Effect of Section Size as a Measure of Cooling Rate on the Solidified Microstructure and Mechanical Properties of Sand-Cast Al-Si Eutectic Alloy

U. Mark
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Abstract

Several factors contribute to the development of structure and properties of aluminiumalloy castings. This study investigated the singular effect of cooling rate on the as-cast structure andmechanical properties of an aluminum-silicon eutectic alloy, keeping other factors such as pouringtemperature, melt treatments, physical and thermal properties of the mould, and alloy compositionconstant. The rate of cooling was varied by employing different casting section sizes, based on thevariation of rate of heat extraction given by solidification time as predicted by the Chvorinov’s rule.Four test bars of section sizes 10, 20, 30, and 40 mm respectively were cast in sand mould using thesame gating system. Spectrometric analysis of the alloy formulated revealed that it could be specifiedapproximately as Al-12.8Si-1.0Cu alloy. The study showed that as section size decreased from 40mm to 10 mm; the solidification time reduced (i.e. the cooling rate increased), the microstructure gotfiner, the silicon flakes became more uniformly distributed, and the mechanical properties generallyimproved. The tensile strength, ductility, and hardness all increased in the order of decreasing sectionsize, i.e. increasing cooling rate. The mechanical properties were found to be linearly correlated withsection size or cooling rate. Whereas the elongations were lower than values for pure aluminium, thestrength and hardness were significantly higher than values for the pure metal. It is concluded thatthe cooling rate modifies the microstructure and improves the mechanical properties of as-cast Al–Sieutectic alloys
断面尺寸作为冷却速度指标对砂铸铝硅共晶合金凝固组织和力学性能的影响
铝合金铸件的组织和性能的发展受多种因素的影响。本研究研究了冷却速度对铝硅共晶合金铸态组织和力学性能的单一影响,同时保持浇注温度、熔体处理、模具的物理和热性能以及合金成分等其他因素不变。根据Chvorinov规则所预测的凝固时间对抽热速率的影响,通过采用不同的铸件截面尺寸来改变冷却速率。采用相同的浇注系统,将截面尺寸分别为10、20、30、40 mm的4根试验棒浇注在砂型中。光谱分析表明,该合金近似为Al-12.8Si-1.0Cu合金。研究表明,随着截面尺寸从40mm减小到10mm;凝固时间缩短(即冷却速度加快),显微组织细化,硅片分布更加均匀,力学性能普遍提高。拉伸强度、延展性和硬度均按截面尺寸减小的顺序增加,即冷却速度增加。力学性能与截面尺寸或冷却速度呈线性相关。虽然拉伸率低于纯铝,但强度和硬度明显高于纯金属。结果表明,冷却速度改变了铸态铝共晶合金的组织,提高了铸态铝共晶合金的力学性能
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