A method for producing a low thermal stress mold core system for directional solidification

Abstract

The application belongs to the field of high-temperature alloy directional solidification, and particularly relates to a preparation method of a low-thermal-stress mold shell core system for directional solidification, characterized by comprising the forming and sintering of a ceramic core, the making of a wax mold, the forming and dewaxing of a mold shell, and the sintering of the mold shell, wherein the first to second layers of the mold shell are surface coatings, and the third to seventh layers are back coatings. The application uses magnesium oxide ceramic with a high expansion coefficient and silicon dioxide with a low expansion coefficient, so that the dynamic thermal expansion coefficients of the ceramic core and the aluminum oxide ceramic mold shell core are consistent or similar, and the system is always in a low-stress state. In the heating process from 100 DEG C to the directional solidification temperature of 1520 DEG C, the thermal expansion coefficient of the ceramic core is obviously higher than that of the silicon dioxide-based core material, and the thermal matching of the aluminum oxide ceramic mold shell is good. The ceramic core in the blade can be removed smoothly by using lye, and the problem that the single-crystal high-temperature alloy turbine hollow blade is not easy to be demolded after pouring is solved.

Description

Invention Field

[0001] This invention belongs to the field of directional solidification of high-temperature alloys, and specifically relates to a method for preparing a low thermal stress mold core system for directional solidification. Background of the Invention

[0002] High-temperature alloys are widely used in the manufacture of turbine blades for aero-engines and ground-based gas turbines due to their excellent mechanical properties, fatigue resistance, oxidation resistance, and resistance to thermal fatigue. Directionally solidified columnar and single-crystal high-temperature alloys, having eliminated transverse grain boundaries or all grain boundaries, exhibit even higher mechanical properties, superior fatigue resistance, and oxidation resistance at high temperatures, and are therefore widely used in the manufacture of advanced aero-engine and ground-based gas turbine blades.

[0003] With the increasing efficiency of aero-engines and ground-based gas turbines, their inlet temperatures are constantly rising. Turbine blade materials alone can no longer withstand such high temperatures, necessitating gas cooling technology. This involves incorporating various cooling channels within the blades, relying on gas cooling within these channels to reduce the blade surface temperature. The rapid development of cooling technology has given blades greater temperature resistance, but it has also made the internal cooling structure of the blades more complex.

[0004] Hollow turbine blades made of directional columnar and single-crystal superalloys are typically fabricated using directional solidification. However, this process first requires the preparation of a complex core (to form the cooling channels within the blade cavity) and a mold shell containing the core, followed by directional solidification. The specific process is as follows: 1) Injection molding and sintering are used to prepare the core (to form the cooling structure within the blade cavity); 2) Injection molding is used to prepare a wax model containing the core; 3) Precision casting is used to prepare the mold shell containing the core; 4) The mold shell containing the core is dewaxed and fired; 5) The alloy is poured and directionally solidified at high temperature; 6) After solidification, the ceramic core is removed using a chemical method, finally yielding the hollow blade structure.

[0005] Generally, the mold shell and core system needs to undergo two processes: a process from room temperature to the firing temperature (850-1050℃) and then cooling back to room temperature (mold shell firing), and a process from room temperature to the directional solidification temperature (1480-1520℃) (directional solidification). Ideally, the mold shell and core should have similar dynamic thermal expansion coefficients during these two processes to ensure no or only low thermal stress between them, ultimately guaranteeing the dimensional accuracy of the blade profile and internal cavity, and the blade's yield rate. Simultaneously, the mold shell and core materials need to operate at the directional solidification temperature (1480-1520℃) for 1-2 hours. Therefore, the ceramic mold shell and core must also possess excellent creep resistance to maintain the dimensional accuracy of the blade and internal cavity. Furthermore, in the mold shell / core system, the ceramic core must also have good chemical removeability to ultimately ensure the blade obtains a complete internal cavity cooling structure.

[0006] In actual production, to ensure creep resistance during directional solidification, the mold shell material is usually made of alumina. However, alumina has a high coefficient of thermal expansion; for example, from 100℃ to the directional solidification temperature of 1520℃, the coefficient of thermal expansion changes from 3.0 to 4.75 × 10⁻⁶. -6 / ℃, during which the maximum value was 8.5×10 -6 / ℃, the thermal expansion curve of the mold shell is shown in the figure. Figure 1 To ensure chemical removal, commercially available core materials typically use silica ceramic cores, which have a low coefficient of thermal expansion, ranging from 3.0 × 10⁻⁶ at 100°C to 1520°C at the directional solidification temperature. -6 / ℃ to -2.56×10 -6 / ℃, with a maximum value of 5.0×10 during the period. -6 / ℃, see Figure 2 It is evident that the dynamic thermal expansion coefficients of these two materials vary significantly in range and value, which can easily lead to thermal stress between the mold shell and the core, causing deformation, bending, or breakage of the core, ultimately resulting in non-compliance or low pass rate of the blade surface and inner cavity dimensions.

[0007] In existing processes, although the use of free-end core technology can ensure that the thermal expansion between the core and the mold shell does not interfere or that the stress between them is very small, it is only suitable for unidirectional adjustment between columnar or rod-shaped ceramic cores and mold shells with simple structures. For complex ceramic cores, it is still impossible to meet the requirements for simultaneous spatial thermal expansion interference and adjustment between the core and the mold shell.

[0008] Patent CN104086161A discloses a method for preparing a silicon-based ceramic core with an adjustable coefficient of thermal expansion. It uses alumina to improve the coefficient of thermal expansion of the silicon dioxide core, but the coefficient of thermal expansion can only be controlled within the range of 0-3.96×10⁻⁶. -6 Within the range of / ℃, it is obvious that the coefficients of thermal expansion of the ceramic core and the alumina mold shell still differ significantly, failing to achieve a low-stress state between the mold shell and the core, thus failing to meet the actual production requirements of directional solidification hollow blades. Furthermore, alumina cores or cores with high alumina content are difficult or impossible to remove; even if removal is possible, it requires a long time and easily results in residual cores, significantly increasing the scrap rate. Summary of the Invention

[0009] The purpose of this invention is to provide a method for preparing a low thermal stress mold shell and core system for directional solidification, which overcomes the shortcomings of the prior art, prepares a ceramic core with a dynamic thermal expansion coefficient similar to that of an alumina ceramic mold shell, ensures that the dynamic thermal expansion of the mold shell and the core is consistent, keeps the mold shell / core system in a low stress state, improves the dimensional accuracy and pass rate of directional solidification blades, and improves the core removal effect.

[0010] To achieve the above objectives, the present invention employs the following solution:

[0011] A method for preparing a low thermal stress mold core system for directional solidification, characterized by comprising the molding and sintering of a ceramic core, wax pattern making, molding and dewaxing of the mold shell, and sintering of the mold shell, wherein the first and second layers of the mold shell are surface coatings, and the third to seventh layers are back coatings. The specific operation steps are as follows:

[0012] 1) Molding and sintering of ceramic cores: The ceramic core blanks are prepared by injection molding. The components are composed of 25-75 parts by weight of magnesium oxide powder, 25-75 parts by weight of quartz glass powder, and 15-18 parts by weight of plasticizer. The plasticizer is a mixture of paraffin wax and polyethylene, with a mass ratio of paraffin wax to polyethylene between 96:4 and 99:1. The molding pressure is 30-80 atmospheres, the injection time is 10-60 seconds, and the holding time is 10-120 seconds. The sintering process of the ceramic cores is as follows: heating rate is 0.5-5℃ / min, heating to the sintering temperature, the sintering temperature is 1200-1300℃, the holding time is 2-10 hours, and cooling is carried out with the furnace.

[0013] 2) Wax model making: After the ceramic core is prepared, a wax model containing the ceramic core is prepared.

[0014] 3) Molding and dewaxing of the mold shell: The mold shell is prepared using a coating-sanding process. The specific steps are as follows: The surface coating of the mold shell is prepared by mixing high-purity corundum powder and silica sol, with a powder-to-liquid weight ratio of 3.3-3.5:1. The surface coating of the mold shell is then sanded with 60-80 mesh sand. The back layer slurry of the mold shell is prepared by mixing high-purity corundum powder and silica sol, with a powder-to-liquid weight ratio of 3.8-4.5:1. The 3rd to 7th back layer coatings are then sanded with 24-36 mesh sand. The relative humidity of the mold shell is controlled at 40-70%, the temperature is controlled at 23-35℃, and the drying time is 4-8 hours. The dewaxing temperature of the mold shell is 160-180℃, the dewaxing pressure is 0.6-0.8MPa, and the time is 10-30 seconds.

[0015] 4) Sintering of the mold shell: The mold shell is fired at a temperature of 850-1000℃ for 2-4 hours.

[0016] Compared with the prior art, the beneficial effects of the present invention are:

[0017] 1) This invention uses magnesium oxide ceramic with a high coefficient of thermal expansion and silicon dioxide with a low coefficient of thermal expansion to prepare ceramic cores. While ensuring that it has good resistance to high temperature creep and removability, it makes its dynamic thermal expansion coefficient consistent with or similar to that of alumina ceramic mold cores, so that the mold / core system is always in a low stress state. During the heating process from 100°C to the directional solidification temperature of 1520°C, the thermal expansion coefficient of the ceramic core is significantly higher than that of the silicon dioxide-based core material, and it has good thermal matching with the alumina ceramic mold.

[0018] 2) Under the conditions of directional solidification temperature of 1500-1520℃ and directional solidification pulling speed controlled at 1-6 mm / min, the ceramic core has a creep deformation of <0.5mm after holding at 1500-1520℃ for 1-2 hours. The ceramic core in the blade can be successfully removed by using alkaline solution, which ensures the smooth removal of the core after casting of single crystal high temperature alloy turbine hollow blades. Attached Figure Description

[0019] Figure 1 This is a thermal expansion curve of an alumina mold shell in the prior art;

[0020] Figure 2 The thermal expansion curve of a silica-based (25% alumina added) ceramic core in the prior art is shown.

[0021] Figure 3 This is a dynamic thermal expansion curve diagram of Embodiment 1 of the present invention;

[0022] Figure 4 This is a dynamic thermal expansion curve diagram of Embodiment 2 of the present invention;

[0023] Figure 5 This is a dynamic thermal expansion curve diagram of Embodiment 3 of the present invention;

[0024] Figure 6 This is a dynamic thermal expansion curve diagram of Embodiment 4 of the present invention. Detailed Implementation

[0025] The technical solutions of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. In order to more clearly illustrate the technical solutions in the specific embodiments of the present invention or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0026] In the following examples, the magnesium oxide particle size is 150-800 mesh, of which 150-200 mesh accounts for 70-80% by weight and 600-800 mesh accounts for 20-30% by weight; the quartz glass particle size is 800-1000 mesh. The plasticizer is industrial paraffin wax or a mixture of paraffin wax and polyethylene. In the mold shell preparation, the sand material is corundum sand with a mesh size of 24-80 mesh, and the Al2O3 content in the corundum sand is >99.9%. In the examples, the dynamic thermal expansion curves of the mold shell and core are measured using a high-temperature thermal expansion tester under the following conditions: heating rate 5℃ / min, temperature range from room temperature to 1520℃. The high-purity corundum powder has an Al2O3 content >99.9% and a particle size of 325 mesh. The average particle size of the silica sol is 8-14 nm, and the SiO2 content is 30-35 wt%.

[0027] Example 1

[0028] The present invention discloses a method for preparing a low thermal stress mold core system for directional solidification, comprising the molding and sintering of a ceramic core, wax pattern making, molding and dewaxing of the mold shell, and sintering of the mold shell. The first and second layers of the mold shell are surface coatings, and the third to seventh layers are back coatings. The specific operation steps are as follows:

[0029] 1) The forming and sintering of the ceramic core: The ceramic core blank is prepared by injection molding. Its components are composed of 70 parts by weight of magnesium oxide powder, 30 parts by weight of quartz glass powder, and 18 parts by weight of plasticizer. The plasticizer is a mixture of paraffin wax and polyethylene with a mass ratio of 96:4. The molding pressure is 80 atmospheres, the injection time is 30 seconds, and the holding time is 60 seconds. The sintering process of the ceramic core is as follows: heating rate 0.5℃ / min, heating to 1200℃, holding time 10 hours, and cooling with the furnace.

[0030] 2) After the molding and sintering of the mold shell surface layer and the preparation of the core product are completed, a wax model containing the ceramic core is prepared;

[0031] 3) Molding and dewaxing of the mold shell: The mold shell is prepared using a coating-sanding process. Specifically: the surface coating is prepared by mixing high-purity corundum powder and silica sol, with a powder-to-liquid ratio of 3.4:1 by mass; the back layer slurry is prepared by mixing high-purity corundum powder and silica sol, with a powder-to-liquid ratio of 4.0:1 by mass; the relative humidity of the mold shell is controlled at 55%, the temperature is controlled at 26℃, and the drying time is 6 hours; the mold shell is dewaxed at a temperature of 160℃, a dewaxing pressure of 0.6MPa, and a time of 15 seconds.

[0032] 4) Firing of the mold shell at 850℃ for 4 hours.

[0033] Example 1 uses a high-temperature thermal expansion tester to measure the dynamic thermal expansion curve of the core. The value of the ceramic core can be controlled within 0-10.8×10 from 100℃ to the directional solidification temperature of 1520℃.-6 Within the range of / ℃, see Figure 3 ;and Figure 1 In comparison, the ceramic core and the mold shell have a better match in terms of dynamic thermal expansion coefficients. Under conditions of 1500-1520℃, the creep deformation of the ceramic core after 1 hour of heat treatment is <0.2mm. The ceramic core can be easily removed from the blade using an alkaline solution, with a blade qualification rate >90%.

[0034] Example 2

[0035] The present invention discloses a method for preparing a low thermal stress mold core system for directional solidification, comprising the molding and sintering of a ceramic core, wax pattern making, molding and dewaxing of the mold shell, and sintering of the mold shell. The first and second layers of the mold shell are surface coatings, and the third to seventh layers are back coatings. The specific operation steps are as follows:

[0036] 1) The forming and sintering of the ceramic core: The ceramic core blank is prepared by injection molding. Its components are composed of 50 parts by weight of magnesium oxide powder, 50 parts of quartz glass powder and 15 parts of plasticizer. The plasticizer is a mixture of paraffin wax and polyethylene with a mass ratio of paraffin wax to polyethylene of 99:1. The molding pressure is 30 atmospheres, the injection time is 60 seconds, and the holding time is 120 seconds. The sintering process of the ceramic core is as follows: heating rate is 2℃ / min, heating to 1240℃, holding time is 4 hours, and cooling is carried out with the furnace.

[0037] 2) After the molding and sintering of the mold shell surface layer and the preparation of the core product are completed, a wax model containing the ceramic core is prepared;

[0038] 3) Molding and dewaxing of the mold shell: The mold shell is prepared using a coating-sanding process. Specifically: the surface coating is prepared by mixing high-purity corundum powder and silica sol, with a powder-to-liquid ratio of 3.5:1 by mass; the back layer slurry is prepared by mixing high-purity corundum powder and silica sol, with a powder-to-liquid ratio of 4.1:1 by mass; the relative humidity of the mold shell is controlled at 59%, the temperature is controlled at 24℃, and the drying time is 6 hours; the mold shell is dewaxed at a temperature of 160℃, a dewaxing pressure of 0.6MPa, and a time of 15 seconds.

[0039] 4) Firing of the mold shell at 850℃ for 4 hours.

[0040] Example 2 uses a high-temperature thermal expansion tester to measure the dynamic thermal expansion curve of the core. The value of the ceramic core can be controlled within 0-7×10 from 100℃ to the directional solidification temperature of 1520℃. -6 Within the range of / ℃, see Figure 4 ;and Figure 1 In comparison, the ceramic core and the mold shell have a better match in terms of dynamic thermal expansion coefficients. Under conditions of 1500-1520℃, the creep deformation of the ceramic core after 1 hour of heat treatment is <0.4mm. The ceramic core can be easily removed from the blade using an alkaline solution, resulting in a blade qualification rate >85%.

[0041] Example 3

[0042] The present invention discloses a method for preparing a low thermal stress mold core system for directional solidification, comprising the molding and sintering of a ceramic core, wax pattern making, molding and dewaxing of the mold shell, and sintering of the mold shell. The first and second layers of the mold shell are surface coatings, and the third to seventh layers are back coatings. The specific operation steps are as follows:

[0043] 1) The forming and sintering of the ceramic core: The ceramic core blank is prepared by injection molding. Its components are composed of 30 parts by weight of magnesium oxide powder, 70 parts by weight of quartz glass powder and 16 parts by weight of plasticizer. The plasticizer is a mixture of paraffin wax and polyethylene with a mass ratio of paraffin wax to polyethylene of 97:3. The molding pressure is 70 atmospheres, the injection time is 10 seconds, and the holding time is 10 seconds. The sintering process of the ceramic core is as follows: heating rate is 5℃ / min, heating to 1250℃, holding time is 6 hours, and cooling is carried out with the furnace.

[0044] 2) After the molding and sintering of the mold shell surface layer and the preparation of the core product are completed, a wax model containing the ceramic core is prepared;

[0045] 3) Molding and dewaxing of the mold shell: The mold shell is prepared using a coating-sanding process. Specifically: the surface coating is prepared by mixing high-purity corundum powder and silica sol, with a powder-to-liquid ratio of 3.5:1 by mass; the back layer slurry is prepared by mixing high-purity corundum powder and silica sol, with a powder-to-liquid ratio of 4.5:1 by mass; the relative humidity of the mold shell is controlled at 60%, the temperature is controlled at 28℃, and the drying time is 6 hours; the mold shell is dewaxed at a temperature of 168℃, a dewaxing pressure of 0.8MPa, and a time of 10 seconds.

[0046] 4) Firing of the mold shell at 1000℃ for 2 hours.

[0047] Example 3 uses a high-temperature thermal expansion tester to measure the dynamic thermal expansion curve of the core. The value of the ceramic core can be controlled within 0-10.8×10 from 100℃ to the directional solidification temperature of 1520℃. -6 Within the range of / ℃, see Figure 5 ;and Figure 1 In comparison, the ceramic core and the mold shell have a better match in terms of dynamic thermal expansion coefficients. Under conditions of 1500-1520℃, the creep deformation of the ceramic core is <0.5mm after 1 hour of heat treatment. The ceramic core can be easily removed from the blade using an alkaline solution, resulting in a blade qualification rate >75%.

[0048] Example 4

[0049] The present invention discloses a method for preparing a low thermal stress mold core system for directional solidification, comprising the molding and sintering of a ceramic core, wax pattern making, molding and dewaxing of the mold shell, and sintering of the mold shell. The first and second layers of the mold shell are surface coatings, and the third to seventh layers are back coatings. The specific operation steps are as follows:

[0050] 1) The forming and sintering of the ceramic core: The ceramic core blank is prepared by injection molding. Its components are composed of 55 parts by weight of magnesium oxide, 45 parts by weight of quartz glass powder and 18 parts by weight of plasticizer. The plasticizer is a mixture of paraffin wax and polyethylene with a mass ratio of 96:4. The molding pressure is 80 atmospheres, the injection time is 30 seconds, and the holding time is 60 seconds. The sintering process of the ceramic core is as follows: heating rate is 0.5℃ / min, heating to 1200℃, holding time is 10 hours, and cooling is carried out with the furnace.

[0051] 2) After the molding and sintering of the mold shell surface layer and the preparation of the core product are completed, a wax model containing the ceramic core is prepared;

[0052] 3) Molding and dewaxing of the mold shell: The mold shell is prepared using a coating-sanding process. Specifically: the surface coating is prepared by mixing high-purity corundum powder and silica sol, with a powder-to-liquid ratio of 3.4:1 by mass; the back layer slurry is prepared by mixing high-purity corundum powder and silica sol, with a powder-to-liquid ratio of 4.0:1 by mass; the relative humidity of the mold shell is controlled at 55%, the temperature is controlled at 26℃, and the drying time is 6 hours; the mold shell is dewaxed at a temperature of 160℃, a dewaxing pressure of 0.6MPa, and a time of 15 seconds.

[0053] 4) Firing of the mold shell at 850℃ for 4 hours.

[0054] Example 1 uses a high-temperature thermal expansion tester to measure the dynamic thermal expansion curve of the core. The value of the ceramic core can be controlled within 0-14.3×10 from 100℃ to the directional solidification temperature of 1520℃. -6 Within the range of / ℃, see Figure 6 ;and Figure 1 In comparison, the dynamic thermal expansion coefficients of the ceramic core and the mold shell are better matched. Under conditions of 1500-1520℃, the creep deformation of the ceramic core after 1 hour of holding is <0.2mm, making it difficult to remove the blades using an alkaline solution.

[0055] Comparative Example 1:

[0056] The ceramic core, commonly used in the market, is composed of alumina powder and quartz glass powder, with a weight ratio of alumina to quartz glass powder of 10:90. The alumina powder is 600 mesh, and the quartz glass powder is 1000 mesh. The ceramic core slurry consists of alumina powder, quartz glass powder, and a plasticizer, with the plasticizer accounting for 18% by weight. The plasticizer is a mixture of paraffin wax and polyethylene, with a mass ratio of paraffin wax to polyethylene of 96:4. The ceramic core is prepared using injection molding, with a molding pressure of 80 atmospheres, an injection time of 30 seconds, and a holding time of 60 seconds. The sintering process for the ceramic core blank is as follows: heating rate of 0.5℃ / min, heating to 1240℃, holding time of 8 hours, and cooling in the furnace. After the core is finished, a wax model containing the ceramic core is prepared, followed by the preparation of the mold shell. The mold shell is prepared using a coating-sand application process. Specifically: the surface coating is prepared by mixing high-purity corundum powder and silica sol. The corundum powder has an Al2O3 content > 99.9%, a powder mesh size of 325 mesh, and the silica sol has an average particle size of 8-14 nm and a SiO2 content of 30-35 wt%. The powder-to-liquid ratio is 3.4:1 by mass. The back layer slurry is also prepared by mixing high-purity corundum powder and silica sol. The corundum powder has an Al2O3 content > 99.9%, a powder mesh size of 325 mesh, and the silica sol has an average particle size of 8-14 nm. -14nm, SiO2 content 30-35wt%, powder-to-liquid ratio 4.5:1; the sand material is corundum sand, mesh size 24-80, Al2O3 content >99.9%; the first and second layers of the mold shell use surface coating, sand with 80 mesh; the third to seventh layers use back coating, sand with 24 mesh; the relative humidity of the mold shell is controlled at 50%, the temperature is controlled at 25℃, and the drying time is 8 hours; the dewaxing temperature of the mold shell is 160℃, the dewaxing pressure is 0.6MPa, and the time is 10 seconds. The mold shell firing temperature is 850℃, and the time is 4 hours; the dynamic thermal expansion curve of the core is measured using a high-temperature thermal expansion tester, with a heating rate of 5℃ / min, and the temperature range is room temperature to 1520℃. The ceramic core value can be controlled from 100℃ to the directional solidification temperature of 1520℃ within the range of 0-4.6×10. -6 Within the specified temperature range, the ceramic core exhibits a low dynamic thermal expansion coefficient, resulting in poor compatibility with the mold shell. At 1500-1520℃, the ceramic core exhibits creep deformation exceeding 2mm after 1 hour of heat treatment, making it prone to breakage. Furthermore, the internal dimensional tolerances are severely out of control, and the blade pass rate is less than 40%.

[0057] Comparative Example 2:

[0058] The ceramic core, commonly used in the market, is composed of zircon powder and quartz glass powder, with a weight ratio of 30:70. The zircon powder is 600 mesh, and the quartz glass powder is 800 mesh. The ceramic core slurry consists of zircon powder, quartz glass powder, and a plasticizer, with the plasticizer accounting for 18% by weight. The plasticizer is a mixture of paraffin wax and polyethylene, with a mass ratio of 96:4. The ceramic core is prepared using injection molding, with a molding pressure of 80 atmospheres, an injection time of 30 seconds, and a holding time of 60 seconds. The sintering process for the ceramic core blank is as follows: heating rate 0.5℃ / min, heating to 1210℃, holding time 10 hours, and cooling in the furnace. After the core is prepared, a wax model containing the ceramic core is prepared, followed by the preparation of the mold shell. The mold shell is prepared using a coating-sand application process. Specifically: the surface coating is prepared by mixing high-purity corundum powder and silica sol. The corundum powder has an Al2O3 content >99.9%, a powder mesh size of 325 mesh, and the silica sol has an average particle size of 8-14 nm and a SiO2 content of 30-35 wt%. The powder-to-liquid ratio is 3.5:1 by mass. The back layer slurry is also prepared by mixing high-purity corundum powder and silica sol. The corundum powder has an Al2O3 content >99.9%, a powder mesh size of 325 mesh, and the silica sol has an average particle size of 8-14 nm and a SiO2 content of 30-35 wt%. The core has a particle size of 8-14 nm, a SiO2 content of 30-35 wt%, and a powder-to-liquid ratio of 4.5:1 (mass ratio). The alumina sand used for sprinkling is 80 mesh, with an Al2O3 content >99.9%. The first and second layers of the mold shell are coated with a surface layer of 80 mesh sand; the third to seventh layers are coated with a back layer of 36 mesh sand. The relative humidity for drying the mold shell is controlled at 50%, the temperature at 35℃, and the drying time is 8 hours. The dewaxing temperature is 160℃, the dewaxing pressure is 0.6 MPa, and the time is 10 seconds. The mold shell is fired at 850℃ for 4 hours. The dynamic thermal expansion curve of the core is measured using a high-temperature thermal expansion tester, with a heating rate of 5℃ / min and a temperature range from room temperature to 1520℃. The ceramic core's thermal expansion value can be controlled within the range of 0-4.2 × 10⁻⁶ from 100℃ to the directional solidification temperature of 1520℃. -6 Within the range of / ℃, the ceramic core has a relatively small dynamic thermal expansion coefficient, resulting in poor matching with the mold shell. At 1500-1520℃, the ceramic core exhibits creep deformation >1.5mm after 1 hour of heat treatment, making it prone to breakage, causing severe dimensional deviations in the internal cavity, and resulting in a blade qualification rate <40%.

Claims

1. A method for preparing a low thermal stress mold core system for directional solidification, characterized in that, This includes the molding and sintering of the ceramic core, wax model making, molding and dewaxing of the mold shell, and sintering of the mold shell. The first and second layers of the mold shell are the surface coating, and the third to seventh layers are the back coating. The specific operation steps are as follows: 1) Forming and sintering of ceramic cores: The ceramic core blanks are prepared by injection molding. The components are composed of 25-75 parts by weight of magnesium oxide powder, 25-75 parts by weight of quartz glass powder and 15-18 parts by weight of plasticizer. The plasticizer is a mixture of paraffin wax and polyethylene. The mass ratio of paraffin wax to polyethylene is between 96:4 and 99:

1. The molding pressure is 30-80 atmospheres, the injection time is 10-60 seconds, and the holding time is 10-120 seconds; the ceramic core sintering process is as follows: heating rate is 0.5-5℃ / min, heating to the sintering temperature, the sintering temperature is 1200-1300℃, the holding time is 2-10 hours, and cooling is carried out with the furnace. 2) Wax model making: After the ceramic core is prepared, a wax model containing the ceramic core is prepared. 3) Molding and dewaxing of the mold shell: The mold shell is prepared using a coating-sanding process. The specific steps are as follows: The surface coating of the mold shell is prepared by mixing high-purity corundum powder and silica sol, with a powder-to-liquid weight ratio of 3.3-3.5:

1. The surface coating of the mold shell is then sanded with 60-80 mesh sand. The back layer slurry of the mold shell is prepared by mixing high-purity corundum powder and silica sol, with a powder-to-liquid ratio of 3.8-4.5:

1. The 3rd to 7th back layer coatings are then sanded with 24-36 mesh sand. The relative humidity of the mold shell is controlled at 40-70%, the temperature is controlled at 23-35℃, and the drying time is 4-8 hours. The dewaxing temperature of the mold shell is 160-180℃, the dewaxing pressure is 0.6-0.8MPa, and the time is 10-30 seconds. 4) Sintering of the mold shell: The mold shell is fired at a temperature of 850-1000℃ for 2-4 hours.

2. The method for preparing a low thermal stress mold core system for directional solidification according to claim 1, characterized in that, After the ceramic core is sintered, the weight ratio of magnesium oxide powder to quartz glass powder is (30-50): (70-50).

3. The method for preparing a low thermal stress mold core system for directional solidification according to claim 1, characterized in that, The magnesium oxide powder has a particle size of 150-800 mesh, of which 150-200 mesh accounts for 70-80% by weight and 600-800 mesh accounts for 20-30% by weight; the quartz glass powder has a particle size range of 800-1000 mesh.

4. The method for preparing a low thermal stress mold core system for directional solidification according to claim 1, characterized in that, The sand-spreading material is corundum sand with a mesh size of 24-80 mesh and an Al2O3 content of >99.9%.

5. The method for preparing a low thermal stress mold core system for directional solidification according to claim 1, characterized in that, The high-purity corundum powder contains Al2O3 content > 99.9% and has a particle size of 325 mesh.

6. The method for preparing a low thermal stress mold core system for directional solidification according to claim 1, characterized in that, The silica sol has an average particle size of 8-14 nm and a SiO2 content of 30-35 wt%.

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