FRICTION STIR WELDING AS A RIVET REPLACEMENT TECHNOLOGY

Abstract: The present invention provides an improved method and apparatus for joining aluminum alloy panels (29, 29)(e.g. 2090-T83 aluminum lithium alloy). The method preliminarily positions two panels (28, 29) next to each other so that a stringer panel (28) overlaps a skin panel (29). A friction stir weld pin tool (10) penetrates through one panel (28) and at least partially into another panel (29). The panel material around the pin tool (10) is frictionally heated.

Description: PATENT APPLICATION Attorney Docket No. A99224WO (46455.27WO) TITLE OF THE INVENTION "FRICTION STIR WELDING AS A RIVET REPLACEMENT TECHNOLOGY" APPLICANT: LOCKHEED MARTIN CORPORATION, a corporation organized under the laws of Maryland with address P.O. Box 29304, New Orleans, Louisiana 29304 US. INVENTORS: DRACUP, Brian, J. a US citizen, of 1101 Pacific Avenue #501, San Francisco, California 94133; ARBEGAST, William, J., a US citizen, of #4 Fred Mallet Road, Picayune, Mississippi 39426. CROSS-REFERENCE TO RELATED APPLICATIONS Priority of US Provisional Patent Application Serial No. 60/152,770, filed 3 September 1999, incorporated herein by reference, is hereby claimed. Priority of US Patent Application Serial No. 09/571,789, filed 16 May 2000, incorporated herein by reference, is hereby claimed. In the US, this is a continuation of US Patent Application Serial No. 09/571 ,789, filed 16 May 2000. STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT The inventions described herein were made in the performance of work under Lockheed Martin Michoud Space Systems IRAD (Internal Research and Development). REFERENCE TO A "MICROFICHE APPENDIX" Not applicable BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the manufacture of aircraft, space vehicles and the like wherein panels are connected using friction stir welding. More particularly, the present invention relates to the construction of aircraft, space vehicles and the like SUBSTITUTE SHEET (RULE 2β> wherein an improved method enables stringer stiffened panels to be joined with friction stir lap welds, in replacement of traditional riveting practice. 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 US Patent No. 5,460,317, incorporated herein by reference. Compared with traditional fusion welding processes, it offers simplified processing, improved mechanical properties, diminished weld defect formation, equivalent corrosion resistance, and reduced distortion, shrinkage, and residual stresses. Using conventional milling equipment with a backside anvil support, a non-consumable, cylindrical pin tool is rotated and plunged into the butt or lap joint of the material to be welded. Pin tools are specifically designed for a given alloy and gauge. Also incorporated herein by reference are US Patent No.5,718,366 and all references disclosed therein. The following additional references of possible interest are incorporated herein by reference: US Patent 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; and German Patent No. 447,084. Publications that discuss friction stir welding include "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. A publication authored by applicants is entitled "Friction Stir Welding as a Rivet Replacement Technology"; Brian Dracup and William Arbegast; SAE Aerofast Conference, October 5, 1999. BRIEF SUMMARY OF THE INVENTION The present invention provides an improved method of constructing structures such as aircraft using friction stir welding, to thereby replaces the traditional riveting practice of previously unweldable aluminum alloys. Friction Stir Welding is a Solid State joining process that now allows the welding of previously unweldable aluminum alloys. Traditionally, these aluminum alloys have found use only in mechanically joined structures such as in aircraft and space vehicles. The present invention provides a method of joining overlapped panels using friction stir welding, replacing the traditional riveting practice. The method of the present invention is a viable, and cost reducing alternative to aluminum riveted structures. The present invention features a non-consumable friction stir weld pin tool (see figures 1- 4) that is preferably constructed of HI 3 tool steel. The tool is rotated, plunged, and traversed along the stringer flanges of a stringer-skin panel to produce a friction stir lap weld. The tool is preferably tilted at an angle of about 2 l degrees. The present invention discloses a method and apparatus that uses a friction stir welding tool that fully penetrates through the top sheet and partially penetrates into the bottom sheet. The material around the pin tool is frictionally heated, plasticized, and extruded/forged to the back of the pin where it consolidates and cools under hydrostatic pressure conditions. Friction stir lap welding stringer-skin panels will eliminate the inter-rivet buckling commonly seen on mechanically joined structures and consequently, increase the buckling strength of the vehicle. In addition, the present invention enables simpler processing as compared with the traditional riveting practice by replacing any touch labor required with an automated process. Eliminating the rivets and other associated parts will also reduce quality control and material handling issues. Consequently, friction stir welding will increase production build rates and reduce production costs. Overall vehicle weight will also be decreased by eliminating the rivets and their associated parts. BRIEF DESCRIPTION OF THE DRAWINGS For a further understanding of the nature, objects, and advantages of the present invention, reference should be had to the following detailed description, read in conjunction with the following drawings, wherein like reference numerals denote like elements and wherein: Figure 1 is a side view of the pin tool portion of the preferred embodiment of the apparatus of the present invention that is used in the method of the present invention; Figure 2 is an end view of the pin tool of figure 1 , taken along lines 2-2 of Figure l; Figure 3 is a side, partial section view of the pin tool of figure 1 , taken along lines 3-3 of Figure 1 ; Figure 4 is a fragmentary, enlarged section view of the pin tool of figure 1; Figure 5 is a perspective view of the preferred embodiment of the apparatus of the present invention; Figure 6 is a sectional view taken along lines 6-6 of Figure 5; Figure 7 is a sectional view taken along lines 7-7 of Figure 5; Figure 8 is a perspective view of the preferred embodiment of the apparatus of the present invention; and Figure 9 is a schematic top view illustrating the method of the present invention. DETAILED DESCRIPTION OF THE INVENTION In figures 1- 4, the pin tool that is used as part of the method of the present invention is shown in detail, and designated generally by the numeral 10 in figure 1. Pin tool 10 provides a first end portion 11, a second end portion 12, and a middle section. The middle section includes frustoconical shaped section 14 and cylindrical larger diameter section 13. Tool 10 has a central longitudinal axis 24. End portion 12 provides a smaller diameter cylindrical section 20 and a frustoconical section 21. The lower smaller diameter cylindrical portion 20 can include a plurality of angular grooves 15 as shown in figure 3. End portion 12 provides a tip 16 that actually penetrates the portions of the materials that are to be stir welded together. The tip 16 can be a generally cylindrical shaped portion 17 and dished end 18 having convex surface 19 (see figure 4). An annular shoulder 22 can surround tip 16 extending radially between the cylindrical section 17 and frustoconical section 21 as shown in figures 3 and 4. The annular shoulder 22 can form an angle 23 of about 8 degrees with a line that is perpendicular to the central longitudinal axis 24 of the pin tool 10. Thus, the annular shoulder defines an annular cavity 25. The generally cylindrically shaped portion 17 can have an external thread 26 as shown in Figures 1 , 3 and 4. This external thread 26 helps produce a uniform stir weld. The concavity 25 enables some material to flow from the parts being welded into cavity 25 rather than laterally away from the weld site. The threads act to push the material down against the backside anvil. The cavity acts as a temporary reservoir that holds plastic metal that has been displaced by the volume of the pin (16). Figures 5-9 show the method and apparatus of the present invention during the lap welding of two stringer panels 28, 29. It should be understood that the method and apparatus of the present invention can be used with any type of lap weld wherein there is full penetration through a top sheet and partial penetration through a bottom sheet. That top sheet can be of any geometry, including flat. In the example drawing of Figures 5 and 6, the top sheet has a "hat stringer" shape. The bottom sheet can be of any geometry, including flat. In the drawings shown such as in Figures 5, 6, 7 and 8, the bottom sheet is a flat sheet and can be called a "skin". In the embodiment shown in Figures 5-9, the method and apparatus of the present invention illustrates a "hat stringer" being joined to a "skin". The resulting panel can be referred to as a "stringer- stiffened panel". In airplane manufacturing for example, the method and apparatus of the present invention would almost solely deal with two flat panels as opposed to the hat stringer and skin illustrations of Figures 5-9. In Figure 5, a welding machine or welding head 27 is shown gripping pin tool 10 at end portion 11. The opposing end portion 12 of pin tool 11 engages each of the stringer panels 28, 29 as shown in Figures 5 and 6. The stringer 29 can be generally flat. The stringer 28 can include one or more inclined portions 30 and flat flange portions 31 as shown in Figure 6. The stir welding machine 27 rotates the pin tool 10 as shown in Figure 9, indicated generally by the arrow 35. The numeral 34 shows the position of the pin tool 10 in hard lines. In Figure 6, curved arrow 35 shows rotational motion that is transmitted from the stir welding machine 27 to the pin tool 10. In addition to rotation that is imparted to the pin tool 10 from the stir welding machine 27, linear motion is also transferred from the stir welding machine 27 to the pin tool 10. In Figures 5, 8 and 9, this linear motion is indicated schematically by the numeral 32. In Figure 7, a weld 30 is shown penetrating an upper stringer panel 28 and part of a lower stringer panel 29. In Figure 7, the weld 33 penetrates the flange 31 portion of stringer panel 28 and a majority of the thickness of the skin panel 29. The following is a list of parts and materials suitable for use in the present invention: PARTS LIST Part Number Description 10 pin tool 11 end portion 12 end portion 13 large cylindrical section 14 large frustoconical section 15 groove 16 tip 17 cylindrical section 18 dished end 19 convex surface 20 small cylindrical section 21 small frustoconical section 22 annular shoulder 23 angle 24 central longitudinal axis 25 annular cavity 26 external thread 27 welding machine 28 stringer panel 29 stringer panel 30 inclined portion 31 flange 32 arrow 33 weld 34 pin tool position 35 arrow All measurements disclosed herein are at standard temperature and pressure, at sea level on Earth, unless indicated otherwise. The foregoing embodiments are presented by way of example only; the scope resent invention is to be limited only by the following claims.

Claims: CLAIMS 1. A method of joining aluminum alloy panels, comprising the steps of : a) preliminary positioning a top panel and a bottom panel such that the top panel overlaps the bottom panel; b) providing a friction stir weld pin tool; c) fully penetrating the pin tool through one of the panels and at least partially into the other panel; and d) wherein in step "c" the panel material around the pin tool is frictionally heated plasticized, and joined. 2. The method of Claim 1 wherein the friction stir weld takes place in either the vertical or horizontal orientations. 3. The method of Claim 1 wherein the panel that is fully penetrated is of any geometry including flat. 4. The method of Claim 1 wherein the panel that is partially penetrated is of any geometry including flat. 5. The method of Claim 2 further comprising the step of allowing the panel material to cool and consolidate under hydraulic pressure. 6. The method of Claim 1 further comprising the step of constructing an air frame of a plurality of panels wherein at least a plurality of the panels are connected together at least in part by using steps "a" through "d". 7. A method of joining aluminum alloy panels comprising the steps of: a) preliminary positioning two panels next to each other so that they overlap; b) providing a friction stir weld pin tool; c) penetrating the pin tool through one of the panels and at least partially d) wherein in step "c" the panel material around the pin tool is frictionally heated until the panels are lap welded together. 8. The method of Claim 7 further comprising the step of constructing an air frame of a plurality of panels wherein at least a plurality of the panels are connected together at least in part by using steps "a" through "d". 9. An air frame comprising: (a) a plurality of connected panels of aluminum alloy; and (b) a plurality of lap welds that connect the panels together with a friction stir welding tool that fully penetrates one panel and partially penetrates another panel, at least a plurality of said lap welds being friction stir welds. 10. The air frame of Claim 9 wherein at least some of the panels are stringer panels. 11. The air frame of Claim 9 wherein at least some of the panels are flat panels. 12. The air frames of Claim 9 wherein the air frame panels are aluminum lithium alloy panels. 13. The air frame of Claim 9 wherein the welds are lap shear joints. 14. The air frame of Claim 9 wherein some of the panels are stringer stiffened panels. 15. The method of Claim 8 further comprising the step of constructing fuel tanks of a plurality of panels wherein at least a plurality of the panels are connected together at least in part by using steps "a" through "d". 16. A fuel tank apparatus comprising: (a) a plurality of connected panels of aluminum alloy; and (b) a plurality of lap welds that connect the panels together with a friction stir welding tool that fully penetrates one panel and partially penetrates another panel, at least a plurality of said lap welds being friction stir welds. 17. The fuel tank of Claim 16 wherein at least some of the panels are stringer panels. 18. The fuel tanks of Claim 16 wherein at least some of the panels are flat panels. 19. The fuel tanks of Claim 16 wherein the fuel tanks panels are aluminum lithium alloy panels. 20. The fuel tanks of Claim 16 wherein the welds are lap shear joints. 21. The fuel tanks of Claim 16 wherein some of the panels are stringer stiffened panels. 22. The inventions substantially as shown and described herein.

Inventors: ARBEGAST WILLIAM J, DRACUP BRIAN J, ARBEGAST, WILLIAM, J., DRACUP, BRIAN, J.
Applicants: ARBEGAST WILLIAM J, DRACUP BRIAN J, LOCKHEED CORP, ARBEGAST, WILLIAM, J., DRACUP, BRIAN, J., LOCKHEED MARTIN CORPORATION
Application year: 2000
Main classification: B23K20
IPC codes: B23K20