Multi-stage martensite transformation behavior in strongly-textured NiTi alloy joint via vacuum laser beam welding

NiTi shape memory alloys have widespread applications in biomedical and aerospace fields. In this study, rapidly-solidified Ni₅₁Ti₄₉ alloy were welded using vacuum laser beam welding (LBW). The resulting microstructure and phase transformation behaviors were systematically characterized. The results show that LBW led to the formation of coarse columnar grains in the fusion zone (FZ) and fine equiaxial grains in the base metal (BM), both exhibiting strong <001>B₂ λ-fiber texture. The hardness distribution revealed a precipitation strengthening effect in the heat-affected zone (HAZ), where a higher hardness (334.6 ± 18.5 HV) compared to the FZ and BM. Multi-stage martensitic transformation (MMT) was observed in the aged FZ, while the aged BM exhibited a normal two-stage martensitic transformation. Additionally, preferential variant selection of Ni₄Ti₃ precipitation occurred in the FZ, whereas the BM showed more homogeneous precipitation. Finite element simulations highlighted the significant role of high tensile residual stresses in the FZ in promoting MMT and facilitating variant selection of Ni₄Ti₃ precipitates. These findings provide valuable insights into the effects of LBW on the microstructure and functional properties of NiTi alloy joints.