is an open access repository that collects the work of Arts et Métiers ParisTech researchers and makes it freely available over the web where possible. During forging and machining manufacturing processes, the material is subject to large strains at high strain rates which provoke local heating and microstructural changes. Modelling of these phenomena requires precise knowledge of the stress-strain constitutive equations for a large range of strains, strain rates and temperatures. An experimental study of the rheology of both hyper-and hypo-eutectoid steels (with different microstructures) over a temperature range from 20 1C to 1000 1C and with strain rates from 10 À2 to 10 5 s À1 has been undertaken. These tests were performed in compression on cylindrical specimens and in shear using hat-shaped specimens. Both a GLEEBLE 3500 thermomechanical testing machine and a Split-Hopkinson Pressure Bar apparatus were used. From these tests, three deformation domains have been identified as a function of the material behaviour and of the changes in the deformed microstructure. Each domain was characterized by its behaviour, including the competition between hardening and softening, strain rate sensitivity on the flow stress and the softening phenomenon (i.e. recrystallisation or recovery, etc.). Finally, based on thermodynamical considerations, the conditions of thermoplastic instability (i.e. shear bands, twinning, heterogeneities, etc.) and microstructural changes are highlighted using process maps of the dissipated power repartition.

is an open access repository that collects the work of Arts et Métiers ParisTech researchers and makes it freely available over the web where possible. a b s t r a c t A review of the different phenomenological thermo-viscoplastic constitutive models often applied to forging and machining processes is presented. Several of the most common models have been identified using a large experimental database (Hor et al., 2013). The latter consists of the tests were done in compression on cylindrical shaped specimens and in shear using hat-shaped specimens. The comparison between these different models is shown that the group of decoupled empirical constitutive models (e.g. the Johnson and Cook (1983) model), despite their simple identification procedures, are relatively limited, especially over a large range of strain rates and temperatures. Recent studies have led to the proposal of coupled empirical models. Three models in this class have also been studied. The Lurdos (2008) model shows the best accuracy but requires a large experimental database to identify its high number of parameters. After this comparison, a constitutive equation is proposed by modifying the TANH model (Calamaz et al., 2010). Coupling between the effects of strain rate and temperature is introduced. This model is easier to identify and does not require knowledge of the saturation stress. Compared to other models, it better reproduces the experimental results especially in the semi-hot and hot domains. In order to study real machining conditions, an orthogonal cutting tests is considered. The comparison between experimental test results and numerical simulations conducted using the previously identified constitutive models shows that the decoupled empirical models are not capable of reproducing the experimental observations. However, the coupled constitutive models, that take into account softening, improve the accuracy of these simulations.

is an open access repository that collects the work of Arts et Métiers ParisTech researchers and makes it freely available over the web where possible. Multiaxial high cycle fatigue modeling of materials is an issue that concerns many industrial domains (automotive, aerospace, nuclear, etc.) and in which many progress still remains to be achieved. Several approaches exist in the literature: invariants, energy, integral and critical plane approaches all of them having their advantages and drawbacks. These different formulations are usually based on mechanical quantities at the micro or mesoscales using localization schemes and strong assumptions to propose simple analytical forms. This study aims to revisit these formulations using a numerical approach based on crystal plasticity modeling coupled with explicit description of microstructure (morphology and texture) and proposes a statistical procedure for the analyses of numerical results in the HCF context. This work has three steps: First, 2.5D periodic digital microstructures based on a random grain sizes distribution are generated. Second, multiaxial cyclic loading conditions corresponding to the fatigue strength at 106 cycles are applied to these microstructures. Third, the mesoscopic Fatigue Indicator Parameters (FIPs), formulated from the different criteria existing in the literature, are identified using the finite element calculations of the mechanical fields. These mesoscopic FIP show the limits of the original criteria when it comes to applying them at the grain scale. A statistical method based on extreme value probability is used to redefine the thresholds of these criteria. These new thresholds contain the sensitivity of the HCF behavior to microstructure attributes. Finally, the biaxiality and phase shift effects are discussed at the grain scale and the loading paths of some critical grains are analyzed.

In the Selective Laser Melting (SLM) process, residual stresses are a major problem because they impact the dimensional accuracy and mechanical properties of the manufactured parts. A new methodology, based on distortion measurements using the bridge curvature method (BCM), is developed for the quantitative assessment of residual stresses. The bending of the surface of the released specimen is approximated by a quadratic polynomial and quantitative criteria are determined on both profiles and surface topographies measured by noncontact 3D optical microscopy. The accuracy of the method is evaluated by a statistical analysis using repeatability tests. Focus variation microscopy (FVM) measurements show better repeatability than extended field confocal microscopy. Compared to the 2D measurements generally reported in the literature, 3D characterization provides relevant information as the orientation of the main distortion, which may help to highlight the effect of SLM process parameters. In fact, the flatness parameters and curvature attributes measured on surface topographies are much more robust and repeatable than the distortion magnitude measured on isolated profiles. In particular, 3D analysis helps to show that the distortions are maximum perpendicular to the path of the laser.

Laser‐based powder bed fusion (LPBF) is attractive to build complex parts in spite of the low machinability of Inconel 625. The LPBF microstructures and tensile properties are well documented in literature. But fatigue properties and impact of the LPBF intrinsic defects and microstructures on the crack initiation and propagation mechanisms are uncommon. Low and high‐cycle fatigue tests at room and service temperature presented in this paper prove that the LPBF defects shorten the crack initiation stage. After annealing heat treatment, the microstructure adapts to the defects, and the fatigue response becomes similar to a free defect material. The fatigue life of as‐built Inconel 625 is lower than 50% of its monotonic yield tensile strength (YTS) while the annealed material reaches 100% of its YTS. The propagation rate is slowed by the homogeneous annealed microstructure and improved ductility resulting from removal of dislocation cells and changes in grain morphology.

Inconel 718 (IN718) is a precipitation hardened nickel-base super-alloy exhibiting high strength and good corrosion resistance at elevated temperatures and on the downside it is characterized by poor machinability. Abrasive Water Jet (AWJ) process, offers a potential method to machining difficult-to-cut materials such as IN718. The present work investigates the influence of Abrasive Water Jet parameters on surface roughness, topography, depth of cut and residual stress when milling IN718. Surface characterization was conducted through 3D optical microscopy and SEM techniques. Residual stresses were measured in longitudinal and transverse directions with respect to the machining path using X-ray diffraction (XRD) technique. The obtained results showed that milled surfaces have a homogeneous texture with embedded abrasive particles and high surface roughness. AWJ process introduced high compressive residual stresses with similar order of level in both directions ( and ). In addition, it was observed that jet pressure is the most influencing parameter on roughness and depth of cut, whilst traverse speed and step-over distance had a significant effect on the residual stress. Based on the experimental analysis, an empirical model to predict the depth of cut was proposed. The validation of the proposed model, has shown around 5% error in the predicted and actual pocket depth.

The parameters describing the elastoplastic behavior of the 316 L austenitic stainless steel are identified through inverse analysis based on finite element modeling of the Berkovich nanoindentation test. The true geometry of the Berkovich indenter is introduced in axisymmetric and 3D finite element models using experimental nanoindentation data obtained by adapting the calibration method proposed by Oliver and Pharr [1] . Then, using these true indenter shape models, the elastoplastic parameters of the 316 L are estimated with high accuracy compared to the parameters obtained from tensile test identification. The indentation curve was correctly described by the numerical model for all the analyzed indentation depths, even for indentations inferior to 100 nm, which is a challenge until today. The 3D indenter model produces a residual imprint very close to the experimental indentation mark. The friction analysis between the indenter and the sample surface reveals small changes in the surface deformation, introducing an increase on the hardness, which disappears as the indentation depth decreases.These studies demonstrate that the most important aspect in the elastoplastic parameter identification is the correct representation of the indenter geometry in the finite element model.

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