Modelling of the bead formation during multi pass hybrid laser/gas metal arc welding

International audienceA three dimensional finite element model has been developed to simulate weld bead formation in multi pass hybrid laser/gas metal arc welding. The model considers both a gas metal arc welding (GMAW) electrode and a laser beam moving along a workpiece. A Eulerian approach is used in which the interface between the metal and the surrounding gas or plasma is defined by a level set function. Therefore heat transfer boundary conditions are applied through a "Continuum Surface Force" model. An original method has been settled to model material supply. A volume expansion source term is added to the right hand side of the mass conservation equation for certain liquid finite elements in the fusion zone. A compressible Navier Stokes equation is then solved. The new obtained velocity field is used to solve the transport of the level set function for the updating of the gas/metal interface. A new topological reinitialization method has been developed to hold the Eikonal property in the compressible framework of the level set transport. The efficiency of this global model enables to simulate industrial multi pass hybrid welding processes

A multi-physics level set approach for the simulation of the hybrid Laser/GMAW process

International audienceA new hybrid welding technique has recently been developed in order to answer to the industry needs to gather high thickness steel sheets. The combination of a laser beam and a gas metal arc enables to develop welds with a high added metal rate and low porosity. Moreover lower deformation and residual stresses are observed after cooling. Nevertheless the mastery of this technique is still in development due to the various physical phenomena which occur and interact during the welding process. A three dimensional finite element model has been developed to simulate this welding process. Metal and air gas domains are both meshed. A Eulerian-Lagrangian approach is used in which the interface between the metal and the surrounding air or plasma is defined by a level set function. Fluid flow phenomena and temperature evolution in the weld pool are simulated. Two moving heat sources are considered at the surface of the metal. As the arc plasma and laser beam are not modeled in the level set framework, the 'Continuum Surface Force' approach is used: a volumetric heat source distribution is applied to the immediate neighbourhood of the interface. The added metal represents an additional heat source. The Navier-Stokes equations are solved in the weld pool regarding the surface Marangoni force, the volumetric buoyancy. After solving the momentum conservation equation, metal / air interface is tracked through the resolution of a convection equation with the calculated velocity field as input. As the hybrid welding technique is usually a multi-pass process applied on high thickness piece with high chamfer, it is of primary importance to develop a correct modeling of solid / solid contact interfaces. It is shown that the present level set approach coupled with an adaptative remeshing tool enables to follow these interfaces to simulate the entire process steps. Weld geometry on a specific steel alloy is shown and compared to the expected result

A level set approach for the simulation of the multipass hybrid laser/GMA welding process

International audienceA new 3D finite element model, developed in a level set approach, is proposed to model hybrid gas metal arc/laser welding in multipass conditions. This heat and mass transfer model couples the resolution of the heat conservation equation, the momentum and mass conservation equations and the weld bead development. The arc welding total power is divided in two parts: one corresponding to the energy transferred to the fusion zone by droplets of melted filler material, the rest being transferred to the workpiece by the surrounding plasma. The droplets energy input is modelled as a volumetric heat source. Regarding the heat surface fluxes associated with plasma and laser heating, the "Continuum Surface Force" approach is used to model them as volumetric heat sources concentrated in the immediate neighbourhood of the metal-gas interface. A resolution scheme, consisting in ignoring high velocity fluid flow in the fusion zone, through the use of an augmented liquid viscosity, is proposed and discussed. Accordingly, the liquid thermal conductivity is enhanced to result in a realistic heat transfer to the workpiece. The formation of the weld bead is obtained through the introduction of a source term in the mass conservation equation, and the application of the normal component of surface tension forces, proportional to the mean curvature of the metal-gas interface. This approach is proposed to reduce computation time. The resolution scheme is applied to the simulation of hybrid welding of 18MND5 (ASME SA 533) steel grade. Results are compared to experimental measurements and observations operated in conditions close to industrial ones. The influence of the enhancement applied to the liquid conductivity coefficient is shown and discussed. A strong sensitivity evolution is demonstrated when varying it from the physical value to the value proposed in the welding literature. As proposed, the simplified resolution scheme leads to a good estimation of the weld bead surface development. Nevertheless, there are still noticeable differences with the whole set of experimental results that are discussed and can be explained by model limitations and insufficient knowledge of material and interfacial properties

Numerical modelling of hybrid arc/laser welding: a coupled approach to weld bead formation and residual stresses

International audienceThe joining of high thickness steel sheets by means of hybrid Laser/GMAW welding processes is studied in this paper. A three dimensional finite element model has been developed to simulate this process. Through an ALE framework, a level set approach is used to model the interface between the metal and the surrounding gas. Even though the physics of the plasma is not modelled, both thermal and material supply phenomena are taken into account: (i) The laser and GMAW heat sources are simulated and applied on the interface through the Continuum Surface Force method, (ii) An original method of volume element expansion has been chosen to simulate the material supply and the bead formation. A thermo mechanical problem resolution has been settled in this model. Depending on the thermal evolution of the mechanical parameters and on the velocity field, the material behaviour will be elastic, elasto-visco-plastic or visco-plastic. The ALE approach enables to compute the stresses inside the workpiece and to obtain the displacements of the workpiece borders. Two finite elements models are presented to illustrate: (i) A hybrid arc/laser welding simulation through the thermal and material supply resolution, (ii) A TIG welding simulation through the stress and strain mechanical resolution

Correlation between thermal properties and aluminum fractions in CrAlN layers deposited by PVD technique

The CrAlN coatings are a good alternative to conventional CrN coatings especially for high temperature oxidation-resistance applications. Different CrAlN coatings were deposited on silicon (100) by PVD (Physical vapor deposition) technique from two targets (chromium and aluminum) in a reactive nitrogen atmosphere at aluminum applied negative voltage ( 300, 500, 700 and 900 V). The composition, structural, mechanical and thermal properties of the as-deposited coatings were systematically characterized by energy dispersive analysis of X-rays, X-ray diffraction, nanoindentation, and the ‘‘Mirage effect’’ experiments. The X-ray diffraction (XRD) data show that in general CrAlN coatings were crystallized in the cubic NaCl B1 structure, with the (1 1 1) and (2 0 0) diffraction peaks observed. Two-dimensional surface morphologies of CrAlN coatings were investigated by atomic force microscope (AFM). The results show that with increasing aluminum proportion the coatings became more compact and denser and their increased correspondingly, showing a maximum hardness of about 36 GPa (30 at% of Al) which is higher than that of CrN. Moreover, the results in this work demonstrate that the variation of aluminum fraction alter the resulting columnar grain morphology and porosity of the coatings. However, the thermal properties are greatly affected by these morphological alterations. The correlation between aluminum fraction in CrAlN coatings and its thermal properties revealed that the conductivity and the diffusivity are influenced primarily by size and shape distribution of the pores and secondarily by a decrease of the stitch parameter dimension

Interactions between columnar solidification and segregation: a comparison of the predictions of the CAFE model with in-situ observations

International audienceA two-dimensional (2D) cellular automaton (CA) - finite element (FE) model has been proposed to simulate the solidification of binary alloys. The time evolution of maps of temperature, composition and fraction of solid are simulated while accounting for the undercooling of the growing structure. The solidification experiment of a Gallium-5wt%Indium alloy developed by Yin and Koster [1-2] is shown to provide a good benchmark for comparison with predictions of the model. Observed and simulated time evolutions of the development of the grain structure and the macrosegregation can be directly compared, together with mesosegregation such as segregated channels. These channels differ from freckle-type segregation described in the literature. They form as a result of the dynamics of the columnar growth front and its interaction with the fluid and solute flows. Comparisons of the predictions of the CAFE model with a purely macroscopic FE model also reveal the influence of accounting for the growth undercooling in numerical modeling of solidification

On the detection of corrosion pit interactions using two-dimensional spectral analysis

A particular thanks to Professor Maxence Bigerelle (UTC, Compiègne, France) and to research and development engineer Dr. Benjamin Fournier (CEA, Saclay, France) for their interesting discussions and useful advices about the scientific problem of pitting corrosion.A statistical methodology for detecting pits interactions based on a two-dimensional spectral analysis is presented. This method can be used as a tool for the exploratory analysis of spatial point patterns and can be advanced as an alternative of classical methods based on distance. One of the major advantages of the spectral analysis approach over the use of classical methods is its ability to reveal more details about the spatial structure like the scale for which pits corrosion can be considered as independent. Furthermore, directional components of pattern can be investigated. The method is validated in a first time using numerical simulations on random, regular and aggregated structures. The density of pits, used in the numerical simulations, corresponds to that assessed from a corroded aluminium sheet. In a second time, this method is applied to verify the independence of the corrosion pits observed on the aforementioned aluminium sheet before applying the Gumbel theory to determine the maximum pit depth. Indeed, the property of independence is a prerequisite of the Gumbel theory which is one of the most frequently used in the field of safety and reliability.International audienceA statistical methodology for detecting pits interactions based on a two-dimensional spectral analysis is presented. This method can be used as a tool for the exploratory analysis of spatial point patterns and can be advanced as an alternative of classical methods based on distance. One of the major advantages of the spectral analysis approach over the use of classical methods is its ability to reveal more details about the spatial structure like the scale for which pits corrosion can be considered as independent. Furthermore, directional components of pattern can be investigated. The method is validated in a first time using numerical simulations on random, regular and aggregated structures. The density of pits, used in the numerical simulations, corresponds to that assessed from a corroded aluminium sheet. In a second time, this method is applied to verify the independence of the corrosion pits observed on the aforementioned aluminium sheet before applying the Gumbel theory to determine the maximum pit depth. Indeed, the property of independence is a prerequisite of the Gumbel theory which is one of the most frequently used in the field of safety and reliability

Comments on the paper “Modification of composite hardness models to incorporate indentation size effects in thin films”, D. Beegan, S. Chowdhury and M.T. Laugier, Thin Solid Films 516 (2008), 3813–3817

Since the original work of Bückle concerning the substrate influence on the hardness measurement of thin film, more than 20 models were proposed to separate the contribution of the substrate. Subsequently to the development of these numerous models, a question arises: Which is the most relevant models among them? Indeed, the authors usually consider that their proposed model leads to the best prediction of the film hardness which is probably correct for a given experimental condition applied to a particular material. In addition, the authors also assume that the other models are not so relevant. But to have a sound discussion about the existing models, it is necessary to correctly apply them according to the author statement. In this paper, we better specified the application of the Jönsson and Hogmark model and that of Chicot and Lesage applied to the results obtained on copper films by Beegan et al. Contrarily to these authors, we show that the above-mentioned models lead to a good representation of the experimental data and a good predicted value of the film hardness

Correlation between thermal properties and aluminum fractions in CrAlN layers deposited by PVD technique

The CrAlN coatings are a good alternative to conventional CrN coatings especially for high temperature oxidation-resistance applications. Different CrAlN coatings were deposited on silicon (100) by PVD (Physical vapor deposition) technique from two targets (chromium and aluminum) in a reactive nitrogen atmosphere at aluminum applied negative voltage ( 300, 500, 700 and 900 V). The composition, structural, mechanical and thermal properties of the as-deposited coatings were systematically characterized by energy dispersive analysis of X-rays, X-ray diffraction, nanoindentation, and the ‘‘Mirage effect’’ experiments. The X-ray diffraction (XRD) data show that in general CrAlN coatings were crystallized in the cubic NaCl B1 structure, with the (1 1 1) and (2 0 0) diffraction peaks observed. Two-dimensional surface morphologies of CrAlN coatings were investigated by atomic force microscope (AFM). The results show that with increasing aluminum proportion the coatings became more compact and denser and their increased correspondingly, showing a maximum hardness of about 36 GPa (30 at% of Al) which is higher than that of CrN. Moreover, the results in this work demonstrate that the variation of aluminum fraction alter the resulting columnar grain morphology and porosity of the coatings. However, the thermal properties are greatly affected by these morphological alterations. The correlation between aluminum fraction in CrAlN coatings and its thermal properties revealed that the conductivity and the diffusivity are influenced primarily by size and shape distribution of the pores and secondarily by a decrease of the stitch parameter dimension

A generic statistical methodology to predict the maximum pit depth of a localized corrosion process

The authors would like to thank V. Hague for her help in English.This paper outlines a new methodology to predict accurately the maximum pit depth related to a localized corrosion process. It combines two statistical methods: the Generalized Lambda Distribution (GLD), to determine a model of distribution fitting with the experimental frequency distribution of depths, and the Computer Based Bootstrap Method (CBBM), to generate simulated distributions equivalent to the experimental one. In comparison with conventionally established statistical methods that are restricted to the use of inferred distributions constrained by specific mathematical assumptions, the major advantage of the methodology presented in this paper is that both the GLD and the CBBM enable a statistical treatment of the experimental data without making any preconceived choice neither on the unknown theoretical parent underlying distribution of pit depth which characterizes the global corrosion phenomenon nor on the unknown associated theoretical extreme value distribution which characterizes the deepest pits. Considering an experimental distribution of depths of pits produced on an aluminium sample, estimations of maximum pit depth using a GLD model are compared to similar estimations based on usual Gumbel and Generalized Extreme Value (GEV) methods proposed in the corrosion engineering literature. The GLD approach is shown having smaller bias and dispersion in the estimation of the maximum pit depth than the Gumbel approach both for its realization and mean. This leads to comparing the GLD approach to the GEV one. The former is shown to be relevant and its advantages are discussed compared to previous methods