Influence of convection on feathery grain formation in aluminum alloys

The influence of convection on the formation of feathery grains, i.e., of columnar grains made of twinned dendrites growing along 〈110〉 directions, in AA1050 aluminum alloys has been studied. Round billets have been semicontinuously cast in a mold equipped with lateral liquid feeding. The fluid flow pattern in the liquid sump has been modeled using Fidap software. Feathery grains have been observed in the region opposite the mold entrance, i.e., in regions where the change of the velocity field (shearing rate) is the highest. Electron back scattered diffraction (EBSD) maps of two feathery regions, which were symmetric with respect to the liquid flow pattern, showed clear symmetry relationships. Furthermore, the 〈110〉 secondary dendrite arms had grown in directions opposite to the fluid flow. This experimental evidence brings more experimental support to the mechanism of feathery grain formation proposed earlier by Henry et al.