Solidification numerical simulation and process optimization of Dia120mm wear resistant casting ball (2)
1.2 Simulation analysis of previous casting process
The software is used to calculate the temperature field of the casting solidification process. Finally, the result of the temperature field is displayed by the post-processing module. The casting temperature during solidification 240s, 780s, 840s, and 900s is selected, as shown in Fig. 6. The casting material is nodular graphite, the casting iron, the liquidus temperature is 1150 °c, the solidus temperature is 1090 °c, the pouring temperature is about 1350 °C. The molten metal is poured into the mold, and the mold has a chilling effect on the molten metal, so the surface temperature of the casting is relatively low at the beginning, and the internal temperature field is relatively uniform, as shown in Fig. 6a.
With the solidification of the casting, the heat of the liquid metal is continuously transferred to the outside. The mold is heated, and the casting’s surface temperature rises again. When the casting is solidified to 780 s, the distribution of temperature field is obvious, the outer layer temperature is lowest, and gradually increases from the outside to the inside, the highest temperature is in the center of the gate, where the outer wall of the casting and the top of the gate have solidified, and the rest of the temperature is above the liquidus because the top of the gate is exposed to the air and is not encased in a sand sleeve, the top metal, therefore, cools relatively quickly, while the rest of the gate is above liquidus temperature and above the ball temperature, so it does not affect the feeding of the molten metal in the gate, as shown in figure 6b.
By 840s, the temperature of the whole casting was significantly reduced; only a small amount of metal temperature in the center of the gate was above the solid line temperature, the whole casting had been completely solidified, so the liquid in the gate played a feeding role successfully, as shown in figure 6c.
At 900s, the whole casting system is completely solidified, and the metal temperature in runner is obviously higher than that in other parts, as shown in Fig. 6d.
It can be seen that, because of the heat preservation effect of the sand sleeve, the metal liquid in the runner solidifies relatively late, by which time the surrounding parts have already solidified, and the metal liquid in the runner has played a feeding role. In summary, by simulating the solidification temperature field of the cast ball, it is found that the shrinkage cavity and porosity defect appear in the runner, and the shrinkage cavity defect does not appear in the cast ball, but in the actual production, the casting defect does exist in the runner.
2.Simulation analysis of improved process
2.1Process optimization analysis
Based on the original casting process analysis and the actual production, the existing casting process can ensure the quality of the cast ball. In order to improve the technological yield of cast ball, the optimization design is carried out under the condition of guaranteeing the quality of existing cast ball. The design of the sprue and die core of the casting system should be changed in order to improve the yield of the casting ball if its size can not be changed.
The die core parameters and sprue parameters were adjusted to reduce the volume of die core and sprue in the whole casting system to increase the ball production rate. The size of the Ingate is not changed to ensure that the metal liquid in the gate can provide enough feeding during the solidification process of the cast ball, so as to reduce the height of the mould core further and optimize the scheme. In addition, the SPRUE is designed as a circular platform body, which is mainly as follows:
(1) if the diameter of the cylinder is the same as the diameter of the bottom surface of the round table, the volume of the round table is smaller and the yield of C can be increased while the flow rate of casting and feeding can be ensured.
(2) In the process of pouring the molten metal, when the molten metal suddenly begins to directly wash the lower part of the inner wall of the cylindrical sprue, as shown in Fig. 7a, the molten metal is flushed directly to point P1 through path a1, due to the strong impact force of the molten metal, it may cause the sand sleeve to collapse and affect the shape and quality of the ball after forming. This will happen more easily when the diameter of the cylindrical sprue is bigger, and the height of the sprue is lower. When the sprue is designed as a circular platform of the same diameter (as d1 = d2 in Fig. 7), and the liquid metal is poured at the same angle as shown in Fig. 7(b), the liquid metal is impinged to point P2 via path a2, the angle is then changed and injected into the cavity via the b2 path, thus reducing or even avoiding the possibility of the bottom of the cylindrical sprue being swept away.
From the above two points of view, it is more practical to design the sprue as a round platform.