Finite element investigations for the role of transformation plasticity on springback in hot press forming process

The phase transformation plasticity model was used to examine the springback of a hot press formed sheet. Plane stress formulations for the shell element were provided to predict the transformation kinetics and thermo-mechanical behavior during mechanical forming at high temperatures and subsequent cooling. Parametric analyses were performed to examine the combined effects of the transformation plasticity, the volumetric strain and cooling rate on the springback. The simple idealized boundary value problems with two different loading conditions were chosen: loading with a constant force and constant displacement. For comparison, conventional finite element analysis using the flow stress at the forming temperature was also carried out. The results show that the transformation plasticity plays a significant role in reducing the level of springback during hot press forming. Considerable amounts of springback were predicted when only conventional plasticity with a lower yield stress at high temperature and volumetric strain due to the phase difference were used, which contradicts the typically reported experimental observations. It was also shown that competition between stress relaxation by the abnormal transformation plasticity and the increase in stress due to emergence of harder phases dominates under constant loading conditions, whereas stress relaxation is dominant under constant displacement boundary conditions.