Effect of microstructure on mechanical properties and fracture toughness of Aluminum alloy die castings

The effect of microstructure, especially DAS II (Secondary Dendrite Arm Spacing) and chill layer in ADC10 aluminum alloy die castings on the tensile properties, the fracture toughness and the crack propergation is examined by preparing the die castings which have different DAS II and different thickness of chill layer cast under several casting conditions by cold chamber die casting machine. The results are as follows. (1) The tensile strength increases with the decrease in DAS II and there is the Hall-Petch relation between DAS II and 0.2% proof stress. While DAS II doesn't affect the fracture strain. (2) As a crack propagates through the boundary between α-phase and eutectic Si crystal, the increase in DAS II makes the fracture surface be rougher and it causes the increase in the fracture toughness. (3) The tensile strength and the surface hardness of chill layer are higher than substrate. It is possible for the reason to be not only relaxation of stress concentration due to small DAS II but also strengthening by solid solution due to the larger amount of occluded copper and magnesium.