On the role of vibration frequency on the solidification of AZ91D magnesium alloys during electromagnetic vibration

In the present paper, we solidified magnesium-based AZ91D alloys in a superconducting magnetic field when an alternating current flowed through the alloy. As the direction of the magnetic field is perpendicular to that of the alternating current, a periodic electromagnetic force is produced to activate an electromagnetic vibration (EMV) on the alloy during solidification. The microstructure formation and microtexture evolution processed by EMV were examined. A significant difference arises in electrical resistivity between a solid and a liquid in the mushy zone of the alloy, making the solid move faster than the liquid and thus generating uncoupled motion, from which melt flow is initiated. The texture evolution obtained by x-ray diffraction and electron backscatter diffraction (EBSD) mapping reveal a strong dependence of melt flow intensity versus vibration frequency. A further analysis reveals that melt flow is rather weak when the vibration frequency is too low and thus the segmentation of growing crystals cannot be thoroughly completed. At medium vibration frequencies, severe fluid flow occurs, which favors fragmentation and thus results in a refined microstructure and a random microtexture. When the vibration frequency is too high, the relative leading distance covered by the mobile solid is rather short and melt flow once again becomes weak. Meanwhile, the static magnetic field makes the crystals orient to their easy magnetization direction and thus yields highly aligned textures. Experimentally, the present systematic observation indicates that the role of melt flow is of substantial importance in revealing the origin of structure formation when the alloy is solidified at various vibration frequencies.