Abstract: This article focuses on the research of investment casting technology for K648 superalloy castings with a hollow wax mold structure. It analyzes the factors affecting the dimensional accuracy of investment castings, especially emphasizing the role of the hollow wax mold structure. Through a series of experiments and analyses, it demonstrates the significant impact of this structure on reducing the linear shrinkage rate of the wax mold and improving the dimensional accuracy of the casting. The research provides valuable references for the design of investment casting processes for high-precision and thick-large castings.
1. Introduction
Investment casting is widely used in the manufacturing of core components in aerospace, gas turbines, and other fields, demanding high dimensional accuracy of castings. The dimensional accuracy of investment castings is affected by multiple factors, and among them, the linear shrinkage rate of the wax mold has a crucial impact. This article aims to study the influence of a hollow wax mold structure on the dimensional accuracy of K648 superalloy castings.
2. Factors Affecting the Dimensional Accuracy of Investment Castings
2.1 Wax Pattern Material Types
Investment castings, especially medium and high-end ones, commonly use non-filler wax materials and filler wax materials to make wax patterns. The linear shrinkage rate of non-filler wax materials is about 1% and that of filler wax materials is about 0.5%. In this study, a 162-brand non-filler wax material with a linear shrinkage rate of 0.9% – 1.0% was used, which has good formability, stable shrinkage, good flexibility, and is suitable for making wax patterns for thin-walled and small and medium-sized parts.
| Wax Pattern Material Type | Linear Shrinkage Rate | Characteristics | Applicable Parts |
|---|---|---|---|
| Non-filler wax material | About 1% | Good formability, stable shrinkage, good flexibility | Thin-walled and small and medium-sized parts |
| Filler wax material | About 0.5% | – | – |
2.2 Molding Process
Higher wax injection temperature, larger injection pressure, faster wax liquid flow rate, shorter cooling time, and higher mold temperature lead to a larger linear shrinkage rate of the wax pattern and poorer casting dimensional accuracy. With the improvement of wax injection equipment manufacturing capabilities, these molding process parameters can be stably controlled. Through multiple pressing tests and experience data obtained from a big data intelligent management system, the impact of the molding process on the dimensional accuracy of the wax pattern can be effectively reduced.
2.3 Mold Manufacturing Precision
With the development of the mechanical processing industry, ultra-precision machining technology has emerged, with a maximum machining dimensional accuracy of 10 nm and a surface roughness of 1 nm, and the minimum machined size reaching 1 μm. The application of intelligent manufacturing and network technologies in the mechanical processing field, along with the development of optical and electronic technologies, has significantly improved machining accuracy. The influence of mold manufacturing precision on the dimensional accuracy of the wax pattern is becoming smaller.
2.4 Wax Pattern Structure
When the cross-sectional thickness of the wax pattern exceeds 13 mm, it is recommended to use a cold wax block to reduce the wall thickness and thus the linear shrinkage rate of the wax pattern. However, this method has several problems, such as increased mold quantity and production costs, reduced production efficiency, uneven shrinkage, and increased wax pattern repair workload. Therefore, a reasonable design of the wax pattern structure is crucial for improving its dimensional accuracy.
3. Experimental Materials and Methods
3.1 Wax Pattern Structure Analysis
A K648 superalloy casting was studied. Its chemical composition includes 0.05 C, 34Cr, 3Mo, 0.85 Ti, 4.45 W, 0.98 Nb, 1.0 Al, and the balance Ni. Although the wall thickness difference of the casting is small except for a “C”-shaped blind hole structure, the wall thickness of key parts exceeds 13 mm. The traditional wax pattern structure is prone to problems such as bending deformation and large linear shrinkage.
3.2 Wax Pattern Structure Design
The wax pattern structure was designed to be hollow. The wall thickness of the hollow section was designed to be 4.5 – 5.0 mm to ensure the collection of linear shrinkage rate data and sufficient room temperature strength of the wax pattern. In some positions where core pulling is difficult, a tapered hollow structure was adopted. The open ends of all hollow structures were set on the gate surface and closed during wax pattern assembly to form a hollow wax pattern structure.
3.3 Mold Preparation
Based on the wall thickness and shrinkage characteristics of the casting, the comprehensive shrinkage rate of the mold cavity for the test casting was preliminarily set to 2.6%. Metal molds were made for both the solid and hollow wax pattern structures. The results showed that the surface offset of the solid wax pattern was -0.695 – +0.735 mm, and that of the hollow wax pattern was -0.44 – +0.475 mm. The solid wax pattern showed obvious bending and greater plane shrinkage than the hollow wax pattern.
3.4 Wax Pattern Preparation
100% new 162 medium-temperature wax material was used, and a 16 t double-station hydraulic wax injection machine was employed to prepare the wax pattern. After being taken out from the metal mold, the wax pattern was placed on a shaping platform and shaped with a weight for more than 2 h to reduce deformation.
3.5 Shell Mold Making
A full silica sol shell mold was made according to a specific process, and the wax was removed using a high-temperature steam dewaxing kettle to obtain the test shell mold.
3.6 Casting Pouring and Post-treatment
The test shell mold was preheated in a box-type resistance sintering furnace, and a 25 kg three-chamber vacuum induction electric furnace was used to remelt the K648 alloy ingot for pouring. After pouring, the casting was obtained through cooling, shell removal, removal of the pouring system, heat treatment, and sandblasting.
4. Experimental Results and Analysis
4.1 Wax Pattern Contour Dimension Analysis
The two types of wax patterns were scanned using a Geomagic Control blue light scanner and compared with the 3D theoretical model. The reduction in the average wall thickness of the hollow wax pattern reduced the heat accumulated during the pressing process, decreased the internal stress during cooling and shrinking, and increased the contact area between the wax pattern and the metal mold, resulting in more stable and uniform cooling. As a result, the deformation of the distal part of the wax pattern and the plane shrinkage of the core part were effectively alleviated.
4.2 Wax Pattern Linear Dimension Analysis
The key dimensions of the two types of wax patterns were detected, and the actual linear shrinkage rates were calculated. The average linear shrinkage rate of the solid wax pattern was 1.16%, while that of the hollow wax pattern was 0.54%. The linear shrinkage rates of all key dimensions of the hollow wax pattern were smaller than those of the solid wax pattern. For dimensions with a larger shrinkage distance during the cooling process of the wax pattern, the reduction in the linear shrinkage rate was more obvious. For dimensions where the original wax pattern wall thickness was less than the minimum hollow design wall thickness of 4.5 mm, the change in the linear shrinkage rate was smaller.
| Wax Pattern Type | Average Linear Shrinkage Rate | Key Dimension Shrinkage Characteristics |
|---|---|---|
| Solid wax pattern | 1.16% | Larger shrinkage in some dimensions, affected by wall thickness |
| Hollow wax pattern | 0.54% | Smaller shrinkage in all key dimensions, more obvious reduction for larger shrinkage distances |
4.3 Casting Dimension Analysis
According to HB 6103, the casting dimension detection results were analyzed. The casting made from the solid wax pattern had a dimensional accuracy of CT7 level with an average linear shrinkage rate of 2.70%, while the casting made from the hollow wax pattern had a dimensional accuracy of CT5 level with an average linear shrinkage rate of 2.41%. The solid wax pattern had larger shrinkage differences in different positions during cooling, resulting in larger casting dimension fluctuations. The hollow design of the wax pattern reduced the wall thickness and shrinkage differences, and the hollow parts were supported by metal cores, resulting in smaller size fluctuations of the wax pattern and the casting.
| Wax Pattern Type for Casting | Casting Dimensional Accuracy Level | Average Linear Shrinkage Rate | Characteristics of Casting Dimension Fluctuations |
|---|---|---|---|
| Solid wax pattern | CT7 | 2.70% | Larger fluctuations due to different shrinkage in different positions |
| Hollow wax pattern | CT5 | 2.41% | Smaller fluctuations as shrinkage differences are reduced |
5. Conclusions
(1) By locally hollowing out the cross-section of the solid wax pattern with a thickness of 4.5 – 5.0 mm and shaping the wax pattern for more than 2 h, the center shrinkage of the larger plane and the distal bending of the wax pattern can be alleviated, and the overall linear shrinkage of the wax pattern can be reduced from 1.16% to 0.54%.
(2) The dimensional accuracy of the investment casting can be improved from CT7 level to CT5 level, and the actual linear shrinkage rate of the casting can be reduced from 2.70% to 2.41%. This research provides a reference for the design of investment casting processes for thick and large castings.
In conclusion, the hollow wax pattern structure has a significant impact on improving the dimensional accuracy of K648 superalloy castings through investment casting. Future research could focus on further optimizing the hollow wax pattern structure and exploring its application in other alloy castings.