Interfacial reactions and joint performances of high-power ultrasonic welding of aluminum to steel

Aluminum and steel dissimilar sound joints were fabricated using a 4.0 kW high-power ultrasonic welder. The microstructure and mechanical properties of the joint resulting from high-power ultrasonic welding were investigated. To further understand the formation mechanism of the joint, macro and micro plastic deformation were also examined using the finite element method. As the welding duration increased to 0.4 s, the conversion of electrical power to heat flow at the interface also increased, with approximately 56% of the electrical power being transformed into the heat flow of the welding interface. The material flowed from the center of the welding interface and accumulated at the edge, resulting in a welding area measuring 9 mm × 7 mm, significantly larger than the sonotrode tip area. The plastic strain in the welding zone reached 3.7, with the aluminum side exhibiting higher plastic strain than the steel side, facilitating Fe atom diffusion towards aluminum. The intermetallic compound (IMC) primarily consisted of FeAl₃ and Fe₂Al₅. The joint strength increased significantly as the thickness of the IMC increased. However, when the IMC thickness exceeded 1.7 μm, the joint strength decreased. Through interfacial metallurgical reactions and intense mechanical interlock, joints with a lap-shear strength of 62.7 MPa were achieved. The fracture mode of the joint was a ductile-brittle hybrid fracture.