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Effect of electromagnetic fields on microstructure and mechanical properties of sub-rapid solidification-processed Al–Mg–Si alloy during twin-roll casting
Materials Science & Engineering A 766 (2019) 138328
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of electromagnetic fields on microstructure and mechanical properties of sub-rapid solidification-processed Al–Mg–Si alloy during twin-roll casting
T
Chen Hea, Yong Lia,∗, Jiadong Lia, Guangming Xub, Zhaodong Wanga, Di Wua a b
State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang, 110819, China Key Laboratory of Electromagnetic Processing of Materials, Ministry of Education, Northeastern University, Shenyang, 110819, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Twin-roll casting 6022 aluminum alloy External fields Segregation Mechanical properties
The main purpose of this study was to investigate the effects of different external physical fields on the microstructure and mechanical properties of sub-rapid solidification-processed Al–Mg–Si alloy. The microstructural evolution, compositional distribution, hardness, and mechanical properties of cast-rolled strips with and without the application of external fields were studied in detail using metallographic microscopy (OM), electron probe microanalysis (EPMA), tensile tests, and hardness tests. When an external field was applied, the structure of α-Al became equiaxed and fine, the area fraction of non-equilibrium eutectic phases greatly decreased; the improvement effect was greatest under application of an electromagnetic oscillation field. By virtue of the electromagnetic braking effect and shock wave effect resulting from application of a static magnetic field and pulsed current field, respectively, the uniformity of the microstructure and composition distribution was improved and the mixing capacity and solid solubility of the alloy elements in matrix were increased. These changes reduced the difference in the hardness in the thickness direction and in the mechanical properties in the width direction, ultimately improving the overall mechanical properties of the AA6022 Al–Mg–Si alloy cast-rolled strips.
1. Introduction Age-hardened Al–Mg–Si aluminum alloys are widely used in transportation, aerospace, and other industries because of their advantages of light weight, high strength, good formability, good corrosion resistance, and recyclability [1,2]. Traditional production methods for aluminum alloy sheets include many processing steps: fabrication of large sized ingots, homogenization, surface scalping, hot rolling, intermediate annealing, and cold rolling [3]. As a near-net-shape processing technique, twin-roll casting (TRC) has been considered one of the most promising technologies in the metallurgical industry in the 21st century because of the short processing time, low energy consumption, and low equipment and running cost [4]. This method can be used to directly produce 3–10 mm strips by pouring the molten metal into the cast–rolling zone, which consists of a nozzle and rollers. Although this technology has been used for decades, core sandwich and segregation defects are one of the difficulties hindering further development of twin-roll casting. Only alloys with low alloying element contents, such as the 1xxx, 3xxx, 8xxx series, can currently be massproduced. With the increasing demand for high-quality and low-cost aluminum alloy sheets for the transportation industry, research on the fabrication of 5xxx, 6xxx, and 7xxx series products is currently being
∗
Corresponding author. E-mail address: [email protected] (Y. Li).
https://doi.org/10.1016/j.msea.2019.138328 Received 16 August 2019; Accepted 24 August 2019 Available online 27 August 2019 0921-5093/ © 2019 Elsevier B.V. All rights reserved.
promoted [5–11]. The initial microstructure quality of cast-rolled strips has always been a concern because most casting defects formed during the subrapid solidification process will not be effectively eliminated by the subsequent hot/cold-rolling and heat-treatment processes, which significantly affects the performance of the final products. Because of the inherent symmetrical crystallization characteristics, element segregation is a typical and inevitable defect of TRC strips [12]. Therefore, to improve the overall mechanical properties, the solidification process of twin-roll casting should be carefully controlled to reduce the segregation degree. In the traditional direct-chill (DC) casting of large sized ingots with high alloying element content, segregation defects are also an issue [13]. To overcome this problem, different methods have been applied in the traditional casting process, such as mechanical vibration [14], compound fields [15], high static magnetic and low alternating current [16], pulse magneto-oscillation treatment [17], electric current pulses [18], and low-frequency electromagnetic fields [19], and the effects of different methods on the microstructure and mechanical properties have been studied. Based on previous success with the application of an external field in the DC casting process, the introduction of external fields in TRC has great potential. Su et al. applied different external
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fields in the twin-roll casting of AA7075 aluminum alloy strip, and the mechanisms of structure refinement and composition homogenization in the field were studied [20–25]. In addition, magnesium alloy strip with thicknesses of approximately 3.6–4 mm were successful fabricated by twin-roll casting with the application of an ultrasonic field; the grain size of α-Mg decreased from 136.3 to 44.7 μm, and the morphology changed from dendritic to globular [26]. These results indicate that these methods are more or less effective in controlling or alleviating segregation in both DC casting and TRC processes; however, the effect is limited and segregation cannot be completely eliminated. Even more, the application of external fields in the TRC process has attracted considerable attention, and some laboratory research results have been obtained. The non-contact type reduces the contamination of the foreign material in the melt. Introducing different physical fields into TRC has great engineering application value. To the best of our knowledge, few studies have been performed on the production of aluminum alloy with a wide solidification range using TRC, especially for automotive body sheet. To fill this research gap, in this study, AA6022 Al–Mg–Si alloy strips were produced using the TRC technique with the application of different external fields. Additionally, the microstructural evolution, alloy element distribution, and mechanical properties were also investigated. Our findings demonstrate that this approach can be applied to produce high-quality and low-cost aluminum alloy sheets for automotive industrial application.
2.2. Twin-roll casting Commercial AA6022 aluminum alloy with the composition of Mg 0.8%, Si 1.0%, Cu 0.1%, Mn 0.1%, Fe 0.1%, Al–Ti–B 0.4%, and balance Al was used in this study. The pure aluminum ingots were placed into the resistance furnace in a TiO2-coated stainless steel crucible and heated to 750 °C. After all the pure aluminum ingots were melted, the Al–20Si, Al–50Cu, and Al–10Mn master alloys were added into the melt, and the temperature held at 720 °C for 1 h to ensure that the alloying elements were completely dissolved and diffused into the melt. Finally, pure Mg was added to the molten melt followed by degassing and removal of the slag. An Al–Ti–B grain refiner was also added into the melt and then reduced to 690 °C to prepare for the TRC experiment. For the electromagnetic TRC process, the initial rolling speed, melt temperature, nozzle width, and cooling water pressure were 1.2 m/min, 690 °C, 200 mm, and 0.4 MPa, respectively. After nearly 200 mm cast-rolled strip was successfully fabricated, the DC power supply and pulse power supply device were turned on. The DC current value was set to 500 A (45 mT). The frequency, peak current value, and duty cycle of the pulse power supply were set to 20 Hz, 15%, and 500 A, respectively. The electromagnetic oscillation field was a compound field consisting of a static magnetic field and pulsed current field with two power supplies switched on simultaneously. 3. Results
2. Experimental procedures
3.1. Metallographic macro-segregation
2.1. Electromagnetic cast-rolling equipment
Fig. 2 shows the microstructure in the thickness direction of the cast-rolled strips with the application of different external fields; the white region is α-Al, and the red region consists of the α-Al and eutectic structure, commonly known as the non-equilibrium eutectic phase. During the solidification process of the aluminum alloy, the area fraction of the non-equilibrium eutectic phase could be used to characterize the segregation degree. With the application of the different external fields, the decrease in the eutectic phase area fraction at grain boundaries indicates that more solute elements were dissolved into the matrix and that the segregation of the cast-rolled strips was eliminated [29]. In Fig. 2 above, the morphology in the thickness direction can be observed. The red part in the figure corresponds to the non-equilibrium eutectic phase. The area fraction of the non-equilibrium eutectic phase without application of an external field was determined to be 8.28% using Image-Pro Plus 6.0. However, with the application of a static magnetic field and pulse current field, the area fraction of the eutectic phase decreased to 5.49% and 4.06%, respectively. When an electromagnetic oscillation field was applied, the area fraction of the non-
As shown in Fig. 1, a horizontal twin-roll caster with different electromagnetic fields was adopted in this study. The caster had two rolls 500 mm in diameter and 500 mm in width. The thickness of the roll sleeve, roll gap, and separation rolling force were 50 mm, 5 mm, and 1000 kN, respectively. A static magnetic field was generated by passing a direct current through the excitation coil. The magnetic induction intensity was controlled by changing the current value. The current of the electromagnetic wire in the coil was controlled to be 0–500 A using a 3 × 9 mm copper flat wire. The coil was placed in a stainless steel tank with cooling water inside and the stainless steel tank as close as possible to the roll gap, such that the maximum magnetic induction intensity of 45 mT could be obtained in the cast-rolling zone. A pure titanium rod from the pulse power supply was placed in the melt of the front box. A pair of pinch rollers were used to achieve close contact between the rollers and cast-rolled strip, thus forming a pulse current loop. The current parameters were controlled by controlling the output voltage, duty cycle, and frequency of the pulse power supply.
Fig. 1. Schematic of twin-roll casting process with different external fields. 2
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Fig. 2. Microstructure in the thickness direction of the cast-rolled strips with the application of different external fields (a) Non-field; (b) SMF field; (c) PCF field; (d) EMOF field.
mainly of Si-containing phases, whereas the slender rods contained Mn, Fe, and a small amount of Si, presumably as insoluble eutectic Al–Fe–Mn–Si phases. The other grains consisted of a relatively uniform distribution of Cu and Mn. Because of the absence of an external field and because the Al–Mg–Si aluminum alloy has a wide solid–liquid interval, when the dendrite arms grow, they “bridge” with each other, enrich a large number of solute elements in the root of the intergranular and secondary dendrite arms, and form intermetallic compounds, which precipitate in the form of coarse skeletal or a rod-like second phase or exist in the form of intergranular non-equilibrium compounds, leading to macro-segregation.
equilibrium eutectic phase decreased to a minimum value of 2.33%. These results indicate that the electromagnetic oscillation field can effectively reduce the precipitation of elements, improve the effective allocation coefficient ke, and increase the solute solubility in the matrix, thus reducing the segregation defects. 3.2. Element distribution in the segregation region The element distribution in the central segregation band of the Al–Mg–Si alloy cast-rolled strip produced without application of an external field is shown in Fig. 3. The alloying elements in the segregation region mainly consisted of Mg and Si, with a small amount of Mn and Fe. The light-gray skeletal compounds at the grain boundaries consisted
Fig. 3. The EPMA images of element distribution in the central segregation band of cast-rolled strips processed without external field. 3
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Fig. 4. Distribution of Mg and Si elements in matrix from periphery to center (a) Distribution of Mg element (b) Distribution of Si element.
external fields are presented in Fig. 5. The XRD data reveal that the cast-rolled strips mainly contained the Mg2Si phase, and the peak intensity of the phase was significantly reduced with the application of the external fields. The DSC data also showed the same result. Under different external field conditions, at approximately 557 °C, the four samples showed obvious endothermic peaks at the same time, indicating that the second phase was dissolved at this temperature. The peak area in the DSC curve corresponds to the thermal effect (i.e., enthalpy change) that occurs. With the application of the static magnetic field, pulse current field, and electromagnetic oscillation field, the thermal enthalpy value was reduced from 2.697 J/g without external field to 0.006 J/g with the electromagnetic oscillation field.
3.3. Element distribution in the matrix As the solidification process proceeds, the dendrite growth direction is perpendicular to the roll surface and points to the center of the strip. At the front of the dendrite, the melt enriched with solute elements solidifies at the center of the strip and results in segregation. As shown in Fig. 4, the contents of Mg and Si in the strip center range from 2.5 to 3 mm increased significantly without application of an external field, indicating that there is serious center-line segregation in traditional TRC. With the application of a single static magnetic field or pulsed current field, we observed that the central segregation defect was suppressed; however, other types of segregation defects appeared, which may be related to the action mechanism of different external fields. The static magnetic field only changes the movement direction of charged particles in melt. The pulse current field has the effect of grain refinement. The segregation defect cannot be completely eliminated from the original large-area macro-segregation to discontinuous and dispersed segregation. When the electromagnetic oscillation field was applied to the TRC process, the distribution of Mg and Si elements from the periphery to the center were more uniform, and the peak content was mainly maintained in a stable range. Thus, with the application of an electromagnetic external field, the effective partition coefficient of the solute elements increases, which reduces the composition difference between the solidified solid and liquid phases. Finally, the formation of segregation defects in the center region was reduced and the mechanical properties of cast-rolled strips was improved. This result is consistent with the previous OM microstructure results in Fig. 2.
3.5. Mechanical properties The mechanical properties of the cast-rolled strips are shown in Fig. 6. As observed in Fig. 6 (b) and (c), with the application of external fields, the strength and elongation of the cast-rolled strips clearly increased. The elongation and strength of the cast-rolled strip processed under the electromagnetic oscillation field were the highest; these results are consistent with those in Fig. 4. The application of an electromagnetic oscillation field can significantly alleviate segregation defects, reduce the cracking probability during the subsequent tensile deformation process, and significantly improve the elongation and strength of cast-rolled strips. Uniaxial tensile tests were performed along the rolling direction at different positions in the width direction (i.e., center position 3#, 1/4 positions 2# and 4#, edge positions 1# and 5#) of the cast-rolled strip. Obvious inhomogeneity of the mechanical properties in the TD direction of the cast-rolled strip was observed without application of an external field. In general, the strength and elongation of the samples at the center of the strip were higher than those at the edge. The
3.4. XRD and DSC analysis XRD patterns and DSC curves of the cast-rolled strips with different
Fig. 5. The XRD and DSC analysis of cast-rolled strips with different external fields (a) XRD patterns with different external fields (b) DSC curves with different external fields (c) The enthalpy of cast-rolled strips with different external fields. 4
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Fig. 6. Mechanical properties of cast-rolled strips with different external fields (a) Stress-strain curves with different external fields (b) Strength with different external fields (c) Elongation with different external fields.
Fig. 7. Mechanical properties in width direction with different external fields (a) Five samples were cut in the width direction of cast-rolled strips (b) Elongation distribution in width direction (c) Strength distribution in width direction.
the application of the electromagnetic oscillation field can significantly reduce segregation defects and increase the number and depth of small dimples. The dimples distribution become more uniform, which is consistent with the previous results. The electromagnetic oscillation field has the best effect on improving the microstructure and properties of cast-rolled strips.
inhomogeneity of the mechanical properties may result from the inhomogeneity of the chemical composition, microstructure and cracks at the edge of cast-rolled strips. As observed in Fig. 7 (b) and (c), the elongation and strength increased with the application of the electromagnetic oscillation field simultaneously. In addition, the differences between the three positions were small, indicating that the uniformity of the electromagnetic oscillation samples in the TD direction was improved. As observed in Fig. 2 (a), a large number of non-equilibrium eutectic phases were accumulated in these defect region, which were prone to crack during subsequent deformation process and reduce the bonding strength. With the application of the external fields, the stirring effect of the Lorentz force on the melt reduced the superheat of the melt, homogenized the distribution of alloy elements in the melt, and reduced the concentration gradient of the solute. The supersaturated solid solubility of the alloying elements in the matrix was improved, which not only reduced the formation of the non-equilibrium brittle eutectic phase in the segregation region but also improved the uniformity of the structure and composition of the cast-rolled strip. Finally, the tensile strength and elongation were improved.
3.7. Hardness distribution As demonstrated in the Fig. 9, the Vickers hardness values of castrolled strips prepared with the application of different external fields were measured in the thickness direction. The average hardness value of traditional cast-rolled strips prepared without the application of an external field in the entire thickness direction was approximately 50.4. At the same position in the thickness direction, the hardness values increased with the application of a static magnetic field, pulse current field, and electromagnetic oscillation field. Compared with other TRC conditions, the uniformity and consistency of the hardness value in the thickness direction of the electromagnetic oscillation field were the best. The average standard deviation was approximately 0.64, and the average hardness value was the highest (61.2), much better than the values of 1.69 and 50.4 for traditional TRC. According to Figs. 5 and 2 (a), the area fraction of non-equilibrium eutectic phases decreases significantly with the application of external fields, indicating that the content of alloying elements dissolved into the matrix increases. The lattice distortion caused by the solute atoms in the solid solution increases the resistance of dislocation movement and makes the slip difficult, thus increasing the strength and hardness of the alloy.
3.6. Fracture morphology The fracture morphologies of the cast-rolled strips fabricated with the application of different external fields are shown in Fig. 8. The fracture surface was composed of some small pits, a typical tough fracture morphology commonly known as dimples. When no external field was applied, serious segregation defects were present in the castrolled strip; the fracture consisted of obvious tearing edges and less dimples. However, as observed in Fig. 6 (c), with the application of SMF and PCF external fields, the elongation of the material increased significantly and the number of dimples increased; however, the size did not change significantly. As observed in Figs. 2 (d) and Fig. 8 (d), with 5
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Fig. 8. Tensile fracture morphology of cast-rolled strips (a) None field (b) Static magnetic field (c) Pulse current field (d) Electromagnetic oscillation field.
the final solidification area of the core, and the segregation layer is formed after solidification. This defect becomes more apparent in the alloys with wide solidification intervals during TRC. At a higher cast-rolling speed, the solidification and deformation processes occur too rapidly; thus, the soluteenriched liquids cannot be extruded from solidified dendrites, and small liquid regions remain between solid grains. These events result in dispersed segregation if the semi-solid region can be deformed without liquid flow, which can minimize two types of segregation. 4.2. Mechanism of external fields during TRC 4.2.1. Static magnetic field The effect of the static magnetic field on the heat-transfer behavior of the metal solidification front consists of two aspects: one is to suppress the convection of the melt, and the other is to produce a thermoelectromagnetic fluid effect. There is a competitive relationship between these two effects. The mechanism of alleviating segregation by applying a static magnetic field mainly involves the interaction between the static magnetic field and melt flow, called the electromagnetic braking effect (EMBR). When the conductive fluid cuts the magnetic induction line, the induced electromagnetic force, induced current, and Lorentz force were generated. The direction of the Lorentz force was opposite to the flow direction of the melt, indicating that the Lorentz force can restrain the melt flow and eliminate the turbulence of the melt. As shown in Fig. 11 (a), the trajectory of charged particles is a helix around the magnetic force line. Thus, the tendency of the solute elements discharged during solidification to segregate at the center region of strip was reduced. The content of alloying elements in the matrix increased from the center to the surface, causing the segregation to change from large-area macro-segregation to less micro-segregation defects. Finally, the effect on the mechanical properties is reduced, as shown in Fig. 11(b) and (c). That is, the EMBR effect increases the running time of solute elements from the periphery to the center and reduces the content of alloy elements gathered in the center region under the same cast-rolling conditions. During the solidification process with separation rolling force, liquid convection, structural fluctuation, and energy fluctuation were
Fig. 9. Hardness distribution of the cast-rolled strips in the thickness direction with different external fields.
4. Discussion 4.1. Formation mechanism of centerline segregation When the molten melt enters the cast–rolling zone, it is chilled by the casting roll. Firstly, fine dendrites are formed on both sides of the roll surface and grow rapidly along the < 001 > direction in the form of columnar crystals. These columnar crystals grow against the roll surface and rotate with the roll at the same time. In the area near the kiss point, the formed solidified shells meet and are “welded” together by the casting roll. Finally, the finished cast-rolled strips are produced. As observed in Fig. 10, during the solidification process, the mixture with low melting point and the solute composition with a partition coefficient ke ≤ 1 in the liquid were pushed to the liquid phase zone of the molten pool at the tip of dendrite and accumulated. Therefore, the solute content in the liquid phase gradually increased with the advance of the solidification interface. Ultimately, the solute composition and low-melting-point substances in the liquid phase are concentrated in 6
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Fig. 10. Formation mechanism of centerline segregation during traditional twin-roll casting.
restrained, and under the action of the magnetic field, the viscosity of the molten metal increased. According to Utech, the band segregation is caused by turbulent action with the solidification interface movement. When a static magnetic field is applied perpendicular to the direction of crystal growth, the convection and associated temperature fluctuations can be suppressed and the band segregation can be alleviated [27]. Therefore, the grain structure of AA6022 aluminum alloy treated by a static magnetic field has not been refined; however, the effect of electromagnetic braking improves the solid solubility of Mg, Si, Fe, and other segregating alloy elements and reduces the formation of a nonequilibrium eutectic phase, thus playing a role in reducing the centerline segregation of the cast-rolled strip.
effect causes the melt to be compressed back-and-forth, as shown in Fig. 12. When the shock wave was large enough, the dendrites will be broken into small pieces; an equally important feature is that if the aggregated grain refiners can be dispersed sufficiently, the nucleation rate increased significantly with increasing number of nucleation particles. Finally, the microstructure becomes refined and uniform. To explore the refinement mechanism, the force on the melt under the action of pulse current was analyzed. When a variable current passes through the melt, a variable magnetic field will be generated. Under the action of the magnetic field, the conductive fluid will shrink to its central axis, that is, the magneto-striction effect. The magnetic pressure P on the melt per unit area is expressed by formula (1) and (2).
P=
4.2.2. Pulse current field For the mechanism of the pulse current field on the solidification process, Misra reported that the electric current enhances the solute gradient at the interface front and the Joule heat effect disturbs the interface, leading to remelting of the dendrites near the interface and refinement of the eutectic structure [28]. Another remarkable characteristic of the pulse current is abrupt change. When the pulse current passes through the conductive melt instantaneously, the electromagnetic force caused by the pulse discharge will cause the directional drift of a large number of electrons, forming a strong shock wave. This
μ 0 I 2r 2 8π 2R 4 R
I = ∫ 2πrj (r ) dr 0
(1)
R
I=
∫ 2πrj (r ) dr 0
(2)
Here, I is the total current through the melt, R is the radius of the conductive melt, μ0 is the vacuum permeability, and r is the distance from any point in the melt to the axis. Under the action of the magnetic
Fig. 11. Solute movement path in static magnetic field (a) Movement path of charged particles in static magnetic field (b) Movement path of charged particles without external field (c) Movement path of charged particles with static magnetic field. 7
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Fig. 12. Shock wave effect of pulse current field.
oscillation produces shear force Fτ at the root or tip of the crystal. According to Fig. 13, when the force is greater than the adhesion force Fa between the grain and roll surface, dendrites separated from the roll surface and broken dendrites are distributed in the molten melt and become heterogeneous nucleation particles, and the nucleation core increases, as stated by formula (4) and (5). Finally, fine dendrites make the distribution of structure and chemical composition more uniform and reduce the formation of macro-segregation bands. As shown in Fig. 14, the charged particles in the melt are mainly subjected to gravity fg, the melt resistance fz, the driving force fS provided by solidification, and the electromagnetic force fm generated by the simultaneous introduction of the pulse current field and static magnetic field into the TRC process. In the traditional TRC method, a slender black segregation band was formed near the center line of the strip, indicating that the driving force fs provided by solidification was dominant. According to Sun [8], when a compound field was applied to the TRC process, the maximum electromagnetic force was approximately 11 N/m3. When the electromagnetic force acts on charged particles, different rates will be generated because of the different charged quantities of each particle. Under the combined action of the driving force and electromagnetic force, the trajectory of charged particles will be an ellipse, moving in the direction of the dendrite arm. Another important effect of electromagnetic fields was described by Chen [29]: under the application of an electromagnetic oscillation field, the temperature of the liquidus and solidus increases, with the temperature of the solidus increasing greatly; thus, the solidification interval decreases. To a certain extent, this process is equivalent to a strengthening secondary cooling process, accelerating the solidification speed and shortening the residence time of alloy elements in the solid–liquid coexistence zone, thus reducing the number of coarse intermetallic compounds and non-equilibrium eutectic phases at grain boundaries. The increase of the solidification speed reduces the secondary dendrite spacing, inhibits the long-distance flow of the solute-rich melt, and weakens the segregation degree. In summary, the increase of the crystalline core in the melt and the inhibition of the magnetic field on dendritic growth conditions resulted in the formation of a uniform and fine non-dendritic microstructure.
pressure, the melt was repeatedly compressed and stretched. The undercooling degree was increased by eliminating the superheat of the melt rapidly. The impact effect of clusters in the melt was produced, the growth of crystal nucleus was restrained, and the dendrite was broken. It can thus be concluded that when the pulse current was introduced into the solidifying melt, it not only increased the number of nucleating particles but also inhibited the growth conditions of grains, which refined the crystalline structure and reduced segregation. 4.2.3. Compound field (combination of SMF and PCF) During the solidification process, the static magnetic field and pulsed current field were added simultaneously, and the Lorentz force was generated by the interaction of external fields, resulting in forced oscillation of the melt. Under the condition of electromagnetic oscillation, the reasons for the disappearance of the segregation defect can be summarized as follows: (1) the increase of nucleation particles in melt by the shock wave effect and forced oscillation effect, (2) the elimination of dendrite growth conditions, and (3) the increase of solubility of alloy elements in the matrix caused by the electromagnetic braking effect. The increasing nucleation core of electromagnetic oscillation results from the interaction between the static magnetic field B and pulse current density J, which produces the electromagnetic force F = J×B. This force has a repeated stretching and compression effect on the melt, increasing the wetting condition of high-temperature solid-phase compounds and the molten melt, thus decreasing the critical free energy of heterogeneous nucleation and significantly increasing the nucleation rate, as indicated in formula (3). 3 3 4 ⎛ Tm ⎞ ⎛ 2 − 3 cos θ + cos θ ⎞ ΔG * = 6πσAL 4 ⎝ ΔHΔT ⎠ ⎝ ⎠
(3)
L 3 Fτ = 6γν ⎛ ⎞ ⎝r ⎠
(4)
⎜
Fa =
2.05γ δi
⎟
(5)
In addition, the forced convection induced by electromagnetic
Fig. 13. Fragmentation of dendrite by electromagnetic oscillation field. 8
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Fig. 14. Mechanism of compound field on element migration in the melt.
The electromagnetic braking effect increased the amount of alloying elements in the matrix and reduced the precipitation of the non-equilibrium eutectic phase in the central region. The results indicate that the segregation defect of AA6022 aluminum alloy can be significantly reduced by applying an electromagnetic oscillation field in the TRC process, and uniform microstructures and good mechanical properties can be obtained.
Foundation of China (Grant No. 51790485) and the State Key Laboratory of Rolling and Automation (RAL) of Northeastern University, Shenyang, China. The author would like to thank Dr. Li, Dr Xu, Tiffany Jain, and some other students for their assistance in this paper.
5. Conclusions
[1] J. Hirsch, Recent development in aluminium for automotive applications, Trans. Nonferrous Metals Soc. China 24 (2014) 1995–2002. [2] J. Zhou, X.M. Wan, Y. Li, Advanced aluminium products and manufacturing technologies applied on vehicles presented at the EuroCarBody conference, Mater. Today: Proceedings 2 (2015) 5015–5022. [3] M.S. Kim, S.H. Kim, H.W. Kim, Deformation-induced center segregation in twin-roll cast high-Mg Al-Mg strips, Scr. Mater. 152 (2018) 69–73. [4] T. Haga, K. Tkahashi, M. Ikawaand, H. Watari, Twin roll casting of aluminum alloy strips, J. Mater. Process. Technol. 153–154 (2004) 42–47. [5] M. Slapakova, M. Zimina, S. Zaunschirm, J. Kastner, J. Bajer, M. Cieslar, 3D analysis of macrosegregation in twin-roll cast AA3003 alloy, Mater. Char. 118 (2016) 44–49. [6] Z. Lv, F.S. Du, Z.J. An, H.G. Huang, Z.Q. Xu, J.N. Sun, Centerline segregation mechanism of twin-roll cast A3003 strip, J. Alloy. Comp. 643 (2015) 270–274. [7] G. Chen, J.T. Li, Z.K. Yin, G.M. Xu, Improvement of microstructure and properties in twin-roll casting 7075 sheet by lower casting speed and compound field, Mater. Char. 127 (2017) 325–332. [8] K.M. Sun, L. Li, S.D. Chen, G.M. Xu, G. Chen, R.D.K. Misra, G. Zhang, A new approach to control centerline macrosegregation in Al-Mg-Si alloys during twin roll continuous casting, Mater. Lett. 190 (2017) 205–208. [9] H.B. Wang, L. Zhou, Y.W. Zhang, Y.H. Cai, J.S. Zhang, Effects of twin-roll casting process parameters on the microstructure and sheet metal forming behavior of 7050 aluminum alloy, J. Mater. Process. Technol. 233 (2016) 186–191. [10] C. Gras, M. Meredith, J.D. Hunt, Microdefects formation during the twin-roll casting of Al-Mg-Mn aluminium alloys, J. Mater. Process. Technol. 167 (2005) 62–72. [11] C. Shi, K. Shen, Twin-roll casting 8011 aluminium alloy strips under ultrasonic energy field, Int. J. Lightweight Mater.Manuf. 1 (2018) 108–114. [12] Y. Birol, Analysis of macro segregation in twin-roll cast aluminium strips via solidification curves, J. Alloy. Comp. 486 (2009) 168–172. [13] R. Nadella, D.G. Eskin, Q. Du, L. Katgerman, Macrosegregation in direct-chill casting of aluminium alloys, Prog. Mater. Sci. 53 (2008) 421–480. [14] J.X. Chen, X. Chen, Z.M. Luo, Effect of mechanical vibration on microstructure and properties of cast AZ91D alloy, Results Phys. 11 (2018) 1022–1027. [15] Z.W. Shao, Q.C. Le, Z.Q. Zhang, J.Z. Cui, A new method of semi-continuous casting of AZ80 Mg alloy billets by a combination of electromagnetic and ultrasonic fields, Mater. Des. 32 (2011) 4216–4224. [16] C.S. Li, S.D. Hu, Z.M. Ren, Y. Fautrelle, X. Li, Effect of the simultaneous application of a high static magnetic field and a low alternating current on grain structure and grain boundary of pure aluminum, J. Mater. Sci. Technol. 34 (2018) 2431–2438. [17] I. Edry, N. Frage, S. Hayun, The effect of pulse magneto-oscillation treatment on the structure of aluminum solidified under controlled convection, Mater. Lett. 182 (2016) 118–120. [18] Y.H. Zhang, X.C. Miao, Z.Y. Shen, Q.Y. Han, C.J. Song, Q.J. Zhai, Macro segregation formation mechanism of the primary silicon phase in directionally solidified Al–Si hypereutectic alloys under the impact of electric currents, Acta Mater. 97 (2015) 357–366. [19] V. Hatic, B. Mavric, N. Kosnik, B. Sarler, Simulation of direct chill casting under the influence of a low-frequency electromagnetic field, Appl. Math. Model. 54 (2018) 170–188. [20] X. Su, G.M. Xu, D.H. Jiang, Abatement of segregation with the electro and static magnetic field during twin-roll casting of 7075 alloy sheet, Mater. Sci. Eng. A 599 (2014) 279–285. [21] X. Su, X.J. Wang, X. OuYang, P. Song, G.M. Xu, D.H. Jiang, Physical and mechanical properties of 7075 sheets produced by EP electro- and electromagnetic cast rolling, Mater. Sci. Eng. A 607 (2014) 10–16.
References
In this study, AA6022 cast-rolled strips were fabricated using TRC with the application of different external fields, and the effect of external fields on microstructure and mechanical properties were investigated. The associated formation mechanism of segregation during the traditional TRC process and the mechanism of external fields on segregation alleviation were also examined. The main conclusions can be summarized as follows: (1) The width of the segregation band was more than 200 μm without application of an external field. By introducing an electric or magnetic field into the TRC process, the segregation defects transformed into discontinuous or dispersed segregation, and the width of segregation band was reduced to less than 100 μm. When the electromagnetic oscillation field was applied to the TRC process, the microstructure was clearly refined, the number of non-equilibrium eutectic phases decreased significantly, and the large segregation bands were basically eliminated. (2) The area fraction of the non-equilibrium eutectic phase was determined to be 8.28% without the application of an external field. With the application of a static magnetic field or pulse current field, the amount of the eutectic phase decreased to 5.49% and 4.06%, respectively. When an electromagnetic oscillation field was applied, the area fraction of the non-equilibrium eutectic phase decreased to a minimum of 2.33%. (3) The application of an external field can improve the uniformity of the element content and hardness distribution in the thickness direction. The amount of precipitation of the non-equilibrium eutectic phase is reduced and the content of alloying elements in the matrix is increased, thus improving the hardness of the cast-rolled strips. (4) Compared with traditional TRC without the application of external fields, the yield strength, tensile strength, and elongation of electromagnetic oscillation cast-rolled strip increased by approximately 30 MPa, 30.6 MPa, and 7%, respectively. The external field not only improves the strength and elongation of the material but also improves the uniformity of the microstructure and chemical composition in the width and thickness direction of the cast-rolled strip. Acknowledgments This study was supported by the National Natural Science 9
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(2014) 2937–2942. [26] J. Zhao, K. Yu, X.Y. Xue, D.H. Mao, J.P. Li, Effects of ultrasonic treatment on the tensile properties and microstructure of twin roll casting Mg-3%Al-1%Zn-0.8%Ce0.3%Mn (wt%) alloy strips, J. Alloy. Comp. 509 (2011) 8607–8613. [27] H.P. Utech, M.C. Flemings, Elimination of solute banding in indium antimonide crystals by growth in a magnetic field, J. Appl. Phys. 37 (5) (1966) 2021–2025. [28] A.K. Misra, Misra technique applied on solidification of cast iron, Metall. Trans. A 17A (1986) 358–359. [29] D.D. Chen, H.T. Zhang, H.X. Jiang, J.Z. Cui, Experimental investigation of microsegregation in low frequency electromagnetic casting 7075 aluminum alloy, Mater. Sci. Eng. Technol. 42 (6) (2011) 500–505.
[22] X. Su, T. Liu, G.M. Xu, Composition homogenization evolution of twin-roll cast 7075 aluminum alloy using electromagnetic field, Rare Metal Mater. Eng. 44 (3) (2015) 0581–0586. [23] X. Su, G.M. Xu, D.H. Jiang, Distribution uniformity of added elements in twin-roll cast Al-Zn-Mg-Cu alloy by multi-electromagnetic fields, Rare Met. 34 (8) (2015) 546–552. [24] X. Su, M.N. Li, A.P. Zhang, Y. Xiao, G.M. Xu, Effect of electromagnetic field on the micromorphology of twin-roll cast 7075 alloy, Rare Metal Mater. Eng. 43 (10) (2014) 2354–2358. [25] X. Su, A.P. Zhang, Y. Xiao, M.N. Li, G.M. Xu, Effect of oscillating field on microstructure of twin-roll cast 7075 aluminum alloy, Rare Metal Mater. Eng. 43 (12)
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of electromagnetic fields on microstructure and mechanical properties of sub-rapid solidification-processed Al–Mg–Si alloy during twin-roll casting
T
Chen Hea, Yong Lia,∗, Jiadong Lia, Guangming Xub, Zhaodong Wanga, Di Wua a b
State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang, 110819, China Key Laboratory of Electromagnetic Processing of Materials, Ministry of Education, Northeastern University, Shenyang, 110819, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Twin-roll casting 6022 aluminum alloy External fields Segregation Mechanical properties
The main purpose of this study was to investigate the effects of different external physical fields on the microstructure and mechanical properties of sub-rapid solidification-processed Al–Mg–Si alloy. The microstructural evolution, compositional distribution, hardness, and mechanical properties of cast-rolled strips with and without the application of external fields were studied in detail using metallographic microscopy (OM), electron probe microanalysis (EPMA), tensile tests, and hardness tests. When an external field was applied, the structure of α-Al became equiaxed and fine, the area fraction of non-equilibrium eutectic phases greatly decreased; the improvement effect was greatest under application of an electromagnetic oscillation field. By virtue of the electromagnetic braking effect and shock wave effect resulting from application of a static magnetic field and pulsed current field, respectively, the uniformity of the microstructure and composition distribution was improved and the mixing capacity and solid solubility of the alloy elements in matrix were increased. These changes reduced the difference in the hardness in the thickness direction and in the mechanical properties in the width direction, ultimately improving the overall mechanical properties of the AA6022 Al–Mg–Si alloy cast-rolled strips.
1. Introduction Age-hardened Al–Mg–Si aluminum alloys are widely used in transportation, aerospace, and other industries because of their advantages of light weight, high strength, good formability, good corrosion resistance, and recyclability [1,2]. Traditional production methods for aluminum alloy sheets include many processing steps: fabrication of large sized ingots, homogenization, surface scalping, hot rolling, intermediate annealing, and cold rolling [3]. As a near-net-shape processing technique, twin-roll casting (TRC) has been considered one of the most promising technologies in the metallurgical industry in the 21st century because of the short processing time, low energy consumption, and low equipment and running cost [4]. This method can be used to directly produce 3–10 mm strips by pouring the molten metal into the cast–rolling zone, which consists of a nozzle and rollers. Although this technology has been used for decades, core sandwich and segregation defects are one of the difficulties hindering further development of twin-roll casting. Only alloys with low alloying element contents, such as the 1xxx, 3xxx, 8xxx series, can currently be massproduced. With the increasing demand for high-quality and low-cost aluminum alloy sheets for the transportation industry, research on the fabrication of 5xxx, 6xxx, and 7xxx series products is currently being
∗
Corresponding author. E-mail address: [email protected] (Y. Li).
https://doi.org/10.1016/j.msea.2019.138328 Received 16 August 2019; Accepted 24 August 2019 Available online 27 August 2019 0921-5093/ © 2019 Elsevier B.V. All rights reserved.
promoted [5–11]. The initial microstructure quality of cast-rolled strips has always been a concern because most casting defects formed during the subrapid solidification process will not be effectively eliminated by the subsequent hot/cold-rolling and heat-treatment processes, which significantly affects the performance of the final products. Because of the inherent symmetrical crystallization characteristics, element segregation is a typical and inevitable defect of TRC strips [12]. Therefore, to improve the overall mechanical properties, the solidification process of twin-roll casting should be carefully controlled to reduce the segregation degree. In the traditional direct-chill (DC) casting of large sized ingots with high alloying element content, segregation defects are also an issue [13]. To overcome this problem, different methods have been applied in the traditional casting process, such as mechanical vibration [14], compound fields [15], high static magnetic and low alternating current [16], pulse magneto-oscillation treatment [17], electric current pulses [18], and low-frequency electromagnetic fields [19], and the effects of different methods on the microstructure and mechanical properties have been studied. Based on previous success with the application of an external field in the DC casting process, the introduction of external fields in TRC has great potential. Su et al. applied different external
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fields in the twin-roll casting of AA7075 aluminum alloy strip, and the mechanisms of structure refinement and composition homogenization in the field were studied [20–25]. In addition, magnesium alloy strip with thicknesses of approximately 3.6–4 mm were successful fabricated by twin-roll casting with the application of an ultrasonic field; the grain size of α-Mg decreased from 136.3 to 44.7 μm, and the morphology changed from dendritic to globular [26]. These results indicate that these methods are more or less effective in controlling or alleviating segregation in both DC casting and TRC processes; however, the effect is limited and segregation cannot be completely eliminated. Even more, the application of external fields in the TRC process has attracted considerable attention, and some laboratory research results have been obtained. The non-contact type reduces the contamination of the foreign material in the melt. Introducing different physical fields into TRC has great engineering application value. To the best of our knowledge, few studies have been performed on the production of aluminum alloy with a wide solidification range using TRC, especially for automotive body sheet. To fill this research gap, in this study, AA6022 Al–Mg–Si alloy strips were produced using the TRC technique with the application of different external fields. Additionally, the microstructural evolution, alloy element distribution, and mechanical properties were also investigated. Our findings demonstrate that this approach can be applied to produce high-quality and low-cost aluminum alloy sheets for automotive industrial application.
2.2. Twin-roll casting Commercial AA6022 aluminum alloy with the composition of Mg 0.8%, Si 1.0%, Cu 0.1%, Mn 0.1%, Fe 0.1%, Al–Ti–B 0.4%, and balance Al was used in this study. The pure aluminum ingots were placed into the resistance furnace in a TiO2-coated stainless steel crucible and heated to 750 °C. After all the pure aluminum ingots were melted, the Al–20Si, Al–50Cu, and Al–10Mn master alloys were added into the melt, and the temperature held at 720 °C for 1 h to ensure that the alloying elements were completely dissolved and diffused into the melt. Finally, pure Mg was added to the molten melt followed by degassing and removal of the slag. An Al–Ti–B grain refiner was also added into the melt and then reduced to 690 °C to prepare for the TRC experiment. For the electromagnetic TRC process, the initial rolling speed, melt temperature, nozzle width, and cooling water pressure were 1.2 m/min, 690 °C, 200 mm, and 0.4 MPa, respectively. After nearly 200 mm cast-rolled strip was successfully fabricated, the DC power supply and pulse power supply device were turned on. The DC current value was set to 500 A (45 mT). The frequency, peak current value, and duty cycle of the pulse power supply were set to 20 Hz, 15%, and 500 A, respectively. The electromagnetic oscillation field was a compound field consisting of a static magnetic field and pulsed current field with two power supplies switched on simultaneously. 3. Results
2. Experimental procedures
3.1. Metallographic macro-segregation
2.1. Electromagnetic cast-rolling equipment
Fig. 2 shows the microstructure in the thickness direction of the cast-rolled strips with the application of different external fields; the white region is α-Al, and the red region consists of the α-Al and eutectic structure, commonly known as the non-equilibrium eutectic phase. During the solidification process of the aluminum alloy, the area fraction of the non-equilibrium eutectic phase could be used to characterize the segregation degree. With the application of the different external fields, the decrease in the eutectic phase area fraction at grain boundaries indicates that more solute elements were dissolved into the matrix and that the segregation of the cast-rolled strips was eliminated [29]. In Fig. 2 above, the morphology in the thickness direction can be observed. The red part in the figure corresponds to the non-equilibrium eutectic phase. The area fraction of the non-equilibrium eutectic phase without application of an external field was determined to be 8.28% using Image-Pro Plus 6.0. However, with the application of a static magnetic field and pulse current field, the area fraction of the eutectic phase decreased to 5.49% and 4.06%, respectively. When an electromagnetic oscillation field was applied, the area fraction of the non-
As shown in Fig. 1, a horizontal twin-roll caster with different electromagnetic fields was adopted in this study. The caster had two rolls 500 mm in diameter and 500 mm in width. The thickness of the roll sleeve, roll gap, and separation rolling force were 50 mm, 5 mm, and 1000 kN, respectively. A static magnetic field was generated by passing a direct current through the excitation coil. The magnetic induction intensity was controlled by changing the current value. The current of the electromagnetic wire in the coil was controlled to be 0–500 A using a 3 × 9 mm copper flat wire. The coil was placed in a stainless steel tank with cooling water inside and the stainless steel tank as close as possible to the roll gap, such that the maximum magnetic induction intensity of 45 mT could be obtained in the cast-rolling zone. A pure titanium rod from the pulse power supply was placed in the melt of the front box. A pair of pinch rollers were used to achieve close contact between the rollers and cast-rolled strip, thus forming a pulse current loop. The current parameters were controlled by controlling the output voltage, duty cycle, and frequency of the pulse power supply.
Fig. 1. Schematic of twin-roll casting process with different external fields. 2
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Fig. 2. Microstructure in the thickness direction of the cast-rolled strips with the application of different external fields (a) Non-field; (b) SMF field; (c) PCF field; (d) EMOF field.
mainly of Si-containing phases, whereas the slender rods contained Mn, Fe, and a small amount of Si, presumably as insoluble eutectic Al–Fe–Mn–Si phases. The other grains consisted of a relatively uniform distribution of Cu and Mn. Because of the absence of an external field and because the Al–Mg–Si aluminum alloy has a wide solid–liquid interval, when the dendrite arms grow, they “bridge” with each other, enrich a large number of solute elements in the root of the intergranular and secondary dendrite arms, and form intermetallic compounds, which precipitate in the form of coarse skeletal or a rod-like second phase or exist in the form of intergranular non-equilibrium compounds, leading to macro-segregation.
equilibrium eutectic phase decreased to a minimum value of 2.33%. These results indicate that the electromagnetic oscillation field can effectively reduce the precipitation of elements, improve the effective allocation coefficient ke, and increase the solute solubility in the matrix, thus reducing the segregation defects. 3.2. Element distribution in the segregation region The element distribution in the central segregation band of the Al–Mg–Si alloy cast-rolled strip produced without application of an external field is shown in Fig. 3. The alloying elements in the segregation region mainly consisted of Mg and Si, with a small amount of Mn and Fe. The light-gray skeletal compounds at the grain boundaries consisted
Fig. 3. The EPMA images of element distribution in the central segregation band of cast-rolled strips processed without external field. 3
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Fig. 4. Distribution of Mg and Si elements in matrix from periphery to center (a) Distribution of Mg element (b) Distribution of Si element.
external fields are presented in Fig. 5. The XRD data reveal that the cast-rolled strips mainly contained the Mg2Si phase, and the peak intensity of the phase was significantly reduced with the application of the external fields. The DSC data also showed the same result. Under different external field conditions, at approximately 557 °C, the four samples showed obvious endothermic peaks at the same time, indicating that the second phase was dissolved at this temperature. The peak area in the DSC curve corresponds to the thermal effect (i.e., enthalpy change) that occurs. With the application of the static magnetic field, pulse current field, and electromagnetic oscillation field, the thermal enthalpy value was reduced from 2.697 J/g without external field to 0.006 J/g with the electromagnetic oscillation field.
3.3. Element distribution in the matrix As the solidification process proceeds, the dendrite growth direction is perpendicular to the roll surface and points to the center of the strip. At the front of the dendrite, the melt enriched with solute elements solidifies at the center of the strip and results in segregation. As shown in Fig. 4, the contents of Mg and Si in the strip center range from 2.5 to 3 mm increased significantly without application of an external field, indicating that there is serious center-line segregation in traditional TRC. With the application of a single static magnetic field or pulsed current field, we observed that the central segregation defect was suppressed; however, other types of segregation defects appeared, which may be related to the action mechanism of different external fields. The static magnetic field only changes the movement direction of charged particles in melt. The pulse current field has the effect of grain refinement. The segregation defect cannot be completely eliminated from the original large-area macro-segregation to discontinuous and dispersed segregation. When the electromagnetic oscillation field was applied to the TRC process, the distribution of Mg and Si elements from the periphery to the center were more uniform, and the peak content was mainly maintained in a stable range. Thus, with the application of an electromagnetic external field, the effective partition coefficient of the solute elements increases, which reduces the composition difference between the solidified solid and liquid phases. Finally, the formation of segregation defects in the center region was reduced and the mechanical properties of cast-rolled strips was improved. This result is consistent with the previous OM microstructure results in Fig. 2.
3.5. Mechanical properties The mechanical properties of the cast-rolled strips are shown in Fig. 6. As observed in Fig. 6 (b) and (c), with the application of external fields, the strength and elongation of the cast-rolled strips clearly increased. The elongation and strength of the cast-rolled strip processed under the electromagnetic oscillation field were the highest; these results are consistent with those in Fig. 4. The application of an electromagnetic oscillation field can significantly alleviate segregation defects, reduce the cracking probability during the subsequent tensile deformation process, and significantly improve the elongation and strength of cast-rolled strips. Uniaxial tensile tests were performed along the rolling direction at different positions in the width direction (i.e., center position 3#, 1/4 positions 2# and 4#, edge positions 1# and 5#) of the cast-rolled strip. Obvious inhomogeneity of the mechanical properties in the TD direction of the cast-rolled strip was observed without application of an external field. In general, the strength and elongation of the samples at the center of the strip were higher than those at the edge. The
3.4. XRD and DSC analysis XRD patterns and DSC curves of the cast-rolled strips with different
Fig. 5. The XRD and DSC analysis of cast-rolled strips with different external fields (a) XRD patterns with different external fields (b) DSC curves with different external fields (c) The enthalpy of cast-rolled strips with different external fields. 4
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Fig. 6. Mechanical properties of cast-rolled strips with different external fields (a) Stress-strain curves with different external fields (b) Strength with different external fields (c) Elongation with different external fields.
Fig. 7. Mechanical properties in width direction with different external fields (a) Five samples were cut in the width direction of cast-rolled strips (b) Elongation distribution in width direction (c) Strength distribution in width direction.
the application of the electromagnetic oscillation field can significantly reduce segregation defects and increase the number and depth of small dimples. The dimples distribution become more uniform, which is consistent with the previous results. The electromagnetic oscillation field has the best effect on improving the microstructure and properties of cast-rolled strips.
inhomogeneity of the mechanical properties may result from the inhomogeneity of the chemical composition, microstructure and cracks at the edge of cast-rolled strips. As observed in Fig. 7 (b) and (c), the elongation and strength increased with the application of the electromagnetic oscillation field simultaneously. In addition, the differences between the three positions were small, indicating that the uniformity of the electromagnetic oscillation samples in the TD direction was improved. As observed in Fig. 2 (a), a large number of non-equilibrium eutectic phases were accumulated in these defect region, which were prone to crack during subsequent deformation process and reduce the bonding strength. With the application of the external fields, the stirring effect of the Lorentz force on the melt reduced the superheat of the melt, homogenized the distribution of alloy elements in the melt, and reduced the concentration gradient of the solute. The supersaturated solid solubility of the alloying elements in the matrix was improved, which not only reduced the formation of the non-equilibrium brittle eutectic phase in the segregation region but also improved the uniformity of the structure and composition of the cast-rolled strip. Finally, the tensile strength and elongation were improved.
3.7. Hardness distribution As demonstrated in the Fig. 9, the Vickers hardness values of castrolled strips prepared with the application of different external fields were measured in the thickness direction. The average hardness value of traditional cast-rolled strips prepared without the application of an external field in the entire thickness direction was approximately 50.4. At the same position in the thickness direction, the hardness values increased with the application of a static magnetic field, pulse current field, and electromagnetic oscillation field. Compared with other TRC conditions, the uniformity and consistency of the hardness value in the thickness direction of the electromagnetic oscillation field were the best. The average standard deviation was approximately 0.64, and the average hardness value was the highest (61.2), much better than the values of 1.69 and 50.4 for traditional TRC. According to Figs. 5 and 2 (a), the area fraction of non-equilibrium eutectic phases decreases significantly with the application of external fields, indicating that the content of alloying elements dissolved into the matrix increases. The lattice distortion caused by the solute atoms in the solid solution increases the resistance of dislocation movement and makes the slip difficult, thus increasing the strength and hardness of the alloy.
3.6. Fracture morphology The fracture morphologies of the cast-rolled strips fabricated with the application of different external fields are shown in Fig. 8. The fracture surface was composed of some small pits, a typical tough fracture morphology commonly known as dimples. When no external field was applied, serious segregation defects were present in the castrolled strip; the fracture consisted of obvious tearing edges and less dimples. However, as observed in Fig. 6 (c), with the application of SMF and PCF external fields, the elongation of the material increased significantly and the number of dimples increased; however, the size did not change significantly. As observed in Figs. 2 (d) and Fig. 8 (d), with 5
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Fig. 8. Tensile fracture morphology of cast-rolled strips (a) None field (b) Static magnetic field (c) Pulse current field (d) Electromagnetic oscillation field.
the final solidification area of the core, and the segregation layer is formed after solidification. This defect becomes more apparent in the alloys with wide solidification intervals during TRC. At a higher cast-rolling speed, the solidification and deformation processes occur too rapidly; thus, the soluteenriched liquids cannot be extruded from solidified dendrites, and small liquid regions remain between solid grains. These events result in dispersed segregation if the semi-solid region can be deformed without liquid flow, which can minimize two types of segregation. 4.2. Mechanism of external fields during TRC 4.2.1. Static magnetic field The effect of the static magnetic field on the heat-transfer behavior of the metal solidification front consists of two aspects: one is to suppress the convection of the melt, and the other is to produce a thermoelectromagnetic fluid effect. There is a competitive relationship between these two effects. The mechanism of alleviating segregation by applying a static magnetic field mainly involves the interaction between the static magnetic field and melt flow, called the electromagnetic braking effect (EMBR). When the conductive fluid cuts the magnetic induction line, the induced electromagnetic force, induced current, and Lorentz force were generated. The direction of the Lorentz force was opposite to the flow direction of the melt, indicating that the Lorentz force can restrain the melt flow and eliminate the turbulence of the melt. As shown in Fig. 11 (a), the trajectory of charged particles is a helix around the magnetic force line. Thus, the tendency of the solute elements discharged during solidification to segregate at the center region of strip was reduced. The content of alloying elements in the matrix increased from the center to the surface, causing the segregation to change from large-area macro-segregation to less micro-segregation defects. Finally, the effect on the mechanical properties is reduced, as shown in Fig. 11(b) and (c). That is, the EMBR effect increases the running time of solute elements from the periphery to the center and reduces the content of alloy elements gathered in the center region under the same cast-rolling conditions. During the solidification process with separation rolling force, liquid convection, structural fluctuation, and energy fluctuation were
Fig. 9. Hardness distribution of the cast-rolled strips in the thickness direction with different external fields.
4. Discussion 4.1. Formation mechanism of centerline segregation When the molten melt enters the cast–rolling zone, it is chilled by the casting roll. Firstly, fine dendrites are formed on both sides of the roll surface and grow rapidly along the < 001 > direction in the form of columnar crystals. These columnar crystals grow against the roll surface and rotate with the roll at the same time. In the area near the kiss point, the formed solidified shells meet and are “welded” together by the casting roll. Finally, the finished cast-rolled strips are produced. As observed in Fig. 10, during the solidification process, the mixture with low melting point and the solute composition with a partition coefficient ke ≤ 1 in the liquid were pushed to the liquid phase zone of the molten pool at the tip of dendrite and accumulated. Therefore, the solute content in the liquid phase gradually increased with the advance of the solidification interface. Ultimately, the solute composition and low-melting-point substances in the liquid phase are concentrated in 6
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Fig. 10. Formation mechanism of centerline segregation during traditional twin-roll casting.
restrained, and under the action of the magnetic field, the viscosity of the molten metal increased. According to Utech, the band segregation is caused by turbulent action with the solidification interface movement. When a static magnetic field is applied perpendicular to the direction of crystal growth, the convection and associated temperature fluctuations can be suppressed and the band segregation can be alleviated [27]. Therefore, the grain structure of AA6022 aluminum alloy treated by a static magnetic field has not been refined; however, the effect of electromagnetic braking improves the solid solubility of Mg, Si, Fe, and other segregating alloy elements and reduces the formation of a nonequilibrium eutectic phase, thus playing a role in reducing the centerline segregation of the cast-rolled strip.
effect causes the melt to be compressed back-and-forth, as shown in Fig. 12. When the shock wave was large enough, the dendrites will be broken into small pieces; an equally important feature is that if the aggregated grain refiners can be dispersed sufficiently, the nucleation rate increased significantly with increasing number of nucleation particles. Finally, the microstructure becomes refined and uniform. To explore the refinement mechanism, the force on the melt under the action of pulse current was analyzed. When a variable current passes through the melt, a variable magnetic field will be generated. Under the action of the magnetic field, the conductive fluid will shrink to its central axis, that is, the magneto-striction effect. The magnetic pressure P on the melt per unit area is expressed by formula (1) and (2).
P=
4.2.2. Pulse current field For the mechanism of the pulse current field on the solidification process, Misra reported that the electric current enhances the solute gradient at the interface front and the Joule heat effect disturbs the interface, leading to remelting of the dendrites near the interface and refinement of the eutectic structure [28]. Another remarkable characteristic of the pulse current is abrupt change. When the pulse current passes through the conductive melt instantaneously, the electromagnetic force caused by the pulse discharge will cause the directional drift of a large number of electrons, forming a strong shock wave. This
μ 0 I 2r 2 8π 2R 4 R
I = ∫ 2πrj (r ) dr 0
(1)
R
I=
∫ 2πrj (r ) dr 0
(2)
Here, I is the total current through the melt, R is the radius of the conductive melt, μ0 is the vacuum permeability, and r is the distance from any point in the melt to the axis. Under the action of the magnetic
Fig. 11. Solute movement path in static magnetic field (a) Movement path of charged particles in static magnetic field (b) Movement path of charged particles without external field (c) Movement path of charged particles with static magnetic field. 7
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Fig. 12. Shock wave effect of pulse current field.
oscillation produces shear force Fτ at the root or tip of the crystal. According to Fig. 13, when the force is greater than the adhesion force Fa between the grain and roll surface, dendrites separated from the roll surface and broken dendrites are distributed in the molten melt and become heterogeneous nucleation particles, and the nucleation core increases, as stated by formula (4) and (5). Finally, fine dendrites make the distribution of structure and chemical composition more uniform and reduce the formation of macro-segregation bands. As shown in Fig. 14, the charged particles in the melt are mainly subjected to gravity fg, the melt resistance fz, the driving force fS provided by solidification, and the electromagnetic force fm generated by the simultaneous introduction of the pulse current field and static magnetic field into the TRC process. In the traditional TRC method, a slender black segregation band was formed near the center line of the strip, indicating that the driving force fs provided by solidification was dominant. According to Sun [8], when a compound field was applied to the TRC process, the maximum electromagnetic force was approximately 11 N/m3. When the electromagnetic force acts on charged particles, different rates will be generated because of the different charged quantities of each particle. Under the combined action of the driving force and electromagnetic force, the trajectory of charged particles will be an ellipse, moving in the direction of the dendrite arm. Another important effect of electromagnetic fields was described by Chen [29]: under the application of an electromagnetic oscillation field, the temperature of the liquidus and solidus increases, with the temperature of the solidus increasing greatly; thus, the solidification interval decreases. To a certain extent, this process is equivalent to a strengthening secondary cooling process, accelerating the solidification speed and shortening the residence time of alloy elements in the solid–liquid coexistence zone, thus reducing the number of coarse intermetallic compounds and non-equilibrium eutectic phases at grain boundaries. The increase of the solidification speed reduces the secondary dendrite spacing, inhibits the long-distance flow of the solute-rich melt, and weakens the segregation degree. In summary, the increase of the crystalline core in the melt and the inhibition of the magnetic field on dendritic growth conditions resulted in the formation of a uniform and fine non-dendritic microstructure.
pressure, the melt was repeatedly compressed and stretched. The undercooling degree was increased by eliminating the superheat of the melt rapidly. The impact effect of clusters in the melt was produced, the growth of crystal nucleus was restrained, and the dendrite was broken. It can thus be concluded that when the pulse current was introduced into the solidifying melt, it not only increased the number of nucleating particles but also inhibited the growth conditions of grains, which refined the crystalline structure and reduced segregation. 4.2.3. Compound field (combination of SMF and PCF) During the solidification process, the static magnetic field and pulsed current field were added simultaneously, and the Lorentz force was generated by the interaction of external fields, resulting in forced oscillation of the melt. Under the condition of electromagnetic oscillation, the reasons for the disappearance of the segregation defect can be summarized as follows: (1) the increase of nucleation particles in melt by the shock wave effect and forced oscillation effect, (2) the elimination of dendrite growth conditions, and (3) the increase of solubility of alloy elements in the matrix caused by the electromagnetic braking effect. The increasing nucleation core of electromagnetic oscillation results from the interaction between the static magnetic field B and pulse current density J, which produces the electromagnetic force F = J×B. This force has a repeated stretching and compression effect on the melt, increasing the wetting condition of high-temperature solid-phase compounds and the molten melt, thus decreasing the critical free energy of heterogeneous nucleation and significantly increasing the nucleation rate, as indicated in formula (3). 3 3 4 ⎛ Tm ⎞ ⎛ 2 − 3 cos θ + cos θ ⎞ ΔG * = 6πσAL 4 ⎝ ΔHΔT ⎠ ⎝ ⎠
(3)
L 3 Fτ = 6γν ⎛ ⎞ ⎝r ⎠
(4)
⎜
Fa =
2.05γ δi
⎟
(5)
In addition, the forced convection induced by electromagnetic
Fig. 13. Fragmentation of dendrite by electromagnetic oscillation field. 8
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Fig. 14. Mechanism of compound field on element migration in the melt.
The electromagnetic braking effect increased the amount of alloying elements in the matrix and reduced the precipitation of the non-equilibrium eutectic phase in the central region. The results indicate that the segregation defect of AA6022 aluminum alloy can be significantly reduced by applying an electromagnetic oscillation field in the TRC process, and uniform microstructures and good mechanical properties can be obtained.
Foundation of China (Grant No. 51790485) and the State Key Laboratory of Rolling and Automation (RAL) of Northeastern University, Shenyang, China. The author would like to thank Dr. Li, Dr Xu, Tiffany Jain, and some other students for their assistance in this paper.
5. Conclusions
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