Abstract
This paper presents a comprehensive study on the numerical simulation and process optimization of steel casting for the ZG20SiMn rocker arm shell used in shearers. The focus is on reducing casting defects such as shrinkage porosity and shrinkage cavities that often arise due to immature casting processes. Two casting process schemes, top-injection and bottom-injection, were designed and analyzed using ProCAST software. Based on the simulation results, the bottom-injection process was optimized to improve the quality of the rocker arm shell casting. The optimization reduced the defect rate significantly and resulted in a more uniform stress distribution. This study provides valuable theoretical and practical insights for improving the casting process of complex steel castings.
1. Introduction
The rocker arm shell is a critical component of shearers used in mining operations. Its complex geometry, including multistage wall thicknesses and variable cross-sections, poses significant challenges during the casting process. Steel casting of such complex components often leads to defects such as shrinkage porosity and shrinkage cavities, which compromise the mechanical properties and service life of the final product. Therefore, optimizing the casting process is essential to ensure high-quality rocker arm shells.
ZG20SiMn is a widely used low-alloy steel with high strength, good plasticity, and toughness, making it suitable for heavy-duty mechanical components like the rocker arm shell. This study aims to develop an optimized casting process for the ZG20SiMn rocker arm shell through numerical simulation and experimental validation.
2. Materials and Methods
2.1 Material Properties
The rocker arm shell is made of ZG20SiMn steel, which has the following chemical composition (wt.%):
| Element | C | Si | Mn | Mo | Cr | Ni | S | P |
|---|---|---|---|---|---|---|---|---|
| Content | 0.18 | 0.71 | 1.13 | 0.12 | 0.09 | 0.05 | 0.021 | 0.017 |
The material properties, including density, specific heat capacity, and thermal conductivity, vary with temperature and are essential for accurate numerical simulation. Table 1 shows the thermal properties of ZG20SiMn steel at different temperatures.
Table 1: Thermal properties of ZG20SiMn steel at various temperatures
| Temperature (°C) | Density (g/cm³) | Specific Heat Capacity (kJ/kg·K) | Thermal Conductivity (W/m·K) |
|---|---|---|---|
| 25 | 7.81 | 460 | 34.80 |
| 65 | 7.65 | 500 | 30.89 |
| 105 | 7.48 | 575 | 38.51 |
| 145 | 7.20 | 720 | 38.59 |
| 185 | 6.83 | 850 | 39.05 |
2.2 Numerical Simulation Method
The casting process was simulated using ProCAST software, which integrates fluid dynamics, heat transfer, and solidification models to predict casting defects and residual stresses. The simulation process included the following steps:
- Geometry Modeling: The rocker arm shell and casting system were modeled using CAD software and imported into ProCAST.
- Mesh Generation: The models were meshed using the MeshCAST module. A finer mesh was used in regions with complex geometry or critical features.
- Material and Boundary Conditions: Material properties and boundary conditions, including pouring temperature, mold temperature, and heat transfer coefficients, were set based on experimental data and literature.
- Simulation Setup: The simulation was configured to track the filling, solidification, and cooling processes.
- Post-processing: Results were analyzed for temperature distribution, solidification behavior, defect prediction, and stress field.
2.3 Experimental Validation
To validate the simulation results, a prototype rocker arm shell was cast using the optimized process parameters. The casting was inspected for defects and mechanical properties were tested to compare with the simulation results.
3. Results and Discussion
3.1 Filling Process Simulation
The filling process was simulated for both top-injection and bottom-injection schemes. the temperature distribution during the filling process for both schemes.
The top-injection scheme resulted in significant turbulence and unequal filling, especially in thin-walled regions. In contrast, the bottom-injection scheme provided a more stable and uniform filling pattern, reducing the risk of entrapped air and inclusions.
3.2 Solidification Process Simulation
The solidification process was simulated to predict defects such as shrinkage porosity and cavities. the solidification fronts at 50% and 90% solid fraction for both casting schemes.
The bottom-injection scheme resulted in a more uniform solidification front, promoting sequential solidification and reducing the likelihood of shrinkage defects. The top-injection scheme showed localized hot spots and slower cooling rates, contributing to shrinkage porosity.
3.3 Defect Prediction
Defects were predicted using the Niyama criterion, which considers temperature gradients and cooling rates. the predicted defect locations for both casting schemes.
The top-injection scheme predicted more shrinkage defects, particularly in thick-walled regions. The bottom-injection scheme showed significantly fewer defects, indicating the effectiveness of the optimized process.
3.4 Stress Field Analysis
Residual stresses were analyzed to assess the mechanical integrity of the castings. the stress distribution in the rocker arm shell for the optimized bottom-injection scheme.
High stresses were observed in thin-walled and variable cross-section regions. However, the optimized process significantly reduced peak stresses, improving the overall mechanical performance of the casting.
3.5 Process Optimization
Based on the simulation results, the bottom-injection process was further optimized by modifying the riser design and adding chillers. the optimized casting system.
The optimized process reduced defects further and improved the overall quality of the rocker arm shell casting. Table 2 summarizes the defect reduction achieved through optimization.
Table 2: Defect reduction through process optimization
| Scheme | Initial Defect Volume (cm³) | Optimized Defect Volume (cm³) | Reduction (%) |
|---|---|---|---|
| Top-Injection | 143.41 | – | – |
| Bottom-Injection (Initial) | 133.57 | 9.01 | 93.3 |
3.6 Experimental Validation
An experimental casting was produced using the optimized bottom-injection process. Non-destructive testing (NDT) confirmed a significant reduction in casting defects, consistent with the simulation results. Mechanical testing showed improved strength and ductility compared to the initial casting process.
4. Conclusion
This study successfully optimized the casting process for the ZG20SiMn rocker arm shell through numerical simulation and experimental validation. The bottom-injection scheme was found to be superior to the top-injection scheme, resulting in fewer casting defects and improved mechanical properties. Further optimization of the riser design and addition of chillers significantly reduced defects and residual stresses. The findings of this study provide valuable insights for improving the casting quality of complex steel components.
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