∼2 μm, and fraction of HAGBs and average misorientation angle determined by the EBSD analysis were 0.47 and 23.21°, respectively. The DSR AZ31 alloy exhibited enhanced superplasticity as compared with the conventionally processed AZ31. A maximum elongation of 830% was obtained at 2 × 10−4 s−1 and 673 K. The strain hardening exponent measured at 2 × 10−4 s−1 and 673 K was as high as 0.71, which could be related to accelerated grain growth in the highly refined microstructure during superplastic flow.