Abstract:
A screen ( 24 A-H) of a specified thickness (T) for insertion in a gap ( 32 ) between surfaces of workpieces ( 32, 34 ) to be joined by brazing. The screen thickness determines and maintains the gap thickness during brazing. The screen has a higher melting point than the braze filler material ( 22 ), is wettable by a melt of the braze filler material, and may have a higher tensile strength than the braze filler material at operating temperatures of the braze joint. The screen may include electrical resistance heating wires ( 52, 62 ) to melt the filler material ( 46 ). The screen may be covered by the filler material, forming a brazing foil ( 20 B). The screen may include electrically conductive insulated wires ( 92, 93 ) connected to a sensor ( 95 ) such as a thermocouple or strain gauge to monitor a condition of the braze joint during subsequent operation.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates generally to the field of brazing, and more specifically to maintaining a specified gap between two surfaces while they are being joined by brazing. 
       BACKGROUND OF THE INVENTION 
       [0002]    Gap control in brazing influences properties and quality of the resulting joint. A known chart by Lucas-Milhaupt Inc. is shown in  FIG. 10 . Tensile strength of a braze joint generally decreases with increasing joint thickness, but many factors affect the relationship. A properly gapped braze joint may be stronger in tension than the braze alloy itself. Explanations for this include: a) Hard faces of adjoining material limit slip along crystallographic planes in the braze; and b) Stretching of the braze under tension must be accompanied by lateral reduction in area, however such lateral reduction is restrained by closely spaced and relatively strong adjoining material. As the joint thickness increases, such lateral restraint becomes less effective and tensile strength decreases toward that of the braze alloy. 
         [0003]    Sometimes a decrease in tensile strength is also found toward the left of the apex  19  of the curve of  FIG. 10  at smaller joint gap thickness. Prior explanations for this include: a) Some brazing processes require flux to clean oxides from the parent materials and to thereby provide good wetting and capillary action of the molten braze. Too small of a gap provides insufficient flux to dissolve oxide films; b) Some brazing processes are done in a reducing atmosphere such as in hydrogen. The hydrogen reacts with metal oxides on the surfaces to produce water. The concentration of water vapor in the immediate vicinity of the surfaces determines if vapor is produced or if oxide is left (or reformed). Very small joint gaps prevent the ingress of reducing gas and egress of vapor; c) Some metals are partially dissolved by some braze alloys. When this occurs the braze alloy melting point may increase to the point of solidification at brazing temperature. With very small joint gaps with limited braze material, this condition is reached quickly. This can prevent flow of braze across the entire joint resulting in incomplete brazing and poor tensile strength. 
         [0004]    Braze joint clearances recommended by researchers such as M. H. Sloboda are typically less than 0.267 mm (0.011 in.), and usually at least 0.051 mm (0.002 in.), or 0.025 mm (0.001 in.), depending on the materials of the braze and the workpiece. Precise joint gap control is important to ensure oxide removal, promotion of wetting, and promotion of capillary-driven coverage of the joint. This ensures good strength of the braze joint and, for certain applications, leak tightness of the braze joint. 
         [0005]    Thus, a need exists for precision and consistency in braze gap control. One existing method is to tack weld at select locations prior to brazing to hold the parts at a desired gap clearance. However, tack welding causes local distortion of the parts and can result in a varied gap across the braze plane of a large part. This is especially common when previous tacks in a sequence cause gap changes that are then frozen in place by subsequent tacks. Tack welds also leave localized oxides that are difficult to remove, especially in the immediate vicinity of the tack. Another existing method is to use fixtures, including dead weights placed on the part. However, during brazing, the molten braze may be compressed by such fixturing, changing the gap width. For some precisely machined parts, a surface finish may provide reasonable gap control, but capillary action of the braze melt must reach, penetrate and wet all gap surfaces including contact locations. A further complication arises when parts having different coefficients of thermal expansion are brazed, which complicates gap maintenance during the brazing process. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The invention is explained in the following description in view of the drawings that show: 
           [0007]      FIG. 1  is an isometric view of a brazing foil including a gap-setting screen, illustrating aspects of an embodiment of the invention. 
           [0008]      FIG. 2  is an isometric view of the brazing foil of  FIG. 1  inserted between opposed surfaces to be brazed. 
           [0009]      FIG. 3  is an isometric view of an embodiment with a single longitudinal wire. 
           [0010]      FIG. 4  is a front sectional view of a brazing screen with overlapping wires in a brazing gap with molten brazing filler. 
           [0011]      FIG. 5  is a front sectional view of a brazing screen with electrically insulated resistance heating wires in a brazing gap with molten brazing filler. 
           [0012]      FIG. 6  is a front sectional view of a brazing screen with longitudinal electrically insulated resistance heating wires in a brazing gap with molten brazing filler. 
           [0013]      FIG. 7  is a top schematic view of a brazing foil having a screen with resistance heating wires connected between electrical terminals. 
           [0014]      FIG. 8  is a top schematic view of a brazing foil having a screen with resistance heating wires connected at one end to a power supply, and grounded at the other end. 
           [0015]      FIG. 9  is a top schematic view of a brazing foil having a screen providing electrical connection to a mid-joint sensor element. 
           [0016]      FIG. 10  is a known chart of brazing joint thickness versus joint tensile strength. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]      FIG. 1  illustrates a brazing foil  20 A having a brazing filler material  22  with an embedded open mesh screen  24 A made of one or more longitudinal wires  26  that intersect  30  one or more transverse wires  28 . The wires have a higher melting point than the brazing filler material  22 , so the thickness T of the screen sets and maintains a predetermined gap between the opposed surfaces being brazed. Examples of wire materials suitable for very high temperature brazing include tungsten (mp 3422° C.), rhenium (mp 3180° C.), tantalum (mp 2996° C.), molybdenum (mp 2620° C.), ceramics, and refractory alloys. Examples of materials suitable for moderate temperature brazing include nickel, nickel alloys, stainless steel, cobalt alloys and iron alloys. The wire material may be the same as the material of the part(s) being brazed. The thickness of the screen is the thickness of the intersections  30 , which in this embodiment may be the thickness of a single wire  26 ,  28  or of the thickest wire  26 ,  28  if they have different thicknesses. Alternately, the wires may overlap at the intersections, as later shown, making the screen thickness the sum of thicknesses of the overlapping wires at the intersections. Herein, the term “intersection” includes wires crossing through the same space as in  FIG. 1  and wires crossing each other with physical contact and overlap as in  FIG. 4 , unless the term is narrowed by further description for a given embodiment. 
         [0018]      FIG. 2  illustrates the brazing foil  20 A inserted in a gap  31  between opposed surfaces of two workpieces  32 ,  34  to be brazed, for example in an oven. During heating, the workpieces may be urged together  36  to close the gap  31  down to the screen thickness T during melting of the braze filler  22 . The gap  31  and the foil  20 A have a length L and a width W for descriptive purposes herein. Most brazed joints are designed for shear loading, and the wires  26 ,  28  of the foil  10 A can provide strengthening of the braze under shear loading, functioning somewhat like rebar in concrete. 
         [0019]      FIG. 3  shows an embodiment of a brazing foil  20 B with an embedded screen  24 B having a wire arrangement that allows the filler material  22  to expand within and laterally beyond the gap during melting without the constraint of intervening longitudinal wires. This screen has only one longitudinal wire  26 , and multiple transverse wires  28  that intersect the longitudinal wire. Other embodiments may include only transverse wires  28  in a region where a braze joint is to be formed, with one or more longitudinal wires  26  interconnecting the transverse wires  28  being located in a region away from where the braze joint is to be formed, thereby providing mechanical support for the transverse wires  28  which function to control the gap size, while still allowing unencumbered flow and expansion of the filler material  22  in a direction parallel to the transverse wires  28  in the region where the braze joint is formed. Thus, the term “screen” as used herein includes an array of wires held together such that at least a portion of some of the wires can be inserted into a region of a braze joint for the purpose of controlling a gap of the braze joint. The term “screen” should not be interpreted so broadly herein to encompass random filaments or non-interconnected rodlets (small wire portions). 
         [0020]      FIG. 4  is a front sectional view of an embodiment of a brazing foil  20 C with a screen  24 C having intersecting longitudinal  42  and transverse  44  wires that lap each other at the intersections  40 . The foil  20 C is inserted in a brazing gap  31 . The wires may interleave over-under each other longitudinally and transversely, for example in a plain weave pattern. Alternately, the longitudinal wires may be bent as shown, and the transverse wires may be straight. The screen thickness T is the sum of the thicknesses of two overlapping wires at each intersection. All intersections  40  within the gap  31  may have the same thickness T, or only a subset of the wires may be thicker to determine the gap  31 . Overlapping wires allow molten filler material  46  to flow across the width of the brazing gap by capillary action, since the bent wires  42  provide flow space as shown. Such a brazing screen  24 C may be provided separately from the filler material. Filler material  46  may be melted and infused from a side of the gap after the screen is in place. Alternately the foil  20 C may be formed as a preform including both the screen  24 C and filler material  46  spanning across the screen  24 C. For example, the screen  24 C may be embedded in the filler material  46 , or the screen  24 C be pressed onto a foil of the filer material  46 , or the screen  24 C may be filled with a paste of the filler material  46 . 
         [0021]      FIG. 5  is a front sectional view of an embodiment of a brazing foil  20 D with a screen  24 D having longitudinal  52  and transverse  54  wires that overlap at the intersections  51 . The wires may be coated with refractory electrical insulation  56  such as silica, hafnium oxide, magnesia, or alumina, allowing the wires to serve as resistance heating elements. For example, the longitudinal wires  52  may be made of tungsten arranged for connection between electrical terminals, as later shown. Tungsten is a resistance heating material used in filaments of incandescent light bulbs. It has a high melting point (3422° C.), low coefficient of thermal expansion, and high tensile strength. Incandescent bulb filaments commonly have an uncoiled length of over 20 inches (580 mm). In this embodiment  20 D the wires  52  may be coated with insulation after forming the screen  24 D, allowing the longitudinal and transverse wires  52 ,  54  to contact each other at the intersections. This facilitates heat conduction from the longitudinal heating wires to the transverse wires. The screen thickness T is the sum of the thicknesses of two overlapping wires at each intersection, including the thickness of the insulation material. 
         [0022]      FIG. 6  is a front sectional view of an embodiment of a brazing foil  20 E with a screen  24 E having longitudinal  62  and transverse wires  64  that overlap at the intersections  61 . The screen thickness T is the sum of thicknesses of two overlapping wires at each intersection. The longitudinal wires may be coated with electrical insulation  66 , allowing them to serve as resistance heating elements. In this embodiment the longitudinal wires  62  may be coated with insulation before forming the screen  24 E, thus separating them electrically from the transverse wires. The transverse wires  64  may also be coated before forming the screen, or they may be non-insulated, as shown. 
         [0023]      FIG. 7  is a top schematic top view of an embodiment of a brazing foil  20 F with a brazing screen  24 F formed of longitudinal  72  and transverse  74  wires. Some or all of the wires  72  are electrically insulated resistance heating wires connected between electrical terminals  76 ,  77 . The terminals  76 ,  77  may be formed integrally with the ends of the longitudinal wires  72 , or may be attached thereto, such as with clamps. 
         [0024]      FIG. 8  is a top schematic view of an embodiment of a brazing foil  20 G with a brazing screen  24 G formed of longitudinal  82  and transverse  84  wires. Some or all of the wires  82  are electrically insulated resistance heating wires connected to an electrical terminal  86  at one end. These wires lack insulation at the second end for electrical connection to the workpiece  34  via electrically conductive filler  22 , completing a circuit to a terminal  87  connected to the workpiece. If the filler is not sufficiently electrically conductive, contacts  89  may be formed on the ends of the wires  82  with the thickness of the gap to contact the workpieces directly when the workpieces are urged together. Alternatively, the second end(s) may be spot welded to the workpiece  34  for electrical connection. Alternate to spot welds a filler such as copper based epoxy could accomplish electrical contacts  89  on the ends of the wires  82  within the thickness of the gap to contact the workpieces directly. These embodiments are especially useful where access to opposed sides of the workpiece is unavailable or limited, but they can be used in any situation in which one or both workpieces is/are electrically conductive. 
         [0025]      FIG. 9  is a top schematic view of an embodiment of a brazing foil  20 H with a brazing screen  24 H formed of longitudinal  92  and transverse  93 ,  94  wires. At least one of the wires may be electrically conductive and insulated and connected to a mid-joint sensor  95  such as a thermocouple or strain gauge that indicates a condition of the brazing within the braze joint, either during the brazing operation or upon later operation of the brazed component. High temperature thermocouples are available, for example, made of tungsten/rhenium or platinum/rhodium with insulation of hafnium oxide, magnesia, or alumina. These can be used in temperatures up to 2315° C. (4200° F.). At least some intersections  96  may be electrically continuous to provide a conductive path  92 ,  93  from the sensor  95  in a middle part of the foil to connections  98  outside the filler material  22 . If these connection wires  92 ,  93  have free ends within the foil, they may be capped  97  with electrical insulation to block electrical continuity with the brazing. Alternately, an electrical grounding path may be provided through the brazing and workpiece. Other embodiments may utilize fiber optic filaments to facilitate communication with a fiber optic sensor or to deliver laser energy for heat. The fiber optic filaments may function as wires of the screen to define the braze gap, or they may be used with larger metal wires which define the gap while the fiber optic filaments function only for signal communication or energy transfer. As shown in this figure, some or all of the wires of any foil embodiment do not necessarily extend to the edges of the foil filler material  22 . 
         [0026]      FIG. 10  is a known chart of brazing joint thickness versus joint tensile strength, and illustrates the importance of precise joint thickness control. Such control is provided by the screens of the present invention. 
         [0027]    While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. For example, the illustrated embodiments have longitudinal and transverse wires disposed at right angles to each other, however, any appropriate relative wire orientation may be used. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.