Laser Increases Lifetime of Jet Engine Fan Blades
Ruth A. Mendonsa
During takeoff, jet engine fan blades can take a heavy beating. Foreign objects such as stones and ice can be sucked into the engine, denting the blades. The damage shortens the blades' useful lifetime and makes it necessary to remove them from service. A well-known manufacturer of aircraft engines is strengthening these components with laser shock peening equipment from LSP Technologies Inc.
The laser shock peening system increases strength and resistance to cracks on jet engine fan blades.
Laser shock peening, a mechanical process for treating metal surfaces, produces deep, residual compressive stresses in metal parts that increase strength and resistance to other service-related material failures. Allan Clauer and Jeff Dulaney, now vice president and president, respectively, of LSP Technologies, developed the technology while at Battelle Memorial Institute in Columbus, Ohio. LSP has dramatically improved the production capabilities of the technology with its proprietary LaserPeen process.
The process uses high-energy laser pulses from a 1.054-µm neodymium phosphate glass laser to generate a shock wave that produces stresses in a metal part. This is achieved by coating the part with an absorbent layer of material, such as black paint, and a transparent layer, such as water. A surface plasma results when the laser pulse interacts with the thin absorbent overlay. As the plasma expands, a shock wave is driven into the part surface. The strength of the shock wave is increased significantly by the transparent overlay.
The laser shock peening system consists of a high-energy pulsed laser, a beam-delivery system and a robotic parts-handling station. It simultaneously can place two beams, each containing 50 J in a 20-ns pulse, on a part. It peens a part at repetition rates up to 0.5 Hz.
The system typically produces stresses in stronger metals of more than 900 MPa at the surface, which penetrate to a depth of a millimeter or more. These stresses increase service lifetime and resistance to surface crack initiation and propagation from fatigue, fretting and stress corrosion. By comparison, conventional shot peening, which is the bombardment of a material with small metal shot, produces residual stresses to a nominal depth of about 0.25 mm and deforms the surface much more than laser shock peening does.
Clauer said the process can be applied to many aerospace products, such as discs, rotors, gear shafts and structural components, as well as to automobile parts, hip implants, industrial equipment and tooling. Air Force tests have shown that severely damaged fan blades treated with laser shock peening maintain fatigue strengths equal to or greater than that of undamaged blades, as compared with a 70 percent fatigue loss on an untreated blade.
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