Showing papers on "Peening published in 2007"

A literature survey on the stability and significance of residual stresses during fatigue

Abstract: Many manufacturing processes can induce residual stresses in components. These residual stresses influence the mean stress during cyclic loading and so can influence the fatigue life. However, the initial residual stresses induced during manufacturing may not remain stable during the fatigue life. This paper provides a broad and extensive literature survey addressing the stability of surface and near-surface residual stress fields during fatigue, including redistribution and relaxation due to static mechanical load, repeated cyclic loads, thermal exposure and crack extension. The implications of the initial and evolving residual stress state for fatigue behaviour and life prediction are addressed, with special attention to fatigue crack growth. This survey is not a critical analysis; no detailed attempt is made to evaluate the relative merits of the different explanations and models proposed, to propose new explanations or models or to provide quantitative conclusions. Primary attention is given to the residual stresses resulting from four major classes of manufacturing operations: shot peening and related surface treatments, cold expansion of holes, welding and machining.

Effect of structure evolution induced by ultrasonic peening on the corrosion behavior of AISI-321 stainless steel

Abstract: A nanocrystalline surface layer was produced on an AISI-321 stainless steel by severe plastic deformation via ultrasonic peening (UP). The microstructural evolution of the surface layer was characterized by means of X-ray diffraction (XRD) analysis and transmission electron microscopy (TEM). The volume fraction of strain-induced α-martensite as a function of the effective strain ( e ¯ ) was evaluated quantitatively using XRD and magnetic measurements. Considering the e ¯ magnitudes and the TEM data obtained, it is concluded that a grain refinement of austenitic structure passes ahead of the α-martensite formation, particularly in the top surface layer. The nanocrystalline austenitic grain structure (mean grain size ∼ 15 nm) was observed at e ¯ = 0.45 , while the startup of the strain-induced martensitic transformation was revealed at the strain extent of 0.62. The nanostructured surface layer formed after straining to e ¯ = 0.8 already contains mainly the martensite nanograins characterized by an average size of about 10 nm. Grain size increased gradually up to 60 nm within the layer containing both austenite and martensite phases at a depth of about 30 μm from the treated surface. Both the microhardness behavior of the stainless steel surface and its corrosion performance in 3.5% NaCl solution can be enhanced by the UP. They are shown to be in correlation with: (i) the grain refinement process and (ii) the increase in the volume fraction of strain-induced α-martensite.

FEM calculation of residual stresses induced by laser shock processing in stainless steels

Abstract: Laser shock processing, also known as laser shock peening, generates through a laser-induced plasma, plastic deformation and compressive residual stresses in materials for improved fatigue or stress corrosion cracking resistances. The calculation of mechanical effects is rather complex, due to the severity of the pressure loading imparted in a very short time period (in the ns regime). This produces very high strain rates (106 s−1), which necessitate a precise determination of dynamic properties.Finite element techniques have been applied to predict the residual stress fields induced in two different stainless steels, combining shock wave hydrodynamics and strain rate dependent mechanical behaviour. The predicted residual stress fields for single or multiple laser processes were correlated with those from experimental data, with a specific focus on the influence of process parameters such as pressure pulse amplitude and duration, laser spot size or sacrificial overlay.Among other results, simulations confirmed that the affected depths increased with pulse duration, peak pressure and cyclic deformations, thus reaching much deeper layers (> 0.5 mm) than with any other conventional surface processing. To improve simulations, the use of experimental VISAR determinations to determine pressure loadings and elastic limits under shock conditions (revealing different strain-rate dependences for the two stainless steels considered) was shown to be a key point.Finally, the influence of protective coatings and, more precisely, the simulation of a thermo-mechanical uncoated laser shock processing were addressed and successfully compared with experiments, exhibiting a large tensile surface stress peak affecting a few tenths of micrometres and a compressive sub-surface stress field.

Comparison of the fatigue behavior and residual stress stability of laser-shock peened and deep rolled austenitic stainless steel AISI 304 in the temperature range 25–600 °C

Abstract: Laser-shock peening and deep rolling were applied to an austenitic stainless steel AISI 304 and the effects on the fatigue life were investigated. Isothermal push–pull fatigue tests were performed in the temperature range 25–600 °C. The results of the investigations, e.g. s/n curves, cyclic deformation curves as well as the residual stress stability of the laser-shock peened condition were compared to untreated and deep rolled conditions. Near-surface regions were characterized using X-ray diffraction methods, transmission electron microscopy (TEM) and focused ion beam microscopy (FIB). The different near-surface microstructures as well as residual stress stabilities of laser-shock peened and deep rolled specimens were investigated and discussed.

Comparison between shot peening and surface nanocrystallization and hardening processes

Abstract: The surface nanocrystallization and hardening (SNH) is a relatively new process that has been developed to enhance fatigue and wear resistances. The SNH is similar to widely used shot peening (SP) in the sense that both processes entail repeated impacts of the work-piece surface with spheres. The difference between them lies in the sizes of spheres and the impact velocities used. Such a difference results in dramatic changes in kinetic energies and thus the thicknesses of the work-hardened layer and the nano-grained surface layer. In this study, finite element modeling is performed to provide quantitative description of these differences. The results show that the kinetic energy in the SNH process is typically 180 times larger than that in shot peening, and the deformation layer in the SNH process is about 10 times thicker than that generated in shot peening. Furthermore, the maximum plastic strain and the maximum residual compressive stresses in the SNH-processed work-piece are 100 and 10 times larger than those in the shot-peened work-piece, respectively. The implication of these differences on fatigue resistance has been discussed.

Effect of surface work hardening on wear behavior of Hadfield steel

Abstract: Shot peening has become an effective method to strengthen alloy. In this investigation, it has been used to work-harden Hadfield steel. It has been regarded that the surface hardness of Hadfield steel has been increased greatly after shot peening. Using electron scanning microscope (SEM), high resolution transmission electron microscope (HRTEM) and X-ray diffraction (XRD) analysis, the microstructure of shot penned surface layer was examined. It has been observed that a nanocrystalline surface layer is formed. The grain sizes of surface were decreased to 11.1–17.4 nm and the maximum hardened layer can reach to 100 μm after the treatment. Surface hardness was also increased with increasing shot peening period. Two-body and three-body abrasive wear experiments were carried out for work hardening and original specimens, separately. The results showed that the three-body wear resistance of the nanocrystallized Hadfield steel has distinctly been improved using soft abrasive particles. For harder abrasive particles like emery paper, the increased hardness and grain refinement by shot peening cannot improve two-body abrasive wear resistance.

Coating residual stress effects on fatigue performance of 7050-T7451 aluminum alloy

Abstract: The tendency of the aircraft industry is to enhance customer value by improving performance and reducing environmental impact. In view of availability, aluminum alloys have a historically tendency to faster insertion due to their lower manufacturing and operated production infrastructure. In landing gear components, wear and corrosion control of many components is accomplished by surface treatments of chrome electroplating on steel or anodizing of aluminum. One of the most interesting environmentally safer and cleaner alternatives for the replacement of hard chrome plating or anodizing is tungsten carbide thermal spray coating, applied by the high velocity oxy fuel (HVOF) process. However, it was observed that residual stresses originating from these coatings reduce the fatigue strength of a component. An effective process as shot peening treatment, considered to improve the fatigue strength, pushes the crack sources beneath the surface in most of medium and high cycle cases, due to the compressive residual stress field induced. The objective of this research is to evaluate a tungsten carbide cobalt (WC–Co) coating applied by the high velocity oxy fuel (HVOF) process, used to replace anodizing. Anodic films were grown on 7050-T7451 aluminum alloy by sulfuric acid anodizing, chromic acid anodizing and hard anodizing. The influence on axial fatigue strength of anodic films grown on the aluminum alloy surface is to degrade the stress-life performance of the base material. Three groups of specimens were prepared and tested in axial fatigue to obtain S–N curves: base material, base material coated by HVOF and base material shot peened and coated. Experimental results revealed increase in the fatigue strength of Al 7050-T7451 alloy associated with the WC 17% Co coating. On the other hand, a reduction in fatigue life occurred in the shot peened and coated condition. Scanning electron microscopy technique and optical microscopy were used to observe crack origin sites, thickness and coating/substrate adhesion.

Microstructure and mechanical property characterizations of metal foil after microscale laser dynamic forming

Abstract: This article discusses the feasibility of a new microforming technique—laser dynamic forming (LDF). LDF is a new hybrid forming process, combining the advantages of laser shock peening, and metal forming, with an ultra high strain rate forming utilizing laser shock waves. Experiments are conducted on copper foils to demonstrate this forming process. After the forming process, the mechanical and microstructure of the formed work piece will be characterized. Electron backscatter diffraction will be used to investigate the grain microstructure and misorientations quantitatively. The residual stress distributions will be measured using x-ray diffraction. The key factors for the improved formability of this high strain rate microforming process will be discussed in detail. With further development, LDF may become an important microforming technology for various materials.

Formation of a nanocrystalline surface layer on steels by air blast shot peening

Abstract: A nanostructured surface layer was fabricated on two kinds of steels by means of air blast shot peening. The nanolayer shows a sharp boundary to the underlying work-hardened area and good thermal stability up to 873 K. It has much higher hardness than the work hardened region in both the as-treated and annealed states. When using small shot sizes, the nano area can be formed in very short treatment times, and the thickness and continuity of the nanolayer is enhanced. On the contrary, the nanocrystalline region is more difficult to synthesize when using large shot particles, even though the deformed area is much thicker. The effect of particle diameter is attributed to the different collision time and different strain rate of the treated materials.

Method of repairing nickel-based alloy articles

Abstract: Methods for repairing nickel based alloy articles such as gas turbine rotors generally includes a removing a damaged portion of the articles and laser cladding a high temperature nickel based alloy powder thereto to form a solid layer. The process can be repeated until a desired thickness is obtained. Optionally, a peening process subsequent to laser cladding can be implemented to introduce compressive stress to the solid layer formed by laser cladding.

Development and Validation of Novel FE Models for 3D Analysis of Peening of Strain-Rate Sensitive Materials

Abstract: In this paper, we provide two different symmetry cells to describe the shot-peening process In this multiple impingement model, we study the dynamic behavior of TI-6Al-4V targets subjected to a large number of shots Three-dimensional elastoplastic finite element analysis (FEA) of the process was conducted using these two symmetry cells for strain-rate sensitive targets and rigid shots The basic symmetry cell is assigned a target surface area CC, where C is one half of separation distance between adjacent shots The second “enhanced” symmetry cell is assigned a target surface area 2C2C thus allowing higher density of impact point locations Average residual stresses inside the target predicted by FEA were compared with experimental measurements using the holedrilling technique In order to do this, a new averaged technique was developed to obtain the stress distribution inside the symmetry cell The results reveal that both symmetry cell models could be used for shot-peening modeling However, the use of the enhanced symmetry cell leads to a better agreement with the measured residual stresses In addition, the enhanced symmetry cell model allowed us to overcome some of the shortcomings of the basic symmetry cell for cases involving high peening velocity and intensity DOI: 101115/12712469

Laser Peening and Shot Peening Effects on Fatigue Life and Surface Roughness of Friction Stir Welded 7075-T7351 Aluminum

Abstract: The effects of laser peening, shot peening, and a combination of both on the fatigue life of Friction Stir Welds (FSW) was investigated. The fatigue samples consisted of dog bone specimens and the loading was applied in a direction perpendicular to the weld direction. Several laser peening conditions with different intensities, durations, and peening order were tested to obtain the optimum peening parameters. The surface roughness resulting from various peening techniques was assessed and characterized. The results indicate a significant increase in fatigue life using laser peening compared to shot peened versus their native welded specimens.

High-frequency eddy current conductivity spectroscopy for residual stress profiling in surface-treated nickel-base superalloys

Abstract: Recent research results indicated that eddy current conductivity measurements can be exploited for nondestructive evaluation of subsurface residual stresses in surface-treated nickel-base superalloy components. Most of the previous experimental studies were conducted on highly peened (Almen 10-16A) specimens that exhibited harmful cold work in excess of 30% plastic strain. Such high level of cold work causes thermo-mechanical relaxation at relatively modest operational temperatures; therefore the obtained results were not directly relevant to engine manufacturers and end users. The main reason for choosing peening intensities in excess of recommended normal levels was that in low-conductivity engine alloys the eddy current penetration depth could not be forced below 0.2 mm without expanding the measurements above 10 MHz which is beyond the operational range of most commercial eddy current instruments. In this paper we report the development of a new high-frequency eddy current conductivity measuring system that offers an extended inspection frequency range up to 50 MHz with a single spiral coil. In addition to its extended frequency range, the new system offers better reproducibility, accuracy, and measurement speed than the previously used conventional system.

Improvement of fatigue strength by using cavitating jets in air and water

Abstract: Peening method using cavitation impact at bubble collapse has been proposed. At the peening, cavitation bubble was normally generated by injecting a high-speed water jet into a water-filled chamber, i.e., a cavitating jet in water. In the present paper, the peening using a cavitating jet in air, which can generate the cavitation bubble without a water-filled chamber was investigated. It was revealed that the improvement of fatigue strength peened by the cavitating jet in air was better than that of the cavitating jet in water.

The effect of laser power density on the fatigue life of laser-shock-peened 7050 aluminium alloy

Abstract: Laser shock peening (LSP) is an innovative surface treatment method that can result in significant improvement in the fatigue life of many metallic components. The process produces very little or no surface profile modification while producing a considerably deeper compressive residual stress layer than traditional shot peening operations. The work discussed here was designed to: (a) quantify the fatigue life improvement achieved by LSP in a typical high strength aircraft aluminium alloy and (b) identify any technological risks associated with its use. It is shown that when LSP conditions are optimal for the material and specimen configuration, a —three to four times increase in fatigue life over the as-machined specimens could be achieved for a representative fighter aircraft loading spectrum when applied at a representative load level. However, if the process parameters are not optimal for the material investigated here, fatigue lives of LSP treated specimens may be reduced instead of increased due to the occurrence of internal cracking. This paper details the effect of laser power density on fatigue life of 7050-T7451 aluminium alloy by experimental and numerical analysis.

Rehabilitation and repair of welded elements and structures by ultrasonic peening

Abstract: The Ultrasonic Peening (UP) is the most efficient technique for increasing the fatigue life of welded elements as compared to such existing improvement treatments as grinding, TIG-dressing, shot peening, hammer peening, etc. The results of experimental investigation of the efficiency of UP for rehabilitation and repair of welded elements and structures with the goal of preventing the origination and propagation of fatigue cracks are considered in this document. UP treatment was applied to large-scale welded specimens in as-welded condition, after 50 % of expected fatigue life and after repair of fatigue cracks. Also, different techniques for restraining and repair of fatigue cracks were analyzed and compared: overloading; drilling of the crack tips; drilling of the crack tips with installation of high strength bolts; local explosive treatment; local heat treatment; welding with and without UP of weld toe zones. As an example, the practical application of UP for rehabilitation and repair of welded elements of highway and railway bridges are also discussed.

Influence of anodization on the fatigue strength of 7050-T7451 aluminium alloy

Abstract: In recent years, with higher demand for improved quality and corrosion resistance, recovered substrates have been extensively used. Consequently residual stresses originated from these coatings reduce the fatigue strength of a component. Due to this negative influence occasioned by corrosion resistance protective coatings, an effective process like shot peening must be considered to improve the fatigue strength. The shot peening treatment pushes the crack sources beneath the surface in most of medium and high cycle cases due to the compressive residual stress field (CRSF) induced. The aim of this study was to evaluate the influence on the fatigue life of anodic films grown on 7050-T7451 aluminium alloy by sulphuric acid anodizing, chromic acid anodizing and hard anodizing. The influence on the rotating and reverse bending fatigue strength of anodic films grown on the aluminium alloy is to degrade the stress life fatigue performance of the base material. A consistent gain in fatigue life in relation to the base material was obtained through the shot peening process in coated specimens, associated to a residual stress field compressive near the surface, useful to avoid fatigue crack nucleation and delay or even stop crack propagation.

Laser shock peening of medical devices

Abstract: A laser shock peening process for producing one or more compressive residual stress regions in a medical device is disclosed. A high-energy laser apparatus can be utilized to direct an intense laser beam through a confining medium and onto the target surface of a workpiece. An absorption overlay disposed on the target surface of the workpiece absorbs the laser beam, inducing a pressure shock wave that forms a compressive residual stress region deep within the workpiece. Medical devices such as stents, guidewires, catheters, and the like having one or more of these compressive residual stress regions are also disclosed.

Two dimensional hydrodynamic simulation of high pressures induced by high power nanosecond laser-matter interactions under water

Abstract: In laser shock peening (LSP) under a water-confinement regime, laser-matter interaction near the coating-water interface can induce very high pressures in the order of gigapascals, which can impart compressive residual stresses into metal workpieces to improve fatigue and corrosion properties. For axisymmetric laser spots with finite size, the pressure generation near the water-coating interface is a two dimensional process in nature. This is in particular the case for microscale LSP performed with very small laser spots, which is a very promising technique to improve the reliability performance of microdevices. However, models capable of predicting two dimensional (2D) spatial distributions of the induced pressures near the coating-water interface in LSP have rarely been reported in literature. In this paper, a predictive 2D axisymmetric model is developed by numerically solving the hydrodynamic equations, supplemented with appropriate equations of state of water and the coating material. The model can p...