Showing papers on "Peening published in 2004"

Role of thermal spray processing method on the microstructure, residual stress and properties of coatings: an integrated study for Ni–5 wt.%Al bond coats

Abstract: Thermal spray offers a variety of sub-sets of processing approaches to produce coatings. The various processes are classified based on the thermal spray source (from low velocity combustion spray to high temperature plasma jets) and method of material injection (in the form of powder, wire or rod). However, it is this intrinsic versatility which sets-up variations in characteristics of the applied coatings. Properties of thermally sprayed coatings, including process induced residual stress, are controlled by various parameters of the spraying process. This study examines three thermal spraying techniques with significantly different particle temperatures and velocities. They are air plasma spraying (APS), twin wire-arc spraying (TWA) and high velocity oxy-fuel (HVOF) spraying. For comparison purposes the recently developed cold spray processed materials were included in the study. For each method, in-flight particle diagnostics was performed; Ni–5 wt.%Al splats and deposits were fabricated and analyzed. Porosity, elastic modulus and thermal conductivity of the deposits were evaluated and correlated to the process variables. Using indentation at different loads and analysis of the indented region, stress–strain relationships for these coatings were obtained. Surprising differences in the properties were observed and were explained based on the fundamental variations in microstructure development. Through-thickness residual stress profiles in Ni–5 wt.%Al coatings on steel substrates were determined non-destructively by neutron diffraction. The stresses range from highly tensile in the APS coating to compressive in the HVOF coating. Various stress generation mechanisms—splat quenching, peening and thermal mismatch—are discussed with respect to process parameters and material properties.

Effect of laser shock processing on fatigue crack growth and fracture toughness of 6061-T6 aluminum alloy

Abstract: Laser shock processing (LSP) or laser shock peening is a new technique for strengthening metals. This process induces a compressive residual stress field which increases fatigue crack initiation life and reduces fatigue crack growth rate. Specimens of 6061-T6 aluminum alloy are used in this investigation. A convergent lens is used to deliver 1.2 J, 8 ns laser pulses by a Q-switch Nd:YAG laser, operating at 10 Hz. The pulses are focused to a diameter of 1.5 mm onto a water-immersed type aluminum samples. Effect of pulse density in the residual stress field is evaluated. Residual stress distribution as a function of depth is assessed by the hole drilling method. It is observed that the higher the pulse density the larger the zone size with compressive residual stress. Densities of 900, 1350 and 2500 pulses/cm 2 with infrared (1064 nm) radiation are used. Pre-cracked compact tension specimens were subjected to LSP process and then tested under cyclic loading with R = 0.1. Fatigue crack growth rate is determined and the effect of LSP process parameters is evaluated. Fatigue crack growth rate is compared in specimens with and without LSP process. In addition fracture toughness is determined in specimens with and without LSP treatment. It is observed that LSP reduces fatigue crack growth and increases fracture toughness in the 6061-T6 aluminum alloy.

Microscale Laser Shock Peening of Thin Films, Part 1: Experiment, Modeling and Simulation

Abstract: Microscale Laser Shock Peening (LSP), also known as Laser Shock Processing, is a technique that can be potentially applied to manipulate residual stress distributions in metal film structures and thus improve the fatigue performances of micro-devices made of such films. In this study, microscale LSP of copper films on single crystal silicon substrate is investigated. Before and after-process curvature measurement verifies that sizable compressive residual stress can be induced in copper thin films using microscale LSP. Improved modeling work of shock pressure is summarized and the computed shock pressure is used as loading in 3D stress/strain analysis of the layered film structure. Simulation shows that the stress/strain distribution in the metal film is close to equi-biaxial and is coupled into the silicon substrate.

Grain size effect on the electrochemical corrosion behavior of surface nanocrystallized low-carbon steel

Abstract: A nanocrystallized surface was fabricated on low-carbon steel by ultrasonic shot peening (USSP) technique. The grain size on the top latter of the surface narrocrystallized (SNC) low-carbon steel was about 20 nm. which increased with distance from the peening surface. The electrochemical corrosion behavior of the SNC low-carbon steel with different grain sizes was studied by electrochemical methods in 0.05 M sulfuric acid (H2SO4) + 0.05 M sodium sulfate (Na2SO4) aqueous solution. Grain sizes <35 am showed a strong effect on the electrochemical corrosion behavior. The corrosion rate of the SNC low-carbon steel increased with the decreasing of grain size. This was attributed to the increased number of the active sites caused by SNC low-carbon steel.

Laser shock peening on fatigue crack growth behaviour of aluminium alloy

Abstract: The effect of laser shock peening (LPS) in the fatigue crack growth behaviour of a 2024-T3 aluminium alloy with various notch geometries was investigated. LPS was performed under a 'confined ablation mode' using an Nd: glass laser at a laser power density of 5 GW cm -2 . A black paint coating layer and water layer was used as a sacrificial and plasma confinement layer, respectively. The shock wave propagates into the material, causing the surface layer to deform plastically, and thereby, develop a residual compressive stress at the surface. The residual compressive stress as a function of depth was measured by X-ray diffraction technique. The fatigue crack initiation life and fatigue crack growth rates of an Al alloy with different preexisting notch configurations were characterized and compared with those of the unpeened material. The results clearly show that LSP is an effective surface treatment technique for suppressing the fatigue crack growth of Al alloys with various preexisting notch configurations.

Comparison of Nanocrystalline Surface Layer in Steels Formed by Air Blast and Ultrasonic Shot Peening

Abstract: Surface nanocrystallization in various steels by shot peening (both air blast (ABSP) and ultrasonic (USSP)) was investigated. In all the shot-peened specimens, the equiaxed nanocrystals with grain size of several 10 nm were observed at the surface regions. The depth of nanocrystalline (NC) layers was several mm. The NC layers have extremely high hardness and were separated from the deformed structure regions just under the NC layers with sharp boundaries. By annealing, the NC layers show the substantially slow grain growth without recrystallization. These characteristics are similar to those observed in the specimens treated by ball milling, ball drop and particle impact deformation. Comparing ABSP and USSP at the similar peening condition, the produced volume of NC region in ABSP is larger than that in USSP.

Assessment of Tensile Residual Stress Mitigation in Alloy 22 Welds Due to Laser Peening

Abstract: This paper examines the effects of laser peening on Alloy 22 (UNS N06022), which is the proposed material for use as the outer layer on the spent-fuel nuclear waste canisters to be stored at Yucca Mountain. Stress corrosion cracking (SCC) is a primary concern in the design of these canisters because tensile residual stresses will be left behind by the closure weld. Alloy 22 is a nickel-based stainless steel that is particularly resistant to corrosion, however, there is a chance that stress corrosion cracking could develop given the right environmental conditions. Laser peening is an emerging surface treatment technology that has been identified as an effective tool for mitigating tensile redisual stresses in the storage canisters. The results of laser-peening experiments on Alloy 22 base material and a sample 33 mm thick double-V groove butt-weld made with gas tungsten arc welding (GTAW) are presented. Residual stress profiles were measured in Alloy 22 base material using the slitting method (also known as the crack-compliance method), and a full 2D map of longitudinal residual stress was measured in the sample welds using the contour method. Laser peening was found to produce compressive residual stress to a depth of 3.8 mm in 20 mm thick base material coupons. The depth of compressive residual stress was found to have a significant dependence on the number of peening layers and a slight dependence on the level of irradiance. Additionally, laser peening produced compressive residual stresses to a depth of 4.3 mm in the 33 mm thick weld at the center of the weld bead where high levels of tensile stress were initially present.@DOI: 10.1115/1.1789957#

Eddy Current Assessment of Near-Surface Residual Stress in Shot-Peened Nickel-Base Superalloys

Abstract: It is shown in this paper that, in contrast with most other materials, shot-peened nickel-base superalloys exhibit an apparent increase in eddy current conductivity at increasing inspection frequencies, which can be exploited for nondestructive residual stress assessment of subsurface residual stresses. It has been found that the primary reason why nickel-base superalloys, which are often used in the most critical gas-turbine engine components, lend themselves easily for eddy current residual stress assessment lies in their favorable electro-elastic behavior, namely that the parallel stress coefficient of the eddy current conductivity has a large negative value while the normal coefficient is smaller but also negative. As a result, the average stress coefficient is also large and negative, therefore the essentially isotropic compressive plane state of stress produced by most surface treatments causes a significant increase in conductivity parallel to the surface. The exact reason for this unusual behavior is presently unknown, but the role of paramagnetic contributions cannot be excluded, therefore the measured quantity will be referred to as “apparent” eddy current conductivity. Experimental results are presented to demonstrate that the magnitude of the increase in apparent eddy current conductivity correlates well with the initial peening intensity as well as with the remnant residual stress after thermal relaxation.

Characterization of Plastic Deformation Induced by Microscale Laser Shock Peening

Abstract: Electron backscatter diffraction (EBSD) is used to investigate crystal lattice rotation caused by plastic deformation during high-strain rate laser shock peening in single crystal aluminum and copper sample on (I 10) and (001) surfaces New experimental methodologies are employed which enable measurement of the in-plane lattice rotation under approximate plane-strain conditions Crystal lattice rotation on and below the microscale laser shock peened sample surface was measured and compared with the simulation result obtained from FEM analysis, which account for single crystal plasticity The lattice rotation measurements directly complement measurements of residual strain/stress with X-ray micro-diffraction using synchrotron light source and it also gives an indication of the extent of the plastic deformation induced by the microscale laser shock peening

Introduction of Compressive Residual Stress Using a Cavitating Jet in Air

Abstract: Cavitation impact from a cavitation jet, which is formed from bubbles induced by a high-speed water jet in water, can be used for surface modification in a similar manner to shot peening. A cavitating jet is normally produced by injecting a high-speed water jet into a water-filled chamber. It is possible to make a cavitating jet in air by injecting a high-speed water jet into a concentric low-speed water jet that surrounds the high-speed jet. In order to demonstrate this, a high-speed water jet with a concentric low-speed water jet was impacted onto an aluminum specimen to observe the pattern of erosion. The mass loss of the specimen was weighed to measure the capability of the jet, since a more powerful jet produces a larger mass loss. It was shown that the combination of high- and concentric low-speed water jets produced a typical erosion pattern such as that obtained using a cavitating jet in a water-filled chamber. When the injection pressure of the concentric low-speed water jet was optimised, the capability of the cavitating jet in air was much greater than that of a cavitating jet in a water-filled chamber. It was demonstrated that an optimized cavitating jet in air introduced more compressive residual stress in the surface of tool steel alloy than that from a cavitating jet in a water-filled chamber. In addition, this stress was larger than that induced by shot peening. The peened surface was also less rough compared with shot peening.

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 and guidewires having one or more of these compressive residual stress regions are also disclosed.

Compressive Residual Stress into Titanium Alloy Using Cavitation Shotless Peening Method

Abstract: Cavitation shotless peening (CSP) method, where impacts are generated by a submerged cavitating jet (without shots), was used to introduce compressive residual stress in titanium alloy, Ti-6Al-4V for the purpose of enhancing the conventional fatigue and fretting fatigue life and strength. This method provided higher compressive stress at surface as well as up to a depth of 40 μm from the surface than that with the shot peening method. Further, the surface treated by CSP was considerably less rough compared to that by the shot peening method, which is a highly desirable feature to improve the fretting fatigue performance.

Fatigue Strength of a Longitudinal Attachment Improved by Ultrasonic Impact Treatment

Abstract: Improvement methods can be divided into two main groups: weld geometry modification and residual stress modification. The former remove weld toe defects and/or reduce the stress concentration while the latter introduce compressive stress fields in the area where fatigue cracks are likely to initiate. Ultrasonic impact treatment belongs to residual stress improvement methods. It makes use of an ultrasonic carrier frequency to accelerate hardened tools that, in turn, impact the weld toe. The fatigue strength of non-load carrying attachments in the as-welded condition has been experimentally compared to the fatigue strength of ultrasonic impact treated welds. Longitudinal attachment specimens made of two thicknesses of steel S355 J0 have been tested for determining the efficiency of ultrasonic impact treatment. Treated welds were found to have about 50% greater fatigue strength, when the slope of the S-N-curve is three. High mean stress fatigue testing based on the Ohta-method did not decrease the degree of weld improvement due to UIT. This indicated that the method could be also applied for large fabricated structures operating under high reactive residual stresses equilibrated within the volume of the structure.

Ultrasonic Peening and Low Transformation Temperature Electrodes used for Improving the Fatigue Strength of Welded Joints

Abstract: The fatigue strengths of welded joints in welded structures are much lower than those of the base metal because there are stress concentrations and tensile residual stresses at weld toes. Ultrasonic peening treatment and the use of low transformation temperature electrodes are possible methods for improving the fatigue strengths of welded joints. It was thought that the latter might be particularly suitable for longitudinal welds containing high tensile residual stresses. Comparative fatigue tests carried out on treated transverse butt welded joints and on plate specimens with longitudinal fillet welded gussets confirmed this.

A novel approach for modelling of water jet peening

Abstract: In this paper, a novel approach, proposed for predicting residual stresses induced on materials treated with high pressure water jets, i.e. water jet peening, is presented. This approach considers the impact pressure distribution due to high velocity droplets impinging on the material surface instead of stationary pressure distribution considered in Trans ASME J Eng Mat Technol 121 (1999) 336 for prediction of residual stresses on water jet peened surfaces. It makes use of Reichardt’s theory for predicting the velocity distribution of droplets and liquid impact theory for predicting the impact pressure and duration of impact of high velocity droplets. For predicting residual stresses on the surface and sub surface of material subjected to water jet peening, finite element modelling approach was adopted by considering the transient, dynamic nature of droplets for analysis. The effectiveness of the proposed approach was demonstrated by comparing the predicted residual stresses with those predicted employing the approach proposed in Trans ASME J Eng Mat Technol 121 (1999) 336. Finally, the practical relevance of the proposed approach was shown by comparing the predicted results with the experimental results obtained by water peening of 6063-T6 aluminium alloy.

Fatigue behaviour of welded joints with cracks, repaired by hammer peening

Abstract: Rehabilitation of a welded structure, which involves repair of cracked joints, is achieved when the local treatment for repair gives a fatigue strength in the joint equal or above the fatigue strength of the uncracked original detail. If the treatment is properly applied the rehabilitation of the detail is assured, and the nature of the weld toe improvement methods can produce a joint, after repair, with a fatigue strength and residual life greater than the initial detail. The paper presents the results obtained on a fatigue study on the rehabilitation of non-load carrying fillet welded joints loaded in bending at the main plate and with fatigue cracking at the weld toes of the attachment in the main plate and though the plate thickness. Residual stresses were measured at the surface, with X-ray diffraction. The residual stresses induced by hammer peening at the weld toe were found to be greater along the longitudinal direction of the plate than in the transverse direction. The peak residual stresses near the weld toe were found to be close to yield in compression, justifying the great benefit of hammer peening. Results of a derived gain factor, g, in fatigue life were obtained as a function of the crack depth repaired by hammer peening.

Three-dimensional elastoplastic finite element model for residual stresses in the shot peening process

Abstract: The fatigue life of mechanical components subjected to variable loading conditions is a crucial parameter in the design of automotive and aerospace mechanical components. The fatigue streng...

Microscale Laser Shock Peening of Thin Films, Part 2: High Spatial Resolution Material Characterization

Abstract: Microscale Laser Shock Peening (LSP) is a technique that can be potentially applied to manipulate the residual stress distributions in metal film structures and thus improve the reliability of micro-devices. This paper reports high-spatial-resolution characterization of shock treated copper thin films on single-crystal silicon substrates, where scanning x-ray microtopography is used to map the relative variation of the stress/strain field with micron spatial resolution, and instrumented nanoindentation is applied to measure the distribution of hardness and deduce the sign of the stress/strain field. The measurement results are also compared with 3-D simulation results. The general trends in simulations agree with those from experimental measurements. Simulations and experiments show that there is a near linear correlation between strain energy density at the film-substrate interface and the X-ray diffraction intensity contrast.

Effects of Surface Treatment on Fretting Fatigue Performance of Ti-6Al-4V

Abstract: : The fretting fatigue performance of Ti-6Al-4V after isothermal exposure was explored in test coupons in low plasticity burnished (LPB), shot peened (SP), and electropolished (ELP) baseline conditions. In the current study, fretting fatigue data and fractography are presented along with in-depth residual stress profiles, both before and after the isothermal exposure. Surface roughness data for each of the three surface conditions are reported. For the studied fretting configuration, it was found that both the shot peening and LPB process improved the fretting fatigue performance over the baseline electropolished condition. In the LPD'd case the fretting damage was largely ameliorated by the burnishing process.

Microscale Laser Shock Peening of Thin Films, Part 2: High Spatial Resolution Material

Abstract: Microscale Laser Shock Peening (LSP) is a technique that can be potentially applied to manipulate the residual stress distributions in metal film structures and thus improve the reliability of micro-devices. This paper reports high-spatial-resolution characterization of shock treated copper thin films on single-crystal silicon substrates, where scanning x-ray microtopography is used to map the relative variation of the stress/strain field with micron spatial resolution, and instrumented nanoindentation is applied to measure the distribution of hardness and deduce the sign of the stress/strain field. The measurement results are also compared with 3-D simulation results. The general trends in simulations agree with those from experimental measurements. Simulations and experiments show that there is a near linear correlation between strain energy density at the film-substrate interface and the X-ray diffraction intensity contrast.

Spatially Resolved Characterization of Residual Stress Induced by Micro Scale Laser Shock Peening

Abstract: Single crystal aluminum and copper of (001) and (110) orientation were shock peened using laser beam of 12 micron diameter and observed with X-ray micro-diffraction techniques based on a synchrotron light source The X-ray micro-diffraction affords micron level resolution as compared with conventional X-ray diffraction which has only mm level resolution The asymmetric and broadened diffraction profiles registered at each location were analyzed by sub-profiling and explained in terms of the heterogeneous dislocation cell structure For the first time, the spatial distribution of residual stress induced in micro-scale laser shock peening was experimentally quantified and compared with the simulation result obtained from FEM analysis Difference in material response and microstructure evolution under shock peening were explained in terms of material property difference in stack fault energy and its relationship with cross slip under plastic deformation Difference in response caused by different orientations (110 and 001) and active slip systems was also investigated