Nd:YAG Laser Welding of NiTi-SS Wires by Creating In-Situ High Entropy Joints

Abstract

Welding of NiTi and stainless steel (SS) wires is of great importance for device fabrication within the medical industry. Devices such as implants, surgical equipment, and even orthodontic arc-wires would benefit from cost effective and efficient welding between NiTi and SS. The production of Ni-Ti and Fe-Ti based intermetallic compounds (IMCs) in the fusion zone (FZ) makes joining extremely challenging. This study aims to eliminate IMCs in the FZ by generating an in-situ high entropy alloy (HEA) FZ of NiTi-SS wires. A custom Co-Cr-Ni powder was created to target a face-center-cubic (FCC) CoCrNiFe HEA FZ. Temporal pulse shaping was investigated by using different laser power/duration during the welding process. It was discovered that long; low-power pulses were effective at obtaining a fully FCC HEA joint. However, IMC precipitation occurred at the NiTi-FZ interface. The results observed a 60% increased tensile strength and 47 % increased strain. Ultrasonic assistance was then applied under the same conditions to mechanically alter the interfacial region. Using the acoustic streaming and cavitation effects, the thickness of the interface was drastically reduced. Instead, a Ti-rich area of the FZ, with lower hardness, served as the point of failure. As a result, the mechanical properties of the joint increased and the welds came remarkably close to the target, the super-elastic plateau of the base NiTi wire. Achieving a weld capable of the functional properties of the base NiTi has rarely been reported in the literature, especially for wire joining. A chemical route was then further developed, using a CoVNi powder. It was observed that the V was successful at migrating into previously Ti-rich regions and forming more ductile IMCs which lowered hardness. However, the V could not diffuse far enough into the interface, and the results matched those observed with the CoCrNi powder.