Steel casting plays a critical role in manufacturing heavy-duty components like shearer rocker arm housings, which require exceptional structural integrity under cyclic stresses. This study investigates the casting process optimization of ZG20SiMn steel for a complex rocker arm housing with variable wall thicknesses (10–80 mm) using numerical simulation. Two gating systems—top-injection and bottom-injection—were evaluated to minimize defects such as shrinkage porosity and residual stress.
Mathematical Modeling of Casting Processes
The fluid dynamics during mold filling were governed by continuity and energy conservation equations. For incompressible flow, the continuity equation simplifies to:
$$ \text{div} \mathbf{F} = \frac{\partial u_x}{\partial x} + \frac{\partial u_y}{\partial y} + \frac{\partial u_z}{\partial z} = 0 $$
Energy transfer during solidification was modeled using:
$$ \rho c_p \frac{\partial T}{\partial t} = \nabla \cdot (\lambda \nabla T) + \dot{E} $$
where \( \rho \), \( c_p \), and \( \lambda \) represent density, specific heat, and thermal conductivity, respectively.
Defect Prediction Using Niyama Criterion
Shrinkage defects were analyzed through the Niyama criterion:
$$ G / \sqrt{R} \leq C_{\text{Niyama}} $$
where \( G \) denotes temperature gradient, \( R \) is cooling rate, and \( C_{\text{Niyama}} \) is a material-specific threshold. The thermal gradient was calculated as:
$$ G = \max \left( \frac{T_{i+m,j,k} – T_{i,j,k}}{m \Delta x}, \frac{T_{i,j+m,k} – T_{i,j,k}}{m \Delta y}, \frac{T_{i,j,k+m} – T_{i,j,k}}{m \Delta z} \right) $$
Material Properties and Process Parameters
ZG20SiMn steel’s thermophysical properties and chemical composition are critical for accurate simulation:
| C | Si | Mn | Mo | Cr | Ni | S | P |
|---|---|---|---|---|---|---|---|
| 0.18 | 0.71 | 1.13 | 0.12 | 0.09 | 0.05 | 0.021 | 0.017 |
| Temperature (°C) | Density (g/cm³) | Enthalpy (kJ/kg) | Conductivity (W/m·K) |
|---|---|---|---|
| 253 | 7.81 | 111.31 | 34.80 |
| 653 | 7.65 | 358.67 | 30.89 |
| 1053 | 7.48 | 650.01 | 38.51 |
| 1453 | 7.20 | 984.52 | 38.59 |
| 1853 | 6.83 | 1508.21 | 39.05 |
Simulation Results and Process Comparison
The bottom-injection system demonstrated superior performance in steel casting:
| Parameter | Top-Injection | Bottom-Injection |
|---|---|---|
| Shrinkage Volume (cm³) | 143.41 | 133.57 |
| Defect Percentage | 0.0775% | 0.0721% |
| Filling Time (s) | 49.63 | 50.35 |
Thermal analysis revealed distinct solidification patterns:
$$ \text{Solid Fraction} = 1 – \left( \frac{T – T_{\text{solidus}}}{{T_{\text{liquidus}} – T_{\text{solidus}}} \right)^n $$
where \( n \) represents the Scheil coefficient. The bottom-injection system maintained better thermal gradients, promoting directional solidification.
Process Optimization Strategy
Key improvements for steel casting quality included:
- Modified riser design with increased dimensions
- Strategic placement of chill plates
- Enhanced mold preheating (250°C → 300°C)
Optimization results demonstrated significant improvements:
$$ \text{Stress Reduction} = \frac{\sigma_{\text{initial}} – \sigma_{\text{optimized}}}{\sigma_{\text{initial}}} \times 100\% $$
| Location | Initial Stress (MPa) | Optimized Stress (MPa) | Reduction |
|---|---|---|---|
| Motor Hole | 850 | 523 | 38.47% |
| Output Port | 632 | 56.4 | 91.08% |
Residual Stress Analysis
The von Mises stress distribution confirmed process effectiveness:
$$ \sigma_{\text{von Mises}} = \sqrt{\frac{1}{2} \left[ (\sigma_1 – \sigma_2)^2 + (\sigma_2 – \sigma_3)^2 + (\sigma_3 – \sigma_1)^2 \right]} $$
Post-optimization stress concentrations decreased significantly, particularly in thin-wall sections prone to cracking.
Conclusion
This study establishes a systematic approach for optimizing steel casting processes of complex geometries. The bottom-injection system with enhanced riser design and thermal management reduced shrinkage defects by 93.3% and residual stresses by up to 91.08%. These findings provide critical insights for improving the manufacturing reliability of heavy machinery components through advanced steel casting techniques.