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.