In sand-lined metal mold casting processes, nitrogen porosity defects frequently manifest as irregular cavities concentrated near the upper outer radius of automotive brake drums. These defects, visible after 1–2 mm of machining or as surface holes in severe cases, significantly compromise component integrity. This study investigates the root causes and implements corrective measures through material analysis and process optimization.
Casting Process Overview
The production utilizes medium-frequency induction furnaces with charge materials comprising 60–70% steel scrap and 30–40% returns. Key process parameters include:
| Parameter | Value |
|---|---|
| Melting temperature | 1,500–1,530°C |
| Pouring temperature | ≥1,360°C |
| Resin content in sand | 2.0–2.5% |
| Hexamine proportion | 15% of resin |
The gating system employs a top-fed design with ceramic filters (15 PPI), achieving a cross-section ratio of $$ \Sigma S_{\text{inner}} : \Sigma S_{\text{runner}} : \Sigma S_{\text{sprue}} = 1 : 4.17 : 2.04 $$
Defect Characterization
Energy-dispersive spectroscopy (EDS) of defective regions revealed elemental segregation:
| Element | Mass Percentage (%) |
|---|---|
| N | 49.5 |
| C | 28.58 |
| Fe | 12.71 |
| O | 8.87 |
The nitrogen concentration gradient during solidification follows Sievert’s law:
$$ S_N = k \sqrt{P_N} $$
where \( S_N \) = nitrogen solubility, \( k \) = equilibrium constant, and \( P_N \) = nitrogen partial pressure.
Mechanism of Nitrogen Porosity Formation
Three primary nitrogen sources contribute to casting defects:
- Steel scrap (N content: 40–120 ppm)
- Coating additives (hexamine decomposition: \( \text{(CH}_2\text{)}_6\text{N}_4 \rightarrow 6\text{CH}_2\text{O} + 4\text{NH}_3 \))
- Recycled sand (LOI ≤ 2.6%, gas evolution ≤20 mL)
The critical nitrogen threshold for defect formation is:
$$ \omega(N)_{\text{critical}} = 120\ \text{ppm} $$
Measured melt nitrogen levels (71.7 ppm) excluded molten metal as the primary defect source.
Process Optimization Strategies
| Parameter | Original | Optimized |
|---|---|---|
| New sand addition | 5–20% | 20–40% |
| Mold temperature | 220–240°C | 230–250°C |
| Vent channels | 8 | 20 |
| Curing time | 10 min | ≥20 min |
Key improvements included:
1. Segregated sand management for shell/core production
2. Enhanced venting through strategic placement of 0.5–1.0 mm diameter vent plugs
3. Modified coating formulation reducing hexamine by 30%
Results and Validation
The implemented changes reduced nitrogen porosity defects from 5.2% to 0.1% in production batches exceeding 100,000 components. The relationship between sand reuse cycles and residual nitrogen demonstrates:
$$ N_{\text{residual}} = N_0 \cdot e^{-0.15k} $$
where \( k \) = number of sand reuse cycles.
Conclusion
This systematic approach to controlling casting defects through material selection and process parameter optimization proves effective for sand-lined metal mold production. Continuous monitoring of sand LOI values below 2.0% and maintaining melt nitrogen under 80 ppm ensures consistent quality in high-volume manufacturing.