Effects of magnesium and copper additions on tensile properties of Al-Si-Cr die casting alloy under as-cast and T5 conditions

Aluminum high pressure die casting (HPDC) technology has evolved in the past decades, enabling stronger and larger one-piece casting with significant part consolidation. It also offers a higher design freedom for more mass-efficient thin-walled body structures. For body structures that require excellent ductility and fracture toughness to be joined with steel sheet via self-piercing riveting (for instance, shock towers and hinge pillars, etc.), a costly T7 heat treatment comprising a solution heat treatment at elevated temperatures (450 degrees C-500 degrees C) followed by an over-ageing heat treatment is needed to optimize microstructure for meeting product requirement. To enable cost-efficient mass production of HPDC body structures, it is important to eliminate the expensive T7 heat treatment without sacrificing mechanical properties. Optimizing die cast alloy chemistry is a potential solution to improve fracture toughness and ductility of the HPDC components. The present study intends to tailor the Mg and Cu additions for a new Al-Si-Cr type die casting alloy (registered as A379 with The Aluminum Association, USA) to achieve the desired tensile properties without using T7 heat treatment. It was found that Cu addition should be avoided, as it is not effective in enhancing strength while degrades tensile ductility. Mg addition is very effective in improving strength and has minor impact on tensile ductility. The investigated Al-Si-Cr alloy with a nominal composition of Al-8.5wt.%Si-0.3wt.%Cr-0.2wt.%Fe shows comparable tensile properties with the T7 treated AlSi10MnMg alloy which is currently used for manufacturing shock towers and hinge pillars.