Showing papers on "Peening published in 2015"

Effects of laser shock peening on stress corrosion behavior of 7075 aluminum alloy laser welded joints

Abstract: 7075 aluminum alloy weldments were processed by an intensive process known as laser shock peening (LSP), meanwhile its stress corrosion behaviors were observed by scanning electron microscopy (SEM) and slow strain rate tensile (SSRT) tests. Results showed that the effect of LSP on corrosion behavior of the joint was fairly useful and obvious. With LSP, the elongation, time of fracture and static toughness after the SSRT test were improved by 11.13%, 20% and 100%, respectively. At the same time, the location of the fracture also changed. LSP led to a transition of the fracture type from transgranular to intergranular The reasons for these enhancements of the joint on corrosion behavior were caused by microstructure, residual stress, micro-hardness, and fracture appearance.

Random-type scanning patterns in laser shock peening without absorbing coating in 2024-T351 Al alloy: A solution to reduce residual stress anisotropy

Abstract: Laser Shock Peening (LSP) is considered as an alternative technology to shot peening (SP) for the induction of compressive residual stresses in metallic alloys in order to improve their fatigue, corrosion and wear resistance. Since laser pulses generated by high-intensity laser systems cover only a small area, laser pulses are generally overlapped and scanned in a zigzag-type pattern to cover completely the surface to be treated. However, zigzag-type scanning patterns induce residual stress anisotropy as collateral effect. The purpose of this paper is to describe and explain, for the first time and with the aid of the numerical model developed by the authors, the influence of the scanning pattern directionality on the residual stress tensor. As an effective solution, the authors propose the application of random-type scanning patterns instead of zigzag-type in order to reduce the mentioned residual stress anisotropy.

Effect of shot peening on metastable austenitic stainless steels

Abstract: In this work, shot peening was performed in a metastable austenitic stainless steel EN 1.4318 (AISI 301LN) in order to evaluate its effect on austenite to martensite phase transformation and also the influence on the fatigue limit. Two different steel conditions were considered: annealed, i.e., with a fully austenitic microstructure, and cold rolled, consisting of a mixture of austenite and martensite. X-ray diffraction, electron back-scattered diffraction and focus ion beam, as well as nanoindentation techniques, were used to elucidate deformation mechanisms activated during shot peening and correlate with fatigue response. Results pointed out that extensive plastic deformation and phase transformation developed in annealed specimens as a consequence of shot peening. However, the increase of roughness and the generation of microcracks led to a limited fatigue limit improvement. In contrast, shot peened cold rolled specimens exhibited enhanced fatigue limit. In the latter case, the main factor that determined the influence on the fatigue response was the distance from the injector, followed successively by the exit speed of the shots and the coverage factor.

Fatigue behavior of cold spray coatings: The effect of conventional and severe shot peening as pre-/post-treatment

Abstract: Cold spray coating is gaining increasing attention as a solid state additive technique for repairing structural components. The fatigue behavior of Al 6082 cold spray coating with the same material as substrate is studied. Furthermore, the possibility of improving the load bearing capacity of the cold spray coated specimens by shot peening process is explored. Conventional and severe shot peening were applied before and after cold spray depositions. The coated, coated-peened and peened-coated specimens were characterized by optical microscopy observation, surface roughness and X-ray diffraction residual stress measurements. The specimens were subjected to rotating bending fatigue tests in order to determine the fatigue strength. Unlike the fixed-cantilever, constant-deflection thin specimens, often used to measure fatigue strength of cold spray coatings, the present approach does not considerably reduce the applied stress at the interface compared to the nominal stress and thus represents a more critical and realistic response of the coatings to fatigue loading conditions. The fractured surfaces were examined by scanning electron microscopy. Cold spray coating increased the fatigue strength by 15% compared to the as-received specimens. Further improvement was achieved by the hybrid peening and coating treatment. It was found that shot peening is more efficient if it is performed prior to cold spray deposition. A 26% improvement in the fatigue strength was obtained by performing severe shot peening prior to cold spray deposition. This improvement was attributed to the integrity of coating/substrate interface without any signs of delamination, crack initiation from the surface instead of the interface, crack propagation mainly by transcrystalline mechanism and not through particle boundaries and finally high depth of compressive residual stresses.

Effect of LASER shock peening on microstructure, mechanical properties and corrosion behavior of interstitial free steel

Abstract: LASER shock peening (LSP) is an effective process of surface modification. This work is concerned with the effect of LSP on the modification of microstructure, hardness, microhardness profile, residual stress, tensile properties and corrosion behavior of interstitial free (IF) steel. In order to study the effect of pulse energy on hardness, samples were subjected to LSP at pulse energy of 170, 230, 290 and 340 mJ respectively. The effect of LSP time on hardness, microhardness profile and tensile properties was investigated by processing the samples for 5, 10, 15 and 20 min at pulse energy of 230 mJ and there was found to be significant increase in tensile strength. There was grain refinement to nano level in the surface region due to LSP. Hardness was observed to increase by LSP up to 10 min and tensile strength increased up to LSP of 5 min. However with the increase in LSP time beyond 10 min there was prominent ablation and melting. The effect of overlapped (OV) LASER shocks was also studied by comparing their behavior with that of single shock. The potentiodynamic polarization study showed significant increase in corrosion resistance of the LSPed IF steel.

Effect of high energy shot peening pressure on the stress corrosion cracking of the weld joint of 304 austenitic stainless steel

Abstract: The weld joint of 304 stainless steel is treated using high energy shot peening(HESP) with various shot peening pressures. The grain size and metallographic microstructure of the specimen surface layer are analyzed using the X-ray diffraction method, and the surface hardness is measured. Slow strain rate tension tests are then performed to investigate the effect of shot peening pressure on the stress corrosion sensitivity. The results show that in the surface layer of the specimen, the grain refinement, hardness and the strain-induced plastic deformation all increase with the increasing shot peening pressure. Martensitic transformation is observed in the surface layer after being treated with HESP. The martensite phase ratio is found to increase with increasing shot peening pressure. The result also shows that the effects of the shot peening treatment on the stress corrosion sensitivity index depend on the shot peening pressure. When the shot peening pressure is less than 0.4 MPa, the grain refinement effect plays the main role, and the stress corrosion sensitivity index decreases with the increasing shot peening pressure. In contrast, when the shot peening pressure is higher than 0.4 MPa, the martensite transformation effect plays the main role, the stress corrosion sensitivity index increases with increasing shot peening pressure.

Laser Shock Peening on Zr-based Bulk Metallic Glass and Its Effect on Plasticity: Experiment and Modeling

Abstract: The Zr-based bulk metallic glasses (BMGs) are a new family of attractive materials with good glass-forming ability and excellent mechanical properties, such as high strength and good wear resistance, which make them candidates for structural and biomedical materials. Although the mechanical behavior of BMGs has been widely investigated, their deformation mechanisms are still poorly understood. In particular, their poor ductility significantly impedes their industrial application. In the present work, we show that the ductility of Zr-based BMGs with nearly zero plasticity is improved by a laser shock peening technique. Moreover, we map the distribution of laser-induced residual stresses via the micro-slot cutting method, and then predict them using a three-dimensional finite-element method coupled with a confined plasma model. Reasonable agreement is achieved between the experimental and modeling results. The analyses of serrated flows reveal plentiful and useful information of the underlying deformation process. Our work provides an easy and effective way to extend the ductility of intrinsically-brittle BMGs, opening up wider applications of these materials.

Surface modification of 17-4 PH stainless steel by laser peening without protective coating process

Abstract: The objective of the present study was to examine the influence of laser peening without protective coating (LPwC) process on surface mechanical properties of precipitation hardened stainless steel 17-4 PH. Pulse densities, 2500 and 6250 pulses cm−2, using a constant laser power density of 5.97 GW cm−2 were chosen and their effect on residual stresses, micro-structure, micro-hardness, surface roughness and topography were investigated. Higher pulse density induced large compressive residual stresses and influenced up to depth of 600 μm. It was confirmed from resulted larger lattice strain in LPwC processed 17-4 PH steel with higher pulse density. No phase transformation and microstructural change were observed, which represented pure mechanical effect of LPwC. Hardness was increased with depth of hardened layer up to 600 μm for higher pulse density. However, the surface roughness was increased by the process, and resulted in more pronounced peening pattern. Surface topography revealed columnar structure after LPwC process.

Inverse determination of the regularized residual stress and eigenstrain fields due to surface peening

Abstract: A modified stress function approach is developed for reconstruction of residual stress and eigenstrain fields from limited experimental measurements. The modified approach is successfully applied to three experimental case studies where residual stresses are introduced by surface peening. The smooth reconstructed residual fields not only minimize the deviation of measurements from its approximation but also satisfy all continuum mechanics requirements. Furthermore, a comprehensive discussion is performed on regularity of the solution in Tikhonov scheme and the regularization parameter is then determined utilizing Morozov discrepancy principle. Newton iterative method is also shown to have an excellent fast convergence to the regularization parameter while effectively reduces the computational cost.

Fatigue Performance of Medical Ti6Al4V Alloy after Mechanical Surface Treatments

Abstract: Mechanical surface treatments have a long history in traditional engineering disciplines, such as the automotive or aerospace industries. Today, they are widely applied to metal components to increase the mechanical performance of these. However, their application in the medical field is rather rare. The present study aims to compare the potential of relevant mechanical surface treatments on the high cycle fatigue (R = 0.1 for a maximum of 10 million cycles) performance of a Ti6Al4V standard alloy for orthopedic, spinal, dental and trauma surgical implants: shot peening, deep rolling, ultrasonic shot peening and laser shock peening. Hour-glass shaped Ti6Al4V specimens were treated and analyzed with regard to the material’s microstructure, microhardness, residual stress depth profiles and the mechanical behavior during fatigue testing. All treatments introduced substantial compressive residual stresses and exhibited considerable potential for increasing fatigue performance from 10% to 17.2% after laser shock peening compared to non-treated samples. It is assumed that final mechanical surface treatments may also increase fretting wear resistance in the modular connection of total hip and knee replacements.

Effect of shot peening on the fatigue resistance of laser surface melted 20CrMnTi steel gear

Abstract: The effect of combined laser surface melting and shot peening (LSMSP for short) on fatigue resistance of 20CrMnTi steel gear is investigated in this paper. After laser surface melting (LSM) treatment to gear tooth flanks, gear was treated by shot peening (SP) with different peening intensities. The residual stresses, full width at half maximum (FWHM), microhardness, retained austenite volume fraction and surface roughness of the LSMSP gear were measured. The Forschungsstelle fur Zahnrader und Getriebebau (FZG) back-to-back spur gear test rig was used for fatigue experiments. Experimental results showed that compressive residual stress in the LSMSP gear tooth firstly increased to a maximum value and then decreased to stress state of the substrate. With improving strength of shot peened specimen, maximum values and depths of compressive residual stresses increased in the LSMSP gear. Owing to grain refinement, FWHM of the LSMSP gear tooth was broadened obviously along the direction of depth. Through LSMSP treatment, the retained austenite volume fraction of the LSMSP gear was lower than 5% in the LSMSP surface layer. The microhardness of the LSMSP gear tooth has been raised above 20%. The fatigue and wear resistance of the LSMSP gear was much better than that of the LSM and SP gears.