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For specialized applications, it is incumbent to develop new materials that enable manufacturers to develop new processes and designs. For better fuel economy, structural integrity, and lightweight applications, the development of bimetallic steel/aluminum (Al) alloys having a strong interfacial bond is required. Therefore, a mild steel/Al-bearing alloy bimetallic composite was investigated in this study. Firstly, a tin (Sn) interlayer was developed between the steel substrate and the Al-bearing alloy by the tinning process. For further improvement in the interfacial integrity, alumina (Al₂O₃) nanoparticles were added to the Sn powder during the tinning process. Four different wt.% of Al₂O₃ nanoparticles of 0.25, 0.5, 1, and 1.5 were added and mixed thoroughly with Sn powder before mixing them with flux prior to the tinning process. Finally, molten Al-bearing alloy (Al–Sn-Si–Cu) was poured over the Al₂O₃ nanoparticles reinforced tinned steel substrate. A cross-section of the steel/Al-bearing alloy bimetallic composite was prepared for optical microscopy (OM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and shear testing. The cross-section microstructure of the steel/Al-bearing alloy bimetallic composite revealed irregular and discontinuous interfacial layers in the case of the low-temperature (170 °C) tinning process. However, a uniform, continuous interfacial layer was fabricated during the tinning process when additional preheat to the steel substrate and tinning process was adopted. It can be reported that low Al₂O₃ nanoparticles loading (0.25%) and steel substrate preheating were recommended for the better interfacial layer in the steel/Al-bearing alloy bimetallic composite.