The effect of the application of forced convection during the continuous solidification of Al-Si alloys has been presented in this work. The investigation has been made on the hypothesis that as an alloy solidifies under the action of a rotating magnetic field, a nondendritic casting structure would be obtained. The Al-Si alloy has been studied in its three different states (original, modified, and modified with a grain refiner), using a magnetic stirring equipment with permanent magnets, a device that has allowed the study of the effect of the rotational speed (degree of stirring) on the micro-and macro-solidification structure, as well as on the material's mechanical properties. With the purpose of validating the hypothesis, different analyses and tests have been carried out, such as thermal analysis of the solidification process, micro-and macrostructural analysis, and hardness measurements for the different process conditions studied. The results have shown that magnetic stirring affects the cooling curves, expanding the solidification range. A microstructural evolution of the alloys has also been observed, from a 100% dendritic structure to a mature rosette type structure, in addition to a notable decreasing of grain size, which results in the improvement of the material's mechanical behavior.

ResumenEn este trabajo se estudia el comportamiento reológico y la evolución microestructural de la aleación AA6063 sometida a dos vías de procesamiento diferentes: deformación en frío y fusión parcial (proceso SIMA) y agitación magneto hidrodinámica durante su solidificación (proceso MHD). Se estudió la evolución de la microestructura durante el mantenimiento isotérmico, con el fin de verificar si los mecanismos de maduración de Ostwald, crecimiento y coalescencia clásicos, son aplicables a aleaciones fabricadas por estas vías de procesado. Las propiedades reológicas se evaluaron utilizando un reómetro de compresión entre placas paralelas con captura digital de datos de posición y tiempo. Los ensayos de compresión se realizaron sobre cilindros cortos extraídos de lingotes que presentaban: una microestructura dendrítica típica de colada, una microestructura típica de un material deformado en frío y una microestructura de un material obtenido por proceso MHD. Se comprobó que una microestructura globular posee el comportamiento típico de un fluido cuando se conforma en estado semisólido, no así una microestructura dendrítica de colada. Además, a través de análisis metalográfico, se verificaron los mecanismos que operan en la evolución microestructural durante los mantenimientos isotérmicos. Palabras claveProceso SIMA.. Proceso MHD. Reología. Aleación de aluminio.Microstructural evolution and rheological properties of AA6063 alloy produced by semisolid processing (SIMA and MHD) AbstractIn this work the rheological behaviour and the microstructural evolution of alloy AA6063 submitted to two different processing routes were studied: cold deformation and partial fusion (SI-MA process) and magneto hydrodynamic stirring during its solidification (MHD process). The microstructural evolution during the isothermal holding was studied to verify if the Ostwald ripening mechanisms, classic growth and coalescence, are applicable to alloys made by these processing routes. The rheological properties were evaluated using a compression rheometer with parallel plates and digital capture of position and time data. Compression tests were made in short cylinders extracted from ingots that showed: a dendritic microstructure typical of as cast material, a typical microstructure of cold deformed material and a microstructure of materials obtained by MHD process. It was found that a globular microstructure has a typical behaviour of a fluid when being formed in semisolid state, contrary to the behaviour of the as cast dendritic microstructure. In addition, the mechanisms that operate in the microstructural evolution during the isothermal holdings were verified, through metallographic analysis. KeywordsSIMA process. MHD process. Rheology. Aluminium alloys * Trabajo recibido el día 8 de marzo de 2006 y aceptado en su forma final el día 17 de noviembre de 2006.

RESUMENEn el siguiente trabajo de investigación se analizó el efecto de la temperatura de tratamiento sub-cero sobre la transformación de austenita retenida a martensita en dos fundiciones blancas de alto cromo ASTM A532 Clase II Tipo B. El tratamiento sub-cero aplicado a cada aleación, en estado de colada, fue realizado a temperaturas de -40, -65 y -180 °C con un tiempo de mantenimiento de 10 minutos; transcurridos los 10 minutos cada probeta fue expuesta a temperatura ambiente. Se realizó análisis metalográfico mediante microscopia óptica, microscopía electrónica de barrido (SEM) y análisis espectroscopia de energía dispersiva (EDS) a ambas aleaciones en estado de colada, con el objetivo de caracterizar los micro-constituyentes presentes en cada aleación. La caracterización metalográfica de las probetas sometidas a tratamiento sub-cero fue realizada mediante microscopia óptica. Además, se midió la dureza Brinell de ambas aleaciones, en estado de colada y sometidas a tratamiento sub-cero.Las aleaciones en estudio presentan diferencias importantes en su composición química, principalmente en el contenido de molibdeno y cobre, elementos determinantes en la microestructura de éstas. Los resultados obtenidos muestran un aumento en la dureza de ambas aleaciones conforme disminuye la temperaturas de tratamiento; esto, producto de un aumento en la cantidad de austenita retenida transformada a martensita a medida que se alcanza la temperatura de fin de la transformación martensítica, Mf. Se observó que un mayor contenido de molibdeno permite la obtención de una matriz austenítica metaestable de colada. Además, el efecto de este elemento en conjunto con los demás elementos de aleación fue estabilizar la austenita, obteniendo bajas cantidades de martensita al final del tratamiento sub-cero, incluso a -180 °C.Palabras clave: fundición blanca alto cromo, tratamiento sub-cero, austenita retenida, martensita. ABSTRACTThe effect of the sub-zero treatment temperature on the transformation of retained austenite to martensite in two high chromium white cast irons (ASTM A532 Class II Type B) was studied. The sub-zero treatment applied to each alloy in the as cast condition was performed at -40, -65 and -180 ° C for 10 minutes; after the holding time was completed the samples were cooled to room temperature. The microstructure of the as cast condition was analyzed by optical microscopy (LOM) and scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis was also performed with the aim to identify the microconstituents initially present in each alloy. The metallographic characterization of the specimens subjected to sub-zero treatment was performed by optical microscopy. Furthermore, Brinell hardness was measured in the as cast and sub-zero treated samples.The alloys studied have important differences in chemical composition, mainly in the molybdenum and cop-

The effect of the application of forced convection during the continuous solidification of A356 aluminum alloy over microstructural parameters of AlFeSi phases has been presented in this work. The investigation has been made on the fact that as an alloy solidifies under the action of a rotating magnetic field; a non-dendritic casting structure will be obtained. The Al-Si alloy has been studied using magnetic stirring equipment with permanent magnets. This device has allowed the study of the rotational speed (degree of stirring) effect on the micro-and macro-solidification structure. Different analyses and tests have been carried out, such as micro-and macro-structural analysis. The results have shown that magnetic stirring affects the cooling curves, expanding the solidification range.Microstructural evolution of the alloys has also been observed, from a 100% dendritic structure to a mature rosette type structure, in addition to a notable grain size decrease, which may improve the material's mechanical behavior. The results show that all the microstructural parameters of the AlFeSi phases (quantity, size, shape factor, particle area, and aspect ratio) decrease when stirring is applied to the alloy during its solidification.