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.