Preparation method of ceramic core of oversized blade of heavy-duty gas turbine

Abstract

The invention relates to the field of precision casting, and particularly discloses a preparation method of a ceramic core of an oversized blade of a heavy-duty gas turbine, which comprises the following steps: preparing a ceramic core material which comprises the following raw materials in percentage by weight: 55-60% of a matrix, 15-20% of a mineralizer, 3-5% of a pore forming agent, 5-10% of a modifier and 15-18% of a plasticizer, and uniformly mixing; performing injection molding; carrying out box filling, carrying out water bath secondary shape correction before box filling, hanging the ceramic core on a box filling tool, uniformly scattering a landfill material, and carrying out vibration tamping; sintering: charging the saggar filled with the ceramic core into a furnace and sintering in sections; strengthening the ceramic core, and soaking the sintered ceramic core in hexyl silicate hydrolysate and an epoxy resin diluted solution; depoling: loading the blade mold shell with the mold core into a depoling kettle body, and then filling alkali liquor into the depoling kettle body; according to the method, batch production of the ceramic core of the oversized blade of the heavy-duty gas turbine is achieved, it is guaranteed that the size and performance meet the requirements, the overall percent of pass reaches 90% or above, core breakage and core leakage are avoided in the casting process of a casting, and the wall thickness size meets the requirements of a design drawing.

Description

Technical Field

[0001] This invention relates to the field of precision casting, specifically a method for preparing ceramic cores for ultra-large blades of heavy-duty gas turbines. Background Technology

[0002] Heavy-duty gas turbines are crucial equipment in the modern energy sector. The performance of their blades directly impacts the efficiency and reliability of the turbine. Blades, located in critical areas, are mostly hollow to withstand higher temperatures and harsher operating environments. Ceramic cores play a key role in forming the complex internal cavity structure during blade manufacturing, and are an indispensable step in creating the complex cavities of hollow blades. Furthermore, ultra-large blades, due to their large size and complex structure, place extremely high demands on the preparation of ceramic cores. The performance of the ceramic core is the core content of quality evaluation and the basis for selection, directly affecting the forming accuracy and quality of the blade. However, existing ceramic core preparation methods have many problems, such as insufficient core strength, low precision, and unstable sintering performance, making it difficult to meet the manufacturing requirements of ultra-large blades for heavy-duty gas turbines. During the use of ceramic cores, the temperature environment undergoes numerous rapid changes. Therefore, thermal expansion performance is one of the most crucial fundamental properties of ceramic cores. During the firing and preheating of the mold shell and the pouring and solidification of the molten metal, significant temperature variations frequently occur. As the core, mold shell, and molten metal expand or contract, resulting in changes in length or volume, the thermal stress generated by their mutual restraint can lead to deformation or damage of the core and mold shell, thus affecting the quality of the casting. This often results in defects such as missing cores, off-center cores, broken cores, and arcing during pouring, leading to unqualified and scrapped castings. Therefore, good thermal expansion matching is essential between the ceramic core, the mold shell, and the molten metal.

[0003] Currently, the mainstream approach in the industry is to use SiO2-Al2O3-based glass ceramics or fused silica as the matrix, and prepare the cores through injection molding and sintering processes. To improve dimensional stability, some processes add low-expansion mineral phases such as zirconium oxide and yttrium oxide to the formulation. However, problems such as dimensional deformation, cracking, low strength, and excessive thermal expansion coefficient still occur during the actual production of ceramic cores. Furthermore, numerous problems still arise during casting and application, such as core leakage, core breakage, and core misalignment. These issues are particularly prominent in ceramic cores used for ultra-large heavy-duty gas turbine blades. a) Ultra-large ceramic cores (length > 1000 mm, wall thickness only 1.5 mm at the thinnest point) warp after sintering due to anisotropic thermal expansion, exceeding 0.5 mm / 100 mm, which cannot meet the tolerance requirement of ±0.3 mm / 100 mm for heavy-duty gas turbines. b) Existing low-expansion formulations undergo a crystal phase transformation above 1200℃, resulting in a sudden increase in expansion (>2×10⁻³ dL / L0), causing the ceramic core to crack or the blade wall thickness to exceed tolerance during casting; c) Traditional ceramic core sintering process has a long cycle, high energy consumption, and incomplete elimination of residual stress in ultra-large ceramic cores, making it impossible to achieve consistency in sintering degree and sintering performance of each cross section. d) The lack of online closed-loop control methods for the thermal expansion coefficient of ceramic cores leads to poor batch stability and a pass rate of <60%. There are many performance indicators for ceramic cores, such as flexural strength, thermal shock resistance, thermal expansion compatibility, chemical compatibility, air permeability, collapsibility, and solubility. Currently, there are no clearly defined performance parameters for each indicator in the industry. Therefore, it is difficult for users to judge the quality of ceramic cores based on their performance alone.

[0004] In response to the above problems, based on numerous experiments and practical experience, this invention proposes a method for preparing ceramic cores for ultra-large blades of heavy-duty gas turbines, in order to solve the aforementioned problems. Summary of the Invention

[0005] To address the existing problems, this invention provides a method for preparing ceramic cores for ultra-large blades of heavy-duty gas turbines, which can effectively solve the problems mentioned in the background art.

[0006] To solve the above problems, the present invention adopts the following technical solution: A method for preparing ceramic cores for ultra-large blades of heavy-duty gas turbines includes the following steps: S1. Preparation of ceramic core materials: The ceramic core raw materials include, by weight ratio: 55-60% matrix, 15-20% mineralizer, 3-5% pore-forming agent and 5-10% modifier. After mixing all the ceramic core raw materials evenly, add plasticizer at a weight ratio of 15-18% and mix well. S2. Injection molding: The ceramic core material is injected into the ceramic core mold under a pressure of 0.5MPa. After cooling and solidification, it is placed in the ceramic core straightening jig for dimensional shaping. S3. The ceramic core is packed and sintered in the sagger. Before packing, it is shaped twice in a water bath at a temperature of 40-45℃ for 4-6 minutes. Then it is placed in the shaping jig for a second shaping for 15-20 minutes. After the second shaping, the ceramic core is suspended on the packing fixture and the filling material is evenly sprinkled on it. The sagger with the ceramic core packed and the filling material is placed on the vibrating table and vibrated to compact it. S4. Sintering: The sagger containing the ceramic core is placed in the furnace for segmented sintering. S5. Strengthen the ceramic core by immersing the sintered ceramic core in a ethyl silicate hydrolysate solution to improve the high-temperature strength of the ceramic core. After high-temperature strengthening, the core is then immersed in a diluted epoxy resin solution for 1-2 hours to improve the room-temperature strength of the ceramic core. S6. Core removal: After the blade shell with the core is loaded into the core removal kettle, alkaline solution is filled in to corrode and dissolve the solid materials or minerals inside the casting.

[0007] As a further embodiment of the present invention: in step S1, the matrix is ​​fused silica powder of various particle sizes, the mineralizer is zirconium oxide powder, the pore-forming agent is porous silica micro powder, and the modifier is nano Al2O3-Y2O3 composite powder.

[0008] As a further embodiment of the present invention: in step S1, a V-type mixer is used to mix the ceramic core raw materials for 4-6 hours, and then the mixture is dried at a temperature of 120-160°C for 2-4 hours, and then a plasticizer with a weight ratio of 15-18% is added.

[0009] As a further embodiment of the present invention: in step S3, the landfill material is a mixed filler and adopts a three-layer crucible filler with upper, middle and lower layers. The upper and lower layers of filler are mixed landfill material with 45% industrial alumina powder with a particle size of 180-250 mesh and 55% kaolin powder with a particle size of 320-400 mesh. The middle layer of filler is mixed landfill material with 35% industrial alumina powder with a particle size of 180-250 mesh and 65% kaolin powder with a particle size of 320-400 mesh.

[0010] As a further embodiment of the present invention: the segmented sintering in step S4 is as follows: the wax removal stage is when the temperature is below 600℃, during which the plasticizer begins to melt, diffuse, decompose and volatilize, and diffuses smoothly through the surface of the ceramic core into the filler until the plasticizer inside the ceramic core is completely removed; the transition stage is when the temperature is between 600-900℃, and the temperature is held at 900℃ for 2 hours to ensure that the temperature of the ceramic core is uniform when it enters the high-temperature sintering stage; the final firing stage is when the temperature is between 1150-1190℃, and the temperature is held for 15-20 hours. After cooling with the furnace, the ceramic core is taken out.

[0011] As a further embodiment of the present invention: the core removal in step S6 adopts a three-stage cyclic operation, namely the pressurization stage, the venting stage, and the pressure holding stage. In the pressurization stage, the alkaline solution is statically injected into the core and the shell. In the venting stage, the alkaline solution boils violently and agitates, impacting the ceramic core. In the pressure holding stage, the alkaline solution continues to boil, thereby carrying away the core fragments and residues in the shell.

[0012] Compared with the prior art, the beneficial effects of the present invention are as follows: The present invention realizes the mass production of ceramic cores for ultra-large-sized blades of heavy-duty gas turbines and meets the dimensional accuracy requirement of ≤0.3 mm for the surface contour of ultra-large-sized ceramic cores with a length of 700-1200 mm after sintering. By controlling the proportion of each component of the mixed filler material, the consistency of the overall performance of the ultra-large-sized ceramic core is achieved, and the thermal expansion performance of each part in the length direction is controlled within ±0.2×10⁻³dL / L0, ensuring that the dimensions and performance meet the requirements. The overall pass rate is over 90%. There are no core breaks or core leaks during the casting process, and the wall thickness meets the design drawing requirements. The addition of pore-forming agent in the ceramic core material significantly accelerates the erosion of ultra-large-sized ceramic cores by alkaline solution, which shortens the core removal cycle and reduces the harshness of the process. It is a necessary means to achieve both "easy removal" and "high strength" for ceramic cores of heavy-duty gas turbine blades. Detailed Implementation

[0013] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0014] This embodiment provides a method for preparing ceramic cores for ultra-large-sized blades of heavy-duty gas turbines, which includes the following steps: S1. Preparation of ceramic core materials: The ceramic core raw materials include, by weight ratio: 55-60% matrix, 15-20% mineralizer, 3-5% pore-forming agent and 5-10% modifier. After mixing all the ceramic core raw materials evenly, add plasticizer at a weight ratio of 15-18% and mix evenly. The matrix is ​​fused silica powder of various particle sizes, the mineralizer is zirconium oxide powder, the pore-forming agent is porous silica micro powder and the modifier is nano Al2O3-Y2O3 composite powder. Use a V-type mixer to mix the ceramic core raw materials for 4-6 hours and dry them at a temperature of 120-160℃ for 2-4 hours. Then add the plasticizer. S2. Injection molding: The ceramic core material is injected into the ceramic core mold under a pressure of 0.5MPa. After cooling and solidification, it is placed in the ceramic core straightening jig for dimensional shaping. S3. Sagger Loading: The ceramic core is loaded into saggers for sintering. Before loading, a secondary water bath is performed at 40-45℃ for 4-6 minutes. Then, it is placed in a shaping fixture for a second shaping time of 15-20 minutes. This second shaping releases residual stress in the wet ceramic core, creating conditions for geometric recovery and thermal stress elimination, reducing the probability of dimensional deformation during sintering. After the second shaping, the ceramic core is suspended on the sagger loading fixture to avoid stress. Filler material is evenly spread. The sagger filled with the ceramic core and sieved filler is placed on a vibrating table for compaction. The vibration frequency is 95-100 Hz, the amplitude is 0.2-0.5 mm, and the vibration time is 30-50 seconds. The sagger uses a three-layer nested structure, with the upper, middle, and lower layers being equally divided and separable. The filler material is a mixed filler. Due to the large size of the sagger, the overall height is >1400 mm. To ensure consistent sintering performance across all parts of the ceramic core, the landfill material needs to be matched and adjusted. The upper and lower layers of the sagger use a mixed landfill material consisting of 45% industrial alumina powder with a particle size of 180-250 mesh and 55% kaolin powder with a particle size of 320-400 mesh. The middle layer of the sagger uses a mixed landfill material consisting of 35% industrial alumina powder with a particle size of 180-250 mesh and 65% kaolin powder with a particle size of 320-400 mesh. S4. Sintering: The sagger containing the ceramic core is placed in the furnace for segmented sintering. The segmented sintering process is as follows: The wax removal stage is below 600℃, during which the plasticizer begins to melt, diffuse, decompose, and volatilize, smoothly diffusing through the surface of the ceramic core into the filler until the plasticizer inside the ceramic core is completely removed. The heating rate in this stage should not be too fast, otherwise defects such as peeling, blistering, and delamination may occur. The transition stage is between 600-900℃. Since the heating rate is controlled during the wax removal stage, deformation and breakage of the ceramic core are prevented. The core is held at 900℃ for 2 hours to ensure a uniform temperature when it enters the high-temperature sintering stage. The final firing stage is between 1150-1190℃, held for 15-20 hours. After cooling in the furnace, the ceramic core is removed. The dimensions and performance meet the requirements, and the overall pass rate is over 90%. S5. Strengthening the ceramic core: Since the ceramic core needs to withstand high-temperature firing and the impact of high-temperature molten metal, it should have sufficiently high high-temperature strength, which is especially important for ultra-large, thin, and complex cores. The strength of the core after firing is 10-15 MPa. Immersing the sintered ceramic core in ethyl silicate hydrolysate solution for 1-2 hours can increase the high-temperature strength of the core to 25-30 MPa, which can increase the high-temperature strength by about 30%. At the same time, the ceramic core needs to withstand the impact force during wax mold injection. To prevent the ceramic core from cracking, especially for ultra-large, thin, and complex ceramic cores, it is necessary to further improve the room temperature strength of the core. The core after high-temperature strengthening is immersed in epoxy resin dilution solution for 1-2 hours. The room temperature strength of the strengthened ceramic core can be increased by 3-4 times. Due to the long length, thin walls, and complex structure of the ceramic core, it is very easy to deform. Therefore, conformal placement fixtures are required during transportation and storage. The thermal expansion coefficient of ceramic cores is controlled by adjusting the types and particle size distribution of the ceramic core powder, sintering parameters, filler component ratio, and holding time. Traditional fused silica-based ceramic cores typically exhibit a cumulative expansion of 2.0~5.5×10⁻³ dL / L₀ in the temperature range of 20℃ to 1500℃. This invention firstly uses the addition of zirconium oxide powder as a mineralizer and nano-Al₂O₃-Y₂O₃ composite powder as a modifier to create a negative expansion constraint on the overall material's thermal expansion performance, thereby compressing the expansion value to a "sub-zero expansion" window, reducing it to 1.2×10⁻³ dL / L₀, equivalent to reducing the expansion coefficient α to 0.9–1.0×10⁻³ dL / L₀. 6 / ℃, entering the "near-zero expansion" range. Secondly, since the overall heating and heat storage of ultra-large ceramic cores within the sintering furnace and sagger cannot be kept consistent through parameter settings, this invention achieves the consistency of the overall performance of ultra-large ceramic cores by controlling the composition and proportion of the filler material in different zones, keeping the thermal expansion performance of each part along the length within ±0.2×10⁻³dL / L0. Practical use has demonstrated that the online expansion coefficient α is controlled within the range of (1.2±0.2)×10⁻³ dL / L0, with no core breakage or leakage during casting, and the wall thickness meets the design drawing requirements.

[0015] S6. Core Removal: After loading the blade shell with the core into the core removal kettle, alkaline solution is filled in to corrode and dissolve the solid materials or minerals inside the casting. Core removal is carried out in three stages of cyclic operation: pressurization stage, venting stage, and pressure holding stage. In the pressurization stage, the alkaline solution is statically pressed into the core and shell. In the venting stage, the alkaline solution boils violently and agitates the ceramic core. In the pressure holding stage, the alkaline solution continues to boil, thereby carrying away the core fragments and residues inside the shell. The casting can be cooled by rinsing or immersion.

[0016] Furthermore, the addition of the pore-forming agent, through a dual "physical-chemical" acceleration mechanism, forms interconnected microporous channels, increasing the specific surface area; it shortens the alkali penetration path and reduces diffusion resistance. The removal of dense ceramic cores mainly relies on slow diffusion from the surface to the interior; a 1000 mm long casting requires 40–48 hours to completely remove the core. With the introduction of interconnected pores, the alkali solution rapidly penetrates along the channels, reducing the effective diffusion distance from the 30 mm level to the 5 mm level, shortening the removal time to 10–15 hours. It also reduces the critical wall thickness and avoids the "closed-end effect," where the thickest wall area of ​​hollow blades often remains due to the inability of the alkali solution to reach it. The interconnected pores formed by the pore-forming agent transform closed ends into "semi-open" ends, allowing areas with wall thicknesses >50 mm to completely dissolve within 8 hours, increasing the pass rate from 65% to 95%. The pore-forming agent significantly accelerates the erosion of ultra-large ceramic cores by alkaline solution through a three-step process of "pore opening-connection-shortened diffusion". This shortens the core removal cycle and reduces the severity of the process, making it a necessary means to achieve both "easy removal" and "high strength" for heavy-duty gas turbine blade ceramic cores.

[0017] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0018] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for producing a ceramic core for a heavy-duty gas turbine oversize blade, characterized in that, The method comprises the following steps: S1, ceramic core material preparation, the ceramic core raw material comprises, by weight ratio, 55-60% base material, 15-20% mineralizer, 3-5% pore former and 5-10% modifier, after mixing all the ceramic core raw materials, 15-18% plasticizer by weight ratio is added and mixed uniformly; S2, injection molding, the ceramic core material is injected into the ceramic core mold under the pressure of 0.5 MPa, and after cooling and forming, it is placed into a ceramic core correction jig for size setting; S3, boxing, the ceramic core is filled and sintered in a box, before boxing, it is subjected to secondary correction in water bath, the water bath temperature is 40-45℃, the water bath time is 4-6 min, then it is placed into a correction jig for secondary correction, the correction time is 15-20 min, after secondary correction, the ceramic core is hung on a boxing tool, the filling material is uniformly scattered, and the box with the ceramic core and the filling material is put on a vibrating table for vibration and filling; S4, sintering, the box with the ceramic core is put into a furnace for segmented sintering; S5, strengthening the ceramic core, after sintering, the ceramic core is soaked in a silicic acid hydrolysis solution to improve the high-temperature strength of the ceramic core, and then the ceramic core after high-temperature strengthening is soaked in an epoxy resin dilute solution for 1-2 hours to improve the room-temperature strength of the ceramic core; S6, core removal, the vane shell with the core is put into a core removal kettle, and then alkali solution is filled in to corrode and dissolve the solid material or mineral in the casting.

2. A method of making a ceramic core for a large size blade for a heavy duty gas turbine engine as defined in claim 1, wherein In the step S1, the base material is fused quartz powder with multiple particle sizes, the mineralizer is zirconia powder, the pore former is porous silica powder, and the modifier is nano Al2O3-Y2O3 composite powder.

3. A method of making a ceramic core for a large size blade for a heavy duty gas turbine engine as defined in claim 1, wherein In the step S1, the ceramic core raw materials are mixed by using a V-shaped mixer, the mixing time is 4-6 hours, and then the mixed materials are dried at a temperature of 120-160℃ for 2-4 hours, and then 15-18% plasticizer by weight ratio is added.

4. A method of making a ceramic core for a large size blade for a heavy duty gas turbine engine as defined in claim 1, wherein In the step S3, the filling material is mixed filling material, and the filling material is filled into the box in three layers, that is, the upper layer and the lower layer of the filling material are mixed filling material with industrial alumina powder with a particle size of 180-250 mesh and kaolin powder with a particle size of 320-400 mesh, and the middle layer of the filling material is mixed filling material with industrial alumina powder with a particle size of 180-250 mesh and kaolin powder with a particle size of 320-400 mesh.

5. A method of making a ceramic core for a large size blade for a heavy duty gas turbine engine as defined in claim 1, wherein In the step S4, the segmented sintering is as follows: the temperature below 600℃ is a wax removal stage, the plasticizer starts to melt, diffuse, decompose and volatilize, and smoothly diffuses from the surface of the ceramic core to the filling material until the plasticizer in the ceramic core is completely removed; the temperature of 600-900℃ is a transition stage, and the temperature is kept at 900℃ for 2 hours to make the temperature of the ceramic core uniform and consistent when entering the high-temperature sintering stage; the temperature of 1150-1190℃ is a final sintering stage, and the temperature is kept at 1150-1190℃ for 15-20 hours, and then the ceramic core is taken out after cooling in the furnace.

6. A method of making a ceramic core for a large size blade for a heavy duty gas turbine engine as defined in claim 1, wherein In the step S6, the core removal adopts three-stage circulation work, and the three stages are a pressurization stage, an exhaust stage and a pressure maintaining stage, in which, in the pressurization stage, the alkali solution is pressurized into the core and the shell, in the exhaust stage, the alkali solution is boiled and stirred to impact the ceramic core, and in the pressure maintaining stage, the alkali solution continues to boil to take away the core fragments and residues in the shell.