U.S. patent application number 09/793118 was filed with the patent office on 2001-10-18 for metal porous preform and manufacturing process for metal composite member using the same.
Invention is credited to Oda, Nobuyuki.
Application Number | 20010030035 09/793118 |
Document ID | / |
Family ID | 18609077 |
Filed Date | 2001-10-18 |
United States Patent
Application |
20010030035 |
Kind Code |
A1 |
Oda, Nobuyuki |
October 18, 2001 |
Metal porous preform and manufacturing process for metal composite
member using the same
Abstract
Provided are a Fe-based metal porous preform capable of ensuring
sufficient adhesiveness between a molten metal and a base material
thereof even when a casting process having a time lag from
completion of pouring of the molten metal till pressure
impregnation is applied; and a manufacturing process for a metal
composite member using such a preform. The Fe-based metal porous
preform is a metal porous preform adopted in a casting process
having a prescribed time lag from completion of pouring of a molten
metal till impregnation of the molten metal when the metal porous
preform set inside a mold is impregnated with the molten metal, and
characterized by that the metal porous preform is made from an
iron-based metal as a base, including chromium in the range of 10
to 40 wt % as a content.
Inventors: |
Oda, Nobuyuki; (Hiroshima,
JP) |
Correspondence
Address: |
NIXON PEABODY, LLP
8180 GREENSBORO DRIVE
SUITE 800
MCLEAN
VA
22102
US
|
Family ID: |
18609077 |
Appl. No.: |
09/793118 |
Filed: |
February 27, 2001 |
Current U.S.
Class: |
164/100 |
Current CPC
Class: |
B22F 3/114 20130101;
F05C 2253/16 20130101; B22D 19/0027 20130101; B22D 27/13 20130101;
B22F 3/26 20130101; B22D 19/14 20130101; B22F 2998/00 20130101;
B22F 2998/00 20130101; F16C 7/023 20130101 |
Class at
Publication: |
164/100 |
International
Class: |
B22D 019/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 20, 2000 |
JP |
2000-093965 |
Claims
What is claimed is:
1. A metal porous preform adopted in a casting process having a
prescribed time lag from completion of pouring of a molten metal
till impregnation of the molten metal when the metal porous preform
set inside a mold is impregnated with the molten metal, wherein the
metal porous preform is made from iron or an iron-based metal as a
base material, including chromium.
2. The metal porous preform according to claim 1, wherein a
chromium content is in the range of 10 to 40 wt %.
3. A manufacturing process for a metal composite member including a
casting process having a prescribed time lag from completion of
pouring of a molten metal till impregnation of the molten metal
when a metal porous preform set inside a mold is impregnated with
the molten metal, comprising the steps of: forming the metal porous
preform, made from iron or an iron-based metal as a base material,
and including chromium; setting the metal porous preform inside a
mold at a prescribed position; and thereafter, pouring the molten
metal into the mold to impregnate the metal porous preform with the
molten metal under pressure, whereby a composite section is formed
in which a casting material and the preform are included.
4. The manufacturing process for a metal composite member according
to claim 3, wherein pores in the preform are filled with the molten
metal by pressurizing the molten metal in the mold with a gas
pressure after pouring of the molten metal is completed and a sprue
of the mold is closed, in the casting process.
5. The manufacturing process for a metal composite member according
to claim 3, wherein the metal porous preform is manufactured by
means of a sintering process.
6. The manufacturing process for a metal composite member according
to claim 3, wherein addition of chromium is performed by means of a
chromizing treatment.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a porous preform made from
metal and a manufacturing process for a metal composite member
using such a preform as a composite forming material.
[0003] 2. Description of the Related Art
[0004] As has been well known, a light alloy such as aluminum alloy
is often used in automotive engine parts and others because of
light weight and good thermal conductivity, but has week points
such as poor heat resistance and low wear resistance as compared
with an iron-base material such as cast iron and steel. Hence, for
example, in a case of an engine piston for use in a diesel engine,
a method or a technique has been developed of forming a composite
reinforcement section in the periphery of a top ring groove using a
metal porous material such as made from nickel in order to
reinforce the top ring groove which requires higher wear
resistivity than the other sections of the piston (see Japanese
Patent Publication No. HEI 2-30790 which corresponds to U.S. Patent
No. 4,966,221).
[0005] As a process for forming such a composite reinforcement
section, a so-called high-pressure casting process was generally
employed in the prior art for the purpose to impregnate molten
aluminum alloy into pores of a preform constituted of a metal
porous material.
[0006] This high-pressure casting process proceeds like this: a
preform made of a reinforcement material having a prescribed
porosity is set inside a mold cavity, molten aluminum alloy is
poured into the mold cavity and thereafter, a high pressure is
applied on the molten aluminum alloy, for example, at the order of
29.4 to 147 MPa by mechanical means such as a pressure punch or a
plunger and the pressure is kept in the state till full
solidification of the molten aluminum alloy.
[0007] In this high-pressure casting process, however, problems
have been present in the actual use since a high pressure is
employed in forming a composite, which are:
[0008] 1. since a pressurizing mechanism and a strong mold clamping
mechanism for applying a high pressure are required, the casting
facility becomes to be large in scale and highly expensive;
[0009] 2. since a molten metal is apt to be infiltrated into a core
at least partially and the core itself is apt to be deformed or
broken by applying a high pressure, it is required to ensure a
sufficient strength of the core and particularly, there is
difficulty in actual use of a disintegrable core (for example, a
salt core and a sand core) for this case; and
[0010] 3. since a metal mold itself is necessary to endure a high
pressure and a mold construction also has many constraints, a
freedom in shape of a product is low, in other words this process
is not suited for manufacture of a product of a complex shape or a
large size.
[0011] Therefore, the applicant of the present application has
proposed a process and apparatus for manufacturing a light alloy
composite material which includes a so-called gas pressure casting
process as a technical base constituted by using pressure means
whose medium is a gas in a normal, metal mold gravity casting
process so as to be able to form a composite reinforcement section
(for example, see Japanese Patent Laid-Open Publication No. HEI
9-122887 which corresponds to U.S. Patent No. 6,035,923).
[0012] In a case of this gas pressure casting process, the process
proceeds this way: a composite forming material including pores is
held inside a mold cavity; a molten metal of a light alloy is
poured into the mold cavity; thereafter, a sprue of the mold is
closed; in that state, the molten metal in the mold cavity is
pressurized by a gas in order to fill the pores of the composite
forming material with the molten metal, with the result that a
composite section in which the light alloy and the composite
forming material are included is formed. Hence, no large scale,
expensive casting facilities as were used in a prior art high
pressure casting process are necessary, which makes it possible to
solve the problems as shown in items 1 to 3 above.
[0013] In the mean time, it is most general that a metal porous
preform for use in forming a composite reinforcement section is
obtained such that electroplating is applied on foamed resin and
thereafter, the resin foam is eliminated to leave a porous core. A
main stream of metal porous preforms obtained by means of such a
manufacturing process is one manufactured using nickel (Ni) or an
alloy thereof which can be electroplated with ease as a base
thereof, since electroplating is adopted in the process.
[0014] Hence, such a metal porous material is more expensive as a
reinforcement material in comparison with that made of cast iron
and thus obtained composite material is poor in machinability,
which leads to complexity in processing for obtaining a preform,
thereby resulting in a problem of increased manufacturing cost.
[0015] It has been conceived in regard to such a preform material
that instead of the Ni-based porous material, a iron (Fe)-based
porous material low in cost and excellent in machinability is used
(for example, see Japanese Patent Publication No. HEI 6-45830 which
corresponds to U.S. Pat. No. 5,028,493).
[0016] In a case where this Fe-based porous material is employed,
however, the material thereof itself is oxidized with ease;
therefore, a problem arises since the gas pressure casting process
is hard to be applied to the Fe-based porous material. That is,
this gas pressure casting process is a composite forming process
based on a metal mold gravity casting process, having been
generally used, as a base and to be detailed, after molten metal
pouring is completed, a sprue is closed and subsequent to this, the
molten metal is pressurized by a gas of a relatively low pressure;
therefore, there exists an inevitable time lag (normally, of the
order of 5 to 10 seconds) from the completion of molten metal
pouring into the mold till impregnation into the porous material
under the pressure. For this reason, the reinforcement material is
forcibly exposed to the atmospheric air for a time in a high
temperature condition produced by the molten metal (for example, in
the range of 700 to 750.degree. C. in a case where a molten
aluminum-based alloy is poured) prior to the impregnation under the
pressure.
[0017] That is, a surface of the framework of a Fe-based
reinforcement material is forcibly exposed to the atmospheric air
at high temperature for a time corresponding to the time lag from
the pouring of the molten metal till the pressure impregnation and
thereby, oxidized, which impairs adhesiveness between the molten
metal (that is a casting material) and a base of the reinforcement
material. Hence, a problem arises since wear resistance of the
reinforcement material after the impregnation decreases, which
makes it impossible to obtain a prescribed wear resistance and
prescribed strength characteristics.
[0018] It is to be noted that in a case of a prior art high
pressure casting process, a process from completion of molten metal
pouring till pressurization of the molten metal is performed in
consecutive steps and furthermore, a pressure is very high;
therefore, no time lag as experienced in the gas pressure casting
process occurs.
SUMMARY OF THE INVENTION
[0019] The present invention has been made to resolve the above
described technical problems and it is accordingly an object of the
present invention to provide a Fe-based metal porous preform
capable of ensuring sufficient adhesiveness between a molten metal
and a base thereof even when a casting process having a time lag
from completion of pouring of the molten metal till pressure
impregnation is applied; and a manufacturing process for a metal
composite member using such a preform.
[0020] Thus, in a first aspect of the present invention, there is
provided a metal porous preform adopted in a casting process having
a prescribed time lag from completion of pouring of a molten metal
till impregnation of the molten metal when the metal porous preform
set inside a mold is impregnated with the molten metal, wherein the
metal porous preform is made from iron or an iron-based metal as a
base material, including chromium.
[0021] According to the first aspect of the present invention,
since a base material of a metal porous preform is iron or a
Fe-based metal, the preform is low in cost. And, it is prevented by
the addition of chromium that the preform is oxidized by a time lag
from the completion of molten metal pouring till the molten metal
impregnation, while facilitating formability. That is, it is
possible that not only are reduction in material cost and
improvement of formability achieved, but also adhesiveness between
a casting material and a preform in a composite reinforcement
section is ensured, and a desired wear resistance is attained.
[0022] Furthermore, in the first aspect of the present invention, a
chromium content is preferably in the range of 10 to 40 wt %.
[0023] The reason why the lower limit of a chromium content is set
to be 10 wt % is that a heat resistance (oxidation resistance) of
the order of about 700.degree. C. is required for the preform
during a casting process, in other words, in order to ensure a wear
resistance equal to or higher than a prior art metal porous preform
prepared by electroplating on nickel (Ni) base, and to satisfy this
requirement, a chromium content of 10 wt % or higher is
necessary.
[0024] On the other hand, the reason why the upper limit of a
chromium content is set to 40 wt % is that when a chromium content
exceeds this value, a .delta. phase as an intermetallic compound is
precipitated in the material structure of the metal porous preform
to make it fragile and thereby, it is difficult to ensure wear
resistance thereof at a desired level.
[0025] By setting a Cr content in the range of 10 to 40 wt %, there
can be ensured a wear resistance equal to or higher than a preform
prepared by means of an electroplating on a nickel (Ni) base having
been generally adopted in the prior art.
[0026] Furthermore, in a second aspect of the present invention,
there is provided a manufacturing process for a metal composite
member including a casting process having a prescribed time lag
from completion of pouring of a molten metal till impregnation of
the molten metal when a metal porous preform set inside a mold is
impregnated with the molten metal, comprising the steps of: forming
the metal porous preform, made from iron or an iron-based metal as
a base material, and including chromium; setting the metal porous
preform inside a mold at a prescribed position; and thereafter,
pouring the molten metal into the mold to impregnate the metal
porous preform with the molten metal under pressure, whereby a
composite section is formed in which a casting material and the
preform are included.
[0027] According to the second aspect of the present invention,
since a base of a metal porous preform as a composite forming
material is iron or a Fe-based metal, the preform is low in cost.
And, it is prevented by the addition of chromium that the preform
is oxidized by a time lag from the completion of molten metal
pouring till the molten metal impregnation, while facilitating
formability. That is, it is possible that not only are reduction in
material cost and improvement of formability achieved, but
adhesiveness between a casting material and a preform in a
composite reinforcement section is ensured, and a desired wear
resistance is attained.
[0028] Furthermore, in the second aspect of the present invention,
it is preferable that the pores in the preform are filled with the
molten metal by pressurizing the molten metal in the mold with a
gas pressure after pouring of the molten metal is completed and a
sprue of the mold is closed, in the casting process.
[0029] In this case, particularly the casting process is a
so-called gas pressure casting process in which the sprue is closed
after molten metal pouring is completed, the molten metal in the
mold is pressurized by a gas pressure and thereby, pores (or cells)
in the preform is filled with the molten metal; therefore, the
problems in a prior art high pressure casting process can be
avoided and in addition to this, pores (cell) in a preform can be
filled with the molten metal simply and surely under a low
pressure.
[0030] Still furthermore, in the second aspect of the present
invention, it is preferable that the metal porous preform is
manufactured by means of a sintering process.
[0031] In this case, since the metal porous preform is manufactured
by means of the sintering process, a preform can be simply molded
as compared with an electroplating process having been generally
adopted in the prior art.
[0032] Yet furthermore, in the second aspect of the present
invention, it is preferable that addition of chromium is performed
by means of a chromizing treatment.
[0033] In this case, since addition of chromium to the metal porous
preform is performed by means of a chromizing treatment, chromium
diffusion into a base (iron or a iron-based metal) can be performed
with simplicity and ease.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] FIG. 1 is a front view showing an aluminum alloy piston
including a view of a partial section thereof relating to an
embodiment of the present invention;
[0035] FIG. 2 is a perspective view of a composite forming material
core ring for partial reinforcement of the piston;
[0036] FIG. 3 is a sectional view schematically showing the entire
casting apparatus used in casting of the piston;
[0037] FIG. 4 is a sectional view schematically showing an example
of a mold of the casting apparatus;
[0038] FIG. 5 is a sectional view taken along line V-V of FIG.
4;
[0039] FIG. 6 is a sectional view schematically showing another
example of the mold used in casting the piston;
[0040] FIG. 7 is a sectional view of a main part of an aluminum
alloy cast body (piston) cast in the casting apparatus;
[0041] FIG. 8 is a graph showing a relationship of a specific
gravity of a composite section with an applied pressure;
[0042] FIG. 9 is a graph showing a relationship between a volume
ratio of a composite forming material and an applied pressure
required for impregnation of the composite forming material with a
molten metal;
[0043] FIGS. 10A to 10E are pictorial representations of a series
of steps of a manufacturing process for a preform relating to the
embodiment of the present invention;
[0044] FIG. 11 is a descriptive view schematically showing a
construction of a tester performing a wear test for a composite
member relating to the embodiment; and
[0045] FIG. 12 is a graph showing test results of the wear
test.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0046] Description will be given of an embodiment of the present
invention in regard to a case where, for example, an automotive
engine piston having a composite reinforcement section in the
periphery of a top ring groove is cast below with reference to the
accompanying drawings.
[0047] Description is started with a manufacturing process for a
piston (a composite member) using a porous preform of a prescribed
porosity made from a metal relating to the embodiment as a partial
reinforcement material for composite formation.
[0048] FIG. 1 shows an aluminum alloy piston 1 obtained by means of
a manufacturing process relating to the embodiment. On the outside
surface of the piston main body 2, formed are a top ring groove 3
in which a top ring is fittingly inserted; a secondary ring groove
4 in which a secondary ring is fittingly inserted; and an oil ring
groove 5 in which an oil ring is fittingly inserted.
[0049] The top ring groove 3 of the piston 1 is provided on a ring
composite section 6 prepared by means of a process described later
and the piston main body 2 other than the composite section 6 is
constituted of an aluminum alloy.
[0050] In manufacture of the piston 1, first formed is a composite
forming material ring 7 having the sectional shape of a rectangle
as shown in FIG. 2, by means of materials and a manufacturing
process described later. In this case, the top ring groove 3 is
finished in after-machining into a final shape; therefore, the
composite forming material ring is simply shaped as a ring without
precise shape. After the ring 7 is placed at a position in a mold
corresponding to the top ring groove 3 and the mold is clamped, a
molten aluminum alloy is poured into the mold to perform casting,
and, thereafter, followed by machining of the top ring groove
3.
[0051] FIG. 3 is a descriptive, sectional view schematically
showing a casting apparatus used in manufacture of an automotive
engine piston, for example used in a diesel engine.
[0052] In the casting apparatus 10, a mold 11 is constructed by
outer mold pieces 12L and 12R constitute a split mold to be
disassembled to the right and left sides, a middle mold piece 13
placed downward and a top mold piece 14 disposed upward with a
feeder section 14a. And, a product section cavity 15 is formed in
the inside of the mold 11. The composite core ring 7 is sustained
inside the mold 11 and a pipe 16 is connected to the feeder section
14a of the top mold piece 14, the pipe 16 being used for pressure
application with air as a working fluid therethrough. A numerical
symbol 17 indicates a pin for a hole as cast forming a piston pin
insert hole.
[0053] Furthermore, the outer mold pieces 12L and 12R and the
middle mold piece 13 are driven by outer mold piece cylinders 18L
and 18R, and a middle mold piece cylinder 19, respectively, and
besides, the top mold piece 14 is driven by a top mold piece
cylinder 20.
[0054] FIG. 4 is a descriptive, sectional view of the mold 11 when
a pressure is applied by air through the sprue thereof and FIG. 5
shows a descriptive, sectional view (partly omitted) taken along
line V-V of FIG. 4. It is to be noted that the section of FIG. 4 is
perpendicular to the section of FIG. 3.
[0055] In the embodiment, an air vent (open in the atmospheric air)
21 is formed in one of the outer mold pieces 12L and 12R, for
example in the outer mold piece 12L. The air vent 21 is located at
the interface 12a between both mold pieces 12L and 12R, and
communicating with a section at which the composite forming
material core ring 7 is sustained inside the mold 11. The air vent
21 is formed as a groove, for example, with a section of about 5 to
10 mm in width and about 0.2 mm in thickness. And, a pipe 13 for
performing of a pressurizing by air as a working gas is mounted to
a cover 23 covering the sprue 22. In a case of this mold 11, the
top mold piece 14 is again a split type to be disassembled into
upper and lower portions and an air vent groove 24 (open in the
atmospheric air) is formed at the interface between the upper and
lower portions. The air vent groove 24 is used to discharge a air
in the cavity in charging a molten metal up to the feeder section
Note that a numerical symbol 25 indicates a runner leading to the
product section cavity 15 starting from the sprue 22 and
furthermore, a numerical symbol 26 is a salt core sustained by a
support, not shown, for forming a cooling oil passage in the
piston.
[0056] In the above described construction, a molten aluminum alloy
(AC8A) is poured through the sprue 22 and the runner 25 into the
cavity 15, thereafter, the cover 23 is lowered to close the sprue
22 airtight. And, simultaneously to this, a compressed air having a
pressure of, for example, 0.49 MPa is supplied through the pipe 16
mounted to the cover 23 to pressurize the molten metal for about 50
sec to 1 min. During application of a pressure by the air, part of
the molten metal intrudes into the grooves 21 and 24 partially and
is cooled and solidified to seal the grooves 21 and 24. The part of
the molten metal solidified in the grooves 21 and 24 is removed as
flashes in company with splitting of the mold. It is to be noted
that, although the application of a pressure by the air has to
start within 10 to 30 sec after pouring of the molten metal, this
time length, generally speaking, has only to be set in a time
period during a pressure can be effectively applied prior to
solidification of the molten metal.
[0057] FIG. 6 shows another example of a casting apparatus for
piston manufacture. It is a descriptive, sectional view of the mold
11 in a case where application of a pressure by the air is
performed through the feeder section 14a provided in the top mold
piece 14, similar to FIG. 3. While the air vent groove 24 may be
formed in the top mold piece 14 as shown in FIG. 4, in the case of
FIG. 6, a valve 27 is provided in the pipe 16 on the way to the
compressed air source. The valve 27 is to communicate selectively
the feeder section 14a with a compressed air source or the
atmospheric air.
[0058] In this case, a molten metal of aluminum alloy is poured
from the sprue in a state where the feeder section 14a is open in
the atmosphere through the valve 27. Thereafter, the cover 23
having a cooling mechanism such as a water-cooled copper block 28
is lowered to close the sprue 22. And, simultaneous to this the
valve 27 is operated to communicate the pipe 16 with the compressed
air source and the factory air has to be supplied onto the mold 11
from the pipe 16 to pressurize the molten metal. In the case of the
construction of FIG. 6, the neighboring spaces of a section in
which a composite is formed can be effectively pressurized.
[0059] According to the above procedure, as shown in FIG. 7,
obtained is an aluminum alloy cast product 8 including a ring
composite section 6 as a part of the piston main body 2. The ring
composite section 6 is obtained by filling pores in the composite
forming material ring 7 made from a metal porous preform with an
aluminum alloy, as explained in detail after.
[0060] In such a way, a so-called gas pressure casting process is
employed in this embodiment. In the gas pressure casting process,
the sprue is closed after molten metal pouring is completed, the
molten metal in the mold is pressurized by a gas pressure and
thereby, pores (or void spaces) in the composite forming material
ring core 7 as a preform is filled with the molten metal.
Therefore, the following problems in the prior art high pressure
casting process can be avoided and in addition to this, pores (void
spaces) in a preform can be filled with the molten metal simply and
surely under a low pressure, which problems are:
[0061] 1. since a pressurizing mechanism and a strong mold clamping
mechanism for applying a high pressure are required, the casting
facility becomes to be large in scale and highly expensive;
[0062] 2. since a molten metal is apt to be infiltrated into a core
at least partially and the core itself is apt to be deformed or
broken by applying a high pressure, it is required to ensure a
sufficient strength of the core and particularly, there is
difficulty in actual use of a disintegrable core (for example, a
salt core and a sand core) for this case; and
[0063] 3. since a metal mold itself is necessary to endure a high
pressure and a mold construction also has many constraints, a
freedom in shape of a product is low, in other words this process
is not suited for manufacture of a product of a complex shape or a
large size.
[0064] Then, the aluminum alloy casting 8 is placed into a heating
furnace to heat at 500.degree. C. for 4.5 hr; thereby, a solution
treatment is given to an aluminum alloy base material and
subsequent to this, water quenching is performed, followed by an
annealing treatment at 180.degree. C. for 5 hr.
[0065] Cutting is performed on the aluminum alloy cast greenbody 8
having been subjected to such a T6 treatment not only to machine
the outer surfaces of the ring composite section 6 and the piston
main body 2 but also to form the top ring groove 3 in the ring
composite reinforcement section 6, the secondary ring groove 4 and
the oil ring groove 5 as shown in FIG. 1.
[0066] It is to be noted that, as apparent from the above
description, in manufacture of a piston, a considerable depth in
the outer surface region of the ring composite section 6 is, after
casting, removed by cutting. The removed portion of the outer
surface region does not have any risk to give an adverse influence
on a quality of the section 6, even if a filling state of the
aluminum alloy in the removed portion is somewhat incomplete.
Therefore, an applied pressure of a gas may be set to a lower value
by a certain degree.
[0067] In the embodiment described above, after the molten aluminum
alloy is poured from the sprue 22, the cover 23 is lowered to close
the sprue 22 airtight. And simultaneous to this, a factory air
having a pressure of the order of 0.49 MPa is supplied through the
pipe 16 mounted to the cover 23 or the feeder section 14a to
pressurize the molten metal. However, as apparently seen from FIG.
8 showing a relationship of a specific gravity of the composite
section with an applied pressure, when an applied pressure is 0.098
MPa or higher, a specific gravity of the composite section stays
constant. This means that pores of the composite forming material
core ring 7 is sufficiently filled with an aluminum alloy by the
above-mentioned pressure.
[0068] In addition, although an applied pressure lower than 0.049
MPa is hard to achieve sufficient formation of a composite, an
applied pressure of 0.049 MPa or higher is sufficient to achieve a
composite to satisfaction. However, an applied pressure exceeding
2.94 MPa unpreferably requires a large mold clamping force in order
to prevent blowing-off of the molten metal from the interface of a
split mold; so an applied pressure is preferably set to 2.94 MPa or
lower. Especially, an applied pressure 0.98 MPa or lower can
prevent the molten metal blowing-off from occurring even if the
mold clamping force is not so large; therefore, an applied pressure
is most preferably set in the range of 0.049 to 0.98 MPa.
[0069] A preferable range of volume ratios of the composite forming
material is suitably from 5 to 20 vol. % on the average, e.g., from
80 to 95% in void ratio (porosity) while altering according to
conditions including kind of the composite forming material,
preheating temperature, molten metal temperature and others.
[0070] Preferable volume ratios of a composite forming material in
an applied pressure range of 0.049 to 2.94 MPa generally fall in
the following ranges according to a kind of composite forming
material:
1 (kind of composite (preferable range forming material) of volume
ratio) metal porous body, metal to 20% fibers inorganic short
fibers, to 10% whiskers inorganic particles to 15%
[0071] FIG. 9 is a graph showing a relationship between an applied
pressure and a volume ratio of a composite forming material. This
graph shows, for example, that in a case of a metal porous material
of the order of 9% in volume ratio, composite formation is possible
under an applied pressure of the order of 0.0196 MPa, and a sound
composite material is obtained under an applied pressure of the
order of 0.196 MPa.
[0072] It is to be noted that in FIGS. 8 and 9, while an applied
pressure represented on the abscissa is plotted using two unit
systems ([MPa] and [kg/cm.sup.2]), the scale thereon are graduated
with logarithmic spacing in the [kg/cm.sup.2] unit.
[0073] Next, description will be given of the composite forming
material core ring 7 for partial reinforcement of the top ring
groove 3 of the piston (composite member) 1.
[0074] In the embodiment, a metal porous preform having a
prescribed porosity is employed as the composite forming material
core ring 7. And, the material of the preform is a metal material
prepared such that iron (Fe) or an iron-based metal is used as a
base and chromium (Cr) is included as an additive.
[0075] In such a way, since the base of the composite forming
material core ring 7 (metal porous preform) is iron or an
iron-based metal, it is low in cost and it is prevented by the
addition of chromium that the preform is oxidized by a time lag
from the completion of molten metal pouring till the molten metal
impregnation, while facilitating formability. That is, it is
possible that not only are reduction in material cost and
improvement of formability achieved, but also adhesiveness between
a casting material and a preform in a composite forming section is
ensured and a desired wear resistance is attained.
[0076] Furthermore, in the embodiment, a chromium content is set to
be in the range of 10 to 40 wt %.
[0077] The reason why the upper limit of a chromium content is set
to be 40 wt % is that the Fe--Cr based metal of a chromium content
over this limit is hard to ensure a desired wear resistance as a
composite section. As well known, when a chromium content exceeds
this limit in a Fe--Cr based metal, the .delta. phase as an
intermetallic compound is precipitated in the material structure
and thereby such a material structure fall to be fragile;
therefore, the Fe--Cr based metal of a chromium content over this
limit is hard to ensure a desired wear resistance as a composite
section.
[0078] On the other hand, the reason why the lower limit of a
chromium content is set to be 10 wt % is that as described later,
heat resistance (oxidation resistance) of the order of about
700.degree. C. is required for the preform in order to ensure a
wear resistance equal to or higher than a prior art metal porous
preform prepared by electroplating on nickel base during a casting
operation, and to satisfy this requirement, a chromium content of
10 wt % or higher is necessary.
[0079] In manufacture of the preform, there have been available
various processes and in the embodiment, a powder sintering process
was more preferably employed. Besides, processes for addition of
chromium are conceived in various ways and in the embodiment,
preferably adopted was a process in which Cr powder is blended into
Fe powder and thus prepared mixed powder is added or a process in
which chromium is diffused into a porous body by a so-called
chromizing treatment.
[0080] FIGS. 10A-10E show an outline of basic steps of the powder
sintering process for a preform adopted in the embodiment. As shown
in the descriptive, pictorial representations of the series of
steps, an foamed urethane sheet M1 of a prescribed thickness is
subjected to punching to prepare an urethane foam ring member M2
and the ring member M2 is immersed in a metal powder slurry stored
in a slurry bath Bs.
[0081] The slurry is obtained by mixing metal powder, water and
water-soluble phenol resin in respective prescribed amounts,
together with a dispersant (surfactant) and the urethane foam ring
member M2 is immersed in the slurry bath Bs, thereby enabling
coating a structural framework of urethane resin with metal
powder.
[0082] After the metal powder coating step in the slurry bath Bs is
finished, burning of the urethane resin is performed under
predetermined condition, followed by sintering of the metal powder
under predetermined condition; thus a metal porous ring M3 is
obtained. The urethane burning and metal powder sintering steps are
performed under temperature conditions of, for example, a heating
temperature of 800.degree. C. .times.a holding time of 10 min-a
heating temperature of 1100.degree. C. .times.a holding time of 30
min, both in a non-oxidizing atmosphere.
[0083] By press molding thus obtained metal porous ring M 3,
obtained is a metal porous preform M of a prescribed size (that is,
a prescribed volume ratio) having a ring-like shape.
[0084] In the above-described series of steps, addition of chromium
(Cr) proceeds in this way. That is, in a case where Cr powder and
Fe powder (or in addition to the powders, another kind of metal
powder according to a kind of alloy) are mixed to form mixed powder
and Cr is added as the mixed powder, the metal mixed powder is
suspended in the slurry liquid of the coating step (see FIG. 10C)
such that weights of the metal powders are adjusted so as to show a
prescribed ratio in weight.
[0085] On the other hand, in a case where Cr is diffused into the
porous body through the so-called chromizing treatment, the ring
preform M is prepared by means of the powder sintering process and
thereafter, the chromizing treatment is applied on the ring preform
M. A diffusing agent and treatment conditions in the chromizing
treatment were as follows:
[0086] diffusing agent: chromium+ammonium chloride+alumina
[0087] diffusion conditions: 900.degree. C. .times.4 hr (note that
in a case of Ni, 1300.degree. C. .times.4 hr)
[0088] It is to be noted that in order to improve wear resistance
of a partial reinforcement composite, it is generally required that
a base material low in wear resistance (in the embodiment, an
aluminum alloy) is dispersed in as narrow an area as possible. It
is recommended to increase a volume ratio of a reinforcement
material in order to decrease of an absolute value of the area.
However, in this case, some problems arise since an infiltrating
property of a molten metal is deteriorated to inhibit satisfactory
formation of a composite and furthermore, an amount of a
reinforcement material in use increases to be costly. Accordingly,
it is preferable that increase in volume ratio of the reinforcement
material is avoided to the lowest level possible.
[0089] For this reason, it is conceived that cell (or a void space)
sizes of a porous material is minimized to finely disperse an
aluminum base material in the porous body. In this case, in an
electroplating process, the minimization of cell sizes of the
porous material can be achieved with smaller cell sizes of a
urethane foam as a substrate for the porous body. However, in the
sintering process, a problem has occurred since a metal slurry is
coated on a urethane foam, clogging of the slurry arises, inviting
deterioration in formability of a composite with extremely small
cell sizes of the urethane foam.
[0090] In the embodiment, the following setting scheme was adopted
with respect to a cell size and a volume ratio of a porous material
in light of the above-described problems.
[0091] First of all, a urethane foam of coarser cell sizes than a
target cell size is prepared and a solid unit-volume weight of a
slurry is set to a value lower than a target value. Thereby, a
clogging-less metal porous body with a volume ratio lower than a
target value can be obtained. Furthermore, a large compressibility
is set on thus obtained metal porous body and sizing is conducted,
for example, by press molding to obtain a final shape. Thereby, a
metal porous preform having a target cell size and a target volume
ratio was able to be obtained successfully.
[0092] To be concrete, a urethane foam having cell sizes in the
range of 37 to 43 cells/inch was adopted and a solid unit-volume
weight of the slurry was set such that a volume ratio after
sintering is of the order of 7%. Furthermore, a compressibility of
thus obtained metal porous body was set to 30% to perform sizing
and attain a final shape. As a result, there could be obtained a
preform having cell sizes in the range of 53 to 60 cells/inch and a
volume ratio of about 10%.
[0093] Note that as a material of a preform, there were used, for
example, Fe--13Cr, Fe--26Cr and Fe--18Cr--8Ni.
[0094] Next, comparative tests to compare wear resistance
properties of various kinds of samples with each other were
conducted in order to confirm an effect of improving wear
resistance of a metal porous preform relating to the
embodiment.
[0095] Description will be given of the comparative tests below:
First of all, test samples were adopted that were made from the
following materials by means of the following manufacturing
processes, wherein the samples were of aluminum base material, two
kinds of comparative examples and four kinds of examples of the
present invention. The samples are detailed:
[0096] 1) An aluminum base material (AC8A base metal/Hv=145): An
aluminum alloy AC8A base material, not reinforced in a composite
structure, in its original state having a Vickers hardness (Hv) of
145 was used, and
[0097] 2) Comparative Example 1 (Ni--30Cr alloy/an electroplating
process/Hv=180): A Ni porous sheet was prepared by means of an
electroplating process and thus obtained Ni porous sheet was
treated in chromizing to diffuse chromium thereinto to prepare a
Ni--30Cr porous sheet. Thus obtained Ni--30Cr sheet was cut into
short strips and each strip is rolled into a porous ring. A porous
ring of Ni--30Cr was set inside a mold and casting was performed as
described above using a molten metal of aluminum alloy AC8A by
means of the gas pressure casting process. Conditions for the gas
pressure casting process were a molten metal temperature of
780.degree. C. and an applied pressure of 0.4 MPa.
[0098] By performing the-above casting process, manufactured was a
piston whose top ring groove was partially reinforced with the
Ni--30Cr porous ring. In this case, a volume ratio of the
reinforcement material was about 8%. Besides, a Vickers hardness
(Hv) of the reinforcement material was 180.
[0099] 3) Comparative Example 2 (Fe--Ca powder sintering
process/Hv=700): A Fe--C porous ring (C:0.8 wt %) was prepared by
means of the sintering process using Fe--C mixed powder. This ring
was used to perform casting like the case of Comparative Example 1
and obtain a piston having a partial composite reinforcement
section. Note that a Vickers hardness (Hv) of the reinforcement
material was 700.
[0100] 4) Example 1 of the present invention (Fe--13Cr/a powder
sintering process+chromizing/Hv=81): A Fe porous ring was prepared
by means of the sintering process using Fe powder. A chromizing
treatment was applied on the Fe porous ring to diffuse Cr into the
porous body and obtain a porous ring of Fe--13Cr. Casting was
performed using this ring by means of the above-mentioned gas
pressure casting process to attain a piston having a partial
composite reinforcement section. Note that a Vickers hardness (Hv)
of the reinforcement material was 81.
[0101] In such a way, by manufacturing a metal porous preform (Fe
porous ring) by means of the sintering process, molding of a
preform can be simply conducted as compared with a electroplating
process having been generally adopted in the prior art.
[0102] 5) Example 2 of the present invention (Fe--26Cr/a powder
sintering process+chromizing/Hv=185): A Fe--26Cr porous ring was
obtained in the same process as above described Example 1 of the
present invention except that only an amount of Cr diffusion was
altered in the chromizing treatment. Note that a Vickers hardness
of the reinforcement material was 185.
[0103] In such a way, by performing addition of Cr to a metal
porous preform by means of a chromizing treatment, Cr diffusion
into a base (Fe-base metal) can be achieved with simplicity and
ease.
[0104] 6) Example 3 of the present invention (Fe--12Cr/a powder
sintering process/Hv=380): Fe powder and Cr powder were mixed so as
to show a weight ratio of Fe--12Cr and a porous ring of Fe--12Cr
was prepared by means of the sintering process using the mixed
powder. Casting was conducted using this ring by means of the above
described gas pressure casting process to attain a piston having a
partial composite reinforcement section. Note that a Vickers
hardness (Hv) of the reinforcement material was 380.
[0105] 7) Example 4 of the present invention (Fe--18Cr--8Ni/a
powder sintering process/Hv=266): Fe powder and Cr powder were
mixed so as to show a weight ratio of Fe--18Cr--8Ni and a porous
ring of Fe--18Cr--8Ni was prepared by means of the sintering
process using the mixed powder. Casting was conducted using this
ring by means of the above described gas pressure casting process
to attain a piston having a partial composite reinforcement
section. Note that a Vickers hardness (Hv) of the reinforcement
material was 266.
[0106] Wear tests were performed on each of the above-described
samples and a wear resistance was evaluated on each of the samples.
FIG. 11 is a descriptive view schematically showing a construction
of a wear tester used in the tests.
[0107] As shown in FIG. 11, the tester 30 includes: a hydraulic
cylinder 32; a cylinder holder 34 connected to the cylinder; and a
cylinder liner 36 having a vertical section of approximately a
Greek letter .pi. provided inside the cylinder 34.
[0108] A test piston ring 40 is fixed on a flat protrusion 38
provided at the bottom of the cylinder liner 36. A heater 42 is
embedded in the cylinder holder 34.
[0109] The cylinder holder 34 is driven by the hydraulic cylinder
32 and reciprocated by a prescribed distance in a prescribed
cycle.
[0110] A piston 50 as a test sample is accommodated in a cavity of
the cylinder liner 36. The top of each piston (test sample) 50 is
removed by machining to expose a composite reinforcement section
50m formed as a composite including the above described preform at
an end surface of the piston 50. Therefore, the composite
reinforcement section 50m faces the piston ring 40 fixed on the
cylinder holder 34 side. Note that a distal end of an oil supply
pipe 44 supplying lubricant oil is located in the vicinity of the
outer periphery of the piston ring 40.
[0111] The piston 50 is fixed onto a rotary shaft 48 driven by a
motor (not shown) at the end of the other side thereof from the
composite reinforcement section 50m and rotated at a prescribed
rotation speed.
[0112] In the above construction, the piston 50 is rotated by the
rotary shaft 48 driven by the motor (not shown), and the hydraulic
cylinder 32 is also driven. Thereby, the piston ring 40 is pressed
in a prescribed cycle to the composite reinforcement section 50m of
the piston 50 in rotation at a prescribed rotation speed. That is,
a pressure is acted in a prescribed cycle according to an applied
pressure of the hydraulic cylinder 32 over the interface between
the composite reinforcement section 50m and the piston ring 40
under a condition of a prescribed relative speed (sliding
speed).
[0113] In the tests, each of the test was conducted for a
prescribed time in a state where a temperature of the cylinder
holder 34 was kept constant by the heater 42. Amount of wear was
measured on the composite reinforcement section 50m after an
operation of the tester was finished, and a wear resistance of each
preform was evaluated.
[0114] In the embodiment, test conditions were set as follows:
[0115] sliding speed: 0.08 m/s
[0116] frequency of pressure application: 20 Hz
[0117] pressure over the interface: 6 MPa
[0118] temperature: 250.degree. C.
[0119] test operation time: 3 hr
[0120] Test results were as shown in the graph of FIG. 12. That is,
in a case where a preform which is simply Fe-based (Fe--C) material
was employed as a base material (Comparative Example 2), since the
preform is very hard, but is fragile, a so-called aggressive wear
is intense and much of a wear is experienced. In contrast to this,
in cases where preforms made from Fe-based metal added with Cr at a
content of 10 wt % or higher (to be concrete, 12 wt % or higher)
were employed (Examples 1 to 4 of the present invention), wear
resistances in all of the cases were found to be equal to or higher
than in a case where a prior art preform obtained by electroplating
on a Ni base material (Comparative Example 1) was employed.
[0121] This is conceived because in Examples 1 to 4, oxidation of a
preform caused by a time lag from completion of molten metal
pouring till impregnation of a molten metal in the gas pressure
casting process is prevented by addition of Cr.
[0122] It is to be noted that when a Cr additive amount exceeds 40
wt %, an intermetallic compound is precipitated in the material
structure; therefore, it is as described above that a Cr content
should be limited to the value or lower.
[0123] As described above, according to the embodiment, since a
base of a metal porous preform is a Fe-based metal, the preform is
low in cost. And it is prevented by the addition of chromium that
the preform is oxidized by a time lag from the completion of molten
metal pouring till the molten metal impregnation in the gas
pressure casting process, while facilitating formability. That is,
it is possible that not only are reduction in material cost and
improvement of formability achieved, but also adhesiveness between
a casting material and a preform in a composite reinforcement
section is ensured, and a desired wear resistance is attained.
Particularly, by setting a Cr content in the range of 10 to 40 wt
%, there can be ensured a wear resistance equal to or higher than a
preform prepared by means of an electroplating on a nickel (Ni)
base having been generally adopted in the prior art.
[0124] In the above described embodiment, the gas pressure casting
process is adopted in manufacture of a composite member (piston)
using a metal porous preform. However, it is to be noted that the
present invention is not limited to such a casting process, but any
of other casting processes can be adopted instead, as long as a
prescribed time lag exists between completion of molten metal
pouring till impregnation of a molten metal.
[0125] Furthermore, a composite member manufactured by means of a
process of the present invention is not limited to a piston of an
automotive engine as described above. Needless to say, it can be
applied to other engine parts such as a bearing cap and a
connecting rod or, besides, parts of other kinds as well. Still
furthermore, in addition to the aluminum alloy casting, a composite
member made from other kinds of light alloy cast, for example made
from magnesium alloy cast can be manufactured according to the
present invention.
* * * * *