Deep Compressive Residual Stresses Research Articles

The effects of stress ultrasonic rolling on the surface integrity and fatigue properties of TiB2/7050 Al composite

In the present work, a new surface mechanical strengthening (SMS) technique named stress ultrasonic rolling (SUR), which is ultrasonic rolling (UR) assisted with a uniaxial tensile elastic stress field, was reported and successfully conducted on 5 % wt TiB2/7050Al composite. Based on the experimental analysis and numerical simulation, the effects of SUR on surface integrity and fatigue performance of the composite were comprehensively investigated. Compared with ordinary UR, SUR further enhanced the fatigue strength at 106 cycles of the composite by 5 % and prolonged the fatigue life of the composite under the stress level of 532.8 MPa by 1.8 to 3.3 times, which are attributed to the comprehensive effect of excellent surface finish, larger and deeper compressive residual stress (RSc) layer, dispersed TiB2 particles distribution in the surface, and superficial nanocrystalline structures. Besides, the assistance of uniaxial tensile elastic stress field did not increase the amount of surface plastic deformation, which indicates the RSc generated by SUR should have similar mechanical and thermal stability to that produced by ordinary UR treatment.

Analysis of the thermal stability of residual stresses induced in Ti-6Al-4 V by high density LSP treatments

Laser Shock Processing (LSP) is increasingly applied as an effective technology for the improvement mechanical and surface properties of metallic materials for different types of components, mostly as a means of enhancement of their fatigue life behavior. As reported in previous contributions, a main effect resulting from the application of the LSP technique consists in the generation of relatively deep compression residual stresses fields allowing an improved mechanical behavior. In this paper, the special case of Ti-6Al-4 V alloy is considered with specific consideration of the microstructural changes and residual stresses fields justifying those macroscopic effects. In particular, 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. Emphasis is placed on the study of the thermal stability of these fields after an aging heat treatment at typical high temperature working conditions. The results show that, according to expectations, a certain level of residual stresses and dislocation density remains after in-work thermal aging. The combination of different material characterization techniques has made possible to obtain a correlated set of results highlighting the complementarity of different scale approaches from surface (X-ray) to bulk (neutrons) methods. A model based on the evolution of immobile dislocation density is proposed to rationalize the distribution of dislocation densities, pointing out that plastic deformation is retained. The results clearly show such material improvement stability under typical heavy-duty conditions, thus endorsing the use of the LSP for Ti-6Al-4 V as a suitable technology for industrial applications in relatively high temperature conditions.

Metallic surface architectures realized through plastically deformed micro‐volumes

AbstractFor developing performing medical devices, the biomechanical adaptation plays a key role for both, the bulk material (at macro scale) and surfaces (at micro/nano scale). As concerning the interaction of living cells with artificial surfaces, the cellular mechano‐sensitivity has direct effects on tissue structure and functions. In this work, surfaces architectures, obtained by self‐nano‐crystallization, were realized on a gum‐alloy as substrate (titanium‐niobium‐zirconium‐iron‐oxygen), using silicone masks with micrometric patterns/perforations, on which a near‐surface‐severe‐plastic‐deformation process was applied. For finding 3D suitable geometries of the perforations and their distribution on the masks, generative design algorithms were developed using dedicated software tools. The masks were obtained by 3D printing using the fused filament fabrication method. Multidirectional repeated mechanical impacts on the sample surface with high velocity balls at high strain rates were applied. Deeper compressive residual stresses are generated, creating a superficial self‐nano‐crystallized structure on un‐covered areas, due to small volume‐multidirectional local plastic deformation. The applied parameters have a direct effect on local micro‐topography and microstructure evolution.

Study on surface properties and fatigue performance of aluminum alloy plate with prestressed ultrasonic peening

Prestressed ultrasonic peening (UP) is beneficial to enlarge bending deformation and improve surface properties of the plate compared to free UP process. The effects of processing parameters on variation of surface topography, surface roughness and hardness, residual stress distribution are analyzed. Then the microstructure and fatigue life with prestressed UP is studied. The results show that smaller surface roughness, larger surface hardness, larger and deeper compressive residual stress are generated in prestressed UP compared to free UP. Small prebending curvature radius and offset distance both contribute to enhancement of surface properties. Large firing pin velocity is beneficial for large surface hardness as well as larger and deeper compressive residual stress; nevertheless, it will increase the surface roughness. The UP process mainly improves the high cycle fatigue life of the material, while there is no difference of the low cycle fatigue life between the peened and un-peened samples. The high cycle fatigue life significantly increases at small stress after peening process and it can be further enhanced after prestressed UP. Deformation band with peened indentations are generated on the surface. With the experiments of distributions for hardness, yield stress and residual stress, the strengthening mechanism in peening process is discussed.

Effect of Laser Peening on Microstructural Changes in GTA-Welded 304L Stainless Steel.

The introduction of tensile residual stress has led to the induction of damage such as fatigue, corrosion fatigue, and stress corrosion cracking (SCC) in stainless steel in association with the influence of environments, components, surface defects, and corrosive factors during its use. Compressive residual stress can be achieved through various techniques. Among several methods, laser peening can be more attractive as it creates regularity on the surface with a high-quality surface finish. However, there is very little research on heavily peened surface and cross-section of stainless steel with very deep compressive residual stress. This work focused on welding and laser peening and the influence of Al coating on the microstructural changes in 304L stainless steel. The specimen obtained by laser peening had a very deep compressive residual stress of over 1 mm and was evaluated based on microstructural and hardness analysis. Therefore, a model for microstructural change by laser peening on welded 304L stainless steel was proposed.

Numerical Analysis of Surface Rolling Effects on Fatigue Life Enhancement of Wire Arc Additively Manufactured Parts

With the advancements in additive manufacturing (AM) technologies, it is expected that fast and efficient production using the AM techniques will gradually replace the traditional manufacturing processes. An important consideration in the design and life assessment of AM-built parts is the asset integrity management and in particular fatigue life enhancement of such components. Surface rolling treatment is known to be an efficient way to introduce deep compressive residual stresses into engineering components, and therefore it has been considered as a practical and effective life enhancement technology. In this study, the surface rolling effects on the fatigue life enhancement of wire arc additively manufactured (WAAM) parts have been investigated. For this purpose, a compact tension specimen geometry was modeled in ABAQUS and the rolling process was simulated by means of finite element simulations to predict the extent of compressive residual stresses induced into a WAAM built part. The simulation results have been discussed in terms of the beneficial effects of surface rolling on fatigue life enhancement of WAAM built parts.

Prediction of fatigue crack propagation behavior of AA2524 after laser shot peening

Laser shot peening is a relatively novel surface treatment technology generating deep compressive residual stress and further retarding fatigue crack growth rate (FCGR). Meanwhile, the load ratio, R, plays an important role in fatigue crack growth behavior. Evaluating the remaining life of components accurately is essential to damage tolerance design, many efforts have been applied to correlate the R-effect. In this paper, the effect of load ratio on the gain of AA2524′s fatigue life after laser shot peening have been thoroughly studied. Initially, the fatigue crack growth rate of AA2524 after laser shot peening was measured under different load ratios. Subsequently, a comparative analysis between the FCGR for AA2524 before and after laser shot peening was thoroughly conducted. Then, the crack closure model has been applied to correlate the R-effect of peened specimens, but the results are unsatisfied due to the ignore of compressive residual stress. To address this issue, a novel method of predicting crack growth life considering compressive residual stress was proposed. The artificial neural network model and support vector regression model based on particle swarm optimization algorithm were trained by the FCGR data, and the fatigue crack growth life is predicted by a cycle-by-cycle method. The result shows the artificial neural network model is more accurate than the support vector regression model, and this intelligent algorithm can be a potentially effective way to predict fatigue crack growth behavior.

Individual and synergistic effects of thermal and mechanical surface post-treatments on wear and corrosion behavior of laser powder bed fusion AlSi10Mg

Materials fabricated by laser powder bed fusion (LPBF) are generally characterized with internal defects, inhomogeneous microstructure and rough surface with multiple forms of irregularities in their as-built state. Post-treatments are needed to resolve the issues associated with these imperfections. In this study, the individual and synergistic effects of different post-treatments including heat treatment, shot peening and ultrasonic nanocrystalline surface modification on microstructure, surface morphology, roughness, mechanical, corrosion and tribological properties of LPBF AlSi10Mg specimens have been investigated. Two different sets of parameters were considered for the surface treatments, both with mechanical energy source. Apart from surface characteristics, various properties including microhardness and residual stresses as well as wear and corrosion behavior of post-processed specimens were analyzed and compared with those of the as-built condition. The results indicated that hybrid post-treatments have the highest positive effects on wear and corrosion resistance as their kinetic energy affected multiple physical aspects; these include the microstructure leading to surface grain refinement paired with deep compressive residual stress field and improved surface morphology offering a more regular surface.

Through-Thickness Microstructures and Yield Strength Enhancement for AZ31 Mg Sheets Treated by Ultrasonic Nanocrystal Surface Modification

An ultrasonic nanocrystal surface modification (UNSM) technique was applied to a 1-mm thick AZ31 magnesium sheet. UNSM is a relatively new surface modification technique in which a hard, hemispherical tip (2.38 mm in diameter) strikes the surface at an ultrasonic frequency to induce plastically deformed gradient microstructures and deep compressive residual stresses through the thickness. After the UNSM treatment, the through-thickness microstructures were thoroughly investigated using electron microscopy and electron backscatter diffraction analysis. The through-thickness microstructures revealed zones that were severely deformed (down to 200 µm from the surface) and twin-dominated (200~300 µm deep from the surface). The severely deformed zone consisted of shear banding, grain subdivision and reorientation, due to the strong plastic deformation, accompanied by the formation of {10<math xmlns="http://www.w3.org/1998/Math/MathML"><mover accent='true'><mn>1</mn><mo>−</mo></mover></math>2} tensile twins (despite compressive strikes by the hemispherical tip), {10<math xmlns="http://www.w3.org/1998/Math/MathML"><mover accent='true'><mn>1</mn><mo>−</mo></mover></math>1}-{10<math xmlns="http://www.w3.org/1998/Math/MathML"><mover accent='true'><mn>1</mn><mo>−</mo></mover></math>2} double twins and {10<math xmlns="http://www.w3.org/1998/Math/MathML"><mover accent='true'><mn>1</mn><mo>−</mo></mover></math>1} compression twins. The cause for tensile twinning was examined through a literature survey. In the twin-dominated zone, the twining activity prevailed as the slip activity gradually decayed through the thickness. The UNSM-induced hardness and microstructure enhancement was found to be effective down to about 300~400 μm deep from the surface. Finally, the source of the increase in yield strength after the UNSM treatment of the AZ31 sheet was analyzed, and focused on individual cases of microstructural enhancement in the severely deformed zone and the twin zone, and the compressive residual stress.

Improvement in Bending Strength of Silicon Nitride through Laser Peening.

This study aimed to improve the bending strength and reliability of ceramics using laser peening (LP). In the experiment, LP without coating (LPwC) and with coating (LPC) were applied to silicon nitride (Si3N4) under various conditions. The surface roughness, residual stress, and bending strength were then measured for the non-LP, LPwC, and LPC specimens. The results show that the LPwC specimen had a greater surface roughness but introduced larger and deeper compressive residual stress when compared with the non-LP and LPC specimens. In addition, the bending strength of the LPwC specimen was higher and scatter in bending strength was less compared with the non-LP and LPC specimens. This may be attributed to the transition of the fracture initiation point from the surface to the interior of the LPwC specimen because of the compressive residual stress introduced near the surface. Thus, it was demonstrated that the application of LP is effective in improving the strength and reliability of ceramics.

小型可搬型レーザーピーニング装置による金属材料の残留応力および疲労特性の改善

The advantage of laser peening is the possibility of fine execution management and the capability to introduce deep compressive residual stresses on the material surfaces. It is well known to be highly effective in inhibiting stress corrosion cracking and fatigue cracking on material surfaces. In addition, laser peening has an excellent effect on improving the fatigue strength of welds, which compensates for the disadvantage of high strength steels when welded. Laser peening has a high potential for enhancing material surface, but the high-power laser used requires clean room facilities, large equipment and severe operating conditions. Therefore, the application of the laser peening has been limited to the mitigation of high cycle fatigue of jet engine fan blades and stress corrosion cracking of nuclear reactor structures. If microchip lasers, which are small and easy to handle, could be used as a light source for laser peening, it would be possible to apply them not only to production processes in factories but also to existing structures such as bridges, airplanes, etc. to which conventional lasers have been difficult to apply for the above reasons. In this presentation, we report the results of the laser peening treatment of A7075 (aluminum alloy) and HT780 (high strength steel) with the laser pulse energy of less than 10 mJ. Even at such a low pulse energy, compressive residual stress was imparted and fatigue properties were improved.

Finite-element analysis of residual stresses in the TС4 titanium alloy treated by laser shock peening

Laser shock peening is an effective tool for generating deep compressive residual stresses (more than 1 mm) near the surface of metallic structures. This can improve their mechanical properties and fatigue performance. For this purpose, it is necessary to find optimal laser shock parameters. In this paper, the effect of laser intensity and peening pattern on the residual stress field in a Ti-6Al-4V titanium alloy is investigated through numerical simulations. The applied approach includes two steps. In the first stage, the modeling of elasto-plastic stress waves is performed. The second stage involves a solution of static equilibrium problem for residual stress distribution taking into account plastic deformation fields. The analysis was carried out for a square plate with a thickness of 3 mm, the central part of which is subjected to a series of square laser pulses. The pulse length was the same for all considered cases, and it was equal to 3 mm. The results of the analysis demonstrate that, when the laser pulse power density increases, the maximum value of compressive stress increases as well, and tensile stresses occur on the opposite side of the sample. The application of the two-sided laser peening (no overlap) or one-sided peening (50% overlap) can eliminate this effect. An increase in the maximum compressive stress is observed on both sides of the specimen in these cases. Due to an increase in the number of layers in the case of the one-sided peening (no overlap), the maximum compressive stress and penetration depth become greater. However, tensile stresses in the volume of the sample also increase.

Response of silicon nitride ceramics subject to laser shock treatment

A comprehensive and novel investigation on multiple-layer, square-beam laser shock treatment (“laser peening”) of Si 3 N 4 ceramics is reported in this work. Surface topography, hardness, fracture toughness (K Ic ), residual stresses, and microstructural changes were investigated. The evaluation of fracture toughness via the Vickers hardness indentation method revealed a reduction in crack lengths produced by the indenter after laser shock treatment (LST). Upon appropriate calculation, this revealed an increase in K IC of 60%. This being attributed to a near-surface (50 μm depth) compressive residual stress measured at −289 MPa. Multiple layer LST also induced beneficial residual stresses to a maximum measured depth of 512 μm. Oxidation was evident, only on the top surface of the ceramic, post LST (<5 μm depth) and was postulated to be due to hydrolyzation . The surface enhancement in K IC and flaw-size reduction was assigned to an elemental change on the surface, whereby, Si 3 N 4 was transformed to SiO 2 , particularly, with multiple layers of LST. Compressive residual stresses measured in the sub-surface were attributed to mechanical effects (below sub-surface elastic constraint) and corresponding shock-wave response of the Si 3 N 4 . This work has led to a new mechanistic understanding regarding the response of Si 3 N 4 ceramics subject to the LST deployed in this resesrch. The findings are significant because inducing deep compressive residual stresses and corresponding enhancement in surface K IC are important for the enhanced durability in many applications of this ceramic, including cutting tools, hip and knee implants, dental replacements, bullet-proof vests and rocket nozzles in automotive, aerospace, space and biomedical industries.

Deep cold rolling of single crystal nickel-based superalloy CMSX-4

Nickel-based superalloys used in the aerospace industry are regularly subjected to high temperature and stress cycles. In order to extend component life, many components are subjected to a cold work process during manufacturing. One such technique is deep cold rolling, a highly controllable process which utilises a roller ball to impart cold work and deep compressive residual stress into a surface. In this study, local misorientation analysis of electron backscattered diffraction data has been used to quantify the surface cold worked layers in single crystal samples of the superalloy CMSX-4®, deep cold rolled with different pressures, ball diameters and roll orientations. As expected, it was found that greater pressure and ball diameter both increased the cold work depth, with pressure also affecting the amount of cold work per depth. Different roll orientations produced very similar distributions of cold work. In comparison with mechanical shot peening, another surface hardening technique, deep cold rolling is shown to produce a much deeper layer of relatively low cold work.

Effects of ablative and non-ablative laser shock peening on AA7075-T651 corrosion and fatigue performance

The fatigue performance from pre-corroded pits was studied in laser-shock-peened AA7075-T651 with and without a protective ablation-layer. Surface and microstructural characterisation showed laser-shock-peening generated residual stresses up to −400MPa, limited hardness and moderate surface roughness increase. The laser-shock-peened specimens were exposed to 3.5 wt-% sodium chloride solution for different levels of galvanostatic control. The compressive residual stresses did not significantly affect corrosion behaviour, or corrosion pit morphology. Laser-shock-peening-induced surface roughness had the most detrimental impact on corrosion performance. Fatigue testing of pre-corroded AA7075-T651 showed pits act as stress concentrations. Cracks initiated shortly after dynamic loading, reducing fatigue life by 50%. Laser-shock-peening increased fatigue life by 400% compared to corroded-untreated AA7075-T651, due to residual stresses effectively counteracting stress concentrations produced by pits. Highlights Pre-corroded laser-peened (LSP and LSPwC) AA7075-T651 fatigue performance is investigated. XRD and incremental hole drilling show deeper compressive residual stresses for LSPwC compared to LSP. Electrochemical tests show no significant changes in corrosion behaviour after laser peening. Fatigue testing and fractography show compressive residual stresses effectively counteract stress concentrations at pits.

Experimental study and fatigue life prediction on high cycle fatigue performance of laser-peened TC4 titanium alloy

Laser shock peening (LSP) is an innovative surface treatment, which has been successfully applied in aero-engine compressor blades to improve high cycle fatigue performance. Deep compressive residual stress layer with a gradient distribution can be produced, which is the special advantage of LSP and plays a main role in high cycle fatigue performance improvement. Therefore, compressive residual stress, and especially the gradient distribution feature should be simultaneously considered in high cycle fatigue life prediction for laser-peened component. In this work, firstly, TC4 titanium alloy, a typical material used for aero-engine compressor blade, was treated by LSP with different power densities, overlapping rates and shocks respectively. High cycle fatigue life of TC4 titanium alloy is enhanced from 1473 × 103 cycles to 6148 × 103 cycles, by 317% increase with LSP treatment. Compressive residual stresses with a gradient distribution induced by LSP were disposed into an equivalent parameter according to critical plane method. At last, high cycle fatigue life of laser-peened TC4 titanium alloy specimens was successfully predicted within twice-fold error band, comprehensively considering equivalent compressive residual stress and FINDLEY model.

Experimental research on global deformation and through-thickness residual stress in laser peen formed aluminum plates

Laser shock peening (LSP) is an emerging technique for effectively forming metal sheets due to its ability to produce higher and deeper compressive residual stress than conventional shot peening. Because it is crucial to understand the evolution of through-thickness residual stress in peening formed components, the purpose of this research was to investigate the correlation between forming deformation and altered residual stress fields in laser-peened Al7055-T7751 plates of distinct thickness. The effect of geometry thickness on global deformation and through-thickness residual stress was investigated experimentally. First, the global deformation and surface residual stress were measured and compared quantitatively. Additionally, the through-thickness of the residual stress was determined using the slitting technique, and the measurements were validated using X-ray diffraction (XRD). The experimental results indicated that the spatial distribution of the residual stress in the laser-peened plate was the superposition of the LSP-induced elastoplastic stress and the elastic bending stress. Geometry thickness not only affected bending stiffness and macroscopic deformation but also played a significant role in the evolution of residual stress across the entire section. Under the identical peening conditions, a 3-mm plate inherited a 0.5-mm compression layer with approximately 180–280 MPa stress magnitude, whereas a 9-mm plate inherited a 1-mm compression layer with approximately 340–480 MPa of stress magnitude. Further analysis revealed that the elastic bending across the section had a certain compensation effect on the compressive stress of the peened surface. Thick plates (6–9 mm) tended to inherit consistent plane stress states because they demonstrated a relatively small deformation compatibility due to the high stiffness.

The effects of shot peening, laser shock peening and ultrasonic nanocrystal surface modification on the fatigue strength of Inconel 718

As most of the failures in engineering components initiate from the surface layer, applying surface treatments can play a crucial role in controlling material performance and lifetime. In this study, different surface severe plastic deformation techniques including severe shot peening, laser shock peening and ultrasonic nanocrystal surface modification have been considered. The effects of process parameters and the kinetic energy of each treatment on the microstructure, mechanical properties and fatigue behavior of nickel-based super-alloy Inconel 718 have been investigated. The results revealed that using the proper parameters to increase the kinetic energy of the applied surface treatments, it is possible to effectively promote surface grain refinement and induce a deep compressive residual stress field in Inconel 718 samples. Among the applied treatments, ultrasonic nanocrystal surface modification was found to be the most efficient one in improving the mechanical properties as it led to the most significant fatigue performance, followed by severe shot peening and laser shock peening.

Fatigue performance of laser shock peened Ti6Al4V and Al6061‐T6 alloys

AbstractPropagation of fatigue crack in laser shock peened metallic component is retarded by introduction of deep compressive residual stresses during the process. Apart from large compressive residual stress build‐up near the surface, surface topography is also significantly altered during laser shock peening. However, a systematic evaluation of the individual effects of residual stresses and surface topography as well as their interaction effects on fatigue crack initiation and growth is still lacking. This study investigates the effect of laser shock peening on near‐surface properties of Ti6Al4V and Al6061‐T6 alloys and how these changes dictate their overall fatigue performance. Microstructure, surface topography, hardness and residual stress state of the alloys are investigated before and after peening, and four‐point bending fatigue tests are performed to obtain Wöhler curves (S‐N curves) for each case. The results show that both surface residual stresses and surface topography influence the fatigue life of a component. Compressive residual stresses were induced to a depth beyond 1 mm for both alloys. Fatigue life of Ti6Al4V alloy increased by at least 1.28 times after laser shock peening due to the introduction of deep compressive residual stresses with minimal change in surface topography. By contrast, the improvement is nominal for Al6061‐T6 alloy due to the surface roughening effect introduced by laser peening, which results in early crack initiation and negates the beneficial effect of compressive residual stresses.

Effects of laser shock peening on the mechanisms of fatigue short crack initiation and propagation of AA7075-T651

A laser shock peening (LSP) treatment was performed on AA7075-T651 for maximum fatigue improvement. Surface and microstructural characterisation techniques (micro-hardness, SEM-EBSD, contact-profilometry) showed LSP surface modification was limited, and LSP generated deep compressive residual stresses above −300 MPa. Fatigue testing showed a two-order magnitude increase in overall life, due to the mechanism of crack initiation changing from surface second-phase particles to subsurface crack initiation dependent on the local stress field. Modelling highlights the sensitive balance between surface roughness (including LSP-induced pits) and residual stress on the micro-mechanism of crack initiation, and how this can be used to maximise fatigue life extension.