Effect of laser offset on microstructure and mechanical properties of Ir–Rh/Ni-based alloy

The increasingly stringent performance requirements for spark ignition engines have necessitated the use of precious metals as spark plug electrodes. However, the welding quality of these electrodes must be improved. An Ir–Rh alloy and a Ni-based material were welded by focusing a fiber laser with different offsets close to the weld centerline. Optical microscopy, scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS) were conducted to analyze the microstructure and bonding mechanism of the welding seam as well as thermal fatigue performance. The welding seam mainly consists of an Ir–Ni solid solution, with a thin intermetallic compound (IMC) layer formed at the Ni-welded metal interface. Thermal fatigue experiment (TFE) results indicated that the thermal fatigue resistance is affected by thermal stress and high-temperature oxidation, and the dominant factor changes with the change in the laser beam offset. The joint exhibited the highest breakdown force of 163.3 N when the laser offset was 0.1 mm on the Ni side. Further, fracture only occurred on the Ir–Rh fusion line area, with different fracture mechanisms being observed. The results showed that the welding quality and thermal fatigue resistance of Ir–Rh/ Ni-based alloy joint were significantly improved by using an appropriate laser offset setting, which also reduced the abnormal expiration probability of the spark plug.