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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. [doi:10.2320/matertrans.F-M2013838]

Change in Molten Metal Pressure and Its Effect on Defects of Aluminum Alloy Die Castings