Abstract: In this work, the A356/AZ91D bimetal composites were prepared by ultrasonic vibration-assisted lost foam compound casting, and the effects of ultrasonic powers on interfacial microstructures and mechanical properties of the Al/Mg interfaces were investigated. Results revealed that the Al/Mg bimetal composites without ultrasonic vibration treatment (UVT) were heterogeneous, and the Al/Mg interface was composed of Al-Mg intermetallic compounds (IMCs, i.e., Al₃Mg₂ and Al₁₂Mg₁₇) area and Al-Mg eutectic structures (δ-Mg+Al₁₂Mg₁₇) area. The Mg₂Si particles were gathered at the IMCs area and an oxide film that mainly composed of Al₂O₃ was existed between the IMCs area and eutectic structures area. With UVT, the oxide film was eliminated and the gathered Mg₂Si particles were refined and dispersed by the acoustic cavitation effect, and part of the Al₃Mg₂ and Al-Mg eutectic structures were transformed into the Al₁₂Mg₁₇ due to the promoted solute interdiffusion, which improved the homogeneity of the Al/Mg interfaces. Besides, the grains of the Al/Mg interface with UVT under ultrasonic power of 75 W were significantly refined. The thickness of Al/Mg interface was increased with the increase of the ultrasonic power. Due to the excessive heat induced by UVT under the further increased ultrasonic power, the cooling rates and the degree of supercooling were reduced, resulting in the coarsening of interfacial grains. The microhardness of the Al/Mg interfaces was increased and got more uniform by UVT. The shear strengths of the Al/Mg bimetal composites with UVT were enhanced to 61.4 MPa from 32.4 MPa, with an increase of 89.5 % compared with that of the Al/Mg bimetal without UVT. This could be ascribed to the removal of the oxide film, the refinement of the interfacial grains and the dispersed and refined Mg₂Si particles achieved by UVT, which hindered the crack propagation during deformation.

Key words: Ultrasonic vibration treatment, Al/Mg bimetal composites, Compound casting, Interfacial microstructure, Mechanical properties