The beginnings of laser shock peening lie in the early 1960’s with the realization that an intense laser pulse could produce a significant pressure on the surface of an irradiated target.1,2 The source of the enhanced pressure, greater than the very small pressure exerted by the photons in the beam itself, was the thermally driven evaporation of material from the target surface caused by the beam energy heating the surface. The rapidly evaporating material exerted a momentum impulse or pressure to the surface. Throughout the remainder of the 1960’s, a number of investigators defined and modelled the laser-material interaction to begin to frame an understanding of this effect. All of these investigations and modelling activities addressed the laser beam interaction with bare surfaces. To get a higher pressure, more intense heating, and therefore a higher intensity laser beam was necessary. This required that the irradiation be performed using a focused laser beam with the target placed in a vacuum chamber. It was observed that the material leaving the target material surface at high beam intensities was a rapidly expanding, high energy plasma plume formed from the evaporated material.
The next big step forward was the discovery that by confining the rapidly expanding plasma against the target surface, much higher pressures could be achieved.3 This was accomplished by placing a quartz overlay, transparent to the laser beam, tightly against the surface to be irradiated. The laser beam passed through the overlay, heated the target surface, and initiated a rapidly expanding plasma, but instead of freely expanding outward away from the target surface, the plasma had to expand against the mating surfaces of the overlay and the target. The result was over an order of magnitude increase in pressure. This pressure increased rapidly, within nanoseconds, and propagated into the target as a shock wave. With this discovery, it became possible to achieve peak pressures of 1 to 8 GPa (145 to 1,100 ksi) or more outside of vacuum and using much larger, unfocused laser beams.
In 1969, Battelle Memorial Institute in Columbus, Ohio installed a large, high energy pulsed laser for laser fusion studies by Drs. Philip Malozzi and Barry Fairand (Figure 1a and b). In 1972, Dr. Fairand approached Dr. Benjamin Wilcox, a metallurgist, as to whether a laser induced shock wave could be used to improve metal properties. The result was the first demonstration of laser shocking to improve metal properties - the strengthening of aluminum tensile specimens using a quartz overlay to confine the plasma.4 At this point, Dr. Allan Clauer, a metallurgist, joined Dr. Fairand to begin to develop an understanding of the breadth and magnitude of laser shock processing by experiment and modeling, primarily on metals. In 1974, Drs. Malozzi and Fairand were granted the first patent for laser shock processing.5 Generous funding of this research by the National Science Foundation was critical at this initial stage of discovery to establish many of the underlying characteristics and the potential range of this process through the mid-1970’s. Later in the 1970’s, Battelle funded much exploratory research, resulting in the application of laser shock processing to improve fatigue life and strength as a laser peening process.6,7 However, the big hurdle to moving laser shock peening beyond the research stage at this time was that Battelle’s laser filled a large room and a very long cycle time between laser pulses. A smaller, faster laser would be necessary for laser peening to be considered for a production process.
In 1983, Wagner Castings Company approached Battelle with an interest in using laser peening to improve the fatigue properties of iron castings and powder metallurgy parts. After successfully demonstrating fatigue life and strength increases in these materials, they agreed to fund the design and development of a laser that would demonstrate an industrial laser was commercially viable. They also licensed the technology from Battelle. A team of Drs. Craig Walters and Jeff Dulaney, and Mr. Steve Toller completed a laser with a small footprint and fast pulse rate in 1987 (Figure 2). At this point, with a viable pre-prototype industrial laser in hand, Wagner Castings Co., then part of Sudbury Holdings, launched a major marketing effort to introduce laser peening to industry through Dr. John Koucky assisted by Dr. Clauer. This effort was critical in familiarizing many in industry with laser peening throughout the late 1980’s and early 1990’s.
In 1991, the Air Force was having a serious problem with foreign object damage on the fan blades of the F101 engine powering the B-1B Bomber. They got GEAE, the manufacturers of the engine, together with Wagner Castings and Battelle to solve this problem with laser peening. It was successful, and GEAE went into production of laser peening the F101 blades in 1997.
Dr. Dulaney started LSP Technologies in 1995, after Sudbury gave up its license to Battelle. LSP Technologies provides laser peening equipment and services to industry and government. Soon after its opening, it built the first four laser peening production lasers for GEAE. Through the late 1990’s, with funding from the US Air Force LSP Technologies built its own laser peening production facility with three lasers and four laser peening cells (Figure 3 the one on the front cover). It began production of integrally bladed rotors (IBRs) for the F119 engine powering the F-22 fighter aircraft for Pratt & Whitney in 2003.
1. G. A. Askaryan and E.M. Moroz, , 1962
2. F. Neuman, , 1964
3. N.C. Anderholm, Appl Phys Ltrs, 16,113-117, 1970
4. B.P Fairand, B.A. Wilcox, W.J. Gallagher and D.N. Williams, J Appl Phys, 43, 3893-3895, 1972
5. P. Mallozi and B. Fairand, US patent 3,850,698, November 26,1974
6. A.H. Clauer, J.H. Holbrook and B.P. Fairand, Shock Waves and High-Strain-Rate Phenomena in Metals, M.A. Meyers and L.E. Murr, eds., 1981, pp675-702
7. A.H. Clauer, C.T. Walters and S.C. Ford, Lasers in Materials Processing, ASM, Metals Park, OH, 1983, pp7-22
For Information: David Lahrman
LSP Technologies, Inc.
6145 Scherers Place, Dublin, Ohio 43016
Tel. +1.614.718-3000 x244, Fax 718-3007
E-mail: email@example.com, www.lspt.com