Enhancing microstructural and mechanical characteristics of laser welded NiTi SMA/304 SS lap joints with medium and high entropy alloy fillers

The demand for high-quality nickel-titanium (NiTi) shape memory alloy (SMA) and stainless steel (SS) welded structures has led to significant challenges in fusion welding technology, which is largely influenced by filler materials. This study explores the application of CoCrNi medium entropy alloy and CoCrNiFe high entropy alloy as filler materials for laser lap welding of NiTi SMA and 304 SS. A self-fusing joint was produced for comparison. The results demonstrate that the interface zone on the NiTi side was the weakest area regarding cracking defects in NiTi/304 SS lap joints. However, using CoCrNi and CoCrNiFe fillers effectively eliminated cracking defects by considerably suppressing the formation of brittle FeTi and Fe₂Ti intermetallic compounds (IMCs). The average microhardness value of the weld zone without filler was higher than with CoCrNi and CoCrNiFe. Furthermore, the addition of CoCrNi and CoCrNiFe fillers to NiTi/304 SS lap joints resulted in a substantial increase in tensile properties, with tensile strength reaching 196 ± 39 MPa without filler, 319 ± 25 MPa with CoCrNi, and 377 ± 33 MPa with CoCrNiFe, respectively.