Heat treatment of laser-additive welded Ti₂AlNb joints: Microstructure and tensile properties

The microstructure evolution and tensile properties of laser-additive welded Ti₂AlNb joints under different heat treatments were investigated in this paper. The heat treatment was conducted in the B2 + O (HT1) and B2 + α₂ + O (HT2) phase field to obtain different microstructural characteristics. For HT1, due to the B2 → O transformation, the microstructure of heat affected zone was B2 + α₂ + O, B2 + residual α₂ + O, and B2 + O as the distance from the base metal increased. As for HT2, the microstructure of heat affected zone was composed of B2 + α₂ + rim-O + primary O + acicular O in the region close to the base metal, B2 + intergranular α₂ + transformed O + primary O + acicular O in the region close to the fusion zone. The fusion zone was composed of B2 + O laths after HT1, and B2 + intergranular α₂ + transformed O + primary O + acicular O after HT2. The joints composed of B2 + O phase exhibited higher tensile strength compared with the as-welded joints due to the strengthening effects of O phase. The intergranular α₂ phase formed during HT2 was detrimental for the tensile strength. The joints exhibited no plastic deformation at room temperature after both heat treatments on account of the lack of independent slip systems in the O phase. The ductility of the heat-treated joints at 650 °C was better than that at room temperature because more slip systems were activated in the O phase. Compared with the joints heat-treated in HT1, the joints after HT2 exhibited better ductility at 650 °C resulting from the coarse primary O laths and lower volume fraction of O phase.