Microstructure, Macrosegregation, and Mechanical Properties of NiTi to Ti6Al4V Dissimilar Laser Welds Using Co Interlayer

Joining alloys exhibiting individual superior properties could yield major benefits in terms of design and production flexibility. Nevertheless, differences in thermophysical properties between the materials to be joined complicate the formation of dissimilar combinations, thus limiting the potential applications of multiple engineering alloys. The formation of brittle Ti₂Ni intermetallic (IMC) in the fusion zone (FZ) is the main challenge in joining NiTi to Ti6Al4V without an interlayer. Hence, the composition of the FZ needs to be changed to ideally suppress brittle IMC phases to form and decrease the likelihood of crack formation upon solidification. In this study, two strategies were used concurrently to reduce brittle Ti₂Ni intermetallic compound: a cobalt interlayer was introduced in a butt joint configuration, and the laser was offset to the Ti6Al4V side. The use of a Co interlayer resulted in a joint free of brittle interaction layer susceptible to microcracks at the NiTi boundary by reducing the amount of brittle Ti₂Ni intermetallic compound. A joint with a lower hardness of 438HV was attained, compared to 515HV for the conventional NiTi/Ti6Al4V joint. The maximum strength and fracture strain of the Co-interlayered joint were improved to 285MPa and 1.67%, respectively, compared to 148MPa and 0.8% for the Co-free joint.