Note: Descriptions are shown in the official language in which they were submitted.
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Method and apparatus for making metal alloy castings
The present application relates to a method and apparatus for forming a
continuous product
from a liquid metal alloy, and in particular for making continuous castings
with fine and
uniform microstructures, which may be used as feedstock for secondary
processing methods,
such as thixoforming, forging and machining. The application also relates to
articles formed
from these methods, such as billets, bars, wires, tubes or strips.
In the metal forming industry, a continuous billet is usually produced by
direct chill (DC)
casting process. In this process, an over-heated liquid alloy is fed
continuously into a water-
cooled cylindrical mould and the solidified alloy is withdrawn continuously to
produce a
continuous billet. The resulting cross-section of the billet exhibits three
characteristic zones:
the chilled zone at the surface, the columnar zone and the coarse equiaxed
zone at the centre.
Such microstructural non-uniformity has limited the direct application of the
DC cast billets.
Extensive secondary processing methods, such as rolling, extrusion and
recrystallisation, are
usually used to achieve microstructural refinement and uniformity. Such
secondary
processing routes are energy-intensive, time-consuming and costly. Therefore,
it is desirable
to develop a continuous casting process, which can produce directly continuous
billet with a
fine and uniform microstructure without involving those secondary processing
routes.
One known process uses DC cast billets as feedstock materials is an extrusion
process to
produce bars with either simple or complex cross-sections. Extrasion of metals
can be
classified into two categories: cold extrusion and hot extrusion. In cold
extrusion, the cold
metal is forced into an open die near room temperature. During the cold
extrusion of metals,
considerable pressure is necessary to force the metal through the die block,
resulting a high
cost of capital equipment, a reduced die life and a low energy-efficiency. In
hot extrusion, an
alloy billet is heated below its solidus and extruded thereby like a cold
extrusion process. In
hot extrusion, the die life is improved but the efficiency of the energy is
still limited.
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Meanwhile the accurate extrusion such as fine wire is difficult with such a
hot extrusion
process. It would be advantageous therefore to develop an extrusion process
which can
directly produce bars and wires from liquid alloys.
Another process which uses DC cast billets as feedstock materials is the
rolling process to
produce strip product by successive rolling of a continuously cast billet to
the required
thickness. Compared with the extrusion process, the rolling process is even
more cost
intensive because of the high-energy consumption, high capital equipment cost
and low
material yield.
An alternative technology for strip production is twin-roll casting process,
which is effective
to a limited extent on a variety of metals. In a twin-roll casting process, a
pair of rolls having
horizontal axes and rotating in opposite directions are disposed parallel to
each other with an
appropriate gap therebetween, a pool of liquid alloy is fonned on the upper
circumferential
surfaces of the rolls above the gap and the liquid alloy is continuously cast
into an alloy strip.
The problems associated with the conventional twin-roll casting process
include the leakage of
liquid alloy in the vicinity of the side dams, the damages of the side dams,
crack formation at
the sides of the cast strip, the short life of rolls, the difficulties in
process control and chemical
segregation in the solidified products.
Yet another process which uses DC cast billets as feedstock materials is the
recently
developed thixoforming process. This is basically a two-step process. In the
first step, a
thixotropic feedstock is produced by a modified DC casting process for
creation of a cast billet
with a non-dendritc microstructure. In the second step, a thixotropic
billet/feedstock is
reheated to its semisolid state (at a temperature between its liquidus and
solidus) and then
shaped into a cavity by either casting (thixocasting) or forging
(thixoforging).
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Ideally, a feedstock material for thixoforming should contain a controlled
volume fraction of
fine and spherical solid particles in a matrix. Thixoforming is currently
limited by the low
quality and high cost of the feedstock materials. Processes for thixotropic
feedstock
production include simple mechanical stirring, electromagnetic stirring,
coarsening an initial
dendritic microstructure, addition of excessive grain refiner during
continuous casting and
application of ultrasonic vibration. However, these processes have
considerable
disadvantages. One important disadvantage of the known methods is that the
microstructure
on the cross-section is not uniform, giving rise to difficulties during
reheating process and
relatively poor mechanical properties of the final components. The other
disadvantage is
lo caused by the non-spherical particle morphology, which requires long
reheating time to
spheroidise the solid particles, compromising the potential benefits offered
by semi-solid
processing. A further disadvantage is the high cost for the feedstock
materials, which
currently accounts for up to 50% of the final component cost.
A number of references disclose thixomoulding processes, in which a solid or
semisolid feed
is first processed (for example by heating the feed to liquefy it whilst
subjecting it to shear)
and then injected into a mould to form a component. Examples of such
references include:
EP 0867246 Al (Mazda Motor Corporation); WO 90/09251 (The Dow Chemical
Company);
US 5,711,366 (Thixomat, Inc.); US 5,735,333 (The Japan Steel Works, Limited);
US
2o 5,685,357 (The Japan Steel Works, Limited); US 4,694,882 (The Dow Chemical
Company);
and CA 2,164,759 (Inventronics Limited).
The disadvantage however with heating solid granules in order to convert them
into the
thixotropic state (thixomoulding) rather than cooling liquid metal into the
thixotropic state
(rheomoulding) is that it is very difficult to control particle size and
particle size distribution in
the sub-structure of the thixotropic slurry. Specifically, particle sizes of
thixomoulded slurries
tend to be an order of magnitude larger than those of rheomoulded slurries,
and to have a
wider sized distribution. This has negative implications for the structural
properties of the
casted components.
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Furthermore, the above-mentioned references employ a standard single screw
extruder for
subjecting the thixotropic slurry to shear. The result is a component of low
quality.
A number of references do disclose rheomoulding processes. For example, WO
97/21509
(Thixomat, Inc.) relates to a process for forming metal compositions in which
an alloy is
heated to a temperature above its liquidus, and then employing a single screw
extruder to shear
the liquid metal as it is cooled into the region of two phase equilibrium.
1o US 4,694,881 (The Dow Chemical Company) relates to a process in which a
material having a
non-thixotropic-type structure is fed in solid form into a single screw
extruder. The material is
heated to a temperature above its liquidus, and then cooled to a temperature
lower than its
liquidus and greater than its solidus whilst being subjected to a shearing
action.
WO 95/34393 (Cornell Research Foundation, Inc.) also discloses a rheomoulding
process in
which super-heated liquid metal is cooled into a semisolid state in the barrel
of a single screw
extruder, where it is subjected to shear whilst being cooled, prior to being
injection moulded
into a cast.
WO 01/21343 (Brunel University) is a co-pending application published after
the priority date
of the present invention. It discloses a method of forming a shaped component
from a liquid
metal alloy by cooling the alloy below its liquidus whilst applying shear in
order to convert the
alloy into its thixotropic state. The alloy is then transferred into a
discrete mould to form a
shaped component. There is no disclosure of the formation of a continuous
product.
WO 01/23124 (Brunel University) is another co-pending application published
after the
priority date of the present invention. It relates specifically to a method of
producing a casting
from a metal alloy having at least two immiscible components, wherein the
components are
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subjected to shear and converted into a semi-solid slurry. The slurry can then
be transferred to
a mould or a pre-heated metal band.
None of the thixomoulding or rheomoulding references describe a process which
enables
5 continuous products of a sufficiently high structural integrity to be
formed.
According to a first aspect of the present invention, there is provided a
method for forming a
continuous product from a liquid metal alloy, comprising the steps of cooling
the alloy to
about the liquidus or from the solidus to the liquidus of the alloy, applying
shear to the alloy at
a sufficiently high shear rate and intensity of turbulence to convert the
alloy into its thixotropic
state, and transferring the sheared liquid alloy or sheared semisolid slurry
into a shaping
device in order to form a solid product, wherein the shaping device is capable
of forming a
continuous product.
By "continuous product" is meant a product which is formed continuously, so
that a product of
any length can be formed, provided that sufficient feedstock is provided. This
is in contrast to
a discrete product, such as is formed in a mould.
The shaping device can be a DC caster (DC rheocasting), or an extrusion die
(rheo-extrusion),
or a twin-roll caster (twin-roll rheocasting). The materials can be processed
according to the
present invention can be either miscible or immiscible alloys. In the case of
immiscible alloy,
the process is referred as rheomixing process.
It has been discovered that most of the disadvantages of the prior art the
thixoforming,
conventional DC casting, twin-roll casting and extrusion processes can be
overcome if the
feeding liquid alloy has lower temperature and higher viscosity, which can be
achieved by
shearing liquid alloy at a temperature close to its liquidus or between its
liquidus and soliduss.
The higher viscosity of the intensively sheared liquid alloy or semisolid
slurry can shorten the
solidification time, prevent the leakage in twin roll casting, reduce the
chemical segregation
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during solidification in continuous casting and extrusion and increase the
production rate. The
lower pouring/feeding temperature can also improve die life, energy efficiency
and product
quality.
In a second aspect of the present invention, there is provided a method for
forming a
continuous product from a liquid metal alloy, comprising the steps of cooling
the alloy to
about the liquidus or from the solidus to the liquidus of the alloy, applying
shear to the alloy at
a sufficiently high shear rate and intensity of turbulence to convert the
alloy into its thixotropic
state, and transferring the sheared liquid alloy or sheared semisolid slurry
into a shaping
device in order to form a solid product, wherein the shaping device is capable
of forming a
continuous product, and wherein the alloy is formed from miscible components.
According to a third aspect of the present invention, there is provided a
method for forming a
continuous product from a liquid metal alloy, comprising the steps of cooling
the alloy to
about its liquidus, applying shear to the alloy at a sufficiently high shear
rate and intensity of
turbulence to convert the alloy into its thixotropic state, and transferring
the sheared liquid
alloy into a shaping device in order to form a solid product. Preferably, the
shaping device is
capable of forming a continuous product. The alloy may be formed from either
miscible or
immiscible components.
In a further aspect of the present invention, there is provided a method for
forming a
continuous product from a liquid metal alloy having immiscible components,
comprising the
steps of cooling the alloy to a temperature below its immiscibility gap,
applying shear to the
alloy at a sufficiently high shear rate and intensity of turbulence to convert
the alloy into its
thixotropic state, and transferring the sheared liquid alloy or sheared
semisolid slurry into a
shaping device in order to form a solid product, wherein the shaping device is
capable of
forming a continuous product but wherein the shaping device is not a pre-
heated metal band.
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Generally, the shearing device is a high shear extruder, comprising the steps
of receiving the
liquid alloy into a temperature-controlled barrel, operating the screw,
positioned in barrel, at a
sufficiently high shear rate to convert the liquid alloy into its intensively
sheared state and/or
thixotropic state. The profile of the screw should be specially designed to
provide high shear
rate and high intensity of turbulence and to achieve positive pumping action
during the
transfer of the liquid alloy from one end to another end of the barrel. The
extruder can be any
kinds of extruder which at least has one screw positioned in the barrel.
Preferably, the extruder is a twin-screw extruder having at least two screws
which are at least
partially intermeshed, and more preferably are substantially fully
intermeshed.
The shaping device can be different kinds of dies or moulds with accompanying
accessories
depending on the requirement of the fmal product to be produced. (a) In the DC
rheocasting
process, the die/mould can be a simple cylinder with an optional start-up
base, in which the
cylinder can have a cooling system. The solidified alloy is continuously
extracted out of the
die/mould via the start-up base, whereby a casting is formed continuously. (b)
In the twin-roll
rheocasting process, the die/mould comprises a pair of rotating rolls and a
pair of side dams
disposed on both axial ends of the rolls so that a pool of sheared liquid
alloy or semisolid
slurry is defined by both the rolls and the side dams, the rolls rotate in
counter-directions so
that the sheared liquid alloy or semisolid slurry is subsequently solidified
to form
solidification shells which are then pressure-bonded to each other as they
pass through the gap
defined between the rolls, thus forming a continuous strip. (c) In the rheo-
extrusion process,
the die/mould can be a simple open hole attached on the end of the extruder.
The positive
pumping action provided by the extruder will force the sheared liquid alloy or
semisolid slurry
through the pre-heated open die to form rods or thin wires or any other
suitable sectional
shapes.
The mould may be heated to or maintained at a predetermined temperature when
the sheared
liquid alloy or semisolid slurry is transferred into it. There may be a
predetermined
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relationship between the mould temperature and the metal shearing temperature,
which is
established by the requirements of the individual art.
The continuous casting method and apparatus of the present invention
preferably employs an
the extruder having an inlet toward one end and having an outlet at another
end, a
temperature-controllable barrel communicating said inlet with said outlet and
at least one
screw located within the said barrel.
The said screw in the said extruder preferably includes a body having at least
one vane
thereon, the said vane at least partially defining a helix around said body to
propel the metal
through said barrel, wherein said screw can be rotated to shear said liquid
alloy at a rate
sufficient to inhibit complete formation of dendritic structures therein while
said metal is in a
semisolid state, rotation of the said screw further causing said metal to be
transported from the
said inlet to the said outlet through the said barrel.
In one embodiment, temperature controllable means are employed for adjusting
the
temperature of the extruder barrel, the screw and the alloy, such that the
alloy is maintained
either at a temperature near its liquidus or at a temperature between its
liquidus and solidus.
The extruder outlet may have a controllable valve for transferring the allow
from the extruder
to the shaping device. The amount of liquid alloy or semisolid slurry
transferred from the
extruder to the shaping device can be controlled by said valve in order to
maintain the
consistency of the semisolid slurry in the shaping device.
In a preferred embodiment, an extrusion die is directly attached to the
extruder for production
of continuous rods with various cross-sections. This process is called rheo-
extrusion.
Alternatively, a single screw extruder may be employed in addition to the high
shear device,
wherein the sheared liquid alloy or semisolid slurry discharged from the said
high shear device
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can be extruded via the continuous rotation of the screw in said single screw
extruder for
production of continuous castings like rods or fine wires or billets.
The continuous products formed by the method of the present invention may be
further
deformed by a conventional extrusion process.
Generally, the materials which can be processed according to this invention
can be any
metallic alloys, such as alloys based on Al, Mg, Zn, Cu, Fe and so on. An
important group of
materials, which can be processed according to this invention, are those with
a liquid
immiscibility gap, for instance, those alloys based on Al-Pb, Al-Bi, Al-In and
Cu-Pb.
An important advantage of the method according to this invention is the
resulting fine and
uniform microstructure throughout the cross-section of the billet, wherein a
controlled volume
fraction of fine and spherical particles are uniformly distributed in a
matrix. Consequently, the
billet produced has a high degree of thixotropy and particularly suitable as
feedstock for
thixoforming.
A further advantage of the process of the present invention resides in the
fact that no
secondary processing procedures, such as extrusion and recrystallisation, are
required for
microstructural control, since the billets produced already have a fine and
uniform
microstructure. Therefore, they can be used directly for solid state
processing, such as
machining, forging and so on.
The cylinder type mould/die for the DC rheocasting may be made from any kind
of material,
but preferably the mould is made of graphite or cooper-based alloy. A cooling
system may be
attached to the cylinder type die/mould, by which the sheared liquid alloy or
semisolid slurry
can be solidified at a proper cooling rate. A start-up base may be used at the
initial stage of
casting.
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The rotating rolls in the twin-roll caster can have any kinds of profiles
which are capable of
providing a narrowest gap during turning. Preferably, the profile of the rolls
is flat.
The extrusion die in the rheo-extrusion process can be any kinds of sectional
shapes, including
5 either simple shapes like round, triangle, square, rectangle or complex
shapes like polygon or
any other suitable shapes. The sectional size can be varied in a large scale,
which means that
the products can be a fine wire or a large rod.
The said method may employ sheared liquid alloy at a temperature close to its
liquidus or
10 semisolid slurry with fine and uniform microstructures with different solid
volume fractions
(0% to 750%) via shearing liquid alloy at temperatures between its liquidus
and solidus.
When shearing is carried out at a temperature near its liquidus, a fine and
uniform
microstructure is produced in the finally solidified product due to the
enhanced effective
nucleation rate in the intensively sheared liquid alloy with a uniform
temperature and chemical
composition. When shearing is carried out at a temperature between liquidus
and solidus,
semisolid slurry with fine and spherical particles can be produced under
intensive shearing.
Such a slurry can be directly used for forming a component or for shaping into
billet as a
feedstock for thixoforming process. The said apparatus and method can also
offer metallic
component with the improved mechanical properties due to the effective
modification of
microstructures, especially for alloys close to eutectic composition.
The said apparatus and method preferably comprises the steps of:
= providing liquid alloy in the liquid state and feeding said liquid alloy to
a temperature-
controlled extruder through an inlet, located toward one end of the said
extruder;
= shearing the said liquid alloy by a sufficiently high shear rate offered by
the extruder with
at least one screw to form a sheared liquid alloy or semisolid slurry;
= transferring said sheared liquid alloy or semisolid slurry from the said
extruder into a
shaping device, which can be either a cylinder type mould for DC rheocasting,
or a twin-
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roll nip for twin-roll rheocasting or a open die for rheo-extrusion by opening
a control
valve located at one end of the said extruder; and
= solidifying the sheared liquid alloy or semisolid slurry in the shaping
device to produce
continuous castings with various cross-sections. The said shaping device can
be a DC
caster, or an extrusion die or a twin-roll caster.
Generally, the extruder, consisting of a barrel, one or more screws and a
driving system, is
adopted to receive liquid alloy through an inlet located generally toward one
end of the
extruder. Once in the passageway of the extruder, liquid alloy is either
cooled down or
1o maintained at a predetermined temperature. In either situation, the
processing temperature is
either at a temperature near liquidus or at a temperature between its solidus
and liquidus.
The processing temperature, which depends upon the liquidus and solidus of the
alloy, will
vary from alloy to alloy. The appropriate temperature is apparent to one
skilled in the art.
Also in the extruder, the liquid alloy is subjected to shearing. The shear
rate is such that it is
sufficient to achieve spherical particles and a fine and uniform
microstructure in the final
product. The shearing action is induced by the screw(s) located within the
barrel and is further
invigorated by helical screw flights formed on the body of the screws.
Enhanced shearing is
generated in the annular space between the barrel and the screw flights and
between the flights
of the screws. The positive displacement in extruder can result in the sheared
liquid alloy or
semisolid slurry to travel from the inlet of the extruder toward the outlet of
the extruder, where
it is discharged.
In the twin-roll rheocasting process, a nip, consisting of a pair of rolls, a
pair of side dams and
a driving system, is preferably adopted to receive sheared liquid alloy or
semisolid slurry
through a pool located above and formed by inner surface of the two side dams
and the
upward surface of two rolls. Once in the pool of twin-roll caster, the sheared
liquid alloy or
semisolid slurry is cooled to form a shell on the surface of the rolls. The
temperature of the
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rolls varies from alloy to alloy. The appropriate temperature will be apparent
to one skilled in
the art, but usually it should be lower than the solidus of the alloy. Also in
the twin-roll
casting process, the sheared liquid alloy or semisolid slurry is subjected to
actions of
solidification, deformation, bonding of the solidified shell and continuous
extraction of the
solid strip. The extraction speed is such that it is sufficient to keep the
continuity of the
process. The deformation and bonding are such that it is sufficient to keep
the effective
bonding in the sheet section.
In the case of the rheo-extrusion process, the pressure needed for extrusion
of the sheared
liquid alloy or semisolid slurry may be controlled by the temperature and
shear rate for a given
composition. The variation of the temperature between extruder barrel and the
open die can
drastically reduce the outer friction between the sheared liquid alloy or
semisolid slurry and
the open die during extrusion. The balance of the outer friction, the
viscosity of the sheared
liquid alloy or semisolid slurry and the positive pumping pressure of the
extruder determine
the extrusion speed through the open die. Extrusion dies with different
sectional shapes can be
used to continuous castings with different cross-sectional shapes.
When undertaking rheomixing, either a homogeneous liquid alloy at a
temperature above its
miscibility gap or a preliminarily mixed liquid mixture at a temperature
within the miscibility
gap can be fed into the extruder for intensive mixing to create a fine and
uniform liquid
mixture. Inside the extruder, the liquid alloy experiences both cooling and
intensive shearing.
The extruder can be operated at temperatures either above or below the
monotectic
temperature. The shaping device can be either an extrusion die, or a twin-roll
caster.
A number of preferred embodiments of the invention are described in detail
below with
reference to the drawings, in which:
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Fig. 1 is a schematic illustration of an embodiment of an apparatus for
converting
liquid alloy into an intensively sheared liquid alloy or semisolid slurry
according to the
principles of the present invention;
Fig. 2 is a schematic illustration of an embodiment of an apparatus for
converting
liquid alloy into an intensively sheared liquid alloy or semisolid slurry and
subsequently producing metal billet with a DC-rheocasting process according to
the
principles of the present invention;
Fig. 3 is a schematic illustration of an alternative embodiment of an
apparatus for
converting liquid alloy into an intensively sheared liquid alloy or semisolid
slurry and
subsequently producing metal rod/wire with a rheo-extrusion process using an
extrusion die according to the principles of the present invention;
Fig. 4 is a schematic illustration of an alternative embodiment of an
apparatus of
extrusion die for a rheo-extrusion process.
Fig. 5 is a schematic illustration of an alternative embodiment of an
apparatus for
converting liquid alloy into an intensively sheared liquid alloy or semisolid
slurry and
subsequently producing metal rod/wire with a rheo-extrusion process using a
single
screw extruder according to the principles of the present invention;
Fig. 6 is a schematic illustration of an embodiment of an apparatus for
converting
liquid alloy into an intensively sheared liquid alloy or semisolid slurry and
subsequently producing metal strip with a twin-roll rheocasting process
according to
the principles of the present invention.
In the description of the preferred embodiment, which follows, the casting is
produced by an
extruder and a shaping device from an AZ91D liquid alloy. The invention is not
limited to
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AZ91D magnesium alloy and is equally applicable to any other types of metals
including
aluminium alloys, magnesium alloys, zinc alloys, copper alloys, ferrous alloys
and any other
alloys possibly suitable for shearing-induced metal processing andlor
semisolid metal
processing. Furthermore, specific temperature and temperature ranges cited in
the description
of the preferred embodiment are only applicable to AZ91D magnesium alloy, but
could be
modified readily in accordance with the principles of the invention by those
skilled in the art
in order to accommodate other alloys, such as those based on Al, Mg, Zn and
Cu.
Fig. 1 illustrates an extruder system according to an embodiment of this
invention. A liquid
1o alloy is supplied to the feeder 10. The feeder 10 is provided with a series
of heating elements
11 disposed around the outer periphery. The heating elements may be of any
conventional
type and operate to maintain the feeder 10 at a temperature high enough to
keep the metal
supplied through the feeder 10 in the liquid state. For AZ91D alloy, this
temperature would be
over 600 C, the liquidus of the alloy. The extruder has a plurality of cooling
channels 12 and
heating elements 13 dispersed along the length of the extruder. The matched
cooling channels
12 and heating elements 13 may form a series of heating and cooling zones
respectively. The
heating and cooling zones make it possible to maintain a complex temperature
profile along
the extruder axis, which may satisfy the special requirement during semisolid
processing. The
temperature control of each individual zone is achieved by balancing the
heating and cooling
power input by a central control system through the thermal couple 20. The
methods of
heating can be resistance heating, induction heating or any other means of
heating. The
cooling media may be water or gas or any other media depending on the process
requirement.
While only one heating/cooling zone is shown in figure 1, the extruder can be
equipped with
between 1 to 10 separately controllable heating/cooling zones.
The extruder also has a physical slope or an inclination. The inclination is
usually between 0-
90 relative to the metal sheet-moving plane. The inclination is designed to
assist the transfer
of the sheared liquid alloy or semisolid slurry from the extruder to the next
steps in different
processes.
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The extruder is also provided with two screws 14 that are driven by an
electric motor or
hydraulic motor 16 through a gearbox 17. The two screws 14 are positioned
within barrel 15
and kept in line with end cap 18, 19. The two screws 14 are designed to
provide high shear
5 rate, which are necessary to achieve fine and uniform solid particles.
Different types of screw
profiles may of course be used. In addition, any device that offers high shear
may also be used
to replace the twin-screw extruder.
The sheared liquid alloy or semisolid slurry existing in the extruder is
transferred to shaping
10 device through a valve 21, connected with the end cap 19. The valve 21
operates in response
to a signal from the central control system. The valve 21 is provided for
supplying a
constantly regulated flow of the semisolid slurry so as to form a proper pool
for next step of
the process. The optional opening of valve 21 should match the process
requirement. The
valve 21 can be opened continuously with a limited flow rate or discretely
without flow rate
15 limitation.
Fig. 2 illustrates a DC rheocasting system. The system has two functional
units: a twin-screw
extruder 1 and a DC caster 2. The extruder 1 has been described in Fig. 1. The
DC caster 2
mainly includes a cylindrical mould 31 and a cooling media 33. The mould is
attached on a
supporter 36 with a predetermined gap to extruder 1. A start-up base is
enclosed so as to be
continuously movable. The discharged sheared liquid alloy or semisolid slurry
is enclosed in
the DC caster. The direct chilling unit 31 is filled by cooling media 33 via
inlet 32 and flow
out via outlet 34. The sheared liquid alloy or semisolid slurry in poo130 can
be therefor
solidified to form a continuously billet 34, which is supported by the base 35
for starting of the
process and continuous drawing the billet.
Fig. 3 illustrates a rheo-extrusion system. The system is modified from an
extruder, as shown
in Fig. 1. A temperature-controlled extrusion die 8 is directly attached to
the outlet valve 21.
The extrusion die 8 has a separate thermal couple 9 to maintain the required
die temperature
CA 02417822 2008-05-27
16
and has an outlet 7 to extrude the alloy. The higher temperature variation
around open die
may significantly reduce the resistant to the flow of the sheared liquid alloy
or semisolid
slurry. The sheared liquid alloy or semisolid slurry is forced out from the
extrusion die to
form a rod or a wire.
Fig. 4 illustrates an alternative extrusion die, in wliich an alternative
cross sectional shape 7 is
incorporated in the extrusion die 8 to produce different cross-sectional
shapes. Generally, the
cross-section of the extrusion die can be any simple or complex shapes, or any
other shapes
suitable for extrusion.
Fig. 5 illustrates an alternative rheo-extrusion system. The system has two
functional units: a
twin-screw extruder and a single screw extruder. The twin-screw extruder has
been described
in Fig. 1. The single screw extruder is used as a shaping device for
production rods/wires
from the sheared liquid alloy or semisolid slurry. The single screw extruder
consists of a
barre142, screw 43, nozzle 41 and heating elements 45 along the barre142 with
a thermal
couple 46 for the required temperature. The sheared liquid alloy or seniisol{d
slurry is
discharged into the single screw extruder via the valve 21. The continuous
rotation of the
screw 43 forced the sheared liquid alloy or semisolid slurry forward to the
nozzle 41 to form a
continuous product. The temperature control of barre142 via heating elements
45 may
significantly reduce the flow-out resistant of the sheared liquid alloy or
semisolid sluiry or
semisolid slurry. The sectional shapes of the nozzle 41 determine the shape of
the extruded
part. The nozzle can be simply a small circle, which forms a metal wire, or
any other possible
shapes. Such a prior art extruder can be modified by application of the
alternative
einbodiment in the present invention wherein at least the open die can be
modified to different
shapes.
Fig. 6 illustrates a twin-roll rheocasting system. The system has two
functional units: a twin-
screw extruder 1 and a DC caster 2. The extruder 1 has been described in Fig.
1. The
DC caster 2 mainly includes a pair of rolls 22 and a pair of side dams 23. The
twin-rolls
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17
are disposed horizontally and parallel with each other with a predetermined
gap, one or both of
the rolls are supported so as to be selectively movable in the radial
direction of the roll. The
rolls 22 rotate in the directions indicated by the arrows. The interior of
each roll 22 may
constitute a cooling jacket or heating jacket. Side dams 23 are disposed in
contact with the
rolls 22 in the axial direction of the roll. A pool 24 for reserving the
semisolid slurry is
formed between upper surfaces of the opposite ends of the rolls 22 and the
inner surface of the
two side dams 23. The transferred semisolid slurry in pool 24 can be further
solidified to form
a shell 25 on the surfaces of rolls 22. Under this condition, the rolls 22 is
rotated in the
direction of the arrows shown in the figure so that the solidified shells 25
formed on the
surfaces of the rolls 22 are pulled down and bonded together by pressure to
form a
continuously cast strip 26.
As noted in Fig. 6, the roll peripheral surfaces are generally flat, i.e., the
rolls are of plain
cylindrical form. Such a prior art caster can be modified by application of
the alternative
embodiment in the present invention wherein at least a portion of the roll
surface is a convex
or concave surface or any other appropriate surfaces.
Generally, the system has a control device to realise all the functions.
Preferably, the control
device is programmable so that the desired characteristics of the metal can be
achieved.
Control device (not shown in the Figures) may, for example, comprise a
microprocessor,
wllich may be easily and quickly reprogrammed to change the relative
parameters depending
on the type of the finished product.
The embodiment may also have a presence device attached to the extruder to
increase the
pressure in the barrel. The embodiment may also have a presence device
attached to the
extruder and relative parts to supply protective gas in order to avoid
oxidation. Such a gas
may be argon, nitrogen or any other suitable gas.
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While particular embodiments according to the invention have been illustrated
and described
above, it will be clear that the invention can take a variety of forms and
embodiments within
the scope of the appended claims. For example, the barrel and screw can be of
a modular
design.
