Superplastic behavior of micro-regions in two-pass friction stir processed 7075Al alloy

Effect of overlapping passes on the microstructure and superplastic behavior of friction stir processed (FSP) 7075Al was subjected to a detailed investigation. Overlapping passes exerted no obvious effect on the size of recrystallized grains. Both single and two-pass FSP 7075Al exhibited similar grain sizes of 5.4–5.7 μm. Compared to single-pass FSP, two-pass FSP resulted in an enhancement in superplastic elongation and a change in superplastic response. A shift to higher optimum temperature was observed in the two-pass FSP 7075Al. Furthermore, overlapping passes led to a shift to higher optimum strain rate in the center region of second pass in the two-pass FSP 7075Al. Maximum superplastic elongation of 1220% was achieved at 480 °C and an initial strain rate of 1 × 10−2 s−1 in the center region of second pass in the two-pass FSP 7075Al. Analyses of superplastic data indicated that grain boundary sliding is the main superplastic deformation mechanism for both single and two-pass FSP 7075Al.