Mechanical Behavior and Low-Cycle Shear Fatigue Life of the Pure Ni Laser-Welded Joints in Optoelectronics Packaging

The aim of this study is to characterize the behaviors of laser-welded joint between pure nickel (Ni200) weld clip and Kovar base metal under various mechanical loadings, i.e. shear and fatigue in shear. A Nd:YAG laser source with 1064 nm was used to weld a Ni200 piece onto a Kovar substrate by single weld spot. These samples were then subjected to shear test. While the shear fatigue cyclic tests on the four welding spots that joined Ni200 saddle shape weld clip onto the Kovar base metal. Results show that power density is the key parameter that determines the beam penetration depth and overlapping area. The pulse width has dominant effects on the weld width while the charge voltage dominant the depth of penetration and thus increases the absorbed power density. The maximum shear force requires to break a joint is ranged between 90-95, 70-75, and less than 45 N when welded by 380, 360, and 340 V, respectively. The shear strength is very much depends on the spot welding diameter and beam penetration depth. In general, higher pulse width increases the joint diameter and thus reduces the shear strength. The condition of dimples observed in the fracture surface after shear test provides important indication on the properties of the weld joints. The fatigue life for the weld formed by using the selected welding condition ranges from 1178 to 1813 cycles. The fatigue ratios obtained show that the stress below endurance limit has about 17%-23% possibility that fatigue failure will never occur for joints formed by using this range of welding conditions. The surface condition of the fracture surface after shear fatigue test provide direct indication on the crack propagation rate. Observations of all fracture surfaces by scanning electron microscope (SEM) help in understanding the deformation and damage mechanisms of Ni200 laser-welded joint at room temperature.