- 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.
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