Development of Al/Mg bimetal prepared by ultrasonic vibration-assisted compound casting: Effects of interface treatment temperatures

In this work, the A356/AZ91D bimetals were fabricated by ultrasonic vibration-assisted solid-liquid compound lost foam casting. The ultrasonic vibration treatment (UVT) was introduced to the Al/Mg interfaces, and the effects of interface treatment temperatures on the formation, microstructures and mechanical properties of the Al/Mg interfaces were investigated. The results revealed that the Al/Mg interface without UVT was composed of intermetallic compounds area (Al3Mg2 + Al12Mg17 + gathered Mg2Si particles) and eutectic area (Al12Mg17 + delta-Mg). With UVT, phase compositions were not changed, but the distributions were got more homogeneous. The Mg2Si particles at the Al/Mg interfaces were refined and formed at eutectic area. Besides, the thickness of the Al/ Mg interfaces were increased. When the UVT performed to the Al/Mg interface of above liquidus temperature of liquid Mg alloy melt, the solid insert was eroded by ultrasonic cavitation, cracks appeared between the Al/Mg interface and AZ91D matrix, which declined the shear strength of Al/Mg interface. When the UVT performed to the Al/Mg interface of temperature close to the formation temperature of it, the Al/Mg interfacial grains were refined and the Al-Mg eutectic structures were formed at intermetallic compounds area, which was not observed in the case of UVT performed to Al/Mg interface of elevated temperatures. The microhardness of the Al/Mg interfaces were increased and became more uniform by UVT, due to the refinement of Mg2Si particles and more homogeneous microstructure. The shear strengths of the Al/Mg bimetals with UVT were increased to 53.9 MPa from 35.5 MPa, with an increase of 51.8%. The fracture morphologies of the Al/Mg interfaces showed typical brittle fracture characteristics. However, the main cracks were hindered and deflected by dispersed Mg2Si particles and transformed to crack branches during deformation and thus enhanced the shear strengths of the Al/Mg bimetal.