Abstract
Two series of experiments were carried out to study the mechanism of equiaxed crystal formation induced by electromagnetic stirring in an ingot.
Microscopic observation of the solidification process in a transparent H2O-32 mass%NH4Cl model alloy revealed that no dendrite branch detachment occurred during the columnar dendrite growth from the bottom while being stirred. When free crystals were allowed to exist in the liquid before the start of the stirring, they acted as hammers on the growing dendrite tips during stirring and promoted the multiplication of equiaxed crystals.
Other experiments were performed on 99.8%Al, Al-0.9 mass%Si, and Al-2 mass%Cu alloys using a magnetic field in the range of 9.0×10−3 T to 2.7×10−2 T, and the hot top was used only when heat extraction from the molten surface was prevented.
The equiaxed crystal zone in Al alloy ingots expanded as the intensity of the magnetic field increased. The equiaxed crystal zone was most predominant when the magnetic field was applied before pouring.
It was considered that the formation of equiaxed crystals in Al alloy ingots during electromagnetic stirring most easily occurred in the initial stage of solidification. It was concluded that the most suitable position for the electromagnetic stirrer to produce a large amount of equiaxed crystals was the top of the mold near the molten surface.
