Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR CASTING A SEMI-SOLID METAL ALLOY
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of United States Provisional
Application
No. 60/527,030, filed on December 4, 2003, the entire contents of which are
incorporated by reference .
BACKGROUND OF THE INVENTION
[0002] This invention relates to industrial metal forming, and more
particularly to a
process for forming metal components from non-dendritic, semi-solid metal
slurries.
[0003] It is well known that when a molten metal alloy is transferred into a
mold to
create a part, the metal undergoes a phase change from liquid to solid. The
metal
density changes during this phase change, resulting in a change in volume. For
most
metals, the density increases during freezing, resulting in a loss in volume.
In casting,
this loss of volume creates a casting defect known as shrinkage. Casting molds
are
designed so that the part freezes in a directional manner that allows for
molten metal to
fill into the voids created by the loss of volume. This step of compensating
for
shrinkage is known as "feeding. "
[0004] Molten alloy will continue to flow even after it begins to solidify.
Under normal
solidification conditions, the solid phase will possess a morphology that is
referred to as
dendritic. Dendritic means that the solid phase has a tree-like structure,
with branches
and side arms of solid protruding into the liquid. Dendrites have a large
surface area to
volume ratio. As the metal freezes, the dendrites occupy a larger volume
fraction of the
alloy, and at some point the dendrites form a network that prevents the
partially
solidified alloy from easily flowing. The partially solidified alloy cannot
feed shrinkage
when this occurs. The amount of solid present in the alloy when the metal
ceases to
flow and accommodate shrinkage is known as the coherency fraction solid. The
coherency fraction solid will occur at varying fraction solids depending on
the
morphology of the freezing alloy. The coherency fraction solid is relatively
low,
usually about 0.20 fraction solid; as a result, normal casting processes use
molten alloy
that is superheated as much as 100°C above the liquidus temperature.
This extra heat
allows the alloy to remain molten for a longer period of time to allow for
feeding of the
shrinkage. A result of the energy used to heat the alloy to higher
temperatures is an
increase in associated cycle time due to the extra heat that must be removed
through the
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casting process during part formation, and die life is reduced by the
increased thermal
shock on the tool.
[0005] Existing semi-solid casting processes rely upon controlling the
morphology of the
solid phase so that the solid phase possesses a globular, spheroidal or
ellipsoidal shape.
Specifically, it has been discovered that various processing and physical
property
advantages can be achieved by casting or otherwise forming metal components
from a
non-dendritic, semi-solid metal slurry. This allows for easier movement of the
solid
phase as it freezes because of the prevention of the formation of a dendritic
network.
The non-dendritic metal particles in the semi-solid slurry provide
substantially reduced
viscosity for a given solids fraction as compared with a semi-solid metal
alloy
composition containing dendritic particles. Often the difference in viscosity
is several
orders of magnitude. Feeding of shrinkage is easier to accommodate with a
globular,
non-dendritic morphology. Heretofore, the majority of semi-solid processes use
a high
pressure injection system to force the semi-solid alloy into an associated
mold.
[0006] When molten alloy is held at a temperature right at or slightly above
the liquidus,
copious nucleation of solid particles form within the melt as the metal is
transferred into
the mold. The resulting solid particles are very small and finely dispersed,
such that, as
the metal freezes in the mold, the metal continues to flow and feed because
the dendrites
are generally more round. Feeding shrinkage is enhanced by this process.
[0007] There are deficiencies to using this process. For example, maintaining
the
molten alloy at a temperature just above the liquidus is challenging from an
industrial
standpoint because of furnace temperature fluctuations. Furnaces are normally
filled
with molten metal periodically either through a central feeding system or from
a
crucible. This molten alloy is superheated above the liquidus to prevent the
metal from
beginning to freeze in the metal transfer system. When the new metal is added
to the
holding furnace the resultant metal temperature is normally well above the
liquidus such
that it is impossible to utilize the aforementioned process. Further, the
solid particles
are very small and dispersed within the liquid, but they are still dendritic
in shape.
Formation of round particles from fine dendrites requires time to allow for a
reduction
in surface area of the particles. The fine dendrites created by low-
temperature pouring
enhances feeding compared with liquid casting processes, but the advantages
that are
possible using globular particles are not fully realized in existing
processes.
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[0008] More recent processes have utilized molten metal that has been modified
with
"grain refiners." Grain refiners are added to a molten metal to promote
copious
nucleation of the solid phase so that the morphology becomes non-dendritic and
globular
in a short amount of time. The time required to allow the dendrites to coarsen
into
round particles makes grain refiners challenging to use in industrial
processes. In an
industrial process, the melt would need to be cooled and held at a temperature
that
allows the solid particles to coarsen into round particles.
[0009] Non-dendritic, semi-solid slurries with fractions solid above
approximately 0.4,
i.e., high fraction solid slurries, possess solid-like properties in the
unsheared state
because of the shear-dependent viscosity of the slurry (e.g., the slurry can
support its
own weight for finite amounts of time). The slurry is typically injected into
a mold via
a high-pressure casting machine with injection forces adequate to create
enough shear to
decrease the viscosity of the slurry and enable filling of a mold cavity.
Typically, the
high-pressure casting machines utilize injection forces of greater than 1000
psi, and at
least greater than 500 psi. Because shear is necessary to initiate flow, high-
pressure
injection style casting machines (injection molding, die casting, or squeeze
casting
machines) have been utilized to cast semi-solid slurries having fractions
solid above
about 0.4.
[0010] Semi-solid slurries at low fractions of solid (about less than 0.20) do
not have
enough viscosity or strength to support their own weight. Heretofore, low
fraction solid
slurries were not widely used in high pressure industrial casting until
recently for a
number of reasons. First, it was believed that the transfer of low fraction
solid slurries
to a relatively cold vessel (e.g., the mold or cold chamber) would return the
microstructure of the slurry to a dendritic state. Flow of the slurry into the
mold would
thus cease at fractions solid much lower than a high fraction solid slurry.
Secondly, the
low-fraction solid slurry has more heat than high fraction solid slurries, so
the full
benefits of using semi-solid rather than liquid casting could not be realized.
For
example, die life and casting cycle time would not be expected to improve as
drastically
as with a high fraction solid slurry. Finally, existing semi-solid processes
were not
designed to create low-fraction solid slurry that was suitable for casting.
Equipment
was designed to handle the more solid-like material, not the lower viscosity,
low
fraction solid slurry.
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[0011] Recent work with low-fraction semi-solid slurries has shown the slurry
will
remain essentially non-dendritic throughout the filling of the mold during
high-pressure
molding processes. High-pressure casting machines now cast low-fraction solid
slurry
and gain many of the benefits of semi-solid processing, such as reduced cycle
time,
increased mold life, and increased casting mechanical properties. Furthermore,
major
changes that were necessary to the casting equipment to use high-fraction
solid slurry
are no longer necessary.
[0012] Many existing liquid alloy (i.e., no solids) casting processes do not
use high-
pressure injection. These low pressure processes include sand casting, gravity
or low-
pressure permanent mold, investment casting, and lost foam casting. These
processes
each have unique advantages for the production of certain castings compared
with high
pressure casting processes. Complex shapes can be produced with sand and lost
foam
casting because molds associated with these processes are not limited in
geometry.
Tooling costs are lower and make casting of low-volumes of parts more
economical.
Casting equipment is less expensive compared with high-pressure casting
processes.
Current automotive parts produced with these processes include closed-deck
internal
combustion engines, intake manifolds, pistons and wheels. In these processes,
the
molten metal fill velocity is much slower than high pressure casting
processes.
Therefore, to allow filling of the entire mold, the wall thickness of the cast
components
is usually thicker than a comparable high pressure casting to ensure the alloy
does not
solidify prior to flowing into the walls. However, there are disadvantages to
low-
pressure molding techniques. For example, the amount of time necessary to
remove
heat from the molten metal is correspondingly longer than in high pressure
processes.
[0013] Use of low pressure processes for production of castings has been
limited
because of the relatively long cycle, in particular the long "dwell time"
relative to high-
pressure casting processes.
[0014] Another problem with low pressure casting processes is the lower
mechanical
strength of the castings compared with high pressure castings. Mechanical
strength for
a given alloy is inversely correlated to the grain size of the solidified
casting. Grain size
is directly related to the cooling rate of the metal within the casting.
SUMMARY OF THE INVENTION
[0015] One aspect of the present invention is a method of forming a metal
part. The
method includes heating a metal alloy to form a liquid that is substantially
free of metal
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solids. The liquid is cooled to form a semi-solid metal alloy slurry having a
weight
percentage of solids in the range of about one percent to about thirty
percent, and
wherein the slurry is substantially free of dendritic solids. The semi-solid
metal alloy
slurry is transferred to a mold at a low pressure, and the metal alloy slurry
is to cast a
substantially solid part.
[0016] Another aspect of the present invention is a method of casting metallic
parts.
The method includes forming a semi-solid metal alloy slurry including globular
solids.
The semi-solid metal alloy slurry is substantially free of dendritic solids.
The metal
alloy slurry is transferred to a mold having a sprue and runner connected to a
mold
cavity, and the metal alloy slurry flows through the sprue and runner into the
mold
cavity. The metal alloy is cooled in the mold cavity to substantially solidify
the metal
alloy and form a part. The metal alloy is pressurized in the mold cavity at a
pressure of
about one hundred pounds per square inch or less during cooling of the metal
alloy.
[0017] Another aspect of the present invention is a system for making cast
metallic
parts. The system includes a furnace having a heated vessel suitable for
holding liquid
metal alloy. The system also includes a semi-solid slurry production machine
having a
movable agitating member configured to agitate metal alloy to produce a semi-
solid
metal alloy slurry having a weight percentage of non-dendritic solids of about
one
percent to about twenty percent. The system further includes a mold having a
mold
cavity and an intake runner and a vent connected to the mold cavity. A metal
transfer
device transfers semi-solid metal alloy slurry from the semi-solid slurry
production
machine to the mold.
[0018] These and other features, advantages, and objects of the present
invention will be
further understood and appreciated by those skilled in the art by reference to
the
following specification, claims, and appended drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] Fig. 1 is a schematic top plan view of a system for making cast
metallic parts
according to one aspect of the present invention; and
[0020] Fig. 2 is a partially schematic view of a mold of the system of Fig. 1.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
[0021] For purposes of description herein, the terms "upper, " "lower, "
"right, " "left, "
"rear " "front " "vertical " "horizontal " and derivatives thereof shall
relate to the
> > > >
invention as oriented in Figs. 1 and 2. However, it is to be understood that
the
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invention may assume various alternative orientations and step sequences,
except where
expressly specified to the contrary. It is also to be understood that the
specific devices
and processes illustrated in the attached drawings and described in the
following
specification are simply exemplary embodiments of the inventive concepts
defined in the
appended claims. Hence, specific dimensions and other physical characteristics
relating
to the embodiments disclosed herein are not to be considered as limiting,
unless the
claims expressly state otherwise.
[0022] The benefits of non-dendritic semi-solid forming include higher speed
part
forming, high speed continuous casting, lower mold erosion, lower energy
consumption,
improved mold filling, reduced oxides for improved machinability of the
finished metal
components, and less gas entrapment resulting in reduced porosity. Other
advantages of
casting or otherwise forming metal components from a semi-solid slurry
includes less
shrinkage during forming of the metal components, i.e., near-net-shape
castings, fewer
voids and lower porosity in the formed metal components, less
macrosegregation, less
susceptibility to hot tearing, and more uniform mechanical properties (e.g.,
strength). It
is also possible to form more intricate parts using non-dendritic, semi-solid
alloy
compositions during casting or other forming techniques. For example, parts
having
thinner walls with improved strength properties are possible.
[0023] The present invention provides an improved process for utilizing low-
fraction
solid slurry with a non-high pressure injection casting process. More
specifically, the
present invention provides a method of utilizing low-fraction solid slurries
with non-high
pressure casting processes that result in a decrease of process cycle times by
as much as
50 % . Also, the decreased heat of semi-solid slurries in low pressure casting
processes
decreases the grain size and improves mechanical properties of the castings.
[0024] As utilized herein, the low-fraction solid slurry refers to a semi-
solid slurry
having a sufficiently low weight fraction of non-dendritic solids so that the
semi-solid
slurry readily flows in a viscous manner with little or no applied shear
stresses.
Conversely, high-fraction solid slurry requires a finite applied shear stress
to initiate
viscous flow. Over time, a high-fraction solid slurry will flow without
applied shear
stress (i.e., as a result of its own weight); however, a relatively long
duration of time is
required to achieve viscous flow, and the amount of viscous flow achieved
after the long
duration of time is negligible. Further, the long duration of time required to
achieve the
viscous flow is significantly greater than cycle times desired~in industrial
metal casting
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and forming processes. The low-fraction solid slurry can achieve viscous flow
in a
duration of time suitable for industrial metal casting and forming processes
and without
any applied shear stresses, and the viscous flow can be enhanced by
application of low
pressure, as will be described in further detail hereinafter. Suitable weight
percentages
of non-dendritic solids will depend upon variables such as the alloy
composition, mold
configuration, and other such process variables. In general, the preferred
weight
percentage of non-dendritic solids will be in the range of about one to
twenty, and more
preferably in the range of about five to fifteen. However, somewhat higher
weight
percentages of solids up to thirty or thirty-five percent may be utilized in
some
applications. A description of forming and processing rheocast structures is
given in
"Formation and Processing of Rheocast Microstructures", PhD Thesis,
Massachusetts
Institute of Technology, by co-inventor Raul A. Martinez-Ayers, published
September
2004, the entire contents of which are incorporated by reference. The weight
percentage of solids in the low-fraction solid slurry is sufficient to permit
casting
utilizing non-high pressure injection casting processes (low-pressure casting
methods)
that utilize pressures that are substantially less than die casting or the
like.
[0025] Examples of the low-pressure casting methods include sand casting,
gravity or
low-pressure permanent mold, investment casting, and lost foam casting.
However,
other low-pressure methods be also be utilized for a particular application.
Such low-
pressure casting methods generally utilize pressures in the range of
atmospheric pressure
to about one hundred pounds per square inch (psi), but can utilize pressures
as high as
500 psi, or as low as 0.1 psi, depending upon the requirements of a particular
application.
[0026] A process according to one aspect of the present invention utilizes a
low-fraction
solid slurry with a low pressure injection casting process. Fig. 1 is a
partially schematic
plan view of a casting system 1 according to another aspect of the present
invention. A
furnace 2 holds molten metal alloy 4 at a temperature above its liquidus
temperature.
The furnace 2 is a commercially available unit that includes a vessel 5
forming a dip
well providing access to the molten metal alloy 4. A robot 10 or other such
conunercially available transfer device includes a movable vessel 11 or the
like for
transferring the molten metal alloy 4 from the furnace 2 to a semi-solid
slurry
production machine 3. Various slurry production machines and processes have
been
developed. One example of a semi-solid slurry production machine and method is
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disclosed in U.S. Patent No. 6,645,323, the entire contents of which are
incorporated by
reference. The metal alloy is agitated and cooled in the semi-solid slurry
production
machine 3 to produce a slurry having non-dendritic solids that approach a
spherical
configuration. The robot 10 transfers the slurry 12 to a mold 13. The slurry
12 has a
weight fraction solid content preferably of from about one percent to about
twenty
percent at the instant the slurry 12 is transferred to the mold 13, and more
preferably of
from about five percent to about ten percent. A plurality of molds 13 may be
mounted
on a rotatable table 14 to provide for simultaneous filling, cooling, and
removal of parts
from the molds 13. Although four molds 13 are shown, it will be understood
that table
14 may include any number of molds (e.g., six).
[0027] The molds 13 comprise low pressure molds having filling mechanisms that
utilize low pressure casting processes having pressures of less than 500 psi,
more
preferably of less than 100 psi, and most preferably of less than 14.7 psi
(atmospheric
pressure). With further reference to Fig. 2, the illustrated, exemplary mold
13 includes
a pouring basin 15, a sprue 16, and a gate 17 that are fluidly connected to a
mold cavity
18. A vent 19 is also fluidly connected to the mold cavity 18. It will be
appreciated
that the mold 13 could include multiple mold cavities, runners, or other
features as
required for a particular application. The mold cavity 18 can have a
relatively complex
shape that produces surfaces on a casting that are finished or close to
finished, such that
additional machining or the like is minimized or eliminated. Also, if required
for a
particular application, a vacuum can be applied to the mold cavity 18 to
promote flow of
the slurry 12, or a mechanical force can be applied to the slurry 12 in the
pouring basin
15, such as by application of a pneumatic piston. Use of the vacuum or the
mechanical
force increases a pressure differential between the pouring basin side and the
vent side
of the slurry 12 to increase the rate of viscous flow of the slurry 12.
[0028] Because the weight percent of solids in the slurry 12 is relatively
low, the slurry
12 in mold 13 must be cooled at a relatively slow rate to prevent the
formation of
dendritic solid particles in the slurry. The allowable cooling rate depends
upon the alloy
used for a particular application. Also, the cooling rate will be affected by
the mold
material and the geometry of the part being cast. Fox example, if the part to
be cast
includes thin wall sections that tend to cool at a quicker rate, the mold
cavity 18 can be
designed to provide a reservoir portion that is initially filled with slurry.
The slurry in
the reservoir will cool at a relatively slow rate, such that the solids formed
are non-
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dendritic. After the percentage of solids increases to a level permitting a
higher cooling
rate, the slurry can be introduced into the thin wall sections of the mold.
[0029] Various suitable casting metals may be utilized in conjunction with the
present
application. For example, aluminum, magnesium, copper, zinc, and iron alloys
can be
utilized.
[0030] In the foregoing description, it will be readily appreciated by those
skilled in the
art that modifications may be made to the invention without departing from the
concepts
disclosed herein. Such modifications are to be considered as included in the
following
claims, unless these claims by their language expressly state otherwise.
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