Abstract:
This paper delves into the structure of a shell casting component for an aerospace engine and the technical challenges associated with its investment casting process. By meticulously formulating process plans and parameters for mold making, shell making, melting, and pouring, we have addressed casting defects such as porosity, cold shuts, size deviations, and deformations. The improved process not only enhances the casting quality of this component but also serves as a reference for the development and optimization of investment casting processes for similar parts.
1. Introduction
The shell casting in question plays a crucial role in aerospace engines, characterized by its complex structure and significant variations in wall thickness. Typically manufactured through the investment casting process using K403 high-temperature alloy, this casting faces challenges such as crack formation, porosity, and insufficient filling during solidification. Given the stringent requirements for dimensional accuracy, metallurgical quality, and comprehensive mechanical properties, the casting process design and manufacture pose considerable difficulties.
2. Analysis of Casting Structure and Technical Challenges
The shell casting adopts K403 alloy, obtained through vacuum melting and pouring processes. The alloy composition is detailed in Table 1. 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, the casting exhibits significant variations in shape and thickness, accompanied by numerous hot spots. This makes it a highly challenging precision casting with no allowance.
Table 1. Main Chemical Components of K403 High-Temperature Alloy
| Element | C | Cr | Co | W | Mo | Ti | Al | Ce | Fe | Si | Mn | S | P | Ni |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Content | 0.11 | 10.0 | 4.50 | 4.80 | 3.80 | 2.30 | 5.30 | ≤0.01 | ≤2.0 | ≤0.5 | ≤0.5 | ≤0.01 | ≤0.02 | Balance |
| (%) | ~0.18 | ~12.0 | ~6.00 | ~5.50 | ~4.50 | ~2.90 | ~5.90 |
Three primary technical challenges arise during production:
- Complex Casting Structure: Difficult to extract the mold, prone to dimensional deformation.
- Pouring Restrictions: Due to structural limitations, the casting is susceptible to shell cracking, fire leakage, and burrs during pouring.
- Multiple Hot Spots: Difficult to fill, prone to porosity, cracks, and cold shuts.
3. Optimization of Casting Process and Results
3.1 Mold Making
Strict control over the mold-making process is crucial for obtaining high-quality wax molds. Key process parameters include wax material temperature, die temperature, injection pressure, and dwell time. These parameters significantly impact the precise shaping, dimensional accuracy, surface roughness, and elimination of surface depressions of the wax mold.
Table 2. Key Parameters for Mold Making
| Parameter | Range/Value |
|---|---|
| Wax Material Temp. | 55~63 °C |
| Die Temp. | 25~35 °C |
| Injection Pressure | 15~25 bar |
| Dwell Time | 15~20 s |
Initially, the wax mold was assembled from three separately pressed parts using a combination fixture. However, variations in positioning led to significant dimensional deviations. To address this, a new integrated mold for the casting blank was designed and manufactured, eliminating the risk of dimensional deviations due to assembly.
3.2 Shell Making
A quality shell is essential for producing a quality casting. The shell for shell-type castings must possess sufficient strength at both room temperature and high temperatures to complete the shell-making process and withstand pouring.
To address porosity defects resulting from poor heat dissipation after pouring, we thinned the shell at thicker sections. After the fourth layer of coating, the wax mold was returned for wax addition operations, plugging the upper and lower holes of the wax mold with soft wax and surrounding each of the eight inner gating ports with a ring of soft wax . The selected shell-making process parameters are outlined in Table 3.
Table 3. Parameters of Shell-Making Process
| Layer | Slurry Viscosity | Sand Type | Drying Method | Time |
|---|---|---|---|---|
| 1 | 40~50 s | White fused alumina WAF70 | Air drying | ≥12 h |
| 2 | 37~42 s | 36-mesh | Ammonia drying | ≥20 min, 10 min抽风 |
| 3-8 | 13~15 s | 24-mesh | Ammonia drying | ≥20 min, 10 min抽风 |
| Sealing | 13~15 s | – | – | ≥12 h |
Preheating the shell before pouring removes residual moisture and ash, preparing it for hot-type pouring. At the same pouring temperature, a higher shell temperature reduces the temperature difference between the alloy melt and the mold, slowing the cooling rate and improving alloy filling capability. However, excessively high shell temperatures can hinder the formation of a large temperature gradient during solidification, leading to porosity and coarse grain structure. Typically, the shell preheating temperature is selected within the range of 950~1,000 °C.
3.3 Melting and Pouring
The pouring system in investment casting not only supports the entire mold assembly and shell but also often functions as a riser. Therefore, it must possess sufficient strength and ensure sequential solidification of the casting, allowing for adequate feeding during solidification and retaining shrinkage cavities in the gating system.
The pouring temperature is critical to ensuring casting quality. Higher pouring temperatures improve fluidity but can increase shrinkage, leading to porosity defects. Lower pouring temperatures can result in cold shuts and incomplete filling. The pouring temperature for castings with complex structures is generally set at the upper limit, and based on production experience, we selected a pouring temperature of 1,430 °C ± 10 °C. Pouring speed directly affects heat exchange between the liquid metal and the mold. Slower pouring speeds can lead to incomplete filling and cold shut defects. Therefore, we aimed to increase the pouring speed as much as possible, as faster pouring speeds increase the dynamic pressure generated by the alloy liquid flow, improving filling and temperature uniformity within the mold cavity. The selected pouring process parameters are shown in Table 4.
Table 4. Parameters of Casting Process
| Parameter | Range/Value |
|---|---|
| Shell Preheating Temp. | 950~1,000 °C |
| Pouring Temp. | 1,430 °C ± 10 °C |
| Pouring Speed | 2~3 s/mold |
4. Verification of Results
After implementing the improved processes for mold making, shell making, melting, and pouring, casting trials were conducted. The metallurgical quality and full dimensions of the trial castings were evaluated according to dedicated acceptance standards. Out of 40 castings poured, 35 were qualified, with a qualification rate of 87.5%, indicating suitability for batch production of this type of component.
5. Conclusion
(1) For shell-type investment castings with complex structures and varying wall thicknesses, selecting an appropriate mold-making process can yield high-quality wax molds, effectively ensuring the casting’s surface finish and dimensional accuracy. For shell-type castings using assembled wax molds, adopting an integrated mold can not only improve production efficiency but also reduce manual errors leading to dimensional deviations and deformations.
(2) During the shell-making process, wax application after the fourth layer of coating and local thinning of the shell improve heat dissipation at hot spots. By selecting appropriate pouring temperatures and pouring speeds, issues such as porosity and cold shuts can be effectively addressed.