Columnar to equiaxed transition during directional solidification in refined Al-based alloys

Generally, solidification leads to two types of dendritic grains: either columnar (often oriented in one direction) or equiaxed (growing in random orientation) rendering different mechanical properties. The understanding and control of the transition from columnar to equiaxed dendrite microstructure (CET) are crucial points to obtain desired final properties of casting products. The general objective of our study is to understand how the formation and the evolution of the CET are influenced by the processing parameters and the natural convection in refined Al-3.5 wt.% Ni alloys and Al-7.0 wt.% Si alloys. Various experiments are carried out to deepen our understanding on CET in a Bridgman-Stockbarger furnace for which the temperature gradient and pulling velocity can be independently controlled. We focus our interest on a precise description of CET, equiaxed grains features (grain size and elongation) after CET and associated segregation which shows that there is an oscillation of solutal concentration at the level of CET depending on applied pulling rate and thermal gradient. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.