Abstract:
Microspray apparatus and methods involve injecting powdered material into a plasma gas stream. The material comprises first and second component powders. The second powder is a majority by the weight of the powdered material. The first powder acts as a melting point depressant. The first and second powders may have similar compositions but with the first powder including a greater quantity of a melting point depressant element.

Description:
BACKGROUND OF THE INVENTION 
     The present disclosure generally relates to spray deposition methods and apparatus and, more particularly, deposition on high temperature components of gas turbine engines. 
     Well developed fields exist regarding plasma spray deposition. U.S. patent application Ser. No. 10/976,560 filed on Oct. 29, 2004 by Zajchowski, et al. and entitled “Method and Apparatus for Microplasma Spray Coating a Portion of a Turbine Vane in a Gas Turbine Engine” discloses an exemplary microplasma spray coating apparatus. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention involves a microplasma spray apparatus. A microplasma gun includes an anode, a cathode, and an arc generator for generating an electric arc between the anode and cathode. A nozzle emits arc gas into the electric arc. The electric arc is operable for ionizing the gas to create a plasma gas stream. At least one reservoir contains powdered material. The material includes first and second component powders. The second powder is by a majority, by weight, of the powdered material. The first powder acts as a melting point depressant. A powder injector is coupled to the reservoir for injecting the powdered material into the plasma gas stream. 
     Another aspect of the invention involves a method for replacing material lost from a site on a substrate. A microplasma spray is formed from a multi-powder combination of at least a first metal powder and a second metal powder. The spray is directed to a substrate to form a deposit. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a microplasma spray gun and a workpiece. 
         FIG. 2  is a partially exploded view of a microplasma spray apparatus. 
         FIG. 3  is a view of the microplasma spray apparatus of  FIG. 2  applying a material to a workpiece. 
         FIG. 4  is a flowchart of a process applying the material. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 1  shows a microplasma spray apparatus  10 . The apparatus may be constructed and operated as in U.S. patent application Ser. No. 10/976,560, the disclosure of which is incorporated herein by reference herein as if set forth at length. The exemplary microplasma spray apparatus  10  includes a microplasma gun  12  having an arc gas emitter  14 , an anode  16 , and a cathode  18 . An electric arc  20  is generated between the anode  16  and cathode  18 . A plasma stream  21  is formed when arc gas is injected from the arc gas emitter  14  through the arc  20 . A powdered material injector  22  dispenses powdered material into the plasma gas stream, which transports the powdered material to the workpiece  24 . As a result, the powdered material forms a deposit on a desired location on the workpiece  24 . 
     The powdered material is not, however, initially provided as a single powder of a single alloy. Rather, powders of multiple alloys are provided either pre-mixed or mixed by the apparatus. The component powders may be selected in view of the workpiece properties. The workpiece may consist of or comprise a nickel-based superalloy substrate. The apparatus may be used to form a deposit for replacing parent material lost from the substrate (e.g., due to damage plus cleaning and preparation) or to augment (e.g., fill a manufacturing defect, coat with a dissimilar material, or otherwise). Exemplary powdered material combinations are disclosed in U.S. Pat. No. 4,008,844, the disclosure of which is incorporated by reference herein as if set forth at length. The exemplary powder material includes a transient liquid phase (TLP)-forming powder and a main powder. The exemplary main powder may have a composition similar to the desired deposit. The TLP powder may have an otherwise generally similar composition but including at least one melting point depressant such as boron. For nickel-based superalloys, exemplary boron concentrations in the main powder are preferably less than 1% (by weight), preferably less than 0.5%, and more preferably essentially zero (or the level in the substrate). For the TLP-forming powder, exemplary boron concentrations are at least 2%, more preferably at least 2.5%. 
       FIGS. 2 and 3  show further details of one exemplary microplasma spray apparatus  10 . The apparatus  10  may be operable for depositing material on many things, including, but not limited to at least a portion of a HPT or LPT vane  72  in a gas turbine engine (not shown). HPT vanes are particularly relevant components. Whereas turbine blades are typically single crystal or directionally solidified (DS) structures, HPT vanes are typically non-crystalline. The present microplasma repair process may provide the repair with properties highly similar to those of the undamaged vane substrate. Thus, the method may advantageously be used to repair localized vane damage (e.g., foreign object damage, erosion, or thermal fatigue damage) along surfaces exposed to the turbine gas path. 
     In the exemplary embodiment, the cathode  18  is held within and extending from an insulated body  26  of a cathode cartridge or assembly  28 . The exemplary cartridge  28  also includes threads  30  for threadingly engaging the microplasma gun body. The exemplary cathode  18  also includes an O-ring seal  32  to seal the leak path that is created at the interface between the cartridge  28  and the microplasma gun body. 
     In operation, an electric arc  20  ( FIG. 1 ) is generated between the anode  16  and cathode  18 . Arc gas such as, but not limited to, argon is emitted into the electric arc  20 . The arc gas can be emitted prior to generating the electric arc. The electric arc ionizes the gas to create the plasma gas stream  21 . The ionization process removes electrons from the arc gas, causing the arc gas to become temporarily unstable. The arc gas heats up to approximately 20,000°-30,000° F. as it re-stabilizes. The plasma stream cools rapidly after passing through the electric arc. 
     A powdered material injector  22  injects powdered material  34  into the plasma gas stream  21 . The powdered material  34  is heated to a super-plastic state in the plasma stream and is deposited on the vane ( FIG. 3 ) where it cools and re-solidifies to form the deposit. The exemplary powdered material injector  22  includes a powder hopper  36  for holding the powdered material  34 . The exemplary hopper  36  is attached to the microplasma gun  12  via a connector  38  formed on the microplasma gun  12 . The powdered material  34  is channeled through a discharge tube  40  and controlled by a valve  42  positioned in the discharge tube  40 . The valve  42  can be mechanical or electromechanical as is known to those skilled in the art. There may be multiple hoppers (e.g., to contain multiple components mixed at discharge/injection. Powder may alternatively be injected into the plasma stream via one or more powder gas lines from one or more remote powder feeders (not shown). 
     A nozzle shroud  46  positioned on a forward wall  48  of the microplasma gun  12  holds a nozzle insert  50  and permits the electrode  28  to extend through a center aperture  52  formed in the nozzle shroud  46 . The nozzle insert  50  can be threadingly attached to an end of the nozzle shroud  46 . A shield gas cap  54  is positioned adjacent the nozzle shroud  46 . An insulator  56  is positioned between the shield gas cap  54  and the nozzle shroud  46  to electrically isolate the shield gas cap  54  from the nozzle shroud  46 . The shield gas cap  54  can be pressed to fit onto the nozzle shroud  46  and over the insulator  56 . The shield gas cap  54  includes a plurality of through apertures  58  for permitting shield gas to flow therethrough and shield the arc gas from ambient atmosphere. A center aperture  60  formed in the shield gas cap  54  permits high velocity arc gas to pass through and into the electric arc. 
     Cooling fluid, such as water or the like, may be utilized to cool the microplasma gun  12 . The cooling fluid is delivered to the microplasma gun  12  via a cooling fluid hose  62 . The cooling fluid traverses through internal passages (not shown) in the microplasma gun  12  and flows through an inlet passage  64 , into an anode holder  66  and back through an outlet passage  68 . The cooling fluid reduces the temperature of the anode  16  during operation of the microplasma gun  12 . The cooling flow rate may be approximately 1.0-1.5 gallons per minute. A second conduit  70  is connected to the microplasma gun  12 . The second conduit may be operable for providing electrical power, arc gas, and shield gas to the microplasma gun  12 . 
       FIG. 3  shows the vane  72  having a localized a damage site  73  along a platform  74 . Such a damage site  73  or other localized area may receive a deposit of the powdered material  34 . The plasma gas stream  21  is directed toward the damage site  73 . The site may be a raw damage site or a treated site (e.g., where further material has been machined from the vane substrate such as to remove contaminants). The added material strengthens the area under repair by substantially reinforcing the eroded/machined repair surface. The repair site, when fully processed (e.g., by heat treatment processing) has mechanical properties approaching those of the parent part surface. 
     The microplasma gun  12  may be operated at a relatively low power range of between approximately 0.5 Kilowatts and 2.5 Kilowatts. The low power output of the microplasma gun  12  significantly reduces the heat flow into the vane  72  over that of conventional coating methods. The maximum surface temperature of the vane  72  caused by the coating process is approximately 200° F. depending on the mass of the vane. The microplasma gun  12  is operable for applying powdered material  34  to a thin wall area of the vane  72  without distorting the vane  72  because the low power output limits the localized stress caused by high thermal gradients. 
     The microplasma gun  12  can apply the material in small spots (e.g., 2-4 mm in diameter) or be swept to apply narrow strips (e.g., 2-4 mm in width). This permits accurate surface coating even with a hand held device. The small spot/strip size may substantially eliminate the need for masking or otherwise covering the vane  72  in areas where the material is unwanted. The nozzle opening size controls the spray pattern. The hand-held version of the microplasma gun  12  may be sufficiently accurate that material can be sprayed on components while they remain installed in an engine or the like. 
     An exemplary arc gas flow rate of the microplasma apparatus  10  may be 1.5-3 liters per minute. As stated above, the arc gas and shield gas are typically argon, but any suitable inert gas can be utilized. An exemplary shield gas flow rate ranges may be 2-4 liters per minute for a typical application. 
     The powder hopper  36  holds the powdered material  34  prior to being injected into the plasma gas stream  21  by the powder injector  22 . The powdered material  34  can be injected into the plasma gas stream  21  either through gravity feed or through a pressurized system (not shown). The shut-off control valve  42  controls the powdered material  34  feed rate into the plasma gas stream  21 . Powdered material  34  is transferred to the vane  72  at an exemplary 1-30 grams per minute. The microplasma gun  12  may apply the material from exemplary distances of 3-16 cm to the vane  72 , but can vary depending on the coating application requirements. The exemplary microplasma spray gun  12  can be oriented between a positive 45° angle and a negative 45° angle relative to a normal axis of the vane and still provide adequate material delivery with a gravity feed system. A pressure feed system may provide greater orientational freedom for the microplasma gun  12 . The microplasma spray gun  12  advantageously generates a relatively low noise level that ranges (e.g., 40-70 dB) due to the low power output, thereby making the apparatus  10  suitable for hand held application. Current U.S. government regulations require hearing protection when environmental noise reaches 85 dB. The microplasma spray apparatus  10  can be hand held or alternatively held in an automated fixture (not shown) that is computer controlled. 
       FIG. 4  shows the operation of the microplasma spray apparatus  10 . Initially, at block  80 , arc gas is emitted from the nozzle insert  50 . An electric potential is generated between the anode  16  and the cathode  18  of the plasma spray gun  12  and is directed through the arc gas, as described in block  82 . Arc gas is directed through the electric potential to create the plasma stream  21 . At block  84 , powdered material  34  is injected into the plasma stream  21 . At block  86 , the plasma stream heats the powdered material  34  to a “super plasticized” condition such that the powdered material  34  is malleable when it is applied to a workpiece. At block  88 , the powdered material  34  is applied to an unmasked substrate. The powdered material  34  then bonds with the substrate and cools to form a solid deposit on the substrate. 
     One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, various existing or yet-developed apparatus may be used. The nature of the substrate and the amount, nature, and physical form of the desired deposit will also influence any particular implementation. While illustrated with respect to nickel-based superalloy substrates and powders, the methods and apparatus may be used with cobalt-based superalloys. Other cast components include blade outer air seals and transition ducts. Accordingly, other embodiments are within the scope of the following claims.