This paper presents the casting process optimization for a split-type 9AT differential housing made of QT600-M ductile iron. The component weighs 3.32 kg as-cast with challenging geometric features including four windows, three asymmetric bosses, and strict dimensional tolerances (maximum mismatch ≤ 0.5 mm). The production utilized DISA molding line technology to achieve high precision and repeatability.
Material Requirements and Challenges
The chemical composition and mechanical properties requirements are shown in Tables 1-3. Critical quality targets included:
- Internal porosity ≤ 3% defect area ratio (D3/1 criteria)
- X-ray inspection ≤ ASTM E446 Level 2
- CT scan compliance for critical sections
| C | Si | Mn | Cu | Mg | S | P | Sn | Ti |
|---|---|---|---|---|---|---|---|---|
| 3.3-3.9 | 1.8-3.0 | 0.2-1.0 | 0.2-1.0 | 0.027-0.06 | ≤0.02 | ≤0.06 | ≤0.06 | ≤0.06 |
| Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) | Hardness (HBW) |
|---|---|---|---|
| ≥650 | ≥405 | ≥3 | 200-265 |
Initial Process Design
The original gating system employed three risers for two castings with stepped runner connections. The riser design followed modulus calculations:
$$
M_{riser} = \frac{V}{A} = \frac{120 \times 43 \times 50}{2(120 \times 43 + 120 \times 50 + 43 \times 50)} = 6\ \text{mm}
$$
where $M_{riser}$ is riser modulus, $V$ is volume, and $A$ is cooling surface area. Simulation revealed shrinkage defects up to 11 mm³ in pin holes (Figure 4), necessitating process modifications.
Process Optimization
Key improvements addressed casting defects and production efficiency:
- Orientation Adjustment: Rotated casting 90° to optimize riser feeding positions
- Gating System Redesign: Implemented top-feeding runners with splash guards
- Venting Enhancement: Added 6 mm venting slots in horizontal runners
- Process Yield Improvement: Reduced runner weight by 18% through sectional optimization
The modified feeding system reduced pouring time from 16s to 10.3s, calculated as:
$$
t = \frac{W}{\rho \cdot A \cdot \sqrt{2gH}} = \frac{6.64\ \text{kg}}{7030\ \text{kg/m³} \cdot 423\ \text{mm²} \cdot \sqrt{2 \cdot 9.8 \cdot 0.3}} \approx 10.2\ \text{s}
$$
where $W$ is metal weight, $\rho$ is density, $A$ is choke area, and $H$ is metallostatic head.
Defect Control Strategy
Critical measures for casting defect reduction included:
| Defect Type | Control Method | Effectiveness |
|---|---|---|
| Shrinkage | Riser modulus optimization | Defect volume ↓82% |
| Gas Porosity | Venting system upgrade | Blowholes ↓95% |
| Sand Inclusion | Runner junction redesign | Surface defects ↓70% |
Production Results
The optimized process achieved:
- Yield improvement: 36.7% → 42.7%
- Scrap rate reduction: 3.29% (vs. initial 6.5%)
- X-ray inspection pass rate: 100%
- CT scan compliance: 98.7%
Final mechanical properties exceeded requirements with pearlite content ≥75% and nodularity ≥85%, satisfying:
$$
\frac{P_{actual}}{P_{required}} = \frac{650\ \text{MPa}}{405\ \text{MPa}} = 1.6 > 1.2\ \text{(safety factor)}
$$
Conclusion
The developed casting process successfully addressed critical casting defect challenges through systematic gating design optimization and process parameter adjustments. The combination of modulus-based riser design, venting system enhancement, and automated DISA line production proved effective for manufacturing high-precision differential housings with complex geometries. Continuous monitoring showed sustained defect control capability, with production scrap rates maintained below 3.5% over six months of mass production.