Effect of surface state on the quality of copper joints produced by ultrasonic welding

The quality of joints produced by ultrasonic welding is affected by a large number of parameters and factors (vibration amplitude, welding duration, clamping pressure, material properties, etc.). One of the factors is the surface state of workpieces to be joined, which significantly affects the contact area, friction coefficient and, consequently, the temperature in the contact zone. The objective of this study was to evaluate the effect of surface roughness and the presence of an oxide film on the quality of copper joints obtained by ultrasonic welding. The study revealed that the linear weld density of joints amounted 60 - 65 % after welding of plates with a surface roughness parameter Ra = 0.10 µm and 30 % after welding of plates with Ra = 0.47 µm. Preliminary fine grinding of copper plates (Ra = 0.10 µm) ensures the formation of joints with the highest strength. At the same value of the roughness parameter, Ra = 0.10 µm, the strength of joints was approximately 10 % lower and fracture energy was twice lower when welding copper plates with an oxide film as compared to the characteristics of welds of plates with a minimized oxide film thickness. An increase in the values of Ra to 0.47 μm (rough grinding) resulted in a 20 % reduction in the joint strength and a five-fold reduction in the fracture energy. The formation of a joint during ultrasonic welding was accompanied by the formation of an ultrafine-grained microstructure in the joint zone for all considered surface states of the plates.