Laser Shock Peening Process Research Articles

Influence of laser strengthening techniques on anti-wear and anti-fatigue properties of rail welding joint

The fatigue damage of an on-site rail welding joint was analysed. The fatigue cracks were longer and deeper on the welding joint than on the normal rail material. The influence of two laser strengthening techniques, laser quenching (LQ) and laser shock peening (LSP), on the anti-wear and anti-fatigue properties of the welding joint were explored through rolling contact fatigue (RCF) tests. The LQ process significantly improved the anti-wear property of the rail welding joint, but the anti-fatigue property got worse. The wear rate of the LQ treated welding joint was decreased by 91.6–92.4%, however, the average crack angle (23–25°) and the average crack length (62–74 μm) on the LQ treated welding joint were larger than those on the untreated rail welding joint (13–15° and 37–59 μm, respectively). The LSP process improved both the anti-wear and anti-fatigue properties of the welding joint. The wear rate of the LSP treated welding joint was decreased by 5.1–10.1%. Meanwhile, the density of cracks (0.9–4.2 mm−1), the average crack angle (10–13°), and the average crack length (16–26 μm) on the LSP treated welding joint were smaller than those on the untreated and LQ treated welding joints.

Surface modification of AA7075 T651 by laser shock peening to improve the wear characteristics

The Enhancement of surface properties of various materials by laser shock peening (LSP) to prolong the service life of the industrial components gained significant importance for the past decade. Though several investigations are made on the different aluminum alloys through LSP route, very limited studies are reported on AA 7075 T651 aluminum alloy, which is extensively used in upper wing cover and fuselages of aerospace industries. In the present investigation, a Q-switched Nd:YAG laser having different pulse energies of 200, 300 and 400 mJ with black paint and water as the ablative layer and confinement medium respectively are used to study the variations in the mechanical properties. A deformed layer thickness of about 500 μm with a peak hardness of 236 Hv at the surface is obtained with pulse energy of 400 mJ. Further, the maximum compressive residual stress induced in the said specimen is 317 MPa and a wear rate of 1.18 × 10−6 g m−1 is measured. The microstructural studies performed using HR-TEM, revealed the enhancement in the properties are attributed to the grain refinement and dislocation strengthening effected by the LSP process.

Application of design of experiments for laser shock peening process optimization

Laser shock peening—a very promising life enhancement technique—has demonstrated great success regarding the improvement of fatigue behavior via deep compressive residual stresses. However, the prediction and adaption of residual stress fields on basis of the laser peening parameters are still not comprehensively established. The aim of the current work is to investigate the effects of the laser pulse energy, the number of treatment overlaps as well as the laser spot size on the resulting residual stress distribution, characterized by following quantities: the residual stress close to the surface, the maximum compressive residual stress, and the integral compressive stress area over the specimen depth. For a systematic investigation of all main and interaction-based process parameter effects, and a subsequent parameter optimization, the general full factorial design is employed. The results show that laser shock peening with different process parameter combinations, inducing residual stresses with comparable integral stress area, can lead to a minimum fatigue life extension of approx. 100,000 cycles, representing a minimum fatigue life of 250% of the base material. The experimental scatter in the number of cycles to failure follows the Weibull distribution which qualitatively correlates with the standard deviation of the integral stress area.

Experimentally validated multi-step simulation strategy to predict the fatigue crack propagation rate in residual stress fields after laser shock peening

Laser shock peening (LSP) is a promising technology to retard the fatigue crack propagation (FCP) in metallic lightweight structures. A multi-step simulation strategy to predict FCP in LSP-induced residual stress fields is proposed and applied. The simulation strategy involves an LSP process simulation, a transfer approach to include the plastic strains in a C(T) specimen model to calculate the residual stresses and an FCP simulation to determine the stress intensity factors. The FCP rate is finally determined via FCP equations. The validity of the simulation strategy including the crack driving quantities prediction is experimentally demonstrated by a novel ‘simulation’ approach.

Laser shock peening of welded joints

The effect of laser shock peening (LSP) process on the mechanical characteristics of aluminum and titanium welding joints was investigated. Since LSP requires special ablative and transparent coatings, efficiency of different coatings was analyzed. It was shown that highest LSP effect was obtained by using aluminum foil attached with adhesive tape as an ablative layer and water as a transparent overlay. Several laser peening conditions with different intensities, durations, and number of shots were tested to determine the applicability of the method for an aluminum and titanium alloys. Surface microhardness of observed materials was increased up to 70 %. It was shown LSP of welded joints of Al-6Mg aluminum alloy allows to change tensile residual stresses in the weld zone, on compressive stresses.

Prediction of residual stress profile and optimization of surface conditions induced by laser shock peening process using artificial neural networks

The purpose of this paper is to predict the compressive residual stress profile induced by the laser shock peening process and to optimize the surface conditions on a Ti-6Al-4V titanium super-alloy using the new technology based on the artificial neural networks. This study covers two principal cases: (i) The numerical analysis: the compressive residual stress distribution induced by the laser peening process is simulated based on the finite element method using ABAQUS software (ABAQUS/Explicit code). (ii) The mathematical modeling analysis: the using of the artificial neural network technique has been proposed to predict the residual stress profile induced by the laser shock processing in order to optimize the laser shock peening surface conditions. To train the artificial neural network, we use different numerical experiments measurements as training and test data. The best fitting training data set was obtained with four neurons in the hidden layers. After training and as seen from the mathematical experiments, the calculated residual stress and the damage state are obviously acceptable. The principal goal of this research paper is to predict the surface treatment state induced by the laser shock peening based on the artificial neural networks technique and on the numerical results without making multiple simulations that can take much more time.

Effects of corrosion-inhibiting surface treatments on irradiated microstructure development in Ni-base alloy 718

The objective of this study is to understand the influence of surface treatments, namely laser shock peening (LSP) and ultrasonic nanocrystal surface modification (UNSM), on the irradiation tolerance of a Ni-base alloy. Surface treatments such as LSP and UNSM have been shown to mitigate the potential for primary water stress corrosion cracking (PWSCC). As such, there is emerging interest in the development and implementation of LSP and UNSM for nuclear reactor components. The LSP process utilizes a laser-generated plasma, while UNSM utilizes mechanical contact at ultrasonic vibration speeds, to create near-surface compressive residual stresses, high dislocation densities, twins and subgrains, and nanostructuring of the workpiece. These resultant microstructural changes can substantially affect the creation and evolution of irradiation damage. Herein, we study precipitation hardened Ni-base Alloy 718+, which is utilized in reactor components exposed to PWSCC-inciting environments. Specimens are treated with LSP or UNSM, then irradiated with 2.0 MeV protons to 7 displacements per atom (dpa) at 500°C. The dislocation line density is roughly an order of magnitude larger in the unirradiated LSP and UNSM specimens than in the baseline (untreated) specimen. Irradiation-induced dislocation loop nucleation results in an increase in the areal density of dislocation-type defects. Irradiation also induces disordering of the γ′ precipitates; this disordering appears more extensive in the baseline than in the LSP and UNSM specimens. Results are considered in the context of overall sink strength. Finally, irradiation-induced softening is observed in all specimens through nanoindentation, and is ascribed to the overall change in sink strength, resulting from the competition between γ′ disordering and dislocation loop nucleation.

Surface characterization and tribological performance of laser shock peened steel surfaces

Laser shock peening (LSP) is a preeminent surface treatment technique that can surpass many of the modern surface modification processes. Though the wear and surface hardening behavior of the LSP treated surfaces has been extensively investigated, the friction behavior and surface morphological changes due to LSP are not well explored. Hence, the present study focuses on the effect of LSP process parameters on surface morphology and tribological behavior of 1045 steel surfaces. More specifically, the influence of laser intensity on surface roughness and its effect on the coefficient of friction (COF) and transfer layer formation were investigated. The results show that the COF decreased with increasing laser intensity up to a threshold intensity, thereafter, the COF increased with increasing laser intensity. These variation in COF was attributed to the change in surface morphology as a result of applied laser intensity. As the laser intensity increased to a threshold value, the COF decreased as a result of surface strengthening and roughening effects. Beyond the threshold laser intensity, the COF increased as a result of the dominant surface roughening effect.

Gradient twinning microstructure generated by laser shock peening in an AZ31B magnesium alloy

Mg alloys are lightweight structural metals that are promising for a variety of engineering applications. However, use of Mg alloys is often restricted by their poor mechanical properties. Recent studies indicate that a novel laser-based surface processing technology, laser shock peening (LSP), is promising to improve the engineering performance of Mg alloys by enhancing their surface strength, biocompatibility, fatigue resistance, and anti-corrosion ability. Despite these experimental efforts, little attention has been paid to study the surface microstructure evolution in the LSP process, particularly the formation of high density deformation twins. Deformation twinning in hexagonal closed-packed (HCP) crystal structure plays a fundamental role in enhancing mechanical performance of Mg alloys. This research is to establish the process-microstructure relationship of Mg alloys as processed by LSP. A focus is placed on understanding the deformation twinning mechanism. LSP experiments are conducted on a rolled AZ31B Mg alloy. The microstructures before and after laser processing are characterized. The effect of laser intensity on the twin volume fraction is investigated. The surface hardness as associated with the twin density is measured. The mechanism responsible for the formation of gradient twinning microstructure and the twinning-induced hardening effect are discussed. The anisotropic response to LSP in terms of grain orientation and the resultant microstructure and hardness improvement in the Mg samples are discussed.

Optimization of peened-surface laser shock conditions by method of finite element and technique of design of experiments

This paper presents a numerical simulation of the laser shock peening (LSP) process using the finite element method. The majority of controlling parameters of the LSP process have been taken into account. The LSP loading has been characterized by the use of a repetitive time Gaussian increment pressure applied uniformly at a circular impacted zone. The utilized model of the treated material behaviour law is the Johnson-Cook’s visco-elastic-plastic coupled with damage. The proposed model leads to determine the LSP surface modifications: (i) the in-depth residual stresses, (ii) the induced plastic strains and (iii) the superficial damage. These modifications can be significantly induced in few cases, particularly when the operating conditions are not well optimized. An application is carried out on the laser peened titanium aero-engine super alloy Ti-6Al-4V. A satisfactory correlation between the computed and experimental results is observed. Also, it is noted that the computed superficial damage values increase with the growth of the maximal peak pressure of the laser spot, which are physically consistent. Otherwise, in order to optimize the laser peening operating conditions, a design of experiments is established. It allows having surface-response relationships between the operating parameters and the three announced induced effects.

Corrosion behavior of as-weld CLAM steel after laser shock peening process and tempering in static molten lithium lead

China low activation martensitic steel was welded by gas tungsten arc welding, after that laser shock peening and tempering treatment are applied to as-weld material, its compatibility with molten lithium lead are studied. Tempering treatment weight loss is 1.32 mg/cm2 compared to LSP is 1.91 mg/cm2 after 3000 h. Tempered martensite is more excellent than coarse martensite, as-weld surface corrosion grooves are serious. Passivation layers protect the internal metal from destroy in molten metal and it firstly appeared in defective areas during LSP process. Main elements markedly reduced through dissolving into molten metal, microstructure and phase transformation took place along the whole corrosion surface.

Mapping residual strain induced by cold working and by laser shock peening using neutron transmission spectroscopy

This paper presents 2D mapping of residual strains, induced by cold expansion and laser shock peening processing of aluminium alloy samples, by using Bragg edge neutron transmission. Neutron transmission uses information contained in the neutron beam transmitted through a sample. It is shown that neutron transmission strain mapping with high spatial resolution can provide important insights into the distribution of residual strains associated with processing of materials. The residual strain field around a cold-expanded hole can be revealed in detail, as can be the residual strain profile associated with laser peening. Results are correlated with measurements obtained by conventional neutron diffraction and incremental hole drilling. The residual strain variation around the cold-expanded hole and the depth of compressive residual strain generated by the peening process were captured with high spatial resolution, showing the advantages of neutron transmission over other well-established strain measurement methods by non-destructively generating a map of residual strains over a large area.

Fatigue Life Extension of AA2024 Specimens and Integral Structures by Laser Shock Peening

The goal of the present study is to understand the effects of laser shock peening (LSP)-induced residual stresses on the fatigue crack propagation (FCP) behaviour of the commonly used aircraft aluminium alloy AA2024 in T3 heat treatment condition. LSP treatment was performed using a pulsed Nd:YAG laser on compact tensile C(T)50-specimens with a thickness of 2.0 mm. LSP-treated specimens reveal a significant retardation of the fatigue crack propagation. The fatigue crack retardation effect can be correlated with the compressive residual stresses introduced by LSP throughout the entire specimen thickness. A possible application of the LSP process on a component like panel with three welded stringers representing a part of a fuselage structure was performed as well. The skin-stringer AA2024-AA7050 Tjoints were realised through stationary shoulder friction stir welding (SSFSW), a variant of the conventional friction stir welding process. In this relatively new process, the shoulder does not rotate and therefore does not contribute to the heat generation. Consequently, a reduced and more homogeneous heat input leads to a less affected microstructure and better mechanical properties. The efficiency of the LSP process has been demonstrated resulting in an increase of 200 – 400% in fatigue lifetime.

Laser shock peening process modelling and experimental validation of AA2198‐T3 and AA2198‐T8

AbstractLaser shock peening (LSP) is a surface treatment which improves the fatigue life of metallic structures by laser‐induced compressive residual stresses. The purpose of this work is the prediction of the measured residual stresses after LSP in sheets consisting of AA2198. Therefore, a finite element model is set‐up. The Johnson‐Cook material model is used to model the strain rate depend material behaviour during plastic deformations. Stress averaging enables the comparison between measured and predicted residual stresses. Residual stress measurements are performed using the incremental hole drilling method. The initially unknown pressure pulse is identified for AA2198‐T3 and validated with AA2198‐T8. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Experimental and numerical investigation of residual stresses in laser shock peened AA2198

Laser shock peening (LSP) is a surface treatment which improves the fatigue performance of metallic structures by introducing compressive residual stresses. The aim of this paper is the investigation of LSP of the aluminium alloy AA2198. This investigation includes the variation of the laser power density (2.78–25 GW/cm2) and the square laser focus (1 mm × 1 mm and 3 mm × 3 mm). Additionally, two different temper stages (T3 and T8) and thicknesses (3.2 mm and 4.8 mm) of AA2198 are considered. The study of the LSP process is split into two parts; at first, LSP experiments are performed to clarify the influence of the temper stage, the focus size, the laser power density and the thickness of the specimen on the residual stress field. Secondly, a process model based on the finite element method is employed which requires in particular the adjustment of a suitable laser induced pressure pulse. Due to the different yield strength and strain hardening behaviour of the different temper conditions, AA2198-T8 shows a lower penetration depth of compressive residual stresses compared to AA2198-T3. A smaller focus size leads to higher compressive residual stresses near the surface but a lower penetration depth. To investigate possible shock wave reflections, different base layers in the LSP process are investigated considering a free, a clamped and a glued back-side of the specimen. No differences in terms of resulting residual stresses were observed. The experimental study provides some preliminary assumptions which are used to simplify the simulation set-up. Residual stresses are measured by the incremental hole drilling method using electronic speckle pattern interferometry (ESPI) as well as synchrotron X-ray diffraction. The calculated residual stresses in the simulation are averaged layer-wise over a sample area for comparison with the measured residual stresses. The model is used to simulate the LSP process for the considered temper stages and focus sizes to predict the resulting residual stresses. Simulated and measured residual stress profiles show for the different cases very good agreement.

Laser shock peening studies on SS316LN plate with various sacrificial layers

Laser shock peening (LSP) has been utilized to modify the surface characteristics of SS316LN plates of 6mm thickness. Laser pulse widths employed are 30ps and 7ns and the laser energy was varied in the range 5–90mJ. Peening was performed in direct ablation mode as well as with various sacrificial layers such as black paint, transparent adhesive tape and absorbing adhesive tape. The surface characteristics were greatly influenced by the type of sacrificial layer employed. The average surface roughness values are about 0.4μm when the black paint and transparent adhesive tape were used as sacrificial layers. In contrast to this, using absorbent adhesive tape as a sacrificial layer has resulted in an average surface roughness of about 0.04μm. Irrespective of pulse durations (30ps or 7ns), absorbent adhesive tape has always resulted in compressive residual stresses whereas other layers appear to be not that effective. In case of 30ps pulse, as the laser energy was increased from 5mJ to 25mJ, there was a texture observed in (111) reflection of X-ray diffractograms and the center of the peak has also gradually shifted to left. X-ray line profile analysis suggests that with the increase in laser energy, lattice microstrain also has increased. This lattice microstrain appears to be resulting from the increased dislocation density in the peened sample as evidenced during transmission electron microscopic investigations. Cross-sectional scanning electron microscopy performed on peened samples suggests that absorbing adhesive tape brings no surface damage to the samples whereas other sacrificial layers have resulted in some surface damage. Based on all these structural and microstructural details, it is recommended that absorbent tape could be used as a sacrificial layer during LSP process which induces surface residual stresses with no damage to the sample surface.

Cavitation bubble oscillation period as a process diagnostic during the laser shock peening process

Laser shock peening (LSP) technology is a laser-induced shock process implemented as a surface enhancement technique to introduce beneficial compressive residual stresses into metallic components. The process employs water to confine and enhance the pressure pulse delivered to the target. For thick water layers, or fully water immersed LSP, a cavitation bubble is generated by the surface vaporization of the LSP laser pulse. This research shows that the first bubble oscillation period of the cavitation bubble can be used to characterize effective and repeatable energy delivery to the target. High-speed shadowgraphy is implemented to show that variations in the bubble period occur before visual observations of dielectric breakdown in water. The diagnostic potential of the first bubble oscillation period is used to identify the dielectric breakdown threshold of water, which shows an increase with increasing water quality measured by water conductivity.

Effect of laser shock peening and its size-dependence on the compressive plasticity of Zr-based bulk metallic glass

In this study, laser shock peening (LSP) has been performed on brick-shaped Zr41.2Ti13.8Cu12.5Ni10Be22.5 (vit1) bulk metallic glass (BMG) with different dimensions but the same aspect ratio of 2:1 to study the effects of LSP treatment and size-dependence on plasticity and compressive strength at room temperature. The results show that: (1) the plasticity of LSP treated specimens is still size-dependent, the smaller the specimen size is, the higher plasticity and compressive strength it has, which means the size effects of BMG can be preserved and inherited during LSP. (2) The same LSP treatment has different effects on different size specimens, the smaller the specimen size is, the greater effect it has, which indicates that LSP process causes much more obvious size-dependence in BMG.The mechanism for plasticity improvement can be attributed to the increase of free volume and the generation of deep compressive residual stress induced by LSP for each size specimen. Therefore, smaller specimen is corresponding with larger volume fraction of LSP affected zones and greater LSP effect.

Comparison of warm laser shock peening and laser shock peening techniques in lengthening the fatigue life of welded joints made of aluminum alloy

Welded joints made of 6061-T6 Al alloy were studied to evaluate warm laser shock peening (WLSP) and laser shock peening (LSP) processes. The estimation model of laser-induced surface residual stress was examined by means of experiments and numerical analysis. The high-cycle fatigue lives of welded joint specimens treated with WLSP and LSP were estimated by conducting tensile fatigue tests. The fatigue fracture mechanisms of these specimens are studied by surface integrity and fracture surface tests. Experimental results and analysis indicated that the fatigue life of the specimens processed by WLSP was higher than that with LSP. The large increase in fatigue life appeared to be the result of the larger residual stress, more uniform microstructure refinement and the lower surface roughness of the WLSP specimens.

3D Laser Shock Peening – A new method for the 3D control of residual stresses in Selective Laser Melting

This paper describes a hybrid additive manufacturing process – 3D Laser Shock Peening (3D LSP), based on the integration of Laser Shock Peening (LSP) with selective laser melting (SLM). The well-known tensile residual stresses (TRS) in the as – built (AB) state of SLM parts in the subsurface region have a detrimental effect on their fatigue life. LSP is a relatively expensive surface post treatment method, known to generate deep CRS into the subsurface of the part, and used for high end applications (e.g. aerospace, nuclear) where fatigue life is crucial. The novel proposed 3D LSP process takes advantage of the possibility to repeatedly interrupt the part manufacturing, with cycles of a few SLM layers. This approach leads to higher and deeper CRS in the subsurface of the produced part, with expected improved fatigue properties. In this paper, 316L stainless steel samples were 3D LSP processed using a decoupled approach, i.e. by moving back and forth the baseplate from an SLM machine to an LSP station. A clear and significant increase in the magnitude and depth of CRS was observed, for all investigated process parameters, when compared to the AB SLM parts, or those traditionally LSP (surface) treated.