Patent Application
20080264897
The present disclosure relates generally to material removal processes, more specifically to a method for incorporating a pattern within a coating deposited upon a substrate, and even more specifically to the substrate being a turbine component, more specifically an airfoil shroud.
In a gas turbine engine, such as may be used for electrical power generation for example, in order to achieve enhanced engine efficiency, it is important that the buckets rotate within a turbine casing or “shroud” with reduced interference to provide the enhanced efficiency relative to the amount of energy available from an expanding working fluid. Typically, increased operation efficiencies can be achieved by maintaining a reduced threshold clearance between the shroud and tips of the buckets, which prevents unwanted “leakage” of hot gas over tips of the buckets. Increased clearances lead to leakage problems and cause significant reduction in overall efficiency of the turbine. However, it should be appreciated that a reduction in clearances that leads to interference between bucket tips and the shroud is generally undesirable.
The need to maintain adequate clearance without significant loss of efficiency is made more difficult by the fact that as the turbine transitions to steady state operating temperature, different components that possess varying thermal expansion properties can expand at different rates. Furthermore, as the turbine rotates, centrifugal forces acting on the turbine components can cause the buckets to expand in an outward direction toward the shroud, particularly when influenced by high operating temperatures. Thus, it is important to establish reduced effective running clearances between the shroud and bucket tips while preventing interference at various operating conditions of the turbine.
Typically, the shrouds are fabricated (for example, by casting and machining) to include a concave profile that mates with a convex contour of a surface of the bucket tips (the rotation of the bucket tip forms a convex contour towards the shroud, though it should be appreciated that the surface of each bucket tip is not necessarily convex, and may be flat). Mating the concavely machined shroud with the convex bucket tip contour in this manner maintains a reduced clearance over the whole surface of the tip. The shrouds often further include coatings such as thermally sprayed MCrAlYs where M is the base metal, Cr is chromium, Al is aluminum, and Y is yttrium, or aluminides for example, to resist oxidation and corrosion of the shroud in the high operating temperatures of the turbine. It has been found that incorporating a pattern within the coatings increases the surface area and reduces airflow between the bucket and the shroud to perform in the same manner as a reduction in clearance between the bucket and the shroud, thereby increasing operating efficiency. One current method to incorporate the pattern is to spray the coating onto the base of the shroud in conjunction with a mask that reflects the desired pattern. Such spray masking methods are slow and have pattern geometry resolution limits. Another method to incorporate the pattern including the concave profile, or any desired profile, is conventional machining, such as computer numerical control (CNC) milling for example. Because of properties of the oxidation-resistant coatings, machining the coatings into the large surface area of the shroud is difficult, and time and labor intensive. For example, each depression or recess of the pattern is formed in a serial fashion, one after another. Furthermore, the size and surface geometry of the pattern may be limited by that of the machining tool. Accordingly, there is a need in the art for an arrangement to incorporate patterns into turbine component surfaces that overcomes these drawbacks.
An embodiment of the invention includes a method of forming a pattern comprising a plurality of recesses within a turbine component. The method includes simultaneously dissolving a plurality of portions of a selected section of the turbine component, thereby defining the plurality of recesses of the pattern.
Another embodiment of the invention includes a method of forming a pattern comprising a plurality of recesses within a coating disposed upon a turbine component. The method includes simultaneously dissolving a plurality of portions of the coating disposed upon a selected section of the turbine component, thereby defining the plurality of recesses of the pattern.
These and other advantages and features will be more readily understood from the following detailed description of preferred embodiments of the invention that is provided in connection with the accompanying drawings.
Referring to the exemplary drawings wherein like elements are numbered alike in the accompanying Figures:
An embodiment of the invention provides a process to incorporate a pattern having a plurality of recesses into a surface of a coating, such as the thermally sprayed MCrAlY coating or aluminides, to resist oxidation and corrosion for example, deposited upon a section of an airfoil shroud. Such coatings typically include elements from the group of Nickel, Cobalt, Chromium, Aluminum, Yttrium, Rhenium, Rhodium, Ruthenium, Palladium, Platinum, Niobium, Molybdenum, Silicon, Hafnium, Iron, Manganese, and at least one from lanthanide series such as Gadolinium, and Lanthanum, for example. In an embodiment, a chemical etch is used to simultaneously remove the plurality of recesses from selected portions of the coating to provide the desired pattern. As used herein the term “portion”, used with respect to the oxidation resistant coating, shall indicate a fraction of an area of the surface that is greater than zero, but less than the entire area. In another embodiment, electrochemical machining (ECM) is utilized to simultaneously remove the plurality of recesses selected portions of the coating to provide the desired pattern. While the embodiment described herein depicts an airfoil shroud as an exemplary substrate, it will be appreciated that the disclosed invention is also applicable to other substrates or turbine components that may incorporate difficult to machine coatings, such as on turbine buckets, nozzles, liners, and transition pieces for example.
Referring now to
In one process known as chemical etching, an etchant, such as one or more of hydrofluoric acid, sulfuric acid, nitric acid, and combinations thereof, for example, is selected, based upon a composition of the coating 58, to dissolve the coating 58.
In an exemplary embodiment, the mask 78 comprises a photoresistant mask 78 that is applied directly to portions of the surface 64 of the coating 58 via a lithography process, for example. For example, the surface 64 of the coating 58 is cleaned, with a cleaner such as one or more of acetone and isopropyl alcohol solvent followed by a rinse with deionized water. Additional lithographic solvents and a plasma etch prepare the surface 64 of the coating 58 for application of a photoresistant material. A mask or cover 82 is disposed between an energy source 86 and the photoresistant material applied to the coating 58. The mask 82 includes a geometry inverse to that desired of the mask 78. As an example of inverse geometry, the mask 82 includes openings 84 that correspond to the geometry of the mask 78 and allow transmission of the energy created by the energy source 86. The remainder of the mask 82 blocks transmission of energy, such as ultraviolet energy for example, from the energy source 86 to the photoresistant material. In response to exposure of the energy, the photoresistant material cures and adheres to the coating 58. Solvents, and optionally, a second plasma etch, remove portions of the photoresistant material that have not cured, and the cured photoresistant material thereby defines the mask 78, which defines the plurality of protrusions 70. In an alternate embodiment, use of the mask 82 is replaced with a directed energy source, such as a laser beam for example, that focuses the energy to the portions of the photoresistant material that represent the desired geometry of the mask 78. Use of the lithography process provides the mask 78 having geometry resolution as fine as 10 microns. Photoresistant materials are selected based upon their ability to chemically resist the etchant and thereby prevent dissolving of the coating 58.
In another embodiment, the mask 78 is produced by an additive manufacturing process known in the art as direct writing. As with lithography, the direct writing process begins with cleaning the surface of the coating 58. An appropriate mask 78 material that is compatible with the direct writing process, such as particles suspended within a liquid or fluid medium for example, is selected. As with lithography, the mask 78 material selected must adhere appropriately to the coating 58, and be chemically resistant to the etchant. The selected mask material is applied directly upon the coating 58, using a suitable direct write tool, such as a pen dispensing tool, a thermal plasma gun, laser transfer, or an ink-jet to deposit the suitable material upon the coating 58, or a laser beam to cause the particles suspended in the medium proximate the coating 58 to locally activate and adhere to the coating 58, for example. The direct write tool is guided, via robotic guidance for example, to apply the mask 78 material in accordance with a desired geometry of the mask 78, as may be provided via a computer generated model, for example. The direct writing process further includes consolidating or curing the mask 78 material applied to the coating 58 via a curing activator appropriate to the selected material, such as one of heat energy, laser energy, plasma energy, electron beam energy, ion beam energy, and combinations thereof.
It will be appreciated that the etchants described above to dissolve the coating 58 are extremely corrosive materials, use of which require specialized handling procedures, and may therefore be desired to be avoided. Referring now to
In an embodiment, the power source 94 is a direct current (DC) power source with the coating 58 in power connection with a positive terminal 106 of the power source 94, either directly or via the base 54. The tool 102 is in power connection with a negative terminal 110 of the power source 94. Accordingly, the coating 58 and the tool 102 represent electrodes, such as an anode and a cathode respectively, of an electrochemical cell, as will be appreciated by one of skill in the art. In response to the application of an electrical potential from the power source 94, current flow or electrical charges passing through the coating 58, via the electrolyte 98 will result in the dissolving of the anode (coating 58). It will be appreciated that a gap 112 is disposed between the coating 58 and the tool 102, such that any current flow occurs through the electrolyte 98, not via contact of the tool 102 with the coating 58. The application of power may be straight DC, or pulsed DC. In an embodiment, the coating 58 includes oxides and metal and the pulsed DC power includes alternating cathodic and anodic biased pulses to preferentially dissolve oxides during one cycle and metal during the other. Power settings such as total machining time, pulse amplitude, pulse on-time, and pulse off-time will determine collectively the total electrical charges passing through the machining areas, which in turn determine the amount of material removal, and therefore, the geometry of the recesses 66.
Because of the use of the power supply 94 to electrochemically dissolve the coating 58 material, the electrolyte 98 of the ECM system 90 is less corrosive than the etchants described above, and therefore requires less specialized handling procedures. The specific electrolyte 98 to be used is related to the material composition of the coating 58 to be dissolved. For example, hydrofluoric silicate, ammonium fluorosilicate ((NH4)2SiF6), fluorosilic acid (H2SiF6), and aqueous solutions of sodium chloride (NaCl), sodium nitrate (NaNO3), and sodium bromide (NaBr), and combinations thereof are examples of such electrolytes 98 that are less corrosive than the etchants described above and are contemplated as appropriate for dissolving coatings 58 applied to airfoil shroud sections 50. While an embodiment is depicted having the tool 102 and the shroud section 50 immersed in the electrolyte 98, it will be appreciated that the scope of the invention is not so limited, and that the invention will also apply to other means to dispose the electrolyte 98 between the tool 102 and the coating 58, such as to use any of gaskets, pumps, and directed flow nozzles to provide and circulate the electrolyte 98, for example.
In one embodiment, the tool 102 includes an inverse of the geometry of the desired pattern 62 to be formed within the coating 58. As an example of inverse geometry, the tool 102 includes protrusions 111 that are disposed to correspond to the desired location of the recesses 66 of the pattern 62 in the airfoil shroud section 50 (best seen with reference to
In another embodiment, the mask 78 is disposed upon the coating 58. The mask 78, for use in conjunction with the ECM system 90, shall be electrically insulating to prevent current flow between the tool 102 and the locations of the coating 58 upon which the mask 78 is disposed, thereby preventing dissolving of the coating 58 and providing the protrusions 70 of the pattern 62. The mask 78 can be applied to the coating 58 via one of the lithographic processes and the direct write process, as described above. It will be appreciated that use of the mask 78 in conjunction with the electrolyte 98 of the ECM system 90 (as compared to the etchants described above) represents a less corrosive environment to which the mask 78 is exposed. Therefore, use of the ECM system 90 allows selection of mask 78 materials suitable for use in conjunction with less corrosive environments. In an embodiment, the geometry of mask 78 is produced separately, such as upon a polyester substrate for example, and subsequently disposed upon or transferred to the coating 58. As a result of use of the mask 78, a finer resolution of the features of the pattern 62, is contemplated, such as the recesses 66 and protrusions 70 having a resolution of greater than or equal to approximately 0.13 mm (0.005 inches) and less than or equal to approximately 0.25 mm (0.010 inches). In conjunction with the mask 78, the tool 102 need not include geometry that is the inverse of the desired pattern 62, as the electrical insulation of the mask 78 prevents dissolving of the coating 58, to provide the protrusions 70.
Referring now to
Referring now to
The method begins at Step 134 with selecting the turbine component 50, such as the airfoil shroud section 50 that includes the coating 58 for incorporation of the pattern 62. The method continues at Step 138 by simultaneously dissolving a first plurality of portions of the coating 58 disposed upon the selected turbine component 50. As used herein, the term “simultaneously dissolving” refers to a process where some dissolving of the plurality of portions of the coating occur at the same time, but does not require that all of the desired dissolving of the plurality of portions is initiated and completed in the exact same time frame. As a result of the simultaneous dissolving of the first plurality of portions of the coating 58, the plurality of recesses 66 of the pattern 62 are thereby defined. In an embodiment, a plurality of components 50 are selected, and the first plurality of portions upon each component 50 of the plurality of components 50 are dissolved simultaneously, thereby providing an increased productivity. In an embodiment, the method further includes applying the mask 78 upon a second plurality of portions of the coating 58, which correspond to the protrusions 70.
The mask 78 may be formed via the lithographic process, which includes cleaning the surface 64 of the coating 58 upon which the mask 78 will be applied, applying the photoresistant material to the cleaned surface 64, and disposing the cover 82 having openings 84 that correspond to a geometry of the mask 78 (to be disposed upon the second plurality of portions of the coating 58) between the energy source 86 and the photoresistant material. In response to exposing the photoresistant material to energy from the source 86 through the openings 84, the photoresistant material is cured. Following removing uncured photoresistant material, the mask 78 is defined by the remaining cured photoresistant material.
The mask 78 may also be formed by the direct write process, which includes cleaning the surface 64 of the coating 58 upon which the mask 78 will be applied, and depositing the material of the mask 78 in the form of particles suspended within the fluid medium directly upon the coating 58 via the robotic guidance control at the desired mask location 78. The direct write process further includes applying the species, such as heat for example, to cure the deposited mask 78 material, and thereby define the mask 78 deposited upon the coating 58.
In one embodiment, the dissolving of the first plurality of portions of the coating 58 includes selecting the etchant to chemically dissolve the coating 58 and exposing the coating 58 and the chemically resistant mask 78 to the etchant. Accordingly, the chemically resistant mask 78 prevents the dissolving of the coating 58 where the mask is applied and thereby defines the plurality of protrusions 70 of the pattern 58.
In another embodiment, using the ECM system 90, the dissolving of the first plurality of portions of the coating 58 includes applying the electric potential via the power supply 94 to the coating 58 and the cathode (tool 102). The electrolyte 98 is conductive, and permits the flow of current between the cathode (tool 102) and coating 58.
One embodiment of the ECM system 90 utilizes the mask 78 as the electrically insulating mask 78 to prevent passing of the charge through the coating 58, which prevents dissolving (at the second plurality of portions) of the coating 58 upon which the mask 78 is disposed, thereby defining the plurality of protrusions 70 of the pattern 62.
Another embodiment of the ECM system 90 utilizes the cathode (tool 102) including the plurality of protrusions 111 in a pattern that defines the plurality of recesses 66. The tool 102 is disposed such that the plurality of protrusions 111 on the tool 102 correspond to the desired location of the plurality of recesses 66 in the coating.
Repair of the used shroud section 50 may be accomplished via the direct write process, which includes selecting the material for depositing upon the coating 58, depositing the material upon one or more worn protrusions 114 of the pattern 62, and applying the appropriate species to the deposited material to cure, or consolidate the deposited material, thereby defining the geometry of unworn protrusions 118 and repairing the worn section 50.
While an embodiment of the invention has been described employing a mask 78 produced by lithography using a negative photoresist that cures in response to exposure to energy, it will be appreciated that the scope of the invention is not so limited, and that the invention will also apply to lithography that may use a positive photoresist, such that exposure to energy renders the photoresist soluble, for example.
As disclosed, some embodiments of the invention may include some of the following advantages: an ability to reduce a cycle time for incorporating patterns into the surface of turbine airfoil shroud sections; an ability to produce a pattern within an airfoil shroud section surface having resolution of 10 microns; an ability to reduce an overall cost of an airfoil shroud section including a patterned surface; and an ability to repair an airfoil shroud section patterned surface without removing any of the coating.
While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best or only mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Also, in the drawings and the description, there have been disclosed exemplary embodiments of the invention and, although specific terms may have been employed, they are unless otherwise stated used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention therefore not being so limited. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. Furthermore, the use of the terms a, an, etc. do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.
