When Boeing’s first 747-8 Freighter is delivered it will be lifted into flight on wings shaped and made stronger by laser shot peening.
The process grew out of Star Wars-era research by the Defense Advanced Research Projects Agency and, over the past decade, has found its way into aviation, most prominently in engines and airframes for combat aircraft. But it is beginning to play a role in commercial aircraft, particularly in engines, where Rolls-Royce uses it to impart deep residual stress into its Trent series, including the new Trent 1000 for the Boeing 787. By mitigating fatigue, laser peening extends the time between required inspections.
|Credit: METAL IMPROVEMENT |
Slowly, laser peening is working its way into airframe manufacturing, which is where the newest 747 comes in.
In 2008, Boeing turned to Curtiss-Wright’s Metals Improvement Co. (MIC) to establish a laser-peen-forming production cell inside the airplane maker’s Frederickson skin and spar facility in Puyallup, Wash. MIC’s ability to impart precise, deep compressive stresses into the wing’s aluminum alloy skins is being used to give the wing its complex curvature.
One of Boeing’s accomplishments on the -8 has been a complete reengining of its wing’s aerodynamic shape to improve the new airplane’s efficiency. That shape is the application for MIC’s laser peening technology, and the company says it has talks under way for other airframe uses.
“We actively promote the use of laser-peen-forming to make aluminum wing panels more competitive with composite wings by enabling designers to conceive of more intense curvature in thick areas to yield better aerodynamics,” says MIC’s director of laser technology, C. Brent Dane. Besides improving a metal surface’s resistance to stress—on the wing, that would include foreign object damage (FOD)—laser peening also can be used to shape metal sections and to join panels. “Laser-peen-forming could potentially reduce original equipment manufacturing costs by enabling the forming of a single, wide panel,” he says.
MIC’s laser peening is part of Curtiss-Wright’s Metals Treatment unit, which includes conventional shot peening. Curtiss-Wright began strengthening Airbus A300 panels via shot peening in the 1980s.
In April, Curtiss-Wright forecast a 20-24% jump in this year’s metal treatment revenues over 2010 results, which will push them to $265-275 million.
MIC grew out of laser fusion research for the National Ignition Facility at Lawrence Livermore National Laboratory, which uses lasers to generate X-rays for nuclear fusion.
MIC’s application relies on a rat-a-tat-tat burst of hundreds of 1-gigawatt laser pulses lasting just 25 nanoseconds. They are shot through a 1-mm-deep flow of water to confine the laser’s energy and then penetrate an ablative layer, usually aluminum tape, that coats the metal surface and creates a plasma. The laser’s optics are computer-controlled and each pulse’s energy is recorded. On the 105-ft.-long section that Boeing peens, MIC imparts some 300,000 spot pulses.
Rolls-Royce was MIC’s first customer and its largest, applying the process initially to repairs of compressor and turbine blades. Later it began using laser peening in new parts manufacturing to strengthen them for extended-life performance. The Trent 500/800/1000 and XWB engines all use laser peening.
One of MIC’s challenges was to turn a lab laser into one robust enough for manufacturing. “We now have a highly, highly reliable system,” says Dane.
Besides compressing metals better than conventional shot peening, laser peening also works at extreme angles—as much as 70 deg., a big advantage for life-extension work on already manufactured structures; MIC is currently working on the fuselage of the Lockheed Martin/Boeing F-22 fighter.
To win the F-22 contract, MIC created a portable laser system in a standard truck trailer. It also has developed movable beams (see time-lapse photo, above), particularly useful for working around the edges of structures like the F-22’s fuselage.
General Electric Aviation in Evendale, Ohio, began researching laser peening in the mid-1990s, turning to technologies originally developed at Battelle Laboratories. But it found the early machines fragile, recalls GE’s laser-peening program manager, Mark Bailey. So GE developed the Gen4 technology in-house, which relies on a solid-state YAG (yttrium, aluminum, garnet) laser system.
In 2004, GE began applying laser peening to compressors to strengthen them against FOD and now produces all of the F110 engine family’s fan blades using the process.
While U.S. firms have gained a global customer base for laser peening, Bailey has no doubts that competition is coming. “I am quite sure the Chinese are nipping at our heels even as we speak,” he says.