Abstract:
A gas turbine engine plenum cover is repaired using a prefabricated repair cover SPAD. The SPAD repair cover has a spoolie boss that is aligned and attached to the body of the repair plenum cover prior to attachment to a plenum casting to form a repaired plenum chamber. Preattachment of the spoolie boss by brazing methods, as well as optional machining of a borescope hole prior to or after attachment of the repair cover, greatly simplifies the repair process, thereby reducing the time required and cost of repair.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 60/612,230 filed Sep. 22, 2004, which application is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to repair methods for gas turbine engines, and more particularly, to a method of replacing a damaged plenum cover using a multiple detail assembly. 
     BACKGROUND OF THE INVENTION 
     In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor and ignited for generating hot combustion gasses which flow downstream through several turbine stages. A turbine stage includes stationary turbine vanes extending radially between inner and outer castings. The vanes are hollow with openings that mate to openings within the castings. The vanes have airfoil configurations for guiding the combustion gasses between corresponding turbine rotor blades disposed downstream of the vanes. The blades are mounted to the perimeter of a rotor disk that rotates during operation for providing power to turn the compressor or fan during operation. 
     Since the turbine vanes are heated during operation by the hot combustion gasses which flow thereover, cooling air bled from the compressor is channeled inside the vanes for cooling thereof. Typically, the cooling air flows toward the center of the turbine as the air passes through the vanes. In order to channel the cooling air through the vanes, the outer casing is fitted with a cover to create a plenum. The cover encases the compressed cooling air openings that mate with the vane openings. The cover has an opening to accommodate a spoolie boss which allows air to enter the plenum. 
     During normal operations, the cover may collapse due to engine cycling. This collapse is associated with cover distortion and elongation as well as spoolie boss distortion. While it is not known exactly why the plenum covers become distorted, it is believed that an adverse thermal gradient is responsible, with pressure changes possibly also an associated factor. 
     In order to repair the cover, a typical procedure involves removing the cover, installing a repair cover, and then attaching a spoolie boss to the cover. Additionally, if a borescope hole is desired, existing methods require machining the repair cover after installation. This method requires multiple repair steps which increase repair costs. What is needed is a method of repairing a plenum cover that requires fewer steps, lowers repair costs, and produces a repaired part that meets or exceeds OEM performance specifications. 
     SUMMARY OF THE INVENTION 
     The present invention is a novel method for repairing a plenum cover for a turbine engine including by fabricating a repair cover including attaching a spoolie boss to the cover to provide a one-piece spare-part detail (hereinafter “SPAD”) for subsequent use as a plenum cover replacement. 
     The present invention provides a method of repairing a gas turbine plenum that includes fabricating a repair cover, wherein the repair cover is fabricated from a sheet metal with an attached spoolie boss, and wherein the spoolie boss is adapted to permit the passage of a cooling gas. The method further includes the step of removing an undesired cover from a vane assembly, wherein the undesired cover is adapted to direct the flow of cooling gas to at least one vane opening, positioning the repair cover in a desired alignment with the vane assembly, and attaching the repair cover to the plenum. The step of attaching preferably includes use a of a first mechanical means to initially secure the part, followed by use of a second means to permanently attach the repair cover to the vane assembly. Optionally, the methods further include providing a borescope hole in the repair cover during the step of fabrication. 
     Advantages of the methods and apparatus of the present invention include a substantial reduction in manufacturing and repair costs due to recycling of previously non-repairable used vane assemblies made possible by the present invention. 
     A corresponding advantage is an increase in the service life of the vane assemblies due to repair and re-use facilitated by the present invention. 
     Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side perspective view of an exemplary prior art nozzle and vane assembly for a gas turbine engine. 
         FIG. 2  illustrates a top perspective view of the prior art nozzle and vane assembly of  FIG. 1 , wherein a plenum cover of the assembly is damaged. 
         FIG. 3  illustrates a top perspective view of the plenum casting of the assembly of  FIG. 1  following removal of the existing plenum cover in accordance with the present invention. 
         FIG. 4  is a repair plenum cover in accordance with the present invention. 
         FIG. 5  illustrates the repair plenum cover of  FIG. 4  attached to the plenum casting of  FIG. 3 . 
         FIG. 6  illustrates a flow chart of one embodiment of the method of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to the repair of low-pressure turbine (hereinafter “LPT”) nozzle and vane assemblies used in stages of gas turbine engines. In particular, the invention provides manual methods of repair of the inner and outer plenum covers of a LPT nozzle and vane assembly. 
     Referring now to  FIGS. 1-6 , the methods of the present invention include providing a SPAD having a multiple detail assembly that includes a repair plenum cover  34  having an attached spoolie boss  32 , the repair plenum cover  34  fabricated from a unitary sheet of material. In the repair method for a vane assembly  20  previously in service in a turbine engine, the nozzle and vane assembly  20  is first removed from the engine. The existing plenum cover  30  is then mechanically removed from the vane assembly to expose the plenum casting  25 . Optionally, the plenum casting  25  is machined as necessary, such as by CNC machining, to receive and retain a repair outer plenum cover  34 . Next, the repair plenum cover  34  is fabricated from sheet material and a spoolie boss  32  is attached using appropriate manual methods such as gas tungsten arc welding (“GTAW”), tack welding, and/or the like. The resulting SPAD is then alloyed to permanently attach the spoolie boss  32 , and is then permanently secured to the plenum casting, preferably using a brazing process. Optionally, a borescope hole is machined into the SPAD, whether before or after attachment of the spoolie boss  32  to the repair plenum cover  34 , as well as before or after the attachment of the SPAD to the plenum casting of the outer housing  26 . Finally, the repaired nozzle and vane assembly is inspected and machined as necessary so as to provide a turbine vane assembly that is meets predetermined specifications. Where a borescope hole  33  is provided, the inspection may include a visual inspection of the interior plenum chamber  25  of the repaired vane assembly using tools known in the art, such as a borescope inserted into a borescope hole  33  to inspect the plenum chamber  28 , the chamber formed by attachment of the repair cover  34  to the plenum casting  25 . 
       FIGS. 1-2  depict an exemplary nozzle and vane assembly  20  for a gas turbine aircraft engine (not shown). In these illustrations, vane assembly  20  includes an inner housing  22 , a plurality of vanes  24 , and an outer housing  26 . Vane assembly  20  has an outer plenum cover  30  attached to the plenum casting  25  of the outer housing  26 . In operation, vanes  24  are heated by hot compressed gasses and channel cooling air through channels (not shown) provided in the interior of the vanes  24  to permit cool, compressed air to cool the structure of vanes  24 . This cool air is directed from the compressor (not shown) through at least one outer spoolie boss  32  provided in the plenum cover  30 , and then to the channels in the vanes  24 . A portion of the outer housing  26 , such as the plenum casting  25 , is adapted to receivably retain the outer plenum cover  30 . 
     In  FIG. 2 , the existing outer plenum cover  30  has been damaged, and has partially collapsed. Collapse of the cover  30  is typically due to thermal and mechanical stress over an extended operating period in a gas turbine engine assembly, but can also be due to manufacturing defects. In any case, the collapse of the cover  30  compromises the volume of the plenum chamber  28 , thereby impeding proper cooling air flow through the assembly  20 . Replacement of the cover  30  only presents a very cost-efficient solution, so long as the other components of the assembly  20  are still within service specifications. An exemplary assembly  20  with the damaged cover  30  removed is shown in  FIG. 3 . The cover  30  can now be replaced with the repair cover  34 , as further described herein. 
     As shown in greater detail in  FIG. 4 , the repair plenum cover  34  includes a body portion  35  and a spoolie boss  32 . Body  35  provides a seal for the repair plenum cover  34  with the plenum casting  25  of the outer housing  26  to form a plenum chamber  28 , and spoolie boss  32  permits airflow from the compressor into the plenum chamber  28 . 
     As shown in detail in  FIG. 6 , in one embodiment, the method of repairing a gas turbine plenum comprises the steps of: in step  110 , fabricating a repair plenum cover  34 , wherein the repair cover  34  (shown in  FIG. 5 ) is fabricated from a sheet material and includes an attached spoolie boss  32 , wherein the spoolie boss  32  is adapted to permit the passage of a cooling gas into a plenum chamber  28  formed by attachment of the cover  34  to an outer plenum casting  26 ; in step  120 , removing an undesired existing plenum cover  30  from a nozzle and vane assembly  20 , wherein the undesired existing cover  30  is adapted to direct the flow of the cooling gas to a plurality of vane openings; in step  130 , positioning the repair plenum cover  34  in contact with and in a desired alignment with the plenum casting  26  of the nozzle and vane assembly; and in step  140 , attaching the repair cover  34  to the plenum casting  25  of the outer housing  26  to form a repaired nozzle and vane assembly meeting predetermined specifications. 
     As shown in  FIGS. 4-5 , the repair cover  34  is provided as a SPAD having predetermined specifications and features suitable for forming a repaired nozzle and vane assembly. Preferably, the repair plenum cover  34  includes a spoolie boss  32  and plenum cover body portion  35  comprising a high-temperature metal or alloy that can endure the environmental demands of operating in the flowpath of a gas turbine engine assembly. The high-temperature material is preferably a nickel-based superalloy, cobalt-based superalloy, and combinations thereof. Preferably, the high-temperature alloy comprises RENE′ 77 or RENE′ 80. RENE™ is a trademark of General Electric Company, Fairfield, Conn. for superalloy metals having nominal compositions as indicated in Table 2 below. Other suitable high-temperature alloys are also shown in Table 1. 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Material 
               
             
          
           
               
                   
                 C 
                 Mn 
                 Si 
                 Cr 
                 Fe 
                 B 
                 Zr 
                 Ni 
                 Co 
                 Al 
                 Ti 
                 Mo 
                 W 
                 Others 
                 V 
                 Hf 
                 Ta 
                 Re 
                 Misc. 
               
               
                   
                   
               
             
          
           
               
                 MA734 
                 0.05 
                   
                   
                 20.0 
                   
                   
                   
                 Bal 
                   
                 0.3 
                 0.5 
                   
                   
                   
                   
                   
                   
                   
                 2.5 
               
               
                 MA754 
                 0.11 
                 — 
                 — 
                 8.9 
                 — 
                 0.015 
                 0.05 
                 Bal 
                 10 
                 4.75 
                 2.5 
                 2 
                 7 
                 Ta 3.8; Hf 1.5 
               
               
                 Rene&#39; 80H 
                 0.17 
                 — 
                 — 
                 14 
                 0.2 
                 0.015 
                 0.013 
                 Bal 
                 9.5 
                 3 
                 4.8 
                 4 
                 4 
                 Hf 0.75 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 Rene&#39; 108 
                 0.08 
                 0.1 
                 — 
                 8.4 
                 — 
                 0.015 
                 — 
                 Bal 
                 9.5 
                 5.5 
                 0.7 
                 0.5 
                 9.5 
                 Ta 3.1; Hf 1.5 
               
               
                 Rene&#39; 125 
                 0.11 
                 — 
                 — 
                 8.9 
                 — 
                 0.015 
                 0.05 
                 Bal 
                 10 
                 4.75 
                 2.5 
                 2 
                 7 
                 Ta 3.8; Hf 1.5 
                 — 
                 1.55 
                 3.8 
                 — 
                 — 
               
               
                 Rene&#39; 142 
                 0.12 
                 0.1 
                 — 
                 6.8 
                 0.2 
                 0.015 
                 — 
                 Bal 
                 12 
                 6.15 
                 — 
                 1.5 
                 4.9 
                 Ta 6.35; 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Hf 1.5; Re 2.8 
               
               
                 Rene&#39; N4 
                 0.06 
                 — 
                 — 
                 9.75 
                 — 
                 0.004 
                 0.02 
                 Bal 
                 7.5 
                 4.2 
                 3.5 
                 1.5 
                 6 
                 Ta 4.8; Hf 0.15 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 Rene&#39; N5 
                 0.05 
                 — 
                 — 
                 7 
                 — 
                 0.004 
                 — 
                 Bal 
                 7.5 
                 6.2 
                 — 
                 1.5 
                 5 
                 Ta 6.5; Re 3; 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Hf 0.15 
               
               
                 Rene&#39; 80 
                 0.17 
                 0.2 
                 0.2 
                 14 
                 0.2 
                 0.005 
                 0.03 
                 Bal 
                 9.5 
                 3 
                 5 
                 4 
                 4 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 Rene&#39; 77 
                 0.07 
                 0.15 
                 0.2 
                 14.2 
                 0.5 
                 0.016 
                 0.04 
                 Bal 
                 15 
                 4.3 
                 3.4 
                 4.2 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 Mar-M-509 
                 0.06 
                 0.1 
                 0.4 
                 22.5 
                 1.5 
                 0.01  
                 0.5 
                 10 
                 Bal 
                 — 
                 — 
                 — 
                 7 
                 Ta 3.5 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 X40 
                 0.5 
                 1 
                 1 
                 25 
                 2   
                 — 
                 — 
                 10.5 
                 Bal 
                 — 
                 — 
                 — 
                 7.5 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 HS188 
                 0.1 
                 1.25 
                 0.4 
                 22 
                 3.5 
                 — 
                 — 
                 22 
                 Bal 
                 — 
                 — 
                 — 
                 15.5 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 L605 
                 0.1 
                 1.5 
                 1 
               
               
                 Hastelloy X 
                 0.1 
                 1 
                 1 
               
               
                 Inconel 625 
                 0.1 
                 0.5 
                 0.5 
               
               
                 Inconel 600 
                 0.15 
                 1 
                 0.5 
               
               
                 Inconel 
                 0.05 
                 0.35 
                 0.35 
                 19 
                 bar- 
                 0.006 
                 — 
                 52.5 
                 — 
                 0.5 
                 0.9 
                 3 
                 — 
                 Nb + Ta = 4.9 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 718(C) 
                   
                   
                   
                   
                 ium. 
               
               
                 17-4PH (C) 
                 0.05 
                 0.6 
                 0.7 
                 17 
                 Bal 
                 — 
                 — 
                 4 
                 — 
                 — 
                 — 
                 — 
                 — 
                 Cu = 3.1 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 Ti 6-4 (C) 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 6 
                 Bal 
                 — 
                 — 
                 Va = 4 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 Rene&#39; 220C 
                 0.03 
                 0.05 
                 0.1 
                 19 
                 0.5 
                 0.004 
                 0.01 
                 Bal 
                 12 
                 0.5 
                 1 
                 3.2 
                 — 
                 Nb 5.3; Ta 3.3 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 GTD 222 
                 0.1 
                 0.1 
                 0.25 
                 22.5 
                  0.35 
                 0.004 
                 0.013 
                 Bal 
                 19 
                 1.2 
                 2.3 
                 0.15 
                 2 
                 Ta 1 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 Rene&#39; 95 
                 0.15 
                 0.15 
                 0.2 
                 14 
                 — 
                 0.01  
                 0.05 
                 Bal 
                 8 
                 3.5 
                 2.5 
                 3.5 
                 3.5 
                 Nb 3.5 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 Rene&#39; 88DT 
                 — 
                 — 
               
               
                 Inconel 718 
                 0.05 
                 0.35 
                 0.35 
                 19 
                 Bal 
                 0.006 
                 — 
                 52.5 
                 — 
                 0.5 
                 0.9 
                 3 
                 — 
                 Nb + Ta = 4.9 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 Direct Age 
                 0.05 
                 0.35 
                 0.35 
                 19 
                 Bal 
                 0.006 
                 — 
                 52.5 
                 — 
                 0.5 
                 0.9 
                 3 
                 — 
                 Nb + Ta = 4.9 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 718 
               
               
                 Hi Strength 
                 0.05 
                 0.35 
                 0.35 
                 19 
                 Bal 
                 0.006 
                 — 
                 52.5 
                 — 
                 0.5 
                 0.9 
                 3 
                 — 
                 Nb + Ta = 4.9 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 718 
               
               
                 Ti 17 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 2 
                 — 
                 — 
                 5 
                 Bal 
                 4 
                 — 
                 Sn 2 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 Ti 6-4 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 6 
                 Bal 
                 — 
                 — 
                 V 4 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 Ti 8-1-1 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 8 
                 Bal 
                 1 
                 — 
                 V 1 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 Ti 6-2-4-2 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 4 
                 — 
                 — 
                 6 
                 Bal 
                 2 
                 — 
                 Sn 2 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                   
               
             
          
         
       
     
     More preferably, the high temperature alloy is HASTELLOY X. HASTELLOY™ is a trademark for alloy metals owned by HAYNES INTERNATIONAL, INC. CORPORATION, KOKOMO Ind. 46904-9013. HASTELLOY X is assigned UNS No. N06002, and is also assigned the following ASM numbers: AMS 5798, AMS 5799, AMS 5888, AMS 5536, AMS 5588, AMS 5754. HASTELLOY X material is recommended especially for use in high-temperature applications because it has unusual resistance to oxidizing, reducing, and neutral atmospheres. For example, furnace rolls made of this alloy were still in good condition after operating for 8,700 hours at 2150° F. (1177° C.). HASTELLOY X also has performed well in jet engine tailpipes, afterburner components, cabin heaters, and other aircraft parts. HASTELLOY X is a nickel base alloy that possesses exceptional strength and oxidation resistance up to 2200° F. It has also been found to be exceptionally resistance to stress-corrosion cracking in petrochemical applications. The alloy has excellent forming and welding characteristics. HASTELLOY X can be forged and, because of its good ductility, can be cold-worked. It can be welded by both manual and automatic welding methods including shielded metal-arc (coated electrodes), gas tungsten-arc (TIG), and gas metal-arc (MIG). HASTELLOY X can also be resistance-welded. HASTELLOY X is further characterized as follows in Table 2 below: 
     
       
         
               
               
               
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Element 
                 Min 
                 Max 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Molybdenum 
                 8.00 
                 10.0 
               
               
                   
                 Chromium 
                 20.5 
                 23.0 
               
               
                   
                 Iron 
                 17.0 
                 20.0 
               
               
                   
                 Tungsten 
                 0.20 
                 1.00 
               
               
                   
                 Cobalt 
                 0.50 
                 2.50 
               
               
                   
                 Carbon 
                 0.05 
                 0.015 
               
               
                   
                 Silicon 
                 — 
                 1.00 
               
               
                   
                 Manganese 
                 — 
                 1.00 
               
               
                   
                 Boron 
                 — 
                 0.01 
               
               
                   
                 Phosphorus 
                 — 
                 0.04 
               
               
                   
                 Sulfur 
                 — 
                 0.03 
               
             
          
           
               
                   
                 Nickel 
                 Balance 
               
               
                   
                   
               
             
          
         
       
     
     In addition to manual attachment methods for initially connecting the repair cover  34  to the spoolie boss  32  and the plenum casting  25  of the housing  26 , a brazing repair material is used to attach the spoolie boss  32  to the body  35  of the repair cover  34 , and the repair cover  34  to the housing  26 , is preferably a braze material that is selected to be compatible with the material of the repair cover  34  and the spoolie boss  32 . The brazing material can be provided in the form of wire, rod, strip, foil, powder, and/or a viscous mixture (paste) including powder in a suitable binder. Where the housing  26 , cover  34  and/or spoolie boss  32  are comprised of stainless steel, the repair material is preferably a nickel alloy brazing material. More preferably, the repair cover  34  and spoolie boss  32  are made of 310 stainless steel or 410 stainless steel, and the brazing material is a nickel alloy brazing filler. Most preferably, the brazing filler is comprised of about 4.5% Si, 7.0% Cr 3.1% B, 3.0% Fe, balance Ni, and characterized by a solidus-liquidus range of between about 1780 to about 1830° F. (about 971 to about 999° C.). An exemplary nickel alloy brazing material having these properties is SAE AMS4777F, as specified, described and published by the Society of Automotive Engineers, Inc. of Warrendale, Penn., USA. However, other nickel alloy brazing materials can be utilized. In this embodiment, the preferred nickel alloy brazing material is capable of joining nonferrous alloys and corrosion and heat resistant steels and alloys, and displays low flow point and corrosion and produces oxidation resistant joints with good strength at elevated temperatures. A further benefit of the preferred nickel alloy brazing material is that it provides a corrosion and oxidation resistant hard coating that can be smoothed and otherwise worked to yield a smooth repair area on turbine engine components that are routinely exposed to high operating temperatures. Because the braze material includes Boron, an element that diffuses very rapidly, and Si, an element that diffuses quickly (but not as quickly as Boron), these elements, which contribute to the solidus temperature of the braze material being lower than the materials being joined, will rapidly diffuse from the braze material upon high temperature exposure, thereby raising the solidus temperature of the braze material. 
     Alternatively, where the material comprising the housing  26 , repair cover  34  and/or spoolie boss  32  is an alloy, such as an iron-based superalloy, cobalt-based superalloy or a nickel-based superalloy, an appropriate repair material can be selected according to the alloy material. Appropriate braze materials and methods for such alloys are disclosed in commonly owned U.S. Pat. No. 6,464,128, which is hereby incorporated by reference in its entirety. For example, in one embodiment, the repair material is a braze material comprising a powder of a first alloy component and a powder of a second alloy component, each component having different solidus temperatures. The repair material that is later formed as a melted mixture of the first powder and the second powder has a solidus temperature less than that of a shroud material that forms a repair region on the assembly  20 , such as the joinder between the repair cover  34  and spoolie boss  32 , and the repair cover  34  and the plenum casting  25  and outer housing  26 . 
     The repair material powders selected for use with cobalt-base replacement elements  34 ,  32  and the housing  26  materials are different from those selected for nickel-base assemblies  20  and elements  34 ,  32 . In one embodiment, the sheet material of the repair cover  34  is a cobalt-base alloy known as Mar M509, which has a nominal composition, in weight percent, comprising about 23.5 percent chromium, about 10 weight percent nickel, about 7 percent tungsten, about 3.5 percent tantalum, about 0.2 percent titanium, about 0.4 percent zirconium, about 0.6 percent carbon, no more than about 2 percent iron, the balance cobalt and impurities. Additionally, in this embodiment, the braze repair material can include melt-depressants such as Boron and Silicon. 
     Preferably, for cobalt-base housing  26  and repair elements  34 ,  32 , the first alloy component of the repair material preferably comprises a prealloyed composition, in weight percent, of from about 10 to about 25 percent nickel, from about 15 to about 25 percent chromium, from about 5 to about 10 percent silicon, from about 2 to about 6 percent tungsten, from about 0.2 to about 0.8 percent carbon, from about 0.4 to about 2.0 percent boron, balance cobalt and impurities. The second alloy component preferably comprises a prealloyed composition, in weight percent of from about 5 to about 15 percent nickel, from about 15 to about 30 percent chromium, about 2.0 percent maximum silicon, from about 5 to about 10 percent tungsten, from about 0.3 to about 0.8 percent carbon, about 1.5 percent maximum manganese, about 3 percent maximum iron, about 0.5 percent maximum zirconium, balance cobalt and impurities. The first fraction is preferably from about 25 weight percent to about 50 weight percent, most preferably about 35 weight percent. The second fraction is preferably from about 75 weight percent to about 50 weight percent, most preferably about 65 weight percent. 
     In another embodiment, the housing  26 , repair cover  34  and/or spoolie boss  32  can comprise a nickel-base superalloy such as Rene N5, which has a nominal composition, in weight percent, of from about 6 to about 6.4 percent aluminum, from about 6.75 to about 7.25 percent chromium, from about 7 to about 8 percent cobalt, from about 0.12 to about 0.18 percent hafnium, from about 1.3 to about 1.7 percent molybdenum, from about 2.75 to about 3.25 percent rhenium, from about 6.3 to about 6.7 percent tantalum, from about 4.75 to about 5.25 percent tungsten, a sum of aluminum plus tantalum about 12.45 percent minimum, balance nickel and impurities. Where the assembly  20  material is a nickel-base superalloy such as Rene N5, the first alloy component preferably comprises a prealloyed composition, in weight percent, of from about 10 to about 20 percent cobalt, from about 14 to about 25 percent chromium, from about 2 to about 12 percent aluminum, from 0 to about 0.2 percent yttrium, balance nickel and impurities. The second alloy component preferably comprises a prealloyed composition, in weight percent of from about 10 to about 20 percent cobalt, from about 14 to about 25 percent chromium, from about 2 to about 12 percent aluminum, from about 2 to about 12 percent silicon, balance nickel and impurities. The first fraction is preferably from about 55 to about 80 weight percent, most preferably about 68.5 weight percent. The second fraction is preferably from about 45 weight percent to about 20 weight percent, most preferably about 31.5 weight percent. 
     Where the braze repair material is comprised of two types of individually prealloyed powders, the powders may be provided in a loose, free-flowing form. Alternatively, they may instead be provided as an unsintered compact or a pre-sintered compact. Both approaches are operable, although the use of the pre-sintered compact can be more practical for production operations. In this latter approach, the powders are mixed together, pressed with a binder into a desired shape before applying to the repair region. Optionally, to form a pre-sintered compact, the mixed powders can be pre-sintered by heating to a temperature where the powders are slightly sintered together to form a compact. It is not necessary that either compact have a high relative density (that is, little porosity), as it is later fully melted. The compacts are more easily handled and positioned than are the free-flowing powders, and there is less compaction and shrinkage in subsequent melting. A combination of these approaches may be desired. 
     The braze repair material is placed into the repair region adjacent the elements  25 ,  32 ,  34  to be joined. The repair material may be the mixture of the free-flowing powders, the compact, or a combination of both. The amount of braze repair material is preferably selected so that, after subsequent melting and machining, the repair regions are restored to approximate a predetermined desired service dimension. However, machining can be performed as an additional step to impart and restore desired dimensions of the original nozzle and vane assembly  20 . 
     To attach the spoolie boss  32  to the repair cover  34 , the spoolie boss  32  is placed through a hole provided in the repair cover  34 . Manual attachment methods such as GTAW are used to position the spoolie boss  32  on the cover  34 . A braze repair material is next placed in the repair region formed by the touching surfaces of the cover  34  and spoolie boss  32 . The repair material is next heated to a brazing temperature to melt at least a portion of the repair material but not the cover  34  material or the spoolie boss  32 . The braze repair material should always be selected so that its melting temperature is below both that of the material of the repair spoolie boss  32  and the repair cover  34 . In the case of the above-discussed preferred repair material for stainless steel components  32 ,  34  and the indicated brazing repair material R15/D80, the brazing temperature is between about 1780° F. to about 1830° F. However, the brazing temperature can be varied between about 1400 and about 2200° F. based on parameters of the particular repair, including material, mass, configuration, and application. Where the elements  32 ,  34 , are comprised of cobalt-base alloys or the nickel-base alloys, the brazing temperature is from about 2190° F. to about 2335° F., and preferably from about 2300° F. to about 2325° F. However, the brazing temperature range can be further varied based on parameters of the particular repair, including mass, configuration, and application. In embodiments wherein the repair material includes two alloy powders, at the brazing temperature, the powder having the lower solidus temperature melts to accelerate the bonding and the densification process, while the powder having the higher solidus temperature remains substantially solid so that the powder mass generally retains its shape. 
     The brazing is preferably performed in a vacuum furnace. The brazing time, defined as the period of time at which the repair region is maintained at a temperature sufficient to braze the repair material, can also be varied based on parameters of the particular repair, including material, mass, configuration, and application. For example, the brazing time is typically on the order of about 20 minutes to about 2 hours. More preferably, the brazing time is between about 1 hour to about 2 hours. After brazing, the repaired assembly is braze cooled to solidify the repair material. The braze repair material solidifies, forming a metallurgical bond between the connected elements  32 ,  34 . The result is a repair cover  34  suitable for permanently affixing to the plenum casting  25 . 
     Attachment to the plenum casting is performed using the same attachment and brazing methods described for attaching the spoolie boss  32  to the repair cover  34 , taking into account the composition of the casting  25  and the repair cover  34 . Additionally, in most cases, the amount of braze repair material is selected so that the repair region will be slightly oversize after the brazing and cooling steps. Although it would be desirable to make the repair exactly the right size after brazing and cooling, it is typically not possible to control the amount and distribution of the repair metal that precisely. Accordingly, the repair region can be made oversize, and then excess material machined away to achieve the correct size and with the necessary details to restore the repair region to operating tolerances. 
     EXAMPLES 
     The invention has been applied to repair of CF6 stage 1 LPT vanes in General Electric CF6-80C2 and CF6-80E nozzles. In those models, 22 plenum covers are provided in each LPT nozzle and vane assembly, with 2 spoolies and 2 covers per nozzle—an inner cover and an outer cover  30 . In this embodiment, a spoolie boss  32  receiver cup comprising HASTELLOY X alloy was attached to the sheet material comprising HASTELLOY X using the Activated Diffusion Healing (“ADH”) method of brazing, at approximately 2210° F. using R80/D15 alloy brazing material to form a repair cover  30 . R80/D15 is a 50/50 mixture by weight of Rene &#39;80 (GE Spec. B50TF183-C1.A) and Braze Alloy D15 (GE Spec. B50TF173-C1.A), and is available from Sulzer Metco, Inc. of Westbury, N.Y. 11590. The resulting assembly was attached to a RENE &#39;77 plenum casting  25  using welding and the ADH method at 2210° F. using R80/D15 alloy brazing material. The composition of R80 and D15, respectively, used in this Example are as follows: 
     GE Spec. B50TF173, D-15 Brazing And Activated Diffusion Bonding Alloy Nickel Base (by Chemical Composition and Weight Percent). 
                                                     Carbon   0.05   Max.   Molybdenum   0.05   Max.       Manganese   0.10   Max.   Tungsten   0.05   Max.       Sulfur   0.01   Max.   Iron   0.5   Max.       Chromium   14.8-15.8       Boron   2.1-2.5       Cobalt   9.5-11.0       Zirconium   0.05   Max.       Titanium   0.05   Max.   Selenium   0.005   Max.       Tantalum   3.0-3.8       Oxygen   0.03   Max.       Aluminum   3.2-3.7       Nitrogen   0.03   Max.       Total Other Impurities   0.1   Max.                    
GE Spec. B50TF183, RENE&#39;80 ALLOY POWDER (by Chemical Composition and Weight Percent).
 
     
       
         
               
               
               
               
               
               
             
           
               
                   
               
             
             
               
                 Carbon 
                 0.15-0.19 
                   
                 Cobalt 
                 9.00-10.00 
                   
               
               
                 Manganese 
                 0.10 
                 Max. 
                 Zirconium 
                 0.03-0.10 
               
               
                 Silicon 
                 0.10 
                 Max. 
                 Iron 
                 0.35 
                 Max. 
               
               
                 Sulfur 
                 0.0075 
                 Max. 
                 Niobium 
                 0.10 
                 Max. 
               
               
                 Phosphorus 
                 0.015 
                 Max. 
                 Tantalum 
                 0.10 
                 Max. 
               
               
                 Chromium 
                 13.70-14.30 
                   
                 Vanadium 
                 0.10 
                 Max. 
               
               
                 Titanium 
                 4.80-5.20 
                   
                 Copper 
                 0.10 
                 Max. 
               
               
                 Boron 
                 0.01-0.02 
                   
                 Hafnium 
                 0.10 
                 Max. 
               
               
                 Aluminum 
                 2.80-3.20 
                   
                 Magnesium 
                 0.01 
                 Max. 
               
               
                 Tungsten 
                 3.70-4.30 
                   
                 Oxygen 
                 0.0300 
                 Max (1) 
               
               
                 Molybdenum 
                 3.70-4.30 
                   
                 Nitrogen 
                 0.0300 
                 Max. (1) 
               
               
                 Tungsten + 
                 7.70 
                 Min. 
                 Nickel 
                   
                 Remainder 
               
               
                 Molybdenum 
                   
                   
                 Nv3 
                 2.32 
                 Max. 
               
               
                   
               
             
          
         
       
     
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.