Low-Convection-Cooling Slope Cast AlSi7Mg Alloy: A Rheological Perspective R. Ritwik, A.K. Prasada Rao, and B.K. Dhindaw

Transcription

1 JMEPEG ÓASM International DOI: /s /$19.00 Low-Convection-Cooling Slope Cast AlSi7Mg Alloy: A Rheological Perspective R. Ritwik, A.K. Prasada Rao, and B.K. Dhindaw (Submitted October 30, 2012; in revised form February 10, 2013) In this paper we made an attempt to assess the solidification and flow behavior of the AlSi7Mg alloy melt flowing down the cooling slope, by calculating the Reynolds number of the flowing melt. It has been found that the length of the laminar regime within the flowing melt (low-convection flow) depends on the angle of slope. The microstructure of as-cast AlSi7Mg alloy processed by low-convection-casting using cooling slope method has been studied. The microstructure reveals dendritic primary a-al phase with fine fibrous eutectic silicon in the interdendritic regions. The modification of eutectic silicon occurs predominately by the shearing of the solute-rich liquid between the primary a-al dendrites prior to eutectic solidification as it flows down the cooling slope. Nucleation and growth of the primary silicon dendrites was also observed, which confirms earlier reports on three-layer theory. The mechanism responsible for the refinement of eutectic phase is the enhanced heterogeneous nucleation in the last liquid to solidify. Keywords 1. Introduction aluminium alloy, casting, convection, Reynolds number, semi-solid processing SpencerÕs (Ref 1) experiments on Sn-15Pb lead to the discovery of the semi-solid processing, i.e., microstructural refinement of alloys in the solidification range, by the application of external shearing. Later, Flemings (Ref 2) demonstrated that shearing action occurs in the semi-solid range, which results in fragmentation of the dendrites, eventually forming rosette or cellular type of microstructure. It was also reported that the dendrite to spheroid formation depends on shear rate, shearing time, and cooling rate. A shearing action is believed to take between the solid dendrites, which shears the liquid besides fragmenting the dendritic arms of the solid phase (Ref 2). This discovery had inspired several other research teams leading to the invention semi-solid processing techniques (Ref 3-5). Twin-screw shearing technology and Dow thixomolding are some of such techniques (Ref 3-5). Hypo- and hyper-eutectic Al-Si alloys have been popularly processed through the semi-solid processing techniques. Literature suggests that such processing techniques result in refinement of both primary and eutectic phases in the Al-Si alloys (Ref 6-9). All the semi-solid processing techniques discussed above involve application of some sort of deliberate application of shearing action. However, a new technique known as cooling R. Ritwik and A.K. Prasada Rao, BCAST, Brunel University, Uxbridge, Middlesex, London UB8 3PH, UK; and Metallurgical and Materials Engineering, I.I.T. Kharagpur, Kharagpur, India; and B.K. Dhindaw, Metallurgical and Materials Engineering, I.I.T. Kharagpur, Kharagpur, India; and School of Minerals, Materials and Metallurgical Engineering, Indian Institute of Technology, Bhubaneswar, India. Contact akprasada@yahoo.com. slope method was developed. This method does not need application of external shearing action. The semi-solid slurry is allowed to flow down an inclined surface (Ref 9-14). During this process, the shearing action takes place within the mush which slides down an inclined cooling slope due to natural gravity. Hence, this process is simple and versatile. Taghavi and Ghassemi (Ref 9) studied the effects of length and angle of the cooling slope plate on the microstructure of the A356 alloy. It was reported that optimum refinement of primary and eutectic phase occurs at 40 cm and 40 inclination. Similar results were reported by Birol (Ref 10) in the case of A390 alloy. In addition, Birol (Ref 10) also reported that the alloy processed by this process exhibits better strength compared to its counterpart processed by conventional die casting. Similar reports observations were reported by others in the case of Al-Si alloys (Ref 11-14). Although literature found till date, qualitatively discussed on the cooling slope casting process, none was found to explain the rheological behavior quantitatively. Few reports were found to mention the three-layer concept in cooling slope process. Generally low-shear occurs in laminar flow regime rather than in turbulent flow regime. Although, Sunitha et al. (Ref 14) had made a mention of low-shear rate in their work, no evidence of flow transition (turbulent-laminar) was shown. Hence in the present work we made an attempt to understand the rheological behavior (flow transition) flowing down the cooling slope. In addition, this work also aims at the evolution of the eutectic phase and macro-segregation issues. 2. Experimental Figure 1 schematically illustrates a specially made siliminide tube (2 m long) tilting furnace, in order to create a laminar flow and simultaneous solidification of the AlSi7Mg alloy. The terminal temperatures of 620 C (T L ) and 577 C (T E ) were maintained such that the temperature decreases linearly from T L to T E. The temperatures were chosen on the basis of liquidus

2 and solidus temperatures of AlSi7Mg alloy, whose composition was analyzed by spark emission spectroscopy and is given in Table 1. The furnace was fixed at a tilting angle of 10, as shown in Fig. 1(a) and (b). About 1 kg of AlSi7Mg alloy was melted to 720 C in a zirconia-coated graphite crucible using a pit-type resistance furnace. The melt was degassed appropriately and was poured into the above-described titling tube furnace at 620 C. A part of the melt is directly poured into water for a reference sample, in order to understand the microstructural evolution during cooling slope process. Remainder of the melt was poured into the cooling slope furnace. The melt thus poured slid through the inclined tube and was cast into a water trough placed at the exit of the tube. The castings obtained were in the form of buttons of 1 cm in size. The microstructures of the castings thus obtained were characterized using a field emission type JEOL SEM and the results are discussed in the following sections. 2.1 Calculation of Reynolds Number of the AlSi7Mg Alloy Mush Flowing Down the Cooling Slope It is well known that Reynolds number is a key parameter used to understand the flow pattern in liquids. The dimensionless number enables the user to predict the laminar or turbulent nature of the fluid and determine reasons for the observed effect. The Reynolds number was calculated using the methodology discussed below. It is usually believed that Reynolds number of a fluid flow is independent. However, in the present case with the drop in the temperature, the fraction of solid increases, which further affects the viscosity and velocity of the melt flowing. On the other hand, it is known that melt viscosity and velocity affect the Reynolds number (Eq 4). Hence present study is based on such dependency relationship of Reynolds number on temperature. Hence, certain assumptions were made for calculating the Reynolds number of the flowing melt at several positions in the siliminide tube as stated below: (i) (ii) Temperature decreases linearly down the cooling slope as plotted in Fig. 2(c). Density of the mushy slurry varies as per the following equation as a function of solid fraction (Ref 15): q i ¼ ðq e f si Þþð1 f si Þq i : ðeq 1Þ (iii) The velocity of the mush at a given point was calculated, by following the law of conservation of energy. Potential energy is gradually converted to kinetic energy as the mush slides down the cooling slope. And the mush velocity is calculated using following equations: q l gh a þ h 0 o þ h 1 o ¼ 2 v2 i q i þ q i gh i þ h 0 o þ h o ; ðeq 2Þ Fig. 1 (a) Tilting tube furnace showing the AlSi7Mg alloy melt flow and casting and (b) schematic of the tilting tube furnace sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ) v i ¼ 2 q lgh a þ h 0 o þ h o qi gh i þ h 0 o þ h o ; q i ðeq 3Þ where h a = (Sin a) 9 L = (Sin 10 ) = mm; h o = 300 mm; h 0 o = 100 mm. (iv) Temperature changes in the mush caused by remelting or solidification during its flow down the cooling slope are ignored. (v) Energy equation is un-altered or independent of remelting or solidification along the slope. (vi) The value of viscosity of the mushy AlSi7Mg alloy at the exit of the tilting tube furnace was adopted from Ref 15. (vii) Velocity of the mush is independent of viscosity changes and depends only on density variations during its travel down the cooling slope. (viii) The values of viscosity of mush at various temperatures were adopted from Ref 16. (ix) Reynolds number at a given point in the cooling slope was calculated using the following formula:

3 Table 1 The chemical composition (wt.%) of the LM25 alloy used in the present study Si Mg Fe Cu Mn Ti Zn Al Balance R e ¼ v iq i l i ; ðeq 4Þ g i where v i is the velocity of mush at a given point (calculated using Eq 3); q i the density of mush at a given point; l i the length traveled from the pouring point up to a given point; g i the viscosity of mush at a given point; g the acceleration due to gravity; q e the density of the mush at solidus temperature (values adopted from Ref 15); q l the density of the liquid at liquidus temperature (values adopted from Ref 15); and f si is the fraction of solid at a given point in the cooling slope (values adopted from Ref 15). 3. Results and Discussion Fig. 2 Sigmoidal fit of Reynolds number calculated using Eq 4 for the (a) inclination of 10 ; (b) inclination of 10, 20, and 30, indicating the critical Reynolds number; and (c) temperature distribution along the cooling slope along the length of 2 m (indicating the point of turbulent to laminar transition, for 10 inclination) The Reynolds number calculated using Eq 4 was plotted with reference to the temperature. From Fig. 2(a), it can be seen that the Reynolds number increases with decrease in temperature from 620 to 600 C. In other words, the flow of the mush is rather turbulent in this regime as expected in any liquid poured on an inclined slope as the velocity of the mush increases due to gravity factor. Interestingly, it is seen that the Reynolds number drops suddenly at a temperature close to 600 C. This could be the stage where, the natural gravity factor is dominated by the solidification process, as the viscosity of the mush raises up with the increase in the fraction of solid down the cooling slope and hence lowering the turbulence which brings the Reynolds number to a lower value. A zoomed view of the temperature range C (Fig. 2a) has been shown in Fig. 2(b) for 10 inclination, along with the Reynolds number values obtained for inclinations of 20 and 30. It is interesting to see that the Reynolds number decreases in a sigmoidal manner as the mush flows from temperature zone of C. A critical Reynolds number of 2100 was found at a temperature of C. When this temperature range is correlated with the length traveled by the mush, it has been found that laminar flow begins after the mush travels a distance of 1.5 m in the cooling slope for 10 inclination. However, as the angle of inclination increases to 20 and 30 ; the temperature at which the critical Reynolds number transpires would decrease; by decreasing the span of the laminar regime as anticipated. The SEM photomicrograph shown in Fig. 3 represents the microstructure of as-cast of AlSi7Mg alloy which was obtained by quenching the melt without cooling slope processing. This microstructure reveals faceted needles of eutectic silicon in the interdendritic regions of a-al dendrites. The microstructure shown in Fig. 3 has been used to compare the microstructures of the same alloy processed by cooling slope processing method. This comparison enables us to understand the effect of

4 the cooling slope processing on the microstructural evolution. Figure 4(a) and (b) depicts the SEM micrographs of AlSi7Mg alloy processed by cooling slope casting process, described above. Figure 4(a) and (b) indicates the formation of ellipsoidal dendrites of a-al surrounded by fine fibrous eutectic silicon in the interdendritic regions. Detailed observation of the eutectic silicon (Fig. 4c), reveals a micron-long fibrous eutectic silicon with an interlamellar spacing of about 100 nm. On the other hand, nucleation and growth of primary silicon was also observed in the same as-cast sample corresponding to Fig. 4(a)-(c), which is shown in Fig. 5. From the microstructures and the rheological characteristics studied, nucleation and growth mechanism responsible for the microstructural evolution can be suggested as below: As soon as the molten AlSi7Mg alloy is poured into the tilting tube furnace, at the T l point (Fig. 1a), the liquid starts flowing down the cooling slope. As the liquid slides down the slope, nucleation and growth of a-al dendrites starts at the interface between the liquid and the tube (substrate) as demonstrated in the previous work (Ref 12, 14). Hence the solute is rejected upwards along with the heat. Further, as the hotter melt flows over the a-al dendrites, the latter detach from the surface of the tube due to necking, according to the phenomenon demonstrated by Robert et al. (Ref 17) and tend to swim in the liquid flowing down the slope. However, substantial dendritic spheroidization is not observed, since the angle of inclination is not enough to induce adequate shearing action. This could be due to inadequate shear rate during initial stages, as the solid fraction is small. Gradually, these a-al dendrites grow, which increases the solid fraction in the mush. Further, when solute-rich liquid comes in between two or more of such growing dendrites, the liquid is sheared by redistributing the solute atoms in the liquid. This process continues till the flow of the mush transforms into a laminar regime where the solid fraction of a-al dendrites increases and leads to squeezing and shearing of the solute-rich liquid which is subjected to further shearing. The heat generated by the shearing action is expected to raise the liquid-solid interface temperature locally, such situation eventually leads to a decrease in the under-cooling (DT value in the following classic nucleation equation for the energy barrier for nucleation, DG ). Hence there would be a suppression in the nucleation of eutectic to some extent DG ¼ 16pc3 SL T 2 m DHv 2 ð DT 2Þ : The growth of eutectic silicon is rather prevented due to the restriction on the mobility of the silicon atoms due to increasing viscosity of the liquid at this stage, consequently resulting in fine fibrous eutectic silicon when the mush is quenched in water Fig. 3 SEM photomicrograph of the melt quenched directly into water without cooling slope processing Fig. 4 SEM photomicrographs of cooling slope processed AlSi7Mg alloy revealing (a) and (b) the a-al dendrites with fine eutectic silicon in the interdendritic regions and (c) fine fibrous eutectic silicon

5 primary silicon according to CampbellÕs concept (Ref 18), above the eutectic temperature. And, since the flow regime turns to laminar, the silicon dendrites remain in the top layer, which grow during their journey down the cooling slope. These primary silicon dendrites eventually drop along with liquid during quenching. Therefore, the resulting microstructure of a low-convection-cast AlSi7Mg alloy consists of primary a-al, eutectic phase, and primary silicon. 4. Conclusions Following conclusions have been drawn from the present work: The flow regime of the mush in the low-convection cooling slope processing primarily depends on the inclination of the slope. Present study also reveals the flow transition point. Shearing action in the laminar regime is more affective in refining the eutectic phase, which is due to the enhanced nucleation and growth restriction due to high viscosity of the last liquid to solidify. This is because the laminar regime occurs close to the eutectic solidification. Results indicate high amount of solute segregation during low-convection cooling slope process, which is attributed to the layer formation. Normally, cooling slope process enables the refinement of both a-al and eutectic silicon, which may be an alternative to Ti, B, Na, Sr additions to hypoeutectic AlSi7Mg alloy, provided substantial developments are made to the process. Fig. 5 (a) SEM photomicrograph of primary silicon dendrites and (b) EDS spectrum taken on the region shown by arrow, found locally in cooling slope processing specimens (evident in Fig. 4a-c). The effect of shearing action on the eutectic phase is clearly revealed, when the microstructures shown in Fig. 3 and 4(a)-(c) are compared, although the quenching is common in both. In another study (Ref 14), it has been demonstrated that the spheroidization effect of the a-al dendrites increases with the increase in the angle of inclination. This can be attributed to the increase in the shear rate with the angle of inclination from 10 to 30 (Ref 14). On the other hand, a typical nucleation and growth process of dendritic primary silicon phase (Fig. 5a and b) was also found in the samples obtained from the above experiment. The formation of the primary silicon phase in a hypoeutectic Al-Si alloy can be explained as below: During the initial stages of the cooling slope casting the primary a-al dendrites stuck to the tube surface of the cooling slope furnace, start growing upwards by pushing away the solute and heat. Since initial solidification starts from the melt-tube interface region, the solute is rejected upwards, as a result the top layer of the melt is rich in silicon and other solute elements. On the other hand, as the melt surface is expected to have oxide film over it, i.e., the region just below the oxide film is rich in solute silicon. In such condition, oxide would promote the heterogeneous nucleation of References 1. D.B. Spencer, R. Mehrabian, and M.C. Flemings, Rheological Behaviour of Sn-15Pb in the Crystallization Range, Metall. Mater. Trans. B, 1972, 3, p M.C. Flemings, Behaviour of Metal Alloys in the Semi-Solid State, Metall. Mater. Trans. A, 1991, 22(5), p Z. Fan, Semisolid Metal Processing, Int. Mater. Rev., 2002, 47, p D.H. Kirkwood, Semisolid Metal Processing, Int. Mater. Rev., 1994, 39(5), p H.V. Atkinson, Modelling the Semisolid Processing of Metallic Alloys, Prog. Mater Sci., 2005, 50, p A.M. Kliauga and M. Ferrante, Liquid Formation and Microstructural Evolution During Re-heating and Partial Melting of an Extruded A356 Aluminium Alloy, Acta Mater., 2005, 53, p E.A. Vieira and M. Ferrante, Prediction of Rheological Behaviour and Segregation Susceptibility of Semi-Solid Al-Si Alloys by a Simple Back Extrusion Test, Acta Mater., 2005, 53, p S. Nafisi, O. Lashkari, R. Ghomashchi, F. Ajersch, and A. Charette, Microstructure and Rheological Behaviour of Grain Refined and Modified Semi-Solid A356 Al-Si Slurries, Acta Mater., 2006, 54, p F. Taghavi and A. Ghassemi, Study on the Effects of the Length and Angle of Inclined Plate on the Thixotropic Microstructure of A356 Aluminium Alloy, Mater. Des., 2009, 30, p Y. Birol, Cooling Slope Casting and Thixoforming of Hypereutectic A390 Alloy, J. Mater. Process. Technol., 2008, 207, p T. Haga and P. Kapranos, Billetless Simple Thixo-Forming Process, J. Mater. Process. Technol., 2002, , p E.C. Legoretta, H.V. Atkinson, and H. Jones, Cooling Slope Casting to Obtain Thyrotrophic Feedstock II: Observations with A356 Alloy, J. Mater. Sci., 2008, 43, p

6 13. T. Haga and P. Kapranos, Simple Rheocasting Processes, J. Mater. Process. Technol., 2002, 118, p J.S. Sunitha, V. Kumar, N.S. Barekar, K. Biswas, and B.K. Dhindaw, Microstructural Evolution Under Low Shear Rates During Rheo Processing of LM25 Alloy, J. Mater. Eng. Perform., 2012, doi: /s K.C. Mills, Recommended Values of Thermo-Physical Properties for Selected Commercial Alloys, 2nd ed., Woohead Publishing Ltd., Cambridge, 2002, p M. Mada and F. Ajersch, Rheological Model of Semi-Solid A356-SiC Composite Alloys. Part-II, Dissociation of Agglomerate Structures During Shear, Mater. Sci. Eng., A, 1996, 212, p M.H. Robert, E.J. Zoqui, F. Tanabe, and T. Motegi, Producing Thixotropic Semi-Solid A356 Alloy-Microstructure Formation x Forming Behaviour, J. Achiev. Mater. Manuf. Eng., 2007, 20(1 2), p J. Campbell, Complete Casting Hand Book-Metal Casting Processes, Techniques and Design, 2011, vol 1, p , B