In gas tungsten arc welding (GTAW), weld pool surface contains crucial information for welding development. In this research, simulate skilled welders to control the welding process and determine the penetration stages based on the weld pool reaction. This study focuses on solving the uncertainty of the liquid weld pool in joint bases. The weld pool penetration process is highly depending on how the weld pool surface shape. To observe the weld pool, reflect the weld pool surface by the laser and image on the shield glass. The experiments show that the penetration can’t be determine by the reflecting grayness due to the variability of base metal. To control the joint bases diversity, fed a tip of the wire after the arc is established. Crate the new pattern of the weld pool penetration. Experiments verified the feasibility of this method.

Human welders have long been able to monitor a weld pool and adjust welding parameters accordingly. Automated welding robots can provide consistent movement during the welding process, but lack the ability to monitor the weld pool. A vision system attached to the welding robot could provide a way to monitor the weld pool substantially faster than a human being. Previous vision systems to monitor weld pool surfaces have been developed, but their uses are limited since the system is fixed in place. The compact vision system developed in this research attaches directly to the welding torch, which provides no limitations in weld pool monitoring. This system takes advantage of the specular surface of a molten weld pool by reflecting a pattern of laser beams from the weld pool surface. The deformation of the laser beam after it reflects from the weld pool surface can provide clues to the weld pool shape, and thus the penetration of the weld. Image processing techniques and geometric optics are used to reconstruct a weld pool surface shape based on the image captured of the deformed laser pattern.

An innovative auxiliary control system is developed to cooperate with an unskilled welder in a manual GTAW in order to obtain a consistent welding performance. In the proposed system, a novel mobile sensing system is developed to non-intrusively monitor a manual GTAW by measuring three-dimensional (3D) weld pool surface. Specifically, a miniature structured-light laser amounted on torch projects a dot matrix pattern on weld pool surface during the process; Reflected by the weld pool surface, the laser pattern is intercepted by and imaged on the helmet glass, and recorded by a compact camera on it. Deformed reflection pattern contains the geometry information of weld pool, thus is utilized to reconstruct its $3$D surface. An innovative image processing algorithm and a reconstruction scheme have been developed for (3D) reconstruction. The real-time spatial relations of the torch and the helmet is formulated during welding. Two miniature wireless inertial measurement units (WIMU) are mounted on the torch and the helmet, respectively, to detect their rotation rates and accelerations. A quaternion based unscented Kalman filter (UKF) has been designed to estimate the helmet/torch orientations based on the data from the WIMUs. The distance between the torch and the helmet is measured using an extra structure-light low power laser pattern. Furthermore, human welder's behavior in welding performance has been studied, e.g., a welder`s adjustments on welding current were modeled as response to characteristic parameters of the three-dimensional weld pool surface. This response model as a controller is implemented both automatic and manual gas tungsten arc welding process to maintain a consistent full penetration.

Fundação de Amparo a Pesquisa do Estado de Minas Gerais / The Thermal Pulse GMAW process seeks the combination of relative high productivity, good weld bead finish and low spatter generation of the Pulsed GMAW with the capability of controlling welding pool of the Pulsed TIG. Nevertheless, its industrial application is still very limited, except for aluminum welding. But this GMAW version may become differential for application if a distribution of the arc/droplet heat and pressure along the joint is applied. Thus, the objective of this study was to evaluate the potential weld pool control of the Thermal Pulsed GMAW for automated welding of root passes, without backing strips. Synchronization between the thermal pulses and torch trapezoidal weaving was initially devised. In this case, the thermal pulse was applied on the joint side, ensuring to melt this region, and the thermal base applied during the transition from one side to the other, in order to minimize the possibility of weld pool collapse, by reducing the heat and pressure of arc and droplets during this period. The maximum root gap was searched by evaluating this technique under different root gaps at several oscillation amplitudes for a given combination of thermal pulse and base parameters (V-groove butt joints of carbon steel welded in flat position, with and without root face). The results showed that this approach, in contrast to the welding without any synchronization, was able to control the weld pool. Root passes with complete penetration, adequate geometry and defect free were produced for gaps ranging from 2,5 to 6 mm. The oscillation amplitude must be larger than the root gap. However, the process does not seem robust enough and requires further investigation to allow keeping the same welding parameters regardless joint geometry variations, especially in the root face and gap. / Com o processo MIG/MAG com Pulsação Térmica busca-se a combinação de uma relativa alta produtividade, bom acabamento do cordão e baixa geração de respingos da soldagem MIG/MAG Pulsada com a capacidade de controle da poça de fusão da soldagem TIG Pulsada. Porém, sua aplicação é ainda muito limitada no meio industrial, com apelo apenas para soldagens de alumínio. Mas esta versão do MIG/MAG pode ter um diferencial de aplicação ao se usar a distribuição de calor e pressão do arco/gotas ao longo da junta. Desta forma, este trabalho teve como objetivo avaliar o potencial da soldagem MIG/MAG com pulsação térmica para controlar a poça de fusão em soldagens automatizadas de passes de raiz, sem utilização de cobrejunta. Para isso, foi inicialmente desenvolvida uma sincronização da pulsação térmica com o tecimento trapezoidal da tocha. Neste caso, o pulso térmico foi aplicado nas laterais da junta, garantindo a fusão desta região, e a base térmica aplicada durante o movimento de transição entre uma lateral e outra, com a finalidade de minimizar o risco de colapso da poça pela redução do calor e pressão do arco e gotas durante este período. Esta abordagem foi avaliada sob diferentes aberturas de raiz em distintas amplitudes de oscilação, para uma dada combinação de parâmetros de pulso e base térmicos (juntas de topo de aço carbono com chanfro em V , posição plana, com e sem altura do nariz). Buscou-se verificar a máxima abertura de raiz possível de ser soldada. Os resultados mostraram que foi possível controlar a poça de fusão com o uso dessa abordagem, em contraste com soldagens sem essa sincronização. Passes de raiz com penetração completa, geometria adequada e livre de defeitos foram obtidos em juntas de topo com abertura variando de 2,5 a 6 mm. Os resultados mostram que a amplitude de oscilação tem de ser maior do que a abertura da raiz. Porém, o processo ainda se mostra pouco robusto, demandando maiores estudos para permitir manter os mesmos parâmetros de soldagem frente a variações da geometria do chanfro, sobretudo a altura e o tamanho da folga na raiz. / Mestre em Engenharia Mecânica

Le procédé de soudage TIG est actuellement le plus utilisé dans l’industrie aéronautique du fait de la qualité des joints de soudure qu’il permet d’obtenir et de sa simplicité d’automatisation. Une opération de soudage provoque des gradients thermiques dus au passage de la source de chaleur sur la pièce qui induisent des déformations et des contraintes résiduelles pouvant impacter la durée de vie de l’assemblage. Ce travail vise à mettre en place un modèle de simulation de ce procédé dans le but d’optimiser les paramètres opératoires.Avant d’envisager un couplage thermomécanique, il convient de représenter convenablement les transferts thermiques au cours du soudage et en particulier l’apport de chaleur lié au procédé. Dans cette étude, on propose une source de chaleur prédictive simplifiée représentative des paramètres opératoires qui permet en particulier d’estimer les dimensions caractéristiques du cordon de soudure et de traduire fidèlement l’évolution thermique dans la pièce. Cette source est définie par un flux de chaleur homogène dépendant d’une puissance P, réparti sur un disque de rayon R, ces 2 paramètres numériques étant liés aux principaux paramètres opératoires de soudage que sont l’intensité I et la hauteur d’arc h.Une campagne d’essais expérimentaux dans laquelle on étudie les variations des dimensions de la Zone Fondue (ZF) pour des cas non pénétrants et pénétrants en fonction des paramètres opératoires (I, h) est présentée. Pour chaque essai, un couple de paramètres d’entrée de la source de chaleur (P, R) permettant de reproduire les dimensions du bain fondu est identifié. La confrontation des résultats obtenus numériquement et expérimentalement permet de mettre en place des relations entre les paramètres opératoires de soudage (I, h) et les paramètres numériques (P, R) conférant un caractère prédictif à la source de chaleur. Ce modèle de source a été validé pour différentes configurations de soudage en termes d’épaisseurs de tôles, de matériaux à assembler, de vitesses d’avance de la torche, ...Notre modèle thermique a ensuite servi de base pour la simulation thermomécanique du procédé. Le modèle est appliqué à l’assemblage de deux composants d’un turbomoteur en superalliage base Nickel. / Gas Tungsten Arc Welding (GTAW) is the most widely used welding process in aeronautics, due to its weld quality. During a welding operation, the thermal source induces thermal gradients causing strains and stresses that could affect assembly’s life duration. The aim of this study is to develop a numerical model of the welding process in order to get optimized process parameters.Before coupling thermal and mechanical phenomena, we must modelize heat transfers during welding. We propose a simplified heat source linked to the process parameters which enables to predict the main dimensions of the weld pool and the thermal evolution in the solid part. This source is defined by an homogeneous heat flux depending on a power P distributed in a R radius disk. These two parameters relate to process parameters, the arc height (h) and the current intensity I.Experiment tests was achieved to study the weld pool dimensions for both cases : incomplete penetration and full penetration weld. For each test, we identified the heat source parameters (P, R) which allow to obtain the experimental weld pool dimensions. The confrontation of numerical and experimental results enables to get links between the heat source parameters (P, R) and the welding parameters (I, h), producing a predictive heat source. The heat source reliability was verified taking into account several welding configurations with various superalloys sheet thickness, welding speed, materials.A coupled thermal-mechanical analysis, based on our thermal model, was applied to an industrial case: a nickel based superalloy components assembly of a gas turbine.

Welding is used extensively in aerospace, automotive, chemical, manufacturing, electronic and power-generation industries. Thermally-induced residual stresses due to welding can significantly impair the performance and reliability of welded structures. Numerical simulation of weld pool dynamics is important as experimental measurements of velocities and temperature profiles are difficult due to the small size of the weld pool and the presence of the arc. From a structural integrity perspective of welded structures, it is necessary to have an accurate spatial and temporal thermal distribution in the welded structure before stress analysis is performed. Existing research on weld pool dynamics simulation has ignored the effect of fluid flow in the weld pool on the temperature field of the welded joint. Previous research has established that the weld pool depth/width (D/W) ratio and Heat Affected Zone (HAZ) are significantly altered by the weld pool dynamics. Hence, for a more accurate estimation of the thermally-induced stresses it is desired to incorporate the weld pool dynamics into the analysis. Moreover, the effects of microstructure evolution in the HAZ on the mechanical behavior of the structure need to be included in the analysis for better mechanical response prediction. In this study, a three-dimensional model for the thermo-mechanical analysis of Gas Tungsten Arc (GTA) welding of thin stainless steel butt-joint plates has been developed. The model incorporates the effects of thermal energy redistribution through weld pool dynamics into the structural behavior calculations. Through material modeling the effects of microstructure change/phase transformation are indirectly included in the model. The developed weld pool dynamics model includes the effects of current, arc length, and electrode angle on the heat flux and current density distributions. All the major weld pool driving forces are included, namely surface tension gradient, plasma drag force, electromagnetic force, and buoyancy. The weld D/W predictions are validated with experimental results. They agree well. The effects of welding parameters (like welding speed, current, arc length, etc.) on the weld D/W ratio are documented. The workpiece deformation and stress distributions are also highlighted. The transverse and longitudinal residual stress distribution plots across the weld bead and their variations with welding speed and current are also provided. The mathematical framework developed here serves as a robust tool for better prediction of weld D/W ratio and thermally-induced stress evolution and distribution in a welded structure by coupling the different fields in a welding process. / Ph. D.

Fusion welding of dissimilar metals constitutes a crucial processing stage in a variety of applications, and the use of high energy beams (HEB) like lasers and electron beams for such welding applications has several advantages, such as, precision, narrow heat affected zone, and consequently, low distortion. An understanding of microstructural evolution in the weld is a prerequisite for producing sound joints with desired properties. HEB welding of similar metals have been studied extensively. In contrast, fewer studies have been directed toward understanding the fundamental aspects of solidification of dissimilar welds. This thesis presents an effort in that direction by exploring microstructural evolution in Ti/Ni dissimilar welds. Welding of Ti/Ni serves to illustrate the fundamental differences that distinguish dissimilar welding from the welding of similar metals. These are: (i) Thermophysical properties of the base metals are, in general, different, and this can have important consequences in the heat transfer conditions. (ii) Composition can vary over an wide range, the extreme being for the case of a pure binary couple, and the solid–liquid interface cannot be defined by a single liquidus isotherm. (iii) In addition to the surface energy driven Marangoni convection, a strong solutal convection can arise due to a large difference in the density of the base metals. (iv) Nucleation of phases assumes greater importance, especially in systems with intermediate phases. We have carried out laser and electron beam welding (LW and EBW) experiments in a butt welding geometry to join Ti/Ni dissimilar couples. Weld microstructures were characterised using scanning and transmission electron microscopy (SEM and TEM); composition information was obtained from energy dispersive spectroscopy (EDS) of Xrays in the SEM. In addition to the pure binary couple, we have also studied electron beam welding of Ti/Ni with a thin Ta interlayer. We summarise our findings in each set of experiments in the following sections. Laser welding of Ti/Ni We have studied partial penetration welds obtained within the range of experimental parameters used in our study. These welds show the following interesting features: 1. The welds are asymmetric with respect to the initial joint. Despite its higher melting point, Ti melts more than Ni due to its lower thermal diffusivity, making the average composition of the weld richer in Ti (Ti–40at.%Ni). 2. Composition changes very steeply near the fusion interfaces in both Ti and Ni with associated microstructural changes. The variation is of much lesser magnitude in the rest of the weld, reflecting a well mixed melt pool on a macroscopic scale. 3. Growth of base metal grains into the weld pool at the fusion interfaces is severely restricted at both Ti and Ni ends. 4. The Ti fusion interface is marked by a band consisting of Ti2Ni dendrites which grow toward the Ti base metal. 5. Layered structures form at the Ni fusion interface. The sequence of the layers is: solid solution (Ni)→ Ni3Ti→ Ni3Ti+NiTi eutectic → NiTi. We note the absence of the (Ni)+Ni3Ti eutectic in this sequence. 6. NiTi and Ti2Ni are the major phases that appear in the bulk of the weld. Volume fraction and morphology of NiTi vary almost periodically to form microstructural bands. 7. Solid state transformation of NiTi results in the formation of the Rphase and martensite, which reflect the composition heterogeneity in the weld. Sometimes, Ni4Ti3 precipitates are observed also, providing indirect evidence of nonequilibrium solidification. 8. Nitrogen pickup from the atmosphere during welding leads to the formation titanium nitride dendrites in the weld. 9. Solutal convection and buoyancy forces manifest themselves through the segregation of the lighter nitride and Ti2Ni phases toward the top surface of the weld; the heavier liquid forms blocky NiTi in the bottom half of the weld. These observations stand in striking contrast with the microstructures of conventional welds. We have proposed a set of composition and temperature profiles in the weld which reflect the diffusive and advective transport processes; when combined with thermodynamic information from the Ti–Ni phase diagram to yield spatial liquidus temperature profiles, these profiles can adequately explain most of the results. Our observations illustrate the importance of (a) nucleation, and (b) the inhomogeneous nature of the melt in which growth takes place. They also highlight the role of convective currents in bringing about local fluctuations in composition and temperature leading to ‘low velocity bands’. Electron beam welding of Ti/Ni We have carried out full penetration EBW of thin plates of Ti and Ni. The major observations are: (i) Average composition of the weld is in the Ni–rich side of the phase diagram (Ni–40at.%Ti). (ii) Fusion interface microstructures are very similar to that in LW exhibiting restricted base metal growth (although little amount of epitaxy can be seen in the Ni side), growth of Ti2Ni dendrites toward the base metal at the Ti fusion interface and the sequence of layers at the Ni interface: (Ni)→ Ni3Ti→ Ni3Ti+NiTi. Unlike LW, however, Ni3Ti, instead of NiTi, reappeared after the third layer on the Ni side. (iii) General microstructure consists of the Ni3Ti+NiTi eutectic, which appears in several anomalous as well as regular morphologies. (iv) Formation of NiTi is restricted mostly to regions near the Ti fusion interface. (v) Segregation of Ni3Ti was observed in a few places. The most prominent change in the microstructure compared to LW is a shift from the Ti2Ni– NiTi phases in the bulk of the weld to a Ni3Ti+NiTi eutectic structure. This is a direct consequence of the shift in the average composition of the weld to the Ni– rich side. The occurrence of different anomalous and regular eutectic structures bear similarity with bulk undercooling experiments conducted on eutectic systems having a strongly faceting phase as one of its constituents. The asymmetric coupled zone, along with composition and temperature fluctuation due to fluid flow, can be attributed to the origin of these structures. Electron beam welding of Ti/Ni with a Ta interlayer Motivated by the report of superior mechanical properties of Ti/Ni welds with an interlayer of Ta, whose melting point is much higher than those Ni and Ti, we performed EBW experiments using a Ni–Ta– Ti configuration. The key observations are: (i) The process is inherently unsteady in nature, and results in partial and irregular melting of the Ta interlayer. This partial melting essentially divides the weld into Ni–rich and Ti–rich halves. (ii) Microstructure near the fusion interface in Ni and Ti show similarities with that of the pure binary Ti/Ni welds; the phases here, however, contain Ta as a ternary addition. (iii) Microstructure in the Ti–rich half consists of dendrites of the Ni(Ti,Ta) phase with a high Ti:Ta ratio, and an eutectic formed between this phase and a (Ti,Ta)2Ni phase having significant amount of Ta. Two Ni(Ti,Ta) type phases dominate the microstructure in the Ni–rich half: the phase having a higher Ti:Ta ratio forms cells and dendrites, whereas the one of a lower Ti:Ta ratio creates an interdendritic network. (iv) Regions near the unmolten Ta layer in the middle show the formation of a sawtoothlike Ta–rich faceted phase of composition (Ta,Ti)3Ni2. Since very scarce thermodynamic data exist for the Ni–Ta–Ti ternary system, we have taken cues from the binary phase diagrams to understand the microstructural evolution. Such extrapolation, although successful to some extent, fails where phases which have no binary equivalents start to appear. In summary, in this thesis, we explore microstructural evolution in the Ti/Ni dissimilar welds under the different settings of laser and electron beam welding processes. This study reveals a variety of phenomena occurring during dissimilar welding which lead to the formation of an extensive range of microstructural features. Although a few questions do remain, most results can be rationalised by drawing from, and extending the knowledge gained from previous studies by introducing physical and thermodynamic arguments.

Le soudage à électrode réfractaire, plus souvent appelé TIG (Tungsten Inert Gas), est un procédé dans lequel un arc électrique est généré entre une électrode en tungstène et les pièces à souder sous un flux gazeux inerte. Le transfert d'énergie entre l'arc et l'anode donne naissance à un bain métallique et à la production de vapeurs. La qualité des soudures obtenues est directement liée au comportement du plasma et aux phénomènes physiques présents dans le bain métallique (effets Marangoni, force de traînée, gravité, forces de Laplace, conduction thermique...). L'objectif de ces travaux de thèse est de mettre en place des méthodes de diagnostic permettant une meilleure compréhension des mécanismes impliqués dans le transfert d'énergie lors de l'interaction arc-matériau sur une configuration de soudage TIG. Il s'agit également d'évaluer et d'analyser l'influence de certains paramètres opératoires impliqués dans le procédé (nature du gaz, intensité du courant, longueur de l'arc...) sur ces mécanismes. Plusieurs axes de recherche ont été dégagés : - l'étude de la colonne plasma pour estimer sa température selon les paramètres opératoires utilisés ; - l'étude de l'interface arc-liquide pour la compréhension des phénomènes de brassage et d'écoulement dans le liquide ainsi que l'influence des vapeurs métalliques issues de l'érosion du matériau sur le plasma ; - l'étude du matériau après son interaction avec l'arc pour faire le lien entre le brassage dans le bain durant le procédé et les caractéristiques de la zone fondue observée après solidification. L'équipe Arc Électrique et Procédés Plasmas Thermiques (AEPPT) du laboratoire LAPLACE s'intéresse à ces phénomènes d'interaction arc-matériau par le biais de la modélisation et de l'expérience. Un modèle 3D a été récemment développé au sein de notre équipe sur cette thématique. La caractérisation expérimentale du procédé devrait permettre également d'apporter des éléments de validation aux modèles déjà existants. / TIG (Tungsten Inert Gas) welding process is achieved by the creation of an electric arc between a tungsten cathode and a piece of metal. This process uses a shielding gas as argon, helium or mixtures at atmospheric pressure. The heat transfer between the arc and the work piece leads to a metallic weld pool and metallic vapours production. Weld quality is related to the plasma behavior and the molten zone motion (Marangoni force, Laplace force, drag force and gravity...).during the process. The aim of this work is to develop diagnostic methods leading to a better understanding of mechanisms involved in heat transfer in TIG welding. The aim is also to evaluate the influence of different experimental conditions (gaz nature, current intensity, arc length...) on theses mechanisms. Several lines of research were defined: - the study of the plasma temperature under different experimental conditions; - the study of the arc-liquid interface to understand the weld pool flow behavior and the impact of metallic vapor on the arc plasma ; - the study of the work piece after interaction to determine the dimension of the melted zone and link it to the flow behavior during the process. Arc-material interaction phenomena can be studied through modeling and experience. A 3D model has recently been designed in our team. The experimental characterization of the process developed in this work should provide a set of data to validate it.

Au cours de ce travail, un modèle physique et numérique 3D du procédé de soudage à l’arc TIG (Tungsten Inert Gas) a été développé dans l’objectif de prédire, en fonction des paramètres opératoires, les grandeurs utiles au concepteur d’assemblages soudés.Le modèle développé, à l’aide du code de calcul aux éléments finis Cast3M, traite les phénomènes physiques agissant dans la pièce et, plus particulièrement, dans le bain de soudage, l’arc étant traité comme une source. Pour ce faire, les équations non-linéaires de la thermohydraulique couplées à celles de l’électromagnétisme sont résolues en régime stationnaire avec un modèle prenant en compte la surface libre déformable du bain de soudage.Une première étape du développement a porté sur la modélisation des phénomènes électromagnétiques par deux méthodes numériques différentes, à comparer les résultats numériques obtenus avec ceux de la littérature. Ensuite, afin de valider le pouvoir prédictif du modèle, des simulations de différentes configurations de soudage d’intérêt ont été étudiées, en variant la composition chimique du matériau, la vitesse de défilement, la pression d’arc imposée et, plus particulièrement, la position de soudage. Des comparaisons avec des expériences et des modèles numériques de la littérature confirment les bonnes tendances obtenues. Enfin, une approche de la modélisation de l’apport de matière a été abordée et des résultats de cette approche ont été montrés. Notre modèle complet constitue donc une base solide pour le développement de modèles de simulation numérique du soudage (SNS) 3D totalement couplés avec l’arc dans le futur et sera intégré dans le logiciel métier WPROCESS. / In this work, we develop a 3D physical and numerical model of the GTA (gas tungsten arc) welding process in order to predict, for given welding parameters, useful quantities for the designer of welded assembly.The model is developed in the Cast3M finite element software and takes into account the main physical phenomena acting in the workpiece and particularly in the weld pool, subject to source terms modeling the arc part of the welding process. A steady solution of this model is thought for and involves the coupling of the nonlinear thermohydaulics and electromagnetic equations together with the displacement of the deformable free surface of the weld pool.A first step in the development consisted in modeling the electromagnetic phenomena with two different numerical methods, in comparing the numerical results obtained with those of the literature. Then, in order to assess the predictive capability of the model, simulations of various welding configurations are performed : variation in the chemical composition of the material, of the welding speed, of the prescribed arc pressure and of the welding positions, which is a focus of this work, are studied. A good agreement is obtained between the results of our model and other experimental and numerical results of the literature. Eventually, a model accounting for metal filling is proposed and its results are discussed. Thus, our complete model can be seen as a solid foundation towards future totally-coupled 3D welding models including the arc and it will be included in the WPROCESS software dedicated to the numerical simulation of welding.