In semi-solid casting, in order to reduce or even eliminate the shrinkage cavity and porosity defect, the surplus slurry is usually forced to flow and feed during the solidification process of slurry. This behavior is called feeding. In gravity casting, riser feeding is usually set to reduce shrinkage cavity, while in semi-solid die casting, feed cake is often set to feed In order to ensure the feeding degree, the thickness of the cake is 1.5-2 times of the casting wall thickness.
In liquid casting, according to the time sequence of solidification feeding, there are five kinds of feeding: liquid feeding slurry feeding, interdendritic feeding, explosive feeding and solid feeding. As shown in Figure 1.
Compared with the traditional liquid die casting, the feeding mechanism of semi-solid slurry does not include liquid feeding mechanism. In fact, the main feeding modes are slurry feeding mechanism and solid feeding mechanism. The shrinkage solidification mechanism can significantly reduce the shrinkage porosity of lock hole in semi-solid die casting.
According to the theoretical derivation of Hu Xiaogang and others, the higher the slurry temperature is, the more close the critical position of shrinkage defects will be. That is to say, with the increase of slurry temperature, the shrinkage defects will gradually decrease or even disappear. There are many conditions affecting the temperature of semi-solid slurry, such as the initial temperature of aluminum alloy solution and the mold Initial set temperature, thermal conductivity of die metal, etc.
In squeeze casting of Al Si alloy, when the pressure is low, the increase of pressure can lead to a large reduction of shrinkage cavity and porosity. However, when the pressure increases to a certain range, the increase of pressure can only make the shrinkage cavity and porosity defect decrease slightly or no more. As shown in Fig. 1, in gravity casting, Li Longfei et al. Studied the effect of die temperature on the shrinkage defects The higher the mold temperature, the greater the formation trend of shrinkage porosity, and the wider distribution range, as shown in Fig. 2. However, in the actual semi-solid die casting process, it is found that the higher the die temperature, the less shrinkage defects or even disappear.
