With the development of aerospace, aviation and industrial gas turbine, the gas temperature of turbine engine is increasing, so the ability of high temperature resistance of turbine blade is required to be increased. For this purpose, the development of superalloy turbine blades from solid to hollow has been realized in structure, and in solidification mode, it has realized the leap from polycrystalline, directional solidification and inverted single crystal.
Operating environment of turbine blades
Turbine blade is one of the most important parts in the aeroengine, which is located in the most complex part of the engine with the highest temperature, the most complex stress and the worst environment. It has a large number, complex shape, high dimensional requirements and difficult processing, which directly affects the performance of the aeroengine. The working temperature of advanced aeroengine can reach over 1700 ℃, and the pressure can reach more than 50 atmospheres after supercharging.
In order to meet the requirements of engine performance, reliability and life, turbine blade materials need to have excellent high temperature strength, good oxidation resistance, thermal corrosion resistance, good fatigue, fracture toughness and other comprehensive properties.
Turbine blade cooling and ceramic core
At present, the cooling structure of blades has developed from traditional convection cooling, impact cooling and film cooling to efficient divergent cooling and laminar cooling. All these cooling methods are related to the shape of the inner cavity of the blade, and the possibility of realizing the shape of the inner cavity depends on the performance of the ceramic core. After the casting blade is cooled, the blade is taken out of the mold, and the ceramic core inside needs to be dissolved. At present, ceramic core is developing in the direction of more complex shape, smaller size and higher performance, which greatly promotes the application of high-performance cast superalloy in gas turbine blades.
Ceramic core material
The raw materials for the preparation of ceramic core must have the following conditions: sufficient fire resistance (melting point or high temperature softening point is higher than 1600 ℃); good thermochemical stability, incompatible with the high temperature of alloy elements; good thermal stability and thermal shock resistance, small coefficient of linear expansion, matching with the shell, easy to remove, no excessive high and low temperature crystal transformation after firing. Currently commonly used materials include:
1) Silicon oxide based ceramic core
Quartz glass is used as matrix material, alumina, mullite and zircon powder are added as strengthening phase, and the performance of ceramic core is guaranteed by controlling the content of cristobalite. The sintering temperature of silicon oxide based ceramic core is usually 1150 ~ 1250 ℃, and the service temperature is 1520 ~ 1550 ℃. When the temperature is only over 1550 ℃, the creep resistance and chemical stability of silicon-based core decrease significantly.
2) Alumina based ceramic core
In the process of roasting and casting, the structure of alumina is stable, without crystal transformation, with low coefficient of thermal expansion and good thermochemical stability, so the chemical stability, high temperature strength and creep resistance of alumina based core are better than those of silica based core. At the same time, due to the high refractoriness of alumina, alumina based cores are usually difficult to sinter and chemical removal, so it is usually necessary to add appropriate mineralizers.
Preparation technology of conventional ceramic core
The preparation methods of ceramic core include hot pressing injection, grouting, extrusion and transfer mold. At present, the hot pressing injection method is mainly used to prepare ceramic core at home and abroad. The thermoplastic material is used as plasticizer to prepare ceramic slurry. The ceramic core body is formed by hot pressing injection, dewaxing and high-temperature roasting are carried out, and then the baked core is reshaped and strengthened to obtain the final size Ceramic core with required accuracy. During the preparation of ceramic core, the composition and particle size distribution of refractory powder, firing temperature, furnace temperature uniformity, filler and strengthening treatment have significant influence on the microstructure and high-temperature mechanical properties of ceramic core.
Work flow chart of hot pressing casting
Ceramic 3D printing process
3D printing (also known as additive manufacturing) technology is based on the "discrete stacking" forming principle, which is driven by the 3D data of components to directly manufacture solid parts, which saves the complex process and expensive mold cost in the traditional forming technology, and transforms the traditional "removed" material manufacturing into "increased" material manufacturing. The technology integrates computer, numerical control, laser and new materials, and has great advantages in the preparation of complex shape and structural parts without mold.
3D printing technology has shown great advantages in the direct forming of complex shape metal and polymer parts. The emerging 3D printing technology has great development potential in the field of high-performance ceramic forming and manufacturing. It is expected to break through the technical bottleneck of traditional ceramic processing and production, and fully meet the rapid manufacturing of personalized, refined, lightweight and complex high-end products The requirement of manufacturing opens up a new way for the application of ceramic key parts. Using 3D printing technology, combined with the excellent characteristics of ceramic materials, it provides the choice of core for the molding of hollow blades with high performance and complex structure, which can save the complex process of mold development and realize the manufacturing of finished products with fast, low cost and complex structure.
The principle of 3D printing of ceramics (taking the most popular 3D printing based on UV curable ceramics as an example) is to mix ceramic powder and photosensitive resin into printing materials of ceramic paste, and then solidify them layer by layer into ceramic green body by laser. Therefore, in the right side of Figure 2.2, after printing, processes such as degreasing and sintering need to be carried out to remove the organic components in the ceramic green to get the final ceramic products.
There are three main steps in 3D printing of UV curable ceramics: preparation of ceramic slurry, UV curable molding and degreasing sintering process. Each step will affect the quality of the final ceramic products.
Stable ceramic printing material: ceramic green body made of high solid content and low particle size ceramic material can obtain good density after heat treatment. The stability and fluidity of the material depend on the interaction between particles, particles and solvents.
UV curing: after the preparation of stable slurry, it needs to be UV curing in the printer. There are a large number of ceramic particles in UV photosensitive resin. As shown in the figure, ceramic particles have absorption and scattering effects on light, which will affect the accuracy of product molding.
Scattering of light by ceramic particles
Debinding sintering: with the decomposition of organic matter and the change of ceramic grain and grain boundary, ceramic products are finally obtained. The residual carbon of decomposition of organic matter will affect the quality of products. Therefore, the selection of appropriate baking process plays an important role in the final ceramic products. The schematic diagram is as follows, in which black represents ceramic particles and red represents resin part:
Ceramic 3D printing solution for ceramic core
Printing materials: according to different casting methods, alloy types and core removal methods, we have developed appropriate ceramic core printing materials, including aluminum-based materials with good chemical stability, silicon-based materials with convenient core removal, aluminum oxide / silicon oxide composite printing materials and zirconia / silicon oxide composite printing materials, which can be determined according to the ceramic powder currently used by customers Manufacturing research and development, to meet the casting process requirements to the maximum extent. Silicon is a printing material specially developed for silicon-based ceramic core. Its characteristics are: high mechanical properties, easy detaching, high temperature stability, suitable for casting a variety of alloys. The specific indicators are as follows
Printing equipment: provide printing equipment of different specifications from 100 * 100mm to 600 * 600mm for different applications to fully meet the requirements.
Technological advantages: because the printer adopts sinking structure and high solid content printing materials, the traditional column support which is directly connected with the sample can be avoided in the forming process, while the non-contact "block support" can be used, so as to facilitate the separation of the support and the parts after printing and the shape maintenance of the parts in the sintering process.
Source: 3dceram