Blow forming of superplastic 7475 Al Alloy sheets

Abstract

The 7475 aluminum alloy sheets were subjected to a thermornechanical treatment to have superplastic characteristics. The superplastic specimens were tested in an Instron testing machine at constant cross head speeds to obtain a constitutive equation for an optimum superplasticity. The constitutive equation which offers the reference data for a constant-apex-strain-rate blow forming was established at 516℃ in the form of 0=K·ε˙^m. The equation was applied to the analyses of blow forming into cylinderical and conical dies. The theoretical models mainly followed Ragab`s analytic model about a constant-pressure blow forming but were modified into the models relevant to a constant-apex-strain-rate blow forming. A number of experiment have been conducted, involving the blow forming of superplastic 7475 Al alloy sheets into cylinderical and conical dies at 516℃ with the apex-strain-rate, 1.15 × 10^(-3)/sec. The theoretical results and experiments were compared and the problems such as cavity occurrence and dynamic grain growth were discussed.