Material modelling data for superplastic forming optimisation

Optimisation of the superplastic forming process involves the determination of the minimum forming time within the constraints of product characteristics such as thickness distribution and cavitation level. However, superplastic forming is a non-linear system, with the material behaviour giving a significant contribution to that non-linearity. For example, dynamic grain growth gives useful strain hardening but can ultimately reduce strain rate sensitivity and, conversely, forming with a decreasing strain rate can enhance ductility. The results of tests on two commercial superplastic alloys, AA5083 and AA7475, have been used to provide data for material modelling. Simple power law descriptions of the mechanical behaviour were not adequate, and a basis for constitutive relationships for these materials is discussed.