This article provides details of a low-temperature joining technology called friction stir welding. Friction stir welding (FSW) 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. According to an engineer, stir welding eliminated 60 percent of the rivets that the plane would have otherwise required. Eclipse Aviation Corp., Albuquerque, NM, is building a separate plant to house its stir welding operations for commercial production, once its plane receives certification by the US Federal Aviation Administration. FSW is a solid-state process, more like forging and extruding than to fusion welding. Since the process is solid state, the joint is not subject to any shrinkage because 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.
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.
Matt Hansen is a systems engineer with MTS Systems Corp. in Eden Prairie, Minn.
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 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% 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.
By Brent Christner
Eclipse Aviation Corp. is developing the Eclipse 500 twin-engine jet to enable an affordable alternative to the airline hub-and-spoke system. With its low acquisition and operating costs, the Eclipse 500 provides the basis for a point-to-point air taxi service that will be cost competitive with full-fare coach tickets on a major airline. One of the enabling technologies to achieving the Eclipse 500’s low cost is friction stir welding.
FSW makes possible joining speeds six times faster than automated riveting or 60 times faster than manual riveting, with improved quality. To make the use of stir welding a reality, however, a number of technical challenges had to be overcome, including distortion control, corrosion protection, and development of material properties for use in the aircraft design. FAA approval of the Eclipse Aviation FSW process specification was received in May 2002.
Stir welding is being used on the Eclipse 500 to build up integrally stiffened skin panels by lap welding stringers and frames to pocketed skins. The pockets are machined or chemically milled into the skins between the stiffeners for weight savings. The aircraft skins currently being welded include the cabin, aft fuselage, and wing skin panels. All the skin panel details were sized to accommodate riveted field repairs, using conventional techniques for aluminum aircraft.
The engine mount beam was designed specifically to take advantage of stir welding to yield an extremely light, stiff structure. Overall, there are a total of 263 welds on the aircraft with a total length of 136 meters (5,354 inches). The welds replace 7,378 conventional fasteners.
Friction stir welding of the Eclipse 500 is being done on a custom 7-axis machine designed and built for Eclipse Aviation by MTS Systems Corp. To date, three aircraft have been completed and a fourth is under way.
A new 50,000-square-foot friction stir welding center, being built in Albuquerque, N.M., is scheduled to be operational in September. The center will house up to three FSW systems, an automated transfer line, and 12 tool-loading stations. At the loading stations, a hoist and trolley system for the stringer and frame mandrels will enable rapid loading and clamping of the aircraft components into the FSW tools. Once a tool has been loaded, the automated transfer line will shuttle it to the next available gantry for welding.
Brent Christner is materials and process engineering lead and friction stir welding manager at Eclipse Aviation Corp. in Albuquerque, N.M.
Tool exchange time will be under one minute. With three gantries, the capacity of the facility will be in excess of four aircraft a day.
Friction stir welding has a limited number of easily controlled variables and can tolerate a significant amount of variation without affecting weld properties. As a consequence, rework and the associated impact on cost and cycle time are significantly reduced over conventional fastening.
Testing performed by Eclipse has shown that stir welding produces joints with more than twice the strength of a single-row riveted joint. Fatigue life at least equals riveted joints for the Eclipse 500 alloys and stress levels.
Barrels simulating the aircraft cabin were cyclically pressurized to characterize the fatigue and damage tolerance of representative structures. Four FSW curved panels were attached to a pressure test fixture to form a cylinder equal to the maximum cabin diameter. Each panel consisted of two stringers and two frames that had been friction stir welded to a roll-formed skin.
The barrel was tested by cyclically pressurizing to 8.33 psi, simulating cabin pressure at 41,000 feet. Three test sequences were conducted. During each one, the barrel was made up of two fatigue panels and two damage tolerance panels. The damage tolerance panels had one or more 51-mm-long artificially induced starter notches machined into the panels prior to testing.
The fatigue panels were monitored for naturally occurring fatigue crack initiation. The damage tolerance panels were monitored for crack growth rates.
Crack initiation in the fatigue panels did not occur until 371,000 cycles, or 18.5 aircraft lifetimes. After 473,000 cycles (23.6 lifetimes) the maximum crack length was just 19.8 mm, or 0.78 inch.
Crack growth data from the barrel panels with 51-mm starter notches placed in the welds was equally encouraging. After initiation, the cracks immediately turned and grew into the parent metal. In no case was crack growth observed along the weld line. The maximum crack length on damage tolerance panels that had received the Eclipse-developed lap joint corrosion protection treatment prior to welding was 94 mm (including the 51-mm starter notch) after 220,000 cycles.
The development and use of FSW at Eclipse Aviation have resulted in reduced cycle time and improved quality over conventional fastening, and have become key factors in the manufacture and sale of a low cost twin-engine personal jet. Our comprehensive online resources are the quickest way to find products that meet your power transmission requirements.
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 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 A1 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.