Observing the change of temperature field in the solidification process of sand castings can accurately predict the location of shrinkage porosity and shrinkage porosity defects, and dynamically express the change of temperature field of the whole sand castings, which can provide a good reference condition for the judgment of shrinkage porosity and shrinkage porosity. In the solidification process, select a section to observe the inside. Figure 1 shows the changes of solidification temperature field after filling the cavity for 10s, 45s, 120s and 220s respectively. As can be seen from Figure 1 (a) and (b), when the sand casting is just filled, the sand casting can form sequential solidification in the direction of the inner gate on the section. The temperature at the inner gate is the highest in the whole sand casting, and the temperature at the side away from the inner gate drops significantly. At this stage, the inner gate provides conditions for liquid metal feeding. As can be seen from Fig. 1 (c) and (d), with the increase of solidification time, the liquid-solid two-phase region begins to appear, and the temperature of the inner gate is still higher than the solidus line, which can feed the casting well. However, after 120s, the riser has solidified, while the solid-liquid coexistence is still at the bottom of the sand casting, so the riser can not feed the casting well.
Figure 2 shows the change of solid rate in the solidification stage of a riser section during the solidification of liquid metal. The setting time is 10s, 45s, 125S and 240s respectively. It can be seen that the molten metal at the inner gate does not solidify within 10s after the sand casting is filled, and solidifies after 45s. At 45s, the solid rate at the inner gate and riser reaches about 40%. In this period of time, the thick parts of the casting can not be shrunk, which is basically in line with the change of section temperature field in the solidification stage of sand casting. When the liquid metal fills the mold cavity for 125S, the solidification rate of the metal at the lower part of the inner gate reaches 85%, and there is an obvious solidification phenomenon at the top. At this time, the solidification rate of the central metal in the thick part of the sand casting is only 45%, which will form an isolated liquid phase area. Finally, due to the volume shrinkage of the metal, it will inevitably lead to the casting defects of shrinkage porosity and shrinkage cavity.
Fig. 3 shows the position of shrinkage cavity and shrinkage porosity after casting solidification. It can be seen that the gating system with single side top injection and inner gate can meet the smooth mold filling of castings. After the mold filling, the liquid metal follows the principle of sequential solidification along the direction of inner gate and riser. However, due to the large volume of castings, the mold filling and solidification time are not very uniform. In the figure, there are some scattered shrinkage porosity away from the lower part of the inner gate, but there are still three obvious shrinkage porosity and shrinkage cavities near the thick part of the middle and lower part of the inner gate, which is consistent with the isolated liquid phase region in the solid rate in Figure 2.
The introduction of molten metal into four inner sprues on one side can realize the smooth filling of molten metal, and the molten metal can meet the principle of sequential solidification after filling. Due to the effect of gravity, the liquid metal cannot be filled horizontally at the beginning, which will affect the quality of sand castings to a certain extent. Due to the use of open risers, the temperature at the riser drops rapidly and presents a certain temperature gradient shortly after the completion of liquid metal filling. In the initial stage, when the solid rate from the two riser sections is still low, the liquid metal can still meet the sequential solidification conditions. However, due to the influence of the large volume and complex shape of the cylinder head, there will be solidification areas in the thick parts, which creates conditions for the isolated liquid phase removal, resulting in obvious shrinkage porosity and shrinkage defects. Therefore, the process needs to be improved.