A finite-strain thermomechanical model for severe superplastic deformation of Ti-6Al-4V at elevated temperature

This paper presents a new finite-strain and thermomechanically-coupled constitutive model for severe superplastic deformation of Ti-6Al-4V at elevated temperature. The kinematic hypothesis of this model is a multiplicative decomposition of the deformation gradient into elastic, superplastic and thermal components, allowing the model to simultaneously take into account the large superplastic flowing deformation, the evident thermal expansion and the local temperature evolution. The model begins with the establishment of a thermodynamically consistent framework, including the Helmholtz free energy function, the entropy inequality and the heat production, from which constitutive equations are derived. The strain rate and temperature dependent material behavior is formulated by using a bilinear interpolation scheme and an exponential function. The ABAQUS/Explicit user-defined material subroutine VUMAT is employed to realize the numerical implementation of the model. The model is validated against the experimental data obtained in tensile tests on Ti-6Al-4V specimens. Finite element simulation of the superplastic gas-blow forming of Ti-6Al-4V sheet well demonstrates the model capabilities of accurately describing large superplastic deformation of Ti-6Al-4V at elevated temperature.