Core for casting high-melting-point alloy and method for manufacturing same

Abstract

The present invention relates to a composition of a core which can be used for casting an alloy having a high melting point, and a method for manufacturing same and, more specifically, to a core that withstands a high casting temperature, has sufficient strength, and does not cause surface defects in a cast product after casting, in order to cast an alloy having a high melting point, and to a method for manufacturing same. To this end, a core for casting a high-melting-point alloy and a method for manufacturing same are provided by applying a ternary inorganic binder, establishing a composition thereof, and providing a suitable heat treatment temperature.

Description

Core for casting high-melting-point alloy and method for manufacturing the same

[0001] The present invention relates to a composition and a method for manufacturing a core that can be used in casting an alloy having a high melting point, and more specifically, to a composition and a method for manufacturing a core that can withstand a high melting point molten metal without changing its shape by maintaining a stable phase even at high temperatures.

[0002]

[0003] The production of hollow machine parts (castings) involves both a ceramic mold, which forms the outer shape, and a ceramic core, which provides the internal space. The ceramic core is a critical component in ensuring the precision and purpose of the machine part. These precision casting cores are manufactured through a series of injection molding processes, which have the disadvantages of low productivity and high manufacturing costs. The thermal and mechanical properties of ceramic cores are highly dependent on factors such as the size and distribution of the starting powder, the composition ratio, and process variables. Accurate data for manufacturing ceramic cores tailored to the required properties is therefore difficult to obtain. Furthermore, tens of hours of heat treatment are required to strengthen ceramic cores for high-temperature parts. Sintering causes shrinkage of the ceramic core, leading to distortion and the vaporization of organic substances, creating voids within the core. To address these issues, a novel organic-inorganic conversion process utilizing an inorganic binder system has been proposed. This process enhances fracture strength and improves dimensional stability. The applied inorganic binder is a complex of silicate and metal alkoxide, and has the advantage of lowering the glass transition temperature due to the added metal alkoxide. However, since it turns into a liquid phase at a temperature of 1000℃ or higher, when casting a high-melting-point alloy, the core may collapse or have dimensional changes due to the temperature of the molten metal. Therefore, in the prior patent, a dual-coating process using both a binary binder and a ternary binder was applied to the core production, thereby effectively increasing the strength and reducing the shape change. However, the dual-coating process requires two repetitions of high-temperature heat treatment, and it is difficult to obtain a uniform reaction due to the ternary binder system to which a solid precursor is applied, resulting in low plastic strength of the core. Therefore, in the present invention, in order to produce a core applicable to casting a high-melting-point alloy, a liquid ternary inorganic binder system is applied to produce a core having a high casting temperature and excellent mechanical properties.

[0004]

[0005] The purpose of the present invention is to establish a composition of a core for casting a high-melting-point alloy that can withstand a high casting temperature for casting a high-melting-point alloy and has effective strength so as not to cause surface defects in the cast product after casting, and to provide a method for manufacturing a ceramic core using the same.

[0006] The technical problems to be solved by the invention are not limited to the technical problems mentioned above, and other technical problems not mentioned can be clearly understood by a person having ordinary skill in the art from the description of the present invention.

[0007]

[0008] In order to achieve the above object, the present invention provides a method for manufacturing a core for casting a high-melting-point alloy, characterized by comprising the steps of: coating a core powder with an inorganic binder; mixing the powder coated with the inorganic binder with an organic binder to form a core molded body; and heat-treating the core molded body to vitrify the coated inorganic binder.

[0009] The above inorganic binder may be a liquid inorganic binder precursor.

[0010] The above inorganic binder may be a ternary inorganic binder precursor.

[0011] The above ternary inorganic binder precursor may be a precursor forming three components of sodium oxide (Na2O)-silica (SiO2)-zirconia oxide (ZrO2).

[0012] The above silica precursor can form a Si-O-Si chain structure by a sol-gel reaction.

[0013] The above sodium oxide precursor may use sodium alkoxide.

[0014] The above zirconia oxide precursor may use zirconium alkoxide.

[0015] The above-mentioned zirconia oxide precursor may be included in an amount of 20 mol% to 33 mol% with respect to 100 mol% of the total precursor composition.

[0016] After the step of heat-treating the above core molded body to vitrify the coated inorganic binder, a three-phase crystal phase of ZrO2, Na2SiO3, and Na2ZrSO5 can be generated.

[0017] The step of coating the above core powder with an inorganic binder can be performed once or twice.

[0018] In the step of heat-treating the core molded body to vitrify the coated inorganic binder, the heat-treatment temperature may be 1300°C to 1600°C.

[0019] In addition, the present invention provides a core for casting a high-melting-point alloy manufactured by the method of any one of claims 1 to 11.

[0020]

[0021] The manufacturing method of the present invention uses a ternary inorganic binder precursor of a silica precursor, a zirconium oxide precursor, and a sodium oxide precursor to form a crystal phase by these precursors, thereby generating an inorganic binder having a high melting point, thereby enabling the production of a core that can withstand high molten metal temperatures during casting and casting a high-quality casting product without shape change.

[0022] According to the present invention, the above ternary inorganic binder composition can reduce internal defects and deterioration of properties of a cast product by forming a crystal phase capable of suppressing reaction with molten metal.

[0023] According to the present invention, by applying a liquid inorganic binder, the coating efficiency on the surface of the core powder can be improved and the particle surface can be uniformly coated.

[0024] According to the present invention, by generating an inorganic crystal phase having a high melting point, a ceramic core for casting a high-melting-point alloy having effective strength even at a casting temperature of 1500°C or higher and a long casting time can be provided.

[0025] According to the present invention, a ceramic core for casting a high-melting-point alloy having high melting point after casting can be provided.

[0026] The effects of the present invention are not limited to the effects mentioned above, and other effects not mentioned will be clearly understood by those skilled in the art from the description of the claims.

[0027]

[0028] Figure 1 is a schematic diagram of a manufacturing process for a core for casting a high-melting-point alloy.

[0029] Figure 2 is a graph of the XRD results of Comparative Examples 1 and 2.

[0030] Figures 3(a) to (d) are XRD result graphs of Examples 1 to 8.

[0031] Figure 4 is a microstructure image before and after heat treatment of Comparative Example 2.

[0032] Figures 5(a) to (d) are microstructural images after heat treatment of Examples 1 to 8.

[0033] Figure 6 is a graph showing the strength characteristics for Examples 1 to 8.

[0034]

[0035] The terms used in this specification have been selected from widely used, current terms, taking into account the functions of the present invention. However, these terms may vary depending on the intentions of those skilled in the art, precedents, the emergence of new technologies, etc. Furthermore, in certain cases, terms may be arbitrarily selected by the applicant, and in such cases, their meanings will be described in detail in the relevant description of the invention. Therefore, the terms used in this invention should not be defined simply as names, but rather based on their inherent meanings and the overall content of the present invention.

[0036] Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Terms defined in commonly used dictionaries should be interpreted as having a meaning consistent with their meaning in the context of the relevant technology, and shall not be interpreted in an idealized or overly formal sense unless explicitly defined herein.

[0037] When a part of a specification is said to “include” a component, this does not mean that it excludes other components, but rather that it may include other components, unless otherwise stated.

[0038] Below, with reference to the attached drawings, embodiments of the present invention are described in detail so that those skilled in the art can easily implement them. However, the present invention may be implemented in various different forms and is not limited to the embodiments described herein.

[0039]

[0040] Hereinafter, the present invention will be described in detail.

[0041]

[0042] A method for manufacturing a core for casting a high-melting-point alloy according to the present invention may include the steps of coating core powder with an inorganic binder; mixing the powder coated with the inorganic binder with an organic binder to form a core molded body; and heat-treating the core molded body to vitrify the coated inorganic binder.

[0043] Figure 1 illustrates a process for manufacturing a core for casting a high-melting-point alloy according to the present invention.

[0044] The above core powder applies ceramic powder, and powders with high mechanical properties such as silica, mullite, and zircon flower can be used independently or in mixture.

[0045] Conventional precision casting ceramic cores achieve core strength by forming a sintering reaction between core powder particles through high-temperature and long-term heat treatment. This not only increases the length of the process time and production cost due to the 24-hour heat treatment, but also causes problems with core dissolution after casting. Therefore, in the present invention, the core powder is coated with the inorganic binder precursor, and the glassy and crystalline phases formed by the inorganic binder can be used to develop core strength. This also allows for easy dissolution in an alkaline solution after casting.

[0046] The above-mentioned inorganic binder may be a liquid inorganic binder precursor. To effectively induce strength through the inorganic binder, the inorganic binder precursor must be uniformly coated on the surface of the core particle, thereby achieving high coating efficiency. Therefore, the inorganic binder precursor may be applied in a liquid form to enhance the coating efficiency on the surface of the powder particle, thereby ensuring effective strength.

[0047] The above-mentioned inorganic binder may be a ternary inorganic binder precursor. Conventional binary inorganic binders composed of SiO2-NaO2 form a glass phase even at a low heat treatment temperature of 1000°C and a short heat treatment time, thereby exhibiting core strength and excellent meltability after casting. However, this has a disadvantage in that in a molten metal having a temperature of 1500°C or higher, the glass phase formed by the molten metal liquefies, causing the shape of the core to collapse. Therefore, the present invention is characterized by applying an inorganic binder precursor that maintains a crystal phase even at a solid solution point by applying the above-mentioned ternary inorganic binder.

[0048] The above ternary inorganic binder precursor may be a precursor forming three components of sodium oxide (Na2O)-silica (SiO2)-zirconia oxide (ZrO2).

[0049] The above silica precursor can form a Si-O-Si chain structure through a sol-gel reaction. More specifically, the above silica precursor can be applied to form a Si-O-Si chain structure through a sol-gel reaction in which a Si-OR structure is hydrolyzed into Si-OH and then condensed.

[0050] The silica precursor may be a mixture of one or more selected from the group consisting of a silicate precursor, a siloxane precursor and a silane precursor, more preferably tetraethyl orthosilicate (TEOS), but is not limited thereto.

[0051] The above sodium oxide precursor may be sodium alkoxide, more preferably sodium methoxide (NaOMe), but is not limited thereto. The sodium alkoxide may be used by dissolving it in a solvent such as alcohol.

[0052] The above zirconia oxide precursor may be zirconium alkoxide, more preferably zirconium butoxide (ZrOBu), but is not limited thereto.

[0053] The above-mentioned zirconia oxide precursor may be included in an amount of 20 mol% to 33 mol% with respect to 100 mol% of the total precursor composition.

[0054] When a zirconia oxide precursor within the above range is included, three crystal phases of ZrO2, Na2SiO3, and Na2ZrSO5 can be produced together by the ternary inorganic binder precursor. This can increase the strength of the core.

[0055] The above-mentioned inorganic binder precursor may include an alcohol-based solvent as a diluent.

[0056] The step of coating the core powder with an inorganic binder may be performed once or twice. The process of coating with the inorganic binder may be repeated to increase strength, but from the perspective of elution properties, it is preferable to coat once or twice.

[0057] After the step of coating the core powder with an inorganic binder, a drying process may be further included to remove H2O and ROH formed in the sol-gel reaction of the inorganic binder precursor. The drying process may be performed at 80°C to 100°C to evaporate water and alcohol.

[0058] In the step of forming a core molded body by mixing the core powder coated with the above-described inorganic binder and the organic binder, the organic binder may be one that forms a polymer compound by heat, light, etc., which provides the molding strength of the core and is decomposed during the heat treatment described below.

[0059] In the step of heat-treating the core molded body to vitrify the coated inorganic binder, the heat-treating temperature may be 1300°C to 1600°C. The step of heat-treating the core molded body to convert the coated ternary inorganic binder into a crystal phase may be performed. As described above, the heat-treating temperature is preferably 1300°C to 1600°C in order to generate all three phases of ZrO2, Na2SiO3, and Na2ZrSO5. During the heat-treating, the organic binder decomposes, and the inorganic binder present between the core powders is converted into a crystal phase, thereby enabling the development of strength.

[0060] The above heat treatment time should be appropriately selected depending on the size of the core and the content of the inorganic binder, and according to one embodiment of the present invention, the heat treatment can be performed for 1 to 3 hours.

[0061] In addition, the present invention provides a core for casting a high-melting-point alloy manufactured by the method of any one of claims 1 to 11.

[0062]

[0063] Hereinafter, to aid understanding of the present invention, examples will be given in detail. However, the following examples are intended only to illustrate the scope of the present invention and are not intended to limit its scope. These examples are provided to more fully explain the present invention to those of average skill in the art.

[0064]

[0065] <Examples 1 and 2>

[0066] Mullite powder was coated with an inorganic binder precursor solution containing TEOS, NaOMe, and ZrOBu in a molar ratio of 16:16:1. The inorganic binder-coated mullite powder was dried at 100°C for 1 hour. 10 wt% of polyvinyl alcohol was mixed with the dried powder and press-molded to produce a molded article. The molded article was heat-treated at 1300°C and 1470°C for 1 hour each to produce a core test piece.

[0067]

[0068] <Examples 3 and 4>

[0069] Mullite powder was coated with an inorganic binder precursor solution containing TEOS, NaOMe, and ZrOBu in a molar ratio of 2:2:1. The inorganic binder-coated mullite powder was dried at 100°C for 1 hour. 10 wt% of polyvinyl alcohol was mixed with the dried powder and press-molded to produce a molded article. The molded article was heat-treated at 1300°C and 1470°C for 1 hour each to produce a core test piece.

[0070]

[0071] <Examples 5 and 6>

[0072] Mullite powder was coated with an inorganic binder precursor solution containing TEOS, NaOMe, and ZrOBu in a molar ratio of 1.16:1.16:1. The inorganic binder-coated mullite powder was dried at 100°C for 1 hour. 10 wt% of polyvinyl alcohol was mixed with the dried powder and press-molded to produce a molded article. The molded article was heat-treated at 1300°C and 1470°C for 1 hour each to produce a core test piece.

[0073]

[0074] <Examples 7 and 8>

[0075] Mullite powder was coated with an inorganic binder precursor solution containing TEOS, NaOMe, and ZrOBu in a molar ratio of 1:1:1. The inorganic binder-coated mullite powder was dried at 100°C for 1 hour. 10 wt% of polyvinyl alcohol was mixed with the dried powder and press-molded to produce a molded article. The molded article was heat-treated at 1300°C and 1470°C for 1 hour each to produce a core test piece.

[0076]

[0077] Table 1 below shows the composition ratio of the inorganic binder precursor applied in Examples 1 to 8.

[0078]

[0079] TEOS:NaOMe:ZrOBu (mol) Content of ZrOBu in precursor (mol%) Example 1, 2 16:16:1.03 Example 3, 42.0:2.0:1.020 Example 5, 6 1.16:1.16:1.030 Example 7, 8 1.0:1.0:1.033

[0080]

[0081] <Comparative Examples 1, 2>

[0082] Comparative Example 1 is a method for producing a core using a binary inorganic binder system. An inorganic binder precursor containing TEOS and NaOMe in a molar ratio of 0.18:1.5 was mixed with 0.08 mol of isobutyl alcohol to adjust the concentration. Then, mullite powder was coated with the inorganic binder precursor solution. The mullite powder coated with the inorganic binder was dried at 100°C for 1 hour. 10 wt% of polyvinyl alcohol was mixed with the dried powder and press-molded to produce a molded article. The molded article was heat-treated at 500°C and 1000°C for 1 hour to produce a core test piece.

[0083]

[0084] <Experimental Example 1> XRD analysis results of Comparative Examples 1 and 2

[0085] Figure 2 shows the XRD results of Comparative Example 1, which show complex peaks of Na2CO3 converted from NaOMe in the mixture of TEOS and NaOMe before heat treatment and amorphous SiO2 generated from TEOS after hydrolysis. These products are then converted to a complex mixture of Na2CO3, amorphous SiO2, and Na2SiO3 through heat treatment at 500°C, and then to glassy Na2SiO3 at 1000°C. The strength of the core is developed by the generated Na2SiO3, and the strength of the core applied with the binary inorganic binder system has a value of 8 to 10 MPa.

[0086]

[0087] <Experimental Example 2> XRD analysis results of Examples 1 to 8

[0088] Figure 3 shows the XRD phase analysis results of Examples 1 to 8. No crystalline phase exists in the inorganic binder before heat treatment. However, after heat treatment, ZrO2, Na2SiO3, and Na2ZrSiO5 were all detected regardless of the temperature and composition of the inorganic binder.

[0089] In compositions with a very low ZrOBu content of about 3%, a large amount of sodium silicate (Na2SiO3) that has not been converted into a three-phase is detected, and in Examples 3 to 8, most of the sodium silicate reacts with zirconia to form a three-phase, and a large amount of ZrO2 is generated. This difference in the content of the generated phase affects the strength of the core.

[0090]

[0091] <Experimental Example 3> Microstructure images before and after heat treatment of Comparative Example 2

[0092] Figure 4 shows microstructural images of Comparative Example 2 before and after heat treatment. Before heat treatment, Na2CO3 and SiO2, which are generated through hydrolysis and condensation reactions, exist on the surface of core particles, and after heat treatment, they generate Na2SiO3 at the particle surface and particle interface, forming a glassy substance as indicated by the arrow. The strength of the core is expressed by this glassy substance.

[0093]

[0094] <Experimental Example 4> Microstructure of the surface and fracture surface of the core after heat treatment in Examples 1 to 8

[0095] Figure 5 shows the microstructure of the surface and fracture surface of the core after the heat treatment of Examples 1 to 8. It was shown that after the heat treatment, glass (white solid arrow) transformed by the inorganic binder was formed on the surface and interface of the mullite particles, and in particular, the presence of the inorganic binder between the particles was clearly confirmed in the fracture surface backscatter image (the dark gray part is the cross-section of the mullite particle, and the light gray part is the ternary inorganic binder), revealing that a core with a dense structure was produced.

[0096]

[0097] <Experimental Example 5> Strength characteristic analysis for Examples 1 to 8

[0098] Figure 6 is a graph showing the strength characteristics for Examples 1 to 8. It was confirmed that the mechanical properties of the core increased as the content of the zirconia precursor increased. There was no significant difference in the rupture strength (white bar) before heat treatment depending on the composition, and it showed about 7 MPa, which was slightly increased compared to the existing binary inorganic binder mold (using binary inorganic binder system: ~5 MPa). However, when the ternary inorganic binder of the present invention was applied, the strength of the core of Example 8 was improved by about three times compared to the strength of the core to which the existing binary inorganic binder was applied, and it is judged that the application of the ternary inorganic binder system is suitable for high-melting-point alloy casting.

[0099]

[0100] We have discussed specific embodiments of the present invention. Those skilled in the art will appreciate that the present invention can be implemented in modified forms without departing from its essential characteristics. Therefore, the disclosed embodiments should be considered illustrative rather than restrictive. The scope of the present invention is set forth in the claims, not the foregoing description, and all differences within the scope equivalent thereto should be construed as being encompassed by the present invention.

Claims

1. A step of coating the core powder with an inorganic binder; A step of forming a core molded body by mixing a powder coated with the above inorganic binder and an organic binder; and A method for manufacturing a core for casting a high-melting-point alloy, characterized in that it comprises a step of heat-treating the core molded body to vitrify the coated inorganic binder.

2. In paragraph 1, A method for manufacturing a core for casting a high-melting-point alloy, characterized in that the above-mentioned inorganic binder is a liquid inorganic binder precursor.

3. In paragraph 1, A method for manufacturing a core for casting a high-melting-point alloy, characterized in that the above-mentioned inorganic binder is a ternary inorganic binder precursor.

4. In paragraph 3, A method for manufacturing a core for casting a high-melting-point alloy, characterized in that the above-mentioned ternary inorganic binder precursor is a precursor forming three components of sodium oxide (Na2O) - silica (SiO2) - zirconia oxide (ZrO2).

5. In paragraph 4, A method for manufacturing a core for casting a high-melting-point alloy, characterized in that the silica precursor forms a Si-O-Si chain structure by a sol-gel reaction.

6. In paragraph 4, A method for manufacturing a core for casting a high-melting-point alloy, characterized in that the sodium oxide precursor uses sodium alkoxide.

7. In paragraph 4, A core for casting a high-melting-point alloy and a manufacturing method thereof, characterized in that the above-mentioned zirconia oxide precursor uses zirconium alkoxide.

8. In paragraph 4, A method for manufacturing a core for casting a high-melting-point alloy, characterized in that the above-mentioned zirconia oxide precursor is contained in an amount of 20 mol% to 33 mol% with respect to 100 mol% of the total precursor composition.

9. In paragraph 1, A method for manufacturing a core for casting a high-melting-point alloy, characterized in that the core molded body is heat-treated to vitrify the coated inorganic binder in the step of forming a three-phase crystal phase of ZrO2, Na2SiO3, and Na2ZrSO5.

10. In paragraph 1, A method for manufacturing a core for casting a high-melting-point alloy, characterized in that the step of coating the core powder with an inorganic binder is performed once or twice.

11. In paragraph 1, A method for manufacturing a core for casting a high-melting-point alloy, characterized in that in the step of heat-treating the core molded body to vitrify the coated inorganic binder, the heat treatment temperature is 1300°C to 1600°C.

12. A high-melting-point alloy casting core manufactured by any one of the methods of clauses 1 to 11 above.

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