Effects of Solidification Behavior during Filling on Surface Defects of Aluminum Alloy Die Casting

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 ITS 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.