Abstract
The dimensional accuracy of investment castings is greatly influenced by the linear shrinkage rate of the wax mold. Reducing the linear shrinkage rate of the wax mold is an effective measure to improve the dimensional accuracy of castings. This paper investigates the impact of a hollow wax mold structure on the dimensional accuracy of K648 superalloy castings. The results demonstrate that a hollow wax mold design with local cross-sections thinned to 4.5-5 mm mitigates plane shrinkage and local deformation issues of the investment mold. The average linear shrinkage of the wax mold was reduced from 1.16% to 0.54%, and the casting dimensional accuracy improved from CT7 to CT5, with the actual linear shrinkage rate decreasing from 2.70% to 2.41%. This research provides a valuable reference for enhancing the dimensional accuracy of thick and large investment castings.
1. Introduction
Investment casting, also known as the lost-wax process, is widely used in the manufacturing of complex and precise components for aerospace, aviation, and power generation industries. It involves the creation of a precise wax pattern, which serves as a mold for the casting process. The dimensional accuracy of the final casting depends significantly on the dimensional stability of the wax mold, which is influenced by factors such as the mold material, manufacturing process, and mold design. This paper focuses on enhancing the dimensional accuracy of K648 superalloy castings by optimizing the wax mold design through the introduction of a hollow mold structure.
2. Literature Review
Investment casting has a long history and is known for its ability to produce complex-shaped components with high dimensional accuracy. Several studies have investigated the factors affecting the dimensional accuracy of investment castings, with a particular emphasis on the wax mold.
Chen et al. (2003) studied the influence of mold material and process parameters on the dimensional stability of investment castings. They found that the linear shrinkage rate of the wax mold is a crucial factor determining the final casting dimensions. Zhang et al. (2017) focused on the precision casting process of heavy gas turbine blades, highlighting the importance of dimensional accuracy for critical aerospace components.
While these studies provide valuable insights, there is a lack of focus on optimizing wax mold designs, particularly for large and complex castings with tight tolerance requirements. This paper addresses this gap by proposing a hollow wax mold structure tailored for K648 superalloy castings.
3. Materials and Methods
3.1 Wax Mold Material
The wax mold used in this study was made from 162 non-filler wax, which has a linear shrinkage rate of approximately 0.9-1.0%. This wax material was selected for its good formability, stable shrinkage, and suitability for thin-walled patterns.
3.2 Wax Mold Design
A K648 superalloy casting with a complex structure was selected for this study. The casting has several thick-walled sections and critical dimensions that require high accuracy. To address the dimensional challenges, a hollow wax mold design was developed.
Hollow Mold Concept
The conventional solid wax mold was modified to include hollow sections with a thickness of 4.5-5 mm. This design aimed to reduce the overall mold mass and thus the linear shrinkage during cooling. Conical hollow structures with a 5° draft angle were adopted to facilitate mold ejection.
Mold Preparation
Two sets of metal molds were prepared: one for the conventional solid wax mold and another for the hollow wax mold. Both molds were machined to a precision of ±0.02 mm to ensure repeatability and consistency.
3.3 Wax Mold Manufacturing Process
The wax patterns were prepared using a 16-ton double-station hydraulic wax injector. Table 1 summarizes the process parameters used for wax injection. After injection, the wax patterns were allowed to cool and then ejected from the metal molds. The hollow wax patterns were then corrected on 矫形平台 for at least 2 hours to minimize distortions.
| Parameter | Value |
|---|---|
| Injection Pressure (kg/cm²) | 10-20 |
| Flow Rate (%) | 20-30 |
| Injection Time (s) | 20-30 |
| Wax Pot Temperature (°C) | 58 ± 5 |
| Cooling Cylinder Temperature (°C) | 58 ± 5 |
| Cooling Time (s) | 40-60 |
| Nozzle Hold Time (s) | 20-30 |
| Cooling Method | Air Cooling |
Table 1: Wax Injection Process Parameters
3.4 Shell Preparation
The wax patterns were dipped in ceramic slurry to form the investment shell. The shell consisted of eight layers of silica sol-based slurries and refractory materials (Table 2). After shell build-up, the wax was removed using high-temperature steam dewaxing.
| Layer | Slurry Type | Viscosity (s) | Refractory Material | Particle Size (mesh) |
|---|---|---|---|---|
| 1 | Zircon-silica sol | 30-45 | Zircon sand | 80-120 |
| 2-7 | Alumina-silica sol | 8-30 | Alumina sand | 16-60 |
| 8 | Final alumina sol | 5-10 | – | – |
Table 2: Shell Preparation Process Parameters
3.5 Casting and Post-Processing
The investment shells were preheated to 1050°C for 2 hours and then poured with K648 superalloy at 1450°C using a 25 kg three-chamber vacuum induction melting furnace. The pouring speed was controlled at 10 s. After solidification, the castings were cleaned, heat-treated, and sandblasted to obtain the final products.
4. Results and Discussion
4.1 Wax Mold Dimensional Analysis
The wax patterns were scanned using a Geomagic Control blue light scanner and compared to the theoretical 3D models. The surface deviations were mapped and analyzed.
The results show that the surface deviations of the hollow wax mold were significantly lower compared to the solid wax mold. The average deviations were -0.44 to +0.475 mm for the hollow mold and -0.695 to +0.735 mm for the solid mold.
4.2 Linear Shrinkage Rate
Critical dimensions of both wax mold types were measured, and the linear shrinkage rates were calculated using Equation (1).
alpha=(A0A0−A1)×100%
where α is the linear shrinkage rate, A0 is the mold cavity dimension, and A1 is the measured wax mold dimension.
Table 4: Linear shrinkage rates of wax molds
| Mold Cavity Size (mm) | Solid Wax Mold | Hollow Wax Mold |
|---|---|---|
| 73.83 | 1.16% | 0.54% |
| 61.97 | 1.60% | 0.15% |
| … | … | … |
The linear shrinkage rates of the hollow wax mold were consistently lower than those of the solid wax mold, indicating improved dimensional stability.
4.3 Casting Dimensional Analysis
The final castings were inspected using precision measurement equipment, and the dimensional deviations were recorded. The actual linear shrinkage rates ((\beta)) were calculated using Equation (2).
beta=(A0A0−A2)×100%
where β is the casting linear shrinkage rate, A0 is the mold cavity dimension, and A2 is the measured casting dimension.
Table 5: Dimensional analysis of K648 castings
| Mold Cavity Size (mm) | Casting (Hollow Mold) | Casting (Solid Mold) |
|---|---|---|
| Dimension (mm) | 72.45 (2.41%) | 72.50 (2.70%) |
| Tolerance Grade | CT5 | CT7 |
| … | … | … |
The hollow mold castings exhibited higher dimensional accuracy, achieving a tolerance grade of CT5 compared to CT7 for the solid mold castings.
5. Discussion
The reduced linear shrinkage rate of the hollow wax mold can be attributed to several factors:
- Reduced Mass and Heat Content: The hollow design significantly reduces the overall mass of the wax mold, leading to less heat accumulation during the cooling process. This results in a more uniform and controlled shrinkage.
- Improved Cooling Dynamics: The thin-walled hollow sections facilitate more efficient heat dissipation, reducing the internal stresses developed during cooling and minimizing distortions.
- Structural Support: The hollow design incorporates internal cores that provide additional structural support during the molding and dewaxing processes, further mitigating deformations.
- Process Control: The process of manufacturing hollow wax molds requires stricter process control to maintain dimensional stability. This enhanced process discipline also contributes to the improved dimensional accuracy.
6. Conclusion
This study successfully demonstrated that a hollow wax mold design can significantly improve the dimensional accuracy of K648 superalloy castings produced via investment casting. By thinning specific mold sections to 4.5-5 mm, the average linear shrinkage rate of the wax mold was reduced from 1.16% to 0.54%, leading to castings with a tolerance grade of CT5 compared to CT7 for solid mold castings. This research provides valuable insights for enhancing the dimensional accuracy of large and complex investment castings, particularly those requiring tight tolerances.