Investigation on laser assisted welding of NiTi shape memory alloy

Abstract

NiTi shape memory alloy (SMA) is widely applied in many industrial domains, such as biomedical, aerospace, automotive and power plants, due to its outstanding functionality including superelasticity (SE) and shape memory effect (SME). The machining process of this material is really challenging with a lot of barriers. Accordingly, joining techniques can be an alternative approach to design shape memory components with more flexibility. Among all methods, laser welding process is a reliable and economic technique for joining of NiTi alloys. However, thermal process influences strongly on the strength and functionality of the NiTi welded joints in the Heat Affected Zone (HAZ) and the Fusion Zone (FZ). Indeed, the transformation temperature of NiTi alloy can be altered due to varying in the material composition. Therefore, controlling the operational parameters, including laser power and scan speed lead to effectively improve the mechanical and the functional behaviour of NiTi joints. It consequently enhances the weldability of this material. This Thesis shows the experiments performed on the laser welding of similar NiTi-NiTi and dissimilar NiTi-Cu shape memory sheets of dimension 30 mm x 30 mm x .5 mm and reports the effect of laser parameters on microstructural, functionality, and mechanical properties of the welded joints. The first part of this study employed a numerical model to estimate the optimum laser parameters, including laser power and scan speed, which can reduce the Heat Affected Zone (HAZ) and the Fusion Zone (FZ) and therefore result in a better weld. The simulation results, including the optimised temperature, and the dimension of HAZ and FZ, were analysed. The second part of this work addressed fibre laser butt welding of similar NiTi-NiTi alloy. Here, the weld quality was evaluated in terms of the weld geometry, microstructures, hardness distributions and the formation of intermetallic phases. Results showed that the welding speed of the laser beam was an important factor affecting the weld quality. The third part of this work focused on fibre laser lap welding of dissimilar NiTi-Cu alloy with different laser welding speed. The mechanical properties, metallurgical effects and performances of welds were analysed.