La fissuration à chaud par liquation peut se produire dans la zone affectée thermiquement lors du soudage. Deux facteurs influent ce phénomène : les contraintes thermiques dues au gradient de température et la perte potentielle de ductilité due à la présence d'un film liquide aux joints de grains en fonction de leur composition chimique. Des essais de ductilité à chaud (Gleeble) ont été utilisés pour étudier l'effet combiné de la teneur en bore et du temps de maintien sur la chute de la ductilité dans la plage de température de liquation pour l’acier inoxydable austénitique de type 316L. Il est démontré que les teneurs en bore élevées et les temps de maintien courts favorisent la perte de ductilité dans cette plage de température. En complément la spectrométrie de masse à ionisation secondaire a été utilisée pour tenter de corréler les variations de ductilité à la distribution du bore aux joints de grains. D'autres essais de fissuration à chaud en soudage (Varestraint, PVR) ont été effectués pour confirmer l'influence de la teneur en bore sur la sensibilité à la fissuration de l’acier 316L. Les résultats indiquent que des fissures apparaissent sur toutes les éprouvettes et que le chargement mécanique externe minimal pour créer les fissures de liquation diminue avec la teneur en bore. Plus la teneur en bore est élevée, plus le matériau est donc sensible à la fissuration à chaud par liquation. Un critère de fissuration à chaud par liquation a été déterminé en se basant sur les résultats des essais de ductilité à chaud et la simulation des essais de soudage. / Liquation cracking may occur in the heat-affected zone during welding. Two factors influence this phenomenon: the tensile stresses generated during welding and the potential loss of ductility due to the presence of a liquid film at grain boundaries depending on their chemical composition. Gleeble hot ductility tests have been used to study the combined effect of boron content and holding time on ductility drop in the liquation temperature range of a 316L type austenitic stainless steel. It is shown that high boron contents and short holding times promote the loss of ductility in this temperature range. Secondary ion mass spectrometry has been used inattempt to correlate mechanical results to boron distribution either at grain boundaries or in the bulk. Other hot cracking tests (Varestraint, PVR) have been performed to confirm the influence of boron content on hot cracking sensitivity of AISI 316L stainless steels during welding. Results indicate that cracks appear on all specimens and that the minimum external mechanical loading for liquation cracking decreases with boron content. The higher the boron content is, the more the specimen exhibits tendency to hot cracking. A liquation hot cracking criterion has been determined, based on the results of the hot ductility tests and the simulation of welding tests.

Aluminum alloys are the subject of increasing interest (in the automotive industry, as well as aircraft industry), aiming to reduce the weight of components and also allowing a profit in term of energy saving. Concerning the assembly, riveting has been widely used in the aircraft industry, whereas welding seems to be promising in the car industry in the case of aluminum alloys. Nevertheless, welding can generate defects, such as porosity or hot cracking, which could limit its development. One of the major problems associated with the welding of aluminum alloys is the formation of gas porosity. Aluminum alloy cleanliness remaining one of the aluminum industry's primary concerns, this project focuses on the formation and distribution of porosity in Al-Si welds. A literature review has been performed, to identify the mechanisms of porosity formation in welds and castings. Porosity distribution in welds has been investigated, based on three different welding techniques: hybrid Laser/MIG welding process, the electron beam welding process, and the MIG dual wire welding process. Porosity distribution results provide information on to the porosity formation mechanisms involved during welding. A complete microstructure, microhardness and EDX analysis have been carried out, to describe and quantify the solidification process within the welds.

Advanced high strength steels (AHSSs) are considered to have a promising future due to the outstanding properties compared with the conventional steel and have been widely adopted as the base materials for the automotive components. Some of the challenges preventing the extensive applications of AHSSs are due the solidification defects, i.e. hot tearing and segregation. In this thesis, a 3D mesoscale and multi-physics model is developed and validated to directly investigate solidification defects for semi-solid steel with dendritic morphology associated with the peritectic transformation. Similar to the prior models [1,2], the current model explicitly considers the solidification behavior of each grain prior to assembling, which allows for the mesoscale simulation within a semisolid containing thousands of grains. Six sub-models are incorporated: (i) microstructure generation model is used to create the fully solidified microstructure of equiaxed grains based on a Voronoi tessellation; (ii) a dendritic solidification module based on an average volume approach is developed for predicting the solidification behavior of a random set of grains, considering the diffusion in different phases along with peritectic transformation. The progressive coalescence to form a solid cluster is predicted by incorporating an interfacial energy determination model; (iii) a fluid flow module is developed for the prediction of both intra-dendritic flow and extra-dendritic flow within the dendritic network induced by solidification shrinkage and deformation; (iv) a semisolid deformation model is used and extended to simulate the semi-solid mechanical behavior of steel using a discrete element method. The solid grains are modeled using a constitutive law and implemented via Abaqus commercial software; (v) a coupled cracking model incorporated with a failure criterion is used and extended to predict the crack formation and propagation in semi-solid steel. This comprehensive model consists of models (i-iv) and considers the interaction between the deformation within the solid phase and pressure drop in the liquid phase; (vi) a one-way coupled solute transportation module is also developed and used to simulate the solute redistribution due to fluid flow and diffusion within the liquid channels assuming the solid grains are fixed. The movement of the solute-enriched liquid in the solute transport model is induced by solidification shrinkage and deformation. The new 3D mesoscale model is then applied to correlate the semisolid behavior during solidification to different physical and process parameters. The results from the dendritic solidification model show the evolution in semi-solid microstructure and consequently liquid film migration. The model is able to predict the solidification of equiaxed grains with either globular and dendritic structure having experiencing primary solidification and the peritectic transformation. The coalescence phenomenon between grains is considered at the end of solidification using Bulatov’s approach[24] for estimating interfacial energy. It is seen that only 0.9% of the grains are attractive based on their orientations within a specific domain, significantly depressing final-stage solidification. The dendritic fluid flow model quantitatively captures both semi-solid morphology and the fluid flow behavior, and provides an alternative to the convectional experiment for the prediction of permeability by using the given surface area concentration. Comparison of the numerical and experimental permeabilities shows a good agreement (within ± 5%) for either extra-dendrite or intra-dendritic flow, and deviation from the conventional Carman-Kozeny equations using simplified Dendritic Sv or Globular Sv are explained in detail. The results quantitatively demonstrate the effect of grain size and microstructure morphology during solidification on the permeability prediction. The localization of liquid feeding under the pressure gradient is also reproduced. Additionally, the fluid flow due to shrinkage and deformation for non-peritectic and peritectic steel grades with dendritic morphology during solidification was captured for the first time. The cracking model allows for the prediction of hot tearing initiation and the progressive propagation during a tensile test deformation and the results are compared with the experimental results conducted by Seol et al.[3]at different solid fractions. Parametric studies of coalescence criteria and surface tension on the constitutive behavior of the semisolid are discussed and the deformation behavior of alloys with different carbon contents under a feedable mushy zone is investigated. Finally, the solute transport model has been applied to the continuous casting process of steel for the investigation of centreline segregation, and results indicate that the grain size has a great impact on the solute distribution and solute partitioning combined with intra-dendritic fluid flow leads eventually to liquid channels enriched with solute. The predicted composition in these discrete liquid channels shows a great match with the experimental measured profile obtained via the microscopic X-Ray fluorescence (MXRF). / Thesis / Doctor of Philosophy (PhD)

Increasing use of aluminum alloys in the automotive industry increases the importance of the production of sheet aluminum. To provide cost effective sheet aluminum to the industry, twin-roll casting (TRC) is becoming more important compared to DC casting. Demand for usage of different aluminum alloys in sheet form introduces some difficulties that should be considered during their applications. The main problem encountered during the welding of aluminum alloys is hot cracking. The aim of this study is to understand the difference in hot cracking susceptibility of two twin roll cast (TRC) aluminum-magnesium alloys (5754 and 5049 alloys) during welding. Varestraint test method was used to evaluate the effect of welding parameters, strain levels, filler alloys and mid-plane segregation on hot cracking susceptibilities. Hot cracking susceptibility of both 5049(Al-2wt%Mg) and 5754(Al-3wt%Mg) alloys increased with increasing strain level. Also, it was observed that hot cracking susceptibility was higher for the alloy having higher magnesium content. Thermal analysis results verified that hot cracking susceptibility indeed can be related to the v solidification range. As is suggested in the solidification range approach, the results of the present study confirm that the extent of solidification and liquation cracking depend on the magnitude of solidification range and the strain imposed during welding. Hot cracking susceptibility of 5754(Al-3wt%Mg) alloy has shown slightly decreasing behavior with addition of 5356 filler alloy. On the other hand, addition of 5183 filler alloy has increased solidification cracking susceptibility of two base alloys. The fracture surfaces of liquation and solidification cracks were investigated by scanning electron microscope with EDS. Liquation crack surfaces of the 5754(Al-3wt%Mg) alloy were found to have high Mg and Si content. For the 5754(Al-3wt%Mg) alloy, a quench test was designed to observe the effect of mid-plane segregation zone. It was observed that there was a eutectic reaction resulting in formation of liquid phase below solidus temperature of 5754(Al-3wt%Mg) alloy. Moreover, internal cracks have formed at the mid-plane segregation zone after Varestraint test. Results show that 5049(Al-2wt%Mg) alloy should be chosen compared to 5754(Al-3wt%Mg) alloy for welding. Moreover, low line energy should be applied and filler alloys with high magnesium content should be used during welding to decrease hot cracking tendency of welds.

7039 aluminum alloys are widely being used in the aerospace, automotive and defense industries in which welding technique is used for their joining. The main problem encountered during the welding of 7039 aluminum alloy is hot cracking. The aim of this study is to understand the effect of welding parameters on the hot cracking behavior of 7039 aluminum alloy by using Modified Varestraint Test (MVT) with Gas Tunsgten Arc Welding (GTAW) technique. During tests, welding speed was selected as varying parameter, welding current was kept constant and to understand the effect of filler materials 5183 and 5356 aluminum alloy filler materials were used. It has been observed that with the change in welding speed hot cracking susceptibility of 7039 aluminum alloy changes. The effect of filler materials is found to be favorable by decreasing the hot cracking susceptibility of 7039 aluminum alloy. Filler material additions also improved the hardness of the weld metal. Based on the cracking mechanism hot cracks were investigated as solidification cracks and liquation cracks. It has been experienced that liquation cracking susceptibility of the filler material added samples has been affected from the magnesium and manganese contents of the weld seams. Effect of solidification range on liquation cracking was also justified with differential thermal analyses. With the micro examinations the intergranular structure of hot cracking is revealed. In addition, the characterization and growth properties of the hot cracks under cyclic load were tried to be understood and the fractography of these cracks were taken.

No description available.

This thesis explores experimentally and theoretically two different aspects of the properties and behaviour of metals: their ability to damp noise and their susceptibility to crack when solidifying. The first part concerns intrinsic material damping, and is motivated by increased demands from society for reductions in noise emissions. It is a material’s inherent ability to reduce its vibration level, and hence noise emission, and transform its kinetic energy into a temperature increase. To design new materials with increased intrinsic material damping, we need to be able to measure it. In this thesis, different methods for measurement of the intrinsic damping have been considered: one using Fourier analysis has been experimentally evaluated, and another using a specimen in uniaxial tension to measure the phase-lag between stress and strain has been improved. Finally, after discarding these methods, a new method has been developed. The new method measures the damping properties during compression using differential calorimetry. A specimen is subjected to a cyclic uniaxial stress to give a prescribed energy input. The difference in temperature between a specimen under stress and a non-stressed reference sample is measured. The experiments are performed in an insulated vacuum container to reduce convective losses. The rate of temperature change, together with the energy input, is used as a measure of the intrinsic material damping in the specimen. The results show a difference in intrinsic material damping, and the way in which it is influenced by the internal structure is discussed. The second part of the thesis examines hot cracks in solidifying shells. Most metals have a brittle region starting in the two-phase temperature range during solidification and for some alloys this region extends as far as hundreds of degrees below the solidus temperature. To calculate the risk of hot cracking, one needs, besides knowledge of the solidifying material’s ability to withstand stress, knowledge of the casting process to be able to calculate the thermal history of the solidification, and from this calculate the stress. In this work, experimental methods to measure and evaluate the energy transfer from the solidifying melt have been developed. The evaluated data has been used as a boundary condition to numerically calculate the solidification process and the evolving stress in the solidifying shell. A solidification model has been implemented using a fixed-domain methodology in a commercial finite element code, Comsol Multiphysics. A new solidification model using an arbitrary Lagrange Eulerian (ALE) formulation has also been implemented to solve the solidification problem for pure metals. This new model explicitly tracks the movement of the liquid/solid interface and is much more effective than the first model. / QC 20100929

Hot cracking is a serious problem that encounters during welding of aluminium-magnesium alloys. In the present study, solidification and liquation type of hot cracks in weld metal and the heat-affected zones of 5086-H32 aluminium alloy were investigated by using Modified Varestraint Test (MVT) with TIG-AC and TIG-DC welding. With determining the size, type and number of cracks, a relation was established between welding line energy and strain on the hot crack formation. This information was used to determine the hot crack safe parameter ranges. The hot cracking tendency as a function of applied parameters were discussed in the frame of temperature fields around the moving heat source. Moreover, the characteristic hot crack locations on the 5086-H32 MVT specimens were generalized. The results of the study indicated that the increase in line energy and strain increased the hot cracking tendency of the specified aluminium alloy. In the low line energy range, the main hot cracking mechanism is the solidification cracking which could be overcome by the use of a suitable filler material. At high line energy range, due to the increased amount of interdendritic liquid, the amount of solidification cracking decreases by healing mechanism. However, because of the enlarged-temperature-field around the weld zone, fraction of HAZ cracking increases. The comparison between the hot cracking tendencies in low and high line energies indicates that the low line energy ranges with low augmented strains resulted in hot crack safer parameters.

Le matériau principal de cette étude est l’acier inoxydable austénitique 316L(N) (X2CrNiMo17-12-2 à teneur en azote contrôlée) envisagé dans la conception de la cuve et des structures du circuit primaire des futurs réacteurs de quatrième génération refroidis au sodium. Pour assembler des composants de forte épaisseur, il faut réaliser des soudages multipasses avec métal d'apport. Lors du soudage, il a parfois été constaté des défauts de fissuration à chaud de solidification au refroidissement dans la zone pâteuse, près du bain de fusion. Ces fissures sont des décohésions du matériau apparaissant à haut température le long des joints de grains lorsque la déformation dépasse un seuil critique. Il est donc nécessaire de prévenir ce risque en utilisant un critère de fissuration à chaud. L'approche utilisée dans cette étude est double : développer un essai de fissuration à chaud à chargement extérieur, puis simuler numériquement ces expériences pour déterminer un seuil critique en déformation en utilisant un critère proposé par Kerrouault. Une version améliorée d’un essai de fissuration à chaud (Controlled Restraint Weldability (CRW) test) a été proposé dans cette étude afin d'analyser la susceptibilité à la fissuration de solidification du matériau 316L(N) et d’un métal d'apport de nuance Thermanit 19-15H. L'objectif de ce test est, en fonction de l'intensité du chargement extérieur, d'amorcer une fissure dans un régime thermique établi, puis d’arrêter la propagation de cette fissure si les conditions thermomécaniques locales sont remplies. Le modèle de comportement du matériau choisi est une loi élasto-visco-plastique à écrouissage mixte. Des essais thermomécaniques sur un simulateur Gleeble ont été réalisés à haute température afin d'identifier et d’améliorer la loi de comportement du matériau 316L(N). Le grossissement des grains dans la zone affectée thermiquement a été modélisé et intégré dans ce modèle. Les intervalles de fusion et de solidification du matériau 316L(N) ont été déterminés par des essais ATD (Analyse Thermique Différentielle). Des analyses des microstructures de solidification ont été également menées afin de mieux comprendre le phénomène de fissuration à chaud. Certains essais CRW ont ensuite été modélisés et simulés par éléments finis en utilisant les logiciels Cast3M et Abaqus afin valider le critère de fissuration à chaud et de déterminer un seuil critique de fissuration pour l'acier 316L(N). / The austenitic stainless steel AISI 316L(N) (X2CrNiMo17-12-2) with controlled nitrogen content is widely used for manufacture of vessel and primary circuit structures of the 4th Generation sodium- cooled fast reactors. Multi-pass welds with an appropriate filler metal is used to assemble thick components. Solidification cracks may occur in the mushy zone near the melting weld poor during solidification when a liquid film is distributed along grains boundaries and interdendritic regions and the shrinkage strains across the boundaries cannot be accommodated. It is therefore necessary to prevent this defect using a hot cracking criterion. The approach used in this study is to initiate experimentally a hot crack by a weldability test, and then to simulate these tests to identify a critical strain using a hot crack criterion for the prediction of solidification cracking. Therefore, a hot cracking test (Controlled Restraint Weldability (CRW) test) is proposed in the present study to analyze the susceptibility to hot cracking for base metal 316L(N) and its filler metal 19-15H Thermanit grade. This test is designed to initiate a hot crack in thermal steady state, and then to stop the crack once the local thermomechanical conditions are met. The initiation and stop of the crack depend on external mechanical preload. The material constitutive equations chosen for the material is a visco-plastic model with isotropic and kinematic hardening. The Gleeble thermomechanical tests have been performed at high temperature in order to identify material parameters. The increase of the grain size in the thermally affected zone was modeled and integrated into constitutive equations. The temperature range of melting and solidification of 316L(N) were determine by using the Differential Thermal Analysis (DTA). The analysis of the solidification microstructures were also carried out in order to better understand the phenomenon of hot cracking. Some CRW tests were then simulated by finite element method using the Cast3M and Abaqus software in order to valid the hot cracking criterion and to determine a thermomechanical criterion of hot cracking for 316L(N).

Aujourd’hui, la fabrication additive de pièces métalliques par le procédé EBM (fusion sélective par faisceau d’électrons) concerne essentiellement les alliages de titane et les alliages cobalt-chrome. Une forte demande du secteur aéronautique pousse à étudier la possibilité d'étendre les champs d’application de ce nouveau procédé d'élaboration à d'autres matériaux à haute valeur ajoutée, notamment les superalliages base Nickel.Après la caractérisation des poudres et la description des particularités du procédé EBM (mise en œuvre, paramètres, thermique…), ce travail s'est attaché à développer une méthodologie permettant de structurer l’utilisation d’un nouveau matériau par EBM. Cette méthodologie a dans un premier temps été validée sur un superalliage base Nickel soudable: l'inconel 625.La mise en œuvre d’un superalliage non-soudable a révélé une problématique de fissuration à chaud. Une partie du travail de thèse a été consacrée à la compréhension de l'origine de la fissuration à partir de caractérisations microstructurales multi-échelles. L'étude de la genèse des microstructures et des défauts hérités de la fabrication a permis de proposer des règles de fabrication afin de limiter, et même d'éviter complètement la fissuration. Une adaptation des paramètres opératoires et des stratégies de fusion lors du procédé EBM est utilisée pour générer des microstructures présentant des structures de grains différentes allant de structures équiaxes jusqu'à la fabrication de monocristaux en passant par des structures colonnaires de différentes tailles.Le couplage entre un modèle de solidification prédisant la transition colonnaire-équiaxe et des simulations éléments finis permettant de quantifier les gradients thermiques et les vitesses de solidification a permis d’établir des liens entre les paramètres procédé et les microstructures résultantes. / Over the last decade, new processing routes based on additive manufacturing (AM) have emerged. Among the AM processes, Electron Beam Melting (EBM) was mainly dedicated to the fabrication of components made of titanium or chromium-cobalt alloys. Aeronautic industry has been a driving force to investigate the possibility to extend the EBM process to other materials and in particular to Ni-based superalloys.The first objective of this work was to develop a methodology to rationalize the use of a new material in the EBM machine. This can be achieved by studying the main characteristics of the EBM process: powder requirements, melting parameters and strategies, thermal aspects.... The methodology was first validated on a weldable Ni-based superalloy: the Inconel 625 grade.The methodology was then extended to the fabrication of a non-weldable Ni-based superalloy, i.e. a grade containing a large fraction of the γ' strengthening phase. Processing such non-weldable superalloys by EBM usually induced cracks in the fabricated components. The microstructures were characterized in order to identify the mechanism at the origin of the cracks. Understanding the mechanism responsible for the development of cracks has allowed to propose new melting strategies limiting or completely avoiding the formation of cracks.Adjusting melting parameters and strategies turns out to be an efficient way for tailoring the grain structure. Equiaxed grains, columnar grains with different sizes as well as single crystals can thus be generated with suitable process parameters.Finally, coupling a solidification model predicting the equiaxed/columnar transition and finite element calculations quantifying the magnitude of the thermal gradient and solidification velocity allowed to establish some links between microstructures and EBM melting parameters.