FIELD OF THE INVENTION
This invention relates generally to the field of casting of materials, and more particularly, to a method of casting a component having convoluted internal passageways.
BACKGROUND OF THE INVENTION
Investment casting is one of the oldest known metal-forming processes, dating back thousands of years to when it was first used to produce detailed artwork from metals such as copper, bronze and gold. Industrial investment castings became more common in the 1940's when World War II increased the demand for precisely dimensioned parts formed of specialized metal alloys. Today, investment casting is commonly used in the aerospace and power industries to produce gas turbine components such as airfoils having complex outer surface shapes and internal cooling passage geometries.
The production of a component using the prior art lost wax investment casting process involves producing a ceramic casting vessel including an outer ceramic shell having an inside surface corresponding to the desired outer surface shape of the component, and one or more ceramic cores positioned within the outer ceramic shell corresponding to hollow interior passages to be formed within the component. Molten metal alloy is introduced into the ceramic casting vessel and is then allowed to cool and to solidify. The outer ceramic shell and ceramic core(s) are then removed by mechanical or chemical means to reveal the cast component having the desired external shape and hollow interior volume(s) in the shape of the ceramic core(s).
The known investment casting process is useful for producing components having a limited number of interior passages of relatively simple shape, such as a turbine blade design which includes relatively straight radially extending cooling passages, such as illustrated in U.S. Pat. No. 7,534,089. However, much more complex three dimensional cooling schemes incorporating convoluted 3-D cooling passages will be needed in the near future for advanced gas turbine blades, and the production and use of ceramic cores reflecting such convoluted cooling passages will surpass existing investment casting process capabilities.
Accordingly, an improved method of casting components with interior passageways is needed.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is explained in the following description in view of the drawings that show:
FIG. 1 is a schematic illustration of a trailing edge portion of a gas turbine airfoil illustrating a convoluted cooling air flow scheme.
FIG. 2 is a plan view of a first laminate sheet of fugitive material.
FIG. 3 is a plan view of a second laminate sheet of fugitive material.
FIG. 4 is a three dimensional structure formed by stacking the sheets of FIGS. 2 and 3.
FIG. 5 is a core die incorporating the three dimensional structure of FIG. 4.
FIG. 6 is the core die of FIG. 5 being used for casting a ceramic core material.
FIG. 7 is the core die and ceramic core material of FIG. 6 after being dipped in a ceramic shell material.
FIG. 8 is a ceramic casting vessel formed by removing the fugitive material from the structure of FIG. 7.
FIG. 9 is the casting vessel of FIG. 8 being used for casting a metal alloy material.
FIG. 10 is a cast metal alloy component revealed by removing the ceramic portions of the structure of FIG. 9.
FIGS. 11-20 illustrate the steps of an alternative embodiment wherein a hollow alloy component is cast without the need for a wax mold or a ceramic dipping process.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a schematic illustration of a trailing edge portion of a gas turbine airfoil 10 illustrating a convoluted cooling air flow scheme 12. This illustration is a planar cross-sectional view of the airfoil, but one will appreciate that the cooling air flow paths (illustrated by arrows) may also include a third dimensional component rising above and falling below the illustrated cross section (above and below the plane of the paper or screen on which the figure is displayed) such that the cooling air progresses along a three dimensional convoluted pathway which weaves left and right as well as up and down around various structures 11 within the airfoil 10 as it moves toward the exit holes 14 formed at the trailing edge 16 of the airfoil 10. The trailing edge 16 may also be formed to include a plurality of unconnected openings 13 between respective exit holes 14, wherein the unconnected openings 13 can then be filled with a ceramic insulating material 15. Prior art ceramic core investment casting techniques would be incapable of producing such a component structure due to the convoluted geometry of the cooling passageways.
The present invention provides for the fabrication of a ceramic core appropriate for casting convoluted structures such as illustrated in FIG. 1 by utilizing a layering process which allows a three dimensional mold for the ceramic core to be constructed from a stacked plurality of layers of fugitive material. This allows the three dimensional structural detail of the mold to be devolved into a plurality of two dimensional layers, where each layer can be conveniently fabricated to include void areas in appropriate regions, such that when the layers are stacked into the desired three dimensional structure, the adjoining void areas define a desired three dimensional passageway within the three dimensional structure. The term “two dimensional” as used herein when referring to the layers of a stacked mold is meant to include a finite third dimension equivalent to the thickness of the sheet of material, where the thickness of the material is selected to be large enough for convenience in handling the sheet and thin enough to achieve a desired degree of detail in the third dimension of the stacked mold.
FIGS. 2 and 3 are plan views of two different designs of sheets of material 18, 20 which have somewhat different shapes of void areas 22, 24. A plurality of each sheet design may be fabricated from a fugitive material, and the sheets then stacked to form a three dimensional structure 26, such as shown in FIG. 4, such that the adjoining void areas 22, 24 define a convoluted path 28 through the three dimensional structure 26. The term “fugitive material” as used herein means a material which can function as a mold for casting a ceramic part within the three dimensional structure and which can then be removed from the ceramic cast part by dissolving, melting and/or vaporization without harming the ceramic cast part. A typical fugitive material used for this invention may be a rubber or plastic material. The material may be selected to achieve desired properties, such as thermal expansion (relative to the ceramic core material) and/or its mode of being made fugitive. The sheets of material 18, 20 may themselves be cast in a respective master mold (not shown) or they may be cut from an integral sheet of the fugitive material such as by laser cutting or water cutting. The sheets of material 18, 20 may be joined together when stacked to form the three dimensional structure by the use of an adhesive or other means.
The three dimensional structure 26 of FIG. 4 is sealed as appropriate such as with top and bottom sheets 30 or other structures to form a core die 32 as shown in FIG. 5. The core die 32 is capable of receiving and retaining ceramic slurry which cures to form a ceramic core 34 as shown in FIG. 6. The structure of FIG. 6 may then be surrounded by a ceramic shell 36 as shown in FIG. 7 such as by using a known dipping process while the fugitive material is still in place. The fugitive material three dimensional structure 26 is then removed, such as by heating or other processing, to reveal a ceramic casting vessel 38 including the external ceramic shell 36 and the internal cast ceramic core 34 as shown in FIG. 8. Molten alloy 40 is then cast into the ceramic casting vessel 38 as shown in FIG. 9, and after the alloy has solidified, the ceramic casting vessel 38 is removed by mechanical and/or chemical means to reveal the final cast alloy component 42 having convoluted interior passageways 44 as shown in FIG. 10.
In prior art investment casting processes for hollow parts, a ceramic casting mold is formed by positioning a ceramic core within the two joined halves of a steel mold (referred to as the wax die or wax pattern tooling) which defines an injection volume that corresponds to the desired outside shape of the part. Melted wax is then vacuum injecting into the wax die around the ceramic core. Once the wax has hardened, the wax die halves are separated and removed to reveal the ceramic core encased inside a wax pattern, with the wax pattern now corresponding to the desired outside shape of the part. The outer surface of the wax pattern is then coated with a ceramic mold material, such as by a dipping process, to form the ceramic shell around the core/wax pattern. Upon hardening of the shell and removal of the wax by melting or other means, the completed ceramic casting mold is available to receive molten steel alloy in the investment casting process. It is known that the use of wax in this manner presents a variety of difficulties and limitations in the investment casting process.
Furthermore, the dipping process typically used in the prior art and described above for forming the outer ceramic shell also presents difficulties and limitations in the investment casting process, since dipping is hard to control and requires the use of a material having different properties than those of the ceramic core material. The process of FIGS. 2-10 can be extended to eliminate the need for the wax die, wax pattern, and shell dipping by incorporating the structure of the shell into the layers of FIGS. 2-4, thereby forming the fugitive three dimensional structure to include the shell features, as described in view of FIGS. 11-20 below.
A three dimensional model is first formed of a casting vessel that may be used to cast a hollow component, and that model is devolved into a plurality of layers. If multiple products are to be produced, master tools 46, 48 may be formed for each respective layer, as illustrated in FIGS. 11 and 12. The master tools may be machined from a relatively soft metal, such as aluminum for example, or may be formed with any process that produces a desired degree of detail in the tool. Respective sheets 50, 52 of fugitive material are then cast using the master tools, as shown in FIGS. 13 and 14, and the sheets 50, 52 are stacked and bonded as necessary to form a three dimensional fugitive mold die 54, as shown in the side cross-sectional view of FIG. 15 and the top cross-sectional view of FIG. 16. A slurry of ceramic material is then cast into the fugitive mold die 54, as shown in FIG. 17, wherein the ceramic material is directed to take the shape of both the ceramic shell 56 and the interior ceramic core 58 of the ceramic casting vessel 60. The ceramic casting vessel 60 is revealed upon the removal of the fugitive material, as shown in FIG. 18. Molten alloy material 62 is then cast into the ceramic casting vessel 60, as shown in FIG. 19, and upon the alloy material solidification, the ceramic casting vessel is removed using known processes to reveal the cast hollow metal component 64 as shown in FIG. 20.
It will be appreciated that the layering process provides a degree of freedom which allows the thickness of the “two dimensional” sheets of material to be varied as desired to achieve a desired degree of fidelity in the profile of the interior cooling passages. For example, if the passageways are small and contain a large degree of curvature in a direction perpendicular to the axis of stacking of the layers of material, then each layer would be formed to be relatively thinner than for an embodiment where the passageways are larger and contain a lesser degree of curvature. The selection of the thickness of the layers can be likened to the process of digitizing an analog signal; i.e. the smaller portions of the signal are represented by each bit of digital data (thinner layers) when a high level of fidelity is desired, and relatively later portions of the signal are represented by each bit of digital data (thicker layers) when a lower level of fidelity is acceptable. The layers of material may be the same thickness throughout the three dimensional stacked structure or they may vary in thickness according to local design conditions.
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. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.
Patent Grant
9216451
