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Variable-Frequency Ultrasonic Treatment on Microstructure and Mechanical Properties of ZK60 Alloy during Large Diameter Semi-Continuous Casting Xingrui Chen, Qichi Le *, Xibo Wang, Qiyu Liao and Chaoyang Chu Key Lab of Electromagnetic Processing of Materials, Ministry of Education, Northeastern University, 314 Mailbox, Shenyang 110819, China; [email protected] (X.C.); [email protected] (X.W.); [email protected] (Q.L.); [email protected] (C.C.) * Correspondence: [email protected]; Tel.: +86-24-8368-3312 Academic Editor: Manoj Gupta Received: 7 April 2017; Accepted: 4 May 2017; Published: 16 May 2017

Abstract: Traditional fixed-frequency ultrasonic technology and a variable-frequency ultrasonic technology were applied to refine the as-cast microstructure and improve the mechanical properties of a ZK60 (Mg–Zn–Zr) alloy during large diameter semi-continuous casting. The acoustic field propagation was obtained by numerical simulation. The microstructure of the as-cast samples was characterized by optical and scanning electron microscopy. The variable-frequency ultrasonic technology shows its outstanding ability in grain refinement compared with traditional fixed-ultrasonic technology. The variable-frequency acoustic field promoted the formation of small α-Mg globular grains and changed the distribution and morphology of β-phases throughout the castings. Ultimate tensile strength and elongation are increased to 280 MPa and 8.9%, respectively, which are 19.1% and 45.9% higher than the values obtained from billets without ultrasonic treatment and are 11.6% and 18.7% higher than fixed-frequency ultrasound treated billets. Different refinement efficiencies appear in different districts of billets attributed to the sound attenuation in melt. The variable-frequency acoustic field improves the refinement effect by enhancing cavitation-enhanced heterogeneous nucleation and dendrite fragmentation effects. Keywords: magnesium alloy; semi-continuous casting; grain refinement; mechanical properties; ultrasonic treatment

1. Introduction Magnesium alloy has a multitude of applications in various domains such as vehicles, aerospace, medicine, computer, communication, and consumer electronics, for its outstanding advantages, including low density, high specific strength and stiffness, positive conductivity, and promising machinability [1–8]. However, it has its own characteristics such as low thermal capacity, melting heat, and thermal conductivity, which can lead to hard heat dissipation and huge temperature difference between solidification interface and the melt center, resulting in coarse grains, developed dendrites, and inhomogeneous structure [9–11]. Nevertheless, refinement grain billets play a significant role in the subsequent deformation process. Consequently, it is indispensable to produce aplitic, homogeneous, pure magnesium alloy billets. Generally, there are two methods to refine grain during solidification process, including chemical stimulation [12–15] and physical induction [16–19]. Chemical elements, such as Zr, C, and Ca are added as grain refiners into Mg alloys and the addition of these grain refiners can effectively refine grain size. However, chemical methods also bring some problems and face a mushrooming number of environmental challenges, for example the addition of iron element Metals 2017, 7, 173; doi:10.3390/met7050173

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can lead to low corrosion resistance [20]. Accordingly, the physical method is a favorable option for grain refinement. A large number of studies show that ultrasound has excellent contributions in degassing [21–23] and grain refinement. Fang et al. suggest that it is possible to obtain non-dendritic and globular α-Mg grains by supplying acoustic vibration to molten Mg–Zn–Y alloy [24]. Globular grains were also obtained in AZ91 alloy subject to high intensity ultrasonic vibration [25] demonstrated by Gao et al. Acoustic cavitation and acoustic streaming are two useful effects born in ultrasonic vibration. Eskin has explained the effect of ultrasound on grain refinement: cavitation-enhanced heterogeneous nucleation and dendrite fragmentation [26]. When melt is treated by high intensity ultrasonic vibrations, a multitude of tiny bubbles appear because of acoustic pressure, which start growing, shrinking, and finally collapse following variation of acoustic pressure. Undercooling occurs in the bubble interface during expansion because of energy absorbed from melt, giving rise to nucleation on the bubble surface. High pressure produced by collapses of cavitation bubbles create acoustic streaming in melt, disturbing nuclei into the surrounding liquid, thus promoting heterogeneous nucleation, and this huge pressure leads to fragmentation of dendritic cells in the mushy zone, which are disturbed by acoustic streaming, adding the number of nuclei. Although grain refinement by ultrasonic vibration technology has succeeded under laboratory conditions, it was rarely used in industrial manufacture. Traditional ultrasonic applications are based on a fixed frequency, well-turned ultrasonic source where a host of design and parameters have to be respected. These basic requirements cannot be satisfied in factories with complex environments. Therefore, the resonance condition of melt cannot be fully satisfied due to variable load conditions. It is a pity that ultrasonic energy as an environmentally friendly and green energy has not been applied in magnesium industrial production. Therefore, in this work, variable-frequency ultrasonic vibration was introduced to ZK60 alloy melt during major diameter semi-continuous casting. Meanwhile, the variable-frequency ultrasonic field propagations in ZK60 alloy melt were calculated by the finite element method. The paper focuses on the effect of grain refinement by variable-frequency and difference between fixed-frequency ultrasound and variable-frequency ultrasound during large diameter semi-continuous casting. 2. Experimental Procedures 2.1. Magnesium Alloy and Experimental Set-Up The composition of ZK60 alloy used in this work is shown in Table 1, obtained by an inductively coupled plasma analyzer. Table 1. Composition of ZK60 alloy used in this work. Element

Zn

Zr

Fe

Cu

Si

Ni

Mg

wt %

5.720

0.644

0.018

0.013

0.026

0.010

Balance

Figure 1 illustrates the experimental set-up for this study. The ultrasonic vibration system comprises an ultrasonic power supply, an ultrasonic converter with a maximum power output of 3000 W, a 35 mm diameter and 960 mm long acoustical wave-guide, and a 35 mm diameter and 120 mm long stainless steel acoustic radiator/horn. The ultrasonic power supply unit is linked to a computer, and consequently the ultrasonic frequency and power are fully controlled by dedicated Windows compatible software. This ultrasonic vibration system can supply variable frequency ultrasonic agitation, which has a 20 kHz center frequency, a 200 kHz changing frequency, and a frequency variation range of ±1 kHz. An air cooler was utilized to guarantee the ultrasonic system has a favorable working environment. The semi-continuous casting system used in this research includes a crystallizer with a 255 mm diameter duraluminum inner sleeve, a tundish, a casting control system, a melting

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control system, a melting a water cooling system. The supporting and positioning furnace, and a water coolingfurnace, system. and The supporting and positioning device of the ultrasonic system deviceachieve of the ultrasonic system could achieve three-dimensional position adjustment. could three-dimensional position adjustment.

Figure 1. of the set-up:set-up: 1—ultrasonic power supply, converter, Figure 1. Schematic Schematic of experimental the experimental 1—ultrasonic power2—ultrasonic supply, 2—ultrasonic 3—acoustical wave-guide, 4—acoustic radiator/horn, 5—liquid melt,melt, 6—billet, 7—tundish, converter, 3—acoustical wave-guide, 4—acoustic radiator/horn, 5—liquid 6—billet, 7—tundish,8 —crystallizer, 9—dummy 8—crystallizer, 9—dummybar barhead, head,10—air 10—air cooler, cooler, 11—positioning 11—positioning device. device.

2.2. Casting Casting Procedure Procedure and and Samples Samples Characterization Characterization 2.2. The melting melting of of ZK60 ZK60 Mg Mg alloy alloy was was carried carried out out in in aa resistance resistance furnace furnace with with an an iron iron crucible crucible The containing approximately approximately 100 100 kg kg of of metal metal and and protected protected by by CO CO2 ++ 0.5% 0.5% SF SF6 atmosphere atmosphere in in order order to to containing 2 6 prevent severe oxidation. The melt was transferred to a tundish from crucible at 710 °C and then ◦ prevent severe oxidation. The melt was transferred to a tundish from crucible at 710 C and then transferredto tothe thecrystallizer. crystallizer.The Thecasting castingstarted started when temperature melt in the tundish ◦ C. transferred when temperature of of melt in the tundish waswas 660 660 °C. Cooling water Casting velocity was mm/min.In Inthe thefirst first casting, casting, the the billet billet Cooling water flowflow waswas 4.634.63 t/h.t/h. Casting velocity was 5050 mm/min. was cast by conventional direct chilling casting (DC) process. After casting about 600 mm length, the was cast by conventional direct chilling casting (DC) process. After casting about 600 mm length, preheated ultrasonic horn at 660 °C was inserted 50 mm under the surface of melting liquid during ◦ the preheated ultrasonic horn at 660 C was inserted 50 mm under the surface of melting liquid the casting process. Thus, Thus, the melt was was treated by by ultrasonic vibration with ultrasonic during the casting process. the melt treated ultrasonic vibration with0 0W W (no (no ultrasonic treatment), fixed-frequency of 20 kHz with 1800 W, and variable-frequency of 20 ± 1 kHz with 1800 treatment), fixed-frequency of 20 kHz with 1800 W, and variable-frequency of 20 ± 1 kHz with 1800 W. W. The treated billet was cooled to room temperature. The treated billet was cooled to room temperature. In order microstructure of each experimental condition, samples were cut at In order totocompare comparethethe microstructure of each experimental condition, samples were positions shown in Figure 2 from different locations in the cross-section of the billets. The cut at positions shown in Figure 2 from different locations in the cross-section of the billets. characterizations of microstructure were obtained from scanning The characterizations of microstructure were obtained froma aLeica Leicaoptical opticalmicroscope microscope and and aa scanning electronic microscope (SEM). Samples were ground by using 400, 1000, 2000, and 3000 SiC papers, electronic microscope (SEM). Samples were ground by using 400, 1000, 2000, and 3000 SiC papers, then polished and etched using a solution of 4% nitric acid in alcohol to reveal their then polished and etched using a solution of 4% nitric acid in alcohol to reveal their microstructures. microstructures. grain size of the billets was byintercept using the mean and linear The average grainThe size average of the billets was measured by using themeasured mean linear method, at intercept method, and at least 110 intercepts were utilized to get the average grain size value. least 110 intercepts were utilized to get the average grain size value.

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Figure 2. Schematic illustration of the positions of specimen taken from billets. Figure 2. Schematic illustration of the positions of specimen taken from billets. Figure 2. Schematic illustration of the positions of specimen taken from billets.

Mechanical Properties Tests Segregation Rate Detection 2.3.2.3. Mechanical Properties Tests andand Segregation Rate Detection 2.3. Mechanical Properties Tests and Segregation Rate Detection Mechanical properties were characterized tensile tests room temperature. Tensile Mechanical properties were characterized by by tensile tests at at room temperature. Tensile Mechanical properties were characterized by tensile tests at room temperature. Tensile specimens specimens were machined along longitudinal direction at the position of center, 1/2R edge specimens were machined along thethe longitudinal direction at the position of center, 1/2R andand edge of of were machined along the of longitudinal direction at the position of center,in1/2R and edge of billetstrain with billet with a gage length 36 mm and a diameter of 6 mm, as shown Figure 3b. The initial billet with a gage length of 36 mm and a diameter of 6 mm, as shown in Figure 3b. The initial strain −3 mm −1 arates gagewere length of 36 and a diameter of 6 mm, as shown in Figure 3b. The initial strain rates were rates were 1 ×110×−310 ·s−1.·s . −3 · s−1 . 1 × 10 Segregation detections were carried inductively coupled plasma analyzer. Strip Segregation raterate detections were carried outout by by inductively coupled plasma analyzer. Strip Segregation rate detections were carriedtests outevery by inductively coupled plasma analyzer. samples were cut from billets and the sample 23 mm, as shown in Figure 3a. In samples were cut from billets and the sample tests every 23 mm, as shown in Figure 3a. In thisthis Strip samples were cutrate from billets and the sample tests everywhich 23 mm, as shown in as Figure 3a. In this work, segregation used to judge segregation, obtained follows. work, thethe segregation rate waswas used to judge thethe segregation, which waswas obtained as follows. work, the segregation rate was used to judge the segregation, which was obtained as follows. i x i  xi / 11  100 − ∑ xi /11 i  i i xi  100 %% x / 11

x 

ηi =

x / 11

x x/ 11  ∑ i /11 i

i

(1) (1) (1)

× 100%

where is the segregation rate, iselement the element content at position i the i the iη where segregation rate, element content position where is the segregation rate, xxi iisxisthe content atat position i. i . i is

(a) (a)

i.

(b) (b)

Figure 3. Schematic illustration of (a) positions of segregation detection; tensile samples size. Figure 3. Schematic illustration of (a) positions of segregation raterate detection; (b) (b) tensile samples size. Figure 3. Schematic illustration of (a) positions of segregation rate detection; (b) tensile samples size.

3. Results Discussion 3. Results andand Discussion 3. Results and Discussion 3.1. Microstructure Refinement by Different Ultrasonic Treatments 3.1.3.1. Microstructure Refinement by by Different Ultrasonic Treatments Microstructure Refinement Different Ultrasonic Treatments The microstructures of the as-cast ZK60 alloy without ultrasonic processing are composed of The microstructures of of the as-cast ZK60 alloy without ultrasonic processing areare composed of of The microstructures the as-cast ZK60 alloy without ultrasonic processing composed uneven coarse α-Mg grains, as shown in Figure 4. The coarse dendrites of the primary phase are uneven coarse α-Mg grains, as as shown in in Figure 4. 4. TheThe coarse dendrites of of thethe primary phase areare uneven coarse α-Mg grains, shown Figure coarse dendrites primary phase presented throughout the whole cross-section of the billets. presented throughout the whole cross-section of the billets. presented throughout the whole cross-section of the billets.

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Figure 4. Microstructures as-castZK60 ZK60alloy alloy without without ultrasonic at at (a)(a) center; (b) (b) 1/2R; Figure 4. Microstructures ofof as-cast ultrasonicprocessing processing center; 1/2R; Figure 4.position Microstructures (c) edge of billet. of as-cast ZK60 alloy without ultrasonic processing at (a) center; (b) 1/2R; (c) edge position of billet.

(c) edge4.position of billet. of as-cast ZK60 alloy without ultrasonic processing at (a) center; (b) 1/2R; Figure Microstructures Figure 5 illustrates the as-cast microstructures of the ZK60 alloy isothermally suffered by 20 (c) edge position of billet.

Figure 5 illustrates the as-cast microstructures of of thethe ZK60 alloy isothermally suffered 20 Figure 5 illustrates the as-cast microstructures ZK60 alloy isothermally suffered by 20kHz kHz fixed-frequency ultrasonic energy at 1800 W electric power. As shown in Figure 5a, theby α-Mg fixed-frequency ultrasonic energy at 1800 W electric power. As shown in Figure 5a, the α-Mg grains kHz fixed-frequency ultrasonic energy at 1800 W electric power. As shown in Figure 5a, the α-Mg grains are refined to somethe degree in the center of the billet with Figure 4a. However, grain Figure 5 illustrates as-cast microstructures of thecompared ZK60 alloy isothermally suffered by 20 are refined to refined some degree in the ofcenter compared with 4a. However, grain size in grains are toedge some in the ofWthe billet compared with Figure 4a. However, grain size in 1/2R and the ofdegree the center billet have no billet large difference. Some coarse dendrites are kHz fixed-frequency ultrasonic energy atthe 1800 electric power. AsFigure shown in Figure 5a,removed. the α-Mg size inthe 1/2Rrefined and of the edge ofdegree the billet have nodifference. large Some dendrites are removed. 1/2Rgrains and edge billet have large Some coarsecoarse dendrites areHowever, removed. are tothe some inno the center of thedifference. billet compared with Figure 4a. grain size in 1/2R and the edge of the billet have no large difference. Some coarse dendrites are removed.

Figure 5. Microstructures of as-cast ZK60 alloy treated by fixed-frequency ultrasonic vibration at (a) Figure 5. Microstructures as-cast ZK60 by by fixed-frequency ultrasonic vibration at (a) at center; (b) 1/2R; (c) edge position of billet. Figure 5. Microstructures ofofas-cast ZK60alloy alloytreated treated fixed-frequency ultrasonic vibration center; (b) 1/2R; (c) edge position of billet. Figure (b) 5. Microstructures as-cast of ZK60 alloy treated by fixed-frequency ultrasonic vibration at (a) (a) center; 1/2R; (c) edge of position billet.

Figure(b) 6 1/2R; presents theposition as-castof billet. microstructures of the ZK60 alloy isothermally treated by center; (c) edge Figureenergy 6 presents microstructures of alloypower. isothermally treated by ultrasonic with 20the ± 1as-cast kHz variable-frequency at the 1800ZK60 W electric There are obvious Figure 6ofpresents thein20 as-cast microstructures theof ZK60 alloy isothermally treated byobvious ultrasonic ultrasonic energy with ± 1whole kHz variable-frequency at 1800 W electric There are changes grain size billet. Coarseof dendrites almost disappeared and globular α-Mg Figure 6 presents the as-cast microstructures the ZK60 alloypower. isothermally treated by energy with 20 ± dominant. 1 kHz variable-frequency at 1800 W at electric power. are obvious changes changes of grain size billet. Coarse dendrites almost disappeared and globular α-Mg of grains become The microstructures of ZK60 alloy consisted ofThere homogeneous grains. ultrasonic energy within 20the ± 1whole kHz variable-frequency 1800 W electric power. There are obvious grain size in the whole billet. Coarse dendrites almost disappeared and globular α-Mg grains become grains become dominant. The microstructures of ZK60 alloy consisted of homogeneous grains. changes of grain size in the whole billet. Coarse dendrites almost disappeared and globular α-Mg

dominant. The microstructures ZK60 alloy consisted of homogeneous grains. grains become dominant. The of microstructures of ZK60 alloy consisted of homogeneous grains.

Figure 6. Microstructures of as-cast ZK60 alloy treated by variable-frequency ultrasonic vibration at Figure 6. Microstructures ofposition as-cast ZK60 alloy treated by variable-frequency ultrasonic vibration at (a) center; (b) 1/2R; (c) edge of billet. (a) center; (b) 1/2R; (c) edge of billet. Figure 6. Microstructures ofposition as-cast ZK60 alloy treated by variable-frequency ultrasonic vibration at

Figure 6. Microstructures of as-cast ZK60 alloy treated by variable-frequency ultrasonic vibration at Figure 7 indicates the average grain size of ZK60 alloy with different ultrasonic processes. It is (a) center; (b) 1/2R; (c) edge position of billet. (a) center; (b)indicates 1/2R; (c) the edgeaverage positiongrain of billet. Figure of to ZK60 alloy with different ultrasonic processes. It is obvious that7ultrasonication can refine thesize grain different degrees in different positions of a billet, obvious that ultrasonication can refine the grain to different degrees in different positions of a billet, and variable-frequency ultrasound reveals powerwith in grain refinement. Fixed-frequency Figure 7 indicates the average grain sizeitsofstrong ZK60 alloy different ultrasonic processes. It is and variable-frequency ultrasound reveals its power ingrain grain refinement. Fixed-frequency Figure 7 indicates average grain ofstrong ZK60 alloythe with different ultrasonic processes. ultrasonication showsthe lower efficiency onsize grain refinement, decrease from 168 8 μm It is obvious that ultrasonication can refine the grain to different degrees insize different positions of a± billet, ultrasonication shows lower efficiency onand grain refinement, the size decrease from 168 ±of 81/2R, μm to 112 ± 7 ultrasonication μm, 201 ± 8 μm tocan 149 ± 9 μm, 186strong ± 10 μm to 165 ±grain 12 μm at the position center, obvious that refine the grain to different degrees in different positions a billet, and variable-frequency ultrasound reveals its power ingrain refinement. Fixed-frequency to 112edge ± 7 μm, ± 8 μm to 149 ± 9reveals μm, 186 ± 10 μm to 165 ± 12 μm atcan the position center, and of 201 billet, respectively. Theonand variable-frequency ultrasound not from only refine the shows lower efficiency grain refinement, the size decrease 168 ± 81/2R, μm and ultrasonication variable-frequency ultrasound its strong power ingrain grain refinement. Fixed-frequency and edge of billet, respectively. The variable-frequency ultrasound can not only refine microstructures of the billets more effectively from edge to center, but it can also make the to 112 ± 7 μm, 201 ± 8 μm to 149 ± 9 μm, and 186 ± 10 μm to 165 ± 12 μm at the position center, 1/2R, ultrasonication shows lower efficiency on grain refinement, the grain size decrease from 168 ± 8 µm microstructures of the billets moreThe effectively from edge ultrasound to center, but canonly also refine make the and edge of billet, respectively. variable-frequency can itnot

to 112 ± 7 µm, 201 ± 8 µm to 149 ± 9 µm, and 186 ± 10 µm to 165 ± 12 µm at the position microstructures of the billetsrespectively. more effectively from edge to center,ultrasound but it can can alsonot make center, 1/2R, and edge of billet, The variable-frequency onlythe refine the microstructures of the billets more effectively from edge to center, but it can also make the

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microstructures much more uniform than fixed-frequency ultrasound. The grain sizes are microstructures muchto more than fixed-frequency ultrasound. The ± grain sizes are dramatically dramatically decreased 70 ±uniform 4 μm (center), 78 ± 5 μm (1/2R), and 101 7 μm (edge). decreased to 70 ± 4 µm (center), 78 ± 5 µm (1/2R), and 101 ± 7 µm (edge). 250

No ultrasonic treatment Fixed-frequeency US treatment Variable-frequency US treatment

Average grain size (μm)

200

150

100

50

0 0

56

112

Distance form center (mm)

Figure 7. Average grain size of ZK60 alloys with different ultrasonic treatments.

Figure 7. Average grain size of ZK60 alloys with different ultrasonic treatments.

effectsof of ultrasonic vibration on grain attribute to cavitation-enhanced heterogeneous The The effects ultrasonic vibration onrefinement grain refinement attribute to cavitation-enhanced nucleation and dendrite fragmentation. At the beginning of solidification, the temperature between heterogeneous nucleation and dendrite fragmentation. At the beginning of solidification, the pouring temperature and the liquidus temperature of the alloy, cavitation-enhanced heterogeneous temperature between pouring temperature and the liquidus temperature of the alloy, nucleation was the main effect of ultrasonic grain refinement, since dendrite fragmentation would not be cavitation-enhanced heterogeneous nucleation was the main effect of ultrasonic grain refinement, possible at those temperatures due to solidification having not started yet. When at a temperature below sincethe dendrite would not be started possible at those due liquidus fragmentation temperature, acoustic streaming to work in thetemperatures melt, which can leadtotosolidification dendrite having not started yet. When at a temperature below the liquidus temperature, fragmentation and disturbance of nuclei. These two simultaneously existed mechanismsacoustic generate astreaming high started to work in in themelt, melt, which can tolead to dendrite offragmentation and disturbance nuclei. density of nuclei which can lead the development a multitude of globular grains with of small grain These twosize. simultaneously existed mechanisms generate a high density of nuclei in melt, which can Owing to the acoustic playing a significant rolewith in grain refinement, in order to study the lead to the development of a pressure multitude of globular grains small grain size. ultrasonic numerical simulations were carried Therefinement, acoustic pressure p can Owing tofield the propagation, acoustic pressure playing a significant role inout. grain in order tobestudy obtained by solving the wave equation as the ultrasonic field propagation, numerical simulations were carried out. The acoustic pressure p as can be obtained by solving the wave 1 1 ∂2equation p ρc2 ∂t2

−∇·

δ ∂(∇ p) ∇p + 2 ρ ∂t ρc

 (2)

1 1 2 p  p         p   (2) 2 2 where ρ is the density of the  melt, c tis the speed c  cin2 themelt, t δis a measure of the acoustic  ofthe sound diffusivity. With the time harmonic assumption, a solution of the form is obtained as

where



is the density of the melt,

c is the speed of the sound in the melt,  is a measure of the iωt

P(r, z, t) = p(r, z)e acoustic diffusivity. With the time harmonic assumption, a solution of the form is obtained as(3) where ω is the angular frequency. Pr , z, t   pr , z eit The intensity distribution I(r,z) can be obtained as

where  is the angular frequency. p2 (r, z) The intensity distribution I(r,z) can beIobtained (r, z) = as 2ρc

(3)

(4)

p 2 r, z 

I finite r, z element  The wave equation was solved by the method. The following boundary conditions (4) 2 c are obtained (as shown in Figure 8a): The pwave was solvedcorresponding by the finite method. The as following (1) = 0 at equation the liquid-air interfaces to aelement total reflection condition soft walls;boundary conditions are obtained (as shown in Figure 8a): (2) ∂p/∂n = 0 at the side walls of the horn which represents hard walls; p0 the sin(liquid-air 2π ( f 0 + sininterfaces at the horn tip, where p0 is reflection the amplitude of the wave, f 0 walls; is the (2π f 1 t))t) corresponding (1) (3) to a total condition as soft p p0= at initial frequency of the wave, f 1 is the changing frequency of wave frequency; (2) p / n  0 at the side walls of the horn which represents hard walls;

1/ρ(∂p/∂n) + iω p/Z = 0 at the walls of the crystallizer and liquid cave which represents the p0 is the horn tip, input whereimpedance amplitude  f 0  sincondition 2f1t tand  atZthe p impedance p0 sin2boundary is the acoustic of the external. of the wave,

(4)

(3)

f 0 is the initial frequency of the wave, f1 is the changing frequency of wave frequency; (4) 1 /  p / n   ip / Z  0 at the walls of the crystallizer and liquid cave which represents the impedance boundary condition and Z is the acoustic input impedance of the external.

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Table 2. Material properties and some critical model parameters.

Table 2 shows the material properties and some critical parameters used in the simulation. Name

Value

Description

c

4000 m/s 0.0279 dB/cm Value

Speed of sound in magnesium alloy Attenuation coefficient Description

20 kg/m kHz 3 1820 4000 m/s 200 Hz 0.0279 dB/cm 3.2 20kHz 106 Pa 200 Hz 4.2  1076Pa·s/m 3.2 × 10 Pa 4.2 × 107 Pa· s/m

Initial frequency of the wave Density of magnesium alloy

3  1820 kg/m Density of magnesium alloy Table 2. Material properties and some critical model parameters.

 Name

ρ f0 c f δ 1 f 0 p0 f1 p0 Z Z

Speed of sound in magnesium alloy Attenuation coefficient Initial pressure frequencyamplitude of the wave Source Changing frequency of wave frequency Acoustic input impedance Source pressure amplitude Acoustic input impedance

Changing frequency of wave frequency

Figure 8 shows the acoustic pressure of two ultrasonic vibration model at positions A, B, and C of the liquid cave. It is noticed that the variable-frequency ultrasonic field has higher acoustic Figure 8 shows the acoustic pressure of two ultrasonic vibration model at positions A, B, and C of pressure than fixed-frequency ultrasonic field. Average acoustic pressure at position A is much the liquid cave. It is noticed that the variable-frequency ultrasonic field has higher acoustic pressure higher than B and C in both acoustic fields. These differences lead to different refinement than fixed-frequency ultrasonic field. Average acoustic pressure at position A is much higher than B efficiencies. and C in both acoustic fields. These differences lead to different refinement efficiencies.

b1

×107

Variable-frequency acoustic pressure in Position A

c1 ×106

Variable-frequency acoustic pressure in Position B

4

d1 ×106

2

0

-2

Variable-frequency acoustic pressure in Position C

0.8

Acoustic pressure (Pa)

Acoustic pressure (Pa)

Acoustic pressure (Pa)

2

1

0

-1

0.4

0.0

-0.4

-2 -4 -0.8

0.0000

0.0004

0.0008

0.0012

-3 0.0000

0.0016

0.0004

0.0012

0.0016

0.0000

0.0004

Time (s)

Time (s)

b2

c2 ×107

0.0008

Fixed-frequency acoustic pressure in Position A

×106

0.0008

0.0012

0.0016

Time (s)

Fixed-frequency acoustic pressure in Position B

d2

×106

Fixed-frequency acoustic pressure in Position C

0.4

2

0

Acoustic pressure(Pa)

Acoustic pressure(Pa)

Acoustic pressure(Pa)

0.2

0.2

0.0

-0.2

0.1

0.0

-0.1

-2 -0.2

-0.4 0.0000

0.0004

0.0008

Time(s)

0.0012

0.0016

0.0000

0.0004

0.0008

Time(s)

0.0012

0.0016

0.0000

0.0004

0.0008

0.0012

0.0016

Time(s)

Figure 8. Variable-frequency acoustic pressure at position A (b1 ); position B (c1 ); and position C Figure 8. Variable-frequency acoustic pressure at position A (b1); position B (c1); and position C (d1) (d1 ) and fixed-frequency acoustic pressure at position A (b2 ); position B (c2 ); and position C (d2 ). and fixed-frequency acoustic pressure at position A (b2); position B (c2); and position C (d2). (a) (a) Boundary conditions. Boundary conditions.

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The different results of grain refinement effect among center, 1/2R, and edge (seen in Figures 4–6) could be ascribed to the sound attenuation in melt. The acoustic intensity decreases following the increase of acoustic propagation distance. Sound intensity (I) along the propagation distance (x) in a melt, according to expression (5): I = I0 e−2αx (5) where α is attenuation coefficient in melt. This attenuation coefficient is heavily depending on the viscosity and thermal conductivity of the melt. Thus, the sound attenuation cannot be neglected in large diameter casting. On one hand, the viscosity at edge of billet is higher than the center due to cooling water in crystallizer, leading to the increase of attenuation coefficient (α), so acoustic intensity decreases heavily at the edge of the liquid cave. On the other hand, according to the results of numerical simulation (seen in Figure 8), the average acoustic pressure (p) at edge and 1/2R of the liquid cave is lower than center, meaning the sound intensity (I) is lower at edge and 1/2R of liquid cave according to Equation (4). Thus, the grain refinement effect is weakened in edge and 1/2R. Based on the earlier discussion, the grain refinement efficiency strongly depends on acoustic cavitation and streaming produced by cavitation bubbles. There is more than one ultrasonic wave with different frequencies in melt generated from a variable-frequency ultrasonic source. These sound waves superpose with each other and affect acoustic pressure distribution, as seen in Figure 8. The compositive wave brings higher acoustic pressure compared with fixed-frequency ultrasound. Xi et al. have demonstrated that the cavitation bubble trajectory depends on the acoustic pressure amplitude and the initial bubble size: an increase of pressure amplitude can increase bubbles’ sizes [27]. Therefore, higher acoustic pressure brings bigger cavitation bulbs, which can strengthen the cavitation effect. Assume that the cavitation bulbs’ collapse process is adiathermic, the highest temperature (T(max) ) and the highest pressure (P(max) ) when a cavitation bulb collapsed is obtained as [28]: P(max) = pi [ p a (γ − 1)/pv ]γ(γ−1)

(6)

T(max) = Tmin [ p a (γ − 1)/pv ]

(7)

where Tmin is the temperature of melt, p a is the extra pressure out of the bulb, pi is the initial pressure in the bulb, pv is the equilibrium vapor pressure, γ is the specific heat capacity. In ultrasonic melt treatment process, the p a relies on the acoustic pressure amplitude. According to Equation (6), the highest pressure (P(max) ) and p a have an exponential relation, which is that the highest pressure (P(max) ) increases following the increase of acoustic pressure. Hence, associated with results shown in Figure 8, the variable-frequency acoustic vibration can lead to higher pressure when a cavitation bulb collapsed than fixed-frequency ultrasound. Consequently, higher impact force is introduced, which reinforce the acoustic streaming effect and dendrite fragmentation phenomenon. Equation (7) shows a linear relation between the highest temperature (T(max) ) and the p a . The highest temperature (T(max) ) increases with the increase of acoustic pressure. Thus, when ZK60 magnesium alloy melt was treated by variable-frequency ultrasound, a higher degree of energy fluctuation occurred in the melt, which can lead to reinforcement of cavitation-enhanced heterogeneous nucleation phenomenon. Vanhille et al. have demonstrated that cavitation bubbles appear easily at high acoustic amplitude [29]. Moreover, Feng et al. have reported that a multi-frequency sonication field can increase the number of cavitation bulbs [30]. Therefore, these high acoustic pressure different sound waves with different frequencies can produce a large volume concentration of bubbles, which could enhance the cavitation effect, increasing grain refinement efficiency. Besides, higher acoustic pressure brought by variable-frequency ultrasound can also reduce the negative impact of sound attenuation. Figure 9 presents the β-phase morphology images of billet with different sonication by SEM. The coarse β-MgZn phases appear on the grain boundary with the shape of nets in no ultrasonic treated casting (seen in Figure 9a). While reticular β-MgZn phases are broken when the fixed-frequency ultrasonic field was applied, and some spherical β-MgZn phases are observed (as shown in Figure 9b). When the

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Figure 9 presents the β-phase morphology images of billet with different sonication by SEM. The coarse β-MgZn phases appear on the grain boundary with the shape of nets in no ultrasonic treated (seen in Figure 9a). While reticular β-MgZn phases are broken when Metals 2017,casting 7, 173 9 ofthe 13 fixed-frequency ultrasonic field was applied, and some spherical β-MgZn phases are observed (as shown in Figure 9b). When the melt was treated by variable-frequency ultrasonic energy, the melt was treated variable-frequency ultrasonic the morphology of β-MgZn phases is morphology ofby β-MgZn phases shrank. It isenergy, obvious that the ultrasonic field can shrank. change Itthe obvious that the ultrasonic field can change the distribution and morphology of β-phase in ZK60 alloy. distribution and morphology of β-phase in ZK60 alloy.

Figure 9. Scanning electronic microscope (SEM) images: (a) no ultrasonic treatment; (b) Figure 9. Scanning electronic microscope (SEM) images: (a) no ultrasonic treatment; (b) fixed-frequency fixed-frequency ultrasonic treatment; (c) variable-frequency ultrasonic treatment. ultrasonic treatment; (c) variable-frequency ultrasonic treatment.

The mechanism of shape change of β-phase on ZK60 alloy is not well understood, and available The mechanism of shape ZK60 alloycan is not understood, and available references are very scarce. Wechange think of theβ-phase possibleonmechanism be well explained as follows: when the references are very scarce. We think the possible mechanism can be explained as follows: cavitation bulbs collapsed under the acoustic field, a strong impact force was introduced when in the the cavitationmelt, bulbs collapsed undertothe acoustic a strong impact forceresulting was introduced the magnesium which was likely fracture thefield, reticular β-MgZn phases, in tinier in MgZn magnesium melt, which was likely to fracture the reticular β-MgZnvariable-frequency phases, resulting inacoustic tinier MgZn phases on the grain boundary. According to previous discussion, fields phases on the grain boundary. According to previous discussion, variable-frequency acoustic fields can can produce higher impact force. Therefore, the β-MgZn phases become tinier under produce higher impact force. Therefore, the β-MgZn phases become tinier under variable-frequency variable-frequency acoustic field. acoustic field. 3.2. Mechanical Properties and Segregation 3.2. Mechanical Properties and Segregation The mechanical properties of the ZK60 Mg alloy billets cast by different processes were The mechanical properties of the ZK60 Mg alloy billets cast by different processes were examined, examined, with the results shown in Figures 10–12. One can notice that ultrasonication can improve with the results shown in Figures 10–12. One can notice that ultrasonication can improve billets’ yield billets’ yield strength and variable-frequency ultrasound has higher efficiency. Figure 11 strength and variable-frequency ultrasound has higher efficiency. Figure 11 demonstrates the ultimate demonstrates the ultimate tensile strength (UTS) at different positions of billet with different tensile strength (UTS) at different positions of billet with different ultrasonic treatments. It is clear ultrasonic treatments. It is clear that the UTS of the billets cast by the conventional DC process were that the UTS of the billets cast by the conventional DC process were lowest. The strength values lowest. The strength values are 235 ± 8 MPa, 221 ± 6 MPa, and 210 ± 7 MPa at the positions of center, are 235 ± 8 MPa, 221 ± 6 MPa, and 210 ± 7 MPa at the positions of center, 1/2R, and edge in billet 1/2R, and edge in billet respectively. The UTS increase to 251 ± 7 MPa, 241 ± 8 MPa, and 217 ± 6 MPa respectively. The UTS increase to 251 ± 7 MPa, 241 ± 8 MPa, and 217 ± 6 MPa when fixed-frequency when fixed-frequency ultrasound played its role during semi-continuous casting, showing good ultrasound played its role during semi-continuous casting, showing good improvement in center improvement in center and 1/2R but poor promoting in edge. While variable-frequency acoustic and 1/2R but poor promoting in edge. While variable-frequency acoustic energy can observably energy can observably promote billets’ UTS, which increase to 280 ± 8 MPa (center), 263 ± 6 MPa promote billets’ UTS, which increase to 280 ± 8 MPa (center), 263 ± 6 MPa (1/2R), and 245 ± 6 MPa (1/2R), and 245 ± 6 MPa (edge). Meanwhile, the elongation is also largely promoted especially the (edge). Meanwhile, the elongation is also largely promoted especially the melt was treated by melt was treated by variable-frequency ultrasound. For example, the elongation in center of the billet variable-frequency ultrasound. For example, the elongation in center of the billet increase from increase from 6.1% to 8.9% with variable-frequency ultrasound treatment, which is 45.9% higher 6.1% to 8.9% with variable-frequency ultrasound treatment, which is 45.9% higher than non-ultrasonic than non-ultrasonic treated sample. Metals 2017,sample. 7, 173 10 of 13 treated No ultrasonic treatment Fixed-frequency US treatment Variable-frequency US treatment

150

Yield strength(MPa)

125

100

75

50

25

0

center

1/2R

edge

Figure Yield strength ZK60 alloy with differentultrasonic ultrasonictreatments. treatments. Figure 10.10. Yield strength ofof ZK60 alloy with different 350 300

Pa)

250 200

No ultrasonic treatment Fixed-frequency US treatment Variable-frequency US treatment

Yi Yi

50 50 25 25 0 0

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center center

1/2R 1/2R

edge edge

Figure 10. Yield strength of ZK60 alloy with different ultrasonic treatments. 350 350

10 of 13

No No ultrasonic ultrasonic treatment treatment Fixed-frequency US Fixed-frequency US treatment treatment Variable-frequency US US treatment treatment Variable-frequency

300 300

UTS(MPa) UTS(MPa)

250 250 200 200 150 150 100 100 50 50 0 0

Center Center

1/2R 1/2R

Edge Edge

Figure 11. 11. Ultimate tensile tensile strength strength of of ZK60 ZK60 alloy alloy with with different different ultrasonic ultrasonictreatments. treatments. Figure 12 12

Non-ultrasonic Non-ultrasonic treatment treatment Fixed-frequency Fixed-frequency US US treatment treatment Variable-frequency US US treatment treatment Variable-frequency

Elongation(%) Elongation(%)

10 10

8 8

6 6

4 4

2 2

0 0

Center Center

1/2R 1/2R

Edge Edge

Figure Figure 12. 12. Elongation Elongation of of ZK60 ZK60 alloy alloy with with different different ultrasonic ultrasonic treatments. treatments.

Theyield yieldstrength strengthofofas-cast as-castZK60 ZK60alloy alloyisisimproved improved variable-frequency, as shown in Figure The byby variable-frequency, as shown in Figure 10. 10. According to the well-known Hall-Petch relation, the yield stress depends on the grain size as According to the well-known Hall-Petch relation, the yield stress depends on the grain size as follows: follows: σy = σ0 + kd−1/2  1 1 // 2 2

 yy   00  kd

(8) (8)

where the friction stress σ0 = 88 MPa [31] and strengthening coefficient k = 316 MPa [32]. By using where the friction stress  00 = 88 MPa [31] and strengthening coefficient k = 316 MPa [32]. By Equation (8) and the measured grain size in Figure 6, it is in a certain agreement for the calculated and using Equation (8)strength and theinmeasured experimental yield this work.grain size in Figure 6, it is in a certain agreement for the calculated andand experimental strength this work. The UTS elongation yield of as-cast ZK60inalloy are remarkably improved by ultrasonic treatment, The UTS and elongation of as-cast ZK60 alloy are remarkably ultrasonic especially with variable-frequency treatment, as shown in Figures 11 and 12. improved Mechanicalby properties of treatment, especially with variable-frequency treatment, as shown in Figures 11 and 12. Mechanical ZK60 alloy depend on several factors. The precipitated phase morphology and distribution should be properties of ZK60 alloypromotion. depend on factors. precipitated morphology and firstly considered for UTS It isseveral well known thatThe the MgZn phase isphase the main strengthening distribution should be firstly considered for UTS promotion. It is well known that the MgZn phase is phase in ZK60 alloys [33]. Ultrasonic vibration could affect the distribution and morphology of the main strengthening phase ZK60inalloys [33]. could affect the globular distribution MgZn phase in ZK60 alloy, as in shown Figure 9. Ultrasonic The MgZn vibration phases become tiny and by and morphology of MgZn phase in ZK60 alloy, as shown in Figure 9. The MgZn phases become tiny acoustic vibration, particularly under variable-frequency ultrasound’s influence. These tiny MgZn and globular acousticonvibration, under variable-frequency ultrasound’s phases evenly by distribute the grainparticularly boundary, increasing the UTS of as-cast ZK60 alloy.influence. Besides, These tiny MgZn phases evenly distribute on the grain boundary, increasing the UTS of as-cast ZK60 the refinement grain also needs to be considered. The UTS is exponentially dependent to the grain size [31], which means that UTS increase with the decreasing of grain size. Thereby, the increase of UTS and elongation in this work can be attributed to the combined effect of the tiny MgZn phase distribution and the globular and small size of grain. Figure 13 indicates the segregation rate of main elements with different ultrasonic treatments. The Zr has less segregation rate than Zn. It is noticed that ultrasonic vibration can reduce the segregation of the billet and the variable-frequency US treatment has a higher efficiency than fixed-frequency US treatment. The maximum segregation rate of Zr reduces from 2.09% to 0.79% and for Zn reduces from 25.17% to 6.97%.

MgZn phase distribution and the globular and small size of grain. Figure 13 indicates the segregation rate of main elements with different ultrasonic treatments. The Zr has less segregation rate than Zn. It is noticed that ultrasonic vibration can reduce the segregation of the billet and the variable-frequency US treatment has a higher efficiency than fixed-frequency US treatment. The maximum segregation rate of Zr reduces from 2.09% to 0.79% Metals 2017, 7, 173 11 of 13 and for Zn reduces from 25.17% to 6.97%.

Figure 13.13. Segregation rate of of ZK60 alloy with different ultrasonic treatments: (a)(a) Zr;Zr; (b)(b) Zn.Zn. Figure Segregation rate ZK60 alloy with different ultrasonic treatments:

The acoustic streaming plays primary role decreasing segregation. According The acoustic streaming plays thethe primary role in in decreasing segregation. According to to thethe Eckart acousticsteaming steamingtheory theory[34–36], [34–36],the the maximum maximum axial velocity Eckart acoustic velocity should shouldbe beproportional proportionaltotothe results of ofsimulation, simulation,the thevariable-frequency variable-frequency ultrasonic thesound soundintensity. intensity. Associating Associating with with the the results ultrasonic treatment is able to bring sound intensity, which canthe enhance acoustic steaming. treatment is able to bring higherhigher sound intensity, which can enhance acousticthe steaming. Therefore, Therefore, variable-frequency has a higher degree decrease ofrate. segregation rate. variable-frequency US treatment US hastreatment a higher degree of decrease ofof segregation 4. 4. Conclusions Conclusions In In this study, traditional ultrasonic technology and a variable-frequency ultrasonic technology this study, traditional ultrasonic technology and a variable-frequency ultrasonic technology were used in large diameter semi-continuous casting of ZK60 alloy, to evaluate the influence of of were used in large diameter semi-continuous casting of ZK60 alloy, to evaluate the influence ultrasonic treatment in microstructure and mechanical properties. In what concerns grain refinement ultrasonic treatment in microstructure and mechanical properties. In what concerns grain and mechanical properties improvement, the results show refinement and mechanical properties improvement, thethat: results show that: (1)(1) Ultrasonic treatment is is a clean, eco-friendly, and efficient physical technology that can bebe Ultrasonic treatment a clean, eco-friendly, and efficient physical technology that can recognized as as a method to to control thethe size and morphology of of α-Mg grains and distribution of of recognized a method control size and morphology α-Mg grains and distribution β-MgZn phases; β-MgZn phases; (2)(2) TheThe different refinement efficiencies appearing in different districts ofdistricts the billetofare attributed different refinement efficiencies appearing in different the billet are to attributed the sound to attenuation melt and refinement reducing follow the increase of distance the sound in attenuation in melt andefficiency refinement efficiency reducing follow the increase of from ultrasonic distance fromradiator; ultrasonic radiator; (3)(3) Variable-frequency ultrasound shows itsits strong power in in grain refinement compared with Variable-frequency ultrasound shows strong power grain refinement compared with traditional fixed-frequency 70 μm µm(center), (center),7878μm µm(1/2R), (1/2R), traditional fixed-frequencyultrasound. ultrasound.Globular Globularα-Mg α-Mg grains with 70 and and 101 µm (edge) are obtained when variable-frequencyultrasound ultrasoundwas wasused; used; 101 μm (edge) are obtained when variable-frequency (4)(4) Microstructure refinement was followed byby promotion in in mechanical properties. The UTS Microstructure refinement was followed promotion mechanical properties. The UTS increase from 235 to 280 MPa (center), 221 to 263 MPa (1/2R), and 210 to 245 MPa (edge) and elongation increase from 235 to 280 MPa (center), 221 to 263 MPa (1/2R), and 210 to 245 MPa (edge) and from 6.1% to 8.9% 7.8% (1/2R), 4.6%(1/2R), to 5.8%and (edge), compared with elongation from(center), 6.1% to4.9% 8.9%to(center), 4.9% and to 7.8% 4.6%respectively, to 5.8% (edge), respectively, non-ultrasonic treatment; compared with non-ultrasonic treatment; (5)(5)The Thevariable-frequency variable-frequencyacoustic acousticfield fieldimproves improvesrefinement refinementeffects effectsbybyenhancing enhancing cavitation-enhanced heterogeneous nucleation and dendrite fragmentation effects. cavitation-enhanced heterogeneous nucleation and dendrite fragmentation effects. Acknowledgments: This research was financially supported by the National Key Research and Development Program of China (Grant Nos. 2016YFB0301101, 2016YFB0301104) and the National Basic Research Program of China (Grant No. 2013CB632203). Author Contributions: Xingrui Chen and Qichi Le conceived and designed the experiments; Xingrui Chen, Qichi Le and Xibo Wang performed the experiments; Qiyu Liao and Chaoyang Chu analyzed the data; Qiyu Liao contributed reagents/materials/analysis tools; Xingrui Chen wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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