Note: Descriptions are shown in the official language in which they were submitted.
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J. Winter et al 2-2-2
BACKGROUND OF THE INVENTION
Thi~ invention relates to an apparatus for forming
semi-601id thixotropic alloy slurries for use in application~
such as rheocasting, thixocasting, or thixoforging.
PRIOR ART STATEMENT
The known methods for producing semi-solid thixotropic
alloy 61urries include mechanical stirring and inductive
electromagnetic stirring. The processes for producing such a
slurry with a proper structure require a balance between the
~hear rate imposed by the ~tirring and the solidification rate
of the material being cast.
The mechanical stirring approach is be6t exemplified by
reference to U.S. Patent Nos. 3,902,544, 3,954,455, 3,94~,650,
all to Flemings et al. and 3,936,298 to Mehrabian et al. The
mechanical stirring approach i~ al80 described in articles
appearing in AFS International Cast Metals Journal, Sept., 1976,
pages 11-22, by Flemings et al. and AFS Cast Metals Research
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 ~omewhat
different arrangement.
In the mechanical stirring proce6s, 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 approache~ 6uffer from several inherent
problems. The annulus formed between the rotor and the mixing
chamber walls provides a low volumetric flow rate of thixotropic
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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.
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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the solidifying alloy melt. While the indirect nature of this
electromagnetic stirring is an improvement over the mechanical
process, there are still limitations imposed by the nature of
the stirring technique.
With AC inductive 6tirring, the maximum electromagnetic
forces and associated shear are limited to the penetration depth
of the induced currents. Accordingly, the section ~ize 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 solidifying shell i6
present. The inductive electromagnetic stirring process also
requires high power consumption and the resistance heating of
the stirred metal is significant. The resistance heating in
turn increases the required amount of heat extraction for
601idification.
The pulsed DC magnetic field technique is also
effective, however, it i6 not as effective as desired because
the force field rapidly diverges as the distance from the DC
electrode increases. Accordingly, a complex geometry is
required to produce the required high shear rates and fluid flow
patterns to insure production of slurry with a proper
structure. Large magnetic fields are required for this proce6s
and, therefore, the equipment i~ co~tly and very bulky.
The abovenoted Flemings et al. patents make brief
mention of the use of electromagnetic stirring as one of many
alternative stirring techniques which could be u~ed to produce
thixotropic slurrie~. They fail, however, to ~uggest any
indication of how to actually carry out such an electromagnetic
stirring approach to produce such a slurry. The German patent
publication to Feurer et al. 6uggest6 that
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J. Winter et al 2-2-2
it is also po6sible to arrange induction coils on the periphery
of the mixing chamber to produce an electromagnetic field 80 a6
to agitate the melt with the aid of the field. However, Feurer
et al. doe6 not make it clear whether or not the electromagnetic
agitation i6 intended to be in addition to the mechanical
agitation or to be a 6ubstitute therefore. In any event, it is
clear that Feurer et al. is 6uggesting merely an inductive type
elec~romagnetic 6tirring approach.
There i6 a wide body of prior art dealing with
electromagnetic stirring techniques applied during the ca6ting
of molten metals and alloy6. U.S. Patent No6. 3,268,963 to
Mann; 3,995,678 to Zavara6 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., a6 well as an article by Szekely et
al. entitled Electromaqneticallv Driven Flows in Metals
Processinq, Sept. 1976, Journal of Metal6, are illustrative of
the art with respect to casting metals using inductive
electromagnetic stirring provided by 6urrounding induction coils.
In order to overcome the disadvantage6 of inductive
electromagnetic 6tirring it ha6 been found in accordance with
the present invention that electromagnetic 6tirring can be made
more effective, with a substantially increased productivity and
with a le66 complex application to continuou6 type ca6ting
technique6, if a magnetic field which move6 tran6ver6ely of the
mold or casting axis such a6 a rotating field i6 utilized.
The use of rotating magnetic fields for 6tirring molten
metal during ca6ting i6 know a6 exemplified in U.S. Patent No6.
2,963,758 to Pe~tel et al. and 2,861,302
1~7t~82~ J. Winter et al 2-2-2
to Manl'L et al. and in U.K. Pantents 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 ield. 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 m~3tal
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.
Hot-tops are known for use in direct chill casting as
exemplified by U.S. Patent Nos. 3,477,494 to Burkart et al.:
3,612,151 to Harrington et al.; and 4,071,072 to McCubbin.
SUMMARY OF l'HE INVENTION
The disadvantages associated with the prior art
approaches for making thixotropic slurries utilizing either
mechanical agitation or inductive electromagnetic stirring
have been overcome in accordance with the invention disclosed
in our companion Canadian application no. 346,381, filed Feb.
25, 1980. In our companion application magnetohydromagnetic
motion associated with a rotating magnetic field generated
by a single two pole multiphase motor stator i9 used to achieve
the required high shear rates of at least 500 sec.~l for
producing thixotropic semi-solid alloy slurries. The mag-
netohydromagnetic process therein disclosed provides high
volumetric flow rates which make the process particularly
adaptable to continuous or semi-continuous rheocasting.
The present invention is concerned with the design of
the rheocasting mold which is used in the process and
apparatus of our companion application. In constructing a
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J. Winter et al 2-2-2
suitable casting system for use in rheocasting it is difficult
to a6sociate the various elements which make up the system in
such a way that the stirring force field generated by the two
pole induction motor stator extends over the entire
~olidification zone. It i~ preferred to have the manifold which
applies the coolant to the mold wall arranged above the stator.
This can result in a portion of the mold cavity which extends
out of the region wherein an effective magnetic stirring force
is provided. That in turn can cause undesired structural
variations in the rheocasting which is formed.
To overcome this problem in accordance with the present
invention a means is provided for postponing solidification
within the mold cavity until the molten metal is within the
effective magnetic field which provides the desired
magnetohydrodynamic stirring force. This is accomplished in
accordance with the invention by providing the mold with a first
region of low thermal conductivity and a second region of high
thermal conductivity. The region of low thermal conductivity
postpones solidification during the mixing step until the molten
metal is within the magnetic field thereby promoting the
formation of a degenerate dendritic structure throughout the
slurry. Preferably, a partial insulating mold liner is inserted
in the upper portion of the mold to provide the region of lo~
thermal conductivity. The mold liner extends down into the mold
cavity for a distance sufficient so that the magnetic stirring
force field is intercepted at lea~t in part by the partial mold
liner.
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 throughout its cross section degenerate
dendrite primary solid particles in a surrounding
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matrix of molten metal, said apparatus comprising means for
containing moltèn metal, said containing means having a de~ired
cro66 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 i6 cooled for forming said slurry: said mixing
means comprising a single two pole stator for generating a
non-zero rotating magnetic field which moves transversly of a
longitudinal axis of said containing means across the entirety
of said cro~s 6ection of said containing means and over ~aid
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 6aid containing means, said magnetic force being of
6ufficient magnitude to provide said fihearing of said dendrites,
said magnetomotive force providing a shear rate of at lea~t 500
sec. 1 the improvement being wherein said containing mean~
includes a first portion of low thermal conductivity and a
second portion of high thermal conductivity, said portion of low
thermal conductivity extending into said magnetic field for
postponing solidification within said containing means until
said molten metal is within said magnetic field, thereby
promoting the formation of a degenerate dendritic structure
throughout the slurry.
The use of a duplex mold in accordance with thi6
invention having an upper portion of low thermal conductivity
and a lower portion of higher thermal conductivity insures that
the molten metal can ~olidify under the influence of the
rotating magnetic field. This helps the resultant rheocast
casting to have a degenerate dendritic structure throughout its
cro6s section even up to its outer 6urface.
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Accordingly, it is an object of thi6 invention to
provide a rheocasting mold apparatus which is capable of forming
a casting having a rheoca6t structure throughout its entire
cro~s section.
It i6 further object of this invention to provide an
apparatu6 as above wherein the mold cavity includes regions of
differing thermal conductivity for preventing premature
solidification.
The~e and other objects will become more apparent from
the following descrietion and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 i6 a 6chematic representation in partial cross
~ection of an apparatu6 in accordance with thi~ invention for
continuou61y or 6emi-continuou61y casting a thixotropic
semi-solid metal slurry.
Figure 2 i6 a 6chematic repre6entation in partial cro66
6ection of the apparatu6 of Figure 1 during a casting operation.
Figure 3 i6 a partial cro66-6ectional view along the
line 3-3 in Figure 1.
Figure 4 is a schematic bottom view of a non-circular
mold and linear induction motor 6tator arrangement in accordance
with another embodiment of thi6 invention.
Figure 5 i6 a 6chematic repre6entation of the line6 of
force at a given in~tant generated by a four pole induction
motor stator.
Figure 6 i6 a 6chematic repre6entation of the line6 of
force at a given instant generated by a two pole motor 6tator.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMæNTS
In the background of thi6 application there have been
described a number of techniques for forming semi-solid
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thixotropic metal slurrie6 for u6e in rheocasting, thixoca6ting,
thixoforging, etc. Rheoca~ting as the term i~ u~ed herein
refer6 to the formation of a semi-601id thixotropic metal
61urry, directly into a desired ~tructure, 6uch a6 a billet for
later proces6ing, or a die ca6ting formed from the 61urry.
Thixoca6ting or thixoforging re6pectively a6 the terms are u6ed
herein refer to proce66ing which begins with a rheoca~t material
which i6 then reheated for further proces6ing 6uch as die
ca6ting or forging.
This invention i6 principally intended to provide
rheoca6t material for immediate proces6ir.g or for later u6e in
various aeplication of ~uch material, such a6 thixoca6ting and
thixoforging. The advantageæ of rheocasting, etc., have been
amply described in the prior art. Tho6e advantages include
improved ca6ting soundness as compared to conventional die
casting. This re6ult6 becau6e the metal i6 partially 601id a6
it enters the mold and, hence, le66 shrinkage poro6ity occurs.
Machine component life i6 al60 improved due to reduced ero6ion
of die6 and molds and reduced thermal 6hock a660ciated with
rheoca6ting.
The metal compo6ition of thixotropic 61urry compri6e6
primary 601id di6crete particle6 and a 6urround~ng matrix. The
surrounding matrix is solid when the metal composition is fully
solidified and i6 liquid when the metal compo6tion is partially
~olid and partially liquid 61urry. The primary solid particle6
comprise degenerate dendrites or nodules which are generally
6pheroidal in shape. The primary solid particles are made up of
a 6ingle phase or a plurality of phases having an average
compo6ition dieferent from the average compo6ition of the
6urrounding matrix in the fully
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solidified alloy. The matrix itself can compri~e one or more
pha6es upon further solidification.
Conventionally 601idified alloys have branched
dendrite6 which develop interconnected networks as the
temperature is reduced and the weight fraction of solid
increases. In contrast thixotropic metal 61urrie~ consist of
discrete primary degenerate dendrite particle6 separated from
each other by a liquid metal matrix, potentially even up to
solid fractions of 80 weight percent. The primary 601id
particles are degenerate dendrites in that they are
characterized by smoother surfaces and a leæ~ branched structure
which approaches a spheroidal configuration. The ~urrounding
solid matrix is formed during 601idification of the liquid
matrix subsequent to the formation of the primary solids and
contains one or more phases of the type which would be obtained
during solidification of the liguid alloy in a more conventional
process. The surroundinq solid matrix comprises dendrite6,
single or multiphased compound6, solid solution, or mixtures of
dendrites, and/or compounds, and~or solid solutions.
Referring to Figures 1 and 2 an apparatus 10 for
continuously or 6emi-continuously rheocasting thixotropic metal
slurries is shown. The cylindrical mold 11 is adapted for such
continuous or semi-continuous rheocasting. The mold 11 may be
formed of any desired nonmagnetic material such as 6tainles6
6teel, copper, copper alloy or the like.
Referring to Figure 3 it can be seen that the mold wall
13 is cylindrical in nature. The apparatus 10 and proce66 of
this invention is particularly adapted for making cylindrical
ingot~ utilizing a conventional two pole polyphase induction
motor stator for stirring. However, it is not limited to the
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J. Winter et al 2-2-2
formation of a cylindrical ingot cro6s 6ection since it i~
po6~ible to achieve a transversely or circumferentially moving
magnetic field with a non-cylindrical mold 11 a6 in Figure 4
In the embodiment of Figure 4 the,mold 11 has a rectangular
cros~ sec~ion surrounded by a polypha6e rectangular induction
motor 6tator 12. The magnetic field move6 or rotates around the
mold 11 in a direction normal to the longitudinal axi6 of the
ca6ting which i6 being made. At thi6 time, the preferred
embodiment of the invention is in reference to the u6e of a
cylindrical mold 11.
The bottom block 13 of the mold 11 iB arranged for
movement away from the mold as the ca6ting form& a solidifying
shell. The movable bottom block 13 compri6es a 6tandard direct
chill ca~ting type bottom block. It is formed of metal and i6
arranged for movement between the position shown in Pigure 1
wherein it 6its up within the confines of the mold cavity 14 and
a position away from the mold 11 a6 6hown in Figure 2. Thi6
movement i8 achieved by 6upporting the bottom block 13 on a
6uitable carriage 15. Lead screw6 16 and 17 or hydraulic mean6
are used to raise and lower the bottom block 13 at a de6ired
ca6ting rate in accordance with conventional practice. The
bottom block 13 is arranged to move axially along the mold axis
18. It include6 a cavity 19 into which the molten metal i~
initially poured and which provide6 a 6tabili~ing influence on
the re~ulting ca~ting a6 it i6 withdrawn from the mold 11.
A cooling manifold 20 i6 arranged circumferentially
around the mold wall 21. The particular manifold 6hown includes
a fir6t input chamber 22, a 6econd chamber ~3 connected to the
fir6t input chamber by a narrow slot 24.
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A discharge slot 25 is defined by the gap between the manifold
20 and the mold 11. A uniform curtain of water is provided
ab~ut the outer surface 26 of the mold 11. A suitable valving
arrangemen~ 27 i8 provided to control the flow rate of the water
or other coolant discharged in order to control the rate at
which the slurry S solidifies. In the apparatu~ 10 a manually
operated valve 27 i6 shown, however, if de~ired this could be an
electrically operated valve.
The molten metal which i~ poured into the mold 11 is
cooled under controlled conditions by means of the water sprayed
upon the outer 6urface 26 of the mold 11 from the encompas6ing
manifold 20. By controlling the rate of water flow against the
mold surface Z6 the rate of heat extraction from the molten
metal within the mold 11 is controlled.
In order to provide a means for stirring the molten
metal within the mold 11 to form the desired thixotropic slurry
a two pole multiphase induction motor stator 28 i5 arranged
surrounding the mold 11. The stator 28 is comprised of iron
laminations 29 about which the desired winding6 30 are arranged
in a conventio~al manner to provide a three-phase induction
motor stator. The motor stator 28 i6 mounted within a motor
hou~ing M. The manifold 20 and the motor stator 28 are arranqed
concentrically about the axis 18 of the mold 11 and casting 31
formed within it,
It i6 preferred in accordance with this invention to
utilize a two pole three-phase induction motor stator 28. One
advantage of the two pole motor stator 28 is that there is a
non-zero field acros6 the entire cross section of the mold 11.
It is, therefore, pos6ible with this invention to solidify a
casting having the desired rheoca6t 6tructure
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over it6 full cro66 section.
Figure 5 show6 the in6tantaneou6 line6 of force for a
four pole induction motor 6tator at a given in6tant in time. It
i6 apparent that the center of the mold does not have a de6ired
magnetic field associated with it. Therefore, the ~tirring
action i6 concentrated near the wall 21 of the mold 11. In
comparison thereto, a two pole induction motor stator a~ shown
in Figure 6 generates instantaneous lines of force at a given
in6tant which provide a non-zero field acro6s the entire cross
section of the mold 11. The two pole induction motor stator 28
also provide6 a higher frequency of rotation or rate of stirring
of the slurry S for a given current frequency than the four pole
approach of Figure 5.
A partially enclosing cover 32 is utilized to prevent
spill out of the molten metal and slurry S due to the stirring
action imparted by the magnetic field of the motor stator 28.
The cover 32 compri6e6 a metal plate arranged above the manifold
20 and 6eparated therefrom by a 6uitable ceramic liner 33. The
cover 32 include6 an opening 34 through which the molten metal
flows into the mold cavity 14. Communicating with the opening
34 in the cover is a funnel 35 for directing the molten metal
into the opening 34. A ceramic liner 36 is u6ed to protect the
metal funnel 35 and the opening 34. As the thixotropic metal
61urry S rotate6 within the mold 11, cavity centrifugal forces
cause the metal to try to advance up the mold wall 21. The
cover 32 with its ceramic lining 33 prevents the metal slurry S
from advancing or spilling out of the mold 11 cavity and
causing damage to the apparatus 10. The funnel portion 35 of
the cover 32 al60 serves a6 a re6ervoir of molten metal to keep
the mold 11 filled in order
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to avoid the formation of a U-shaped cavity in the end of the
casting due to centrifugal force~.
Situated directly above the funnel 35 i6 a down6pout 37
through which the molten metal flows from a 6uitable furnace
38. A valve member 39 associated in a coaxial arrangement with
the downspout 37 is u6ed in accordance with conventional
practice to regulate the flow of molten metal into the mold 11.
The furnace 38 may be of any conventional design, it i6
not e6sential that the furnace be located directly above the
mold 11. In accordance with convention direct chill ca6ting
proce66ing the furnace may be located laterally displaced
therefrom and be connected to the mold 11 by a serie6 of troughs
or launder6.
Referring again to Figure 3, a further ad~antage of the
rotary magnetic field stirring approach in accordance with thi6
invention i6 illu6trated. In accordance with the Flemings
right-hand rule for a given current J in a direction normal to
the plane of the drawing the magnetic flux vector B extend~
radially inwardly of the mold 11 and the magnetic 6tirring force
vector F extends generally tangentially of the mold wall 21.
Thi6 6et6 up within the mold cavity a rotation of the molten
metal in the direction of arrow R which generates the de6ired
6hear for producing the thixotropic slurry S. The force vector
F i6 also tangential to the heat extraction direction and i8
normal to the direction of dendrite growth. This maximize6 the
shearing of the dendrite6 as they grow.
It i6 preferred in accordance with thi6 invention that
the 6tirring force field generated by the 6tator 28 extend
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over the full solidification zone of molten metal and
thixotropic metal slurry S. Otherwise the structure of the
casting will comprise regions within the field of the 6tator 28
having a rheocast structure and regions outside the sta~or field
tending to have a non-rheoca6t 6tructure. In the embodiment of
Figures 1 and 2 the solidification zone preferably comprises the
6ump of molten metal and slurry S within the mold 11 which
extend6 from the top surface 40 to the solidification front 41
which divides the solidified casting 31 from the ~lurry S. The
solidification zone extends at least from the region of the
initial onset of solidification and slurry formation in the mold
cavity 14 to the solidification front 41.
Under normal solidification conditions, the periphery
of the ingot 31 will exhibit a columnar dendritic grain
structure. Such a 6tructure is undesireable and detracts from
the overall advantages of the rheocast structure which occupies
most of the ingot cross section. In order to eliminate or
substantially reduce the thickness of this outer dendritic layer
in accordance with this invention the thermal conductivity of
the upper region of the mold 11 is reduced by mean6 of a partial
mold liner 42 formed from an insulator such as a ceramic. The
ceramic mold liner 42 extends from the ceramic liner 33 of the
mold cover 32 down into the mold cavity 14 for a distance
sufficient so that the magnetic 6tirring force field of the two
pole motor stator 28 is intercepted at least in part by the
partial ceramic mold liner 42. The ceramic mold liner 42 i8 a
shell which conform~ to the internal shape of the mold 11 and is
held to the mold wall 21. The mold 11 compri6es a duplex
structure including a low heat conductivity upper
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portion defined by the ceramic liner 42 and a high heat
conductivity portion defined by the exposed portion of the mold
wall 21.
The liner 4Z po6tpones solidification until the molten
metal is in the region of the strong magnetic stirring force.
The low heat extraction rate associated with the liner 42
generally prevents in that portion of the mold 11. Generally
solidification does not occur except towards the downstream end
of the liner 42 or just thereafter. The 6hearing process
resulting from the applied rotating magnetic field will further
override the tendency tO form a solid shell in the region of the
liner 42. This region 42 or zone of low thermal conductivity
thereby he}ps the re6ultant rheocast casting 31 to have a
degenerate dendritic structure throughout it6 cros6 section even
up to it8 outer surface.
Below the region of controlled thermal conductivity
defined by the liner 42, the normal type of water cooled metal
casting mold wall 21 is present. The high heat tran6fer rates
associated with thi6 portion of the mold 11 promote ingot shell
formation. However, because of the zone 42 of low heat
extraction rate even the peripheral shell of the casting 31
should con6ist of degenerate dendrites in a 6urrounding matrix.
It is preferred in order to form the desired rheocast
structure at the surface of the ca~ting to effectively shear any
initial solidified gro~th from the mold liner 42. This can be
accomplished by insuring that the field a~ociated with the
motor ~tator 28 extends over at least that portion of the liner
42 at which solidification i~ first initiated.
The dendrites which initially form normal to the
periphery of the casting mold 11 are readily sheared off due to
the
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metal flow resulting from the rotating magnetic field of the
induction motor ~tator 28. The dendrite~ which are sheared off
continue to be ~tirred to form degenerate dendrite~ until they
are trapped by the 601idifying interface 41. Degenerate
dendrites can also form directly within the 61urry because the
rotating ~tirring action of the melt doe~ not permit
preferential growth of dendrite6. To in~ure this the stator Z8
length should ereferably extend over the full length of the
solidification zone. In particular the 6tirring force field
associated with the stator 28 6hould preferably extend over the
full length and cross section of the 601idification zone with a
sufficient magnitude to generate the desired 6hear rate6.
To form a rheocasting 31 utilizing the apparatus 10 of
Figures 1 and 2 molten metal is poured into the mold cavity 14
while the motor stator 28 is energized by a suitable three-phase
AC current of a desired magnitude and frequency. After the
molten metal is poured into the mold cavity it is stirred
continuously by the rotating maqnetic field produced by the
motor stator 28. Solidification begins from the mold wall 21.
The highest 6hear rate~ are generated at the 6tationary mold
wall 21 or at the advancing solidification front 41. By
properly controlling the rate of 601idification by any desired
mean6 as are known in the prior art the desired thixotropic
61urry S i~ formed in the mold cavity 14. As a 601idifying
6hell is formed on the casting 31, the bottom block 13 is
withdrawn downwardly at a desired casting rate.
The 6hear rate6 which are obtainable with the process
and apparatu6 10 of thi6 invention are much higher than tho6e
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reported for the mechanical stirring proces6 and can be achieved
over much larger cro66-6ectional area6. The~e high 6hear rate6
can be extended to the center of the ca~ting cross section even
when the 601id shell of the solidifying slurry S is already
present.
The induction motor stator 28 which provide6 the
6tirring force needed to produce the degenerate dendrite
rheocast 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 motor
6tator 28 and mold 11 are located below the cooling manifold 20.
The continuous casting apparatus 10 of this invention
is particularly advantageous as compared to the proces6e6 and
apparatuses described in the prior art. In those processes the
stirrinq chamber i8 located above a continuou6 ca~ting mold and
the thixotropic slurry S is delivered to the mold. This has the
disadvantage that the mold i~ hard to fill and entrainment of
oxides is enhanced. In accordance with this invention the
stirring chamber comprises continuous casting mold 11 itself.
Thi6 process doe6 not 6uffer from the transfer of contamination
problems of the prior art continuous ca6ting process.
It is preferred in accordance with the process and
apparatus of this invention that the entire casting solidify in
the stator 28 field in order to produce castings with proper
rheocast structure through their entire cross section.
; Therefore, the casting apparatus 10 in accordance with this
invention should preferably be designed to insure that the
entire 601idification zone is within the stator 28 field.
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This may require extra long ~tators 28 to be provided to handle
some types of casting.
In accordance with thi6 invention two competing
processe6 shearing and golidification are controlling. The
6hearing produced by the electromagnetic proces6 and apparatu6
of thi6 invention can be made equivalent to or greater than that
obtainable by mechanical stirring. The interaction between
shear rate6 and cooling rate6 cau6e6 higher 6tator currents to
be required for continuou~ type casting then are required for
~tatic casting.
It ha6 been found in accordance with this invention
that the effect6 of the experimental variable6 in the proce~
can be predicted from a consideration of two dimensionless
groups, namely Band N a6 follows:
B = ~/~UJ ~H~ R2 (1)
N R2 B2 (2)
~0
where j = ~ -1
w = angular line frequency
6 = melt electrical conductivity
H~ = magnetic permeability
R = melt radius
B~ = magnetic induction at the mold wall
n~ = melt visco6ity.
The fir6t group,B , is a measure of the field geometry effects,
while the 6econd group. N, appear6 as a coupling coefficient
between the magnetomotor body forceæ and the a660ciated velocity
field. The computed velocity and 6hearing fields for a single
value of ~ as a function of the parameter N can be determined.
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J. Winter et al 2-2-2
From these determinations it has been found that the
shear rate increases sharply toward the outside o~ 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 6hould still be shear stresses in the melt and
they should be maximal at the liquid solid interface 41.
Further because there are always ~hear stre~ses at the advancing
interface 41 it is possible to make a full section ingot 31 with
the appropriate degenerate dendritic rheocast structure.
The stator current and shear rates required to achieve
the desired degenerate dendritic thixotropic slurry S are very
much higher than those required to achieve fine dentritic grains
in accordance with the prior art as set forth in the background
of this application. The process and apparatus 10 of this
invention offer several unique advantages in contrast to the
processes of the prior art. For example, the loss of magnetic
field strength due to the presence of solidifying metal is small
due to the low frequency which is used. The equipment
associated with the apparatus 10 of this invention is relatively
easy to fabricate since two pole induction motor stators 28 are
well-known in the art. The apparatus 10 of this invention has a
relatively 1GW 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 this invention i6
indirect and, therefore, has insignificant associated erosion
problems. Another advantage
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J. Winter et al 2-Z-2
of the present process and apparatus is the high volumetric flow
rates which are obtainable. This i6 particularly important if
one desires to carry out the rheoca6ting proce~6 continuou61y or
semi-continuously. The duplex mold arrangement comprising
region6 of low and high thermal conductivity produces casting
having the desired rheocast ~tructure throughout while allowing
flexibility in the arrangment of various components of the
ca6ting 6y~tem.
EXAMPLE I
Ingot6 2.5 inche6 in diameter of alloy 6061 were cast
using an apparatus 10 similar to that shown in Figures 1 and 2.
The bottom bloc~ 13 wa6 lowered and the casting was drawn from
the mold 11 at 6peeds of from about 8 to 14 inches per ~inute.
The two pole three-pha6e induction motor stator 28 current was
varied between 5 and 35 amps. It wa6 found that at the low
current end of thi6 range, a fine dendritic grain 6tructure wa~
produced but not the characteristic 6tructure of a rheocast
thixotropic slurry. At the high current end of the range
particularly in and around lS amps fully non-dendritic
structures were generated having a typical rheocast structure
comprising generally spheroidal primary solids surrounded by a
solid matrix of different compo6ition.
The mold cover 32 by enclosing the mold cavity 14
except for the 6mall centrally located opening 34 6erves 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 least
partially fill the funnel 35 it i6 possible to in~ure that the
mold cavity 14 is completely
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filled with molten metal and slurry. The cover 32 offsets the
centrifugal forces and prevents the formation of the U-~haped
cavity on solidification. By completely filling the mold oxide
entrainment in the re~ulting casting is 6ubstantially reduced.
While it is preferred in accordance with this invention
that the ~tirring force due to the magnetic field extend over
the entire solidification zone it i~ recognized that the
shearing action on the dendrites results from the rotating
movement of the melt. This metal 6tirring movement can cause
6hearing of dendrites outside the field if the moving molten
metal pool extends outside the field.
Dendrites will initially attempt to grow from the 6ides
or wall 21 of the mold 11. The solidifying metal at the bottom
of the mold may not be dendritic because of the comparatively
low heat extraction rate which promote6 the formation of more
equiaxed grains.
Suitable stator currents for carrying out the process
of this invention will vary depending on the 6tator which is
used. The currents mu6t be sufficiently high to provide the
de~ired magnetic field for generating the desired shear rates.
Suitable shear rates for carrying out the proces6 of
this invention comprise from at lea6t about 100 sec. to
about 1500 ~ec. -1, and preferably from at least about 500
sec. to about 1200 sec. . For aluminum and it6 alloys a
shear 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 ~hould be
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J. Winter et al 2-2-2
from about 0.1 C per minute to about 1000 C per minute and
preferably from about 10 C per minute to about 500 C per
minute. For aluminum and its alloy6 an average cooling rate of
from about 40C per minute to about 500 C per minute ha~
been found to be 6uitable. The efficiency of the
magneto-hydrodynamic 6tirring allows the use of higher cooling
rate6 than with prior art stirring proces6es. Higher cooling
rates yield highly de6irable finer grain 6tructure~ in the
re6ulting rheoca6ting. Further, for continuous rheoca6ting
higher throughput follow6 from the u6e of higher cooling rate6.
The parameter ¦B ¦ (~defined by eguation (1)) for
carrying out the proce66 of thi6 invention 6hould compri6e 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 proces6 of thi6 invention 6hould comprise from
about 1 to about 1000 and preferably from about 5 to about 200.
The angular line frequency ~ for a casting having a
radius of from about 1" to about 10" 6hould be from about 3 to
about 3000 hertz and preferably from about 9 to about 2000 hertz.
The magnetic field strength which i6 a function of the
angular line frequency and the melt radiu6 6hould comprise from
about 50 to 1500 gau66 and preferably from about 100 to about
600 gaus~.
The particular parameters employed can vary from metal
sy6tem to metal 6y6tem in order to achieve the desired shear
rate6 for providing the thixotropic 61urry. The appropriate
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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 aking place. Magnetohydro-
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 movinq 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.
It is apparent that there has been provided in accordance
with this invention an apparatus for making thixotropic
metal slurries which fully satisfies the objects, means and
advantages sot forth hereinbefore. While the invention has
been described in combination with specific embodiments
thereof, it is evident that many alternatives, 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 claims.
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