Axial Static Magnetic Field Research Articles

Numerical Investigation About the Effect of Axial Static Magnetic Field on Improvement of Metal Pool Profile During Electroslag Remelting Process

Numerical Investigation About the Effect of Axial Static Magnetic Field on Improvement of Metal Pool Profile During Electroslag Remelting Process

Morphology Control of Metal Pool and Eutectic Carbides in Electroslag Remelted M2 HSS with an External Axial Static Magnetic Field

This study investigates the influence of a superimposed axial static magnetic field (ASMF) on the morphology of metal pool and eutectic carbides (ECs) in electroslag remelted M2 high-speed steel (HSS). The application of ASMF caused the metal pool to become shallower, and the solidified structure to expand axially, along with finer (i.e., more uniformly distributed and crystallographically oriented) ECs. Lorentz force-driven unidirectional circulation in slag pool was the primary cause of the metal pool’s morphological adjustment; this resulted in a more homogenous temperature distribution in slag pool. Thus, the heat transfer from the slag pool to the metal pool became more uniform, creating a metal pool that is shallower. Additionally, local solidification time (LST) became shorter, while the number of (Ti, V)N-Al2O3 inclusions serving as heterogeneous nuclei for EC formation increased due to ASMF, enabling finer EC with more crystallographic orientations in ESR ingots.

Effect of axial static magnetic field on microstructure evolution, performance, and melt pool signals of AlSi10Mg fabricated by laser powder bed fusion

Static magnetic field (SMF) assisted laser powder bed fusion (LPBF) has significant potential to improve the mechanical properties of AlSi10Mg parts. In this study, the tensile properties of AlSi10Mg are improved under the SMF, mostly due to the increased relative density and finer microstructure. Under the SMF, the epitaxial growth in the building direction is inhibited, indicating a weakening 〈001〉 texture. However, the mechanism of the magnetic field within the melt pool remains unknown. The melt pool area, temperature gradient, radiation emission, and spatter extracted by in-situ monitoring technology provide an excellent opportunity for increasing the understanding of the SMF-assisted LPBF process. The Marangoni convection in the melt pool is limited by the magnetic damping effect, which improves the melt pool and radiation stability and reduces spatters. Further, thermoelectric magnetic force at the cell surface in the mushy zone improves the local convection, increasing the thermal diffusion rate in the melt pool boundary during the solidification. This results in smaller melt pool areas, reduced radiation emission magnitude, and a steep temperature gradient. As a result, the directional solidification in the thermal diffusion direction is inhibited, and the microstructure is refined. Finally, the thermoelectric magnetic convection also facilitates the escape of the entrapped pores in the melt pool, increasing relative density.

Azimuthal surface waves in circular metal waveguides entirely filled by two layers of plasma in axial static magnetic field

The dispersion properties of surface-type electromagnetic waves propagating along azimuthal angles in circular metal waveguides entirely filled by two layers of plasma in an axial static magnetic field are studied. The three-component structure models plasma radial inhomogeneity and generalizes two research which was two researches which were accomplished earlier. One is a circular metal waveguide entirely filled by cold plasma in an axial static magnetic field. The other is a coaxial metal waveguide entirely filled by plasma. Introducing the additional interface between the plasma column and coaxial plasma layer causes arising of the additional root of the dispersion relation which can be of interest for practical applications. The disappearance of additional wave branches with decreasing the difference in the plasma particle densities is carefully investigated. The wave dispersion properties are demonstrated to be mostly determined by the plasma with smaller particle density.

Twisted plasma waves driven by twisted ponderomotive force

We present the results of twisted plasma waves driven by twisted ponderomotive force. With the beating of two, co-propagating, Laguerre-Gaussian (LG) orbital angular momentum (OAM) laser pulses with different frequencies and also different twist indices, we can obtain the twisted ponderomotive force. Three-dimensional particle-in-cell simulations are used to demonstrate the twisted plasma waves driven by lasers. The twisted plasma waves have an electron density perturbation with a helical rotating structure. Different from the predictions of the linear fluid theory, the simulation results show a nonlinear rotating current and a static axial magnetic field. Along with the rotating current is the axial OAM carried by particles in the twisted plasma waves. A detailed theoretical analysis of twisted plasma waves is also given.

Long-wavelength electromagnetic waves of surface type in circular metal waveguides partially filled by plasma in presence of an axial static magnetic field

The method of successive approximations is applied to solve the Maxwell equations in cylindrical plasma waveguide geometry for electromagnetic waves with arbitrary azimuthal wave index and small axial wavenumber. The theory of surface flute waves is used as zeroth approximation. This study generalizes previous investigations whose results are utilized for verification of newly obtained conclusions. The influences of several plasma waveguide parameters such as magnitude and sign of the azimuthal wave index, the width of the dielectric layer between a plasma and a waveguide wall and the magnitude of its dielectric constant, the radii of the plasma column and the metal wall, and the external axial static magnetic field on the wave dispersion properties are analyzed.

Carbides refinement and mechanical properties improvement of H13 die steel by magnetic-controlled electroslag remelting

The effect of an axial static magnetic field (ASMF) on the carbides and mechanical properties of electroslag remelted H13 die steel was studied in current work. Optical microscope (OM) and scanning electron microscope (SEM) were used to analyze the solidification structure and carbide morphology of electroslag remelting (ESR) ingots. The FEI Aspex Explorer was utilized to detect the inclusions in H13 ESR ingots. Compared with conventional ESR, the application of ASMF in this study made the metal molten pool shallower and flatter, resulting in shorter local solidification time and uniform distribution of solute atoms, which alleviated the element segregation degree, and further inhibited the formation of primary carbides. The magnetic-controlled electroslag remelting (MC-ESR) process not only refined the dendrite structure (tended to grow parallel to the ingot axis) and the carbides, but also improved the inclusion removal efficiency and mechanical properties of H13 ESR ingots.

Multi‐Stage Enhanced Removal of Inclusions During Electroslag Remelting Process by a Static Magnetic Field

An axial static magnetic field (ASMF) is superimposed during the electroslag remelting (ESR) process to regulate the inclusion removal (GCr15 bearing steel) in the current study. After applying a 50 mT ASMF, the metal pool of the ESR ingot becomes shallower and the solidified dendritic structure tends to grow axially, together with a higher inclusion removal efficiency. The number of inclusions in ESR ingot decreases, and the inclusion with larger size (>10 μm) is almost eliminated. By tailoring the morphology of the liquid melt film (LMF), droplet, and metal pool, the ASMF applied in this study strengthens the kinetic conditions (inclusion migrated to slag/metal interface) for the inclusion removal, thus realizing the multi‐stage enhanced removal of inclusion during ESR process.

Formation mechanism of ring-like segregation and structure during directional solidification under axial static magnetic field

• The ring-like structure forms under a moderate axial magnetic field. • The ring-like convection and segregation form under a moderate axial magnetic field. • The formation of the ring-like structure is attributed to solute redistribution under magnetic field. The effect of the axial static magnetic field on the macro-segregation and structure in the Al-Cu and Ni-Mn-Ga alloys during directional solidification is investigated experimentally and numerically. It is found that the ring-like segregation and structure in the above-mentioned two alloys form during directional solidification at a certain growth speed under a moderate magnetic field. For the Al-Cu and Ni-Mn-Ga alloys, the moderate values of the magnetic field under which the ring-like structure forms are about 0.5 T and 1.0 T at respective growth speed of 10 μm/s and 5 μm/s. Further, the distributions of the flow and solute in the Al-Cu alloy during directional solidification under the axial static magnetic field is numerically simulated. Numerical results reveal that the rotary thermoelectric (TE) magnetic convection forms in the mushy zone during directional solidification under an axial magnetic field. This flow will induce the formation of the ring-like macro-segregation and structure. Changes in structures under the magnetic field in the experimental results are in good agreement with the distributions of the TE magnetic convection and solute in the numerical results. Therefore, the formation of the ring-like structure and segregation under the magnetic field should be attributed to the solute redistribution induced by the TE magnetic convection.

Enhancement of Inclusion Removal in Electroslag Remelted M2 High-Speed Steel Assisted by Axial Static Magnetic Field

Enhancement of Inclusion Removal in Electroslag Remelted M2 High-Speed Steel Assisted by Axial Static Magnetic Field

Effect of axial static magnetic field on cleanliness and microstructure in magnetically controlled electroslag remelted bearing steel

Effect of axial static magnetic field on cleanliness and microstructure in magnetically controlled electroslag remelted bearing steel

AZIMUTHAL SURFACE WAVES IN LOW-DENSITY PLASMA LOADED, COAXIAL HELIX TRAVELING-WAVE-TUBE-LIKE WAVEGUIDES

The dispersion properties of flute electromagnetic waves propagating across an external axial static magnetic field in traveling-wave-tube-like waveguides with low-density plasma filling are investigated. The cylindrical structure consists of a central dielectric rod, placed inside a plasma coaxial layer with a metallic helix sheath on its outer interface, and a metal screen separated from the helix by another dielectric layer.

Zeroth radial modes of azimuthal surface waves in dense plasma-loaded, coaxial helix traveling-wave-tube-like waveguides

An analytical model of coaxial traveling-wave-tube-like waveguides with plasma filling has been justified and utilized to analyze the eigenmodes. Very often, introducing plasma into vacuum electronic devices leads to essential advantages as compared with evacuated tubes. The cylindrical structure under the present consideration consists of a central dielectric rod, placed inside a plasma coaxial layer with a metallic helix sheath on its outer interface, and a metal screen separated from the plasma by another dielectric layer. The dispersion properties of electromagnetic waves propagating across the external axial static magnetic field in such traveling-wave-tube-like waveguides are studied and summarized. The presence of a dense plasma coaxial layer makes the media nontransparent for waves in the electron cyclotron frequency range. However, surface type electromagnetic waves can propagate in this case. These waves are called azimuthal surface waves (ASWs). The helix sheath causes coupling of ordinarily and extraordinarily polarized ASWs. The zeroth radial ASW modes have been found to be most dangerous for parasitic wave excitation in dense plasma-loaded, coaxial traveling-wave-tube-like waveguides.

Carbides Modification and Mechanical Properties Enhancement of Cr12MoV Die Steel by Magnetically Controlled Electroslag Remelting

The effects of an axial static magnetic field (ASMF) on the carbides and mechanical properties of electroslag-remelted (ESR) Cr12MoV die steel were investigated. The superimposition of a 30 mT ASMF induced unidirectional rotation in the molten slag pool, resulting in a more uniform temperature distribution at the front of the dendrite solid–liquid interface. The size of the grains and carbides in the ESR Cr12MoV steel ingot became smaller. The sulfur content of the ingots decreased and the mechanical properties (hardness and impact energy) increased after applying the ASMF. A new mechanism was proposed that the unidirectional flow was mainly due to the coupled thermoelectric magnetic force induced in the melt between dendrites, which promoted the refinement of grains and carbides, as well as the desulfurization process, thereby resulting in the enhanced mechanical properties of the Cr12MoV die steel.

HIGHER RADIAL MODES OF AZIMUTHAL SURFACE WAVES ABOVE THE UPPER-HYBRID FREQUENCY IN CYLINDRICAL WAVEGUIDES PARTIALLY FILLED BY PLASMA

Azimuthal surface waves (ASWs) are known to be eigen waves of cylindrical metal waveguides partially filled by magnetoactive plasma. Zeroth radial modes were under study earlier. Their dispersion properties are known to be significantly influenced by the plasma column properties: its particle density, external axial static magnetic field, geometric dimensions, – rather than properties of the dielectric layer which separates the plasma column from the metal wall. Application of higher order ASWs in the low-frequency range was shown earlier to make it possible to get advantage of exciting ASWs with higher frequency than in the case of zeroth order ASWs without no change in the waveguide design. The present study generalises those investigation for the case of the waves above the upperhybrid frequency.

Effects of Axial Static Magnetic Field on Inclusions Removal in the Liquid Melt Film During Electroslag Remelting Process

Magnetic-controlled Electroslag Remelting (MC-ESR) process has been proposed to improve the inclusion removal efficiency in recent years. In this study, the effects of axial static magnetic field (ASMF) on liquid melting film (LMF) and inclusion removal during this period in ESR process of GCr15 steel were studied by a series of MC-ESR experiments. The thickness of the characteristic position of LMF was reduced by 18.6 to 90.3 pct when a 50 mT ASMF was applied. The number and size of the inclusions in LMF were also decreased. In addition, the mechanisms of LMF thinning and inclusion removal efficiency improvement enhanced by the ASMF were interpreted in detail. Furthermore, a numerical simulation of the inclusion migration in LMF was also carried out to support the proposed mechanisms to some extent.

Numerical simulation of Marangoni effect on the deformation behavior and convection of electromagnetic levitated silicon droplets under static magnetic fields

The application of electromagnetic levitation for the measurement of thermophysical properties, container-less processing of materials and evaluation of metallurgical reactions, can be adversely affected by instability phenomena, such as droplet oscillation, deformation and ejection. In this work, the influence of the Marangoni effect and static magnetic fields on the deformation, flow velocities and temperature profiles of electromagnetic levitated silicon droplets are investigated by a series of numerical simulations based on the Arbitrary Lagrangian-Eulerian method. According to the results, in the absence of a static magnetic field, droplet deformation is increased when the Marangoni effect is taken into account. Static magnetic fields can partially restrain droplet deformation and the restraining effect is enhanced by the Marangoni effect. For a silicon droplet of 4mm radius, when a static magnetic field of 1T was added and the Marangoni effect was taken into account, the maximum deformation ratio decreased from 1.413 to 1.373. In the absence of the Marangoni effect, the ratio only decreased from 1.372 to 1.352. In addition, within the droplet, the 1T axial static magnetic field has a significant suppression effect on the internal flow while the velocity near the droplet surface is substantially reduced. Under these conditions, the heat transfer mode changes from convection to conduction. Based on the findings from this work, operational parameters can be optimized in order to improve the stability and deformation behavior of levitated materials.

Surface and bulk plasmons in cylindrical electric-gyrotropic wires

The effects of an axial static magnetic field on surface and bulk plasmons of a cylindrical electric-gyrotropic (or ε -anisotropic) wire are investigated. Solving a modified Laplace’s equation in conjunction with the appropriate boundary conditions, the dispersion relations, power flows, energy densities, and group velocities are obtained for both types of modes, and some numerical solutions are presented. The effects of external magnetic field on plasmonic properties of the system are analyzed and show significant influence on the behavior of the modes.

Azimuthal surface waves in cylindrical metal waveguides partially filled by magnetoactive plasma: Analysis of energy transfer

Azimuthal surface waves are well known to be eigenwaves of cylindrical waveguide structures—plasma–dielectric–metal—placed into an external axial static magnetic field. The angular velocity of energy transfer along with the angular group velocity is applied to analyze electromagnetic energy rotation in these structures. Optimal magnitudes of effective azimuthal wavenumbers are determined for which the angular velocity reaches its maximum. The angular velocity is shown to decrease with the increasing absolute value of the azimuthal wavenumber and with the decreasing strength of the external axial static magnetic field.

Gradient Coil Design and Realization for a Halbach-Based MRI System

In this paper we design and construct gradient coils for a Halbach permanent magnet array magnetic resonance (MR) scanner. The target field method, which is widely applied for the case of axial static magnetic fields, has been developed for a transverse static magnetic field as produced by a Halbach permanent magnet array. Using this method, current densities for three gradient directions are obtained and subsequently verified using a commercial magneto-static solver. Stream functions are used to turn the surface current densities into wire patterns for constructing the gradient coils. The measured fields are in good agreement with simulations and their prescribed target fields. Three dimensional images have been acquired using the constructed gradient coils with very low degree of geometric distortion.