Solidification Behavior of AZ31 Magnesium Alloys Using an Electromagnetic Vibration Technique

Abstract

The commercial magnesium-based AZ31 alloy was solidified in a static magnetic field when an alternating electric current, perpendicular to the direction of the magnetic field, passes through the alloy. In this case, the periodical Lorentz force is generated, making the conductor vibrate centering on the initial equilibrium position. In this paper, we investigated the microstructure evolution of the alloy as a function of vibration frequency, magnetic flux density and electric current, respectively. The solidification behavior was discussed and the mechanism for the formation of the microstructure was proposed when considering the electrical properties of solid and liquid at high temperature. Because the electrical resistivity of liquid in the mushy zone is about twice that of the solid, this significant difference drives the solid to move faster than the liquid and thus generating a leading displacement for the solid over the liquid even within one vibration cycle. The uncoupled movement between the solid and liquid also gives rise to melt flow, which may be the reason to segment dendrites into fine particles. Meanwhile, the uncoupled movement makes it difficult to establish a steady state for solute redistribution during solidification and thus favoring equiaxed structures instead of dendrites. Considering these two factors, we examine the solidification behavior of the alloy separately when vibration frequency, magnetic flux density, and electric current are set as independent variables and the microstructure evolution as a function of these processing parameters can be well interpreted.KEY WORDS: AZ31 magnesium alloy; microstructure formation; electromagnetic vibration; melt flow; solute redistribution. structure and thus having a fewer slip planes than those with a cubic structure. The anisotropic characteristic in deformation is rather pronounced for alloys with coarse structures and it is not favorable for a uniform deformation. However, the anisotropic feature can be greatly minimized by refined grains that randomly distribute throughout the entire volume, which is beneficial to have a uniform deformation during plastic processing. Moreover, a refined structure makes it possible to process at ambient temperature and thus reducing the production cost. StJohn et al. 17) summarized the influence of refined microstructures in Mg alloys on mechanical and chemical properties. Hence, a grain-refined structure of a solidified ingot is desirable from the viewpoint of industrial application even for the wrought AZ31 alloy.The EMV technique has been demonstrated to be effective in producing grain-refined structures for alloys. In comparison with other approaches for the production of fine structures, e.g., rapid melt quenching and external incubation, a bulk ingot can be fabricated without any external additives during EMV processing. Furthermore, no attenuation in vibration intensity can be inferred throughout the entire volume of the sample provided that electric current can pass through the conductor uniformly,...

Modification of Primary Mg<SUB>2</SUB>Si Crystals in Hypereutectic Mg-Si Alloy by Application Alternating Current

“…[24][25][26][27] In the present study, when the alternating current was applied into the hypereutectic Mg-Si melt at high temperatures of 770 C and 740 C, it was interesting that obvious agglomerations of refined Mg 2 Si crystals occurred in these two samples and the Mg 2 Si crystals distributed non-uniformly, as shown in Figs. 7(a) and 7(b).…”

“…Therefore, no obvious agglomerations of the Mg 2 Si crystals were found in the samples treated by application alternating current from starting temperatures of 700 and 670 C. Usually, it is widely accepted that the refinement of the structures caused by EMV attributes to the fragmentation by mechanical stress and by the re-melting of the primary solid phase due to constitutional undercooling. 26,27) For the latter, it is not necessary to think of the bonding among atoms. For the former, the present authors think that the dendritic Mg 2 Si crystals were fragmented from some weak parts subjected to the EMV.…”

“…[16][17][18][19][20][21][22][23][24][25][26][27] This route has an excellent advantage of making no change to the composition of the treated alloys. Many studies have been focused on the modification effect of electromagnetic vibration on the eutectic and primary Si crystals in the AlSi alloys.…”

Effects of Operating Parameters on Modification of Primary Mg<SUB>2</SUB>Si Crystals in Hypereutectic Mg-Si Alloy Treated by Imposition of Alternating Current

“…The refining effect could be obtained only when EMV was imposed in the initial stage of the solidification. More recently, Li et al 27) reported that the effects of the operating parameters of EMV on the solidification behavior of AZ31 alloy and the best refining efficiency could be obtained under the condition of the optimum operating parameters.…”

“…[16][17][18][19] In particular, it has long been established that the electromagnetic vibration (EMV) can effectively refine the microstructures for a large range of metals prepared by the ingot metallurgy process. [20][21][22][23][24][25][26][27][28] For this route, the operating parameters of EMV were proved to be very important to determine the refining efficiency, including vibrating frequency, 22,23,[25][26][27] current density, 24,26,27) magnetic flux density, 26,27) imposition timing, etc. 24,28) For example, Mizutani et al 22) found that the vibration frequency near 1 kHz was most effective for the refinement of the primary silicon in the Al-17 mass%Si treated by EMV.…”