The present invention relates to repairs of gas turbine engine components, and more particularly to repairs for gas turbine engine components having aluminide coatings.
Components of gas turbine engines, such as airfoils, transition ducts and other parts, frequently are provided with aluminide coatings to promote corrosion and oxidation resistance. Aluminide coatings include a broad variety of coating compounds that include aluminum with at least one other more electropositive element. The parent materials of these coated engine components are frequently nickel- or cobalt-based superalloys. When the aluminide coatings are applied to the parent alloys, a diffusion layer is formed in the parent alloys beneath the exterior aluminide coating layers.
During operation of the engine, the coated components may become worn or damaged, due to oxidation, erosion, foreign object damage, or other factors. Over time, it may become necessary to repair or replace the aluminide coating in order to continue using the worn or damaged components. Over the full useful life of a particular component, numerous coating repairs may need to be performed in order to allow continued use. These processes typically involve first stripping any remaining coatings. When the remaining coatings are removed, the diffusion layer must also be removed in order to prevent the formation of a deleterious microstructure in the replacement coating. A new, replacement aluminide coating is then applied.
A problem with known repairs for aluminide coated components is that removal of the diffusion layer results in a reduction of the component contour (or envelope) from original blueprint dimensions. On average, about 1.5 mils of parent alloy is lost on each exterior surface of the parent material where such coating repairs are performed that remove the diffusion layer (see
It is desired to provide a repair method that expands repairable limits for gas turbine engine components and lessens the need to reduce structural contours (or envelopes) of gas turbine engine components in order to repair or replace aluminide coatings.
A method of repairing a component of a gas turbine engine that includes a metallic substrate, an existing coating, and a diffusion layer formed in the metallic substrate adjacent to the coating. The method includes removing at least a portion of the existing aluminide coating, removing material forming the diffusion layer, applying a new metallic layer to the metallic substrate, and applying a new aluminide coating over the new metallic layer to form a new diffusion layer in the new metallic layer. The new metallic layer is a substantially homogeneous material that is substantially similar in chemical composition to that of the metallic substrate, and the new metallic layer forms a structural layer having a thickness selected to provide a specified contour to the component.
In general, the present invention relates to repairs of gas turbine engine components having aluminide coatings, such as coatings that include the beta NiAl phase, the gamma-prime Ni3Al phase, the gamma Ni phase, and any combination thereof, and each of those phases can be solid solutions that contain, for example, aluminum, cobalt, chrome, yttrium, hafnium, silicon, tantalum, tungsten, rhenium, molybdenum or ruthenium. The repair involves removing existing coatings as well as a diffusion layer formed in a metallic parent material. A new, structural “build-up” material is applied to the remaining parent material to restore an original blueprint contour of the component, which helps to compensate for the loss of material in the diffusion layer. Then a new aluminide coating is applied over the new “build-up” material, creating a new diffusion layer. The new “build-up” material can be a substantially homogeneous material of substantially the same chemical composition as the metallic parent material. The new “build-up” material can be applied using directed vapor deposition (DVD) or plating processes.
The turbine blade 20 can be placed in service in a gas turbine engine. As a result of such use, the airfoil section 22 of the blade 20 is prone to wear and damage. After being placed in service, the blade 20 can be inspected in order to make a determination as to whether replacement of the aluminide coating layer 28 is necessary or desired.
Once repair has been undertaken, a preliminary step is to remove any remaining portions of the aluminide coating layer 28, as well as to remove material forming the diffusion layer 30. It is generally necessary to remove the material of the diffusion layer 30 in order to prevent the formation of a deleterious microstructure in the replacement aluminide coating. Material removal can be accomplished using known chemical or mechanical methods. Removal of the original aluminide coating layer 28 and the diffusion layer 30 can be accomplished with a single removal process that removes both layers 28 and 30, or as separate steps using either identical or distinguishable processes to remove those layers 28 and 30 individually.
At this point it is helpful to understand prior art repair methods.
According to the present invention, the airfoil 22 is essentially restored to the original contour boundary 26 before a new aluminide coating is applied. The original contour boundary 26 is restored by applying a new structural “build-up” layer upon the surface of the parent material 24 at the reduced contour boundary 26A. The new structural layer is made up of a substantially homogeneous metallic material that is substantially similar in chemical composition to the parent material 24. For example, the new structural layer and the parent material 24 can both be made of the same nickel-based superalloy.
The new structural layer 32 can be deposited in a number of different ways in alternative embodiments of the present repair. Directed vapor deposition (DVD) is one suitable process that involves vaporizing a material from multiple crucibles using an electron beam and then condensing the vaporized material on a desired component inside a chamber, much like with electron beam physical vapor deposition (EB-PVD) processes. The component on which the vapor condenses can be rotated to provide even coating. DVD further involves the use of a carrier gas jet of an inert carrier gas (e.g., helium) to direct vaporized material to surfaces of the target component where condensation occurs. The carrier gas jet is typically a single gas stream provided either coaxially with the vaporized material or perpendicular to the flow of the vaporized material. An advantage of the DVD process is that it permits complex alloy chemistries of the new structural layer 32 to be deposited on the parent material 24, making the process well-suited for applying nickel-based superalloy materials without disrupting the complex chemistries of those alloys. In addition, applying material of the new structural layer 32 using a DVD process can involve a number of unique steps that can be used as desired with particular components. For example, a mask can be positioned relative to a selected first portion of a surface where material will be applied while leaving another portion of the surface uncovered in order to reduce the amount of material applied to the first portion of the surface covered by the mask. As another example, a secondary carrier gas jet can be provided to direct material vapor to areas that would otherwise be concealed or hidden from a single coaxial carrier gas jet, which may be helpful when “build-up” material is applied to components having complex geometries. Also, a component where condensation will occur can be charged and the material vapor cloud ionized in order to facilitate the DVD process.
Plating is a well-known process that provides an alternative method for depositing material of the new structural layer 32. Plating is well suited to applications involving materials comprising single-element metals or relatively simple alloys. Known types of plating process include electroplating, sputtering, and other thin film deposition techniques. Electroplating is perhaps the most basic type of plating process, and, in the present context, involves supplying a metallic coating material that acts as an anode and charging the parent material 24 such that it acts as a cathode. When placed in an ionic aqueous solution and current is applied between the anode and cathode, material is plated onto the cathode to form the new structural layer 32 on the parent material 24.
With any method used to apply the material of the new structural layer 32, thickness of the new structural layer 32 can be controlled using weight gain analysis. The process of weight gain analysis involves performing a material application to a scrap part (e.g., using the DVD process) and destructively analyzing the scrap part to correlate the thickness of the applied material as a function of weight gain to the scrap part. Thickness of the new structural layer 32 as applied can be determined through nondestructive weight gain measurements of the turbine blade 20 that are correlated to measurements from the scrap part. The weight gain analysis correlation can be periodically re-determined to ensure desired process tolerances are met over time. In this way, application of the new structural layer 32 can be controlled so as to produce the new contour boundary 26B at substantially the same dimensions as the original contour boundary 26.
After the new structural layer 32 has been applied, a new aluminide coating layer is applied to the surface of the new structural layer 32 defined at the new contour boundary 26B. The new aluminide coating layer can be applied in a well-known manner, and can be applied in substantially the same manner and to substantially the same depth as during original fabrication of the blade 20. Application of the new aluminide coating layer forms a new diffusion layer. Heat treatment can be performed on the airfoil 22 before and/or after application of the new aluminide layer, in order to provide desired microstructures and other properties. For example, heat treatment can help provide substantially the same microstructure in the new structural layer 32 as in the parent material 24.
Upon completion of repairs according to the present invention, the turbine blade 20 can be returned to service. It is contemplated that upon further use in service, the turbine blade 20 may require further repairs to replace the aluminide coating again. In that instance, the repair process described above can be repeated. In that context, any new structural layer 32 from previous repairs can be considered an integral part of the parent material 24 subject to partial or complete removal and the reapplication of additional new layers thereupon.
If at step 112 it is determined that the reduced contour is not below allowed limits, a new aluminide coating can be applied to the parent material (step 118) without the application of any “build-up” material. Thus, the steps required to apply the “build-up” material can be avoided in some situations to reduce costs and simplify repairs.
Once all repairs are completed, the component can be returned to service (step 102). After the repaired component has returned to service, subsequent repairs can be repeatedly performed on the component, at later occasions, in substantially the same manner.
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and scope of the invention. For instance, the aluminide coatings used according to the present invention can have nearly any composition, and replacements coatings can differ from original coatings as desired. Moreover, the processes used for repair steps such as applying new structural “build-up” layers and applying the aluminide coatings can vary as desired for particular applications. In addition, it should be recognized that the present repair can be performed in conjunction with any other repairs desired to be performed on a particular component.