Microstructure and mechanical properties of dissimilar laser welding of NiTi to copper

This paper characterises the dissimilar laser welding of 0.5 mm thick plate of shape memory alloy (NiTi) and copper (Cu) at different welding speeds. Properties of laser welds are characterised by microhardness testing, tensile-shear testing, XRD analysis, differential scanning calorimetry (DSC), and optical microscopy. XRD analysis of welded zone demonstrates the presence of NiTi, Cu, and equiatomic NiTi-Cu phases due to diffusion of NiTi and Cu at the interfacial zone of NiTi-Cu joint. DSC analysis shows the start and finish temperature of austenite phase during heating and martensitic phase during cooling for NiTi and laser welded sample. Microhardness studies reveal that weld zone possesses much higher hardness than NiTi and Cu base alloy. Microhardness value at NiTi-Cu joint interface (fusion zone) increases to 280HV and 150HV because of the presence of brittle and hard IMC phases (i.e. NiTiCu). The welded joints deliver higher joint strength than Cu base alloy but lower strength than NiTi SMA. Maximum joint strength of 335MPa (strain of 13.6%) is achieved at 1400W laser power and 1 m/min welding speed due to improved mixing and material flow. Microstructures of the fusion zone have dendrites grains and show the presence of Cu and NiTi constituents in the weld region.