Influence of laser power on precipitate formation and multiple step transformation kinetics in NiTi shape memory alloy weld joints

Abstract NiTi shape memory alloys are considered to be one of the fastest-growing materials with vast potential. However, the broader applications in intricately designed components are limited due to difficulties faced during welding processes such as degradation of functional and mechanical properties, precipitate formation, and selective vaporization of elements. In this study, the effect of laser power on precipitates formation and subsequent transformation temperatures on NiTi shape memory alloy was investigated by testing 0.8 kW, 1.0 kW and 1.2 kW welded specimens. The microstructures revealed the formation of Ni4Ti3 precipitates in the fusion zone and energy dispersive X-ray spectroscopy assessment exhibited preferential Ni evaporation. Differential scanning calorimetry analysis revealed the evolution of multiple step transformations (3-step / 3-step) after laser welding. Furthermore, enthalpy change in peak transformations was evaluated and increasing order of enthalpy change was recorded for weld joints with increasing laser power. The kinetics of martensite transformation was evaluated by placing emphasis on average activation energy. High laser power welded joints showed easy martensitic transformation compared to low laser power welded joints. X-ray diffraction assessment also validated the existence of Ni4Ti3 precipitates and martensitic phase in NiTi alloy after laser welding. The electron backscattered diffraction maps of welded joints revealed the grain growth in the fusion zone and heat affected zone, and the unindexed black region emphasized the formation of distorted interphase of austenite/martensite and additional martensite phase at the fusion zone/heat affected zone interface.