Microstructural evolution and numerical simulation of laser-welded Ti₂AlNb joints under different heat inputs

Abstract: The influence of heat input on the microstructural evolution of laser-welded Ti₂AlNb joints was investigated in this study. The thermal cycles during welding process were analyzed by numerical simulation. In the heat affected zone (HAZ), the amount of α₂ and O phases decreased with laser power increasing. During the heating period, α₂ → B2 and O → B2 transformations occurred, but the decomposition of the B2 phase into α₂ and O phases was suppressed during the cooling period. The heat transfer in the HAZ generated more equiaxed B2 grains, fewer LAGBs and a weaker {001}<11¯0> texture due to recovery, recrystallization and grain growth. The phase composition of the fusion zone remained single with only the B2 phase with the increase in heat input, but the mode of grain growth transformed from cellular growth into cellular dendritic growth. A finite element model was established to simulate the thermal cycles during the welding process. Higher heat input induced higher peak temperature, leading to higher temperatures in the HAZ for longer periods of time, which was beneficial for the α₂ → B2 and O → B2 transformations. The calculated cooling rates in both the HAZ and in the fusion zone were faster than the critical cooling rate for B2 → α₂ and B2 → O transformations. Graphic abstract: [Figure not available: see fulltext.]