Patent Publication Number: US-11391165-B2

Title: Apparatus and method for masking under platform areas of airfoil components

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. patent application Ser. No. 15/412,482, filed Jan. 23, 2017. 
    
    
     BACKGROUND 
     The present disclosure relates generally to masking and, more particularly, to a method an apparatus to mask turbine components. 
     Gas turbine engines, such as those that power modern commercial and military aircraft, generally include a compressor section to pressurize an airflow, a combustor section to burn hydrocarbon fuel in the presence of the pressurized air, and a turbine section to extract energy from the resultant combustion gases. 
     The compressor and turbine include one or more arrays of blades extending radially outwardly from a rotatable rotor hub. Each blade has a root that mates with a slot in the rotor hub to retain the blade. Each blade also has a platform that partly defines a circumferential inner boundary of an engine flowpath, and an airfoil that extends radially across the flowpath. During engine operation, a working medium, which flows axially along the flowpath, receives energy from the compressor blade arrays and provides energy to the turbine blade arrays. Those portions of the blades that come in direct contact with the working medium are subjected to a demanding operational environment. This is particularly true of the turbine blades, which are exposed to the elevated temperature and damaging effects of combustion products discharged from the engine&#39;s combustion chamber. 
     Various protective coatings are applied to the flowpath exposed surfaces of the blades to extend their useful life. Application of such coatings to other portions of the blades may be undesirable, since the presence of a layer of coating can interfere with the installation of the blades into the hub. Accordingly, various fixtures facilitate the application of a protective coating to selected portions of a blade, while shielding non-selected portions of the blade from the application of the coating. These fixtures are normally used in conjunction with a coating application apparatus such as a low pressure plasma spray coater or a physical vapor deposition coater. 
     Existing fixtures lasts only a limited number of coating cycles. During each coating cycle, a quantity of the coating accumulates on the fixture itself. After a number of coating cycles, further use of the fixture results in the formation of a coating “bridge” between the fixture and the blade platform. Once this bridge is established, it may become difficult to remove the blade from the fixture without chipping the coating from the platform and rendering the blade unsuitable for service. The blade must then be stripped and recoated. 
     To avoid the potential chipped coating damage to the blades, each fixture is typically used only a limited number of times and then is temporarily removed from service and refurbished by stripping the accumulated coating from the fixture with an acidic solution. This maintenance is relatively time consuming and expensive. Moreover, because each fixture is serviceable for only a limited number of coating cycles, a relative significant inventory of fixtures must be available so that the supply of serviceable fixtures is sufficient to support uninterrupted production runs. 
     SUMMARY 
     A fixture for masking a gas turbine engine blade, the blade having a root, a platform and an airfoil, the fixture according to one disclosed non-limiting embodiment of the present disclosure can include a base with a receptacle for receiving the root of the blade; a removable sidewall mountable to the base to form a masking box with the receptacle that shields the root of the blade; a first removable Z plane detail mountable to the base, the first removable Z plane detail providing a first electrical contact point to the root of the blade; and a second removable Z plane detail mountable to the removable sidewall, the second removable Z plane detail providing a second electrical contact point to the root. 
     A further embodiment of the present disclosure may include that the first and second removable Z plane detail each includes a tab that provides an electrical contact point to the root. 
     A further embodiment of the present disclosure may include that the tab has a rectilinear polygon shaped end surface. 
     A further embodiment of the present disclosure may include that the base includes a shank that defines a longitudinal axis, the longitudinal axis being parallel to an axis of the blade. 
     A further embodiment of the present disclosure may include that the Z-plane is a reference surface upon which the root is referenced with a turbine rotor disk. 
     A further embodiment of the present disclosure may include that the removable sidewall includes a first and a second edge rail that at least partially surrounds a forward wall and an aft wall of the receptacle to orient the removable sidewall to form the masking box. 
     A further embodiment of the present disclosure may include that the removable sidewall, the first removable Z plane detail, and the second Z plane detail are mountable to the base via a fastener. 
     A further embodiment of the present disclosure may include that at least one of the first removable Z plane detail and the second Z plane detail includes a hexagonal aperture for a head of the fastener. 
     A further embodiment of the present disclosure may include that the first removable Z plane detail includes a tab that extends through a slot in the base to provide the first electrical contact point to the root. 
     A further embodiment of the present disclosure may include that the second removable Z plane detail includes a tab that extends through a slot in the removable sidewall to provide the second electrical contact point to the root. 
     A further embodiment of the present disclosure may include that the base includes an indentation to receive the first removable Z plane detail. A further embodiment of the present disclosure may include that the removable sidewall includes an indentation to receive the second removable Z plane detail A method for selectively applying a coating to a gas turbine engine blade, the blade having a root, a platform and an airfoil, the method according to one disclosed non-limiting embodiment of the present disclosure can include locating the root into a partial receptacle formed by a base and mounting a removable Z plane detail to the base to complete the receptacle of the masking box to shield the root. 
     A further embodiment of the present disclosure may include mounting the removable Z plane detail to the base with a fastener. 
     A further embodiment of the present disclosure may include mounting a removable sidewall to the base to form the masking box with the receptacle that shields the root. 
     A further embodiment of the present disclosure may include mounting a second removable Z plane detail to the removable sidewall with a fastener. 
     A further embodiment of the present disclosure may include mounting the removable Z plane detail to provide an electrical contact point to the root. 
     A further embodiment of the present disclosure may include removing the removable Z plane detail from the base; stripping the accumulated coating from the base with an acidic solution. 
     A further embodiment of the present disclosure may include stripping the accumulated coating from the removable sidewall with an acidic solution. 
     A further embodiment of the present disclosure may include replacing the removable Z plane detail after a predetermined number of coating cycles. 
     The foregoing features and elements may be combined in various combinations without exclusivity, unless expressly indicated otherwise. These features and elements as well as the operation thereof will become more apparent in light of the following description and the accompanying drawings. It should be understood, however, the following description and drawings are intended to be exemplary in nature and non-limiting. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various features will become apparent to those skilled in the art from the following detailed description of the disclosed non-limiting embodiment. The drawings that accompany the detailed description can be briefly described as follows: 
         FIG. 1  is a general schematic view of an exemplary actively cooled component as a representative workpiece; 
         FIG. 2  is an exploded view of the fixture according to one disclosed non-limiting embodiment; 
         FIG. 3  is an assembled view of the fixture of  FIG. 2 ; 
         FIG. 4  is a schematic view of a coating apparatus for use with that can be used to coat an actively cooled turbine blade while it is disposed in the fixture of the  FIG. 2 ; 
         FIG. 5  is a top perspective view of the fixture of  FIG. 2 ; 
         FIG. 6  is an expanded view of a Z plane detail interface with a blade root; and 
         FIG. 7  is a method for selectively applying a coating to a gas turbine engine blade. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically illustrates a general perspective view of an exemplary component  10 , e.g., an actively cooled turbine blade of a gas turbine engine. It should be understood that although a particular component type is illustrated in the disclosed non-limiting embodiment, other components, such as blades, vanes, exhaust duct liners, nozzle flaps nozzle seals as well as other actively cooled components will also benefit herefrom. These components operate in challenging high-temperature environments such as a hot section of a gas turbine engine and have aggressive requirements in terms of durability and temperature allowances. It should be appreciated that not all turbine components as defined herein may be identical to that illustrated, and that other turbine components such as vanes and static structures that are to be masked will also benefit herefrom. 
     The component  10 , for example a turbine rotor blade defined along a spanwise axis  12 , generally includes a root  14 , a platform  16  with an inner surface  20 , an outer surface  22 , peripheral faces  24 ,  26 ,  28 ,  30 , and an airfoil  34 . In one embodiment, the component  10  is manufactured from a high temperature superalloy. The root  14  need not be plated and the platform  16  segregates the airfoil  34  from the root  14 . The root  14  also includes openings to cooling passages to communicate a coolant through the airfoil  34 . 
     The root  14  may have a fir-tree, dovetail, or other convoluted shape, which is precision machined to fit within a correspondingly shaped slot in a rotor disk (not shown). Because of the precision machining, the addition of even small amounts of plating to the root  14  may adversely affect the tight tolerances in the assembly process. In addition, the plating materials may instigate fretting and thereby undesirably affect the fatigue life of the root  14 . 
     When installed in a gas turbine engine, the root  14  mates with the correspondingly shaped slot in a rotatable hub so that the blade projects radially outwardly from the hub. The platform cooperates with platforms of adjacent blades installed in the hub to define a circumferential inner boundary of an engine flowpath. The airfoil extends radially across the flowpath so that both the airfoil and the outer surface  22  of each platform are directly exposed to the damaging influences of a working medium gas flowing through the flowpath. The root, the platform inner surface and the faces are not directly exposed to the working medium. 
       FIG. 2  illustrates one exemplary embodiment of a fixture  100  for applying a protective coating, such as a thermally insulating, oxidation resistant or corrosion resistant coating to the airfoil and the platform outer surface. The illustrated fixture  100  is capable of holding two blades at a time, however, fixtures with other capacities will benefit herefrom. 
     In an embodiment, the fixture  100  is used in conjunction with a coating application apparatus such as the low pressure plasma spray (LPPS) coater  60  shown in  FIG. 4 . The coater includes a vacuum chamber  62  with a plasma spray gun  64  and a nozzle  66  projecting through one wall of the chamber. The nozzle is connected to a hopper  70  containing a supply of powder metal  72 . A gripper  74  extends into the interior of the chamber. One end of a shaft  76  is removably connected to the gripper and the fixture  100  is removably connected to the other end of the shaft. During coating operations, the gun  64  generates a high temperature flame that vaporizes metal particles metered through the nozzle. The resultant metallic mist coats the exposed portions of the component  10  and accumulates on the fixture as well. An example of a powder metal used in the above-described coater is one that includes a nickel with significant amounts of cobalt, chromium and aluminum. Such a material, when deposited as a coating on selected surfaces of a turbine blade, forms a corrosion and oxidation resistant barrier and serves as a foundation for the subsequent application of a ceramic thermal barrier coating. 
     With continued reference to  FIG. 2 , the fixture  100  generally includes a base  102  with a shank  104  having a longitudinal axis Y. The base  102  defines a plane transverse to the longitudinal axis Y and the longitudinal axis Y is generally parallel to the axis  12  of each component  10 . The base  102  includes, in this embodiment, two receptacles  106 , each of which receives the root  14 . Each receptacle  106  includes a forward wall  108 , an aft wall  110 , a first sidewall  112 , a removable sidewall  114 , and a first and second removable Z plane detail  116  associated with the first sidewall  112  and the removable sidewall  114 . As defined herein “Z-plane” is a reference surface upon which the root  14  is referenced with the rotor. It should be appreciated that other reference surfaces may be utilized. 
     Each removable sidewall  114  includes edge rails  115  that at least partially surround the forward wall  108  and the aft wall  110  to properly orient that removable sidewall  114  to form a masking box  152  ( FIGS. 3 and 5 ) that enshrouds and shields the area below the platform  16  during the, for example, Low Pressure Plasma Spray (LPPS) operation ( FIG. 4 ). 
     The first sidewall  112 , the removable sidewall  114 , and the removable Z plane details  116 , each include a respective aperture  118 , to receive a fastener  124  such as a bolt and nut to secure the first sidewall  112 , the removable sidewall  114 , and the removable Z plane details  116  together to retain or otherwise sandwich the rotor blade in position by the root  14  for the coating process ( FIG. 4 ). At least one of the removable Z plane details  116  may include a hexagonal indentation  117  ( FIG. 3 ) to receive a head of the fastener  124  to rotationally restrain the fastener  124  and thereby facilitate assembly of the fixture  100 . 
     The first sidewall  112  and the removable sidewall  114  each include a respective slot  126  within an indentation  128  shaped to receive the respective removable Z plane details  116  such that a tab  130  from the respective removable Z plane details  116  can extend through the respective slot  126  to interface with the appropriate Z-plane serration of the root  14  ( FIG. 6 ). The removable Z plane details  116  are shaped to interface with an allowable electrical contact point on the root serration. The tabs  130  may include a rectilinear polygon shaped end surface  131  ( FIG. 6 ) The tabs  130  are so shaped and configured so as to not interface with a non-reference surface of the Z-plane slot in the serrations of the root  14 . That is, the tabs  130  do not influence the critical reference surfaces of the root  14  should electrical arcs occur during the Low Pressure Plasma Spray (LPPS) operation. 
     The removable Z plane details  116  facilitate stripping of the main masking box when subjected to acid stripping, while protecting removable Z plane details  116  from the corrosive effects of the acid. One benefit of not stripping the removable Z plane details  116  is that proper electrical contact with the root  14  ensures the component  10  is suitably retained in the fixture  100  as a result of clamping in the masking box  152  by the removable Z plane details  116 . Since the removable Z plane details  116  are produced independently of the masking box, there is a cost savings as when the removable Z plane details  116  require replacement, only the removable Z plane details  116  need be replaced. The removable Z plane details  116  always remain in a desired electrical contact zone since the tabs  130  of the removable Z plane details  116  need not be subjected to the acid stripping and thus do not change dimensional tolerances over time. 
     With reference to  FIG. 7 , a method  300  for selectively applying a coating to the component  10 , is schematically disclosed in terms of a functional block diagram flowchart. It should be appreciated that alternative or additional steps may be provided without departing from the teaching herein. 
     Initially, the component  10  is inserted into the partially formed receptacle  150  ( FIG. 2 ) of the fixture  100  (step  302 ). The removable sidewall  114  and the removable Z plane details  116  are then assembled to the base  102  to complete the receptacle  150  of the masking box  152  ( FIG. 5 ) that shields the root  14  (step  304 ). Respective fasteners  124  secure the first sidewall  112 , the removable sidewall  114 , and the removable Z plane details  116  together to form the receptacle  150  ( FIG. 5 ) that retains the component  10  such that the tab  130  extends through the respective slot  126  to interface with the Z-plane in a serration of the root (step  306 ). 
     Next, the component  10  is coated (step  308 ;  FIG. 4 ). Various coating processes may be utilized, but here, a Low Pressure Plasma Spray (LPPS) operation is utilized. 
     After a predetermined number of coating cycles, the accumulated coating may be stripped from the fixture  100  with an acidic solution (step  310 ). It should be appreciated that the acidic solution tends to slowly eat away, and thus change, the dimensions of the material from which the fixture  100  is manufactured. The removable Z plane details  116  need not be stripped with the acidic solution at the same frequency as the other components of the fixture  100  thus avoiding dimensional degradation. Further, the removable Z plane details  116  may simply and relatively inexpensively be replaced at select coating cycle intervals to assure a proper interface with the component  10  (step  312 ). 
     The use of the terms “a” and “an” and “the” and similar references in the context of description (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or specifically contradicted by context. The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (e.g., it includes the degree of error associated with measurement of the particular quantity). All ranges disclosed herein are inclusive of the endpoints, and the endpoints are independently combinable with each other. It should be appreciated that relative positional terms such as “forward,” “aft,” “upper,” “lower,” “above,” “below,” and the like are with reference to the normal operational attitude of the vehicle and should not be considered otherwise limiting. 
     Although the different non-limiting embodiments have specific illustrated components, the embodiments of this invention are not limited to those particular combinations. It is possible to use some of the components or features from any of the non-limiting embodiments in combination with features or components from any of the other non-limiting embodiments. 
     It should be appreciated that like reference numerals identify corresponding or similar elements throughout the several drawings. It should also be appreciated that although a particular component arrangement is disclosed in the illustrated embodiment, other arrangements will benefit herefrom. 
     Although particular step sequences are shown, described, and claimed, it should be understood that steps may be performed in any order, separated or combined unless otherwise indicated and will still benefit from the present disclosure. 
     The foregoing description is exemplary rather than defined by the limitations within. Various non-limiting embodiments are disclosed herein, however, one of ordinary skill in the art would recognize that various modifications and variations in light of the above teachings will fall within the scope of the appended claims. It is therefore to be understood that within the scope of the appended claims, the disclosure may be practiced other than as specifically described. For that reason the appended claims should be studied to determine true scope and content.