Laser Peening Process Research Articles

Numerical investigation of the effect of laser peen forming process parameters on 1060 aluminium sheets under preload

Abstract Laser peen forming process changes the shape of the metal sheet or plate by plastic deformation caused by laser induced mechanical shock wave. The bending behavior of 1060 aluminium sheets under laser peen forming (LPF) are studied numerically taking into account the various laser peening process parameters for two distinct cases; with and without preload. Modal analysis has been performed on 3D finite element model (FEM) to stop post-shock oscillations. A time-dependent and space-dependent pressure distribution is applied to the laser-peened area along with preload at the free end. The numerical comparison of bending angles and surface profile of 1060 aluminium sheets under laser peen forming are studied for both the cases, with and without preload. Preloading provides better surface profile for same bending angle for a no laser shock overlap.

Forming of monoaxially curved thin-walled T-section integral panels by double-sided laser peening

Thin-walled T-section integral panels are widely used in the aerospace industry for achieving lightweight and high performance, but their strict manufacturing requirements present challenges to current forming processes. This study proposes the double-sided laser peening (DSLP) process and develops corresponding models to achieve the forming of monoaxially curved T-section integral panels. An eigen-force model is derived to describe deformation behavior and reveal the underlying forming mechanism. The results indicate that bending deformation is driven by the moment originally induced by the in-plane eigen-force deviating from the neutral plane. Full-covered DSLP on the skin results in bending deformation towards the stiffeners, and vice versa. Additionally, DSLP on complementary regions induces identical bending deformations in opposite directions. The undesired bending distortion appeared in single-sided laser peening is eliminated by DSLP, as the action point of the eigen-force migrates to the neutral plane of the skin or stiffeners after DSLP, thereby preventing the generation of bending moments. Subsequently, drawing inspirations from topology optimization methods, a customized process planning model is developed for DSLP of monoaxially curved T-section panels. The globally convergent method of moving asymptotes is applied to solve the process planning model. The formed T-section panels, guided by the solution of the process planning model, exhibit good consistency with the objective shape. By unleashing the potential of double-sided laser peening in driving bending, this study provides a novel strategy for forming large-scale complex panels with stiffeners.

Effects of high energy laser peening followed by pre-hot corrosion on stress relaxation, microhardness and fatigue life and strength of single crystal nickel CMSX-4® superalloy

This study investigated the stress relaxation and fatigue life and strength of laser peened single crystal nickel superalloy specimens following hot corrosion exposure and then fatigue testing compared to un-peened and shot peened specimens. The specimens were treated by conventional laser peening and a new cyclic laser peening plus thermal microstructure engineering process. Stress measurements by slitting showed the plastic penetration depth of laser peening exceeded shot peening by a factor of 24. Un-peened and peened specimens were exposed to sulphate corrosives at 700°C for 300 hours and then fatigue tested. Tests of five non-laser peened specimens all failed in low cycle fatigue regime whereas three identically tested laser peened specimens all achieved multi-million-cycle runout without failure, indicating fully consistent large benefit for life by laser peening. Additional tests showed fatigue strength improvement of 2:1 by laser peening.

Enhancement of surface characteristics of additively manufactured γ-TiAl and IN939 alloys after laser shock processing

The enhancement in laser technology opened up new methods such as additive manufacturing (AM) of metals. While AM offers high design flexibility and reduced material usage, it can also produce problematic parts due to poor surface quality. A variety of post-processing techniques are available to address the drawbacks associated with the surface properties of AM. Laser Shock Processing (LSP) is one of the unique methods that induces compressive residual stress (CRS) on the surface and subsurface by creating severe plastic deformation. In this study, two critical aerospace alloys (γ-TiAl and IN939) were manufactured through two different methods called Powder Bed Fusion with Electron Beam (PBF-EB) and Powder Bed Fusion with Laser Beam (PBF-LB). Subsequently, AM samples were investigated to observe single layer LSP effects on the surface. The results of the laser peening process were determined by residual stress, microhardness and surface roughness measurements. The residual stress profiles showed that LSP significantly induces CRS on the surfaces of AM alloys. For the γ-TiAl and IN939 samples, the maximum values of CRS and depth of CRS were −460 MPa/1000 µm and −516 MPa/700 µm, respectively. Similarly, the microhardness of the materials was increased by 44.4 % for γ-TiAl and 18.2 % for IN939 by laser post-processing. In addition, a comparison of the roughness of the unground and ground AM samples was carried out. Depending on the surface condition of the AM samples, LSP had different outcomes. For example, the extreme roughness of the AM samples was partially reduced by the thermal effects of the high-energy laser shots. When comparing the roughness values in terms of Ra and Rz, there were decreases in the range of 14.7–21.3 % and 3.34–39.3 % for unground IN939 samples. However, for unground γ-TiAl samples, depending on the direction of measurement, roughness variations were observed as a 0.99 % decrease − 5.3 % increase for Ra and a 2.2 % decrease − 14.2 % increase for Rz. The roughness values for ground samples of both alloys were drastically increased, varying between 42.1 % and 7x increase. Sa values were recorded on unground AM samples. For IN939 and γ-TiAl samples these values decreased by 8.41 % and 15.8 % respectively.

Effects of high-energy laser peening followed by pre-hot corrosion on stress relaxation, microhardness, and fatigue life and strength of single-crystal nickel CMSX-4® superalloy

This study investigated the stress relaxation and fatigue life and strength of laser-peened single-crystal nickel superalloy specimens compared to unpeened and shot-peened specimens following hot corrosion exposure and then fatigue testing. The specimens were treated by conventional laser peening and a new cyclic laser peening plus thermal microstructure engineering process. The latter treatment supports the benefit of a unique process involving application of layers of laser peening using high energy with large footprint spots combined with interspersed cyclic annealing. Stress measurements by slitting showed the plastic penetration depth of laser peening exceeded shot peening by a factor of 24. Unpeened and peened specimens were exposed to sulphate corrosives at 700 °C for 300 h and then fatigue tested. Tests of five non-laser-peened specimens all failed in low-cycle fatigue regime, whereas three identically tested laser-peened specimens all achieved multi-million-cycle runout without failure, indicating fully consistent large benefit for life by laser peening. Additional tests also showed fatigue strength improvement of 2:1 by laser peening. Residual stress measurements post hot-corrosion exposure and fatigue testing showed notable 5 mm depth retention of residual eigenstress in a laser-peened specimen.

Multi-cycling nanoindentation in additively manufactured Inconel 625 before and after laser peening

In this research, a room temperature multicycle nanoindentation technique was implemented to evaluate the effects of the laser peening (LP) process on the surface mechanical behavior of additively manufactured (AM) Inconel 625. Repetitive deformation was introduced by loading-unloading during an instrumented nanoindentation test on the as-built (No LP), 1-layer, and 4-layer laser peened (1LP and 4LP) conditions. It was observed that laser-peened specimens had a significantly higher resistance to penetration of the indenter and lower permanent deformation. This is attributed to the pre-existing dislocation density induced by LP in the material which affects the dislocation interactions during the cyclic indentation. Moreover, high levels of compressive stresses, which are greater in the 4LP specimen than the 1LP specimen, lead to more effective improvement of surface fatigue properties. The transition of the material response from elastic-plastic to almost purely elastic in 4LP specimens was initiated much earlier than it did in the No LP, and 1LP specimens. In addition to the surface fatigue properties, hardness and elastic modulus were also evaluated and compared.

Residual stress distribution of aluminium-lithium alloy in hybrid process of friction stir welding and laser peening

Using laser peening (LP) to strengthen the welding zone of aluminium-lithium alloy after friction stir welding (FSW) is great important to promote the application of FSW in aerospace field. The simulation model of the “FSW + LP” hybrid process was established by finite element method and verified by the experiment. The effect of laser impact number, laser power density and spot overlap rate on the residual stress distribution in the FSW zone of 2195 aluminum-lithium alloy was studied in detail. It was found that the increase of laser impact number and laser power density impart higher compressive residual stresses. However, the induced compressive residual stress decreases with the increase of impact number due to the work hardening effect. With the increase of overlap rate, the uniformity of the residual stress in the welding zone gradually becomes better, and the value of the residual stress gradually decreases. In this study, the optimal combination of LP parameters are as follows: the impact number, the laser power density and the laser spot overlap rate are 3, 10GW/cm2 and 50%-75%, respectively.

Bending analysis and residual stress measurement of thin stainless-steel alloy

LPF (laser peen forming) is a non-contact sheet metal forming technology that uses a laser detonated shockwave to bend, shape, and fix engineering components accurately. To understand the bending behavior of SS-304 steel samples in the form of thin sheets of various thicknesses (0.3, 0.5, and 0.8 mm), laser peening at various power densities (3, 6, and 9 GW /cm2) was used. After the peening process, laser peened samples show convex and concave bending. The bending that occurs as a result of residual stress distribution across the sample's cross section during the laser peening process. As a result, to better understand the role of residual stress in LPF, the residual stress was studied using the usual X-ray based cos approach. The significance of these findings for a possible laser forming process known as laser peen forming are also investigated.

Improving Fatigue Limit and Rendering Defects Harmless through Laser Peening in Additive-Manufactured Maraging Steel

Additive-manufactured metals have a low fatigue limit due to the defects formed during the manufacturing process. Surface defects, in particular, considerably degrade the fatigue limit. In order to expand the application range of additive-manufactured metals, it is necessary to improve the fatigue limit and render the surface defects harmless. This study aims to investigate the effect of laser peening (LP) on the fatigue strength of additive-manufactured maraging steel with crack-like surface defects. Semicircular surface slits with depths of 0.2 and 0.6 mm are introduced on the specimen surface, and plane bending-fatigue tests are performed. On LP application, compressive residual stress is introduced from the specimen surface to a depth of 0.7 mm and the fatigue limit increases by 114%. In a specimen with a 0.2 mm deep slit, LP results in a high-fatigue-limit equivalent to that of a smooth specimen. Therefore, a semicircular slit with a depth of 0.2 mm can be rendered harmless by LP in terms of the fatigue limit. The defect size of a 0.2 mm deep semicircular slit is greater than that of the largest defect induced by additive manufacturing (AM). Thus, the LP process can contribute to improving the reliability of additive-manufactured metals. Compressive residual stress is the dominant factor in improving fatigue strength and rendering surface defects harmless. Moreover, the trend of the defect size that can be rendered harmless, estimated based on fracture mechanics, is consistent with the experimental results.

Surface strengthening of stainless steels by nondestructive laser peening

Laser peening is an advanced technology for surface engineering. However, the unintentional surface destruction due to melting degrades the quality and reduces the corrosion resistance. Here we have introduced the nondestructive laser peening using femtosecond lasers with ultrahigh pulse density and ultralow pulse energy by a combined approach of experiment, finite element analysis, and molecular dynamics simulations, taken stainless steel as a paradigm. A reinforcement of 33.6% was achieved on surface hardness of American National Standards Institute (ANSI) 301 stainless steel with the pulse energy of 0.375 μJ (fluence of 0.45 J cm−2) and pulse density of 2 × 108 mm−2, without penalty in surface roughness compared with conventional nanosecond laser peening. Two-temperature finite element analysis indicates the non-melting of the peening process. Molecular dynamic simulations reveal the pinning mechanism on dislocation movement by defect meshes generated during femtosecond laser peening process. Dislocation pinning, multiplication, and intertwining enhance both the linear and nonlinear mechanical properties, agreeing well with experiment. With the atomistic insights, our results imply promising applications of nondestructive laser peening on various surfaces.

Atom probe tomography analysis of hydrogen distribution in laser peened Ti6Al4V alloy to control hydrogen embrittlement

Hydrogen embrittlement of Ti6Al4V alloy was investigated under electrochemical cathodic hydrogen charging. Advanced surface treatment and material characterisation, laser shock peening (LSP) or laser peening (LP), atom probe tomography, slow strain rate tensile (SSRT) test, and XRD residual stress are employed for analysis. Results obtained exposed the opportunity to implement surface treatment for controlling hydrogen embrittlement. Induced grain refinement and compressive residual stress restrict the hydrogen uptake in Ti6Al4Valloy. Furthermore, APT results gave wt. % of TiH (titanium hydride) in laser peened hydrogen charged and untreated hydrogen-charged specimens. Thus, it is concluded that compressive stress induced by laser peening process controlled uptake of hydrogen by Ti6Al4V alloy.

Crystal Plasticity Modeling of Laser Peening Effects on Tensile and High Cycle Fatigue Properties of 2024-T351 Aluminum Alloy

Abstract This study aims to model the effects of multiple laser peening (LP) on the mechanical properties of AA2024-T351 by including the material microstructure and residual stresses using the crystal plasticity finite element method (CPFEM). In this approach, the LP-induced compressive residual stress distribution is modeled through the insertion of the Eigenstrains as a function of depth, which is calibrated by the X-ray measured residual stresses. The simulated enhancement in the tensile properties after LP, caused by the formation of a near-surface work-hardened layer, fits the experimentally obtained tensile curves. The model calculated fatigue indicator parameters (FIPs) under the following cyclic loading application show a decrease in the near-surface driving forces for the crystal slip deformation after the insertion of the Eigenstrains. This leads to a higher high cycle fatigue (HCF) resistance and the possible transformation of sensitive locations for fatigue failure further to the depth after LP. Experimental observations on the enhancement in the HCF life, along with the relocation of fatigue crack nucleation sites further to the depth, reveal the improvement in the HCF properties due to the LP process and validate the numerical approach.

Influence of laser peening on the high-temperature fatigue life and fracture of Inconel 718 nickel-based alloy

High-temperature fatigue test was performed to study the effects of laser peening (LP) on the fatigue behavior of Inconel 718 nickel-based alloy at high temperature. Compressive residual stress (CRS) induced by LP was investigated at both room and high temperature. Microstructural evolution was also studied before and after LP process. The macro- and micro-fracture morphologies in fatigue crack initiation (FCI) region, fatigue crack growth (FCG) region and finial rupture (FR) region of LPed samples with different laser power densities at different temperatures were systematically analyzed. The results showed that LP process can enhance the fatigue life of IN718 nickel-based alloy at both room and high temperature. Excessively high temperature (e.g. 800 °C) may result in a sudden decrease in the fatigue life of LPed samples due to the weakening of grain boundaries and oxidation damage. The losing of coherent strengthen between γ″ phase and matrix was another reason for the rapid reducing of fatigue life. Compared to the untreated samples, the crack source transferred from the surface to deeper layer indicated that LP can retard the crack initiation on the metal surface, which is conducive to enhance the final fatigue life. Some direct evidence, such as the decreased distance between the parallel fatigue striations, the tortuous crack growth path in the LPed specimen confirmed the retarding effects of LP on the FCG. The mechanism of microcracking and the dynamic plastic behavior of LPed specimens during high-temperature fatigue test of IN718 nickel-based alloy were also discussed.

Experimental study and finite element simulation of hydrogen permeation resistance of Ti-6Al-4V alloy strengthened by laser peening

The hydrogen permeation resistance of Ti-6Al-4V alloy treated by laser peening (LP) was investigated. Electrochemical hydrogen charging, hydrogen permeation experiments were carried out respectively. The residual stress, micro-hardness of hydrogenated and non-hydrogenated specimens were analyzed. The apparent hydrogen diffusion coefficient, lattice hydrogen concentration and hydrogen trap density were compared. LP process and hydrogen diffusion process were numerically simulated by ABAQUS software. The results show that LP can effectively increase the compressive residual stress (CRS) and micro-hardness on the surface of Ti-6Al-4V alloy. LP also promoted the closure of the original micro-cracks by forming a dense strengthening layer on the surface of the material, which reduced the hydrogen concentration and the diffusion coefficient in the crystal lattice. The laser power was inversely related to the hydrogen diffusion coefficient. The numerical simulation results reveal that the hydrogen concentration in the depth direction of LPed specimen decreases significantly, and the maximum drop of 50% for hydrogen concentration was appeared at the depth of 0.6 mm from the top surface. Moreover, the distribution trend of hydrogen concentration in the LP area is consistent with that of CRS, which indicates that CRS plays a positive role in the inhibition of hydrogen diffusion.

Post-treatment of arc-welded aluminum alloy joints by laser shock peening

The possibility to improve aluminum alloy welded joints with laser shock penning was demonstrated. It was shown that microhardness of peened area in heat affected zone is higher than one without post-treatment. Also in this area the residual tensile stresses turns into residual compressive stresses. Laser peening process is proposed to carry out using low-energy lasers. In this case were analysed different types of ablative overlays.

Effect of Laser Peening Process Parameters and Sequences on Residual Stress Profiles

Laser Peening (LP) is a surface modification technology that can induce high residual stresses in a metallic material. The relation between LP process parameters, in particular laser sequences, as well as pulse parameters and the resulting residual stress state was investigated in this study. The residual stress measurements, performed with the hole drilling technique, showed a non-equibiaxial stress profile in laser peened AA2024-T3 samples with a clad layer for certain parameter combinations. Shot overlap and applied energy density were found to be crucial parameters for the characteristic of the observed non-equibiaxial residual stress profile. Furthermore, the investigation showed the importance of the advancing direction, as the advancing direction influences the direction of the higher compressive residual stress component. The direction of higher residual stresses was parallel or orthogonal to the rolling direction of the material. The effect was correlated to the microstructural observation obtained via electron backscattered diffraction. Additionally, for peening with two sequences of different advancing directions, the study showed that the order of applied advancing directions was important for the non-equibiaxiality of the resulting residual stress profile.

Preventing Stress Corrosion Cracking of Spent Nuclear Fuel Dry Storage Canisters

Multi-purpose canisters (MPCs) are being employed for dry canister storage of spent fuel at nuclear plant sites as a temporary approach until an interim or permanent dry storage site(s) is available. For storage in coastal or lakeside regions and even humid environments, the corrosive nature of the moist air with entrained chlorides can make the welded regions of the canisters susceptible to pitting and chloride induced stress corrosion cracking (CISCC). In this report we evaluate CISCC lifetimes of welded 316L stainless steel canister plates showing in excess of 19 times increase of laser peened panel sections vs. those left as-welded using ASTM G36 (2013) accelerated corrosive testing. Specifically cracks never developed or propagated into the laser peening area. We also provide measurements of residual stress in test plates and related calculations of stress intensity and depth expected in the full canister geometry. We discuss the relevance of stress depth to pitting depth and crack growth rates. For this project welded 316L stainless steel panels were configured to MPC geometry and laser peened in the same manner as deployed for actual canisters. Our two-dimensional stress mapping shows that high energy laser peening provides deep (>5 mm) plastic compression. Literature information is not clear as to specific dependence of crack initiation and growth rates on variables including temperature, specifics of chlorine exposure, pit nucleation and residual stress intensity and depth. In our CISCC testing, although extensive cracking developed quickly (less than 18 hours) in the unpeened area, no cracking developed at all in the laser peened sections after 340 hour exposure. We further show that cracking that initiated and grew in unpeened regions arrested upon propagating to the laser peened boundary. We performed detailed finite element stress analysis (FEA) to evaluate the difference in expected retention of stress generated by the peening of test panels and the more constrained geometry of actual storage canisters. Our analysis shows that the 4 mm depth of stress measured in unconstrained test panels correlates to actual stress depth in the confined canister geometry of 6 mm, that is approximately 2 mm deeper thus satisfying a key stress depth requirement. The accelerated CISCC tests indicate that MPC design life can be dramatically improved by laser peening thereby making the MPCs a viable technical and economic solution to prevent CISCC. Following qualification testing and system configuration, a high energy laser peening process was deployed to peen welds of the dry fuel canisters for the San Onofre Nuclear Power Plant. The laser peening thereby helps ensure that the storage canisters will remain free from chlorine induced stress corrosion cracking.

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.

Investigation on mechanical properties and microstructural evolution of TC6 titanium alloy subjected to laser peening at cryogenic temperature

The aim of this study was to investigate the influence of the synergistic function of cryogenic temperature and ultrahigh strain rate deformation during the process of Cryogenic Laser Peening (CLP) on the mechanical properties and microstructural evolution of TC6 titanium alloy. The measurements of tensile properties at room and elevated temperatures, as well as micro-hardness on the cross-sectional direction were carried out. Meanwhile, the microstructural evolution was characterizated by Electron Backscattered Diffraction (EBSD) analysis and Transmission Electron Microscopy (TEM) observation. The experimental results demonstrated that cryogenic laser peening could significantly improve the strength and ductility, as well as the stability at elevated temperature condition of TC6 titanium alloy. At the same time, cryogenic laser peening could provide higher surface micro-hardness than room temperature laser peening (RT-LP), the surface micro-hardness of the specimen subjected to cryogenic laser peening increased by 4.86% than that of the specimen treated by room temperature laser peening. Additionally, cryogenic laser peening could generate finer grains and higher-density of dislocation structures, while large numbers of sub-grains and mechanical twins were also observed in the surface layer. Finally, the microscopic strengthening mechanism of cryogenic laser peening for better mechanical properties on TC6 titanium alloy was also analyzed in detail.

Improvement in fatigue properties of 2024-T351 aluminum alloy subjected to cryogenic treatment and laser peening

The aim of this study was to investigate the effects of Cryogenic Treatment (CT) and Laser Peening (LP) on the fatigue properties of 2024-T351 aluminum alloy. The measurements of tensile properties and microhardness were carried out, and the residual stress tests were also conducted through x-ray diffraction (XRD) technology. Examination of microstructure was executed by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) observations. Fatigue tests were conducted and the fatigue fracture morphologies were also analyzed by SEM. Experimental results found that both CT and LP could enhance the fatigue properties of 2024-T351 aluminum alloy. The process of CT prior to LP could provide a higher fatigue life compared with single CT or LP process. After CT prior to LP, the fatigue life of the treated specimen was 28,077 cycles under gradient loading, which obtained an increase of 4640 cycles compared to the untreated specimen. Meanwhile, compared with single CT and LP, CT prior to LP could produce finer grains, more precipitated second phases, higher density dislocation and compressive residual stresses. Beneficial microstructure evolution and compressive residual stress induced by CT prior to LP were considered to be the main factors that contribute to the improvement of the fatigue properties.