(1) Simulation results of mold filling process
Figure 1 (a) – (f) shows the change process of mold filling temperature field with bottom injection. As shown in figure (a), it takes 2.55 seconds for the molten metal to flow into the casting cavity. Compared with the original process, the mold filling time is much shorter. This is because the shape of the sprue is changed into a cylinder with thick top and thin bottom. During pouring, more molten metal can fill the sprue. On the other hand, the inner sprue is added, It shortens the flow from the molten metal in the sprue to the casting cavity and reduces the heat loss of the molten metal.
From figures (a) – (f), it can be found that the time taken for each 20% of mold filling forward is 2.55s, 3.58s, 16.81s, 20.73s, 41.57s and 48.96s. From the time interval spent for each 20% of mold cavity filling, it can be seen that the filling speed in the whole pouring process tends to be slow first, then fast and then slow, and there is no flow interruption and retention.
The reason why metal liquid filling is slow at the beginning is that the temperature of the molten steel poured into the gate cup is higher. The main product of the decomposition of the foam pattern in the molten steel is the gaseous product. The gas volume is large, and the air permeability of the coating and molding sand on the outer surface of the mold is limited, thus creating a rapid increase in the gas back pressure between the steel front and the shape, which hinders the further filling of the molten steel. Then the metal liquid drops slightly. With the continuous emission of gaseous products, the air gap pressure between the liquid steel and the foam pattern reaches a constant value, which makes the filling speed slightly increase. At the last filling stage, the liquid steel reaches the farthest from the gate end, and a lot of heat is lost during the filling process, which makes the filling capacity worse and the filling time longer. From the filling process of the whole molten steel, the metal liquid in the bottom pouring system is uniform and gentle, and the foam pattern is fully degraded.
(2) Simulation results of solidification process
Fig. 2 is the solidification process diagram after the improved process. From Fig. a to Fig. C, it can be seen that the solidification mode of liquid metal is gentle cooling and solidification from far away from the inner sprue to close to the inner sprue, which improves the feeding of the upper and lower large toroidal hot joint areas. The liquid metal solidifies to 80.1% during the solidification process, and the feeding of liquid metal at the inner sprue can also be obtained. Two small cylindrical cylinder walls outside the casting cylinder wall form an isolated liquid phase area, and shrinkage cavity and porosity defects may occur. Figure 3 shows the distribution of shrinkage cavity and porosity defects of the casting. It can be seen that the shrinkage cavity and porosity defects are significantly reduced, and the shrinkage rate is 13.355%.
