Controlling interfacial composition and improvement in bonding strength of compound casted Al/steel bimetal via Cr interlayer

The fabrication of Al/steel bimetal with a good metallurgical interface is always a great challenge, particularly for the improvement of bonding strength due to the inevitably extensive formation of brittle Al–Fe intermetallic compounds (IMCs) at the interface. Therefore, in this work, the electroplated Cr coating with thickness of 5 μm was firstly used as the interlayer of Al/steel bimetal to suppress Al–Fe IMCs and further improve the interfacial bonding strength. Al/steel bimetal was prepared by compound casting combined with a hot-dip aluminizing process. The results showed that the interface of Al/steel bimetal with Cr interlayer contained an unmolten Cr coating and a reaction layer, in which the latter consisted mainly of Al13Cr4Si4, Al5Cr, Al-rich solid solution and CrSi2 phases. Besides, the morphology of these new Al–Cr–Si and Al–Cr phases was granular rather than the tongue-shaped of Al–Fe phase and the polygonal blocky of Al–Fe–Si phase. Both the microstructure characterization and kinetics calculation results revealed that the application of Cr interlayer could avoid the contact between Fe atoms from steel side and Al atoms from Al alloy side, thereby suppressing the formation of brittle Al–Fe phases at the interface. Moreover, the reaction layer showed a lower nano-hardness than that of without Cr interlayer. With assistance of Cr interlayer, Al/steel bimetal with maximum shear strength of 115.5 MPa was obtained and the interface failed through the Al alloy side. The morphology characteristic and lower nano-hardness of these new phases, the composite structure consisting of Al–Cr–Si and Al–Cr layer together with the Al-rich solid solution layer greatly contributed to the enhancement of the interfacial strength for the Al/steel bimetal.