Abstract
The rocker arm shell, an essential component of a shearer, exhibits complex geometric features such as multi-stage wall thickness and variable cross-sections. This study aims to improve the casting quality of the rocker arm shell made from ZG20SiMn steel by addressing casting defects like shrinkage porosity and shrinkage cavity due to immature casting processes. Using ProCAST software, two pouring schemes—top-injection and bottom-injection—were designed and simulated to analyze the filling and solidification processes of the rocker arm shell casting. Based on the Niyama criterion and stress field distribution, the bottom-injection process was optimized. The results showed that the optimized bottom-injection process maintained an incremental temperature gradient during solidification, effectively promoting sequential solidification and reducing casting defects. The shrinkage cavity volume accounted for only 0.0049% of the rocker arm shell volume, with stress optimization of 38.47% at the thin-wall end face of the motor hole and 91.08% at the output port. This study provides a theoretical basis and data support for enhancing the casting technology of shearer rocker arm shells.
Keywords: Steel casting, casting defect, process optimization, ZG20SiMn, rocker arm shell, stress field, sequential solidification
1. Introduction
The rocker arm shell is a critical component in coal mining machinery, particularly in shearers, where it serves as a connection between the spiral drum and the main body. The rocker arm shell experiences high levels of cyclic loading and complex impact stresses during operation, leading to the accumulation of residual stresses in areas of low stiffness and variable cross-sections. These stresses can cause local deformations or even fractures if not managed properly (Zhao et al., 2023).
ZG20SiMn steel is commonly used in heavy machinery due to its high strength, good ductility, and toughness. However, the rocker arm shell, being a large and complex cast part, is prone to casting defects such as shrinkage porosity and shrinkage cavity if the casting process is not carefully controlled. Therefore, optimizing the casting process to reduce these defects and minimize residual stresses is crucial for improving the quality and reliability of the rocker arm shell.
In this study, numerical simulations using ProCAST software were conducted to investigate and optimize the casting process of a ZG20SiMn rocker arm shell. The objectives were to compare the filling and solidification behaviors of top-injection and bottom-injection schemes, identify casting defects, and optimize the bottom-injection process based on the Niyama criterion and stress field analysis.
2. Literature Review
Several studies have addressed casting process optimization for large and complex components, including those made from ZG20SiMn steel. Kan et al. (2021) improved the casting quality of a rocker arm shell by optimizing the material and casting process through field tests and finite element analysis. Yu (2022) proposed a casting process for the rocker arm shell of a 3050 shearer, while Lyu et al. (2015) developed a high-strength rocker arm shell and optimized its casting process. Wang et al. (2012) also conducted studies on the casting process for shearer rocker arms.
Moreover, studies on similar materials and components have shown the effectiveness of numerical simulations in predicting and mitigating casting defects. Jiang et al. (2022) used ProCAST to simulate the local pressurization casting process of A356.2 aluminum alloy wheel hubs, while Ma (2021) investigated the evolution of residual stresses in marine diesel engine blocks during casting and subsequent heat treatment processes.
This study builds upon these findings by focusing specifically on the casting process optimization of ZG20SiMn rocker arm shells through comprehensive numerical simulations and process modifications.
3. Materials and Methods
3.1 Material Properties
The rocker arm shell was cast using ZG20SiMn steel, which has excellent mechanical properties suitable for heavy machinery applications. The chemical composition and thermophysical properties of ZG20SiMn steel are provided in Tables 1 and 2, respectively.
Table 1: Chemical Composition of ZG20SiMn Steel (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 |
Table 2: Thermophysical Properties of ZG20SiMn Steel
| Temperature (°C) | Density (g/cm³) | Enthalpy (kJ/kg) | Thermal Conductivity (W/m·K) |
|---|---|---|---|
| 25 | 7.81 | 111.3 | 34.80 |
| 65 | 7.65 | 358.67 | 30.89 |
| 105 | 7.48 | 650.01 | 38.51 |
| 145 | 7.20 | 984.52 | 38.59 |
| 185 | 6.83 | 1508.21 | 39.05 |
3.2 Modeling and Simulation Setup
The 3D model of the MG325 shearer rocker arm shell was developed using CAD software and imported into ProCAST for mesh generation and simulation. The shell dimensions were 1742 mm × 1040 mm × 524 mm, with a net weight of 1462 kg. Two pouring schemes—top-injection and bottom-injection—were designed to compare their filling and solidification behaviors.
The mesh for the rocker arm shell and pouring systems was generated using the MeshCAST module in ProCAST. The primary mesh size for the shell was 32 mm, with a finer mesh size of 8 mm for thin-walled regions. The total number of mesh elements for the top-injection and bottom-injection models was 1,622,421 and 1,085,291, respectively. The mold mesh size was set to 80 mm to reduce computational time.
The casting parameters were set based on standard practices for ZG20SiMn steel castings. The pouring temperature was 1580 °C, with a pouring speed of 35 kg/s. The mold was preheated to 250 °C, and heat transfer coefficients were set based on the materials in contact (resin-bonded sand, cold iron, and air).
3.3 Mathematical Models
The filling and solidification processes were simulated using fluid dynamics and heat transfer models implemented in ProCAST.
3.3.1 Filling Process Model
The filling process was modeled using the Navier-Stokes equations for incompressible fluids:
∂x∂ux+∂y∂uy+∂z∂uz=0
ρ(∂t∂ux+ux∂x∂ux+uy∂y∂ux+uz∂z∂ux)=−∂x∂p+μ(∂x2∂2ux+∂y2∂2ux+∂z2∂2ux)
(Similar equations for uy and uz apply.)
The energy equation was used to model the temperature distribution during filling:
ρcp(∂t∂T+ux∂x∂T+uy∂y∂T+uz∂z∂T)=∇⋅(k∇T)+q˙
where cp is the specific heat capacity, k is the thermal conductivity, and q˙ is the heat source term.
3.3.2 Solidification Process Model
The solidification process was modeled using the heat conduction equation:
ρcp∂t∂T=∇⋅(k∇T)+E˙
where E˙ represents the internal heat generation or absorption during phase change.
3.3.3 Defect Prediction
The Niyama criterion was used to predict shrinkage porosity and shrinkage cavity:
Niyama=RCG
where G is the temperature gradient, R is the cooling rate, and C is a material-dependent constant.
3.3.4 Stress Field Model
The stress field was modeled using the von Mises equivalent stress:
σe=21[(σ1−σ2)2+(σ2−σ3)2+(σ3−σ1)2]+3(τxy2+τyz2+τzx2)
where σ1, σ2, and σ3 are the principal stresses, and τxy, τyz, and τzx are the shear stresses.
4. Results and Discussion
4.1 Filling Process Analysis
The filling processes for the top-injection and bottom-injection schemes were simulated, and their temperature fields were compared.
The bottom-injection scheme showed a more stable filling process with better venting and reduced defects such as entrapped gas and inclusions. The temperature distribution was also more uniform, indicating a more efficient heat transfer during filling.
4.2 Solidification Process Analysis
The solidification processes were simulated to analyze the solid fraction and temperature distributions.
Both schemes exhibited similar solidification behaviors, with solidification starting from the bottom and progressing upwards. However, isolated liquid regions were observed at the thick-walled sections near the output port, indicating potential areas for shrinkage defects.
4.3 Defect Prediction
The Niyama criterion was applied to predict shrinkage defects in both schemes.
The bottom-injection scheme had fewer predicted defects, with a lower volume of shrinkage cavities (133.57 cm³ compared to 143.41 cm³ for the top-injection scheme).
4.4 Stress Field Analysis
The stress fields were analyzed to identify areas of high residual stress concentration.
High stress concentrations were observed at the motor hole, connection axle hole, idler axle hole, and output port, with stress values exceeding the material’s yield strength in some regions.
4.5 Process Optimization
Based on the simulation results, the bottom-injection scheme was optimized by modifying the riser design, adding chillers, and increasing the mold preheat temperature.
The optimized scheme significantly reduced defects and stress concentrations.
The optimized process resulted in a 93.3% reduction in shrinkage cavity volume and substantial stress reductions at critical locations.
5. Conclusion
This study investigated the casting process of a ZG20SiMn rocker arm shell using numerical simulations. The bottom-injection scheme was found to be superior to the top-injection scheme in terms of filling stability, defect formation, and stress distribution. By optimizing the bottom-injection process through riser modifications, chiller addition, and mold preheat temperature adjustment, significant improvements were achieved in casting quality. The optimized process reduced shrinkage cavity volume by 93.3% and substantially reduced residual stresses at critical locations. These findings provide valuable insights into the casting process optimization of large and complex steel castings.