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An experimental study has been carried out to evaluate the microstructural and microhardness evolution on the directionally solidifi ed binary Al-Cu and multicomponent Al-Cu-Si alloys and the infl uence of Si alloying. For this purpose specimens of Al-6wt.%Cu and Al-6wt.%Cu-8wt.%Si alloys were prepared and directionally solidifi ed under transient conditions of heat extraction. A water-cooled horizontal directional solidifi cation device was applied. A comprehensive characterization is performed including experimental dendrite tip growth rates (V L ) and cooling rates (T R ) by measuring Vickers microhardness (HV), optical microscopy and scanning electron microscopy with microanalysis performed by energy dispersive spectrometry (SEM-EDS). The results show, for both studied alloys, the increasing of T R and V L reduced the primary dendrite arm spacing (λ 1 ) increasing the microhardness. Furthermore, the incorporation of Si in Al-6wt.%Cu alloy to form the Al-6wt.%Cu-8wt.%Si alloy infl uenced signifi cantly the microstructure and consequently the microhardness but did not affect the primary dendritic growth law. An analysis on the formation of the columnar to equiaxed transition (CET) is also performed and the results show that the occurrence of CET is not sharp, i.e., the CET in both cases occurs in a zone rather than in a parallel plane to the chill wall, where both columnar and equiaxed grains are be able to exist.

In order to analyze the effect of the growth direction on dendrite arm spacing (1) and microhardness (HV) during horizontal directional solidification (HDS), experiments were carried out with the Al-3wt.%Cu-5.5wt.%Si alloy and the results compared with others from the literature elaborated for upward directional solidification (UDS). For this purpose, a water-cooled directional solidification experimental device was developed, and the alloy investigated was solidified under unsteady-state heat flow conditions. Thermal parameters such as growth rate (VL) and cooling rate (TR) were determined experimentally and correlations among VL, TR, 1 and HV has been performed. It is observed that experimental power laws characterize 1 with a function of VL and TR given by: 1=constant(VL) -1.1 and 1=constant(TR) -0.55 . The horizontal solidification direction has not affected the power growth law of 1 found for the upward solidification. However, higher values of 1 have been observed when the solidification is developed in the horizontal direction. The interrelation of HV as function of VL, TR and 1 has been represented by power and Hall-Petch laws. A comparison with the Al-3wt.%Cu alloy from literature was also performed and the results show the Si element affecting significativaly the HV values.

A liga Al-6%Cu foi solidificada direcionalmente sob condições transitórias de extração de calor e microestruturas dendríticas, variáveis térmicas de solidificação, ou seja, velocidade de deslocamento da isoterma líquidus (VL), taxa de resfriamento (TR) e gradiente de temperatura à frente da isoterma liquidus (GL) foram caracterizadas, determinadas experimentalmente e correlacionadas com os espaçamentos dentríticos terciários (λ3). Para tanto, foi projetado, construído e aferido um dispositivo de solidificação direcional horizontal. Os resultados encontrados mostram que leis de potência -1,1 e -0,55 caracterizam a variação dos espaçamentos terciários com a velocidade de deslocamento da isoterma liquidus (VL) e a taxa de resfriamento (TR), respectivamente. Finalmente, é realizado um estudo comparativo entre os resultados obtidos neste trabalho e aqueles publicados na literatura para ligas Al-Cu solidificadas direcionalmente sob condições transientes de fluxo de calor nos sistemas verticais ascendente e descendente.