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Effect of currents on the microstructure of directionally solidified Al–4.5 wt% Cu alloy
Journal of Crystal Growth 324 (2011) 235–242
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Effect of currents on the microstructure of directionally solidified Al–4.5 wt% Cu alloy Changjiang Song a, Yuanyi Guo a, Yunhu Zhang a, Hongxing Zheng a, Meng Yan a, Qingyou Han b, Qijie Zhai a,n a b
Shanghai Key Laboratory of Modern Metallurgy and Materials Processing, School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China Mechanical Engineering Technology Department, Purdue University, West Lafayette, IN 47907-2021, USA
a r t i c l e i n f o
abstract
Article history: Received 30 September 2010 Received in revised form 16 February 2011 Accepted 14 March 2011 Communicated by M. Rettenmayr Available online 21 March 2011
This paper describes a comparative study on the effect of electric current pulse (ECP) and the semisinusoidal current (SSC) on the microstructure of directionally solidified Al–4.5 wt% Cu alloy. The experimental results indicate that both ECP and SSC can make the convex solid/liquid interface flat, reduce the primary spacing and the depth of mushy zone, and change the interface morphology from cellular to planar (i.e. delay the planar/cellular transition). Under the same heating power, SSC produces a greater influence on the microstructure than ECP. Limited effect of the current direction on microstructure is also observed. Generally, current flowing from the liquid to the solid have a stronger effect on the microstructure than those from the solid to the liquid. Possible mechanisms by which the current altes the microstructure of Al–4.5 wt% Cu alloy under directional solidification conditions are suggested. & 2011 Elsevier B.V. All rights reserved.
Keywords: A1. Directional solidification A1. Interface A1. Morphological stability B1. Alloys
1. Introduction Electromagnetic processing of materials (EPM) has been extensively studied in the past two decades. It has been documented that EPM can be used to improve the properties of the metallic materials by refining grain size, eliminating defect, and improving chemistry homogeneity. As a result, EPM has been considered as an effective method to produce high-performance materials and a potential direction to develop novel material processing techniques [1–4]. Recently, much attention has been paid to the applications of electric current pulse (ECP) to the metal solidification process due to its high instantaneous current intensity. Many studies have demonstrated that ECP can significantly refine the solidification structure and improve the properties of alloys [3–6]. Our previous experiments demonstrated that during the directional solidification, ECP can decrease the primary dendrite spacing and delay planar/cellular and cellular/dendrite transitions in certain metals and alloys [7]. The application of ECP is capable of inducing both heat (including Peltier, Thomson, and Joule effects) and force effects in metals and alloys during their directional solidification process [8]. These two effects are related to the frequency and intensity of the current. This article focuses on addressing the
n
Corresponding author. Tel./fax: þ 86 21 56331218. E-mail addresses: [email protected] (C. Song), [email protected] (Q. Zhai).
0022-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2011.03.025
force effect and the heating effect of current on the microstructure of directionally solidified alloy. The force effect of ECP is significant due to its instantaneously sharp change of current intensity. In contrast, SSC usually generates a weak force effect. Thus, these types of current (ECP and SSC) were used to investigate the mechanisms by which the currents influence the microstructure of metals and alloys. In addition the Peltier and Thomson effects are also related to the direction of an electric current, which may result in a change of the temperature field ahead of the freezing front. Therefore, this paper also discusses the effect of current direction on the microstructure of directional solidified alloys.
2. Experimental Fig. 1 depicts the experimental apparatus. It consists of a resistance furnace, a power supply system, a pulling mechanism, high purity corundum tubes, and a cooling system using liquid Ga–In–Sn as the coolant. The power supply system included two separate units for ECP and SSC. The pulling unit was controlled by a servo-actuator. The alloy used in this study was Al–4.5 wt% Cu alloy, which was prepared using commercial purity aluminum (99.7 wt%) and electrolytic copper (99.99 wt%). The sample was placed in a tube of high purity corundum with inner diameter of 4 mm and length of 420 mm before being slowly heated (and melted) up to 1200 1C
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C. Song et al. / Journal of Crystal Growth 324 (2011) 235–242
Consumable
Electric current pulse
electrode (Al-4.5Cu)
Intensity
Resistance furnace
Power
L
Semi-sinusoidal current
supply
5
10
15
20
Time, ms
Experimental table
Fig. 2. Schematic illustration of ECP and SSC used in the experiments.
Pulling mechanism
Coolant Power supply
Movement direction Fig. 1. Sketch of the experimental apparatus.
in the resistance furnace. The power supply was then switched on and current was applied to the sample for 20 min to establish steady state conditions. Subsequently, the sample was pulled downward for 30 mm at a constant rate and then quenched into the liquid Ga–In–Sn alloy by mowing downwards at a high pulling rate to preserve the morphology of the solid/liquid (S/L) interface. The pulling rates were 1.2 mm/s for planar solidification and 2.0 mm/s for cellular solidification. These two pulling rates were chosen because both were near planar/cellular transition conditions. The quenched samples were then sectioned along the central axis, ground, polished, and etched for microstructure examination. The etching agent was a solution of HNO3:HCl:HF:H2O¼ 2.5:1.5:1.0:95.0. The microstructure was examined using a light microscope, and the solute content near the solid/liquid (S/L) interface was measured using JEOL JSM-6700F scanning electron microscopy (SEM) with energy dispersive spectrum (EDS).
Thermal insulating materials
Al-4.5Cu sample
Thermocouple
Light fire brick
Fig. 3. Schematic diagram of the experimental setup used to determine the corresponding intensity of two types of current under the same heating power.
samples used in Fig. 3 were the same as those used in the directional solidification experiments. The experimental results are shown in Fig. 4. Table 1 lists the corresponding intensities of the current with the same heating power.
4. Experimental results 3. Currents used in the experiments 4.1. Cellular growth under the influence of ECP The previous experiments show that ECP can significantly affect the microstructure of metals directionally solidified [7]. When ECP is applied on a sample under directional solidification, it produces two effects: heat and force effects. It is difficult to determine which factor is more dominant because these two factors always occur simultaneously. To understand the mechanism of current on microstructure, SSC was chosen for comparative study on the alloy during its directional solidification. The relationship of these two types of current is schematically shown in Fig. 2 The force effect is difficult to evaluate either theoretically or experimentally because it induces a forced convection in the liquid during the solidification of the specimen. In this work, the comparative experiments were conducted under the same heating effect (i.e. equivalent power) of both types of current. An experimental setup was designed to determine the corresponding intensity of these two types of current, as shown in Fig. 3. The
Fig. 5 demonstrates the effect of ECP on the microstructure of Al– 4.5 wt% Cu directionally solidified at a pulling rate of 2.0 mm/s: Fig. 5(a) depicts the microstructure of the sample not subjected to ECP. The images at the left side of Fig. 5 show the microstructure of the sample with the electric current flowing from the solid to the liquid. The images at the right side of Fig. 5 are the microstructure of the sample with the electric current flowing from the liquid to the solid. It can be seen clearly that the microstructure of the sample without ECP is typically a cellular one, and the S/L interface obviously protrudes into the melt. The application of ECP changes the convex interface into a flat one. In addition, the cellular spacing and the depth of the mushy zone gradually decrease with increasing intensity of the current. The cellular interface transits to planar interface when the current intensity is 5.6 103 A/cm2, as shown in Fig. 5(e) and (i). It was also noted that the electric current flowing from the liquid to the
C. Song et al. / Journal of Crystal Growth 324 (2011) 235–242
237
Fig. 4. Heating curves of two types of current.
Table 1 Intensities of two types of current under the same heating power. ECP (A/cm2)
SSC (A/cm2)
0.8 103 2.4 103 4.0 103 5.6 103
0.2 103 0.4 103 0.7 103 1.0 103
solid has marginally greater effect than that flowing from the solid to the liquid. 4.2. Cellular growth under the influence of SSC The above experimental results indicate that ECP significantly affects the microstructure of the directionally solidified Al–4.5 wt% Cu alloy. To identify the dominant influencing factor to the microstructure, comparative SSC experiments were performed under the same experimental conditions. Fig. 6 shows the microstructure of the alloy directionally solidified at a pulling rate of 2.0 mm/s under the influence of SSC. It indicates that SSC is more effective than ECP in promoting the transition from cellular to planar growth when their heating power is the same. For example, the interface became planar when SSC with intensity of 0.7 103 A/cm2 was applied to the sample, but it was still cellular when an equivalent ECP with intensity of 4.0 103 A/cm2 was applied. The effect of the direction of the electric current on the interface morphology by SSC is similar to that of ECP. 4.3. Planar growth under the influence of either ECP or SSC Fig. 7 shows the microstructure of a Al–4.5 wt% Cu sample directionally solidified at a pulling rate of 1.2 mm/s: Fig. 7(a) shows
the microstructure of a sample not subjected to a current. The images at the left side of Fig. 7 show the microstructure of a sample with ECP, and the images at the right side of Fig. 7 depict the microstructure of a sample with SSC (the direction of the current was from the solid to the liquid). The S/L interface of the sample without a current was mostly planar except for the regions near the tube wall, where cellular interface forms due to a solute enrichment locally. The planar interface markedly protruded into the melt. When a current (either ECP or SSC) was applied, the convex interface became flat. It is interesting to note that the planar interface becomes cellular when a small current (for example, 0.8 103 A/cm2 ECP and 0.2 103 A/cm2 SSC) is applied, as shown in Fig. 7(b) and (e). With the increase of the current intensity, the cellular interface became planar interface again. Furthermore, the interface was very flat compared to that of the sample not subjected to a current, shown in Fig. 7(d) and (g). Under the same heating power, SSC also had greater effect on the microstructure of the alloy than ECP in these experiments similar to the experimental results obtained under cellular growth conditions.
5. Discussion Our experimental results demonstrate that both ECP and SSC can significantly affect the microstructure of Al–4.5 wt% Cu alloy directionally solidified, and turn a cellular interface into a planar one, i.e. delay planar/cellular transition. Compared with the current flowing from the solid to the liquid, the current flowing from the liquid to the solid has a marginally greater effect. On the other hand, SSC has a greater effect than ECP when the heating power is the same. Considering that both ECP and SSC have the force effect and the heat effect during the directional solidification of Al–4.5 wt% Cu alloy, we can analyze the individual effect based on our experimental results.
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Without current
-0.8×10 3 A/cm 2
+0.8×10 3 A/cm 2
-2.4×10 3 A/cm 2
+2.4×10 3 A/cm 2
Current direction
3
-4.0×10 A/cm
2
-5.6×10 3 A/cm 2
3
+4.0×10 A/cm
2
Current direction
+5.6×10 3 A/cm 2
Fig. 5. Effect of ECP on the microstructure of directionally solidified Al–4.5 wt% Cu at the pulling rate of 2.0 mm/s.
5.1. Force effect of currents The Lorentz force produced by a current will lead to a forced convection in the liquid ahead of the S/L interface. It is well known that convection stabilizes the S/L morphology and delays the planar/cellular transition during directional solidification of an alloy [9,10]. Due to low frequency and low intensity of the currents, convection caused by SSC is weaker than that caused by ECP. Consequently, ECP should have a stronger effect on microstructure transition than SSC under the same heating power.
However, our experimental results suggest otherwise. It seems unreasonable if we explain the experimental results only using the force effect, i.e. the force effect should not be the single main reason for the change in microstructure of the directionally solidified Al–Cu alloy. To examine whether the Lorentz force caused by currents has any effect on the microstructure of the directionally solidified Al–4.5 wt% Cu alloy, the solute content near the S/L interface was measured by using EDS. Tables 2 and 3 list the Cu content near the S/L interface of some samples solidified at the pulling rate of
C. Song et al. / Journal of Crystal Growth 324 (2011) 235–242
-0.2×103 A/cm2
+0.2×103 A/cm2
-0.4×103 A/cm2
+0.4×103 A/cm2
-0.7×103 A/cm2
+0.7×103 A/cm2
239
Current direction
Current direction
-1.0×103 A/cm2
+1.0×103 A/cm2
Fig. 6. Effect of SSC on the microstructure of directionally solidified Al–4.5 wt% Cu at the pulling rate of 2.0 mm/s.
2.0 mm/s and 1.2 mm/s, respectively. The Cu content listed in the tables is the average value of data measured at three positions about 50 mm away from the S/L interface as schematically shown in Fig. 8. The measured results reveal that the Cu content becomes lower both in the liquid and in the solid near the S/L interface when a current of high intensity is applied, such as 5.6 103 A/cm2 ECP, 1.0 103 A/cm2 SSC at the pulling rate of 2.0 mm/s, and 4.0 103 A/cm2 ECP, 0.7 103 A/cm2 SSC at the pulling rate of 1.2 mm/s. This is a strong indication that high intensity current decreases the effective partition coefficient of solute element. According to the literature, an electric current can lead to solute migration in the melt, and the solute atom with high electrical resistance moves toward the anode, a phenomenon termed as electrotransport or electromigration [11,12]. In the samples subjected to current, the effective partition coefficient (ke) should be related to the current direction if electromigration has an effect on the solute redistribution. But our experimental results suggest that the effective partition coefficient of Cu is independent of the current direction. During directional
solidification of an alloy, the effective partition coefficient of the solute decreases with increasing intensity of convection at a constant growth rate. In the samples subject to current, the growth rate is constant; thus, the forced convection should be the reason for the decrease of the effective partition coefficient. Therefore, the force effect caused by current certainly has an effect on the microstructure of the directionally solidified Al– Cu alloy. Convection is inevitable during the directional solidification of an Al–Cu sample in a 4 mm ID crucible due to radial temperature difference [10]. When an electrical current is applied on the sample, the resultant Lorentz force enhances the convection in the liquid. With regard to the effect of convection on the microstructure during the directional solidification process, Lehmann et al. [13] found that a change in terms of convection intensity has a significant effect when a diffusive solute transport regime is approached, while it has a weak effect in a well-established convective regime. The Lorentz force produced by ECP is greater than that by SSC, but
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Without current
ECP
SSC
-0.8×103 A/cm2
-0.2×103 A/cm2
-2.4×103 A/cm2
-0.4×103 A/cm2
-4.0×103 A/cm2
-0.7×103 A/cm2
Fig. 7. Effect of ECP and SSC on the microstructure of directionally solidified Al–4.5 wt% Cu at the pulling rate of 1.2 mm/s.
Table 2 Cu contents near the S/L interface in samples solidified at the pulling rate of 2.0 mm/s.
Table 3 Cu contents near the S/L interface in samples solidified at the pulling rate of 1.2 mm/s.
Current ( 103 A/cm2)
Liquid CL (Cu wt%)
Solid CS (Cu wt%)
k0 ¼ Cs/CL
ke ¼Cs/C0
Current, ( 103 A/cm2)
Liquid CL (Cu wt%)
Solid CS (Cu wt%)
k0 ¼Cs/CL
ke ¼Cs/C0
0 2.4(ECP) þ2.4(ECP) 5.6(ECP) þ5.6(ECP) 1.0(SSC) þ1.0(SSC)
12.2 17.8 21.0 7.6 8.3 5.7 8.8
3.0 4.3 3.1 1.4 1.5 1.2 1.7
0.25 0.24 0.15 0.18 0.18 0.21 0.19
0.67 0.96 0.69 0.31 0.33 0.27 0.38
0 2.4(ECP) 4.0(ECP) þ 4.0(ECP) 0.7(SSC)
21.5 20.9 9.5 5.6 5.9
4.1 4.5 2.3 1.2 1.1
0.19 0.22 0.24 0.21 0.19
0.91 1.00 0.51 0.27 0.24
it cannot give rise to a stronger influence on the microstructure in a well-established convection regime. The solute content near the S/L interface increases when a small intensity of ECP (for
example 2.4 103 A/cm2) is applied, which may contribute to the change of interface morphology from a convex interface to a flat one.
C. Song et al. / Journal of Crystal Growth 324 (2011) 235–242
5.2. Heat effects of current During the directional solidification, there are three kinds of heat effect when a current is applied along the axial line: The Joule heat, the Thomson effect in the bulk (i.e. the solid and the liquid) phase, and the Peltier effect at the S/L interface [8]. Both the Thomson effect and the Peltier effect are related to the current direction as well as the current intensity and actuation time. They become cooling effect (or endothermic effect) when the current flows from the solid to the liquid, and heating effect (or exothermal effect) when the flow direction of the current is reversed. In the experiments, the current flowing from the liquid to the solid has a slightly greater effect on interface morphology than that flowing from the inverse direction. This phenomenon should result from the Peltier and Thomson effects, indicating that heating effect is favorable for interface stability. The Joule heat is always a heating effect, which plays a dominant role in affecting the microstructure of directionally solidified
241
samples using SSC and ECP. During the directional solidification of an Al–4.5 wt% Cu sample the Joule heat increases the temperature of the sample and makes the S/L interface move downwards, as shown in Fig. 9. As a result, the distance between the S/L interface and the coolant becomes shorter, leading to a high temperature gradient both in the solid (GS) and in the liquid (GL) near the S/L interface. High temperature gradients delay the planar/cellular transition and enhance the stability of the planar interface. Due to the lower electrical resistance of the solid phase than that of the liquid phase the current density through the solid phase should be higher than that through the liquid phase in the mushy zone, as schematically illustrated in Fig. 10. As a result
Current
Liquid
Mushy zone
Solid
Fig. 8. Schematic illustration of the positions where the Cu content were measured.
Fig. 10. Schematic of the current distribution in a directionally solidifying sample with the current.
Distance
L
Melting point L
S
S/L interface S
Coolant
Coolant
Temperature
Fig. 9. Schematic illustration of the effect of the Joule heat on the interface position and the temperature gradient in a directionally solidifying sample. (a) Without current and (b) with current.
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C. Song et al. / Journal of Crystal Growth 324 (2011) 235–242
more Joule heat is produced in the solid phase of the mushy zone, leading to a non-uniform temperature distribution in the solid cells and the liquid among solid cells. The high current density in a solid cell will lead to a local temperature increase within the solid cell, making the cellular tip split. Hence uneven current distribution caused by the electrical resistance difference between the solid phase and the liquid phase decreases the primary cellular spacing. For the planar growth with convex interface at the pulling rate of 1.2 mm/s, the uneven current distribution gives rise to the split of the planar interface and changes the planar interface into a cellular one when a small current is applied to the sample as shown from Fig. 7(a) to (b) or (e). The cellular interface then gradually splits until a flat planar interface is obtained as the current intensity is increased as shown from Fig. 7(b) to (c) and (d).
direction is very small. The experimental results are explained using the force effect (Lorentz force) and heating effect caused by these two types of current in the solidifying alloy.
Acknowledgments This work was supported by China National Natural Science Foundation (Nos. 50701030 and 50734008) and China National Basic Research Development Project (973 Program no. 2010CB630802). Instrumental Analysis and Research Center of Shanghai University provided facility for measuring the copper content near the S/L interface. Dr. Ke Han of National High Magnetic Field Laboratory (Tallahassee, USA) helped us modify the manuscript regarding English.
5.3. Periodic characteristic of currents Both ECP and SSC used in this research are not steady current. The resultant Lorentz force and the heat effect changes periodically. The Lorentz force and the heat effect induced by ECP have high vibration amplitudes and rapid change rates because the current changes discretely and sharply. Thus, in the samples subjected to ECP, the convection pattern and the temperature alter periodically near the S/L interface, which would give rise to a very unstable condition for the advancement of the S/L interface. However, change of the SSC occurs more continuously and gently than that of the ECP current. Still the convection pattern and the temperature field alter periodically. Effect of vibration on the morphological stability and microstructure has not been well understood, but it is certain that the vibration can affect solidification microstructure, and enhance the morphological stability during the directional solidification process of alloys [14–16]. The vibration pattern caused by SSC may be more favorable to the stability of the S/L interface than that of ECP. This might be the reason that SSC has a stronger effect on the interfacial stability and on the microstructure of the directionally solidified Al–4.5 wt% Cu alloy than that of ECP under the same heating power.
6. Conclusions The effect of electric current pulse (ECP) and semi-sinusoidal current (SSC) on the interface morphology of Al–4.5 wt% Cu alloy under the directional solidification conditions has been investigated. Experimental results suggest that both the current can affect the microstructure of the directionally solidified alloy by making the convex interface flat, decreasing the primary spacing and the depth of mushy zone, and delaying the planar/cellular transition. Under the same heating effect (i.e. equivalent power) conditions, SSC has a greater influence on the microstructure than ECP. Moreover the current flowing from the liquid to the solid have a slightly stronger effect on microstructure than that flowing along the reverse direction, but the difference caused by current
References [1] C. Song, Q. Li, H. Li, Q. Zhai, Effect of pulse magnetic field on microstructure of austenitic stainless steel during directional solidification, Mater. Sci. Eng. A 485 (2008) 403–408. [2] Q. Li, C. Song, H. Li, Q. Zhai, Effect of pulsed magnetic field on microstructure of 1Cr18Ni9Ti austenitic stainless steel, Mater. Sci. Eng. A 466 (2007) 101–105. [3] X. Liao, Q. Zhai, J. Luo, W. Chen, Y. Gong, Refining mechanism of the electric current pulse on the solidification structure of pure aluminum, Acta Mater. 55 (2007) 3103–3109. [4] M. Gao, G.H. He, F. Yang, J.D. Guo, Z.X. Yuan, B.L. Zhou, Effect of electric current pulse on tensile strength and elongation of casting ZA27 alloy, Mater. Sci. Eng. A 337 (2002) 110–114. [5] L.N. Brush, R.N. Grugel, The effect of an electric current on rod-eutectic solidification in Sn–0.9 wt% Cu alloys, Mater. Sci. Eng. A 238 (1997) 176–181. [6] F. Li, L.L. Regel, W.R. Wilcox, The influence of electric current pulses on the microstructure of the MnBi/Bi eutectic, J. Cryst. Growth 223 (2001) 251–264. [7] X. Liao, Q. Zhai, C. Song, W. Chen, Y. Gong, Effects of electric current pulse on stability of solid/liquid interface of Al–4.5 wt%Cu alloy during directional solidification, Mater. Sci. Eng. A 466 (2007) 56–60. [8] L.N. Brush, A phase field model with electric current, J. Cryst. Growth 247 (2003) 587–596. [9] B. Kauerauf, G. Zimmermann, L. Murmann, S. Rex, Planar to cellular transition in the system succinonitrile–acetone during directional solidification of a bulk sample, J. Cryst. Growth 193 (1998) 701–711. [10] R. Trivedi, H. Miyahara, P. Mazumder, E. Simsek, S.N. Tewari, Directional solidification microstructures in diffusive and convective regimes, J. Cryst. Growth 222 (2001) 365–379. [11] H. Conrad, Influence of an electric or magnetic field on the liquid–solid transformation in materials and on the microstructure of the solid, Mater. Sci. Eng. A 287 (2000) 205–212. [12] G. Guo, Liquid Metals, Science Press, Beijing, 1987 in Chinese. [13] P. Lehmann, R. Moreau, D. Camel, R. Bolcato, A simple analysis of the effect of convection on the structure of the mushy zone in the case of horizontal Bridgman solidification comparison with experimental results, J. Cryst. Growth 183 (1998) 690–704. [14] L.L. Regel, W.R. Wilcox, D. Popov, F. Li, Influence of freezing rate oscillations and convection on eutectic microstructure, Acta Astronaut. 48 (2001) 101–108. [15] C.A. Chung, K.H. Ho, P.S. Chou, Morphological stability during directional solidification into an oscillatory molten zone, J. Cryst. Growth 276 (2005) 289–298. [16] D.V. Lyubimov, T.P. Lyubimova, A.A. Cherepanov, N.I. Lobov, V.V. Vasilyev, B. Roux, Vibration effect on morphological instability of planar solidification front, J. Cryst. Growth 303 (2007) 269–273.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Effect of currents on the microstructure of directionally solidified Al–4.5 wt% Cu alloy Changjiang Song a, Yuanyi Guo a, Yunhu Zhang a, Hongxing Zheng a, Meng Yan a, Qingyou Han b, Qijie Zhai a,n a b
Shanghai Key Laboratory of Modern Metallurgy and Materials Processing, School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China Mechanical Engineering Technology Department, Purdue University, West Lafayette, IN 47907-2021, USA
a r t i c l e i n f o
abstract
Article history: Received 30 September 2010 Received in revised form 16 February 2011 Accepted 14 March 2011 Communicated by M. Rettenmayr Available online 21 March 2011
This paper describes a comparative study on the effect of electric current pulse (ECP) and the semisinusoidal current (SSC) on the microstructure of directionally solidified Al–4.5 wt% Cu alloy. The experimental results indicate that both ECP and SSC can make the convex solid/liquid interface flat, reduce the primary spacing and the depth of mushy zone, and change the interface morphology from cellular to planar (i.e. delay the planar/cellular transition). Under the same heating power, SSC produces a greater influence on the microstructure than ECP. Limited effect of the current direction on microstructure is also observed. Generally, current flowing from the liquid to the solid have a stronger effect on the microstructure than those from the solid to the liquid. Possible mechanisms by which the current altes the microstructure of Al–4.5 wt% Cu alloy under directional solidification conditions are suggested. & 2011 Elsevier B.V. All rights reserved.
Keywords: A1. Directional solidification A1. Interface A1. Morphological stability B1. Alloys
1. Introduction Electromagnetic processing of materials (EPM) has been extensively studied in the past two decades. It has been documented that EPM can be used to improve the properties of the metallic materials by refining grain size, eliminating defect, and improving chemistry homogeneity. As a result, EPM has been considered as an effective method to produce high-performance materials and a potential direction to develop novel material processing techniques [1–4]. Recently, much attention has been paid to the applications of electric current pulse (ECP) to the metal solidification process due to its high instantaneous current intensity. Many studies have demonstrated that ECP can significantly refine the solidification structure and improve the properties of alloys [3–6]. Our previous experiments demonstrated that during the directional solidification, ECP can decrease the primary dendrite spacing and delay planar/cellular and cellular/dendrite transitions in certain metals and alloys [7]. The application of ECP is capable of inducing both heat (including Peltier, Thomson, and Joule effects) and force effects in metals and alloys during their directional solidification process [8]. These two effects are related to the frequency and intensity of the current. This article focuses on addressing the
n
Corresponding author. Tel./fax: þ 86 21 56331218. E-mail addresses: [email protected] (C. Song), [email protected] (Q. Zhai).
0022-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2011.03.025
force effect and the heating effect of current on the microstructure of directionally solidified alloy. The force effect of ECP is significant due to its instantaneously sharp change of current intensity. In contrast, SSC usually generates a weak force effect. Thus, these types of current (ECP and SSC) were used to investigate the mechanisms by which the currents influence the microstructure of metals and alloys. In addition the Peltier and Thomson effects are also related to the direction of an electric current, which may result in a change of the temperature field ahead of the freezing front. Therefore, this paper also discusses the effect of current direction on the microstructure of directional solidified alloys.
2. Experimental Fig. 1 depicts the experimental apparatus. It consists of a resistance furnace, a power supply system, a pulling mechanism, high purity corundum tubes, and a cooling system using liquid Ga–In–Sn as the coolant. The power supply system included two separate units for ECP and SSC. The pulling unit was controlled by a servo-actuator. The alloy used in this study was Al–4.5 wt% Cu alloy, which was prepared using commercial purity aluminum (99.7 wt%) and electrolytic copper (99.99 wt%). The sample was placed in a tube of high purity corundum with inner diameter of 4 mm and length of 420 mm before being slowly heated (and melted) up to 1200 1C
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C. Song et al. / Journal of Crystal Growth 324 (2011) 235–242
Consumable
Electric current pulse
electrode (Al-4.5Cu)
Intensity
Resistance furnace
Power
L
Semi-sinusoidal current
supply
5
10
15
20
Time, ms
Experimental table
Fig. 2. Schematic illustration of ECP and SSC used in the experiments.
Pulling mechanism
Coolant Power supply
Movement direction Fig. 1. Sketch of the experimental apparatus.
in the resistance furnace. The power supply was then switched on and current was applied to the sample for 20 min to establish steady state conditions. Subsequently, the sample was pulled downward for 30 mm at a constant rate and then quenched into the liquid Ga–In–Sn alloy by mowing downwards at a high pulling rate to preserve the morphology of the solid/liquid (S/L) interface. The pulling rates were 1.2 mm/s for planar solidification and 2.0 mm/s for cellular solidification. These two pulling rates were chosen because both were near planar/cellular transition conditions. The quenched samples were then sectioned along the central axis, ground, polished, and etched for microstructure examination. The etching agent was a solution of HNO3:HCl:HF:H2O¼ 2.5:1.5:1.0:95.0. The microstructure was examined using a light microscope, and the solute content near the solid/liquid (S/L) interface was measured using JEOL JSM-6700F scanning electron microscopy (SEM) with energy dispersive spectrum (EDS).
Thermal insulating materials
Al-4.5Cu sample
Thermocouple
Light fire brick
Fig. 3. Schematic diagram of the experimental setup used to determine the corresponding intensity of two types of current under the same heating power.
samples used in Fig. 3 were the same as those used in the directional solidification experiments. The experimental results are shown in Fig. 4. Table 1 lists the corresponding intensities of the current with the same heating power.
4. Experimental results 3. Currents used in the experiments 4.1. Cellular growth under the influence of ECP The previous experiments show that ECP can significantly affect the microstructure of metals directionally solidified [7]. When ECP is applied on a sample under directional solidification, it produces two effects: heat and force effects. It is difficult to determine which factor is more dominant because these two factors always occur simultaneously. To understand the mechanism of current on microstructure, SSC was chosen for comparative study on the alloy during its directional solidification. The relationship of these two types of current is schematically shown in Fig. 2 The force effect is difficult to evaluate either theoretically or experimentally because it induces a forced convection in the liquid during the solidification of the specimen. In this work, the comparative experiments were conducted under the same heating effect (i.e. equivalent power) of both types of current. An experimental setup was designed to determine the corresponding intensity of these two types of current, as shown in Fig. 3. The
Fig. 5 demonstrates the effect of ECP on the microstructure of Al– 4.5 wt% Cu directionally solidified at a pulling rate of 2.0 mm/s: Fig. 5(a) depicts the microstructure of the sample not subjected to ECP. The images at the left side of Fig. 5 show the microstructure of the sample with the electric current flowing from the solid to the liquid. The images at the right side of Fig. 5 are the microstructure of the sample with the electric current flowing from the liquid to the solid. It can be seen clearly that the microstructure of the sample without ECP is typically a cellular one, and the S/L interface obviously protrudes into the melt. The application of ECP changes the convex interface into a flat one. In addition, the cellular spacing and the depth of the mushy zone gradually decrease with increasing intensity of the current. The cellular interface transits to planar interface when the current intensity is 5.6 103 A/cm2, as shown in Fig. 5(e) and (i). It was also noted that the electric current flowing from the liquid to the
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Fig. 4. Heating curves of two types of current.
Table 1 Intensities of two types of current under the same heating power. ECP (A/cm2)
SSC (A/cm2)
0.8 103 2.4 103 4.0 103 5.6 103
0.2 103 0.4 103 0.7 103 1.0 103
solid has marginally greater effect than that flowing from the solid to the liquid. 4.2. Cellular growth under the influence of SSC The above experimental results indicate that ECP significantly affects the microstructure of the directionally solidified Al–4.5 wt% Cu alloy. To identify the dominant influencing factor to the microstructure, comparative SSC experiments were performed under the same experimental conditions. Fig. 6 shows the microstructure of the alloy directionally solidified at a pulling rate of 2.0 mm/s under the influence of SSC. It indicates that SSC is more effective than ECP in promoting the transition from cellular to planar growth when their heating power is the same. For example, the interface became planar when SSC with intensity of 0.7 103 A/cm2 was applied to the sample, but it was still cellular when an equivalent ECP with intensity of 4.0 103 A/cm2 was applied. The effect of the direction of the electric current on the interface morphology by SSC is similar to that of ECP. 4.3. Planar growth under the influence of either ECP or SSC Fig. 7 shows the microstructure of a Al–4.5 wt% Cu sample directionally solidified at a pulling rate of 1.2 mm/s: Fig. 7(a) shows
the microstructure of a sample not subjected to a current. The images at the left side of Fig. 7 show the microstructure of a sample with ECP, and the images at the right side of Fig. 7 depict the microstructure of a sample with SSC (the direction of the current was from the solid to the liquid). The S/L interface of the sample without a current was mostly planar except for the regions near the tube wall, where cellular interface forms due to a solute enrichment locally. The planar interface markedly protruded into the melt. When a current (either ECP or SSC) was applied, the convex interface became flat. It is interesting to note that the planar interface becomes cellular when a small current (for example, 0.8 103 A/cm2 ECP and 0.2 103 A/cm2 SSC) is applied, as shown in Fig. 7(b) and (e). With the increase of the current intensity, the cellular interface became planar interface again. Furthermore, the interface was very flat compared to that of the sample not subjected to a current, shown in Fig. 7(d) and (g). Under the same heating power, SSC also had greater effect on the microstructure of the alloy than ECP in these experiments similar to the experimental results obtained under cellular growth conditions.
5. Discussion Our experimental results demonstrate that both ECP and SSC can significantly affect the microstructure of Al–4.5 wt% Cu alloy directionally solidified, and turn a cellular interface into a planar one, i.e. delay planar/cellular transition. Compared with the current flowing from the solid to the liquid, the current flowing from the liquid to the solid has a marginally greater effect. On the other hand, SSC has a greater effect than ECP when the heating power is the same. Considering that both ECP and SSC have the force effect and the heat effect during the directional solidification of Al–4.5 wt% Cu alloy, we can analyze the individual effect based on our experimental results.
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Without current
-0.8×10 3 A/cm 2
+0.8×10 3 A/cm 2
-2.4×10 3 A/cm 2
+2.4×10 3 A/cm 2
Current direction
3
-4.0×10 A/cm
2
-5.6×10 3 A/cm 2
3
+4.0×10 A/cm
2
Current direction
+5.6×10 3 A/cm 2
Fig. 5. Effect of ECP on the microstructure of directionally solidified Al–4.5 wt% Cu at the pulling rate of 2.0 mm/s.
5.1. Force effect of currents The Lorentz force produced by a current will lead to a forced convection in the liquid ahead of the S/L interface. It is well known that convection stabilizes the S/L morphology and delays the planar/cellular transition during directional solidification of an alloy [9,10]. Due to low frequency and low intensity of the currents, convection caused by SSC is weaker than that caused by ECP. Consequently, ECP should have a stronger effect on microstructure transition than SSC under the same heating power.
However, our experimental results suggest otherwise. It seems unreasonable if we explain the experimental results only using the force effect, i.e. the force effect should not be the single main reason for the change in microstructure of the directionally solidified Al–Cu alloy. To examine whether the Lorentz force caused by currents has any effect on the microstructure of the directionally solidified Al–4.5 wt% Cu alloy, the solute content near the S/L interface was measured by using EDS. Tables 2 and 3 list the Cu content near the S/L interface of some samples solidified at the pulling rate of
C. Song et al. / Journal of Crystal Growth 324 (2011) 235–242
-0.2×103 A/cm2
+0.2×103 A/cm2
-0.4×103 A/cm2
+0.4×103 A/cm2
-0.7×103 A/cm2
+0.7×103 A/cm2
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Current direction
Current direction
-1.0×103 A/cm2
+1.0×103 A/cm2
Fig. 6. Effect of SSC on the microstructure of directionally solidified Al–4.5 wt% Cu at the pulling rate of 2.0 mm/s.
2.0 mm/s and 1.2 mm/s, respectively. The Cu content listed in the tables is the average value of data measured at three positions about 50 mm away from the S/L interface as schematically shown in Fig. 8. The measured results reveal that the Cu content becomes lower both in the liquid and in the solid near the S/L interface when a current of high intensity is applied, such as 5.6 103 A/cm2 ECP, 1.0 103 A/cm2 SSC at the pulling rate of 2.0 mm/s, and 4.0 103 A/cm2 ECP, 0.7 103 A/cm2 SSC at the pulling rate of 1.2 mm/s. This is a strong indication that high intensity current decreases the effective partition coefficient of solute element. According to the literature, an electric current can lead to solute migration in the melt, and the solute atom with high electrical resistance moves toward the anode, a phenomenon termed as electrotransport or electromigration [11,12]. In the samples subjected to current, the effective partition coefficient (ke) should be related to the current direction if electromigration has an effect on the solute redistribution. But our experimental results suggest that the effective partition coefficient of Cu is independent of the current direction. During directional
solidification of an alloy, the effective partition coefficient of the solute decreases with increasing intensity of convection at a constant growth rate. In the samples subject to current, the growth rate is constant; thus, the forced convection should be the reason for the decrease of the effective partition coefficient. Therefore, the force effect caused by current certainly has an effect on the microstructure of the directionally solidified Al– Cu alloy. Convection is inevitable during the directional solidification of an Al–Cu sample in a 4 mm ID crucible due to radial temperature difference [10]. When an electrical current is applied on the sample, the resultant Lorentz force enhances the convection in the liquid. With regard to the effect of convection on the microstructure during the directional solidification process, Lehmann et al. [13] found that a change in terms of convection intensity has a significant effect when a diffusive solute transport regime is approached, while it has a weak effect in a well-established convective regime. The Lorentz force produced by ECP is greater than that by SSC, but
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Without current
ECP
SSC
-0.8×103 A/cm2
-0.2×103 A/cm2
-2.4×103 A/cm2
-0.4×103 A/cm2
-4.0×103 A/cm2
-0.7×103 A/cm2
Fig. 7. Effect of ECP and SSC on the microstructure of directionally solidified Al–4.5 wt% Cu at the pulling rate of 1.2 mm/s.
Table 2 Cu contents near the S/L interface in samples solidified at the pulling rate of 2.0 mm/s.
Table 3 Cu contents near the S/L interface in samples solidified at the pulling rate of 1.2 mm/s.
Current ( 103 A/cm2)
Liquid CL (Cu wt%)
Solid CS (Cu wt%)
k0 ¼ Cs/CL
ke ¼Cs/C0
Current, ( 103 A/cm2)
Liquid CL (Cu wt%)
Solid CS (Cu wt%)
k0 ¼Cs/CL
ke ¼Cs/C0
0 2.4(ECP) þ2.4(ECP) 5.6(ECP) þ5.6(ECP) 1.0(SSC) þ1.0(SSC)
12.2 17.8 21.0 7.6 8.3 5.7 8.8
3.0 4.3 3.1 1.4 1.5 1.2 1.7
0.25 0.24 0.15 0.18 0.18 0.21 0.19
0.67 0.96 0.69 0.31 0.33 0.27 0.38
0 2.4(ECP) 4.0(ECP) þ 4.0(ECP) 0.7(SSC)
21.5 20.9 9.5 5.6 5.9
4.1 4.5 2.3 1.2 1.1
0.19 0.22 0.24 0.21 0.19
0.91 1.00 0.51 0.27 0.24
it cannot give rise to a stronger influence on the microstructure in a well-established convection regime. The solute content near the S/L interface increases when a small intensity of ECP (for
example 2.4 103 A/cm2) is applied, which may contribute to the change of interface morphology from a convex interface to a flat one.
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5.2. Heat effects of current During the directional solidification, there are three kinds of heat effect when a current is applied along the axial line: The Joule heat, the Thomson effect in the bulk (i.e. the solid and the liquid) phase, and the Peltier effect at the S/L interface [8]. Both the Thomson effect and the Peltier effect are related to the current direction as well as the current intensity and actuation time. They become cooling effect (or endothermic effect) when the current flows from the solid to the liquid, and heating effect (or exothermal effect) when the flow direction of the current is reversed. In the experiments, the current flowing from the liquid to the solid has a slightly greater effect on interface morphology than that flowing from the inverse direction. This phenomenon should result from the Peltier and Thomson effects, indicating that heating effect is favorable for interface stability. The Joule heat is always a heating effect, which plays a dominant role in affecting the microstructure of directionally solidified
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samples using SSC and ECP. During the directional solidification of an Al–4.5 wt% Cu sample the Joule heat increases the temperature of the sample and makes the S/L interface move downwards, as shown in Fig. 9. As a result, the distance between the S/L interface and the coolant becomes shorter, leading to a high temperature gradient both in the solid (GS) and in the liquid (GL) near the S/L interface. High temperature gradients delay the planar/cellular transition and enhance the stability of the planar interface. Due to the lower electrical resistance of the solid phase than that of the liquid phase the current density through the solid phase should be higher than that through the liquid phase in the mushy zone, as schematically illustrated in Fig. 10. As a result
Current
Liquid
Mushy zone
Solid
Fig. 8. Schematic illustration of the positions where the Cu content were measured.
Fig. 10. Schematic of the current distribution in a directionally solidifying sample with the current.
Distance
L
Melting point L
S
S/L interface S
Coolant
Coolant
Temperature
Fig. 9. Schematic illustration of the effect of the Joule heat on the interface position and the temperature gradient in a directionally solidifying sample. (a) Without current and (b) with current.
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more Joule heat is produced in the solid phase of the mushy zone, leading to a non-uniform temperature distribution in the solid cells and the liquid among solid cells. The high current density in a solid cell will lead to a local temperature increase within the solid cell, making the cellular tip split. Hence uneven current distribution caused by the electrical resistance difference between the solid phase and the liquid phase decreases the primary cellular spacing. For the planar growth with convex interface at the pulling rate of 1.2 mm/s, the uneven current distribution gives rise to the split of the planar interface and changes the planar interface into a cellular one when a small current is applied to the sample as shown from Fig. 7(a) to (b) or (e). The cellular interface then gradually splits until a flat planar interface is obtained as the current intensity is increased as shown from Fig. 7(b) to (c) and (d).
direction is very small. The experimental results are explained using the force effect (Lorentz force) and heating effect caused by these two types of current in the solidifying alloy.
Acknowledgments This work was supported by China National Natural Science Foundation (Nos. 50701030 and 50734008) and China National Basic Research Development Project (973 Program no. 2010CB630802). Instrumental Analysis and Research Center of Shanghai University provided facility for measuring the copper content near the S/L interface. Dr. Ke Han of National High Magnetic Field Laboratory (Tallahassee, USA) helped us modify the manuscript regarding English.
5.3. Periodic characteristic of currents Both ECP and SSC used in this research are not steady current. The resultant Lorentz force and the heat effect changes periodically. The Lorentz force and the heat effect induced by ECP have high vibration amplitudes and rapid change rates because the current changes discretely and sharply. Thus, in the samples subjected to ECP, the convection pattern and the temperature alter periodically near the S/L interface, which would give rise to a very unstable condition for the advancement of the S/L interface. However, change of the SSC occurs more continuously and gently than that of the ECP current. Still the convection pattern and the temperature field alter periodically. Effect of vibration on the morphological stability and microstructure has not been well understood, but it is certain that the vibration can affect solidification microstructure, and enhance the morphological stability during the directional solidification process of alloys [14–16]. The vibration pattern caused by SSC may be more favorable to the stability of the S/L interface than that of ECP. This might be the reason that SSC has a stronger effect on the interfacial stability and on the microstructure of the directionally solidified Al–4.5 wt% Cu alloy than that of ECP under the same heating power.
6. Conclusions The effect of electric current pulse (ECP) and semi-sinusoidal current (SSC) on the interface morphology of Al–4.5 wt% Cu alloy under the directional solidification conditions has been investigated. Experimental results suggest that both the current can affect the microstructure of the directionally solidified alloy by making the convex interface flat, decreasing the primary spacing and the depth of mushy zone, and delaying the planar/cellular transition. Under the same heating effect (i.e. equivalent power) conditions, SSC has a greater influence on the microstructure than ECP. Moreover the current flowing from the liquid to the solid have a slightly stronger effect on microstructure than that flowing along the reverse direction, but the difference caused by current
References [1] C. Song, Q. Li, H. Li, Q. Zhai, Effect of pulse magnetic field on microstructure of austenitic stainless steel during directional solidification, Mater. Sci. Eng. A 485 (2008) 403–408. [2] Q. Li, C. Song, H. Li, Q. Zhai, Effect of pulsed magnetic field on microstructure of 1Cr18Ni9Ti austenitic stainless steel, Mater. Sci. Eng. A 466 (2007) 101–105. [3] X. Liao, Q. Zhai, J. Luo, W. Chen, Y. Gong, Refining mechanism of the electric current pulse on the solidification structure of pure aluminum, Acta Mater. 55 (2007) 3103–3109. [4] M. Gao, G.H. He, F. Yang, J.D. Guo, Z.X. Yuan, B.L. Zhou, Effect of electric current pulse on tensile strength and elongation of casting ZA27 alloy, Mater. Sci. Eng. A 337 (2002) 110–114. [5] L.N. Brush, R.N. Grugel, The effect of an electric current on rod-eutectic solidification in Sn–0.9 wt% Cu alloys, Mater. Sci. Eng. A 238 (1997) 176–181. [6] F. Li, L.L. Regel, W.R. Wilcox, The influence of electric current pulses on the microstructure of the MnBi/Bi eutectic, J. Cryst. Growth 223 (2001) 251–264. [7] X. Liao, Q. Zhai, C. Song, W. Chen, Y. Gong, Effects of electric current pulse on stability of solid/liquid interface of Al–4.5 wt%Cu alloy during directional solidification, Mater. Sci. Eng. A 466 (2007) 56–60. [8] L.N. Brush, A phase field model with electric current, J. Cryst. Growth 247 (2003) 587–596. [9] B. Kauerauf, G. Zimmermann, L. Murmann, S. Rex, Planar to cellular transition in the system succinonitrile–acetone during directional solidification of a bulk sample, J. Cryst. Growth 193 (1998) 701–711. [10] R. Trivedi, H. Miyahara, P. Mazumder, E. Simsek, S.N. Tewari, Directional solidification microstructures in diffusive and convective regimes, J. Cryst. Growth 222 (2001) 365–379. [11] H. Conrad, Influence of an electric or magnetic field on the liquid–solid transformation in materials and on the microstructure of the solid, Mater. Sci. Eng. A 287 (2000) 205–212. [12] G. Guo, Liquid Metals, Science Press, Beijing, 1987 in Chinese. [13] P. Lehmann, R. Moreau, D. Camel, R. Bolcato, A simple analysis of the effect of convection on the structure of the mushy zone in the case of horizontal Bridgman solidification comparison with experimental results, J. Cryst. Growth 183 (1998) 690–704. [14] L.L. Regel, W.R. Wilcox, D. Popov, F. Li, Influence of freezing rate oscillations and convection on eutectic microstructure, Acta Astronaut. 48 (2001) 101–108. [15] C.A. Chung, K.H. Ho, P.S. Chou, Morphological stability during directional solidification into an oscillatory molten zone, J. Cryst. Growth 276 (2005) 289–298. [16] D.V. Lyubimov, T.P. Lyubimova, A.A. Cherepanov, N.I. Lobov, V.V. Vasilyev, B. Roux, Vibration effect on morphological instability of planar solidification front, J. Cryst. Growth 303 (2007) 269–273.