COOLING SCHEDULE EFFECTS ON THE MICROSEGREGATION IN AL-MG-SI ALLOYS

By means of a numerical simulation technique the extent of microsegregation during dendritic solidification for given cooling conditions can be predicted. A model including dendrite arm coarsening and solid state back diffusion has been developed in which a thermodynamic formulation of the ternary phase diagram is used. Following up the results for binary alloys, the influence of the shape of the cooling curve on the extent of microsegregation and the solidification path is investigated for aluminium alloys containing 0.8 to 1.2 wt.% Mg and 2.8 to 3.2 wt.% Si. The varying amounts of binary and ternary eutectic phases, dendrite arm spacings and the exact position of the solidification path on the liquidus surface are calculated for solidification times ranging from 0.1 to 10 000 seconds with three types of cooling curves (slow/fast, linear, fast/slow). Extreme changes of the cooling curve shape for a given solidification time affect the microstructural parameters as much as changes in the solidification time by several orders of magnitude for a given cooling curve shape. Therefore, predictions of microsegregation and microstructure from solidification time are valid only for a specified shape of the cooling curve.