Columnar to equiaxed transition in directional solidification of inoculated melts

Experiments on transient directional solidification were carried out with Al–3% Si and Al–7% Si alloys to study the modification by melt inoculation of the as-cast ingot macrostructure with columnar and equiaxed zones into a completely refined equiaxed structure. Without inoculation the macrostructure consists of typical columnar and equiaxed zones, separated by a relatively thin region of columnar to equiaxed transition. As the inoculant Al–3% Ti–1% B was added to the melt in a series of experiments relatively small equiaxed grains were observed in the as-cast macrostructure. For larger inoculant additions the number of these equiaxed grains increased. The macrostructure became completely refined and equiaxed as the Ti concentration in the melt increased in the small range 0.002 < %Ti < 0.01 for Al–3% Si, and of 0.029 < %Ti < 0.075 for Al–7% Si. The larger inoculant additions for the Al–7% Si alloy are attributed to Si poisoning of the inoculant. This macrostructure modification occurred with an increasingly large fraction of fine equiaxed grains mixed with columnar grains, rather than by a significant decrease in the length of the columnar grains. Solidification paths were calculated from the measured cooling curves and superimposed on dimensionless growth maps, enabling good qualitative predictions of the macrostructure, especially the existence of the mixed region. The maps, however, predict that the position where columnar grains are completely blocked is closer to the ingot base than that observed experimentally. This discrepancy might be related to the restrictive hypothesis assumed to construct the growth maps, not valid in the present experiments, of steady-state solidification conditions.