Phase transformation dynamics during welding of Ti–6Al–4V

In situ time-resolved x-ray diffraction (TRXRD) experiments were used to track the evolution of the α→β→L→β→α/α′ phase transformation sequence during gas tungsten arc welding of Ti–6Al–4V. Synchrotron radiation was employed for the in situ measurements in both the fusion zone (FZ) and the heat-affected zone (HAZ) of the weld, providing information about transformation rates under rapid heating and cooling conditions. The TRXRD data were coupled with the results of computational thermodynamic predictions of phase equilibria, and numerical modeling of the weld temperatures. The results show that significant superheat is required above the β transus temperature to complete the α→β transformation during weld heating, and that the amount of superheat decreases with distance from the center of the weld where the heating rates are lower. A Johnson–Mehl–Avrami phase transformation model yielded a set of kinetic parameters for the prediction of the α→β phase transformation during weld heating. Corresponding TRXRD measurements were made during weld cooling. In the HAZ, the β→α transformation during weld cooling was shown to initiate at the β transus temperature and terminate below the Ms temperature, resulting in a microstructure containing a substantial fraction of α′ martensite. In the FZ, the β→α transformation during weld cooling was shown to initiate below the Ms temperature, and to completely transform the microstructure to α′ martensite.