PNNL

Abstract

In recent years, high-pressure die-cast (HPDC) Al-alloys have been increasingly used in structural body-in-white (BIW) components for glider weight reduction and part unitizing to achieve greater fuel efficiency and lower carbon emission. However, HPDC Al-alloys based structural components must demonstrate a good combination of strength, ductility, fracture toughness, and high fatigue properties for long-term durability and reliability. Typical HPDC Al-alloys (e.g., A380) use a high weight percentage of Fe (~1 wt.%) to avoid die soldering during the HPDC process, which usually leads to the formation of needle-shaped ß-FeSiAl5 phase that causes inferior mechanical properties. In addition, the usual microstructure of HPDC Al-alloys consists of gas/shrinkage porosity, dendritic microstructure with Al-Si eutectic colonies, and large second-phase particulates. These features also contribute to poor mechanical properties. The usual research trend in the HPDC community is to develop better alloy composition and HPDC process optimization to improve mechanical properties. However, premium quality alloy composition and improved HPDC method carry a cost penalty. Alternatively, microstructure modification by post-processing can be a solution to improve mechanical properties by eliminating defects/other microstructural features responsible for mechanical property depression. This study uses friction stir processing (FSP) for thermomechanical processing of HPDC castings to modify microstructure. FSP is carried out on general-purpose HPDC A380 alloy and premium quality HPDC Aural-5 alloy in thin wall, flat plate geometry. Subsequent mechanical testing shows ~45% and ~80% enhancement in yield strength and tensile ductility. In addition, FSP leads to ~10 times improvement in fatigue life for A380 alloy and ~70% improvement in fracture toughness for Aural-5 alloy.