Effects of processing variables on microstructure formation in AZ31 magnesium alloys solidified with an electromagnetic vibration technique

In the present study, we solidified magnesium-based AZ31 alloys by an electromagnetic vibration technique in a superconducting magnetic field at a vibration frequency of 500 Hz. Two groups of processing variables were used to carry out experiments; one is that the electric current is set as 60 A so as to testify to the influence of magnetic flux density on microstructure development from 1 up to 10 T. The other is that the electric current increases from 10 up to 120 A in the static magnetic field of 10 T, from which the dependence of structure formation on electric current is revealed. It is found that with the increase of both magnetic flux density and the level of electric current, solidified structures experience a transition from coarse dendrites to equiaxed grains. The melt fluid induced by the vibration force during solidification may promote the dendrite to a fragment. Meanwhile, the solids can be driven to move out of the operating region of the solute redistribution boundary. These effects make it difficult to form a complete dendrite but a refined structure. Furthermore, the vibration force can result in the formation of deformation twins in the alloy that has a low critical stress for basal slip. Regarding the effect of the electric current on microstructure, heat (measured in joules) can be produced when a large electric current is imposed, which can ripen the microstructure and induce a nonuniform structure. The slow cooling rate also makes the number fraction of deformation twinning decrease due to a rapid migration rate of atoms at high temperatures.