In the production of mining flatbed truck wheel castings, the complexity of the structure often leads to defects such as shrinkage porosity and shrinkage cavities during the lost wax investment casting process. These defects significantly impact the quality and performance of the final product. Through numerical simulation, I analyzed the pouring process and identified critical areas prone to defects, particularly at the bottom of the wheel rim. To address these issues, I focused on optimizing key process parameters, including pouring temperature, pouring speed, and shell preheating temperature, using orthogonal experiments to determine the optimal combination. This approach not only reduced the shrinkage porosity rate but also enhanced the overall process efficiency and casting quality.
The wheel casting, made of ZG35CrMnSi alloy, is a typical disk-shaped component with a mass of approximately 33.46 kg and overall dimensions of 350 mm in diameter and 114 mm in height. Its structure includes a hub, rim, spokes, and a flange, with varying wall thicknesses: 36 mm at the hub, 15 mm at the rim, and 10 mm at the spokes. The presence of six uniformly distributed slots (30 mm × 20 mm) adds to the complexity. Key hot spots, where defects are likely to occur, are located at the junctions of the rim base and spokes, as well as the flange and hub areas. These regions require careful design to ensure proper solidification and minimize defects.
In lost wax investment casting, the selection of the pouring position is crucial for achieving directional solidification. I positioned the casting with the wheel base facing downward to facilitate a shorter flow path for the molten metal and placed the gates at the thickest sections to promote sequential solidification from the casting to the gating system. The gating system was designed as a side-pouring type, which minimizes turbulence and improves venting. It consists of a sprue cup, vertical runners, horizontal runners, and ingates, with two additional ingates added at the wheel base to enhance feeding and reduce defects. The gating system was modeled for four castings per mold to increase productivity.
The minimum cross-sectional area of the gating system was determined using the Oseen formula to ensure proper flow and avoid defects like misruns or air entrapment. The formula is given by:
$$ F_{\text{min}} = \frac{G}{\rho \cdot \tau \cdot \mu \cdot H} $$
where \( F_{\text{min}} \) is the minimum cross-sectional area in cm², \( G \) is the total mass of molten metal (244.913 kg), \( \rho \) is the density of the molten metal (6.962 g/cm³ at 1580°C), \( \tau \) is the filling time in seconds, \( \mu \) is the flow coefficient, and \( H \) is the average effective head in meters. Based on calculations, the阻流截面 area ranged between 44 and 47 cm², and the sectional ratios were set as \( M_{\text{sprue}} : M_{\text{runner}} : M_{\text{ingate}} = 1.15 : 1.05 : 1 \).
To determine the optimal pouring speed, I applied the Kalkin formula, which accounts for casting height, wall thickness, and pouring temperature:
$$ v = \frac{h \cdot \delta \cdot T}{k} $$
where \( v \) is the pouring speed in cm/s, \( h \) is the casting height in cm, \( \delta \) is the wall thickness in cm, \( T \) is the pouring temperature in °C, and \( k \) is a constant. For this casting, with a height of 11.4 cm, an average wall thickness of 1.5 cm, and a pouring temperature of 1580°C, the calculated pouring speed was 278.416 mm/s, which I rounded to 280 mm/s for initial simulations.
The initial process parameters were set as follows: pouring temperature of 1580°C, pouring speed of 280 mm/s, and shell preheating temperature of 1000°C. The shell material was quartz sand with a thickness of 6 mm and five layers. Numerical simulations using Cast software revealed the filling and solidification behavior. The filling process completed within 11 seconds, with minimal turbulence. However, the solidification analysis showed that the rim base and spoke junctions were the last to solidify, leading to isolated liquid zones and shrinkage defects. The initial shrinkage porosity rate was 13.13%, primarily concentrated in the rim bottom area.
To improve the process, I modified the gating system by adding two more ingates at the wheel base and incorporating vent holes to enhance gas escape. This redesign aimed to improve feeding and reduce defect formation. Subsequent simulations showed a reduction in shrinkage porosity to 8.65%, but some defects persisted in the spoke areas, indicating the need for further parameter optimization.
I conducted an orthogonal experiment to optimize the three key parameters: pouring temperature (A), pouring speed (B), and shell preheating temperature (C). The factors and levels are summarized in the table below:
| Level | A: Pouring Temperature (°C) | B: Pouring Speed (mm/s) | C: Shell Preheating Temperature (°C) |
|---|---|---|---|
| 1 | 1530 | 270 | 750 |
| 2 | 1555 | 280 | 900 |
| 3 | 1580 | 290 | 1000 |
The orthogonal array L9 was used, with shrinkage porosity rate as the evaluation index. The experimental design and results are shown in the following table:
| Experiment | A | B | C | Shrinkage Porosity Rate (%) |
|---|---|---|---|---|
| 1 | 1 | 1 | 1 | 3.10 |
| 2 | 1 | 2 | 2 | 3.00 |
| 3 | 1 | 3 | 3 | 2.97 |
| 4 | 2 | 1 | 2 | 3.03 |
| 5 | 2 | 2 | 3 | 3.08 |
| 6 | 2 | 3 | 1 | 3.13 |
| 7 | 3 | 1 | 3 | 3.04 |
| 8 | 3 | 2 | 2 | 3.08 |
| 9 | 3 | 3 | 1 | 3.30 |
Analysis of variance (ANOVA) was performed to determine the significance of each factor. The results are summarized below:
| Source | Sum of Squares | Degrees of Freedom | Mean Square | F-value | p-value |
|---|---|---|---|---|---|
| A | 0.036 | 2 | 0.018 | 32.860 | 0.030* |
| B | 0.019 | 2 | 0.010 | 17.256 | 0.055 |
| C | 0.058 | 2 | 0.029 | 52.228 | 0.019* |
| Error | 0.001 | 2 | 0.001 |
The ANOVA results indicate that factors A (pouring temperature) and C (shell preheating temperature) have significant effects on shrinkage porosity (p < 0.05), while factor B (pouring speed) is less significant. The order of influence is: shell preheating temperature > pouring temperature > pouring speed. The optimal combination was identified as A1B3C3, corresponding to a pouring temperature of 1530°C, pouring speed of 290 mm/s, and shell preheating temperature of 1000°C. Simulations with this combination showed a further reduction in shrinkage porosity, with defects primarily confined to the sprue area, not affecting the casting quality.
Production trials were conducted using the optimized parameters, resulting in wheel castings with improved surface quality and minimal defects. The successful application of lost wax investment casting with these adjustments demonstrates the effectiveness of numerical simulation and orthogonal experiments in process optimization. This approach not only enhances product quality but also reduces development time and costs, making it valuable for industrial applications in mining equipment manufacturing.
In conclusion, the lost wax investment casting process for mining flatbed truck wheels was significantly improved through gating system modifications and parameter optimization. The use of numerical simulation allowed for precise identification of defect-prone areas, while orthogonal experiments provided a systematic way to determine the best process conditions. The optimized parameters—pouring temperature of 1530°C, pouring speed of 290 mm/s, and shell preheating temperature of 1000°C—resulted in a lower shrinkage porosity rate and higher casting integrity. This methodology can be extended to other complex castings in the lost wax investment casting field, promoting efficiency and reliability in production.