Papers by Keyword: Laser Peening

Abstract: Due to the recent increase in energy consumption, global environmental issues have attracted attentions in order to continue the sustainable development of society [1]. Currently, scrap-and-build becomes a mainstream in the field of constructions, which would use and /or dispose a large amount of materials. This situation results in massive energy consumption with a significant environmental impact [2]. Therefore, it is important to consider sustainable systems in the construction industry. In many countries, a lot of structures had been constructed during the period of rapid economic growth in the middle of the last century, and their constructions are reaching the end of their design lives. In particular, due to the significant increase in the number of automobiles, numerous cracks above the estimated numbers has been observed in the bridges of motorways. Therefore, the improvement of fatigue life is strongly required to keep a good condition of constructions for a long time. In addition, the nuclear power plant is one of important infrastructures in the modern society, and the proper maintenance of nuclear reactor has been required to ensure their safety after the accident at the Fukushima Daiichi Nuclear Power Plant caused by the Great East Japan Earthquake. Stress corrosion cracking is considered as the most important damage event in the maintenance of nuclear reactor. Stress corrosion cracking occurs, when three factors are combined: tensile residual stress due to welding, material sensitization in the heat-affected zone, and the corrosive environment. In other words, eliminating one of these factors might prevent from generating stress corrosion cracks, and it is expected to extend the fatigue life of nuclear reactors [3]. Therefore, shot peening has been conventionally used as a method to reduce fatigue cracks, because the compressive residual stress can be obtained by the surface’s deformation generated by the high-speed impact of shot grain. However, quantitative control of residual stress is not easy in the shot peening process. Because the shot peening treatment involves the combinations of many factors in the collision between shot grains and the specimen’s characteristics, and the mechanism of micro-structural transformation is also important point to consider the process’s effect. In addition, shot grains possibly remains on the specimen surface, and the contamination of shot grains might deteriorate the functionality of metal surface [4].

Abstract: Stress corrosion cracking (SCC) is one of serious aging degradation problems for the Alloy 600 components of pressurized water reactors (PWRs). In order to prevent SCC, various methods such as water jet peening (WJP), laser peening (LP), surface polishing have been used to introduce compressive stresses at the surfaces of the PWR components. However, it has been reported that such compressive residual stress introduced by these methods might be relaxed during the practical operation, because of high temperature environment. In this study, the hardness reduction behavior of the Alloy 600 processed by LP, Buff and WJP in the thermal aging process has been investigated to estimate the stability of the residual stress improving effect by each method, based on the fact that there is a correlation between the compressive residual stress relaxation and the decrease of hardness. The behavior of the residual stress relaxation in the processed materials in the high temperature environment has been discussed with kinetic analysis.

Abstract: Laser peening is a novel surface treatment process that generates deep compressive residual stresses and microstructural changes and thereby dramatically improves fatigue strength of critical metal aircraft engine parts. In order to study the effects of laser peening on properties of TiAl alloy, Surface micro-hardness, microstructural, residual stress and pole figures before and after laser peening were tested. The experimental results showed that surface micro-hardness increased by 23%. The compressive residual stress increased 20 times. The texture in the normal direction of 8J peened sample showed a strong fiber texture components 10o away. In conclusion, the laser peening could improve properties of TiAl alloy.

Abstract: Microstructure and residual stress in AISI316 stainless steels processed via shot peening (SP) and laser peening (LP) were evaluated using X-ray line profile analysis and residual stress measurements. Although both specimens exhibited similar compressive residual stress profiles in the depth direction, the dislocation density in the SP specimen was greater than that in the LP specimen, while the crystallite size in the SP specimen was less than that of the LP specimen. Thus, the variation in the microstructural features in the samples subjected to the two processes differed.

Abstract: Residual stress measurements are very challenging in thin aluminium plates. Rolling-induced crystallographic texture can lead to an S-shape fit when using the sin²ψ method for surface X-ray diffraction. Peak broadening and missing peaks can also be observed for synchrotron X-ray diffraction with conventional θ/2θ scanning due to texture. In addition, when measuring near the plate surfaces, partially-filled gauge volumes in diffraction experiments will lead to “pseudo-strains”, an apparent shift between measured and actual positions for the diffraction peak. Obtaining a meaningful value of d0 for strain calculations is another issue for diffraction experiments in thin plates. The low thickness also offers challenges for destructive methods including incremental hole drilling, i.e. there is no defined ASTM standard for measuring non-uniform residual stress profile for thin plates. In this work, 2-mm-thick Al2024-T351 plate was investigated for residual stress fields due to laser peening. Neutron diffraction measurements were carried out at POLDI (Pulse Overlap time-of-flight Diffractometer) in PSI, Switzerland and the results are compared with incremental hole drilling.

Abstract: Laser peening offers potential advantages over conventional peen technologies in terms of the depth of the residual stresses that can be induced, and improvements in surface micro-hardness. The present study was undertaken to understand the effect of laser penning on the properties of titanium alloy, a TC4 titanium alloy work-piece was processed with ND: YAG laser with the wavelength of 1064 nm, pulse energy of 0-10J and pulse width of 12ns, and micro-hardness and residual stress for different laser peening parameters were examined and analyzed by micro-hardness tester and X-ray diffraction. Results are presented and discussed of the residual stress profiles and the micro-hardness profiles, The experimental results show that the satisfying laser peening appearance can be achieved when the pulse energy was 6J, water tamping layer thickness was 1.8mm and ablative layer thickness was 100μm, surface micro-hardness increased by up to 33% and the compressive residual stress on the surface of laser shocked area reached up to-327.8MPa, laser peening improved hardness and residual stress of titanium alloy significantly. The experiment results show that the effect of laser peening was evidently.

Abstract: Laser peening offers potential advantages over conventional peen technologies in terms of the depth of the residual stresses that can be induced, and improvements in surface micro-hardness. The present study was undertaken to understand the effect of laser penning on the properties of titanium alloy, a TC4 titanium alloy work-piece was processed with ND: YAG laser with the wavelength of 1064 nm, pulse energy of 0-10J and pulse width of 12ns, and micro-hardness and residual stress for different laser peening parameters were examined and analyzed by micro-hardness tester and X-ray diffraction. Results are presented and discussed of the residual stress profiles and the micro-hardness profiles, The experimental results show that the satisfying laser peening appearance can be achieved when the pulse energy was 6J, water tamping layer thickness was 1.8mm and ablative layer thickness was 100μm, surface micro-hardness increased by up to 33% and the compressive residual stress on the surface of laser shocked area reached up to-327.8MPa, laser peening improved hardness and residual stress of titanium alloy significantly. The experiment results show that the effect of laser peening was evidently.

Abstract: The mechanical properties of 1Cr11Ni2W2MoV stainless steel after laser peening (LP) and ultrasonic shot peening (USP) were examined and compared. The stainless steel specimens were treated with the two different surface processing techniques. X-ray diffractometry (XRD), scanning electron microscope (SEM), microhardness tester were used to investigate microstructure and mechanical properties. Vibration fatigue tests of untreated, LPed and USPed samples were also conducted. The results indicated that LP treatment can improved the fatigue life of 1Cr11Ni2W2MoV stainless steel more effectively. The increases of the compressive residual stress depth and microhardness in surface layer after LP were greater than that of USP. The SEM studies showed that USP treatment had an advantage in microstructure refinement. The compressive residual stresses make great contributions to the superiority of LP in the improvement of fatigue life of 1Cr11Ni2W2MoV stainless steel to USP.

Abstract: Laser shock peening (LSP) is widely known as a cold-worked surface treatment, and this technology has been to greatly improve the fatigue life of many metallic components. Our works focused on laser shock peening with Nd: glass laser system (pulse duration 30ns) and square laser spot size of 4mm×4mm for TC17 titanium alloy. Surface morphology, residual stresses and fatigue performance had been studied for TC17 alloy specimens and blades processed by LSP treatment. The results show that plastic strains in shocked dents become more homogeneous than ones produced by original circle spot with gaussian energy distribution. Surface residual stresses which measured using x-ray diffraction method showed different characteristic as varying specimen thickness, and LSP with overlapping ratio of 8% provided uniform residual stresses on peened surface. Low fluence peening which was implemented at borderline of peened surface was effective to diminish the stress gradient. Compared with mechanical shot peening, LSP attained smoother surface, lower microhardness and better fatigue performance. In a word, Square-spot LSP is an excellent way to improve fatigue life of titanium blade.

Abstract: Laser shock peening offers potential advantages over conventional peen technologies in terms of the depth of the residual stresses that can be induced, and improvements in surface roughness. In this study the application of laser peening to thin aluminium plates such as are used in aerospace applications is investigated. Peening of thin plates presents challenges in balancing the peen intensity to prevent overpeening that will actually lower the stress field. Strain profiles for different laser peening parameters were obtained using synchrotron X-ray diffraction at the ESRF, France. Results are presented and discussed of the residual strain profiles in terms of the laser power density and the number of peen passes. When the power density and number of passes are increased the compressive strain magnitudes are also increased, as has been observed in previous studies. However, the strain components longitudinal and transverse to the peen line are not identical to each other, with the transverse component being much less compressive.