Laser microwelding of NiTi/PtIr alloys with laser beam offset

The NiTi and PtIr alloy joint has been employed in biomedical devices to combine the superelasticity of NiTi alloy with the X-ray visibility of PtIr alloy. Laser microwelding is usually used for the joints, but there is a risk of forming brittle intermetallic compounds (e.g., Ni₃Ti, Ti₂Ni, and Ti₃Pt) in the fusion zone (FZ), which could deteriorate joint strength. In this study, laser beam offset (laser offset) was implemented for a butt joint of Ni-49.8 at.% Ti and Pt-10.0 at.% Ir alloy wires to control the intermetallic compound formation in the FZ. Welding with 300 μm laser offset on the NiTi side achieved 2.3 times higher joint breaking stress and 13.0 times higher joint breaking strain than welding without laser offset. The joint breaking stress and strain were enhanced from 221 MPa and 0.9 % to 502 MPa and 11.7 % by 300 μm laser offset on the NiTi side, respectively. In the absence of laser offset, the dissolution of Pt and Ir into the FZ facilitated the M₃Ti (M = Ni, Pt, Ir) formation in the FZ, resulting in crack propagation within the M₃Ti. In contrast, the 300 μm offset on the NiTi side inhibited the M₃Ti formation by mitigating Pt and Ir dissolution into the FZ. Laser offset on the NiTi side can be an attractive option to enhance the strength and ductility of NiTi and PtIr butt joints.