New Insights into the Characterization and Formation of the Interface of A356/AZ91D Bimetallic Composites Fabricated by Compound Casting

In the present study, the A356/AZ91D bimetallic composites were prepared by the lost foam casting (LFC) solid–liquid compound process, and the characterization and formation of the interface of the A356/AZ91D bimetallic composites were investigated. The crystallographic orientations of intermetallic compounds in the interface layer were also studied. The results obtained showed that the interface layer was constituted by four regions: Mg2Si + Al3Mg2, Mg2Si + Al3Mg2 + Al12Mg17, Mg2Si + Al12Mg17, and Al12Mg17 + δ-Mg eutectic + Mg2Si. The formation of the interface layer was attributed to fusion bonding and diffusion bonding, and the Al3Mg2, Al12Mg17, and α-Al12Mg17 dendritic crystals and Al12Mg17 + δ-Mg eutectic intermetallic compounds successively formed in the interface layer. The Al3Mg2 and Al12Mg17 phases grew, respectively, with {0001} and {111} preferred crystallographic orientation, while the texture of the Mg2Si phase was essentially random in the interface. The interface layer of the A356/AZ91D bimetallic composites had a higher hardness than the substrates, and the Mg2Si phase obtained the highest hardness in the intermetallic phases. The shear strength and tensile strength of the A356/AZ91D bimetallic composites reached 47.67 and 48.17 MPa, respectively. The fracture surface of the bimetallic composites exhibited a brittle fracture morphology with a partial plastic deformation, and the fracture mainly initiated with the junction zone between the Mg2Si + Al3Mg2 and Mg2Si + Al12Mg17 intermetallic layers.