Investment Casting Process of High Temperature Alloy Castings

Investment casting is widely used in the manufacturing of core components in the fields of aviation, aerospace, gas turbines, etc., and has high requirements for the dimensional accuracy of castings.High temperature alloy castings often use medium temperature wax and full silica sol shell processes, resulting in dimensional accuracy of CT4-CT7 levels.The study found that the factors affecting the dimensional accuracy of investment castings mainly include the structure of the casting, the shrinkage during the solidification process of the metal, and the temperature changes caused by the mould Shell size changes, mold size accuracy, etc., and mold size accuracy can affect the size of the casting by more than 50%.

Factors affecting the dimensional accuracy of the investment mold

Medium and high-end investment castings generally use non-filler wax and filler wax to make investment molds. The linear shrinkage rate of non-filler wax is about 1%, while the linear shrinkage rate of filler wax is about 0.5%. Research has shown that reducing the linear shrinkage rate is an important way to improve the dimensional accuracy of investment molds. This study uses 162 brand non-filler wax, which has a linear shrinkage rate of 0.9% to 1.0%, good formability, stable shrinkage, good flexibility and toughness, and is suitable for thin-walled and small-sized investment molding.

Mold making process

The study found that the higher the wax pressing temperature, the greater the injection pressure, the faster the wax flow rate, the shorter the cooling time, and the higher the mold temperature, the greater the linear shrinkage rate of the investment mold, and the worse the dimensional accuracy of the casting. Conversely, the dimensional accuracy of the casting is higher. With the gradual improvement of wax pressing equipment manufacturing capabilities, these molding process parameters can be stably controlled. After multiple pressing tests, combined with empirical data obtained from a big data intelligent management system, the impact of molding processes on investment mold dimensional accuracy can be effectively reduced.

Mold manufacturing accuracy

With the rapid development of the mechanical processing industry, traditional precision machining can no longer meet the development needs of cutting-edge technology. On this basis, ultra-precision machining technology has been derived, with the highest machining dimensional accuracy reaching 10nm, surface roughness up to 1nm, and the smallest machining size of 1μm. At present, more and more intelligent manufacturing technology and network technology are applied to the field of mechanical processing. At the same time, with the development of optical and electronic technology, the accuracy of mechanical processing has been greatly improved. In recent years, some foreign specialized mold equipment has entered China, making China’s mold manufacturing more high-speed, precision, composite and intelligent. The influence of mold making accuracy on the dimensional accuracy of the investment mold is becoming smaller and smaller.

Molded structure

When the section thickness of the investment mold exceeds 13mm, it is recommended to use cold wax blocks to reduce the wall thickness and thus reduce the linear shrinkage rate of the investment mold. However, using cold wax block technology often encounters the following problems: ① Increasing the number of molds, thereby increasing production costs; ② Due to the need for pre-fabrication of cold wax blocks in advance, the production efficiency of the product is reduced; ③ When the structural design of the cold wax block is unreasonable, it will lead to uneven shrinkage of the investment mold, resulting in dimensional fluctuations, which usually requires multiple trial productions and adjustments to determine the final cold wax block structure; ④ Cold wax blocks are prone to being wrapped in place or having exposed support points, which increases the workload of investment mold repair and reduces the surface quality of the investment mold. The main factors affecting the dimensional accuracy of the investment mold are concentrated on the mold material and investment mold structure. In the case where it is impossible to change the mold material and casting structure, rational design of the investment mold structure is the best solution to improve the dimensional accuracy of the investment mold. This article designs a hollow investment mold structure to reduce the section thickness of the investment mold part, so as to obtain a more uniform wall thickness of the investment mold and achieve the goal of reducing the linear shrinkage rate of the investment mold. By reducing the linear shrinkage rate of the investment mold, it can effectively improve the dimensional accuracy of the casting. The hollow investment mold structure only requires appropriate addition of core-pulling structures to the mold, without extending the production cycle of the investment mold, and is suitable for investment castings with large wall thickness differences, high dimensional accuracy requirements, and large batch sizes.

Structural analysis of the mold

The material designation is K648, and the chemical composition is (mass fraction, %) 0.05C, 34Cr, 3Mo, 0.85Ti, 4.45 W, 0.98 Nb, 1.0 Al, with the balance being Ni. This casting is a typical structural investment casting. In addition to a “C” type blind hole structure in one place, although the wall thickness difference of the casting is small, the wall thickness of key parts exceeds 13mm. After investment molding, the cooling rate of the contact surface with the mold cavity is the fastest, and a solid wax shell of 3~5mm is formed first. The heat of the investment core is difficult to dissipate. When the investment is removed from the mold, the heat of the core gradually transfers outward to the surface of the investment mold, causing the surface of the investment mold to “soften” again, which easily leads to bending and deformation of the investment mold. During the process of transferring heat from the thick and large part of the core outward, large stresses will also be generated, resulting in large linear shrinkage in traditional investment structures, causing fluctuations in the final casting dimensions.

Mold structure design

The characteristic of investment casting process is to prepare castings by replicating the surface shape of the fusible mold, and the internal structure of the mold has no effect on the structure of the casting. Considering the problems of the cold wax block scheme, the mold structure of the experimental castings was designed as hollow. HB5350 stipulates that the thickness of the test sample for the mold shrinkage test of investment casting is 4mm, and the thickness of the sample for the bending strength test of mold material at room temperature is 6mm. In order to effectively collect the data on the linear shrinkage rate of the mold and ensure that the mold has sufficient room temperature strength, the section wall thickness of the hollow part of the mold is designed to be 4.5~5.0mm, which also facilitates the flow of wax and heat transfer of wax materials. The hollow structure of the mold needs to be formed by core pulling with a metal mold. The location where it is difficult to use a uniform wall thickness structure is designed with a tapered hollow structure, that is, a single side wall thickness of 4.5mm at the mouth, with an inward 5° draft angle added to facilitate mold core pulling. The open ends of all hollow structures are arranged on the gate surface, and when the mold is assembled, the open ends are closed, finally forming a mold structure with a “cavity” inside.

Mold preparation

The wall thickness of the key parts of the solid structure of the investment mold exceeds 13mm, and it belongs to free shrinkage. According to the HB5350.1 test method, the linear shrinkage rate of 162 wax from the mold to the investment mold is 0.5% to 1.0%. According to production practice, the comprehensive shrinkage rate of small and medium-sized K648 high-temperature alloy castings from the investment mold to the casting is 1.6% to 2.0%. Therefore, the preliminary preset test casting mold cavity comprehensive shrinkage rate is 2.6%, and metal molds are made with two structures shown in Figures 2 and 3 respectively.

Melt molding preparation

The ash content of the new 162 medium-temperature wax material is generally less than 0.05% (mass fraction). With the increase of recycling times and addition ratio, the ash content of the wax material will increase to more than 0.10% (mass fraction), which will ultimately affect the metallurgical quality of the casting. At the same time, the linear shrinkage rate of the recycled wax material will also increase to more than 1.0%, which will reduce the dimensional accuracy of the investment mold. Therefore, this study uses 100% new 162 wax material and uses a 16t dual-station hydraulic wax injection machine to prepare the investment mold. After removing the investment mold from the metal mold, it is placed on an orthopedic platform and corrected with a weight for more than 2 hours to reduce the deformation of the investment mold.

Analysis of the contour dimensions of the investment mold

Using the Geomagic Control blue light scanner to scan the two structural investment molds and compare them with the 3D theoretical model. The surface offset of the solid investment mold is -0.695~+0.735mm, and the surface offset of the hollow investment mold is -0.44~+0.475mm. At the same time, due to the large wall thickness of the section, the solid investment mold shows significant bending after the distal end shrinks, and the depression degree of the middle plane is significantly higher than that of the hollow investment mold. This is due to the reduction in the average wall thickness of the hollow investment mold, which reduces the heat accumulated during the investment molding process and reduces the internal stress generated during cooling and shrinkage of the investment mold. At the same time, due to the increase in the contact area between the investment mold and the metal mold, the cooling of the investment mold is more stable and uniform. It can be seen that reducing the wall thickness of the investment mold from more than 13mm to 4.5~5.0mm After that, the deformation of the flexible and distorted part of the distal end of the mold was effectively alleviated, and the collapse of the planar core was also alleviated, which improved the mold’s strength and accuracy The dimensional accuracy of the mold is of great significance.

Casting size analysis

According to the analysis of the dimensional inspection results of the castings based on HB6103, the dimensional accuracy of the castings prepared with solid investment meets CT7 level, and the average linear shrinkage rate is calculated as 2.70% according to the formula. The dimensional accuracy of the castings prepared with hollow investment meets CT5 level, and the average linear shrinkage rate is calculated as 2.41% according to the formula.

During the cooling and shrinkage process of the solid mold of the test casting, except for the parts in contact with the orthopedic tooling, all parts are free to shrink, and the shrinkage is greatly affected by the wall thickness. The larger the wall thickness, the greater the shrinkage stress, resulting in significant shrinkage differences in different parts of the entire mold, leading to large fluctuations in casting dimensions. After the mold is hollowed out, the wall thickness of the mold is artificially reduced, reducing the shrinkage difference in different parts. Moreover, the hollow parts are supported by metal cores, which are blocked during the shrinkage process before the mold is removed from the die. Therefore, the dimensional fluctuations of the mold and casting are relatively small.

Conclusion

(1) By partially hollowizing the section thickness of the solid investment mold to 4.5-5.0mm and orthotizing the investment mold for more than 2 hours,It can alleviate the central collapse and distal bending of the larger surface of the investment mold, and the overall linear contraction of the investment mold is reduced from 1.16% to 0.54%.
(2) The dimensional accuracy of investment castings has been improved from CT7 to CT5, and the actual linear shrinkage rate of castings has been reduced from 2.70% to 2.41%. This result This study can provide a reference for the casting process design of thick and large investment castings.

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