The effects of silicon content on abnormal microstructures such as coarse
α phase, massive structure and scattered structure, and the tensile strength of die castings, were investigated with Al-XSi binary alloys (X=5, 8, 11 mass %) and commercial alloys such as AC2A, ADC 10 and ADC 12 die castings fabricated by a 90 metric ton locking force cold chamber type die casting machine. In the microstructures of both Al-XSi binary alloys and commercial alloys, coarse
α phase decreases and scattered structure increases with increasing silicon contents and shot-time-lag. In the case of wide temperature range between Al-Si eutectic and liquidus line such as Al-5 mass % Si and AC2A alloy, the abnormal microstructures mainly consist of coarse
α phases and massive structures. On the other hand, in the case of narrow temperature range such as Al-11 mass % Si and ADC 12 alloy, the abnormal structure mainly consists of scattered structures. The tensile strength of both AC2A and ADC 10 alloys decreases with increasing shot-time-lag because of increasing coarse
α phase. In the case of ADC 12 alloy die castings, the amount of dispersion for values of tensile strength is larger than that of the other two alloys because ADC 12 alloy die castings contain many scattered structures.
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