Abstract:
A method for restoring a turbine engine blade includes removing material from a wear/damage site on an OD shroud of the blade. Additional material is laser cladded to the site and then machined to restore the shroud.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention relates to gas turbine engines. More particularly, the invention relates to gas turbine engines having turbine blades with outer diameter (OD) shrouds.  
         [0002]     Typical gas turbine engines include interspersed stages of rotating airfoils (blades) and non-rotating airfoils (vanes) in each of the compressor and turbine sections. Many different blade and vane configurations exist. Among blades, many typical configurations include an airfoil extending from an airfoil inboard end at a platform to a free tip. A mounting root (e.g., a convoluted so-called “fir tree”) depends from the platform for attaching the blade to a separate disk. In such configurations, the tips of the mounted blades may rotate in close facing proximity to an associated circumferential blade outer air seal (BOAS) assembly carried by the engine case.  
         [0003]     In some configurations, the airfoils carry mid-span and/or OD shrouds. The term “shroud” is often used interchangeably to denote the individual segment carried by an individual blade and the resulting full circumferential structure provided by the combination of segments of the stage of blades as attached to their associated disk.  
         [0004]     A particular example of a blade stage having an OD shroud is the HPT first stage (T1) of the Pratt &amp; Whitney (division of United Technologies Corporation, East Hartford, Connecticut) JT8D long-used, for example, in Boeing 727, 737, and DC-9/MD80 aircraft. Damage has been observed to the outboard surfaces of the OD shrouds.  
         [0005]     A variety of restoration techniques have been proposed for turbine engine components. These include welded prostheses, and various build-up repairs including brazing, welding, and depositions. U.S. Patent Application Publication 20050178750A1 discloses laser cladding remanufacturing of sulphidation-attacked turbine engine parts, with specific reference to platforms. U.S. Patent Application Publication 20040086635A1 discloses laser cladding remanufacturing of a damaged gas turbine engine stationary (non-rotating) shroud.  
       SUMMARY OF THE INVENTION  
       [0006]     One aspect of the invention involves a method for restoring a turbine engine blade. Material is removed from a wear/damage site on an OD shroud of the blade. Additional material is laser cladded to the site and then machined to restore the shroud.  
         [0007]     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is view of a prior art HPT blade.  
         [0009]      FIG. 2  is a side view of the blade of  FIG. 1 .  
         [0010]      FIG. 3  is an outboard end view of the blade of  FIG. 1 .  
         [0011]      FIG. 4  is a cutaway side view of the blade of  FIG. 1  installed in an engine.  
         [0012]      FIG. 5  is an outboard end view of the blade of  FIG. 1  in a worn condition.  
         [0013]      FIG. 6  is an outboard end view of the blade of  FIG. 1  in a first intermediate condition of restoration according to the present invention.  
         [0014]      FIG. 7  is an outboard end view of the blade of  FIG. 1  in a second intermediate condition of restoration according to the present invention. 
     
    
       [0015]     Like reference numbers and designations in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0016]      FIG. 1  shows an exemplary shrouded blade  20 . The exemplary blade is generally representative of one prior art first stage HPT blade used on various members of the Pratt &amp; Whitney JT8D engine family. However, the methods described below may be applied to other blades.  
         [0017]     The blade may be made as a superalloy casting (e.g., of a nickel-base superalloy such as MAR-M-200+Hf originally developed by Lockheed Martin or PWA1447 of Pratt &amp; Whitney), optionally coated (e.g., with a thermal barrier coating such as PWA70/73 dual coating, PWA 270/273 dual coating, or PWA 36095 platinum aluminide, all of Pratt &amp; Whitney). The exemplary blade  20  has an airfoil  22  extending radially outward from an inboard end  24  at an outboard surface  26  of a platform  28 . The radial direction is defined relative to an engine centerline when the airfoil is mounted to a disk (not shown). The blade includes a fir tree attachment root  30  depending from an inboard surface (underside)  32  of the platform  28 . The blade includes an OD shroud  34  at the outboard end  36  of the airfoil. The shroud underside  38  and platform outboard surface  26  locally define respective outboard and inboard extremes of the engine core flowpath.  
         [0018]     The airfoil includes a leading edge  40 , a trailing edge  42 . The airfoil has a generally concave pressure side  44  and a generally convex suction side  46  extending between the leading edge  40  and the trailing edge  42 .  
         [0019]     For reference,  FIG. 2  shows an afterward/generally downstream direction  500  and the radial (radially outward) direction  502 .  FIG. 2  shows the shroud  34  as having a central radially-outwardly projecting spoiler  50  generally formed as a segment of an annular flange. The spoiler  50  has an outboard surface  52 , a radially-extending forward/upstream/leading surface  54 , and a radially-extending aft/downstream/trailing surface  56 . On a leading side of the spoiler, the shroud includes a leading portion  60  extending to a leading rim  62  and having an outboard surface  64 . On a trailing side of the spoiler, the shroud includes a trailing portion  70  extending to a trailing rim  72  and having an outboard surface  74 .  
         [0020]      FIG. 3  shows the spoiler  50  as including a lightening compartment  80  extending inward from the surface  52 .  FIG. 3  further shows the shroud as including first and second circumferential ends  82  and  84 , respectively associated with pressure and suction sides of the airfoil. The ends  82  and  84  are convoluted to permit a preloaded interlocking nesting between shrouds of adjacent blades in the blade stage. The interlocking aligns the surfaces  52 ,  54 ,  56 ,  64 , and  74  with their counterparts of the remaining blades in the stage. The rim  62  is essentially annular so that the assembled shroud formed by the shrouds  34  has an annular leading rim formed by the rims  62 . The rim  72  is mostly annular (e.g., annular along a majority of its circumferential span) but protrudes along a protruding portion  90  at a trailing portion of the airfoil. Thus, the assembled shroud formed by the shrouds  34  has a generally annular trailing rim formed by the rims  72  with an array of protrusions. A direction of rotation  504  of the blade stage is also shown.  
         [0021]      FIG. 4  shows the blade  20  in an installed position within an engine case  100 . The case carries a circumferentially segmented seal carrier  102 . The seal carrier carries honeycomb seal elements  104 ,  106 , and  108  respectively facing and sealing with the leading rim  62 , the leading portion outer surface  64 , and the spoiler outer surface  52 .  
         [0022]      FIG. 5  shows wear patterns that have been observed on the shroud  34 . A significant region of wear is a region  110  in the surface  64  adjacent a corner  112  of the shroud leading portion  60  formed by the rim  62  and the end  82 . Wear in the region  110  (shown approximately bounded by a dashed line) is characterized by a combination of deep circumferential scoring (e.g.,  120  and  122 ) and more generalized thinning wear  126 ). A less significant wear region  130  is shown in the surface  64  adjacent a corner  132  of the shroud leading portion  60  formed by the rim  62  and the end  84 . When the stage is assembled, this region  130  is contiguous with the adjacent region  110  of the adjacent blade. Scoring  140  and  142  in this region may be continuations of the scoring of the adjacent region  110  of the adjacent blade. There may also be thinning wear  144 . Wear has also been observed in a region  150  shown in the surface  74  adjacent a corner  152  of the shroud trailing portion  70  formed by the rim  72  and the end  84 .  
         [0023]     Due to factors not fully understood, the wear in the region  110  may be particularly significant. There may be a relationship to the relative thinness of the shroud in this region as further influenced by dynamic factors.  
         [0024]      FIGS. 6 and 7  show exemplary details of a restoration process. After any cleaning and inspection (e.g., to assess damage and confirm restorability), the worn areas may be machined to create a base surface for laser cladding. In  FIG. 6 , the region  110  has been completely removed by such machining (e.g. machining all the way through to the underside  38 ). The exemplary machining is down to facets  180  and  182  to define a notch  184  relative to the original platform contour. The exemplary facet  182  extends to the rim  62  so that an exemplary 10-33% (more narrowly, 17-27%) of the rim  62  is removed by the notch  184 . Similarly, the facet  182  extends from the end  82 , just ahead of the surface  54  to the facet  180 . The exemplary facet  182  is located so that the notch  180  removes an exemplary 60-100% (more narrowly 75-95%) of the end  82  along the leading portion  60 .  
         [0025]     Although not required,  FIG. 6  also shows an exemplary machining to remove the area  150 . This machining involves a single facet  190  having removed a corner region.  
         [0026]     After any further cleaning, restoration material may be built up atop the machined facets.  FIG. 7  shows a buildup  200  filling the notch  184 . The buildup  200  is formed by a series of laser clad beads starting with a first bead  202  formed atop the facets  180  and  182 . Exemplary second, third, fourth and fifth/final beads  204 ,  206 ,  208 , and  210 , respectively are formed on atop the other until sufficient material is applied. Depending upon the damage extent, exemplary restoration may involve an exemplary 2-10 beads (more narrowly 3-7).  FIG. 7  also shows a buildup  220  applied atop the facet  190  and including beads  222 ,  224 , and  226 .  
         [0027]     Exemplary laser cladding techniques and apparatus are disclosed in U.S. Patent Application Publication 20050178750A1, the disclosure of which is incorporated by reference herein as if set forth at length. Exemplary cladding material has a composition that is preferably essentially the same as the base material of the blade at the facets.  
         [0028]     After buildup, the buildups may be machined to restore the original local contour. The machining may involve a slight machining along non-built-up areas (e.g., intact portions of the surfaces  64  and  74  for continuous circularity). After machining, the blade may be locally or generally recoated.  
         [0029]     Relative to tungsten inert gas (TIG) welding laser cladding is believed to generate a substantially smaller heat affected zone in the area being repaired. As a result, there is a reduction in post-weld stress and the structural integrity of the part is not compromised. There is also reduced or eliminated chances of distortion of the part which may be encountered with TIG welding. Laser cladding also offers a fast cycle time and high repeatability.  
         [0030]     One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, the nature of particular damage may influence the appropriate repair. The choice of any particular known or yet-developed laser cladding apparatus may also influence details. Accordingly, other embodiments are within the scope of the following claims.