Effect of friction stir processing on the kinetics of superplastic deformation in an Al-Mg-Zr alloy

The effect of friction stir processing on the superplastic behavior of extruded Al-4Mg-1Zr was examined at 350 degrees C to 600 degrees C and at initial strain rates of 1 X 10(-3) to 1 s(-1). A combination of a fine grain size of 1.5 mu m and high-angle grain boundaries in the friction stir-processed (FSP) alloy led to considerably enhanced superplastic ductility, much-reduced flow stress, and a shift to a higher optimum strain rate and lower optimum temperature. The as-extruded alloy exhibited the highest superplastic ductility of 1015 pct at 580 degrees C and an initial strain rate of 1 X 10(-2) s(-1), whereas a maximum elongation of 1280 pct was obtained at 525 degrees C and an initial strain rate of 1 X 10(-1) s(-1) for the FSP alloy. The FSP alloy exhibited enhanced superplastic deformation kinetics compared to that predicted by the constitutive relationship for superplasticity in fine-grained aluminum alloys. A possible origin for enhanced superplastic deformation kinetics in the FSP condition is proposed.