Note: Descriptions are shown in the official language in which they were submitted.
WO 92/01$21 r r ~ ~ PCT/CA9i/00242
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CAST COMPOSITE MATERIALS
TECHNICAL FIELD
This invention relates to the preparation of
cast composite materials having matrix alloys that do not
readily wet the reinforcement particles, and to cast
metal-matrix composite materials, particularly composites
having a matrix alloy tailored to avoid the formation of
harmful intermetallic phases.
BACKGROUND ART
Cast composite materials are conventionally
formed by melting a matrix alloy in a reactor and then
adding particles. The mixture is vigorously mixed to
encourage wetting of the matrix alloy to the particles,
and after a suitable mixing time the mixture is cast into
molds or forms. The mixing is conducted while minimizing
the introduction of gas inter the mixture. The cast
composite materials have fully wetted particles, few
voids, arid a generally uniformly mixed structure.
Complete wetting is necessary to realize the full
composite strength and other mechanical properties.
Such cast composite materials are much less
expensive to prepare than other types of metal-matrix
composite materials such as those produced by powder
metallurgical technology. Composite materials produced by
this approach, as described in US Patents 4,759,995 and
4,786,467, have enjoyed commercial success in only a few
years after theirs first introduction.
As the cast composite materials have entered
commercial production, customers have sometimes requested
particle/matrix alloy combinations wherein the matrix does
not readily wet the particles. In other instances, new
metallic alloys have been identified that produce
unexpectedly superior performance when used as the matrix
phase of the composite materials, except for the problem
that the composite materials are difficult to produce
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commercially due to the inability of the matrix alloy to
wet and mix with the particles readily.
There are a number of techniques that can be
applied to enhance wetting, which may work in some
circumstances. The particles can be modified with special
coatings, but the coating operation can significantly
raise the cost of the particles and the composite
material. Small amounts of reactive gases can be
introduced into the mixing chamber, but the improved
wetting may only be achieved at the cost of increased
porosity in the cast composite material. Special reactive
alloying ingredients can be added to the melt, but these
are often expensive and may have adverse consequences in
the production of undesired minor phases in the cast
composite material. Another approach is to raise the
temperature at which the mixing to achieve wetting is
accomplished, but increased temperature may also result in
the acceleration of the production of deleterious minor
phases where such phases are thermodynamically favored but
kinetically slow in forming at lower temperatures.
There therefore exists a continuing need for an
improved technique for producing cast composite materials
from particle/matrix alloy combinations wherein the matrix
does not inherently readily wet the particle. Desirably,
any such technique would not add substantially to the cost
of the product or have detrimental effects.
.one potential application of cast composite
materials is in foundry remelt alloys. The composite
materials are prepared by a supplier and cast into ingots
at the supplier's plant. The cast ingots are transported
to a commercial foundry, where they are remelted and cast
to the final shape required by the customer. This foundry
remelt approach is commonplace throughout industry for the
processing of conventional aluminum alloys, and the
introduction of aluminum-based cast composite materials
into many applications is practical only where they can
conform to this approach.
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Experience has shown that, with the proper
mi~;ing technique, a wide variety of cast composite
materials can be mixed by the suppliers. In the mixing
stesp, the maximum temperature to which the molten
composite may be heated is normally limited to avoid the
production of unwanted reaction products between the
matrix alloying elements and the reinforcement particles.
Some reaction products can reduce the mechanical
properties of the composite material and cause porosity in
the composite material, and are therefore to be avoided.
However, many of these cast composite materials
are not compatible with commercial foundry remelt
practices. Cast composite materials used in remelt
applications must permit high remelt temperatures,
typically greater than those used in the composite mixing
operation, and long remelt holding times. The casting of
metallic composite materials into complex shapes requires
that the molten material can be superheated above its
melting point and be highly fluid so that it can flow into
cold mold cavities for a considerable distance before the
superheat is removed and the metal freezes. The greater
is the remelt temperature permitted for the material and
the fluidity of the material, the greater is the distance
the molten composite material may flow into mold cavities
before it solidifies, and the more intricate the products
that can be cast.
Additionally, present foundry techniques usually
call for the melting of large masses of the casting alloy
to reach a stable temperature distribution, and casting
articles from the large melted mass. The remelted
material may remain at elevated temperature for extended
periods of time, such as up to 24 hours, before casting.
During this holding period, the castability of the
composite material may degrade, so that a composite
material may be much less castable after such a holding
period than if cast immediately upon remelting. It is
important that the composite material be castable by such
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commercial practices that have been long established, to
accelerate the acceptance of the composite material by
foundrymen.
In one specific example, aluminum-7 weight
percent silicon alloys have been used in industry for
years as remelt alloys, because the alloy has good
fluidity and acceptable mechanical properties after
casting. A satisfactory composite material of, for
example, 15 volume percent of silicon carbide particles in
an aluminum-7 weight percent silicon alloy may be prepared
and cast by the supplier with a maximum temperature of
685°C in the mixing process. Ingots of this alloy are
furnished to a foundry remelter, who remelts the ingots
and holds the molten composite at a conventional remelt
temperature of about 788'C for 8 hours before casting.
The molten composite material casts very poorly, has low
fluidity, and results in unacceptable product. The
composite material is therefore rejected for the
particular application, even though it might otherwise
provide important benefits to the final product.
There therefore exists a need for an improved
approach to the preparation of cast composite materials,
particularly those for use in foundry remelt applications.
The present invention fulfills this need, and further
provides related advantages.
DISCLOSURE OF INVENTION
One feature of the present invention provides a
process modification that permits the production of many
cast composite material particle/matrix alloy compositions
that are difficult to prepare because the matrix alloy
does not wet the particles. No new alloying ingredients
or atmospheric additions are required, the particles need
not be coated, and the temperature is not raised over that .
normally used. The cost of the production operation
remains essentially unchanged from that of conventional
procedures. In some instances the quality of the
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resulting composite materials is surprisingly improved
over anything previously known.
In accordance with this feature of the
invention, a process for preparing a cast composite
5 material having particles embedded in an aluminum-alloy
matrix of a preselected composition that does not readily
wet the particles comprises the steps of providing a
molten mixture of the particles and a wetting alloy having
a composition that readily wets the particles and has no
alloying elements present in an amount substantially in
excess of the preselected matrix composition; mixing
together the molten mixture under conditions such that the
wetting alloy is wetted to the particles; adding
additional alloying ingredients to the melt to adjust the
composition of the matrix to the preselected composition
and distributing the additional alloying ingredients
throughout the melt; and casting the resulting melt.
This technique rests upon the realization that
some matrix alloy compositions enhance wetting of
particular particle types, and other compositions impede
wetting. When a difficult-to-wet combination of particle
and matrix alloy composition is to be prepared, according
to the present invention the matrix alloy is evaluated for
the presence of either wettability enhancing elements,
combinations, or amounts, or wettability inhibiting
elements, combinations, or amounts.
If such enhancing or inhibiting compositions can
be identified for a composite system, a wetting alloy
composition is designed to take advantage of that
situation. The wetting alloy must contain not more than
the required amount of each element in the final matrix
alloy composition, but can contain less or none. Wetting
of the matrix to the particles is then achieved with the
wetting alloy. After wetting is accomplished, the
composition of the matrix is adjusted with further
alloying additions to reach the desired final matrix
composition.
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The present invention also provides a cast
composite material having a metallic alloy matrix
component whose composition is carefully selected to avoid
the formation of unwanted and deleterious phases during
preparation, remelt, and final casting. The amount of one
alloying ingredient is carefully controlled to prevent
degradation of properties during remelting, formation of
unwanted phases, and good castability. Other conventional
alloying ingredients can be varied as necessary to attain
other desirable properties of the final product. The
particulate need not be altered or specially selected in
order to attain good composite remelt properties.
The novel composite material comprises a mixture
of from about 5 to about 35 volume percent of nonmetallic
reinforcing particles and from about 95 to about 65 volume
percent of a matrix alloy, the matrix alloy being an
aluminum-based alloy containing from about 8.5 to about
12.6 weight percent silicon. Other conventional aluminum
alloying elements can be added to the matrix alloy as
needed, and do riot interfere with the beneficial effects
of the silicon. Such other alloying elements include, for
example, copper, nickel, magnesium, iron and manganese.
This composite material can be prepared by a
procedure which comprises the steps of preparing a molten
mixture of from about 5 to about 35 volume percent of
free-flowing nonmetallic reinforcing particles and from
about 95 to about 65 volume percent of a matrix alloy, the
matrix alloy being an aluminum-based alloy containing from '
about 8.5 to about 12.6 weight percent silicon; mixing the
molten mixture to wet the matrix alloy to the particles
and to distribute the particles throughout the volume of
the melt, the mixing to occur while minimizing the
introduction of gas into and retention of gas within the
molten mixture; and casting the molten mixture.
The composite material of the invention is
particularly useful in remelt applications. It can be
remelted to a conventional foundry remelt practice
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temperature of greater than 700'C, and typically 788°C or
more, and held for 24 hours, and then cast with good
results. In a preferred embodiment having from about 9.5
to about 11.0 weight percent silicon, the same good
casting results are attained with even further improved
microstructures in the cast final product.
The present invention provides an important
advance in the art of preparation of cast composite
materials. Composite materials can be prepared from
materials combinations that are otherwise not commercially
feasible, without adding special alloying ingredients or
gases that might adversely affect the final product,
without specially coating the particles, and without
raising the temperature to unacceptably high levels.
The present invention also provides an important
advance in the art of cast composite materials, by
providing a foundry remelt alloy that can be readily cast
by conventional remelt practices. Other features and
advantages of the invention will be apparent from the
following more detailed description of the preferred
embodiment, taken in conjunction with the accompanying
drawings, which illustrate, by way of example, the
principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic illustration of a
fluidity testing apparatus;
Figure 2 is a graph of fluidity test results as
a function of silicon content:
Figure 3 is a micrograph of a cast composite
material having an aluminum-based matrix containing 7
weight percent silicon;
Figure 4 is a micrograph of a cast composite
material having an aluminum-based matrix containing 10
weight percent silicon:
Figure 5 is a micrograph of a cast composite
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material having aluminum-based matrix containing 10 weight
percent silicon and additional alloying elements; and
Figure 6 is a micrograph of another cast
composite material having an aluminum-based matrix
containing l0 weight percent silicon and additional ' .
alloying elements.
BEST MODES FOR CARRYING OUT THE INVENTION
In accordance with the first embodiment of the
invention, a process for preparing a cast composite
material having particles embedded in a matrix of a
preselected composition that wets the particles only with
great difficulty comprises the steps of providing a molten
mixture of the particles and a wetting alloy having a
composition of the preselected matrix composition but with
a deficiency in a wettability inhibiting element, the
wetting alloy being readily wetted to the particles during
mixing; mixing together the molten mixture to wet the
wetting alloy to the particles under conditions that the
particles are distributed throughout the volume of the
melt and the particles and the metallic melt are sheared
past each other to promote wetting of the particles by the .
melt, the mixing to occur while minimizing the
introduction of any gas into, and while minimizing the
retention of any gas within, the mixture of particles and
molten metal, and at a temperature whereat the particles
do not substantially chemically degrade in the molten
metal in the time required to complete said step of
mixing: adding the wettability inhibiting elements to the
melt so that the matrix has the preselected composition
and casting the resulting melt at a casting temperature
sufficiently high that substantially no solid metal is
present.
For the purposes of describing the preferred
embodiments of the present invention, cast composite
materials can be classified into two groups, those with
chemically highly reactive particles and those with
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chemically nonreactive particles. The principal obstacle
with forming cast composite materials containing reactive
pari~icles is to prevent particle dissolution and unwanted
formation of intermetallic compounds, while achieving
wetting. The commercially most important reactive
particle is silicon carbide. The principal problem with
forming cast composite materials containing nonreactive
particles is achieving some degree of reactivity and
wetting. The commercially most important nonreactive
particle is aluminum oxide. In each case, sufficient
fluidity must be exhibited by the melt for casting.
According to one preferred embodiment of the
invention for dealing with reactive particles, a process
for preparing a cast composite material having particles
embedded in an aluminum-alloy matrix having more than
about 7 weight percent silicon comprises the steps of
providing a molten mixture of the particles, arid an
aluminum-based wetting alloy having no more than about 7
weight percent silicont mixing together the molten mixture
under conditions such that the aluminum wetting alloy is
Wetted to the particles making an addition of silicon
and other elements as needed to adjust the silicon content
of the melt to its desired final composition which has
more than about 7 weight percent silicon, and dissolving
and distributing the addition throughout the melt; and
casting the resulting melt.
A particularly useful cast composite material
has reactive silicon carbide particles embedded in an
aluminum-alloy matrix with about l0 weight percent
silicon. Castings of this alloy can be made only with
great difficulty using the approach of combining all of
the ingredients together and mixing. Although the alloy
can be mixed, the particulate matter enters the melt
slowly, and the melt becomes so viscous that it is
difficult to cast.
To prepare such an alloy by the preferred
approach, a wetting alloy of aluminum plus about 7 weight
WO 92/01821 ~ ~ ~ ~ j ~ ~ PCT/CA91/00242,~s,
percent silicon is prepared and mixed with silicon carbide
particles using the approach discussed in US patents
4,759,995 and 4,786,467. Wetting of the wetting alloy to
the particles is readily accomplished in about 1 hour of
5 mixing, and the viscosity is acceptable. An addition of
the remaining silicon and any other alloying additions
required to adjust the matrix to the required alloy
content is then made, those additions are dissolved and
distributed throughout the volume of the melt, and the
10 melt is cast.
The improvement achieved by the above process is
quite surprising. Normally, the fluidity of aluminum-
silicon alloys increases with increasing silicon content.
Achieving better wetting with a lower silicon content
alloy is not expected.
The cast composite material containing silicon
preferably comprises a mixture of from about 5 to about 35
volume percent of silicon carbide particles and from about
95 to about 65 volume percent of a cast matrix alloy, the
matrix alloy being an aluminum-based alloy containing from
about 9.5 to about 11.0 weight percent silicon. Most
preferably, the silicon content is about 10 percent by
weight of the matrix.
In accordance with a processing aspect of the
invention, a method for preparing a cast composite
material comprises the steps of preparing molten mixture
of from about 5_to about 35 volume percent of free-flowing
nonmetallic reinforcing particles and from about 95 to
about 65 volume percent of a matrix alloy, the matrix
alloy being an aluminum-based alloy containing from about
8.5 to about 12.6 weight percent silicon; mixing the
molten mixture to wet the matrix alloy to the particles
and to distribute the particles throughout the volume of
the melt, the mixing to occur while minimizing the
introduction of gas into and retention of gas within the
molten mixture; casting the molten mixture; remelting the
cast mixture at a temperature that reaches at least about
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700°C; and recasting the remelted mixture.
The particles are preferably silicon carbide,
because of their light weight and inexpensive commercial
availability in suitable forms and sizes. Other
nonmetallic reinforcing particles such as other carbides,
oxides, nitrides, solicides, and borides, may also be
used. The particles must be "free-flowing" in the sense
that they are not constrained against movement to reach a
uniform distribution throughout the composite material,
and are not attached or constrained by a substrate or each
other, as is the case for elongated fibers.
The particles constitute from about 5 to about
35 volume percent of the composite material. If less than
about 5 volume percent is present, the composite material
does not achieve properties superior to those of the
conventional, non-composite material. Potentially
incurring the problems of a composite material is
therefore not justified by superior properties. If more
than about 35 volume percent is present, the composite
material is so viscous that it cannot be cast. This upper
limit to the amount of particulate material can vary
somewhat with the shape and type of the particulate
material.
The remainder of the composite material, from
about 95 to about 65 percent by volume, is the matrix
alloy. The matrix alloy is an aluminum-based alloy
containing from about,8.5 to about 12.6 weight percent
silicon, balance aluminum and other alloying ingredients
selected to impart particular mechanical and physical
properties to the final solid composite material.
Preferably, the silicon content of the matrix is from
about 9.5 to about 11.0 weight percent.
The high silicon level has several important
functions. The silicon influences the formation of the
intermetallic compound aluminum carbide, A14C3, and in
particular suppresses its formation. When molten aluminum
is contacted to a carbide such as silicon carbide, the
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formation of aluminum carbide is thermodynamically
favoured by a negative free energy of formation. The
aluminum carbide is hygroscopic and will absorb moisture.
The result is porosity in the final cast product.
The kinetics of aluminum carbide formation have
been discovered to be such that melting and mixing of a
cast composite material haivng a conventional aluminum-7
(or less) weight percent silicon alloy matrix at a
controlled temperature of, for example, about 718°C for a
relatively. short period of time of about one hour, permits
only a small and acceptable amount of the aluminum carbide
to form. Thus, an acceptable cast composite material can
be prepared by a carefully controlled melting and mixing
procedure.
If, however, the cast composite material having
such a conventional matrix alloy is thereafter remelted in
a foundry practice of 788°C for 24 hours, the aluminum
carbide formation continues at an accelerated rate.
Aluminum carbide intermetallic compound grows from silicon
carbide particles as outwardly projecting needles, which
can break off to form particles in the melt.
one potential solution would be to place tight
operational controls on foundries. Such controls would
not be accepted by some foundries, and in all cases would
inhibit the introduction and use of the cast composite
materials in applications where they would otherwise be
useful.
It has now been discovered that carefully
selected larger amounts of silicon in the matrix alloy
3o suppress aluminum carbide formation even during extended
holding periods at very high temperatures sufficiently to
permit casting of the composite material. The remelted
composite materials having the higher silicon content
within particular ranges simultaneously achieve
exceptional castability and fluidity, without occurrence
of primary phases such as pure silicon. Amounts of
silicon greater than about 8.5 percent result in
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significantly reduced aluminum carbide formation and
improved remelt fluidity, and amounts of silicon greater
than about 9.5 percent eliminate aluminum carbide
formation entirely and result in the greatest remelt
fluidity for the selected particulate content and remelt
temperature and holding conditions.
Castability and fluidity of remelt alloys are of
direct interest to foundrymen, because improvements on
these characteristics have direct consequences in the
ability to cast intricate parts in a reproducible manner.
The composite materials of the invention have been
comparatively tested to measure their fluidity under
typical foundry remelt conditions. Figure 1 illustrates a
device 10 for measuring fluidity. Molten composite
material 12 is held in a heated crucible 14, with the
temperature measured by a thermocouple 16. One end of a
hollow pyrex glass tube 18, here about 5 millimetres
inside diameter, is inserted vertically into the melt 12.
A vacuum of about 63.5 cm of mercury is applied to the
other end of the tube 18 by a vacuum pump 20. Molten
composite material is drawn up the inside of the tube 18
until the metallic portion of the composite material
freezes. The tube 18 is removed from the melt, and the
distance of travel of the composite material up the tube
prior to freezing is measured.
A number of specimens of composite material were
evaluated using the apparatus of Figure 1. Two kilogram
heats were prepared with 20 volume percent silicon carbide
particles in an aluminum-alloy matrix containing varying
amounts of silicon as an alloying ingredient. Melts were
prepared with matrix silicon contents of 7, 8, 9, 10, 11,
12, and 13 weight percent silicon. The melts were
prepared in a mixer like that disclosed in US Patents
4,759,995 and 4,786,467. The melts were cast into molds
and solidified. The castings were remelted in crucibles
under air at a temperature of 788°C and held for 24 hours.
The temperature of the melt was reduced to 690°C +/6°C,
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and tested for fluidity using the apparatus of Figure 1.
Figure 2 presents the height rise for the
composite materials in inches above the melt level. The
greater the height rise, the greater the fluidity. The
fluidity increases from a low value at 7 weight percent
silicon, to a level at 10 weight percent silicon that
remains nearly constant with further increases in silicon
content to 13 weight percent. Specimens were~cut from the
tubes and examined metallographically. The amount of
aluminum carbide in the 7 weight percent silicon material
was large. A much smaller amount was visible in the
sample containing 8 weight percent silicon. There was no
aluminum carbide visible in the alloys containing 9 weight
percent or more of silicon.
From these data, it was concluded that the
minimum silicon content for suppression of aluminum
carbide formation, in conditions of extended exposure,
together with attainment of acceptable fluidity was about
8.5 percent. This value is marginal, as the aluminum
carbide is nearly completely absent, but the fluidity has
not reached its greatest value. A preferred minimum
silicon content was therefore selected to be about 9.5
weight percent, a level at which no aluminum carbide is
present and the fluidity has nearly reached its highest
level.
Although the fluidity appears to increase with
ever-increasing silicon content within this general range,
there is a maximum limit to the silicon content of the
matrix alloy. The maximum silicon content of the matrix
alloy according to the present invention is about 12.6
weight percent. This is the value of the aluminum-silicon
eutectic composition. For greater amounts of silicon,
there are two undesirable results. First, the liquidus
temperature rises so that the superheat for a selected
remelt temperature is reduced. Second, primary silicon
particles are precipitated in the matrix upon
solidification. The silicon particles reduce the
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ductility of the matrix. A preferred maximum silicon
content is slightly lower, at 11.0 percent.
Metallographic studies reveal that, in the range 11.0-12.6
weight percent silicon, there can be some precipitation of
5 primary silicon in the final structure, regardless of the
expected equilibrium phase diagram. Also, there is
observed some shrinkage of the matrix alloy during
solidification.
The minimum silicon content of the matrix of the
10 present composite material is therefore about 8.5 weight
percent, and the preferred minimum is about 9.5 weight
percent. The maximum silicon content of the matrix of the
present composite material is about 12.6 percent, and the
preferred maximum is about 11.0 percent. These values are
15 selected because of, and in conjunction with, the presence
of the free-flowing reinforcement particles in the
composite material and the potential for chemical
interaction between the matrix alloy and the particles as
has been discussed herein. The choice of silicon content
in non-composite alloys, and alloys that are not to be
cast, is therefore not pertinent to the selection of
silicon levels for the matrices of cast composite
materials.
Most preferably, the silicon content is about 10
weight percent of the matrix, to provide a margin of error
between the preferred limits of 9.5 and 11.0 weight
percent, and to achieve close to the maximum fluidity
possible in this general range.
The silicon in the matrix appears to suppress
the formation of aluminum carbide by altering the
thermodynamic equilibria of the system. To a good
approximation, these equilibria are not affected by the
presence of metallic alloying elements commonly provided
in aluminum alloys to achieve'specific properties such as
strength, toughness, corrosion resistance, and the like in
the final cast product. Thus, superior casting
' performance can be achieved by maintaining the proper
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silicon content, and other alloying elements can be added
to achieve specific properties in the final product. Such
alloying elements include, for example, copper, nickel,
magnesium, iron, and manganese.
The following examples are presented in addition
to those discussed previously to illustrate aspects of the
invention, but are not intended to limit the invention in
any aspect.
Example 1
To prepare a cast composite material of 20
volume percent of silicon carbide particles in a matrix
alloy of 1o weight percent silicon, 1 weight percent
magnesium, balance aluminum, a wetting alloy of 7 weight
percent silicon, 1 weight percent magnesium, balance
aluminum was prepared. The appropriate amounts of silicon
carbide and the wetting alloy were mixed according to the
procedures disclosed in US Patent 4,759,995 and 4,786,467.
Mare specifically, the wetting alloy was melted at 671°C,
and the appropriate amount of silicon carbide particles
was added to the surface of the melt under vacuum over a
period of 35 minutes, while the melt was mixed with an
impeller. After all the silicon carbide was added, mixing
was continued for another 25 minutes under vacuum. This
procedure produced full wetting of the aluminum-7 weight
percent silicon, 1 weight percent magnesium alloy to the
particles. The mixing was stopped, the chamber vented to
air, and a sufficient amount of silicon was added to
adjust the matrix composition to 10 weight percent silicon
and 1 weight percent magnesium. The chamber was sealed
and a vacuum drawn, and mixing was continued for another
15 minutes to dissolve the alloying additions and
distribute them throughout the melt. The composite
material was cast into pigs. The pigs were provided to a
foundry for remelt, and the remelted composite material
was observed to have excellent fluidity for casting into
narrow mold passages.
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Example 2
Example 1 was repeated, except that stepped
alloying was not used. That is, the conventional practice
was followed wherein the final matrix alloy of 10 weight
percent silicon, 1 weight percent magnesium, balance
aluminum was prepared. Silicon carbide particulate in the
appropriate amount was added to the melt, and the melt and
particles mixed together for the same amount of time as in
Example 1. The resulting melt was very viscous and could
not be cast into small-diameter passages in molds.
Example 3
Example 1 is repeated, except that the composite
was made to contain 10 volume 10 volume percent of silicon
carbide particles. The final melt was fluid and could be
cast into molds with both large and small passageways.
Example 4
Example 2 was repeated, Except that the
composite was made to contain 10 volume percent of silicon
carbide particles.
Example 5
The mechanical properties of cast specimens of
the stepped-alloy addition cast composite material of
Example 3 and the conventionally prepared cast composite
material of Example 4 were tested. The following table
reports the results in ksi, thousands of pounds per square
inch.
Table 1
Yield Strength Tensile Strength
Process (ksi) [M_pal ~(ksi) (MpaZ
Prior 42 290 47 324
Stepped 49 338 56 386
The composite materials produced with the
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stepped addition of alloying ingredients exhibit
significantly improved post-casting properties as compared
with those produced by the conventional approach.
The second class of particles is nonreactive
particles such as aluminum oxide particles. According to
another preferred embodiment of the invention for dealing
with nonreactive particles, a process for preparing a cast
composite material having particles embedded in an
aluminum-alloy matrix comprises the steps of providing a
molten mixture of the particles, and an aluminum-based
wetting alloy having about 1 weight percent silicon and
about 0.6 weight percent magnesium; mixing together the
molten mixture under conditions such that aluminum wetting
alloy is wetted to the particles; making an addition of
elements as needed to adjust the alloy content of the melt
to its desired final composition, and dissolving and
distributing the addition throughout the melt; and casting
the resulting melt.
To prepare a cast composite material containing
2o nonreactive particles, the particles are first mixed with
a wetting alloy which is known to wet the particles and
also has sufficient fluidity for mixing. For example, it
is known that aluminum alloys containing about 1 weight
percent silicon and 0.6 weight percent magnesium readily
wet aluminum oxide particles during mixing. Many aluminum
matrix alloys of interest contain at least 1 weight
percent silicon and at least 0.6 weight percent magnesium,
so initial wetting can be accomplished with an aluminum
alloy of that composition. After wetting, the composition
of the matrix is adjusted with further additions of
alloying elements. The initial wetting is accomplished
using the wetting alloy and the procedure of US Patents
4,759,995 and 4,786,467.
The following example is intended to illustrate'
aspects of the invention as related to wetting of
nonreactive particles.
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Example 6
This example illustrates aspects of the
invention as related to wetting of nonreactive particles.
A cast composite material was prepared of 10
volume percent aluminum oxide particles in an aluminum
alloy containing 1o weight percent silicon, 0.6 weight
percent magnesium, 0.7 weight percent iron, and 0.4 weight
percent manganese. This cast composite material is
exceedingly difficult to prepare by conventional methods,
because the particles wet only slowly. The molten
composite material is so viscous that it is nearly
impossible to cast into a mold. To prepare the cast
composite material using the.approach of the invention, a
matrix alloy of 1 weight percent silicon, 0.6 weight
percent magnesium, 0.7 weight percent iron, and 0.4 weight
percent manganese, balance aluminum, was melted in a
crucible at 674°C under vacuum, and the appropriate amount
of aluminum oxide particles added over a period of 20
minutes. The aluminum-based matrix alloy contains 1
weight percent silicon and 0.6 weight percent magnesium, a
composition known to achieve wetting to aluminum oxide
particles. After all of the particulate matter was added,
the melt was mixed under vacuum for another 20 minutes,
following the approach of US Patents 4,759,995 and
4,786,467. This combination of matrix alloy composition
and mixing conditions produced good wetting of the matrix
alloy to the particles. Mixing was stopped, the chamber
was vented to air, and sufficient silicon added to adjust
the matrix content to l0 weight percent silicon (with the
amounts of the other alloying additions essentially ,
unchanged). The vacuum was reapplied, and mixing
continued for another 15 minutes. The composite was then
cast into foundry pigs. The cast composite material
exhibited excellent fluidity, and was suitable for
preparation of castings having narrow passageways.
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Example 7
A cast composite material was prepared from 20
volume percent silicon carbide particles and 80 volume
percent of an alloy meeting a specification of 7 weight
5 percent silicon, 0.3-0.45 weight percent magnesium,
balance aluminum. This matrix alloy is not within the
scope of the invention, and is presented for comparative
purposes. The cast composite material was prepared by the
prbcedures discussed previously. The cast composite
10 material was remelted at a temperature of over 760°C. A
sample was taken of the remelted composite material, and
its microstructure is illustrated in Figure 3. Aluminum
carbide intermetallic compound is found extensively
throughout the microstructure as a dark-appearing phase.
15 In Figure 3, circles have been drawn around some of the
aluminum carbide particles and regions for illustrative
purposes.
Example 8.
Example 7 was repeated, except using a matrix
20 alloy that meets a specification of l0 Weight percent
silicon, 0.8-1.0 weight percent magnesium, balance
aluminum. Except for the higher silicon content within
the preferred range of the invention and a minor
difference in magnesium content, this matrix alloy has the
same composition as that of Example 7. The microstructure
of this alloy is shown in Figure 4. There is no aluminum
carbide visible in the microstructure.
Example 9
Example 7 was repeated, except using a matrix
alloy that meets a specification of 10 weight percent
silicon, 0.6-1.0 weight percent iron, 3.0-3.5 weight
percent copper, 0.2-0.6 weight percent manganese, 0.3-0.6
weight percent magnesium, 1.0-1.5 weight percent nickel,
balance aluminum. This matrix alloy is within the scope
of the invention, having 10 weight percent silicon. It is
:" WO 92/01821 ~ ~ ~ ~ ~ ~ ~ PCTlCA91/00242
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a more complex alloy in that it also contains iron,
copper, manganese, and nickel. This cast composite
material is suitable as a die casting alloy. Figure 5
illustrates the microstructure of this cast and then
remsalted alloy. There is no aluminum carbide visible in
the microstructure.
example l0
Example 7 was repeated, except using a matrix
alloy that meets a specification of 10 weight percent
silicon, 2.8-3.2 weight percent copper, 0.8-1.2 weight
percent magnesium, 1.0-1.5 weight percent nickel, balance
aluminum. This matrix alloy contains copper and nickel in
addition to the silicon within the range of the invention
and magnesium. This composite material is suitable as a
high temperature sans and permanent mold casting alloy.
Figure 6 illustrates the microstructure of this cast and
remelted alloy. There is no aluminum carbide visible in
the microstructure.
Example 8 demonstrates that the addition of
silicon to within the range of the invention suppresses '
aluminum carbide formation, as compared with the alloy of
Example 7. Examples 9 and 10 demonstrate that additions
of other alloying elements do not interfere with the
suppression of aluminum carbide formation by the high '.
silicon content. The results of Figure 2 demonstrate that
the 10 weight percent silicon alloy has excellent fluidity
and it was observed to have good castability.
In the preceeding disclosure and Examples 1-6,
procedures for achieving acceptable wetted cast composite
materials of both reactive and nonreactive particles have
been demonstrated. In each case, composite materials that
are difficult to make by the conventional approach are '
prepared using the conventional mixing procedure and a
matrix alloy that is known to be operable to first achieve
wetting of the matrix alloy to the particles, and
thereafter adjusting the composition of the matrix to the
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preselected alloying level. In each case, no coatings on
the particles, special alloying elements, special
atmo:apheric additions, or overly elevated temperatures are
required. The only changes to the processing procedures
are to add some alloying elements after wetting is
complete, and to extend the mixing for a short time to
incorporate these later additions into the melt.
The composite material of the invention provides
an important commercial advance in the art of cast
composite materials. The material can be mixed and cast
by a primary composite material supplier, and shipped as a
cast ingot to a foundry for remelting and casting into
precise shapes as required. The composition of the matrix
alloy is selected so that the remelting practice at the
foundry may be similar to conventional remelt practices,
which would not be possible for a cast composite material
of conventional alloying content. Excellent fluidity is
retained by the molten composite material even when it is
held in a remelt crucible for extended periods of time and
at temperatures previously thought to be unacceptably high
because they produce undesirable reaction products.
Although particular embodiments of the invention
have been described in detail for purposes of
illustration, various modifications may be made without
departing from the spirit and scope of the invention.
Accordingly, the invention is not to be limited except as
by the appended claims.
