Rheological law and constitutive model for superplastic deformation of Ti-6Al-4V

Abstract Superplastic tensile tests of Ti-6Al-4V were performed at 890, 920 and 950 °C with different strain rates to explore the superplastic flow behavior of Ti-6Al-4V alloy. A new superplastic constitutive model was proposed considering the unique hardening characteristic of Ti-6Al-4V. The mean error of the developed constitutive model is about 35% lower than visco-plastic model and 63% lower than Johnson-Cook model. Furthermore, the effect of temperature on the material constants of the developed constitutive equation was discussed. It shows that the hardening behavior of Ti-6Al-4V is strengthened by the deforming temperature, meanwhile the strain rate affects the hardening rate significantly under the condition of large strain at 950 °C. In addition, the strain rate sensitive exponent increases as forming temperature increases from 890 to 950 °C, which indicates that the Ti-6Al-4V alloy has better necking resistance at 950 °C. A superplastic forming experiment of a rectangular box was conducted and the thickness distribution of the deformed box was measured. The relative error of thickness between simulation with the developed constitutive model and experimental data is about 2.1%. Thus, it could be concluded that the established constitutive model can accurately predict the superplastic deformation behavior of Ti-6Al-4V alloy.