Engineering Concepts: Laser Shock Processing
Lasers have been used to selectively heat treat metals for more than 25 years, but lasers also can be used to selectively induce compressive residual stresses in metal surfaces. In 1974, Battelle Laboratory (Columbus, OH) researchers patented (US1982000378975) a process to induce compressive residual stresses using shock waves generated by laser pulses. Results of this treatment, called laser shock processing (LSP), are similar to those obtained using traditional shot peening.
LSP does not involve the direct heating of the metal surface, but rather, involves directing a laser beam pulse onto an opaque overlay applied to the area being treated. Any paint that absorbs laser radiation can be used for the overlay; paints containing iron oxide or carbon typically are used. Black electrical tape also can be used. A small portion of the overlay is vaporized when the laser beam pulse hits it, creating an expanding gas release, which is further heated by the laser pulse. A pressure-induced shock, or stress, wave created at the surface is transmitted through the metal. A compressive residual stress is produced when the maximum stress of the shock wave exceeds the dynamic yield strength of the metal. The magnitude of the stress wave can be amplified by constraining the gas release with a laser transparent cover. Water typically is used for this purpose (fig. 1) and can simply be poured over the surface during LSP. Using water as a physical constraint generates a pressure in the range of 0.9 to 1.5 x10^6 psi (6 to 10 GPa), which is more than adequate to plastically yield the metal surface and create the beneficial compressive residual stresses.
The depth of the induced compressive stress depends on the attenuation of the shock wave, which is material dependent. Affected depth typically ranges from 0.02 to 0.06 in. (0.5 to 1.5 mm). The induced compressive stress decreases with increasing distance from the surface. Harder materials tend to develop deeper affected depths because stress waves are not attenuated as rapidly. Multiple laser pulses can be used to drive the compressive stresses deeper below the surface (fig. 2) by work hardening the metal surface during each pulse, lowering the attenuation of the shock wave. By comparison, the residual stress developed using traditional shot peening is limited to a depth of approximately 0.01 in. (0.25 mm).
The laser used in LSP typically is a Q-switched neodymium-glass laser having a pulse width (full width at half maximum) of 6 to 40 nsec with energy outputs of 50 J per pulse. Power densities greater than 109 W/cm^2 are required to produce the preferred compressive stresses. A 50-J pulse is adequate to treat an area as large as 0.15 in.^2 (1 cm^2); to treat larger areas, the laser is optically scanned to create overlap between successive pulses applied to the surface.
As with traditional shot peening, laser shock processing can be used to improve fatigue, fretting fatigue and stress corrosion cracking resistance of most materials. While both processes induce plastic strain into the surface being treated, the amount of plastic strain is quite different between the two methods. Shot peening produces strains as high as 20-40% compared with 1-8% strain induced by LSP. This difference is most apparent with respect to the treated surface finish. In most cases, LSP can be used on surfaces after final machining. Laser shock processing of softer materials such as aluminum produces 25 to 50 Km deep impressions, which may require subsequent finishing. Laser shock processing currently is used on the leading edge of the first stage fan blades of General Electric F101 and F110 jet engines to improve blade resistance to damage from foreign objects.