In the field of transportation machinery, vacuum wheel rim castings are critical safety components used in commercial and engineering vehicles. These ductile iron castings, typically made from QT450-15 grade, feature a complex structure with annular grooves on the periphery and multiple ventilation and mounting holes on the web. The average wall thickness ranges from 10 to 15 mm, and the casting weight is approximately 60 kg. Ensuring high dimensional accuracy, mechanical properties, and internal quality is essential for safety and performance. Traditional casting methods, such as two-part molding, struggle with the peripheral grooves, often requiring additional cores that lead to defects like wall thickness variations and increased costs. To address these challenges, we developed a lost foam casting process, which offers advantages in surface quality, dimensional precision, and cost-effectiveness for producing ductile iron castings.
The lost foam casting process involves creating a foam pattern (typically EPS) that matches the casting geometry, coating it with refractory material, and embedding it in unbonded sand under vacuum. During pouring, the metal replaces the vaporized pattern, resulting in a precise replica. This method is particularly suitable for ductile iron castings like vacuum wheel rims, as it eliminates the need for complex cores and reduces machining. In our development, we focused on optimizing the process to achieve near-net-shape casting for holes and grooves, minimizing post-processing. Key aspects included pattern design, gating system configuration, vacuum-assisted pouring, and solidification control. Through iterative trials and computer simulations, we refined the process to meet technical requirements, emphasizing the repeated application of ductile iron castings in automotive safety parts.
Our process design began with a detailed analysis of the vacuum wheel rim’s structure. The annular groove and thin-walled sections posed significant challenges for conventional methods, as they could lead to misalignment and wall thinning. By adopting lost foam casting, we leveraged its ability to form intricate shapes without cores, ensuring uniform wall thickness and enhanced mechanical properties. The EPS foam pattern was designed to incorporate ventilation holes and valve holes as-cast, eliminating machining steps. Bolt holes were cast as near-net-shape to reduce machining allowance. The gating system was configured as a top-pouring design with multiple ingates to facilitate smooth metal flow and minimize turbulence. Additionally, we implemented vacuum adsorption during pouring to improve mold filling and feeding capacity, which is crucial for achieving dense microstructures in ductile iron castings.
To prevent pattern deformation, especially in the rim area, we reinforced the EPS foam with glass fiber strips in a cross-shaped pattern before coating. The foam density was controlled between 26–32 g/L to balance strength and gas evolution. The coating process involved dipping or brushing a water-based quartz sand coating twice, with drying at temperatures below 55°C for 8–10 hours between applications. This ensured a uniform refractory layer that could withstand the thermal shock during pouring. The coated pattern was then assembled with the gating system, which included 6–8 slag traps to collect impurities. The entire cluster was placed in a flask and embedded in chromite sand, vibrated to compact the sand, and subjected to a vacuum of 0.04–0.06 MPa during pouring. After solidification, the castings were cooled, shaken out, and cleaned by shot blasting.
The theoretical foundation of our process involves fluid dynamics and heat transfer principles. For instance, the mold filling velocity in vacuum-assisted pouring can be described by the equation: $$v = \frac{Q}{A}$$ where \(v\) is the flow velocity, \(Q\) is the volumetric flow rate, and \(A\) is the cross-sectional area of the gating system. This ensures rapid and complete filling, reducing defects like cold shuts. The solidification process in ductile iron castings follows Chvorinov’s rule: $$t = B \left( \frac{V}{A} \right)^2$$ where \(t\) is solidification time, \(B\) is a mold constant, \(V\) is volume, and \(A\) is surface area. By optimizing the gating and riser design, we achieved directional solidification, minimizing shrinkage porosity. The vacuum pressure enhances the feeding effect, as described by Darcy’s law for flow through porous media: $$Q = \frac{k A \Delta P}{\mu L}$$ where \(k\) is permeability, \(\Delta P\) is pressure difference, \(\mu\) is viscosity, and \(L\) is length. This contributed to the dense microstructure of the ductile iron castings.
| Parameter | Value | Description |
|---|---|---|
| Foam Density | 26–32 g/L | EPS pattern density to reduce deformation |
| Coating Thickness | 0.5–1.0 mm | Refractory coating after two dips |
| Drying Temperature | < 55°C | Controlled drying to prevent pattern warping |
| Vacuum Pressure | 0.04–0.06 MPa | Applied during pouring for improved filling |
| Pouring Temperature | 1350–1400°C | Typical for QT450-15 ductile iron |
| Cooling Time | 2–2.5 hours | In-mold cooling before shakeout |
In the implementation phase, we produced multiple batches of ductile iron castings to validate the process. The EPS patterns were fabricated using molded foaming to achieve consistent dimensions, with ventilation and valve holes formed directly. Pattern reinforcement was critical; without it, the rim section deformed during coating and drying. The gating system, as shown in the design, featured a central sprue with radial runners to distribute metal evenly. Vacuum-assisted pouring proved effective in enhancing fluidity, as the negative pressure draws metal into the mold cavity, reducing air entrapment. The use of unbonded chromite sand allowed for easy reclamation and minimal environmental impact. After pouring, the castings were inspected for dimensional accuracy, surface defects, and mechanical properties. Results confirmed that the as-cast holes met installation requirements, and the bolt holes required only minimal machining, highlighting the efficiency of this approach for ductile iron castings.
Mechanical testing revealed that the ductile iron castings achieved the required QT450-15 properties: tensile strength ≥ 450 MPa, elongation ≥ 15%, and hardness of 150–200 HB. Microstructural analysis showed a predominantly pearlitic matrix with nodular graphite, ensuring good ductility and strength. The vacuum轮辋 process eliminated common defects like shrinkage and gas porosity, thanks to the controlled solidification and vacuum feeding. Table 2 summarizes the mechanical properties compared to standards, demonstrating the suitability of lost foam casting for high-performance ductile iron castings.
| Property | Standard Requirement | Measured Value |
|---|---|---|
| Tensile Strength | ≥ 450 MPa | 460–480 MPa |
| Elongation | ≥ 15% | 16–18% |
| Hardness | 150–200 HB | 160–190 HB |
| Impact Toughness | ≥ 12 J (at 20°C) | 13–15 J |
The economic and environmental benefits of this process are significant. By casting holes near-net-shape, we reduced machining costs by approximately 30% and decreased material waste. The lost foam process also minimized cleaning efforts, as the castings had smooth surfaces with minimal flash. Moreover, the use of recyclable sand and the absence of binders made the process greener. In terms of production efficiency, the cycle time was shorter compared to traditional methods, allowing for higher throughput. These advantages make lost foam casting an ideal choice for mass-producing ductile iron castings like vacuum wheel rims, where quality and cost are paramount.
In conclusion, the development of the lost foam casting process for vacuum wheel rim ductile iron castings has proven highly successful. It addresses the limitations of conventional methods by ensuring dimensional accuracy, superior surface quality, and enhanced mechanical properties. The integration of vacuum adsorption during pouring improved mold filling and feeding, resulting in dense and defect-free castings. Key innovations included pattern reinforcement, optimized gating, and controlled solidification. Future work could focus on automating the pattern assembly and coating processes to further increase efficiency. Overall, this approach underscores the versatility and effectiveness of lost foam casting for complex ductile iron castings in the automotive industry, paving the way for broader applications.