JP2000179400A - Forged piston for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a forged piston for internal combustion engine to increase strength and reduce weight through forging, while evading reduction of strength under high temperatures without using a special material, and to improve piston durability thereby. SOLUTION: This forged piston for internal combustion engine is equipped with a generally disk-like head portion 2 having a top surface 2a that forms the bottom surface of the combustion chamber, a pair of pin boss portions extending downward from two places on the piston pin axis on the bottom surface of the head portion 2, and a skirt portion extending downward from at least both sides with respect to the piston pin axis of the outer peripheral edge of the head portion 2. The piston is manufactured by closed die forging with aluminum alloy containing at least silicone. Fiber flows f are formed on the head portion 2 extending in the radial direction from the center. Between the fiber flows f on the top surface 2a of the head portion 2, heat radiator grooves 7a are formed by shot peening.

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 formed by die forging.

[0002]

2. Description of the Related Art As a piston for a two-cycle engine or a four-cycle engine, a billet made of an aluminum alloy is forged using a mold to produce a piston material having a head portion, a pin boss portion, a skirt portion, and the like. After that, a forged piston may be used, which is formed by forming a cylinder sliding surface, a piston pin hole, a ring groove, and the like on the piston material by machining.

[0003] Since the above-mentioned forged piston has a high strength due to its manufacturing method, the thickness of the head portion where a large stress is generated due to the explosion pressure can be reduced as compared with the conventional one, and the entire piston is lightened accordingly. Can be

[0004]

However, in a piston for an internal combustion engine, the temperature of the top surface of the head portion tends to rise due to contact with high-temperature burned gas, and depending on the material, the strength is increased by the temperature rise of the head portion. Decrease. In particular, in the case of an aluminum alloy, the strength tends to decrease at high temperatures, and improvement in this respect is required. For this purpose, in addition to increasing the strength of the piston by forging, it is necessary to use a material with a small decrease in strength even at a high temperature, or to increase the heat dissipation to suppress the rise in the temperature of the head.

However, the use of a material having a small decrease in strength even at high temperatures generally has a problem of high cost. On the other hand, in the case of the method in which the external cooling is strongly reduced to suppress the rise in the temperature of the piston, there is a concern that the temperature of the burned gas in the entire combustion chamber is reduced and the engine performance is reduced.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and it is possible to avoid a decrease in strength at high temperatures without depending on a special material, and to improve the strength and reduce the weight of a forged piston for an internal combustion engine. It is intended to provide.

[0007]

According to the present invention, a substantially disk-shaped head portion having a top surface forming a bottom surface of a combustion chamber, and a lower portion of the lower surface of the head portion are formed from two portions on a piston pin axis. A forged piston for an internal combustion engine having a pair of extended pin boss portions and a skirt portion extending downward from both sides of at least the piston pin axis of the outer peripheral edge of the head portion, wherein the piston is at least silicon. The head is formed with a die forging made of an aluminum alloy, and a fiber flow extending radially from the central portion is formed in the head portion, and the fiber flow is radially formed by shot peening between the fiber flows on the top surface of the head portion. Are formed.

A second aspect of the present invention is characterized in that, in the first aspect, the average particle size of the silicon is 50 μm or less.

[0009]

According to the forged piston of the present invention, a piston material made of an aluminum alloy lump or aluminum alloy powder containing Si is forged into a piston shape by a forging die, and then subjected to predetermined machining. Manufactured.

Due to the difference in deformation resistance between the aluminum alloy and Si with respect to the forging force during the die forging, Si is converged to form a flow of the material fiber structure, that is, a fiber flow (forging line). This fiber flow is formed radially from the center of the head part in the radial direction, is continuously formed from the head part to the land part where the ring groove is formed and further to the skirt part, and in the skirt part is parallel to the piston central axis, In addition, a part is exposed on the top surface of the head portion.
The top surface of the head thus formed has a structure in which a fiber flow with high hardness and low thermal conductivity and an aluminum alloy material with lower hardness and high thermal conductivity are radially striped. Will be provided.

Then, shot peening is applied to the top surface of the head. By this processing, the aluminum alloy ground softer than the fiber flow in the top surface structure is preferentially shaved, and thus the radial heat radiation groove of the present invention is formed.

As described above, according to the forged piston of the present invention, a fiber flow extending radially from the center portion is formed in the head of a die forged piston made of an aluminum alloy containing Si, and the fiber flow is reduced by shot peening. Since the heat radiation grooves extending in the radial direction are formed between the heat radiation grooves, the heat radiation grooves can increase the surface area of the aluminum alloy base on the top surface. While increasing the amount of heat received by this area increase,
Through the aluminum alloy having high radial thermal conductivity, heat can be efficiently guided from the exposed aluminum alloy ground to the inner peripheral wall of the cylinder and from the skirt to the inner peripheral wall of the cylinder, so that heat dissipation can be enhanced. In this way, particularly, the heat of the burned gas that comes into contact with the top surface is taken into the head portion from the heat radiation groove of the head portion, conducted in the radial aluminum alloy ground and in the fiber flow, and transmitted along the piston ring and the skirt portion. It will be transmitted to the cylinder wall.

As a result, the temperature of the layer in contact with the top surface of the burned gas can be lowered, and the temperature rise of the head can be suppressed accordingly. In this case, since the temperature of the burned gas in the entire combustion chamber does not decrease so much, a decrease in engine performance can be avoided.

As described above, since the rise in the temperature of the head portion can be suppressed, the decrease in the strength of the aluminum alloy due to the heat of the burned gas is reduced, and the strength of the head portion is improved by the radial fiber flow. In addition, there is an effect that strength reduction can be avoided without depending on the above-mentioned special material, and thus, the durability of the piston can be improved.

According to the second aspect of the present invention, the average grain size of Si is 5
0 μm or less, S
The i-particles can be prevented from falling off, and the fatigue strength can be improved, and from this point, the life of the piston can be further extended. Further, by setting the Si particles to 50 μm or less, ductility during forging can be increased, and there is an effect that surface roughness can be prevented.

[0016]

Embodiments of the present invention will be described below with reference to the accompanying drawings. 1 to 8 are for explaining a forged piston for an internal combustion engine according to an embodiment of the present invention, and FIGS. 1 to 4 are a plan view, a right side view, a front view, and a bottom view of the forged piston, respectively. 5 and FIG. 6 show FIG.
7 is a cross-sectional view taken along line VV, VI-VI, FIG. 7 is a schematic view of a fiber flow formed on a forged piston, and FIG.
FIG. 8 is a schematic sectional view taken along line -VIII. The front, rear, left and right in the present embodiment are as shown in FIG. 1, and the exhaust side is the front and the intake side is the rear in plan view.

In the drawing, reference numeral 1 denotes a forged aluminum alloy piston for a four-stroke, five-valve engine.
Denotes a substantially disk-shaped head portion 2, a pair of pin boss portions 4, 4 extending downward from two portions on the lower surface of the head portion 2 on the piston pin axis C, and an outer peripheral edge of the head portion 2. Skirt portions 3, 3 extending downward from the front and rear portions of the piston pin axis C, and a lower surface 2f of the head portion 2.
Each of the pin bosses 4 and the skirt 3
And four rib portions 5 for integrally connecting the lower portion 2f of the head portion 2 with the lower portion 2f.

The pin bosses 4 and 4 are connected to the head 2
The lower surface 2f extends downward on two axially indented portions from the outer peripheral surface of the head portion 2 on the piston pin axis C. Piston pin holes 4a are formed in the pin boss portions 4, 4 so as to penetrate coaxially. In addition, 4
Reference numeral b denotes a retaining ring groove in which a retaining ring for the piston pin is mounted, and reference numeral 4c denotes a tool insertion hole for attaching / detaching the retaining ring to / from the retaining ring groove 4b.

The head 2 has a substantially disk shape.
On the top surface 2a forming the bottom surface of the combustion chamber, three relief recesses 2b for intake valves and two relief recesses 2c for exhaust valves are formed. On the side surface 2h of the head portion 2, two pressure ring grooves 2d extending in the circumferential direction and one oil ring groove 2e located below the two pressure ring grooves 2d are formed by machining. A plurality of oil holes 2j communicating with the inside of the piston 2 are formed at the bottom of the lowest oil ring groove 2e.

The skirt portions 3, 3 are for receiving piston side pressure generated with the rotation of the engine, and are connected to the front and rear sides of the piston pin axis C, that is, a connecting rod (not shown) connected to the piston pin. Is provided in a portion opposed to the swing direction of. This skirt part 3,3
The outer peripheral surface is in sliding contact with the inner peripheral surface of the cylinder bore, and is machined.

The thickness of the skirt 3 is, as shown in FIG. 5, an inclined thickness that gradually decreases from the upper end 3a adjacent to the oil ring groove 2e to the lower end 3b located at the lower end of the piston. Is set. The skirt 3
Has an outer peripheral surface parallel to the piston axis A and an inner peripheral surface inclined inward. The boundary 2g between the upper end 3a of the skirt 3 and the lower surface 2f of the head 2 is thick.
The ring grooves 2d and 2e are formed in the boundary 2g.

As described later, the piston 1 is formed by forging a piston material made of an aluminum alloy lump containing Si having an average particle diameter of 50 μm or less into a piston shape using a forging die, and subjecting the piston material to shot peening. And then subject to a predetermined machining process. The shot ball used in this shot peening has a hardness higher than the hardness of the aluminum alloy ground and lower than the hardness of the fiber flow portion. Si
Is mainly about Hv = 1200, the hardness of the aluminum alloy ground whose hardness is increased by forging is about Hv = 200, and Hv = 30 as a shot ball.
Approximately 0 to 1000 steel balls, glass balls and the like are selectively used.

As shown in FIGS. 7 and 8, the head portion 2 is formed with a fiber flow f radially extending from the center of the piston (the portion of the piston axis A) in the radial direction. The fiber flow f means a flow of a material fiber structure appearing in die forging, that is, a forging line. A part of the fiber flow f is exposed on the top surface 2a of the head portion 2, and the fiber flow f and the aluminum alloy material Al form radial stripes on the top surface 2a.

On the top surface 2a of the head portion 2, there is formed a heat radiation groove 7 which also extends in the radial direction between the fiber flows f. The heat radiation groove 7 is formed by shaving off an aluminum alloy material Al having a lower hardness than the fiber flow f during the shot peening process.

Next, one method of manufacturing the piston 1 will be described with reference to FIGS.
This will be described with reference to FIG. Here, in FIG.
Reference numeral 0 denotes a continuous casting device for producing the piston blank W1, and reference numeral 21c denotes an outlet. An ingot G of an aluminum alloy and Si powder having an average particle size of 50 μm or less are charged,
Alternatively, an ingot G of an aluminum alloy containing Si powder having an average particle diameter of 50 μm or less is charged, and at a temperature equal to or higher than the melting temperature of the aluminum alloy and equal to or lower than the melting temperature of Si,
After melting, continuous casting is carried out by continuously taking out from the outlet 21c and cooling and solidifying, and the taken-out continuous cast body is cut into a predetermined size to produce a forged material (burette) W1 for a piston. It is desirable to control the amount of the Si powder charged so that the Si content is 10 to 22% by weight, or to feed the aluminum alloy ingot G having the Si content of this weight ratio.

In the continuous casting apparatus shown in FIG. 9, a temperature difference occurs between the periphery and the center of the continuous cast material in the process of cooling and solidifying, and the solidification of the center is delayed, so that the crystal grain size of the aluminum alloy increases at the center. And the particle size tends to be uneven inside and outside. FIG. 10 shows a continuous casting apparatus capable of making the particle size uniform inside and outside. This casting apparatus 20 includes a melting furnace 21 having an inlet 21a at an upper portion, and a heater 21b wound around the melting furnace 21. A stirrer 22 provided with an outlet 21c formed at the bottom of the furnace 21 and formed of an electromagnet
And a roller 23 for continuously drawing the casting.

In the continuous casting apparatus shown in FIG. 10, the aluminum alloy ingot G and the average
0 μm or less of Si powder,
An ingot G of an aluminum alloy containing Si powder of not more than μm is charged, and the molten metal is stirred by the stirrer 22 to disperse the nuclei (Si particles) while preventing the difference in cooling temperature between inside and outside. The solidified and cooled continuous cast body W is drawn out by the roller 23 and cut into a size corresponding to one piston to produce a piston raw material W1. Similarly to the above, it is desirable to control the content of Si in the continuous casting W to be 10 to 22% by weight.

FIGS. 11 and 12 show another method of manufacturing three piston blanks W1. In the process (1) of FIG. 11, an ingot G of an aluminum alloy containing Si having an average particle diameter of 50 μm or less is prepared. In the process (2),
This ingot G is put into a melting furnace and is heated to about 700 ° C.
After melting at a temperature equal to or lower than the melting temperature of Si, the aluminum alloy containing Si is rapidly dispersed and sprayed in a mist state and rapidly cooled and solidified at a cooling rate of 100 ° C./sec or more (atomizing method). Solidified powder (powder metal). An ingot G of an aluminum alloy containing no Si was melted, and a rapidly solidified powder of an aluminum alloy was produced by an atomizing method.
The powder may be mixed and stirred to form a powder material of an aluminum alloy containing Si.

Thereafter, the process of (3) in FIG.
Either the process (1) or the process (3 '') shown in FIG. 12 is employed. In the step (3), the cylindrical rod-shaped body formed by heating and extruding the powdered material of the aluminum alloy containing Si is cut into a predetermined size in the step (4) to produce the piston material W1.

When the step (3 ') is adopted, the mold is filled with a powder material of an aluminum alloy containing Si, and then pressed (cold pressed) at room temperature to be solidified. , 4
By heating to 00 to 500 ° C. and pressing (hot pressing 4 ′), a piston material W1 of a desired size is manufactured.

When the step (3 '') is adopted, the aluminum alloy powder material containing Si is accommodated in a thin aluminum alloy cylindrical container, vacuum degassed, and then (4 ''). In the process, the cylindrical container is housed in a mold and extruded in a heated state, and in the process (5 ''), the extruded rod is cut into a predetermined size to produce a piston material W1.

13 and 14, reference numeral 25 denotes a forging die for forming the piston material W1 into a forming material 10 by die forging. The mold 25 includes a first mold (upper mold) 11 having a shape corresponding to a part of the surface shape of the molding material 10 having substantially the same shape as the piston 1, and a remaining portion of the surface shape of the molding material 10. Second of corresponding shape
And a mold (lower mold) 12. The first and second dies 11, 12 are relatively movable with a predetermined forging force.

Then, in the second mold 12 of the mold 25,
A release agent is applied to the outer surface of the piston material W1 and placed thereon, and the first mold 11 is lowered with a predetermined forging force.
In this case, the piston material W1 is heated to a predetermined temperature by a heater or the like and hot forged. By performing the hot forging, the forming material 10 can be forged with high dimensional accuracy by sufficiently utilizing the ductility of the aluminum alloy (see FIGS. 15 (1) to (4)).

In this manner, the piston material W1 is
It is formed into a forming material 10 corresponding to the piston 1 by crushing while applying an impact force by a mold 11. Here, the mold surface 11 a of the first mold 11 corresponds to the top surface 2 a ′ of the head part 2 ′, and the mold surfaces 12 a, 1 of the second mold 12.
2b is a side outer surface 2h 'and a lower surface 2f' of the head 2 '.
It corresponds to. The mold surface 12c corresponds to the skirt 3 ', and the mold surface 12d corresponds to the rib 5'.

In the above-described die forging, the first forging force is applied.
When the piston material W1 is plastically deformed by being pressed by the mold surface 11a of the mold 11 and the mold surface 12b of the second mold 12, Si accumulates due to a difference in deformation resistance between the aluminum alloy and Si. A radial fiber flow is formed. Accordingly, a fiber flow f having high hardness and small thermal conductivity and an aluminum alloy base Al having low hardness and high thermal conductivity are alternately arranged on the top surface 2a 'of the head portion 2'. (See FIGS. 7 and 8A).

Next, shot peening is applied to the top surface of the head portion of the molding material 10 (see FIG. 15 (5)).
This shot peening process is a process in which a shot (steel grain or the like) is pressurized by air pressure or centrifugal force and sprayed onto the above-mentioned top surface. The purpose is mainly to increase the surface hardness and extend the wave working life. And By this shot peening, the soft aluminum alloy ground on the top surface is preferentially scraped off, thereby forming a radiation groove 7 extending in the radial direction between the fiber flows f, f (see FIG. 8B).

During the shot peening, the side surface 2h, ie, the upper side surface on which the ring grooves 2d and 2e are formed, is masked to protect the ring grooves 2d and 2e and to scrape off the aluminum alloy ground exposed on the side surface 2h. Not to be.

Thereafter, the molding material 10 is subjected to machining, and the sliding surface, each of the ring grooves 2d and 2e, and the piston pin holes 4 are formed.
a etc. are formed. Thereafter, a surface treatment is performed, for example, an inner chemical conversion film of Al ₂ O ₃ of about 1 μm is formed or tin plating is applied (see FIG. 15 (6)). Thereby, the forged piston 1 of the present embodiment is manufactured. The inner surface of the skirt portion 3 of the piston 1 and the surfaces of the pin boss portions 4 and the rib portions 5 are subjected to a surface treatment without forging without machining. Regarding the head portion 21, the heat radiation groove 7 by shot peening is used even when the forged skin is left as it is or when machining is performed.
So that it remains.

Here, a center boss (not shown) for machining is formed by forging at a portion corresponding to the center of the top surface of the head portion of the molding material 10. This center boss was removed during machining. Due to the removal of the center boss, a boss mark A ′ remains at the center of the top surface, and the terminal end of the fiber flow f is exposed to the boss mark A ′.

Next, the operation and effect of this embodiment will be described. The forged piston 1 of the present embodiment is slidably inserted into a cylinder bore of a four-cycle engine. The head portion 2 of the piston 1 is exposed to the high heat of the burned gas and rises in temperature during the explosion combustion stroke.
Transmitted to the inside of the piston, is transmitted from the piston ring and the skirt portion 3 to the inner peripheral wall of the cylinder bore, thereby preventing the head portion 2 from overheating.

According to the present embodiment, a fiber flow f extending radially in the radial direction from the center is formed in the head 2 of the forged piston 1 made of an aluminum alloy containing Si, and the head 2 is formed by shot peening. Top 2
a, the heat dissipation groove 7 is formed on the top surface 2 by the heat dissipation groove 7.
a, the surface area of the aluminum alloy ground can be increased, the amount of heat received is increased by the increase in area, and heat is efficiently transferred from the exposed aluminum alloy ground on the side surface 2h through the aluminum alloy having high radial thermal conductivity. Furthermore, the heat can be guided from the skirt portion to the inner peripheral wall of the cylinder, so that heat radiation can be improved. Thereby, the temperature of the layer in contact with the top surface 2a of the burned gas can be reduced, and the temperature rise of the head portion 2 can be suppressed accordingly. In this case, since the temperature of the burned gas in the entire combustion chamber does not decrease so much, a decrease in engine performance is avoided.

As described above, since the rise in the temperature of the head portion 2 due to the heat of the burned gas can be suppressed, a decrease in the strength of the piston can be avoided without using a special material, and the durability of the piston can be improved.

In this embodiment, since the end of the fiber flow f is exposed at the center boss mark A ', more heat of the burned gas in contact with the top surface is transferred from the exposed portion to the cylinder wall. The temperature of the boss mark portion of the burned gas in contact with the top surface is more reliably reduced.

On the other hand, a greater stress is generated at the center of the piston top surface due to the explosion pressure than at other portions. However, in the present embodiment, as described above, since a rise in temperature at the center of the piston top surface is suppressed, a decrease in strength can be reduced, a large stress can be endured, and durability can be improved.

In this embodiment, the average particle size of Si is 50 μm.
m or less, the Si particles can be prevented from falling off during shot peening, and the fatigue strength can be improved. In this respect, the durability can be improved and the life of the piston can be extended. In addition, since the Si particle size is set to 50 μm or less, ductility during die forging can be increased, and surface roughness can be prevented.

FIG. 16 is a characteristic diagram showing the relationship between the average Si grain size and the fatigue life to show the results of an experiment performed to confirm the effect of the present embodiment. In this experiment, a piston sample with a high outer surface hardness and a piston sample with a lower surface hardness were prepared with different Si particle sizes.
The fatigue life when shot peening was performed on each sample was measured and performed.

As is clear from the figure, when the average grain size of Si exceeds 50 μm, the fatigue strength of all samples is extremely reduced. This is considered to be because if the Si average particle diameter exceeds 50 μm, the hardness reduction rate due to shot peening sharply increases. On the other hand, when the average particle size of Si is set to 50 μm or less, the rate of decrease in hardness does not increase because there is no dropout of Si by shot peening, and as a result, fatigue strength can be improved. Conceivable.

[0048]

[Table 1]

Table 1 shows the results of measuring the hardness and fatigue strength of the forged piston of the present invention. Column II of Table 1 above shows the piston of the present invention obtained by die forging an aluminum alloy (AHSG) lump having a Si particle size of 30 μm manufactured by continuous casting.
Column III shows the Si particle size of 10 μm produced by atomization.
The piston of the present invention obtained by die-forging the following aluminum alloy (PM1) powder is shown. Column IV shows the piston of the present invention (PM2) having a different chemical composition by the same manufacturing method as that of column III. I
The column shows a conventional die casting piston (AC9B).

As is clear from the table, the conventional piston has a low hardness of 65 HRB and a low fatigue strength of 140 Mpa (see column I). On the other hand, each of the pistons of the present invention has a hardness of 70 to 80 HRB and a fatigue strength of 180 to 22.
It can be seen that it is 0 Mpa, which is much higher than that of the conventional piston (see columns II to IV).

In the continuous casting shown in FIGS. 9 and 10, silicon carbide (Sic) and aluminum oxide (Al
_The burette may be manufactured by adding powder having an average particle size of 50 μm or less of a hard component such as ₂ O ₃ ) or aluminum oxide (AlN) singly or in a plurality of melting furnaces. In this case, the total content of these components is about 5% by weight.

In the same manner as described above, powder having an average particle size of a hard component such as silicon carbide (Sic) of 50 μm or less alone or in addition to the rapidly solidified powder of the aluminum alloy shown in FIG. The burette may be manufactured through the process shown in FIG. 11 or FIG.

Even when a burette containing a hard component in addition to the above-mentioned Si is subjected to the die forging shown in FIGS. 13 and 14, the fiber flows shown in FIGS. 7 and 8 are formed. This fiber flow is formed by gathering Si particles and hard components. Thereby, the strength of the head portion 2 can be further improved, and the temperature rise of the head portion 2 can be suppressed.

[Brief description of the drawings]

FIG. 1 is a plan view illustrating a forged piston according to an embodiment of the present invention.

FIG. 2 is a right side view of the forged piston.

FIG. 3 is a front view of the forged piston.

FIG. 4 is a bottom view of the forged piston.

5 is a sectional side view of the forged piston (FIG. 1)
VV line cross section).

6 is a sectional front view of the forged piston (FIG. 1)
VI-VI sectional view).

FIG. 7 is a schematic view showing a fiber flow of the forged piston.

FIG. 8 is a schematic view showing a heat radiation groove of the forged piston (a sectional view taken along line VIII-VIII in FIG. 7).

FIG. 9 is a schematic view showing a manufacturing process of the forged piston.

FIG. 10 is a schematic view of a continuous casting apparatus in a process of manufacturing the forged piston.

FIG. 11 is a schematic view showing a manufacturing process of the forged piston.

FIG. 12 is a schematic view showing a manufacturing process of the forged piston.

FIG. 13 is a schematic diagram showing a state of die forging of the forged piston.

FIG. 14 is a schematic diagram showing a state of die forging of the forged piston.

FIG. 15 is a schematic view showing a manufacturing process of the forged piston.

FIG. 16 is a characteristic diagram showing the effect of the embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Forged piston 2 Head part 2a Top surface 2f Lower surface 3 Skirt part 4 Pin boss part 7 Heat dissipation groove f Fiber flow C Piston pin axis

Continued on front page F-term (reference) 3J044 AA01 AA06 AA08 CA01 CA40 DA09 EA10 4E087 AA02 BA01 BA04 BA23 CA08 CA11 CB01 CB03 DB12 DB13 DB18 DB22 DB24 EC12 EC46 HA62

Claims (2)

[Claims] 1. A substantially disk-shaped head portion having a top surface forming a bottom surface of a combustion chamber, and a pair of pin boss portions extending downward from two portions on a piston pin axis of a lower surface of the head portion. A forged piston for an internal combustion engine having a skirt portion extending downward from at least both side portions of the piston pin axis on the outer peripheral edge of the head portion, wherein the piston is made of an aluminum alloy containing at least silicon. The head portion is formed with a fiber flow extending radially from the center portion, and a radial heat radiation groove is formed by shot peening between the fiber flows on the top surface of the head portion. A forged piston for an internal combustion engine, comprising: 2. The forged piston for an internal combustion engine according to claim 1, wherein the silicon has an average particle size of 50 μm or less.