Inert ceramic core for high-temperature alloy hollow castings and method for manufacturing same

Abstract

This invention relates to the field of high-temperature alloy investment casting technology, and discloses an inert ceramic core for high-temperature alloy hollow castings and its preparation method. The preparation method includes the following steps: using yttrium-stabilized zirconia as the matrix material, yttrium oxide powder as a mineralizer, alumina-toughened zirconia powder as a toughening agent, yttrium aluminum garnet powder as an additive, and thorium oxide as a grain inhibitor, the above powders are mixed evenly, a paraffin-based plasticizer is added, and the mixture is vacuum stirred to form a slurry with fluidity and thermoplasticity; the slurry is added to a hot-pressing core-casting machine, and a core blank is prepared using the hot-pressing method; the core blank is embedded with sand and then sintered at high temperature, and cleaned after sintering to obtain the ceramic core. This invention improves the chemical inertness, high-temperature strength, and dimensional stability of the ceramic core through the synergistic effect of yttrium-stabilized zirconia, yttrium oxide powder, alumina-toughened zirconia powder, yttrium aluminum garnet powder, and thorium oxide, making it suitable for the internal cavity forming of high-temperature alloy hollow castings.

Description

Technical Field

[0001] This invention relates to the field of investment casting technology for high-temperature alloys, and in particular to an inert ceramic core for high-temperature alloy hollow castings and its preparation method. Background Technology

[0002] With the continuous improvement of thrust-to-weight ratio and thermal efficiency in aero-engines and gas turbines, the internal cavity structure of high-temperature alloy hollow castings is becoming increasingly complex. Furthermore, highly reactive metallic elements such as Y, Ti, Hf, Re, and Ru are constantly being added to high-temperature alloys to increase service temperature. However, during investment casting, traditional silicon-based or aluminum-based ceramic cores are prone to interfacial reactions with these highly reactive elements, leading to defects such as sand adhesion, porosity, and chemical segregation on the inner surface of the casting, severely reducing yield and service life. For example, silicon oxide-based cores can react with C, Mg, and Y in the alloy at high temperatures to generate gaseous or low-melting-point phases, damaging the quality of the casting's internal cavity.

[0003] Zirconia is considered an ideal inert core material due to its high melting point, good thermochemical stability, and low reactivity with high-temperature alloys. However, pure zirconia cores suffer from problems during sintering, such as easy grain growth, difficulty in densification, insufficient strength at both room and high temperatures, and poor high-temperature creep resistance, which limit their engineering applications. While existing technologies have attempted to add mineralizers or toughening agents, it remains difficult to simultaneously meet the comprehensive requirements of precision casting for core dimensional accuracy, high-temperature strength, chemical inertness, and process stability. Therefore, there is an urgent need to develop a novel inert ceramic core and its preparation method to solve these problems. Summary of the Invention

[0004] To address the problems of existing zirconia ceramic cores, such as easy grain growth during sintering, difficulty in densification, insufficient strength at room temperature and high temperature, poor high-temperature creep resistance, and the tendency of traditional silicon-based or aluminum-based ceramic cores to undergo interfacial reactions with highly reactive high-temperature alloys, this invention provides a method for preparing an inert ceramic core for high-temperature alloy hollow castings, comprising the following steps:

[0005] Using yttrium-stabilized zirconia as the matrix material, yttrium oxide powder as the mineralizer, alumina-toughened zirconia powder as the toughening agent, yttrium aluminum garnet powder as the additive, and thorium oxide as the grain inhibitor, the yttrium-stabilized zirconia, yttrium oxide powder, alumina-toughened zirconia powder, yttrium aluminum garnet powder, and thorium oxide are mixed evenly to obtain ceramic powder.

[0006] A paraffin-based plasticizer is added to the ceramic powder, and the mixture is vacuum stirred to form a slurry with fluidity and thermoplasticity.

[0007] The slurry is added to a hot press molding machine, and a core blank is prepared by hot press molding.

[0008] The core blank is embedded in sand and then sintered at high temperature. After sintering, it is cleaned to obtain a ceramic core.

[0009] Preferably, the yttrium-stabilized zirconia accounts for 50-80 wt% of the total mass of the ceramic powder, the yttrium oxide powder accounts for 10-30 wt% of the total mass of the ceramic powder, the alumina-toughened zirconia powder accounts for 5-15 wt% of the total mass of the ceramic powder, the yttrium aluminum garnet powder accounts for 3-10 wt% of the total mass of the ceramic powder, and the thorium oxide accounts for 0.01-1 wt% of the total mass of the ceramic powder.

[0010] Preferably, the yttrium-stabilized zirconium oxide includes 320-mesh yttrium-stabilized zirconium oxide and 1000-mesh yttrium-stabilized zirconium oxide, and the mass ratio of the 320-mesh yttrium-stabilized zirconium oxide to the 1000-mesh yttrium-stabilized zirconium oxide is (4-6):1.

[0011] Preferably, the yttrium-stabilized zirconium oxide is prepared by an electrofusion process, wherein the ZrO2 content is ≥88% and the Y2O3 content is 8.8-9.8%.

[0012] Preferably, the yttrium oxide powder has a particle size of 400 mesh and a Y2O3 content of ≥99%.

[0013] Preferably, the alumina-toughened zirconia powder has a particle size of 1–3 μm, wherein the γ-ZrO2 content is 90% and the Al2O3 content is 10%.

[0014] Preferably, the yttrium aluminum garnet powder is 320 mesh, wherein the Y2O3 content is 52-58% and the Al2O3 content is 42-48%; the thorium oxide has a particle size of 0.5-2 μm and a purity of ≥99%.

[0015] Preferably, the paraffin-based plasticizer accounts for 15-20% of the total mass of the ceramic powder, and the vacuum stirring time is 20-30 h.

[0016] Preferably, the high-temperature sintering includes the following steps:

[0017] Heat to 200-300℃ at a heating rate of 1-4℃ / min, and hold for 1-2 hours;

[0018] Heat to 500-600℃ at a heating rate of 2-4℃ / min, and hold for 1-2 hours;

[0019] Heat to 900-1000℃ at a heating rate of 2-5℃ / min, and hold for 1-2 hours;

[0020] Heat to 1500–1700℃ at a heating rate of 5–10℃ / min, and hold for 5–10 h;

[0021] Then it was cooled to room temperature in the furnace.

[0022] An inert ceramic core for high-temperature alloy hollow castings, the ceramic core being formed by sintering yttrium-stabilized zirconium oxide, yttrium oxide powder, alumina-toughened zirconium oxide powder, yttrium aluminum garnet powder, and thorium oxide, and prepared by the preparation method according to any one of claims 1 to 9, wherein the ceramic core has a firing shrinkage rate ≤1%, a high-temperature deflection ≤1 mm, a room temperature bending strength ≥20 MPa, and a bending strength at 1550℃ ≥15 MPa.

[0023] Compared with the prior art, the present invention provides an inert ceramic core for high-temperature alloy hollow castings and its preparation method, which has the following beneficial effects:

[0024] 1. This invention uses yttrium-stabilized zirconia as the matrix material, combined with yttrium oxide mineralizer, alumina-toughened zirconia toughening agent, yttrium aluminum garnet additive, and thorium oxide grain inhibitor. Through the synergistic effect of multiple components, the chemical inertness of the ceramic core is significantly improved. Practical verification shows that after contacting high-temperature alloys such as DD6 and Inconel 718 at 1550℃ for 30 minutes, no obvious interfacial reaction layer was observed, and the inner surface of the casting was smooth and free of adhering sand, effectively solving the problem of internal cavity forming in high-activity alloy hollow castings.

[0025] 2. The ceramic core prepared by this invention has excellent high-temperature mechanical properties and dimensional stability. By optimizing the powder particle size distribution (e.g., mixing 320 mesh and 1000 mesh yttrium-stabilized zirconia) and the gradient sintering regime, the core's firing shrinkage rate is no more than 1%, the high-temperature deflection is no more than 1 mm, the room temperature bending strength is greater than 20 MPa, and the 1550℃ bending strength is greater than 15 MPa. It can meet the requirements of investment casting for core strength, stiffness, and dimensional accuracy, and is suitable for the production of single-crystal and directionally solidified blades with complex internal cavities.

[0026] 3. This invention effectively inhibits the abnormal growth of zirconia grains during sintering by adding thorium oxide grain inhibitors, maintaining a fine-grained structure and thus improving the overall mechanical properties of the core. Simultaneously, the introduction of yttrium aluminum garnet powder helps improve particle bonding and microstructure densification during sintering, enhancing the core's high-temperature creep resistance.

[0027] 4. This invention employs hot-press casting combined with embedded sand sintering, offering a wide process window and good reproducibility, making it suitable for industrial mass production. The prepared ceramic core is less prone to harmful interfacial reactions with the high-temperature alloy melt, which helps improve the internal cavity quality of high-temperature alloy hollow castings, enhances casting stability, and reduces rework costs. Attached Figure Description

[0028] Figure 1 This is a process flow diagram for preparing the inert ceramic core for high-temperature alloy hollow castings according to the present invention. Detailed Implementation

[0029] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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.

[0030] Please see Figure 1 The present invention provides a technical solution:

[0031] The inert ceramic core of this invention uses yttrium-stabilized zirconia as the matrix material, yttrium oxide powder as the mineralizer, alumina-toughened zirconia powder as the toughening agent, yttrium aluminum garnet powder as the additive, and thorium oxide as the grain inhibitor. By optimizing the particle size, proportion, and sintering process of each component, a ceramic core with a firing shrinkage rate ≤1%, high-temperature deflection ≤1 mm, room temperature flexural strength ≥20 MPa, and 1550℃ flexural strength ≥15 MPa is obtained, which is suitable for investment casting of high-temperature alloy hollow castings.

[0032] Example 1

[0033] This embodiment provides a method for preparing an inert ceramic core for high-temperature alloy hollow castings, the specific steps of which are as follows:

[0034] (1) Weigh the following powders: 12 kg of 320 mesh fused yttrium stabilized zirconium oxide (ZrO2≥88%, Y2O3 8.8-9.8%), 3 kg of 1000 mesh fused yttrium stabilized zirconium oxide (weight ratio 4:1); 3 kg of 400 mesh yttrium oxide powder (Y2O3≥99%); 1.2 kg of alumina toughened zirconium oxide powder (Y-ZrO2 about 90%, Al2O3 about 10%) with a particle size of 1-3 μm; 0.79 kg of 320 mesh yttrium aluminum garnet powder (Y2O3 52-58%, Al2O3 42-48%); and 0.01 kg of thorium oxide with a particle size of 0.5-2 μm and a purity ≥99%.

[0035] (2) Mixing and pulping: The above powders are mixed evenly in a mixer and then dried. 15% of the total weight of the powder is added to a vacuum mixer, heated and melted, and then stirred. The dried mixed powders are added gradually. After all the powders are added, stirring is continued for 24 hours. Then, a vacuum is drawn to obtain a slurry with good fluidity and thermoplasticity.

[0036] (3) Molding: Add the slurry into the hot press core injection machine and press it to form the core blank.

[0037] (4) Sintering: The core blank is buried in sand and placed in a high-temperature furnace and sintered according to the following system: the temperature is increased to 200℃ at 1.5℃ / min and held for 1 h; the temperature is increased to 500℃ at 2℃ / min and held for 1.5 h; the temperature is increased to 1000℃ at 2.5℃ / min and held for 2 h; the temperature is increased to 1500℃ at 5℃ / min and held for 10 h; and then cooled to room temperature with the furnace.

[0038] (5) Cleaning: Remove the sintered ceramic core and remove the surface sand to obtain the finished product.

[0039] The ceramic core prepared in this embodiment has a firing shrinkage rate of 0.8%, a high-temperature deflection of 0.6 mm, a room temperature bending strength of 22 MPa, and a bending strength of 16 MPa at 1550 °C. After being held at 1550 °C for 30 min with the high-temperature alloy DD6, there is no interface reaction layer, and the inner cavity of the casting is smooth.

[0040] Example 2

[0041] The examples are basically the same as Example 1, except for the powder ratio and sintering process:

[0042] (1) Weigh out: 11 kg of 320 mesh yttrium-stabilized zirconium oxide, 2.5 kg of 1000 mesh yttrium-stabilized zirconium oxide (weight ratio of the two is 4.4:1); 4 kg of yttrium oxide powder; 1 kg of alumina-toughened zirconium oxide powder; 1.4 kg of yttrium aluminum garnet powder; and 0.1 kg of thorium oxide.

[0043] (2) The plasticizer accounts for 15.5% of the total weight of the powder, and the stirring time is 20h.

[0044] (3) Sintering process: Heat up to 300℃ at 3℃ / min and hold for 1.5h; heat up to 550℃ at 4℃ / min and hold for 2h; heat up to 950℃ at 5℃ / min and hold for 1.5h; heat up to 1600℃ at 6℃ / min and hold for 8h; cool with the furnace.

[0045] The ceramic core prepared in this embodiment has a firing shrinkage rate of 0.7%, a high-temperature deflection of 0.5 mm, a room temperature bending strength of 24 MPa, and a bending strength of 17.5 MPa at 1550 °C. No obvious chemical reaction was observed after holding it at 1550 °C for 30 min with the high-temperature alloy Inconel 718. The surface quality of the inner cavity of the casting is good.

[0046] Example 3

[0047] This embodiment is basically the same as Embodiment 1, except for the powder ratio and sintering process:

[0048] (1) Weigh out: 10 kg of 320 mesh yttrium-stabilized zirconium oxide, 2 kg of 1000 mesh yttrium-stabilized zirconium oxide (weight ratio of the two is 5:1); 5 kg of yttrium oxide powder; 2 kg of alumina-toughened zirconium oxide powder; 0.8 kg of yttrium aluminum garnet powder; and 0.2 kg of thorium oxide.

[0049] (2) The plasticizer accounts for 16% of the total weight of the powder, and the stirring time is 30h.

[0050] (3) Sintering process: Heat up to 280℃ at 4℃ / min and hold for 2h; heat up to 600℃ at 3.5℃ / min and hold for 1h; heat up to 900℃ at 4℃ / min and hold for 1h; heat up to 1700℃ at 10℃ / min and hold for 5h; cool with the furnace.

[0051] The ceramic core prepared in this embodiment has a firing shrinkage rate of 0.9%, a high-temperature deflection of 0.7 mm, a room temperature bending strength of 21 MPa, and a bending strength of 15.5 MPa at 1550℃. It is suitable for casting single-crystal blades with complex internal cavities. No obvious sand adhesion or obvious porosity defects were observed on the inner wall of the casting.

[0052] Comparative Example 1

[0053] To illustrate the effect of thorium oxide grain inhibitors on the performance of ceramic cores, Comparative Example 1 was set up: except that no thorium oxide grain inhibitors were added, everything else was the same as in Example 1. The results showed that after sintering, the core grains were coarse, the room temperature flexural strength dropped to 12 MPa, the flexural strength at 1550℃ was only 8 MPa, and the high-temperature deflection reached 2.5 mm, which could not meet the requirements of precision casting.

[0054] Comparative Example 2

[0055] Comparative Example 2: Except for the absence of yttrium aluminum garnet powder, it was identical to Example 1. The results showed insufficient core density, a firing shrinkage rate of 2.5%, substandard dimensional accuracy, and a bending strength of only 9 MPa at 1550°C.

[0056] Results Summary

[0057] As demonstrated in Examples 1-3 and Comparative Examples 1-2, the present invention, through the combined use of yttrium-stabilized zirconia, yttrium oxide powder, alumina-toughened zirconia powder, yttrium aluminum garnet powder, and thorium oxide, can improve the room temperature strength, high temperature strength, dimensional stability, and high temperature deformation resistance of ceramic cores. Thorium oxide, as a grain inhibitor, helps suppress abnormal zirconia grain growth during sintering; yttrium aluminum garnet powder, as an additive, helps improve particle bonding and microstructure densification during sintering. Compared with Comparative Example 1 (without thorium oxide) and Comparative Example 2 (without yttrium aluminum garnet powder), the ceramic cores prepared in the embodiments of the present invention exhibit better comprehensive performance in terms of room temperature flexural strength, 1550℃ flexural strength, firing shrinkage, and high temperature deflection, meeting the requirements for strength, dimensional stability, and chemical inertness in investment casting of high-temperature alloy hollow castings.

[0058] 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 preparing an inert ceramic core for high-temperature alloy hollow castings, characterized in that, Includes the following steps: Using yttrium-stabilized zirconia as the matrix material, yttrium oxide powder as the mineralizer, alumina-toughened zirconia powder as the toughening agent, yttrium aluminum garnet powder as the additive, and thorium oxide as the grain inhibitor, the yttrium-stabilized zirconia, yttrium oxide powder, alumina-toughened zirconia powder, yttrium aluminum garnet powder, and thorium oxide are mixed evenly to obtain ceramic powder. A paraffin-based plasticizer is added to the ceramic powder, and the mixture is vacuum stirred to form a slurry with fluidity and thermoplasticity. The slurry is added to a hot press core injection molding machine, and a core blank is prepared by hot press injection. The core blank is embedded in sand and then sintered at high temperature. After sintering, it is cleaned to obtain a ceramic core.

2. The method for preparing an inert ceramic core for high-temperature alloy hollow castings according to claim 1, characterized in that, The yttrium-stabilized zirconia accounts for 50-80 wt% of the total mass of the ceramic powder, the yttrium oxide powder accounts for 10-30 wt% of the total mass of the ceramic powder, the alumina-toughened zirconia powder accounts for 5-15 wt% of the total mass of the ceramic powder, the yttrium aluminum garnet powder accounts for 3-10 wt% of the total mass of the ceramic powder, and the thorium oxide accounts for 0.01-1 wt% of the total mass of the ceramic powder.

3. The method for preparing an inert ceramic core for high-temperature alloy hollow castings according to claim 1, characterized in that, The yttrium-stabilized zirconium oxide includes 320-mesh yttrium-stabilized zirconium oxide and 1000-mesh yttrium-stabilized zirconium oxide, and the mass ratio of the 320-mesh yttrium-stabilized zirconium oxide to the 1000-mesh yttrium-stabilized zirconium oxide is (4-6):

1.

4. The method for preparing an inert ceramic core for high-temperature alloy hollow castings according to claim 3, characterized in that, The yttrium-stabilized zirconium oxide is prepared by an electrofusion process, wherein the ZrO2 content is ≥88% and the Y2O3 content is 8.8-9.8%.

5. The method for preparing an inert ceramic core for high-temperature alloy hollow castings according to claim 1, characterized in that, The yttrium oxide powder has a particle size of 400 mesh and a Y2O3 content of ≥99%.

6. The method for preparing an inert ceramic core for high-temperature alloy hollow castings according to claim 1, characterized in that, The alumina-toughened zirconia powder has a particle size of 1–3 μm, with a Y-ZrO2 content of 90% and an Al2O3 content of 10%.

7. The method for preparing an inert ceramic core for high-temperature alloy hollow castings according to claim 1, characterized in that, The yttrium aluminum garnet powder is 320 mesh, with a Y2O3 content of 52-58% and an Al2O3 content of 42-48%; the thorium oxide has a particle size of 0.5-2 μm and a purity of ≥99%.

8. The method for preparing an inert ceramic core for high-temperature alloy hollow castings according to claim 1, characterized in that, The paraffin-based plasticizer accounts for 15-20% of the total mass of the ceramic powder, and the vacuum stirring time is 20-30 h.

9. The method for preparing an inert ceramic core for high-temperature alloy hollow castings according to claim 1, characterized in that, The high-temperature sintering includes the following steps: Heat to 200-300℃ at a heating rate of 1-4℃ / min, and hold for 1-2 hours; Heat to 500-600℃ at a heating rate of 2-4℃ / min, and hold for 1-2 hours; Heat to 900-1000℃ at a heating rate of 2-5℃ / min, and hold for 1-2 hours; Heat to 1500–1700℃ at a heating rate of 5–10℃ / min, and hold for 5–10 h; Then it was cooled to room temperature in the furnace.

10. An inert ceramic core for high-temperature alloy hollow castings, characterized in that, The ceramic core is formed by sintering yttrium-stabilized zirconium oxide, yttrium oxide powder, alumina-toughened zirconium oxide powder, yttrium aluminum garnet powder and thorium oxide, and is prepared by the preparation method according to any one of claims 1 to 9. The ceramic core has a firing shrinkage rate ≤1%, a high-temperature deflection ≤1 mm, a room temperature bending strength ≥20 MPa, and a bending strength at 1550℃ ≥15 MPa.

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