Composite core for use in precision investment casting

A composite core for an investment casting process, the core including both a ceramic portion and a refractory metal portion, with the refractory metal portion being so disposed as to perform the function of a plurality of such refractory metal elements. In particular, a refractory metal element attached to a trailing edge of a ceramic element extends beyond the plane of a tip end of the ceramic element so as to replace the refractory metal element otherwise extending from the ceramic tip edge. The refractory metal element also extends beyond the space to be occupied by the wax casting, both in the direction of the tip end and the trailing edge such that improved placement and securing of the core is facilitated during the casting process. A further embodiment uses a single refractory metal element that extends into both the airfoil portion and an orthogonal extending platform portion thereof.

BACKGROUND OF THE INVENTION

The present invention relates to investment casting cores, and in particular to investment casting cores which are formed of a composite of ceramic and refractory metal components.

Investment casting is a commonly used technique for forming metallic components having complex geometries, such as turbine blades for gas turbine engines which are widely used in aircraft propulsion, electric power generation, and ship propulsion.

In all gas turbine engine applications, efficiency is a prime objective. Improved gas turbine engine efficiency can be obtained by operating at higher temperatures. However current operating temperatures are at such a level that, in the turbine section, the superalloy materials used have limited mechanical properties. Consequently, it is a general practice to provide air cooling for components in the hottest portions of gas turbine engines, typically in the turbine section. Cooling is provided by flowing relatively cool air from the compressor section of the engine through passages in the turbine components to be cooled. It will be appreciated that cooling comes with an associated cost in engine efficiency, and consequently, there is a strong desire to provide enhanced specific cooling to, maximize the amount of cooling benefit obtained from a given amount of cooling air.

While turbine blades and vanes are some of the most important components that are cooled, other components such as combustion chambers and blade outer air seals also require cooling, and the invention has application to all cooled turbine hardware, and in fact to all complex cast articles.

Traditionally cores used in the manufacture of airfoils having hollow cavities therein have been fabricated from ceramic materials, but such ceramic cores are fragile, especially the advanced cores used to fabricate small intricate cooling passages in advanced hardware. Such ceramic cores are prone to warpage and fracture during fabrication and during casting. In some advanced experimental blade designs, casting yields of less than 10% are achieved, principally because of core failure.

Conventional ceramic cores are produced by a molding process using a ceramic slurry and a shaped die; both injection molding and transfer-molding techniques may be employed. The pattern material is most commonly wax, although plastics, low melting-point metals, and organic compounds such as urea, have also been employed. The shell mold is formed using a colloidal silica binder to bind together ceramic particles which may be alumina, silica, zirconia and alumina silicates.

To briefly describe the investment casting process for producing a turbine blade using a ceramic core, a ceramic core having the geometry desired for the internal cooling passages is placed in a metal die whose walls surround but are generally spaced away from the core. The die is filled with a disposable pattern material such as wax. The die is removed, leaving the ceramic core embedded in a wax pattern. The outer shell mold is then formed about the wax pattern by dipping the pattern in a ceramic slurry and then applying larger, dry ceramic particles to the slurry. This process is termed stuccoing. The stuccoed wax pattern, containing the core, is then dried and the stuccoing process repeated to provide the desired shell mold wall thickness. At this point the mold is thoroughly dried and heated to an elevated temperature to remove the wax material and strengthen the ceramic material.

The result is a ceramic mold containing a ceramic core which in combination define a mold cavity. It will be understood that the exterior of the core defines the passageway to be formed in the casting and the interior of the shell mold defines the external dimensions of the superalloy casting to be made. The core and shell may also define casting portions such as gates and risers which are necessary for the casting process but are not a part of the finished cast component.

After the removal of the wax, molten superalloy material is poured into the cavity defined by the shell mold and core assembly and solidified. The mold and core are then removed from the superalloy casting by a combination of mechanical and chemical means such as leaching.

As previously noted, the traditional ceramic cores tend to limit casting designs because of their fragility and limitations regarding acceptable casting yields, especially with cores having small dimensions.

In order to overcome the limitations, the use of refractory metal elements for use in cores was introduced. The refractory metal elements can be used either by themselves or in combination with the ceramic elements to form a composite. This approach is described in U.S. Patent Publication No. US 2003/0075300 A1, now U.S. Pat. No. 6,637,500 which is assigned to the common assignee of the present invention and which is incorporated herein by reference.

One of the problems that has been encountered with use of refractory metal elements is that, as the total number of refractory metal elements is increased, so do the complexities of locating and attaching them to associated ceramic elements. Further, some of these refractory metal elements are small and fragile so as to be easily damaged and thereby reduce the yield rate.

Another problem associated with such composite cores is that of properly locating and maintaining their position within the die prior to the filling of the die with wax. Heretofore this has accomplished by the use of so called “print outs”, or handles, which are extensions of the ceramic core which extend beyond the cavity that is to be filled with wax. Generally, the number and locations of these ceramic printouts has been very limited because of the brittleness and fragility of the ceramic material which is necessarily in a cantilevered disposition.

SUMMARY OF THE INVENTION

Briefly, in accordance with one aspect of the invention, the number of refractory metal elements used in the core is reduced by the combining of a plurality of refractory metal elements into a single refractory metal element. In this way, the cost of manufacturing is substantially reduced because of the reduced number of the refractory metal elements and their need to be individually located and attached to associated ceramic elements.

In accordance with another aspect of the invention, refractory metal elements that are small and fragile are replaced by other refractory metal elements that are extended to their locations so as to serve the purpose of both refractory metal elements. In one embodiment, this is accomplished by replacing a refractory metal element from the tip of a ceramic element by extending the refractory metal element at a trailing edge of the ceramic element to extend into that area associated with the tip of the ceramic element.

In accordance with another embodiment of the invention, a refractory metal element can serve as a printout by extending it beyond the area of the cavity in which the wax will be inserted for purposes of making a wax pattern. In one form, plural printouts extend into adjacent edges to thereby enhance the process of locating and holding the core in position during the wax casting process.

In the drawings as hereinafter described, a preferred embodiment is depicted; however, various other modifications and alternate constructions can be made thereto without departing from the true spirit and scope of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now toFIG. 1, the invention is shown generally at10as applied to a composite core11which includes a ceramic element12and a refractory metal element13.

As is typical for the investment casting process, the core is placed within a metal die whose molds surround the core and the space therebetween is filled with wax. The die is then removed and the composite core11is embedded in a wax pattern14as is shown inFIG. 1.

As will be seen inFIGS. 1–4, the composite core element11has a tip edge16and an adjacent trailing edge17. A slot18is formed in the trailing edge17as shown inFIG. 4so as to receive a front edge19of the refractory metal element13. The refractory metal element leading edge19is secured in the slot18by any of various methods such as by an adhesive or the like.FIGS. 3 and 4show the combination of the ceramic element12and the refractory metal element13prior to the casting process, andFIGS. 1 and 2show the combination after the casting process.

As will be seen inFIG. 2, most of the refractory metal element13is disposed within the wax pattern14, but there are portions which extend beyond the wax pattern14. That is, trailing edge portion21extends beyond the trailing edge22of the wax pattern14, and a tip portion23extends beyond the tip edge24of the wax pattern14. The trailing edge portion21and tip portion23are referred to as “printout” and are used for positioning and securing the composite core in position during the casting process. In this regard, it should be recognized that a single refractory metal element13provides both a trailing edge portion21and a tip portion23, with the two extending in substantially orthogonal directions, to be used for this purpose. This provides not only improved positioning and holding capabilities but also improved strength capabilities.

As will be seen inFIGS. 1 and 2, the tip portion23of the refractory metal element13includes a portion26which is embedded in the wax pattern14and another portion27that extends beyond the tip edge24of the wax pattern14. The non-embedded portion27serves the purpose of locating and holding the core as described hereinabove. The embedded portion26serves as a portion of the ceramic core which, when removed by a leaching process or the like, forms a cavity within the superalloy casting. To understand the significance of this embedded portion26, reference is made to the prior art design as shown inFIG. 5.

As shown inFIG. 5is a composite core28is embedded in a wax pattern29. The composite core includes a ceramic core element31and a refractory metal element32. The ceramic core element31has a tip edge33and a trailing edge34. The refractory metal element32is attached to the ceramic core element31at its tip edge33as shown and has a portion36that is cantilevered out over the trailing edge34of the ceramic core element31. It will therefore be seen that the prior art design includes a fragile cantilevered portion36which is very susceptible to being damaged during the casting process.

Referring again to the present design as shown inFIGS. 1–4, it will be seen that the refractory metal element32ofFIG. 5, which was attached to the ceramic element tip edge33and included a fragile cantilevered portion36, was replaced by the embedded portion26of the refractory metal element13of the present invention. This portion26is the robust portion that is disposed between a substantial main body of the refractory metal element13and the rather robust non-embedded portion27thereof. In this way, the single refractory metal element13provides for an extension to the ceramic core element at its trailing edge while, at the same time, extending beyond the tip edge16of the ceramic element12to replace the refractory metal element32which would otherwise project from its tip edge33.

It should be recognized that the refractory metal element13may use any of a variety of shapes to create pedestals, trip strips, pins, fins or other heat transfer enhancement features in the final casting. As shown inFIGS. 1–3, an array of small cylinders37project from the main body for this purpose.

As shown inFIGS. 1–3, the tip portion23of the refractory metal element13is a single projecting element.FIG. 6shows a variation thereof wherein the tip portion23includes a pair of spaced extensions38and39with each having embedded and non-embedded portions as shown.

In the process of forming the airfoil with superalloy materials, after the wax pattern has been removed and replaced with the molten superalloy metal the composite core, including both the ceramic element and the refractory metal element, are removed by a leaching process or the like. The resulting airfoil is as shown inFIG. 7wherein the airfoil41includes a tip exit slot42as shown. The cooling air therefore passes into the internal cavity formerly occupied by the refractory metal element13and passes out the tip exit slot42.

InFIG. 8, there is shown a cross section as seen along lines8—8ofFIG. 7wherein a counter-bore type feature43has been incorporated to reduce the potential for the tip exit slot42to become plugged during engine running conditions. (i.e. smearing over of the blade as a result of frictional contact with the mating surface.)

Referring now toFIGS. 9–11, there is shown another embodiment of the present invention wherein a composite core element45as shown is incorporated into wax pattern for a blade and has an airfoil portion44and a platform portion46. The platform portion, of course, is that portion which serves to secure the blade to a rotating member such as a disk (not shown). The composite core element45includes both a ceramic element47and a refractory metal element48. The combination of the two, which forms the composite core element45is embedded within the wax pattern49.

As will be seen, the ceramic core element47is a single element that includes both the airfoil portion44and platform portion46. Further, rather than each of the airfoil portion44and platform portion46having its individual refractory metal portions, a single L-shaped refractory metal element50extends through the airfoil portion44of the ceramic core element47and then outwardly in an orthogonal direction to pass through the platform portion46of the ceramic core element47as shown inFIG. 10. In this way a single L-shaped refractory metal element50serves on both the airfoil portion44and the platform portion46such that the final blade will have exit slots on both the platform gas path surfaces as well as on the blade gas path surface. Since the platform leg of the refractory metal element50would be tied to the blade portion thereof, the platform portion would be held directly to the ceramic core element47for increased casting stability.

As shown inFIG. 11the refractory metal element51has its one end52secured in a slot53of the ceramic core element47. The refractory metal element48then passes through the wax pattern49, which will become the airfoil wall, and then projects through the wax pattern49to form the extension54. Subsequently, when the wax pattern49has been removed and replaced with the superalloy metal, and the refractory metal element51has been leached out, a passage will be left for the flow of cooling air therethrough.