Abstract:
A process for heating a braze alloy by microwave radiation so that heating of the alloy is selective and sufficient to cause complete melting of the alloy and permit metallurgical bonding to a substrate on which the alloy is melted, but without excessively heating the substrate so as not to degrade the properties of the substrate. The process entails providing metallic powder particles having essentially the same metallic composition, with at least some of the particles being sufficiently small to be highly susceptible to microwave radiation. A mass of the particles is then applied to a surface of a substrate, after which the mass is subjected to microwave radiation so that the particles within the mass couple with the microwave radiation and sufficiently melt to metallurgically bond to the substrate. The microwave radiation is then interrupted and the mass is allowed to cool, solidify, and form a solid brazement.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention generally relates to brazing methods, including processes and materials for use in the manufacturing, coating, repair, and build-up of superalloy components. More particularly, this invention relates to a method of brazing a superalloy, in which microwave energy is employed to melt a braze alloy that contains minimal or no melting point depressants relative to the superalloy being brazed. 
         [0002]    Nickel, cobalt, and iron-base superalloys are widely used to form high temperature components of gas turbine engines. While some high-temperature superalloy components can be formed as a single casting, others are preferably or required to be fabricated by other processes. As an example, brazing is widely used to fabricate gas turbine components, as in the case of high pressure turbine nozzle assemblies. Brazing techniques conventionally encompass joining operations performed at an elevated temperature but sufficiently below the melting points of the superalloys being joined so as not to cause grain growth, incipient melting, recrystallization, or unfavorable phase formation that can lead to degradation of the alloys. In carrying out the brazing process, an appropriate braze alloy is placed between the interface (faying) surfaces to be joined, and the faying surfaces and the braze alloy therebetween are heated in a vacuum to melt the braze alloy. The braze alloy melts at a lower temperature than the superalloy base material as a result of containing one or more melting point depressants, such as boron and/or silicon in an amount greater than the superalloy(s) being brazed. For example, whereas superalloys containing intentional amounts (above impurity levels) of boron and/or silicon typically contain less than 0.1 weight percent of each, braze alloys that contain boron and/or silicon as melting point depressants typically contain at least 2.0 weight percent boron, or at least 6.0 weight percent silicon, or both silicon and boron at a ratio of about 3:1. On cooling, the braze alloy solidifies to form a permanent metallurgical bond. 
         [0003]    During engine operation, gas turbine engine components are subject to strenuous high temperature conditions under which various types of damage or deterioration can occur. As examples, erosion and oxidation reduce wall thicknesses of turbine nozzles and vanes, and cracks can initiate at surface irregularities and propagate as a result of stresses that are aggravated by thermal cycling. Because the cost of components formed from superalloys is relatively high, it is often more desirable to repair these components rather than replace them. In response, brazing techniques have been developed for crack repair and wall thickness build-up that entail placing a braze alloy filler metal on the surface area requiring repair, and then heating the filler metal in a vacuum to above its melting point, but below that of the surface substrate, so that the molten filler metal wets, flows, and fills the damaged area. 
         [0004]    While widely employed to fabricate and repair gas turbine engine components, conventional brazing processes have notable disadvantages. For example, the entire component must be subjected to a vacuum heat treatment, which is a very lengthy process in a production environment, unnecessarily exposes undamaged regions of the component to high temperatures, and can potentially remelt joints in other sections of the component. Furthermore, though braze alloys typically have compositions similar to the base metal of the component being brazed, the presence of boron and/or silicon in a braze alloy as a melting point suppressant reduces the mechanical and environmental properties of the resulting brazement as a result of the minimal ductility of the borides and silicides they form by reaction with refractory elements. Boron and silicon can also diffuse into the base metal repaired by the brazement to adversely affect the mechanical and environmental properties of the component. 
         [0005]    Microwave brazing has been investigated as a potential candidate for eliminating these issues, as heating can be localized to selected areas of a component. The general approach has been to use a susceptor (e.g., SiC enclosure) that is heated when exposed to microwave energy and, in turn, transfers the heat to the component by radiation. Drawbacks include the lack of local heating of the braze alloy only, as an entire region of the component is inevitably heated, and significant heat loss from radiation in directions away from the intended brazement. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    The present invention generally provides a process for heating a braze alloy by microwave radiation so that heating of the alloy is selective and sufficient to cause complete melting of the alloy and permit metallurgical bonding to a substrate on which the alloy is melted, but without excessively heating the substrate so as not to degrade the properties of the substrate. The invention is particularly beneficial for use in brazing operations for the purpose of metallurgically joining superalloys and coating, repairing, and building up of superalloy surfaces. 
         [0007]    The process generally entails providing metallic powder particles having essentially the same metallic composition, with at least some of the metallic powder particles being sufficiently small to be highly susceptible to microwave radiation. A mass of the metallic powder particles is then applied to a surface of a substrate, after which the mass is subjected to microwave radiation so that the metallic powder particles within the mass couple with the microwave radiation and sufficiently melt to metallurgically bond to the substrate. The microwave radiation is then interrupted and the mass is allowed to cool, solidify, and form a solid brazement. 
         [0008]    According to a preferred aspect of the invention, the metallic powder particles may have the same metallic composition as the substrate on which the mass of particles was applied and melted. According to another aspect of the invention, all of the metallic powder particles are sufficiently small to be significantly more susceptible to absorbing microwave energy than the substrate, which predominantly reflects the microwaves. As a result, complete melting of the particles can be achieved accompanied by only surface melting of the substrate caused by heat transfer from the particles to the substrate by thermal conduction. Such a result may be obtained even if the metallic powder particles have the same or even higher melting temperature than the substrate. Finally, melting of the particles can be achieved even if their metallic composition is free of melting point depressants, such as boron and silicon, beyond amounts conventionally used in superalloys. 
         [0009]    From the above, it can be appreciated that the process of this invention can be applied to various processes in which heating of a powdered material is desired, for example, the forming of coatings including the repair or build-up of a damaged surface and the metallurgical joining of components by brazing. Because heating is by microwave radiation, the heating rate and melting of the powder particles are influenced by susceptibility to microwave radiation instead of location relative to a heating source or relative to any surface contacted by the powder mass. This aspect of the invention enables the powder mass to melt prior to melting of the surface contacted by the mass. As a result, the powder particles can be formed of an alloy having the same melting temperature (for example, within 150° C.) as the surface contacted by the powder mass. 
         [0010]    Other objects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  schematically represents a mass of powder metal particles between a pair of substrates, in which the powder metal particles are susceptible to microwave heating to enable microwave brazing in accordance with an embodiment of the present invention. 
           [0012]      FIG. 2  schematically represents a mass of powder metal particles similar to that of  FIG. 1  but deposited on a surface of a substrate containing a defect in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    The invention will be described with specific reference to processing of components for a gas turbine engine, and particularly the fabrication and repair of such components with a braze material. However, the invention has application to a variety of components, materials, and processes other than those discussed, and such variations are within the scope of this invention. 
         [0014]      FIGS. 1 and 2  depict embodiments of this invention, in which consistent reference numbers are used to identify functionally similar structures.  FIG. 1  schematically represents a mass  10  of powder metal particles  12  between and contacting opposing surfaces of two substrates  14  and  24  to be metallurgical joined by the particles  12 , and  FIG. 2  schematically represents a mass  10  of powder metal particles  12  deposited on a surface of a substrate  14  for the purpose of repairing a defect in the surface. As discussed below, the invention provides a process by which the particles  12  are melted and resolidified to form brazements that join the substrates  14  and  24  of  FIG. 1  and repair the surface defect of  FIG. 2 . Though the mass  10  of particles  12  is shown as being directly placed between the substrates  14  and  24  in  FIG. 1 , and the mass  10  of particles  12  is shown as placed directly within the defect in  FIG. 2 , it will be understood by those skilled in the art that the particles  12  could be placed adjacent a gap between the substrates  14  and  24  in  FIG. 1  or over the defect in  FIG. 2 , and allowed to flow into the gap and defect by capillary action when molten. In both  FIGS. 1 and 2 , the particles  12  are shown as being contained within a binder  30  that, according to known brazing practices with braze pastes, burns off during the brazing process, which is preferably performed in an inert or low pressure atmosphere to minimize oxidation of the particles  12  and any surfaces (e.g., substrates  14  and  24 ) to which the particles  12  are bonded. Either or both substrates  14  and  24  may be formed of a superalloy, whose composition or compositions will depend on the particular type of component and its anticipated operating conditions. As will be noted below, various other metallic materials are also possible for the substrates  14  and  24 , and therefore within the scope of the invention. 
         [0015]    According to the invention, the mass  10  of particles is melted as a result of being subjected to microwave radiation  26 , as discussed in more detail below. The powder particles  12  can be formed of a variety of materials, limited only by the requirement that the particles  12  have a composition that is capable of being heated by microwave radiation  26  and is compatible with the materials of the substrates  14  and  24  while at the maximum heating temperature induced by microwave radiation  26 . Materials capable of being heated when subjected to microwave radiation include non-conductors and conductors under appropriate conditions. Microwave radiation has varying electric and magnetic fields that are believed to cause direct electric heating and heating through magnetic effects, respectively. For compatibility with the metallic substrates  14  and  24 , the particles  12  employed by this invention are metallic and are believed to be heated by a combination of electrical and magnetic effects, with the latter possibly being the dominant effect. Compatibility is assured if the particles  12  have the very same composition as that of the substrates  14  and  24 , though suitable compatibility can also be achieved if the particles  12  and substrates  14  and  24  do not have compositions prone to detrimental interdiffusion at elevated temperatures that would lead to loss of desired mechanical or environmental properties. The particles  12  may be a conventional braze alloy that contains significant amounts of one or more melting point depressants, such as up to two weight percent boron or up to six weight percent silicon, or some combination of both. Conversely, the particles  12  may contain elements capable of serving as a melting point depressant at only impurity levels or in only limited amounts that do not exceed the levels for those same elements in the substrate  14  or substrates  14  and  24  being brazed, for example, less than 0.1 weight percent for both boron and silicon in typical nickel-base superalloys. Another alternative is to form the particles  12  to have a composition that contains lower amounts of boron and/or silicon than typically added as a melting point depressant, for example, less than one weight percent of boron and/or less than three weight percent of silicon. As such, it is possible for the particles  12  to contain one or more melting point suppressants, though not at levels that would lead to an unacceptable loss of properties in the substrates  14  and  24  as a result of diffusion of the suppressant into the substrates  14  and  24  during heating of the particles  12  and later during the life of the substrates  14  and  24 . Furthermore, the particles  12  can be formed of a superalloy such as of the type used in turbine applications, or an alloy whose base composition is similar to that of the substrates  14  and  24  but modified to contain alloying constituents different from or at different levels than the substrates  14  and  24 . Though all of the particles  12  are not required to have the same composition, the present invention permits such uniformity. 
         [0016]    According to a preferred aspect of the invention, at least some and preferably all of the powder particles  12  must be sufficiently small to be highly susceptible to microwave radiation  26 , thereby preferentially coupling with the microwave radiation  26  (as compared to the substrates  14  and  24 ) to significantly enhance heating and melting of the particles  12  by the microwave radiation  26 . Coupling with the microwave radiation  26  is believed to be the result of the metallic particles  12  being sufficiently conductive to generate eddy currents induced by the magnetic field of the microwave radiation  26 , while possibly also possessing a level of electrical resistivity capable of generating joule heating from the eddy currents. It is known that the magnetic loss component of susceptibility for a material in very fine powder size is dependent on factors such as microwave power and frequency. Conversely, for a given microwave power and frequency, the interaction between microwave and individual metals or alloys will be optimum at a distinct particle size, usually on the order of a few tens of nanometers for conventional microwave conditions (about 2.45 GHz and about 1 to about 10 kW power). Particle sizes above or below that size will not couple as well with the microwave radiation. Consequently, suitable and preferred maximum sizes for the particles  12  will depend on the particular application, temperatures, and materials involved. Generally speaking, it is believed that a maximum particle size is on the order of about 150 mesh (about 100 micrometers), more preferably less than 325 mesh (about 44 micrometers). Minimum particle sizes can be as little as nanoscale, e.g., less than 100 nanometers such as on the order of about 10 nanometers. 
         [0017]    In contrast to the particles  12 , bulk metals such as the substrates  14  and  24  tend to reflect microwave radiation. As noted above, this aspect of the present invention makes possible the brazing of superalloy substrates  14  and/or  24  with alloys having the very same composition as the substrate  14 / 24 , as well as alloys with the same or even higher melting point as the substrate  14 / 24 . For example, a nickel-base superalloy component can be joined or repaired with a braze material of the same nickel-base superalloy composition or another nickel-base alloy, in other words, an alloy whose base metal is the same as the base metal of the substrate  14 / 24 . In this manner, degradation of the properties of the substrate  14 / 24  resulting from interdiffusion with the braze material can be essentially if not entirely avoided. In view of the capability of melting particles  12  formed of an alloy having a melting point above that of the substrate  14 / 24 , it should be appreciated that the term “brazing” as used herein is not limited to the conventional limitation of a joining operation performed at a temperature below the melting point of the metals being joined. 
         [0018]    Microwave radiation is preferably applied to the powder mass  10  in a multi-mode cavity, which as known in the art provides for a microwave field that does not establish a standing wave, but instead provides a uniform amplitude of both its magnetic and electric components. Alternatively, a single-mode cavity can be used, in which case a standing or traveling wave is propagated, enabling imposition, to a certain extent, the relative amplitudes of the electric and magnetic components of the microwave field. A wide range of microwave frequencies could be used with the present invention, though regulations generally encourage or limit implementation of the invention to typically available frequencies, e.g., 2.45 GHz and 915 MHz, with the former believed to be preferred. However, it should be understood that other frequencies are technically capable of use. A benefit of using a lower frequency is the greater associated wavelength, which may be better suited for higher power transmission or processing of larger components. Suitable microwave power levels will depend on the size and composition of the particles  12 , but are generally believed to be in a range of about 1 to about 10 kW, though lesser and greater power levels are also foreseeable. 
         [0019]    In an experiment using a multi-mode microwave cavity, about 5 grams of a nanoscale-sized nickel powder and about 25 g of a nickel powder sieved to −325 mesh were subjected to microwave radiation at frequencies of about 2.45 GHz and power levels of about 1 kW. A maximum temperature of about 1140° C. (about 2085° F.) was obtained for the finer nickel powder, while the coarser nickel powder only attained a temperature of about 817° C. (about 1500° F.), evidencing the greater susceptibility of the finer powder particles to heating by microwave radiation. 
         [0020]    While the invention has been described in terms of particular embodiments, it is apparent that other forms could be adopted by one skilled in the art. Accordingly, the scope of the invention is to be limited only by the following claims.