Abstract
The wheel castings of mining flatbed trucks are structurally complex and prone to defects such as porosity and shrinkage during the investment casting process. This study aims to reduce the porosity rate of these castings by analyzing the pouring process using numerical simulation software and optimizing the process scheme. Additionally, three process parameters—pouring temperature, pouring speed, and mold shell preheating temperature—were optimized through orthogonal testing. The optimized process parameters significantly improved the quality and production efficiency of the wheel castings.
1. Introduction
The wheel castings of mining flatbed trucks, characterized by a diameter much larger than their height, are typical disk-shaped castings commonly used in automotive manufacturing, aerospace, mining machinery, and transportation equipment. Investment casting, known for producing products with good surface roughness, complex shapes, and high dimensional accuracy, is utilized in the production of these wheel castings. However, during the investment casting process, defects such as porosity and shrinkage can occur, especially in complex structures. Traditional process design relies heavily on trial and error, which is both time-consuming and labor-intensive. To address this, numerical simulation software is employed to shorten the trial period and visualize the casting process, predicting the location of potential defects.
2. Structural Analysis of Wheel Castings
The wheel casting analyzed in this study is a component of a mining flatbed truck, made of ZG35CrMnSi material, which is characterized by high strength, impact resistance, and abrasion resistance. The casting, weighing approximately 33.46 kg and with dimensions of 350 mm × 400 mm × 114 mm, is a rotating body disk-shaped casting with a convex flange on one end. Its structure is complex, with significant variations in wall thickness; the hub is 36 mm thick, the rim is 15 mm thick, and the intermediate spoke plate is 10 mm thick. There are six uniformly distributed rectangular slots of 30 mm × 20 mm between the spoke plates. This component must be free from obvious casting defects, particularly in the wheel rim bottom and spoke connections, where hot spots are located.
Table 1. Chemical Composition of ZG35CrMnSi
| Element | Composition (%) |
|---|---|
| C | 0.32-0.40 |
| Si | 0.80-1.10 |
| Mn | 1.20-1.50 |
| Cr | 0.80-1.10 |
| S | ≤0.035 |
| P | ≤0.035 |
3. Investment Casting Process Design
3.1 Pouring Position Selection
The pouring position refers to the orientation of the casting in the mold during pouring, which is a crucial and early consideration in process design. For the wheel casting, the pouring position is chosen at the bottom of the wheel disk to minimize the flow path for the molten metal. The inner gate is positioned at the thickest part of the casting to ensure sequential solidification from the casting to the pouring system.
3.2 Gating System Design
The gating system in investment casting generally consists of a pouring cup, sprue, runner, and ingate. Due to the multiple spokes on the wheel casting, high fluidity of the molten metal is required, and considerations must be given to the resistance during pouring. A side-gating system is selected due to its advantages, including reduced impact on the mold wall, convenient mold welding, good feeding, and better venting performance compared to top-gating. The minimum section area of the gating system is calculated using the Ozen formula, and the design adopts a runner-sprue-ingate configuration with one mold producing four castings. A circular funnel-shaped pouring cup is chosen, with two ingates (A and B) placed at the bottom of the wheel casting for better feeding and reducing defects in the wheel rim bottom.
4. Simulation of Filling and Solidification Process
Numerical simulation software is used to analyze the filling and solidification process of the wheel casting, as well as the distribution of porosity and shrinkage. The simulation results show that the casting completes filling at t=13s and begins to solidify at t=337s. The solidification sequence starts from the upper rim and spokes, proceeding towards the inner circle of the wheel and the junction between the spokes and the rim. The final solidification occurs at the wheel rim bottom and the connection with the ingate and pouring cup center.
5. Optimization of Casting Process Parameters
5.1 Initial Defect Analysis
The initial simulation results reveal that porosity and shrinkage defects are severe at the wheel rim bottom, with a porosity rate of 13.13%. These defects are caused by inadequate feeding during solidification due to the volume contraction of the casting.
5.2 Improved Process Scheme
Based on the initial scheme, two additional ingates (C and D) are added at the bottom of the wheel casting to expand the feeding area, eliminating porosity and shrinkage defects in both the upper and lower parts of the wheel. Additionally, vent holes are added, and the improved scheme is subjected to numerical simulation analysis. The porosity rate after improvement is 8.65%, indicating a reasonable design of the improved process scheme. However, minor porosity defects still exist around the spokes and wheel rim chassis, necessitating parameter optimization.
5.3 Orthogonal Test for Process Parameter Optimization
Three casting process parameters—pouring temperature, pouring speed, and mold shell preheating temperature—are studied. Based on relevant literature and production experience, the pouring temperatures selected are 1530°C, 1555°C, and 1580°C. The mold shell preheating temperatures are chosen within the range of 750°C to 1000°C, specifically 750°C, 900°C, and 1000°C. The pouring speeds are set at 270 mm/s, 280 mm/s, and 290 mm/s.
Selection of Parameter Levels
Based on relevant literature and production experience, the following levels were selected for each parameter:
- Pouring Temperature: 1530°C, 1555°C, and 1580°C
- Mold Shell Preheating Temperature: 750°C, 900°C, and 1000°C
- Pouring Speed: 270 mm/s, 280 mm/s, and 290 mm/s
Orthogonal Test Design
An orthogonal test design was employed to systematically evaluate the impact of these parameters on the porosity rate of the castings. The orthogonal test factors and levels are summarized in Table 2.
Table 2: Orthogonal Test Factor Level Table
| Level | Factor A: Pouring Temperature (°C) | Factor B: Pouring Speed (mm/s) | Factor C: Mold Shell Preheating Temperature (°C) |
|---|---|---|---|
| 1 | 1530 | 270 | 750 |
| 2 | 1555 | 280 | 900 |
| 3 | 1580 | 290 | 1000 |
Experimental Results and Analysis
Nine experimental schemes were conducted according to the orthogonal test design, and the porosity rates of the castings were measured. The experimental results are summarized in Table 3.
Table 3: Orthogonal Test Protocol and Results
| Run | Factor A | Factor B | Factor C | Porosity Rate (%) |
|---|---|---|---|---|
| 1 | 1 (1530) | 1 (270) | 1 (750) | 3.10 |
| 2 | 1 (1530) | 2 (280) | 2 (900) | 3.00 |
| 3 | 1 (1530) | 3 (290) | 3 (1000) | 2.97 |
| 4 | 2 (1555) | 1 (270) | 2 (900) | 3.03 |
| 5 | 2 (1555) | 2 (280) | 3 (1000) | 3.08 |
| 6 | 2 (1555) | 3 (290) | 1 (750) | 3.13 |
| 7 | 3 (1580) | 1 (270) | 3 (1000) | 3.04 |
| 8 | 3 (1580) | 2 (280) | 1 (750) | 3.08 |
| 9 | 3 (1580) | 3 (290) | 2 (900) | 3.30 |
Variance Analysis
Variance analysis was conducted to determine the significance of each factor on the porosity rate. The results are summarized in Table 4.
Table 4: Results of Three-Way ANOVA
| Variance Source | Factor A (Pouring Temperature) | Factor B (Pouring Speed) | Factor C (Mold Shell Preheating Temperature) | Error |
|---|---|---|---|---|
| Sum of Squares | 0.036 | 0.019 | 0.058 | 0.001 |
| Mean Square | 0.018 | 0.010 | 0.029 | 0.001 |
| Degrees of Freedom | 2 | 2 | 2 | 2 |
| F-Value | 32.860 | 17.256 | 52.228 | |
| p-Value | 0.030* | 0.055 | 0.019* |
*Note: *p < 0.05 indicates significance.
The variance analysis revealed that both factors A (pouring temperature) and factor C (mold shell preheating temperature) had significant impacts on the porosity rate (p < 0.05). The order of importance of the factors was determined as: mold shell preheating temperature > pouring temperature > pouring speed.
Optimal Process Parameter Combination
Based on the experimental results and variance analysis, the optimal process parameter combination was identified as: pouring temperature of 1530°C, pouring speed of 290 mm/s, and mold shell preheating temperature of 1000°C. This combination resulted in the lowest porosity rate and improved casting quality.
In conclusion, the orthogonal test effectively optimized the casting process parameters for the mine flatbed truck wheel castings, leading to improved product quality and reduced porosity rates.