Metal fusion bonding – Process – With condition responsive – program – or timing control
Utility Patent
1999-04-21
2001-01-02
Ryan, Patrick (Department: 1725)
Metal fusion bonding
Process
With condition responsive, program, or timing control
C228S103000, C228S112100, C228S234100, C228S002100
Utility Patent
active
06168066
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to friction stir welding, and more particularly to controlling friction stir welders.
BACKGROUND OF THE INVENTION
Friction-stir heating or welding is a relatively new technique, which involves plunging a moving nonconsumable tool into the material to be heated or welded, as described in U.S. Pat. No. 5,460,317, issued Oct. 24, 1995 in the name of Thomas et al. The moving tool plasticizes portions of the workpiece immediately adjacent to the tool. An advantage of the friction-stir weld technique is that the material which is heated is not exposed to combustion products, which avoids chemical changes due to interactions of the workpiece with the fuel and its byproducts. Another advantage is that the heated regions tend to coalesce and solidify immediately upon removal of the tool. Also, the temperature of the workpiece, even in the heated region, tends not to be as high as that which may occur during conventional welding, which reduces oxidation of the workpiece due to ambient atmosphere, and this in turn results in a reduced need to provide an inert atmosphere at the weld location.
In the particular case of butt-welding of metal plates along juxtaposed sides to form a seam, the moving tool is a “pin” tool extending into the region between the plates. The plates are restrained from moving away from the rotating tool, but are not otherwise pressed toward each other, according to the Thomas et al. patent. Relative motion between the rotating tool and the joint to be welded is provided, as for example by moving the workpieces so that the rotating tool tracks along the joint.
It has been discovered, that producing a high-quality welded joint requires that the penetration of the pin tool into the joint region between the plates be precisely controlled. If the pin tool penetrates by only a small amount into the joint, only a portion of the joint will be heated and joined, and the weld will not have as much strength as if the weld penetrated fully through the joint region, from one face of the plates to the other. In addition to this lack of strength, the material will not be fully plasticized in the entire welded seam, so that a portion of the apparently welded region will be in the form of a lap weld. The lap weld is simply a region in which the material from one of the workpieces overlaps onto, but does not fully bond with, the material of the adjacent workpieces. The weakness of such a weld will not always be obvious.
One way to guarantee full welding is to make the pin penetrate far enough to extend beyond the reverse side of the plates being welded. In another arrangement, described in U.S. Pat. No. 5,718,366, issued Feb. 17, 1998 in the name of Colligan, the pin tool and the shoulder are connected by a spring, so that they are capable of relative movement. Consequently, the shoulder can be maintained in contact with the obverse or upper surface of the workpieces, while the tip of the pin tool is able to penetrate through to the reverse or lower side of the workpiece, to thereby guarantee a complete weld. This, however, raises the possibility of damaging the pin tool or the underlying anvil by contact therebetween. U.S. Pat. No. 5,769,306, issued Jun. 23, 1998 in the name of Colligan, describes an arrangement in which this problem is solved by supporting the workpieces being welded by a roller having a groove. Such a roller support, however, gives rise to the possibility of imparting an unwanted curvature to the welded plates.
It has been discovered that it is not necessary for the pin tool to extend beyond the lower surface of the joint in order to obtain a good weld; instead, it is only necessary for the tip of the rotating pin to lie within a particular distance (the ligament distance) from the reverse side of the plate, such that the heating effect results in satisfactory metallurgical changes on the reverse surface. An arrangement for tending to maintain precise depth of a rotating pin tool is described in U.S. patent application Ser. No. 09/006,915, filed Mar. 9, 1998 in the name of Gentry. The Gentry arrangement includes a set of adjustable rollers which rotate with the pin tool, and bear on the upper surface of the plates being welded. This has the advantage of tending to force the workpieces into uniform contact with the underlying anvil. Another arrangement for maintaining the pin tool penetration is described in U.S. patent application Ser. No. 09/249,680, filed Feb. 12, 1999 in the name of Adams et al. In the Adams et al. arrangement, the pin tool is provided with a shoulder which bears against the upper surface of the workpieces, so that the forces required for maintaining the pin tool at a particular depth vary with the depth. A feedback arrangement senses the forces, and adjusts the drive to tend to maintain the shoulder in contact with the upper surface of the workpieces, which in turn tends to maintain a constant penetration of the pin tool into the joint.
Improved pin tool penetration control is desired.
SUMMARY OF THE INVENTION
A method for welding or heating a seam between two workpieces according to an aspect of the invention includes the steps of rotating a friction-stir weld pin tool, and plunging the pin tool to a depth in the seam under the control of a control signal. The electrical or magnetic characteristics (or both) across the seam is sensed to produce a conductivity or magnetic value signal. In one embodiment, the conductivity or magnetic value signal is compared with a standard value representing the sensed electrical conductivity or magnetic characteristic of a properly welded seam of the same material. The difference is taken between the conductivity or magnetic value and the standard value to produce the control signal. In a preferred mode of practicing the method of the invention, the step of sensing the electrical conductivity or magnetic characteristic is performed in a noncontacting manner. In a most preferred mode, the step of sensing the electrical conductivity is performed by applying alternating signals to a first winding located adjacent the seam, sensing the alternating signals received by a second winding coupled to the first winding, and processing the sensed signals to generate the conductivity value signal. In one version of the mode, the step of sensing the electrical conductivity is performed by using a meandering winding magnetometer.
A friction-stir welding or heating apparatus according to an aspect of the invention, for welding at least a portion of an elongated seam between juxtaposed workpieces, includes a friction-stir pin tool, and also includes a mounting, positioning and rotation arrangement coupled to the pin tool, (a) for holding the pin tool, (b) for rotating the pin tool about an axis, and (c) for controlling the position of the pin tool in the direction of the axis in response to a control signal. An anvil is provided for supporting the juxtaposed workpieces. The workpieces are supported in a manner which tends to maintain a portion of the seam on the axis. A seam conductivity or magnetic sensing arrangement is located adjacent the seam, for producing a first signal representing the conductivity or magnetic characteristics of at least that portion of the seam near the axis. A signal processing arrangement is coupled to the sensing arrangement and to the mounting, positioning and rotation arrangement, for comparing the first signal with a standard signal representing the desired conductivity or magnetic characteristic of the seam, and for generating the control signal in response to the difference between the first and standard signals. The apparatus also includes a coupling arrangement for coupling the control signal from the signal processing arrangement to the mounting, positioning and rotation arrangement, for controlling the positioning of the pin tool so as to tend to maintain the desired conductivity.
In a particular embodiment of the apparatus, an arrangement is provided for generating relative motion between the anvil and the pin tool along, or in the direction
Fierke T. G.
Johnson Jonathan
Lockheed Martin Corp.
Meise W. H.
Ryan Patrick
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