HEMISPHERE FREE-BULGING BEHAVIOR OF SUPERPLASTIC 8090 Al-Li SHEETS DURING THE LOW-STRAIN REGIME

Hemisphere free bulging of a superplastic 8090 Al-Li sheet has been conducted, emphasizing the superplastic behavior over the low strain regime ε=0-0.7. The constant-pressure condition for hemisphere free bulging of the dynamically recrystallized sheet is not only easier for industry operation but also a more favorable route compared with the constant-strain-rate condition. The consideration of initial work hardening contribution (n-value) into the constitutive equation seemed to offer better simulated results. A multiple-strain-rate bulging path, including a high initial rate followed by several lower strain rates, will be able to fully utilize the higher values of m and n during each straining stage. The thickness uniformity as well as the total forming time can be improved and shorten. The material superplasticity characteristics extracted from uniaxial tensile tests were seen to be reliably applicable to the equibiaxial diaphragm bulging.