The increasing demand for powerful miniaturized products for all industrial applications has prompted the industry to develop new and innovative manufacturing processes to fabricate miniature parts. One of the major challenges facing the industry is the dynamic market which requires continuous improvements in design and fabrication techniques. This means providing products with complex features while sustaining high functionality. As a result, microfabrication has gained a wide interest as the technology of the future, where tabletop machine systems exist. Microforming processes have the capability of achieving mass production while minimizing material waste. Microforming techniques can produce net-shape products with intricacy in fewer steps than most conventional microfabrication processes. Despite the potential advantages, the industrial utilization of microforming technology is limited. The deformation and failure modes of materials during microforming is not yet well understood and varies significantly from the behavior of materials in conventional forming operations. In order to advance the microforming technology and enable the effective fabrication of microparts, more studies on the deformation and failure of materials during microforming are needed. In this research work, an effort to advance the current status of microforming processes for technologies of modern day essentials, is presented. The main contribution from this research is the development of a novel method for characterizing thin sheet formability by introducing a micro-mechanical bulge-forming setup. Various aspects of analyzing microscale formability, in the form of limiting strains and applied forces, along with addressing the well known size effects on miniaturization, were considered through the newly developed method. A high temperature testing method of microformed thin sheets was also developed. The aim of high temperature microforming is to study the material behavior of microformed thin sheets at elevated temperatures and to explore the capability of the known enhancement in formability at the macroscale level. The focus of this work was to develop a better understanding of tool-sheet metal interactions in microforming applications. This new knowledge would provide a predictive capability that will eliminate the current time-consuming and empirical techniques that, and this in turn would be expected to significantly lower the overall manufacturing cost and improve product quality.

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Os Aços Avançados de Alta Resistência ou AHSS (do inglês Advanced High Strength Steels) apresentam muitas vantagens mecânicas em relação aos aços convencionais. Seu uso crescente na indústria automotiva deve-se principalmente à sua capacidade de possibilitar a redução de peso e, ao mesmo tempo, o aumento da segurança aos ocupantes do veículo em caso de colisões. No entanto, apresentam maiores dificuldades no que se refere à conformabilidade (maiores níveis de solicitação e desgaste das ferramentas, menor deformabilidade plástica, etc). Assim, alguns testes para avaliar a conformabilidade destes materiais ganham maior importância. É o caso do Teste de Expansão de Furos, cuja propriedade medida é a Razão de Expansão de Furos (REF). Neste trabalho investiga-se o processo de expansão de furos para o aço avançado de alta resistência (AHSS) martensítico DOCOL 190M, sob as seguintes condições de processo: duas formas de obtenção do furo (jato d’água e usinagem); duas geometrias distintas de punções (cônico de 60º e elíptico); diversos diâmetros do furo inicial; com e sem o uso de lubrificante; com acabamento diferenciado da borda do furo; e expansão com deslocamento do punção em etapas. Os experimentos demonstram que a expansão de furos possui uma estreita relação com a geometria do punção utilizado para a expansão, bem como com o diâmetro do furo inicial, acabamento da borda e condições de lubrificação. A partir dos resultados experimentais de expansão de furos foi possível realizar a calibração de um software de simulação computacional em relação ao dano crítico do material no momento da fatura na borda do furo. / Advanced High Strength Steels (AHSS) offer many mechanical advantages over conventional steels. Its increasing use in the automotive industry is mainly due to its ability to reduce weight and, at the same time, increase occupant safety in the event of collisions. However, they present greater difficulties with respect to the formability (higher levels of solicitation and wear of the tools, lower plastic formability, etc). Thus, some tests to evaluate the formability of these materials come to have greater importance. This is the case of the Hole Expansion Test, whose measured property is the Hole Expansion Ratio (REF). This work investigates the hole expansion process for a martensitic advanced high-strength steel (AHSS), DOCOL 190M, under the following process conditions: two ways of obtaining the hole (water jet and machining); two different geometries of punctures (conical of 60º and elliptical); various diameters of the initial hole; with and without the use of lubricant; with differentiated finishing of the hole edge; and expansion with punch displacement in steps. The experiments demonstrate that the hole expansion has a close relationship with the geometry of the punch used for the expansion, as well as the initial hole diameter, edge finish and lubrication conditions. From the experimental hole expansion results it was possible to carry out the calibration of a computational simulation software in relation to the critical damage of the material at the moment of hole edge rupture.

Os Aços Avançados de Alta Resistência ou AHSS (do inglês Advanced High Strength Steels) apresentam muitas vantagens mecânicas em relação aos aços convencionais. Seu uso crescente na indústria automotiva deve-se principalmente à sua capacidade de possibilitar a redução de peso e, ao mesmo tempo, o aumento da segurança aos ocupantes do veículo em caso de colisões. No entanto, apresentam maiores dificuldades no que se refere à conformabilidade (maiores níveis de solicitação e desgaste das ferramentas, menor deformabilidade plástica, etc). Assim, alguns testes para avaliar a conformabilidade destes materiais ganham maior importância. É o caso do Teste de Expansão de Furos, cuja propriedade medida é a Razão de Expansão de Furos (REF). Neste trabalho investiga-se o processo de expansão de furos para o aço avançado de alta resistência (AHSS) martensítico DOCOL 190M, sob as seguintes condições de processo: duas formas de obtenção do furo (jato d’água e usinagem); duas geometrias distintas de punções (cônico de 60º e elíptico); diversos diâmetros do furo inicial; com e sem o uso de lubrificante; com acabamento diferenciado da borda do furo; e expansão com deslocamento do punção em etapas. Os experimentos demonstram que a expansão de furos possui uma estreita relação com a geometria do punção utilizado para a expansão, bem como com o diâmetro do furo inicial, acabamento da borda e condições de lubrificação. A partir dos resultados experimentais de expansão de furos foi possível realizar a calibração de um software de simulação computacional em relação ao dano crítico do material no momento da fatura na borda do furo. / Advanced High Strength Steels (AHSS) offer many mechanical advantages over conventional steels. Its increasing use in the automotive industry is mainly due to its ability to reduce weight and, at the same time, increase occupant safety in the event of collisions. However, they present greater difficulties with respect to the formability (higher levels of solicitation and wear of the tools, lower plastic formability, etc). Thus, some tests to evaluate the formability of these materials come to have greater importance. This is the case of the Hole Expansion Test, whose measured property is the Hole Expansion Ratio (REF). This work investigates the hole expansion process for a martensitic advanced high-strength steel (AHSS), DOCOL 190M, under the following process conditions: two ways of obtaining the hole (water jet and machining); two different geometries of punctures (conical of 60º and elliptical); various diameters of the initial hole; with and without the use of lubricant; with differentiated finishing of the hole edge; and expansion with punch displacement in steps. The experiments demonstrate that the hole expansion has a close relationship with the geometry of the punch used for the expansion, as well as the initial hole diameter, edge finish and lubrication conditions. From the experimental hole expansion results it was possible to carry out the calibration of a computational simulation software in relation to the critical damage of the material at the moment of hole edge rupture.

The desire to reduce weight in automotive products is driven by a need to improve efficiency. As such, to allow further weight reduction, higher performance aluminium alloys are in demand for sheet metal body structures. Due to their high strength to weight ratio 7xxx alloys are seen as an ideal candidate for this, however their use to date has been limited by poor formability. Previous work indicated that by moving to high temperatures (>350°C) or by using a soft temper (W), good formability could be achieved but the samples required further heat treatment post-forming. This work explored the warm forming temperature range to improve formability whilst developing the required properties during processing. The performance of a 7xxx candidate alloy, 7021, has been assessed in stretching and drawing operations, both at room temperature and over the elevated temperature range of 150-250°C. The microstructure and other properties of the alloy were investigated in W, T4 and T6 tempers, before and after testing, through a range of techniques, including DSC, DMTA, SEM, EBSD and TEM.In the T4 temper, UTS and proof stress increased with temperature up to 190°C, due to dynamic precipitation. Increasing temperature only provided a modest increase in strain to failure for both the T4 and T6 temper. Cup height was not significantly improved in the warm forming temperature range during Erichsen cup testing. By deep drawing at 250°C it was possible to fully draw a cup (with an LDR of 2.2) in both the T4 and T6 temper of 7021, with both tempers having comparable post-forming hardness. This indicates that at 250°C the starting condition has no impact on drawability. Although full drawability is achieved at 250°C the final product would require further heat treatment if it were to replace 6016. However, by deep drawing 7021-T4 at 190°C, a fully formed cup was produced with a hardness between that of the T4 and T6 temper. The microstructure of the formed cup showed no grain boundary precipitation and a fine distribution of the strengthening phase η', suggesting there is a dynamic effect on the precipitation during deep drawing at this temperature. In conclusion, the work has shown that warm forming does not significantly improve stretching behaviour of 7021, but by using warm forming temperatures deep drawing is improved.

Les tubes métalliques de précision sont largement utilisés pour des applications biomédicales. De tels tubes sont fabriqués par étirage à froid car ce procédé garanti le meilleur aspect de surface, le plus grand contrôle des dimensions du tube et le contrôle des propriétés mécaniques. L'objet de cette étude est de modéliser le procédé d'étirage de tube sur mandrin afin d'en améliorer la compréhension et de construire un outil permettant l'optimisation du procédé et de prédire la rupture des tubes en étirage. La construction du modèle élément finis s'appuie sur la réalisation d'essais expérimentaux afin de caractériser les propriétés mécaniques des matériaux et le frottement entre le tube et les outils d'étirage (mandrin, filière). Le comportement mécanique des alliages est caractérisé par des essais de traction sur tube, des essais de traction sur des éprouvettes découpées dans différentes orientations dans un tube déplié et des essais de gonflement de tube. Pour ces derniers, une machine et un outillage de gonflement de tubes ont été développés spécifiquement. Par le biais de ces essais différents aspects ont été étudiés : la viscoplasticité, l'anisotropie plastique, l'hétérogénéité des propriétés dans l'épaisseur du tube, la thermomécanique. Les coefficients de frottements ont été identifiés par analyse inverse sur des essais d'étirage instrumentés par des cellules d'effort. Des essais d'étirage ont été spécifiquement conçus en modifiant la géométrie du mandrin afin de conduire à la rupture des tubes lors de l'étirage. L'objectif de tels essais étant d'identifier la limite de formabilité des tubes. L'approche choisie pour prédire de la rupture a été d'utiliser des critères de ruptures qui pouvaient être calibrés sur des essais de traction uniquement. Les critères ont été calculés au cours de la simulation numérique de l'étirage sur mandrin et ils ont été évalués par rapport à leur capacité à prédire les réductions de section et d'épaisseur maximales. Enfin, des méthodes analytiques de calcul d'effort d'étirage ont été développées et comparées à la modélisation éléments finis. / Precision metallic tubes are widely used for biomedical applications. The requirements of such tubes in term of surface quality, precise dimensions and mechanical properties can be achieved by cold tube drawing only. The purpose of this study is to model the mandrel tube drawing in order improve the process understanding and to build a tool both for process optimisation and for failure prediction during drawing. Building the finite element modelling requires to perform a series of experimental tests in order to characterise the material mechanical behaviour and the friction between the tube and the forming tools (mandrel, die). The materials mechanical behaviour is characterized by means of tube tensile tests, tensile tests of oriented samples cut in different directions from flattened tubes and tube bulge test. For the latter, a tube bulge test device was specifically designed. Different aspects were covered by these tests: viscoplasticity, plastic anisotropy, materials properties heterogeneity in the tube thickness, thermomechanics. Friction coefficients were identified by inverse analysis on instrumented tube drawing tests. A specific drawing test was designed in order to identify the tube fracture during drawing by modifying the mandrel geometry. The goal of such test was to identify the tube formability limit. Among the different techniques available to predict tube failure, the approach of failure criterion was chosen. Different failure criteria that could be calibrated on tensile test were selected. Failure criteria were computed during the simulation of the mandrel tube drawing and they were evaluated in term of predictability of the maximum section and thickness reductions before fracture. Finally, analytical methods that enable to compute the drawing force were developed and compared with the finite element modelling.

<div>The proliferation of single-use plastics in the foodware and packaging sector has stimulated interest in sustainable material substitutes that can be processed efficiently and which possess sufficient structural integrity. Herein, we study the structure, mechanical response and diffusion properties of leaf-sheath from a representative palm species—<i>Areca catechu</i>—widely cultivated in the Indian sub-continent and Southeast Asia. The study of this material system, and the specific attributes, are motivated by the use of this material in foodware applications. Foodware such as plates and bowls can be produced from the areca sheath, directly, in a single step, by stretch forming analogous to sheet metals. The material is eco-friendly, biodegrading in ~100 days. Formability and water diffusion are two key attributes of relevance for foodware, since the former attribute determines the shape change capability of the material and range of producible shapes; and the latter, foodware product (structural) integrity and life.<br></div><div><br></div><div>We characterize the morphology (external structure) and anatomy (internal structure) of the sheath using imaging techniques and composition analysis. The sheath is shown to resemble a composite material, with structural characteristics intermediate between those of the palm leaf and stem. By measuring the mechanical response of the sheath to various types of 1D and 2D loading (e.g., uniaxial tension and compression, biaxial stretching, rolling), and hydration conditions, we show that the sheath material has high stretch-formability, especially when hydrated. This formability is similar to that of the most ductile sheet metals. The formability is shown to be further enhanced by addition of small quantities of NaOH (~5%) during hydration. Local deformation measurements in biaxial stretching, based on analysis of distortion of grid-markers inscribed onto sheath samples, have enabled characterization of strain-field anisotropy and mode of failure in the sheath. By consolidating the mechanical test results, we present a forming limit diagram for the leaf-sheath.<br></div><div><br></div><div>The structural integrity and life of foodware products produced from the leaf-sheath are directly determined by diffusion of liquids (e.g., water, oils) through the sheath wall thickness. Water and oils are important constituents of semi-solid and solid foods. Diffusion of water is also important for designing the hydration cycle to enhance formability. The diffusion of water through the sheath material process is studied using mass gain measurements and<i> in situ</i> imaging of water transport.<br></div><div><br></div><div>We determine the diffusion coefficient for water, which is critical for estimating product life. The diffusion coefficient for the matrix is shown to be one order of magnitude greater than for the fiber. We vary salt concentration in the water by controlled additions of NaCl and note a non-monotonic dependence of the diffusion on concentration. By subjecting the leaf-sheath to a short-time (~ 3 minutes) thermal treatment (~ 80⁰C), a hydrophobic wax layer can be made to secrete onto the leaf surface. This wax coating is found to significantly reduce the water diffusion, enabling the sheath foodware life to be increased. <br></div><div><br></div><div>Lastly, we argue, that since the leaf-sheath is a “waste product” of the palm, it has negligible embodied energy (4 to 5 orders of magnitude smaller) compared to paper and plastics based foodware.<br></div><div><br></div><div>We discuss the implications of the results for single-step forming of high-aspect ratio products and structures from the palm sheath, methods to reduce diffusion of liquids and improve foodware product life, and some directions for future research into mechanical behavior of plant leaf materials from a forming perspective.<br></div>