Caractérisation et modélisation des métaux fondus

The behaviour of molten metals involved in high temperature processes (welding, additive manufacturing, etc.) can be studied using two main approaches: experimental and numerical. The first axe of research that I am developing concerns the development of multiphysics models involving heat transfer, fluid flow and interface tracking in order to describe these processes in a finite element code. This activity is presented through examples of representative studies in the field of welding (laser and arc), metal additive manufacturing (arc) and cutting (laser). The second axe of research that I am developing concerns the characterization of molten metals at high temperature, thanks to a platform for the characterization of liquid metals which uses aerodynamic levitation. This device allows the measurement of many thermophysical properties such as surface tension or density at temperature levels that can reach 3000°C, without contact and in a controlled atmosphere. Recent results show how these two distant axes of research (numerical and experimental) complement and feed each other extremely well.Le comportement des métaux fondus impliqués dans les procédés hautes températures (soudage, fabrication additive…) peut être étudié selon deux principales approches : expérimentale et numérique. Le premier axe de recherche que je développe concerne la mise au point de modèles multiphysiques faisant intervenir la thermique, la mécanique des fluides et le suivi d’interface afin de décrire ces procédés dans un code éléments finis. Cette activité est présentée à travers des exemples d’études représentatives dans le domaine du soudage (laser et arc), de la fabrication additive métallique (arc) et la découpe (laser). Le second axe de recherche que je développe porte sur la caractérisation des métaux fondus à hautes températures, grâce à une plateforme de caractérisation des métaux liquides qui emploie, entre autres, la lévitation aérodynamique. Ce dispositif permet la mesure de nombreuses propriétés thermophysiques comme la tension de surface ou la densité à des niveaux de température pouvant atteindre les 3000 °C, sans contact et en atmosphère contrôlée. De récents résultats montrent enfin que ces deux axes de recherche apriori éloignés (numérique et expérimental) se complètent et s’alimentent extrêmement bien

Use of a µ-Scale Synthetic Gas Bench for Direct Comparison of Urea-SCR and NH3-SCR Reactions over an Oxide Based Powdered Catalyst

International audienceThe selective catalytic reduction (SCR) of NOx by NH3 has been extensively studied in the literature, mainly because of its high potential to remediate the pollution of diesel exhaust gases. The implementation of the NH3-SCR process into passenger cars requires the use of an ammonia precursor, provided by a urea aqueous solution in the conventional process. Although the thermal decomposition and hydrolysis mechanisms of urea are well documented in the literature, the influence of the direct use of urea on the NOx reduction over SCR catalysts may be problematic. With the aim to evaluate prototype powdered catalysts, a specific synthetic gas bench adjusted to powdered material was developed, allowing the use of NH3 or urea as reductant for direct comparison. The design of the experimental setup allows vaporization of liquid urea at 200 °C under 10 bar using an HPLC pump and a micro injector of 50 μm diameter. This work presents the experimental setup of the catalytic test and some remarkable catalytic results towards further development of new catalytic formulations specifically dedicated to urea-SCR. Indeed, a possible divergence in terms of DeNOx efficiency is evidenced depending on the nature of the reductant, NH3 or urea solution. Particularly, the evaluated catalyst may not allow an optimal NOx conversion because of a lack in ammonia availability when the urea residence time is shortened. This is attributed to insufficient activity of isocyanic acid (HNCO) hydrolysis, which can be improved by addition upstream of an active solid for the hydrolysis reaction such as ZrO2. Thus, this µ-scale synthetic gas bench adjusted to powdered materials enables the specific behaviour of urea use for NOx reduction to be demonstrated

Transient infrared thermography to characterize thermal properties of millimeter-sized low conductivity materials

International audienceThis work deals with the thermal characterization of a low conductivity material of millimeter-sized used mainly in building insulation, the hemp shiv. Experimentally, the samples are positioned in a stainless steel chamber, in which it is possible to control the environment. They are heated during few seconds on one side and an infrared camera measures the temperature field of the sample surface. The experimental thermograms are compared with a theoretical estimation model derived from the fin model in order to estimate simultaneously two parameters by ordinary least-squares regression: the thermal diffusivity and a modified Biot number

Urea vs NH3 in NOx selective catalytic reduction at laboratory scale

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A new approach to compute multi-reflections of laser beam in a keyhole for heat transfer and fluid flow modelling in laser welding

International audienceIt is widely accepted that laser reflections can play a critical role during keyhole laser welding. The energy concentration, the mask effects and the laser polarization can directly affect the molten pool dynamic. In this paper a new approach to compute laser reflections is proposed which consists of treating laser under its wave form by solving Maxwell's equations. The method has the advantage to be easily coupled with heat transfer and fluid flow equations and can be immediately transposable in any 2D, 2D axi or 3D configurations. The reliability and limits of this approach are discussed through different numerical examples. The complete model takes into account the three phases of the matter: the vaporized metal, the liquid phase and the solid base. To predict the evolution of these three phases, coupled equations of energy, continuity, momentum and Maxwell are solved. The liquid/vapour interface is tracked using the level-set method. All these physics are solved simultaneously with the commercial code COMSOL Multiphysics®. The calculated temperatures, velocities and free surface deformation are analysed. Examples of simulations leading to the formation of porosity are also presented. Finally, melt pool shapes evolution are compared to experimental macrographs

Design and development of an induction furnace to characterize molten metals at high temperatures

International audienceThis work presents the first step concerning the design of a very high temperature inductive furnace dedicated to the characterization of molten metals in the 1 600 K-2 800 K temperature range. One major constraint is to provide homogeneous temperature within the tested sample and thus avoid thermal gradients as well as magnetic fields. For that, the choice of materials for the furnace design has been made and validated through both a magneto-thermal simulation and some in situ temperature measurements of the sample with both thermocouples until 1 600 K, and with a visible-NIR pyrometer used for the upper temperatures. Experimentally, the pyrometer has been calibrated using the melting temperature measurement of a pure nickel sample placed in the furnace. Finally, the furnace heating capacity limit has been tested and reached during the niobium sample melting. During this test, technical difficulties occurred due to physico-chemical contamination from the crucible

A complete model of keyhole and melt pool dynamics to analyze instabilities and collapse during laser welding

International audienceA complete modeling of heat and fluid flow applied to laser welding regimes is proposed. This model has been developed using only a graphical user interface of a finite element commercial code and can be easily usable in industrial R&D environments. The model takes into account the three phases of the matter: the vaporized metal, the liquid phase, and the solid base. The liquid/vapor interface is tracked using the Level-Set method. To model the energy deposition, a new approach is proposed which consists of treating laser under its wave form by solving Maxwell's equations. All these physics are coupled and solved simultaneously in Comsol Multyphysics®. The simulations show keyhole oscillations and the formation of porosity. A comparison of melt pool shapes evolution calculated from the simulations and experimental macrographs shows good correlation. Finally, the results of a three-dimensional simulation of a laser welding process are presented. The well-known phenomenon of humping is clearly shown by the model

Guidelines in the experimental validation of a 3D heat and fluid flow model of keyhole laser welding

During the past few years, numerous sophisticated models have been proposed to predict in a self-consistent way the dynamics of the keyhole, together with the melt pool and vapor jet. However, these models are only partially compared to experimental data, so the reliability of these models is questionable. The present paper aims to propose a more complete experimental set-up in order to validate the most relevant results calculated by these models. A complete heat transfer and fluid flow three-dimensional (3D) model is first proposed in order to describe laser welding in keyhole regimes. The interface is tracked with a level set method and fluid flows are calculated in liquid and gas. The mechanisms of recoil pressure and keyhole creation are highlighted in a fusion line configuration chosen as a reference. Moreover, a complete validation of the model is proposed with guidelines on the variables to observe. Numerous comparisons with dedicated experiments (thermocouples, pyrometry, highspeed camera) are proposed to estimate the validity of the model. In addition to traditional geometric measurements, the main variables calculated, temperatures, and velocities in the melt pool are at the center of this work. The goal is to propose a reference validation for complex 3D models proposed over the last few years

A novel apparatus dedicated to the estimation of the thermal diffusivity of metals at high temperature

International audienceIn order to characterize thermophysical properties of high conductivity metallic materials, this work proposes a novel experimental apparatus supported by an analytical solution and a numerical simulation. The experimental set-up measures simultaneously the temperature evolutions on both the front and rear faces of a sample using a single detector following the high-speed thermography principle through a dedicated system of six mirrors. This innovative configuration allows to reach high temperatures in a controlled atmosphere without pollution by contact. A transient 2D-axisymmetric model is developed, considering heat transfer along the radius and thickness directions. In a first stage, the model is compared to a finite element model, then the estimation procedure is validated by a noised direct model. Afterwards, the experimental temperatures versus time are used to estimate the thermal diffusivity of pure iron and stainless steel 304 in the solid state at high temperature in a pollution-free environment. The estimated values for a temperature range between 1300 K and 1600 K are then compared with published data

Surface tension measurements of liquid pure iron and 304L stainless steel under different gas mixtures

International audienceIn this paper, the surface tension of molten pure iron and 304L stainless steel is measured under different gas mixtures (pure argon and Ar-2.5 %vol. H-2). Measurements are realized by aerodynamic levitation with an acoustic excitation. The surface tension is measured between 1756 K and 2227 K. The addition of H-2 in the surrounding gas is supposed to create a desorption of oxygen and modify surface tension. The effect of oxygen on surface tension is then analyzed.Measurements show a linear variation of surface tension with temperature for pure iron under Ar-2.5 % vol. H-2 and a nonlinear relationship for the 304L stainless steel even when an active gas is used. This phenomenon can be explained by the short time at high temperature which does not allow oxygen desorption. Increasing the time at high temperature may change the surface tension but at the risk of evaporation of some alloy elements. The intensification of the evaporation would also disturb the surface tension measurements. In this study, the time spent at the liquid state is equivalent to that of welding and additive manufacturing processes, allowing to determine the thermocapillary coefficient (Marangoni effect) for both metals considered in these conditions. The comparison between our measurements and different data are consistent