Abstract: This paper delves into the study of the investment casting process for K403 shell castings, which are crucial components in aerospace engines. Due to their complex structure and varying wall thicknesses, these castings are prone to defects such as cracks, porosity, and misruns during solidification. By analyzing the technical challenges associated with the casting process and conducting experimental research on mold making, shell making, melting, and pouring, an improved solution and optimal process parameters were identified to address porosity and dimensional deviations. The refined process not only enhances the casting quality but also serves as a reference for the development and optimization of investment casting processes for similar components.
1. Introduction
Shell castings, particularly those made from K403 high-temperature alloy, play a pivotal role in aerospace engines. Their intricate design, varying wall thicknesses, and high performance requirements pose significant challenges during the casting process. The K403 alloy, known for its susceptibility to cracks, porosity, and misruns during solidification, necessitates precise process control to ensure dimensional accuracy, metallurgical quality, and robust mechanical properties.
In initial development and trial production phases, issues such as inconsistent metallurgical quality, significant dimensional deviations, and porosity severely hindered production progress. Therefore, this study aimed to investigate and optimize the mold making, shell making, melting, and pouring processes to address these challenges.
2. Analysis of Casting Structure and Technical Difficulties
2.1 Casting Material and Structure
The shell casting in question is fabricated using K403 alloy through vacuum melting and pouring. The alloy’s chemical composition is detailed in Table 1. The casting features a complex structure with a pillar height of 129 mm, a length of 111 mm, a maximum diameter of 32 mm, a minimum diameter of 14 mm, and a wall thickness of 6.5 mm (Figure 1).
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Table 1: Main chemical components of K403 high-temperature alloy
| Element | C | Cr | Co | W | Mo | Ti | Al |
|---|---|---|---|---|---|---|---|
| Content | 0.11-0.18 | 10.00-12.00 | 4.50-6.00 | 4.80-5.50 | 3.80-4.50 | 2.30-2.90 | 5.30-5.90 |
| Element | Ce | Fe | Si | Mn | S | P | Ni |
| Content | ≤0.01 | ≤2.00 | ≤0.50 | ≤0.50 | ≤0.01 | ≤0.02 | Balance |
The casting’s shape and varying thicknesses, coupled with numerous hot spots, classify it as a high-difficulty, no-allowance precision casting. Three primary technical challenges arise during production:
- Complex Structure and Difficult Mold Removal: The casting’s intricate design makes mold removal challenging, increasing the risk of dimensional deformation.
- Shell Cracking and Defects: Structural limitations can lead to shell cracking, fire running, and burrs during pouring.
- Hot Spots and Filling Difficulties: Multiple hot spots and the casting’s complexity can cause porosity, cracks, and cold shuts.
3. Optimization of Casting Process and Results
3.1 Mold Making
Achieving high-quality wax molds is crucial for obtaining high-quality castings. The pressing pressure and dwell time of the mold material significantly impact the precise formation, dimensional accuracy, surface roughness, and elimination of surface sink marks of the wax mold.
3.1.1 Mold Making Process Parameters
Key process parameters affecting wax mold pressing quality include wax material temperature, die temperature, injection pressure, and dwell time.
- Wax Material Temperature: Too low, the wax material has poor fluidity, leading to cold shuts and incomplete filling. Too high, the wax material experiences greater shrinkage, resulting in flow lines and sink marks.
- Injection Pressure: Increasing injection pressure reduces wax mold shrinkage and improves dimensional accuracy. Excessive pressure can compress gases within the mold material, causing sticking, cracks, and blisters. Insufficient pressure leads to incomplete corners and bubbles.
After comprehensive consideration, the optimal process parameters for the mold making process are as follows:
Wax Material Temperature: Controlled within a range of 55~63°C. This ensures adequate fluidity of the wax material to avoid defects such as cold shuts and incomplete filling, while minimizing shrinkage to prevent flow lines and sink marks.
Die Temperature: Maintained between 25~35°C. This temperature range helps in achieving a good balance between the wax material’s ability to flow and solidify, ensuring accurate and smooth mold formation.
Injection Pressure: Adjusted to between 15~25 bar. An appropriate injection pressure reduces the shrinkage rate of the wax mold and enhances its dimensional accuracy. Avoiding excessive pressure prevents sticking, cracks, and blisters, while insufficient pressure is guarded against to ensure complete filling and prevent bubbles.
Dwell Time: Set at 15~20 seconds. This dwell time allows sufficient time for the wax material to properly fill the mold and solidify, ensuring a high-quality wax mold with good surface finish and dimensional stability.
By strictly controlling these parameters, high-quality wax molds can be obtained, which is crucial for achieving precise dimensions, smooth surfaces, and eliminating surface defects in the subsequent casting process. The optimized mold making process parameters not only ensure the accuracy and surface roughness of the wax molds but also improve the overall casting quality of the shell castings made from K403 alloy.