Microstructure and phase transformation behavior of rapidly solidified Ni₅₁Ti₄₉ shape memory alloy joint by laser beam welding

In this paper, the laser beam welding (LBW) process of rapidly solidified (RS) Ni₅₁Ti₄₉ shape memory alloy strips with a thickness of 1 mm was investigated. The formation of weld, microstructure evolution, hardness distribution and martensitic transformation behavior of the as-fabricated RS Ni₅₁Ti₄₉ shape memory alloy LBW joint were also studied. The results show that the LBW process parameters, such as laser power and welding speed, have a significant influence on the formation of weld. With a laser power of 700 W and welding speed of 8 mm/s, the weld zone is completely penetrated and a high-quality joint with moderate fusion area and less defects can be obtained. After LBW, the NiTi alloy joint exhibite an obvious microstructural inhomogeneity in different regions of the joint. Base metal (BM) remain a fine-grain and strong-texture microstructure, which is initially formed during rapid solidification process. Mixed microstructure of coarse equiaxed grains and columnar grains appear in heat-affected zone (HAZ). The central area of fusion zone (FZ) is consisted of coarse columnar grains. Hardness of HAZ and FZ are significantly lower than that of BM. In particular FZ shows the lowest average hardness, which is 311 HV ± 14 HV. Undergoing a stress-free aging treatment at 500 ℃ for 1 h, different martensitic transformation behaviors are detected in different regions of the joint. BM shows a conventional two-step martensitic transformation behavior (i.e., B2–R–B19′) on cooling, while FZ exhibits a multi-step martensitic transformation behavior, namely one-step B2–R transformation followed by two individual R–B19′ transformations.