KR100348713B1 - Alumina-base investment casting shell mold and manufacturing method thereof - Google Patents

Abstract

The present invention relates to an alumina mold for precision casting and a method for producing the same, which are easily baked and have excellent high temperature deformation characteristics by being produced by including an aluminum metal powder in a wax or a plastic coating slurry. The mold of the present invention forms a model of a product to be made of wax or plastic, and the surface of the wax or plastic model alternately coated a refractory slurry containing a colloidal silica and alumina powder and alumina sand several times, and then only the slurry thereon The coating is applied once more, and then the final coating is completely dried, dewaxed and calcined, wherein the refractory slurry is characterized in that the aluminum metal powder is included. The aluminum metal powder preferably has a particle size of 10 to 50 µm, and includes 1.0 to 4.0 wt% based on the sum of the weights of the alumina powder and colloidal silica in the slurry. The firing step is preferably performed at 1000 to 1500 ° C for 1 hour. The alumina mold of the present invention prepared as described above can be usefully used for casting of precision castings having excellent surface condition and dimensional stability, in particular for directional solidification of nickel-based super heat-resistant alloys.

Description

Alumina mold for precision casting and manufacturing method thereof {Alumina-base investment casting shell mold and manufacturing method}

The present invention relates to an alumina mold and a method for manufacturing the same, which are required during a process of making a precision casting by the lost wax method (or an investment method), which is a type of precision casting. The present invention relates to an alumina mold for precision casting and a method for producing the mold by including a powder, which is easy to be baked and has excellent high temperature deformation characteristics.

Looking at the general process of the lost wax method (or investment method), which is a precision casting method, first, a model of a metal product to be manufactured using wax or plastic is made, and the models are formed in a cluster or a tree form. Assemble Then, the surface of the wax cluster is repeatedly coated seven times to ten times according to the shape and size of the final metal product with slurry refractory and powder refractory. The refractory sand is applied before the applied refractory slurry is dried and then sufficiently dried. The dried mold is heated in a steam autoclave to dissolve the wax model in the mold, and then calcined at a suitable temperature for a certain time to maintain the strength of the refractory to complete the casting mold for precision casting. After the molten metal is injected into the mold thus prepared and solidified and cooled, the outer mold is removed to leave only the metal in the cluster state, and the metal is then cut and polished to produce the desired metal product.

Among the metal products manufactured by the precision casting method, alumina molds are commonly used for precision casting of nickel-based superheat-resistant alloys, particularly nickel-based superheat-resistant alloys. When preparing an alumina mold according to the lost wax method, the refractory slurry used in the coating process is usually used by mixing fine alumina powder into a colloidal silica, which is a caking agent, and applying powder on the slurry. As the nasal material, alumina sand is usually used. The type and the characteristics of the refractory powder cause a difference in the degree of fire resistance and disintegration of the mold, and the breathability of the mold depends on the stucco material. Meanwhile, the colloidal silica is a liquid in which colloidal silica particles of negatively charged Si-O-Si structure are dispersed in water, and the silica particles are usually 6 × 10 ^-3 to 100 × 10. ^-3 micron ultrafine particles.

In addition, the composition and viscosity of the refractory slurry used in the coating process and the particle size of the refractory sand may vary slightly depending on the order of coating. Currently, the primary slurry is colloidal silica, alumina powder, defoaming agent. ) And a surfactant (surfactant) and the like are used in combination, their properties and components are shown in Table 1 below.

Conventional Primary Slurry Al ₂ O ₃ : SiO ₂ 87: 13 Furtherance Colloidal Silica 13 wt% Particle size: 0.008 to 0.013 µm SiO ₂ Content: 30 wt% pH (at room temperature): 10.5 Alumina powder 87 wt% Particle size: 320 mesh · Surfactant 1.0 wt% (to colloidal silica) Addition amount)-defoamer 0.4 wt% (addition amount to colloidal silica) Viscosity (Zahn # 5) 30 to 35 sec

In addition, as the refractory sand used for the primary coating, alumina sand having a particle size of 100 mesh having the same chemical composition as that of the alumina powder used in the primary slurry is used.

Secondary slurry is also used by mixing colloidal silica, alumina powder, antifoaming agent and surfactant, the properties and components of which are shown in Table 2 below.

Conventional Secondary Slurry Al ₂ O ₃ : SiO ₂ 87: 13 Furtherance Colloidal Silica 13 wt% Particle Size: 0.008 to 0.013 µm SiO ₂ Content: 30 wt% pH (at room temperature): 10.5Alumina Powder 87 wt% Particle Size: 320 mesh, Surfactant 1.0 wt% (Colloidal Silica) Addition amount to the antifoaming agent) 0.4 wt% (addition amount to colloidal silica) Viscosity (Zahn # 4) 20-25 sec

As the refractory sand used for the secondary coating, the same alumina sand as that used for the primary coating was used.

Back-up slurry is a slurry commonly used from the tertiary coating to the final coating, and is used by mixing colloidal silica, alumina powder, antifoaming agent and surfactant, and the properties and components thereof are shown in Table 3 below. Same as

Conventional backup slurry Al ₂ O ₃ : SiO ₂ 87: 13 Furtherance Colloidal Silica 13 wt% Particle Size: 0.008 to 0.013 µm SiO ₂ Content: 30 wt% pH (at room temperature): 10.5Alumina Powder 87 wt% Particle Size: 320 mesh, Surfactant 1.0 wt% (Colloidal Silica) Addition amount to the antifoaming agent) 0.4 wt% (addition amount to colloidal silica) Viscosity (Zahn # 4) 10 to 17 sec

As the refractory sand used for the backup coating, alumina sand having the same chemical composition as that of the alumina powder used in the backup slurry and having a particle size of 28 × 48 mesh is commonly used, and the particle size is different depending on the order of coating. .

After seven to ten backup coatings are finished, only the backup slurry is applied once more and completely dried to prevent the sand particles from adhering to the surface. This is called a finish coating.

On the other hand, after coating each layer, the coating layer should be dried for at least 5 hours in a constant temperature-humidity chamber maintained at a temperature of 23 ± 1 ° C. and a relative humidity of 50-60%, followed by the next order of coating. This is because the cracking of the finished mold can be prevented only when the drying is performed sufficiently. In particular, during the final coating is dried for at least 12 hours in a drying chamber under the above conditions.

After the repetitive coating process as described above, the completely dried mold is heated in a steam autoclave to melt the wax model in the mold, and is then calcined at a temperature of about 1500 ° C. or higher for 4 hours or more to obtain a nickel-based super heat-resistant alloy. The alumina mold for vacuum precision casting is completed.

The conventional alumina mold prepared as described above is excellent in that the high temperature deformation is small, the proper strength is maintained, and the reactivity with the molten metal is small even when the molten metal is injected, that is, molten metal is maintained for a long time at a high temperature of about 1400 to 1600 ° C. Therefore, it has been widely used as a precision casting mold for directional solidification of nickel-based super heat-resistant alloys. However, excellent properties of such alumina-based molds can be obtained only when the colloidal silica used as a caking agent through the calcining process is sufficiently changed to mullite by reacting with alumina refractory to minimize the residual silica in the mold. However, since the high temperature stability of the alumina itself is very excellent, in order for the colloidal silica to react with the alumina refractory to be converted to mullite, the firing process must be performed for a long time of 4 hours or longer at a high temperature of 1500 ° C or higher. If the calcination is insufficient, the residual silica content is increased and the bonding between the alumina refractory materials is not sufficient, and the high temperature strength of the mold is reduced at a temperature of 1500 ° C. or higher, which is the actual casting temperature, and the mold is damaged and the dimensional stability is maintained. Undesirable casting defects are caused, and the residual silica reacts with the molten metal, resulting in a poor surface condition of the metal product and loss of elements in the molten metal. In addition, since the coefficient of thermal expansion of alumina is large, if the shape of the mold is complex or large, thermal expansion and contraction effect by heating or cooling is increased during firing at a high temperature of 1500 ° C. or higher for a long time, thereby causing cracks in the mold. The problem appears. Cracked molds are difficult to use as sound molds, and in particular in the case of directional solidification of nickel-based superheat-resistant alloys, serious casting defects lead to a significant decrease in the production yield of castings. In addition, since the firing process that is continued for 4 hours or more at a high temperature of 1500 ° C. or more requires excessive consumption of thermal energy, it also acts as a cause of raising the manufacturing cost of the product.

Accordingly, the present invention has been made to solve the above-mentioned problems, the object of the present invention is easy to be fired by coating the surface of the product model made of wax or plastic with alumina refractory slurry containing aluminum metal powder. It is to provide an alumina mold for precision casting excellent in high temperature deformation characteristics.

Another object of the present invention is to provide a method for producing alumina-based molds for precision casting, which comprises a step of coating and firing a surface of a product model made of wax or plastic with an alumina refractory slurry containing aluminum metal powder.

1 is a scanning electron microscope (SEM) photograph showing the microstructure of the surface of the alumina-based template.

In order to achieve the above object, the precision casting alumina-based mold of the present invention is to make a model of the product to be made of wax or plastic, a refractory slurry and alumina containing colloidal silica and alumina powder on the surface of the wax or plastic model After coating the sand several times alternately, only the slurry is applied once more to finish coating and then completely dried, dewaxed and calcined, wherein the refractory slurry includes aluminum metal powder.

Aluminum metal powder contained in the refractory slurry is preferably a particle size of 10 ~ 50㎛. The reason for limiting the size of the aluminum metal powder particles to 10-50 μm is that powder having a particle size of less than 10 μm is expensive, which is undesirable in terms of mold production cost, and powders having a particle size of more than 50 μm are uniformly mixed in the slurry. Because you can not. In addition, the aluminum metal powder is included in an amount of 1.0 to 4.0 wt%, preferably 1.0 to 3.0 wt%, most preferably 3.0 wt%, based on the sum of the weights of the alumina powder and colloidal silica in the refractory slurry. The reason for limiting the content of aluminum metal powder to 1.0 to 4.0 wt% is that when it is less than 1.0 wt%, it is not possible to shorten the firing time of the mold and to improve the high temperature deformation characteristics. This is because the strength is too weak. On the other hand, the alumina powder and colloidal silica and powdered alumina sand used for the said refractory slurry use a well-known thing.

In addition, the mold of the present invention coated with a refractory slurry containing an aluminum metal powder as described above is characterized in that it is fired for 1 hour at 1000 ~ 1500 ℃. Conventional alumina-based molds were fired at 1500 ° C. for at least 4 hours to form mullite microstructures of the alumina-silica interface needed to prevent high temperature deformation of the finished mold. However, since the mullite reaction is promoted at a relatively low temperature by the aluminum contained in the slurry, the mullite may be fired only for 1 hour at a temperature of 1000 to 1500 ° C, more preferably at 1000 ° C for 1 hour. The tissue is sufficiently formed.

On the other hand, the method of manufacturing alumina casting mold for precision casting of the present invention (i) a process of making a model of the product to be made of wax or plastic; (ii) coating the wax or plastic model surface several times with alternating refractory slurry containing colloidal silica and alumina powder and alumina sand; (iii) applying only the slurry once more on the coating layer to finish coating and then drying the mold completely; And (iv) removing the wax from the dried mold and firing it, wherein the refractory slurry used in the coating process includes aluminum metal powder. The aluminum metal powder preferably has a particle size of 10 to 50 µm, and the content of the aluminum metal powder is preferably 1.0 to 4.0 wt% based on the sum of the weights of the alumina powder and colloidal silica in the refractory slurry. The mold production method of the present invention is further characterized in that the firing step is carried out at 1000 to 1500 ° C for 1 hour. If the firing time is shortened, it is possible to prevent cracking of the mold generated during a long firing process in advance. Therefore, the alumina-based mold prepared by the method of the present invention is a nickel-based very sensitive to the cracking of the mold, especially among nickel-based superheat-resistant alloys. It is very useful for the casting of directional solidified products of heat resistant alloys. In addition, since the amount of residual silica in the mold is minimized due to the promotion of mullite formation by aluminum during firing, the reaction between the mold and the molten metal may be reduced during casting to prevent damage to the mold and deterioration of the final casting. In addition, the room temperature strength of the mold decreases in proportion to the amount of aluminum metal powder contained. Therefore, by varying the aluminum content, the room temperature strength of the mold is appropriately controlled, thereby reducing the manpower and time required for the die removal process of the mold after precise casting. The cost of the product can be reduced.

Hereinafter, the present invention will be described in more detail with reference to Examples. These examples are only for illustrating the present invention, and it will be apparent to those skilled in the art that the scope of the present invention is not limited to these examples according to the gist of the present invention.

Example 1 Preparation of Alumina Mold

To 100 parts by weight of the mixture consisting of 13 wt% of colloidal silica and 87 wt% of alumina powder, each containing aluminum (Al) metal powder in different ratios, and adding an antifoaming agent and a surfactant to colloidal silica in a constant ratio, Three slurry for primary and secondary coatings having the composition of Table 4 were prepared.

Refractory slurries for primary and secondary coatings of the present invention Slurry type Alumina + 1.0 wt% Al Alumina +3.0 wt% Al Alumina + 4.0 wt% Al Furtherance Al ₂ O ₃ : SiO ₂ 87: 13 87: 13 87: 13 Colloidal silica particle size: 0.008 ~ 0.013㎛SiO ₂ content: 30wt% pH (room temperature): 10.5 13 wt% 13 wt% 13 wt% Alumina powder particle size: 320 mesh 87 wt% 87 wt% 87 wt% Aluminum metal powder particle size: 10 to 50㎛ 1.0 wt% 3.0 wt% 4.0 wt% ·Surfactants 1.0 wt% 1.0 wt% 1.0 wt% Antifoam 0.4 wt% 0.4 wt% 0.4 wt% Viscosity (Zahn # 5) 20-25 sec 20-25 sec 20-25 sec

In addition, the aluminum (Al) metal powder is contained in different ratios with respect to 100 parts by weight of the mixture composed of 13 wt% of colloidal silica and 87 wt% of alumina powder, and an antifoaming agent and a surfactant are added at a constant ratio with respect to colloidal silica. , To prepare three slurry for the backup coating having a composition of Table 5.

Refractory Slurry for Backup Coatings of the Present Invention Slurry type Alumina + 1.0 wt% Al Alumina +3.0 wt% Al Alumina + 4.0 wt% Al Furtherance Al ₂ O ₃ : SiO ₂ 87: 13 87: 13 87: 13 Colloidal silica particle size: 0.008 ~ 0.013㎛SiO ₂ content: 30wt% pH (room temperature): 10.5 13 wt% 13 wt% 13 wt% Alumina powder particle size: 320 mesh 87 wt% 87 wt% 87 wt% Aluminum metal powder particle size: 10 to 50㎛ 1.0 wt% 3.0 wt% 4.0 wt% ·Surfactants 1.0 wt% 1.0 wt% 1.0 wt% Antifoam 0.4 wt% 0.4 wt% 0.4 wt% Viscosity (Zahn # 5) 17 to 20 sec 17 to 20 sec 17 to 20 sec

Subsequently, in order to prepare the cast specimen of the present invention, the wax model was repeatedly coated by a conventional lost wax method using the slurry of Tables 4 and 5. In this case, as the refractory sand to be applied after the refractory slurry treatment, alumina sand having a particle size of 100 mesh was used for the first and second coatings, and 28 × 48 mesh alumina sand was used for the backup coating after the third round. A total of seven back-up coatings were applied, followed by a finish coating and complete drying. The dried mold was heated in a steam autoclave to melt the wax model in the mold, and then calcined at 1000 ° C. for 1 hour to complete a mold specimen for high temperature deformation test.

On the other hand, except that the conventional refractory slurry that does not contain aluminum powder was used to prepare a test specimen specimen for the comparative test in the same manner as described above. At this time, the composition of the slurry used for the primary and secondary coating is as described in Table 1 and Table 2, respectively, and the slurry of Table 3 was used for the backup coating.

Example 2 High Temperature Strain Test

After casting the cast material specimens obtained in Example 1 for 4 hours at 1500 ℃, the actual casting temperature, the amount of deformation of the mold when and when the aluminum metal powder was included in the coating slurry was measured, respectively. As a result, as shown in Table 6, in the case of the cast specimen prepared using a conventional slurry, the high strain at 1500 ℃ was 4.5mm, the high strain was 1.0mm when using a slurry containing 1.0 wt% of aluminum powder In this case, when the slurry containing 3.0 wt% or more of aluminum powder was used, no high temperature deformation occurred.

High temperature deformation test (1500 ℃, 4 hours) Conventional alumina mold Alumina Mold of the Invention Alumina Alumina + 1.0 wt% Al Alumina +3.0 wt% Al Alumina + 4.0 wt% Al Firing conditions 1000 ℃ / 1hr 1000 ℃ / 1hr 1000 ℃ / 1hr 1000 ℃ / 1hr Deformation amount (mm) 4.5 1.0 none none

In addition, the specimens collected after the high temperature deformation test were collected and observed by scanning electron microscopy (SEM). See Figure 1). In Figure 1, (a) is 0.0 wt%, (b) is 1.0 wt%, (c) is 2.5 wt%, and (d) is 3.0 wt% when the slurry contains aluminum powder Respectively.

As a result, the mold of the present invention coated with a slurry containing aluminum powder has excellent high temperature deformation characteristics even after being fired for a short time at a low temperature as compared with the conventional mold, and as a result, it is possible to obtain a precision casting that requires high dimensional stability. It has been found to be useful for manufacture. In addition, the result of FIG. 1 shows that the above-described excellent properties of the mold of the present invention are the result of promoting mullite formation by aluminum and that the residual silica content in the mold is very low. It is possible to prevent the deterioration of the final casting and the composition of the composition due to the reaction between the molten metal.

Example 3 Room temperature strength test

Except for varying the temperature and time in the firing process, the specimen specimens were prepared in the same manner as in Example 1, and the room temperature strength test was performed. As a result, as shown in Table 7 below, in the case of the cast material specimen containing aluminum powder, the room temperature strength decreased proportionally as the aluminum content increased under the same test conditions as compared with the conventional cast material. From the results, it was confirmed that the alumina-based mold of the present invention containing aluminum powder has excellent collapsability after casting as compared with the conventional mold, thereby reducing the manpower and time required for the post-casting desalting process.

Room temperature strength test Firing conditions Room temperature strength (㎏ / ㎠) Conventional alumina mold Alumina Mold of the Invention Alumina Alumina + 1.0 wt% Al Alumina +3.0 wt% Al 1400 ℃ / 1hr 2.4 2.1 2.0 1500 ℃ / 1hr 2.6 2.5 2.1 1550 ℃ / 1hr 2.6 2.6 2.2

As described in detail above, the precision casting alumina-based mold of the present invention is coated with a slurry containing aluminum, so that sufficient mullite structure is generated even after firing at 1000 ° C. for 1 hour, i) It can greatly reduce the time, ii) prevent cracking of the mold due to long time firing at high temperature, iii) excellent high temperature deformation characteristics at the casting temperature, and iv) do not react with the molten metal during casting. v) It is easy to collapse the mold after casting. Therefore, by using the alumina mold of the present invention, it is possible to easily cast precision castings having excellent surface condition and dimensional stability, especially directional solidified products of nickel-based super heat-resistant alloys at low cost.

Claims (5)

Model the product to be made of wax or plastic, coat the surface of the wax or plastic with a refractory slurry containing colloidal silica and alumina powder and alumina sand several times, and then apply only the slurry thereon once more. In the alumina mold for precision casting manufactured by the final coating after complete coating, dried, dewaxed and calcined, The refractory slurry is an alumina mold for precision casting, characterized in that it comprises an aluminum metal powder. The method of claim 1, The aluminum metal powder has an alumina mold for precision casting, characterized in that the particle size of 10 ~ 50㎛. The method of claim 1, The aluminum metal powder is a precision casting alumina-based mold, characterized in that 1.0 to 4.0 wt% based on the sum of the weight of the colloidal silica and alumina powder in the refractory slurry. The method of claim 1, The firing process is alumina-based casting for precision casting, characterized in that carried out for 1 hour at 1000 ~ 1500 ℃. (i) modeling the product to be manufactured from wax or plastic; (ii) coating the wax or plastic model surface several times with alternating refractory slurry containing colloidal silica and alumina powder and alumina sand; (iii) applying only the slurry once more on the coating layer to finish coating and then drying the mold completely; And, (iv) a method for producing an alumina mold, comprising the step of removing the wax in the mold and firing the same; A method for producing alumina casting molds for precision casting, characterized in that the refractory slurry used in the coating process comprises aluminum metal powder.