Showing papers on "Peening published in 2010"

Improvement of fatigue life of Ti–6Al–4V alloy by laser shock peening

Abstract: The aim of this paper was to address the effects of multiple laser peening and laser peening intensity used in laser shock peening (LSP) on the residual stress, micro-hardness and three-point bending fatigue performance of Ti–6Al–4V alloy. The multiple laser peening was accomplished by using the successive laser shocks at the same spot and the laser peening intensity was changed through changing the number of overlapped laser spots. The microstructure, which was characterized by highly tangled and dense dislocation arrangements due to high strain rate, can be found near the surface of the laser-peened specimen. By comparing with the as-received specimen, high micro-hardness and compressive residual stress were introduced at the surface of the laser-peened specimen. With increasing the number of overlapped laser spots, the fatigue life of the laser-peened specimen increased, reached a local maximum and then decreased. The specimen treated by using three overlapped laser spots exhibited the highest fatigue life. When the number of overlapped laser spots was kept to be three, the LSP treatments with one single laser shock and two successive laser shocks respectively provided a 22.2% and 41.7% increase in the fatigue strength as compared with the as-received specimens.

A numerical model of severe shot peening (SSP) to predict the generation of a nanostructured surface layer of material

Abstract: Generation of a surface layer of material characterized by grains with dimensions up to 100 nm by means of severe plastic deformation is one of the most interesting methods to improve the mechanical behaviour of materials and structural elements. Among the ways to obtain a surface layer with this characteristic, shot peening is one of the most promising processes, since it is applicable to very general geometries and to all metals and metal alloys without high-tech equipments. Notwithstanding the fact that the ability of shot peening to obtain nanostructured surfaces by using particular process parameters (mainly high impact energy and long exposure time) is proved, deep knowledge of the correct choice of quantitative values of process parameters and their relation to the grain size and the thickness and uniformity of the nanostructured layer is still lacking. In this paper a finite element model of severe shot peening (SSP) is developed with the aim of predicting the treatment conditions that lead to surface nanocrystallization. After having assessed the accuracy of the model as regards mesh parameters and constitutive law of the material, the results are discussed and interpreted in terms of induced residual stresses and surface work hardening. A method to assess the formation of nanostructured layer of materials based on the value of the equivalent plastic strain is developed. The comparison with experimental results allow to affirm that the model is a useful tool to predict the generation of a nanostructured surface layer by shot peening and to relate the peening parameters with the treated surface layer in terms of residual stresses, work hardening, and depth of the nanostructured layer.

The Effects of Laser Peening and Shot Peening on High Cycle Fatigue in 7050-T7451 Aluminum Alloy

Abstract: The high cycle fatigue performance of 7050-T7451 aluminum was investigated for untreated as-machined, laser peened, and shot peened conditions Constant amplitude, smooth (Kt = 1) fatigue tests were conducted in four-point bending at a stress ratio of R = 01 Results show that laser peening induces a layer of compressive residual stress more than three times deeper than for shot peening Both treatments significantly increase fatigue performance At a moderate level of stress, peened specimens outlast as-machined specimens, by a factor of 79 for laser peening and 29 for shot peening At higher stress, life improvements are lower, a factor of 33 for laser peening and 21 for shot peening At a 100,000-cycle lifetime, fatigue strength of laser peened specimens is 41% higher than as-machined specimens and the fatigue strength of shot peened specimens 30% higher than as-machined A form of pitting was noted on the laser peened surfaces and follow-on tests assessed the effects of the pitting on fatigue performance Results indicate that the pitting does not significantly influence fatigue performance

The effect of shot peening on fatigue and corrosion behavior of 316L stainless steel in Ringer's solution

Abstract: Stainless steel 316L is one of the most common biomaterials utilized for producing orthopedic implants. But it has low resistance to fatigue and wear. Therefore surface treatments such as shot peening are used to modify the surface properties. In the present research, the influence of shot peening treatment on hardness, fatigue and corrosion behavior of 316L stainless steel in Ringer's solution was investigated. For this purpose, the steel specimens were shot peened for 5, 10, 15, 20 and 25 min. Hardness, fatigue and electrochemical tests were performed on each specimen before and after shot peening treatment. The open circuit potential (OCP) of the specimens, after 2 h of equilibrium time, was measured in Ringer's solution for 300 s. The cyclic potentiodynamic polarization tests were performed with 5 mV/s scan rate. According to the results, the shot peening treatment increases the surface hardness and fatigue resistance. In addition, this treatment decreases the break-down potential of the passive layer and increases the corrosion current density in shot peened specimens up to 10 min, which shows a reduction in resistance to pitting corrosion. However, the break-down potential of the passive layer begins to increase and the corrosion current density decreases at upper times. This trend continues such that even the conditions of resistance to pitting corrosion improve in comparison with un-shot peened specimens at longer times of shot peening. The morphology of the fractured surfaces of samples was investigated by scanning electron microscopy (SEM).

An analytical approach to relate shot peening parameters to Almen intensity

Abstract: Shot peening is widely used in the automotive and aerospace industries to improve the fatigue strength of metal components by introducing near-surface plastic strains and compressive residual stresses. This mechanical treatment is primarily controlled by monitoring Almen (peening) intensity, which corresponds to the arc height at saturation of standardized test strips exposed to the shot stream. However, the same Almen intensity may be obtained by using small shots impacting the surface at high velocity or by using large shots impacting the surface at low velocity. This paper describes a model for predicting Almen intensity based on an analytical model for shot peening residual stresses. Theoretical results for different sets of peening parameters were consistent with published experimental results and revealed that although different combinations of shot peening parameters can produce the same Almen intensity, each combination resulted in a different through thickness residual stress distribution.

Plasma nitriding of AISI 304 austenitic stainless steel with pre-shot peening

Abstract: AISI 304 austenitic stainless steel was plasma nitrided at the temperature ranging from 410 to 520 °C with pre-shot peening. The structural phases, micro-hardness and electrochemical behavior of the nitrided layer were investigated by optical microscopy, X-ray diffraction, micro-hardness testing and anodic polarization testing. The effects of shot peening on the nitride formation, nitride layer growth and corrosion properties were discussed. The results showed that shot peening enhanced the nitrogen diffusion rate and led to a twice thicker nitrided layer than the un-shot peening samples under the same plasma nitriding conditions (410 °C, 4 h). The nitrided layer was composed of single nitrogen expanded austenite (S-phase) when nitriding below 480 °C, which had combined improvement in hardness and corrosion resistance.

Surface integrity and process mechanics of laser shock peening of novel biodegradable magnesium-calcium (Mg-Ca) alloy

Abstract: Current permanent metallic biomaterials of orthopedic implants, such as titanium, stainless steel, and cobalt–chromium alloys, have excellent corrosive properties and superior strengths However, their strengths are often too high resulting in a stress shielding effect that is detrimental to the bone healing process Without proper healing, costly and painful revision surgeries may be required The close Young’s modulus between magnesium-based implants and cancellous bones has the potential to minimize stress shielding while providing both biocompatibility and adequate mechanical properties The problem with Mg implants is how to control corrosion rates so that the degradation of Mg implants matches that of bone growth Laser shock peening (LSP) is an innovative surface treatment method to impart compressive residual stress to a novel Mg–Ca implant The high compressive residual stress has great potential to slow corrosion rates Therefore, LSP was initiated in this study to investigate surface topography and integrity produced by sequential peening a Mg–Ca alloy Also, a 3D semi-infinite simulation was developed to predict the topography and residual stress fields produced by sequential peening The dynamic mechanical behavior of the biomaterial was modeled using a user material subroutine from the internal state variable plasticity model The temporal and spatial peening pressure was modeled using a user load subroutine The simulated dent agrees with the measured dent topography in terms of profile and depth Sequential peening was found to increase the tensile pile up region which is critical to orthopedic applications The predicted residual stress profiles are also presented

Engineering the residual stress state and microstructure of stainless steel with mechanical surface treatments

Abstract: Four mechanical surface treatments have been considered for the application to austenitic stainless steel structures. Shot peening (SP), laser shock peening (LSP), ultrasonic impact treatment (UIT) and water jet cavitation peening (WJCP), also known as cavitation shotless peening (CSP), have been applied to 8 mm thick Type 304 austenitic stainless steel coupons. This study considers the merits of each of these mechanical surface treatments in terms of their effect on the surface roughness, microstructure, level of plastic work and through thickness residual stress distribution. Microstructural studies have revealed the formation of martensite close to the treated surface for each process. Residual stress measurements in the samples show compressive stresses to a significantly greater depth for the LSP, UIT and WJCP samples compared to the more conventional SP treated sample.

Warm Laser Shock Peening Driven Nanostructures and Their Effects on Fatigue Performance in Aluminum Alloy 6160

Abstract: Warm laser shock peening is an innovative manufacturing process that integrates laser shock peening and dynamic aging to improve materials' fatigue performance. Compared to traditional laser shock peening (LSP), warm laser shock peening (WLSP) - i.e., LSP at elevated temperatures - provides better performance in many aspects. WLSP can induce nanoscale precipitation and high density dislocation arrangement, resulting in higher surface strength and lower surface roughness than LSP, which are both beneficial for fatigue life improvement. Due to pinning of the dislocation structure by nanoscale precipitates - so-called dislocation pinning effects - the relaxation of residual stress and surfaces dislocation arrangement is significantly reduced. In this study, AA6061 alloy is used to evaluate the WLSP process. It is found that the fatigue life improvements after WLSP are not only caused by large compressive residual stress and high density dislocations but also by the higher stability of the residual stresses and surface strength during cyclic loading.

A direct comparison in the fatigue resistance enhanced by surface severe plastic deformation and shot peening in a C-2000 superalloy

Abstract: In this study, we present the first evidence and modeling efforts showing that surface severe plastic deformation (S2PD) can be more effective in producing metallic components with superior fatigue properties than shot peening (SP). With the aid of a wide battery of characterization techniques (i.e., X-ray diffractometry, optical microscopy, scanning electron microscopy, transmission electron microscopy, and 3-dimensional non-contact optical profilometry), micro- and nano-hardness testing, and finite element modeling, we have identified the underlying mechanism for the fatigue improvement. It is shown that the enhancement in the fatigue limit is derived from a nanocrystalline surface layer, a work-hardened surface region, and residual compressive stresses at the surface, all of which are introduced by S2PD and more substantial than that introduced by SP.

Process Control and Characterization of NiCr Coatings by HVOF-DJ2700 System: A Process Map Approach

Abstract: The concept of ‘process maps’ has been utilized to study the fundamentals of process-structure-property relationships in high velocity oxygen fuel (HVOF) sprayed coatings. Ni-20%Cr was chosen as a representative material for metallic alloys. In this paper, integrated experiments including diagnostic studies, splat collection, coating deposition, and property characterization were carried out in an effort to investigate the effects of fuel gas chemistry (fuel/oxygen ratio), total gas flow, and energy input on particle states: particle temperature (T) and velocity (V), coating formation dynamics, and properties. Coatings were deposited on an in situ curvature sensor to study residual stress evolution. The results were reconciled within the framework of process maps linking torch parameters with particle states (1st order map) and relating particle state with deposit properties (2nd order map). A strong influence of particle velocity on induced compressive stresses through peening effect is discussed. The complete tracking of the coating buildup history including particle state, residual stress evolution and deposition temperature, in addition to single splat analysis, allows the interpretation of resultant coating microstructures and properties and enables coating design with desired properties.

Mixed-Variable Optimization Strategy Employing Multifidelity Simulation and Surrogate Models

Abstract: A progressive simulation-based design optimization strategy is developed that can be applied to highly nonlinear impulse-type processes such as shot peening, laser peening, and bullet impacts on aircraft structural components. The design problems entail the use of multiple fidelities in simulation, time-consuming elastic-plastic analysis, and mixed types of optimization variables. An optimization strategy based on progressively increasing the complexity and fidelity is developed, along with suitable surrogate models. Multilevel fidelity models include axisymmetric, symmetric three-dimensional, and full-scale simulations to enable design optimization. The first two models are used to perform parametric studies and to localize the potential design space. This creates a reduced design space and an effective starting point for the subsequent optimization iterations, using the proposed modified particle swarm optimization for mixed variables. In the third step, the full-scale model is employed to find an optimum solution. The design methodology is demonstrated on laser peening of a structural component. Laser peening is a surface enhancement technique that induces compressive residual stresses at the peened surface by generating elastic-plastic deformation. These stresses improve the surface fatigue life. The parameters, pressure pulse and spot dimensions, impulse locations (all continuous), number of shots (integer), and location of shots (discrete) are the optimization variables with stress constraints.

Nucleation of highly dense nanoscale precipitates based on warm laser shock peening

Abstract: Warm laser shock peening (WLSP) is an innovative thermomechanical processing technique, which combines the advantages of laser shock peening (LSP) and dynamic aging (DA). It has been found that a unique microstructure with highly dense nanoscale precipitates surrounded by dense dislocation structures is generated by WLSP. In order to understand the nucleation mechanism of the highly dense precipitates during WLSP, aluminum alloy 6061 (AA6061) has been used by investigating the WLSP process with experiments and analytical modeling. An analytical model has been proposed to estimate the nucleation rate in metallic materials after WLSP. The effects of the processing temperature and high strain rate deformation on the activation energy of nucleation have been considered in this model. This model is based on the assumption that DA during WLSP can be assisted by the dense dislocation structures and warm temperature. The effects of the working temperature and dislocation density on the activation energy of precipitation have been investigated. This model is validated by a series of experiments and characterizations after WLSP. The relationships between the processing conditions, the nucleation density of precipitates and the defect density have been investigated.

Estimation of residual stress and its effects on the mechanical properties of detonation gun sprayed WC–Co coatings

Abstract: It is essential to analyze the residual stresses of thick coatings since they seriously affect coatings' performance during their service. It is also important to understand the mechanisms by which the stresses arise, to predict and control the stresses for improving coating properties. Because the Stoney formula is commonly used to relate stress to curvature for thin coatings, a new calculation formula was developed to estimate the residual stresses of thick coatings that represent a comprehensive stress state of the coated specimen. Based on the deduced formula and accurate curvature measurements, the residual stresses of detonation gun (D-Gun) sprayed WC-Co coatings with different thickness were obtained. With increasing the coating thickness, the residual stress changed gradually from the tensile nature to a compressive nature. Meanwhile, the coating was in an approximately stress-free state at the thickness of around 365 mu m. The analysis results emphasized the significance of peening stress in controlling the final stress state of the coated specimen, due to the high spraying velocity and kinetic energy during the D-Gun spraying process. Finally, the effects of residual stress on the mechanical properties of the coating were understood, namely, the compressive stress could significantly improve the coating properties, whereas the tensile stress impaired the coating properties. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.

Structure, microhardness and damping characteristics of Al matrix composite reinforced with AlCuFe or Ti using ultrasonic impact peening

Abstract: Ultrasonic impact peening (UIP) is used to modify the near-surface layers of cp aluminum. The effects of icosahedral quasicrystalline (QC) AlCuFe or hcp Ti fine powders added to the zone of severe plastic deformation at the UIP process on microstructure, phase composition, microhardness of near-surface layers and damping properties of aluminum are studied. The results show that composite layers, which are characterized by relatively uniform distribution of reinforcing particulates with similar volume fraction of about 0.17 are formed. While semi-coherent particulate/matrix interface is observed for QC reinforcements, the Ti particulates seem to be strongly adhered to the aluminum matrix due to formation of Ti 3 Al interlayer. While a dislocation-cell structure is formed after the UIP only, highly-misoriented fine grain structure with mean grain size of 0.1–0.5 μm is observed in the AlCuFe reinforced composite layer, and the Ti reinforced layer is characterized by mean grain size of 0.5–2 μm. Observed microsructural features predetermine significant enhancement of microhardness and damping properties of as-treated aluminum specimens. Much higher magnitudes of microhardness (about 1.3 GPa) and logarithmic decrement (about 12 × 10 − 4 ) are observed in Al specimens covered with the QC reinforced composite layer in comparison to those for specimens contained the Ti reinforced layer (about 1 GPa and 3.6 × 10 − 4 ) and to the as-peened aluminum specimen (0.58 GPa and 1.4 × 10 − 4 ). It is due to (i) the smallest grain size, (ii) semi-coherent particulate/matrix interface and (iii) high hardness and specific stiffness of the AlCuFe QC phase. Relatively high level of microhardness (about 1.1 GPa and 0.8 GPa) and logarithmic decrement (about 5 × 10 − 4 and 2 × 10 − 4 ) are conserved for Al specimens covered with the QC and Ti reinforced composite layers even after heating to 623 K.

Wear and Corrosion Behavior of HVOF-Sprayed WC-CoCr Coatings on Al Alloys

Abstract: WC-CoCr coatings were HVOF-sprayed onto an AA6082T6 substrate. Thickness values between 50 and 150 μm were produced by stepwise increase of the number of torch scans. This increase made the coatings not only thicker but also denser. This was due both to peening effects and by modifications to the splat formation mechanism, investigated by focused ion beam technique. Thanks to such densification, the hardness, the wear and impact resistance, and the corrosion protectiveness of the layers increased with the number of torch scans. The largest improvement occurred from 2 to 3 torch scans. These coatings were also compared to anodized films: cermets had superior wear and impact resistance but offered less corrosion protection.

Numerical Simulation of Residual Stress Relaxation in Shot Peened High-Strength Aluminum Alloys Under Reverse Bending Fatigue

Abstract: The mechanism of the residual stress relaxation during the fatigue life of shot peened high-strength aluminum alloys was investigated. Experiments were conducted on specimens subjected to three different shot peening treatments and tested under reverse bending fatigue. x-ray diffraction (XRD) measurements were carried out to determine the initial and stabilized residual stress fields. The residual stress field created by the surface treatments has been introduced into a finite element (FE) model by means of a fictitious temperature distribution. The elastic-plastic response of the superficial layers affected by the shot peening treatments has been derived through reverse strain axial testing combined with microhardness tests and implemented in the FE model. The proposed numerical/experimental approach is able to satisfactorily predict the residual stress field evolution. Notably, relaxation has been correctly simulated in the low-cycle fatigue regime and imputed to plastic flow in compression when the superposition of compressive residual and bending stresses exceeds the local cyclic yield strength of the material. Conversely the residual stress field remains stable at load levels corresponding to the 5 × 10 6 cycles fatigue endurance.

Investigation of shot-peened austenitic stainless steel 304L by means of magnetic Barkhausen noise

Abstract: Different shot peening conditions were applied to an austenitic stainless steel AISI 304L in order to transform austenite to martensite α′ at different depths. Magnetic Barkhausen noise measurements performed on this steel reveal a correlation between the strength of the signal and the depth of the treatment. The combined effect of the volume fraction of martensite and the residual stress in martensite determined using X-ray diffraction analysis were found to be responsible for the evolution of the Barkhausen noise response. Using tensile plastic deformation, the residual stress in martensite was changed, giving rise to a strong increase of the Barkhausen noise activity. This variation was correlated to a modification of the sign and amplitude of the residual stress in the martensite phase. Directional measurements of the Barkhausen noise revealed the anisotropy of the residual stresses induced by the tensile plastic deformation. It is concluded that the Barkhausen noise activity recording could lead to the determination of the residual stresses in martensite induced by shot peening processes.

Damage tolerant functionally graded WC–Co/Stainless Steel HVOF coatings

Abstract: In this paper, effective damage tolerance of a functionally graded coating (FGC) deposited by high velocity oxygen fuel (HVOF) spraying is observed. The thick FGC (≈ 1.2 mm) consists of 6 layers with a stepwise change in composition from 100 vol.% ductile AISI316 stainless steel (bottom layer) to 100 vol.% hard WC–12Co (top layer) deposited onto an AISI316 stainless steel substrate. Damage tolerance is observed via 1) an increase in compliance with depth, and 2) an increase in fracture resistance by containment, arrest and deflection of cracks. A smooth gradation in the composition and hardness through the coating thickness is found by scanning electron microscopy and depth-sensing microindentation, respectively. The in-situ curvature measurement technique reveals that during the deposition of the FGC, compressive stresses exist in the lower, metallic layers owing to peening effect of successive impact, and these gradually evolve to high tensile, in the top layers. Tensile stresses appear to be due to quenching alone; thermal stresses are low because of the gradation. All of this is beneficial for the deposition of a thick coating. The FGC structure shows the ability to reduce cracking with increased compliance in the top layer during static and dynamic normal contact loading, while retaining excellent sliding wear resistance (ball-on-disk tests). Results are discussed in comparison to the behavior and properties of coatings of similar individual compositions and thicknesses, as well as a thick monolithic WC–12Co sprayed coating. Further improvements in the processing are proposed to enhance the adhesion strength and avoid coating delamination under high load contact-fatigue conditions.