Industrial thixoforming processes are carried out within a second. Characterisation of the rheological behaviour of the semisolid metal slurry during this short space of time is therefore important. Rheological experiments were carried out on Sn 15%Pb alloy (fraction solid 0.2-0.5) in a HAAKE cylindrical rotational viscometer using ~ 1 kHz data collection rate to obtain the shear stress response to a rapid change in shear rate. Results and conclusions based on them are valid over a limited range of 0 s·I-200 S·l. The slurry undergoes an initial rapid structural breakdown during a jump upwards in shear rate, followed by a more gradual breakdown. The former occurs within a second of the start of the jump. The metal slurry breaks down more rapidly with a higher final shear rate, but is independent of the initial shear rate. The reverse is found with downward jumps in shear rate: recovery times increase with increasing final shear rate. Again, it is independent of the starting shear rate. The rheological behaviour of aluminium alloys with solid fraction >0.5 at 1 to 100 s'\ was studied by rapidly compressing cylindrical slugs against a load cell. The slurry exhibits a near-Newtonian behaviour at the thixoforming temperature, after an appropriate soaking time is employed and a near-spheroidal microstructure develops. Use of image analysis to quantify the spheroidicity indicates that particle shape affects the flow of the slurry, a higher spheroidicity corresponding to a lower load. Using solid-state mechanical deformation to break up the dendrites, produces thixoforming feedstock with lower resistance to flow than magnetohydrodynamic stirring. Power law index values were obtained for both rapid compression and viscometry experiments. Results from this work have shown the importance of understanding the rheological behaviour of a semisolid slurry in the transient state.

<p>In the last forty years, most of researches in casting fields especially in semi-solid metal state were dedicated to find new ways to enable near net shaped casting of Al alloys to improve the product properties and decreases the product cost. The thixoforming and rheocasting processes are presented as a ways by which the microstructure of the alloys can be changed to non-dendritic microstructure leading to improve the mechanical properties by mitigating the defect associated with the dendritic microstructure. Unfortunately, these processes have proved to be capital cost prohibitive and complicated for commercial production. Further, near net shaped casting of Al wrought alloys along with the superior properties and performance of these alloys have been a challenge for conventional casting routes due to the main disadvantage of hot tearing or hot cracking during solidification, which renders the cast component ineffective. To overcome the disadvantages of thixoforming and rheocasting processes, Controlled diffusion solidification (CDS) process was innovated to enable casting aluminum alloys with a non-dendritic morphology of the primary Al phase in the resultant cast microstructure and thus alleviating the problem of hot tearing and obtaining a cost effective product with improved mechanical properties. The CDS is a simple process involving mixing of two precursor alloys of different thermal masses (temperature and solute) and subsequently cast the resultant mixture of the desired solute composition and temperature as a near net shaped cast product. The process lends itself to easy commercialization with a marginal capital cost required for set up such as the addition of an extra holding furnace. Further, the CDS process would prove itself to be unique in its ability to cast Al based wrought alloys into near net shaped components without additional processes and cost.</p><p>The CDS process has been proven to yield a cast product with a non-dendritic Al phase morphology and this dissertation presents the in-depth details and analysis of the various events occurring during the process to obtain a successful cast part. The process involves various inter-related events such as mixing, re-distribution of thermal field, redistribution of solute field, three types of nucleation events and growth of these different nuclei. Further the dissertation aims to present a study of the critical parameters such as temperatures of the two pre-cursor alloys, initial mass ratio of these alloys and the rate of mixing them on the effectiveness of the CDS process.</p> <p>The results from this study shows that mixing two precursor alloys to form the final desired alloy presents a natural environment for copious nucleation events aided by distribution of these nuclei by forced convection followed by the formation of unique cells in the resultant mixture (micro-scale) with significant thermal and solute gradients. The solidification in the CDS process is unique and different from conventional casting process in that initial growth of the nuclei takes place with the solute diffusing towards and temperature diffusing away from the solid/liquid interface which presents a favorable environment for a stable unperturbed growth of the solid/liquid interface resulting in a non-dendritic morphology of the primary AI phase.</p><p>The proposed events in the CDS process has been verified with a few Al based wrought alloys and organic alloy systems.</p> / Thesis / Doctor of Philosophy (PhD)

Techn. Hochsch., Diss.--Aachen, 2006.

Zugl.: Aachen, Techn. Univ., Diss., 2008

Aachern, Techn. Hochsch., Diss., 2003.

Techn. Hochsch., Diss., 2004--Aachen.

Ce travail vise à mettre en évidence la robustesse du procédé de thixoformage des aciers via létude qualitative et quantitative de linfluence des différents paramètres impliqués dans le processus. Le thixoformage est la mise en forme des métaux à létat semi-solide et à microstructure globulaire. Dans ces conditions, il a été montré que la consistance du matériau lors de la mise en forme diminue quand la vitesse de cisaillement augmente et que les paramètres régissant son comportement sont dépendants du temps. Etape-clé de cette technologie, le chauffage est étudié au même titre que la déformation. Linfluence de la stratégie de chauffage, de la géométrie de linducteur et des conditions extérieures est déterminée à laide de modélisations et dessais de réchauffage de lopins. Le transport entre le four et la presse est également abordé. Pour étudier la sensibilité de létape de déformation, trois géométries sont utilisées : le filage direct, une pièce détude rhéologique et une pièce industrielle. Ici encore, les essais pratiques et la modélisation sont employés de manière complémentaire. Les effets de la température du lopin et de celle des outils, de la vitesse de déformation, de la lubrification et des effets thermiques sont ainsi analysés. Comme pour le chauffage, la répétabilité de la déformation est mise en évidence. Par ailleurs, les défauts et structures obtenus dans les pièces sont présentés avant de discuter de la thixoformabilité des différentes nuances dacier. En particulier, la qualification dalliages spécialement dessinés pour le thixoformage ou au contraire réputés difficiles à mettre en oeuvre par cette technique est discutée. Finalement, le problème de la durée de vie des outillages est abordé puisque celui-ci est le dernier obstacle au développement de la technologie. / This work aims to assess the robustness of the steel thixoforming process by a qualitative and quantitative study of the influence of the parameters involved in the process. Thixoforming is the shaping of metals in the semi-solid state and with a globular microstructure. In this case, one shows that consistency decreases when shear rate increases and that the process parameters leading the behaviour are time dependent. As a key-step of the technology, heating is studied as well as forming. Influences of heating strategy, inductors geometry and external conditions are determined by the help of modelling and heating experiments. Handling system between heating furnace and press is also tackled. To study the formings relability and sensitivity, three geometries are used : direct extrusion, a rheological study part and an industrial part. Again, practical experiments and modelling are used in a complementary way. Effects of slugs temperature, forming speed, lubrication and tools temperature are analysed. Repeatability of heating and forming are highlighted. The defects and structures obtained in the parts are presented before discussing the thixoformability of steel grades. In particular, qualification of especially designed grades as well as non adapted grades is discussed. Finally, the tooling wear and lifetime issues are detailed as it is one of the mean locks of the technology industrialisation.

Orientador: Maria Helena Robert / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-07-23T20:58:55Z (GMT). No. of bitstreams: 1 Bubenik_RubensLuiz_M.pdf: 9820006 bytes, checksum: 74e4259834492d5dafe6eb6b382dbc4f (MD5) Previous issue date: 1998 / Resumo: Este trabalho trata do projeto e construção de um equipamento capaz de alterar a morfologia da fase primária de ligas metálicas em solidificação, diferenciando-a das estruturas dendríticas produzidas na solidificação convencional. Neste processo uma força externa provoca movimentos no metal líquido com objetivo de alterar a morfologia de crescimento do sólido, resultando em estruturas pré-reofundidas. No equipamento idealizado é utilizado um campo magnético girante para fornecer a força necessária ao movimento. O trabalho envolve projeto, construção e montagem de circuitos elétricos, circuitos eletrônicos e dispositivos mecânicos, bem como testes de eficiência do equipamento na produção de estruturas pré-reofundidas da liga AA 2011. Para testes foram variados o tempo de agitação do líquido (230 e 300 s), e o modo de agitação (normal e com reversão), sendo mantidas constantes a potência (1040 W) e a taxa de resmamento ('20 GRAUS¿/min.). Os resultados obtidos atestam a eficiência do equipamento na obtenção de lingotes com macroestrutura equiaxial refinada, homogênea em todo o seu volume, e microestrutura apresentando dendritas fragmentadas, com fator de forma de 2 a 3 para todas as condições analisadas. Estes valores de fator de forma podem ser considerados excelentes para materiais a serem utilizados como matéria prima para a conformação no estado pastoso reofundido / Abstract: This work relates the design and development of an equipment capable of modifying the morphology of primary phase during solidification of metal alloys, differentiating such structures ftom those dendritic obtained in conventional solidification. In this process turbulence is imposed to the liquid metal by means of a external force with the purpose of modifying the morphology of the growing solid, resulting in pre-rheocast structures. In the idealized equipment a rotating magnetic field is used to provide the necessary stirring in the Liquid. This work involves design, building and assembling of the equipment, as well as preliminarytests to analyse the efficiency of the equipment to produce ingots of the alloy AA 2011 with pre-rheocast structures. Different stirring times (230 and 330 s) and type of movement (normal or reverted) were utilized in the tests, while power and freezing rate in the material were kept constant (1040 W and '20 DEGREES¿/min., respectively). Results showed the efficiency of the designed equipment in the production of ingots with fine, equiaxial and homogeneous macrostructure and microstructures presenting highly ftagmented dendrites, with values of morphology index varying ftom 2 to 3 for all operational conditions tested. Those values of morphology index can be considered excellent for a material to be utilized as raw material for forming in the semi-solid state or thixoforming / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica

Orientador: Maria Helena Robert / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-11-08T18:39:47Z (GMT). No. of bitstreams: 1 Silva_RenatoRafaelda_M.pdf: 15646556 bytes, checksum: 19ff8cd2fd9081ac7985bc44c94795f5 (MD5) Previous issue date: 2008 / Resumo: Este trabalho teve como objetivo estudar o processo de produção de esponjas metalicas por tixoconformacao em pré-formas removiveis e caracterizacao mecanico-metalurgica do produto, visando a compreensão da influencia de parametros de processo nas suas caracteristicas estruturais e propriedades. A liga empregada, AA2011, foi infiltrada sobre pre-formas de particulas de NaCl de granulometrias distintas, sinterizadas ou soltas. As esponjas metalicas produzidas foram caracterizadas quanto a sua arquitetura, propriedades fisicas (densidade real e relativa, condutividade e difusividade termicas) e mecanicas (tensão de compressão, modulo de Young, tensão no plato, deformação e energia de impacto absorvida); foi analisada a influencia do tipo de pre-forma utilizada para produção das esponjas, nestas caracteristicas e propriedades. Os resultados mostraram boa reprodutibilidade do processo, principalmente quando da utilização de pre-formas de partículas medias e grosseiras nao sinterizadas. A densidade da esponja e a espessura da parede celular apresentam tendencia a aumentar com o aumento das dimensões das celulas produzidas em preformas de particulas mais grosseiras. Todas as amostras produzidas apresentaram condutividade termica da ordem de 10x inferior e difusividade termica da ordem de 4x superior as do metal maciço. O comportamento das esponjas em compressão estatica ou dinamica se mostrou tipico de materiais celulares, com reduzidos valores de modulo de Young e de tensão de compressão, grande plato de deformação plástica sem acréscimo de tensão, e elevados valores de deformação total e energia absorvida no impacto / Abstract: The aim of this work was the analysis of the thixoforming process to produce metallic sponges and the mechanical and metallurgical characterization of the product, searching for better understanding the influence of processing parameters in the material structural characteristics and properties. The aluminium alloy AA2011 was infiltrated in the semi-solid state into performs of NaCl particles with different sizes and in sintered and non sintered conditions. Cellular products were characterized concerning internal architecture, physical properties (actual and relative densities, thermal conductivity and diffusivity) and mechanical properties (Young's modulus, compressive strength, plateau stress and absorbed impact energy); it was analyzed the influence of processing parameters on the properties and characteristics of the produced sponges. Results showed good reproducibility of the process, mainly when medium and coarse non sintered space holder particles were employed. Density of the product as well as cell wall thickness increases as space holder particles sizes increase. All the sponges produced showed low thermal conductivity (~10x inferior compared to the bulk material) and high thermal diffusivity (~3x superior compared to the bulk alloy). Results of static and dynamic compression tests showed typical cellular material behaviour in all cases, presenting low values for Young's modulus and compressive strength, a well defined plateau of plastic deformation, high plastic deformation and high capacity of energy absorption in impact events / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica

Orientador: Maria Helena Robert / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-11-08T18:37:47Z (GMT). No. of bitstreams: 1 Delbin_Daniel_M.pdf: 8551361 bytes, checksum: 770e9d00eeb959cc6e40cf3f314faa4e (MD5) Previous issue date: 2006 / Resumo: Neste trabalho é estudada a viabilidade de produção de esponjas metálicas pela conformação de pasta tixotrópica da liga A2011, sobre camada de agente bloqueador (NaCl) posteriormente removido para formação da porosidade. Investiga-se a influência da temperatura de tratamento térmico, para obtenção da pasta semi-sólida, e da granulometria do agente bloqueador, na estrutura formada do material celular (aspecto geral, caracterização qualitativa e quantitativa dos poros, microestrutura da parede celular e densidade) e nas características de processo (forças de tixoforjamento e capacidade de penetração da pasta). São produzidos cilindros de material poroso metálico tipo sanduíche, compostos de camada porosa entre camadas maciças da liga, com três diferentes classes de porosidade: fina, média e grosseira. As esponjas foram submetidas à tomografia computadorizada e à análise metalográfica para sua caracterização estrutural. Os resultados obtidos mostram a viabilidade de obtenção de esponjas metálicas utilizando a tecnologia de semi-sólidos e o tipo de bloqueador utilizado. O sucesso do processo depende da fração líquida presente na pasta metálica tixotrópica, reduzida fração líquida pode resultar em incompleta infiltração e compressão das partículas do agente bloqueador. Nas condições analisadas a granulometria do agente bloqueador não teve influência sensível na qualidade do produto. A densidade do material poroso aumenta com o aumento da temperatura de processo, devido ao aumento da espessura de paredes metálicas na estrutura porosa / Abstract: The work analyses the possibility of the production of cellular material by pressing the A2011 alloy in the thixotropic semi-solid state, into a space holder pre-form. Space holder particles used are NaCl, which are removed from the product after the forming operation, resulting the porous material. It is investigated the influence of the thixoforming temperature and the size of space holder particles, in the structure of the obtained porous material (general aspect, quantitative and qualitative characterization of porosity, microstructure of cell walls and density of the product), as well as in the processing characteristics (required forces for infiltration, penetration ability of the slurry in the salt pre-form). Cylindrical samples, sandwich type, with a porous layer inserted between layers of compact alloy are produced, presenting three different ranges of porosity. The cellular material obtained contains opened porosity, being characterized as sponge. Products were analyzed by tomography and metallographic techniques. Results show that the proposed process is able to produce acceptable porous material, with a simple and low cost technique. The quality of the product depends rather on the processing temperature than on the size of space holder particles. Low liquid fraction in the thixotropic slurry can lead to incomplete infiltration and deformation of the pre-form. In the analyzed conditions, influence of the size of space holder particles could be observed neither in the processing ability nor in the quality of the product. Density of produced porous material increases as processing temperature increases, due to the increase of thickness of cell walls / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica