GB2202542A

GB2202542A - Core molding composition

Info

Publication number: GB2202542A
Application number: GB08804152A
Authority: GB
Inventors: Hilton Andrew Roth
Original assignee: United Technologies Corp
Current assignee: Raytheon Technologies Corp
Priority date: 1987-02-24
Filing date: 1988-02-23
Publication date: 1988-09-28
Anticipated expiration: 2008-02-23
Also published as: IL85506A0; FR2611150B1; JPS63268536A; GB2202542B; GB8804152D0; IL85506A; FR2611150A1

Abstract

A ceramic core molding composition, which includes alumina fibers, zircon, and fumed silica in an amorphous silica base, reduces shrinkage and shrinkage stresses during the core production process. Cores produced utilizing the disclosed composition and method exhibit an improved resistance to macrocracking over conventionally produced cores.

Description

Ceramic Core Molding Composition This invention relates to ceramic cores utilized in the investment casting process and more particularly to a core moulding composition which reduces shrinkage stresses during the core production process.

Backqround Art Investment casting is extensivelv used in the production of nickel and cobalt base superalloy blades and vanes for qas turbine engines, particularly those requiring internal coolinq passaqes, providing relatively precise dimensional tolerances and excellent surface finishes. In investment casting, a ceramic shell mold is formed around a wax pattern with a ceramic core or cores precisely positioned within the wax pattern to simulate the reauired holes and passages. The wax pattern is removed during a firing operation while the mold and cores remain in place, thus providing a mold cavity.Molten metal is poured into and solidified in the cavity, and the ceramic cores chemically removed such as by leaching with a hot alkali solution. Utilizinq removable ceramic cores avoids machining or drillions operations which may be impossible to perform on superalloy materials.

Ceramic cores are typically manufactured by injection molding a ceramic material into a core shape and firing to effect sinterinq. The core material includes ceramic particles dispersed in a binder which enhances moldability, with the binder removed prior to sintering. Generally, additives or mechanical restraints are required to maintain the core shape between binder removal and particle sinterinq. In U.S. Patent No. 3,234,308 to Herrmann, a core composition is disclosed which includes both an organic binder and a thermoset resin. The resin maintains the core shape between binder removal (debindering) and sintering, with the resin burnt out during the heat up to the sinterinq temperature. Of course, such a composition necessitates additional formation and processinq, and may leave additional residue in the core.

Debinderinq is critical to successful ceramic core production. It is essential that debindering be performed at temperatures high enough to minimize the debinderinq period yet low enouqh to avoid rapid qas formation or vaporization within the core and surface blistering. In addition, shrinkage must be controlled to prevent dimensional faults in the final metal product. This usually requires lowerinq temperatures and further extendinq the debindering period, to upwards of 20 hours. Such extensions add siqnificantly to the cost of producing a core.

In addition to dimensional faults, crackling of the core body can occur as the core material shrinks along different stress axes during debindering and sintering. For example, a core may shrink to a greater deqree across its width than across its length, and may shrink more alonq different planes across its width. This stress is relieved during intering by cracking. While microcracking may be tolerable and in fact desirable, macrocracking precludes usability in an investment casting process.

Presently, it is not uncommon to encounter macrocracks in up to 50% of the ceramic cores produced.

Amorphous silica base formulations, which may incorporate zircon or other materials to enhance properties, have been used in manufacturing investment casting cores for various alloys because of their stability during casting, low cost and availability.

One limitation in the use of amorphous silica base cores is devitrification, i.e., transformation of the amorphous silica into crystalline forms at a transition temperature; in particular, crystobalite and, to a lesser extent, quartz and tridymite, with consequent crystal volume changes. The formation of these crystalline forms produces cracks during the post-sinterinq cool down. Generally, this is avoided by sintering at temperatures and for time periods that provide a crystobalite content of from about 15-30%.

For example, intering for 18 hours at about 21000F achieves a desirable crystobalite content. For amounts over 30%, the cores generally become excessively cracked and cannot be used. Therefore, either sintering must be performed at low temperature for extended periods or other means utilized to inhibit crystobalite formation during core production.

Consequently, what is needed in the art is a core composition which reduces shrinkage and limits devitrification during the core production process.

Disclosure of Invention It is an object of the present invention to provide a core molding composition with controllable shrinkage properties.

It is a further object of the present invention to provide a core molding composition which limits macrocracking during core production.

These and other objects of the present invention are achieved by utilizing a core molding composition comprising high temperature stable fibers disposed in an amorphous ceramic mixture which is suitable for use in forming investment casting cores. The fibers are added in an amount sufficient to reduce shrinkage stresses below a level at which macrocracking would occur. The mixture also includes a very fine particle size ceramic material, which is added in an amount sufficient to promote sintering at low temperatures.

The combination of fibers and very fine particle size material synergisticially combine to inhibit low temperature devitrification, further preventing crack formation.

In a preferred embodiment, the core molding composition comprises alumina fiber, up to 6.5 percent by weight; zircon, up to 35 percent; fumed silica, up to 5 percent; balance amorphous silica, with a binder added in an amount sufficient to mold and maintain a molded shape before firing. Utilizing the inventive composition, a ceramic core is produced by the steps of mixing the above composition inqredients, molding the mixture to a desired shape, and firing the molded shape such that the binder is removed and the ceramic mixture is sintered.

Brief Description of Drawings The sole Figure is a graph illustrating the shrinkage that occurs with silica/zircon cores containing no alumina fiber, 1.5% alumina fiber and 6.5% alumina fiber, respectively, with firing times of 1.5 and 5 hours.

Best Mode for Carrying Out the Invention The inventive core molding compound includes a ceramic mixture as a base. For example, a typical ceramic mixture may comprise up to 35 percent by weight zircon powder, balance amorphous silica, with a binder included in an amount sufficient to mold and maintain the core molded shape before sinterinq.

While an exemplary ceramic mixture, it will be understood by those skilled in the art that other ceramic mixtures, which include silica, alumina and zircon ceramic particles and mixtures thereof, may be used in the inventive compound and that other additives may be included to adjust properties.

To provide dimensional stability and reduce shrinkage stresses, high temperature stable fibers are added to the ceramic mixture. Such high temperature stable fibers are herein defined as those capable of s-urvivin the metal casting temperatures as fibers.

For example, alumina or silica fibers exhibit high temperature stability and may be used in the inventive core molding compound. The presence of the high temperature stable fibers, in particular, fibers having a length of about 0.10-0.25 inches and a diameter of about 3-5 microns, homogenizes shrinkage stresses, reduces macrocracking and restricts propagation of cracks throuqh the core body.

Generally, up to 6.5% by weight fibers are added to the ceramic mixture. Above 6.5%, the composition becomes difficult to injection mold.

In addition to the ceramic mixture and high temperature stable fiber, a very fine particle size ceramic material is added to the core molding composition. Fumed silica, such as CAB-O-SILe fumed silica, manufactured by CABOT Corporation, Tuscola, Illinois, is an exemplary material having a particle size of approximately 0.007-0.014 microns. The high surface energy of the submicron particles enables the initiation of sinterinq to occur at lower temperatures than would occur with larger micron sized particles.

The large silica and zircon particles therefore beqin bridging and bondinq at lower temperatures which appears to limit devitrification. The combination of temperature stable fibers and submicron particles is believed to also shift upward the temperature at which devitrification occurs, wallowing sintering to be completed at a higher temperature in a short period, thereby reducing processing time.

Referring to the Fiqure, it is shown that a decrease in shrinkage occurs as the amount of fibers employed in the core molding compound is increased.

With no fibers in the molding compound, represented by lines A and B, the total shrinkage may vary from about 3-5 percent. Line A represents shrinkage after 1 1/2 hours of firing, while line B represents shrinkage after 5.0 hours of firing. However, by adding about 1.5 weight percent alumina fiber to the core molding compound, represented by lines C and D, total shrinkage is reduced to between 1 1/2 and 2 1/2 percent, with the shrinkage differential less affected by changes in the firing time or temperature. By increasing the fiber content to about 6.5%, shrinkage can be reduced to between 0.5 and 1.58, represented by lines E and F, with firing times of 1.5 and 5 hours, respectively.While the interaction is not completely understood, the presence of the alumina fiber in the core may have the effect of reducing or inhibiting the degree of sintering and, therefore, interact synergistically with the fumed silica to reduce shrinkage stresses and inhibit devitrification, thereby producing low shrinkage cores. With the inventive core molding composition, over 95% of the cores produced are dimensionally reproducible, free of macrocracks and, therefore, suitable for use in investment casting without requirinq resin impregnation.

A typical core production process comprises molding compound preparation, core molding, debindering and sintering. Other processing steps may be included, such as powder surface treatment, binder preparation, and pelletization of the core moldinq compound to improve feed flow through a molding machine. Generally, core molding is performed using conventional injection molding equipment, such as either a conventional plunger or screw-type molding machine. While either may be used, best results are generally obtained using a machine with electronic feedback process control of the injection temperature, injection rate and injection pressure. It will be understood by those skilled in the art that universal molding conditions are not obtainable and that the optimal molding conditions must be determined by trial.However, molding temperatures between lSO0F and 2300F and pressures between 4000 psi and about 10,000 psi are common.

Example A 6 pound batch of core molding composition was prepared for the injection molding of a ceramic core.

The proportions of each ingredient are disclosed is Table I.

Table I Inqredient Concentration Weiqht Percent amorphous silica 62.64 zircon 27.84 fumed silica 4.12 alumina fiber (5 micron 4.16 dia x 0.125 inches lonq) silane coupling agent 1.24 The silane coupling agent converts the essentially polar ceramic surface of the compound to a surface of a nonpolar nature which is easily wetted by a nonpolar binder material. For illustrative purposes, the silane coupling agent is Union Carbide A1108.

After blending the ingredients with the silane coupling agent, the moist powder mixture is dried for three hours at about 1800F. The powder is then mixed with a binder in a vacuum blender. For this example, 13.5% of a binder was added to the core molding composition. The binder comprises 33.3% paraffin wax having a meltinq point of 131-136 C, 33.3% paraffin wax having a melting point of 144-1490C and 33.3% mineral wax having a melting point of 163-1720C with aluminum stearate, oleic acid and beeswax added as deflocculants, lubricants and plasticizers . A vacuum atmosphere accelerates the mixing within the blender through air removal, promoting intimate contact between the liquid and solid materials. Mixing time is batch dependent and, for this example, is three hours at a temperature of 2200F.The molding compound is then extruded, pelletized and stored in a low humidity chamber until required for use.

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 alumina sand contained in a firing sagger.

The sagger, sand and cores are then placed in a furnace and subjected to a debindering and sintering firing cycle, with binder removal occurring during a temperature increase from ambient to 7500F at 1350F per hour. The cores are then further heated, up to the sintering temperatures of 22500F at a rate of 2750F per hour. The cores are held at the sinterinq temperature for about three hours and then furnace cooled. In applicant's copendin application titled "Method for Manufacturing Ceramic Cores", Attorney Docket No. R-3133, herein incorporated by reference, a method is disclosed for the sinqle cycle debindering and intering of molded cores.

While the preferred embodiment of the present invention is described in relation to a core molding composition having particular quantities of ingredients it will be understood by those skilled in the art that various changes in the ceramic mixture, moulding conditions, firing time and temperatures may be made without varyinq from the scope of the present invention.

Having thus described the invention, what is claimed is:

Claims

Claims 1. A core molding composition for producing a ceramic core suitable for use in an investment casting process characterized by: a ceramic mixture comprising ceramic particles dispersed in a binder, said binder added in an amount sufficient to mold and maintain a molded shape before firing; high temperature stable fibers, distributed in the ceramic mixture, said fibers added in an amount sufficient to reduce shrinkage stresses during core manufacturing; and a very fine particle size ceramic material, added in an amount sufficient to effect low temperature sinterinq and interact with said fibers to reduce shrinkage stresses and control devitrification.
2. The composition of claim 1 wherein said high temperature stable fibers comprise refractory fibers from the group consisting essentially of alumina, silica and zircon fibers.
3. The composition of claim 2 wherein said refractory fibers comprise alumina fibers.
4. A core molding composition for producing ceramic core suitable for use in an investment casting process comprising: up to 35 percent by weight zircon; up to 5% fumed silica; up to 6.5% alumina fiber; balance amorphous silica with a binder added in an amount sufficient to mold and maintain a molded shape before firing.
5. The composition of claim 4 further comprising: a silane coupling agent added in an amount sufficient to produce optimal surface wetting of the silica and zircon with the binder.
6. The composition of claim 4 wherein said composition comprises 1-30% by weight zircon, 1-4.5% fumed silica, 1-5.5% alumina fiber, balance amorphous silica, with up to 15% of a binder added thereto.
7. The composition of claims 6 further comprising: a silane coupling agent added in an amount sufficient to produce optimal surface wetting of the silica and zircon with the binder.
8. A ceramic core production process characterized by: preparing a molding composition having up to 35% by weight zircon, up to 5% fumed silica, up to 6.5% alumina fiber, balance amorphous silica, with a binder added in an amount sufficient to mold and maintain a molded shape before firing; molding said composition into a desired shape; and firing the molded shape at controlled temperatures wherein the binder is removed and the remaining composition is sintered, thereby providing a ceramic core.
9. A ceramic core produced according to the process of claim 8.
10. The process of claim 8 wherein said molding composition comprises 1-30% by weight zircon, 1-4.5% fumed silica, 1-5.5% alumina fiber, amorphous balance silica.
11. A ceramic core produced according to the process of claims 10.
12. In a core production process for producing ceramic cores, wherein the process includes the steps of preparing a ceramic powder composition, blending with a binder, molding the blended composition into a desired shape and firing at controlled temperatures, the improvement characterized by: preparing a molding composition having up to 35% by weight zircon, up to 5% fumed silica, up to 6.5% alumina fiber, balance amorphous silica.
13. A ceramic core produced according to the process of claim 12.
14. The process of claim 12 wherein said molding composition comprises 1-30% by weight zircon, 1-4.5% fumed silica, 1-5.5% alumina fiber, up to 2% silane coupling agent, balance amorphous silica.
15. A ceramic core produced according to the process of claim 14.
16. A ceramic core suitable for use in an investment casting process, said core produced by a ceramic core production process characterized by: preparing a moulding composition having up to 35% by weight zircon, up to 5% fumed silica, up to 6.5% alumina fiber, balance amorphous silica; blending said composition with a binder which is added in an amount sufficient to maintain a molded shape before firing; molding said blended composition into a desired shape; and firing the molded shape at controlled temperatures wherein the binder is removed and the remaining composition is sintered.
17. The ceramic core of claim 16 wherein said molding composition comprises 1-30% by weight zircon, 1-4.5% fumed silica, 1-5.5% alumina fiber, balance amorphous silica.
18. The ceramic core of claim 17 wherein said molding composition further includes a coupling agent added in an amount sufficient to achieve uniform surface wetting.
19. A ceramic core produced by a ceramic core production process which includes the steps of preparing a molding composition, blending with a binder, molding the blended composition into a desired shape and firing at controlled temperatures to provide a debindered and sintered ceramic core, the improvement characterized by: preparing a molding composition having up to 35% by weight zircon, up to 5% fumed silica, up to 6.5% alumina fiber, balance amorphous silica.
20. The ceramic core of claim 19 wherein said molding composition comprises 1-30% by weight zircon, 1-4.5% fumed silica, 1-5.5% alumina fiber, balance amorphous silica.
21. The ceramic core of claim 20 wherein said molding composition further includes a coupling agent added in an amount sufficient to achieve uniform surface wetting.