1. Introduction
Lost foam casting is a modern casting process with unique advantages. In the production of heavy transmission housing, it plays a crucial role. However, there are also some challenges in ensuring the quality of castings. This article focuses on the quality control of heavy transmission housing in lost foam casting, aiming to reduce defects such as inclusions, slag inclusions, and iron penetration.
1.1 The Importance of Heavy Transmission Housing
The heavy transmission is a key component in vehicles, especially for heavy-duty vehicles. The transmission housing not only provides protection for internal components but also affects the overall performance and reliability of the transmission. High-quality transmission housing is essential for the proper functioning of the transmission system.
1.2 Challenges in Lost Foam Casting of Heavy Transmission Housing
Lost foam casting has its own characteristics and challenges. In the production of heavy transmission housing, problems such as inclusions, slag inclusions, and iron penetration are common. These defects can significantly affect the mechanical properties and service life of the castings. Understanding the causes of these defects and finding effective control methods is the key to improving the quality of castings.
2. Defect Analysis
2.1 Inclusion Defects
2.1.1 Causes of Inclusion Defects
Inclusion defects in lost foam casting can be caused by various factors. During the filling process, the destruction and peeling of the coating layer can enter the molten metal, resulting in inclusions. Additionally, the solid and liquid products formed after the high-temperature pyrolysis of the model may not be discharged in time, also causing inclusion defects.
| Cause | Description |
|---|---|
| Coating destruction | During filling, coating on the model surface may be damaged and enter the molten metal. |
| Pyrolysis products | High-temperature pyrolysis of the model generates solid and liquid products that may not be removed in time. |
2.1.2 Analysis of Inclusion Sources
Analysis of the composition of normal and defective parts shows that the coating materials used for the gate cup and the model are the main sources of inclusion defects. The scanning electron microscope results indicate that the normal part has a high percentage of “Fe” (96.01%), while the defective part has a significant amount of “O”, “Si”, and “Al”, which are components of the coating materials.
2.2 Slag Inclusion Defects
2.2.1 Causes of Slag Inclusion Defects
In the original process, the control of slag inclusions mainly relied on slag removal in the furnace and ladle. However, the casting pouring system did not have slag blocking measures, and the ladle condition was not included in the process control points. As a result, the slag in the molten iron and the ladle lining could not be discharged and remained inside the casting, forming slag inclusion defects.
| Cause | Description |
|---|---|
| Lack of slag blocking | The pouring system did not have effective slag blocking measures. |
| Ladle condition | The ladle lining and slag could not be effectively controlled. |
2.2.2 Sources of Slag Inclusion Defects
The main sources of slag inclusion defects are the slag in the molten iron and the ladle lining materials. If these substances enter the casting mold, they will cause slag inclusions.
2.3 Iron Penetration Defects
2.3.1 Causes of Iron Penetration Defects
In the original process, the iron penetration part was vibrated once, and the first pouring temperature was 1520 °C. Considering the casting structure and production process, it is found that the iron penetration part is on the back sand surface of the casting. During vibration molding, the molding sand cannot be filled compactly, which is the main cause of iron penetration defects. Additionally, a higher pouring temperature also increases the risk of iron penetration defects.
| Cause | Description |
|---|---|
| Molding sand problem | The molding sand on the back sand surface cannot be filled compactly. |
| Pouring temperature | High pouring temperature increases the risk of iron penetration. |
2.3.2 Analysis of Iron Penetration Locations
The iron penetration usually occurs on the back sand surface of the casting, where the molding sand has a problem of insufficient compaction.
3. Quality Control Measures
3.1 Control of Inclusion Defects
3.1.1 Model Drying Time Optimization
The original drying time of the model after molding was 8 hours. To further improve the drying degree of the model, the drying time was extended to 16 hours, and then it was dried for another 8 hours before bonding. This can ensure that the model has a better drying effect and reduces the risk of moisture-related defects.
| Original Drying Time | Extended Drying Time | Bonding Drying Time |
|---|---|---|
| 8 hours | 16 hours | 8 hours (before bonding) |
3.1.2 Automatic Gluing Machine Application
Manual gluing may lead to uneven gluing and glue leakage on the mold joint surface. By using a gluing machine with a bonding fixture, the quality of the bonding seam can be ensured, and the risk of coating entering the joint can be eliminated.
| Manual Gluing | Automatic Gluing |
|---|---|
| Uneven gluing, glue leakage | Even gluing, no leakage |
3.1.3 Pouring Negative Pressure Control
The state of the sand box directly affects the actual negative pressure during the casting pouring process. By formulating requirements for sand box and sand net replacement, the pouring negative pressure can be maintained within 0.04 MPa – 0.07 MPa.
| Pouring Negative Pressure Range | Sand Box and Sand Net Requirements |
|---|---|
| 0.04 MPa – 0.07 MPa | Regular replacement |
3.1.4 Gate Cup Coating Quality Control
Since the gate cup is used after being brushed with coating and recycled, over time, the surface coating cannot be completely cleaned, and the new coating on the old coating may not bond well and is prone to falling off, causing inclusion defects. To solve this problem, a regular shot blasting cleaning system for the gate cup was established to ensure a smooth surface for coating application and good coating quality.
| Gate Cup Problem | Solution |
|---|---|
| Coating not clean, bonding poor | Regular shot blasting cleaning |
3.2 Control of Slag Inclusion Defects
3.2.1 Use of Ceramic Filter
A ceramic filter was installed at a distance of 220 mm from the top of the runner (φ70 mm, 10 PPI). The ceramic filter can effectively prevent slag in the molten iron from entering the casting mold, reducing slag inclusion defects.
| Filter Location | Filter Specifications |
|---|---|
| 220 mm from runner top | φ70 mm, 10 PPI |
3.2.2 Reasonable Slag Removal Process
Before the molten iron is discharged from the furnace, slag removal should be carried out at least 3 times, and in the pouring ladle, slag removal should be carried out at least 2 times. This can effectively reduce the amount of slag in the molten iron.
| Slag Removal Location | Number of Slag Removal Times |
|---|---|
| Furnace | At least 3 times |
| Pouring ladle | At least 2 times |
3.2.3 Pouring Ladle Use Requirements
The pouring ladle is only allowed to repair the ladle nozzle and ladle edge, and the ladle lining is not allowed to be repaired. The pouring ladle should be replaced every ten days to ensure the quality of the ladle and reduce the risk of slag inclusions caused by ladle problems.
| Ladle Repair Allowed | Ladle Replacement Frequency |
|---|---|
| Ladle nozzle and edge | Every ten days |
3.3 Control of Iron Penetration Defects
3.3.1 Casting Structure Optimization
The root fillet of the back sand surface was increased to R10. This can improve the compaction of the molding sand on the back sand surface and reduce the risk of iron penetration.
| Original Fillet Radius | Optimized Fillet Radius |
|---|---|
| Smaller value | R10 |
3.3.2 Additional Vibration and Manual Sand Draining
An additional vibration was added above the back sand surface, and during the vibration process, manual sand draining was carried out. This can further improve the compaction of the molding sand and ensure the quality of the casting.
| Original Vibration Times | Optimized Vibration Times | Manual Sand Draining |
|---|---|---|
| 1 time | 2 times | During vibration |
3.3.3 Pouring Temperature Control
The first pouring temperature was strictly controlled not to exceed 1510 °C. By controlling the pouring temperature, the risk of iron penetration due to high temperature can be reduced.
| Original Pouring Temperature | Optimized Pouring Temperature |
|---|---|
| 1520 °C | Not higher than 1510 °C |
4. Process Digitalization Control
In lost foam casting, there are many manual operations and human factors have a significant impact on the process. To ensure effective process control, digitalization control measures were adopted.
4.1 Online Monitoring of Drying Chamber Temperature and Humidity
By adding an online monitoring system for the temperature and humidity of the drying chamber, the drying process of the model can be better controlled. This can ensure that the model has a proper drying environment and reduces the risk of defects caused by improper drying.
4.2 Automatic Recording of Pouring Temperature and Pouring Negative Pressure
An automatic recording system for the pouring temperature and pouring negative pressure of the molding line was established. This can ensure the stability of key process parameters and provide data support for process control and quality improvement.
5. Effect Verification
After implementing the above quality control measures and process digitalization control, the production of 12 -gear transmission housing from January to December 2023 was evaluated. A total of 60,181 pieces were produced, with 2,367 defective pieces, resulting in a defect rate of 3.93%. This shows that the casting defect rate has been significantly reduced, demonstrating the effectiveness of the quality control measures.
6. Conclusion
6.1 Summary of Quality Control Measures
In this study, a series of quality control measures for lost foam casting of heavy transmission housing were proposed and implemented. These measures include optimizing the model drying time, applying an automatic gluing machine, controlling the pouring negative pressure, ensuring the quality of the gate cup coating, using a ceramic filter, optimizing the casting structure, controlling the pouring temperature, and implementing process digitalization control.
6.2 Significance of Quality Control
The successful implementation of these quality control measures has significantly reduced the defect rate of heavy transmission housing castings, improving the quality and reliability of the castings. This is of great significance for the production of high-quality transmissions and the development of the automotive industry.
6.3 Future Research Directions
Although significant progress has been made in the quality control of lost foam casting of heavy transmission housing, there are still some areas that need further research. For example, more in-depth research on the interaction between the coating and the molten metal, the optimization of the vibration process, and the improvement of the digitalization control system can be carried out to further improve the quality of castings.