For the purpose of the prediction of casting structures, heterogeneous nucleation rate in the
undercooled melt of solififying Al-Si alloys were evaluated by comparing experimentally observed
macrostructures of solidified ingots with numerically simulated ones. Molten alloys were
unidirectionally solidified in an adiabatic mold from a steel chill block located at the bottom of the
mold. In the experiment, columnar to equiaxed transition (CET) was observed. A numerical
simulation for grain structure formation of the sample ingots was carried out using a cellular
automaton (CA) method, and heterogeneous nucleation rate in the solidifying alloys were evaluated
by producing the similar structures to experimental ones. An attempt was made to predict the grain
structure of conventionally cast ingots using the evaluated heterogeneous nucleation rate. However,
the simulation could not predict the structure of ingot with low superheat due to crystal
multiplication near the mold wall. The crystal multiplication mechanism, so-called "Big Bang
mechanism", was introduced into the simulation and the simulation could predict the grain
macrostructure composed of columnar and equiaxed crystals that were similar to experimentally
observed one.
Grain structure formation during friction stir welding observed by the ‘stop action technique’
