Fig. 1 is a sectional view of the solidification time distribution during bottom injection. It can be seen that the slag inclusion riser, fork tip and sprue solidify the earliest and the solidification time is the shortest; The solidification time at the junction of the two fork arms and the cylinder is the longest, which corresponds to the intersection of the thick wall of the nodular cast iron long fork and the transition of wall thickness. A large hot spot area will be formed during the solidification process. Although the large hot spot area at the lower part of the nodular cast iron long fork is connected with the inner sprue, the feeding at the hot spot cannot be provided due to the early solidification of the whole pouring system, Like the upper large hot spot region, a large range of isolated liquid phase region will be formed.
Fig. 2 shows the results of selecting the solid rate lower than 70% to show the change of the solid rate of the long fork of nodular cast iron. From the figure, we can clearly see the position and size of the isolated liquid phase zone at the large and small hot joints on the fork arm and cylinder wall, as well as its formation process. Figure a) it can be seen that the sprue and inner sprue solidify too early, the gating system can not feed the bottom of the long fork of nodular cast iron, and there is a large isolated liquid phase area in the large hot spot area. At this time, the expanded self feeding of graphite precipitated from nodular cast iron becomes the only way to feed the long fork of nodular cast iron; At the same time, thelong fork solidifies early at the thin wall of the cylinder, the channels in the upper and lower head areas of the cylinder are disconnected, and the ductile iron long fork cannot realize the convection of molten iron in the upper and lower hot spot areas; Figures b) ~ d) show the formation of isolated liquid phase zone on the long fork arm of nodular cast iron and the thick wall of the cylinder. When the feeding is insufficient, the isolated liquid phase zone will develop into shrinkage cavity and porosity casting defects.
Fig. 3 shows the simulation results of shrinkage cavity and loose mold of bottom injection trial production scheme, and the porosity greater than 1% is taken as the minimum display score. It can be seen from figure a) that the isolated liquid phase area in the solidification process of nodular cast iron long fork is determined to have shrinkage and porosity casting defects. Among them, the shrinkage and porosity are relatively concentrated at the junction of the upper and lower fork arms and the thick wall area of the cylinder, which is because the isolated liquid phase area formed here is very large in the previous solid rate change, and there is no further external feeding, When the self feeding of nodular cast iron long fork cannot meet the demand, serious solidification pore casting defects will be formed. Figure b) and figure C) show the shrinkage and porosity of nodular cast iron long fork in cross-section and longitudinal section respectively. From the simulation results of the whole shrinkage cavity and porosity casting defects, the shape, size and position of the shrinkage cavity in the rear wall area of the lower part of the cylinder are in line with the trial production results, but there are no casting defects on the upper part of the cylinder and the fork arm in the trial production process. In the simulation results, the reason for the shrinkage cavity and porosity at the small hot joint on the fork arm may be the insufficient estimation of the graphite spheroidization expansion effect. Realistic results can be obtained by reducing the fading parameter value (other expansion parameters have been adjusted to the maximum expansion value) or increasing the cutoff value of observing the shrinkage cavity and porosity; The reason for the significant shrinkage cavity in the upper part of the cylinder and the long fork of nodular cast iron is not clear, which needs to be comprehensively analyzed together with the top injection simulation results.
The bottom injection type can smoothly fill the mold, filling process is continuous, but during the filling process, it is possible that the metal liquid wrap bubble phenomenon occurs. It is necessary to set up the slag collecting riser at the top of the cylinder. The riser slag collecting and exhausting effect at the front fork of the upper fork arm is good, and the slag quantity is not large enough to be properly changed. In terms of solidification, the feeding scheme without riser can not meet the internal feeding requirements of nodular cast iron long fork. The internal sprue and sprue condense prematurely, resulting in the inability of metal liquid to feed the bottom of nodular cast iron long fork from the outside. It is necessary to improve the design of transverse sprue and internal sprue.