- Laser peening
Laser peening is the process of hardening or
peeningmetal using a powerful laser. Laser peening can impart a layer of residual compressive stresson a surface that is four times deeper than that attainable from conventional shot peeningtreatments." [cite web | last= | first= | authorlink= | coauthors= | year=2004 | url=http://www.metalimprovement.com/laser_peening.php | title=Laser Peening | work=Metal Improvement Company | pages= | publisher= | accessdate=2006-10-16 ] Laser peening is often used to improve the fatigue resistance of highly stressed critical turbine enginecomponents, and the laser (or component) is typically manipulated by an industrial robot.
Prototype laser peening machines were developed in the 1970s, but they and subsequent versions over the past two decades were not cost effective because the lasers lacked the high repetition rate required for treating parts rapidly. [http://www.llnl.gov/str/Hackel.html]
LSP is a novel surface treatment using laser energy to induce compressive residual stresses deep into a metallic component's surface layer [1-3] . LSP is currently being applied with an aerospace to treat critical rotating titanium components for commercial turbine engines. LSP is also improving the resistance to foreign object damage (FOD) of aero-engine fan and compressor blades [4 -5] . Other applications future uses for laser peening include components used in automotive applications, nuclear power generation and medical implants [6-7] . LSP has been intensively investigated in the last two decades. Most studies and investigations are based on experimental approaches, focusing on understanding mechanisms of LSP and its influences on mechanical behaviours and in particular enhanced fatigue performance of treated metallic components [4-5] . In most cases, there was a lack of comprehensive documentation in the relevant information in applications of LSP for various metallic alloys, such as materials properties, component geometry, laser sources, LSP parameters, and distribution of 3-D residual stresses. However, some comprehensive modelling capacities based on analytical models [8, 12] and dynamic finite element models (FEM) have been established for simulating LSP in the last decade [8-15] , which provide unique tools for evaluation of LSP and optimization of residual stress distributions in relation to materials properties, component geometry, laser sources, and LSP parameters. Those approaches can play significant roles in design and optimization of LSP processes in practical applications. The objective of this study is therefore to predict the residual stresses and plastic deformation induced by LSP process using developed finite element model. This paper is divided into 5 sections. Following this introduction, Section 2 presents the mechanics of LSP superficial treatment. Section 3 describes the finite element model used. In section 4, we verify the finite element results and discuss the effect of the impact pressure upon the plastic zone developed and unloading residual stresses. Finally, in Section 5, we conclude the work.
Mechanics of LSP superficial treatment
The principle of laser shock processing is shown in Figure 1. The workpiece is covered with a protective ablative layer (organic paint, tape, or thin metallic foil) and an inertial tamping layer (water or glass). When a metallic sample is irradiated by in intense Nd:YAG laser pulse spot 5-15 GW/cm², 10 to 30 nanoseconds long, having a wavelength of 1.06 μm, with an energy per pulse of 50 joules or more and range from 5mm to 1 mm in diameter, it forms high-pressure plasma on the surface of the part, causing a shock wave to travel through the depth and plastically deforming material. The undeformed material attempts to restore the original shape of the surface, causing inplane compressive residual stress fields to be setup in the near surface region of the target [16-20] . The coating used in LSP also prevents any melting of the target metal surface and thus the metal is “cold worked”. LSP is primarily a mechanical process rather than a thermal treatment  .
Comparing to conventionally shot peening treatment, the LSP provide four-times or more the depth of residual compressive stress, together with similar magnitudes  . These deeper levels of stress provide greater resistance to failure mechanisms such as fatigue, fretting fatigue and stress corrosion [21-22] . This gives a more damage-tolerant component, with increased resistance to various forms of stress-related failures, achieved with minimal cold working. LP can be controlled and adjusted in real-time and the energy per pulse can be recorded for every discrete location peened on the component  . Component areas inaccessible to shot peening can be selectively laser peened by directing the beam to fatigue-crucial areas.
 David W. Sokol, Laser Shock Processing, Technical Bulletin No.1, LSP Technologies, Inc. (2002)  B.P. Fairand, and A.H. Clauer, Use of laser generated shocks to improve the properties of metals and alloys, Industrial Applications of High Power Laser Technology, Vol. 86, (1977).  C.S. Montross, T.Wei, L. Ye, G. Clark, and Y.W. Mai, Laser shock processing and its effects on microstructure and properties of metal alloys: a review, International Journal of fatigue 24 (2002), pp.1021-1036.  D. Richard and F. David, Preventing Fatigue Failures with Laser Peening, LSP Technologies, Inc, the AMPTIAC Quarterly, Volume 7, Number 2 (2003).  G. Hammersley, L. A. Hackel and F. Harris, Surface prestressing to improve fatigue strength of components by laser shot peening, Optics and Lasers in Engineering 34 (2000) pp. 327-337.  B.P. Fairand and A.H. Clauer, Application of laser-induced stress waves, Laser in modern industry Seminar, May 23-25, 1978, Cambridge, Massachusetts.  P. Laurens, C. Dubouchet and D. Kechemair, Applications des lasers aux traitements de surfaces, Techniques de l’ingénieur ; traité matériaux métalliques, M1 643 (2000).  Patrick BALLARD, Contraintes résiduelles induites par impact rapide. Application au choc –Laser. Thèse de l’école polytechnique 1991.  W. Braisted, R. Brockman, « Finite element simulation of laser shock peening », International Journal of Fatigue 21 (1999), pp. 719-724.  Wenweu Zhang and Y. Lawrence Yao, Micro scale laser shock processing of metallic components, Journal of Manufacturing Science and Engineering, Vol.124, pp. 369-378, (May 2002).  P. Peyre, A. Sollier, I. Chaieb, L. Berthe, E. Bartnicki, C. Braham & R. Fabbro, « FEM simulation of residual stresses induced by laser peening », Eur. Phys. J. AP 23 (2003), pp. 83-88.  Abul Fazal M. Arif, Numerical prediction of plastic deformation and residual stresses induced by laser shock processing, Journal of Materials Processing Technologiy 136 (2003), pp. 120-138.  Iheb Chaieb, Analyse et simulation des contraintes résiduelles induites par des traitements mécaniques de précontrainte en grenaillage et choc laser, Thèse de l’Université de Reims Champagne-Ardenne 2004.  Wenweu Zhang , Y. Lawrence Yao and I.C.Noyan, Microscale Laser Shock Peening of thin films, Part1: Experiment, Modeling and Simulation, Journal of Manufacturing Science and Engineering, Vol. 126 (February 2004).  K. Ding and Lye, Laser shock peening: Performance and process simulation, Woodhead Publishing, (2004).  A.H. Clauer and B.P. Fairand, Interaction of laser-induced stress waves with metals, Application of laser in material processing, 1979.  B.P. Fairand, A.H. Clauer, R.G. Jung, and B.A. Wilcox, Quantitative assessment of laser –induced stress waves generated at confined surfaces, Applied Physics Letters, 25 (8), pp. 431-433, (1974).  A.H. Clauer, J.H. Holbrook, and B.P. Fairand, Effects of laser induced shock waves on metals, Book: Shock waves and high –strain-rate phenomena in metals(1981), chapitre 38,Edited by Marc A. Meyers and Lawrence E. Murr.  B.P. Fairand and A.H Clauer, Laser generation of high-amplitude stress waves in materials, Journal of Applied Physics, 50 (3), pp. 1497-1502, (1979).  B.P. Fairand and A.H. Clauer, Laser generated stress waves: Their characteristics and their effects to materials, conference Proceeding No.50: Laser-Solid Interactions and Laser Processing, pp.27-42, (1978).  R.A. Everett, Jr., W. T. Matthews, R. Prabhakarn, J.C. Newman and M.J. Dubberly, The effects of shot and laser peening on fatigue life and crack growth in 2024 Aluminium alloy and 4340 steel, NASA /TM- 2001- 210834 , ARL-TR- 2363, December 2001.  G. Banas and F.V. Lawrence, Jr., Shot peening versus laser shock processing, Paper presented at ICSP-4 in Tokyo, Japan, 1990  J. E. Rankin, M. R. Hill and L. A. Hackel, the effects of process variations on residual stress in laser peened 7049 T73 aluminium alloy, Material Science and Engineering A349 (2003) pp. 279-291.
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