Abstract:
A method of forming an internal channel within an article, such as a cooling channel in an air-cooled blade, vane, shroud, combustor or duct of a gas turbine engine. The method generally entails forming a substrate to have a groove recessed in its surface. A solid member is then placed in the groove, with the solid member being sized and configured to only partially fill the groove so that a void remains in the groove. The void is then filled with a particulate material so that the groove is completely filled. A layer is then deposited on the surface of the substrate and over the solid member and the particulate material in the groove, after which at least the solid member is removed from the groove to form the channel in the substrate beneath the layer.

Description:
BACKGROUND OF THE INVENTION 
     Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through formulation of iron, nickel and cobalt-based superalloys, though components formed from such alloys often cannot withstand long service exposures if located in certain sections of a gas turbine engine, such as the turbine, combustor or augmentor. 
     A common solution is to provide internal cooling of turbine, combustor and augmentor components, at times in combination with a thermal barrier coating. Airfoils of gas turbine engine blades and vanes often require a complex cooling scheme in which cooling air flows through cooling channels within the airfoil and is then discharged through carefully configured cooling holes at the airfoil surface. Convection cooling occurs within the airfoil from heat transfer to the cooling air as it flows through the cooling channels. In addition, fine internal orifices can be provided to direct cooling air flow directly against inner surfaces of the airfoil to achieve what is referred to as impingement cooling, while film cooling is often accomplished at the airfoil surface by configuring the cooling holes to discharge the cooling air flow across the airfoil surface so that the surface is protected from direct contact with the surrounding hot gases within the engine. 
     In the past, cooling channels have typically been integrally formed with the airfoil casting using relatively complicated cores and casting techniques. More recently, U.S. Pat. Nos. 5,626,462 and 5,640,767, both to Jackson et al. and commonly assigned with the present invention, teach a method of forming a double-walled airfoil by depositing an airfoil skin over a separately-formed inner support wall (e.g., a spar) having surface grooves filled with a sacrificial material. After the airfoil skin is formed, preferably by such methods as electron-beam physical vapor deposition (EBPVD) or plasma spraying, the sacrificial material is removed to yield a double-walled airfoil with cooling channels that circulate cooling air against the interior surface of the airfoil skin. 
     The sacrificial material must be carefully selected to withstand the skin deposition temperature, typically about 1000 to 1200° C., and have adequate thermal conductivity to prevent overheating of the deposition (spar) surface. In addition, the sacrificial material must be deposited to completely fill the surface grooves of the spar, and have a coefficient of thermal expansion (CTE) that is at least as high as that of the spar to prevent shrinkage of the sacrificial material away from the groove wall during deposition of the airfoil skin. If incomplete fill or shrinkage occurs, a gap will be present between the sacrificial material and the groove wall during skin deposition, which, if sufficiently large, leads to an unacceptable surface defect in the airfoil skin. Airfoil skins deposited by EBPVD are particularly sensitive to surface discontinuities due to the atom-by-atom manner in which the coating is built up. For example, gaps of less than up to about 2 mils (about 50 Fm) can be tolerated by plasma-sprayed airfoil skins, while gaps should not be larger than about 0.5 mil (about 13 Fm) for airfoil skins deposited by EBPVD. 
     For the above reason, sacrificial materials in the form of a solid shaped to fit a surface groove of a spar are unacceptable, because of the inherent likelihood of a gap between the solid and the groove wall. Consequently, and as shown in FIGS. 1 through 3, Jackson et al. teach that a sacrificial material  12  is deposited in the surface groove  14  of a spar  10  in excess amounts, with the excess being removed by machining or another suitable technique so that the surface of the sacrificial material  12  is flush with the surrounding surface of the spar  10 . Depending on its composition, the sacrificial material  12  can be removed after deposition of the airfoil skin  16  by melting/extraction, chemical etching, pyrolysis or another suitable method. 
     As is evident from FIG. 3, though a sacrificial material  12  having the desired physical properties outlined above can be deposited to completely fill the groove  14 , a problem arises because the groove  14  inevitably has rounded corners and edges, particularly if formed by casting or machining. As a result, even if the sacrificial material  12  completely fills the groove  14  and does not shrink to form a gap during deposition of the airfoil skin  16 , removal of the material  12  leaves a sharp (&lt;90°) notch  18  between the skin  16  and the wall of the groove  14 . Cracks will tend to initiate from this notch  18 , and thereafter propagate between the airfoil skin  16  and spar  10 . 
     In view of the above, it can be seen that it would be desirable if a method were available that could ensure adequate filling of the groove prior to deposition of the airfoil skin, while also eliminating notches that could serve as the source of cracks between the skin and spar. 
     BRIEF SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a method of forming an internal channel in an article, and particularly a cooling channel in an air-cooled component, such as a blade, vane, shroud, combustor or duct of a gas turbine engine. The method generally entails forming a substrate to have a groove recessed in its surface. A solid member is then placed in the groove, with the solid member being sized and configured to only partially fill the groove so that a void remains in the groove. The void is then filled with a particulate fill material so that the groove is completely filled. A layer is then deposited on the surface of the substrate and over the solid member and the fill material in the groove, after which at least the solid member is removed from the groove to form a channel in the substrate beneath the layer. 
     In one embodiment of the invention, the solid member and fill material are consolidated so that the fill material forms a solid mass that is bonded to the substrate, and only the solid member is removed from the groove so that the solid mass remains and forms a wall of the channel. In an alternative embodiment, the groove is formed so that its edge at the surface of the substrate projects over the groove, and the solid member and fill material are both removed from the groove to form the channel. With each embodiment, the tendency is greatly reduced for a sharp notch to be present within the channel between the substrate and layer. In the first embodiment, the fill material fills the notch formed between the layer and the rounded edge of the groove, while in the second embodiment the groove is configured to eliminate the notch. Accordingly, with each embodiment, removal of a sacrificial filler (i.e., the solid member and optionally the fill material) from the groove does not leave a sharp notch from which cracks are likely to initiate and propagate between the layer and substrate. 
     In another embodiment, a channel is formed in an article by steps of forming a substrate having a surface and a groove recessed in the surface, placing a solid member in the groove so as to partially fill the groove and leave a void therein, filling the void with a fill material so that the groove is completely filled, depositing a layer on the surface of the substrate and over the solid member and the fill material in the groove and then removing at least the solid member from the groove so as to form a channel in the substrate and beneath the layer. The fill material can be removed from the groove after the depositing step. The groove can be formed to have an edge at the surface of the substrate, the edge projecting over the groove. 
     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 through 3 illustrate process steps when forming a cooling channel below the skin of a double-walled airfoil in accordance with the prior art; 
     FIGS. 4 through 6 illustrate process steps when forming a cooling channel below the skin of a double-walled airfoil in accordance with a first embodiment of the present invention; 
     FIGS. 7 through 9 illustrate process steps when forming a cooling channel in accordance with a second embodiment of the present invention; and 
     FIG. 10 illustrates an airfoil and solid filler employed by the method of this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is generally applicable to any component in which an internal channel is desired. The advantages of this invention are particularly applicable to gas turbine engine components that employ internal cooling to maintain their service temperatures at an acceptable level. Notable examples of such components include shrouds, combustors, ducts and airfoils of high and low pressure turbine vanes and blades. 
     Referring to FIGS. 4 through 6, the method of this invention will be described in terms of a component, such as a turbine vane or blade airfoil, having a double-walled construction formed by the deposition of an airfoil skin  112  over a substrate  110 , such as a cast spar, having a groove  114  formed in its outer surface  116 . The groove  114  will typically have a rectilinear cross-section and extend in the longitudinal direction of the airfoil, though it is foreseeable that the groove  114  could have any configuration suitable for a particular component. 
     The method is generally an improvement of the method disclosed in U.S. Pat. Nos. 5,626,462 and 5,640,767, both to Jackson et al., which are incorporated herein by reference. As seen in FIG. 4, the edge of the groove  114  is rounded as a result of the manner in which the groove  114  was formed, e.g., casting or machining. As previously noted, the rounded edge would define a sharp (less than 90°) notch with the skin  112  overlying the groove  114 . In accordance with a first embodiment of this invention, sharp notches are prevented by the combined use of a sacrificial solid filler  118  and a particulate filler  120 , which together completely fill the groove  114  as shown in FIG.  5 . After deposition of the skin  112 , the solid filler  118  is removed to form a cooling channel  122 . The particulate filler  120  remains in the groove  114  to form a permanent wall portion of the channel  122 . 
     As is evident from FIGS. 4 and 5, the solid filler  118  is slightly undersized for the groove  114 , creating a void or gap between the filler  118  and the walls of the groove  114 . The use of a preformed solid filler  118  is advantageous because the final cross-sectional shape of the channel  122  can then be predetermined, particularly for the purpose of eliminating the aforementioned sharp notch. Suitable materials for the solid filler  118  must be compatible with that of the substrate  110  in terms of CTE, thermal conductivity and composition. Specifically, the solid filler  118  must have a CTE sufficiently close to the substrate  110  to prevent the formation of an excessive gap during deposition of the skin  112 , a sufficiently high thermal conductivity to prevent overheating of its surface during the deposition step, and a composition that is compatible with the substrate  110  to the extent that detrimental interdiffusion during the consolidation and deposition steps is minimized. If the substrate  110  is formed of a nickel-based alloy, including nickel-based superalloys, suitable materials for the solid filler  118  include other nickel-based alloys and certain iron alloys, including steel. In a preferred embodiment, the solid filler  118  is removed by etching, necessitating that the solid filler  118  is formed of a material that can be preferentially etched from the substrate  110 . For substrates  110  formed of nickel-based superalloys, a preferred material for the solid filler  118  is MONEL7, having a nominal composition of 70Ni-30Cu, though iron and steel alloys can also be preferentially etched from a nickel-based superalloy. MONEL7 and suitable iron and steel alloys have melting temperatures greater than 1200° C. and high thermal conductivities, e.g., at least 0.5 cal/cm·s·K at temperatures above 1000K. MONEL7 has a CTE that is comparable to that of nickel-based alloys, though bcc iron and steels are notably lower and may not be suitable for use with a nickel-based substrate. Austenitic iron-based alloys have higher CTEs than bcc iron, and may be adequate. Steels, bcc iron alloys and other lower-CTE materials may be appropriate fill materials for substrates  110  formed of intermetallic and composite materials. 
     As shown in FIG. 5, the particulate filler  120  fills the gap between the solid filler  118  and the walls of the groove  114  so that the groove  114  is completely filled prior to deposition of the skin  112 . The groove  114  is preferably filled so that the material within forms a surface substantially coplanar with the surrounding surface  116  of the substrate  110 , such that the skin  112  deposited on the substrate  110  will not form a depression over the groove  114 . As shown in FIGS. 4 and 5, the thickness of the solid filler  118  is such that its upper surface is substantially coplanar with the surrounding surface  116  of the substrate  110 , so that the skin  112  is deposited directly on the surface of the solid filler  118 , so that the particulate filler  120  is limited to surrounding the solid filler  118  within the groove  114 . 
     According to this embodiment of the invention, the particulate filler  120  forms a permanent wall portion of the channel  122 , and therefore must be compatible with the substrate  110  in terms of composition and physical properties. In particular, the particulate filler  120  must be capable of being consolidated within the groove  114  to yield a fill that can withstand the operating temperatures of the skin  112 , and will not degrade the surrounding substrate  110  from interdiffusion. Suitable materials for the particulate filler  120  include powders of NICHROME7 (nominally 60Ni-24Fe-16Cr-0.1C), NiAl intermetallic and NiCrAIY, each of which are high temperature materials having compositions that are sufficiently similar to that of nickel-based superalloys for use with substrates  110  formed of such materials. 
     Alternatively, the particulate filler  120  could be formed of a partitioned alloy braze powder, such as a blend of a nickel-based superalloy powder and a second powder of the same or similar superalloy powder with additions of a melting point depressant, such as boron or silicon. Upon heating to a temperature above the melting point of the second powder but below that of the first powder and substrate, the second powder melts and flows by capillarity to fill interstices between the unmelted powder particles. There is at least a partial dissolution of the higher-melting powder and the substrate into the lower-melting powder until the composition of the melt is altered enough that its melting point is increased and freezing occurs. This approach can be used where the substrate  110  and skin  112  are formed of a niobium-based intermetallic composite, in which case the filler  120  is a partitioned alloy that includes a lower-melting powder high in silicon and boron. 
     Consolidation of the particulate filler  120  within the groove  114  can be achieved using various techniques. Notable methods include an electroconsolidation process commercially available through Superior Graphite, a process referred to as pneumatic isostatic forging (PIF) developed by Forged Performance Products, Inc. (FPPI), and a fluidized powder hot load transfer medium process available through Ceracon, Inc. The first mentioned consolidation process employs graphite as both a pressure transmitting medium and an electrical resistor for heat generation. This process advantageously does not require dies and enables rapid powder consolidation without the need for canning or cladding. The PIF process uses liquefied argon to quickly apply pressures of up to about 60 ksi (about 4000 bar), enabling densification and consolidation to occur at relatively low temperatures (about 800 to 900° C.). Finally, the Ceracon process uses a powder medium to transmit pressure to the material being consolidated. 
     Each of the above consolidation processes entails similar pre-processing steps. In the processing of an airfoil intended to have multiple cooling channels, as shown in FIG. 10, the solid filler  118  would be manufactured to have a number of sections that nest within an appropriate number of grooves  114  on the substrate (spar)  110 . The particulate filler  120  would then be used to fill the gaps within the grooves  114  around the filler  118 , and then the assembly appropriately fixtured for consolidation using the desired consolidation process. To facilitate filling of the gaps, the particulate filler  120  is preferably dispersed in a binder that is burned off during consolidation and does not leave a detrimental residue. Typically, surplus particulate filler  120  is used to allow for compaction of the filler  120  during consolidation. Consolidation is carried out to densify the particulate filler  120  and bond the resulting mass to the substrate  110 . As shown in FIG. 5, the final level of the fill within the groove  114  is level with the surrounding surface  116  of the substrate  110 . 
     After consolidation (FIG.  5 ), the skin  112  is deposited by any suitable method. For airfoils and other gas turbine engine components having a nickel-base superalloy spar (substrate  110 ), preferred materials for the skin  112  include nickel-base superalloys, NiAI and NiAI-intermetallic composite alloys, and superalloy matrix composites. For niobium-base suicide composite spars, preferred materials for the skin  112  include similar niobium-base silicide composites, with composition and phase fractions adjusted to achieve the greater oxidation resistance required for the higher service temperature skin. Skins  112  of these materials can be deposited by plasma spraying and physical vapor deposition (PVD) techniques such as electron beam physical vapor deposition (EBPVD). A preferred EBPVD technique is that disclosed in U.S. Pat. No. 5,474,809, incorporated herein by reference. After deposition of the skin  112 , the solid filler  118  can be removed by etching without damaging the substrate  110  or skin  112 . A suitable etchant for preferentially etching a MONEL7 solid filler  118  from a nickel-based superalloy substrate  110  is 10% to 50% nitric acid in water. 
     In the second embodiment of the invention, represented by FIGS. 7 through 9, a skin  212  is again deposited over a substrate (spar)  210  having a groove  214  formed in its surface  216 . In contrast to the groove  114  of FIGS. 4 through 6, the groove  214  intentionally incorporates an inwardly-projecting extension  215  along its edge with the surface  216  of the substrate  210 . The extension  215  prevents the formation of a sharp (less than 90°) notch between the skin  212  and the wall of the groove  214 . Also contrary to the first embodiment of this invention, though a solid filler  218  and a particulate filler  220  are again used to completely fill the groove  214  as shown in FIG. 8, both are sacrificial to the process, and are removed following deposition of the skin  112  to form a cooling channel  222  having the same cross-sectional shape as the original groove  214 . 
     As is evident from FIG. 7, the solid filler  218  is again undersized for the groove  214 , creating a void or gap between the filler  218  and the walls of the groove  214 . The shape of the solid filler  218  is not as critical as that of the solid filler  118  of the first embodiment, since the former does not determine the shape of the channel  222 . As shown in FIG. 8, the particulate filler  220  fills the gap between the solid filler  218  and walls of the groove  214  so that the groove  214  is completely filled prior to deposition of the skin  212 . As before, the groove  214  is preferably filled so that, after consolidation, the material within forms a surface substantially coplanar with the surrounding surface  216  of the substrate  210 . Also as before, the upper surface of the solid filler  218  is substantially coplanar with the surrounding surface  216  of the substrate  210 , so that the skin  212  is deposited directly on the surface of the solid filler  218 . Alternatively, the solid filler  218  could be thinner than the depth of the groove  214 , in which case the particulate filler  220  would also be required to fill that portion of the groove  214  above the solid filler  218 . Preferably, a minimal amount of the particulate filler  220  is used in order to minimize the affect of shrinkage during consolidation. Otherwise, the groove  214  would be required to be overfilled with the filler  220  and then the excess removed after consolidation by machining, which could potentially damage the substrate/filler interface continuity. 
     Suitable materials for the solid and particulate fillers  218  and  220  must again be compatible with that of the substrate  210  in terms of CTE to prevent the formation of an excessive gap during deposition of the skin  212 , thermal conductivity to prevent overheating of the fill surface during deposition, and compositional compatibility to avoid detrimental interdiffusion during the consolidation and deposition steps. The solid filler  218  and particulate filler  220  are both preferably removed by etching, necessitating that each is formed of a material that can be preferentially etched from the substrate  210 . Consolidation of the particulate filler  220  within the groove  214  and deposition of the skin  212  can be performed using the techniques discussed in reference to the first embodiment of the invention. 
     While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims.