Study on weld formation and segregation mechanism for dissimilar pulse laser welding of NiTi and Cu wires

In this study, laser microwelding was adopted to join NiTi and Cu wires using different laser pulse duration. The cross section of welded samples showed no visible defect, but some intermetallic compounds were formed inside the fusion zone. An excellent metallurgic bonding was achieved during laser welding for NiTi and Cu wires with increasing pulse duration. Local element segregation like peninsulas and beaches was observed in the fusion zone for the pulse duration of 5 and 8 ms, but the segregation was absent with the pulse duration increased to 15 ms. The liquidus temperatures of weld metals in molten pool had a significant effect on the segregation formation. If TLW < TLNiTi/TLCu, a NiTi/Cu peninsula can form near the fusion boundary of NiTi/Cu side, while a NiTi/Cu beach can form if TLW > TLNiTi/TLCu. With increasing pulse duration, the fraction of Ni-Ti brittle intermetallic compounds in the fusion zone decreased, but Cu-Ti intermetallic compounds with lower hardness increased. Little hardness variation was achieved and peak hardness values decreased (~630 → ~560 → ~490 HV). The joint with the pulse duration of 5 ms displayed poor mechanical properties due to uncompleted penetration. Fracture occurred in Cu base metal for the pulse duration of 8 ms and 15 ms. This study showed that it was potential to diminish the brittle intermetallic compounds and improve the mechanical properties of fusion zone by adjusting the laser pulse duration for the dissimilar laser welding of NiTi and Cu wires.