Stock material or miscellaneous articles – All metal or with adjacent metals – Honeycomb – or with grain orientation or elongated elements...
Reexamination Certificate
2001-03-27
2003-03-25
Zimmerman, John J. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Honeycomb, or with grain orientation or elongated elements...
C428S598000, C428S629000, C428S654000
Reexamination Certificate
active
06537682
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to structural assemblies and, more particularly, relates to the application of friction stir welding to superplastically formed structural assemblies.
BACKGROUND OF THE INVENTION
Superplastic forming (“SPF”) is a process used to form structural assemblies having complex three-dimensional shapes, such as the two- and three-sheet assemblies
10
,
11
illustrated in
FIGS. 1A and 1B
, respectively. These assemblies are formed from metal alloys, such as aluminum and titanium alloys (particularly Zn-22Al and Ti-6Al-4V) that exhibit superplastic behavior at certain temperatures, i.e., large elongation (up to 2000 percent) under low strain rates. During the SPF process, a multi-sheet SPF pack is placed into a shaping die and heated to a sufficiently high temperature to soften the sheets of material. Pressurized heated gas is then injected into the SPF pack, causing the pack to inflate and fill the die. The assembly is then cooled and removed from the die and final machining steps are performed, such as edge trimming, to form the finished structural assembly.
As illustrated in
FIG. 2A
, the SPF pack
12
used to form the structural assembly is constructed by stacking two or more sheets
13
of material (a three-sheet SPF pack is illustrated in
FIG. 2
) and joining the sheets by forming partial-penetration weld joints
14
making a pre-selected pattern using any conventional fusion welding processes such as oxyfuel, arc, and resistance welding. A partial-penetration weld joint joins two or more adjacent sheets in a stack, but generally does not join all the sheets in the stack. The partial-penetration weld joints define areas therebetween where the adjacent sheets
13
remain in contact after the SPF pack
12
has been inflated to form the structural assembly. As illustrated in
FIG. 2B
, prior to inflating the SPF pack
12
, the sheets of material
13
in the stack are joined by full-penetration weld joints
16
along the periphery of the stack to thereby form a sealed pack
12
. Plumbing fittings
17
are connected to the interior of the pack
12
through gas passages (not shown) machined into or between the sheets of material so that pressurized heated gas can be injected into the pack. The SPF pack
12
is typically sealed around the plumbing fittings
17
by fillet welds formed between the fittings
17
and the edge of the pack
12
using conventional fusion welding processes.
The SPF process is particularly advantageous since complex shapes can be formed with lower tooling costs. Additionally, structural assemblies formed using the SPF process have minimal residual stresses. Notwithstanding these benefits, the materials used during the SPF process are generally limited to those that are readily weldable using conventional fusion welding techniques, such as oxyfuel, arc, and resistance welding, due to the necessity of forming partial-penetration and full-penetration welds in preparing the SPF packs. Thus, “unweldable” materials are unavailable to designers for use with the SPF process, as these materials produce relatively weak weld joints. “Unweldable” materials are materials that possess high conductivity and quickly dissipate heat away from the weld joint and/or that exhibit cracking along the weld joint as a result of stresses caused by thermal expansion. Such materials include aluminum and some aluminum alloys, particularly some AA 2000 and 7000 series alloys. The exclusion of these materials from use with the SPF process has been problematic, as many of these materials possess special corrosion, fatigue, strength, density or ductility characteristics that are desired in certain applications.
In seeking better methods for forming SPF packs and, in particular, forming the partial-penetration and full-penetration welds between the individual sheets in the pack, a relatively new welding process known as friction stir welding has been proposed. As illustrated in
FIGS. 3 and 3A
, friction stir welding is a solid state process in which the probe
18
of a rotating friction stir welding tool
15
, which is attached to a friction stir welding machine (not shown), is forced into or between workpieces
19
that are to be joined. The frictional heat generated by the rotating probe
18
and the shoulder
15
a
of the friction stir welding tool
15
creates a plasticized region or joint between the workpieces
19
that subsequently solidifies thereby joining the workpieces. See U.S. Pat. No. 5,460,317 to Thomas et al. for a general discussion of friction stir welding, the contents of which are incorporated herein by reference.
Although friction stir welding is a solid state process that can be used to join materials that were previously considered unweldable using conventional fusion welding techniques, the use of friction stir welding to form weld joints between stacked sheets of material during the construction of SPF packs presents several problems. First, as illustrated in
FIG. 4
, the frictional heat conducted to the interface between the sheets
20
by the rotating friction stir welding probe
18
and the tool shoulder
15
a
, when combined with the pressure exerted on the sheets by the shoulder, can cause thermo-compression welding
21
of the interface between the adjacent sheets resulting in weld joints as wide as the diameter D of the shoulder. In this regard, the diameter D can vary, depending on the thickness of the material being welding, from approximately 0.2 inches to approximately 1 inch, and even as much as approximately 1.6 inches for relatively thick sheets. Thermo-compression welding
21
is particularly a problem when friction stir welding thin sheets of material, on the order of 1.5 mm in thickness. Ideally, in order to maintain the tolerances of the finished structural assembly and minimize stock material usage, the weld joints should only be as wide as the diameter P of the friction stir welding probe
18
, which typically is about as large as the thickness of the plate or plates to be welded. For example, for a 1.5 mm plate, a 1.5 mm diameter probe would be acceptable. Secondly, as illustrated in
FIG. 2B
, to contain the pressurized heated gas that is injected into the SPF pack
12
during the SPF process, the pack is sealed by forming full-penetration weld joints
16
around the periphery of the pack. However, on the side of the SPF pack
12
where the plumbing fittings
17
are attached, friction stir welding cannot be used as the rotating probe
18
will impinge upon and damage the plumbing fittings and/or obstruct the internal passages intended for delivery of gas to the interior portion of the SPF pack.
Thus, there is a need for improved methods of forming SPF packs, and particularly, for friction stir welding SPF packs. Such manufacturing methods should be cost effective, minimize thermo-compression welding of the interface between adjacent sheets of material and prevent damage to the plumbing fittings of SPF packs.
SUMMARY OF THE INVENTION
The present invention provides a superplastically formed structural assembly and an associated method for manufacturing. The structural assembly includes first and second structural members having facing surfaces. The first and second structural members can include a first outer structural member, a second outer structural member or one or more intermediate structural members. In one embodiment, the first and second structural members include first and second outer structural members. In another embodiment, the first and second structural members include first and second intermediate structural members. The first and second structural members may be formed of titanium, aluminum, or alloys thereof. In one embodiment, the first and second structural members are formed of dissimilar metals.
The structural assembly includes at least one friction stir weld joint joining the first and second structural members. The structural assembly may include a plurality of friction stir weld joints joining the first and second structural members. In one embodiment, the plurality
Alston & Bird LLP
The Boeing Company
Zimmerman John J.
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