Precessing rivet and method for friction stir riveting
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.
1. A friction stir rivet having a cap and an elongated stirring body extending downward from the cap, the body including a lower end and sidewalls that join the lower end with an underside of the cap, wherein;
the rivet has a predetermined rotational axis;
the body has a centerline that angles downward and outward from the rotational axis; and
the body has a cross-section that increases smoothly from the cap to the lower end to create a re-entrant area along at least one side of the body.
2. A rivet as in claim 1 wherein the body has angled fluting along the sidewalls operative during rotation of the rivet to force flowable material downward toward the lower end of the rivet.
3. A rivet as in claim 2 wherein the cap is a disk and the underside is inwardly recessed to direct material inward toward an upper end of the fluting.
4. A rivet as in claim 1 including a rotary drive feature on the cap.
5. A rivet as in claim 1 wherein the elongated body is pear shaped to create a re-entrant portion around the sidewalls.
6. A rivet as in claim 1 wherein the elongated body is asymmetrical relative to the rotational axis.
7. A rivet as in claim 1 wherein the lower end of the body is generally hemispherical.
8. A method of joining fusible workpieces at a point of engagement, comprising the steps of;
providing a rivet with an elongated body formed of a material with a substantially higher melting point than that of the workpieces wherein the body angles outward from a rotational axis which causes the body to rotate in a precession like motion;
rotating the rivet about the rotational axis and simultaneously driving the precessing body through a first workpiece and into an engaged second workpiece, thereby creating friction between the rivet and the workpieces, that locally plasticizes an enlarged volume of material around the rivet;
stopping further rotation and inward driving of the rivet after a desired depth is reached; and
cooling the workpieces and the rivet, until solidification of plasticized material occurs and the fusible workpieces are held together by the hardened or solidified material and by the rivet retained within the workpieces at the point of engagement.
9. A method as in claim 8 including forming the rivet of a material compatible with that of the fusible workpieces such that during cooling of the plasticized material surrounding the rivet a diffusion bond is created between the rivet and the surrounding material.
10. A method as in claim 8 wherein the cooling step causes interwelding of the fusible workpieces.
11. A method as in claim 8 wherein the cooling step embeds the rivet to form a mechanical bond between the joined workpieces.
12. A method as in claim 8 wherein the fusible workpiece are metal.
13. A method as in claim 8 including providing a cap on the body with a recessed underside adjacent the body to force displaced material back into a stir area around the rivet.
14. A method as in claim 8 including providing angled fluting around sidewalls of the body to direct displaced material down toward and end of the rivet.
15. A method as in claim 8 wherein the body of the rivet is asymmetrical relative to the rotational axis.