Surface engineering process
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 , 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.
Practical laser peening (1980s).
Laser shocking during the initial development stages was severely limited by the laser technology of the time period. The pulsed laser used by Battelle encompassed one large room and required several minutes of recovery time between laser pulses. To become a viable, economical, and practical industrial process, the laser technology had to mature into equipment with a much smaller footprint and be capable of increased laser pulse frequencies. In the early 1980s, Wagner Castings Company located in Decatur, Illinois became interested in laser peening as a process that could potentially increase the fatigue strength of cast iron to compete with steel, but at a lower cost. Laser peening of various cast irons showed modest fatigue life improvement, and these results along with others, convinced them to fund the design and construction of a pre-prototype pulsed laser in 1986 to demonstrate the industrial viability of the process. This laser was completed and demonstrated in 1987. Although the technology had been under investigation and development for about 15 years, few people in industry had heard of it. So, with the completion of the demonstration laser, a major marketing effort was launched by Wagner Castings and Battelle engineers to introduce laser peening to potential industrial markets.
Also in the mid 1980s, Remy Fabbro of the Ecole Polytechnique was initiating a laser shock peening program in Paris. 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. In 1998, they measured using VISAR (Velocimeter Interferometer System for Any Reflector) pressure loadings in water confinement regime as function of wavelength. They demonstrate the detrimental effect of breakdown in water limiting maximum pressure at the surface of material.
Creation of an industry (1990s).
In the early 1990s, the market was becoming more familiar with the potential of laser peening to increase fatigue life. In 1991, the U.S. Air Force introduced Battelle and Wagner engineers to GE Aviation to discuss the potential application of laser peening to address a foreign object damage (FOD) problem with fan blades in the General Electric F101 engine powering the Rockwell B-1B Lancer Bomber. The resulting tests showed that laser peened fan blades severely notched after laser peening had the same fatigue life as a new blade. After further development, GE Aviation licensed the laser shock peening technology from Battelle, and in 1995, GE Aviation and the U.S. Air Force made the decision to move forward with production development of the technology. GE Aviation began production laser peening of the F101 fan blades in 1998.
The demand for industrial laser systems required for GE Aviation to go into production attracted several of the laser shock peening team at Battelle to start LSP Technologies, Inc. in 1995 as the first commercial supplier of laser peening equipment. Led by founder Jeff Dulaney, LSP Technologies designed and built the laser systems for GE Aviation to perform production laser peening of the F101 fan blades. Through the late 1990s and early 2000s, the U.S. Air Force continued to work with LSP Technologies to mature the laser shock peening production capabilities and implement production manufacturing cells.
In the mid 1990s, independent of the laser peening developments ongoing in the United States and France, Yuji Sano of the Toshiba Corporation in Japan initiated the development of a laser peening system capable of laser peening welds in nuclear plant pressure vessels to mitigate stress corrosion cracking in these areas. The system used a low-energy pulsed laser operating at a higher pulse frequency than the higher powered lasers. The laser beam was introduced into the pressure vessels through articulated tubes. Because the pressure vessels were filled with water, the process did not require a water overlay over the irradiated surface. However, the beam had to travel some distance through the water, necessitating using a shorter wavelength beam, 532 nm, to minimize dielectric breakdown of the beam in the water, instead of the 1054 nm beam used in the United States and France. Also, it was impractical to consider using an opaque overlay. This process is now known as Laser Peening without Coating (LPwC). It began to be applied to Japanese boiling water and pressurized water reactors in 1999.
Also in the 1990s a significant laser peening research group was formed at the Madrid Polytechnic University by José Ocaña. Their work includes both experimental and theoretical studies using low-energy pulsed lasers both without and with an opaque overlay.
Supplier foundation and industry growth (1990s – 2000s).
With the major breakthrough of commercial application of laser peening on the F101 engine to resolve a major operational problem, laser peening attracted attention around the globe. Researchers in many countries and industries undertook investigations to extend understanding of the laser shock peening process and material property effects. As a result, a large volume of research papers and patents were generated in the United States, France, and Japan. In addition to the work being done in these countries and Spain, laser peening programs were initiated in China, Britain, Germany and several other countries. The continuing growth of the technology and its applications led to the appearance of several commercial laser shock peening providers in the early 2000s.
GE Aviation and LSP Technologies were the first companies performing laser peening commercially, having licensed the technology from Battelle. GE Aviation performed laser peening for its aerospace engine components and LSP Technologies marketed laser shock peening services and equipment to a broader industrial base. 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. Toshiba also redesigned the laser and beam delivery into a compact system, enabling the entire system to be inserted into the pressure vessel. This system was ready for commercial use in 2013 MIC developed and adapted laser shock peening for forming the wing shapes on the Boeing 747-8.
The growth of industrial suppliers and commercial proof of laser peening technology lead to many companies adopting laser peening technology to solve and prevent problems. Some of the companies who have adopted laser peening include: GE, Rolls-Royce, Siemens, Boeing, Pratt &amp; Whitney, and others.
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. High costs in the laser peening process were previously attributable to laser system complexity, processing rates, manual labor and overlay applications. Numerous ongoing advancements addressing these challenges have reduced laser peening costs dramatically: laser peening systems are designed to handle robust operations; pulse rates of laser systems are increasing; routine labor operations are increasingly automated; application of overlays are automated in many cases. These reduced operational costs of laser peening have made it a valuable tool for solving an extended range of fatigue and related applications.
Process description.
Laser peening uses the dynamic mechanical effects of a shock wave imparted by a laser to modify the surface of a target material. It does not utilize thermal effects. Fundamentally, laser peening can be accomplished with only two components: a transparent overlay and a high-energy pulsed laser system. The transparent overlay confines the plasma formed at the target surface by the laser beam. It is also often beneficial to use a thin overlay, opaque to the laser beam, between the water overlay and the target surface. This opaque overlay can provide either or each of three benefits: protect the target surface from potentially detrimental thermal effects from the laser beam, provide a consistent surface for the laser beam-material interaction and, if the overlay impedance is less than that of the target surface, increase the magnitude of