Method of manufacturing a piston from an aluminium alloy.

Abstract: An aluminium alloy for pistons contains Si + SiC by an amount in the range of 6% to 35% by mass, and SiC by an amount of 1% or more by mass. An ingot containing the above-described aluminum alloy is melted, sprayed in the state of mist, rapidly cooled and solidified into rapidly cooled powder. The rapidly cooled powder is heated and solidified into a workpiece for the piston having the size corresponding to the piston. The workpiece for the piston is forged, heat-treated, and machined into the piston piece. This enables casting without sacrificing the ease of casting, restricting deformation and melting at high temperatures. Moreover, resistance to fatigue and wear caused by high speed sliding movement will be improved.

Description: This invention relates to a method of manufacturing a piston for an internal combustion engine using an aluminium alloy. A piston subject to high temperatures and high pressures during its reciprocation in a cylinder of an internal combustion engine is required to have a high strength and a high resistance against wear and to be lightweight. As a material for such a piston, an Al (aluminium) alloy containing Si (silicon) is widely used. The main reasons for adding Si here is to (1) improve casting property by lowering the melting point and by facilitating the flow of molten metal, (2) restrict deformation at high temperatures by lowering the coefficient of thermal expansion, and (3) improve resistance against wear and fatigue due to high speed sliding movement. Concerning the lowering of the coefficient of thermal expansion (2) and the improvement of the resistance against wear (3), the effect is greater in proportion to the amount of Si contained in the alloy. Therefore, the higher the output of an engine and thermal load on the piston, the more the amount of Si added. However, since Si has a considerably lower thermal conductivity than Al, the aluminium alloy containing a large amount of Si has inevitably a low thermal conductivity. As a result, heat cannot be appropriately dissipated, or the heat dissipating action is poor. Therefore, the piston head area in particular is likely to overheat and liable to suffer melting damage. EP 0 539 172 discloses a method of producing a piston from an aluminium alloy. According to this prior art method, an aluminium powder is prepared from an alloy containing between 10 to 25 % by weight of Si, 5 to 20 % by weight of Ni, 1 to 5 % by weight of Cu and the balance being aluminium and impurity elements. This powder is mixed with a dispersant being at least one selected from the group consisting of 0.5 to 10 % of nitride, boride, carbide or oxide. The dispersant has an average powder diameter of between 0.2 to 20 µm. This mixture is extruded at around 450°C and mechanically processed to the final required shape of the piston. The object of the invention made in view of the disadvantages above is to provide a method for manufacturing a piston that may restrict deformation or melting at high temperatures and deterioration in fatigue or wear property due to high speed sliding movement. To accomplish the above object, the invention provides a method as defined in claim 1. An alternate solution to the above object according to the present invention is defined in claim 2. Preferred embodiments of the inventive method are contained in the claims 3 and 4. Further advantageous embodiments will become apparent from the following description of examples of the invention and the appended drawings. In the drawings FIG. 1 is a graph of comparison of hardness property data between conventional aluminium alloys and two embodiments of alloys according to the invention. FIG. 2 is a graph of comparison of thermal conductivity between the conventional aluminium alloys and two embodiment of alloys according to the invention. FIG. 3 illustrates the shape of the piston formed using an aluminium alloy according to the invention. FIG. 4 is a flow chart of a manufacturing method for the piston shown in FIG. 3. FIG. 5 illustrates a forging process for manufacturing the piston shown in FIG. 3. The ingredient Si is added to improve wear resistance and heat resistance by producing hard crystal silicon grains of initial or eutectic crystals in the metallic composition. The ingredient Fe (iron) is added to produce a dispersed metallic composition so as to provide a high strength at temperatures over 200 °C. The ingredients Cu (copper) and Mg (magnesium) are added to increase strength at temperatures under 200 °C. The intended resistance against wear and seizure, and the necessary strength at high temperatures are not attained outside the ranges of the above-described embodiments. Table 1 shows the ingredients of aluminium alloys AC8A and AC9B specified in JIS and conventionally used for pistons, and the ingredients of the Alloy 1 and Alloy 2 as aluminium alloy examples of the present invention. (in % by mass) Si Cu Mg Ni Fe SiC Si+SiC JIS AC8A 12 1 1 1 - - 12 JIS AC9B 19 1 1 1 - - 19 Alloy 1 10 1 0.5 - 5 2 12 Alloy 2 17 1 0.5 - 5 2 19 FIG. 1 shows comparison of hardness property data between conventional aluminium alloys AC8A, AC9B, and the alloys 1, 2 of the invention. As seen from the figure, the hardness properties of the alloys 1 and 2 of the invention are superior to those of the alloys AC8A and AC9B. FIG. 2 shows the comparison of thermal conductivity (in watt per meter per Kelvin) between the aluminium alloys of the invention respectively containing 8 % and 20 % of Si + SiC by mass, and the above-mentioned alloys AC8A and AC9B. As seen from the figure, the alloys of the invention are higher in thermal conductivity than the alloys AC8A and AC9B which do not contain SiC. Therefore, the alloys of the invention, when used for pistons, improve heat dissipation property, and enable the use under conditions of high output at high temperatures. Now the method of manufacturing the piston using the aluminium alloys of the invention will be described. FIG. 3 shows an embodiment of an internal combustion engine piston piece made by forging according to the invention; (A) showing a side view as seen in the piston pin bore axis direction, (B) showing a top view as seen from above the piston, and (C) showing a vertical cross-sectional view as seen along the line C-C in FIG. 3(B). The piston piece 1 is a finished, final product made by machining a primary workpiece formed by forging a thick cylindrical workpiece to form piston ring grooves 5 and a piston pin bore 6 and to cut off unnecessary part, and further processed with surface treatment such as plating, and having integrally a head portion 2 exposed to the combustion chamber and a skirt portion 3 which is thicker in the area around the piston pin boss 4 and gradually thinner downward away from the piston pin boss. FIG. 4 shows an example of the method of manufacturing a piston piece 1 as an embodiment of the invention. First in the process (1), an aluminium alloy ingot is prepared in which a mother material of aluminium (Al) contains silicon (Si), iron (Fe), and other ingredients. Next in the process (2), one or several kinds of ingots are melted at a temperature of 700 °C or higher, atomized in the mist state, and rapidly cooled at a rate of 100 °C per second to solidify into rapidly cooled powdered metal of aluminium alloy. In the process (3), the rapidly cooled powdered metal of aluminium alloy is heated up to 400 - 500 °C, and extruded to solidify into a round aluminium alloy bar. Then in the process (4), the round aluminium alloy bar is cut into thick disk-shaped workpieces, each having an appropriate amount corresponding to the piston made by forging according to the present embodiment. Here, in addition to the above-described method of forming the workpieces for the forged pistons by cutting the extruded round aluminium alloy bar into pieces of intended shape and size, it is also possible to form the workpieces of intended shape and size more directly for example by packing a mold with the aluminium alloy powder, and heating up to 400 - 500 °C under pressure. Also it is possible to form the workpieces of the thick disk shape for the forged pistons as the aluminium alloy powder is heated up to 400 - 500 °C under pressure, introduced and rolled between a pair of pressing rolls, and press-punched. Or it is also possible to cut the rolled material into rectangular workpieces of a desired size for the forged pistons, and the rectangular workpieces may be preliminarily forged into thick disk-shaped workpieces for the forged pistons. A primary formed workpiece of the piston piece integrally having the head portion and the skirt portion is formed from the workpiece for the forged piston made as described above through processes (5) of applying a parting agent to the outside surface of the workpiece, (6) of heating for improving ease of forming, and (7) of forging by squeezing with paired upper and lower molds. The primary formed workpiece as a single piece formed by forging as described above is then subjected to the process (8) of heat treatment for increasing strength and the final process (9) of machining to form piston ring grooves and the piston pin bore, and to cut off unnecessary part to provide the final shape of the piston piece. Furthermore, if required, the piston piece finished as described above is processed by surface treatment such as plating on the skirt portion side surface for improving the sliding property and wear resistance. According to the present embodiment described above, forging the workpiece for the piston in the processes (6) and (7) comprise the steps of, first as shown in FIG. 5(A), placing a workpiece 10 of a thick disk shape for example in the recessed portion of a lower mold 11 preheated up to a controlled temperature between 200 and 500 °C, and second as shown in FIG. 5(B), pressing into the shape of the piston with an upper mold (punch) 12 pre-heated up to a controlled temperature between 200 and 500 °C. In this way, the primary formed workpiece of the piston piece may be formed by hot forging using the upper and lower molds 11 and 12 preheated to the controlled temperature with good dimensional accuracy while making good use of the ductility of the aluminium alloy. Also, the workpiece 10 may be heated up to a temperature between 200 and 500 °C before being placed in the forging molds, then placed in the recess of the lower mold 11, and immediately forged with the upper mold 12. In that case too, the forging is carried out while controlling the temperature of the upper and lower molds 11 and 12 between 200 and 500 °C. In this way, the forging time may be shortened with the separate, parallel processes of forging and workpiece heating. As described above, the workpiece for the forged piston of the aluminium alloy is made by melting and spraying the aluminium alloy, solidifying by rapid cooling to produce solidified powder, and then forming and solidifying the powder. As a result, the average grain diameter of the aluminium alloy power is about 100 µm. The average grain diameter of the ingredients Si and SiC contained in the aluminium alloy is as small as 20 µm or less and distributed to each grain of the aluminium alloy, while the initial crystal silicon grains contained in the melt-produced aluminium alloy material supposed to be used for forging are much larger. As a result, the forged piston for internal combustion engines of the present embodiment primarily forged using the workpiece for the forged piston of the present embodiment containing the ingredients of Si and SiC in dispersed fine grains is free from cracks as a result of fracture of grains of initial crystal silicon in the skirt portion 3 even if the skirt portion in particular is extended to be thin-walled when the primarily formed workpiece is forged into the piston piece 1, and therefore has a high fatigue strength in the skirt portion. Here, to disperse the Si and SiC in fine grains in the aluminium alloy, it may also be arranged that the aluminium alloy is rapidly cooled and solidified to produce the aluminium alloy powder. Then Si and SiC of the average grain diameter of 1 -20 µm are mixed by an amount that produces the mixture ratio of the aluminium alloy of the present invention, and formed directly to a required size by pressing and heating at a temperature below 700 °C. This results in Si and SiC of average grain diameter smaller than 20 µm dispersed in the boundary area of the aluminium alloy powder composition. In the case the primary forming of the piston piece is made by a normal casting process using an aluminium alloy as a workpiece containing a large amount of iron as an additive, coarse grains of iron compound are produced as the material is cooled after casting, resulting in lowering in strength. Unlike the above, in the present embodiment, since the aluminium alloy is made into powder by rapid cooling and made into the workpiece for the forged piston by heating under pressure, the coarse grains of iron compound are prevented from being produced, a uniform metallic composition is provided free from coarse iron compound grains which may cause stress concentration. As a result, iron may be added in a large amount to provide an alloy having a high fatigue strength. The workpiece for the forged piston and the forged piston itself for internal combustion engines of the present embodiment according to the invention containing SiC as described above contains a specified amount of SiC which is harder than Si so as to increase the wear resistance. Another embodiment of the workpiece for the forged piston and the forged piston itself for internal combustion engines of the present invention containing SiC as described above may be effected as follows: For example in the process (2) shown in FIG. 4, an aluminium alloy ingot not containing SiC is melted and sprayed in the state of mist, rapidly cooled and solidified into powder (powdered metal). A specified amount of SiC having an average grain diameter of 1 - 20 µm is mixed into the powdered metal so that the workpiece for the piston made with the rapidly cooled, solidified powder contains SiC and that SiC and Si having an average grain diameter of 20 µm or less are distributed in the boundary area of the aluminium alloy powder composition having average grain diameter of about 100 µm. As described above, the aluminium alloy of the present invention has a high thermal conductivity, an improved wear resistance, and a high fatigue strength, without increase in the coefficient of thermal expansion. Forming an engine piston using such an aluminium alloy, it is possible to reduce the wall thickness and weight of the piston so as to successfully stand high temperatures free from wear and melting while permitting high speed sliding movement of the piston at a high output. Furthermore, when the piston is made by forging or the like with the average grain diameter of SiC and Si contained in the aluminium alloy 20 µm or less, cracks is prevented from occurring due to fracture of Si and SiC when the workpiece is extended. Since grains of Si and SiC are uniformly distributed among aluminium alloy grains, fatigue strength is enhanced. When the aluminium alloy containing Si ad SiC as described above is used to form a piston through a conventional method of casting, a desired shape of piston is hard to made due to poor ease of casting. With the present invention, however, the piston may be formed by forging or casting by making the rapidly cooled and solidified powder from the aluminium alloy containing Si and SiC. As a result, the piston is provided with restricted deformation at high temperatures, a good thermal conductivity, and a high strength and wear resistance.

Claims: A method of producing a piston comprising the steps of: melting an ingot, said ingot being composed of an aluminum alloy containing Si in a range of 5-25% by mass, SiC 1-10%; Fe 1-10%; Cu 0,5-5%; Mg 0,5-5%; Mn ≤ 2%; Ni ≤ 2%; Cr ≤ 2%; Zr ≤ 2%; Mo ≤ 2% and the rest being Al; atomizing said melt and subsequently rapidly cooling said atomized melt thereby producing a solidified powder; and heating said solidified powder to temperatures in the range of 400° to 500°C and extruding the heated solidified powder to solidify a round aluminum alloy bar and cutting portions from the extruded bar in a size approximately corresponding to the size of the piston and subsequently forging said portions of extruded material for the piston thereby forming a piston having a head and a skirt portion made from said powder. A method of producing a piston comprising the steps of: melting an ingot, said ingot being composed of an aluminum alloy atomizing said melt and subsequently rapidly cooling said atomized melt thereby producing a powdered aluminum alloy; mixing Si and/or SiC to said powder aluminum alloy to obtain a solidified powder containing Si in a range of 5-25% by mass; SiC 1-10%, Fe 1-10%; Cu 0,5-5%; Mg 0,5-5%; Mn ≤ 2%; Ni ≤ 2%; Cr ≤ 2%; Zr ≤ 2%; Mo ≤ 2% and the rest being Al; heating said solidified powder to temperatures in the range of 400° to 500°C and extruding the heated solidified powder to solidify a round aluminum alloy bar and cutting portions from the extruded bar in a size approximately corresponding to the size of the piston and subsequently forging said portions of extruded material for the piston thereby forming a piston having a head and a skirt portion made from said powder. Method according to claim 1 or 2, characterized in that said atomized melt is rapidly cooled at a rate of 100°C per second or more. Method according to any of the preceding claims wherein the average grain diameter of the ingredients Si and SiC contained in the aluminium alloy is 20 µm or less.

Inventors: KOIKE TOSHIKATSU, TSUCHIDA NAOKI, KOIKE, TOSHIKATSU, TSUCHIDA, NAOKI
Applicants: YAMAHA MOTOR CO LTD, YAMAHA HATSUDOKI KABUSHIKI KAISHA
Application year: 1998
Main classification: C22C1
IPC codes: C22C1, C22C21, F02F3, F16J1, C22C32