KR101763122B1 - Manufacturing method of ceramic core, ceramic core, precision casting method and precision casting products - Google Patents

Abstract

The present invention relates to a method to manufacture a ceramic core with improved dimensional stability. According to the present invention, the method comprises: (S1) a step of manufacturing a ceramic mixture including fused silica powder, zircon powder, and wax; (S2) a step of pouring the ceramic mixture in a mold to manufacture a molded body; and (S3) a step of sintering the molded body at high temperature to manufacture the ceramic core. In a high temperature sintering process, an amount of crystallization of fused silica is adjusted based on an X-ray diffraction (XRD) analysis peak of cristobalite formed by crystallization of the fused silica and zircon. According to the present invention, since an amount of crystallization of the fused silica incapable of being directly measured or digitized is digitized based on zircon which is a crystalline material, the dimensional stability of the ceramic core is indirectly evaluated and determined based on an amount of cristobalite crystallized from the fused silica in the ceramic core using the fused silica; thereby providing the manufacture of the ceramic core having excellent dimensional stability and strength.

Description

Technical Field [0001] The present invention relates to a method of manufacturing a ceramic core, a ceramic core produced by the method, a precision casting method, and a precision casting product manufactured thereby.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a ceramic core and a ceramic core produced thereby, and more particularly, to a method of manufacturing a ceramic core having improved dimensional stability and a ceramic core.

Generally, a ceramic core (core) is inserted into a mold at the time of precise casting for manufacturing a high-temperature part (for example, a blade, a vane, etc.) for a gas turbine having a cooling flow path. By melting the ceramic core after casting, it is a core material that has a direct relationship with the improvement of the efficiency of the gas turbine together with the super-heat-resistant alloy for high-temperature components because it can manufacture hollow turbine parts.

Such a ceramic core has been studied for its strength and excellent elution characteristics that can withstand molten metal in precision casting process.

Since the ceramic core is placed in a high temperature state during the precise casting process and it undergoes a large temperature change, there is a problem that the dimension of the ceramic core is changed by heat shrinkage and thermal expansion, ) Is required.

However, since there is no standard for quantifying and evaluating the dimensional stability, there is no technique for improving the dimensional stability in addition to the application of the method using the fused silica known to have a low coefficient of thermal expansion.

Korean Patent No. 10-1481479

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems of the prior art, and it is an object of the present invention to provide a ceramic core having improved dimensional stability and a ceramic core.

According to an aspect of the present invention, there is provided a method of manufacturing a ceramic core, comprising: (S1) preparing a ceramic mixture comprising a fused silica powder, a zircon powder and a wax; Pouring the ceramic mixture into a mold to produce a molded body (S2); And a step (S3) of preparing a ceramic core by high-temperature sintering the formed body, wherein the fused silica in the high-temperature sintering step is fused by melting based on the XRD analysis peak of the crystallized cristobalite and the XRD analysis peak of the zircon, And the amount of crystallization of silica is controlled.

The inventors of the present invention have confirmed that the dimensional stability of a ceramic core can be indirectly evaluated and judged based on the amount of fused silica crystallized in cristobalite in a ceramic core using fused silica and can be directly measured or quantified by melting A method of manufacturing a ceramic core having excellent dimensional stability and strength was developed by quantifying the amount of silica crystallized based on zircon as a crystalline material.

In this case, in the step S1, the weight ratio of the fused silica powder to the zircon powder is in the range of 2: 1 to 4: 1, and the intensity ratio of the XRD analysis peak of the cristobalite crystallized fused silica to the XRD analysis peak of the zircon is in the range of 0.05 to 0.7 . When the weight ratio of the fused silica powder to the zircon powder is 3: 1, when the intensity ratio of the XRD analysis peak of the crystallized cristobalite to the XRD analysis peak of the zircon falls within the range of 0.08 to 0.5, the fracture strength and dimensional stability An excellent ceramic core can be manufactured and the intensity ratio of the XRD analysis peak of the cristobalite and the XRD analysis peak of the zircon which is the crystallized fused silica according to the change of the weight ratio of the fused silica powder and the zircon powder is partially changed.

In step S1, the fused silica powder comprises 10 to 20% of fused silica powder having a particle size of 0.2 to 0.5 mm; 10 to 20% of fused silica powder having a particle size of 0.1 to 0.2 mm; 10 to 20% fused silica powder having a particle size of 0.025 to 0.045 mm; 20 to 30% of fused silica powder having a particle size of 0.010 to 0.025 mm; 20 to 30% of fused silica powder having a particle size of 0.001 to 0.005 mm; And 10 to 20% of fused silica powder having a particle size of 0.0004 to 0.0005 mm.

In the step S1, it is preferable that the particle size of the zircon powder is 0.001 to 0.005 mm and the wax is in the range of 5 to 40% by weight based on the total weight of the ceramic mixture.

In step S1, the wax may be selected from the group consisting of paraffin wax, microcrystalline wax, polyolefin wax, beeswax, carnauba wax, and mixtures thereof. The wax may be prepared by mixing two kinds of waxes having different melting points in a ratio of 6: 4 to 9: By weight based on the total weight of the composition.

In the step S1, the ceramic mixture may further contain additives selected from the group consisting of alumina, silicon carbide, polyvinyl alcohol, cellulose, polyvinyl butyral, acrylate and ethyl silicate.

A ceramic core according to another aspect of the present invention is manufactured by molding and high-temperature sintering a ceramic mixture composed of fused silica powder, zircon powder and wax, wherein the fused silica has an XRD analysis peak of crystallized cristobalite and an XRD analysis peak of zircon And the amount of cristobalite is controlled based on the intensity ratio. In this case, the weight ratio of the fused silica powder to the zircon powder used in the manufacturing process is in the range of 2: 1 to 4: 1, the XRD analysis peak of the cristobalite in which the fused silica is crystallized and the XRD analysis peak ratio of the zircon are in the range of 0.05 - 0.7, the dimensional stability of the ceramic core is excellent. The ceramic core preferably has a line shrinkage of 3.5% or less and a fracture strength of 10 MPa or more.

According to another aspect of the present invention, there is provided a precision casting method comprising the steps of: preparing a ceramic core by the above manufacturing method; Forming a product-shaped wax pattern on the exterior of the ceramic core; Forming a shell mold using the wax patterned ceramic core; And casting the product using the shell mold.

The precision casting method of the present invention has the effect of improving the precision of the manufacturing process by applying the ceramic core manufacturing method with improved dimensional stability.

The precision casting product according to another embodiment of the present invention is manufactured by the above-described precision casting method and is manufactured using a ceramic core having excellent dimensional stability, thereby being characterized by high precision.

The present invention constituted as described above makes it possible to quantify the amount of fused silica crystallized into cristobalite in a ceramic core using fused silica by quantifying based on the amount of crystallized fused silica which can not be directly measured or quantified, The dimensional stability of the ceramic core can be indirectly evaluated and judged based on the criteria, and the ceramic core having excellent dimensional stability and strength can be manufactured.

Fig. 1 shows the results of measurement of fracture strength and line shrinkage of the ceramic core produced in Example 1. Fig.
FIG. 2 is an electron micrograph of the microstructure of the ceramic core produced in Example 1. FIG.
Fig. 3 shows the results of XRD analysis of the ceramic core produced in Example 1. Fig.
FIG. 4 shows the results of measurement of fracture strength and line shrinkage of the ceramic core produced in Example 2. FIG.
5 is a result of XRD analysis of the ceramic core produced in Example 2. Fig.
6 is a graph showing changes in strength and shrinkage according to the ratio of the peak intensity of cristobalite to that of zircon in the ceramic cores produced by Example 1 and Example 2. Fig.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the accompanying drawings, embodiments of the present invention will be described in detail.

≪ Example 1 >

As a first step (S1) for producing a ceramic core, a ceramic mixture (feedstock), which is a raw material for injection molding, is prepared by mixing materials such as silica powder, zircon powder and wax.

The flowability and powder packing density of the feedstock for the injection of the ceramic core were increased by mixing various sizes of fused silica as the silica powder, and the fused silica powder and the zircon powder were mixed with 3 : 1. (Nippon-Seiro, Japan) and microcrystalline wax (Nippon-Seiro, Japan) were used as thermoplastic binders and stearic acid (C ₁₉ H ₃₆ O ₂ , Samchun Pure Chemical, Korea) as a lubricant. The feedstock was prepared by mixing oleic acid (C ₁₉ H ₃₄ O ₂ , Samchun Pure Chemical, Korea) with the fused silica and zircon powder prepared above. The content of the fused silica and zircon mixed powder contained in the feedstock was fixed to 85 wt%.

Table 1 shows the particle size and content of fused silica and zircon mixed powder.

In the feedstock manufacturing process, uniform mixing of the fused silica and zircon mixed powder was performed using a zirconia ball having a diameter of 6 mm for 6 hours at room temperature, and the mixed powder after ball milling was fused with a thermoplastic binder in a vacuum planetary vacuum mixer for 6 hours.

In the next step (S2), the ceramic mixture is poured into a mold to produce a molded body.

In this embodiment, injection molding, which is most commonly used for manufacturing a ceramic core, is performed for the production of a molded body. The ceramic mixture (feedstock) obtained in the step S1 is introduced into a ceramic injection molding machine (CTM-CI-CF- Cleveland, United States) and stirred at 80 ° C for 3 hours. Injection molding was carried out under the conditions shown in Table 2.

The ceramic core manufactured in this embodiment is used for a two-stage blade for a gas turbine. In order to measure the physical properties of the specimen, a hexagonal specimen having a size of 6 × 8 × 90 mm (ASTM C1161) Respectively.

As the last step (S3) for producing a ceramic core, a ceramic core was produced by sintering the molded body at a high temperature.

The injected molded body was heat treated in a state where it was placed in a fused silica powder to prevent the thermoplastic binder from being degreased and collapsed at the time of firing, and the degreasing process and the firing process for the heat treatment of the ceramic core were continuously performed It went on.

First, the temperature of the binder was raised to 300 deg. C, which is a temperature for degreasing the binder, at a rate of 0.2 K / min. After degreasing, the molded article was heat-treated at a temperature range of 1150 to 1400 deg.

The heat treated specimens were wax patterned in the form of castings using a wax injection molding machine (MPI, United States), then shell molds for precision casting were fabricated and fired at temperatures above 1000 ° C.

The fracture strength of the ceramic core samples prepared according to the above examples was measured using a universal testing machine (UTM, H10SK, Hounsfield, England) with a three-point bending fixture. The length of the specimen before and after the heat treatment was measured to confirm the linear shrinkage of the specimen. The microstructures of the ceramic cores after firing were observed using a scanning electron microscope (FE-SEM, S-4700 and S-4800, Hitachi, Japan) and analyzed using an X-ray diffractometer XRD, D / Max-2200, Rigaku, Japan). X-ray nondestructive testing was also carried out to check whether the ceramic cores were defective or damaged after wax patterning and shell mold sintering.

Fig. 1 shows the results of measurement of fracture strength and line shrinkage of the ceramic core produced in Example 1. Fig.

The fracture strength was selected as the physical property reflecting the characteristics of the ceramic core in the precision casting process and the linear shrinkage was selected as the physical property reflecting the dimensional stability characteristic that the size of the ceramic core does not change during the precision casting process.

When the ceramic core was sintered at 1150 ℃ for 12 hours, it showed low linear shrinkage and fracture strength was slightly larger than 10 MPa. When sintered at 1200 ℃ for 12 hours, it showed the highest value of 22 MPa and the linear shrinkage was about 2.5%. As the temperature is increased, the linear shrinkage ratio increases steadily, and the strength increases to 1200 ° C, and then decreases.

FIG. 2 is an electron micrograph of the microstructure of the ceramic core produced in Example 1. FIG.

Microcracks were not observed in the silica region of the ceramic core heat treated at 1200 ° C, but microcracks indicated by the arrows in the silica core were observed in the ceramic core heat treated at 1300 ° C and 1400 ° C. .

These microcracks are expected to occur in the crystallization process of amorphous fused silica. The fused silica is crystallized into β-cristobalite having a cubic system structure at 1300 ° C. or higher, and the produced β-cristobalite is crystallized at a temperature of 300 ° C. to 200 ° C. in the cooling process after the heat treatment, Cristobalite having a structure of [alpha] -cristobalite. At this time, 5% volume shrinkage occurs due to the size difference existing in the unit cell of each crystal phase, and microcracks are generated due to the stress generated at the time of contraction. As a result, the strength of the ceramic core is lowered and the shrinkage ratio is increased by such microcracks.

In micrographs, microcracks are mostly formed on the surface of silica particles, because the crystallization of cristobalite in fused silica occurs from the surface.

Fig. 3 shows the results of XRD analysis of the ceramic core produced in Example 1. Fig.

As shown, the ceramic cores heat-treated at 1200 ° C or less had no or very weak cristobalite peaks, but the cristobalite peaks were strongly observed in the case of the ceramic cores annealed at 1300 ° C or higher. From this, it can be confirmed that the cause of the decrease in the strength and the shrinkage rate of the ceramic core is due to the formation of cristobalite.

From the above results, it can be seen that the shrinkage ratio (dimensional stability) of the ceramic core can be indirectly evaluated and judged based on the amount of the fused silica crystallized in cristobalite in the ceramic core using the fused silica. However, there is a problem that the amount of crystallized fused silica can not be directly measured or quantified, but the present invention has developed a method of quantifying the amount of crystallized fused silica based on zircon, which is a crystalline material used together with fused silica . Specifically, in the XRD analysis, the peaks (21.68 °, (110), JCPDS no. 39-1425) of the cristobalite crystal phase in which the fused silica crystallized and the peak of the crystalline zircon (26.86 °, (200) -266), the amount of crystallized fused silica was quantified.

Table 3 shows the ratio between the peak intensity of cristobalite, which is a crystalline phase in which the fused silica crystallized in the XRD analysis of FIG. 2, and the peak intensity of crystalline zircon.

≪ Example 2 >

In the step (S3) of manufacturing the ceramic core by the high temperature sintering of the formed body by the same process as the above-mentioned step, the heat treatment was performed at the same temperature of 1200 캜 for 6 to 48 hours.

FIG. 4 shows the results of measurement of fracture strength and line shrinkage of the ceramic core produced in Example 2. FIG.

The fracture strength and shrinkage ratio with increasing firing time showed similar tendency to physical properties (Fig. 1) as a function of firing temperature. The ceramics core fired for 6 hours had a low shrinkage but a low fracture strength of less than 10 MPa and showed the highest fracture strength values after 12 hours of heat treatment. On the other hand, in the case of firing over 24 hours, the fracture strength decreased and the shrinkage rate increased as the firing time increased, as in the case of firing at 1300 ° C. or higher.

5 is a result of XRD analysis of the ceramic core produced in Example 2. Fig.

As a result of X-ray diffraction analysis, when the firing time was increased to 24 hours or more, it was confirmed that the fused silica was crystallized and the peak due to cristobalite was rapidly increased as in the case where the firing temperature was increased. It can be confirmed that crystallization of the fused silica in the ceramic core is promoted when the firing time is prolonged at 1200 ° C, resulting in lowering of the strength and shrinkage ratio. As a result, the change of the strength and shrinkage rate according to the firing time also causes the fused silica to crystallize, As shown in FIG.

The intensity and shrinkage according to the peak intensity ratio of crystalline cristobalite peak and crystalline zircon crystal phase in which the fused silica crystallized in the XRD analysis applied as a criterion for evaluating the amount of crystallization of fused silica into cristobalite in the ceramic core using the fused silica in the present invention The ceramic cores of Example 1 and Example 2 are shown in Fig. 6, and these are summarized in Table 4.

As the C / Z ratio of the cristobalite peak to the zircon peak increases, the fracture strength decreases and the linear shrinkage increases. This is because the ratio of the peak of cristobalite to the peak of zircon is higher than that of fused silica And reflects the degree of crystallization accurately. At this time, when the ratio of the peak of cristobalite to the peak of zircon is in the range of 0.08 to 0.8, it can be confirmed that it has a generally known fracture strength range. However, when the linear shrinkage is taken as a reference, the ratio of the peak of cristobalite to the peak of zircon is In the range of 0.08 ~ 0.5, the linear shrinkage was 3.5% or less, indicating that the dimensional stability is excellent. Furthermore, when the ratio of the peak of cristobalite to the peak of zircon is in the range of 0.08 to 0.3, a ceramic core having a linear shrinkage of 3.0% or less can be produced.

From the above results, it can be confirmed that the dimensional stability is more easily evaluated in the case where the degree of crystallization is determined by the ratio between the peak of cristobalite and the peak of zircon as in the present invention.

While the present invention has been particularly shown and described with reference to preferred embodiments thereof, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. Those skilled in the art will understand. Therefore, the scope of protection of the present invention should be construed not only in the specific embodiments but also in the scope of claims, and all technical ideas within the scope of the same shall be construed as being included in the scope of the present invention.

Claims (15)

(S1) producing a ceramic mixture comprising a fused silica powder, a zircon powder and a wax;
(S2) pouring the ceramic mixture into a mold to produce a molded body; And
(S3) high-temperature sintering the shaped body to produce a ceramic core,
Wherein the amount of crystallization of the fused silica is controlled based on an XRD analysis peak of cristobalite, which is a crystalline material in which the fused silica is crystallized during the high-temperature sintering process, and an intensity ratio of an XRD analysis peak of the zircon.
The method according to claim 1,
The weight ratio of the molten silica powder to the zircon powder in the step S1 is in the range of 2: 1 to 4: 1,
Wherein an intensity ratio of the XRD analysis peak of the crystallized cristobalite to the XRD analysis peak of the zircon is in the range of 0.05 to 0.7.
[Claim 3 is abandoned upon payment of the registration fee.] The method according to claim 1,
In the step (S1), the fused silica powder may contain,
A fused silica powder having a particle size of 0.2 to 0.5 mm;
A fused silica powder having a particle size of 0.1 to 0.2 mm;
A fused silica powder having a particle size of 0.025 to 0.045 mm;
A fused silica powder having a particle size of 0.010 to 0.025 mm;
A fused silica powder having a particle size of 0.001 to 0.005 mm; And
And a fused silica powder having a particle size of 0.0004 to 0.0005 mm.
The method of claim 3,
In the step (S1), the fused silica powder may contain,
10 to 20% fused silica powder having a particle size of 0.2 to 0.5 mm;
10 to 20% of fused silica powder having a particle size of 0.1 to 0.2 mm;
10 to 20% fused silica powder having a particle size of 0.025 to 0.045 mm;
20 to 30% of fused silica powder having a particle size of 0.010 to 0.025 mm;
20 to 30% of fused silica powder having a particle size of 0.001 to 0.005 mm; And
And 10 to 20% of fused silica powder having a particle size of 0.0004 to 0.0005 mm.
The method according to claim 1,
Wherein the zircon powder in the step (S1) has a particle size of 0.001 to 0.005 mm.
The method according to claim 1,
Wherein the wax is in the range of 5 to 40% by weight based on the total weight of the ceramic mixture in the step S1.
The method according to claim 1,
Wherein the wax is selected from the group consisting of paraffin wax, microcrystalline wax, polyolefin wax, beeswax, carnauba wax, and mixtures thereof.
The method of claim 7,
Wherein the wax is a mixture of two kinds of waxes having different melting points in a weight ratio of 6: 4 to 9: 1.
The method according to claim 1,
Wherein the ceramic mixture further comprises an additive selected from the group consisting of alumina, silicon carbide, polyvinyl alcohol, cellulose, polyvinyl butyral, acrylate and ethyl silicate in the step S1 .
Which is prepared by molding and hot sintering a ceramic mixture composed of fused silica powder, zircon powder and wax,
Wherein the amount of crystallization of the fused silica in the high-temperature sintering process is controlled based on the XRD analysis peak of cristobalite, which is a crystalline material in which the fused silica is crystallized, and the intensity ratio of the XRD analysis peak of the zircon.
The method of claim 10,
The weight ratio of the fused silica powder to the zircon powder is in the range of 2: 1 to 4: 1,
Wherein an intensity ratio of an XRD analysis peak of the cristobalite crystallized in the fused silica to an XRD analysis peak of the zircon is in a range of 0.05 to 0.7.
The method of claim 10,
Wherein a linear shrinkage of the ceramic core is 3.5% or less.
The method of claim 10,
Wherein the ceramic core has a fracture strength of 10 MPa or more.
Preparing a ceramic core having controlled crystallization amount of fused silica by the method of claim 1;
Forming a product-shaped wax pattern on the exterior of the ceramic core;
Forming a shell mold using the wax patterned ceramic core; And
And casting the product using the shell mold.
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