JP2003035198A - Piston for internal combustion engine and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a forged piston for an internal combustion engine of which head surface and piston pin portion have superior high-temperature strength property, of which skirt portion has superior forgeability, and of which oil ring groove portion has stable mechanical processability and abrasion resistance. SOLUTION: The forged piston for an internal combustion engine, made of an aluminum alloy containing 6-25 mass % of silicon, has the ratio (A/B) for an eutectic silicon average particle diameter (A) of the oil ring groove portion to an eutectic silicon average particle diameter (B) of the skirt top end portion being 1.5 or more, and the eutectic silicon average particle diameter (A) of the oil ring groove portion is 4 μm or more, or/and is characterized in that the ratio (C/D) of a primary crystal phase silicon average particle diameter (C) of the oil ring groove portion to a primary crystal phase silicon average particle diameter (D) of the skirt top end portion is 1.3 or more and the primary crystal phase silicon average particle diameter (C) of the oil ring groove portion is 15 μm or more.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a forged piston for an internal combustion engine using an aluminum-silicon alloy and a method for manufacturing the forged piston.

[0002]

2. Description of the Related Art The manufacture of pistons for internal combustion engines has heretofore been cast by die casting. In the die casting method, the piston is manufactured as follows. The molten aluminum alloy melt is poured into a mold to form a piston shape.
Then, after performing necessary heat treatment such as artificial aging treatment, necessary machining is performed to finish the product.

Recently, some parts have been manufactured by the forging method. In this forging method, a piston is manufactured as follows. First, the molten aluminum-silicon alloy is continuously cast into a billet for extrusion. After that, heat treatment (homogenization treatment) to homogenize internal stress generated by segregation of solute elements and shrinkage during solidification, and then extruded into a thin round bar, or continuous casting A small-diameter continuous cast rod is manufactured by the method, and then homogenized and face-cut into a small-diameter round rod, which is then cut as a forging material. The forging material is preheated and then forged by a hot forging machine to form the piston shape. Then, after heat treatment such as artificial aging treatment, mechanical processing is applied to finish the piston. Depending on the application, in order to improve wear resistance and heat resistance, the piston head and the side surface on the piston head side of the top ring may be subjected to alumite treatment or coating formation treatment.

[0004]

Recently, there has been a demand for further improvement in fuel efficiency of an internal combustion engine such as an automobile. Therefore, the weight of the vehicle body is being reduced, and the development of a lightweight engine is being promoted as one of them. For example, in a piston used in an engine, the material is being changed to aluminum, and the structure of the piston is being made thinner.

There is also a demand for a piston having a quality capable of withstanding a high performance engine.

However, in the piston manufactured by the conventional die casting method, it is difficult to reduce the thickness of the skirt portion due to the technical limitation of the casting method. For this reason, it is common to perform a thin machining by machining after casting. The casting piston is
Since the cooling rate during the solidification in casting is slow, the cast structure becomes rough, so the workability in the machining process is excellent.
However, it is difficult to control the accuracy of the final product because the cast body has large variations in wall thickness and dimensions. In addition, the cast body may have internal defects such as cavities and micro-shrinkage, which may reduce the strength of the cast body itself. We are responding by molding and increasing the rib thickness. Therefore, the piston produced by the casting method is disadvantageous for improving engine performance. Further, due to these measures, variation in piston performance becomes large, and further improvement is required in terms of reliability of engine performance.

On the other hand, in the piston produced by the forging method, the forging material has almost no internal defects and the mechanical characteristics are constant and stable. Therefore, the piston formed by the forging method has a constant wall thickness at each part. be able to. In that respect, it is possible to supply a piston of stable quality. However, since the material has a fine structure, it is suitable for forgeability, especially for thin parts such as skirts that have a long distance, whereas machinability is insufficient. There is a face. For example, since the cutting powder is a continuous type that is not finely divided, the cutting powder processing property is poor, and the machining productivity is accordingly poor. In addition, the surface roughness of the oil ring groove portion of the product after machining was insufficient. Further, the continuous casting method has a limitation on the alloy composition that can be produced in order to suppress ingot cracking due to solidification shrinkage stress generated inside the ingot during casting. Therefore, higher strength than required for the piston,
A component aiming at higher wear resistance and higher strength at higher temperatures cannot be freely manufactured as a forging material.

The present invention has been developed in view of the above situation, and the machinability of the oil ring groove is good, and the dimensions such as the oil ring groove surface roughness and the oil ring groove flatness are stable, and the head is Provided is a forged piston for an internal combustion engine, which has excellent mechanical properties in the vicinity of the head part, for example, high-temperature strength properties of the head surface and piston pin part, excellent forgeability of the skirt part, and stable wear resistance of the oil ring groove part. is there.

[0009]

Means for Solving the Problems The present inventor has conducted earnest research on the product characteristics of a new forging material and a forged piston for an internal combustion engine, and has completed the present invention based on the findings. 1) A first invention for solving the above problems is a forged piston for an internal combustion engine, which is made of an aluminum alloy containing 6 to 25 mass% of silicon, and has a eutectic silicon average particle diameter (A) of an oil ring groove portion. And the ratio (A / B) of the eutectic silicon average particle diameter (B) at the tip of the skirt portion is 1.5 or more, and the eutectic silicon average particle diameter (A) at the oil ring groove portion
Is 4 μm or more, which is a forged piston for an internal combustion engine. 2) A second invention for solving the above-mentioned problem is a forged piston for an internal combustion engine, which is made of an aluminum alloy containing 6 to 25% by mass of silicon, and has a eutectic silicon average particle diameter (A) of an oil ring groove portion. And the eutectic silicon average particle size (B) at the tip of the skirt (A / B) is 1.5 or more, and the eutectic silicon average particle size (A) at the oil ring groove is
Is 4.0 μm or more, and the ratio (C / D) of the primary crystal silicon average particle diameter (C) at the oil ring groove portion to the primary crystal silicon average particle diameter (D) at the tip of the skirt portion is 1. 3 or more and primary silicon average particle size (C) in oil ring groove
Of 15 μm or more is a forged piston for an internal combustion engine. 3) A third invention for solving the above-mentioned problems is that an aluminum alloy contains Cu in an amount of 0.3 to 7 mass% and Mg in an amount of 0.1 to 0.1% by mass.
The forged piston for an internal combustion engine according to 1) or 2), which contains 2% by mass. 4) A fourth invention for solving the above-mentioned problems is characterized in that the aluminum alloy contains Ni in an amount of 0.1 to 2.5% by mass. It is a forged piston for an internal combustion engine. 5) A fifth invention for solving the above-mentioned problem is a forging material having a structure in which an average grain size of silicon is different on the upper surface and the lower surface by a unidirectional solidification casting method of an aluminum alloy melt containing 6 to 25 mass% of silicon. And a step of preheating the forging material, and the forging material is housed in a mold with the surface having a large average silicon particle diameter and the surface forming the piston head of the mold facing each other. Forming a piston blank by a forging method, a step of artificially aging the piston blank, and a step of machining the piston blank to obtain a forged piston for an internal combustion engine. A method for manufacturing a forged piston for an internal combustion engine, which is characterized. 6) A sixth aspect of the invention for solving the above-mentioned problems is that the cooling conditions for unidirectional solidification are that the eutectic silicon average particle size (A) on the upper side of the ingot and the eutectic silicon average on the cooling plate side. The condition (5) is characterized in that the ratio (A / B) to the particle size (B) is 1.5 or more and the eutectic silicon average particle size (A) is 4 μm or more. Is a method of manufacturing a forged piston for an internal combustion engine. 7) A seventh aspect of the invention for solving the above-mentioned problems is that the cooling condition for unidirectional solidification is such that the cooling rate (E) is 0 at a position e which is 5 mm from the upper surface of the solidification mold and 5 mm from the outer periphery. A position f of 5 ° C./sec or more and 1 mm from the position e and the bottom surface of the mold for coagulation and 5 mm from the outer periphery.
The ratio (E / F) to the cooling rate (F) at 0.85 is 0.85 or less. 8) The eighth invention for solving the above-mentioned problems is characterized in that the temperature condition of the preheating treatment is in the range of 350 ° C to (solidus temperature of aluminum alloy-10) ° C. The method for manufacturing a forged piston for an internal combustion engine according to any one of 1.

In the present invention, a sample prepared according to the method of observing a metallographic structure is microscopically observed, and an image analysis processing device is used for the obtained observation surface to determine the cross-sectional shape of each particle exposed on the observation surface. An average value of HEYWOD diameters, in which the diameter of the circle was used as the diameter of the particle and the diameter of the circle was replaced with the equivalent circle, was used as the average particle diameter.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of a forged piston for an internal combustion engine of the present invention will be described.

The forged piston for an internal combustion engine of the present invention comprises a head surface having a valve recess, a thick skirt portion, a rib, an oil ring groove portion, and a piston pin hole.

FIG. 1 is a sectional view of an example of a forged piston for an internal combustion engine of the present invention. FIG. 1A is a longitudinal sectional view including a skirt portion. FIG. 1B is a vertical cross section including a pin hole (piston pin hole (14)) into which a piston pin for connecting the piston to the connecting rod is inserted. The upper surface of the piston is a head surface (11) having a valve recess. The oil ring groove (12) is a groove for incorporating the piston ring, and the oil ring groove is required to be perpendicular to the outer peripheral surface of the piston, that is, the oil ring groove is required to be perpendicular to the depth direction. The skirt (13) is a guide for maintaining the posture in the engine liner of the piston, and has high strength and wear resistance,
It is required to be thin to reduce the weight. The shape of the two-dot broken line shown by reference numeral 15 is the shape of the rough piston material which is a forged product, and the shape of the solid line shown by reference numeral 16 is the shape of the finished piston product after machining. Reference numeral 17 is a rib. Reference numeral 18 is the tip of the skirt. The tip of the skirt portion is a skirt portion in the range of 40% from the bottom with respect to the height of the entire piston. This is a place where a large plastic flow occurs during forging and good forgeability is required.

The forged piston for an internal combustion engine of the present invention is a forged piston for an internal combustion engine which is made of an aluminum alloy containing 6 to 25% by mass of silicon, and has a eutectic silicon average particle diameter (A) in the oil ring groove and a skirt. The ratio (A / B) to the eutectic silicon average particle size (B) at the tip of the part is 1.5 or more (preferably 1.6 or more), and the eutectic silicon average particle size ( A) is 4 μm or more.

The average grain size of eutectic silicon in the oil ring groove is 4 μm or more (preferably 4.5 μm or more).
The reason is as follows. If the eutectic silicon average particle size of the oil ring groove is less than 4 μm, during the cutting of the oil ring groove, the cutting powder is likely to be connected for a long time and become entangled with the tool or the chuck of the cutting machine during the cutting. Moreover, the entangled cutting chips not only scratch the piston's machined surface, but also the cutting chips collected in a cotton shape at the bottom of the cutting machine are dragged up and the entire chuck part is wrapped with cutting chips, and more. It becomes impossible to continue operation, resulting in poor productivity. Alternatively, the forged piston for an internal combustion engine processed by being entangled with the cutting chips has a large surface roughness and cannot satisfy the quality.

Since the entire circumference of the oil ring groove is continuous, the cutting chips must be divided by the cutting chips themselves, and it is desired that the cutting chips have a good property of treating the cutting chips.

In response to this demand, the forged piston for an internal combustion engine of the present invention has a eutectic silicon average particle size of 4 μm or more. Therefore, in the machining process in manufacturing the forged piston, the cutting powder crystallizes silicon particles. It becomes easy to be cut by the object, and the cutting powder becomes a cutting powder with no connection. as a result,
The swarf can be prevented from being entangled with the cutting tool or the chuck, and it does not remain in the machine like cotton, so the workability of the swarf is greatly improved. In addition, since it is possible to prevent the cutting chips from being entangled with the blade or the product being processed, the surface roughness after processing becomes stable.

Further, since the eutectic silicon average particle size of the oil ring groove is 4 μm or more, the wear resistance is excellent. When the engine operates, the upper and lower surfaces of the oil ring groove are rubbed by the piston rings, so the oil ring groove portion is required to have high wear resistance. Further, on the piston head side (the surface exposed to the combustion chamber of the engine), the temperature near the piston head becomes high due to the high-temperature combustion gas generated when the fuel burns. Compared to the skirt, the oil ring groove part that contacts the inner surface of the engine cylinder near the piston head is
Since it slides by being exposed to a more severe environment, the wear resistance of that part must be excellent. If it is less than 4 μm, the wear resistance is insufficient.

In the forged piston for an internal combustion engine of the present invention, since the eutectic silicon average particle size of the oil ring groove is 4 μm or more, a wear-resistant coating agent near the oil ring groove as used in a high performance engine. It has sufficient abrasion resistance without any treatment for improving the abrasion resistance by the coating treatment with or the hard alumite treatment. Therefore,
Since it is not necessary to carry out these expensive treatments, the unit price of the piston can be kept low, and an inexpensive engine can be provided. Eutectic silicon average grain size (A) in the oil ring groove and eutectic in the skirt tip. Ratio to average silicon particle size (B) (A /
B) is 1.5 or more (more preferably 1.6 or more), in other words, the eutectic silicon average particle diameter B of the skirt tip is smaller than the eutectic silicon average particle diameter (A) of the oil ring groove, The reason why it is 0.67 times or less of A is as follows.

Since the skirt is not formed over the entire circumference of the piston, but the circumference is cut at the piston pin, the skirt is automatically interrupted when cutting in the circumferential direction in the machining process. Cutting is done. Therefore, since there is no concern that the cutting powder will be entangled with the blade, it is sufficient that the eutectic silicon average particle size B at the tip of the skirt portion is 0.67 times or less for the workability of the cutting powder during machining.

On the other hand, when the eutectic silicon average particle size (B) at the tip of the skirt exceeds 0.67 times the eutectic silicon average particle size (A) at the oil ring groove, the wear resistance of the oil ring groove is increased. It becomes impossible to secure good plastic flow during hot forging while securing. For example, there are cases where the wear resistance of the oil ring groove portion is inferior but good plastic workability of the skirt portion can be secured, or conversely, wear resistance is excellent but plastic workability is poor. As a result, it becomes difficult to supply an object having both good characteristics as a piston.

However, in the piston for internal combustion engine of the present invention, the eutectic silicon average particle size (B) at the tip of the skirt is 0.67 times or less than the eutectic silicon average particle size (A) at the oil ring groove. Even when the skirt portion is formed thin, cracks do not occur at the tip of the skirt filled in the mold, and plastic flowability into the mold is not deteriorated. For this reason, the skirt portion can be formed to be thin, so that the weight of the piston can be easily reduced. Since it can be formed into a thin wall by forging, and the machining allowance for reducing the wall thickness by machining can be reduced, the productivity and the material yield are improved.

The piston of the present invention is a forged piston for an internal combustion engine, which is made of an aluminum alloy containing 6 to 25% by mass of silicon, and has a eutectic silicon average particle diameter (A) in the oil ring groove and a skirt tip. Of the eutectic silicon average particle diameter (B) is 1.5 or more, and the eutectic silicon average particle diameter (A) of the oil ring groove is 4.0 μm.
That is, the ratio (C / D) of the primary crystal silicon average particle diameter (C) at the oil ring groove portion to the primary crystal silicon average particle diameter (D) at the tip of the skirt portion is 1.3 or more. It is preferable that the forged piston for an internal combustion engine is characterized in that the primary crystal silicon average particle diameter (C) of the oil ring groove is 15 μm or more.

The ratio (A / B) of the eutectic silicon average particle diameter (A) at the oil ring groove portion to the eutectic silicon average particle diameter (B) at the tip of the skirt portion is 1.5 or more, and the oil The reason why the eutectic silicon average particle size (A) of the ring groove portion is 4 μm or more is as described above.

On the other hand, in the case of an aluminum alloy containing 6 to 25% by mass of silicon, primary crystal silicon may exist in the eutectic silicon structure depending on the cooling rate.

In such a case, the primary crystal silicon average particle diameter C of the oil ring groove is 15 μm or more (more preferably 1
7 μm or more. ) Is preferred. The reason is that the machinability and wear resistance due to the grain size of primary crystal can be further enhanced, and if it is less than 15 μm, the effect cannot be sufficiently obtained.

Since the forged piston for internal combustion engine of the present invention has the eutectic silicon average particle size of 4.0 μm or more and the primary crystal silicon average particle size C of the oil ring groove portion is 15 μm or more,
In the machining step when manufacturing this, the cutting chips are easily cut by the crystallized substances of these silicon particles,
The cutting chips are divided type cutting chips with no connection. As a result, the swarf can be prevented from being entangled with the cutting tool or the chuck, and the swarf does not remain in the machining device like cotton, so that the workability of the swarf treatment is greatly improved. In addition, since it is possible to prevent the cutting chips from being entangled with the blade or the product being processed, the surface roughness after processing becomes stable. Further, since the primary crystal silicon average particle diameter C of the oil ring groove is 15 μm or more, the wear resistance of the oil ring groove is improved.

The ratio (C / D) of the primary crystal silicon average particle diameter (C) in the oil ring groove portion to the primary crystal silicon average particle diameter (D) in the tip of the skirt portion is 1.3 or more (more preferably). 1.4 or more) is preferable. The reason is (C / D)
When the ratio is 1.3 or more, it becomes possible to further improve the wear resistance and the machinability of the oil ring groove portion and to secure the plastic fluidity of the tip portion of the skirt portion. But,
When it is less than 1.3, either the wear resistance and the machinability of the oil ring groove portion or the plastic fluidity of the tip portion of the skirt portion becomes insufficient.

In the aluminum alloy used for the piston, Cu is added in an amount of 0.3 to 7% by mass and Mg is added in an amount of 0.1 to
2 mass% (more preferably, 0.4 to 6.5 mass% Cu and 0.15 to 1.8 mass% Mg) are preferably contained. The inclusion of these alloying elements increases the hardness of the piston, resulting in mechanical strength such as tensile strength, 0.2%.
The yield strength and fatigue strength are increased, and the weight of the piston structure can be reduced by thinning the entire structure. If it is less than this, the effect is not obtained, and if it is more than this, the effect is saturated and the effect does not appear, so that not only the material cost increases but also the forgeability becomes insufficient.

The aluminum alloy used for the piston preferably further contains 0.1 to 2.5 mass% of Ni (more preferably 0.2 to 2.0 mass% of Ni). This is because the inclusion of Ni increases the high temperature strength and improves the durability of the oil ring groove portion that comes into contact with the inner surface of the engine cylinder near the piston head under a high load of the engine. If it is less than this, the effect cannot be obtained, and if it is more than that, the effect is saturated. In addition, Ni is an expensive material, and the manufacturing cost is unnecessarily increased.

An example of the piston manufacturing method according to the present invention will be described. The production method of the present invention comprises a step of producing a forging material having a structure in which the average grain size of silicon is different on the upper surface and the lower surface by an unidirectional solidification casting method of an aluminum alloy melt containing 6 to 25% by mass of silicon, and the forging A process for accommodating the raw material for use with the surface with a large average silicon particle diameter and the face for molding the piston head of the mold facing each other and forming into a piston raw material by forging, and an artificial aging treatment for the piston raw material And a step of machining the piston blank to obtain a forged piston for an internal combustion engine.

The piston manufactured by the manufacturing method of the present invention is
The piston has the characteristics described above.

The details will be described below. The material for forging is obtained by casting an aluminum alloy as a raw material by a unidirectional solidification casting method. The casting apparatus used is, for example, the apparatus shown in FIG. 2 disclosed in JP-A-9-174198.

Reference numeral 201 is a cooling plate on which the main mold 2 is placed.
02 is arranged. A receiving tank 203 for a molten aluminum alloy 207 from a melting furnace or the like (not shown) is provided on the upper part of the main mold 202, and in FIG. 2, the bottom part thereof is integrated with the upper part of the mold. The molten metal receiving tank 203 and the casting mold are in communication with each other through an injection port 204. The inlet 204 is provided with an opening / closing plug 205, and the molten metal is injected into the mold by pulling up the opening / closing plug by a device (not shown) that drives the opening / closing plug up and down. After the temperature rises and the injection is completed, or after a lapse of a predetermined time, the open / close stopper is lowered to shut off the molten metal. Reference numeral 208 is a lid,
Reference numeral 209 is an electric furnace for maintaining the molten metal at a predetermined temperature. The cooling plate 201 is cooled by spraying water or the like under the cooling plate 201 with a spray nozzle 210. Reference numeral 211 is a case,
Reference numeral 212 is a drainage port.

The molten aluminum alloy poured into the mold is unidirectionally solidified toward the upper mold wall by heat removal through the cooling plate. The metal structure is affected by the cooling rate,
Eutectic silicon particles and primary crystal silicon particles (also collectively referred to as silicon particles) become finer as the cooling rate becomes faster, and coarser as the cooling rate becomes slower. When this device is used, the cooling rate on the cooling plate side is the fastest, and the vicinity of the upper mold wall surface is the slowest, so the size of the silicon particles crystallized by the solidification of the aluminum-silicon alloy is the cooling plate side. An ingot having a structure that is small, increases on the side of the upper mold, and has a graded grain size distribution is obtained.

The raw material aluminum alloy contains 6 to 25 mass% of silicon. Silicon amount is 6
When it is less than 25% by mass, the wear resistance is inferior, and when it exceeds 25% by mass, the further effect of the wear resistance is not exposed and becomes excessive, and rather the forgeability is deteriorated such as cracking during forging. In addition, the life of the cutting tools is significantly reduced.

In addition to silicon, 0.3 to 7% by mass of Cu and 0.1 to Mg of Mg which can enhance the hardness and mechanical properties of the piston by making the aluminum alloy have age hardenability.
It is preferable to add 2% by mass alone or in combination. Further, it is preferable to add Ag or Sc in an amount of 1.5% by mass or less.

Since the internal combustion engine piston is exposed to high temperatures inside the engine due to the combustion of fuel, it is necessary to secure high temperature strength. Therefore, Ni, which is generally known as a high temperature strength improving metal, is added to 0.1%. It is preferable to add about 2.0% by mass. Furthermore, Fe, Mn, Zr, Ti,
It is also effective to add W, Cr, V, Co, Mo or the like individually or in combination.

Further, it is preferable to add an improving agent such as Na, Ca, Sr or Sb, which is effective in reducing the grain size of eutectic silicon. The addition can be performed alone or in combination. This is because it is possible to prevent the grain size of the eutectic silicon from becoming coarse and adversely affecting forgeability and tool wear during machining.

Further, when primary crystal silicon particles are generated, P is generally added to refine the primary crystal silicon particle size. However, when Na, Ca, etc. are present in the molten metal. The effect of P is obstructed and the grain size of primary crystal silicon does not become fine. Therefore, Na and Ca are 50 mass ppm or less. If it is contained more than that, the grain size of primary crystal grains becomes extremely coarse, so that not only the forgeability deteriorates, but also the life of the cutting tool during cutting is shortened.

The ingot used in the present invention uses the above alloy as a molten metal to remove heat from the cooling plate so that the eutectic silicon particles and the primary crystal silicon particles have a small particle size on the cooling plate side and the upper mold side. It can be manufactured by controlling so as to obtain an ingot having a large and inclined structure.

As the inclined structure, for example, the cooling conditions for unidirectional solidification are that the eutectic silicon average grain size (A) on the upper side of the ingot and the eutectic silicon average grain size (B) on the cooling plate side. And the eutectic silicon average particle size (A) is 4.0 μm or more.

The following conditions can be mentioned as a method of controlling the cooling rate for obtaining the above-mentioned graded structure. For example, as shown in FIG. 3, the cooling conditions for unidirectional solidification are 5 mm from the upper surface of the mold for solidification and 5 m from the outer circumference.
The cooling rate (E) at the position e inside the m is 0.5 ° C /
Second or more, and the ratio (E / F) of the position e to the cooling rate (F) at the position f within 1 mm from the bottom surface of the solidification mold and 5 mm from the outer periphery is 0.85 or less. .

Within the above range, a forging material having such a structure can be produced, and the forging method using the forging material is excellent in workability during forging and is excellent in the machining process. This is because it is possible to obtain a forged piston for an internal combustion engine that has excellent workability and excellent wear resistance.

As long as the ingot shape satisfies the above-mentioned gradient structure, in addition to the disk having the upper and lower surfaces parallel to each other, the shape of the ingot has a plane which is not parallel to the upper and lower surfaces in accordance with the piston shape. It may be an ingot, or an ingot having an uneven surface in which one or both of the upper and lower surfaces are not parallel. This is because it is advantageous for reducing the load on the forging die and forming a complicated piston shape by forging.

The ingot can be subjected to a forging process after being subjected to a mechanical process to form it, if necessary.

Alternatively, the ingot can be subjected to forging after being machined as necessary to cut out the surface of the structure in a required state from the inclined structure. If the average grain size at the end face of the ingot is not the desired average grain size, forging the ingot with a sloping average grain size distribution cut to have silicon grains with the desired average grain size It is preferable to use it as a raw material.

The obtained ingot is used as a forging material. The forging material is preheated before being forged. The temperature condition of the preheating treatment is in the range of 350 ° C to (solid phase temperature of aluminum alloy-10) ° C. The processing time is until the temperature of the entire forging material reaches the preheating temperature range, and then the forging process is started. If it is less than 350 ° C, sufficient plastic flow cannot be obtained when hot forging the forging material, and if it exceeds (solidus temperature of aluminum alloy-10) ° C, burning (local melting) may occur in the forging material. There is. If burning occurs, the strength of the forged product will deteriorate sharply,
It is not preferable because the product causes defects such as blisters and micro-shrinkage due to local dissolution.

Since forging is usually performed while hot, not only the material is preheated but also the die is heated. The heating temperature can be 100 to 400 ° C. The heating temperature is selected depending on the shape to be forged, the type of forging equipment, the type of alloy of the material used, and other forging factors.
If the temperature is too low, heat removal from the material will be large, resulting in poor workability and insufficient plastic flow of the material. If the temperature is too high, the strength of the mold is lowered, and damage such as abrasion and chipping easily occurs, which is not preferable from the viewpoint of mold life. It is preferable to apply a lubricant to the die before forging.

Forging is die forging. An example of the structure of the forging device used in the present invention will be described with reference to FIG. The forging device includes a forging machine 101, an upper die 103 attached to an upper bolster 102, and a lower die 105 attached to a lower bolster 106. The mold used in the present invention includes an upper mold, a lower mold, and a knockout pin 107. In this figure, the mold used is a combination of molds in which the head of the piston is the upper mold and the skirt is the lower mold, but the mold used is the lower mold for the head and the upper mold for the skirt. Combinations of molds can also be used. If necessary, a lubricant application device having a spray front-rear transfer device 108, a spray rotation device 109 and a lubricant spray nozzle 104 attached to the spray front-rear device can be arranged via a shaft 110. .

Here, when the material is put into the mold, in the present invention, the surface of the mold for molding the piston head and the surface of the forging material having a large average silicon particle diameter are oriented so that they face each other. Put them all together. For example, in the case of using the ingot described above, the ingot is charged in the same direction so that the surface of the die for molding the piston head and the upper side of the ingot face each other. If the directions are opposite, the average particle size of the silicon particles near the tip of the skirt becomes large and the silicon particles near the oil ring groove have a fine average particle size, making it impossible to obtain the effect of the present invention. The eutectic silicon average particle diameter and the primary crystal silicon average particle diameter of the oil ring groove a portion are each less than 4 μm,
The ratio (A / B) of the eutectic silicon average particle diameter (A) in the oil ring groove portion to the eutectic silicon average particle diameter (B) in the tip portion of the skirt portion is less than 1.5 and is less than 15 μm. The ratio (C / D) of the primary crystal silicon average particle diameter (C) in the groove portion to the primary crystal silicon average particle diameter (D) in the tip of the skirt portion is 1.3.
This is because the effect of good machinability and good wear resistance of the oil ring groove portion, and good plastic fluidity at the time of forging of the tip portion of the skirt portion cannot be obtained because it is less than 1.

In the manufacturing method of the present invention, the dies of the die for molding the piston head and the surface of the material for forging having a large average silicon grain size are placed in such a manner that they face each other. The average silicon particle size is preferably 3 μm or less. This is because the workability during hot forging becomes better. Average particle size of eutectic silicon at the tip of the skirt is 3 μm
This is because, even when the skirt portion is formed into a thin wall, cracks do not occur at the tip end portion to be filled in the mold, and the filling property of the mold is not deteriorated because of the following.

The piston blank obtained by forging is
It can be used as it is, but Cu, Mg, Sc,
In an alloy to which Ag or the like is added, the heat treatment improves the mechanical properties of the material, and therefore it is preferable to perform artificial aging treatment as the heat treatment. The artificial aging treatment is, for example, a solution treatment treatment in which the piston material is immediately immersed in water after a heating temperature of 400 to 550 ° C. and a holding time of 0.2 to 10 hours, and then 150 to 250 ° C. 0.2 to 2
It is preferable to perform tempering for 0 hours. This allows
Including hardness, mechanical properties (for example, tensile strength, 0.
2% yield strength) and fatigue strength can be increased.

After that, the forged piston blank is machined, and for example, a forged piston for an internal combustion engine is formed by performing drilling for piston pins, piston surface machining, oil ring groove machining, and other machining. Is finished.

According to the manufacturing method of the present invention, since the surface of the die for forming the piston head and the side of the forging material having a large silicon grain size are aligned in the same direction, the oil is introduced into the oil ring groove portion. Eutectic silicon particle size (A) and primary crystal silicon particle size (C) are 4 μm or more and 15 μm or more, respectively, and the eutectic silicon particle size (B) and primary crystal silicon particle size (D) at the tip of the skirt portion The ratio (A / B) is 1.5 or more, and the ratio (C / D) is 1.3 or more. As a result, during machining of the oil ring groove portion, it is possible to prevent the cutting chips from being entangled for a long time and entangled with the tool or the chuck portion of the cutting machine during the cutting process. Therefore, it is possible to prevent the machining surface of the piston from being scratched, and to prevent the cutting dust accumulated in a cotton shape from being dragged up on the bottom of the cutting machine and wrapping the entire chuck part with the cutting dust, thus improving productivity. Be improved.

Since the skirt does not exist all around the piston but is divided by the piston pin hole for inserting the pin for connecting the piston with the connecting rod, the skirt is naturally cut intermittently. . Therefore, there is no concern that the cutting chips will become entangled with the blade. Therefore, regarding the size of the eutectic silicon grain size, the eutectic silicon average grain size B of the skirt part
Of the eutectic silicon average particle size (A) of the oil ring groove is 0.
67 times or less is sufficient. Further, the ratio (C / D) of the primary crystal silicon average particle diameter (C) at the oil ring groove portion to the primary crystal silicon average particle diameter (D) at the tip of the skirt portion is 1.3 or more.

On the other hand, since the skirt portion is formed into a thin wall by forging, the cutting step by machining for thinning is unnecessary, or the cutting allowance in the machining can be reduced, so that the material yield is high. The productivity is improved by improving the machining time and shortening the machining time.

Before subjecting to hot forging, if the forgeability and the artificial aging treatability after forging are further improved, it is preferable to homogenize the material. Homogenization treatment refers to the state where microsegregation of added metal such as Cu or Mg that is added to increase the mechanical strength of the piston or the strength during high temperature use in the engine occurs during casting. That is, the heat treatment is performed to uniformly disperse the aluminum base. As a result, the workability during forging and the uniformity of the mechanical properties after the artificial aging treatment in the subsequent process can be ensured. The conditions of the homogenization treatment conditions are a temperature range of 400 ° C. to (solid phase temperature of alloy used −10) ° C., holding time 1 to 30.
I can give you time.

On the other hand, depending on the alloy components used or the shape of the piston, it is possible to obtain the same effect as the homogenization treatment by utilizing the preheating of the material to be applied before forging. By increasing the holding time of the preheating step before forging to 1 hour or more, the same effect as the homogenizing treatment of the material can be obtained. Also, the alloy components used,
Alternatively, depending on the shape of the piston, it is possible to obtain the same effect as the homogenization treatment by using the heat treatment process performed after forging.In this case, increase the holding time during the solution treatment of the artificial aging treatment process. With this, it is possible to obtain the same effect as the homogenization treatment of the material.

[0060]

[Example] Next, a forging material by a unidirectional solidification casting method,
The effects of the present invention will be described in detail with reference to specific examples using a forged molten material by a continuous casting method and a forging material by a die casting. [Examples 1 and 2] Alloy 1 in the alloy list shown in Table 1 was replaced with
An ingot having an outer diameter of 77 mm and a thickness of 30 mm was obtained by the unidirectional solidification casting method using the apparatus shown in FIG. 2 and used as a forging material (Example 1). Further, alloy 2 in Table 1 was cast by the same casting method to have an outer diameter of 110 mm and a thickness of 30 mm to obtain a material for forging (Example 2). The casting conditions are shown in Table 2. In both Examples, a K thermocouple was inserted into the position shown in FIG. 3 in the mold to measure the cooling rate during solidification.

The forging materials thus obtained were subjected to a homogenizing treatment under the conditions of holding each at 490 ° C. for 8 hours, and then,
A piston blank was formed by the hot forging method using the forging device shown in FIG. When the material is put into the die of the forging machine, the upper surface of the ingot which is the material for forging (the surface which is not the cooling plate side when solidified by the above casting device) is on the piston head side and the bottom surface of the ingot ( The charging direction was aligned so that the surface on the cooling plate side when solidified by the above casting device was on the skirt side. The forging conditions were as shown in Table 3.

The outer diameter of the forged piston blank was 78 mm in Example 1, and the skirt wall thickness was 3.5 mm.
The forging load during forging was 430 t. Example 2 was a piston blank having an outer diameter of 111 mm and a skirt portion having a wall thickness of 4 mm, and the forging load during forging was 670 t.

Cracking occurs in the skirt portion of the obtained piston blank in the same direction as plastic flow, insufficient plastic flow at the tip of the skirt due to insufficient plastic flow during forging, or hair filling failure. The formability of the skirt portion was evaluated by visually inspecting for generation of cracks.

The obtained piston blank was subjected to artificial aging treatment under the conditions shown in Table 4.

The hardness of the obtained piston blank was measured using a Rockwell hardness meter. The eutectic silicon average particle diameter and the primary crystal silicon average particle diameter of the skirt tip and the oil ring groove were measured and the ratio was calculated. On the other hand, for reference, the maximum chord length (MAXLNG) was also measured by the image processing device.
This value means that the maximum length part of the particle shape is sandwiched with calipers, and even if the HEYWOD diameter is the same,
If the value of MAXLNG is large, it means that the larger value has a flatter shape or acicular crystals.

The obtained piston blank was subjected to a cutting test around the oil ring groove under the conditions shown in Table 5 to investigate the shape of the cutting powder and the processing property of the cutting powder. Next, the surface roughness of the inner surface of the oil ring groove was evaluated under the conditions shown in Table 6. As the cutting tool used in the cutting test, a Compax bite (artificial diamond cutting tool) as shown in FIG. 4 was used. The surface roughness of the obtained piston is measured by cutting out the side surface of the oil ring groove from the piston and using a surface roughness meter to measure the roughness in a direction parallel to the cutting tool cutting direction (that is, parallel to the piston head surface). Was measured.

Next, a test piece was sampled from the vicinity of the oil ring groove and subjected to a pin-on-disk friction and wear test at room temperature under the conditions of Table 7 to measure the amount of pin wear.

Table 8 shows the results of the cooling rates measured above. The eutectic silicon average particle diameter and the primary crystal silicon average particle diameter of the obtained ingot were as shown in Table 9. Hardness (HR
B), formability of the skirt portion, swarf shape around the oil ring groove portion, swarf processing ability, roughness of the inner surface of the oil ring groove,
Table 12 shows the results of the amount of wear of the test piece around the oil ring groove.

[Comparative Examples 1 and 2] As shown in Table 1, a continuous cast rod having a diameter of 82 mm (Comparative Example 1) using alloy 1 as a raw material and a continuous cast rod having a diameter of 115 mm (Comparative Example 2) using alloy 2 as a raw material were manufactured. did. The continuous casting method is Japanese Patent Publication Sho 54-4284.
According to the gas pressure hot-top casting method disclosed in Japanese Patent Laid-Open No. The casting conditions are as shown in Table 10. The obtained ingot was homogenized by holding it at 495 ° C. for 8 hours and then chamfered to give a diameter of 77 mm in Comparative Example 1 and 1 in Comparative Example 2.
It was set to 10 mm. After that, each was cut into a thickness of 30 mm and used as a material for forging. The forging conditions were the same as in Examples 1 and 2, respectively. However, the orientation of the material was not used properly during forging. The artificial aging condition after forging is also
The machining conditions were also the same as in Examples 1 and 2.

Table 11 shows the measurement results of the eutectic silicon average particle diameter and the primary crystal silicon average particle diameter, and Table 12 shows the evaluation results obtained in the same manner as in Examples 1 and 2.

[0071]

[Table 1]

[0072]

[Table 2]

[0073]

[Table 3]

[Table 4]

[Table 5]

[0074]

[Table 6]

[0075]

[Table 7]

[Table 8]

[Table 9]

[Table 10]

[0076]

[Table 11]

[0077]

[Table 12]

As is clear from Example 1 in Table 8, the cooling rate at the measurement site e was 2.6 ° C./sec and the E / F was 0.42.
In the material for forging cast under the conditions, the structure is such that primary silicon does not exist, and the average grain size of eutectic silicon (HEYW
The OD diameter) was 4.7 μm at the oil ring groove portion, 2.3 μm at the tip of the skirt portion, and the ratio (A / B) was 2. According to Table 11, the results of alloy 1 of Comparative Example 1 using a small diameter continuous casting rod show that the average grain size of eutectic silicon is 2.
0μm, 2.0μm at the tip of the skirt
The ratio (A / B) was 1, and there was no tendency to incline the structure in the plate thickness direction. The average grain size of eutectic silicon in the oil ring groove was smaller in Comparative Example 1. From Table 12, the moldability of the skirt portion at this time was good in both Example 1 and Comparative Example 1. However, the swarf shape of the oil ring groove portion was divided type in Example 1 whereas the swarf type in Comparative Example 1 was continuous type and had poor swarf processing property. Further, in Comparative Example 1, the roughness of the inner surface of the oil ring groove was large, and the wear resistance of the oil ring groove portion was poor. If the inner surface of the oil ring groove has a rough surface, the piston ring wears the oil ring groove, causing the piston ring to become slanted and causing galling on the inner surface of the cylinder. If the clearance between the piston and the cylinder becomes large due to deterioration of wear resistance, not only the engine oil will be consumed more severely, but also the airtightness cannot be maintained and the output of the engine will be reduced. Therefore, in Comparative Example 1,
It is considered to be insufficient when the piston is molded from this rough material.

Further, when the example 2 and the comparative example 2 are compared in the same manner, the cooling rate of the measuring portion e of the example 2 is 4.1.
C./second, ratio (E / F) was 0.75. At that time, the eutectic silicon average particle size (A) in the oil ring groove was 6.9 μm.
m, the eutectic silicon average grain size B at the tip of the skirt is 2.8.
And the ratio (A / B) was 2.5. The average primary crystal grain diameter C of the oil ring groove portion is 23.9 μm, and the average primary crystal grain diameter D of the skirt tip portion is 15.7 μm.
m, and its ratio (C / D) was 1.5. In Comparative Example 2 of Table 11, eutectic silicon average particle size A in the oil ring groove portion
Is 2.6 μm, the eutectic silicon average particle size B at the skirt tip is 2.5 μm, the ratio (A / B) is 1.0, and the primary crystal silicon average particle size C in the oil ring groove is 17. 7μ
The average grain size D of primary silicon at the tip of the skirt is 18.
3 μm, the ratio (C / D) was 1.0, and there was no tendency to incline the structure in the plate thickness direction. Thus, both the eutectic silicon average particle diameter and the primary crystal silicon average particle diameter in the oil ring groove were smaller in Comparative Example 2. The moldability was good in both Example 2 and Comparative Example 2, but the cutting powder shape of the oil ring groove portion was divided type in Example 2 whereas Comparative Example 2 was continuous type. Comparative Example 2 was inferior in terms of chipping property and surface roughness. Since the average particle size of the primary crystal silicon was fine, the shape of the cutting chips was continuous, the processing property of the cutting chips was poor, and the roughness of the inner surface of the oil ring groove and the wear of the test piece were inferior.

[Comparative Example 3] Alloys 1 and 2 shown in Table 1 were used as raw materials and cast into a piston by die casting. As the mold, two types, that is, the A type having the same shape as that used for forging in Example 1 and having a skirt portion with a wall thickness of 3.5 mm and the B type having 6 mm, were used. The size and shape of the inner diameter of the B type were the same as those of the A type.

Using these two kinds of A and B dies, the piston was cast under the casting conditions shown in Table 13, and the soundness of the skirt portion of the obtained piston was evaluated. The results are shown in Table 14. Table 15 shows the measurement results of the average particle sizes of eutectic silicon and primary crystal silicon.

[0082]

[Table 13]

[0083]

[Table 14]

[0084]

[Table 15]

From these results, in the die casting, if the thickness is 3.5 mm, which is the same as the thickness of the forged product, a flesh thickness is generated at the tip of the skirt due to defective spinning, and a thickness equivalent to that of forging is secured for both alloys. I could not do it. A healthy product was obtained with a wall thickness of 6 mm. In this way, it is clear that the same die casting as the forged product cannot be obtained by die casting. Both the eutectic silicon and primary crystal silicon average particle diameters in the oil ring groove portion were measured in Examples 1 and 2.
It has the same size as that of the oil ring, and it can be expected that the oil ring groove part has good machinability, but the other purpose is that the skirt cannot be thinned and the cutting allowance is large, resulting in high productivity. I found that I couldn't get the piston.

[Comparative Example 4] The alloy 2 shown in Table 1 was melted in a melting facility to obtain an outer diameter of 300 mm and a total length of 350 mm.
Then, it was poured into a cylindrical iron mold having an inner diameter of 115 mm and a depth of 250 mm to obtain a cylindrical ingot having an outer diameter of 115 mm and a sound part length of 150 mm with no cavities. The molten metal temperature during casting was 800 ° C, and the mold was preheated to 300 ° C. The obtained cylindrical ingot was heated and held at 490 ° C. for 8 hours to be homogenized, and then the outer periphery was chamfered to remove unhealthy portions to obtain a round bar having an outer diameter of 110 mm. 30mm thick from the round bar
The cut piece was taken out and used as a forging material.

In forging, the piston has the same outer diameter, but the skirt has a wall thickness of 4.0 mm and a die C of 6 mm.
Two types, a mold D and a mold D, were prepared. Before performing hot forging, the preheating temperature of the material was set to 450 ° C. and then forging was performed to evaluate whether or not the skirt portion can be formed. The forging conditions are the same as in Example 2 in Table 3, and the results are shown in Table 16. A test piece was taken from the tip of the skirt and the metal structure was observed to measure the eutectic silicon average particle diameter and the primary crystal silicon average particle diameter. The results are shown in Table 17.

[0088]

[Table 16]

[0089]

[Table 17]

From these results, if the eutectic silicon average particle size and the primary crystal silicon average particle size are large, the forgeability is deteriorated, and the product can be completed as a product even if the wear resistance and the chip breaking property are aimed at. It can be seen that there is no or, as in the case of die casting, the wall thickness must be increased and then machined to be thinly removed.

[0091]

As described above, according to the present invention, the average grain sizes of eutectic silicon and primary crystal silicon are fine at the tip of the skirt portion and coarse at the oil ring groove portion, so that the skirt portion has a continuous small diameter. It has the same forgeability as the one using a casting rod and can be thinned, and at the same time, the oil ring groove part has excellent chip disposability during cutting like a cast piston, the groove surface roughness is small, and the oil ring A forged piston for an internal combustion engine having excellent groove wear resistance was obtained.

Such a piston cannot be obtained by hot forging using a conventional continuous casting small diameter continuous casting rod or casting by die casting, and forging for an internal combustion engine having both advantages of the present invention is provided by the present invention. It is now possible to provide pistons.

This piston is obtained by a manufacturing method in which hot forging and artificial aging treatment are combined using a unidirectional solidification casting material.

As the aluminum alloy used, in addition to silicon, Cu, and Mg, which are the main additive elements, Ag and Sc, which have a further effect on artificial age hardening, and Fe, Ni, and Ti, which increase the heat resistance. , Cr, V, Zr, Mn,
By adding Co, Nb, Mo, etc. alone or in combination, not only the piston groove portion and the skirt portion but also the characteristics of the head surface and the piston pin portion can be improved as compared with other portions.

As a result, a high-performance and inexpensive forged piston can be provided, and a high-performance engine with stable quality can be provided.

[Brief description of drawings]

FIG. 1 is a schematic sectional view of an example of a forged piston for an internal combustion engine of the present invention. (A) is a vertical sectional view including a skirt portion, and (b) is a vertical sectional view including a piston pin hole.

FIG. 2 is a schematic view of an example of a casting apparatus used in the unidirectional solidification casting method.

FIG. 3 is an explanatory diagram of cooling rate measurement points in a mold of a unidirectional solidification apparatus.

FIG. 4 is a diagram of a Compax subbite used for a cutting test. (A) is a side view, (b) is a plan view, and (c) is a front view.

FIG. 5 is a schematic view of a forging device used in the present invention.

[Explanation of symbols]

11: head surface, 12: oil ring groove, 13: skirt portion, 14: piston pin hole, 15: shape of rough piston material, 16: shape of finished piston product after machining, 1
7: rib, 18: skirt tip, 101: forging machine,
102: upper bolster, 103: upper mold, 106: lower bolster, 105: lower mold, 108: spray forward / backward transfer device, 109: spray rotating device, 110: shaft, 1
04: Lubricant spray nozzle, 201: Cooling plate, 20
2: main mold, 203: receiving tank, 204: inlet, 205:
Opening / closing plug, 206: ingot, 207: molten aluminum alloy, 208: lid, 209: electric furnace, 210: spray nozzle, 211: case, 212: drainage port

Front page continuation (51) Int.Cl. ⁷ Identification code FI theme code (reference) C22F 1/043 C22F 1/043 F16J 1/01 F16J 1/01 // C22F 1/00 630 C22F 1/00 630D 630J 630K 650 650A 651 651B 681 681 682 682 683 683 694 694B (72) Inventor Masahiro Sato 7840 Showa Denko Stock Association In-house F-term (reference) 3J044 AA01 AA02 AA18 BA04 DA09

Claims (8)

[Claims] 1. A forged piston for an internal combustion engine made of an aluminum alloy containing 6 to 25% by mass of silicon, wherein the eutectic silicon average particle size (A) in the oil ring groove and the eutectic silicon average in the tip of the skirt are average. Ratio with particle size (B) (A / B)
Is 1.5 or more, and the eutectic silicon average particle size (A) of the oil ring groove is 4 μm or more, the forged piston for an internal combustion engine. 2. A forged piston for an internal combustion engine made of an aluminum alloy containing 6 to 25% by mass of silicon, wherein the eutectic silicon average particle diameter (A) in the oil ring groove and the eutectic silicon average in the tip of the skirt are average. Ratio with particle size (B) (A / B)
Is 1.5 or more, and the eutectic silicon average particle diameter (A) of the oil ring groove portion is 4 μm or more, and the primary crystal silicon average particle diameter (C) of the oil ring groove portion and the skirt tip portion are Internal combustion characterized in that the ratio (C / D) to the average primary silicon particle diameter (D) is 1.3 or more, and the average primary silicon particle diameter (C) in the oil ring groove is 15 μm or more. Forged piston for engines. 3. The forged piston for an internal combustion engine according to claim 1, wherein the aluminum alloy contains 0.3 to 7 mass% Cu and 0.1 to 2 mass% Mg. 4. Ni to 0.1-2.
The forged piston for an internal combustion engine according to any one of claims 1 to 3, wherein the forged piston contains 5% by mass. 5. A step of producing a forging material having a structure in which the average grain size of silicon is different on the upper surface and the lower surface of an aluminum alloy melt containing 6 to 25% by mass of silicon by a unidirectional solidification casting method, and the forging material. And heat-treating the forging material with the surface having a large average silicon particle diameter and the surface forming the piston head of the die facing each other and housed in the die to form a piston raw material by the forging method. And a step of artificially aging the piston blank, and a step of machining the piston blank to form a forged piston for an internal combustion engine. Method. 6. The cooling condition for unidirectional solidification is such that the eutectic silicon average particle size (A) on the upper side of the ingot and the eutectic silicon average particle size (B) on the cooling plate side (A). / B) is 1.5 or more and the eutectic silicon average particle size (A) is 4 μm or more, and the conditions are such that the method for producing a forged piston for an internal combustion engine according to claim 5. . 7. The cooling condition for unidirectional solidification is such that the cooling rate (E) at a position e of 5 mm from the upper surface of the casting mold for solidification and 5 mm from the outer periphery is 0.5 ° C./second or more, and 1 mm from the position e and the bottom surface of the mold for solidification, 5 mm from the outer periphery
Ratio of cooling rate (F) at position f inside (E /
F) is 0.85 or less, The manufacturing method of the forged piston for internal combustion engines of Claim 5 characterized by the above-mentioned. 8. The internal combustion engine according to claim 5, wherein the temperature condition of the preheat treatment is in the range of 350 ° C. to (solidus temperature of aluminum alloy −10) ° C. Forged piston manufacturing method.