Abstract: In order to reduce the defects of inclusion, slag inclusion, and iron leakage in transmission housings, based on the principles of lost foam casting and the structural characteristics of transmission housings, the causes of these defects were analyzed. Through measures such as optimizing model drying time, applying automatic glue-coating machines, selecting appropriate pouring temperatures, optimizing casting structures, using ceramic filter screens, and adopting digital process control, the overall rejection rate of transmission housings can be controlled within 4%.
1. Introduction
The 12-speed transmission, developed by Shaanxi Fast Gear Co., Ltd., is a heavy-duty transmission product with high power, high load capacity, and low failure rates. It can be adapted to heavy-duty trucks, dump trucks, container transport trucks, and various specialized and special vehicles with a rated output torque of 1600 Nm.
The transmission housing, as one of the key assemblies of a vehicle, is particularly important, especially for heavy-duty vehicle transmissions, which have large input torques and complex operating conditions, thereby demanding higher quality for their housings. The transmission housing is one of the largest production batches of 12-speed transmission housings produced by our company. It has a complex structure with a maximum thickness of 48 mm and a minimum wall thickness of 8 mm, and is produced using the lost foam casting process.
When producing such transmission housings using the lost foam casting process, the rejection rate has been around 8%. The main defects are inclusions, slag inclusions, and iron leakage, accounting for more than 80% of all defects. Based on the principles of lost foam casting and the structure of the transmission housing, an analysis of the causes of these defects was conducted. By selecting appropriate pouring temperatures, optimizing model drying time, optimizing the bonding process, optimizing casting structures, and adopting digital process control, the overall rejection rate of transmission housings can be controlled within 4%.
2. Control of Inclusion Defects
2.1 Causes of Inclusion Defects
Inclusion defects, are caused by the coating being destroyed and peeled off into the molten metal during the mold-filling process, or by solid and liquid products formed after the model undergoes pyrolysis at high temperatures not being able to be discharged in time. After the casting solidifies, strip or flocculent inclusion defects are formed.
Samples were taken from the inclusion defect areas and normal areas, and sent for scanning electron microscope (SEM) analysis. The results, indicate that the normal composition is 96.01% Fe; however, in the defective composition, Fe only accounts for 4.11%, while O, Si, and Al account for 48.14%, 32.16%, and 9.64%, respectively. The coatings used for brushing the gating cup and the model during production contain Al2O3 and SiO2. Therefore, it can be confirmed that the coatings for the gating cup and the model are the main sources of inclusion defects.
<img src=”https://example.com/sem_results.jpg” />
| Element | Normal Part (%) | Defective Part (%) |
|---|---|---|
| C | – | 1.74 |
| O | – | 48.14 |
| Na | – | 0.79 |
| Si | – | 32.16 |
| Cr | – | 0.12 |
| Mn | 0.78 | 0.29 |
| Fe | 96.01 | 4.11 |
| Al | – | 9.64 |
| Table 1: SEM Analysis Results |
2.2 Control of Inclusion Defects
Analysis revealed that the bonding quality of the model’s mold-closing surface, the brushing quality of the gating cup coating, and the smoothness of the pouring process, as well as the scouring of molten iron on the coating, are key factors affecting inclusion defects. Therefore, improving the drying degree and bonding strength of the model, reducing the scouring of molten iron on the coating, controlling the pouring negative pressure, and ensuring the brushing quality of the gating cup are the main control points for addressing inclusion defects.
2.2.1 Model Drying Time
The original process involved drying the model for 8 hours after molding. To further improve the drying degree of the model, the drying time was extended to 16 hours, followed by an additional 8 hours of drying before bonding.
2.2.2 Automatic Glue-Coating Machine
Manual bonding involves uneven coating and leakage at the mold-closing surface. By investing in bonding fixtures and using a glue-coating machine for bonding, the bonding seam quality is ensured, eliminating the risk of coating ingress.
2.2.3 Pouring Negative Pressure
The state of the sand box directly relates to the actual negative pressure during the pouring process of the casting. Requirements for the replacement of sand boxes and sand screens were established to ensure a pouring negative pressure of 0.04 MPa to 0.07 MPa.
2.2.4 Gating Cup Brushing Quality
The gating cup is made of iron and coated before use, and is used circularly. After prolonged use, the surface coating cannot be completely cleaned, making it difficult for the new coating to adhere properly when applied over the old coating, leading to easy peeling and inclusion defects. Measures taken include establishing a regular shot blasting cleaning system for the gating cup to ensure a smooth surface, ease of coating application, and quality of the gating cup coating.
3. Control of Slag Inclusion Defects
3.1 Causes of Slag Inclusion Defects
Slag inclusion defects, are mainly controlled through slag removal within the furnace and ladle in the original process. However, no slag-blocking measures were taken in the pouring system of the casting, and the condition of the ladle was not included in the process control points. During pouring, furnace slag and ladle lining that enter the molten iron cannot be discharged, remaining inside the casting and forming slag inclusion defects.
3.2 Control of Slag Inclusion Defects
The main sources of slag inclusion defects are slag in the molten iron and ladle lining material. Reducing the ingress of slag and ladle lining material into the mold is key to avoiding the occurrence of slag inclusion defects. Therefore, the use of ceramic filter screens, reasonable slag removal processes, and correct ladle usage requirements are the main control points for addressing slag inclusion defects.
3.2.1 Use of Ceramic Filter Screens
Ceramic filter screens can significantly prevent slag in the molten iron from entering the mold. A φ70 mm, 10 PPI ceramic filter screen is installed 220 mm from the top of the cross gate.
3.2.2 Reasonable Slag Removal Process
Slag removal within the furnace should be conducted no less than 3 times before tapping, and within the pouring ladle no less than 2 times.
3.2.3 Ladle Usage Requirements
The pouring ladle is only allowed to be repaired at the spout and rim, with the lining not allowed to be repaired. The pouring ladle should be replaced every ten days.
4. Control of Iron Leakage Defects
4.1 Causes of Iron Leakage Defects
Iron leakage defects, primarily occur on the back sand surface of the casting. In the original process, this surface underwent a single vibration step, with an initial pouring temperature set at 1520°C. Upon analyzing the casting structure and production process, it is determined that the primary cause of iron leakage defects lies in the inability of the molding sand to be adequately compacted during the vibration process. Additionally, a pouring temperature that is too high also increases the risk of iron leakage defects.
Specifically, the back sand surface, being the exterior facing away from the molten metal during pouring, may not receive sufficient compaction due to the vibration method or intensity used. If the molding sand is not tightly packed, gaps or voids may form, allowing molten metal to seep through and cause iron leakage defects. Furthermore, a high pouring temperature can exacerbate this issue by increasing the fluidity of the molten metal, making it more likely to penetrate through any imperfections in the sand mold.
4.2 Control Measures for Iron Leakage Defects
Ensuring tight compaction of the molding sand and selecting a reasonable pouring temperature are crucial for controlling iron leakage defects. Therefore, the following measures are taken to address this issue:
- Optimization of Casting Structure: The root radius of the back sand surface is increased to R10. This modification helps to distribute the pressure more evenly during vibration, improving the compaction of the molding sand.
- Improvement of Molding Process: An additional vibration step is added above the back sand surface, accompanied by manual insertion and drainage of the molding sand during vibration. This ensures that the molding sand is fully compacted and any voids or gaps are minimized.
- Strict Control of Pouring Temperature: The initial pouring temperature is strictly controlled to not exceed 1510°C. Lowering the pouring temperature slightly reduces the fluidity of the molten metal, making it less likely to seep through imperfections in the sand mold.
By implementing these measures, the risk of iron leakage defects can be effectively reduced. The optimization of the casting structure and molding process ensures better compaction of the molding sand, while the strict control of pouring temperature further mitigates the risk of molten metal seeping through.
In summary, through comprehensive analysis of the causes of iron leakage defects and the adoption of targeted control measures, the quality of heavy-duty transmission housing lost foam castings can be significantly improved.