2018 Volume 59 Issue 10 Pages 1603-1609
In the present study, a three-phase induction coil was used to generate a rotating magnetic field (RMF) when three-phase alternating current flowed through the coil. We solidified the 7150 aluminum alloy (main composition: Al–6.1 mass%Zn–2.2 mass%Mg) in the RMF where electromagnetic torque could be exerted upon the alloy and thus electromagnetic stirring (EMS) could be generated upon the crystallizing melt. The imposition pattern and time for EMS processing were tailored, making it possible for the liquid column to be rotated along clockwise (CW) and counterclockwise (CCW) alternatively. We solidified the alloy when periodic CW/CCW rotation was imposed and then examined the microstructure as function of reverse imposition time, exhibiting a strong dependence on imposition time. The microstructure formation was discussed when considering the substantial difference in electrical resistivity between the primary solid solution and remaining liquid, which led to uncoupled movement between the primary mobile solid and remaining sluggish liquid. The movement of the solid was further diagnosed when considering the fluid resistance that lagged the movement and thus lowered the velocity difference between the mobile solid and the sluggish liquid when the imposition time was extended. The lowered velocity difference could generate reduced interaction between the solid and liquid and thus in some areas, coarse dendritic embryos could be observed. This effect was further verified when continuous one-way rotation was applied, which resulted in the formation of developed dendrites in the alloy.