Albuquerque, NM—Friction stir welding, a patented method for joining tough-to-weld aluminum alloys, got its start on the high seas as a way to put together marine decking. But Eclipse Aviation Corp. has now adopted the method for a loftier task—assembling a new jet's fuselage and wing structures.
To form joints, friction stir welding (FSW) relies on the frictional heat generated by a rotating tool spinning on the surface of the aluminum. A pin-shaped protrusion on the tool carries heat from the surface down into the joint, creating a plasticized region that consolidates in the pin's wake.
Unlike traditional welding, the friction-stir process doesn't melt the material but stays roughly 100C below its melting temperature to avert distortions, porosity, and a loss of mechanical properties, according to Jerry Gould, chief engineer at the Edison Welding Institute (Columbus, OH).
Aside from marine decking, the method has so far been applied to various fuel tanks and air ducts. Boeing has also used the process to build the body for its Delta rocket. "However, these applications have all used butt joints and relatively thick sheets or plates," notes Dr. Oliver Masefield, Eclipse's vice president of engineering.
Eclipse will use FSW a bit differently. To build primary structures for its forthcoming Eclipse 500 jet, the company will rely on lap joints that attach the aircraft's thin aluminum skin, measuring 0.040 to 0.050-inch thick, to underlying frame members. According to Masefield, these continuous welded joints have about three times the static strength offered by a single row of rivets. "And the welded joints have a fatigue strength that is at least as good as a riveted joint," he adds.
Eclipse plans to assemble the jet's fuselage by first welding up three 12-foot-long sub-sections—one for the floor and two for the sides. These sections will be riveted together, but the overall use of rivets still drops dramatically. "Friction stir welding eliminates about 65% of the riveted joints that we would otherwise need," Masefield says.
FSW also reduces the weight of the plane, shortens process cycle time, and decreases cost. Masefield estimates that the welded joints shave at least 50 lbs from the Eclipse 500, a savings that translates into an additional range of roughly 50 nautical miles. This weight reduction comes not from the loss of rivets but from welding's contribution to strong structures: Unlike laser or fusion welding, FSW enables the selective use of high-strength 7000 Series high-strength aluminum alloys to supplement the 2000 Series aluminum used throughout the plane. Better-performing materials, in turn, permitted thinner, lighter wall sections and reduced flange widths, Masefield explains.
And FSW is speedy. "The real story here is cycle time," Masefield says. While manual riveting operations crawl along at less than 2 inches/min, and automated riveting walks along at 6 inches/min, FSW sprints at speeds greater than 20 inches/min. "Our experiments show that we can go even faster—up around 40 inches/min," Masefield says. As for cost, Masefield calculates that the fast, highly automated FSW will save between $50,000 and $100,000 per plane.
The only problem with FSW, which was invented at the British Welding Institute ten years ago, comes down to a lack of experience in aircraft applications. "Riveted structures have well-known design allowables," Masefield says. But FSW does not, forcing Eclipse engineers to generate the reams of process and performance data required for FAA certification, which Masefield expects in December 2003.
With the new jet's commercial launch slated for 2004, Eclipse has already cranked up its $1 million FSW gantry from Eden Prairie, MN-based MTS Systems Corp. Pre-production runs for the company's flight-test aircraft will start this November.