Controlling microstructures of AZ31 magnesium alloys by an electromagnetic vibration technique during solidification: From experimental observation to theoretical understanding

This paper examines the microstructure formation of magnesium-based AZ31 alloys solidified in a magnetic field with the imposition of an alternating current, from which an electromagnetic vibration force is yielded. The grain structure was characterized, revealing that refined equiaxed grains could be yielded at a frequency range from ∼500 Hz to less than 2000 Hz. When the vibration frequency was too low or too high, coarse structures could be obtained. In the mushy zone, a significant difference in electronic resistivity between a solid and a liquid drives the solid to move much faster than the surrounding liquid, thus yielding relative velocity and relative displacement, due to which a dendrite may be segmented into pieces. This motion generates agitation in the semisolid stage, thus making the microstructure more random and resulting in deformation twins. At low frequencies, the coarse structure may be due to the suppression of macrofluid flow by a high magnetic field. Grain refinement occurs at the frequency interval where the mobile leading solid is vibrated beyond the solute operating region, in which the relative displacement covered by the solid is larger than the thickness of solute equivalent boundary layer. At high frequencies, the relative displacement is so small that it is always less than the thickness of the solute boundary layer and the vibration cannot alter the solute pile-up ahead of the solid/liquid interface of the growing crystals. Thus, it is similar to that in normal casting and always produces very coarse structures.