JP2013071169A - Ceramic core for precision casting, and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ceramic core for precision casting, withstanding a strength load received in each process of casting and excelling in an elution property of the ceramic core after casting at the same time.SOLUTION: A sintered body structured to have high density at the outermost surface part of the ceramic core and to incline to low density toward the inside is used as the ceramic core. The outermost surface part is made high in density to have an effect of securing strength, and since the inside maintains a porous property, the elution property is excellent. The use of this ceramic core enables an improvement in the yield of a cast product.

Description

  The present invention relates to a ceramic core for precision casting used for forming an internal cooling passage such as a gas turbine blade manufactured by precision casting, and a method for manufacturing the same.

  In order to improve the power generation efficiency of energy equipment typified by gas turbines, the combustion gas temperature has been increasing year by year. Currently, the turbine inlet temperature is mainly 1300 ° C, but research and development of members aiming at the 1700 ° C class is underway. In the gas turbine blade manufacturing field, a unidirectional solidification process and a single crystal growth process in which crystal grain boundaries, which cause a decrease in strength reliability at high temperatures, are reduced as much as possible. The feature of these manufacturing methods is that the process is performed at a high temperature for a long time as compared with the conventional casting method in which a conventional molten metal is cast. The gas turbine blade tends to use a Ni-based alloy, which is an ultra-high heat resistant alloy, and a cooling gas flow path is provided inside the gas turbine blade for reducing the surface temperature. A ceramic core is used to form this gas flow path.

  A gas flow path forming process using a ceramic core will be briefly described with reference to FIG. A ceramic core is placed inside a turbine blade-shaped mold, and wax is injected into this mold to produce a turbine blade wax model including the ceramic core. A mold made of a heat-resistant material is formed on this, and then the wax is lost with high-temperature steam (lost wax). A Ni-based alloy is cast on the mold in this state by various processes such as ordinary casting and unidirectional solidification. Thereafter, the core inside the turbine blade is eluted with a chemical method, specifically, a strong alkaline solution.

  In other words, the characteristics required for ceramic cores for precision casting are that there is no trouble such as deformation when interacting with wax injection or molten metal, that is, some strength is required. These include a point that can be eluted from the inside and a point that is made of a material that does not react with the Ni-based alloy. From this point, the ceramic core for precision casting in this field is a ceramic member that is substantially composed of fused silica powder and controlled to a relative density of about 70% by molding and sintering it. It is known.

  The ceramic core for precision casting as described above is necessary only for casting, and after that it requires a special function to be excluded from the casting, so it is porous that is advantageous for elution and has a certain degree of strength. Is required. As a certain level of strength, the bending strength is approximately 10 MPa as one standard. In order to develop strength, it is advantageous to improve the density by heat treatment at a high temperature. However, when the density exceeds, for example, 90%, there is sufficient reliability in terms of strength, but elution is a subsequent process. It takes a lot of time to cause deterioration of the alkaline solution used for elution, and there is a problem in terms of blade productivity. The density is expressed as a relative density with respect to the theoretical density determined by the ceramic material used. Conversely, if the sintered compact density is kept low, elution is improved, but strength reliability cannot be guaranteed, and core deformation is likely to occur during wax model production, mold production, and casting. The shape of the passage changes greatly, and problems such as significant reduction in cooling efficiency occur. In addition, there is a high possibility that the core breaks during the manufacturing process. In other words, the ceramic core for precision casting requires two points of robustness that is porous and does not cause deformation of the core itself, in other words, strength, in order to ensure dissolution.

Japanese Patent Laid-Open No. 9-67662

  In order to deal with such a problem, the conventional technique addresses this problem by making the entire ceramic core uniform and controlling the sintering density. Such a conventional technique is difficult to cope with when the thermal environment loaded on the environment becomes severe.

  Accordingly, an object of the present invention is to provide a ceramic core for precision casting that is excellent in elution in a subsequent process while ensuring strength as a core.

  The ceramic core for precision casting of the present invention is a ceramic core for precision casting containing 80% by weight or more of fused silica, and the sintered density of the core surface portion in contact with the molten metal is 75 of the theoretical density of the core. It is characterized by comprising a surface layer in the range of -80%, an intermediate layer consisting of at least one layer in the range of 74-68%, and an inner layer in the range of 64-67%.

  As mentioned above, as the core for precision casting, the most stressed surface part has a high density and with a slant structure such as low density as it goes inside, it can withstand the strength in each precision casting process. And it becomes the ceramic core excellent also in the elution property of the ceramic core after completion | finish of casting. By using a ceramic core having this characteristic, the yield of cast products can be improved.

Schematic diagram showing the role of the ceramic core (precision casting process). Sectional drawing of the ceramic core which has the inclination structure showing this invention.

  In order to achieve the above-mentioned object, the present inventor has made extensive studies. 1. The sintered body density can be controlled by appropriately selecting the particle size of the fused silica powder. 2. The density is increased as the setting progresses. 2. If the core surface, that is, the part in contact with the wax or the molten metal can secure a high strength, the inside of the core is sufficiently porous so as not to obstruct the shape. 3. It has been found that the relative density of the surface portion should be about 80% in order to ensure the strength, and it has been found that the density at this level has little influence on the elution which is a subsequent process, and the present invention has been achieved. Is. That is, the content is (1) a ceramic core containing 80% by weight or more of fused silica, and the sintered density of the surface portion in direct contact with the molten metal is in the range of 75 to 80% of the theoretical density of the core. (2) A ceramic core for precision casting characterized by comprising at least three layers: a surface layer, an intermediate layer in the range of 68 to 74%, and an inner layer in the range of 64 to 67%. A ceramic core characterized in that the average particle size of the fused silica is fine in the surface layer and becomes larger as it goes to the intermediate layer and the inner layer, and is a gradient core. (3) Starting Slurry with a different raw material average particle diameter is poured, that is, the slurry for the surface portion is first poured into the space shaped like a core, and after surfacing, the excess slurry is discarded, and then the second portion for the intermediate portion slurry Pour from the top of the first slurry inking layer, after the inking again, discard the second slurry, repeat this finally, pouring the slurry that is the inner layer and inking, take out the molded body consisting of multiple layers, A method for producing a ceramic core for precision casting, comprising a step of sintering this.

  Hereinafter, the present invention will be described in detail.

  In general, the ceramic strength increases as the sintering density progresses. According to the experiments by the present inventors, when the relative density of fused silica is in the range of 63% to 80%, the strength (three-point bending strength) can be changed from about 6 to 30 MPa. The thickness of the core varies widely, but is approximately 1 mm to 20 mm in order to follow the turbine blade shape. If the strength is 30 MPa, the thickness of the core is 50 to 100 microns, and it can sufficiently withstand the load during wax injection and precision casting. On the other hand, when the relative density is about 80%, the reaction takes time when the core is eluted in the subsequent step, and at the same time, the alkaline solution is deteriorated. If the relative density of the ceramic core is about 65%, the alkaline solution easily reacts with the fused silica and elution proceeds in a short time. Since the reaction takes place in a short time, the deterioration of the alkaline solution is suppressed, and the reaction is more likely to proceed. However, strength reliability is inferior in a sintered body of about 65%. That is, with a uniform type as a sintered body, it is difficult to ensure the necessary minimum strength as a ceramic core and to ensure the elution properties in the subsequent process. Therefore, after making the high-strength portion as thin as possible and ensuring the minimum strength, if the shape is secured with a porous sintered body as much as possible, the demand as a ceramic core can be met.

  The portion of the ceramic core that requires the most strength is the surface portion that is directly touched by the wax or molten metal. The relative density here is increased to ensure the strength. In order to increase the relative density, sintering may be performed using a raw material powder having a small particle diameter. Similarly, in order to control the density, it is preferable to use a powder having a coarse particle diameter, but it is not always necessary to stick to the particle diameter. For example, even if the sintering temperature and the sintering time are devised to control the desired density, the effect of the present invention remains unchanged.

  However, as will be described later, in the present invention, in order to sinter a molded body consisting of multiple layers, in order to give a difference in density under one sintering condition, the starting material powder particle size is controlled to reduce the sinterability. It is realistic to control. It is not preferable that the density of the surface portion is 75-80% because elution becomes difficult if the density is further increased. If it is lower than 75%, strength reliability is lost. It is not preferable that the internal density is defined as 64-67% because if it is larger than 67%, the elution property is impaired. On the other hand, if it is lower than 64%, it is difficult to form the member. The reason why the intermediate portion is defined as 68-74% is that it is necessary to be in the middle between the surface portion and the inside, so that structural distortion due to the difference in the degree of sintering between the surface portion and the inside can be alleviated. .

  FIG. 2 shows a cross-sectional view of a ceramic core according to the present invention. In addition, although it was set as three layers here, as long as a density is in the range of the present invention, it may be more than three layers.

  Various methods known in the ceramics industry can be used singly or in combination to produce a ceramic core for precision casting having such a density gradient. However, in practice, a “casting method” is used in which ceramic slurry is poured into a mold. Among cast molding methods, a slurry made of fine particles hitting the core surface layer is first poured into a mold made of gypsum or the like (desired core female mold). Therefore, when the desired thickness is reached, the slurry remaining in the mold is thrown away (sludge casting), and then the second ceramic slurry having a particle size corresponding to the intermediate layer is poured from above the inking layer. After a certain period of time, when the desired thickness is reached, discard the excess slurry, and finally pour a third ceramic slurry having a coarse particle size corresponding to the core inner layer. It is solid without discharging.

  By repeating this “casting molding method”, a core molded body composed of three layers having different particle diameters from the surface can be obtained. After drying, this is sintered by adjusting the desired temperature, holding time, heating rate and the like. In general, a ceramic core for precision casting having a gradient of the present invention can be obtained under sintering conditions of 1150-1300 ° C., 100 ° C./h for about 2 hours. Although it is a slurry for casting, it is generally prepared by mixing water as a main component and a dispersing agent such as a surfactant. As long as it is possible to discard the slurry, there is no limitation on the slurry preparation method. In addition, a gypsum mold has been described as an example, but any type of porous alumina, refractory, resin mold, or the like may be used as long as the fleshing-discharging process is possible. Although it is complicated as a manufacturing method, an appropriate “nesting” may be set in a core female mold, and a ceramic slurry using an organic resin as a binder may be injection molded there. In this case, when the surface layer is formed and then the nest is removed to form the intermediate layer, another nest is used, but this is possible as a manufacturing method. In addition, a rubber mold or the like is prepared, and a powder whose particle size is adjusted is pressed (CIP) with isotropic hydrostatic pressure. As in the case of injection molding, it is actually difficult to produce a gradient material, such as requiring “nesting” many times, but it is possible as a method. Also, once the core is sintered in the test tube (first surface layer), the intermediate layer is formed there by the sludge method and fired again. Finally, the inner layer is provided and fired again. It doesn't matter.

  Hereinafter, the present invention will be described in detail with reference to specific embodiments. In addition, although the commercially available industrial powder was used for the starting raw material powder, the inevitable impurities contained therein, for example, iron oxides and the like, do not change the effect of the present invention as long as their total is 2% by weight or less.

Fused silica powders having different average particle diameters were prepared. Using these powders, an appropriate amount was mixed in order to control the particle size, and a predetermined amount of alumina powder and zirconium silicate powder was mixed to obtain a ceramic core composition. The amount of fused silica powder in the ceramic core composition was fixed at 85% by weight, alumina at 3% by weight, and zirconium silicate at 12% by weight. The average particle size was determined by the 50% value (d ₅₀ ) in accordance with a generally well-known particle size distribution measurement method such as a laser diffraction method or a sieving method. As a result, the average particle size of composition 1 was 2 microns, composition 2 was 15 microns, and composition 3 was 30 microns.

  First, as a basic physical property evaluation of a ceramic, a plate-like sample was prepared, and its density and three-point bending strength were examined. In addition, as a basic study of dissolution, a plate sample was immersed in a 70 ° C. potassium hydroxide solution (concentration 30%) and evaluated by the remaining weight ratio after 30 minutes (complete dissolution at 0%, but not at 100%). did. As for the sample shape, a 10 mm × 50 mm × 3 mm mold was filled with ceramic powder, a molded body was prepared by a pressure molding method, and this was heat-treated in the atmosphere at 1200 ° C. for 2 hours. The density was determined by an underwater replacement method for the sintered body. The three-point bending strength was determined by a weight test using a digital force gauge as a simple method. The results are shown in Table 1.

  As can be seen from Table 1, with the composition 1 having a small average particle size, the sintered density is improved, and as a result, sufficient strength is developed. However, complete dissolution was not possible in the 30 minute dissolution test. On the other hand, with composition 3 having a large average particle size of 30 microns, sintering hardly proceeds under this heat treatment condition, and the porosity is kept (the strength is weak). On the contrary, there was no problem with the dissolution property. Composition 2 has an intermediate property. The particle size after sintering of each sample was almost the same as the average particle size of the starting raw material powder. By controlling the particle size in this way, it was found that the density of the ceramic core can be controlled, which is also reflected in characteristics such as bending strength and elution with respect to an alkaline solution.

  A ceramic core was produced using the composition 1-3 used in Example 1. First, the female shape of FIG. 2 was produced with a plaster mold. 60-120 g of pure water and 0.1-1.0 g of a polyacrylic acid dispersant were added to 300 g of the ceramic powder adjusted to composition 1, and mixed well to obtain a slurry. The slurry of composition 1 was poured into a gypsum mold and allowed to stand for about 20 seconds, and then the slurry was discharged. By this treatment, a 0.1 mm thick inking layer was formed inside the gypsum mold. When the inking layer of composition 1 was dried, the slurry of composition 2 was poured from above and allowed to stand for about 1 minute, and then the excess slurry was discharged as described above. The thickness of composition 2 was 1.2 mm. When the inking layer of composition 2 was dried, the slurry of composition 3 was poured and the slurry was replenished so as to be solid. When the slurry of composition 3 was dried, the gypsum mold was separated, and a molded body composed of 3 layers having composition 1 to composition 3 was taken out. The molded body was sufficiently dried and then sintered under the same heat treatment conditions as in Example 1 to obtain a ceramic core having a high density in the surface layer and a low density gradient as going to the inside.

  This ceramic core was placed in a turbine blade mold, and precision casting wax was injected. The injection pressure at this time was 3 MPa. When this wax model was taken out and an internal transmission image was confirmed by X-ray CT examination, no breakage or deformation was observed in the ceramic core. Next, mold powder was applied to the wax model with colloidal silica and aggregate (alumina, zircon sand). After casting the wax with high temperature steam, the mold was fired at 1100 ° C. Next, a Ni-based alloy (Rene 80) melt was poured into the mold to produce a turbine blade. At this point, since the ceramic core is inside the blade, the internal transmission image was observed by X-ray CT as before. As a result, no breakage or deformation was observed in the ceramic core. Next, the ceramic core was eluted under the same conditions as in Example 1 (time was 10 hours). As a result of observing the internal transmission image again by X-ray CT, no core remained.

  The amount of fused silica powder used in the slurry was controlled so that the average particle size of composition 1 corresponding to the core surface was 7 microns. A ceramic core was manufactured through the same process as in Example 2. However, when the Ni-based alloy molten metal was poured, the ceramic core cracked and became a casting defect. Thus, if the strength of the surface portion is insufficient, reliability as a ceramic core cannot be ensured.

  In the same process as in Example 3, a slurry was prepared in which the average particle size of composition 1 corresponding to the surface layer was 1.5 microns, and an attempt was made to produce a ceramic core. Cracks ran through the entire child and did not become a sintered body. Thus, even if the particle size of the surface layer is too small, the intended ceramic core cannot be obtained.

  The amount of fused silica powder used in the slurry was controlled so that the average particle size of composition 3 falling inside the core was 17 microns. A ceramic core was produced by the same process as in Example 2, and a Ni-based alloy was cast. An attempt was made to elute the ceramic core, but in about 10 hours, the ceramic core could not be completely eluted, and the prescribed amount (air volume) could not be satisfied in a predetermined cooling test. As described above, if the degree of sintering of the ceramic core is excessively advanced, the properties of a cast product using the ceramic core cannot be guaranteed.

1 Wax model 2 Mold 3 Ceramic core 4 Molten metal 5 Precision casting (finished product)
6 Outermost surface layer 7 Intermediate layer 8 Inner layer

Claims (4)

A ceramic core for precision casting containing 80% by weight or more of fused silica,
A surface layer in which the sintered density of the core surface portion in contact with the molten metal is in the range of 75 to 80% of the theoretical density of the core, an intermediate layer comprising at least one layer in the range of 68 to 74%, and 64 to A ceramic core for precision casting, comprising an inner layer in a range of 67%.   2. The precision casting ceramic core according to claim 1, wherein the average particle size of the fused silica increases in the order of the surface layer, the intermediate layer, and the inner layer. In claim 1, the average particle size of the fused silica powder of the starting material, 1-5 microns at 50% average value of the particle size distribution (d ₅₀₎ in the surface layer, 6-18 microns in intermediate layer, 19 is an internal layer A ceramic core for precision casting characterized by being 40 microns. The first step of pouring the slurry for the surface portion containing the fused silica into the space shaped like the core shape, throwing away the excess slurry after the fleshing,
A second step of pouring a slurry for the intermediate part larger than the average particle diameter of the fused silica of the slurry for the surface part from above the slurry parting layer for the surface part; ,
Repeating the first step and the second step, a third step;
A fourth step of pouring a slurry for an inner layer larger than the average particle diameter of the fused silica of the slurry for the intermediate portion, and after taking out the meat, taking out a molded body consisting of multiple layers, and sintering this;
A method for producing a ceramic core for precision casting, comprising: