Microstructure characteristic, formation mechanism and mechanical properties of AZ91D/A356 bimetallic interface with a novel Ni-Cr composite coating

In this study, a cost-effective and innovative Ni-Cr composite coating was applied to AZ91D/A356 bimetal to enhance its properties. The influence of coating thickness on the interfacial structure, formation mechanism, and mechanical properties of the AZ91D/A356 bimetal was investigated for the first time. The results reveal that variations in coating thickness significantly affect the interfacial thickness, phase compositions, formation mechanism, and properties of the bimetal. When the coating is thin (0–10 μm), the thickness of the interface layer is approximately 1021.2–1676.2 μm with a formation mechanism characterized by 'melting and diffusion', and its primary phase compositions are Al-Mg intermetallic compounds (IMCs) and Al-Mg eutectic structure. As the coating thickness increases to 45–80 μm, the formation mechanism shifts to 'diffusion' due to the large thickness, high melting point and low reactivity of the coating, resulting in a significant reduction in the interface layer thickness to approximately 346.8–394.2 μm. The predominant phase compositions in this case are Ni-Cr solid solution (Ni-Cr SS) and non-Al-Mg IMCs, such as Al₃Ni, Mg₂Ni, and Ni₂Mg₃Al. These Ni-Cr SS and non-Al-Mg IMCs exhibit superior fracture toughness than the Al-Mg IMCs, leading to a marked improvement in shear strength. The AZ91D/A356 bimetal with an 80 μm composite coating achieves a maximum shear strength of 56.9 MPa, which is 120.5 % higher than the bimetal without the composite coating.