Laser Shock Peening Research Articles

Fatigue Properties of Spring Steels after Advanced Processing.

This article deals with the effect of strain-assisted tempering (SAT) on the fatigue properties of 54SiCr6 steel used for spring steel wires in a wide variety of automotive applications, including coil springs. This steel spring wire is extremely strong, having a high elastic limit and yield point, giving the steel excellent energy accumulation and fatigue properties. This combination opens up new possibilities in helical and cylindrical coil spring design, resulting in the reduction of both size and weight. Lightweight coil springs lead to improvements in fuel consumption, stability and vehicle traction. A large plastic deformation and SAT were applied to enhance the yield point of the study material. Improvements in the static and cyclic properties of steel springs were investigated using tensile tests and 3PB fatigue tests at ambient temperature. In addition, an advanced laser shock peening (LSP) process was employed to increase the fatigue resistance of the SAT material. The results presented here show great improvements in the static and fatigue properties over commercial steel treatment. The material quality of the wires was evaluated to be insufficient for further processing with cold coiling.

Tribological Performance of Laser Shock Peened Cold Spray Additive Manufactured 316L Stainless Steel

Abstract In recent years, cold spray additive manufacturing (CSAM) has become an attractive technology for surface modification and protection. However, due to the intrinsic porous nature of CSAM coatings, they suffer from rapid material degradation due to premature brittle fracturing induced by tribological interactions. In this work, laser shock peening (LSP) was utilized as a post-processing technology to mitigate the surface porosity and augment the surface characteristics of CSAM 316L stainless steel (SS). Due to the synergistic influence of severe plastic deformation and rapid surface heating, the surface porosities were effectively healed, thus reducing the surface roughness. Combined with the surface-strengthening effects of LSP, the frictional resistance and transfer layer formation on the CSAM LSP surfaces were reduced. The underlying mechanisms for these findings were discussed by correlating the atomic, microstructural, and physical features of the LSP surfaces. Based on these findings, it can be suggested that LSP is indeed a useful technique to control the surface characteristics of CSAM 316L SS coatings.

Femtosecond Laser Shock Peening Residual Stress and Fatigue Life of Additive Manufactured AlSi10Mg

Femtosecond Laser Shock Peening Residual Stress and Fatigue Life of Additive Manufactured AlSi10Mg

Superior effects of hybrid laser shock peening and ultrasonic nanocrystalline surface modification on fatigue behavior of additive manufactured AlSi10Mg

The surface texture of metallic parts produced by laser powder bed fusion (L-PBF) in the as-built condition detrimentally affects their mechanical properties, especially fatigue behavior. Accordingly, applying surface post-treatments has become an attractive approach to improve the mechanical performance of these materials. In the present study, both the individual and combined effects of post-processing methods, i.e., laser shock peening (LSP) and ultrasonic nanocrystalline surface modification (UNSM) with the same intensity of 10–12 A [0.001 in.], were systematically investigated on mechanical properties and fatigue behavior of L-PBF AlSi10Mg specimens. A wide range of experiments involving microstructural characterization, hardness and residual stresses measurements, porosity and surface texture analyses, tensile tests, and rotating bending fatigue tests were conducted. The results revealed that the hybrid LSP + UNSM process resulted in significant improvement in mechanical properties and fatigue behavior due to (i) sub-surface pores closure up to the depth of 517 μm, (ii) 60 % surface hardness improvement, (iii) inducing −420 MPa surface compressive residual stresses, and (iv) surface roughness reduction up to 70 %. The fatigue life was improved up to 75, 56, and 35-fold compared to the as-built state after applying LSP + UNSM, UNSM, and LSP treatments, respectively.

Effects of power density on residual stress and microstructural behavior of Ti-2.5Cu alloy by laser shock peening without coating

The present study concerns the micro structural changes of Ti-2.5 Cu after LPwC at various laser intensities (i.e., ≈3 GW cm−2, 6 GW cm−2 and 9 GW cm−2). X-ray diffraction (XRD), electron backscattered diffraction (EBSD), transmission electron microscope (TEM) were used to analyze the microstructure. X-ray photoelectron spectroscopy (XPS) was used to characterize the oxidation behavior of the samples after peening. The depth profile of residual stress was obtained using the sin2ѱ X-ray diffraction technique, and the compressive residual stress increased for all the peened conditions in the surface and sub-surface regions, and the maximum obtained was −402.44 MPa at 50 μm depth for the 9 GW cm−2. Furthermore, the cross-sectional micro hardness of the peened samples improved at a depth of 50 μm with increasing laser intensity. For the highest energy, the maximum hardness at a depth of 50 μm was 439.40 HV (0.1), which was higher than the unpeened sample (315.56 HV0.1). TEM studies showed nano precipitates (50–170 nm), mechanical twins with a width ranging from (27.20–118.51 nm) and cell size dislocation structure. EBSD results conformed the high angle grain boundaries increased to 43% with the formation of mechanical twins at the higher energy.

Effects of high-energy laser peening followed by pre-hot corrosion on stress relaxation, microhardness, and fatigue life and strength of single-crystal nickel CMSX-4® superalloy

This study investigated the stress relaxation and fatigue life and strength of laser-peened single-crystal nickel superalloy specimens compared to unpeened and shot-peened specimens following hot corrosion exposure and then fatigue testing. The specimens were treated by conventional laser peening and a new cyclic laser peening plus thermal microstructure engineering process. The latter treatment supports the benefit of a unique process involving application of layers of laser peening using high energy with large footprint spots combined with interspersed cyclic annealing. Stress measurements by slitting showed the plastic penetration depth of laser peening exceeded shot peening by a factor of 24. Unpeened and peened specimens were exposed to sulphate corrosives at 700 °C for 300 h and then fatigue tested. Tests of five non-laser-peened specimens all failed in low-cycle fatigue regime, whereas three identically tested laser-peened specimens all achieved multi-million-cycle runout without failure, indicating fully consistent large benefit for life by laser peening. Additional tests also showed fatigue strength improvement of 2:1 by laser peening. Residual stress measurements post hot-corrosion exposure and fatigue testing showed notable 5 mm depth retention of residual eigenstress in a laser-peened specimen.

Nanoengineered Laser Shock Processing Via Pulse Shaping for Nanostructuring in Metals: Multiscale Simulations and Experiments

AbstractModulating the heating and cooling during plastic deformation has been critical to control the microstructure and phase change in metals. During laser shock peening under optimal elevated temperatures, high-density dislocations and nanoprecipitates can be generated to greatly enhance material strength and fatigue life in metals. Currently, heating control during laser shock is limited to steady-state heat transfer, such as hot plate, irradiative heating, or far-infrared heating, which is slow for practical treatment and does not provide the transient conditions for generating nanostructures during shock processing. In this paper, we propose a general methodology to modulate the heating and cooling during laser shock processing via temporal pulse shaping, namely dual pulse laser shock peening (DP-LSP), which combines both ultrafast-heating and laser shock peening in one operation to generate desired microstructure and mechanical property. By modulating the duration of pulses as well as the spacing between pulses, different processing temperatures can be achieved. To test the feasibility of this novel process, DP-LSP has been applied to an Al matrix nanocomposite. Single pulse laser shock peening was able to remelt large second phase precipitates due to fast cooling, resulting in smaller grains (500 nm), while using DP-LSP with the appropriate pulse duration, dynamic precipitation effects can generate nanosized (30 nm) intermetallic phase Al3Ti with high density. By generation of grain size refinement, high-density nanoscale precipitates, and dislocations after DP-LSP, the yield strength increases by 18% and 102% compared with single pulse processing, and original sample respectively. Finite element method modeling was used to simulate the temperature profile in the alloy during the temporal modulated dual laser pulsing. A phase-field model and multiscale dislocation dynamics were applied to study dislocation dynamics and nanoprecipitation generation during DP-LSP, and their interactions at elevated temperatures. The work provides the basis for controlling microstructure in DP-LSP to enhance mechanical properties in metals.

The effect of surface treatment and orientation on fatigue crack growth rate and residual stress distribution of wire arc additively manufactured low carbon steel components

The directed energy deposition (DED) processes, such as laser metal deposition or Wire Arc Additive Manufacturing (WAAM), are gradually becoming the preferred method for fabrication of large-scale components using metal additive manufacturing (AM) technology. In this work, the possibility of fatigue life enhancement in WAAM built low carbon steel components, by means of rolling and laser shock peening surface treatment techniques, was investigated. A series of fatigue crack propagation tests were performed on surface treated ER70S-6 and ER100S-1 WAAM built specimens, and the results were analysed and compared with the untreated materials tested under the same loading conditions. The obtained results were interpreted in terms of the sensitivity of the cracking behaviour to the specimen orientation and extraction location. Furthermore, the residual stress profiles were measured, before and after applying the surface treatment techniques, and the effects of locked-in residual stresses on the fatigue performance of WAAM built components were discussed. Finally, a detailed texture analysis was performed on the surface treated and untreated regions of both WAAM built materials considered in this work. The obtained results from this study provide an insight into the advantages and disadvantages of various surface treatment techniques for fatigue life enhancement of WAAM built components with the view to extend the application of this advanced manufacturing technology to a wider range of industrial applications.

Laser shock peening rounds influencing microstructural and mechanical properties of 300M steel

Laser shock peeing (LSP) was introduced to produce surface gradient structures on 300M steel to improve its performance, service life and related workpiece safety. The influences of LSP rounds on microstructural and mechanical properties were investigated to optimise the LSP processing. Smoother surface, surface nanocystallization, and gradient structure, hardness, compressive residue stress was observed LSP. Strength and plasticity of 300M steel improved significantly due to the surface changes induced by one round of LSP. However, excessive LSP rounds tend to deteriorate the surface roughness and mechanical properties despite increased surface nanocrystallization, hardening zone depth and compressive residual stress.

Promoted phase transformation of α2 to O in Ti2AlNb alloy with improved mechanical performance via laser shock peening

Phase transformation and microstructure change are important ways for the mechanical property improvement of Ti2AlNb alloys, such as ductility, tensile strength, and fracture toughness. However, the brittle α2 phase is stable, and phase transformation is difficult to be introduced at a lower temperature. This seriously affects the mechanical properties of Ti2AlNb alloy. Here, we propose a new method to promote phase transformation in Ti2AlNb alloy by deformation-induced defects through laser shock peening (LSP). The results show that after LSP, high-density defects were observed in the α2 phase, such as dislocations and stacking faults (SFs). After heat treatment at 600 ℃, the phase transformation from α2 phase to O phase was observed in the Ti2AlNb alloy treated by LSP, forming twinned and chessboard-like morphologies with nanoscale grain size. High-density defects induced by LSP reduced the thermodynamic barrier of phase transformation from α2 phase to O phase. LSP promoted the lattice distortion and Nb element diffusion in Ti2AlNb alloy at high temperatures. This work provides a new method to promote phase transformation in Ti2AlNb alloy, which may be used to improve other materials' mechanical properties.

Wear resistance of AISI 5140 steel with Micro-laser shock peening

AISI 5140 steel is widely used in automobile steering shafts, whereby its abrasion resistance directly affects the reliability of transmission components. This paper proposes Micro-laser shock peening (Micro-LSP) as a new laser shock peening (LSP) treatment. The surface of the AISI 5140 steel was examined after Micro-LSP, and its influence on the wear resistance was determined. Compared with LSP, Micro-LSP exhibited a better strengthening effect on the wear and surface hardness. After Micro-LSP treatment, the surface hardness of the sample was 368.7 HV0.5, the depth of the hardened layer was 580 μm, and the surface roughness was 1.40 μm. This improved wear resistance is attributed to the higher microhardness and formation of a surface oxide layer.

Investigation on the effect of low pulsed laser shock peening on corrosion behavior of 316 L stainless steel

A 316L alloy finds its application in diverse industries most profoundly used for marine application. Effect of corrosion becomes an inevitable negative attribute in the marine industry, particularly, the development of oxide layer formation restricted by the NaCl which creates a broken surface, developing into an inevitable failure of the surface and thereby the machinery. This study aims to develop a processing technique, namely the low pulse energy, laser shock peening process to slow down and inhibit the growth of the oxide layer. 316L alloys are primarily subjected to a low pulse laser shock peening process which is then treated in 3.5% of NaCl solution to analyze the corrosion behavior. The treated and untreated samples are subjected to various characterization techniques such as SEM to identify microstructural variation, XRD to evaluate the compressive residual stress induced, potentiostat galvanic device to understand the corrosion resistance of treated 316L. Promising results were observed through the polarization curve, wherein no 40% lower corrosion rate was obtained for the low pulse energy LSP of 400 mJ.

Correlation between Wear Resistance of High-Speed Steel R6M5 and Deformations Arising during Laser Hardening

Correlation between Wear Resistance of High-Speed Steel R6M5 and Deformations Arising during Laser Hardening

Correlation of residual stress, hardness and surface roughness with crack initiation and fatigue strength of surface treated additive manufactured AlSi10Mg: Experimental and machine learning approaches

Post-processing methods are widely used to address the issues caused by surface imperfections and bulk defects in additive manufactured materials. In our previous studies, we analysed the effects of different peening-based treatments of shot peening (SP), severe vibratory peening (SVP) and laser shock peening (LSP) on fatigue performance of V-notched laser powder bed fusion AlSi1Mg samples. Herein, the fracture surfaces of failed samples were further analyzed and obtained experimental data were further elaborated by machine learning (ML)-based approach to identify the correlation between residual stress, hardness and surface roughness (all affected by the applied post-treatments) with the depth of crack initiation site and fatigue life of the post-treated samples. ML-based model was developed via a six layer deep neural network (DNN) as well as using stacked auto-encoder (SAE) for pre-training of the used data set. Taking the advantages of SAE, the accuracies of more than 0.96 were obtained for the predicted results. Correlations were obtained by performing parametric analyses and the importance of each input factor was assessed through sensitivity analyses. The obtained results revealed that by enhancing surface hardening and inducing higher compressive residual stresses as well as more efficient surface roughness reduction, deeper crack initiation site and superior fatigue life can be obtained. In addition, it was found that the depth of sub-surface crack initiation had direct relation with fatigue life improvement in the samples.

Effect of laser shock peening on austempered ductile iron

In this study, laser shock peening (LSP) has been applied on austempered ductile iron (ADI) to determine the optimum laser power and spot overlapping rate required for automotive gear applications. The surface roughness and phases present were studied to understand the effect of the laser shock wave application on the material. An increase in the mean surface roughness was achieved when the laser spot overlap increased from 20% to 60%. A significant increase in the microhardness was obtained in the surface and sub-surface, indicating an increased dislocation density of the material. Compressive residual stress depth and magnitude characterized by X-ray Diffraction (XRD) and Incremental Hole Drilling (IHD) were found to increase with laser power density. This work significantly identifies the pathway toward the engineering manufacture of austempered ductile iron and the application of LSP for the automotive drive-train industries.

Effects of nanosecond laser shock peening on residual stress, corrosion and tribocorrosion behavior of WE43 magnesium alloys

Biodegradable magnesium (Mg) alloys are promising candidates for use as next-generation implants due to their similar mechanical properties to natural bones, excellent biocompatibility, and the ability to completely degrade in vivo. However, a great challenge of Mg alloys in physiological environments is their fast corrosion rate and low tribocorrosion resistance, which makes it difficult to ensure adequate structural integrity over the required time for complete tissue and bone healing. In this work, the effects of nanosecond laser shock peening (ns-LSP) processing parameters on the surface tribocorrosion resistance of WE43 Mg alloys was investigated. The laser power densities and overlapping ratios were tuned to control the magnitude and depth of residual stresses and the amount of cold work. The tribocorrosion behaviors of WE43 Mg after the ns-LSP treatment were measured in stagnant blood bank buffered saline (pH of 7.0–7.2) under ∼ 37 °C. The tribocorroded surfaces were characterized via X-ray diffraction, scanning electron microscope and energy dispersive spectroscopy. It was found that the optimum peening effect was achieved at lower laser power density and overlapping ratio, where the corrosion cathodic reaction rate was decreased and tribocorrosion rate was reduced by 47.8%, compared to those of the untreated samples. The established processing-performance (i.e., surface hardness, corrosion and tribocorrosion) relationship in this study could shed light on the future LSP process optimization for enhancing surface properties of Mg alloys.

Peening pattern optimization with integer eigen-moment density for laser peen forming of complex shape

Peening pattern optimization with integer eigen-moment density for laser peen forming of complex shape

An investigation on the effect of change in experimental sequence of warm laser shock peening and laser shock peening on the performance of DD6 nickel-based single-crystal superalloy

Incorporated with the benefits of dynamic strain aging (DSA) and laser shock peening (LSP), warm laser shock peening (WLSP) is able to induce a compressive residual stress (CRS) layer with high thermal stability on the near-surface of the material. It has been widely used in recent years as a new surface modification technique to prolong the fatigue life of aero-engine turbine blades. Single WLSP has limited effect on material modification, while multiple WLSP when the confining layer is quartz glass will affect the thickness and thermal stability of the CRS layer during replacement of the confining layer, absorbing layer, and cooling and heating. To address this issue, this study investigates the mechanism of the CRS response, mechanism of strengthening, mechanism of action, and change of electrochemical corrosion performance of DD6 superalloy samples to different surface treatments after double LSP, single WLSP plus single LSP and double WLSP surface treatments by changing the experimental sequence of WLSP and LSP.The experimental results show that all three surface treatments deform the sample by inducing 〈110〉 cross-slip bands on the sample surface. Also all three surface treatments strengthen the sample surface by pipeline diffusion of elements to form a dislocation network similar to the γ/γ′ substructure.Meanwhile, single WLSP plus single LSP treatments can effectively prevent the thermal relaxation of the previously induced CRS layer. The final induced CRS layer thickness, dislocation distribution and dislocation density are better than those of double LSP and double WLSP. Pipeline diffusion of elements can improve the electrochemical and thermal corrosion resistance of the single-crystal superalloy.

Influence of laser shock peening on microstructure and property of Ni60 cladding layer and the combined area of 20CrNiMo alloy

The effects of laser shock peening (LSP) on the microhardness, residual stress, and microstructure of the Ni60 cladding layer and the combined area of the 20CrNiMo alloy for high-speed rail brake disks are investigated to address the problem of coarse columnar crystals and residual tensile stress, which affect the coating–substrate bonding performance and the application development of the laser cladding repair process. The results indicate that LSP can improve the microhardness, prefabricate residual compressive stress field, and refine the microstructure of the cladding layer and the combined area. The surface hardness of the cladding layer increased by 14.55%, 34.92%, and 40.21% after the energy impact of 6, 9, and 12 J, with an impact depth of about 1.2, 2.2, and 2.5 mm, respectively. The roughness result showed that the effect is more satisfactory under the 9 J energy impact. The grain refinement is significant compared to the Clad specimen, with the average grain area at the molten layer's top, middle, and bottom reduced by 35.5%, 79.6%, and 85.8%, respectively. A residual compressive stress of −538 MPa is introduced on the surface of the clad layer, with a compressive stress value of −30 MPa at a depth of 2 mm. After the LSP, the microstructure and properties of the clad layer and the combined area are significantly improved. Still, no new phases appear and do not significantly affect the diffusion of elements between the dendrites and the dendrites.

Laser shock peening strengthens additively manufactured high-entropy alloy through novel surface grain rotation

Additive manufacturing has flourished as an advanced technique to process metals and alloys. However, this strategy usually introduces undesired defects that deteriorates the mechanical performance of structural materials. Herein laser shock peening (LSP) is proposed as an efficient strengthening approach to reshape the surface morphology of a prototypical dual-phase AlCoCrFeNi high-entropy alloy (HEA) after additive manufacturing, in which remarkable strengthening is achieved. Combined electron back scatter diffraction and transmission electron microscope characterizations reveal that the mechanical enhancement is attributed to the grain refinement and accumulation of dislocations at the impact surface. In extreme condition of LSP, the grain refinement is not accommodated by the conventional dynamic recrystallization anymore, but a novel mechanism of parental columnar grain rotation which can be rationalized by a continuum-level theory from a geometrical perspective. The new mechanism is verified by large-scale atomistic simulations which further recognizes the critical role of multiple unstable dislocation slip and amorphization in formation of smaller grains under shock. Our strategy offers a promising pathway toward polishing morphology of HEAs and thus, prohibiting the potential intrinsic defected induced-mechanical degradation of the additively manufactured metals and alloys via novel microscopic mechanism.