Made available in DSpace on 2014-10-09T12:40:44Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:09:39Z (GMT). No. of bitstreams: 1 02751.pdf: 7124848 bytes, checksum: 953b44ed12302b6f4d35b091dd91cef3 (MD5) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Orientador: Ruís Camargo Tokimatsu / Co-orientador: Maria Aparecida Larosa / Banca: Vicente Afonso Ventrella / Banca: Cecília Amelia de Carvalho Zavaglia / Resumo: A liga Ti-6A-4V é um biomaterial que tem se mostrado muito adequado na fabricação de implantes para reparar lesões craniofaciais. Entre outros requisitos, é importante que o implante resista a solicitação mecânica para garantir uma resposta clínica satisfatória de modo a proporcionar conforto e segurança ao paciente. O propósito específico do presente trabalho de pesquisa é determinar a influência de diferentes ciclos térmicos na microestrutura da amostra da Ti-6Al-4V produzida pelo processo de sinterização direta de metal por laser (DMLS) e no micromecanismo de fratura resultante de tensionamento uniaxial em tração. Para isso foram fabricados corpos-de-prova de tração, utilizando a técnica DMLS, e os mesmos foram submetidos a diferentes ciclos térmicos de 650 °C/1h, 650 °C/3h, 800 °C/4h e 850 °C/2h. Posteriormente, os corpos-de-prova foram submetidos a ensaios de tração. A caracterização microestrutural foi realizada com auxílio de microscopia óptica (MO) e eletrônica de varredura (MEV). As superfícies de fratura resultantes para as diferentes condições de ciclos térmicos foram analisadas por microscopia eletrônica de varredura. A microestrutura final da amostra como produzida resultante do processo de manufatura aditiva por DMLS revelou uma microestrutura martensita hexagonal α'. Para a amostra A6501H, a microestrutura martensítica resultante do processo DMLS não se decompôs. Na amostra A6503H a microestrutura martensítica começa a se decompor, pois são observados precipitados da fase β. Já nas amostras A8004H e A8502H observa-se uma estrutura (α +β), com cristais de fase α e fase β precipitada nos contornos. De acordo com a análise fractográfica todas as condições estudadas apresentaram fratura dúctil caracterizada pela formação e coalescência de microcavidades com forma e tamanhos variados / Abstract: The Ti-6A-4V alloy is a biomaterial that has proved very suitable for the manufacture of prostheses to repair craniofacial injuries. Among other requirements, it is important that the prosthesis resist mechanical stress to ensure a satisfactory clinical response in order to provide comfort and safety to patients. The specific purpose of this research is to determine the influence of thermal cycles - as prototyped, 650 °C (1h); 650 °C (3h), 800 °C (2h) and 850 °C (4h) - on the microstructure and fracture micromechanisms resulting from uniaxial tension in traction. Therefore it was manufactured by DMLS technique bodies - of - proof traction which were subsequently subjected to different thermal cycles. Then, the bodies -specimens were subjected to tensile test. Microstructural characterization was carried out using optical and scanning electron microscopy (SEM). The resulting fracture surfaces of the five conditions for thermal cycling were analyzed by SEM. The final sample microstructure produced as a result of the additive manufacturing process DMLS revealed a hexagonal martensitic microstructure α '. For A6501H sample, the resulting martensitic microstructure DMLS process is not decomposed. In A6503H martensitic microstructure sample begins to decompose, as are observed precipitates of β phase. Already in the samples A8004H and A8502H observe a structure (α + β) with α and β phase crystal phase precipitated in the contours. According to fractographic analysis of all studied showed ductile fracture conditions characterized by the formation and coalescence wells with varying shape and sizes / Mestre

Made available in DSpace on 2014-10-09T12:24:22Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:04:23Z (GMT). No. of bitstreams: 1 01040.pdf: 2700481 bytes, checksum: 1f84b09901d3ff97219652b8e79365be (MD5) / Dissertacao (Mestrado) / IEA/D / Escola Politecnica, Universidade de Sao Paulo - POLI/USP

Made available in DSpace on 2014-10-09T12:40:44Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:09:39Z (GMT). No. of bitstreams: 1 02751.pdf: 7124848 bytes, checksum: 953b44ed12302b6f4d35b091dd91cef3 (MD5) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

M.Ing. / The principle of the laser sintering process as applied to stainless steel powder for rapid prototyping is discussed. This work attempts to present and explain the process-modelling concept with respect to potential analytical and empirical relationships between the input and output parameters. It considers aspects related to process behavior, material characteristics, laser parameters and product accuracy. A comprehensive experimental set- up with empirical results is presented.

Studies have been made of both the solid state and supersolidus sintering characteristics of spherical cupronickel powders. Observations were made of the structural changes and shrinkage rates in specimens sintered in vacuum and in hydrogen. It was concluded that the early stage of solid state sintering (up to one hour at 1200°C) was dominated by Nabarro-Herring creep. Calculations of the stresses at necks during sintering were consistent with the proposed mechanism. No solute segregation to necks occurred during sintering, contrary to earlier observations by Kuczynski with other copper alloys. When pre-sintered cupronickel powder ( 68 μm) aggregates were heated to a temperature above the equilibrium solidus, melting was nucleated first at high angle grain boundaries (necks) and particle surfaces (voids). Most melting was intragranular, nucleated at interdendritic sites of above-average copper content. Solid-liquid equilibrium was established in less than one minute at the supersolidus temperature. The dihedral angle in the system was less than or equal to zero. Growth of solid grains during supersolidus sintering obeyed a parabolic rate law consistent with a model of growth due to phase boundary reaction-controlled solution and precipitation. Shrinkage during supersolidus sintering proceeded in several distinct stages. Prior to attainment of equilibrium; i.e. within the first minute above the solidus (Stage 1), contraction could be attributed to a melting and melt accommodation sequence, plus flattening by the the local operation of solution and precipitation. Beyond this (Stages 2 and 3) all densification was attributed to solution-precipitation, including grain growth. In the final stage of shrinkage (Stage 3) the rate of contraction was controlled by the rate of escape of gas from closed pores. Comparisons have been made between the supersolidus sintering of cupronickel and the liquid-phase sintering of iron-copper. The processes are seen to have little in common. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

No

Spent Nuclear Fuel (SNF) is a by-product of existing nuclear reactors; SNF consists of long-lived radioactive actinides which have an average half-life of several thousand years (e.g. Plutonium-239 with a half-life of 24,000 years, and Americium-243 with a half-life of 7,360 years). Several multinational organizations are making an attempt to extract the energetic value out of these nuclear stockpiles in order to minimize the risk of nuclear proliferation and reduce waste volume. The Inert Matrix Fuel (IMF) concept is being considered as an option to reuse the radioactive actinides present in spent nuclear fuel by means of a transmutation process. Due to the volatile nature of these radioactive actinides, it is expected that the high-temperature conventional processing of IMFs will result in a significant loss of material. This study investigates microwave sintering of inert matrix material (excluding actinide fuel) as an alternative route to conventional processing. It was observed that microwave sintering showed a reduction of 300°C in temperature required for full densification when compared to conventional sintering. The reduction in sintering temperatures did not show any significant variation in the resulting properties (hardness and grain size). While these results satisfy the need for the application, it is important to understand why microwaves enhance the sintering phenomena. It is speculated (by many researchers) that the electric field associated with microwave energy is enhancing flux leading to accelerated densification during microwave sintering. This study has observed a decrease in the activation energy (for sintering 8YZ) with the increase in the magnitude of the applied electric field. / Ph. D.

The purpose of this work is to extend the functionality of the Master Sintering Curve (MSC) such that it can be used as a practical tool for predicting sintering schemes that combine both a constant heating rate and an isothermal hold. Rather than just being able to predict a final density for the object of interest, the extension to the MSC will actually be able to model a sintering run from start to finish. Because the Johnson model does not incorporate this capability, the work presented is an extension of what has already been shown in literature to be a valuable resource in many sintering situations. A predicted sintering curve that incorporates a combination of constant heating rate and an isothermal hold is more indicative of what is found in real-life sintering operations. This research offers the possibility of predicting the sintering schedule for a material, thereby having advanced information about the extent of sintering, the time schedule for sintering, and the sintering temperature with a high degree of accuracy and repeatability. The research conducted in this thesis focuses on the development of a working model for predicting the sintering schedules of several stabilized zirconia powders having the compositions YSZ (HSY8), 10Sc1CeSZ, 10Sc1YSZ, and 11ScSZ1A. The compositions of the four powders are first verified using x-ray diffraction (XRD) and the particle size and surface area are verified using a particle size analyzer and BET analysis, respectively. The sintering studies were conducted on powder compacts using a double pushrod dilatometer. Density measurements are obtained both geometrically and using the Archimedes method. Each of the four powders is pressed into 1/4 inch diameter pellets using a manual press with no additives, such as a binder or lubricant. Using a double push-rod dilatometer, shrinkage data for the pellets is obtained over several different heating rates. The shrinkage data is then converted to reflect the change in relative density of the pellets based on the green density and the theoretical density of each of the compositions. The Master Sintering Curve (MSC) model is then utilized to generate data that can be utilized to predict the final density of the respective powder over a range of heating rates. The Elton Master Sintering Curve Extension (EMSCE) is developed to extend the functionality of the MSC tool. The parameters generated from the original MSC are used in tandem with the solution to a specific closed integral (discussed in document) over a set range of temperatures. The EMSCE is used to generate a set of sintering curves having both constant heating rate and isothermal hold portions. The EMSCE extends the usefulness of the MSC by allowing this generation of a complete sintering schedule rather than just being able to predict the final relative density of a given material. The EMSCE is verified by generating a set of curves having both constant heating rate and an isothermal hold for the heat-treatment. The modeled curves are verified experimentally and a comparison of the model and experimental results are given for a selected composition. Porosity within the final product can hinder the product from sintering to full density. It is shown that some of the compositions studied did not sinter to full density because of the presence of large porosity that could not be eliminated in a reasonable amount of time. A statistical analysis of the volume fraction of porosity is completed to show the significance of the presence in the final product. The reason this is relevant to the MSC is that the model does not take into account the presence of porosity and assumes that the samples sinter to full density. When this does not happen, the model actually under-predicts the final density of the material.

Le "Spark Plasma Sintering" est un procédé innovant qui permet de densifier, assembler, forger... tous types de matériaux avec des cinétiques très rapides. Cependant, il nécessite des améliorations pour le contrôle des températures, l'homogénéité microstructurale pour des pièces de formes complexes... et de productivité industrielle. Pour résoudre ces problèmes, un modèle électro-thermo-mécanique-microstructural est identifié : i) pour la partie Thermo-Electrique une instrumentation fine a permis par une approche inverse d'évaluer les résistances de contacts, ii) par essais in-situ de fluage et de compression pour la partie mécanique-microstructurale. Il a permis de trouver des solutions pertinentes pour élaborer des pièces - de microstructure homogène - simultanément en grand nombre (modification du passage du courant électrique) - de formes complexes (intervention de pièces sacrificielles). / The "Spark Plasma Sintering" process allows very high consolidation kinetics (densification, assembly, forging) of materials (powder, porous, nanostructured). However, some difficulties remains on this innovative process, particularly in terms of temperature control, microstructural homogeneity especially for complex shapes ... and industrial productivity. To solve these problems, an electro-thermo-mechanical-microstructural model is identified: i) using a thin instrumentation of the machine for the Thermo-Electric part including a reverse approach to evaluate the contact resistances, ii) by in situ creep and compression tests for mechanical microstructural-part. The resulting model has helped to find solutions for microstructural homogenization of the parts, for simultaneously densify of large numbers of parts (modifying the flow of electric current) and/or complex shapes (intervention sacrificial parts).