The laser shock peening (LSP) process using a Q-switched pulsed laser beam for surface modification has been reviewed. The development of the LSP technique and its numerous advantages over the conventional shot peening (SP) such as better surface finish, higher depths of residual stress and uniform distribution of intensity were discussed. Similar comparison with ultrasonic impact peening (UIP)/ultrasonic shot peening (USP) was incorporated, when possible. The generation of shock waves, processing parameters, and characterization of LSP treated specimens were described. Special attention was given to the influence of LSP process parameters on residual stress profiles, material properties and structures. Based on the studies so far, more fundamental understanding is still needed when selecting optimized LSP processing parameters and substrate conditions. A summary of the parametric studies of LSP on different materials has been presented. Furthermore, enhancements in the surface micro and nanohardness, elastic modulus, tensile yield strength and refinement of microstructure which translates to increased fatigue life, fretting fatigue life, stress corrosion cracking (SCC) and corrosion resistance were addressed. However, research gaps related to the inconsistencies in the literature were identified. Current status, developments and challenges of the LSP technique were discussed.

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Laser Peening Process and Its Impact on Materials Properties in Comparison with Shot Peening and Ultrasonic Impact Peening

Microstructural response and grain refinement mechanism of commercially pure titanium subjected to multiple laser shock peening impacts

The effects of ultrasonic impact peening (UIP) and laser-shock peening (LSP) without protective and confining media on microstructure, phase composition, microhardness and residual stresses in near-surface layers of an austenitic stainless steel AISI 321 are studied. An X-ray diffraction analysis shows both significant lines broadening and formation of strain-induced e- and α-martensite after UIP with additional peaks found near austenite ones in the low-angle part after LSP supposedly due to formation of a dislocation-cell structure in the surface layer. TEM studies demonstrate that a nano-grain structure containing either only austenitic grains with e-martensite (at strains up to 0.42) or both austenite and α-martensite grains (at higher strains) can form in the surface layer after UIP. Highly tangled and dense dislocation arrangements and even cell structures in fully austenitic grains are revealed both at the surface after LSP and in the layer at a depth of 80 μm after UIP. UIP is found to produce a sub-surface layer 10 times thicker and about 1.4 times harder than that formed by LSP. A mechanism of formation of the dislocation-cell structure in such steels (with a low stacking fault energy) is discussed. A nucleation process of α-martensite is discussed with respect to strain, strain rate, local heating and mechanical energy accumulated/applied to the surface layer under conditions of UIP and the LSP and compared to literature data for different loading schemes.

Characterization of ultrasonically peened and laser-shock peened surface layers of AISI 321 stainless steel

Laser shock processing is an innovative surface treatment technique similar to shot peening. It can impart compressive residual stresses in material for improving fatigue, corrosion and wear resistance of metals. FEM simulation is an effective method to predict mechanical effects induced in the material treated by laser shock processing. An analysis procedure including dynamic analysis performed by LS-DYNA and static analysis performed by ANSYS is presented in detail to attain the simulation of the single and multiple laser shock processing to predict the residual stress field and the surface deformation. History of the energies during dynamic analysis is analyzed and validated by the theoretical calculation. The predicted residual stress field for single laser shock processing is well correlated with the available experimental data and a homogeneous depression with little roughness modification in the action zone of the shock pressure is induced on the treated surface according to the simulation of surface deformation. Simulation of multiple laser shocks is also performed, which indicates that compressive residual stresses and plastically affected depth can be extensively increased and gradually reach the saturated state with the increase of laser shock number.

3-D FEM simulation of laser shock processing

Thermal and mechanical modeling analysis of laser-assisted micro-milling of difficult-to-machine alloys

Laser shock processing, also known as laser shock peening, generates through a laser-induced plasma, plastic deformation and compressive residual stresses in materials for improved fatigue or stress corrosion cracking resistances. The calculation of mechanical effects is rather complex, due to the severity of the pressure loading imparted in a very short time period (in the ns regime). This produces very high strain rates (106 s−1), which necessitate a precise determination of dynamic properties.Finite element techniques have been applied to predict the residual stress fields induced in two different stainless steels, combining shock wave hydrodynamics and strain rate dependent mechanical behaviour. The predicted residual stress fields for single or multiple laser processes were correlated with those from experimental data, with a specific focus on the influence of process parameters such as pressure pulse amplitude and duration, laser spot size or sacrificial overlay.Among other results, simulations confirmed that the affected depths increased with pulse duration, peak pressure and cyclic deformations, thus reaching much deeper layers (> 0.5 mm) than with any other conventional surface processing. To improve simulations, the use of experimental VISAR determinations to determine pressure loadings and elastic limits under shock conditions (revealing different strain-rate dependences for the two stainless steels considered) was shown to be a key point.Finally, the influence of protective coatings and, more precisely, the simulation of a thermo-mechanical uncoated laser shock processing were addressed and successfully compared with experiments, exhibiting a large tensile surface stress peak affecting a few tenths of micrometres and a compressive sub-surface stress field.

https://iopscience.iop.org/article/10.1088/0965-0393/15/3/002/pdf

FEM calculation of residual stresses induced by laser shock processing in stainless steels

Benefits from laser Peening have been demonstrated several times in fields like fatigue, wear or stress corrosion cracking. However, in spite of recent work on the calculation of residual stresses, very few authors have considered a finite element method (FEM) approach to predict laser-induced mechanical effect. This comes mainly from the high strain rates involved during LP (10 6  s -1 ), that necessitate the precise determination of dynamic properties, and also from the possible combination of thermal and mechanical loadings in the case of LP without protective coatings. In this paper, we aim at presenting a global approach of the problem, starting from the determination of loading conditions and dynamic yield strengths, to finish with FEM calculation of residual stress fields induced on a 12% Cr martensitic stainless steel and a 7075 aluminium alloy.

FEM simulation of residual stresses induced by laser Peening

In the current study, deep rolling treatment was applied to enhance low-cycle fatigue behavior of the AISI 304 stainless steel. Desirability function approach was applied to determine the process parameters offering the optimal surface roughness and hardness as these surface characteristics are supposed to control the fatigue cracks initiation and growth. The enhancement of the low-cycle fatigue behavior was investigated using strain-controlled fatigue tests applied to machined and deep rolled specimens associated to experimental evaluation of surface topography, microhardness, and residual stress. Findings of this work show that an increase of the fatigue lifetime of the AISI 304 components can be achieved by the application of deep rolling to machined surfaces. Moreover, the application of an intermittent deep rolling leads to a significant extension of service life especially when it is performed at low strain amplitudes. The improvement of the residual lifetime of deep rolled components is explained based on evaluations of the surface texture changes, residual stresses, cold work hardening, and strain-induced martensite transformation.

Improvement of AISI 304 austenitic stainless steel low-cycle fatigue life by initial and intermittent deep rolling

The finite element simulation of LSP induced surface modifications (residual stresses and deformations, bending effect on thin targets), is of primary importance to predict accurate LSP conditions for a given application. We have carried out such simulations with the use of RADIOSS TM and ABAQUS TM commercial softwares, mainly on Ti6Al4V titanium alloys and 7150 aluminum for aeronautical applications, and various kind of steels. Mostly, FEM simulations are shown to provide a rather good agreement with experimental data, and to reproduce precisely the residual stress fields and / or bending effects induced by LSP, for one or a large number of local impacts. Last, a thermo-mechanical simulation was carried out successfully to reproduce the effects of a LSP treatment without protective overlay [2].

Finite element modelling of laser peening and laser peen forming of materials

Shot peening is a major finishing treatment aiming to enhance parts’ life. However, the recourse to simulation to master the process and its effects still suffers from problems due to its complexity. The current paper presents a finite element-based approach that considers the random aspect of the shot peening process in association with a large number of shots, making it more realistic. The developed method is found to allow a computing time gain up to 70 % with comparable results to those obtained by the discrete element method, which has been reported as the most suitable for shot peening simulation. The comparison of both simulations results to experiment returned a good agreement. Simulations and experiences were concerned with C75 Almen-like strips.

Iheb Chaieb

Papers

FEM calculation of residual stresses induced by laser shock processing in stainless steels

FEM simulation of residual stresses induced by laser Peening

Improvement of AISI 304 austenitic stainless steel low-cycle fatigue life by initial and intermittent deep rolling

Finite element modelling of laser peening and laser peen forming of materials

An innovative contactless finite element simulation of the shot peening process