Comparison of the effects of submerged laser peening,cavitation peening and shot peening on the improvement of the fatigue strength of magnesium alloy AZ31

To investigate the improvement in the fatigue strength of magnesium alloy by peening methods,magnesium alloy AZ31 was treated by submerged laser peening(SLP),cavitation peening(CP),and shot peening(SP),and the fatigue properties were evaluated by a plane bending fatigue test.In the case of SLP,both the impact induced by laser ablation(LA)and that caused by laser cavitation(LC),which developed after LA,were used.In the present study,the fatigue life at a constant bending stress was examined to determine the suitable coverage.It was found that the fatigue strengths at N=10^(7)for the SLP,CP,and SP specimens treated by each optimum condition were 56%,18%,and 16%higher,respectively,than that of the non-peened(NP)specimen,which was 97 MPa.The key factors in the improvement of fatigue strength by peening methods were work hardening and the introduction of compressive residual stress.

Improving Tribological Performance of Gray Cast Iron by Laser Peening in Dynamic Strain Aging Temperature Regime

A high and stable brake disc friction coefficient is needed for automobile safety, while the coefficient degrades due to elevated temperature during the braking process. There is no better solution except changes in material composition and shape design optimization. In the dynamic strain aging（DSA） temperature regime of gray cast iron, micro-dimples with different dimple depth over diameter and surface area density are fabricated on the material surface by laser peening（LP） which is an LST method. Friction behavior and wear mechanism are investigated to evaluate the effects of surface texturing on the tribological performance of specimens under dry conditions. Through LP impacts assisted by DSA, the friction coefficients of the LPed specimens increase noticeably both at room temperature and elevated temperature in comparison to untreated specimens. Moreover, the coefficient of specimen with dimple depth over diameter of 0.03 and surface area density of 30% is up to 0.351 at room temperature, which dramatically rises up to 1.33 times that of untextured specimen and the value is still up to 0.3305 at 400℃ with an increasing ratio of 35% compared to that of untreated specimen. The surface of textured specimen shows better wear resistance compared to untreated specimen. Wear mechanism includes adhesive wear, abrasive wear and oxidation wear. It is demonstrated that LP assisted by DSA can substantially improve wear resistance, raise the friction coefficient as well as its stability of gray cast iron under elevated temperatures. Heat fade and premature wear can be effectively relieved by this surface modification method.

Influence of laser peening on microstructure and fatigue lives of Ti-6Al-4V

The influence of low energy laser peening on fatigue lives of Ti-6Al-4V was investigated. Laser peening was carried out on Ti-6Al-4V samples. Laser peened samples were characterized by residual stress analysis, surface roughness measurements, X-ray diffraction, optical microscopy, nanoindentation hardness tests, scanning and transmission electron microscopy and fatigue testing. Laser peening resulted in the formation of nanocrystallites on the surface and near surface regions with associated increase in hardness and introduction of compressive residual stress. Owing to positive influence of nanostructured surface and compressive residual stress, fatigue lives of the laser peened samples were significantly increased compared to the unpeened samples.

Cyclic Oxidation Resistance of In718 Superalloy Treated by Laser Peening

The aim of this study was to improve the cyclic oxidation resistance of In718 superalloy by laser peening（LP）. Specimens were treated by LP from one to three times, respectively. The cyclic oxidation tests at 900 ℃ for periods up to 2 h were conducted. Changes of the top surface morphology and microstructure were analyzed by scanning electron microscope （SEM）, energy-dispersive spectra （EDS）, transmission electron microscope （TEM） and X-ray diffraction technique （XRD）, respectively. The weights were measured between the oxidation cycles to assess the oxidation of the specimens. The top surface microstructure after LP was characterized by highly tangled and dense dislocation arrangements and a high amount of twins. Protective oxidation layer was generated more quickly on the surface treated by LP. The average oxidation rate was about 50 % lower. A tiny homogeneous oxidation layer containing （Fe,Cr）2O3, NiCrO3 and Ni（A1,Cr）2O4 spinel was generated on the surface. The experimental results of cyclic oxidation tests show that specimens treated by LP have a better high temperature oxidation resistance, and the antistrip performance of the oxidation layer improves. Moreover, the effects of LP are strengthened with the increase of laser peening.

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.

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.

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 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.

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.

Impact toughness of a gradient hardened layer of Cr5Mo1V steel treated by laser shock peening

Laser shock peening（LSP） is a widely used surface treatment technique that can effectively improve the fatigue life and impact toughness of metal parts.Cr5Mo1 V steel exhibits a gradient hardened layer after a LSP process.A new method is proposed to estimate the impact toughness that considers the changing mechanical properties in the gradient hardened layer.Assuming a linearly gradient distribution of impact toughness,the parameters controlling the impact toughness of the gradient hardened layer were given.The influence of laser power densities and the number of laser shots on the impact toughness were investigated.The impact toughness of the laser peened layer improves compared with an untreated specimen,and the impact toughness increases with the laser power densities and decreases with the number of laser shots.Through the fracture morphology analysis by a scanning electron microscope,we established that the Cr5Mo1 V steel was fractured by the cleavage fracture mechanism combined with a few dimples.The increase in the impact toughness of the material after LSP is observed because of the decreased dimension and increased fraction of the cleavage fracture in the gradient hardened layer.

Surface profile and microstructure of laser peened Ti-6Al-4V

The surface profile of laser peening with square spots was compared with that of circle spots, and the microstructure of laser peened titanium alloy Ti-6Al-4V in the center of a square spot and at the edge of the square spot was investigated in this paper. The results show that a smaller size crystal is produced at the edge of square spots because the shearing strain produces nanoscale crystals in laser peening titanium alloy Ti-6Al-4V.

PROPERTY AND THERMOSTABLITY STUDY ON TC6 TITANIUM ALLOY NANOSTRUCTURE PROCESSED BY LSP

TC6 titanium alloy samples are processed by laser shock peening （LSP）. Then, some samples are vacu- um annealed at 623 K for 10 h for the study on the thermost.ablity of the nanostructure produced by LSP. The characteristics of the strengthened layer and nanostructure are studied by atomic force microscopy（AFM）, scan- ning electron microscope （SEM）, electron backscatter diffraction（EBSD）, X-ray diffraction（XRD）, and transmis- sion electron microscopy（TEM） appliances, meanwhile the enhanced microhardness is tested at cross section. AFM of the processed surface indicates that the deformation is approximately uniform, and LSP slightly increases the roughness. SEM and EBSD of the strengthened cross section show that a phases are compressed to strip- shaped, a proportion of a and ~ phases is shattered to smaller phases from surface to 200 ttm in depth. The sur- face XRD shows that although there is no new produced phase during LSP, the grain size refinement and the in- troduction of lattice micro-strains lead to the broadened peak. The TEM photographs and diffraction patterns in- dicate that the shock wave provides high strain rate deformation and leads to the formation of nanocrystal. Com- pared with the samples before annealing, the dislocation density is lower and the grain-boundary is more distinct in the annealed samples, but the nanocrystal size does not grow bigger after annealing. The microhardness measurement indicates that LSP improves the microhardness of TC6 for about 12.2% on the surface, and the layer affected by LSP is about 500/~m in depth. The microhardness after annealing is 10 HVo.5 lower, but the affected depth does not change. The thermostable study shows that the strengthened layer of TC6 processed by LSP is stable at 623 K. The strengthened thermostable layer can significantly improve the fatigue resistance, wear resis- tance and stress corrosion resistance of the titanium alloy. The study results break the USA standard AMS2546 that titanium parts after LSP are subjected in subsequent processing within 589 K.

Study and development of high peak power short pulse Nd:YAG laser for peening applications

Because short pulse Nd:YAG laser of nanosecond pulse-width and high peak power has a unique capability to improve the mechanical properties of metal parts,a study on the development of high peak power short pulse from Nd:YAG laser along with its peening application has been performed.The design scheme of laser and the characteristic of laser beam transmission are presented and discussed.A pulse energy of 25 J with 15 ns pulse-width and a maximum peak power of 1660 k W laser system which use one oscillation and eight amplifiers has been achieved.Laser beam has a max divergence angle of 0.03 mrad,a pulse-to-pulse pulse-width stability of±0.1 ns,and the pulse-to-pulse energy stability factors of less than±2.8%.A low value of divergence means an easier modification of a nearly hat-top laser beam intensity profile and an easier transmission of laser beam.To evaluate the performance of the laser system,several metal materials are processed.Laser peening quality and efficiency are analyzed by using an optical microscope,a transmission electron microscope,and an X-ray diffraction device.The processing results show that the performance of this laser system is excellent.

Microstructural evolution and stress state related to mechanical properties of electron beam melted Ti-6Al-4V alloy modified by laser shock peening

This work characterizes microstructural evolutions of electron beam melted(EBM) Ti-6 Al-4 V alloy modified via laser shock peening(LSP).The depth stress distribution and tensile properties of EBM Ti-6 Al-4 V alloy were measured before and after LSP.The results indicate that microstructure consists of β phase with 7.2%±0.4% vol.% and balance α lamellar in EBM sample,and the α lamella was refined into nano-equiaxed grains and submicro-equiaxed grains after LSP.The dominant refinement mechanism is revealed during LSP.Stacking faults were found in the LSP-treated sample,and their corresponding planes were determined as(0001) basal plane,(1010) prismatic plane,and(1011) pyramidal plane obtained by high resolution transmission electron microscopy.The subgrains and high-angle grains formed during dynamic recrystallization were identified by selected area electron diffraction pattern.The LSP treatment produces a significantly residual compressive stress approximately-380 MPa with the depth of compressive stress layer reaching 450 μm.Strength and elongation of the EBM sample were significantly increased after LSP.The strength and ductility enhancements are attributed to compre s sive stress,grain refinement and grain gradient distribution of α phase.

Influence of laser shock peening on surface integrity and tensile property of high strength low alloy steel

Laser Shock Peening(LSP)is a well-established surface treatment commonly used to improve mechanical properties of material’s surfaces.To further understand the relationship between tensile property and fatigue life improvement of high strength low alloy steel in the LSP process,LSP treatment of 32 CrNi high strength low alloy steel was carried out by YAG laser with pulse energy of 15 J,and tensile property was tested by electronic universal material testing machine.Surface morphology,residual stress and tensile fracture of the specimens before and after LSP were observed by white light interferometer(WLI),X-ray measuring apparatus and scanning electron microscope(SEM).Result shows that LSP did not change tensile strength of 32 CrNi steel but cause yield characteristic transform from obvious yield point to no yield phenomenon which is the only factor benefiting fatigue life,indicating that the increment of fatigue life was probably related to the disappearance of yield phenomenon.Formation mechanisms of tensile fractures and yield phenomenon induced by LSP at room temperature were also discussed and completely revealed.Deeper compressive residual stress and flat grains contributed to the transition of yield characteristic and lower elongation rate of 32 CrNi steel subjected to LSP.

Investigation of Surface Integrity on TC17 Titanium Alloy Treated by Square-spot Laser Shock Peening

Laser shock peening（LSP） is an innovative surface treatment method,which has been shown to greatly improve the fatigue life of many metallic components.This work investigates surface integrity of TC17 titanium alloy treated by LSP with innovative square laser spot.Nd：glass laser with duration of 30 ns and spot size of 4 mm×4 mm is applied.The surface morphology and surface residual stress of the TC17 titanium alloy,treated with varying peening parameters such as laser power density and overlapping ratio,have been studied in detail.The results show that laser pulse energy greatly influences surface morphology and surface residual stress around single-spot treated areas,and compressive residual stresses are saturated as laser pulse energy is over 55 J.There are significantly different surface morphologies and residual stress distributions at the overlapped areas with different overlapping ratios.A relative smooth surface is produced with uniform compressive residual stress distribution at an overlapping ratio of 8 %.The experiment of residual stress relaxation is implemented by measuring residual stress at the center of four overlapped spots and by four point bending fatigue test at the frequency of 105 Hz.The compressive residual stresses induced by LSP are found to relax quite slowly under cyclic fatigue loading.

Effect of laser shock peening on combined low- and high-cycle fatigue life of casting and forging turbine blades

Laser shock peening （LSP） is a novel effective surface treatment method to improve the fatigue performance of turbine blades. To study the effect of LSP on combined low- and high-cycle fatigue （CCF） life of turbine blades, the CCF tests were conducted at elevated temperatures on two types of full-scale turbine blades, which were made of K403 by casting and GH4133B by forging. Probabilistic analysis was conducted to find out the effect of LSP on fatigue life of those two kinds of blades. The results indicated that LSP extended the CCF life of both casting blades and forging blades obviously, and the effect of LSP on casting blades was more evident; besides, a threshold vibration stress existed for both casting blades and forging blades, and the CCF life tended to be extended by LSP only when the vibration stress was below the threshold vibra- tion stress. Further study of fractography was also conducted, indicating that due to the presence of compressive residual stress and refined grains induced by LSP, the crack initiation sources in LSP blades were obviously less, and the life of LSP blades was also longer; since the compressive residual stress was released by plastic deformation, LSP had no effect or adverse effect on CCF life of blade when the vibration stress of blade was above the threshold vibration stress.

Advances in Microscale Laser Shock Peening

The response of materials after microscale laser shock peening (μLSP) was experimentally characterized and compared with the theoretical prediction from the finite element method (FEM) analysis in microlength level. X-ray micro-diffraction technique was applied to the post-peened single crystal aluminum of (001) and (110) orientations, and X-ray profile was analyzed by sub-profiling and Fourier analysis method. Spatially resolved residual stress and strain deviation was quantified and explained in terms of the hetero- geneous dislocation cell structure. In-plane crystal lattice rotation induced by μLSP was measured by elec- tron backscatter diffraction (EBSD) and compared with the FEM simulation. Average mosaic size was evaluated from X-ray profile Fourier analysis and compared with the result from EBSD. Surface strength in- crease and dislocation cell structure formation were studied. The systematical characterization will lay the ground work for better understanding the effect of μLSP in microlength level and developing more realistic simulations.