Showing papers on "Peening published in 2003"

On the influence of mechanical surface treatments—deep rolling and laser shock peening—on the fatigue behavior of Ti–6Al–4V at ambient and elevated temperatures

Abstract: It is well known that mechanical surface treatments, such as deep rolling, shot peening and laser shock peening, can significantly improve the fatigue behavior of highly-stressed metallic components. Deep rolling (DR) is particularly attractive since it is possible to generate, near the surface, deep compressive residual stresses and work hardened layers while retaining a relatively smooth surface finish. In the present investigation, the effect of DR on the low-cycle fatigue (LCF) and high-cycle fatigue (HCF) behavior of a Ti–6Al–4V alloy is examined, with particular emphasis on the thermal and mechanical stability of the residual stress states and the near-surface microstructures. Preliminary results on laser shock peened Ti–6Al–4V are also presented for comparison. Particular emphasis is devoted to the question of whether such surface treatments are effective for improving the fatigue properties at elevated temperatures up to ∼450 °C, i.e. at a homologous temperature of ∼0.4 T/T m (where T m is the melting temperature). Based on cyclic deformation and stress/life ( S / N ) fatigue behavior, together with the X-ray diffraction and in situ transmission electron microscopy (TEM) observations of the microstructure, it was found that deep rolling can be quite effective in retarding the initiation and initial propagation of fatigue cracks in Ti–6Al–4V at such higher temperatures, despite the almost complete relaxation of the near-surface residual stresses. In the absence of such stresses, it is shown that the near-surface microstructures, which in Ti–6Al–4V consist of a layer of work hardened nanoscale grains, play a critical role in the enhancement of fatigue life by mechanical surface treatment.

Nanocrystallization of Steels by Severe Plastic Deformation

Abstract: The formation of nanocrystalline structure (NS) in steels by various severe plastic deformation processes, such as ball milling, a ball drop test, particle impact deformation and air blast shot peening are demonstrated. Layered or equiaxed nanograined region appeared near the specimen surface and dislocated cell structured region appeared interior of specimens. These regions are separated with clearly defined boundaries. The deformation induced nanograined regions have the following common specific characteristics: 1) with grains smaller than 100 nm and low dislocation density interior of grains, 2) extremely high hardness, 3) dissolution of cementite when it exist and 4) no recrystallization and slow grain growth by annealing. The deformation conditions to produce NS was discussed based on the available data in literatures. It was suggested that the most important condition is to impose a strain larger than about 7. High strain rates, low deformation temperature, multidirectional deformation, hydrostatic pressure are considered to be favorable conditions to produce NS. Introducing alloying elements, precipitates and second phase also enhance nanocrystallization by suppressing recovery. The mechanisms of the formation of sharply defined boundaries which separate nanograined structure region from dislocated cell structured region were discussed with respect to impurities, martensitic transformation and deformation. It was suggested several mechanisms may operate simultaneously in the formation of the clear boundaries.

Three-dimensional Dynamic Finite Element Analysis of Multiple Laser Shock Peening Processes

Abstract: Laser shock peening (LSP) is an innovative surface treatment technique successfully applied to improving fatigue performance of metallic materials. The fatigue strength and fatigue life of the laser peened material can be significantly improved by deep compressive residual stresses being introduced into the material. The compressive residual stress distribution along the depth of the material is attributed to a high amplitude stress wave induced by a high energy laser pulse. The present paper describes a finite element method for simulating the residual stress distribution in a metal alloy 35CD4 50 HRC steel in single and multiple LSP processes. The process used a three-dimensional dynamic finite element model impacted by a square laser spot. The predicted results for single LSP were well correlated with the available experimental data. Meanwhile, the effects of multiple LSP processes, pressure magnitude and duration, and laser spot sizes on the compressive stress field in the metal alloy were eva...

FEM simulation of residual stresses induced by laser Peening

Abstract: Benefits from laser Peening have been demonstrated several times in fields like fatigue, wear or stress corrosion cracking. However, in spite of recent work on the calculation of residual stresses, very few authors have considered a finite element method (FEM) approach to predict laser-induced mechanical effect. This comes mainly from the high strain rates involved during LP (10 6 s -1 ), that necessitate the precise determination of dynamic properties, and also from the possible combination of thermal and mechanical loadings in the case of LP without protective coatings. In this paper, we aim at presenting a global approach of the problem, starting from the determination of loading conditions and dynamic yield strengths, to finish with FEM calculation of residual stress fields induced on a 12% Cr martensitic stainless steel and a 7075 aluminium alloy.

Effect of controlled shot peening and laser shock peening on the fatigue performance of 2024-T351 aluminum alloy

Abstract: The influence of shot peening, laser shock peening, and dual (shot and laser peening) treatment on the fatigue behavior of 2024-T351 was investigated. Tests showed a fatigue life improvement in all three cases with laser shock peening and dual treatment displaying fatigue performance superior to shot peening. Fractographic analysis showed that the relatively poor performance of the shot peening is caused by ductility loss.

FEM Simulation of Two Sided Laser Shock Peening of Thin Sections of Ti-6Al-4V Alloy

Abstract: Laser shock peening (LSP) is a fatigue enhancement surface treatment for metallic materials. The key beneficial characteristic of LSP is the presence of compressive residual stresses mechanically produced by shock waves within the metallic materials, which can significantly improve their fatigue life and fatigue strength. During LSP, a laser beam can be directed to impact on either one side or two sides of a target. Generally, when treating a thin section, two sided peening is used to avoid harmful effects such as spalling and fracture, which may occur when only one side of a thin section is laser peened. The present work focuses on applying the finite element method to predict the distribution of residual stresses along the surface and depth of thin sections of a Ti-6Al-4V alloy in single and multiple LSP. In order to understand the effects of the target geometry on the distribution of residual stresses, the residual stress profiles corresponding to various target thicknesses were carefully evalu...

The effects of process variations on residual stress in laser peened 7049 T73 aluminum alloy

Abstract: This paper reports measurements of the distribution of residual stress with depth from the surface in laser peened coupons made of a high-strength aluminum alloy. Residual stresses were measured using slitting (also known as the crack compliance method). Measurements were made on several coupons to: compare laser peening (LP) and shot peening (SP) residual stresses; ascertain the influence of LP parameters on residual stress; determine whether tensile residual stress existed outside the peened area; assess the variation of residual stress with in-plane position relative to the layout of the laser spots used for peening; and, determine the importance of a uniform spatial distribution of laser energy within the spot. Residual stress 0.1 mm from the surface due to LP and SP were comparable and the depth of the compressive stress for LP was far greater than for SP. Variations of most LP parameters did not significantly alter residual stress at shallow depths, but greater laser energy and larger layer overlap increased residual stress at depths between 0.2 and 0.6 mm from the surface. Residual stresses adjacent to the peened area were found to be compressive. Decreased levels of surface residual stress were found when laser spots had a non-uniform distribution of laser intensity.

Case Studies of Fatigue Life Improvement Using Low Plasticity Burnishing in Gas Turbine Engine Applications

Abstract: Surface enhancement technologies such as shot peening, laser shock peening (LSP), and low plasticity burnishing (LPB) can provide substantial fatigue life improvement. However, to be effective, the compressive residual stresses that increase fatigue strength must be retained in service. For successful integration into turbine design, the process must be affordable and compatible with the manufacturing environment. LPB provides thermally stable compression of comparable magnitude and even greater depth than other methods, and can be performed in conventional machine shop environments on CNC machine tools. LPB provides a means to extend the fatigue lives of both new and legacy aircraft engines and ground-based turbines. Improving fatigue performance by introducing deep stable layers of compressive residual stress avoids the generally cost prohibitive alternative of modifying either material or design. The X-ray diffraction based background studies of thermal and mechanical stability of surface enhancement techniques are briefly reviewed, demonstrating the importance of minimizing cold work. The LPB process, tooling, and control systems are described. An overview of current research programs conducted for engine OEMs and the military to apply LPB to a variety of engine and aging aircraft components are presented. Fatigue performance and residual stress data developed to date for several case studies are presented including: * The effect of LPB on the fatigue performance of the nickel based super alloy IN718, showing fatigue benefit of thermal stability at engine temperatures. * An order of magnitude improvement in damage tolerance of LPB processed Ti-6-4 fan blade leading edges. * Elimination of the fretting fatigue debit for Ti-6-4 with prior LPB. * Corrosion fatigue mitigation with LPB in Carpenter 450 steel. *Damage tolerance improvement in 17-4PH steel. Where appropriate, the performance of LPB is compared to conventional shot peening after exposure to engine operating temperatures.

Automated positioning of mobile laser peening head

Abstract: The present invention enables the processing head to locate itself precisely on the surface of the structure being processed, and to then reposition itself correctly for the next laser spot. Further, the present invention will complete processing a laser peened area, the area including a multiplicity of spots arranged in a specific pattern, and correctly laser peen each spot in the area under control of a controller including control linkages with the laser. The invention further provides an automated laser peening processing head encompassing spatial position sensing and locating means, as well as programmed spatial positioning, application of overlay materials, verification of proper overlay condition and positioning, and notification of the laser to pulse the surface of the structure.

Effect of surface properties on high cycle fatigue behaviour of shot peened ductile steel

Abstract: Experimental investigations into shot peened ductile steel have been carried out, applying three surface finishing conditions: as machined, standard shot peening using 100% coverage and severe shot peening with 1000% coverage (high exposure time). The properties of the shot peened surfaces were examined and characterised, and specimens were then submitted to three point bending tests. The fatigue limit was determined for each case. In this way, the dependence of fatigue behaviour on initial surface finishing properties was determined, and a relationship is suggested to describe and correlate fatigue limits with initial surface properties. A phenomenological approach is proposed to characterise and to correlate qualitatively and quantitatively the influence of local shot peened surface properties on fatigue limit of treated specimens. The Crossland multiaxial failure high cycle fatigue criterion is used in this approach to model the influence of each surface property.

The effect of excimer laser surface treatment on the pitting corrosion fatigue behaviour of aluminium alloy 7075

Abstract: An excimer laser (KrF) operating at a wavelength of 248 nm was used to modify the surface microstructure of 7075-T651 aluminium alloy. The aim was to improve both the corrosion resistance and the pitting corrosion fatigue resistance of the alloy by means of laser surface melting (LSM). The microstructure and the phases of the modified surface structure were analysed, and the corrosion behaviour of the untreated and the laser-treated specimens were evaluated by immersion test. The fatigue resistance of the 7075 alloy has been presented in the form of S/N curves. A microscopical examination and the transmission electron microscopy (TEM) study revealed that LSM caused a reduction both in number and size of constituent particles and a refinement of the grain structure within the laser melted zone. As a result, the corrosion resistance of the aluminium alloy was improved. There was a significant reduction in the number of corrosion pits and shallow attack occurred. The fatigue test results showed that under dry fatigue conditions, the total fatigue life of the laser treated specimens, in which the crack initiation period is of considerable significance, was lower than that of the untreated specimens. However, after shot peening, the fatigue life of the laser treated specimens was recovered. This was primarily attributed to the elimination of surface defects, but also be in part, due to the introduction of compressive residual stresses in the surface layer of the specimen. The fatigue resistance of the shot peened laser-treated specimens, tested in 3.5 wt% NaCl solution with 48 hrs prior immersion, was greater than the untreated specimens with an increase of two orders of magnitude in fatigue life. This was primarily due to the elimination of surface defects and the reduction of corrosion pits.

Residual stress distribution on surface-treated Ti-6Al-4V by X-ray diffraction

Abstract: The x-ray diffraction technique has been used to measure surface residual stress in Ti-6Al-4V samples subjected to shot peening (SP), laser shock peening (LSP) and low plasticity burnishing (LPB). The magnitude, spatial and directional dependence and uniformity of the surface residual stresses have been investigated. The results show that residual stresses due to SP are uniform and independent of direction. LSP has been observed to produce non-uniform residual stress varying from one region to another, and also within a single laser shock. In the case of LPB, residual stresses have uniform spatial distribution but have been observed to be direction-dependent. Various components of the residual stress tensor in the LPB sample have been determined following the Dolle-Hauk method. The results of the residual stress due to three surface treatments are compared, and possible reasons for spatial and directional dependence are discussed.

Mechanical Surface Treatment Technologies for Gas Turbine Engine Components

Abstract: Various mechanical surface treatment technologies have been invented and developed to induce a protective layer of compressive residual stress at the surface of gas turbine engine components where the operating loads are tensile dominated. The benefits of these surface treatments are to prevent crack initiation, to retard propagation of small cracks, and even to resist corrosion and wear damage. In this paper the literature on these technologies and their effects on component reliability and durability is reviewed, with an emphasis on shot peening (SP), laser shock peening (LSP), and low plasticity burnishing (LPB). The relative advantages of these three surface treatment technologies, as well as their limitations, are identified and evaluated. The most important issues for these three technologies, and the relative merits of the resultant residual stress fields, are presented and discussed with a view to enhancing gas turbine engine component operating lives.

Monitored laser shock peening

Abstract: A pulse laser is configured for projecting a pulsed laser beam at a target site on a fluid film atop a workpiece for laser shock peening the workpiece. The fluid film is monitored by a probe laser which projects a probe laser beam at the target site, and an optical detector which detects reflection of the probe beam from the target site. The pulse laser is coordinated by the detector in order to emit the pulsed beam in response to the condition of the monitored film.

Laser peening method and apparatus using tailored laser beam spot sizes

Abstract: A laser shock processing treatment enables a selectively adjustable and customized compressive residual stress distribution profile to be developed within a workpiece by tailoring the size and shape of the laser beam spots One peening operation applies to the workpiece a first pattern having relatively large laser beam spots and then applies a second pattern having relatively small laser beam spots The composite use of such small and large beam spots enables the stress distribution profile to be tailored to the part specifications The large beam spots maximize the depth of compressive residual stress in the part, while the small beam spots optimize the surface compressive residual stresses of the part The use of small spot beam patterns allows untreated or improperly processed areas to be laser peened

Enhanced Shot Peening Effect for Steels by Using Fe-based Glassy Alloy Shots

Abstract: The use of Fe-Co-Ni-Mo-B-Si glassy alloy balls as peening shots was found to cause a significantly enhanced shot peening effect for steel sheets, i.e., the increase in the thickness of the shot region, higher hardness and higher compressive residual stress in the shot region, and the generation of much distinct crater-like pattern on the shot surface in comparison with those for conventional cast steel shots and high speed steel shots. It was also found that the endurance life time of the glassy alloy shots is 8 to 10 times longer than those for the conventional crystalline alloy shots. The enhanced effect was interpreted to originate from unique mechanical properties of the Fe-based glassy alloy shots such as lower Young’s modulus, larger elastic elongation limit and higher tensile strength which cannot be obtained for the conventional crystalline steel shots. The finding of the effectiveness of the Fe-based glassy alloy as peening shots is promising for future applications.

Peen conditioning of titanium metal wood golf club heads

Abstract: A golf club head is provided with a residual compressive stress layer on the inside surface of the club face using a peening treatment. The peening treatment also may remove material from the club head, such as unwanted alpha case on a titanium club head. The body of the club head further may be subjected to a peen treatment.

Residual Stress Induced by Waterjet Peening: A Finite Element Analysis

Abstract: The concept of multiple droplet impacts resulting from ultra high-pressure waterjet (UHPWJ) was used to develop a mathematical model to describe the effect of interfacial pressure on the underlying workpiece material. A non-linear elastic-plastic finite element analysis (FEA) was carried out in this study using the interfacial pressure model to predict residual compressive stresses. This three-dimensional FEA model was based on quasi-static considerations to provide prediction of both magnitude and depth of residual stress fields in a 7075-T6 aluminum alloy (A17075-T6). Results of the FEA modeling were in good agreement with experimental measurements. Effects of applied pressures on the residual stress fields are also presented and discussed as a method of estimating high-pressure waterjet induced compressive stresses under varying process conditions for peening.Copyright © 2003 by ASME

Surface Roughness Influence on Eddy Current Electrical Conductivity Measurements

Abstract: : The measurement of eddy current conductivity, in view of its frequency dependent penetration depth, has been suggested as a possible means to allow the nondestructive testing (NDT) of subsurface residual stresses in shot peened specimens. This paper addresses the apparent reduction of the near surface electrical conductivity measured by the eddy current method in the presence of surface roughness. Experimental results are presented on shot peened pure (C11000) copper, in which the effect is particularly strong and readily measurable because of the low penetration depth caused by the very high electric conductivity of the material. Eight shot peened samples between almen intensities 2 and 16 were thermally treated and tested by X-ray diffraction measurements until the residual stress and cold work fully dissipated, leaving only the surface roughness. Eddy current electrical conductivity measurements were carried out on each fully relaxed shot peened copper specimen over a wide frequency range from 1 kHz to 10 MHz. Our results show that surface roughness, acting alone, causes a srong apparent reduction of up to 10 to 20% of the measured electrical conductivity in shot peened copper. These results can be used to estimate the much smaller effect of surface roughness on the testing of shot peened turbine engine materials, which typically have approximately 100 times lower conductivity than pure copper and therefore exhibit 10 times higher penetration depth at the same test frequency. A comparison of the predicted underestimation of the electrical conductivity in the presence of surface roughness to the expected less than 1% conductivity increase due to compressive near surface residual stresses indicates that the described artifact can significantly affect eddy current testing of shot peened turbine engine alloys, such as Ti-6A1-4V or IN100.

Effects of shot peening on fatigue properties of 0Cr13Ni8Mo2Al steel

Abstract: The influence of shot peening on the fatigue properties of 0Cr13Ni8Mo2Al steel has been studied. Changes in surface roughness, surface topography and residual compressive stress field were determined by experiments. The experimental results show that shot peening improves the fatigue property and the fatigue crack sources are pushed to the region beneath the hardened layer. Low Almen intensities should be used when 0Cr13Ni8Mo2Al steel is shot peened because of its sensitiveness to the surface roughness.

Cavitation Shotless Peening for surface modification of alloy tool steel

Abstract: Cavitation Shotless Peening (CSP) is a new method of surface modification. Cavitation impacts induced by the collapse of cavitation bubbles produce compressive residual stress and work hardens the material surface. In the case of CSP, shots are not required and therefore we call it Cavitation Shotless Peening. In CSP, cavitation is induced by a submerged high-speed water jet, i. e., a cavitating jet, for which the intensity and region of cavitation impact can be controlled by parameters such as the upstream pressure and nozzle size. The authors have already shown that the lifetime of forging die treated by CSP can be extended by about 50% compared with non-peened forging die. In this paper, in order to make clear the mechanism by which the lifetime of forging die is increased, an alloy tool steel (JIS SKD61) was tested both in peened and non-peened conditions. Compressive residual stress was measured by an X-ray diffraction method. It was evident from a comparison between the non-peened specimen and the cavitation shotless peened specimen that CSP improved the mechanical properties of the forging die thus giving it a longer lifetime.

Real time laser shock peening quality assurance by natural frequency analysis

Abstract: A real time method for quality control testing of a laser shock peening process of production workpieces by analysis of natural frequency shifts during the laser shock peening process. One particular embodiment includes laser shock peening surface of the production workpiece by firing a plurality of laser beam pulses on the surface and forming a plurality of corresponding plasmas, each one of the plasmas pulses having a duration in which the plasma causes a region having deep compressive residual stresses to form beneath the surface, measuring at least one natural frequency of the workpiece for each of the laser beam pulses, calculating natural frequency shifts from a baseline natural frequency for the measured natural frequencies for at least a portion of the laser beam pulses, and using the natural frequency shifts for accepting or rejecting the workpiece with respect to pass or fail criteria.

Measurement of Thickness-Average Residual Stress Near the Edge of a Thin Laser Peened Strip

Abstract: This paper presents measurements of the thickness-average residual stress imposed by laser peening near the edge of a thin sheet of a common titanium alloy. The slitting (or crack compliance) method was extended beyond its typical form to make residual stress measurements in this geometry. An analytical method was employed to optimize the experiment design for the near-edge, thin material geometry, where the design included the optimal number and position of strain gages and the most effective set of basis functions for stress computation. Two experiments were performed on a titanium strip that had been laser peened near the edge, using an optimal experiment design. Residual stress was found to be large and compressive near the edge of the sheet, with the compressive stress extending over 38% of the laser peened area.

Method and apparatus for abrasive fluid jet peening surface treatment

Abstract: An abrasive water treatment method and apparatus includes supporting a metal workpiece on a workpiece support and arranging a nozzle above a target surface of the workpiece so that the nozzle is pointed towards the target surface of the workpiece. A pressurized fluid having entrained abrasive particles is then generated and discharged through the nozzle and toward the target surface of the workpiece. The nozzle is located a texturing standoff distance from the target surface such that the periphery of the pressurized fluid stream discharged from the nozzle expands after being discharged from the nozzle and prior to impinging upon the target surface of the workpiece. As a result, a textured surface is created on the workpiece.