Heat Exchanging Process of Metal Target under Millisecond Pulsed Laser Shocking

Laser plays an important role in synthesizing nanometer material. A three-dimensional mathematical model is established in this paper when single pulsed millisecond laser shocks the surface of the metal target at a liquid-solid interface. By changing laser power density and target size, the temperature field variation of the metal target is investigated. Results show that the generation process of nanoparticles includes heating, melting and boiloff.

A comprehensive review on the processing-property relationships of laser strengthened magnesium

Among the existing series of softer metals,magnesium(Mg)has attracted much attention due to its impressive strength-to-weight ratio.However,due to its ease of deformability,Mg tends to suffer from rapid degradation in a wide variety of abrasive and electrochemical environments.One method of improving its surface properties is through surface modification techniques.Among the existing techniques,laser shock peening(LSP)has been one of the most widely utilized processes due to its surface-hardening-like effects.Despite this understanding,a comprehensive review has yet to exist that encapsulates the strengthening mechanism of LSP for Mg and its influence in degradation environments.This review aims to encapsulate the existing research around the LSP field for Mg.Specifically,an understanding of the surface-strengthening effects in relation to its mechanical,tribological,corrosion,and tribo-corrosion characteristics is elucidated.Additionally,the feasibility of LSP for Mg materials in critical industries is also discussed.Through this work,a novel understanding of LSP for Mg can be understood,which can provide a future direction for research in this field.

Microstructural evolution and mechanical properties of duplex-phase Ti6242 alloy treated by laser shock peening

The effects of laser shock peening(LSP)on the microstructural evolution and mechanical properties of the Ti6242 alloy,including the residual stress,surface roughness,Vickers microhardness,tensile mechanical response,and high-cycle fatigue properties,were studied.The results showed that the LSP induced residual compressive stresses on the surface and near surface of the material.The maximum surface residual compressive stress was−661 MPa,and the compressive-stress-affected depth was greater than 1000μm.The roughness and Vickers micro-hardness increased with the number of shocks,and the maximum hardness-affected depth was about 700μm after three LSP treatments.LSP enhanced the fraction of low-angle grain boundaries,changed the grain preferred orientations,and notably increased the pole density ofαphase on the near surface from 2.41 to 3.46.The surface hardness values of the LSP samples increased with the increase of the number of shocks due to work hardening,while the LSP had a limited effect on the tensile properties.The high-cycle fatigue life of the LSP-treated sample was significantly enhanced by more than 20%compared with that of the untreated sample,which was caused by the suppression of the initiation and propagation of fatigue cracks.

Effect of laser shock processing on fatigue life of fastener hole

The fatigue properties of laser shock processing （LSP） on both side surfaces of fastener hole with diameter of 3 mm in the LY12CZ aluminum alloy specimens were investigated. The superficial residual stress was measured by X-ray diffraction method. Fatigue experiments of specimens with and without LSP were performed, and the microstructural features of fracture of specimens were characterized by scanning electron microscopy （SEM）. The results indicate that the compressive residual stress can be induced into the surface of specimen, and the fatigue life of the specimen with LSP is 3.5 times as long as that of specimen without LSP. The location of fatigue crack initiation is transferred from the top surface to the sub-surface after LSP, and the fatigue striation spacing of the treated specimen during the expanding fatigue crack is narrower than that of the untreated specimen. Furthermore, the diameters of the dimples on the fatigue crack rupture zone of the specimen with LSP are relatively bigger, which is related to the serious plastic deformation in the material with LSP.

Improving impact wear resistance of Ti-6Al-4V alloy treated by laser shock peening

The effects of laser shock peening (LSP) on the impact wear behavior of Ti-6Al-4V alloys were investigated by a homemade impact wear test rig. The microstructure and mechanical properties of the peened samples were studied. During the impact wear test, the energy absorption, impact force, wear contact time and wear mechanism of all the test samples were investigated in terms of the influence of the impact kinetic energy. The results showed that microhardness, elastic modulus and residual compressive stress of the treated samples were markedly improved. The wear resistances of both treated samples were highly improved after LSP, and a higher pulse energy corresponded to a more obvious effect. Besides, the wear in all test samples involved a combination of abrasive and oxidation wear and fatigue spalling.

Effects of Laser Shock Processing on Mechanical Properties of Laser Welded ANSI 304 Stainless Steel Joint

With the rapid development of engineering component with integration,high-speed and multi-parameter,traditional techniques haven＇t met practical needs in extreme service environment.Laser welding,a new welding technology,has been widely used.However,it would generate the drop of mechanical properties for laser welded joint due to its thermal effect.Laser shock processing（LSP） is one of the most effective methods to improve the mechanical properties of laser welded ANSI 304 stainless steel joint.In this paper,the effects of LSP on the mechanical properties of laser welded ANSI 304 stainless steel joint have been investigated.The welded joint on the front of the tensile samples is treated by LSP impacts,and the overlapping rate of the laser spot is 50%.The tensile test of the laser welded joint with and without LSP impacts is carried out,and the fracture morphology of the tensile samples is analyzed by scanning electron microscope（SEM）.Compared with the yield strength of 11.70 kN,the tensile strength of 37.66 kN,the yield-to-tensile strength ratio of 0.310 7,the elongation of 25.20%,the area reduction of 32.68% and the elastic modulus of 13 063.876 MPa,the corresponding values after LSP impacts are 14.25 kN,38.74 kN,0.367 8,26.58%,42.29% and 14 754.394 MPa,respectively.Through LSP impacts,the increasing ratio of the yield strength and tensile strength are 121.79% and 102.87%,respectively;the elongation and area reduction are improved by 5.48% and 29.38%,respectively.By comparing with coarse fracture surface of the welded joint,the delamination splitting with some cracks in the sharp corner of the welded joint and asymmetric dimples,LSP can cause brighter fracture surface,and finer and more uniform dimples.Finally,the schematic illustration of dimple formation with LSP is clearly described.The proposed research ensures that the LSP technology can clearly improve the yield strength,tensile strength,yield-to-tensile strength ratio,elongation,area reduction and elastic modulus of the welded joint.The enhancement mechanism of LSP on laser welded ANSI 304 stainless steel joint is mainly due to the fact that the refined and uniform dimples effectively delay the fracture of laser welded joints.

FEM SIMULATION OF RESIDUAL STRESSES INDUCED BY LASER SHOCK WITH OVERLAPPING LASER SPOTS

The finite element method is presented to attain the numerical simulation of the residual stresses field in the material treated by laser shock processing. The distribution of residual stresses generated by a single laser shock with square and round laser spot is predicted and validated by experimental results. With the Finite Element Method （FEM） model, effects of different overlapping rates and impact sequences on the distribution of residual stresses are simulated. The results indicate that： （1） Overlapping laser shock can increase the compressive residual stresses. However, it is not effective on the growth of plastically affected depth; （2） Overlapping rate should be optimized and selected carefully for the large area treatment. Appropriate overlapping rate is beneficial to obtain a homogeneous residual stress field; （3） The impact sequence has a great effect on the residual stress field. It can greatly attenuate the phenomenon of the “residual stress hole” to obtain a homogeneous residual stress field.

Prediction about residual stress and microhardness of material subjected to multiple overlap laser shock processing using artificial neural network

In this work,the nickel-based powder metallurgy superalloy FGH95 was selected as experimental material,and the experimental parameters in multiple overlap laser shock processing(LSP)treatment were selected based on orthogonal experimental design.The experimental data of residual stress and microhardness were measured in the same depth.The residual stress and microhardness laws were investigated and analyzed.Artificial neural network(ANN)with four layers(4-N-(N-1)-2)was applied to predict the residual stress and microhardness of FGH95 subjected to multiple overlap LSP.The experimental data were divided as training-testing sets in pairs.Laser energy,overlap rate,shocked times and depth were set as inputs,while residual stress and microhardness were set as outputs.The prediction performances with different network configuration of developed ANN models were compared and analyzed.The developed ANN model with network configuration of 4-7-6-2 showed the best predict performance.The predicted values showed a good agreement with the experimental values.In addition,the correlation coefficients among all the parameters and the effect of LSP parameters on materials response were studied.It can be concluded that ANN is a useful method to predict residual stress and microhardness of material subjected to LSP when with limited experimental data.

Mechanical Properties and Microstructure of Bionic Non-Smooth Stainless Steel Surface by Laser Multiple Processing

Laser multiple processing, i.e. laser surface texturing and then Laser Shock Processing (LSP), is a new surface processing technology for the preparation of bionic non-smooth surfaces. Based on engineering bionics, samples of bionic non-smooth surfaces of stainless steel 0Crl 8Ni9 were manufactured in the form of reseau structure by laser multiple processing. The mechanical properties (including microhardness, residual stress, surface roughness) and microstructure of the samples treated by laser multiple processing were compared with those of the samples without LSP The results show that the mechanical properties of these samples by laser multiple processing were clearly improved in comparison with those of the samples without LSP The mechanisms underlying the improved surface microhardness and surface residual stress were analyzed, and the relations between hardness, comnressive residual stress and roughness were also presented.

Thermal fatigue crack growth behavior of ZCuAlFeMn alloy strengthened by laser shock processing

The effect of laser shock processing(LSP) on the hardness, surface morphology, residual stress, and thermal fatigue properties of a ZCuAl10Fe3Mn2 alloy was investigated to improve the thermal fatigue performance and decrease the surface crack of high-temperature components. The microstructure and crack morphology were analyzed by scanning electron microscopy(SEM) and energy dispersive spectroscopy(EDS). The results showed that laser shock could significantly improve the thermal fatigue performance of the alloy at a pulse energy of 4 J. Under the effect of thermal stress and alternating stress, microstructure around the specimen notch was oxidized and became porous, leading to the formation of multiple micro-cracks. The micro-cracks in the vertical direction became the main cracks, which mainly expanded with the conjoining of contiguous voids at the crack tip front. Micro-cracks in other directions grew along the grain boundaries and led to material shedding.

A Study on Super Speed Forming of Metal Sheet by Laser Shock Waves

Metal sheet plastic deformation or forming is gener at ed through a mechanical pressure or a thermal variation. These pressure variatio ns or thermal variations can be created by a variety of means such as press form ing, hydroforming, imploding detonation and so on. According to the magnitude of the strain rates all these forming methods can be divided into quasi-static fo rming and dynamical forming. Up to now there are no reports of forming methods w ith the strain rates above 10 5sec -1, even though the exploding forming. In this article, we work on a dynamic super-speed forming method driven by lase r shock waves and advanced a novel concept of laser shock forming. The initial o bservation of the laser shock forming is done through a bugle testing with speci mens of SUS430 sheet metal, using a neodymium-glass laser of pulse energy 10J～ 3 0J and duration of 20 ns (FWHM). The investigation revealed that the plastic de formation during the laser shock forming is characterized as ultrahigh strain ra te up to 10 7sec -1. We indicate that plastic deformation increases nonlin early when the energy density of the laser varies. By investigating the hardness and residual stress of the surfaces, we conclude that laser shock forming is a combination technique of laser shock strengthening and metal forming for introdu cing a strain harden and a compressive residual stress on the surface of the wor k-piece, and the treated surface by laser shock forming has good properties in fatigue and corrosion resistance. This technique can achieve forming wit h or without mould.

Effects of laser shock peening on fatigue crack growth rate and fracture properties of AA2524 aluminum alloy

In order to prolong the service life of aircraft skin made from AA2524, the effects of laser shock peening(LSP) on fatigue crack growth(FCG) rate and fracture toughness(K_(c)) of AA2524 were investigated. Multiple LSP treatment was performed on compact tension(CT) specimen from single side and double sides. The surface integrity was measured with Vickers hardness tester, X-ray diffractometer and confocal laser scanning microscope, respectively. FCG rate test and fracture toughness test under plane stress were carried out after LSP treatment. The microstructure features of cross-sections were observed with scanning electron microscope. The results showed that the micro-hardness and residual stress of CT specimens were increased dramatically after LSP treatment. Compared to the base metal(BM), the fatigue life was prolonged by 2.4 times and fracture toughness was increased by 22% after multiple LSP.

Femtosecond laser shockwave peening ablation in liquids for hierarchical micro/nanostructuring of brittle silicon and its biological application

This paper presents a new technique,termed femtosecond laser shock peening ablation in liquids(fs-LSPAL),which can realize simultaneous crack micro/nanomanufacturing and hierarchical micro/nanolaser ablation,giving rise to the formation of diverse multiscale hierarchical structures,such as macroporous ratcheted structures and enéchelon microfringes decorated with parabolic nanoripples.Through analysis of surface morphologies,many phenomena have been confirmed to take place during fs-LSPAL,including enéchelon cracks,nanostriation,ripple densification,crack branching,and selective formation of high spatial frequency laser-induced periodic surface structures of 100–200 nm in period.At a high laser power of 700 mW,fs-LSPAL at scanning speeds of 0.2 mm s^-1 and 1 mm s^-1 enables the generation of height-fluctuated and height-homogeneous hierarchical structures,respectively.The height-fluctuated structures can be used to induce‘colony’aggregates of embryonic EB3 stem cells.At 200 mW,fs-LSPAL at 1 mm s^-1 is capable of producing homogeneous tilt macroporous structures with cracked structures interleaved among them,which are the synergistic effects of bubble-induced light refraction/reflection ablation and cracks.As shown in this paper,the conventional laser ablation technique integrated with its self-driven unconventional cracking under extreme conditions expands the horizons of extreme manufacturing and offers more opportunities for complex surface structuring,which can potentially be used for biological applications.

Effects of laser pulse energy on surface microstructure and mechanical properties of high carbon steel

Surface microstructure and mechanical properties of pearlitic Fe–0.8%C(mass fraction) steel after laser shock processing(LSP) with different laser pulse energies were investigated by scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD) and microhardness measurements.After LSP,the cementite lamellae were bent,kinked and broken into particles.Fragmentation and dissolution of the cementite lamellae were enhanced by increasing the laser pulse energy.Due to the dissolution of carbon atoms in the ferritic matrix,the lattice parameter of α-Fe increased.The grain size of the surface ferrite was refined,and the microstructure changed from lamellae to ultrafine micro-duplex structure(ferrite(α)+cementite(θ)) with higher laser pulse energy,accompanied by the residual stress and microhardness increase.

Effective femtosecond laser shock peening on a Mg-3Gd alloy at low pulse energy 430 μJ of 1 kHz

In this paper,microstructure evolution and hardness of Mg-3 Gd alloy treated by femtosecond(fs)laser shock peening(LSP)with direct and confined ablation m odes were investigated in detail.Under a relatively low pulse energy of 430μJ with a repetition of 1 kHz,the surface hardness of sample has been enhanced by 70%effectively.Comp a r e d with ns-LSP with pulse fluence of 71.7J/cm2,fs-LSP with pulse fluence of 34.2 J/cm2 is superior in the hardness increment,both of which are in the same order of magnitude.A distinct grain refinement of surface layer has been discovered and results in the increase of hardness.Nonuse of absorption and confining layers and the employment of the industry commercial fs laser with high repetition can inspire big potential L S P application in special metal material.©2019 Published by Elsevier B.V.on behalf of Chongqing University.

Influence of multiple laser shock peening treatments on the microstructure and mechanical properties of Ti-6Al-4V alloy fabricated by electron beam melting

Laser shock peening(LSP)is an attractive post-processing method to tailor surface microstructure and enhance mechanical performances of additive manufactured(AM)components.The effects of multiple LSP treatments on the microstructure and mechanical properties of Ti-6Al-4V part produced by electron beam melting(EBM),as a mature AM process,were studied in this work.Microstructure,surface topography,residual stress,and tensile performance of EBM-manufactured Ti-6Al-4V specimens were systematically analyzed subjected to different LSP treatments.The distribution of porosities in EBM sample was assessed via X-ray computed tomography.The results showed that EBM samples with two LSP treatments possessed a lower porosity value of 0.05%compared to the value of 0.08%for the untreated samples.The strength of EBM samples with two LSP treatments was remarkably raised by 12%as compared with the as-built samples.The grains ofαphase were refined in near-surface layer,and a dramatic increase in the depth and magnitude of compressive residual stress(CRS)was achieved in EBM sample with multiple LSP treatments.The grain refinement ofαphase and CRS with larger depth were responsible for the strength enhancement of EBM samples with two LSP treatments.

Numerical simulation of residual stress field induced by laser shock processing with square spot

Laser shock processing（LSP）,also known as laser peening,is a novel surface treatment technique in the past few years.Compressive residual stresses which imparted by LSP are very important for improving fatigue,corrosion and wea rresistance of metals.Finite element analysis（FEA） simulation using ABAQUS software has been applied to predict residualstresses induced by LSP on Ti-6Al-4V titanium alloy with laser pulse duration 30 ns and water confined ablation mode.The residual stress field generated by different shape laser spots was studied,and the square laser spot is shown the most suitability for avoiding stress lack phenomenon and overlapping LSP.Surface residual stresses and plastically affected depth within single square spot both increased with the increase of laser intensity and laser shock times.Furthermore,compared with circle and ellipse spot,the residual stress distribution in overlapping square spots is very uniform only with small overlapping ratio.LSP with square spot can process advantageous residual stress field,and this technique will be used widely.

Laser Shock Processing of an Austenitic Stainless Steel and a Nickel-base Superalloy

An austenitic stainless steel 1Cr18Ni9Ti and a solid solution-strengthened Ni-base superalloy GH30 were shock processed using a Q-switched pulsed Nd-glass laser. Microstructure, hardness and residual stress of the laser shock processed surface were investigated as functions of laser processing parameters. Results show that high density of dislocations and fine deformation twins are produced in the laser shock processed surface layers in both the austenitic stainless steel and the nickel-base superalloy. Extensive strain-induced martensite was also observed in the laser shock processed zone of the austenitic steel. The hardness of the laser shock processed surface was significantly enhanced and compressive stress as high as 400 MPa was produced in the laser shock processed surface.

Effect of laser shock processing on erosive resistance of 6061-T6 aluminum

Application of laser shock processing （LSP） on 6061-T6 aluminum was made in order to evaluate its response to the erosive wear by silica sand. Impact angles of 15° , 30° , 60° and 90° were tested, two particle speeds （37 and 58 m/s） and two LSP irradiation conditions were used. Erosion marks were characterized by 3D profilometry and SEM analysis was conducted to identify the erosion mechanisms for each tested angle. The results showed a maximum erosive wear at low impact angles （ductile type behavior）. Erosion strength and the erosion mechanisms were not affected by the application of LSP and they were attributed to the high strain rate of the erosion phenomena. A few differences encountered on the erosion plots were explained on the basis of the surface roughness left by the LSP process. The maximum mass loss and the maximum erosion penetration happened in different impact angles （15° and 30° , respectively）. Finally, a well-defined erosion mechanism transition was observed, from cutting action at low impact angle, to crater formation at 90° of incidence.

Low-cycle Fatigue Behavior of Ni-based Superalloy GH586 with Laser Shock Processing

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.