Piston for internal combustion engine and method for manufacturing the same

Abstract

To provide a piston for an internal combustion engine in which heat conductivity and volume specific heat capacity are low, durability is excellent, and an anodic oxide film capable of suppressing heat from being accumulated even during continuous operation of the internal combustion engine is provided on a piston crown surface, and a manufacturing method thereof.SOLUTION: A manufacturing method according to the present invention includes the steps of: forming a recess 10 by irradiating a piston crown surface 2 of a piston body for an internal combustion engine made of an aluminum ally with a laser beam X; and sealing an opening 13 of the recess 10 by forming an anodic oxide film 20 by applying AC / DC superposition voltage to the piston crown surface 2. By this, a piston for an internal combustion engine according to the present invention has the plurality of recesses 10 on the piston crown surface 2, the respective openings 13 of the plurality of recesses 10 are sealed with an anodic oxide film 25, and has gaps 26 sealed with the anodic oxide films 20 provided inside the plurality of recesses 10.SELECTED DRAWING: Figure 5

Description

Technical Field

[0001] The present invention relates to a piston for an internal combustion engine and a method for manufacturing the same, and more particularly, to a piston for an internal combustion engine having an anodic oxide film on a piston crown surface and a method for manufacturing the same.

Background Art

[0002] Conventionally, when an aluminum alloy is used for a component forming a combustion chamber of an internal combustion engine or a part of a piston for an internal combustion engine, in order to improve the thermal efficiency in the combustion chamber, it has been required to form a film on the surface of the aluminum alloy to improve the heat insulation property.

[0003] For example, Patent Document 1 discloses an internal combustion engine in which an anodic oxide film is formed on a part or all of an aluminum-based wall surface facing a combustion chamber. The anodic oxide film has a film thickness in the range of 30 to 170 μm. The anodic oxide film has first micro-holes with a diameter in the micro-size range and nano-holes with a diameter in the nano-size range that extend in the thickness direction or substantially the thickness direction of the anodic oxide film from the surface toward the inside, and second micro-holes with a diameter in the micro-size range inside the anodic oxide film. The first micro-holes and the nano-holes are sealed with a sealant formed by conversion of a sealant, and the second micro-holes are not sealed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In a gasoline engine, an air-fuel mixture is compressed and spark ignition is performed by a spark plug. The flame propagates and burns so as to spread around the spark plug, and the generated combustion gas expands. Then, the unburned air-fuel mixture (end gas) located far from the spark plug is pressed against the piston and the cylinder wall surface, and becomes high temperature and high pressure by adiabatic compression. When this high temperature and high pressure exceed the limit, the end gas self-ignites all at once, and a shock wave (knocking) is generated at that time. If a heat insulating film is provided over the entire combustion chamber like an internal combustion engine, heat is less likely to escape overall in the combustion chamber, so the combustion temperature becomes uniformly high. As a result, although it contributes to the improvement of thermal efficiency, on the other hand, there is a problem that knocking is likely to occur especially at high loads.

[0006] In order to suppress the occurrence of such knocking, attempts have been made to reduce the thermal conductivity and the volumetric specific heat capacity of the anodic oxide film which is a heat insulating film. For example, by increasing the porosity or void fraction of the film, a low thermal property such as a thermal conductivity of 0.1 W / m·K and a volumetric specific heat capacity of 0.1×10 3 kJ / m 3 ·K is required, but the durability and good surface properties that can be adopted for internal combustion engines have not been obtained. Further, in the anodic oxide film in which the micropores and nanopores in the film are sealed as in Patent Document 1, even if good thermal properties are obtained, when it is adopted on the crown surface of the piston for an internal combustion engine, since the pores in the film are sealed, the original thermal properties cannot be maintained during the continuous operation of the internal combustion engine, and it becomes a factor for the occurrence of knocking due to heat accumulation.

[0007] Therefore, in view of the above problems, the present invention aims to provide a piston for an internal combustion engine provided with an anodic oxide film having a low thermal conductivity and a low volumetric specific heat capacity, excellent durability, and capable of suppressing heat accumulation even during continuous operation of the internal combustion engine, and a method for manufacturing the same.

Means for Solving the Problems

[0008] In order to achieve the above object, in one aspect, the present invention provides a method for manufacturing a piston for an internal combustion engine, the method comprising: irradiating a laser beam onto the piston crown surface of a piston body for an internal combustion engine made of an aluminum alloy, and forming a recess in a portion of the piston crown surface irradiated with the laser beam; anodizing the piston crown surface to form an anodic oxide film; and closing the opening of the recess, thereby manufacturing a piston for an internal combustion engine having voids closed by the anodic oxide film inside the recesses.

[0009] In another aspect, the present invention provides a piston for an internal combustion engine, comprising: a piston body for an internal combustion engine made of an aluminum alloy and having a piston crown surface; and an anodic oxide film covering the piston crown surface. The piston crown surface of the piston body for the internal combustion engine has a plurality of recesses. The cells of the anodic oxide film extend in random directions with respect to the surface of the piston crown surface and the surfaces inside the plurality of recesses, and the cells branch in random directions and surround the silicon inside the anodic oxide film. Each opening of the plurality of recesses is closed by the anodic oxide film, and each of the plurality of recesses has a void closed by the anodic oxide film inside.

Advantages of the Invention

[0010] According to the present invention as described above, by forming a plurality of recesses in the piston crown surface of a piston body for an internal combustion engine made of an aluminum alloy and forming an anodic oxide film on the piston crown surface so as to close each opening of the plurality of recesses, voids closed by the anodic oxide film respectively exist inside the plurality of recesses. Therefore, an anodic oxide film having a low thermal conductivity and a low volumetric specific heat capacity and excellent durability can be formed on the piston crown surface. In addition, since the pores in this anodic oxide film are not sealed, heat trapping can be suppressed even during continuous operation of the internal combustion engine.

Brief Description of the Drawings

[0011]

Figure 1

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Figure 10

Embodiments for Carrying Out the Invention

[0012] Hereinafter, with reference to the accompanying drawings, an embodiment of a piston for an internal combustion engine and a method for manufacturing the same according to the present invention will be described. Note that the drawings are drawn with priority given to ease of understanding and are not drawn to scale.

[0013] The manufacturing method of the piston for an internal combustion engine according to this embodiment is, as shown in FIGS. 5(a) and 5(b), to irradiate the piston crown surface 2 of the piston body for an internal combustion engine made of an aluminum alloy with laser light X, and form a recess 10 in the portion of the piston crown surface 2 irradiated with the laser light X; and, as shown in FIGS. 5(c) to 5(e), apply an AC superimposed voltage to the piston crown surface 2 to form an anodic oxide film 20, and block the opening 13 of the recess 10 with the anodic oxide film 20. By this, a piston for an internal combustion engine having a void 26 blocked with the anodic oxide film 20 inside the recess 10 of the piston crown surface 2 is manufactured. Each step will be described in more detail.

[0014] [Laser Irradiation Step] The piston body for an internal combustion engine having the piston crown surface 2 to be irradiated with laser is formed of an aluminum alloy material 4. Generally, silicon (Si) is contained in the aluminum alloy material 4 as a component contributing to wear resistance and aluminum seizure resistance. Examples of such aluminum alloy materials 4 include AC materials such as AC4, AC8, AC8A, and AC9, ADC materials such as ADC10 to ADC14, and A4000, etc. as pistons.

[0015] The laser light X for forming a recess in the piston crown surface 2 is not particularly limited as long as it is a laser for metal processing. For example, a CO2 laser, a YAG laser, a fiber laser, etc. can be used alone or in combination. By irradiating the piston crown surface 2 of the aluminum alloy material 4 with the laser light X, the aluminum alloy material in the portion irradiated with the laser irradiation X sublimates or evaporates, and the recess 10 can be formed.

[0016] The size of the recess 10 is not particularly limited as long as it is within a range that allows a void to be formed inside the recess 10 when the opening of the recess 10 is blocked by an anodic oxide film in the subsequent anodic oxidation treatment step. For example, assuming the thickness of the anodic oxide film to be formed is 100, it is preferable that the depth of the recess is in the range of 50 to 300, and the width of the recess is in the range of 100 to 200. More preferably, the depth of the recess is in the range of 100 to 200, and the width of the recess is in the range of 130 to 160. That is, the size of the void formed inside the recess 10 can be adjusted according to the size of the recess 10 and the thickness of the anodic oxide film. The depth and width of the recess 10 can be controlled by the wavelength and output of the laser light X, etc.

[0017] The shape or pattern on the surface of the piston crown surface 2 of the recess 10 is not particularly limited. For example, as shown in FIGS. 1 to 3, a plurality of groove-shaped recesses 10 may be formed radially from the center of the piston crown surface 2 of the piston body 1 for an internal combustion engine toward the outer peripheral portion 3. In this case, each recess 10 may be formed such that the left and right wall surfaces 11 of the groove-shaped recess 10 expand from the center of the piston crown surface 2 toward the outer peripheral portion 3 as indicated by the arrow lines R1 and R2 in FIG. 2. That is, the width of the groove-shaped recess 10 expands from the center of the piston crown surface 2 toward the outer peripheral portion 3. The bottom surface 12 of the groove-shaped recess 10 may be formed with a constant depth.

[0018] [Anodic Oxidation Treatment Step] The piston crown surface 2 having such a recess 10 formed on its surface is subjected to anodic oxidation treatment to form an anodic oxide film 20 on the piston crown surface 2. In the anodic oxidation treatment step, a conventional anodic oxidation treatment capable of forming an anodic oxide film on the surface of an aluminum alloy can be widely employed. For example, in an acidic treatment bath such as sulfuric acid, oxalic acid, phosphoric acid, chromic acid, or a basic treatment bath such as sodium hydroxide, sodium phosphate, sodium fluoride, an electrode plate such as titanium or carbon is used as the cathode, and the piston crown surface 2 of the piston body 1 for an internal combustion engine is immersed as the anode, and electrolysis is performed to oxidize the aluminum alloy material 4 on the surface of the piston crown surface 2 to form the anodic oxide film 20.

[0019] As electrolysis methods, there are generally direct current electrolysis methods, alternating current and direct current superimposed electrolysis methods, etc. In this embodiment, by using the alternating current and direct current superimposed electrolysis method, as will be described in detail later, a void 26 can be formed inside the recess 10. The alternating current and direct current superimposed electrolysis method is a method of performing anodic oxidation treatment by repeating a step of applying a positive voltage and a step of removing charges to an aluminum alloy material to be electrolyzed. When performing anodic oxidation treatment by the alternating current and direct current superimposed electrolysis method, as shown in FIG. 6, the anodic oxide film (alternating current and direct current superimposed electrolytic film) 20 formed by the alternating current and direct current superimposed electrolysis method grows in a random direction with respect to the surface of the aluminum alloy material 4 and has no orientation. Therefore, silicon (not shown) contained in the aluminum alloy material 4 to be electrolyzed grows while enclosing it in a state of being branched in random directions, and thus, a dense and smooth-surfaced anodic oxide film 20 can be formed.

[0020] In the anodic oxidation treatment step, as shown in FIG. 5(c), an anodic oxide film 23 is formed on the surface of the piston crown surface 2, an anodic oxide film 21 is also formed on the wall surface 11 of the recess 10, and an anodic oxide film 22 is also formed on the bottom surface 12 of the recess 10. In the anodic oxidation treatment, since the aluminum alloy material 4 is oxidized to form a film, about half of the film thickness of the formed anodic oxide film 20 is a penetrated film that has penetrated the surface of the piston crown surface 2, the wall surface 11, and the bottom surface 12 of the recess 10, and the remaining about half of the film thickness becomes a grown film that has grown from the surface of the piston crown surface 2, the wall surface 11, and the bottom surface 12 of the recess 10.

[0021] In the alternating current and direct current superimposed electrolysis method, as described above, the cells of the anodic oxide film 20 grow in a random direction with respect to the surface of the aluminum alloy material 4. Therefore, when the anodic oxidation treatment of the piston crown surface 2 is continued, as shown in FIG. 5(d), the growth speed of the anodic oxide film 24 at the opening 13 of the recess 10 is faster than the growth speed of the anodic oxide films 21 and 22 on the wall surface 11 and the bottom surface 12 of the recess 10. Therefore, as shown in FIG. 5(e), when the opening 13 of the recess 10 is blocked by the anodic oxide film 25, a void 26 can be generated inside the recess 10.

[0022] On the one hand, the DC electrolysis method is a method of performing anodic oxidation treatment by applying a certain DC voltage to an aluminum alloy material to be electrolytically treated. When performing anodic oxidation treatment by the DC electrolysis method, as shown in FIG. 7, the anodic oxide film (DC electrolysis film) 30 formed by the DC electrolysis method grows in a direction perpendicular to the surface of the aluminum alloy material 4. In addition, in the DC electrolysis method, the growth of the anodic oxide film 30 is inhibited by silicon (not shown) contained in the aluminum alloy material to be electrolytically treated, so the surface roughness of the film surface of the DC electrolysis film 30 is larger than that of the film surface of the AC / DC superimposed electrolysis film 20.

[0023] Therefore, when the piston crown surface 2 with the concave portion 10 formed on the surface is anodized by the DC electrolysis method that grows in a direction perpendicular to the aluminum alloy material 4 in this way, as shown in FIG. 8(a), an anodic oxide film 33 is formed on the surface of the piston crown surface 2, and an anodic oxide film 31 is also formed on the wall surface 11 of the concave portion 10, and an anodic oxide film 32 is also formed on the bottom surface 12 of the concave portion 10. However, as shown in FIG. 8(b), the growth speed of the anodic oxide film 34 at the opening 13 of the concave portion 10 is slow. Therefore, as shown in FIG. 8(c), when the opening 13 of the concave portion 10 is blocked by the anodic oxide film 35, the inside of the concave portion 10 is filled with the anodic oxide films 31 and 32, and it is difficult to generate voids.

[0024] In this embodiment, since the anodic oxide film 20 is formed by the AC / DC superimposed electrolysis method, voids 26 are respectively formed inside the plurality of concave portions 10 of the piston crown surface 2. The shape of the voids 26 depends on the shape of the concave portion 10. Therefore, these plurality of voids 26 extend radially so as to expand from the center of the piston crown surface 2 toward the outer peripheral portion 3. Since the anodic oxide film 20 of this embodiment has voids 26 inside the concave portion 10 in this way, the thermal conductivity and the volume specific heat capacity can be lowered. In addition, since the voids 26 can be formed in the aluminum alloy material 4, the anodic oxide film 20 has excellent durability. Furthermore, since the pores (not shown) in the anodic oxide film 20 itself are not sealed, heat accumulation can be suppressed even during the continuous operation of the internal combustion engine.

[0025] Here, when an anodic oxide film with low thermal conductivity and low volumetric specific heat capacity is evenly formed on the piston crown surface, during the operation of the internal combustion engine, although the effect of suppressing heat trapping is high at low rotational speeds and low loads, at high rotational speeds and high loads, the heat at the center becomes significantly high, so that the effect of suppressing heat trapping cannot be fully exerted. In the present embodiment, the voids 26 extend radially so as to expand from the center of the piston crown surface 2 toward the outer peripheral portion 3, which means that the porosity of the voids in the piston crown surface 2 gradually increases from the center of the piston crown surface 2 toward the outer peripheral portion 3.

[0026] The higher the porosity of the anodic oxide film, the lower the thermal conductivity and the volumetric specific heat capacity. The lower the thermal conductivity, the more difficult it is for heat to be transmitted, and the higher the heat insulation effect. Also, the lower the volumetric specific heat capacity, the easier it is to warm up, the easier it is to cool down, and the less likely heat is to be trapped. That is, the surface temperature is likely to follow the change in the gas temperature in the combustion chamber. Therefore, as the porosity of the voids 26 increases from the center of the piston crown surface 2 toward the outer peripheral portion 3, the thermal conductivity and the volumetric specific heat capacity become lower, so that heat easily escapes from the center of the piston crown surface 2 toward the outer peripheral portion 3, and heat trapping can be effectively suppressed even at high rotational speeds and high loads.

[0027] In addition, if fuel adheres and adheres (carbon deposits accumulate) to the piston crown surface 2, it may cause problems in the operation of the internal combustion engine. In the present embodiment, even if fuel adheres to the piston crown surface 2, it can penetrate into the voids 26 through the pores of the anodic oxide film 20 and be fixed in the voids 26. Due to the heat load at the start of the internal combustion engine, the piston body 1 for internal combustion engine and the anodic oxide film 20 expand. Due to the difference in the coefficient of thermal expansion between the aluminum alloy, which is the material of the piston body 1 for internal combustion engine, and the anodic oxide film, there is a difference in the degree of expansion and contraction. By repeating expansion and contraction, the deposits in the voids 26 are peeled off, and since the voids 26 extend to the outer peripheral portion 3, the deposits can be discharged through the voids 26.

[0028] In addition, in the present embodiment, the case where the void 26 extends radially so as to expand from the center of the piston crown surface 2 toward the outer peripheral portion 3 has been described. However, the present invention is not limited to this. For example, as shown in FIG. 4, in the laser irradiation step, a flat circular recess 10A may be formed on the surface of the piston crown surface 2 of the piston body 1 for an internal combustion engine, so that a flat circular void is formed inside the recess 10A. Even with a void having such a shape, an anodic oxide film having a low thermal conductivity and a low volumetric specific heat capacity, excellent durability, and capable of suppressing heat trapping can be obtained. Further, in FIG. 4, the recess is flat circular, but the shape is not limited to such a shape, and for example, it may be a polygon such as a quadrilateral or an ellipse. Furthermore, in FIG. 4, the recesses of the same size are evenly arranged on the piston crown surface 2, but the pattern is not limited to such a pattern. By increasing the size of the recesses or increasing the number of recesses from the center of the piston crown surface 2 toward the outer peripheral portion 3, the porosity of the voids in the piston crown surface 2 can be gradually increased from the center of the piston crown surface 2 toward the outer peripheral portion 3, and the effect of suppressing heat trapping even at high rotation and high load described above can be obtained.

[0029] The film thickness of the anodic oxide film 20 of the present embodiment is preferably, for example, 40 μm or more, more preferably 50 μm or more on the surface of the piston crown surface 2 in order to enhance the heat shielding effect. The upper limit of the film thickness is preferably, for example, 100 μm or less, more preferably 80 μm or less in order to prevent heat trapping (knock resistance).

[0030] In addition, in this embodiment, the anodizing process is performed immediately after the laser irradiation process. However, the present invention is not limited to this. For example, as another embodiment of the method for manufacturing a piston for an internal combustion engine of the present invention, before the anodizing process, a masking application step of applying a masking agent into the recess formed on the surface of the piston crown surface, and after the anodizing process, a masking removal step of removing the masking agent applied into the recess may be further included. As still another embodiment, before the anodizing process, an aluminum alloy remelting step of remelting the aluminum alloy in the recess formed on the surface of the piston crown surface may be further included. Hereinafter, each step of these embodiments will be described.

[0031] [Masking Application Step and Masking Removal Step] As shown in FIG. 9(a), before the anodizing process, a masking application step is performed, and a masking agent 40 is applied into the recess 10 of the piston crown surface 2. In the anodizing process of this embodiment, an anodic oxide film 20A is formed on the surface of the piston crown surface 2 and on the portion of the wall surface 11 of the recess 10 that is not covered by the masking agent 40 (that is, the portion on the opening 13 side from the exposed surface 41 of the masking agent 40).

[0032] As the masking agent 40, for example, a masking agent used in semiconductor manufacturing can be used. Specifically, a photosensitive material called a photoresist can be mentioned. Also, a masking agent can be applied into the recess 10 using photolithography technology or the like used in semiconductor manufacturing.

[0033] When the anodizing process continues, as shown in FIG. 9(b), the opening 13 of the recess 10 is also blocked by the anodic oxide film 25A. Then, by performing the masking removal step and removing the masking agent 40 in the recess 10, a void 26A can be generated inside the recess 10. Also in this embodiment, since the anodizing process 20A has a void 26A inside the recess 10 of the piston crown surface 2, an anodic oxide film having a low thermal conductivity and volumetric specific heat capacity, excellent durability, and capable of suppressing heat trapping can be obtained.

[0034] Since the anodized layer 20A in the anodizing process is a porous body, the masking agent 40 can be removed from the recess 10 by dissolving it with a masking removal agent used in semiconductor manufacturing or by baking it by heating. Note that the masking removal step may not be performed, and in the piston for an internal combustion engine in which the masking agent 40 remains in the recess 10, when the internal combustion engine is started after being incorporated into the internal combustion engine, voids are generated by baking and removing it by heat.

[0035] In this embodiment, the electrolysis method employed in the anodizing process is not limited to the AC / DC superimposed electrolysis method, and voids 26A can be formed even by the DC electrolysis method. However, since the anodized film 20A is formed by oxidizing the aluminum alloy material 4 as described above, in the DC electrolysis method in which the anodized film grows in the direction perpendicular to the surface of the aluminum alloy material 4, the height of the anodized film 20A on the exposed surface 41 of the masking agent 40 becomes significantly lower. Therefore, the AC / DC superimposed electrolysis method is preferred. Also, smoothness is required for the piston crown surface 2, and the better the surface roughness of the anodized film 20A, the better the flow in the combustion chamber, the less the fuel adhesion amount to the piston, the better the fuel efficiency, and the lower the exhaust gas. The AC / DC superimposed electrolysis method grows in a random direction with respect to the surface of the aluminum alloy material 4 as described above, and an anodized film with a smooth surface can be obtained. Therefore, the AC / DC superimposed electrolysis method is also preferred from this point.

[0036] [Aluminum Alloy Remelting Process] As shown in FIG. 10(a), before the anodizing process, an aluminum alloy remelting process is performed to remelt the aluminum alloy on the lower part of the wall surface 11 and the bottom surface 12 of the recess 10 of the piston crown surface 2. The remelted part 50 of the aluminum alloy has a finer structure (silicon particle size) than the non-remelted part. Therefore, as shown in FIG. 10(b), in the anodizing process, in the remelted part 50 of the aluminum alloy, the growth speed of the anodized film 20B can be made slower than that of the non-remelted part.

[0037] Then, by continuing the anodizing treatment, as shown in Fig. 10(c), the opening 13 of the recess 10 can be closed with the anodic oxide film 25B, and a void 26B can be generated inside the recess 10. Therefore, since the anodizing treatment 20B has the void 26B inside the recess 10 of the piston crown surface 2, an anodic oxide film with low thermal conductivity and volumetric specific heat capacity, excellent durability, and capable of suppressing heat trapping can be obtained.

[0038] In this embodiment, since the growth rate of the anodic oxide film 25B in the remelted portion 50 of the aluminum alloy can be slowed down, due to the synergistic effect with the above-described anodizing treatment by the AC / DC superimposed electrolysis method, the void 26 can be formed more easily. Examples of the method for remelting the aluminum alloy include irradiating the recess 10 of the piston crown surface 2 with laser light again, performing arc treatment, or irradiating with an ion beam. When irradiating with laser light again, the structure (silicon particle size) of the remelted portion 50 of the aluminum alloy can be made finer than that of the non-remelted portion by increasing the laser output compared to the irradiation of the laser light when forming the recess, and the growth rate of the anodic oxide film 20B can be slowed down.

Explanation of Reference Numerals

[0039] 1 Piston body for internal combustion engine 2 Piston crown surface 3 Piston outer peripheral surface 4 Aluminum alloy material 10 Recess 11 Wall surface 12 Bottom surface 13 Opening 20 - 25 Anodic oxide film (AC / DC superimposed electrolytic film) 26 Void 27 Cell 30 - 34 Anodic oxide film (DC electrolytic film) 37 Cell 40 Masking agent 41 Masking agent surface 50 Remelted portion of aluminum alloy

Claims

1. A step of irradiating a piston crown surface of a piston body for an internal combustion engine made of an aluminum alloy with laser light, the step of forming a recess in a portion of the piston crown surface irradiated with the laser light, A step of anodizing the piston crown surface to form an anodic oxide film, the step of closing the opening of the recess Including, as the anodizing treatment, a method for manufacturing a piston for an internal combustion engine in which an alternating current and direct current superposed voltage is applied to the piston crown surface and the recess has a void filled with the anodic oxide film inside.

2. A step of irradiating a piston crown surface of a piston body for an internal combustion engine made of an aluminum alloy with laser light, the step of forming a recess in a portion of the piston crown surface irradiated with the laser light, A step of anodizing the piston crown surface to form an anodic oxide film, the step of closing the opening of the recess Including, a method for manufacturing a piston for an internal combustion engine having a void filled with the anodic oxide film inside the recess, Before the step of forming the anodic oxide film, a step of applying a masking agent into the recess, After the step of forming the anodic oxide film, a step of removing the masking agent applied into the recess A method for manufacturing a piston for an internal combustion engine further including.

3. The method for manufacturing a piston for an internal combustion engine according to claim 1, further including a step of remelting the aluminum alloy in the recess before the step of forming the anodic oxide film.

4. A piston for an internal combustion engine having a piston crown surface and a piston body for an internal combustion engine made of an aluminum alloy, An anodic oxide film covering the piston crown surface Comprising, a piston for an internal combustion engine, The piston crown surface of the piston body for the internal combustion engine has a plurality of recesses, The cells of the anodic oxide film extend in random directions with respect to the surface of the piston crown surface and the surfaces inside the plurality of recesses, and the cells are branched in random directions and surround the silicon in the anodic oxide film, Each opening of the plurality of recesses is closed by the anodic oxide film, and the piston for an internal combustion engine has voids filled with the anodic oxide film inside the plurality of recesses respectively.

5. The piston for an internal combustion engine according to claim 4, wherein the porosity of the voids in the piston crown surface gradually increases from the center to the outer peripheral portion of the piston crown surface.

6. The piston for an internal combustion engine according to claim 4 or 5, wherein the gap extends radially so as to expand from the center of the piston crown surface toward the outer peripheral portion.

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