Macrosegregation Mechanism of Primary Silicon Phase in Cast Hypereutectic Al-Si Alloys Under Alternating Electropulsing

Controlling the macrosegregation induced by electropulsing is of high commercial importance. This study investigates the macrosegregation of the primary Si phase in casting Al-Si hypereutectic alloys via the different solidification stages and components of alloys treated with electropulsing. Experimental results show that a serious gradient macrosegregation of the primary Si phase occurs, and four types of primary Si regions are formed: coarse plate-like, refined plate-like, and fine polygon primary Si, as well as a eutectic structure. The wider the solidification temperature range, the more serious the macrosegregation. A near-eutectic structure occurs at the center of ingots when the solidification temperature range exceeds a certain threshold. With an increasing current density of electropulsing, the segregation degree of primary Si increases initially and then decreases for different Al-Si hypereutectic alloys, but the current density with regard to the most serious segregation is closely related to the Si content. Furthermore, it is proved that the migration behavior of primary Si particles plays an important role in macrosegregation. A special casting experiment under the condition of limited heat flux along the radial direction was performed to clarify the macrosegregation mechanism of primary Si under electropulsing. After nucleation during the solidification process, the primary Si particles move to the front of the solid-liquid interface due to secondary flow in bulk liquid and then are easily captured due to the electromagnetic repulsive force or its component. The force flow in the bulk liquid and mush zone and the secondary flow in front of the solid-liquid interface make obvious solute redistribution and promote the growth of the primary Si phase, which is maintained until the solute concentration in the bulk liquid approaches the eutectic composition.