Strength of joints produced by ultrasonic spot welding of copper plates using tools with different tooth heights

Ultrasonic welding of metals is an energy-efficient, environmentally friendly technology that allows producing solid-state joints between thin blanks. The widespread use of this technology is hampered by the low strength of the resulting joints and the instability of their properties. One of the ways to improve strength characteristics is to develop a welding tool that ensures stable transmission of ultrasonic vibration energy to the joint zone. For this purpose, a relief with teeth or pyramids of different shapes and heights is applied to the surface of the welding tip and anvil. This paper presents data on the fracture load and fracture energy of lap joints produced by ultrasonic spot welding of copper plates using tools with a tooth height of 0.1 and 0.4 mm. Ultrasonic welding was carried out with a frequency of 20 kHz and a vibration amplitude of 18–20 μm, the welding duration was 2 and 3 s, the clamping force was 2.5 kN. The paper considers the features of the fracture of the produced joints and the distribution of normal strains in the weld spot, and results of calculation of stress intensity factors in its vicinity. It is shown that after ultrasonic welding for 3 s, the strength characteristics of the joints produced with different tools reach the highest values, they are close in magnitude, but the experimental data scatter is half as much after welding with a tool with small teeth. The joints produced with such a tool fractured along the interface of the joint, and after welding with a tool with large teeth, the fracture developed with nugget pull-out, which is explained by an increase in the stress intensity factor at the tip of the concentrator surrounding the weld spot.