ABSTRACT 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.

ABSTRACT 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.

ABSTRACT The 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 copper content, which mainly determine its microstructure. The results showed an increase in the hardness of both alloys with decreasing treatment temperature; which is due to the fact that the amount of retained austenite transformed to martensite increases as the martensite finish temperature (Mf) is reached. It was also observed that a higher molybdenum content produces a metastable austenitic matrix in the as cast samples. Furthermore, the effect of Mo and other alloying elements substantially stabilizes the austenite, obtaining low amounts of martensite after the sub-zero treatment, even at -180 ° C.

RESUMEN En 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.

Este estudio explora varios factores con el fin de establecer cuáles son los más importantes en motivar a los lectores de periódicos locales a comprar, visitar tiendas y buscar información adicional acerca de los productos o servicios promovidos en los anuncios. El comportamiento durante el proceso de compra es consecuencia de una compleja interacción de dimensiones culturales, sociales, personales y psicológicas. Este proceso -el cual se produce antes de la acción- tiene implicaciones relevantes y los departamentos de mercadeo deben prestar atención a ello. Una serie de hipótesis basadas en la forma como la publicidad atrae a los consumidores y en cómo afecta la toma de decisiones al momento de la compra fueron puestas a prueba usando una encuesta que fue administrada a una muestra de 1.333 personas encuestadas en Chile. También se realizó un análisis discriminante para averiguar por qué algunos lectores de periódicos se ven motivados a comprar bienes o servicios, visitar una tienda o buscar más información. Los resultados muestran que el atractivo de la oferta anunciada es el factor más importante para explicar el comportamiento posterior del consumidor.

This study explores several factors in order to establish which are the most important in driving local newspaper readers to buy, visit shops and look for additional information about products or services promoted by ads. The behavior in the process of buying is a consequence of a complex interplay among cultural, social, personal and psychological dimensions. This process -which occurs prior to the action- has relevant implications and marketing departments should pay attention to it. A series ofhypotheses based on how advertising appeals to consumers and how it affects decision making at the time of buying are tested using a survey administered to a sample of 1,333 respondents in Chile. A discriminant analysis is also performed to find out why some newspapers readers are driven to buy goods or services, visit a shop or search for more information. The results show that the appeal of the deal advertised is the single most important factor in explaining subsequent consumer behavior.

Este estudo explora vários fatores com o objetivo de estabelecer quais são os mais importantes em motivar os leitores de jornais locais a comprarem, visitarem lojas e buscarem informação adicional sobre os produtos ou serviços promovidos nos anúncios. O comportamento durante o processo de compra é consequência de uma complexa interação de dimensões culturais, sociais, pessoais e psicológicas. Esse processo -o qual se produz antes da ação- tem implicações relevantes e os departamentos de marketing devem prestar atenção a isso. Uma série de hipóteses baseadas na forma como a publicidade atrai os consumidores e em como afeta a tomada de decisões no momento da compra foi provada usando um questionário que foi administrado a uma amostra de 1.333 pessoas no Chile. Uma análise discriminadora também se realizou para averiguar por que alguns leitores de jornais são motivados a comprarem bens ou serviços, visitarem uma loja ou buscarem mais informação. Os resultados mostram que o atrativo da oferta anunciada é o fator mais importante para explicar o comportamento posterior do consumidor.