RESUMO Este artigo apresenta um estudo teórico-experimental com o objetivo de determinar o coeficiente de transferência de calor interfacial, as variáveis térmicas de solidificação, os espaçamentos dendríticos primários e secundários bem como a microdureza Vickers da liga Al-11%Si solidificada direcionalmente, sob condições transientes de fluxo de calor, em um dispositivo de configuração horizontal refrigerado a água. Para o monitoramento dos perfis de temperatura utilizou-se um conjunto de termopares tipo K localizados em diferentes posições a partir da interface metal-molde, considerando a linha central correspondente ao eixo longitudinal do lingote. Uma técnica numérica, que compara perfis térmicos teóricos e experimentais, foi utilizada no cálculo do coeficiente de transferência de calor na interface metal/molde durante a solidificação cuja condição transiente é representada por uma equação analítica na forma de potência. Foram aplicadas técnicas experimentais para o cálculo das variáveis térmicas de solidificação, ou seja, deslocamento da frente de solidificação, velocidade da isoterma liquidus e taxa de resfriamento. Os resultados experimentais e simulados apresentaram uma concordância muito boa e possibilitaram estabelecer correlações analíticas entre tais variáveis térmicas e os espaçamentos dendríticos primários e secundários obtidos, por meio de equações na forma de potência representadas por expoentes característicos. Finalmente, são também apresentadas expressões experimentais capazes de correlacionar os valores dos espaçamentos dendríticos estudados com o comportamento da microdureza Vickers da referida liga.

ABSTRACT This paper presents a theoretical-experimental study aiming to determine the interfacial heat transfer coefficient and solidification thermal parameters from experimentally obtained data correlating these with the primary and secondary dendritic spacings as well as microhardness during the horizontal solidification of the Al-11wt.%Si alloy under transient heat flow conditions. For the experimental procedure, a water-cooled solidification device was used which allows the horizontal directional solidification and the monitoring of the temperature profiles in different positions from the metal-mold interface considering the center line corresponding to the longitudinal axis of the ingot. A numerical technique which compares theoretical and experimental thermal profiles has permitted the evaluation of the variation of metal-mold heat transfer coefficient along the solidification. The interfacial heat transfer coefficient of studied alloy is represented by a equation which show the time dependence during the process. The experimental and calculated values have shown a very good agreement. Experimental techniques were applied to calculate the following solidification thermal variables: solidification front displacement, growth rate and cooling rate. The experimental results allowed establishing a quantitative correlation between such thermal variables and the studied structural parameters by means of power type equations represented by characteristic exponents. Finally, correlation between the values of both primary and secondary dendritic spacings found with Vickers microhardness behavior for the studied alloy is also presented.

Aluminum alloys of the ANSI series 319 present Si and Cu as the main alloying elements and the mechanical strength of these alloys can be improved by the precipitation of the metastable Al2Cu phase during the ageing heat treatment. In this paper, the Al-5.5wt.%Si-3wt.%Cu alloy was elaborated and solidified in a water-cooled horizontal directional solidification device. The as-cast ingot was subjected to the precipitation hardening heat treatment (T6 heat treatment), which consisted of: solution for 5 h at 490°C±2°C, followed by quenching in water at 60°C±2°C and ageing for 3 h at 155°C±2°C, and cooling-air. Secondary dendrite arm spacing (λ2) measurements were carried out before and after T6 heat treatment. The mechanical strength of the alloy was investigated by the microhardness test. It has been found that the heat treatment did not influence the λ2 values, however, highest HV values have been observed for the heat-treated samples.

Abstract This work presents an analytical method for the study of the solid state diffusion process in binary systems of two phases with cylindrical radial atomic flux. The method is developed from the differential equation that describes Fick's second law that is modified by geometric function and suitable changes of variables. The modified differential equation is solved by using a well-known closed form solution based on the error function, and then analytical equations are obtained to analyze the diffusion interface position as a function of time, and the concentration profiles as a function of time and position. The predictions provided by the analytical method are compared with numerical results.

In order to investigate the effect of natural convection in columnar to equiaxed transition (CET), Al-3.0wt.%Cu and Al-3.0wt.%Cu-5.5wt.%Si alloys ingots were obtained during the transient horizontal directional solidification (THDS). Aiming to analyze the effect of superheat in the formation of the macrostructure in ternary Al-Cu-Si alloy, the experiments were conducted with three superheat temperatures above the liquidus temperature of the ternary alloy. A water-cooled solidification experimental device was used. Continuous temperature measurements were made during solidification at different positions in the casting and the data were automatically acquired. Thermal analysis has been applied to determine the thermal parameters such as growth rate (VL), cooling rate (TR) and temperature gradient (GL), whose values have been interrelated with the CET. The observation of the macrostructures has indicated that the resulting thermosolutal convection combined with superheat seem to favor the transition, which did not occur in a single plane, for all ingots obtained, i.e., it has been seen in a range of positions in ingots. The addition of Si element in binary Al-Cu alloy anticipates the CET. A comparison with experimental results for CET occurrence in different growth directions has been carried out.