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Investigation of the tensile and fatigue properties of cast magnesium alloys, created by the heated mold continuous casting process (HMC), was conducted. The mechanical properties of the Mg-HMC alloys were overall higher than those for the Mg alloys, made by the conventional gravity casting process (GC), and especially excellent mechanical properties were obtained for the Mg97Y2Zn1-HMC alloy. This was because of the fine-grained structure composed of the α-Mg phases with the interdendritic LPSO phase. Such mechanical properties were similar levels to those for conventional cast aluminum alloy (Al84.7Si10.5Cu2.5Fe1.3Zn1 alloys: ADC12), made by the GC process. Moreover, the tensile properties (σUTS and εf) and fatigue properties of the Mg97Y2Zn1-HMC alloy were about 1.5 times higher than that for the commercial Mg90Al9Zn1-GC alloy (AZ91). The high correlation rate between tensile properties and fatigue strength (endurance limit: σl) was obtained. With newly proposed etching technique, the residual stress in the Mg97Y2Zn1 alloy could be revealed, and it appeared that the high internal stress was severely accumulated in and around the long-period stacking-order phases (LPSO). This was made during the solidification process due to the different shrinkage rate between α-Mg and LPSO. In this etching technique, micro-cracks were observed on the sample surface, and amount of micro-cracks (density) could be a parameter to determine the severity of the internal stress, i.e., a large amount to micro-cracks is caused by the high internal stress.

The influence of the cooling rate on the extent of precipitation hardening of cast aluminum alloy (ADC12) was investigated experimentally. This study explored the cooling rate of the solidification of Cu in the α-Al phase to improve the mechanical properties of ADC12 after an aging process (Cu based precipitation hardening). The solid solution of Cu occurred in the α-Al phases during the casting process at cooling rates exceeding 0.03 °C/s. This process was replaced with a solid solution process of T6 treatments. The extent of the solid solution varied depending on the cooling rate; with a higher cooling rate, a more extensive solid solution was formed. For the cast ADC12 alloy made at a high cooling rate, high precipitation hardening occurred after low-temperature heating (at 175 °C for 20 h), which improved the mechanical properties of the cast Al alloys. However, the low-temperature heating at the higher temperature for a longer time decreased the hardness due to over aging. [ABSTRACT FROM AUTHOR]

Material properties of two different aluminium alloys (ADC12 and AC4C) were investigated using cast samples produced by conventional gravity and pressure casting at various solidification rates. The microstructural characteristics and defect density vary depending on the cast sample. Those material characteristics affect directly the mechanical properties. The mechanical properties, including tensile strength and fatigue strength, are improved with increasing solidification rate, due to the fine grains, very small eutectic structures and low dislocation density. Tensile strength and ductility for the AC4C alloy are overall higher than those for the ADC12 alloy. The reason for this is attributed to mainly the lower volume fraction of brittle Si- and Fe-based eutectic structures. The mechanical properties are more sensitive to the eutectic structures than to the grain size, although both factors affect the properties significantly. Dimple based ductile fracture is the main feature for the AC4C alloy, while cleavage based brittle failure is the dominant feature for the ADC12 alloy. Severe lattice strain occurs around the cracks in the AC4C alloy because of the high ductility, whereas weak lattice strain occurs in the brittle ADC12 alloy. On the basis of the microstructural and defect characteristics of the cast aluminium alloys, their tensile properties could be approximated using a proposed formula. [ABSTRACT FROM AUTHOR]