In the field of metal casting, defects such as sand washing are common challenges that impact product quality and production efficiency. As a researcher focused on metal materials and liquid forming technologies, I have encountered numerous cases where metal casting defects arise due to improper process design. This article delves into the analysis and optimization of sand washing defects in sparse-body castings produced via vacuum process casting, a method known for its environmental benefits and high-quality outputs. Sparse-body castings are characterized by large, flat-plate structures with significant overall dimensions, low weight, and high ratios of dimension to wall thickness, making them prone to specific metal casting defects during production. Through numerical simulation and practical adjustments, I aim to address these issues and enhance casting reliability.
The vacuum process casting method, often referred to as V-process casting, utilizes dry silica sand without binders or moisture, sealed with an EVA film and evacuated to create a pressure differential that provides mold strength. This technique is celebrated for reducing environmental pollution and improving dimensional accuracy, but it is not immune to metal casting defects like sand washing, which occurs when molten metal scours the mold wall, leading to surface imperfections. In this study, I focus on a specific sparse-body casting component used in engineering machinery, with an overall size of 1479 mm × 1022 mm × 349 mm and a weight of 330 kg. The casting material is gray iron HT200, which has a minimum wall thickness of 10 mm and an average wall thickness of 24.19 mm. Such geometries exacerbate the risk of metal casting defects, particularly in areas with high fluid flow during filling.
To better understand the component, I conducted a detailed part analysis. The casting features multiple installation platforms and threaded holes for assembly, which require precise dimensional control. The sparse-body nature means that the casting has a high surface-area-to-volume ratio, increasing the likelihood of defects like cold shuts and sand washing if the filling process is not optimized. Table 1 summarizes the key parameters of the casting, highlighting its structural characteristics that contribute to metal casting defects.
| Parameter | Value |
|---|---|
| Overall Dimension | 1479 mm × 1022 mm × 349 mm |
| Weight | 330 kg |
| Material | Gray Iron HT200 |
| Minimum Wall Thickness | 10 mm |
| Average Wall Thickness | 24.19 mm |
| Dimensional Tolerance | DCTG11 Grade |
| Weight Tolerance | MT11 Grade (±15 kg) |
| Machining Allowance | 8 mm (RMAG 8 Grade) |
| Shrinkage Rate | 0.7% in All Directions |
| Draft Angle | 0°35′ (Increased Wall Thickness) |
In designing the casting process, I employed a vacuum process casting approach with a split along the maximum contour of the casting thickness, simplifying mold making and reducing costs. The sandbox dimensions were 2300 mm × 1600 mm × 600/850 mm, accommodating one casting per box. To calculate the gating system, I used established formulas to determine the pouring time and choke area, ensuring a controlled fill. The pouring time \( t \) was derived from the equation \( t = S \sqrt{G_L} \), where \( G_L \) is the total weight of molten metal in the mold, and \( S \) is a coefficient dependent on wall thickness. For this casting, \( S \) was selected based on the average wall thickness, resulting in a pouring time of approximately 40 seconds. The choke area \( A_{\text{choke}} \) was calculated using the formula:
$$ A_{\text{choke}} = \frac{G_L}{\rho_L \mu t \sqrt{2g H_P}} $$
where \( \rho_L \) is the density of the molten metal, \( \mu \) is the flow coefficient, \( g \) is gravitational acceleration, and \( H_P \) is the average pressure head. This yielded a choke area of 11.75 cm². I designed a closed-open top-pouring gating system with a ratio of \( A_{\text{vertical}}: A_{\text{horizontal}}: A_{\text{choke}}: A_{\text{ingate}} = 2:1.5:1:2 \), incorporating ceramic filters at the horizontal runner junctions to reduce velocity and purify the metal. Additionally, six side risers were placed around the casting to vent gases, and a tilt pouring angle of 7° was applied to facilitate filling and gas escape. This design aimed to minimize turbulence and prevent metal casting defects, but initial production runs revealed issues.
During production validation, I manufactured multiple castings using the designed process. The chemical composition of HT200 was strictly controlled, as shown in Table 2, to meet mechanical property requirements. The molten metal was tapped at 1500–1540°C, held for 10 minutes to allow slag removal, and poured at 1410–1450°C. The vacuum system was maintained at a low negative pressure of 0.024–0.042 MPa before pouring, switched to high pressure (0.050–0.078 MPa) during pouring, and held for specified times post-pouring. Despite these precautions, sand washing defects occurred near the ingates in 10 out of 50 castings, along with other metal casting defects like cold shuts and gas pores, leading to a reject rate of 26%. The most severe sand washing defects manifested as rough, irregular metal protrusions, indicating mold erosion during filling.
| Element | Required Range | Actual Composition |
|---|---|---|
| C | 3.2–3.4 | 3.4 |
| Si | 1.6–2.0 | 1.77 |
| Mn | 0.65–0.9 | 0.71 |
| P | ≤0.15 | 0.095 |
| S | ≤0.12 | 0.027 |
To analyze these metal casting defects, I employed numerical simulation technology to model the filling process. The simulation involved meshing with 20 million elements, using silica sand as the mold material at an initial temperature of 25°C, and setting the alloy composition to the actual values. The heat transfer coefficient between the casting and mold was defined as 500 W/(m²·K), with ambient air cooling. The results illustrated the relationship between filling speed and time, revealing that metal entered the mold through the ingates after passing through filters, with lower velocities in the ingates compared to the runners, aligning with the closed-open system design. At t=3.5 seconds, the gating system was fully filled, and metal began冲刷 the mold walls, reaching maximum velocity at the bottom corners. By t=5 seconds, metal converged at the bottom, potentially leading to cold shuts due to lower-temperature metal from the ladle bottom. The filling proceeded steadily, with complete bottom fill by t=10 seconds and full mold fill by t=38 seconds, showing no significant turbulence or air entrapment.
Further analysis focused on metal flow rates at the ingates, where points 1–4 were monitored. The flow rate \( Q \) as a function of time \( t \) was plotted, showing an initial increase followed by a decrease due to reduced pressure head. The maximum flow rate of 1.23 g/s occurred at ingate 4, near the defect sites, indicating intense scouring action at around 27 seconds. This correlation confirmed that sand washing defects were primarily caused by metal flow erosion and trapped gases from film combustion disrupting the shell-like crust formed during the vacuum process. The EVA film evaporates upon contact with molten metal, creating a crust that maintains negative pressure; however, high flow rates and unvented gases can fracture this crust, allowing sand grains to wash into the casting. This mechanism underscores the susceptibility of vacuum process casting to such metal casting defects when process parameters are not optimized.
Based on this analysis, I optimized the casting process to mitigate these metal casting defects. The gating system was relocated to the central window frame of the casting, reducing direct冲刷 on vulnerable mold areas. The risers were repositioned to the defect-prone zones to enhance gas venting. The modified gating ratio and layout maintained the same choke area but improved flow distribution. After implementing these changes, 55 castings were produced with no rejections, demonstrating the effectiveness of the optimization in eliminating sand washing and other related metal casting defects. The improved process not only increased the yield to 100% but also reduced material waste, highlighting the importance of numerical simulation in diagnosing and resolving metal casting defects.
In conclusion, this study addresses the pervasive issue of metal casting defects in sparse-body castings produced by vacuum process casting. Through detailed numerical simulation and practical adjustments, I identified that sand washing defects stem from combined effects of metal flow erosion and gas entrapment. The optimized process, which repositions the gating system and risers, successfully prevents these metal casting defects, enhancing production efficiency and sustainability. This approach can be applied to similar casting scenarios, underscoring the value of integrated analysis in overcoming metal casting defects. Future work could explore advanced materials or real-time monitoring to further reduce the incidence of metal casting defects in industrial applications.