U.S. patent application number 10/344209 was filed with the patent office on 2004-05-13 for method and apparatus for making metal alloy castings.
Invention is credited to Bevis, Michael John, Fan, Zhongyung, Ji, Shouxun.
Application Number | 20040089437 10/344209 |
Document ID | / |
Family ID | 27255843 |
Filed Date | 2004-05-13 |
United States Patent
Application |
20040089437 |
Kind Code |
A1 |
Fan, Zhongyung ; et
al. |
May 13, 2004 |
Method and apparatus for making metal alloy castings
Abstract
A method and apparatus are provided for fabricating of
continuous castings with fine and uniform microstructure, which can
be used as feedstock for secondary processing routes, such as
thixoforming, forging and machining or direct application in
industry. An overheated liquid alloy is fed into a high shear
device (for example, a twin-screw extruder) and sheared intensively
to produce a sheared liquid alloy or a semisolid slurry, wherein
the sheared liquid alloy is at a temperature close to its liquidus
and the semisolid slurry is then transferred to a shaping device
for production of continuous castings with fine and uniform
microstructures through a solidification process. The shaping
device is any device capable of forming continuous (i.e. infinite
length) products, such as a direct chill (DC) caster (DC
rheocasting) for production of continuous billets, an extrusion die
(rheo-extrusion) for production of continuous bars or wires, or a
twin-roll caster (twin-roll rheocasting) for producing of
continuous strips. In all those cases, the cross-section of the
continuous castings exhibits a microstructure in which a controlled
volume fraction of fine and spherical primary particles are
uniformly distributed in a fine structured matrix.
Inventors: |
Fan, Zhongyung; (Uxbridge,
GB) ; Ji, Shouxun; (Uxbridge, GB) ; Bevis,
Michael John; (Uxbridge, GB) |
Correspondence
Address: |
Dewitt Ross & Stevens
Intellectual Property Department
Firstar Financial Centre
8000 Excelsior Drive Suite 401
Madison
WI
53717-1914
US
|
Family ID: |
27255843 |
Appl. No.: |
10/344209 |
Filed: |
June 27, 2003 |
PCT Filed: |
August 9, 2001 |
PCT NO: |
PCT/GB01/03596 |
Current U.S.
Class: |
164/459 ;
164/113; 164/71.1; 164/900 |
Current CPC
Class: |
B22D 17/007 20130101;
B22D 11/112 20130101; B22D 11/00 20130101; B22D 11/0622
20130101 |
Class at
Publication: |
164/459 ;
164/113; 164/071.1; 164/900 |
International
Class: |
B22D 025/00; B22D
017/08 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 11, 2000 |
GB |
0019856.4 |
Aug 11, 2000 |
GB |
0019855.6 |
Jul 26, 2001 |
GB |
0118289.8 |
Claims
1. 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.
2. A method as claimed in claim 1, wherein shear is applied to the
alloy by means of an extruder having at least one screw, the screw
having at least one vane thereon, the vane at least partially
defining a helix around the screw to propel the alloy through the
extruder, wherein the at least one screw can be rotated to shear
said liquid alloy at a rate sufficient to inhibit complete
formation of dendritic structures therein while the alloy is in a
semisolid state.
3. A method as claimed in claim 1 or 2, wherein shear is applied to
the alloy by means of a twin screw extruder having at least two
screws which are at least partially intermeshed.
4. A method as claimed in claim 3, wherein the screws are
substantially fully intermeshed.
5. A method as claimed in any preceding claim, wherein the shaping
device is a direct chill caster, and extrusion die, or a twin roll
caster.
6. A method as claimed in any preceding claim, wherein the alloy
comprises at least two immiscible components, and wherein said
components are provided separately in liquid form, and pre-mixed
before being subjected to shear.
7. A method as claimed in claim 6, wherein the alloy is subjected
to sufficient shear to be converted into a liquid suspension in
which the minor immiscible component is dispersed in the major
immiscible component in the liquid phase.
8. A method as claimed in claim 7, wherein the liquid suspension is
cooled to its monotectic temperature or below whilst being sheared
in order to form a semisolid slurry.
9. A method as claimed in claim 8, wherein the viscosity of the
slurry is sufficiently high to prevent coarse segregation of the
immiscible system.
10. A method as claimed in any of claims 6 to 9, wherein the
shaping device is not a pre-heated metal band.
11. A method as claimed in any of claims 1 to 6, wherein the alloy
does not include any immiscible components.
12. An article formed from a metal alloy, which article is
obtainable by means of a method as claimed in any preceding
claim.
13. Apparatus for forming a continuous product from a liquid metal
alloy, comprising a temperature-controlled shearing device capable
of imparting sufficient shear and intensity of turbulence to a
liquid metal alloy to convert it into its thixotropic state, and a
shaping device in fluid communication with the shearing device,
wherein the shaping device is capable of forming a continuous
product.
14. Apparatus as claimed in claim 13, wherein the shearing device
is an extruder having at least one screw, the screw having at least
one vane thereon, the vane at least partially defining a helix
around the screw to propel the alloy through the extruder, wherein
the at least one screw can be rotated to shear said liquid alloy at
a rate sufficient to inhibit complete formation of dendritic
structures therein while the alloy is in a semisolid state.
15. Apparatus as claimed in claim 13 or 14, wherein the shearing
device is a twin screw extruder having at least two screws which
are at least partially intermeshed.
16. Apparatus as claimed in claim 15, wherein the screws are
substantially fully intermeshed.
Description
[0001] 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.
[0002] 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.
[0003] 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.
[0004] One known process uses DC cast billets as feedstock
materials is an extrusion process to produce bars with either
simple or complex cross-sections. Extrusion 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.
[0005] 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.
[0006] 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.
[0007] 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 formed on the upper
circumferential surfaces of the rolls above the gap and the liquid
alloy is continuously cast into an alloy strip.
[0008] 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.
[0009] 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).
[0010] 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 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.
[0011] 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 A1 (Mazda Motor Corporation); WO
90/09251 (The Dow Chemical Company); U.S. Pat. No. 5,711,366
(Thixomat, Inc.); U.S. Pat. No. 5,735,333 (The Japan Steel Works,
Limited); U.S. Pat. No. 5,685,357 (The Japan Steel Works, Limited);
U.S. Pat. No. 4,694,882 (The Dow Chemical Company); and CA
2,164,759 (Inventronics Limited).
[0012] 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.
[0013] 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.
[0014] 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.
[0015] U.S. Pat. No. 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.
[0016] 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.
[0017] 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.
[0018] 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 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.
[0019] None of the thixomoulding or rheomoulding references
describe a process which enables continuous products of a
sufficiently high structural integrity to be formed.
[0020] 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.
[0021] 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.
[0022] 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.
[0023] 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 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.
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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.
[0029] The shaping device can be different kinds of dies or moulds
with accompanying accessories depending on the requirement of the
final 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.
[0030] 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 relationship
between the mould temperature and the metal shearing temperature,
which is established by the requirements of the individual art.
[0031] 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.
[0032] 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.
[0033] 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.
[0034] 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.
[0035] 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 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.
[0036] The continuous products formed by the method of the present
invention may be further deformed by a conventional extrusion
process.
[0037] 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.
[0038] 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.
[0039] 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.
[0040] 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.
[0041] The rotating rolls in the twin-roll caster can have any
kinds of profiles which are capable of providing a narrowest gap
during tuning. Preferably, the profile of the rolls is flat.
[0042] The extrusion die in the rheo-extrusion process can be any
kinds of sectional shapes, including 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.
[0043] The said method may employ sheared liquid alloy at a
temperature close to its liquidus or semisolid slurry with flue 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
[0044] The said apparatus and method preferably comprises the steps
of:
[0045] 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;
[0046] 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;
[0047] 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-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
[0048] 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.
[0049] 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 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.
[0050] 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.
[0051] 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.
[0052] 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 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.
[0053] 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.
[0054] 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.
[0055] A number of preferred embodiments of the invention are
described in detail below with reference to the drawings, in
which:
[0056] 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;
[0057] 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;
[0058] 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;
[0059] FIG. 4 is a schematic illustration of an alternative
embodiment of an apparatus of extrusion die for a rheo-extrusion
process.
[0060] 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;
[0061] 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.
[0062] 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
[0063] 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 and/or 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.
[0064] FIG. 1 illustrates an extruder system according to an
embodiment of this invention. A liquid 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.degree. 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 FIG.
1, the extruder can be equipped with between 1 to 10 separately
controllable heating/cooling zones.
[0065] The extruder also has a physical slope or an inclination.
The inclination is usually between 0-90.degree. 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.
[0066] 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 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.
[0067] The sheared liquid alloy or semisolid slurry existing in the
extruder is transferred to shaping 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 limitation.
[0068] 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 pool 30 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.
[0069] 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 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.
[0070] FIG. 4 illustrates an alternative extrusion die, in which 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.
[0071] 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 barrel 42, screw
43, nozzle 41 and heating elements 45 along the barrel 42 with a
thermal couple 46 for the required temperature. The sheared liquid
alloy or semisolid slurry is discharged into the single screw
extruder via the valve 22. 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
barrel 42 via heating elements 45 may significantly reduce the
flow-out resistant of the sheared liquid alloy or semisolid slurry
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 embodiment in the present invention wherein at
least the open die can be modified to different shapes.
[0072] FIG. 6 illustrates a twin-roll rheocasting system. The
system has two functional units: a twin-screw extruder 1 and a
twin-roll caster 2. The extruder 1 has been described in FIG. 1.
The twin-roll caster 2 mainly includes a pair of rolls 22 and a
pair of side dams 23. The twin-rolls 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.
[0073] 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.
[0074] 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, which may be easily and quickly
reprogrammed to change the relative parameters depending on the
type of the finished product.
[0075] 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.
[0076] 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.
* * * * *