Abstract:The effects of vacuum assistance on the microstructure and mechanical properties of high pressure die cast A390 alloy at different slow shot speeds were evaluated. Plate-shaped specimens of hypereutectic A390 aluminum alloy were produced on a TOYO BD-350V5 cold chamber die casting machine incorporated with a self-improved TOYO vacuum system. According to the results, the vacuum pressure inside the die cavity increased linearly with the increasing slow shot speed at the beginning of mold filling. Meanwhile, tensile properties of vacuum die castings were deteriorated by the porosity content. In addition, the average primary silicon size decreased from 23 to 14 μm when the slow shot speed increased from 0.05 to 0.2 m/s, which has a binary functional relationship with the slow shot speed. After heat treatment, microstructural morphologies revealed that needle-shaped and thin-flaked eutectic silicon particles became rounded while Al2Cu dissolved into α(Al) matrix. Furthermore, the fractography revealed that the fracture mechanism has evolved from brittle transgranular fracture to a fracture mode with many dimples after heat treatment.

Key words: A390 aluminum; shot speed; vacuum assisted high pressure die casting (VHPDC); Si distribution; tensile strength; heat treatment