The components of precision casting rail transit braking system have the characteristics of good chip processing performance, shock absorption and noise reduction, and low cost, and have become widely used components in high-speed train braking units. With the acceleration of high-speed rail speed and the improvement of safety requirements, precision castings of high-speed train braking units are not allowed to have defects such as slag inclusion, porosity, shrinkage porosity, and cracks. Numerical calculation software can better obtain the distribution of filling speed, filling temperature, solidification and shrinkage during the precision casting process, predict the occurrence of defects in precision castings in a timely manner, and adopt better precision casting processes to achieve cost savings and improve the quality of precision castings. Lv Yunqi and others used AnyCasting software to numerically simulate the precision casting process of complex cavities under different risers, predicted the shrinkage and porosity of complex cavities, and made process improvements. Pu Sheng et al. used AnyCasting software to numerically simulate the filling and solidification process of impeller precision castings, predicting the occurrence of shrinkage porosity and porosity defects in the center of the impeller precision castings. Zhang Yunping conducted numerical simulation on the precision casting process of butterfly valve body using ProCAST software, and obtained the velocity field, temperature field, and solid fraction distribution of the valve body precision casting. He predicted the defect location of the valve body precision casting and optimized the process, resulting in a defect volume reduction of over 95%.
ZHY Casting takes the high-speed train brake unit box as the research object, and uses AnyCasting software to numerically simulate the filling, solidification, and shrinkage during the precision casting process of the box. It predicts and judges whether there are defects in the casting of the high-speed train brake unit box. Based on the numerical analysis results, the precision casting process is improved to obtain high-performance precision castings of the high-speed train brake unit box.
1. Structure and Model
The high-speed rail braking unit box is an important supporting component of the high-speed rail braking system, as shown in Figure 1. The box, along with the clamp lever assembly, brake shoe support assembly, suspension seat assembly, etc., forms a complex unit with parking brake clamps, and its performance directly affects the safety of high-speed rail trains. The weight of the brake unit box component is 14 kg, and the precision casting size is 302 mm x 287 mm x 235 mm. Its structure is complex, as shown in Figure 2 (a), with a total of three sand cores forming the inner cavity structure. According to the structural characteristics of the braking unit box, a gate is set in the middle of the disc, with a diameter of 50mm and a height of 60mm; Place filter blocks at the bottom of the pouring channel, with a size of 50 mm x 50 mm x 22 mm; The lower part of the filter block is the riser, with a diameter of 75 mm and a height of 90 mm, as shown in Figure 2 (b). Risers are set on both sides of the flange to supplement the thickness, with a diameter of 60 mm and a height of 90 mm.
2. Numerical simulation of precision casting
Based on the design of the riser and runner of the high-speed rail braking unit box, numerical simulation of precision casting forming of the high-speed rail braking unit box was carried out using AnyCasting analysis software, in order to predict the distribution of filling, solidification, and shrinkage in precision casting of the high-speed rail braking unit box, and evaluate whether there are precision casting defects in the high-speed rail braking unit box.
Simulation condition: The pouring temperature is set to 1400 ℃. The chemical composition control is as follows: ω (C) 3.6%~3.7% ω (Si) 2.55%~2.65% ω (Mn) 0.65%~0.75% ω (P) ≤ 0.02% ω (S) ≤ 0.015% ω (Cu) 0.3%~0.4% ω (Mg) 0.035%~0.05%. The pouring time for each type is 9 seconds.
2.1 Distribution of Mold Filling in Precision Casting
The filling capacity of the high-speed rail brake unit box is particularly crucial for obtaining castings with precise dimensions, clear contours, and complete shapes during precision casting. The distribution of precision casting filling in the high-speed rail brake unit box obtained using AnyCasting analysis software is shown in Figure 2. From the filling speed, it can be seen that during the filling process, the speed of molten iron entering the precision casting cavity is relatively fast. As shown in Figure 2 (a), there is a risk of sand flushing in precision castings, and it is necessary to provide sufficient sand mold strength. From the overall filling perspective, the flow of molten iron is relatively turbulent, as shown in Figure 2 (b), where turbulence occurs when the liquid level rises from the bottom of the precision casting cavity. From the perspective of filling temperature, during the filling process, when the molten iron finally completes filling, the minimum surface temperature of precision castings is around 1300 ℃, as shown in Figure 2 (c). At the end of filling, the temperature of the molten metal is above the solidus line, and the probability of forming a cold shut in precision castings is relatively low.
From the analysis of the oxide situation during the filling process, as shown in Figure 2 (d), during the pouring process, the first metal liquid undergoes an oxidation reaction with air, which easily forms oxide slag (indicated by the yellow circle).
2.2 Solidification and shrinkage distribution in precision casting
The solidification and shrinkage distribution of the high-speed rail brake unit box obtained by using AnyCasting analysis software is shown in Figure 3. From the solidification sequence, there are three large isolated liquid phase zones in the precision casting during the solidification process, as shown in Figure 3 (a), which cannot effectively supplement the iron liquid and pose a risk of producing shrinkage porosity and porosity defects. From the perspective of probability defects, as shown in Figure 3 (b), there are a total of 5 areas of the precision casting body that have the risk of shrinkage and porosity, with a probability of more than 60% of defects occurring. Improvement is needed from the process aspect.
3. Improvement of precision casting process
3.1 Process improvement plan
According to the numerical analysis results of the original process, it was found that there are certain defects in the precision casting of the high-speed rail brake unit box, which need to be improved from a process perspective. The improvement plan is shown in Figure 4. Considering the reduction of sand flushing and oxides, the pouring system is improved to reduce the turbulence of molten iron and the impact speed of molten iron on the precision casting cavity. The middle riser is changed to a heating riser (using Shengquan Group’s FT500-M50 × 75 heating riser sleeve), and an additional runner is added on the outer side to connect the two risers, increasing the filling effect of the two risers at the flange. The added cross-sectional area size of the sprue is ϕ 35 mm, the size of the filter block at the bottom of the sprue is 50 mm x 50 mm x 22 mm. The added cross-sectional area size of the transverse runner is 25 mm x 35 mm. The cross-sectional area of the inner sprue connecting the riser with the added transverse sprue is 50 mm x 7 mm. At the same time, subsidies will be added to the riser neck at the flange, with an additional subsidy size of 20 mm in length and 15 mm in width, and an additional riser shrinkage channel will be added.
3.2 Numerical analysis of process improvement
Simulation condition: The pouring temperature is set to 1 360 ℃. Due to the addition of runners, the pouring time for each type has increased to 10.7 seconds.
3.2.1 Filling distribution
Using AnyCasting analysis software to numerically calculate the filling distribution of precision casting for high-speed rail brake unit box, as shown in Figure 5. From the filling speed, when the molten iron enters the riser from the runner, the flow rate increases by 0.6 m/s due to the small cross-sectional area. After entering the precision casting cavity, the speed decreases again. As shown in Figure 5 (a), the overall filling speed is not fast and there is no risk of sand flushing. From the overall filling perspective, the flow of molten iron is relatively stable, as shown in Figure 5 (b). The liquid level rises steadily from the bottom of the precision casting cavity without turbulence. Simulate the temperature field changes during the filling process with the last box temperature of each pack of molten iron at 1360 ℃, as shown in Figure 5 (c). When the molten iron completes the filling process, the minimum surface temperature of the precision casting is around 1275 ℃. At the end of the filling process, the temperature of the molten metal is above the solidus line, and the probability of forming a cold shut in the precision casting is relatively low. From the oxide situation during the filling process, as shown in Figure 5 (d), no obvious oxide defects affecting product quality were formed during the filling process.
3.2.2 Solidification and shrinkage distribution
Using AnyCasting analysis software, numerical calculations were conducted on the solidification and shrinkage distribution of precision casting of high-speed rail brake unit box, as shown in Figure 6. From the solidification sequence, the middle riser can achieve sequential solidification of the disc position, as shown in Figure 6 (a). The risers on both sides of the flange can achieve sequential solidification of the flange position, and no isolated liquid phase zone was found throughout the solidification process. From probabilistic defect analysis, there are 5 defects in the entire pouring system, all located on the riser and runner, as shown in Figure 6 (b). No shrinkage porosity or shrinkage defects were found in the precision casting body.
4. Precision casting specimens and performance test results
4.1 Precision casting specimens
According to the improved process plan for the high-speed rail braking unit box, the high-speed rail braking unit box was poured and the obtained sample is shown in Figure 7. In order to detect defects in the high-speed rail braking unit box, the sample was dissected every 5-10 mm and subjected to dye penetration testing. The results showed that no shrinkage or shrinkage defects were found inside the high-speed rail braking unit box.
4.2 Performance testing of precision casting specimens
Mechanical performance tests were conducted on precision cast specimens of high-speed rail brake unit boxes using testing equipment, and the results are shown in Table. The spheroidization rate, pearlite content, and metallographic structure are shown in Figure 8, with a spheroidization rate of 85% and a pearlite content of 45%. It can be seen that the tensile strength, yield strength, elongation, hardness, and spheroidization rate of the precision cast specimens of the high-speed rail braking unit box all meet the requirements for use.
| Technical indicators | Tensile strength/MPa | Yield strength/MPa | Elongation rate (%) | Hardness (HBW) |
| Customer standards | ≥ 500 | ≥ 320 | ≥ 7 | 170~230 |
| Measurement value | 566 | 368 | 14 | 198 |
5. Conclusion
(1) The AnyCasting numerical simulation software can be used to obtain the distribution of filling, solidification, and shrinkage during the casting process of high-speed rail brake unit box precision casting samples, and predict and judge the forming status of the box precision casting.
(2) Through numerical simulation of the precision casting process of the original high-speed rail brake unit box, the filling speed, filling temperature, oxide, solidification, and shrinkage distribution were obtained. It was predicted that there would be defects such as shrinkage porosity and shrinkage holes in the precision casting of the box, and process improvement is needed.
(3) By improving the pouring system, reducing the turbulence of molten iron and the impact speed of molten iron on the precision casting cavity, the middle riser is changed to a heating riser, and the outer side is connected to two risers by adding a runner, increasing the filling effect of the two risers at the flange; At the same time, measures were taken to improve the process, such as increasing subsidies for the flange neck and adding feeding and shrinking channels for the riser. The filling, solidification, and shrinkage of the box were analyzed again using AnyCasting numerical simulation software, and the predicted results showed that there were no defects in the precision casting of the box.
(4) The precision cast sample of the high-speed rail braking unit box obtained was dissected and subjected to dye penetrant testing, and there were no internal defects, which is consistent with the numerical simulation results of the improved process. In addition, performance testing was conducted on precision cast specimens, and the tensile strength, yield strength, elongation, hardness, and spheroidization rate all met the customer’s standard requirements. The combination of theoretical calculation and simulation can effectively avoid the occurrence of precision casting defects, which is conducive to reducing the cost of precision casting for high-speed rail brake unit boxes and improving the quality of precision casting for boxes.