In the manufacturing of heavy machinery, such as coal mining equipment, the rocker arm is a critical transmission component that operates under extreme conditions, requiring high mechanical performance and structural integrity. These shell castings are characterized by complex geometries, significant variations in wall thickness, and large planar surfaces, which often lead to casting defects like shrinkage porosity, cracks, and deformation. Traditionally, the casting process for such shell castings has faced challenges with low yield rates, increasing production costs and material waste. In this study, we focus on improving the casting process yield rate for rocker arm shell castings using Magma simulation software. By simulating and optimizing the casting process, we aim to predict defect distributions and enhance efficiency, thereby reducing costs and ensuring quality. The shell castings under investigation are made of ZG30Mn steel, with a weight of approximately 4 tons and dimensions of 2745 mm × 1160 mm × 1254 mm, featuring main wall thicknesses of 76 mm and maximum thicknesses of 130 mm. Key areas, such as motor shaft holes and hinge ear roots, are prone to shrinkage defects, necessitating precise control during casting.
The initial casting process for these shell castings involved a two-box molding method with resin-bonded quartz sand, using wooden patterns and a bottom-gating system with six ingates across two levels. Seven blind risers were placed to mitigate shrinkage, but the process yield rate was only 52.0%, resulting in a rough casting weight of 7.8 tons. This low yield indicated significant material loss and high production costs. To address this, we employed Magma simulation to analyze the process, focusing on fluid flow, solidification, and defect formation. The simulation parameters included a pouring temperature of 1560–1580°C, chill temperatures of 25°C, and mold preheating to 150–200°C. Chemical composition of the shell castings is detailed in Table 1, which aligns with standard ZG30Mn requirements.
| Element | C | Si | Mn | P ≤ | S ≤ |
|---|---|---|---|---|---|
| Standard Range | 0.20–0.30 | 0.30–0.45 | 1.10–1.30 | 0.04 | 0.04 |
| Actual Composition | 0.24 | 0.38 | 1.11 | 0.035 | 0.031 |
The Magma simulation for the initial process revealed that shrinkage porosity was primarily concentrated in the risers, with dispersed defects in critical zones of the shell castings, as shown in the simulation results. The yield rate, defined as the ratio of casting weight to total poured metal weight, was calculated using the formula:
$$ \text{Yield Rate} = \frac{W_{\text{casting}}}{W_{\text{total}}} \times 100\% $$
where \( W_{\text{casting}} \) is the weight of the final shell casting (4 tons) and \( W_{\text{total}} \) is the total metal poured (7.8 tons). For the initial process, this yielded 52.0%. The simulation also predicted potential crack formation in thick sections due to thermal stresses, which can be modeled with the thermal stress equation:
$$ \sigma = E \alpha \Delta T $$
where \( \sigma \) is the thermal stress, \( E \) is Young’s modulus, \( \alpha \) is the coefficient of thermal expansion, and \( \Delta T \) is the temperature gradient. These insights highlighted the need for optimization to improve the integrity of the shell castings.
Based on the simulation findings, we optimized the casting process by reorienting the shell castings to a vertical position, which promotes directional solidification and reduces defect formation. The revised design included four open risers and one blind riser at the top, along with chills placed at the bottom to enhance cooling rates. Additionally, chromite sand was used as facing sand in key areas to improve chill effects and resist sand burning. This optimization aimed to increase the yield rate while maintaining the quality of the shell castings. The new process layout is illustrated below, showing the strategic placement of risers and chills for these complex shell castings.
The optimized process was simulated in Magma, demonstrating a significant improvement in yield rate. The total poured metal weight decreased to 5.3 tons, resulting in a yield rate of 74.8%, calculated as:
$$ \text{Yield Rate}_{\text{optimized}} = \frac{4}{5.3} \times 100\% = 74.8\% $$
This represents a 22.8% increase compared to the initial process. The simulation also showed reduced shrinkage porosity, with defects mostly confined to risers and minimal dispersion in critical zones of the shell castings. The solidification time, a key factor in defect formation, was optimized using Chvorinov’s rule:
$$ t_s = B \left( \frac{V}{A} \right)^n $$
where \( t_s \) is the solidification time, \( V \) is volume, \( A \) is surface area, \( B \) is a mold constant, and \( n \) is an exponent typically around 2. By adjusting riser and chill placements, we achieved a more uniform cooling profile for the shell castings, reducing thermal gradients and associated stresses. Table 2 summarizes the comparison between initial and optimized processes for these shell castings.
| Parameter | Initial Process | Optimized Process |
|---|---|---|
| Casting Weight (tons) | 4.0 | 4.0 |
| Total Poured Metal Weight (tons) | 7.8 | 5.3 |
| Yield Rate (%) | 52.0 | 74.8 |
| Number of Riser | 7 (blind) | 5 (4 open, 1 blind) |
| Chill Usage | Limited | Extensive (bottom placement) |
| Predicted Shrinkage Volume | High dispersion | Low, concentrated in risers |
To validate the simulation results, we conducted production trials using the optimized process for the shell castings. The castings were produced in a foundry setting, with careful monitoring of pouring parameters and solidification behavior. After rough machining, the shell castings were inspected using magnetic particle testing (MT) and ultrasonic testing (UT) to assess defect levels. The results confirmed that critical areas, such as motor seat holes and hinge ears, were free from significant shrinkage, porosity, or cracks. Furthermore, pressure tests showed no leakage, meeting the stringent technical requirements for these shell castings. The overall reduction in metal usage—saving 2.5 tons per casting—translates to substantial cost savings in large-scale production.
The success of this optimization highlights the effectiveness of Magma simulation in refining casting processes for complex shell castings. By simulating fluid dynamics and thermal behavior, we can predict defect formation and adjust parameters proactively. For instance, the fluid flow during pouring can be modeled with the Navier-Stokes equations:
$$ \rho \left( \frac{\partial \mathbf{u}}{\partial t} + \mathbf{u} \cdot \nabla \mathbf{u} \right) = -\nabla p + \mu \nabla^2 \mathbf{u} + \mathbf{f} $$
where \( \rho \) is density, \( \mathbf{u} \) is velocity, \( p \) is pressure, \( \mu \) is viscosity, and \( \mathbf{f} \) is body force. This allows for optimizing gating systems to minimize turbulence and oxide inclusion in shell castings. Additionally, the use of chromite sand in critical zones improved heat dissipation, as described by the heat transfer equation:
$$ q = -k \nabla T $$
where \( q \) is heat flux, \( k \) is thermal conductivity, and \( \nabla T \) is temperature gradient. These technical adjustments collectively enhanced the quality and yield of the shell castings.
In conclusion, this study demonstrates that Magma simulation is a powerful tool for optimizing the casting process of rocker arm shell castings, leading to a significant increase in yield rate from 52.0% to 74.8%. The optimization involved reorienting the casting, using open risers and chills, and employing chromite sand, all validated through simulation and production trials. The reduction in rough casting weight from 7.8 to 5.3 tons per shell casting underscores the economic benefits, while the improved defect distribution ensures mechanical integrity. Future work could explore further refinements, such as advanced feeding systems or alloy modifications, to push the yield rate even higher for these demanding shell castings. Ultimately, integrating simulation into foundry practices enables more sustainable and cost-effective production of high-performance shell castings for industrial applications.