Effect of NiTi sleeve on controlling fusion zone characteristics and improving mechanical performance of NiTi-SS dissimilar laser joints
Welding of NiTi shape memory alloys with stainless steel is challenging due to the formation of brittle intermetallic compounds (IMCs) that arise from their compositional differences and properties disparities. In the present work, a comparative study was carried out to study the effect of using NiTi sleeve on the metallurgical and mechanical properties of NiTi-SS laser welded joints. A conventional butt weld was first fabricated between NiTi wire and AISI 316L stainless steel wire of 400 µm diameter each, using pulsed Nd:YAG laser. Subsequently, a NiTi sleeve (with an outer diameter of 635 µm and an inner diameter of 417 µm) was placed over the NiTi-SS weld and double-sided laser welding was performed. Microstructural and compositional analyses of both these welds were carried out using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) respectively. The findings indicate that melting and subsequent mixing of the NiTi sleeve enriched the FZ with Ni and Ti, which helped suppressing Ni vaporization and promoted homogenization of the microstructure of the FZ. In addition, significant microstructural differences were observed due to higher heat input used during sleeve reinforced double sided welding. The fusion zone (FZ) of the initial NiTi-SS weld got partially remelted leading to microstructural grain refinement in it, which further resulted into homogenization of the microstructure, thus lowering its chemical heterogeneity. Consequently, sleeve-reinforced joints exhibited improved tensile strength and ductility as compared to conventional NiTi–SS butt welds. This work demonstrates that the use of a NiTi sleeve is an effective approach to control fusion zone composition, suppress deleterious IMC formation, and enhance the mechanical integrity of dissimilar NiTi–SS laser welded joints.
Key words: NiTi shape memory alloy, Stainless steel, Laser welding, Sleeve reinforcement, Microstructure, Mechanical properties