Deep Compressive Residual Stresses Research Articles

Laser Shock Processing as an Advanced Technique for the Surface and Mechanical Resistance Properties Modification of Bioabsorbable Magnesium Alloys

Laser shock processing (LSP) is increasingly applied as an effective technology for the improvement of metallic materials’ mechanical and surface properties in different types of components, mostly as a means of enhancement of their fatigue life behavior. As reported in previous contributions by the authors, a main effect resulting from the application of the LSP technique consists in the generation of relatively deep compression residual stresses fields into metallic components allowing an improved mechanical behaviour. On the other hand, Mg and its alloys have gained increasing relevance as natural biomaterials as their mechanical properties are in the same range as those corresponding to natural bone as well as due to their inherent bioabsorbable properties. In the present paper, the application of the LSP technology to biocompatible bioabsorbable Mg alloys suitable for chirurgical implementation is envisaged, the experimental verification of the residual stresses fields induced and the experimental characterization of the surface properties introduced by means of the treatment being specifically considered.

Residual Stress Field of a Single Edge Notch Specimen after Laser Shock Peening and Shot Peening Treatment

Abstract Laser shock peening (LSP) is a reliable, repeatable, and successful surface technique for introducing high magnitude, deep compressive residual stresses that can significantly increase the fatigue life of metallic components. However, depending upon how the LSP treatment is applied, the induced residual stresses can result in the undesirable deformation of the part. In this work, traditional shot peening has been applied over LSP as a means to optimize the stress distribution at the surface of a part while constraining deformation. A single edge notch test specimen of AA7075 was laser peened local to the notch region and then shot peened over the entire central region. The resulting residual stress distribution has been characterized using neutron diffraction to measure the stress distribution in the bulk, and it was compared with (1) incremental center hole drilling to measure the stress distribution up to depths of ∼1 mm and (2) near-surface stresses obtained in a previous X-ray diffraction (XRD) study on nominally identical specimens subjected to the same surface treatments. For regions where the two techniques overlap, the residual stresses are in good agreement (within uncertainty). Comparing the bulk stresses obtained from neutron diffraction in this study and XRD data published elsewhere, it can be shown that shot peening applied after LSP has a profound effect on near-surface stresses; however, these effects disappear at depths of ∼0.7 mm or more.

A micromechanical model to study the closure stress effect on fatigue life of Ti6Al4V subjected to laser shock peening

With the wide application of titanium alloy in various engineering fields, the service life of Ti alloy product has drawn much more attention. It’s been proven that the laser shock peening (LSP) is an effective technology to improve the fatigue performance of titanium alloy by introducing a large and deep compressive residual stress. In this paper, a microstructural fracture mechanics model incorporating the crack closure stress effect is presented to investigate the reinforcement mechanism of laser shock peened (LSPed) titanium alloy. Firstly, a rigid-plastic simplified model is utilized to describe the residual stress characteristics of LSPed Ti6Al4V. Then the extended Navarro-Rios model incorporating crack closure stress effect is established to systematically analyze the crack propagation behavior. Finally, the effect of crack closure stress on fatigue life can be accurately predicted. Result shows that the crack propagation processing is a behavior reflecting the deceleration and acceleration of the crack growth rate. A higher threshold stress is required for LSPed Ti6Al4V to promote the propagation of fatigue crack. The closure stress effect reduces the barrier stress and critical crack length in each grain leading to the highly raised fatigue life of LSPed Ti6Al4V. Besides, the closure stress effect has a greater improvement in fatigue performance for a sample with a shorter crack length and thus can be neglected for a long crack.

Simulation of prestressed ultrasonic peen forming on bending deformation and residual stress distribution

Elastic prestressed ultrasonic peen forming (UPF) is beneficial to obtain sufficient bending deformation and decrease spherical deformation. Numerical models of free UPF and prestressed UPF are established in this study to analyze effects of forming parameters (prebending curvature, firing pin velocity, offset distance, and plate thickness) on bending deformation and residual stress distribution. Meanwhile, the comparisons between free UPF and prestressed UPF are studied. The results show that increasing prebending curvature is beneficial to enlarge spanwise bending deformation and decrease spherical deformation of the plate. Large firing pin velocity, small offset distance, and small plate thickness play positive roles in the increase of spanwise bending deformation. Larger and deeper compressive residual stress can be obtained in prestressed UPF due to prebending deformation before shot peening process. Large prebending curvature, large firing pin velocity, and small plate thickness significantly increase surface and maximum compressive residual stress and total depth of compressive residual stress, while descending offset distance only slightly increases these values. Experiments are conducted and the simulated results agree with experiments. This study provides guidance for parameter selection in prestressed UPF and sufficient deformation for wing plate with large thickness.

Microstructural Response and Strain Hardening in Deep Cold Rolled Nickel-based Superalloy for Aerospace Application

Fatigue crack resistance is a critical factor for the performance of aero-engine components. Deep cold rolling (DCR) is one promising surface enhancement technique, as it generates deep compressive residual stresses with the good surface finish. In this work, the influence of DCR process on the microstructure and mechanical properties of nickel-based superalloy (Udimet 720Li) was investigated. After DCR process, a deep layer of compressive residual stress up to 1mm was observed, which agrees well with the depth profile of Vickers micro-hardness. Statistical analysis of Grain Orientation Spread (GOS) in the Electron Back Scattered Diffraction (EBSD) map also shows the presence of strain hardening layer after the DCR process. Results are discussed with a focus on the strengthening mechanism through grain refinement, the addition of Low Angle Grain Boundaries (LAGBs), and intragranular deformation in the sub-surface. Overall, this fundamental understanding could shed light on a new pathway to develop nickel-based superalloys with an excellent mechanical performance for aerospace applications.

Effect of laser shock peening and its size-dependence on the compressive plasticity of Zr-based bulk metallic glass

In this study, laser shock peening (LSP) has been performed on brick-shaped Zr41.2Ti13.8Cu12.5Ni10Be22.5 (vit1) bulk metallic glass (BMG) with different dimensions but the same aspect ratio of 2:1 to study the effects of LSP treatment and size-dependence on plasticity and compressive strength at room temperature. The results show that: (1) the plasticity of LSP treated specimens is still size-dependent, the smaller the specimen size is, the higher plasticity and compressive strength it has, which means the size effects of BMG can be preserved and inherited during LSP. (2) The same LSP treatment has different effects on different size specimens, the smaller the specimen size is, the greater effect it has, which indicates that LSP process causes much more obvious size-dependence in BMG.The mechanism for plasticity improvement can be attributed to the increase of free volume and the generation of deep compressive residual stress induced by LSP for each size specimen. Therefore, smaller specimen is corresponding with larger volume fraction of LSP affected zones and greater LSP effect.

Effect of texture on the residual stress response from laser peening of an aluminium-lithium alloy

Laser shock peening can improve the damage tolerance of metallic materials by introducing deep compressive residual stress that inhibits crack initiation and growth. This study investigates the laser peening of aluminium alloy in a product form – an extruded T-section – that has different crystallographic textures in different locations. The alloy studied is Al 2099, an aluminium-lithium alloy that shows anisotropy in the mechanical properties when texture is present. Specimens extracted from different regions of the extrusion were laser shock peened with a power density of 3 GW/cm2 in single shocks as well as in a pattern. Residual stresses were characterized primarily using incremental hole drilling. The results show 20% higher residual stresses in the web area of the extrusion compared to the flange after peening with a single laser shock, with this difference decreasing as the number of shocks increases. This effect can be explained by the difference in yield strength between those locations. No significant differences were observed in the residual stresses from peening onto different planes of a textured sample at a given location.

Fatigue behavior of superferritic stainless steel laser shock treated without protective coating

The laser shock peening (LSP) is a new technique that improves the fatigue life of metallic components by inducing deep compressive residual stresses through the surface. However, the beneficial effects of LSP depend on the persistence and stability of such residual stress fields under cyclic loading and temperature. Moreover, if no absorbent coating is used in LSP operation, thermal effects can occur on the metallic substrate. The purpose of this work is to study the influence of LSP, without protective coating and with different pulse densities, on the low cyclic fatigue behavior of a superferritic stainless steel UNS S 44600. These results are correlated with observations performed by means of transmission electron microscopy (TEM) and scanning electron microscopy (SEM) with electron diffraction spectroscopy (EDS). The hole-drilling method is used to measure residual stresses. The micro-hardness and roughness profiles are also presented. This paper shows that LSP without coating produces beneficial compression residual stresses. However, in the first 10μm beneath the surface, thermal effects occur that induce intergranular corrosion. This intergranular corrosion deteriorates the fatigue properties of a superferritic stainless steel UNS S 44600.

Compressive Residual Stresses and Associated Surface Modifications Induced in Ti6Al4V by Laser Shock Processing

Laser shock processing (LSP) is increasingly applied as an effective technology for the improvement of metallic materials’ mechanical and surface properties in different types of components, mostly as a means of enhancement of their fatigue life behavior. As reported in previous contributions by the authors, a main effect resulting from the application of the LSP technique consists in the generation of relatively deep compression residual stresses fields into metallic components allowing an improved mechanical behaviour. In this paper, the special case of Ti6Al4V alloy is considered from a more fundamental point of view, with specific consideration of the microstructural changes and residual stresses fields justifying those macroscopic effects. From a concrete point of view, the effect of the application of different typical LSP intensities on the microstructure and residual stresses fields introduced in this material and their possible correlation to the associated surface effects are analyzed.

Improvement of surface and mechanical properties of high strength metallic alloys by laser shock processing

Profiting by the increasing availability of laser sources delivering intensities above 109 W/cm2 with pulse energies in the range of several Joules and pulse widths in the range of nanoseconds, laser shock processing (LSP) is being consolidated as an effective technology for the improvement of surface mechanical and corrosion resistance properties of metals and is being developed as a practical process amenable to production engineering. The main acknowledged advantage of the LSP technique consists on its capability of inducing a relatively deep compression residual stresses field into metallic alloy pieces allowing an improved mechanical behaviour. Following a short description of the theoretical/computational and experimental methods developed by the authors for the predictive assessment and experimental implementation of LSP treatments, experimental results on the residual stress profiles and associated surface properties modification successfully reached in typical materials (specifically Al and Ti alloys) under different LSP irradiation conditions are presented.

Laser shock peening enhanced corrosion properties in a nickel based Inconel 600 superalloy

Laser peening without coating (LPwC) is an advanced surface modification treatment applied nowadays for many of the industrial alloys, as it tremendously enhance the fatigue and corrosion properties. We report the influence of LPwC on residual stresses, hardness, microstructure, surface roughness and topography of a nickel based superalloy, Inconel 600. LPwC parameter optimization for Inconel 600 that results in larger and deeper compressive residual stresses with relatively smaller surface roughness was carried out. It was noted with optimized LPwC condition that maximum compressive residual stresses was nearly −672 MPa and depth of work-hardening was about 1400 μm, with slight increase in the hardness with a case depth of about 400 μm. The 3D topography indicated roughening at LPwC treated surface and represented the column or steps like feature whilst the average roughness was found to be smaller (≤1 μm). Grain refinement was not observed. The corrosion resistance of LPwC specimen showed tremendous increase with a 106 fold decrease in corrosion rate from that of untreated condition. Consequently, larger and deeper pits were observed in the case of untreated specimen while LPwC treated specimen showed no pitting. The chemical composition and chemical state analyzed with XPS indicated a better passivation of the surface film on LPwC specimen.

Effect of jet velocity in co-flow water cavitation jet peening

Water cavitation jet peening (WCP) uses cavitation caused by the shear layer created by two concentric co-flowing jets with large velocity difference to introduce compressive residual stresses in the surface layers of metal components subjected to fatigue loading or a corrosive environment. Mass loss and surface alteration in WCP have been shown to be minimal compared to other mechanical surface enhancement techniques, such as shot peening (SP). This paper investigates the effect of concentric jet velocities in cavitation jet peening in a co-flow configuration on cavitation intensity and peening performance, which are characterized by accelerated erosion on Al 1100-O and Al 7075-T6 and a strip curvature test on Al 7075-T6. Accelerated erosion tests reveal that cavitation intensity and associated erosion (measured by mass loss) are greatly affected by the combination of the inner (Vin) and outer (Vout) jet velocities and the normalized standoff distance (sn). Two characteristic erosion patterns are found depending on the relative magnitudes of the jet velocities: one that is focused at the jet center (termed center regime) and another that is concentrated in the surrounding annular region (termed ring regime). Erosion tests on Al 1100-O and Al 7075-T6 give unexpectedly different results in terms of the maximum mass loss as a function of the jet velocities and standoff distance. When compared to strip curvature tests, it is found that the accelerated erosion tests on Al 1100-O do not capture the influence of inner jet velocity Vin and imply misleading trends with regard to outer flow velocity Vout. Erosion and curvature tests on Al 7075-T6 are found to be in good agreement and therefore are believed to be better suited to identify the optimum process conditions in WCP. Notwithstanding the higher mass loss density values observed in the center regime, the resultant strip curvature is found to be higher in the ring regime for a higher inner jet velocity Vin, potentially leading to higher and deeper compressive residual stresses.

Analysis of Shear Banding in Zr-Based Metallic Glass under Laser Shock Peening and Bending

Metallic glasses have been expected to be used as structural materials since their high strength and high hardness. Unfortunately, their catastrophic brittle fracture behavior with poor plasticity becomes the major weakness for structural application. It has been recognized that the mechanism of plastic deformation in metallic glasses is through the formation of shear bands, which provide them with limited ductility. Laser shock peening (LSP) is an innovative surface treatment technique which can introduce deep compressive residual stress layer to materials for improving their mechanical behavior. In this work, a finite-element model has been developed to numerically simulate the pure bending process, LSP and subsequent bending process. An advanced constitutive equation was established based on the large deformation theory of nonlinear mechanics, the free volume model and the Coulomb-Mohr yield criterion. The model is able to capture the following results: (i) for a given bending deflection, the shear band spacing increases with increasing plate thickness; (ii) for a given plate thickness, the free volume increases with the bending deflection; (iii) for a given thickness and a given deflection, the shear bands increase under the effect of LSP.

Surface Improvement of Shafts by Turn-Assisted Deep Cold Rolling Process

It is well recognized that mechanical surface enhancement methods can significantly improve the characteristics of highly-stressed metallic components. Deep cold rolling is one of such technique which 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 this paper, the effect of turn-assisted deep cold rolling on AISI 4140 steel is examined, with emphasis on the residual stress state. Based on the X-ray diffraction measurements, it is found that turn-assisted deep cold rolling can be quite effective in retarding the initiation and initial propagation of fatigue cracks in AISI 4140 steel.

Iron GH2036 alloy residual stress thermal relaxation behavior in laser shock processing

Abstract Laser shock processing (LSP) has a significant effect on the fatigue performance of Iron GH2036 alloy by inducing a deep compressive residual stress field. The effects of isothermal annealing treatments on Iron GH2036 alloy compressive residual stress after laser shock processing at various temperatures ranging from 200 °C to 650 °C were studied comprehensively using experimental and simulation methods. The thermal stability of compressive residual stress induced by LSP was investigated. The residual stress distributions of original and processed specimens were measured by the X-ray diffraction method. The results showed that the experimental measurements were well consistent with the simulation data. Residual stress of laser shock processing Iron GH2036 alloy had released but not completely released at the selected temperatures and during the initial exposure period (less than 30 min); the thermal relaxation magnitude was large and increased with the rise of applied temperature. We infer that the main mechanism of thermal relaxation is the mechanism involving rearrangement and annihilation of dislocation.

Application of the eigenstrain approach to predict the residual stress distribution in laser shock peened AA7050-T7451 samples

Laser Shock Peening allows the introduction of deep compressive residual stresses into metallic components. It is applicable to most metal alloys used for aerospace applications. The method is relatively expensive in application, and therefore development studies often rely heavily on Finite Element Modelling to simulate the entire process, with a high computational cost. A different approach has been used recently, the so-called eigenstrain approach. The present study looks at the feasibility of applying the eigenstrain method for prediction of the residual stress in a sample that contains curved surface features. The eigenstrain is determined from a simple geometry sample, and applied to the more complex geometry to predict the residual stress after Laser Shock Peening. In particular the prediction of residual stress at a curved edge, and for different values of material thickness, have been studied. The research has demonstrated that the eigenstrain approach gives promising results in predicting residual stresses when both the thickness and the geometry of the peened surface is altered.

Study of Laser Peening on TiAl Alloy Properties with Different Parameters

Laser peening is a novel surface treatment process that generates deep compressive residual stresses and microstructural changes and thereby dramatically improves fatigue strength of critical metal aircraft engine parts. In order to study the effects of laser peening on properties of TiAl alloy, Surface micro-hardness, microstructural, residual stress and pole figures before and after laser peening were tested. The experimental results showed that surface micro-hardness increased by 23%. The compressive residual stress increased 20 times. The texture in the normal direction of 8J peened sample showed a strong fiber texture components 10o away. In conclusion, the laser peening could improve properties of TiAl alloy.

Research on Laser Peening of TC21 Titanium Alloy with High Energy Laser

Abstract Laser peening (LP) also known as laser shock processing (LSP), is a surface enhancement technique that induces intensive plastic deformation, high dislocation density and deeper compressive residual stress to improve the surface performance of materials. The microhardness, surface profiles, roughness, and the residual stress of TC21 alloy were tested by LSP with high energy laser. The results show that the microhardness of the shocked area is improved apparently compared with that of unshocked area. The magnitude of dent is related with the diameter and the distribution of laser spot. The roughness Ra of shocked area is less than 0.8 μm. The compressive residual stress is enhanced greatly. The investigations show that the technology of LSP could further improve the performance of TC21 alloy.

Evolution of a laser shock peened residual stress field locally with foreign object damage and subsequent fatigue crack growth

Foreign object damage (FOD) can seriously shorten the fatigue lives of components. On the other hand, laser shock peening improves fatigue life by introducing deep compressive residual stress into components. In this paper we examine how the non-uniform steep residual stress profile arising from FOD of laser peened aerofoil leading edges varies as a function of fatigue crack growth under high cycle fatigue and mixed high and low cycle fatigue conditions. The ballistic FOD impacts were introduced by impacting a cube edge head-on (at an angle of 0°) to the leading edge. The residual stress distributions have been mapped by synchrotron X-ray diffraction prior to cracking and subsequent to short (∼1mm) and long (up to 6mm) crack growth. The results suggest that the local residual stress field is highly stable even to the growth of relatively long cracks.

Deep surface rolling for fatigue life enhancement of laser clad aircraft aluminium alloy

Deep surface rolling can introduce deep compressive residual stresses into the surface of aircraft metallic structure to extend its fatigue life. To develop cost-effective aircraft structural repair technologies such as laser cladding, deep surface rolling was considered as an advanced post-repair surface enhancement technology. In this study, aluminium alloy 7075-T651 specimens with a blend-out region were first repaired using laser cladding technology. The surface of the laser cladding region was then treated by deep surface rolling. Fatigue testing was subsequently conducted for the laser clad, deep surface rolled and post-heat treated laser clad specimens. It was found that deep surface rolling can significantly improve the fatigue life in comparison with the laser clad baseline repair. In addition, three dimensional residual stresses were measured using neutron diffraction techniques. The results demonstrate that beneficial compressive residual stresses induced by deep surface rolling can reach considerable depths (more than 1.0mm) below the laser clad surface.