Showing papers on "Peening published in 2013"

Electron Beam Additive Manufacturing of Titanium Components: Properties and Performance

Abstract: This research evaluates the fatigue properties of Ti-6Al-4V specimens and componentsproduced by Electron Beam additive manufacturing. It was found that the fatigue per-formance of specimens produced by additive manufacturing is significantly lower thanthat of wrought material due to defects such as porosity and surface roughness. However,evaluation of an actual component subjected to design fatigue loads did not result in pre-mature failure as anticipated by specimen testing. Metallography, residual stress, staticstrength and elongation, fracture toughness, crack growth, and the effect of post process-ing operations such as machining and peening on fatigue performance were alsoevaluated. [DOI: 10.1115/1.4025773]Keywords: additive manufacturing, electron beam, titanium, fatigue, fracture

Comparison of mechanisms of advanced mechanical surface treatments in nickel-based superalloy

Abstract: Mechanical surface enhancement techniques are used to introduce compressive residual stresses to enhance the fatigue life and corrosion resistance of metallic components. In this study, we compare the effects of three advanced mechanical surface enhancements treatments: laser shock peening, cavitation shotless peening and ultrasonic nano structure modification on residual stress, hardness, plastic deformation and changes in near surface microstructure introduced in a Ni-Base superalloy, IN718 SPF(Super plastic forming). Coupons of the alloy were peened using two different conditions of each treatment and results compared to achieve a better understanding of underlying mechanisms of these techniques. Results indicate that there are significant differences in mechanisms of these surface treatments leading to differences in material response.

Laser peening without coating on aluminum alloy Al-6061-T6 using low energy Nd:YAG laser

Abstract: The present study investigates the effect of laser peening without coating on aluminum alloy Al-6061-T6 with a 300 mJ infrared laser The surface topography, microstructure, surface topography, surface residual stress and micro-hardness of peened and unpeened surfaces were studied The study shows that laser peening without coating can significantly improve surface compressive stress and micro-hardness with trivial increase in surface roughness Microstructure evaluation confirmed there was no near surface solidification after LPwC

Deforming TC6 titanium alloys at ultrahigh strain rates during multiple laser shock peening

Abstract: The deformation mechanism of TC6 titanium alloys at ultrahigh strain rates (>10 6 s −1 ) during multiple LSP was investigated. When nanosecond pulse and 1000 MW laser irradiated on the materials, the high pressure plasma shock wave was induced and ultrahigh strain rates response was caused in the material. The TEM observation of the surface layer indicates that a layer of nanocrystalline has been formed and is unevenly distributed after a single impact. Increasing impact times could provide longer time and more energy to dislocation movement which refines the grains into smaller size and makes the distribution uniform. The hardness of TC6 titanium alloy has been improved by a single laser shock peening, forming a severe plastic deformted layer with a depth of 200 μm. Increasing impact times will improve the hardness and effective depth. The XRD test shows that the position of diffraction peak did not show any change, but the Bragg diffraction peak was broadened. Dislocation activity is a prevalent deformation mechanism of TC6 titanium alloy in the grain refinement study.

Simulation-based optimization of laser shock peening process for improved bending fatigue life of Ti–6Al–2Sn–4Zr–2Mo alloy

Abstract: Laser shock peening (LSP) induced residual stresses significantly affect the high cycle fatigue behavior of certain metals and alloys. Residual stress distribution is a function of various laser parameters (energy, laser pulse width, and spot diameter), the geometry, the material and the laser shot sequencing. Considering the wide range of parameters involved in the LSP process, a numerical approach based on 3D nonlinear finite element method has been employed to explore the relation between the processing parameters and the residual stress distribution. This methodology is applied to a thin coupon of Ti–6Al–2Sn–4Zr–2Mo (Ti-6242) alloy, with a view towards establishing conditions for obtaining through-thickness compressive residual stresses and hence improved bending fatigue life. Material response at very high strain rates in the LSP process is effectively represented using the modified Zerilli–Armstrong material model. The numerical approach is verified by comparison with the experimental results. Effects of laser parameters and laser shot sequencing on final residual stress distribution are studied by performing full scale simulations of LSP patches constituting a large number of laser shots. Based on simulation studies, optimal set of parameters is obtained that produces through thickness compression, which leads to a substantial improvement in bending fatigue life. Fatigue testing results support the recommendations made based on simulation results.

Life assessment methodologies incoroporating shot peening process effects: mechanistic consideration of residual stresses and strain hardening Part 1 – effect of shot peening on fatigue resistance

Abstract: Shot peening is a well known process applied to components in order to improve their fatigue resistance. In recent years, there has been an increasing interest in including the effects of the shot peening process in life assessment models since this would allow a reduction in conservatism compared to those in current application. The present paper seeks to review firstly the effects of the shot peening process (surface roughening, strain hardening and compressive residual stresses) and how the magnitude of these effects can be determined both experimentally and numerically. The reasons for the beneficial effect of shot peening on fatigue resistance are reviewed; this includes consideration of how different operating conditions can affect the magnitude of the benefit. The second part of the review details the life assessment approaches which have been developed to date incorporating these effects and seeks to identify the areas in which further development is still required before the models can be...

Structurally induced enhancement in corrosion resistance of Zr–2.5%Nb alloy in saline solution by applying ultrasonic impact peening

Abstract: Near surface layers in Zr–2.5%Nb alloy are modified using severe plastic deformation process induced by ultrasonic impact peening (UIP). The effect of the UIP processing time on the microstructural formation, microhardness and corrosion is analyzed. XRD analysis and TEM observations allow establishing the links between microstructure in the surface layers formed at different extents of the effective strain e ¯ and their microhardness and corrosion resistance. Structural formation occurs under influence of deformation induced heating, which facilitates dynamic recrystallization and omega precipitation in beta grains. Subdivision of alpha grains is observed to be a dislocation-mediated process mainly. Deformation twinning may be effective in the early stages of the UIP process till the strain extents of approx. e ¯ ≈0.2. Generation and rearrangement of dislocations result in formation of elongated cell structure at e ¯ ≈0.43. Further straining to e ¯ ≈0.8 leads to formation of ultrafine-grained (UFG) structure through the subdivision of elongated subgrains into equiaxed ones and subsequent increase in their misorientation. Average grain size in the formed UFG structure was about 150–200 nm. XRD analysis reveals high compressive residual stresses and strong basal texture after the UIP process. It is suggested that the sliding character of the impact loads occurred at the UIP process plays an important role in the crystallites' reorientation in superficial layer. Formed UFG surface layer with strong basal texture provides an essential increase in surface microhardness from 2 to 3.15 GPa and marked enhancement of the corrosion resistance of Zr–2.5%Nb alloy in saline solution due to the broadening of passivity region on the anodic potentiodynamic polarization curve. Decreased surface roughness and large compressive residual stresses also promote higher corrosion resistance.

Enhanced fatigue durability of Al-6Mg alloy by applying ultrasonic impact peening: Effects of surface hardening and reinforcement with AlCuFe quasicrystalline particles

Abstract: Composite layer reinforced with quasicrystalline (QC) Al63Cu25Fe12 particles was fabricated on the surface of Al–6 Mg alloy specimens by ultrasonic impact peening (UIP). Stress-controlled fatigue response of the specimens was studied and compared with those for the annealed and UIP-treated specimens. The notch effect of the UIP induced surface roughness calculated in terms of the stress concentration factor does not exceed∼10%. XRD, OM and SEM analyses were used to characterize formed surface layers and fatigue fracture surfaces. Surface composite layer of 40–50 μm thick contains the homogeneously dispersed QC particles (the volume fraction Vf∼0.15) trapped by high compressive residual stresses. The layer demonstrates almost triple increase in microhardness comparing to that for the annealed alloy and twice exceeding of that for the UIP-treated specimen. Superior fatigue endurance of Al–6 Mg alloy after the UIP process and the UIP-induced formation of the composite layer is explained by sub-surface fatigue cracks’ initiation promoted with high compressive residual stresses and tight interfacial bonding of QC reinforcement and the matrix alloy. The improved fatigue behavior of the UIP-treated specimens in both the low cycle and high cycle regimes can be ensured by combination of the following favorable characteristics: (i) sufficiently high ductility and resistance to fatigue damage and crack growth in the core parent material along with (ii) superior fatigue strength supported by high microhardness and compressive stresses in the surface layer, which contains fine grained matrix and/or uniformly distributed and tightly bonded QC reinforcements.

Effects of curved geometry on residual stress in laser peening

Abstract: Prevention of failure in materials has been extensively researched for centuries by people all over the world. Sustained or cyclic loading is a major cause of failure in many situations. There can be other contributing factors, such as manufacturing defects and the presence of unfavorable residual stresses. But residual stresses can also play a positive role and can enhance the life of the component, if applied properly. Laser peening (LP) is a surface enhancement technique that can impede crack initiation and propagation by inducing favorable compressive residual stresses in the peened components. Laser peening can generate deeper compressive stresses compared with other surface enhancement techniques, such as shot peening, and it has been used to improve the fatigue life of components in aerospace, automotive and medical applications. In this work, the effects of laser peening are studied by using two dimensional finite element simulation models created with ABAQUS©. Crack initiation often occurs along the curved regions or fillets of structural components because of the presence of high stress concentrations. These critical regions are modeled as a curved geometry to capture the curvature effects using simulation models. Concave and convex simulation models are created and compared with flat geometry to investigate the effects of curvature in a laser peening problem. A mechanism of residual stress generation in curved models is used to explain the residual stress results obtained from finite element models. The results predict that increasing the radius of curvature in a concave model decreases the compressive residual stress generated in the component while increasing the radius of curvature of a convex model increases the compressive residual stress induced in the material when compared with a flat component.

Experimental and numerical investigation of material heterogeneity in shot peened aluminium alloy AA2024-T351

Abstract: Shot peening is a mechanical surface treatment that consists of projecting numerous small particles onto a ductile surface. Repeated and random hammering leads to compressive residual stresses as well as work-hardening near the surface resulting in a potentially significant improvement of the fatigue performance. On the other hand, shot peening also increases surface roughness and can produce surface damage which are detrimental to fatigue properties. Fatigue life prediction models have been proposed to predict the combined influence of these factors. Most of these models assume that shot peening leads to a uniform material state parallel to the surface. The current work investigated the heterogeneities present in peened aluminium alloy AA2024-T351 after repeated random impacts in terms of residual stresses, work-hardening, surface roughness, and damage while also considering the initial non-uniformities in the material. Experimental and numerical methods were combined to obtain a detailed description of the material state at various length scales. The implications of the heterogeneities in terms of fatigue performance as well as the potential contributions of numerical simulation were also discussed.

Eigenstrain modelling of residual stress generated by arrays of laser shock peening shots and determination of the complete stress field using limited strain measurements

Abstract: This paper presents a hybrid explicit finite element (FE)/eigenstrain model for predicting the residual stress generated by arrays of adjacent/overlapping laser shock peening (LSP) shots where the use of a completely explicit FE analysis may be impractical. It shows that for a given material, the underlying eigenstrain distribution (in contrast to the resulting stress field) representing a laser shock peen is primarily dependent on the parameters of the laser pulse and the number of overlays rather than the precise component geometry. Consequently the residual stress introduced by complex laser peening treatments can be built up by using static FE models and superposition of individual eigenstrain distributions without recourse to further computationally demanding explicit FE analyses. It is found that beneath a small LSP array the magnitude of the compressive residual stress is higher than for a wider array of LSP shots and that with increasing numbers of layers the compressive stress increases as does the depth of the compressive zone. The model predictions for the eigenstrain distributions are compared well with experimental measurements of plastic strain (full-width-at-half-maximum) obtained by neutron diffraction. The eigenstrain method is also extended to construct the full residual stress field using measured residual elastic strains at a finite number of measurement locations in a component.

Effect of waterjet peening on aluminum alloy 5005

Abstract: The present study addresses the effect of waterjet peening parameters on aluminum alloy 5005. The approach was based on the response surface methodology utilizing the Box–Behnken experimental design. Workable empirical models were developed to predict surface roughness (R a ) and hardness (HV). Increasing the number of passes, pressure, and standoff distance produces a higher surface roughness as well as a higher hardness. On the contrary, increasing the feedrate produces a lower surface roughness and hardness. The developed empirical models for R a and HV have reasonable correlations between the measured and predicted responses with acceptable coefficients of determinations. A different set of optimum parameters was generated based on different desirability functions for each response. The predicted and the actual responses for optimized R a and HV are satisfactory with good reliability. It is shown that the models are workable in predicting the responses of R a and HV in the present research. A proper selection of peening parameters can be formulated to be used in practical works.

Residual stresses caused by head-on and 45 foreign object damage for a laser shock peened Ti?6Al?4V alloy aerofoil

Abstract: This paper investigates the effect on the residual stresses of foreign object damage (FOD) to a previously laser shock peened (LSP) leading edge (LE). FOD was introduced onto the LE of the aerofoil-shaped specimen through ballistic impacts of a cube edge at angles of 0° and 45° to the leading edge. The residual stress distribution was mapped around the FOD notch by synchrotron X-ray diffraction. The results suggest that for both impact angles, the FOD event superimposed a significant additional residual stress on top of the pre-existing stress associated with the LSP process. In particular, the compressive stress was found to be largest directly beneath the notch and the tensile region, seen previously for unpeened aerofoils beneath the compressive zone, was absent due to the pre-existing peening stress field. This may help to explain the improved fatigue strength observed previously. It is shown that the FOD notch created by 45° impact was asymmetric in shape and smaller in depth compared to that created at 0°. The residual stresses were somewhat larger for the 0° impact condition than for 45° partly due to the larger notch depth introduced in the former case.

Estimation of the Yield Stress of Stainless Steel from the Vickers Hardness Taking Account of the Residual Stress

Abstract: In this paper, a method that uses the Vickers hardness to estimate the yield stress of a metallic material with taking account of residual stress is proposed. Although the yield stress of bulk metal can be evaluated by a tensile test, it cannot be applied to local yield stress varied by surface modification methods, such as the peening technique which introduces high compressive residual stress at the surface. Therefore, to evaluate the local yield stress employing a relatively easy way, the Vickers hardness test was conducted in this paper. Since the Vickers hardness depends on both the residual stress and the yield stress, the relationship between the residual stress and the Vickers hardness was experimentally examined. It was concluded that the yield stress of the surface treated by several peening techniques can be estimated from the Vickers hardness once this has been corrected for residual stress.

Experiment investigation on microstructure and mechanical properties of TC17 titanium alloy treated by laser shock peening with different laser fluence

Abstract: In this paper, microstructure, microhardness, and residual stress of TC17 titanium alloy treated by laser shock peening (LSP) with different laser fluence were examined and compared by x-ray diffraction, scanning electron microscope, transmission electron microscope, microhardness test, and residual stress test. The results of microstructure point out that microstrains and grain-refinement are generated in the material surface layer which is a severe plastic deformation layer. In relation to the microhardness and residual stress, the results indicate that LSP can obviously improve the hardness and introduce a great compressive residual stress which also has a good thermal stability.