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

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of U.S. patent application Ser. No. 09/571,789, filed May 16, 2000 now abandoned and incorporated herein by reference. 
     Priority of U.S. Provisional Patent Application Ser. No. 60/152,770, filed Sep. 3, 1999, incorporated herein by reference, is hereby claimed. 
    
    
     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 wherein an improved method enables stringer stiffened panels to be joined with friction stir lap welds, in replacement of the 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 U.S. Pat. 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 U.S. Pat. No. 5,718,366 and all references disclosed therein. The following additional references of possible interest are 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; 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&#39;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 &#39;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, Oct. 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 replace the traditional riveting practice of previously unweldable aluminum alloys. Possible applications include the intertank of the External Tank of the Space Shuttle and airplane manufacturing. 
     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 FIGS. 1-4) that is preferably constructed of H13 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 2½ degrees. 
     As the pin tool initially plunges into the weld jointline, the material is frictionally heated and plasticized at a temperature below that of the alloy&#39;s melting temperature and typically within the material&#39;s forging temperature range. When the metal becomes sufficiently soft and plastic, and the appropriate penetration depth has been reached, the tool is traversed along the weld line. As the tool is traversing, metal flows to the back of the pin tool where it is extruded/forged behind the tool. It then consolidates and cools under hydrostatic pressure conditions [2-8]. 
     Unlike fusion welding processes in which there are numerous inputs to the welding schedule, friction stir welding requires only three: spindle speed (RPM), travel speed (IPM), and the penetration depth of the tool in the material (heel plunge or penetration ligament). Penetration depth can be monitored either through load control or displacement [9]. 
     The present invention thus 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: 
     FIG. 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; 
     FIG. 2 is an end view of the pin tool of FIG. 1, taken along lines  2 — 2  of FIG. 1; 
     FIG. 3 is a side, partial section view of the pin tool of FIG. 1, taken along lines  3 — 3  of FIG. 1; 
     FIG. 4 is a fragmentary, enlarged section view of the pin tool of FIG. 1; 
     FIG. 5 is a perspective view of the preferred embodiment of the apparatus of the present invention; 
     FIG. 6 is a sectional view taken along lines  6 — 6  of FIG. 5; 
     FIG. 7 is a sectional view taken along lines  7 — 7  of FIG. 5; 
     FIG. 8 is a perspective view of the preferred embodiment of the apparatus of the present invention; 
     FIG. 9 is a schematic top view illustrating the method of the present invention; 
     FIG. 10 is a schematic view showing typical microstructure of a full penetration friction stir weld in 0320″ 2195-T8 alloy; 
     FIG. 11 is an illustration of typical microstructure of a partial penetration friction stir lap weld with no faying surface intermixing in 0.160″ 2024-T83 alloy; 
     FIG. 12 is a graphical representation of mechanical properties of friction stir welding vs. fusion welds; 
     FIG. 13 is an illustration of a friction stir weld with tunnel defect; 
     FIG. 14 is an illustration of a friction stir weld with a lack of penetration defect; 
     FIG. 15 is an illustration of a friction stir lap weld with cold lap defect in 2024-T83 alloy; 
     FIG. 16 is a graphical illustration showing shear strength in relation to processing parameters for 2090-T83 alloy friction stir lap welds; 
     FIG. 17 is a schematic view showing scalloped fracture surface in 2090-T83 alloy friction stir weld; 
     FIG. 18 is a schematic view showing sheared fracture surface in 2090-T83 alloy friction stir weld; 
     FIG. 19 is a schematic illustration showing the metallurgy of stringer-stiffened friction stir welded 2090-T83 alloy panels; 
     FIG. 20 is a graphical illustration showing compression buckling results for each strain gauge showing initial buckling and ultimate failure load (shown are results obtained from friction stir weld panel and riveted panel); 
     FIG. 21 is a schematic view showing initial buckling on a riveted panel; 
     FIG. 22 is a schematic illustration showing compression failure of a riveted panel; 
     FIG. 23 is a schematic view showing initial buckling of a friction stir welded panel; and 
     FIG. 24 is a schematic view showing a compression failure of a friction stir welded panel. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIGS. 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 FIG.  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 FIG.  3 . During use, the pin tool  10  is rotated, plunged, and traversed along the stringer flanges of a stringer-skin panel to produce a friction stir lap weld. The tool  10  is preferably tilted at an angle of about 2½ degrees. The tool  10  fully penetrates through the stringer and partially penetrates into the skin. The material around the pin tool  10  is frictionally heated, plasticized, and extruded/forged to the back of the pin where it consolidates and cools under hydrostatic pressure conditions. 
     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 FIG.  4 ). An annular shoulder  22  can surround tip  16  extending radially between the cylindrical section  17  and frustoconical section  21  as shown in FIGS. 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  22  defines an annular cavity  25 . 
     The generally cylindrically shaped portion  17  can have an external thread  26  as shown in FIGS. 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  26  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 ). 
     FIGS. 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 FIGS. 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 FIGS. 5,  6 ,  7  and  8 , the bottom sheet is a flat sheet and can be called a “skin”. In the embodiment shown in FIGS. 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 FIGS. 5-9. In FIG. 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 FIGS. 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 FIG.  6 . 
     The stir welding machine  27  rotates the pin tool  10  as shown in FIG. 9, indicated generally by the arrow  35 . The numeral  34  shows the position of the pin tool  10  in hard lines. In FIG. 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 FIGS. 5,  8  and  9 , this linear motion is indicated schematically by the numeral  32 . 
     In FIG. 7, a weld  30  is shown penetrating an upper stringer panel  28  and part of a lower stringer panel  29 . In FIG. 7, the weld  33  penetrates the flange  31  portion of stringer panel  28  and a majority of the thickness of the skin panel  29 . 
     As the pin tool initially plunges into the weld jointline, the material is frictionally heated and plasticized at a temperature below that of the alloy&#39;s melting temperature and typically within the material&#39;s forging temperature range. When the metal becomes sufficiently soft and plastic, and the appropriate penetration depth has been reached, the tool is traversed along the weld line. As the tool is traversing, metal flows to the back of the pin tool where it is extruded/forged behind the tool. It then consolidates and cools under hydrostatic pressure conditions. 
     Unlike fusion welding processes in which there are numerous inputs to the welding schedule, friction stir welding requires only three: spindle speed (RPM), travel speed (e.g., inches per minute or IPM), and the penetration depth of the tool in the material (heel plunge or penetration ligament). Penetration depth can be monitored either through load control or displacement. 
     As shown in FIG. 10, several distinct metallurgical zones have been identified for both full penetration butt welds and partial penetration lap welds. Within the weld nugget, there exists a dynamically recrystallized microstructure consisting of fine, equiaxed grains on the order of 3-6 microns in size. Closer to the surface is a re-heated dynamically recrystallized zone (flow arm) where the trailing edge of the pin tool&#39;s shoulder drags parent material from the retreating side toward the advancing side after the pin tip has passed through. Further away from the weld jointline, there is insufficient heating and strain energy to cause complete recrystallization of the grains. This thermal-mechanical zone (TMZ) shows some degree of plastic deformation and grain boundary coarsening. The heat affected zone (HAZ) separates the TMZ from the parent metal. 
     The basic metallurgy in friction stir lap welds is similar to full penetration butt welds. A dynamically recrystallized zone, a thermal mechanical zone, a re-heated surface zone (flow arm), and a heat affected zone are all apparent in lap welds (FIG.  11 ). Of particular interest is the path of the interlayer&#39;s faying surface through the weld nugget. 
     Friction Stir Welding offers several mechanical benefits over its fusion counterparts (FIG.  12 ). Both ultimate tensile strengths and yield strengths are significantly higher over a broad range of temperatures and thicknesses. In addition, friction stir welds show improved fatigue and fracture properties over VPPA/SPA welded plate. Like wrought aluminum products, friction stir welds experience an increase in elasticity and strength with decreasing testing temperature. There is also a marked increase in ductility as compared with fusion welding. 
     Weld preparation and cleanliness are much less stringent than that required for fusion welding. A simple Scotchbrite® rub of the area to be welded, coupled with an alcohol wipe and deburring of the root side is sufficient to produce quality welds. In addition, friction stir welding is a very robust process with a large operating parameter box. 
     There are two types of defects that may occur in butt joints when welds are processed far from normal operating conditions. The first characteristic is a “wormhole” or “tunnel” that runs in the longitudinal direction through the length of the weld (FIG.  13 ). This defect occurs when there is insufficient forging pressure under the tool shoulder, preventing the material from consolidating. This is normally caused by too quick a welding speed. This defect is readily detectable through radiographic inspection. The second type of defect is a “root lack of penetration” or “root lack of fusion” that occurs when the dynamically recrystallized zone fails to penetrate fully to the bottom surface of the joint (FIG.  14 ). It can be caused by inadequate tool penetration, insufficient heat and pressure, or improper pin tool geometry. This defect can have an effect on mechanical properties and may be difficult to detect through conventional non-destructive examination techniques. Root lack of penetration is not a concern with partial penetration lap welds. 
     A third type of defect that has been identified specifically for friction stir lap welds can exist when the faying surface between the two sheets becomes stirred into the weld, producing a cold lap defect (FIG.  15 ). Hot parameters, produced by a relatively fast spindle speed and low travel speed, are required to break up this faying surface and prevent a reduction in the weld&#39;s effective thickness. However, one must take caution in not producing too hot a weld, characterized by excessive flash and a powdery surface finish (galling). Alternative pin tool designs are being investigated in an attempt to break up the faying surface within the nugget while operating under colder parameters. 
     Successful butt and lap friction stir welds have been made at Lockheed Martin Michoud Space Systems (LMMSS) in multiple aluminum alloys including: 2014, 2024, 2219, AFC-458, 2090, 2195, 5083, 6061, 7050, 7075, and 7475. Table 1 shows the average strengths of the welds for some of these alloys. Large grain size extrusion, metal matrix composites (Al—SiC, Al—Al2O3), and dissimilar metal (2219 to 2195) friction stir welds have also been produced that exhibit good strength and quality. Various thicknesses have been joined ranging from 0.063″ 2024-T3 sheet to 2″ thick 6082-T6 plate welded in a single pass and 3″ thick 6082-T6 plate welded in a double pass. Welds up to 43 feet long have been successfully joined with no weld defects and no tool wear. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Friction stir butt weld joint efficiencies for various aluminum alloys 
               
             
          
           
               
                   
                   
                 Parent 
                 Friction Stir 
                 Joint 
               
               
                   
                 Alloy 
                 Metal UTS 
                 Weld UTS 
                 Efficiency 
               
               
                   
                   
               
               
                   
                 AFC458-T8 
                 79.0 [18] 
                 52.5 
                 66% 
               
               
                   
                 2014-T651 
                 70.0 [19] 
                 49.0 
                 70% 
               
               
                   
                 2024-T351 
                 70.0 [18] 
                 63.0 
                 90% 
               
               
                   
                 2219-T87 
                 69.0 [19] 
                 45.0 
                 65% 
               
               
                   
                 2195-T8 
                 86.0 [18] 
                 59.0 
                 69% 
               
               
                   
                 5083-O 
                 42.0 [19] 
                 43.0 
                 102%  
               
               
                   
                 6061-T6 
                 47.0 [18] 
                 31.5 
                 67% 
               
               
                   
                 7050-T7451 
                 79.0 [18] 
                 64.0 
                 81% 
               
               
                   
                 7075-T7351 
                 68.5 [18] 
                 66.0 
                 96% 
               
               
                   
                   
               
             
          
         
       
     
     Prior shear testing was performed at LMMSS on 2090 lap shear joints mechanically joined with {fraction (3/16)}″ diameter 2017 solid “icebox” rivets. Test specimens were manufactured and tested in accordance with MIL-STD-1312-4. Two different sheet thicknesses were tested (t=0.063″ and 0.083″) at room temperature. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Lap shear strengths for 2090-T83 sheet joined 
               
               
                 with a 2017 “icebox” aluminum rivet 
               
             
          
           
               
                 2090 Sheet 
                 Avg. Strength 
                 St. Dev. 
               
               
                   
               
               
                 0.063″ to 0.063″ 
                 1083 
                 46.3 
               
               
                 0.080″ to 0.080″ 
                 1056 
                  8.1 
               
               
                   
               
             
          
         
       
     
     Lap shear results on 2090-T83 sheet mechanically joined with {fraction (3/16)}″ diameter 2017 aluminum rivets showed little variation between the 0.063″ and 0.080″ sheet thicknesses (Table 2). Consequently, it can be presumed that the test was appropriately determining the shear strength of only the rivet. Failure occurred by shear through the rivet. 
     EXAMPLE 1 
     2090-T83 is an Al—Cu—Li alloy that has been solution heat treated, cold worked and artificially aged. The specified chemical composition and general mechanical properties are given in Tables 3 and 4. 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Chemical composition for 2090-T83 aluminum alloy (wt %) [20] 
               
             
          
           
               
                 Alloy 
                 Cu 
                 Fe 
                 Li 
                 Mg 
                 Mn 
                 Si 
                 Ti 
                 Zn 
                 Zr 
                 Others 
                 Al 
               
               
                   
               
               
                 2090-T83 
                 2.4-3.0 
                 .12 
                 1.9-2.6 
                 .25 
                 .05 
                 .10 
                 .15 
                 .10 
                 .08-.15 
                 .20 
                 Rem 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 As tested parent metal properties for 2090-T83 
               
             
          
           
               
                 Sheet Thickness (in.) 
                 Grain Direction 
                 UTS (ksi) 
                 YS (ksi) 
                 % EL (2″) 
               
               
                   
               
               
                 0.060 
                 Longitudinal 
                 86.5 
                 78.9 
                 6.0 
               
               
                 0.082 
                 Longitudinal 
                 85.8 
                 78.0 
                 6.0 
               
               
                   
               
             
          
         
       
     
     Friction stir lap welds were produced on flat panels of 2090-T83 with the top and bottom sheets having thicknesses of 0.063″ and 0.083″, respectively. The welds fully penetrated through the top sheet and partially penetrated the bottom sheet. They were done at various spindle and travel speeds in an attempt to achieve the highest weld quality as determined through lap shear strength, metallurgy and non-destructive evaluation. Welds were examined for internal defects using radiography to Grade 1 requirements for manned flight. Of particular interest was the path of the panels&#39; interlayer faying surface. 
     Lap welded panels were cut for metallographic examination and mechanical shear testing. Shear samples were tested at room temperature on a 20 kip MTS testing machine at a constant cross head deflection of 0.05 in/min. In contrast to the riveted lap shear specimens, the sheet thicknesses of the friction stir welded lap joint varied. Consequently, although the specimens were pin loaded, they were additionally held by offset friction grips to account for the difference in top and bottom sheet thicknesses to ensure loading through the specimen interlayer. 
     Once desirable friction stir weld parameters had been set, a 21″ long, 0.063″ thick 2090-T83 stringer  28  having a cross-section shown in FIGS. 5-6, was lap welded to a 0.083″ 2090-T83 skin sheet  29 . Welds were performed on a conventional mill with a steel backing anvil and traditional finger clamps. A specially designed pin tool  10  (see FIGS. 1-4) was contoured to accommodate the specific geometry of the stringer  28 . A duplicate test set was created by mechanically joining the stringer  28  to the skin  29  with {fraction (3/16)}″ diameter 2017 solid “icebox” rivets spaced 1″ apart. 
     Two friction stir welded and two riveted compression buckling samples (L/p=11.4) were produced, all identical except for their joining method. Both friction stir welded panels were non-destructively examined using ultrasonic inspection and radiography. These four panels were prepared for compression testing by potting their ends with Hysol Epoxy EA9394. The compression tests were performed at room temperature on a 200 kip MTS testing machine at a constant cross head deflection rate of 0.05 inches per minute. 
     Larger scale panels having 5 stringers across their width and having dimensions of 60.05″×33.2″ are being fabricated. One riveted and one friction stir welded panel will be compression tested at NASA Langley Research Center (LaRC) for comparison. 
     Lap Shear Results 
     Friction stir lap welds on 2090-T83 were done at various spindle speeds, travel speeds, and heel plunges. Metallurgical examination of the weld revealed remnant interlayer faying surface across the width of the nugget in all welds. The extent at which the faying surface remained across the weld nugget varied from weld to weld, and is currently being quantified to compare its effect on shear strength results. 
     In general, higher strengths were found on welds produced at faster spindle speeds (Table 5). An increase in pin tool rotation enhances the break up of the interlayer surface within the weld nugget. Furthermore, as shown in FIG. 16, for a given rotation speed, the faster travel speed also produced the stronger weld. Lengthening the time at temperature experienced by the panel, resulting from a slower travel speed, increases the heat input into the weld. This additional heat input may cause a softening of the weld zone and consequently, lower shear strengths. Within this experiment, the weld that produced the highest shear strength was that which was processed at the highest spindle speed and the highest travel speed of all those fabricated (970 RPM and 9 inches per minute). Although the friction stir lap welds possessed cold lap defects, they still had shear strengths approximately 50%-100% higher on average than that of the mechanically joined specimens (Table 2). Future work will include testing friction stir welds processed at even higher spindle and travel speeds to produce defect free welds and determine the upper limit of shear strength. 
     
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Processing parameters and strengths for 2090-T83 friction stir lap welds 
               
             
          
           
               
                   
                   
                   
                 Weld Pitch 
                 Heel Plunge 
                 Peak Load Avg. 
                 St. Dev. 
                 Typical Fracture 
               
               
                 Weld No. 
                 RPM 
                 IPM 
                 (RPM/IPM) 
                 (in) 
                 (lbs/in) 
                 (lbs/in) 
                 Location 
               
               
                   
               
             
          
           
               
                 3 
                 440 
                 4.5 
                 98 
                 0.010 
                 1277 
                 37.1 
                 Interface Shear 
               
               
                 10  
                 645 
                 5.25 
                 123  
                 0.007 
                 1441 
                 7.0 
                 0.080″ Sheet 
               
               
                 6 
                 542 
                 6.5 
                 83 
                 0.006 
                 1517 
                 27.5 
                 Interface Shear 
               
               
                 5 
                 542 
                 6.5 
                 83 
                 0.010 
                 1524 
                 59.5 
                 Interface Shear 
               
               
                 8 
                 815 
                 7.5 
                 109  
                 0.007 
                 1529 
                 41.4 
                 0.080″ Sheet 
               
               
                 4 
                 542 
                 5.25 
                 103  
                 0.010 
                 1581 
                 30.7 
                 0.080″ Sheet 
               
               
                 2 
                 542 
                 4.5 
                 120  
                 0.010 
                 1610 
                 108.3 
                 0.080″ Sheet 
               
               
                 1 
                 542 
                 3.75 
                 145  
                 0.010 
                 1619 
                 20.7 
                 0.080″ Sheet 
               
               
                 7 
                 645 
                 6.5 
                 99 
                 0.007 
                 1624 
                 131.0 
                 0.080″ Sheet 
               
               
                 11  
                 970 
                 7.5 
                 129  
                 0.007 
                 1668 
                 15.2 
                 0.080″ Sheet 
               
               
                 9 
                 815 
                 9 
                 91 
                 0.007 
                 1909 
                 38.2 
                 0.080″ Sheet 
               
               
                 12  
                 970 
                 9 
                 108  
                 0.007 
                 2175 
                 89.2 
                 0.080″ Sheet 
               
               
                   
               
             
          
         
       
     
     Two types of fracture surfaces were seen in the shear specimens. In the first type of failure, the friction stir weld shears at the interlayer between the 2090 sheets, with neither of the sheets ultimately failing. The interlayer is strongly present within the nugget, and consequently, the weld is of poor quality with only a weak diffusion bond occurring between the two sheets. The fracture surface reveals a scalloped pattern and the resultant shear strengths are low (FIG.  17 ). 
     In the second type of failure, fracture initiation occurs within the thicker, bottom sheet where the weld nugget, TMZ, and faying surface all intersect. The fracture traverses along this faying surface and up to the interlayer. The true minimum thickness of the specimen is now the distance from the faying surface to the bottom edge of the 0.083″ sheet. Consequently, fracture also moves down from the initiation point and through the thickness of this sheet, where the specimen ultimately fails (FIG.  18 ). 
     Compression Buckling Results 
     Stringer stiffened panels of 2090-T83 were successfully fabricated using friction stir lap welding in place of traditional riveting methods. Visual inspection of the weld showed good bonding, limited flash, and a smooth rippled surface with no galling or excess heat. All welds passed both ultrasonic and radiographic inspection. Metallurgic examination showed that the majority of the interlayer&#39;s faying surface was consumed by the fast rotation action of the pin tool at the weld nugget (FIG.  19 ). 
     Prior to testing, predicted crippling compression loads for both the riveted and friction stir welded panels were obtained using two different techniques that were based upon specimen geometry (Table 6). It was calculated that ultimate failure for both types of panels would occur at similar loads. 
     From the data for each strain gauge, a straight line was drawn along the elastic region of the compression test. Initial buckling was determined to occur at the tangential point of this line with the data, i.e. when inelastic deformation began (FIG.  20 ). 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 6 
               
               
                   
               
               
                 Predicted failure loads for compression buckling tests 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Ultimate Compression = 32.3 kips 
                 (Johnson-Euler 
               
               
                   
                 Load (Pc) 
                 method) 
               
               
                   
                 Ultimate Compression = 32.7 kips 
                 (Gerard Crippling 
               
               
                   
                 Load (Pc) 
                 method) 
               
               
                   
                 Panel Deflection (d @ Pc) = 0.078 in. 
               
               
                   
                   
               
             
          
         
       
     
     Although the riveted specimens ultimately failed at a slightly higher load on average then the friction stir welded panels, initial buckling first occurred in the riveted specimens at approximately 16% (3700 lbs.) lower than the welded panels as determined for each strain gauge. Results are shown in Tables 7 and 8. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 7 
               
             
             
               
                   
               
               
                 Initial buckling loads for FSW Panel 1 and Riveted Panel 1 
               
             
          
           
               
                 Strain Gauge 
                 FSW (kips) 
                 Riveted (kips) 
               
               
                   
               
               
                 1 
                 26.1 
                 24.3 
               
               
                 2 
                 26.8 
                 22.7 
               
               
                 3 
                 25.1 
                 19.8 
               
               
                 4 
                 26.6 
                 23.1 
               
               
                 Average 
                 26.2 
                 22.5 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 8 
               
             
             
               
                   
               
               
                 Failure loads and deflections for FSW and Riveted Panels 
               
             
          
           
               
                   
                   
                 Ultimate Compression 
                 Panel Deflection 
               
               
                   
                 Panel 
                 Load (Pc) - kips 
                 (d @ Pc) - in. 
               
               
                   
                   
               
               
                   
                 Friction Stir Weld 1 
                 30.2 
                 0.092 
               
               
                   
                 Friction Stir Weld 2 
                 29.5 
                 0.084 
               
               
                   
                 FSW Avg. 
                 29.9 
                 0.088 
               
               
                   
                 Riveted 1 
                 32.1 
                 0.110 
               
               
                   
                 Riveted 2 
                 32.7 
                 0.095 
               
               
                   
                 Riveted Avg. 
                 32.4 
                 0.103 
               
               
                   
                   
               
             
          
         
       
     
     Both riveted panels failed in the same manner. Initial buckling occurred in the skin panel (FIG. 21) and as the buckling continued, the energy was transferred directly into the riveted flanges. As shown in FIG. 22, failure ultimately occurred along the stringer flange, near the row of rivets. Both friction stir welded panels also failed in a similar manner but unlike the riveted panels. Here, the specimen is a more isotropic configuration by virtue of the continuous weld, and consequently the panel acted more as a single “cross-section”. As the skin underwent initial buckling (FIG.  23 ), the load was transmitted to the overall cross-section causing the outer part of the stringer to fail in a combined bending and axial compression mode (FIG.  24 ). 
     The welded panel was less stiff at the flange/weld location than the riveted specimen. Since loads seek the stiffest path, more load was transmitted to the riveted flange at a faster rate than the welded specimen. The stringer wall on the riveted panel was not strong enough to distribute the load any further and ultimately failed at the row of rivets in the bend radius of the stringer flange. The welded panel attracted a somewhat lesser load at a slower rate allowing the stringer wall to transfer the stress to the location furthest from the neutral axis. Consequently, once initial buckling of the skin plate and stringer flanges occurred, the overall specimen performed as a column in axial compression and bending, induced by lateral deflection. Results of the large scale panel compression tests are to be reported at a later date. 
     In Summary 2090-T83 sheet can be successfully joined using friction stir welding in place of traditional mechanical joining processes. High spindle speeds aid in the break up of the interlaying faying surface within the weld nugget. Low travel speeds appear to have a detrimental effect on weld shear strength. Favorable friction stir lap welded shear specimens of 2090-T83 thin sheet had maximum peak loads approximately 100% higher than mechanically joined 2090-T83 using a {fraction (3/16)}″ diameter 2017 solid rivet. Calculated predictions of failure load and associated deflections for compression buckling tests were nearly identical to the actual test values obtained. Small scale compression buckling tests performed on friction stir welded stringer-stiffened panels had initial buckling loads approximately 16% higher than identical panels mechanically joined with 2017 solid rivets spaced 1″ apart. The failure mechanism for these two types of panels varied. 
     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 
                 stinger 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. All materials used or intended to be used in a human being are biocompatible, unless indicated otherwise. 
     The foregoing embodiments are presented by way of example only; the scope of the present invention is to be limited only by the following claims.