Additive-formed ceramic core and method for manufacturing the ceramic core

Abstract

A laminate molded ceramic core is provided which has strength, disintegration properties and meltability, and which has improved surface roughness. This laminate molded ceramic core, used as the core when producing a metal casting, is configured from a center part, which is laminate molded fired body of a prescribed ceramic powder, a first layer, which covers at least part of the center part, and a second layer, which is formed on the outer surface of the first layer. In the ceramic core, defining Ag as the amount of projection particles projected in a brittleness test and Bμm as the erosion depth, when using the average erosion rate, which is the average value of the erosion rate calculated by the expression erosion rate (μm / g) = B / A, the average erosion rate of the first layer is lower than that of the center part and the average erosion rate of the second layer is higher than that of the first layer. Further, the surface roughness Ra of the second layer is less than or equal to 10μm.

Description

Technical Field

[0001] The present invention relates to a laminated ceramic core and a method for manufacturing the ceramic core. This application claims priority based on Japanese Patent Application No. 2020-193179 filed on November 20, 2020, and the entire contents of that application are incorporated herein by reference.

Background Art

[0002] Conventionally, additive manufacturing (also referred to as three-dimensional printing), in which a powder material is combined with a binder to form a powder solidified layer having a predetermined cross-sectional shape, and the powder solidified layers are sequentially laminated to form a shaped object having a desired three-dimensional shape, is known. In recent years, in additive manufacturing of powder materials (powder lamination molding), powder materials made of ceramic materials that are difficult to perform precision machining after molding are also widely used. According to such powder lamination molding, a ceramic core used as a core when forming a metal casting having a complex shape can be manufactured at low cost and in a short period of time.

[0003] As characteristics required for such a ceramic core, generally, when pouring molten metal, strength that can withstand molten metal at around 1500 degrees and sufficient surface roughness to smooth the surface of the casting to be manufactured are required. When the molten metal thermally contracts during the process of solidification, collapsibility that does not cause recrystallization defects is required. And when removing the ceramic core from the metal casting, fusibility that can be easily removed by an alkaline solution is required. <​Various proposals have been made to achieve a good balance of the properties of such ceramic cores, and examples of these include Patent Documents 1 to 3. Patent Documents 1 and 2 disclose techniques for improving the strength of the mold. Patent Document 1 describes a technique in which the surface of a silica-containing powder is coated with an organic binder, a laminated object is formed, and then fired. Patent Document 2 describes a technique in which an inorganic binder is impregnated into the laminated object multiple times before firing. Furthermore, Patent Document 3 discloses a technique in which a molded object having a slurry layer and a stucco layer is formed, and then fired, in order to achieve both mold strength and self-collapsibility. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Application No. 2015-171725 [Patent Document 2] Japanese Patent Application No. 2016-159324 [Patent Document 3] Japanese Patent Application No. 2015-171724 [Overview of the project] [Problems that the invention aims to solve]

[0006] Incidentally, additively manufactured objects using powder materials made from ceramic materials tend to have larger voids between particles and higher porosity compared to ceramic molded objects made using molds. In addition, the layering steps that occur when stacking the powder materials tend to increase surface roughness (i.e., the surface becomes rougher). The surface roughness of the ceramic core is directly related to the surface roughness of the metal casting. For this reason, there has been a need for technology to obtain ceramic cores with improved surface roughness (i.e., smoother surfaces).

[0007] The present invention has been made in view of these points, and its main objective is to provide an additively fabricated ceramic core that combines strength, collapsibility, and meltability, and furthermore, has improved surface roughness. Another objective is to provide a method for manufacturing such a ceramic core. [Means for solving the problem]

[0008] To achieve the above objective, an additively manufactured ceramic core is provided that can be used as a core when manufacturing metal castings. Specifically, the additively manufactured ceramic core disclosed herein comprises a central part which is an additively manufactured and fired body of a predetermined ceramic powder, a first layer covering at least a part of the central part, and a second layer formed on the surface of the first layer. Here, when the amount of projected particles projected in a brittleness test is Ag and the erosion depth is B μm, and the average erosion rate, which is the average value of the erosion rate calculated by the following formula: erosion rate (μm / g) = B / A, is used, the average erosion rate of the first layer is lower than the average erosion rate of the central part, and the average erosion rate of the second layer is higher than the average erosion rate of the first layer. This configuration makes it possible to create a layered ceramic core that achieves both strength and collapsibility while also improving surface roughness.

[0009] In one preferred embodiment of the ceramic core disclosed herein, the average erosion rate of the central part is 5 times or more the average erosion rate of the first layer, and the average erosion rate of the second layer is 2.5 times or more the average erosion rate of the first layer. Within this range of average erosion rates, it is possible to provide additively fabricated ceramic cores that achieve a higher level of balance between strength and collapsibility.

[0010] In one preferred embodiment of the ceramic core disclosed herein, the arithmetic mean surface roughness Ra of the second layer is 10 μm or less. Such a ceramic core may have a surface roughness sufficient to smooth the surface of the metal casting.

[0011] In one preferred embodiment of the ceramic core disclosed herein, the central part and the second layer are each composed of at least one selected from the group consisting of silica, alumina, zircon, and magnesia. The above-mentioned effects can be more favorably achieved by using oxides containing such metallic and metalloid elements.

[0012] In one preferred embodiment of the ceramic core disclosed herein, the first layer is characterized by containing silica as a main component. A ceramic core is provided that contains silica as the most abundant component, and is easily removed by dissolution in an alkaline solution.

[0013] To achieve the above-mentioned other objectives, a method for manufacturing an additively manufactured ceramic core is provided. Specifically, the manufacturing method disclosed herein includes: forming an additively manufactured object by additive manufacturing using a first ceramic powder having an average particle size D1; firing the additively manufactured object to obtain the central part which is an additively manufactured fired body; immersing the additively manufactured fired body in a ceramic sol containing a second ceramic having an average particle size D2 to form the first layer on at least a part of the additively manufactured fired body; and immersing the additively manufactured fired body with the first layer applied to it in a ceramic slurry containing a third ceramic powder having an average particle size D3 to form the second layer on the surface of the first layer. According to this method of manufacturing, it is possible to produce an additively fabricated ceramic core that achieves both strength and collapsibility, while also having improved surface roughness.

[0014] In one preferred embodiment of the method for manufacturing a ceramic core disclosed herein, the average particle sizes D1, D2, and D3 of the first ceramic powder, the second ceramic powder, and the third ceramic powder are D1 > D3 > D2. This configuration makes it possible to manufacture a ceramic core that possesses both appropriate strength and collapsibility, and also has improved surface roughness.

[0015] In a preferred embodiment of the method for manufacturing the ceramic core disclosed herein, the arithmetic mean surface roughness Ra of the second layer is 10 μm or less. The ceramic core manufactured by the manufacturing method of such an embodiment has a surface roughness sufficient to smooth the surface of the metal casting.

[0016] In a preferred embodiment of the method for manufacturing the ceramic core disclosed herein, the first ceramic powder and the third ceramic powder are each composed of at least one selected from the group consisting of silica, alumina, zircon, and magnesia. Also, in a preferred embodiment of the method for manufacturing the ceramic core disclosed herein, the second ceramic powder contains silica as a main constituent component. According to such a configuration, the above-described effects can be more effectively exhibited.

Brief Description of the Drawings

[0017] [Figure 1] FIG. 1 is a SEM observation image of the surface of Example 1. [Figure 2] FIG. 2 is a SEM observation image of the surface of Comparative Example 1. [Figure 3] FIG. 3 is a SEM observation image of the surface of Comparative Example 2. [Figure 4] FIG. 4 is a SEM observation image of a cross section perpendicular to the surface of Example 1 (a cross section along the thickness direction). [Figure 5] FIG. 5 is a SEM observation image of a cross section perpendicular to the surface of Comparative Example 2 (a cross section along the thickness direction). [Figure 6A] FIG. 6A is a graph showing the result of measuring the surface roughness Ra of Example 1. [Figure 6B] FIG. 6B is a graph showing the result of measuring the surface roughness Ra of Comparative Example 1. [Figure 6C] FIG. 6C is a graph showing the result of measuring the surface roughness Ra of Comparative Example 2.

Modes for Carrying Out the Invention

[0018] Preferred embodiments of the present invention will be described below. Matters other than those specifically mentioned herein that are necessary for carrying out the present invention can be understood as design matters for those skilled in the art based on the prior art. The present invention can be carried out based on the contents disclosed herein and common technical knowledge in the art.

[0019] In this specification, "containing A as a major component" in the composition of the first layer and the first to third ceramic powders may mean that A is the most abundant component in the first layer and the first to third ceramic powders. While not particularly limited, the proportion of A in the first layer and the first to third ceramic powders is typically 60% or more by mass (preferably 70% or more, more preferably 80% or more, even more preferably 90% or more, for example 99% or more). It may also include cases where the entire composition (100%) is A by mass.

[0020] The additively manufactured ceramic core disclosed herein comprises a central part, a first layer, and a second layer. The central part is a porous additively manufactured and fired body formed by additive manufacturing and firing of a first ceramic powder. The first layer contains a second ceramic powder and covers at least a portion of the central part. The first layer is formed by immersing the central part in a dispersion liquid (ceramic sol) consisting of the second ceramic powder and a dispersion medium, and removing the dispersion medium by drying and heat treatment. The second layer contains a third ceramic powder and is a porous layer formed on the surface of the first layer. The second layer is formed by coating the surface of the first layer with a ceramic slurry consisting of the third ceramic powder and a solvent, and removing the solvent by high-temperature firing.

[0021] The additively manufactured ceramic core disclosed herein has an average erosion rate of the first layer that is lower than that of the center layer, and an average erosion rate of the second layer that is higher than that of the first layer. Such average erosion rates can be measured and calculated using commercially available equipment. For example, a brittleness test of the additively manufactured ceramic core can be performed using equipment manufactured by Palmeso Corporation (e.g., MSE-A203). This allows for the calculation of the average erosion rate in each structure of the additively manufactured ceramic core. Specifically, a test specimen of an substantially rectangular parallelepiped additively manufactured ceramic core comprising a center layer, a first layer, and a second layer, and a test specimen (cross-section) obtained by cutting the test specimen along the width direction at a position 4 mm from the surface along the thickness direction are prepared. A predetermined amount of projection particles (e.g., 3μm spherical alumina MSE-BA-3-3 manufactured by Parmeso Co., Ltd.) is projected onto the surface of the test specimen and the cross-section of the test specimen at a projection power value such that the projection particles achieve a predetermined erosion rate (e.g., 0.18μm / g) relative to a standard test specimen (e.g., HRC-45 manufactured by Parmeso Co., Ltd.). The test specimen and the cross-section of the test specimen are abraded from the surface in the thickness direction to a depth of 180μm. The relationship between the projection amount of projection particles (Ag) and the erosion depth (Bμm) is obtained continuously (at least 3 locations, and more than 10 locations). This allows for the acquisition of a graph (erosion progress graph) showing the relationship between the projection amount of projection particles (Ag) and the erosion depth (Bμm). The erosion rate is calculated based on A and B above using the following formula: Erosion rate (μm / g) = B / A. By plotting the change in erosion rate at various depths from the surface against the erosion depth at at least three, and more than ten, locations, the erosion rate distribution in the thickness direction from the surface can be obtained. The erosion rate is a parameter that indicates the erosion rate (ease of erosion), and a smaller value indicates that the test specimen is harder. Of the plotted erosion rate distribution, the values ​​from the surface of the specimen (0 μm) to 60 μm are defined as the second layer, the values ​​from 100 μm to 180 μm as the first layer, and the values ​​from the surface of the specimen (cross-section) (0 μm) to 180 μm as the erosion rate corresponding to the center. For each structure, the average value of the erosion rates at three or more locations (and even ten or more locations) is calculated, and this value is defined here as the average erosion rate for each structure.

[0022] The average erosion rates of the core, the first layer, and the second layer can be determined as described above. The additively fabricated ceramic core disclosed herein ensures a certain level of strength and collapsibility required when used as a ceramic core, as the average erosion rate of the first layer is lower than that of the core, and the average erosion rate of the second layer is higher than that of the first layer.

[0023] Furthermore, the average eroticism rate of the central layer is required to be higher than that of the first layer. Preferably, the average eroticism rate of the central layer is about 5 to 10 times that of the first layer, more preferably 5.5 to 9.5 times, and even more preferably 6 to 9 times. Furthermore, the average eroticism rate of the second layer is required to be higher than that of the first layer. Preferably, the average eroticism rate of the second layer is 2.5 to 8.5 times that of the first layer, more preferably 3 to 8 times, and even more preferably 3.5 to 7.5 times. The technology disclosed herein can preferably be implemented in a manner that provides average eroticism rates within the above-described ranges.

[0024] The first to third ceramic powders used in the central layer, the first layer, and the second layer, respectively, have different average particle sizes (D1 to D3). The average particle size of the ceramic powders is D1 > D3 > D2. By using ceramic powders with such different average particle sizes in each structure, it is possible to realize an additively fabricated ceramic core that combines strength and collapsibility while also having improved surface roughness.

[0025] Furthermore, unless otherwise specified, the "average particle size" for the first and third ceramic powders refers to the particle size at 50% of the cumulative value (50% volume average particle size; D50) in the volume-based particle size distribution measured by a particle size distribution analyzer based on the laser scattering and diffraction method. The "average particle size" for the second ceramic powder, unless otherwise specified, refers to the average particle size calculated from the specific surface area of ​​the second ceramic powder obtained by the BET method (e.g., the BET one-point method). This average particle size is calculated assuming that the primary particle size of the second ceramic powder coincides with the diameter (equivalent spherical diameter) of a spherical particle that can achieve the specific surface area. This average particle size D2 can be determined, for example, based on the following equation: D2 = 6 / (ρS), where S is the specific surface area of ​​the second ceramic powder and ρ is the density of the second ceramic powder.

[0026] The average particle size D1 of the first ceramic powder used here is required to be larger than the average particle size D2 of the second ceramic powder and the average particle size D3 of the third ceramic powder. The average particle size D1 of the first ceramic powder is not particularly limited, but for example, it is 20 μm or more, preferably 25 μm or more, and more preferably 30 μm or more. On the other hand, ceramic particles that are too coarse are undesirable because they can reduce the molding accuracy of the ceramic core. Therefore, D1 is generally 100 μm or less, more preferably 80 μm or less, and more preferably 60 μm or less. For example, the average particle size D1 of the first ceramic powder is preferably 20 μm or more and 100 μm or less. By using a first ceramic powder having such an average particle size D1, for example, a core with both strength and appropriate collapsibility can be obtained.

[0027] The average particle size D2 of the second ceramic powder used here is required to be smaller than the average particle size D1 of the first ceramic powder and the average particle size D3 of the third ceramic powder. The average particle size D2 of the second ceramic powder is not particularly limited, but for example, it is 25 nm or less, preferably 20 nm or less, and more preferably 15 nm or less. There is no particular limit to the lower limit of the average particle size D2 of the second ceramic powder, and for example, one of 1 nm or more, typically 5 nm or more can be used. The average particle size D2 of the second ceramic powder is preferably between 1 nm and 25 nm. By using a second ceramic powder having such an average particle size D2, for example, it can be more suitably arranged in the gaps of the first ceramic powder, thereby increasing the strength of the ceramic core.

[0028] The average particle size D3 of the third ceramic powder used here is required to be smaller than the average particle size D1 of the first ceramic powder and larger than the average particle size D2 of the second ceramic powder. The average particle size D3 of the third ceramic powder is not particularly limited, but for example, it is 15 μm or less, and preferably 10 μm or less. The lower limit of the average particle size D3 of the third ceramic powder is not particularly limited, but for example, it is 0.5 μm or more, typically 1 μm or more. The average particle size D3 of the third ceramic powder is preferably 0.5 μm or more and 15 μm or less. By using a third ceramic powder having such an average particle size D3, for example, when a ceramic slurry containing the third ceramic powder is applied to the surface of an additively manufactured object and fired, a moderately porous coating layer can be obtained.

[0029] The shape (outer form) of the first to third ceramic powders is not particularly limited. They may be spherical, or they may be non-spherical, such as elliptical, granular, or angular (e.g., crushed). From the viewpoint of mechanical strength and ease of manufacture, substantially spherical ceramic powders can be preferably used. For example, the aspect ratio of the first to third ceramic powders is preferably close to 1. For example, 1.3 or less is preferred, and 1.2 or less is more preferred. In this specification, the aspect ratio is the value obtained as (a / b) when the smallest rectangle circumscribing the first to third ceramic powders is drawn, with a being the longer side and b the shorter side.

[0030] There are no particular restrictions on the material and properties of the first to third ceramic powders. For example, the first to third ceramic powders may be either inorganic particles or organic-inorganic composite particles. As ceramic particles constituting the first to third ceramic powders, inorganic particles are preferred, and among these, particles made of metal or metalloid compounds are preferred. For example, in addition to oxide ceramics made of oxides of any element belonging to groups 1 to 14 of the periodic table (e.g., groups 4 to 14), non-oxide ceramics made of nitrides, carbides, borides, silicides, phosphoric acid compounds, etc. of various metal elements, and ceramic particles containing composite ceramics thereof as major components can be suitably used. Among these, ceramic particles containing oxides, nitrides, carbides, etc. containing any metal or metalloid element among Al, Zr, Mg, and Si as major components are preferred. Alternatively, ceramic particles containing metals or alloys thereof containing any element belonging to groups 1 to 13 of the periodic table (e.g., groups 4 to 13) as major components may be used.

[0031] Specifically, alumina (Al2O3), zirconia (ZrO2), magnesia (MgO), silica (SiO2), titania (TiO2), ceria (CeO2), yttria (Y2O3), hafnia (HfO2), barium titanate (BaTiO3), manganese dioxide (MnO2), lime (CaO), zinc oxide (ZnO), red iron oxide (Fe2O3), zircon (ZrSiO4), mullite (Al6O 13The ceramics may be oxide-based ceramics such as Si2, aluminum silicate, strontium oxide (SrO), barium oxide (BaO), and niobium oxide (Nb2O5), or non-oxide-based ceramics such as silicon nitride (Si3N4), boron nitride (BN), aluminum nitride (AlN), silicon carbide (SiC), and boron carbonitride, or composite materials containing at least one of these ceramics. Depending on the application of the ceramic core and the required properties, one of these ceramics may be used alone or in combination of two or more. Among these, silica, alumina, zircon, and magnesia are preferred for their excellent flame retardancy. The chemical formulas shown in parentheses after the names of the substances above indicate the representative composition of the substance and are not intended to limit the actual composition of the ceramic to those of the chemical formulas shown.

[0032] The first ceramic powder is the material that constitutes the core. Specific examples of particles that can be preferably used as this first ceramic powder include silica particles, alumina particles, zircon particles, and magnesia particles. These particles can be used individually or in combination of two or more. The content of the first ceramic powder in the central part is not particularly limited, but when the total amount of the central part is 100 parts by mass, it is usually 60 parts by mass or more, preferably 65 parts by mass or more, more preferably 75 parts by mass or more, for example 80 parts by mass or more, and typically 90 parts by mass or more, from the viewpoint of improving mechanical strength. The upper limit of the content of the first ceramic powder is not particularly limited, but is preferably 99 parts by mass or less, more preferably 98 parts by mass or less, for example 96 parts by mass or less. When the content of the first ceramic powder is within this range, the effects of this configuration can be exhibited at a higher level.

[0033] The second ceramic powder is a material for forming the first layer. A specific example of particles that can be preferably used as the second ceramic powder is silica particles. Using silica as the ceramic component is preferable because, for example, when such a layered ceramic core is used as a core, dissolution in an alkaline solution after casting can be done easily and quickly (i.e., it has excellent solubility). Furthermore, silica is also preferable because it can be obtained relatively inexpensively and easily in a form with more precisely adjusted particle size.

[0034] The third ceramic powder is a material for forming the second layer. Specific examples of particles that can be preferably used as the third ceramic powder include silica particles, alumina particles, zircon particles, and magnesia particles. These particles can be used individually or in combination of two or more.

[0035] The additively manufactured ceramic core disclosed herein is not limited by its manufacturing method, but can be suitably produced by, for example, the manufacturing method described below. That is, the manufacturing method for such an additively manufactured ceramic core comprises the following steps. (1) Prepare a central part (layer-formed fired body) by forming the first ceramic powder using an additive manufacturing method and firing it. (2) The central part (layer-formed fired body) is immersed in a ceramic sol containing the second ceramic powder, and the dispersion medium is removed by drying and heat treatment to form the first layer. (3) A ceramic slurry containing a third ceramic powder is coated onto the surface of the first layer, and the solvent is removed by high-temperature sintering to form a second layer.

[0036] (1) Preparation of the central area The central part of the additively manufactured ceramic core disclosed herein is prepared by an embodiment including, for example, the following method: an additive manufacturing powder containing a first ceramic powder is prepared, and an additively manufactured object is fabricated using this additive manufacturing powder in a conventionally known manner. The additive manufacturing powder may optionally contain components other than the first ceramic powder. Such components include binders and surfactants. Examples of binders include thermoplastic resins such as isobutylene resins, polyamide resins, polyester resins, polyether resins, polyvinyl alcohol resins, polyvinyl butyral resins, and polyethylene glycol resins, thermosetting resins such as melamine resins, and polysaccharides such as cellulose derivatives.

[0037] To improve the mechanical strength of the above-mentioned additively manufactured product, it may be immersed in a solution containing a compound that can be ceramicized by chemical reaction or heating. Examples of compounds that can be ceramicized by chemical reaction or heating include coupling agents composed of any of the following: a silane coupling agent containing silicon (Si), an aluminum coupling agent containing aluminum (Al), a titanium coupling agent containing titanium (Ti), and a zirconium coupling agent containing zirconia (Zr); and metal alkoxides composed of any of the following: a silane alkoxide containing silicon (Si), an aluminum alkoxide containing aluminum (Al), a titanium alkoxide containing titanium (Ti), and a zirconium alkoxide containing zirconia (Zr).

[0038] Furthermore, preparing the above-mentioned central part may include firing the additively manufactured object. For example, the manufactured additively manufactured object may be fired at a predetermined firing temperature (e.g., 1000°C to 1500°C). The above-mentioned manufacturing method and firing method are not characteristic of the present invention, so a detailed explanation is omitted.

[0039] The average erosion rate of the central layer prepared as described above is required to be higher than that of the first layer. In a preferred embodiment, the average erosion rate of the central layer is approximately 30 μm / g to 80 μm / g when 3 μm spherical alumina (MSE-BA-3-3, manufactured by Parmeso Co., Ltd.) is projected onto an MSE standard test piece (HRC-45, manufactured by Parmeso Co., Ltd.) at a projection power value that results in an erosion rate of 0.18 μm / g for the standard test piece. The average erosion rate of the central layer may be, for example, 40 μm / g or more, or 50 μm / g or more. The average pore diameter of the central layer is approximately 1 μm to 30 μm. The average pore diameter may be, for example, 5 μm or more, or 10 μm or more. The porosity of the central layer is approximately 30% to 60%. The porosity may be, for example, 35% or more, or 40% or more. Furthermore, the arithmetic surface roughness Ra of the central part may be, for example, 10 μm or more, typically 12 μm or more. While such a high erosion rate is advantageous in terms of collapse resistance, there is still room for improvement in terms of strength. Also, there may still be room for improvement in terms of surface roughness to smooth the surface of the metal casting. For this reason, the effects of applying the first and second layers of this configuration can be better realized. In this specification, unless otherwise specified, "average pore diameter" refers to the value measured using the mercury intrusion method. "Porosity" refers to the value calculated from the pore volume measured using the mercury intrusion method.

[0040] (2) Formation of the first layer A dispersion (ceramic sol) is prepared in which the second ceramic powder is dispersed in a dispersion medium. The second ceramic powder can be suitably introduced into the central pores through this ceramic sol. This ceramic sol is in a colloidal state (i.e., a colloidal solution) in which the second ceramic powder is uniformly suspended or independently suspended in the dispersion medium without agglomeration. Note that the term "colloidal solution" encompasses sols, suspensions, and the like.

[0041] The composition and properties of the second ceramic powder have been explained in detail above and will therefore be omitted here. The second ceramic powder is dispersed in a suitable dispersion medium. The dispersion medium is not particularly limited and either an aqueous solvent or a non-aqueous solvent may be used. As the aqueous solvent, water or a mixed solvent containing water is preferred. As the solvent other than water that constitutes the mixed solvent, one or more organic solvents (lower alcohols, lower ketones, etc.) that can be homogeneously mixed with water can be appropriately selected and used. For example, it is preferable to use an aqueous solvent in which 80% by mass or more (more preferably 90% by mass or more, and even more preferably 95% by mass or more) is water. A particularly preferred example is an aqueous solvent that is substantially water (e.g., water). In addition, if necessary, the dispersion may contain additives (stabilizers) such as dispersants and thickeners.

[0042] To facilitate the introduction of the second ceramic powder into the central pores, it is preferable to adjust the amount (concentration) of the second ceramic powder in the ceramic sol to approximately 10% by weight or more and 40% by weight or less. The ceramic sol in which the second ceramic powder is dispersed may be prepared by, for example, reacting a predetermined metal salt with dilute hydrochloric acid and performing dialysis, or it may be purchased commercially and used.

[0043] Next, the central part formed as described above is immersed in the ceramic sol to impregnate the pores in the central part with the ceramic sol and to introduce the second ceramic powder into the pores of the central part. The duration of this immersion in the ceramic sol of the central part cannot be generalized as it depends on the shape of the pores formed in the central part, as well as the concentration and viscosity of the ceramic sol, but it can be done for approximately 1 minute to 1 hour as a guideline.

[0044] The first layer disclosed herein can be obtained by removing the dispersion medium from the additively fabricated object after immersion as described above. Removal of the dispersion medium can be achieved by drying and heat treatment. Examples of drying methods include natural drying and forced-air drying. Heat treatment is performed by holding the object in an atmospheric environment at, for example, 400-500°C for 1-3 hours. This allows the first layer to be formed in at least a portion of the central part. Alternatively, the process of immersing the object in ceramic sol, drying, and heat treatment may be repeated multiple times to form the first layer.

[0045] In the ceramic core prepared above, comprising a central core and a first layer, the average erosion rate of the first layer is required to be lower than that of the central core and the second layer. In a preferred embodiment, the average erosion rate of the first layer is approximately 3 μm / g to 16 μm / g when 3 μm spherical alumina (MSE-BA-3-3, manufactured by Parmeso Co., Ltd.) is projected onto an MSE standard test piece (HRC-45, manufactured by Parmeso Co., Ltd.) at a projection power value that results in an erosion rate of 0.18 μm / g for the standard test piece. The average erosion rate of the first layer is preferably, for example, 13 μm / g or less, and more preferably 10 μm / g or less. Furthermore, the average pore diameter of the additively manufactured ceramic core comprising a central core and a first layer is generally 1 μm to 30 μm. The average pore diameter may be, for example, 5 μm or more, or 10 μm or more. The porosity of the additively manufactured ceramic core comprising a central core and a first layer is generally between 10% and 40%. Preferably, the porosity is 35% or less, and typically 30% or less. Furthermore, the arithmetic surface roughness Ra of the additively manufactured ceramic core comprising a central core and a first layer may be, for example, 10 μm or more, and typically 12 μm or more. In this way, by forming a first layer with a low erosion rate (i.e., high mechanical strength) on at least a portion of the central core, the strength of the additively manufactured ceramic core can be improved by the first layer while the central core maintains a moderate degree of collapsibility. However, there is still room for improvement regarding surface roughness. Therefore, the effect of further adding a second layer to the surface of the first layer can be achieved.

[0046] (3) Formation of the second layer A ceramic slurry is prepared in which a third ceramic powder is dispersed in a solvent. This ceramic slurry is used to form a porous coating layer. Typically, a porous second layer is formed by coating the surface of a first layer with the slurry and then firing it.

[0047] The composition and properties of the third ceramic powder have been explained in detail above and will therefore be omitted here. The solvent used in the ceramic slurry contained in the second layer is not particularly limited, as long as it can disperse the third ceramic powder described above. For example, an aqueous solvent can be used. As the aqueous solvent, water or a mixed solvent containing water is preferably used. As the solvent component other than water that constitutes such a mixed solvent, one or more organic solvents (lower alcohols, lower ketones, etc.) that can be uniformly mixed with water can be appropriately selected and used. For example, it is preferable to use an aqueous solvent in which 80% by mass or more (more preferably 90% by mass or more, and even more preferably 95% by mass or more) is water. A particularly preferred example is an aqueous solvent that is substantially composed of water. The solvent used in the ceramic slurry is not limited to an aqueous solvent and may also be a non-aqueous solvent (organic solvent). As a non-aqueous solvent, for example, alcohols such as ethyl alcohol and isopropyl alcohol can be used.

[0048] The ratio of the content of the third ceramic powder to the solvent in the ceramic slurry (third ceramic powder:solvent) is not particularly limited, but is preferably in the range of 1:5 to 5:1 by mass, and more preferably 1:4 to 4:1. Within this range of the content ratio of the third ceramic powder to the solvent, the aforementioned tax efficiency improvement effect can be better realized.

[0049] The ceramic slurry disclosed herein may contain a dispersant. The dispersant is an ingredient added for the purpose of stably dispersing the third ceramic powder in the slurry, and is typically a surfactant. Examples of dispersants include high molecular weight polycarboxylic acids. The content of the dispersant in the ceramic slurry is not particularly limited, but is usually appropriate to be between 0.1% and 3% by mass.

[0050] The ceramic slurry disclosed herein may further contain known additives such as thickeners, rust inhibitors, preservatives, and fungicides, as long as they do not impair the effects of the present configuration. The content of the above additives can be appropriately set according to their purpose of addition and is not a defining feature of the present invention, so a detailed explanation is omitted.

[0051] The method for preparing the ceramic slurry is not particularly limited. For example, the components contained in the ceramic slurry may be mixed using a well-known mixing method. The manner in which these components are mixed is not particularly limited; for example, all components may be mixed at once, or they may be mixed in an order set as appropriate.

[0052] Here, when firing the additively manufactured ceramic core after the second layer has been applied, it is necessary to fire the additively manufactured object at a high temperature of 1000°C or higher (preferably 1200°C or higher). For this reason, it is desirable to use firing jigs (e.g., topdressing sand) made of highly heat-resistant metal compounds such as alumina, mullite, cordulite, and silicon carbide when firing the additively manufactured object. However, firing jigs made of these metal compounds may react with the third ceramic powder contained in the ceramic slurry at firing temperatures of 1000°C or higher. If the third ceramic powder and the firing jig react, the third ceramic powder and the firing jig may adhere to each other, and when the fired object is removed from the firing jig, it may not be possible to obtain a ceramic core with good surface roughness. For this reason, it is desirable to appropriately suppress the reaction between the third ceramic powder contained in the ceramic slurry and the firing jig during the firing process. By such firing, an additively manufactured ceramic core having the central part, the first layer and the second layer disclosed herein can be produced.

[0053] In the additively manufactured ceramic core comprising the central core, the first layer, and the second layer, the average erosion rate of the second layer is required to be higher than that of the first layer. In a preferred embodiment, the average erosion rate of the second layer is approximately 20 μm / g to 60 μm / g when 3 μm spherical alumina (MSE-BA-3-3, manufactured by Parmeso Co., Ltd.) is projected onto an MSE standard test piece (HRC-45, manufactured by Parmeso Co., Ltd.) at a projection power value that results in an erosion rate of 0.18 μm / g for the standard test piece. The average erosion rate of the second layer is preferably, for example, 25 μm / g or more, and more preferably 30 μm / g or more. Furthermore, the average pore diameter of the additively manufactured ceramic core comprising the central core, the first layer, and the second layer is approximately 1 μm to 30 μm. The average pore diameter may be, for example, 5 μm or more, or 10 μm or more. The porosity of the additively manufactured ceramic core, comprising a central core, a first layer, and a second layer, is generally between 20% and 50%. The porosity of the second layer may be, for example, 25% or more, and is typically 30% or more. Furthermore, the arithmetic surface roughness Ra of the additively manufactured ceramic core, comprising a central core, a first layer, and a second layer, may be, for example, 15 μm or less, and typically 10 μm or less. Such an additively manufactured ceramic core may have a surface roughness sufficient for use as a core when manufacturing metal castings.

[0054] The following describes examples relating to the present invention, but it is not intended to limit the present invention to those shown in the examples.

[0055] As the first ceramic powder, a mixed powder of silica, zircon, and alumina (average particle size D1: 34 μm) was prepared. The weight ratio of silica, zircon, and alumina was 75:23:2. This mixed powder and PVA (Kurarepovar 205) as a binder were weighed in a mass ratio of 90:10 and mixed in a dry mixer for 20 minutes to prepare the additive manufacturing powder. This additive manufacturing powder was placed in a 3D Systems ProJet460Plus and an additively shaped object (8 mm wide x 40 mm deep x 6 mm thick) was fabricated and dried at room temperature for 16 hours and at 65°C for 1 hour. Next, a coupling solution containing a coupling agent (3-aminopropyltriethoxysilane) was prepared, and the fabricated object was impregnated in the coupling solution for 1 minute, and then dried at room temperature for 1 hour and at 65°C for 1 hour. The laminated material, after drying, was fired at 1250°C in alumina sand to obtain the central part (laminated laminated body) (Comparative Example 1).

[0056] Next, a silica sol (silica concentration 20% by weight, average particle size D2: 10 nm) was prepared as a ceramic sol containing the second ceramic powder. The core (additive-formed sintered body) was immersed in the silica sol for 25 minutes, then dried at room temperature for 1 hour and at 80°C for 1 hour. After drying, heat treatment was performed at 450°C to obtain an additive-formed body consisting of the core and the first layer. The process of immersion in silica sol, drying, and heat treatment was repeated four times. This was designated as Comparative Example 2.

[0057] Next, a mixed powder of silica, zircon, and alumina (average particle size D3: 6.7 μm) was prepared as the third ceramic powder. The weight ratio of silica, zircon, and alumina was 75:23:2. A ceramic slurry was prepared by mixing this mixed powder with a high-molecular-weight polycarboxylic acid as a dispersant and ethanol as a solvent. This slurry was applied to the surface of the additively fabricated object (Comparative Example 2), which consisted of the above-mentioned central core and the first layer, by dip-coating, and then dried. The dried additively fabricated object was fired at 1300°C in alumina sand to obtain an additively fabricated ceramic core (Example 1).

[0058] <Surface observation using a scanning electron microscope> The surfaces of the above examples and comparative examples were observed using a scanning electron microscope (JSM-6610LA) manufactured by JEOL Ltd. SEM images were acquired for each at a magnification of 50x. Figures 1 to 3 are examples of SEM images, with Figure 1 being from Example 1, Figure 2 from Comparative Example 1, and Figure 3 from Comparative Example 2.

[0059] <Observation of cross-sections using a scanning electron microscope> SEM images were obtained at a magnification of 50x in the same manner for the cross-section perpendicular to the surface (cross-section along the thickness direction) of Example 1 and Comparative Example 2. Figures 4 and 5 are examples of SEM images, with Figure 4 being from Example 1 and Figure 5 being from Comparative Example 2.

[0060] <Calculation of average erosion rate> Brittleness tests were conducted using MSE-A203 manufactured by Parmeso. Three specimens each of Example 1 and Example 1 (cross-section), which was obtained by cutting Example 1 along the width direction at a position of 4 mm along the thickness direction, were prepared as test specimens. The brittleness test was performed using 3 μm spherical alumina (MSE-BA-3-3, manufactured by Parmeso Co., Ltd.) as projection particles, at a projection power value that resulted in an erosion rate of 0.18 (μm / g) for the MSE standard test specimen (HRC-45, manufactured by Parmeso Co., Ltd.). The projection particles were continuously projected from the surface (0 μm) of Example 1 to a depth of 180 μm, and from the surface (0 μm) of Example 1 (cross-section) to a depth of 180 μm (thickness direction). The relationship between the projection amount (Ag) of 3 μm spherical alumina and the erosion depth (B μm) was obtained at 3 to 20 locations. Based on this value, the erosion rate was calculated using the following formula: Erosion rate (μm / g) = B / A. The erosion rate was calculated from the value measured at a position 60 μm from the surface (0 μm) of Example 1, and the average value of this was taken as the average erosion rate of the second layer. The erosion rate was calculated at a distance of 100 μm to 180 μm from the surface of Example 1, and the average value of these measurements was taken as the average erosion rate of the first layer. The erosion rate calculated at a position 180 μm from the surface (0 μm) of Example 1 (cross section) was determined, and the average value of this value was taken as the average erosion rate of the center section. The brittleness test was carried out three times with different samples under the same conditions, and the average value at this time was shown in Table 1 as the average erosion rate.

[0061] <Evaluation of surface roughness Ra> The surface roughness Ra of each surface of the above Examples and Comparative Examples was calculated. The surface roughness Ra was calculated as the arithmetic mean roughness (μm) from the roughness curve at a cut-off value of 2.5 mm in accordance with JIS B0601:1982 using Surfcom manufactured by Tokyo Seimitsu Co., Ltd. (see FIGS. 6A to 6C). The scanning distance of the surface properties was set to 10 mm. The results are shown in Table 2.

[0062] <Three-point bending strength> The three-point bending strength of each of the above Examples and Comparative Examples was measured. The three-point bending strength was measured in accordance with JIS B1601:2008 using a three-point bending tester (EZ-TEST) manufactured by Shimadzu Corporation. The results are shown in Table 2.

[0063] <Porosity and average pore diameter> The porosity and average pore diameter of the above Examples and Comparative Examples were measured. For the measurement of the porosity and average pore diameter, an AutoPore V9600 manufactured by Micromeritics was used and measured by the mercury intrusion method. The results are shown in Table 2.

[0064] <Analysis of the elemental distribution of Si, Al, and Zr by EDX> Using the scanning electron microscope (JSM-6610LA) described above, energy dispersive X-ray analysis (EDX) was performed at positions of 0.15 mm (the first layer), 1 mm (center section), and so on, from the surface of a cross section perpendicular to the surface (a cross section along the thickness direction) in Example 1. The results of oxide weight conversion are shown in Table 3.

[0065] As shown in Figure 4, it was confirmed that Example 1 has a first layer covering the central part and at least a portion thereof, and a second layer formed on the surface of the first layer. It was also confirmed that the second layer had a moderate porosity and covered the layering steps observed in Figures 3 and 5.

[0066] [Table 1]

[0067] As shown in Table 1, it was confirmed that the average erosion rate of the first layer was lower than that of the central layer, and that the average erosion rate of the second layer was higher than that of the first layer. Furthermore, the average erosion rates of the central and second layers were higher than those of the first layer, with the central layer having an average erosion rate more than five times higher than that of the first layer, and the second layer having an average erosion rate more than 2.5 times higher than that of the first layer. A laminated ceramic core with such average erosion rates can be a ceramic core that achieves both strength and collapsibility.

[0068] [Table 2]

[0069] As shown in Table 2 and Figures 6A-6C, Example 1 had a surface roughness Ra of 10 μm or less, which was a significant improvement compared to Comparative Examples 1 and 2. Also, as shown in Table 2, Example 1 and Comparative Example 2 showed a significant improvement in strength compared to Comparative Example 1. Example 1, which included a second layer, showed even greater strength compared to Comparative Example 2. In other words, it was confirmed that Example 1, which consisted of a central part, a first layer, and a second layer, showed improved surface roughness Ra and strength compared to Comparative Example 1, which consisted only of a central part, and Comparative Example 2, which consisted of a central part and a first layer.

[0070] [Table 3]

[0071] As shown in Table 3 and Figure 4, it was confirmed that silica was unevenly distributed at a position of 0.15 mm from the surface (first layer) in Example 1 compared to 1 mm from the surface (center) and 4 mm from the surface (center). By having a first layer in which silica is unevenly distributed, the additively fabricated ceramic core can exhibit appropriate meltability.

[0072] Although a detailed explanation has been given above with specific embodiments, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes to the specific examples illustrated above.

Claims

1. A laminated ceramic core used as a core when manufacturing metal castings, It is composed of a central part which is a laminated and fired body of a predetermined ceramic powder, a first layer which covers at least a part of the central part, and a second layer which is formed on the surface of the first layer. Here, when the average erosion rate, which is the average value of the erosion rates calculated by the following formula: Erosion rate (μm / g) = B / A; in the brittleness test for the additively manufactured ceramic core, where the amount of projected particles is Ag and the erosion depth is Bμm, is used, A laminated ceramic core in which the average erosion rate of the first layer is lower than the average erosion rate of the central part, and the average erosion rate of the second layer is higher than the average erosion rate of the first layer.

2. The average erosion rate of the central part is five times or more the average erosion rate of the first layer. The additively manufactured ceramic core according to claim 1, wherein the average erosion rate of the second layer is 2.5 times or more the average erosion rate of the first layer.

3. The additively fabricated ceramic core according to claim 1 or 2, wherein the arithmetic mean surface roughness Ra of the second layer is 10 μm or less.

4. The additively fabricated ceramic core according to any one of claims 1 to 3, wherein the central part and the second layer are each composed of at least one selected from the group consisting of silica, alumina, zircon, and magnesia.

5. The additively fabricated ceramic core according to any one of claims 1 to 4, wherein the first layer comprises silica as a main component.

6. A method for manufacturing a laminated ceramic core comprising a central part, a first layer covering at least a portion of the central part, and a second layer formed on the surface of the first layer, To fabricate an additively manufactured object using a first ceramic powder having an average particle size D1, The aforementioned laminated material is fired to obtain the central part which is a laminated fired body. The additively manufactured and fired body is immersed in a ceramic sol containing a second ceramic powder having an average particle size D2 to form the first layer on at least a portion of the additively manufactured and fired body. The laminated sintered body, to which the first layer has been applied, is immersed in a ceramic slurry containing a third ceramic powder having an average particle size D3, thereby forming the second layer on the surface of the first layer. It includes, A method for manufacturing an additively fabricated ceramic core, wherein the average particle sizes D1, D2, and D3 of the first ceramic powder, the second ceramic powder, and the third ceramic powder are D1 > D3 > D2.

7. The method for manufacturing a laminated ceramic core according to claim 6, wherein the arithmetic mean surface roughness Ra of the second layer is 10 μm or less.

8. The method for manufacturing an additively fabricated ceramic core according to claim 6 or 7, wherein the first ceramic powder and the third ceramic powder each consist of at least one selected from the group consisting of silica, alumina, zircon, and magnesia.

9. The method for manufacturing an additively fabricated ceramic core according to any one of claims 6 to 8, wherein the second ceramic powder contains silica as a main component.

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