The formability of Mg alloy sheet in the as-hot rolled condition depends on the microstructure developed during hot rolling. In general, the formability of Mg alloys is improved by finer grain sizes. 'Microalloying' levels of calcium (Ca), strontium (Sr), and cerium (Ce) have been found to refine the as-cast structure, but there is no information as to whether this effect will be reflected in the as-hot worked structure and formability. Thus, in this work, the effects of microalloying levels of calcium (Ca), strontium (Sr), and cerium (Ce) on the microstructures (from as-cast to as-hot rolled) and subsequent hot deformation behavior of AZ31, nominally 3% Al, 1% Zn, and 0.3%Mn, were systematically investigated. / To include the effect of solidification rate these alloys were cast in different moulds (preheated steel mould, Cu-mould, and water cooled Cu-mould). One-hit compression testing at temperatures between 250°C ∼ 400 °C, strain rates of 0.001, 0.01, 0.1 s-1 and strains from 0.2 up to 1.0, was performed to investigate the basic hot compression behavior, while two-hit compression testing was conducted to determine the static softening behavior. Hot rolling of the microalloyed AZ31 alloys was then carried out to study the effects of microalloying on as-hot rolled structure under two sets of rolling schedules. To investigate the formability of these microalloyed sheets, tensile tests were completed over a temperature range between ambient and 450°C, at strain rates between 0.1 and 0.0003 s-1. / Results show that Ca and Sr act to refine the as cast grain size and the second phases, consistently promoting fine and uniform as-hot rolled grain structure. With regard to grain refinement, calcium has the strongest effect, whereas Ce is most effective for second phase refinement. In addition, microalloying retards grain growth during hot tensile testing. Multiple alloying presents a combined and complementary effect. / A refined and uniform grain structure combined with well dispersed and thermally stable second phases significantly improves the hot formability of AZ31 sheets by promoting dynamic recrystallization (DRX) in the matrix, resisting grain coarsening, and retarding the development of cavitation and necking. Under the superplastic condition of 450°C and 0.0003 s -1, the elongation was improved by 17% with Ca only, 26% with Ca and Ce, 51% with Ca and Sr, and 59% with Ca, Sr and Ce.

This numerical investigation of an advanced Gurson-Tvergaard-Needleman (GTN) model is an extension of the original work of Ben Bettaieb et al. (2011). The model has been implemented as a user-defined material model subroutine (VUMAT) in the Abaqus/explicit FE code. The current damage model extends the previous version by integrating the three damage mechanisms: nucleation, growth and coalescence of voids. Physically based void nucleation and growth laws are considered, including an effect of the kinematic hardening. These new contributions are based and validated on experimental results provided by high-resolution X-ray absorption tomography measurements. Also, the numerical implementation of the kinematic hardening in this damage extension has obliged to readapt the classical triaxiality definition. Besides, a secondary fracture initiation criterion based on the ultimate average inter-cavities distance has been integrated to localize and quantify with good accuracy the strain distribution just before the material fails apart. The current damage model is applied in industrial conditions to predict the damage evolution, the stress state and the fracture initiation in various tensile thin flat sheet geometries and the cross-die drawing tests.

Les aciers Dual-Phase sont largement utilisés dans le secteur de l’automobile enraison de leurs propriétés mécaniques remarquables et du bon compromis résistanceductilité qui lui donne d’intéressante potentialités comme absorbeur d’énergiemécanique. Cependant, la recherche de bons compromis entre les propriétésmécaniques en traction et celles de formabilité nécessite une optimisation desparamètres microstructuraux. Ce travail de thèse s’inscrit dans cet optique. Dans unepremière partie, l’étude bibliographique proposée permet de mieux cerner lesparamètres influençant la formation des microstructures ainsi que les propriétés desaciers DP. Dans une seconde partie, nous proposons un travail expérimental originalpermettant de mieux comprendre la formation des microstructures des aciers DP etde découpler l’effet de certains paramètres microstructuraux sur les propriétés deces aciers. Enfin, la modélisation micromécanique proposée permet de compléter etd’interpréter les données expérimentales acquises. Ce travail ouvre des voiesintéressantes de « design » des microstructures des aciers DP en vue de développerdes aciers de nouvelles générations possédant des propriétés optimisées. / Ferrite-martensite dual-phase (DP) steels have been widely used in automotiveindustry due to their excellent mechanical properties, such as high work-hardeningrate and a good compromise between strength and ductility allowing high energyabsorbing performance. In order to fully exploit the potential of DP steels and extendthe application, the dual-phase microstructure has to be optimized for bettercombination of strength and formability that is characterized by uniform strainand/or fracture strain. As a starting point, detailed literature review is made on themicrostructure development and mechanical properties of DP steels, and the keyfactors controlling microstructural features and determining mechanical propertiesare identified. Through experimental investigation, microstructures are developed inorder to decouple the effects of various microstructural features, and themicrostructure—mechanical properties relationship is systematically studied.Micromechanical modeling is used to further understand the experimental resultswithin a quantitative framework, and to provide a support for microstructurerefinement of DP steels by parametric study. Strategies of designing DP steels tofulfill specific forming operation have been proposed, and the concept of DP steelswith graded martensite islands has been discussed with FEM analysis as a possibilityof improving strength—formability trade-off.

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In the last two decades or so, hybrid structures from dissimilar materials and/or sheet gauges have been developed to achieve weight reduction while maintaining or even improving structural performance such as stiffness, crash and impact behavior. In particular, welded and brazed sheet materials in the form of tailor blanks (TBs) are being increasingly used or considered for future applications in different applications such as automotive, aerospace and marine constructions as they offer attractive combination of strength and performance in applications where weight reduction is desirable. However, technical problems are often encountered during forming of TBs from dissimilar base sheet materials with different thickness and/or strength. These include weld line movement and non-uniform deformation. Additionally, there are premature weld failures due to the presence of softening zone (as in TBs made from advanced high strength steels), and brazed interface failure due to insufficient bonding and wetting (as in TBs made from steel and aluminum). These areas of forming of TBs need to be scientifically studied to advance the use of dissimilar materials. The current research involves an understanding of deformation and forming behavior of steel-to-steel tailor welded blanks (TWBs) made from advanced high strength steel (AHSS) such as dual phase (DP780) steel. The research also involves a study of deformation behavior of steel-to-aluminum tailor brazed blanks (TBBs). TWBs have been successfully joined using a relatively new welding techniques such as defocused fiber laser welding. TBBs, on the other hand, have been successfully produced by fiber Laser/MIG hybrid brazing and Cold Metal Transfer brazing (CMT). In addition, the formability of TWBs of different gauges and/or strengths was tested by using a new, simple and reproducible method of formability testing using a double-layer blank method. This method was devised and assessed for testing various steel combinations in different strain paths such that the weld line stayed in position with respect to forming tools and is subjected to the same stress and strain state as the parent material in the weld and its vicinity. Moreover, results from conventional stretch forming tests, single-layer blank, and the double-layer method were compared at the macroscopic level (such as weld line movement, forming limit etc.) as well as at the microscopic level (such as failure location within the weld and failure mode) to isolate the advantages of the proposed double-layer method. With regard to TBBs made by fiber Laser/MIG and CMT brazing methods, a fundamental knowledge and understanding of the local deformation behavior and material plastic ow in and around the brazed steel-aluminum interfaces were obtained by conducting miniature tensile mechanical tests that focus on continuous observation of the brazed region under a high magnification optical microscope to assess the ductility of the brazed joint and its capacity to carry the load during a material shaping process. / Thesis / Doctor of Philosophy (PhD) / In the last two decades or so, hybrid structures from dissimilar materials and/or sheet gauges have been developed to achieve weight reduction while maintaining or even improving structural performance such as stiffness, crash and impact behavior. In particular, welded and brazed sheet materials in the form of tailor blanks (TBs) are being increasingly used or considered for future applications in different applications such as automotive, aerospace and marine constructions as they offer attractive combination of strength and performance in applications where weight reduction is desirable. However, technical problems are often encountered during forming of TBs from dissimilar base sheet materials with different thickness and/or strength. These include weld line movement and non-uniform deformation. Additionally, there are premature weld failures due to the presence of softening zone (as in TBs made from advanced high strength steels), and brazed interface failure due to insufficient bonding and wetting (as in TBs made from steel and aluminum). These areas of forming of TBs need to be scientifically studied to advance the use of dissimilar materials.

The aim of this research work is to study the inter-relationship, under controlled industrial conditions, among skin pass reductions, surface topography characterized by 2D and 3D roughness parameters, stampability and painted surface finish quality for automotive steel sheet stampings. Different surface textures obtained from cold rolling finishing have been evaluated in terms of paint appearance (rating and spectral curve) and tentatively related to roughness parameters (2D and 3D) obtained from the cold finished sheets. Some relevant tendencies have been established amongst these parameters. The results presented here are in accordance with other recently published research showing that there is a clear relationship between these parameters, and that further detailed studies are needed. / O foco do presente trabalho é o estudo, em condições industriais, da inter-relação entre grau de redução do passe de acabamento, topografia superficial caracterizada pelos parâmetros de rugosidade 2D e 3D, estampabilidade e aparência de pintura de chapas de aço para painéis automotivos. Diferentes texturas superficiais de chapas de aço foram analisadas em termos de aparência de pintura (rating e curvas espectrais) e tentativamente relacionadas com os parâmetros de rugosidades (2D e 3D) obtidos na chapa antes de estampar. Algumas tendências relevantes foram estabelecidas entre estes parâmetros. Os resultados presentes aqui estão de acordo com publicações recentes mostrando uma clara relação entre estes parâmetros e que trabalhos futuros ainda são necessários.

Este trabalho trata da avaliação da técnica de conformação por expansão fluidostática (bulge forming) através de uma matriz com múltiplas cavidades. Duas ligas de especificação AA5083 e Pb-60Sn foram submetidas ao processo de superplasticidade (Superplastic Forming) para a verificação de diferentes parâmetros de trabalho e comparação com os resultados por simulação numérica. Uma das principais conclusões obtidas é que o método multidomo foi válido para o estudo do fenômeno superplástico. Os testes foram realizados através do método de pressão constante, onde foi possível obter variáveis como tensão, taxa de deformação e índice de sensibilidade a taxa de deformação. Uma importante implicação deste processo é o controle correto do tempo de trabalho com cavidades conformadas em ensaios distintos. / This study aims to evaluate the forming technique fluidostatic expansion (bulge forming) through a die with multiple cavities. Two AA5083 alloy and Pb-60Sn specification were submitted to superplasticity process (superplastic forming) for checking different working parameters and comparison with the results in numerical simulation. One of the main conclusions is that the multidomo method was valid to study the superplastic phenomenon. The tests were performed using the constant pressure method, where it was possible to obtain variables such as stress, strain rate and the strain rate sensitivity index. An important implication of this process is the correct control of working time with shaped cavities in separate trials.

O processo de estampagem é influenciado por diferentes variáveis, tais como, propriedades mecânicas e rugosidade da chapa, lubrificação do blank, coeficiente de atrito entre o blank e a ferramenta, parâmetros da prensa (força relativamente constante e velocidade), bem como variações nas ferramentas de conformação (deformação elástica). Este trabalho se dedica a investigação da influência do atrito entre a ferramenta e o blank. Em particular, será investigado o desenvolvimento de padrões artificiais de rugosidade de chapas metálicas (textura de superfície). O foco do presente trabalho é o estudo, em condições industriais, da inter-relação entre topografia superficial caracterizada pelos parâmetros de rugosidade 2D e 3D e estampabilidade de chapas de aço para painéis automotivos. Diferentes texturas superficiais de chapas de aço foram analisadas em termos de estampabilidade (medido pelo deslocamento do blank durante a estampagem) e tentativamente relacionadas com os parâmetros de rugosidades (2D e 3D) obtidos na chapa antes de estampar. Algumas tendências relevantes foram estabelecidas entre estes parâmetros. Os resultados aqui apresentados estão em concordância com publicações recentes mostrando uma clara relação entre estes parâmetros e que trabalhos futuros são necessários. / The stamping process is influenced by several variables such as sheet mechanical properties and roughness, blank lubrication, friction coefficient, press parameters (press force and speed), as well as variations in the forming tools (elastic strain). This work is dedicated to study the influence of friction between the tool and the blank. In this sense, it is necessary to study the development of artificial roughness patterns of sheet metals (surface textures). The aim of this research work is to study the inter-relationship, using the surface topography characterized by 2D and 3D roughness parameters and the formability of steel sheets for automotive panels. Different surface textures of sheet metal have been evaluated in terms of formability (measured by material flow during the process stamping) and tentatively related to roughness parameters (2D and 3D) obtained in the sheet metal prior to stamping. Some relevant tendencies have been established amongst these parameters. The results presented here are in accordance with other recently published research work showing that there is a clear relationship between these parameters, and that further detailed studies are needed.

Este trabalho trata da avaliação da técnica de conformação por expansão fluidostática (bulge forming) através de uma matriz com múltiplas cavidades. Duas ligas de especificação AA5083 e Pb-60Sn foram submetidas ao processo de superplasticidade (Superplastic Forming) para a verificação de diferentes parâmetros de trabalho e comparação com os resultados por simulação numérica. Uma das principais conclusões obtidas é que o método multidomo foi válido para o estudo do fenômeno superplástico. Os testes foram realizados através do método de pressão constante, onde foi possível obter variáveis como tensão, taxa de deformação e índice de sensibilidade a taxa de deformação. Uma importante implicação deste processo é o controle correto do tempo de trabalho com cavidades conformadas em ensaios distintos. / This study aims to evaluate the forming technique fluidostatic expansion (bulge forming) through a die with multiple cavities. Two AA5083 alloy and Pb-60Sn specification were submitted to superplasticity process (superplastic forming) for checking different working parameters and comparison with the results in numerical simulation. One of the main conclusions is that the multidomo method was valid to study the superplastic phenomenon. The tests were performed using the constant pressure method, where it was possible to obtain variables such as stress, strain rate and the strain rate sensitivity index. An important implication of this process is the correct control of working time with shaped cavities in separate trials.