Effect of Overlapping of Peen Spots on Residual Stresses in Laser-Peened Aluminium Sheets
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ning is a relatively new surface treatment technique, used to prevent fatigue failure particularly in safety-critical applications including aerospace and nuclear energy. Laser peening uses 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 compressive residual stresses. Owing to compressive residual stress fields, crack initiation and propagation can be inhibited or
M. BURAK TOPARLI is with the Department of Engineering, Izmir University of Economics, Balcova, Izmir, Turkey and also with the Norm Fasteners R&D Center, Izmir, Turkey. Contact e-mails: [email protected]; [email protected] MICHAEL E. FITZPATRICK is with the Centre for Manufacturing and Materials Engineering, Coventry University, Priory Street, Coventry CV1 5FB, UK. Manuscript submitted March 25, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
delayed, resulting in prolonged fatigue life. However, it is very challenging to apply laser peening to thin sheets without inducing tensile near-surface residual stress.[1,2] In the literature, it was reported that tensile near-surface residual stresses after laser peening lead to a reduction in fatigue performance.[3,4] Therefore, selection and modification of the laser peening process parameters is crucial to obtain a fatigue life improvement. In this study, one of the most important and easy-to-control process parameters, i.e., the amount of spot overlapping, was investigated. Two different laser peening systems were employed to have 200 and 400 pct coverage for LP System 1; and 18 and 54 shots/mm2 for LP System 2. The effect of overlapping on the residual stress fields in 2.0-mm-thick Al2024-T351 sheets were measured by incremental hole drilling. 2.0-mm-thick Al2024-T351 sheets with Al-cladding were laser peened in this study. This is an aerospacegrade material, with a 50–80-lm-thick layer of pure aluminium roll bonded to both surfaces of the plates to increase the corrosion resistance. Tensile tests were carried out according to ASTM B557M-07e1, with samples extracted perpendicular to the rolling direction. Averages from three tests are presented in Table I. The size of the samples peened by LP System 1 and LP System 2 were 80 mm 9 80 mm and 120 mm 9 60 mm, respectively, with peen patches as shown in Figures 1 and 2. The residual stress measurements were carried out at the center of the laser-peened regions. During the peening, water overlay was used for both systems. The detailed laser peening process parameters can be seen in Table II. The schematic overlapping pattern for each system can be seen in Figures 1 and 2. The overlapping procedure of many round spots of LP System 2 resulted in a complex pattern. However, repeated incremental hole drilling experiments showed that the resulting residual stress profiles were uniform, owing to very small laser spot size and high amou
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