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The effect of the melt-spinning processing parameters on the rate of cooling
Materials Science and Engineering A323 (2002) 91 – 96 www.elsevier.com/locate/msea
The effect of the melt-spinning processing parameters on the rate of cooling Victor I. Tkatch *, Alexander I. Limanovskii, Sergey N. Denisenko, Sergey G. Rassolov Physics and Engineering Institute of National Academy of Sciences of Ukraine, Rose Luxemburg street, 72, Donetsk 83114, Ukraine Received 5 June 2000; received in revised form 27 February 2001
Abstract The effect of the melt-spinning processing variables such as wheel speed, gas ejection pressure and melt temperature on cooling rates measured by a thermoelectric technique has been investigated. It is found that cooling rates during melt spinning of Fe40Ni40P14B6 ribbons increase in the range 4 ×104 – 6.9×106 K s − 1 while ribbon thickness decreases from 92 to 18 mm with increasing wheel speed from 9 to 30.5 m s − 1. The resultant strong dependence of the rate of cooling on ribbon thickness (in this case T: 8d − 3.1) is obtained. Cooling rates and ribbon thickness are found to increase with increasing ejection pressure and to decrease as melt superheat grows. As a result of improving thermal contact at the wheel– ribbon interface with increasing gas ejection pressure, the cooling rates of melt-spun ribbons tend to increase with their thickness. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Melt spinning; Rate of cooling; Ribbon thickness; Wheel speed; Ejection pressure; Melt temperature
1. Introduction The unique properties of metallic alloys with metastable structures produced by rapid solidification techniques make this class of materials attractive both for fundamental research and for numerous industrial applications. However, there are several obstacles limiting a proper understanding of the nature and properties of the non-equilibrium structures as well as extended practical application of rapidly solidified alloys. One such limitation is due to the strong dependence of structure and physical properties of rapidly solidified metallic alloys on their thermal history, which in turn, is a function of the parameters of the cooling process. It is clear, therefore, that thorough control of the cooling rates during rapid solidification processing is an important problem both from scientific and engineering points of view. Among various rapid solidification techniques the melt-spinning technique is the most com* Corresponding author. Tel.: + 380-622-557726; fax: + 380-622510703. E-mail address: [email protected] (V.I. Tkatch).
monly used at present. In this process, a jet of liquid metal is ejected from a nozzle and impinges on the outer surface of a moving substrate (rotating wheel), where a thin layer is formed from a melt puddle and rapidly solidifies as a continuous ribbon. Depending on the nozzle-to-substrate distance, g, the melt-spinning process may be divided on two modifications: chillblock melt spinning (CBMS) [1] and planar flow casting (PFC) [2]. In the CBMS process, a free jet of liquid metal is ejected from a circular nozzle positioned at a sufficiently large (above several millimeters) nozzle – wheel gap, while in the PFC technique rectangularshaped nozzles at small gap distances (gB 1 mm) are used. In the latter process (used on an industrial scale) the melt puddle is constrained between the nozzle lips and the wheel which allows the production of ribbons up to a few tens of centimeters in width. The problem of the influence of melt-spinning process variables on the structure and physical properties of rapidly quenched (mainly amorphous) alloys has been discussed in many papers and reviews (e.g. [3,4]). It has been shown that many physical properties of melt-spun ribbons as well as the thermal stability of
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non-equilibrium structures sensitively depend on the values of the processing parameters such as the wheel speed, VS, gas ejection pressure, PE, melt temperature, TE, nozzle –wheel gap etc. In turn, it has also been established that variations of these parameters change the geometric dimensions and primarily the thickness of the ribbons, d [1,2,5– 10]. For example, Fiedler et a1. [8] have obtained for the PFC method the empirical rela1 1/2 1/4 tionship d8 V − . But this relationship does not S PE g account for the effect of TE and, besides, the experimental data of Huang [7] show that this expression is valid only in a limited range of ejection pressure values. It should be noted, that for many reasons the thickness is the most important parameter of melt-spun ribbons and its value determines the rate of cooling to a great extent [11]. Certainly, the most correct way to control the thermal history of the melt-spun ribbons during processing is the direct measurement of cooling rates. However, experimental studies of thermal regimes in the meltspinning process are difficult due to both short times of ribbon solidification and high velocities of the quenching wheel. Therefore, a few direct measurements using mainly photocalorimetric methods have been made (e.g. [9,10,12,13]). This technique allows the temperature to be recorded (within 9 50 K) at the free (upper) side of the ribbon in the CBMS method (when the nozzle– wheel distance is relatively large). It has been established that the rates of cooling in the melt-spinning process range from 4× 104 to 5×106 K s − 1 and depend mainly on the wheel velocity, while variations of PE and TE have no observable effect on T: [10]. As far as we know, the thermal regime of the PFC process has not been studied systematically until now. Recently, we have presented the results of direct measurements of the cooling rate in the melt-spinning process by a thermoelectric method [14]. This technique allows the measurements at the contact side of the ribbon during its formation and solidification and may be used, in particular, for the analysis of the thermal regime in the PFC process. The studies carried out at small values of the nozzle– wheel gap have confirmed the strong influence of the wheel velocity on the ribbon thickness, rate of cooling and on the heat transfer coefficient of the wheel– ribbon interface. The aim of the present paper is to provide further experimental information concerning the effects of the variables of the melt-spinning process on the rate of cooling.
2. Experimental procedure The experimental studies have been performed using a laboratory scale single roller melt-spinning device operating in open air [14]. Bar charges of about l5 g of easy glass-forming Fe40Ni40P14B6 alloy were placed into
quartz crucibles of internal diameter l4 mm, melted in a resistance furnace under flowing argon and then gas ejected through the nozzle with a circular or slotshaped orifice onto the outer surface of the rotating quenching wheel (22 cm in diameter) made of aluminum bronze. In these experiments particular attention has been paid to control the main processing variables. As a rule, every experimental set has been carried out using a single quartz crucible to minimize the influence of non-strictly controlled dimensions of the nozzle orifice on the rate of cooling. Before each melt spinning run, the outer surface of the quenching wheel was polished with 1200 grit silicon carbide paper and, after the run, the nozzle was cleaned to maintain the original orifice size. The diameter (or slot breadth) of the nozzles was about 1– 1.5 mm and the distance from the tip of the nozzle to the wheel surface was kept at 150 mm throughout the study. The angular speed of the wheel was recorded by a tachometer and the melt temperature was measured by a Pt–PtRh thermocouple immersed in the melt. The thermal regimes of the melt-spinning process were studied by a thermoelectric technique in which the casting wheel was used as one element of the thermocouple [14]. It facilitates a continuous record of the variations of temperature on the underside of the melt puddle under the nozzle and at the contact side of the liquid layer during its cooling and solidification, until the ribbon departs from the wheel surface. The output of the thermocouple was recorded by a computer through a high-frequency analog–digital converter. The rates of cooling were calculated by numerical differentiation with respect to time of the experimentally obtained ‘temperature-time’, T(t), curves. As the measured cooling rates were found to decrease linearly with falling temperature, and the values of T: in the vicinity of the melting point of the alloy investigated (1173 K) were taken for subsequent analysis. A detailed description of this technique has been presented previously [14]. The ribbon thickness was measured by an optical micrometer to 9 1 mm. It should be noted that despite an eccentricity of the wheel, which did not exceed 10 mm, variations of thickness along the ribbon length of 92 mm were observed. The structure of the asquenched ribbons was checked by X-ray diffraction (Co Ka-radiation) and the wetting patterns on the ribbon roll-contact surface were examined in each case by optical microscopy at low magnifications. In order to study the influence of the processing conditions on the thickness of the melt-spun ribbons and on the cooling rates, a number of experimental sets have been carried out, in which one parameter (wheel surface speed, gas ejection overpressure or melt temperature) was changed while the others were fixed. In these sets, the wheel surface speed was varied from
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9 to 30.5 ms − 1, the gas ejection pressures were 12 to 92 kPa and the melt temperature ranged from 1210 to 1380 K, i.e. from 37 to 207 K above the melting temperature for Fe40Ni40P14B6 alloy. The resultant melt-spun ribbons were about 5– 10 m in length, 1.5– 5.0 mm wide, and 18– 92 mm thick depending on the values of the processing variables.
3. Results and discussion Fig. 1 shows the thickness of as-quenched ribbons as a function of the wheel surface speed (open symbols). During these castings, the ejection pressure, PE, and melt temperature, TE, were kept at 30 kPa and 1273 K, respectively. As can be seen from Fig. 1, increasing the wheel surface speed from 9 to 30.5 ms − 1 results in
Fig. 1. Variation of thickness, d, ( ) and cooling rate at melting temperature, T: , () of as-quenched Fe40Ni40P14B6 ribbons with wheel surface speed, VS, at constant ejection pressure PE = 30 kPa and melt ejection temperature TE = 1273 K. The dashed lines are drawn for clarity.
Fig. 2. The data from Fig. 1 plotted as log –log dependence T: vs d (). The dashed line with a slope − 3.1 90.2 is the least-square best fit. The open symbols show the cooling rates of 316L stainless steel () and Ni–5 wt.% Al alloy () measured by photocalorimetric technique [16].
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decrease of the ribbon thickness from 92 to 18 mm. These data are in reasonable agreement with those obtained by both the CBMS and PFC techniques (e.g. [5,8–10]). It should be noted, however, that replotting of the data in Fig. 1 as d against 1/VS (not shown) gives a straight line, but in contrast with the data of Fiedler et al. [8] and with the above-mentioned empirical relationship for the PFC method, this line does not pass through the origin of the plot. A better fit for the 1.3 present data is found to be d8 V − . The rates of S cooling estimated at the melting temperature from experimental T(t) curves increase (almost by three orders of magnitude) with increasing surface wheel speed, as shown in Fig. 1 (solid symbols). Using the experimentally obtained dependencies of the ribbon thickness as well as the rate of cooling on the wheel speed presented in Fig. 1, we have plotted the curve T: versus d on a logarithmic scale (Fig. 2). The straight line in Fig. 2 is the least-square best fit of the experimental data. The slope of this line is −3.190.1. In other words, in the case when the thickness of melt-spun ribbons was varied by changing of the wheel surface speed, T: 8d − 3.1. This experimentally derived dependence of the cooling rate on the ribbon thickness is stronger than that predicted from the numerical analysis [11] for the case of ideal cooling (T: 8d − 2). Such a strong dependence results from two effects: increasing of the wheel speed leads to decreasing of the ribbon thickness and simultaneously to enhancing the heat transfer coefficient at the wheel–ribbon interface [6,9,10,14]. Greer [15] has estimated the value of the exponent in the T: (d) dependence to be between 2 and 4 in order to describe experimental data for the crystallization kinetics of Fe80B20 amorphous melt-spun ribbons of various thicknesses. The rates of cooling at the roll-contact (bottom) surface of the Fe40Ni40P14B6 melt-spun ribbons measured in the present study by a thermoelectric technique are compared in Fig. 2 with values of T: at the free surfaces of 316L stainless steel [9] and Ni–5 wt.% Al alloy [10] ribbons, derived from photocalorimetric measurements and presented as functions of d in Ref. [16]. Accounting that the quoted data are obtained with a copper wheel with a higher thermal conductivity than is used in our studies the agreement is quite reasonable. X-ray diffraction studies of the rapidly solidified Fe40Ni40P14B6 ribbons obtained at various wheel speeds showed that up to d 50 mm the ribbons had an amorphous structure, in the range from 50 to 65 mm — a partially crystallized structure, while the 92 mm-thick ribbon was completely crystalline. The T(t) cooling curves of ribbons with d\55 mm show a clear evidence of recalescence which follows the initial undercooling [14]. The observed values of the initial undercooling were relatively small (75–110 K), which implies that crystallization of the Fe40Ni40P14B6 melt during rapid
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Fig. 3. Variation of thickness of Fe40Ni40P14B6 melt-spun ribbons with ejection pressure ( ) at constant wheel speed of 24 m s − 1 and melt temperature of 1273 K and with melt superheat ( ) at VS =24 m s − 1 and PE =30 kPa. The dashed lines are drawn for better visualization.
Fig. 4. Variation of cooling rate at melting temperature of Fe40Ni40P14B6 melt-spun ribbons with ejection pressure () and with melt superheat () at the other process variables as in Fig. 3. The dashed lines are drawn for better visualization.
cooling occurs by heterogeneous nucleation. Note that the phase composition of the partially and completely crystallized as-cast ribbons {g-(Fe,Ni) + (Fe,Ni)3(PB)} was similar to that of ribbons crystallized from a glassy state [17]. The effects of the ejection overpressure and the melt superheat on the thickness of the Fe40Ni40P14B6 meltspun ribbons are given in Fig. 3. Both these sets of experiments have been carried out at a wheel speed of 24 m s − 1 and in the former set the melt ejection temperature was maintained at 1273 K while the gas ejection pressure of 30 kPa was applied in the latter set. As seen from the data presented in Fig. 3 by open symbols, the ribbon thickness monotonically increases from 23 to
40 mm with applied ejection pressure ranging from 12 to 91 kPa, in accordance with previously published data [5,7,8] although, without saturation, as observed in Ref. [7]. Note that the ribbons cast at the lower ejection pressures become porous, while at PE \ 100 kPa the melt puddle was ‘blown out’ from the nozzle–wheel gap. It should also be noted that the experimental data d(PE) presented in Fig. 3 may be rather satisfactorily approximated by a linear function of d versus P 1/2 E , in accordance with the predictions of Bernoulli’s equation used in Refs. [5,8]. The ribbon thickness as a function of the melt ejection temperature is presented in Fig. 3 by solid symbols. As can be seen, these experimental data have a relatively large scatter, partially due to the impossibility of using one nozzle in this experimental set. Nevertheless, it is apparent that d tends to decrease with the melt superheat. Such behavior of d versus TE was qualitatively noted earlier [8]. Fiedler et al. tried to explain this effect on the basis of an increasing surface tension of several Fe-based liquid alloys (including Fe40Ni40P14B6 melt) with temperature. On the other hand, as has been experimentally shown [14], the ribbon formation under the present experimental conditions is dominated by a momentum transfer mechanism firstly proposed by Vincent and Davies [18]. In such a case one may expect that a thickness of the melt layer extracted by a quenching wheel from the melt puddle with lower viscosity would be smaller. Note that all ribbons obtained in these two last experimental sets were amorphous. The effect of the variations of gas ejection pressure and melt superheat on the rate of cooling of melt-spun ribbons are presented in Fig. 4. It follows from the experimental data that the value of T: (open symbols) increases with applied pressures up to 40 kPa and then tends to saturation. Such behavior may result from two effects. The observations by optical microscopy of the wetting pattern at the contact sides of the ribbons showed that at higher ejection pressures both the total area and the sizes of the so-called ‘air-pockets’ [6] (places where liquid alloy did not wet the quenching wheel surface) decrease. This effect of ejection pressure (as well as other processing parameters) on the ribbon surface quality has been described in a number of papers [6–8]. We also observed at experimental T(t) curves that increasing of the ejection pressure resulted in increasing the contact time (length) between the ribbon and the wheel surface which provides further evidence of improved heat transfer conditions. Hence, increasing the ejection pressure, on one hand, leads to an increasing heat transfer rate, but, on the other hand, the monotonically increasing ribbon thickness tends to decrease T: . The estimation of the cooling rates of the melt-spun ribbon quenched from various temperatures shows that the values of T: decrease with melt superheat (solid
V.I. Tkatch et al. / Materials Science and Engineering A323 (2002) 91–96
symbols in Fig. 4). This result is at first sight somewhat unexpected in view of approximate twofold decrease in ribbon thickness with increasing in TE, as shown in Fig. 3. On the other hand, from analysis of the original T(t) cooling curves it follows that the values of the initial rate of cooling (at T near TE) increase with the melt superheat, but then rapidly decrease with temperature while the values of T: presented in Fig. 4 were calculated at T=Tm. The other possible reason for the cooling rate decrease with superheat may be increasing roughness of the contact surface of the ribbon as has been observed by Ferrara et al. [19]. We are also aware that a relatively large scatter of the T: versus TE data due to a possible influence of weakly controlled variations in the nozzle dimensions as well as to oxidation effects in these experiments must be taken into account in further studies. The detailed studies of these effects are now in progress. Nevertheless, the data presented in Fig. 4 clearly indicate that variations both in the ejection pressure and in the melt temperature allow control of the cooling rates in melt-spinning process with small nozzle–wheel gap sizes. It is interesting to note that the similar investigations for the CBMS process showed no measurable effect of variations in PE and TE on the cooling rates [9,10]. In Fig. 5 the experimental data shown in Figs. 3 and 4 are presented as dependencies of the rates of cooling on ribbon thickness for cases when d was varied by changing the ejection pressure or the melt temperature. In these cases the T: (d) behavior is quite different from that presented in Fig. 2: the cooling rates, in general, tend to increase with ribbon thickness. The data presented in Fig. 5 indicate, in part, that estimations of the T: variations in melt-spun ribbons, based solely on the variations of the thickness, may in some cases be erroneous. Moreover, from these data it follows that using enhanced melt ejection pressures and reduced superheat
Fig. 5. The data from Figs. 3 and 4 plotted as functions of log T: vs d changed by variation in gas ejection pressure () and in melt ejection temperature (). The dashed lines are drawn for better visualization.
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temperatures allows one to reach higher cooling rates in thicker ribbons when produced by PFC method. The last problem is important, for example, in manufacturing ribbons for stacked core power transformers [20].
4. Conclusions A thermoelectric method has been used to study the effect of the main melt-spinning process variables on cooling rates of Fe40Ni40P14B6 alloy ribbons. It is found that the cooling rate at the vicinity of the melting temperature monotonically increases from 4×104 to 6.9× 106 by increasing the wheel surface speed from 9 to 30.5 m s − 1, while the ribbon thickness decreases from 92 to 18 mm. This results in a dependence of the cooling rate on ribbon thickness as strong as T: 8 d − 3.1. Both the thickness of melt-spun ribbons and the cooling rates increase with applied ejection pressure at fixed wheel speed and melt temperature. Due to improved thermal contact at the wheel–ribbon interface in this case the rate of cooling grows with ribbon thickness. Increasing the melt temperature leads to smaller ribbon thicknesses and lower cooling rates at the melting temperature.
Acknowledgements The authors would like to thank V.P. Khalievskii for the preparation of the alloys and technical assistance. This work was partially supported by the Foundation of Fundamental Investigations of Ukraine through Grant No. 2.4/220-97.
References [1] H.H. Liebermann, IEEE Trans. Magn. MAG: B 22 D 11/06, 22.10.76/6.03.79-15 (1979) 1393. [2] M.C. Narasimhan, US Patent No. 4 142 571, March 6, 1979. [3] A. Lovas, E. Kisdi-Koszo, L. Potocky, L. Novak, J. Mater. Sci. 22 (1987) 1535. [4] M. Calvo-Dahlborg, Mater. Sci. Eng. A226 – 228 (1997) 833. [5] S. Takayama, T. Oi, J. Appl. Phys. 50 (1979) 4962. [6] S. Huang, H. Fiedler, Mater. Sci. Eng. 51 (1981) 39. [7] S. Huang, in: T. Masumoto, K. Suzuki (Eds.), Rapidly Quenched Metals, vol. 1, The Japan Institute of Metals, Sendai, 1982, p. 65. [8] H. Fiedler, H. Muhlbach, G. Stephani, J. Mater. Sci. 19 (1984) 3229. [9] B.P. Bewlay, B. Cantor, Int. J. Rapid Solidification 2 (1986) 107. [10] A.G. Gillen, B. Cantor, Acta Metall. 33 (1985) 1813. [11] R.C. Ruhl, Mater. Sci. Eng. 1 (1967) 313. [12] D.H. Warrington, H.A. Davies, N. Shohoji, in: T. Masumoto, K. Suzuki (Eds.), Rapidly Quenched Metals IV, vol. 1, The Japan Institute of Metals, Sendai, 1982, p. 69. [13] M.J. Tenwick, H.A. Davies, in: S. Steeb, H. Warlimont (Eds.), Rapidly Quenched Metals V, vol. 1, North-Holland, Amsterdam, 1985, p. 67.
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[14] V.I. Tkatch, S.N. Denisenko, O.N. Beloshov, Acta Mater. 45 (1997) 2821. [15] A.L. Greer, Acta Metall. 30 (1982) 171. [16] W.T. Kim, B. Cantor, Scripta Metall. Mater. 24 (1990) 633. [17] V.P. Naberezhnykh, A.I. Limanovskii, V.I. Tkatch, L.V. Kuksa, V.Yu. Kameneva, Fizika Metall. Metalloved. 66 (1988) 169 (in Russian).
[18] J.H. Vincent, H.A. Davies, Solidification Technology in the Foundry and Casthouse 1980, The Metals Society, London, 1983 (p. 153). [19] E. Ferrara, A. Stantero, R. Tiberto, M. Baricco, D. Janickowic, L. Kubicar, P. Duhaj, Mater Sci. Eng. A226 – 228 (1997) 326. [20] S.K. Das, R.L. Bye, E.P. Barth, F. Spaepen, V.R.V. Ramanan, T. Vu, Mater. Sci. Eng. A226 – 228 (1997) 1.
The effect of the melt-spinning processing parameters on the rate of cooling Victor I. Tkatch *, Alexander I. Limanovskii, Sergey N. Denisenko, Sergey G. Rassolov Physics and Engineering Institute of National Academy of Sciences of Ukraine, Rose Luxemburg street, 72, Donetsk 83114, Ukraine Received 5 June 2000; received in revised form 27 February 2001
Abstract The effect of the melt-spinning processing variables such as wheel speed, gas ejection pressure and melt temperature on cooling rates measured by a thermoelectric technique has been investigated. It is found that cooling rates during melt spinning of Fe40Ni40P14B6 ribbons increase in the range 4 ×104 – 6.9×106 K s − 1 while ribbon thickness decreases from 92 to 18 mm with increasing wheel speed from 9 to 30.5 m s − 1. The resultant strong dependence of the rate of cooling on ribbon thickness (in this case T: 8d − 3.1) is obtained. Cooling rates and ribbon thickness are found to increase with increasing ejection pressure and to decrease as melt superheat grows. As a result of improving thermal contact at the wheel– ribbon interface with increasing gas ejection pressure, the cooling rates of melt-spun ribbons tend to increase with their thickness. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Melt spinning; Rate of cooling; Ribbon thickness; Wheel speed; Ejection pressure; Melt temperature
1. Introduction The unique properties of metallic alloys with metastable structures produced by rapid solidification techniques make this class of materials attractive both for fundamental research and for numerous industrial applications. However, there are several obstacles limiting a proper understanding of the nature and properties of the non-equilibrium structures as well as extended practical application of rapidly solidified alloys. One such limitation is due to the strong dependence of structure and physical properties of rapidly solidified metallic alloys on their thermal history, which in turn, is a function of the parameters of the cooling process. It is clear, therefore, that thorough control of the cooling rates during rapid solidification processing is an important problem both from scientific and engineering points of view. Among various rapid solidification techniques the melt-spinning technique is the most com* Corresponding author. Tel.: + 380-622-557726; fax: + 380-622510703. E-mail address: [email protected] (V.I. Tkatch).
monly used at present. In this process, a jet of liquid metal is ejected from a nozzle and impinges on the outer surface of a moving substrate (rotating wheel), where a thin layer is formed from a melt puddle and rapidly solidifies as a continuous ribbon. Depending on the nozzle-to-substrate distance, g, the melt-spinning process may be divided on two modifications: chillblock melt spinning (CBMS) [1] and planar flow casting (PFC) [2]. In the CBMS process, a free jet of liquid metal is ejected from a circular nozzle positioned at a sufficiently large (above several millimeters) nozzle – wheel gap, while in the PFC technique rectangularshaped nozzles at small gap distances (gB 1 mm) are used. In the latter process (used on an industrial scale) the melt puddle is constrained between the nozzle lips and the wheel which allows the production of ribbons up to a few tens of centimeters in width. The problem of the influence of melt-spinning process variables on the structure and physical properties of rapidly quenched (mainly amorphous) alloys has been discussed in many papers and reviews (e.g. [3,4]). It has been shown that many physical properties of melt-spun ribbons as well as the thermal stability of
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non-equilibrium structures sensitively depend on the values of the processing parameters such as the wheel speed, VS, gas ejection pressure, PE, melt temperature, TE, nozzle –wheel gap etc. In turn, it has also been established that variations of these parameters change the geometric dimensions and primarily the thickness of the ribbons, d [1,2,5– 10]. For example, Fiedler et a1. [8] have obtained for the PFC method the empirical rela1 1/2 1/4 tionship d8 V − . But this relationship does not S PE g account for the effect of TE and, besides, the experimental data of Huang [7] show that this expression is valid only in a limited range of ejection pressure values. It should be noted, that for many reasons the thickness is the most important parameter of melt-spun ribbons and its value determines the rate of cooling to a great extent [11]. Certainly, the most correct way to control the thermal history of the melt-spun ribbons during processing is the direct measurement of cooling rates. However, experimental studies of thermal regimes in the meltspinning process are difficult due to both short times of ribbon solidification and high velocities of the quenching wheel. Therefore, a few direct measurements using mainly photocalorimetric methods have been made (e.g. [9,10,12,13]). This technique allows the temperature to be recorded (within 9 50 K) at the free (upper) side of the ribbon in the CBMS method (when the nozzle– wheel distance is relatively large). It has been established that the rates of cooling in the melt-spinning process range from 4× 104 to 5×106 K s − 1 and depend mainly on the wheel velocity, while variations of PE and TE have no observable effect on T: [10]. As far as we know, the thermal regime of the PFC process has not been studied systematically until now. Recently, we have presented the results of direct measurements of the cooling rate in the melt-spinning process by a thermoelectric method [14]. This technique allows the measurements at the contact side of the ribbon during its formation and solidification and may be used, in particular, for the analysis of the thermal regime in the PFC process. The studies carried out at small values of the nozzle– wheel gap have confirmed the strong influence of the wheel velocity on the ribbon thickness, rate of cooling and on the heat transfer coefficient of the wheel– ribbon interface. The aim of the present paper is to provide further experimental information concerning the effects of the variables of the melt-spinning process on the rate of cooling.
2. Experimental procedure The experimental studies have been performed using a laboratory scale single roller melt-spinning device operating in open air [14]. Bar charges of about l5 g of easy glass-forming Fe40Ni40P14B6 alloy were placed into
quartz crucibles of internal diameter l4 mm, melted in a resistance furnace under flowing argon and then gas ejected through the nozzle with a circular or slotshaped orifice onto the outer surface of the rotating quenching wheel (22 cm in diameter) made of aluminum bronze. In these experiments particular attention has been paid to control the main processing variables. As a rule, every experimental set has been carried out using a single quartz crucible to minimize the influence of non-strictly controlled dimensions of the nozzle orifice on the rate of cooling. Before each melt spinning run, the outer surface of the quenching wheel was polished with 1200 grit silicon carbide paper and, after the run, the nozzle was cleaned to maintain the original orifice size. The diameter (or slot breadth) of the nozzles was about 1– 1.5 mm and the distance from the tip of the nozzle to the wheel surface was kept at 150 mm throughout the study. The angular speed of the wheel was recorded by a tachometer and the melt temperature was measured by a Pt–PtRh thermocouple immersed in the melt. The thermal regimes of the melt-spinning process were studied by a thermoelectric technique in which the casting wheel was used as one element of the thermocouple [14]. It facilitates a continuous record of the variations of temperature on the underside of the melt puddle under the nozzle and at the contact side of the liquid layer during its cooling and solidification, until the ribbon departs from the wheel surface. The output of the thermocouple was recorded by a computer through a high-frequency analog–digital converter. The rates of cooling were calculated by numerical differentiation with respect to time of the experimentally obtained ‘temperature-time’, T(t), curves. As the measured cooling rates were found to decrease linearly with falling temperature, and the values of T: in the vicinity of the melting point of the alloy investigated (1173 K) were taken for subsequent analysis. A detailed description of this technique has been presented previously [14]. The ribbon thickness was measured by an optical micrometer to 9 1 mm. It should be noted that despite an eccentricity of the wheel, which did not exceed 10 mm, variations of thickness along the ribbon length of 92 mm were observed. The structure of the asquenched ribbons was checked by X-ray diffraction (Co Ka-radiation) and the wetting patterns on the ribbon roll-contact surface were examined in each case by optical microscopy at low magnifications. In order to study the influence of the processing conditions on the thickness of the melt-spun ribbons and on the cooling rates, a number of experimental sets have been carried out, in which one parameter (wheel surface speed, gas ejection overpressure or melt temperature) was changed while the others were fixed. In these sets, the wheel surface speed was varied from
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9 to 30.5 ms − 1, the gas ejection pressures were 12 to 92 kPa and the melt temperature ranged from 1210 to 1380 K, i.e. from 37 to 207 K above the melting temperature for Fe40Ni40P14B6 alloy. The resultant melt-spun ribbons were about 5– 10 m in length, 1.5– 5.0 mm wide, and 18– 92 mm thick depending on the values of the processing variables.
3. Results and discussion Fig. 1 shows the thickness of as-quenched ribbons as a function of the wheel surface speed (open symbols). During these castings, the ejection pressure, PE, and melt temperature, TE, were kept at 30 kPa and 1273 K, respectively. As can be seen from Fig. 1, increasing the wheel surface speed from 9 to 30.5 ms − 1 results in
Fig. 1. Variation of thickness, d, ( ) and cooling rate at melting temperature, T: , () of as-quenched Fe40Ni40P14B6 ribbons with wheel surface speed, VS, at constant ejection pressure PE = 30 kPa and melt ejection temperature TE = 1273 K. The dashed lines are drawn for clarity.
Fig. 2. The data from Fig. 1 plotted as log –log dependence T: vs d (). The dashed line with a slope − 3.1 90.2 is the least-square best fit. The open symbols show the cooling rates of 316L stainless steel () and Ni–5 wt.% Al alloy () measured by photocalorimetric technique [16].
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decrease of the ribbon thickness from 92 to 18 mm. These data are in reasonable agreement with those obtained by both the CBMS and PFC techniques (e.g. [5,8–10]). It should be noted, however, that replotting of the data in Fig. 1 as d against 1/VS (not shown) gives a straight line, but in contrast with the data of Fiedler et al. [8] and with the above-mentioned empirical relationship for the PFC method, this line does not pass through the origin of the plot. A better fit for the 1.3 present data is found to be d8 V − . The rates of S cooling estimated at the melting temperature from experimental T(t) curves increase (almost by three orders of magnitude) with increasing surface wheel speed, as shown in Fig. 1 (solid symbols). Using the experimentally obtained dependencies of the ribbon thickness as well as the rate of cooling on the wheel speed presented in Fig. 1, we have plotted the curve T: versus d on a logarithmic scale (Fig. 2). The straight line in Fig. 2 is the least-square best fit of the experimental data. The slope of this line is −3.190.1. In other words, in the case when the thickness of melt-spun ribbons was varied by changing of the wheel surface speed, T: 8d − 3.1. This experimentally derived dependence of the cooling rate on the ribbon thickness is stronger than that predicted from the numerical analysis [11] for the case of ideal cooling (T: 8d − 2). Such a strong dependence results from two effects: increasing of the wheel speed leads to decreasing of the ribbon thickness and simultaneously to enhancing the heat transfer coefficient at the wheel–ribbon interface [6,9,10,14]. Greer [15] has estimated the value of the exponent in the T: (d) dependence to be between 2 and 4 in order to describe experimental data for the crystallization kinetics of Fe80B20 amorphous melt-spun ribbons of various thicknesses. The rates of cooling at the roll-contact (bottom) surface of the Fe40Ni40P14B6 melt-spun ribbons measured in the present study by a thermoelectric technique are compared in Fig. 2 with values of T: at the free surfaces of 316L stainless steel [9] and Ni–5 wt.% Al alloy [10] ribbons, derived from photocalorimetric measurements and presented as functions of d in Ref. [16]. Accounting that the quoted data are obtained with a copper wheel with a higher thermal conductivity than is used in our studies the agreement is quite reasonable. X-ray diffraction studies of the rapidly solidified Fe40Ni40P14B6 ribbons obtained at various wheel speeds showed that up to d 50 mm the ribbons had an amorphous structure, in the range from 50 to 65 mm — a partially crystallized structure, while the 92 mm-thick ribbon was completely crystalline. The T(t) cooling curves of ribbons with d\55 mm show a clear evidence of recalescence which follows the initial undercooling [14]. The observed values of the initial undercooling were relatively small (75–110 K), which implies that crystallization of the Fe40Ni40P14B6 melt during rapid
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Fig. 3. Variation of thickness of Fe40Ni40P14B6 melt-spun ribbons with ejection pressure ( ) at constant wheel speed of 24 m s − 1 and melt temperature of 1273 K and with melt superheat ( ) at VS =24 m s − 1 and PE =30 kPa. The dashed lines are drawn for better visualization.
Fig. 4. Variation of cooling rate at melting temperature of Fe40Ni40P14B6 melt-spun ribbons with ejection pressure () and with melt superheat () at the other process variables as in Fig. 3. The dashed lines are drawn for better visualization.
cooling occurs by heterogeneous nucleation. Note that the phase composition of the partially and completely crystallized as-cast ribbons {g-(Fe,Ni) + (Fe,Ni)3(PB)} was similar to that of ribbons crystallized from a glassy state [17]. The effects of the ejection overpressure and the melt superheat on the thickness of the Fe40Ni40P14B6 meltspun ribbons are given in Fig. 3. Both these sets of experiments have been carried out at a wheel speed of 24 m s − 1 and in the former set the melt ejection temperature was maintained at 1273 K while the gas ejection pressure of 30 kPa was applied in the latter set. As seen from the data presented in Fig. 3 by open symbols, the ribbon thickness monotonically increases from 23 to
40 mm with applied ejection pressure ranging from 12 to 91 kPa, in accordance with previously published data [5,7,8] although, without saturation, as observed in Ref. [7]. Note that the ribbons cast at the lower ejection pressures become porous, while at PE \ 100 kPa the melt puddle was ‘blown out’ from the nozzle–wheel gap. It should also be noted that the experimental data d(PE) presented in Fig. 3 may be rather satisfactorily approximated by a linear function of d versus P 1/2 E , in accordance with the predictions of Bernoulli’s equation used in Refs. [5,8]. The ribbon thickness as a function of the melt ejection temperature is presented in Fig. 3 by solid symbols. As can be seen, these experimental data have a relatively large scatter, partially due to the impossibility of using one nozzle in this experimental set. Nevertheless, it is apparent that d tends to decrease with the melt superheat. Such behavior of d versus TE was qualitatively noted earlier [8]. Fiedler et al. tried to explain this effect on the basis of an increasing surface tension of several Fe-based liquid alloys (including Fe40Ni40P14B6 melt) with temperature. On the other hand, as has been experimentally shown [14], the ribbon formation under the present experimental conditions is dominated by a momentum transfer mechanism firstly proposed by Vincent and Davies [18]. In such a case one may expect that a thickness of the melt layer extracted by a quenching wheel from the melt puddle with lower viscosity would be smaller. Note that all ribbons obtained in these two last experimental sets were amorphous. The effect of the variations of gas ejection pressure and melt superheat on the rate of cooling of melt-spun ribbons are presented in Fig. 4. It follows from the experimental data that the value of T: (open symbols) increases with applied pressures up to 40 kPa and then tends to saturation. Such behavior may result from two effects. The observations by optical microscopy of the wetting pattern at the contact sides of the ribbons showed that at higher ejection pressures both the total area and the sizes of the so-called ‘air-pockets’ [6] (places where liquid alloy did not wet the quenching wheel surface) decrease. This effect of ejection pressure (as well as other processing parameters) on the ribbon surface quality has been described in a number of papers [6–8]. We also observed at experimental T(t) curves that increasing of the ejection pressure resulted in increasing the contact time (length) between the ribbon and the wheel surface which provides further evidence of improved heat transfer conditions. Hence, increasing the ejection pressure, on one hand, leads to an increasing heat transfer rate, but, on the other hand, the monotonically increasing ribbon thickness tends to decrease T: . The estimation of the cooling rates of the melt-spun ribbon quenched from various temperatures shows that the values of T: decrease with melt superheat (solid
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symbols in Fig. 4). This result is at first sight somewhat unexpected in view of approximate twofold decrease in ribbon thickness with increasing in TE, as shown in Fig. 3. On the other hand, from analysis of the original T(t) cooling curves it follows that the values of the initial rate of cooling (at T near TE) increase with the melt superheat, but then rapidly decrease with temperature while the values of T: presented in Fig. 4 were calculated at T=Tm. The other possible reason for the cooling rate decrease with superheat may be increasing roughness of the contact surface of the ribbon as has been observed by Ferrara et al. [19]. We are also aware that a relatively large scatter of the T: versus TE data due to a possible influence of weakly controlled variations in the nozzle dimensions as well as to oxidation effects in these experiments must be taken into account in further studies. The detailed studies of these effects are now in progress. Nevertheless, the data presented in Fig. 4 clearly indicate that variations both in the ejection pressure and in the melt temperature allow control of the cooling rates in melt-spinning process with small nozzle–wheel gap sizes. It is interesting to note that the similar investigations for the CBMS process showed no measurable effect of variations in PE and TE on the cooling rates [9,10]. In Fig. 5 the experimental data shown in Figs. 3 and 4 are presented as dependencies of the rates of cooling on ribbon thickness for cases when d was varied by changing the ejection pressure or the melt temperature. In these cases the T: (d) behavior is quite different from that presented in Fig. 2: the cooling rates, in general, tend to increase with ribbon thickness. The data presented in Fig. 5 indicate, in part, that estimations of the T: variations in melt-spun ribbons, based solely on the variations of the thickness, may in some cases be erroneous. Moreover, from these data it follows that using enhanced melt ejection pressures and reduced superheat
Fig. 5. The data from Figs. 3 and 4 plotted as functions of log T: vs d changed by variation in gas ejection pressure () and in melt ejection temperature (). The dashed lines are drawn for better visualization.
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temperatures allows one to reach higher cooling rates in thicker ribbons when produced by PFC method. The last problem is important, for example, in manufacturing ribbons for stacked core power transformers [20].
4. Conclusions A thermoelectric method has been used to study the effect of the main melt-spinning process variables on cooling rates of Fe40Ni40P14B6 alloy ribbons. It is found that the cooling rate at the vicinity of the melting temperature monotonically increases from 4×104 to 6.9× 106 by increasing the wheel surface speed from 9 to 30.5 m s − 1, while the ribbon thickness decreases from 92 to 18 mm. This results in a dependence of the cooling rate on ribbon thickness as strong as T: 8 d − 3.1. Both the thickness of melt-spun ribbons and the cooling rates increase with applied ejection pressure at fixed wheel speed and melt temperature. Due to improved thermal contact at the wheel–ribbon interface in this case the rate of cooling grows with ribbon thickness. Increasing the melt temperature leads to smaller ribbon thicknesses and lower cooling rates at the melting temperature.
Acknowledgements The authors would like to thank V.P. Khalievskii for the preparation of the alloys and technical assistance. This work was partially supported by the Foundation of Fundamental Investigations of Ukraine through Grant No. 2.4/220-97.
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