Abstract:
The present invention relates to a method for depositing braze alloy on a surface of a component that will be subsequently joined to a second component. In one form a braze alloy is sprayed by a thermal process. The braze alloy is sprayed in such a fashion that substantially all of the powder braze alloy is unmolten, but is heated to a state deformable enough to bond to the surface. One form of the spraying operation utilizes a High Velocity Oxygen Fuel spray gun.

Description:
BACKGROUND OF THE INVENTION  
         [1]    1. The present invention relates in general to the braze joining of metal components with a braze alloy. More particularly, one embodiment of the present invention relates to the thermal spraying, of a nickel braze alloy on surfaces of metallic components that will subsequently be joined together. While one embodiment of the present invention was developed for the manufacture of gas turbine engine components, certain applications may be outside of this field.  
           [2]    2. It is generally well known that a high temperature causing the brazing alloy to completely melt will produce on the depositing surface a resistant oxide membrane, which renders the surface less brazeable. Thus, the resistant oxide membrane will limit the effectiveness of the brazing agent layer to join two metal components together. In the case of thermal spraying of the brazing agent at extremely high temperatures, a considerable amount of the brazing agent may evaporate thereby creating difficulties in achieving uniformity in coating deposition. Further, certain brazing agents contain brazing alloy that may pre-react during or after melt deposition on the target surface. This pre-reaction of braze alloy elements impairs the subsequent braze joining, of the metal components.  
           [3]    3. Methods are generally known to produce brazeable aluminum components using aluminum-silicon, aluminum-zinc, and aluminum-silicon-zinc alloys. The prior methods were often multi-step processes that deposited the alloy in a molten or semi-molten state, while delivering the flux emulsion in a separate spray. Further, the prior deposition methods generally recommend spraying the brazing agent in a non-oxidizing atmosphere such as N 2  gas, and required extensive surface preparation prior to deposition. Additionally, lower spray velocities previously used were not very effective. When the particles struck the surface at slower speeds they tended to peel away even though they were softened by the heating and quenched by contact with the base material surface.  
           [4]    4. In one particular method an aluminum-brazing agent was delivered by plasma arc spraying. A number of difficulties occurred including that the high temperatures associated with plasma arc spraying pre-reacted the fluxing agent. The high temperatures causing the aluminum alloy to be in a molten state produced on the surface a resistant oxide membrane, which renders the material less brazeable. There were also difficulties in achieving uniformity of deposition due to evaporation of the thermally sprayed alloy at such high temperatures.  
           [5]    5. Prior methods of joining components together by brazing have included placing the braze alloy with a tape/powder/prewet technique. This method has a litany of limitations that include limitations on the component geometry, long prewet time, and surface preparation that generally required grit blasting.  
           [6]    6. Although the prior methods of depositing braze alloys and braze joining are steps in the right direction, the need for additional improvements still remains. The present invention satisfies this need in a novel and unobvious way.  
         SUMMARY OF THE INVENTION  
         [7]    7. One form of the present invention contemplates a method for depositing a braze alloy. The method comprising: thermally spraying a powder braze alloy onto a surface. The powder braze alloy is in such a state that substantially all of the powder braze alloy is unmolten but is heated to a state deformable enough to bond to the surface.  
           [8]    8. Another form of the present invention contemplates a method for depositing braze alloy, comprising: thermally spraying a powder braze alloy at a high velocity onto a surface in such a state that substantially all the powder braze alloy is unmolten, but is heated to a state malleable enough to bond to the surface.  
           [9]    9. Another form of the present invention contemplates a method for depositing braze alloy, comprising; flame spraying a powder braze alloy at a high velocity onto a surface. The flame spraying occurs with a stoichiometrically neutral or fuel rich flame. The flame spraying process is such that the powder braze alloy is in such a state that substantially all the powder braze alloy is unmolten but is heated to a state malleable enough to bond to the surface.  
           [10]    10. One object of the present invention is to provide an improved technique to apply braze alloy to components for subsequent joining.  
           [11]    11. Related objects and advantages of the present invention will be apparent from the following description.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [12]    12.FIG. 1 is an illustrative view of a gas turbine engine.  
         [13]    13.FIG. 2 is a perspective view of a gas turbine engine airfoil, comprising a portion of FIG. 1.  
         [14]    14.FIG. 3 is a diagrammatic view of one form of the present invention comprising a High Velocity Oxygen Fuel (HVOF) spray process.  
         [15]    15.FIG. 4 a  is a view of a braze alloy deposited by a tape/powder/prewet on a surface.  
         [16]    16.FIG. 4 b  is a view of a braze alloy applied with one form of the High Velocity Oxygen Fuel spray process of FIG. 3.  
         [17]    17.FIG. 5 a  is a view of a braze joint created with the prior art tape/powder/prewet applied braze alloy.  
         [18]    18.FIG. 5 b  is a view of a braze joint created with HVOF preplaced braze alloy.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [19]    19. For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates.  
         [20]    20. Referring the FIGS. 1 and 2, there is illustrated a gas turbine engine  20  which includes a compressor  21 , a combustor  22 , and a power turbine  23 . The three components have been integrated together to produce an aircraft flight propulsion engine. The term aircraft is generic and includes helicopters, airplanes, missiles, unmanned spaced devices and any other substantially similar devices. It is important to realize that there are a multitude of ways in which the gas turbine engine components can be linked together. Additional compressors and turbines can be added with intercoolers connecting between the compressors and reheat combustion chambers could be added between the turbines.  
         [21]    21. Further, the gas turbine engine is equally suited to be used for an industrial application. Historically, there has been widespread application of industrial gas turbine engines, such as pumping sets for gas and oil transmission lines, electricity generation, and naval propulsion. A plurality of turbine blades  24  are coupled to a rotor disk that is affixed to a shaft rotatable within the gas turbine engine  20 . A plurality of vanes  25  are conventionally joined together to collectively form a complete 360° nozzle. It is understood herein that gas turbine engine blades and/or vanes are often referred to as airfoils. Other products utilizing the present invention are contemplated herein including but not limited to combustion liners, exhaust nozzles, exhaust liners, airframe wing leading edges and/or other fabricated components. One form of the present invention allows the joining of dissimilar metals with a braze alloy that is compatible with complimenting metallurgical structures. An alternate form of the present invention allows the braze joining of similar metals with a braze alloy.  
         [22]    22. Referring to FIG. 3, a thermal coating spraying apparatus  30  is positioned relative to a base material  40  for delivering a coating  41  of braze alloy material thereon. In the preferred embodiment, the spraying apparatus  30  is a HVOF (High Velocity Oxygen Fuel) spray gun. It is well known that a HVOF process is a flame spray coating deposition process that can apply a dense, very low porosity coatings. The controlled BTU output and high gas velocity imparts both thermal and kinetic energy to the powder braze alloy particles. One spraying apparatus of this type is disclosed in U.S. Pat. No. 4,999,225 to Rotolico incorporated herein by reference, and is available from the Perkin Elmer Corporation of Norwolk, Conn. Other HVOF spraying apparatuses of this general type are available in the market place and are known to those of ordinary skill in the art. In a preferred embodiment the Diamond Jet HVOF spray gun, manufactured by Sulzer Metco (US) Inc., with the DJ 2600 hybrid air/water cooled air cap assembly enhancement package that produces higher spray velocities than the standard Diamond Jet hardware is utilized. The DJ 2600 is intended for use only with hydrogen as the fuel gas to produce premium quality HVOF coatings. The DJ 2600 option provides the Diamond Jet spray gun with gas velocities of up to 7000 ft/sec whereas gas velocities typically approach 4500 ft/sec on the standard Diamond Jet spray gun. However, the present invention is not intended to be limited to the use of this particular spray gun. The use of other HVOF spray guns is contemplated herein.  
         [23]    23. The thermal spraying process associated with HVOF guns heats and/or melts ceramic or metal stock/powder coating. The powder or stock is then fed through the center of the HVOF gun  30  and the heated coating material is carried by the high velocity fluid stream to the surface for deposition. In a preferred embodiment, the HVOF process delivers the heated coating material particles  45  to the base surface material  40  at supersonic speed. Delivery of the heated coating material particles  45  to the base material surface  40  at supersonic speeds creates a mechanical interlock between the base surface material  40  and the sprayed coating material  41 . It is preferred that the heated material be delivered at speeds equal to Mach  2  or greater. However, in an alternate embodiment, the delivery of the coating material occurs at speeds that are about equal to the speed of sound.  
         [24]    24. The high velocity of the malleable impinging particle spray  45  onto the base material  40  produces a dense, uniform, low porosity coating  41 . The braze alloy coatings applicable for base material braze joining include a wide variety of materials. The following coatings and base materials, however, have been found to provide preferred results in conjunction with the HVOF conditions discussed below.  
         [25]    25. One preferred embodiment is the deposition of braze alloy Amdry DF- 4 B onto base metal AF 2 - 1 DA- 6 . A more preferred embodiment is the deposition of Amdry BRB braze alloy onto base metal AF 2 - 1 DA- 6 . It is understood herein that other braze alloys such as nickel or cobalt superalloys may be deposited using this method as long as they are homogenous compounds. The DF-4B braze alloy composition is: 14Cr, Bal.Ni, 10Co, 3.5Al, 2.75B, 2.5Ta, 0.02Y. The AF2-IDA-6 base metal composition is: 12Cr, Bal.Ni, 10Co, 2.75Mo, 6.7W, 2.8Ti, 4.6Al, 0.015B, 0.0Zr, 1.5Ta. The Amdry BRB braze alloy composition is: 13.5Cr, Bal.Ni, 9.5Co, 4.0Al, 2.5B, 0.1Y. With these powder braze alloys, the following parameters were developed for the HVOF system to utilize a neutral or fuel rich flame condition to retard the pre-reaction of braze with base material. All flow rates have units of S.C.F.H. (standard cubic feet hour). The fuel used in the Diamond Jet 2600 spray gun is H 2 . The primary gas type is oxygen (O 2 ) at 175 psi and primary flow of 26 S.C.F.H. Secondary gas is H 2  at 140 psi and secondary flow of 62 S.C.F.H. The airflow is 46 S.C.F.H. at 20 psi. The carrier flow was of N 2  gas at 150 psi with a carrier flow of 55 S.C.F.H. The spray powder was fed into the gun at a rate of 5 lbs./hr. Referring to FIG. 3, the preferred spray distance  50  is about nine inches. In another preferred embodiment, the base surface material  40  is prepared with sixty-grit silicon carbide paper and is degreased with Acetone.  
         [26]    26. In an alternative embodiment the base surface material  40  is preheated to 150 degrees Fahrenheit. This can be accomplished by directing the HVOF gun&#39;s flame onto the base surface material  40  to preheat it. Upon reaching the appropriate temperature for deposition of the selected material, the powder braze alloy is fed through the HVOF gun. It is understood that using the HVOF gun is one technique of preheating base surface material  40 , however, other preheating techniques, such as preheating in an oven, are contemplated herein. The braze alloy used is homogenous. Both the Amdry BRB and DF-4B alloy are preferably used with particle sizes in the range of ASTM mesh size −140+325 or −106+45 microns. The particle sizes were selected based on their superior resistances to melting and entraining oxides therein. Additionally, particle size affects compression during the secondary braze operation utilized to join the metallic components together.  
         [27]    27. In the preferred mode the oxygen, fuel, and carrier gas pressures and flow rates previously given result in a neutral or slightly fuel rich flame. This type of flame retards oxidation of the braze alloy by creating a circular flame which shapes the powder stream and accelerates the particles to the surface.  
         [28]    28. In one embodiment flame velocity is about 4,000 fl/sec. and imparts both thermal and kinetic energy to the impinging powder braze alloy particles. In one embodiment the partical velocity being in the range of about 1,200-1,600 ft/sec. The particles are malleable but substantially unmolten. This results in mechanical adhesion to the surface. Since the adhesion is mechanical, minimal surface preparation is necessary prior to depositing the braze alloy coating. Since the particles are not in a semi-molten condition there is little loss or uneven distribution due to evaporation. In this method, the braze alloy is delivered in a single spray stream. The environment surrounding the process can be the ambient shop floor condition and need not be a non-oxidizing or an evacuated atmosphere.  
         [29]    29. Prewetting involves heating the base material and braze alloy to a temperature that causes the braze alloy to partially melt. A high temperature and softened braze alloy result in some of melting point suppressants in the braze alloy diffusing into the base material. This results in an increase in the braze alloy melting point, which is limited by how hot a material can be taken during brazing where the braze alloy does not react the same due to the loss of certain elements.  
         [30]    30. In contrast to the tape/powder/prewet cycle, there is no substantial loss of braze alloy potential due to diffusion. This results in both better braze flow and wettability as well as better joint filling capability. Additionally there are time and cost savings in part preparation and vacuum furnace operation by using the HVOF spraying process instead of the labor intensive tape/powder/prewet cycle. The braze alloy can be preplaced on contoured and complex surfaces and the base material is not subjected to high prewet temperatures. Thermal spraying also allows for the possibility of applying braze alloy in patterns by masking certain areas. Also, there is no concern of a residue of an adhesive as in the tape/powder/prewet cycle.  
         [31]    31. With reference to FIG. 4 a  and  4   b,  these two photographs show the advantages of uniformity of deposition achieved by HVOF applied braze alloy powder in contrast to braze alloy powder deposited by the prior art tape/powder/prewet method. FIGS. 5 a  and  5   b  show a braze joint created with the prior art tape/powder/prewet applied braze alloy and braze joint created with HVOF preplaced braze alloy.  
         [32]    32. While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.