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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.

An original approach is introduced to discriminate the effects of residual stress and surface roughness on turned Inconel 718 alloy parts. This could be used for assessing or studying the performance of these specimens during fatigue tests. This method consists of building surface integrity maps in terms of surface roughness parameters and residual stress in order to select specific turning conditions with adequate machining parameters. The surface integrity maps were generated by using response surface methodology based empirical models and precision turning tests on coupons. The proposed approach was therefore successfully applied to manufacture rotating bending fatigue specimens with the surface axial residual stress varying from − 239 to 446 MPa while keeping the arithmetic average roughness and the surface hoop residual stress constant. The effect of the surface axial residual stress on the fatigue life could thus be discriminated from the other surface integrity characteristics effects. The proposed methodology is independent of the manufacturing process and can be extended to other machining processes such as milling, grinding, and broaching.

In order to fully exploit the benefits of shot peening process in terms of fatigue performance, the proper choice of peening conditions is indispensable, which is largely material specific. In this study, the influence of 3 different peening conditions on the axial fatigue life of high strength 300M steel (modified AISI 4340) was investigated in the high cycle fatigue regime. A statistical estimate of 90% survival life at 95% confidence interval was made to rigorously compare the fatigue performance of the material with different surface conditions. The peening induced roughness, compressive residual stresses and their relaxation during cyclic loading were quantified. It was however revealed, that shot peening had no significant effect on this material when tested at 55% of yield strength ( σ y , 0.2 ). The fatigue life was dominated by the larger non-metallic inclusions within the bulk material. An empirical relationship between inclusion size and fatigue life was proposed based on this observation. Furthermore, the inclusion size distribution was well described using Gumbel extreme value distribution allowing the largest inclusion to be predicted in a given volume of steel. The prediction results were in good agreement with the experimentally observed value of the largest inclusion size observed at a fatigue crack initiation site.

Fatigue lives of as-polished and shot peened specimens were predicted using the original Navarro-Rios model (monotonic damage model) and a more recent version of the same model (cyclic damage model). Crack closure, residual stress relaxation, cold work and cyclic yield stress were experimentally characterized and implemented within the cyclic damage model. Two shot peening conditions were studied (S230 and Z425 shots at 8A and 100% coverage). The in-depth profiles of the relaxed residual stress and cold work were experimentally characterized and implemented within both versions of the model. Two stress amplitudes leading to high cycle fatigue (HCF) and low cycle fatigue (LCF) were studied at a stress ratio of 0.1. Predictions of the cyclic damage model were globally closer to experimental fatigue lives than those predicted by the monotonic damage model. The monotonic damage model significantly overestimated fatigue life of shot peened samples in LCF. A detailed list of the used parameters and their characterization is provided. Finally, potential improvements and uses of the model for future studies are proposed.

The role of twin boundaries (TBs) in the dynamic recrystallization (DRX) of Ni-30Cu (Monel400) was investigated through hot compression tests in the temperature interval of 750–1150 °C and strain rate of 0.01 s−1. Electron Back Scattered Diffraction (EBSD) analysis showed that DRX at 750 and 950 °C was limited to bulging and sparse nucleation on random high angle grain boundaries (HAGBs) while the primary coherent twin boundaries (TBs), with a length density of 55% in the starting microstructure, did not take part in the bulging process before conversion into HAGBs. The sluggish DRX was therefore attributed to the necessary conversion of primary TBs into HAGBs. The investigation also revealed that a new variant of TBs formed behind the protruding grain boundaries separated the bulges from the parent grains, indicating that unlike primary TBs, the newly formed TBs contributed to the occurrence of DRX. The formation of the new TBs was related to the dissociation of well-developed subgrains and the migration of random HAGBs. A mechanism for the occurrence of DRX in hot deformed Monel 400 is proposed that takes into account the contribution from the new TBs.

International audience; The paper demonstrates how defects inherited from the deposition processes can severely impair the lifetime of MCrAlY coatings in service. The oxidation behavior of two NiCoCrAlY coatings was investigated at 1150 °C up to 500 h. The coatings had the same nominal composition but were processed by two different projection techniques: air plasma spray (APS) and high velocity oxy fuel (HVOF). Freestanding coating specimens were extracted from the coated system and thinned down to different thicknesses ranging from 400 to 15 μm in order to investigate size effects inherent to the oxidation response. The oxidation rate of the APS coating was found to be insensitive to the specimen thickness, while that of the HVOF coating increased with the specimen thickness, due to greater intersplat oxidation. APS specimens thinner than 60 μm experienced intrinsic chemical failure (InCF) due to Al consumption to form the Al2O3 scale. In comparison, HVOF specimens with a thickness of 367 μm were subject to InCF after 250–350 h oxidation. This first stage of InCF resulted in the formation of a Cr2O3 layer at the Al2O3/metal interface once Al activity in the MCrAlY coating was low enough to thermodynamically allow Cr2O3 to form. In addition, thick HVOF specimens developed mechanically induced chemical failure (MICF) resulting in the formation of (Ni,Co)(Cr,Al)2O4 spinels on top of the Al2O3 scale and within oxide intrusions. The occurrence of MICF was associated with the concomitant effects of Al consumption due to intrusive oxidation and the spallation of the external Al2O3 scale.

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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The mechanical behaviour of porous lattice materials for potential orthopaedic applications was investigated at a fine scale by means of digital image correlation (DIC). Specimens with cubic, body cubic-centered reinforced (BCCZ), and diamond mesostructures were tested in quasi-static compression up to failure. Images were continuously recorded by an imaging setup and processed by a custom DIC program, OpenDIC. The resulting strain maps were analyzed in both spatial and temporal scales, displaying the onset and evolution of strain heterogeneities. The three geometries show different failure modes, i.e. collective buckling of an entire row for cubic, diagonal shearing band for BCCZ, and generalized crushing for diamond. The strain maps correlate well with these patterns. Most importantly, they show early strain localization below the macroscopic elastic limit. After failure, a phenomenon of strain release was witnessed and evaluated in the parts of the specimen that do not fracture. In the cubic geometry, the vertical struts sustain most of the deformation. Further analysis shows that the rows of vertical struts have similar, yet scattered, evolutions until failure. Interestingly, the row that leads to specimen failure is not necessarily the first one to deform. In addition to these experimental results, the uncertainties of the method were thoroughly assessed by means of calibration procedures.

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.

In this paper we consider the numerical solution of stiff problems in which the eigenvalues are separated into two clusters, one containing the “stiff”, or fast, components and one containing the slow components, that is, there is a gap in their eigenvalue spectrum. By using exponential fitting techniques we develop a class of explicit Runge–Kutta methods, that we call stability fitted methods, for which the stability domain has two regions, one close to the origin and the other one fitting the large eigenvalues. We obtain the size of their stability regions as a function of the order and the fitting conditions. We also obtain conditions that the coefficients of these methods must satisfy to have a given stiff order for the Prothero–Robinson test equation. Finally, we construct an embedded pair of stability fitted methods of orders 2 and 1 and show its performance by means of several numerical experiments.

Shot peening is commonly used in the aerospace industry to improve mechanical components fatigue life. It introduces compressive residual stresses and cold work at the surface which tend to close short fatigue cracks and delay their propagation, respectively. However, shot peening also creates surface irregularities that can be detrimental to fatigue. The effect of different shot peening conditions on Inconel 718 tested in low and high cycle fatigue is presented in this study. An analysis of the fatigue life, crack initiation mechanisms, residual stress relaxation, process induced strain hardening and surface roughness showed that, in high cycle fatigue, shot peening can increase Inconel 718 fatigue life from 2 to 20 times, depending on the shot peening conditions. This observation suggests that careful selection of peening parameters is crucial. In low cycle fatigue the roughness resulting from shot peening is to be considered while in high cycle fatigue, it is the presence of significant residual stresses.

The beneficial effect of using cryogenic temperatures for the ultrasonic surface mechanical attrition treatment (SMAT) is depicted in the case of the 304 L austenitic stainless steel. The cryogenic temperatures induce an additional driving force for the formation of martensite at depth where the effect of shot peening tends to vanish, raising the subsurface hardness by 20 to 30%. The maximum amount of α′ martensite was always located within the subsurface. This is explained by different mechanisms operating at different stages during the SMAT process: (i) higher shear stresses at the subsurface due to the Hertzian contact stress, (ii) local heating due to the shot impacts and (iii) surface martensite destabilization when the microstructure enters the nanometric range. Consequently, the gradient structure formed by the SMAT superimposes different natures of gradients in terms of grain size and martensitic fraction. The steel being harder to deform at cryogenic temperatures, a significant decrease of the apparent surface roughness (down to 44%) is also obtained. An appropriate selection of processing parameters allows to tailor the surface towards a broad range of hardness and roughness values: 425 to 560 HV and 1.6 to 3.3 μm, respectively.

The high-cycle fatigue performance and crack growth pattern of transient liquid phase-brazed joints in a nickel-based superalloy Inconel 625 were studied. Assemblies with different geometries and types of overlaps were vacuum-brazed using the brazing paste Palnicro-36M in conditions such as to generate eutectic-free joints. This optimal microstructure provides the brazed assemblies with static mechanical strength corresponding to that of the base metal. However, eutectic micro-constituents were observed in the fillet region of the brazed assembly due to an incomplete isothermal solidification within this large volume of filler metal. The fatigue performance increased significantly with the overlap distance for single-lap joints, and the best performance was found for double-lap joints. It was demonstrated that these apparent changes in fatigue properties according to the specimen geometry can be rationalized when looking at the fatigue data as a function of the local stress state at the fillet radii. Fatigue cracks were nucleated from brittle eutectic phases located at the surface of the fillet region. Their propagation occurred through the bimodal microstructure of fillet and the diffusion region to reach the base metal. High levels of crack path tortuosity were observed, suggesting that the ductile phases found in the microstructure may act as a potential crack stopper. The fillet region must be considered as the critical region of a brazed assembly for fatigue applications.

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 performance and integrity of coated engineering components rely on the time required for the appearance of defects and cracks and their propagation, leading to delamination of the coating and degradation of the substrate. Optimizing the coating's mechanical properties such as hardness, residual stress (RS), and adhesion is essential to delay crack onset and propagation. In the present work, we investigate the mechanical properties, and the failure mechanisms of model TiN coatings reactively sputtered onto Ti-6Al-4V substrates while comparing three interface engineering approaches to enhance the system durability: a) Argon plasma treatment, b) plasma surface nitriding, and c) Titanium implantation. The TiN coatings possessed a hardness of ~29 GPa and a Young's modulus of ~350 GPa. Multi-reflection grazing incidence X-ray diffraction was used to assess the compressive RS depth profile. Each interface engineering process induced RS variation in the coating and the adjacent interfacial area and in the substrate's near-surface layer ranging from −1 GPa to −2.2 GPa with different gradients. Cohesive failure, crack evolution and fracture toughness were studied using micro-scratch and micro-tensile tests, while the surface grain deformation behaviour and surface cracks on fractured Ti-6Al-4V with various interface treatments and TiN coatings were identified by SEM image and elasto-plastic property analyses. We found a close correlation between the RS of the coatings or the interface layers and the fracture mechanism. Specifically, higher compressive RS led to an enhanced interfacial shear strength and a critical energy release rate of around 550 to 790 MPa, and of ~18 J/m2, respectively.

Additive Manufacturing (AM) is gaining widespread attention over the last decade in the areas of product development and prototyping, across aerospace and medical domains. Nevertheless, the built parts are near-net shaped that usually require a machining finish to improve the surface quality and operational life. The current study aims to document the machinability behaviour of the Ti6Al4V parts fabricated by the AM process, which is based on powder bed fusion, namely, Selective Laser Melting (SLM) and Electron Beam Melting (EBM). Further, machinability parameters are correlated with the microstructure and mechanical properties of the material. Face milling operation is performed under finishing conditions to study the response on the cutting forces. The effect of Hot Isostatic Pressing (HIP) and the influence of the build direction on microstructure and machinability have been investigated for the two AM processes. The effect of crystallographic textures and microstructure morphology of α and reconstructed prior β phases on the mechanical properties was studied using shear tests. It was observed that the SLM process primarily produced homogenous and fine α lath structures with less grain boundary α (αGB) grains, leading to low shear resistance and hence generated low cutting forces. On the other hand, the EBM process produced microstructures with variable morphologies, including more grain boundary alpha (αGB) grains and widmenstatten structures which resulted in a higher shear resistance and forces. The results show a striking influence of the microstructure induced by the type of the AM process, build orientation, and the subsequent heat treatment on the shear resistance and their ability to be machined. Further, the crystallographic textures were not strong for both processes and seem not to impact the machining process. The results highlight a strong “process – property” link which demands optimization of the cutting process for the type of the AM process and the resulting microstructure.

The effect of laser wave modes on the fatigue behavior of laser cold-wire welding made of 4.8 mm thick AA6005-T6 aluminum alloy was investigated using continuous and pulsed wave lasers. Due to the inherent differences in these two wave laser modes, different welding parameters were used, while keeping the interaction time constant. The mechanical properties of welded joints were measured using tensile tests, while their fatigue performances were quantified using a constant amplitude force-controlled technique to obtain S-N curves. The pulsed wave laser mode produced higher fatigue resistance as compared to the continuous wave mode. The fatigue strength corresponding to the run-out condition (i.e., 107 cycles, in this study) was about 28% higher for the pulsed wave mode than for the continuous wave laser mode. At a high stress amplitude (30 MPa), the lifespan of the pulsed wave joints was about twice as high as that with continuous wave joints. These fatigue results were cross-referenced with a 3D topographic map, a 2D microhardness map, a metallographic study, and a fractographic analysis to better understand the crack nucleation and crack propagation mechanisms. A microhardness analysis performed along the cross-section of the joints did not reveal any significant difference between pulsed wave and continuous wave modes. A fractographic analysis confirmed crack propagation within the fusion zone and that 83 to 90% of cracks nucleated from the root undercuts. Topographic maps of the joints before fracture revealed that the continuous wave laser mode produces deeper and narrower (i.e., more acute) undercut defects than does the pulsed wave mode. A Weibull approach based on the biggest defects found on the fracture surfaces also confirmed that the continuous wave process produces larger defects at the root. Top defects are significantly small (55%), but they have a larger size dispersion than do root defects. Since the root undercuts act as the main stress concentrators, they are mainly responsible for the lower fatigue performance of the joints, and their sizes and shapes should be minimized during further process development of the welding process.