Microscale Laser Shock Peening Research Articles

Investigation on femtosecond laser shock peening of commercially pure copper without ablative layer and confinement layer in air

A femtosecond pulse laser was employed to perform micro-scale laser shock peening (LSP) on commercially pure copper directly in air without any ablative layer or confinement layer. The effect of femtosecond LSP on surface morphology, microstructure, surface residual stress and microhardness was investigated. Regular dents are produced at the surface with low pulse energy of 12 μJ and 28 μJ, and a macro ripple structure begins to appear at 51 μJ. All samples exhibit residual compressive stress which increases from the untreated 23.0 ± 9.7 MPa to a maximum 108.3 ± 10.8 MPa at 51 μJ. The surface microhardness increases from the untreated 113.32 ± 1.45 HV to a maximum 127.34 ± 3.42 HV by 12.4% at 28 μJ. The affected thickness of femtosecond LSP is about 5 μm as indicated by the nano-indentation tests. With low pulse energy, plastic deformation, grain refinement and residual compressive stress contribute to the surface strengthening, which is inhibited by surface oxidation especially with high energy.

Study on Simulation of Residual Stress Distribution in Microscale Laser Shock Peening Based on Crystal Orientation

Study on Simulation of Residual Stress Distribution in Microscale Laser Shock Peening Based on Crystal Orientation

Study on the Numerical Simulation and Statistical Optimization of Micro-Scale Laser Shock Peening

Study on the Numerical Simulation and Statistical Optimization of Micro-Scale Laser Shock Peening

Numerical Simulation and Response Analysis of Residual Stress Induced by Micro-Scale Laser Shock Peening in TiN Film

The ABAQUS software was used to analyze the residual stress of TiN film treated by the single point micro-scale laser shock peening (μLSP). In view of the multi-factor effect of μLSP, the response surface methodology (RSM) of Design-Expert software was utilized to analyze the influence of laser process parameters on the residual stress in TiN film, based on the Box-Behnken experimental design methods, as a result, optimal combination of the laser process parameters was obtained. The results showed that μLSP can transform the tensile residual stress in the TiN film into the compressive residual stress, the compressive residual stress was gradually increasing with the increased laser power density, when the laser power density was 8 GW/cm2, the maximum compressive residual stress of the film surface was up to -350.48 MPa. In addition, as the laser power density increased, the maximum compressive residual stress was moving away from the spot center. The optimal combination of the laser process parameters of μLSP was obtained by the RSM, the laser power density was 7.6 GW/cm2, laser spot diameter was 283 μm, and the number of shocking was 2 times. Simulation results of the average residual stress was -248.76 MPa, while the predicting result of regression model was -245.31 MPa, the error was just 1.38 %. The results showed that μLSP was feasible for improving the residual stress distribution of TiN film, and the RSM can effectively optimize the process parameters of μLSP.

Numerical Simulation and Response Analysis of Residual Stress Induced by Micro-Scale Laser Shock Peening in TiN Film

The ABAQUS software was used to analyze the residual stress of TiN film treated by the single point micro-scale laser shock peening (μLSP). In view of the multi-factor effect of μLSP, the response surface methodology (RSM) of Design-Expert software was utilized to analyze the influence of laser process parameters on the residual stress in TiN film, based on the Box-Behnken experimental design methods, as a result, optimal combination of the laser process parameters was obtained. The results showed that μLSP can transform the tensile residual stress in the TiN film into the compressive residual stress, the compressive residual stress was gradually increasing with the increased laser power density, when the laser power density was 8 GW/cm2, the maximum compressive residual stress of the film surface was up to -350.48 MPa. In addition, as the laser power density increased, the maximum compressive residual stress was moving away from the spot center. The optimal combination of the laser process parameters of μLSP was obtained by the RSM, the laser power density was 7.6 GW/cm2, laser spot diameter was 283 μm, and the number of shocking was 2 times. Simulation results of the average residual stress was -248.76 MPa, while the predicting result of regression model was -245.31 MPa, the error was just 1.38 %. The results showed that μLSP was feasible for improving the residual stress distribution of TiN film, and the RSM can effectively optimize the process parameters of μLSP.

微尺度激光喷丸强化TiN涂层的表面性能

A TiN coating was modificated by microscale Laser Shock Peening(μLSP) based on the laser spot with a diameter of 300 μm to further improve its surface properties.An optical profiler and a nanomechanical test system were used to observe and measure the surface morphology,nano-hardness and the elastic modulus of the TiN coating in a peening area with different μLSP parameters.Experimental results show that a conical dent is formed in the peening area after μLSP,and the biggest depth is about 1.2 μm.The nano-hardness and elastic modulus are significantly changed in the center of peening area,and they are enhanced with the increase of laser energy in a certain range.When the laser energy is 250 mJ,both the nano-hardness and elastic modulus reach the maximum,which are 50.21 GPa and 402.8 GPa,respectively.Compared with the initial state of the TiN coating,they have been increased by 140% and 43.9%,respectively.

Experimental Study of Micro-Scale Laser Shock Peening on Copper Foils

Micro-scale laser shock peening (μLSP) is a flexible and precise process that can potentially be applied to metallic structures in micro devices to improve strength and reliability performance. In order to understand the mechanism of μLSP process, a typical experiment was carried out for copper foils specimen with various process parameters. Surface morphology, deformation and hardness of the specimens were observed and characterized by 3D microscope system and situ nano-mechanical test system respectively. It was found that overlapping rate of laser spot has a little effect on microscopic deformation depth which increases slowly with the increasing of laser energy, and micro-hardness of the laser treated specimens was improved significantly.

Study on Characteristics of Stress and Parameters Influence in Microscale Laser Shock Peening

Microscale laser shock peening (μLSP) can generate beneficial compressive stress distribution in the targets, as the used beam diameter in μLSP is at the order of micron equivalent with grain size, the treated material must be considered as anisotropic and inhomogeneous, this causes an asymmetrical distribution of residual stress. In this paper, shape factor σSF was introduced and defined to characterize the asymmetrical distribution of stress, optimum conditions of factors and the influence degree were explored based on Taguchi design with the optimal object of stress characterization values. The results show that shape factor is a significant characteristic of residual stress induced by μLSP, crystal orientation is the most important influence factor, but laser energy and peening number have significant influence on stress characterization values.

Progress in Theory and Application Research on Microscale Laser Shock Peening

Micro-scale laser shock peening (μLSP) is a novel surface modification technique utilizing mechanical effect of shock wave induced by high intensity pulsed laser with micron spots. μLSP can introduce the beneficial residual compressive stress distribution in surface layers of metal with micron-level spatial resolution, and thus enhance wear resistance and fatigue performances of metallic micro-structures. The characteristics and influence factors of μLSP were briefly introduced, and progress in μLSP research fields was reviewed and presented, including laser induced shock pressure, material constitutive relation, changes of mechanical properties and microstructure evolution of materials. Finally, proposals on further investigations of μLSP were brought forward. The systematical characterization will lay the ground work for better understanding the effect of μLSP in microlength level and developing a more practical simulation method.

Study on Strengthening Mechanism of Microscale Laser Shock Peening

Microscale laser shock peening (μLSP) is a novel surface treating technology which oriented to microscale metal components in MEMS. Beneficial compressive residual stress is induced at the shocked region to improve the performance of microstructure based on wave-solid interactions. In this paper, the basic principle of μLSP and mechanism of wave-solid coupling were introduced, the influence factors on strengthening effects, such as micro-size effect, anisotropy, dislocation, stacking fault, grain boundary and surface energy were discussed from the microscopic point of view, the results provide theoretical guidance for further study.

Spatially Resolved Characterization of Geometrically Necessary Dislocation Dependent Deformation in Microscale Laser Shock Peening

As the laser spot size in microscale laser shock peening is in the order of magnitude of several microns, the anisotropic response of grains will have a dominant influence on its mechanical behavior of the target material. Furthermore, conventional plasticity theory employed in previous studies needs to be re-examined due to the length scale effect. In the present work, the length scale effects in microscale laser shock peening have been investigated. The crystal lattice rotation underneath the shocked surface was determined via electron backscatter diffraction. From these measurements, the geometrically necessary dislocation (GND) density that the material contains has been estimated. The yield strength increment was then calculated from the GND distribution by using the Taylor model and integrated into each material point of the finite element method (FEM) simulation. Finite element simulations, based on single crystal plasticity, were performed for the process both with and without considering the GND hardening, and the comparison has been conducted.

Comparative study of symmetric and asymmetric deformation of Al single crystal under microscale laser shock peening

Laser shock peening by a micron sized laser beam is a process in which compressive residual stresses are induced in order to improve material fatigue life of micro scale components. The size of the laser target interaction zone is of the same order of magnitude as the target material grains and thus the effects of anisotropic material response must be taken into account. Single crystals are therefore chosen to study such anisotropy. It is also of interest to investigate the response of symmetric and asymmetric slip systems with respect to the yield surface. In presented work, analytic, numerical and experimental investigations of two different orientations, (110) and (1 1 4) of aluminum single crystals are studied. Anisotropic slip line theory is employed for the construction of slip line fields for both orientations and compared with numerical results. Theory is further used to explain the difference in plastic deformation for two different orientations. Lattice rotations on the top surface and cross section are also measured using Electron Backscatter Diffraction (EBSD), while residual stress is measured using X-ray microdiffraction. Both the analytical and numerical models are then validated via experimental results.

Microscale laser peen forming of single crystal

As the result of quickly increased requirement in many industrial products resulting from microtechnology, laser thermal microforming and microsurface treatment [microscale laser shock peening (μLSP)] have been well studied. By combining the beneficial effects of these two processes with a controlled bending deformation, microscale laser peen forming (μLPF) attracts more attention recently since it not only improves the fatigue life of the material but also shapes microscale metallic parts at the same time. In the present study, μLSP of single crystal aluminum was presented to study anisotropic material response. Local plastic deformation was characterized by lattice rotation measured through electron backscatter diffraction. Residual stress distributions of both sides of a peened sample, characterized by x-ray microdiffraction, were compared with the results obtained from finite element method simulation. μLPF anisotropic behavior was investigated in three effective slip systems via both the anisotropic slip line theory and numerical method. Also, the work hardening effect resulted from self-hardening, and latent hardening was analyzed through comparing the results with and without considering hardening.

Analytical solution of anisotropic plastic deformation induced by micro-scale laser shock peening

Laser shock peening (LSP) is a process to improve material fatigue life by introducing compressive residual surface stress in a target. The residual stresses are introduced when a high-intensity laser impinges on an ablative layer deposited on the surface of the target material. The interaction between laser and the ablative layer creates a high pressure plasma that leads to plastic deformation. If the laser spot size is of the order of a few micrometers, the potential exists to use this process to enhance the fatigue life of micro-scale components or to selectively treat highly localized regions of macroscale components. However, for such micro-scale laser shock peening (μLSP), the laser spot size is likely to be of the order of the material grain size. Therefore the material properties must be treated as anisotropic and heterogeneous rather than isotropic and homogeneous. In the present work, anisotropic slip line theory is employed to derive the stress and deformation fields caused by μLSP on single crystal aluminum which is oriented so that plane strain conditions are admitted. The predicted size of the deformed region is compared with deformation measurement by atomic force microscopy (AFM) and with lattice rotation measurement by electron backscatter diffraction (EBSD). In addition, single crystal plasticity finite element simulations are performed for the process. The results suggest that the analytical solution captures the salient features of the deformation state and is able to predict the size of the resulting plastically deformed region.

Two dimensional hydrodynamic simulation of high pressures induced by high power nanosecond laser-matter interactions under water

In laser shock peening (LSP) under a water-confinement regime, laser-matter interaction near the coating-water interface can induce very high pressures in the order of gigapascals, which can impart compressive residual stresses into metal workpieces to improve fatigue and corrosion properties. For axisymmetric laser spots with finite size, the pressure generation near the water-coating interface is a two dimensional process in nature. This is in particular the case for microscale LSP performed with very small laser spots, which is a very promising technique to improve the reliability performance of microdevices. However, models capable of predicting two dimensional (2D) spatial distributions of the induced pressures near the coating-water interface in LSP have rarely been reported in literature. In this paper, a predictive 2D axisymmetric model is developed by numerically solving the hydrodynamic equations, supplemented with appropriate equations of state of water and the coating material. The model can produce 2D spatial distributions of material responses near the water-coating interface in LSP, and is verified through comparisons with experimental measurements. The model calculation shows that the effect of radial release wave on pressure spatial distributions becomes more significant as the laser spot size decreases, indicating the importance of a 2D model, particularly for microscale LSP.

Study of anisotropic character induced by microscale laser shock peening on a single crystal aluminum

The beam spot size used in microscale laser shock peening is of the same order as grain size in many materials. Therefore, the deformation is induced in only a few grains so that it is necessary to treat the material as being anisotropic and heterogeneous. In order to investigate the corresponding anisotropic features, different experimental techniques and three-dimensional finite element simulations are employed to characterize and analyze anisotropic responses for single crystal aluminum under single pulse shock peening at individual locations. X-ray microdiffraction techniques based on a synchrotron light source affords micron scale spatial resolution and is used to measure the residual stress spatial distribution along different crystalline directions on the shocked surface. Crystal lattice rotation due to plastic deformation is also measured with electron backscatter diffraction. The result is experimentally quantified and compared with the simulation result obtained from finite element analysis. The influence of crystalline orientation is investigated using single crystal plasticity in finite element analysis. The results of the finite element simulations of a single shock peened indentation are compared with the finite element results for a shocked line of plain strain deformation assumption. The prediction of overall characters of the anisotropic characters associated with microscale laser shock peening will lay the ground work for the practical application of microscale laser shock peening.

Response of Thin Films and Substrate to Micro-Scale Laser Shock Peening

Micro-scale laser shock peening (μLSP) can potentially be applied to metallic structures in microdevices to improve fatigue and reliability performance. Copper thin films on a single-crystal silicon substrate are treated by using μLSP and characterized using techniques of X-ray microdiffraction and electron backscatter diffraction (EBSD). Strain field, dislocation density, and microstructure changes including crystallographic texture, grain size and subgrain structure are determined and analyzed. Further, shock peened single crystal silicon was experimentally characterized to better understand its effects on thin films response to μLSP. The experimental result is favorably compared with finite element method simulation based on single-crystal plasticity.

Numerical Investigation of Opposing Dual Sided Microscale Laser Shock Peening

Laser shock peening (LSP) is an innovative process which imparts compressive residual stresses in the processed surface of metallic parts to significantly improve fatigue life and fatigue strength of this part. In opposing dual sided LSP, the workpiece can be simultaneously irradiated or irradiated with different time lags to create different surface residual stress patterns by virtue of the interaction between the opposing shock waves. In this work, a finite element model, in which the hydrodynamic behavior of the material and the deviatoric behavior including work hardening and strain rate effects were considered, was applied to predict residual stress distributions in the processed surface induced under various conditions of the opposing dual sided microscale laser shock peening. Thus the shock waves from each surface will interact in different ways through the thickness resulting in more complex residual stress profiles. Additionally, when treating a thin section, opposing dual sided peening is expected to avoid harmful effects such as spalling and fracture because the pressures on the opposite surfaces of the target balance one another and prohibit excessive deformation of the target. In order to better understand the wave–wave interactions under different conditions, the residual stress profiles corresponding to various workpiece thicknesses and various irradiation times were evaluated.

Fourier analysis of X-ray micro-diffraction profiles to characterize laser shock peened metals

X-ray micro-diffraction profiles using a synchrotron light source were analyzed via Fourier transformation for single crystal Aluminum and Copper samples subjected to micro-scale laser shock peening. Specifically, the asymmetric and broadened diffraction profiles registered across the shock peen region were observed and analyzed by classic Warren and Averbach (W–A) method [Warren, B.E., Averbach, B.L., 1950. The effect of cold-work distortion on X-ray patterns. Journal of Applied Physics 21, 595–599] and modified W–A method [Ungar, T., Borbely, A., 1996. The effect of dislocation contrast on X-ray line broadening: A new approach to line profile analysis. Applied Physics Letters 69, 3173–3175]. Average strain deviation, mosaic size and dislocation density were estimated for the first time with a spatial resolution of 5 μm. The results compare well with the simulation results obtained from FEM analysis and from electron backscatter diffraction (EBSD) measurements. Differences in response caused by different materials and crystalline orientations (1 1 0 and 0 0 1) were also studied.

Systematical Characterization of Material Response to Microscale Laser Shock Peening

The response of materials after microscale laser shock peening (μLSP) was experimentally characterized and compared with the theoretical prediction from FEM analysis in microlength level. Since μLSP is predominantly a mechanical process instead of a thermal process, the characterization focuses on mechanical properties and associated microstructures. An X-ray microdiffraction technique was applied on the postpeened single crystal aluminum of (001) and (110) orientations, and an X-ray profile was analyzed by subprofiling and Fourier analysis method. Spatially resolved residual stress and strain deviation was quantified and explained in terms of the heterogeneous dislocation cell structure. In-plane crystal lattice rotation induced by μLSP were measured by electron backscatter diffraction (EBSD) and compared with the FEM simulation. Average mosaic size was evaluated from X-ray profile Fourier analysis and compared with the result from EBSD. Surface strength increase and dislocation cell structure formation were studied. The systematical characterization helps develop more realistic simulation models and obtain better understanding in microlength level.