Effects of Melt-to-Solid Insert Volume Ratio on the Microstructures and Mechanical Properties of Al/Mg Bimetallic Castings Produced by Lost Foam Casting

Abstract

A356 aluminum and AZ91D magnesium alloys, which act as a solid insert and a melt, respectively, were used to prepare Al/Mg bimetallic castings using the lost-foam casting process, and the effects of the melt-to-solid volume ratio (VR) on the microstructures, mechanical properties, and fractographies of the Al/Mg bimetallic castings were investigated in this paper. Obtained results show that the average thickness of the reaction layer between aluminum and magnesium significantly increased with increasing VR, and a compact and uniform interface was obtained with a VR of 14.6. The reaction layers of all the bimetallic castings obtained by different VRs mainly consisted of the Al12Mg17 + δ eutectic layer close to the magnesium matrix, the Al12Mg17 + Mg2Si intermediate layer as well as the Al3Mg2 + Mg2Si layer next to the aluminum base. The microhardnesses of reaction layers of all the bimetallic castings with different VRs were considerably higher than those of the magnesium and aluminum matrix alloys, particularly the Al3Mg2 layer. Excessive thick reaction layer and pore defects remarkably weakened the bonding strength of the bimetallic castings, especially pore defects, and a relative maximum shear strength was obtained with a VR of 14.6. The fractographs of the push-out samples showed a change in the fracture surface from a mixed brittle and ductile fracture nature to that of a brittle fracture nature with the increase of VR.