US20150218707A1

US20150218707A1 - Method for preparing aluminum matrix composite using no pressure infiltration

Info

Publication number: US20150218707A1
Application number: US14/426,278
Authority: US
Inventors: Jung Moo Lee; Su Hyeon Kim; Young Hee Cho; Sang Kwan Lee; In Hyuck Song; Jong Jin Kim; Jing Jing Zhang
Original assignee: Korea Institute of Machinery and Materials KIMM
Current assignee: Korea Institute of Machinery and Materials KIMM
Priority date: 2012-09-06
Filing date: 2013-08-21
Publication date: 2015-08-06
Also published as: KR101409294B1; WO2014038802A1; KR20140033586A

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.

Description

TECHNICAL FIELD

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

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.
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.
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.
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° C. and the infiltration time is as long as 24 hours.

DISCLOSURE

Technical Problem

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

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.
The infiltrating of the molten aluminum may further include allowing the preform to be ignited.
Further, the method may further include immersing the entire preform in the aluminum molten metal after the infiltrating of the molten aluminum.
The raw powder mixtures may be any one of a mixture of Ti and B₄C powders, a mixture of Al, B₂O₃, and C, a mixture of Al, B₂O₃, and TiO₂, a mixture of Ti and C, a mixture of Al, TiO₂, and C, a mixture of Al, Ti, and B₂O₃, and a mixture of Al, TiO₂, and B₄C.
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.
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.
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 B₄C, SiC, TiC, and Al₂O₃.
Meanwhile, the temperature of the aluminum melt may be in a range of 750° C. to 950° C.

Advantageous Effects

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

FIG. 1 is a schematic view of a method for preparing an aluminum matrix composite according to the present invention.

FIG. 2 is a schematic view illustrating an infiltration process in a preform.

FIG. 3 is a graph illustrating a change in temperature of the preform in an aluminum molten metal.

FIG. 4 is a graph in which a relationship between the infiltration length-infiltration time is theoretically calculated.

FIGS. 5 to 8 are results of microstructures for aluminum matrix composites prepared according to experimental examples of the present invention.

FIG. 9 is a perspective view of a ring-type holder for fixing the preform.

FIG. 10 illustrates immersing the preform in the aluminum molten metal using the ring-type holder.

BEST MODE

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.
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.
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.
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.
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.
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.
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.
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.
(1) Generation of Pressure Difference Inside and Outside of Preform 100
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.
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.
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.
(2) Action of Capillary Pressure
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).
$\begin{matrix} P_{c} = \frac{2 r_{1 v} \cos θ}{r_{c}} & [Equation 1] \end{matrix}$
Here, P_cis a capillary pressure, γ_1vis a surface tension of the liquid, θ is a contact angle between the liquid and the solid, and r_cis 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 θ is larger than 90°, P_chas 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 θ is smaller than 90°, P_chas 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.
In general, the contact angle between molten aluminum and ceramic particle has a value larger than 90°. 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 B₄C, the contact angle is 100° in the case where the contact is maintained at 900° for 1 second, but the contact angle is decreased to 90° in the case where the contact is maintained at the same temperature for 1 hour, and the contact angle is further decreased to 60° or less in the case where the contact is maintained at 1,200° 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° or less, and the molten metal can be spontaneously infiltrated into the preform due to the decrease in contact angle.
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.
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.
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.
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+B₄C→2TiB₂+TiC (Equation 2)
4Al+2B₂O₃+C→B₄C+2Al₂O₃ (Equation 3)
10Al+3TiO₂+3B₂O₃→2TiB₂+5Al₂O₃ (Equation 4)
Ti+C→TiC (Equation 5)
4Al+3TiO₂+3C→3TiC+2Al₂O₃ (Equation 6)
2Al+Ti+B₂O₃→TiB₂+Al₂O₃ (Equation 7)
4Al+3TiO₂+B₄C→2TiB₂+TiC+2Al₂O₃ (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 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

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.
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 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

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→Al₃Ti+(1/3)Al₄C₃→TiC+(13/3)Al (Equation 9)
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.
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 3

	Equation	Adiabatic Temperature (K)

	Cu oxide	3044
	Ni oxide	3183
	Mn oxide	2474
	Fe oxide	3133

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→Al₂O₃+3Cu (Equation 10)
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.
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 Al₂O₃.
In the present invention, the temperature of the aluminum melt may be maintained in a range of 750° C. to 950° C. At less than 750° 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° 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° C. or less, rigorously 920° C. or less.
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.
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.
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.
For example, when the B₄C powder is used as a reactant as in Equations 2 and 8, it is also possible to add B₄C in an excessive amount to react the B₄C partially (see the following Equations 11 and 12), and allow the excessively added B₄C to remain in the preform.
3Ti+(1+x)B₄C→2TiB₂+TiC+xB₄C (Equation 11)
4Al+3TiO₂+(1+x)B₄C→2TiB₂+TiC+2Al₂O₃+xB₄C (Equation 12)
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), B₄C or SiC, TiC, Al₂O₃, or the like is added in an excessive amount. The following Equations 13 and 14 exemplify the case where B₄C and SiC are added to (Equation 3).
4Al+2B₂O₃+C+xB₄C→(1+x)B₄C+2Al₂O₃ (Equation 13)
4Al+2B₂O₃+C+SiC→SiC+B₄C+2Al₂O₃ (Equation 14)
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.
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.
Table 4 shows the compositions of the raw powders which follow Experimental Examples 1 and 2 of the present invention.

TABLE 4

Basically added component	Excessively added component
and content thereof (mole)	and content thereof (mole)	Result

No.	Ti	B₄C	Al	CuO	B₄C	Infiltration	Produced phase

Experimental Example 1	3	1	6	0.4	3	Successful	TiB₂, B₄C
Experimental Example 2	3	1	1	0.1	0	Successful	TiB₂, B₄C, TiC

The raw powders with the compositions shown in Experimental Example 1 were maintained at a temperature of 180° 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.
The prepared preforms were introduced into aluminum melt which was maintained at 900° 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.
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° 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.
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 B₄C powder and the size of final B₄C 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.
FIGS. 5 a and 5 b 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 B₄C remained in the microstructure, and it can be also confirmed that fine TiB2 phase (brown color) was formed around B₄C due to the reactions. The properties of the sample are summarized in Table 5. The sample was a composite composed of Al-TiB₂—B₄C. 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_f/mm²(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 5

	Item	Measured value

	Density	2.94 g/cc
	Elastic coefficient	158 GPa
	Hardness	166 kg_f/mm²(1.63 GPa)
	Thermal expansion	9.4 ppm/K
	coefficient

For the composition which followed Experimental Example 2, a preform was prepared in the same manner as in Experimental Example 1. FIGS. 6 a and 6 b 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 B₄C was remained in the microstructure; whole B₄C was reacted to form reaction products and it can be seen that fine TiB₂phase (brown color, approximately 1 μm in size) and TiC phase (gray color, approximately 1 μm in size) had been successfully formed in the microstructure.
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. 7 a and 7 b 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 Al₂O₃phase (dark gray) was present in a network form in the microstructure by the reaction, and B₄C phase (light gray) having a square shape had been produced along the border of the network.

TABLE 6

Basically added component	Excessively added component
and content thereof (mole)	and content thereof (mole)	Result

No.	Al	B₂O₃	C	Al	CuO	B₄C	Infiltration	Produced phase

Experimental Example 5	4	2	1	3	1.5	0	Successful	Al₂O₃, B₄C
Experimental Example 6	4	2	1	5	1	1	Successful	Al₂O₃, B₄C

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. 8 a and 8 b 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 Al₂O₃phase (dark gray) and a brown TiB₂phase, which is as fine as 1 had been produced in the microstructure by the reaction.

TABLE 7

Basically added component	Excessively added component
and content thereof (mole)	and content thereof (mole)	Result

No.	Al	TiO₂	B₂O₃	Al	CuO	B₄C	Infiltration	Produced phase

Experimental Example 8	10	3	3	8	1.5	0	Successful	Al₂O₃, TiB₂,
Experimental Example 9	10	3	3	8	1.5	1	Successful	Al₂O₃, TiB₂, B₄C

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

- 100: Preform
- 110: Aluminum melt
- 120: Crucible

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 B₄C powders, a mixture of Al, B₂O₃, and C, a mixture of Al, B₂O₃, and TiO₂, a mixture of Ti and C, a mixture of Al, TiO₂, and C, a mixture of Al, Ti, and B₂O₃, and a mixture of Al, TiO₂, and B₄C.

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 B₄C, SiC, TiC, and Al₂O₃.

11. The method of claim 1, wherein a temperature of the aluminum melt is in a range of 750° C. to 950° C.