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Obtaining a strong bond between aluminum and steel is challenging due to poor wettability between aluminum melt and steel and brittle intermetallic phases forming in the interface. In this research, a novel coating method, namely hot dipping of Sn, has been developed to treat the steel insert surfaces. Results show that without preheating the mold or Sn-coated insert, a thin, crack-free, and continuous metallurgical bonding layer was achieved in the A356 aluminum/steel compound castings. Intermetallic structures forming in the interface have been characterized in detail. The Sn-coating layer completely melted and mixed with the liquid aluminum during the casting process. The reaction layer at the aluminum/steel interface is composed of ternary Al–Fe–Si particles and a thin layer of binary Al5Fe2 phase with thickness less than 1 µm. A small fraction of dispersed Sn-rich particles was observed distributing in the reaction layer and adjacent to eutectic Si particles in the A356 alloy. A sessile drop wetting test showed that Sn-coated steel substrates can be well wetted by aluminum melt. The improved wettability between A356 alloy melt and steel was attributed to the penetration and breaking of the aluminum oxide layer at the surface of the aluminum droplets by liquid Sn. Graphic Abstract

Compound castings between aluminum and steel have great potential for applications in the automotive industry. However, due to large differences in thermal and mechanical properties between steel and aluminum, and the formation of stable aluminum oxides at the interface, it is difficult to form high strength metallic bonding between the two metals. In this work, A356/steel compound castings were produced through a gravity casting process. Various metal coatings, including galvanizing, aluminizing and brass-coating, were applied on the steel inserts to ensure that the A356 aluminum melt could react sufficiently with an oxide-free steel surface, resulting in a high-quality metallurgical bond. The reaction layer formed between the alloys was investigated using Optical Microscopy (OM), Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS). In addition, Vickers Micro-hardness was measured across the aluminum-steel interface. Results showed that metallurgical bonding could be achieved with all three coatings. However, for the brass-coated components only local bonding areas were found. In the aluminized and galvanized components, thick reaction layers consisting of binary Al-Fe and ternary Al-Fe-Si phases formed in the aluminum-steel interface. Between the A356 aluminum and aluminized layer, nearly no reaction layer formed. The mechanism for the formation of the various intermetallic phases at the reaction layers are discussed.

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Compound casting is an attractive approach to create multi-material components and thus reduce the overall weight, while maintaining both the functional and mechanical properties. In this work, Al7SiMg alloy/copper compound castings were produced by a low-pressure die casting process. A flux coating was applied on copper pipes to reduce the oxide layer present in the interface between Al and Cu. The interface layer formed between the two alloys was investigated using optical microscopy, scanning electron microscopy and energy-dispersive X-ray spectroscopy. Vickers micro-hardness was also measured across the interface. Results showed that a continuous metallurgical bond formed between copper and aluminum without use of surface treatment. In the bond layer, various Al-Cu intermetallic phases were detected, as well as primary silicon particles and the quaternary phase Al 5 Cu 2 Mg 8 Si 6. Flux coating prevented formation of any metallic bond between copper and aluminum. Instead, high concentrations of potassium, magnesium and fluorine, indicative of formation of KMgF 3 and MgF 2 , were detected in the interface. The mechanism for the formation of the intermetallic phases and the strength of the interface layer have been discussed.