Note: Descriptions are shown in the official language in which they were submitted.
CA 02382104 2002-03-O1
SPECIFICATION
PREFORM, MOLD ARTICLE AND INTERNAL-COMBUSTION ENGINE
COMPONENT
TECI-INICAL FIELD
The present invention relates to a preform formed by solidifying
alloy powder and a mold article formed by plastic working of the preform.
More particularly, the invention relates to an internal-combustion engine
component such as a piston, formed as such alloy powder molded article.
BACKGROUND ART
Aluminum alloy has a low specific gravity as low as about 1/3 of
that of iron. For this reason, it is extensively used not only in internal-
combustion engine components, but also as aircraft material. Further, as
aluminum alloy has a high heat conductivity, it is sometimes used as heat
radiating material (heat sink).
In such applications, it is often desired to provide a predetermined
function to a certain limited portion of the aluminum alloy product. For
instance, it may be desirable to provide a portion of its surface with a
functionally gradient layer having superior high temperature strength, so
as to take advantage of the high temperature strength of the functionally
gradient layer without sacrificing the weight advantage of the material as a
light metal material. Or, it may be desired to form the other or remaining
portion of the product than a portion requiring high temperature strength of
a light weight alloy, so as to achieve lightness in the product as a whole
without losing the high temperature strength of its surface.
Especially, in the case of a piston which is one such aluminum alloy
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powder molded article forming the surface of a combustion chamber of an
internal-combustion engine and which itself effects a reciprocating
movement, while it is advantageous to form the piston of an light-weight
aluminum alloy or magnesium alloy in the respect of e~ciency, it is
necessary for the top surface of the piston facing the combustion chamber to
have an enhanced high temperature strength to be able to withstand
combustion at the combustion chamber. For instance, for the top of the
piston, there is high temperature strength requirement of 150 MPa/300°C
or more for a spark ignition engine or 250 MPal300°C or more for a
diesel
engine.
Conventionally, as a piston having a functionally gradient layer
formed of an aluminum alloy having a high temperature strength at least at
a portion thereof, there is known a piston in which the functionally gradient
layer is formed of an Fe material, Al-Fe alloy material or Al alloy material
mixed with ceramic particles and this functionally gradient layer is
coordinate-cast with an Al alloy used for forming the piston body and then
these are welded together.
The piston manufactured by such welding technique has a
durability problem at its welded portion and this piston also requires a
number of steps for its manufacture, thus inviting cost increase.
Further, if such functionally gradient layer is caused to contain a
large amount of Fe, this will result in increase in the weight of the
component.
In particular, in recent years, with view to e.g. energy saving, there
has been a demand for further weight reduction for internal-combustion
engine components. For the purpose of such weight reduction, even if only
the top surface of the piston requiring high temperature strength is formed
of Al-Fe alloy material having high temperature strength and the other
portion not requiring high temperature strength is formed of a relatively
light-weight aluminum alloy or the like, its effect for weight reduction is
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small.
DISCLOSURE OF THE INVENTION
In view of the above-described state of the art, an object of the
present invention is to manufacture and provide a light-weight mold article
such as a piston and a preform to be molded into the mold article easily and
inexp ensively.
Namely, for accomplishing the above-noted object, a preform of the
present invention relating to claim 1 is characterized in that the preform is
formed by solidifying more than two kinds of aluminum alloy powder into a
monolithic construction, said each kind of aluminum alloy powder
containing 1-15 wt% of one or more elements selected from a group of
transition metals consisting of Fe, Cr, Ni, Zr, Mn, Mo and Ti; 10-30 wt% of
Si, 0.5-5 wt% of Cu, 1-5 wt% of Mg and the rest substantially of Al, having a
crystal size equal to or greater than 0.05 a m and equal to or less than 2 ,u
m
and a powder particle size equal to or greater than 30 a m and equal to or
less than 1000 ,u m, said two kinds of aluminum alloy powder having
different amounts of said transition metals) from each other; and at least a
portion of an outer surface of the preform is formed as a functionally
gradient layer having a greater amount of said transition metals) than the
other main body portion of the preform.
Further, for accomplishing the above-noted object, a preform of the
present invention relating to claim 2 is characterized in that the preform is
formed by solidifying an aluminum alloy layer comprising aluminum alloy
powder and a magnesium alloy layer comprising magnesium alloy powder
into a monolithic construction, said aluminum alloy powder including 1-15
wt% of one or more elements selected from a group of transition metals
consisting of Fe, Cr, Ni, Zr, Mn, Mo and Ti; 10-30 wt% of Si, 0.5-5 wt% of Cu,
1-5 wt% of Mg and the rest substantially of Al, having a crystal size equal to
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or greater than 0.05 a m and equal to or less than 2 a m and a powder
particle size equal to or greater than 30 a m and equal to or less than 1000
,u m, said magnesium alloy powder having a greater amount of Mg than
said aluminum alloy powder.
Further, a mold article of the present invention relating to claim 3
is characterized in that the mold article is formed by plastic working of the
preform according to claim 1 or 2.
In the case of aluminum alloy containing a large amount of
transition metal such as Fe, for its rigidity and heat resistance, the plastic
working of crystal material generally requires a high working force of 200
MPa or more at a high temperature range of 500°C or higher.
Further,
even when a working process utilizing superplasticity (crystal size: about 10
to 100 a m) is employed, its strain working speed is as low as about 10-3 to
10-9/sec. A high-speed working higher than 10-2/sec is infeasible, hence, the
productivity is poor.
Then, in the case of the aluminum alloy powder employed by the
present invention and its preform, the powder or preform is provided with a
high-speed superplasticity due to the above-described chemical composition
of the alloy powder, super-fine crystal structure and the size of the powder
particles. As will be described later, it allows a high-speed working higher
than 10-2/sec of strain working speed ( F ). Under such working condition,
the powder or its preform exhibits high ductility equal to or greater than
about 200% with an extremely low deformation flow stress of about 20 MPa
or lower. Hence, the plastic working process can be carried out efficiently
at a high speed and under a low pressure application condition.
Therefore, such aluminum alloy powder can contain a large
amount of transition metal such as Fe as much as 5 to 15 wt% for example.
And, the aluminum alloy powder containing a large amount of such
transition metal as Fe (to be referred to as "Al-Fe alloy powder" hereinafter)
provides superior high-temperature strength and abrasion resistance.
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However, with its high content of e.g. Fe, the specific gravity is
increased. Then, if a preform or mold article is formed of this Al-Fe alloy
powder alone, it becomes impossible to achieve satisfactory weight reduction
effect which is an advantage of aluminum alloy.
Then, in the case of the preform and the mold article formed by
plastic working of this preform proposed by the present invention, its main
boty portion other than the aluminum alloy layer is formed as a magnesium
alloy layer comprising a magnesium alloy powder containing a greater
amount of Mg than said aluminum alloy powder. Alternatively, the main
body portion is foxmed of e.g. an aluminum alloy powder of Al-Si or the like
and at least a portion of its outer surface requiring higher temperature
strength can be a functionally gradient layer and the monolithic
construction in which the two kinds of alloy powder are graded from each
other is solidified by the spark plasma sintering (SPS) method, thereby to
form a preform in which only its surface portion requiring high temperature
is formed as the functionally gradient layer made of the aluminum alloy
layer containing a large amount of transition metals) while the remaining
main body portion of the preform is formed as the magnesium alloy layer.
Further, as this preform has high-speed supexplasticity as described above,
it may be formed by an efficient plastic working at a high speed and low
pressure. Moreover, with this invention's mold article formed in the
manner described above, since the article is sintered sufficiently at is
graded
portion in the vicinity of the interface between the respective layers, there
occurs no such problem as poor welding.
If such mold article is provided as an internal-combustion engine
component, this may be constructed as follows. Namely, an internal-
combustion engine component of the invention relating to claim 4, is
characterized in that the component is formed by plastic working of a
preform, said preform being formed by solidifying more than two kinds of
aluminum alloy powder into a monolithic construction, said each kind of
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aluminum alloy powder containing 1-15 wt% of one or more elements
selected from a group of transition metals consisting of Fe, Cr, Ni, Zr, Mn,
Mo and ~; 10-30 wt% of Si, 0.5-5 wt% of Cu, 1-5 wt% of Mg and the rest
substantsally of Al, having a crystal size equal to or greater than 0.05 ~ m
and equal to or less than 2 a m and a powder particle size equal to or greater
than 30 a m and equal to or less than 1000 ,u m, said two kinds of
aluminum alloy powder having different amounts of said transition metals)
from each other; and a portion of the preform to be exposed to a combustion
chamber is formed as a functionally gradient layer having a greater amount
of said transition metals) than the other main body portion of the preform.
If such mold article is provided as an internal combustion engine
component, this may be alternatively constructed as follows. Namely, an
internal-combustion engine component of the invention relating to claim 5,
is characterized in that the component is formed by plastic working of a
preform, said preform being formed by solidifying an aluminum alloy layer
comprising aluminum alloy powder and a magnesium alloy layer
comprising magnesium alloy powder into a monolithic construction, said
aluminum alloy powder containing 1-15 wt% of one or more elements
selected from a group of transition metals consisting of Fe, Cr, Ni, Zr, Mn,
Mo and Ti; 10-30 wt% of Si, 0.5-5 wt% of Cu, 1-5 wt% of Mg and the rest
substantially of Al, having a crystal size equal to or greater than 0.05 ~c m
and equal to or less than 2 a m and a powder particle size equal to or greater
than 30 a m and equal to or less than 1000 ~c m, said magnesium alloy
powder having a greater amount of Mg than said aluminum alloy powder,
and at least a portion of the preform to be exposed to a combustion chamber
is formed of said aluminum alloy powder layer and the other main body
portion of the preform is formed of said magnesium alloy powder layer.
In these manners, by forming such components having a portion
exposed to the combustion chamber, such as a piston, a cylinder liner, an
intake or exhaust valve, etc as an internal combustion engine component
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according to the present invention, it is possible to form the entire top of
the
piston or a cavity formed on the piston top for initial combustion, the inner
surface of the cylinder line, the valve head, etc. as the functionally
gradient
layer as the aluminum alloy layer containing a large amount of Fe as the
transition metal while forming the other main body portion as the super
light weight magnesium layer, with the respective layers being formed into
a monolithic construction. By adapting the high temperature strength of
said aluminum alloy layer to about 250 MPa/300°C, there is obtained an
internal-combustion engine component which is light weight as a whole and
in which at least a portion of e.g. the piston top has superior high
temperature strength. Incidentally, in case an exhaust valve is
constructed as an internal combustion engine component according to the
present invention, Al-~ alloy may be employed in addition to the A1 alloy
containing a large amount of Fe.
Further, since the internal-combustion engine component of the
invention constructed as above substantially comprises the mold article
according to claim 3, substantially same functionleffect as the above-
described mold article of the invention may be achieved.
As the functionally gradient layer described above, e.g. Al-Fe alloy
powder of Al-l2Si-5 to 15 Fe is employed preferably. And, in the remaining
main body portion, Al-l2Si or Al-l7Si commonly used in the conventional
pistons can be used.
Also, for forming the magnesium alloy layer in the main body
portion, as this magnesium alloy layer, it is possible to use magnesium alloy
powder or magnesium alloy billet (fine crystals smaller than 2 a m)
containing 0.1-15 wt% of Al, 0.1-10 wt% of Zn, Ga, 0.01-5 wt% of Zr, Mn, Si,
Cu, Ni, Fe, Ca, ~; 0.1-10 wt% of more than one kind of rare earth element
(Nd, Pr, etc.) and the rest substantially of Mg.
This magnesium alloy layer is light weight, but has low high
temperature strength. However, in the case of the present invention, by
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combining this magnesium alloy layer with the aluminum alloy layer or the
functionally gradient layer, it may be employed in an engine component or
the like.
Moreover, in order to obtain overall uniformity in the linear
expansion coefficient, it is also possible to make adjustment by
appropriately correlating the content of Fe, Si, etc. with the linear
expansion coefficient.
Further, as recited in claim 6, the internal combustion engine
component of the invention is constructed preferably as a piston having a
piston top as the portion faang the combustion chamber. By constructing
the piston requiring both overall weight reduction and high temperature
strength at the piston top surface as the internal combustion engine
component of the present invention, it is possible, for example, to form the
piston top as the functionally gradient layer for higher temperature
strength and to achieve the overall weight reduction at the same time by
forming the main body portion thereof of the magnesium layer.
Also, the internal combustion engine component of the present
invention relating to claim 7, in addition to the construction of the internal
engine component according to any one of claims 4-6, the component is
formed by plastic working of the preform, wherein a portion of the preform
formed as the at least one portion of the surface is formed by solidifying the
aluminum alloy powder together with ceramics-containing powder
containixig 1-30 vol% of ceramics powder having a particle diameter equal to
or less than 5 ,u m, said at least one portion of the surface being
constructed
as an abrasion resistant portion containing the ceramics.
The aluminum alloy powder provided at the at least one portion of
the surface is mixed, if necessary, with the ceramics powder as abrasion-
resistant material. The ceramics particles are dispersed within the
aluminum alloy matrix, thus serving not only to enhance the abrasion
resistance of the component product, but also to restrict crystal growth of
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the matrix aluminum alloy. The ceramics can be oxide type, nitride type,
carbide type, boride type, etc, or one or more kinds of which may be
appropriately selected for use. In particular, use of silicon carbide (SiC),
alumina (A120~, silicon nitride (Si3N4) singly or in combination is effective.
Further, it is also effective to use an Fe compound, instead of the ceramics.
The ceramics particles need to be fine particles of a particle size
equal to or less than 5 a m. If the particle size is greater than this, this
will result in deterioration in the superplasticity of the aluminum alloy
powder, thus making its superplastic working difficult and the finishing
(machining) process also di~cult. The reason why its addition amount is
set as 1-30 vol% is that if it is smaller than 1 vol%, the addition effect
will be
poor, whereas if the amount exceeds 30 vol%, this will invite embrittlement
of the alloy, thus not being able to ensure the superplasticity.
Such ceramic powder can be added to the aluminum alloy powder
used for forming a portion of the lateral face of the piston preform which is
then formed by e.g. compression plastic working, into a piston as an
internal-combustion engine component according to the present invention
or to the aluminum alloy powder used for forming grooves of the piston ring.
The resultant piston as an internal-combustion engine component is
provided with the favorable abrasion resistance at a portion requiring such
abrasion resistance.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a graph showing relationship between high temperature
strength of an aluminum alloy and its Fe content,
Fig. 2 is a section view showing a piston preform as a first mode of
embodiment of the present invention relating to an internal-combustion
engine component,
Fig. 3 is a schematic illustrating a compression plastic working of
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the preform shown in Fig. 2,
Fig. 4 is a perspective view of an internal-combustion engine piston
formed by the compression plastic worldng of the preform shown in Fig. 2,
Fig. 5 is a section view showing a preform according to a further
embodiment of the present invention,
Fig. 6 is a section view showing a preform according to a further
embodiment of the present invention,
Fig. 7 is a section view showing a cylinder liner as a further
embodiment of the internal-combustion engine component relating to the
present invention, and
Fig. 8 is a section view showing a valve as a further embodiment of
the internal-combustion engine component relating to the present
invention.
BEST MODE OF EMBODYING THE INVENTION
The reason why the chemical composition of the aluminum alloy
powder employed in the present invention is defined as described above is to
secure the mechanical properties required as e.g. a structural member and
also to ensure superplasticity. That is, such elements as Si, Cu,Mg, Mo, Ti
are elements used for enhancing the strength, heat resistance, abrasion
resistance, etc. If its/their content is below the above-defined upper limit,
the property improving effect will be insufficient. On the other hand, if it
exceeds the above-defined upper limit, it will result in hardening
embrittlement of the material, thus becoming unable to ensure the
superplasticity.
The transition metal elements of Fe, Cr, Ni, Zr, Mn and Ti are
elements which can contribute to improvement of mechanical properties.
The present invention aims at enhancing the superplasticity as the result of
its/their addition. Namely, these elements combine with Al and deposit as
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fine chemical phase, thereby to restrict crystal growth of the aluminum
alloy. As a result, a fine crystal structure needed for realizing the
superplasticity can be obtained. The reason why the content (or the total
content in the case of more than two kinds of them are added) is set as
equal to or greater than 1 wt% is to obtain su~.cient addition effect. The
reason why the upper limit is set as 10 wt% is that if the content exceeds
this value, this will result in hardening of the material, thus impairing the
superplasticity.
Further, the aluminum alloy powder employed by the present
invention needs to have a crystal size equal to or greater than 0.05 ,u m and
equal to or less than 2 ~c m. The crystal size is set as equal to or gzeater
than 0.05 ,u m, because it is difficult with the currently available technique
to manufacture powder having a crystal size equal to or less than 0.05 ,u m.
Also, the crystal size is set as equal to or less than 2 a m in order to
improve
the compressive performance, workability and plastic deformability of the
powder. In the case of powder manufactured by ultra-rapidly solidification,
the finer the powder, the greater its strain hardening. Also, the friction
resistance at the particle interface during the plastic working will increase,
whereby the plastic deformability is deteriorated. The reason why the
particle size of the powder is limited to be equal to or less than 1000 a m is
that if the powder particle size exceeds 1000 a m, it becomes di~cult to
realize the superplasticity and also that the yield will be deteriorated and
the particle becomes too large to be manufactured by the SWAP (Spinning
Water Atomization Process) method described later. The aluminum alloy
powder having the above-described super fine crystal structure and the
above-defined particle size can be obtained afficiently by the atomization
process (cooli_ng speed: 104°C/sec or higher) of the SWAP method.
In general, when aluminum alloy contains a transition metal: Fe,
the high temperature strength of the alloy will increase, depending on its
addition amount. This relationship is illustrated in Fig. 1.
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Generally, however, if it contains a large amount of Fe, its
hardness and heat resistance will increase also, thus leading to
deterioration in productivity in the subsequent plastic working process.
However, in the case of the aluminum alloy powder employed by the present
invention, even when it contains a large amount of Fe such as by 9-15 wt%,
the alloy effectively retains its superplasticity; hence, there will occur no
productivity deterioration in the subsequent plastic working process. And,
such Al-Fe alloy powder containing a large amount of Fe has a high
temperature strength of 250 MPal300°C or more.
The magnesium alloy powder employed by the present invention
preferably has a crystal size equal to or greater than 0.05 ~c m and equal to
or less than 10 a m and a powder particle size equal to or greater than
30 ,u m and equal to or less than 500 ,u m.
The crystal size is set as equal to or greater than 0.05 a m because
it is difficult with the presently available technique to manufacture powder
having a crystal size equal to or less than 0.05 a m. The crystal size is set
also as equal to or less thanl0 a m in order to ensure the superplasticity.
Further, the reason why the powder particle size is set as equal to
or greater than 30 a m is to improve the compressive performance,
workability, plastic deformability and the handling of the powder.
In the case of powder manufactured by ultra-rapidly solidification,
the finer the powder, the greater its strain hardening. Also, the friction
resistance at the particle interface during the plastic working will increase,
whereby the plastic deformability is deteriorated. There is also the danger
of combustive explosion.
The reason why the particle size of the powder is limited to be
equal to or less than 500 ,u m is that if the powder particle size exceeds 500
,u m, it becomes difficult to realize the superplasticity and also that the
yield
too will be deteriorated and the particle becomes too large to be
manufactured by the method described later.
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For obtaining the mold article of the invention, prior to the molding
(compression plastsc working such as forge) of the product component,
there is obtained a preform (sintered compact) having an appropriate shape.
In the course of the above, by solidifying (sintering) more than two
kinds of the above-described aluminum alloy powder different in their Fe
contents from each other into a monolithic construction, it is possible to
obtain a preform in which at least a portion of the outer surface of the
preform is provided as a functionally gradient layer formed of the Al-Fe
alloy powder having a large amount of Fe and the remaining main body
portion of the preform is formed of e.g. Al-Si alloy powder having a smaller
amount of Fe than the functionally gradient layer. The preform thus
manufactured exhibits the superplasticity as described above. It is also
light weight as a whole and has superior high temperature strength in the
functionally gradient layer as well.
Further, if at least a portion of the outer surface of the preform is
provided as e.g. the above-described functionally gradient layer formed of
the A1 alloy containing a large amount of Fe and the remaining main body
portion thereof is formed as a magnesium alloy layer which is rendered light
weight by containing a large amount of Mg, such preform too exhibits the
superplasticity as descxzbed above. Moreover, since the magnesium is the
lightest metal among the structural metals in practical use, it becomes
possible to form the entire preform lighter than one formed entirely of the
aluminum alloy layer and with good high temperature strength as the
border portion thereof as well.
This pre-forming operation is implemented preferably by the spark
plasma sintering method. The spark plasma sintering is effected to carry
out pressure sintering by using supply of pulsed current. It is a kind of
internal heating sintering method which utilizes high energy of high
temperature plasma resulting from momentary intermittent spark
discharge generated at inter-particle gaps in the powder. The discharging
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points within the powder material will be moved and dispersed within the
entire material in association with repeated ON/OFF of the currentlvoltage
application. With a uniform heating effect possible with such internal
heating method, it is possible to realize uniform sintering within a short
time and under a low-temperature (capable of restricting and preventing
crystal grain growth and its enlargement) processing condition.
Preferably, the temperature of the above-described sintering
process is limited to be lower than 500°C. This is done for preventing
growth enlargement of the crystal grains so as to effectively retain the
superplasticity due to fine crystal structure. The processing temperature
can be easily controlled by way of e.g. the pulse current, ON/OFF switching
interval, processing time period, etc. Further, the pressure preferably
ranges between about 50 MPa and 180 MPa. If the pressure is lower than
this, it becomes necessary to effect the sintering at a higher temperature,
thus inviting the inconvenience of growth enlargement of the crystal grains.
On the other hand, it is not necessary to increase the pressure higher than
180 MPa. Further increase of the pressure is not desirable as it promotes
wear of the mould. With the spark plasma sintering method (SPS method),
the A1 alloy and the Mg alloy can be sintered and joined together effectively.
In the spark plasma sintering process, within the crystal of the
aluminum alloy powder, various kinds of intermetallic compounds (Cu-Al,
Mg-Si, Al-Cu-Fe, Al-Mn, etc.) will be deposited and produced. In the case
of the aluminum alloy powder employed by the present invention, although
it contains a large amount of alloy-constituting elements, as it is
manufactured by the ultra-rapidly solidification (cooling speed:
104°C/sec or
higher) such as the SWAP method, there hardly occurs generation of such
deposits. Even if does, the amount of deposits will be small and the powder
will be maintained under a supersaturated solid solution condition. In the
spark plasma sintering process, these elements will deposit as intermetallic
compounds. As this sintering process is accomplished under the low
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temperature and low pressure application conditions, the deposited
compounds phase is fine (particle size of 1 a m or less), thus not impairing
the superplasticity of the powder and contributing to reinforcement of the
mechanical properties of the aluminum alloy product.
In the present invention, the plastic working of the preform is
carried out in a temperature range directly under a liquidus curve of the
alloy and under a strain working speed condition of 10-Z/sec or higher. The
optimum range for the plastic working temperature T is: Tliq - 35°C<-
_T~
Tliq -10°C. With a high strain speed working in the temperature
range
(about 515-540°C) directly under the liquidus curve, it exhibits a high
ductility of 200% or more and its deformation flow stress is extremely low as
20MPa or lower.
Therefore, the preform according to the present invention allows
e~cient plastic working at high speed and under low pressure application
condition, thus improving the productivity of various components in powder
metallurgical production, lessening wear of the mould for improvement of
its durability and allowing also the form accuracy of a component having a
complex shape. The mold article of the invention formed in this manner is
advantageous in terms of costs and dimensional accuracy.
[first embodiment]
As a first mode of embodiment of the present invention relating to
an internal combustion engine component, details of a piston will be
described with reference to the drawings.
The internal-combustion engine piston according to the present
invention is foz~ned, like the preform of the invention described above, by
sintering more than two kinds of aluminum alloy powder having differing
contents of transition metal elements) such as Fe into a monolithic
construction as a piston prefoxm and then subjecting this piston preform to
CA 02382104 2002-03-O1
a compression plastic working by e.g. a backward extruder to form it into
the mold article. In this article, at least a portion of the piston top is
formed as a functionally gradient layer containing a large amount of the
transition metal such as Fe, so that the article achieves superior high
temperature strength in this functionally gradient layer and the entire
article is formed light weight by restricting the content of the transition
metals) such as Fe.
More particularly, there is prepared Al-Fe alloy powder such as Al-
12 Si-5 to 15 Fe which comprises an aluminum alloy powder containing 1-15
wt% of one or more elements selected from a group of transition metals
consisting of Fe, Cr, Ni, Zr, Mn, Mo and Ti; 10-30 wt% of Si, 0.5-5 wt% of Cu,
1-5 wt% of Mg and the rest substantially of Al, having a crystal size equal to
or less than 2 a m and a powder particle size equal to or greater than
30 ~ m. There is also prepared Al-Si alloy powder such as Al-12 Si-5 or
Al-l7Si which contains a smaller amount of Fe. Then, these materials are
charged into e.g. a mould in such a manner that the Al-Fe alloy powder
forms the piston top and the Al-Si alloy powder forms the remaining main
body portion and they are sintered together by the above-described spark
plasma sintering process, thereby to form a piston preform 10. In the
resultant piston preform 10, its piston top 1 may be formed as the
functionally gradient layer containing a greater amount of transition metal
element, Fe, than the main body portion 2.
Next, this piston preform 10 is set into a mould 21 of a backward
extruder as shown in Fig. 3 in such a manner that its piston top 1 is located
downward. Then, a punch 20 is driven to effect a compression plastic
working on the piston preform 10.
As this piston preform 10 has superplasticity as described
hereinbefore, the compression plastic working can be carried out e~ciently
at high speed and under low pressure application condition, thus reducing
the time required for the working.
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Then, in this mold article of the invention formed in the manner
described above, such portions as piston pin boss 30, piston ring grooves 40,
etc. are formed, as shown in Fig. 4, whereby an internal-combustion engine
piston 100 of the invention is obtained, having its piston top 1 formed as the
functionally gradient layer containing a greater amount of Fe or the like
than the main body portion 2. For instance, the piston top 1 may have a
high temperature strength of 250 MPa/300°C. And, as the piston top 1
containing the greater amount of Fe is smaller than the main body portion 2
of the piston, which is formed of the light weight Al-Si alloy, the internal
combustion engine piston 100 may be formed light weight as a whole.
(second embodiment)
As a second mode of embodiment of the present invention relating
to an internal-combustion engine component, details of a further piston will
be described with reference to the drawings.
The piston relating to the present invention is a mold article
formed by sintex~ng aluminum alloy powder and magnesium alloy powder
into a monolithic construction as a piston preform and then subjecting this
piston preform to a compression plastic working by e.g. a backward extruder,
like the first mode of embodiment. At least a portion of the piston top is
formed as a functionally gradient layer containing a large amount of
transition metal element such as Few and the remaining main body portion
thereof is formed as a super light-weight magnesium alloy layer. So that,
this piston has superior high temperature strength at the piston top facing a
combustion chamber and the piston as a whole is formed light weight.
More particularly, there are prepared an aluminum alloy powder
containing a large amount of Fe as a transition metal element, such as Al-12
Si-5 to 15 Fe, and a magnesium alloy powder. With these respective alloy
powders, a piston preform 10 shown in Fig. 2 is formed such that its piston
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CA 02382104 2002-03-O1
top 1 is formed of the aluminum alloy layer of e.g. Al-12 Si-8 Fe and the
remaining piston main body portion 2 is formed of the magnesium alloy
layer of Mg-Al-Zn-Mn-Si alloy powder. This sintered piston preform 10
consists of the piston top 1 which is the functional gradient layer containing
a large amount of Fe as the transition metal element and of the piston main
body portion 2 which is the super light-weight magnesium alloy layer.
Then, like the first mode of embodiment described above, the piston
preform 10 is subjected to a compression plastic working as illustrated in
Fig. 3.
As this piston preform 10 too has superplasticity, it allows efficient
compression plastic working at high speed and under low pressure
application condition, so that the working time may be reduced.
For obtaining the mold article of the invention, like the first mode
of embodiment described above, as illustrated in Fig. 4, such portions as a
piston pin boss 30, piston ring grooves 40, etc. are formed, whereby an
internal-combustion engine piston 100 of the invention is obtained In this
piston, the piston top 1 may be formed as the functionally gradient layer
containing a large amount of Fe or the like whereas the piston main body
portion 2 may be formed as the super light-weight magnesium alloy layer.
For instance, the piston top may have a high temperature strength of 250
MPa/300°C. And, as the piston top 1 is smaller than the piston
main body
portion 2 which is formed entirely of the light weight magnesium alloy, the
internal combustion engine piston 100 may be formed still lighter as a
whole.
Next, Table 1 below shows results of measurement of tensile
strengths of the piston preform 10 sintered as the monolithic construction
described above, the measurement being done at the joined portion between
the aluminum alloy layer and the magnesium alloy layer of the preform.
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Table 1
temperature tensile strength of joined
portion
a)
RT (room tem erature) 240
100C 230
200C 200
As may be understood from Table 1, the piston preform 10 of the
invention exhibits high tensile strengths at the joined portion,
demonstrating the magnesium alloy layer and the aluminum alloy layer
sintered and integrated together effectively.
Next, Table 2 below shows results of measurement of tensile
strengths of the piston 100 formed by the plastic working of the above-
described piston preform 10, the measurement being done at the piston top
1 (functionally gradient layer), the piston main body portion 2 (magnesium
alloy layer) and the respective joined portion, respectively.
Table 2
temperature tensile strengthtensile strengthtensile strength
of joined portionof piston top of piston main
1
(lVIPa) (lVIPa) body portion
2
a)
RT (room 386 434 400
temperature)
100C 350 420 370
200C 300 380 320
As may be understood from Table 2, this piston 100 of the present
invention exhibits superior high temperature strength especially at its
piston top to be exposed to a combustion chamber and exhibits good high
temperature strengths also at the joined portion and the main body portion.
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(other embodiments)
<1> In the foregoing embodiments, the entire piston top 1 of the piston
preform 10 is formed as the functionally gradient layer. Alternatively, the
piston preform 10 of the invention may be formed as shown in Fig. 5.
In the piston preform 10 shown in Fig. 5, the outer periphery 3 of
the piston including the piston top is formed as the functionally gradient
layer containing a greater amount of transition metal element such as Fe
than the remaining main body portion 2. The internal combustion engirxe
piston formed by compression plastic working of such piston preform has
distinguished high temperature strength or abrasion resistance in its outer
periphery, and since its inner portion can be formed of the light weight Al-Si
alloy layer, the piston is light weight as a whole. Further, if the inner
portion is formed of the magnesium alloy layer, further weight reduction is
made possible.
Moreover, the outer peripheral portion 3 may be formed as a
functionally gradient layer containing a large amount of a transition metal
element having abrasion resistance.
Further, in the case of a further piston preform 10 shown in Fig. 6,
a center portion as a portion of the piston top is formed as the functionally
gradient layer containing a greater amount of transition metal element such
as Fe than the remaining main body portion 2. Then, an internal-
combustion engine piston formed by compression plastic working of such
piston preform may be foxmed with e.g. a cavity at the center of the piston
top for allowing initial combustion to take place at this cavity. The cavity
may be formed as the above-described functionally gradient layer for
enhanced high temperature strength, while the remaining main body
portion may be formed as the light weight Al-Si alloy for forming the piston
light weight. Also, in this case too, further weight reduction will be made
CA 02382104 2002-03-O1
possible if the main body portion is formed of the magnesium alloy layer.
Further, as shown in Fig. 6, 1-30 vol% of ceramics powder having a
particle size of 5 ~ m or less may be added to the aluminum alloy powder for
forming a portion of the piston preform 10, so that at least a portion (5) of
the lateral side of the piston, such as the groove forming portion for forming
the piston ring grooves, may be formed as an abrasion resistant portion
containing the ceramics powder, so that the piston preform 10 may be
provided with abrasion resistance without reduction in the superplasticity
of the preform.
<2> Next, as internal-combustion engine components relating to the
present invention, constructions of other components than the pistons 100
described in the foregoing embodiments will be described.
First, in case a cylinder liner 200 shown in Fig. 7 is constructed as
an internal-combustion engine component relating to the present invention,
its inner face portion 101 facing a combustion chamber may be formed as
the functionally gradient layer described above for obtaining high
temperature resistance whereas the other outer face portion 102 may be
formed as the magnesium alloy layer for achieving overall weight reduction.
Further, in case a valve 200 shown in Fig. 8 is constructed as an
internal-combustion engine component relating to the present invention, its
head portion 201 facing the combustion chamber may be formed as the
magnesium alloy layer or the Al alloy layer for achieving overall weight
reduction.
INDUSTRIAL APPLICABILITY
The preform and the mold article formed by plastic working of the
preform according to the present invention are useful as a piston, a cylinder
liner or a valve especially for an internal-combustion engine having a high
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compression ratio in which its combustion chamber is exposed to a high
temperature. Further, it is useful also as e.g. a piston for an internal-
combustion engine in which stratified charge is effected for combusting
high-concentration fuel adjacent a plug of the combustion engine. And, it is
useful as a piston of such component which requires both high temperature
strength and weight reduction.
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