Numerical simulation of the mold filling process of the brake lever for an electric multiple unit

The brake lever for an electric multiple unit (EMU) of a train is an important supporting component that directly affects the usability and safety of EMU. In this study, the mold filling, solidification, and shrinkage processes of the brake lever of the EMU were simulated using Anycasting software, and the distributions of the mold filling speed, mold filling temperature, and shrinkage defects were analyzed. The results show that the brake lever for the EMU exhibits casting defects such as shrinkage porosity and shrinkage cavities. To eliminate these casting defects, a chilled-iron process was developed; however, the appearance of cementite increased the brittleness of the castings, which deteriorated their strength and plastic toughness. Hence, a snap-chilling sheet process was designed to fabricate the brake lever for the EMU, and a numerical simulation of the process was performed using the Anycasting software. The shrinkage defects of the castings were effectively eliminated; simultaneously, the cementite disappeared, and the microstructure of the thick parts of the brake lever showed no obvious changes. The prepared brake lever satisfied the requirements of the EMU.


Introduction
Precision casting parts are inexpensive and exhibit good performance, shock absorption, and noise reduction; hence, this form of casting has become a widely used processing technology for electric multiple units (EMUs) of trains.With the accelerated development of high-speed railways and the increasing levels of safety requirements, the brake lever for EMUs needs to be without any defects such as slag inclusion, porosity, shrinkage porosity, and cracks [1][2][3][4][5].
In recent years, numerical simulation software has been used to obtain the distribution of the mold filling speed, mold filling temperature, solidification, and shrinkage in the casting molding process.This technique can predict the presence of any defects in the casting to screen better casting parameters, improve the casting quality, and realize cost savings.Li et al [6] used a SiC ceramic/Al-Si alloy composite as the research object and simulated the microstructure of the composite made by stir casting.The simulation results showed that particle clustering/ accumulation was caused by particle pushing, and the particles were distributed more uniformly as the pouring temperature increased.Yeh et al [7] performed a numerical simulation of the mold filling and solidification processes and predicted the occurrence of relative casting defects in rotor hub casting using an integrated numerical model.Then, to alleviate the casting defects in the rotor hub casting, a better alloy design was proposed based on the simulated results to alleviate casting defects of the rotor hub casting.Hu et al [8] found that numerical simulation is a cost-effective tool in the design of runner and gating systems for visibly analyzing the mold filling process.A thin-walled magnesium telecommunication component was selected as the hotchamber die cast, and a numerical simulation technique was applied to optimize the runner and gating.The new design provided a homogenous mold filling pattern, with the last filled areas located at the upper edge of the part, where overflows and vents were conveniently attached.Good-quality thin-walled telecommunications parts with sound microstructures were produced using this optimized runner and gating system.Guo et al [9] developed a comprehensive finite element model in ProCAST to simulate the microporosity fraction and distribution in a multicomponent alloy casting.This model simulated the transport of gas solutes such as hydrogen and nitrogen and the solubility of these elements as a function of temperature and alloy composition.It can accurately identify the location of micropores and calculate the volume fraction and size of pores.Wang et al [10] presented a simulation of the low-pressure die casting process of a magnesium wheel that adopts the finite difference method (FDM).Air gas pores and shrinkage at the top of the rim were effectively eliminated by reducing the pouring velocity.Setting the cooling pipe system in the side mold alone was found to be a valid method for enhancing the cooling capacity at the rim/spoke junction areas.Strength analysis further confirmed the effectiveness of the new cooling method.
In this study, the brake lever of the EMU was considered as the research object, and the filling, solidification, and shrinkage of the brake lever of the EMU in the casting process were simulated using Anycasting software to predict and judge whether the casting has defects.Based on the results of the numerical simulation analysis, the casting process was improved to obtain a brake lever for the EMU with good performance.

Prototype structure and model structure
The product investigated in this study was the brake lever of the EMU, which is the key connector of the rail transit braking system.Figure 1(a) illustrates the assembly drawing method.The brake lever was connected to the brake seat of the rail transit braking system, and the brake seat was connected to the brake housing assembly.The wall thickness of the brake lever for the EMU is uneven (figure 1(b)); there are six positions with thick walls in the middle and at both ends, which can easily produce internal defects such as shrinkage porosity and cavity.As shown in figure 1(c), riser feeding is required for the middle-thick hot joint according to the following formula:  Parameters of D , R H , R A, S , 1 L, and h were 80, 120, 28, 38, 28, and 32, respectively.Based on the riser size calculated above and the bottle riser theory, a 30% reduction of the upper diameter of the riser was considered to ensure a sufficient pressure head and heat capacity in the riser center.Thus, the riser weight could be reduced to the extent possible while ensuring feeder feeding, and the product yield could be improved.Figure 1(d) shows the optimized process scheme.

Numerical simulation of the mold filling process
Based on the riser and gate design of the brake lever for the EMU, the numerical simulation of the brake lever for the EMU was carried out by Anycasting analysis software, which is also suitable for casting analysis of ductile iron, gray cast iron, and other alloy metal materials.The brake lever of the EMU was made of ductile iron QT500-7, with a density, tensile strength, yield strength, and fracture elongation of 7.3 g cm −3 , 593 MPa, 385 MPa, and 13%, respectively; the spheroidization level was level 3. When casting the brake lever for the EMU, the sand mold was made of aluminum and the sand mold was made of clay sand.The pouring system material was QT500-7.To evaluate whether the castings had defects, the casting filling, solidification, and shrinkage distribution of the brake lever for the EMU were analyzed 3.1.Casting filling process Regardless of the ability of the casting process to obtain brake lever castings for EMU with accurate size, clear contours, and complete shapes, the filling capacity of the castings is particularly critical.The casting filling distribution of the brake lever castings for the EMU was obtained using Anycasting analysis software.When the pouring temperature is 1400 °C, the lowest temperature during mold filling is located at the thin rib and four round bosses (figure 2).When 98% of the mold filling is completed, the lowest temperature in the mold cavity is approximately 1350 °C, which is higher than the liquidus temperature of the casting (1186 °C), and the risk of cold shut defect is avoided.From the perspective of the filling speed, the maximum speed of molten iron in the DC channel exceeds 1.5 m s −1 (figure 3).After passing through the filter block at the bottom of the DC channel,  the flow rate of the molten iron decreases to less than 1 m s −1 .When the cross-flow channel enters the casting cavity, the flow rate of the molten iron is approximately 0.7-0.8m s −1 .This flow rate does not have a significant impact on the molding sand, and there is no risk of sand flushing.Figure 4 indicates that 2.5 g of oxide per cubic centimeter will appear temporarily at the thin rib position when 82% of the mold filling is completed but will   soon dissolve in the body fluid.When 98% of the mold filling is completed, no high oxide is found in the entire mold cavity, and there is no risk of the casting oxidizing the slag.

Casting solidification and shrinkage distribution
During the solidification process of the casting (figure 5), the molten iron of the four bosses at the ends of the casting cuts off from the middle molten iron to form a residual melt, which cannot be fed effectively.From the defect probability analysis, six defects were found in the entire gating system and casting, and four shrinkage porosity and shrinkage cavity defects were found on two risers and runners with a 60% probability.Although the production rate of this scheme is high, there is a risk of shrinkage for the four bosses (circles in figure 6).The defect position was fixed and related to the thicker structure of the four bosses.

Improved casting process 4.1. Chilled iron process
According to the numerical analysis results of the original process, the casting of the brake lever for the EMU has some defects; hence, the casting has to be improved from the perspective of the casting process.AnyCasting analysis software was used to conduct numerical simulations of the filling distribution of the brake lever for the EMU.The casting process was adjusted considering the shrinkage risk of the four bosses.To eliminate shrinkage, chilled iron was added to the four bosses (figure 7) at the far end of the process.No changes were made to the mold filling method and the addition of the cold iron had no effect on the mold filling of the molten iron.However, there were differences in the solidification sequence between the original process and the modified one; hence, the solidification sequences were compared and analyzed.Anycasting analysis software was used to conduct numerical simulations of the solidification and shrinkage distribution of the brake lever for the EMU (figure 8).From the perspective of the solidification sequence, the cooling of the iron in the modified process is  significantly accelerated at the four boss positions at both ends compared to that in the process in figure 1(d), and the four boss positions have no isolated melt participation.Thus, the riser can achieve sequential solidification because the speed of cooling is the reason for the formation of cementite, and increasing the cooling iron accelerates the cooling rate in the contact area of the cooling iron, reducing the hotspot modulus in that area [11].From the defect probability analysis (figure 9), in contrast to the probability of defects found in the castings formed in the original process, the probability of defects of shrinkage porosity and shrinkage cavity exist only in the DC channel and riser positions; there are no defects in the position the casting body.
Experiments were conducted at different pouring temperatures under the set size of the cold iron state (figure 10(a).When the pouring temperature was below 1360 °C, there was 100% penetration in the contact area of the cold iron.The addition of cold iron led to the appearance of cementite in the metallographic structure.Carburizing reduces the strength and plastic toughness of the casting, increases its brittleness, and affects the safety of the parts of the rail transit braking system.However, when cold iron is not added (figure 10(b)), cementite is not found in the metallographic structure of the boss; thus, the metallographic structure meets the requirements of the safety standards.Additionally, the management cost of cold iron is high and sand hole defects occur easily in the production process.Therefore, cold iron processing is infeasible.

Snap-chilling sheet process
Considering the advantages and disadvantages of these two schemes, we redesigned the process scheme.The cold iron was replaced by a 2 mm-thick chilled sheet at the two ends of the boss (figure 11).The principle is that the cooling surface area of the four bosses is increased by the presence of the chilling sheet, and the hotspot modulus of the bosses is reduced to prevent shrinkage.From the perspective of the solidification sequence, when the chilled sheet is added, the four bosses at both ends can achieve the same rapid cooling effect as that when cold iron is added (figure 12), and no isolated residual melt is formed, which causes the intermediate riser to realize sequential solidification.From a macro perspective, the feeding effect of the middle riser was good (figure 13), and the casting body was free of shrinkage porosity and cavity defects.From a microscopic point of view, no shrinkage porosity or cavity defect was observed at the position of the casting body; there is a 60%-80% probability of a shrinkage porosity defect on the chilled sheet connecting the casting boss.No cementite microstructure appears on the metallographic samples (figure 14).Through numerical simulation and process improvement, a sample of the brake lever for the EMU obtained (figure 15); the performance indicators of the sample meet the usage requirements.
mm R T, D , R H , R and h represent the casting thickness, riser diameter, riser height, and riser socket height, respectively.

Figure 1 .
Figure 1.Prototype structure and model structure of the brake lever for EMU.

Figure 2 .
Figure 2. Distribution of temperature in casting filling process.

Figure 3 .
Figure 3. Distribution of speed in casting filling process.

Figure 4 .
Figure 4. Distribution of oxidation in casting filling process.

Figure 5 .
Figure 5. Solidification process of the brake lever for EMU.

Figure 6 .
Figure 6.Possible casting defects in the brake lever for EMU.

Figure 7 .
Figure 7. Chilled iron casting process in the brake lever for EMU.

Figure 9 .
Figure 9. Possible casting defects in chilled iron casting process.

Figure 10 .
Figure 10.Microstructures in casting process with chilled iron (a) and without chilled iron (b).

Figure 11 .
Figure 11.Snap-chilling sheet process in the brake lever for EMU.

Figure 13 .
Figure 13.Possible casting defects in snap-chilling sheet process.