U.S. patent application number 14/426278 was filed with the patent office on 2015-08-06 for method for preparing aluminum matrix composite using no pressure infiltration.
The applicant listed for this patent is KOREA INSTITUTE OF MACHINERY & MATERIALS. Invention is credited to Young Hee Cho, Jong Jin Kim, Su Hyeon Kim, Jung Moo Lee, Sang Kwan Lee, In Hyuck Song, Jing Jing Zhang.
Application Number | 20150218707 14/426278 |
Document ID | / |
Family ID | 50237371 |
Filed Date | 2015-08-06 |
United States Patent
Application |
20150218707 |
Kind Code |
A1 |
Lee; Jung Moo ; et
al. |
August 6, 2015 |
METHOD FOR PREPARING ALUMINUM MATRIX COMPOSITE USING NO PRESSURE
INFILTRATION
Abstract
The present invention provides a method for preparing an
aluminum matrix composite by infiltrating aluminum into a preform
within a short period of time without a pressurization
configuration using a special device as compared to the existing
pressure infiltration method. According to one aspect of the
present invention, provided is a method for preparing an aluminum
matrix composite using pressureless infiltration, the method
including: preparing a preform formed of a mixture of raw powders
capable of forming ceramic through a combustion synthesis reaction;
immersing the preform in an aluminum melt, in which a part of the
preform is exposed to an external environment without being
immersed in the aluminum melt; and infiltrating molten aluminum
into the preform while causing a combustion synthesis reaction in
the preform.
Inventors: |
Lee; Jung Moo; (Changwon-si,
KR) ; Kim; Su Hyeon; (Changwon-si, KR) ; Cho;
Young Hee; (Changwon-si, KR) ; Lee; Sang Kwan;
(Changwon-si, KR) ; Song; In Hyuck; (Changwon-si,
KR) ; Kim; Jong Jin; (Changwon-si, KR) ;
Zhang; Jing Jing; (Changwon-si, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KOREA INSTITUTE OF MACHINERY & MATERIALS |
Daejeon |
|
KR |
|
|
Family ID: |
50237371 |
Appl. No.: |
14/426278 |
Filed: |
August 21, 2013 |
PCT Filed: |
August 21, 2013 |
PCT NO: |
PCT/KR2013/007495 |
371 Date: |
March 5, 2015 |
Current U.S.
Class: |
427/431 |
Current CPC
Class: |
B22D 23/04 20130101;
C22C 2001/1057 20130101; C23C 30/005 20130101; B22D 19/14 20130101;
C23C 2/12 20130101; C23C 26/02 20130101; C23C 22/73 20130101; C23C
22/70 20130101; B22D 21/04 20130101; C22C 1/1042 20130101; C22C
32/00 20130101 |
International
Class: |
C23C 30/00 20060101
C23C030/00; C23C 26/02 20060101 C23C026/02; C23C 22/73 20060101
C23C022/73; C23C 2/12 20060101 C23C002/12; C23C 22/70 20060101
C23C022/70 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 6, 2012 |
KR |
10-2012-0099026 |
Claims
1. A method for preparing an aluminum matrix composite using
pressureless infiltration, the method comprising: preparing a
preform formed of a mixture of raw powders capable of forming
ceramic through a combustion synthesis reaction; immersing the
preform in an aluminum melt, in which a part of the preform is
exposed to an external environment without being immersed in the
aluminum molten metal; and infiltrating molten aluminum into the
preform while causing a combustion synthesis reaction in the
preform.
2. The method of claim 1, wherein the infiltrating of the molten
aluminum further comprises allowing the preform to be ignited.
3. The method of claim 1, further comprising: immersing the entire
preform in the aluminum melt after the infiltrating of the molten
aluminum.
4. The method of claim 1, wherein the mixture of raw powders is any
one of a mixture of Ti and B.sub.4C powders, a mixture of Al,
B.sub.2O.sub.3, and C, a mixture of Al, B.sub.2O.sub.3, and
TiO.sub.2, a mixture of Ti and C, a mixture of Al, TiO.sub.2, and
C, a mixture of Al, Ti, and B.sub.2O.sub.3, and a mixture of Al,
TiO.sub.2, and B.sub.4C.
5. The method of claim 1, wherein the mixture of raw powders
further comprises an aluminum powder in an excessive amount in
addition to a stoichiometric amount required for a combustion
synthesis reaction among the raw powders constituting the
mixture.
6. The method of claim 5, wherein the mixture of raw powders
further comprises an activating powder capable of causing an
exothermic reaction with aluminum.
7. The method of claim 6, wherein the activating powder comprises
one or more of a copper oxide, a manganese oxide, an iron oxide,
and a nickel oxide.
8. The method of claim 1, wherein in order to induce a partial
combustion synthesis reaction, a non-metallic raw powder among the
raw powders constituting the mixture is added in the mixture of raw
powders in an excessive amount equal to or more than a
stoichiometric amount required for the combustion synthesis
reaction.
9. The method of claim 1, wherein the mixture of raw powders
further comprises a stable compound powder which does not
participate in the combustion synthesis reaction.
10. The method of claim 9, wherein the stable compound comprises
one or more of B.sub.4C, SiC, TiC, and Al.sub.2O.sub.3.
11. The method of claim 1, wherein a temperature of the aluminum
melt is in a range of 750.degree. C. to 950.degree. C.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for preparing an
aluminum matrix composite by distributing a non-metallic material,
such as ceramic, as a reinforcing phase (or a reinforcing material)
on an aluminum matrix to enhance mechanical properties, and more
particularly, to a method for preparing an aluminum matrix
composite using pressureless infiltration.
BACKGROUND ART
[0002] An aluminum matrix composite is a material in which a
non-metallic material such as ceramic is distributed as a
reinforcing phase in a matrix formed of pure aluminum or an
aluminum alloy, is light-weight and has high strength and rigidity
and excellent resistance to wear and high-temperature properties,
and thus has potential to be used as a structural material for
transportation, and machinery and electric devices and so on.
[0003] Mechanical properties of a metal matrix composite are
greatly affected by the type, size, shape, and volume fraction of a
reinforcement, characteristics of interface between matrix and
reinforcement. When a composite is prepared by ex-situ method
(introducing ceramic reinforcements into a molten metal), it was
not easy to introduce the ceramic reinforcements into a molten
metal due to low wettability between the ceramic reinforcements and
the matrix molten metal.
[0004] In order to solve these problems, a pressure infiltration
process has been developed, and the process is a method in which a
preform is first prepared using a reinforcing material powder,
aluminum (described as only aluminum for convenience, and
hereinafter, aluminum will refer to aluminum and an aluminum alloy)
molten metal is injected thereinto, and then the preform is filled
with the aluminum molten metal by high pressure (using a mechanical
or gas pressure, and the like). The method is advantageous in that
the composite may be prepared within a short time, but has a
problem in that a large complex apparatus is required for applying
high pressure.
[0005] In order to improve demerits of the pressure infiltration
process, pressureless infiltration processes have been developed,
and among the processes, a direct melt oxidation (DIMOX) process
developed by Lanxide Corp., is a representative process. The
process is a method of preparing a composite of metal/ceramic by
inducing an oxidation reaction at an interface between a molten
metal and a preform to simultaneously produce and grow an oxidation
product (Urquhart, Mat. Sci. Eng. A144, 1991, 75-82). However, the
process is disadvantageous in that the molten metal temperature is
as high as 1,200.degree. C. and the infiltration time is as long as
24 hours.
DISCLOSURE
Technical Problem
[0006] The present invention has been made in an effort to solve
various problems including the aforementioned problems and provides
a method for preparing an aluminum matrix composite by infiltrating
aluminum into a preform within a short period of time without a
pressurization configuration using a special device as compared to
the existing pressure infiltration method. However, this problem is
illustrative only, but the scope of the present invention is not
limited thereby.
Technical Solution
[0007] According to one aspect of the present invention, provided
is a method for preparing an aluminum matrix composite using
pressureless infiltration, the method including: preparing a
preform of a raw powders mixture capable of forming ceramic
reinforcements through a combustion synthesis reaction; immersing
the preform in an aluminum molten metal, in which a part of the
preform is exposed to an external environment without being
infiltrated into the aluminum molten metal; and infiltrating molten
aluminum into the preform while causing a combustion synthesis
reaction in the preform.
[0008] The infiltrating of the molten aluminum may further include
allowing the preform to be ignited.
[0009] Further, the method may further include immersing the entire
preform in the aluminum molten metal after the infiltrating of the
molten aluminum.
[0010] The raw powder mixtures may be any one of a mixture of Ti
and B.sub.4C powders, a mixture of Al, B.sub.2O.sub.3, and C, a
mixture of Al, B.sub.2O.sub.3, and TiO.sub.2, a mixture of Ti and
C, a mixture of Al, TiO.sub.2, and C, a mixture of Al, Ti, and
B.sub.2O.sub.3, and a mixture of Al, TiO.sub.2, and B.sub.4C.
[0011] The raw powder mixtures may further include an aluminum
powder in an excessive amount in addition to a stoichiometric
amount required for a combustion synthesis reaction among the raw
powders constituting the mixture. In this case, the mixture of raw
powders may further include activating powders which can promote
the exothermic reactions, and the activating powders may include
one or more of a copper oxide, a manganese oxide, an iron oxide,
and a nickel oxide.
[0012] In order to induce a partial combustion synthesis reaction,
a non-metallic raw powder among the raw powders constituting the
mixture may be added in the mixture of raw powders in an excessive
amount equal to or more than a stoichiometric amount required for
the combustion synthesis reaction.
[0013] Alternatively, the mixture of raw powders may further
include a compound powder which does not participate in the
combustion synthesis reaction, and the compound may include one or
more of B.sub.4C, SiC, TiC, and Al.sub.2O.sub.3.
[0014] Meanwhile, the temperature of the aluminum melt may be in a
range of 750.degree. C. to 950.degree. C.
Advantageous Effects
[0015] According to the present invention configured as described
above, an aluminum matrix composite may be prepared by using a
combustion synthesis reaction of a preform to be infiltrated by
aluminum melt into the preform within a short period of time
without a special device as compared to the existing pressure
infiltration method. Accordingly, the present invention is
economically efficient in terms of device and cost as compared to
the existing pressure infiltration process. In addition, since the
process is completed at a low aluminum melt temperature in the
atmosphere within a short time of a several minutes, the process is
advantageous in that the process time may be significantly reduced
as compared to the existing pressureless infiltration process which
requires a long period of time, and the process temperature may
also be reduced. A metal matrix composite prepared by the process
is light-weight and excellent in mechanical properties such as
elastic modulus and hardness, and excellent in thermal stability,
and thus, may be utilized in parts which require high hardness,
high stiffness and thermal stability. The effects of the present
invention are not limited to those mentioned above, and other
effects which are not mentioned may be clearly understood by a
person with ordinary skill in the art to which the present
invention pertains from the following description.
DESCRIPTION OF DRAWINGS
[0016] FIG. 1 is a schematic view of a method for preparing an
aluminum matrix composite according to the present invention.
[0017] FIG. 2 is a schematic view illustrating an infiltration
process in a preform.
[0018] FIG. 3 is a graph illustrating a change in temperature of
the preform in an aluminum molten metal.
[0019] FIG. 4 is a graph in which a relationship between the
infiltration length-infiltration time is theoretically
calculated.
[0020] FIGS. 5 to 8 are results of microstructures for aluminum
matrix composites prepared according to experimental examples of
the present invention.
[0021] FIG. 9 is a perspective view of a ring-type holder for
fixing the preform.
[0022] FIG. 10 illustrates immersing the preform in the aluminum
molten metal using the ring-type holder.
BEST MODE
[0023] Hereinafter, embodiments of the present invention will be
described in detail as follows with reference to the accompanying
drawings. However, the present invention is not limited to the
embodiments to be disclosed below, but may be implemented in
various forms different from each other, and the following
embodiments are provided to make the disclosure of the present
invention complete and to completely inform a person with ordinary
skill in the art of the scope of the present invention. Further, in
the drawings, sizes of constituent elements may be exaggerated or
reduced for convenience of explanation.
[0024] FIG. 1 illustrates a schematic view of a method for
preparing an aluminum matrix composite according to the present
invention. Referring to FIG. 1, provided is a preform 100 prepared
by using a mixture of raw powders capable of causing a combustion
synthesis reaction (or also referred to as a self-propagating high
temperature synthesis). As a product of the combustion synthesis
reaction, a hard ceramic material such as carbide, oxide, and
boride may be formed. When the combustion synthesis reaction is
used to produce a hard ceramic material as a reinforcing phase in
an aluminum matrix, the interfacial bonding strength of the
matrix/reinforcing phase is excellent because the ceramic material
is thermodynamically stable and the interface of the reinforcing
phase is clean. For this reason, mechanical properties of a metal
matrix composite prepared by using a combustion synthesis reaction
are better than those of a composite prepared by the ex-situ
method.
[0025] The preform 100 is prepared using a mixture of raw powders
capable of forming a hard ceramic reinforcement through a
combustion synthesis reaction. For example, the preform 100 may be
prepared in a form of pellet by cold pressing after blending or
ball-milling using raw powders.
[0026] The preform 100 is put into a crucible 120 in which an
aluminum (or aluminum alloy) melt 110 is placed, and is immersed in
the aluminum melt 110. In the present specification and the claims,
the aluminum melt 110 all refers to a melt of pure aluminum, or a
melt of an aluminum alloy to which additional elements (Mg, Si, Cu,
Mn, Cr, Zn, Ni, Ti, Fe, Sn, Li, and the like) of a typical aluminum
alloy are added.
[0027] Here, part of the preform is immersed so as to be exposed to
an external environment, for example, the atmosphere without being
immersed in the aluminum molten metal 110. Representatively, as in
FIG. 1, the upper surface of the preform 100 may not be immersed in
the aluminum melt 110 by exposing the upper surface of the preform
100 above the surface of the melt.
[0028] The preform 100 injected into the aluminum melt 110 receives
heat from the aluminum melt 110, and a combustion synthesis
reaction occurs within several ten seconds and up to several
minutes while the preform 100 is heated. Simultaneous with the
combustion synthesis reaction, molten aluminum is infiltrated into
the preform 100 due to a pressure difference between the inside and
the outside of the preform 100. When the preform 100 is completely
infiltrated, an aluminum matrix composite is prepared by taking the
preform 100 out of the aluminum melt 110, and solidifying the
preform 100.
[0029] According to the present embodiment, the preform 100 having
cavities therein forms a hard ceramic, such as carbide, oxide, and
boride by a combustion synthesis reaction, and as molten aluminum
is infiltrated through the cavities inside the preform 100, an
aluminum matrix composite in which a hard ceramic is distributed in
an aluminum matrix is prepared.
[0030] In the present embodiment, an important factor for inducing
infiltration of the molten metal into the preform 100 is the
pressure difference, and a basic principle of generating the
difference is due to the following two factors.
[0031] (1) Generation of Pressure Difference Inside and Outside of
Preform 100
[0032] A preform formed by a raw powder mixture usually has a
density which is 50 to 80% of a theoretical density. That is, 20 to
50% of the preform is occupied inevitably by the air and gas.
Further, moisture or gas, and the like are adsorbed on the surface
of the raw powders. When the preform is in contact with the molten
metal, or combustion synthesis reaction occurs inside of the
preform, the internal temperature of the preform is increased, and
accordingly, the air, moisture, gas, and the like present inside
the preform are thermally activated. In conditions where the
activated air, moisture, adsorbed gas and the like are easily
removed from the preform, the inside of the preform may be
temporarily in a lower pressure condition which is close to the
vacuum.
[0033] Furthermore, when the combustion synthesis reaction is
occurred by reactions of the powder mixtures composing the preform,
more cavities can be formed inside the preform due to volume
contraction. In general, the volume of reacted powder is smaller
than that of the raw powders. Due to this reason, an additional
empty space is created inside the preform. Accordingly, a pressure
difference between the inside and the outside of the preform is
generated by the aforementioned two factors, and the aluminum melt
is spontaneously infiltrated into the empty space.
[0034] In the related art, a pressure difference was generated
artificially by using a vacuum generation device. In comparison
with the related art, in the present invention, it is possible to
make inside of the preform nearly vacuum state just simple contact
with melt or reaction synthesis heat.
[0035] (2) Action of Capillary Pressure
[0036] When a rigid body having cavity therein is brought into
contact with a liquid, the liquid is sucked into the inside of the
rigid body by a capillary pressure, and accordingly, the rigid body
may be infiltrated into the liquid. In this case, the acting
pressure may be expressed by the following Equation (1).
P c = 2 r 1 v cos .theta. r c [ Equation 1 ] ##EQU00001##
[0037] Here, P.sub.c is a capillary pressure, .gamma..sub.1v is a
surface tension of the liquid, .theta. is a contact angle between
the liquid and the solid, and r.sub.c is a radius of the capillary.
FIG. 2 illustrates a schematic view in which the molten metal is
infiltrated by capillary pressure. As shown in Equation 1, when the
contact angle .theta. is larger than 90.degree., P.sub.c has a
negative value, and the liquid cannot be infiltrated into the
inside the rigid body by capillary pressure. That is, the molten
metal cannot be infiltrated into the preform spontaneously, and
additional external pressure is needed to infiltrate the molten
metal into the preform. On the contrary, when the contact angle
.theta. is smaller than 90.degree., P.sub.c has a positive value,
and the liquid can be infiltrated spontaneously into the rigid body
by to the capillary pressure. That is, the molten metal can be
spontaneously infiltrated into the preform.
[0038] In general, the contact angle between molten aluminum and
ceramic particle has a value larger than 90.degree.. However, the
contact angle is not a constant value, and is a value which varies
according to time and temperature. For example, when molten
aluminum is in contact with B.sub.4C, the contact angle is
100.degree. in the case where the contact is maintained at
900.degree. for 1 second, but the contact angle is decreased to
90.degree. in the case where the contact is maintained at the same
temperature for 1 hour, and the contact angle is further decreased
to 60.degree. or less in the case where the contact is maintained
at 1,200.degree. C. for 1 second (Q. Lin, Scripta Mat., 60, 2009,
960-963). That is, when the temperature of the liquid (or the
molten metal) is increased, the contact angle may be decreased to
90.degree. or less, and the molten metal can be spontaneously
infiltrated into the preform due to the decrease in contact
angle.
[0039] The present embodiment may easily produce nearly vacuum
state and capillary pressure using a combustion synthesis reaction,
and may prepare an aluminum matrix composite by spontaneously
infiltrating the aluminum molten metal into the preform using the
pressure difference.
[0040] Referring to FIG. 1 as described above, part of the preform
100 in the aluminum melt 110 is exposed to the external environment
without being immersed in the aluminum melt 110, and the other
parts thereof need to be immersed in the aluminum melt 110 to be
directly in contact with the aluminum melt 110. This can help the
exit of the air, adsorbed gas present inside of the preform 100 to
outside easily, and molten aluminum is more easily infiltrated into
the preform 100 through a surface contacted with the aluminum melt
110. The infiltration step may be performed at a pressure lower
than the atmosphere or under a vacuum atmosphere as well as in the
case where the external environment is the atmosphere.
[0041] In the step of infiltrating molten aluminum into the
preform, the preform may be ignited while the combustion synthesis
reaction rapidly proceeds. When the ignition occurs, molten
aluminum is rapidly infiltrated by a combustion synthesis
reaction.
[0042] The mixture of raw powders which compose the preform is not
limited as long as the mixture itself enables the combustion
synthesis reaction, and a product produced in an aluminum alloy
matrix after the combustion synthesis reaction includes a product
formed of at least one combination of hard ceramics, such as
carbide, oxide, and boride. Equations (2) to (8) show examples of
the combustion synthesis reaction which may be used in the present
embodiment.
3Ti+B.sub.4C.fwdarw.2TiB.sub.2+TiC (Equation 2)
4Al+2B.sub.2O.sub.3+C.fwdarw.B.sub.4C+2Al.sub.2O.sub.3 (Equation
3)
10Al+3TiO.sub.2+3B.sub.2O.sub.3.fwdarw.2TiB.sub.2+5Al.sub.2O.sub.3
(Equation 4)
Ti+C.fwdarw.TiC (Equation 5)
4Al+3TiO.sub.2+3C.fwdarw.3TiC+2Al.sub.2O.sub.3 (Equation 6)
2Al+Ti+B.sub.2O.sub.3.fwdarw.TiB.sub.2+Al.sub.2O.sub.3 (Equation
7)
4Al+3TiO.sub.2+B.sub.4C.fwdarw.2TiB.sub.2+TiC+2Al.sub.2O.sub.3
(Equation 8)
The left side of the reaction equations of (Equation 2) to
(Equation 8) indicates raw materials constituting the preform, and
the right side of the reaction equations corresponds to the
reinforcing phase of a composite as a product produced by a
combustion synthesis reaction. Table 1 is a calculated result for
volume contraction due to the reaction.
TABLE-US-00001 TABLE 1 Equation Ratio of volume contraction (%)
Equation 2 19.7 Equation 3 26.7 Equation 4 33.0 Equation 5 23.5
Equation 6 21.7 Equation 7 28.9 Equation 8 20.0
[0043] It can be seen that volume contraction occurs by
approximately 19 to 33% due to the combustion synthesis reaction,
and the aluminum melt may be spontaneously infiltrated into the
empty spaces after the combustion synthesis reaction.
[0044] Table 2 illustrates an adiabatic temperature due to heat
generated by the reaction, and it can be seen that the adiabatic
temperature is increased by heat of the exothermic reaction.
TABLE-US-00002 TABLE 2 Equation Adiabatic Temperature (K) Equation
2 3304 Equation 3 2323 Equation 4 2682 Equation 5 3441 Equation 6
2368 Equation 7 3054 Equation 8 2522
[0045] In the present embodiment, the mixture of raw powders may
further include an aluminum powder in an excessive amount in
addition to a stoichiometric amount required for a combustion
synthesis reaction among the raw powders mixture. This is because
the reactions of Equations (2) to (8) are reactions using aluminum
as an intermediate, and as an example, the reaction of Equation (5)
is finally completed via an intermediate reaction as in Equation
(9), and Al introduced as an intermediate is reduced in an amount
which is the same as the amount of the initial stage.
(13/3)Al+Ti+C.fwdarw.Al.sub.3Ti+(1/3)Al.sub.4C.sub.3.fwdarw.TiC+(13/3)Al
(Equation 9)
[0046] Accordingly, the combustion synthesis reaction may be more
actively induced by adding an aluminum powder in an excessive
amount to the left side (that is, raw powder mixture) of (Equation
2) to (Equation 8). The amount of excess aluminum powder may be in
a range of 0.5 mol to 15 mol according to the type of reaction.
[0047] When an aluminum powder is included in excessive in the
mixture of raw powders, the present invention may further include
activating powders which can promote the exothermic reactions. For
example, the reactions of (Equation 2) to (Equation 8) may be
further promoted in an aluminum melt by further adding activating
powders, which have high reactivity with aluminum, to the reactions
of (Equation 2) to (Equation 8). The activating powders may be a
metal oxide, and may include one or more of, for example, Cu oxide
(CuO), Mn oxide (MnO), Fe oxide (FeO), and Ni oxide (NiO). Table 3
shows a change in adiabatic temperature by the reaction of the
oxide with aluminum.
TABLE-US-00003 TABLE 3 Equation Adiabatic Temperature (K) Cu oxide
3044 Ni oxide 3183 Mn oxide 2474 Fe oxide 3133
[0048] As an example, the Cu oxide CuO is an oxide which exhibits a
high exothermic reaction when reacted with aluminum, and the
following reaction occurs.
2Al+3CuO.fwdarw.Al.sub.2O.sub.3+3Cu (Equation 10)
[0049] When the adiabatic temperature by the reaction of (Equation
10) is calculated, it can be seen that the temperature may reach
3,044 K (see Table 3), and the reactions of (Equation 2) to
(Equation 8) may be promoted by the addition of CuO.
[0050] As for the content of activating powder to be added, it is
preferred to add a content of 0.01 to 3 moles based on the mole
content. The higher the content of activating powder to be added
is, the more rapid the reaction is, but when the activating powder
is added in an extremely large amount, in the case of a metal
component produced by decomposition of the activating powder, for
example, CuO, Cu remains in the aluminum melt, and may
unnecessarily increase the content of Al.sub.2O.sub.3.
[0051] In the present invention, the temperature of the aluminum
melt may be maintained in a range of 750.degree. C. to 950.degree.
C. At less than 750.degree. C., infiltration by molten aluminum may
rarely occur. In particular, when the mixture of raw powders
includes an aluminum powder in an excessive amount, the excess
aluminum powder absorbs the reaction heat, thereby decreasing the
adiabatic temperature. In this case, the combustion synthesis
reaction is prolonged to a longer time, or even no reaction may
occur. Accordingly, in consideration of this point, it is preferred
to maintain the temperature of the aluminum melt at 750.degree. C.
or more. Meanwhile, the content of hydrogen gas in the aluminum
melt is inevitably increased when the temperature of the aluminum
melt is increased, so possibility of presence of pores inside the
infiltrated preform is increased after the process is completed.
Further, due to a need for an additional device for increasing the
temperature, production costs are increased, and the process
becomes complicated. Accordingly, in the embodiment, it is
preferred to perform the process at the temperature of the aluminum
melt of 950.degree. C. or less, rigorously 920.degree. C. or
less.
[0052] Meanwhile, after the infiltrating of molten aluminum into
the preform 100 is completed while a part of the preform 100 is
exposed to an external environment, the embodiment may further
include stabilizing the preform 100 by completely immersing the
entire preform 100 in the aluminum melt 110 and maintaining the
preform 100 in the aluminum melt 110 for a predetermined time
before finally taking the preform 100 out of the aluminum melt 110
and solidifying the preform 100. This may be a final step for more
securely infiltrating molten aluminum into the preform 100.
[0053] In order to perform a series of processes more easily
according to the present invention, it is possible to use a holder
for holding the preform 100 stably. As an example, as illustrated
in FIGS. 9 and 10, it is possible to manufacture and use a ring
type holder which supports only the border of the preform in order
to facilitate the work. In the case of the holder, the contact
surface with the molten metal is so wide that it is easy to
infiltrate the molten metal, and the holder is easily applied even
to an arbitrary shape of preform and the upper portion of the
holder may be filled with the molten metal. As another example, it
is also possible to use a tube type holder in which only the upper
and lower portions of the holder is open. When the holder is used,
there is an advantage in that the shape of the preform may be more
completely maintained.
[0054] In addition, in the present invention, in order to induce a
partial combustion synthesis reaction, a non-metallic raw powder
may be added in the mixture of raw powders in an excessive amount
equal to or more than a stoichiometric amount required for the
combustion synthesis reaction. And, it is also possible to allow
the reactant to remain by inducing a partial reaction in some
cases.
[0055] For example, when the B.sub.4C powder is used as a reactant
as in Equations 2 and 8, it is also possible to add B.sub.4C in an
excessive amount to react the B.sub.4C partially (see the following
Equations 11 and 12), and allow the excessively added B.sub.4C to
remain in the preform.
3Ti+(1+x)B.sub.4C.fwdarw.2TiB.sub.2+TiC+xB.sub.4C (Equation 11)
4Al+3TiO.sub.2+(1+x)B.sub.4C.fwdarw.2TiB.sub.2+TiC+2Al.sub.2O.sub.3+xB.s-
ub.4C (Equation 12)
[0056] As another example, the mixture of raw powders may further
include a compound powder which does not participate in the
combustion synthesis reaction. For example, in (Equation 3),
B.sub.4C or SiC, TiC, Al.sub.2O.sub.3, or the like is added in an
excessive amount. The following Equations 13 and 14 exemplify the
case where B.sub.4C and SiC are added to (Equation 3).
4Al+2B.sub.2O.sub.3+C+xB.sub.4C.fwdarw.(1+x)B.sub.4C+2Al.sub.2O.sub.3
(Equation 13)
4Al+2B.sub.2O.sub.3+C+SiC.fwdarw.SiC+B.sub.4C+2Al.sub.2O.sub.3
(Equation 14)
[0057] A stable compound to be added in an excessive amount in
order to induce a partial reaction as described above is not
directly involved in the reaction and may maintain the shape of the
preform more stably as illustrated in FIG. 2, and thus may serve to
remove the air, moisture, adsorbed gas and the like in the preform
more easily, and the compound itself is a reinforcing phase having
excellent properties, and thus serves to enhance properties of a
metal composite to be prepared.
[0058] Hereinafter, experimental examples will be provided in order
to help understand the present invention. However, the following
experimental examples described below are only for helping to
understand the present invention, and the present invention is not
limited by the experimental examples below.
[0059] Table 4 shows the compositions of the raw powders which
follow Experimental Examples 1 and 2 of the present invention.
TABLE-US-00004 TABLE 4 Basically added component Excessively added
component and content thereof (mole) and content thereof (mole)
Result No. Ti B.sub.4C Al CuO B.sub.4C Infiltration Produced phase
Experimental Example 1 3 1 6 0.4 3 Successful TiB.sub.2, B.sub.4C
Experimental Example 2 3 1 1 0.1 0 Successful TiB.sub.2, B.sub.4C,
TiC
[0060] The raw powders with the compositions shown in Experimental
Example 1 were maintained at a temperature of 180.degree. C. for 1
hour and dried, and then mixed by using a ball-mill. A preform was
prepared by compressing the mixed powder using a press device, and
the applied pressure was controlled during the preparation to allow
a density of the preform to be 60% of the theoretical density. The
shape of preform was 35 mm in diameter and 28 mm in thickness, and
several identical preforms were prepared.
[0061] The prepared preforms were introduced into aluminum melt
which was maintained at 900.degree. C. in the atmosphere, and
during the introduction, the surface of the preform was allowed to
be exposed to the surface of the molten metal as in FIG. 1. After
the preform was ignited in the aluminum melt, the preform was kept
for 3 minutes by immersing the preform in the aluminum melt
completely, and then the preform was taken out of the aluminum
melt, and solidified in the atmosphere.
[0062] For some of the preforms, the upper portion of the preform
was perforated, and then a thermocouple was mounted to directly
measure a change in temperature of the preform and the aluminum
melt. FIG. 3 illustrates the results, and it can be seen that when
the preform was introduced into the molten metal, the temperature
of the preform was gradually increased (heating stage), and when
about 74 seconds elapsed, the temperature of the preform was
sharply increased (about 1,156.degree. C.) while the preform was
ignited due to reaction in the preform (ignition and infiltration
stage). In the next stabilization stage, it can be seen that the
temperature of the preform becomes close to the temperature of the
aluminum melt.
[0063] Some of the preforms were taken out of the aluminum melt,
immediately before the ignition (A of FIG. 3) and immediately after
the ignition (B of FIG. 3), and quenched in water to observe
whether the aluminum was infiltrated thereinto. As a result of the
observation, it can be confirmed that aluminum melt was not
infiltrated into the preform for the preform taken out of
immediately before ignition. But aluminum melt was completely
infiltrated into the preform for the preform taken out of
immediately after ignition. That is, when the preform is ignited,
it can be confirmed that aluminum has been infiltrated into the
preform in a very short time, and it can be confirmed that the
entire process may be easily completed in the atmosphere within 5
minutes. In order to verify whether the test results as described
above are theoretically feasible, the relationship between the
infiltration length-infiltration time was theoretically calculated
based on the size of initial B.sub.4C powder and the size of final
B.sub.4C powder for the composition of the preform in Experimental
Example 1. FIG. 4 is a view illustrating the result, and the
calculation result shows that the infiltration can be completed
within 2 to 3 seconds.
[0064] FIGS. 5a and 5b illustrate a microstructure observed at low
magnification and high magnification, respectively, for the preform
in Experimental Example 1. It can be seen that the aluminum melt
had been successfully infiltrated, so that the preform had a sound
structure having few pores therein. Since a partial reaction was
used in the present experimental example, it can be observed that a
large amount of B.sub.4C remained in the microstructure, and it can
be also confirmed that fine TiB2 phase (brown color) was formed
around B.sub.4C due to the reactions. The properties of the sample
are summarized in Table 5. The sample was a composite composed of
Al-TiB.sub.2--B.sub.4C. The composite samples exhibited lower
density as 2.94 g/cc, excellent mechanical properties such as an
elastic modulus of 158 GPa and a hardness of 166 kg.sub.f/mm.sup.2
(1.63 GPa), and had a coefficient of thermal expansion (CTE) of 9.4
ppm/K. Thus it is expected to utilize the composites as parts where
high hardness, high stiffness and thermal stability are
required.
TABLE-US-00005 TABLE 5 Item Measured value Density 2.94 g/cc
Elastic coefficient 158 GPa Hardness 166 kg.sub.f/mm.sup.2 (1.63
GPa) Thermal expansion 9.4 ppm/K coefficient
[0065] For the composition which followed Experimental Example 2, a
preform was prepared in the same manner as in Experimental Example
1. FIGS. 6a and 6b illustrate a microstructure observed at low
magnification and high magnification, respectively, for the preform
in Experimental Example 2. It can be seen that the aluminum melt
had been successfully infiltrated, so that the preform had a sound
structure having few pores therein. Since a complete reaction was
used in the present experimental example, no B.sub.4C was remained
in the microstructure; whole B.sub.4C was reacted to form reaction
products and it can be seen that fine TiB.sub.2 phase (brown color,
approximately 1 .mu.m in size) and TiC phase (gray color,
approximately 1 .mu.m in size) had been successfully formed in the
microstructure.
[0066] Table 6 shows the compositions of the raw powders which
follow Experimental Examples 5 and 6. A test, which is the same as
in Experimental Example 2, was performed by using a raw powder with
the composition shown in Table 6. FIGS. 7a and 7b illustrate a
microstructure observed at low magnification and high
magnification, respectively, for Experimental Example 5. It can be
seen that the aluminum melt had been successfully infiltrated, so
that the preform had a sound structure having few pores therein,
and it can be seen that Al.sub.2O.sub.3 phase (dark gray) was
present in a network form in the microstructure by the reaction,
and B.sub.4C phase (light gray) having a square shape had been
produced along the border of the network.
TABLE-US-00006 TABLE 6 Basically added component Excessively added
component and content thereof (mole) and content thereof (mole)
Result No. Al B.sub.2O.sub.3 C Al CuO B.sub.4C Infiltration
Produced phase Experimental Example 5 4 2 1 3 1.5 0 Successful
Al.sub.2O.sub.3, B.sub.4C Experimental Example 6 4 2 1 5 1 1
Successful Al.sub.2O.sub.3, B.sub.4C
[0067] Table 7 shows the compositions of the raw powders which
follow Experimental Examples 8 and 9. A test, which is the same as
in Experimental Example 2, was performed by using a raw powder with
the composition shown in Table 7. FIGS. 8a and 8b illustrate a
microstructure observed at low magnification and high
magnification, respectively, for Experimental Example 8. It can be
seen that the aluminum melt had been successfully infiltrated, so
that the preform had a sound structure having few pores therein,
and it can be seen that coarse Al.sub.2O.sub.3 phase (dark gray)
and a brown TiB.sub.2 phase, which is as fine as 1 had been
produced in the microstructure by the reaction.
TABLE-US-00007 TABLE 7 Basically added component Excessively added
component and content thereof (mole) and content thereof (mole)
Result No. Al TiO.sub.2 B.sub.2O.sub.3 Al CuO B.sub.4C Infiltration
Produced phase Experimental Example 8 10 3 3 8 1.5 0 Successful
Al.sub.2O.sub.3, TiB.sub.2, Experimental Example 9 10 3 3 8 1.5 1
Successful Al.sub.2O.sub.3, TiB.sub.2, B.sub.4C
[0068] The present invention has been described with reference to
the embodiments illustrated in the drawings, but the embodiments
are only illustrative, and it would be appreciated by those skilled
in the art that various modifications and other equivalent
embodiments can be made. Therefore, the true technical scope of the
present invention shall be defined by the technical spirit of the
appended claims.
EXPLANATION OF REFERENCE NUMERALS AND SYMBOLS
[0069] 100: Preform [0070] 110: Aluminum melt [0071] 120:
Crucible
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