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.
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.
[2][3] 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.
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.
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.
He and Jean Fournier of the Peugeot Company visited Battelle in 1986 for an extended discussion of laser shock peening with Allan Clauer.
The programs initiated by Fabbro and carried forward in the 1990s and early 2000s by Patrice Peyre, Laurent Berthe, and co-workers have made major contributions, both theoretical and experimental, to the understanding and implementation of laser peening.
[14][15][16] In 1998, they measured using VISAR (Velocimeter Interferometer System for Any Reflector) pressure loadings in water confinement regime as function of wavelength.
Researchers in many countries and industries undertook investigations to extend understanding of the laser shock peening process and material property effects.
The continuing growth of the technology and its applications led to the appearance of several commercial laser shock peening providers in the early 2000s.
In the late 1990s, Metal Improvement Company (MIC is now part of Curtis Wright Surface Technologies) partnered with Lawrence Livermore National Laboratory (LLNL) to develop its own laser peening capabilities.
In Japan, Toshiba Corporation expanded the commercial applications of its LPwC system to pressurized water reactors, and in 2002 implemented fiber optic beam delivery to the underwater laser peening head.
This system was ready for commercial use in 2013[24] MIC developed and adapted laser shock peening for forming the wing shapes on the Boeing 747-8.
In the 1990s and continuing through present day, laser peening developments have targeted decreasing costs and increasing throughput to reach markets outside of high-cost low-volume components.
These reduced operational costs of laser peening have made it a valuable tool for solving an extended range of fatigue and related applications.
However, there are situations where an opaque overlay is not used; in the Toshiba process, LPwC, or where the tradeoff between decreased cost and possibly somewhat lowered surface residual stress allows superficial grinding or honing after laser peening to remove the thin thermally effected layer.
Quartz or glass overlays produce much higher pressures than water, but are limited to flat surfaces, must be replaced after each shot and would be difficult to handle in a production setting.
By varying the laser power density, pulse duration, and number of successive shots on an area, a range of surface compressive stress magnitudes and depths can be achieved.
The deep compressive stresses are due to the shock wave peak pressure being maintained above the HEL to greater depths than for other peening technologies.
There may be instances where it is cost effective not to apply the opaque overlay and laser peen the bare surface of the work piece directly.
These detrimental effects of bare surface processing, both aesthetic and metallurgical, can be removed after laser peening by light grinding or honing.
Laser pulse shapes are customizable to circular, elliptical, square, and other profiles to provide the most convenient and efficient processing conditions.
Because the process originally was intended to operate in large water-filled vessels, the wave frequency was doubled to halve the wavelength to 532 nm.
This factor, combined with the small spot size, requires many shots to achieve a significant surface compressive stress and depths of 1 mm.
This enables the laser peened compressive stresses to be retained at higher operating temperatures during long exposures than is the case for the other technologies.
By selectively laser shocking areas on the surface of metal sheets or plates, or smaller items such as airfoils, the associated compressive residual stresses cause the material to flex in a controllable manner.
[27] Careful tailoring of the shockwave shape and intensity has also enabled the inspection of bonded composite structures via laser shocking.
By controlling the pressure of the tensile wave this procedure is capable of reliably locally testing adhesion strength between bonded joints.
Fundamental issues are also studied to characterize and quantify the effect of shock wave produced by laser inside these complex materials.