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 when laser technology began to proliferate across the globe and be investigated. One of these early studies of laser interaction with materials by Gurgen Askaryan and E.M. Moroz documented high 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 alone. Research into the phenomenon indicated the high pressure was a result of 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 the stress waves in the material were generated from rapidly expanding plasma created when the laser beam struck the target. Emphasis on the process was placed on achieving higher pressures to increase the stress wave intensity. To achieve higher pressures it was necessary to focus (concentrate the energy) the laser beam and perform the operation in a vacuum chamber. These operating conditions limited study of the process 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 was able to confine the plasma by placing a quartz overlay against the target surface. The quartz was transparent to the laser beam and was pressed tight to the target surface. This method of confining the plasma generated pressure peaks of 1 to 8 GPa (145 to 1,100 ksi) that were over an order of magnitude greater than unconfined plasma pressure measurements. Anderholm’s research enabled laser shocking with larger, less concentrated laser beams and processing outside of a vacuum.

Laser shocking as a metallurgical process (1970's)[edit]

Using a high energy pulsed laser, researchers at Battelle Memorial Institute in Columbus, Ohio began investigating whether the laser shocking process could improve metal mechanical properties. In 1972, the first documentation of the beneficial effects of laser shocking metals was published: the strengthening of aluminum tensile specimens using a quartz overlay to confine the plasma.[5] Subsequently, the first patent on laser shock peening was granted to Dr. Phillip Mallozzi and Dr. Fairand of Battelle in 1974.[6] Research continued throughout the 1970s and 1980s, supported by funding from the National Science Foundation, the US Department of Defense, and internally by Battelle.[7][8][9] In 1983, the first definitive laser shock peening patent was issued to Dr. Allan Clauer and other researchers at Battelle.[10] This patent has served as the standard for laser shock peening since being issued.

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 two large rooms and required several minutes of recovery time between laser pulses .[11] For economical and practical applications as an industrial process the laser technology was required to mature into equipment with smaller footprints and increased laser pulse frequencies. In the mid-1980s, Battelle researches began constructing a small footprint laser system with a pulse rate capable of production laser peening.

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

With industrial style laser peening equipment in place, a team of engineers launched a major marketing effort in the late 1980s and early 1990s to familiarize key companies throughout different industries with the benefits of laser shock peening. The first significant and public application of laser peening was a result of foreign object damage (FOD) sustained on F-101 fan blades powering the B-1 Bomber. General Electric Aircraft Engines (GEAE) licensed the laser shock peening technology from Battelle to investigate the effects of the process on preventing FOD engine failures. In 1995, the United States Air Force made the decision to move forward with production development of the laser peening technology.

The demand for industrial laser systems drove several of the laser shock peening researchers from Battelle to start LSP Technologies, Inc. in 1995 as the first commercial supplier of laser peening equipment. Led by founder Dr. Jeff Dulaney, LSP Technologies designed and built the laser systems for GEAE to being production laser peening of the F-101 turbine blades. Laser shock peening the F-101 turbine blades reduced the sensitivity of the blades to FOD damage up to 0.25 inch deep on the leading edge. The Air Force continued to work with LSP Technologies to mature the laser shock peening production capabilities and implement production manufacturing cells .[12][13]

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

With the major success of laser peening on the F-101 engine, laser peening garnered significant attention across the globe. Researches from many countries and industries began further investigations of the laser shock peening process. A large volume of research papers and patents were generated from France and Japan during the 1990s. The continuing growth of the technology and its applications led to the formation of several commercial laser shock peening providers.

GEAE and LSP Technologies were the premiere 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.[14]

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 affects 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 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. JETP Letters. pp. no. 16, p. 1638, 1963. 
  2. ^ D. W. Gregg and S. J. Thomas. Journal of Applied Physics. pp. no. 37, p. 2787, 1966. 
  3. ^ F. Neuman. Applied Physics Letters. pp. vol. 16, pp. 113–117, 1970. 
  4. ^ N. Anderholm. Applied Physics Letters. pp. no. 4, p. 167, 1964. 
  5. ^ B. P. Fairand, B. A. Wilcox, W. J. Gallagher and D. N. Williams. "Laser Shock-Induced Microstructural and Mechanical Property Chances in 7075 Aluminum", "Journal of Applied Physics". pp. vol. 43, no. 9, pp. 3893–3895, 1972. 
  6. ^ P. J. Mallozi and B. P. Fairand. "Altering Material Properties". pp. US patent Patent 3,850,698, 26 November 1974. 
  7. ^ S. C. Ford, B. P. Fairand, A. H. Clauer and R. D. Galliher. ""Investigation of Laser Shock Processing - Executive Summary (AFWAL-TR-80-3001)," Air Force Wright Aeronautical Laboratories, Wright-Patterson Air Force Base, 1980. 
  8. ^ 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". 
  9. ^ A. H. Clauer, J. H. Holbrok and B. P. Fairand. Effects of Laser Induced Shock Waves on Metals," in Shock Waves and High-Strain-Rate Phenomena in Metals. pp. 675–702. 
  10. ^ A. H. Clauer, B. P. Fairand, S. C. Ford and C. T. Walters. Laser shock processing," in Shock Waves and High-Strain-Rate Phenomena in Metals. pp. USA Patent 4401477, 30 August 1983. 
  11. ^ A. H. Clauer. "A Historical Perspective on Laser Shock Peening," Metal Finishing News, vol. 10. 
  12. ^ Air Force Research Laboratory. "Laser Shock Peening - The Right Technology at The Right Time". "DoD Manufacturing Technology Program". Retrieved 2006-10-16. 
  13. ^ Air Force Research Laboratory (2001). "Increasing the Life Cycle of Gas Turbine Engine Airfoils". "AF SBIR/STTR Success Story,". Retrieved 2006-10-16. 
  14. ^ "Laser Peening". LSP Technologies. 2004. Retrieved 2013-10-22.