Metal fusion bonding – Including means to provide heat by friction between...
Reexamination Certificate
2002-02-12
2004-01-13
Dunn, Tom (Department: 1725)
Metal fusion bonding
Including means to provide heat by friction between...
C228S112100, C029S889210
Reexamination Certificate
active
06676004
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains to friction stir welding and more particularly to a friction stir weld tool with an improved probe structure for welding high-strength aluminum alloys and other hard materials such as steel, copper, nickel, titanium and their alloys.
2. Background and Objects of the Invention
Friction stir welding is a relatively new welding technique discovered in the mid, 1990s that was developed primarily for welding aluminum and soft aluminum alloys that were difficult to weld using traditional welding techniques. The technique uses a rotating shouldered cylindrical tool with a projecting pin to generate heat in the workpiece. The mechanical friction of the rotating tool contacts the workpiece and plasticizes (softens) the metal as it is plunged into the bondline. At this stage, there is a substantial amount of plasticized metal in a column about the rotating pin beneath the shoulder of the cylinder portion of the tool. The tool is then moved along the bondline relative to the workpiece. As the pin rotates and moves in a transverse direction, the metal is plasticized at the front of the pin and extruded to the back of the pin while undergoing a mechanical stirring and forging action imparted by the pin surface profile and confined from above by the pressure exerted on the material by the shoulder of the cylindrical tool. The plasticized metal is transferred from the front of the pin around the periphery of both sides of the pin and subsequently reconstituted at the back of the pin to produce the weld. The rotational and translational (travel) speed are controlled to maintain a plasticized metal state.
The stir weld tool is formed as a cylindrical piece with a shoulder face that meets a pin (probe) that projects from the shoulder face at a right angle (U.S. Pat. Nos. 5,460,317 and 6,029,879). In some instances, the probe actually moves in a perpendicular direction in an aperture formed in the face of the shoulder (U.S. Pat. No. 5,697,544; U.S. Pat. No. 5,718,366; and U.S. Pat. No. 5,893,507). The face of the shoulder can be formed with an upward dome that is perpendicular to the pin (U.S. Pat. No. 5,611,479; U.S. Pat. No. 5,697,544; and U.S. Pat. No. 6,053,391). The dome region and an unobstructed dome (shoulder face)/pin interface are considered essential for the proper frictional heating of the workpiece material. The dome region constrains plasticized material for consolidation at the trailing edge of the friction stir tool rather than permitting it to extrude out from under the sides of the tool. A clean, unobstructed shoulder face/pin interface are considered critical for proper mechanical flow, stirring and mixing of the plasticized workpiece material around the pin during welding and the subsequent mechanical and metallurgical properties of the resultant joint.
Another factor influencing friction stir weld design is the translational (travel) speed at which the tool moves along the bond line. Although tool speeds of 7-15 inches/minute (180-380 mm/min are common in ¼ inch (6 mm) aluminum-alloy stock (BS6082; U.S. Pat. No. 5,460,317), the pin of the tool has a tendency to break as the travel speed or the thickness and/or hardness of the material to be joined increases. Although typically the friction stir weld material is a tool steel material such as H13 (U.S. Pat. No. 6,053,391), increasing the hot shear strength of the tool material alone or the diameter of the pin has not been the answer to welding thick sections of material, i.e., greater than about 1 inch (25 mm) in thickness (“Development of Improved Tool Designs for FSW [Friction Stir Weld] Aluminum”, Chris Dawes and Wayne Thomas, The Welding Institute (TWI), Friction Stir-Welding Symposium” June 1999, Thousand Oaks, Calif.).
Several attempts have been made to avoid the pin breakage problem. When welding thick or hard-material sections, travel speed can be reduced to avoid tool breakage. However, this brings about its own problems. At such slow speeds, the workpiece metal overheats and a dramatic loss in joint efficiency, often of the order of 10-15%, occurs. In addition, the welding process may become so slow as to be economically infeasible. Another solution is to use a double pass welding technique in which the pin is only plunged into the workpiece to slightly over half its thickness and then welded again from the opposite side of the workpiece. Although tool speed and breakage problems are decreased, material handling time and cost increase substantially and the technique may not be applicable where the workpiece configuration prevents access to the second side. Finally considerable effort has been put into the design of the tool pin (probe) with a variety of pin designs having been made (U.S. Pat. Nos. 6,053,391; 6,029,879; 5,460,317 and “Development of Improved Tool Designs for FSW [Friction Stir Weld] Aluminum”, Chris Dawes and Wayne Thomas, The Welding Institute (TWI), Friction-Stir Welding Symposium” June 1999, Thousand Oaks, Calif.). However, none of the designs satisfy the need for friction stir welding tool pins that allow for high-speed welding in thick, strong, or hard materials without breakage. In fact, the introduction of sharp angled threads and bosses into the tool may well be one of the sources of high-stress concentration that leads to pin failure.
As such, friction stir welding has been limited to relatively soft materials such as plastics and soft aluminum alloys in the 1xxx, 5xxx, and 6xxx aluminum series for joints with thickness in the ranges of {fraction (1/32)} in (0.79 mm) to ½ in (13 mm). For stronger aluminum alloys such as the 2xxx and 7xxx series, workpieces of a thickness less than about ½ in (13 mm) are the limit for good stir friction welds.
Unfortunately, as the joint thickness is increased above about ½ in (13 mm) thick on aluminum alloys with ultimate tensile strengths greater than about 50 ksi at welding speeds above 2 inches/min, friction stir welding becomes a less effective joining process due to tool failure or short tool life. Typically, most tools built according to the teachings of the state of the art, fail after only a few inches of travel when welding in harder materials. Typically the length of travel before pin fracture is less than 12-inches of travel leaving the fractured pin solidified in the workpiece.
Accordingly, it is an object of the present invention to provide a friction stir welding tool. capable of welding high strength materials while providing high strength in the resulting weld without pin fracture.
It is another object of the present invention to provide a friction stir welding tool capable of welding thick materials while providing high strength in the resulting weld without pin fracture.
It is another object of the present invention to provide a friction stir welding tool capable of welding thick high-strength materials while providing high strength in the resulting weld without pin fracture.
It is another object of the present invention to provide a friction stir welding tool capable of welding high-strength and/or thick materials at relatively high speed.
It is another object of the present invention to provide a friction stir welding tool with a thread pattern that reduces areas of high-stress concentration.
It is another object of the present invention to provide a friction stir welding tool that avoids the need for double pass welding.
It is another object of the present invention to provide a friction stir welding tool that increases the speed of the welding process.
It is another object of the present invention to provide a friction stir welding tool that decreases the cost of the friction .stir welding process.
It is yet another object of the present invention to provide a friction stir welding tool that results in high joint efficiency.
SUMMARY OF THE INVENTION
In order to meet these objects, the present invention of a friction stir weld tool comprises a cylinder having a first end for attachment to a rotating drive, a longi
Hunt Mark L.
Mohr William C.
Skilliter Mathew
Stotler Timothy
Trapp Timothy J.
Cooke Colleen P.
Dawsey David J.
Dunn Tom
Edison Welding Institute, Inc.
Gallagher Michael J.
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