Laser peening (LP), or laser shock peening (LSP), is a surface engineering process used to impart beneficial residual stresses in materials. The deep, high-magnitude compressive residual stresses induced by laser peening increase the resistance of materials to surface-related failures, such as fatigue, fretting fatigue, and stress corrosion cracking. Laser shock peening can also be used to strengthen thin sections, harden surfaces, shape or straighten parts (known as laser peen forming), break up hard materials, compact powdered metals and for other applications where high-pressure, short duration shock waves offer desirable processing results.
History
Discovery and development (1960s) Initial scientific discoveries towards modern-day laser peening began in the early 1960s as pulsed-laser technology began to proliferate around the world. In an early investigation of the laser interaction with materials by Gurgen Askaryan and E.M. Moroz, they documented pressure measurements on a targeted surface using a pulsed laser. The pressures observed were much larger than could be created by the force of the laser beam alone. Research into the phenomenon indicated the high-pressure resulted from a momentum impulse generated by material vaporization at the target surface when rapidly heated by the laser pulse. Throughout the 1960s, a number of investigators further defined and modeled the laser beam pulse interaction with materials and the subsequent generation of stress waves. These, and other studies, observed that stress waves in the material were generated from the rapidly expanding plasma created when the pulsed laser beam struck the target. Subsequently, this led to interest in achieving higher pressures to increase the stress wave intensity. To generate higher pressures it was necessary to increase the power density and focus the laser beam (concentrate the energy), requiring that the laser beam-material interaction occur in a vacuum chamber to avoid dielectric breakdown within the beam in air. These constraints limited study of high-intensity pulsed laser–material interactions to a select group of researchers with high-energy pulsed lasers. In the late 1960s a major breakthrough occurred when N.C. Anderholm discovered that much higher plasma pressures could be achieved by confining the expanding plasma against the target surface. Anderholm confined the plasma by placing a quartz overlay, transparent to the laser beam, firmly against the target surface. With the overlay in place, the laser beam passed through the quartz before interacting with the target surface. The rapidly expanding plasma was now confined within the interface between the quartz overlay and the target surface. This method of confining the plasma greatly increased the resulting pressure, generating pressure peaks of 1 to 8 gigapascals (150 to 1,200 ksi), over an order of magnitude greater than unconfined plasma pressure measurements. The significance of Anderholm's discovery to laser peening was the demonstration that pulsed laser–material interactions to develop high-pressure stress waves could be performed in air, not constrained to a vacuum chamber.
Laser shocking as a metallurgical process (1970s) The beginning of the 1970s saw the first investigations of the effects of pulsed laser irradiation within the target material. L. I. Mirkin observed twinning in ferrite grains in steel under the crater created by laser irradiation in vacuum. S. A. Metz and F. A. Smidt, Jr. irradiated nickel and vanadium foils in air with a pulsed laser at a low power density and observed voids and vacancy loops after annealing the foils, suggesting that a high concentration of vacancies was created by the stress wave. These vacancies subsequently aggregated during post-iradiation annealing into the observed voids in nickel and dislocation loops in vanadium. In 1971, researchers at Battelle Memorial Institute in Columbus, Ohio began investigating whether the laser shocking process could improve metal mechanical properties using a high-energy pulsed laser. In 1972, the first documentation of the beneficial effects of laser shocking metals was published, reporting the strengthening of aluminum tensile specimens using a quartz overlay to confine the plasma. Subsequently, the first patent on laser shock peening was granted to Phillip Mallozzi and Barry Fairand in 1974. Research into the effects and possible applications of laser peening continued throughout the 1970s and early 1980s by Allan Clauer, Barry Fairand, and coworkers, supported by funding from the National Science Foundation, NASA, Army Research Office, U.S. Air Force, and internally by Battelle. This research explored the in-material effects in more depth and demonstrated the creation of deep compressive stresses and the accompanying increase in fatigue and fretting fatigue life achieved by laser peening.
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