Patent Application
20070029369
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.
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).
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.
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.
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.
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.
Like reference numbers and designations in the various drawings indicate like elements.
The wall 40 may be assembled by integrating a multi-layer sandwich structure.
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).
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.
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.
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 %).
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.
