Abstract: The hot stamping of titanium alloy sheets is a non-isothermal forming process while hot blank is formed and cooled in cold forming tools. The stress evolution is complex that involves not only the thermal-mechanical coupled deformation during forming but also the time-dependent stress relaxation during die quenching, which affects the final springback. To predict the stress relaxation accurately, it is critical to establish a universal stress relaxation model of titanium alloy under hot stamping conditions. Based on the GLEEBLE system, stress relaxation tests at temperature of 550-850℃ and different initial stress levels are conducted. The short-term stress relaxation of TA15 alloy is rapid at elevated temperature. The initial stress levels mainly affect the stress relaxation rate while the temperature affects the amount of stress release. Based on the stress relaxation mechanism of titanium alloy, which mainly include the dislocation climbing and sub-grain boundary migration, a macro-micro coupled model taken the dislocation density as internal variable is established. The model parameters are identified based on experimental results. Good agreement is achieved between the experimental values and the predicted ones, which indicates the proposed model is capable of describing stress relaxation behaviors of TA15 alloy comprehensively under complex conditions.

Key words: titanium alloy, hot stamping, stress relaxation, constitutive model