This paper develops a new micro-mechanical damage model for fatigue crack propagation in tensile (Mode I) cyclic loading. The double-scale fatigue damage model is entirely obtained through small-scale yielding descriptions of micro-crack propagation. Based on the Paris fatigue crack growth law, the stress intensity factor range is expressed thanks to an appropriate micro-mechanical energy release rate analysis coupled with asymptotic homogenization developments. The macroscopic fatigue crack evolution law is established. Numerical simulations are presented. The reliability of the model to reproduce stress-strain shielding is demonstrated. The influence of the microstructural length on fatigue damage evolution is illustrated. The numerical results are compared with fatigue crack propagation tests performed on Ti-6Al-4V titanium fashioned by additive layer manufacturing (ALM). Through the calibration process, the model agrees with the experimental results.

In this study, Al2024-T3 aluminium alloy and Ti-6Al-4V titanium alloy were lap jointed by friction stir welding (FSW). New insights are given into the multiscale effects of the pin tip shape and length on both the interface microstructure and the tensile-shear properties of dissimilar joints. The correlation between the stress distribution at the joint interface, governed by the pin shape, and the phase formation sequence was notably studied. The microstructure of the joints was examined from macro-to-nanoscale by optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM), while their mechanical properties were investigated with tensile-shear tests, coupled with digital image correlation (DIC). It was found that with a semispherical pin tip, the joint interface was sharp and semi-coherent. Al and Ti solid solutions were identified by HRTEM analysis over a distance of 10 nm on both sides of this interface. Conversely, a longer flat pin tip generated a 15 mu m thick multi-layered interface containing Ti3Al, TiAl and TiAl3 intermetallic compounds. The effect of the welding conditions on the reaction kinetics during friction stir welding of Al/Ti alloys has been investigated. It was found that the intermetallic compounds were formed by atomic diffusion favoured by both temperature and plastic deformation. The interfacial joint microstructures were explained by the different stress and strain fields generated by the distinct pin tip shapes and by their different penetration depths in the Ti-6Al4V bottom plate. Although these microstructural features at the joints interface very likely governed the tensileshear properties of safe joints, they were, in this case, less critical than the hooks and the deformed Alclad layer which led to the joint fracture. Stronger and more ductile joints were obtained with the semi-spherical tip shorter pin. A maximum joint coefficient of 50 % compared to Al2024 base material was found. Some ways of optimization were finally proposed.

The continuous growth in the manufacture of aerospace components such as blisks has led to an increase in the application of different hybrid materials fabricating methods, and thus the requirements for joining and strengthening of dissimilar welds. According to this goal, selective laser melted (SLM) Inconel 718 was joined with forged AD730™ Nickel-based superalloy through linear friction welding (LFW) in this study. Microstructure variation, specifically with respect to secondary phases precipitation was investigated. The microhardness and strengthening mechanisms of the weldment were also studied. The precipitation (volume fraction and size of particles) at different regions of both sides of the weld line was characterized. Close to the weld line, the dissolution of γ'/γ" and Laves phases and grain refinement occurred which reveals the effects of both compression strain and high temperature on recrystallization and high degree of elemental diffusion in the weld zone (WZ). It is shown that the size, volume fraction, and shape of secondary phases increased and changed (from spherical to long-striped for Laves particles) as we went from the WZ toward the base metal. However, the measured microhardness indicated that the strength of AD730™ alloy depends significantly on the grain size, while strength in SLM Inconel 718 was dominated by shape (or size) and the presence of secondary phases (γ'/γ" and Laves).

Nitinol is used in dental tools for its superelasticity, shape memory and biocompatibility. Selective laser melting (SLM) on a powder bed is a potential process for manufacturing such as dental tools. Microstructural studies are conducted on NiTi-SLM using a parallel strip strategy and 90 degrees laser beam rotation. The multi-scale (nano-, micro- and mesoscopic) microstructure is revealed. Similar "cast" type microstructure features are found. The SLM melt is casted in a "micro-mold". Fine micro-grains with random crystallographic orientation are formed near the mold walls and served as seats for the rapid growth of columnar dendritic grains along the < 001 > direction with epitaxial crystallographic relationships. The growth of the columnar grains curves towards the center of the "micro-mold", coinciding with the center of the laser beam. It is proposed that SLM processing resembles "Metallic Self Micro-Mold Casting" (MS mu MC) and/or "Self Micro-Molding" (S2M).

During friction stir lap joint welding, hooks are systematically formed at the interface of dissimilar Al2024/Ti-6Al-4V plates. Various hooks dimensions and shapes are observed according to the welding parameters (welding speed, rotational speed and double-pass). Based on digital image correlation (DIC) during tensile shear tests, it is found that hooks features govern the fracture behaviours and, hence, the mechanical properties of the overall welds. In particular, three fracture behaviours are obtained: at the interface, in the Al2024 and across the Ti-6Al-4V plate, which was never reported in dissimilar Al/Ti studies. The aims of this work are (i) to study the influence of FSW parameters on the mechanisms of hooks formation and (ii) to understand the role of both AS and RS hooks on the mechanical behaviour of the welds, by the means of two new approaches. First, hooks are investigated by X-ray tomography which provided some innovative insight into the material flow (3D viewing). Based on these qualitative and quantitative results, hooks are used as tracers and two new scenarios of AS and RS generation are proposed. Then, the contribution of both hooks on the stress distribution is clearly revealed by some calculations of the stresses reached around AS and RS hooks. Contrary to RS hook, AS hook is sharp and, hence, the stress reached at the top of AS hook is characterized by a stress concentration. The results showed that the fracture behaviours are greatly influenced by hooks size and the formation of microstructural defects inside the Ti-6Al-4V, which depend on the welding parameters. In conclusions, this work revealed the relationship between the achievable parameters, the resulting hooks dimensions and the fracture behaviours. In addition, based on the understanding of the material flow, another FSW parameters are proposed to optimize the mechanical properties.

Selective Laser Melted (SLM) Inconel718 (IN718) superalloy was linear friction welded (LFWed) to forged AD730™ Nickel-based superalloy. Successful joints free of micro-porosity, micro-cracking, and oxides were obtained. Microstructure variations across the weld line developed during LFW were examined using different techniques, including laser confocal microscopy, scanning electron microscopy, energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD). The microstructure was also evaluated, particularly in terms of Grain size and misorientation changes were determined and correlated with microhardness evolution in different regions of the weld joint. The characteristics of the microstructure on both sides of the weld joint was analyzed and related to the deformation and temperature paths imposed during the LFW process. Dynamic recrystallization (DRX) occurred on both sides of the dissimilar weld line and it was found that Discontinuous DRX (DDRX) and Continuous DRX (CDRX) took place in the WZ and in the TMAZ, respectively. In order to study the influence of the starting microstructure in the LFW experiments, LFW of a homogenized SLM IN718 sample was analyzed and compared with the non-homogenized sample. A clear change in the size and grain misorientation levels of the heat and thermomechanical affected zones were observed between the two conditions. The differences were related to a greater degree of strain induced in homogenized sample and the increasing effect of the solid solution strengthening mechanism caused by a partial dissolution of the second-phase strengthening particles in the matrix.

A new post-weld heat treatment (PWHT) cycle was designed for novel dissimilar linear friction welding (LFW) of selective laser melted (SLM) Inconel 718 (IN718) to AD730 forged nickel-based superalloy. The microstructure and hardness of the joints after the PWHT are investigated and compared with those of as-linear friction welded samples. The precipitation of γ′ + γ″ is determined as the main mechanism to increase the mechanical properties of SLM IN718 alloy. These particles coarsened during heat treatment at 1253 K and double aging. The results show that the thermomechanical history of linear friction welded joints can affect the microstructure of IN718 alloy such as the morphology of δ phase after solution treatment (ST) from the platelike in the weld zone (WZ) to the needlelike in the base material (BM). It was found that in AD730, nanometric size γ′ particles reprecipitated close to the weld line during rapid cooling after welding. The presence of ultrafine γ′ particles and coarsening of the remaining particles in the microstructure of the alloy, during PWHT, can enhance the strength and hardness. The developed PWHT resulted in uniform hardness across the new dissimilar joint.

Taking into account the interaction between the engine oil and the crankshaft to model crankshaft thermomechanical behavior under dynamic loading is very important. In particular, when the crankshaft is working in severe conditions. This paper deal with an air cooled direct injection-type engine crankshaft thermomechanical FEM modelling account for engine oil-cranks half interaction in severe working conditions. As case of application we consider the diesel engine Deutz F8L413. The model takes into account 2 forced convectives heat flux: engine oil and crankcase air. The severe mechanical and thermal characteristics of engine are experimentally measured on a bench test equipped with a hydraulic brake. The temperature distribution inside the crankshaft was computed using the measured temperature as boundary conditions. The most thermo-mechanical stressed zones of the crankshaft have been determined. The fatigue resistance of the crankshaft under thermo-mechanical conditions was examined using Dang-Van multi-axial fatigue criteria. To prove our model efficiency, we have compared crankshaft damage in service to the numerical simulation results. It was found the breakage occurred in an area where the numerical simulations give the highest stresses.

Inconel 718 (IN718) superalloy samples fabricated by selective laser melting (SLM) were submitted to different heating cycles and their microstructural characteristics were investigated. The selected heating rates, ranging from 10 °C/min to 400 °C/s, represent different regions in the heat-affected zone (HAZ) of welded additively manufactured specimens. A combination of differential thermal analysis (DTA), high-resolution dilatometry, as well as laser confocal and electron microscopy were used to study the precipitation and dissolution of the secondary phases and microstructural features. For this purpose, the microstructure of the additively manufactured specimen was investigated from the bottom, in contact with the support, to the top surface. The results showed that the dissolution of γ″ and δ phases were delayed under high heating rates and shifted to higher temperatures. Microstructural analysis revealed that the Laves phase at the interdendritic regions was decomposed in specific zones near the surface of the samples. It was determined that the thickness and area fraction of these zones were inversely related to the applied heating rate. A possible mechanism based on the influence of heating rate on Nb diffusion in the interdendritic regions and core of the dendrites is proposed to interpret the observed changes in the microstructure.

In hot forming processes, steel tools are subjected to rapid temperature transients and cyclic oxidation. AnT(2)COK, a new analytical model based on diffusion-controlled oxide growth in cyclic conditions, is proposed to predict the parabolic oxidation kinetics in spallation-free conditions. Compared to cyclic oxidation models of the literature (COREST, COSP, DICOSM, etc.), it is applicable for any shape of thermal cycle, fully transient or with dwell time. The model is applied to thermal fatigue tests on X38CrMoV5 steel, for T-max between 500 and 685 degrees C. Its validity is demonstrated above 550 degrees C, using frequency factor and activation energy calculated from isothermal tests.

This paper examines plastic mechanisms that lead to damage to the sub-surfaces of ductile metallic contact areas and, by extension, to the creation of wear particles. The word "damage" is used here as a generic term to designate all topological, morphological and/or microstructural modifications to the surfaces that are the consequence of interfacial shear stress under friction. During friction, the accommodation mechanisms of plastic deformation by the M3 mode in the contact area can be different, depending on the microstructures of the alloy and the first sites (bulk materials). For example, for tempered martensitic steels under frictional stresses, plasticity strain occurs in Tribologically Transformed Surfaces (TTS) under dislocation gliding. The creation of new dislocations and the rearrangement of all these dislocations into a "bamboo"-type structure leads to a decrease in hardness, or softening of the steel. Otherwise, depending on the stacking fault energy in alloy microstructures, plasticity may occur through mechanisms involving either perfect dislocation gliding and/or partial dislocation gliding. In addition to hardening, the mobility and morphology of dislocations are also related to stacking fault energy, which can promote phase transformation leading to shear strain in TTS. These shear strain mechanisms have a great influence on the internal flow of wear particles and on the formation of the "Third-body layers". These findings are given in a review of tribological results of measurements carried out on a wrought X38CrMoV5 grade steel and on cobalt-base thick coatings, using tribometers, under various loadings, test temperatures and sliding speeds. The cobalt-base thick coatings are deposited on steel by several processes which modify the nominal chemical composition of the cobalt alloy and make it possible to study the influence of the iron content on shear strain under friction stresses.

This paper deals with the influence of the mechanisms of plasticity on the evolution of the friction coefficient in cobalt-based hardfacings. Particularly depending on the alloying element content and so on the stacking fault energy, plasticity in Co-based alloys may occur through mechanisms involving either perfect dislocations gliding and/or partial dislocation gliding. As the coalescence of stacking faults by partial dislocations promotes the phase transformation of the Co, this study focus on the impact of this phase transformation on the evolution of the friction coefficient. Stellite 21 (Co-27Cr-5Mo-0.25C) hardfacings deposited on a steel substrate by two different processes, namely metal inert gas (MIG) and laser, are studied. The tribological properties are evaluated with a ring on disc tribometer under high load (800 daN) and continuous sliding (5 mm/s) at room and high temperatures (450 degrees C and 600 degrees C). The wear volume is characterized by confocal microscope. From micro-hardness measurements and SEM observations the work-hardening and the plastic strain of the Tribological Transformations of Surfaces are identified in relationship with chemical analysis (by EDS). Moreover X-rays diffraction and EBSD reveal the crystal structure evolution. As-deposited Co-based hardfacings have a face-centered cubic (FCC) structure. Meanwhile depending on the process of deposition on the steel substrate, the nominal composition of the Stellite 21 can be modified due to dilution effect. So, the iron content is found to be higher in the MIG hardfacing than in the laser one, leading to different mechanisms of plasticity, respectively by perfect dislocations gliding and by phase transformation (FCC to hexagonal closed-packed (HCP)). Moreover, a significant influence of the phase transformation on the friction coefficient has been evidenced: without phase transformation the cobalt remains in the FCC structure and the friction coefficient is stable during the test, while a decrease of its value occurs during the FCC to HCP phase transformation.

The interaction between thermal fatigue and aluminizing and/or oxidation is investigated using an experimental approach based on decoupling of mechanisms. Virgin and pre-aluminized steel specimens are tested in air and nitrogen between 100 and 650 degrees C. Homogeneous uniaxial micro-crack network forms on the oxidised or pre-aluminized surface in air, with a better resistance to micro-cracking for the intermetallic coating. The propagation of the micro-cracks is delayed in nitrogen, whilst no evidence of micro-cracking is observed on the virgin specimen. The premature cracking of the steel depends on the formation of the superficial micro-crack network, and the crack propagation is assisted by oxidation.

The present work aims to model the influence of microstructural features of Ti-6Al-4V titanium alloy on its mechanical behavior. A multi-scale approach based on crystal plasticity is considered. The elasto-viscoplastic constitutive equations of Meric-Cailletaud are modified to take into consideration the effect of the grain size by introducing the Hall-Petch relationship at the local scale. This modified model is coupled with finite element calculations under small strain assumption to simulate the monotonic mechanical behavior of Ti-6A-4V at local and global scales. It is shown that the mechanical behavior of Ti-6Al-4V is drastically dependent upon the material features. Strong crystallographic texture can result in the formation of hardened and less hardened areas. Moreover, by increasing the grain size scattering, the heterogeneously deformed areas are multiplied. By decreasing the average grain size, the yield strength increases. It is observed that the effects of grain size, grain size scattering and crystallographic texture are coupled.

Ce travail consiste a etudier l'effet de la taille, la dispersion et la texture cristallographique des grains sur le comportement mecanique d'un alliage de titane Ti-6Al-4V. L'investigation des champs mecaniques (contraintes et deformations) est effectuee aux differentes echelles, sous chargement quasi-statique monotone de traction. Le modele de comportement utilise est developpe dans le cadre de l'approche de plasticite cristalline. Douze volumes elementaires representatifs (VER) permettant la prise en compte des parametres steriologiques (taille, dispersion et texture cristallographique) sont construits, puis la simulation du comportement mecanique du Ti-6Al-4V est effectuee en utilisant le modele de meric-Cailletaud [1] en corporation avec la metode des elements finis. Enfin, l'influence des elements microstructurales du materiau est analysee. Les resultats montrent que le comportement mecanique macroscopique du Ti-6Al-4V est fortement lie a la texture cristallographique des grains, cette derniere donne a la deformation plastique de l'alliage Ti-6Al-4V un caractere heterogene ainsi qu'un comportement mecanique macroscopique anisotrope. La taille moyenne des grains influe egalement sur les proprietes mecaniques du Ti-6Al-4V, en particulier sur la limite d'elasticite; en diminuant la taille moyenne des grains, la limite d'elasticite augmente. Enfin la distribution des tailles des grains influe sur le comportement local du materiau, elle donne aux champs de deformation une heterogeneite suffisante lorsque la dispersion augmente.

The present investigation proposes an experimental device able to assess the thermo-mechanical behavior of Ti-6Al-4V Titanium alloy throughout the die-forging operation. Constitutive equations are developed to assess the influence of the process (die-forging temperature, cooling rate) and the microstructure parameters on the mechanical response of the alloy. For this purpose, a non-unified behavior model formulation is implemented, which defines two main mechanisms related to alpha and beta phases and allows the prediction of hardening, strain rate sensitivity and temperature, combined with the phase evolution that is dependent on the cooling conditions and which can greatly affect the mechanical behavior. This identification strategy is then applied for die-forging temperatures below the beta-transus temperature, which requires microstructural information provided by SEM (Scanning Electron Microscopy) observations and image analysis. Finally, the approach is extended to die-forging temperatures above the beta-transus temperature.

Tensile behaviour of the alpha + beta Ti-6Al-4V titanium alloy is investigated while quenching. A new in-situ thermal and mechanical treatments facility is developed. Different time-temperature and timemechanical strain histories are coupled to investigate the uniaxial tensile properties and strength of this alloy during quenching. The microstructural evolutions are reported and their influences on the mechanical behaviour are addressed. (C) 2017 Elsevier B.V. All rights reserved.