Formation and evolution of the interfacial structure in al/steel compound castings during solidification and heat treatment

In this work, Al7SiMg/steel compound castings were produced through a low-pressure die casting process. All steel inserts were galvanized, where half of them were flux-coated to further improve the wettability and remove interfacial oxide layers during casting. The reaction layer formed in the Al7SiMg/steel interface was examined using Optical Microscopy (OM), Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS). In addition, Vickers Micro-hardness was measured across the interface. Results show that successful metallurgical bonding can be achieved between aluminum and galvanized steel, both with and without additional flux coating. A large fraction of intermetallic particles formed at the reaction layer, where ternary Al_4·5FeSi particles were the dominating phase. The influence of T6 heat treatment (solution treatment at 540 °C, followed by artificial ageing) on the interfacial microstructure was also studied. After heat-treatment, the thickness of the interfacial layer increased significantly, due to the growth of β-Al_4.5FeSi and Al–Fe binary particles into the bulk of steel. Consequently, cracks formed and propagated through the inner binary intermetallic layer. Formation mechanisms of various intermetallic phases at the interface during solidification and heat treatment have been discussed.