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Laser Peening

The effect of high intensity laser induced stress waves on the hardness and tensile strength of 2024 and 7075 aluminum and on the fatigue properties of 7075 aluminum were investigated. Laser shocking of these alloys increases the hardness of the underaged 2024-T351 but has little or no effect on the peak aged 2024-T851 and 7075-T651 or the over-aged 7075-T73. The largest increases in tensile strength were observed in 7075-T73, lesser increases in 2024-T351 and none in 2024-T851 or 7075-T651. The fretting fatigue life of fastener joints of 7075-T6 was increased by orders of magnitude by laser shocking the region around the fastener hole before drilling and assembling. Also the fatigue crack propagation rates were significantly decreased by laser shocking.
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A high-energy, pulsed laser beam combined with suitable transparent overlays can generate pressure pulses of up to 6 to 10 GPa on the surface of a metal. The propagation of these pressure pulses into the metal in the form of a shock wave produces changes in the materials microstructure and properties similar to those produced by shock waves caused in other ways. This paper reviews the mechanism of shock wave formation, calculations for predicting the pressure pulse shape and amplitude, in-depth microstructural changes and the property changes observed in metals. These property changes include increases in hardness, tensile strength and fatigue life. The increases in fatigue life appear to result from significant residual surface stresses introduced by the shock process.
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Laser shock processing (LSP) produces a surface compressive residual stress in the metal part being treated that can significantly improve those properties which are affected by the initiation and propagation of surface cracks. The properties of greatest interest are fatigue life and fatigue strength. But the process also can reduce fretting fatigue and stress-corrosion cracking as well as strengthen thin sections. The potential advantages of LSP include the possibility of direct integration into manufacturing production lines with a high degree of automation, use on machined surfaces, increased quality assurance, treatment of localized fatigue critical areas without masking, and the ability to make design changes that would not be possible using alternative methods for increasing fatigue resistance. Among the applications that have been identified are the manufacture of blades, disks, and vanes for aircraft gas turbine engines; gears, connecting rods, and crankshafts for automotive engines; and medical implants.
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Laser shock peening produces a compressive residual stress in the surface of metallic materials, which significantly increases fatigue life in applications where failure is caused by surface-initiated cracks. Laser shock peening is applied by using a high energy pulsed laser to create a high amplitude stress wave or shock wave on the surface to be treated. This stress wave propagates into the material, causing the surface layer to yield and plastically deform, and thereby, develop a residual compressive stress. Where comparisons have been made to shot peening, the magnitude of the residual stresses at the surface are similar, but the compressive stresses from laser peening extend much deeper below the surface than those from shot peening. The resulting fatigue life enhancement is often greater for laser peering than it is for shot peening. In addition to fatigue strength improvement, laser peering can also locally strain harden thin sections of parts or strain harden a surface.
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The feasibility of using a high energy, pulsed laser beam to shock-harden weld zones in 5086- H32 and 6061-T6 aluminum sheet was investigated. The tensile strength, hardness, and microstructure of samples 0.3 cm thick were studied before and after laser shocking. After laser shocking, the tensile yield strength of 5086-H32 was raised to the bulk level and the yield strength of 6061-T6 was raised midway between the welded and bulk levels. The increases in ultimate tensile strength and hardness were smaller than the increases in the yield strength. The microstructures after shocking showed heavy dislocation tangles typical of cold working.
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1992 by Atlanta Technology Publications except for articles by US government employees. All rights reserved. This work may not be translated or copied in whole or part without the written permission of the publisher (Atlanta Technology Publications, P.O. Box 77032, Atlanta, GA 30357, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaption, computer software, or by similar or by dissimilar methodology now known or hereafter developed is forbidden.
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The plastic deformation produced by laser induced stress waves was investigated on an Fe-3 wt pct Si alloy. The intensity and duration of the stress waves were varied by changing the intensity and pulse length of the high energy pulsed laser beam, and also by using different overlays on the surfaces of the specimens. The resulting differences in the distribution and intensity of the deformation caused by the stress waves within the samples were determined by sectioning the specimens and etching (etch pitting) the transverse sections. The magnitude of the laser shock induced deformation depended on the laser beam power density and the type of surface overlay. A combination transparent plus opaque overlay of fused quartz and lead generated the most plastic deformation. For both the quartz and the quartz plus lead overlays, intermediate laser power densities of about 5 x 108 w/cm2 caused the most deformation. The shock induced deformation became more uniform as the thickness of the material decreased, and uniform shock hardening, corresponding to about 1 pct tensile strain, was observed in the thinnest specimens (0.02 cm thick). 200 ns laser pulses caused some surface melting, which was not observed for 30 ns pulses, the pulse length used in most of the experiments. Deformation of the Fe-3 wt pct Si alloy occurred by both slip and twinning.
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Reprinted with permission from Lasers in Materials Processing, copyright 1983, ASM International, Materials Park, OH 44073-0002. Although this article is being used with permission, ASM did not prepare the version for Web display. For information about ASM International or the purchase of books, visit ASM International on the Web at http://www.asm-intl.org or call 1-800-336-5152, ext. 5900.
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When light from a pulsed laser is incident on the surface of an absorbent material, part of the light is absorbed and vaporizes a small amount of surface material. The rapid vaporization and blow off of this material generates a stress wave at the surface. As this pressure pulse propagates into the material, it changes the metal's microstructure, which is the source of the observed improvements in material properties. This laser shock process has been successfully used to increase the strength and hardness of stainless steel. The strength properties of heat-affected zones in welded aluminum structures have been increased to values up to the strength of the parent material. Recent studies have demonstrated that laser shock processing can also be used to improve the fatigue life and stress corrosion of some aluminum alloys. In general, all alloys which are strain hardenable have a good chance of responding favorably to the laser shock process.
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When the energy from a powerful pulsed laser is trained on the surface of an absorbent material, a high-amplitude stress wave is generated. If the surface is covered with a material which is transparent to the incident laser light, peak pressure environments are significantly enhanced compared to free surface conditions. Experimental pressure measurements using piezoelectric pressure transducers have demonstrated that peak pressures up to approximately 10 GPa can be generated in this manner. The rise time of these pressure waves, which is controlled by the temperature of the laser heated absorbent material, approximates the shape of the incident laser pulse. The decay time of the pressure waves is slower than the laser pulse because it is governed by the rate at which work is done on surrounding materials and the rate at which heat is conducted out of the heated vapor into colder adjacent materials. Theoretical calculations of the pressure environments using a one-dimensional radiation hydrodynamic computer code are in good agreement with the experimental measurements.
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The generation of high-amplitude stress waves with short bursts of laser radiation was first investigated a few years after the first laser became operational.1-4 These early studies predicted that high amplitude stress waves could be generated in materials by impinging the laser beam on an unconfined surface of the body and vaporizing a small amount of surface material. Later work which involved direct measurements of pressure showed this was not the case and peak pressures typically were less than 1 GPa.5 Subsequently, methods to enhance the pressure environments over the free-surface conditions by modifications in the target surface conditions proved to be successful.6-11 Our interest in this area was stimulated by the need to generate pressures greater than 1 GPa in order to produce significant changes in the in-depth microstructures and mechanical properties of metal alloys.12-14 A comprehensive understanding of the stress-wave environments needed to produce these changes is required for their effective application in altering the properties of materials. This paper presents our studies of laser induced stress-wave environments with particular attention given to methods of enhancing the magnitude of the stress waves over free-surface conditions. The effects on stress-wave environments from placing transparent confining media on the target surface and addition of absorbent films to the surface are treated. Our results are based on experimental measurements of pressure and theoretical calculations using a one-dimensional radiation hydrodynamic computer code.
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Laser-induced stress waves in iron samples were analyzed by measuring the pressure environment at the back surface of various sample thicknesses. These results were compared with numerical calculations obtained from a one-dimensiona1 radiation hydrodynamics computer code. The experiments were conducted in an air environment under ambient conditions and the metal surfaces were confined by transparent overlays. Peak pressures exceeding 50 kbar were measured with quartz pressure transducers at a laser power density of about 109 W/cm2. Computer predictions agreed favorably with the experimental results and indicated that peak pressures exceeding 100 kbar could be generated by appropriate modifications in the laser environment and target overlay configuration.
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This paper was published in Vol. 86-Industrial Applications of High Power Laser Technology and is made available as an electronic reprint with permission of SPIE. Single print or electronic copies for personal use only are allowed. Systematic or multiple reproduction, or distribution to multiple locations through an electronic list server or other electronic means, or duplication of any material in this paper for a fee or for commercial purposes is prohibited. By choosing to view or print this document, you agree to all the provisions of the copyright law protecting it.
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The laser shock peening (LSP) process was used to increase life of pilger dies made of A2 tool steel by imparting compressive residual stresses to failure prone areas of the dies. The result of X-ray diffraction analysis indicated that deep, high-magnitude compressive residual stresses were generated by the laser shock peening process, and the peened dies exhibited a significant increase of in-service life. Fractography of the failed dies indicates that the failure mechanism was altered by the peening process.
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Laser peening has been a commercial surface enhancement process for over six years, and has been gradually expanding the number of applications being laser peened in production ever since. LSP Technologies has been a major developer of the process and new applications for laser peening. It has developed production laser peening systems and innovative laser peening technology to increase throughput and reduce cost. Some of these production and technology developments will be discussed in this paper. Also, an evaluation of applying laser peening to increase the fretting fatigue resistance of titanium alloys, based on Ti-6Al-4V has been made. Included in this evaluation is the use of small spot laser peening to enable the processing of the inside of small, generally inaccessible areas such as the insides of holes and slots. Laser peening with either large or small spots dramatically increased the fretting fatigue life under both R=0.5 and R=0 fatigue conditions with three different contact pad pressures. Fretting fatigue life was increased by at least 25 times. Actual increases in fatigue life and fatigue strength could not be determined because most specimens ran to the run out life of 106 cycles without failure. The laser peening does not appear to affect the fretting behavior, but instead inhibits the initiation of fatigue cracks at the fretting cracks developed from the fretting process. The compressive residual stress from laser peening also would slow the growth rate of any fatigue crack that does eventually initiate at a fretting crack.
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Laser peening is an innovative surface enhancement process used to increase the resistance of aircraft gas turbine engine compressor and fan blades to foreign object damage (FOD) and improve high cycle fatigue (HCF) life. [1,2,3,4] The process creates residual compressive stresses deep into part surfaces – typically five to ten times deeper than conventional metal shot peening. These compressive surface stresses inhibit the initiation and propagation of fatigue cracks. Laser peening has been particularly effective in aircraft engine titanium alloy fan and compressor blades, however the potential application of this process is much broader, encompassing automotive parts, orthopedic implants, tooling and dies, and more. Significant progress has been made to lower the cost and increase the throughput of the process, making it affordable for numerous applications from gas turbine engines to aircraft structures, land vehicles, weapon systems, as well as general industrial use. Laser peening may also be referred to as laser shock processing (LSP), and various other commercial trade names. This paper reviews the status of laser peening technology, material property enhancements, and potential applications.
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Neodymium-glass laser pulses (1.06-mm wavelength, 25-ns pulse width) have been used to generate shock waves with peak pressures in the 5- to 120-kbar range at the front surface of solids. Relatively uniform irradiance levels were employed with circular beam areas in the 0.4- to 1.5-cm2 range and single pulse energy up to 800 J (fluences ranged from 200-2000 J/cm2). At 1000 J/cm2, the resulting peak shock pressure is about 35 kbar. By confining the plasma with a transparent glass overlay, this peak pressure was raised to 120 kbar. The nature of the plasma initiation process has been revealed through careful simultaneous temporal resolution of the beam-power, temperature, and stress-wave details.
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The effects of intense single pulses of 1.06 mm radiation on structural composite materials have been investigated. Fluences in the 1000 - 3000 J/cm2 range were delivered in single pulses with 20 ns pulse widths (FWHM) to thermal coupon and tensile bar type samples in vacuum. Materials studied included Kevlar/epoxy, fiberglass/epoxy, and graphite epoxy uniaxial composites in coated and uncoated conditions. Diagnostics were employed to assess energy partitioning in the interaction and stress wave histories in the material. Post-test sample examination and strength tests were conducted on the tensile bar samples. The diagnostics indicated that most of the beam energy goes into a very hot plasma (300,000 K) which drives a shock wave into the material. The shock wave has a peak amplitude of about 30-40 kbars and attenuates as it propagates through the sample. A synergistic damage effect was discovered wherein the sample fails in tension due to addition of the sample preload stress and the axial component of stress due to the shock wave reflected from the rear surface of the sample. Details of the beam energy partitioning and strength degradation in the samples will be presented.
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Laser Bond Inspection

Advancements in manufacturing and assembly methods to reduce cost and increase rate are pushing for the application of adhesive bonding in carbon fiber reinforced plastic (CFRP) primary structure applications. Both CFRP to CFRP and CFRP to metal bonds are under consideration. In these cases, the certification of the structure requires total confidence in the as manufactured strength of the adhesion in the joint. Unfortunately, there are no standard nondestructive inspection (NDI) methods that can return a value for the bond strength.
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The use of controlled, localized stress waves in materials and bonded joints offers new opportunities for the characterization of structures. Bonded joints in particular benefit from this application because there is no nondestructive inspection method to measure bond strength. Laser bond inspection using a high peak power short pulse laser can perform a localized proof test of a bond. The method can be applied nondestructively to strong bonds but will fail a weak bond, creating an internal disbond detectable by the laser bond inspection system itself or by post test ultrasonic inspection. To date numerous tests have shown the method to be sensitive to weak bonds in carbon fiber reinforced polymer composites structure bonds created by poor adhesive mixing, improper surface preparation or contamination.
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The Laser Bond Inspection technology is a nondestructive method to characterize the adhesive bond strength in bonded structures through the use of controlled, laser-induced localized stress waves. Structures with adhesive bonded joints can realize a particular benefit from this unique inspection method because of its ability to evaluate adhesive bond strength. The inspection method is nondestructive to strong adhesive bonds.
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Extensive experimental development, supported by 1-and 2-D hydrodynamic code simulations has demonstrated that the strength of bonds can be tested using calibrated weak shock waves (stress waves) generated at the surface of composite (and other) joints. Previously full-scale proof testing of bonded structure has been the only sure method of detecting "kissing" or weak bonds. Laser bond inspection (LBI), using high-intensity stress waves, has been shown to provide a method for localized testing of bond strength.
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