Laser welding-brazing of NiTi/304 stainless steel wires with beam defocus and large offset

The brittleness of dissimilar NiTi-stainless steel (SS) welds caused by intermetallic compounds (IMCs) network limits their application in medical field. Inserting interlayer and special assembly are high-cost but still hard to eradicate the formation of IMCs. This work realized NiTi-SS wire joints with a high strength using laser welding-brazing mode through beam defocus and large offset. The effect of laser beam offset on joint appearance, microstructure, tensile strength and fractography was investigated. In welded-brazed joints, a fusion zone (FZ) of only one base metal formed while multilayered reaction microstructure at the brazed interface was observed. Without beam offset, a typical FZ was dominated by Ni3Ti+Fe2Ti IMCs network. The tensile strength was improved from 176.6 MPa of conventional welding mode without offset to 329.5 MPa of welding-brazing mode with 0.4-mm offset to SS side. The joint welded with large offset fractured at the brazed interface with a mixed ductile-brittle fracture; while brittle fracture inside the IMCs network of FZ was observed in the joint welded without beam offset. The enhancement of joint strength was attributed to the eradication of IMCs network in the FZ and a reaction layer with less Ti–Fe brittle IMCs. This welding-brazing mode can be applied to suppress the overall brittleness of NiTi-SS joints without using any interlayer.