Effect of electric current on the microstructural refinement of pure aluminum

The effect of electric current on the refinement of pure aluminum during solidification was investigated based on microstructural observations and numerical simulations. Coarse columnar grains are observed without the application of direct current (DC off), while equiaxed grains clearly exist under the application of electric current during solidification; the area fraction of the equiaxed zone increased up to 66% under electric current, whereas it is negligible in DC off condition. Also, grain size decreased drastically to 221 μm under electric current compared to that of several mm in DC off condition. The grain refinement was affected by the electrical conditions such as the intensity and duration of applied electric current. The flow in liquid aluminum was observed experimentally from the homogenization of the cooling curves measured at different heights and the oscillations in the cooling curves, when electric current was applied during solidification. Through numerical analysis, it was found that the circulating flow of liquid aluminum was developed by the Lorentz force induced by a combination of the electric current density field and magnetic flux density field under electric current. Moreover, the circulating flow could affect grain refinement by providing additional nuclei through dendrite fragmentation. However, under extreme conditions of relatively high electrical energy, the refining effect could disappear because of the retardation of solidification or remelting of nuclei, induced by the Joule heating effect. This study proves that electric current plays an important role in the control of solidification structure.