Biaxial stress state deformation behavior and constitutive modeling of Ti65 sheets under superplastic forming conditions

To precisely elucidate the deformation characteristics of Ti65 sheets under uniaxial and biaxial stress states during the superplastic forming (SPF) process, a series of uniaxial hot tensile and biaxial bulging tests were conducted to explore the superplastic deformation behavior within a temperature range of 900–960 °C and a strain rate range of 0.001–0.03 s⁻1. The deformation behavior and uniform strain under uniaxial stress states were characterized through the DIC real-time strain measurement system. In addition, the key microstructures evolutions during different stress states were characterized and analyzed to determine the deformation mechanisms. Based on the test results, the constitutive model for both uniaxial and biaxial behavior of Ti65 sheets was developed and calibrated. The results of this research indicated that Ti65 exhibited superplastic deformation at 940 °C—0.0014 s⁻1 and 960 °C—0.0075 s⁻1, which led to an enlargement of its forming limit. Simultaneously, the evolution mechanism of the microstructure under biaxial stress was revealed. As the temperature increased, the proportion of high angle grain boundaries rose, the grain size decreased, and the forming limit increased accordingly. The established constitutive model, which takes into account the evolution of the microstructure, successfully captured the forming limit points. The accuracy of the predicted uniaxial stress–strain and the bulging grain size reached 91.2% and 96.24%, respectively. This research provides theoretical guidance for the selection of the process window of titanium alloys.