Origin of Showering Crystals in the Molten Metal in a Mold cooled from the Top

Several experiments were carried out in order to study the formation mechanism of equiaxed crystals in an ingot cooled from the top surface, using an ammonium chloride-water model and Al alloys.Microscopic observation of the solidification phenomena of the ammonium chloride-water model revealed that the solid skin formed on the top surface undulated because of the release of a gas film at the interface between the solid and the liquid, and that many dendrite fragments of ammonium chloride crystals formed by the breaking of the solid skin by the undulation were scattered into the bulk liquid. Visible free dendrites growing from those fragments appeared in the liquid just below the columnar dendrites, which grew from the top. No dendrite remelting was observed during the columnar growth from the top.Experiments performed with 99.8%Al, Al-Cu, and Al-Ti alloys suggested that the breaking of an unstable solid skin at the top surface in the initial stage of solidification was the primary source of showering crystals when the molten surface was cooled by gas blowing. Beside the cooling of the top surface, when the heat of the molten metal was extracted through the side wall of the mold as well, a U-shaped equiaxed zone was obtained in alloy ingots, showing that crystals were mostly separated at the edge of the mold and the molten surface. Showering of crystals from the top surface of molten metal occurred only when the solid skin was mechanically broken.