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

Miwa

2008

ISIJ Int.

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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,...

Section: Discussionmentioning

confidence: 99%

Solidification Behavior of AZ31 Magnesium Alloys Using an Electromagnetic Vibration Technique

Miwa

2008

ISIJ Int.

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“…We recently solidified AZ31 15) and AZ91D 12) alloys using the EMV technique and examined microstructure development as a function of vibration frequency. In order to elucidate the origin of structural formation, it is considered that the electrical resistivity of solid is only about half of that liquid, therefore, the electric current in solid is about twice of that in liquid and thus the solid is driven to move faster than the liquid in mushy zone due to the stronger Lorentz force.…”

Section: Discussionmentioning

confidence: 99%

Effect of Vibration Timing on the Microstructure and Microtexture Formation of AZ91D Magnesium Alloys during Electromagnetic Vibration

Omura

et al. 2009

Mater. Trans.

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For an electric conductor in a static magnet, electromagnetic vibration (EMV) is activated to take place when an alternating current flows through the conductor in the direction perpendicular to that of the magnetic field. Following the principle, in the present study we solidified the AZ91D magnesium alloys at various vibration timing moments and then examined the microtexture and microtexture. It is revealed that the decrease of the starting vibration temperature in the mushy zone results in coarse microstructures with large dendritic fragments and eventually with fully developed dendrites that are well oriented with their basal planes being perpendicular to the direction of magnetic field. It has been demonstrated that melt flow is responsible for structural refinement during EMV processing due to the electrical resistivity difference of the solid and liquid. In mushy zone, when vibration is imposed at a low temperature, the fraction solid of the primary phase prior to vibration has been large, which makes the semisolid slurry rather viscous. The intensity and scale of the melt flow is weakened and thus yielding coarse structures. Meanwhile, as the magnetic field is imposed throughout the entire experimental operation, the magnetization torque takes into effect to orient dendrites. The bridging of dendrites may have been accomplished at low vibration temperature before EMV is activated. In this case, the magnetization torque becomes predominant to align crystals along their preferential crystallographic orientation and thus resulting in highly oriented textures.

“…Electromagnetic vibrations are generated in a molten metal when alternating current is passed through it perpendicular to the static magnetic field, and it has been reported that electromagnetic vibrations refine the grain size by the induced motion of the solid and liquid phases. 9) Under the assumption that the rare earth elements, if they were present, would migrate from the Fe-rich liquid to the Cu-rich liquid, electromagnetic vibrations (60 A, 500 Hz) were applied for 10 seconds just before the end of temperature holding time. When the electromagnetic forces were applied to the sample, the magnetic field of the superconducting magnet was increased to the desired value during the sample heating.…”

Section: Methodsmentioning

confidence: 99%

Effect of Electromagnetic Forces on Separation of Two Immiscible Liquids in the Fe–Cu–C System

2016

Mater. Trans.

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The recovery of rare earth elements, especially Nd and Dy, from used products is important for maintaining stable supplies. Two immiscible liquids in the Fe-Cu-C system, one that is Fe-rich and the other Cu-rich, are proposed for recycling motors that use Nd magnets. In this recovery system, mechanical separation of the Fe-rich and Cu-rich phases is important. This study aims to investigate the effect of electromagnetic forces on the separation of two immiscible liquids in FeCuC systems. The degree of mechanical separation of the Fe-rich and Cu-rich phases increases with increasing strength of the electromagnetic separation force. It was found that low magnetic field and high direct current are advantageous for the separation when the strength of the electromagnetic separation force is equal. Moreover, it was found that the electromagnetic separation force can remove the primary crystalized Fe particles from the Cu-rich liquid immediately.