Time-dependent springback prediction with stress relaxation effect for non-isothermal hot stamping of titanium alloy sheets

The hot stamping of titanium alloy sheets is a non-isothermal forming process due to the interfacial heat transfer while hot blank is formed and cooled in cold forming tools. Due to the condition of high temperature, the material experiences a thermal-mechanical coupled deformation and a time-dependent stress relaxation subsequently. The control on shape accuracy of hot-stamped titanium alloy parts is of great challenge owing to the complicated stress evolution mechanism. The accurate prediction of springback requires a comprehensive description on the temperature and strain rate sensitive deformation, and also on the stress relaxation during die quenching. The precise testing methods and constitutive models are crucial for accurate simulation. In this work, tensile tests with different temperature and strain rates, and stress relaxation tests with different temperature and initial stress levels are performed in Gleeble system. An integrated physically based constitutive model is developed to describe both the thermo-mechanical behavior and time-dependent stress relaxation in hot stamping process. Furthermore, the proposed model is implemented in the hot stamping simulations of U-shape part through user subroutines. The essential effect of stress relaxation on stress evolutions of U-shape part is analyzed. The simulation results show promising agreement with experiments when stress relaxation is taken into account. Finally, based on the principle of increasing temperature to enhance stress relaxation, increasing the forming speed is found to be an effective way to reduce the springback of titanium alloy in hot stamping.