Abstract:
A method for repairing a turbine component includes identifying a crack in a surface of the component and applying a fluoride mixture to the surface of the component containing the crack. The method also includes exposing the portion of the component including the fluoride mixture to a controlled atmosphere and returning the repaired surface of the component to predetermined dimensions.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to gas turbine engine components, and more particularly, to methods for repairing gas turbine engine components.  
         [0002]     During operation of gas turbine engines, at least some components, for example shrouds, air foils, and/or turbine nozzles, within the engine, may be exposed to high temperature gases. Accordingly, to facilitate reducing the effects of exposure to such temperatures, at least some known turbine components are coated with a protective thermal barrier coating. For example, known thermal barrier coatings, such as, aluminide coatings, facilitate providing the components with an effective barrier against oxidation and/or corrosion of the component. However, over time, cycling of the engine and continuous exposure to high temperature gases can cause the aluminide layer to erode and may cause stress cracks to develop within the component.  
         [0003]     Typically, during repair of cracked components, reactive cleaning chemicals may be applied to facilitate removing deposits of oxides and combustion products form the surface of the component being repaired. However, with at least some known cleaning methods, when the component is exposed to the highly reactive chemical gases may undesirably remove or denigrate the protective aluminide layer from areas of the component other than the cracked surface to be repaired, thus degrading the structural properties of the components.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0004]     In one aspect, a method for repairing a turbine component is provided. The method includes identifying a crack in a surface of the component and applying a fluoride mixture to the surface of the component containing the crack. The method also includes exposing the portion of the component including the fluoride mixture to a controlled atmosphere and returning the repaired surface of the component to pre-determined dimensions.  
         [0005]     In another aspect, method for repairing a component is provided. The method includes applying a fluoride mixture to a first portion of the component such that the fluoride mixture is compresses into a crack in the component and such that the remainder of the component is not contacted by the fluoride mixture and heat-treating the portion of the component including the fluoride mixture for a pre-determined period of time. The method also includes repairing the crack and removing excess material from the component until the repaired surface of the component is returned to pre-determined dimensions. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  is a cross-sectional side view of an exemplary gas turbine engine;  
         [0007]      FIG. 2  is an enlarged cross-sectional side view of a shroud assembly segment that may be used with the gas turbine engine in  FIG. 1 ;  
         [0008]      FIG. 3  is an enlarged a cross-sectional view of the shroud assembly segment shown in  FIG. 2  during an initial stage of repair; and  
         [0009]      FIG. 4  is a flow chart illustrating an exemplary method for use in repairing a damaged component. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0010]      FIG. 1  is a schematic illustration of an exemplary gas turbine engine  10  including a fan assembly  12 , a high pressure compressor  14 , and a combustor  16 . Engine  10  also includes a high pressure turbine  18 , a low pressure turbine  20 , and a booster  22 . Fan assembly  12  includes an array of fan blades  24  extending radially outward from a rotor disc  26 . Engine  10  has an intake side  28  and an exhaust side  30 . In one embodiment, gas turbine engine  10  is a CF6-80 engine commercially available from General Electric Company, Cincinnati, Ohio.  
         [0011]     In operation, air flows through fan assembly  12  and compressed air is supplied to high pressure compressor  14 . The highly compressed air is delivered to combustor  16 . Airflow from combustor  16  drives turbines  18  and  20 , and turbine  20  drives fan assembly  12 .  
         [0012]      FIG. 2  is an enlarged cross sectional view of a shroud assembly segment  50  that may be used with a gas turbine engine, such as gas turbine engine  10  (shown in  FIG. 1 ). Shroud assembly segment  50  includes a forward mounting flange  52  and an aft mounting flange  54  used to couple shroud assembly segment  50  to case segment (not shown).  
         [0013]     Shroud assembly segment  50  includes a forward mounting hook  56  and an aft mounting hook  58  use to couple a shroud segment  60  to shroud assembly segment  50 . In the exemplary embodiment, hook  56  extends radially inward and afterward from flange  52 , and hook  58  extends radially inward and afterward from flange  54 .  
         [0014]     A forward flange  62  is positioned between flange  52  and hook  56  for coupling shroud assembly segment  46  to an adjacent stator assembly (not shown). Specifically, flange  62  extends axially forward of hook  56  such that a contact surface  64  is defined between flange  62  and hook  56 . A flange face  66  is defined between flange  62  and flange  52 . An aft flange  68  extends afterward between flange  54  and hook  58 . An aft flange face  70  is defined between flange  68  and flange  54 .  
         [0015]      FIG. 3  An exemplary embodiment of a turbine engine component is described below in detail. Specifically, shroud assembly segment  50  is an example of a gas turbine engine component, and has been selected for illustrative purposes only. Each assembly segment  50  is not limited to the specific embodiments described herein. In the exemplary embodiment, shroud assembly segment  50  is fabricated from a high temperature super-alloy based on at least one of iron-base alloy, a nickel-base alloy, a cobalt-base alloy, and a titanium-base alloy, including a metallic environmental resistant overlay coating containing aluminum.  
         [0016]      FIG. 4  is a flow chart  100  illustrating an exemplary method for repairing a damaged component. The invention can be practiced on any turbine engine component, which is also known as a substrate article. In the exemplary embodiment, the invention is practiced on shroud assembly segment  50 . During engine operation, segment  50  is exposed to high temperatures and high stresses. As a result, cracks, fissures, breaks, or openings  80  may develop in the shroud assembly segments  50 . Generally, such cracks extend from the barrier coating outer surface generally inwardly through the barrier coating and into the super-alloy material.  
         [0017]     The first step in repairing the damaged component is to identify  102  cracks  80  in the outer surface of the shroud assembly segment  50 . In the exemplary illustrative embodiment, cracks  80  have formed in the outer surfaces of flange  62 , contact surface  64 , channel  66 , and channel  70 .  
         [0018]     Prior to repair, the component must be cleaned adjacent the identified cracks to facilitate removing oxides which may have built up on the surface of the component adjacent the crack to repaired. In the exemplary embodiment, only the specific locations of cracks  80  are cleaned rather than subjecting the entire component to the cleaning process. Specifically, the outer surface of crack  80  is initially cleaned using conventional cleaning methods including, but not limited to, grit blasting.  
         [0019]     Following surface cleaning, the fluoride mixture is applied  104  to the component crack  80 . The fluoride mixture includes at least one of a fluoride, chromium, aluminum chromium alloy, silicon aluminum alloy, titanium aluminum alloy, vanadium, vanadium aluminum alloy, cobalt aluminum and/or any combination thereof In the exemplary embodiment, the fluoride mixture is formed into a paste and/or highly compressed tape  82 .  
         [0020]     Tape  82  is positioned against the crack  80  and compressed into and against the crack  80  for a pre-determined time and at a pre-determined temperature. Tape  82  facilitates providing fluoride ions which can penetrate the crack  80  and facilitate removing oxides from the inside surfaces of the crack. By limiting exposure to just cracks  80 , fluoride tape  82  facilitates reducing the detrimental effects of cleaning to the surrounding undamaged component surfaces. Additionally, reduced exposure substantially improves the component quality thus facilitating an increase in product life, durability, and reduced cycle time.  
         [0021]     The portion of the component including the fluoride mixture is then exposed  106  to a reactive gas at a pre-determined temperature for a pre-determined time period. In the exemplary embodiment, the gas is hydrogen gas, that is at a temperature of between approximately between 1800° F. and 2000° F. In one embodiment, the component is exposed to the gas for a time period of between approximately two and six hours. In alternative embodiments, shroud assembly segment  50  is exposed to any reactive gas that enables the repair methods described herein to be performed as described herein. In one embodiment, known heat-treatment equipment, such as, for example, a hydrogen atmosphere retort in an air furnace is used. In alternative embodiments, it will be readily recognized by those skilled in the art that any means can be used to provide gaseous hydrogen and heat for a period of time.  
         [0022]     Following the heat-treatment, the area of the component surface adjacent the crack  80  is then cleaned with a water-based compound and subject to a light grit blast. Crack  80  is then ready for repair. It will be readily recognized by those skilled in the art that any repair method can be used to repair crack  80 . In one embodiment, repair includes flowing a healing-type alloy into crack  80  under a vacuum and them allowing the alloy to wet and diffuse with walls of crack  80 . In another embodiment, the repair method includes heating and isostatically pressing the walls of cleaned crack  80  together. Finally, once crack  80  has been repaired, the surface of the component is machined and returned  108  to pre-determined dimensions.  
         [0023]     The present invention provides a method for cleaning/removing complex oxides from within a narrow crack in an air turbine component through the application of a fluoride mixture formed into paste or tape ions on the location of the crack and then repairing such cleaned crack. Gas turbine components are costly to manufacture and constant exposure to potentially harsh chemical gases degrade the chemical/mechanical properties of the components, a localized and tailored method preferable than a generalized method.  
         [0024]     The present invention has been described in connection with specific examples and combinations of materials and structures. However, it should understood that they are intended as exemplary, rather than in any way limiting the scope of the invention. The methods described herein may be utilized independently and separately with other components other than those described herein. Moreover, the methods can also be used to repair components other than turbine components.  
         [0025]     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.