Abstract
In the initial development stage of a steel casting convex ring for mining machinery, two lost foam casting (LFC) process schemes were designed based on structural analysis. Numerical simulations of filling and solidification processes were conducted using InteCAST software to evaluate and optimize the schemes. The results indicated that the second scheme, where the large cross-section of the convex ring was positioned upward, demonstrated smoother filling dynamics, superior feeding effects, and minimal shrinkage defects on critical surfaces. This scheme also simplified foam pattern assembly. Practical production trials confirmed that the final castings met stringent quality requirements. This study highlights the efficacy of numerical simulation in optimizing lost foam casting processes for complex steel castings.
1. Introduction
Lost foam casting (LFC) is a modern, eco-friendly casting technique that employs expendable foam patterns to produce high-precision components with minimal post-processing. Its advantages include reduced sand usage, excellent surface finish, and adaptability to diverse alloys such as steel casting, aluminum, and titanium. Despite these benefits, designing lost foam casting processes for geometrically intricate parts—especially those with uneven wall thicknesses or stringent quality requirements—remains challenging.
This paper focuses on optimizing the lost foam casting process for a steel casting convex ring used in mining equipment. The ring’s structural complexity, including varying wall heights (21–114 mm) and a critical wear-resistant surface, necessitated a systematic approach combining numerical simulation and empirical validation. Two process schemes were evaluated, with the goal of minimizing defects like shrinkage porosity and ensuring dimensional accuracy.
2. Structural Analysis of the Convex Ring
The convex ring (Figure 1) has an outer diameter of Φ435 mm, height of 114 mm, and nominal wall thickness of 25 mm. Key design challenges include:
- Critical Surface Requirements: The upper surface, featuring a渐开线 (involute) profile, must be free of defects such as shrinkage cavities, porosity, and cracks.
- Dimensional Constraints: Three internal arc columns require tight positional tolerances for assembly.
- Material Properties: The alloy ZG50CrMnSiA (chemical composition in Table 1) demands precise thermal management to avoid defects.
Table 1. Chemical Composition of ZG50CrMnSiA (wt%)
| Element | C | Si | Mn | S | P | Cr |
|---|---|---|---|---|---|---|
| Range | 0.45–0.55 | 0.40–0.60 | 0.70–1.00 | ≤0.020 | ≤0.030 | 0.80–1.20 |
3. Lost Foam Casting Process Design
3.1. Key Considerations
- Pattern Orientation: Vertical placement was chosen to position the critical surface sideways, avoiding upward-facing defects.
- Gating System: A stepped gating system was adopted to ensure sequential filling and minimize turbulence.
3.2. Two Process Schemes
Scheme 1:
- Narrow section positioned upward, leveraging gravity for feeding the lower thick section.
- Challenges: Difficulty in attaching the pouring cup to the narrow upper section.
Scheme 2:
- Thick section positioned upward, simplifying cup attachment and enhancing feeding via prolonged solidification of the gate.
4. Numerical Simulation Setup
4.1. Material Properties
Thermophysical parameters for the steel casting and EPS foam are summarized in Table 2.
Table 2. Thermophysical Parameters
| Material | Density (g/cm³) | Specific Heat (J/g·K) | Thermal Conductivity (W/m·K) | Latent Heat (J/g) |
|---|---|---|---|---|
| ZG50CrMnSiA (casting) | 7.6 | 0.828 | 22.015 | 251.2 |
| EPS foam | 2.5 | 3.7 | 0.15 | 100 |
4.2. Boundary Conditions
- Pouring temperature: 1640°C
- Vacuum pressure: 0.06 MPa
- Heat transfer coefficients:
- Casting-mold interface: 800 W/m²·K
- Foam-mold interface: 100 W/m²·K
- Ambient air: 10 W/m²·K
4.3. Governing Equations
The filling and solidification processes were modeled using the following energy equation:ρCp∂T∂t=∇⋅(k∇T)+ρL∂fs∂tρCp∂t∂T=∇⋅(k∇T)+ρL∂t∂fs
where ρρ, CpCp, kk, LL, and fsfs represent density, specific heat, thermal conductivity, latent heat, and solid fraction, respectively.
5. Simulation Results and Analysis
5.1. Filling Process
Scheme 1:
- Metal initially filled the narrow upper section (t = 1.66 s), while the lower thick section lagged due to delayed foam degradation (Figure 4a–d).
- Complete filling at t = 12.88 s, with minimal turbulence.
Scheme 2:
- Rapid filling of the lower narrow section (t = 3.52 s), followed by upward flow into the thick section (Figure 6a–e).
- Final filling at t = 12.06 s, with smoother thermal gradients.
Table 3. Filling Time Comparison
| Scheme | Time to Full Fill (s) | Key Observations |
|---|---|---|
| 1 | 12.88 | Slower thick-section filling |
| 2 | 12.06 | Faster narrow-section filling |
5.2. Solidification Process
Scheme 1:
- The thick section solidified last, leading to isolated liquid pools and potential microporosity (Figure 5d).
- Shrinkage defects predicted in the upper narrow section.
Scheme 2:
- Prolonged solidification of the thick section allowed effective feeding from the gate (Figure 7c–e).
- Minimal defects in critical regions.
Table 4. Defect Prediction
| Scheme | Shrinkage Porosity Location | Severity |
|---|---|---|
| 1 | Upper narrow section | Moderate |
| 2 | Gate and lower narrow section | Low |
6. Production Validation
Scheme 2 was implemented for batch production. Key outcomes:
- Dimensional Accuracy: All arc columns met positional tolerances (±0.2 mm).
- Surface Quality: No visible defects on the involute surface.
- Mechanical Performance: Post-heat treatment hardness reached 280–320 HB, satisfying wear resistance requirements.
7. Conclusion
This study demonstrates the critical role of numerical simulation in optimizing lost foam casting processes for steel castings with asymmetric geometries. By comparing two schemes, the upward placement of the thick section (Scheme 2) proved superior in defect mitigation and operational feasibility. Future work will explore advanced alloy systems and multi-objective optimization algorithms to further enhance lost foam casting efficiency.