In order to investigate the solute distribution and freckles formation during
directional solidification of superalloy ingots, a mathematical model with
coupled solution of flow field, solute and temperature distribution was
developed. Meanwhile, the reliability of this model was verified by the
experimental and simulation results in relevant literatures. The
three-dimensional directional solidification process of
Ni-5.8wt%Al-15.2wt%Ta superalloy ingot was simulated, and then the dynamic
growth of solute enrichment channels was demonstrated inside the ingot.
Freckles formation under different cooling rates was studied, and the local
segregation degree inside the ingot was obtained innovatively after
solidification. The results show that the number of freckles formed at the
top gradually decreases, and so do the degree of solute enrichment at these
freckles with the increase of cooling rate. Moreover, the relative and
volume-averaged segregation ratio is defined to describe the segregation
degree inside the ingot. The span of relative segregation ratio for positive
segregation is wider than that for negative segregation, but it accounts for
less of total volume. As the cooling rate increases from 0.1 K/s to 1.0 K/s,
the proportion of weak segregation (-20%~20%) increases significantly from
26% to 41%, so that the segregation degree is weakened in general. By
analyzing the freckles formation and segregation degree inside the ingot,
the numerical simulation results can provide a theoretical basis for
optimizing the actual production process to suppress the freckle defects.
Numerical investigations on solute transport and freckle formation during directional solidification of nickle-based superalloy ingot