1. Cause analysis
The modified ductile iron casting process is drawn into a three-dimensional diagram by 3D software, and then the STL format is exported and input into the magma simulation software for temperature field and solidification feeding simulation analysis. The finite difference method is used for grid division, and the number of grids is 5 million, as shown in Figure 1.
The database material is gjs-700, the solidus temperature is 1166 ℃, the chemical composition of nodular cast iron casting is set according to the actual production chemical composition, the liquidus temperature is 1169 ℃, the crystallization latent heat is 200 kJ / kg, the pouring temperature is 1400 ℃, the mold filling time is 21 s, the sand mold is green sand, the initial temperature is 40 ℃, the moisture is 3.5%, and the sand core is coldbox_ Silica, the initial temperature is 20 ℃, the cold iron is set as gjs-600, and the initial temperature is 35 ℃. The interface thermal conductivity between nodular cast iron casting and mold, nodular cast iron casting and sand core adopts the default tempiron in the software data, the thermal conductivity between sand core and sand mold adopts the constant C1000 in the database, and the filter is set as foam_ 15 ppi（75 mm × 75 mm × 22 mm), the thermal conductivity of chilled iron and ductile iron castings is set as c1200.
2. Mold filling process simulation
The simulation of mold filling process is shown in Figure 2. It can be seen from the simulation results that the mold filling process is stable without turbulence. The molten iron in each inner sprue enters the mold cavity in a consistent manner without splashing.
3. Solidification process simulation
The simulation of solidification process of nodular cast iron casting is shown in Figure 3. From the simulation results, it can be seen that the boss of installation hole solidifies first and the riser solidifies last, realizing sequential solidification. Because the mounting holes of the intermediate shaft and frame are relatively thick and large, the cooling is slow, and there is a large isolated liquid phase area. According to the results of shrinkage criterion, there is shrinkage defect on the side of the frame mounting hole close to the inner sprue, as shown in Figure 4.
Cause analysis: the cooling capacity of the cold iron here is insufficient. Due to the thickness of the mounting hole of the frame up to 47.5 mm, a large hot spot will be formed here, and due to the structure of nodular cast iron castings, it is impossible to place a larger cold iron to increase the cooling capacity here. It is found from the solidification rate of different stages in the solidification process, The feeding channel between the frame mounting hole and the ingate has been disconnected when the solidification rate is 20% (as shown in Figure 3 (b)). Due to the thin iron inlet thickness of the ingate (only 5 mm), most of the frame hole thickness can not be fed at the later stage of solidification, forming an isolated liquid phase area, as shown in Figure 5.
4. Improvement scheme
Through the analysis of solidification results, an improvement scheme is put forward: a riser is placed at the mounting hole of the frame for feeding, and the introduction of molten iron from the original sheet is modified to the introduction of molten iron from the riser, and the size of the riser is φ 80 mm × 100 mm, riser neck size 20 mm × 20 mm, as shown in Figure 6. The solidification simulation results show that sequential solidification can be realized, and the shrinkage porosity criterion shows that there is no shrinkage porosity defect on the side of the frame mounting hole close to the inner sprue.
5. Production verification
After modifying the mold according to the simulation results, 10 pieces were trial manufactured again and all of them were processed. The results showed that the shrinkage and looseness inside the frame mounting hole were eliminated, which was consistent with the simulation results, as shown in Figure 7. The improved process achieves the purpose of eliminating shrinkage defects in the frame mounting hole. Although the process yield decreases from 60% to 57.9% and increases the post-treatment workload of nodular cast iron castings, it reduces the quality claims caused by poor processing and reduces the quality complaints of customers.