High-strain-rate superplastic gas pressure forming of an Al–4.4Cu–1.5Mg/21SiCW composite under variable strain rate paths

The deformation and cavitation behavior of a Al–4.4Cu–1.5Mg/21SiCW composite under biaxial stress states with variable strain rate paths were investigated in this paper. The composite was bulged in dies with aspect ratios of 1:1, 4:3, 2:1, and 8:3 at the optimum temperature of 793 K. For each die, a two-stage strain rate path was used. A stress of 5.5 MPa was first applied to the composite until it became deformed to a polar height of about 8.5 mm. A second stress of 4 MPa was then applied to a large strain or to a fracture. The strain distribution of the composite diaphragm deformed under this two-stage variable strain rate path in different dies was determined. It was found that the distribution is similar to that deformed at a constant strain rate path. The cavitation behavior and the limit strains of the composite under the two-stage strain rate path were also determined and compared with that under a constant strain rate path. The cavity growth rate parameter under the variable path was found to be slightly greater than the constant one, but smaller limit strains were observed in the sheet under the two-stage path. Using the experimental cavity growth rate parameter, the limit strains of the composite bulged at different stress ratios and the variable strain rate path were predicted using the model recently developed by the author. The trend of the prediction was in agreement with the experimental findings.