Abstract:
A system is disclosed for reducing an internal diameter of a hole in a metal substrate. The system includes an electrospark device having an electrospark torch and an electrode holder rotatable about an axis. An electrode is removably supported in the electrode holder. The electrospark device configured to apply a coating of a material of which the electrode is formed, to the substrate on an inner wall of the hole when the electrode is rotated within the hole.

Description:
BACKGROUND OF THE INVENTION 
     The application generally relates to sizing of cooling holes on turbine components. The application relates more specifically to an electrospark deposition (ESD) process for reducing oversized holes in gas turbine engine components. 
     Electrospark deposition (ESD) is a pulsed-arc, micro-welding process that uses short-duration, high-current electrical pulses to deposit a consumable electrode material on a conductive workpiece. ESD processes generally involve very high spark frequencies with the spark duration lasting only a few microseconds. ESD generally requires manual control or preprogramming of the process parameters. Significantly, depositions result in very little heat input because heat is generated during less than 1% of a weld cycle and dissipated during 99% of the cycle. ESD coatings are extremely dense and metallurgically bonded to the workpiece. Conventional electrospark deposition devices employ an electrode rod, which is required to have a sharp tip for generating electrical discharges or sparks. When using conventional ESD devices it is difficult to repair or build up confined surfaces such as an inner diameter of a hole in a metal part. 
     Many component parts of a gas turbine engine include cooling holes for active cooling of engine sections located downstream of the turbine section. The rising combustor exit temperatures in gas turbine engines necessitate active cooling to avoid thermal failure. In some parts of the gas turbine engine, hole dimensions are not as important, but cooling hole configurations may be critical on such parts as buckets, vanes, shrouds and nozzles, for example. 
     In manufacturing gas turbine component parts, drilling and machining operations may on occasions result in cooling holes being larger than specified, e.g., when an incorrect hole diameter is used when setting up the tooling, or when an operator error occurs during manufacturing. When a hole drilled in an engine component is too large, the component may not pass inspection, and will be returned for quality control reasons for correction. Currently, repair of oversized holes may be a costly and time consuming project, and may not be possible at all, resulting in the loss of valuable engine parts. Oversizing of holes can occur during turbine operation due to erosion, oxidation, and other operational conditions. 
     Currently, one solution for dealing with oversized cooling holes in engine components is to weld the oversized holes until they are closed, and machine new cooling holes in the engine component using electrode discharge machining (EDM) process. EDM is a process whereby material is removed from the workpiece by a series of rapidly recurring current discharges between an electrode and the workpiece, separated by a dielectric liquid and subject to an electric voltage. 
     Intended advantages of the disclosed systems and/or methods satisfy one or more of these needs or provide other advantageous features. Other features and advantages will be made apparent from the present specification. The teachings disclosed extend to those embodiments that fall within the scope of the claims, regardless of whether they accomplish one or more of the aforementioned needs. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The disclosed method is an ESD process using a rotary electrode. A ceramic tube spacer is placed around a portion of the electrode to maintain the desired length of electrode positioned in the hole constant during the ESD process. The ceramic tube spacer also protects the top surface adjacent the cooling holes from being damaged, e.g., by heat, during the ESD process. Selection of the electrode material depends on the substrate material. 
     One embodiment relates to a method of reducing an internal diameter of a hole in a metal substrate. The method includes the steps of providing the metal substrate having a hole penetrating the metal substrate; providing an electrospark deposition device, the ESD device configured to receive a rotary electrode supported axially in the ESD device; centering the electrospark torch over the hole in the substrate; inserting the rotary electrode into the hole to a predetermined length below a top surface of the metal substrate; maintaining constant the predetermined length of insertion of the rotary electrode in the hole; applying the electrode disk to the substrate on an inner surface of the hole; rotating the rotary electrode in contact with an inner wall of the hole; applying an electrospark process; and depositing an alloyed coating on at least a portion of the inner wall along the inner wall to reduce the diameter of the hole by a thickness of the coating. 
     Another embodiment relates to a system for reducing an internal diameter of a hole in a metal substrate. The system includes an electrospark device having an electrospark torch and an electrode holder rotatable about an axis. An electrode is removably supported in the electrode holder. The electrospark device configured to apply a coating of a material comprising the electrode to the substrate on an inner wall of the hole when the electrode is rotated within the hole. 
     An advantage of the present disclosure is an electrospark deposition process that is simple to operate; characterized by low heat input; flexible, e.g., with respect to electrode length which depends on the application. 
     Another advantage is that the repair process may be completed without the use of EDM, as only slight grinding of the repaired site is sufficient to finish the substrate. 
     Alternative exemplary embodiments relate to other features and combinations of features as may be generally recited in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an ESD process for reducing an oversized hole in a substrate. 
         FIG. 2  is a photograph of an oversized hole and an adjacent hole reduced by the ESD process of the present disclosure, adjacent to an oversized hole. 
         FIG. 3  is a flow diagram of an ESD hole reduction process, according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The ESD process is known to have a very low heat input. The ESD transfers stored energy to a consumable electrode, e.g., carbides (W, Ti, Cr etc) stainless steel, nickel alloys, cobalt alloys, aluminum, and other electrode compositions. The temperature on the tip of the electrode may be in a range of about 8000 to about 25000° C. The electrode material is ionized and transferred to the substrate surface, producing an alloy with the substrate and a deposition on the alloyed electrode-substrate interface. The deposited layer adheres metallurgically on the alloyed substrate and electrode material. 
     Referring to  FIG. 1 , an exemplary arrangement for an ESD process for reducing an internal diameter of a hole is shown. An electrospark torch  10  is positioned over a substrate  12 , and centered over a hole  14  through the substrate  12 . Electrospark torch  10  may be supported by a robotic frame or other conventional mounting frame (not shown). At one end electrospark torch  10  includes a rotary electrode holder  16  configured to hold a consumable electrode  20 . Electrode  20  is supported axially within rotary electrode holder  16 . A cylindrical ceramic tube spacer  18  having a hollow bore is positioned above hole  14  on substrate  12 , substantially coaxial with the hole. Electrode  20  is inserted through hollow bore of the ceramic tube spacer  18  and into hole  14 . Electrode  20  may penetrate partially or completely through the thickness of substrate  12 . The length L, indicated by arrows, indicates the length of electrode  20  penetrating into hole  14  below substrate top surface  22 . Ceramic tube spacer  18  acts as a spacer between electrode holder  16  and substrate top surface  22 , to ensure that the length L of electrode  20  in hole  14  remains constant throughout the ESD operation. Length L may be adjusted for different applications, e.g., different substrate materials, substrate thicknesses and hole sizes. 
     Electrospark torch  10  deposits a coating of electrode material from electrode  20  along an inner wall  24  of hole  14 . The deposited coating of electrode material reduces the inner diameter of hole  14 , using the ESD process. 
     Referring to  FIG. 2 , an exemplary test substrate is shown. In the example test plate, a pair of holes  32 ,  34  were originally drilled with a diameter of 0.117″ representing an oversized diameter. Hole  34  was then reduced by the ESD process described above. The inside diameter D of the reduced hole  34  after ESD is 0.0710″, indicating that the hole diameter has been reduced by 0.046″ overall from the original diameter of 0.117″. The repair site on the top surface  22  of substrate  12  was then ground to remove any surface irregularities caused by the ESD process, to leave a substantially smooth finish. The hole diameter may be reduced by the ESD process in the range of 0.001″ to 0.100″, for example, the deposition thickness depending on the length of time the ESD process is applied. 
     Referring next to  FIG. 3 , a flow diagram of the disclosed ESD hole reduction process is shown. The method begins at step  100 , by providing an electrospark torch configured for a rotary electrode, and a substrate having at least one hole therethrough. At step  110 , the method proceeds by holding a consumable rotary electrode in the electrospark torch, with the rotary electrode axially supported within an electrode holder. Next at step  112 , the method proceeds by centering the electrospark torch over the hole through the substrate, which hole is to be reduced in diameter. Next at step  114 , a hollow ceramic tube spacer is positioned above the hole on the substrate. At step  116 , the depth of penetration of the substrate by the rotary electrode is maintained constant by inserting the rotary electrode through the ceramic tube spacer, which provides a predetermined space between the electrode holder and the substrate top surface throughout the ESD operation. Then at step  118 , the method proceeds with the ESD to deposit an alloyed coating of electrode material and fusion bonding the deposited coating with the substrate at the inner wall of the hole to reduce the inner diameter of the original hole. Finally, at step  120 , the electrospark torch and associated electrode and ceramic tube spacer are removed and, optionally, the substrate surface surrounding the reduced hole is ground down to a smooth surface. 
     Substrate  12  may be any metal used in turbine components. In particular, the materials used for substrate  12  may include carbon steels, low alloy steels, stainless steels (martensitic, ferritic austenitic and precipitation hardened), solid solution nickel/cobalt base alloys such as INCONEL® 617, INCONEL® 625, Hastelloy X, HAYNES® 25, HAYNES® 188 and similar Ni/Co alloys, Precipitation hardened superalloys such as, for example, GTD111, GTD444, Rene 108, Rene N4, Rene N5, FSX414 (trademarks of GE Corporation) and similar superalloys; wear resistant alloys such as TRIBALOY® (T800), various stellites, and similar wear resistant alloys; and titanium alloys. “INCONEL” is a federally registered trademark of alloys produced by Huntington Alloys Corporation, Huntington, W. Va. “HAYNES” and “HASTELLOY” are federally registered trademarks of alloys produced by Haynes International, Inc., Kokomo, Ind., “TRIBALOY” is a federally registered trademark of alloys produced by Deloro Stellite Holdings Corporation, St. Louis, Mo. 
     In one embodiment, the substrate may have a composition, by weight, of about 14% chromium, about 9.5% cobalt, about 3.8% tungsten, about 1.5% molybdenum, about 4.9% titanium, about 3.0% aluminum, about 0.1% carbon, about 0.01% boron, about 2.8% tantalum, and a balance of nickel. 
     In another embodiment, the substrate may have a composition, by weight, of about 9.75% chromium, about 7.5% cobalt, about 3.5% titanium, about 4.2% aluminum, about 6.0% tungsten, about 1.5% molybdenum, about 4.8% tantalum, about 0.08% carbon, about 0.009% zirconium, about 0.009% boron, and a balance of nickel. 
     In another embodiment, the substrate may have a composition, by weight, of about 7.5% cobalt, about 7.0% chromium, about 6.5% tantalum, about 6.2% aluminum, about 5.0% tungsten, about 3.0% rhenium, about 1.5% molybdenum, about 0.15% hafnium, about 0.05% carbon, about 0.004% boron, about 0.01% yttrium, and a balance of nickel. 
     In another embodiment, the substrate may have a composition, by weight, of about 9.75% chromium, about 7.5% cobalt, about 4.2% aluminum, about 3.5% titanium, about 1.5% molybdenum, about 6.0% tungsten, about 4.8% tantalum, about 0.5% niobium, about 0.15% hafnium, about 0.05% carbon, about 0.004% boron, and a balance of nickel. 
     In another embodiment, the substrate may have a composition, by weight, of about 5% iron, about 20% to about 23% chromium, about 0.50% silicon, about 8% to about 10% molybdenum, about 0.50% manganese, about 0.10% carbon and a balance nickel. 
     In another embodiment, the substrate may have a composition, by weight, of about 20% to about 24% chromium, about 10% to about 15% cobalt, about 8% to about 10% molybdenum, about 0.8% to about 1.5% aluminum, about 0.05% to about 0.15% carbon, about 3.0% iron, about 1.0% manganese, about 0.015% silicon, about 0.015% sulfur, about 0.6% titanium, about 0.5% copper, about 0.006% boron and a balance nickel. 
     In another embodiment, the substrate may have a composition, by weight, of about 16.5% to about 18.5% chromium, about 1.5% nickel, about 3% iron, about 27% to about 30% molybdenum, about 0.08% carbon, about 0.07% nitrogen, about 0.05% oxygen, about 3.0% to about 3.8% silicon, about 0.03% phosphorus, about 0.03% sulfur and a balance cobalt. 
     In one embodiment, the substrate has a composition, by weight, of about 0.07-0.10% carbon, about 8.0-8.7% chromium, about 9.0-10.0% cobalt, about 0.4-0.6% molybdenum, about 9.3-9.7% tungsten, about 2.5-3.3% tantalum, about 0.6-0.9% titanium, about 5.25-5.75% aluminum, about 0.01-0.02% boron, about 1.3-1.7% hafnium, about 0.1% maximum manganese, about 0.06% maximum silicon, about 0.01% maximum phosphorus, about 0.004% maximum sulfur, about 0.005-0.02% zirconium, about 0.1% maximum niobium, about 0.1% maximum vanadium, about 0.1% maximum copper, about 0.2% maximum iron, about 0.003% maximum magnesium, about 0.002% maximum oxygen, about 0.002% maximum nitrogen, and a balance nickel and incidental impurities. 
     In another embodiment, the substrate may have a composition, by weight, of about 10% nickel, about 29% chromium, about 7% tungsten, about 1% iron, about 0.25% carbon, about 0.01% boron and a balance cobalt. 
     In another embodiment, the substrate may have a composition, by weight, of about 22% chromium, about 18% iron, about 9% molybdenum, about 1.5% cobalt, about 0.6% tungsten, about 0.10% carbon, about 1% manganese, about 1% silicon, about 0.008% boron and a balance nickel. 
     In another embodiment, the substrate may have a composition, by weight, of about 20% chromium, about 15% tungsten, about 10% nickel, about 3% iron, about 1.5% manganese and a balance cobalt. 
     In another embodiment, the substrate may have a composition, by weight, of about 0.015% boron, about 0.05% to about 0.15% carbon, about 20% to about 24% chromium, about 3% iron, about 0.02% to about 0.12% lanthanum, about 1.25% manganese, about 20% to about 24% nickel, about 0.2% to about 0.5% silicon, about 13% to about 15% tungsten, and a balance cobalt. 
     It should be understood that the application is not limited to the details or methodology set forth in the following description or illustrated in the figures. It should also be understood that the phraseology and terminology employed herein is for the purpose of description only and should not be regarded as limiting. 
     It is important to note that the construction and arrangement of the ESD system as shown in the various exemplary embodiments is illustrative only. Although only a few embodiments have been described in detail in this disclosure, those who review this disclosure will readily appreciate that many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter recited in the claims. For example, elements shown as integrally formed may be constructed of multiple parts or elements, the position of elements may be reversed or otherwise varied, and the nature or number of discrete elements or positions may be altered or varied. Accordingly, all such modifications are intended to be included within the scope of the present application. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. In the claims, any means-plus-function clause is intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. Other substitutions, modifications, changes and omissions may be made in the design, operating conditions and arrangement of the exemplary embodiments without departing from the scope of the present application. 
     It should be noted that although the figures herein may show a specific order of method steps, it is understood that the order of these steps may differ from what is depicted. Also two or more steps may be performed concurrently or with partial concurrence. Such variation will depend on the software and hardware systems chosen and on designer choice. It is understood that all such variations are within the scope of the application. Likewise, software implementations could be accomplished with standard programming techniques with rule based logic and other logic to accomplish the various connection steps, processing steps, comparison steps and decision steps. 
     While the exemplary embodiments illustrated in the figures and described herein are presently preferred, it should be understood that these embodiments are offered by way of example only. Accordingly, the present application is not limited to a particular embodiment, but extends to various modifications that nevertheless fall within the scope of the appended claims. The order or sequence of any processes or method steps may be varied or re-sequenced according to alternative embodiments.