Muriel Carin

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

It 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 Maxwells 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.

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

A 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 Maxwells 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.

2D longitudinal modeling of heat transfer and fluid flow during multilayered direct laser metal deposition process

Derived from laser cladding, the direct laser metal deposition (DLMD) process is based upon a laser beam–powder–melt pool interaction and enables the manufacturing of complex 3D shapes much faster than conventional processes. However, the surface finish remains critical, and DLMD parts usually necessitate postmachining steps. Within this context, the focus of our work is to improve the understanding of the phenomena responsible for deleterious surface finish by using numerical simulation. Mass, momentum, and energy conservation equations are solved using comsol multiphysics® in a 2D transient model including filler material with surface tension and thermocapillary effects at the free surface. The dynamic shape of the molten zone is explicitly described by a moving mesh based on an arbitrary Lagrangian–Eulerian method (ALE). This model is used to analyze the influence of the process parameters, such as laser power, scanning speed, and powder feed rate on the melt pool behavior. The simulations of a single layer and multilayer claddings are presented. The numerical results are compared with experimental data, in terms of layer height, melt pool length, and depth of penetration, obtained from high speed camera. The experiments are carried out on a widely used aeronautical alloy (Ti–6Al–4 V) using a Nd:YAG laser. The results show that the dilution ratio increases with increasing the laser power and the scanning velocity or with decreasing the powder feed rate. The final surface finish is then improved.

The Osmotic Migration of Cells in a Solute Gradient

The effect of a nonuniform solute concentration on the osmotic transport of water through the boundaries of a simple model cell is investigated. A system of two ordinary differential equations is derived for the motion of a single cell in the limit of a fast solute diffusion, and an analytic solution is obtained for one special case. A two-dimensional finite element model has been developed to simulate the more general case (finite diffusion rates, solute gradient induced by a solidification front). It is shown that the cell moves to regions of lower solute concentration due to the uneven flux of water through the cell boundaries. This mechanism has apparently not been discussed previously. The magnitude of this effect is small for red blood cells, the case in which all of the relevant parameters are known. We show, however, that it increases with cell size and membrane permeability, so this effect could be important for larger cells. The finite element model presented should also have other applications in the study of the response of cells to an osmotic stress and for the interaction of cells and solidification fronts. Such investigations are of major relevance for the optimization of cryopreservation processes.

A model comparison to predict heat transfer during spot GTA welding

Abstract The present work deals with the estimation of the time evolution of the weld fusion boundary. This moving boundary is the result of a spot GTA welding process on a 316L stainless steel disk. The estimation is based on the iterative regularization method. Indeed, the three problems: direct, in variation and adjoint, classically associated with this method, are solved by the finite element method in a two-dimensional axisymmetric domain. The originality of this work is to treat an experimental estimation of a front motion using a model with a geometry including only the solid phase. In this model, the evolution of this solid domain during the fusion is set with the ALE moving mesh method (Arbitrary Lagrangian Eulerian). The numerical developments are realized with the commercial code Comsol Multiphysics ® coupled with the software Matlab ®. The estimation method has been validated in a previous work using theoretical data ( [1] ). The experimental data, used here for this identification are, temperatures measured by thermocouples in the solid phase, the temporal evolution of the melt pool boundary observed at the surface by a fast camera and the maximal dimensions of the melted zone measured on macrographs. These experimental data are also compared with numerical results obtained from a heat and fluid flow model taking into account surface tension effects, Lorentz forces and the deformation of the melt pool surface under arc pressure.

Estimation of a source term in a two-dimensional heat transfer problem: application to an electron beam welding

This article is concerned with the estimation of a heat source applied in the electron beam welding process by using the micrographic information (hardness, optical micrograph, etc.) and temperature measurements in solid phase. The aim is to identify the energy distribution that is applied in the liquid and vapor zones. This identification is realized at each time in a transversal plane perpendicular to the welding axis. For this work, the goal is to analyze the feasibility of the estimation. So we do not use noise with the theoretical measurements. At last, the iterative regularization method will be used for this two-dimensional metallurgical inverse heat transfer problem.

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, high-speed 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.

Surface Finish Issues after Direct Metal Deposition

Derived from laser cladding, the Direct Metal Deposition (DMD) laser process, is based upon a laser beam – projected powder interaction, and allows manufacturing complex 3D shapes much faster than conventional processes. However, the surface finish remains critical, and DMD parts usually necessitate post-machining steps. In this context, the focus of our work was: (1) to understand the physical mechanisms responsible for deleterious surface finishes, (2) to propose different experimental solutions for improving surface finish. Our experimental approach is based upon: (1) adequate modifications of the DMD conditions (gas shielding, laser conditions, coaxial or off-axis nozzles), (2) a characterization of laser-powder-melt-pool interactions using fast camera analysis, (3) a precise check of surface aspects using 3D profilometry, SEM, (4) preliminary thermo-convective simulations to understand melt-pool hydrodynamics. Most of the experimental tests were carried out on a Ti6Al4V titanium alloy, widely investigated already. Results confirm that surface degradation depends on two aspects: the sticking of non-melted or partially melted particles on the free surfaces, and the formation of menisci with more or less pronounced curvature radii. Among other aspects, a reduction of layer thickness and an increase of melt-pool volumes to favor re-melting processes are shown to have a beneficial effect on roughness parameters.

Numerical Simulation of Electron Beam Welding and Instrumental Technique

In the present work, thermal cycles measured with thermocouples embedded in specimens are employed to validate a numerical thermometallurgical model of an Electron Beam welding process. The implemented instrumentation techniques aim at reducing the perturbations induced by the sensors in place. The numerical model is based on the definition of a heat source term linked to the keyhole geometry predicted by a model of pressure balance using the FEMLAB code. The heat source term is used by the thermometallurigal simulation carried out with the finite element code SYSWELD. Kinetics parameters are extracted from dilatometric experiments achieved in welding austenitization conditions at constant cooling rates.

Simulations of joule effect heating in a bulge test

This work focuses on the integration of an electrical conduction heating of circular blank in a bulge test device. This device will be used to characterize the thermomechanical behaviour of Usibor®1500 under biaxial deformation at very high temperature (to 930°C). First a thermoelectric model using COMSOL Multiphysics® was developed to study the heating of a rectangular blank. This model is validated by comparing the calculated temperatures with thermocouples measurements. Secondly electrical field optimization is approached to obtain a fast and uniform heating of a circular blank.