Abstract: This paper analyzes the structure of K403 housing castings used in aerospace engines and the technical difficulties associated with investment casting. By optimizing the process parameters of mold making, shell making, melting, and pouring, casting defects such as porosity, cold shuts, dimensional deviations, and deformations were addressed. The improved process not only enhances the casting quality but also provides a reference for the development and optimization of investment casting processes for similar parts.
1. Introduction
Housing castings, as critical components in aeroengine systems, demand precise dimensions, high metallurgical quality, and robust comprehensive mechanical properties. These castings are often produced using the investment casting process with K403 superalloy, which is prone to defects such as cracks, porosity, and misruns during solidification. This study focuses on addressing these challenges through experimental research on mold making, shell making, melting, and pouring processes.
2. Analysis of Casting Structure and Technical Difficulties
2.1 Casting Structure
The K403 housing casting features a complex structure with varying wall thicknesses, 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.
Table 1. Main Chemical Components of K403 High-Temperature Alloy (Mass Fraction, %)
| 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 | ≤2.00 | ≤0.50 | ≤0.50 | ≤0.01 | ≤0.02 | Bal. |
2.2 Technical Difficulties
- Complex Structure and Difficult Mold Removal: The casting’s intricate design makes it challenging to remove the mold without causing dimensional distortions.
- Shell Cracking and Defects: Structural limitations increase the risk of shell cracking, fire running, and burrs during pouring.
- Hot Spots and Difficult Filling: Multiple hot spots make it challenging to ensure complete filling, leading to porosity, cracks, and cold shuts.
3. Optimization of Casting Process and Results
3.1 Mold Making
To obtain high-quality wax molds, the mold-making process must be tightly controlled. Key process parameters include wax material temperature, die temperature, injection pressure, and dwell time.
Table 2. Mold Making Process Parameters
| Parameter | Range |
|---|---|
| Wax Material Temperature | 55~63 °C |
| Die Temperature | 25~35 °C |
| Injection Pressure | 15~25 bar |
| Dwell Time | 15~20 s |
Addressing dimensional deviations, a new integrated mold was designed and manufactured, replacing the previous method of assembling separately pressed wax molds, which eliminated dimensional inaccuracies due to assembly.
3.2 Shell Making
A high-quality shell is essential for producing quality castings. For housing castings, the shell must possess sufficient strength at both room and high temperatures to withstand the shell-making process and pouring.
To enhance heat dissipation and address porosity in thick sections, the shell was thinned after the fourth layer of coating. Additional wax was applied to the upper and lower holes of the wax mold and around the eight ingates.
Table 3. Shell-Making Process Parameters
| Layer | Slurry Viscosity | Sand Type | Drying Method | Drying Time |
|---|---|---|---|---|
| 1 | 40~50 s | White Fused Alumina WAF70 | Air Drying | ≥12 h |
| 2 | 37~42 s | 36-mesh | Ammonia Drying +抽风 | ≥20 min + 10 min |
| 3-8 | 13~15 s | 24-mesh | Ammonia Drying +抽风 | ≥20 min + 10 min |
| Sealing | 13~15 s | – | Air Drying | ≥12 h |
Preheating the shell before pouring reduces moisture and ash content, preparing it for hot-mold pouring. The optimal shell preheating temperature is 950~1000 °C.
3.3 Melting and Pouring
The pouring system not only supports the mold assembly and shell but also acts as a riser, ensuring sequential solidification and adequate feeding of the casting.
Table 4. Pouring Process Parameters
| Parameter | Range |
|---|---|
| Shell Preheating | 950~1000 °C |
| Pouring Temperature | 1430 °C ± 10 °C |
| Pouring Speed | 2~3 s/mold |
An appropriate pouring temperature enhances fluidity, while a faster pouring speed improves filling and temperature uniformity within the mold cavity.
4. Results and Validation
After implementing the optimized processes, 40 castings were poured, with 35 passing quality inspection, resulting in a pass rate of 87.5%. This rate is suitable for mass production of such components.
5. Conclusion
- Mold Making: For complex, thick-walled housing castings, selecting the right mold-making process ensures high-quality wax molds, guaranteeing surface finish and dimensional accuracy.
- Shell Making: Thinning the shell in hot spots and optimizing pouring temperature and speed effectively addressed porosity and cold shut defects.
- Integrated Mold: Adopting an integrated mold improved production efficiency and reduced dimensional deviations and distortions.
The optimized investment casting process for K403 housing castings not only enhances casting quality but also serves as a reference for developing and refining similar casting processes.