Die castings are prone to contain considerable porosities due to the entrapment of air or gases in the molten metal during mold filling. Reducing the die filling velocity is effective for reducing the entrapment, but it increases surface defects, such as surface folds and cold shut on die castings.In this research, the solidification behavior of molten metal during mold filling was investigated by developing a highly sensitive thermosensor with a response time of 0.015 s that can correctly measure the temperature of flowing molten metal. The criterion for the formation of surface defects was further examined based on the solidification behavior of molten metal during mold filling.It was found that the type of surface defects varies with the solidification manner of aluminum alloys. Surface fold defects occur on die castings made of JIS AD12.1 alloy with skin-formation type solidification. The occurrence of surface folds can be predicted by the thickness of the solidified layer of the molten metal from the surface of cavity. The critical thickness for the formation of surface folds increases with increasing casting pressure. On the other hand, cold shut defects occur on die castings made of JIS AC4C alloy with mushy-formation type solidification. The molten metal temperature drops toward the tip of the molten metal flow. The occurrence of cold shut defects can be estimated by the temperature of this molten metal flow tip at the time this flow converges with other flows.

Die castings generally contain a large quantity of porosities due to the entrapment of air or gas in molten metal during mold filling. Although the entrapped air or gas is compressed by high casting pressure during pressurization, it will eventually remain in the castings as defects after solidification. Therefore, it is important to clarify the relation between the volume of gas defects and the pressure applied to the molten metal so as to optimize the casting design.In this study, we investigated the compression behavior of entrapped gas during casting. We determined the volume of gas defects and gas content in die castings by density measurement and vacuum fusion extraction method respectively. Then we calculated the gas pressure in the defects from the above volume of defects and gas content, and compared with the die casting pressure. The calculated gas pressure in the defects was found to be not equal to the die casting pressure, but equal to the pressure of the molten metal just before it dropped abruptly due to the complete blocking of the liquid metal channel by solidification. From the experimental results, the behavior of the entrapped gas can be inferred as follows. Immediately after the mold was filled with molten metal, the entrapped gas was instantly compressed. After that, the pressure of molten metal decreased gradually with the progress of solidification of the molten metal channel, and the volume of entrapped gas increased correspondingly until the pressure of the molten metal dropped abruptly. Then the volume of the entrapped gas showed a slight expansion equal to the solidification shrinkage of the enclosed molten metal.The above inference was verified by measuring the volume of the entrapped gas defects in castings made with intentional depressurization carried out at the time when mold filling just finished or halfway through the solidification of the molten metal channel.

Surface defects of die castings, such as surface folds, cold shuts and misruns, etc., are thought to occur due to the solidification of molten metal during mold filling. In the die casting process, mold filling and solidification are completed under high pressure. Thus, varying the casting pressure may also have some effects on surface defects.In this study, we investigated the effects of solidification on mold filling and pressure transmission and elucidated the influences of casting pressure on the quality of die castings.In the mold filling process of JIS AC4C (A356) alloy (hereinafter called AC4C alloy), flow resistance was found to increase with the increase of the amount of solid phases, and thus decreased the velocity of molten metal flow. Cold shut occurred due to the rapid drop of the temperature of molten metal as a result of the decrease in flow velocity. A similar phenomenon was also observed for JIS AD12.1 (A38X) alloy (hereinafter called AD12.1 alloy), although this alloy showed a skin-formation type solidification. The pressure transmission in molten metal weakened gradually with the increase in the amount of solid phases and eventually stopped completely to cause casting defects as the result. In addition, resistance to molten metal flow due to the back pressure, shape of mold cavities, etc., was also noticed. These results indicate that a high plunger pressure is necessary to obtain die castings without defects.

A cold crack criterion for JIS ADC12 aluminum alloy die casting is proposed. Through investigating the temperature dependence of the fracture strain of JIS ADC12 aluminum alloy die casting, it was found that the fracture strain features a turning point at a temperature, T c (we called ''critical temperature to the ductility'', about 573 K for the present composition), i.e. stays low while below T c , rises rapidly to a high level beyond T c . Focusing on this character of the fracture strain, we analyzed the equivalent plastic strain (" c ) of the castings introduced below T c in casting processes by thermal stress simulations and compared with the occurrence of cold cracks in the die casting experiments. It was found that the " c of the cracking positions in the castings exceeded, while the " c of the castings without crack were much lower than the fracture strain of JIS ADC12 aluminum alloy die casting below T c . That is to say, the occurrence of the cold crack in a die casting can be judged by comparing the " c with the fracture strain below T c . Based on this proposed criterion, it is possible to predict the appearance of the cold cracks in ADC12 die castings by thermal stress simulations.

This study investigated the strain variation of JIS ADC12 aluminum alloy die castings by heat treatment and its relation with the strain arising from the precipitation of silicon, copper, and magnesium so that the most effective measures can be adopted to ensure dimensional precision in various service environments. Expansion strain of over 0.1% was produced in JIS ADC12 aluminum alloy die castings by heat treatment, which resulted in a simultaneous decrease in the half-width angles of the X-ray diffraction (XRD) peaks of the aluminum phase in the die castings. Moreover silicon, copper, and magnesium concentrated zones appeared in the aluminum phase after heat treatment. Thus, the decrease in the half-width angles can be regarded as a result of the improved crystallinity of the aluminum phase because of the relaxation of the lattice strain by the precipitation of silicon, copper, and magnesium. Hence, the strain variation in the ADC12 alloy die castings due to the heat treatment can be attributed to the precipitation of silicon, copper, and magnesium from the supersaturated aluminum phase. For quantitative veri cation of the above relation, growth attributable to the precipitation of silicon, copper, and magnesium from the aluminum phase and the transformation of the precipitated metastable Cu-Al compounds was estimated theoretically and compared with the measured strain variation caused by the heat treatment of the ADC12 alloy die castings. The results con rmed that the strain variation of the ADC12 alloy die castings by heat treatment corresponds well to the growth resulting from the precipitation of silicon, copper, and magnesium from the aluminum phase and the transformation of the precipitated metastable Cu-Al compounds. The results also revealed that silicon, copper, and magnesium precipitated at the early stage of heat treatment and that silicon precipitation contributed the most to the growth. The precipitated metastable Cu-Al compound, θ , transformed into another metastable compound, θ , and nally into the stable compound, θ, thereby resulting in expansion and contraction growth.