Piston for an internal combustion engine and a method for producing same

Abstract: A piston for an internal combustion engine comprises a piston main body (1) being formed of at least two different materials (1A,1B) having different strengths. Said materials being integrally bonded by forging.

Description: This invention relates to a piston for an internal combustion engine according to the preamble of independent claim 1 and to a method of manufacturing a piston for an internal combustion engine according to the preamble of independent claim 9. With regard to a piston for use in a reciprocating engine for internal combustion engine, such as a 2 cycle or 4 cycle gasoline engine or diesel engine, there is a demand for an improvement in strength and in abrasion resistance. There is also a demand for further reduction of the weight of the piston so as to reduce the reciprocating force of inertia thereof with a view toward an increase of an output and a reduction of vibration of the engine. Thus, a material for the piston is required to be light in, to permit the formation into a thin wall, to be low in permanent set at a high temperature when molded into a thin wall, to be high in strength and to be high in abrasion resistance. As such a material for pistons, for example, an aluminum alloy containing light weight aluminum (Al) as a substrate, silicon (Si) for increasing abrasion resistance and resistance to baking and copper (Cu) and magnesium (Mg) for increasing strengths has been hitherto used. Such an aluminum alloy is generally cast into a primary molded article of a piston main body. In a piston for an internal combustion engine formed of such an aluminum alloy, the reaction force from a conrod during the operation of the engine acts on a pin bore portion of the piston main body from a piston pin, so that a periphery of the pin bore portion which provides a sliding contacting surface of the piston pin is deteriorated (plastic deformation). Thus, there is a fear that the piston is broken as a result of the creation of rattling. To cope with this problem, the pin bore portion has been conventionally reinforced by casting a high strength member in the pin boss section of the piston main body. On the other hand, in a piston for internal combustion engine used for a reciprocating engine, a head section thereof which is exposed in a combustion chamber is required to have a very high heat resistance, while a skirt section thereof which is adapted for slidably contacting with an inside wall of a cylinder is required to have a very high abrasion resistance. Additionally, the material cost should be saved and the weight should be reduced. Thus, various proposals have been hitherto made to form a composite piston main body having different portions made of different materials, rather than to uniformly improve the piston as a whole using the same material. (Refer, for example, JP-A-Sho-63-126661, JP-A-Hei-1-180927 and JP-A-Hei-5-320788) In the above-described conventional piston for an internal combustion, when a piston main body formed of a composite material having different material portions is manufactured, a method is adopted in which respective portions made of different materials are first produced and then integrated by welding or in which one portion is first shaped using one material and placed in a mold into which the other material after fusing is poured to surround the one portion and cast to integrate them. Therefore, as compared with a case in which a piston main body is manufactured by a single material, process steps are increased and the production time is increased, so that the manufacturing costs are increased. In a case where respective parts are first produced using different materials and then integrated by welding, the strength of the materials per se is occasionally lowered at positions near the bonding portion due to the heat of the welding. In a case where one part formed of one material is covered by casting with the other material into a unitary molding, there is a fear that the bonding strength at the bonding boundaries between the different materials is not sufficient. A piston of an internal combustion engine and such a method of manufacturing a piston as above described are known from US-A-4 364 159, wherein two different aluminium alloys are used for forming said piston by forging. Accordingly, it is an objective of the present invention to provide an improved piston as indicated above having an improved strength and abrasion resistance as well as an excellent bonding strength between the different materials. According to the present invention, this objective is solved by a piston according to independent claim 1. In the above-mentioned piston for an internal combustion engine, since the pin bore portion may be reinforced by casting a high strength member in the pin boss section of the piston main body, it is necessary to enlarge the pin boss section. This is not advantageous in attempting to reduce the weight of the piston main body by making the pin boss section compact. Additionally, there is a problem that the bonding strength between the piston base material and the high strength member because of casting is adopted. On the other hand, with a structure in which a piston main body is composed by partially changing the material thereof, a head section and a ring groove portion(or only the ring groove portion) are made of a high strength material because of limitations in manufacturing such as friction welding and weld padding, while the pin boss section is retained as the piston base material. As a consequence, the pin bore portion serving as a sliding contacting surface of the piston pin, which is subjected to a high temperature because of the flow of heat from a head section, is not improved in its strength and abrasion resistance at a high temperature. Moreover, in the above-described piston for an internal combustion, since a reaction force from a conrod at the time of explosion combustion acts on a piston main body during the operation of the engine from a piston pin or since a reaction force, although a weak force, from a cylinder side generated as a result of the force acting from the conrod during the up stroke of the piston acts on the piston main body, the outer peripheral surface of the piston main body is strongly pressed to the cylinder wall. Namely, as shown in Fig. 23, an explosion pressure P mainly acts on an upper surface of a head section of the piston main body at the time of explosion combustion. As a result, when a crank shaft disposed below the piston main body is rotated rightward, a reaction force F from the conrod acts on the pin hole portion, so that a reaction force from the cylinder acts on the outer peripheral surface of a right half of the piston main body as a distribution load f. During the stroke in which the piston main body moves from the bottom dead point to the top dead point, a driving force F' from the conrod side mainly acts on the pin hole portion from the lower right side. As a result, a force of inertia Ma (a product of the mass M of the piston main body 1 and an acceleration a) acts on the center of gravity of the piston main body, so that a reaction force from the cylinder acts on the outer peripheral surface of a left half of the piston main body as a distribution load f'. When the top land of the piston main body is enlarged (elongated in the direction of the sliding movement of the piston) for the purpose of improving the strength for withstanding such a force of pressing the outer peripheral surface of the piston main body to the cylinder wall, the amount of the exhaust gas remaining in the gap between the top land and the cylinder wall is increased, so that the exhaust gas from the engine has an increased amount of HC, etc. Hitherto, an attempt has been made to increase the strengths and the abrasion resistance of a portion adjacent a ring groove section by partially changing the material of which the piston main body is made. With this measure, however, the workability is lowered because the material of such a portion is integrated into the piston main body by welding, etc. Further, the heat at the time of welding reduces the strength surrounding the welded portion. Moreover, since only the portion adjacent the ring groove section is strengthened, the strength for withstanding the above-described pressing to the cylinder wall and the abrasion resistance cannot be improved in the skirt section of the piston main body. The material of the piston main body is partially changed to a material having high strength and abrasion resistance. Such a material has generally a high material unit cost for reasons of special constituting elements. The alloy has a large specific gravity because of a large content of large specific gravity elements. Thus, it is necessary to reduce the amount of such a material per one piston in order to save the material costs and to reduce the weight of the piston. Therefore, according to a preferred embodiment, one of said materials having a higher strength extends in an outer periphery of said piston main body from an upper end of a head section to at least a skirt section below a ring groove portion and in that said piston main body is prepared by forging such that the length of said material having a higher strength from said upper end of said head section is greater in an intermediate portion between a pair of pin bosses than in a vicinity of each of said pair of pin bosses. Moreover, it is possible that said skirt section has a lower end provided with cut away portions so that the entire region of the outer peripheral surface of said piston main body is made of said material having a higher strength. According to a further preferred embodiment, at least an outer peripheral portion of said upper end of said head section is made of said material having a higher strength. According to still another preferred embodiment, said material having a higher strength is an aluminum alloy which is obtained by solidifying a rapidly solidified powder, which contains silicon (Si) in an amount of 10-22 % by weight and which has initial crystal silicon with an average particle diameter of not greater than 10 µm. According to still another preferred embodiment, said aluminum alloy obtained by solidifying the rapidly solidified powder contains non-metallic component particles, harder than silicon (Si) and having an average particle diameter of not greater than 10 µm, in an amount of 1-10 % by weight. According to still another preferred embodiment, said material having a higher strength is an aluminum alloy obtained by solidifying the rapidly solidified powder and containing non-metallic component particles, harder than silicon (Si) and having an average particle diameter of not greater than 10 µm, in an amount of 1-10 % by weight. According to still another preferred embodiment, said material having a higher strength is an aluminum alloy which is obtained by solidifying the rapidly solidified powder, which contains iron (Fe) in an amount of 1-10 % by weight and in which the average particle diameter of a compound of the iron is not greater than 10 µm. It is a further objective of the present invention to provide an improved method as indicated above facilitating an improved strength and abrasion resistance of said piston as well as ensuring an excellent bonding strength between the different materials. According to the present invention, this objective is solved by a method of manufacturing a piston for an internal combustion engine as indicated above according to independent claim 9. According to an preferred embodiment, while extruding an aluminum alloy continuous casting material through a first die, a rapidly solidified powder of an aluminum alloy provided around said extruded continuous casting material is coextruded, with heating and under a pressure, together with said continuous casting material through a second die having a greater diameter than said first die to obtain a columnar body composed of a core material made of said material of said continuous casting material and an outer peripheral material integrally bonded to said core and made of the material of said rapidly solidified powder, in that said columnar body is cut into a predetermined size to obtain a raw material for forging, and in that said raw material for forging is subjected to a primary molding step and to a succeeding processing, thereby obtaining said piston main body as a finished article. According to another preferred embodiment, said rapidly solidified powder of an aluminum alloy contains silicon (Si) in an amount of 10-22 % by weight and in that the average particle diameter of initial crystal silicon is not greater than 10 µm. According to still another preferred embodiment, said rapidly solidified powder of an aluminum alloy contains a non-metallic component particles, harder than silicon (Si) and having an average particle diameter of not greater than 10 µm, in an amount of 1-10 % by weight. According to still another preferred embodiment, said component particles harder than silicon (Si) are at least one of those selected from silicon carbide (SiC), aluminum oxide (Al 2 O 3 ) and aluminum nitride (AlN). According to still another preferred embodiment, said rapidly solidified powder of an aluminum alloy contains iron (Fe) in an amount of 1-10 % by weight and in that the average particle diameter of a compound of the iron is not greater than 10 µm. Other preferred embodiments of the present invention are laid down in the further dependent claims. In the following, the present invention is explained in greater detail with respect to several embodiments thereof in conjunction with the accompanying drawings, wherein: Fig. 1 shows one example (first embodiment) of a piston main body according to a piston for an internal combustion engine of the present invention, (A) being a side view, (B) being a top view and (C) being a vertical cross-sectional view taken along the line C-C in Fig. (B). Fig. 2 is a vertical cross-sectional view showing a material for producing the piston main body shown in Fig. 1. Fig. 3 is a sectional view showing an example of the state for the molding of a portion of a material having a high strength of a composite piston material shown in Fig. 2. Fig. 4 is a sectional view showing an example of the state for forging a primary molded article of a piston main body from the composite piston material shown in Fig. 2. Fig. 5 shows another embodiment (second embodiment) of a piston main body according to a piston for an internal combustion engine of the present invention, (A) being a side view, (B) being a top view and (C) being a vertical cross-sectional view taken along the line C-C in Fig. (B). Fig. 6 is a vertical cross-sectional view showing a material for producing the piston main body shown in Fig. 5. Fig. 7 is a sectional view showing an example of the state for the molding of a portion of a material having a high strength of a composite piston material shown in Fig. 6. Fig. 8 is a sectional view showing an example of the state for forging a primary molded article of a piston main body from the composite piston material shown in Fig. 2. Fig. 9 shows a further embodiment (third embodiment) of a piston main body according to a piston for an internal combustion engine of the present invention, (A) being a side view, (B) being a top view and (C) being a vertical cross-sectional view taken along the line C-C in Fig. (B). Fig. 10 is a vertical cross-sectional view showing a material for producing the piston main body shown in Fig. 9. Fig. 11 is a sectional view showing an example of the state for the molding of a portion of a material having a high strength of a composite piston material shown in Fig. 10. Fig. 12 is a sectional view showing an example of the state for forging a primary molded article of a piston main body from the composite piston material shown in Fig. 10. Fig. 13 are graphs showing a difference in abrasion resistance according to a difference in material for producing a piston main body in an example of using a material having a high strength (embodiment A-1 containing SiC, embodiment A-2 without SiC) and an example of using a material having a lower strength (embodiment B). Fig. 14 are graphs showing a difference in fatigue strength at temperatures of 25, 150 and 250 according to a difference in material for producing a piston main body in an example of using a material having a high strength (embodiment A-1 containing SiC, embodiment A-2 without SiC) and an example of using a material having a lower strength (embodiment B). Fig. 15 shows one example (first embodiment) of a piston main body according to a piston for an internal combustion engine of the present invention, (A) being a side view, (B) being a top view, (C) being a vertical cross-sectional view taken along the line C-C in Fig. (B) and (D) being an unfolded view. Fig. 16 is a sectional view showing an example of the state for forging a primary molded article of a piston main body as shown in Fig. 16 from a composite piston material. Fig. 17 a vertical cross-sectional view showing an example of a composite piston material for the primary molding of the piston main body shown in Fig. 15 by forging. Fig. 18 is a sectional view showing an example of the state for producing a portion having a high strength of the composite piston material shown in Fig. 17. Fig. 19 shows another embodiment (second embodiment) of a piston main body according to a piston for an internal combustion engine of the present invention, (A) being a side view, (B) being a top view and (C) being a vertical cross-sectional view taken along the line C-C in Fig. 15(B). Fig. 20 is a sectional view showing an example of the state for forging a primary molded article of a piston main body as shown in Fig. 19 from a composite piston material. Fig. 21 is a vertical cross-sectional view showing an example of a composite piston material for the primary molding of the piston main body shown in Fig. 19 by forging. Fig. 22 is a sectional view showing a state of producing the composite piston material shown in Fig. 21. Fig. 23 is a side view explanatory of the forces acting on a piston main body during operation of an engine. Fig. 24 shows one example of a piston main body produced according to a method of the present invention, (A) being a side view, (B) being a top view, and (C) being a vertical cross-sectional view taken along the line C-C in Fig. (B). Fig. 25 shows an illustration explanatory of one example of a method of producing a continuous casting material of an aluminum alloy used as a material for the manufacturing method of the present invention. Fig. 26 shows an illustration explanatory of one example of a method of producing a rapidly solidified powder of an aluminum alloy used as a material for the manufacturing method of the present invention. Fig. 27 shows a state of producing a composite piston forging material from a continuous casting material and a rapidly solidified powder in one embodiment of the method of the present invention, (A) being a cross-sectional side view taken along the line A-A in Fig. B, and (B) being a cross-sectional plan view taken along the line B-B in Fig. A. Fig. 28 shows an example of a composite piston forging material produced in the state shown in Fig. 27, (A) being a plane view, and (B) being a sectional view taken along the line B-B in Fig. A. Fig. 29 shows another example of a composite piston forging material produced in the state shown in Fig. 27, (A) being a plane view, and (B) being a sectional view taken along the line B-B in Fig. A. Fig. 30 is a cross-sectional side view showing an example of the state for forging a composite piston material as shown in Fig. 28 into a primary molded article of a piston main body. Embodiments of a piston of an internal combustion engine according to the present invention will be described below with reference to the drawings. Fig. 1 shows a piston main body of one embodiment of the piston of an internal combustion engine according to the present invention; (A) is a side view as seen in the axial direction of a pin bore portion, (B) shows an upper surface of a head section as seen from above, and (C) shows a vertical cross-section taken along the line C-C in Fig. (B). The piston main body 1 is obtained by integrally molding by forging a thick cylindrical material into a primary molded article having a head section 2 having an upper surface to be exposed in a combustion chamber and a skirt section 3 to be slidably contacted with an inside surface of a cylinder such that the thickness of the wall is large in a side provided with a pin boss 4 and gradually decreases downward from the pin boss 4 in a side having no pin boss 4, the primary molded article being mechanically processed to chip unnecessary portions and to form a piston ring groove 5 and a pin bore portion 6, followed by, if necessary, a surface treatment such as plating, thereby finishing into a final product. The whole piston main body 1 is made of two kinds of materials 1A and 1B having different strengths and integrally bonded by forging in their boundaries. In the embodiment shown, the high strength material 1A distributes to occupy from an upper surface of the head section 2 to a ring groove portion 5, while the material 1 B with a lower strength than the material 1A distributes to occupy from a back surface of the head section 2 to the skirt section 3. At the same time, the high strength material 1A extends to an upper part of the pin boss 4 located below the ring groove portion 5, so that a part of an upper side of the pin bore portion 6 formed in the pin boss 4 is made of the high strength material 1A. The piston main body 1 of the present embodiment is produced by, as shown in Figs. 4(A) and 4(B), primary molding by forging of a composite piston raw material 10 obtained, as shown in Fig. 2, by simply superimposing two kinds of different materials 1A and 1B or by directly pressure bonding materials 1A and 1B after destroying oxidized surfaces at their boundaries by expanding small outer diameter materials by primary forging. As a result, in the piston main body 1, the high strength material 1A and the lower strength material 1B are extended by forging at their bonding surfaces, so that the area of the direct contact between them except the oxidized surfaces is increased, providing more tightly integrated state than that before forging. With regard to the forging of a primarily molded piston, as shown in Figs. 4(A) and 4(B), from the composite raw material 10, the forging is performed with a lower mold 22 previously heated while controlling the temperature between 250-450°C and an upper mold (punch) 21 similarly heated while controlling the temperature between 250-450°C. By such heat forging with the use of upper mold 21 and lower mold 22 previously heated to a controlled temperature, a primary molded article of the piston main body can be molded with a good dimensional accuracy by sufficient utilization of the ductility of the aluminum alloy. In the present embodiment, as the material having a high strength 1A for forming the above-described piston main body, there may be used, for example, an aluminum alloy obtained by solidification of rapidly solidified powder and containing aluminum (Al) as a base material and, additionally, 10-22 % by weight of silicon (Si), 1-10 % by weight of iron (Fe), 0.5-5 % by weight of copper (Cu), 0.5-5 % by weight of magnesium (Mg), 1 % by weight or less of manganese (Mn), 1 % by weight or less of nickel (Ni), 1 % by weight or less of chromium (Cr), 2 % by weight or less of zirconium (Zr) and 1 % by weight or less of molybdenum (Mo). One example of such material an aluminum alloy obtained by solidification of rapidly solidified powder and containing 17 % by weight of silicon (Si), 5 % by weight of iron (Fe), 1 % by weight of copper (Cu), 0.5 % by weight of magnesium (Mg), 0.01 % by weight of manganese (Mn), 0.01 % by weight of nickel (Ni), 0.01 % by weight of chromium (Cr), 1 % by weight of zirconium (Zr) and 0.01 % by weight of molybdenum (Mo). Further, in the present embodiment, as another example of the material having a high strength 1A, there may be used an aluminum alloy obtained by solidification of rapidly solidified powder and containing aluminum (Al) as a base material and 10-22 % by weight of silicon (Si), 1-10 % by weight of iron (Fe), 0.5-5 % by weight of copper (Cu), 0.5-5 % by weight of magnesium (Mg), 1 % by weight or less of manganese (Mn), 1 % by weight or less of nickel (Ni), 1 % by weight or less of chromium (Cr), 2 % by weight or less of zirconium (Zr), 1 % by weight or less of molybdenum (Mo) and, additionally, 1-10 % by weight of silicon carbide (SiC) which is used for improving the abrasion resistance and which is a component harder than silicon (Si). As one example of such an alloy, there may be mentioned an aluminum alloy obtained by solidification of rapidly solidified powder and containing 17 % by weight of silicon (Si), 5 % by weight of iron (Fe), 1 % by weight of copper (Cu), 0.5 % by weight of magnesium (Mg), 0.01 % by weight of manganese (Mn), 0.01 % by weight of nickel (Ni), 0.01 % by weight of chromium (Cr), 1 % by weight of zirconium (Zr), 0.01 % by weight of molybdenum (Mo) and 5 % by weight of silicon carbide (SiC). In each of the above-described materials having a high strength 1A, silicon (Si) and silicon carbide (SiC) are added to improve the abrasion resistance and resistance to baking by the existence of hard particles in the metal texture. Iron (Fe) is added to obtain a high strength at 200°C or more by dispersing and strengthening the metal texture. Copper (Cu) and magnesium (Mg) are added to improve the strength at 200°C or less. The amounts of these components outside the above-described ranges fail to obtain desired abrasion resistance, resistance to baking and required strengths at high temperatures. As the material having a lower strength 1B for forming the piston main body 1 together with the above-described material having a high strength 1A, there may be used an aluminum alloy conventionally used for casting as a melt production-type (continuous casting material), namely an aluminum alloy of a melt production-type containing aluminum (Al) as a base material and additionally 10-22 % by weight of silicon (Si), 1 % by weight or less of iron (Fe), 0.5-5 % by weight of copper (Cu), 0.5-2 % by weight of magnesium (Mg), 1 % by weight or less of manganese (Mn), 1 % by weight or less of nickel (Ni) and 1 % by weight or less of chromium (Cr). One specific example of such a material is an aluminum alloy of the melt production-type containing 19 % by weight of silicon (Si), 0.2 % by weight of iron (Fe), 4 % by weight of copper (Cu), 1 % by weight of magnesium (Mg), 0.1 % by weight of manganese (Mn), 0.1 % by weight of nickel (Ni) and 0.1 % by weight of chromium (Cr). With regard to the above-described rapidly solidified powder aluminum alloy as examples of the material having a high strength 1A, an ingot of an aluminum alloy is melted at about 700°C or more and the melt is sprayed like a fog to rapidly cool and solidify same at a cooling rate of at least 100°C/sec, thereby obtaining rapidly solidified powder (powder metal) of the aluminum alloy having an average particle diameter of about 100 µm. After being incorporated with powders of necessary constituent components, the rapidly solidified powder of the aluminum alloy is, as shown in Fig. 3, heated to 400-500°C and extruded into a round rod and solidified. The rod is cut into a predetermined size, thereby obtaining a disc-like material 1A, as shown in Fig. 2, of a composite piston material 10. In each of the examples of the material having a high strength 1A used in the present embodiment, each of the constituents such as silicon (Si) and iron (Fe) is dispersed in a finely divided state of an average particle diameter of 10 µm or less. Thus, for example, since silicon (Si) is dispersed in such a finely divided state of in the aluminum alloy texture that the particle diameter of the primary crystal silicon is 10 µm or less, even when the material 1A is stretched into a thin wall during the forging molding of the primary molded article of the piston main body 1, no cracks are formed in the primary silicon particles in the skirt section and, hence, the wrought piston main body 1 has an improved fatigue strength. In the case of the example in which silicon carbide (SiC) is contained, since the silicon carbide (SiC) is dispersed uniformly in the texture of the aluminum alloy in a finely divided state, an improved abrasion resistance can be obtained. With regard to iron (Fe), since a rapidly solidified powder aluminum alloy containing iron (Fe) in a finely divided state is molded by forging, the formation of coarse iron compounds is prevented so that a uniform metal texture free of coarse iron compounds which would cause stress concentration is obtainable. Therefore, the iron component can be added in a larger amount in comparison with the case in which a piston main body is primary molded by a conventional casting process, enabling the preparation of an alloy having a high strength. On the other hand, when a piston main body is subjected to primary molding by the conventional casting method using, as a material, an aluminum alloy containing a large amount of iron, coarse iron compounds are formed in the alloy upon cooling after the casting, so that the strengths are apt to be lowered. Further, the other constituting components are also contained in the aluminum allow powder as fine powder and since the aluminum alloy powder is formed in to a dense crystal texture through solidification and forging, these components do not cause a decrease in strength due to concentration of the stress in the crystal grain boundaries, thereby to improve the fatigue strength. Each of the examples of the above-described material having a high strength 1A and an example of the material having lower strength 1B are subjected to comparative tests with respect to the abrasion resistance and the fatigue strength. The results are as follows. Namely, Fig. 13 shows the results of fretting abrasion test (A test sample is used as a rotor). A rider of a predetermined material is repeatedly pressed against the rotor which is maintained in a swung state. The area of the abrasion marks in the contact surfaces represents the degree of abrasion) performed at a test temperature of 250°C to compare the abrasion resistance of each example of the material 1A having a high strength (Example A1 containing SiC, and Example A-2 containing no SiC) with an example (Example B) of the material having a lower strength 1B, from which it is appreciated that both materials 1A (Example A1 and Example A2) give higher abrasion resistance at a high temperature as compared with the material 1B (Example B). Fig. 14 shows the results of fatigue test (A sinuous load is applied to a test sample). The fatigue limit represents the number of repetition (one number represents one period of the sinuous curve) until the test sample has been broken) performed at test temperatures of 25°C, 150°C and 250°C to compare the fatigue strength of each example of the material having a high strength 1A (Example A1 containing SiC, and Example A-2 containing no SiC) with an example (Example B) of the material having a lower strength 1B, from which it is appreciated that both materials 1A (Example A1 and Example A2) give higher fatigue strength as compared with the material 1B (Example B) at any test temperature. Described above is one embodiment (first embodiment) of the piston for an internal combustion engine according to the present invention. Figs. 5 and 9 illustrate other embodiments (second and third embodiments) of the piston for an internal combustion engine according to the present invention. In these embodiments, the same materials 1A and 1B as those shown as examples in the above embodiment (first embodiment) are used. But the state of the distribution of the materials 1A and 1B of the piston main body is different. Namely, in the second embodiment according to the present invention, as shown in Fig. 5, the material having a high strength 1A distributes to occupy an area from the peripheral portion of the head section 2 to the ring groove portion 5 of the piston main body 1, whereas the material having a lower strength 1B distributes to occupy an area from the central portion of the head section 2 to the skirt section 3. The material having a high strength 1A extends up to an upper portion of the pin boss 4 located below the ring groove portion 5, so that a part of the upper side of the pin bore 6 formed in the pin boss 4 is made of the material having a high strength A. The piston main body 1 according to the second embodiment is obtained by hot forging of a composite piston material 10 composed of the two kinds of the materials 1A and 1B having different strengths as shown in Fig. 6 between a lower mold 22 preheated under a controlled state to 250-450°C and an upper mold (punch) 21 preheated under a controlled state to 250-450°C, as shown in Fig. 8, thereby to obtain a primary molded article. This is subjected to a machining treatment by mechanical processing such as for the chipping of unnecessary portions and for the formation of a piston ring groove 5 and a pin bore portion 6 and, if desired, to a surface treatment such as plating, thereby to form the final product. A composite piston material 10 as shown in Fig. 6 may be produced by heating and extruding a rapidly solidified powder of an aluminum alloy (containing powder of respective constituting components) into a hollow bar as shown in Fig. 7. After solidification, the bar is cut into a predetermined size to obtain a ring material 1A. This is then fitted to the material 1 B. The fitting is suitably interference fitting using an interference. However, transition fitting or running fitting may be adopted. In this case, the fitting engagement surfaces are extended by the generation of a surface pressure in the direction normal to the fitting engagement surfaces by forging, so that the oxidized film on the surfaces are broken to permit the direct bonding of the material 1A and 1B. In the third embodiment according to the present invention, as shown in Fig. 9, the material having a high strength 1A distributes to occupy an area from the peripheral portion of the head section 2 to the skirt section 3 located outside the ring groove portion 5 and the pin boss 4 of the piston main body 1, whereas the material having a lower strength 1 B distributes on the axis side of the piston main body 1 to occupy an area from the central portion of the head section 2 to the inside of the pin boss 4. The material having a high strength 1A extends the entire periphery of the outside the pin bore portion 6 formed in the pin boss 4. The piston main body 1 according to the third embodiment is obtained by hot forging of a composite piston material 10 composed of the two kinds of the materials 1A and 1B having different strengths as shown in Fig. 10 between a lower mold 22 preheated under a controlled state to 250-450°C and an upper mold (punch) 21 preheated under a controlled state to 250-450°C, as shown in Fig. 12, thereby to obtain a primary molded article. This is subjected to a machining treatment by mechanical processing such as for the chipping of unnecessary portions and for the formation of a piston ring groove 5 and a pin bore portion 6 and, if desired, to a surface treatment such as plating, thereby to form the final product. A composite piston material 10 as shown in Fig. 10 may be produced by extruding an aluminum alloy of a melt production material with heating into a round rod as shown in Fig. 11 and, at the same time, heating and extruding a rapidly solidified powder of an aluminum alloy (containing powder of respective constituting components) so as to cover the periphery of the round rod of an aluminum alloy of a melt production material. The two extruded portions are integrally solidified. The resulting composite column having central and peripheral portions made of different materials is cut into a predetermined size to obtain the composite material. According to each of the embodiments of the piston for internal combustion engines of the present invention, since that portion of a pin bore portion 6 which serves as a sliding contacting surface of a piston pin is formed of a material having a high strength 1A, the pin bore portion has improved strength and abrasion resistance without resorting to casting of a specific reinforcing member into the pin boss 4. The material 1A used in each of the above-described embodiments has a higher unit cost as compared with the material 1B, because of the mixing with silicon carbide (SiC) and iron (Fe), because of a high content of silicon (Si), because of necessity of mixing a variety of ingredients while controlling the amounts thereof, because of the use of special constituting components and because necessity of manufacturing step for producing rapidly solidified powder. Therefore, it is necessary to save the amount of the material used per one piston. Additionally, since the material 1A contains greater amount of elements having a high specific gravity and, hence, the resulting alloy has a greater specific gravity in comparison with the material 1B, it is necessary to save the amount of the material used per one piston. In this regard, each of the above-described embodiments can reduce the material costs and the weight of the piston main body 1 by using the material 1A only in a portion surrounding the pin bore portion 6. Further, in the first embodiment, the material 1A is used in the head section 2 and a peripheral portion of the head section 2 of the pin bore portion 6 on which the reaction force of the conrod at the time of explosion combustion and the operating force from the conrod during the up stroke of the piston, so that the piston main body 1 has a reduced weight while improving the rigidity and strengths around the ring groove portion 5 and the pin bore portion 6. In each of the above-described embodiments, since the top land portion extending from a vicinity of the ring groove portion 5 to the head section 2 is made of the material having a high strength 1A, the top land portion can withstand the force caused when the piston main body is swung about the piston pin by the reaction force of the conrod and piston slap at the time of the explosion combustion and when the top land is thereby strongly pressed against the cylinder wall. Because of the top land has a high strength, it is possible to make the top land portion small, so that the amount of an exhaust gas remaining in a gap between the top land and the cylinder wall can be reduced, resulting in the reduction of HC. In the second embodiment, the piston main body 1 has a more reduced weight because only the peripheral portion of the head section is made of the material 1A in view of the fact that the inside stress and the inside distortion generated in the head section 2 by the explosion combustion pressure acted on the upper surface of the head section 2 is larger in the outer peripheral portion than in the central portion of the head section 2. In each of the above-described embodiments, since a rapidly solidified powder aluminum alloy having a composition shown in the above examples is used as the material having a high strength 1A, it is possible to extend the service life of the piston main body 1 due to the abrasion resistance and the resistance to baking of the material 1A. Further, since the material 1A has a small coefficient of expansion, the thermal deformation of the piston main body 1 can be reduced. In particular, in the case of the third embodiment, since the skirt section 3 is made of the material 1A, the abrasion resistance of the piston main body 1 can be surely improved. Further, in each of the above-described embodiment, since a rapidly solidified powder aluminum alloy having a low thermal conductivity is used as the material having a high strength 1A, the conduction of heat from the head section having an upper surface exposed to the combustion chamber to the pin bore portion 6 is reduced, so that the abrasion resistance of the piston main body 1 which is subjected to high temperature is improved. In the second embodiment, an aluminum alloy of an ingot material having a high thermal conductivity is used as a material having a low strength 1B which is distributed to occupy an area from the central portion of the head section 2 to the skirt section 3, the heat of the head section 2 can be transmitted through the material 1B from the skirt section 3 to the cylinder wall with high efficiency, so that the temperature of the head section is lowered to release the thermal strength of the head section. As a result, it is possible to make the head section 2 thin so that the piston main body 1 can be reduced in weight. In each of the above-described embodiments, the piston main body 1 is primary molding obtained by forging a composite piston material 10 obtained by integrally bonding or simply superimposing the two kinds of materials 1A and 1B having different strengths, the bonding interface of the material 1A and material 1B of the piston main body 1 is extended by forging so that the resulting article is in a more tightly integrated state as compared with that before forging. Each of the embodiments of a piston for an internal combustion engine according to the present invention has been described in the foregoing. The present invention is, however, not limited to the above embodiments. For example, the two kinds of materials 1A and 1B having different strengths are not limited to those exemplified in the above embodiments. The present invention can be practiced using any other suitable materials. Also, the method of producing the piston main body 1 is not limited to the specific forging method as illustrated in the above embodiments, but, rather, any other suitable method can be utilized for the manufacture thereof. According to the piston for an internal combustion engine of the present invention as explained above, the strengths and abrasion resistance of a pin bore portion which provides a sliding contacting surface for a piston pin can be improved without preventing the reduction of the weight of the piston main body which would be caused by enlarging a pin boss. In the following, another embodiment of the present invention will be explained. Fig. 15 shows a piston main body of one embodiment of the piston of an internal combustion engine according to the present invention; (A) is a side view as seen in the axial direction of a pin bore portion, (B) shows an upper surface of a head section as seen from above, and (C) shows a vertical cross-section taken along the line C-C in Fig. (B). The piston main body 1 is obtained by integrally molding by forging a thick cylindrical material into a primary molded article having a head section 2 having an upper surface to be exposed in a combustion chamber and a skirt section 3 to be slidably contacted with an inside surface of a cylinder such that the thickness of the wall is large in a side provided with a pin boss 4 and gradually decreases downward from the pin boss 4 in a side having no pin boss 4, the primary molded article being mechanically processed to chip unnecessary portions and to form a piston ring groove 5 and a pin bore portion 6, followed by, if necessary, a surface treatment such as plating, thereby finishing into a final product. The piston main body 1 is made of two kinds of materials 1A and 1B having different strengths and integrally bonded by forging in their boundaries. In the embodiment, as shown in Fig. 15(D), the high strength material 1A extends from an upper surface of the head section 2 to a skirt section 3 below a ring groove portion 5, such that the length from the upper portion of the head section 2 is longer in an intermediate portion between two pin hole portions (pin bosses 4) than that in a vicinity of the pin bore portions (pin bosses 4). The high strength material 1A which is exposed on the outer peripheral surface of the piston main body 1 distributes, as shown in Fig. 15(C), in a periphery (outer peripheral surface side) of the piston main body, while the material with a lower strength 1B than the material 1A distributes to occupy from a center portion (axis side) to a lower end portion of the skirt section 3. The piston main body 1 of the present embodiment may be produced by primary molding by forging, as shown in Fig. 16, from a composite piston material 10, as shown in Fig. 17, composed of two kinds of materials 1A and 1 B fitted to each other and having different strengths. As a result, in the piston main body 1, the material having a high strength 1A and the material having a low strength 1B are bonded to each other such that the fitting engagement surfaces by forging are extended while being subjected to a tangential load and tightly integrated into a unitary structure. The fitting of the material 1A into the material 1B is suitably by interference fitting using an interference. However, transition fitting or running fitting may be adopted. In this case, the fitting engagement surfaces are extended by the generation of a surface pressure in the direction normal to the fitting engagement surfaces by forging, so that the oxidized film on the surfaces are broken to permit the direct bonding of the materials 1A and 1B. The forging of the composite piston material 10 into a primary molding of the piston as shown in Fig. 16 is carried out in a lower mold 22 composed of side molds 22a and a bottom mold 22b and preheated under a controlled state to 250-450°C and an upper mold (punch) 21 preheated under a controlled state to 250-450°C. By the hot forging using the upper and lower molds 21 and 22 each preheated to a controlled temperature, a primary molded article of the piston main body having a precise dimension can be obtained sufficiently utilizing the ductility of the aluminum alloy. In the present embodiment, as the material having a high strength 1A for forming the above-described piston main body, there may be used, for example, an aluminum alloy obtained by solidification of rapidly solidified powder and containing aluminum (Al) as a base material and, additionally, 10-22 % by weight of silicon (Si), 1-10 % by weight of iron (Fe), 0.5-5 % by weight of copper (Cu), 0.5-5 % by weight of magnesium (Mg), 1 % by weight or less of manganese (Mn), 1 % by weight or less of nickel (Ni), 1 % by weight or less of chromium (Cr), 2 % by weight or less of zirconium (Zr) and 1 % by weight or less of molybdenum (Mo). One example of such material is an aluminum alloy obtained by solidification of rapidly solidified powder and containing 17 % by weight of silicon (Si), 5 % by weight of iron (Fe), 1 % by weight of copper (Cu), 0.5 % by weight of magnesium (Mg), 0.01 % by weight of manganese (Mn), 0.01 % by weight of nickel (Ni), 0.01 % by weight of chromium (Cr), 1 % by weight of zirconium (Zr) and 0.01 % by weight of molybdenum (Mo). Further, in the present embodiment, as another example of the material having a high strength 1A, there may be used an aluminum alloy obtained by solidification of rapidly solidified powder and containing aluminum (Al) as a base material and 10-22 % by weight of silicon (Si), 1-10 % by weight of iron (Fe), 0.5-5 % by weight of copper (Cu), 0.5-5 % by weight of magnesium (Mg), 1 % by weight or less of manganese (Mn), 1 % by weight or less of nickel (Ni), 1 % by weight or less of chromium (Cr), 2 % by weight or less of zirconium (Zr), 1 % by weight or less of molybdenum (Mo) and, additionally, 1-10 % by weight of silicon carbide (SiC) which is used for improving the abrasion resistance and which is a component harder than silicon (Si). As one example of such an alloy, there may be mentioned an aluminum alloy obtained by solidification of rapidly solidified powder and containing 17 % by weight of silicon (Si), 5 % by weight of iron (Fe), 1 % by weight of copper (Cu), 0.5 % by weight of magnesium (Mg), 0.01 % by weight of manganese (Mn), 0.01 % by weight of nickel (Ni), 0.01 % by weight of chromium (Cr), 1 % by weight of zirconium (Zr), 0.01 % by weight of molybdenum (Mo) and 5 % by weight of silicon carbide (SiC). In each of the above-described materials having a high strength 1A, silicon (Si) and silicon carbide (SiC) are added to improve the abrasion resistance and resistance to baking by the existence of hard particles in the metal texture. Iron (Fe) is added to obtain a high strength at 200°C or more by dispersing and strengthening the metal texture. Copper (Cu) and magnesium (Mg) are added to improve the strength at 200°C or less. The amounts of these components outside the above-described ranges fail to obtain desired abrasion resistance, resistance to baking and required strengths at high temperatures. As the material having a lower strength 1B for forming the piston main body 1 together with the above-described material having a high strength 1A, there may be used an aluminum alloy conventionally used for casting as a melt production-type (continuous casting material), namely an aluminum alloy of a melt production-type containing aluminum (Al) as a base material and additionally 10-22 % by weight of silicon (Si), 1 % by weight or less of iron (Fe), 0.5-5 % by weight of copper (Cu), 0.5-2 % by weight of magnesium (Mg), 1 % by weight or less of manganese (Mn), 1 % by weight or less of nickel (Ni) and 1 % by weight or less of chromium (Cr). One specific example of such a material is an aluminum alloy of the melt production-type containing 19 % by weight of silicon (Si), 0.2 % by weight of iron (Fe), 4 % by weight of copper (Cu), 1 % by weight of magnesium (Mg), 0.1 % by weight of manganese (Mn), 0.1 % by weight of nickel (Ni) and 0.1 % by weight of chromium (Cr). With regard to the above-described rapidly solidified powder aluminum alloy as examples of the material having a high strength 1A, an ingot of an aluminum alloy is melted at about 700°C or more and the melt is sprayed like a fog to rapidly cool and solidify same at a cooling rate of at least 100°C/sec, thereby obtaining rapidly solidified powder (powder metal) of the aluminum alloy having an average particle diameter of about 100 µm. After being incorporated with powders of necessary constituent components, the rapidly solidified powder of the aluminum alloy is, as shown in Fig. 18, heated to 400-500°C and extruded into a hollow cylinder and solidified. The cylinder is cut into a predetermined size, thereby obtaining a ring-like material 1A. The material 1B is then fitted in the ring material 1A to obtain a composite piston material 10 as shown in Fig. 17. In each of the examples of the material having a high strength 1A used in the present embodiment, each of the constituents such as silicon (Si) and iron (Fe) is dispersed in a finely divided state of an average particle diameter of 10 µm or less in the powder aluminum alloy having an average particle diameter of about 100 µm. Thus, for example, since silicon (Si) is dispersed in such a finely divided state of in the aluminum alloy texture that the particle diameter of the primary crystal silicon is 10µm or less, even when the material 1A is stretched into a thin wall during the forging molding of the primary molded article of the piston main body 1, no cracks are formed in the primary silicon particles in the skirt section and, hence, the wrought piston main body 1 has an improved fatigue strength. In the case of the example in which silicon carbide (SiC) is contained, since the silicon carbide (SiC) is dispersed uniformly in the texture of the aluminum alloy in a finely divided state, an improved abrasion resistance can be obtained. With regard to iron (Fe), since a rapidly solidified powder aluminum alloy containing iron (Fe) in a finely divided state is molded by forging, the formation of coarse iron compounds is prevented so that a uniform metal texture free of coarse iron compounds which would cause stress concentration is obtainable. Therefore, the iron component can be added in a larger amount in comparison with the case in which a piston main body is primary molded by a conventional casting process, enabling the preparation of an alloy having a high strength. On the other hand, when a piston main body is subjected to primary molding by the conventional casting method using, as a material, an aluminum alloy containing a large amount of iron, coarse iron compounds are formed in the alloy upon cooling after the casting, so that the strengths are apt to be lowered. Further, the other constituting components are also contained in the aluminum allow powder as fine powder and since the aluminum alloy powder is formed in to a dense crystal texture through solidification and forging, these components do not cause a decrease in strength due to concentration of the stress in the crystal grain boundaries, thereby to improve the fatigue strength. Each of the examples of the above-described material having a high strength 1A and an example of the material having lower strength 1B are subjected to comparative tests with respect to the abrasion resistance and the fatigue strength. The results are as follows. Namely, Fig. 13 again shows the results of fretting abrasion test (A test sample is used as a rotor. A rider of a predetermined material is repeatedly pressed against the rotor which is maintained in a swung state. The area of the abrasion marks in the contact surfaces represents the degree of abrasion) performed at a test temperature of 250°C to compare the abrasion resistance of each example of the material 1A having a high strength (Example A1 containing SiC, and Example A-2 containing no SiC) with an example (Example B) of the material having a lower strength 1B, from which it is appreciated that both materials 1A (Example A1 and Example A2) give higher abrasion resistance at a high temperature as compared with the material 1B (Example B). Fig. 14 shows the results of fatigue test (A sinuous load is applied to a test sample. The fatigue limit represents the number of repetition (one number represents one period of the sinuous curve) until the test sample has been broken.) performed at test temperatures of 25°C, 150°C and 250°C to compare the fatigue strength of each example of the material having a high strength 1A (Example A1 containing SiC, and Example A-2 containing no SiC) with an example (Example B) of the material having a lower strength 1B, from which it is appreciated that both materials 1A (Example A1 and Example A2) give higher fatigue strength as compared with the material 1B (Example B) at any test temperature. Described above is one embodiment of the piston for an internal combustion engine according to the present invention. Fig. 19 illustrates another embodiment of the piston for an internal combustion engine according to the present invention. In this embodiment, the same materials 1A and 1B as those shown as examples in the above embodiment are used. But the state of the distribution of the materials 1A and 1B of the piston main body is different. Namely, in the piston main body according to the first embodiment, the skirt section 3 is continuously formed in an elongated state throughout the periphery of the piston main body, with a lower end side of the skirt section 3 being formed of the material 1 B having a low strength. On the other hand, in the piston main body according to the second embodiment, the skirt section 4 is provided with portions cut away from the lower end at positions corresponding to a pair of pin bosses 4 so that the skirt section 3 does not exist at a greater part of the portions outside the pin bosses 4. As a result, the piston main body as a whole is light in weight while, in the whole side wall of the piston main body 1, the outer peripheral surface is made of the material having a high strength 1A. The piston main body 1 according to the second embodiment is obtained by hot forging of a composite piston material 10, composed of the two kinds of the materials 1A and 1B having different strengths as shown in Fig. 21, between a lower mold 22 preheated under a controlled state to 400-500°C and an upper mold (punch) 21 preheated under a controlled state to 400-500°C, as shown in Fig. 20, thereby to obtain a primary molded article. This is subjected to a machining treatment by mechanical processing such as for the chipping of unnecessary portions and for the formation of a piston ring groove 5 and a pin bore portion 6 and, if desired, to a surface treatment such as plating, thereby to form the final product. A composite piston material 10 as shown in Fig. 21 may be produced by extruding an aluminum alloy of an ingot material with heating into a round rod and, at the same time, heating and extruding a rapidly solidified powder of an aluminum alloy (containing powder of respective constituting components) so as to cover the periphery of the round rod of the ingot aluminum alloy. The two extruded portions are integrally solidified. The resulting composite column having central and peripheral portions made of different materials is cut into a predetermined size to obtain the composite material. According to each of the embodiments of the piston for internal combustion engines of the present invention, since the outer peripheral side of the piston main body 1 is formed of the material having a high strength 1A in the long area from the head section 2 to the lower portion of the skirt section 3 through the ring groove section 5 on an intermediate portion side between the both pin bosses 4 of the piston main body 1, namely on the side at which the piston main body is pressed against the cylinder wall by a reaction from a conrod at the time of explosion combustion and by an operating force, although a weak force, from the conrod during the up stroke of the piston, it is possible to ensure a sufficient strength for withstanding the pressing of the piston main body 1 to the cylinder wall. Because a sufficient strength for withstanding the pressing of the piston main body 1 to the cylinder wall is ensured, it is possible to make the top land portion small, so that the amount of an exhaust gas remaining in a gap between the top land and the cylinder wall can be reduced, resulting in the reduction of HC. At the same time, since the thickness of the outer peripheral portion of the head section 2 can be made small, the weight of the piston main body 1 can be correspondingly reduced. In each of the above-described embodiments, since a rapidly solidified powder aluminum alloy having a composition shown in the above examples is used as the material having a high strength 1A, it is possible to improve the abrasion resistance of the skirt section 1 and to extend the service life of the piston main body 1 due to the abrasion resistance and the resistance to baking of the material 1A. Further, since the material 1A has a small coefficient of expansion, the thermal deformation of the piston main body 1 can be reduced. The material 1A used in each of the above-described embodiments has a higher unit cost as compared with the material 1B, because of the mixing with silicon carbide (SiC) and iron (Fe), because of a high content of silicon (Si), because of necessity of mixing a variety of ingredients while controlling the amounts thereof, because of the use of special constituting components and because necessity of manufacturing step for producing rapidly solidified powder. Therefore, it is necessary to save the amount of the material used per one piston. Additionally, since the material 1A contains greater amount of elements having a high specific gravity and, hence, the resulting alloy has a greater specific gravity in comparison with the material 1B, it is necessary to save the amount of the material used per one piston. In this regard, each of the above-described embodiments can reduce the material costs and the weight of the piston main body 1 as compared with the case in which a portion made of the material 1A is elongated throughout the circumference of the outer peripheral surface of the piston main body by elongating a portion made of the material 1A only in an intermediate portion between the two pin bosses 4. Yet, as described above, the strength for withstanding the pressing of the piston main body to the cylinder wall can be sufficiently ensured. In each of the above-described embodiments, while ensuring the rigidity and strength of the skirt section to withstand the pressing force to the cylinder wall, the weight of the skirt section 3 can be more reduced by reducing the thickness thereof as compared with the case in which the skirt section is made only of the material B. Since at least a small thickness is required in the production by forging, there is a limitation in reducing the thickness of the skirt section when the skirt section is made only of the material A. As in the above-described embodiments, by using the material B in the inner side (axis side of the piston main body) while using the material A in a surface (outer peripheral surface side of the piston main body) of the skirt section 3, the reduction of the weight of and the material costs for the piston main body can be achieved while ensuring abrasion resistance, at least necessary degree of strength and rigidity. In the other embodiment, the piston main body 1 is primary molding obtained by forging a composite piston material 10 obtained by integrally bonding the two kinds of materials 1A and 1B having different strengths, the bonding interface of the material 1A and material 1 B of the piston main body 1 is extended by forging so that the resulting article is in a more tightly integrated state as compared with that before forging. Each of the embodiments of a piston for an internal combustion engine according to the present invention has been described in the foregoing. The present invention is, however, not limited to the above embodiments. For example, the piston main body having an appearance and a shape as shown in Fig. 15 may be prepared by forging the composite piston material 10 as shown in Fig. 21 obtained by the step shown in Fig. 22 with the use of a forging mold as shown in Fig. 16. In this case, since the pin boss 4 is thick, a structure is adopted in which the material 1A does not sufficiently extend to the skirt section 3 below the pin bore portion 6. Similarly, the piston main body having an appearance and a shape as shown in Fig. 19 may be prepared by forging the composite piston material 10 as shown in Fig. 17 obtained by the step shown in Fig. 18 with the use of a forging mold as shown in Fig. 20. In this case, too, though the portion made of the material 1A does not extend to a lower end of the skirt section 3, it is possible that the surface of the skirt section made of the material 1A extend to the lower portion of the pin bore portion 6. In this case, by making the inside diameter of the ring of the ring-like material 1A of the composite piston material 10 shown in Fig. 17 small or by increasing the height thereof, it is possible to let the portion made of the material 1A extend near the lower end of the skirt section. Further, in the piston for internal combustion engines according to the present invention, the two kinds of materials 1A and 1B having different strengths are not limited to those exemplified in the above embodiments. The present invention can be practiced using any other suitable materials. Also, the method of producing the piston main body 1 is not limited to the specific forging method as illustrated in the above embodiments, but, rather, any other suitable method can be utilized for the manufacture thereof. According to the piston for an internal combustion engine of the present invention as explained above, the strengths for withstanding the pushing of a piston main body to a cylinder wall and the abrasion resistance of a skirt section can be improved while saving material costs for the production of the piston main body and reducing the weight of the piston main body without increasing a top land of the piston main body and increasing the amount of a residual exhaust gas. Other embodiments of the present invention will be described below with reference to the drawings. Fig. 24 shows a piston main body of one embodiment of the piston of an internal combustion engine according to the present invention; (A) is a side view as seen in the axial direction of a pin bore portion, (B) shows an upper surface of a head section as seen from above, and (C) shows a vertical cross-section taken along the line C-C in Fig. (B). The piston main body 1 is obtained by integrally molding by forging a thick disc-like piston forging material into a primary molded article having a head section 2 having an upper surface to be exposed in a combustion chamber and a skirt section 3 to be slidably contacted with an inside surface of a cylinder such that the thickness of the wall is large in a side provided with a pin boss 4 and gradually decreases downward from the pin boss 4 in a side having no pin boss 4, the piston forming material thus primary molded (primary molding article) being mechanically processed to chip unnecessary portions and to form a piston ring groove 5 and a pin bore portion 6, followed by, if necessary, a surface treatment such as plating, thereby finishing into a final product. The piston main body 1 is made of two kinds of materials 1A and 1B having different strengths and integrally bonded by forging in their boundaries. The material 1A formed of a rapidly solidified powder (powder metal) of an aluminum alloy, as a high strength material, distributes in an outer peripheral portion (side wall portion) of the head section 2 to occupy from a periphery of the head section 2 to a ring groove portion 5, an outer side of the pin boss 4 and the skirt section 3, while the material 1B formed of a continuous casting material (ingot material) of an aluminum alloy, as material with a lower strength than the material 1A, distributes in a center part (axis portion) of the piston main body 1 to occupy from a center part of the head section 2 to an inner side of the pin boss 4. In the present embodiment, as the rapidly solidified powder for forming one material 1A of the above-described piston main body, there may be used, for example, an aluminum alloy obtained by solidification of rapidly solidified powder and containing aluminum (Al) as a base material and, additionally, 10-22 % by weight of silicon (Si), 1-10 % by weight of iron (Fe), 0.5-5 % by weight of copper (Cu), 0.5-5 % by weight of magnesium (Mg), 1 % by weight or less of manganese (Mn), 1 % by weight or less of nickel (Ni), 1 % by weight or less of chromium (Cr), 2 % by weight or less of zirconium (Zr) and 1 % by weight or less of molybdenum (Mo). One example of such a material is an aluminum alloy obtained by solidification of rapidly solidified powder and containing 17 % by weight of silicon (Si), 5 % by weight of iron (Fe), 1 % by weight of copper (Cu), 0.5 % by weight of magnesium (Mg), 0.01 % by weight of manganese (Mn), 0.01 % by weight of nickel (Ni), 0.01 % by weight of chromium (Cr), 1 % by weight of zirconium (Zr) and 0.01 % by weight of molybdenum (Mo). Further, in the present embodiment, as another example of the rapidly solidified powder of an aluminum alloy for forming the part made of the material 1A, there may be used an alloy containing aluminum (Al) as a base material and 10-22 % by weight of silicon (Si), 1-10 % by weight of iron (Fe), 0.5-5 % by weight of copper (Cu), 0.5-5 % by weight of magnesium (Mg), 1 % by weight or less of manganese (Mn), 1 % by weight or less of nickel (Ni), 1 % by weight or less of chromium (Cr), 2 % by weight or less of zirconium (Zr), 1 % by weight or less of molybdenum (Mo) and, additionally, 1-10 % by weight of silicon carbide (SiC) which is used for improving the abrasion resistance and which is a component harder than silicon (Si). As one example of such an alloy, there may be mentioned an aluminum alloy obtained by solidification of rapidly solidified powder and containing 17 % by weight of silicon (Si), 5 % by weight of iron (Fe), 1 % by weight of copper (Cu), 0.5 % by weight of magnesium (Mg), 0.01 % by weight of manganese (Mn), 0.01 % by weight of nickel (Ni), 0.01 % by weight of chromium (Cr), 1 % by weight of zirconium (Zr), 0.01 % by weight of molybdenum (Mo) and 5 % by weight of silicon carbide (SiC). In each of the above-described rapidly solidified powder of an aluminum alloy, silicon (Si) and silicon carbide (SiC) are added to improve the abrasion resistance and resistance to baking by the existence of hard particles in the metal texture. Iron (Fe) is added to obtain a high strength at 200°C or more by dispersing and strengthening the metal texture. Copper (Cu) and magnesium (Mg) are added to improve the strength at 200°C or less. The amounts of these components outside the above-described ranges fail to obtain desired abrasion resistance, resistance to baking and required strengths at high temperatures. In the present embodiment, as the continuous casting material (ingot material) for forming the other part made of the material 1B of the piston main body, there may be used an aluminum alloy of continuous casting material conventionally used for casting, namely an alloy containing aluminum (Al) as a base material and additionally 10-22 % by weight of silicon (Si), 1 % by weight or less of iron (Fe), 0.5-5 % by weight of copper (Cu), 0.5-2 % by weight of magnesium (Mg), 1 % by weight or less of manganese (Mn), 1 % by weight or less of nickel (Ni) and 1 % by weight or less of chromium (Cr). One specific example of such a material is an aluminum alloy of a continuous casting material containing 19 % by weight of silicon (Si), 0.2 % by weight of iron (Fe), 4 % by weight of copper (Cu), 1 % by weight of magnesium (Mg), 0.1 % by weight of manganese (Mn), 0.1 % by weight of nickel (Ni) and 0.1 % by weight of chromium (Cr). Each of the examples of the material 1A of the above-described rapidly solidified powder of an aluminum alloy and an example of the material 1B of a continuous casting material are subjected to comparative tests with respect to the abrasion resistance and the fatigue strength. The results are as follows. Namely, Fig. 13 shows the results of fretting abrasion test (A test sample is used as a rotor. A rider of a predetermined material is repeatedly pressed against the rotor which is maintained in a swung state. The area of the abrasion marks in the contact surfaces represents the degree of abrasion) performed at a test temperature of 250°C to compare the abrasion resistance of each example of the material 1A of a rapidly solidified powder (Example A1 containing SiC, and Example A-2 containing no SiC) with an example (Example B) of the material 1B of a continuous casting material, from which it is appreciated that both materials 1A (Example A1 and Example A2) give higher abrasion resistance at a high temperature as compared with the material 1B (Example B). Fig. 14 shows the results of fatigue test (A sinuous load is applied to a test sample. The fatigue limit represents the number of repetition (one number represents one period of the sinuous curve) until the test sample has been broken.) performed at test temperatures of 25°C, 150°C and 250°C to compare the fatigue strength of each example of the material 1A of the rapidly solidified powder (Example A1 containing SiC, and Example A-2 containing no SiC) with an example (Example B) of the material 1B of the continuous casting material, from which it is appreciated that both materials 1A (Example A1 and Example A2) give higher fatigue strength as compared with the material 1B (Example B) at any test temperature. A method of manufacturing a piston for an internal combustion engine of the present embodiment will be described below, in which the whole piston main body 1 is made of a composite of two kinds of materials 1A and 1B having different strengths is manufactured using, as raw materials, the above-described rapidly solidified powder of an aluminum alloy and the continuous casting material of an aluminum alloy. The continuous casting material of an aluminum alloy used as a raw material for the production of the piston main body 1 is obtained by, for example, as shown in Fig. 25, (B) melting and continuously casting (A) an aluminum alloy ingot and (C) cutting into a predetermined size. The rapidly solidified powder of an aluminum alloy used is produced by, for example, as shown in Fig. 26, (B) melting (A) an aluminum alloy ingot and spraying like a fog to rapidly cool at a cooling rate of at least 100°C/sec and to solidify, thereby obtaining a rapidly solidified powder (powder metal) having an average particle diameter of about 100 µm. Respective constituting components in the rapidly solidified powder of an aluminum alloy are contained therein in the form of a fine powder formed either by rapidly solidifying an aluminum alloy ingot containing such components or by mixing fine powder of such components with rapidly solidified powder of an aluminum alloy. In the manufacturing method of the present invention, a composite piston forging material 10 is produced by means of a material producing device 11 as shown in Fig. 27 using, as a raw material, the above-described rapidly solidified powder and the continuous casting material. The material producing device 11 has a first accommodating chamber 12 for containing the continuous casting material 1B and a second accommodating chamber 13 for containing the rapidly solidified powder 1A in juxtaposition with the first chamber. A first die 14 opens in a partition wall between the first and second accommodating chambers 12 and 13, while a second die 15 having a diameter greater than that of the first die 14 opens outward in the second accommodating chamber 13 at a position opposite the first die 14. With such a device 11, the continuous casting material 1 B accommodated in the first accommodating chamber 12 is heated at 400-500°C and pressurized and extruded through the first die 14 toward the second accommodating chamber 13 to form a columnar body. At the same time, the rapidly solidified material 1A accommodated in the second accommodating chamber 13 is heated at 400-500°C and pressurized and extruded through the second die 15 having a larger diameter than the first die 14. As a result, a columnar body of a composite material composed of a core material made of the material 1B of the continuous casting material and an outer peripheral material made of the material 1A of the rapidly solidified powder is coextruded through the second die 15 out of the device 11. The columnar body of the composite material is cut into a predetermined size to obtain a raw forging material 10 of a composite piston for the formation of a primary molded article of the piston main body by forging. In the composite piston forging material 10 produced by the above-described device 11, the oxidized film thereof is destroyed during the passage of the continuous casting material 1B through the first die 14 so that the base material of the continuous casting material 1B is exposed. Since the heated, rapidly solidified powder 1A is bonded to the exposed portion under a pressure, the bonding strength between the columnar core material 1B and the outer peripheral material 1A has a high value. Therefore, in the composite piston forging material 10, the bonding between the core material 1B and the outer peripheral material 1A is not easily destroyed during transportation or during the preparation of the succeeding forging step, thereby providing good handling property. The composite piston forging material 10 is shaped to have a lower height (thickness) than the that of the piston main body 1 after forging. The outer peripheral material (rapidly solidified powder 1A) has a smaller thickness than the diameter of the core material (continuous casting material 1B). In the present embodiment, as shown in Fig. 28, the core material 1B and the outer peripheral material 1A are aligned concentrically. However, as desired, the thickness of the outer peripheral material 1A may be partly changed or a protrusion 10a for use in determining the position thereof relative to a mold for forging may be formed, as shown in Fig. 29. Such a change in the shape of the core material 1B and the outer peripheral material 1A can be easily achieved by changing the shape of the first and second dies 14 and 15 of the above-described device 11. The thus produced composite piston forging material 10 is, in the present embodiment, subjected to hot forging using, as shown in Fig. 30, a lower mold 22 preheated under a controlled state to 250-450°C and an upper mold (punch) 21 preheated under a controlled state to 250-450°C, thereby to obtain a primary molded article. This is subjected to a machining treatment by mechanical processing such as for the chipping of unnecessary portions and for the formation of a piston ring groove 5 and a pin bore portion 6 and, if desired, to a surface treatment such as plating, thereby to form the final product. By such hot forging with the use of upper mold 21 and lower mold 22 previously heated to a controlled temperature, a primary molded article of the piston main body can be molded with a good dimensional accuracy by sufficient utilization of the ductility of the aluminum alloy. Further, since the bonding boundaries between the material 1A and the material 1B of the composite piston forging material are extended by forging, the area of the direct contact between the base materials of the material 1A and the material 1B is increased, so that, in the forged piston main body 1, the bonding boundaries between the material 1A and the material 1B is in a more enhanced state than that before forging. In the method of manufacturing a piston for an internal combustion engine according to the above-described embodiment, the step of molding the rapidly solidified powder of an aluminum alloy by solidification can simultaneously produce a composite material piston forging material 10 composed of a core material of the continuous casting material 1B and an outer peripheral material of the rapidly solidified powder 1A without significantly increasing manufacturing steps and manufacturing time so that the cost up is prevented. Further, since the bonding boundaries between the material 1A (of the rapidly solidified powder) and the material 1B (of the continuous casting material) of the thus produced composite piston forging material 10 are extended by forging, the bonding strength between the material 1A and the material 1B of the forged piston main body 1 can be. Also, since, in the forged piston main body 1 obtained from such a composite piston forging material 10, the material 1A forged by solidifying the rapidly solidified powder distributes in an outer peripheral portion of the piston main body 1, the strength and the abrasion resistance of the piston main body 1 can be improved. In the portion of the piston main body 1 made of the material 1A used in the present embodiment, each of the constituents such as silicon (Si) and iron (Fe) is dispersed in the aluminum alloy powder having an average particle diameter of about 100 µm and is in a finely divided state of an average particle diameter of 10 µm or less. Thus, for example, since silicon (Si) is dispersed in such a finely divided state in the aluminum alloy texture that the primary crystal silicon is 10 µm or less, even when the material 1A is stretched into a thin wall during the forging molding of the primary molded article of the piston main body 1, no cracks are formed in the primary silicon particles in the skirt section and, hence, the wrought piston main body 1 has an improved fatigue strength. With regard to iron (Fe), since a rapidly solidified powder aluminum alloy containing iron (Fe) in a finely divided state is molded by forging, the formation of coarse iron compounds is prevented so that a uniform metal texture free of coarse iron compounds which would cause stress concentration is obtainable. Therefore, the iron component can be added in a larger amount in comparison with the case in which a piston main body is primary molded by a conventional casting process, enabling the preparation of an alloy having a high strength. On the other hand, when a piston main body is subjected to primary molding by the conventional casting method using, as a material, an aluminum alloy containing a large amount of iron, coarse iron compounds are formed in the alloy upon cooling after the casting, so that the strengths are apt to be lowered. In the case where silicon carbide (SiC) is contained in the rapidly solidified powder 10A, since the silicon carbide (SiC) is dispersed uniformly in the texture of the aluminum alloy in a finely divided state, an improved abrasion resistance can be obtained. Further, the other constituting components are also contained in the aluminum alloy powder as fine powder when the material 1A is the rapidly solidified powder of an aluminum alloy. As a result, since the aluminum alloy powder is formed in to a dense crystal texture through solidification and forging, these components do not cause a decrease in strength due to concentration of the stress in the crystal grain boundaries, thereby to improve the fatigue strength. In connection with each of the above-described points, since such a strengthened material 1A distributes in an outer peripheral portion of the piston main body 1 and, for example, since the side wall of the piston main body 1 is made of the material having high abrasion resistance and the resistance to baking, it is possible to extend the service life of the piston main body 1. Further, since the material 1A has a small coefficient of thermal expansion, the thermal deformation of the piston main body 1 can be reduced. Moreover, even when the material 1A is thinned by forging at its skirt section 3, there occur neither breakage of silicon (Si) particles nor formation of cracks at those portions; i.e. the fatigue strength is improved. Further, since the top land portion extending from the vicinity of the ring groove portion 5 to the head section 2 is made of the material 1A, even when the top land of the piston is strongly pressed to the cylinder wall during the operation of the engine, the top land can withstand such a pressing force. Thus, it is possible to make the top land portion small, so that the amount of an exhaust gas remaining in a gap between the top land and the cylinder wall can be reduced, resulting in the reduction of HC. One embodiment of a method of manufacturing a piston for an internal combustion engine according to the present invention has been described in the foregoing. The present invention is, however, not limited to the above embodiment. For example, the rapidly solidified powder of an aluminum alloy and the continuous casting material of an aluminum alloy used as a raw material are not limited to those specifically shown as an example in the above embodiment. Further, the specific method for forging the composite piston forging material into a primary molded article of a piston main body and the specific shape of the piston main body finally produced are not limited to those shown in the above-described embodiment. According to the method of manufacturing a piston for an internal combustion engine of the present invention as explained above, the strength and abrasion resistance of a piston main body are improved by using a rapidly solidified powder aluminum alloy as a material for constituting the piston main body. Further, a composite piston forging material composed of such a rapidly solidified powder is produced without significantly increasing manufacturing steps and manufacturing time so that the cost up is prevented. At the same time, such a composite piston forging material is integrally forged so that the bonding strength between different materials of the piston main body is improved.

Claims: Piston for an internal combustion engine, comprising a piston main body (1) being formed of at least two different materials (1A,1B) having different strengths wherein said materials (1A,1B) are integrally bonded by forging, characterized in that said piston main body (1) is obtained by primary molding by forging of the composite material (10) composed of a piston base material and a material having a higher strength than said piston base material, such that at least a portion of a pin bore portion (6) of said piston main body (1) that represents a sliding contact surface of a piston pin is formed of said material having a higher strength. Piston according to claim 1, characterized in that one of said materials having a higher strength extends in an outer periphery of said piston main body (1) from an upper end of a head section (2) to at least a skirt section (3) below a ring groove portion (5) and in that said piston main body (1) is prepared by forging such that a length of said material having a higher strength from said upper end of said head section (2) is greater in an intermediate portion between a pair of pin bosses (4) than in a vicinity of each of said pair of pin bosses (4). Piston according to claim 2, characterized in that said skirt section (3) has a lower end provided with cut away portions so that an entire region of an outer peripheral surface of said piston main body (1) is made of said material having a higher strength. Piston according to claim 2 or 3, characterized in that at least an outer peripheral portion of said upper end of said head section (2) is made of said material having a higher strength. Piston according to at least one of the preceding claims 1 to 4, characterized in that said material having a higher strength is an aluminum alloy which is obtained by solidifying a rapidly solidified powder, which contains silicon (Si) in an amount of 10-22% by weight and which has initial crystal silicon with an average particle diameter of not greater than 10 µm. Piston according to at least one of the preceding claims 1 to 5, characterized in that said material having a higher strength is an aluminum alloy obtained by solidifying the rapidly solidified powder and containing non-metallic component particles, harder than silicon (Si) and having an average particle diameter of not greater than 10 µm, in a amount of 1-10% by weight. Piston according to claim 6, characterized in that said non-metallic component particles are at least one of those selected from silicon carbide (SiC), aluminum oxide (Al 2 O 3 ) and aluminum nitride (AIN). Piston according to at least one of the preceding claims 1 to 7, characterized in that said material having a higher strength is an aluminum alloy which is obtained by solidifying the rapidly solidified powder, which contains iron (Fe) in an amount of 1-10% by weight and in which the average particle diameter of a compound of the iron is not larger than 10 µm. Method of manufacturing a piston for an internal combustion engine, having a piston main body (1) formed from at least two materials (1A,1B) having different strengths and comprising the steps of joining and integrally bonding said materials (1A,1B) together by forging, characterized by forging of a composite material (10) composed of a piston base material and a material having a higher strength than said piston base material, such that at least a portion of a pin bore portion (6) of said piston main body (1) that represents a sliding contact surface of a piston pin is formed of said material having a higher strength. Method according to claim 9, characterized in that said materials (1A,1B) are forged in a lower mold (22) by means of an upper mold or punch (21). Method according to claim 10, characterized in that both molds (22,21) are preheated to a controlled temperature of 250° to 450°C. Method according to at least one of the preceding claims 9 to 11, characterized in that while extruding an aluminum alloy continuous casting material through a first die (14), a rapidly solidified powder of an aluminum alloy provided around said extruded continuous casting material is coextruded, with heating and under a pressure, together with said continuous casting material through a second die (15) having a greater diameter than said first die (14) to obtain a columnar body composed of a core material made of said material of said continuous casting material and an outer peripheral material integrally bonded to said core and made of the material of said rapidly solidified powder, and in that said columnar body is cut into a predetermined size to obtain a raw material (10) for forging, and in that said raw material (10) for forging is subjected to a primary molding step and to a succeeding processing, thereby obtaining said piston main body (1) as a finished article. Method according to at least one of the preceding claims 9 to 12, characterized In that one of said materials having a higher strength extends in an outer periphery of said piston main body (1) from an upper end of a head section (2) to at least a skirt section (3) below a ring groove portion (5) and in that said piston main body (1) is forged such that a length of said material having a higher strength from said upper end of said head section (2) is greater in an intermediate portion between a pair of pin bosses (4) than in a vicinity of each of said pair of pin bosses (4). Method according to claim 13, characterized in that said skirt section (3) has a lower end provided with cut away portions so that an entire region of an outer peripheral surface of said piston main body (1) is made of said material having a higher strength. Method according to claim 13 or 14, characterized in that at least an outer peripheral portion of said upper end of said head section (2) is made of said material having a higher strength. Method according to at least one of the preceding claims 9 to 15, characterized in that said material having a higher strength is an aluminum alloy which is obtained by solidifying a rapidly solidified powder, which contains silicon (Si) in an amount of 10-22% by weight and which has initial crystal silicon with an average particle diameter of not greater than 10 µm. Method according to at least one of the preceding claims 9 to 16, characterized in that said material having a higher strength is an aluminum alloy obtained by solidifying the rapidly solidified powder and containing non-metallic component particles, harder than silicon (Si) and having an average particle diameter of not greater than 10 µm, in a amount of 1-10% by weight. Method according to claim 17, characterized in that said non-metallic component particles are at least one of those selected from silicon carbide (SiC), aluminum oxide (Al 2 O 3 ) and aluminum nitride (AIN). Method according to at least one of the preceding claims 9 to 18, characterized in that said material having a higher strength is an aluminum alloy which is obtained by solidifying the rapidly solidified powder, which contains iron (Fe) in an amount of 1-10% by weight and in which the average particle diameter of a compound of the iron is not larger than 10 µm.

Inventors: INOUE SEIJI, KOIKE TOSHIKATSU, KURITA HIROTAKA, MIYAZAWA KAZUO, YAMAGATA HIROSHI, INOUE, SEIJI, KOIKE, TOSHIKATSU, KURITA, HIROTAKA, MIYAZAWA, KAZUO, YAMAGATA, HIROSHI
Applicants: YAMAHA MOTOR CO LTD, YAMAHA HATSUDOKI KABUSHIKI KAISHA
Application year: 1998
Main classification: F02F3
IPC codes: F02F3