Metal fusion bonding – Including means to provide heat by friction between...
Reexamination Certificate
2000-05-02
2002-10-08
Dunn, Tom (Department: 1725)
Metal fusion bonding
Including means to provide heat by friction between...
C228S112100, C029S402010
Reexamination Certificate
active
06460750
ABSTRACT:
REFERENCE TO A “MICROFICHE APPENDIX”
Not applicable
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to friction welding. More particularly, the present invention provides an improved method and apparatus that relates to friction plug welding suitable for flight hardware usage, wherein the pull plug has an enlarged heat with a chamfered heat sink.
2. General Background of the Invention
Friction stir welding (FSW) is a solid state joining process developed by The Welding Institute (TWI), Cambridge, England and described in U.S. Pat. No. 5,460,317, incorporated herein by reference. Also incorporated herein by reference are U.S. Pat. No. 5,718,366 and all references disclosed therein.
The following references are also incorporated herein by reference: U.S. Pat. Nos. 3,853,258, 3,495,321, 3,234,643, 4,087,038, 3,973,715, 3,848,389; British Patent Specification No. 575,556; SU Patent No. 660,801; German Patent No. 447,084, “New Process to Cut Underwater Repair Costs”, TWI Connect, No.29, January 1992; “Innovator's Notebook”, Eureka Transfer Technology, October 1991, p. 13; “Repairing Welds With Friction-Bonded Plugs”, NASA Tech. Briefs, September 1996, p. 95; “Repairing Welds With Friction-Bonded Plugs”, Technical Support Package, NASA Tech. Briefs, MFS-30102; “2195 Aluminum-Copper-Lithium Friction Plug Welding Development”, AeroMat '97 Abstract; “Welding, Brazing and Soldering”, Friction welding section: “Joint Design”, “Conical Joints”, Metals Handbook: Ninth Edition, Vol. 6, p. 726.
Friction plug welding (FPW), also referred to as plug welding and friction taper plug welding (FTPW), is a process in which initial defective weld material is located, removed and replaced by a tapered plug, which is friction welded into place. This process is similar to friction stud welding, in which a plug is welded to the surface of a plate, end of a rod, or other material. The primary difference is that FPW is designed to replace a relatively large volume of material containing a defect whereas friction stud welding is a surface-joining technique.
Friction plug welding could be used to repair weld defects in a wide variety of applications; however, it would most likely be used where weld strength is critical. This is due to the fact that manual weld repairs result in strengths much lower than original weld strengths, as opposed to friction plug welds (FPWs) whose typical mechanical properties exceed that of the initial weld. In applications where high strength is not required, manual welding would be less expensive and would not require specialized equipment.
An extension of FPW is known as stitch welding or friction tapered stitch welding (FTSW) and has been developed to repair defects longer than what a single plug can eliminate. Stitch welding is the linear sequential welding of several plugs such that the last plug weld partially overlaps the previous plug. Defects of indefinite length can be repaired with this process, limited only to the time and cost of performing multiple plug welds. These welds have undergone the same testing procedures as single FPWs, including NDI and destructive evaluation. The strengths for stitch welds are similar to those for single plug welds.
Stagger stitch welding is a process best defined as stitch welding in a non-linear fashion. Areas wider than one plug length can be completely covered by staggering plugs side to side as they progress down the length of an initial weld. This process is being developed for plug welds whose minor diameter is on the crown side of the initial weld, and where replacement of the entire initial weld is desired.
While friction plug welding might be a preferred method of repairing defects or strengthening initial welds, there are some applications where heretofore it has been extremely difficult to use friction plug welding. The main cause is due to the logistics of setting up the equipment and/or support tooling to perform friction plug welding, and the geometry of the work piece to be welded.
BRIEF SUMMARY OF THE INVENTION
The apparatus of the present invention solves the problems confronted in the art in a simple and straightforward manner. What is provided is an apparatus for and a method of friction plug welding an article using a plug that has a geometry that provides less heat sink and facilitates a good weld when the plug is pulled. As demonstrated through several single variable experiments, a larger plug mass acting as a heat sink at the top of the weld has deleterious affects on the bonding of the plug top. Thermodynamically, plug mass acts to conduct heat away from the interface, while the atmosphere insulates the interface, increasing the heating capabilities of the welding process. With less plug mass, or heat sink, left on top of the plate surface after the completion of a weld, the heat produced by the weld process is not conducted away from the interface as rapidly.
Due to the nature of the Friction Pull Plug Welding process, “cold” plug material is always being pulled into a “hot” interface, with the plug shaft being the first location of the plug to be heated. This situation poses a problem to ensuring complete plug/plate interfacial bonding at the top of the weld, the last location of bonding during the welding process. Since the lack of bonding defect at the topside of the weld is easily detected through dye-penetrant non-destructive inspection, it has been thoroughly characterized and analyzed. The problem of bonding this last interfacial location is quite complex, involving a combination of both loading and heat flow.
Finite Element Thermal Modeling has demonstrated the validity of the above thesis of heat flow within the pull plug. Two plug models have been described by the thermal model, Model 1 with 0.500 inches and Model 4 with 0.100 inches of plug mass left above the plate surface after welding. The divergence of the temperature versus distance (distance from the bottom of the plate to the top of the heat sink) curves at the region of final bonding, exhibits the effects of heat conduction. The plug with less heat sink, maintains a higher temperature at the plug top, due to atmospheric insulation and less heat conduction through plug. At a radial distance of 0.100 to 0.200 inches from the plug interface, the divergence of the curves reaches a maximum, indicating the overall importance of plug mass heat sink at this location.
Thermal modeling and experimental testing have both demonstrated the need to minimize the plug mass heat sink remaining above the plate surface to ensure complete interfacial bonding at the plug top. However, three major problems can arise from a minimized heat sink, the entire plug could be pulled through the plate hole, the central portion of the plug could be separated along grain boundaries, or with a top hat plug, the plug top hat can be separated from the body. Each of these conditions is completely unacceptable for a production acceptable process.
The forging load can completely pull a low angle plug through the plate hole because of plug overheating (i.e. does not have enough mass heat sink) or because of too little enough plug mass resistance (i.e. no top hat). A central pull-through can occur in higher angle plugs without a substantial heat sink, where the angle of the plug prevents complete plug pull through. In high angle plugs, the loading is transferred to the high heat region at a radial distance of 0.100 to 0.200 inches from the interface. The heated plug grain boundaries do not have adequate strength to support the forging load and a central plug pull-through occurs. A similar situation arises during a top hat separation, where the interfacial bonding and the geometry of the top hat, act to transfer loading to the temperature weakened plug material at the hat/plug body transition region. The forging load overcomes the shear strength of the heated grain boundaries leading to a top hat separation, which is usually followed by central plug pull-through.
The present invention includes a method of friction plug welding an articl
Cantrell Mark A.
Coletta Edmond R.
Dunn Tom
Garvey, Smith, Nehrbass & Doody, L.L.C.
Pittman Zidia
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