<p>The integration of aluminium and steel into a single component by solid--liquid compound casting offers a pathway to producing lightweight, high-strength components. In this study, AA6061-T6 aluminium alloy (AA6061) was cast onto mild steel (MS) substrates pretreated with Zn, Cu, and Ni coatings to tailor interfacial reactions and control the morphology of intermetallic compounds (IMCs). Microstructural characterization using SEM and EDS revealed that each coating uniquely influenced the IMC evolution. Zn-coated MS developed a moderately thick (~185 µm) IMC layer dominated by Fe₂Al₅, and Fe₄Al₁₃ with tongue-shaped features, voids, and cracks, leading to poor bonding strength (~18 MPa). Cu coating promoted the thickest IMC layer (&gt;&#xa0;290 µm), composed primarily of Fe₂Al₅ and Fe₄Al₁₃, along with Cu enrichment at the interface, resulting in intermediate shear strength (~36 MPa). The Ni coating produced the thinnest (~135 µm) and continuous IMC layer, effectively restricting interdiffusion and thus yielding the highest shear strength (~56 MPa). In all the joints, the fracture occurred at the IMC interface, confirming IMC brittleness as the critical factor for governing failure. The findings demonstrate that Ni coating provides superior control of IMC growth compared with Zn and Cu, improving interfacial microstructure and joint strength.</p>