Model based optimization criteria for the generation of deep compressive residual stress fields in high elastic limit metallic alloys by ns-laser shock processing

doi:10.1016/j.surfcoat.2007.12.007

Surface and Coatings Technology

Volume 202, Issue 11, 25 February 2008, Pages 2257-2262

https://doi.org/10.1016/j.surfcoat.2007.12.007 Get rights and content

Abstract

Laser Shock Processing (LSP) is based on the application of a high intensity pulsed Laser beam (I > 1 GW/cm²; τ < 50 ns) on a metallic target forcing a sudden vaporization of its surface into a high temperature and density plasma that immediately develops inducing a shock wave propagating into the material.

The main acknowledged advantages of LSP consist on its capability of inducing a relatively deep compression residual stresses field into metallic alloy pieces allowing an improved mechanical behavior, explicitly, the life improvement of the treated specimens against wear, crack growth and stress corrosion cracking. Due to these specific advantages, Laser Shock Processing is considered as a competitive alternative technology to classical treatments for improving fatigue, corrosion cracking and wear resistance of metallic materials, and is being developed as a practical process amenable to production technology.

In this paper, a model based systematization of process optimization criteria and a practical assessment on the real possibilities of the technique is presented along with practical results at laboratory scale on the application of LSP to characteristic high elastic limit metallic alloys, showing the induced residual stresses fields and the corresponding results on mechanical properties improvement induced by the treatment. The homogeneity of the residual stress fields distribution following the laser treatment spatial density will be specially analyzed.

Introduction

Following its first developments in the 1970s laser shock processing (LSP) is being consolidating as an effective technology for the improvement of surface mechanical and corrosion resistance properties of metals and is being developed as a practical process amenable to production engineering [1].

However, although significant work from the experimental side has contributed to explore the optimum conditions of application of the treatments [2], [3], [4], only limited attempts have been developed in the way of full comprehension and predictive assessment of the characteristic physical processes [5], [6], [7]. Additionally, some relevant work has been made in the line of prediction and characterization of the mechanical properties enhancement of material treated by the LSP technique [8], [9], [10].

A fundamental reason for the referred lack of predictive capability of LSP processes is their inherent physical complexity, specially stemming on the coexistence of different material phases (including plasma) developing and interacting under the action of the high intensity laser beam.

In the present paper, a review on the physical issues dominating the development of LSP processes from a high intensity laser-matter interaction point of view is presented along with the theoretical and computational methods developed by the authors for their predictive assessment, and practical results at laboratory scale on the application of the technique are shown along with corresponding results on the mechanical properties improvement.

Section snippets

Background: physical basis of LSP processes

During a first step (during which the laser beam is active on the piece), the laser energy is deposited at the interface between the target and the surrounding medium (normally a transparent

Model results

The described model has been applied to the simulation of the material behaviour of relevant metal alloys subject to LSP conditions. The final effect on the solid material of gaussian laser pulses incident with given maximum intensity and duration is analyzed. With the aid of HELIOS and LSPSIM, the resulting plasma pressure applied to the (coated and supposed undamaged) solid material is obtained for representative breakdown-free conditions (see Fig. 2), the influence of the characteristic

Experimental setup of LSP applications

The practical irradiation system used as experimental in LSP treatments conducted at CLUPM is schematically and photographically shown in Fig. 7. The LSP experiments reported in this paper were performed on Al2024-T351 alloy at 1064 nm laser wavelength using a Q switched Nd:YAG laser operating at 10 Hz and providing 9.4 ns FWHM, 1.2 J pulses. A convergent lens was used to deliver the laser energy over a 1.5 mm spot diameter. The confining layer was provided by a water jet incident close to the

Experimental results

Residual stress distribution was determined according to the ASTM E837-01 Standard test method for determining residual stresses by the hole drilling strain gage method. Al2024-T351 specimens 8 mm thick were used for the experiments. Fig. 8 shows, as an example, the depth profiles obtained for LSP-induced residual stresses and a comparison with simulation results.

Discussion

The phenomenology involved in LSP processes is complicated mostly because of their characteristic laser–plasma interaction dynamics. The need for a practical capability of process control in practical applications has led to the development of comprehensive theoretical/computational models for the predictive assessment of the complex phenomenology involved.

From the practical point of view, the LSP technology allows the effective induction of residual stresses fields in metallic materials.

Acknowledgements

Work partly supported by MCYT (Spain; Project DPI2005-09152-C02-01) and EADS-Spain.

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Laser shock processing (LSP) is a mechanical surface treatment to induce a compressive residual stress state into the near surface region of a metallic component. The effect of the cyclic deformation properties of ductile materials on the final residual stress fields obtained by LSP is analysed. Conventional modelling approaches either use simple tensile yield criteria, or isotropic hardening models if cyclic straining response is considered for the material during the peen processing. In LSP, the material is likely to be subject to cyclic loading because of reverse yielding after the initial plastic deformation. The combination of experiment and modelling shows that the incorporation of experimentally-determined cyclic stress-strain data, including mechanical hysteresis, into material deformation models is required to correctly reflect the cyclic deformation processes during LSP treatment and obtain accurate predictions of the induced residual stresses.
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Citation Excerpt :
They reached the conclusion that the overlap between two laser surface locations improves the magnitude and the affected depth of the residual stress. Morales et al also reported [17] that from the practical point of view, the LSP technology allows the effective induction of residual stresses fields in metallic materials. Cvetkovski et al showed in [18] that the development of residual stresses in medium carbon steels during their laser treatment depends on heating rate, temperature and mainly two material properties, thermal expansion/contraction and yield strength.
The paper contains the results of the structural analysis, hardness tests and fatigue tests conducted for the medium carbon structural steel with low content of Cr and Ni after its processing with CO₂ laser beam. Pre-cracks were made in the round compact tension (RCT) specimen used for fatigue test. Next, four paths, parallel to each other, were melted on both sides of the samples using a laser beam. The paths were perpendicular to the direction of the axis of the cut notch. The first melted path ran at a distance of about 2.5 mm from the pre crack tip. Fatigue test results were compared with the sample which was not subjected to laser treatment. The fatigue tests showed that the sample with no laser treatment fractured after 270,000 cycles and the laser treated sample was able to withstand 7.2 million cycles for the same load and during this time the crack length increased only 0.4 mm. Hardness tests to estimate the residual internal stresses in the melted zone, in the heat affected zone (HAZ) and in the native material were carried out using nanoindenter. It was shown that compressive residual stress, in native material, close to HAZ in half-length of the melted path, just before the front of the crack, was 1165 MPa. These residual stresses contribute to the stopping of the development of the fatigue crack. It was also shown that the longitudinal manganese sulfide inclusions reduce crack development rate probably by blunting the crack blade.

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Model based optimization criteria for the generation of deep compressive residual stress fields in high elastic limit metallic alloys by ns-laser shock processing

Abstract

Introduction

Section snippets

Background: physical basis of LSP processes

Model results

Experimental setup of LSP applications

Experimental results

Discussion

Acknowledgements

Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms

Appl. Surf. Sci.

Int. J. Fatigue

Appl. Surf. Sci.

J. Quant. Spectrosc. Radiat. Transfer

J. Appl. Physi.

Surf. Eng.