Primary Dendrite Growth of Al–Cu Alloy Films between Solid Plates

Abstract

Bulk and film-like Al-15% Cu alloy specimens were unidirectionally solidified in a graphite crucible to investigate the effects of thermal conditions and sample geometry on the solute redistribution around dendrite tips and the formation process of solidification structure. Primary dendrite arm spacings, d 1 , of bulk Al-15%Cu alloy specimens are given by Kurz-Fisher's and Hunt's equations as a function of alloy composition, growth rate and temperature gradient. On the other hand, d 1 in film-like specimens are 2∼ 3 times as large as those in bulk specimens. This enlargement of d 1 is explained from the viewpoint of the geometrically constrained solute diffusion layer in the film-like specimen. The Cu concentration at the dendrite tip is also estimated using the measured Cu distribution in a primary arm. The experimental effective distribution coefficient k e , the ratio of Cu content at the dendrite tip to the initial Cu content of alloy, is a little higher than the predicted ones from Kurz-Fisher's and Trivedi's theories, indicating that a small shift should be made on equilibrium partition coefficient k in the Al-Cu phase diagram. Besides, Cu distribution is higher in the film specimen than that in the bulk specimen, which should occur because of very limited solute diffusion ahead of the freezing front of the film specimen.