Friction stir welding is a solid-state joining technique that is well known to those of ordinary skill in the art. Typically, friction stir welding is used to join difficult-to-weld metals, metal alloys (such as aluminum alloys, titanium alloys, nickel alloys, and the like), and other materials. For example, certain aluminum alloys are sensitive in a plasticized heat-affected zone, where the base metal reaches temperatures between solidus and liquidus during welding. In this zone, partial melting at grain boundaries forms a network containing brittle compounds. As a result, weld ductility is substantially reduced. Likewise, other conventional joining techniques may create geometric distortions near a weld joint due to high temperature gradients induced in a workpiece during welding. These distortions may cause warping and other dimensional defects in the workpiece, as well as residual stresses that may cause premature failure by cracking in the heat-affected zone or weld joint, lamellar tearing, or by stress-corrosion cracking in some metals and metal alloys. In addition, some metals, metal alloys, other materials, and types of weld joints are difficult to join except in a flat position.
Friction stir welding techniques overcome many of the problems encountered with other conventional joining techniques. In friction stir welding, a cylindrical, non-consumable, rotating pin tool is plunged into a rigidly clamped workpiece and traversed along the joint to be welded. The pin tool is specially designed to provide a combination of frictional heat and thermo-mechanical working to accomplish the weld. As the pin tool is traversed along the joint to be welded, the plasticized metal, metal alloy, or other material is transferred from the leading edge of the pin tool to the trailing edge of the pin tool, forming a strong solid-state weld joint in the wake of the pin tool. During the friction stir welding of hard metals and metal alloys, such as steel, titanium alloys, and nickel alloys, high temperatures are generated in the pin tool, as well as the pin tool holder. Under such conditions, pin tool degradation is a serious problem. Pin tool wear and pin tool debris entrapment are issues that must be addressed in order to obtain a defect-free weld joint.
Conventional friction stir welding apparatuses and methods have been designed, at least in part, to address some of these problems, with limited success. For example, U.S. Patent Application No. 2003/0075584 (Sarik et al.) discloses an apparatus for use in friction stir welding including a friction stir tool, having a shoulder, a non-consumable welding pin extending downward centrally from the shoulder, a first workpiece disposed on a backing workpiece, a second workpiece located a predetermined distance from the first workpiece on the backing workpiece, and a transition strip disposed on the backing workpiece between the first workpiece and the second workpiece, wherein the contact area or a space between the transition strip and the first workpiece defines a first interface and the contact area or a space between the transition strip and the second workpiece defines a second interface, wherein the non-consumable welding pin is rotated over the first interface and the second interface to weld the first workpiece to the second workpiece with the transition strip material incorporated as part of the weld.
U.S. Pat. No. 6,543,671 (U.S. Patent Application No. 2003/0042292) (Hatten et al.) discloses a friction stir welding tool that includes a body having an upper surface defining an entrance opening and a lower surface, and a pin having a lower surface defining an exit opening, wherein the pin extends from the lower surface of the body. The friction stir welding tool also includes a passageway defined by the body and the pin from the entrance opening to the exit opening and is capable of allowing a filler material to pass therethrough. A friction stir welding method includes applying a frictional heating source to a workpiece to plasticize a volume of the workpiece and applying the frictional heating source to a filler material to plasticize the filler material. The friction stir welding method also includes introducing the filler material into the volume of the workpiece and incorporating the filler material into the volume of the workpiece.
U.S. Pat. No. 6,206,268 (Mahoney) discloses a friction stir welding pin having internal flow cavities. The pin is adapted to be driven by a conventional friction stir welding machine, and may include external threads for forcing plasticized material toward the weld root. An internal cavity located along the centerline and open to the distal end facilitates deformation of the workpiece material at the weld root. One or more flow channels extending from the sidewall of the pin to the internal flow cavity induce a continuous path of plasticized material through the pin. The internal cavity may include internal threads to further help force plasticized material toward the weld root. The pin is particularly useful in welding aluminum workpieces where the tolerance of the workpiece thickness is not critical.
In general, conventional friction stir welding apparatuses and methods fail to address the problem of pin tool degradation. In some cases, pin tools are manufactured from relatively hard refractory metal alloys, such as molybdenum alloys, tungsten alloys, and the like, in order to minimize pin tool degradation. However, pin tool wear and pin tool debris entrapment still occur, resulting in weld joints with defects. The present invention provides friction stir welding apparatuses and methods that address these problems and issues.