Abstract
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 Al4·5FeSi 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-Al4.5FeSi 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.
Highlights
Increasing demand in reducing climate gas emission, has led to development in the automotive industry towards more lightweight vehicles
Successful metallurgical bonding has been achieved in compound castings between aluminum alloy A356 and galvanized steel by an industrial scale low pressure die casting method
Additional flux coating of the galvanized steel inserts showed no significant improvement on the final interfacial microstructure, likely due to the higher melting temperature needed for melting of the flux coating than the galvanized layer
Summary
Increasing demand in reducing climate gas emission, has led to development in the automotive industry towards more lightweight vehicles. During the compound casting process, a reaction layer will form between the two materials, resulting in a metallurgical bond. Due to large differences in mechanical properties and melting temperature of the two metals, and the ease of aluminum oxide forming at the joint interface, a defect-free metallic bond between aluminum and steel is difficult to achieve. The oxides on the steel insert and liquid aluminum surfaces are thermodynamically stable and have melting points greatly exceeding casting temperatures [6,7]. This will reduce the wettability of the steel surfaces to liquid aluminum, and inhibit formation of metallurgical bonding [8]. The formation of brittle intermetallic phases at the aluminum/steel interface during solidification will reduce the bonding strength [9]
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