Abstract:
A method for repairing a foot of a stator vane segment includes the step of locating a cast stator vane segment with a foot requiring repair. A damaged portion of the foot requiring repair is then removed from the cast stator vane segment. A replacement foot section is then secured onto the cast stator vane segment using an electron beam weld. Lastly, the replacement foot section is machined to a suitable shape.

Description:
TECHNICAL FIELD  
       [0001]     This invention relates to a cast article, such as a stator assembly of the type used in rotary machines, and more particularly, to the repair of areas of such articles having limited access.  
       BACKGROUND ART  
       [0002]     Typical axial flow rotary machines, such as those found in gas turbine engines for aircraft, have a series of sections. Each section has an array of cast stator vane segments disposed upstream of an array of rotor blades in both the compressor section and turbine section of the engine. Stator assemblies normally include an outer casing with inwardly directed vanes that end in sealing elements, such as an inner shroud assembly. Both the outer casing and inner shroud extend circumferentially around the working medium flow path through the machine. The arrays of cast stator vane segments found in a stator assembly are designed to interact with the working medium gases that flow through the engine.  
         [0003]     The extreme temperatures found in the compressor section of the engine along with other operating conditions (e.g. vibration) can cause wear and damage to parts within this section. Due to the difficulty of accessing various areas of these parts that may have been worn or damaged during operation, conventional practice has been to replace rather than attempting to repair these parts. Additionally, the conditions these parts will encounter in service preclude the use of brazing or standard welding techniques to repair them. A repair technique that can return damaged cast stator vane assemblies to service in such an environment is greatly desired.  
       SUMMARY OF THE INVENTION  
       [0004]     One embodiment of the present invention is directed towards a method for repairing a foot of a cast stator vane segment. This method includes the steps of removing a portion of a foot from a cast stator vane segment and securing a replacement foot section onto the cast stator vane segment using an electron beam weld.  
         [0005]     Another embodiment of the present invention discloses a method for repairing an inaccessible area of an article. This method includes the steps of removing a section of an article located in an inaccessible area and securing a replacement section to the article.  
         [0006]     Other aspects and advantages of the present invention will become apparent upon consideration of the following detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is a side elevational view in partial section of an axial flow rotary machine, such as a gas turbine aircraft engine;  
         [0008]      FIG. 2  is an enlarged detailed view of a portion of the high pressure compressor section of the engine shown in  FIG. 1 ;  
         [0009]      FIG. 3  is an enlarged isometric view of a portion of the high pressure compressor section of the engine shown in  FIG. 1 ;  
         [0010]      FIG. 4  is a schematic view of a foot requiring repair;  
         [0011]      FIG. 5  is a schematic view of the foot of  FIG. 4  with an area removed;  
         [0012]      FIG. 6  is a schematic view of the foot of  FIG. 5  with a replacement foot section;  
         [0013]      FIG. 7  is a schematic view of the foot and the replacement foot section of  FIG. 6  welded together; and  
         [0014]      FIG. 8  is a schematic view of the foot and the replacement foot of  FIG. 7  after machining. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]      FIGS. 1-3  show portions of an axial flow rotary machine, such as a gas turbine aircraft engine  20 . The engine  20  includes a compressor section  22 , a high pressure compressor section  23 , a combustion section  24 , and a turbine section  26 . The engine is disposed about an axis of symmetry A and has a flow path  28  for working medium gases that extends axially through the sections of the engine  20 .  
         [0016]     With reference to  FIG. 2 , a stator assembly  30  including an array of cast stator vane segments  32  (only one vane  33  is shown) is located in the engine  20 . A rotor assembly  34  includes an array of rotor blades  35  (only two blades are shown) that extends radially outward across the working medium flow path  28 . The rotor assembly  34  is supported by various static bearing assemblies within the engine  20 . The stator assembly  30  includes an outer case (not shown) that acts as a pressure vessel to confine the working medium gases to the working medium flow path  28 .  
         [0017]     With particular reference to  FIG. 3 , each cast stator vane segment  32  has an outer platform  36 , an inner platform  37 , and one or more vanes (“airfoils”)  33  extending between the platforms  36 ,  37 . The inner platform  37  is typically comprised of feet  38  that are disposed circumferentially with respect to the axis of symmetry A. The cast stator vane segment  32  extends between the outer case and an inner shroud  40 . The inner shroud  40  has corresponding projections that extend into the grooves formed by the feet  38 . By combining a plurality of stator assemblies  30  circumferentially throughout the compressor, high pressure compressor, and turbine sections  22 ,  23 ,  26  of the engine  20 , the cast stator vane segment  32  interacts with the working medium gases to direct the flow downstream to the rotor blades  36 . The stator vane segment can be made from any suitable alloy, such as a high temperature nickel-based alloy.  
         [0018]     During use, aerodynamic and thermal forces can cause stresses in the stator assemblies  30 . Such stresses may cause the stator assembly  30  to be worn down or crack. If there is limited access to such worn areas, conventional weld build-up techniques cannot repair the location. In particular, a narrow groove portion  41  on the foot  38  is not accessible with conventional welding techniques.  FIG. 4  shows an example of a damaged portion  100  on the foot  38  of the stator assembly.  
         [0019]     Therefore, one advantage to the disclosed embodiment of the present invention is the ability to repair inaccessible locations. In a preferred embodiment, the inaccessible location would be the feet  38  of later stage stator assemblies  42  in the high pressure compressor section  23  of twin spool turbine engines. For example, the feet  38  of the 12 th  and 13 th  stator vane segments in a Pratt &amp; Whitney 4000 high pressure compressor could be repaired. It is also envisioned that any stator assembly  30  within the engine  20  of like material or configuration can be repaired with the present embodiment of the invention. Additionally, because typical welding and brazing process temperatures would exceed the heat treatment specifications of the materials and weaken the repaired part, typical welding and brazing cannot be successfully used to repair the above stator vane segments  32 .  
         [0020]     The disclosed embodiment of the present invention utilizes electron beam welding to realize new advantages in stator foot  38  repairs. Electron beam welding is a fusion joining process that produces a weld by impinging a beam of high energy electrons to heat the weld joint. Some of the distinct advantages to using the electron beam welding process are the low distortion effects and the ability to have a narrow heat affected zone. By using an electron beam welding process, the feet  38  of later stage stator assemblies  42  in the high pressure compression section  23  can now be repaired instead of being replaced. Additionally, the use of an electron beam weld allows an increased area of the foot  38  to be replaced in both compressor sections  22 ,  23  of the engines  20  by creating a stronger and more stress resistant weld than other conventional techniques. An example of a suitable electron beam welding apparatus is a Leybold-Heraeus W-3 electron beam welder.  
         [0021]     In one embodiment of the present invention, an operator locates the foot  38  on the cast stator vane segment  32  requiring repair. It is envisioned that each foot  38  may have different tolerance limitations for the determination of whether the foot  38  is damaged enough to necessitate being repaired, is too damaged and must be replaced, or can be reused without repair.  
         [0022]     Once it is determined that the foot  38  can be repaired, the damaged portion  100  is then removed from the foot  38  using, for example, conventional machining techniques. When removing the damaged portion  100 , an area  110  is removed that is typically of greater size than the damaged portion  100 , as can be seen in  FIG. 5 . The area  110  is separated from the foot  38  at a cut line  112 . By removing a larger portion of the foot  38 , the operator can also remove any localized defects that may exist near the damaged portion  100 . Another benefit of removing a larger area  110  is the ability to position the cut line  112  in the foot  38  so as to maximize the effectiveness of the welding to be performed on the foot  38 .  
         [0023]      FIG. 6  shows how in one embodiment a replacement foot section  120  is moved towards the cut line  112  of the foot  38 . The replacement foot section  120  is then tack welded onto the foot  38  using conventional low temperature welding techniques. The joint at cut line  112  is then electron beam welded to secure the replacement foot section  120  onto the foot  38 , as can be seen in  FIG. 7 . A groove  122  may form in the replacement foot section  120  during the electron beam welding process. This is because the material has been drawn in between the replacement foot section  120  and the foot  38  at the cut line  112 . After welding, the vane segment  32 , including the replacement section  120 , are heat treated to relieve any stress build-up during the electron beam welding process. One skilled in the art will know how to weld and how to heat treat the pertinent areas for repair of the foot  38 .  
         [0024]      FIG. 7  shows the electron beam welded replacement foot section  120  attached to the foot  38 . Cut-out lines  130  are shown where the replacement foot section  120  is to be machined. Those skilled in the art will know how to machine the replacement foot section  120  to the appropriate tolerances.  FIG. 8  shows the foot  38  after having been successfully repaired using the present method.  
         [0025]     Numerous modifications to the present invention will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only and is presented for the purpose of enabling those skilled in the art to make and use the invention and to teach the best mode of carrying out same. The exclusive rights to all modifications which come within the scope of the appended claims are reserved.