Metal fusion bonding – Process – Using dynamic frictional energy
Reexamination Certificate
2001-08-14
2002-11-26
Elve, M. Alexandra (Department: 1725)
Metal fusion bonding
Process
Using dynamic frictional energy
C228S002100, C228S103000
Reexamination Certificate
active
06484924
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to friction stir welding and, more particularly, relates to backing up a weld joint during friction stir welding.
BACKGROUND OF THE INVENTION
Friction stir welding is a relatively new process using a rotating tool to join in a solid state two workpieces or to repair cracks in a single workpiece. At present, the process is applied almost exclusively in straight-line welds. For example, such a process is described in U.S. Pat. No. 5,460,317 to Thomas et al., the contents of which are incorporated herein by reference. During friction stir welding a rotating tool is plunged into a workpiece or between two workpieces by a friction stir welding machine to produce the required resistance force to generate sufficient frictional heating to form a region of plasticized material. Upon solidification of the plasticized material, the workpieces are joined along the weld joint. The magnitude of force exerted by the friction stir welding tool on a workpiece is applied over a small, but well defined area of the workpiece, and must be maintained above a prescribed minimum in order to generate the required frictional heating.
To prevent deformation of a workpiece by the force exerted by the friction stir welding tool and maintain dimensional tolerances, the workpiece must have support behind the weld joint over an area at least as large as the imprint of the welding tool. Additionally, because the frictional heat generated by the welding tool plasticizes the material within the weld joint, the plasticized material must be constrained to prevent the material from flowing out of the weld joint and to provide a weld joint having the desired surface finish. When friction stir welding relatively flat workpieces, the weld joint can be supported by a continuous planar surface, such as a steel plate, positioned underneath the workpieces to be joined.
When friction stir welding workpieces having curvilinear geometries, providing adequate support to the weld joint becomes problematic because the curvilinear geometry makes it more difficult to provide a continuous support surface. Such welds are often necessary when fabricating military and commercial aircraft and rocket fuel tanks. In certain instances, a built-up structure can be secured to the interior surfaces of the curvilinear workpieces prior to friction stir welding. However, weight restrictions and/or design parameters often require a finished assembly having a smooth interior surface. As such, the built-up structure must be removed, for example, by machining, which is time consuming and labor intensive and increases the manufacturing cost of the finished assembly.
In seeking better methods for welding curvilinear geometries, a solid backing wheel has been proposed. For example, referring to
FIGS. 1A and 1B
, there is illustrated one design of a welding apparatus
11
having a solid wheel
13
for backing a friction stir weld joint between two curvilinear structural members
15
, as is known in the art. In order to produce a curvilinear weld joint between a pair of curvilinear structural members
15
, the friction stir weld tool
17
is positioned in the joint opposite the solid wheel
13
. Assuming ideal structural members
15
without deflection or deformation, the joint between the structural members will meet the solid wheel
13
at a tangent point
19
, leaving slender gaps
21
on either side of the wheel directly under the shoulder
17
a
of the friction stir welding tool
17
. As illustrated in
FIG. 1B
, the friction stir welding tool
17
is typically positioned at an angle a with the plane extending from the center of the solid wheel
13
through the tangent point
19
. During friction stir welding, the welding force crushes the gaps between the structural members
15
and the solid wheel
13
. As the joint cools from welding temperatures, the joint can retain an instantaneous and permanent set in the shape the structural members
15
were welded, i.e., the curvature of the solid wheel
13
, which can result in a distorted joint that adversely affects the tolerances of the resulting structural assembly.
In addition, referring to
FIG. 1B
, the curvature of the solid wheel
13
typically results in some amount of material
23
being extruded through the joint away from the shoulder
17
a
of the friction stir welding tool
17
. As the material
23
is extruded, the shoulder
17
a
of the friction stir welding tool
17
forms a trough
25
on the side of the joint facing the shoulder, which can weaken the joint. When the centerline of the friction stir welding tool
17
is in line with the center of the solid wheel
13
, the tool is aligned at the top dead center (“TDC”) of the wheel. Referring to
FIGS. 1C
and
1
D, if the tool
17
is ahead or behind of TDC of the wheel, the tool receives less support on the slope of the wheel. Both cases cause extrusion of material
23
away from the shoulder
17
a
of the tool
17
. As illustrated in
FIG. 1C
, in the worst case the tool is ahead of TDC and the extrusion
23
builds a “ramp” that wedges under the tangent point of the wheel
13
and lifts the structural members
15
away from the wheel. If the tool
17
is ahead of TDC, the shoulder
17
will also dig a relatively deep trough
25
in the surface of the structural members
15
facing the shoulder. Where the tool is behind TDC, the shoulder
17
a
extrudes metal
23
away from the shoulder, but the extent of distortion of the structural members
15
is typically not as severe. Reducing the diameter of the solid wheel
13
typically increases the distortion of the structural members
15
, as well as the extrusion
23
of material from the backside of the joint, since the slope of the wheel becomes steeper. Increasing the diameter of the solid wheel
13
results in heavier, more complex tooling and can, in certain instances, result in difficulty extracting the wheel from the resulting structural assembly.
Referring to
FIGS. 1E and 1F
, where there is illustrated a comparison of a friction stir weld joint
27
a
formed by welding flat plates
15
a
with a flat backing and a weld joint
27
b
formed by welding curvilinear plates
15
b
using a solid backing wheel. As illustrated in
FIG. 1E
, the backside of the weld joint
27
a
of the flat plates
15
a
is flush with the original profile of the structural members
15
. In contrast, as illustrated in
FIG. 1F
, the weld joint formed between the curvilinear members
15
exhibits extrusion
23
on the backside of the joint away from the shoulder and a severe depression or trough
25
on the side of the joint facing the shoulder.
Thus, there is a need for an improved backing device for weld joints formed by friction stir welding workpieces having curvilinear geometries. The backing device should be capable of effectively supporting a weld joint and constraining the plasticized material within the weld joint during friction stir welding. Additionally, the backing device should be easily adaptable to varying workpiece geometries.
SUMMARY OF THE INVENTION
The present invention provides an apparatus and an associated method for backing up weld joints formed by friction stir welding workpieces having curvilinear geometries. The backing device includes a backing ring defining a contact zone. Advantageously, the backing ring has sufficient flexibility such that at least a portion of the contact zone defined by the backing ring engages the weld joint opposite the friction stir welding tool to thereby support the weld joint and to constrain the plasticized material within the weld joint.
The backing device also includes a restraining member in operable communication with the backing ring to urge the central axis of the backing ring and, thus, the contact zone of the backing ring, toward the weld joint. According to one embodiment, the restraining member is in operable communication with the backing ring through a spring member and a plurality of bogies. The plurality of bogies rotatively engage at least a portion of the backing ring and urge the
Alston & Bird LLP
Elve M. Alexandra
Stoner Kiley
The Boeing Company
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