Abnormal segregation of impurity elements in reverse squeeze casting

On the whole, the impurity segregation of reverse squeeze casting belongs to positive segregation. The last position of reverse filling is position 8 and the thin-walled structure at the bottom of the cavity is position 4, which is the first solidification position. However, abnormal segregation of Ca was found in the experiment. The equilibrium distribution coefficient of Ca and Al is only 0.001, which belongs to the easily segregated element with K0 < 1. The content of Ca and Al should be lower at the first solidification position. However, it is found that the content of Ca and Al is very high at position 8 at the first solidification position, as shown in Table 1.

The origin of the abnormal segregation can be explained by the existence of ca. According to the Al CA phase diagram, the equilibrium solubility of Ca in Al is very low, only 0.01%. That is to say, when Al alloy solidifies, CA is rarely solidified in the solidified layer, but is free in the liquid phase. The density of Ca is only 1.54g/cm3, which is only 50% of that of al. The free Ca in the liquid phase will float and gather in the upper position 8, while it is relatively poor in the lower position 5. The larger the casting, the more obvious this phenomenon will be. Therefore, the anomalous segregation of Ca is essentially “density segregation” caused by the density difference between Ca and al.

This “density segregation” can also be found by carefully observing the Cu content at different positions in Table 1. The density of Cu is 8.91g/cm3, which is more than three times of that of al. Therefore, it will appear “sedimentation” in the liquid phase, resulting in the Cu content in the lower part of the final solidification position (position 6) being significantly higher than that in positions 7 and 8. This kind of density segregation will be more prominent in the workpiece with larger casting height and wall thickness.