Effect of deformation schedule on cavitation in superplastic 7475 Al alloy

Cavitation is a major concern in parts fabricated by the superplastic forming process, partly because of the damage produced within the parts and also because of the possibility of flow localization and fracture during the forming operation. The deformation schedule to minimize forming time is also important. Cavitation, however, usually increases with increasing strain and strain rate for superplastic metals. Thus, it is important to investigate the effect of strain-rate-steps during superplastic forming on cavitation and elongation to determine optimum deformation schedules. Constant strain-rate tensile tests were conducted at a temperature of 788 K in a strain-rate range from 8x10(-5) s(-1) to 2x10(-3) s(-1) for a commercial 7475 aluminum alloy. Three deformation schedules were selected, in which the strain rate was decreased step by step at predetermined strains depending on the plastic instability parameter of I=0, 1 and 2. The value of I was calculated according to the Hart theory (I=(1-m-gamma)/m), where gamma is a strain-hardening parameter and m is a strain-rate sensitivity. Variations in cavity volume fraction as a function of true strain were examined for the each deformation schedule.