Heat-resistant aluminum-base composites and process of making same

A heat-resistant aluminum-base composite which includes an aluminum matrix of not smaller than 99.0% purity, Si particles whose average diameter falls in the range of 0.1 to 100 .mu.m, and Al.sub.2 O.sub.3 and Al.sub.4 C.sub.3 particles, the particles being dispersed in the aluminum matrix at volume percents Vf(Si) and V.sub.f (Al.sub.2 O.sub.3 +Al.sub.4 C.sub.3) wherein Vf(Si).gtoreq.9%, Vf(Al.sub.2 O.sub.3 +Al.sub.4 C.sub.3).ltoreq.20% and Vf (Al.sub.2 O.sub.3 +Al.sub.4 C.sub.3)+Vf(Si).ltoreq.40%.

BACKGROUND OF THE INVENTION 
The present invention relates to a heat-resistant aluminum-base composite 
and process of producing same, the composite being adapted for making 
components for internal combustion engines such as pistons, and more 
particularly to a heat-resistant aluminum-base composite containing evenly 
dispersed reinforcing particles in the aluminum matrix and process for 
making same. 
It is generally known that components for internal combustion engines such 
as pistons are used under severe physical conditions such as at elevated 
temperatures as 150.degree. to 400.degree. C. To withstand the hard 
conditions the components are made of highly heat- and wear-resistant 
material which has good thermal conductivity and low coefficient of 
thermal expansion. 
On the other hand, there is a strong demand for vehicles to be lightweight 
which requires individual components to be as light as possible. In 
addition, they must be easy to machine so as to increase the production 
efficiency and reduce the cost. 
To satisfy such demands the components for internal combustion engines are 
made of Al--Si alloy made by an I/M method, such as AC8A and AC8B, but 
these materials are not sufficiently strong at elevated temperatures. For 
example, the tensile strength thereof is 17 kgf/mm.sup.2 at 200.degree. 
C., and 7 kgf/mm.sup.2 at 300.degree. C. As a result, it is difficult to 
make a thin and lightweight components with these materials. 
To overcome the difficulty encountered by Al--Si alloy made by an I/M 
method, there is a proposal for using another type of Al--Si alloys made 
by a P/M method but they are costly and is not satisfactory in the 
heat-resistant property. There is another proposal for using aluminum 
alloys containing dispersed reinforcing particles of Al.sub.2 O.sub.3 and 
SiC in the aluminum matrix. It is found that this reinforced alloys 
increases the heat-resistant property but disadvantageously shortens the 
life of a cutting tool because of its excessive hard quality. 
OBJECTS AND SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide an 
aluminum-base composite having enhanced heat- and wear-resistant 
properties and workability and a process for making same. 
Another object of the present invention is to provide an aluminum-base 
composite adapted for making machine components used under severe physical 
conditions, and a process for making same. 
In achieving these objects the inventors have made the present invention: 
According to one aspect of the present invention there is provided a 
heat-resistant aluminum-base composite which comprises an aluminum matrix 
of not smaller than 99.0% purity, Si particles whose average diameter 
falls in the range of 0.1 .mu.m to 100 .mu.m, and Al.sub.2 O.sub.3 and 
Al.sub.4 C.sub.3 particles, the particles being dispersed in the aluminum 
matrix at a volume percent Vf(Si) for the Si particles and a total volume 
percent V.sub.f (Al.sub.2 O.sub.3 +Al.sub.4 C.sub.3) for the Al.sub.2 
O.sub.3 and Al.sub.4 C.sub.3 particles wherein: 
EQU Vf(Si).gtoreq.9% 
and 
EQU Vf(Al.sub.2 O.sub.3 +Al.sub.4 C.sub.3).ltoreq.20% 
and 
EQU Vf(Al.sub.2 O.sub.3 +Al.sub.4 C.sub.3)+Vf(Si).ltoreq.40%. 
According to another aspect of the present invention there is provided a 
process of producing a heat-resistant aluminum-base composite, the process 
comprising mixing an aluminum powder of not smaller than 99.0% purity for 
matrix with Si particles whose average diameter falls in the range of 0.1 
to 100 .mu.m; ball milling the powdery mixture of aluminum and Si 
particles into a powdery complex, during which the aluminum in the powdery 
complex is allowed to react with the oxygen in the atmosphere and the 
carbon in an organic anti-seizure agent added to the powdery complex, 
thereby dispersing Al.sub.2 O.sub.3 and Al.sub.4 C.sub.3 particles in the 
aluminum matrix at the following volume percents Vf(Si) and V.sub.f 
(Al.sub.2 O.sub.3 +Al.sub.4 C.sub.3): 
EQU Vf(Si).gtoreq.9% 
and 
EQU Vf(Al.sub.2 O.sub.3 +Al.sub.4 C.sub.3).ltoreq.20% 
and 
EQU Vf(Al.sub.2 O.sub.3 +Al.sub.4 C.sub.3)+Vf(Si).ltoreq.40%. 
The purity of the aluminum matrix must be at least 99.0%, which is required 
to secure the high thermal conductivity of the composite. 
The reinforcing Si particles serve to achieve the low coefficient of 
thermal expansion and enhance the wear resistance of the composite. To 
reduce the weight of the composite the specific weight of the reinforcing 
particles is preferably not greater than 2.7 of the matrix. To this end Si 
(specific weight: 2.3) and B.sub.4 C (specific weight: 2.5) can be used. 
However B.sub.4 C is as hard as 3700 Hv, thereby shortening the life of a 
cutting tool. Whereas, Si is as hard as 1200 Hv which is softer than the 
known hard-alloy cutting tools (about 1800 Hv). Actually the long life of 
Al--Si alloy cutting tools is generally appreciated. In addition, Si has a 
thermally conductivity of 0.20 cal/.degree.C.multidot.cm.multidot.s, and 
owing to the good conductivity the Al--Si alloy is used for making 
pistons. Si particles can increase the thermal conductivity of the 
composite, wherein they are preferably 0.1 to 100 .mu.m in diameter on 
average. If the diameters of the particles are smaller than 0.1 .mu.m the 
wear resistance of the resulting composite will become insufficient. 
Whereas, if they have a diameter of larger than 100 .mu.m the resulting 
composite will be unsuitable for making components for internal engines 
because of the possibility that the components are likely to crack during 
forging. The optimum range is 0.1 to 100 .mu.m for achieving the adequate 
wear resistance and workability. 
The Al.sub.2 O.sub.3 particles are formed through the reaction of the 
aluminum reacts with oxygen in the atmosphere while the powdery complex of 
Al and Si is treated by a ball mill. The Al.sub.4 C.sub.3 particles are 
formed through the reaction of the aluminum with the carbon content in an 
organic anti-seizure agent added to the powdery complex while it is 
treated by the ball mill. 
The amounts of these Al.sub.2 O.sub.3 and Al.sub.4 C.sub.3 reinforcing 
particles are required to fall in the ranges mentioned above so as to 
achieve the desired heat resistance and low coefficient of thermal 
expansion. If Vf(Si) is smaller than 9%, it is difficult to obtain the 
desired low coefficient of thermal expansion. Preferably it is in the 
range of 10 to 20%. If Vf(Al.sub.2 O.sub.3 +Al.sub.4 C.sub.3) exceeds 20% 
the resulting composite will become too brittle to forge it into 
components for internal combustion engines, etc. Preferably it is in the 
range of 3 to 11%, and more preferably in the range of 3 to 8%. As 
described above, the Si particles serve to achieve the low coefficient of 
thermal expansion and increase the wear resistance of the composite. To 
this end it is required to limit the total amount Vf(Al.sub.2 O.sub.3 
+Al.sub.4 C.sub.3)+Vf(Si) to 40%. If it exceeds 40%, the resulting 
composite will become too brittle to decrease the workability of the 
composite. 
Briefly, the heat-resistant aluminum-base composite of the present 
invention is produced by obtaining a powdery complex of aluminum and 
reinforcing particles by a ball mill treatment, and placing the powdery 
complex in a pressure vessel to degasify it. The degasified powdery 
complex is hot compacted into a mass which is subjected to hot processing 
such as hot extrusion, hot forging, or hot rolling as desired. The 
above-mentioned process is carried out under a batch system. Of course a 
continuous line process is possible where subsequently to the ball mill 
process the transporting, degasifying, filling of particles in the 
pressure vessel, and compacting consecutively follow on the line. 
The ball mill process is preferably carried out at an atmosphere in which 
the concentration of oxygen is controlled to not larger than 1.0%. The 
amount of Al.sub.4 C.sub.3 particles forming through the ball milling is 
adjusted by controlling the amount of an organic anti-seizure agent to be 
added, wherein the organic anti-seizure agent can be selected from organic 
solvents such as ethanol.

EXAMPLE (1) 
This example was carried out to see the relationship between the heat 
resistance and forgeability of the composite and the equations Vf(Al.sub.2 
O.sub.3 +Al.sub.4 C.sub.3) and Vf(Al.sub.2 O.sub.3 +Al.sub.4 
C.sub.3)+Vf(Si). 
Aluminum powder having a grain size of 45 .mu.m on average produced by an 
air atomizing, and Si particles of 98% purity having a grain size of 1 
.mu.m were mixed by a mixing apparatus at 2,000 rpm for four minutes at 
volume percent Vf (Si) varied as shown in Table (1), wherein the total 
weight was 1 kg. 
The mixture was ground by steel balls of 40 kg in total amount, each ball 
having a diameter of 3/8" at an atmosphere of Ar (argon) in which the 
concentration of air was adjusted as shown in Table (1). The ball milling 
continued at 280 rpm for an hour. In this way the specimens were obtained 
in powder. During the ball mill process ethanol as an anti-seizure agent 
was added at the rates shown in Table (1). The total volume percent 
Vf(Al.sub.2 O.sub.3 +Al.sub.4 C.sub.3) are shown in Table (1). 
Each of the specimens obtained in this way was filled in a pressure vessel 
of aluminum at an atmosphere of Ar (argon), and degasified at a vacuum at 
a pressure of 3.times.10.sup.-3 torr for five hours. Then each specimen 
was pulverized by a hot press at 500.degree. C. at a pressure of 7,000 
kgf/cm.sup.2 to form a billet, which 7 was extruded into a cylindrical bar 
at a ratio of 10:1 at a temperature of 450.degree. C. 
Each specimen was tested to see how the tensile strength was maintained at 
300.degree. C., and what the limiting upsetting percent was at 500.degree. 
C. The results are shown for comparison with the AC8A-T5 die casting: 
TABLE (1) 
__________________________________________________________________________ 
Vf(Al.sub.2 O.sub.3 + 
Ethanol .sigma.B 
Limiting 
Specimen 
Vf(Si) 
Al.sub.4 C.sub.3) 
added 
Con. of 
300.degree. C. 
Upsetting 
No. (%) (%) (%) O.sub.2 (%) 
(kgf/mm.sup.2) 
500.degree. C. (%) 
__________________________________________________________________________ 
Comp. 
1 0 6 40 0.1 less 
21 65 
2 0 19 40 1.0 30 55 
3 0 23 80 1.0 34 25 
Inv. 
4 15 9 80 0.1 less 
28 58 
5 25 11 80 0.1 less 
31 53 
Comp. 
6 35 10 80 0.1 less 
36 20 
AC8T-T.sub.5 7 67 
__________________________________________________________________________ 
(Note) Comp. stands for comparative specimen. 
It will be appreciated from Table (1) that the specimens containing 
Al.sub.2 O.sub.3 and Al.sub.4 C.sub.3 dispersed particles is superior in 
heat resistance to the AC8A-T.sub.5 die casting. However, if Vf(Al.sub.2 
O.sub.3 +Al.sub.4 C.sub.3) exceeds 20% like specimen No. 3, and 
Vf(Al.sub.2 O.sub.3 +Al.sub.4 C.sub.3)+Vf(Si) exceeds 40% like specimen 
No. 6, the limiting upsetting percent is as poor as 25% and 20%, 
respectively, the forging is almost impossible. 
EXAMPLE (2) 
This experiment was carried out to see the relationship between Vf(Si) and 
the coefficient of thermal expansion of the composite. 
In obtaining the specimens Nos. 7 to 10 the values of Vf(Si) were varied as 
shown in Table (2) and the ball milling was carried out under the same 
conditions as in Example (1) except that Vf(Al.sub.2 O.sub.3 +Al.sub.4 
C.sub.3) was set to not larger than 6% wherein the concentration of oxygen 
in the atmosphere of Ar (argon) was below 0.1% and the added ethanol was 
40 cc. The process of obtaining the specimens was the same as that of 
Example (1). 
The specimens were examined on their coefficients of thermal expansion, and 
compared with the AC8A-T.sub.5 die casting. The results are shown in Table 
(2): 
TABLE (2) 
______________________________________ 
Coefficient of 
Specimen Vf(Si) Thermal Expansion 
No. (%) (10.sup.-6 /.degree.C.) 
______________________________________ 
comp. 7 0 23.9 
8 5 22.4 
Inv. 9 9 19.9 
10 15 18.0 
AC8T-T.sub.5 -- 19.5 
______________________________________ 
It will be appreciated from Table (2) that when Vf(Si) is smaller than 9%, 
the coefficient of thermal expansion will become large, and that when it 
is not smaller than 9% the specimens have the same as or a larger 
coefficient of thermal expansion than the AC8A-T.sub.5. 
EXAMPLE (3) 
This experiment was carried out to see the relationship among the average 
grain size of Si particles, the resulting wear resistance and 
forgeability. 
In obtaining the specimens Nos. 11 to 16 the average grain sizes of Si 
particles were varied as shown in Table (3), and the ball milling was 
carried out under the same conditions as in Example (1) except that 
Vf(Al.sub.2 O.sub.3 +Al.sub.4 C.sub.3) was set to 6% wherein Vf(Si) was 
15%, with the concentration of oxygen in the atmosphere of Ar being below 
0.1% and the added ethanol being 40 cc. The specimens were obtained in the 
same manner as Example (1). 
The specimens were examined with respect to the specific wearing amounts 
and limiting upsetting percents at 500.degree. C., and compared with the 
AC8A-T.sub.5 die casting. The wear resistance was tested by an Ohgoshi 
testing machine wherein no lubricant was used, the mating piece was FC30, 
the abrading speed was 1.99 m/s, and the abrading distance was 600 m, and 
the final load was 2.1 kg. The results are shown in Table (3): 
TABLE (3) 
______________________________________ 
Si particles 
Specific Wearing 
Specimen 
diameter Amount Limiting Up- 
Nos. (.mu.m) (mm2 .multidot. kg.sup.-1) 
Setting 500.degree. C. (%) 
______________________________________ 
comp. 11 0.05 51 65 
Inv. 12 0.1 34 66 
13 1.0 30 63 
14 10.0 28 57 
15 100.0 20 50 
comp. 16 200.0 19 27 
AC8T-T.sub.5 
-- 35 67 
______________________________________ 
It will be noted from Table (3) that when the average grain size of Si 
particles is smaller than 0.1 .mu.m, the resulting specific wearing amount 
become large, which means a decreased wear resistance, and that when it 
exceeds 100 .mu.m, the limiting upsetting percents become low, which means 
a decreased forgeability. 
EXAMPLE (4) 
This experiment was carried out to see the relationship between the purity 
of aluminum used for the matrix and the thermal conductivity of the 
specimens. 
In obtaining the specimens Nos. 11 to 16 the purity of aluminum was varied 
as shown in Table (4), and the experiment was carried out under the same 
conditions as in Example (1) except that Vf(Al.sub.2 O.sub.3 +Al.sub.4 
C.sub.3) Vf(Si) was set to 6% wherein Vf(Si) was 15%, with the 
concentration of oxygen in the atmosphere of Ar being below 0.1% and the 
added ethanol being 40 cc. The specimens were obtained in the same manner 
as Example (1). 
The specimens were examined on their thermal conductivity, and compared 
with the AC8A-T.sub.5. The results are shown in Table (4): 
TABLE (4) 
______________________________________ 
Specimen Thermal Conductivity 
No. Purity of Al (%) 
(cal/cm.sup.2 .multidot. .degree.C. .multidot. 
______________________________________ 
s) 
comp. 17 99.99 0.35 
18 99.0 (A1100) 0.32 
Inv. 19 97.1 (A3003) 0.27 
20 96.6 (A6061) 0.24 
AC8T-T.sub.5 0.29 
______________________________________ 
It will be appreciated from Table (4) that when the purity of aluminum is 
smaller than 99%, the specimens have a low thermal conductivity than the 
AC8A-T.sub.5 specimen, and that when it is equal to or larger than 99%, 
they have a higher thermal conductivity than the AC8A-T.sub.5. 
EXAMPLE (5) 
This experiment was carried out to see the specific weight of the specimen 
No. 10 of Table (2) and the life of a cutting tool affected by it, and the 
results are shown in Table (5) for comparison with the AC8A-T.sub.5 
specimen. The cutting test was conducted under the following conditions: 
The length of a specimen: 23 mm (diameter).times.200 mm 
______________________________________ 
Cutting Tool: K10 
Cutting Speed: 247 m/s 
Feed: 0.2 mm/rev. 
Depth of Cut: 1 mm 
Number of Cutting: 8 times 
Lublicant: Not used 
______________________________________ 
The widths of wear on the clearance surface of the cutting tool were 
measured. The results are shown in Table (5): 
TABLE (5) 
______________________________________ 
Specimen Specific Specific Wearing 
No. Weight Amount (.mu.m) 
______________________________________ 
Inv. 10 2.66 28.0 
AC8T-T.sub.5 2.72 55.0 
______________________________________ 
It will be appreciated from Table (5) that the specimen No. 10 has a 
lighter specific weight than the AC8A-T.sub.5, and that it abrades the 
cutting tool less than AC8A-T.sub.5 does.