Solidification of metal alloys under the influence of pulse-modulated magnetic fields

Alternating current (AC) magnetic fields are commonly used in industrial practice for melt stirring. The requirements arising from the particular metallurgical or casting operation can be manifold. For instance, the electromagnetic stirring is applied to provide an efficient mixing of metallic melts, to control the flow at the mold region in the continuous casting process or to achieve a purposeful alteration of the microstructure of casting ingots.
In the present study we introduce a novel type of electromagnetic stirring using a pulse-modulated rotating magnetic field (RMF). Capabilities of this approach are exemplarily demonstrated for controlling the melt flow during the unidirectional solidification of an Al-7wt%Si alloy. Two variants have been examined. Firstly, we use a succession of pulses that always have the same rotational direction (RMF-PSCD). Secondly, we use an RMF pulse sequence of alternating direction (RMF-PSAD). The latter method is related to a periodic inversion of the sense of rotation between two consecutive pulses. The characteristic period of the inversion for RMF-PSAD and of the pulses for RMF-PSCD is derivable from the spin-up dynamics of a continuously applied RMF.
Numerical and experimental investigations of different configurations applying an RMF either continuously or in form of RMF-PSAD and RMF-PSCD reveal the development of different flow pattern. It was found that the application the time-modulated RMF with suitable modulation frequencies delivers a homogeneous, equiaxed microstructure. Typical segregation pattern or flow-induced gas porosity usually arising from a continuous RMF application can be avoided with the new stirring methods. Essential features of the interplay between the electromagnetically-driven melt flow and solidification parameters such as temperature and mixture mass concentration of Si are discussed. Our results demonstrate that the success of the presented RMF-PSAD technique requires a precise tuning of the magnetic field parameter with respect to the material properties of the alloy, the geometry of the casting or process parameters like the cooling rate.