Liquidus casting of a wrought aluminum alloy 2618 for thixoforming

A wrought aluminium alloy 2618 was cast from its liquidus temperature to obtain a fine structure of equiaxed, nondendritic primary grains. In contrast, the structures obtained from casting the alloy from above its liquidus temperature displayed the usual dendritic grains. The primary grains were characterized by image analysis which showed that the average grain size in two dimensions was similar to 44 mu m and the average circularity factor was similar to 18, indicating that the grains were both fine and globular. Some particles were also viewed in three dimensions and this showed mostly round and smooth surfaces although some protrusions could be seen to emerge on some particles. The misorientation angles between adjacent particles were measured by the electron back-scattered diffraction (EBSD) and most were > 15 degrees, indicating genuinely separate grains. The grains in the as-cast material remained fine for up to 30 min at 610 degrees C during reheating, although particle coalescence led to eventual coarsening. The liquidus cast billets were thixoformed at 610 degrees C and the microstructures after forming were examined. It is concluded that the liquidus casting is capable of producing thixotropic materials without resorting to stirring, grain refining or severe deformation and reheating. (C) 1998 Elsevier Science S.A. All rights reserved.