ALUMINUM AND AL-4.5CU ALLOY END-CHILL - STRUCTURAL OBSERVATION AND HEAT-FLOW ANALYSIS

End chill experiments were performed on aluminium and Al-4-5Cu (wt-%) in order to study the effect of melt superheat (20-150 K), chill mater ial (copper, iron, or sand), and specimen length (890-230 mm) on the t ype and size of macrostructure. Increasing melt superheat increases th e length of columnar zone, which is shorter for the alloy than for the commercial purity metal. The columnar fraction increases with the the rmal conductivity of the chill material and the heat transfer coeffici ent. The results are correlated with the temperature gradient, solidif ication rate, and growth rate obtained from a heat flow model. The col umnar to equiaxed transition is found to occur at a critical temperatu re gradient and growth rate. These critical values differ with alloy c omposition. The grain size of columnar and equiaxed grains is found to follow a power relationship with solidification rate.