Deep cup forming by superplastic punch stretching of friction stir processed 7075 Al alloy

Multiple overlapping passes of friction stir processing (FSP) were used to cover 80 mm wide region for punch stretching of 7075 Al alloy plate. The forming experiments were performed at 723 K. While a maximum depth of 52 mm (depth/punch diameter ∼ 2.7) could be achieved by punch stretching at a slow strain rate of 10−3 s−1, the maximum depth obtained at a higher strain rate of 10−2 s−1 was 40.4 mm. However, tolerable variations in thickness can be maintained up to a depth of 18.5 mm which is nearly equal to the punch diameter. FEM simulation closely predicts the load and thickness variation till the point of initiation of cavities and instabilities.