Note: Descriptions are shown in the official language in which they were submitted.
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SOLIDIFICATION MICROSTRUCTURE OF AGGREGATE MOLDED SHAPED
CASTINGS
FIELD OF THE INVENTION
[0001] The present invention relates to metal castings. More particularly, the
present
invention relates to aggregate shaped metal castings having a fine
solidification
microstructure.
BACKGROUND
[0002] In the traditional casting process, molten metal is poured into a mold
and
solidifies, or freezes, through a loss of heat to the mold. When enough heat
has been lost
from the metal so that it has frozen, the resulting product, i.e., a casting,
can support its own
weight. The casting is then removed from the mold.
[0003] Different types of molds of the prior art offer certain advantages. For
example, green sand molds are composed of an aggregate, sand, that is held
together with a
binder such as a mixture of clay and water. These molds may be manufactured
rapidly, e.g.,
in ten (10) seconds for simple molds in an automated mold making plant. In
addition, the
sand can be recycled for further use relatively easily.
[0004] Other sand molds often use resin based chemical binders that possess
high
dimensional accuracy and high hardness. Such resin-bonded sand molds take
somewhat
longer to manufacture than green sand molds because a curing reaction must
take place for
the binder to become effective and allow formation of the mold. As in clay-
bonded molds,
the sand can often be recycled, although with some treatment to remove the
resin.
[0005] In addition to relatively quick and economical manufacture, sand molds
also
have high productivity. A sand mold can be set aside after the molten metal
has been poured
to allow it to cool and solidify, allowing other molds to be poured.
[0006] The sand that is used as an aggregate in sand molding is most commonly
silica. However, other minerals have been used to avoid the undesirable
transition from alpha
quartz to beta quartz at about 570 degrees Celsius ( C), or 1,058 degrees
Fahrenheit ( F), that
include olivine, chromite and zircon. Some of these sands exhibit minor
differences in
thermal conductivity from common silica sand and are sometimes mixed in as
mold or core
sections with silica sand or each other in order to try and help achieve
directional
solidification. These minerals possess certain disadvantages, as olivine is
often variable in its
chemistry, leading to problems of uniform control with chemical binders.
Chromite is
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typically crushed, creating angular grains that lead to a poor surface finish
on the casting and
rapid wear of tooling. Zircon is heavy, increasing the demands on equipment
that is used to
form and handle a mold and causing rapid tool wear. Mixing these sands as
different
components of a single mold also complicates efforts at recycling of the sand.
[0007] In addition, the disadvantages created by the unique aspects of silica
and
alternative minerals, sand molds with clay and chemical binders typically do
not allow rapid
cooling of the molten metal due to their relatively low thermal conductivity.
Rapid cooling
of the molten metal is often desirable, as it is known in the art that such
cooling improves the
mechanical properties of the casting. In addition, rapid cooling allows the
retention of more
of the alloying elements in solution, thereby introducing the possibility of
eliminating
subsequent solution treatment, which saves time and expense. The elimination
of solution
treatment avoids the need for the quench that typically follows, removing the
problems of
distortion and residual stress in the casting that are caused by the quench.
Related to the
mechanical properties, the fineness of the cast microstructure is related to
the rate of cooling
and solidification. Generally, as the rate of cooling and solidification
increases, the
solidification microstructure of the casting becomes finer.
[0008] As an alternative to sand molds, molds made of metal or semi-permanent
molds or molds with chills are sometimes used. These metal molds are
particularly
advantageous because their relatively high thermal conductivity allows the
cast molten metal
to cool and solidify quickly, leading to advantageous mechanical properties in
the casting.
For example, a particular casting process known as pressure die casting
utilizes metal molds
and is known to have a rapid solidification rate. Such a rapid rate of
solidification is
indicated by the presence of fine dendrite arm spacing (DAS) in the casting.
As known in the
art, the faster the solidification rate, the smaller the DAS. However,
pressure die casting
often allows the formation of defects in a cast part because extreme surface
turbulence occurs
in the molten metal during the filling of the mold. The presence of fine
dendrite arm spacing
may also be achieved by cooling the casting by a local chill or fin. Such
techniques include
the localized application of solid chill materials, such as metal lump chills
or moldable
chilling aggregates, and the like, that are integrated into the mold adjacent
to the portion of
the casting that is to be chilled. These methods, however, only provide a
localized effect in
the region where the chill is applied. This localized effect contrasts with
the benefits of the
invention discussed in this application, in which the benefits of fine
microstructure can apply,
if the invention is implemented correctly, extensively throughout the casting.
This is an
important aspect of the current application, because the ultimate benefit is
that the casting
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displays properties that are not only generally higher, but are also
essentially uniform
throughout the product, and thus of great benefit to the designer of the
product. The product
now essentially enjoys the uniformity normally associated with forgings.
[0009] One variety of known permanent mold process in which the residual
liquid
phase in the structure may be subject to rapid cooling includes some types of
semi-solid
casting. In this process the metallic slurry is formed exterior to the mold,
and consists of
dendritic fragments in suspension in the residual liquid. The transfer of this
mixture into a
metal die causes the remaining liquid to freeze quickly, giving a fine
structure, but
surrounded by relatively coarse and separated dendrites, often in the form of
degenerate
dendrites, rosettes, or nodules.
[0010] However, all molds made from metal possess a significant economic
disadvantage. Because the casting must freeze before it can be removed from
the mold,
multiple metal molds must be used to achieve high productivity. The need for
multiple molds
in permanent mold casting increases the cost of tooling and typically results
in costs for
tooling that are at least five (5) times more than those associated with sand
molds.
(0011] Another common feature of the internal structure of conventional shaped
castings, well known and well understood throughout the casting. industry, is
that those
regions of larger geometric modulus (i.e., regions having a larger ratio of
volume to cooling
area) generally have a coarser structure. Such regions of the casting
typically have
significantly lower mechanical properties. Further, such regions commonly
exhibit shrinkage
cavities or pores because they are more easily isolated from feed metal at a
late stage of
freezing. Such regions are often seen, for instance, at hot spots formed by an
isolated boss on
a relatively thin plate, or in the hot spot that is found at the T -junction
between two similar
sections. Complicated castings are often full of such features, resisting the
attainment of any
degree of uniformity of properties. This problem greatly complicates the work
of the designer
of the product. For instance, the thickening of a section intended to increase
its strength will
lower properties and in the worst instance may even lead to defects, and so
is, often to some
indeterminate degree, counter productive.
[0012] In the locations of the casting where solidification is slow, not only
is (i) the
structure coarse, typified by a coarse DAS, but (ii) porosity is also present,
and (iii) for those
Al alloys that commonly suffer iron as an impurity, large plate-like crystals
of iron-rich
phases can form. All these factors are greatly damaging to the ductility of
the alloy.
[0013] Extremely fine cast microstructures have been produced as laboratory
curiosities in various scientific studies (for instance, the research paper by
G.S. Reddy and
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J.A. Sekhar in Acta Metallurgica, 1989, vol. 37, Number 5, pp. 1509-1519 and
the research
paper by L. Snugovsky, J.F. Major, D. D. Perovic and J. W. Rutter; "Silicon
segregation in
aluminium casting alloy" Materials Science and Technology 2000 16 125 - 128.).
[0014] However, in contrast to such laboratory studies, the invention
described in this
application provides the unique conditions in which the solidification
microstructures
described herein are produced routinely by a production process that can be
operated to
produce one-off or volume-produced castings that are shaped in three-
dimensions.
[0015] It is less generally known that the interior of castings can experience
accelerated freezing as a result of a geometrical effect in shaped castings.
The early
solidification near the skin of the casting occurs substantially
unidirectionally, and typically
varies at a rate that decreases parabolically with time; i.e. the
solidification rate reduces in
speed as the thickness of the solidified layer increases. In contrast, the
volume of remaining
liquid in the center of a casting dwindles with time, and experiences
increasing heat
extraction from additional directions, so that the speed of freezing can be
greatly increased.
This effect is well described by one of the inventors. See Castings, John
Campbell, pp. 125-
126(2 d Edition, 2003), published by Butterworth Heinemann, Oxford, UK, the
entire
disclosure of which is incorporated herein by reference. The behavior explains
the so-called
reverse chill effect in cast iron castings, in which the center of the
casting, seemingly
inexplicably, sometimes exhibits a white, apparently chilled, structure in
contrast to the outer
regions of the casting that remain grey, signifying a slow cooling rate.
[0016] As a result, it is desirable to develop a casting process and related
apparatus
that have the advantage of rapid solidification of metal molds, while also
having the lower
costs, high productively and reclaim-ability associated with sand molds.
[0017] It is also desirable to provide an aggregate molded shaped casting
exhibiting a
region of fine solidification microstructure over extensive regions of the
casting, so as to
promote substantially uniform properties akin to those of forgings. (Because
of the relative
insensitivity of properties to the variations in cooling rate at the high
cooling rates used in this
application, the variations that are discussed later, for instance in Figure
4, do not
significantly affect properties, conferring substantial uniformity of
properties in the cast
product). In particular, it is desirable to provide an aggregate molded shaped
casting having
a fine solidification microstructure that is finer than a structure produced
by a conventional
aggregate casting method, and possibly even finer than that produced by a
permanent mold.
[0018] It is further desirable to provide an aggregate molded shaped casting
having a
fine solidification microstructure region that is substantially continuous
from a distal point of
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the casting to the feeder or riser.
SUMMARY
[0019] The disclosure provides, in various embodiments, a shaped metal casting
formed in an aggregate mold by an ablation casting process, the casting
comprising a fine
solidification microstructure that is finer than the microstructure of a
casting of a similar
metal having a similar weight or section thickness that is produced by a
conventional
aggregate casting process, wherein the fine microstructure comprises one or
more of grains,
dendrites, eutectic phases, or combinations thereof.
[0020] The disclosure also provides, in various embodiments, a metal casting
exhibiting a cast microstructure, the microstructure comprising a first region
located adjacent
a surface of the metal casting, the first region comprising a coarse
solidification
microstructure; and a second region located internal to the first region, the
second region
comprising a fine solidification microstructure.
[0021] In a further aspect, the disclosure provides, in various embodiments, a
metal
casting formed from a eutectic-containing alloy, the casting comprising a dual
solidification
microstructure region, wherein the dual solidification microstructure region
comprises (i) one
or more regions containing coarse dendrites; and (ii) one or more regions
containing fine
eutectic.
[0022] Additionally, the disclosure provides a shaped metal casting made in a
mold
that is at least a partially aggregate mold, the casting comprising a dual
solidification
microstructure region, wherein the dual solidification microstructure region
comprises at least
one coarse solidification microstructure portion having a grain size and/or
dendrite arm
and/or eutectic spacing in the range commonly to be expected in a conventional
aggregate or
metal mold; and at least one fine solidification microstructure having a grain
size and/or
dendrite arm and/or eutectic spacing of less than one third of the
conventional spacing for
that portion of the casting.
[0023] In still another aspect, the disclosure provides a shaped metal casting
made in
an aggregate mold, the casting comprising a dual solidification microstructure
region,
wherein the dual solidification microstructure region comprises: at least one
coarse
solidification microstructure portion having a dendrite arm spacing in the
range of about 50 to
about 200 micrometers; and,at least one fine solidification microstructure
portion having a
dendrite arm spacing of less than about 15 micrometers.
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5a
[0023a] In accordance with one aspect of the present invention, there is
provided a
metal casting formed of an alloy comprising aluminum in a mold comprising an
aggregate and
exhibiting a cast microstructure, the microstructure comprising: a first
region located adjacent
a surface of the metal casting, the first region comprising a coarse
solidification
microstructure; and a second region located internal to the first region, the
second region
comprising a fine solidification microstructure.
[0023b] In accordance with another aspect of the present invention, there is
provided a metal casting, having a pre-defined shape, formed of an alloy
comprising
aluminum in a mold comprising an aggregate and exhibiting a cast
microstructure, the
microstructure comprising: a first region located adjacent a surface of the
metal casting, the
first region comprising a coarse solidification microstructure; and a second
region located
internal to the first region, the second region comprising a fine
solidification microstructure.
[0023c] In accordance with yet another aspect of the present invention, there
is provided a
cast metal part, formed of an alloy comprising aluminum in a mold comprising
an aggregate via a
rapid cooling process, the part exhibiting a cast microstructure, the
microstructure comprising: a
first region located adjacent a surface of the metal casting, the first region
comprising a coarse
solidification microstructure; and a second region located internal to the
first region, the second
region comprising a fine solidification microstructure.
[0024] In another aspect, the disclosure provides a shaped metal casting
formed in an
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aggregate mold by an ablation casting process, the casting comprising a fine
solidification
microstructure having a dendrite arm spacing that is finer than the dendrite
arm spacing of a
casting having a similar metal of a similar weight or section thickness that
is produced by a
conventional aggregate molded or permanent molded casting process.
[0025] Further, the disclosure provides a shaped metal casting with
substantial
soundness and with high and substantially uniform properties, to some extent
resembling
those features normally associated with forgings.
[0026] Other features and advantages of castings in accordance with the
disclosure
are further understood in view of the drawings, detailed description,
examples, and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIGURE 1 is a cooling curve for a solid solution alloy that undergoes
dendritic solidification;
[0028] FIGURE IA is a graph showing the relationship between dendrite arm
spacing
and freezing or solidification rate;
[0029] FIGURE 2 is a micrograph (at X 200) showing the microstructure of a
solid
solution alloy cast by conventional casting methods;
[0030] FIGURE 3 is a micrograph of a solid solution alloy comprising a fine
solidification microstructure region (Dual DAS structure) produced by
ablation;
[0031] FIGURES 4A-4E are schematic representations of metal castings
comprising
fine solidification microstructure regions;
[0032] FIGURE 5 is a cooling curve of a mixed dendrite and eutectic alloy;
[0033] FIGURE 6 is a micrograph (at X 200) depicting the microstructure of a
356
alloy showing coarse eutectic silicon particles cast by conventional methods;
[0034] FIGURE 7 is a micrograph (at X 200) depicting an ablation-frozen A356
alloy
having regions of coarse plus fine dendritic material (dual DAS structure) and
fine eutectic
material;
[0035] FIGURE 8 is a micrograph (at X 200) of a portion of an ablation-frozen
A356
alloy showing uniform coarse DAS and uniform fine eutectic phase;
[0036] FIGURE 9 is a micrograph (at X 200) of an ablation-frozen A356 alloy
having
fine eutectic regions, but which are somewhat coarsened after a solution heat
treatment;
[0037] FIGURE 10 is a micrograph (at X 200) of an ablation-frozen A356.alloy
exhibiting coarse dendritic microstructure and mainly fine eutectic
microstructure, but
containing a trace of coarse eutectic phase;
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[0038] FIGURE 11 is a detail of the micrograph (at X 1000) of FIGURE 10 at
higher
magnification;
[0039] FIGURE 12 depicts the solidification rate of various portions of a
dendritic
alloy formed in accordance with an exemplary embodiment;
[0040] FIGURE 13 is a graph depicting the cooling rate of various sections of
an
exemplary embodiment casting;
[0041] FIGURE 14 is a table showing the mechanical properties of various
exemplary
embodiment castings;
[0042] FIGURE 15 is a bar graph and table comparing the mechanical properties
of
metal castings formed by various casting methods;
[0043] FIGURE 16 is a chart showing the relationship between dendrite cell
size and
solidification rate for aluminum alloys;
[0044] FIGURE 17a is a micrograph (at X 100) of a conventional cast permanent
mold microstructure of a 2.75" diameter bar of an aluminum alloy;
[0045] FIGURE 17b is a micrograph (at X100) of such a microstructure made with
the ablation process for the same alloy at the start of the ablated part
(first part in); and,
[0046] FIGURE 17c is a micrograph (at X 100) of the ablated last part.
[0047] FIGURE 18 is a photograph of an ablation frozen B206 alloy automotive
control arm showing the location (boxed) from which tensile test bars were
machined after
heat treatment.
[0048] FIGURE 19 is a table showing mechanical properties obtained from a very
thick section in a specific exemplary embodiment B206 aluminum alloy casting.
[0049] FIGURE 20 is a table showing data from the literature for A206
separately
cast tensile bars produced via various casting methods.
[0050] FIGURE 21 is a micrograph taken from an as-cast (F-temper) B206 casting
obtained from the center of the thick section shown in FIGURE 18 and having
both coarse
and fine dendritic material (dual DAS structure).
[0051] The figures are merely for the purpose of illustrating various
embodiments of
a development in accordance with the present disclosure and are not intended
to be limiting
embodiments of the development.
DETAILED DESCRIPTION
[0052] The disclosure relates to an aggregate molded, shaped casting
comprising at
least a fine solidification microstructure region. An aggregate molded shaped
metal casting
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8
in accordance with the disclosure includes a solidification microstructure
region that is finer
than the solidification microstructure obtained by conventional aggregate
molding methods.
In some embodiments, an aggregate molded shaped metal casting in accordance
with the
disclosure has a solidification microstructure that is substantially free of
shrinkage porosity.
[0053] The type or nature of the solidification microstructure will vary and
depend on
the metal and/or metal alloys undergoing solidification. Various
microstructures include
dendrites, eutectic phases, grains, and the like. In one embodiment, for
example, a shaped
casting may comprise only a single type of microstructure. Further, an alloy
may exhibit a
solidification microstructure comprising one or more different
microstructures. For example,
in one embodiment, a shaped casting may exhibit a microstructure comprising a
combination
of dendrites and grains. In another embodiment, a shaped casting may exhibit a
combination
of dendrites and eutectic phases. In still another embodiment, a shaped
casting may exhibit a
combination of dendrites, eutectic phases, and grains. These embodiments are
not limiting
embodiments, as other combinations and/or other microstructures may be
possible.
[0054] As used herein, eutectic alloys includes any alloy that forms eutectic
phases,
including hypo-eutectic, near-eutectic or hyper-eutectic alloys.
(0055] An aggregate molded, shaped metal casting in accordance with the
present
disclosure may be formed by a method such as that described in U.S. Patent
7,216,691 that
was filed on July 7, 2003. Generally, U.S. Patent 7,216,691 discloses a
process for the rapid
cooling and solidification of aggregate molded shaped castings. The method
also provides
for the removal of the mold. The process described in U.S. Patent 7,216,691 is
referred to
herein in as "ablation".
[0056] Upon solidifying during an ablation process, a metal casting exhibits a
fine
solidification microstructure that is finer than the microstructure of a
similar metal having a
similar weight or section thickness that is produced by a conventional
aggregate casting
process. The fineness of a microstructure may be defined in terms of the size
or spacing
exhibited by a particular type of microstructure. For example, grains exhibit
a grain size,
dendrites exhibit a dendrite arm spacing, and eutectic phases exhibit a
eutectic spacing.
[0057] With reference to FIGURE 1, a cooling or solidification curve for a
solid
solution alloy is shown. Solid solution type alloys form only grains and/or
dendrites during
solidification. The cooling curve shows the cooling of a solid solution alloy
with time, from
the pouring temperature (Ti,) through the liquidus temperature (TL) to the
solidus temperature
(Ts), which is the point at which solidification is complete.
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[0058] Cooling of a solid solution type alloy in a conventional aggregate mold
is
represented by the time/temperature profile "abcdef". The total time for
cooling using
conventional methods is in the range of from minutes to hours and is, of
course, particularly
dependent on the thickness of the casting, and the rate at which heat can
transfer into the
mold. The rate of cooling slows at the liquidus temperature (TL) as a result
of the latent heat
evolution during the formation of dendrites. Solidification is complete at the
solidus
temperature (Ts) and the rate of fall of temperature increases once the
evolution of latent heat
has subsided.
[0059] At point "e" on FIGURE 1, the application of any rapid cooling is too
late to
have any effect on the solidification microstructure. Thus, for example, the
cooling profile
"el" would not have any effect on the solidification microstructure and is not
a part of this
patent application. Cooling profiles such as "el," however, are commonly
utilized in the
casting industry where castings are taken from a metal die and quenched
directly into water.
[0060] The solidified structure of solid solution alloys typically consists
almost
entirely of dendrites that are outlined by a negligible thickness of residual
inter-dendritic
material. The secondary dendrite arm spacing (often referred to more simply in
this
application as dendrite arm spacing, or DAS) is dependent on the freezing
time, i.e. the
solidification time, which is the time that a fixed location in the casting
exists at a
temperature between the liquidus TL and the solidus temperature Ts of the
alloy. In terms of
the period of time in Figure 1 a, it is seen to be is = tt - tz.
[0061] Figure 1 a shows the approximate logarithmic relation between the local
DAS
and the local is in many common Al alloys. This figure illustrates that to
reduce DAS by a
factor of 10, is is required to be reduced by a factor of approximately 1000.
(Unfortunately
such a relationship has not been investigated for grains and eutectic spacing,
so that a clear,
quantitative description of the refinement of these other features of the
solidification structure
of some alloys cannot easily be made. Thus the quantitative predictions of
refinement of
structure by ablation described in this application concentrate on DAS.
However, it is to be
understood that similar but unquantified refinements are paralleled in grain
size and eutectic
spacing) Thus very large increases in cooling rate are required to
substantially affect the
fineness of the solidification microstructure.
[0062] While the quantitative relationship exemplified in Figure 1 is
described with
respect to dendrites and dendrites arm spacing, similar parallel refinements
are expected in
grain size and/or eutectic spacing in alloys comprising grains and/or eutectic
phases.
[0063] For conventional aggregate casting methods, for a small aluminum alloy
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casting weighing a few pounds or kilograms, the local solidification time is
typically of the
order of about 1,000 seconds. These conventional aggregate casting methods
produce
castings having a DAS of around 100 micrometers, and sometimes in the range of
from about
50 micrometers to about 200 micrometers. As used herein, a solidification
microstructure
having a DAS of greater than 50 micrometers is referred to as "coarse"
microstructure.
FIGURE 2, is a micrograph depicting the coarse microstructure of a solid
solution cast alloy
A206 (a nominal Al - 4.5wt%Cu alloy) that was cast by conventional methods.
[0064] A casting in accordance with the present disclosure comprises the
presence of
fine solidification microstructure in at least a portion of the casting. That
is, a casting may
comprise from greater than 0% to up to 100% of fine solidification
microstructure. In one
embodiment, the casting is substantially free of any coarse solidification
microstructure and
comprises up to 100% fine solidification microstructure that is continuous
throughout the
casting. In another embodiment, a casting comprises a first region adjacent
the surface of the
casting that comprises up to 100% coarse solidification microstructure, and a
second region
internal to the first region wherein the second region comprises up to 100%
fine solidification
microstructure. In still another embodiment, a casting in accordance with the
present
disclosure comprises a continuous or at least a substantially continuous,
region of fine
solidification microstructure extending from a distal end of the casting to a
proximal end
thereof, i.e., the end adjacent the feeder or riser.
[0065] In other embodiments, a casting comprises a dual solidification
microstructure
region intermediate a coarse solidification microstructure region and a fine
solidification
microstructure region. As used herein, a dual microstructure region is a
region that includes
areas of coarse microstructure having one or more areas of fine microstructure
interspersed
therein. In still another embodiment, the dual solidification microstructure
in a casting is
substantially continuous throughout from a distal end of the casting to the
feeder.
[0066] The dendrite arm spacing is generally dependent on the time over which
solidification occurs. As shown in FIGURE lA, the log/log relationship of
dendrite arm
spacing to freezing time is linear and, for aluminum alloys, for example, has
a slope of
approximately 1/3. The graph shows there is approximately a factor of 5
spacing reduction
for each factor of 100 in freezing time. Thus, a particular casting section in
a conventional
mold may experience solidification in 1000 seconds giving a corresponding DAS
of 100
micrometers. In the ablation casting techniques described in U.S. Application
Serial No.
10/614,601, the same casting section would result in a local solidification
time of only
approximately 10 seconds, giving a dendrite arm spacing of about 20
micrometers. The
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relationship between spacing and freezing time remains constant over all
experimental times.
FIGURE 3 is a micrograph showing regions of both fine microstructure and
regions*of coarse
microstructure.
[0067] With reference to FIGURES 4A-4E, various exemplary embodiments of
aggregate molded, shaped metal castings comprising a fine solidification
microstructure
region are shown.
[0068] With reference to FIGURE 4A, an embodiment is shown in which a casting
comprises a small percentage of fine solidification microstructure. The
casting in FIGURE
4A represents a situation in which the rapid cooling process, such as
ablation, is applied late
in the casting process. In the casting in FIGURE 4A, some solidification has
occurred prior
to rapid cooling (e.g., ablation), and portions of the casting, such as those
having a small
geometric modulus (volume to cooling area ratio) conventionally freeze. Dual
solidification
microstructure regions occur where some portions have solidified prior to
ablation but other
portions remain liquid at the time ablation begins. Even in an example such as
FIGURE 4A
where a portion of the casting has solidified prior to a rapid cooling
process, and thus
constituting a far from optimum application of this invention, the presence of
even a small
amount of fine solidification microstructure is advantageous. Specifically, in
conventional
processes, the small percentage of residual liquid in metal alloys that
remains is particularly
difficult to freeze without creating defects such as shrinkage porosity.
Applying a rapid
cooling technique such as ablation, however, converts these regions from
defective zones to
fine structure zones. The presence of even small zones of fine structure
provide good
mechanical properties compared to those defective zones of a conventionally
solidified
casting. Although the trapped region of residual liquid illustrated in Figure
4A will be
expected to demonstrate some shrinkage porosity, ablation freezing of this
region will reduce
the extent of the shrinkage, and. will replace it with a corresponding region
of strong, sound
material. The solidification microstructure profile of FIGURE 4A would be
expected where
the rapid cooling step is applied rather late, closer to, but before, point
"e" on the graph of
FIGURE I.
[0069] In another embodiment, an aggregate molded shaped casting comprises a
fine
solidification microstructure region and a dual solidification microstructure
region wherein
the dual solidification microstructure region is substantially continuous from
a distal end of
the casting to a proximal end of the casting. The embodiment of FIGURE 4B is
an example
of an embodiment of a casting having a substantially continuous dual
solidification
microstructure region. The solidification microstructure profile of FIGURE 4B
is a profile
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that would be expected from following the cooling profile "abcdjk" in FIGURE
1. In
FIGURE 4B, the freezing point arrives at and passes the point at which the
natural freezing of
the constricted section reaches the center of the section. The casting is
frozen by a rapid
cooling procedure, such as ablation, from the more distant parts of the
casting up to the center
point. If the fine structure zone produced by rapid freezing is terminated on
reaching the
modulus constriction (as it does in the embodiment in FIGURE 4B) the casting
will freeze
soundly up to this point. Even though the dual solidification microstructure
is absent in the
local region of the constriction in the embodiment in FIGURE 4B, the rapid
local
solidification time throughout the remainder of the casting creates a
substantially continuous
zone of fine and sound alloy, free from shrinkage defects, throughout the
remainder of the
casting. Thus, by driving the solidification directionally from distal regions
to those proximal
to the feeder, the more distant portions of the casting exhibit fine
solidification microstructure
and mechanical soundness that would not have been achieved by conventional
methods.
[0070] In the embodiment in FIGURE 4C, the casting includes a greater
percentage
of fine solidification microstructure, and the region of dual solidification
microstructure is
continuous through the constricted region of the casting.. Such a desirable
solidification
microstructure may be achieved by applying a rapid cooling procedure, such as
ablation, at a
time earlier than that of FIGURE 4B.
[0071] In another embodiment, such as that of FIGURE 4D, a metal casting
comprises a desirable fine solidification microstructure region that is
substantially continuous
from the distal ends of the casting to the feeder. Such a solidification
microstructure may be
achieved by applying a rapid cooling procedure early in the cooling profile.
In this
embodiment, the casting may also include regions of dual solidification
microstructure and
coarse solidification microstructure. The solidification microstructure
profile of FIGURES
4C and 4D would be expected from applying a rapid cooling at an early time,
such that the
narrowest part of the casting would follow a path starting at a point between
c and d on the
cooling profile in FIGURE 1.
[0072] In still another embodiment, such as. the embodiment of FIGURE 4E, the
entire solidification microstructure comprises a fine solidification
microstructure. Such a
desirable structure might be achieved by applying a rapid cooling method at
Point "b" in the
cooling curve of FIGURE 1 and following the profile "abghi." This would occur
if no
freezing occurs due to loss of heat to the mold and the freezing occurs
totally unidirectionally
and at a high rate. Such a structure, however, is not easily achieved and has
yet to be
experimentally achieved by the inventors. Difficulties in achieving this
solidification
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microstructure arise from the impingement of the liquid coolant directly on
the surface of the
still liquid casting. A casting comprising 100% fine solidification
microstructure may be
achievable under certain conditions such as using a highly insulating mold,
and applying a
highly directional solidification process.
[0073] An aggregate molded, shaped metal casting may comprise from about 1 to
about 100% fine solidification microstructure. Even a small amount of
solidification
microstructure is desirable for enhancing the mechanical properties of a
casting. This is
especially the case where the creation of small amounts of fine solidification
microstructure,
denoting as it does in this invention the action of directional
solidification, and thus optimal
feeding, prevents defects such as shrinkage porosity from occurring in the
casting.
[0074] Castings in accordance with the present disclosure that include a fine
solidification microstructure region may be formed from any solid solution
alloy that
solidifies dendritically. These include both ferrous materials and non-ferrous
materials. The
dendrite arm spacing of both the coarse and fine solidification microstructure
regions will
vary depending on the metal that is used. With respect to aluminum alloys,
coarse
solidification microstructure regions typically have a dendrite arm spacing of
greater than
about 50 micrometers. In some embodiments the coarse solidification
microstructure has a
dendrite arm spacing of from about 50 to about 200 micrometers. Also in
aluminum alloys,
the fine solidification microstructure has a dendrite arm spacing of less than
about 15
micrometers, and, in some embodiments, is from about 5 to about 15
micrometers.
[0075] Alloys in which solidification occurs partly by dendritic
solidification and
partly by eutectic solidification, as is typical of many Al-Si alloys,
exemplified by Al-7Si-
0.4Mg (A356) alloy, may also exhibit fine dendritic and/or fine eutectic
microstructure. The
conventional cooling curve for mixed dendrite/eutectic alloys is illustrated
in FIGURE 5 as
curve. "a-h." Starting at the pouring temperature (Tp) at point "a" the liquid
alloy cools to the
liquidus temperature (TO, at point "c," which is the point at which dendrites
start to form.
The dendrite growth is complete at point "e," which is the eutectic
temperature (TE). The
fall in temperature is arrested, forming a plateau until the completion of the
eutectic
solidification at point "g". At this point the casting is completely frozen,
and further cooling
to room temperature follows "gh." A second example alloy of an AI-Si alloy
that benefits
powerfully from the application of this invention is the widely used A319
alloy. This alloy
also contains some copper. The alloy differs somewhat from A356 in having a
eutectic
formation region "eg" that is not isothermal, the horizontal plateau "eg" of
Figure 5 being
replaced by a steady downward slope. However, the same principles apply
precisely.
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[0076] This slow conventional cooling in, for instance, a silica sand mold,
results in a
structure of dendrites, having a DAS in the region of 200 down to 50
micrometers. The
dendrites are surrounded by eutectic, which is characterized by a spacing in
the region of 20
down to 2 micrometers. This is denoted the conventional or "coarse" eutectic
microstructure for purposes of our description (FIGURE 6).
[0077] If it were possible. to subject the alloy to total cooling by ablation,
the cooling
profile would follow the path "bijkl- so that the whole solidification
microstructure would
consist of fine dendrites and very fine eutectic. However, as discussed above,
although this
structure is not easily obtained, and has yet to be tested by the inventors,
it may be
achievable in special conditions. These conditions might include circumstances
in which
the mold is highly insulating, and the freezing is highly directional.
Castings having
excellent mechanical properties are, nevertheless, achievable without resort
to these special
conditions.
[0078] In general, a more practical cooling profile is illustrated by the
cooling curve
"abcdmno." In this situation the prior cooling from "cd" creates coarse
dendrites to
strengthen the casting in its hot, partially solidified, and therefore weak
state, prior to the
application of the coolant. The subsequent dendrites and the eutectic are both
then subjected
to rapid cooling, so that the fine solidification microstructure includes both
fine dendrites
(DAS in the region of 30 down to 5 micrometers) and fine eutectic (not
resolvable at 1000 X
magnification, since its spacing is around only 1 micrometer).
[0079] The two regions of the consequently dual microstructure form visually
highly
distinct areas when viewed under the microscope. FIGURE 7 shows a structure in
which the
ablation was applied in time to freeze some dendritic material, followed by
the rapid freezing
of all of the eutectic. The eutectic is so fine that it is not resolvable in
this image, but
appears as a uniform light grey phase (in this case the alloy had no refining
action of the
additions of chemical modifiers such as Na or Sr). Additionally, because all
of the eutectic
freezes along the path "mn," the whole of the eutectic phase, between both the
coarse and the
fine dendrites, is seen to be uniformly fine in FIGURE 7.
[0080] The uniform and extremely fine eutectic is a common feature of ablated
solidification microstructures and is unique to ablation cooled alloys that
have received no
benefit from chemical modification by Na or Sr as an aid to refine the
microstructure. It is
seen in FIGURE 8, in which some finely distributed dross and associated pores
can also be
seen in the structure. The mechanical properties of the castings appear to be
remarkable
insensitive to most defects of this variety and size. FIGURE 9 illustrates a
similar fine
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eutectic after a solution heat treatment. In FIGURE 9, the eutectic has
coarsened somewhat
to reduce its interfacial energy as is common for two-phase structures
submitted to high
temperature treatment.
[0081] In principle, although not normally desirable, it would be possible to
allow all
of the dendrites to freeze with the coarse structure, making only a late
application of the
ablation cooling, such as, for instance, at point "f" in FIGURE 5. In this
case some of the
eutectic would have frozen with a coarse eutectic structure. Such a structure
is shown in
FIGURE 10. The final regions of the eutectic that freezes with the benefit of
ablation cooling
adopt the extremely fine structure and are generally free from porosity and
exhibit only fine
iron-rich phases, which are generally too small to be seen in FIGURE 10. At
higher
magnification, a few iron-rich phases can be seen, as shown in FIGURE 1 1 .
[0082] For mixed dendrite/eutectic alloys, most of the benefits of ablation
are
enjoyed by those structures seen in FIGURES 10 and 11 because progressive
solidification
of the residual liquid will still be effective to feed the casting
directionally. The alloy in
FIGURE 10, for example, has been heat treated, therefore to some extent
coarsening all of the
silicon particles in the eutectic phase.
[0083] In practice, however, it is desirable, and easily achieved, for some
coarse
dendritic structure to be formed prior to the application of the ablation
(starting point "d" in
FIGURE 5). The usual resulting dendritic microstructure is therefore dual in
the sense
described above and includes relatively uniformly fine eutectic.
[0084] As before, if coolant is applied extremely late, for instance following
path
"gq" in FIGURE 5, the action of ablation cannot influence the solidification
microstructure
of the casting because, of course, the casting has fully solidified prior to
any application of a
coolant. Such cooling of a casting does not form part of this patent
application, and falls
into the casting production regime well known to those skilled in the art.
[0085] For conventionally cooled castings (those that adopt the cooling path
ending in
"h" or "q") the last regions of the casting to solidify often contain
porosity. In addition, such
castings when made in a typical aluminum alloy such as A356 alloy, often
contain thin
platelets of beta-iron precipitates that further impair properties.
[0086] Ablation-cooled castings, including both dendritic castings and
dendritic/eutectic castings, comprising a fine solidification microstructure
are generally free
of defects that are often found in castings formed by conventional casting
methods. In one
embodiment, a casting comprising a fine solidification microstructure portion
is substantially
free from porosity. The rapid freezing and directional feeding created by
ablation reduces
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both gas and shrinkage porosity. In another embodiment, a casting comprising a
fine
solidification microstructure portion is substantially free of large damaging
iron-rich
platelets. In other embodiments, a casting is substantially free of both
porosity and iron-rich
platelets. Without being bound to any particular theory, the reduction in size
of the iron-rich
platelets may be the result of the more rapid quench of the liquid alloy. The
reduction of
porosity also benefits from this speed. In addition, it is significantly aided
by the naturally
progressive action of the ablation process, in which the cooling action of
water (or other
fluid) is moved steadily along the length of the casting to drive the
solidification in a highly
directional mode towards the source of feed metal. Furthermore, the
maintenance of a
relatively narrow pasty zone by the imposition of a high temperature gradient
in this way is
highly effective in assisting the feeding of the casting.
[0087] The substantial reduction or elimination of shrinkage porosity is
significant,
and may be restated as follows. Shrinkage porosity would normally be expected
in regions
of the casting such as an unfed hot spot. In principle, however, these regions
can be fed if the
freezing process is carried out directionally. The water or other cooling
fluid is applied to
ablate the mold and cool and cause solidification in the casting progress
systematically,
creating a uniquely strong directional temperature gradient. Thus, those
regions that would
have been isolated from feed liquid in a conventional casting are easily and
automatically fed
to soundness, or greatly improved soundness, when the benefits of the
invention are
correctly applied.
[0088] For this reason, alloys that cannot normally be cast as shaped castings
because of hot-shortness problems, such as the wrought alloys 6061 and 7075,
etc., or with
long freezing range such as alloys 7075 and 852, can easily and beneficially
be cast into a
shaped form via ablation techniques. In addition, the ablated castings are
characterized by a
solidification microstructure that is immediately identifiable as being unique
in a shaped
casting.
[0089] An aggregate molded shaped metal casting comprising a fine
solidification
microstructure region is further described with reference to the following
examples. The
examples are merely for the purpose of illustrating potential embodiments of a
shaped metal
casting having a fine solidification microstructure region and are not
intended to be limiting
embodiments thereof.
EXAMPLES
[0090] In one example of the application of the technique described in this
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17
application, a single test bar was molded of diameter 20 mm and length 200 mm
furnished
with a small conical pouring basin at one end that was filled to act as a
feeder. The mold
material was silica sand bonded with a water-soluble inorganic binder as
described in
U.S. Patent 7,216,691.
(0091]. Thermocouples were inserted into the mold cavity at the base of the
feeder,
and at the base of the cavity. Two additional thermocouples were located at
equal intervals
along the axis. These four thermocouples were labeled TC I (riser), TC2 (top
midsection),
TC3 (bottom midsection) and TC4 (bottom).
[0092] An aluminum alloy 6061 at a temperature of 730 C (1350 F) was poured
into
the cavity, arranged with its axis vertical. Within approximately 10 seconds,
water at 20 C
(68 F) was then applied from nozzles directed at the base of the mold so as to
start the
ablation of the mold from the base upward. The rate of upward progression of
the ablation
front was approximately 25 mm/s.
[0093] Cooling traces of the four thermocouples were recorded as seen in
FIGURE
12. The thermocouple TC4 is seen to cool rapidly, signaling the freezing and
cooling to
below the boiling point of water in only about 2 seconds. At this time the
thermocouple
immediately above, TC3, still records that the metal is still molten, and that
cooling has only
just begun. This pattern is repeated successively up the mold. (The jump in
temperature for
TC2 records the unintentional momentary loss of cooling water in this
experiment). The
thermal traces confirm that the temperature gradient caused by the application
of ablation was
sufficient to freeze the melt and cool it to near ambient temperatures within
a distance of less
than the spacing between thermocouples (50 mm). Furthermore, the effect was
easily and
accurately sustainable for the length of an average automotive casting.
[0094] In a second example an automotive knuckle casting was made in alloy
A356.
Many knuckle castings have a reputation for being difficult to cast because of
their
complexity, having heavy sections distant from points where feeders can be
added. This
casting was no exception. The casting was filled with metal at 750 C (1385 F)
on a tilt
pouring station, taking 8 seconds to fill, resulting in an excellent surface
finish. The feeder
(riser) was located at the far end of the casting.from the pouring cup and
down-sprue.
Thermocouples in the spree and feeder are illustrated in Figure 13. It is seen
that freezing
took place in the sprue, being the first to ablate, in approximately 20
seconds. Freezing was
then caused to progress across the casting, arriving at the feeder
approximately 90 seconds
later, at which point the feeder is caused to freeze at a similar time of only
about 20 seconds.
[0095] To achieve ablation for this casting, three banks of water spray
nozzles were
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employed, with water pressure at approximately 0.7 bar (approximately 10 psi)
and
temperature approximately 40 C (approximately 100 F).
[0096] The casting was found to be completely sound, and with
properties.exceeding
specification.
[0097] In a third example a control arm casting, a steering/suspension
component of
an automobile, was molded in the mold material as specified for Example 1. The
mold was
poured with A356 alloy of approximately composition Al-7Si-0.35Mg-0.2Fe at
approximately 700 C (approximately 1400 F). Ablation cooling of this mold
produced a
casting that was-subsequently cut up and machined to produce tensile test bars
that were
subject to a T6 heat treatment. The solution treatment was 538 C (.1000 F) for
0.5 hours,
water quench at 26 C (78 F) and age at 182 C (360 F) for 2.5 hours. Four test
bars were cut
from each of three castings numbered 45, 46 and 47. The bars were subjected to
tensile
testing and the results are listed, together with averages, in Figure 14. (The
one low extension
value of 9% was attributed to a large oxide inclusion since the control of the
melt quality was
known to be less than optimum on this occasion.) The results are compared with
those from
competitive casting processes in FIGURE 15. The properties are clearly
attractive, exceeding
those of all current best competitive processes.
[0098] It should be appreciated that certain potential process conditions
could result
in a conventional microstructure from an ablated mold when using the ablation
casting
process. As an example, an 852 aluminum alloy (Al-6Sn-2Cu- lNi-0.75Mg alloy)
is known
as a long range freezing alloy, wherein the eutectic freezes approximately at
the melting point
of tin (232C, 610F). This alloy was ablated using the ablation casting
process. The mold was
symmetrical, allowing the identical mold halves to be produced from a single
sided pattern
and then assembled. The metal was poured at or near 700C (1275F). The casting
section
thickness was approximately 75mm (3 inch) in diameter. The pouring of the mold
by gravity
was achieved in 10 seconds. The mold was then left to sit for a period of
nearly 180 seconds,
to achieve a mostly solidified alpha phase. After this period of normal
solidification rate
being controlled by the molding aggregate (in this case silica sand), the mold
was ablated.
[0099] The ablation conditions were as follows. Water pressure used for
ablation was
approximately I bar (15 psi). The spray volume was limited by the spray
nozzles. However,
the water volume is nearly insignificant as the pressure controls the water
impingement
against the as cast surface. This procedure captured a conventional but
uniform cooling rate
that yielded similar properties to that produced by a metal mold (i.e., a
permanent mold, see
FIG. 16). In this connection, FIGURE 16 shows the spectrum of various casting
processes
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and the relationship between dendrite cell size and solidification rate for
aluminum alloys. A
conventional permanent mold microstructure is illustrated in FIGURE 17a.
FIGURES 17b
and c show the same alloy but now created using the ablation process. In all
three of
FIGURES 17a-c, the same magnification, 100X, was employed. The final eutectic
structure
was closely similar to that produced by a permanent mold. Although such a
conventional
microstructure can be achieved in the ablation process, in some conditions,
those
microstructures unique to ablation, including extremely fine phases possibly
of dendrites, but
more often of extremely fine eutectic, can also be observed.
[00100] To use the ablation process to capture a conventional microstructure
(such as
that resulting from a permanent mold), several variables have significant
potential. An
important parameter is the mold aggregate itself. In addition, the volume,
pressure and
temperature of the cooling medium used in removing the mold while
simultaneously causing
solidification of the metal are, of course, also important. During their
advance along the
length of the casting, the dwell time of the cooling sprays upon the casting
can be beneficially
adjusted to allow for the local surface to volume ratio (the geometrical
modulus). The rate of
dissolution of the mold binder can be reduced to slow the rate of ablation of
the mold, and so
reduce the rate of thermal extraction. This could limit the rate so as to
produce a
conventional microstructure. Furthermore, the water pressure can be varied. At
first, a
higher pressure can be used to remove the aggregate of the mold. Then, the
pressure can be
reduced to create a cooling rate that would be akin to that of a conventional
metal mold
process.
[00101] Although a conventional microstructure may be attained in this way,
there are
significant additional benefits offered by the ablation process that make the
ablation process
uniquely desirable. First, there is improved control of the residual stress
within the casting
since the final solidification occurs with sufficient liquid ahead of the
final solidification front
to allow for a complete accommodation of thermal strains. Second, the porosity
in the
casting is significantly reduced (practically to zero in the majority of
cases) because of the
excellent directional solidification. This occurs as a result of the high
applied temperature
gradient that ablation uniquely achieves.
[00102] In a fourth example an automotive suspension control arm casting was
molded
in the material as specified for Example 1. Molds were poured with B206 alloy
of
approximate composition Al-4.8Cu-0.4Mn-0.28Mg-0.07Fe at approximately 1265F
plus or
minus 15F. Ablation cooling of these molds produced castings that were
subsequently
subjected to T4 and T7 heat treatments. A photograph of the part in question
appears in
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Figure 18 with a box showing the position from which tensile samples were
subsequently
machined. Tensile properties obtained from the parts are shown in Figure 19
and are
compared to standard tabulated values obtained from separately cast test bar
data from the
open literature for this type of alloy in Figure 20. A micrograph taken from
the center of the
thick section of one of the parts prior to heat treatment appears in Figure 21
. The properties
are again; clearly attractive, almost equaling those produced from separately
cast permanent
mold test bars and exceeding those in all other molding medias listed.. These
values are
particularly attractive considering that they were obtained from an area with
a section
thickness of over 2 inches. Under conventional sand.and permanent mold
processes, those
skilled in the art would expect to achieve far lower values of ductility from
bars machined
from a section this massive than are listed for the separately cast test bar
data in Figure 20 in
which 1/2 inch thick gauge diameter bars with an as-cast surface are the norm.
It should be
noted that these comparisons also do not take into account the fact that
manufacturing a part
the size and geometry of the one shown in Figure 18 would be a severe
engineering challenge
in any AA 2XX series alloy using a permanent mold due to the increased
tendency towards
hot tearing inherent in that casting process/alloy combination. Such could, in
fact, be
physically impossible without measures which would make the resultant part
prohibitively
expensive.
[00103] The micrograph in Figure 21 displays the aforementioned dual
microstructure
seen in ablatively solidified parts. A course DAS averaging 43um with spacings
as high as
85um is host to much finer patches with a DAS averaging on the order of 22um.
The
mechanical properties reported in Figure 17 are more typical of those that
would be expected
from the finer DAS. The intermetallics visible in Figure 19 are, in the main,
Cu Aluminides
which dissolve during the solution heat treatment applied as part of both the
subsequent T4
and 77 tempers. Those skilled in the art of producing 200 series aluminum
castings will
recognize the advantage of a fine structure with respect to economy of heat
treatment. The
three stage solutionizing operation needed to dissolve the aforementioned Cu
Aluminides for
thick sand castings may be foregone in favor of the two stage treatment most
commonly
applied in the case of thin and/or rapidly solidified casting.
[00104] Aggregate molded or shaped castings having time solidification
microstructure
have been described with reference to the present disclosure and various
exemplary
embodiments. It will be appreciated that variations or modifications may be
within the
capabilities of a person skilled in the art and that the present application
and claims are
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intended to encompass such modifications. It is intended that the development
be construed
as including all such modifications and alterations insofar as they come
within the scope of
the appended claims and the equivalents thereof.
