1. Introduction
Investment casting is a highly precise and complex manufacturing process widely used in the production of various components, especially those with high quality requirements. In the context of locomotive manufacturing, the axle box body is a crucial component that demands excellent mechanical properties, dimensional accuracy, and internal quality. This article focuses on the investment casting process of the axle box body for locomotive bogie, exploring the challenges and solutions in achieving a high-quality casting.
1.1 Importance of Axle Box Body in Locomotive
The axle box body serves as a vital connection between the axle and the bogie frame in a locomotive. It not only bears the load transmitted from the axle but also provides a stable support and guidance for the axle’s movement. Any defect or inaccuracy in the axle box body can lead to serious consequences, such as abnormal wear of the axle, reduced running stability of the locomotive, and even potential safety hazards.
1.2 Overview of Investment Casting Process
Investment casting, also known as the lost-wax process, involves several key steps. Firstly, a wax pattern is created, which is an exact replica of the final part. This wax pattern is then coated with a refractory material to form a shell. After dewaxing, the shell is heated to a high temperature and molten metal is poured into it. Once the metal solidifies, the shell is removed, and the final casting is obtained. This process allows for the production of complex shapes with high precision and good surface finish.
2. Product Requirements and Characteristics
2.1 Technical Requirements
The axle box body for the SDA – 80 type bogie has specific technical requirements. It is made of B – grade steel (TB/T 2942.1 – 2020). The maximum outer dimension of the part is 700 mm, with a significant variation in wall thickness. The maximum wall thickness is 55 mm, and the minimum is 12 mm. The single part weighs 64.1 kg, and the casting weighs 92 kg. In terms of quality inspection, key parts require a radiographic inspection level of 2 or higher, and 100% of the axle box bodies in the first train are subject to radiographic inspection. The magnetic particle inspection requires a quality level of 1.
2.2 Structural Characteristics and Challenges
The structure of the axle box body presents several challenges for the investment casting process. The large difference in wall thickness, especially in areas such as the 8 bolt hole positions, makes it difficult to provide proper feeding during solidification, resulting in potential shrinkage porosity and shrinkage cavity defects. Additionally, the large size of the part and the requirement for high shell strength to prevent sand inclusions pose further difficulties.
3. Initial Investment Casting Process Design and Trial
3.1 Process Scheme
Based on the structure and requirements of the axle box body part, an initial investment casting process was designed. The linear shrinkage rate was set at 2.5%. The process design incorporated the use of patches and large risers. Due to the large contour size of the part, a mechanism for demolding was included in the model making. The wax pattern assembly and welding scheme is shown in Figure 2. The shell making process consisted of 1 surface layer, 1 transition layer, and 7 reinforcement layers. After hanging 3 layers of sand on the reinforcement layer, 3 mm iron wire was tied around to increase the shell strength. Patches were placed at the bearing hole positions, and triangular gates were used for feeding. Small risers were used for feeding at both ends, and two 50 kg ladles were used to pour the molten metal from the middle triangular gate.
3.2 Trial Results
The investment casting were subjected to a series of tests, including full – dimensional layout, magnetic particle inspection, radiographic inspection, and physical and chemical property testing. The results were as follows:
| Test Items | Results |
|---|---|
| Dimensional Layout | Center distance of 460 mm was found to be larger, with a measured value of 463.7 mm |
| Magnetic Particle Inspection | Qualified |
| Radiographic Inspection | Sheets 3, 7, and 8 showed a level of 4, exceeding the technical requirements |
| Physical and Chemical Properties Testing | Qualified |
| Sand Inclusion (after rough machining) | A large number of small sand inclusions were found on the large flat surface below the triangular gate, with a depth of about 1 mm |
3.3 Analysis of Trial Results
The main defects identified in the first trial were dimensional deviation, excessive radiographic inspection results, and sand inclusions. The reasons for these defects were analyzed as follows:
| Defects | Reasons |
|---|---|
| Dimensional Deviation | The shrinkage ratio of the wax pattern and molten steel was smaller than the designed 2.5% due to the hindrance of the cross – rib structure during solidification |
| Radiographic Inspection Failure | Insufficient feeding from the square gate near the end cover end |
| Sand Inclusions | The large amount of molten steel and the large flat surface structure caused the surface layer sand of the shell to be flushed into the cavity during pouring, and it was determined that pouring from the triangular gate was not ideal |
4. Process Improvement and Verification
4.1 Improvement Measures
(1) For the dimensional deviation problem, a shrinkage rate of 1.5% was considered more reasonable. The specific measure was to revise the mold size and change the shrinkage rate of the 460 mm center distance dimension to 1.5%.
(2) To address the insufficient feeding of the square riser at the end cover end of the axle box body, the amount of molten steel feeding and the feeding distance were recalculated. The square riser was changed to a waist – shaped riser, as the waist – shaped riser has a better feeding effect than the square riser.
(3) To solve the problem of sand inclusions on the large flat surface, the gate position was changed so that the molten steel did not pour into the large flat surface structure. Pouring from the large flat surface caused the high – temperature molten steel to continuously erode the large flat surface of the shell, resulting in shell peeling and sand inclusions.
4.2 Improved Process Trial
In the improved process, a direct gate assembly and welding was used as a riser at the square lug of the axle box body. A triangular gate was used for assembly and welding at the φ230 mm circle. A 150 mm high waist – shaped riser and a spherical gate were used for assembly and welding at the semicircular arc. The molten steel was poured from the spherical gate. The assembly and welding process is shown in Figure 5.
4.3 Results of Improved Process Trial
After the process improvement, the investment casting were again tested for full – dimensional layout, magnetic particle inspection, radiographic inspection, and physical and chemical property testing. The results showed that all tests were qualified, and no sand inclusions were found after machining. The investment casting produced after the process improvement have passed the customer’s acceptance.
5. Key Findings and Conclusions
5.1 Shrinkage Rate Adjustment
In the investment casting process design, the shrinkage rate needs to be carefully considered, especially in areas with structures that hinder contraction, such as long rib plates. A reduced shrinkage rate may be required to ensure dimensional accuracy.
5.2 Avoiding Sand Inclusions
To reduce sand inclusions, it is essential to avoid pouring molten steel from large flat surfaces. This helps maintain the integrity of the shell and prevents sand from being introduced into investment casting.
5.3 Optimal Riser Selection
The use of waist – shaped and spherical risers is more beneficial in terms of feeding effect compared to square risers. Appropriate selection of risers can improve the internal quality of investment casting.
In conclusion, through a series of process design, trial, analysis, and improvement steps, a successful investment casting process for the axle box body of the locomotive bogie has been achieved. This study provides valuable insights and practical experience for the production of high – quality axle box bodies and other similar components using the investment casting method.
6. Future Research Directions
Although the current study has achieved satisfactory results, there are still some areas that can be further explored. For example, the optimization of the refractory material used in the shell making process to improve the shell strength and heat resistance. Additionally, the investigation of more advanced techniques for wax pattern making to enhance the precision and surface quality of the patterns could be beneficial. Further research on the influence of different pouring parameters on the quality of investment casting, such as pouring temperature and pouring speed, may also lead to better process control and higher quality products.