Relación entre la microestructura de la aleaciones Al-Cu solidificadas direccionalmente

Abstract

Tesis para optar al título de Magister en Ciencia y Tecnología de Materiales

En general, en un proceso de solidificación de piezas metálicas se espera que la estructura de los granos del material solidificado sea completamente columnar o completamente equiaxial. Sin embargo, bajo determinadas condiciones de solidificación se produce una zona donde se encuentran presentes simultáneamente los granos columnares y los equiaxiales, esta zona donde coexisten ambos tipos de granos se denomina zona de transición de estructura columnar a equiaxial 'TCE'. El objetivo de esta Tesis es evaluar la relación entre la macroestructura y la microestructura de aleaciones Al-Cu solidificadas unidireccionalmente, en forma ascendente, con los valores de la microdureza Vickers y los parámetros térmicos de las aleaciones. Para ello, se prepararon probetas de aleaciones Al-Cu con composiciones hipoeutécticas [Al-1 por ciento Cu, Al-4,5 por ciento Cu y Al-15 por ciento Cu] y eutécticas [Al-33,2 por ciento Cu], obteniéndose en todas las probetas la TCE. Se obtuvieron las curvas de enfriamiento de las distintas aleaciones y se determinaron los parámetros térmicos. A las probetas se le realizaron cortes longitudinales donde se midieron los parámetros metalográficos y la variación en la composición de los principales elementos de la aleación, tanto a escala macroscópica como microscópica. Además, se midieron los valores de microdureza Vickers en la longitud, en el ancho y en las distintas fases presentes. Se observó que al momento en que se produce la TCE los valores de los gradientes de temperatura alcanzan un valor mínimo y crítico, que varía entre 0,99 grados C/cm y 5,11 grados C/cm. Mientras que los valores de las velocidades de las interfases líquidas se encuentran entre 1,2 cm/min y 1,9 cm/min. Asimismo, para aleaciones de composiciones hipoeutécticas se determinó que existe un valor crítico de la velocidad de enfriamiento por debajo del cual la macroestructura de las probetas es completamente equiaxial. Además, los valores de microdureza aumentan a medida que se incrementa el contenido de soluto en la aleación y las correlaciones entre el espaciamiento dendrítico secundario y el espaciamiento eutéctico con los valores de microdureza son independientes del tipo de estructura de grano. Igualmente, se encontró que el valor de la microdureza es superior en la fase intermetálica 'Al2Cu' que en la fase alfa. Por último, se obtuvo que a medida que aumentan los valores de la microdureza se incrementan los valores de las velocidades de las interfases; en cambio, los valores de los gradientes de temperatura disminuyen a medida que se incrementan los valores de la microdureza.|In general, in a solidification process of metal pieces, it is expected that the structure of the grains becomes completely columnar or equiaxed. However, under certain conditions of solidification, a zone where equiaxed and columnar grains coexist occurs. This area is called zone of columnar-to-equiaxed transition 'CET'. The objective of this Thesis was to evaluate the relationship between the macrostructure and microstructure of Al-Cu alloys unidirectionally solidified, in an ascending way, with the Vickers microhardness values and the thermal parameters of the alloys. To do this, samples of Al-Cu alloys with hypoeutectic [Al-1per cent Cu, Al-4. 5 per cent Cu and Al-15 per cent Cu] and eutectic [Al-33. 2 per cent Cu] compositions were prepared, obtaining the CET in all specimens. Also, the cooling curves of the different alloys were obtained and the thermal parameters were determined. The samples were longitudinally cut and metallographic parameters and the variation in the composition of the main alloying elements, both in macroscopic and microscopic scale, were measured. In addition, the Vickers microhardness values were measured in the length, the width and the different phases present. Results show that the CET occurs when the values of temperature gradients achieve a minimum critical value which varies between 0.99 °C/cm and 5.11 °C/cm and that the values of the velocities of the liquid interphases are between 1.2 cm/min and 1.9 cm/min. Also, for alloys of hypoeutectic compositions, results show that there is a critical value of the cooling rate below which the macrostructure of the specimen is fully equiaxed. Further, microhardness values increased as the solute content in the alloy increased and the correlations between the secondary dendritic spacing and eutectic spacing with microhardness values were independent of the type of grain structure. Results also show that the value of microhardness is higher in the intermetallic phase 'Al2Cu' than in the alpha phase. Finally, the velocities of the interphases increased as the microhardness values increased, whereas the temperature gradients values decreased as the microhardness values increased.

In general, in a solidification process of metal pieces, it is expected that the structure of the grains becomes completely columnar or equiaxed. However, under certain conditions of solidification, a zone where equiaxed and columnar grains coexist occurs. This area is called zone of columnar-to-equiaxed transition 'CET'. The objective of this Thesis was to evaluate the relationship between the macrostructure and microstructure of Al-Cu alloys unidirectionally solidified, in an ascending way, with the Vickers microhardness values and the thermal parameters of the alloys. To do this, samples of Al-Cu alloys with hypoeutectic [Al-1per cent Cu, Al-4. 5 per cent Cu and Al-15 per cent Cu] and eutectic [Al-33. 2 per cent Cu] compositions were prepared, obtaining the CET in all specimens. Also, the cooling curves of the different alloys were obtained and the thermal parameters were determined. The samples were longitudinally cut and metallographic parameters and the variation in the composition of the main alloying elements, both in macroscopic and microscopic scale, were measured. In addition, the Vickers microhardness values were measured in the length, the width and the different phases present. Results show that the CET occurs when the values of temperature gradients achieve a minimum critical value which varies between 0.99 °C/cm and 5.11 °C/cm and that the values of the velocities of the liquid interphases are between 1.2 cm/min and 1.9 cm/min. Also, for alloys of hypoeutectic compositions, results show that there is a critical value of the cooling rate below which the macrostructure of the specimen is fully equiaxed. Further, microhardness values increased as the solute content in the alloy increased and the correlations between the secondary dendritic spacing and eutectic spacing with microhardness values were independent of the type of grain structure. Results also show that the value of microhardness is higher in the intermetallic phase 'Al2Cu' than in the alpha phase. Finally, the velocities of the interphases increased as the microhardness values increased, whereas the temperature gradients values decreased as the microhardness values increased.