Revealing microstructural evolution and mechanical properties of resistance spot welded NiTi-stainless steel with Ni or Nb interlayer.

• Join NiTi and 316 L stainless steel sheets using resistance spot welding with Ni or Nb interlayer. • Uncover microstructural evolutions and mechanical properties of RSWed NiTi-steel joints. • Overcome the overall brittleness by suppressing the mixing with the help of a high-melting-point interlayer. • Provide first-hand observations on NiTi-steel RSW and new insights for dissimilar welding. Dissimilar welding of NiTi and stainless steel (SS) for multifunctional device fabrication is challenging due to the brittle nature of intermetallic compounds (IMCs) that are formed in the weld zone. In this work, Ni and Nb interlayers were applied for the resistance spot welding (RSW) of NiTi and SS to replace the harmful Fe 2 Ti phase and to restrict the mixing of dissimilar molten metals, respectively. Microstructural evolution and mechanical properties of the joints were investigated. It was shown that a conventional weld nugget was created in the absence of any interlayer in the welded joint suffering from traversed cracks due to the formation of brittle IMCs network in the fusion zone (FZ). By the addition of Ni from the interlayer, Fe 2 Ti dominated weld nugget was efficaciously replaced by Ni 3 Ti phase; however, the presence of the large pore and cracks reduced the effective joining area. The use of a Nb interlayer resulted in a fundamentally different joint, in which FZs at NiTi and SS sides separated by the unmolten Nb would suppress the mixing of dissimilar molten metals. Nb-containing eutectic structures with low brittleness formed at the interfaces, contributing to the enhancement of joint strength (increased by 38% on fracture load and 460% on energy absorption). A high-melting-point interlayer showed great potential to realize a reliable and high-performing RSWed NiTi-SS joint. [Display omitted] [ABSTRACT FROM AUTHOR]