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In this work the feasibility test of friction stir processing (FSP) of 1.5 mm thick austenitic stainless steel cold spray coating deposited on 304L stainless steel substrate was performed using tungsten carbide tool. Applied FSP parameters (advance speed 50 mm/min, rotation speed 300 rpm, axial force 20 kN, tilt angle 1.5°) allowed to perform FSP treatment with a higher depth than the coating thickness. As a result, the material mixing at the coating/substrate interface was observed. The microstructure observation revealed that the coating microstructure in the stir zone was significantly modified. EBSD analysis confirmed that full material recrystallization during FSP allowed to formation of dense and uniform fine-grain structure with the mean grain size 1.9 mm. Average coating microhardness was decreased from 406 HV to 299 HV. Further FSP parameters optimization should be carried out in order to improve the process reliability and avoid any coating failure during treatment.

International audience; Micromechanics damage models applied to composites predict stresses and strains in the matrix and fibers as a function of the microstructure, constituting phases mechanical properties and load histories. Material parameters, like interface properties, are identified through inverse methods based on macroscopic stress-strain curves. Predictions are also benchmarked against macroscopic measurements. This situation does not capture local phenomena and hinders the robustness of the indentification/validation process. The purpose of this work is to provide full displacement and strain fields at the scale of a single fibre embedded into a matrix to allow the modelling community to either develop and identify micromechanics damage models or to benchmark their own predictions. Such data is critically lacking in the community. To that end, we have investigated three single fibers having radically different bonding strength with epoxy in addition to a bundle of about a hundred carbon fibers that were used as reinforcements of standard “dogbone” epoxy specimens. A laser scanning confocal microscope (LSCM) is used for micro digital image correlation (μDIC) during in-situ quasi-static tests of single-reinforcement dogbone specimens. For all specimens, damage initiated with fiber debonding at the free surface along the tensile direction. The crack then propagates around the interface while slightly growing along the fiber. The interfacial crack is shown to grow faster for couples with weak interfacial bonding. Strong fiber / matrix bonding is shown to stop Mode II transverse interfacial debonding which significantly delays specimen failure. Analysis of the LSCM micrographs with μDIC is used to provide measurements of displacements, strains, and measure depth during each test. The importance of out of plane displacements in interfacial debonding is highlighted. Out of plane displacement is shown to play a role in interfacial crack opening and growth and ought to be considered when studying or modeling damage in FRCs. μDIC is shown to be a promising technique to provide a better understanding of the damage mechanisms at the fiber or bundle scales and to determine interfacial toughness of a specific fibre / matrix couple in order to perform accurate damage modeling in FRCs. Displacement, strain, and confidence field results for each pixel from each experiment and at each time step are also provided in an extensive data package for detailed comparison with simulation results.

Conventional machining especially hard turning is commonly used in aerospace industry to produce the final shape of cylindrical parts. However, the process generates damages and microstructural alterations on the surface and at the subsurface layer, in particular the cracking of hard carbides particles. The present paper focuses on investigating the probable mechanisms responsible for carbide cracking and the formation of a softened layer at a depth between 10 and 30 µm below the machined surface. Advanced techniques, such as laser confocal microscopy and field emission gun scanning electron microscope equipped with in situ picoindenter and electron backscatter diffraction, were used to characterize and analyze the evolution of the microstructure in the affected layer and propose the possible governing mechanisms for the observations.

This paper presents the development of a Cellular Automaton (CA) capable of describing polycrystalline structures heterogeneous behavior in the elastic domain. Based on Eshelby’s inclusion problem, this model is the first step to a better consideration of heterogeneities in polycrystals by including the neighborhood effect in grain’s behavior. Neighborhood effects have been defined, quantified, and approximated from observations made in the first of this two-part paper, using finite element method (FEM). Considering these approximations, and based on FEM simulations results, an analytical model of the neighborhood effect was proposed in the present paper on which the CA model was built. As a first step in the model development, a regular aggregate structure (Kelvin structure) is used where grains are considered spherical and having identical size. The stress field predictions obtained with the FEM for a polycrystalline aggregate submitted to an elastic loading were compared with the CA predictions, grain by grain, for two different crystal structures (Fe and Ti). Considering the FEM as a reference tool, CA model predictions show an excellent propensity at predicting the local stress fields in polycrystals by capturing the neighborhood effect induced by grain orientations for a low computation time cost. The CA model has also shown its facility to evaluate quickly a grain influence on another grain stress level, and therefore, the orientations generating high stress concentrations in a grain can be easily identified. Using this capacity, the neighborhood effect has been shown to be able to at least double a grain stress level without the material showing any particular texture.

Polycrystals are full of all kinds of heterogeneities which introduce stress concentrations. The local stress field in a grain does not depend solely on its crystallographic orientation. In fact, its neighborhood has also been shown to play a significant role. A definition and quantification method of the neighborhood effect was proposed and a finite element study was performed to evaluate the elastic strain variations of a given grain surrounded by a heterogeneous neighborhood composed of one or several grains inserted in an infinite homogeneous matrix. A regular structure was used to generate the aggregates and annihilate any grain size and shape ratio. Grains crystallographic orientations influences on a grain’s strain tensor were studied with respect to their relative positions and the loading axis. A grain strain variations due to its neighborhood were found to be independent of its orientation, and a grain’s influence on another grain’s mean strain tensor was shown to be independent of other neighboring grains. From these observations, some simplifications were proposed to better describe the neighborhood effect in order to develop the analytic model presented in a second part of the paper.

Ultrafine austenite-ferrite duplex medium Mn steels often show a discontinuous yielding phenomenon, which is not commonly observed in other composite-like multiphase materials. The underlying dislocation-based mechanisms are not understood. Here we show that medium Mn steels with an austenite matrix (austenite fraction ∼65 vol%) can exhibit pronounced discontinuous yielding. A combination of multiple in situ characterization techniques from macroscopic (a few millimeters) down to nanoscopic scale (below 100 nm) is utilized to investigate this phenomenon. We observe that both austenite and ferrite are plastically deformed before the macroscopic yield point. In this microplastic regime, plastic deformation starts in the austenite phase before ferrite yields. The austenite-ferrite interfaces act as preferable nucleation sites for new partial dislocations in austenite and for full dislocations in ferrite. The large total interface area, caused by the submicron grain size, can provide a high density of dislocation sources and lead to a rapid increase of mobile dislocations, which is believed to be the major reason accounting for discontinuous yielding in such steels. We simultaneously study the Luders banding behavior and the local deformation-induced martensite forming inside the Luders bands. We find that grain size and the austenite stability against deformation-driven martensite formation are two important microstructural factors controlling the Luders band characteristics in terms of the number of band nucleation sites and their propagation velocity. These factors thus govern the early yielding stages of medium Mn steels, due to their crucial influence on mobile dislocation generations and local work hardening.

This research is performed by 2D axis-symmetric finite element simulation based on the coupling of electromagnetic fields and heat transfer applied on 4340 steel specimen with flux concentrators heated by induction process. The model is built using COMSOL software based on an adequate formulation taking into account the material properties and process parameters. The obtained induced currents and temperature distributions are analyzed versus the geometrical dimensions of the model. The originality of this paper lies in the exploitation of simulation data using classical modeling and the optimization techniques to optimize the hardness profile according to geometrical factors. The proposed method is based on finite element simulations and adequate objective function able to converge to the optimal linear hardness profile. The results demonstrate that the final hardness profile can be quasi-uniform with a narrow flux concentrator gap when the gap between the master part and the inductor is larger. This overall study allows a good exploration of hardness profile linearity under various geometrical dimensions and permits a good comprehension of induction heating with flux concentrator behavior.

Hard turning technology was proven effective in enhancing the surface finish and dimensional accuracy of hardened steel. For this reason, it is used for finishing operations in the manufacturing of some critical aircraft landing gear components where bulk material hardness exceeds 45 HRC such as 300M ultra-high-strength steel. Nonetheless, the selection of cutting conditions remains critical to ensure generated surface characteristics meet the performance requirements. In this context, this study focuses on the optimization of hard turning cutting conditions to reduce surface roughness, increase compressive surface residual stresses, and reduce surface white layer thickness of 300M ultra-high-strength steel (55 HRC). To this end, a design of experiments (DoE) is used to investigate the effects of cutting tool (T), cutting speed (V), feed rate (f), and depth of cut (D) during hard turning. The cutting tool materials investigated include physical vapor deposited AlTiN–coated cemented carbide, mixed ceramic, and polycrystalline cubic boron nitride (PCBN), all commonly used in hard turning. Three different optimization approaches are evaluated and compared, namely the single-response Taguchi method, the multi-response grey relational analysis (GRA), and the proportion quality loss reduction (PQLR) methodologies. Because responses were found to be correlated, the selected multi-response optimization approaches were combined with the principal component analysis (PCA) method to transform correlated parameters into uncorrelated ones. The effect of cutting parameters and their interactions on surface roughness (Ra) and white layer thickness (WL) were considered in detail, particularly the effect of cutting tool material. Results demonstrate that the PCA-based GRA approach is a very suitable optimization technique for hard turning of 300M steel. Under those optimized conditions, Ra was decreased by 50%, and WL was decreased by 6% while compressive axial surface residual stress (ASRS) and hoop surface residual stress (HSRS) were increased by 307% and 74%, respectively.

The aim of the study is to determine the effect of each parameter involved in the induction heating process on the final temperature distribution and case depth dispersion of a spur gear placed between two other gears having identical shapes and acting as a flux concentrator using two different approaches. The purpose of flux concentrators is to adjust the heat distribution in the part at the end of the heating process and to produce a better case depth between the middle and the edge of the gear. Mechanical properties of the gear could be improved by minimizing the edge effect at the tooth; thus, the optimization of temperature gradient between the middle and the edge plan by varying geometrical and machine parameters was studied. Two structured and comprehensive approaches to design an efficient model based on analysis of variance (ANOVA) and artificial neural networks (ANN) for the estimation of quality and the prediction of temperature profiles and edge effect was developed. The obtained results demonstrate that the statistical model was able to predict accurately the behavior of temperature and case depth distribution. In the final phase, several experimental tests were conducted on the induction machine to validate the simulation results and the prediction model.

International audience; The 3rd generation of aluminum–lithium (Al–Li) alloys provides a desirable combination of high mechanical properties and low density compared to their traditional counterparts, exempt of lithium. Therefore, providing a reliable joint between new and conventional aluminum alloys is crucial for making hybrid structures. The present study focuses on improving the mechanical properties of dissimilar AA2198/AA2024 joints using different heat treatments before and after welding. Tensile tests paired with digital image correlation (DIC) techniques and micro-hardness maps were performed to document the macro-scale and local mechanical behavior of the joints. As-welded joints demonstrated a similar yield strength, 30% lower than that of the base metals in T3 and T8 metallurgical states. As-welded joints failed at the AA2198 side in the heat affected zone (HAZ), parallel to the thermo-mechanically affected zone (TMAZ), region experiencing intense strain concentration and minimal hardness values. Post welding-heat treatments (PWHT) was found to successfully strengthen HAZ on the AA2198 side, without abnormal grain growth in the nugget and impairing the hardness properties on AA2024 side. This improvement in local mechanical properties on the AA2198 side was related to the re-precipitation of dissolved T1 (Al2CuLi) and θ (Al2Cu) during welding as characterized by differential scanning calorimetry (DSC) and microscopy analyses. However, PWHT joint variants demonstrated a reduction in total elongation and ultimate tensile strength due to intense strain localization on the AA2024 retreating side compared to a much more homogeneous strain distribution in the as-welded joints.

International audience; Effects on fatigue behaviors and microstructures of Surface Mechanical Attrition Treatment (SMAT) before or after precipitation aging have been investigated on two hardenable Al alloys. The aluminum alloy with high notch sensitivity (7075) should not be processed by SMAT as the generated low surface integrity is always detrimental to fatigue performances. On less notch sensitive alloys (2024), SMAT before aging formed smaller and denser precipitates, resulting in a high hardened depth, and microstructures more resistant to residual stress relaxation than after conventional shot-peening. SMAT after aging resulted in a significant improvement of fatigue performance with only subsurface crack nucleation sites.

In this work, the microstructure and mechanical properties of an additively manufactured X3NiCoMoTi18-9-5 maraging steel were determined. Optical and electron microscopies revealed the formation of melt pool boundaries and epitaxial grain growth with cellular dendritic structures after the laser powder bed fusion (LPBF) process. The cooling rate is estimated to be around 106 &deg, C/s during solidification, which eliminates the nucleation of any precipitates. However, it allows the formation of austenite with a volume fraction of about 5% and dendritic structures with primary arm spacing of 0.41 &plusmn, 0.23 &micro, m. The electron backscatter diffraction analysis showed the formation of elongated grains with significant low-angle grain boundaries (LAGBs). Then, a solutionizing treatment was applied to the as-printed samples to dissolve all the secondary phases, followed by aging treatment. The reverted austenite was evident after heat treatment, which transformed into martensite after tensile testing. The critical plastic stresses for this transformation were determined using the double differentiation method. The tensile strength of the alloy increased from 1214 MPa to 2106 MPa after the aging process due to the formation of eta phase. The experimental data were complemented with thermodynamic and mechanical properties simulations, which showed a discrepancy of less than 3%.

By comparing the behavior of 13%Cr–4Ni and 13%Cr–6Ni alloys, this paper investigated the effect of Ni and Mn enrichment on the amount, mechanical stability, and microstructure of reformed austenite. 4%Ni and 6%Ni–1.5%Mn weld multilayer deposits were made and heat treated to stabilize various amounts of austenite content at room temperature. The maximum austenite content has been obtained at the same temperature (630 °C for 1 h) for both alloys, but 3.6 times more austenite content was formed in the 6%Ni steel. For both alloys similar austenite contents were obtained by post-weld heat treating at different temperatures and were compared experimentally. Results showed that with the addition of Ni and Mn, thinner and lower Ni content austenite was formed. Moreover, austenite generated at lower temperature was mechanically more stable under low cycle fatigue loading. Its rate of transformation has been reduced by a factor of 2.3, resulting in the stabilization of twice more austenite after 20 cycles at 2% strain considering the results of the present study, it is concluded that Ni and Mn contents do significantly affect the mechanical stability of reformed austenite; furthermore austenite lamellae morphology and thickness also seem to play a significant role in stabilizing this phase.

The tribological behavior of pure titanium having coarse-grained or nanostructured surfaces has been investigated against a steel ball moved with an alternative motion. The nanostructures were obtained by Surface Mechanical Attrition Treatment (SMAT) both at room and at cryogenic temperatures. An unexpected wear behavior was revealed: the hard steel ball was abraded for all cases even if it was several times harder compared to the Ti surface. This was due to the formation of a third body consisting of hard Ti oxides. Interestingly, important variations of the coefficient of friction were also revealed during the rubbing process. These variations could be separated into three successive stages, each with its specific wear mechanisms. The wear regimes were related to changes in the third body layer formed between the Ti and steel surfaces. SMAT changed the formation kinetics of the third body. The temperature at which the SMAT was conducted also introduced different third body formation kinetics. Important variations in the wear resistance were consequently observed between each surface condition.

International audience; A quantitative investigation of the porosity and its effects on mechanical properties was conducted on brazed materials presenting macroscopically brittle behavior with various defect levels. Three different techniques for evaluating the porosity and the fracture mechanisms were compared: (i) pre-mortem X-ray computed tomography (XCT), (ii) post-mortem laser scanning confocal microscopy (LSCM), and (iii) post-mortem scanning electron microscopy (SEM). The apparent surface area of the pores was evaluated by segmenting the height maps produced by LSCM and the electron backscattered images produced by SEM. The results show good agreement for different porosity levels. The LSCM height maps for opposite fracture faces were virtually stitched together in order to reconstruct three-dimensional (3D) images of the whole brazed joints. These proved to be similar to the XCT 3D segmented images, yet with some discrepancies due to local plastic deformation. Quantitative volume fraction and spatial distribution of pores were assessed as well as the identification of fracture mechanisms. Interestingly, it was noted that two specimens with different ultimate tensile strengths presented the same porosity amount, but different distributions. Two distinctive features were identified in the broken specimens: plastically teared pillars and walls that were separating the pores before fracture, and plateaus of approximately constant height with micro-sized dimples showing interfacial fracture. Although the specimens exhibited a macroscopic brittle behavior, these features are characteristic of ductile fracture. The most probable scenario for fracture was conjectured as follows: (i) Final fracture occurs in the near-surface region of the samples and conversely early damage develops in the volume, (ii) Interfacial fracture within plateaus is responsible of the rupture/strength of the brazed specimen but detrimental effect is induced by the tearing if highly voided regions are present in the joint.

The potential of ultrasonic surface mechanical attrition treatment (SMAT) at different temperatures (including cryogenic) for improving the fatigue performance of 304L austenitic stainless steel is evaluated along with the effect of the fatigue loading conditions. Processing parameters such as the vibration amplitude, the size, and the material of the shot medias were fixed. Treatments of 20 min at room temperature and cryogenic temperature were compared to the untreated material by performing rotating&ndash, bending fatigue tests at 10 Hz. The fatigue limit was increased by approximately 30% for both peening temperatures. Meanwhile, samples treated for 60 min at room temperature were compared to the initial state in uniaxial fatigue tests performed at R = &minus, 1 (fully reversed tension&ndash, compression) at 10 Hz, and the fatigue limit enhancement was approximately 20%. In addition, the temperature measurements done during the tests revealed a negligible self-heating (∆t &lt, 50 &deg, C) of the run-out specimens, whereas, at high stress amplitudes, temperature changes as high as 300 &deg, C were measured. SMAT was able to increase the stress range for which no significant local self-heating was reported on the surface.

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Shot peening is a cold-working process widely used to enhance the fatigue life of metallic components. This process consists of propelling thousands of spherical particles onto a ductile metallic surface. This paper presents a sequential coupled discrete element (DE)-periodic cell based finite element (FE) model to predict the shot peening effects, in terms of residual stresses and surface roughness. Shot stream expansion was accounted for and distribution of shots impacting velocities was extracted in discrete element model (DEM). Shot peening effects were computed on an impacting zone represented by a periodic cell. A cyclic material model was used to describe the plastic properties of the targets. Shot peening on IN718, 300M and AA7050 with CW14, S230 and Z425 shots at 4A and 8A Almen intensities were simulated to investigate their influences on the distribution of shot impacting velocities, residual stress profiles and surface roughness at full coverage. Numerical results show that accounting for shot stream expansion leads to residual stress profiles that are closer to those experimentally measured than assuming a cylindrical shot stream. The predicted residual stress profiles and surface roughness are comparable to the experimentally measured values, for the three materials studied.

International audience; Different tool geometries were used to investigate the joining of aluminum alloys (AA2198 to AA2024) by friction stir welding (FSW). Three shoulder profiles (flat, raised spiral, and raised fan) and five different pin profiles (cone, half threaded cylindrical, straight cylindrical, tapered cylindrical and square) were selected. Preliminary investigations were conducted by moving the tool into a seamless sheet made of the AA2024-T3 in order to select the tools that produce defect-free joints. Preliminary investigations showed the raised fan shoulder profile helps the material flow from the edge of shoulder to the pin creating a smooth surface finish with no flash in comparison with flat and raised spiral shoulder profiles. Pins with a minimum diameter equal to half the plate thickness produced lack of penetration (LOP) defects, while increasing minimum pin diameter to the plate thickness eliminates the LOP defects. Half threaded cylindrical pin produced tunneling defect, whereas defect free joint made by straight cylindrical, tapered cylindrical and cubic pin profiles. So they were selected for joining AA2024 to AA2198. Fracture locations of different joint variants were observed the vicinity of the thermomechanical affected zone (TMAZ) of AA2198-T3 alloy, and in the nugget on the AA2198-T3 side which have the minimum hardness and highest strain localization as confirmed by hardness maps and digital image coronation (DIC). Higher measured temperature than dissolution temperature of AA2198 main strengthening precipitates could be the reason of low hardness and fracture in TMAZ and center of nugget. Furthermore a raised fan shoulder with a tapered cylindrical pin produced highest elongation and yield strength and it was selected as the best candidate for optimization of the welding parameters. It was found that higher rotational and traverse speeds enhance the formation of tunneling and kissing bond defects, suggesting that longer pins have to be used for higher traverse speeds. Welding speed 750 rpm with 450 mm min−1 could create joint with highest yield strength.

Contact fatigue is an important issue in gear industry as it influences the reliabilities of mechanical equipment. Case-hardening processes could provide better mechanical properties by increasing the surface hardness and generating compressive residual stress. Yet these processes still cannot avoid contact fatigue problems and careful modeling is required. These processes generate gradients of mechanical properties below the surface, increasing complexities of contact analysis and fatigue life prediction. A numerical model is proposed to study the effect of the hardening layer on the carburized gear contact fatigue behavior. The gradients of the residual stress and the hardness are measured and the contact fatigue risk is evaluated using the concept of the material exposure. The effects of residual stress distribution, effective case depth, and surface-core hardness discrepancy on the contact fatigue performance of the gear are investigated. Results show that the contact fatigue of subsurface and the case- core transition area should both be evaluated to estimate the risk of pitting and tooth flank fracture.

The kinetics of cementite coarsening in tempered martensite AISI 4340 steel system has been investigated in order to understand over-tempering behavior. Microstructural response during tempering at 550 and 650 °C has been recorded, analyzed, and modeled. A statistical analysis of metallographic sections was performed to yield measurements such as average particle size, volume fraction, morphology, volume number density. The coarsening behavior of this multicomponent alloy system has been modeled with the classical coarsening theory for multicomponent alloys. These results highlight the remarkable coarsening resistance of 4340 steel even for prolonged tempering.

The machining of nickel-based superalloys used for aeronautical applications generates damage, deformation, and carbide cracking in machined surface and subsurface layers as a result of microstructural heterogeneities which reduce the fatigue life of aeronautic machined components. In this study, Inconel 718 was hard turned with a carbide tool using different cutting conditions according to a Roquemore 311B hybrid design of experiments (DOE) method. The main objective of the study was to model the effect of cutting parameters on the evolution of the microstructure and to accurately predict the alterations induced by machining, especially the deformed layer thickness (DL) and the average number of cracked carbides (ACC). The material removal rate (MRR) and the deformation power (E) were calculated in order to obtain a strong correlation between controlled cutting parameters and microstructure alterations. Damages were quantified using a confocal laser-digital microscope and were validated with the proposed models. These models showed a direct relation between both MRR and E with DL and ACC, with good prediction at a 95% confidence interval (CI).

The correlation between the strain-induced martensite transformation (SIMT) and the Portevin-Le Chatelier (PLC) effect during tensile testing of a ferrite-austenite duplex medium manganese (Mn) steel was investigated in this study. A discontinuous SIMT phenomenon (D-SIMT) was observed in the steel by in situ magnetic induction measurements complemented with ex situ X-ray diffraction analysis. This unique behavior was due to the localized martensite formation in the PLC bands forming and propagating continuously during tensile straining. The PLC bands were characterized by the digital image correlation technique and their origin is further discussed.

Microstructures and inclusion characteristics of cast and wrought 13% Cr-4% Ni martensitic stainless steels were related to tensile, fracture toughness JIc, and Charpy V-notch absorbed energy. A lower volume fraction of inclusions, a larger number of inclusions smaller than 10 µm, and a more resistant inclusion type against rupture and against micro-void formation were the key parameters explaining why the wrought steel has better mechanical properties than the cast steel. It has been shown that TRIP effect occurs only in a small region of 6 mm length from the crack plane in JIc-test.

The purpose of this work was to investigate tensile and fatigue behaviors of Inconel 625 superalloy brazed joints after transient liquid-phase bonding process. Brazing was performed in a vacuum furnace using a nickel-based filler metal in a form of paste to join wrought Inconel 625 plates. Mechanical tests were carried out on single-lap joints under various lap distance-to-thickness ratios. The fatigue crack initiation and crack growth modes were examined via metallographic analysis, and the effect of local stress on fatigue life was assessed by finite element simulations. The fatigue results show that fatigue strength and endurance limit increase with overlap distance, leading to a relatively large scatter of results. Fatigue cracks nucleated in the high-stressed region of the weld fillets from brittle eutectic phases or from internal brazing cavities. The present work proposes to rationalize the results by using the local stress at the brazing fillet. When using this local stress, all fatigue-obtained results find themselves on a single S-N curve, providing a design curve for any joint configuration in fatigue solicitation.

International audience; High-resolution digital image correlation (HR-DIC) techniques have become essential in material mechanics to assess strain measurements at the scale of the elementary mechanisms responsible of the deformation in polycrystalline materials. The purpose of this study is to demonstrate the use of laser scanning confocal microscopy (LSCM) coupled with DIC techniques to deepen knowledge on the deformation process of polycrystalline nickel-based superalloy at room temperature. The LSCM technique is capable of detecting both in-plane and out-of-plane strain localization within slip bands at the sub-grain level. The LSCM observations are consistent with previous in-situ scanning electron microscopy (SEM) studies: The onset of crystal plasticity occurs primarily near Σ3 twin boundaries with bulk locations in the elastic domain (macroscopic stress as low as 80% of the 0.2 % offset yield strength (Y.S.0.2%)). This intense irreversible strain localization occurs with either a high Schmid factor (μ > 0.43) or a significant elastic modulus difference between the pair of twins (ΔΕ > 100 GPa). In the plastic deformation domain, transgranular slip activity following slip systems with the highest Schmid factor is mostly responsible for the deformation at the grain level, thus leading to strain percolation. The simultaneous in-plane and out-of-plane deformation assessment via the HR-LSCM-DIC technique was found to be essential for the identification of active slip systems. Finally, the HR-LSCM-DIC technique enabled the quantification of the real glide amplitude involved in the three-dimensional shearing process at the grain level that solely in-plane measurements cannot provide.

International audience; The analysis of the tribological behavior of a steel ball on pure titanium has been carried out under alternative motion on both, a coarse grained Ti surface and its nanostructured derivative obtained by surface mechanical attrition treatment (SMAT). Along the duration of the tribology test, variations in friction behavior were interpreted as a three stages sequence of different wear mechanisms that were controlled by the successive formation of Ti-rich and Fe-rich oxides. Surface nanostructures appeared to change the oxide formation kinetics and delay the formation of a protective Fe-rich third body oxide layer on Ti. The results indicate that apparently contradicting literature results can be explained by taking into account the contamination induced during the SMAT process.

International audience; Surface Mechanical Attrition Treatment (SMAT) has been carried out at cryogenic and room temperatures on a 5553 β-metastable Ti alloy to describe its effects on the modification of the microstructure and fatigue properties. The cryogenic temperature promoted the martensitic transformation and the width of the kink bands produced by SMAT. The kink bands formation was promoted by the high strain rates of the SMAT process. The samples SMATed at cryogenic temperature showed an increase in fatigue resistance, by about 8%, compared to samples processed by SMAT at room temperature. This improvement was possibly due to a combination of several factors: compressive residual stresses, lower roughness, deeper formation of martensite and larger kink bands. On the contrary, the SMAT at room temperature did not provide any improvement in terms of fatigue compared to the polished condition. The large grain size provoked the appearance of subsurface crystallographic defects on which primary cracks initiated. This greatly limited the effectiveness of the reinforced surface layer produced by SMAT.

Tension and fracture toughness JIc tests were performed on both as-welded and heat-treated conditions. The softer tempered martensitic matrix and the transformation-induced plasticity (TRIP) of reformed austenite improved significantly the strain hardening behavior of the heat-treated material. These matrix characteristics were found to retard micro-void nucleation during straining. The onset of necking, the elongation at fracture, and the fracture toughness JIc were improved by 55%, 28% and 90%, respectively. These results provide an opportunity to document how the matrix mechanical properties influence the fracture behavior of material which rupture is controlled by the nucleation of micro-voids.

Dry machining is one of the reliable eco-friendly techniques since it enables avoiding the use of harmful/expensive cutting fluids, reducing part costs, and increasing productivity. This study attempts to optimize the dry peripheral milling conditions with regard to the surface finish and residual stress (RS) of machined aeronautical Al–Zn–Mg–Cu alloy. The proposed approach involves a multi-objective optimization of five machining parameters, including cutting tool geometry, milling mode, feed per tooth, cutting speed, and radial depth of cut. The multi-objective optimization approach is developed based on the Taguchi method combined with the Grey relational analysis (GRA). An improvement of the multiple performance characteristics by 104%, as compared to the initial cutting conditions, has been obtained using the proposed approach. The analysis of variance (ANOVA) has shown that the feed per tooth, the cutting speed, and the interaction between cutting tool and milling mode significantly influence the surface quality of dry peripheral milled parts. This study demonstrated that the dry peripheral milling of aluminum parts can be an efficient sustainable process in terms of part quality if the machining conditions are adequately selected and optimized.

The local mechanical properties of a dissimilar friction stir welded AA-2024-T3/AA-2198-T3 joint were documented during a uniaxial tensile test. High-resolution digital image correlation was performed during monotonic tensile tests to capture the local in-plane strain fields of the heterogeneous macrostructure of the weld. In the shoulder-affected region, banded macrostructures with heterogeneous mechanical properties were found. They were related to pronounced textures regions, which can be associated to strain-rate gradient during one rotation of the tool. The banded macrostructures in the nugget region were observed to be responsible for early plasticity in the joint and ultimately to be the fracture location of the weld. The heterogeneous mechanical response of the joint was also investigated by microhardness measurements. Differences were found between hardness and local tensile properties, demonstrating microhardness measurements can be misleading and only direct high-resolution digital image correlation techniques can document the mechanical behavior of materials having complex and heterogeneous micro-/macrostructures. Keywords: Friction stir welding (FSW), Digital image correlation (DIC), Electron backscattered diffraction (EBSD), Texture, Aluminum alloys, Banded macrostructures, Microhardness, Dissimilar joint

The machining of high strength materials used in aeronautical applications generates damage on the subsurface layer, which can significantly affect the fatigue life of the machined components. It is then important to distinguish between the damages due to machining from those caused by mechanical polishing operations used for sample observation. In this study, a new method is proposed to characterize and quantify properly the affected layer by machining and eliminating the impact from the defects originated during mechanical polishing. A protective layer of nickel coating was deposited on the machined surface. An optimum thickness of 100 μm was determined for the nickel layer to avoid any damage to the subsurface layer during sample preparation. The subsurface layer was analyzed using an automatic Knoop microhardness machine, laser-digital microscope, and electron backscatter diffraction (EBSD) microscopy.

In recent years, there has been growing interest for the identification of material constitutive equations using machining tests (inverse method). However, the inverse method has shown some drawbacks that could affect the accuracy of the identified material constants. On one hand, this approach requires the use of analytical model to estimate the cutting temperature. Nevertheless, the used temperature models lead to large discrepancies for the calculated temperatures even for the same work material and cutting conditions. On the other hand, some computation issues were observed when all material constants were determined, in the same time, using machining tests data. Therefore, this study attempts to provide a methodology for identifying the coefficients of the Marusich’s constitutive equation (MCE) which demonstrated a good capability for the simulation of the material behavior in high speed machining. The proposed approach, which is based on an analytical inverse method together with dynamic tests, was applied to aluminum alloys AA6061-T6 and AA7075-T651, and induction hardened AISI 4340 steel (60 HRC). The response surface methodology was used in this approach. Two sets of material coefficients, for each tested work material, were determined using two different temperature models (Oxley and Loewen-Shaw). The obtained constitutive equations were validated using dynamic tests and finite element simulation of high speed machining. The predictions obtained are also compared to those performed with the corresponding Johnson and Cook constitutive equations (JCE) from the literature. The sensitivity analysis revealed that the selected temperature models used in the analytical inverse method can affect significantly the identified material constants and thereafter predicted dynamic response and machining data. Moreover, the MCE obtained using the hybrid method performed better than the JCE obtained by only dynamic tests.

This paper examines the metallurgical evolution of AISI Stainless Steel 321 (SS 321) during multi-step forming, a process that involves cycles of deformation with intermediate heat treatment steps. The multi-step forming process was simulated by implementing interrupted uniaxial tensile testing experiments. Evolution of the mechanical properties as well as the microstructural features, such as twins and textures of the austenite and martensite phases, was studied as a function of the multi-step forming process. The characteristics of the Strain-Induced Martensite (SIM) were also documented for each deformation step and intermediate stress relief heat treatment. The results indicated that the intermediate heat treatments considerably increased the formability of SS 321. Texture analysis showed that the effect of the intermediate heat treatment on the austenite was minor and led to partial recrystallization, while deformation was observed to reinforce the crystallographic texture of austenite. For the SIM, an Olson-Cohen equation type was identified to analytically predict its formation during the multi-step forming process. The generated SIM was textured and weakened with increasing deformation.

A method is proposed for evaluating the geometrical parameters of metallic cellular structures produced by additive manufacturing. It is based on the segmentation and the skeletonization of plane images of the specimens. The medial axis transform is the key operator used to quantify the geometry of the structure. The analysis discriminates the struts and the nodes by modeling the latter as polygons. The average thickness, length, and orientation of the struts are individually estimated. The method is implemented as a program that can process multiple images almost automatically. It was validated on a CAD model, and then applied to several Ti6Al4V specimens manufactured by Direct Metal Laser Sintering with varying geometrical parameters. The irregularities of the specimens are readily put into evidence thanks to the comprehensiveness of the measurements. It is concluded that the process is orthotropic with respect to the laser beam. In particular, the struts parallel to the laser beam tend to be thinner than perpendicular ones. The extension of the method to other geometries is discussed. Keywords: Additive manufacturing, Selective laser sintering, Image analysis, Porous materials, Medial axis transform

In the aeronautical industry, many critical components are made of high-strength steels. These steels have high tensile strength but are sensitive to corrosion and to the hydrogen embrittlement phenomenon. This study examines how acoustic emission could be used to detect hydrogen embrittlement of high-strength steels and reduce the duration of the standard test. Acoustic emission (AE) was coupled with sustained load testing at 75% of the material fracture stress for 200 h as prescribed in ASTM standard F519. Tensile strength tests were also conducted to determine the mechanical characteristics and acoustic signature of the mechanisms of damage that progress to material fracture. The results show that the time required for sustained load tests can be shortened, based on the emission of acoustic events by the hydrogen-embrittled test coupons from the beginning of the test and after certain lengths of time. Scanning electron microscopy imaging of the fracture surfaces showed a very good correlation with the recorded acoustic event signatures.

International audience; The fatigue properties of gas metal arc welded and friction stir welded assemblies made of aluminum alloy AA6061-T6 structural extrusions were examined. The mechanical performances of welded joints were obtained using uniaxial tensile and force-controlled constant amplitude axial fatigue tests. Microstructural and fractographic analyses were conducted to document the influence of the process on microstructure evolution, fatigue crack initiation sites, and propagation mechanisms leading to the final rupture of the assemblies. Microhardness measurements and digital image correlation techniques paired with interrupted tensile tests were also used to investigate the complex heterogeneous local mechanical behavior and to highlight the fact that the crack initiation mechanism was driven by the microstructural state of the joint as well as by the structural-contact-fretting occurring at the notch root. The corresponding fatigue strengths at 2 million and 10 million cycles were evaluated respectively at 10% and 20% higher for friction stir welded assemblies versus gas metal arc welded assemblies. Fractographic analyses revealed that the fatigue cracks were initiated from microstructural features (pores for the GMAW configuration and banded structure on the crown side for the FSW configuration), or from large sub-surface grains in a shallow region below the structural-contact-fretting occurring at the notch root.