ABSTRACT The effect of heat and strain during the semisolid processing route on the microstructural evolution and mechanical performance of the Al-5.5wt%Si-5wt%Zn-0.2wt%Mg alloy was evaluated. The as cast alloy was heated and partially melted to 588 °C (fs = 0.40), thixoformed (in an open die forged) and solution heat-treated (SHT) at 525 °C for 1 to 6 h. The microstructural evolution and chemical distribution (SEM-EDS) of the main alloying elements were evaluated at each step of the processing route. The SHT for 2.5 h after thixoforming showed to be adequate considering (a) the spheroidization of Si crystals, (b) the formation of Fe- and Mn-rich platelet-like particles in the eutectic phase, (c) the Mg dissolution from the Mg2Si conglomerates and (d) the homogeneous Zn diffusion along the entire part. As a consequence of the higher circularity of secondary particles, the possibility of crack formation and propagation under stress was minimized. The possible formation of solute clusters and GP zones containing Zn, Mg, Si and Cu was enabled due to the dissolution and diffusion of secondary elements during SHT, which increased the alloy’s mechanical strength. Larger Si crystals and the homogeneous spatial spread of Zn resulted in the hardness increase of the SHT alloy.

Abstract Al-Si-Zn-Mg quaternary alloys represent a potential range of feedstock's for semisolid processing (SSM) that combine the advantages of the thixoforming process with the possibility of heat treatment to improve mechanical performance. In this work, the suitability of the Al-5wt%Si-5wt%Zn-0.5wt%Mg alloy for use in thixoforming was assessed by thermodynamic characterization and by an analysis of its microstructural evolution and rheological behavior. Thermodynamic analysis was performed by numerical simulation and by application of the differentiation method to Differential Scanning Calorimetry (DSC) curves for heating cycles up to 700 °C at 5 and 25 °C/min. Microstructural evolution during partial melting at 594 °C for 0, 30, 60 and 90 s and the corresponding rheological behavior were analyzed. The findings indicate good agreement between the thermodynamic behavior predicted by DSC and Calculation of Phase Diagrams (CALPHAD) and the data obtained from semisolid forging. Although the alloy has a coarse microstructure (GSmax = 200 μm), its rheological behavior is appropriate for thixoforming (σmax = 1.06 MPa and μmax = 1.79x104 Pa.s), indicating that the microstructure only plays a secondary role in determining viscosity when enough liquid (fl = 0.63) is present in the slurry to enhance the flow. The alloy is suitable for use as raw material for semisolid processing.

RESUMO O presente trabalho visa a análise microestrutural de ligas de alumínio da série 7XXX, a saber: AA7004 e AA7075, em vista de sua aplicação nos processos de tixoconformação. As ligas foram reaquecidas às temperaturas semissólidas, ou seja, as temperaturas correspondentes às frações sólidas de 60% e 45% e mantidas nos tempos de tratamento térmico de reaquecimento de 0, 30, 90 e 210 s e na sequência submetidas a ensaios de compressão a quente entre placas paralelas. Como resultado, a condição de 60% de fração sólida de ambas as ligas apresentaram diferenças significativas na aparência das amostras tixoconformadas, com bordas mal preenchidas e quebradiças, quando comparadas com a condição de 45% de fração sólida, isto devido às ligações existentes na microestrutura que possivelmente ainda estavam fortemente ligadas entre si, exigindo maior tensão para sua deformação; a fração sólida de 45% adotada durante os ensaios foi satisfatória, pois as bordas das amostras não apresentaram trincas e/ou quebras intensas como também apresentou um bom acabamento superficial. Quanto ao seu comportamento microestrutural, as ligas semissólidas AA7004 e AA7075, apresentaram morfologia globular na região da borda das amostras com grãos/glóbulos primários bastante esferoidizados, devido aos fenômenos de ostwald ripening e coalescência e na região central das amostras grãos/glóbulos primários totalmente deformados, com pouca presença de líquido e grande presença de fase sólida, devido à deformação imposta.

ABSTRACT The present work aims at the microstructural analysis of 7XXX aluminum alloys series, namely: AA7004 and AA7075, in view of their application in thixoforming processes. The alloys were reheated to semi-solid temperatures, that is, the temperatures corresponding to 60% and 45% solid fractions and maintained to heat reheat treatment of 0, 30, 90 and 210 s and subsequently submitted to parallel plate hot compression tests. As a result, the 60% solid fraction condition of both alloys showed significant differences in the appearance of thixoforming samples, with to poorly filled and brittle edges, when compared to the 45% solid fraction condition, due to the microstructure bonds that were possibly still strongly bonded between themselves, requiring bigger tension to the forming; the 45% solid fraction adopted during the tests was satisfactory, since the edges of the samples did not present intense cracks and/or breaks, as well as a good surface finish. Regarding their microstructural behavior, the semisolid alloys AA7004 and AA7075 showed globular morphology in the edge region of the samples with very spheroidized grains/primary globules, due to the phenomena of ostwald ripening and coalescence and in the central region of the grain/primary globules samples fully deformed, with small liquid presence and large solid phase presence due to imposed deformation.

Abstract This work describes the design, construction and testing of a continuous casting device coupled to an electromagnetic stirring system for the production of thixoforming feedstock using Al-Si alloys. The following processing and construction parameters were tested: pouring temperature (660-800 °C), mould cavity roughness (Ra = 0.17-1.06 µm), casting speed (2-4 mm/s), use of a combined (primary and secondary) cooling system and the intensity and radial penetration of electromagnetic stirring (0, 1500 and 3000 W stator power). The macro-appearance of the ingots and the longitudinal and radial microstructural parameters were then evaluated. The results indicate that the surface finish of the mould cavity was an important construction parameter as it had a strong influence on heat transfer and, consequently, feed speed; use of a secondary cooling system for the mould allowed greater grain refinement and microstructural homogeneity to be achieved for all the conditions tested; optimal electromagnetic stirring power was 1500 W (EMFmax = 22 Gauss); this resulted in a grain size-to-dendritic arm spacing ratio of close to 1, which is suitable for subsequent SSM processing.

RESUMO O presente trabalho tem como objetivo contribuir para o desenvolvimento da tecnologia semissólida ao analisar as ligas Al2Si2.5Cu e Al4Si2.5Cu produzidas via deformação plástica severa pelo método ECAP (extrusão em canal angular) a fim de compreender o comportamento microestrutural destes materiais. As ligas foram submetidas a tratamentos térmicos de globularização à fração sólida de 60% e tempo de tratamento de 210 s. A caracterização microestrutural ocorreu via microscopia óptica através da análise de tamanho de glóbulo primário, tamanho de grão, fator de forma de circularidade e RQI (rheocast quality index). As ligas semissólidas processadas via ECAP foram comparadas com as técnicas tradicionais de processamento, a saber, agitação eletromagnética e técnica de ultra-refino de grãos, para a liga Al2Si2.5Cu não houve redução de tamanho de glóbulos primários e de grãos, porém para a liga Al4Si2.5Cu houve uma redução total de 53 μm para o tamanho de glóbulos primários e de aproximadamente 50μm para o tamanho de grãos, característica favorável ao processo de tixoconformação.

ABSTRACT The present work aims to contribute to the development of semisolid technology by analyzing the Al2Si2.5Cu and Al4Si2.5Cu alloys produced by severe plastic deformation - ECAP method (equal channel angular pressing) in order to understand the microstructural behavior of these materials. The alloys submitted to reheating treatment in condition of 60% solid fraction at treatment times of 210 s. Microstructural characterization by optical microscopy through analysis of primary globule size, grain size, circularity shape factor and rheocast quality index (RQI). The semisolid alloys produced by ECAP were compared with the traditional processing techniques, namely electromagnetic stirring and grain ultra-refining technique, the Al2Si2.5Cu alloy there was no reduction in size of primary globules and grains, but the Al4Si2.5Cu alloy there was a total reduction of 53 μm for the primary globules size and approximately 50 μm for the grains size, favors characteristic the thixoforming process.

RESUMO Este trabalho tem como objetivo analisar o processo de tixoforjamento em prensa excêntrica das ligas aeronáuticas AA7004 e AA7075 e compreender o comportamento do material semissólido durante o preenchimento da matriz através de análises microestruturais. As ligas foram submetidas a tratamentos térmicos de globularização pelo tempo de 210 s e fração líquida de 55% e na sequência tixoforjadas. As temperaturas de trabalho foram determinadas via ensaios de DSC (differential scanning calorimetry). O tratamento térmico de globularização tem como objetivo efetuar a globularização parcial ou total das ligas, permitindo analisar a evolução microestrutural. A força máxima aplicada no ensaio de tixoforjamento foi de aproximadamente 500 kg, sendo que a capacidade da prensa excêntrica segundo seu fabricante é de 25 toneladas, demonstrando que a carga utilizada para a realização dos ensaios foi irrisória em comparação com a capacidade do equipamento. O produto final apresentou total preenchimento da matriz e qualidade dimensional. A região localizada no centro da peça, onde ocorreu a maior deformação apresentou grãos deformados e menores em relação às outras regiões, a saber, a liga AA7004 apresentou tamanho de grãos de 142 µm e a liga AA7075 apresentou tamanho de grãos de 105 µm.

ABSTRACT The aim of this work was to analyze the thixoforging process in an eccentric press of the AA7004 and AA7075 aeronautical alloys and to understand the behavior of the semisolid material during the filling of the die through microstructural analysis. The alloys were subjected to globularization heat treatments for 210 seconds and liquid fraction of 55% and in the sequence thixoforged. Working temperatures were determined via DSC tests (differential scanning calorimetry). The globularization heat treatments aim to effect the partial or total globularization of the alloys, allowing the analysis of microstructural evolution. The maximum force applied in the thixoforging process was approximately 500 kg, and the capacity of the eccentric press according to its manufacturer is 25 tons, demonstrating that the force used to perform the tests was negligible compared of the capacity of the equipment. The thixoforged final product presented total filling of the die and dimensional quality. The part in the center of the final product, where the biggest deformation occurred, presented deformed and smaller grains in relation to the other parts, namely the AA7004 alloy presented a grain size of 142 µm and the AA7075 alloy presented a grain size of 105 µm.

ABSTRACT This work aims to evaluate different routes of raw material production to be used in thixoforming process. The semisolid materials exhibit particular structural characteristics, such the characteristic non dendritic structure. The studies for the production of near-net-shape parts have progressed together with the studies of material weight reduction on the automobile industries production lines. In general the advantages are energy saving, manufacturing steps reduction, productivity and quality increasing. Five processing routes have been investigated (conventional casting, electromagnetic stirring, grain refinement technique and different cooling rates) through microstructural characterizations, conventional metallographic and colour metallography. The material used in this study was the A356 alloy (commercial) having a hypoeutectic structure and grain size of about 750 μm and primary dendrite arm spacing size of around 80 μm, the combination of electromagnetic stirring + grain refining + high cooling rate provided the best route processing to obtain a refined structure, with grain sizes of around 240 μm, being feasible at the thixoforming process. Note also that using conventional metallography (black and white) all routes tested showed almost the same primary dendrite arm spacing size values ie had identical characteristics. Importantly, only with the characterization via polarized colour metallography is that was achieved in fact the best processing route definition.

This study involved a complete evaluation of the thixoformability of AA7004 and AA7075 alloys, from their microstructural characterization to their viscous behavior. The alloys were subjected to globularization heat treatments for 0, 30, 90 and 210 seconds in two conditions of solid fractions, 45 and 60%, and to viscosity assays under the same conditions. Heat treatments promote the globularization of primary phase particles; hence, the best viscosity results were achieved for alloys with low solid fractions heat-treated for 210 seconds. Alloys AA7004 and AA7075 showed an apparent viscosity of 10(4) to 10(5) (Pa.s). The behavior of materials in this range is similar to that of molten glass and they show high formability. However, the AA7075 alloy showed a better performance than the AA7004 due to the smaller size of its primary particles and original grains, their lower growth during reheating, and depending on the condition, their viscosity of 10(4) Pa.s, which is extremely low for thixoforming standards.