MULTI-WALL CORE AND PROCESS

Abstract: Method making a multi-wall ceramic core for use in casting airfoils, such a turbine blades and vanes, wherein a fugitive pattern having multiple thin wall pattern elements corresponding to internal wall-forming spaces of a final core is formed, the pattern is placed in a core molding die cavity having a desired core configuration, a fluid ceramic material is introduced into the die cavity about the pattern and between the pattern elements to form a ceramic core, and the core is removed from the die cavity. The fugitive pattern is selectively removed from the core to provide a multi-wall green core. The green core then is fired to develop core strength for casting and used to form an investment casting mold for casting an airfoil.

Description: MULTI-WALL CORE AND PROCESS FIELD OF THE INVENTION The present invention relates to a method for making multi- wall ceramic cores for casting multi-wall metal castings. BACKGROUND OF THE INVENTION Most manufacturers of gas turbine engines are evaluating advanced multi-thin-walled turbine airfoils (i.e. turbine blade or vane) which include intricate air cooling channels to improve efficiency of airfoil internal cooling to permit greater engine thrust and provide satisfactory airfoil service life. U.S. Patents 5 295 530 and 5 545 003 describe advanced multi- walled, thin-walled turbine blade or vane designs which include intricate air cooling channels to this end. In U.S. Patent 5 295 530, a multi-wall core assembly is made by coating a first thin wall ceramic core with wax or plastic, a second similar ceramic core is positioned on the first coated ceramic core using temporary locating pins, holes are drilled through the ceramic cores, a locating rod is inserted into each drilled hole and then the second core then is coated with wax or plastic. This sequence is repeated as necessary to build up the multi-wall ceramic core assembly. This core assembly procedure is quite complex, time consuming and costly as a result of use of use of the connecting rods, pins and the like and drilled holes in the cores to receive the rods as well as tooling requirements to assemble the core components with required dimensional accuracy. A method is needed for making a multi-wall ceramic core that avoids the need for core element connecting or locating rods, pins and the like as well as to bypass tooling constraints imposed by current core manufacturing technology. An object of the invention is to satisfy this need. SUMMARY OF THE INVENTION The present invention provides, in an illustrative embodiment, a method making a multi-wall ceramic core for use in casting airfoils, such a turbine blades and vanes, wherein a fugitive pattern having multiple thin wall pattern elements defining therebetween core wall-forming spaces is formed, the pattern is placed in a core molding die cavity having a desired core configuration, a fluid ceramic material is introduced into the die cavity about the pattern and between the pattern elements to form a multi-wall ceramic core, and the core is removed from the die cavity. The fugitive pattern is selectively removed from the core to provide a multi-wall green core. The green core then is fired to develop core strength for casting in an investment casting shell mold. The pattern elements can be formed in three dimensional pattern configuration by sterolithographic deposition of pattern material, injection molding and other techniques . The multi-wall ceramic core so produced comprises a plurality of spaced apart thin core walls connected together by other integral regions of the molded core. The invention reduces core assembly costs and provides high dimensional accuracy and repeatability of core walls. DESCRIPTION OF THE DRAWINGS Figure 1 is a sectional view of a fugitive pattern used to make a multi-wall ceramic core pursuant to an illustrative embodiment of the invention. Figure 2 is a sectional view showing the pattern in a core molding die cavity. Figure 3 is a sectional view showing the multi-wall core formed about the fugitive pattern in the core die cavity. Figure 4 is a sectional view showing the multi-wall core invested in a ceramic investment casting shell mold with wax pattern removed. DESCRIPTION OF THE INVENTION Referring to Figures 1-3, the present invention provides in the illustrative embodiment shown a method of making a multi- wall ceramic core 10 for use in casting a multi-thin-walled airfoil (not shown) which includes a gas turbine engine turbine blade and vane. The turbine blade or vane can be formed by casting molten superalloy, such as a known nickel or cobalt base superalloy, into ceramic investment shell mold M in which the core 10 is positioned as shown in Figure 4. The molten superalloy can be directionally solidified as is well known in the mold M about the core 10 to produce a columnar grain or single crystal casting with the ceramic core 10 therein. Alternately, the molten superalloy can be solidified in the mold M to produce an equiaxed grain casting as is well known. The core 10 is removed by chemical leaching or other suitable techniques to leave a multi-wall cast airfoil with internal passages between the walls at regions formerly occupied by the core walls Wl, W2, W3, W4 as explained below. Referring to Figure 1, an exemplary fugitive core pattern 20 comprises a plurality (3 shown) of individual thin airfoil shaped fugitive pattern elements PI, P2, P3 that are assembled or molded integrally together to form the multi-wall pattern 20. The pattern elements will have a general airfoil cross-sectional profile each with concave and convex sides and leading and trailing edges complementary to the airfoil to be cast as those skilled in the art will appreciate. The pattern elements PI, P2, P3 are formed of plastic, wax, or other fugitive material and to desired three dimensional airfoil shape by injection molding, sterolithographic, and other techniques. Plastic or wax pattern elements PI, P2, P3 can be made with the airfoil configuration using a commercially available sterolithographic machine (e.g. model SLA500 sterolithographic machine made by 3D Systems) that deposits plastic material, such as epoxy resin, in successive layers to buildup the pattern. Individual pattern elements PI, P2, P3 can be made in this manner and joined together by suitable adhesive to form pattern assembly 20. Alternately, the pattern 20 can be formed as one piece by injection molding with the pattern elements PI, P2, P3 integrally interconnected at molded pattern regions. The pattern elements PI, P2, P3 can be formed with locating features, such as recesses 22 and posts 24, that mate with one another, by which the patterns can be positioned relative to one another with three dimensional accuracy. The pattern elements also can be formed with holes or other apertures 26 that will be filled with ceramic material when the core is formed. Other features which can be formed on the pattern elements include, but are not limited to, pedestals, turbulators, turning vanes and similar features used on turbine blades and vanes. The spaces Si, S2 formed between pattern elements Pi, P2, P3 and the apertures 26 ultimately will be filled with ceramic core material when the core is formed about the pattern 20 in a core die cavity. In production of a core 10 for casting a superalloy airfoil, such as a gas turbine engine blade or vane, the pattern elements PI, P2, P3 will have a general airfoil cross-sectional profile with concave and convex sides and leading and trailing edges complementary to the airfoil to be cast as mentioned hereabove. Pattern 20 is placed in a core molding die cavity 30 having a desired core configuration and fluid ceramic material, such as ceramic slurry, is introduced into the die cavity about the pattern 20 and between the pattern elements PI, P2, P3. The invention is not limited to this core forming technique and can be practiced as well using poured core molding, slip-cast molding, transfer molding or other core forming techniques. U.S. Patent 5 296 308 describes injection molding of ceramic cores and is incorporated herein by reference. The ceramic core can comprise silica based, alumina based, zircon based, zirconia based, or other suitable core ceramic materials and mixtures thereof known to those skilled in the art. The particular ceramic core material forms no part of the invention, suitable ceramic core materials being described in U.S. Patent 5 394 932. The core material is chosen to be chemical leachable from the airfoil casting formed thereabout as described below. Ceramic slurries suitable for injection into the core die cavity include a liquid vehicle and/or binder, such as wax or silicone resin, to render the slurry flowable enough to fill about and between the patterns PI, P2, P3 in the core die cavity 30. Ceramic powders are mixed with the liquid vehicle, binder, and a catalyst to form the slurry. The ceramic slurry is injected under pressure into the core die cavity 30 and allowed to cure or harden therein to form a green core body. Then, the green (unfired) core 10 is removed from the die cavity 30 and visually inspected prior to further processing in order that any defective cores can be discarded. Following removal from the respective core die cavity 30, the pattern 20 is selectively removed from the green core by thermal, chemical dissolution or other pattern removal treatment, leaving a multi-wall core. The thermal treatment involves heating the green core with the pattern thereon in a furnace to an elevated temperature to melt, vaporize or burn off the pattern material. Then, the green core 10 is fired at elevated temperature on a ceramic setter support, or sagger comprising a bed of ceramic powder, such as alumina, (not shown) . The ceramic setter support includes an upper support surface configured to support the adjacent surface of the core resting thereon during firing. The bottom surface of the ceramic setter support is placed on conventional support furniture so that multiple core elements can be loaded into a conventional core firing furnace for firing using conventional core firing parameters dependent upon the particular ceramic material of the core element. The fired multi-wall ceramic core 10 so produced comprises a plurality of spaced apart thin wall, airfoil shaped core walls Wl, W2, W3, W4 integrally joined by molded core regions and posts PP where ceramic material fills apertures 26. The multi-wall ceramic core 10 then is used in further processing to form an investment shell mold thereabout for use in casting superalloy airfoils. In particular, expendable pattern wax, plastic or other material is introduced about the core 10 and in the spaces between the core walls Wl, W2, W3, W4 in a pattern injection die cavity (not shown) to form a core/pattern assembly. Typically, the core 10 is placed in a pattern die cavity to this end and molten wax is injected about the core 10 and into spaces between the core walls. The core/pattern assembly then is invested in ceramic mold material pursuant to the well known "lost wax" process by repeated dipping in ceramic slurry, draining excess slurry, and stuccoing with coarse grain ceramic stucco until a shell mold is built-up on the core/pattern assembly to a desired thickness. The pattern is selectively removed from the shell mold M by thermal or chemical dissolution techniques, leaving the shell mold M having the core assembly 10 therein, Figure 4. The shell mold then is fired at elevated temperature to develop mold strength for casting. Molten superalloy is introduced into the fired mold M with the core 10 therein using conventional casting techniques. The molten superalloy can be directionally solidified in the mold M about the core 10 to form a columnar grain or single crystal airfoil casting. Alternately, the molten superalloy can be solidified to produce an equiaxed grain airfoil casting. The mold M is removed from the solidified casting using a mechanical knock-out operation followed by one or more known chemical leaching or mechanical grit blasting techniques. The core 10 is selectively removed from the solidified airfoil casting by chemical leaching or other conventional core removal techniques. The spaces previously occupied by the core walls Wl, W2, W3, W4 comprise internal cooling air passages in the airfoil casting, while the superalloy in the spaces between the core walls forms internal walls of the airfoil separating the cooling air passages . The present invention is advantageous in that the ceramic core can be formed without the need for core element connecting or locating rods, pins and the like as well as to bypass tooling constraints imposed by current manufacturing technology. It will be apparent to those skilled in the art that various modifications and variations can be made in the embodiments of the present invention described above without departing from the spirit and scope of the invention as set forth in the appended claims.

Claims: CLAIMS WE CLAIM 1. A method of making a multi-wall ceramic core for casting an airfoil, comprising forming a fugitive pattern having multiple thin wall pattern elements corresponding to internal wall- forming spaces of a final core, placing the pattern in a core molding die cavity having a desired core configuration, introducing fluid ceramic material into the die cavity about the pattern and between the pattern elements to form a ceramic core, removing the core from the die cavity, and selectively removing the pattern from the core to provide a multi-wall core. 2. The method of claim 1 including firing the core to develop core strength for casting. 3. The method of claim 1 wherein the fugitive pattern comprises multiple pattern elements assembled together. 4. The method of claim 1 wherein the fugitive pattern comprises multiple pattern elements molded integrally together. 5. The method of claim 1 wherein the pattern comprises plastic material . 6. The method of claim 5 wherein the plastic comprises epoxy resin. 7. The method of claim 1 wherein the pattern elements are formed by sterolithographic deposition. 8. A method of casting an airfoil wherein the core of claim 2 is positioned in an investment mold and molten superalloy is cast in the mold about the core. 9. A multi-wall ceramic core and pattern assembly comprising a ceramic core molded about and between a fugitive pattern having multiple thin wall airfoil shaped pattern elements corresponding to internal wall-forming spaces of a final core. 10. The core of claim 9 wherein the pattern comprises plastic material .

Inventors: SIKKENGA WILLIAM E, WOODRUM HARRY A, SIKKENGA, WILLIAM, E., WOODRUM, HARRY, A.
Applicants: HOWMET RES CORP, HOWMET RESEARCH CORPORATION
Application year: 2000
Main classification: B22D
IPC codes: B22C7, B22C9, B28B7, F01D5, F02C7