US20150122450A1 - Ceramic casting core having an integral vane internal core and shroud backside shell for vane segment casting - Google Patents

Abstract

A cast ceramic core (110), including: an airfoil portion (116) shaped to define an inner surface (56) of an airfoil (52) of a vane segment (50); and a shell portion (122) having a backside-shaping surface (120) shaped to define a backside surface (68) of a shroud (62) of the vane segment. The backside-shaping surface has a higher elevation (132) and a lower elevation (134). The higher elevation is set apart from a nearest point (138) on the airfoil portion by the lower elevation. The airfoil portion and the shell portion are cast as a monolithic body during a single casting pour.

Description

FIELD OF THE INVENTION
• The invention is related to casting of a cooled vane segment used in a gas turbine engine. Specifically, the invention relates to a monolithic ceramic casting core having a conventional core portion used to form an internal cooling channel in the airfoil of the vane segment plus a shroud external shell portion used to form a backside surface of the shroud of the vane segment.
BACKGROUND OF THE INVENTION
• Industrial gas turbine engine include a compressor for compressing air, a combustor arrangement for combusting a mixture of fuel and the compressed air, and a turbine for extracting energy from the combustion gases. The turbine section includes rows of blades secured to a rotor shaft, all of which are turned by the combustion gases in the energy extraction process. Between the rows of turbine blades are rows of stationary vanes that properly orient the combustion gases as the combustion gases travel within the turbine. Each row of vanes includes vane segments, each of which has an inner shroud and an outer shroud secured to opposite ends of at least one airfoil. In many turbines the airfoils may be cooled via an internal cooling channel and backsides of the shrouds may also be cooled. These cooled airfoils may be part of the first and even second rows of turbine blades. Cooling may include convective cooling via a flow of cooling air over the surface to be cooled, and/or impingement cooling via an impingement plate inserts placed inside the airfoil's internal cooling channel and adjacent the backside of the shroud.
• The airfoil and shrouds may be cast together thereby forming a monolithic vane segment. Alternately, the airfoil and shrouds may be case separately and then welded together. The airfoil's internal channel requires the use of a ceramic core to create the channel and define the internal surface configuration during the casting process. The remainder of the vane segment surfaces, including those of the inner and outer shrouds, is typically defined by a ceramic shell that is formed around a wax pattern of the vane segment, where the wax pattern is formed around the ceramic core. The wax pattern is then removed, leaving a void in the shape of the vane segment, where the ceramic core defines surface of the airfoil's interior channel and the ceramic shell defines the remainder of the surface of the vane segment.
• Small features are desirable on some of the surfaces of the vane segment, including the airfoil's internal channel and the backside surface of the shroud, because they can be used in conjunction with impingement jets to improve the impingement cooling. The small features can be readily formed in the surface of the airfoil's internal channel by the ceramic core because the small features on the ceramic core survive the steps leading to the final casting, and are impressed directly onto the final vane segment. The small features on the backside surface of the shroud, however, would be formed by the wax pattern, since the wax pattern defines the backside surface of the shroud. The wax pattern is a soft material. For this reason if the small features are impressed on the surface of the wax pattern and then subject to the dipping process, during which the ceramic shell is formed, the small features are distorted and/or lost. Since small features in wax patterns cannot survive the dipping process, the surfaces of the vane segment that are defined by the shell, (which are, in turn, defined by the wax pattern), cannot have small features when the small features would result from a wax pattern that is exposed to the dipping process.
• One technique that has been used to overcome this problem is to use a separately-cast, discrete ceramic insert to form the small features on the shroud backside surfaces. The wax pattern is formed around the ceramic core and the ceramic insert and the shell removed, leaving a void for the vane segment. In this method the ceramic core define the airfoil's internal surface and its small features, and the ceramic insert defines the shroud backside surface and its small features, and the remainder of the vane segment surface is formed by the shell. However, the position of the ceramic insert is difficult to control precisely and the quality of the casting is less than acceptable when a ceramic insert is used. For the foregoing reasons there is room in the art for improvement.
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 cross-section of a prior art vane segment casting.
• FIG. 2 is a schematic cross-section of a vane segment using the casting core and method disclosed herein.
• FIG. 3 is a schematic cross-section of a prior art ceramic core as cast in a prior art core die.
• FIG. 4 is a schematic cross-section of an integral casting core having an airfoil portion and a shroud shell portion as cast in a core die having a flexible core die liner.
• FIG. 4A is a close up of the detail ofFIG. 4.
• FIG. 5 is a schematic cross-section of the prior art ceramic core ofFIG. 3.
• FIG. 6 is a schematic cross-section of the integral casting core ofFIG. 4.
• FIG. 7 is a schematic cross-section of the prior art ceramic core ofFIG. 5 positioned inside a prior art wax die, and a prior art wax pattern formed using the assembly.
• FIG. 8 is a schematic cross-section of the integral casting core ofFIG. 6 positioned inside a wax die disclosed herein, and a wax pattern formed using the assembly.
• FIG. 9 is a schematic cross-section of the prior art ceramic core and prior art ceramic pattern ofFIG. 7.
• FIG. 10 is a schematic cross-section of the integral casting core and the wax pattern ofFIG. 8.
• FIG. 11 is a schematic cross-section of the prior art ceramic core and prior art ceramic pattern ofFIG. 9 and a prior art ceramic shell formed around the assembly.
• FIG. 12 is a schematic cross-section of the ceramic core and the wax pattern ofFIG. 10 and a ceramic shell formed around the assembly.
• FIG. 13 is a schematic cross-section of the prior art ceramic core and prior art ceramic shell ofFIG. 11, with a void for the prior art vane segment.
• FIG. 14 is a schematic cross-section of the integral casting core and ceramic shell ofFIG. 12, with a void for the vane segment.
• FIG. 15 is a schematic cross-section of the prior art ceramic core and the prior art ceramic shell ofFIG. 11, and a prior art vane segment cast therein.
• FIG. 16 is a schematic cross-section of the integral casting core and ceramic shell ofFIG. 14, and a vane segment cast therein.
DETAILED DESCRIPTION OF THE INVENTION
• The present inventors have devised a unique method and integral casting core through which fine features can be formed on a backside of a shroud of a cooled vane segment when an airfoil portion and the shroud portion (or portions) of the vane segment are simultaneously cast to form a monolithic, cooled, cast vane segment. The fine features may be heat transfer features that can be used in conjunction with impingement cooling jets to more effectively cool the shroud backsides. The method and casting core are enabled in some embodiments through the use of a flexible core die liner. The flexible liner makes it possible to incorporate features into a surface of a casting that are not possible when a rigid core die is used. This is because rigid core dies must be separated along a pull plane. When the die must be pulled apart while sliding along a surface of the casting the two surfaces cannot be shaped such that they interfere with that sliding. Due to the geometry of many parts, such as vane segments, this limits the places where fine feature can be formed into the casting, such as the shroud backside surface. The flexible liner inside the rigid core die obviates this problem because the flexible liner is used to form the fine features and the flexible liner can flex around the fine features as it is removed.
• The inventors have taken advantage of this flexible liner to create the integral casting core that innovatively forms the cooling channel of the airfoil portion of the vane segment while simultaneously forming the backside surface of the shroud, which was previously formed either by a ceramic shell or a discrete, ceramic insert. Fine features may also be formed on the backside surface using the integral casting core because in this casting method the shroud backside, and hence any shroud backside surface features, are formed directly in the vane segment during casting by the integral core. Since the integral core is forming the fine features there is no concern about loss of the fine features when formed via the wax pattern or misalignment when formed via the discrete, ceramic inserts. Since the flexible liner can be pulled out from around small features that would prevent the separation of rigid core die liners, there is no concern about core die separation.
• FIG. 1 is a schematic cross-section of a priorart vane segment10 having anairfoil12 with an airfoilinternal channel14, an airfoilinner surface16, and an airfoilouter surface18. Aninner shroud20 and anouter shroud22 are disposed at aninner end24 and anouter end26 of theairfoil12. The shrouds each have arespective backside surface28 which is smooth, i.e. devoid of fine heat transfer features. An airfoil impingement insert30 may be sued to form impingement jets used to cool the airfoil inner surface. Ashroud impingement plate32 may be used to form impingement jets used to cool theshroud backside surfaces28.
• FIG. 2 is a schematic cross-section of a high-temperaturealloy vane segment50 formed using the teachings herein. Thevane segment50 includes anairfoil52 with an airfoilinternal channel54, an airfoilinner surface56, and an airfoilouter surface58. Aninner shroud60 and anouter shroud62 are disposed at aninner end64 and anouter end66 of theairfoil52. The shrouds each have arespective backside surface68. Small features70 may be formed into the airfoilinner surface56 and/or one or both of the shroud backside surfaces68. Thesesmall features70 may be heat transfer features designed to work together with impingement jets formed by theairfoil impingement insert30 and/or theshroud impingement plate32. Thesesmall features70 may take on any shape known to improve heat transfer, including arrays of repeated geometry such as an array of dimples. Alternately there may by various different sizes and shapes to thesmall features70 that can be locally tailored as necessary to maximize heat transfer.
• FIGS. 3-16 schematically compare the prior art vane segment casting process and associated core to the process and associated core disclosed herein.FIG. 3 schematically depicts the formation of a prior artceramic casting core100 within a prior art rigid core die102.FIG. 4 is a schematic cross-section of theintegral casting core110 as formed using aflexible liner112 and an associated rigid core die114. Theintegral casting core110 includes acore airfoil portion116 that defines the airfoilinternal channel54, and a shell portion118 (outer shell portion) extending laterally from thecore airfoil portion116 and having a core shroud backside-shapingsurface120 that defines thebackside surface68 of theouter shroud62. Theintegral casting core110 may also include an opposing shell portion122 (inner shell portion) extending laterally from thecore airfoil portion116 and having an opposing core shroud backside-shapingsurface124 that defines thebackside surface68 of theinner shroud60. Thecore shroud portions118,122 may have core shell features130 that are configured to form the small features70. Thecore airfoil portion116 may have core airfoil features140 that are configured to form thesmall features70 on the airfoilinner surface56. The core shell features130 may include ahigher elevation132 and alower elevation134 adjacent to thehigher elevations132, where the elevation is relative to the respectivecore shroud portion118.
• Due to the geometry of theintegral casting core110, if a rigid core die were used the rigid core die would need to be separated alongline136. However, when the core shell features130 are configured as shown, where there is alower elevation134 between ahigher elevation132 and anearest point138 on asurface142 of thecore airfoil portion116, an interference between the core shell features130 and the opposite features in the rigid core die would prevent lateral movement between thecore shroud portion118 and the rigid core die. This would necessarily prevent movement alongline136. However, the flexible liner is sufficiently flexible that it can be removed from around the core shell features130 without causing any damage to them. Any pattern in the core shroud backside-shapingsurface120 that causes an interference that prevents separation of a rigid core die in this manner but which can be formed with theflexible liner112 is envisioned. One such example would be an array of dimples, or recesses, or trip strips that are not aligned withline136 etc.
• FIG. 5 is a schematic cross-section of the prior artceramic casting core100 ofFIG. 3 with the prior art rigid core die102 remove. The prior art ceramic casting core is removed as a green body and may be sintered at this point.FIG. 6 is a schematic cross-section of theintegral casting core110 with theflexible liner112 and the associated rigid core die114 removed. Likewise, theintegral casting core110 is removed as a green body and may be sintered at this point.
• FIG. 7 is a schematic cross-section of the prior artceramic casting core100 ofFIG. 5 placed inside a prior art wax die150, between which a priorart wax pattern152 has been formed. The priorart wax pattern152 is in the shape of the priorart vane segment10 to be formed. Due to the geometry of thevane segment50 to be formed, the geometry of the prior art wax die150 includes aprojection154 into the priorart wax pattern152. Thisprojection154 is a complexity of the prior art wax die150 that makes its removal more difficult.
• FIG. 8 is a schematic cross-section of theintegral casting core110 ofFIG. 6 inside awax die160, between which awax pattern162 has been formed. Thewax pattern162 is in the shape of thevane segment50 to be formed. Due to the different shape of theintegral casting core110, theprojection154 of the prior art wax die150 is not present. The result is an interior shape of the wax die160 that is much simpler. This increased simplicity makes it easier to remove the wax die160, and thereby increases available process options.
• FIG. 9 is a schematic cross-section of the prior artceramic casting core100 and the prior art wax die150 ofFIG. 7 with the prior art wax die150 removed. Similarly,FIG. 10 is a schematic cross-section of theintegral casting core110 and thewax pattern162 ofFIG. 8 with the wax die160 removed.FIG. 11 is a schematic cross-section of the prior artceramic casting core100 and the prior art wax die150 ofFIG. 9 after a dipping process where a prior artceramic shell170 formed. Similar to theprojection154 of the prior art wax die150, the prior artceramic shell170 includes aprojection172.FIG. 12 is a schematic cross-section of theintegral casting core110 and thewax pattern162 ofFIG. 10 after the dipping process where aceramic shell180 is formed. Theprojection172 of the prior artceramic shell170 is no longer present. In those instances where theceramic shell180 forms a lower quality surface than does theintegral casting core110, eliminating theprojection172 reduces the amount of thevane segment50 that is formed by theceramic shell172, and this represents an improvement to thevane segment50.
• FIG. 13 is a schematic cross-section of the prior artceramic casting core100 and the prior artceramic shell170 ofFIG. 11, with the priorart wax pattern152 removed. This leaves aprior art void190 into which molten alloy will be poured during a single casting pour in order to form the priorart vane segment10.FIG. 14 is a schematic cross-section of theintegral casting core110 and theceramic shell180 ofFIG. 12, with thewax pattern162 removed. This leaves a void200 into which molten alloy will be poured during a single casting in order to form the monolithic,integral casting core110.
• FIG. 15 is a schematic cross-section of the prior artceramic casting core100 and the prior artceramic shell170 ofFIG. 13 and the priorart vane segment10 that has been cast in theprior art void190. In this prior art investment casting process (lost-wax casting) the prior artceramic shell170 forms allexterior surfaces210 as well as the backside surfaces28 of theshrouds20,22. The prior artceramic casting core100 is limited to forming the airfoilinner surface16.
• FIG. 16 is a schematic cross-section of theintegral casting core110 and theceramic shell180 ofFIG. 14 and thevane segment50 that has been cast into thevoid200. In the method disclosed herein theintegral casting core100 now not only forms the airfoilinner surface56, but it also forms the backside surfaces68 of theshrouds60,62. Theceramic shell180 is now limited to formingexternal surfaces210. This change allows for the surface features70 to be formed on the backside surfaces68 of theshrouds60,62, via the same casting that forms the airfoilinternal channel54, which has not been done before. The surface features70 will not wash out as may happen when the surface features70 are formed into the prior artceramic shell170 via the priorart wax pattern152, and will not reposition as may happen when the surface features70 are formed using a prior art ceramic insert. For this reason the surface features70 may be made more fine than has been possible until now. Consequently, this represents an improvement in the art.
• 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.

Claims (16)

The invention claimed is:

1. A cast ceramic core, comprising:

an airfoil portion shaped to define an inner surface of an airfoil of a vane segment; and

a shell portion comprising a backside-shaping surface shaped to define a backside surface of a shroud of the vane segment, the backside-shaping surface comprising a higher elevation and a lower elevation, the higher elevation being set apart from a nearest point on the airfoil portion by the lower elevation,

wherein the airfoil portion and the shell portion are cast as a monolithic body during a single casting pour.

2. The cast ceramic core ofclaim 1, wherein the shell portion defines the backside surface of an outer shroud of the vane segment, and wherein the cast ceramic core further comprises an opposing shell portion comprising an opposing backside-shaping surface shaped to define a backside surface of an inner shroud of the vane segment, the opposing backside-shaping surface comprising the higher elevation and the lower elevation, the higher elevation being set apart from a respective nearest point of the airfoil portion by the lower elevation.

3. A cast ceramic core, comprising an airfoil portion shaped to define an inner surface of an airfoil of a vane segment and a shell portion shaped to define a backside surface of a shroud of the vane segment, wherein the airfoil portion and the shell portion are formed as a single casting.

4. The cast ceramic core ofclaim 3, wherein the shell portion further comprises core shell features shaped to define heat transfer features on the backside surface of a vane segment shroud.

5. The cast ceramic core ofclaim 4, wherein the airfoil portion comprises core airfoil features shaped to define heat transfer features on the inner surface of a vane segment airfoil.

6. The cast ceramic core ofclaim 4, wherein the core shell features form a higher elevation and a lower elevation in the shell portion, the higher elevation being set apart from a nearest point on the airfoil portion.

7. The cast ceramic core ofclaim 3, wherein the shell portion defines the backside surface of an outer shroud of the vane segment, and wherein the cast ceramic core further comprises an opposing shell portion shaped to define a backside surface of an inner shroud of the vane segment.

8. The cast ceramic core ofclaim 7, wherein the shell portion further comprises core shell features shaped to define heat transfer features on the backside surface of the outer shroud, wherein the opposing shell portion further comprises core shell features shaped to define heat transfer features on the backside surface of the inner shroud.

9. The cast ceramic core ofclaim 8, wherein the core shell features on at least one of the shell portions form a higher elevation and a lower elevation in the at least one shell portion, the higher elevation being set apart from a nearest point on the airfoil portion.

10. The cast ceramic core ofclaim 8, wherein the core shell features on both shell portions comprises a plurality of dimples.

11. A cast ceramic core, comprising:

a monolithic body comprising:

an airfoil portion shaped to define an inner surface of an airfoil of a vane segment;

an outer shell portion shaped to define a backside surface of an outer shroud of the vane segment; and

an inner shell portion comprising a core backside-shaping surface shaped to define a backside surface of an inner shroud of the vane segment.

12. The cast ceramic core ofclaim 11, wherein at least one of the core shell portions is further shaped to define a heat transfer feature on a respective backside surface of the vane segment.

13. The cast ceramic core ofclaim 12, wherein the further shaping forms a higher elevation and a lower elevation in the at least one shell portion, the higher elevation being set apart from a nearest point on the airfoil portion.

14. The cast ceramic core ofclaim 12, wherein the further shaping forms a plurality of dimples on the respective backside surface.

15. The cast ceramic core ofclaim 12, wherein both backside-shaping surfaces are further shaped to define heat transfer features on respective backside surfaces of the vane segment, the further shaping forming a higher elevation and a lower elevation in each backside-shaping surface, the higher elevations being set apart from respective nearest point of the airfoil portion.

16. The cast ceramic core ofclaim 15, wherein the further shaping forms a plurality of dimples.

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