Contribution in comprehension of hybrid laser welding : equivalent heat source approach and phenomenological approach

Les travaux présentés concernent l’étude du soudage hybride laser/MIG d’aciers de fortes épaisseurs pour deux configurations différentes : soudage monopasse d’acier inoxydable superduplex en chanfrein « Y » et soudage multipasse d’acier ferritique en chanfrein étroit. Une étude expérimentale a été réalisée visant la compréhension et l’investigation des mécanismes régis par le procédé, ainsi que la validation des résultats numériques des modèles développés (caractérisation du bain fondu et obtention de thermogrammes). Une recherche paramétrique a permis de déterminer les paramètres optimaux de soudage pour la configuration en chanfrein « Y ». L’étude numérique menée a pour objectif de déterminer l’histoire thermique des pièces soudées. Ses simulations numériques basées sur deux approches différentes ont été développées à l’aide du logiciel COMSOL Multiphysics. La première, dite approche par sources équivalentes, repose sur un calcul purement thermique, et la deuxième est basée sur un calcul thermohydraulique des écoulements dans le bain fondu avec prise en compte de la convection naturelle et de la thermocapillarité. La méthode des plans d’expériences numériques a été utilisée pour déterminer les paramètres analytiques des sources de chaleur équivalentes ainsi que des paramètres inconnus tels que les rendements des sources et les propriétés thermophysiques de la phase liquide intervenant dans le modèle multiphysique. L’analyse comparative entre résultats expérimentaux et numériques permettant de juger la qualité des modèles à reproduire les dimensions et la forme de la zone fondue et de valider le choix des hypothèses formulées, a fourni des résultats satisfaisants.The present study is dedicated to hybrid laser/MIG welding of high thickness steels for two different configurations: one hybrid single pass welding of superduplex steel UR2507Cu with Y-shaped chamfer geometry and a multipass hybrid welding of ferritic steel 18MND5 in narrow gap. An experimental study was carried for understanding and investigation of mechanisms governed by the process and validation of numerical models results (characterization of the weld pool and thermocouples measurements). A parametric research allows the determination of the optimal welding parameters for the Y-shaped chamfer configuration. The numerical study aims to determine the thermal history in the solid part of the work piece. Those numerical simulations which are based on two different approaches were developed using the COMSOL Multiphysics software. The first, called equivalent heat sources approach, consists only in the resolution of the heat transfer problem, and the second one, is based on the resolution of heat transfer and fluid flow problem in the weld pool by taking into account the natural convection and thermocapillarity effect. A numerical exploratory designs method is used to identify the equivalent heat sources parameters as well as some unknown parameters such as the thermophysical properties of the metal liquid and the thermal efficiency involved in the thermohydraulic models. The comparative analysis between experimental and numerical results, which permits judging the models quality to reproduce the melted zone size and shape and to validate the assumptions choice, has provided satisfying results

On the powers of quasihomogeneous Toeplitz operators

summary:We present sufficient conditions for the existence of $p$th powers of a quasihomogeneous Toeplitz operator $T_{{\rm e}^{{\rm i} s\theta }\psi }$, where $\psi$ is a radial polynomial function and $p$, $s$ are natural numbers. A large class of examples is provided to illustrate our results. To our best knowledge those examples are not covered by the current literature. The main tools in the proof of our results are the Mellin transform and some classical theorems of complex analysis

Influence of the first weld bead on strain and stress states in wire+arc additive manufacturing

International audienceWAAM (Wire+Arc Additive Manufacturing) allows manufacturing mechanical components by adding successive layers of molten metallic wire using electrical arc. The WAAM process, compared to other processes using metallic powders, presents some advantages such as: high deposition rate (2-4kg/hour), manufacturing of large scales components and cheaper industrial installations. WAAM is then an interesting candidate for manufacturing components often CNC machined. However, the main disadvantages of this process are: high surface roughness requiring a post machining, strains and stresses states generated during the deposition process [1]. A better understanding of the relation between the welding parameters and the state of stresses can contribute to minimize residual stresses, eventually in relation with a deposition strategy [2]. As a first approach, the effects of the first deposition of molten metal on the base plate is investigated. This work focuses on finite element method, based on Code Aster solver, with a nonlinear thermo-mechanical model. Concerning the thermal aspects, the GMAW heat input is modeled by a Gaussian distribution [3]. The temperature fields are used to solve the mechanical problem. The material behavior laws are assumed to be elastoplastic with different hardening configurations: no hardening, linear isotropic or kinematic hardening and non-linear isotropic hardening. Based on the results from these elastoplastic models, the influence of the hardening is presented

A WAAM benchmark: From process parameters to thermal effects on weld pool shape, microstructure and residual stresses

International audienceA two-dimensional experimental setup has been designed to investigate the influence of process parameters during WAAM operation. This experimental study has been also designed as a WAAM benchmark for numerical investigations such as thermal-mechanical ones. Indeed, several measurements have been done in-situ such as welding current and voltage, temperature, weld pool images and local displacement. Some measurements have been realized ex-situ: micro-structure and residual stresses. Residual stresses were investigated by neutron diffraction technique on two specimens. The experiments consisted in the deposition of 5 successive layers of 316L stainless steel filler-wire. 3 sets of process parameters were investigated based on variations of travel speed and linear energy from 147.1 to 189.7 J/mm. The welding thermal cycle was clearly visible on the time evolution of the displacement, also varying cyclically by a few tenths of a millimeter, as well as on the weld pool size. The weld pool size increased (up to 5 mm) with the number of layer increasing as the initial temperature of the specimen was higher due to the previous deposited layer. The micro-structure is textured with columnar dendrites of few millimeter long. The effect of studied process parameters on the residual stresses is slightly perceptible in the melted zone

The numerical challenges in multiphysical modeling of laser welding with arbitrary Lagrangian-Eulerian method

International audienceThe interaction of high power laser beam with metallic materials produces a number of interconnected phenomena that represent a serious challenge for numerical modeling, especially for creation of auto-consistent models. Additional difficulty consists in lack of data on materials properties at the temperatures superior to their melting point. The present work summarizes the numerical challenges in creation and validation of free-surface models using ALE moving mesh coupled with heat transfer equation and Navier-Stokes fluid flow

The Numerical Challenges in Multiphysical Modeling of Laser Welding with ALE

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Observations in-situ des mouvements du bain de fusion et des mécanismes de solidification au cours de ligne de fusion TIG

Ce document est un rapport d’étude qui présente les travaux effectués par l'équipe Assemblages Soudés (AS) du LMGC (Univ. Montpellier - CNRS, UMR 5508) dans le cadre du projet ANR Nemesis (Projet ANR-17-CE08-0036 du Programme AAPG 2017). Il s’agit d’une étude expérimentale réalisée principalement au cours du post-doctorat de Nicolas Blanc. Le but de cette étude est de pouvoir observer la solidification et le comportement du bain de soudage in-situ pour deux types d’alliages d’intérêt industriel : l’acier 316L et l’acier 22MnB5, intéressant de façon prioritaire respectivement EDF et Arcelor Mittal, partenaires du projet ANR. Les expériences doivent permettre d’obtenir des informations sur la taille et la forme du bain, sur les écoulements existants en son sein et sur le processus de solidification. Pour cela un dispositif similaire à celui utilisé par Alexis Chiocca [1] est utilisé. Le document est organisé autour de quatre chapitres. Le premier est dédié à un succinct état de l’art afin de rappeler le contexte scientifique général et l’utilité des observations réalisées dans la compréhension des phénomènes de solidifications pendant le soudage. Le deuxième chapitre est consacré à la présentation du dispositif expérimental spécifiquement conçu pour ce type de mesures et à la présentation des méthodologies d’étude. Les deux derniers chapitres sont consacrés à la présentation des résultats obtenus respectivement pour l’acier inoxydable 316L et les alliages 22MnB5

The numerical simulation of heat transfer during a hybrid laser–MIG welding using equivalent heat source approach

International audienceThe present study is dedicated to the numerical simulation of an industrial case of hybrid laser-MIG welding of high thickness duplex steel UR2507Cu with Y-shaped chamfer geometry. It consists in simulation of heat transfer phenomena using heat equivalent source approach and implementing in finite element software COMSOL Multiphysics. A numerical exploratory designs method is used to identify the heat sources parameters in order to obtain a minimal required difference between the numerical results and the experiment which are the shape of the welded zone and the temperature evolution in different locations. The obtained results were found in good correspondence with experiment, both for melted zone shape and thermal history

Microstructural and micro-electrochemical study of a tantalum-titanium weld interface

International audienceLaser welding offers an array of advantages compared to conventional fusion welding techniques, such as a higher welding speed and lower thermal distortion. However, information about the key aspects of the welding of dissimilar materials remains limited. Therefore, we chose to study the weld interface between tantalum and Ti-6Al-4V, which have good metallurgical compatibility but highly different thermophysical properties that can lead to pronounced chemical heterogeneities. An SEM analysis of the microstructure clearly revealed the formation of an unmixed zone in the melted area and a DRX analysis highlighted the presence of a beta(Ta,Ti) solid solution, tantalum islets and possibly alpha'-titanium where betagenic tantalum was absent. In terms of electrochemical behaviour, the titanium content was the main indicator of oxidation phenomena: a content greater than 50% resulted in a stable passive film, while lower Values led to an oxidation peak at approximately 1.7V vs Ag/AgCl. These results suggest that a reliable laser welding process.must promote a high titanium content in the melted area. This may be achieved through am optimized process with a heat source that shifts away from the tantalum in order to minimize tantalum levels in the melted area