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Initial stages of solidification during semisolid processing of a transparent model material
Materials Chemistry and Physics 135 (2012) 738e748
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Initial stages of solidification during semisolid processing of a transparent model material Mehdi Reisi, Behzad Niroumand* Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran
h i g h l i g h t s < Direct observation of the morphological evolution of succinonitrile crystals under stirring. < Evidences of the role of turbulence on the morphological evolution of primary particles. < Proposal of a new mechanism for the formation of spherical particles in SSR process.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 October 2011 Received in revised form 26 March 2012 Accepted 16 May 2012
Growth morphology of succinonitrile crystals under different cooling and stirring conditions was experimentally investigated by real time microscopic observation of the solidifying melt. It was found that under laminar fluid flow conditions, the solidified crystals exhibited equiaxed dendritic morphology immediately after the beginning of solidification. On the other hand, under turbulent fluid flow conditions the growth of solid particles followed a spherical mode with a high growth velocity from the initial stages of solidification. When the stirring was combined with rapid heat extraction from the rotor, a mushy layer was observed to form around the rotating chill. Under turbulent conditions, detachment of dendrite arms from this layer of rapidly coarsened dendrites is suggested to be the origin of the spherical particles in the microstructure. Formation of the layer was experimentally confirmed for an AleSi alloy. The new findings can be used for more effective refinement of rheocast microstructures. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Alloys Crystal growth Solidification Microstructure
1. Introduction Microstructure is the strategic link between the materials processing and the materials behavior. Microstructure control is therefore, essential for any successful processing activity. One of the most promising technological developments in recent years in metal casting is that of semisolid metal (SSM) processing where the alloys are especially treated and cast at temperatures between their liquidus and solidus temperature [1]. Understanding and controlling the microstructure formation in this process is an inherent part of the foundry technology, which has brought about numerous scientific and technological challenges up to present. The main characteristic of SSM processing is the drastic changes in the solidification microstructure from dendritic under normal conditions to non-dendritic or globular morphology under SSM processing [2,3]. In spite of the importance of such microstructures, the
* Corresponding author. Tel.: þ98 311 3915731; fax: þ98 311 3912752. E-mail addresses: [email protected] (M. Reisi), [email protected] (B. Niroumand). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.05.053
mechanisms by which SSM treatments, especially the imposed forced convection, enhance the microstructural evolution still remain largely unclear. The most widely accepted explanation for the formation of nondendritic morphologies in SSM processed materials is dendrite fragmentation theory initially proposed by Flemings [1]. The conventional belief is that under forced convection, the initial dendrites would be fragmented either by bending of the dendrite arms followed by liquid penetration into their high-angle grain boundaries [4,5] or through remelting at the root of the dendrite arms due to solute enrichment [6]. The detached dendrite arms then undergo a coarsening process to obtain the non-dendritic microstructure. Although the dendrite fragmentation theory could well explain the grain multiplication observed in semisolid microstructures, it was difficult to explain why the fragmented dendrite arms would grow in spherical rather than dendritic mode [3]. Validity of this theory has been increasingly questioned in recent years. Mullis proposed that liquid flow causes a rotation in the dendrite tip growth direction and thus creates bent dendrite arms [7]. His model showed that this dendrite arm bending could lead to rosette formation without invoking mechanical effects [7].
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For mechanically stirred AleCu alloys, Niroumand and Xia showed through serial sectioning that apparently isolated globular particles in the two dimensional microstructures studies were in fact complex three dimensional interconnected ripened structures, and concluded that fragmentation of dendrite arms under fluid flow may not be as effective in evolution of semisolid microstructures as commonly thought [8]. Browsing the literature shows that many of the theories proposed so far are incompatible with each other and do not suggest any comprehensive elaboration of the multitude of crystals morphologies observed under different flow conditions. It should be also noted that the inability to directly observe the microstructure formation at the initial stages of solidification (during nucleation and early growth) adds to the difficulties in correlating the morphological development to a possible explanation. Most of the proposed hypotheses are therefore based on indirect observations of the as-solidified microstructures. Although the mechanisms of the microstructural formation are not agreed upon, from a process development standpoint, it has been generally accepted in recent years that the critical temperature range for creation of non-dendritic microstructure is at and just below the liquidus temperature and that further processing after this time during cooling has a less significant effect on morphology of solid particles [9,10]. This idea has been employed for developing new processes such as cooling slope [11], Semisolid Rheocasting (SSR) [12], Gas-Induced Semisolid (GISS) [13], and low superheat pouring with a shear field (LSPSF) [14] processes. Among these processes, SSR process has attracted much attention since its invention in 2001. In this process, a short vigorous agitation is combined with rapid heat extraction only over a very narrow temperature range close to the liquidus temperature [12]. SSR involves immersing a cold rotating copper or graphite rod into a melt held just above its liquidus temperature. The rod rapidly cools the alloy below its liquidus temperature to initiate solidification while vigorously stirring the melt. The rod remains in the melt only for a few seconds, just long enough to cool the melt a few degrees below the liquidus temperature and is then removed [9,12]. Although a number of studies can be found on various aspects of the SSR process [9,12], mechanisms of evolution of non-dendritic morphology during the early stages of solidification still remain unclear. It has been hypothesized that most of the events responsible for formation of non-dendritic microstructure take place during the first 5 s of solidification [9,12]. However, obtaining definitive experimental data concerning the mechanisms of particles generation during SSR processing in such a short time is very difficult for metallic alloys. Experimental evidence regarding the events occurring during this period can only be obtained by
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performing real-time microscopic observation during semisolid processing using transparent organic substances whose solidification behavior resembles that of metals. Such transparent materials have been extensively used for studying the morphological development under directional solidification [15]. However, their application for the study of solidification under forced convection is limited to a few investigations conducted on semisolid processing of succinonitrile (SCN) [16], neopentyl alcohol (NPA) [17], NH4CleH2O [18] and SCNe5at.% H2O [19]. This is mainly due to the difficulty of imaging rapidly moving particles of microscopic dimensions, which necessitates employing special high speed imaging systems. The work of Smeulders et al. [17] appears to be one of the earliest and the most complete studies in this field. They performed real time microscopic observations during stir casting process using model organic alloy NPA in water and studied the growth of primary particles as small as 50 mm and as large as 2 mm. Based on their observations, they concluded that neither the fragmentation nor clustering mechanisms but the coarsening of the equiaxed dendrites led to the typical nondendritic particle morphologies after semisolid casting [17]. Recently, Li et al. studied the morphological evolution of SCNe5at.% water alloy under mechanical stirring [19]. Their results showed that the primary solid particles would solidify with a globular shape when mechanical stirring induced a strong shear flow. They suggested that the rotation of solid particles in a shear flow induced a stabilizing effect on the morphology of the solideliquid interface and promoted the globular growth of solid primary particles after they were nucleated in the melt [19]. In the study reported in the present work, a contribution is made toward the explanation of the mechanisms involved in the formation and evolution of semisolid microstructures through the experiments on a transparent model material under different stirring and cooling conditions. 2. Experimental procedures 2.1. SSM apparatus An optical semisolid simulating device was developed for the study of solidification of the transparent model materials as shown in Fig. 1. The growth cell consists of two concentric copper cylinders; its center was a rotating cylinder (rotor) with a diameter of Ri ¼ 12.5 (mm). Between this rotating inner and a static temperature controlled outer cylinders was an annular gap with a width of d ¼ 27 (mm) which was filled with a model material (Fig. 1(a)). A fundamental difference between the work described in the present study with those published previously [17e19] is that, in all the
Fig. 1. (a) Horizontal cross section of the growth cell; (1) rotating copper inner cylinder (rotor) with cooling water jacket, (2) annular gap containing the solidifying slurry, (3) static outer cylinder. (b) and (c) Photographs of the SSM apparatus and observation system.
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previous works the rotor has been at approximately the same temperature as the melt and therefore has had little chilling effect during solidification of the alloy. In the present investigation, although the rotor (inner cylinder) is in thermal balance with the melt during cooling from above the liquidus temperature, but it is possible to create a sudden local chilling zone around it during the first stages of solidification by provisions made for external cooling of the inner cylinder.
of the melt near the inner cylinder at this stage was about 0.06 ( C s1). Each of the experiments was repeated 3e4 times to ensure the reproducibility of the observations. Up to 3000 microphotographs were taken during each experiment, from which about 1200e1500 were analyzed to study the time evolution of solid particles morphology under various cooling and stirring conditions. b) Experiment with Ale7.1wt.%Si alloy
2.2. Observation system During the rheocasting experiments, the solidifying particles can move in the melt with velocities in the order of 1 (m s1) [17]. An imaging system with a very short exposure time is therefore, necessary to freeze the motion of solid particles. In the present investigation, a high performance digital 12Bit ‘Pixelfly CCD camera’ (PCO, Germany) was used. With this system, exposure times of as short as 5 (ms) were feasible. The images captured by the camera head, were transferred via a high speed data transfer system to the PCI-Board in the computer. The visualization and acquisition of digital images and camera control were performed using ‘Camware software’ supplied by PCO. By setting the camera parameters in Camware, the maximum number of frames that the camera could acquire per second was 23. To obtain high resolution images of the microscopic particles solidified under forced convection, a Navitar high magnification lens zoom (Navitar, USA) was used. Using this combination, details as small as 5 (mm) could be discriminated in the focal plane of the imaging system. The rapidly moving particles were illuminated with a (3 3) array of LED lamps in transmission mode.
To validate the solidification behavior observed during semisolid rheocasting of SCN alloy, an Ale7.1wt.%Si alloy was subjected to SSR processing conditions. The procedure for this experiment was as follows. 450 g of the alloy which resides within cylindrical graphite crucible with a diameter of 78 mm was heated in a resistance furnace to 650 C. The melt was then cooled and when the melt temperature reached to 624 C (about 6 above its measured liquidus temperature), a cold graphite rod (27 mm in diameter and 70 mm long) rotating at 1000 rpm was inserted into the melt. The rod was kept in the melt for about 7 s, cooling the melt to a temperature corresponding to about 0.05 fraction of solid. The rod was then removed and the alloy was allowed to cool down quiescently inside the crucible in the furnace to room temperature. The cooling rate of the melt during its passage through the liquidus temperature (Tl 1 C) and within the entire semisolid zone was about 0.13 and 0.04 C s1, respectively. The solidified alloy was then cut from the middle height of the ingot for microstructural investigation. 3. Results
2.3. Model material
3.1. Experiments without rapid heat extraction from the rotor
The model material used in the present investigation was a 99% pure SCN (NC(CH2)2CN) alloy. The 1% impurity content causes a freezing range of several degrees in which the solid fraction can be varied. SCN has been widely used as a model substance in the study of the solidification of metals due to its transparency, low melting point and its characteristic non-faceted solideliquid interface [15].
The SCN alloy was initially solidified under static condition. As shown in Fig. 2, under this condition columnar dendrites were observed to grow out from the surface of the copper rotor. Fig. 3(a)e(d) depict the time evolution of the solid particles morphology at solidification times (tl) between 0 and 40 s when the inner cylinder was rotated at 6 rpm. At later stages of solidification, it was difficult to focus on the growing particles without any disfiguration by other unfocussed large moving particles. tl was defined from the time that the first solid particles were observed in the field of view of the imaging system. As can be seen in the figure, during this experiment equiaxed dendritic particles appear to have formed and separated from the
2.4. Experimental conditions Two sets of experiments were carried out: 2.4.1. Experiments without rapid heat extraction from the rotating inner cylinder In the first set, the transparent material was heated to about 60 C (about 11 C above its measured liquidus temperature) and held for 2e3 min for compositional homogenization. The melt was then cooled at a cooling rate of about 0.02 ( C s1) below its liquidus temperature under different stirring speeds of 0, 6, 20, 60, 200 and 500 rpm. 2.4.2. Experiments with rapid heat extraction from the rotating inner cylinder a) Experiments with SCN alloy In the second set of experiments, similar experiments with only a small difference were carried out. The difference was that when the melt temperature reached to 2 C above the liquidus temperature, the rotating inner cylinder was cooled by pumping 20 C water to a water jacket positioned around it (Fig. 1(b) and (c)) to create a local high undercooling near the cylinder. The cooling rate
Fig. 2. Columnar dendritic morphology of SCN grown without stirring and water cooling on the rotor (tl ¼ 3.6 s).
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Fig. 3. Microstructures of solidifying SCN particles at a stirring speed of 6 rpm: (a) tl ¼ 0.9 (b) tl ¼ 15 (c) tl ¼ 27 and (d) tl ¼ 40 s.
surface of the copper rotor at the start of solidification (Fig. 3(a) and (b)). But their initial morphology is gradually changed by spheroidization of dendrite arms during subsequent stages of solidification (Fig. 3(c) and (d)) giving rise to a pseudo-cluster [8] or rosette-type [20] morphologies. Similar results were observed at higher stirring speed of 20 rpm. The time evolution of SCN solid particles morphology at this stirring speed is shown in Fig. 4. The only difference between the microstructures in Figs. 3 and 4 is the smaller size of the equiaxed dendrites formed at the higher stirring speed of 20 rpm. It is believed that increasing the stirring speed enhances the detachment of solid crystals nucleated on the surface of the rotor and facilitates the conditions for the new nucleation events to occur on the previously nucleated sites and therefore increases the overall nucleation rate. Comparison of Figs. 3(a) and 4(a) reveals the higher density of the nucleated and growing solid particles at higher stirring speed of 20 rpm. Fig. 5(a)e(d) presents the microstructures of SCN crystals during different stages of solidification at a stirring rate of 60 rpm. The figures show that under the conditions used in this experiment, SCN particles formed on the surface of the rotor took globular morphology from the very early stage of solidification (Fig. 5(a)) and continued to grow with stable interfaces after separation from the rotor and during further cooling. This is completely different from the observations at stirring rates of 6 and 20 rpm, which indicated dendritic growth following by coarsening from the start of solidification (Figs. 3 and 4). To further investigate the influence of stirring on the generation of spherical crystals, two more experiments at stirring speeds of 200 and 500 rpm were performed. The experimental results at the initial stage of solidification are shown in Fig. 6. The lower quality of these images was due to the fact that under high stirring speeds the solid particles moved with such a high velocity and chaotic fashion that it was very difficult to obtain a good and clear focused on particles. Nevertheless, Fig. 6 shows that during continuous cooling experiments with stirring speeds of 200 and 500 rpm, the SCN
crystals also took a globular morphology similar to the microstructures shown in Fig. 4. It must be also noted that the average size of SCN crystals decreased with the increase in the stirring speed. 3.2. Experiments with rapid heat extraction from inner cylinder To investigate the influence of combined high local cooling and convection on the microstructure of SCN crystals, a series of experiments were carried out in which the shearing conditions were similar to the previous ones, but when the melt temperature reached to about 2 C above its liquidus temperature, the rotating inner cylinder was cooled using an external cooling system to create a local chilling effect. The condition is similar to that occurs during semisolid rheocasting (SSR) process of metallic alloys. At unstirred condition, the columnar dendrites were seen to directionally solidify out from the surface of the copper rotor (Fig. 7(a)). The difference with Fig. 2 was that in this case, the interdendritic regions were quickly filled with solidification of the alloy due to the high local cooling rate on the surface of the rotor and the dendritic structure is much finer than that of Fig. 2. A similar trend was also observed at low stirring speed of 6 rpm, i.e. solidification starting with the columnar dendritic growth from the surface of chilled rotor. During further cooling, solidification continued by continuous radial growth of this layer into the melt. Fig. 7(b) presents the microstructure of the dendritic layer formed around the rotating chill after the stirring and cooling was applied for 1.3 s below the liquidus temperature. Comparing Fig. 7(a) and (b) indicates that at a stirring speed of 6 rpm the dendrite arms are somewhat bent due to the imposed fluid flow. Thickness of the dendritic layer reached to about 500 mm after 3 s. In the next experiment, stirring rate was increased to 20 rpm while the other solidification variables were held constant. Fig. 8 reveals that increasing the stirring rate from 6 to 20 rpm created a distinct solidification behavior. As shown in Fig. 8 under this
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Fig. 4. Microstructures of solidifying SCN particles at a stirring speed of 20 rpm: (a) tl ¼ 0.8 (b) tl ¼ 14 (c) tl ¼ 28 and (d) tl ¼ 41 s.
condition, solidification started with the columnar dendritic growth from the surface of chilled rotor similar to what occurred at stirring speeds of 0 and 6 rpm. However, within about 2.7 s the thickness of the mushy layer reached to about 300 mm and then its growth practically stopped. Subsequent solidification progressed
with the formation of spherical SCN crystals in front of the mushy layer. At longer solidification times, much more spherical particles were observed in front of the mushy layer which grew in size. This solidification behavior is completely different from those observed at the same stirring speed but without heat extraction from the
Fig. 5. Microstructures of solidifying SCN particles at a stirring speed of 60 rpm: (a) tl ¼ 1 (b) tl ¼ 9 (c) tl ¼ 15 and (d) tl ¼ 25 s.
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Fig. 6. Microstructures of solidifying SCN particles at stirring speeds of (a) 200 rpm (tl ¼ 3 s) and (b) 500 rpm (tl ¼ 6 s). The white arrows show the solid particles.
copper disc (Fig. 4). Similar trend was observed at higher stirring speed of 60 rpm. The microstructural evolution of SCN crystals at this stirring speed is depicted in Fig. 9. At this stirring speed, it was observed that the growth of the mushy layer stopped at a shorter time from the start of the solidification. To further test the microstructural effects of liquid flow, the experiment was repeated at stirring rates of 200 and 500 rpm. The corresponding results are shown in Fig. 10 which shows spherical particles in front of the mushy layer formed around the rotor. The lower quality of the images was explained before. Comparing the microstructures of Figs. 8e10 indicated that with increasing the stirring speed from 20 to 500 rpm, the thickness of the mushy layer decreased from 300 to about 70 mm. 4. Discussion 4.1. Flow patterns evaluation Based on the experimental results presented in Section 3, it was clear that morphology of solid crystals formed under forced convection markedly correlated to the stirring rate and hence, the nature of flow pattern during stirring. In this section an attempt is made to employ a criterion for comparing the flow patterns induced under different stirring rates. For a fluid between two concentric cylinders in which inner cylinder rotates and the outer one is at rest (similar to which used in present study), it has been shown that the critical angular velocity of the inner cylinder (Ucrit), above which the systems become turbulent, is correlated to the specifications of the setup and fluid through the following relationship:
Ucrit ¼
400n R20 Kð1
KÞ3=2
(1)
where K is the radius ratio of inner to outer cylinder and n is the kinematic viscosity of liquid [21,22]. Inputting the relevant parameters of the growth cell in Eq. (1) and considering the kinematic viscosity of SCN as 2.6 106 m2 s1 [23], one may easily calculate the critical velocity of the inner cylinder to be about 25 rpm. This means that for the lower stirring speeds of 6 and 20 rpm, the flow pattern in the growth cell is laminar. But higher stirring speeds of 60, 200 and 500 rpm induce fully turbulent fluid flow. This finding is used in the following sections for interpretation of various particles morphologies observed under different stirring rates. 4.2. Experiments without rapid heat extraction from the inner cylinder In the range of stirring rates employed in this work, two types of morphologies were observed for the SCN particles formed under stirred solidification. At stirring rates of 6 and 20 rpm which creates laminar fluid flow (Section 4.1), the morphology of solid crystals was equiaxed dendritic from the start of solidification (Figs. 3(a,b) and 4(a,b)). This morphology was modified to a large extent during the subsequent stages of solidification by coarsening mechanism, giving rise to morphologies resembling those of pseudo-clusters [8] or rosettes [20]. The driving force for the coarsening is lowering the surface energy of the particles. Therefore the surfaces with small radii of curvature (and therefore, a high surface energy) tend to melt and feed the growth of the surfaces with larger radii of curvature. The induced fluid flow in the semisolid slurry will help to accelerate this mechanism. During coarsening, rejection of solute between the dendrite arms retarded the growth of the arms at their roots. However, the body of the dendrite arms can be coarsened into large spherical shapes (Figs. 3(c) and 4(c)). As a result, an initial dendrite with its
Fig. 7. Micrograph of the fine dendritic mushy layer radially grown from the chilled copper rotor at (a) 0 rpm (tl ¼ 0.6 s), (b) 6 rpm (tl ¼ 1.3 s).
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Fig. 8. Microstructure of SCN crystals at a stirring speed of 20 rpm and water cooling condition: (a) tl ¼ 3 (b) tl ¼ 9 (c) tl ¼ 20 and (d) tl ¼ 32 s.
primary, secondary and higher orders arms will gradually evolve into a large globular particle apparently consisting of many smaller spherical particles as shown in Figs. 3(d) and 4(d). Such a morphology of coarsened dendritic particles resembles those described by Smeulders et al. as ‘bunches of grapes’ [17] and
denominated by Niroumand et al. [8,24] as ‘Pseudo-clusters’. The fact that equiaxed dendrites such as those shown in Figs. 3(a,b) and 4(a,b) have not been observed in metallic alloys under low shearing condition is probably due to very high surface curvature of these particles and therefore, their rapid coarsening giving rise to
Fig. 9. Microstructure of SCN crystals at a stirring speed of 60 rpm and water cooling condition: (a) tl ¼ 2 (b) tl ¼ 12 (c) tl ¼ 17 and (d) tl ¼ 27 s.
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Fig. 10. Microstructure of SCN crystals at the initial stages of solidification at rotation speeds of (a) 200 (tl ¼ 10 s) and (b) 500 rpm (tl ¼ 13 s), when the copper rotor was cooled with water. The white arrows show the solid particles.
morphologies resemble those of rosettes or clusters when studied in a two dimensional view. In other words, it is not possible to preserve the solid particles morphology at low solid fractions by any kind of rapid quenching. For instance, Pompe and Rettenmayr showed that in order to freeze the initial microstructure of Ale6.8wt.% Cu alloy by quenching, cooling rates in the order of 150 ( C s1) or higher were necessary [25]. In another study, Martinez et al. demonstrated that for a rheocast Ale4.5wt.%Cu alloy, the primary spherical particles with 40 mm diameter grew almost 40 mm in diameter during quenching with a cooling rate of about 400 ( C s1) [26]. For Ale7.1wt.%Si alloys, the present authors also suggested that a cooling rate of more than 60 ( C s1) was required to minimize the morphological changes of primary particles during quenching [27]. These findings clearly show the rapid coarsening of solid particles in metallic systems. Achieving such high cooling rates is difficult for samples of reasonable size under normal processing conditions. Therefore, under laminar fluid flow condition, normally rosette type morphology of solid particles such as those shown in Figs. 3(c,d) and 4(c,d) are observed during stirred solidification of metallic alloys. At higher stirring rates of 60, 200 and 500 rpm inducing turbulent fluid flow (Section 4.1), spherical SCN crystals were observed in the microstructure from the early stages in solidification processing (Figs. 5 and 6). These results are in good agreement with in situ observations made by Li et al. on semisolid processing of SCNe5at.% H2O [19]. They reported that under forced convection, the morphology of solid particles was spherical from the early stages of solidification. But, they did not offer any explanation for the rosette type morphology, which is normally observed during semisolid processing of metallic alloys. Another interesting point which was clear from the in situ observations was that in none of the experiments conducted in this work, any crystals with a size less than 15 mm was observed at the initial stages of solidification. Considering the image acquisition rate (23 frames per second) and resolution (5 mm) of the imaging system used in this study, it can be argued that the nuclei formed under such turbulent fluid flow conditions have undergone a spheroidal growth route with a very high growth velocity in the order of 200 (mm s1) during the initial stages of their growth. Study of the time evolution of the morphology of solid particles at a stirring speed of 60 rpm (Fig. 5) indicated that not only the globular crystals grew in size during continuous cooling condition, but also much more globular particles gradually appeared in the melt. These observations are in good agreement with the continuous nucleation mechanism proposed by Hellawell for solidification under forced convection [6]. According to Hellawell’s theory, the critical size of clusters is below the boundary layer thickness
around the moving particles. Under forced convection, these clusters will be swept away with the boundary layers. These unfixed nuclei alter the recalescence during solidification and induce continuous fluctuations in the local temperature. This promotes a continuous nucleation event. 4.3. Influence of laminar and turbulent fluid flow on growth pattern of solid crystals during stirred solidification The experimental results presented in previous sections suggested that laminar fluid flow supported the dendritic growth (Figs. 3 and 4) while turbulent fluid flow stabilized the solideliquid interface and promoted spherical growth from the initial stages of solidification (Figs. 5 and 6). In this section, the influence of different flow patterns on the growth morphology of solidified crystals under forced convection is discussed. It has been established that the morphological evolution of solid crystals during stirred solidification depends mainly on the shape and depth of the diffusion boundary layer and thus on the influence of the fluid flow characteristics on the diffusion boundary layer [28]. Because of the partitioning of solute during solidification, a diffusion boundary layer always exists around the growing crystals. Within this layer, diffusion is the only efficient mechanism of solute transport, while outside this layer the melt is completely homogenized by strong convection [29]. The present investigation showed that when the fluid flow is laminar, the crystals exhibited equiaxed dendritic morphology immediately after nucleation (Figs. 3(a) and 4(a)). This finding is in a good agreement with the stability analysis based on stagnant boundary layer given by Vogel and Cantor [29]. Their theoretical study showed that contrary to the experimental observation on rheocast materials, shearing flow was expected to destabilize the solidifying interface and encourage dendritic growth. To rationalize the formation of non-dendritic particles under forced convection, they proposed a dendrite fragmentation mechanism. But it should be considered that in the work of Vogel and Cantor, the diffusiondominant area in liquid was modeled as a spherical outer boundary layer around a dendritic particle. This assumption is clearly only valid for laminar flow conditions [29]. When the fluid flow is laminar, the liquid trapped between dendrite arms was hardly affected by the weak convective currents associated with stirring. But, when the fluid flow becomes turbulent and more complex, the external boundary of the diffusion layer does not remain spherical as supposed by Vogel and Cantor and is drawn between the dendrite arms [30]. Thus under turbulent flow conditions, it is expected that stirred liquid penetrates between the dendrite arms more effectively. Theoretical stability analyses using various
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techniques such as boundary element method [31], Monte Carlo simulation [28] and phase field approach [30] have shown that penetration of liquid between dendrite arms has stabilizing effects on the growth of solideliquid interface. It is concluded that penetration of solute between dendrite arms increases the local constitutional undercooling (and thus growth rate) at the sides and roots of the arms of the solideliquid interface and promotes the formation of spherical particles [28,30]. This argument is well supported by the experimental results presented in previous sections. Therefore, the penetration of the liquid phase of uniform concentration is believed to be the main reason for non-dendritic growth under turbulent fluid flow conditions. Although the transition in growth morphology from dendritic to spherical mode, due to altering the flow pattern from laminar to turbulent flow, was previously predicted by theoretical studies [31,32], the in situ experimental results presented in previous sections are believed to be the first experimental confirmation of such a change in the growth pattern of growing crystals under stirred solidification. 4.4. Experiments with rapid heat extraction from the rotor The aim of this set of experiments was to assess the mechanism of the formation of spherical particles during semisolid rheocasting (SSR) of metallic alloys. Martinez et al. studied the semisolid rheocasting of Ale4.5wt.%Cu alloy and showed that primary particles in quenched rheocast alloy were already spheroidal after 5 s of solidification time [12]. Although, their study could not obtain any evidence regarding the mechanism of generation of primary particles, they proposed that rapid cooling created such extremely fine and fragile dendrites adjacent to the rotating chill that when the convection was present, they were fragmented immediately after forming. They believe that all these processes occur at a time period of as short as 5 s from the start of solidification [13,26]. The experimental results presented in Section 3.2 showed that when the condition similar to SSR is applied on SCN melt held just above its liquidus temperature, a mushy layer was formed around the chilled rotor for all stirring rates used in this investigation (Figs. 7e10). The main difference was that at a low stirring rate of 6 rpm, i.e. under laminar fluid flow, this skeleton continued to grow and the resultant microstructure was a columnar dendritic body, part of which was shown in Fig. 7(b). But at higher stirring speeds of 60, 200 and 500 rpm, the formation of the mushy layer was followed by the appearance of spherical particles in the field of view of the imaging system (Figs. 8e10). In other words, only when the fluid flow around the rotor became turbulent, the spherical particles were observed around the mushy layer. It was not possible to focus on this mushy layer during the rotation of the copper disc to observe its microstructure. But it was possible to detect the morphology of this layer when the stirring was stopped. Fig. 11 presents a micrograph of the mushy layer just when the stirring of 60 rpm was stopped after 10 s of stirred solidification (tl ¼ 10 s). As this figure shows, the mushy layer is a compact layer of coarsened dendrite arms where all the arms have evolved into a rounded morphology only after 10 s since their formation. The mushy layer is attached to the copper rotor and rotates at the same speed as the rotor does. Therefore, one can easily calculate the fluid flow velocity in front of mushy layer to be about 8 cm s1. Obviously; the flow velocity in front of the mushy layer increases, as the layer grows outward and therefore becomes more turbulent. This is believed to be the reason for rapid coarsening of the dendrite arms at such a short time. The fact that no dendrite fragments were observed right in front of the chilled rotor during the present study suggests that the
Fig. 11. Microstructure of mushy layer formed around the copper rotor after 10 s of stirred solidification and cooling.
fragmentation mechanism proposed by Martinez et al. [12,13] is unlikely to be responsible for the formation of spherical particles during SSR processing of the SCN alloy. Furthermore, the present findings indicated that a stable mushy layer of rapidly coarsening dendrites, and not transitory fragmenting dendrites, was formed around the rotor at very early stages of solidification. Although it is not possible to capture the moments of detachment of the dendrite arms from this layer, it is believed that this coarsened dendritic network is responsible for the formation of spherical particles seen in front of the layer when the fluid flow is turbulent. It is hypothesized that this mushy layer will rapidly become an effective source for the release of fresh solid growth centers into the melt. Thus, it is concluded that turbulent liquid flow may be a required element for detachment of globular crystals from the mushy layer formed around the rotor during SSR process. Under laminar flow condition, i.e. stirring rate of 6 rpm, the mushy layer continuously grew out from the rotor without detachment of any solid particle and therefore, was unable to produce a non-dendritic microstructure (Fig. 7(b)). Direct observation of the formation of this mushy layer around the rotor during SSR processing of metallic alloys is not possible. Nevertheless, to examine the possibility of obtaining indirect evidence for the formation of the mentioned mushy layer in SSRprocessed metallic alloys, an experiment described in Section 2.4.2(b), was performed on semisolid rheocasting of Ale7.1wt.%Si alloy. Fig. 12 shows the locations and microstructures of different regions in the ingot. The rigorous study of different locations of the SSR-processed ingot showed that although most of the volume of the alloy processed by immersing the rotating graphite rod had a non-dendritic microstructure (Fig. 12(a)), the central region of the ingot had a dendritic microstructure (Fig. 12(b)). The dendritic region corresponds to the location where the graphite rod had been immersed. The dendritic morphology of large particles observed in this location suggests that it was solidified under different solidification conditions than the rest of ingot. This region seems to have been created by solidification of liquid, which flowed into the void left when the rod was removed. It may be speculated that when the rod is removed, the liquid phase can quickly fill the void, flowing through the mushy layer, which was formed around the rod. It is believed that solid particles, formed and distributed during stirring, cannot flow through this mushy layer and therefore, the solidification microstructure of the liquid which fills the void will become dendritic.
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Fig. 12. Microstructures of different locations of a SSR-processed Ale7.1wt%Si ingot.
Considering that the rotating chill was immersed into the melt for only 7 s, it is reasonable to expect that the mushy layer was formed around the rotor once it is immersed into the melt due to the very high local chilling potency of the rotor at that moment. Therefore, formation of non-dendritic particles observed in SSRprocessed organic (Figs. 8e10) and metallic alloys (Fig. 12(a)) may be interpreted based on their detachment from the mushy layer. The new findings can be used for more effective refinement of SSR microstructures. For example, addition of small amounts of alloying elements with high negative segregation tendency seems a viable way to encourage faster arm separation from the mushy layer and therefore smaller and more numerous primary particles. This is the subject of our future investigations. 5. Conclusion Real time in situ microstructural observations were carried out for transparent 99% pure succinonitrile alloy in order to study the formation mechanism of the semisolid structures. The following conclusions could be drawn from the experimental observations under different cooling and fluid flow conditions: (1) Under laminar fluid flow conditions, the solidified crystals exhibited equiaxed dendritic morphology immediately after the beginning of solidification. The equiaxed dendrites underwent coarsening induced by forced convection to create the typical rosette or pseudo-cluster morphologies during the subsequent stages of solidification. (2) Solidification under turbulent fluid flow conditions produced spherical particles from the start of solidification. No evidence of dendrites or dendrite fragments was observed at this stage. This indicated that particle growth under turbulent fluid flow followed a spheroidal growth route rather than dendrite fragmentation followed by ripening as commonly believed. (3) When the stirring was combined with rapid heat extraction from the rotor, a mushy layer was observed to form around the rotating chill at very early stages of solidification. The experimental results show that existence of this layer is decisive in the growth pattern developed during subsequent stages of solidification.
(4) It was postulated that under turbulent fluid flow conditions, this layer acted as an effective source for the release of new solid growth centers into the melt. (5) The formation of the mushy layer around the rotating chill during processing of metallic alloys was also confirmed by finding a dendritic region at the location where the rotating chill had been immersed. (6) The formation of the non-dendritic particles observed in SSRprocessed organic and metallic alloys was explained based on solid particles detachment from the mushy layer when the fluid flow was turbulent. References [1] [2] [3] [4] [5]
[6]
[7] [8] [9] [10]
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M.C. Flemings, Metall. Trans. 22A (1991) 957e981. D.H. Kirkwood, Int. Mater. Rev. 39 (1994) 173e189. Z. Fan, Int. Mater. Rev. 47 (2002) 49e85. A. Vogel, Met. Sci. 12 (1978) 576e578. A. Vogel, R.D. Doherty, B. Cantor, Proceedings of the International Conference on Solidification and Casting of Metals, The Metals Society, Sheffield, UK, 1977, pp. 518e525. A. Hellawell, in: D.H. Kirkwood, P. Kapranos (Eds.), Proceedings of the 4th International Conference on Semisolid Processing of Alloys and Composites, University of Sheffield, Sheffield, UK, 1996, pp. 60e65. A.M. Mullis, Acta Mater. 47 (1999) 1783e1789. B. Niroumand, K. Xia, Mater. Sci. Eng. 283A (2000) 70e75. M. Reisi, B. Niroumand, J. Alloy Compd. 470 (2009) 413e419. J.A. Yurko, R.A. Martinez, M.C. Flemings, in: Y. Tsutsui, M. Kiuchi, K. Ichikawa (Eds.), Proceedings of the 7th International Conference on Semisolid Processing of Alloys and Composites, Japan Society for Technology of Plasticity, Tsukuba, Japan, 2002, pp. 659e664. T. Haga, P. Kapranos, J. Mater. Process. Tech. 130e131 (2002) 594e598. R.A. Martinez, M.C. Flemings, Metall. Mater. Trans. 36A (2005) 2205e2210. J. Wannasin, R.A. Martinez, M.C. Flemings, Scr. Mater. 55 (2006) 115e118. H.M. Guo, X.J. Yang, Mater. Sci. Tech. 24 (2008) 55e63. S.C. Huang, M.E. Glicksman, Acta Metall. 29 (1981) 717e734. J.C. Van dam, F.H. Mischgofsky, J. Mater. Sci. 17 (1982) 989e993. R.J. Smeulders, F.H. Mischgofsky, H.J. Frankena, J. Cryst. Growth 76 (1986) 151e169. A.M. Mullis, S.E. Battersby, H.L. Fletcher, Scr. Mater. 39 (1998) 147e152. T. Li, X. Lin, W. Huang, Acta Mater. 54 (2006) 4815e4824. J.M.M. Moleenar, F.W.H.C. Salemans, L. Katgerman, J. Mater. Sci. 20 (1985) 4335e4344. R.B. Bird, W.E. Stewart, E.N. Lightfoot, Transport Phenomena, Wiley, New York, 1960. H. Schlichting, Boundary-layer Theory, sixth ed.. McGraw-Hill, New York, 1968. M. Serefoglu, R.E. Napolitano, Acta Mater. 56 (2008) 3862e3873. P. Falak, B. Niroumand, Scr. Mater. 53 (2005) 53e57.
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[25] O. Pompe, M. Rettenmayr, J. Cryst. Growth 192 (1998) 300e306. [26] R.A. Martinez, A. Karma, M.C. Flemings, Metall. Mater. Trans. 37A (2006) 2807e2815. [27] M. Reisi, B. Niroumand, J. Alloy Compd. 475 (2009) 643e647. [28] A. Das, S. Ji, Z. Fan, Acta Mater. 50 (2002) 4571e4585. [29] A. Vogel, B. Cantor, J. Cryst. Growth 37 (1977) 309e316.
[30] R.S. Qin, E.R. Wallach, Mater. Sci. Eng. 357 (2003) 45e54. [31] R.S. Qin, Z. Fan, in: Y. Tsutsui, M. Kiuchi, K. Ichikawa (Eds.), Proceedings of the 7th International Conference on Semisolid Processing of Alloys and Composites, Japan Society for Technology of Plasticity, Tsukuba, Japan, 2002, pp. 819e824. [32] S. Wu, X. Wu, Z. Xiao, Acta Mater. 52 (2004) 3519e3524.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Initial stages of solidification during semisolid processing of a transparent model material Mehdi Reisi, Behzad Niroumand* Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran
h i g h l i g h t s < Direct observation of the morphological evolution of succinonitrile crystals under stirring. < Evidences of the role of turbulence on the morphological evolution of primary particles. < Proposal of a new mechanism for the formation of spherical particles in SSR process.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 October 2011 Received in revised form 26 March 2012 Accepted 16 May 2012
Growth morphology of succinonitrile crystals under different cooling and stirring conditions was experimentally investigated by real time microscopic observation of the solidifying melt. It was found that under laminar fluid flow conditions, the solidified crystals exhibited equiaxed dendritic morphology immediately after the beginning of solidification. On the other hand, under turbulent fluid flow conditions the growth of solid particles followed a spherical mode with a high growth velocity from the initial stages of solidification. When the stirring was combined with rapid heat extraction from the rotor, a mushy layer was observed to form around the rotating chill. Under turbulent conditions, detachment of dendrite arms from this layer of rapidly coarsened dendrites is suggested to be the origin of the spherical particles in the microstructure. Formation of the layer was experimentally confirmed for an AleSi alloy. The new findings can be used for more effective refinement of rheocast microstructures. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Alloys Crystal growth Solidification Microstructure
1. Introduction Microstructure is the strategic link between the materials processing and the materials behavior. Microstructure control is therefore, essential for any successful processing activity. One of the most promising technological developments in recent years in metal casting is that of semisolid metal (SSM) processing where the alloys are especially treated and cast at temperatures between their liquidus and solidus temperature [1]. Understanding and controlling the microstructure formation in this process is an inherent part of the foundry technology, which has brought about numerous scientific and technological challenges up to present. The main characteristic of SSM processing is the drastic changes in the solidification microstructure from dendritic under normal conditions to non-dendritic or globular morphology under SSM processing [2,3]. In spite of the importance of such microstructures, the
* Corresponding author. Tel.: þ98 311 3915731; fax: þ98 311 3912752. E-mail addresses: [email protected] (M. Reisi), [email protected] (B. Niroumand). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.05.053
mechanisms by which SSM treatments, especially the imposed forced convection, enhance the microstructural evolution still remain largely unclear. The most widely accepted explanation for the formation of nondendritic morphologies in SSM processed materials is dendrite fragmentation theory initially proposed by Flemings [1]. The conventional belief is that under forced convection, the initial dendrites would be fragmented either by bending of the dendrite arms followed by liquid penetration into their high-angle grain boundaries [4,5] or through remelting at the root of the dendrite arms due to solute enrichment [6]. The detached dendrite arms then undergo a coarsening process to obtain the non-dendritic microstructure. Although the dendrite fragmentation theory could well explain the grain multiplication observed in semisolid microstructures, it was difficult to explain why the fragmented dendrite arms would grow in spherical rather than dendritic mode [3]. Validity of this theory has been increasingly questioned in recent years. Mullis proposed that liquid flow causes a rotation in the dendrite tip growth direction and thus creates bent dendrite arms [7]. His model showed that this dendrite arm bending could lead to rosette formation without invoking mechanical effects [7].
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For mechanically stirred AleCu alloys, Niroumand and Xia showed through serial sectioning that apparently isolated globular particles in the two dimensional microstructures studies were in fact complex three dimensional interconnected ripened structures, and concluded that fragmentation of dendrite arms under fluid flow may not be as effective in evolution of semisolid microstructures as commonly thought [8]. Browsing the literature shows that many of the theories proposed so far are incompatible with each other and do not suggest any comprehensive elaboration of the multitude of crystals morphologies observed under different flow conditions. It should be also noted that the inability to directly observe the microstructure formation at the initial stages of solidification (during nucleation and early growth) adds to the difficulties in correlating the morphological development to a possible explanation. Most of the proposed hypotheses are therefore based on indirect observations of the as-solidified microstructures. Although the mechanisms of the microstructural formation are not agreed upon, from a process development standpoint, it has been generally accepted in recent years that the critical temperature range for creation of non-dendritic microstructure is at and just below the liquidus temperature and that further processing after this time during cooling has a less significant effect on morphology of solid particles [9,10]. This idea has been employed for developing new processes such as cooling slope [11], Semisolid Rheocasting (SSR) [12], Gas-Induced Semisolid (GISS) [13], and low superheat pouring with a shear field (LSPSF) [14] processes. Among these processes, SSR process has attracted much attention since its invention in 2001. In this process, a short vigorous agitation is combined with rapid heat extraction only over a very narrow temperature range close to the liquidus temperature [12]. SSR involves immersing a cold rotating copper or graphite rod into a melt held just above its liquidus temperature. The rod rapidly cools the alloy below its liquidus temperature to initiate solidification while vigorously stirring the melt. The rod remains in the melt only for a few seconds, just long enough to cool the melt a few degrees below the liquidus temperature and is then removed [9,12]. Although a number of studies can be found on various aspects of the SSR process [9,12], mechanisms of evolution of non-dendritic morphology during the early stages of solidification still remain unclear. It has been hypothesized that most of the events responsible for formation of non-dendritic microstructure take place during the first 5 s of solidification [9,12]. However, obtaining definitive experimental data concerning the mechanisms of particles generation during SSR processing in such a short time is very difficult for metallic alloys. Experimental evidence regarding the events occurring during this period can only be obtained by
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performing real-time microscopic observation during semisolid processing using transparent organic substances whose solidification behavior resembles that of metals. Such transparent materials have been extensively used for studying the morphological development under directional solidification [15]. However, their application for the study of solidification under forced convection is limited to a few investigations conducted on semisolid processing of succinonitrile (SCN) [16], neopentyl alcohol (NPA) [17], NH4CleH2O [18] and SCNe5at.% H2O [19]. This is mainly due to the difficulty of imaging rapidly moving particles of microscopic dimensions, which necessitates employing special high speed imaging systems. The work of Smeulders et al. [17] appears to be one of the earliest and the most complete studies in this field. They performed real time microscopic observations during stir casting process using model organic alloy NPA in water and studied the growth of primary particles as small as 50 mm and as large as 2 mm. Based on their observations, they concluded that neither the fragmentation nor clustering mechanisms but the coarsening of the equiaxed dendrites led to the typical nondendritic particle morphologies after semisolid casting [17]. Recently, Li et al. studied the morphological evolution of SCNe5at.% water alloy under mechanical stirring [19]. Their results showed that the primary solid particles would solidify with a globular shape when mechanical stirring induced a strong shear flow. They suggested that the rotation of solid particles in a shear flow induced a stabilizing effect on the morphology of the solideliquid interface and promoted the globular growth of solid primary particles after they were nucleated in the melt [19]. In the study reported in the present work, a contribution is made toward the explanation of the mechanisms involved in the formation and evolution of semisolid microstructures through the experiments on a transparent model material under different stirring and cooling conditions. 2. Experimental procedures 2.1. SSM apparatus An optical semisolid simulating device was developed for the study of solidification of the transparent model materials as shown in Fig. 1. The growth cell consists of two concentric copper cylinders; its center was a rotating cylinder (rotor) with a diameter of Ri ¼ 12.5 (mm). Between this rotating inner and a static temperature controlled outer cylinders was an annular gap with a width of d ¼ 27 (mm) which was filled with a model material (Fig. 1(a)). A fundamental difference between the work described in the present study with those published previously [17e19] is that, in all the
Fig. 1. (a) Horizontal cross section of the growth cell; (1) rotating copper inner cylinder (rotor) with cooling water jacket, (2) annular gap containing the solidifying slurry, (3) static outer cylinder. (b) and (c) Photographs of the SSM apparatus and observation system.
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previous works the rotor has been at approximately the same temperature as the melt and therefore has had little chilling effect during solidification of the alloy. In the present investigation, although the rotor (inner cylinder) is in thermal balance with the melt during cooling from above the liquidus temperature, but it is possible to create a sudden local chilling zone around it during the first stages of solidification by provisions made for external cooling of the inner cylinder.
of the melt near the inner cylinder at this stage was about 0.06 ( C s1). Each of the experiments was repeated 3e4 times to ensure the reproducibility of the observations. Up to 3000 microphotographs were taken during each experiment, from which about 1200e1500 were analyzed to study the time evolution of solid particles morphology under various cooling and stirring conditions. b) Experiment with Ale7.1wt.%Si alloy
2.2. Observation system During the rheocasting experiments, the solidifying particles can move in the melt with velocities in the order of 1 (m s1) [17]. An imaging system with a very short exposure time is therefore, necessary to freeze the motion of solid particles. In the present investigation, a high performance digital 12Bit ‘Pixelfly CCD camera’ (PCO, Germany) was used. With this system, exposure times of as short as 5 (ms) were feasible. The images captured by the camera head, were transferred via a high speed data transfer system to the PCI-Board in the computer. The visualization and acquisition of digital images and camera control were performed using ‘Camware software’ supplied by PCO. By setting the camera parameters in Camware, the maximum number of frames that the camera could acquire per second was 23. To obtain high resolution images of the microscopic particles solidified under forced convection, a Navitar high magnification lens zoom (Navitar, USA) was used. Using this combination, details as small as 5 (mm) could be discriminated in the focal plane of the imaging system. The rapidly moving particles were illuminated with a (3 3) array of LED lamps in transmission mode.
To validate the solidification behavior observed during semisolid rheocasting of SCN alloy, an Ale7.1wt.%Si alloy was subjected to SSR processing conditions. The procedure for this experiment was as follows. 450 g of the alloy which resides within cylindrical graphite crucible with a diameter of 78 mm was heated in a resistance furnace to 650 C. The melt was then cooled and when the melt temperature reached to 624 C (about 6 above its measured liquidus temperature), a cold graphite rod (27 mm in diameter and 70 mm long) rotating at 1000 rpm was inserted into the melt. The rod was kept in the melt for about 7 s, cooling the melt to a temperature corresponding to about 0.05 fraction of solid. The rod was then removed and the alloy was allowed to cool down quiescently inside the crucible in the furnace to room temperature. The cooling rate of the melt during its passage through the liquidus temperature (Tl 1 C) and within the entire semisolid zone was about 0.13 and 0.04 C s1, respectively. The solidified alloy was then cut from the middle height of the ingot for microstructural investigation. 3. Results
2.3. Model material
3.1. Experiments without rapid heat extraction from the rotor
The model material used in the present investigation was a 99% pure SCN (NC(CH2)2CN) alloy. The 1% impurity content causes a freezing range of several degrees in which the solid fraction can be varied. SCN has been widely used as a model substance in the study of the solidification of metals due to its transparency, low melting point and its characteristic non-faceted solideliquid interface [15].
The SCN alloy was initially solidified under static condition. As shown in Fig. 2, under this condition columnar dendrites were observed to grow out from the surface of the copper rotor. Fig. 3(a)e(d) depict the time evolution of the solid particles morphology at solidification times (tl) between 0 and 40 s when the inner cylinder was rotated at 6 rpm. At later stages of solidification, it was difficult to focus on the growing particles without any disfiguration by other unfocussed large moving particles. tl was defined from the time that the first solid particles were observed in the field of view of the imaging system. As can be seen in the figure, during this experiment equiaxed dendritic particles appear to have formed and separated from the
2.4. Experimental conditions Two sets of experiments were carried out: 2.4.1. Experiments without rapid heat extraction from the rotating inner cylinder In the first set, the transparent material was heated to about 60 C (about 11 C above its measured liquidus temperature) and held for 2e3 min for compositional homogenization. The melt was then cooled at a cooling rate of about 0.02 ( C s1) below its liquidus temperature under different stirring speeds of 0, 6, 20, 60, 200 and 500 rpm. 2.4.2. Experiments with rapid heat extraction from the rotating inner cylinder a) Experiments with SCN alloy In the second set of experiments, similar experiments with only a small difference were carried out. The difference was that when the melt temperature reached to 2 C above the liquidus temperature, the rotating inner cylinder was cooled by pumping 20 C water to a water jacket positioned around it (Fig. 1(b) and (c)) to create a local high undercooling near the cylinder. The cooling rate
Fig. 2. Columnar dendritic morphology of SCN grown without stirring and water cooling on the rotor (tl ¼ 3.6 s).
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Fig. 3. Microstructures of solidifying SCN particles at a stirring speed of 6 rpm: (a) tl ¼ 0.9 (b) tl ¼ 15 (c) tl ¼ 27 and (d) tl ¼ 40 s.
surface of the copper rotor at the start of solidification (Fig. 3(a) and (b)). But their initial morphology is gradually changed by spheroidization of dendrite arms during subsequent stages of solidification (Fig. 3(c) and (d)) giving rise to a pseudo-cluster [8] or rosette-type [20] morphologies. Similar results were observed at higher stirring speed of 20 rpm. The time evolution of SCN solid particles morphology at this stirring speed is shown in Fig. 4. The only difference between the microstructures in Figs. 3 and 4 is the smaller size of the equiaxed dendrites formed at the higher stirring speed of 20 rpm. It is believed that increasing the stirring speed enhances the detachment of solid crystals nucleated on the surface of the rotor and facilitates the conditions for the new nucleation events to occur on the previously nucleated sites and therefore increases the overall nucleation rate. Comparison of Figs. 3(a) and 4(a) reveals the higher density of the nucleated and growing solid particles at higher stirring speed of 20 rpm. Fig. 5(a)e(d) presents the microstructures of SCN crystals during different stages of solidification at a stirring rate of 60 rpm. The figures show that under the conditions used in this experiment, SCN particles formed on the surface of the rotor took globular morphology from the very early stage of solidification (Fig. 5(a)) and continued to grow with stable interfaces after separation from the rotor and during further cooling. This is completely different from the observations at stirring rates of 6 and 20 rpm, which indicated dendritic growth following by coarsening from the start of solidification (Figs. 3 and 4). To further investigate the influence of stirring on the generation of spherical crystals, two more experiments at stirring speeds of 200 and 500 rpm were performed. The experimental results at the initial stage of solidification are shown in Fig. 6. The lower quality of these images was due to the fact that under high stirring speeds the solid particles moved with such a high velocity and chaotic fashion that it was very difficult to obtain a good and clear focused on particles. Nevertheless, Fig. 6 shows that during continuous cooling experiments with stirring speeds of 200 and 500 rpm, the SCN
crystals also took a globular morphology similar to the microstructures shown in Fig. 4. It must be also noted that the average size of SCN crystals decreased with the increase in the stirring speed. 3.2. Experiments with rapid heat extraction from inner cylinder To investigate the influence of combined high local cooling and convection on the microstructure of SCN crystals, a series of experiments were carried out in which the shearing conditions were similar to the previous ones, but when the melt temperature reached to about 2 C above its liquidus temperature, the rotating inner cylinder was cooled using an external cooling system to create a local chilling effect. The condition is similar to that occurs during semisolid rheocasting (SSR) process of metallic alloys. At unstirred condition, the columnar dendrites were seen to directionally solidify out from the surface of the copper rotor (Fig. 7(a)). The difference with Fig. 2 was that in this case, the interdendritic regions were quickly filled with solidification of the alloy due to the high local cooling rate on the surface of the rotor and the dendritic structure is much finer than that of Fig. 2. A similar trend was also observed at low stirring speed of 6 rpm, i.e. solidification starting with the columnar dendritic growth from the surface of chilled rotor. During further cooling, solidification continued by continuous radial growth of this layer into the melt. Fig. 7(b) presents the microstructure of the dendritic layer formed around the rotating chill after the stirring and cooling was applied for 1.3 s below the liquidus temperature. Comparing Fig. 7(a) and (b) indicates that at a stirring speed of 6 rpm the dendrite arms are somewhat bent due to the imposed fluid flow. Thickness of the dendritic layer reached to about 500 mm after 3 s. In the next experiment, stirring rate was increased to 20 rpm while the other solidification variables were held constant. Fig. 8 reveals that increasing the stirring rate from 6 to 20 rpm created a distinct solidification behavior. As shown in Fig. 8 under this
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Fig. 4. Microstructures of solidifying SCN particles at a stirring speed of 20 rpm: (a) tl ¼ 0.8 (b) tl ¼ 14 (c) tl ¼ 28 and (d) tl ¼ 41 s.
condition, solidification started with the columnar dendritic growth from the surface of chilled rotor similar to what occurred at stirring speeds of 0 and 6 rpm. However, within about 2.7 s the thickness of the mushy layer reached to about 300 mm and then its growth practically stopped. Subsequent solidification progressed
with the formation of spherical SCN crystals in front of the mushy layer. At longer solidification times, much more spherical particles were observed in front of the mushy layer which grew in size. This solidification behavior is completely different from those observed at the same stirring speed but without heat extraction from the
Fig. 5. Microstructures of solidifying SCN particles at a stirring speed of 60 rpm: (a) tl ¼ 1 (b) tl ¼ 9 (c) tl ¼ 15 and (d) tl ¼ 25 s.
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Fig. 6. Microstructures of solidifying SCN particles at stirring speeds of (a) 200 rpm (tl ¼ 3 s) and (b) 500 rpm (tl ¼ 6 s). The white arrows show the solid particles.
copper disc (Fig. 4). Similar trend was observed at higher stirring speed of 60 rpm. The microstructural evolution of SCN crystals at this stirring speed is depicted in Fig. 9. At this stirring speed, it was observed that the growth of the mushy layer stopped at a shorter time from the start of the solidification. To further test the microstructural effects of liquid flow, the experiment was repeated at stirring rates of 200 and 500 rpm. The corresponding results are shown in Fig. 10 which shows spherical particles in front of the mushy layer formed around the rotor. The lower quality of the images was explained before. Comparing the microstructures of Figs. 8e10 indicated that with increasing the stirring speed from 20 to 500 rpm, the thickness of the mushy layer decreased from 300 to about 70 mm. 4. Discussion 4.1. Flow patterns evaluation Based on the experimental results presented in Section 3, it was clear that morphology of solid crystals formed under forced convection markedly correlated to the stirring rate and hence, the nature of flow pattern during stirring. In this section an attempt is made to employ a criterion for comparing the flow patterns induced under different stirring rates. For a fluid between two concentric cylinders in which inner cylinder rotates and the outer one is at rest (similar to which used in present study), it has been shown that the critical angular velocity of the inner cylinder (Ucrit), above which the systems become turbulent, is correlated to the specifications of the setup and fluid through the following relationship:
Ucrit ¼
400n R20 Kð1
KÞ3=2
(1)
where K is the radius ratio of inner to outer cylinder and n is the kinematic viscosity of liquid [21,22]. Inputting the relevant parameters of the growth cell in Eq. (1) and considering the kinematic viscosity of SCN as 2.6 106 m2 s1 [23], one may easily calculate the critical velocity of the inner cylinder to be about 25 rpm. This means that for the lower stirring speeds of 6 and 20 rpm, the flow pattern in the growth cell is laminar. But higher stirring speeds of 60, 200 and 500 rpm induce fully turbulent fluid flow. This finding is used in the following sections for interpretation of various particles morphologies observed under different stirring rates. 4.2. Experiments without rapid heat extraction from the inner cylinder In the range of stirring rates employed in this work, two types of morphologies were observed for the SCN particles formed under stirred solidification. At stirring rates of 6 and 20 rpm which creates laminar fluid flow (Section 4.1), the morphology of solid crystals was equiaxed dendritic from the start of solidification (Figs. 3(a,b) and 4(a,b)). This morphology was modified to a large extent during the subsequent stages of solidification by coarsening mechanism, giving rise to morphologies resembling those of pseudo-clusters [8] or rosettes [20]. The driving force for the coarsening is lowering the surface energy of the particles. Therefore the surfaces with small radii of curvature (and therefore, a high surface energy) tend to melt and feed the growth of the surfaces with larger radii of curvature. The induced fluid flow in the semisolid slurry will help to accelerate this mechanism. During coarsening, rejection of solute between the dendrite arms retarded the growth of the arms at their roots. However, the body of the dendrite arms can be coarsened into large spherical shapes (Figs. 3(c) and 4(c)). As a result, an initial dendrite with its
Fig. 7. Micrograph of the fine dendritic mushy layer radially grown from the chilled copper rotor at (a) 0 rpm (tl ¼ 0.6 s), (b) 6 rpm (tl ¼ 1.3 s).
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Fig. 8. Microstructure of SCN crystals at a stirring speed of 20 rpm and water cooling condition: (a) tl ¼ 3 (b) tl ¼ 9 (c) tl ¼ 20 and (d) tl ¼ 32 s.
primary, secondary and higher orders arms will gradually evolve into a large globular particle apparently consisting of many smaller spherical particles as shown in Figs. 3(d) and 4(d). Such a morphology of coarsened dendritic particles resembles those described by Smeulders et al. as ‘bunches of grapes’ [17] and
denominated by Niroumand et al. [8,24] as ‘Pseudo-clusters’. The fact that equiaxed dendrites such as those shown in Figs. 3(a,b) and 4(a,b) have not been observed in metallic alloys under low shearing condition is probably due to very high surface curvature of these particles and therefore, their rapid coarsening giving rise to
Fig. 9. Microstructure of SCN crystals at a stirring speed of 60 rpm and water cooling condition: (a) tl ¼ 2 (b) tl ¼ 12 (c) tl ¼ 17 and (d) tl ¼ 27 s.
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Fig. 10. Microstructure of SCN crystals at the initial stages of solidification at rotation speeds of (a) 200 (tl ¼ 10 s) and (b) 500 rpm (tl ¼ 13 s), when the copper rotor was cooled with water. The white arrows show the solid particles.
morphologies resemble those of rosettes or clusters when studied in a two dimensional view. In other words, it is not possible to preserve the solid particles morphology at low solid fractions by any kind of rapid quenching. For instance, Pompe and Rettenmayr showed that in order to freeze the initial microstructure of Ale6.8wt.% Cu alloy by quenching, cooling rates in the order of 150 ( C s1) or higher were necessary [25]. In another study, Martinez et al. demonstrated that for a rheocast Ale4.5wt.%Cu alloy, the primary spherical particles with 40 mm diameter grew almost 40 mm in diameter during quenching with a cooling rate of about 400 ( C s1) [26]. For Ale7.1wt.%Si alloys, the present authors also suggested that a cooling rate of more than 60 ( C s1) was required to minimize the morphological changes of primary particles during quenching [27]. These findings clearly show the rapid coarsening of solid particles in metallic systems. Achieving such high cooling rates is difficult for samples of reasonable size under normal processing conditions. Therefore, under laminar fluid flow condition, normally rosette type morphology of solid particles such as those shown in Figs. 3(c,d) and 4(c,d) are observed during stirred solidification of metallic alloys. At higher stirring rates of 60, 200 and 500 rpm inducing turbulent fluid flow (Section 4.1), spherical SCN crystals were observed in the microstructure from the early stages in solidification processing (Figs. 5 and 6). These results are in good agreement with in situ observations made by Li et al. on semisolid processing of SCNe5at.% H2O [19]. They reported that under forced convection, the morphology of solid particles was spherical from the early stages of solidification. But, they did not offer any explanation for the rosette type morphology, which is normally observed during semisolid processing of metallic alloys. Another interesting point which was clear from the in situ observations was that in none of the experiments conducted in this work, any crystals with a size less than 15 mm was observed at the initial stages of solidification. Considering the image acquisition rate (23 frames per second) and resolution (5 mm) of the imaging system used in this study, it can be argued that the nuclei formed under such turbulent fluid flow conditions have undergone a spheroidal growth route with a very high growth velocity in the order of 200 (mm s1) during the initial stages of their growth. Study of the time evolution of the morphology of solid particles at a stirring speed of 60 rpm (Fig. 5) indicated that not only the globular crystals grew in size during continuous cooling condition, but also much more globular particles gradually appeared in the melt. These observations are in good agreement with the continuous nucleation mechanism proposed by Hellawell for solidification under forced convection [6]. According to Hellawell’s theory, the critical size of clusters is below the boundary layer thickness
around the moving particles. Under forced convection, these clusters will be swept away with the boundary layers. These unfixed nuclei alter the recalescence during solidification and induce continuous fluctuations in the local temperature. This promotes a continuous nucleation event. 4.3. Influence of laminar and turbulent fluid flow on growth pattern of solid crystals during stirred solidification The experimental results presented in previous sections suggested that laminar fluid flow supported the dendritic growth (Figs. 3 and 4) while turbulent fluid flow stabilized the solideliquid interface and promoted spherical growth from the initial stages of solidification (Figs. 5 and 6). In this section, the influence of different flow patterns on the growth morphology of solidified crystals under forced convection is discussed. It has been established that the morphological evolution of solid crystals during stirred solidification depends mainly on the shape and depth of the diffusion boundary layer and thus on the influence of the fluid flow characteristics on the diffusion boundary layer [28]. Because of the partitioning of solute during solidification, a diffusion boundary layer always exists around the growing crystals. Within this layer, diffusion is the only efficient mechanism of solute transport, while outside this layer the melt is completely homogenized by strong convection [29]. The present investigation showed that when the fluid flow is laminar, the crystals exhibited equiaxed dendritic morphology immediately after nucleation (Figs. 3(a) and 4(a)). This finding is in a good agreement with the stability analysis based on stagnant boundary layer given by Vogel and Cantor [29]. Their theoretical study showed that contrary to the experimental observation on rheocast materials, shearing flow was expected to destabilize the solidifying interface and encourage dendritic growth. To rationalize the formation of non-dendritic particles under forced convection, they proposed a dendrite fragmentation mechanism. But it should be considered that in the work of Vogel and Cantor, the diffusiondominant area in liquid was modeled as a spherical outer boundary layer around a dendritic particle. This assumption is clearly only valid for laminar flow conditions [29]. When the fluid flow is laminar, the liquid trapped between dendrite arms was hardly affected by the weak convective currents associated with stirring. But, when the fluid flow becomes turbulent and more complex, the external boundary of the diffusion layer does not remain spherical as supposed by Vogel and Cantor and is drawn between the dendrite arms [30]. Thus under turbulent flow conditions, it is expected that stirred liquid penetrates between the dendrite arms more effectively. Theoretical stability analyses using various
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techniques such as boundary element method [31], Monte Carlo simulation [28] and phase field approach [30] have shown that penetration of liquid between dendrite arms has stabilizing effects on the growth of solideliquid interface. It is concluded that penetration of solute between dendrite arms increases the local constitutional undercooling (and thus growth rate) at the sides and roots of the arms of the solideliquid interface and promotes the formation of spherical particles [28,30]. This argument is well supported by the experimental results presented in previous sections. Therefore, the penetration of the liquid phase of uniform concentration is believed to be the main reason for non-dendritic growth under turbulent fluid flow conditions. Although the transition in growth morphology from dendritic to spherical mode, due to altering the flow pattern from laminar to turbulent flow, was previously predicted by theoretical studies [31,32], the in situ experimental results presented in previous sections are believed to be the first experimental confirmation of such a change in the growth pattern of growing crystals under stirred solidification. 4.4. Experiments with rapid heat extraction from the rotor The aim of this set of experiments was to assess the mechanism of the formation of spherical particles during semisolid rheocasting (SSR) of metallic alloys. Martinez et al. studied the semisolid rheocasting of Ale4.5wt.%Cu alloy and showed that primary particles in quenched rheocast alloy were already spheroidal after 5 s of solidification time [12]. Although, their study could not obtain any evidence regarding the mechanism of generation of primary particles, they proposed that rapid cooling created such extremely fine and fragile dendrites adjacent to the rotating chill that when the convection was present, they were fragmented immediately after forming. They believe that all these processes occur at a time period of as short as 5 s from the start of solidification [13,26]. The experimental results presented in Section 3.2 showed that when the condition similar to SSR is applied on SCN melt held just above its liquidus temperature, a mushy layer was formed around the chilled rotor for all stirring rates used in this investigation (Figs. 7e10). The main difference was that at a low stirring rate of 6 rpm, i.e. under laminar fluid flow, this skeleton continued to grow and the resultant microstructure was a columnar dendritic body, part of which was shown in Fig. 7(b). But at higher stirring speeds of 60, 200 and 500 rpm, the formation of the mushy layer was followed by the appearance of spherical particles in the field of view of the imaging system (Figs. 8e10). In other words, only when the fluid flow around the rotor became turbulent, the spherical particles were observed around the mushy layer. It was not possible to focus on this mushy layer during the rotation of the copper disc to observe its microstructure. But it was possible to detect the morphology of this layer when the stirring was stopped. Fig. 11 presents a micrograph of the mushy layer just when the stirring of 60 rpm was stopped after 10 s of stirred solidification (tl ¼ 10 s). As this figure shows, the mushy layer is a compact layer of coarsened dendrite arms where all the arms have evolved into a rounded morphology only after 10 s since their formation. The mushy layer is attached to the copper rotor and rotates at the same speed as the rotor does. Therefore, one can easily calculate the fluid flow velocity in front of mushy layer to be about 8 cm s1. Obviously; the flow velocity in front of the mushy layer increases, as the layer grows outward and therefore becomes more turbulent. This is believed to be the reason for rapid coarsening of the dendrite arms at such a short time. The fact that no dendrite fragments were observed right in front of the chilled rotor during the present study suggests that the
Fig. 11. Microstructure of mushy layer formed around the copper rotor after 10 s of stirred solidification and cooling.
fragmentation mechanism proposed by Martinez et al. [12,13] is unlikely to be responsible for the formation of spherical particles during SSR processing of the SCN alloy. Furthermore, the present findings indicated that a stable mushy layer of rapidly coarsening dendrites, and not transitory fragmenting dendrites, was formed around the rotor at very early stages of solidification. Although it is not possible to capture the moments of detachment of the dendrite arms from this layer, it is believed that this coarsened dendritic network is responsible for the formation of spherical particles seen in front of the layer when the fluid flow is turbulent. It is hypothesized that this mushy layer will rapidly become an effective source for the release of fresh solid growth centers into the melt. Thus, it is concluded that turbulent liquid flow may be a required element for detachment of globular crystals from the mushy layer formed around the rotor during SSR process. Under laminar flow condition, i.e. stirring rate of 6 rpm, the mushy layer continuously grew out from the rotor without detachment of any solid particle and therefore, was unable to produce a non-dendritic microstructure (Fig. 7(b)). Direct observation of the formation of this mushy layer around the rotor during SSR processing of metallic alloys is not possible. Nevertheless, to examine the possibility of obtaining indirect evidence for the formation of the mentioned mushy layer in SSRprocessed metallic alloys, an experiment described in Section 2.4.2(b), was performed on semisolid rheocasting of Ale7.1wt.%Si alloy. Fig. 12 shows the locations and microstructures of different regions in the ingot. The rigorous study of different locations of the SSR-processed ingot showed that although most of the volume of the alloy processed by immersing the rotating graphite rod had a non-dendritic microstructure (Fig. 12(a)), the central region of the ingot had a dendritic microstructure (Fig. 12(b)). The dendritic region corresponds to the location where the graphite rod had been immersed. The dendritic morphology of large particles observed in this location suggests that it was solidified under different solidification conditions than the rest of ingot. This region seems to have been created by solidification of liquid, which flowed into the void left when the rod was removed. It may be speculated that when the rod is removed, the liquid phase can quickly fill the void, flowing through the mushy layer, which was formed around the rod. It is believed that solid particles, formed and distributed during stirring, cannot flow through this mushy layer and therefore, the solidification microstructure of the liquid which fills the void will become dendritic.
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Fig. 12. Microstructures of different locations of a SSR-processed Ale7.1wt%Si ingot.
Considering that the rotating chill was immersed into the melt for only 7 s, it is reasonable to expect that the mushy layer was formed around the rotor once it is immersed into the melt due to the very high local chilling potency of the rotor at that moment. Therefore, formation of non-dendritic particles observed in SSRprocessed organic (Figs. 8e10) and metallic alloys (Fig. 12(a)) may be interpreted based on their detachment from the mushy layer. The new findings can be used for more effective refinement of SSR microstructures. For example, addition of small amounts of alloying elements with high negative segregation tendency seems a viable way to encourage faster arm separation from the mushy layer and therefore smaller and more numerous primary particles. This is the subject of our future investigations. 5. Conclusion Real time in situ microstructural observations were carried out for transparent 99% pure succinonitrile alloy in order to study the formation mechanism of the semisolid structures. The following conclusions could be drawn from the experimental observations under different cooling and fluid flow conditions: (1) Under laminar fluid flow conditions, the solidified crystals exhibited equiaxed dendritic morphology immediately after the beginning of solidification. The equiaxed dendrites underwent coarsening induced by forced convection to create the typical rosette or pseudo-cluster morphologies during the subsequent stages of solidification. (2) Solidification under turbulent fluid flow conditions produced spherical particles from the start of solidification. No evidence of dendrites or dendrite fragments was observed at this stage. This indicated that particle growth under turbulent fluid flow followed a spheroidal growth route rather than dendrite fragmentation followed by ripening as commonly believed. (3) When the stirring was combined with rapid heat extraction from the rotor, a mushy layer was observed to form around the rotating chill at very early stages of solidification. The experimental results show that existence of this layer is decisive in the growth pattern developed during subsequent stages of solidification.
(4) It was postulated that under turbulent fluid flow conditions, this layer acted as an effective source for the release of new solid growth centers into the melt. (5) The formation of the mushy layer around the rotating chill during processing of metallic alloys was also confirmed by finding a dendritic region at the location where the rotating chill had been immersed. (6) The formation of the non-dendritic particles observed in SSRprocessed organic and metallic alloys was explained based on solid particles detachment from the mushy layer when the fluid flow was turbulent. References [1] [2] [3] [4] [5]
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