Showing papers on "Residual stress published in 2020"
TL;DR: In this article, the impact of the angle between LSW and the deposited plane on tensile properties of both SLM-LSPed specimens was analyzed, and two kinds of LSW-induced atomic diffusion processes at the interfaces between both adjacent deposited layers were presented.
Abstract: High-performance integrated additive manufacturing with laser shock peening (LSP), is an innovative selective laser melting (SLM) method to improve mechanical properties, and refine microstructure in the surface layer of metallic components. Phase, residual stress distribution, surface micro-hardness, tensile properties and microstructural evolution of SLMed and SLM-LSPed specimens in horizontal and vertical directions were examined. In particular, typical microstructural features in the surface layer were characterized by transmission electron microscopy (TEM) observations. Results indicated that surface micro-hardness subjected to massive LSP treatment had significantly improved, tensile residual stress was transformed into compressive residual stress by LSP-induced plastic deformation, and both SLMed specimens in two directions exhibited a good combination of the ultimate tensile strength (UTS) and ductility. Meanwhile, high-density dislocations and a large number of mechanical twins were generated in the coarse α′ martensites by laser shock wave (LSW), and gradually evolved into refined α′ martensites. Furthermore, according to the included angle between LSW and the deposited plane, two kinds of LSW-induced atomic diffusion processes at the interfaces between both adjacent deposited layers were presented, and the influence mechanisms of the included angle between LSW and the deposited plane on tensile properties of both SLM-LSPed specimens were revealed. The hybrid additive manufacturing technology combined SLM with LSP realizes the high-efficiency and high-quality integrated manufacturing of the formed metallic components for practical applications.
182 citations
TL;DR: In this article, the effect of powder feedstock, fabrication parameters, and post fabrication treatments on the resulting microstructure, defect characteristics, and surface quality of the fabricated Ti-6Al-4V parts is evaluated.
133 citations
TL;DR: In this article, a series of conceptual methods are discussed on mitigating residual stress to make some practical inspiration for developing a systematical residual stress balancing technique for selective laser melting (SLM) process.
Abstract: The undesirable residual stress accumulated in the parts during the melting and solidification of the metal powder layer by layer retards the further application of the selective laser melting (SLM) process. This paper focuses on reviewing the recent illuminating achievements about physical modeling, experimental characterizing, and active adjusting of the residual stress in the parts fabricated by SLM. The advantages and disadvantages of the mainstream and emerging models or approaches are further analyzed. Based on the status and prospect of the relative techniques, a series of conceptual methods are discussed on mitigating residual stress to make some practical inspiration for developing a systematical residual stress balancing technique for SLM.
124 citations
TL;DR: In this article, the effects of various surface treatments on surface topography, porosity, hardness, and residual stress on turbine blade test parts were studied. And the effect of accumulated energy on the outcomes of the applied surface treatments is also addressed.
Abstract: The turbine blade test parts were manufactured by the selective laser melting (SLM) process using a nickel-based pre-alloyed Inconel (IN) 718 powder. Various mechanical post-processing techniques, such as barrel finishing (BF), shot peening (SP), ultrasonic shot peening (USP), and ultrasonic impact treatment (UIT), were applied to improve the surface layer properties of the SLM-built specimens. Effects of mechanical surface treatments on surface topography, porosity, hardness, and residual stress were studied. In comparison with the SLM-built state the surface roughness (Sa = 5.27 μm) of the post-processed specimens were respectively decreased by 20.6%, 26.2%, and 57.4% after the BF, USP, and UIT processes except for the SP-treated ones. The Sz parameter was reduced in all treated SLM-built specimens except for the SP-treated ones. The surface microhardness of the SLM-built specimen (~390 HV0.025) was increased after the BF (by 14.2%), USP (by 23.8%), UIT (by 50%), and SP (by 66.5%) processes. The deepest hardened layers were formed after the UIT and SP processes. Residual porosity of the SLM-built specimen was decreased by 23.1%, 40.6%, 55%, and 84% after the BF, SP, USP and UIT processes, respectively. The UIT process formed a densified subsurface layer of significantly reduced porosity (0.118%). All mechanical surface treatments successfully transformed the tensile residual stresses generated in SLM-built specimen into the compressive residual stresses (−201.4...510.7 MPa). The thickness of hardened, densified and compressed near-surface layers ranges from ~80 μm after BF to ~140 μm after USP, and ~180 μm after SP and UIT processes, which correlates to the accumulated energy and deformation extent of the treated surface. The effect of the accumulated energy on the outcomes of the applied surface treatments is also addressed.
123 citations
TL;DR: In this paper, the effects of a wide range of process parameters on three part properties; density, surface roughness, and surface residual stresses simultaneously for selective laser melting of Inconel 718 were investigated.
97 citations
TL;DR: In this article, the effect of residual stress on corrosion of 316 L using electrochemical methods was investigated and it was shown that compressive stresses lower the film growth and repassivation kinetics but slightly enhances the pitting resistance of SLM 316´L.
95 citations
TL;DR: In this paper, the ABAQUS AM module is employed to simulate two important metal additive manufacturing processes: (a) Laser Direct Energy Deposition (LDED) and (b) Selective Laser Melting (SLM).
93 citations
07 Apr 2020-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, the effect of deposition patterns on the microstructure, mechanical performance and residual stress of 316L cubes was evaluated through the Primary Cellular Arm Spacing (PCAS) analysis.
Abstract: In this work, 316L cubes were produced by Directed Energy Deposition (DED) process. To evaluate the effect of deposition patterns on the microstructure, mechanical performance and residual stress of 316L samples, two different deposition strategies are selected (67° and 90°). The general microstructure is revealed, and then the effect of deposition pattern on the microstructure of 316L alloy is evaluated through the Primary Cellular Arm Spacing (PCAS) analysis. The cooling rate in each sample is estimated according to the PCAS values. Interestingly, it is found that by increasing the rotation angle per layer, the PCAS value decreases as a consequence of increment in the cooling rate. On the other hand, in both cases, by increasing the distance from the substrate, due to the changes in cooling mechanisms, the cooling rate at first decreases and then at the last layers increases again. The phase composition analysis of 316L samples confirms the predictions that suggested the presence of residual δ-ferrite in the final microstructure. In fact, the final microstructure of samples is characterized by austenitic dendrites together with some residual δ-ferrite in the interdendritic regions. Moreover, the microstructural evaluations exhibit that during the DED process, some metallic inclusions are formed within the 316L samples that consequently deteriorates their mechanical properties. Tensile results show that the samples with 90° rotation per layer have a better mechanical performance such as slightly higher ultimate tensile strength and almost 35% higher elongation to fracture, mainly owing to their finer microstructure and slightly less oxide content. However, in both cases, the elongation of the 316L samples is lower than the typical elongation of this material produced via DED. This discrepancy is found to be as a result of higher inclusions contents in the samples produced in this work with respect to those of literature. Lastly, it is found that the residual stresses on the top surfaces are similar for both deposition patterns, although higher stress values are observed on the lateral surfaces of the cubes produce using the 90° rotation per layer.
90 citations
TL;DR: In this article, the residual stress and surface topography obtained by various shot peening treatments were experimentally and theoretically examined, and it was found that the ratio between the layer thickness of compressive stress and the critical depth of maximum stress generally lies in the regime of 3.6-4.3 after shot-peening.
86 citations
TL;DR: In this article, the microstructure, residual stress, and mechanical properties of the as-printed specimen and specimens annealed at 773-1573 K for 2'h were compared.
Abstract: To widen the applications of FeCoCrNi high-entropy alloys (HEAs) fabricated via selective laser melting, their mechanical properties must be improved, and annealing plays an important role in this regard. In this study, the microstructure, residual stress, and mechanical properties of the as-printed specimen and specimens annealed at 773–1573 K for 2 h were compared. As the annealing temperature increased, the specimen structure recrystallized from all columnar grains to equiaxial grains containing numerous annealing twins. The dislocation network, which formed during the solidification process under considerable shrinkage strain, decomposed into dislocations. The residual stress, yield strength, and hardness decreased, while the plasticity and impact toughness increased. During the deformation of as-printed and low-temperature-annealed specimens, the dislocation network remained unchanged and provided resistance to the dislocations moving within it, thus strengthening the specimen. The tensile strength remained largely unchanged owing to the reduction in the residual stress during low-temperature annealing, as well as the formation of the twinning network and dislocation wall under large deformation upon high-temperature annealing. Meanwhile, the ductility greatly increased, thus increasing the potential for industrial application of HEAs.
85 citations
TL;DR: In this article, the effects of the as-built condition on the fatigue properties of an AlSi 10 Mg aluminium alloy manufactured by Selective Laser Melting (SLM) in different orientations were investigated.
TL;DR: In this article, a series of triaxial discontinuous cyclic compression tests were conducted on salt and the residual stress was reproduced through numerical simulations, showing that the interval effect decreases with the increase of the confining pressure.
Abstract: When rock salt is subjected to discontinuous fatigue, time intervals (periods during which the stress remains constant) can accelerate plastic deformation and reduce the fatigue life due to the internal residual stress generated between various defects and the host material. However, the time interval effect on rock salt in a three-dimensional stress state has rarely been investigated despite its great significance in the use of underground salt caverns as storages. In this research, a series of triaxial discontinuous cyclic compression tests were conducted on salt and the residual stress was reproduced through numerical simulations. Experimental results show that the interval effect decreases with the increase of the confining pressure. The change in plasticity caused by time intervals is small and constant when the stress level (ratio of the maximum stress to strength) is below “60%” but then increases as a linear function of the stress level. During simulations, it was observed that the greater the difference in the elasticity modulus between the host material and impurities, the larger the residual stress. The moderate effect of time interval under confinement is the result of the fact that a higher confining pressure strengthens the host material, reduces the volume of impurities and thus leads to a smaller residual stress. In addition, when the stress level is below a certain threshold (which is shown to be at 0.8–0.85 of the critical yield stress), the residual stress is relatively low in magnitude and area of influence. Above that threshold, the residual stress grows rapidly. This is the reason why the interval effect displays a distinct behavior before and after dilatancy point.
TL;DR: In this paper, a massive laser shock peening treatment (MLSPT) with different pulse energies and effects of MLSPT on residual stress and corrosion fatigue properties in NaCl solutions with different pH values were investigated by microstructural observation, residual stress measurement, corrosion fatigue test, and fractural morphology.
TL;DR: In this article, the effect of massive laser shock peening (MLSP) on surface integrity of cast dumbbell shaped and cylindrical cast specimens of commercially pure titanium (CP-Ti) and to compare with those of untreated CP-Ti was investigated.
TL;DR: In this article, a detailed 3D microstructural analysis involving focused ion beam/scanning electron microscopy tomography was performed to describe the connectivity and size of the Si-rich eutectic network and link it to the strength and fracture strain.
TL;DR: In this article, an investigation was conducted to understand how different parameters, namely laser power, scan speed and hatch spacing, influence the printing qualities, i.e. surface roughness, porosity and crack density of the nickel-based superalloy Inconel 738LC during laser powder bed fusion.
TL;DR: In this paper, a 3D finite element model (FEM) is established to investigate the relationship between the process parameters and the residual stress during SLM, and the magnitude of the parametric effect on residual stress is obtained.
TL;DR: In this paper, the authors compare the properties of the planes parallel (side) and perpendicular (top) to the building direction of the fabricated sample to achieve a clear understanding of relationships between the anisotropy in the microstructure and corrosion behavior.
TL;DR: In this article, the influence of residual in-plane stress on the performance of InGaN-based red light-emitting diodes (LEDs) by changing the thickness of the underlying n-GaN layers was investigated.
Abstract: This work investigates the influence of residual stress on the performance of InGaN-based red light-emitting diodes (LEDs) by changing the thickness of the underlying n-GaN layers. The residual in-plane stress in the LED structure depends on the thickness of the underlying layer. Decreased residual in-plane stress resulting from the increased thickness of the underlying n-GaN layers improves the crystalline quality of the InGaN active region by allowing for a higher growth temperature. The electroluminescence intensity of the InGaN-based red LEDs is increased by a factor of 1.3 when the thickness of the underlying n-GaN layer is increased from 2 to 8 μm. Using 8-μm-thick underlying n-GaN layers, 633-nm-wavelength red LEDs are realized with a light-output power of 0.64 mW and an external quantum efficiency of 1.6% at 20 mA. The improved external quantum efficiency of the LEDs can be attributed to the lower residual in-plane stress in the underlying GaN layers.
TL;DR: In this article, the surface of the FeCoNiCrMn high-entropy alloy (HEA) coatings were prepared by high-speed laser cladding (HSLC) technology, and the surface was subjected to ultrasonic surface mechanical rolling treatment (SMRT).
TL;DR: In this paper, an alternative for measuring residual stress both during and after fabrication is presented. But, the authors do not specify the type of residual stress that should be measured, only that the residual stress should be defined.
Abstract: Additive manufacturing has become a major growing field in materials engineering, following a new tendency for custom, high precision and on-demand fabrication. Residual stresses are prone to appear in any production technique, which remain a challenge to be measured. These stresses can lead to a reduction on mechanical performance and even cause premature failure. Thus, a wide understanding of residual stress is critical for greater part reliability. Among Non-destructive Testing (NDT) techniques, acoustic and ultrasonic waves remain widely used to determine stresses, voids and defects in a wide array of parts. In this contribution, Ultrasonic Testing (UT) is highlighted as an alternative for measuring residual stress both during and after fabrication.
TL;DR: In this paper, a mesoscale model was developed to examine thermal and mechanical responses in a single layer and a numerical-based process window was constructed to show the correlation among process parameters, surface temperature, and residual stress.
TL;DR: In this article, a decoupled 3D LSP process was used, i.e. LSP treatments were applied during the SLM process, by removing the sample from the standard SLM chamber whenever needed.
Abstract: Although Selective Laser Melting (SLM) currently revolutionizes the way parts are being manufactured, certain process inherent limitations such as accumulation of residual stresses and increased porosity content can lead to a decrease in fatigue life of produced parts. 3D Laser Shock Peening is a new hybrid additive manufacturing process whereby a periodic Laser Shock Peening (LSP) treatment is added to the standard SLM process. LSP can be applied selectively in the 3D volume of metallic parts, and is shown to influence the bulk internal stress state and hardness, as well as the distribution of porosities in the material. Here, a decoupled 3D LSP process was used, i.e. LSP treatments were applied during the SLM process, by removing the sample from the SLM chamber whenever needed. We report fatigue lives of 316 L stainless steel multiplied by more than 15 times compared to standard SLM parts, and more than 57 times compared to conventional manufacturing, with a significant improvement over other state-of-the art methods. The effect of the 3D LSP treatment on SLM parts and the consequence on crack initiation and propagation is investigated by measuring the near surface residual stresses, imaging the fractured surface, measuring the porosity content and analyzing the microstructure and microhardness.
TL;DR: In this article, the effect of shot peening coverage on residual stress, surface roughness, microhardness and microstructure of rollers is investigated, and it is shown that the shot-peening leads to a slight increase of surface and near-surface hardness from 690 HV to 740 HV, and an appreciably increasing of the subsurface maximum compressive residual stress.
Abstract: Shot peening is currently becoming a widely used surface strengthening technique that can refine the material grain, increase the hardness and introduce a certain depth of residual compressive stress layer. At the same times it changes the surface topography which may have a deleterious effect on the contact fatigue life. A fully understanding on the mechanism of shot peening for high-strength steels is to be explored. In this paper, the effect of shot peening coverage on residual stress, surface roughness, microhardness and microstructure of rollers are experimentally investigated. Experimental results show that the shot peening leads to a slight increase of surface and near-surface hardness from 690 HV to 740 HV, and an appreciably increasing of the subsurface maximum compressive residual stress. Moreover, some retained austenite in the the near-surface layer are transformed to martensite after shot peening, and wherein the grain refinement occurs.
TL;DR: In this paper, the authors combine thermomechanical simulations with in situ high-speed video of microcracking in single laser-melted tracks, visualizing the ductile-to-brittle transition.
TL;DR: In this article, the microstructure evolution during LSP was investigated via electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) observations, and the micro-hardness distributions on cross-sections and subsurface residual stress states as functions of the laser energy were also determined.
TL;DR: In this article, a post-treatment method combining heat treatment (HT) and laser shock peening (LSP) was employed to alter the microstructure and mechanical properties of WAAM Ti17 titanium alloy.
Abstract: Wire and arc additive manufactured (WAAM) metal parts usually contain large columnar grains and detrimental tensile residual stress, affecting their mechanical performance. In this work, a post-treatment method combining heat treatment (HT) and laser shock peening (LSP) was employed to alter the microstructure and mechanical properties of WAAM Ti17 titanium alloy. The results show severe plastic deformation was induced in the surface layer, which, in turn, led to a high-level surface compressive residual stress (~−763 MPa) by combination treatment of HT and LSP. Meanwhile, high-density dislocations and mechanical twins were observed in coarse α phases after treatment by laser shock wave, and gradually evolved into refined α phases. The elongation of samples was significantly improved by 15% while ensuring original ultimate tensile strength (UTS, 1153 ± 13 MPa) after HT and LSP treatment. This combined HT and LSP method helps enhance the mechanical performance of WAAM parts through changing their microstructure and residual stress distribution.
TL;DR: Comparisons between numerical and experimental results reveal that a reliable and computationally-efficient model captures well the key aspects of the response, such as the SFRC tension softening, the tension stiffening effect, the bending moment–curvature envelope, and the favorable contribution of the steel fibers on the residual response.
Abstract: This paper investigates the ability of steel fibers to enhance the short-term behavior and flexural performance of realistic steel fiber-reinforced concrete (SFRC) structural members with steel reinforcing bars and stirrups using nonlinear 3D finite element (FE) analysis. Test results of 17 large-scale beam specimens tested under monotonic flexural four-point loading from the literature are used as an experimental database to validate the developed nonlinear 3D FE analysis and to study the contributions of steel fibers on the initial stiffness, strength, deformation capacity, cracking behavior, and residual stress. The examined SFRC beams include various ratios of longitudinal reinforcement (0.3%, 0.6%, and 1.0%) and steel fiber volume fractions (from 0.3% to 1.5%). The proposed FE analysis employs the nonlinearities of the materials with new and established constitutive relationships for the SFRC under compression and tension based on experimental data. Especially for the tensional response of SFRC, an efficient smeared crack approach is proposed that utilizes the fracture properties of the material utilizing special stress versus crack width relations with tension softening for the post-cracking SFRC tensile response instead of stress-strain laws. The post-cracking tensile behavior of the SFRC near the reinforcing bars is modeled by a tension stiffening model that considers the SFRC fracture properties, the steel fiber interaction in cracked concrete, and the bond behavior of steel bars. The model validation is carried out comparing the computed key overall and local responses and responses measured in the tests. Extensive comparisons between numerical and experimental results reveal that a reliable and computationally-efficient model captures well the key aspects of the response, such as the SFRC tension softening, the tension stiffening effect, the bending moment-curvature envelope, and the favorable contribution of the steel fibers on the residual response. The results of this study reveal the favorable influence of steel fibers on the flexural behavior, the cracking performance, and the post-cracking residual stress.
TL;DR: In this paper, the influence of laser power and scanning strategies on residual stress distribution in 316L steel by a metal additive manufacturing (AM) process, namely, selective laser melting (SLM), was investigated.
Abstract: Residual stress control in the metal components by additive manufacturing (AM) has been a major challenge. To mitigate this challenge, proper selection of AM process parameters is of great importance. In this study, we investigate the influence of laser power and scanning strategies on residual stress distribution in 316L steel by a metal AM process, namely, selective laser melting (SLM). Finite element simulation and experimental verification are conducted by using the identical process parameters and part geometry to ensure that the results are indeed comparable and can shed light on the challenging issue of residual stress control. With two levels of laser power (i.e., 160 W and 200 W) and two scanning strategies (i.e., stripe scanning and chessboard scanning), four process conditions are investigated. For all four conditions, both simulation and experiment show that the tensile residual stress in the area of interest (the center area of each layer) tends to gradually increase along the depth into surface. Also, the increase of laser power from 160 W to 200 W and the adoption of stripe scanning (instead of chessboard scanning) generally lead to the increase of tensile residual stress in the area of interest. The trends are also confirmed by both simulation and experiment. In addition, the laser power increase from 160 W to 200 W appears to have more significant effect, compared with the switch of two scanning strategies.
TL;DR: In this article, a finite element model that simulates the movement and application of thermo-mechanical loading and phase transitions on the surface and subsurface of the machined material was developed to predict the residual stress generated by micro-grinding.