Abstract:
A friction stir rivet is rotated and driven through a first fusible workpiece into an engaged second fusible workpiece, causing local portions of the first and second workpieces to plasticize. A slideable cap contacts the exposed surface of the first workpiece shortly after the process begins. The contact causes the cap to act as a retaining element, limiting the escape of plasticized material from stir site. Once the rivet is driven into the first and second workpieces, rotation ceases and the plasticized material hardens around the rivet. A weld is thus created, joining the workpieces and encompassing the rivet, which provides additional mechanical strength.

Description:
TECHNICAL FIELD 
     This invention relates to friction stir welding and riveting, more particularly, to methods of joining multiple workpieces using a stir rivet to create a mechanical weld, an interweld, and a diffusion bond. 
     BACKGROUND OF THE INVENTION 
     Friction stir welding (FSW) is a method used to join metal workpieces. The method generally uses a cylindrical, shouldered tool with a profiled pin that is rotated at the joint line between two workpieces while being traversed along the joint line. The rotary motion of the tool generates frictional heat which serves to soften and plasticize the workpieces. This softened material, contributed by both workpieces, intermingles and is consolidated by the pin shoulder. As the pin moves laterally the frictional heating is reduced and the softened material hardens, creating a bond between the two workpieces. The best current understanding of the process is that no melting occurs and the weld is left in a fine-grained, hot worked condition with no entrapped oxides or gas porosity. 
     Stir rods used in conventional FSW are typically symmetrical cylinders having an enlarged fixed cap located on their upper side. The fixed cap used in conventional FSW does not engage a workpiece until the end of tool insertion, allowing a majority of the initially plasticized material to be expelled from the cavity before the cap creates a seal around the worksite. Current methods used in FSW do not teach or suggest methods of engaging a cap and a workpiece at the beginning of the process to retain the maximum amount of plasticized material in the weld zone. 
     SUMMARY OF THE INVENTION 
     This invention is based on a newly developed method which we call friction stir riveting. This method improves friction stir welding by using a stir rivet having a slideable cap. The stir rivet is rotated and advanced into a pair of workpieces to plasticize material around the rivet for stir welding the workpieces together. Near the beginning of the process, the slideable cap contacts the first workpiece. The contact between the cap and the first workpiece creates a partial seal, limiting the amount of plasticized material displaced out of the stir site. The rivet is then left in place to form a weld between the rivet and the solidified material. 
     The present invention utilizes a friction stir rivet having a body including an elongated cylindrical section and upper and lower stops at opposite ends of the cylindrical section. The cylindrical section of the body extends through a cap. A spring may extend between the cap and the upper stop, or the cap and a driving apparatus. An interlocking guide extends longitudinally along a portion of the cylindrical section. The interlocking guide on the cylindrical section may be a flat surface. 
     The cap has a central opening surrounding the cylindrical section. The central opening of the cap has an interlocking guide compatible with an interlocking guide of the cylindrical section, which causes the cap to rotate with the body. The interlocking guide in the central opening of the cap may be a flat surface. Alternatively, a threaded surface may be used to form a guide between the cylindrical section and the opening of the cap. 
     The upper stop forms the head of the rivet and provides a physical barrier, which can be used to compress the spring against the slideable cap, biasing the cap toward the lower stop. If the upper stop is not used to compress the spring, a retainer located on a rotary drive apparatus can compress the spring against the cap, biasing the cap against the lower stop. The upper stop limits upward travel of the cap. 
     The lower stop limits downward travel of the cap. The underside of the lower stop forms a lower end of the rivet which contacts the workpieces to be joined. The lower stop may be applied or formed after the cap is slid over the cylindrical section of the rivet. Once the cap is on the cylindrical section of the body the lower stop can be created or applied in any suitable manner, such as peening the lower end of the cylindrical section, deforming the lower end, pinning the lower end to act as the stop or to secure separate stop member, or by enlarging the lower end of the rivet with extra material. 
     A recessed socket is centrally located on the upper portion of the upper stop and is aligned with the rotational axis of the rivet. To rotate the rivet, a rotational rotary device is inserted into the recessed socket of the rivet. 
     The rivet, when rotated, locally softens and penetrates the workpieces, creating a cavity filled with plasticized material. Shortly after the lower end of the rivet penetrates the first workpiece, the slideable cap contacts the first workpiece to create a seal around the stir site, thereby limiting the amount of plasticized material displaced out of the cavity, ensuring that the plasticized material fills the cavity, and promoting intimate contact between the rivet and the plasticized material 
     As the rivet advances into the workpieces, the cap slides up the cylindrical section of the rivet toward the upper stop, while the bias of the spring continues to press the cap against the first workpiece. 
     Upon reaching a desired depth, the rotary motion is stopped and the stir site is cooled to provide an internally welded joint maintained together partially by the shape of the rivet and partially by the welding of the components together. 
     Preferably, the cylindrical section of the rivet body has a smaller radial thickness than the lower stop to create a re-entrant portion along the cylindrical section. Alternatively, threads on the cylindrical section may be used to create re-entrant portions along the cylindrical section of the body. The re-entrant portion allows plasticized material to fill in above the lower stop, thereby, increasing the mechanical retention of the rivet in the workpieces. 
     The slideable cap limits oxygen access to the rivet during the stirring process by creating a seal between the rivet and the first workpiece. The reduced oxygen supply around the rivet reduces the formation of oxides on the body of the rivet. Reducing oxidation allows a better bond to form between the rivet and the workpieces. 
     The rivet should be formed of a relatively high melting point metal or refractory metal so that the rivet has a higher melting point than the workpieces to be joined. Preferably, the rivet should have a melting point that is at least 100° Fahrenheit higher and more preferably at least 200° Fahrenheit higher than workpieces, such as aluminum. Further, the rivet should be formed of a metal of substantially greater hardness than the metal workpieces to be joined. Exemplary metals include high carbon steel, titanium (e.g. titanium 6-4) and the like. Preferably, the rivet should be formed of a metal that is capable of forming a diffusion bond with the metal workpieces to be joined. 
     A driving apparatus is used to rotate and press the rivet into the metal workpieces to be joined. The rivet penetrates best when it is rotated at speeds between 4,500 and 27,000 revolutions per minute. The amount of pressure needed to allow the rivet to penetrate the metal workpiece depends upon the speed of rotation. The rate of penetration is increased when the amount of pressure applied is increased, or when the revolutions per minute are increased. Under good conditions, a friction stir rivet can penetrate aluminum at up to 27 millimeters per minute. 
     The foregoing description is directed, as an example, to joining aluminum metal workpieces with a stir rivet made of metal with a higher temperature melting point. However, it should be understood that other fusible materials may be joined using the same process with a proper selection of compatible materials. Thus, other metals and thermoplastics may also be successfully joined with a stirring rivet and process within the guidelines above described. 
     These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a side view of an exemplary embodiment of a friction stir rivet according to the invention; 
         FIG. 2  is a cross-sectional view from the line  2 — 2  of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view showing the friction stir rivet of  FIG. 1  at the conclusion of rotation during stir riveting of two workpieces together; 
         FIG. 4  is a cross-sectional view showing the combination of an alternative embodiment of friction stir rivet with associated rotary drive and biasing apparatus; and 
         FIG. 5  is a cross-sectional view showing the combination of  FIG. 4  at the conclusion of rotation of the rivet during stir riveting of two workpieces together. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to  FIGS. 1 and 2  of the drawings in detail, numeral  10  generally indicates a friction stir rivet  10 . Rivet  10  includes an elongated body  11  having a cylindrical section  12  with enlarged upper and lower stops  14 ,  16  at opposite ends of the cylindrical section  12 . The cylindrical section  12  extends through a cap  18  and a spring  20 . An interlocking guide  22  extends longitudinally along the cylindrical section  12 . Preferably, the interlocking guide of cylindrical section  12  is a flat surface. 
     Cap  18  has a generally round central opening  24  fitted over the cylindrical section  12 . The central opening  24  of the cap  18  has an interlocking guide  26  that mates with the interlocking guide  22  of the cylindrical section  12  and causes the cap  18  to rotate with the cylindrical section  12 . Preferably, the interlocking guide  26  of the cap  18  is a flat surface. 
     The upper stop  14  forms the head of the rivet  10  and provides a physical barrier which compresses the spring  20  against the slideable cap  18 , biasing the cap  18  toward the lower stop  16 . A recessed socket  28  is centrally located on an upper portion  30  of the upper stop  14  and is aligned with a rotational axis  32  of the rivet  10 . To rotate the rivet  10 , a driving apparatus is inserted into the recessed socket  28  of the rivet  10 . 
     Referring to  FIG. 3 , the rivet  10  is shown in use, forming an assembly  33  by stir riveting a first workpiece  34 , such as a fusible aluminum sheet or plate, to a second workpiece  36 , such as a fusible aluminum frame or other substrate. In operation, the rivet  10  is rotated around its rotational axis  32 . 
     During rotation, downward force is applied to the rivet  10  causing a lower surface  38  of the lower stop  16  to frictionally contact an exposed surface  40  of the first workpiece  34 . The downward force and rotation of the rivet  10  cause a portion of the first workpiece  34  to plasticize, allowing the rivet  10  to penetrate the workpiece  34  and create a cavity  42 . As the rivet  10  is driven through an unexposed surface  44  of the first workpiece  34 , rivet  10  frictionally contacts an unexposed surface  46  of the second workpiece  36 . The downward force and rotation of rivet  10  cause a portion of the second workpiece  36  to plasticize, allowing rivet  10  to continue penetrating cavity  42 . As the rivet  10  is driven through the first workpiece  34  into the second workpiece  36 , the plasticized material  48  in cavity  42  is intermixed. 
     Shortly after the lower surface  38  of the rivet  10  penetrates the first workpiece  34 , the underside  50  of the slideable cap  18  contacts the first workpiece  34  to create a seal around the stir site, thereby limiting the amount of plasticized material displaced out of the cavity  42 . As the rivet  10  advances into the workpieces  34 ,  36 , the cap  18  slides up the cylindrical section  12  of the rivet  10 , against the force of spring  20  which forces the cap  18  to press against the first workpiece  34 . The force of the cap  14  against the first workpiece  34  maintains the seal while the cap  18  travels up the cylindrical section  12  of the rivet  10 . The cap  18  acts as a retaining element, limiting the amount of plasticized material escaping throughout the process. 
     Upon reaching a desired depth, motion is stopped as shown in FIG.  3  and the stir site is cooled to harden the plasticized material and provide an internally welded joint. The resulting assembly  33  is then held together partially by the mechanical shape of the rivet  10  and partially by the welding of the workpieces  34 ,  36 , together with bonding to the rivet to form the assembly  33 . 
     Preferably, rivet  10  is driven though the first workpiece  34  and partially into the second workpiece  36  until the cap  18  of the rivet  10  is partially recessed into the exposed surface  40  of the first workpiece  34 . Thereafter, the rotary motion of rivet  10  is stopped, allowing locally plasticized material  48  to harden and form several welds. Rivet  10  forms a mechanical bond between the first workpiece  34  and the second workpiece  36 . Plasticized material  48  preferably forms a diffusion bond between the rivet  10  and the first and the second workpieces  34 ,  36 . Furthermore, the plasticized material  48  forms an interweld between the first workpiece  34  and the second workpiece  36 . 
     The cylindrical section  12  of the body  11  of rivet  10  has a smaller radial thickness than the lower stop  16 , to create a re-entrant section  52  along the cylindrical section  12 . When the rivet  10  is embedded into the workpieces  34 ,  36  the re-entrant section  52  extends from the lower stop  16  up to the underside  50  of the cap  18  when the cap  18  is compressed against the upper stop  14 . Allowing plasticized material  48  to fill in between the underside  50  of the cap  18  and the lower stop  16  of the rivet  10  increases the strength of the mechanical retention around the cylindrical section  12  of the rivet  10 . 
     During the process, the slideable cap  18  restricts oxygen access to the rivet  10  by creating a seal between the rivet  10  and the first workpiece  34 . The reduced oxygen supply around the rivet  10  reduces the formation of oxides on the cylindrical section  12  of the rivet  10 , which provides a clean surface to form a bond with the plasticized material  48 . Allowing formation of an oxide layer would interfere with bonding between the cylindrical section  12  and the plasticized material  48 . 
       FIGS. 4 and 5  show a combination  53  of an alternative embodiment of friction stir rivet  54  with an associated rotary drive and biasing apparatus  55 . Rivet  54  includes an elongated body  56  having a cylindrical section  58 , an enlarged upper stop  60  and a lower stop  62 . The upper stop  60  has an angled side  64 . A receiver, such as a recessed socket  66 , is located on the upper stop  60 . Threads  68 , having a long lead, extend longitudinally along the cylindrical section  58  of the body  56 . 
     A slideable cap  70  is carried on the threaded cylindrical section  58 . Cap  70  has a central opening  72  including threads  74  engaging the threads  68  of the cylindrical section  58 . The cap  70  has an angled side  76  mateable with the angled side  64  of the upper stop  60 . During operation, the cap  70  slides up the threads  68  of the cylindrical section  58 , rotating slightly until the cap  70  engages the upper stop  60 . 
     To rotate the rivet  54 , a driving apparatus  55 , including a driver  80  and a biasing device  82 , engages the rivet  54 . The biasing device  82  surrounds the driver  80  and includes a telescoping retainer  84 , housing a biasing spring  86 . 
     In operation, the driver  80  engages the receiver  66  located on the upper stop  60  of the rivet  54 , while the biasing device  82  urges the cap  70  towards the lower stop  62 . As the rivet  54  is driven into the workpieces  88  the cap  70  rotates slightly as it slides up the threads  68  of cylindrical section  58  of the rivet  54 , which compresses the biasing device  82  against the driver  80 . The biasing spring  86 , housed inside the retainer  84 , continuously urges the cap  70  toward the lower stop  62 , causing the cap  70  to maintain contact with the exposed surface  90  of the first workpiece  92 . Upon reaching a desired depth, motion is stopped as shown in FIG.  5 . Once rotational motion stops the driving apparatus  55 , the retainer  84 , and the spring  86  are disengaged from the rivet  54 , leaving the rivet  54  fixed in the joined workpieces  88 . 
     The threads  68  along the cylindrical section  58  of the rivet  54 , create re-entrant portions  94  along the cylindrical section  58 . When the rivet  54  is embedded in the workpieces  88 , the re-entrant portions  94  receive some of the plasticized material  96  that fills in between the underside  98  of the cap  70  and the lower stop  62  of the body  54  to increase the strength of the mechanical retention around the cylindrical section  58  of the rivet  54 . 
     During the process, the slideable cap  70  restricts oxygen access to the rivet  54  by creating a partial seal between the rivet and the first workpiece  92 . The reduced oxygen supply around the rivet  54  reduces the formation of oxides on the cylindrical section  58  of the rivet  54 , which provides a clean surface to form a bond with the plasticized material  96 . Allowing formation of an oxide layer would interfere with bonding between the cylindrical section  58  and the plasticized material  96 . 
     While the invention has been described by reference to certain preferred embodiments, it should be understood that numerous changes could be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the disclosed embodiments, but that it have the full scope permitted by the language of the following claims.