ORIGINAL ARTICLE

Improvement of AA5052 sheet propertiesby electromagnetic twin-roll casting

J. T. Li1,2 & G. M. Xu1& H. L. Yu2

& G. Chen1& H. J. Li2 & C. Lu2

& J. Y. Guo3

Received: 6 April 2015 /Accepted: 11 October 2015# Springer-Verlag London 2015

Abstract Electromagnetic fields were used in twin-roll cast-ing (TRC) of aluminum alloy 5052 (AA5052) for improve-ment of the microstructure and mechanical properties. A staticmagnetic field induces an inhibiting effect on the melt in thecast-rolling area and reduces diffusion of the solutes. It alsoresults in more nucleating opportunities and less segregation,thus enhancing the mechanical properties. However, the staticmagnetic field does not change the orientation of crystalgrowth and columnar crystals still exist in microstructure.On the other hand, an oscillating magnetic field can refinethe suspended particles and induce strong convection. Thisleads to more uniform distribution of temperature and soluteelements, simultaneously increasing nucleating opportunitiesand decreasing segregation, thereby enhancing themechanicalproperties. An oscillating magnetic field also inhibits the ori-entation of crystal growth and makes finer and equiaxedgrains.

Keywords AA5052 . Twin-roll casting . Segregation .

Inhibiting effect . Oscillating effect

1 Introduction

Twin-roll casting (TRC) is proposed as an alternative to con-ventional direct chill (DC) casting followed by hot rolling, aproven technology for economical production of thin alumi-num sheets directly from the melt [1–3]. TRC is a shorterprocess route that combines casting and dynamic hot defor-mation in a single step and is suitable for the production of thinsheets (mm gauge). The method involves pouring the meltinto the gap between two rotating and water-cooled cylindricalrollers. The metal solidifies just before reaching the bite of therollers and is then rolled as it passes through the rollers. Com-pared with conventional DC casting, the TRC technique canlead to reduced capital and operating costs, less energy con-sumption, and reduced scrap rate.

In the TRC process, several methods have been used forcontrolling the microstructure and thus the mechanical prop-erties. Berg et al. [4] accomplished gauge reduction from 5 to1.9 mm. The thicker 5-mm strip did indeed show the typicaldual-grain microstructure expected of TRC material in the as-cast state. However, the 1.9-mm strip exhibited purelyequiaxed grains finer in size as compared to the 5-mm strip.No cell structure region was seen within the grains, and solid-ification was apparently completed by equiaxed grain growthin the mushy zone. Haga et al. [5] combined low superheatcasting and semisolid casting with an unequal diameter twinroll caster with a long solidification length. The macrostruc-ture of the as-cast strip was equiaxed and spherical, not co-lumnar. Haga et al. [6] also carried out semisolid strip castingwith a twin roll caster equipped with a cooling slope, leadingto improvement in sheet elongation. Das et al. [7] comparedthe microstructure resulting from TRC with that produced bymelt-conditioned TRC (MC-TRC). A uniform, fine, andequiaxed grain structure was observed in as-cast MC-TRCsamples. However, coarse columnar grains with centerline

* G. M. Xuxu_gm@epm.neu.edu.cn

* H. L. Yuhailiang@uow.edu.au

1 Key Laboratory of Electromagnetic Processing of Materials(Ministry of Education), Northeastern University, Shenyang 110819,China

2 School of Mechanical, Materials and Mechatronic Engineering,University of Wollongong, Wollongong, NSW 2500, Australia

3 Hongyanhe Project Department, China Guangdong Nuclear PowerEngineering Co. Ltd, Dalian 116319, China

Int J Adv Manuf TechnolDOI 10.1007/s00170-015-7963-8

segregation were observed in the as-cast TRC samples. Kimet al. [8] investigated the feasibility of producing high-strengthAl sheets with high solute content using a twin roll strip casterequipped with an asymmetric nozzle. The centerline segrega-tion and hot tear of the sheets could be reduced, thereby in-creasing the casting speed. Cheon et al. [9] successfully fab-ricated Al alloy sheets using a combination of twin roll stripcasting and asymmetric rolling. The Al sheets exhibited ex-cellent formability and mechanical properties, far exceedingthose in commercially available Al sheets. Sun et al. [10]homogenized the AA3105 alloy prior to cold rolling and an-nealing and found that homogenization reduced the supersat-uration and coarsened the constituent particles, thus reducingthe total number of particles. Homogenization also resulted ina marked reduction in the recrystallized grain size and fasterrecrystallization kinetics after cold rolling and annealing.Birol [11] also investigated the effect of homogenization onTRC-processed thin Al-Mn strips and obtained similar results.

Imposition of an electromagnetic field can provide morepossibilities to control the development of the microstructure.Hicher et al. [12] designed a new mirror furnace crystalgrowth device using a static external electric field. They founda significant change in the melting temperature and a strongchange in the separation between the liquid and solid statesduring growth upon application of an electric field. Ma et al.[13] analyzed the thermal and fluid effects caused by applyingalternating electric fields during BiMn/Bi eutectic directionalsolidification. They found that the microstructure formationdepends on the frequency of the applied alternating currentand that it changes spontaneously as the alternating electricfield is applied. Li et al. [14, 15] investigated semi-continuouscasting of Al alloy with the application of a static magneticfield. They found that the static magnetic field changes themicrostructure from regular columnar grains to twinned la-mellas. Jie et al. [16] used a rotating magnetic field (RMF)during solidification of hypereutectic Al-Si alloy and foundthat it resulted in efficient congregation of the primary Siphase near the inner wall of the crucible and formation of aSi-rich layer. This suggested that a forced intense melt flowcombined with proper cooling conditions can greatly changethe solidification structure of alloys, which could be beneficialfor microstructure control. Zhang et al. [17, 18] studied theinfluence of a low-frequency electromagnetic field on the mi-crostructure and macrosegregation in continuous casting. Inthe presence of an electromagnetic field, a substantial reduc-tion of grain size and macrosegregation of the alloy elementswas achieved. The frequency of the applied electromagneticfield was found to play a significant role in grain refinementand macrosegregation inhibition. Liotti et al. [19] developedan in situ technique for studying the effect of a pulsed electro-magnetic field on dendrite fragmentation. They used synchro-tron X-ray imaging, involving the passage of an oscillatingcurrent through a foil specimen placed in a static magnetic

field. The application of a pulsed electromagnetic field led toan increase in the dendrite fragmentation rate. Yu et al. [20]directly applied alternative current (AC) to the melt in themold through the launder during electromagnetic continuouscasting (EMCC) of copper billets. The imposition of the ACcurrent on the melt during EMCC significantly increased theconvection and vibration in the melt, which are very beneficialfor the solidification of the alloy.

Electromagnetic fields obviously have an effect on grainrefinement and microstructure improvement during melt so-lidification. However, there have been few studies of the effectof an electromagnetic field on TRC. The AA5052 alloy hasbeen attracting increasing attention in recent years because ofits desirable attributes for processing, machining, and welding[21–24], making it suitable for use in the automobile, ship-ping, machinery manufacturing, and other industries [25–27].However, in the forming process of AA5052 sheets, segrega-tion has always been a critical hindrance to improvement ofthe microstructure and mechanical properties [28, 29]. In thepresent study, static and oscillating magnetic fields were im-posed on the AA5052 melt with a view to abating segregationand improving the microstructure and mechanical properties.The effects of the electromagnetic field on the microstructure,segregation, and mechanical properties such as tensilestrength are examined.

2 Experimental investigation

AA5052 sheets were produced by twin-roll casting, both withand without an electromagnetic field. The twin-roll caster hastwo counter-rotating rollers, which are water-cooled from theinside. The roller shell is made of heat-resistant alloy steel.The roller gap is adjusted by hydraulic pressure. The twin-roll caster dimensions are listed in Table 1. An AA5052 alloy(composed of 2.8 % Mg, 0.40 % Mn, 0.50 % Cr, 0.25 % Si,0.10 % Cu, 0.40 % Fe, and 95.5 % Al) was used in the TRCexperiments. An Al ingot was completely melted in an elec-trical resistance furnace. After the furnace temperaturereached 700 °C, Mg, Mn, and Cr were added to the melt.The well-stirred melt was then heated up to 800 °C. Thiswas followed by standing and filtration in the temperaturerange 720–740 °C. When the temperature reduced to690 °C, the TRC experiment commenced. Figure 1 presentsthe flow chart of the melting process of AA5052.

The melt was poured into a sodium silicate thermal insula-tion nozzle which was pre-heated to 400 °C. The twin-rollcaster with copper coil excitation apparatus on the upper rollerwas prepared for the AA5052 sheet production and is sche-matically illustrated in Fig. 2. The experimental rolling speedwas 1.4 m/min. The cross section of the AA5052 sheets was5×400 mm.

Int J Adv Manuf Technol

A static magnetic field was applied to cast-rolling area witha nominal strength of 0.3 T. Three sheets were produced underdifferent conditions: (1) without the applied field (B=0 T andI=0 A), (2) with a static magnetic field (B=0.3 T), and (3)with a combination of three-phase AC (I=200 A) and staticmagnetic field (B=0.3 T).

After the TRC process, the sheet macrostructure wasinspected visually by etching using an etchant (5 % nitric acidand 17 % hydrofluoric (HF) acid) for about 15 s prior torinsing by water. Specimens measuring 5×15×20 mm select-ed from a quarter of the thickness or width were prepared bystandard metallographic polishing with MgO paste followedby etching with 4 % HF acid reagent for about 20 s to observethe microstructures in the vertical cross section, using a LeicaDMI5000M optical microscope (OM). In addition, the distri-bution of Mg was investigated using a Hitachi S-3400N scan-ning electron microscope (SEM). Specimens were also select-ed from the sample cores (half thickness or width) to analyzethe macrosegregation.

Three 200mm long pieces were cut out from the cast-rolledsheets and processed sequentially: 5 mm→annealing at360 °C for 3 h→cold rolling to 2 mm→annealing at 360 °Cfor 3 h→cold rolling to 1 mm→annealing at 360 °C for 3 h.Tensile tests were conducted to evaluate the mechanical prop-erties of the AA5052 sheets. Tensile testing at room tempera-ture in the longitudinal direction was carried out using acomputer-controlled electronic universal testing machineCMT5105 at a constant deformation speed of 1 mm/min.

3 Results

3.1 Structure

The macrostructures of the AA5052 sheets produced using nofield, a static magnetic field, and an oscillating magnetic field,respectively, are shown in Fig. 3. In all three cases, the macro-structure at the edges consists of fine grains. However, the coregrains are comparatively larger for all three conditions. The “nofield” core macrostructure (Fig. 3a) is coarse and unevenlydistributed. The core macrostructure with the static magneticfield (Fig. 3b) is relatively refined but shows grain growth alongcertain orientations. The macrostructure induced by the oscil-lating magnetic field (Fig. 3c) is obviously refined, and thegrain size is uniform with no clear orientation preference.

Figure 4 shows the microstructures of the rolled AA5052sheets: (a) no field, (b) with a static magnetic field, and (c)with an oscillating magnetic field. The no field microstructure(Fig. 4a) has coarse columnar crystals which are a clear pref-erence for growth direction. The grain size was measuredusing a mean linear intercept method and the value is∼24 μm. The “static magnetic field” microstructure (Fig. 4b)is significantly refined compared to the no field microstructure(Fig. 4a), with restrained secondary dendrites and thinner pri-mary dendrites. The dendrites become more uniform anddenser, but the growth of dendrites is perpendicular to theroller surface. The grain size is ∼18 μm. The “oscillating mag-netic field” microstructure (Fig. 4c) shows the maximum re-finement. The dendrites become extremely small with no pre-ferred orientation and very few rod-like crystals. In someareas, the dendrites have degenerated into spherical crystalsand some secondary dendrites are suppressed completely. Thegrain size is ∼15 μm. For comparison, grain sizes of AA 5052obtained using other methods are also listed. The microstruc-ture produced by homogenization at 550 °C for 30 minfollowed by air cooling has coarse grains (∼95 μm) [30].The microstructure of an AA5052 produced by rheo-squeezecasting with a solid fraction of 0.17 solidified under 100 MPaconsists of the relatively large grains (∼75 μm) and fine grains(∼32 μm) [31]. In the l.6-mm thick cold rolled sheet of acontinuous-cast AA 5052, the average length and averagethickness of the elongated grains in the near-surface regionare 18 and 1.5 μm, respectively, whereas the correspondingvalues in the core region are 20 and 2.0 μm, respectively [29].The microstructure resulting from four-pass differential speedrolling is characterized by nearly equiaxed nanostructuredgrains of size ∼0.7 μm [30]. The grain size resulting fromequal channel angular extrusion (ECAE) changed from 0.3

Table 1 Dimensions of twin-rollcaster Diameter of upper roller Diameter of lower roller Minimum rolling speed Maximum rolling speed

500 mm 500 mm 0.5 m/min 7 m/min

g

Stirring

Standing

Filtration

Adding the Al ingot

Adding the M , the Mn, and the Cr

Fig. 1 Flow chart of melting process of AA5052 alloy

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to 2.0 μm following an increase in the extrusion temperaturefrom 50 to 300 °C [32]. The microstructure of the sheets after3 cycles of accumulative roll bonding (ARB) consists of near-ly equiaxed grains whose diameters range from 0.2 to 0.5 μm[33]. Cryogenic deformation with 80 % reduction leads to theformation of parallel bands of elongated substructures, 0.05–0.15 μm in width and 0.4–0.8 μm in length [34].

3.2 Segregation

Solidification proceeds from the outside to the inside as theheat is dissipated during the TRC process, so that the melt inthe core solidifies last and segregates readily. Figure 5a showsthe severe segregation following the no field TRC process.Various precipitates with different physical properties, suchas Mg2Si and Al3Fe [35], are formed by the added elements.This is accompanied by the packing of massive dendrites inthe microstructure of the AA5052 sheet, which can be seenusing an optical microscope. Following application of thestatic magnetic field, the microstructure of the second sheetis mostly composed of thinner, denser, and more uniform pri-mary dendrites, as shown in Fig. 5b. Segregation is obviouslyreduced, compared with the no field case. Figure 5c shows therefinement in the microstructure due to the oscillating mag-netic field. A few precipitates can be spotted, and they arealmost completely replaced by equiaxed grains.

Apart from the severe centerline segregation in the core of theAA5052 sheet, intergranular segregation also occurs due to se-quential crystallization. Figure 6a shows the cross-sectional

microstructure at a quarter of the thickness in the no field sheet,and the Mg content revealed by linear scanning. The maximumMg content was measured to be 5.0 % by weight at the grainboundary and 1.1 % by weight in the grain interior. It can beshown that the segregation ratio (SR) is given by

SR ¼ ωmax

ωminð1Þ

where ωmax is the maximum solute content and ωmin is the min-imum solute content. This yields SR=4.5 in the case above.

With the static magnetic field (Fig. 6b), the maximum Mgcontent by weight at the grain boundary is 4.0 % and theminimum Mg content is 1.5 %, giving a segregation ratio of2.7. Similarly, under the oscillating magnetic field (Fig. 6c),the maximum and minimum Mg contents are 4.4 and 2.1 %,giving a segregation ratio of 2.1.

The influence of the electromagnetic field on the Mg seg-regation ratio is shown in Fig. 7. It can be seen from the chartthat the Mg content inside and outside of the grains tendstowards equilibrium and that the Mg segregation ratio forthe no field AA5052 sheet is much higher than those for thesheets with the fields applied. In other words, the applicationof the static and oscillating magnetic fields contributes to anincrease in the solute solubility.

3.3 Mechanical properties

In general, the AA5052 sheets produced by the TRC processusing the static and oscillating magnetic fields are much

Fig. 2 Schematic diagram ofelectromagnetic TRC of AA5052.1 nozzle, 2 electrode, 3 metallicmelts, 4 casting roller, 5 rollcasting sheet, 6 pinch roll(electrode), 7 coiler, 8 coil, 9 DCpower supply, 10 AC powersupply

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stronger. Figure 8 shows the engineering stress vs engineeringstrain curves for the samples after cold rolling in the longitu-dinal direction for three different TRC conditions during ten-sile processing. In the tensile processing, the serrated defor-mation called “Lüders strain” occurs. This is an inherent fea-ture of Al-Mg alloys [36, 37]. As seen in the figure, the TRCsheets produced with the fields imposed have improved prop-erties. Without the magnetic field, the tensile strength is200 MPa and the elongation is 21.9 %. With the static mag-netic field, although the tensile strength increases only slightlycompared with that in the no field case, the elongation shows asubstantial increase to 23.8 %. The maximum tensile strengthof 241 MPa is obtained in the longitudinal direction under anoscillating magnetic field, an increase of 20.5 % over the “nofield” specimen. In addition, the elongation of the samples

also increased to 25.7 %. The improvement in the propertiesdue to the effect of static magnetic field is smaller than thatdue to the oscillating magnetic field.

4 Discussion

The sheet macrostructure at the edge is distinctly differentfrom that in the core for all three conditions, as shown inFig. 3. The fine crystal zone at the edge is formed due to rapidcooling (cooling rate 100–1000 °C/s), causing the generationof numerous nuclei and restricting the available space for theindividual nuclei. The grains in the core are comparativelylarge because of the lesser heat dissipation there. The no fieldmacrostructure (Fig. 3a) is larger than the “with field” macro-structure (Fig. 3b, c). That indicates that this is unfavorable fornucleation and that every nucleus has sufficient space to growif no field is applied. However, application of electromagneticfield improves the nucleating environment. The no field mi-crostructure (Fig. 4a) at quarter thickness is formed at regionsof comparatively high temperature gradient, generating co-lumnar crystals and causing the grains to grow along certainorientations. The orientation of grain growth induces the ex-pulsion of alloy element particles, leading to segregation. Coresegregation (Fig. 5a) is the result of delayed solidification ofthe melt with a high content of alloy element particles. Inter-granular segregation (Fig. 6a) occurs because the expelledalloy element particles solidify in the intergranular regions.The large rolling deformation during the TRC process alsocontributes to the segregation because dislocation anddeformation-induced vacancies enhance the mobility of theMg atoms [38]. Rolling stress also induces grain boundarymigration and grain rotation. However, second-phase particlesand solute atoms segregated at the grain boundaries slowdown grain boundary migration and grain rotation [39]. Bothsolidification and deformation conditions result in the forma-tion of non-uniform microstructure, affecting the mechanicalproperties (Fig. 8). The microstructure resulting from the ap-plication of the electromagnetic field (Fig. 4b, c) is character-ized by finer grains and slightly reduced orientation. Theseresult in the abatement of centerline segregation (Fig. 5b, c)and intergranular segregation (Fig. 6b, c) and, furthermore,influence the mechanical properties as shown in Fig. 8.

In the TRC process, the region most influenced by theelectromagnetic field should be the cast-rolling region, wherethe solidification and deformation of the metal occur. Appli-cation of a static magnetic field to the TRC process amounts toinducing flow in the melt. Atoms lose their valence electronsand become ions in the metal. If no field is applied, every ionundergoes random thermal motion characterized by a velocityv!, as shown in Fig. 9a. When a static magnetic field is ap-

plied, a Lorentz force f!¼ qv⊥

!� B!

is generated. Here, v⊥! is

a

b

c

Fig. 3 Macrostructures of cast-rolling area of AA5052: a no field, bwitha static magnetic field, and c with an oscillating magnetic field

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the velocity component perpendicular to the applied magnetic

field, q is the particle charge, and B!

is the magnetic fieldintensity. The ion rotates round the magnetic field line under

the action of the Lorentz force f!. The velocity component

v==�! parallel to the magnetic field causes the ion to move alongthe magnetic field line, as shown in Fig. 9b. The radius of theorbit in the plane perpendicular to the magnetic field is mv⊥/

qB, where m is the particle mass. The frequency of gyration isqB/m. Compared with random motion, spiral motion inhibitsparticle diffusion, especially the diffusion of solute particles inthe solid phase. Considering the melt flow perpendicular tothe magnetic field at a rolling speed of 1.4 m/min in the TRCprocess, v⊥ is considerably greater than v//, resulting in a verystrong inhibiting effect. This inhibiting effect is equivalent toan effective increase in the fluid viscosity. The effectiveness of

a b

c

Fig. 4 Microstructures ofAA5052 sheets: a no field, bwitha static magnetic field, and c withan oscillating magnetic field

a

c

bFig. 5 Macrosegregation ofAA5052 sheets: a no field, bwitha static magnetic field, and c withan oscillating magnetic field

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the magnetic field in increasing the viscosity varies with the

Hartmann number H ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiσB2r2=μc2

p, where σ is the electri-

cal conductivity, r the radius of the crystal, μ the ordinaryviscosity (in the absence of the magnetic field), and c thevelocity of light [40]. Based on this theory, convection in themelt is reduced. Reduced convection results in reduction inthe diffusion of the solute element. So, evenly distributed highmelting particles can be core of crystal nucleus. This increasesthe nucleation rate. However, the heat transfer to the rollersalso reduces, thereby reducing the undercooling of the bulkliquid. The arrangement of the primary trunks with the highconcentration of added elements in the rolling direction formsthe intermediate morphology between the dendrites and the

equiaxed grains, as shown in Fig. 4b. Moreover, the segrega-tion is abated due to the reduction in heat and mass transfercapacity (Figs. 5b and 6b). Additionally, since a local currentdensity occurs in the dendrite network, due to thermoelectric(TE) effect, a thermoelectric magnetic force (TEMF) is creat-ed [41, 42]. This also makes the particles follow a spiral path,inducing an inhibiting effect in the mushy zone and reducingthe diffusion of the solute element. This force becomes espe-cially important when the magnetic field is applied duringsolidification under a high-temperature gradient, like inthe TRC process. The inhibiting effect improves themechanical properties of the AA5052 sheet to a certainextent (Fig. 8).

1 2

1.1% 1

keV

5.0% 2

keV

4 3

1.5% 3

keV

4.0% 4

keV

6 5

2.1% 5

keV

4.4% 6

keV

a

b

c

Fig. 6 Scanning of AA5052 in different TRC processes. a no field, b static magnetic field, and c oscillating magnetic field

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Simultaneous imposition of a static magnetic field B!

and

an alternating electric field with current density J!

on the non-solid area results in an oscillating electromagnetic body force

F!¼ J

!� B!, with a frequency equal to that of the applied

electric field and a direction perpendicular to the plane of thetwo fields (Fig. 10). This force acts on the particles of the non-solid area and cause oscillation and convection. On the otherhand, the alternating electric current induces an alternatingmagnetic field with the same frequency, which interacts withthe electric current itself and induces an alternating

electromagnetic force. Compared with the oscillating forcementioned previously, the latter force is small. When electro-magnetic oscillation is imposed upon the molten metal, cyclicforces are induced which act as periodic compressive/tensileforces, causing oscillations in the liquid. Consequently, forcesof tension and compression are alternately developed on thesurface of suspended particles. This can result in the formationof cavities. These cavities may grow in size as vacuum bub-bles and, partly, by absorbing gas from the melt during thetension period of the cyclic force and releasing a part of itduring the compression period (rectified diffusion).

Fig. 7 Statistical histogram ofMg element segregation ratiodisplays the reduction of Mgelement content according to theno field and static and oscillatingmagnetic fields

Fig. 8 Engineering stress vsengineering strain curves ofAA5052 sheets under varioustreatments

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Eventually, the bubbles collapse to produce powerful shockwaves by involution and jetting [43]. The shock waves in theliquid can crush the suspended particles and smash them intoeach other, causing further refinement. Strong convection canpromote heat transfer and enhance the fluidity of melt, whichwill lead to the temperature and solute concentration distribu-tion. Uniform distribution of nucleation particles and temper-ature is very beneficial for increasing the nucleation rate. Thisresults in grain refinement as shown in Figs. 3c and 4c. Strongconvection is unfavorable for gathering solute particles tocause segregation, so oscillating effect can obviously reducethe centerline segregation (Fig. 5c) and intergranular segrega-tion (Fig. 6c). All of these influences of the oscillating fieldcause the mechanical properties to improve (Fig. 8).

5 Conclusions

& AA5052 alloy sheets were manufactured using the TRCtechnique under three conditions: (1) no field applied, (2)with a static magnetic field, and (3) with an oscillating

magnetic field. For the no field case, the tensile strengthwas 200 MPa and the elongation 21.9 %. With the staticmagnetic field, the tensile strength increases slightly to204 MPa and the elongation to 23.8 %. With an oscillatingmagnetic field, both the tensile strength and elongation weregreatly improved to 241 MPa and 25.7 %, respectively.

& A static magnetic field induces an inhibiting effect on themelt, reduces diffusion of the solute elements, and resultsinmore nucleating opportunities and less segregation, thusenhancing the mechanical properties. However, theinhibiting effect cannot change the orientation of crystalgrowth and columnar crystals still exist in themicrostructure.

& An oscillating magnetic field can “crush” the suspendedparticles and cause strong convection, leading to moreuniform distribution of temperature and solute elements,simultaneously increasing nucleating opportunities anddecreasing segregation, thus enhancing the mechanicalproperties. An oscillating field basically eliminates theorientation of crystal growth and makes the grains finerand rounder.

Fig. 9 a The relationship of ionvelocity and magnetic directionand (b) motion trace of ion

Fig. 10 Oscillating forcedeveloped by interaction ofalternating electric and staticmagnetic fields

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& In the future, design of experiment methods such asTaguchi or response surface methods will be used to dothe experimental arrangement.

Acknowledgments The authors wish to thank Dr. Ajit Godbole at theUniversity of Wollongong for the constructive discussions. The authorsgratefully acknowledge the financial support from the Vice-Chancellor’sFellowship Grant and URC small grant at the University of Wollongong,and the Open Research Fund of key Laboratory of High PerformanceComplex Manufacturing, Central South University.

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