Abstract:
In a method for forming a component, a braze material is assembled between first and second wall portions to form a sandwich. The first wall portion consists essentially of copper or a copper-based alloy. The second wall portion comprises at least one non-copper-based alloy. The sandwich is heated. The heating melts the braze material to cause a transient liquid phase bonding of at least a portion of the first wall portion to the second wall portion.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention relates to transient liquid phase bonding. More particularly, the invention relates to transient liquid phase bonding of a copper alloy to a non-copper alloy.  
         [0002]     The difficulty of blind welding has plagued the field of milled channel heat exchangers. One example of a milled channel heat exchanger is the wall of a rocket nozzle as shown in Damgaard et al. “Laser Welded Sandwich Nozzle Extension for the RL60 Engine” (AIAA-2003-4478), AIAA, Reston, Va., 2003, the disclosure of which is incorporated by reference herein as if set forth at length. In an exemplary milled channel heat exchanger, an array of channels are milled in a base material leaving ribs between the channels. A cover sheet or panel is placed atop the ribs and welded thereto (e.g., via laser or e-beam from the side of the sheet facing away from the base layer).  
         [0003]     Copper alloys have been proposed for heat exchanger use. US Patent Application 20040011023-A1 references use of NASA Glenn Research Center alloy GRCop-84 (Cu-8Cr-4Nb nominal composition by atomic percent) for heat exchanger use. U.S. patent application Ser. No. 11/011,314 discloses a heat exchanger wall structure formed as a composite of such a copper alloy and a dissimilar material.  
       SUMMARY OF THE INVENTION  
       [0004]     One aspect of the invention involves a method for forming a component. A braze material is assembled between first and second wall portions to form a sandwich. The first wall portion consists essentially of copper or a copper-based alloy. The second wall portion comprises at least one non-copper-based alloy. The sandwich is heated. The heating melts the braze material to cause a transient liquid phase bonding of at least a portion of the first wall portion to the second wall portion.  
         [0005]     In various implementations, the method may further include milling the relieved areas in the first wall portion. The first wall may consist essentially of Cu-8Cr-4Nb. The second wall may consist essentially of a nickel-based superalloy, stainless steel, or iron-based superalloy. The component may comprise a heat exchanger for a rocket chamber or nozzle.  
         [0006]     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  
       [0007]      FIG. 1  is a schematic view of a rocket engine combustion chamber and exhaust nozzle.  
         [0008]      FIG. 2  is a cross-sectional view of a heat exchanger wall of the nozzle of the engine of  FIG. 1 .  
         [0009]      FIG. 3  is an exploded pre-integration view of the wall of  FIG. 2 .  
         [0010]      FIG. 4  is a photomicrograph of an integration region of the wall of  FIG. 2 .  
         [0011]      FIG. 5  is a sectional view of an alternate heat exchanger.  
     
    
       [0012]     Like reference numbers and designations in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0013]      FIG. 1  shows a rocket engine  20  having a combustion chamber  22 . A nozzle  24  extends downstream from the combustion chamber to an outlet  26 . The nozzle may be bell-shaped and generally symmetric about a central longitudinal axis  500  of the engine.  FIG. 2  schematically shows a heat exchanger wall structure  40  of the chamber  22  and nozzle  24 . The wall  40  has an outboard surface  42 . Opposite the outboard surface  42  is an inboard surface  44  along the chamber/nozzle interior and exposed to the exiting exhaust gases. The wall  40  includes internal channels  46 . In engine operation, the channels accommodate a flow of a heat exchange fluid. An exemplary heat exchange fluid is pre-combustion propellant or a component thereof (e.g., a monopropellant or one of a fuel and oxidizer). The fluid receives heat from the exhaust gases to cool the wall  40 .  
         [0014]     The wall  40  may be assembled by integrating a multi-layer sandwich structure.  FIG. 3  is an exploded view of exemplary sandwich components. A first layer  50  has a first surface that ultimately forms the wall outboard surface  42 . The first layer  50  has a second surface  52  opposite the first surface. The first layer  50  may be selected for structural or environmental properties (e.g., strength, corrosion resistance, thermal conductivity, and/or erosion resistance, etc.).  
         [0015]     A second layer  60  has a first surface that ultimately forms the wall inboard surface  44 . The second layer  60  has a second surface  62  opposite the first surface  44 . In an exemplary non-limiting method of manufacture, one or more open channels  64  are milled below the surface  62 . The channels  64  define relieved/recessed areas separated by intact raised/elevated areas or ribs  66  joined by intact material. The material of the second layer  60  may be selected for ease of machining or other forming, high heat transfer, light weight, and the like. Exemplary materials are copper-based alloys. The layers and sandwich may be flat or shaped otherwise. For example, the layers and sandwich may be frustoconical with the channels running longitudinally as in a rocket nozzle precursor (subsequently formed into a bell shape).  
         [0016]     To integrate the first and second layers, the sandwich includes a bonding layer  70  between the first layer  50  and second layer  60 . The bonding layer  70  has first and second opposed surfaces  72  and  74 . When the sandwich is assembled, the surfaces  72  and  74  contact the surfaces  52  and  62 , respectively. Exemplary bonding material is a transient liquid phase-forming diffusion braze material. TLP diffusion bonding of nickel-based superalloys to each other is well known (see, e.g., U.S. Pat. No. 3,678,570). Upon heating, one or more components of the braze material diffuse into the adjacent materials. The diffusing components temporarily depress the melting points of the adjacent materials forming a transient liquid phase. As further diffusion reduces the concentration of these components, the depressed melting points return toward the original melting points forming an integrated solid structure. Exemplary braze materials for bonding the present combination of dissimilar materials include nickel-based superalloys having boron concentrations of 1-4% by weight and silicon concentrations of 4-8% by weight, typically in inverse proportion. Exemplary thicknesses of the braze materials are 37-50 μm, more broadly 25-150 μm.  
         [0017]      FIG. 4  shows an exemplary junction between two such materials. A first material  80  may be essentially microstructurally unaltered precipitation-hardenable iron-based superalloy. The illustrated first material is alloy A286 (UNS S66286, nominal composition 25.5 Ni, 15 Cr, 1.25 Mo, 2.1 Ti, 0.3 V, balance Fe by weight %). A second material  82  may be essentially unaltered GRCop-84 copper alloy. The two materials have been joined by a transient liquid phase bonding process utilizing a 75 μm thick braze material of MBF-20 (AMS 4777, nominal composition 7 Cr, 3 Fe, 4.5 Si, 3.2 B, balance Ni by weight %). Heating was by immersion in an electrical resistance vacuum oven to a peak temperature of 1010° C. In the resulting junction microstructure, it is believed that  FIG. 4  shows a fine layer  84  of generally intact braze material. On either side of the braze material  84  is a diffusion region  86  and  88 .  
         [0018]     Within each of the diffusion regions  86  and  88 , differential transport of various components is believed to cause a layered appearance. It is known that boron diffuses rapidly in solid solution, and that boron reacts with chromium to form chromium borides of various stoichiometries. The string-like structures in region  86  are believed to be chromium borides resulting from diffusion of boron from the original braze material into the iron-based alloy. It is believed that similar boron diffusion and reaction with chromium occur in region  88 , in which the string-like structures appear heavier and thicker.  
         [0019]     Destructive strength testing has produced mostly failures within the copper alloy rather than joint separation. This confirms joint integrity. Exemplary measured tensile strengths were about 400 Mpa. Even the failed joints had exemplary measured tensile strengths in the vicinity of 90% of the ultimate tensile strength of the GRCop-84 copper alloy. Similar microstructure has been observed with first materials of nickel-based superalloys (e.g., nickel alloy 625 (UNS N06625)) and stainless steel (e.g., SS 347 (UNS S34700)). Similar microstructure also been observed with MBF-30 braze material (AMS 4778, nominal composition 4.5 Si, 3.2 B, balance Ni by weight %).  
         [0020]      FIG. 5  shows an alternate heat exchanger  100  that may be formed by similar methods. A first group of channels  102  is milled on one side of a copper alloy layer  110  and a second group of channels  104  is milled on the opposite side, leaving a web therebetween. Non-copper layers  112  and  114  (e.g., similar to the first layer  50  of  FIG. 3 ) are TLP bonded to the respective sides of the layer  110  to enclose the respective channels  102  and  104 . Such a configuration may be used to provide heat exchange between first and second fluid flows in the channels  102  and  104 , respectively. Although the illustrated channels  102  and  104  are parallel such as in a parallel flow or counterflow heat exchanger, other configurations, including crossflow, are possible.  
         [0021]     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, details of the particular component to be formed may influence details of any particular implementation. Furthermore, while heat exchangers for rocket applications were described in some embodiments herein, this invention is not limited to such. This invention relates to any copper-based alloy being transient liquid phase bonded to a non-copper-based alloy. Accordingly, other embodiments are within the scope of the following claims.