The effect of rotating magnetic fields on the microstructure of directionally solidified Al-Si-Mg alloys

The effect of magnetically controlled convective conditions on the microstructure formation in cast Al-alloys is investigated. Samples of the technical Al-7 wt.% Si-0.6 wt.% Mg alloy (A357) have been directionally solidified under medium temperature gradient (3 K/mm) in the aerogel based furnace facility ARTEMIS. Three pairs of Helmholtz coils around the sample induce a homogeneous rotating magnetic field being able to generate a controlled fluid flow in the melt close to the growing solid-liquid interface. The application of rotating magnetic fields during directional solidification results in pronounced segregation effects, leading to a deformation of the solidification front. For high magnetic field strengths, a change to pure eutectic solidification at the axis of the sample is observed. The investigations show that with increasing fluid flow the primary dendrite spacing decreases whereas the secondary dendrite arm spacing increases.