Comparative study of a high-strain-rate superplastic Al-4.4Cu-1.5Mg/21SiCW sheet under uniaxial and equibiaxial tension

In this paper, the strain rate sensitivity exponent and the cavitation behavior of a high-strain-rate superplastic Al-4.4Cu-1.5Mg/21SiCW sheet were examined under uniaxial and equibiaxial tension at the optimal temperature of 793 K. A maximum elongation of about 335% was obtained at the initial strain rate of 0.17 s−1. It was found that the strain rate sensitivity of the composite under uniaxial and equibiaxial tension was 0.34 and 0.36, respectively. The reasons for a large discrepancy between uniaxial and equibiaxial stress versus strain rate curves were analyzed. It was concluded that the flow stress–strain rate relationship determined from uniaxial tension could be used for forming processes subject to complex stress states after proper adjustment. The amount of cavity was found to increase with increasing strain and the amount of cavity obtained under equibiaxial tension was slightly larger than that under uniaxial tension at the same effective strain rate. A slightly higher cavity growth rate parameter was also observed under equibiaxial tension. The experimental results revealed that the cavity growth was essentially plastic-controlled. It was suggested that the presence of liquid phase was beneficial to nucleation at the interfaces and limited the cavity growth rate.