U.S. patent application number 09/827606 was filed with the patent office on 2001-10-11 for metal matrix composite and piston using the same.
This patent application is currently assigned to NHK Spring Co., Ltd.. Invention is credited to Katsuya, Akihiro, Shiraishi, Tohru, Takehana, Toshihiro.
Application Number | 20010028948 09/827606 |
Document ID | / |
Family ID | 26527137 |
Filed Date | 2001-10-11 |
United States Patent
Application |
20010028948 |
Kind Code |
A1 |
Takehana, Toshihiro ; et
al. |
October 11, 2001 |
Metal matrix composite and piston using the same
Abstract
A part of a piston is composed of a metal matrix composite. The
metal matrix composite is composed of a matrix of a light metal
alloy and reinforcements formed of metallic fibers mixed in the
matrix. The reinforcements are formed of an alloy that consists
mainly of Fe and Cr and contains Al and/or Si. The Cr content and
the Al and/or Si content of the reinforcements range from 5 to 30%
and from 3 to 10%, respectively. The fiber diameter of the
reinforcements ranges from .o slashed.20 .mu.m to .o slashed.100
.mu.m. The reinforcements are formed by the melt extraction method
and have irregular peripheral surfaces. Solution-treatment of the
metal matrix composite is carried out within a temperature range
from 470.degree. C. to 500.degree. C. such that formation of
intermetallic compounds is restrained.
Inventors: |
Takehana, Toshihiro;
(Yokohama-shi, JP) ; Katsuya, Akihiro;
(Yokohama-shi, JP) ; Shiraishi, Tohru;
(Yokohama-shi, JP) |
Correspondence
Address: |
FRISHAUF, HOLTZ, GOODMAN &
LANGER & CHICK, PC
767 THIRD AVENUE
25TH AVE
NEW YORK
NY
10017-2023
US
|
Assignee: |
NHK Spring Co., Ltd.
Yokohama-shi
JP
|
Family ID: |
26527137 |
Appl. No.: |
09/827606 |
Filed: |
April 5, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09827606 |
Apr 5, 2001 |
|
|
|
PCT/JP00/05373 |
Aug 10, 2000 |
|
|
|
Current U.S.
Class: |
428/293.1 ;
428/614 |
Current CPC
Class: |
C22C 38/06 20130101;
Y02T 10/12 20130101; F02B 23/0672 20130101; F02F 3/045 20130101;
Y10T 428/12757 20150115; B22F 2003/248 20130101; F05C 2251/042
20130101; C22C 38/02 20130101; C22C 38/18 20130101; B22D 19/0027
20130101; Y10T 428/12486 20150115; B22D 18/02 20130101; F05C
2201/021 20130101; F05C 2253/16 20130101; B22F 1/062 20220101; Y10T
428/12958 20150115; Y10T 428/249927 20150401; B22D 19/14 20130101;
C22C 49/14 20130101 |
Class at
Publication: |
428/293.1 ;
428/614 |
International
Class: |
B32B 015/14 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 10, 1999 |
JP |
11-226359 |
Mar 13, 2000 |
JP |
2000-069001 |
Claims
What is claimed is:
1. A metal matrix composite having a metallic matrix and
reinforcements mixed in said matrix, the metal matrix composite
characterized in that said reinforcements are metallic fibers of an
alloy consisting mainly of Fe and Cr and containing Al and/or
Si.
2. A metal matrix composite having a metallic matrix and
reinforcements mixed in said matrix, the metal matrix composite
characterized in that said reinforcements are metallic fibers of an
alloy consisting mainly of Fe and Cr and containing Al and/or Si,
and the Cr content and the Al and/or Si content of said
reinforcements range from 5 to 30% and from 3 to 10%,
respectively.
3. A metal matrix composite according to claim 1, wherein
solution-treatment is carried out at temperatures such that
formation of reactants on the interfaces between said matrix and
the reinforcements is restrained.
4. A metal matrix composite according to claim 2, wherein
solution-treatment is carried out at temperatures such that
formation of reactants on the interfaces between said matrix and
the reinforcements is restrained.
5. A metal matrix composite according to claim 3, wherein said
reactants are intermetallic compounds, and said solution-treatment
is carried out within a range from 470.degree. C. to 500.degree.
C.
6. A metal matrix composite according to claim 4, wherein said
reactants are intermetallic compounds, and said solution-treatment
is carried out within a range from 470.degree. C. to 500.degree.
C.
7. A metal matrix composite according to claim 1, wherein said
reinforcements are preformed into a given shape by sintering.
8. A metal matrix composite according to claim 2, wherein said
reinforcements are preformed into a given shape by sintering.
9. A metal matrix composite according to claim 1, wherein said
reinforcements are metallic fibers with fiber diameters of .o
slashed.20 .mu.m to .o slashed.100 .mu.m formed by the melt
extraction method and having irregular peripheral surfaces.
10. A metal matrix composite according to claim 2, wherein said
reinforcements are metallic fibers with fiber diameters of .o
slashed.20 .mu.m to .o slashed.100 .mu.m formed by the melt
extraction method and having irregular peripheral surfaces.
11. A metal matrix composite according to claim 1, wherein the
volume content of said reinforcements ranges from 10% to 40%.
12. A metal matrix composite according to claim 2, wherein the
volume content of said reinforcements ranges from 10% to 40%.
13. A metal matrix composite having a metallic matrix and
reinforcements mixed in said matrix, the metal matrix composite
characterized in that said reinforcements are metallic fibers of an
alloy consisting mainly of Fe and Cr and containing Al and/or Si,
the Cr content and the Al and/or Si content of said reinforcements
range from 5 to 30% and from 3 to 10%, respectively,
solution-treatment is carried out at temperatures of 470.degree. C.
to 500.degree. C. such that formation of intermetallic compounds on
the interfaces between said matrix and the reinforcements is
restrained, said reinforcements are metallic fibers with fiber
diameters of .o slashed.20 .mu.m to .o slashed.100 .mu.m formed by
the melt extraction method and having irregular peripheral
surfaces, the volume content of said reinforcements ranges from 10%
to 40%, and said reinforcements are preformed into a given shape by
sintering.
14. A piston including a metal matrix composite having a metallic
matrix and reinforcements mixed in said matrix, the piston
characterized in that said reinforcements are metallic fibers of an
alloy consisting mainly of Fe and Cr and containing Al and/or
Si.
15. A piston including a metal matrix composite having a metallic
matrix and reinforcements mixed in said matrix, the piston
characterized in that said reinforcements are metallic fibers of an
alloy consisting mainly of Fe and Cr and containing Al and/or Si,
and the Cr content and the Al and/or Si content of said
reinforcements range from 5 to 30% and from 3 to 10%,
respectively.
16. A piston according to claim 15, wherein solution-treatment is
carried out at temperatures of 470.degree. C. to 500.degree. C.
such that formation of reactants on the interfaces between said
matrix and the reinforcements is restrained.
17. A piston according to claim 15, wherein said reinforcements are
metallic fibers with fiber diameters of .o slashed.20 .mu.m to .o
slashed.100 .mu.m formed by the melt extraction method and having
irregular peripheral surfaces.
18. A piston according to claim 15, wherein said reinforcements are
preformed into a given shape by sintering.
19. A piston including a metal matrix composite having a metallic
matrix and reinforcements mixed in said matrix, the piston
characterized in that said reinforcements are metallic fibers of an
alloy consisting mainly of Fe and Cr and containing Al and/or Si,
the Cr content and the Al and/or Si content of said reinforcements
range from 5 to 30% and from 3 to 10%, respectively,
solution-treatment is carried out at temperatures of 470.degree. C.
to 500.degree. C. such that formation of reactants on the
interfaces between said matrix and the reinforcements is
restrained, said reinforcements are metallic fibers with fiber
diameters of .o slashed.20 .mu.m to .o slashed.100 .mu.m formed by
the melt extraction method and having irregular peripheral
surfaces, the volume content of said reinforcements ranges from 10%
to 40%, and said reinforcements are preformed into a given shape by
sintering.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is a Continuation Application of PCT Application No.
PCT/JP00/05373, filed Aug. 10, 2000, which was not published under
PCT Article 21(2) in English.
[0002] This application is based upon and claims the benefit of
priority from the prior Japanese Patent Applications No. 11-226359,
filed Aug. 10, 1999; and No. 2000-069001, filed Mar. 13, 2000, the
entire contents of both of which are incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0003] The present invention relates to a metal matrix composite,
including a light metal alloy, e.g., aluminum alloy, magnesium
alloy, etc., as a matrix (base metal), and a piston using the
same.
[0004] Conventionally, steel materials have been used as materials
for mechanical element components. However, light metal alloys,
such as Al (aluminum) alloy, Mg (magnesium) alloy, etc., are used
for components that require reduction in weight. For some of
components that require high-temperature strength, as well as
reduction in weight, moreover, a metal matrix composite (abbrev. as
MMC) may be used in the case where required characteristics cannot
be obtained with use of a simple light metal alloy with a low
melting point (i.e., with low high-temperature strength) or if the
required characteristics cannot be obtained with use of a simple
light metal alloy with poor wear resistance. The metal matrix
composite is composed of a metallic matrix and reinforcements.
Carbon fibers or ceramic fibers, such as SiC (silicon carbide),
Al.sub.2O.sub.3 (alumina), etc., are used for the reinforcements,
for example.
[0005] Components such as automotive parts and aircraft parts of
which the weight is closely associated with the fuel-efficiency
eagerly require reduction in weight. Materials for the components
of this type are being changed from the conventional steel over to
light metal alloys, such as Al alloy, Mg alloy, etc. To meet this
requirement, materials for internal-combustion engines that exposed
to high temperature and their peripheral parts (engine parts such
as pistons, cylinder heads, cylinder blocks, connecting rods, etc.)
are being changed over to light metal alloys. With the progress of
development of higher-output internal-combustion engines, however,
high-temperature strength and wear resistance have ceased to be
ensured with use of a simple light metal alloy with a low melting
point (i.e., with low high-temperature strength) or a simple light
metal alloy with poor wear resistance. The following is a
description of a piston for a diesel engine of an automobile as an
example.
[0006] Direct-injection engines have recently been becoming
prevailing. The load on the side of the combustion chamber of the
piston is expected to increase as the development of higher-output
versions will advance hereafter. The combustion chamber for forming
eddies of air called swirls is formed in an end face of the piston.
Since the edge (lip portion) that requires machining for finishing
is thin-walled, in particular, it is hard to secure satisfactory
fatigue strength in a high-temperature zone (e.g., at 300.degree.
C. or thereabout) with use of conventional aluminum alloys (AC8A,
etc.) for castings. The following is a description of the chemical
ingredients of AC8A. In this specification, the chemical
ingredients of the alloys are given by % by weight unless otherwise
specified.
[0007] Cu: 0.8 to 1.3
[0008] Si: 11.0 to 13.0
[0009] Mg: 0.7 to 1.3
[0010] Zn: 0.15 or less
[0011] Fe: 0.8 or less
[0012] Mn: 0.15 or less
[0013] Ni: 0.8 to 1.5
[0014] Ti: 0.20 or less
[0015] Pb: 0.05 or less
[0016] Sn: 0.05 or less
[0017] Cr: 0.10 or less
[0018] Al: Remainder
[0019] Composites that use these Al alloys as their base metal
(matrices) may possibly be subjected to surface treatment to
improve their high-temperature fatigue strength. Since the effect
of the strength improvement by the surface treatment is small,
however, a metal matrix composite (MMC) is expected to be used.
[0020] Feasible reinforcements for the metal matrix composite
include metallic fibers, carbon fibers, and ceramic fibers, and
besides, porous structures and whiskers (crystal whiskers) formed
of these materials, etc. Under the present conditions, fibers that
are used as the reinforcements of the metal matrix composite are
ceramic fibers, such as SiC, Al.sub.2O.sub.3, etc., and metallic
fibers have not reached the level of practical use yet. This is so
because no manufacturing technique has been established yet for
metallic fibers of fiber diameters (several micrometers to tens of
micrometers) that are required of reinforcements of a metal matrix
composite, so that low-cost metallic fibers to serve for practical
use cannot be obtained.
[0021] With the recent advance of performance that is required of
various apparatuses, in particular, there is a growing tendency for
higher fatigue strength or higher wear resistance to be required.
Metallic fibers that can meet this high-level requirement are very
hard and fragile, though not harder or more fragile than ceramics,
so that they cannot be manufactured by the conventional wire
drawing.
[0022] Usually, the casting method is used to compound a matrix and
reinforcements. In the casting method, a preform (preformed piece
previously molded to have a given shape and volume content) of
fibers that serve as reinforcements is set in a mold. Thereafter, a
molten matrix metal is poured into the mold. The preform is
compulsorily impregnated with the matrix metal under a given
pressure. A metal matrix composite is obtained by hardening the
matrix metal.
[0023] In the case where carbon fibers or ceramic fibers are used
for the reinforcements, they involve the following problems.
[0024] Carbon fibers and ceramic fibers have poor wettability with
a light metal alloy that forms a base metal (matrix). Therefore,
the light metal alloy of the matrix, e.g., Al alloy, fails to get
well into spaces between the fibers, so that a large number of
cavities (voids) are created inevitably. These defects lower the
initial strength of the metal matrix composite and worsen the
durability against corrosion or the like.
[0025] In order to improve the wettability with the matrix metal,
therefore, the surface quality of the reinforcements that are
formed of carbon fibers or ceramic fibers may be improved by
plating or the like. However, the improvement of the surface
quality requires many processes and much time, thus resulting in an
increase in cost. Metallic fibers have a great advantage over
carbon fibers and ceramic fibers with respect to the wettability
with the matrix metal. As mentioned before, however, metallic
fibers that are suited for reinforcements are expensive. It is hard
for fibers of relatively low-priced stainless steel (SUS) to
fulfill the high-level requirement for the high-temperature fatigue
strength, wear resistance, etc.
[0026] Moreover, a composite that uses carbon fibers or ceramic
fibers as its reinforcements must be preformed in order to prevent
deformation of the reinforcements during casting operation.
Preforming the carbon fibers or ceramic fibers requires a binder
for use as an adhesive agent, and this binder causes the
performance of the metal matrix composite to worsen.
[0027] A mold pressing method, extrusion molding method, and
centrifugal molding method are known methods for manufacturing a
preform with use of a binder. Any of these method requires many
processes including a process for loosening fibers, process for
applying the binder, temporary molding process, drying process,
sintering process, etc.
[0028] There is also a problem that the composite using carbon
fibers or ceramic fibers cannot be machined with ease. Primarily,
carbon fibers and ceramic fibers are unworkable materials, so that
it is natural that the metal matrix composite that uses them as its
reinforcements cannot be worked with ease. Accordingly, there is a
problem that the composite using carbon fibers or ceramic fibers
entails prolonged working time or requires an expensive cutting
tool. It is to be desired also in consideration of these
circumstances that metallic fibers should be used for the
reinforcements.
[0029] A metal matrix composite in which metallic fibers or ceramic
fibers for use as reinforcements are mixed in a matrix metal of Al
alloy has been developed as means for reducing the weight of and
enhancing the strength of an engine piston. The casting method is
adopted as a method for manufacturing the metal matrix composite of
this type. Normally, heat treatment is carried out to enhance the
mechanical strength of the composite after casting operation. There
are close relations between conditions for the heat treatment and
the chemical ingredients of the matrix (Al alloy). The Japanese
Industrial Standards (JIS.H5202) provide the heat treatment
conditions for the Al alloy of this type.
[0030] The aforesaid heat treatment includes a solution-treatment
process for solidly solving additive elements in the alloy at high
temperature and an age hardening process for extracting again the
additive elements that are conducive to the improvement of the
mechanical strength of the alloy after the solution-treatment
process. According to a study made by the inventors hereof,
however, it was recognized that the properties (e.g., fatigue
strength) of the metal matrix composite worsen if the aforesaid
heat treatment provided by JIS.H5202 is executed for the metal
matrix composite in which the reinforcements of metallic fibers are
mixed.
[0031] The aforesaid AC8A material that is a typical Al alloy for
casting, for example, is loaded with Si, Ni, Mg, Cu. etc. as
additive elements in order to restrain the coefficient of thermal
expansion and improve the mechanical strength. According to JIS,
heat treatment conditions for the AC8A material include 510.degree.
C. and 4 hours for the solution-treatment process and 170.degree.
C. and is 10 hours for the age hardening process. Hereinafter, this
heat treatment will be referred to as T6 treatment. The following
problems were aroused when the T6 process was applied to a metal
matrix composite.
[0032] Let it be supposed, for example, that reinforcements, formed
of FeCr metallic fibers of stainless steel or the like, are
compounded with an Al alloy (matrix) and subjected to the T6
treatment. In this case, reactions occur on the interfaces between
the matrix and the reinforcements, and intermetallic compounds such
as FeAl, FeAl.sub.3, etc. are formed. Although these intermetallic
compounds are very hard, they are fragile, so that the fatigue
strength of the composite is adversely affected.
[0033] On the other hand, the heat treatment is supposed to be
executed after ceramic fibers of B.sub.2Al.sub.2O.sub.6 or the
like, for use as reinforcements, are compounded. In this case,
reactions also occur on the interfaces between the matrix and the
reinforcements, and an oxide compound such as MgAl.sub.2O.sub.4 is
formed. Since this oxide compound, like the intermetallic
compounds, is very fragile, so that the fatigue strength or the
like of the composite is adversely affected.
[0034] The amount of formation of the intermetallic compounds of
the composite that uses the metallic fibers is much greater than
the amount of formation of the oxide compound obtained when the
ceramic fibers are used, and the level of the bad influence is
higher.
[0035] A method for coating the surfaces of the fibers used in the
reinforcements with a film that cannot easily react with the
matrix, e.g., an Al.sub.2O.sub.3 film that is chemically stable,
can be adopted as means for solving the above problems. However,
this method is not preferable because of its high cost.
[0036] Accordingly, the object of the present invention is to
provide a metal matrix composite, using metallic fibers for
reinforcements and enjoying excellent strength, wear resistance,
etc. without subjecting the reinforcements to any surface treatment
such as coating, and a piston using the same.
BRIEF SUMMARY OF THE INVENTION
[0037] In order to achieve the above object, according to the
present invention, there is provided a metal matrix composite
having a metallic matrix and reinforcements mixed in the matrix, in
which the reinforcements are formed of an alloy consisting mainly
of Fe and Cr and containing Al and/or Si. The Cr content and the Al
and/or Si content of metallic fibers that meet the object of the
present invention range from 5 to 30% and from 3 to 10%,
respectively.
[0038] Fe (iron), Ni (nickel), or Ti (titanium) may possibly be
used as a metal to serve as the base of an alloy that constitutes
the reinforcements (metallic fibers) of the present invention.
Since Ni and Ti are too expensive to be adopted, however, Fe is
used as the base metal. The oxidation resistance is improved by
adding Cr. Normally, compounding a matrix metal and reinforcements
requires preheating of preformings of the reinforcements.
[0039] The preheating is carried out in order to improve the
wettability with the matrix metal first. The higher the
temperature, in general, the better the wettability of
reinforcements with a matrix is. In the process of cooling the
matrix metal and the reinforcements after they are joined together,
defects are liable to be caused if the difference in shrinkage
between them is great, so that preheating is required. In the case
of ceramic fibers, preheating to 600.degree. C. to 800.degree. C.
is necessary in order to prevent occurrence of defects, even if the
surface quality is improved by plating or the like.
[0040] On the other hand, metallic fibers is higher in wettability
with the matrix metal than ceramic fibers, and their coefficient of
thermal expansion is relatively close to that of the matrix metal.
Accordingly, the metallic fibers have an advantage over the ceramic
fibers in being satisfactorily preheated to a lower temperature
(500.degree. C. or below) than the ceramic fibers is. However, the
metallic fibers have a problem of oxidation by preheating.
[0041] For example, an oxide film may possibly be formed on the
surface of each metallic fiber during the preheating process. If
this oxide film is an oxide of Fe (Fe.sub.2O.sub.3), the
wettability with the matrix metal is poor. Thus, the matrix metal
cannot easily get into spaces between the fibers. Since the oxide
film easily separates from each metallic fiber, moreover, defects
are caused. In order to improve the oxidation resistance during the
preheating process, according to the present invention, therefore,
oxidation of the base metal (Fe) of the reinforcements is prevented
by adding Cr.
[0042] The inventors hereof manufactured a plurality of types of
test pieces with varied quantities of Cr added to the base metal
(Fe), by arc solution-treatment, and conducted an oxidation test.
According to the method of the oxidation test, the manufactured
test pieces were left to stand for two hours in the atmosphere in
electric ovens at different ambient temperatures. After these test
pieces were taken out of the electric ovens, the colors of the
respective surfaces of the test pieces were visually observed and
further observed by means of an electron microscope (SEM), and the
surface conditions were checked to see if they were changed by
heating. By an analysis by means of an EDX (energy dispersed X-ray
spectrometer), moreover, the presence of oxides in the test pieces
was examined.
[0043] The results of the above examinations are shown in TABLE 1.
Based on these examinations, it was confirmed that an oxidation
preventing effect is produced with the Cr content at 5% or more. A
very small quantity of oxygen was detected in the EDX analysis with
the Cr content at 5%. However, this is negligible because it is a
dense, very thin Cr oxide that has good wettability with the matrix
metal and good adhesion to the metallic fibers. Preferably,
therefore, the Cr content should be 5% or more. In order to improve
the safety (reliability) further, however, the Cr content should
preferably be 10% or more.
1TABLE 1 Cr Surface content after heat Observation (%) treatment by
SEM EDX analysis 1 Turned dark Degenerated Oxygen detected blue 3
Turned Degenerated Oxygen detected light blue 5 No change No change
Oxygen (very by heat by heat small quantity) treatment treatment
detected 10 No change No change No oxygen by heat by heat detected
treatment treatment 20 No change No change No oxygen by heat by
heat detected treatment treatment 30 No change No change No oxygen
by heat by heat detected treatment treatment
[0044] Performance that meets high-level requirements for the
high-temperature fatigue strength and wear resistance cannot be
obtained by only adjusting the Cr content to the aforesaid value
(5% or more). According to the present invention, therefore, Al
and/or Si that is low-priced is added as an element for improving
the strength, hardness, and thermal resistance. Test pieces for
which the quantity of Al or Si added to FeCr for use as a base
metal is changed variously were manufactured by arc
solution-treatment, and the oxidation resistance was evaluated by
carrying out the same oxidation test as aforesaid. At the same
time, a tensile test on the test pieces was conducted at the
ambient temperature of 300.degree. C. The ambient temperature of
300.degree. C. is equivalent to the working atmosphere temperature
of internal-combustion engine parts (e.g., pistons, etc.). Further,
the degree of difficulty of fiberization based on drawing of the
alloy material and the degree of difficulty of fiberization by the
melt extraction method were examined.
[0045] TABLE 2 shows the results of the above tests for the case
where Si was added. The same tendency was observed for the case
where Al was added. In TABLE 2, .largecircle., .DELTA., and x
represent good, passable, and failure, respectively.
2TABLE 2 Tensile Fiberization Cr Si strength (melt content content
Oxidation (at 300.degree. C.) Fiberization extraction [%] [%]
resistance [MPa] (drawing) method) 5 1 .smallcircle. 702
.smallcircle. .smallcircle. 3 .smallcircle. 802 .smallcircle.
.smallcircle. 5 .smallcircle. 917 .DELTA. .smallcircle. 10
.smallcircle. 991 X .smallcircle. 15 .smallcircle. 1010 X X 10 1
.smallcircle. 710 .smallcircle. .smallcircle. 3 .smallcircle. 815
.smallcircle. .smallcircle. 5 .smallcircle. 932 .DELTA.
.smallcircle. 10 .smallcircle. 1003 X .smallcircle. 15
.smallcircle. 1033 X X 20 1 .smallcircle. 715 .smallcircle.
.smallcircle. 3 .smallcircle. 830 .DELTA. .smallcircle. 5
.smallcircle. 951 X .smallcircle. 10 .smallcircle. 1015 X .DELTA.
15 .smallcircle. 1064 X X 30 1 .smallcircle. 736 .DELTA.
.smallcircle. 3 .smallcircle. 842 X .smallcircle. 5 .smallcircle.
972 X .DELTA. 10 .smallcircle. 1022 X .DELTA. 15 .smallcircle. 1082
X X 40 1 .smallcircle. 745 X X 3 .smallcircle. 849 X X 5
.smallcircle. 988 X X 10 .smallcircle. 1030 X X 15 .smallcircle.
1092 X X
[0046] As seen from TABLE 2, metallic fibers that can display
higher strength (800 MPa or more) than stainless steel and can be
fiberized preferably have Cr contents of 5 to 30% and Al and/or Si
contents of 3 to 10%.
[0047] TABLE 3 shows the diameters and cuttability of metallic
fibers that can be manufactured by the melt extraction method.
Preferably, as shown in TABLE 3, the lower limit value of the fiber
diameter should not be lower than .o slashed.20 .mu.m, which is the
lower limit for the fiberization by the melt extraction method. As
for the upper limit value of the fiber diameter, it is expected to
be not higher than .o slashed.100 .mu.m in consideration of the
post-workability (cuttability) after compounding. Symbol .o
slashed.1 represents the diameter of each fiber. The cross section
of each reinforcement (metallic fiber) may be perfectly circular.
Preferably, however, the cross section should be irregular in the
circumferential direction, like those of the metallic fibers that
are manufactured by the melt extraction method, since the bite
(anchor effect) on the matrix is then improved. By the melt
extraction method, moreover, even metallic fibers that are formed
of an unworkable material can be manufactured at relatively low
costs.
3TABLE 3 Application of Metallic fiber melt extraction diameter
[.mu.m] method Machinability .phi. 10 Inapplicable -- .phi. 20
Applicable Non-defective .phi. 30 Applicable Non-defective .phi. 40
Applicable Non-defective .phi. 50 Applicable Non-defective .phi. 60
Applicable Non-defective .phi. 70 Applicable Non-defective .phi. 80
Applicable Non-defective .phi. 90 Applicable Non-defective .phi.
100 Applicable Non-defective .phi. 110 Applicable Cracked .phi. 120
Applicable Cracked .phi. 130 Applicable Cracked and chipped
[0048] According to the present invention, there may be obtained a
metal matrix composite that enjoys outstanding fatigue strength and
wear resistance at high temperature, in particular. Since the metal
matrix composite of the present invention is dissolved at
470.degree. C. to 500.degree. C., reactants such as intermetallic
compounds can be restrained from being formed on the interfaces
between the matrix and the reinforcements, and the fatigue strength
can be improved further.
[0049] Since the metal matrix composite of the present invention is
substantially formed of metals only, it can be recycled with ease.
Since the respective mechanical properties of the matrix and the
reinforcements are relatively similar, moreover, cutting that is
carried out after casting is easy, and the working time and working
cost can be reduced considerably. Since the reinforcements
(metallic fibers) can be preformed without using any binder,
furthermore, an application process, temporary molding process,
drying process, etc. for a binder can be omitted. A piston that
uses the metal matrix composite of the present invention for a lip
portion of its combustion chamber has excellent machinability and
enhanced high-temperature strength, and can be recycled.
[0050] Additional objects and advantages of the invention will be
set forth in the description which follows, and in part will be
obvious from the description, or may be learned by practice of the
invention. The objects and advantages of the invention may be
realized and obtained by means of the instrumentalities and
combinations particularly pointed out hereinafter.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0051] The accompanying drawings, which are incorporated in and
constitute a part of the specification, illustrate presently
preferred embodiments of the invention, and together with the
general description given above and the detailed description of the
preferred embodiments given below, serve to explain the principles
of the invention.
[0052] FIG. 1 is a perspective view, partially in section, showing
a piston using a metal matrix composite according to one embodiment
of the present invention;
[0053] FIG. 2 is an enlarged sectional view showing a part of the
metal matrix composite shown in FIG. 1;
[0054] FIG. 3A is a sectional view of a reinforcement used in the
metal matrix composite shown in FIG. 1;
[0055] FIG. 3B is a sectional view showing another example of the
cross-sectional shape of the reinforcement;
[0056] FIG. 4 is a vertical sectional view of a metallic fiber
manufacturing apparatus for carrying out the melt extraction
method;
[0057] FIG. 5 is a sectional view of the metallic fiber
manufacturing apparatus taken along line F5-F5 of FIG. 4;
[0058] FIG. 6A is a perspective view of a preform used in the metal
matrix composite shown in FIG. 1;
[0059] FIG. 6B is a perspective view showing another example of the
preform;
[0060] FIGS. 7A to 7D are sectional views showing a mold and a
preform for manufacturing a piston including the metal matrix
composite shown in FIG. 1 in manufacturing processes,
individually;
[0061] FIG. 8 is a diagram showing relations between the volume
content of metallic fibers and the time of breakdown of the metal
matrix composite;
[0062] FIG. 9 is a sectional view of a casting mold for piston
molding and a preform held therein;
[0063] FIG. 10 is a sectional view of a semi-finished piston cast
by means of the casting mold shown in FIG. 9;
[0064] FIG. 11 is a sectional view of a piston obtained by
machining the semi-finished piston shown in FIG. 10;
[0065] FIG. 12 is a diagram showing relations between
solution-treatment temperature for an AC8A material and the metal
matrix composite and Rockwell B hardness (HRB);
[0066] FIG. 13 is a diagram showing relations between
solution-treatment temperature for the AC8A material and the metal
matrix composite and number of cycles to failure;
[0067] FIG. 14 is a 1,500-magnification photomicrograph of a cross
section of the metal matrix composite before heat treatment;
[0068] FIG. 15 is a 1,500-magnification photomicrograph of a cross
section of the metal matrix composite having cavities attributable
to solution-treatment at 510.degree. C.;
[0069] FIG. 16 is a 500-magnification photomicrograph of a cross
section of the metal matrix composite was performed
solution-treatment at 490.degree. C.;
[0070] FIG. 17 is a 500-magnification photomicrograph of a cross
section of the metal matrix composite was performed
solution-treatment at 500.degree. C.; and
[0071] FIG. 18 is a 500-magnification photomicrograph of a cross
section of the metal matrix composite with intermetallic compounds
formed by solution-treatment at 510.degree. C.
DETAILED DESCRIPTION OF THE INVENTION
[0072] One embodiment of the present invention will now be
described with reference to FIGS. 1 to 8.
[0073] FIG. 1 shows an example of a piston 1 for an
internal-combustion engine. A combustion chamber 2, a recess for
producing air currents such as swirls, is formed in a top face 1a
of the piston 1 by machining. That portion of the piston 1 which is
relatively thin and is exposed to high temperature, that is, a
portion including a lip portion 3 of the combustion chamber 2, is
composed of a metal matrix composite 4. The piston 1 is connected
to a connecting rod 6 (only a part of which is shown) by means of a
piston pin 5. The piston pin 5 is inserted in a piston pin socket
hole 7.
[0074] The metal matrix composite 4, a part of which is shown in
the enlarged view of FIG. 2, includes a matrix 10 formed of an
aluminum alloy and reinforcements 11 formed of metallic fibers with
diameters of .o slashed.20 .mu.m to 100 .mu.m mixed in the matrix
10. The material of the reinforcements 11 is an alloy that consists
mainly of Fe and Cr and contains Al and/or Si. The Cr content of
the reinforcements 11 ranges from 5 to 30% for the aforesaid
reason, and the Al and/or Si content ranges from 3 to 10%.
[0075] If the reinforcements (metallic fibers) 11 are manufactured
by the melt extraction method described below, the cross section of
each reinforcement 11 is not perfectly circular and has
irregularity in the circumferential direction. As shown in FIGS. 3A
and 3B, for example, a V-shaped depression 12 or a flat portion 13
is formed on the peripheral surface of each reinforcement 11. The
diameter is given by d=(d1+d2)/2. These reinforcements 11 are
directed at random as they are mixed in the matrix 10. If the
composite 4 is cut in the manner shown in FIG. 2, the respective
cross sections of the reinforcements 11 on their cut surfaces have
various shapes. If the irregular reinforcements 11 are used in this
manner, the bond strength of the matrix 10 and the reinforcements
11 is enhanced owing to an anchor effect of the reinforcements 11
in the matrix 10.
[0076] The piston 1 is manufactured undergoing a metallic fiber
manufacturing process for obtaining the reinforcements 11,
preforming process, casting process, and machining process. In the
metallic fiber manufacturing process, the reinforcements 11 are
manufactured by the melt extraction method using a metallic fiber
manufacturing apparatus 20 that is schematically shown in FIGS. 4
and 5. The manufacturing apparatus 20 comprises an apparatus body
portion 22, which includes a chamber 21, and a material supply
mechanism 23, a metallic fiber recovery portion 24, etc. that are
attached to the apparatus body portion 22. Arranged in the chamber
21 are a holder 31 for holding a rod-shaped material metal 30 as a
material of the reinforcements 11, a high-frequency induction coil
32 that serves to melt the upper end portion of the material metal
30, thereby forming a molten metal 30a, and a disk 34 that is
rotated in a fixed direction (direction indicated by arrow R in
FIG. 4) around a shaft 33.
[0077] The disk 34 is formed of a metal with high thermal
conductivity, such as copper or copper alloy, or a high-melting
point material, such as molybdenum or tungsten, and has a
peripheral edge 35 that is brought into contact with the molten
metal 30a. If the disk 34 is viewed sideways, as shown in FIG. 5,
the peripheral edge 35 of the disk 34 forms a V-shaped sharp edge
that covers the whole circumference of the disk 34. The disk 34 is
rotated at high speed by means of a rotating mechanism 36.
[0078] An unoxidized atmosphere generator 41 is attached to the
chamber 21 so that a vacuum atmosphere (decompressed atmosphere,
exactly) or an unoxidized atmosphere, such as an inert gas, can be
kept in the chamber 21. The apparatus 41 is provided with an on-off
valve 40, a vacuum pump or inert gas source, etc. A high-frequency
generator 46 is connected to the high-frequency induction coil 32
through a current control device 45 (shown in FIG. 5). Further
provided is a radiation thermometer 47 for detecting the
temperature of the molten metal 30a in a noncontact manner. The
radiation thermometer 47 is connected electrically to the
high-frequency generator 46 through the current control device
45.
[0079] In the metallic fiber manufacturing apparatus 20 constructed
in this manner, the disk 34 is rotated at a given peripheral speed
by means of the rotating mechanism 36. As the material metal 30
held by means of the holder 31 is gradually pushed up by means of
the material supply mechanism 23, moreover, the upper end portion
of the material metal 30 moves to the level of the high-frequency
induction coil 32.
[0080] Then, the upper end portion of the material metal 30 is
heated by means of the high-frequency induction coil 32, whereupon
the molten metal 30a is formed on the upper end of the material
metal 30. The temperature of the molten metal 30a is continually
detected by means of the radiation thermometer 47. As a detection
signal from the thermometer 47 is fed back to the high-frequency
generator 46, the output of the high-frequency generator 46 is
adjusted, and the temperature of the molten metal 30a is kept
constant.
[0081] The molten metal 30a, which is brought into contact with the
sharp peripheral edge 35 of the disk 34, is rapidly cooled to be
solidified as the disk 34 rotates, and at the same time,
continuously flies as metallic fibers (reinforcements 11) with
diameters of 20 .mu.m to 100 .mu.m in the tangential direction. The
metallic fibers (reinforcements 11) are introduced into the
metallic fiber recovery portion 24. Since the material metal 30 is
gradually pushed up by means of the material supply mechanism 23 as
the quantity of the molten metal 30a decreases, the state of
contact between the peripheral edge 35 of the disk 34 and the
molten metal 30a can be kept constant.
[0082] The cross section of each reinforcement 11 manufactured by
the melt extraction method has irregularity in the circumferential
direction, as shown in FIG. 3A or 3B, for example, depending on the
state of the disk 34 or the molten metal 30a. In some cases,
moreover, the cross-sectional shape of each reinforcement 11 may
vary in the longitudinal direction.
[0083] In the preforming process, a disk-shaped preform 11a, such
as the one shown in FIG. 6A, or a ring-shaped preform 11a', such as
the one shown in FIG. 6B, is obtained by preforming the
reinforcements 11 into a desired shape by using suitable forming
means such as sintering. These preforms 11a and 11a' are compressed
into a desired cubic shape so that the reinforcements 11 are
intertwined with one another, and are heated so that nodes of the
fibers are sintered. Thus, the preforms 11a and 11a' in the form of
porous blocks with stable shapes are obtained. For example, a
preform with a desired shape can be obtained by forming fleecy webs
(fibrous aggregates like nonwoven fabric) from the fibrillated
reinforcements 11, superposing these webs in tens of layers, and
compressively sintering them.
[0084] The casting process is carried out using a casting mold 61
with a heater 60 for preheating and a heater 62 for heating the
preform 11a, as shown in FIG. 7A. The casting mold 61 is preheated
in advance to a given temperature by means of the heater 60. As
shown in FIG. 7B, the preform 11a is loaded into the casting mold
61, and a molten alloy 10a for a matrix is poured into the casting
mold 61, whereby the preform 11a is impregnated with the molten
alloy 10a.
[0085] Thereafter, the molten alloy 10a is pressurized under a
pressure P of, e.g., 100 MPa by means of a pressure member 63, as
shown in FIG. 7C. As the molten alloy 10a penetrates into spaces
between the reinforcements 11 of the preform 11a and hardens, a
semi-finished piston 1' is obtained partially having the metal
matrix composite 4, as shown in FIG. 7D. The volume content (Vf) of
the reinforcements 11 in the metal matrix composite 4 ranges from
10% to 50%, and preferably from 10% to 30%.
[0086] The combustion chamber 2 having the lip portion 3 with a
desired shape is formed as the obtained semi-finished piston 1' is
machined (mainly cut) in the machining process. Further, the top
face la of the piston 1 and the outer peripheral surface of the
piston 1 are finished, and the piston pin socket hole 7 is
machined.
[0087] Metallic fibers with diameters of less than .o slashed.20
.mu.m cannot be easily manufactured by the aforesaid the melt
extraction method. If the diameter is not smaller than .o
slashed.100 .mu.m, moreover, the molten metal of the matrix cannot
be loaded well into the gaps between the fibers (reinforcements),
so that defects (voids) are liable to be caused between the matrix
and the reinforcements. If the diameter exceeds .o slashed.100
.mu.m, furthermore, the influence of the reinforcements, compared
with that of the matrix, becomes too great when the composite is
machined (or cut), so that it is hard to set working
conditions.
[0088] FIG. 8 shows the result of examination of the relation
between the volume content (Vf) of the reinforcements 11 of the
metal matrix composite 4 and the time of breakdown. The volume
content (Vf) is a value that is given by
Vf=(V.sub.2/V.sub.1).times.100 (%), where V.sub.1 and V.sub.2 are
the total volume of the metal matrix composite 4 and the volume of
the reinforcements 11, respectively. Aluminum alloy AC8A
(JIS.H5202) for casting was used as the material of the matrix 10
and subjected to heat treatment T6 (JIS.H5202) after casting
operation. FeCrSi alloy (Cr: 20%, Si: 5%) was used for the
reinforcements 11, which were manufactured by the aforementioned
the melt extraction method. This metal matrix composite 4 was
subjected to a rotary bending test (JIS.Z2274) at an ambient
temperature of 300.degree. C. and under repeated stress of 60 MPa.
A metal matrix composite as a comparative example is a metallic
fiber that is composed of AC8A as its matrix and stainless steel
(SUS 304) as its reinforcements.
[0089] If the volume content of the reinforcements reaches 10% or
more, as shown in FIG. 8, both the product of the present invention
and the comparative example enjoy the development of an effect on
their time of breakdown. While the time of breakdown of the
comparative example is about 10.sup.6 cycles when the volume
content is not lower than 20%, however, the time of breakdown of
the metal matrix composite 4 of the present invention, which uses
the reinforcements 11 of FeCr(Al, Si) alloy, is 10.sup.7 or more,
displaying a considerable improvement. If the content of the
reinforcements 11 exceeds 40%, cavities (voids) are created while
the matrix 10 is being cast. Therefore, 40% is the upper limit of
the content of the reinforcements 11.
[0090] FIGS. 9 to 11 show processes for manufacturing the piston 1
using the ring-shaped preform 11a' shown in FIG. 6B. After the
preform 11a' is loaded into a casting mold 61', in the casting
process shown in FIG. 9, a molten Al alloy to form the matrix 10 is
poured into the casting mold 61'. The molten Al alloy poured into
the casting mold 61' hardens and forms a piston body portion 1b. At
the same time, the molten Al alloy penetrates into spaces between
the reinforcements 11 of the preform 11a' and hardens, whereupon
the metal matrix composite 4 is formed. In this manner, the
semi-finished piston 1' that includes the metal matrix composite 4,
as shown in FIG. 10, is manufactured.
[0091] The semi-finished piston 1' is formed with the combustion
chamber 2 that has the lip portion 3 with the desired shape as its
workable portion including the metal matrix composite 4 is machined
in the machining process, as shown in FIG. 11. Further, the top
face la of the piston 1 and a piston periphery 1c are finished, and
the piston pin socket hole 7 is machined.
[0092] In the process of developing the metal matrix composite 4,
the inventors hereof conducted various tests for intermetallic
compounds that are formed on the interfaces between the matrix 10
and the reinforcements 11. The matrix of the metal matrix composite
used in the tests is AC8A, and the reinforcements are metallic
fibers (diameter: about .o slashed.30 .mu.m, volume content Vf:
20%) that are formed of FeCrSi alloy (Cr: 20%, Si: 5%) manufactured
by the melt extraction method.
[0093] First, in the case where the conventional heat treatment
conditions (T6 treatment) were carried out, the composition of the
composite was observed before heat treatment, after
solution-treatment, and after age hardening. FIG. 14 is a
1,500-magnification photomicrograph of a cross section of the metal
matrix composite before heat treatment. A substantially circular
cross section appearing in the center of FIG. 14 indicates a
reinforcement. Very minute intermetallic compounds are observed
around this reinforcement. The presence of these minute
intermetallic compounds implies that a reaction is already caused
on the interface between the matrix and the reinforcement when the
matrix and the reinforcement are compounded (in the casting process
of the matrix). Since the quantity of the intermetallic compounds
is very small, however, they never have any bad influences upon the
properties of the composite.
[0094] FIG. 15 is a photomicrograph (power: 1,500 magnifications)
obtained after the composite was performed solution-treatment at
510.degree. C. As shown in FIG. 15, the whole circumference of a
reinforcement is covered with a large quantity of intermetallic
compounds. These intermetallic compounds are supposed to be grown
versions of the minute intermetallic compounds shown in FIG. 14 as
nuclei. Further, creation of cavities (voids) in the matrix was
recognized. The cavities are supposed to have been created because
the growth of the intermetallic compounds, which are denser than
the matrix, caused a substantial change of density in the matrix.
Thus, the abundant intermetallic compounds that cover the whole
circumference of the reinforcement are so fragile that they are
expected to exert a bad influence upon the fatigue strength of the
composite. Further, the creation of the cavities may possibly
adversely affect the composite. No growth of the intermetallic
compounds was observed in the age hardening (at 170.degree. C. for
10 hours).
[0095] The inventors hereof examined temperatures at which the
intermetallic compounds grow in the metal matrix composite. FIG. 16
is a photomicrograph (500 magnifications) obtained when the
composite was performed solution-treatment at 490.degree. C. When
the solution-treatment temperature was at 490.degree. C., as shown
in FIG. 16, neither intermetallic compounds nor cavities were
observed at all around the reinforcement.
[0096] FIG. 17 is a photomicrograph (500 magnifications) obtained
when the composite was performed solution-treatment at 500.degree.
C. When the solution-treatment temperature was at 500.degree. C. or
thereabout, as shown in FIG. 17, a slight growth of the
intermetallic compounds was recognized. However, these
intermetallic compounds are not all, and creation of cavities was
not recognized. FIG. 18 is a photomicrograph (500 magnifications)
obtained when the composite was performed solution-treatment at
510.degree. C. When the solution-treatment temperature was at
510.degree. C., as shown in FIG. 18, growth of the intermetallic
compounds covering the whole circumference of the reinforcement was
recognized.
[0097] The inventors hereof examined the way the hardness of the
composite changes depending on the solution-treatment temperature.
The result is shown in FIG. 12. The axis of ordinate of FIG. 12
represents Rockwell B hardness (test load: 100 kg). When the
solution-treatment temperature was at 470.degree. C. or above, as
shown in FIG. 12, higher hardness than that of an AC8A material
that contains no reinforcements was obtained. The hardness
drastically increased at 510.degree. C. or thereabout, since a
plenty of intermetallic compounds were produced. The intermetallic
compounds constitute a factor that exerts a bad influence upon the
fatigue strength of the metal matrix composite. The metal matrix
composite can obtain its maximum hardness at temperatures of about
490.degree. C. to 500.degree. C. except for the temperature
(510.degree. C.) at which the intermetallic compounds grow.
[0098] When the solution-treatment temperature is at the
conventional level of 510.degree. C. or above, as described above,
the intermetallic compounds grow and inevitably exert a bad
influence upon the composite. If the solution-treatment temperature
is lower than 470.degree. C., however, the reinforcements cannot
provide the effect of improvement in strength. Thus, it is
concluded that the solution-treatment temperature preferably ranges
from 470.degree. C. to 500.degree. C., and especially, from
490.degree. C. to 500.degree. C.
[0099] In order to examine the durability of the composite
described above, the rotary bending fatigue test (JIS.Z2274) was
conducted at the ambient temperature of 300.degree. C. and under
repeated stress of 60 MPa. The matrix of the composite used in the
test is AC8A, and the reinforcements are metallic fibers (diameter:
about .o slashed.30 .mu.m, volume content Vf: 20%) that are formed
of FeCrSi alloy (Cr: 20%, Si: 5%). As shown in FIG. 13, the
composite was performed solution-treatment at 490.degree. C.
enjoyed about 10 times as high durability as that of the AC8A
material that contains no reinforcements. For these reasons, it was
confirmed that by keeping the solution-treatment temperature of the
metal matrix composite within the aforesaid appropriate range
(470.degree. C. to 500.degree. C.), the formation of intermetallic
compounds was able to be restrained without subjecting the
reinforcements to any surface treatment such as coating, and the
fatigue strength of the metal matrix composite was improved.
[0100] As is evident from the above description, the metal matrix
composite of the present invention can be suitably used in
components that require reduction in weight, high-temperature
strength, etc., including engine parts such as pistons of
internal-combustion engines. It is to be understood, in carrying
out this invention, that the elements that constitute the present
invention, including the matrix and the reinforcements, can be
suitably modified according to the applications of the metal matrix
composite.
[0101] Additional advantages and modifications will readily occur
to those skilled in the art. Therefore, the invention in its
broader aspects is not limited to the specific details and
representative embodiments shown and described herein. Accordingly,
various modifications may be made without departing from the spirit
or scope of the general inventive concept as defined by the
appended claims and their equivalents.
* * * * *