Method of friction stir welding of surfaces with polymers sealant

Field of the Invention

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0001This invention relates to a method of friction stir welding a sealant between two workpieces.

1. Background of the invention

0002Welded joints, such as lap joints, are used to join metal parts. Lap joints are used to join overlapping surfaces using one or more welds at the overlap of the materials. The surfaces of each piece of joined metal adjacent the fasteners or welds that are mated by the lap joint, called "fay" surfaces, are often not fully bonded by the weld or other fasteners and are often protected from corrosion by conventional fay surface sealants, such a polysulfide or polythioether, applied to the mating surfaces prior to joining. Such fay surface sealants may also be used to reduce mechanical and fatigue problems resulting from rubbing between the fay surfaces, vibration and the like.

0003Fay surface sealants are used in welding to prevent or reduce corrosion in large part by reducing moisture which may be trapped between the fay surfaces and/or brought in by capillary action. Conventional fay surface sealants may degrade the properties of the weld and are often degraded by the heat and mechanical activities involved in the welding and working of the materials as well as by vibration during use. Corrosion of fay surfaces within welds is often very difficult to detect by inspection.

0004Welding techniques typically use elevated temperatures to bond metals and metal alloys. Friction stir welding (FSW) is a welding technique in which the shoulder of a rotating tool is applied to the materials to be joined to heat and soften the materials by friction. The tool includes a rotating pin which penetrates the joint and stirs the materials together. Solid state joints are produced thereby without the addition of filler or the use of shielding gases. Friction stir welding is advantageously used in aircraft construction, for example, to weld a stringer or other support to the surface of the aircraft skin. Conventional FSW welding processes apply a zone of corrosion resistant material to cover the welded joint during and after welding as a fay surface sealant. An example of this technique is disclosed in U. S. Patent 6,045,028, which represents the most relevant state of the art. Corrosion of fay surfaces within a FSW weld in an aircraft is a very dangerous condition because of the reduction in strength of the aircraft structure, yet it is very difficult to detect by inspection.

0005What is needed is a new fay surface sealant technique for producing FSW welded components with improved corrosion resistance for use, for example, in the production of airplanes and parts of airplanes.

SUMMARY OF THE INVENTION

0006A method of function stir welding according to claim 1 is defined.

BRIEF DESCRIPTION OF THE DRAWINGS

0007Fig. 1 is a cross sectional side view of a portion of an aircraft or similar structure including a lap joint between metal surfaces, showing typical locations of FSW welding joint lines, with a polymeric fay surface sealant for improved corrosion resistance.

0008Fig. 2 is a cross sectional side view of the structure of Fig. 1 showing an alternate stiffener welded to a surface and protected with a polymeric fay surface sealant.

0009Fig. 3 is a cross sectional view of an aircraft wing and a representative stiffener with a polymeric fay surface sealant.

0010Fig. 4 is a cross sectional view of generic aircraft structure and a representative stiffener with a polymeric fay surface sealant.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT(S)

0011Referring now to Fig. 1, a cross sectional side view of a portion of an aircraft or similar structure is shown including lap joint 10 between an aluminum, or aluminium alloy surface 12, such as an aircraft wing or body skin, with a stringer, such as an "L" shaped aluminum or aluminium alloy support member 14. The lap joint is formed by using FSW at one or more of the welding locations generally indicated as FSW welding joint lines 16, 18 or 20. These joint lines are perpendicular to the plane of the drawing, and are formed by rotational of FSW welding tool 22 against support member 14 and/or surface 12. The rotation of tool 22 causes friction to heat and soften the materials to be joined, and mechanical mixing of the plasticised materials to form the joint. The elevated temperatures from this process may polymerize a sealant and/or adhesive monomer layer which, when cured, forms a corrosion protection layer, fay surface sealant layer 24.

0012The monomer layer preferably has characteristics suitable for the FSW welding process as well as suitable for the intended use of the jointed assembly. For example, the monomeric layer should be easy to apply to the critical area, and cured but not substantially damaged by the elevated temperatures produced by the welding. When cured, sealant layer 24 should provide corrosion resistance for the fay surfaces of the joint by, for example, resisting the intrusion of moisture by capillary action. A particularly useful fay surface sealant material for sealant layer 24, is formed by the application of a layer which when cured forms a fluoroelastomeric polymer to create a protective corrosion shield between the adjacent fay surfaces 26 and 28 of support member 14 and surface 12, respectively. It may be desirable to extend the coverage of sealant layer 24 beyond fay surfaces 26 and 28 in many applications.

0013Lap joint 10 is formed by the following process. The surfaces of 12 and 14 are prepared normally for FSW welding, thereafter, sealant layer 24 is prepared for application to the surfaces to be welded. One particularly useful polymeric fay surface sealant may be formed by use of a fluoroelastomer adhesive PLV 2100 available from Pelseal Technologies, LLC of Newtown, PA. PLV 2100 is a 2 part adhesive which may be mixed in the ratio of 25-27 parts by weight of PLV 2100 base material to one part by weight of Accelerator #4, which is added to the based material and mixed thoroughly for about 5 minutes. For spray application, the mixed coating may then be thinned by adding methylisobutylketone (MIBK) or methylethylketone (MEK) in a 1:1 ration by weight. It is desirable to avoid introducing substantial amounts of air into the mixture and it may be desirable to off-gas the mixture for 10 minutes prior to the application. Other materials shown to be suitable include Pelseal PLV 6032 and Thermodyne THP-2000.

0014Additional surface treatments may also be used with the process of the present disclosure. As device components are aluminum or aluminum alloy and increased joint corrosion resistance is desired, a conventional chemical conversion coating may be applied on at least the surfaces to be welded before application of the surface sealant.

0015The mixture may be applied to surface 26 of stiffener 14, and/or to surface 28 of skin 12, by using a hard rubber roller, brush, or preferably an HPLV sprayer as an applicator 29. An appropriate adhesive wet film thickness for layer 24 may be in the order of up to 0.0127 to 0.254 mm (0.0005 to 0.0100 inches), preferably from 0.0254 to 0.127 mm (0.001 to 0.005 inches), to avoid problems such as causing FSW tool 22 to dive during the welding process, potentially creating an unacceptable weld. After the mixture is applied, surface 26 of support member 14 may then be clamped, preferably within about 20 minutes, to surface 28 of skin surface 12. If the clamping is not accomplished within 20 minutes, it may be desirable to rewet the surface by spraying with a fine mist of MIBK or MEK for up to about 1 hour after coating. Thereafter, the application of a new coating may be desirable if clamping has not been accomplished.

0016The PLV 2100 fluoroelastomer adhesive will cure at a temperature of 23.89°C ± 5.55°C (75°F ± 10°F) in approximately 24 hours. The application of rotating FSW tool 22 to form FSW welding joint lines 16, 18 and/or 24 serves to create the elevated temperatures necessary to cause the polymeric sealant adhesive to cure more quickly. In addition to the heat generated by the friction of the rotating contact between FSW tool 22 and weld points line 16, 18 or 20, additional heat may be provided to reduce cure time prior to or as part of the FSW welding process by the use of laser 30, in a process known as laser-assisted FSW (LAFSW) or by use of a heater, such as induction heater 32. After FSW welding to form joint 10 with polymeric fay surface sealant layer 24, the sealant may be sufficiently cured to permit other work, such as priming, on adjacent non-welded surfaces within the order of about 4 hours.

0017Referring now to Fig.s 2, 3 and 4, there are many different types of stiffeners and surfaces to be joined that may be improved by the use of a sealant cured by the welding process. In Fig. 2, "Z" shaped stiffener 15 may be welded, by FSW or similar processes by lap joint or other known welding technique, to surface 12 at joints 16, 18 or 20 by the application of a suitable adhesive or sealant which is cured to form fay surface sealant 24.

0018Referring now to Fig. 3, a cross section of aircraft wing assembly 34 is shown in which stiffener 36 is joined to an appropriate surface by a welding process which cures a sealant layer to form an appropriate polymerized fay surface sealant 24.

0019Referring now to Fig. 4, a cross section of generic aircraft structure 38 is shown in which Z shaped stiffener 37 is jointed to an appropriate surface by a welding process which cures a sealant layer to form an appropriate polymerized fay surface sealant 24.

0020Although PLV 2100 fluoroelastomer adhesive was used as an example of a suitable material to be applied to form fay surface sealant layer 24, other polymers cured but not damaged by the heat of the welding process to form a layer having appropriate corrosion resistant properties may be used. Fluoroelastomers are particularly useful for this purposes and include Viton branded materials from Dupont/Dow.

0021In addition to lap joints, fillets and any other joint which may produce a fay or unbonded surface area, may be protected by fay sealants cured by the welding process. The use of fay surface sealants cured at least in part by the welding process is particularly useful in the construction of aircraft and aircraft sub-assemblies, many other assemblies may benefit from the use of heat cured fay surface sealants.