Effect of microstructure on fracture toughness of cast and forged aluminum casting alloys

Tensile and fracture toughness properties of three typical aluminum casting alloys, A (0.7%Si)-T6, B (4%Si)-T6 and C (7%Si) -T6, were evaluated in both as-cast and as-forged conditions. Cast and forged alloys showed increased elongation and fracture toughness with increase in forging ratio, especially in A (0.7%Si)-T6 alloy. The tensile and yield strengths of cast and forged alloys of B (4%Si)-T6 and C (7%Si)-T6 increased with forging ratio, but decreased in 50% forging ratio. Mechanism of the increased fracture toughness are considered to be a decrease in the size of casting defects with forging ratio. On the other hand, it is also considered to change microstructures of α phase and eutectic Si particle. In the Al-Si-Mg alloys, Mg₂Si is a major precipitate, however, it is considered that the excess Si also contributes to the increase of the strength.