Improving mechanical properties of AZ91D magnesium/A356 aluminum bimetal prepared by compound casting via a high velocity oxygen fuel sprayed Ni coating

In this paper, a Ni coating was deposited on the surface of the A356 aluminum alloy by high velocity oxygen fuel spraying to improve the performance of the AZ91D magnesium/A356 aluminum bimetal prepared by a compound casting. The effects of the Ni coating as well as its thickness on microstructure and mechanical properties of the AZ91D/A356 bimetal were systematically researched for the first time. Results demonstrated that the Ni coating and its thickness had a significant effect on the interfacial phase compositions and mechanical properties of the AZ91D/A356 bimetal. The 10µm's Ni coating cannot prevent the generation of the Al-Mg intermetallic compounds (IMCs) at the interface zone of the AZ91D/A356 bimetal, while the Ni coating with the thickness of 45 µm and 190 µm can avoid the formation of the Al-Mg IMCs. When the Ni coating was 45 µm, the Ni coating disappeared and transformed into Mg-Mg2Ni eutectic structures+Ni2Mg3Al particles at the interface zone. With a thickness of 190µm's Ni coating, part of the Ni coating remained and the interface layer was composed of the Mg-Mg2Ni eutectic structures+ Ni2Mg3Al particles, Mg2Ni layer, Ni solid solution (SS) layer, Al3Ni2 layer, Al3Ni layer and sporadic Al3Ni+Al-Al3Ni eutectic structures from AZ91D side to A356 side in sequence. The interface layer consisting of the Mg-Ni and Al-Ni IMCs obtained with the Ni coating had an obvious lower hardness than the Al-Mg IMCs. The shear strength of the AZ91D/A356 bimetal with a Ni coating of 45 µm thickness enhanced 41.4% in comparison with that of the bimetal without Ni coating, and the fracture of the bimetal with 45µm's Ni coating occurred between the Mg matrix and the interface layer with a mixture of brittle fracture and ductile fracture.