Magnetic field assisted lost foam compound casting of Al/Mg bimetal: Interfacial microstructure control, strengthening mechanisms and fracture behavior

The objective of this study is to propose a novel magnetic field assisted lost foam compound casting method that simultaneously improve the interfacial and matrix microstructures of Al/Mg bimetals, and to explore the optimal electromagnetic stirring (EMS) treatment duration. The effects of the EMS duration on the interfacial microstructure and mechanical properties of the Al/Mg bimetals were investigated. The Al/Mg interface consisted of the intermetallic compounds (IMCs) area and the eutectic (E) area. The oxide film defect existed in the Al/Mg interface without EMS, which hindered the solute diffusion, thereby resulting in non-uniform distribution of Mg 2 Si phase and the formation of Al 12 Mg 17 dendrites. The EMS initially promoted the dispersion and refinement of Mg 2 Si particles, leading to their wider distribution at the interface. However, excessive EMS duration caused Mg 2 Si particles to agglomerate and coarsen. When the EMS duration reached 8 min, the oxide film was broken. This enabled Mg 2 Si particles to distribute within the E area, while simultaneously preventing the formation of Al 12 Mg 17 dendrites. With increasing EMS duration, δ-Mg dendrites initially formed at the edge of the E area, then fragmented and transformed into equiaxed grains. When the EMS duration reached 10 min, the IMCs area was directly bonded to the AZ91D matrix, and cracks emerged at their bonding region. The shear strength of the interface with 8 min EMS reached 52.3 MPa, representing a 60 % improvement over the interface without EMS (32.9 MPa). This enhancement primarily resulted from the oxide film fragmentation and the elimination of the Al 12 Mg 17 dendrites.