Silicon crystal morphologies during solidification refining from Al-Si melts

To obtain a sound basis for further development of a refining process in which Si is precipitated from aluminium melts, a series of experiments have been performed in a Bridgman furnace. Compositions studied are in the range of 17-38% Si, and experiments have been carried out with the hot zone up and with the hot zone down to study the effects of flotation and sedimentation as well as those of different convection conditions on silicon precipitation. The similarity in density between liquid aluminium and silicon crystals implies that the melt convection is important for the possible separation of the crystals in a gravity field, but the crystal morphologies also have to be controlled to avoid aluminum inclusions. At lower silicon contents the silicon morphologies have a fish-bone or star-like shape but as the Si content increases the morphology changes to a plate-like structure, which becomes coarser with increasing Si concentration. The growth pattern for coarse plates growing as (1 1 1) platelets tend to form pockets of aluminium, which will end up as inclusions during dissolution of the matrix. (C) 2010 Elsevier B.V. All rights reserved.