Comparison of thixoformability of AA6082 reheated from the as-cast and extruded states

It is of great technological interest to reduce the feedstock costs in order to establish thixoforming in the manufacture of wrought aluminium parts. Since thixoforming temperatures are much higher than those employed in homogenization treatments, thixoforming experiments with cast AA6082 alloy were performed to investigate if the homogenization step in the production of 6082 thixotropic feedstock could be omitted. Cast AA6082 billets without a homogenization treatment were thixoformed directly in the as-cast state and after extrusion. While the extruded 6082 feedstock was readily thixoformed at 644 degrees C, it took a slightly higher forming temperature, 646 degrees C, to completely fill the die with the as-cast material. The former enjoyed the extrusion deformation which helped to improve the grain structure of the feedstock upon heating to the semisolid temperature range, producing polygonal alpha-Al grains of uniform size. The as-cast material, on the other hand, suffered substantial grain coarsening in the semisolid temperature range. The heterogeneous grain structure with some very coarse grains thus obtained was not as thixoformable as the extruded material. The part thixoformed from the cast 6082 alloy was inferior with respect to that from the extruded material which revealed a homogeneous structure and was perfectly sound with no evidence of macro and microporosity. (C) 2007 Elsevier B. V. All rights reserved.