Microstructure and damage based constitutive modelling of hot deformation of titanium alloys

Development of a hybrid constitutive model for modelling of the flow behavior, microstructure evolution, damage initiation and fracture formation is crucial in study of hot deformation of titanium alloys. In tandem with this, a microstructure and damage based constitutive model for modelling of hot working of Ti–6Al–2Zr–1Mo–1V (TA15) alloy was developed. The hot tension deformation of the alloy was first conducted to analyze the microstructure evolution, damage and flow behaviors. It was found that increasing β phase fraction and dynamic recrystallization (DRX) fraction suppress the damage initiation and propagation in the way of void nucleation, growth and coalescence, and thus foster the increase of fracture strain in an exponential form. Based on the experimental results, the microstructure evolution, including phase transformation and DRX occurrence, was modelled by using the method represented by physically-based internal state variables. The damage and fracture behaviors were modelled by considering the effects of microstructure and stress state via introducing β phase fraction, DRX fraction and stress triaxiality into the classical Gurson-Tvergaard-Needleman damage model. The constitutive law considering both the microstructure and damage evolution was then given and further the microstructure and damage based constitutive model was established. The parameters in the model were calibrated by comparing the predicted and experimental results. Finally, the developed model was successfully applied in finite element simulation of hot spinning of TA15 alloy tube for unified prediction of macroscopic deformation, microstructure, damage and fracture. The research thus provides a basis for tailoring and control of microstructure and damage in hot working of titanium alloys.