Interfacial Microstructure Formation in Al7SiMg/Cu Compound Castings

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 Al5Cu2Mg8Si6. Flux coating prevented formation of any metallic bond between copper and aluminum. Instead, high concentrations of potassium, magnesium and fluorine, indicative of formation of KMgF3 and MgF2, were detected in the interface. The mechanism for the formation of the intermetallic phases and the strength of the interface layer have been discussed.