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Visualization of freckle formation induced by forced melt convection in solidifying GaIn alloys
Materials Letters 64 (2010) 1340–1343
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Visualization of freckle formation induced by forced melt convection in solidifying GaIn alloys S. Boden, S. Eckert ⁎, G. Gerbeth Forschungszentrum Dresden-Rossendorf, MHD Department, P.O. Box 510119, 01314 Dresden, Germany
a r t i c l e
i n f o
Article history: Received 25 January 2010 Accepted 18 March 2010 Available online 27 March 2010 Keywords: Directional solidification Dendritic growth Convection Segregation X-ray radioscopy
a b s t r a c t A bottom-up solidification of a Ga–25 wt.%In alloy under the influence of buoyancy-driven and electromagnetically driven convection was investigated by X-ray radioscopy. The main effect of the flow on the solidification is determined by the flow-induced redistribution of solute concentration which results in a change of the growth direction of the dendrites and the preference of secondary branches for an accelerated or decelerated growth. The experiments demonstrate clearly how the interdendritic flow contributes to the formation of spacious segregation freckles. © 2010 Elsevier B.V. All rights reserved.
1. Introduction It is well-known that the flow in a solidifying melt can significantly affect the evolution of the microstructure in metal alloys and hence the mechanical properties of the solid material. For instance, unstable density stratifications in the presence of thermal and/or concentration gradients at the solidifying interface generate thermosolutal convection in the mushy zone which is responsible for the formation of freckles and segregation channels [1–4]. The control of melt convection is therefore inevitable to achieve high-quality castings. External magnetic fields offer an efficient way for a contactless influence on the melt flow in solidifying melts. Electromagnetic stirring is proposed as a tool to achieve grain refinement and to counteract the natural convection, respectively [5–8]. A successful concept of flow control requires an improved knowledge with respect to the details of the flow structure, the heat and mass transfer properties of the flow during solidification. Previous experimental investigations of melt flow effects in metal alloys are mainly restricted to a post-process analysis of the solidified microstructure [3–8] or analogous examinations of transparent organic alloys [9–11]. Though the latter delivered valuable information for verification of theories on cellular and dendritic morphology, these experiments do not really represent the situation in a solidifying metal alloy in all details. For instance, fully different thermal boundary layers have to be expected in case of melt flow at the solidification front due to the significant difference in the dimensionless Prandtl number. Furthermore, electromagnetically driven flows can only be simulated in electrically conducting liquids. The present study is concerned with the influence of melt flow on the solidification process of a Ga–25 wt.%In alloy, if the buoyancy-driven ⁎ Corresponding author. E-mail address: [email protected] (S. Eckert). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.03.044
convection is interfered by a much stronger electromagnetically driven flow. The growth of In dendrites causes an accumulation of rejected Ga in the mushy zone. In the case of the bottom-up solidification considered here the solute-rich melt is less dense as compared with the initial composition. This unstable density stratification provokes an intense solutal convection in the form of rising plumes ahead of the solidification front [12]. An undesirable phenomenon associated with natural convection in the mushy zone is the formation of segregation channels, the so-called “chimneys” [13]. The problem of nonlinear buoyancy-driven convection inside the mushy zone was investigated by Amberg and Homsy [14]. They suggested to consider the chimney formation as a subcritical bifurcation which may be triggered by finiteamplitude disturbances from the plumes above the mushy zone. Recent X-ray visualization experiments of the solutal convection in Ga–30 wt.% In [12] confirmed that the rise of solute-rich plumes above the dendrite tips causes a suction effect in the subjacent mushy zone resulting in a detectable interdendritic flow. The flow pattern in the mushy zone with fully developed chimneys shows an upward plume motion inside the chimneys into the melt region above the solidification front which is fed by a downward flow in the neighbouring interdendritic region and a horizontal motion towards the chimney [10]. Electromagnetic stirring is potent to suppress the typical thermosolutal flow pattern and to prevent therewith the formation of respective segregation channels in the mushy zone, however, the forced melt stirring itself may also cause the occurrence of pronounced segregation pattern in the solidified structure [15,16]. This problem could probably be solved by the application of time-modulated magnetic fields, which can create a flow with periodic alternating direction within the mushy zone [17]. Provided that the magnetic field parameters are carefully adjusted, such a stirring method has recently been shown to deliver grain-refined microstructures without remarkable segregation [18].
S. Boden et al. / Materials Letters 64 (2010) 1340–1343
2. Experimental X-ray techniques turned out to be an important diagnostic tool for solidification studies in metallic alloys. Real-time and in-situ observations of the density distribution within the sample reach a spatial resolution of a few microns and contribute essentially to an improved understanding of dendritic growth processes [12,19,20]. The present paper employs the X-ray radioscopy for investigating the directional solidification of a Ga–25 wt.%In alloy (TLiquidus = 25 °C, TSolidus = 15.3 °C) in the presence of solutal and forced convection. The experimental setup is schematically depicted in Fig. 1. The alloy was prepared from 99.999% Ga and 99.999% In and processed in a Hele-Shaw cell made from quartz glass with a surface area of about 35 × 30 mm2 and a parallel gap of approx. 200 µm. The solidification cell is equipped with two pouring nozzles. A suction pressure was generated to fill the liquid alloy into the cavity between the glass plates. An infrared radiant heater was utilised to heat and melt the alloy before solidification. The cooling of the metal alloy was realised by a linear arrangement of Peltier elements, which were positioned at the bottom of the Hele-Shaw cell. The forced convection was produced by a rotating wheel with two parallel disks (∅52 mm) containing at their inner sides an assembly of permanent magnets (NdFeB) with alternating polarisation. The upper end of the solidification cell was positioned in the air gap (width 7 mm) between the disks as shown in Fig. 1(a). A remarkable magnetic field is imposed to the sample only in the domain between the disks. The magnetic flux is rather low in the region of the solidification front, so that we can neglect a direct influence of magnetic forces on the dendritic growth. Fig. 1(b) displays the X-ray radioscopy setup. A microfocus X-ray tube (XS225D, Phoenix|X-ray) generates a horizontally aligned divergent beam which passes through the broad face of the solidification cell and impinges an X-ray image intensifier (TH9438HX 9″, Thales), where the X-rays are converted into a two-dimensional visible light distribution which is recorded by a CCD camera (CF8/1, Kappa) with a scan rate
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of 50 half frames per second. Single images were integrated to reduce the noise level during acquisition. An integration time of 6000 ms was found to be optimal to ensure a sufficient temporal sampling rate while keeping the amount of stored data manageable. Parallel to the image acquisition the CCD camera also delivered a live frame providing an online control of the process. The solidification experiments were carried out as follows. At first, the binary Ga–25 wt.%In alloy was melted and heated above the liquidus temperature. The melting process was controlled by real-time radioscopic observation to ensure a completely melted and homogenously distributed material before starting the solidification experiment. The remelting and homogenisation were supported by additional melt stirring until transient changes of local intensities in the live frame have disappeared. A reference image is computed by averaging the images captured during this quasisteady state. Thereafter, the cooling of the melt and the image acquisition was started. In this initial state the magnetic wheel was at rest. After a certain time if the solidification front has passed the opaque Peltier cooler and reached the observation window, the magnetic wheel was set into rotation at a speed of 50 rotations per minute. The time t=0 s was assigned to the onset of melt stirring. 3. Results and discussion Fig. 2 shows a sequence of pictures obtained from a solidification experiment with forced convection, whereas Fig. 2(a) displays the initial situation just before the onset of wheel rotation. Plume-like flow pattern associated with buoyancy-driven convection can be observed in the melt above the mushy zone. The flow field in the melt can be determined using the optical flow approach [12]. The velocity inside the ascending plumes was found in the range between 20 and 50 μm/s. A counter-clockwise rotation of the magnetic stirrer has been initiated at t=0. The rotating arrangement of permanent magnets acts on the melt as a travelling magnetic field, which produces a single-vortex melt flow in clockwise
Fig. 1. Schematic drawing of the experimental setup: rotating magnetic wheel at the Hele-Shaw solidification cell (a), and X-ray diagnostic system (b).
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S. Boden et al. / Materials Letters 64 (2010) 1340–1343
Fig. 2. Solidification without stirring at t = −25 s (a) and development of the dendritic network under the influence of a magnetically induced convection at t = 25 s (b), t = 100 s (c) and t = 300 s (d).
direction as shown in Fig. 2(b). The magnetic stirrer induces a dominating, almost horizontal fluid motion ahead the solidification front leading immediately to a complete disappearance of the plume-like flow pattern. Estimations of local velocities using the optical flow approach showed values up to approximately 1 mm/s in the vicinity of the solidification front. It becomes obvious that the solute boundary layers are significantly modified by the forced flow, whereas local effects are strongly determined by the roughness of the growing front. A depletion of solute can be noticed in the upstream region of growing dendrites, whereas solute-rich melt is accumulated in the downstream zone. The growth rate of the dendrites is governed by the solute concentration at the dendrite tips [12], hence, the asymmetry of the concentration field provokes an asymmetric development of the dendrites as shown in Fig. 2(c). A reinforced growth of the secondary arms towards the flow occurs at the upstream side of the primary trunks whereas the high solute concentration at the downstream side leads to a stagnation of the growth or inhibits the formation of secondary branches completely. The images reveal a deep infiltration of the electromagnetically driven convection into the mushy zone. The interdendritic flow transports solute towards the left side of the container, where the formation of freckles becomes apparent (see Fig. 2(d)). Zhang and Maxworthy studied the dynamics of a solidifying interface in the transparent material SCN–1 wt.% acetone with and without an externally imposed flow perpendicular to the growth direction of the solidification front [11]. The authors observed a tilting of growing cells or dendrites towards the incoming flow direction. The uneven thickness of the concentration boundary layers at both sides of the dendritic tips (also shown in Fig. 2) is responsible for this behaviour. With respect to the growth of the secondary arms the study of Zhang and Maxworthy shows seemingly the opposite behaviour as seen in our experiment. The secondary branch appeared first on the downstream side and later on the upstream side. The authors explain this behaviour by a mixing effect which is expected to be much stronger at the upstream side than at the downstream side. Such an assumption cannot be confirmed by our study. Photographs in [11] show a rather tight gap between the growing dendrites. The formation of the secondary side arms might be interfered by the solute pile-up caused by the neighbouring crystal. The flow-
induced tilting leads to an orientation of the upstream branch towards the mushy zone whereas the downstream branches are more oriented towards the melt region. In comparison to our experiment such a specific situation may cause a reverse solute distribution around the dendrite which would explain the observed behaviour of the secondary branches. The local In-concentration was derived from the relative brightness in the image frames [12]. Fig. 3 reflects the results for situations just before the onset of the forced melt convection and at an advanced state of the solidification process, respectively. The red-coloured regions disclose a high solute concentration whereas the In-rich dendrites appear in blue colour. The electromagnetically driven flow enters the mushy zone on the right hand side of the solidification cell and reduces drastically the solute concentration in this area. This draining effect results in a reinforced growth of the dendritic structures which leads in turn to an augmented release of solute and a cumulative solute enrichment of the interdendritic flow. An outpouring of the flow from the mushy zone into the melt region can be observed at the left hand side of the sample. However, the rejected solute in large part is not discharged from the mushy zone. The solute deposition causes a partly remelting of the dendritic structures and the development of segregation channels. Medina et al. [15] described such a mushy zone “washing phenomenon” with respect to the solidification of a Pb-10 wt.%Sn alloy in the presence of electromagnetic stirring. For the superposition of thermosolutal convection and a single-vortex flow their simulations showed the formation of a main segregation channel on the downstream side of the solidification cell accompanied by a system of subchannels. Our experiments (not all shown here) disclose the appearance of several segregation channels. Number, shape and location of these channels are hard to predict. Apart from such specific discrepancies our experiments generally confirm the suggestions by Medina et al. [15]. Fig. 4 displays linear plots of the relative extinction E = −log(I / I0) along both horizontal lines shown in Fig. 3, where I denotes the X-ray intensity measured at distinct time and position during solidification and I0 stands for the X-ray intensity related to the initial composition C0. The extinction curves have been obtained from the image sequences
S. Boden et al. / Materials Letters 64 (2010) 1340–1343
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Fig. 3. Two-dimensional solute distribution just before the initiation of the stirring (a) and in an advanced state of solidification 325 s after the onset of the forced convection (b).
Fig. 4. Linear profiles of the relative extinction E as a measure of the solute concentration along the two horizontal lines A and B as shown in Fig. 3. The time step between the successive curves is 25 s (negative values relate to In depletion).
applying a Gaussian low pass filter to reduce the noise level. Line A represents the region adjacent to the dendrite tips just before the onset of melt agitation. Fig. 4(a) reveals an uneven dendrite growth, characterized by a distinct increase of E on the inflow side of the mushy zone whereas solute-rich zones are formed in the outflow area. Remarkable modifications of the local composition can also be noticed for deep regions of the mushy zone (line B). Fig. 4(b) shows clearly a consolidation of the dendritic phase at the inflow side accompanied by an increase of eutectic phase in the left part of the solidification cell. 4. Conclusions The X-ray visualization enables a general intuitional understanding of the flow impact on the bottom-up solidification of a Ga–25 wt.%In. An important advantage of this diagnostic approach is the possibility for the in-situ observation of the flow and the concentration field. The main effect of the flow is determined by the flow-induced redistribution of solute concentration which results in a change of the growth direction of the dendrites and the preference of secondary branches for an accelerated or decelerated growth. The present study demonstrates how a specific flow pattern generated by electromagnetic stirring provokes a unidirectional solute transport in the mushy zone which in turn causes the formation of spacious segregation pattern. The melt flow control during solidification of metal alloys by means of electromagnetic fields is still at the beginning. Further investigations, including the X-ray visualization, will be necessary for a better understanding of the complex interplay between melt flow and solidification process.
Acknowledgements This work was financially supported by Deutsche Forschungsgemeinschaft (DFG) in the framework of the collaborative research centre SFB 609 “Electromagnetic Flow Control in Metallurgy, Crystal Growth and Electrochemistry”.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
Sample AK, Hellawell A. Metall Trans 1984;15A:2163. Sarazin JR, Hellawell A. Metall Trans 1988;19A:1861. Tewari SN, Shah R. Metall Trans 1992;23A:3383. Bergman MI, Fearn DR, Bloxham J, Shannon MC. Metall Mater Trans 1997;28A:859. Vives C. Int J Heat Mass Transf 1990;33:2585. Zhang B, Cui J, Lu G. Mater Lett 2003;57:1707. Willers B, Eckert S, Michel U, Haase I, Zouhar G. Mater Sci Eng A 2005;402:55. Yan Z, Li X, Cao Z, Zhang X, Li T. Mater Lett 2008;62:4389. Murray BT, Wheeler AA, Glicksman ME. J Cryst Growth 1995;154:386. Chen CF. J Fluid Mech 1995;293:81. Zhang M, Maxworthy T. J Fluid Mech 2002;470:247. Boden S, Eckert S, Willers B, Gerbeth G. Metall Mater Trans 2008;39A:613. Worster MG. Annu Rev Fluid Mech 1997;29:91. Amberg G, Homsy GH. J Fluid Mech 1993;252:79. Medina M, Terrail YD, Durand F, Fautrelle Y. Metall Mater Trans 2004;35B:743. Nikrityuk PA, Eckert K, Grundmann R. Int J Heat Mass Transf 2006;49:1501. Eckert S, Nikrityuk PA, Räbiger D, Eckert K, Gerbeth G. Metall Mater Trans 2007;38B:977. Willers B, Eckert S, Nikrityuk PA, Räbiger D, Dong J, Eckert K, et al. Metall Mater Trans 2008;39B:304. [19] Mathiesen RH, Arnberg L. Acta Mater 2005;53:947. [20] Yin H, Koster JN. J Cryst Growth 1999;205:590.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Visualization of freckle formation induced by forced melt convection in solidifying GaIn alloys S. Boden, S. Eckert ⁎, G. Gerbeth Forschungszentrum Dresden-Rossendorf, MHD Department, P.O. Box 510119, 01314 Dresden, Germany
a r t i c l e
i n f o
Article history: Received 25 January 2010 Accepted 18 March 2010 Available online 27 March 2010 Keywords: Directional solidification Dendritic growth Convection Segregation X-ray radioscopy
a b s t r a c t A bottom-up solidification of a Ga–25 wt.%In alloy under the influence of buoyancy-driven and electromagnetically driven convection was investigated by X-ray radioscopy. The main effect of the flow on the solidification is determined by the flow-induced redistribution of solute concentration which results in a change of the growth direction of the dendrites and the preference of secondary branches for an accelerated or decelerated growth. The experiments demonstrate clearly how the interdendritic flow contributes to the formation of spacious segregation freckles. © 2010 Elsevier B.V. All rights reserved.
1. Introduction It is well-known that the flow in a solidifying melt can significantly affect the evolution of the microstructure in metal alloys and hence the mechanical properties of the solid material. For instance, unstable density stratifications in the presence of thermal and/or concentration gradients at the solidifying interface generate thermosolutal convection in the mushy zone which is responsible for the formation of freckles and segregation channels [1–4]. The control of melt convection is therefore inevitable to achieve high-quality castings. External magnetic fields offer an efficient way for a contactless influence on the melt flow in solidifying melts. Electromagnetic stirring is proposed as a tool to achieve grain refinement and to counteract the natural convection, respectively [5–8]. A successful concept of flow control requires an improved knowledge with respect to the details of the flow structure, the heat and mass transfer properties of the flow during solidification. Previous experimental investigations of melt flow effects in metal alloys are mainly restricted to a post-process analysis of the solidified microstructure [3–8] or analogous examinations of transparent organic alloys [9–11]. Though the latter delivered valuable information for verification of theories on cellular and dendritic morphology, these experiments do not really represent the situation in a solidifying metal alloy in all details. For instance, fully different thermal boundary layers have to be expected in case of melt flow at the solidification front due to the significant difference in the dimensionless Prandtl number. Furthermore, electromagnetically driven flows can only be simulated in electrically conducting liquids. The present study is concerned with the influence of melt flow on the solidification process of a Ga–25 wt.%In alloy, if the buoyancy-driven ⁎ Corresponding author. E-mail address: [email protected] (S. Eckert). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.03.044
convection is interfered by a much stronger electromagnetically driven flow. The growth of In dendrites causes an accumulation of rejected Ga in the mushy zone. In the case of the bottom-up solidification considered here the solute-rich melt is less dense as compared with the initial composition. This unstable density stratification provokes an intense solutal convection in the form of rising plumes ahead of the solidification front [12]. An undesirable phenomenon associated with natural convection in the mushy zone is the formation of segregation channels, the so-called “chimneys” [13]. The problem of nonlinear buoyancy-driven convection inside the mushy zone was investigated by Amberg and Homsy [14]. They suggested to consider the chimney formation as a subcritical bifurcation which may be triggered by finiteamplitude disturbances from the plumes above the mushy zone. Recent X-ray visualization experiments of the solutal convection in Ga–30 wt.% In [12] confirmed that the rise of solute-rich plumes above the dendrite tips causes a suction effect in the subjacent mushy zone resulting in a detectable interdendritic flow. The flow pattern in the mushy zone with fully developed chimneys shows an upward plume motion inside the chimneys into the melt region above the solidification front which is fed by a downward flow in the neighbouring interdendritic region and a horizontal motion towards the chimney [10]. Electromagnetic stirring is potent to suppress the typical thermosolutal flow pattern and to prevent therewith the formation of respective segregation channels in the mushy zone, however, the forced melt stirring itself may also cause the occurrence of pronounced segregation pattern in the solidified structure [15,16]. This problem could probably be solved by the application of time-modulated magnetic fields, which can create a flow with periodic alternating direction within the mushy zone [17]. Provided that the magnetic field parameters are carefully adjusted, such a stirring method has recently been shown to deliver grain-refined microstructures without remarkable segregation [18].
S. Boden et al. / Materials Letters 64 (2010) 1340–1343
2. Experimental X-ray techniques turned out to be an important diagnostic tool for solidification studies in metallic alloys. Real-time and in-situ observations of the density distribution within the sample reach a spatial resolution of a few microns and contribute essentially to an improved understanding of dendritic growth processes [12,19,20]. The present paper employs the X-ray radioscopy for investigating the directional solidification of a Ga–25 wt.%In alloy (TLiquidus = 25 °C, TSolidus = 15.3 °C) in the presence of solutal and forced convection. The experimental setup is schematically depicted in Fig. 1. The alloy was prepared from 99.999% Ga and 99.999% In and processed in a Hele-Shaw cell made from quartz glass with a surface area of about 35 × 30 mm2 and a parallel gap of approx. 200 µm. The solidification cell is equipped with two pouring nozzles. A suction pressure was generated to fill the liquid alloy into the cavity between the glass plates. An infrared radiant heater was utilised to heat and melt the alloy before solidification. The cooling of the metal alloy was realised by a linear arrangement of Peltier elements, which were positioned at the bottom of the Hele-Shaw cell. The forced convection was produced by a rotating wheel with two parallel disks (∅52 mm) containing at their inner sides an assembly of permanent magnets (NdFeB) with alternating polarisation. The upper end of the solidification cell was positioned in the air gap (width 7 mm) between the disks as shown in Fig. 1(a). A remarkable magnetic field is imposed to the sample only in the domain between the disks. The magnetic flux is rather low in the region of the solidification front, so that we can neglect a direct influence of magnetic forces on the dendritic growth. Fig. 1(b) displays the X-ray radioscopy setup. A microfocus X-ray tube (XS225D, Phoenix|X-ray) generates a horizontally aligned divergent beam which passes through the broad face of the solidification cell and impinges an X-ray image intensifier (TH9438HX 9″, Thales), where the X-rays are converted into a two-dimensional visible light distribution which is recorded by a CCD camera (CF8/1, Kappa) with a scan rate
1341
of 50 half frames per second. Single images were integrated to reduce the noise level during acquisition. An integration time of 6000 ms was found to be optimal to ensure a sufficient temporal sampling rate while keeping the amount of stored data manageable. Parallel to the image acquisition the CCD camera also delivered a live frame providing an online control of the process. The solidification experiments were carried out as follows. At first, the binary Ga–25 wt.%In alloy was melted and heated above the liquidus temperature. The melting process was controlled by real-time radioscopic observation to ensure a completely melted and homogenously distributed material before starting the solidification experiment. The remelting and homogenisation were supported by additional melt stirring until transient changes of local intensities in the live frame have disappeared. A reference image is computed by averaging the images captured during this quasisteady state. Thereafter, the cooling of the melt and the image acquisition was started. In this initial state the magnetic wheel was at rest. After a certain time if the solidification front has passed the opaque Peltier cooler and reached the observation window, the magnetic wheel was set into rotation at a speed of 50 rotations per minute. The time t=0 s was assigned to the onset of melt stirring. 3. Results and discussion Fig. 2 shows a sequence of pictures obtained from a solidification experiment with forced convection, whereas Fig. 2(a) displays the initial situation just before the onset of wheel rotation. Plume-like flow pattern associated with buoyancy-driven convection can be observed in the melt above the mushy zone. The flow field in the melt can be determined using the optical flow approach [12]. The velocity inside the ascending plumes was found in the range between 20 and 50 μm/s. A counter-clockwise rotation of the magnetic stirrer has been initiated at t=0. The rotating arrangement of permanent magnets acts on the melt as a travelling magnetic field, which produces a single-vortex melt flow in clockwise
Fig. 1. Schematic drawing of the experimental setup: rotating magnetic wheel at the Hele-Shaw solidification cell (a), and X-ray diagnostic system (b).
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S. Boden et al. / Materials Letters 64 (2010) 1340–1343
Fig. 2. Solidification without stirring at t = −25 s (a) and development of the dendritic network under the influence of a magnetically induced convection at t = 25 s (b), t = 100 s (c) and t = 300 s (d).
direction as shown in Fig. 2(b). The magnetic stirrer induces a dominating, almost horizontal fluid motion ahead the solidification front leading immediately to a complete disappearance of the plume-like flow pattern. Estimations of local velocities using the optical flow approach showed values up to approximately 1 mm/s in the vicinity of the solidification front. It becomes obvious that the solute boundary layers are significantly modified by the forced flow, whereas local effects are strongly determined by the roughness of the growing front. A depletion of solute can be noticed in the upstream region of growing dendrites, whereas solute-rich melt is accumulated in the downstream zone. The growth rate of the dendrites is governed by the solute concentration at the dendrite tips [12], hence, the asymmetry of the concentration field provokes an asymmetric development of the dendrites as shown in Fig. 2(c). A reinforced growth of the secondary arms towards the flow occurs at the upstream side of the primary trunks whereas the high solute concentration at the downstream side leads to a stagnation of the growth or inhibits the formation of secondary branches completely. The images reveal a deep infiltration of the electromagnetically driven convection into the mushy zone. The interdendritic flow transports solute towards the left side of the container, where the formation of freckles becomes apparent (see Fig. 2(d)). Zhang and Maxworthy studied the dynamics of a solidifying interface in the transparent material SCN–1 wt.% acetone with and without an externally imposed flow perpendicular to the growth direction of the solidification front [11]. The authors observed a tilting of growing cells or dendrites towards the incoming flow direction. The uneven thickness of the concentration boundary layers at both sides of the dendritic tips (also shown in Fig. 2) is responsible for this behaviour. With respect to the growth of the secondary arms the study of Zhang and Maxworthy shows seemingly the opposite behaviour as seen in our experiment. The secondary branch appeared first on the downstream side and later on the upstream side. The authors explain this behaviour by a mixing effect which is expected to be much stronger at the upstream side than at the downstream side. Such an assumption cannot be confirmed by our study. Photographs in [11] show a rather tight gap between the growing dendrites. The formation of the secondary side arms might be interfered by the solute pile-up caused by the neighbouring crystal. The flow-
induced tilting leads to an orientation of the upstream branch towards the mushy zone whereas the downstream branches are more oriented towards the melt region. In comparison to our experiment such a specific situation may cause a reverse solute distribution around the dendrite which would explain the observed behaviour of the secondary branches. The local In-concentration was derived from the relative brightness in the image frames [12]. Fig. 3 reflects the results for situations just before the onset of the forced melt convection and at an advanced state of the solidification process, respectively. The red-coloured regions disclose a high solute concentration whereas the In-rich dendrites appear in blue colour. The electromagnetically driven flow enters the mushy zone on the right hand side of the solidification cell and reduces drastically the solute concentration in this area. This draining effect results in a reinforced growth of the dendritic structures which leads in turn to an augmented release of solute and a cumulative solute enrichment of the interdendritic flow. An outpouring of the flow from the mushy zone into the melt region can be observed at the left hand side of the sample. However, the rejected solute in large part is not discharged from the mushy zone. The solute deposition causes a partly remelting of the dendritic structures and the development of segregation channels. Medina et al. [15] described such a mushy zone “washing phenomenon” with respect to the solidification of a Pb-10 wt.%Sn alloy in the presence of electromagnetic stirring. For the superposition of thermosolutal convection and a single-vortex flow their simulations showed the formation of a main segregation channel on the downstream side of the solidification cell accompanied by a system of subchannels. Our experiments (not all shown here) disclose the appearance of several segregation channels. Number, shape and location of these channels are hard to predict. Apart from such specific discrepancies our experiments generally confirm the suggestions by Medina et al. [15]. Fig. 4 displays linear plots of the relative extinction E = −log(I / I0) along both horizontal lines shown in Fig. 3, where I denotes the X-ray intensity measured at distinct time and position during solidification and I0 stands for the X-ray intensity related to the initial composition C0. The extinction curves have been obtained from the image sequences
S. Boden et al. / Materials Letters 64 (2010) 1340–1343
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Fig. 3. Two-dimensional solute distribution just before the initiation of the stirring (a) and in an advanced state of solidification 325 s after the onset of the forced convection (b).
Fig. 4. Linear profiles of the relative extinction E as a measure of the solute concentration along the two horizontal lines A and B as shown in Fig. 3. The time step between the successive curves is 25 s (negative values relate to In depletion).
applying a Gaussian low pass filter to reduce the noise level. Line A represents the region adjacent to the dendrite tips just before the onset of melt agitation. Fig. 4(a) reveals an uneven dendrite growth, characterized by a distinct increase of E on the inflow side of the mushy zone whereas solute-rich zones are formed in the outflow area. Remarkable modifications of the local composition can also be noticed for deep regions of the mushy zone (line B). Fig. 4(b) shows clearly a consolidation of the dendritic phase at the inflow side accompanied by an increase of eutectic phase in the left part of the solidification cell. 4. Conclusions The X-ray visualization enables a general intuitional understanding of the flow impact on the bottom-up solidification of a Ga–25 wt.%In. An important advantage of this diagnostic approach is the possibility for the in-situ observation of the flow and the concentration field. The main effect of the flow is determined by the flow-induced redistribution of solute concentration which results in a change of the growth direction of the dendrites and the preference of secondary branches for an accelerated or decelerated growth. The present study demonstrates how a specific flow pattern generated by electromagnetic stirring provokes a unidirectional solute transport in the mushy zone which in turn causes the formation of spacious segregation pattern. The melt flow control during solidification of metal alloys by means of electromagnetic fields is still at the beginning. Further investigations, including the X-ray visualization, will be necessary for a better understanding of the complex interplay between melt flow and solidification process.
Acknowledgements This work was financially supported by Deutsche Forschungsgemeinschaft (DFG) in the framework of the collaborative research centre SFB 609 “Electromagnetic Flow Control in Metallurgy, Crystal Growth and Electrochemistry”.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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