Piston for internal combustion engine and method for producing same

Abstract: A piston for an internal combustion engine comprises a head portion exposable to a combustion chamber, and a skirt portion slidingly receivable within a cylinder bore. At least said skirt portion is a forged part the material of which containing an aluminum alloy and silicon (Si) in an amount of 10% to 22% by weight and having an average particle diameter of initial crystal silicon (Si) of not more than 10 µm.

Description: This invention relates to a piston for an internal combustion engine comprising a head portion exposable to a combustion chamber, and a skirt portion slidingly receivable within a cylinder bore, wherein at least said skirt portion being a forged part the material of which containing an aluminum alloy and silicon in an amount of 10% to 22% by weight and having an average particle diameter of initial crystal silicon of not more than 10 µm and to a method of producing a piston for an internal combustion engine, said piston comprising a head portion exposable to a combustion chamber and a skirt portion slidingly receivable within a cylinder bore, comprising the steps of: forging at least said skirt portion (3) the material of which containing an aluminum alloy and silicon in an amount of 10% to 22% by weight and having an average particle diameter of initial crystal silicon (Si) of not more than 10 µm. Such an internal combustion engine as well as method are known from US-A-4 923 676. However, the longevity of the related parts is not sufficient. With regard to a piston for use in a reciprocating engine such as a 2 cycle or 4 cycle gasoline engine or diesel engine, there is a demand for an improvement in fatigue 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 weight (low specific gravity), to permit the formation into a thin wall, to be low in permanent set (high in rigidity) at a high temperature, to be high in fatigue strength and to be high in abrasion resistance. As such a material for pistons, an aluminum alloy containing aluminum (Al) as a light weight 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. On the other hand, in a piston for a reciprocating engine, a head portion thereof which is exposed in a combustion chamber is required to have a very high heat resistance, while a skirt portion thereof which is adapted for slidably contacting with an inside wall of a cylinder is required to have a very high abrasion resistance. 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. As a material for such a piston, it is known to add, to an aluminum alloy, a ceramic fiber such as aluminum oxide (Al 2 O 3 ) or silicon carbide (SiC) which is a harder component as compared with silicon for the purpose of improving an abrasion resistance thereof. (Refer, for example, JP-A-Sho-63-126661, JP-A-Hei-1-180927 and JP-A-Hei-5-320788) In a piston for an internal combustion engine, an especially high abrasion resistance is required in a skirt portion thereof which is adapted to slidably contact with an inside surface of a cylinder, as described above. Therefore, it is necessary to incorporate a predetermined amount of silicon (Si) into an aluminum alloy of which at least the skirt portion of the piston main body is made, in order to improve the abrasion resistance and resistance to baking. On the other hand, it is desirable to primarily mold the piston main body by forging a material for the piston, since molding of the piston by casting is disadvantageous from the standpoint of strength because solidified texture becomes coarse. However, most of primary molding of piston main bodies is generally carried out by casting. Since silicon (Si) contained in a material for such cast pistons of an aluminum alloy obtained from a molten material has an average particle diameter, as a primary crystal silicon, of 10 µm or more, the primary crystal silicon particles of a skirt portion are apt to be broken when the piston main bodies are formed by primary molding by forging with the use of such an aluminum alloy material obtained from a molten material, because the forging causes the material of the skirt portion to be extended into a thin layer. As a consequence, cracks are formed in the boundaries between the silicon particles and the matrix to cause a problem that the fatigue strength of the skirt portion is considerably lowered. In a material for pistons to form a primary molded article of such pistons, predetermined amount of silicon (Si) or silicon carbide (SiC), as well as iron (Fe), shall be contained to increase abrasion resistance and fatigue strength of the finished piston fabricated from the primary molded article. It is, however, difficult for a conventional ingot For a piston to be cast that the optimum amounts of primary crystal metals, respectively, could be contained so as to be uniformly dispersed, because the contents of metal ingredients having different melting points are limited, respectively. On the other hand, if the amount of a component such as iron is excessively added in aluminum alloy as a material of a primary article for a piston main body, formed by casting process, coarse texture of iron compound is formed in aluminum alloy, causing the strength to be reduced. It is therefore desirable to primarily mold the piston main body by forging a material for the piston, since molding of the piston by casting is disadvantageous from the standpoint of strength because solidified texture becomes coarse. However, in aluminum alloy as a conventional ingot for a piston prepared by casting process, a component, for example silicon (Si), contained in the material has an average particle diameter, as a primary crystal silicon, of 10µm or more, the primary crystal silicon particles of a skirt portion are apt to be broken when the piston main bodies are formed by primary molding by forging with the use of such an aluminum alloy material obtained from an ingot, because the forging causes the material of the skirt portion to be extended into a thin layer. As a consequence, cracks are formed in the boundaries between the silicon particles and the matrix to cause a problem that the fatigue strength of the skirt portion is considerably lowered. Accordingly, it is an objective of the present invention to provide an improved piston for an internal combustion engine as indicated above, all parts of which having a high abrasion resistance, a high resistance to baking and a high fatigue strength. According to the present invention, this objective is solved by a piston according to claim 1 combustion engine as indicated above in that said aluminum alloy containing non-metallic component particles being harder than silicon (Si) and having an average particle diameter of not more than 10 µm in an amount of 1% to 10% by weight. According to an advantageous as well as preferred embodiment of the present invention, said aluminum alloy comprises a solidification of powder, wherein said powder may be a rapidly solidified powder. According to another advantageous embodiment, said aluminum alloy containing non-metallic component particles being harder than silicon (Si) and having an average particle diameter of not more than 10 µm in an amount of 1% to 10% by weight, wherein said non-metallic component particles may consist of at least one selectable from silicon carbide (SiC), aluminum oxide (Al 2 ,O 3 ) and aluminum nitride (AIN). Other advantageous embodiments are given in further dependent claims. It is another objective of the present invention to provide an improved method for producing a piston as indicated above facilitating that all of the respective parts of said piston have a high abrasion resistance, a high resistance to baking and a high fatigue strength. According to the present invention, this objective is solved by a method according to claim 5. According to an advantageous as well as preferred embodiment of the present invention, said aluminum alloy is obtained by solidifying a powder. Said powder may be solidified by extruding into a rod or by heating in a respective mold under pressure or by introducing the heated powder into a gap between a pair of rolls to perform rolling, respectively. According to another advantageous embodiment of the present invention, said aluminum alloy containing non-metallic component particles being harder than silicon (Si) and having an average particle diameter of not more than 10 µm in an amount of 1% to 10% by weight, wherein said non-metallic component particles may consist at least one selectable from silicon carbide (SiC), and aluminum nitride (AlN). Other preferred embodiments of the present invention are given in further dependent claims. Therefore, a wrought piston for an internal combustion engine is provided having a head portion exposed in a combustion chamber and a skirt portion adapted for slidably contacting with an inside wall of a cylinder, wherein : at least said skirt portion is formed by forging with the use of, as a raw material, an aluminum alloy obtained by solidifying a rapidly solidified powder such that the content of silicon (Si) having an average particle diameter of initial crystal silicon of not greater than 10 µm is in the range of 10-22 % by weight. Thus, a wrought piston for an internal combustion engine is provided wherein said aluminum alloy obtained by solidifying the rapidly solidified powder contains iron (Fe) having an average particle diameter of not greater than 10µm in an amount of 1-10 % by weight. In addition, a wrought piston for an internal combustion engine is provided wherein 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 in an amount of 1-10 % by weight. Moreover, a wrought piston for an internal combustion engine is provided wherein said non-metallic component particles harder than silicon (Si) is at least one of those selected from silicon carbide (SiC), and aluminum nitride (AlN). Accordingly a material for a wrought piston, is suggested wherein said material is obtained by mixing an aluminum alloy powder formed by rapidly solidifying a molten aluminum alloy with silicon (Si) powder having an average particle diameter of not greater than 10µm in such an amount that the final content thereof is 10-20 % by weight, followed either by direct molding of said mixture into a desired shape with heating at a temperature lower than 700°C under pressure or by heating of said mixture at a temperature lower than 700°C under pressure and succeeding molding thereof into a desired shape. Further, a material for a wrought piston, is provided wherein said material is obtained by rapidly solidifying a molten aluminum alloy containing silicon (Si) in such an amount that the final content thereof is 10-20 % by weight to form aluminum alloy powder having silicon crystals with an average particle diameter of not greater than 10 µm followed either by direct molding of said aluminum alloy powder into a desired shape with heating at a temperature lower than 700°C under pressure or by heating of said aluminum alloy powder at a temperature lower than 700°C under pressure and succeeding molding thereof into a desired shape. In addition, a material for a wrought piston is obtained by mixing said aluminum alloy powder formed by rapid solidification with a powder, having an average particle diameter of not greater than 10 µm and being at least one of those selected from silicon carbide (SiC), aluminum oxide (Al 2 O 3 ) and aluminum nitride (AlN), in such an amount that the final content thereof is 1-10 % by weight, followed either by direct molding of said mixture into a desired shape with heating at a temperature lower than 700°C under pressure or by heating of said mixture at a temperature lower than 700°C under pressure and succeeding molding thereof into a desired shape. Moreover, a material for a wrought piston is given wherein said aluminum alloy powder formed by rapid solidification is mixed with iron (Fe) powder having an average particle diameter of not greater than 10 µmin such an amount that the final content thereof is 1-10 % by weight. 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 of a piston main body according to a wrought 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] an illustration explanatory of the steps for producing a piston main body according to a wrought piston for an internal combustion engine of the present invention. [Fig. 3] an illustration explanatory of the forging step of the steps for producing a piston main body according shown in Fig. 2. [Fig. 4] photographs showing metal components of a material for piston for the production of a piston main body; (A) being an example according to the embodiment containing SiC, (B) being an example according to the embodiment without SiC, and (C) being Comparative Example. [Fig. 5] graphs showing a difference in abrasion resistance according to a difference in material for piston for producing a piston main body in Example A according to the embodiment containing SiC, Example B according to the embodiment without SiC, and Comparative Example C. [Fig. 6] graphs showing a difference in fatigue strength at temperatures of 25C, 150C and 250C according to a difference in material for piston for producing a piston main body in Example A according to the embodiment containing SiC, Example B according to the embodiment without SiC, and Comparative Example C. The wrought piston of an internal combustion engine and the material for the wrought piston 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 wrought 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 hole, (B) shows an upper surface of a head portion 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 portion 2 having an upper surface to be exposed in a combustion chamber and a skirt portion 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 form a piston ring groove 5 and a pin hole 6 and to chip unnecessary portions, followed by, if necessary, a surface treatment such as plating, thereby finishing into a final product. Fig. 2 shows an example of a process for producing the piston main body 1 according to the above embodiment. First, in step (1), an ingot of an aluminum alloy containing aluminum (Al) as a base material, silicon (Si), iron (Fe), etc. is provided. In step (2), one or more kinds of ingots are 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 100C/sec, thereby obtaining rapidly solidified powder (powder metal) of the aluminum alloy. In step (3), the thus prepared rapidly solidified powder of the aluminum alloy is heated to 400-500°C and extruded into a round rod. After cooling, the round rod of the aluminum alloy obtained by solidifying the rapidly solidified powder is, in step (4), cut into thick discs each having an adequate size corresponding to one piston, thereby obtaining a material for a wrought piston according to the present embodiment. The preparation of a material for a wrought piston is not limited to the above described method in which the aluminum alloy powder is extruded into a round rod, followed by solidification and cutting. It is possible to directly shaping a material for a wrought piston having a desired size and shape by placing the aluminum alloy powder in a mold, followed by heating at 400-500°C under pressure. It is also possible to mold into a rectangular material for a wrought piston by introducing the aluminum metal powder heated to 400-500°C into a gap between a pair of rolls to perform rolling, the rolled product being punched by a press or cut into a desired size by shearing. It is further possible to subject the rectangular shaped product to a preliminary forging to obtain a material for the wrought piston having a thick disc-like shape. A mold release agent is then applied to an outer periphery of the thus produced material for the wrought piston, in step (5). Then, in step (6), this is heated to improve moldability. Thereafter, in step (7), the heated material is integrally molded into a primary molded article having a head portion and a skirt portion by forging in which the heated material is sandwiched between a pair of upper and lower heated molds under a high pressure. The primary molded article of the piston main body thus integrally molded by forging is subsequently heat treated in step (8) to improve the strength. Finally, in step (9), this is subjected to a machining treatment by mechanical processing such as for the formation of a piston ring groove and a pin hole and for the chipping of unnecessary portions, thereby to form the final shape of the piston main body. If desired, the finished piston main body may be thereafter subjected to a surface treatment, for example, plating of a side surface of the skirt portion so as to improve the sliding property and abrasion resistance. With regard to the forging of the material for the piston in step (6) according to the present embodiment, the thick disc-like material 10 for the piston is placed, as shown in Fig. 3(A), in a cavity of a lower mold 11 preheated under a controlled state to 400-500°C. Then, as shown in Fig. 3(B), the material is pressed and forged into a piston shape with an upper mold (punch) 12 preheated under a controlled state to 400-500°C. By the hot forging using the upper and lower molds 12 and 11 each preheated to a controlled temperature, a primary molded article of the piston main body 1 having a precise dimension can be obtained sufficiently utilizing the extension property of the aluminum alloy. Incidentally, the material 10 for the piston prior to the placement into the forging mold can be heated to 400-500°C and then accommodated in the cavity of the lower mold 11, followed by immediate forging with the upper mold 12. In this case, the forging is carried out while preheating the lower and upper molds 11 and 12 between 400-500°C. As described previously, the piston main body 1 according to the present embodiment produced through the above-described steps is a primary molded article by forging of the material for the piston made of the aluminum alloy obtained by solidification of rapidly solidified powder. The material for the piston of the present embodiment contains 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). In the material for the wrought piston of the above aluminum alloy, silicon (Si) is added to improve the abrasion resistance and resistance to baking by crystallizing silicon particles in the form of hard primary crystals or eutectic crystals 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 one specific embodiment of the material for the wrought piston according to the above-described embodiment, there may be mentioned an aluminum alloy obtained by solidification of rapidly solidified powder 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). In the material for the wrought piston according to the present embodiment using the aluminum alloy obtained by solidifying rapidly solidified powder, the melted aluminum alloy is sprayed into a fog-like state to rapidly solidify same into powder. Thereafter, the powder is molded and solidified. Therefore, the aluminum alloy powder has an average particle diameter of about 100µm The silicon (Si) contained in the alloy is, as shown in Fig. 4(B), such that the hard primary crystal silicon crystallized in the metal texture of the aluminum alloy solidified while being made into powder is finely divided into an average particle diameter of 10µm or less and is dispersed in every aluminum alloy particles, in contrast with the primary crystal silicon as shown in Fig. 4(C) which is contained in an aluminum alloy as a material for melting for casting. As a consequence, the wrought piston for an internal combustion engine according to the present embodiment which is a product obtained by primary molding by the forging of a material for the piston according to the present embodiment in which the silicon is contained in a finely divided and dispersed state, even when the material especially is stretched in the skirt portion 3 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 portion and, hence, the skirt portion has an improved fatigue strength. For the purpose of finely dividing silicon (Si) and dispersing same in the aluminum alloy, it is possible to add silicon (Si) having an average particle diameter of 1-10µm to aluminum alloy powder, obtained by rapidly solidifying a melt of the aluminum alloy, in such an amount that the final content thereof is 10-22% by weight, followed either by direct molding of the mixture into a desired shape with heating at a temperature lower than 700°C under pressure or by heating of the mixture at a temperature lower than 700°C under pressure and succeeding molding thereof into a desired shape. By this, silicon (Si) having an average particle diameter of 10µm or less can be dispersed in the boundary of each aluminum alloy powder texture. 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 contrast, in the present embodiment, since the aluminum alloy is rapidly cooled to form powder which is thereafter heated and pressed to obtain a material for the wrought piston, the formation of coarse iron compounds is prevented during the course of the above steps. Thus, a uniform metal texture free of coarse iron compounds which would cause stress concentration. Therefore, the iron component can be added in a large amount, enabling the preparation of an alloy having a high fatigue strength. The above embodiment of the wrought piston for an internal combustion engine and the material for wrought piston according to the present invention includes another embodiment of a material for wrought piston of an aluminum alloy obtained by solidifying the rapidly solidified powder and a wrought piston for an internal combustion engine prepared using the material for the wrought piston, in which for example silicon carbide harder than silicon (Si) is contained in a predetermined amount. In such another embodiment of the wrought piston for an internal combustion engine and the material for the wrought piston containing silicon carbide (SiC) according to the present invention, a predetermined amount of powder of silicon carbide (SiC) having an average particle diameter of not greater than 10µm is incorporated into rapidly solidified powder (powder metal) of the aluminum alloy obtained by spraying a molten ingot of an aluminum alloy material in a fog-like state in step (2) shown in Fig. 2. As a result, the material for the piston obtained by solidifying the rapidly solidified powder contains silicon carbide (SiC), so that the non-metallic component (silicon carbide) having an average particle diameter of not greater than 10µm is dispersed in the boundaries of respective aluminum alloy powder texture having an average particle diameter of about 100 µm. The method for dispersing the silicon carbide having an average particle diameter of not greater than 10µm into the material for the piston obtained by solidifying the rapidly solidified powder is not limited only to the above method. For example, in step (1) shown in Fig. 2, a predetermined amount of silicon carbide (SiC) having an average particle diameter of not greater than 10µm is previously incorporated into the ingot of the aluminum alloy material. The ingot is then melted and sprayed into a fog-like state to form rapidly solidified powder in step (2), so that silicon carbide (SiC) having an average particle diameter of not greater than 10µm is dispersed in the rapidly solidified powder of the aluminum alloy. In either method, the embodiment in which silicon carbide (SiC) is contained provides an aluminum alloy for a material for pistons contains, similar to the embodiment containing no silicon carbide (SiC), 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) and additionally 1-10 % by weight of silicon carbide (SiC). In one specific embodiment of the material for the wrought piston according to the above-described embodiment containing silicon carbide (SiC), there may be mentioned an aluminum alloy obtained by solidification of rapidly solidified powder 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 the material for the wrought piston containing silicon carbide SiC), the silicon (Si) contained is, as shown in Fig. 4(A), finely divided such that the primary crystals having an average particle diameter of 10µm or less are dispersed and, at the same time, finely divided silicon carbide (SiC) is dispersed in the metal texture in a state finely divided to have an average particle size of 10 µm or less so as to further improve the abrasion resistance and resistance to baking. Since the wrought piston for an internal combustion engine made of such a material for the wrought piston is produced by solidifying and forging the aluminum alloy powder containing silicon carbide (SiC) which is harder than silicon (Si), which is an infusible non-metallic component, which finely divide to have an average particle size of 10 µm or less and which is dispersed between the textures of the aluminum alloy, the wrought piston produced contains finely divided silicon carbide (SiC) uniformly dispersed in the aluminum alloy texture and, hence, has an improved abrasion resistance. Each of the materials for wrought piston (Embodiment A contains SiC and Embodiment B contains no SiC) shown as examples according to the present invention in the above embodiments was compared with Comparative Example C which is a piston material of an aluminum alloy for melt production-type with respect to the abrasion resistance and the fatigue strength. The results are as follows. The material for piston of the melt production-type of Comparative Example C contains 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). Namely, one specific example of Comparative Example C is a material for pistons of an aluminum alloy of the melt-production-type for casting 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). Fig. 5 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) at performed at a test temperature of 250°C to compare the abrasion resistance of Examples A and B and Comparative Example C, from which it is appreciated that both Example A containing SiC and Example B containing no SiC give higher abrasion resistance as compared with Comparative Example C. Fig. 6 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 Examples A and B and Comparative Example C, from which it is appreciated that both Example A containing SiC and Example B containing no SiC give higher fatigue strength as compared with Comparative Example C at any test temperature. In the material for wrought piston in each of the above embodiments, since not only the silicon (Si) and iron (Fe) as described above but also the other constituent components contained in the aluminum alloy powder having an average particle diameter of about 100µm have a small average particle diameter of 10µm or less as a result of the pulverization by the rapid solidification, the wrought piston for an internal combustion engine produced from such a material for the wrought piston can have a dense crystal texture through the solidification by molding and forging of the material and does not cause a reduction of the strength due to concentration of stress in the boundaries of the crystal grains. For this reason, too, the fatigue strength is improved. The wrought piston for an internal combustion engine and the material for the wrought piston according to the present invention have been described in the foregoing with respect to the embodiments thereof. The present invention is, however, not limited to the above embodiments. For example, in the wrought piston for an internal combustion engine according to the present invention, the piston main body 1 is formed as a whole of a single material in any of the above embodiments. The present invention is not limited to this. As long as at least the skirt portion is made by forging using the material for piston of the aluminum alloy obtained by solidifying the above-described rapidly solidified powder, the present invention is applicable to a wrought piston for an internal combustion engine made of a composite material wherein the piston main body has different portions made of different materials. With regard to the material for wrought pistons obtained by solidifying rapidly solidified powder, when, for example, iron having an average particle diameter of 10µm or less is incorporated in an amount of 1-10 % by weight into the material for piston obtained by solidifying rapidly solidified powder of an aluminum alloy, not only the above described embodiment in which an aluminum alloy ingot into which iron (Fe) has been previously incorporated is melted and rapidly solidified into powder, but also a method in which, as a step prior to the step (3), powder of iron (Fe) having an average particle diameter of 10µm or less is mixed and stirred with aluminum powder such that the content of the iron is 1-10 % by weight in the final material for wrought piston. As component particles harder than silicon (Si) and added to further improve the abrasion resistance, not only silicon carbide (SiC) shown in the above embodiment but also one or more component particles harder than silicon (Si), such as silicon carbide (SiC), and aluminum nitride (AlN), may also be added to improve the abrasion resistance. In the following, another embodiment of the present invention will be described. Fig. 2 schematically shows an example of a process for producing the material for the wrought piston according to the present invention: in the process (1) first, preparing an aluminum ingot whose base material is aluminum, followed by (2) melting the ingot at the temperature of 700°C then spraying to mist the molten material, and then rapidly cooling the misty material at the cooling rate of 100°C/sec to obtain rapidly cooled solidified powder (powder metal). The thus prepared rapidly solidified powder of the aluminum alloy is mixed with a silicon carbide powder having an average diameter of approximately 5µm. In step (3), the predetermined amount of the mixed powder is put into a solidifying apparatus to be heated to 400-500°C and extruded into a round rod. After cooling, the round rod of the aluminum alloy obtained by solidifying the rapidly solidified powder is, in step (4), cut into thick discs each having an adequate size corresponding to one piston, thereby obtaining a desired size of material for a piston. As described previously, the blank for the wrought piston according to the present embodiment produced in the above-described steps is an article obtained by solidifying the rapidly cooled and solidified powder which contains 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), I % by weight or less of chromium (Cr), 2 % by weight or less of zirconium (Zr), I % by weight or less of molybdenum (Mo) and 1-10 % by weight of silicon carbide (SiC). In the blank for the wrought piston of this embodiment, silicon (Si) is added to improve the abrasion resistance and resistance to baking by crystallizing silicon particles in the form of hard primary crystals or eutectic crystals 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 one specific embodiment according to the above-described embodiment, there may be mentioned a blank for a piston obtained by solidification of rapidly solidified powder 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). The process for produce the piston main body from such blank described above, as shown in fig.2 alternatively comprises: (a) a mold release agent is applied to an outer periphery of the thus produced piston blank, then (b) this is heated to improve moldability, and thereafter (7), the heated blank is integrally molded into a primary molded article having a head portion and a skirt portion by forging in which the heated blank is sandwiched between a pair of upper and lower heated molds under a high pressure. The primary molded article of the piston main body thus integrally molded by forging is subsequently heat treated in (8) to improve the strength. Finally, in (9), this is subjected to a machining treatment by mechanical processing such as for the formation of piston ring grooves and a guide pin hole and for the machining off of unnecessary portions, thereby to form the final shape of the piston main body. If desired, the finished piston main body may be thereafter subjected to a surface treatment, for example, plating of a side surface of the skirt portion so as to improve the sliding property and abrasion resistance. In the blank for the wrought piston according to the present embodiment, the blank is formed by solidifying material which is finely powdered and therefore , the silicon (Si) contained in such aluminum alloy is, as shown in Figs. 4(A) and 4(B), such that the hard primary crystal silicon crystallized in the metal texture of the aluminum alloy solidified while being made into powder is finely divided into an average particle diameter of 10 µm or less and is dispersed in every aluminum alloy particles, in contrast with the primary crystal silicon as shown in Fig.4 (C) which is contained in an aluminum alloy as an ingot for casting. As a consequence, the main body piston which is a product obtained by the forging process of a piston blank according to the present embodiment in which the silicon is contained in a finely divided and dispersed state, even when the material is stretched especially in the skirt portion into a thin wall during the forging of the primary article of the piston main body, no cracks are formed in the primary silicon particles in the skirt portion and, hence, the skirt portion has an improved fatigue strength. When a piston main body is subjected to primary moulding 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 contrast, in the blank for a piston according to the present embodiment, since the aluminum alloy to which iron is added is rapidly cooled to form powder which is thereafter heat extruded to be formed and solidified, an is cut in suitable size, the formation of coarse iron compounds is prevented during the course of the above process. Thus, a uniform metal texture free of coarse iron compounds which would cause stress concentration. Therefore, the iron component can he added in a large amount, enabling the preparation of an alloy having a high fatigue strength. Further, in the blank for the piston of the present embodiment, as shown in Fig.4 (A), the silicon (Si) finely divided is contained such that the average particle size is 10 µm or less, at the same time, finely divided silicon carbide (SiC) is dispersed in the metal texture in a state finely divided to have an average particle size of approximately 5 µm so as to further improve the abrasion resistance and resistance to baking. The above embodiment of the blank for wrought piston shown as Example A according to the present invention, which contains finely divided silicon carbide was compared with Example B which contained the same components, exclusive of silicon carbide, and with comparative C which was formed from an ingot of aluminum alloy, with respect to the abrasion resistance. The ingot blank for a piston of Comparative Example C contains 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). Namely, one specific example of Comparative Example C is a blank for pistons of an aluminum alloy of ingot for casting 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). Fig. 5 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) at performed at a test temperature of 250°C to compare the abrasion resistance of such Example and Comparative Examples B and C, from which it is also appreciated that the Example containing SiC gives higher abrasion resistance than that of Comparative Example B containing no SiC and Comparative Example C of ingot. Fig. 6 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 Examples A and B and Comparative Example C, from which it is appreciated that both Example A containing SiC and Example B containing no SiC give higher fatigue strength as compared with Comparative Example C at any test temperature. According to the wrought piston for an internal combustion engine and the material for wrought piston of the present invention as explained above, the piston main body produced through a primary molding by forging can exhibit improved abrasion resistance and resistance to baking and, additionally, improved fatigue strength at high temperatures. Thus, the piston meets with the realization of a high output and high speed engine. According to the blank for wrought piston of the present invention as explained above, the piston main body produced through a primary article by forging can exhibit improved abrasion resistance and resistance to baking, and thus the piston meets with the realization of a high output and high speed engine.

Claims: Piston for an internal combustion engine, made of an aluminum alloy and comprising a head portion (2) exposable to a combustion chamber, and a skirt portion (3) slidingly receivable within a cylinder bore, characterized in that at least said skirt portion (3) is a forged part formed as a thin wall, and in that said piston consists of an aluminum alloy from a rapidly solified aluminum alloy powder comprising silicon (Si) in an amount of 10% to 22% by weight, and non-metallic component particles selectable from silicon carbide (SiC) and aluminum nitride (AIN) in an amount of 1% to 10 % by weight, these and the silicon particles having an average particle diameter of not more than 10 µm. Piston according to claim 1, characterized by said aluminum alloy containing iron (Fe) particles having an average particle diameter of not more than 10 µm in an amount of 1% to 10% by weight. Piston according to claim 1 or 2, characterized by said aluminum alloy comprising aluminum (Al) as a base material, copper (Cu) in an amount of 0.5% to 5% by weight, magnesium (Mg) in an amount of 0.5% to 5% by weight, manganese (Mn) in an amount of not more than 1% by weight, nickel (Ni) in an amount of not more than 1% by weight, chromium (Cr) in an amount of not more than 1% by weight, zirconium (Zr) in an amount of not more than 2% by weight and molybdenum (Mo) in an amount of not more than 1% by weight. Piston according to claims 1 or 2, characterized by said aluminum alloy comprising aluminum (Al) as a base material, silicon (Si) in an amount of 17% by weight, iron (Fe) in an amount of 5% by weight, copper (Cu) in an amount of 1% by weight, magnesium (Mg) in an amount of 0.5% by weight, manganese (Mn) in an amount of 0.01% by weight, nickel (Ni) in an amount of 0.01% by weight, chromium (Cr) in an amount of 0.01% by weight, zirconium (Zr) in an amount of 1% by weight and molybdenum (Mo) in an amount of 0.01% by weight. Method of producing a piston according to claim 1, comprising the steps of: heating and melting a raw material of an aluminum alloy comprising aluminum (Al) as a base material and silicon (Si) at a temperature at about 700°C or more, rapidly cooling and solidifying the melt at a cooling rate of at least 100°C/sec., forging at least said skirt portion (3) of said piston (1) of a material comprising said solidified melt, thereby stretching said material especially in the skirt portion (3) into a thin wall during the forging procedure, wherein said material contains an aluminum alloy powder comprising silicon (Si) in an amount of 10% to 22% by weight, and non-metallic component particles selectable from silicon carbide (SiC) and aluminum nitride (AIN) in an amount of 1% to 10 % by weight, these and the silicon particles having an average particle diameter of not more than 10 µm. Method of producing a piston according to claim 5, characterized in that said powder is solidified by extruding into a rod or by heating in a respective mold under pressure or by introducing the heated powder into a gap between a pair of rolls to perform rolling, respectively. Method of producing a piston according to claims 5 or 6, characterized in that said powder is obtained by rapidly cooling sprayed molten alloy material. Method of producing a piston according to one of the claims 5 to 7, characterized by said aluminum alloy containing iron (Fe) having an average particle diameter of not more than 10 µm in an amount of 1% to 10% by weight. Method of producing a piston according to one of the claims 5 to 8, characterized by said aluminum alloy comprising aluminum (Al) as a base material, copper (Cu) in an amount of 0.5% to 5% by weight, magnesium (Mg) in an amount of 0.5% to 5% by weight, manganese (Mn) in an amount of not more than 1% by weight, nickel (Ni) in an amount of not more than 1% by weight, chromium (Cr) in an amount of not more than 1% by weight, zirconium (Zr) in an amount of not more than 2% by weight and molybdenum (Mo) in an amount of not more than 1% by weight. Method of producing a piston according to one of the claims 5 to 8, characterized by said aluminum alloy comprising aluminum (Al) as a base material, silicon (Si) in an amount of 17% by weight, iron (Fe) in an amount of 5% by weight, copper (Cu) in an amount of 1% by weight, magnesium (Mg) in an amount of 0.5% by weight, manganese (Mn) in an amount of 0.01% by weight, nickel (Ni) in an amount of 0.01% by weight, chromium (Cr) in an amount of 0.01% by weight, zirconium (Zr) in an amount of 1% by weight and molybdenum (Mo) in an amount of 0.01% by weight. Method of producing a piston according to one of the claims 5 to 10, characterized in that the heating temperature for the extrusion step or for the molding step under pressure is lower than 700°C. Method of producing a piston according to one of the claims 5 to 11, characterized in that said sprayed molten alloy contains silicon (Si) for said powder and that said powder is mixed with silicon carbide (SiC) powder having an average particle diameter of about 5 µm, followed by solidifying through hot extrusion or hot molding process and then forming into a desired size.

Inventors: KOIKE TOSHIKATSU, KURITA HIROTAKA, YAMAGATA HIROSHI, KOIKE, TOSHIKATSU, KURITA, HIROTAKA, YAMAGATA, HIROSHI
Applicants: YAMAHA MOTOR CO LTD, YAMAHA HATSUDOKI KABUSHIKI KAISHA
Application year: 1998
Main classification: C22C1
IPC codes: F02F3, C22C32, B22F9, C22C1