U.S. patent application number 10/753651 was filed with the patent office on 2004-12-30 for method and apparatus for producing components from metal and/or metal matrix composite materials.
This patent application is currently assigned to CYCO SYSTEMS CORPORATION PTY LTD.. Invention is credited to Withers, Graham Rex.
Application Number | 20040261970 10/753651 |
Document ID | / |
Family ID | 33541871 |
Filed Date | 2004-12-30 |
United States Patent
Application |
20040261970 |
Kind Code |
A1 |
Withers, Graham Rex |
December 30, 2004 |
Method and apparatus for producing components from metal and/or
metal matrix composite materials
Abstract
Method and apparatus for producing semi-finished or finished
products from metal-based material. The apparatus includes a mixing
furnace to receive a metal-based material to be formed; temperature
control means to maintain the metal-based material in a thixotropic
semi-solid state in the mixing furnace; rotatable mixing means
operable in the mixing furnace to subject the metal-based material
to a mixing and shearing action while imparting a centrifugal
force; and supply means to move the material to a delivery site.
Optionally, injection means are provided to inject the material
from an introduction chamber of a casting machine into a mold or
die cavity while the material is in a thixotropic semi-solid
state.
Inventors: |
Withers, Graham Rex;
(Cheltenham, AU) |
Correspondence
Address: |
BROOKS KUSHMAN P.C.
1000 TOWN CENTER
TWENTY-SECOND FLOOR
SOUTHFIELD
MI
48075
US
|
Assignee: |
CYCO SYSTEMS CORPORATION PTY
LTD.
Mentone
AU
|
Family ID: |
33541871 |
Appl. No.: |
10/753651 |
Filed: |
January 8, 2004 |
Current U.S.
Class: |
164/97 ; 164/113;
164/900 |
Current CPC
Class: |
B22D 17/007
20130101 |
Class at
Publication: |
164/097 ;
164/113; 164/900 |
International
Class: |
B22D 017/10; B22D
023/00; B22D 025/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 16, 2003 |
AU |
2003905040 |
Jun 27, 2003 |
AU |
2003903273 |
Claims
1. A method of producing semi-finished or finished products from a
metal-based material, the method including the steps of: (i)
maintaining the material in a mixing furnace in a thixotropic
semi-solid state; (ii) subjecting the material to a continuous
shearing and mixing action and centrifugal force while in the
thixotropic semi-solid state within the mixing furnace to form at
least in part a fine, globular microstructure; and (iii) delivering
the material involutely from the mixing furnace while in the
thixotropic semi-solid state to a delivery site for
solidification.
2. A method according to claim 1 including stirring the metal-based
material in the thixotropic semi-solid state within the mixing
furnace with shearing and mixing means acting about an upright axis
to provide the continuous shearing action, the material in the
thixotropoic semi-solid state being delivered tangentially through
discharge means leading from the mixing furnace under forces
applied by the shearing and mixing means.
3. A method according to claim 1 wherein the metal-based material
includes a metal matrix composite material, the method further
including the step of introducing a particulate material into the
mixing furnace while the metal-based material is subjected to the
continuous shearing and mixing action to form the metal matrix
composite material therein, the particulate material being mixed
substantially evenly through the metal matrix composite
material.
4. A method according to claim 1 wherein the metal in the
metal-based material selected from the group consisting of
superplastic alloys, aluminium, magnesium, tin, copper, zinc,
alloys of the aforesaid metals, and mixtures thereof.
5. A method according to claim 3 wherein the particulate material
is selected from the group consisting of ceramic balls, ceramic
particles, micro spheres, fly ash, and mixtures thereof.
6. A metallic material-making apparatus for producing semi-finished
or finished parts from a metal-based material, the apparatus
including: i) a mixing furnace to receive a metal-based material;
ii) temperature control means associated with the mixing furnace to
maintain the material in a thixotropic semi-solid state; iii)
rotatable mixing and shearing and propelling means operable in the
mixing furnace to subject the material in the thixotropic
semi-solid state to a shearing and mixing action and a centrifugal
force; iv) supply means to duct the material involutely in the
thixotropic semi-solid state from the mixing furnace to a delivery
site for solidification into a near-net shape.
7. A metallic material-making apparatus according to claim 6
further including delivery means to meter a desired volume of
particulate material into the mixing furnace to form the
metal-based material therein, the particulate material being
substantially evenly distributed through the metal-based
material.
8. A metallic material-making apparatus according to claim 6
wherein the rotatable mixing and shearing and propelling means
includes a plurality of upright blade means spaced radially from an
axis of rotation, each the blade member being angled relative to a
circumferential direction, with a rear edge zone relative to the
direction of rotation of each blade member being a radially
outermost dimension of each blade member.
9. A metallic material-making apparatus according to claim 8
wherein each blade member forms an angle of about 30 degrees with
the circumferential direction.
10. A metallic material-making apparatus according to claim 8
wherein the rear edge zone is spaced from an inner wall surface of
the mixing furnace by a distance of between 10 and 30 mm.
11. A metallic material-making apparatus according to claim 7
wherein the delivery means is positioned to deliver the particulate
material to a circumferential zone traversed by the upright blade
means.
12. A metallic material-making apparatus according to claim 6
wherein the rotatable means includes a central body section adapted
to occupy a central zone of the mixing furnace, the central body
section having a cylindrical outer surface with a diameter at least
10% of an internal diameter of the mixing furnace.
13. A metallic material-making apparatus according to claim 12
wherein the diameter of the central body section has a diameter of
between 15% and 35% of the diameter of the mixing furnace.
14. A metallic material-making apparatus according to claim 6,
further including first pump means to move the metal-based material
in the thixotropic state from the mixing furnace to the delivery
site.
15. A metallic material-making apparatus according to claim 6,
wherein the supply means has an exit passage leading from the
mixing furnace in a tangential direction whereby flow along the
exit passage of the metal-based material is achieved by rotation of
the mixing means.
16. A metallic material-making apparatus according to claim 6
wherein the supply means includes an exit passage leading from a
central zone of the mixing furnace.
17. A metallic material-making apparatus according to claim 6
wherein the supply means includes an exit passage leading from a
side zone of the mixing region.
18. A metallic material-making apparatus according to claim 15,
wherein the exit passage is located proximate a mid region of a
side zone of the mixing furnace.
19. A metallic material-making apparatus according to claim 6,
further including: intermediate pump means to move the metal-based
material in a thixotropic semi-solid state from the mixing furnace
to an intermediate holding furnace, the material being maintained
in the intermediate holding furnace in a thixotropic semi-solid
state, and casting pump means to move the material from the
intermediate holding furnace while in a thixotropic semi-solid
state to the delivery site.
20. A metallic material-making apparatus according to claim 19
wherein the intermediate and casting pump means include a device
selected from the group consisting of a piston, a screw, an
electromagnetic pump, and combinations thereof.
21. A metallic material-making apparatus according to claim 6,
further including a holding furnace to which the metal-based
material is delivered and is held in a molten or semi-molten state
prior to being delivered therefrom to the mixing furnace.
22. A metallic material-making apparatus according to claim 6
wherein return means is provided to duct any metal-based material
that is not required for casting from the delivery site to the
mixing furnace.
23. A metallic material-making apparatus according to claim 6,
further including injection means to inject the material from an
introduction chamber of a casting machine into the casting machine
while in a thixotropic semi-solid state.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of Australian
provisional application Serial No. 2003905040 filed Sep. 16, 2003
and Australian provisional application Serial No. 2003903273 filed
Jun. 27, 2003.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to improvements in a
continuous or semi-continuous processing method and apparatus for
producing thixotropic-conditioned metals such as aluminum and other
lightweight metals and/or metal matrix composite materials to
produce ingots of metals or metal parts. Such parts may be commonly
be used for vehicle or general engineering applications.
[0004] 2. Background Art
[0005] It is known from U.S. Pat. No. 4,888,054 issued to Pond to
produce metal matrix composite materials including those which
contain fly ash or other reinforcement ceramic particulate material
dispersed relatively uniformly in a metal. The '054 patent
discloses that various particulate materials--such as ceramic
balls, microspheres and the like--can be used in the production of
metal matrix composite materials. The relatively uniform mixing of
such reinforcement materials in semi-solid or liquid metal has been
successfully achieved when the particulate material is relatively
coarse. But extremely fine particulate materials, including fine
fly ash materials, have proven to be quite difficult to uniformly
disperse in a molten or semi-molten metal material such that they
can be cast into ingot form, or semi-finished or finished product
form with a reasonably even dispersion of the fine particulate
material through the metal base material. As disclosed in the '054
patent, which is incorporated herein by reference, the metal base
material could include aluminium, magnesium, tin, copper and zinc
and alloys thereof.
[0006] U.S. Pat. No. 5,881,796 issued to Brown et al. discloses an
apparatus for producing semi-solid material from molten material by
three-dimensional mixing. The semi-solid material is removed from a
container by a removal tube that extends through a chamber cover or
a side wall. Effectuating semi-solid flow from the container is
achieved by vacuum or gravity, or other transfer methods utilizing
mechanical means, such as submerged pistons, helical rotors, or
other positive displacement actuators. Id., col. 5, ll 57-65. The
'796 patent is also incorporated herein by reference.
[0007] It is also known that parts produced from metal matrix
composites can have light weight, high strength and optimum wear
resistance. They can be made at low cost, provided a convenient and
effective production method and apparatus are available. Parts
which are particularly suited to being produced from metal matrix
composite materials include wear resistant vehicle components such
as brake drums, brake disc rotors, other brake parts, engine
blocks, cylinder heads, con rods, pistons, front end accessory
drive parts, belt pulley wheels, auto transmission pump parts, oil
pump bodies, rotors, scrolls, and other rotary compressor parts
used in air conditioning, refrigeration systems, and any other
component in which wear resistance may be a desirable property.
[0008] Many of these parts are currently made from cast iron or a
hypereutectic aluminium alloy which contains free silicon. These
materials suffer from certain disadvantages, such as:
[0009] 1. cast iron components are heavy and susceptible to
corrosion; and
[0010] 2. high silicon content aluminium alloy components are
difficult and expensive to machine because the silicon rich
constituents of high silicon content parts can cause interference
with a tool path when machining to final dimensions.
[0011] Against this background, semi-solid metal processing
involves semi-solid slurries in which non-dendritic solid particles
are dispersed in a liquid matrix. Z. Fan, SEMI-SOLID METAL
PROCESSING, Int'l Materials Reviews, Vol. 47, No. 2 (2002). It is
known that when a dendritic structure is broken up, the partially
solidified alloy has the fluidity of machine oil and exhibits
thixotropic behavior. Id. It is also recognized that rheocasting
involves the application of shearing during solidification to
produce a non-dendritic semi-solid slurry that can be transferred
directly into a mold or die to give a final product. Id. That paper
is incorporated herein by reference.
SUMMARY OF THE INVENTION
[0012] One objective of the present invention is to provide a
method and apparatus for the production of metals and/or
metal-based materials and/or metal matrix composite materials
(collectively referenced herein--unless the context suggests
otherwise--as "metal-based materials") which have a fine globular
or spherical micro structure in an effective and economical manner,
so that dross and otherwise wasted material are minimized.
[0013] Accordingly, in one aspect, the present invention provides a
method of producing semi-finished or finished parts from such
metal-based materials. The method includes the steps of:
[0014] (i) maintaining a metal-based material in a mixing furnace
in a thixotropic semi-solid state (a liquid-like slurry);
[0015] (ii) subjecting the material to a continuous shearing and
mixing action and a centrifugal force while in a thixotropic
semi-solid state state within the mixing furnace to form a fine,
globular microstructure (down to about 0.5 microns in
diameter);
[0016] (iii) delivering the material involutely from the mixing
furnace while in the thixotropic semi-solid state to a delivery
site, such as a casting head or into the introduction chamber of a
molding machine; and
[0017] (iv) transporting the material in the thixotropic semi-solid
state into a mold or die cavity of the molding machine from the
delivery site to form the semi-finished or finished part or
parts.
[0018] Preferably, the finished or semi-finished part or parts
exiting the molding machine are as near net shape as possible to
minimize further machining requirements.
[0019] In accordance with a particularly preferred embodiment,
metal matrix composite materials are used to form the semi-finished
or finished parts. A particulate material may be introduced into
the mixing furnace while the metal-based material is subjected to a
continuous shearing and mixing action and turbulence to form a
metal matrix composite material. The particulate is mixed
substantially evenly through the melt.
[0020] In accordance with a second aspect of the present invention,
there is provided an apparatus for producing semi-finished or
finished parts from the metal-based materials. The apparatus
includes:
[0021] (i) a mixing furnace having a mixing region to receive a
metal-based material;
[0022] (ii) temperature control means associated with the mixing
furnace to maintain the material in a thixotropic semi-solid
state;
[0023] (iii) rotatable mixing means operable in the mixing furnace
to subject the material in the thixotropic semi-solid state to a
shearing and mixing action and centrifugal force, which may cause
the formation of small solid particles which entrap a liquid phase
therewithin, the particles probably being caused in part by a rapid
coalescence of broken dendritic arms; and
[0024] (iv) supply means to propel the material involutely in the
thixotropic semi-solid state from the mixing furnace to a delivery
site, such as an introduction chamber of a molding machine;
and/or
[0025] (v) injection means to move the material from the
introduction chamber into at least one mold or die cavity of the
molding machine while in the thixotropic semi-solid state.
[0026] Accordingly, one objective of the present invention is to
provide a rotatable shearing and mixing means that will
satisfactorily mix a fine particulate material such as fine fly ash
into a molten metal or metal-based material with adequate
dispersion of the particulate material through the sheared melt. It
will be appreciated that the mixer in accordance with this
invention may also be used with coarser particulate materials that
might be satisfactorily mixed with other equipment.
[0027] Preferably, the apparatus when used to produce parts from a
metal matrix composite material, further includes delivery means to
meter a desired volume of particulate material into the mixing
furnace to form a metal matrix composite material so that the
particulate material is substantially evenly distributed through
the metal.
[0028] The present invention is particularly adapted to processing
light weight metals such as superplastic alloys, aluminium,
magnesium, tin, copper and zinc and alloys of the aforesaid metals.
However, it is not limited to such metals. Other heavier metals
including brass can also be processed as described herein. The
invention is also particularly adapted to producing parts from
metal matrix composite materials, but is not limited thereto. In
particular, processing metals to form a fine, globular
microstructure therein improves their performance in producing
sound die cast parts.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] A further understanding of the present invention will be
apparent from the following description of various embodiments
given in relation to the accompanying drawings, in which:
[0030] FIG. 1 is a schematic flow diagram of process steps that may
be taken in practicing the disclosed invention;
[0031] FIG. 2 is a schematic drawing of the disclosed apparatus
constructed in accordance with a first embodiment;
[0032] FIG. 3 is a view similar to FIG. 2 showing a second
embodiment;
[0033] FIGS. 4 and 4a are cross-sectional views of a mixing furnace
and a representative delivery site;
[0034] FIG. 5 is a cross-sectional side view of an alternate
embodiment of the mixing furnace depicted in FIG. 4a;
[0035] FIG. 6 is a plan view, with certain parts omitted for
clarity, of the shearing, mixing and impelling device shown in FIG.
5 that imparts a centrifugal force to the melt;
[0036] FIG. 7a is a perspective view (partially broken away) of the
mixing furnace which contains the mixing device;
[0037] FIG. 7b is a top view thereof;
[0038] FIG. 7c is a side view thereof;
[0039] FIG. 7d is another side view thereof;
[0040] FIG. 7e is a sectional view taken along the line A-A of FIG.
7c;
[0041] FIG. 7f is a sectional view taken along the line B-B of FIG.
7d; and
[0042] FIG. 7g is a sectional view taken along the line C--C of
FIG. 7c.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
[0043] Before turning to the process steps that are schematically
illustrated in FIG. 1, it will be helpful to consider details of
the apparatus that is involved in practicing those steps.
[0044] Referring first to the drawings, an apparatus according to
this invention may be constructed as shown in FIGS. 2 and 3. The
apparatus includes a primary holding furnace 30, that in one
preferred aspect, may receive recycled lightweight metal in the
molten state delivered at 31 from recyclers of such metal.
Alternatively, furnace 30 may be constructed to melt such metal
material from the solid state--either from ingots or the like or
from scrap solid metal material. Typically the metal will be
aluminium or aluminium alloys, but may also include other metals
and their alloys, including magnesium, tin, copper, and zinc.
Collectively alternative source materials are referred to herein as
"metal-based materials."
[0045] The primary holding furnace 30 includes a rotating mixing
and device 32 and further includes means 33 to deliver molten
metal-based material to a mixing furnace 10.
[0046] Optionally, the mixing and shearing furnace 10 includes
heating means and/or cooling means (not shown) to ensure that the
material in a mixing region 34 is maintained in a thixotropic
semi-solid state. The nature and effect of the heating and/or
cooling means will depend upon the state of the material supplied
to the mixing furnace 10. Generally, the term "thixotropic" refers
to "the ability of certain colloidal gels (or slurries) to liquify
when agitated (as by shaking . . . ) and to return to the gel form
when at rest." HAWLEY'S CONDENSED DICTIONARY, p. 1152 (7.sup.th Ed.
1987). As used in this definition, the term "gel" includes "a
colloid in which the dispersed phase has combined with the
continuous phase to produce a viscous jelly-like product." Id., p.
555 (sometimes referred to herein as a "slurry"). The British
Standards Institution defines thixotropy as a "decrease in
viscosity under stress, followed by a gradual return when the
stress is removed." In the metallurgical context, semi-solid metal
alloys are thixotropic. The slurry viscosity is shear-rate and
time-dependent provided the microstructure in the semi-solid state
is nondendritic. T. Y. Liu, et al., "Rapid Compression of Aluminum
Alloys and Its Relationship to Thixoformability", 34A Metallurgical
And Materials Transactions A, p. 1545, July 2003 (incorporated
herein by reference).
[0047] The mixing and shearing furnace 10 includes a rotatable
mixing and shearing device 16 which may be constructed similarly to
that which is shown and described hereinafter with reference to
FIGS. 4 and 5. The stirring and shearing action applied to the
thixotropic metal-based material in the mixing region conditions
the microstructure of the material to a fine, globular state. The
material in this state (whether or not it contains particulate
material mixed evenly through it) has improved capability for being
die cast to produce sound die cast parts with thinner sections than
has been possible to date with conventional processing techniques.
Without wishing to be bound by any particular theory, it is thought
that the ability to make parts of a thinner section is explained by
a continuous reconditioning of the metal that is transported to a
delivery site, in combination with a shearing action that imparts
high velocity to the melt that is propelled peripherally and
involutely in the form of fine, liquid-filled particles that may
occupy a cavity in a delivery site with a high packing density.
[0048] Particulate delivery means 35 is provided to deliver the
desired particulate material to the mixing region 34, specifically
to a region of high shear created by the blades or impellers of the
mixing and shearing device 16. Stirring of the metal in a
thixotropic state in the mixing region 34 improves the globular
microstructure of the metal and enables moldings or die cast parts
having thinner wall sections to be made. Further, enhanced material
properties result following solidification of the cast or molded
product. The ability to mix, relatively uniformly, the particulate
material (typically ceramic particles or fly ash particles) into
the metal in a thixotropic state is a supplementary benefit. This
allows the production of metal, metal alloy or metal matrix
composite materials, or end products made from such materials with
an optimum microstructure exhibiting the best possible
properties.
[0049] A flow path 36 may be provided from the mixing region 34.
Optionally, the flow path 36 includes pumping means 37 (FIG. 2) to
deliver the metal-based material in the thixotropic state directly,
for example, to a delivery site such as the introduction chamber 38
of a pressure die casting machine 39. A "Vision 66N Buhler" die
casting machine manufactured and marketed by Buhler AG is believed
to be suitable in the performance of the present invention.
[0050] Other squeeze die casting process machines might also be
employed where semi-solid metal is introduced without turbulence
into die cavities. High pressures are maintained throughout the
solidification process to produce sound and heat treatable
parts.
[0051] In an alternative arrangement, the pumping means 37 delivers
the metal-based materials in the thixotropic state to an
intermediate holding furnace (not shown). This furnace may include
an additional mixing and shearing device which may be constructed
similar to the mixing device 16 to continue to stir or subject the
thixotropic material to shear forces, before being transferred via
a further flow passage and pumping means to, for example, the
introduction chamber 38 of the pressure die casting machine 39.
[0052] The pressure die casting machine 39 includes a single die or
mold cavity. Optionally, the machine includes multiple molds or die
cavities, whereby multiple parts might be produced. It will be
recognized that the flow passages 36 might have associated with
them heating/cooling means to ensure that the material carried by
the flow passages is kept in a thixotropic state. Similarly, the
introduction chamber 38 of the pressure die casting machine 39 may
have heating/cooling means.
[0053] The pumping means can be any suitable means for moving metal
or metal composite materials in a thixotropic state along the
desired flow path and may include electromagnetic, piston, screw or
similar pumping means.
[0054] In a preferred embodiment (FIGS. 4, 4a), the discharge line
from the mixing furnace 10 exits involutely or tangentially from
the mixing furnace, such that the mixing rotor 16 serves as a
turbine or centrifugal pump to impel the thixotropic metal-based
material towards a desired delivery site, such as a die casting
machine 39. Optionally, the blades are removable from the mixing
furnace 10 to allow their orientation to be reconfigured. In this
way, any wear along their edges can be uniformly distributed among
the four edges because the blades can be inverted laterally or
longitudinally. Thus, what was a leading edge can, after
reconfiguration, become a trailing edge, and vice versa. A
preferred blade composition is titanium or an alloy thereof.
[0055] The discharge line 36, preferably shaped as a curved
involute, is made of an appropriate material such as titanium, or a
titanium-containing metal alloy with at least 50% titanium content
by mass. It may have a round cross section. As used herein, unless
the context suggests otherwise, the term "titanium" includes
titanium metal or a titanium-containing alloy. Alternatively, the
discharge casing may have a cross section which is rectangular in
shape, thereby enabling it to be assembled from flat sheet pieces
that are welded together, through which the material is freely
discharged into large volume casting molds.
[0056] Preferably, the discharge line 36 is heated to a temperature
(such as 585.degree. C.) between the solidus and liquidus of the
metal-based material to ensure that its contents do not freeze
after emerging from the mixing furnace 10 toward a delivery site,
such as for example, a die casting machine or a smelter casthouse.
In this way, the metal-based materials may be continuously
recirculated so as to prevent the metal from losing its thixotropic
characteristics. Such characteristics may be lost if the semi-solid
material becomes stationary in the discharge line. In practice, the
metal remains in motion on the outer periphery of the mixing
furnace into the involute curve before becoming rechanneled into
the discharge line at the urging of centrifugal force imparted by
the mixing rotor 16.
[0057] The pumping and storage system maintains the metal within a
thixotropic semi-solid temperature range so that a casting made by
the process exhibits not only the desired mechanical properties but
also the lowest shrinkage and the closest possible approach to a
desired net molding shape. This lowest possible temperature
approach has a supplementary benefit of prolonging die life, since
operation in the semi-solid temperature range reduces surface
cracking, soldering and die erosion. Morever, viscous alloys can be
handled using the disclosed process because the metal is pumped
rather than ladled, so alloys which previously were not suitable
for die casting can now be used.
[0058] FIG. 3 shows an alternative arrangement, but similar to that
which is shown in FIG. 2. In this embodiment, the mixing furnace 10
includes a series of cooling fins 40 around its periphery and base
to assist with cooling of molten metal to its thixotropic state. If
desired, cooling by various means, such as fan means (not shown)
might also be provided to increase the cooling effect. As is
further illustrated, a return flow path 41 including pumping means
42 is provided to return thixotropic material from the die
introduction chamber 38 that is not required in a particular
casting process step, to the mixing furnace 10.
[0059] Alternatively, if an intermediate holding furnace is
provided between the furnace 10 and the casting machine 39, unused
material can be returned via the intermediate holding furnace.
[0060] Referring to FIG. 5, one mixing furnace embodiment 10 is
illustrated. The mixing furnace 10 includes a fabricated container
11, preferably made from a suitable material such as titanium or a
titanium-containing metal having a base wall 12 and a cylindrical
upright side wall 13 connected to the base wall 12. A horizontal
radially outwardly extending flange 14 may be provided adjacent the
upper end 15 of the container 11. Heating means such as electrical
elements or induction heaters (or any other suitable heating means)
may be provided outwardly of, but adjacent to the wall 13 to
maintain the metal within the container in a thixotropic state
during a mixing operation. As discussed above, cooling means such
as cooling fins 40 might also be required if the metal supplied to
the furnace is initially molten (liquid) to reduce the metal to a
temperature range in which it will be in a thixotropic (semi-solid)
state. The cooling means might include fans cooperating with the
cooling fins 40. The cooling/heating means have not been
illustrated in the drawing for clarity. Similarly, the lifting and
handling means are not illustrated, as they are within the
knowledge of the skilled artisan.
[0061] The apparatus includes a rotatable mixing member 16
including a rotatable shaft 17 which, in FIG. 6, would be rotated,
preferably, in a clockwise direction (arrow A). An adjustable drive
means (not shown) is provided for the shaft 17 such that the shaft
may be selectably rotated to cause the blades to travel at a blade
speed measured at an outer diameter of between about 3 meters/sec
and about 10 meters/sec as the particulate material is metered onto
a rolling surface of the semi-solid metal by a delivery means
illustrated schematically at 35 (FIG. 5). The particulate material
delivery means 35 is desirably positioned so as to deposit the
particulate material to a region of greatest shear on the surface
of the semi-solid material, i.e., adjacent to the peripheral path
traveled by the blades 21. The delivery means 35 are further
controlled such that the particulate material on the surface of the
molten metal does not exceed about 30 mm in depth.
[0062] By suitable adjustment of the distance between the periphery
of the blades and the side walls of the mixing furnace and rotation
speed, the needs of different applications can be met. These
include for example, the desire to achieve a small versus a large
grain size, or provide a pressure-restricted flow to a die casting
machine, versus a free discharge operation for feeding an ingot
casting machine.
[0063] The rotatable mixing device 16 includes a central body
section 18 with a cylindrical outer wall 19 and a closed base wall
20. The body section 18 is provided to occupy the central zone of
the mixing chamber which otherwise would be occupied by semi-solid
metal that would have low or no velocity. It therefore would not be
readily subjected to particulate mixing and a shearing effect. The
relative dimensions of the central body section are such that the
section has a cylindrical surface with a diameter that is at least
10% of the internal diameter of the mixing region of the mixing
furnace. Preferably, the diameter of the central body section is
between 15% and 35% of the diameter of the mixing region.
[0064] As best shown in FIGS. 5 and 6, uniformly spaced (radially
and circumferentially) blade members 21 are provided such that they
are rotated in an upright configuration about the axis of rotation
22 defined by the shaft 17. Each blade member 21 is formed by a
flat sheet that in a preferred embodiment is approximately 70 mm
wide, 10 mm thick and 435 mm in length. The blade members 21 are
preferably angled relative to the circumferential direction at an
angle of 30 degrees.+-.7 degrees (preferably 30 degrees) such that
a trailing edge zone 23 is disposed closest to an inner wall
surface 24 of the container upright side wall 13. Preferably the
trailing edge zone 23 of each blade member is located between 10
and 30 mm from the inner container surface 24. Still more
preferably, this spacing is about 20 mm. As can be seen in FIG. 5,
each blade member 21 is connected to a mid to lower region of the
central body section 18 by support arms 27 such that the upper ends
25 of the blade members 21 are located below the upper end of the
central body section 18 and just below the upper surface of the
metal-based material when a mixing operation is undertaken. The
lower ends 26 of each of the blade members 21 are spaced only a
short distance above the base of the container 12.
[0065] It has been found that the angle of the upright blade
members and the minimum distance between the blade members and the
inner container wall influence the centrifugal forces imparted to
the melt and the shear and mixing effect between the blades and the
container wall and thus promote a suitable, relatively uniform
mixing in of the particulate material and its distribution through
the molten metal-based material.
[0066] In an alternative embodiment, a series of holes may be
provided in one or more of the blade members 21 so as to cause a
further shearing action in the mixing furnace as the blades rotate.
Alternatively, bars (preferably of titanium or an alloy thereof)
could be arranged lengthwise and parallel to a major axis
(vertically). In this embodiment, the bars would be detachably
affixed for ease of maintenance so that they are vertically aligned
perpendicularly to a radially outward direction. In another
embodiment, some or all of the blades are replaced by a mesh-like
material, such as a wire mesh.
[0067] FIGS. 7a to 7g illustrate a further modified mixing furnace
10 which contains a mixing and shearing device 16. In FIG. 7c, the
function of the pipe 70 and the flange 71 shown at the base of the
mixing furnace 10 is to allow molten metal-based material contained
within the furnace 10 to be drained from the mixing furnace 10
through a gate or stop valve (not shown) attached to the flange 71.
The top of the furnace 10 may be formed by a removable lid 72 that
can be bolted to a lower furnace section 73, whereby any desired
atmosphere (gas) may be supplied over the material in the furnace
via pipe 63. The removable lid 72 allows the mixing and shearing
device 16 to be withdrawn from the furnace 10 for maintenance. The
return pipe 41 on the right side of FIG. 7f may be in practice, be
proportionally larger than depicted.
[0068] The funnel 35 shown on the left of FIGS. 7c and 7g allow
particles to be fed into the furnace 10 at a position adjacent to
the passage of the paddle or blade members 21 where the highest
shear conditions are likely to be experienced by the semi-solid
metal-based matrix material in the furnace 10. In practice, the
pipe leading from the funnel 35 need not to be as large as depicted
in diameter as depicted in the drawings.
[0069] As with the embodiment of FIGS. 4, 4a, the involute
discharge passage 64 leading to the flow passage 36 exits the
mixing furnace 10 approximately midway between its base and its top
where the pressure of the thixotropic metal or metal matrix
material is likely to be at or approaching its highest. As
illustrated in FIGS. 7a to 7g, the involute discharge passage 64
may be square or rectangular in cross-section. The paddles or
blades 21 of the mixing and shearing device 16 may extend from a
position adjacent the base wall 12 to a position adjacent the top
wall 74.
[0070] In operation, as indicated earlier, there may usefully be
provided, in alternate embodiments of the system, heat dissipation
cooling fins around the perimeter of the furnace. Additionally, an
area may be provided for installation of an induction heating coil
for rapid heating if temperature correction is required.
[0071] Reference will now be made to the process steps that are
practiced in using the disclosed apparatus. They are schematically
illustrated in FIG. 1. In this process illustration, molten
aluminium or aluminium alloy is conveniently supplied at 15 via a
recycler of such metal material. The aluminum may be in form of
pure aluminum, or an aluminum-containing metal alloy with at least
50% of aluminum content by mass. Other examples of source materials
include magnesium, tin, copper, zinc and alloys and mixtures
thereof. Alternatively, molten aluminium might be produced as part
of the process from solid material (either recycled or not). The
molten metal material is supplied to a primary holding furnace 30
from whence it is delivered in liquid form to a mixing furnace 10.
The mixing furnace 10 is arranged to cool and then maintain the
metal in its thixotropic state. In the mixing furnace 10, ceramic
particulate material or fly ash material in metered quantities are
supplied and mixed into the thixotropic metal-based material formed
in the furnace 10. This composite metal matrix material, still in
the thixotropoic state, may be delivered to an intermediate holding
furnace 50 and from there delivered to a desired delivery site,
such as a high pressure die casting machine 39.
[0072] The parts produced by the high pressure die casting machine
may be inspected, trimmed and machined to a finished product as
required and then packaged and shipped to an end customer as may be
required via steps 51, 52 and 53. Any reject or scrap material is
minimized because it may be returned as solid or semi-solid
material to an intermediate of the mixing furnace for
reprocessing.
[0073] In accordance with the invention disclosed herein, the
technology so described provides a lower cost, more economical and
more efficient process than has been available in the prior
art.
[0074] Thus, there has been described a furnace in which a stirrer
rotates between smooth walls. This action stirs and subjects a
cooling metal-based mix to a mixing and shearing action that
enables thixotropic, substantially non-dendritic, semi-solid alloys
to be produced. The stirrer includes an array of blades made of an
appropriate material that rotate within a central closed furnace
and subject the melt to centrifugal force. The furnace may be
heated or cooled. The blades promote a complex, three-dimensional
movement of the metal-based material, and provide a turbine action
or impeller-like pumping force that urges the effluent to a
delivery site located outside the mixing furnace.
[0075] As a result of recycling and continuous rejuvenation of the
metal-based material, material loss is low (e.g., below 2%). In
contrast, prior art approaches usually involve an allowance for
material loss in a die-casting plant, which is about 2-3% of the
metal consumed.
[0076] While embodiments of the invention have been illustrated and
described, it is not intended that these embodiments illustrate and
describe all possible forms of the invention. Rather, the words
used in the specification are words of description rather than
limitation, and it is understood that various changes may be made
without departing from the spirit and scope of the invention.
* * * * *