Influence of convection on feathery grain formation in aluminum alloys
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7/10/04
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Influence of Convection on Feathery Grain Formation in Aluminum Alloys S. HENRY, G.-U. GRUEN, and M. RAPPAZ 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.[1]
I. INTRODUCTION
FEATHERY grains are a specific growth morphology that has been reported in aluminum[1–7] and more recently in copper alloys.[8] A detailed description of this type of morphology has been given in Reference 1, and thus only a brief summary will be given here. At an intermediate scale, a given feathery grain is made of a series of parallel lamellae that are alternately separated by coherent (111) twin planes and incoherent wavy twin boundaries. These lamellae can be revealed with different colors under polarized light after Barker’s etching or by electron back scattered diffraction (EBSD) orientation imaging. At the scale of the microstructure, it has been shown that the twin planes cut primary dendrite trunks, which grow along [011] directions.[1] The wavy incoherent boundaries correspond to the impingement of secondary dendrites arms, which also grow along 110 directions. Besides a high thermal gradient (typically 100 °C/cm) and growth rate (typically 1 mm/s),[9,10,11] feathery grains are known to be favored by the presence of strong fluid flow[2,12] and a minimal concentration of certain solute elements.[10] Despite these experimental facts, the complete mechanism of feather grain formation is still unclear. A plausible mechanism was proposed recently by two of the present authors[1] and can be summarized as follows. First, under certain conditions, the preferred dendrite growth directions in aluminum are changed from the normal 100 to 110 directions. This occurs as a result of a modification of the anisotropy of the solid/liquid interfacial energy or atom attachment kinetics. Indeed, the solid/liquid interfacial energy of aluminum alloys is known to have a very weak anisotropy[13] and furthermore strongly depends on composition.[14] This anisotropy modification could thus be due to the addition of S. HENRY, Senior Research Engineer, is with the Alcan Pechiney Centre de Recherches de Voreppe, Centr’ Alp, F-38341 Voreppe Cedex, Franc
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