FR2611150A1 - COMPOSITION FOR MOLDING A CERAMIC CORE, METHOD FOR MANUFACTURING SUCH CORE AND CERAMIC CORE THUS OBTAINED - Google Patents

Abstract

THE PRESENT INVENTION RELATES TO A CORE MOLDING COMPOSITION FOR THE PRODUCTION OF A CERAMIC CORE SUITABLE FOR USE IN A LOST WAX MOLDING PROCESS. THIS COMPOSITION IS CHARACTERIZED IN THAT IT INCLUDES A CERAMIC MIXTURE INCLUDING CERAMIC PARTICLES DISPERSE IN A BINDER, THIS BINDER BEING ADDED IN SUFFICIENT QUANTITY TO MOLD AND MAINTAIN A MOLDED SHAPE BEFORE BAKING, STABLE FIBERS IN HIGH TEMPERATURE. CERAMIC MIXTURE, THESE FIBERS BEING ADDED IN AN AMOUNT SUFFICIENT TO REDUCE THE REMOVAL CONSTRAINTS DURING THE MANUFACTURING OF THE CORE, AND A CERAMIC MATERIAL IN THE FORM OF VERY FINE PARTICLES WHICH IS ADDED IN A SUFFICIENT QUANTITY OF THE LOW TEMPERATURE PERFORMANCE AND TO INTERACT WITH FIBERS TO REDUCE REMOVAL CONSTRAINTS AND TO CONTROL DEVITRIFICATION.

Description

The present invention relates to ceramic cores

  The process of the present invention is directed to the use of the casting method and more particularly to a core molding composition which reduces shrinkage stress during the core production process.

  The lost wax casting is used in a manner

  extensively in the manufacture of nickel and cobalt-based superalloy vanes and vanes for gas turbine engines and in particular for those requiring internal cooling passages. and it provides precise dimensional tolerances and excellent surface finishes. In lost wax casting a ceramic shell mold is formed around a mass of wax in

  which are placed. in a precise way. one or more

  ceramic cores which are intended to simulate the holes and passages required. The mass of wax is removed during a cooking operation while the mold and the cores remain in place. which leads to obtaining a mold cavity. Molten metal is then poured into

  the cavity and solidified in it and the cores in

  For example, ramic acid is chemically removed by etching with a hot alkaline solution. The use of

  eliminable ceramic cores makes it possible to avoid

  machining or drilling operations that may be impossible to

  perform on superalloy type materials.

  Ceramic cores are usually made

  by injection molding a ceramic material into a core shape and baking to produce sintering. The core material has

  ceramic particles dispersed in a binder which favors

  moldability, this binder being removed before the frittaqe.

  Generally, additives or mechanical holding elements

  They are required to maintain the shape of the core between binder removal and particle sintering. In US Pat. No. 3,234,308 there is disclosed a core composition which comprises both an organic binder and a thermosetting resin. This resin maintains the shape of the core between binder removal and sintering. this resin

  being burnt during heating at the frying temperature

  tsge.Naturellement such a composition requires a form-

  additional treatment and treatment and may lead to

  leave additional residues in the core.

  The removal of the binder is critical in the perfect production of ceramic cores. It is essential that the binder be removed at sufficiently high temperatures to minimize the binder removal period. however, these temperatures are low enough to prevent formation of

  gas or rapid vaporization into the nucleus and a

  superficial flow. In addition, the shrinkage must be controlled in order to prevent the appearance of dimensional defects in the final metal product. This usually requires lowering temperatures and further extending the binder removal period up to 20 hours. Such extensions contribute so significantly to the increased cost of manufacturing a core. In addition to dimensional defects, cracks in the core body may occur as the core material shrinks along different stress axes during binder removal and sintering. For example, a nucleus can retract further along its width than

  along its length and it can also retract

  vantage following different plans across its lar-

  geur. This stress is eliminated during the frittaqe by the appearance of cracks While the microcracking can

  to be tolerable and indeed desirable, macrofissuring

  the use of the core in a molding process.

  At present it is not unusual to meet

  crofes in a percentage of up to 50% of

Ceramic yaux manufactured.

  Formulas based on amorphous silica that may include zircon or other materials to improve properties. have been used in the manufacture of lost wax casting cores for various alloys. because of their stability during casting their low cost

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  and their availability. A limitation in the use of

  Amorphous silica-based nuclei is devitrification.

  that is, the transformation of amorphous silica into

  my crystalline at a transition temperature, and in particular in crystobalite and to a lesser extent. in quartz and tridymite, with consequent changes in crystal volume. The appearance of these crystalline forms

  leads to the production of cracks during cooling

  after sintering. Generally this is avoided by

  killing sintering at temperatures and for periods

  of time that provide a crystallobalite content of about

  15-30%. For example, frittaqe for 18 hours at a temperature of about 1150 ° C makes it possible to obtain a desirable level of crystobalite. For contents above 30%. the nuclei usually become too cracked and they can not be used. Therefore, the sintering must be carried out at low temperature for periods

  extended time or other means should be used.

  to prevent the formation of crystobalite during the

manufacture of the cores.

  Therefore what is needed in this technique is a core composition that reduces shrinkage

  and limits devitrification during the manufacturing process.

cation of the nuclei.

  An object of the present invention is to provide a core molding composition having

controllable removal.

  Another object of the present invention is to provide a core molding composition which limits the onset of

  macrocracks during the manufacture of the cores.

  These and other objects of the invention are achieved by using a core molding composition comprising high temperature stable fibers which are disposed in an amorphous ceramic blend suitable for use in forming wax casting cores. lost. Fibers are added in an amount sufficient to reduce shrinkage stresses below a level at which macrofissures would occur. The mixture may also comprise a ceramic material in the form of

  very fine particles which is added in sufficient quantity

  to promote sintering at low temperatures. The

  combination of fibers and very fine particulate material

  has a synergistic effect to prevent the devitrifica-

  low temperature, while at the same time preventing

the formation of cracks.

  In a preferred embodiment, the core molding composition comprises alumina fibers up to 6.5% by weight, zircon up to 35%. fumed silica

  up to 5%, the remainder being amorphous silica

  phe. a binder being added in sufficient quantity to mold and maintain a molded shape prior to baking. In

  using the composition according to the invention, it is possible to

  a ceramic core by the steps of

  change the ingredients of the aforementioned composition. to mold the mixture into the desired shape and to cure the molded form such that the binder is removed and the mixture is

ceramic is sintered.

  The single figure in the drawing is an illustrative diagrams.

  the shrinkage that appears with cores of the type si-

  lice / zircon respectively containing no alumina fiber.

  1.5% of alumina fibers and 6.5% of alumina fibers, for

  cooking periods of 1.5 and 5 hours.

  The core molding composition according to the invention

  It comprises a ceramic mixture as a base. For example, a typical ceramic mixture may comprise up to 1% by weight of zircon powder, the remainder being

  by amorphous silica, with a binder in sufficient quantity

  to mold and maintain the molded shape of the core prior to sintering. Although this ceramic mixture is given as an example. those skilled in the art will understand that other ceramic mixtures, which include ceramic particles

  silica, alumina and zircon and mixtures thereof.

  can be used in the composition according to the invention

  tion and that other additives may be included in order to

to juster the properties.

  To ensure dimensional stability and reduce

  shrinkage stresses, stable fibers at high tempe-

  are added to the ceramic mixture. These high temperature stable fibers are presently defined as being able to resist as fibers at the casting temperatures of the metal. For example, alumina or silica fibers exhibit high temperature stability and can be used in the core molding composition according to the invention. The presence of high temperature stable fibers, particularly fibers having a length of about 2.54 to 6.35 mm and a diameter of about 3-5 microns, results in

  a homogenization of the withdrawal constraints. a reduction

  macrofissures and limits the propagation of cracks through the core body. In general, a fiber content of up to 6.5% by weight is added to the mixture

  ceramic. Above 6.5% the composition becomes difficult.

Injection molding.

  In addition to the ceramic mixture and high temperature stable fibers, a ceramic material in the form of very fine particles is added to the core molding composition. The fumed silica, such as the fumed silica known as "CAB-0-SIL", manufactured by CABOT Corporation, Tuscola. Illinois. is an example of material

  having a particle size of about 0.007-0.014mi-

  cromètre. The high surface energy of the sub-particles

  micronic allows the initiation of sintering to occur

  at lower temperatures than in the case of

  the largest of the order of a micrometer. Large areas

  ticles of silica and zircon therefore begin to

  build bridges and bond with each other at times

  lower temperatures, which appears to limit the devitrifi-

  cation. The combination of stable fibers at high temperatures

  and submicron particles is considered to be

  as also an increase in the temperature at which devitrification occurs. which makes it possible to complete the

  sintering at a higher temperature in a short time

  time, thus reducing the processing time. Referring to the figure of the drawing, it will be seen that this figure shows that shrinkage decreases as the amount of fiber used in the core molding composition is increased. If no fiber is present in the core molding composition. as represented by curves A and B, the total shrinkage can vary from about 3 to 5%. Curve A represents the shrinkage after 1 1/2 h of cooking while curve B represents shrinkage after 5 h of cooking. However, adding about 1.5% by weight of alumina fibers to the

  core molding composition, the total shrinkage is

  as represented by the curves C and D at a range of between 1.5 and 2.5%. 1 differential shrinkage being affected by changes in cooking time or temperature. If the fiber content is increased to about 6.5%, the shrinkage can be reduced to

  a range of between 0.5 and 1.5%, as represented by

  Eet F. curves for 1.5 and 5 hour cooking times, respectively. Although the interaction does not

  not completely understood. the presence of aluminum fibers

  mine in the nucleus may have the effect of reducing or

  the degree of sintering and therefore these fibers interact with a synergistic effect. with silica

  smoke to reduce shrinkage and stress

  fishing for devitrification, thus producing low shrinkage kernels. With the core molding composition according to the invention more than 95% of the cores produced are dimensionally reproducible by being free of

  crofissurea and therefore they are suitable for use

  lost wax casting without exerting a

impregnation with a resin.

  A typical core manufacturing process comprises the steps of preparing the core molding, core molding, binder removal and

  sintering. Other processing steps may be included

  its, such as a surface treatment of the powder. a pre-

  binder and granulation of the core molding composition to improve flow through a molding machine. Generally the core molding is performed using conventional injection molding equipment such as a conventional plunger or screw molding machine. Although one or the other

  these machines can be used, the best results

  states are usually obtained using a machine

  equipped with an electronic control system with temperature response

  injection rate, injection speed and pressure.

  injection. Those skilled in the art will understand that

  Universal molding conditions can not be obtained and optimum molding conditions must be determined following tests. However molding temperatures between 820C and 110oC and pressures included

  between 27.6MPa and about 68.9MPa are common.

EXAMPLE

  A mass of 2.7kg of a core molding composition was prepared for injection molding of a ceramic core. The proportions of each ingredient are given in Table I below:

TABLE I

  Ingredient Grade (wt.%) Amorphous Silica 62.64 Zircon 27, 84 Smoke Silica 4.12 Alumina Fiber 4.16 (Diameter 5 micrometers Length 3.17mm) Silane Coupling Agent 1.24

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  The silane coupling agent converts the surface of the substantially polar ceramic of the composition into a non-polar surface which is readily wetted by a non-polar binder. By way of illustration, the silane coupling agent is that known under the reference A1108 of

Union Carbide.

  After mixing the ingredients with the silene coupling agent, the wet powder mixture is dried for three hours at a temperature of about 82 ° C. The powder is then mixed with a binder in a vacuum mixer. In the case of this example 13.5% of a binder has been

  added to the core molding composition. The binder

  33.3% of paraffin wax having a melting point of

  131-1360oC. 33.3% of paraffin wax having a melting point

  of 144-1490C and 33.3% of mineral wax having a melting point of 163-1720c, aluminum stearate, oleic acid and beeswax being added as lubricating and plasticizing deflocculants. . A vacuum atmosphere accelerates mixing in the mixer as a result of air evacuation, which promotes intimate contact between liquid and solid materials. The mixing time depends on the mass of the composition and in this example it is three hours at a temperature of 1040C. The molding composition is then extruded, granulated and stored in a low-grade chamber.

  humidity until required for its use.

  tion. Cores are injection molded using a conventional plunger type molding machine and conventional molds. The molded cores are then removed from the molds and embedded in an alumina sand contained in a cooking pot. The pot, the sand and the cores are then placed in an oven and they are subjected to a binder removal and sintering cycle, the binder removal taking place during an increase in temperature from the temperature. ambient up to 4000C. at a rate of 750C per hour. The nuclei are then heated additionally until the

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  sintering temperature of 1235oC. with a temperature increase of 1500C per hour. The cores are held at the sintering temperature for about three hours and then cooled in the oven. In a parallel patent application of the applicant filed on the same day is described a process for the removal of the binder and the sintering of

cores molded in a single cycle.

  Although the preferred embodiment of the preface

  this invention has been described in connection with a

  core molding having particular amounts of

  grédients. it will be clear to those skilled in the art that various modifications in the ceramic mixture. molding conditions, cooking time and temperatures can

  without departing from the scope of this invention.

tion.

Claims (21)

  A core molding composition which is intended for the production of a ceramic core suitable for use in a lost wax molding process. characterized in that it comprises a ceramic mixture comprising ceramic particles dispersed in a binder, this binder being added in an amount sufficient to mold and maintain a molded shape before baking. stable high temperature fibers distributed in the mixture   ceramic fiber, these fibers being added in sufficient quantities   to reduce shrinkage stresses during core fabrication, and a ceramic material in the form of very fine particles which is added in an amount sufficient to permit low-temperature sintering.   temperature and to interact with the fibers to re-   the constraints of withdrawal and to control the devit- fication.   2.- composition according to claim I cnrac-   characterized in that the high temperature stable fibers comprise refractory fibers from the group   consisting essentially of alumina fibers, lice and zircon.   3. Composition according to claim 2,   characterized in that the refractory fibers are made by alumina fibers.   4. A core molding composition which is intended for the production of a ceramic core suitable for use in a lost wax molding process, characterized in that it comprises zircon up to 35%, fumed silica up to 5%, alumina fibers up to 6.5% by weight, the remainder being constituted by amorphous silica, a binder being added in sufficient quantity to   to mold and maintain a molded shape before cooking.   5. Composition according to claim 4,   characterized in that it further comprises a silane coupling agent added in an amount sufficient to provide optimum surface wetting of the silica and zircon with the binder.   6. Composition according to claim 4,   in that it comprises from 1 to 30% by weight of   con, from 1 to 4.5% of fumed silica, from 1 to 5.5% of fibers   alumina, the remainder being amorphous silica   phe, with an added amount of binder up to 15%.   7. Composition according to claim 6,   characterized in that it further comprises a silane coupling agent added in an amount sufficient to provide optimum surface wetting of silica and zircon with the binder.   8. A method of manufacturing a ceramic core characterized in that it comprises the steps of preparing a molding composition having up to 35% by weight of zircon, up to 5% of fumed silica, up to 6.5% alumina fiber. the remainder being amorphous silica, with a binder added in an amount sufficient to mold and maintain a molded shape prior to firing, to mold the composition into a desired shape and to bake the molded form at controlled temperatures so that the   The binder is removed and the remaining composition is frit-   tee, providing a ceramic core.   9. Ceramic core made by the following process claim 8.   10. A process according to claim 8, characterized in that the molding composition comprises from 1 to 30% by weight of zircon, from 1 to 4.5% of fumed silica, from 1 to 5.5% of alumina, the rest being constituted by amorphous silica.   11.- Ceramic core made by the process according to claim 10.   12.- Method of manufacturing a core for the pro-   ceramic cores, in which a powdery ceramic composition is prepared, mixed with a   binder, the mixed composition is molded into a   and cooked at controlled temperatures, characterized   in that a molding composition having   Up to 35% by weight of zircon, up to 5% of fumed silica.   up to 6.5% of alumina fibers, the rest being by amorphous silica.   13.- Ceramic core made by the process according to claim 12.   14. A process according to claim 12 characterized in that the molding composition comprises from 1 to 30% by weight of zircon. from 1 to 4, 5% of fumed silica, from 1 to 5.5% of alumina fibers, up to 2% of silane coupling agent,   remained being amorphous silica.   15. Ceramic core made by the following process claim 14.   16. Ceramic core suitable for use in a lost wax casting process, said core being   produced by a process for producing ceramic cores   characterized in that a molding composition having up to 35% by weight of zircon, up to 5% of fumed silica, up to 6.5% of alumina fibers is prepared, the remainder being by amorphous silica, this composition is mixed with a binder added in an amount sufficient to maintain a molded shape prior to firing, the mixed composition is molded into a desired shape and the molded form is baked at controlled temperatures so that the   The binder is removed and the remaining composition is frit-   ted. 17. Ceramic core according to claim 16 characterized in that the molding composition comprises from 1 to 30% by weight of zircon, from 1 to 4.5% of fumed silica. from 1 to 5.5% of alumina fibers, the remainder being amorphous silica.   18. Ceramic core according to claim 17 characterized in that the molding composition further comprises a coupling agent added in a sufficient amount   to obtain a uniform surface wetting.   19.- Ceramic core produced by a method of making ceramic cores which comprises the steps   comprising preparing a molding composition. to the   binder, molding the mixed composition into a desired shape and baking it at controlled temperatures to form a ceramic core removed from the   binder and sintered. characterized by preparing a composition   molding composition having up to 35% by weight of zircon,   up to 5% fumed silica, up to 6.5% aluminum fiber   ne, the rest consisting of amorphous silica.   20. Ceramic core according to claim 19, characterized in that the molding composition comprises from 1 to 30% by weight of zircon, from 1 to 4.5% of fumed silica, from 1 to 5.5% of alumina. the rest being constituted by amorphous silica.   21. A ceramic core according to claim 20 characterized in that the molding composition further comprises a coupling agent added in a sufficient amount.   to obtain a uniform surface wetting.