Abstract:
A method and masking assembly for masking a dovetail portion of a turbine blade during coating of an airfoil portion of the blade. The masking assembly comprises at least two masking members, each having an exterior surface and an oppositely-disposed undulatory surface complementary to one of oppositely-disposed undulatory surfaces of the dovetail portion. By mating the masking members, the undulatory surfaces thereof define an interior cavity within the masking assembly that accommodates the dovetail portion, and the undulatory surfaces of the masking members contact the undulatory surfaces of the dovetail portion to entrap the dovetail portion within the interior cavity of the masking assembly.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a Division Patent Application of co-pending U.S. patent application Ser. No. 11/276,745, filed Mar. 13, 2006. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention generally relates to coating deposition processes and equipment. More particularly, this invention relates to a method and masking assembly for selectively depositing a coating on a turbine airfoil while preventing deposition of the coating on a dovetail of the airfoil. 
         [0003]    Components of gas turbine engines, such as the blades and vanes (nozzles) of the turbine section within a gas turbine engine, are often formed of an iron, nickel, or cobalt-base superalloy. A turbine blade has an airfoil against which hot combustion gases are directed during operation of the gas turbine engine, and whose surface is therefore subjected to severe attack by oxidation, corrosion and erosion. The blade further includes a root section separated from the airfoil by a platform. Turbine blades are commonly anchored to the perimeter of a rotor or wheel by forming the rotor to have slots with dovetail cross-sections, and forming the root section of each blade to have a complementary dovetail profile whose oppositely-disposed undulatory surfaces, generally characterized by alternating lobes and recesses, interlock with the dovetail slot of the rotor. 
         [0004]    Due to the severity of their operating environments, turbine blades often require environmentally protective coatings on the surfaces of their airfoils and platforms exposed to the hot gas path. Diffusion coatings such as chromide, aluminide, and platinum aluminide coatings are widely used as environmental coatings in gas turbine engine applications because of their oxidation resistance. Such coatings, which are typically applied to the internal and external surfaces of a blade, are produced by a thermal/chemical reaction process that takes place in a reduced and/or inert atmosphere at a specified temperature. Common processes include pack cementation and noncontact vapor (gas phase deposition) techniques, and typically take place at processing temperatures of about a 1900° F. (about 1040° C.) or more. The dovetail of a turbine blade is typically machined prior to the diffusion coating process, and is not coated during coating of the airfoil so that the dovetail will properly assemble with the dovetail slot in the rotor during engine build. 
         [0005]    Slurries, putties, and tapes have been widely used as masks to prevent coating deposition on the machined surfaces of blade dovetails. One approach is to cover the dovetail surfaces with a mask formed from a slurry paste, such as a mixture of nickle powders and an organic binder. The slurry paste may be applied with pneumatic injection equipment and then dried to form a solid mask. Alternatively, the blade dovetails can be dipped into the masking slurry, with multiple dips typically being required to form an effective mask with sufficient thickness. The masked dovetails are then often wrapped in a metal foil to contain the maskant during the coating process. With either approach, the solid mask must be mechanically removed after the coating process, such as by grit blasting, rotating wire brush, etc. To avoid the requirement of removing a solid maskant, the dovetail can simply be buried in a nickle powder without any binder, so that the powder forms a loose maskant that covers the dovetail during the coating operation. Still another alternative is to cast the slurry into thin film tapes that can be individually applied to the blade. While this approach is well suited for masking localized areas, tapes are not typically used as a primary method for masking the undulatory machined surfaces of a dovetail. 
         [0006]    Significant shortcomings associated with the above-noted approaches include the preparation, application, and removal of the masking materials, which can be labor intensive and require the services of a skilled individual. As such, alternative masking techniques have been proposed. On such approach is taught in commonly-assigned U.S. Pat. Nos. 6,224,673, 6,579,567, and 6,821,564 to Das et al. These patents teach the use of a reusable fixture to enclose those portions of an article, such as a gas turbine blade, on which a coating is not desired. The fixture has an internal cavity and at least one aperture whose cross-section is substantially the same as a cross-section of the article to be coated. In the case of a blade, the aperture is sized to enable the entire dovetail of the blade to be inserted through the aperture into the internal cavity of the fixture, so that the platform seals against the exterior of the fixture. The fixture may include a holder to stabilize the dovetail within the internal cavity. 
         [0007]    While the teachings of Das et al. overcome the shortcomings associated with the use of masking tapes, slurries, and other types of coatings, further improvements would still be desirable. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    The present invention provides a method and masking assembly for masking a dovetail portion of a turbine blade during coating of an airfoil portion of the blade. The masking assembly comprises at least two masking members, each having an exterior surface and an oppositely-disposed undulatory surface complementary to one of oppositely-disposed undulatory surfaces of the dovetail portion. By mating the masking members, the undulatory surfaces thereof define an interior cavity within the masking assembly that accommodates the dovetail portion, and the undulatory surfaces of the masking members contact the undulatory surfaces of the dovetail portion to entrap the dovetail portion within the interior cavity of the masking assembly. 
         [0009]    The method of this invention generally entails placing the dovetail portion in the masking assembly so that the airfoil portion of the blade remains outside the masking assembly, the interior cavity of the masking assembly accommodates the dovetail portion, and the undulatory surfaces of the masking members contact the undulatory surfaces of the dovetail portion and entrap the dovetail portion within the interior cavity. A coating vapor is then supplied to deposit a coating on the airfoil portion of the blade while preventing deposition of coating on the dovetail portion with the masking assembly. Following coating deposition, the masking members are separated to release the blade from the masking assembly. 
         [0010]    In view of the above, the present invention provides a simplified method for masking the dovetail portion of a turbine blade, without the requirement for masking the dovetail portion with a masking slurry or tapes as conventionally done in the past. As such, the present invention eliminates the labor required to prepare and apply a masking slurry over the entire dovetail portion, and avoids the additional labor required to mechanically remove a solidified mask formed by the masking slurry at the conclusion of the coating operation. As a result, both the masking assembly and method made possibly with the masking assembly are considerably less complicated than prior art masking methods, yet achieves the object of preventing coating of the dovetail portion of a turbine blade. 
         [0011]    Other objects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0012]      FIG. 1  is a perspective view of a masking member in accordance with a preferred embodiment of this invention. 
           [0013]      FIG. 2  is a perspective view of a turbine blade installed in a masking assembly formed by mating two masking members of the type shown in  FIG. 1 , such that a dovetail portion of the blade is enclosed within the masking assembly and an airfoil portion of the blade is exposed outside the masking assembly. 
           [0014]      FIG. 3  is a cross-sectional view through the masking assembly of  FIG. 2 . 
           [0015]      FIG. 4  represents the blade of  FIGS. 2 and 3  after removal from the masking assembly. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    The present invention provides a method for preventing the deposition of a coating on surfaces of the dovetail portion of a gas turbine engine blade, particularly a turbine blade. While the advantages of this invention will be illustrated and described with reference to a turbine blade on which an environmental coating is to be deposited to protect the blade from its hostile operating environment, the teachings of this invention are generally applicable to other components having surfaces and on which a coating and still other surfaces on which a coating is not desired. 
         [0017]      FIGS. 1 through 3  depict a masking shell  12  and a turbine blade  14  installed in a masking assembly  10  formed by mating the masking shell  12  with a second and essentially identical masking shell  12 , resulting in the assembly  10  having a clam shell-like construction. Blades of the type represented in the Figures are typically formed of an iron, nickel, or cobalt-base superalloy, though the use of other materials is within the scope of this invention. The blade  14  includes an airfoil  16  against which hot combustion gases are directed during operation of the gas turbine engine, and whose surfaces are therefore subjected to severe attack by oxidation, corrosion and erosion. The blade  14  is configured to be anchored to a turbine disk (not shown) with a dovetail  18  formed on a root section of the blade  14 . A platform  20  is between the airfoil  16  and dovetail  18 . The dovetail  18  has opposing surfaces  22  that may be termed undulatory, wavy, etc., in other words, generally characterized by alternating lobes and recesses. As is known in the art, the dovetail surfaces  22  are complementary to surfaces of a slot formed in the disk into which the dovetail  18  will be inserted to interlock the blade  14  with the disk. 
         [0018]    The airfoil  16  is intended to be protected from the hostile environment of the turbine section by an environmentally-resistant coating, for example, a diffusion coating such as a chromide, aluminide, or platinum aluminide coating. As is understood in the art, these types of coatings are formed by such processes as pack cementation or noncontact vapor (gas phase deposition) techniques, in which a vapor of a desired coating element (e.g., chromium, aluminum, etc.) is generated and caused to contact the surfaces of the blade  14  on which the coating is desired. The vapor reacts with the surface to deposit the desired coating element(s), which are then diffused into the surface. Such processes are well known to those skilled in the art, and therefore will not be discussed in further detail here. 
         [0019]    As noted above, the present invention is intended to prevent deposition of the coating on surfaces of the dovetail  18 , particularly its undulatory surfaces  22  that are required to subsequently mate with the dovetail slot in a rotor. For this purpose, most of the dovetail  18  is shown in  FIGS. 2 and 3  as being enclosed within an interior cavity  24  formed by mating the masking shells  12  to yield the masking assembly  10 . The cavity  24  is defined by opposing interior surfaces  26  of the masking shells  12 . As evident from  FIG. 3 , the interior surfaces  26  of the masking shells  12  are complementary to the undulatory surfaces  22  of the dovetail  18 , so that the surfaces  26  of the shells  12  contact and interlock with the dovetail surfaces  22  to secure and essentially immobilize the dovetail  18  within the masking assembly  10 . While the surfaces  26  of the masking shells  12  are represented as having a close and continuous surface-to-surface fit with the surfaces  22  of the dovetail  18 , it is foreseeable that the benefits of the invention could be realized without such a continuous surface-to-surface fit, as long as sufficient contact exists to interlock and secure the dovetail  18  to the masking assembly  10 . 
         [0020]      FIG. 3  shows the masking assembly  10  as including an optional passage  40  through which coating vapors can enter the interior of the blade  10  to enable deposition of the coating on any internal cooling passages within the blade  10 . If necessary, a maskant material (not shown) of a type used in the prior art as described previously, such as a tape, putty, or slurry, can be applied to mask any exterior surfaces of the dovetail  18  exposed by the passage  40  in the masking assembly  10 . 
         [0021]    The masking assembly  10  is also represented in  FIGS. 2 and 3  as further having a retaining ring  28  to secure the masking shells  12  together. The ring  28  preferably forces the mating surfaces of the shells  12  together with sufficient force to close the split line  30  between the shells  12  and prevent entrance of the coating vapors into the cavity  24 . For this purpose, the retaining ring  28  preferably has a draft angle machined into its surface contacting the masking shells  12  to ease its installation and removal from the masking assembly  10  while locking the ring  28  in place.  FIG. 2  further shows a sealing material  32  deposited along the interfaces  34  between the masking shells  12  and the blade  14  to further inhibit coating penetration. The sealing material  32  may be, for example, a maskant material of a type used in the prior art as described previously, such as a tape, putty, or bead of slurry. 
         [0022]    Suitable materials for the masking shells  12  and retaining ring  28  include metallic and ceramic materials. In practice, the nickel-base superalloy commercially known as Inconel 600 has been shown to be a durable and reusable material for both the shells  12  and ring  28 , though it is foreseeable that other materials could be used. The surfaces of the masking shells  12  and ring  28  that contact other components of the assembly  10  or the blade  14  are preferably machined to ensure an appropriate fit. 
         [0023]    In view of the above, masking of the dovetail  18  with the masking assembly  10  simply involves placing the dovetail  18  in the assembly  10  by mating the masking shells  12  so that the undulatory surfaces  26  of the shells  12  contact and entrap the dovetail  18  within the interior cavity  24  of the masking assembly  10 . After installing the retaining ring  28 , the entire blade and masking assembly can then be placed in a suitable coating apparatus (not shown) and ran through a coating cycle as required by the particular coating material and coating process being employed. Once the coating cycle is complete, the retaining ring  28  is removed and the masking shells  12  separated to release the blade  14 . Reuse of the masking shells  12  and retaining ring  28  may generally involve removing the sealing material  32  and any residual coating material from the exterior surfaces of the assembly  10 . The blade  14  is represented in  FIG. 4  as having been removed from the assembly  10 , and as having a coating  38  on only the surfaces of the airfoil  16  and platform  20  and limited surface portions of the dovetail  18  exposed to the coating vapor, with the dovetail  18  being free of coating below a coating boundary  36  corresponding to the uppermost extent of the masking assembly  10  on the blade  14 . From the location of the coating boundary  36 , it is evident that the masking assembly  10  of this invention enables the entire platform  20 , including its upper and lower surfaces, to be coated for oxidation and corrosion protection. 
         [0024]    While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. For example, the physical configuration of the masking assembly  10 , shells  12 , blade  14 , and ring  28  could differ from that shown. Therefore, the scope of the invention is to be limited only by the following claims.