Constitutive Modeling on the Ti-6Al-4V Alloy during Air Cooling and Application

The flow behavior of a Ti-6Al-4V alloy has been investigated and modeled, with the aim of exploring the damage mechanism and distortion of a sandwich structure during the air cooling process after superplastic forming (SPF). The selected temperature range was 930–700^◦C, and the strain rates were 10⁻², 10⁻³, and 10⁻⁴/s. An Arrhenius model was employed to describe the yield stress at a strain of 0.1, and a simple generalized reduced gradient refinement was applied to optimize the parameters for a constitutive model. The mean error between the predicted and experimental flow stress was 65% and 16% before and after parameter optimization, respectively. The effects of strain on flow stress showed a linear relationship, so a strain compensation method was proposed. The modified Arrhenius model developed in this paper provided a good agreement between the predicted stresses and the experimental data. Finally, a finite element analysis (FEA) with a “UHARD” subroutine was employed, and the results indicated that the inner plate of the sandwich structure was the most vulnerable location during the air cooling process, and that the engineering strain due to a non-uniform temperature was calculated as 0.37%.