Effects of an oscillation electromagnetic field on grain refinement and Al8Mn5 phase formation during direct-chill casting of AZ31B magnesium alloy

Journal Pre-proof Effects of an oscillation electromagnetic field on grain refinement and Al8 Mn5 phase formation during direct-chill casting of AZ31B magnesium alloy Yonghui Jia (Conceptualization) (Methodology) (Data curation) (Writing - original draft) (Writing - review and editing), Jian Hou (Methodology) (Investigation), Hang Wang (Software) (Methodology), Qichi Le (Supervision) (Project administration) (Writing - review and editing), Qing Lan (Formal analysis) (Methodology), Xingrui Chen (Writing - review and editing), Lei Bao (Formal analysis)

PII:

S0924-0136(19)30515-1

DOI:

https://doi.org/10.1016/j.jmatprotec.2019.116542

Reference:

PROTEC 116542

To appear in:

Journal of Materials Processing Tech.

Received Date:

8 June 2019

Revised Date:

27 November 2019

Accepted Date:

1 December 2019

Please cite this article as: Jia Y, Hou J, Wang H, Le Q, Lan Q, Chen X, Bao L, Effects of an oscillation electromagnetic field on grain refinement and Al8 Mn5 phase formation during direct-chill casting of AZ31B magnesium alloy, Journal of Materials Processing Tech. (2019), doi: https://doi.org/10.1016/j.jmatprotec.2019.116542

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Effects of an oscillation electromagnetic field on grain refinement and Al8Mn5 phase formation during direct-chill casting of AZ31B magnesium alloy

Yonghui Jia, Jian Hou, Hang Wang, Qichi Le*, Qing Lan, Xingrui Chen, Lei Bao Key Lab of Electromagnetic Processing of Materials, Ministry of Education, Northeastern University, 314 Mailbox, Shenyang 110819, People’s Republic of China *Corresponding author: Qichi Le

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E-mail address: [email protected] Tel: 0086-24-83683312 Fax: 0086-24-83681758

Abstract

The effects of an oscillation electromagnetic field (EMF) generated by low frequency pulse current on

the grain refinement and phase formation in direct-chill (DC) casting of AZ31B magnesium alloy have been investigated experimentally. The macrostructure evolution was quantitatively examined in terms of the grain size and its distribution, and fine equiaxed grains can be obtained. Given the Lorentz force and

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velocity variations, temperature distribution and heat extraction process during DC in different

electromagnetic conditions, the grain refinement mechanism and the phase composition and formation of AZ31B magnesium alloy were discussed in detail. The grain sizes decrease from 549 ~ 1094 μm

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(without EMF) to 402 ~ 486 μm at the frequency of 15 Hz, and columnar grain region decreases significantly. Forced convection induced by EMF can strengthen the heat extraction along the diametrical

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direction of billet, weaken the contact heat transfer between melt and mold wall. For the formation of Mn-containing phases, eutectic transformations are dominant in DC casting process of AZ31B magnesium alloy, and the area fraction of eutectic Al8Mn5 phase decreases from center to edge of billet, and its distribution is more homogeneous in the presence of EMF.

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Keywords: Oscillation electromagnetic field; Grain refinement; Al8Mn5 phase; AZ31B magnesium alloy; Direct-chill casting 1. Introduction

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Magnesium and its alloys have very impressive properties, such as low density, high specific strength and stiffness, positive conductivity, and promising machinability, etc. Therefore, demand for magnesium alloys especially for wrought magnesium alloys is increasing each year particularly in automotive,

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electronics, and aerospace industries where weight-saving engineering solutions are continuously being searched for reducing the cost for energy consumption. Simultaneously, higher requirements for the quality and size of magnesium alloys billet for further deformation are also put forward. Caron and Wells (2009) reported that 80% of the DC-cast products are pure magnesium for other applications (e.g. aluminum, ductile iron, etc.), while the remaining 20% are wrought magnesium alloys such as AZ31B, AZ61, or ZK60 alloy. Eskin (2008) did a lot of work on the solidified structures, proposed that DC casting technics is a major technological route in production of large-size billets for further deformation, such as extrusion or rolling, and about 20% of magnesium alloys were cast using the DC casting process among the many casting routes. Guo et al. (2005) investigated the effects of alternating magnetic field on properties of AZ91 billets prepared by DC casting technology, indicating that conventional DC cast 1

billets with large diameter always exhibit coarser dendritic and columnar structures, causing poor deformability and mechanical properties. An important limitation to the conventional DC casting process is the formation of defects such as hot tears, macrosegregation, and surface folds, etc. These defects will result in low yield and poor formability. One of the most effective ways to improve the deformability and mechanical properties during subsequent processing is to use a billet made up of fine and homogeneous structures. There are many techniques to achieve fine-grained structure during solidification, such as rapid solidification, ultrasonic melt treatment, and electromagnetic casting. However, rapid solidification can’t be used in the casting of large-sized billet; ultrasonic melt treatment process is complex and the ultrasonic rod will be eroded by high temperature melt and the melt would be polluted. Electromagnetic casting technology has widely used in commercial process, since the electromagnetic casting (EMC) technique and casting, refining, electromagnetic (CREM) processing were developed by Getselev and Vivès, respectively. Trindade et al. (2007) claimed that a low frequency alternating magnetic field is propitious to impurity removal and

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grain refinement. It was found that the solidification structure, tensile strength and surface quality of the

billets are improved significantly. Wang et al. (2008) investigated the heat transport and solidification

during low frequency electromagnetic hot-top casting of 6063 aluminum alloy. The results showed that the temperature profile in the ingot was modified, and the ingot with fine and homogeneous

macrostructure was obtained. Zhao et al. (2007) applied the low frequency electromagnetic field in

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horizontal DC casting process. It revealed that the low frequency electromagnetic field can effectively

improve surface quality, reduce inhomogeneous microstructures and macrosegregation in horizontal DC products. Although the low frequency magnetic field plays a certain role in refining the microstructure,

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the magnetic field action area is small in the large size ingot casting due to its attenuation in the melt. Recently, many researches revealed that oscillation pulsed magnetic field casting is an effective process for the grain refinement of magnesium alloys due to the advantages of energy conservation and

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weakening skin effect. Compared with low frequency alternating magnetic field, under the same electromagnetic conditions, pulse current can generate larger induced magnetic fields and the distribution uniformity of differential phase magnetic fields is better. Fu and Yang (2012) reported that the solidified microstructure of Mg-Al-Zn alloy can be markedly refined with low-voltage pulsed magnetic field. Luo

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et al. (2015) studied as-cast structure and tensile properties of AZ80 magnesium alloy DC cast with lowvoltage pulsed magnetic field, the results showed that the solidified structure can be refined obviously and the morphology of the dendritic was transformed from coarse dendritic to fine rosette. Chen and

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Shen (2018) studied the evolutions of the melt flow, temperature field and mushy zone as well as the solidification rate, which can strengthen the melt convection and heat extraction. However, most of results are obtained from laboratory conditions and small-sized billets (≤ 90 mm). Compared with the

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production of large-sized billets, there are significant differences in process and results. Thus, a new method of electromagnetic DC casting for large-sized magnesium alloy billet was developed in this work. In the present study, an oscillation EMF generated by low frequency pulse current was employed

in DC casting of AZ31B magnesium alloy under industrial conditions. A large-sized billet with diameter of 320 mm was cast under different electromagnetic frequency to observe and investigate the effects of EMF on the solidified microstructures. Grain refinement mechanism and the effects of electromagnetic frequency on the grain size and its distribution were analyzed. Finally, the effect of EMF on Al8Mn5 phase formation of AZ31B magnesium alloy was discussed.

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2. Experimental apparatus and methods 2.1 Experimental set-up and material Figure 1 shows the experimental apparatus used in the present study. The experimental set-up consists of two systems including a crystallizer system of direct chill (DC) electromagnetic casting and an electromagnetic control system. The crystallizer system comprised a metal delivery tube, a metal feed control and distributor devices, a water-cooled system, excitation coil, inner ring (Φ320mm), also known as inner sleeve, and dummy bar installation. The mold (inner ring) is filled with molten metal through a feeder. The removal of heat through the mold wall is called primary cooling. The cooling water exits the mold bottom through an array of holes to produce a series of water jets that are in direct contact with the billet surface and constitute secondary cooling. Caron and Wells (2009) reported that the large amount (circa 80%) of heat is extracted through secondary cooling during steady-state operation, it is vitally important for the formation of solidification structure. The excitation coil is connected to the

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electromagnetic control system, consequently, the PLC unit of electromagnetic control system is able to

modify electromagnetic parameters, including electromagnetic frequency (ƒ), current intensity (I) and duty cycle (D). Electromagnetic force (Lorentz force) caused by the interaction between excitation current (J) and magnetic flux density (B) forms forced vibration and convection in the melt. When the time varying current passes in the coil, the passage will induce a magnetic field and an eddy current

simultaneously in the melt, of which the interaction generates corresponding electromagnetic force (or

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Lorentz force) in the melt. The electromagnetic force will agitate the liquid metal, resulting in the change of flow field. In conventional Direct-chill casting process, the melt is poured into the crystallizer from

metal delivery tube, and when the metal melt reaches the bottom of liquid sump, it flows upward by

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reaction force, forming a circulation. In this case, melt flow is natural convection, the melt that flows with high velocity is in the center of liquid sump. In the presence of electromagnetic field, the Lorentz

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force will agitate the melt, resulting in forced convection and vibration. In this case, the forced convection dominates melt flow, when the melt flows into the crystallizer, it flows to the edge under the agitation of Lorentz force. Due to the largest Lorentz force at the edge of crystallizer, the melt flow velocity at the edge is larger, which makes the heat transfer between the molten metal and primary cooling wall stronger,

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changing the melt temperature distribution in the liquid sump.

Fig. 1 Schematic of the experimental set-up. 1- electromagnetic control system, 2- induction coils, 3feed control & distributor, 4- counter Weight, 5- liquid metal, 6- cooling water inlet, 7- water-cooled mould, 8- cooling water outlet, 9- solid metal, 10- dummy bar. 3

Table 1 Composition of AZ31B alloy in wt. % Elements

Al

Zn

Mn

Si

Fe

Ni

Cu

Mg

wt. %

2.79

0.74

0.31

0.012

0.001

0.009

0.002

Bal.

To prepare specimens for processing in the electromagnetic field, commercial AZ31B alloy (see Table 1 for composition) was melted in a resistance heating furnace under the protection of a gas mixture with a proportion of 0.5% SF6 and balanced CO2. Since liquidus temperature of the alloy is 903 K, the melt was overheated to 968 ± 10 K. The melt was kept 25 min, and then poured into the inner ring of crystallizer to form a billet with a diameter of 320 mm. The casting speed was 0.001m/s. Meanwhile, cooling water flow was 11.3 m3/h. An electromagnetic field was applied when the casting process was stable. The billet was cast to sufficient length (about 600 ~ 900 mm) under each electromagnetic condition before adjusting to the next electromagnetic parameter. The whole casting process was continuous. Electromagnetic parameters are shown in Table 2. Finally, all samples were taken from

Table 2 Electromagnetic parameters used in present experiment. No.

1

2

3

ƒ/Hz

0

5

8

I (time-average value)/A

0

60

60

4

5

6

10

15

20

60

60

60

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2.2 Microstructure observation

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casting billets. The billet did not receive any additional heat treatment after casting.

To reveal the effect of EMF on solidified structures with different electromagnetic frequencies,

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several sheet samples having a thickness of about 15 mm were cut from the billet along the cross section at different position under different electromagnetic conditions. As shown in Fig. 2, the samples were cut along the diametric direction. All samples were ground and polished by traditional approach for

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metallographic preparation and then etched with a solution of ethanol (70 ml), picric acid (4.2 g), acetic acid (10 ml) and distilled water (10 ml). The grain sizes were measured using the Image-Pro plus (IPP) software. Directly measure in the length and width of the grain that can be identified in each region, obtaining the size of each grain. More than 70% (about 200 ~ 300) grains in each region were counted.

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Considering the inhomogeneity of solidified structure, the samples were divided into 5 parts (from A to E) along the radial direction (as shown in Fig. 2). The characterization of macrostructures was captured

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by Olympus DP74 optical microscopy.

Fig. 2 Schematic illustration of samples for microstructure observation. To understand the average grain size and discrete grain size distribution under different electromagnetic conditions, mathematical statistical methods were employed in this study. Chen et al. (2019) measured grain size distribution using normal distribution probability density functions. Normal 4

distributions are important in statistics and used in the natural and social sciences to represent real-valued random variables whose distributions are not known, and Thus, the normal (or Gauss or Gaussian) distribution probability density function (as shown in Eq. (1)) was used to fit real and fine-grain polycrystalline materials in this study, where 𝑑 is the grain size, 𝑑𝑐 is the average grain size of each part, σ is the standard deviation (SD), and 𝜎 2 is the variance. 𝑓(𝑑) =

A σ√2𝜋

𝑒

−

(𝑑−𝑑𝑐 )2 2𝜎2

(1)

This method can improve solving accuracy of 𝑑𝑐 compared with traditional estimation method. To investigate the uniformity of the grain size distribution along radial direction and refinement efficiency for each electromagnetic condition, linear fitting was carried out. Finally, the microstructure, phase compositions, and morphologies were analyzed by scanning electron microscopy (SEM, Zeiss ULTRA PLUS) using an accelerating voltage of 20 kV with the imaging mode of backscattered electron images.

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2.3 Numerical simulation model and procedures To further explain and understand the effect of frequency on the grain refinement of AZ31B alloy

during DC casting under oscillation electromagnetic field, based on the experimental pulse current, a transient 2D axisymmetric mathematical model that couples the electromagnetic fields with fluid flow

and solidification was established to investigate the effect of oscillation electromagnetic frequency on

solidification process using the COMSOL multiphysics software. Fig. 3, Tables 3 and 4 show the

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geometry model and boundary conditions used in numerical simulations. Numerical simulation procedures for coupling transient electromagnetic field, flow field, and temperature field are shown in

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Fig. 5.

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Fig. 3 The geometry model (a) and thermal boundary (b) used in numerical simulation of DC casting. Table 3 Physical properties and dimension of model used in the electromagnetic simulations. Material

Dimension (mm)

Melt

AZ31B

160×400

1

Mold

Al alloy

12×160

1

3.0E-8

Coil

Copper

40×50

1

1.75E-8

Air domain

Air

Radius = 800

1

-

Far field

Air

Radius = 850

1

-

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Component

Relative permeability

Resistivity (Ω•m) 2.98E-7

Table 4 Boundary conditions and relevant parameters used in flow and temperature fields simulations. Boundary zone Inlet

Parameter and value 𝑈𝑖𝑛 = 0.036 m/s, 𝑇𝑖𝑛 = 968 K, k = 1.0913×10-5 m2/s2, 𝜀 = 1.1847×10-7 m2/s3 5

Free surface

𝑈 = 0, h = 10 W/(m2•K), k = 0, 𝜀 = 0

Primary cooling

𝑈 = 0, ℎ in Fig. 6

Second cooling

𝑈 = 𝑈𝑐𝑎𝑠𝑡 , ℎ in Fig. 6

Outlet

𝑈𝑐𝑎𝑠𝑡 = 0.001 m/s, ℎ = 25 W/(m2•K)

1600

22500

Secondary cooling 20000

1400

17500

1200

15000 1000 12500 800 10000 600 7500 400

5000

200

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Heat transfer coefficient (W/(m^2*K))

Primary cooling

2500

0 0

200

400

600

800

1000 0

200

400

600

Temperature (K)

800

0 1000

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Fig. 4 The heat transfer coefficient of primary cooling and secondary cooling zones.

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Fig. 5 Numerical simulation procedures for coupling transient electromagnetic field, flow field, and temperature field.

2.4 Phase amounts calculation procedures

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Laser et al. (2006) studied the influence of the Mn content on the microstructural evolution and mechanical properties, and suggested that the presentation of the amount of phase vs. temperature has proven to be the best way to illustrate a solidification process. Sun et al. (2016) investigated the

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solidification of AE42 (Mg-4Al-2RE, wt. %) alloys using CALPHAD simulation and experimental validation to understand the non-equilibrium solidification microstructure of AE42 alloy using CALPHAD simulation (PANDAT software Scheil model) and experimental validation. In this work, the melt flow and heat transfer can be changed in the absence of electromagnetic field, resulting in a larger cooling rate, and the melt solidifies in a few minutes. In order to predict the amounts and formation temperature of precipitated phase in this condition, the calculated phase amounts of AZ31B alloy based on the classical non-equilibrium solidification Scheil-Gulliver model. This model assumes: (1) no diffusion in solid and complete mixing in liquid; and (2) equilibrium at the solid–liquid interface. The PANDAT software developed by CompuTherm LLC (version 2016.1) and its PanMagnesium database are used in the present calculation. The only inputs for the calculations are the calculation start and end 6

temperature (720 ℃ and 300 ℃, respectively), maximum temperature step size (1 ℃), and alloy composition (AZ31B alloy). No kinetic parameters are needed in the Scheil solidification simulation. 3. Model validation In order to further prove the correctness of the model, the depth of liquid sump along the radius was measured during the production of Φ300mm billet in factory. The mold structure is the same as that used in the present study. With the same casting conditions and parameters used in the production and referring to the model in this study, the corresponding model for melt flow under magnetic field was established, and the shape of liquid sump (the solid/liquid interface shape) at stable casting stage predicted by this model was compared with the experimental data, as shown in Fig. 6. It shows a specific agreement between the simulation and experimental results. The discrepancies at the edge and 1/2R (R, radius) liquid sump may be attributed to the 2D approximation, measurement errors, and constant physical properties used in this simulation. In the real DC casting process, melt feed control & distributor devices

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are usually used to disperse the melt when preparing large-size billet, melt outlets are space alternate

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distribution. The model validation has been added and highlighted in the current manuscript.

Fig. 6 Measured liquid sump depth and calculated liquid sump shape.

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4. Results and discussion

4.1 Macrostructure evolution under EMF with different frequencies Figure 7 shows the macrostructures of as-cast AZ31B alloy. For an easy description, edge areas were

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indicated at the right of diagram, as highlighted by red dashed line, fine-grain areas were indicated in blue box, coarse columnar grain areas were indicated in yellow box. Regardless of whether EMF is applied or not, it can be seen that there is a fine-grain region with the width of about 5mm at the edge of

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billet. In addition to ƒ = 15 Hz, the adjacent to the fine-grain region is a coarse columnar crystal region with the width of about 15 ~ 25 mm. In the absence of EMF, macrostructures are highly inhomogeneous, consist of typical fine equiaxed grains, coarse equiaxed grains, and coarser columnar grain. In terms of samples processed by EMF, the billet has a wide columnar grains region with the width of approximate 25 mm at the edge of billet (expect 15 Hz). However, the columnar grain region decreases to about 12 mm at frequency of 5 Hz, the central structures are fine and homogeneous, indicating that EMF is effective in refining coarse grains into small ones and in reducing the columnar grains region. When the frequency is increased to 8 Hz, the macrostructure consists of fine equiaxed grains and several columnar grains, and the structure is subtly finer than observed previously, more homogeneous. When the frequency is further up to 10 Hz, the grains became coarser, the homogeneity of structure is not improved. 7

With the further increase of frequency to 15 Hz, fine and uniform macrostructures with more equiaxed grains and few developed columnar grains can be obtained, exhibiting little difference in morphology. At the frequency of ƒ = 20 Hz, coarse structures and a wide columnar grains region appear again, even

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coarser than in the absence of magnetic field.

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Fig. 7 Macrostructures of DC casting AZ31B billet near the mold showing fine crystalline zone, ~5mm from edge showing coarse columnar crystal zone, coarse equiaxed grain region, and enlarge fine equiaxed grain area.

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The solidified structures are closely related to the cooling condition. In conventional DC casting of magnesium alloy billet, a solid shell can be formed in primary cooling zone. It is vitally important for the surface grain structures. A chill zone, made of some small equiaxed grains in Fig. 7, is the result of fast convective heat transfer between the melt in the sump and the mold. However, only a minor amount

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of heat (circa 20%) is extracted through primary cooling compared to the secondary cooling. Thus, the central temperature in the liquid sump is higher than the liquidus temperature. In this region, the grains that grow in the opposite direction to the heat extraction, grow faster and form columnar regions, while

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other grains are eliminated by competition. After the columnar grains grow to a certain stage, the branches detached from the dendrites can grow independently, and the latent heat is derived from the supercooled liquid. The grains tend to grow in an equiaxed manner, forming the center equiaxed crystal

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region. Since the transition from columnar to equiaxed growth is largely dependent the convection in the liquid, Li et al. (2012) suggested that electromagnetic vibration and stirring are often used in DC casting to facilitate this transition. In this study, the columnar regions and grain size are significantly reduced in the presence of electromagnetic field during alloy solidification. Stefanescu (2009) proposed the columnar to equiaxed transition condition. It suggested that the

probability of formation of an equiaxed structure increases as the nucleation potential, and the undercooling increase, and as the coherency solid fraction and liquid convection decrease, see in Eq. (2). 𝐺𝑇 ≤ 3.22 [

̅ 𝑁 𝑓𝑠𝑐𝑜ℎ

(1 +

𝑉𝐿 𝑉

1⁄3 𝜇 𝑒

)]

𝜇𝑐

∆𝑇

(2)

̅ is the average volumetric grain density, 𝑓𝑠𝑐𝑜ℎ is the solid fraction of dendrite coherency Where, 𝑁

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point, the 𝑉𝐿 is the flow x-direction, 𝑉 is the solidification velocity, 𝜇𝑒 and 𝜇𝑐 are the growth coefficient of the equiaxed dendrites and columnar dendrites, respectively, and ∆𝑇 is the undercooling in the volume element. Research indicates that the difference in resistivity is ubiquitous for both solid and liquid. Thus, Li et al. (2007) proposed that there exists a relative movement between solid and liquid in mushy zone, which can break dendrites into pieces. Simultaneously, forced convection induced by EMF disperses the inclusions and distributes them uniformly in the melt, which provides sufficient nucleation substrates. The melt is cooled down directly below the liquidus temperature. Zuo et al. (2012) showed that this high cooling rate results in large undercooling and activates more particles in the melt to become sites for the heterogeneous nucleation. In addition, Ohno (1987) put forward the “separation theory”, it revealed that the nucleation on the mold wall and the detachment of the nuclei also contribute to the nucleation rate increasing. Zhang et al. (2007) reported that in the presence of electromagnetic field, the forced convection can decrease the depth of sump, causing the melt temperature to be lower than liquidus temperature, consistent with the results of simulations in the present study, it will be proved

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in the follow-up discussion (chapter 3.2). Most of the formed nuclei from the mold wall and elsewhere

can survive to act as effective nucleation sites. The reasons for the appearance of structural characteristics

are shown in Fig. 7. Hatic et al. (2018) the effect of EMF on the liquid sump depth. It proved that the depth of liquid sump decreases in the presence of EMF, resulting in the shape change of solid-liquid interface. An increase angle between interface of solid-liquid and mold results in the direction of heat

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extraction, changing growth directions of columnar grains. Li et al. (2017) studied effect of magnetic field on the solidified structures of aluminum alloy; and Cui et al. (2010) carried much work on the

relationship between electromagnetic field and microstructure of DC cast magnesium alloy. Both of them

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proved that the columnar grains regions decrease with the change of electromagnetic frequency; and the orientation of columnar grains can also be changed.

4.2 Effect of electromagnetic frequency on grain size distribution

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Li et al. (2010) emphasized that the average grain size depicts an overall concept regarding to the macrostructural morphology, it cannot reveal the distribution of the specific size of individual grain, especially for the large diameter of billet. Therefore, to evaluate the quotient of grains with different average

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diameters, we counted the number of grains within a certain size interval, and then the number fraction corresponding to the grain size interval could be determined. Fig. 8 shows the frequency distributions statistical results of grain number fraction, which is a function of grain size of AZ31B alloy. Figs. 8 (a)(f) show the number fraction in different intervals of grain size of the alloy solidified at 0 Hz (without

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EMF), 5 Hz, 8 Hz, 10 Hz, 15 Hz, and 20 Hz, respectively. As can be seen in Fig. 8 (a), without EMF, the maximum grain size is close to 3500 μm, the average grain size of each specimen is in the range from 536 to 1094 μm. This indicates that the microstructure is uneven, scattering a large interval. At frequency

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of 5 Hz (Fig. 8 (b)), the specimens have the average grain size in the range from 357 to 673 μm. When the frequency is increased to 8 Hz (Fig. 8 (c)), the distribution of average grain size spans a small range from about 455 to 592 μm, and structure uniformity is further improved. However, with the increase of frequency to 10 Hz (Fig. 8 (d)), the average grain size is in the range from 559 to 752 μm, the structure is obviously coarser than observed previously. At the electromagnetic frequency of 15 Hz, the relative frequency of grains size has the highest quotient, see in Fig. 8(e). The distribution of average grains size spans a smaller range from about 402 to 486 μm, revealing that the macrostructure is rather homogeneous, coarser columnar grain almost approaches to naught. When the frequency is further up to 20 Hz (Fig. 8 (f)), the distribution of average grains size spans a small range from about 519 to 681 μm, but the maximum and the number fraction of grains size is larger than that without EMF. 9

In order to accurately describe the uniformity of the grain size distribution and the refinement effect at different electromagnetic frequencies. The average grain size linear fitting and its histogram of the specimens at different positions with different electromagnetic frequencies were carried out, as shown in Figs. 8 (g) and (h). Regression line of the average grain size without EMF has a positive slope (3.87), which means that the grain size increases progressively from center to edge and the structures are extremely inhomogeneous. Regarding the EMF treated billet at frequency of 5 Hz, the slope transforms to negative, is reduced to -1.69, indicating that the grain sizes are smaller and macrostructure homogenization is improved. With the increase of electromagnetic frequency, the slope is 0.14, -1.72, 0.67, and 0.69 at frequency of 8, 10, 15, and 20 Hz, respectively. Thus, with EMF treated at 8 Hz, the structures are the most uniform, but grain sizes are coarser than others. Beyond that, at the frequency of

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15 Hz, the macrostructures are not only homogeneous but also fine in the whole billet.

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Fig. 8 Frequency distributions statistical results of grain size of AZ31B alloy showing grain number fraction versus grain size, (a) without EMF; (b) f = 5 Hz; (c) f = 8 Hz; (d) f = 10 Hz; (e) f = 15 Hz; (f) f along with radial direction. 4.3 Effect of electromagnetic frequency on macro-physical fields

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= 20 Hz, (g) and (h) are the average grain size distribution under different electromagnetic conditions

Le et al. (2007) studied the flow pattern and temperature field of DC casting of magnesium alloys with

and without EMF by numerical simulation, revealed that the vortex agitation produced by EMF reduces the temperature gradient and shallows liquid sump depth during DC casting, and the surface quality of

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the billet could be improved owing to the lower contact pressure. In the present study, the variations of

Lorentz force under different electromagnetic frequency were obtained by means of numerical simulation. Fig. 9 shows the transient Lorentz forces r-direction and z-direction components (at point b in Fig. 3(a))

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under different electromagnetic frequency in several periods. The magnitude of Lorentz force varies periodically with time. It is clear that the Lorentz force is mainly along the radial direction of the billet, and the r-direction component of Lorentz is far greater than that of z-direction component. The

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distribution of Lorentz forces along the radial direction (line a-b in Fig. 3(a)) in the melt with different frequencies was shown in Fig. 10. The direction of Lorentz force on the given path is mainly perpendicular to the mold wall and points to the center of the billets, resulting in forced convection. With

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the increase of frequency, r-directional Lorentz force and the attenuation rate of Lorentz force along the

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radial direction of billet decrease gradually. It is vitally important for melt flow and solidification process.

Fig. 9 Transient Lorentz forces r-direction and z-direction components with different electromagnetic frequency in several periods.

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Fig. 10 Maximum r-direction (a) and z-direction (b) Lorentz force from the center to the edge of billet. Fig. 11 shows the variations of velocity at the different stages of solidification in the absence and

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presence of EMF. It can be found that as the DC casting process goes on, the flow gradually tends to be stable. In the absence of EMF, the maximum flow velocity is mainly at the center of the liquid sump, resulting in a deeper liquid sump, which is not conducive to obtaining a uniform temperature field

distribution in the radial direction. In the presence of EMF, the maximum flow velocity is transferred to the upper surface of the melt. The reason for this is that the Lorentz force can drive the melt convection

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along the radial direction, which can strengthen the heat extraction. Therefore, time to reach stable casting stage in the presence of EMF is shorter. Fig. 12 shows the variations of velocity at the center and edge of liquid sump under different casting conditions at the stable casting stage. It is worth noting that

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the velocities of the center and the edge of the melt fluctuate with different magnitudes in the presence of EMF, but the velocities of the center and the edge of the melt are almost unchanged during the conventional casting process. With the increase of electromagnetic frequency, the maximum speed

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decreases, while the fluctuation frequency increases. In addition, when the EMF is applied, the velocity at the entrance of the center (point a) decreases obviously, and the edge melt flow strengthens, which

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means that the application of EMF can significantly enhance melt oscillation and convection.

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Fig. 11 Velocity vector at the different stages of solidification in the absence and presence of EMF; (a),

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(b), (c), and (d) are in the absence of EMF, (e), (f), (g), and (h) are in the presence of EMF (15 Hz) at t = 50 s, t = 150, t = 300 s, and t = 400 s, respectively. The solidus and liquidus are denoted by blue and

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red lines, respectively.

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Fig. 12 Variation of velocity at the center (a) and edge (b) of liquid sump under different casting conditions.

Fig. 13 shows the temperature distribution at the different stages of solidification in the absence and

presence of EMF. Due to the force convection generated by EMF, the melt temperature in the presence of EMF decreases rapidly, and the heat transfer efficiency is higher. Therefore, the casting process first reaches to a stable stage under EMF, obtaining a shallower depth of the liquid sump. Fig. 14 shows the temperature distribution along the radial and axial direction of billet under different electromagnetic frequencies at the stable casting stage. The results of radial and axial temperature distribution of billet in DC casting process show that the forced convection induced by EMF can significantly enhance the heat extraction of melt. When the frequency is 20 Hz, the fluctuation of melt velocity is small and the

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convection of heat transfer is weak, therefore, the overall structures of billet section are coarse. Although a large velocity fluctuation range is obtained at the frequency of 8 Hz or 10 Hz, the fluctuation rate is smaller, resulting in weak convection heat transfer inside the liquid sump, the temperature is slightly lower than that of 15 Hz (as shown in Fig. 14(c)). Therefore, the solidified structures at the frequency of 8 Hz or 10 Hz are similar. At the frequency of 5 Hz, the fluctuation of melt velocity further increases, and the lower fluctuation rate and larger radial restraining force are not conducive to the contact heat transfer between melt and mold, therefore, the developed columnar grain structures appear at the edge of billet. When the frequency is 15 Hz, the melt temperature is lower and its distribution is relatively uniform while the melt speed fluctuations maintain a large amplitude and frequency. Under the circumstances, the melt can be subjected to strong convection and oscillation at lower temperatures,

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which is beneficial to obtain a uniform and fine macrostructure.

Fig. 13 Temperature distribution patterns at the different stages of solidification in the absence and

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presence of EMF; (a), (c), (e), and (g) are in the absence of EMF, (b), (d), (f), and (h) are in the

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presence of EMF (15 Hz) at t = 50 s, t = 150, t = 300 s, and t = 400 s, respectively.

Fig. 14 Variation of temperature along radial (a) and axial (b and c) direction of billet at the stable

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casting stage. 4.4 Effect of EMF on Al8Mn5 phases formation and transition The optimal structures were obtained at the frequency of 15 Hz. Therefore, a detailed discussion on the morphology and distribution of phases in the absence and presence (15 Hz) of EMF was carried out. To reveal the effect of EMF on phase distribution, backscattered electron (BSE) images of the alloy were depicted in Fig. 15. There are massive, long rod-like, and lamellar structures. The size of long rod-like phases is about 20 ~ 50 μm. The amount of long rod-like phases reduces from the center to the edge in the absence of EMF. With the application of EMF, the amount of long rod-like intermetallic compounds is slightly reduced. Similarly, the amount of central long rod-like intermetallic compounds is less than the edge. Compositions of phases and element mapping were carried out for Mg, Al, Mn, and Zn using the EDS, as depicted from Fig. 16. It is noteworthy that the long rod-like phases have a considerable thickness deep into the matrix, like a lath-shaped, as highlighted in red dashed dimension box in Fig. 16.

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The lath-shaped phases are manganese-rich region, the massive and lamellar phase are the coexistence region of Al and Zn elements. Stanford and Atwell (2013) reported that under equilibrium conditions,

the ternary system Mg-3Al-1Zn can form the Mg17Al12 phase known as γ, along with a third phase known

as φ at room temperature. Thus, the coexistence region of Al and Zn elements might form γ-Mg17Al12 or

φ or both of them. However, the commercial alloy is complicated than the ternary basis. Gröbner et al. (2005) observed that Mn addition can decrease the solubility of Fe in Mg drastically and therefore leads

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to the precipitation of Fe, which settles down in the Mg melt. Moreover, Mn forms a protecting layer

during oxidation of Mg. Therefore, AZ31B alloy is the commercial designation of AZ31 with 0.31 wt. % manganese (Mn) added to improve corrosion resistance in this work. Thermodynamic predictions

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indicated that Mn and Al were able to formed several intermetallic compounds in AZ31B in the solid state. The manganese-rich region is likely to form Al-Mn phases. Therefore, further semi-quantitative analysis of different fine spots, revealing that there are two kinds of phases of manganese-rich and

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manganese-poor phases, as shown in Table 5.

Fig. 15 BSE images of the as-cast AZ31B alloy displaying the different phase distributions. (a), (b), and (c) were obtained without EMF; (d), (e), and (f) were obtained with EMF at the frequency of 15 Hz. (a) and (d) are at the center of billet (E sample in Fig. 2), (b) and (e) are at the mid-radius (1/2R, C sample in Fig. 2), (c) and (f) are at the edge (A sample in Fig. 2).

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Fig. 16 Depicts the BSE images of several phases mentioned above in AZ31B alloy. The corresponding EDS mapping of the area with K-alpha peak of Mg, Al, Mn, and Zn reveals the distribution of these elements in compounds and matrix.

Positions

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Table 5 Chemical composition determined by EDS analysis for the phases in Fig. 11 (at. %). Element Al

Mn

Zn

Total

A

44.23

34.37

20.64

0.76

100

B

59.37

22.44

0

18.19

100

C

61.93

24.94

0.05

13.09

100

D

68.84

24.06

0.07

7.04

100

E

56.38

21.86

0.06

21.7

100

F

96

3.41

0

0.6

100

G

62.6

21.1

0

16.3

100

H

61.17

30.7

0

8.13

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Mg

The non-equilibrium solidification phase compositions and amounts of AZ31B alloy in DC casting process based on the Scheil-Gulliver model were predicted, as shown in Fig. 17. Considering the binary

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phase diagram of Al-Mn, a Mn content between 34 at. % and 53 at. % corresponds to Al8Mn5 and Mn content above 59 at. % corresponds to β-Mn. Based on this, microstructure evolution of the alloy was predicted. The massive particles are determined to be Mg17Al12 phase. There are not only Mg17Al12 phase

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but also φ phase in lamellar phases. The lath-shaped phases are determined to be Al8Mn5.

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Fig. 17 Phase amounts (atomic fractions) calculated for the solidification under Scheil-Gulliver model the left side of the diagram.

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vs. temperature for AZ31B alloy. The final amount of frozen-in solid phases is indicated in the box at

The reasons for the formation of lath-shaped Al8Mn5 phase are as follows. As shown in Fig. 17, Al8Mn5 and α-Mg precipitate almost at the same time, even prior to α-Mg at a large cooling rate.

Therefore, reactions involving Mn element occur in the solidification process are as follows:

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Liquid→Al8Mn5 (first step), Liquid→α-Mg + Al8Mn5 (second step), and Liquid→α-Mg + Al8Mn5 + Al11Mn4 (third step). Zeng et al. (2017) found that the eutectic Al8Mn5 grew with a complex faceted morphology ranging from rod to sheet and folded plate-like, often with growth steps on the largest facets.

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In electromagnetic DC casting process, forced convection can significantly reduce the melt temperature, as shown in Fig. 18. The first reaction is generally in a very narrow temperature range, therefore, due to rapid cooling rate in electromagnetic DC casting process, the reaction time of Liquid→Al8Mn5 is very

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short, and then eutectic reaction occurs. Therefore, eutectic transformation is dominant in the

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electromagnetic DC casting process, resulting in rod or lath-shaped Al8Mn5 phase formation.

Fig. 18 Variation of melt temperature (the point at half of the path a-b) in the casting process in the absence and presence of EMF. IPP software was used to count the area of eutectic Al8Mn5 phase. Results (in Fig. 19) show that

from center to edge of billet, the fraction of Al8Mn5 phase decreases form 29.3%, 14.4%, and 7.1% (in the absence of EMF) to 12.3%, 7.5%, and 2.6% (in the presence of EMF), respectively. It reveals that the Al8Mn5 phase was inhomogeneous, reduced from the center to edge of billet in the absence of EMF

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while Al8Mn5 phases reduced slightly in the presence of EMF. The mechanisms can be explained as follows. On the one hand, during the smelting process, the Fe element is usually removed by standing and dropping temperature to ~ 680 ℃ after Mn addition at 730 ~ 740 ℃. In this process, the temperature distribution is uneven, which may cause local formation of Al8Mn5 phase in the crucible. When the melt was poured into the mold, melt convection at the center of liquid sump is not conducive to the complete diffusion of the Mn element along the diametrical direction of billet, especially for large-sized billets. Mn element tends to gather the center of billet. Luo et al. (2015) studied the macrosegregation of main solute elements Al, Zn and Mn in the DC casting ingots of AZ80 alloy, found that without EMF, the content of Mn element in the center of the billet is higher than that in the edge. In this work, the macro segregation results of Al and Mn results show that that without EMF, the content of Al element in the 1/2R (R, radius) of the billet is higher than that in the edge and center of billet, and the distribution of Al element is relatively uniform in the presence of EMF at 15 Hz. Moreover, it can be found that the distribution of Mn element at the center of billet is slightly higher than that at the edge of billet whether

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EMF is applied or not. The contents of Al and Mn in the billet prepared with EMF is higher than that of

conventional DC casting (without EMF), as shown in Fig. 20. The temperature dropped to 640 ~ 660 ℃ when the melt was poured into the mold. Due to the small temperature difference between the formation

of α-Mg and Al8Mn5 phases (about 640 ℃, as shown in Fig. 12), Al8Mn5 particles formed firstly at the center of liquid sump, even prior to the formation of α-Mg. These are the reasons why the phase amount

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of Al8Mn5 is large, and the reasons why the central amount of Al8Mn5 phase are higher than the edge in conventional DC casting. On the other hand, in the presence of EMF, forced convection can strengthen the heat extraction and increase the cooling rate, as shown in Fig. 18, the time for the casting process to

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reach a stable state is about 60 s, which is much smaller than that without EMF (113 s). Compared to that in the absence of EMF, the melt temperature in the presence of EMF is reduced by 19 K (in Fig. 18) and the uniformity of temperature distribution is improved (in Fig. 14(a)). Low temperature could restrain

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the reactions of Mn element, promoting the solid solution of Al in matrix. Therefore, the amount of

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Al8Mn5 phase is generally lower than that in the absence of EMF.

Fig. 19 Phase areas of AZ31B alloy billet prepared by DC casting in the absence and presence of EMF.

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5. Conclusions

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Fig. 20 Macro segregation of billet in the absence and presence (at 15 Hz) of EMF.

The AZ31B magnesium alloy billet with diameter 320 mm was solidified under an oscillation EMF with the imposition of pulse current. The microstructure evolution was quantitatively examined in terms of the grain size and its distribution. The formation mechanisms of the microstructure and the phase transition were discussed. The main conclusions are drawn as follows:

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(1) The grain size decreases from 549 ~ 1094 μm (without EMF) to 402 ~ 486 μm at the frequency of

15 Hz. Columnar grains region decreases significantly. As the frequency decreases or increases,

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columnar and coarse structures can be obtained.

(2) With the decrease of frequency, greater Lorentz force can be obtained, which can strengthen the forced convection and heat extraction in the melt, obtaining larger cooling rate and uniform

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temperature distribution.

(3) For the formation of Mn-containing phases, eutectic transformations are dominant in DC casting process of AZ31B magnesium alloy. From center to edge of billet, the fraction of eutectic Al8Mn5 phase decreases form 29.3%, 14.4%, and 7.1% (without EMF) to 12.3%, 7.5%, and 2.6% (in the

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presence of EMF), respectively.

Author Contribution Statement

Yonghui Jia: Conceptualization, Methodology, Data curation, Writing- Original draft preparation,

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Writing - Review & Editing

Jian Hou: Methodology, Investigation Hang Wang: Software, Methodology

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Qichi Le: Supervision, Project administration, Writing - Review & Editing Qing Lan: Formal analysis, Methodology Xingrui Chen: Writing - Review & Editing Lei Bao: Formal analysis Declaration of Interest statement No conflict of interest exits in the submission of this manuscript. The work described has not been submitted elsewhere for publication, in whole or in part, and all the authors listed have approved the manuscript that is enclosed.

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