Multiple Laser Shock Research Articles

Microstructure Evolution and Microhardness of Ultrafine-grained High Carbon Steel during Multiple Laser Shock Processing

Surface microstructure and microhardness of (ferrite + cementite) microduplex structure of the ultrafine-grained high carbon steel after laser shock processing (LSP) with different impact times were investigated by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and microhardness measurements. Equiaxed ferrite grains were refined from 400 to 150 nm, and the cementite lamellae were fully spheroidized, with a decrease of the particle diameter from 150 to 100 nm as the impact times increased. The cementite dissolution was enhanced significantly. Correspondingly, the lattice parameter of α-Fe and microhardness increased with the impact times.

Investigation of Corrosion Behaviour of Aluminium Alloy Subjected to Laser Shock Peening without a Protective Coating

The effect of shock waves and strain hardening of laser shock peening without protective coating (LSPwC) on alloy AA 6082-T651 was investigated. Analysis of residual stresses confirmed high compression in the near surface layer due to the ultrahigh plastic strains and strain rates induced by multiple laser shock waves. Corrosion tests in a chloride environment were carried out to determine resistance to localised attack, which was also verified on SEM/EDS. OCP transients confirmed an improved condition, that is, a more positive and stable potential after LSPwC treatment. Moreover, polarisation resistance of the LSPwC treated specimen was by a factor of 25 higher compared to the untreated specimen. Analysis of voltammograms confirmed an improved enhanced region of passivity and significantly smaller anodic current density of the LSPwC specimen compared to the untreated one. Through SEM, reduction of pitting attack at the LSPwC specimen surface was confirmed, despite its increased roughness.

A Study of Multiple Laser Shock Micro-Adjustment Using Numerical Simulation

Laser shock micro-adjustment is a precise and noncontact adjustment technique using laser-shock-waves to adjust the curvature of micro-components. The experimental studies have indicated that: when laser shock region is located at the free end of cantilevers, multiple impacts are applied to achieve a large bending degree; meanwhile, different bending directions can be obtained with multiple impacts in the junction position. Efforts should been made to understand the mechanisms of multiple laser shock micro-adjustment. Two mechanisms have been proposed for describing the laser shock micro-adjustment in different laser shock regions, namely shock inertia mechanism and material flow mechanism. The proposed micro-adjustment mechanisms can predict bending angles and directions. To validate the proposed micro-adjustment mechanisms, numerical simulations were carried out based on the FEM method using the ANSYS/LS-DYNA software and the corresponding results demonstrate the proposed mechanisms.

Effect of Multiple Laser Shock Processing on Microstructure and Mechanical Properties of Ti-5Al-4Mo-4Cr-2Sn-2Zr Titanium Alloy

Abstract The effects of the impact times and laser energy on the mechanical properties and microstructures of the Ti-5Al-4Mo-4Cr-2Sn-2Zr titanium alloy during multiple laser shock processing (LSP) were investigated. Residual stress and microhardness were tested on the samples to analyze the mechanical properties, and X-ray diffraction (XRD) and transmission electron microscope (TEM) observation was conducted to study micro-structural evolution. It's found that laser energy and impact number have an influence on the residual stresses and the hardness. Both the hardness and the residual stress of Ti-5Al-4Mo-4Cr-2Sn-2Zr titanium alloy have been improved, forming a severe plastic deformation layer, and increasing of impact number or laser energy will improve the amplitude and enhance affected depth. XRD tests of the specimen surfaces with different impacts and power density show that the positions of diffraction peaks do not change, but the Bragg diffraction peaks broaden. The TEM observation indicates that a layer of nanocrystalline with a size of 30∼60 nm has been fabricated on the surface of the titanium alloy after 3 impacts, and adopts random orientation. Dislocation activity is the main reason of the grain refinement for Ti-5Al-4Mo-4Cr-2Sn-2Zr titanium alloy.

Laser shock cleaning of radioactive particulates from glass surface

Efficient removal of Uranium-di-oxide (UO2) particulates from glass surface was achieved by Nd–YAG laser induced airborne plasma shock waves. The velocity of the generated shock wave was measured by employing the photo-acoustic probe deflection method. Experiments were carried out to study the effect of laser pulse energy, number of laser exposures and the separation between the substrate surface and the onset point of the shock wave on the de-contamination efficiency. The efficacy of the process was estimated monitoring the alpha activity of the samples before and after laser shock cleaning using a ZnS (Ag) scintillation detector. Significant cleaning efficiency could be achieved when the substrate was exposed to multiple laser shocks that could be further improved by geometrically confining the plasma. No visual damage or loss in optical quality was observed when the shock cleaned surfaces were analysed by optical microscopy and spectrophotometry. The area cleaned by laser shock cleaning was found to be significantly larger than that possible by conventional laser cleaning. Theoretical estimate of the shock force generated has been found to exceed the van der Waal`s binding force for spherical contaminant particulate.

Deforming TC6 titanium alloys at ultrahigh strain rates during multiple laser shock peening

The deformation mechanism of TC6 titanium alloys at ultrahigh strain rates (>106s−1) during multiple LSP was investigated. When nanosecond pulse and 1000MW laser irradiated on the materials, the high pressure plasma shock wave was induced and ultrahigh strain rates response was caused in the material. The TEM observation of the surface layer indicates that a layer of nanocrystalline has been formed and is unevenly distributed after a single impact. Increasing impact times could provide longer time and more energy to dislocation movement which refines the grains into smaller size and makes the distribution uniform. The hardness of TC6 titanium alloy has been improved by a single laser shock peening, forming a severe plastic deformted layer with a depth of 200μm. Increasing impact times will improve the hardness and effective depth. The XRD test shows that the position of diffraction peak did not show any change, but the Bragg diffraction peak was broadened. Dislocation activity is a prevalent deformation mechanism of TC6 titanium alloy in the grain refinement study.

Effects of laser shock processing on surface microstructure and mechanical properties of ultrafine-grained high carbon steel

Ultrafine micro-duplex structure (ferrite (α)+cementite (θ)) formed in a pearlitic Fe–0.8C steel after four passes of equal channel angular pressing (ECAP) at 923K via route Bc. Subsequently the surface of the ultra-micro-duplex structure was treated by multiple laser shock processing (LSP) impacts with different laser pulse energies. The microstructure was obviously refined due to the ultra-high plastic strain induced by LSP. The lattice parameter of α-Fe increased with the laser pulse energy, indicating that the cementite dissolution was enhanced. In addition, residual stress and microhardness increased with the LSP pulse energy.

Nanodiamond powder compaction via laser shockwaves: Experiments and finite element analysis

Abstract Laser shock sintering (LSS) is an innovative technique employing mechanical forces to densify powders. It can impart compressive stresses into material to improve the relative density and generate residual stresses to enhance the fatigue strength. In this work, LSS of nanodiamond powders embedded in polycrystalline cubic boron nitride (PCBN) substrate was experimentally investigated using a 1064 nm, 10 ns Q-switched Nd:YAG laser with a peak power density of 6.37 GW/cm2. Finite element simulation (FEM) was applied to predict the mechanical deformations induced by LSS. The dynamic analysis by ABAQUS/Explicit and static analysis by ABAQUS/Standard were performed in detail to predict the residual stress field and densification of nanodiamond powder compacts. The predicted densification of nanodiamond powder compacts is in agreement with the experimental data for single and multiple laser shocks.

Investigated on Fatigue Properties of Aluminum Ally 7050T7451 with Fastener Holes by 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.

Effects of multiple laser shock processing (LSP) impacts on mechanical properties and wear behaviors of AISI 8620 steel

The aim of this paper was to address the effects of multiple laser shock processing (LSP) impacts with different pulse energy on mechanical properties and wear behaviors of AISI 8620 steel. Wear analyses were conducted by means of calculation of volume loss and scanning electron microscope (SEM) of the wear surface. Surface profiles, roughness and micro-hardness were measured. The micro-structures in the surface layer of the untreated and LSPed samples (treated by multiple LSP impacts) were investigated by using transmission electron microscopy (TEM) observations. Experimental results and analyses indicated that multiple LSP impacts can remarkably improve the wear resistance of AISI 8620 steel, and the wear mechanism of multiple LSP impacts on AISI 8620 steel was also entirely revealed. The wear process of the unpolished sample subjected to multiple LSP impacts can be described as follows: the wear rate was big at the beginning of sliding dry wear, but then decreased after the micro-indention in the sample surface was polished to the disappear. This phenomenon can be attributed to the fact that multiple LSP impacts generate many micro-indents in the sample surface.

Laser shock processing on microstructure and hardness of polycrystalline cubic boron nitride tools with and without nanodiamond powders

High amplitude, short duration shock waves created by a 1064nm, 10ns Q-switched Nd:YAG laser were used to increase the hardness as well as build successive layers of nanodiamond on sintered polycrystalline cubic boron nitride (PcBN) tools. Multiple scans of laser shocking were applied. Scanning electron microscopy, Raman spectroscopy, Tukon microhardness tester, and optical surface profilometer were used to evaluate the microstructure, phase change, Vicker’s microhardness and surface roughness. Results indicated that laser shock processing of plain PcBN changed the binder concentration, caused phase transition from cubic to hexagonal form, increased the hardness, and almost unaffected surface roughness. Laser shock wave sintering of nanodiamond powders on PcBN resulted in deagglomeration and layer-by-layer build-up of nanoparticles for a thickness of 30μm inferring that a novel solid freeform technique designated as “shock wave induced freeform technique (SWIFT)” is being discovered for making micro-tools. Depending on the number of multiple laser shocks, the hardness of nanodiamond compact was lower or higher than that of PcBN. It is hypothesized that nanodiamond particles could serve as crack deflectors, increasing the fracture toughness of PcBN.

Micro-structural strengthening mechanism of multiple laser shock processing impacts on AISI 8620 steel

Micro-structural evolution in the near-surface region of AISI 8620 steel subjected to multiple laser shock processing (LSP) impacts were investigated by means of cross-sectional optical microscopy (OM) and transmission electron microscopy (TEM) observations. Micro-structural evolution process subjected to multiple LSP impacts can be described as follows: (i) the parallel lamellar pearlites are broken into bitty pearlites, and dislocation activities simultaneously led to the formation of dislocation lines (DLs) and dislocation pile-ups in original grains; (ii) bitty pearlites were all broken into Fe 3C granules, and dislocation movement made Fe 3C granules disperse near subgrain boundaries and led to subgrain boundaries separating individual cells, and (iii) subgrain boundaries were refined to grain boundaries. Multiple LSP impacts on AISI 8620 steel had dual-function: the refinement of coarse grains in the near-surface region by dislocation movement and dispersion strengthening of C atoms which cut cementite and diffused into the ferrite by moving dislocations.

Investigated on Residual Stress Field of Aluminum Alloy 7050T7451 with Fastener Holes by Laser Shock Processing

Laser shock processing (LSP) is a new technique for surface strengthening of fastener holes. The process of LSP before hole-drilling was adopted. A finite element model was established to study the effects of laser shock parameters on the residual stress field of aluminum alloy7050T7451 with Fastener Holes. The results indicate that increasing the laser power density until a fixed value results in a large peak in the hole-edge surface residual compressive stress. The hole-edge surface residual compressive stress and the depth of residual compressive stress are both increased with the increase of laser pulse width. Multiple laser shock processing can improve the residual compressive stress greatly, and with the increasing number of shot, the strengthening effect is gradually diminished.

Grain refinement mechanism of multiple laser shock processing impacts on ANSI 304 stainless steel

Micro-structural evolution and grain refinement in ANSI 304 stainless steel subjected to multiple laser shock processing (LSP) impacts were investigated by means of cross-sectional optical microscopy and transmission electron microscopy observations. The plastic strain-induced grain refinement mechanism of the face-centered cubic (fcc) materials with very low stacking fault energy was identified. The micro-structure was obviously refined due to the ultra-high plastic strain induced by multiple LSP impacts. The minimum grain size in the top surface was about 50–200 nm. Multidirectional mechanical twin matrix (MT)–MT intersections led to grain subdivision at the top surface during multiple LSP impacts. Furthermore, a novel structure with submicron triangular blocks was found at the top surface subjected to three LSP impacts. The grain refinement process along the depth direction after multiple LSP impacts can be described as follows: (i) formation of planar dislocation arrays (PDAs) and stacking faults along multiple directions due to the pile up of dislocation lines; (ii) formation of submicron triangular blocks (or irregularly shaped blocks) by the intersection of MT–MT (or MT–PDA or PDA–PDA) along multiple directions; (iii) transformation of MTs into subgrain boundaries; (iv) evolution by continuous dynamic recrystallization of subgrain boundaries to refined grain boundaries. The experimental results and analyses indicate that a high strain with an ultra-high strain rate play a crucial role in the grain refinement process of fcc materials subjected to multiple LSP impacts.

Grain refinement of LY2 aluminum alloy induced by ultra-high plastic strain during multiple laser shock processing impacts

The plastic deformation behavior and the effects of the impact time on the LY2 aluminum (Al) alloy during multiple laser shock processing (LSP) impacts were investigated. The residual stress in the near-surface region was determined by X-ray diffraction. In addition, the micro-structural features of the hardening layer were characterized by scanning electron microscopy, optical microscopy and transmission electron microscopy. It was found that the micro-structure was obviously refined due to the ultra-high plastic strain induced by multiple LSP impacts. The minimum grain size in the top surface after multiple LSP impacts was about 100–200 nm. The grain refinement process after multiple LSP impacts can be described as follows: (i) the formation and development of dislocation lines in original grains; (ii) dislocation tangles (DTs) and the formation of dense dislocation walls (DDWs); (iii) transformation of DTs and DDWs into subgrain boundaries; and (iv) evolution of the continuous dynamic recrystallization in subgrain boundaries to refined grain boundaries.

Simulation of multiple laser shock peening of a 35CD4 steel alloy

Laser shock peening (LSP) treatment of a metal surface is proving to be superior to conventional treatments such as shot peening in many engineering products. The LSP process, which introduces compressive residual stresses deep into a metal alloy, has led to some impressive results such as a significant improvement in fatigue life. This paper focuses on applying finite element analyses, instead of a complicated experimental procedure, to predict the development, magnitude and distribution of residual stresses induced by multiple LSP impacts on a metal alloy, 35CD4 30 HRC steel. The single peening results calculated by the finite element method are correlated well with the available experimental results. The simulated results clearly show that the compressive residual stresses induced by the multiple LSP impacts can be driven deeper below the surface of target than that by the single LSP impact. The simulated results also reveal that adjusting the laser spot size appropriately in a LSP process can result in an increase in compressive residual stresses in the target.

3-D FEM simulation of laser shock processing

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

Generation of a double shock driven by laser

The feasibility and reliability of a multiple laser shock generation to study the equation of state surface off the principal Hugoniot curve and to approach an isentropic compression has been demonstrated. The technique is based on the use of a double laser pulse. A strong shock was generated in iron targets precompressed by a first weak shock. The effect of precompression was studied. The experiment was performed at the Laboratoire pour l'Utilisation des Lasers Intenses laboratory.

Three-dimensional Dynamic Finite Element Analysis of Multiple Laser Shock Peening Processes

Laser shock peening (LSP) is an innovative surface treatment technique successfully applied to improving fatigue performance of metallic materials. The fatigue strength and fatigue life of the laser peened material can be significantly improved by deep compressive residual stresses being introduced into the material. The compressive residual stress distribution along the depth of the material is attributed to a high amplitude stress wave induced by a high energy laser pulse. The present paper describes a finite element method for simulating the residual stress distribution in a metal alloy 35CD4 50 HRC steel in single and multiple LSP processes. The process used a three-dimensional dynamic finite element model impacted by a square laser spot. The predicted results for single LSP were well correlated with the available experimental data. Meanwhile, the effects of multiple LSP processes, pressure magnitude and duration, and laser spot sizes on the compressive stress field in the metal alloy were evaluated for the purpose of optimising the process.