A State Variable Material Model for Superplastic Titanium-6Al-4V

Analysis of superplastic forming requires that the material behavior be accurately quantified. A model has been developed which captures the changing strain rate sensitivity occurring between superplastic regions II and III and which accounts for strain path dependence of the flow stress. Isothermal compression tests have been conducted for strain rates of 10^-5 to 10^-3 s^-1 and temperatures of 875 to 950 °C for nine different equiaxed microstructural states. Tension and compression tests were found to give the same stress response. Static and deformation enhanced grain growth have been measured and their relation to strain hardening discussed. The application of superplastic material behavior to the production of complex parts requires that pressure cycles, temperatures, and other processing conditions be designed to minimize tensile localization. Characterization of the material behavior can be divided into two parts: the kinetics of flow at constant microstructural state and the kinetics of the evolution of state, with the choice of proper state variables being crucial for accurate modeling.