Investment casting process for hollow components

Abstract

An investment casting process for a hollow component such as a gas turbine blade utilizing a ceramic core (10) that is cast in a flexible mold (24) using a low pressure, vibration assisted casting process. The flexible mold is cast from a master tool (14) machined from soft metal using a relatively low precision machining process, with relatively higher precision surfaces being defined by a precision formed insert (22) incorporated into the master tool. A plurality of identical flexible molds may be formed from a single master tool in order to permit the production of ceramic cores at a desired rate with a desired degree of part-to-part precision.

Claims

1. A method for forming a ceramic core for an investment casting process, the method comprising: defining a geometry of a ceramic core; forming first and second flexible mold parts, each formed from a tooling surface comprising a hybrid of a machined surface and an insert surface to cooperatively define a negative of the ceramic core geometry, the forming of the first and second flexible mold parts comprising embedding vibrators in the first and second flexible mold parts; combining the first and second flexible mold parts to define a cavity; activating the vibrators to vibrate the first and second flexible mold parts while filling the cavity with a ceramic composition; from the ceramic composition, forming the ceramic core for investment casting having a green body state modulus of rupture of at least 4,000 psi within the first and second flexible mold parts; and removing the ceramic core from the cavity in a green body state without damaging the ceramic core.

2. The method of claim 1, further comprising: forming the first flexible mold part by casting the first flexible mold part in a master tool; and forming the second flexible mold part by casting the second flexible mold part in the master tool, wherein the master tool comprises a first portion defined by a machining process to form the machined surface and a second portion defined by an insert incorporated into the master tool to form the insert surface.

3. The method of claim 1, further comprising: forming a plurality of identical production molds from a single master tool; casting ceramic core material into each of the identical production molds in a parallel process to form a plurality of identical ceramic cores; wherein a number of molds in the plurality of production molds is selected to achieve a predetermined production rate.

4. The method of claim 1, wherein: the ceramic core has a green body state modulus of rupture of at least 8,000 psi within the first and second flexible mold parts.

5. The method of claim 1, wherein: the ceramic core has an outer envelope dimension with an aspect ratio of at least 20:1 and a length of 30 inches or more.

6. The method of claim 1, wherein: the ceramic core defines two geometric details defining non-parallel pull planes.

7. The method of claim 1, wherein the forming of the ceramic core includes curing the ceramic composition in the first and second flexible mold parts at 110 C. for 3 hours.

8. The method of claim 1, wherein the ceramic composition includes an epoxy content between 3 weight to 28 weight in a silica based slurry.

9. The method of claim 8, wherein the ceramic composition includes a silicon resin content of 3 weight % to 30 weight %.

10. The method of claim 4, wherein the forming of the ceramic core further includes curing the ceramic composition in the first and second flexible mold parts at 120 C. for one hour.

11. The method of claim 1, wherein the ceramic composition is a slurry during the filling of the mold and comprises: an epoxy content between 3 weight to 28 weight %; a silicon resin content of 3 weight % to 30 weight %; and silica of 200 mesh or more.

12. A method for forming a ceramic core for an investment casting process, the method comprising: forming first and second rubber mold parts, each formed from a tooling surface comprising a hybrid of a machined surface and an insert surface to cooperatively define a negative of a ceramic core geometry, the forming of the first and second rubber mold parts comprising embedding vibrators in the first and second rubber mold parts; combining the first and second rubber mold parts to define a cavity; activating the vibrators to vibrate the first and second rubber mold parts while filling the cavity with a ceramic slurry; heating the ceramic slurry within the cavity to form the ceramic core; and removing the ceramic core from the cavity in a green body state without damaging the ceramic core.

13. The method of claim 12, wherein the embedding of the vibrators in the first and second rubber mold parts comprises embedding a substantial entirety of each of the vibrators in multiple distinct locations throughout each of the first and second rubber mold parts.

14. The method of claim 12, wherein the ceramic slurry comprises: an epoxy content between 3 weight % to 28 weight %; a silicon resin content of 3 weight % to 30 weight %; and silica of 200 mesh or more.

15. The method of claim 12, wherein the heating of the ceramic slurry comprises heating the ceramic slurry such that the ceramic core is formed with a green body state modulus of rupture of at least 4,000 psi.

16. The method of claim 12, wherein the heating of the ceramic slurry comprises heating the ceramic slurry such that the ceramic core is formed with a green body state modulus of rupture of at least 8,000 psi.

17. A method for forming a ceramic core for an investment casting process, the method comprising: forming first and second flexible mold parts, each formed from a tooling surface comprising a hybrid of a machined surface and an insert surface to cooperatively define a negative of a geometry of a ceramic core; combining the first and second flexible mold parts to define a cavity; filling the cavity with a ceramic composition; forming the ceramic core for use in an investment casting process from the ceramic composition within the cavity, wherein the ceramic composition and the execution of the forming are provided such that the ceramic core is formed within the cavity in a green body state with a modulus of rupture of at least 4,000 psi; and removing the ceramic core from the cavity in the green body state without damaging the ceramic core.

18. The method of claim 17, wherein the ceramic composition is a slurry during the filling of the first and second flexible mold parts and comprises: an epoxy content between 3 weight % to 28 weight %; a silicon resin content of 3 weight % to 30 weight %; and silica of 200 mesh or more.

19. The method of claim 18, wherein the ceramic core is formed within the cavity in the green body state with a modulus of rupture of at least 8,000 psi.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is explained in detail in the following description in view of the drawings that show:

(2) FIG. 1 illustrates a ceramic core as may be produced in accordance with aspects of the present invention.

(3) FIG. 2 illustrates a prior art computerized design system as may be used during steps of the present invention.

(4) FIG. 3 illustrates two halves of a master tool incorporating precision inserts.

(5) FIG. 4 illustrates a flexible mold being cast in the master tool.

(6) FIG. 5 illustrates the flexible mold being assembled to define a cavity corresponding to the shape of the ceramic core.

(7) FIG. 6 illustrates the ceramic core being cast in the flexible mold.

DETAILED DESCRIPTION OF THE INVENTION

(8) FIGS. 1-6 illustrate steps of a process for manufacturing ceramic cores for investment casting applications. A digital model of a part such as a ceramic core 10 having a desired shape, as shown in FIG. 1, is formed using any known computerized design system 12 as in FIG. 2. That model is digitally sliced into at least two parts, usually in half, and master tools 14 are produced from the digital models using traditional machining processes and relatively low cost and easy to machine material including any soft metal such as aluminum or soft steel. Alignment features 16 may be added to the digital model for subsequent joining of the two halves If a desired surface feature of the master tool cannot be formed using a traditional machining process, a precision formed insert 22 may be installed into the master mold to incorporate the desired surface feature. The insert may be formed using a Tomo process, stereo lithography, direct metal fabrication or other high precision process. The overall tooling surface is then a hybrid of the machined surface 18 and the insert surface 20, as shown in FIG. 3 where each master tool section contains a precision formed insert. Flexible molds 24 are then cast from the master tools, as shown in FIG. 4. The flexible molds are then co-aligned and drawn together to define a cavity 26 corresponding to the desired core shape, as shown in FIG. 5. The cavity is filled with a slurry of ceramic casting material 28, as shown in FIG. 6. The flexible molds are separated once the ceramic casting material has cured to a green state to reveal the ceramic core 10. The ceramic core replicates surface features that were first produced in the precision mold inserts, such as a complex surface topography or a precision formed joint geometry. For example, a dovetail joint may be formed in a first of two ceramic core segments for mechanical joining with a corresponding geometry formed in a second mating core segment. Master tool inserts may also be useful for rapid prototype testing of alternative design schemes during development testing where the majority of a core remains the same but alternative designs are being tested for one portion of the core. In lieu of manufacturing a completely new master tool for each alternative design, only a new insert need be formed.

(9) Prior art investment casting processes require the use of high cost, difficult to machine, hard, tool steel material for the master tool because multiple ceramic cores are cast directly from a single master tool using a high pressure injection process. The high cost results in part because the tool is a highly engineered, multi-piece system due to the need to be able to remove the rigid tool from the cast core in multiple pull planes. The hard tool steel is required because the ceramic material will abrade the tool during the high pressure injection process. In contrast, the present invention uses the master tool only for low pressure or vacuum assisted casting of flexible (e.g. rubber) mold material, as described in the above-cited U.S. Pat. Nos. 7,141,812 and 7,410,606 and 7,411,204. Thus, low strength, relatively soft, easy to machine materials may be used for the master tool, for example, a series 7000 aluminum alloy in one embodiment. This results in a significant time and cost savings when compared to prior art processes.

(10) Another technology which can be exploited in the present invention is described in pending International Patent Application PCT/US2009/58220 also assigned to Mikro Systems, Inc. of Charlottesville, Va., and incorporated by reference herein. That application describes a ceramic molding composition that mimics existing ceramic core molding materials in its fully sintered condition, but that provides significantly improved green body strength when compared to the existing materials. Incorporating such an improved molding composition into the present casting regiment facilitates the production of core geometries that would not previously have survived handling in their green state without an unacceptably high failure rate. Improved green state strength is particularly important during the removal of a ceramic core from a flexible mold when the shape of a core feature is such that the mold must be deformed around the cast material in order to remove the core from the mold. The ceramic material cast into the flexible mold should have adequate green body strength to allow such cast features to be removed from the mold even when they contain protruding undercuts or non-parallel pull plane features requiring some bending of the flexible mold during removal of the green body ceramic core.

(11) A ceramic casting material described in International Patent Application PCT/US2009/58220 exhibits a lower viscosity as a slurry than prior art ceramic core casting materials, thereby allowing the step of FIG. 6 to be performed at low pressure, defined for use herein as no more than 30 psi (gauge), and in one embodiment 10-15 psi., for example. Such low pressures are suitable for injection into flexible molds. In contrast, prior art ceramic core material injection is typically performed at pressures an order of magnitude higher. The present inventors have found that a vibration assisted injection of the casting material is helpful to ensure smooth flow of the material and an even distribution of the ceramic particles of the material throughout the mold cavity. The flexibility of the molds facilitates imparting vibration into the flowing casting material. In one embodiment, one or more small mechanical vibrators 30 as are known in the art are embedded into the flexible mold itself during production of the molds in the step of FIG. 3. The vibrators may then be activated during the FIG. 6 injection of the ceramic molding material in a pattern that improves the flow of the material and the distribution of the ceramic particles of the slurry throughout the mold. Other types of active devices 32 may be embedded into the flexible mold, for example any type of sensor (such as a pressure or temperature sensor), a source of heat or a source of cooling, and/or telemetry circuitry and/or antenna for data transmission.

(12) In one embodiment, the epoxy content of the ceramic casting material could range from 28 weight % in a silica based slurry to as low as 3 weight %. The silicone resin may be a commercially available material such as sold under the names Momentive SR355 or Dow 255. This content could range from 3 weight % to as high as 30 weight %. The mix may use 200 mesh silica or even more coarse grains. Solvent content generally goes up as other resins decrease to allow for a castable slurry. The solvent is used to dissolve the silicon resin and blend with the epoxy without a lot of temperature. The Modulus of Rupture (MOR) of the sintered material is on the norm for fired silica, typically 1500-1800 psi with 10% cristobalite on a 3 point test rig. The sintered material MOR is tightly correlated to the cristobalite content, with more cristobalite yielding weaker room temperature strength. The green state MOR depends on the temperature used to cure the epoxy, as it is a high temperature thermo cure system. The curing temperature may be selected to allow for some thermo-forming, i.e. reheating the green state material to above a reversion temperature of the epoxy to soften the material, then bending it from its as-cast shape to a different shape desired for subsequent use. The reheated material may be placed into a setting die within a vacuum bag such that the part is drawn into conformance with the setting die upon drawing a vacuum in the bag. Alignment features may be cast into the core shape for precise alignment with the setting die. Advantageously, a green body MOR of at least 4,000 psi will permit the core to be removed from a flexible mold and handled with a significantly reduced chance of damage, and to provide adequate strength for it to undergo standard machining operations for adding or reshaping features either before or after reshaping in a setting die. Following such thermo-forming or in the absence of it, additional curing may be used to add strength. In one embodiment the Modulus of Rupture achieved was:

(13) MOR cured at 110 C. for 3 hours=4000 psi

(14) MOR cured as above and then at 120 C. for 1 hour=8000 psi.

(15) A 10% as-fired cristobalite content may be targeted. This may be altered by the mineralizers present and the firing schedule. The 10% initial cristobalite content may be used to create a crystalline seed structure throughout the part to assure that most of the rest of the silica converts to cristobalite in a timely fashion when the core is heated prior to pouring molten metal into the ceramic mold. It also keeps the silica from continuing to sinter into itself as it heats up again.

(16) Another parameter of concern in the investment casting business is porosity. Prior art ceramic casting material typically has about 35% porosity. The material described above typically runs around 28% porosity. The danger of a low porosity is that the cast metal cannot crush the ceramic core as it shrinks and cools, thereby creating metal crystalline damage that is referred to in the art as hot tear. The material described above has never caused such a problem in any casting trial.

(17) The above described regiment for producing investment casting ceramic cores compares favorably with known prior art processes, as summarized in the following Table 1.

(18) TABLE-US-00001 TABLE 1 Prior Art Invention Characteristic Characteristic Prior Art Capability Invention Capability Hard Precision Soft Precision Tooling Single pull plane per Multiple pull planes Tooling (high (aluminum master, section necessitating reduces # of tool hardness machine tool flexible derived mold) multiple tool sections. sections, increases steel) design freedom Linear extraction Curvilinear extraction only. capability. Single cross section Multiple cross section pull plane. pull planes. Provides rigid, Flexible consumable durable (high wear casting cavity for low resistance) casting pressure, vibration cavity (for HP and IP assisted molding. injection molding processes) Low green body High green body Limited aspect ratio Substantially enhanced strength of core strength aspect ratio capability material Yield losses related Green strength losses to low green strength eliminated Limited join-ability Join-ability of sub of core sub assemblies enhanced assemblies (butt through structural joint joints only). designs. High viscosity of Low viscosity of core Requires pressurized Low pressure injection core material slurry material slurry injection, prone to (vacuum assisted), segregation (section promotes particle size thickness sensitive) homogeneity throughout structure, section thickness insensitive Promotes non- Promotes uniform uniform shrinkage shrinkage during during thermal thermal processing processing Dimensional Potentially improves tolerance of fired dimensional tolerance of parts tailored to fired parts process limitations No Green body Thermo-formable None Green body can be flexibility after green body adjusted/modified using formation simple form tools Precision machined Aluminum master Very high cost and Low cost and short lead tool steel die to form tool with high long lead time time mold cavity definition inserts applied, used to generated flexible mold, then used to form mold cavity Inflexible tool set, Low cost modular high cost to modify. modifications/alterations allowed Rigid mold cavity Flexible mold cavity for good for high low pressure and pressure injection vibration assisted injection. Green body Versatile tool ejection extraction requires due to flexible nature of enhanced tooling mold. features

(19) Once the ceramic core is produced, it is incorporated into a ceramic casting vessel and a metal part is cast therein using known processes.

(20) The above-described regiment enables a new business model for the casting industry. The prior art business model utilizes very expensive, long lead time, rugged tooling to produce multiple ceramic casting vessels (and subsequently cast metal parts) from a single master tool with rapid injection and curing times. In contrast, the new regiment disclosed herein utilizes a less expensive, more rapidly produced, less rugged master tool and an intermediate flexible mold derived from the master tool to produce the ceramic core with much slower injection and curing times. Thus, the new casting regiment can be advantageously applied for rapid prototyping and development testing applications because it enables the creation of a first-of-a-kind ceramic core (and subsequently produced cast metal part) much faster and cheaper than with the prior art methods. Furthermore, the new regiment may be applied effectively in high volume production applications because multiple identical flexible molds may be cast from a single master tool, thereby allowing multiple identical ceramic cores to be produced in parallel to match or exceed the production capability of the prior art methods, in spite of the longer casting time required per core due to low pressure injection and potentially longer curing times. The time and cost savings of the present regiment include not only the reduced cost and effort of producing the master tool, but also the elimination of certain post-casting steps that are necessary in the prior art, such as drilling trailing edge cooling holes, since such features may be cast directly into the metal part using a ceramic core formed in accordance with the present invention due to the degree of precision achievable with the precision inserts and the ability to remove the flexible mold in multiple pull planes. The present invention not only produces high precision parts via a flexible mold, but it also enables part-to-part precision to a degree that was unattainable with prior art flex mold processes. Finally, the present regiment provides these cost and production advantages while at the same time enabling the casting of design features that heretofore have not been within the capability of the prior art techniques. thereby for the first time allowing component designers to produce the hardware features that are necessary to achieve next generation gas turbine design goals. For example, the prevent invention facilitates the production of a ceramic core having an overall outer envelope dimension aspect ratio of 20:1 or higher, and/or having an overall length of 30 inches or more. Thus, the present invention permits the commercial production of next generation actively cooled 4.sup.th stage turbine blades which is impossible with prior art techniques. It is also now possible to incorporate such large hollow regions in large cast components in order to reduce weight even if cooling is not a requirement.

(21) While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein.