Laser peening

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Laser peening (LP), or laser shock peening (LSP), is a surface engineering processes 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. The physics of the laser shock peening process can also be used to strengthen thin sections, work-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[edit]

Discovery and development (1960's)[edit]

Initial scientific discoveries towards modern day laser peening began in the early 1960s as pulsed laser technology began to proliferate across the globe. 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.[1] 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.[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. 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.[4] 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 GPa (145 to 1,100 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 (1970's)[edit]

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.[5] 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.[6]

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.[7] Subsequently, the first patent on laser shock peening was granted to Phillip Mallozzi and Barry Fairand in 1974.[8] 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.[9][10][11][12]

Practical laser peening (1980's)[edit]

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.[13] 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 and co-workers have made major contributions, both theoretical and experimental, to the understanding and implementation of laser peening.[14][15][16]

Creation of an industry (1990's)[edit]

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.[17] 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 F-101 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.[18][19]

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.[20] 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 μm, to minimize dielectric breakdown of the beam in the water, instead of the 1054 μm 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.[21]

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. [22][23]

Supplier Foundation and Industry Growth (1990's - 2000's)[edit]

With the major breakthrough of commercial application of laser peening on the F-101 engine to resolve a major operational problem, laser peening attracted attention around the globe. Researchers in many countries and industries began investigating of the laser shock peening process and material property effects. 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.

GEAE and LSP Technologies were the leading companies providing laser peening services having licensed the technology from Battelle. GEAE performed laser peening for its aerospace engines and LSP Technologies marketed laser shock peening services and equipment to a broader industrial base. In the late 1990s, Metal Improvement Company (now part of Curtis Wright Surface Technologies) partnered with Lawrence Livermore National Laboratory (LLNL) to begin developing laser peening capabilities. Toshiba Corporation developed fiber optic based laser peening specifically for the prevention of stress corrosion cracking in nuclear reactors.

The growth of industrial suppliers and commercial proof of laser peening technology via published literature convinced many companies to adopt laser peening technology to solve and prevent problems. Some of the companies who have adopted laser peening include: GE, Rolls-Royce, Siemens, Boeing, Pratt & Whitney, etc.

Starting in the 1990s and continuing through present day, laser peening developments have been targeted towards decreasing cost 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 advancements have been developed to address these challenges and have reduced laser peening costs dramatically: laser peening systems were designed to handle robust operations; pulse rates of laser systems were increased; routine labor operations were automated; application of overlays were automated. The reduced operational costs of laser peening have made it a valuable tool for an extended range of applications.[24]

Process description[edit]

Fundamental laser peening can be accomplished with only two components: a transparent overlay and a high energy, pulsed laser system. For protection of the surface from detrimental heat effects it is also recommended that an opaque overlay be used; however, it is not required. Despite the ability of the process to impart heat affects to the workpiece, the laser shock peening process is based upon shockwave mechanics and not thermal effects in the material.

During processing the laser beam is targeted to the workpiece. The workpiece surface to be processed is covered by a transparent overlay. The transparency refers to the ability of the laser beam to pass through the overlay without having any appreciable level of absorption. When the laser is triggered and the beam strikes the target surface a thin layer of material is vaporized to form a small cloud. The continued delivery of energy in the laser beam pulse rapidly heats the vapor and converts it to a plasma plume. This plasma rapidly expands and, when confined by the transparent overlay, reaches pressures to drive a shockwave into the target. The most common transparent overlay used in the laser peening process is water; however, other overlays including quartz and tape may also be used.

Without the transparent overlay the unconfined plasma pressures are incapable of generating a shockwave with enough magnitude to affect the mechanical properties of the workpiece material. The required magnitude of the shockwave to affect mechanical properties is based on the Hugoniot Elastic Limit (HEL). The HEL is also referred to as the dynamic yield strength of a material. Shockwave pressure amplitudes must be above the HEL value to coldwork the material. Most metallic engineering alloys have HEL values in the 0.5 – 4 GPa range.

As the shockwave propagates into the workpiece, the shockwave coldworks the material and attenuates. When the magnitude of the shockwave is no longer above the HEL, no further work is being performed despite the continuation of the shockwave. Shockwaves are capable of coldworking a material to greater depths than conventional surface impact technologies.

The initial vaporization that occurs results in a small layer of surface material that is lost and also rapid, intense temperature increases of short duration. When processing a bare workpiece surface it is common that superficial melting and staining of the workpiece may occur. Staining on the surface is typically a result of oxidation products generated during the peening process. Due to the detrimental effects of bare surface processing, both aesthetic and metallurgical, an opaque overlay is typically used to protect the surface.

The use of opaque overlays protects the workpiece material from direct contact with the laser beam, provides a consistent interaction with the laser beam, helps to increase shock pressure, and isolates the target from the plasma. Opaque overlays may be any material that absorbs the laser beam energy. Common overlays include tape, paint, and proprietary blends of materials.

Laser pulses are generally applied sequentially on the target to treat a specified area. Laser pulse shapes are customizable to circular, elliptical, square, and other profiles to provide the most efficient and effective processing conditions. The size of the area treated by each individual pulse depends on a number of factors that include material, laser system and other processing factors.

Quality systems for laser peening[edit]

The laser peening process using computer control is described in AMS 2546. Like many other surface enhancement technologies, direct measuring of the results of the process on the workpiece is not practical. Laser peening quality is frequently monitored by measuring the energy and pulse width details of the laser system per laser shot. Other quality control systems are also available that rely on EMAT, PVDF, plasma radiometers. If desired, laser peening can also be evaluated via Almen strips. It should be noted that Almen strips are intended to be a comparison tool and not a definitive guideline of laser peening intensity.

Laser systems[edit]

The initial laser systems used during the development of laser peening were large and featured slow processing rates. Since the mid-late 1990s, lasers have achieved compact size and processing frequencies ideal for production environments. These modern laser peening systems have many different configurations but can be separated into two primary groups: 1. High energy, lower frequency lasers; and 2. Low energy, high frequency lasers. Most commercial laser peening systems operate with equipment from Group 1 with pulse energies capable of 50 Joules or more and pulse lengths between 7 and 30 nanoseconds. These higher energy laser systems can provide energy distribution in spot sizes of approximately 2 – 7 mm in diameter that are capable of generating shockwaves above the HEL of most materials. Laser peening systems from Group 2 typically require smaller spot sizes to reach the equivalent energy density to create a shockwave magnitude above the HEL.

The output from the laser systems is directed via an optical chain of mirrors and/or lenses to the surface of the workpiece. The workpiece, or sometimes the laser beam, are repositioned for subsequent shots via robots. This process is repeated until the workpiece has reached the designated amount of coverage.

Process effect[edit]

The shockwave generated coldwork (plastic strain) in the workpiece material creates residual stresses to maintain an equilibrium state of the material. These residual stresses are compressive at the workpiece surface and gradually fade into low tensile stresses within the core of the workpiece. Compressive residual stresses from laser peening have been shown to prevent and mitigate high cycle fatigue (HCF), low cycle fatigue (LCF), corrosion, wear, fretting, corrosion pitting, and stress corrosion cracking. One of the founding applications, as noted in the History section, was designed for increased damage tolerance.

By enhancing material performance, laser peening enables more efficient designs that reduce weight, extend component lifetimes, and increase performance. The magnitude of residual stress induced by laser peening approaches the physical limit of materials and is capable of extending deep into parts. Experimentation has indicated compressive residual stresses can be generated that extend greater than 8 mm into part surfaces. Experiments have also proven that laser peening induced residual stresses have a high level of stability at prolonged elevated temperature exposure and may have added applications in hot sections of reactors and turbines.

Other applications of laser peening technologies[edit]

Shockwave mechanics generated via laser sources have found uses in other industrial applications outside of surface enhancement technologies. The growth of the technology has permitted the list of applications to continue to grow and expand.

Laser shocking can be used to impart residual stresses at specific locations on a component. These residual stress fields create a predictable corresponding amount of workpiece deformation. By selectively controlling the distribution of residual stresses complex part geometries can be brought into a specified geometric field. This process is capable of bringing manufactured parts back into design tolerance limits and form shaping large bulk products.

Shockwaves from laser shocking are also used for spallation testing of materials. As the shockwave reflects off the backside of a workpiece it can become a tensile wave. When desired, this rebounding tensile wave is capable of reaching magnitudes high enough to spall brittle materials. This spallation testing is useful in verifying the integrity and performance of ballistic materials.

Careful tailoring of the shockwave shape and intensity has also enabled the inspection of bonded structures via laser shocking. The technology, termed Laser Bond Inspection initiates a shockwave that reflects of the backside of a bonded structure and returns as a tensile wave. The tensile wave has a predicted strength and is capable of locally testing adhesion strength between bonded joints. This technology is most often found in application to bonded composite structures but has also been shown to be successful in evaluating bonds between different materials.

See also[edit]

References[edit]

  1. ^ G. Askaryan and E. M. Moroz, "Pressure on Evaporation of Matter in a Radiation Beam", JETP Letters, no. 16, pp. 1638-1639, 1963
  2. ^ D. W. Gregg and S. J. Thomas,"Momentum Transfer Produced by Focused Laser Giant Pulses", Journal of Applied Physics, no. 37, pp. 2787-2789, 1966
  3. ^ F. Neuman, "Momentum Transfer and Cratering Effects Produced by Giant Laser Pulses", Appl. Phys. Ltrs., vol. 16, pp. 113–117, 1970
  4. ^ =N. Anderholm, Appl. Phys. Ltrs., no. 4, p. 167, 1964
  5. ^ L. I. Mirkin, "Plastic Deformation of Metals Caused by a 10-8-sec Laser Pulse", Soviet Physics - Doklady, vol. 14, pp. 11281130, 1970
  6. ^ S. A. Metz and F. A. Smidt, "Production of Vacancies by Laser Bombardment", Appl. Phys. Ltrs., vol. 19, pp. 207-208, 1971
  7. ^ B. P. Fairand, B. A. Wilcox, W. J. Gallagher and D. N. Williams, "Laser Shock-Induced Microstructural and Mechanical Property Changes in 7075 Aluminum", Journal of Applied Physics, vol. 43, no. 9, pp. 3893–3895, 1972
  8. ^ P. J. Mallozzi and B. P. Fairand,"Altering Material Properties", US patent Patent 3,850,698, 26 November 1974
  9. ^ A.H. Clauer, B. P. Fairand and B. A. Wilcox, "Pulsed Laser Induced Deformation in an Fe-3 Wt Pct Si Alloy", Met. Trans. A, vol. 8A, pp. 119-125, 1977
  10. ^ B. P. Fairand and A. H. Clauer, "Laser generation of high-amplitude stress waves in materials", J. Appl. Phys., vol. 50, pp.1497-1502, 1979
  11. ^ A. H. Clauer, C. T. Walters and S. C. Ford, "The Effects of Laser Shock Processing on the Fatigue Properties of 2024-T3 Aluminum," in Lasers in Materials Processing, ASM International, Metals Park, Ohio, 1983
  12. ^ A. H. Clauer, J. H. Holbrook and B. P. Fairand, "Effects of Laser Induced Shock Waves on Metals", in Shock Waves and High-Strain-Rate Phenomena in Metals, M. A. Meyers and L. E. Murr, Eds., pp. 675-702. 1981
  13. ^ A. H. Clauer. "A Historical Perspective on Laser Shock Peening," Metal Finishing News, vol. 10. 
  14. ^ R. Fabro, J. Fournier, P. Ballard, D. Devaux and J. Virmont, "Physical Study of Laser Produced Plasma in Confined Geometry",J. Appl. Phys., no. 2, vol. 68, pp. 775-784, 1990
  15. ^ P. Peyre, R. Fabbro, P. Merrien, H. P. Lieurade, "Laser Shock Processing of Aluminum Alloys. Application to High Cycle Fatigue Behavior", Mat. Sci. and Engr., vol. A210, pp. 102-113, 1996
  16. ^ P. Peyre, L. Berthe, X. Scherpereel, R. Fabbro, E. Bartniki, "Experimental Study of Laser-Driven Shock Waves in Stainless Steels", J. Appl. Phys., no. 11, vol 84, pp. 5985-5992
  17. ^ S. D. Thompson, D. E. See, C. D. Lykins and P. G. Sampson, in Surface Performance of Titanium, J. K. Gregory, H. J. Rack and D. Eylon, Eds, The Minerals, Metals &Materials Society, pp. 239-251, 1997
  18. ^ Air Force Research Laboratory, http://www.dtic.mil/dtic/tr/fulltext/u2/a487687.pdf,"Laser Shock Peening - The Right Technology at The Right Time"."DoD Manufacturing Technology Program", Retrieved 2006-10-16
  19. ^ Air Force Research Laboratory (2001). "Increasing the Life Cycle of Gas Turbine Engine Airfoils". "AF SBIR/STTR Success Story". Retrieved 2006-10-16. 
  20. ^ Y. Sano, N. Mukai, A. Sudo and C. Konagai, "Underwater Laser Processing to Improve Residual Stress on Metal Surface, Proc. of the 6th Int. Symp. Japanese Welding Society, 1996
  21. ^ Y. Sano, M. Kimura, K. Sato, M. Obata, A. Sudo, Y. Hamamoto, S. Shima, Y. Ichikawa, H. Yamazaki, M. Naruse, S. Hida, T. Watanabe, and Y. Oono, Proc. 8th Int. Conf. on Nuclear Eng., (ICONE-8), Baltimore, 2000.
  22. ^ J. L. Ocaña, C. Molpeceres, M. Morales and A. Garcia-Beltran, "A Model for the Coupled Predictive Assessment of Plasma Expansion and Material Compression in Laser Shock Processing Applications", in High-Power Laser Ablation II, C. R. Phipps and M. Niino, Eds., SPIE Proc., vol. 3885, pp252-263, 2000
  23. ^ J. L. Ocaña, C. Molpeceres, J. A. Porro, G. Gómez and M. Morales, "Experimental Accessment of the Influence of Irradiation Parameters on Surface Deformation and Residual Stresses in Laser Shock Processed metallic Alloys", Appl. Surf. Sci., vol. 238, pp. 501-505, 2004
  24. ^ "Laser Peening". LSP Technologies. 2004. Retrieved 2013-10-22. 

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