EP1231286A1

EP1231286A1 - Preliminarily formed article and formed article and parts for internal-combustion engine

Info

Publication number: EP1231286A1
Application number: EP00962851A
Authority: EP
Inventors: Eiji Kubota Corporation Yasu; Noushi Kubota Corporation Kuroishi; Setsuo Kubota Corporation Fujino
Original assignee: Kubota Corp
Current assignee: Kubota Corp
Priority date: 1999-09-27
Filing date: 2000-09-25
Publication date: 2002-08-14
Also published as: WO2001023629A1; AU7444800A; KR20020029402A; CA2382104A1

Abstract

In the present invention, a preform (10) to be formed into an internal-combustion engine component (100) by plastic working of 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 µm and equal to or less than 2 µm and a powder particle size equal to or greater than 30 µm and equal to or less than 1000 µm, said two kinds of aluminum alloy powder having different amounts of said transition metal(s) from each other; and at least a portion (1) of an outer surface of the preform is formed as a functionally gradient layer having a greater amount of said transition metal(s) than the other main body portion (2) of the preform.

Description

TECHNICAL 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 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 efficiency, 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 MPa/300°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 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 inexpensively.
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 0.05µm and equal to or less than 2 µm and a powder particle size equal to or greater than 30 µm and equal to or less than 1000 µm, said two kinds of aluminum alloy powder having different amounts of said transition metal(s) 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 metal(s) 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 or greater than 0.05µm and equal to or less than 2µm and a powder particle size equal to or greater than 30 µm and equal to or less than 1000 µm, 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 100µm) is employed, its strain working speed is as low as about 10^-3 to 10^-4/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 (ε). 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.
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 formed 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 metal(s) while the remaining main body portion of the preform is formed as the magnesium alloy layer. Further, as this preform has high-speed superplasticity 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 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 0.05µm and equal to or less than 2µm and a powder particle size equal to or greater than 30 µm and equal to or less than 1000 µm, said two kinds of aluminum alloy powder having different amounts of said transition metal(s) 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 metal(s) 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 µm and equal to or less than 2µm and a powder particle size equal to or greater than 30 µm and equal to or less than 1000 µm, 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 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-Ti alloy may be employed in addition to the Al 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 function/effect 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-12Si-5 to 15 Fe is employed preferably. And, in the remaining main body portion, Al-12Si or Al-17Si 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 µm) 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, Ti; 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 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 facing 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 containing 1-30 vol% of ceramics powder having a particle diameter equal to or less than 5 µm, 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 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 (Al₂O₃), 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 µm. 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 difficult. 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 the preform shown in Fig. 2,
Fig. 4 is a perspective view of an internal-combustion engine piston formed by the compression plastic working 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 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 lands of them are added) is set as equal to or greater than 1 wt% is to obtain sufficient 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 µm and equal to or less than 2 µm. The crystal size is set as equal to or greater than 0.05 µm, because it is difficult with the currently available technique to manufacture powder having a crystal size equal to or less than 0.05 µm. Also, the crystal size is set as equal to or less than 2 µm 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 µm is that if the powder particle size exceeds 1000 µm, it becomes difficult 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 efficiently by the atomization process (cooling 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.
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 MPa/300°C or more.
The magnesium alloy powder employed by the present invention preferably has a crystal size equal to or greater than 0.05 µm and equal to or less than 10 µm and a powder particle size equal to or greater than 30 µm and equal to or less than 500 µm.
The crystal size is set as equal to or greater than 0.05 µm because it is difficult with the presently available technique to manufacture powder having a crystal size equal to or less than 0.05 µm. The crystal size is set also as equal to or less than 10 µm in order to ensure the superplasticity.
Further, the reason why the powder particle size is set as equal to or greater than 30 µm 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 µm is that if the powder particle size exceeds 500 µm, 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.
For obtaining the mold article of the invention, prior to the molding (compression plastic 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 Al 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 described 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 points within the powder material will be moved and dispersed within the entire material in association with repeated ON/OFF of the current/voltage 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 Al 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 temperature and low pressure application conditions, the deposited compounds phase is fine (particle size of 1 µm 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^-2/sec or higher. The optimum range for the plastic working temperature T is: T_liq - 35°C≦T≦ T_liq -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 efficient 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 formed, like the preform of the invention described above, by sintering more than two kinds of aluminum alloy powder having differing contents of transition metal element(s) such as Fe 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 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 metal(s) 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 µm and a powder particle size equal to or greater than 30 µm. There is also prepared Al-Si alloy powder such as Al-12 Si-5 or Al-17Si 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 efficiently at high speed and under low pressure application condition, thus reducing the time required for the working.
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 sintering 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 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.

temperature tensile strength of joined portion (MPa)

RT (room temperature) 240

100°C 230

200°C 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.

temperature	tensile strength of joined portion (MPa)	tensile strength of piston top 1 (MPa)	tensile strength of piston main body portion 2 (MPa)
RT (room temperature)	386	434	400
100°C	350	420	370
200°C	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.

(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 engine 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 formed 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 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 µm 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 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.

Claims

A preform (10) 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 µm and equal to or less than 2 µm and a powder particle size equal to or greater than 30 µm and equal to or less than 1000 µm, said two kinds of aluminum alloy powder having different amounts of said transition metal(s) from each other; and at least a portion (1) of an outer surface of the preform is formed as a functionally gradient layer having a greater amount of said transition metal(s) than the other main body portion (2) of the preform.
A preform (10) 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 or greater than 0.05 µm and equal to or less than 2 µm and a powder particle size equal to or greater than 30 µm and equal to or less than 1000 µm, said magnesium alloy powder having a greater amount of Mg than said aluminum alloy powder.
A mold article (100) characterized in that the mold article is formed by plastic working of the preform (10) according to claim 1 or 2.
An internal-combustion engine component (100), characterized in that the component is formed by plastic working of a preform (10), said preform being 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 µm and equal to or less than 2 µm and a powder particle size equal to or greater than 30 µm and equal to or less than 1000 µm, said two kinds of aluminum alloy powder having different amounts of said transition metal(s) from each other; and at least a portion (1) of the preform to be exposed to a combustion chamber is formed as a functionally gradient layer having a greater amount of said transition metal(s) than the other main body portion (2) of the preform.
An internal-combustion engine component (100), characterized in that the component is formed by plastic working of a preform (10), 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 0.05µm and equal to or less than 2 µm and a powder particle size equal to or greater than 30 µm and equal to or less than 1000 µm, said magnesium alloy powder having a greater amount of Mg than said aluminum alloy powder, and at least a portion (1) of the preform to be exposed to a combustion chamber is formed of said aluminum alloy powder layer and the other main body portion (2) of the preform is formed of said magnesium alloy powder layer.
An internal combustion engine component (100) according to claim 4 or 5, characterized in that said component is constructed as a piston (100) having a piston top (1) as said portion to be exposed a combustion chamber.
An internal combustion engine component (100) according to any one of claims 4-6, characterized in that the component is formed by plastic working of the preform (10), wherein a portion (5) of the preform (10) formed as the at least one portion of the surface is formed by solidifying said aluminum alloy powder together with ceramics-containing powder containing 1-30 vol% of ceramics powder having a particle diameter equal to or less than 5 µm, said at least one portion of the surface being constructed as an abrasion resistant portion containing the ceramics.