Effect of Laser Positioning on the Microstructure and Properties of NiTi-Copper Dissimilar Laser Welds

Dissimilar pulsed laser welding of 400-μm NiTi and Cu wires was used to investigate the effect of laser positioning on NiTi-Cu joint performance. Laser positioning was found to have a significant role in the microstructure due to the difference in mixing patterns and composition distributions. Homogenous element distribution was observed in both the NiTi offset and the centerline joints, while complex mixing patterns were observed in the Cu offset joints. Changing the laser position resulted in a change in alloy content in the weld pool that affected the mixing patterns, and the phases that were formed in the weld zone of each sample. By moving the laser beam from NiTi toward Cu, NiTi and NiTiCu in the weld zone were replaced by pure Cu which resulted in a decreased average hardness from ~ 512 to ~ 158 HV. However, high hardness values were found inside the weld zone of Cu offset welds due to the presence of hard and brittle ternary intermetallic compounds. The 100-μm Cu offset joint displayed the worst mechanical response due to its inhomogeneous microstructure and the presence of cracks and pores. It was shown that positioning the laser beam on the NiTi or on the centerline resulted in a better transition in microstructure that led to acceptable mechanical properties without optimizing laser parameters or inserting any interlayers.