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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]

The corrosion and mechanical properties of cast aluminium alloys [ADC6(Al-Mg) and ADC12(Al-Si-Cu)] have been investigated. Specifically, cast samples with different microstructural characteristics were produced using two different casting techniques, namely, gravity casting (GC) and heated mould continuous casting (HMC). For ADC6, intergranular corrosion occurs mainly through the eutectic Mg2Si and Al6(Mn,Fe) phases and the precipitate free zone around the eutectic structures. On the other hand, pitting corrosion in the α-Al phase is the dominant feature for ADC12. Such corrosion characteristics can be interpreted from the material properties (polarisation resistance and electrode potential) of the individual structures, such as the α-Al phase, eutectic α-Al phase and precipitate phase. It also appeared that the extent of corrosion is affected by the size of the microstructure; the ADC6 sample with small second phases has a high corrosion resistance due to the limited area for corrosion. In addition, the higher proportion of solid solution in the α-Al phase of ADC6, arising from the high cooling rate, makes a high corrosion resistance because of the change of electrode potential. A different trend is obtained for ADC12 due to the pitting corrosion, where low corrosion resistance is obtained for the sample with tiny grain. The tensile strength and ductility for both HMC samples (ADC6 and ADC12) are higher than those for the GC equivalents, which is caused by the presence of tiny microstructures and uniform crystal orientation. However, the mechanical properties of HMC samples decrease dramatically during corrosion, although these are still high compared to those of the GC samples without corrosion. [ABSTRACT FROM AUTHOR]

This paper reports an investigation of the mechanical properties of cast aluminium AC4CH alloys produced by a unique casting method known as the heated mould continuous casting (OCC) process. The tensile and fatigue strengths and material ductility of the OCC samples are much higher than that for the same alloy produced by conventional gravity casting. The high tensile and high fatigue strengths of the OCC samples are reflected by the presence of tiny round α-Al grains, and the high ductility of OCC is a result of the regularly organized crystal orientation. [ABSTRACT FROM AUTHOR]

In this paper, the fatigue strengths of cast aluminium alloys have been examined using the S-N and da/dN-ΔK approaches on various cast samples having different microstructural characteristics. These were made by several different casting techniques, namely gravity casting, die casting and twin rolled continuous casting. Owing to the different microstructures, the fatigue properties varied. The S-N relations, e.g. the endurance limit, of the twin-rolled casting were apparently high compared to the other cast samples. The high fatigue strength was related to the high tensile strength and high ductility caused by the tiny spherical α-Al phase and fine eutectic structure. On the other hand, the low fatigue strength for the gravity and die cast samples was caused by the high stress concentration caused by the α-Al phase and needle-like eutectic silicon. Unlike the fatigue strength in the S-N approach, the da/dN-ΔK relationships for the twin rolled casting sample, i.e. the resistance to fatigue crack propagation, were low compared to the gravity casting and the die cast samples. Such a difference in the crack growth resistance was influenced by the crack growth characteristics. For example, for the gravity cast and the die cast samples, the crack growth occurs in the grain boundaries of the coarse α-Al phase and along the long eutectic silicon grains (zigzag crack path), which promotes a strong crack closure tendency, leading to high crack growth resistance. The difference in the fatigue strength determined from the S-N relation and da/dN-ΔK relation was interpreted by experimental and numerical examination of the stress-strain distribution. [ABSTRACT FROM AUTHOR]

The crystallisation characteristics of primary silicon particles in a cast hypereutectic Al-Si alloy were examined using various casting processes. The size of the primary silicon particles depends on the cooling rate. However, the primary silicon particles cannot be crystallised completely when the cooling rate is >20 K s-1, since uncrystallised Si atoms migrate to the regions at low cooling rates, producing large primary Si particles. Moreover, turbulent metal flow under conditions of high cooling rate leads to the macrosegregation of the primary Si particles, resulting in poor mechanical properties. From the crystallisation characteristics obtained, a suitable solidification criterion can be proposed. [ABSTRACT FROM AUTHOR]