Showing papers on "Peening published in 2000"

Surface modifications induced in 316L steel by laser peening and shot-peening. Influence on pitting corrosion resistance

Abstract: The influence of laser peening (LP) on the electrochemical behavior of AISI type 316L stainless steel in a saline environment was evaluated. Surface modifications were investigated as they might have beneficial effects on the corrosion behaviour. Low residual stress and work hardening levels were found, when compared with a conventional shot-peening (SP) treatment, mainly because of the absence of martensite transformation in the case of LP. Surface changes were accompanied by small roughening effects and a global preservation of the surface chemistry after treatment. Therefore, electrochemical tests performed on samples after LP and SP treatments showed increases in rest potentials, reductions of passive current densities and anodic shifts of the pitting potentials evidenced by a stochastic approach of pitting. The better pitting resistance was observed after LP treatment, which seems to reflect a reduction or an elimination of active sites for pitting at lower potentials. Even though the deleterious surface state of shot peened surfaces possibly counterbalances the beneficial influence of residual stresses, a beneficial influence of mechanical surface treatments has been demonstrated regarding the localized corrosion properties.

Laser shock peening for fatigue resistance

Abstract: Laser shock peening produces a compressive residual stress in the surface of metallic materials, which significantly increases fatigue life in applications where failure is caused by surface-initi ated cracks. Laser shock peening is applied by using a high energy pulsed laser to create a high amplitude stress wave or shock wave on the surface to be treated. This stress wave propagates into the material, causing the surface layer to yield and plastically deform, and thereby, develop a residual compressive stress. Where comparisons have been made to shot peening, the magnitude of the residual stresses at the surface are similar, but the compressive stresses from laser peening extend much deeper below the surface than those from shot peening. The resulting fatigue life enhancement is often greater for laser peering than it is for shot peening. In addition to fatigue strength improvement, laser peering can also locally strain harden thin sections of parts or strain harden a surface.

The Effect of Cold Work on the Thermal Stability of Residual Compression in Surface Enhanced IN718

Abstract: Surface enhancement, the creation of a layer of residual compression at the surface of a component, is widely used to improve the fatigue life in the automotive and aerospace industries. The compressive layer delays fatigue crack initiation and retards small crack propagation. The benefits of surface enhancement are lost if the compressive layer relaxes at the operating temperature of the component. Surface enhancement methods producing minimal cold work are shown to produce the most thermally stable compression. The residual stress and cold work distributions developed in IN718 by shot peening, gravity peening, laser shock peening (LSP) and low plasticity burnishing (LPB) are compared. Estimation of cold work (equivalent true plastic strain) from x-ray diffraction line broadening is described. Thermal relaxation at temperatures ranging from 525C to 670C is correlated to the degree of cold working of the surface, independent of the method of surface enhancement. Highly cold worked (> 15%) shot peened surfaces are found to relax to half the initial level of compression in minutes at all temperatures investigated. The rapid initial relaxation is shown to be virtually independent of either time or temperature from 525C to 670C. The LPB process is described with application to IN718. High cycle fatigue performance after elevated temperature exposure is compared for surfaces treated by LPB and conventional (8A intensity) shot peening.

Effects of warm peening on fatigue life and relaxation behaviour of residual stresses in AISI 4140 steel

Abstract: A new device has been built which allows shot peening in an air blast machine at elevated temperatures. The effects of conventional shot peening and peening at elevated temperatures on the characteristics of regions close to the surface, on the stability of residual stresses and half widths of X-ray interference lines and on the fatigue strength are presented for a quenched and tempered AISI 4140 steel (German grade 42CrMo4). The alternating bending strength is increased by warm peening compared with conventional shot peening. Additional investigations of samples conventionally peened and then annealed confirm that these effects are due to the stability of the dislocation structure, which is highly affected by strain ageing effects. This causes an additional benefit owing to higher stability of the residual stresses induced.

A combined finite/discrete element simulation of shot peening processes – Part II: 3D interaction laws

Abstract: Following earlier work on the combined finite/discrete element simulation of shot peening process in 2D case, 3D representation of the problem is established with respect to DE modelling and contact interaction laws. An important relevant computational issue regarding the critical time step is carefully studied, and a new time stepping scheme that can ensure both short and long term stability of the contact models is developed. Numerical tests are performed to evaluate the proposed normal and frictional contact interaction laws with various model parameters. The influences of single and multiple shot impact, as well as element sizes are also numerically investigated. The established contact interaction laws can also be applied to other multi‐body dynamic simulations.

A combined finite/discrete element simulation of shot peening processes – Part I: studies on 2D interaction laws

Abstract: In the first part of this series of papers on the combined finite/discrete element simulation of shot peening processes, different contact interaction laws for 2D cases are extensively studied with special attention given to the proper selection of the parameter values involved, which is one of the key issues for successful direct simulation. In addition, computational issues including contact forces, partial contact, energy dissipation, and rheological representation are addressed. Numerical examples for a single shot impact system simulated by the coupled finite/discrete element method using different interaction laws are provided to verify the proposed approaches. The results are also compared with those obtained by using only finite element methods. Findings obtained by performing 2D simulations will, in the subsequent article, be used in realistic computational simulations of 3D shot peening processes.

Fod resistance and fatigue crack arrest in low plasticity burnished in718

Abstract: Surface enhancement methods induce a layer of residual compressive stress to improve fatigue life. Shot peening is inexpensive and widely used, but the associated cold work accelerates relaxation of the compressive layer at turbine temperatures and increases sensitivity to overload relaxation. “Deep rolling” burnishing methods produce deep compression, but with cold work comparable to shot peening. Laser shock peening (LSP) produces deep compression with minimal cold work and impressive FOD resistance, but is costly and presents logistical problems in manufacturing. Low Plasticity Burnishing (LPB) has been investigated as a rapid, inexpensive surface enhancement method. Preliminary results indicate depth and magnitude of compression comparable to LSP. Compression reaching the alloy yield strength and extending to a depth of 1.2 mm (0.047 in.) is achievable with cold work of less than 4%. Excellent surface finish can be achieved with no detectable metallurgical damage. Ease of adaptation to CNC machine tools allows LPB processing at costs and speeds comparable to machining operations. The LPB process is described with application to IN718. Thermal stability at engine temperatures is compared to conventional shot peening. Resistance to 0.13 and 0.25 mm (0.005 and .010 in.) deep sharp notch FOD was achieved, even after exposure to engine temperatures. Elevated temperature crack growth data are presented showing the arrest of existing 0.46 mm x 0.91 mm (0.018 x 0. 036 in.) fatigue cracks by the deep compressive layer.

Abrasive waterjet peening: a new method of surface preparation for metal orthopedic implants.

Abstract: Abrasive waterjet (AWJ) peening is a new mechanical surface treatment process envisioned for use on metal orthopedic implants. The process utilizes an abrasive waterjet to simultaneously texture and work harden the surface of a metal substrate through controlled hydrodynamic erosion. In this study, a titanium alloy (Ti6Al4V) was subjected to AWJ peening over a range of parametric conditions. The textured surfaces were quantified in terms of the apparent interdigitation volume (Vi), the effective stress concentration factor ( Kt) posed by the surface topography, and the magnitude of residual stress (sr). Topographical features of the prepared surfaces were determined using contact profilometry, and X-ray diffraction was used in evaluating the in-plane residual stress. It was found that a large range in Vi (9.4 - 43.8 mm 3 /mm 2 ) and Kt (1.3-2.7) are available through selection of the AWJ peening process parameters. Furthermore, a compressive residual stress (2409 6 sr 62 33) was found to result within the surface of the Ti6Al4V substrates regardless of treatment condi- tions. When compared to a titanium plasma spray coating used for cementless fixation, the AWJ peened Ti6Al4V exhibited a surface topography with significantly lower effective stress concentration and higher compressive residual stress. Based on results from this study, AWJ peening may serve as a new method of surface treatment for metal orthopedic implants, which supports the development of stable primary fixation and simultaneously enhances the com- ponent fatigue strength. © 2000 John Wiley & Sons, Inc. J Biomed Mater Res (Appl Biomater) 53: 536 -546, 2000

Further improvements in some properties of shot peened components using the burnishing process

Abstract: Shot peening and burnishing are considered to be cold working processes, where both processes are used to improve some metallic surface properties, due to the plastic deformation induced into these surfaces. The first process causes an increase in the roughness, while the other, in opposition to the first process, improves the smoothness of metallic surfaces. The present work is an attempt to apply the burnishing process onto shot peened components, in order to decrease the roughness of the metallic surface caused by the shot peening process. From the results obtained it was found that the burnishing process not only improves the surface finish, but it also introduces further improvements in surface hardness, fatigue strength and corrosion resistance for the two non-ferrous metals considered in this work, namely aluminium and brass. Accordingly, it can be suggested that burnishing of shot peened components may be used with benefits in industry for further improvements in the properties of metallic surfaces.

Use of the hole-drilling method for measuring residual stresses in highly stressed shot-peened surfaces

Abstract: The same shot-peening treatment was applied to five steels with different mechanical properties. The induced residual stress profiles were analyzed using X-ray diffraction and incremental hole drilling (IHD). The results of both techniques showed that IHD can still be successfully used for measuring shot-peening residual stresses, even if these exceed the yield strength of the bulk material. Expected errors due to the plasticity effect are reduced by the strain hardening of the surface. For an assessment of the reliability of IHD data, strain-hardening variation was quantified by microhardness measurements to estimate the yield strength of the plastified layer. All the main calculation methods for IHD were applied. The results were compared and discussed with respect to the characteristics of each method.

Waterjet Machining and Peening of Metals

Abstract: An experimental study was conducted to determine the influence of high-pressure waterjet (WJ) peening and abrasive waterjet (AWJ) machining on the surface integrity and texture of metals. A combination of microstructure analysis, microhardness measurements, and profilometry were used in determining the depth of plastic deformation and surface texture that result from the material removal process. The measurement and evaluation of residual stress was conducted with X-ray diffraction. The residual stress fields resulting from treatment were analyzed to further distinguish the influence of material properties on the surface integrity. It was found that waterjet peening induces plastic deformation at the surface layer of metals as good as shot peening. The degree of plastic deformation and the state of material surface were found to be strongly dependent on the peening conditions applied.

Contour forming of metals by laser peening

Abstract: A method and apparatus are provided for forming shapes and contours in metal sections by generating laser induced compressive stress on the surface of the metal workpiece. The laser process can generate deep compressive stresses to shape even thick components without inducing unwanted tensile stress at the metal surface. The precision of the laser-induced stress enables exact prediction and subsequent contouring of parts. A light beam of 10 to 100 J/pulse is imaged to create an energy fluence of 60 to 200 J/cm2 on an absorptive layer applied over a metal surface. A tamping layer of water is flowed over the absorptive layer. The absorption of laser light causes a plasma to form and consequently creates a shock wave that induces a deep residual compressive stress into the metal. The metal responds to this residual stress by bending.

Modelling fatigue crack growth in shot‐peened components of Al 2024‐T351

Abstract: Microstructural fracture mechanics concepts are used to develop a model to incorporate shot-peening effects into crack propagation laws and fatigue life predictions. Shot peening produces a residual stress which resists opening of the crack and also produces a work-hardened layer within which the flow stress is raised. The model takes account of these effects to give an accurate prediction of the increase in fatigue life. The model was also used to derive the conditions for crack arrest, and the results are presented in the form of a fatigue damage map (FDM). The FDM can be used for the determination of safe loads in durability and maintainability analyses.

Corrosion reactivity of laser-peened steel surfaces

Abstract: Laser-shock processing or laser peening (LP) is a novel process used to reinforce surfaces by generating compressive residual stresses that has been investigated to change the surface mechanical state and modify the electrochemical properties of three commercial steels. The first part of this paper relates to experiments where LP has been applied to G10380 and G41400 steels for corrosion testing in an acid HKSO4-0.3 M solution. Only in the case of G41400 martensitic steel is a reduction of the corrosion current observed, depending on the degree of work hardening and the amplitude of compressive stresses. This indicates a small mechanochemical effect of LP, which seems to be restricted to martensitic structure. Second, the effect of LP on stress corrosion cracking (SCC) of AISI 316L stainless steel is demonstrated by static tests in MgCl2 44% – 153 °C solution. The results confirm the applicability of LP to suppress cracks on all the areas processed without occurrence of any problems in the treated-nontreated transitions zones.

Effect of power density and pulse repetition on laser shock peening of Ti-6Al-4V

Abstract: Laser shock peening (LSP) was applied to Ti-6Al-4V (wt. %) simulated airfoil specimens using a Nd:Glass laser. Laser shock peening processing parameters examined in the present study included power density (5.5, 7, and 9 GW/cm2) and number of laser pulses per spot (one and three pulses/spot). The LSP’d Ti-6Al-4V samples were examined using x-ray diffraction techniques to determine the residual stress distribution and percent cold work as a function of depth. It was found that the residual stress state and percent of cold work were relatively independent of LSP power density. However, the number of laser pulses per spot had a significant effect on both residual stress and percent of cold work for a given power density level. In addition, there was a strong correlation between the magnitude of residual compressive stresses generated and the percent cold work measured.

Method for setting up and controlling confinement media flow in laser shock peening

Abstract: A method for setting up a transparent confinement media nozzle for flowing the confinement media during laser shock peening of a work piece includes the following steps: Step A) flowing a confinement media and setting a flow rate of the confinement media through a confinement media flow nozzle; Step B) positioning the nozzle to flow the confinement media through the nozzle and onto the correlation surface; and Step C) measuring a confinement media layer thickness on the correlation surface using an ultrasonic transducer attached to a side of the test piece opposite that of the correlation surface. The ultrasonic transducer is attached to a side of the test piece opposite that of the correlation surface. One embodiment of the invention employs the work piece as the test piece and the correlation surface is a first laser shock peening surface on a first side of the work piece. Further embodiments includes Step D) comparing the measured and recorded confinement media layer thickness from Step C) against a predetermined value or range of values for confinement media layer thickness and Step E) determining whether to reset the confinement media flow rate and/or nozzle position or proceeding with the laser shock peening of the work piece with the confinement media flow rate and nozzle position set in

Identification marking by means of laser peening

Abstract: The invention is a method and apparatus for marking components by inducing a shock wave on the surface that results in an indented (strained) layer and a residual compressive stress in the surface layer. One embodiment of the laser peenmarking system rapidly imprints, with single laser pulses, a complete identification code or three-dimensional pattern and leaves the surface in a state of deep residual compressive stress. A state of compressive stress in parts made of metal or other materials is highly desirable to make them resistant to fatigue failure and stress corrosion cracking. This process employs a laser peening system and beam spatial modulation hardware or imaging technology that can be setup to impress full three dimensional patterns into metal surfaces at the pulse rate of the laser, a rate that is at least an order of magnitude faster than competing marking technologies.

Corrosion fatigue behavior of the high-strength magnesium alloy AZ 80

Abstract: The fatigue performance of the high-strength magnesium alloy AZ 80 is studied in air as well as in aqueous 0.5 and 3.5% NaCl solutions. The effect of mechanical surface treatments, specifically mechanical polishing, shot peening, and roller burnishing on the S-N curves, is investigated using an electropolished surface as a reference. While mechanical polishing as well as shot peening improves fatigue performance in air, no improvement is observed in NaCl solutions. However, roller burnishing, which combines a smooth surface finish with residual compressive stresses in sufficient depths, leads to outstanding fatigue performance even in 3.5% NaCl solution.

Improvement in Fatigue Strength of Silicon Manganese Steel SUP7 by Using a Cavitating Jet.

Abstract: It is possible to strengthen metallic materials by using a cavitating jet to introduce compressive residual stress in the material surface, since the impact of collapsing cavitation bubbles peen the surface in the same way as shot peening. In order to demonstrate the improvement in the fatigue strength of a material by using a cavitating jet, an experimental study was carried out. Silicon manganese steel JIS SUP7 was chosen as a test material, since JIS SUP7 is used as a spring material after shot peening. The specimens were exposed to the cavitating jet with upstream pressure p1=20 MPa, downstream pressure p2=0.28 MPa, the cavitation number σ〓p2/p1=0.014, the nozzle throat diameter d=0.842 mm and the atandoff distance s=31 mm. The scanning speed v at which the compressive residual stress took the most significant value was 0.25 mm/s. The compressive residual stress was introduced in the entire surface peened by the cavitating jet under the above conditions. The fatigue strength of the specimen was investigated by a four point bending test. The minimum bending stress σm/n was fixed at 123 MPa and the amplitude of the load was varied. The fatigue strength of material peened by the cavitating jet is shown to be about 440 MPa, which is about 10% stronger than the strength without peening.

The Effect of Shot Peening Coverage on Residual Stress, Cold Work and Fatigue in a Ni-Cr-Mo Low Alloy Steel

Abstract: : The underlying motivation for this work was to test the conventional wisdom that 100% coverage by shot peening is required to achieve full benefit in terms of compressive residual stress magnitude and depth as well as fatigue strength. Fatigue performance of many shot peened alloys is widely reported to increase with coverage up to 100%, by many investigators and even in shot peening manuals.(1) The fatigue strength of some alloys is reported to be reduced by excessive coverage(2) Aerospace(3,4), automotive(5), and military(6) shot peening specifications require at least 100% coverage. Internal shot peening procedures of aerospace manufacturers may require 125% to 200% coverage. Most of the published fatigue data supporting the 100% minimum coverage recommendation was developed in fully reversed axial loading(2,7) or bending(8,9 ) with a stress ratio, R= Smin / Smax, of -1.

High Power Laser for Peening of Metals Enabling Production Technology

Abstract: Laser shot peening, a surface treatment for metals, is known to induce compressive residual stresses of over 1 mm (0.040 inch) depth providing improved component resistance to various forms of failure. Additionally recent information suggests that thermal relaxation of the laser induced stress is significantly less than that experienced by other forms of surface stressing that involve significantly higher levels of cold work. We have developed a unique solid state laser technology employing Nd:glass slabs and phase conjugation that enables this process to move into high throughput production processing.

Processing tip treatment bars in a gas turbine engine

Abstract: Tip treatment bars 16 of a rotor casing 2 for a gas turbine engine are subjected to a process which induces compressive stress within the bars 16. The process may be a laser shock peening process applied to opposite sides 20 of each bar 16. The processing of the tip treatment bars 16 renders them less susceptible to fatigue failure.

Improving fatigue strength of welded joints by hammer peening and tig-dressing

Abstract: The effectiveness of fatigue strength improvement methods, namely TIG-dressing and hammer peening, were examined using out-of-plane gusset welded joints of high strength steel (SM570). In addition, the mechanism of each improvement method was discussed with respect to the decrease in stress concentration and residual stress at the weld toe. Finally, various methods for improving the effectiveness of these fatigue strength improvement methods are proposed.

Method for the ultrasonic peening of large sized annular surfaces of thin parts

Abstract: A method of so-called “ultrasonic” peening is described for peening large sized annular surfaces on thin parts. The method uses a microbead mist vibrated in a chamber with an opening so that the surface that is to be peened makes at least five movements past the opening of the peening chamber during peening so as to reduce deformation of the part.

Tests of large girders treated to enhance fatigue strength

Abstract: Two large-scale fatigue test studies of plate girders are reported on. One study involved a series of coverplated bridge girders with small fatigue cracks that were retrofitted in 1976 as described in NCHRP Report 206. The second study involved plate girders fabricated from HPS-485W (HPS-70W) steel with welded attachments. The Category E' coverplated beams that were removed from the I-95 Yellow Mill Pond Bridge in 1997 had been retrofitted in 1976 by either air hammer peening or gas tungsten arc (GTA) remelting. All details had small fatigue cracks at the time of retrofit. No further fatigue cracking was observed at the coverplate ends after 20 years of service and an estimated 56 million truck passages. The beams were tested at a stress range of 69 MPa (10 ksi). Cracks developed from the root of the transverse end weld and propagated through the weld throat. The fatigue resistance of the treated weld toe details improved to Category C except for one GTA-remelted detail, which exceeded Category D. Another...

3D Finite Element Models of Shot Peening Processes

Abstract: The principle of shot peening is to hit the surface of a metallic part with spherical shots to introduce a compressive residual stress field by plastification. The compressive stress introduced by shot peening lowers the number of cracks initiation on the material's surface. This process is thus used to increase fatigue life of many mechanical parts. The efficiency of shot peening depends on parameters such as the coverage rate or the size and speed of the shots. The values chosen for these parameters should be accurate to obtain the required fatigue life; investigations on the influence of the shot peening parameters, should be efficient and easy. A few analytical models have been proposed to better evaluate the residual stress field of a shot peened part, but most of the knowledge on shot peening processes today is empirical. In this study we chose to use a finite element model. Indeed, finite element codes are now powerful enough to simulate shot-peening processes extensively and to analyze the influence of the relevant parameters. This paper presents finite element simulations of shot peening processes and the residual stress fields obtained. We validated the model with a confrontation between numerical and experimental results. Two types of materials often used in aircraft and automobile industry were chosen for the study. The mechanical properties of these two materials were experimentally measured. In the same way, the coverage rate, the diameter and the speed of shots used to treat the surface of the materials were evaluated. Selected data were extracted from the literature and experimental measurements carried out in our laboratory. Three-dimensional models reproducing the experimental conditions were then investigated to simulate the shot peening of these materials. Results such as the displacement and the residual stress field are presented and compared to experimental results when possible. The models proposed show how the finite element simulation is able to predict the residual stress field of a shot peened part. The possibility to vary parameters values easily confirms the interest of shot peening numerical simulations.

Effect of stress and heat treatment on magnetic barkhausen emission in case carburized steels

Abstract: There is currently much interest in the use of magnetic barkhausen emission for monitoring the surface integrity of hardened steel components finished by grinding. However, the magnetic barkhausen emission response could vary with composition, and there is a need to calibrate magnetic barkhausen emission response for changes in residual stress and tempering. Magnetic barkhausen emission was investigated in three commercial mechanically equivalent case carburized steels with a range of compositions. Specimens were tested to determine the effect of tempering and macroscopic stress. Increased tempering produced increased emission, but this was found to be greatest when a tensile stress was acting. Although the three steels have closely similar mechanical properties, the sensitivity of magnetic barkhausen emission to tempering was found to vary substantially with composition and was greatest in the steel with the highest nickel content. The effect of glass bead peening after heat treatment was also examined. After allowing for the effect of the residual compressive stress, it was found that the plastic deformation produced by peening had a significant effect on the magnetic barkhausen emission stress relation.

Method for modifying stoichiometric NiAl coatings applied to turbine airfoils by thermal processes

Abstract: A method for producing a turbine airfoil that is coated with a beta phase, high aluminum content coating, such as substantially stoichiometric NiAl, and which has a surface finish suitable for application of a ceramic topcoat. The method involves impacting the coating with preselected particles of a preselected size so that the brittle coating is not adversely affected by chipping or breakage. The impacting produces a surface finish of 120 micro-inches or better so that a ceramic thermal barrier layer can be applied over the coating. The preferred method of improving the surface finish utilizes steel balls having a diameter of about 0.033″ and a peening intensity of no greater than about 6 A.