Abstract:
A friction stir rivet is rotated in a precessing motion and driven through a first fusible workpiece into an engaged second fusible workpiece, causing local portions of the first and second workpieces to plasticize. When the rivet is driven through the first workpiece and into the second workpiece, rotation is stopped and the plasticized material solidifies around the rivet creating an enlarged weld joining the metal workpiece and encompassing the rivet, which provides additional mechanical strength.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to friction stir welding and riveting, more particularly, to methods of joining multiple workpieces using a precessing stir rivet to create a mechanical bond, an interweld, and a diffusion bond.  
         BACKGROUND OF THE INVENTION  
         [0002]    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.  
           [0003]    A common design of FSW stir rods is that the stirring element is substantially symmetrical with some irregularity to induce a stirring motion. Frequently the stir rod has a threaded appearance similar to a bolt. However, to promote intermingling and to retain the plasticized material in the weld zone for as long as possible the direction of rotation of the rod is such that the threads carry the plasticized material downward to create as turbulent a flow and as efficient an intermingling as possible. Particularly for metal workpieces the high thermal conductivity strongly localizes the region, which is plastic enough to be deformed by the stirring action. Thus, the width of the stirred region is substantially equal to the width of the stirring rod.  
         SUMMARY OF THE INVENTION  
         [0004]    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 rod as a rivet. The stir rivet is rotated and advanced into multiple workpieces to plasticize material around the rivet for stir welding the workpieces together. The rivet is then left in place to form a weld between the rivet and the hardened or solidified material.  
           [0005]    The present invention provides a modified stir rivet which includes an angled body that is asymmetrical about a rotational axis. The body preferably lies on a centerline that extends outward and downward from the rotational axis so that the body centerline processes as it rotates on the rotational axis.  
           [0006]    The rivet has an upper and lower portion. The upper portion of the rivet includes a cap which serves as the head of the rivet and includes an upper side, an underside, and an outer face. A recessed socket is centrally located along the rotational axis of the upper side of the cap. The underside of the cap is inwardly recessed and joins with an elongated body.  
           [0007]    The lower portion of the rivet includes the elongated body having sidewalls and a lower end surface. The body has a cross-section that increases smoothly along the length of the elongated body from the cap down to the lower end, causing the body of the rivet to have re-entrant features. The lower end of the body is bulbous, having a pear like shape. Alternative shapes for the body of the rivet include conical, cylindrical, and spherical shapes. A portion of the sidewall angles inward toward the rotational axis of the rivet, which creates a re-entrant portion along at least one side of the elongated body. The lower portion of the rivet has helical flutes which run the length of the elongated body to redirect displaced material to the lower surface of the rivet.  
           [0008]    The cap acts as a retaining element to prevent plasticized material from displacing out of the cavity. Specifically, when the cap comes in contact with the first workpiece the inwardly recessed underside of the cap forces displaced material back into the cavity. As the material re-enters the cavity, helical flutes located on the elongated section of the rivet push material down to the lower surface of the rivet to pack material around the lower end of the rivet.  
           [0009]    To rotate the rivet, a rotational apparatus is inserted into the recessed socket of the rivet. The recessed socket is centrally located on the upper surface of the cap and is aligned with the rotational axis of the rivet. The lower portion of the rivet is aligned along the precession axis which runs at an angle to the rotation axis. Offsetting the alignment of the lower portion of the rivet relative to the axis of rotation causes the lower portion of the rivet to move in a precessing motion when rotated.  
           [0010]    The precessing motion of the rivet creates more contact around the sidewalls of the rivet and increases stir radius around the rivet. The extra contact between the sidewall and the workpieces to be welded promotes the stir welding process by stirring up a greater area around the rivet. As a result, more plasticized material is intermingled and inter melted. Also, the extra friction created by the precession motion of the rivet creates extra heat to further aid the process.  
           [0011]    Scrubbing the sidewalls of the rivet removes oxidation from the rivet which allows a better bond to form between the rivet and the stirred material. If the oxidation is not removed from the sidewalls of the rivet the bond that forms between the rivet and stirred material will be adversely affected by the oxidation layer around the rivet.  
           [0012]    Weld strength is further increased by the re-entrant section of the rivet. The elongated body of the rivet creates a re-entrant section along the angled sidewalls of the rivet. The re-entrant section extends from the lower portion of the rivet up to the underside of the cap. This design allows plasticized material to fill in between the cap and the lower surface of the rivet, thereby, increasing the volume of mechanical retention around the re-entrant section of rivet.  
           [0013]    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.  
           [0014]    A suitable rotational device 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, the friction stir rivet can penetrate aluminum at up to 27 millimeters per minute.  
           [0015]    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  
       [0016]    In the drawings:  
         [0017]    [0017]FIG. 1 is a side view of an exemplary embodiment of a friction stir rivet according to the invention;  
         [0018]    [0018]FIG. 2 is a side view of the friction stir rivet of FIG. 1 rotated 90°; and  
         [0019]    [0019]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. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0020]    Referring first to FIGS. 1 and 2 of the drawings in detail, numeral  10  generally indicates a friction stir rivet. Rivet  10  includes an elongated stirring body  12  and a cap  14 . Elongated body  12  includes a sidewall  16  and a lower end  18 . The cross-section of the body  12  increases smoothly from the cap  14  down to the lower end  18 , which optionally has a hemispherical shape. Sidewall  16  has an angled portion  20  that slopes inward toward the rotational axis  22  of rivet  10  to create a re-entrant section  24 . The elongated body  12  has helical flutes  26 .  
         [0021]    Cap  14  acts as the head of rivet  10  and includes an upper side  28 , an underside  30 , and an outer face  32 . A recessed socket  34  is centrally located along rotational axis  22  of the upper side  28  of the cap  14 . A rotational apparatus engages recessed socket  34  to rotate and drive rivet  10 . The underside  30  of the cap  14  has an inwardly recessed portion  36  which joins with the elongated body  12 . The body  12  Us offset relative to the rotational axis  22  of the rivet and aligned along precession axis  38  that forms an angle with the rotational axis  22 .  
         [0022]    Referring to FIG. 3, the rivet  10  is shown in use for stir riveting a first workpiece  40 , such as a fusible aluminum sheet or plate, to a second workpiece  42 , such as a fusible aluminum frame or other substrate. In operation, the rivet  10  is rotated around its rotational axis  22  while the elongated body  12 , extending along the precession axis  38 , rotates with the axis  38  in a precession like motion.  
         [0023]    During rotation, downward force is applied to the rivet  10  causing the lower end  18  to frictionally contact an exposed surface  44  of the first workpiece  40 . The downward force and rotation of the rivet  10  cause a portion of the first workpiece  40  to plasticize, allowing the rivet  10  to penetrate and create a cavity  46  partially or completely filled with plasticized material  52 . As the rivet  10  is driven through an unexposed surface  48  of the first workpiece  40 , rivet  10  frictionally contacts an unexposed surface  50  of the second workpiece  42 . The downward force and rotation of rivet  10  cause a portion of the second workpiece  42  to plasticize, allowing rivet  10  to continue penetrating cavity  46 . As the rivet  10  is driven through the first workpiece  40  into the second workpiece  42 , the plasticized material  52  is intermixed.  
         [0024]    As rivet  10  is further driven into workpieces  40 ,  42  the underside  30  of cap  14  contacts exposed surface  44 , causing the cap  14  to act as a retaining element restricting plasticized material  52  from escaping during the friction stir riveting process. Specifically, the inwardly recessed portion  36  of underside  30  forces plasticized material  52  into the helical flutes  26 . As the rivet is rotated in a clockwise direction as shown by direction arrow  54 , the flutes  26  push plasticized material  52  down the length of rivet  10  to the lower end  18 , which packs material around the lower end  18  of the rivet  10 .  
         [0025]    The precession motion of body  12  increases contact between the sidewall  16  of the rivet  10  and the cavity  46 . The extra contact created by the sidewall  16  of the rivet  10  promotes the welding process by stirring up a greater area around the rivet  10 , causing more plasticized material  52  to be intermingled.  
         [0026]    The extra contact between the sidewall  16  and the cavity  46  abrades oxidation from sidewall  16 . Removing oxidation around the rivet allows a better bond to form between rivet  10  and the plasticized material  52 . If the oxidation is not removed from the sidewall  16  of rivet  10 , the oxidation layer will interfere with chemical bonding between rivet  10  and the plasticized material  52 .  
         [0027]    Preferably, rivet  10  is driven though the first workpiece  40  and partially into the second workpiece  42  until the cap  14  of the rivet  10  is partially recessed into the exposed surface  44  of the first workpiece  40 . Thereafter, the rotary motion of rivet  10  is stopped, allowing locally plasticized material  52  to solidify and form several welds. Rivet  10  forms a mechanical bond between the first workpiece  40  and the second workpiece  42 . Plasticized material  52  preferably forms a diffusion bond between the rivet  10  and the first and the second workpieces  40 ,  42 . Furthermore, the plasticized material  52  forms an interweld between the first workpiece  40  and the second workpiece  42 .  
         [0028]    The weld strength is further increased by the re-entrant sections of rivet  10 . The elongated body  12  of the rivet  10  creates re-entrant sections around the sidewalls  16 ,  20  of the rivet  10 . Re-entrant section  24  extends from the lower portion  18  of the rivet up to the underside  30  of the cap  14 . This design allows plasticized material  52  to fill in between the cap  14  and the lower surface  18  of the rivet  10 , thereby, increasing the strength of mechanical retention around the sidewalls  16 ,  20  of the rivet  10 .  
         [0029]    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.  
         [0030]    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.