Showing papers on "Peening published in 2012"

Investigation of laser shock peening effects on residual stress state and fatigue performance of titanium alloys

Abstract: Laser shock peening can potentially enhance fatigue life of titanium components by inducing compressive residual stresses in surface layers much deeper than caused by traditional shot peening (SP). In the present study, the high cycle fatigue (HCF) performance of α Ti-alloy Ti–2.5Cu, (α + β) Ti-alloy TIMETAL 54M and the metastable β Ti-alloy TIMETAL LCB was investigated after laser shock peening without coating (LPwC). The fatigue results were interpreted by examining the changes of surface morphology, microhardness and residual stress generated in the surface layer. Furthermore, thermal stability of residual stresses in aged Ti–2.5Cu, as an example, was evaluated after annealing LPwC-treated material at various elevated temperatures and exposure times by applying a Zener–Wert–Avrami approach. The depth profiles of residual stresses were obtained by means of synchrotron X-ray diffraction or by incremental hole drilling method. Results revealed that the HCF performance of Ti–2.5Cu and TIMETAL LCB was markedly improved after LPwC, while it was deteriorated in TIMETAL 54M. Compared to LPwC, better 10 7 fatigue strength of Ti–2.5Cu was obtained after ball-burnishing (BB). Moreover, LPwC-induced residual stresses are thermally more stable than shot peening-induced ones.

Effect of laser shock peening on residual stress and fatigue life of clad 2024 aluminium sheet containing scribe defects

Abstract: Laser peening at a range of power densities has been applied to 2-mm-thick sheets of 2024 T351 aluminium. The induced residual stress field was measured using incremental hole drilling and synchrotron X-ray diffraction techniques. Fatigue samples were subjected to identical laser peening treatments followed by scribing at the peen location to introduce stress concentrations, after which they were fatigue tested. The residual stresses were found to be non-biaxial: orthogonal to the peen line they were tensile at the surface, moving into the desired compression with increased depth. Regions of peen spot overlap were associated with large compression strains; the centre of the peen spot remaining tensile. Fatigue lives showed moderate improvement over the life of unpeened samples for 50 μm deep scribes, and slight improvement for samples with 150 μm scribes. Use of the residual stress intensity Kresid approach to calculate fatigue life improvement arising from peening was unsuccessful at predicting the relative effects of the different peening treatments. Possible reasons for this are explored.

Laser Surface Engineering of Magnesium Alloys: A Review

Abstract: Magnesium (Mg) and its alloys are well known for their high specific strength and low density. However, widespread applications of Mg alloys in structural components are impeded by their insufficient wear and corrosion resistance. Various surface engineering approaches, including electrochemical processes (plating, conversion coatings, hydriding, and anodizing), gas-phase deposition (thermal spray, chemical vapor deposition, physical vapor deposition, diamond-like coatings, diffusion coatings, and ion implantation), and organic polymer coatings (painting and powder coating), have been used to improve the surface properties of Mg and its alloys. Recently, laser surface engineering approaches are attracting significant attention because of the wide range of possibilities in achieving the desired microstructural and compositional modifications through a range of laser–material interactions (surface melting, shock peening, and ablation). This article presents a review of various laser surface engineering approaches such as laser surface melting, laser surface alloying, laser surface cladding, laser composite surfacing, and laser shock peening used for surface modification of Mg alloys. The laser–material interactions, microstructural/compositional changes, and properties development (mostly corrosion and wear resistance) accompanied with each of these approaches are reviewed.

Residual stresses in cold spray Al coatings: The effect of alloying and of process parameters

Abstract: Al and Al alloy cold spray coatings were deposited on Mg substrates using two different cold spray systems - a Kinetic Metallization system (convergent barrel, sonic nozzle) and a CGT system (convergent-divergent barrel, supersonic nozzle). The residual stress profiles in the coatings were measured using neutron diffraction with high spatial resolution. In the first part of the study, the residual stress profile in pure Al coatings was compared when sprayed using the Kinetic Metallization system (using both helium and nitrogen as the driving gas), and with the CGT system using nitrogen gas. In this way the effect of impact velocity and process temperature was studied. In the second part of the study, the residual stress profile was compared in coatings of pure Al, 7075 Al and 6061 Al sprayed using the same process conditions in the CGT system. The residual stress profiles depend more on the alloy content, i.e. intrinsic resistance to plastic deformation, than on the processing conditions, and this is interpreted using a simple model that incorporates the effect of peening stresses in the cold spray process.

Laser shock peening of Ti-17 titanium alloy: Influence of process parameters

Abstract: The influence of the process parameters of laser shock peening was investigated on specimens made of an aeronautic titanium alloy: Ti–5Al–2Sn–2Zr–4Cr–4Mo (Ti-17). In order to quantify the effect of relevant process parameters, an experimental design was carried out. It is based on a full factorial design with four factors (laser fluence, pulse duration, number of impacts and thickness of the sample) and two levels for each factor. The process is characterised with the following variables: the depth of the impacts, the roughness of the treated surface, the hardening of the material (itself evaluated with the hardness and X-ray diffraction peak width), the residual stresses left in the sample and the global curvature of the sample. It is found that all the parameters have an influence on the residual stresses and that laser shock peening has no influence on roughness and low influence on work-hardening. The variables are then analysed in order to evaluate correlations. The increase in hardness is found to be essentially due to compressive residual stresses, cold work-hardening having only a small effect. In thin specimens, the stress redistribution due to self-equilibrium leads to tensile residual stresses at the treated surface and to large deformations of the specimens.

Ultrafine-grained textured surface layer on Zr–1%Nb alloy produced by ultrasonic impact peening for enhanced corrosion resistance

Abstract: An ultrafine-grained (UFG) surface layer was produced on Zr–1%Nb alloy using severe plastic deformation process induced by ultrasonic impact peening (UIP). XRD analysis and TEM observations allow establishing the links between the microstructure formed at different extents of the effective strain ē and superficial microhardness and corrosion resistance. In the topmost surface layer about 10 μm thick, average grain size diminishes down to approx. 100 nm after the UIP process for 4 min ( ē ≈ 0.86). The mechanism of the grain subdivision is discussed on the basis of TEM observations of microstructure in surface layer at different strain extents with taking into account the dynamic recrystallization process facilitated by deformation induced heating. XRD analysis reveals high compressive residual stresses and strong basal texture. Two features of the surface layer formed after the UIP process, viz. UFG structure and strong basal texture, are considered to play the major roles in essential increase in microhardness (from 1 to 2.35 GPa) and corrosion resistance of the Zr-1%Nb alloy in saline solution. Decreased surface roughness and large compressive residual stresses also promote higher corrosion resistance.

Laser shock peening without absorbent coating (LSPwC) effect on 3D surface topography and mechanical properties of 6082-T651 Al alloy

Abstract: The influence of nanosecond laser pulses applied by laser shock peening without absorbent coating (LSPwC) with a Q-switched Nd:YAG laser operating at a wavelength of λ = 1064 nm on 6082-T651 Al alloy has been investigated. The first portion of the present study assesses laser shock peening effect at two pulse densities on three-dimensional (3D) surface topography characteristics. In the second part of the study, the peening effect on surface texture orientation and micro-structure modification, i.e. the effect of surface craters due to plasma and shock waves, were investigated in both longitudinal (L) and transverse (T) directions of the laser-beam movement. In the final portion of the study, the changes of mechanical properties were evaluated with a residual stress profile and Vickers micro-hardness through depth variation in the near surface layer, whereas factorial design with a response surface methodology (RSM) was applied. The surface topographic and micro-structural effect of laser shock peening were characterised with optical microscopy, InfiniteFocus® microscopy and scanning electron microscopy (SEM). Residual stress evaluation based on a hole-drilling integral method confirmed higher compression at the near surface layer (33 μm) in the transverse direction (σmin) of laser-beam movement, i.e. − 407 ± 81 MPa and − 346 ± 124 MPa, after 900 and 2500 pulses/cm2, respectively. Moreover, RSM analysis of micro-hardness through depth distribution confirmed an increase at both pulse densities, whereas LSPwC-generated shock waves showed the impact effect of up to 800 μm below the surface. Furthermore, ANOVA results confirmed the insignificant influence of LSPwC treatment direction on micro-hardness distribution indicating essentially homogeneous conditions, in both L and T directions.

Thermal relaxation of residual stress in laser shock peened Ti–6Al–4V alloy

Abstract: Laser shock peening (LSP) induced residual stresses in Ti–6Al–4V, and their thermal relaxation due to short-term exposure at elevated temperatures are investigated by an integrated modeling/simulation and experimental approach. A rate and temperature-dependent plasticity model in the form of Johnson–Cook (JC) has been employed to represent the nonlinear constitutive behavior under both LSP and thermal loads. By comparing the simulation results with experimental data, model parameters for Ti–6Al–4V are first calibrated and subsequently applied in analyzing the thermal stability of the residual stress in LSP-treated Ti–6Al–4V. The analysis shows that the magnitude of stress relaxation increases with the increase of applied temperature due to material softening. Most of stress relaxation occurs before 10 min to 20 min exposure in this study, and stress distribution becomes more uniform after thermal exposure. An analytical model based on the Zener–Wert–Avrami formula is then developed based on the simulation results. The activation enthalpy of the relaxation process for laser shock peened Ti–6Al–4V is determined to be in the range of 0.71 eV to 1.37 eV.

Significant improvement of corrosion resistance of biodegradable metallic implants processed by laser shock peening

Abstract: Biodegradable magnesium–calcium alloys are attractive new orthopedic biomaterials compared to conventional permanent implant alloys. However, magnesium–calcium alloys corrode too fast in human body fluids. This study explores the process capability of laser shock peening (LSP) to control the corrosion of magnesium–calcium implants by tailoring the surface integrity. LSP induced unique surface topographies, highly compressive residual stresses, and extended strain hardening significantly enhanced the corrosion resistance of the alloy by more than 100-fold in simulated body fluid. Furthermore, corrosion of the peened implants was controllable by varying the laser power and peening overlap ratio.

Improving surface hardness of austenitic stainless steel using waterjet peening process

Abstract: The present study addresses the effect of multiple jet passes and other parameters namely feedrate and pressure in waterjet peening (WJP) of austenitic stainless steel 304. An analysis of surface integrity was used to evaluate the performance of different parameters in the WJP process. An increase in the number of jet passes as well as pressure leads to a higher roughness and more erosion of the surface. However, the feedrate shows a reverse effect on the surface roughness and erosion. The surface microstructures also show the mechanism of material removal process involving initial and evolved damages. The subsurface hardness shows that treating the surface with a higher number of passes and pressure produces a higher increase of hardness and also a deeper hardening layer. But, a reverse effect on the subsurface hardness was found for the feedrate. Furthermore, cross-sectional microstructures show a higher density of slip bands in the deformed grains of the specimen treated with a higher number of jet passes and pressure. However, the amount of slip bands in the deformed grains is lower with increasing feedrate.

Fatigue in laser shock peened open-hole thin aluminium specimens

Abstract: An experimental study was performed in order to determine the influence of the sequence of operations on the effectiveness of Laser Shock Peening (LSP) treatment in increasing the fatigue performances of open-hole aluminium specimens. Residual stress measurements, fractographic analysis and FEM analysis were performed, indicating the presence of compressive residual stresses on the surface of the treated specimens and tensile residual stresses in the mid-section along the thickness of the specimens. Negative effects on fatigue lives were encountered on the specimens with the hole already present, while positive effect were observed in specimens in which the hole was drilled after LSP treatment. These results indicate that LSP can be a good solution for “in production” application, in which open holes are to be drilled after the LSP treatment. The application in which LSP is used “in service” on structures with pre-existing cut-outs, has proven to be impracticable in the investigated configuration.

Deformation-induced martensite and nanotwins by cryogenic laser shock peening of AISI 304 stainless steel and the effects on mechanical properties

Abstract: Laser shock peening (LSP) of stainless steel 304 was carried out at room and cryogenic temperature (liquid nitrogen temperature). It was found that the deformation-induced martensite was generated by LSP only when the laser-generated plasma pressure is sufficiently high. Compared to room temperature laser shock peening (RT-LSP), cryogenic laser shock peening (CLSP) generates a higher volume fraction of martensite at the same laser intensity. This is due to the increase in the density of potential embryos (deformation bands) for martensite nucleation by deformation at cryogenic temperature. In addition, CLSP generates a high density of deformation twins and stacking faults. After CLSP, an innovative microstructure, characterised by networks of deformation twins, stacking faults and composite structure (martensite and austenite phases), contributes to material strength and microstructure stability improvement. The combined effect of higher surface hardness and a more stabilised microstructure results in gre...