- Email: [email protected]
Adhesion interaction on melt extraction from pendant drop
Materials Science and Engineering A304–306 (2001) 598–603
Adhesion interaction on melt extraction from pendant drop W. Archangelsky, S.V. Prischepov, V.A. Vasiliev∗ MATI-Russian State Technology University, Polbina street 45-2, Moscow 109383, Russia
Abstract A technique of pendant drop melt extraction (PDME) is reported and certain results describing thin stainless steel fibers obtained by means of PDME were studied. Adhesion interaction between metal melt and disc-crystallizer was also considered. The adhesion work was established to be approximately 1–2 J/m2 . It was shown that melt adhesion to the disc-crystallizer is determined both by physical interaction and chemical processes in contact region. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Melt extraction; Pendant drop technique; Disc-crystallizer materials and regimes of operation; Stainless steel fibers; Advantages and drawbacks
1. Introduction The PDME technique is a variation of traditional melt extraction [1] widely used industrially in many countries. The PDME technique (its plan is shown in the Fig. 1) is a fast melt cooling and setting, which take place at the edge of a vertical fast rotating disc forming fibers. Unlike traditional melt extraction methods, the melt is yielding to the disc from the top in the form of a pendant drop produced by melting rod of material. For the majority of alloys resistive heaters (e.g., graphite, churlish metals) are used as a source of heat, though, lasers or some cathode-ray tubes can also be used. Through high disc thermal conductivity and its high wettability for the melt, a very high coefficient of thermal conduction is provided at the boundary of disc and melt. That is why the melt cooling and setting has a very high speed, 105 degree/s [1]. This technique allows to produce metal fibers, alloys or ceramics with the gauge from 200 up to 15 micron and with a factor of cross-section circularity up to 0.8–0.9. Solidified melt is separated through centrifugal force and thermal contraction of the fiber during cooling. The cooling process has three stages. During extraction it can be divided into following (Fig. 1): (1) cooling at the disc decreasing contact with the melt, (2) cooling at the disc with decreasing contact to the melt and (3) cooling in the ambient gas. The highest rate of cooling seems to be achieved during the second stage. Forced convective parameters of fiber formation (thickness, gauge, length, the circularity factor) depend on technological conditions (such as a rate of disc speed, rod velocity
feed, geometry of the disc edge), thermal parameters and physical properties (viscosity, surface tension, oxygen affinity, thermal capacity and conductivity, melting thermal interval) of the melt and disc. The selection of “melt — disc material” is very important. Inability of the melt to dampen the disc results in splattering and in another extreme at angle near 0◦ melt adhesion to the disc takes place. The difficulty of “melt — disc material” selection is even more sophisticated by high dynamic characteristics of the PDME and its irregularity. The disc is continuously cooled by water and the temperature at its edge remains significantly lower compared with the temperature of the melt drop. These circumstances explain why the selection of “melt — disc pair” in most cases has to be done experimentally only. Continuous melt extraction can be interrupted by incisions along the edge of the disc, which allow to obtain discrete fibers. The PDME technique’s prime advantage compared with any traditional melt extraction consists in significantly lower level of impurity which can be achieved owing to potless melting. Moreover, the precision contact between the disc and the melt forms more stable fibers in shape and gauge. However, this method has an essential drawback, namely, extremely unstable drop position at the edge of melting rod which often causes drop disruption and its pouring (Fig. 2). Despite on the PDME apparent simplicity its application necessarily demands safe, reliable and perfect automation in an industrial environment, which implicitly increase the equipment costs. 1.1. Experimental methods
∗
Corresponding author. Present address: MATI-Russian State Technology University, Russian Innovation Consortium, CHIEF Group, Petrovka 27, 103767 Moscow, K31, Russia.
In this work stainless steel fibers were obtained at the pilot-industrial plant (shown in Fig. 3) consisting of a high
0921-5093/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 1 5 4 2 - 2
W. Archangelsky et al. / Materials Science and Engineering A304–306 (2001) 598–603
599
0.03%, silicium: 1%, manganese: 2%, phosphorus: 0.045%, sulfur: 0.03%, nickel: 12–15%, chromium: 16–18%, molybdenum: 2–3%. Necessity of obtaining the fibers with diameters 20 micron and less is dictated by the demand in fine purification filter devices in various aggressive media.
2. Results 2.1. Influence of ambient atmosphere
Fig. 1. Scheme of the PDME technique.
Fig. 2. Contact of melt drop with disc-crystallizer: (1) graphite heater; (2) rod and (3) disk.
speed spindle, equipped with two 200 mm diameter discs cooled by water and rods feed mechanism, two graphite heaters and a dressing device to cleaning discs edges. A vacuum system allows to run the fiber production either under vacuum (> to 10−3 Pa), or in inert gas atmosphere with pressure = 105 Pa. The plant permits simultaneous operating with 10 base melt rods of up to 12 mm diameters and length up to 1 m. Stainless steel (SUS316L) rods of 12 mm diameter have been used, with the following composition — carbon:
The influence of ambient gas on the extraction process was investigated using fiber fabrication in vacuum or in argon atmosphere. In vacuum reduced oxides layers are observed on the drop surface increasing the drop stability. Higher surface-tension results in fiber cross-section very close to a circle. Argon airborne gas absorbed on the surface of the disc increases thermal transmission factor along the “melt — disc” bound, which augments the rate of melt cooling and causes a more disperse structure of fibers. This allows to obtain over-saturated solid solutions having an even distribution of alloy elements and to improve the mechanical properties of the fibers. Due to fast vacuum cooling the fiber is obtained with less gauge, for its onflow is controlled by its crystallization. Experiments were carried in vacuum at residual pressures of 01 and 10 Pa. No significant difference connected with reduced pressure was noticed. Melt drops were produced clear and devoid of any oxides upon it’s surface even at reduced pressure of 50 Pa. At disc rotation 3500 rpm the fiber has thickness 24 micron and width of 30 micron. In vacuum liquid metal evaporates and condenses in the disc resulting in increase of melt adhesion on the disc because there is no actual difference between the disc material and the melt resulting in spattering of the drop. It has to bear in mind that increased adhesion along the edge of the disc increases with the heating and it makes purging useless. Applying of abrasives causes rebating of the disc. As a result of fibers adhesion to disc the process has to be stopped after 10–15 min of running time. Thus, further experiments were conducted in argon atmosphere. An argon pressure of 500 Pa was sufficient for inhibition of liquid metal evaporation. 2.2. Influence of disc material
Fig. 3. The plant for metal fibers producing.
At first discs made out of the following materials were tested, aluminum–bronze, brass (composition-metal) and stainless steel. The best results were obtained with the discs of aluminum–bronze. PDME with brass disc resulted in crumbling of disc edges and subsequent claming the melt after 1–2 min of working time. Thus, subsequent experiments were performed with aluminum–bronze discs. The contact angle of fiber jump and disc temperature during the PDME process were changing. Longer contact angle fiber resulting in more efficient fiber cooling. After certain
600
W. Archangelsky et al. / Materials Science and Engineering A304–306 (2001) 598–603
periods the process has to be stopped because the fiber adhesion on the disc. The minimum time of a stable run depends on the speed of disc rotation, faster rotation results in shorter working time. This can be explained by cyclic heating of the disc and by increased adhesion with overall temperature rise. In some cases it was difficult to detach the set fiber from the disc even after the latter was cooled down. The experiments have shown that the disc is heating up mainly due to the contact with the melt drop. 2.3. Influence of disc geometry In order to decrease the temperature of the disc working edge and to elongate the working period the thickness of the disc wall was diminished and the angle of the working flange was increased up to 60◦ . 2.4. Fiber geometry The Influence of rotation speed on thickness and width of the fiber obtained has been studied as well. These parameters decreased with increase of rotation speed up to 5500 rpm. At higher rates, drops even of minimum volume got unstable which results in frequent splattering. But even at 5500 rpm average gauge was not less than 25 micron. Measurements have shown that as a rule, the fibers were thinner by its ends to compare with their middle part. Occasionally, the fiber ends gauges were 15 or even 10 micron. Furthermore, the gauges along long continuous fibers were changing periodically with a period approximately 65 cm. This is associated with radial oscillations of the disc as it’s circumference is equal to this value. Disc radial movements caused small amplitude undulation of the melt drop. This effect of drop oscillation has been used to diminish the cross-section of fibers. The simplest approach to dissolve the problem of induced drop oscillations has proved to be by incisions at the disc edge. Three incisions in the disc edge readily brought the fiber gauge of 20 micron and increased circularity factor to 08, at rotation speed of 5500 rpm resulting in average fiber length of 15 cm. The velocity of rod feed was 8 mm/min. To decrease the fiber length six incisions in the disc edge were made. The average fiber length decreased to 6 cm, but expected diminishing of cross gauges under the equal conditions did not occur. This can be explained as follows, a melt drop, like any other oscillation system has it’s own period of attenuation (damping). On the strength of obtained experimental data, it can be determined that the steel SUS316L under the given melt-temperature provided no process stability at feed speed 8 mm/min has the attenuation period more than 10−6 s. Provided that under these conditions some oscillations with higher frequency are superimposed, they will not be imposed by the melt drop. Decreasing of the drop attenuation period can be achieved by means of it’s viscosity abatement and by reduction of it’s surface tension. If a drop do not change chemical properties, then this can be achieved
Fig. 4. Fibers of stainless steel.
only by overheating the drop. But overheating increased the drop volume which leads to it’s dismantling. Therefore, the only way is to decrease the feed speed. And indeed at feed speed 4 mm/min and rate rotation 5500 rpm the fiber has thickness 18 micron with it’s width of 22. At a feed speed of 2.5 mm/min the fiber thickness was 16 micron with width of 20 micron. Though, the performance in this case was cut to only 180 g/rod/h. The appearance of the fibers obtained in this study is shown in Fig. 4. Fig. 5 demonstrates forms of cross-sections of the fibers obtained by means of the PDME technique. To obtain thin fibers of steel (SUS316L), a copper disc with wall thickness of 6 mm and working edge angle of 60◦ has been also used. Due to the lower temperature of the working edge, compared to the brass disc, it was possible to increase substantially the duration of period of operating. For example, at the rotation rate 4500 rpm after 1.5 h of running only about 5% of fibers were brushed off by the device for purging. The gap angle and fiber temperature practically has no alterations. Moreover, decreasing of the disc speed is necessary for obtaining thinner fibers. The fiber from cooper disc has the gauge of 20 micron already at a speed of 4500 rpm. This means that at lower disc temperature a higher rate of melt cooling can be achieved that results in
Fig. 5. Appearance of cross-sections of the fibers: (a) bronze disc (2000 rpm), argon; (b) copper disc (3500 rpm), vacuum; (c) copper disc (1000 rpm), argon; (d) brass disc (3500 rpm), vacuum; (e) brass disc (2000 rpm), argon and (f) copper disc (3500 rpm), argon.
W. Archangelsky et al. / Materials Science and Engineering A304–306 (2001) 598–603
601
more limited melt fluxing along the disc edge. It has also been noted that the fiber produced on the copper disc was much harder compared with those produced on the bronze disc despite of their identical cross-dimensions. However, the performance of the process with a copper disc did not increase.
Increasing of the drop temperature to abate the melt viscosity, is also restrained by stability considerations. Firstly, with temperature increase, the surface tension abates and secondly, due to the rod’s heating up the drop volume increases. Unlike melt extraction from a bath, in PDME high overheating is impermissible, which put bounds to the thickness of fibers obtained.
3. Conclusions and discussion
3.2. Adhesion
3.1. Mechanical interactions between disc and melt
The adhesion interaction is the prime and principal factor determining fiber formation and structure during melt extraction from pendant drop because it determines the properties of the disc and melt contact. The disc speed rotation and rod feed speed can control the contact only in inconsiderable extent. The adhesion interaction and crystallization during melt extraction from pendant drop are two competitive processes. Increasing of melt cooling rate allows partly to block the melt flowing over the disc surface. Increasing of cooling, like in traditional extraction, can be achieved by improving of “disc — melt” thermal contact, then by decreasing of disc edge temperature, by applying a disc with higher thermal conductivity and also by decreasing of the melt viscosity. Adhesion interaction within “disc — melt” depends on the melt nature, disc material and it’s surface condition. During the first stage the contact between a solid body (a disc) and a fluid (a melt) occurs resulting in damping and flowing over the working edge of the disc, i.e., forming of adhesion contact takes place. The area of adhesion contact determines width of the fiber formed. Therefore, the fiber width to a great extent depends on adhesion between disc and melt. Sharpness of the disc edge and disc’s speed also influence on the fiber width, though in much less degree. Size of disc recess into melt determining by the rod feed speed practically does not affect the fiber width, especially at low disc speed rotation. During melt consolidation the pattern of adhesion gets different because now two solid bodies take part in the contact. This interaction determines duration of fiber being attached to the disc and rate of it’s cooling. To a significant extent this interaction controls how easily the fiber detaches from the disc. According to thermodynamics, the adhesion can be described by loss of free surface energy, which is equal to the adhesion work
During disc and melt contact several events of various nature occur. The disc mechanically influences on the melt drop and transmits it the impact impulse equal to Z τ E FE dt (1) S= 0
where, τ is the impact duration, determined as τ = I /v (I, duration of contact, v, disc speed), and F the centrifugal force. Impact influence and adhesion interaction cause disc damping by the melt. Due to melt viscosity the “disc — melt drop” boundary layers get impelling, which results in extraction process. Good thermal contact and high disc thermal conductivity with temperature gradient along the border, “melt — disc” provide fast cooling and setting for the melt layer which is extracted from the drop. Melt crystallization begins which has a directed character being that it’s cooling realizes only in one-side way. Solidified melt layer suffers contraction, the contact with disc is dislocated and the fiber is detached from the disc by centrifugal force. Under identical conditions impact influence of the disc on the drop depends on the disc speed. At higher v, the impact causes detaching of the drop because the surface tension is insufficient to keep it on the rod. That explains the importance of keeping the drop volume minimal and on the contrary, at low disc speed the impact impulse is too small and unable to influence significantly upon the process. For instance, when the steel fibers are produced at disc speed of 5 m/s the drop is quite stable even with the disc edge recession into the drop up to 3 mm. With increasing of the disc speed (like at traditional melt extraction with feeding from the bottom) the gauge of obtaining fibers decreases which can be explained by the following reasons. The contact duration shortens, the thickness of the melt dynamic bordering layer abates, particularly this is manifested with decreasing of the melt viscosity. Due to impact influence, the thermal contact between disc and melt increased. But in PDME the range of disc speeds is limited by the drop stability. For example, when producing copper fiber at speed up to 3500 rpm certain abatement of fiber gauge has been registered. Further augmentation of the disc speed results in instability of the drop and therefore, in increasing of fiber average gauge and in increasing of it’s dispersion.
Wa = (σ13 + σ23 ) − σ12
(2)
where, σ ij is the value of surface tension ((1) disc, (2) melt and (3) atmosphere). Therefore, under the same conditions, adhesion is increased with rising of melt surface tension and with decreasing of interfacial energy σ 12 . To describe disc damping by the melt the Jung equation can be applied, though highly approximately cos θ =
(σ13 + σ23 ) σ12
(3)
602
W. Archangelsky et al. / Materials Science and Engineering A304–306 (2001) 598–603
where, θ is the edge angle of damping. This equation is applicable for equilibrium conditions. Adhesion is accompanied by thermal effects, namely during this process heat of adhesion is exuded which have to be taken into account when considering processes of melt cooling and setting. When contemplating melt adhesion to a disc certain balance and disbalance processes within interfacial borders have to be considered. Under equilibrium conditions, adhesion between metals is determined by interaction of corpuscular (atomic) forces. Near the surface of a metal the electronic gas is quite irregular resulting in gradient of electronic density. Besides, in solid bodies superficial atoms situated near the surface are characterized by unsaturated (broken) valence links and this rises their surface energy over against the volume. Interfacial energy can be described as following sum [2] σ12 = σe + σij + σei
(4)
where, σ i is the energy of electron system (kinetic component), σ ij the energy of electrostatic interaction between ions and σ ei the energy of electrons and ions interaction. Thus, interfacial energy and therefore, work of adhesion, depends on the gap size between the phases, or in our case, between the disc and the melt (a fiber). When this gap is increased up to inter-atomic size, σ 12 value is slowly decreasing and then abruptly falls. This is caused by the fact that electron–ionic energy determining the force of attraction between phases falls more quickly with enlargement of the gap, compared to gain of the kinetic component, which controls repulsion. In other words, at small distances electronic “tail” in solidifying fiber, having thermally actuated electron gas and which, therefore, has higher kinetic energy, enters into the disc and is attracted by the latter. When the gap is increased electronic “tail” comes out from zone of interaction with ionic disc frame and interacts with it’s electronic “tail”, which causes repulsion and adhesion abatement. During adhesion in the contact zone, due to energetic irregularity along surface caused by temperature gradient and also connected with different density of electronic gas (if disc and melt are made from different materials), a dual electric layer is formed. The melt (a fiber) appears there to be a donor of electrons and the disc plays a role of their acceptor. During the contact disc’s and melt’s electronic energy levels are aligning and so the adhesion here can be considered as a donor–acceptor process. Presence of dual electric layer is confirmed by potential difference registered between disc and a melt drop. The value of potential difference during stainless steel fibers producing with the aid of a bronze disc is ca. 0.2 V, and the melt was charged positively against the disc. When defining value of interfacial energy [4], a bias of superficial atomic surfaces in regard to volumetric position, or so called surface reconstruction, has to be taken into account. This can play a significant role for fiber detaching from the disc, because slight shift of ionic plains can lead
to a marked change in energy of links between the atoms along superficial ionic surface. As a result, instead of adhesion break at the border of disc — melt division, there can be a cohesion rupture in the volume of one metal, which characterized by extreme displacement effects (dilatation) into superficial ionic surfaces. When disc and melt are of the same material the interfacial energy value is significantly lower compared to the contact of dissimilar matters because in the latter case contribution of ion-ionic interaction is sufficiently less resulting in increased adhesion work, which can lead to fiber claming to the disc. At interfacial gap more than 100 Å, the Coulomb forces cease their action. At these distances the adhesion is caused by long-distance interaction, associated with fluctuations of electronic density and has quantum-mechanical nature. This interaction is significantly weaker than the Coulomb forces and for this reason they can be ignored when considering melt — disc adhesion. 3.3. Physical and chemical processes in contact zone Calculated by electron–ionic interaction and affirmed by experimental data the work of adhesion for majority of metals has a maximum value of 4–5 J/m2 when interfacial gap is approximately equal to a distance between atoms. In real conditions the disc and oxide film always have some surface inequalities. Due to this, a gap size usually is about 102 of a distance between atoms, and work of adhesion matching to this distance has to be not more than 10−1 J/m2 . There is a high surface tension of about 1–2 J/m2 in most of the metals. That’s why, according to the Eq. (2), the work of adhesion during interaction between disc and melt has an order of 1–2 J/m2 . Such work of adhesion cannot be provided only by inter-atoms interaction. Adhesion interaction between disc and melt beside inter-atomic forces is also determined by physical and chemical processes that take place at the division border. There is a feature among physical and chemical characteristics observed sometimes, namely sharp augmentation of adhesion work in a certain temperature interval [3]. For example, copper fiber can be produced before a copper disc is heated up, but afterwards melt is clamed to the disc. And contrariwise, fibers of nickel cannot be produced until bronze or copper discs are heated up. The following items ought to be included into the physical and chemical processes at the disc — melt contact border; corrosion, absorptive reduction of disc durability due to falling of free energy along the surface of division disc — melt, dissolution of disc material in the melt, diffusion within disc volume together with diffusion along grains and defects borders, chemical interaction. One of these processes or some of their combinations can be developed, depending on disc and melt natures, atmosphere in a chamber and technological parameters. It should be noted that the disc temperature is always less than the melt one and that the process of disc impacting the melt runs under gravitation. For these reasons and due
W. Archangelsky et al. / Materials Science and Engineering A304–306 (2001) 598–603
being a surface active substance concentrates along the melt drop interfacial border and interacts with metal atoms converting them into positive ions forming ionic lattice containing oxygen along the disc surface. This enhances dumping. But practically in the most cases this comes along with over-enhancing of adhesion and results in fiber claming to the disc. Dissolution of disc material in the melt as a rule is insignificant and does not cause any observable fiber pollution. Nevertheless, it has to be accounted during producing of absolutely pure substances, e.g., semiconductors. As far as it concerns migration of atoms into the disc, this can be manifested only at very low speeds, when contact duration is enough to realize diffusion processes.
Fig. 6. Contact surface of a fiber.
to physical and chemical interactions non-equilibrium adhesion between disc and melt takes place resulting in inequality of disc and melt surface tensions before and after their contact. The edge angle of damping differs from it’s equilibrium value. During non-equilibrium adhesion this parameter is always bigger and interfacial energy is always less by the quantity of 1σ , which meets the following equation of Jukhovitski 1σ = M1µ
(5)
where, M is a constant, 1µ chemical potential change during damping. Thus, under non-equilibrium conditions channeling of the melt over the disc surface is better. As the disc surface is always (even in vacuum) covered with oxide film, it can be thought that chemical interaction makes the main contribution to melt adhesion leading to ion link formation according to the following reaction Mep + Med O ⇔ Med + Mep O
603
4. Conclusions Crucial factor influencing on fiber formation in PDME is adhesion interaction between melt and disc. Obtained experimental data have proved that the work of adhesion increases with rising of the disc temperature. The work of adhesion is determined not only by inter-atomic interaction, but also by various physical and chemical processes taking place within the region of contact between melt and disc-crystallizer. From our point of view, the main among these processes is oxides reduction by melt on the surface of disc.
Acknowledgements The authors thank Prof. B.S. Mitin and M.M. Serov for their assistance in preparation of this work.
(6)
where, Mep is melt metal, Med disc metal. A high correlation between adhesion work and energy of Med O formation has been noted, increasing of latter, decreases the adhesion work [4]. By the way this is the reason why using of aluminium disc for producing various alloys is a problematic thesis. On the fiber surface contacting with the disc some reaction products are observed (6) (Fig. 6). Increasing of melt adhesion to disc caused by the reaction (6) can be achieved by oxygen elevation within a chamber (for example, by elevation of residual pressure). Oxygen
References [1] J.O. Strom-Olsen et al., Fine metallic and ceramic fibers by melt extraction, Mater. Sci. Eng. A179/A180 (1994) 158–162. [2] V.F. Ukhov, R.M. Kobeleva, G.V. Dedkov, A.I. Temrokov, Electron-statistical theory of metals and ionic crystals, Moscow Science 1982, p. 160. [3] M.A. Maurakh, B.S. Mitin, Liquid infusible oxides, Moscow Metallurgy, 1979, p. 288. [4] A.A. Appen, Thermo-stable inorganic coatings, Leningrad Chemistry, 1976, p. 295.
Adhesion interaction on melt extraction from pendant drop W. Archangelsky, S.V. Prischepov, V.A. Vasiliev∗ MATI-Russian State Technology University, Polbina street 45-2, Moscow 109383, Russia
Abstract A technique of pendant drop melt extraction (PDME) is reported and certain results describing thin stainless steel fibers obtained by means of PDME were studied. Adhesion interaction between metal melt and disc-crystallizer was also considered. The adhesion work was established to be approximately 1–2 J/m2 . It was shown that melt adhesion to the disc-crystallizer is determined both by physical interaction and chemical processes in contact region. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Melt extraction; Pendant drop technique; Disc-crystallizer materials and regimes of operation; Stainless steel fibers; Advantages and drawbacks
1. Introduction The PDME technique is a variation of traditional melt extraction [1] widely used industrially in many countries. The PDME technique (its plan is shown in the Fig. 1) is a fast melt cooling and setting, which take place at the edge of a vertical fast rotating disc forming fibers. Unlike traditional melt extraction methods, the melt is yielding to the disc from the top in the form of a pendant drop produced by melting rod of material. For the majority of alloys resistive heaters (e.g., graphite, churlish metals) are used as a source of heat, though, lasers or some cathode-ray tubes can also be used. Through high disc thermal conductivity and its high wettability for the melt, a very high coefficient of thermal conduction is provided at the boundary of disc and melt. That is why the melt cooling and setting has a very high speed, 105 degree/s [1]. This technique allows to produce metal fibers, alloys or ceramics with the gauge from 200 up to 15 micron and with a factor of cross-section circularity up to 0.8–0.9. Solidified melt is separated through centrifugal force and thermal contraction of the fiber during cooling. The cooling process has three stages. During extraction it can be divided into following (Fig. 1): (1) cooling at the disc decreasing contact with the melt, (2) cooling at the disc with decreasing contact to the melt and (3) cooling in the ambient gas. The highest rate of cooling seems to be achieved during the second stage. Forced convective parameters of fiber formation (thickness, gauge, length, the circularity factor) depend on technological conditions (such as a rate of disc speed, rod velocity
feed, geometry of the disc edge), thermal parameters and physical properties (viscosity, surface tension, oxygen affinity, thermal capacity and conductivity, melting thermal interval) of the melt and disc. The selection of “melt — disc material” is very important. Inability of the melt to dampen the disc results in splattering and in another extreme at angle near 0◦ melt adhesion to the disc takes place. The difficulty of “melt — disc material” selection is even more sophisticated by high dynamic characteristics of the PDME and its irregularity. The disc is continuously cooled by water and the temperature at its edge remains significantly lower compared with the temperature of the melt drop. These circumstances explain why the selection of “melt — disc pair” in most cases has to be done experimentally only. Continuous melt extraction can be interrupted by incisions along the edge of the disc, which allow to obtain discrete fibers. The PDME technique’s prime advantage compared with any traditional melt extraction consists in significantly lower level of impurity which can be achieved owing to potless melting. Moreover, the precision contact between the disc and the melt forms more stable fibers in shape and gauge. However, this method has an essential drawback, namely, extremely unstable drop position at the edge of melting rod which often causes drop disruption and its pouring (Fig. 2). Despite on the PDME apparent simplicity its application necessarily demands safe, reliable and perfect automation in an industrial environment, which implicitly increase the equipment costs. 1.1. Experimental methods
∗
Corresponding author. Present address: MATI-Russian State Technology University, Russian Innovation Consortium, CHIEF Group, Petrovka 27, 103767 Moscow, K31, Russia.
In this work stainless steel fibers were obtained at the pilot-industrial plant (shown in Fig. 3) consisting of a high
0921-5093/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 1 5 4 2 - 2
W. Archangelsky et al. / Materials Science and Engineering A304–306 (2001) 598–603
599
0.03%, silicium: 1%, manganese: 2%, phosphorus: 0.045%, sulfur: 0.03%, nickel: 12–15%, chromium: 16–18%, molybdenum: 2–3%. Necessity of obtaining the fibers with diameters 20 micron and less is dictated by the demand in fine purification filter devices in various aggressive media.
2. Results 2.1. Influence of ambient atmosphere
Fig. 1. Scheme of the PDME technique.
Fig. 2. Contact of melt drop with disc-crystallizer: (1) graphite heater; (2) rod and (3) disk.
speed spindle, equipped with two 200 mm diameter discs cooled by water and rods feed mechanism, two graphite heaters and a dressing device to cleaning discs edges. A vacuum system allows to run the fiber production either under vacuum (> to 10−3 Pa), or in inert gas atmosphere with pressure = 105 Pa. The plant permits simultaneous operating with 10 base melt rods of up to 12 mm diameters and length up to 1 m. Stainless steel (SUS316L) rods of 12 mm diameter have been used, with the following composition — carbon:
The influence of ambient gas on the extraction process was investigated using fiber fabrication in vacuum or in argon atmosphere. In vacuum reduced oxides layers are observed on the drop surface increasing the drop stability. Higher surface-tension results in fiber cross-section very close to a circle. Argon airborne gas absorbed on the surface of the disc increases thermal transmission factor along the “melt — disc” bound, which augments the rate of melt cooling and causes a more disperse structure of fibers. This allows to obtain over-saturated solid solutions having an even distribution of alloy elements and to improve the mechanical properties of the fibers. Due to fast vacuum cooling the fiber is obtained with less gauge, for its onflow is controlled by its crystallization. Experiments were carried in vacuum at residual pressures of 01 and 10 Pa. No significant difference connected with reduced pressure was noticed. Melt drops were produced clear and devoid of any oxides upon it’s surface even at reduced pressure of 50 Pa. At disc rotation 3500 rpm the fiber has thickness 24 micron and width of 30 micron. In vacuum liquid metal evaporates and condenses in the disc resulting in increase of melt adhesion on the disc because there is no actual difference between the disc material and the melt resulting in spattering of the drop. It has to bear in mind that increased adhesion along the edge of the disc increases with the heating and it makes purging useless. Applying of abrasives causes rebating of the disc. As a result of fibers adhesion to disc the process has to be stopped after 10–15 min of running time. Thus, further experiments were conducted in argon atmosphere. An argon pressure of 500 Pa was sufficient for inhibition of liquid metal evaporation. 2.2. Influence of disc material
Fig. 3. The plant for metal fibers producing.
At first discs made out of the following materials were tested, aluminum–bronze, brass (composition-metal) and stainless steel. The best results were obtained with the discs of aluminum–bronze. PDME with brass disc resulted in crumbling of disc edges and subsequent claming the melt after 1–2 min of working time. Thus, subsequent experiments were performed with aluminum–bronze discs. The contact angle of fiber jump and disc temperature during the PDME process were changing. Longer contact angle fiber resulting in more efficient fiber cooling. After certain
600
W. Archangelsky et al. / Materials Science and Engineering A304–306 (2001) 598–603
periods the process has to be stopped because the fiber adhesion on the disc. The minimum time of a stable run depends on the speed of disc rotation, faster rotation results in shorter working time. This can be explained by cyclic heating of the disc and by increased adhesion with overall temperature rise. In some cases it was difficult to detach the set fiber from the disc even after the latter was cooled down. The experiments have shown that the disc is heating up mainly due to the contact with the melt drop. 2.3. Influence of disc geometry In order to decrease the temperature of the disc working edge and to elongate the working period the thickness of the disc wall was diminished and the angle of the working flange was increased up to 60◦ . 2.4. Fiber geometry The Influence of rotation speed on thickness and width of the fiber obtained has been studied as well. These parameters decreased with increase of rotation speed up to 5500 rpm. At higher rates, drops even of minimum volume got unstable which results in frequent splattering. But even at 5500 rpm average gauge was not less than 25 micron. Measurements have shown that as a rule, the fibers were thinner by its ends to compare with their middle part. Occasionally, the fiber ends gauges were 15 or even 10 micron. Furthermore, the gauges along long continuous fibers were changing periodically with a period approximately 65 cm. This is associated with radial oscillations of the disc as it’s circumference is equal to this value. Disc radial movements caused small amplitude undulation of the melt drop. This effect of drop oscillation has been used to diminish the cross-section of fibers. The simplest approach to dissolve the problem of induced drop oscillations has proved to be by incisions at the disc edge. Three incisions in the disc edge readily brought the fiber gauge of 20 micron and increased circularity factor to 08, at rotation speed of 5500 rpm resulting in average fiber length of 15 cm. The velocity of rod feed was 8 mm/min. To decrease the fiber length six incisions in the disc edge were made. The average fiber length decreased to 6 cm, but expected diminishing of cross gauges under the equal conditions did not occur. This can be explained as follows, a melt drop, like any other oscillation system has it’s own period of attenuation (damping). On the strength of obtained experimental data, it can be determined that the steel SUS316L under the given melt-temperature provided no process stability at feed speed 8 mm/min has the attenuation period more than 10−6 s. Provided that under these conditions some oscillations with higher frequency are superimposed, they will not be imposed by the melt drop. Decreasing of the drop attenuation period can be achieved by means of it’s viscosity abatement and by reduction of it’s surface tension. If a drop do not change chemical properties, then this can be achieved
Fig. 4. Fibers of stainless steel.
only by overheating the drop. But overheating increased the drop volume which leads to it’s dismantling. Therefore, the only way is to decrease the feed speed. And indeed at feed speed 4 mm/min and rate rotation 5500 rpm the fiber has thickness 18 micron with it’s width of 22. At a feed speed of 2.5 mm/min the fiber thickness was 16 micron with width of 20 micron. Though, the performance in this case was cut to only 180 g/rod/h. The appearance of the fibers obtained in this study is shown in Fig. 4. Fig. 5 demonstrates forms of cross-sections of the fibers obtained by means of the PDME technique. To obtain thin fibers of steel (SUS316L), a copper disc with wall thickness of 6 mm and working edge angle of 60◦ has been also used. Due to the lower temperature of the working edge, compared to the brass disc, it was possible to increase substantially the duration of period of operating. For example, at the rotation rate 4500 rpm after 1.5 h of running only about 5% of fibers were brushed off by the device for purging. The gap angle and fiber temperature practically has no alterations. Moreover, decreasing of the disc speed is necessary for obtaining thinner fibers. The fiber from cooper disc has the gauge of 20 micron already at a speed of 4500 rpm. This means that at lower disc temperature a higher rate of melt cooling can be achieved that results in
Fig. 5. Appearance of cross-sections of the fibers: (a) bronze disc (2000 rpm), argon; (b) copper disc (3500 rpm), vacuum; (c) copper disc (1000 rpm), argon; (d) brass disc (3500 rpm), vacuum; (e) brass disc (2000 rpm), argon and (f) copper disc (3500 rpm), argon.
W. Archangelsky et al. / Materials Science and Engineering A304–306 (2001) 598–603
601
more limited melt fluxing along the disc edge. It has also been noted that the fiber produced on the copper disc was much harder compared with those produced on the bronze disc despite of their identical cross-dimensions. However, the performance of the process with a copper disc did not increase.
Increasing of the drop temperature to abate the melt viscosity, is also restrained by stability considerations. Firstly, with temperature increase, the surface tension abates and secondly, due to the rod’s heating up the drop volume increases. Unlike melt extraction from a bath, in PDME high overheating is impermissible, which put bounds to the thickness of fibers obtained.
3. Conclusions and discussion
3.2. Adhesion
3.1. Mechanical interactions between disc and melt
The adhesion interaction is the prime and principal factor determining fiber formation and structure during melt extraction from pendant drop because it determines the properties of the disc and melt contact. The disc speed rotation and rod feed speed can control the contact only in inconsiderable extent. The adhesion interaction and crystallization during melt extraction from pendant drop are two competitive processes. Increasing of melt cooling rate allows partly to block the melt flowing over the disc surface. Increasing of cooling, like in traditional extraction, can be achieved by improving of “disc — melt” thermal contact, then by decreasing of disc edge temperature, by applying a disc with higher thermal conductivity and also by decreasing of the melt viscosity. Adhesion interaction within “disc — melt” depends on the melt nature, disc material and it’s surface condition. During the first stage the contact between a solid body (a disc) and a fluid (a melt) occurs resulting in damping and flowing over the working edge of the disc, i.e., forming of adhesion contact takes place. The area of adhesion contact determines width of the fiber formed. Therefore, the fiber width to a great extent depends on adhesion between disc and melt. Sharpness of the disc edge and disc’s speed also influence on the fiber width, though in much less degree. Size of disc recess into melt determining by the rod feed speed practically does not affect the fiber width, especially at low disc speed rotation. During melt consolidation the pattern of adhesion gets different because now two solid bodies take part in the contact. This interaction determines duration of fiber being attached to the disc and rate of it’s cooling. To a significant extent this interaction controls how easily the fiber detaches from the disc. According to thermodynamics, the adhesion can be described by loss of free surface energy, which is equal to the adhesion work
During disc and melt contact several events of various nature occur. The disc mechanically influences on the melt drop and transmits it the impact impulse equal to Z τ E FE dt (1) S= 0
where, τ is the impact duration, determined as τ = I /v (I, duration of contact, v, disc speed), and F the centrifugal force. Impact influence and adhesion interaction cause disc damping by the melt. Due to melt viscosity the “disc — melt drop” boundary layers get impelling, which results in extraction process. Good thermal contact and high disc thermal conductivity with temperature gradient along the border, “melt — disc” provide fast cooling and setting for the melt layer which is extracted from the drop. Melt crystallization begins which has a directed character being that it’s cooling realizes only in one-side way. Solidified melt layer suffers contraction, the contact with disc is dislocated and the fiber is detached from the disc by centrifugal force. Under identical conditions impact influence of the disc on the drop depends on the disc speed. At higher v, the impact causes detaching of the drop because the surface tension is insufficient to keep it on the rod. That explains the importance of keeping the drop volume minimal and on the contrary, at low disc speed the impact impulse is too small and unable to influence significantly upon the process. For instance, when the steel fibers are produced at disc speed of 5 m/s the drop is quite stable even with the disc edge recession into the drop up to 3 mm. With increasing of the disc speed (like at traditional melt extraction with feeding from the bottom) the gauge of obtaining fibers decreases which can be explained by the following reasons. The contact duration shortens, the thickness of the melt dynamic bordering layer abates, particularly this is manifested with decreasing of the melt viscosity. Due to impact influence, the thermal contact between disc and melt increased. But in PDME the range of disc speeds is limited by the drop stability. For example, when producing copper fiber at speed up to 3500 rpm certain abatement of fiber gauge has been registered. Further augmentation of the disc speed results in instability of the drop and therefore, in increasing of fiber average gauge and in increasing of it’s dispersion.
Wa = (σ13 + σ23 ) − σ12
(2)
where, σ ij is the value of surface tension ((1) disc, (2) melt and (3) atmosphere). Therefore, under the same conditions, adhesion is increased with rising of melt surface tension and with decreasing of interfacial energy σ 12 . To describe disc damping by the melt the Jung equation can be applied, though highly approximately cos θ =
(σ13 + σ23 ) σ12
(3)
602
W. Archangelsky et al. / Materials Science and Engineering A304–306 (2001) 598–603
where, θ is the edge angle of damping. This equation is applicable for equilibrium conditions. Adhesion is accompanied by thermal effects, namely during this process heat of adhesion is exuded which have to be taken into account when considering processes of melt cooling and setting. When contemplating melt adhesion to a disc certain balance and disbalance processes within interfacial borders have to be considered. Under equilibrium conditions, adhesion between metals is determined by interaction of corpuscular (atomic) forces. Near the surface of a metal the electronic gas is quite irregular resulting in gradient of electronic density. Besides, in solid bodies superficial atoms situated near the surface are characterized by unsaturated (broken) valence links and this rises their surface energy over against the volume. Interfacial energy can be described as following sum [2] σ12 = σe + σij + σei
(4)
where, σ i is the energy of electron system (kinetic component), σ ij the energy of electrostatic interaction between ions and σ ei the energy of electrons and ions interaction. Thus, interfacial energy and therefore, work of adhesion, depends on the gap size between the phases, or in our case, between the disc and the melt (a fiber). When this gap is increased up to inter-atomic size, σ 12 value is slowly decreasing and then abruptly falls. This is caused by the fact that electron–ionic energy determining the force of attraction between phases falls more quickly with enlargement of the gap, compared to gain of the kinetic component, which controls repulsion. In other words, at small distances electronic “tail” in solidifying fiber, having thermally actuated electron gas and which, therefore, has higher kinetic energy, enters into the disc and is attracted by the latter. When the gap is increased electronic “tail” comes out from zone of interaction with ionic disc frame and interacts with it’s electronic “tail”, which causes repulsion and adhesion abatement. During adhesion in the contact zone, due to energetic irregularity along surface caused by temperature gradient and also connected with different density of electronic gas (if disc and melt are made from different materials), a dual electric layer is formed. The melt (a fiber) appears there to be a donor of electrons and the disc plays a role of their acceptor. During the contact disc’s and melt’s electronic energy levels are aligning and so the adhesion here can be considered as a donor–acceptor process. Presence of dual electric layer is confirmed by potential difference registered between disc and a melt drop. The value of potential difference during stainless steel fibers producing with the aid of a bronze disc is ca. 0.2 V, and the melt was charged positively against the disc. When defining value of interfacial energy [4], a bias of superficial atomic surfaces in regard to volumetric position, or so called surface reconstruction, has to be taken into account. This can play a significant role for fiber detaching from the disc, because slight shift of ionic plains can lead
to a marked change in energy of links between the atoms along superficial ionic surface. As a result, instead of adhesion break at the border of disc — melt division, there can be a cohesion rupture in the volume of one metal, which characterized by extreme displacement effects (dilatation) into superficial ionic surfaces. When disc and melt are of the same material the interfacial energy value is significantly lower compared to the contact of dissimilar matters because in the latter case contribution of ion-ionic interaction is sufficiently less resulting in increased adhesion work, which can lead to fiber claming to the disc. At interfacial gap more than 100 Å, the Coulomb forces cease their action. At these distances the adhesion is caused by long-distance interaction, associated with fluctuations of electronic density and has quantum-mechanical nature. This interaction is significantly weaker than the Coulomb forces and for this reason they can be ignored when considering melt — disc adhesion. 3.3. Physical and chemical processes in contact zone Calculated by electron–ionic interaction and affirmed by experimental data the work of adhesion for majority of metals has a maximum value of 4–5 J/m2 when interfacial gap is approximately equal to a distance between atoms. In real conditions the disc and oxide film always have some surface inequalities. Due to this, a gap size usually is about 102 of a distance between atoms, and work of adhesion matching to this distance has to be not more than 10−1 J/m2 . There is a high surface tension of about 1–2 J/m2 in most of the metals. That’s why, according to the Eq. (2), the work of adhesion during interaction between disc and melt has an order of 1–2 J/m2 . Such work of adhesion cannot be provided only by inter-atoms interaction. Adhesion interaction between disc and melt beside inter-atomic forces is also determined by physical and chemical processes that take place at the division border. There is a feature among physical and chemical characteristics observed sometimes, namely sharp augmentation of adhesion work in a certain temperature interval [3]. For example, copper fiber can be produced before a copper disc is heated up, but afterwards melt is clamed to the disc. And contrariwise, fibers of nickel cannot be produced until bronze or copper discs are heated up. The following items ought to be included into the physical and chemical processes at the disc — melt contact border; corrosion, absorptive reduction of disc durability due to falling of free energy along the surface of division disc — melt, dissolution of disc material in the melt, diffusion within disc volume together with diffusion along grains and defects borders, chemical interaction. One of these processes or some of their combinations can be developed, depending on disc and melt natures, atmosphere in a chamber and technological parameters. It should be noted that the disc temperature is always less than the melt one and that the process of disc impacting the melt runs under gravitation. For these reasons and due
W. Archangelsky et al. / Materials Science and Engineering A304–306 (2001) 598–603
being a surface active substance concentrates along the melt drop interfacial border and interacts with metal atoms converting them into positive ions forming ionic lattice containing oxygen along the disc surface. This enhances dumping. But practically in the most cases this comes along with over-enhancing of adhesion and results in fiber claming to the disc. Dissolution of disc material in the melt as a rule is insignificant and does not cause any observable fiber pollution. Nevertheless, it has to be accounted during producing of absolutely pure substances, e.g., semiconductors. As far as it concerns migration of atoms into the disc, this can be manifested only at very low speeds, when contact duration is enough to realize diffusion processes.
Fig. 6. Contact surface of a fiber.
to physical and chemical interactions non-equilibrium adhesion between disc and melt takes place resulting in inequality of disc and melt surface tensions before and after their contact. The edge angle of damping differs from it’s equilibrium value. During non-equilibrium adhesion this parameter is always bigger and interfacial energy is always less by the quantity of 1σ , which meets the following equation of Jukhovitski 1σ = M1µ
(5)
where, M is a constant, 1µ chemical potential change during damping. Thus, under non-equilibrium conditions channeling of the melt over the disc surface is better. As the disc surface is always (even in vacuum) covered with oxide film, it can be thought that chemical interaction makes the main contribution to melt adhesion leading to ion link formation according to the following reaction Mep + Med O ⇔ Med + Mep O
603
4. Conclusions Crucial factor influencing on fiber formation in PDME is adhesion interaction between melt and disc. Obtained experimental data have proved that the work of adhesion increases with rising of the disc temperature. The work of adhesion is determined not only by inter-atomic interaction, but also by various physical and chemical processes taking place within the region of contact between melt and disc-crystallizer. From our point of view, the main among these processes is oxides reduction by melt on the surface of disc.
Acknowledgements The authors thank Prof. B.S. Mitin and M.M. Serov for their assistance in preparation of this work.
(6)
where, Mep is melt metal, Med disc metal. A high correlation between adhesion work and energy of Med O formation has been noted, increasing of latter, decreases the adhesion work [4]. By the way this is the reason why using of aluminium disc for producing various alloys is a problematic thesis. On the fiber surface contacting with the disc some reaction products are observed (6) (Fig. 6). Increasing of melt adhesion to disc caused by the reaction (6) can be achieved by oxygen elevation within a chamber (for example, by elevation of residual pressure). Oxygen
References [1] J.O. Strom-Olsen et al., Fine metallic and ceramic fibers by melt extraction, Mater. Sci. Eng. A179/A180 (1994) 158–162. [2] V.F. Ukhov, R.M. Kobeleva, G.V. Dedkov, A.I. Temrokov, Electron-statistical theory of metals and ionic crystals, Moscow Science 1982, p. 160. [3] M.A. Maurakh, B.S. Mitin, Liquid infusible oxides, Moscow Metallurgy, 1979, p. 288. [4] A.A. Appen, Thermo-stable inorganic coatings, Leningrad Chemistry, 1976, p. 295.