CA1176819A - Process and apparatus for making thixotropic metal slurries - Google Patents
Process and apparatus for making thixotropic metal slurriesInfo
- Publication number
- CA1176819A CA1176819A CA000346381A CA346381A CA1176819A CA 1176819 A CA1176819 A CA 1176819A CA 000346381 A CA000346381 A CA 000346381A CA 346381 A CA346381 A CA 346381A CA 1176819 A CA1176819 A CA 1176819A
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- CA
- Canada
- Prior art keywords
- mold
- molten metal
- slurry
- casting
- section
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
- B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
- B22D11/10—Supplying or treating molten metal
- B22D11/11—Treating the molten metal
- B22D11/114—Treating the molten metal by using agitating or vibrating means
- B22D11/115—Treating the molten metal by using agitating or vibrating means by using magnetic fields
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C1/00—Making non-ferrous alloys
- C22C1/12—Making non-ferrous alloys by processing in a semi-solid state, e.g. holding the alloy in the solid-liquid phase
Abstract
ABSTRACT OF THE DISCLOSURE
A process and apparatus for forming a semi-solid thixotropic alloy slurry and preferably a rheocasting therefrom. Molten metal in a mold is cooled under controlled conditions while it is mixed under the influence of a moving magnetic field. A non-zero, magnetic field is provided across the full cross section of the mold and over the entire solidification zone. This results in magnetomotive stirring force of sufficient magnitude to provide mixing of the molten metal to form the slurry. Preferably, a two pole induction motor stator is used to generate the magnetic field.
A process and apparatus for forming a semi-solid thixotropic alloy slurry and preferably a rheocasting therefrom. Molten metal in a mold is cooled under controlled conditions while it is mixed under the influence of a moving magnetic field. A non-zero, magnetic field is provided across the full cross section of the mold and over the entire solidification zone. This results in magnetomotive stirring force of sufficient magnitude to provide mixing of the molten metal to form the slurry. Preferably, a two pole induction motor stator is used to generate the magnetic field.
Description
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J. Winter et al 1-1-1 BACKGROUND OF THE INVENTION
This invention relates to a process and apparatus for forming semi-solid thixotropic alloy slurries for use in applications such as rheocasting, thixocasting, or thixoforging.
PRIOR ART STATEMENT
The known methods for producing semi-~olid thixotropic alloy slurries include mechanical stirring and inductive electromagnetic stirring. The processes for producing such a slurry with a proper structure require a balance between the shear rate imposed by the stirring and the ~olidification rate of the material being cast.
The mechanical stirring approach is best exemplified by reference to U.S. Patent Nos. 3,902,544, 3,954,455, 3,948,650, all to Flemings et al. and 3,936,298 to Mehrabian et al. The mechanical stirring approach is also de~cribed in articles appearing in AFS Interna~ional Cast Metals Journal, Sept., 1976, page~ 22, by Flemings et al. and AFS Cast Metals Researc~
Journal, Dec., 1973, pages 167-171, by Fascetta et al. In German OLS 2,707,774 published September 1, 1977 to Feurer et al. the mechanical stirring approach is shown in a somewhat different arrangement.
In the mechanical stirring process, the molten metal flows downwardly into an annular space in a cooling and mixing chamber. Here the metal is partially solidified while it is agitated by the rotation of a central mixing rotor to form the desired thixotropic metal slurry for rheocasting. The mechanical stirring approaches suffer from several inherent problems. The annulus formed between the rotor and the mixing chamber walls provides a low volumetric flow rat~ of thixotropic
J. Winter et al 1-1-1 BACKGROUND OF THE INVENTION
This invention relates to a process and apparatus for forming semi-solid thixotropic alloy slurries for use in applications such as rheocasting, thixocasting, or thixoforging.
PRIOR ART STATEMENT
The known methods for producing semi-~olid thixotropic alloy slurries include mechanical stirring and inductive electromagnetic stirring. The processes for producing such a slurry with a proper structure require a balance between the shear rate imposed by the stirring and the ~olidification rate of the material being cast.
The mechanical stirring approach is best exemplified by reference to U.S. Patent Nos. 3,902,544, 3,954,455, 3,948,650, all to Flemings et al. and 3,936,298 to Mehrabian et al. The mechanical stirring approach is also de~cribed in articles appearing in AFS Interna~ional Cast Metals Journal, Sept., 1976, page~ 22, by Flemings et al. and AFS Cast Metals Researc~
Journal, Dec., 1973, pages 167-171, by Fascetta et al. In German OLS 2,707,774 published September 1, 1977 to Feurer et al. the mechanical stirring approach is shown in a somewhat different arrangement.
In the mechanical stirring process, the molten metal flows downwardly into an annular space in a cooling and mixing chamber. Here the metal is partially solidified while it is agitated by the rotation of a central mixing rotor to form the desired thixotropic metal slurry for rheocasting. The mechanical stirring approaches suffer from several inherent problems. The annulus formed between the rotor and the mixing chamber walls provides a low volumetric flow rat~ of thixotropic
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~76~ J. Winter et al l-i-l slurry. There are material problems due to the erosion of the rotor. It is difficult to couple mechanical agitation to a continuous casting system.
In the continuous rheocasting processes described in the art the mixing chamber is arranged above a direct chill casting mold. The transfer of the metal from the mixing chamber to the mold can result in oxide entrainment. This is a particularly acute problem when dealing with reactive ~ ~ alloys such as aluminum, which are susceptible to oxidation.
; 10 The volumetric flow rates achievable by this approach are inadequate for commercial application.
The slurry is thixotropic, thus requiring high shear rates to effect flow into the continuous casting mold. Using the mechanical approach, one is likely to get flow lines due to interrupted flow and/or discontinuous solidification. The mechanical approach is also limited to producing semi-solid ; slurries, containing from about 30 to 60% solids. Lower fractions of solids improve fluidity but enhance undesired coarsening and dentritic growth during completion of solidifi-cation. It is not possible to get significantly higher fractions of solids because the agitator is immersed in the slurry.
In order to overcome the aforenoted problems inductive electromagnetic stirring has been proposed in U.S. Patent 4,229,210 to Winter et al.entitled "Method for the Preparation of Thix-otropic Slurries". In that patent two electromagnetic stirring techniques are suggested to overcome the limitations of mechanical stirring. Winter et al. use either AC induction or pulsed DC magnetic fields to produce indirect stirring of
~76~ J. Winter et al l-i-l slurry. There are material problems due to the erosion of the rotor. It is difficult to couple mechanical agitation to a continuous casting system.
In the continuous rheocasting processes described in the art the mixing chamber is arranged above a direct chill casting mold. The transfer of the metal from the mixing chamber to the mold can result in oxide entrainment. This is a particularly acute problem when dealing with reactive ~ ~ alloys such as aluminum, which are susceptible to oxidation.
; 10 The volumetric flow rates achievable by this approach are inadequate for commercial application.
The slurry is thixotropic, thus requiring high shear rates to effect flow into the continuous casting mold. Using the mechanical approach, one is likely to get flow lines due to interrupted flow and/or discontinuous solidification. The mechanical approach is also limited to producing semi-solid ; slurries, containing from about 30 to 60% solids. Lower fractions of solids improve fluidity but enhance undesired coarsening and dentritic growth during completion of solidifi-cation. It is not possible to get significantly higher fractions of solids because the agitator is immersed in the slurry.
In order to overcome the aforenoted problems inductive electromagnetic stirring has been proposed in U.S. Patent 4,229,210 to Winter et al.entitled "Method for the Preparation of Thix-otropic Slurries". In that patent two electromagnetic stirring techniques are suggested to overcome the limitations of mechanical stirring. Winter et al. use either AC induction or pulsed DC magnetic fields to produce indirect stirring of
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J. Winter et al 1-1-1 the solidifying alloy melt. While the indirect nature of thi6 el0ctromagnetic stirring i6 an improvement over the mechanical proce6s, there are 6till limitations imposed by the nature of the stirring technique.
With AC inductive 6tirring, the maximum electromagnetic forces and as~ociated shear are limited to the penetration depth of the induced current6. Accordingly, the 6ection 6ize that can be effectively stirred is limited due to the decay of the induced forces from the periphery to the interior of the melt.
This is particularly aggravated when a 601idifying shell i6 present. The inductive electromagnetic stirring proce6s also requires high power consumption and the resistance heating of the 6tirred metal is significant. The resi6tance heating in turn increa6e6 the required amount of heat extraction for solidification.
The pulsed DC magnetic field technigue is also effective, however, it is not a6 effective a6 desired because the force field rapidly diverges as the distance from the DC
electrode increa6es. Accordingly, a complex geometry i6 required to produce the required high shear rates and fluid flow patterns to insure production of 61urry with a proper ~tructure. Large magnetic fields are required for this process and, therefore, the equipment is c06tly and very bulky.
The abovenoted Flemings et al. patent6 make brief mention of the u6e of electromagnetic stirring as one of many alternative ~tirring technique6 which could be u6ed to produce thixotropic 61urries. They fail, however, to suggest any indication of ho~ to actually carry out 6uch an electromagnetic stirring approach to produce such a slurry. The German patent publication to Feurer et al. 6uggests that
J. Winter et al 1-1-1 the solidifying alloy melt. While the indirect nature of thi6 el0ctromagnetic stirring i6 an improvement over the mechanical proce6s, there are 6till limitations imposed by the nature of the stirring technique.
With AC inductive 6tirring, the maximum electromagnetic forces and as~ociated shear are limited to the penetration depth of the induced current6. Accordingly, the 6ection 6ize that can be effectively stirred is limited due to the decay of the induced forces from the periphery to the interior of the melt.
This is particularly aggravated when a 601idifying shell i6 present. The inductive electromagnetic stirring proce6s also requires high power consumption and the resistance heating of the 6tirred metal is significant. The resi6tance heating in turn increa6e6 the required amount of heat extraction for solidification.
The pulsed DC magnetic field technigue is also effective, however, it is not a6 effective a6 desired because the force field rapidly diverges as the distance from the DC
electrode increa6es. Accordingly, a complex geometry i6 required to produce the required high shear rates and fluid flow patterns to insure production of 61urry with a proper ~tructure. Large magnetic fields are required for this process and, therefore, the equipment is c06tly and very bulky.
The abovenoted Flemings et al. patent6 make brief mention of the u6e of electromagnetic stirring as one of many alternative ~tirring technique6 which could be u6ed to produce thixotropic 61urries. They fail, however, to suggest any indication of ho~ to actually carry out 6uch an electromagnetic stirring approach to produce such a slurry. The German patent publication to Feurer et al. 6uggests that
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J. Winter et al l-l-l it i8 al60 possible to arrange induction coil~ on the periphery of the mixing chamber to produce an electromagnetic field ~o as to agitate the melt with the aid of the field. However, Feurer et al. does not make it clear whether or not the electromagnetic agitation i8 intended to be in addition to the mechanical agitation or to be a 6ubstitute therefore. In any event, it i6 clear that Feurer et al. is suggesting merely an inductive type electromagnetic 6tirring approach.
There is a wide body of prior art dealing with electomagnetic stirring techniques agplied during the casting of molten metals and alloys. U.S. Patent Nos. 3,268,963 to Mann:
3,995,678 to Zavaras et al.; 4,030,534 to Ito et al.; 4,040,467 to Alherny et al.: 4,042,007 to Zavaras et al.; and 4,042,008 to Alherny et al., as well as an article by Szekely et al. entitled ElectromaaneticallY Driven Flows in Metals Processina, 5ept, 1976, Journal of Metals, are illu6trative of the art with re6pect to ca6ting metal6 u6ing inductive electromagnetic stirring provided by 6urrounding induction coil6.
In order to overcome the di~advantage6 of inductive electromagnetic stirring it ha6 been found in accordance with the pre6ent invention that electromagnetic stirring can be made more effective, with a 6ub6tantially increa6ed productivity and with a les6 complex application to continuou6 tyge casting techniques, if a magnetic field which move6 tran6ver6ely of the mold or casting axi6 such a6 a rotating field is utilized.
The u6e of rotating magnetic fields for stirring molten metal6 during ca6ting is known as exemplified in U.S. Patent Nos. 2,963,758 to Pestel et al. and 2,861,302 ~7~ J. winter et al 1~
to Mann et al. and in U.K. Patents 1,525,036 and 1,525,545.
Pestal et al. disclose both static casting and continuous casting wherein the molten metal is electromagnetically stirred by means of a rotating field. One or more multipoled motor stators are arranged about the mold or solidifying casting in order to stir the molten metal to provide a fine grained metal casting. In the continuous casting embodiment disclosed in the patent to Pestal et al. a 6 pole stator is arranged about the mold and two two pole stators are arranged sequentially there-after about the solidifying casting.
SUMMARY OF THE INVENTION
.
This invention overcomes the disadvantages associated with the prior art approaches for making thixotropic slurries utilizing either mechanical agitation or inductive electromag-nstic stirring. In accordance with this invention magneto-hydromagnetic motion associated with a rotating magnetic field generated by a single two pole multiphase motor stator is used to achieve the required high shear rates of at least 500 sec. -1 for producing thixotropic semi-solid alloy slurries. Two pole induction motor stators are fabricated such that a magnetic field is always present between opposing poles of the motor. It has been found in accordance with this invention that a two pole motor stator is required to provide proper stirring of a thixo-tropic metal slurry. A two pole motor stator provides a non-zero magnetic field which moves transversely of a longitudinal axis of the mold across the full cross section of the melt that is to be stirred and over the entire solidification zone. The force field is also tangential to the mold wall which maximizes the effectiveness of the shearing off of dendrites as they grow and it is in a direction generally normal to the dendrite growth direction.
_~_ i~76~ J. Winter et al 1~
More specifically, the invention is directed to an improvement in an apparatus for continuously or semi-continuously forming a semi-solid thixotropic alloy slurry, said slurry comprising tllroughout its cross section dègenerate dendrite primary sOlid particles in a surrounding matrix of molten metal, said apparatus comprising means for containing molten metal, said containing means having a desired cross section;
means for controllable cooling saia molten metal in said containing means; and means for mixing said molten metal for shearing dendrites formed in a solidification zone as said molten metal is cooled for forming said slurry; the improvement being w~erein said mixing means comprises a single two pole stator for generating a non-zero rotating magnetic field which moves transversely of a longitudinal axis of sald containing means across the entirety of said cross section of said containing means and over said entire solidification zone, said moving magnetic field providing a magnetomotive stirring force directed tangetially of said containing means for causing said molten metal and slurry to rotate in said containing means, said magnetomotive force being of sufficient magnitude to provide said shearing of said dendrites, said magnetomotive force providing a shear rate of at least 500 sec. -1.
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J. Wintex et al l-l-l Using the rotating magnetic field of this invention as compared to the induced magnetic field of the above noted Winter et al. patent the loss of magnetic field stxength due to the presence of solidifying metal i~ small due to the low frequency that is used. The apparatus of the present invention has a fairly low power consumption so that there is very little resistance heating of the melt being sti~red. The shear rates obtainable by the electromagnetic stirring apparatus and process of this invention are much higher than those recorded for the mechanical stirring process and can be achieved over much larger cross-sectional areas. These high shear rates can be extended to the centers of the cross section even when the solidifying shell is present. In contrast to the prior art high volumetric flow rates are readily obtainable with the process and apparatus of this invention.
In accordance with one embodiment of the invention, a static casting system is provided wherein a mold is arranged with a two pole polyphase induction motor stator about it.
The motor stator is arranged circumferentially about the mold.
To insure proper mixing of the slurry the stator length is preferably selected to provide a sufficient magnetic force field which extends over the full length of the solidification zone. To form the desired semi-solid slurry molten metal is poured into the mold and cooled under controlled conditions while the rotating electromagnetic field provided by the stator is present during the entire casting process. All dendrites which are formed at the mold surface or solidifi-cation fro~t are readily sheared off due to the flow of the molten metal and slurry produced by the rotating magnetic field.
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J. Winter et al A partially enclosing cover mean6 i8 preferably provided to prevent 6pillout of t~e ~lurry or molten me~al as it is stirred.
In accordance with another embodiment of the invention, the thixotropic slurry i8 cast in a continuous or semi-continuous manner. In this embodiment the molten metal i~
poured into a continuous casting mold which i~ surrounded by a two pole multipha6e induction motor 6tator in the same manner a6 in the previous embodiment. The molten metal ifi poured into the top of the mold. It i6 6tirred by the rotating electromagnetic field as it i6 cooled under controlled condition6 to produce the desired thixotropic 61urry. The solidifying slurry i8 then withdrawn from the bottom of the mold in a continuou6 or semi-continuous manner. Preferably, the continuous casting mold also includes a cover to prevent spillout of the molten metal and slurry as it is stirred. Further, it is preferred that the continuous casting mold include an upper portion or hot-top having a low rate of heat extraction wherein the molten metal is contained in a molten condition with little if any 601idification occuring, followed by a 6econd portion having a higher rate of heat extraction wherein solidification under the influence of the rotatinq magnetic field produce6 the desired 6emi-solid thixotropic slurry.
Accordingly, it is an object of thi6 invention to provide an improved method and apparatu6 for forming 6emi-601id thixotropic metal 61urrie6 for use in rheocasting or thixocasting type application.
It is a further object of this invention to provide a proces6 and apparatu6 a6 above wherein the thixotropic metal 1176~1~ J. Winter et al 1-1-1 61urry is ca~t continuou61y or 6emi-continuously.
The~e and other object~ will become more apparent from the following de6cription6 and drawinga.
BRIEF DESCRIPTION OF THE DRAW I NGS
Figure 1 is a schematic cro6s-sectional view of a 6tatic casting mold in accordance with one embodiment of this invention.
Figure Z i6 a partial cros6-sectional view along the line 2-2 in Figure 1.
Figure 3 is a schematic bottom view of a non-circular mold and linear induction motor stator arran~ement in accordance with another embodiment of this invention.
Figure 4 is a ~chematic representation of the lines of force at a given instant generated by a four pole induction motor stator.
Figure 5 is a schematic representation of the line6 of force at a given in6tant generated by a two pole motor 6tator.
Figure 6 is a schematic representation in partial cross section of an apparatus in accordance with thi6 invention for continuously or ~emi-continuously casting a thixotropic ~emi-~olid metal slurry.
Figure 7 is a 6chematic representation in partial cross section of the apparatus of Figure 6 during a casting operation.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the background of ~his application there have been described a number of techniques for forming semi-solid thixotropic metal ~lurries for use in rheocasting, thixocasting, thixoforging, etc. Rheocasting as the term i6 u6ed herein refers to the formation of a 6emi-solid thixotropic metal slurry, directly into a de~ired structure, such as a _g_ 1~76~9 9064-MB
J. Winter et al 1-1-1 billet for later proce6sing, or a die casting formed from the slurry. Thixocasting or thixoforging respectively a~ the terms are used hereiD refer to processing which begins with a rheocast material which is then reheated for further proce~6ing such a~
die ca6ting or forging.
This invention i~ principally intended to provide rheocast material for immediate proces~ing or for later use in various application of such material, such as thixocasting and thixoforging. The advantages of rheocasting, etc., have been amply described in the prior art. Those advantage6 include improved casting soundness as compared to conventional die casting. This results because the metal i6 partially solid as it enters the mold and, hence, les~ shrinkage porosity occurs.
Machine component life is also improved due to reduced erosion of dies and molds and reduced thermal shock as~ociated with rheocasting.
The metal composition of a thixotropic slurry comprises primary solid discrete particles and a ~urrounding matrix. The surrounding matrix is solid when the metal composition i8 fully solidified and is liquid when the metal compostion is a partially solid and partially liquid slurry. The primary solid particles comprise degenerate dendrites o$ nodules which are generally spheeoidal in shape. The primary 601id parSicles are made up of a single phase or a plurality of phases having an average composition different from the average composition of the surrounding matrix in the fully solidified alloy. The matrix itself can comprise one or more phases upon further solidification.
Conventionally solidified alloys have branched dendrites which develop interconnected networks a~ the temperature is 7~ ~ ~ 9 9064-MB
J. Winter et al 1-1-1 reduced and the weight fraction of 601id increase6. In contrast thixotropic metal ~lurries consi6t of discrete primary deg~nerate dendrite particles 6eparated from each other by a liquid metal matrix, potentially even up to 601id fractions of 80 weight percent. The primary solid particles are degenerate dendrite6 in that they are characterized by 6moother surface6 and les6 branched structure which approaches a 6pheroidal configuration. The 6urrounding 601id matrix is formed during 601idification of the liquid matrix 6ubsequent to the formation of the primary solid6 and contains one or more pha6e6 of the type which would be obtained during 601idification of the liquid alloy in a more conventional proces~. The surrounding 601id matrix compri6es dendrite6, single or multipha6ed compounds, solid solution, or mixtures of dendrites, and/or compounds, and/or 601id 601utions.
Referring now to Figure 1 there i6 shown an apparatu~
10 in accordance with one embodiment of the pre6ent invention.
The apparatus 10 shown in Figure 1 compri6e6 a cylindrical mold 11 for rheocasting a thixotropic metal 61urry a6 de6cribed above in a 6tatic or non-continuou6 manner. The mold 11 is formed of any de6ired nonmagnetic material, such as copper, copper alloy, 6tainle66 6teel or the like. The bottom 12 of the mold 11 compri6es a plate 6ealingly 6ecured a6 by a tight mechanical fit to the tapered cylindrical wall 13. The top end of the mold 11 include6 a partially enclo6ing cover plate 14 similarly 6ecured to the mold wall 13. The cover plate 14 includes a ceramic liner 15 internally of the mold 11 and a ceramic funnel 16 communicating with an opening 17 in the cover 14 throug~
which molten metal is introduced into the mold 11. The purpose of the cover 1~68~9 9~64-MB
J. Winter et al 1-1-1 plate 14 and liner 15 i6 to prevent spillage of molten metal from the mold during the 6tirring operation. The funnel 16 serves to direct the molten metal into the mold 11.
Referring to Figure 2 it can be seen that the mold wall 13 i6 cylindrical in nature. The apparatu6 10 and proce6s of this invention i8 particularly adapted for making ~ylindrical ingots utilizing a conventional two pole polyphase induction motor stator for 6tirring. However, it i8 not limited to the formation of a cylindrical ingot cro6s ~ection 6ince it is pos6ible to achieve a transversely or circumferentially moving magnetic field with a non-cylindrical mold 11 as in Figure 3.
In the embodiment of Figure 3 the mold 11 has a rectangular cro6s 6ection surrounded by a polyphase rectangular induction motor stator la. The magnetic field moves or rotates around the mold 11 in a direction normal to the longitudinal axis of the casting which is being made. At this time, the preferred embodiment of the invention is in reference to the u~e of a cylindrical mold 11.
Referring again to Figure6 1 and 2, the molten metal which is poured into the mold Il through the opening 17 is cooled within the mold 11 under controlled conditions by means of water sprayed upon the outer surface 19 of the mold 11 from an encompassing manifold 20. By controlling the rate of water flow against the mold æurface 19 the rate of heat extraction from the molten metal within the mold 11 can be controlled. The coolant application manifold 20 i6 of a conventional de6ign comprising an inlet chamber 21 connected by a relatively narrow slot 22 to an output chamber 23 which di6charge6 the water or other desired coolant through a di6charge slot 24. The discharge 610t 24 is angled to direct the water against ~76~19 J. Winter et al 1-1-1 the outer surface 19 of the mold 11. A valve 25 in the inlet connection Z6 to the inlet chamber 21 of the manifold 20 is used to control the rate of water flow from the manifold 20 and thereby the rate of heat extraction. In the apparatu6 10 a manually operated valve ZS is shown, however, if de~ired this could be an electrically operated valve.
In order to provide a mean~ for &tirring the molten metal within the mold 11 to form the desired thixotropic 61urry a two pole multipha6e induction motor stator 27 is arranged surrounding the mold 11. The stator 27 is compri~ed of iron lamination~ 28 about which the de6ired windings 29 are arranged in a conventional manner to provide a three-phase induction motor stator. The motor stator 27 i6 mounted within a motor hou~ing 30. The manifold 20 and the motor stator 27 are arranged concentrically about the axis 31 of the mold 11 and casting 32 formed within it.
It is preferred in accordance with thi6 invention to utilize a two pole three-pha6e induction motor stator 27. One advantage of the two pole motor 6tator 27 i8 that there is a non-zero field acro66 the entire cros6 6ection of the mold 11.
It i6, therefore, po66ible with thi6 invention to solidify a ca6ting having the de6ired rheoca~t 6tructure over its full cross section.
Figure 4 shows the in~tantaneous lines of force for a four pole induction motor 6tator at a given instant in time. It i6 apparent that the center of the mold does not have a desired magnetic field as60ciated with it. Therefore, the stirring action is concentrated near the wall 13 of the mold 11. In compari60n thereto, a two pole induction motor 6tator as shown in Figure S generates in6tantaneous lines il ~6~ 9064-MB
J. Winter et al 1-1-1 of ~orce at a given instant which provide a non-zero field acro~ the entire cro6~ section of the mold 11. The two pole induction motor stator 27 also provides a higher frequency of rotation or rate of ~tirring of the slurry S for a given current frequency than the four pole approach of Figure 4.
Referring again to Figure 2, a further advantage of the rotary magnetic field stirring approach in accordance with thi~
invention is illustrated. In accordance with the Fleming~
right-hand rule for a given current J in a direction normal to the plane of the drawing the magnetic flux vector B extends radially inwardly of the mold 11 and the magnetic stirring force vector F extends generally tangentially of the mold wall 13.
This sets up within the mold cavity a rotation of the molten metal in the direction of arrow R which generates the desired shear for producing the thixotropic slurry S. The force vector F is also tangential to the heat extraction direction and i~
normal to the direction of dendrite growth. Thi~ maximizes the shearing of the dendrites as they grow.
It is preferred in accordance with this invention that the stirring force field generated by the ~tator 27 extend over the full solidification zone 33 of molten metal and thixotropic metal slurry S. Otherwise the structure of the casting will comprise regions within the field of the ~tator 27 having a rheocast structure and regions outside the stator field tending to have a non-rheoca6t ~tructure. In the embodiment of Figure 1 the solidification zone 33 preferably comprises the sump of molten metal and ~lurry S within the mold 11 which extend~ from the top surface 35 to the solidification front 34 which divides the solidified casting ~7~819 9064-MB
J. Winter et al 1-1-1 32 from the slurry S. The ~olidification zone 33 extend~ at least from the region of the inital onset of solidification and slurry formation in the sump to the solidifica~ion front 35.
To form a rheocasting 32 utilizing the apparatus 10 of Figure 1 molten metal is poured into the mold cavity while the motor stator 27 is energized by a ~uitable three-phase AC
current of a desired magnitude and frequèncy. After the molten metal is poured into the mold cavity it is stirred continuously by the rotating magnetic field produced by the motor stator 27.
Solidification begins from the mold wall 13. The highest shear rates are generated at the stationary mold wall 13 or at the advancing ~olidification front 35. By properly controlling the rate of solidification by any desired means as are known in the prior art the desired thixotropic slurry S is formed.
The ~hear rates which are obtainable with the process and apparatus 10 of this invention are much higher than those reported for the mechanical stirring process and can be achieved over much larger cross-sectional areas. As previously noted, these high shear rates can be extended to the center of the casting cross section even when the solid shell of the solidifying slurry ~ is already present.
The induction motor stator 27 which provides the stirring force needed to produce the degenerate dendrite rheoca6t structure can be readily placed either above or below the primary cooling manifold 20 as desired. Preferably, however, in accordance with this invention, the induction mo~or stator 27 and mold 11 are located below the cooling manifold 20.
~1 ~76~319 9064-MB
J. Winter et al 1~
The stator current and shear rates required to achieve the de6ired degenerate dendritic thixotropic slurry S are very much higher than tho6e required to achieve fine dendritic grains in accordance with the prior art as set forth in the background of this application. The process and apparatus 10 of thi6 invention offer several unique aavantages in contrast to the processes of the prior art. For example, the 10~8 of magnetic field ~trength due to the presence of 601idifying metal i6 6mall due to the low frequency which is u6ed. The equipment associated with the appartus 10 of this invention is relatively easy to fabricate since two pole induction motor stators 27 are well-known in the art. The apparatus 10 of this invention ha6 a relatively low power consumption and because of the relatively low current as compared to the AC induction method there is little resistance heating of the melt being stirred. The rotating magnetic field stirring method of thi6 invention is indirect and, therefore, has insignificant associated erosion problems. Another advantage of the present process and apparatu6 i6 the high volumetric flow rates which are obtainable. This is particularly important if one de6ires to carry out the rheocasting process continuously or semi-continuously.
Referring to Figures 6 and 7 an apparatus 10' for continuously or 6emi-continuously rheocasting thixotropic metal slurrie6 i6 6hown. While at first glance the mold 36 in accordance with thi6 embodiment appears to be similar to the mold 11 of Figure 1 there are some very unique differences. The mold 36 is adapted for continuous or semi-continuous rheocasting. The mold 36 may be formed of any desired nonmagnetic material 6uch a stainle66 steel, J. Winter et al 1-1-1 copper, or copper alloy as in ~he previous embodiment. However, the bottom block 37 of the mold 36 is arranged for movement away from the mold 36 as the casting forms a 601idifying shell. The movable bottom block 37 compri6e6 a standard direct chill casting type bottom block.
The bottom block 37 i6 formed of metal and i~ arranged for movement between the po6ition shown in Figure 6 wherein it sits up within the confines of the mold wall 38 and a position away from the mold 36 as 6hown in Figure 7. This movement is achieved by supporting the bottom block 37 on a suitable carriage 39. Lead screws 40 and 41 or hydraulic means are u6ed to rai6e and lower the bottom block 37 at a desired casting rate in accordance with conventional practice. The bottom block 37 is arranged to move axially along the mold axis 42. It include6 a cavity 43 into which the molten metal i6 initially poured and which provides a 6tabilizing influence on the re6ulting ca6ting as it i6 withdrawn from the mold 36.
A cooling manifold 44 i6 arranged circumferentially around the mold wall 38. The particular manifold 6hown include6 a first input chamber 45, a second chamber 46 connected to the fir6t input chamber by a narrow slot 47. A discharge slot 4~ is defined by the gap between the manifold 44 and the mold 36. A
unifor~ curtain of water iæ provided about the outer 6urface 49 of the mold 36. A suitable valving arrangement 50 is provided to control the flow rate of the water di6charged in order to control the rate at which the 61urry S 601idifie6.
As in the previous embodiment, a two pole three-phase inductor motor stator 51 i6 arranged concentrically about ~L176~19 9 o 6 4 -MB
J. Winter et al 1-1-1 the mold 36 ~o that the magnetic force6 generated by the stator act upon the slurry S over its complete zone of 601idification.
The 6tator compri6e6 lamination6 52 and three-phase windings 53.
A partially enclosing cover 54 is utilized to prevent 8pill out of the molten metal and slurry S due to the ~tirring action imparted by the magnetic field of the motor stator 51.
The cover 54 comprises a metal plate arranged above the manifold 44 and ~eparated therefrom by a ~uitable ceramic liner 55. The cover 54 include6 an opening 56 through which the molten metal flows into the mold cavity. Communicating with the opening 56 in the cover 54 is a funnel 57 for directing the molten metal into the opening 56. A ceramic liner 58 i6 u6ed to protect the metal funnel 57 and the opening 56. As the thixotropic metal slurry S rotate6 within the mold 36, cavity centrifugal force6 cause the metal to try to advance up the mold wall 38. The cover 54 with its ceramic lining 55 prevent6 the metal slurry from advancing or 6pilling out of the mold 36 cavity and causing damage to the apparatus 10'.
Situated directly above the funnel 57 i~ a downspout 59 through which the molten metal flows from a 6uitable furnace 60. ~ valve member 61 a6sociated in a coaxial arrangement with the downspout 59 i8 u6ed in accordance with conventional practice to regulate the flow of molten metal into the mold 36.
The furnace 60 may be of any conventional design, it i6 not e66ential that the furnace be located directly above the mold 36. In accordance with conventional direct chill casting proce66ing the furnace may be located laterally ..
J. Winter et al 1~
di6placed therefrom and be connected to the mold 36 by a seLie6 of trough6 or launders.
Under normal 601idification conditions, the periphery of the ingot 32' will exhibit a columnar dendritic grain structure. Such a structure i6 unde~irable and detract~ from the overall advantage~ of the rheocast 6tructure which occupies most of the ingot cro66 section. In order to eliminate or substantially reduce the thickness of this outer dendri~ic layer the thermal conductivity of the upper region of the mold 36 i6 reduced by mean6 of a partial mold liner 6Z formed from an insulator such a6 a ceramic. The ceramic mold liner 62 extends from the ceramic liner 55 of the mold cover 54 down into the mold 36 cavity for a distance 6ufficient 60 that the magnetic stirring force field of the two pole motor stator 51 is intercepted at least in part by the partial ceramic mold liner 62. The ceramic mold liner 62 is a 6hell which conforms to the internal shape of the mold 36 and is held to the mold wall 38.
The mold 36 comprise6 a duplex ~tructure including a low heat conductivity portion defined by the ceramic liner 62 and a relatively higher heat conductivity portion defined by the exposed portion of the mold wall 38.
The liner 62 postpone6 601idification until the molten metal i6 in the region of the 6trong magnetic 6tirring force.
The low heat extraction rate a660ciated with the liner 62 generally prevents 601idification in that portion of the mold.
Generally solidification does not occur except towards the downstream end of the liner 62 or just thereafter. The shearing proce6s re6ulting from the applied rotating magnetic field will further override the tendency to form a 601id 6hell in the region of the liner 62. Thi6 region 62 or zone ~4 ~76 ~9 9064-MB
J. Winter et al 1-1-1 of low thermal conductivity thereby helps ~he resultant rheocast casting 32' to have a degenerate dendritic structure throughout its cross section even up to its outer surface.
Below ~he region of controlled ~hermal conduc~ivity defined by the liner 62, the normal type of water cooled metal casting mold wall 38 is present. The high heat transfer rates associated with this portion of the mold 36 promote ingot shell formation. However, because of the zone 62 of lo~J heat extraction rate even the peripheral shell of the casting 32' should consist of degenerate dendrites in a surrounding matrix.
It is preferred in order to form the desired rheocast structure at the surface of the casting to effectively shear any initial solidiEied growth from the mold liner 6~. This can be accomplished by insuring that the Eield associated with the motor stator 51 extends over at least that portion of the liner 62 at which solidification is firs~ initiated.
The dendrites which initially form normal to the periphery of the casting mold 36 are readily sheared off due ~o the metal flow resulting from the rot~ting magnetic field of the induction motor stator 51. The dendrites which are sheared off continue to be s~irred to form degenerate dendrites until they are trapped by the solidifying interface 63. Degenerate dendrites can also form directly within the slurry because the rotating s~irring action of the melt does not permit preferential growth of dendrites. To in~ure this the stator 51 length should preferably extend over the full length of the solidification zone, In particular the stirring force field associated wi~h the stator 51 should preferably extend over ~he full length and cross section of the solidification zone with a sufficient magnitude to genarate the desired shear rates.
~17~ 9064-MB
J. Winter et al 1-1-1 The continuous casting apparatus 10' and proce~s of this invention i6 particularly advantageous a~ compared to the proces6es and apparatuses described in the prior art. In those processes the stirring chamber is located above a continuous casting mold and the thixotropic slurry S is delivered to the mold. This has the disadvantage that the mold is hard to fill and entrainment of oxides is enhanced. In accordance with thi~
invention the ~tirring chamber comprises continuous casting mold 36 itself. This process does not suffer from the transfer of contamination problem~ of the prior art continuous casting proce6s .
It is preferred in accordance with the process and apparatus of this invention that the entire casting solidify in the stator 51 field in order to produce casting6 with proper rheocast structure through their entire cross section.
Therefore, the casting apparatus 10 or 10' in accordance with this invention should preferably be designed to insure that the entire solidification zone or sump region is within the stator 51 field. This may require extra long stators 51 to be provided to handle some types of casting.
The method and apparatus 10' of this invention can be extended to non-circular cross section molds 36 by constructing non-circular induction motor stator6 to provide stirring similar to that described by reference to Figure 3.
In accordance with this invention two competing processes shearing and solidification are controlling. The shearing produced by the electromagnetic erocess and apparatus of this invention can be made equivalent to or greater than that obtainable by mechanical stirring. The interaction between shear rates and cooling rates cause6 higher stator -21~
117~
J. Winter et al 1-1-1 current6 to be required for continuous type casting then are required for 6tatic casting.
It has been found in accordance with thi6 invention that the effects of the experimental variable6 in the process can be predicted from a consideration of two dimensionless groups, namely B and N as follows:
B = ~ ~ ~oR2 (1) N = ~R B~ (2) ho where j = ~
= angular line frequency = melt electrical conductivity = magnetic permeability R = mele radius B~ magnetic induction at the mold wall n~, = melt vi8c06ity, The first group,~ , is a measure of the field geometry effects, while the second group, N, appears as a coupling coefficient between the magnetomotor body forces and the associated velocity field. The computed velocity and shearing fields for a single value of 8 as a function of the parameter N can be determined.
From the6e determinations it has been found that the shear rate increases sharply toward the outside of the mold where it reaches its maximum. This maximum shear rate increases with increasing N. It has been concluded that the shearing is produced in the melt because the peripheral boundary or mold wall is rigid. Therefore, even when a solidifying shell is present, there should still be shear stresses in the melt and they should be maximal at the liquid ~1~6~19 9064-MB
J. Winter et al 1-1-1 solid interface 35 or 63. Further because there are alway~
~hear 6tre66e6 at the advancing interface 35 or 63 it i~
possible to make a full ~ection ingot 32 or 32' with the appropriate degenerate dendritic rheoca6t 6tructure.
EXAMPLE I
Using an apparatus 10 6imilar to that ~hown in Figures 1 and 2 a 6emi-solid thixotropic alloy slurry wa6 made from each of two separate aluminum alloy6, 6061 and A 356. The mold compri6ed a stainle6s 6teel crucible. The mold was charged with molten metal corresponding to the re6pective alloy. The molten metal was cooled at an average cooling rate of 50C per minute while under the influence of a rotating magnetic field generated, when a current of 15 amp6 at 60 hertz wa6 pa6sed through the two pole three-phase induction motor 6tator 27. The magnetic induction at the crucible wall 13 was 300 gau6s. The re6ulting alloys had a typical rheocast structure comprising generally spheroidal primary solids surrounded by a ~olid matrix of different compostion.
EXAMPLE II
Ingot6 2.5 inche~ in diameter of alloy 6061 were cast u6ing an apparatus 10' similar to that 6hown in Figure6 6 and 7. The bottom block 37 wa6 lowered and the ca6ting wa6 drawn from the mold 36 at speed6 of from about 8 to 14 inche6 per minute. The two pole three-pha~e induction motor 6tator 51 current was varied between 5 and 35 amps. It was found that at the low current end of this range, a fine dendritic grain 6tructure was produced but not the characteristic structure of a rheocast thixotropic slurry. At the high current end of the range particularly in and around 15 amps fully non-dendritic structures were generated ~ i . .
~7~g 9064-M~
J. Winter et al 1-1-1 having a typical rheocast 6tructure comprising generally gpheroidal primary solid6 surrounded by a solid matrix of different compostion.
The mold covers 14 and 54 by enclosing the mold cavity except for the 6mall centrally located opening 17 or 56 serve not only to prevent spillage of molten metal but also to prevent the formation of a U-shaped cavity in the end of the rheocasting. By adding sufficient molten metal to the mold to at lea6t partially fill the funnel 16 or 57 it is possible to insure that the mold cavity i8 completely filled with molten metal and slurry. The cover 14 or 54 offset~ the centrifugal forces and prevent6 the formation of the U-6haped cavity on solidification. By completely filling the mold oxide entrainment in the resulting casting iB substantially reduced.
While it is preferred in accordance with this invention that the stirring force due to the magnetic field extend over the entire solidification zone it is recognized that the shearing action on the dendrite6 results from the rotating movement of the melt. This metal stirring movement can cause ~hearing of dendrites outside the field if the moving molten metal pool extends outside the field.
Dendrites will initially attempt to grow from the sides or wall of the mold. The solidifying metal at the bottom of the mold may not be dendritic because of the comparatively low heat extraction rate which promotes the formation of more equiaxed grain~.
Suitable stator currents for carrying out the process of thi6 invention will vary depending on the 6tator which is u~ed. The currents mu~t be sufficiently high to provide :~, i~7~9 J. Winter et al 1-1-1 the desired magnetic field for generating the desired shear rates.
Suitable shear rates for carrying out the procesæ of this invention comprise from at lea6t about 100 sec. 1 to about 1500 ~ec. and preferably from at least about 500 sec. to about 1200 sec. . For aluminum and it~ alloys a 6hear rate of from about 700 sec. to about 1100 sec. has been found desirable.
The average cooling rates through the solidification temperature range of the molten metal in the mold should be from about 0.1C per minute to about 1000 C per minute and preferably from about 10 C per minute to about 500C per minute. For aluminum and it6 alloys an average cooling rate of from about 40 C per minute to about 500 C per minute has been found to be suitable. The efficiency of the magneto-hydrodynamic stirring allows the use of higher cooling rates than with prior art stirring processe6. Higher cooling rates yield highly desirable finer grain structures in the resulting rheocasting. Further, for continuous rheocasting higher throughput follows from the use of higher cooling rates.
The parameter~ defined by equation (1)) for carrying out the process of this invention should comprise from about 1 to about 10 and preferably from about 3 to about 7.
The parameter in N (defined by equation (2)) for carrying out the proces~ of this invention 6hould comprise from about 1 to about 1000 and preferably from about 5 to about 200.
1176~9 J. Winter et al 1-1-1 The angular line frequency w for a casting having a radius of from about 1" to about 10" should be from about 3 to about 3000 hertz and preferable from about 9 to about 2000 hertz.
The magnetic field strength which is a function of the angular line frequency and the melt radius should comprise from about 50 to 1500 gauss and preferably from about 100 to about 600 gauss.
The particular parameters employed can vary from metal system to metal system in order to achieve the desired shear rates f~r providing the thixotropic slurry. The appropri-ate parameters for alloy systems other than aluminum can be determined by routine experimentation in accordance with the principles of this invention.
Solidification zone as the term is used in this appli-cation refers to the zone of molten metal or slurry in the mold wherein solidification is taking place. Ma~netohydro-dynamic as the term is used herein refers to the process of stirring molten metal or slurry using a moving or rotating magnetic field. The magnetic stirring force may be more appropriately referred to as a magnetomotive stirring force which is provided by the moving or rotating magnetic field of this invention.
The process and apparatus of this invention is applicable to the full range of materials as set forth in the prior art including but not limited to aluminum and its alloys, copper and its alloys and steel and its alloys.
B
~17681~ 9064-M8 J. Winter et al 1-1-1 It is apparent that there has been provided in accordance with this invention a process and apparatus for making thixotropic metal slurries which fully 6ati6fy the objects, mean6 and advantages set forth hereinbefore. While the invention has been described in combination with specific embodiment~ thereof, it is evident that many alternative~, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications and variations as fall within the spirit and broad scope of the appended claim6.
J. Winter et al l-l-l it i8 al60 possible to arrange induction coil~ on the periphery of the mixing chamber to produce an electromagnetic field ~o as to agitate the melt with the aid of the field. However, Feurer et al. does not make it clear whether or not the electromagnetic agitation i8 intended to be in addition to the mechanical agitation or to be a 6ubstitute therefore. In any event, it i6 clear that Feurer et al. is suggesting merely an inductive type electromagnetic 6tirring approach.
There is a wide body of prior art dealing with electomagnetic stirring techniques agplied during the casting of molten metals and alloys. U.S. Patent Nos. 3,268,963 to Mann:
3,995,678 to Zavaras et al.; 4,030,534 to Ito et al.; 4,040,467 to Alherny et al.: 4,042,007 to Zavaras et al.; and 4,042,008 to Alherny et al., as well as an article by Szekely et al. entitled ElectromaaneticallY Driven Flows in Metals Processina, 5ept, 1976, Journal of Metals, are illu6trative of the art with re6pect to ca6ting metal6 u6ing inductive electromagnetic stirring provided by 6urrounding induction coil6.
In order to overcome the di~advantage6 of inductive electromagnetic stirring it ha6 been found in accordance with the pre6ent invention that electromagnetic stirring can be made more effective, with a 6ub6tantially increa6ed productivity and with a les6 complex application to continuou6 tyge casting techniques, if a magnetic field which move6 tran6ver6ely of the mold or casting axi6 such a6 a rotating field is utilized.
The u6e of rotating magnetic fields for stirring molten metal6 during ca6ting is known as exemplified in U.S. Patent Nos. 2,963,758 to Pestel et al. and 2,861,302 ~7~ J. winter et al 1~
to Mann et al. and in U.K. Patents 1,525,036 and 1,525,545.
Pestal et al. disclose both static casting and continuous casting wherein the molten metal is electromagnetically stirred by means of a rotating field. One or more multipoled motor stators are arranged about the mold or solidifying casting in order to stir the molten metal to provide a fine grained metal casting. In the continuous casting embodiment disclosed in the patent to Pestal et al. a 6 pole stator is arranged about the mold and two two pole stators are arranged sequentially there-after about the solidifying casting.
SUMMARY OF THE INVENTION
.
This invention overcomes the disadvantages associated with the prior art approaches for making thixotropic slurries utilizing either mechanical agitation or inductive electromag-nstic stirring. In accordance with this invention magneto-hydromagnetic motion associated with a rotating magnetic field generated by a single two pole multiphase motor stator is used to achieve the required high shear rates of at least 500 sec. -1 for producing thixotropic semi-solid alloy slurries. Two pole induction motor stators are fabricated such that a magnetic field is always present between opposing poles of the motor. It has been found in accordance with this invention that a two pole motor stator is required to provide proper stirring of a thixo-tropic metal slurry. A two pole motor stator provides a non-zero magnetic field which moves transversely of a longitudinal axis of the mold across the full cross section of the melt that is to be stirred and over the entire solidification zone. The force field is also tangential to the mold wall which maximizes the effectiveness of the shearing off of dendrites as they grow and it is in a direction generally normal to the dendrite growth direction.
_~_ i~76~ J. Winter et al 1~
More specifically, the invention is directed to an improvement in an apparatus for continuously or semi-continuously forming a semi-solid thixotropic alloy slurry, said slurry comprising tllroughout its cross section dègenerate dendrite primary sOlid particles in a surrounding matrix of molten metal, said apparatus comprising means for containing molten metal, said containing means having a desired cross section;
means for controllable cooling saia molten metal in said containing means; and means for mixing said molten metal for shearing dendrites formed in a solidification zone as said molten metal is cooled for forming said slurry; the improvement being w~erein said mixing means comprises a single two pole stator for generating a non-zero rotating magnetic field which moves transversely of a longitudinal axis of sald containing means across the entirety of said cross section of said containing means and over said entire solidification zone, said moving magnetic field providing a magnetomotive stirring force directed tangetially of said containing means for causing said molten metal and slurry to rotate in said containing means, said magnetomotive force being of sufficient magnitude to provide said shearing of said dendrites, said magnetomotive force providing a shear rate of at least 500 sec. -1.
-6a-.,~, ',~
1176~
J. Wintex et al l-l-l Using the rotating magnetic field of this invention as compared to the induced magnetic field of the above noted Winter et al. patent the loss of magnetic field stxength due to the presence of solidifying metal i~ small due to the low frequency that is used. The apparatus of the present invention has a fairly low power consumption so that there is very little resistance heating of the melt being sti~red. The shear rates obtainable by the electromagnetic stirring apparatus and process of this invention are much higher than those recorded for the mechanical stirring process and can be achieved over much larger cross-sectional areas. These high shear rates can be extended to the centers of the cross section even when the solidifying shell is present. In contrast to the prior art high volumetric flow rates are readily obtainable with the process and apparatus of this invention.
In accordance with one embodiment of the invention, a static casting system is provided wherein a mold is arranged with a two pole polyphase induction motor stator about it.
The motor stator is arranged circumferentially about the mold.
To insure proper mixing of the slurry the stator length is preferably selected to provide a sufficient magnetic force field which extends over the full length of the solidification zone. To form the desired semi-solid slurry molten metal is poured into the mold and cooled under controlled conditions while the rotating electromagnetic field provided by the stator is present during the entire casting process. All dendrites which are formed at the mold surface or solidifi-cation fro~t are readily sheared off due to the flow of the molten metal and slurry produced by the rotating magnetic field.
i~6~ 9064_MB
J. Winter et al A partially enclosing cover mean6 i8 preferably provided to prevent 6pillout of t~e ~lurry or molten me~al as it is stirred.
In accordance with another embodiment of the invention, the thixotropic slurry i8 cast in a continuous or semi-continuous manner. In this embodiment the molten metal i~
poured into a continuous casting mold which i~ surrounded by a two pole multipha6e induction motor 6tator in the same manner a6 in the previous embodiment. The molten metal ifi poured into the top of the mold. It i6 6tirred by the rotating electromagnetic field as it i6 cooled under controlled condition6 to produce the desired thixotropic 61urry. The solidifying slurry i8 then withdrawn from the bottom of the mold in a continuou6 or semi-continuous manner. Preferably, the continuous casting mold also includes a cover to prevent spillout of the molten metal and slurry as it is stirred. Further, it is preferred that the continuous casting mold include an upper portion or hot-top having a low rate of heat extraction wherein the molten metal is contained in a molten condition with little if any 601idification occuring, followed by a 6econd portion having a higher rate of heat extraction wherein solidification under the influence of the rotatinq magnetic field produce6 the desired 6emi-solid thixotropic slurry.
Accordingly, it is an object of thi6 invention to provide an improved method and apparatu6 for forming 6emi-601id thixotropic metal 61urrie6 for use in rheocasting or thixocasting type application.
It is a further object of this invention to provide a proces6 and apparatu6 a6 above wherein the thixotropic metal 1176~1~ J. Winter et al 1-1-1 61urry is ca~t continuou61y or 6emi-continuously.
The~e and other object~ will become more apparent from the following de6cription6 and drawinga.
BRIEF DESCRIPTION OF THE DRAW I NGS
Figure 1 is a schematic cro6s-sectional view of a 6tatic casting mold in accordance with one embodiment of this invention.
Figure Z i6 a partial cros6-sectional view along the line 2-2 in Figure 1.
Figure 3 is a schematic bottom view of a non-circular mold and linear induction motor stator arran~ement in accordance with another embodiment of this invention.
Figure 4 is a ~chematic representation of the lines of force at a given instant generated by a four pole induction motor stator.
Figure 5 is a schematic representation of the line6 of force at a given in6tant generated by a two pole motor 6tator.
Figure 6 is a schematic representation in partial cross section of an apparatus in accordance with thi6 invention for continuously or ~emi-continuously casting a thixotropic ~emi-~olid metal slurry.
Figure 7 is a 6chematic representation in partial cross section of the apparatus of Figure 6 during a casting operation.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the background of ~his application there have been described a number of techniques for forming semi-solid thixotropic metal ~lurries for use in rheocasting, thixocasting, thixoforging, etc. Rheocasting as the term i6 u6ed herein refers to the formation of a 6emi-solid thixotropic metal slurry, directly into a de~ired structure, such as a _g_ 1~76~9 9064-MB
J. Winter et al 1-1-1 billet for later proce6sing, or a die casting formed from the slurry. Thixocasting or thixoforging respectively a~ the terms are used hereiD refer to processing which begins with a rheocast material which is then reheated for further proce~6ing such a~
die ca6ting or forging.
This invention i~ principally intended to provide rheocast material for immediate proces~ing or for later use in various application of such material, such as thixocasting and thixoforging. The advantages of rheocasting, etc., have been amply described in the prior art. Those advantage6 include improved casting soundness as compared to conventional die casting. This results because the metal i6 partially solid as it enters the mold and, hence, les~ shrinkage porosity occurs.
Machine component life is also improved due to reduced erosion of dies and molds and reduced thermal shock as~ociated with rheocasting.
The metal composition of a thixotropic slurry comprises primary solid discrete particles and a ~urrounding matrix. The surrounding matrix is solid when the metal composition i8 fully solidified and is liquid when the metal compostion is a partially solid and partially liquid slurry. The primary solid particles comprise degenerate dendrites o$ nodules which are generally spheeoidal in shape. The primary 601id parSicles are made up of a single phase or a plurality of phases having an average composition different from the average composition of the surrounding matrix in the fully solidified alloy. The matrix itself can comprise one or more phases upon further solidification.
Conventionally solidified alloys have branched dendrites which develop interconnected networks a~ the temperature is 7~ ~ ~ 9 9064-MB
J. Winter et al 1-1-1 reduced and the weight fraction of 601id increase6. In contrast thixotropic metal ~lurries consi6t of discrete primary deg~nerate dendrite particles 6eparated from each other by a liquid metal matrix, potentially even up to 601id fractions of 80 weight percent. The primary solid particles are degenerate dendrite6 in that they are characterized by 6moother surface6 and les6 branched structure which approaches a 6pheroidal configuration. The 6urrounding 601id matrix is formed during 601idification of the liquid matrix 6ubsequent to the formation of the primary solid6 and contains one or more pha6e6 of the type which would be obtained during 601idification of the liquid alloy in a more conventional proces~. The surrounding 601id matrix compri6es dendrite6, single or multipha6ed compounds, solid solution, or mixtures of dendrites, and/or compounds, and/or 601id 601utions.
Referring now to Figure 1 there i6 shown an apparatu~
10 in accordance with one embodiment of the pre6ent invention.
The apparatus 10 shown in Figure 1 compri6e6 a cylindrical mold 11 for rheocasting a thixotropic metal 61urry a6 de6cribed above in a 6tatic or non-continuou6 manner. The mold 11 is formed of any de6ired nonmagnetic material, such as copper, copper alloy, 6tainle66 6teel or the like. The bottom 12 of the mold 11 compri6es a plate 6ealingly 6ecured a6 by a tight mechanical fit to the tapered cylindrical wall 13. The top end of the mold 11 include6 a partially enclo6ing cover plate 14 similarly 6ecured to the mold wall 13. The cover plate 14 includes a ceramic liner 15 internally of the mold 11 and a ceramic funnel 16 communicating with an opening 17 in the cover 14 throug~
which molten metal is introduced into the mold 11. The purpose of the cover 1~68~9 9~64-MB
J. Winter et al 1-1-1 plate 14 and liner 15 i6 to prevent spillage of molten metal from the mold during the 6tirring operation. The funnel 16 serves to direct the molten metal into the mold 11.
Referring to Figure 2 it can be seen that the mold wall 13 i6 cylindrical in nature. The apparatu6 10 and proce6s of this invention i8 particularly adapted for making ~ylindrical ingots utilizing a conventional two pole polyphase induction motor stator for 6tirring. However, it i8 not limited to the formation of a cylindrical ingot cro6s ~ection 6ince it is pos6ible to achieve a transversely or circumferentially moving magnetic field with a non-cylindrical mold 11 as in Figure 3.
In the embodiment of Figure 3 the mold 11 has a rectangular cro6s 6ection surrounded by a polyphase rectangular induction motor stator la. The magnetic field moves or rotates around the mold 11 in a direction normal to the longitudinal axis of the casting which is being made. At this time, the preferred embodiment of the invention is in reference to the u~e of a cylindrical mold 11.
Referring again to Figure6 1 and 2, the molten metal which is poured into the mold Il through the opening 17 is cooled within the mold 11 under controlled conditions by means of water sprayed upon the outer surface 19 of the mold 11 from an encompassing manifold 20. By controlling the rate of water flow against the mold æurface 19 the rate of heat extraction from the molten metal within the mold 11 can be controlled. The coolant application manifold 20 i6 of a conventional de6ign comprising an inlet chamber 21 connected by a relatively narrow slot 22 to an output chamber 23 which di6charge6 the water or other desired coolant through a di6charge slot 24. The discharge 610t 24 is angled to direct the water against ~76~19 J. Winter et al 1-1-1 the outer surface 19 of the mold 11. A valve 25 in the inlet connection Z6 to the inlet chamber 21 of the manifold 20 is used to control the rate of water flow from the manifold 20 and thereby the rate of heat extraction. In the apparatu6 10 a manually operated valve ZS is shown, however, if de~ired this could be an electrically operated valve.
In order to provide a mean~ for &tirring the molten metal within the mold 11 to form the desired thixotropic 61urry a two pole multipha6e induction motor stator 27 is arranged surrounding the mold 11. The stator 27 is compri~ed of iron lamination~ 28 about which the de6ired windings 29 are arranged in a conventional manner to provide a three-phase induction motor stator. The motor stator 27 i6 mounted within a motor hou~ing 30. The manifold 20 and the motor stator 27 are arranged concentrically about the axis 31 of the mold 11 and casting 32 formed within it.
It is preferred in accordance with thi6 invention to utilize a two pole three-pha6e induction motor stator 27. One advantage of the two pole motor 6tator 27 i8 that there is a non-zero field acro66 the entire cros6 6ection of the mold 11.
It i6, therefore, po66ible with thi6 invention to solidify a ca6ting having the de6ired rheoca~t 6tructure over its full cross section.
Figure 4 shows the in~tantaneous lines of force for a four pole induction motor 6tator at a given instant in time. It i6 apparent that the center of the mold does not have a desired magnetic field as60ciated with it. Therefore, the stirring action is concentrated near the wall 13 of the mold 11. In compari60n thereto, a two pole induction motor 6tator as shown in Figure S generates in6tantaneous lines il ~6~ 9064-MB
J. Winter et al 1-1-1 of ~orce at a given instant which provide a non-zero field acro~ the entire cro6~ section of the mold 11. The two pole induction motor stator 27 also provides a higher frequency of rotation or rate of ~tirring of the slurry S for a given current frequency than the four pole approach of Figure 4.
Referring again to Figure 2, a further advantage of the rotary magnetic field stirring approach in accordance with thi~
invention is illustrated. In accordance with the Fleming~
right-hand rule for a given current J in a direction normal to the plane of the drawing the magnetic flux vector B extends radially inwardly of the mold 11 and the magnetic stirring force vector F extends generally tangentially of the mold wall 13.
This sets up within the mold cavity a rotation of the molten metal in the direction of arrow R which generates the desired shear for producing the thixotropic slurry S. The force vector F is also tangential to the heat extraction direction and i~
normal to the direction of dendrite growth. Thi~ maximizes the shearing of the dendrites as they grow.
It is preferred in accordance with this invention that the stirring force field generated by the ~tator 27 extend over the full solidification zone 33 of molten metal and thixotropic metal slurry S. Otherwise the structure of the casting will comprise regions within the field of the ~tator 27 having a rheocast structure and regions outside the stator field tending to have a non-rheoca6t ~tructure. In the embodiment of Figure 1 the solidification zone 33 preferably comprises the sump of molten metal and ~lurry S within the mold 11 which extend~ from the top surface 35 to the solidification front 34 which divides the solidified casting ~7~819 9064-MB
J. Winter et al 1-1-1 32 from the slurry S. The ~olidification zone 33 extend~ at least from the region of the inital onset of solidification and slurry formation in the sump to the solidifica~ion front 35.
To form a rheocasting 32 utilizing the apparatus 10 of Figure 1 molten metal is poured into the mold cavity while the motor stator 27 is energized by a ~uitable three-phase AC
current of a desired magnitude and frequèncy. After the molten metal is poured into the mold cavity it is stirred continuously by the rotating magnetic field produced by the motor stator 27.
Solidification begins from the mold wall 13. The highest shear rates are generated at the stationary mold wall 13 or at the advancing ~olidification front 35. By properly controlling the rate of solidification by any desired means as are known in the prior art the desired thixotropic slurry S is formed.
The ~hear rates which are obtainable with the process and apparatus 10 of this invention are much higher than those reported for the mechanical stirring process and can be achieved over much larger cross-sectional areas. As previously noted, these high shear rates can be extended to the center of the casting cross section even when the solid shell of the solidifying slurry ~ is already present.
The induction motor stator 27 which provides the stirring force needed to produce the degenerate dendrite rheoca6t structure can be readily placed either above or below the primary cooling manifold 20 as desired. Preferably, however, in accordance with this invention, the induction mo~or stator 27 and mold 11 are located below the cooling manifold 20.
~1 ~76~319 9064-MB
J. Winter et al 1~
The stator current and shear rates required to achieve the de6ired degenerate dendritic thixotropic slurry S are very much higher than tho6e required to achieve fine dendritic grains in accordance with the prior art as set forth in the background of this application. The process and apparatus 10 of thi6 invention offer several unique aavantages in contrast to the processes of the prior art. For example, the 10~8 of magnetic field ~trength due to the presence of 601idifying metal i6 6mall due to the low frequency which is u6ed. The equipment associated with the appartus 10 of this invention is relatively easy to fabricate since two pole induction motor stators 27 are well-known in the art. The apparatus 10 of this invention ha6 a relatively low power consumption and because of the relatively low current as compared to the AC induction method there is little resistance heating of the melt being stirred. The rotating magnetic field stirring method of thi6 invention is indirect and, therefore, has insignificant associated erosion problems. Another advantage of the present process and apparatu6 i6 the high volumetric flow rates which are obtainable. This is particularly important if one de6ires to carry out the rheocasting process continuously or semi-continuously.
Referring to Figures 6 and 7 an apparatus 10' for continuously or 6emi-continuously rheocasting thixotropic metal slurrie6 i6 6hown. While at first glance the mold 36 in accordance with thi6 embodiment appears to be similar to the mold 11 of Figure 1 there are some very unique differences. The mold 36 is adapted for continuous or semi-continuous rheocasting. The mold 36 may be formed of any desired nonmagnetic material 6uch a stainle66 steel, J. Winter et al 1-1-1 copper, or copper alloy as in ~he previous embodiment. However, the bottom block 37 of the mold 36 is arranged for movement away from the mold 36 as the casting forms a 601idifying shell. The movable bottom block 37 compri6e6 a standard direct chill casting type bottom block.
The bottom block 37 i6 formed of metal and i~ arranged for movement between the po6ition shown in Figure 6 wherein it sits up within the confines of the mold wall 38 and a position away from the mold 36 as 6hown in Figure 7. This movement is achieved by supporting the bottom block 37 on a suitable carriage 39. Lead screws 40 and 41 or hydraulic means are u6ed to rai6e and lower the bottom block 37 at a desired casting rate in accordance with conventional practice. The bottom block 37 is arranged to move axially along the mold axis 42. It include6 a cavity 43 into which the molten metal i6 initially poured and which provides a 6tabilizing influence on the re6ulting ca6ting as it i6 withdrawn from the mold 36.
A cooling manifold 44 i6 arranged circumferentially around the mold wall 38. The particular manifold 6hown include6 a first input chamber 45, a second chamber 46 connected to the fir6t input chamber by a narrow slot 47. A discharge slot 4~ is defined by the gap between the manifold 44 and the mold 36. A
unifor~ curtain of water iæ provided about the outer 6urface 49 of the mold 36. A suitable valving arrangement 50 is provided to control the flow rate of the water di6charged in order to control the rate at which the 61urry S 601idifie6.
As in the previous embodiment, a two pole three-phase inductor motor stator 51 i6 arranged concentrically about ~L176~19 9 o 6 4 -MB
J. Winter et al 1-1-1 the mold 36 ~o that the magnetic force6 generated by the stator act upon the slurry S over its complete zone of 601idification.
The 6tator compri6e6 lamination6 52 and three-phase windings 53.
A partially enclosing cover 54 is utilized to prevent 8pill out of the molten metal and slurry S due to the ~tirring action imparted by the magnetic field of the motor stator 51.
The cover 54 comprises a metal plate arranged above the manifold 44 and ~eparated therefrom by a ~uitable ceramic liner 55. The cover 54 include6 an opening 56 through which the molten metal flows into the mold cavity. Communicating with the opening 56 in the cover 54 is a funnel 57 for directing the molten metal into the opening 56. A ceramic liner 58 i6 u6ed to protect the metal funnel 57 and the opening 56. As the thixotropic metal slurry S rotate6 within the mold 36, cavity centrifugal force6 cause the metal to try to advance up the mold wall 38. The cover 54 with its ceramic lining 55 prevent6 the metal slurry from advancing or 6pilling out of the mold 36 cavity and causing damage to the apparatus 10'.
Situated directly above the funnel 57 i~ a downspout 59 through which the molten metal flows from a 6uitable furnace 60. ~ valve member 61 a6sociated in a coaxial arrangement with the downspout 59 i8 u6ed in accordance with conventional practice to regulate the flow of molten metal into the mold 36.
The furnace 60 may be of any conventional design, it i6 not e66ential that the furnace be located directly above the mold 36. In accordance with conventional direct chill casting proce66ing the furnace may be located laterally ..
J. Winter et al 1~
di6placed therefrom and be connected to the mold 36 by a seLie6 of trough6 or launders.
Under normal 601idification conditions, the periphery of the ingot 32' will exhibit a columnar dendritic grain structure. Such a structure i6 unde~irable and detract~ from the overall advantage~ of the rheocast 6tructure which occupies most of the ingot cro66 section. In order to eliminate or substantially reduce the thickness of this outer dendri~ic layer the thermal conductivity of the upper region of the mold 36 i6 reduced by mean6 of a partial mold liner 6Z formed from an insulator such a6 a ceramic. The ceramic mold liner 62 extends from the ceramic liner 55 of the mold cover 54 down into the mold 36 cavity for a distance 6ufficient 60 that the magnetic stirring force field of the two pole motor stator 51 is intercepted at least in part by the partial ceramic mold liner 62. The ceramic mold liner 62 is a 6hell which conforms to the internal shape of the mold 36 and is held to the mold wall 38.
The mold 36 comprise6 a duplex ~tructure including a low heat conductivity portion defined by the ceramic liner 62 and a relatively higher heat conductivity portion defined by the exposed portion of the mold wall 38.
The liner 62 postpone6 601idification until the molten metal i6 in the region of the 6trong magnetic 6tirring force.
The low heat extraction rate a660ciated with the liner 62 generally prevents 601idification in that portion of the mold.
Generally solidification does not occur except towards the downstream end of the liner 62 or just thereafter. The shearing proce6s re6ulting from the applied rotating magnetic field will further override the tendency to form a 601id 6hell in the region of the liner 62. Thi6 region 62 or zone ~4 ~76 ~9 9064-MB
J. Winter et al 1-1-1 of low thermal conductivity thereby helps ~he resultant rheocast casting 32' to have a degenerate dendritic structure throughout its cross section even up to its outer surface.
Below ~he region of controlled ~hermal conduc~ivity defined by the liner 62, the normal type of water cooled metal casting mold wall 38 is present. The high heat transfer rates associated with this portion of the mold 36 promote ingot shell formation. However, because of the zone 62 of lo~J heat extraction rate even the peripheral shell of the casting 32' should consist of degenerate dendrites in a surrounding matrix.
It is preferred in order to form the desired rheocast structure at the surface of the casting to effectively shear any initial solidiEied growth from the mold liner 6~. This can be accomplished by insuring that the Eield associated with the motor stator 51 extends over at least that portion of the liner 62 at which solidification is firs~ initiated.
The dendrites which initially form normal to the periphery of the casting mold 36 are readily sheared off due ~o the metal flow resulting from the rot~ting magnetic field of the induction motor stator 51. The dendrites which are sheared off continue to be s~irred to form degenerate dendrites until they are trapped by the solidifying interface 63. Degenerate dendrites can also form directly within the slurry because the rotating s~irring action of the melt does not permit preferential growth of dendrites. To in~ure this the stator 51 length should preferably extend over the full length of the solidification zone, In particular the stirring force field associated wi~h the stator 51 should preferably extend over ~he full length and cross section of the solidification zone with a sufficient magnitude to genarate the desired shear rates.
~17~ 9064-MB
J. Winter et al 1-1-1 The continuous casting apparatus 10' and proce~s of this invention i6 particularly advantageous a~ compared to the proces6es and apparatuses described in the prior art. In those processes the stirring chamber is located above a continuous casting mold and the thixotropic slurry S is delivered to the mold. This has the disadvantage that the mold is hard to fill and entrainment of oxides is enhanced. In accordance with thi~
invention the ~tirring chamber comprises continuous casting mold 36 itself. This process does not suffer from the transfer of contamination problem~ of the prior art continuous casting proce6s .
It is preferred in accordance with the process and apparatus of this invention that the entire casting solidify in the stator 51 field in order to produce casting6 with proper rheocast structure through their entire cross section.
Therefore, the casting apparatus 10 or 10' in accordance with this invention should preferably be designed to insure that the entire solidification zone or sump region is within the stator 51 field. This may require extra long stators 51 to be provided to handle some types of casting.
The method and apparatus 10' of this invention can be extended to non-circular cross section molds 36 by constructing non-circular induction motor stator6 to provide stirring similar to that described by reference to Figure 3.
In accordance with this invention two competing processes shearing and solidification are controlling. The shearing produced by the electromagnetic erocess and apparatus of this invention can be made equivalent to or greater than that obtainable by mechanical stirring. The interaction between shear rates and cooling rates cause6 higher stator -21~
117~
J. Winter et al 1-1-1 current6 to be required for continuous type casting then are required for 6tatic casting.
It has been found in accordance with thi6 invention that the effects of the experimental variable6 in the process can be predicted from a consideration of two dimensionless groups, namely B and N as follows:
B = ~ ~ ~oR2 (1) N = ~R B~ (2) ho where j = ~
= angular line frequency = melt electrical conductivity = magnetic permeability R = mele radius B~ magnetic induction at the mold wall n~, = melt vi8c06ity, The first group,~ , is a measure of the field geometry effects, while the second group, N, appears as a coupling coefficient between the magnetomotor body forces and the associated velocity field. The computed velocity and shearing fields for a single value of 8 as a function of the parameter N can be determined.
From the6e determinations it has been found that the shear rate increases sharply toward the outside of the mold where it reaches its maximum. This maximum shear rate increases with increasing N. It has been concluded that the shearing is produced in the melt because the peripheral boundary or mold wall is rigid. Therefore, even when a solidifying shell is present, there should still be shear stresses in the melt and they should be maximal at the liquid ~1~6~19 9064-MB
J. Winter et al 1-1-1 solid interface 35 or 63. Further because there are alway~
~hear 6tre66e6 at the advancing interface 35 or 63 it i~
possible to make a full ~ection ingot 32 or 32' with the appropriate degenerate dendritic rheoca6t 6tructure.
EXAMPLE I
Using an apparatus 10 6imilar to that ~hown in Figures 1 and 2 a 6emi-solid thixotropic alloy slurry wa6 made from each of two separate aluminum alloy6, 6061 and A 356. The mold compri6ed a stainle6s 6teel crucible. The mold was charged with molten metal corresponding to the re6pective alloy. The molten metal was cooled at an average cooling rate of 50C per minute while under the influence of a rotating magnetic field generated, when a current of 15 amp6 at 60 hertz wa6 pa6sed through the two pole three-phase induction motor 6tator 27. The magnetic induction at the crucible wall 13 was 300 gau6s. The re6ulting alloys had a typical rheocast structure comprising generally spheroidal primary solids surrounded by a ~olid matrix of different compostion.
EXAMPLE II
Ingot6 2.5 inche~ in diameter of alloy 6061 were cast u6ing an apparatus 10' similar to that 6hown in Figure6 6 and 7. The bottom block 37 wa6 lowered and the ca6ting wa6 drawn from the mold 36 at speed6 of from about 8 to 14 inche6 per minute. The two pole three-pha~e induction motor 6tator 51 current was varied between 5 and 35 amps. It was found that at the low current end of this range, a fine dendritic grain 6tructure was produced but not the characteristic structure of a rheocast thixotropic slurry. At the high current end of the range particularly in and around 15 amps fully non-dendritic structures were generated ~ i . .
~7~g 9064-M~
J. Winter et al 1-1-1 having a typical rheocast 6tructure comprising generally gpheroidal primary solid6 surrounded by a solid matrix of different compostion.
The mold covers 14 and 54 by enclosing the mold cavity except for the 6mall centrally located opening 17 or 56 serve not only to prevent spillage of molten metal but also to prevent the formation of a U-shaped cavity in the end of the rheocasting. By adding sufficient molten metal to the mold to at lea6t partially fill the funnel 16 or 57 it is possible to insure that the mold cavity i8 completely filled with molten metal and slurry. The cover 14 or 54 offset~ the centrifugal forces and prevent6 the formation of the U-6haped cavity on solidification. By completely filling the mold oxide entrainment in the resulting casting iB substantially reduced.
While it is preferred in accordance with this invention that the stirring force due to the magnetic field extend over the entire solidification zone it is recognized that the shearing action on the dendrite6 results from the rotating movement of the melt. This metal stirring movement can cause ~hearing of dendrites outside the field if the moving molten metal pool extends outside the field.
Dendrites will initially attempt to grow from the sides or wall of the mold. The solidifying metal at the bottom of the mold may not be dendritic because of the comparatively low heat extraction rate which promotes the formation of more equiaxed grain~.
Suitable stator currents for carrying out the process of thi6 invention will vary depending on the 6tator which is u~ed. The currents mu~t be sufficiently high to provide :~, i~7~9 J. Winter et al 1-1-1 the desired magnetic field for generating the desired shear rates.
Suitable shear rates for carrying out the procesæ of this invention comprise from at lea6t about 100 sec. 1 to about 1500 ~ec. and preferably from at least about 500 sec. to about 1200 sec. . For aluminum and it~ alloys a 6hear rate of from about 700 sec. to about 1100 sec. has been found desirable.
The average cooling rates through the solidification temperature range of the molten metal in the mold should be from about 0.1C per minute to about 1000 C per minute and preferably from about 10 C per minute to about 500C per minute. For aluminum and it6 alloys an average cooling rate of from about 40 C per minute to about 500 C per minute has been found to be suitable. The efficiency of the magneto-hydrodynamic stirring allows the use of higher cooling rates than with prior art stirring processe6. Higher cooling rates yield highly desirable finer grain structures in the resulting rheocasting. Further, for continuous rheocasting higher throughput follows from the use of higher cooling rates.
The parameter~ defined by equation (1)) for carrying out the process of this invention should comprise from about 1 to about 10 and preferably from about 3 to about 7.
The parameter in N (defined by equation (2)) for carrying out the proces~ of this invention 6hould comprise from about 1 to about 1000 and preferably from about 5 to about 200.
1176~9 J. Winter et al 1-1-1 The angular line frequency w for a casting having a radius of from about 1" to about 10" should be from about 3 to about 3000 hertz and preferable from about 9 to about 2000 hertz.
The magnetic field strength which is a function of the angular line frequency and the melt radius should comprise from about 50 to 1500 gauss and preferably from about 100 to about 600 gauss.
The particular parameters employed can vary from metal system to metal system in order to achieve the desired shear rates f~r providing the thixotropic slurry. The appropri-ate parameters for alloy systems other than aluminum can be determined by routine experimentation in accordance with the principles of this invention.
Solidification zone as the term is used in this appli-cation refers to the zone of molten metal or slurry in the mold wherein solidification is taking place. Ma~netohydro-dynamic as the term is used herein refers to the process of stirring molten metal or slurry using a moving or rotating magnetic field. The magnetic stirring force may be more appropriately referred to as a magnetomotive stirring force which is provided by the moving or rotating magnetic field of this invention.
The process and apparatus of this invention is applicable to the full range of materials as set forth in the prior art including but not limited to aluminum and its alloys, copper and its alloys and steel and its alloys.
B
~17681~ 9064-M8 J. Winter et al 1-1-1 It is apparent that there has been provided in accordance with this invention a process and apparatus for making thixotropic metal slurries which fully 6ati6fy the objects, mean6 and advantages set forth hereinbefore. While the invention has been described in combination with specific embodiment~ thereof, it is evident that many alternative~, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications and variations as fall within the spirit and broad scope of the appended claim6.
Claims (27)
1. In an apparatus for continuously or semi-continuously forming a semi-solid thixotropic alloy slurry, said slurry comp-rising throughout its cross section degenerate dendrite primary solid particles in a surrounding matrix of molten metal, said apparatus comprising:
means for containing molten metal, said containing means having a desired cross section;
means for controllable cooling said molten metal in said containing means; and means for mixing said molten metal for shearing dendrites formed in a solidification zone as said molten metal is cooled for forming said slurry;
the improvement wherein said mixing means comprises:
a single two pole stator for generating a non-zero rotating magnetic field which moves transversely of a longitudinal axis.
of said containing means across the entirety of said cross section of said containing means and over said entire solidification zone, said moving magnetic field providing a magnetomotive stirring force directed tangentially of said containing means for causing said molten metal and slurry to rotate in said containing means, said magnetomotive force being of sufficient magnitude to provide said shearing of said dendrites, said magnetomotive force providing a shear rate of at least 500 sec. -1.
J. Winter et al 1-1-1
means for containing molten metal, said containing means having a desired cross section;
means for controllable cooling said molten metal in said containing means; and means for mixing said molten metal for shearing dendrites formed in a solidification zone as said molten metal is cooled for forming said slurry;
the improvement wherein said mixing means comprises:
a single two pole stator for generating a non-zero rotating magnetic field which moves transversely of a longitudinal axis.
of said containing means across the entirety of said cross section of said containing means and over said entire solidification zone, said moving magnetic field providing a magnetomotive stirring force directed tangentially of said containing means for causing said molten metal and slurry to rotate in said containing means, said magnetomotive force being of sufficient magnitude to provide said shearing of said dendrites, said magnetomotive force providing a shear rate of at least 500 sec. -1.
J. Winter et al 1-1-1
2. An apparatus as in claim 1 wherein said magnetomotive stirring force is directed normal to a growth direction of said dendrites.
3. An apparatus as in claim 1 wherein said rotating magnetic field generating means comprises a multiphase, two pole induction motor stator.
4. An apparatus as in claim 1 wherein said motor stator comprises a three-phase motor stator.
5. An apparatus as in claim 1 wherein said containing means comprises a mold for forming a casting from said slurry, said stator being arranged surrounding said mold, said mold defining a desired longitudinal casting axis.
6. An apparatus as in Claim 5 wherein said mold has a circular cross section and said stator is arranged concentrically about said mold and said casting axis.
7. An apparatus as in claim 5 wherein said mold has a non-circular cross section.
8. An apparatus as in claim 7 wherein said mold has a rectangular cross section and said stator comprises a rectangular induction motor stator.
9. An apparatus as in claim 5 wherein said mold is formed of metal and includes a mold wall and wherein said cooling means comprises a manifold arranged surrounding said mold for directing water against said mold wall.
J. Winter et al 1-1-1
J. Winter et al 1-1-1
10. An apparatus as in claim 9 wherein said mold comprises a static casting mold.
11. An apparatus as in claim 9 wherein said mold comprises a continuous or semi-continuous casting mold.
12. An apparatus as in claim 5 wherein said cooling means provides an average cooling rate through a solidification temperature range of said molten metal of from about 0.1°C/min. to about 1000°C/min.
13. An apparatus as in claim 5 wherein said magnetomotive force provides shear rates of from about 500 sec. -1 to about 1500 sec. -1.
14. An apparatus as in claim 5 further including means for preventing said molten metal or slurry from spilling out of said mold and for preventing the formation of a solidification cavity in the resulting casting.
15. An apparatus as in claim 14 wherein said spilling and cavity preventing means comprises a mold cover member which substantially encloses said mold except for a central opening therein through which molten metal is introduced into said mold.
16. In a process for continously or semi-continuously forming a semi-solid thixotropic alloy slurry, said slurry com-prising throughout its cross section degenerate dendrite primary J. Winter et al 1-1-1 solid particles in a surrounding matrix of molten metal, said process comprising:
providing a means for containing molten metal having a desired cross section;
controllably cooling said molten metal in said containing means; and mixing said contained molten metal for shearing dendrites formed in a solidification zone as said molten metal is cooled for forming said slurry;
the improvement wherein said mixing step comprises:
generating solely with a two pole stator a non-zero rotating magnetic field which moves transversely of a long-itudinal axis of said containing means across the entirety of said cross section of said containing means and over said entire solidification zone, said moving magnetic field providing a magnetomotive stirring force directed tangentially of said containing means for causing said molten metal and slurry to rotate in said containing means, said magnetomotive force being of sufficient magnitude to provide said shearing of said dendrites, said magnetomotive force providing a shear rate of at least 500 sec. -1.
J. Winter et al 1-1-1
providing a means for containing molten metal having a desired cross section;
controllably cooling said molten metal in said containing means; and mixing said contained molten metal for shearing dendrites formed in a solidification zone as said molten metal is cooled for forming said slurry;
the improvement wherein said mixing step comprises:
generating solely with a two pole stator a non-zero rotating magnetic field which moves transversely of a long-itudinal axis of said containing means across the entirety of said cross section of said containing means and over said entire solidification zone, said moving magnetic field providing a magnetomotive stirring force directed tangentially of said containing means for causing said molten metal and slurry to rotate in said containing means, said magnetomotive force being of sufficient magnitude to provide said shearing of said dendrites, said magnetomotive force providing a shear rate of at least 500 sec. -1.
J. Winter et al 1-1-1
17. A process as in claim 16 wherein said magnetomotive stirring force is directed normal to a growth direction of said dendrites.
18. A process as in claim 16 wherein said step of generating said magnetic field includes providing a multiphase, induction motor stator.
19. A process as in claim 16 wherein said containing means comprises a mold and further including the step of forming a casting from said slurry.
20. A process as in claim 19 wherein said casting has a circular cross section.
21. A process as in claim 19 wherein said casting has a non-circular cross section.
22. A process as in claim 21 wherein said casting has a rectangular cross section.
23. A process as in claim 19 wherein said step of forming said casting is carried out statically.
24. A process as in claim 19 wherein said step of forming said casting is carried out con-tinuously or semi-continuously.
25. A process as in claim 16 wherein said cooling means provides an average cooling rate through a solidification temperature range of said molten metal of from about 0.1°C/min. to about 1000°C/min.
J. Winter et al 1-1-1
J. Winter et al 1-1-1
26. A process as in claim 16 wherein said magnetomotive force provides shear rates of from about 500 sec.-1 to about 15500 sec.-1.
27. A process as in claim 19 further in-cluding the step of preventing said molten metal or slurry from spilling out of said mold and preventing the formation of a solidification cavity in the resulting casting.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US1525079A | 1979-02-26 | 1979-02-26 | |
| US015,250 | 1987-02-17 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1176819A true CA1176819A (en) | 1984-10-30 |
Family
ID=21770356
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000346381A Expired CA1176819A (en) | 1979-02-26 | 1980-02-25 | Process and apparatus for making thixotropic metal slurries |
Country Status (10)
| Country | Link |
|---|---|
| JP (1) | JPS55117556A (en) |
| BR (1) | BR8001053A (en) |
| CA (1) | CA1176819A (en) |
| DE (1) | DE3006588A1 (en) |
| ES (1) | ES488945A0 (en) |
| FR (1) | FR2449499A1 (en) |
| GB (1) | GB2042386A (en) |
| IT (1) | IT1141383B (en) |
| MX (1) | MX152822A (en) |
| SE (1) | SE8001284L (en) |
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| US4494461A (en) * | 1982-01-06 | 1985-01-22 | Olin Corporation | Method and apparatus for forming a thixoforged copper base alloy cartridge casing |
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| CN102172771A (en) * | 2011-03-25 | 2011-09-07 | 天津福来明思铝业有限公司 | Remelting heating process in semisolid thixotropic processing of aluminium alloy |
| JP6384872B2 (en) * | 2015-06-12 | 2018-09-05 | アイダエンジニアリング株式会社 | Method and apparatus for producing semi-solid metal material |
| CN106216618A (en) * | 2016-09-18 | 2016-12-14 | 华北理工大学 | A kind of pour into a mould the method that double metallic composite material is prepared in continuous casting |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2324397B1 (en) * | 1975-09-19 | 1979-06-15 | Siderurgie Fse Inst Rech | METHOD AND DEVICE FOR ELECTROMAGNETIC BREWING OF CONTINUOUS CASTING PRODUCTS |
| JPS5522340A (en) * | 1978-08-03 | 1980-02-18 | Morio Tanaka | Inside collecting device of raw material to be pulverized in pulverizer by stoneepar |
-
1980
- 1980-02-18 SE SE8001284A patent/SE8001284L/en not_active Application Discontinuation
- 1980-02-19 GB GB8005621A patent/GB2042386A/en not_active Withdrawn
- 1980-02-22 DE DE19803006588 patent/DE3006588A1/en active Granted
- 1980-02-22 BR BR8001053A patent/BR8001053A/en unknown
- 1980-02-25 CA CA000346381A patent/CA1176819A/en not_active Expired
- 1980-02-25 IT IT20140/80A patent/IT1141383B/en active
- 1980-02-26 MX MX181329A patent/MX152822A/en unknown
- 1980-02-26 FR FR8004176A patent/FR2449499A1/en active Granted
- 1980-02-26 JP JP2234180A patent/JPS55117556A/en active Granted
- 1980-02-26 ES ES488945A patent/ES488945A0/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| ES8100932A1 (en) | 1980-12-01 |
| JPS6225464B2 (en) | 1987-06-03 |
| GB2042386A (en) | 1980-09-24 |
| BR8001053A (en) | 1980-11-04 |
| DE3006588C2 (en) | 1988-12-01 |
| ES488945A0 (en) | 1980-12-01 |
| IT8020140A0 (en) | 1980-02-25 |
| IT1141383B (en) | 1986-10-01 |
| FR2449499A1 (en) | 1980-09-19 |
| DE3006588A1 (en) | 1980-09-25 |
| FR2449499B1 (en) | 1983-12-30 |
| SE8001284L (en) | 1980-08-27 |
| MX152822A (en) | 1986-06-17 |
| JPS55117556A (en) | 1980-09-09 |
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