Local Plastic Deformation and Quality of Cu-Cu Joints Obtained by Ultrasonic Welding

Joints of copper sheets with a thickness of 0.8 mm were produced by ultrasonic welding. To assess the quality of the joints, tensile lap-shear strength, area fraction of bonding, distributions of normal strains in the cross sections of welded samples, linear weld density at a magnification of x1000, and the microstructure and microhardness of welded samples were analyzed. It was proved that the arrangement of microbonds and length of gaps in joint zones significantly depended on the local normal strains of welded samples caused by the penetration of tool ridges under the clamping pressure. Joint regions with a linear weld density of more than 70% were observed if the local compression strains of the sample exceeded 15%. The appearance of local tensile strains was accompanied by a drop in the linear weld density of the joints in some regions, down to 5%. The distribution of normal strains depends on the mutual positions of the ridges of the welding tip and anvil. It is concluded that in order to improve the quality of joints obtained by ultrasonic welding and reduce the scatter of their strength values, welding tools should provide sufficiently high normal compression strains in the weld spot area.