Effect of Treatment Area on Residual Stress and Fatigue in Laser Peened Aluminum Sheets
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r peening is a proven method to enhance the fatigue performance of engineering materials. Compared to conventional shot peening, laser peening can induce deeper compressive residual stresses while maintaining a smoother surface finish resulting in improved fatigue performance through retardation of crack initiation and propagation.[1] However, it is challenging to obtain beneficial compressive residual stress fields in thin samples.[2,3] Laser peening employs high-energy laser pulses to create a high-temperature and high-pressure plasma at the sample surface. The plasma-induced shock waves are constrained by a water overlay, which directs them into the material, leading to plastic deformation and the formation of beneficial residual stresses. A sacrificial ablative layer may be used during the peening process to protect the surface. However, depending on the process parameters, laser peening may actually cause a M. BURAK TOPARLI is with the Cemer Kent Ekipmanlari San. ve Tic. A.S., 35375, Torbali, Izmir, Turkey. Contact e-mails: [email protected], [email protected] NIALL SMYTH and MICHAEL E. FITZPATRICK are with the Centre for Manufacturing and Materials Engineering, Coventry University, Priory Street, Coventry, CV1 5FB, U.K. Manuscript submitted July 4, 2016. Article published online January 18, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A
reduction in fatigue performance. Therefore, selection of the laser peening process parameters is crucial. In the open literature, there are studies investigating the effects of laser peening process parameters such as laser energy, laser pulse duration, laser spot size and shape as well as the amount of overlapping of the laser spots. The peen pattern has also been shown to affect the final residual stresses obtained: for example, we showed previously that a line of large peen spots has a highly non-biaxial stress field,[4] but a patch made up of smaller spots, with the same overall dimensions, has a biaxial residual stress field.[5] Therefore the size of the patch relative to the spot size will affect the final residual stresses obtained. Therefore in this work, two Al2024-T351 sheets (Figures 1(a) and (b)) were laser peened with the same process parameters, but one of the sheets was laser peened over a larger area to investigate the effect of laser peening area on final residual stress fields and fatigue performance. The resulting residual stress fields were measured by three different techniques: incremental hole drilling, surface X-ray diffraction, and synchrotron X-ray diffraction. Fatigue tests were also carried out to investigate the effect of induced residual stress field. A correlation between the residual stresses and the fatigue performance was obtained. Aerospace Al2024-T351 with Al-cladding was used in this study. The 2.0-mm-thick samples were clad with a layer of pure aluminum (with a thickness of around 50 to 80 lm), for increased corrosion resistance. The mechanical properties of the samples were obtained by standard tensile tests according to ASTM B557M-07e1 standard
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