Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2001-12-26
2004-08-24
Stoner, Kiley (Department: 1725)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C228S002100, C228S112100, C148S694000
Reexamination Certificate
active
06780525
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to high strength friction stir welding and, more particularly, relates to reducing material property degradation of friction stir weld joints during subsequent heat treatments.
BACKGROUND OF THE INVENTION
Friction stir welding is utilized to join workpieces to form structural assemblies that can be used in the manufacture of military and commercial aircraft, as well as in other applications requiring high strength weld joints. As illustrated in
FIG. 1
, friction stir welding involves inserting the threaded pin
10
a
of a rotating friction stir welding tool
10
between the opposing faces of a pair of workpieces
12
,
14
while urging the workpieces together. Friction stir welding can also be used to repair cracks or other defects in a single workpiece. The rotation of the threaded pin
10
a
between the opposing faces of the workpieces
12
,
14
, or within a single workpiece, creates friction that generates sufficient heat energy to plasticize the workpiece material in the weld zone
16
. The friction stir welding tool
10
also includes a concave shoulder adapted to consolidate the plasticized workpiece material within the weld zone
16
as the friction stir welding tool is moved along the interface
11
between workpieces or through a single workpiece. A friction stir weld joint
18
forms, joining the workpieces together in a unitary assembly, as the plasticized regions of the workpieces
12
,
14
flow together and cool in the weld zone
16
. See U.S. Pat. No. 5,460,317 to Thomas et al. for a general discussion of friction stir welding, the entire contents of which are incorporated herein by reference.
One particular benefit of friction stir welding is that the formation of the weld joint
18
is autogenous and is created by the solidification of the plasticized parent materials rather than a filler material, as is commonly used in conventional welding processes. In addition, as illustrated in
FIG. 2A
, the friction stir weld joint
18
comprises a nugget having a refined grain structure with grains having an equiaxed shape and grain sizes ranging in order of magnitude from approximately 0.0001 to 0.0002 inches (approximately 3 to 5 microns). As a result of the improved grain structure, the friction stir weld joint
18
resists the formation and propagation of micro-cracks and exhibits improved strength, ductility and toughness, as well as improved corrosion and fatigue resistance.
The frictional heat necessary to plasticize the workpiece material during friction stir welding can degrade the material properties of the parent materials. As shown in
FIG. 1
, during friction stir welding, the frictional heat created by the rotating friction stir welding tool
10
is conducted from the weld zone
16
through the workpieces
12
,
14
into the ambient environment, creating a heat-affected region
20
around the weld zone
16
. The elevated temperatures associated with the friction stir welding process can degrade the material properties of the parent materials, including the strength, stiffness, and ductility of the workpieces
12
,
14
.
Material property degradation is particularly problematic when friction stir welding precipitation hardened parent materials, which have improved mechanical properties obtained through solution and precipitation heat treatments. When friction stir welding precipitation hardened workpieces
12
,
14
, the joined workpieces commonly require additional precipitation hardening or a resolution heat treatment to recover the parent material properties. The resolution heat treatment includes solution heat treating the workpieces
12
,
14
at a predetermined temperature schedule and then rapidly cooling the workpieces by quenching. The solution heat treating process is then followed by a precipitation heat treatment involving either natural or artificial aging at a second predetermined temperature schedule to recover the parent material properties. While resolution heat treating improves the material properties of the joined workpieces
12
,
14
, the resolution heat treatment typically results in appreciable grain growth in the friction stir weld joint
18
, as illustrated by a comparison of
FIGS. 2A and 2B
. For example, friction stir weld joints
18
commonly have grain sizes of up to 0.25 inches after the resolution heat treatment. The large grains in the friction stir weld joint
18
resulting from the resolution heat treatment adversely affect the material properties of the weld joint, including reducing the hardness, ductility, resistance to intergranular corrosion, and fatigue resistance.
In seeking to minimize the degradation of the material properties of friction stir weld joints
18
during post weld heat treatments, several alternative approaches have been proposed, including shortening the duration of the solution heat treatment, post-weld annealing prior to solution heat treatment, and surface peening. However, these approaches have not been effective in reducing the grain growth of friction stir weld joints
18
during post-weld solution heat treatments.
Thus, there is a need for improved methods and apparatus for friction stir welding heat treated materials and, in particular, precipitation hardened materials. Such manufacturing methods and apparatus should realize the improved material properties associated with resolution heat treating while minimizing degradation of the material properties of the friction stir weld joint during such heat treatments.
SUMMARY OF THE INVENTION
The present invention provides an improved precipitation hardened structural assembly formed by friction stir welding and a method and apparatus of forming the same. According to one embodiment of the present invention, a precipitation hardened structural assembly is provided, including a first structural member and a second structural member positioned adjacent to the first structural member such that the first and second structural members define an interface therebetween. At least one friction stir weld joint joins the first structural member to the second structural member at least partially along the interface. The first and second structural members and the friction stir weld joint are solution heat treated at a first predetermined temperature schedule and precipitation heat treated at a second predetermined temperature schedule and wherein the friction stir weld joint comprises a refined grain structure having a grain size of less than about 5 microns. In one embodiment, the first and second structural members comprise dissimilar materials. In another embodiment, at least one of the first and second structural members is formed from aluminum, aluminum alloys, titanium, or titanium alloys.
The present invention also provides an apparatus for attachment to a rotatable spindle for forming a friction stir weld joint. In one embodiment, according to the present invention, the apparatus includes a friction stir welding tool in rotatable communication with the spindle. The friction stir welding tool defines a cavity. The apparatus includes at least one heater adapted to thermally communicate with the friction stir welding tool to thereby heat the tool and wherein the at least one heater is at least partially received in the cavity of the friction stir welding tool. The at least one heater can include a resistance heating coil, an induction heating coil, a quartz lamp, a gas torch, or a laser. In one embodiment, the at least one heater thermally communicates with the friction stir welding tool through convection, conduction, irradiation or induction. In another embodiment, the apparatus includes a sensor in thermal communication with the friction stir welding tool for measuring the temperature of the friction stir welding tool. In yet another embodiment, the apparatus includes a controller in electrical communication with the sensor and in operable communication with the at least one heater. The controller is configured to automatically modify the heat output of the at least one heater to modify the temp
Alston & Bird LLP
The Boeing Company
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