Interfacial liquid control to realize superior comprehensive properties of microwelded NiTi-stainless steel joints for multifunctional biomedical device fabrication

Dissimilar joining of NiTi and stainless steel (SS) is important in biomedical applications but poses significant challenges due to brittle intermetallic compounds (IMCs) formation in the welds. Replacing harmful phases in fusion welding cannot fully eliminate brittle IMCs and may introduce toxic elements, while the mixing restriction in solid-state welding increases the process complexity and results in large plastic deformation that degrades NiTi functional properties. In this work, we present a novel methodology that achieves a solid-state joined interface in NiTi-SS fusion welding (i.e., resistance microwelding) through in-situ interfacial liquid control. By combining the advantages of both welding techniques, the current method produced NiTi-SS joints with superior strength, superelasticity and biocompatibility compared to NiTi joints or base metal. The ultrathin reaction layer at the solid-state joined interface contributed to a strong metallurgical bonding, while Joule heating effects and interfacial reactions enhanced superelasticity and biocompatibility of the joint. By demonstrating complete superelasticity on NiTi side, flexible deformation capacity on SS side, superior resistance to hydrogen embrittlement and electrochemical corrosion, and reduced Ni ion release and cytotoxicity, the welded joint shows great potential for the fabrication of multifunctional biomedical devices. Our work not only provides a comprehensive study of NiTi-SS joining under the biomedical background, but also introduces a new strategy for controlling material interface and dissimilar-metal welding process.