Grain refinement in Al-Si alloys caused by electric current pulses and the role of melt convection

The application of the so-called Electric Current Pulse technique (ECP) unlocks a remarkable potential to influence the solidified macrostructure of metals during the solidification process. Many studies have shown that beneficial effects like a distinct grain refinement or the promotion of the transition from a columnar to an equiaxed dendritic growth (CET) can be achieved (see for instance [1-3]). However, the physical mechanism of the grain refinement effect caused by ECP has not been understood so far. Various effects are under discussion, such as the fragmentation of dendrite induced by the electric current [1], the reduction of the nucleation activation energy [2], or the break out and the transport of little grains from the boundary by the periodic Lorentz force [3]. However, the previous studies did not consider the possibility that intense Lorentz forces resulting from the interaction between the strong electrical current and the self-induced magnetic field can create significant melt flows.
This paper presents an experimental study which focuses on the effect of forced melt flow during solidification under the influence of a strong electric current. A first set of experiments was conducted to obtain quantitative information about the isothermal flow field exposed to various electrical parameters like the frequency and amplitude of the current as well as the pulse length. Flow measurements were carried out by the ultrasound Doppler method. In a second step solidification experiments were carried out using a binary Al-Si alloy to verify the effect of certain current parameters on the solidified structure. In our experimental setup the electrical current was supplied through two parallel electrodes at the free surface. The results demonstrate that the grain refining effect observed in our experiments can be ascribed to the forced melt flow driven by the Lorentz force.