A high-inert TiAl alloy directional solidification shell and a preparation method thereof

Abstract

This invention provides a mold shell for directional solidification of highly inert TiAl alloys and its preparation method. The mold shell provided by this invention consists of a surface layer, a transition layer, and a back layer from the inside out. The surface layer is filled with a yttrium oxide sand layer, the transition layer is filled with a yttrium oxide sand layer, and the back layer is filled with a zirconia sand layer. All sand layers are tightly bonded and crack-free. In the coating process of the mold shell, yttrium oxide is used as the refractory material for the surface and transition layers, zirconia as the refractory material for the back layer, and yttrium sol as the binder. Simultaneously, specific types and proportions of dispersants, surfactants, and defoamers are added, resulting in a uniform slurry with good wettability, effectively eliminating porosity and air bubbles in the mold shell, and increasing the surface density and inertness of the mold shell. The mold shell provided by this invention is thin, uniform, dense, and has good resistance to melt reactivity, making it suitable for precision casting processes of highly reactive alloys such as TiAl alloys, and has significant industrial application value.

Description

Technical Field

[0001] This invention relates to the field of lightweight, high-temperature, and high-strength structural material preparation technology, and in particular to a mold shell for directional solidification of high-inert TiAl alloy and its preparation method. Background Technology

[0002] The development of next-generation high thrust-to-weight ratio and high power-to-weight ratio aero-engines has placed higher demands on turbine inlet temperature, making the development of lightweight structural materials with excellent high-temperature performance and high specific strength an urgent need in the field of aerospace materials. Directional solidification TiAl alloys possess high high-temperature strength, good plasticity, and good creep resistance, making them suitable as materials for next-generation engine turbine blades. The combination of precision casting and directional solidification techniques to prepare large-size TiAl alloys allows for the one-time molding of multiple complex structural components, meeting the needs of industrialized mass production and facilitating rapid iterative improvements in part shape and structure.

[0003] For mold shells with complex cross-sectional shapes, when they are pulled into liquid metal for cooling, the volume of the mold shell immersed in liquid metal per unit time will change due to the different cross-sectional areas at different heights. This will cause changes in the liquid level height, which will alter the thermal and flow fields at the solid-liquid interface. This will result in uneven temperature gradients during the directional solidification process, leading to problems such as severe solute segregation, poor grain continuity, and inconsistent alloy lamellar orientation.

[0004] TiAl alloy melt is highly reactive at high temperatures and readily reacts with the mold shell. Due to the high temperature and long contact time (typically 1–3 hours) between the mold shell and the molten TiAl alloy during directional solidification, conventional refractory materials such as SiO2 or Al2O3 are unsuitable as the surface layer refractory material for the mold shell during TiAl alloy directional solidification. Furthermore, mold shell particles entering the melt can cause inclusions, porosity, and other defects, severely affecting the directional solidification process and leading to the growth of impurity crystals or heterooriented crystals. Therefore, the surface layer of the mold shell used for TiAl alloy directional solidification requires a dense surface refractory material that prevents molten metal from penetrating into the mold shell and is stable at high temperatures. Summary of the Invention

[0005] The technical problem solved by this invention is to provide a mold shell for directional solidification of highly inert TiAl alloys, which can effectively enhance the stability of the mold shell and stabilize the temperature gradient during the directional solidification process.

[0006] In view of this, this application provides a mold shell for directional solidification of a high-inert TiAl alloy, comprising a surface layer, a transition layer and a back layer arranged sequentially; the surface of the surface layer is filled with a yttrium oxide sand layer, the surface of the transition layer is filled with a yttrium oxide sand layer, and the surface of the back layer is filled with a zirconium oxide sand layer;

[0007] The surface layer is prepared from yttrium oxide powder, yttrium sol, dispersant, surfactant and defoamer;

[0008] The transition layer is prepared from yttrium oxide powder, yttrium sol, dispersant, surfactant and defoamer;

[0009] The backing layer is prepared from zirconium oxide powder, yttrium sol, dispersant, surfactant and defoamer.

[0010] Preferably, the yttrium oxide powder in the surface layer has a particle size of 400-500 mesh, the yttrium oxide powder in the transition layer has a particle size of 325-395 mesh, and the zirconium oxide powder in the back layer has a particle size of 325-500 mesh.

[0011] Preferably, the thickness of the mold shell is 8 to 10 layers, and the thickness is 2 to 5 mm.

[0012] Preferably, the number of layers in the surface layer is 2 to 3, the number of layers in the transition layer is 2, and the number of layers in the back layer is 4 to 5.

[0013] Preferably, in the surface layer, the mass ratio of yttrium oxide powder to yttrium sol is (2-2.5):1, the yttrium oxide content in the yttrium sol is 10-15 wt%, the dispersant content is 0.1-0.5 wt%, the surfactant content is 0.1-0.5 wt%, and the defoamer content is 0.1-0.5 wt%.

[0014] Preferably, in the transition layer, the mass ratio of yttrium oxide powder to yttrium sol is (2-2.5):1, the yttrium oxide content in the yttrium sol is 10-15 wt%, the dispersant content is 0.1-0.5 wt%, the surfactant content is 0.1-0.5 wt%, and the defoamer content is 0.1-0.5 wt%.

[0015] Preferably, in the backing layer, the mass ratio of the zirconium oxide powder to the yttrium sol is (4-5):1, the yttrium oxide content in the yttrium sol is 10-15 wt%, the dispersant content is 0.1-0.5 wt%, the surfactant content is 0.1-0.5 wt%, and the defoamer content is 0.1-0.5 wt%.

[0016] This application also provides a method for preparing the aforementioned mold shell, comprising the following steps:

[0017] A) The design of the assembly method and the preparation of the wax model are based on the shape and number of parts, and the layout of the parts and the shape of the central connecting rod are designed.

[0018] B) Mix yttrium oxide powder, yttrium sol, dispersant, surfactant and defoamer to obtain a slurry. Immerse the wax mold in the slurry, remove it, sprinkle yttrium oxide sand on it, and dry it to obtain a surface layer.

[0019] C) Mix yttrium oxide powder, yttrium sol, dispersant, surfactant and defoamer to obtain a slurry. Immerse the surface layer in the slurry, remove it, sprinkle yttrium oxide sand on it, and dry it to obtain a transition layer.

[0020] D) Mix zirconium oxide powder, yttrium sol, dispersant, surfactant and defoamer to obtain a slurry. Immerse the transition layer in the slurry, remove it, sprinkle zirconium oxide sand on it, and dry it to obtain the back layer.

[0021] E) After dewaxing the mold shell obtained in step D), bake it to obtain the mold shell.

[0022] Preferably, step B) is repeated 2 to 3 times, step C) is repeated 2 times, and step D) is repeated 4 to 5 times.

[0023] Preferably, the dewaxing method is steam dewaxing, with a temperature of 150-250℃, a pressure of 0.5-0.8MPa, and a time of 5-10min; the calcination temperature is 1500-2000℃, and the time is 2-5h.

[0024] This invention provides a mold shell for the directional solidification of highly inert TiAl alloys, comprising a face layer, a transition layer, and a back layer arranged sequentially. Yttrium oxide and zirconium oxide refractory materials serve as the matrix materials, exhibiting high inertness and good high-temperature stability. The yttrium sol system provides excellent bonding and exhibits minimal reaction with the melt at high temperatures. An antifoaming agent effectively prevents bubble formation, thereby eliminating porosity in each layer and preventing melt leakage. The yttrium oxide or zirconium oxide powder used in the slurries of the face layer, transition layer, and back layer have similar particle sizes, as do the yttrium oxide or zirconium oxide sand used for filling, resulting in a tighter bond between the layers and preventing detachment during shell formation. Therefore, the mold shell provided by this invention is thin, uniform, dense, and has good resistance to melt reactivity. It effectively improves the grain growth process of directionally solidified TiAl alloys, optimizes lamellar orientation, and reduces solute segregation. It also effectively enhances the chemical stability of the mold shell, reduces melt contamination, and increases the yield of directionally solidified TiAl single-crystal or columnar alloy samples. Furthermore, the mold shell of this invention has 8 to 10 layers with a total thickness of 2 to 5 mm, making it suitable for directional solidification processes and easily maintaining the melt temperature at high temperatures inside the furnace. In addition, the method provided by this invention has good versatility and can adapt to various part shapes, particularly suitable for the preparation of new turbine blades for aero-engines, and has broad industrial application value. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the mold shell preparation process of the present invention;

[0026] Figure 2 A photograph of the mold shell prepared in Example 1 of this invention;

[0027] Figure 3 Macroscopic micrographs of large-sized test bars prepared using the mold shell of Example 1;

[0028] Figure 4 A photograph of the actual mold shell prepared for Comparative Example 1. Detailed Implementation

[0029] To further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, and not for limiting the scope of the claims of the present invention.

[0030] In view of the problem that the mold shell affects the preparation of directionally solidified TiAl alloys, the present invention provides a mold shell comprising a face layer, a transition layer, and a back layer. By selecting the components of each layer, the density and inertness of the precision casting mold shell are increased, preventing melt penetration and reaction during directionally solidified processes. Preferably, the mold shell has an 8-10 layer ceramic structure, which prevents melt leakage and facilitates heat conduction during directionally solidified processes, promoting the growth of columnar or single crystals. Specifically, the present invention provides a mold shell for directionally solidified TiAl alloys, comprising a face layer, a transition layer, and a back layer arranged sequentially; the surface of the face layer is filled with a yttrium oxide sand layer, the surface of the transition layer is filled with a yttrium oxide sand layer, and the surface of the back layer is filled with a zirconium oxide sand layer.

[0031] The surface layer is prepared from yttrium oxide powder, yttrium sol, dispersant, surfactant and defoamer;

[0032] The transition layer is prepared from yttrium oxide powder, yttrium sol, dispersant, surfactant and defoamer;

[0033] The backing layer is prepared from zirconium oxide powder, yttrium sol, dispersant, surfactant and defoamer.

[0034] The mold shell provided by this invention is a yttrium oxide and zirconium oxide-based composite mold shell of yttrium sol system. By using fine-grained yttrium oxide powder and yttrium sol, and adding specific dispersants, surfactants and defoamers, the prepared mold shell has good density and inertness, which can significantly reduce the degree of reaction between the melt and the mold shell.

[0035] Specifically, the surface layer is prepared from yttrium oxide powder, yttrium sol, dispersant, surfactant, and defoamer. In the surface layer, the yttrium oxide powder has a particle size of 400-500 mesh, and its surface is filled with yttrium oxide sand, the yttrium oxide sand having a particle size of 40-100 mesh. The particle size of the yttrium oxide sand can vary depending on the number of layers. According to the present invention, the surface layer has 2-3 layers. In the surface layer, the mass ratio of yttrium oxide powder to yttrium sol is (2-2.5):1, the yttrium sol contains 10-15 wt% yttrium oxide, 0.1-0.5 wt% dispersant, 0.1-0.5 wt% surfactant, and 0.1-0.5 wt% defoamer; more specifically, the mass ratio of yttrium oxide powder to yttrium sol is (2.1-2.4):1, the yttrium sol contains 11-14 wt% yttrium sol, the dispersant contains 0.2-0.4 wt% dispersant, 0.1-0.3 wt% surfactant, and 0.1-0.3 wt% defoamer. In this invention, the dispersant is specifically selected from polyacrylamide, the surfactant is specifically selected from OP-10, and the defoamer is specifically selected from isooctanol.

[0036] The transition layer is prepared from yttrium oxide powder, yttrium sol, dispersant, surfactant, and defoamer. In the transition layer, the yttrium oxide powder has a particle size of 325-395 mesh, and its surface is filled with yttrium oxide sand with a particle size of 60-120 mesh. The particle size of the yttrium oxide sand can vary depending on the number of layers. According to the present invention, the transition layer has three layers. In the transition layer, the mass ratio of yttrium oxide powder to yttrium sol is (2-2.5):1, the yttrium oxide content in the yttrium sol is 10-15 wt%, the dispersant content is 0.1-0.5 wt%, the surfactant content is 0.1-0.5 wt%, and the defoamer content is 0.1-0.5 wt%. More specifically, the mass ratio of yttrium oxide powder to yttrium sol is (2.1-2.4):1, the yttrium oxide content in the yttrium sol is 11-14 wt%, the dispersant content is 0.2-0.4 wt%, the surfactant content is 0.1-0.3 wt%, and the defoamer content is 0.1-0.3 wt%. In this invention, the dispersant is specifically selected from polyacrylamide, the surfactant is specifically selected from OP-10, and the defoamer is specifically selected from isooctanol.

[0037] According to the present invention, the backing layer is prepared from zirconium oxide powder, yttrium sol, dispersant, surfactant, and defoamer. In the backing layer, the zirconium oxide powder has a particle size of 325–500 mesh, and the zirconium oxide sand filling its surface has a particle size of 40–100 mesh. The particle size of the zirconium oxide sand can vary depending on the number of layers. According to the present invention, the backing layer has 2–3 layers. In the backing layer, the mass ratio of zirconium oxide powder to yttrium sol is (4-5):1, the yttrium oxide content in the yttrium sol is 10-15 wt%, the dispersant content is 0.1-0.5 wt%, the surfactant content is 0.1-0.5 wt%, and the defoamer content is 0.1-0.5 wt%. More specifically, the mass ratio of zirconium oxide powder to yttrium sol is (4.2-4.8):1, the yttrium oxide content in the yttrium sol is 11-14 wt%, the dispersant content is 0.1-0.3 wt%, the surfactant content is 0.1-0.3 wt%, and the defoamer content is 0.1-0.3 wt%. In this invention, the dispersant is specifically selected from polyacrylamide, the surfactant is specifically selected from OP-10, and the defoamer is specifically selected from isooctanol.

[0038] In the aforementioned layers, the mass ratio of yttrium oxide powder to yttrium sol or the mass ratio of zirconium oxide powder to yttrium sol affects the manufacturing process and quality of the mold shell. For example, when the powder ratio is high, the mold shell is prone to cracking during coating drying and firing, leading to mold shell cracking or unusability. When the powder ratio is low, the coating cannot achieve proper slurry application. The mass ratio determines the density of the mold shell, thus ultimately affecting its inertness. The aforementioned mass ratio is beneficial for simultaneously avoiding surface porosity and material accumulation, while also increasing the firing performance of the mold shell, ensuring that the internal and interlayer structures of yttrium oxide and zirconium oxide in different layers are dense and crack-free after firing.

[0039] This invention also provides a method for preparing the mold shell, the process of which is as follows: Figure 1 As shown, specifically:

[0040] A) The design of the assembly method and the preparation of the wax model are based on the shape and number of parts, and the layout of the parts and the shape of the central connecting rod are designed.

[0041] B) Mix yttrium oxide powder, yttrium sol, dispersant, surfactant and defoamer to obtain a slurry. Immerse the wax mold in the slurry, remove it, sprinkle yttrium oxide sand on it, and dry it to obtain a surface layer.

[0042] C) Mix yttrium oxide powder, yttrium sol, dispersant, surfactant and defoamer to obtain a slurry. Immerse the surface layer in the slurry, remove it, sprinkle yttrium oxide sand on it, and dry it to obtain a transition layer.

[0043] D) Mix zirconium oxide powder, yttrium sol, dispersant, surfactant and defoamer to obtain a slurry. Immerse the transition layer in the slurry, remove it, sprinkle zirconium oxide sand on it, and dry it to obtain the back layer.

[0044] E) After dewaxing the mold shell obtained in step D), bake it to obtain the mold shell.

[0045] The method provided by this invention, through pre-designing a wax model, ensures that the cross-sectional area of ​​the central connecting rod portion of the mold shell varies with the cross-sectional area of ​​the parts, providing a certain degree of compensation. This ensures that the cross-sectional area is the same at different heights of the mold shell, thereby controlling the liquid level in the cooling tank to remain constant and maintaining a essentially constant temperature gradient. This effectively improves the high-temperature mechanical properties of directionally solidified TiAl alloys. According to this invention, during mold assembly, 2-3 parts are arranged on the tension base, with the parts in a centrally symmetrical relationship and connected to the central connecting rod.

[0046] The present invention then prepares the surface layer by mixing yttrium oxide powder, yttrium sol, dispersant, surfactant, and defoamer to obtain a slurry. The wax mold is then immersed in the slurry, removed, sprinkled with yttrium oxide sand, and dried to obtain the surface layer. The immersion time of the wax mold in the slurry is 7–10 seconds, and the drying time is 12–24 hours. According to the present invention, the above steps for preparing the surface layer are repeated 2–3 times to obtain 2–3 surface layers.

[0047] After the surface layer is prepared, a transition layer is prepared on its surface. Specifically, yttrium oxide powder, yttrium sol, dispersant, surfactant, and defoamer are mixed to obtain a slurry. The surface layer is then immersed in the slurry, removed, sprinkled with yttrium oxide sand, and dried to obtain the transition layer. During this process, the surface layer is immersed in the slurry for 5–7 seconds, and the drying time is 8–16 hours. According to the present invention, the preparation of the transition layer is repeated twice to obtain two transition layers.

[0048] The present invention then proceeds to prepare the back layer by mixing zirconia powder, yttrium sol, dispersant, surfactant, and defoamer to obtain a slurry. The transition layer is then immersed in the slurry, removed, sprinkled with zirconia sand, and dried to obtain the back layer. During this process, the immersion time of the transition layer in the slurry is 6–10 seconds, and the drying time is 10–20 hours. According to the present invention, the preparation of the back layer is repeated 4–5 times to obtain 4–5 back layers.

[0049] According to this invention, the obtained mold shell is finally dewaxed and then fired to obtain the mold shell. The dewaxing method is steam dewaxing at a temperature of 150-250℃, a pressure of 0.5-0.8MPa, and a time of 5-10 min; the firing temperature is 1500-2000℃ and the time is 2-5 h; more specifically, the dewaxing temperature is 180-230℃, the pressure is 0.6-0.8MPa, and the time is 6-10 min; the firing temperature is 1600-1700℃ and the time is 2-4 h.

[0050] To further understand the present invention, the following detailed description of the mold shell and its preparation method provided by the present invention is provided in conjunction with the embodiments. The scope of protection of the present invention is not limited by the following embodiments.

[0051] Example 1

[0052] The embodiments of the present invention are described in detail below:

[0053] Step 1: Prepare the master alloy with a nominal composition of Ti-45-8Nb-0.3C. Select analytically pure sponge Ti, AlNb, C powder and other raw materials, and melt the master alloy in a vacuum induction furnace under an argon protective atmosphere according to the alloy composition ratio. Cast the master alloy into a master alloy ingot for later use.

[0054] Step 2: Design the assembly method and prepare the wax model. The wax model consists of three cylindrical rods and connecting parts. The bottom of the cylindrical rods is equipped with a spiral crystal selector. The overall cross-sectional area of ​​the wax model is the same at different heights.

[0055] Step 3: Prepare the surface layer slurry and mold shell surface layer. The composition ratio by weight is as follows: yttrium oxide powder 71.1%, yttrium sol (yttrium oxide content 15wt%) 28.5%, dispersant ammonium polyacrylate 0.2%, surfactant OP-10 0.1%, and defoamer isooctanol 0.1%. First, add yttrium sol to a container, then add 500-mesh yttrium oxide powder and mix. Stir until there are no obvious lumps of solids, then add the slurry to a stirrer and stir at a speed of 600 r / min for 20 min. During the stirring process, add ammonium polyacrylate, OP-10 and isooctanol in sequence to obtain the surface layer slurry. Immerse the wax mold of the casting shape in the slurry for 7 seconds, then remove it and evenly sprinkle 80-mesh yttrium oxide sand on it. Dry it in air at 25°C for 20 h. Repeat the above steps 3 times to obtain 3 layers.

[0056] Step 4: Prepare the transition layer slurry and prepare the mold shell transition layer. The slurry composition and preparation method are the same as in Step 3. The particle size of the yttrium oxide powder is 325 mesh. Immerse the mold shell surface layer prepared in Step 3 into the slurry for 6 seconds and then take it out. Sprinkle 120 mesh yttrium oxide sand evenly on it and dry it in air at 25°C for 12 hours. Repeat the above steps twice to obtain two transition layers.

[0057] Step 5: Prepare the back layer slurry and fabricate the mold shell back layer. The composition ratio by weight is as follows: 79.6% zirconia powder, 20% yttrium sol (yttrium content is 15 wt%), 0.2% ammonium polyacrylate dispersant, 0.1% JFC surfactant, and 0.1% isooctanol defoamer. First, add yttrium sol to a container, then add 500-mesh zirconia powder and mix. Stir at 500 r / min for 30 min, and add ammonium polyacrylate, JFC, and isooctanol in sequence during stirring to obtain the back layer slurry. Immerse the mold shell transition layer prepared in Step 4 into the slurry for 7 seconds, then remove it and evenly sprinkle 80-mesh zirconia sand on it. Dry it in air at 25°C for 12 h. Repeat the above steps 4 times to obtain 4 back layers.

[0058] Step 6: Dewaxing and calcination. Place the mold shell in a steam dewaxing kettle and dewax for 8 minutes at 200℃ and 0.75Mpa. After dewaxing, place the mold shell in a resistance calcination furnace and calcinate at 1700℃. After reaching the calcination temperature, keep it at that temperature for 3 hours and then let it cool naturally in the furnace to obtain a high-inertia precision casting mold shell.

[0059] Figure 2 This is a photograph of the actual mold shell prepared for this embodiment.

[0060] The mold shell prepared using this embodiment can be used to prepare 20mm large-size test rods. The test rods have good orientation and can effectively obtain columnar crystal structures, as detailed below. Figure 3 As shown.

[0061] Example 2

[0062] The embodiments of the present invention are described in detail below:

[0063] Step 1: Prepare the master alloy with a nominal composition of Ti-46Al-8Nb; select analytically pure sponge Ti and AlNb raw materials, and according to the alloy composition ratio, use a vacuum induction furnace to melt the master alloy under an argon protective atmosphere, and cast it into a master alloy ingot for later use.

[0064] Step 2: Design the assembly method and prepare the wax model. The wax model consists of three turbine blade shapes and a central connecting rod. The cross-sectional area of ​​the central connecting rod varies with the cross-sectional area of ​​the blades, and the overall cross-sectional area of ​​the wax model is the same at different heights. The wax model can be formed using 3D printing technology.

[0065] Step 3: Prepare the surface layer slurry and mold shell surface layer. The composition ratio by weight is as follows: yttrium oxide powder 66.4%, yttrium sol (yttrium oxide content 13wt%) 33.2%, dispersant ammonium polyacrylate 0.2%, surfactant OP-10 0.1%, and defoamer isooctanol 0.1%. First, add yttrium sol to a container, then add 500-mesh yttrium oxide powder and mix. Stir until there are no obvious lumps of solids, then add the slurry to a stirrer and stir at a speed of 500 r / min for 20 min. During the stirring process, add ammonium polyacrylate, OP-10 and isooctanol in sequence to obtain the surface layer slurry. Immerse the wax mold of the casting shape in the slurry for 8 seconds, then remove it and evenly sprinkle 100-mesh yttrium oxide sand on it. Dry it in air at 25°C for 24 h. Repeat the above steps 3 times to obtain 3 layers.

[0066] Step 4: Prepare the transition layer slurry and prepare the mold shell transition layer. The slurry composition and preparation method are the same as in Step 3. The particle size of the yttrium oxide powder is 325 mesh. Immerse the mold shell surface layer prepared in Step 3 into the slurry for 6 seconds and then remove it. Sprinkle 120 mesh yttrium oxide sand evenly on it and dry it in air at 25°C for 16 hours. Repeat the above steps twice to obtain two transition layers.

[0067] Step 5: Prepare the back layer slurry and prepare the mold shell back layer. The composition ratio by weight is as follows: 83% zirconia powder, 16.6% yttrium sol (yttrium content is 13wt%), 0.2% ammonium polyacrylate dispersant, 0.1% JFC surfactant, and 0.1% isooctanol defoamer. First, add yttrium sol to a container, then add 500-mesh zirconia powder and mix. Stir at 500 r / min for 30 min, and add ammonium polyacrylate, JFC and isooctanol in sequence during stirring to obtain the back layer slurry. Immerse the mold shell transition layer prepared in Step 4 into the slurry for 8 seconds, then remove it and evenly sprinkle 100-mesh zirconia sand on it. Dry it in air at 25℃ for 15 h. Repeat the above steps 4 times to obtain 4 back layers.

[0068] Step 6: Dewaxing and calcination. Place the mold shell in a steam dewaxing kettle and dewax it for 10 minutes at 250℃ and 0.6Mpa. After dewaxing, place the mold shell in a resistance calcination furnace and calcinate it at 1700℃. After reaching the calcination temperature, keep it at that temperature for 4 hours and then let it cool naturally in the furnace to obtain a high-inertia precision casting mold shell.

[0069] Example 3

[0070] The embodiments of the present invention are described in detail below:

[0071] Step 1: Prepare the master alloy, with a nominal composition of Ti-48Al-2Nb-2Cr; select commercially available 4822 alloy, use a vacuum induction furnace, and remelt the master alloy under an argon protective atmosphere to improve the purity of the alloy, and cast it into a master alloy ingot for later use.

[0072] Step 2: Design the model assembly method and prepare the wax model. The wax model consists of two turbine blade shapes and a central connecting rod. The cross-sectional area of ​​the central connecting rod varies with the cross-sectional area of ​​the blades. The overall cross-sectional area of ​​the wax model is the same at different heights.

[0073] Step 3: Prepare the surface layer slurry and mold shell surface layer. The composition ratio by weight is as follows: yttrium oxide powder 66.4%, yttrium sol (yttrium oxide content 12wt%) 33.2%, dispersant ammonium polyacrylate 0.2%, surfactant OP-10 0.1%, and defoamer isooctanol 0.1%. First, add yttrium sol to a container, then add 400-mesh yttrium oxide powder and mix. Stir until there are no obvious lumps of solids, then add the slurry to a stirrer and stir at 500 r / min for 20 min. During the stirring process, add ammonium polyacrylate, OP-10 and isooctanol in sequence to obtain the surface layer slurry. Immerse the wax mold of the casting shape in the slurry for 10 seconds, then remove it and evenly sprinkle 100-mesh yttrium oxide sand on it. Dry it in air at 25°C for 16 h. Repeat the above steps twice to obtain two layers.

[0074] Step 4: Prepare the transition layer slurry and prepare the mold shell transition layer. The slurry composition and preparation method are the same as in Step 3. The particle size of the yttrium oxide powder is 350 mesh. Immerse the mold shell surface layer prepared in Step 3 into the slurry for 5 seconds and then remove it. Sprinkle 120 mesh yttrium oxide sand evenly on it and dry it in air at 25°C for 10 hours. Repeat the above steps twice to obtain two transition layers.

[0075] Step 5: Prepare the back layer slurry and fabricate the mold shell back layer. The composition ratio by weight is as follows: 83% zirconia powder, 16.6% yttrium sol (yttrium content is 12 wt%), 0.2% ammonium polyacrylate dispersant, 0.1% JFC surfactant, and 0.1% isooctanol defoamer. First, add yttrium sol to a container, then add 325 mesh zirconia powder and mix. Stir at 500 r / min for 30 min, and add ammonium polyacrylate, JFC and isooctanol in sequence during stirring to obtain the back layer slurry. Immerse the mold shell transition layer prepared in Step 4 into the slurry for 8 s, then remove it and evenly sprinkle 100 mesh zirconia sand on it. Dry it in air at 25°C for 12 h. Repeat the above steps 5 times to obtain 5 back layers.

[0076] Step 6: Dewaxing and calcination. Place the mold shell in a steam dewaxing kettle and dewax for 10 minutes at 220℃ and 0.7Mpa. After dewaxing, place the mold shell in a resistance calcination furnace and calcinate at 1650℃. After reaching the calcination temperature, keep it at that temperature for 3 hours and then let it cool naturally in the furnace to obtain a high-inertia precision casting mold shell.

[0077] Comparative Example 1

[0078] The preparation method is the same as in Example 1, except that a transition layer is missing. A photograph of the actual product is shown below. Figure 4 As shown, the two-layer structured mold shell prepared in this comparative example is prone to generating a large number of cracks after heat treatment, and is not suitable for the directional solidification of TiAl alloy.

[0079] The above description of the embodiments is only for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

[0080] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A mold shell for directional solidification of a high-inert TiAl alloy, comprising a face layer, a transition layer and a back layer arranged sequentially; the face layer is filled with a yttrium oxide sand layer, the transition layer is filled with a yttrium oxide sand layer, and the back layer is filled with a zirconium oxide sand layer; The surface layer is prepared from yttrium oxide powder, yttrium sol, dispersant, surfactant and defoamer; The transition layer is prepared from yttrium oxide powder, yttrium sol, dispersant, surfactant and defoamer; The backing layer is prepared from zirconium oxide powder, yttrium sol, dispersant, surfactant and defoamer; The yttrium oxide powder in the surface layer has a particle size of 400-500 mesh, the yttrium oxide powder in the transition layer has a particle size of 325-395 mesh, and the zirconium oxide powder in the back layer has a particle size of 325-500 mesh. The dispersant is selected from polyacrylamide; In the surface layer, the mass ratio of yttrium oxide powder to yttrium sol is (2-2.5):1, the yttrium oxide content in the yttrium sol is 10-15 wt%, the dispersant content is 0.1-0.5 wt%, the surfactant content is 0.1-0.5 wt%, and the defoamer content is 0.1-0.5 wt%. In the transition layer, the mass ratio of yttrium oxide powder to yttrium sol is (2-2.5):1, the yttrium oxide content in the yttrium sol is 10-15 wt%, the dispersant content is 0.1-0.5 wt%, the surfactant content is 0.1-0.5 wt%, and the defoamer content is 0.1-0.5 wt%. In the backing layer, the mass ratio of the zirconium oxide powder to the yttrium sol is (4.2-5):1, the yttrium oxide content in the yttrium sol is 10-15 wt%, the dispersant content is 0.1-0.5 wt%, the surfactant content is 0.1-0.5 wt%, and the defoamer content is 0.1-0.5 wt%.

2. The mold shell according to claim 1, characterized in that, The thickness of the mold shell is 8 to 10 layers, and the thickness is 2 to 5 mm.

3. The mold shell according to claim 1, characterized in that, The surface layer has 2 to 3 layers, the transition layer has 2 layers, and the back layer has 4 to 5 layers.

4. The method for preparing the mold shell according to claim 1, comprising the following steps: A) The design of the assembly method and the preparation of the wax model are carried out according to the shape and number of parts, and the layout of the parts and the shape of the central connecting rod are designed. B) Mix yttrium oxide powder, yttrium sol, dispersant, surfactant and defoamer to obtain a slurry. Immerse the wax mold in the slurry, remove it, sprinkle yttrium oxide sand on it, and dry it to obtain a surface layer. C) Mix yttrium oxide powder, yttrium sol, dispersant, surfactant and defoamer to obtain a slurry. Immerse the surface layer in the slurry, remove it, sprinkle yttrium oxide sand on it, and dry it to obtain a transition layer. D) Mix zirconium oxide powder, yttrium sol, dispersant, surfactant and defoamer to obtain a slurry. Immerse the transition layer in the slurry, remove it, sprinkle zirconium oxide sand on it, and dry it to obtain the back layer. E) After dewaxing the mold shell obtained in step D), bake it to obtain the mold shell.

5. The preparation method according to claim 4, characterized in that, Step B) Repeat 2-3 times, Step C) Repeat 2 times, Step D) Repeat 4-5 times.

6. The preparation method according to claim 4, characterized in that, The dewaxing method is steam dewaxing at a temperature of 150–250°C, a pressure of 0.5–0.8 MPa, and a time of 5–10 min; the calcination temperature is 1500–2000°C, and the time is 2–5 h.

Images