The enactement of the Energy Policy and conservation Act of 1975, led to a paradigm shift in material selection and design in the automotive industry. The net effect was an increased focus by the automotive industry toward the use of light metals leading for the reduction of weight and hence, the dependence of imported oil. Increasing use of aluminum was a transition in that direction. However, raw aluminum on an average is 1.5 - 2 times as expensive as steel. Near net shape manufacturing processes (Die casting, Thixo-forging, etc) provided the much needed competitive advantage vis-à-vis steel / iron parts by permitting the manufacturing of Al components. Semi solid processing involves the net shape manufacturing of alloys in a two phase region (liquid + solid). The reduced turbulence (during casting), less entrapped gases and lower operating temperatures (processes below the liquidus) make semi solid processing ideal for the manufacturing of high integrity Al parts. Traditionally, semi solid processing involved the heating of billets to a two phase region (called Thixcasting). Rheocasting is a new semi solid processing technique wherein the alloy is cooled from a liquid state (a combination of controlled heat / nucleation and growth phenomena) to yield structures similar to the Thixocasting process. Rheocasting or Slurry-On-Demand is in its early stages of development (the first industrial prototype of rheocasting was invented in the late 1990's) and forms the central point of interest in this work. Much research is underway around the globe to understand the controlling mechanism as well as the structure - property relationships in rheocast parts, primarily limited to the hypoeutectic Al-Si alloys (less than 12.6% Si). This work is dedicated in the development of novel methods for the rheocasting of hypereutectic Al-Si alloys (greater than 12.6% Si) and Al based wrought alloys (alloys with Cu, Zn, Mg and Si as alloying elements). The thesis presents the problems associated with microstructure control of hypereutectic Al-Si (primary Si coarsening and accelerated growth) and Al based wrought alloys (dendritic structures and hot tearing) with currently available technologies. Novel processing techniques are presented for the casting of hypereutectic Al-Si alloys and Al based wrought alloys with a combination of industrial trials and a through analysis of the underlying mechanisms.

"Semi-solid metal (SSM) processing has emerged as a preferred manufacturing method due to the superior quality associated with semi-solid castings. In recent years, the driving force to reduce process cost has led to the development of a few rheocasting (also termed slurry-on-demand) processes. These include UBE’s New Rheocasting (NRC) process [1], Idra Prince’s Semi-Solid Rheocasting (SSR) process [2], and THT’s Sub-Liquidus Casting (SLC®) process [3]. A novel slurry-making SSM process developed at ACRC/MPI, termed the “Continuous Rheoconversion Process” (CRP), is a passive liquid mixing technique in which the nucleation and growth of the primary phase are controlled using a specially designed “reactor”. The reactor provides heat extraction, copious nucleation and forced convection during the initial stage of solidification, leading to the formation of thixotropic structures. In these studies, the critical issues/challenges to optimize the CRP for industrial applications have been addressed through validation experiments and pre-industrial trials."

The occurrence of hot tearing during solidification is one of the major factors influencing both the quality and productivity of aluminum castings. In order to reduce the formation of hot tears, quantitative information regarding both hot tearing formation and semi-solid deformation is essential. In this study, the mechanisms of hot tearing and semi-solid deformation have been investigated via two novel techniques: x-ray micro-tomography on material deformed in the semi-solid region, and development of a three phase microstructural model based on a geometry derived from a Voronoi diagram with rounded corners and porosity. Numerical techniques were utilized to quantify both the size evolution and orientation of internal damage relative to void growth. In order to conduct the above research, a new semi-solid tensile deformation methodology was devised which uses a two thermocouple control technique to enable accurate measurement of semi-solid tensile strength and ductility. The experimental work was conducted on the aluminum – magnesium alloy AA5182 in the as-cast and hot isostatic pressing (HIP) states. The x-ray micro-tomography technique was used to observe that semi-solid deformation is accommodated by internal damage via growth of as-cast porosity and the nucleation of new damage-based voids. As the volume fraction of damage increases, the growth of voids occurs in an orientation perpendicular to the loading direction, both through expansion within the grain boundary liquid and via coalescence between voids. The damage then localizes, causing failure. The finite element semi-solid microstructural model was used to explore the effects of fraction solid, fraction porosity, and grain size on semi-solid constitutive behaviour. The simulations revealed that increased grain size and fraction porosity lead to a reduction in flow stress for a given fraction solid. Furthermore, local strain accumulation was linked to hot tearing, since strain localizes in the liquid very early in the deformation process. Based on the model predictions, a new constitutive relationship was developed over the range 0.75 < fs < 0.95. Together, these two techniques have provided powerful new insight, such as the critical role played by as-cast porosity, on the phenomena of hot tearing and semi-solid deformation in aluminum alloys.

The occurrence of hot tearing during solidification is one of the major factors influencing both the quality and productivity of aluminum castings. In order to reduce the formation of hot tears, quantitative information regarding both hot tearing formation and semi-solid deformation is essential. In this study, the mechanisms of hot tearing and semi-solid deformation have been investigated via two novel techniques: x-ray micro-tomography on material deformed in the semi-solid region, and development of a three phase microstructural model based on a geometry derived from a Voronoi diagram with rounded corners and porosity. Numerical techniques were utilized to quantify both the size evolution and orientation of internal damage relative to void growth. In order to conduct the above research, a new semi-solid tensile deformation methodology was devised which uses a two thermocouple control technique to enable accurate measurement of semi-solid tensile strength and ductility. The experimental work was conducted on the aluminum – magnesium alloy AA5182 in the as-cast and hot isostatic pressing (HIP) states. The x-ray micro-tomography technique was used to observe that semi-solid deformation is accommodated by internal damage via growth of as-cast porosity and the nucleation of new damage-based voids. As the volume fraction of damage increases, the growth of voids occurs in an orientation perpendicular to the loading direction, both through expansion within the grain boundary liquid and via coalescence between voids. The damage then localizes, causing failure. The finite element semi-solid microstructural model was used to explore the effects of fraction solid, fraction porosity, and grain size on semi-solid constitutive behaviour. The simulations revealed that increased grain size and fraction porosity lead to a reduction in flow stress for a given fraction solid. Furthermore, local strain accumulation was linked to hot tearing, since strain localizes in the liquid very early in the deformation process. Based on the model predictions, a new constitutive relationship was developed over the range 0.75 < fs < 0.95. Together, these two techniques have provided powerful new insight, such as the critical role played by as-cast porosity, on the phenomena of hot tearing and semi-solid deformation in aluminum alloys.

Nanocápsulas poliméricas têm sido propostas como carreadores para filtros solares com o objetivo de prolongar o tempo de resistência desses no estrato córneo. O presente trabalho teve por objetivo avaliar comparativamente a permeação cutânea in vitro da benzofenona-3 (BZ3) a partir de nanocápsulas poliméricas incorporadas em diferentes formulações semi-sólidas. Primeiramente, suspensões de nanocápsulas contendo concentrações crescentes de BZ3 foram preparadas, caracterizadas físico-quimicamente e sua estabilidade foi determinada. A suspensão contendo 5 mg/mL de BZ3 apresentou diâmetro médio de 247 ± 4 nm, potencial zeta de -9,50 ± 1,02 mV, pH de 6,6 ± 0,1, teor e taxa de associação próximos a 100 % e foi selecionada para a continuidade do estudo. Na segunda etapa do trabalho a suspensão de nanocápsulas contendo 5 mg/mL de BZ3 foi incorporada em diferentes formulações semi-sólidas, hidrogel (HGNCBZ3), creme-gel (CGNCBZ3) e emulsão água em silicone (SNCBZ3). Estas formulações semi-sólidas foram caracterizadas segundo teor, pH, diâmetro médio das nanocápsulas incorporadas e comportamento reológico. As formulações semi-sólidas apresentaram pH adequado para aplicação cutânea, teores experimentais próximos aos valores teóricos e comportamento reológico não-newtoniano pseudoplástico. A permeação cutânea do filtro solar foi avaliada através de células de difusão de Franz e as formulações contendo BZ3 nanoencapsulada apresentaram maior quantidade e tempo de resistência do filtro no estrato córneo, comparando com as formulações contendo o ativo livre. A SNCBZ3 apresentou maior quantidade da BZ3 no estrato córneo quando comparada ao HGNCBZ3 e ao CGNCBZ3. Em uma última etapa do trabalho avaliou-se a fotoestabilidade da BZ3 nanoencapsulada e livre frente à irradiação por UVA e concluiu-se que a nanoencapsulação aumenta a fotoestabilidade da BZ3. Também foi avaliada a alergenicidade do filtro solar químico e observou-se que, nem o filtro nanoencapsulado ou livre, nem as nanocápsulas sem o ativo, apresentaram sensibilização cutânea. Em conclusão, o conjunto dos resultados obtidos demonstra que estas formulações são sistemas promissores para a aplicação cutânea de filtros solares. / Polymeric nanocapsules have been proposed as carriers for sunscreens in order to prolong the residence time of these substances in the stratum corneum. The aim of this work was to evaluate in vitro the skin permeation of benzophenone-3 (BZ3) from polymeric nanocapsules incorporated in different semi-solid formulations. Nanocapsule suspensions containing different concentrations of BZ3 were prepared, physico-chemically characterized and their stability was determinated. The suspension containing 5 mg/mL of the BZ3 presented average size of 247 ± 4 nm, zeta potential of - 9,50 ± 1,02 mV and pH values of 6,6 ± 0,1. This sample showed an amount of BZ3 and drug entrapment of 100 %. The nanocapsule suspension containing 5 mg/mL was incorporated in different semi-solid formulations, hydrogel (HGNCBZ3), cream-gel (CGNCBZ3) and emulsion water in silicon (SNCBZ3). The amount of BZ3, pH, average size and rheological properties of these formulations were evaluated. The formulations presented pH adequate for skin application and content of BZ3 close to 100 %. All semi-solid formulations showed pseudoplastic reological behavior. The skin permeation of BZ3 was evaluated by Franz diffusion cells and the formulation containing nanoencapsulated BZ3 showed an increase in the residence time of sunscreen in the stratum corneum and high amount of BZ3 compared to the formulations containing the free sunscreen. SNCBZ3 presented greater BZ3 quantity in the stratum corneum compared to HGNCBZ3 and CGNCBZ3. Finally, the photostability of free and nanoencapsulated BZ3 under UVA irradiation was evaluated and the results indicated that the nanoencapsulation increased the BZ3 photostability. The allergenicity of free and nanoencapsulated BZ3 was also determinated. Nanoencapsulated or free BZ3, and empty nanocapsules, did not present skin sensibilization. In conclusion, the results indicated that these formulations are promising delivery systems for the cutaneous applications of sunscreens.

Nanocápsulas poliméricas têm sido propostas como carreadores para filtros solares com o objetivo de prolongar o tempo de resistência desses no estrato córneo. O presente trabalho teve por objetivo avaliar comparativamente a permeação cutânea in vitro da benzofenona-3 (BZ3) a partir de nanocápsulas poliméricas incorporadas em diferentes formulações semi-sólidas. Primeiramente, suspensões de nanocápsulas contendo concentrações crescentes de BZ3 foram preparadas, caracterizadas físico-quimicamente e sua estabilidade foi determinada. A suspensão contendo 5 mg/mL de BZ3 apresentou diâmetro médio de 247 ± 4 nm, potencial zeta de -9,50 ± 1,02 mV, pH de 6,6 ± 0,1, teor e taxa de associação próximos a 100 % e foi selecionada para a continuidade do estudo. Na segunda etapa do trabalho a suspensão de nanocápsulas contendo 5 mg/mL de BZ3 foi incorporada em diferentes formulações semi-sólidas, hidrogel (HGNCBZ3), creme-gel (CGNCBZ3) e emulsão água em silicone (SNCBZ3). Estas formulações semi-sólidas foram caracterizadas segundo teor, pH, diâmetro médio das nanocápsulas incorporadas e comportamento reológico. As formulações semi-sólidas apresentaram pH adequado para aplicação cutânea, teores experimentais próximos aos valores teóricos e comportamento reológico não-newtoniano pseudoplástico. A permeação cutânea do filtro solar foi avaliada através de células de difusão de Franz e as formulações contendo BZ3 nanoencapsulada apresentaram maior quantidade e tempo de resistência do filtro no estrato córneo, comparando com as formulações contendo o ativo livre. A SNCBZ3 apresentou maior quantidade da BZ3 no estrato córneo quando comparada ao HGNCBZ3 e ao CGNCBZ3. Em uma última etapa do trabalho avaliou-se a fotoestabilidade da BZ3 nanoencapsulada e livre frente à irradiação por UVA e concluiu-se que a nanoencapsulação aumenta a fotoestabilidade da BZ3. Também foi avaliada a alergenicidade do filtro solar químico e observou-se que, nem o filtro nanoencapsulado ou livre, nem as nanocápsulas sem o ativo, apresentaram sensibilização cutânea. Em conclusão, o conjunto dos resultados obtidos demonstra que estas formulações são sistemas promissores para a aplicação cutânea de filtros solares. / Polymeric nanocapsules have been proposed as carriers for sunscreens in order to prolong the residence time of these substances in the stratum corneum. The aim of this work was to evaluate in vitro the skin permeation of benzophenone-3 (BZ3) from polymeric nanocapsules incorporated in different semi-solid formulations. Nanocapsule suspensions containing different concentrations of BZ3 were prepared, physico-chemically characterized and their stability was determinated. The suspension containing 5 mg/mL of the BZ3 presented average size of 247 ± 4 nm, zeta potential of - 9,50 ± 1,02 mV and pH values of 6,6 ± 0,1. This sample showed an amount of BZ3 and drug entrapment of 100 %. The nanocapsule suspension containing 5 mg/mL was incorporated in different semi-solid formulations, hydrogel (HGNCBZ3), cream-gel (CGNCBZ3) and emulsion water in silicon (SNCBZ3). The amount of BZ3, pH, average size and rheological properties of these formulations were evaluated. The formulations presented pH adequate for skin application and content of BZ3 close to 100 %. All semi-solid formulations showed pseudoplastic reological behavior. The skin permeation of BZ3 was evaluated by Franz diffusion cells and the formulation containing nanoencapsulated BZ3 showed an increase in the residence time of sunscreen in the stratum corneum and high amount of BZ3 compared to the formulations containing the free sunscreen. SNCBZ3 presented greater BZ3 quantity in the stratum corneum compared to HGNCBZ3 and CGNCBZ3. Finally, the photostability of free and nanoencapsulated BZ3 under UVA irradiation was evaluated and the results indicated that the nanoencapsulation increased the BZ3 photostability. The allergenicity of free and nanoencapsulated BZ3 was also determinated. Nanoencapsulated or free BZ3, and empty nanocapsules, did not present skin sensibilization. In conclusion, the results indicated that these formulations are promising delivery systems for the cutaneous applications of sunscreens.

La fissuration à chaud est un défaut majeur intervenant en fin de solidification d'alliages métalliques, lorsque le matériau est encore à l'état semi solide. L'objectif de cette thèse est d'apporter une contribution à la prévision de ce phénomène. Pour cela il est nécessaire de caractériser la déformabilité de la phase solide d'un matériau à l'état semi solide et d'apporter des informations au niveau local sur la microstructure lors de sa déformation (fractions de liquide et de pores, épaisseur de films de liquide, vitesses d'écoulement du liquide, indice de coalescence des pores...). Le comportement macroscopique de la phase solide sur la ligne de solidus a été déterminé et validé par des essais de compression à chaud. Cette loi de comportement a également été comparée avec plusieurs modèles analytiques. La différence de comportement mécanique entre une microstructure de composition homogène et une microstructure à gradient de composition a été mise en évidence. Des informations microscopiques en temps réel sur des échantillons à l'état semi solide soumis à un effort de traction ont été obtenues par microtomographie in situ standard en conditions isothermes et en solidification. Un modèle analytique prédictif de la fissuration à chaud a été modifié pour mieux rendre compte des valeurs expérimentales. Pour gagner en résolutions temporelle et spatiale, un dispositif de traction destiné à l'utilisation de la microtomographie in situ rapide ou ultra rapide a été conçu et réalisé. Les essais en conditions isothermes et en solidification sont désormais reproductibles sur une microstructure obtenue par refusion. Des phénomènes rapides au sein d'une microstructure très fine sont maintenant observables. / Hot tearing is a defect which forms at high solid fraction in solidifying alloys, when the material is still semi solid. This work aims at bringing a contribution to the prevision of this phenomenon. To be able to do that, it is necessary to characterize deformability of the semi-solid alloy solid phase, and also bring local information on microstructure during its straining (liquid and pore fractions, liquid films thickness, liquid flow velocity…). Solid phase macroscopic behaviour on solidus line has been determined and validated by hot compression tests. The calculated behaviour law has also been compared with several analytical models. Mechanical behaviour differences between homogeneous and composition gradient microstructure has been put in evidence. Real time microscopic information on tensile tested semi solid samples have been obtained by in situ standard microtomography, in isothermal and solidification conditions. An analytic hot tearing predictive model has been modified to better fit the experimental data. An experimental device has been developed and realized to be able to carry out high and ultra high speed microtomography. These two methods bring better both time and space resolution. High velocity phenomenons in very fine microstructures can thus be visible. Isothermal and solidification conditions tests and now reproducible on remelted samples.

Depuis ces dernières décennies, l'administration ophtalmique est devenue une voie privilégiée des molécules en vue d'une action locale. La sécurité, l'efficacité et l'acceptabilité de ces médicaments imposent une formulation stérile et facilement applicable pour l'amélioration de l'observance du patient. L'objectif de ce travail a été de développer une forme ophtalmique topique efficace, facilement applicable et contenant un principe actif (PA) d'intérêt. Les études d'encapsulation du PA dans des particules de polymères en vue de sa protection mettent en évidence la difficulté de formuler des vecteurs avec un PA hydrosoluble. Son instabilité en solution aqueuse a été démontrée lors des études de stabilité et a orienté le choix galénique vers des préparations semi-solides. L'absence, ou la faible quantité d'eau présente, améliore la stabilité du PA. L'association des cyclodextrines dans la formulation montre une nette stabilisation du PA dans les préparations semi-solides. Les études de caractérisation de ces formes mettent en évidence la stabilité du PA dans une pommade hydrophobe et dans une émulsion en présence de cyclodextrine. De plus, les essais de microbiologie démontrent l'intérêt de l'utilisation d'une émulsion dans l'efficacité de la forme pharmaceutique contre Staphylococcus aureus. Les formes ophtalmiques mise au point présentent un intérêt majeur dans le traitement d'infections oculaires localisées en remplacement des thérapeutiques actuellement sur le marché / Over the last few decades, ophthalmic administration has become a preferred route for molecules for local action. The safety, efficacy and acceptability of these drugs require a sterile and easily applicable formulation for improved patient compliance. The objective of this work was to develop an effective, easily applicable topical ophthalmic form containing a drug of interest. Studies of the encapsulation of API in polymer particles for protection show the difficulty of formulating vectors with a water-soluble API. The instability of the drug in aqueous solution has been demonstrated in stability studies and guided galenic choice to semi-solid preparations. The absence, or small amount of water, improves the stability of the API. The association of cyclodextrins in the formulation shows a marked stabilization of the drug in semi-solid preparations. Characterization studies of these forms demonstrate the stability of the drug in a hydrophobic ointment and in an emulsion in the presence of cyclodextrin. Moreover, microbiological tests demonstrate the advantage of using an emulsion in the efficacy of the pharmaceutical form against Staphylococcus aureus. The ophthalmic forms developed are major interest in the treatment of localized eye infections in place of the therapeutics currently on the market

Nanocápsulas poliméricas têm sido propostas como carreadores para filtros solares com o objetivo de prolongar o tempo de resistência desses no estrato córneo. O presente trabalho teve por objetivo avaliar comparativamente a permeação cutânea in vitro da benzofenona-3 (BZ3) a partir de nanocápsulas poliméricas incorporadas em diferentes formulações semi-sólidas. Primeiramente, suspensões de nanocápsulas contendo concentrações crescentes de BZ3 foram preparadas, caracterizadas físico-quimicamente e sua estabilidade foi determinada. A suspensão contendo 5 mg/mL de BZ3 apresentou diâmetro médio de 247 ± 4 nm, potencial zeta de -9,50 ± 1,02 mV, pH de 6,6 ± 0,1, teor e taxa de associação próximos a 100 % e foi selecionada para a continuidade do estudo. Na segunda etapa do trabalho a suspensão de nanocápsulas contendo 5 mg/mL de BZ3 foi incorporada em diferentes formulações semi-sólidas, hidrogel (HGNCBZ3), creme-gel (CGNCBZ3) e emulsão água em silicone (SNCBZ3). Estas formulações semi-sólidas foram caracterizadas segundo teor, pH, diâmetro médio das nanocápsulas incorporadas e comportamento reológico. As formulações semi-sólidas apresentaram pH adequado para aplicação cutânea, teores experimentais próximos aos valores teóricos e comportamento reológico não-newtoniano pseudoplástico. A permeação cutânea do filtro solar foi avaliada através de células de difusão de Franz e as formulações contendo BZ3 nanoencapsulada apresentaram maior quantidade e tempo de resistência do filtro no estrato córneo, comparando com as formulações contendo o ativo livre. A SNCBZ3 apresentou maior quantidade da BZ3 no estrato córneo quando comparada ao HGNCBZ3 e ao CGNCBZ3. Em uma última etapa do trabalho avaliou-se a fotoestabilidade da BZ3 nanoencapsulada e livre frente à irradiação por UVA e concluiu-se que a nanoencapsulação aumenta a fotoestabilidade da BZ3. Também foi avaliada a alergenicidade do filtro solar químico e observou-se que, nem o filtro nanoencapsulado ou livre, nem as nanocápsulas sem o ativo, apresentaram sensibilização cutânea. Em conclusão, o conjunto dos resultados obtidos demonstra que estas formulações são sistemas promissores para a aplicação cutânea de filtros solares. / Polymeric nanocapsules have been proposed as carriers for sunscreens in order to prolong the residence time of these substances in the stratum corneum. The aim of this work was to evaluate in vitro the skin permeation of benzophenone-3 (BZ3) from polymeric nanocapsules incorporated in different semi-solid formulations. Nanocapsule suspensions containing different concentrations of BZ3 were prepared, physico-chemically characterized and their stability was determinated. The suspension containing 5 mg/mL of the BZ3 presented average size of 247 ± 4 nm, zeta potential of - 9,50 ± 1,02 mV and pH values of 6,6 ± 0,1. This sample showed an amount of BZ3 and drug entrapment of 100 %. The nanocapsule suspension containing 5 mg/mL was incorporated in different semi-solid formulations, hydrogel (HGNCBZ3), cream-gel (CGNCBZ3) and emulsion water in silicon (SNCBZ3). The amount of BZ3, pH, average size and rheological properties of these formulations were evaluated. The formulations presented pH adequate for skin application and content of BZ3 close to 100 %. All semi-solid formulations showed pseudoplastic reological behavior. The skin permeation of BZ3 was evaluated by Franz diffusion cells and the formulation containing nanoencapsulated BZ3 showed an increase in the residence time of sunscreen in the stratum corneum and high amount of BZ3 compared to the formulations containing the free sunscreen. SNCBZ3 presented greater BZ3 quantity in the stratum corneum compared to HGNCBZ3 and CGNCBZ3. Finally, the photostability of free and nanoencapsulated BZ3 under UVA irradiation was evaluated and the results indicated that the nanoencapsulation increased the BZ3 photostability. The allergenicity of free and nanoencapsulated BZ3 was also determinated. Nanoencapsulated or free BZ3, and empty nanocapsules, did not present skin sensibilization. In conclusion, the results indicated that these formulations are promising delivery systems for the cutaneous applications of sunscreens.

Les métaux à l’état semi-solide présentent un comportement rhéofluidifiant caractérisé par une forte chute de la viscosité avec la vitesse de déformation facilitant le remplissage des matrices lors de la mise en forme. L’exploitation de ce comportement donne lieu à un procédé de mise en forme appelé « thixoforgeage ». L’état semi-solide est obtenu en refusion partielle à partir de l’état solide et la fraction de solide est supérieure à 0.8. Les plus hautes températures mises en jeu dans le cas des matériaux à haut point de fusion comme les aciers rendent la maîtrise du procédé plus complexe. Cependant, les travaux antérieurs sur le thixoforgeage d’aciers ont montré le potentiel de ce procédé pour réduire la consommation en matière première et en énergie. La thèse s’inscrit dans le cadre du projet TACA «Thixoforgeage d’Aciers pour la fabrication de Composants Automobiles» piloté par l’IRT-M2P. Il ambitionne le développement industriel du thixoforgeage des aciers pour la fabrication de pièces automobiles. La maîtrise et le développement du thixoforgeage nécessite une bonne connaissance du comportement mécanique du matériau à l’état semi-solide et le développement d’outils de simulation numérique adaptés. Ce travail de thèse a pour objectifs de (i) caractériser expérimentalement le comportement thermomécanique des aciers à l’état semi-solide, (ii) développer un modèle de comportement destiné à être implanté dans le logiciel FORGE® pour simuler le thixoforgeage. Une attention particulière a été portée sur le comportement en traction pour caractériser l’intervalle de température dans lequel le matériau devient très sensible à la fissuration à chaud. Des essais en traction menés à différentes températures ont permis de déterminer la température à partir de laquelle la résistance et la ductilité du matériau chutent drastiquement. Les mécanismes conduisant à ces chutes ont été identifiés et sont conformes à ceux décrits dans la littérature. Un modèle basé sur une approche d’homogénéisation qui prend en compte explicitement le rôle mécanique des phases liquide et solide a été développé. Ce modèle repose sur une approche viscoplastique établie antérieurement puis enrichie afin (i) d’intégrer le comportement élastique du squelette solide saturé en liquide et (ii) distinguer les évolutions de distribution spatiale des phases liquide et solide selon le trajet de chargement en traction ou en compression. Il permet de décrire avec succès, pour la première fois, les trois stades de la réponse mécanique en traction (augmentation, stabilisation puis chute de la contrainte en fonction du déplacement). Le modèle a ensuite été implanté dans le code éléments finis FORGE®. Les simulations des essais de traction GLEBBLE ont permis d’identifier les paramètres du modèle. Des comparaisons des résultats expérimentaux et numériques ont permis de reproduire des phénomènes de localisation de la déformation réelle. Après la validation sur des essais de traction, des simulations de procédés de thixoforgeage industriel, tels que le forgeage de U, ont été étudiées et comparées aux résultats expérimentaux réalisés sur la plateforme Vulcain de l'ENSAM de Metz. Un critère permettant de définir des zones sensibles à la fissuration à chaud a été proposé. La comparaison avec des observations expérimentales a montré que ce critère constitue une première approche encourageante pour prédire les zones de fragilité de la pièce en thixoforgeage. / Semi-solid metals and alloys exhibit a shear thinning behavior characterized by a sharp drop in viscosity with increasing strain rate. This property promotes a smooth die filling during forming. To exploit this advantage, several semi-solid forming process have been developed. Among these processes, we find the thixoforging when the semi-solid state is obtained by a partial remelting from solid state. The solid fraction is above 0.8. Thixoforging of high melting point alloys such as steels is particularly challenging because of about 1400°C temperatures involved. However, previous works showed that this process reveals high potential to reduce material as well as energy consumption. The present PhD thesis is part of a French research project named TACA «Thixoforging of steels for fabrication of automative parts» leaded by IRT-M2P. It aims at industrial development of steel thixoforging for manufacturing automotive components. The mastery and the development of steel thixoforging require a good knowledge of the mechanical behavior of semi-solid steels and appropriate numerical tools to simulate the process. The PhD work aims to (1) characterize the thermomechanical behavior of semi-solid steels and (2) develop constitutive equations that have to be implemented into the commercial code FORGE® to simulate thixoforging. A special attention was paid to the tensile behavior to investigate the temperature range in which the material is very sensitive to hot cracking. Tensile tests provided the temperature from which the material lost its tensile strength and its ductility. Mechanisms leading to the drop of these two properties were identified and were found to be consistent with mechanisms described in literature. A model based on homogenization approach, namely taking explicitly into account the mechanical role of the liquid and solid phases was developed. This model is based on a viscoplastic approach previously developed that was enhanced to (1) include the elastic response of the solid skeleton saturated with liquid and to (2) distinguish the evolution of the spatial liquid/solid distribution according to the tensile or compressive loading path. It successfully describes the three stages of the response in tension: increase, stabilization and decrease of the stress with increasing displacement. The model was implemented in the FORGE® finite element code. The experimental tensile tests were simulated to provide identification of the model parameters. The simulation results showed that strong deformation localization zones were predicted consistently with experiments. Simulations of thixoforging industrial processes such as forging U were studied and compared with experimental results achieved on the Vulcan platform (ENSAM Metz). In addition, a criterion determining the zones without any tensile strength and so sensitive to hot cracking was proposed. Comparison with experimental observations showed that this criterion is an encouraging first approach to predict the brittle zones of thixoforging parts.