RESILIENT RESISTANCE WELDING ELECTRODE

Abstract: Resistance welding of a delicate honeycomb sheet (10) to a substrate (12) without damage to the honeycomb is accomplished even when there are irregularities in alignment of the workpieces and electrodes. A generally T-shaped resistance welding electrode (14) is attached to the arm (16) of a welding machine by an attachment tang (20). It has a transversely extending hollow portion (22) made of thin sheet metal. The hollow portion (22) is filled with a material (26) such as an elastomer. The elastomer may comprise silicone rubber. The electrode is thus adapted to make good contact with, and provide even force to the surface (11) of a delicate workpiece (10) being welded to a rigid substrate (12).

Description: Resilient Resistance Welding Electrode Technical Field The present invention relates to resistance welding, particularly to electrodes for resistance welding machines. Background The present invention grew out of the need to resistance weld fine honeycomb structures to the surface of solid metal structures. Specifically, as shown in U.S. Pat. No. 4,218,066 to Ackerman, honeycomb metal grids bonded to circumferential metal structures are useful as gas turbine engines for rotary seals. Brazing is a preferred way of making such structures, a fragment 10, 12 of which is illustrated in Figure 2 herein. To lessen fixturing requirements and to insure the proper placement of the honeycomb 10 with respect to the solid metal substructure 12, the honeycomb is first resistance tack welded to the solid substrate. After this is accomplished the welded structure is placed in a furnace and braze is caused to flow about the joint, including the resistance welded joint, to thus produce the final product. However, because of the relatively fragile structure of a typical honeycomb, where the cell wall thickness is of the order of 0.13-0.38 mm, the con¬ ventional procedures associated with capacitor OMPI iVIFO discharge resistance welding processes have been unsuitable for forming the tack welds. Specifically, rigid electrodes, whether they be in the simple conventional form or wheels, provide highly con- centrated loads and localized current flows which tend to both crush and overheat the honeycomb cell structure. Of course, the electrode pressure can be adjusted to a low level. But still, the align¬ ment of the machine electrodes with respect to each other and the workpiece surface can result in non-uniform contact with the surface of the honeycomb, producing the aforementioned defects. This is the same sort of problem which also results when the surface of the honeycomb is either irregular or not exactly the same shape as the electrode which con¬ tacts it. Therefore, unless electrodes are changed and good alignment is attained, the aforementioned defects will result, especially when the cell material is of the order of 0.1 mm thick. Others have addressed these types of problems previously. For example, Humphrey in U.S. Pat. No. 2,266,424 shows a resistance spot welding electrode wherein a multiplicity of flexible electrode fingers are urged toward the workpiece by a rubber pad. Hardesty in U.S. Pat. No. 2,892,922 discloses a method and apparatus for both resistance welding and forging honeycomb material. In the Hardesty patent, the honeycomb is resistance welded between two metal sheets, compared to the prime object of the present invention where only one sheet is attached to the honeycomb and where one end of the honeycomb must be maintained in the open condition. A resilient pad, or a gas bladder, is positioned to urge the multi¬ plicity of- movable electrodes towards the workpiece. In both the Hardesty and Humphrey patents the electrodes are rigid in one planar dimension and flexible in the opposing planar direction, with re¬ spect to the plane of the workpiece surface. In still another approach, layers of fine copper wire mesh have been laid across the end of a rigid electrode to provide a more compliant surface. But, while the prior methods will carry out the objects to which they are suited, they are not entirely useful for the purposes of the present invention in obtaining good joints without damage to the fine surface of a honeycomb. Disclosure of the Invention An object of the invention is to provide an im¬ proved electrode for resistance welding. Particularly desired is one which applies an even force to fragile structures such as honeycomb and which is compliant to irregularities in the alignment of the workpiece surface with respect to the welding machine. According to the invention, the electrode is generally T-shaped in cross section and is connected to the welding machine by the tang which constitutes the vertical of the tee. Extending transversely from the tang is a hollow shell portion made of thin sheet metal. Within the hollow is a material such as silicone rubber which generally maintains the shape of the electrode when the compliant shell is pressed against the workpiece. The hollow shell portion of the electrode is preferably adapted to flex about the tang, thus enabling the electrode contact surface to comply readily a non-parallel workpiece surface. Preferably the electrode is made of 0.14-0.38 mm copper alloy which is folded so that the paired opposing ends of the sheet form the tang. In an alternate embodiment of the invention the hollow interior of the electrode is filled with an entrapped fluid instead of a solid material. The electrode provides good contact with work- pieces having surfaces which are uneven or variably inclined with respect to the normal plane of the welding machine. It has been found to be durable and able to facilitate the making of good honeycomb joints. The foregoing and other objects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments and accompanying drawings. Brief Description of Drawings Figure 1 shows a resistance welding electrode comprising a hollow sheet metal portion with a contained rubber block, as it is used between the arms of a welding machine to join two workpieces. Figure 2 is a partial vertical cross section through the apparatus of Figure 1. Figure 3 shows an electrode having a rigid tang. Figure 4 shows an electrode having a fluid containing bladder filling the interior of the hollow portion. Figure 5 shows an electrode where the tang and upper part of the hollow shell portion are both rigid. Best Mode for Carrying Out the Invention The present invention is described in terms of the welding of honeycomb material to the surface of a superalloy substrate, to make a structure like that shown in U.S. Pat. No. 2,814,717, the disclosure of which is hereby incorporated by reference. The honeycomb is made of the alloy Hastelloy X (by weight percent 22 Cr, 1.5 Co, 18.5 Fe, 9 Mo, 0.6 W, 0.1 C, balance Ni) sheet metal about 0.091-0.11 mm thick. The honeycomb cells are hexagonal in shape with a cross-dimension or width of about 1.65 mm, and a height of about 15 mm. The resistance weld is formed suitably by using a Benchmark Industries Inc. (Londonderry, N.H., USA) capacitor discharge resistance spot welder with a Model 3000 power supply and Model M head. The unit has an output capacity of up to 3000 watt seconds. The power setting is varied according to the weld joint area. It will be appreciated that the invention will be useful generally for resistance welding of structures and materials other than those specifically discusssed herein, and with other machines. Figure 1 shows an electrode 14 in accord with the present invention as it is used to secure a piece of honeycomb 10 to a solid substrate 12, both shown in phantom, at a joint line 13. The electrode 14 is captured in the upper arm 16 of the resistance welding machine. A vertically downward force on the arm pushes the electrode 14 against the workpieces 10, 12 and thus clamps them against the bottom OMPϊ opposing arm 18 of the welding machine (which is not shown) . The electrode 14 is generally shaped like an inverted T. It is made of thin copper sheet, such as copper-1% chromium or cobalt-beryllium-copper (U.S. Resistance Welder Manufactuers Association Class 2 or 3 coppers) , materials well-known for use in resistance welding electrodes. The foil is very thin, of the order of 0.135-0.38 mm thick to ensure that the electrode structure is sufficiently flexible, The electrode has a tang portion 20 which is held in the arm 16 and an attached lower shell portion 22, hollow in cross section and extending transversely from the tang. The lower surface 24 of the electrode 14 contacts the upper surface 11 of the honeycomb. Contained within the hollow cross section of the shell 22 is a resilient block 26 made of an elastomer such as General Electric RTV 630 silicone rubber having a Durometer hardness of about 50-60. Figure 2 shows a vertical plane cross section of the apparatus illustrated in Figure 1 to enable an appreciation of the features of the electrode. The elastomer block 26 maintains the general shape of the electrode when the arms 16, 18 move towards each other and force the electrode 14 against the workpieces. If the upper surface 11 of the workpiece contacted by the electrode is uneven, then the lower sheet metal surface 24 of the electrode will tend to comply to the surface, and will be urged into contact with it by action of the rubber block 26. If the upper surface of the workpiece is not aligned with the electrode surface (i.e., if it is not generally perpendicular to the vertical motion plane of the arms) , there will be an eccentric force on the shell 22 and it will flex about the tang 20, as illustrated by the phantom line 28. In the drawing, the tang is shown in its simplest fashion, being comprised of the paired opposing ends of the sheet metal which forms the shell portion of the electrode. This is desirable because the tang 20 will be adapted to easy flexing to carry out the objects mentioned above. But an embodiment as shown in Figure 3 will also be within the scope of the invention. In Figure 3, the shell 28a is brazed at joint 34 to a solid tang 20a. The flexible construction of the shell and the elastomer contained therein will still permit flexure motion about the vertical axis of the tang 20a. Figure 3 also illustrates how the shell shape can be varied to increase the compliance of the vertical sidewalls, specifically by angling the sidewalls 36. Of course, the electrode may have still other cross sections than those shown herein. Circular cross sections, and those which are non-regular will be within the scope of the invention. Another embodiment of the invention is illus¬ trated in Figure 4. Here, the elastomer block is replaced by a bladder 30 contained within the shell 28b of the electrode. The bladder may be made out of an elastomer, polymer, or metallic material having _OMPJ suitable temperature capability; it is pressurized by means of the tube 32. In still another embodiment of the invention the bladder may be eliminated and the ends of the electrode closed to create a gas tight cavity of sheet metal therein. While it is most desirable that the shell fully flex about the tang as described, a more basic usable mode of the invention is illustrated by Figure 5. The tang 20c has attached to it a transversely extending rigid member 38. The hollow shell is comprised of the member 38 and attached sheet metal part 40. The resilient block 26c is contained within the hollow. Various other structures, such as springs and other materials, may be used to fill the hollow cross section of the electrode so long as they tend to maintain the general shape of the shell part. While compressible materials are preferred, liquids will also carry out the object of the invention since the thin shell can distend in places away from where it may be impressed. It is only required that the material create sufficient pressure to urge the surface 24 against the workpiece, in view of the applied forces which are used. Gas or liquid cooling can be applied to the electrode in combina¬ tion with the purposes for filling the shell interior, or independently thereof. In a preferred use of the invention, the electrode will have a rectangular surface 24. Silicone rubber filled electrodes of 19 x 38 mm and 32 x 64 mm surf ce area and about 13 mm interior height have been used, with applied forces of 20-135 kilograms between the arms. The nominal workpiece pressures produced by these electrodes range from 275-662 kPa. The electrodes have been found to be durable and when they do eventually erode, they are economical to replace. In use, the electrode is pressed against the workpiece and held there while an electrical poten- tial is applied to the arms of the welding machine. The electric current pulse flows through the con¬ ductive electrode to the joint, thus locally heating the workpieces at the joint 13 and forming the weld. The electrode is flexible yet the structure has sufficient structural integrity so that when the arms of the machine are separated the electrode maintains its shape and lifts readily off of the workpiece surface. Although this invention has been shown and described with respect to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail thereof may be made without departing from the spirit and scope of the claimed invention. OMPI

Claims: I claim : 1. An electrode for resistance welding characterized by a tang for attaching the electrode to a welding machine; a hollow shell portion attached to the tang and extending transversely thereto, the hollow shell comprised of conductive sheet; and means contained within the hollow shell portion for generally maintaining the shape of the shell portion when force is applied along the tang. 2. The electrode of claim 1 characterized by a shell which flexibly bends about the tang when eccentric force is applied to the shell. 3. The electrode of claim 1 characterized in that the means contained within the hollow shell portion is an elastomer block. 4. The electrode of claim 1 characterized by a hollow shell portion made of copper alloy sheet metal of 0.14-0.38 mm thickness. 5. The electrode of claim 1 characterized by a shell portion which is essentially closed and by a fluid being the said means contained within the hollow shell portion. 6. The electrode of claim 4 characterized in that the tang comprises the paired ends of the sheet metal which defines the hollow shell portion. 7. The assembly of claim 3 characterized by an elastomer which is silicone rubber of about 50-60 Durometer.

Inventors: PADDOCK JORDAN H, PADDOCK, JORDAN, H.
Applicants: UNITED TECHNOLOGIES CORP, UNITED TECHNOLOGIES CORPORATION
Application year: 1983
Main classification: B23K35
IPC codes: B23K11, B23K35