This paper presents a comprehensive methodology for designing and manufacturing complex aluminum alloy bridge brackets through sand casting, integrating 3D printing (3DP) sand mold technology and low-pressure casting. The process addresses critical challenges in achieving dimensional accuracy (DCTG7 per GB/T6414-2017) and internal quality (Class II per GB/T9438-2013) for ZL114A alloy components.
1. Process Design Fundamentals
The casting process utilizes bottom gating with slit runners to ensure stable filling, complemented by 30 strategically placed chill plates (20 mm thickness) to control solidification. The gating system design follows the hydrodynamic principles of sand casting:
$$ Q = \sum_{i=1}^{n} A_i \cdot v_i \cdot t $$
Where:
$Q$ = Total metal volume (m³)
$A_i$ = Cross-sectional area of runner $i$ (m²)
$v_i$ = Flow velocity at runner $i$ (m/s)
$t$ = Filling time (s)
2. Mold Design Optimization
The 3DP sand mold structure features three modular components with enhanced positioning accuracy:
| Mold Section | Critical Parameters | Optimization Features |
|---|---|---|
| Upper Mold | Chill plate slots: 30 locations | 8° draft angle on locating pins |
| Middle Mold | Core wall thickness: 35 mm | Reinforced ribs for core stability |
| Lower Mold | Runner dimensions: 90×50 mm | 0.5 mm clearance on locating features |
3. Solidification Control Strategy
The thermal management system combines chill plates and gating design to achieve directional solidification. The Niyama criterion for sand casting defect prediction is applied:
$$ N = \frac{G}{\sqrt{\dot{T}}} $$
Where:
$G$ = Temperature gradient (K/m)
$\dot{T}$ = Cooling rate (K/s)
Critical value: $N_{critical} > 1$ (K1/2·s1/2/m)
| Process Parameter | Initial Value | Optimized Value |
|---|---|---|
| Pouring Temperature | 690°C | 680°C |
| Filling Pressure | 50 kPa | 55 kPa |
| Chill Plate Count | 30 | 40 |
4. Quality Validation
Post-optimization inspection results demonstrate significant improvements in sand casting quality:
| Quality Parameter | Initial Batch | Optimized Batch |
|---|---|---|
| Dimensional Accuracy | DCTG8 | DCTG7 |
| Surface Defects | 3.2 defects/m² | 0.8 defects/m² |
| Internal Porosity | Class III | Class II |
5. Mechanical Performance
The final sand cast components meet stringent aerospace requirements:
$$ \sigma_b \geq 290\text{MPa}, \delta \geq 2\% $$
Experimental results from 10 samples:
| Sample | Tensile Strength (MPa) | Elongation (%) |
|---|---|---|
| 1 | 316 | 3.0 |
| 2 | 303 | 3.0 |
| 3 | 312 | 3.5 |
6. Process Economics
The integration of 3DP sand casting technology demonstrates significant production advantages:
$$ C_{total} = C_{material} + C_{energy} + C_{labor} + C_{tooling} $$
Cost comparison (per unit):
| Cost Component | Traditional Casting | 3DP Sand Casting |
|---|---|---|
| Tooling | $1,200 | $380 |
| Lead Time | 28 days | 7 days |
| Material Yield | 68% | 83% |
This systematic approach to sand casting process optimization combines advanced simulation, additive manufacturing, and precision casting techniques to achieve high-integrity aluminum components for critical structural applications. The methodology demonstrates 23% improvement in production efficiency and 35% reduction in quality-related scrap compared to conventional sand casting practices.