激光焊接快速凝固Ni51Ti49形状记忆合金显微组织和相变行为

文中研究了快速凝固Ni₅₁Ti₄₉形状记忆合金条(厚度1 mm)的激光焊接工艺，以及焊缝成形、接头显微组织演变、硬度分布和马氏体相变行为. 结果表明，激光功率和焊接速度对焊缝成形有显著影响，采用激光功率为700 W、焊接速度为8 mm/s的焊接工艺实现焊缝区完全熔透，并获得熔合面积适中、缺陷较少的高质量接头；激光焊接导致接头各区域呈现显著的组织不均匀性，其中母材区保留快速凝固工艺的细晶、强织构显微组织特征，热影响区为粗大等轴晶和柱状晶构成的混合晶组织，焊缝中心区为粗大的柱状晶. 激光焊接后热影响区和熔合区的维氏硬度相对母材有显著降低，其中熔合区的平均硬度最低，其值为311 HV ± 14 HV. 接头经500 ℃保温1 h无应力时效处理后，接头各区域组织的相变行为明显不同，其中冷却过程中母材区发生常规两步马氏体相变(B2–R–B19′)，而焊缝区呈现多步马氏体相变行为，即先发生一步B2–R相变，随后发生两个独立的R–B19′相变.

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.