Metal fusion bonding – Process – Using dynamic frictional energy
Reexamination Certificate
2000-07-13
2001-03-27
Ryan, Patrick (Department: 1725)
Metal fusion bonding
Process
Using dynamic frictional energy
C228S002100
Reexamination Certificate
active
06206268
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to friction stir welding and, in particular, to an apparatus and method for friction stir welding two pieces together with a reduced incidence of weld root defect.
BACKGROUND OF THE INVENTION
Friction welding is based on the principal of “rubbing” together two articles to be joined so as to generate a sufficient amount of heat and plasticize the adjacent surfaces. The frictional heat is generated solely by the adjacent work pieces to be joined.
A more robust version of friction “stir” welding utilizing a rotating, non-consumable probe or pin has been developed. The pin is of a harder material than the work pieces to be joined, and relative cyclic movement of the pin generates the frictional forces to plasticize the workpiece material. Because of this movement of the non-consumable pin, the method has been named friction stir welding; with the pin “stirring” the material. Aluminum is a particularly suitable soft material that can be readily friction stir welded. The aerospace industry is aggressively pursuing friction stir welding of aluminum panels for the outer skins of launch vehicles because of the weld quality and repeatability.
In a conventional butt joint, for example, the pin spins or linearly reciprocates and travels along the interface between the adjacent or “faying” work piece surfaces to create the plasticize weld joint. Typically, the two work pieces are placed on a backing surface and are tightly held together to prevent separation during the friction stir welding process. Further, a stationary shoulder surrounding the moving pin applies forging pressure to the area around the faying surfaces on the opposite side from the backing surface to prevent the plasticized material from extruding out of the joint. The result is a relatively smooth and void-free joint.
The reciprocating probe or pin is preferably configured to force the plasticize material downward and backward in its “wake.” When the pin translates between the faying surfaces, plasticized material flows from the front of the pin to the rear. The pin extends almost to the bottom of the material to be joined, but cannot extend all the way through so as to avoid contact with the backing surface, which might pull support tooling debris into the weld. Ideally, the rotating pin is held a fixed distance from the backing surface, typically about 0.254 mm (0.010 inch), and the “throwing power” of the rotating pin is relied on to fill the remaining gap. “Throwing power,” in this sense, is defined as the extent of deformation achieved beyond the actual dimension of the pin itself. Of course, there is a limit to the throwing power of any particular stir welding pin, and thus the spacing from the backing surface should be minimized. Unfortunately, variations in material thickness or deflection of the stir welding head can displace the pin a greater distance from the backing surface than intended, potentially resulting in lack of root notch penetration and reduced integrity compared to the remaining weld.
In most cases, the distal face of a rotating pin is convex with a constant radius, and thus the minimum spacing from the backing surface is located almost on the centerline. Because of this configuration, the centerline, being located along the axis of rotation, generates very little or no friction or “stirring” action. The throwing power of the pin results from the stirring action of the radially outer edge of the pin, which action must also induce deformation along the centerline. Depending on the tolerances of the weld process, there is a risk that the throwing power of the rotating pin will not span the entire distance to the backing surface along the centerline of the pin.
U.S. Pat. No. 5,611,479 discloses a friction stir welding technique to ensure complete penetration of the weld. In the '479 patent, the shape of the faying surfaces are modified to create a gap into which the plasticized material will flow for the purpose of reducing root weld defects. Specifically, a chamfer is formed along the faying surfaces of the work pieces running along the weld line and facing the backing surface. Unfortunately, the formation of the chamfers on the work pieces adds a time-consuming and costly step in the weld process.
Consequently, there is a need for a friction stir welding technique that improves the throwing power of the tool or otherwise reduces the possibility of lack of weld root notch penetration.
SUMMARY OF THE INVENTION
The present invention provides a friction stir welding pin having improved throwing power, comprising a pin body rotatable about an axis, the pin having a distal end and an internal flow cavity open to the distal end. The pin may include external threads oriented so as to force plasticized material created during use of the pin toward the distal end. Preferably, the pin also includes internal threads partly defining the flow cavity, the threads oriented so as to force plasticized material created during use of the pin toward the distal end. The pin may further include one or more flow channels extending from an external surface of the pin body to the internal flow cavity. The flow channel(s) are desirably angled toward the distal end from the external surface to the internal flow cavity.
In another aspect, the present invention provides a method of friction stir welding that increases weld root penetration. The method includes providing a friction stir welding pin rotatable about an axis and having a distal end and an internal flow cavity open to the distal end. The pin is rotated at the interface between two work pieces at a speed to plasticize the material of the work pieces. The plasticized workpiece material is then caused to flow through the internal flow cavity toward the distal end. The pin may include a flow channel extending from an external surface thereof to the internal cavity, so that the method includes causing plasticized material to flow through the flow channel to the internal flow cavity. Internal threads partly defining the internal flow cavity may be provided and oriented so as to force plasticized material flowing through the flow channel toward the distal end of the pin.
In a still further aspect, the present invention provides a method of friction stir welding including providing a friction stir welding pin having an internal flow cavity, and operating the friction stir welding pin such that material plasticized by the pin flows through the internal flow cavity. Preferably, the internal flow cavity is located along the centerline of the pin and open to a distal end thereof, wherein the plasticized material flows axially through the internal flow cavity toward the root of the weld. The internal flow cavity may be partly defined by internal threads adapted to force plasticized material toward the distal end of the pin. The internal flow cavity preferably comprises a dead-end cavity open to a distal end of the pin, wherein the pin further includes at least one flow channel extending from a sidewall of the pin to the internal cavity. The method then may involve inducing a continuous flow of plasticized material from the outside of the pin through the flow channel(s) in the sidewall and through the internal cavity to be expelled out of the distal end
A further understanding of the nature advantages of the invention will become apparent by reference to the remaining portions of the specification and drawings.
REFERENCES:
patent: 4811887 (1989-03-01), King et al.
patent: 5460317 (1995-10-01), Thomas et al.
patent: 5611479 (1997-03-01), Rosen
patent: 5682677 (1997-11-01), Mahoney
patent: 5713507 (1998-02-01), Holt et al.
patent: 5718366 (1998-02-01), Colligan
patent: 5813592 (1998-09-01), Midling et al.
patent: 5893507 (2000-04-01), Ding et al.
patent: 6029879 (2000-02-01), Cocks
patent: 6045027 (2000-04-01), Rosen et al.
patent: 6053391 (2000-04-01), Heidman et al.
patent: 2306366 (1997-07-01), None
Cooke Colleen P.
Ryan Patrick
Stout Donald E.
Stout, Uxa Buyan & Mullins, LLP
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