In gray iron casting production, shrinkage porosity remains one of the most persistent casting defects, particularly in rotational components like brake drums. This article presents a systematic approach to resolving shrinkage defects through optimized solidification control, supported by thermodynamic modeling and production validation.
1. Problem Characterization
The investigated brake drum casting (6 kg mass) exhibited 100% defect rate at mounting surface roots after machining. Key dimensional characteristics include:
| Feature | Dimension (mm) |
|---|---|
| Primary wall thickness | 14.7 |
| Maximum section thickness | 27.1 |
| Minimum section thickness | 6.0 |
| Critical transition radius | 4.4 |
X-ray analysis revealed dendritic structures in defect zones, confirming shrinkage porosity formation. The characteristic shrinkage defect pattern follows the thermal gradient equation:
$$ \nabla T = \frac{\partial T}{\partial x} + \frac{\partial T}{\partial y} + \frac{\partial T}{\partial z} $$
2. Process Analysis
Original process parameters:
| Parameter | Value |
|---|---|
| Pouring temperature | 1,360-1,460°C |
| Gating ratio (ΣFsprue:ΣFrunner:ΣFgate) | 0.9:1.2:1 |
| Mold type | Green sand (FCMX system) |
| Riser height differential | 4 mm |
The inadequate feeding distance (Lf) from side riser to critical section followed:
$$ L_f = 4.5 \times \sqrt{t_{solid}} $$
Where tsolid represents solidification time of the thickest section.
3. Thermodynamic Modeling
Using Chvorinov’s rule for solidification time comparison:
$$ t = \left(\frac{V}{A}\right)^n $$
| Section | Volume (cm³) | Surface Area (cm²) | Solidification Time Ratio |
|---|---|---|---|
| Thick section (B) | 317.4 | 215.8 | 1.00 |
| Transition zone | 84.3 | 92.6 | 0.32 |
The critical solidification gradient (G) and growth rate (R) relationship:
$$ G \times R = \frac{\alpha}{\beta} $$
Where α represents thermal diffusivity and β material constant.
4. Process Optimization
Modified parameters achieved defect elimination:
| Improvement Measure | Original | Optimized |
|---|---|---|
| Gating orientation | Bottom | Top |
| Gate velocity (m/s) | 0.62 | 0.43 |
| Gate cross-section (mm²) | 256.2 | 317.2 |
| Yield improvement | 75.8% | 86.3% |
The feeding efficiency (η) improvement followed:
$$ η = \frac{V_{feed}}{V_{shrink}} \times 100\% $$
5. Quality Validation
Production results after optimization:
| Batch | Quantity | Defect Rate | Yield |
|---|---|---|---|
| Initial trial | 188 | 100% | 89.7% |
| Process validation | 60 | 0% | 92.6% |
| Mass production | 1,304 | 0.23% | 93.1% |
The successful resolution demonstrates that systematic analysis of casting defects through solidification control can significantly improve production quality while maintaining cost efficiency.