Laser Peened Specimens Research Articles

Impact of laser shock peening on microstructure hardness and corrosion properties of AA6061 alloy

In recent times, laser shock peening (LSP) has emerged as a pioneering technique to modify the surface properties of various aluminium-based alloys. In this study, AA6061 was treated by single and multi-pass LSP, and its microstructure, hardness, and corrosion behaviour were investigated. Microstructural studies were performed using X-ray diffraction, electron back-scattered diffraction, and transmission electron microscopy methods. Texture studies were conducted using the neutron diffraction method, while hardness measurements were carried out using a Vickers hardness tester. Corrosion studies were conducted using the potentiodynamic method, and surface examination was performed using scanning electron microscopy. The results indicate that laser peening led to the dissolution of strengthening precipitates, resulting in a microstructure characterized by a greater volume of low-angle grain boundaries and dense dislocations. The texture index of laser-peened specimens decreased drastically to 1.1952 and 1.0970, with a significant rise in the volume fraction of Goss, S, and brass components. Specimens subjected to a single LSP pass exhibited a maximum hardness of 132.4 HV and better resistance to pitting, with Ecorr and Icorr values of −0.6721 V and 0.0043 A/cm2, respectively, while multiple LSP passes resulted in reduced hardness and pitting resistance due to excess heat.

Analysis of the effect of deep-profile compressive residual stress induced by laser peening on fatigue strength of 7075 aluminum alloy

The effect of laser peening-induced compressive residual stress on the fatigue strength improvement of A7075 specimens was investigated. Rotary bending fatigue tests and residual stress distribution measurements using the sequential polishing method were conducted on untreated, shot-peened, and laser-peened specimens. The results revealed that laser peening provides a deeper compressive stress and a deeper fracture origin, resulting in higher fatigue strength. Furthermore, controlling the laser parameters could extend the depth of the compressive stress layer by approximately 5 mm, indicating the possibility of applying laser peening to thicker materials to improve the fatigue strength.

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.

Effect of laser shock peening on the plasticity of Zr-based metallic glass under compression

The surface of Zr41.2Ti13.8Cu12.5Ni10Be22.5 BMG was modified in this study using LSP to enhance its compressive mechanical properties.The microstructures and thermal properties of the as-cast and laser-peened specimens were characterized using X-ray diffraction and differential scanning calorimetry, respectively. The results indicated that crystallization did not occur in the laser-peened area. Compared to the as-cast metallic glass, the LSP-treated specimen had a higher free volume. The LSP-treated specimen, as compared to the as-cast specimen with a slight plastic strain of 8.1 % prior to final fracture, exhibited an improved compressive plastic strain of 18.8 %. Based on results of the hardness tests, a significant change in microhardness was observed in the specimen after the LSP treatment. The improved plasticity and decreased hardness of the laser-peened specimen were due to the excess free volume in the laser treated surface zone. Scanning electron microscopy observations of the fracture surfaces revealed that the LSP-treated specimen formed multiple shear bands and highly dense vein-like patterns. This study provides a novel method for increasing the plasticity of metallic glasses to promote their industrial application.

A comparative assessment of the effects of laser shock peening and ultrasonic shot peening on surface integrity and ratcheting fatigue performance of HSLA steel

In this work, a comparative assessment of the effects of laser shock peening (LSP) and ultrasonic shot peening (USP) on surface integrity and ratcheting fatigue performance of ASTM A588 Grade D high strength low alloy steel was done. Surface roughness, microhardness, microstructure, residual stress, and micro-texture analyses were used to describe the unpeened and peened surfaces. Three sets of tension-compression asymmetric cyclic loading tests were carried out at room temperature on unpeened and peened specimens to evaluate the effects of the applied treatment on strain accumulation and fatigue life. The quantitative dislocation densities of selected specimens were calculated in order to examine the mechanism underlying the softening behavior of USP-treated specimens during fatigue testing. The results are presented in light of the substantial surface roughness, lower compressive residual stress, and weakening of texture intensity revealing an unexpectedly large drop in fatigue strength following USP treatment compared to the unpeened and laser-peened specimens.

Microstructure-crystallographic texture and substructure evolution in unpeened and laser shock peened HSLA steel upon ratcheting deformation

ABSTRACT Uniaxial ratcheting behaviours associated with microstructural development of unpeened and laser shock peened ASTM A 588 Grade D High Strength Low Alloy (HSLA) steel were studied in this investigation. The mechanism of plastic deformation and crystallographic texture evolution during ratcheting was also studied. For this, the primary experimental works involved were stress-controlled ratcheting fatigue tests (on unpeened/laser-peened specimens) at room temperature. Followed by this, microstructure and texture evolution on the surfaces and/or cross sections of the deformed specimens, using electron back scattered diffraction (EBSD) were studied in detail. Additionally, the substructural evolution of some selected samples was also studied using transmission electron microscopy (TEM). The results indicated that the average grain size of all the unpeened ratcheted specimens was marginally reduced after fatigue tests. The laser-peened specimen, however, showed a marginal increase in average grain size after ratcheting. Continuous dynamic recovery and recrystallisation (CDRR) during ratcheting was thought to be the cause of the observed reduction in average grain size in unpeened specimens, whereas continuous dynamic recrystallisation (CDRX) was believed to be the controlling factor for marginal accretion of grain size in laser-peened specimens. The plastic deformation of investigated steel was qualitatively explained by the observed dislocation patterns and their evolution.

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.

Effect of Laser Shock Peening without Coating on Grain Size and Residual Stress Distribution in a Microalloyed Steel Grade

This study aimed to identify the optimal combination of wavelength and laser pulse density to achieve the optimal pulse pressure that can induce the maximum compressive residual stress at the subsurface of microalloyed steel. For this, laser shock peening without coating (LSPwC) was performed on microalloyed steel samples at the fundamental wavelength (1064 nm) with pulse densities of 3, 6, 9, and 12 GW/cm2 and at the second harmonic wavelength (532 nm) with pulse densities of 3, 6, and 9 GW/cm2. The residual stress distributions were studied to a depth of 500 µm in the laser-treated samples. Tensile residual stress was observed at the surface of laser-peened specimens in both wavelength conditions (1064 and 532 nm). The significant impartment of compressive residual stress across the depth was achieved at the fundamental wavelength (1064 nm). The maximum compressive residual stress was attained with a laser pulse density of 9 GW/cm2 in the 1064nm wavelength condition. The optical micrographic analysis in the subsurface regions of the LSPwC specimen at 1064 nm and 9 GW/cm2 shows evidence of a high degree of plastic deformation. Electron backscatter diffraction (EBSD) analysis shows that there is grain refinement due to plastic deformations in samples subjected to the fundamental wavelength. Microhardness distribution analysis across the subsurface region shows work-hardening effects in the laser-processed samples in the 1064 nm condition. This study also shows that there is an indication of a thermal softening effect in the samples treated with the 532 nm wavelength, and it is correlated with lower compressive residual stress across the depth.

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.

Effect of laser shock peening on ratcheting strain accumulation, fatigue life and bulk texture evolution in HSLA steel

The effect of laser shock peening (LSP) on the surface properties and ratcheting behavior of a high strength low alloy (HSLA) ASTM A 588 Grade D steel was studied in this investigation. Bulk texture evolution in the unpeened and laser peened specimens was also studied using X-ray diffraction analysis. The results show that the LSP improves the surface mechanical properties significantly without much deterioration of the surface roughness. Accumulation of the ratcheting strain reduced considerably associated with enhancement of the fatigue lives: about twice, if compared with the unpeened specimens. Strengthening of {011} 〈100〉 and {011} 〈111〉 texture components were observed with increased ratcheting strain in both unpeened and laser peened specimens. The induced compressive residual stresses and marginal grain refinement during the LSP caused the improvement in fatigue lives of the specimens.

Laser Shock Peening of SiCp/2009Al Composites: Microstructural Evolution, Residual Stress and Fatigue Behavior.

SiC particle reinforced aluminum alloy has a wide application in the aerospace industries. In this study, laser shock peening (LSP), an advanced surface modification technique, was employed for SiCp/2009Al composite to reveal its microstructure, microhardness and residual stress evolution. After peening, high densities of dislocations were induced in the aluminum substrate, and stacking faults were introduced into the SiC particle. The microhardness was increased from 155–170 HV to 170–185 HV, with an affected depth of more than 1.5 mm. Compressive residual stresses of more than 200 MPa were introduced. The three-point bending fatigue of the base material, laser peened and milled after laser peened specimens with artificial crack notch fabricated by a femtosecond laser was investigated. The average fatigue lives of laser peened and milled after laser peened specimens were increased by up to 10.60 and 2.66 times, compared with the base material. This combined fundamental and application-based research seeks to comprehensively explore the applicability of LSP on metal matrix composite.

Effects of laser peening on tensile properties and martensitic transformation of AISI 316L stainless steel in a hydrogen-rich environment

Slow strain rate tensile test in a hydrogen-rich environment was performed to study the effects of laser peening (LP) on the tensile properties and martensitic transformation of AISI 316L stainless steel. Underlying physio-chemical processes involved in hydrogen embrittlement of 316L steel in the hydrogen-rich environment has been proposed. Results show that the grain refinement, mechanical twins and high-density dislocations were found in the laser peened specimens. The movement of hydrogen-carrying dislocations and martensite transformation were prevented by complex grain boundaries and high-density dislocations during tensile process. The ultimate tensile strength of the laser peened specimen were increased by 7.73% and 8.45% in air and hydrogen-rich environment respectively compared with that of non-laser peened specimen. What's more, the elongation loss of laser peened sample was far less than that of non-LP sample in a hydrogen-rich environment. LP induced compressive residual stress (CRS), grain refinement, high-density dislocations and a dense surface layer on the treated surface are beneficial for suppressing hydrogen penetration.

Effect of laser shock peening on mechanical and microstructural aspects of 6061-T6 aluminum alloy

Abstract Laser shock peening (LSP) of 6061-T6 aluminum alloy was performed and parametric effects post LSP on mechanical aspects and microstructural evolution are meticulously studied using various means of characterization techniques such as residual stress analysis, surface roughness, Vickers microhardness, tensile testing, X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM) and electron back scattered diffraction (EBSD). Work hardened layer of ∼1500 μm depth is obtained with significant improvement in cross-sectional microhardness up to 33.04 %. Beneficial compressive residual stress of maximum magnitude up to -273 MPa was induced in laser peened specimens concentrating its overall effect around the depth of 100 μm along the effective depth region. Second phase Mg5Si6 (βʺ) precipitates were observed post LSP while analyzing XRD profiles along with the peak broadening and peak shifting towards higher 2θ angle justifying the results obtained in residual stress profile. Mg5Si6 (βʺ) precipitates are attributed as contributing precipitates in improving the mechanical properties of LSPed specimens along with the dense dislocation density caused by severe plastic deformation during LSP. High angle grain boundaries (HAGBs) fraction was increased in LSPed specimens and its effect is noticed in microhardness profile. The collective contribution of strain hardening, second phase precipitates, peak broadening, dislocation density and increased fraction of HAGBs is observed in mechanical and microstructural aspects of LSPed specimens. The results are discussed in detailed and are strongly correlated with each other.

Corrosion Study on Laser Shock Peened 316L Stainless Steel in Simulated Body Fluid and Chloride Medium

In the present work, corrosion behavior of laser shock peened 316 L stainless steel was evaluated and compared with unpeened specimen in Hank solution (simulated body fluid) and chloride medium. Laser shock peening (LSP) was used for surface modification using Q-switched Nd-YAG laser. In the corrosion study, different electrochemical experiments such as open circuit potential (OCP)-time measurements, potentiodynamic anodic polarization, electrochemical impedance spectroscopy (EIS) measurements were performed in 0.5 M NaCl and Hank solution to examine the corrosion resistance of unpeened and laser peened 316 L SS specimens. The results of corrosion study demonstrated that LSP improved the pitting corrosion resistance despite marginally higher roughness (Ra = 1.01 μm) for laser peened surface than unpeened surface (Ra = 0.54 μm). Improvement in pitting corrosion resistance was indicated by ennoblement of OCP, more noble (anodic) value of pitting potential (Epit) and lower corrosion current density (Icorr). Conductivity and pH of NaCl and Hank solution increased after every polarization experiments which indicate the ionic dissolution during corrosion experiments. Qualitative analysis of Nyquist plots obtained from electrochemical impedance spectroscopy (EIS) results also showed higher arc radius (higher polarization resistance) for laser peened specimen than unpeened, which is indicative of more protective passive film and hence improved corrosion resistance. Further, microstructural examination after polarization experiments in both NaCl and Hank solution showed less pitting sites in laser peened specimens than unpeened surfaces. These microstructural observations are in line with the results obtained in polarization experiments. The results of the study are important with respect to medical implants and their biocompatibility in the human body.

レーザピーニング施工面の圧縮残留応力に及ぼす外部応力負荷の影響

Peening is the process which is able to be generated compressive residual stress and is known to be effective for preventing SCC initiation and improvement of fatigue strength. Laser peening is used for the nuclear power plant components in order to prevent SCC initiation. Although it is reported that the compressive residual stress decreases due to applied stresses under general operating condition, the change of residual stress might be large under excessive loading such as an earthquake. The objectives of this study are to evaluate the relaxation behavior of the compressive residual stress due to laser peening and to confirm the surface residual stress after loading. Therefore laser peened round bar test specimens of SUS316L which is used for the reactor internals of nuclear power plant were loaded at room temperature and elevated temperature and then surface residual stresses were measured by X-ray diffraction method. In the results of this test, it was confirmed that the compressive residual stress remained after applying uniform stress larger than 0.2 % proof stress, and the effect of cyclic loading on the residual stress was small. The effect of applying compressive stress on the residual stress relaxation was confirmed to be less than that of applying tensile stress. Plastic deformation through a whole cross section causes the change in the residual stress distribution. As a result, the surface compressive residual stress is released. It was shown that the effect of specimen size on residual stress relaxation and the residual stress relaxation behavior in the stress concentration region can be explained by assumed stress relaxation mechanism.

Effect of advancing direction on fatigue life of 316L stainless steel specimens treated by double-sided laser shock peening

Laser shock peening (LSP) is considered as a well-established technology for inducing compressive residual stresses under the surface of metallic components. As a result, fatigue life is increased and the material becomes more resistant to corrosion and wear. This paper studies the effect of a significant parameter of LSP, the advancing direction of the laser scanning pattern, on the induced residual stress fields and the fatigue life of stainless steel 316L samples using experiments and 3D finite element analysis. The chosen material is of interest for nuclear and biomedical applications. Two different laser shock processing strategies (two different pulse sequences varying the advancing direction) were performed on the specimens. Their fatigue lives were compared revealing the decisive role played by the advancing direction of the treatment and the generated residual stresses. Fatigue life in laser peened specimens was increased from +166% to +471% by optimizing the pulse sequence. This improvement is explained by stress–strain analysis.

Effect of Laser Peening without Coating on 316L austenitic stainless steel

Laser Peening without Coating (LPwC) is an innovative surface modification technique used for the in-suit preventive maintenance of nuclear reactor components using frequency doubled (green) laser. The advantage of LPwC is that the laser required for this technique is in milli joule range and the processes can perform in aqueous environment. This paper discussed the effect of LPwC on 316L austenitic stainless steel using low energy Nd: YAG laser with various laser pulse density. The base specimen and laser peened specimen were subjected to surface residual stress, surface morphology, micro hardness and potentiodynamic polarization studies. The laser peened surface exhibit significant improvement in surface compressive residual stress. The depth profile of micro hardness revealed higher strain hardening on laser peened specimens. Though corrosion potential reported an anodic shift,current density is found to be increased after LPwC for the specimen peened with higher pulse density.

Studies on fatigue life enhancement of pre-fatigued spring steel specimens using laser shock peening

SAE 9260 spring steel specimens after enduring 50% of their mean fatigue life were subjected to laser shock peening using an in-house developed 2.5J/7ns pulsed Neodymium-doped Yttrium Aluminum Garnet (Nd:YAG) laser for studying their fatigue life enhancement. In the investigated range of process parameters, laser shock peening resulted in the extension of fatigue life of these partly fatigue damaged specimens by more than 15times. Contributing factors for the enhanced fatigue life of laser peened specimens are: about 400μm thick compressed surface layer with magnitude of surface stress in the range of −600 to −700MPa, about 20% increase in surface hardness and unaltered surface finish. For laser peening of ground steel surface, an adhesive-backed black polyvinyl chloride (PVC) tape has been found to be a superior sacrificial coating than conventionally used black paint. The effect of repeated laser peening treatment was studied to repair locally surface melted regions and the treatment has been found to be effective in re-establishing desired compressive stress pattern on the erstwhile tensile-stressed surface.

Studies towards development of laser peening technology for martensitic stainless steel and titanium alloys for steam turbine applications

Fatigue is a major life limiting factor affecting service life of low pressure steam turbine blades which, depending on turbine rating, are generally made of martensitic stainless steel or Ti6Al4V alloy. The results of the present study has demonstrated that with respect to conventional shot peening, laser peening brought more than two times longer mean fatigue life of Ti6Al4V alloy. Enhanced fatigue performance of laser peened specimens was attributed to the deeper peened layer and smoother finish. In the case of DIN X20Cr13 stainless steel, laser peening did not register significant improvement in fatigue resistance over shot peening. At maximum test stress of 550MPa, DIN X20Cr13 stainless steel specimens, shot peened and laser peened specimens exhibited comparable fatigue lives. However, at lower magnitude of maximum test stress of 400MPa, laser peened specimens displayed about 30% longer fatigue lives than their shot peened counterparts. Similar fatigue lives of shot peened and laser peened specimens of DIN X20Cr13 stainless steel specimens is attributed to comparable magnitude of surface residual stress and case depth produced by the two peening treatments.

Ti-6Al-4V合金の回転曲げ疲労特性に及ぼすレーザピーニング処理の影響

In order to investigate the effects of laser peening on high cycle fatigue properties of (α+β) type Ti-6Al-4V alloy, rotating bending fatigue tests were carried out. The laser peening conditions were a spot size of 0.8mm in diameter, a pulse energy of 200mJ and an irradiation pulse density of 38 pulse/mm2. The fatigue properties were not improved with these conditions of laser peening ; even though the material was hardened to some extent and high level compressive residual stresses were induced in the near-surface layer. Because the reason that the fatigue properties were not improved was considered the notch effect of surface roughness introduced by laser peening, further fatigue tests were performed with laser-peened specimens followed by surface polishing. However, the fatigue lives of polished specimens were shorter than those of unpolished specimens, unexpectedly. Considering the peening effects such as surface roughness, hardness and residual stress, the reasons for arriving at the unexpected results would be the influence of the strain-induced transformation of β phase into α' martensite and a larger yield strength in tension than that in compression due to the HCP structure of α phase of titanium alloy.