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Effects of electromagnetic vibrations on the microstructure of continuously cast aluminium alloys
Materials Science and Engineering, A173 (1993) 169-172
169
Effects of electromagnetic vibrations on the microstructure of continuously cast aluminum alloys Ch. Viv~s Laboratoire de Magn&ohydrodynamique, Facult~ des Sciences, Universit~ d'A vignon, 33 Rue Louis Pasteur, F-84000 Avignon (France)
Abstract The influence of electromagnetic vibrations imposed during solidification on grain refinement in the 1085 and 2214 aluminium alloys, characterized by a narrow and a wide freezing range respectively, has been examined. The vibrations were produced by the simultaneous application of a stationary magnetic field B 0' and a variable magnetic fidd B of frequency 50 Hz in the sump of continuously cast ingots. Extensive grain refinement has been observed in both alloys with increasing magnetizing force. This study shows that the mean grain size obtained by this vibrational technique is always smaller than that produced by the recently developed CREM (carting, refining, electromagnetic) process.
1. Introduction
The grain structure of an alloy is of great importance, since many of its mechanical properties are directly related to the size, shape and distribution of the grains. It is well known that the production of a fine-grained equiaxed structure leads to a substantial improvement in the quality of the metal and allows an increase in the ingot drop rate and a reduction in cracking. For instance, several dynamic methods, producing a vigorous forced convection in the melt during freezing, lead to substantial grain refinements [1-3]. In such processes heat and fluid flow are controlled by various externally applied forces. These methods primarily include the use of electromagnetic or mechanical stirring. Moreover, many investigators have found that mechanical vibrations of both sonic and ultrasonic character, when applied during solidification of metals and alloys, modify conventionally obtained macrostructures and microstructures [4, 5]. The most commonly observed effect is the suppression of undesirable columnar zones and the development of a fine-grained equiaxed structure. In fact, the effects produced when high intensity sonic or ultrasonic waves are propagated through molten metals can be listed under three headings, namely grain refinement, dispersive effects and degassing, with the result that porosity is reduced. However, such a technique presents several disadvantages. The oscillating rods are very rapidly dissolved when they are immersed into molten aluminium alloys and this circumstance provokes an undesirable 0921-5093/93/$6.00
pollution of the metal. Moreover, the intensity of cavitation is greatest close to the tranducer or the coupling rod face; thus the use of such a system is principally justified for the treatment of metal mixtures on a small scale. Furthermore, in view of both the cost and bulkiness of the equipment, the production of large aluminium alloy ingots by continuous casting seems unrealistic. The aim of this investigation is to present an alternative approach based on the simultaneous application of stationary and variable magnetic fields.
2. The electromagnetic caster
2.1 Working principle In our work the vibrations are produced by an electromagnetic vibrational method and without any mechanical contact with the melt. The principle consists of the simultaneous application of a stationary magnetic field B 0' and a variable magnetic field B in the vicinity of the molten metal during the course of solidification (Figs. 1 and 2); these magnetic fields are nearly parallel to the vertical axis of the ingot. The stationary magnetic fields is generated by at least one coil, which is supplied with direct current. The variable magnetic field is created by another inductor, which is of similar geometry, supplied with an alternating current of frequency N. Under the effect of the periodic current the inductor generates in the melt a variable field B, which in turn gives rise to an induced current density J. The combined action of the collinear fields generates vibrations in the metal pool, which are of dual origin: the interaction of B 0' and J engenders a vibrating force of © 1993 - Elsevier Sequoia. All rights reserved
Ch. VivOs /
170
Electromagnetic effects on AI alloy microstructure
frequency N, while the J × B force consists of a timeindependent component and an oscillatory component of frequency 2N. 2.2. Experimental apparatus Experiments were done on ingots of circular and rectangular cross-sections. The main features of the apparatus are presented schematically in Fig. 2. First, billets (diameter 3 2 0 m m ) of 1085 and 2214 aluminium alloys were case at a drop rate of 50 mm min ~. The induction electromagnetic field was generated by an inductor traversed by an electric current of frequency 50 Hz and located below the top of the ingot mould; the magnetizing force was modulated at will, from 0 to 12600 A-turns (At) by an autotransformer. An externally imposed stationary magnetic field B0', mainly parallel to the gravitational force field vector, was created by a coil situated above the ingot mould and excited by a magnetizing force that could vary between 0 and 50000 At, while the corresponding magnetic field strength B,)' was in the range 0-0.2 T. The slabs of aluminium alloy 2214 of cross-section 700 x 200 mm 2 were cast at a drop rate of 50 mm s ~. The device was similar to that adopted for the billets, the variable and stationary magnetic fields being 11 000 and 60 000 At respectively. The set of probes for electric current density, magnetic field and velocity measurements has already been described [6].
3. Electromagnetic and fluid flow phenomena 3.1. Electromagnetic parameter For sinoidally varying fields one can assume that the values of the axial component at a given point and time are given approximately by J(t)=J,,exp
B(t) = B,, exp
cos cot+~,
-
cos o t + + # ( J , B 1 6
where .1o and B o are peak values on the wall, r is the distance along a radius with respect to the wall, ~b(J,B) is the phase difference between the periodic vectors J and B and 6 is the electromagnetic skin depth.
i c
!
""
4 i/a -
L~
%
~l B(t)
-
,.b
dJ
(a)
B°L
Fig. 1. Principle of production of electromagnetic vibrations.
J Inductor [Bo]~
Pole
Sump Ingot mold
.
I
(b) Induction
co. Is(t)]
~ ,,~v~.~_ ;'~'1 /~
'
~
Stilring~ ~ . J Z ~
~Water
cooling
~E:~ct{°°~agnetic
I '~lngot Fig. 2. Schematic diagram of continuous casting of aluminium alloy ingots using an electromagnetic vibrational method.
Fig. 3. Evolution of casting with total d.C. power input (2214 aluminium alloy billet of diameter 320 ram): (a) conventional casting, P = 0; (b) electromagnetic process, I'= 2.2 kW. Key: a, liquid metal; b, pasty zone; c, solidified metal; d, segregation zone: e, exudations; f, estimated heat flux profile; g, smooth surface; h, height of solidifying metal in contact with ingot mould; i, inductors; j, iron core; n, nuclei.
Ch. VivOs /
15IF° (N'm-3) x 10-5 10
o doB~x e_r//5. (50 Hz) • JoBox e-2r/6"(100Hz)
\
tcmm
\~
0
Electromagnetic effects on AI alloy microstructure
12600 At (AC)
g
1~)
--
Fig. 4. Penetration of electromagnetic vibrating forces of freq/aencies N and 2N.
171
3.2. Electromagnetic stirring The effect of the induction electromagnetic field here is identical to that observed in the C R E M (casting, refining, electromagnetic) process [7]. The time-mean electromagnetic body force ( J x B ) may be resolved into a radial component (principally irrotational) and a vertical component (primarily rotational). The potential forces, balanced by a pressure gradient, result in the formation of a convex surface meniscus, while the rotational forces are responsible for an electromagnetic stirring similar to that encountered in a coreless induction furnace (Fig. 3). As in the C R E M process, the action of such vigorous forced convection results in refinement of the grains and a uniformly distributed structure.
Fig. 5. Macrostructures of 1085 aluminium alloy billets of diameter 320 mm: (a) conventional casting; (b) B0', d.c. magnetizing force of 30000 At; (c) CREM process, B(t), a.c. magnetizing force of 5600 At; (d) vibrational method--B0' , d.c. magnetizing force of 10000 At; B(t), a.c. magnetizing force of 5600 At.
172
Ch. VivOs /
Electromagnetic effects on A l alloy microstructure
3.3. Electromagnetic vibrations
An example of the penetration of electromagnetic vibrating forces of frequencies 50 and 100 Hz is shown in Fig. 4. It appears that the magnitude of the 50 Hz vibrations is largely predominant, particularly outside the electromagnetic skin depth, i.e. in the bulk liquid. Moreover, methodical measurements allow one to obtain a rough estimate of the peak of the oscillating electromagnetic pressure, which is of the order of 0.03 bar. On the other hand, subsidiary measurements allowed the determination of the maximum of the fluctuating pressure of frequency 50 Hz, which was of the order of 0.16 bar, while the corresponding acceleration was more than 100 g.
macrostructure is coarsened by the application of a steady magnetic field B 0, (Fig. 5(b)), while inspection of Fig. 5(c) reveals that the refinement is substantially improved by the CREM process. However, the superior efficiency of the vibrational method is clearly apparent here, where the mean grain size of the equiaxed grains is of the ordder of 150 ~m. This superiority is confirmed by methodical experiments carried out on billets and slabs of 2214 aluminium alloy, which is characterized by a wide freezing range. Moreover, the thickness of the peripheral segregation zone is practically reduced to zero and the surface aspect significantly improved. These effects are explained by the designed modification of the heat flow distribution (Fig. 3(b)).
4. Metallurgical results References The macrostructures of the materials investigated have been revealed in order to provide information on variations in structure, such as grain size and columnar and equiaxed crystals. To this end, the ingots were sectioned, mechanically polished and then immersed in specific etching solutions. Figure 5 displays macrographs obtained from 1085 aluminium alloy billets of diameter 320 mm. These ingots were produced using a graphite mould and without inoculation of grain-refining master alloys. Because of its relatively high degree of purity (99.85%), this alloy, which is characterized by a very narrow freezing range (i.e. interval between liquidus and solidus temperatures), is among the most difficult to grain refine by means of electromagnetic stirring. In agreement with previous work [8, 10] it also appears that the
1 B. Chalmers, Principles of Solidification, Krieger, Malabar, FL, 1982. 2 M. Flemings, Solidification Processing, McGraw-Hill, New York, NY, 1974. 3 C. Viv~s, US Patent13219, 1983. 4 J. Campbell, Int. Met. Rev., 2 (1981) 71. 5 D. Goel, D. Shunkla and P. Pandey, Trans. Indian Inst. Met., 33 (1980) 196. 6 R. Ricou and C. Viv~s, Int. J. Heat Mass Transfer, 25 (1982) 1579. 7 C. Viv~s, Metall. Trans. B, 20 (1989) 623. 8 J. Hunt, Proc. 2nd Beer-Sheva Seminar on MHD Flows and Turbulence, Israel University Press, Jerusalem, 1980, pp. 249-269. 9 D. Uhlmann, T. Seward and B. Chalmers, Trans. Metall. Soc., 236 (1966) 527. 10 C. Vivbs and C. Perry, Int. J. HeatMass Transfer, 30 (1987) 479.
169
Effects of electromagnetic vibrations on the microstructure of continuously cast aluminum alloys Ch. Viv~s Laboratoire de Magn&ohydrodynamique, Facult~ des Sciences, Universit~ d'A vignon, 33 Rue Louis Pasteur, F-84000 Avignon (France)
Abstract The influence of electromagnetic vibrations imposed during solidification on grain refinement in the 1085 and 2214 aluminium alloys, characterized by a narrow and a wide freezing range respectively, has been examined. The vibrations were produced by the simultaneous application of a stationary magnetic field B 0' and a variable magnetic fidd B of frequency 50 Hz in the sump of continuously cast ingots. Extensive grain refinement has been observed in both alloys with increasing magnetizing force. This study shows that the mean grain size obtained by this vibrational technique is always smaller than that produced by the recently developed CREM (carting, refining, electromagnetic) process.
1. Introduction
The grain structure of an alloy is of great importance, since many of its mechanical properties are directly related to the size, shape and distribution of the grains. It is well known that the production of a fine-grained equiaxed structure leads to a substantial improvement in the quality of the metal and allows an increase in the ingot drop rate and a reduction in cracking. For instance, several dynamic methods, producing a vigorous forced convection in the melt during freezing, lead to substantial grain refinements [1-3]. In such processes heat and fluid flow are controlled by various externally applied forces. These methods primarily include the use of electromagnetic or mechanical stirring. Moreover, many investigators have found that mechanical vibrations of both sonic and ultrasonic character, when applied during solidification of metals and alloys, modify conventionally obtained macrostructures and microstructures [4, 5]. The most commonly observed effect is the suppression of undesirable columnar zones and the development of a fine-grained equiaxed structure. In fact, the effects produced when high intensity sonic or ultrasonic waves are propagated through molten metals can be listed under three headings, namely grain refinement, dispersive effects and degassing, with the result that porosity is reduced. However, such a technique presents several disadvantages. The oscillating rods are very rapidly dissolved when they are immersed into molten aluminium alloys and this circumstance provokes an undesirable 0921-5093/93/$6.00
pollution of the metal. Moreover, the intensity of cavitation is greatest close to the tranducer or the coupling rod face; thus the use of such a system is principally justified for the treatment of metal mixtures on a small scale. Furthermore, in view of both the cost and bulkiness of the equipment, the production of large aluminium alloy ingots by continuous casting seems unrealistic. The aim of this investigation is to present an alternative approach based on the simultaneous application of stationary and variable magnetic fields.
2. The electromagnetic caster
2.1 Working principle In our work the vibrations are produced by an electromagnetic vibrational method and without any mechanical contact with the melt. The principle consists of the simultaneous application of a stationary magnetic field B 0' and a variable magnetic field B in the vicinity of the molten metal during the course of solidification (Figs. 1 and 2); these magnetic fields are nearly parallel to the vertical axis of the ingot. The stationary magnetic fields is generated by at least one coil, which is supplied with direct current. The variable magnetic field is created by another inductor, which is of similar geometry, supplied with an alternating current of frequency N. Under the effect of the periodic current the inductor generates in the melt a variable field B, which in turn gives rise to an induced current density J. The combined action of the collinear fields generates vibrations in the metal pool, which are of dual origin: the interaction of B 0' and J engenders a vibrating force of © 1993 - Elsevier Sequoia. All rights reserved
Ch. VivOs /
170
Electromagnetic effects on AI alloy microstructure
frequency N, while the J × B force consists of a timeindependent component and an oscillatory component of frequency 2N. 2.2. Experimental apparatus Experiments were done on ingots of circular and rectangular cross-sections. The main features of the apparatus are presented schematically in Fig. 2. First, billets (diameter 3 2 0 m m ) of 1085 and 2214 aluminium alloys were case at a drop rate of 50 mm min ~. The induction electromagnetic field was generated by an inductor traversed by an electric current of frequency 50 Hz and located below the top of the ingot mould; the magnetizing force was modulated at will, from 0 to 12600 A-turns (At) by an autotransformer. An externally imposed stationary magnetic field B0', mainly parallel to the gravitational force field vector, was created by a coil situated above the ingot mould and excited by a magnetizing force that could vary between 0 and 50000 At, while the corresponding magnetic field strength B,)' was in the range 0-0.2 T. The slabs of aluminium alloy 2214 of cross-section 700 x 200 mm 2 were cast at a drop rate of 50 mm s ~. The device was similar to that adopted for the billets, the variable and stationary magnetic fields being 11 000 and 60 000 At respectively. The set of probes for electric current density, magnetic field and velocity measurements has already been described [6].
3. Electromagnetic and fluid flow phenomena 3.1. Electromagnetic parameter For sinoidally varying fields one can assume that the values of the axial component at a given point and time are given approximately by J(t)=J,,exp
B(t) = B,, exp
cos cot+~,
-
cos o t + + # ( J , B 1 6
where .1o and B o are peak values on the wall, r is the distance along a radius with respect to the wall, ~b(J,B) is the phase difference between the periodic vectors J and B and 6 is the electromagnetic skin depth.
i c
!
""
4 i/a -
L~
%
~l B(t)
-
,.b
dJ
(a)
B°L
Fig. 1. Principle of production of electromagnetic vibrations.
J Inductor [Bo]~
Pole
Sump Ingot mold
.
I
(b) Induction
co. Is(t)]
~ ,,~v~.~_ ;'~'1 /~
'
~
Stilring~ ~ . J Z ~
~Water
cooling
~E:~ct{°°~agnetic
I '~lngot Fig. 2. Schematic diagram of continuous casting of aluminium alloy ingots using an electromagnetic vibrational method.
Fig. 3. Evolution of casting with total d.C. power input (2214 aluminium alloy billet of diameter 320 ram): (a) conventional casting, P = 0; (b) electromagnetic process, I'= 2.2 kW. Key: a, liquid metal; b, pasty zone; c, solidified metal; d, segregation zone: e, exudations; f, estimated heat flux profile; g, smooth surface; h, height of solidifying metal in contact with ingot mould; i, inductors; j, iron core; n, nuclei.
Ch. VivOs /
15IF° (N'm-3) x 10-5 10
o doB~x e_r//5. (50 Hz) • JoBox e-2r/6"(100Hz)
\
tcmm
\~
0
Electromagnetic effects on AI alloy microstructure
12600 At (AC)
g
1~)
--
Fig. 4. Penetration of electromagnetic vibrating forces of freq/aencies N and 2N.
171
3.2. Electromagnetic stirring The effect of the induction electromagnetic field here is identical to that observed in the C R E M (casting, refining, electromagnetic) process [7]. The time-mean electromagnetic body force ( J x B ) may be resolved into a radial component (principally irrotational) and a vertical component (primarily rotational). The potential forces, balanced by a pressure gradient, result in the formation of a convex surface meniscus, while the rotational forces are responsible for an electromagnetic stirring similar to that encountered in a coreless induction furnace (Fig. 3). As in the C R E M process, the action of such vigorous forced convection results in refinement of the grains and a uniformly distributed structure.
Fig. 5. Macrostructures of 1085 aluminium alloy billets of diameter 320 mm: (a) conventional casting; (b) B0', d.c. magnetizing force of 30000 At; (c) CREM process, B(t), a.c. magnetizing force of 5600 At; (d) vibrational method--B0' , d.c. magnetizing force of 10000 At; B(t), a.c. magnetizing force of 5600 At.
172
Ch. VivOs /
Electromagnetic effects on A l alloy microstructure
3.3. Electromagnetic vibrations
An example of the penetration of electromagnetic vibrating forces of frequencies 50 and 100 Hz is shown in Fig. 4. It appears that the magnitude of the 50 Hz vibrations is largely predominant, particularly outside the electromagnetic skin depth, i.e. in the bulk liquid. Moreover, methodical measurements allow one to obtain a rough estimate of the peak of the oscillating electromagnetic pressure, which is of the order of 0.03 bar. On the other hand, subsidiary measurements allowed the determination of the maximum of the fluctuating pressure of frequency 50 Hz, which was of the order of 0.16 bar, while the corresponding acceleration was more than 100 g.
macrostructure is coarsened by the application of a steady magnetic field B 0, (Fig. 5(b)), while inspection of Fig. 5(c) reveals that the refinement is substantially improved by the CREM process. However, the superior efficiency of the vibrational method is clearly apparent here, where the mean grain size of the equiaxed grains is of the ordder of 150 ~m. This superiority is confirmed by methodical experiments carried out on billets and slabs of 2214 aluminium alloy, which is characterized by a wide freezing range. Moreover, the thickness of the peripheral segregation zone is practically reduced to zero and the surface aspect significantly improved. These effects are explained by the designed modification of the heat flow distribution (Fig. 3(b)).
4. Metallurgical results References The macrostructures of the materials investigated have been revealed in order to provide information on variations in structure, such as grain size and columnar and equiaxed crystals. To this end, the ingots were sectioned, mechanically polished and then immersed in specific etching solutions. Figure 5 displays macrographs obtained from 1085 aluminium alloy billets of diameter 320 mm. These ingots were produced using a graphite mould and without inoculation of grain-refining master alloys. Because of its relatively high degree of purity (99.85%), this alloy, which is characterized by a very narrow freezing range (i.e. interval between liquidus and solidus temperatures), is among the most difficult to grain refine by means of electromagnetic stirring. In agreement with previous work [8, 10] it also appears that the
1 B. Chalmers, Principles of Solidification, Krieger, Malabar, FL, 1982. 2 M. Flemings, Solidification Processing, McGraw-Hill, New York, NY, 1974. 3 C. Viv~s, US Patent13219, 1983. 4 J. Campbell, Int. Met. Rev., 2 (1981) 71. 5 D. Goel, D. Shunkla and P. Pandey, Trans. Indian Inst. Met., 33 (1980) 196. 6 R. Ricou and C. Viv~s, Int. J. Heat Mass Transfer, 25 (1982) 1579. 7 C. Viv~s, Metall. Trans. B, 20 (1989) 623. 8 J. Hunt, Proc. 2nd Beer-Sheva Seminar on MHD Flows and Turbulence, Israel University Press, Jerusalem, 1980, pp. 249-269. 9 D. Uhlmann, T. Seward and B. Chalmers, Trans. Metall. Soc., 236 (1966) 527. 10 C. Vivbs and C. Perry, Int. J. HeatMass Transfer, 30 (1987) 479.