Effect of Alloying Element on Si Phase Precipitation from Si-Supersaturated α-Al Solid Solution in JIS ADC12 Alloy Die Castings during Heat Treatment

The growth of aluminum alloy die castings can be eliminated with a suitable heat treatment. To elucidate the parameters affecting the heat treatment conditions, we investigated the effects of alloying elements on the precipitation behavior of supersaturated silicon in die castings during heat treatment using JIS ADC12 alloy (hereafter, referred to as ADC12 alloy) and Al-11 mass%Si alloy, considering that the former contains several alloying elements while the latter no other elements except for silicon. Most of the supersaturated silicon in ADC12 alloy die castings precipitated with a short time heat treatment, resulting in a large number of fine silicon precipitates dispersed in the primary aluminum phase. However, the supersaturated silicon in Al-11Si alloy die castings needed a long time of heat treatment to precipitate and resulted in fewer and larger silicon precipitates in the primary aluminum phase than that of ADC12 alloy die castings. The concentration of magnesium, copper and silicon etc. was analyzed in the silicon particle near the interface with aluminum matrix in ADC12 alloy die castings using a three dimensional atom probe. These alloying elements of magnesium, copper, etc. are considered to have formed clusters suitable as the nucleuses of silicon precipitation during die casting or at the early stage of heat treatment thus promoting the precipitation of the supersaturated silicon during heat treatment. To confirm this assumption, we examined the growth behavior of the die castings of Al-11Si alloy with an addition of magnesium. The growth of the die castings with magnesium addition was faster than that of Al-11Si alloy die castings during heat treatment, and the promoting effect of magnesium was verified on the precipitation of the supersaturated silicon in ADC12 alloy die castings. Therefore, the presence and the content of alloying elements such as magnesium, copper, etc. should be considered when deciding the heat treatment conditions for removing the growth of the die castings of Al-Si system alloys.