By Mark Shortt
Editorial Director, Design-2-Part Magazine

Experienced users of friction stir welding (FSW) invariably describe the process as a significant advancement in aluminum joining, capable of producing stronger, more durable joints with less distortion than conventional fusion welding. The technique uses the high rotational speed of a tool and the resulting frictional heat created from contact to crush, stir together, and forge a bond between two metal alloys. Since being patented in 1991 by The Welding Institute (TWI), Cambridge, England, friction stir welding has been the focus of considerable research and development that continues to produce innovations in machinery, tooling, and process development.

Many benefits of friction stir welding derive from its being a solid-state process, in which joining occurs without fusion (below the melting point) of the metal alloys to be welded. The absence of melting eliminates many of the problems normally associated with conventional metal welding, such as fumes, spatter, porosity, solidification cracking, and shrinkage. As a result, the process can be used to join materials such as 2000 and 7000 aluminum alloys that are difficult to fusion weld.

"The process joins materials that are generally considered unweldable, with high reliability and properties that approach the parent material," says Jack Thompson, chief engineer, General Tool Company (GTC), Cincinnati, Ohio. "Friction stir welding is penetrating critical applications for welded aluminum because it is an extremely robust, reliable process. If the process parameters are achieved, the weld will exhibit very predictable properties."

Aluminum alloys are not the only materials that can be joined by friction stir welding. Other low-temperature metalssuch as copper, lead, and magnesiumare currently being welded, and the joining of higher-temperature alloys, such as steel, titanium, and Inconel has also been demonstrated.

"Theoretically, almost any metal or thermoplastic can be friction stir welded," says Brent Christner, friction stir welding manager, Eclipse Aviation Corp., Albuquerque, New Mexico. "The key to the higher-temperature alloys is finding a pin tool material that retains stiffness, strength, fatigue, and wear resistance properties at the temperatures at which the materials being welded become plastic. Significant progress has been made in high-temperature tool materials over the last several years, and will soon be transitioned to production."

In conventional friction welding (a related, solid-state process), heat is generated as the two faying surfaces are rubbed together. However, friction stir welding goes one step further by employing a third piecea profiled, wear-resistant pin toolto create added heat and friction in the weld joint. During FSW, the pin of a shouldered tool is slowly plunged into the joint between the two materials to be welded, and rotated at high speed. The resulting friction creates a plasticized shaft of material around the pin. As the pin moves forward in the joint, it "stirs," or crushes, the plasticized material to create a solid-phase weld. The process uses a non-consumable tool that requires no filler wire or gas shielding for welding aluminum. Straight-line and complex-shape welds can be created while welding in virtually all positions.

Stronger, Lighter Suspension Links

Tower Automotive Inc. (Grand Rapids, Michigan) produces structural components, assemblies, and modules for original equipment manufacturers of passenger cars and light trucks. The firm recently worked in partnership with Ford Motor Company to develop a lightweight replacement suspension link with the increased durability necessary to withstand required loads. According to Tower Automotive, its testing confirmed the ability of friction stir welding to reduce weight, lower costs, and increase joint efficiency and fatigue life over gas metal arc welding, the method traditionally used by the automotive industry. During the design validation and process validation phases of the FSW link development, the company conducted tensile, compression, and fatigue tests to compare the FSW link design to tubular steel and stamped steel link designs.

"The fatigue test is where the FSW link outperforms the other two," says Art Scafe, advanced new product development leader for body and suspension structures. "On average, the tubular steel link failed around three lives, or 450,000 cycles, and the stamped link didn't make one life cycle. The friction stir-welded link lasted one million cycles, and when the load was increased by 15%, it went another 200,000 cycles without failure."

According to Scafe, the greatest challenges to producing friction stir-welded suspension links were in refining a manufacturing process and building the machine. Tower Automotive worked with ESAB to develop what it calls "the first synchronized, two-sided machine for producing lightweight suspension links." The synchronized, two-sided machine reportedly permits faster welding of 12.7-mm material (welding over 20 mm/sec), produces a stronger, double-sided weld with no void, and is less susceptible to variation in extrusion tolerances.

"Tower Automotive saw the market shifting to low-weight suspension components and found a suitable approach to produce low-weight components and still maintain good tolerance control," says Scafe. "The friction stir-welded link is 43% lighter than a comparable steel tubular link." By enabling the replacement of steel automotive parts with lighter-weight aluminum, friction stir welding allows new features to be added to a vehicle design without increasing overall weight. "If OEMs want to add a new gadget like a DVD player, heads-up display, or even a 'cushier' seat," Scafe says, "weight savings need to be incorporated in the overall design to compensate for the resulting weight gain from adding [new] features."

Generally, the level of quality generated by a friction stir weld is superior to other types of welding, says Scafe, who adds that product design "needs to be created with FSW in mind." In addition to producing butt welds, lap welds, and tee welds, "friction stir welding is capable of welding dissimilar-thickness materials," he says.

Until recently, however, the efficacy of the process had been hindered by two limiting factors: the requirement for pin tools of different lengths when welding materials of varying thickness; and the reliance on a single-piece pin tool that leaves a "keyhole" when retracted at the end of the weld.

RPT Adjusts to Variable Thickness, Prevents Keyholes

These limitations were overcome when the NASA Marshall Space Flight Center (MSFC) developed an automatic, retractable pin tool (RPT), which uses a computer-controlled motor to automatically retract the pin into the shoulder of the tool at the end of the weld. According to Jeff Ding, the NASA welding engineer who developed the idea of a retractable pin tool, the RPT has made it possible to use friction stir welding on materials that change in thickness during the weld.

"These welds are typical on the Space Shuttle External Tank, where thickness will transition from 0.310 inch to 0.650 inch to 1.00 inch," Ding says. "The RPT also 'closes out' the keyhole left by the departing pin at the end of the weld. An example would be circumferential welds where the pin retracts after it passes the starting point of the weld. The RPT is also required for the self-reacting pin tool currently being developed for NASA manufacturing programs."

A simple, hand-crank RPT was originally designed and fabricated to demonstrate the concept, according to Ding. The prototype made use of a manually turned wheel to retract the pin; its success led to more robust RPTs, which currently weld up to 0.650-inch-thick material, he says. Already, the innovation has contributed to the development of customized FSW that has proven to provide routinely reliable welds.

"This feature allows for variable-thickness materials to be completely joined, as well as contoured surfaces with variable thickness, when used in machines with sufficient degrees of freedom to track and orient the tool to the surface," says Jack Thompson of General Tool Company. In the friction stir welding equipment that General Tool developed for NASA /Lockheed Martin to create longitudinal welds in the barrels of the Space Shuttle's external tank, GTC devised what Thompson calls a "significant improvement." The patented GTC system employs a fused quartz measuring rod, inserted down the center of the RPT pin, to measure the length of the RPT pin independently of external loads and thermal expansion.

"Fused quartz exhibits a coefficient of thermal expansion ¹/₃₀ of the pin material," says Thompson. "An LVDT, affixed to the RPT spindle housing, bears on the measuring rod string and lets the machine track the end of the pin to 0.001 inch, regardless of loads and expansion of the pin. This compensation system keeps track of the exact length of the pin as its length grows thermally about 0.025 inch, and the pin is compressed by welding loads by about 0.010 inch. Prior to implementing this system, these pin length errors were compensated by trial and error, open-loop offsets."

One of the first companies to commercialize NASA Marshall's auto-adjustable technology last year was MTS Systems Corporation, a supplier of mechanical testing and simulation equipment based in Eden Prairie, Minnesota. The company has been awarded a patent for a self-reacting pin tool that uses front- and back-side shoulders to pinch the weld material. According to the company, advantages of self-reacting welds include the elimination of stiff anvil-side tooling, reduced process loads as a result of increased heat, and increased travel speed due to increased production of heat from the two shoulders.

Process Has Key Role in Assembly of Lightweight Jet

MTS Systems has been working closely with Eclipse Aviation Corp. to develop, validate, and certify the friction stir welding process for lightweight materials used in the Eclipse 500 jet. The six-person, twin-engine jet is currently undergoing a 16-month testing program that is expected to culminate with FAA certification in December of this year. Eclipse Aviation, reported to be the first company to use FSW in production on thin-gauge aircraft aluminum, is using the process to lap weld stiffeners (stringers and frames) to the aircraft skins of the Eclipse 500. Friction stir welding is slated to replace rivets in most major assemblies of the aircraft, including the cabin, aft fuselage, wings, and engine mounts.

For this project, the company has had to develop the technology for application to thin-gauge lap welds. Its objectives include corrosion protection of the mating surfaces, control of distortion, material property characterization, and the development, with MTS Systems Corp., of a friction stir welding system capable of welding complex contours. Eclipse received U.S. Federal Aviation Administration (FAA) approval of its friction stir welding process specification in May 2002, one year ahead of schedule.

At press time, some 9,488 inches of production welding had been completed for assembly of test/certification aircraft. Use of the technology has eliminated nearly 7,000 fasteners and associated hole drilling, and increased joint strength by up to three times, Christner says. It is also said to have resulted in rapid joining speeds four times faster than automated riveting and 20 times faster than manual riveting.

"On the alloys and thickness that Eclipse is using, tensile strength is up to three times greater than mechanical fastening," Christner says. "Fatigue life is comparable to or better than mechanical fastening. Testing of a full-size barrel simulating the aircraft fuselage showed fatigue life in excess of 23 aircraft lifetimes."

"In aircraft applications, friction stir welding has been evaluated in great depth with respect to yield and ultimate strength, fatigue life, da/dn, and corrosion," says General Tool Company's Thompson. "To date, these issues have been addressed for each particular weld geometry, tool, process parameters, and material pair. Friction stir welding generally does as well or better than a typical riveted or mechanically fastened joint."

General Tool recently replaced a cast panel with a fabricated panel composed of extruded plate and extruded shapes, a substitution that was made practical by the use of FSW. While weld strength was never an issue due to the high quality of the welds, the lack of distortion ensured consistent flatness and straightness of the weldment, thereby eliminating significant post-weld machining. Thompson also credits the "fast and predictable" nature of the process.

"While the operator of the FSW work station needs to be careful and consistent in loading material into the weld fixture, once the start button is pushed, the process is not dependent on the technique and undivided attention of a skilled artisan," he explains. "This material and process substitution saved General Tool over $1 million in producing about 400 of these highly finished, 72-in x 89-in and 72-in x 59-in panels. These savings were realized after building a FSW machine, buying a process license, and developing a proven set of process parameters."

According to NASA's Jeff Ding, the cost savings associated with friction stir welding are a result of the reliability and repeatability of the process. "When the proper weld parameters are established for a particular thickness and a specific aluminum alloy, the process as compared to fusion welding produces fewer defects," says Ding. "This, in turn, reduces the costs of Material Review Board dispositions. (The Material Review Board evaluates specific defects and determines if a repair is required, and if so, how the repairs are to be completed.) Also, as important to the External Tank Program as the cost savings, is the reliability issue. The friction stir welding process improves hardware safety margins."

Tools, Materials, & Process Developments

The recent emergence of industrial robots as integral components of friction stir welding systems has produced another option for companies seeking to reduce costs of capital equipment, operations, or outsourcing. Greater flexibility and operating efficiencies, as well as major cost reductions, are among the advantages that robotic friction stir welding can bring to providers and users of friction stir welding services.

Ding says that a design concept for a hand-held friction stir welding device was recently introduced, and is undergoing further development. Additional work is focusing on pin tools: one redesigned retractable pin tool is said to be capable of welding and closing out material two inches thick. Other activity includes the development of a new processthermal stir weldingthat separates the primary elements of friction stir welding (heating, forging, and stirring) for independent control.

Significant advancements have also been made in the development of five-axis, gantry-style FSW machines and general-purpose FSW machinery, according to General Tool's Thompson. Currently in development are machines that can handle up to 35,000 pounds of thrust within a working envelope of 8 ft x 10 ft x 26 ft. The machines, which typically include retractable pin tool heads, can weld a general path in space, thereby enabling the welding of aircraft skin shapes. They also have tool-tilt servo axes, which allow the tool to be oriented to the work to meet path tangency and process control requirements.

Thompson says that the development of friction stir weld tool materials, such as tungsten-rhenium and polycrystalline cubic boron nitride (PCBN), allow the FSW process to "broaden into applications involving steel, nickel base alloys, and stainless steels." Although these tool materials are still in the research phase, the success demonstrated to date "indicates that the process will soon be employed all across the materials spectrum," he says.

Modifying the Microstructure by Friction Stir Processing

"Another very exciting development is the research into employing the same tools, equipment, and process to modify the microstructure to improve material properties of cast and wrought materials," Thompson says. "DARPA (Defense Advanced Research Projects Agency) is funding research that seeks to remove porosity and convert the cast microstructure to a hot-worked, wrought microstructure in a broad range of materials by 'friction stir processing' (FSP) the material. In many circumstances, FSP can be used to improve tensile and yield strength, as well as increase elongation. Friction stir processing is also being used to alter the microstructure in thick-section aluminum alloys (0.2 inch thick) for enhanced super-plasticity. Similarly, FSP is used to improve room temperature formability of very thick (>1 inch) aluminum plate."

Challenges to Overcome

Despite the improved mechanical properties inherent in the process, the use of friction stir welding has been limited by the scarcity of FSW machinery within industry, Thompson says, and by the significant tooling requirements for holding and backing a workpiece. Also, without the benefit of referring to long-established experience and handbook guidance that is widely available for traditional welding processes, engineers need to do job-specific process development and qualification for friction stir welding.

Another limitation, according to Eclipse Aviation's Brent Christner, is that friction stir welding requires access to both sides of the joint. He notes that while conventional FSW requires welding against an anvil, self-reacting FSW requires that a shouldered tool be used on both sides of the weld. "Friction stir welding is less forgiving to poor fit-up than some conventional welding processes," says Christner. "Joints, in some cases, have to be redesigned if applied to existing hardware." He adds that savings from the process should be significant enough to offset the high capital investment and annual licensing fee required by TWI, the owner of the intellectual property rights to friction stir welding.

NASA's Jeff Ding believes that friction stir welding will be unable to realize its full potential until the intellectual property rights expire and there is greater availability of lower-cost welding systems and tooling. "The TWI license is too expensive for the 'mom and pop' shops to secure a license," says Ding. "Investment costs are also high: tooling is expensive for the smaller operations to be able to afford it. These costs need to be reduced."

Future Directions

According to Christner, the process would benefit from increasing "public domain" knowledge of material properties, improved understanding of weld metallurgy and the effect of FSW on corrosion resistance, and the development of pin tool materials for higher-temperature alloys. Some of the R&D work currently being done, he says, involves the application of the process to higher-temperature materials, thin-gauge lap welding, and process modeling. Further work is under way in thick-section welding, designing pin tools to increase welding speeds, and pre- and post-weld thermal treatments to improve corrosion resistance.

Tower Automotive has researched using FSW to produce tailor-welded blanks and is evaluating the potential for using FSW to spot-weld aluminum components. "Friction stir welding applications are limitless, so growth potential is enormous," says Scafe. "Things that were not viewed as a potential application are being considered today."