Abstract:
A method of applying an aluminide coating to an internal cavity of a metal part, such as an airfoil. The method includes inserting a perforated sealed container containing a source of aluminide into the internal cavity and vaporizing the aluminide source to form an aluminide coating on the wall surface of the cavity.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates in general to a coating method and, more specifically, to a method of coating the internal cavities of an airfoil. 
       BACKGROUND OF THE INVENTION 
       [0002]    Aluminide coating of the airfoils is a common process in airfoil repair. The repair requires the coating of both the external and internal airfoil surfaces. The process of coating requires maintaining the airfoil in a mist of hot aluminide vapours. The coating of external surfaces of the airfoil by this process is standard in the industry. 
         [0003]    The process of coating the internal surfaces of the vane is difficult. To coat the cavity of the vane, the coating vapours must travel to the internal cavity by using specially designed fixtures. 
         [0004]    There is therefore a need for coating the internal surfaces of the vane without having to make complex coating pans and other complex fixturing. 
       SUMMARY OF THE INVENTION 
       [0005]    It is therefore an object of the present invention to overcome the problems of the prior art described above. 
         [0006]    It is another object of the present invention to provide a method of coating the surface of internal cavities of metal parts. 
         [0007]    It is yet another object of the present invention to provide a method for forming an aluminide coating on the internal surface of a metal part. 
         [0008]    It is a further object of the present invention to provide a method for forming an aluminide coating on surface of an airfoil. 
         [0009]    It is another object of the present invention to provide a device which enables the efficient aluminde coating of internal cavities of metal parts. 
         [0010]    It is a further object of the present invention to provide a device for aluminide coating internal cavities in airfoils which is inserted in the cavity during the coating process 
         [0011]    The invention is directed to the application of a vapour phase coating of aluminide to an internal airfoil surface. The process includes coating the external and internal surface of an airfoil simultaneously through the use of specially fabricated granule holders. The granule holders are designed to be placed inside the vanes and eliminate the need for the complex prior art hardware, such as coating pans and the required control of the coating vapour stream to the internal cavity through the use of specially designed fixtures. In one embodiment, the internal granule holder is made of metal in the form of an elongated hollow cylinder with a plurality of holes or orifices through its side wall. The holder may optionally be encased in an outer cylindrical fixture which is inserted into the cavity to be in position to coat the inner surface of the cavity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    For a further understanding of these and objects of the invention, reference will be made to the following detailed description of the invention which is to be read in connection with the accompanying drawing, where: 
           [0013]      FIG. 1  is a side view of the granule holders of the present invention. 
           [0014]      FIG. 2  is a perspective view showing the granule holders of  FIG. 1  partially inserted into the internal cavities of a turbine airfoil. 
           [0015]      FIG. 3  is a second perspective view showing the granule holders in place within the airfoil cavities. 
           [0016]      FIG. 4  is a third perspective view with the granule holders partially inserted in the internal cavities. 
           [0017]      FIG. 5  is a perspective view of the component setup inside a coating pan. 
           [0018]      FIG. 6  is an enlarged perspective view of  FIG. 5  showing the granule holders in place. 
           [0019]      FIG. 7  is a perspective top view of the coating pan showing the position of the airfoil to be coated. 
           [0020]      FIG. 8  is an enlarged view of the airfoil in  FIG. 7  showing the position of a granule holders in the cavities. 
           [0021]      FIG. 9  is a graphical illustration of the process of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]      FIG. 1  illustrates the granule holders of the present invention as two separate pairs having a slightly different size and configuration. Holders  10  have a thicker wall thickness and are larger than holder  20 . Holder  10  is in the form of an elongated hollow metal cylinder  12  having an open end  14  and opposite closed end. Holder  10  further contains a plurality of holes  16  through its side wall to allow for the passage of aluminide vapor during the coating process. Flanges  18 A and  18 B aid in stopping and positioning the holder within the airfoil cavity. Holders  20  are slightly smaller in size, have a smaller wall thickness, but have similar holes  26  through the side wall of cylinder  22 , and also an open end  24 . The holders are normally sealed after loading with the granular material with sealing caps  14   a  and  24   a.  Holders  10  and  20  both function to hold an appropriate load of granular aluminide material which when heated under the proper conditions, will vaporize and form an aluminide coating on the internal cavity surfaces of an airfoil. The holder may be made of any suitable high temperature alloy. Inconel 600 has been found to be a suitable material. 
         [0023]      FIG. 2  illustrates a turbine airfoil  30  having internal cavities  32  and  34  with holders  10  partly inserted to illustrate holder positioning. In aluminiding, both the internal surfaces of cavities  32  and  34 , and the outside surface of airfoil  30 , i.e.  36 ,  38 , etc. are simultaneously coated with an aluminide coating. 
         [0024]      FIG. 3  is another view of airfoil  30  showing holder  10  in place containing granular aluminide material  44 . 
         [0025]      FIG. 4  is a further view of airfoil  30  with granular holders  10  partially inserted in the airfoil cavities. 
         [0026]      FIG. 5  illustrates the coating pan  40  and components set up for the process of the present invention. The coating pan  40  contains a support floor for the parts to be treated. The coating pan contains a center ring  44  which supports a granular holder  46  which contains the aluminide granules  48  and any other component(s) such as an activator. 
         [0027]    Vapor phase aluminide coating is well know in the art and has been used to form a protective coating on airfoil surface to inhibit oxidation. The surface to be treated is coated with a diffusion aluminide An activator or precursor material such as an aluminum halide optionally can be used with other modifying elements. 
         [0028]    The diffusion aluminide coating is typically in the range of about 0.005 to 0.003 inches thick. When exposed to a high temperature oxidizing environment, the aluminum enriched region at the airfoil surface oxidizes to form a highly adherent layer of protective aluminum oxide (Al 2 O 3 ) which functions to inhibit oxidation damage. Reactive and noble aluminide modifying elements such as Hf, Zr, Yt, Si, Ti, Ta, Co, Cr, Pt and Pa and combinations thereof may also be used in forming the aluminide layer. 
         [0029]    U.S. Pat No. 6,616,969 is representative of the state of the art with regard to vapor phase aluminide coating and is incorporated in its entirety herein by reference. 
         [0030]    In one embodiment a CrAl alloy is used as the specific aluminide material in CrAl 55/45% concentration. The following example represents one embodiment of a process sequence of the present invention which is suitable in carrying out the present invention. 
         [0000]    
       
         
               
               
             
               
               
             
           
               
                   
                   
               
               
                   
                 Example 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 1. 
                 Clean turbine airfoil to be coated with abrasive grit blasting. 
               
               
                 2. 
                 Insert internal granule holder with filled premixed granules and activator into 
               
               
                   
                 parts&#39; cavity for internal coating. The blended mix contains approximately 
               
               
                   
                 100 grams of fine CrAl granules 2 to 3 mm in size, and 50 grams of aluminum fluoride. 
               
               
                 3. 
                 Prepare the ring type-coating pan (FIGS. 5 &amp; 6) with approximately 1200 
               
               
                   
                 grams of new granules. There are 3 layers of coating media at the bottom of 
               
               
                   
                 the coating pan. The layer of aluminum fluoride is sandwich between the top 
               
               
                   
                 and bottom layer of granules. 
               
               
                 4. 
                 The turbine airfoil 30 together with the internal granule holder inserted is then 
               
               
                   
                 loaded in the coating pan. 
               
               
                 5. 
                 Place the coating pan into a retort, and connect gas piping to the coating pan 
               
               
                   
                 and retort. Both Argon, and Hydrogen will be used before the heating cycle. 
               
               
                   
                 The flow rates of hydrogen and argon are at about 150 CFH for retort and 50 
               
               
                   
                 CFH for coating pan. Argon is used only during purging. 
               
               
                 6. 
                 Load the retort into furnace, and heat up to 1975 ± 25° F. During the heating 
               
               
                   
                 up, supply a 15 CFH of Hydrogen is maintained for the first 7.5 hours. Flow 
               
               
                   
                 rate for retort is 150 FCH. 
               
               
                 7. 
                 The coating cycle will start once the temperature reaches 1975 ± 25° F., and is 
               
               
                   
                 maintained at this temperature for about 8.5 hours. 
               
               
                 8. 
                 The coating pan, and retort are then cooled by increasing Hydrogen flow after 
               
               
                   
                 the thermocouple reaches 600° F. to complete the coating cycle. 
               
               
                   
               
             
          
         
       
     
         [0031]    The flow chart in  FIG. 9  graphically illustrates the process of the present invention. 
         [0032]    While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawing, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims.