Optimization of Residual Stresses Induced by Multiple Laser Shock Processing

Laser shock processing of aluminium alloys. Application to high cycle fatigue behaviour

To investigate the effect of power density on the residual stress distribution on the surface of 17-7 PH stainless steel welded joints during laser shock process, we utilized ABAQUS finite element software for analyzing its distribution under different laser shock power densities. The material was tested for residual stress using an X-ray stress diffractometer to verify the accuracy of the simulation results and lay the foundation for finding the optimal process parameters. The simulation results show that when the laser power density is 2.79 GW/cm2, the maximum residual compressive stress value on the surface of the sample after laser shock peening is −37.2 MPa; When the laser power density is 2.79 GW/cm2, the maximum residual compressive stress value on the surface of the sample is −94.1 MPa; When the power density reaches 5.17 GW/cm2, the maximum residual stress value on the surface of the laser shock peening sample is −144.8 MPa, and residual stress cavity is formed. The experimental results show that when the laser power density is 2.79 GW/cm2, the maximum residual compressive stress on the surface of the sample is −37.3 MPa; When the power density is 3.98 GW/cm2, the uniformity of compressive stress distribution on the surface of the sample is optimal, with a maximum residual compressive stress value of −99.0 MPa; When the power density is 5.17 GW/cm2, the maximum residual compressive stress value is −146.1 MPa, and residual stress cavity is formed on the material surface. The simulation results are close to the experimental results, and the model error ranges from 0.2% to 4.9%. The simulation results are accurate and reliable.

Chapter Three - Shock Processing

3-D FEM simulation of laser shock processing

Based on the FEM code ABAQUS and MSC.Fatigue, the process of LSP before hole-drilling was adopted to study the residual stress field of aluminum alloy7050T7451 with Fastener Holes after Laser shock processing (LSP), and the fatigue life of the specimens by LSP was analyzed in this paper. The results indicate that multiple laser shock processing can improve the residual compressive stress and fatigue life to a certain degree, and with the increasing number of shot, the strengthening effect gradually decreases. The ratio of the fatigue life of specimens treated by LSP to the fatigue life of untreated specimens is gradually decreased as the mean stress σm increases, and when the σm is 67.3MPa, the fatigue life of specimens treated by LSP advances 719%, compared with that of untreated specimens.

Herein, gradient nanostructures (GNs) are created in 300M ultrahigh‐strength (UHS) steel by laser shock processing (LSP). Microstructure evolution and properties of GNs subjected to LSP with different pulse energies are thoroughly characterized on 3D profiler, scanning electron microscope (SEM), transmission electron microscope (TEM), X‐ray diffractometer (XRD), X‐ray stress analyzer, nanoindenter, and tensile tester. Results show successful creations of GNs in 300M steel after LSP treatments. With the increase in pulse energy, the size of the surface layer is refined from 15 nm (3 J) to 10 nm (7 J), and the corresponding grains are amorphized to some extent. Meanwhile, many substructure defects such as dislocation tangles and deformation twins (DTs) are noted in the subsurface. The dislocation density and the number of DTs increase with the pulse energy. Further, the high compressive residual stress is introduced to the 300M steel surface after LSP, and the corresponding hardness is improved substantially. The compressive residual stress, depth of the affected layer, and the hardness rise significantly with the pulse energy. Apart from improvements in strength and plasticity, the fracture morphology is changed from a typical ductile fracture to quasicleavage and ductile mixed fracture.

Ti-6Al-4V (Ti-64) simulated airfoils were laser shock processed with two laser power densities (4 and 9 GW/2) for each of three pulse repetition treatments (1, 3, and 5 shocks/spot). The microstructural effects of laser shock processing (LSP) on the Ti-64 were studied via scanning electron microscopy (SEM). Ultrasonic nondestructive inspection (NDI) was conducted to ensure that the LSP treatments resulted in no internal damage to the simulated airfoils. In-depth residual stress and cold work measurements were made using x-ray diffraction. No substantial changes due to LSP were found in the microstructure, and no internal damage was detected during NDI or metallographic sectioning. It was found that the in-depth residual stress and cold work states induced by LSP were a function of laser power density and pulse repetition. It was possible to induce compressive residual stresses in the direction most critical for the prevention of fatigue-crack growth throughout the thickness of the simulated airfoil leading edge.

Laser shock processing (LSP) is an innovative approach, which effectively improves the mechanical behavior of metallic structures by introducing compressive residual stress. To evaluate the residual stress evolution in low-carbon 13Cr4Ni martensitic stainless steel multi-pass butt-welded joints induced by LSP, a two-step numerical simulation including welding analysis, at first, followed by LSP calculation with the simulated welding stress results being taken into account, was performed based on ABAQUS software. Effects of LSP parameters such as power density, spot size, overlapping rate and numbers of laser shock on the residual stress variations, were systematically investigated. To validate the reliability and accuracy of the numerical simulation, experiments of welding and LSP were conducted in sequence. The residual stress after welding and LSP were investigated by x-ray diffraction method. Results demonstrate that the simulated results show a good agreement with the experimental datas. The welding residual stress distribution is uneven. Larger tensile stresses appear on the weld surface and its adjacent heat-affected zone, which could be converted into high-level compressive stress after LSP. Furthermore, an ideal residual stress field can be obtained after two successive laser shocks with an overlap rate of 75% when the power density, spot diameter, and pulse width are 7.6 GW cm−2, 4 mm, and 25 ns, respectively.

The effects of laser shock processing (LSP) on the fatigue crack properties of Ti-17 titanium alloy are investigated. Surfaces on either side of a fatigue slot are subjected to LSP. The residual stress of the irradiated surface is measured by x-ray diffraction measurement and fatigue crack growth testing of the treated and untreated specimens. The fatigue fracture morphology and microstructure are examined by scanning electron microscopy and transmission electron microscopy. Proliferation and tangles of dislocations occur in the Ti-17, and the density of dislocation increases after the LSP treatment. The fine spacing of the fatigue striations indicates that LSP produces residual compressive stress on the irradiated surfaces which can delay micro-crack formation and expansion. Consequently, the fatigue propagation life of the specimen increases considerably after LSP.

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.

Low-cycle fatigue behavior of Ni-based superalloy GH586 with laser shock processing (LSP) was investigated. The residual stress of the specimens treated with LSP was assessed by X-ray diffraction method. The microstructure and fracture morphology were characterized by using an optical microscope (OM), a scanning electron microscope (SEM), and a transmission electron microscope (TEM). The results indicated that the maximum residual compressive stress was at about 1 mm from the shocking spot center, where the residual compressive stress was slightly lower. High density tangling dislocations, dislocation walls, and dislocation cells in the microstructure of the specimens treated with LSP effectively prevented fatigue cracks propagation. The fatigue life was roughly twice as long as that of the specimens without LSP. The fatigue crack initiation (FCI) in specimens treated with LSP was observed in the lateral section and the subsurface simultaneously. The fatigue striation in the fracture treated with LSP was narrower than that in the untreated specimens. Moreover, dimples with tear ridges were found in the fatigued zones of the LSP treated specimens, which would be caused by severe plastic deformation.

The stress corrosion cracking(SCC) of austenitic stainless steel and nickel-based alloy is a primary concern for stable operation in Boiling Water Reactors(BWRs) and Pressurized Water Reactors (PWRs). Laser peening is one of the promising technologies for prevention of SCC initiation to induce compressive residual stress near the surface region. However, the compressive residual stress by laser peening is likely affected due to applied stress. In this study, the effect of mechanical loading on the compressive residual stress by laser peening was investigated. The specimens of low carbon austenitic stainless steel (Type316L), nickel-based alloy (Alloy600) and their weldments were prepared, and several levels of cyclic loading were applied to their specimens. The surface residual stress at surface was measured by X-ray diffraction method before and after loading. The relaxation behaviour of the compressive surface residual stress by laser peening was evaluated at the base metal and heat affected zone (HAZ) of the weldment. It was confirmed that compressive surface residual stress remained after stress loading which corresponds to 0.2% proof stress, and that the relaxation of the surface residual stress remarkably observed after initial loading during cyclic loading test. These relaxation behaviour were similar in Type316L, Alloy600 and their HAZ.

Effects of laser shock processing on corrosion resistance of AISI 304 stainless steel in acid chloride solution

Laser shock processing (LSP) is an advanced material surface hardening technology that can significantly improve mechanical properties and extend service life by using the stress effect generated by laser-induced plasma shock waves, which has been increasingly applied in the processing fields of metallic materials and alloys. With the rapidly development of modern industry, many new technologies developed from LSP have emerged, which broadens the application of LSP and enriches its technical theory. In this work, the technical theory of LSP was summarized, which consists of the fundamental principle of LSP and the laser-induced plasma shock wave. The new technologies, developed from LSP, are introduced in detail from the aspect of laser shock forming (LSF), warm laser shock processing (WLSP), laser shock marking (LSM) and laser shock imprinting (LSI). The common feature of LSP and these new technologies developed from LSP is the utilization of the laser-generated stress effects rather than the laser thermal effect. LSF is utilized to modify the curvature of metal sheet through the laser-induced high dynamic loading. The material strength and the stability of residual stress and micro-structures by WLSP treatment are higher than that by LSP treatment, due to WLSP combining the advantages of LSP, dynamic strain aging (DSA) and dynamic precipitation (DP). LSM is an effective method to obtain the visualized marks on the surface of metallic materials or alloys, and its critical aspect is the preparation of the absorbing layer with a designed shape and suitable thickness. At the high strain rates induced by LSP, LSI has the ability to complete the direct imprinting over the large-scale ultrasmooth complex 3D nanostructures arrays on the surface of crystalline metals. This work has important reference value and guiding significance for researchers to further understand the LSP theory and the new technologies developed from LSP.

Laser shock peening (LSP) is a powerful technique for improving the fatigue performance of metallic components by customizing compressive residual stresses in the desired near-surface regions. In this study, the residual stress distribution characteristics of 6061-T6 aluminum alloy induced by LSP were identified by the X-ray diffraction method, and their dependent factors (i.e., LSP coverage, LSP energy, and scanning path) were evaluated quantitatively by numerical simulations, exploring the formation mechanism of LSP residual stresses and the key role factor of the distribution characteristics. The results show that LSP is capable of creating anisotropic compressive residual stresses on the specimen surface without visible deformation. Compressive residual stresses are positively correlated with LSP coverage. The greater the coverage, the higher the residual stress, but the longer the scanning time required. Raising LSP energy contributes to compressive residual stresses, but excessive energy may lead to a reduction in the surface compressive residual stress. More importantly, the anisotropy of residual stresses was thoroughly explored, identifying the scanning path as the key to causing the anisotropy. The present work provides scientific guidance for efficiently tailoring LSP-induced compressive residual stresses to improve component fatigue life.