Effect of controlled forced convection on macrosegregation and structure in direct-chill casting of an aluminium alloy
Published by Maney Publishing (c) W. S. Maney & Son Limited
D. G. Eskin*1, A. N. Turchin1 and L. Katgerman2 Effects of upward or downward forced flow in the centre of the sump of a 195 mm Al-alloy billet on structure and macrosegregation were studied. Introduction of forced flow in the liquid part of the sump resulted in some changes in the structure and dramatic changes in macrosegregation. Upward flow coarsened the structure and increased negative centreline segregation. Downward flow made the structure finer and more uniform across the billet, and suppressed the macrosegregation. Strong downward flow produced positive macrosegregation in the billet. The results are discussed in terms of macrosegregation mechanisms. Keywords: Macrosegregation, Melt flow, Direct-chill casting, Microstructure, Floating crystals, Convection
Introduction The fundamental reason behind macrosegregation (uneven composition on the scale of a casting) is the relative movement of solid and liquid phases in the twophase region of the casting. The two-phase region can be conditionally divided by a coherency isotherm to the upper, semi-liquid (fluid) part (slurry) and the lower, semi-solid part (mush).1 The mechanisms of such a relative movement can be many, e.g. natural or forced convection in the semi-liquid slurry, transport and settling of solid crystals in the slurry, and shrinkageor contraction-induced flow in the semi-solid mush.1–3 Natural convection during solidification of alloys is of thermo-solutal nature, i.e. the melt flow is driven by temperature and concentration gradients. These gradients exist in the liquid (or more correctly – in the fluid part that includes liquid and semi-liquid regions) part of a casting (billet) due to uneven cooling of the whole volume. In the case of a direct-chill (DC) cast billet, the sides are cooled faster than the bottom and there is a noticeable temperature difference between the central part and the periphery of the billet sump. Temperature difference drives the difference in density for the liquid. As a result of this thermal convection, the cooler liquid sinks at the periphery and creates the momentum that forces the liquid in the centre to rise. In the liquid part of the sump only thermal convection is active (if we neglect the solutal effects brought by washing out of liquid from the slurry zone). As soon as the liquidus isotherm is passed, the partitioning of alloying elements starts to produce the difference in composition and corresponding difference in density, causing the solutal convection. 1
Materials Innovation Institute, Mekelweg 2, 2628CD Delft, The Netherlands Delft University of Technology, Dept. Materials Science and Engineering, Mekelweg 2, 2628CD Delft, The Netherlands
2
*Corresponding author, email
[email protected]
ß 2009 W. S. Maney & Son Ltd. Received 17 June 2008; accepted 12 September 2008 DOI 10.1179/136404609X367425
In aluminium alloys and during DC casting, the directions of flow resulted from both solutal and thermal convection coincide with each other, giving the overall flow pattern shown in Fig. 1.4 The main reason why this flow may affect the distribution of alloying elements in the billet crosssection is the penetration of this flow into the slurry zone and washing out of the liquid with the composition already changed by the solidification process. The interaction between the liquid pool and the transition zone of the billet was noted as the main reason for convection-driven segregation by Tageev in 1949.5 In addition, the thermo-solutal flow may assist in transporting the solid phase within the slurry region and to the liquid pool. These solid crystals known as ‘floating crystals’, ‘free-floating dendrites’ or ‘daisies’ generally contribute to the negative centreline segregation.6,7 Figure 1 shows that the penetration of the melt flow into the slurry zone occurs in the outer quarter of the billet cross-section. Thus the solute-rich liquid from this part of the billet is mixed with the bulk liquid and the enriched mixture is brought to the centre of the billet. The result is centreline positive segregation. On the other hand, there is an upward flow in the central part of the billet that may transport the enriched liquid back to the liquid part, therefore diminishing the positive centreline segregation. On top of this the floating crystals can accumulate (settle) in the central portion of the billet decreasing the average alloy concentration there. Different means of controlling the melt flow in the sump of DC cast billets and ingots have been proposed and used in practice. To mention just a few: distribution inserts into the mould or hot top (bags, plates, spouts) and mechanical or electromagnetic stirrers.8–11 In most cases the idea is to suppress the melt flow and by that homogenize the structure and composition. However, the distribution systems do not change the direction of the flow but merely redistribute the flow, which is still
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Effect of controlled forced convection on macrosegregation and structure in direct-chill casting of an aluminium alloy
part of the billet sump. The diameter of the pump was 50 mm and the height 110 mm. The melt temperature in the furnace was 725uC and in the hot top – 695–700uC. The water flow was 150–160 L min21 and the casting speed was 120 mm min21. The melt level throughout the casting was controlled by the laser and it variation was within 2 mm. During casting the pump was completely submerged in the melt, with 75 mm of melt being above the top of the pump. A schematic illustration is given in Fig. 1. The operation mode of the flow-control device was first tested on a water model and by computer simulations. The maximum local flow velocities were about 40 mm/s. After commencing the casting, 300 mm of length were cast with the pump being idle. Then the pump was switched-on directing the flow upwards in the centre of the billet, acting therefore in the direction of natural convection (regime ‘forced aligned’). After 230 mm of length were produced, the direction of the flow in the centre was changed to the opposite, pushing the melt downwards (regime ‘forced opposite’) with the same speed as in the ‘forced aligned’ regime. After another 200 mm of length were cast, the forced-flow intensity was increased twice without changing the direction (regime ‘strong forced opposite’). Then another 200 mm of length were produced before casting was stopped. The billet was then cut in sections reflecting the end of each of the regimes to assure that the steady state was obtained. Next, horizontal rectangular in cross-section samples were cut along the diameter of the billet, measuring 156156195 mm. These samples were used for macrosegregation evaluation. Chemical analysis was performed in an optical spark spectrometer Spectromax. The measurements were taken each 8–10 mm along the diameter, after grinding the series of measurements was repeated. Totally 3 to 4 measurements were taken for each position. In this paper the average values are reported in the form of relative segregation. i.e. the deviation of the current Cu concentration from the average alloy composition. Grain size and dendrite arm spacing (DAS) were examined on three samples cut along the diameter: one close to the surface of the billet, one at the mid-radius, and one in the centre of the billet.
1 A typical flow pattern in the sump of a 195-mm DC cast billet with the location of the flow control device, mould and hot top. Half of the billet is shown with the centreline on the left
controlled by gravity and natural convection. Moreover, the stirrers do not change the flow pattern in a controlled way but rather in a chaotic manner. This paper describes the results of experiments when the flow direction along the axis of the billet is unidirectionally changed with a help of a mechanical pump* inserted into the liquid part of the billet. The effects of the flow direction are reported and discussed. The primary goal of the study is to demonstrate that the direction of the flow essentially affects the macrosegregation.
Results Structure examination showed that forced flow aligned with natural convection does not dramatically affect the structure with tendency to some coarsening of grains in the centre of the billet. The macrostructure exhibits a clear duplex grain structure with fine-DAS grains and coarse-DAS grains. The latter represent ‘floating crystals’. At the same time, the volume fraction of floating crystals halved upon forced aligned convection, from 0?33 to 0?16. The examination of microstructure shows that the coarse-DAS dendrites tend to form agglomerates. When the forced flow changes direction and becomes generally opposite to the natural convection, the grain structure in the centre refines both in terms of the grain size and DAS. Moreover the structure generally becomes more uniform across the billet section. The fraction of ‘floating grains’ remains virtually the same as in the ‘forced aligned’ case, 0?17. Further increase of the ‘forced opposite’ flow reverses the grain-size distribution; finer grains are now in the
Experimental An Al–3?7% Cu alloy was cast to a 195-mm round billet in a DC caster at Delft University of Technology. The installation is described in detail elsewhere.1 The hot-top aluminium-alloy mould is supplied by the melt through the side opening of the hot top, hence the level pour system is in use. Previous studies showed that this does not affect the flow pattern close to the mould which remains axisymmetric.12 Before the beginning of the casting, a mechanical flow control device (pump) was inserted along the vertical axis in the centre of the hot top. The bottom of the device was in level with the bottom of the ceramic ring of the hot top, in order to assure that the melt flow is controlled only in the liquid *The design and construction of the device is not the subject of this paper and is currently under patent application NL 2001248, February 15, 2008.
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centre of the billet. The DAS remains quite uniformly distributed in the billet cross section. The coarse-DAS dendrites are still present in the central part of the billet with the fraction 0?17, but now they are clearly agglomerated. The results of structure examination are briefly summarized in Table 1. The macrosegregation patterns in Fig. 2 are presented as trend lines. The ‘unaffected’ DC cast billet with natural convection (solid line) demonstrates the typical picture of negative centreline segregation with positive mid-radius and positive surface segregation. The forced flow aligned with the natural convection causes shifts the segregation curve to lower concentrations and, generally, the negative centreline segregation is enhanced (dashed line in Fig. 2). When the direction of the forced flow is reversed, the copper concentrations scatter more or less evenly along the diameter, making the macrosegregation less pronounced as shown in Fig. 2 by the thick solid line. The strong forced opposite flow dramatically changes the macrosegregation pattern that becomes profoundly positive (dotted line in Fig. 2). There is a scatter in experimental measurements and sometimes the presented results seem to contradict the mass conservation. However, the observed trends are obvious.
Discussion It has been demonstrated in this work that macrosegregation can be controlled by the direction of a forced flow in the central part of the liquid bath to the extent that the segregation can be enhanced, suppressed, or reversed. The macrosegregation curves obtained in billets with forced convection sometimes seem to contradict the mass conservation, showing either too positive or too negative overall segregation (see Fig. 2). The reason for that could be the unsteady flow regime introduced by the forced flow and the loss of the symmetry of the flow pattern that is reflected along a randomly chosen billet diameter. However, these unsteady effects do not affect the trend in the change of the macrosegregation pattern, the observation of which was the goal of this work. The structure modifications that result from the induced forced flow cannot explain the observed changes in macrosegregation, as shown in Table 1. The overall structure pattern remains the same – equiaxed grains with duplex grain structures in the centre of the billet. It is, therefore, not possible to ascribe the suppression of the negative centreline segregation or its reversal to the positive centreline segregation to the presence or absence of the ‘floating’ grains. The analysis of possible changes in the sump depth and the related changes in the
2 Macrosegregation trend lines in ‘unaffected’ billet with natural convection; after forced convection aligned with natural; after forced convection opposite to natural; and after strong forced convection opposite to natural. Relative segregation C5(Ci2Calloy)/Calloy is shown
macrosegregation8,13 cannot explain the observed changes either. And finally, the changes in the structure and corresponding changes in the permeability are too subtle to noticeably alter macrosegregation. It is known that the transition from negative (inverse) to positive (normal) segregation is possible during DC casting, especially in the presence of forced convection (mechanical or electromagnetic stirring).14 This transition is believed to reflect the ratio between the convection in the slurry part of the billet that washes solute-rich melt from the mushy zone and brings it to the centre and the shrinkage-induced flow that takes the solute-rich liquid inside the mushy zone from the centre towards the surface, and also to the magnitude of solidification shrinkage of the alloy. The higher the intensity of forced convection in the sump and the smaller the solidification shrinkage, the more the possibility of the normal (positive) macrosegregation. In our case, the solidification shrinkage did not change (the alloy was the same) and the changes in the structure or sump profile did not match the observed variation in macrosegregation. Hence, the movement of liquid appears to be the main cause of the observed effects. At first glance, the enhancement of natural convective flows (‘forced aligned’ regime) should decrease the negative centreline segregation by bringing more solute-rich liquid within the slurry zone to the centre. The experimental results attest to the contrary. The reason can be that the forced flow in our experiment is induced in the central part of the sump, therefore affecting mostly the movement of liquid along the billet
Table 1 Summary of structure and sump changes and corresponding effects on centreline macrosegregation. Structure feature
Observed change in structure with forced convection
Potential effect on macrosegregation
Floating crystals Amount decreases from 33 vol.% Less negative centreline segregation to 16–157 vol.% in the centre irrespective of flow direction Finer structure Structure refinement in the centre Slightly enhanced negative centreline of the billet with downward flow segregation due to permeability decreased by 20–60% Deeper sump Downward melt flow can deepen Enhanced negative due to the sump the larger shrinkage-induced flow
Observed change in macrosegregation Enhanced negative, negligible or positive in dependence on the flow direction Negligible or positive
Negligible or positive
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centreline. With reference to Fig. 1, we can say that the upward liquid flow, that is usually weak under natural conditions, is either enhanced, or suppressed, or reversed in the course of our experiment. Therefore, the forced, aligned flow actually extracted solute-rich liquid from the slurry zone, prevented its transport deeper into the mushy zone, and depleted the centre of the billet of copper. The forced, opposite flow suppressed the upward melt flow in the slurry and facilitated the penetration of the solute-rich liquid coming from the sides of the sump to the central part of the slurry and mush, compensating for the negative contribution of shrinkage-induced flow and floating grains. And the strong forced, opposite flow apparently further increased the penetration of the solute rich liquid into the mush over a broader region, inducing the positive segregation in the billet.
Acknowledgements The work is performed within the framework of Research Program of the Materials Innovation Institute (www.m2i.nl), former Netherlands Institute for Metals Research, project 02134. Authors would like to thank Ms S. Virdhian and Mr J.J.H. van Etten for active participation in experiments.
References 1. D. G. Eskin, V. I. Savran and L. Katgerman: Metall. Mater. Trans. A, 2005, 36A, 1965–1976. 2. M. C. Flemings: ISIJ Intern., 2000, 40, 833–841. 3. C. Beckermann: Intern. Mater. Rev., 2002, 47, 243–261. 4. Q. Du, D. G. Eskin and L. Katgerman: Mater. Sci. Eng. A, 2005, 413–414, 144–150. 5. V. M. Tageev: Doklady Akad. Nauk SSSR, 1949, 67, 491–494. 6. C. J. Vreeman and F. P. Incropera: Intern. J. Heat Mass Transfer, 2000, 43, 687–704. 7. D. G. Eskin, R. Nadella and L. Katgerman: Acta Mater., 2008, 56, 1358–1365. 8. V. I. Dobatkin: ‘Ingots of aluminium alloys’, 119–136; 1960, Sverdlovsk, Metallurgizdat. 9. V. A. Livanov, R. M. Gabidullin and V. S. Shepilov: ‘Direct-chill casting of aluminium alloys’, 98–110; 1977, Moscow, Metallurgizdat. 10. B. Gariepy and Y. Caron: in ‘Light metals 1991’, (ed. E. L. Rooy), 961–971; 1991, Warrendale, TMS. 11. B. Zhang, J. Cui and G. Lu: Mater. Sci. Eng. A, 2003, 355, 325– 330. 12. D. G. Eskin, J. Zuidema jr., V. I. Savran and L. Katgerman: Mater. Sci Eng. A, 2004, 384, 232–244. 13. D. G. Eskin, Q. Du and L. Katgerman: Scr. Mater., 2006, 55, 715– 718. 14. V. I. Dobatkin and N. F. Anoshkin: Mater. Sci. Eng. A, 1999, 263, 224–229.
Summary Published by Maney Publishing (c) W. S. Maney & Son Limited
A dedicated experiment has been developed for the demonstration of the effect of the direction of melt flow in the sump of a direct-chill cast billet on structure and macrosegregation. It is shown that the control of the melt flow along the centreline of the billet can enhance, suppress or reverse the macrosegregation pattern. Observed changes in the structure cannot adequately explain the observed phenomena. It is suggested that the changes in macrosegregation patterns are caused by the effect of the forced flow on the flow pattern in the central part of the sump.
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