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 Al11 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 Al11Si 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 Al11Si alloy with an addition of magnesium. The growth of the die castings with magnesium addition was faster than that of Al11Si 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 AlSi system alloys.

In this study, to obtain guidelines for inhibiting the combustion of the molten metal in magnesium alloys, AZ91D magnesium alloy and AZ91 magnesium alloy with added calcium were exposed to the atmosphere, and the surfaces of the alloys were observed. In the case of AZ91D magnesium alloy, aggregated products were observed on the surface of the molten alloy. In the case of AZ91 alloy with added calcium, aggregated products were not observed. A CaO layer was formed on the outermost surface of the molten alloy, and a MgO layer was formed below the CaO layer. With increasing exposure time, the thicknesses of these layers remained almost constant. It was considered that the CaO layer, in which aluminum did not appear, inhibited combustion by limiting the ow of magnesium vapor from the molten alloy to the atmosphere and the oxidation of magnesium over a long time.

In this study, to obtain guidelines for inhibiting the combustion of the molten metal in magnesium alloys, AZ91D magnesium alloy and AZ91 magnesium alloy with added calcium were exposed to the atmosphere, and the surfaces of the alloys were observed. In the case of AZ91D magnesium alloy, aggregated products were observed on the surface of the molten alloy. In the case of AZ91 alloy with added calcium, aggregated products were not observed. A CaO layer was formed on the outermost surface of the molten alloy, and a MgO layer was formed below the CaO layer. With increasing exposure time, the thicknesses of these layers remained almost constant. It was considered that the CaO layer, in which aluminum did not appear, inhibited combustion by limiting the ow of magnesium vapor from the molten alloy to the atmosphere and the oxidation of magnesium over a long time.

In a previous study, we reported that adding tin to Mg-Al-Ca alloys inhibits cast cracking while maintaining excellent heat resistance. In this study, to clarify the mechanism resulting in the improved castability, Mg-Al-Ca alloys with different compositions were cast by high-pressure die casting into an I-shaped die with strain gauge instrumentation. The solidi cation shrinkage and thermal contraction of the alloys during the solidi cation and cooling process were investigated. The addition of tin decreased the solidi cation shrinkage of the alloys, and the tensile stress in the alloys was relaxed. On the basis of these results, the addition of tin is considered to improve the castability.

The size of defects in castings affects mechanical properties such as fatigue strength. These properties are the most important properties used for designing aluminum alloy casting components, and usually increase with decreasing size of internal defects. Large internal defects mainly form due to the volumetric shrinkage of melt at the last solidified position. In this study, we therefore examined the possibility of generating microbubbles in aluminum alloy melts for dispersing internal defects in castings.Ultrasonic treatment was used for generating nitrogen or argon microbubbles in the melt and was found to be suitable for forming fine gas porosities dispersed in castings. The fine gas remained in the melt for 3600 s, and the generated argon gas was detected from the porosities in the castings. The ultrasonic treatment could also remove inclusions and hydrogen in the melt. Moreover, the fine gas porosities in castings did not decreases the tensile strength of the castings.