a cooler weld
An emerging joining technology lets manufacturers rethink how products fit together.
By Matt Hansen
The Eclipse 500, which made its first test flight last August, is designed as a means to get around the system. It is a personal twinjet aircraft that can carry four to five passengers. The manufacturer says it will have a range of 1,300 nautical miles, or about 2,400 kilometers, and cruising speed of 355 knots, or more than 650 km an hour. That performance will give the jet access to thousands of municipal airports across the United States. These are the airports that don't have the runways or facilities to handle the large commercial jets (read: no traffic or crowds).
Distinguished By Price
Other small jets can do that, too. What distinguishes this one is its
price: under $1 million.
The manufacturer, Eclipse Aviation Corp. of Albuquerque, N.M., says that
is approximately one-fourth of the cost of a comparable small jet aircraft.
The company has attributed the low price partly to design and manufacturing
efficiencies made possible by a low-temperature joining technology called
friction stir welding. Major assemblies are being manufactured with the
process.
The
manufacturer of the Eclipse 500 twinjet says friction stir welding of
major components will bring the airplane in at a purchase price under
$1 million; 263 welds replaced more than 7,000 fasteners.
Friction stir welding uses a cylindrical, shouldered tool with a profiled
pin that is rotated and slowly plunged into the joint line between two
pieces of sheet or plate material. Frictional heat between the wear-resistant
welding tool and the workpiece causes the metal to soften without reaching
the melting point and allows the tool to traverse the weld line.
As it does, the plasticized material is extruded around the pin. A solid-phase
bond with extremely fine-grain structure is the result.
Without the shoulder the extruded material would move up along the pin
and create a rooster or fantail, since the material is solid and not self-leveling.
MTS Systems Corp. of Eden Prairie, Minn., is one of a handful of companies
licensed to market equipment for friction stir welding, which is often
called FSW, and has been Eclipse's supplier. The technology was developed
about a dozen years ago by The Welding Institute, a membership-based research
and technology organization in Cambridge, England.
Fewer Riveted Joints
According to Brent Christner, the materials and process engineering lead
at Eclipse, stir welding eliminated 60 percent of the rivets that the
plane would have otherwise required. In a presentation to the International
Council of Aeronautical Sciences last year, Christner reported that in
side-by-side tests, friction stir welded joints had two or more times
the static strength of a comparable yet conservatively designed riveted
joint. Fatigue properties of stir welded joints were found to be at least
as good as those of riveted joints.
Eclipse estimates that FSW reduces process time for assemblies by two-thirds;
that is, stir welded assemblies will take 1.2 shifts to complete, versus
3.6 shifts for automatically riveted assemblies. Stir welding also eliminates
rivet costs, as well as any handling and overhead costs associated with
fasteners.
Eclipse is building a separate plant to house its stir welding operations
for commercial production, once its plane receives certification by the
U.S. Federal Aviation Administration. The 50,000-square-foot plant, which
is due to be completed this spring, will be able to house equipment to
turn out as many as 1,500 airplanes a year.
FSW for use in commercial and private aircraft has been an area of interest
for a number of companies. Boeing and Airbus both have been developing
the process for aircraft for a number of years, and Airbus has plans to
use the process for skin-to-skin butt welds on various aircraft. Many
smaller aircraft manufacturers and components suppliers are also developing
FSW for various
applications.
In the past decade, friction stir welding has been developed for many
diverse industries, not only in aerospace, but also in automotive, marine,
and nuclear assemblies.
Marine Aluminum of Norway uses FSW to join long aluminum extrusions into
flat panels and was the first manufacturer to use stir welding in production.
The panels most commonly are used as decks and bulkheads in fast ferries.
A
stir welding system used by Eclipse Aviation has completed part of a aircraft
cabin skin. The company says it also uses the process on the fuselage
and wing skin, and each aircraft has a total 136 meters of welds.
A number of automotive companies have been developing stir welding for
various applications. Stir welding has been used successfully to create
suspension components, crash boxes, and wheel rims. One of the production
applications is the joining of two extruded panels for a seat frame. Tower
Automotive Inc., based in Grand Rapids, Mich., is using FSW to create
tailored blanks by joining extrusions. These blanks are then sliced perpendicular
to the weld direction, and the resulting component is used as a suspension
arm.
SKB, the Swedish Nuclear Fuel and Waste Management Co., has purchased
a stir welding system that seals vessels used for containing spent nuclear
fuel rods.
Meanwhile, Boeing has been using stir welding for the past few years to
build its Delta rockets.
The rockets were originally manufactured using gas metal arc welding and
variable polarity arc welding, depending on the joint. While it's been
successful, the fusion welding process had high costs associated with
rework and joint preparation. The fusion welding processes averaged a
weld defect every 330 inches, or every 8 3/8 meters.
According to a paper by Dave Nicholas of The Welding Institute, during
the first four years in which Boeing used stir welding to manufacture
Delta rockets, it produced over 2.5 km of continuous defect-free welds.
This improvement resulted in a cost savings of several hundred thousand
dollars per year.
There are additional savings in the preparation stage; fusion welded edges
need to be etched prior to welding, while FSW requires only a solvent
wipe.
Friction stir welding is a solid-state process, more similar to forging
and extruding than to fusion welding.
No Filler Material Required
FSW does not require consumables in the same sense as fusion welding.
The process is autogenous, meaning that filler material is not required.
Cover gases are also not required in many cases, although the use of a
cover gas has proved beneficial for titanium and steels.
Unlike fusion welding, stir welding does not emit radiation, so welding
curtains and face shields are not required. Some people have said that
FSW is one of the most boring processes to watch. Without sparks, fumes,
or chips, the only thing to watch is a machine quietly stir welding parts
together.
Stir welding does not rely on an experienced operator with the right touch,
because the process has a near-zero defect rate. Given a well-designed
and integrated FSW system, the process is extremely robust.
Various sources have cited joint strength increases as high as 30 percent
when compared to fusion welding. The joints also have much higher elongation
than fusion welding, as much as two or three times more, depending on
the alloy and heat treatment. This higher elongation increases the energy
absorption in the weld prior to failure, which is beneficial for high
shock applications.
Since the process is solid state, the joint is not subject to any shrinkage
as a result of phase changes. The process also introduces minimal heat
into the weld, so the heat-affected zone is relatively small in comparison
to arc welding.
Welding the Unweldable
Potentially, the biggest advantage for FSW is the ability to join unweldable
aluminum alloys. FSW is routinely used to join 2XXX and 7XXX series aluminums.
Fusion welds in these alloys can have high defect rates or are too brittle
to be useful. Stir welding can be used to join dissimilar aluminum alloys
as well.
The better material properties can also cut expenses by reducing material
costs. Sources at Boeing have said that stir welded joints have shown
30 to 50 percent increases in tensile strength, fracture toughness, and
fatigue strength at both room and cryogenic temperatures. The increase
in strength may permit an increase in payload, a decrease in joint thickness
to save weight, or a reduction in processing costs for the rocket skins.
Boeing has also been able to salvage a rocket that was built using fusion
welding that had been declared unusable from a quality standpoint. To
repair the rocket, Boeing decided to stir weld directly through the fusion
weld. The repair procedure was successful, and the rocket was declared
launch-worthy.
NASA
and Lockheed Martin plan to install a large stir welding system to build
the Space Shuttle's external fuel tank, shown in this rendering as the
dome at right, of a superior alloy that resisted conventional fusion welding.
NASA has a plan, in the works before the loss of Columbia and its crew,
to use stir welding to improve the external fuel tank of the Space Shuttle.
The new tank will use a superior alloy, Al-Li 2195.
Although the Al-Li 2195 was known to offer strength and density improvements
over the Al 2219 alloy currently used in the tank, difficulties encountered
during the fusion welding process adversely affected productivity.
Engineers from NASA's Marshall Space Flight Center and Lockheed Martin
Space Systems Co. have successfully demonstrated the fabrication by friction
stir welding of a full-scale tank barrel using Al-Li 2195.
A universal welding system from MTS Systems is being installed at NASA's
Michoud assembly facility in New Orleans. The system will be operated
and maintained by Lockheed Martin Space Systems.
As part of NASA's Next Generation Launch Technologies Program, the Michoud
unit will stir weld full-size test panels representative of a dome section
of a reusable cryogenic tank. The five-axis friction stir welding system
with a horizontal-boom configuration will have a welding envelope of 16
feet by 20.5 feet by 10 feet, the largest envelope of any stir welder
in the world. After completion of the representative dome section, Lockheed
will use the system for various production programs for itself and other
equipment manufacturers.
Friction stir welding has gained a significant level of acceptance in
a relatively short time. You may already be using something that contains
stir welded parts and not even realize it—maybe that commuter train
in Japan or ferry in Europe, or even a pair of B&O stereo speakers.
In the coming years, MTS Systems expects the process to become even more
widely accepted as manufacturers take advantage of the benefits provided
by a simple and robust joining process—welding that really isn't.
Matt Hansen is a systems engineer with MTS Systems
Corp. in Eden Prairie, Minn.