O processo de conformação de ligas no estado semi-sólido (ESS) é uma tecnologia que, devido às vantagens, é uma realidade na indústria automobilística em especial para ligas de Al-Si. Um dos aspectos mais importante, nesse processo, é o comportamento reológico dessas ligas, pois, tal comportamento afeta diretamente as condições de fluxo e, também, as propriedades mecânicas finais das peças tixoconformadas. Uma maneira empírica para avaliar esse parâmetro é através do teste de fluidez. O objetivo do presente trabalho foi estudar a influência do tempo de espera sobre a fluidez da liga A356, no estado semi-sólido (ESS). Amostras foram semifundidas até uma temperatura de 590ºC, processo este seguido de tempos de espera de 10min e 30min. A liga refundida foi injetada em uma matriz com um longo canal contínuo. Os resultados obtidos foram discutidos, tendo, como base, teorias preexistentes para sistemas de coexistência sólido/líquido. Parâmetros microestruturais foram medidos, com o intuito de explicar os fenômenos observados. O grau de fluidez da liga A356 sofre intensa influência do tempo de espera no ESS.

Semi-solid state forming process (SS) is a technology that, due its benefits, is a reality for automotive industry applications, especially for aluminum silicon alloys. In this forming process, one of the most important aspects is alloy rheological behavior because it directly affects the flux condition and mechanical properties of tixoformed parts. An empiric way for measuring this parameter is the fluidity test. The aim of this work was to study the influence of waiting time on the fluidity of an A356 alloy in a semi-solid state. Samples were heated until partial melting at 590ºC, followed by waiting times of 10min and 30min. The alloy was injected into a long and continuous channel. The results obtained were discussed using pre-existent theories for semi-solid state systems with coexisting solid and liquid phases. Some microstructural parameters were measured to explain theobserved phenomena. The degree of fluidity of A356 alloy is strongly affected by waiting time.

The present work examines microstructure, tensile properties and quality (porosity level) of two thixoformed products, namely an assembly of three cylindrical rods from which tensile samples were machined out, and a cup-shaped part. The material is an A356 aluminium alloy and the microstructural conditioning was carried out by the deformation - recrystallization technique. Additionally, the microstructures of an A356 ingot produced by the magnetic stirring process was also investigated. Results showed that solid fraction and soaking time in the semi-solid state determine the thixoforming forces, that very low porosity levels can be achieved and minimized by the maintenance of high pressures during solidification, and that the material flow during injection in the mould is relatively homogeneous, except for the material conditioned by magnetic stirring. Physical simulation experiments of the thixoforming process (rods) showed a strong correlation between liquid segregation and porosity level. Finally, the tensile properties were shown to be much higher that those of permanent cast samples.