Sand washing defects remain a persistent challenge in vacuum process casting (VPC), particularly for sparse-body castings characterized by large planar geometries and high dimension-to-thickness ratios. This study investigates the root causes of sand erosion during VPC production of a 330 kg HT200 gray iron component (1,479 mm × 1,022 mm × 349 mm) and proposes systematic process optimizations.
1. Process Design and Initial Defect Manifestation
The casting features 6 mounting platforms and multiple threaded holes, requiring dimensional accuracy of DCTG11 grade and surface roughness of RMAG8. Initial process parameters included:
| Parameter | Value |
|---|---|
| Pouring temperature | 1,410-1,450°C |
| Vacuum pressure (pre-pour) | 0.024-0.042 MPa |
| Vacuum pressure (post-pour) | 0.050-0.078 MPa |
| Solidification time | 4 hours |
The gating system employed a closed-open design with ceramic filters, calculated using:
$$A_{\text{choke}} = \frac{G_L}{\rho_L \mu t \sqrt{2gH_p}}$$
$$t = S\sqrt{G_L}$$
where \(A_{\text{choke}}\) represents choke area, \(G_L\) total metal weight, and \(H_p\) average pressure head.
2. Defect Analysis Through Numerical Simulation
Finite element analysis revealed critical flow dynamics contributing to casting defects:
| Time (s) | Maximum Velocity (m/s) | Critical Region |
|---|---|---|
| 3.5 | 1.23 | Gate #4 entrance |
| 15 | 0.89 | Lower fillet |
| 30 | 0.67 | Riser base |
The velocity field equation demonstrates localized turbulence:
$$v(x,y,z) = \sqrt{v_x^2 + v_y^2 + v_z^2} \geq 1.2\ \text{m/s}$$
Regions exceeding this threshold showed direct correlation with sand erosion locations. Mold wall shear stress analysis confirmed:
$$\tau_w = \mu\frac{\partial v}{\partial y}\bigg|_{y=0} > 450\ \text{Pa}$$
3. Gas Entrapment and Film Degradation
Thermogravimetric analysis of EVA film combustion revealed:
$$m_{\text{residue}} = m_0 \times e^{-0.023T}$$
where \(T\) represents temperature (°C). At 1,400°C, only 12% of original film mass remained as protective crust. Gas generation from film pyrolysis reached:
$$Q_{\text{gas}} = 0.18m_{\text{film}}^{1.2}\ \text{(NL/kg)}$$
4. Process Optimization Strategy
The modified gating system achieved 100% yield through:
- Relocation of choke area to central window frame
- Implementation of stepped velocity control:
$$v_{\text{step}} = \begin{cases}
0.8\ \text{m/s} & t \leq 15\ \text{s} \\
1.1\ \text{m/s} & t > 15\ \text{s}
\end{cases}$$ - Enhanced venting capacity:
$$A_{\text{vent}} = 1.25A_{\text{choke}}$$
| Parameter | Before | After |
|---|---|---|
| Sand erosion frequency | 26% | 0% |
| Surface roughness (μm) | 25.4 | 12.7 |
| Yield strength (MPa) | 195 | 210 |
This comprehensive approach demonstrates that systematic analysis of casting defects through numerical modeling combined with empirical process adjustments can effectively eliminate sand washing issues in vacuum process casting of large sparse-body components.