This article delves into the casting process of nodular cast iron motor shells. It begins with an introduction to the product and its technical requirements, followed by a detailed exploration of the casting process design, including ，and the application . The trial production phase, along with the corresponding improvement measures, is presented. Finally, conclusions are drawn, highlighting the importance of simulation software in the casting process and the need to consider various factors to prevent casting defects.
1. Introduction
With the continuous development of urbanization, the demand for urban rail transit is on the rise. The motor shell, a crucial component of the urban rail transit traction system, is typically manufactured through casting. The casting process of the motor shell requires careful design and optimization to ensure the quality of the final product. In this study, we focus on the casting process of a nodular cast iron motor shell, aiming to develop a reliable and efficient casting process.
2. Product Introduction and Technical Requirements
2.1 Product Overview
The motor shell for urban rail transit traction systems is made of nodular cast iron with the material grade of QT500 – 7. It has a thin – walled barrel – shaped structure. The outline dimensions are 760 mm×500 mm×580 mm, and the mass is approximately 160 kg. The main wall thickness is 10 mm, while the maximum wall thickness reaches 90 mm. The resin sand manual molding process is employed for production. The 3D model of the motor shell is shown in Figure 1.
| Feature | Details |
|---|---|
| Material | QT500 – 7 nodular cast iron |
| Structure | Thin – walled barrel – shaped |
| Dimensions | 760 mm×500 mm×580 mm |
| Mass | About 160 kg |
| Wall Thickness | Main: 10 mm, Maximum: 90 mm |
| Molding Process | Resin sand manual molding |
2.2 Technical Requirements
The motor shell is required to undergo X – ray flaw detection. The acceptance criteria are Level 2 for key areas and Level 3 for non – key areas. Preliminary analysis indicates that there are multiple isolated hot spots in the product, mainly distributed in the following areas: ① the contact area between the inner ring ribs and the barrel wall; ② the contact area between the upper and lower mounting brackets and the barrel wall; ③ the contact area between the safety bracket and the barrel wall; ④ the contact area between the junction box and the barrel wall. The specific locations and modulus values of the hot spots are determined through the assistance of MAGMA simulation software.
3. Casting Process Design
3.1 Determination of Parting Method
3.1.1 Analysis of Pouring Position Options
From the product structure analysis, there are two possible pouring position options: ① vertical placement with the axis of the round barrel perpendicular to the ground (referred to as vertical placement); ② horizontal placement with the axis of the round barrel parallel to the ground (referred to as horizontal placement).
| Pouring Position | Advantages | Disadvantages |
|---|---|---|
| Vertical Placement | The number of sand cores needed around the barrel wall is large. The gating system uses bottom – pouring, ensuring smooth molten iron filling, and it is easier to solve the hot spots at the inner ring ribs. | It is difficult to solve the hot spots at the mounting brackets. |
| Horizontal Placement | The number of sand cores is relatively less compared to vertical placement. It is easier to solve the hot spots at the mounting brackets. | It is difficult to solve the hot spots at the inner ring ribs. |
3.1.2 Selection of the Final Parting Method
Considering the product’s usage requirements, the horizontal HALF parting method is ultimately chosen. This method balances the challenges of hot – spot resolution and the complexity of the sand – core setup, providing a more suitable solution for the casting process.
3.2 Gating System Design
3.2.1 Options for Molten Iron Inlet
After determining the parting method, there are two options for the molten iron inlet: ① setting annular horizontal runners at both ends of the lower – type round barrel and using flat runners to introduce molten iron; ② setting horizontal runners along the parting surface at the barrel wall and using flat runners to introduce molten iron.
| Option | Advantages | Disadvantages |
|---|---|---|
| Option 1 | Smooth filling | Low casting process yield |
| Option 2 | High process yield | Turbulence is likely to occur during filling |
3.2.2 Selection of the Final Option and Parameter Calculation
After comprehensive consideration, the second option is selected. The existing sand box dimensions are: upper type 1000 mm×980 mm×480 mm, lower type 1000 mm×980 mm×340 mm. One casting is produced per mold. The gating system is designed with 2 ingates to introduce molten iron into the cavity. After determining the process plan, specific process parameters are calculated as follows:
| Parameter | Formula | Calculation Process | Result |
|---|---|---|---|
| Pouring Time (t) | \(t = K\sqrt{G}\) | \(G\approx180 kg\), \(K = 1.85\) | \(t = 25 s\) |
| Liquid Level Rising Speed (V) | \(V=\frac{c}{t}\) | \(c = 48 cm\), \(t = 25 s\) | \(V = 1.92 cm/s\) |
| Choke Area (\(S_{E}\)) | \(S_{E}=\frac{G}{0.3\mu t\sqrt{H}}\) | \(\mu = 0.48\), \(H = 35 cm\), \(G = 180 kg\), \(t = 25 s\) | \(S_{E}=8.5 cm^{2}\), corrected to \(12 cm^{2}\) |
| Cross – sectional Areas of Each Component of the Gating System | \(\sum S_{直}:\sum S_{横}:\sum S_{内}=1.2:1.5:1\) | Based on \(S_{内}=6 cm^{2}\) (single ingate), \(S_{横}=18 cm^{2}\), \(S_{直}=14 cm^{2}\) |
3.3 Application of Risers and Chills
3.3.1 Initial Simulation Analysis
Without the use of chills and risers, the solidification process of the casting is initially simulated using MAGMA software based on the calculated process parameters. The simulation results show that there are multiple large hot spots in the casting, as shown in Figure 2 and Figure 3. The predicted shrinkage porosity defects are presented in Figure 4.
| Simulation Stage | Description |
|---|---|
| Solidification Simulation | Shows the progress of solidification at different solid – phase ratios, such as 固相 5%, 10%, etc. |
| Hot – Spot Distribution | Identifies areas with concentrated heat, indicating potential problem areas. |
| Shrinkage Porosity Defect Prediction | Predicts the location of shrinkage porosity based on thermal analysis. |
3.3.2 Selection of Risers and Chills
Based on the simulation results and in combination with the product technical requirements, process measures such as placing chills and exothermic insulating risers at the corresponding hot – spot locations are adopted. The initial casting process plan is shown in Figure 5. Exothermic insulating risers are selected according to the specifications and the modulus of the hot spots at the placement locations. It should be noted that after placing the exothermic insulating risers, the modulus of the casting hot – spot at the placement location will increase. When selecting risers, they are generally chosen with a modulus 1.2 – 1.3 times larger than that of the casting hot – spot area. For example, as shown in Figure 6, the modulus of the hot – spot at the upper mounting bracket is 1.6 cm before setting the riser, and it increases to 1.9 cm after setting the riser.
| Component | Modulus Before Riser Placement | Modulus After Riser Placement |
|---|---|---|
| Upper Mounting Bracket Hot – Spot | 1.6 cm | 1.9 cm |
4. Trial Production and Results
4.1 Pre – trial Production Simulation
Before the trial production, the solidification process of the initially determined casting process plan is simulated, as shown in Figure 7. The simulation indicates that the selection and placement of the risers form a temperature gradient field from low to high, achieving local sequential solidification. The last solidification part of the casting is in the top riser, suggesting a low possibility of shrinkage porosity defects in the casting.
| Simulation Stage | Observation |
|---|---|
| Solidification Simulation Before Trial Production | Shows the solidification process and the role of risers in forming a temperature gradient. |
4.2 First – trial Production Results
After the first trial production, X – ray inspection of the casting reveals that there are shrinkage porosity defects in the contact area between the junction box and the barrel wall, as shown in Figure 8. These defects exceed the Level 3 standard for non – key areas specified in the technical requirements, failing to meet the customer’s acceptance requirements. The cause of the defects is analyzed as the presence of an isolated liquid – phase region in this area, which cannot be sufficiently fed with molten iron, resulting in shrinkage porosity defects.
| Inspection Method | Defect Location | Defect Severity |
|---|---|---|
| X – ray Inspection | Contact area between junction box and barrel wall | Exceeds Level 3 standard for non – key areas |
4.3 Improvement Measures and Second – trial Production
After analyzing the causes of the defect formation, corresponding solutions are implemented. Two chills are added to the contact position between the casting junction box and the barrel wall to accelerate the solidification speed of this area, reduce the isolated liquid – phase region, and minimize the area of shrinkage porosity defects. The optimized casting process plan is shown in Figure 9. The second – trial production casting passes all inspections, including X – ray and mechanical property tests, meeting the customer’s acceptance standards. The chemical composition of the casting and the mechanical properties of the single – cast specimens are presented in Table 1 and Table 2.
| Element | Content (%) |
|---|---|
| C | 3.64 |
| Si | 2.83 |
| Mn | 0.38 |
| P | 0.029 |
| S | 0.01 |
| Cu | 0.40 |
| Mg | 0.045 |
Table 1: Chemical composition of casting (mass fraction, %)
| Property | Technical Requirement | Measured Value |
|---|---|---|
| Tensile Strength (MPa) | ≥500 | 570 |
| Yield Strength (MPa) | ≥320 | 423 |
| Elongation (%) | ≥7 | 15.5 |
| Hardness (HBW) | 170 – 230 | 200 |
5. Conclusions
The application of simulation software in the casting process design simplifies the design process. In particular, in modulus calculation, it eliminates the cumbersome division calculations in the past, allowing for direct acquisition of results from the simulation. When determining casting defects such as shrinkage porosity, it is necessary to consider the isolated liquid – phase regions that occur during the solidification of the molten metal. These regions cannot be ignored even if the possibility of shrinkage porosity defects is low, as they are related to factors such as the chemical composition, pouring temperature, and inoculation conditions set during the simulation. Through the design, optimization, and trial production of the casting process of the nodular cast iron motor shell, a qualified casting can be rapidly developed, meeting the requirements of urban rail transit traction systems. Future research can focus on further optimizing the casting process, exploring new materials and technologies to improve the quality and performance of motor shells.
