Constitutive equation for superplastic Ti–6Al–4V alloy

Superplasticity is the capability of some materials to exhibit large plastic deformations prior to failure. Structural superplasticity is observed in fine-grained alloys (the average grain size does not exceed 10 μm) under proper conditions of:
• high temperature (greater than about one-half the absolute melting point);
• a controlled strain-rate (within the strain-rate range 10−4 to 10−2 s−1).Commercial applications of superplastic forming are restricted to aluminium and titanium alloys. In the aircraft and automotive industries, superplastic forming shows promise as a main approach for producing light, complex-shaped parts.This paper describes a method to determine the material constants of superplastic alloys from a free forming test at constant pressure. In the finite element simulation the constitutive equation based on power law with hardening variables containing significant physical elements is chosen to fit the true stress, true strain and true strain-rate obtained from experimental data for Ti–6Al–4V at 1200 K. The finite element simulation of the free forming process is used to examine the validity of the suggested method.