Effect of Ultrasonic Welding Energy on the Forming of Copper/Aluminum Conductor Joints

The cross-sectional morphology of the joint of copper/aluminum conductors under different ultrasonic welding energies is shown in the figure.

It can be seen from the figure that as the welding energy increases, the cross-sectional thickness of the joint gradually decreases.

When the welding energy is 200 J, the cross-sectional thickness of the joint is 2.77 mm. The top of the aluminum wire is flattened by the tip of the welding head. Under the action of ultrasonic softening, the aluminum wire is severely plastically deformed and flows around the copper wire. A small amount of aluminum is embedded in the gap of the copper wire to form a mechanical fit with the copper wire and is easily peeled off at the bonding interface when stretched.

When the welding energy is 300 J, the aluminum wire is further compressed, and the cross-sectional thickness of the joint is about 2.12 mm, which is about 23.5% less than that of 200 J. However, a larger interlocking area is formed between copper and aluminum, and the interface bonding force between copper and aluminum increases, which helps to improve the tensile strength of the joint. In addition, the gap between copper wires is further reduced, and atomic bonding is formed between some copper wires, which also helps to increase the tensile strength of the joint. Due to the severe plastic deformation of the aluminum wire, the effective bearing area is reduced. When the copper/aluminum interface bonding force is greater than the tensile strength of the aluminum wire, the fracture occurs on the aluminum side.

When the welding energy is 400 J, the joint section thickness is about 1.82 mm, which is 14.1% less than that of 300 J welding energy. Under the action of ultrasonic vibration, the aluminum wire undergoes plastic deformation along the surface of the copper wire, filling the gaps between the copper wires to form a tighter bond, and the interfacial bonding force is further increased.

Conclusions

(1) As the ultrasonic welding energy increases, the interface temperature of the copper/aluminum wire increases, but it is still significantly lower than the melting point of the copper/aluminum wire, which belongs to the solid phase connection; at the same time, the mutual diffusion distance of Cu/Al atoms at the interface increases with the increase of ultrasonic welding energy, and no intermetallic compound is formed.

(2) The tensile strength of the joint increases first and then decreases with the increase of ultrasonic welding energy. When the welding energy is 300 J, the joint obtains the peak tensile load, the tensile shear strength of the joint exceeds the parent material, and the fracture occurs on the aluminum side.

(3) When the ultrasonic welding energy is low, the tensile strength of the joint depends on the metallurgical bonding area of ​​the copper/aluminum wire. When the ultrasonic welding energy exceeds 300 J, the tensile strength of the joint depends on the effective bearing area of ​​the aluminum side.