In the field of aerospace engineering, the production of high-performance shell components is critical for engine systems. These parts, often fabricated using K403 high-temperature alloy through lost wax investment casting, face significant challenges due to their intricate geometries, varying wall thicknesses, and susceptibility to defects such as cracks, porosity, and incomplete filling. The demanding requirements for dimensional accuracy, metallurgical quality, and mechanical properties make the casting process highly complex. In our research, we address these issues by optimizing key parameters in pattern making, shell building, and melting-pouring processes. This study focuses on overcoming common defects like shrinkage and dimensional deviations, which have hindered production efficiency. The lost wax investment casting method, central to this work, involves creating precise wax patterns, building ceramic shells, and melting alloys under vacuum conditions. Through systematic experimentation, we developed improved techniques that enhance casting quality and yield, providing a reference for similar components.
The shell component under investigation is characterized by a complex structure with pillars reaching 129 mm in height, lengths of 111 mm, and diameters varying from 14 mm to 32 mm, with a uniform wall thickness of 6.5 mm. This geometry results in multiple hot spots and uneven cooling, which exacerbate defects in the lost wax investment casting process. Key technical difficulties include challenges in pattern removal leading to dimensional distortions, shell cracking during pouring causing leaks and burrs, and hot spots promoting porosity, cold shuts, and cracks. To analyze these issues, we consider the alloy composition, primarily K403, which contains elements like carbon, chromium, cobalt, and nickel, as detailed in Table 1. The high reactivity and solidification characteristics of this alloy necessitate precise control in lost wax investment casting to achieve defect-free parts.
| Element | Content |
|---|---|
| C | 0.11–0.18 |
| Cr | 10.00–12.00 |
| Co | 4.50–6.00 |
| W | 4.80–5.50 |
| Mo | 3.80–4.50 |
| Ti | 2.30–2.90 |
| Al | 5.30–5.90 |
| Ce | ≤ 0.01 |
| Fe | ≤ 2.00 |
| Si | ≤ 0.50 |
| Mn | ≤ 0.50 |
| S | ≤ 0.01 |
| P | ≤ 0.02 |
| Ni | Balance |
In the lost wax investment casting process, pattern making is the first critical step. The quality of wax patterns directly influences the final casting’s surface finish and dimensional precision. We optimized parameters such as wax temperature, mold temperature, injection pressure, and holding time to minimize defects. For instance, the wax temperature must balance fluidity and shrinkage; too low a temperature causes cold shuts, while too high leads to flow marks and shrinkage cavities. The relationship between injection pressure and pattern quality can be expressed using a simplified model for volumetric shrinkage: $$ \Delta V = V_0 \cdot \beta \cdot (T_{\text{injection}} – T_{\text{room}}) $$ where $$ \Delta V $$ is the volume change, $$ V_0 $$ is the initial volume, $$ \beta $$ is the thermal expansion coefficient of the wax, and $$ T $$ denotes temperatures. Based on experimental data, we set the wax temperature at 55–63°C, mold temperature at 25–35°C, injection pressure at 15–25 bar, and holding time at 15–20 seconds. These parameters ensure high pattern integrity in lost wax investment casting.
Dimensional control of wax patterns posed a significant challenge. Initially, patterns were produced as three separate sections and assembled using fixtures, but this introduced human errors and dimensional deviations, such as misalignments of up to 2.2 mm between cylindrical features. To address this, we redesigned the pattern-making process to use a unified mold, eliminating assembly steps and reducing variability. This approach in lost wax investment casting not only improved accuracy but also streamlined production. The transition to a one-piece pattern design is a key advancement in optimizing the lost wax investment casting method for complex geometries.
Shell building is another vital phase in lost wax investment casting, where the ceramic shell must possess adequate strength at room and high temperatures to withstand handling and pouring. To mitigate porosity in thick sections, we implemented a shell-thinning strategy by applying soft wax to specific areas after the fourth coating layer, as illustrated in the process. This localized thinning enhances heat dissipation, promoting faster solidification in hot spots. The shell-building parameters are summarized in Table 2, including slurry viscosity, sand types, and drying times. For example, the first layer uses a silica sol-zirconia flour slurry with 40–50 seconds viscosity and white corundum sand, while subsequent layers employ ethyl silicate-based coatings. This meticulous layering in lost wax investment casting ensures shell integrity and defect reduction.
| Layer | Slurry Type | Viscosity (s) | Sand Type | Drying Time | Ammonia Drying | Ventilation |
|---|---|---|---|---|---|---|
| 1 | Silica Sol-Zirconia Flour | 40–50 | White Corundum WAF70 | ≥ 12 h | – | – |
| 2 | Ethyl Silicate Hydrolysate | 37–42 | Shangdian Sand 36 mesh | ≥ 20 min | 10 min | ≥ 10 min |
| 3–8 | Ethyl Silicate Hydrolysate | 13–15 | Shangdian Sand 24 mesh | ≥ 20 min | 10 min | ≥ 10 min |
| Sealing | Ethyl Silicate Hydrolysate | 13–15 | – | ≥ 12 h | – | – |
Preheating the shell before pouring is essential in lost wax investment casting to remove moisture and ash, ensuring optimal metal flow. The shell temperature affects the thermal gradient during solidification; higher temperatures improve fluidity but may cause coarse grains and porosity. We selected a preheat temperature of 950–1000°C to balance these factors. The heat transfer during preheating can be modeled using Fourier’s law: $$ q = -k \cdot \frac{dT}{dx} $$ where $$ q $$ is the heat flux, $$ k $$ is the thermal conductivity, and $$ \frac{dT}{dx} $$ is the temperature gradient. This control is crucial in lost wax investment casting for achieving uniform cooling and minimizing defects.
Melting and pouring parameters are pivotal in the lost wax investment casting process. The gating system must support the mold and act as a riser to facilitate directional solidification, preventing shrinkage defects. Pouring temperature and speed are optimized to ensure complete filling without excessive turbulence. The pouring temperature, set at 1430°C ± 10°C, is approximately 50–100°C above the alloy’s liquidus temperature, enhancing fluidity while avoiding shrinkage issues. The pouring speed, maintained at 2–3 seconds per mold, minimizes cold shuts and ensures rapid cavity filling. These parameters are detailed in Table 3. The dynamics of metal flow can be described by the Bernoulli equation for incompressible fluids: $$ P + \frac{1}{2} \rho v^2 + \rho g h = \text{constant} $$ where $$ P $$ is pressure, $$ \rho $$ is density, $$ v $$ is velocity, $$ g $$ is gravity, and $$ h $$ is height. This principle underpins the optimization of pouring speed in lost wax investment casting to achieve defect-free castings.
| Shell Preheat Temperature (°C) | Pouring Temperature (°C) | Pouring Speed (s/mold) |
|---|---|---|
| 950–1000 | 1430 ± 10 | 2–3 |
To validate the optimized lost wax investment casting process, we conducted production trials, evaluating metallurgical quality and dimensional accuracy against specialized standards. Out of 40 castings produced, 35 met all criteria, yielding a qualification rate of 87.5%. This demonstrates the effectiveness of our improvements in addressing porosity, cold shuts, and dimensional errors. The success highlights the importance of integrated process control in lost wax investment casting for high-integrity components.
In conclusion, our research on lost wax investment casting for K403 shell components reveals that precise pattern making, shell building, and pouring parameter optimization are essential for mitigating defects. The use of unified molds in pattern production enhances dimensional accuracy, while localized shell thinning and controlled preheating improve thermal management. The lost wax investment casting process, when finely tuned, achieves high yields and quality, offering a scalable solution for complex aerospace parts. Future work could explore advanced materials and real-time monitoring to further refine lost wax investment casting techniques.