Development of unified constitutive model for hot deformation behavior of TC4 with and without diffusion bonding

The hot deformation behavior of TC4 with and without diffusion bonding (DB) is studied. Firstly, the hot tensile testes of TC4 specimens with and without DB were carried out at temperature of 750℃ and strain rate of 0.1 ~ 0.0001 s-1. It was found that the peak stress and fracture strain of DB specimens are lower than those of base metal (BM) specimens. The quasi-cleavage fracture appeared in the DB specimens at 750℃, and the mechanism could be explained as straight grain boundary formed by incomplete recrystallization and insufficient grain boundary migration energy and the joints weak left over from the DB process causing rapid voids nucleation at the DB interface. The shorter distance of voids coalescence within the interface leads to quasi-cleavage fracture. In addition, a unified constitutive model based on internal variables was developed by introducing the weld-dependent fracture coefficients and damage tolerance coefficient. The model can accurately predict the hot deformation behavior of TC4 with and without diffusion bonding, describing the influence behavior of DB area on hot deformation, as well as the evolutions of internal variables, including dislocation density, reserve fraction, dynamic recrystallization (DRX) fraction and damage. It provides a theoretical basis for further accurate simulation of the hot forming of titanium alloy with diffusion bonding.