Piston for internal combustion engine and manufacturing method thereof

Abstract

To provide a piston for an internal combustion engine which is excellent in both heat insulation performance and gas temperature followability and which can suppress cracks and exfoliation even in a situation where a thermal shock such as abrupt acceleration and deceleration is repeatedly applied to the piston, and a method of manufacturing the piston.SOLUTION: When a piston crown face 11 of a piston main body made of aluminum alloy for an internal combustion engine is irradiated with a laser beam, an average grain diameter of an eutectic Si existing on a surface in the piston crown face is gradually increased from an intake side 5 of the piston crown face toward an exhaust side 6 by changing an application output of the laser beam, after which an anodic oxide coating 12 is formed by applying anodic oxide treatment to the piston crown face. This results in a piston for an internal combustion engine in which a film thickness of the anodic oxide coating 12 is gradually thicker toward the exhaust side 6 from the intake side 5, an average grain diameter of Si particulates in the anodic oxide coating 12 and the average grain diameter of the eutectic Si immediately below the coating in the piston crown face is gradually larger, and a ratio of needle-shape Si occupied in the eutectic Si immediately below the coating in the piston crown face gradually increases.SELECTED DRAWING: Figure 1

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 the 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, a heat insulating film is formed on the surface of the aluminum alloy. The heat insulating film is composed of a substance and structure having low thermal conductivity and low volumetric specific heat capacity, and suppresses heat conduction to the aluminum alloy base material in the combustion cycle of the internal combustion engine, and the heat insulating film itself has a function of following the change in the surrounding gas temperature. Due to this function, the difference ΔT between the combustion chamber wall surface including the piston crown surface and the in-cylinder gas temperature can be made smaller, so that the cooling loss can be improved by suppressing the outflow of heat energy to the outside, contributing to the reduction of fuel consumption.

[0003] Further, since the performance of following the change in the surrounding gas temperature depends on the heat capacity (specific heat × density × volume) of the heat insulating film, if the substance and structure of the heat insulating film are uniform, the film thickness thereof becomes dominant. In this case, the thicker the film thickness of the heat insulating film, the better the heat insulating performance, while the gas temperature followability tends to deteriorate. Conversely, the thinner the film thickness of the heat insulating film, the lower the heat insulating performance, while the gas temperature followability tends to be good. This indicates that the heat insulating performance and the gas temperature followability represent a trade-off relationship, and in the case of a specification with a uniform film thickness, one of the performances must be prioritized.

[0004] Therefore, for example, Patent Document 1 describes that in a compression ignition engine, a combustion chamber is defined by a cylinder block, a cylinder head, and a piston. With respect to the central axis of this combustion chamber, an opening of an intake port is located on one side and an opening of an exhaust port is located on the other side. And a heat insulating material layer is formed on the crown surface of the piston. Among this heat insulating material layer, the heat capacity of the exhaust side heat insulating material layer on the side facing the opening of the exhaust port is set to be higher than the heat capacity of the intake side heat insulating material layer on the side facing the opening of the intake port. As a specific example, a configuration is described in which the thickness of the intake side heat insulating material layer is made smaller than the thickness of the exhaust side heat insulating material layer.

[0005] Also, for example, Patent Document 2 describes that in an internal combustion engine, a combustion chamber is constituted by the bottom surface of the cylinder head, the bottom surfaces of an intake valve in the intake port and an exhaust valve in the exhaust port opened in the cylinder head, the bore of the cylinder block, and the top surface of the piston sliding in the bore. A heat insulating film is formed on part or all of the wall surfaces constituting the combustion chamber. An ignition plug is located between the intake piston and the exhaust piston on the bottom surface of the cylinder head and faces the combustion chamber. And when the combustion chamber is divided into an intake valve side region and an exhaust valve side region with the ignition plug as a boundary, the heat insulation performance of at least part of the heat insulating film on the wall surface of the intake valve side region is made higher than at least that of the heat insulating film on the wall surface of the exhaust valve side region. As a specific example, a configuration is described in which the film thickness of the heat insulating film in the intake valve side region is made thicker than that of the heat insulating film in the exhaust valve side region.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, in a portion where the thickness of the heat insulating material layer or the heat insulating film on the piston crown surface is reduced, there is a problem that fine cracks may occur due to thermal fatigue when thermal shock (thermal shock) is repeatedly applied to the piston during rapid acceleration or deceleration. In particular, eutectic silicon in the aluminum alloy usually has a large size and a needle-shaped metal structure, so there is a problem that cracks tend to progress along the longitudinal direction due to this.

[0008] In addition, when a heat insulating material layer or a heat insulating film having different film thicknesses is provided on the piston crown surface, a step is formed at the boundary portion where the film thicknesses are different. Due to this, in the situation where the engine is under heat load, particularly when the above thermal shock is applied, there is a problem that cracks and peeling may occur at the step portion due to the difference in the linear expansion coefficient and the thermal deformation distribution of the piston crown surface. In particular, at high engine loads, there is a problem that abnormal combustion such as knocking may frequently occur during combustion.

[0009] Patent Document 2 describes, as a method of forming a heat insulating film having different film thicknesses, masking a region on the exhaust valve side of the piston crown surface and growing an anodic oxide film on the region on the intake valve side to form a heat insulating film on the intake valve side. Then, the masking is peeled off, masking is applied to the formed heat insulating film, and an anodic oxide film is grown on the region on the exhaust valve side to form a heat insulating film on the exhaust valve side. However, in such a method, there is a problem that it is impossible to avoid the formation of a step at the boundary portion where the film thicknesses are different.

[0010] Therefore, in view of the above problems, an object of the present invention is to provide a piston for an internal combustion engine and a method for manufacturing the same, which are excellent in both heat insulating performance and gas temperature followability, and can suppress cracks and peeling even in a situation where thermal shock is repeatedly applied to the piston during rapid acceleration or deceleration.

Means for Solving the Problems

[0011] In order to achieve the above object, in one aspect of the present invention, there is provided a piston for an internal combustion engine. The piston for the internal combustion engine has a piston crown surface and includes a piston body for the internal combustion engine made of an aluminum alloy, and an anodic oxide film that covers the piston crown surface and contains silicon particles. The thickness of the anodic oxide film gradually increases from the intake side to the exhaust side of the piston crown surface. The average particle diameter of the silicon particles in the anodic oxide film and the average particle diameter of eutectic silicon immediately below the anodic oxide film in the piston crown surface gradually increase from the intake side to the exhaust side of the piston crown surface. The ratio of acicular silicon in the eutectic silicon immediately below the anodic oxide film in the piston crown surface gradually increases from the intake side to the exhaust side of the piston crown surface.

[0012] In another aspect of the present invention, there is provided a method for manufacturing a piston for an internal combustion engine. The method includes a step of irradiating a laser on the piston crown surface of a piston body for an internal combustion engine made of an aluminum alloy, and by changing the irradiation output of the laser, gradually increasing the average particle diameter of eutectic silicon on the surface in the piston crown surface from the intake side to the exhaust side of the piston crown surface, and a step of anodizing the piston crown surface to form an anodic oxide film.

Advantages of the Invention

[0013] According to the present invention as described above, the thickness of the anodic oxide film, which is a heat insulating film, gradually increases from the intake side to the exhaust side of the piston crown surface, and the average particle diameter of the silicon particles in the anodic oxide film and the average particle diameter of eutectic silicon immediately below the anodic oxide film in the piston crown surface gradually increase from the intake side to the exhaust side of the piston crown surface. Therefore, it is excellent in both heat insulation performance and gas temperature followability, and can suppress cracks and peeling even in a situation where thermal shock is repeatedly applied to the piston.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Mode for Carrying Out the Invention

[0015] 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. In the following embodiment, the internal combustion engine is a gasoline engine, but the present invention is not limited thereto, and the internal combustion engine may be a diesel engine.

[0016] As shown in FIG. 1, the piston 10 for an internal combustion engine according to this embodiment is a component that constitutes the internal combustion engine 1. The combustion chamber 2 of the internal combustion engine 1 is defined by a cylinder block (not shown), a cylinder head (not shown), and the piston 10 for an internal combustion engine. The internal combustion engine 1 includes an intake port 3 and an exhaust port 4 that open into the combustion chamber 2. An intake valve 5 for opening and closing the opening is provided in the intake port 3 so as to be movable up and down, and an exhaust valve 6 for opening and closing the opening is provided in the exhaust port 4 so as to be movable up and down. Two intake ports 3 are provided at intervals in the crankshaft direction (the direction perpendicular to the plane of FIG. 1). Similarly, two exhaust ports 4 are provided at intervals in the crankshaft direction. And a fuel injection valve 7 is disposed at the center of these.

[0017] The main body of the piston 10 for an internal combustion engine is formed of an aluminum alloy material. Generally, silicon (Si) is contained in the aluminum alloy material as a component that contributes to wear resistance and aluminum seizure resistance. Examples of such aluminum alloy materials include AC materials such as AC4, AC8, AC8A, and AC9, ADC materials such as ADC10 to ADC14, and A4000 as pistons.

[0018] An anodic oxide film 12 is formed as a heat insulating film on the surface of the piston 10 for an internal combustion engine on the combustion chamber 2 side, that is, on the piston crown surface 11. As shown in FIG. 1, the film thickness of this anodic oxide film 12 gradually increases from the intake side where the intake port 3 is located toward the exhaust side where the exhaust port 4 is located.

[0019] Also, in FIG. 2 which shows the anodic oxide film 12 when viewed from the combustion chamber 2 side, the difference in the film thickness of the anodic oxide film 12 is indicated by the darkness of the color. The thicker the film thickness, the darker the color is represented. Also, the positions 3a and 3b of the openings of the intake port 3 into the combustion chamber 2 and the positions 4a and 4b of the openings of the exhaust port 4 into the combustion chamber 2 are shown by broken lines. As shown in FIG. 2, the film thickness of the anodic oxide film 12 gradually increases from the intake side where the openings 3a and 3b of the intake port are located toward the exhaust side where the openings 4a and 4b of the exhaust port are located. In other words, the film thickness of the anodic oxide film 12 gradually decreases from the exhaust side toward the intake side.

[0020] The anodic oxide film 12 is porous. By reducing the film thickness of the anodic oxide film 12, the volumetric specific heat capacity can be made lower. Therefore, the surface temperature of the anodic oxide film 12 can follow the gas temperature change in the combustion chamber 2 with a smaller time lag and a smaller temperature difference. On the intake side of the combustion chamber 2 with the intake port 3, the in-cylinder gas temperature decreases due to the introduction of fresh air into the combustion chamber 2, and accordingly, the surface temperature of the anodic oxide film 12 decreases. Therefore, by reducing the film thickness of the anodic oxide film 12 on the intake side, the volumetric efficiency can be increased, and the engine output can be improved.

[0021] On the other hand, on the exhaust side of the combustion chamber 2 with the exhaust port 4, since the in-cylinder gas temperature is usually higher than that on the intake side of the combustion chamber 2 with the intake port 3, by increasing the film thickness of the anodic oxide film 12 on the exhaust side, the volumetric specific heat capacity is increased to enhance the heat insulation performance. Therefore, the cooling loss of the piston can be suppressed.

[0022] Since the film thickness of the anodic oxide film 12 changes gradually, there is no step at the boundary between the thick part and the thin part of the film thickness, and the anodic oxide film 12 has a shape inclined from the intake-side end to the exhaust-side end of the piston crown surface 11. Therefore, since there is no starting point where cracks or peeling can occur, even when thermal shock is repeatedly applied to the piston during sudden acceleration or deceleration, the occurrence of cracks or peeling can be suppressed, and abnormal combustion such as knocking during high engine load can also be suppressed.

[0023] From the viewpoint of suppressing the turbulent flow in the combustion chamber, the gradual change in the film thickness of the anodic oxide film 12 preferably changes linearly, that is, linearly in proportion to the distance between the intake side and the exhaust side of the piston crown surface, as shown in FIG. 1. However, the present invention is not limited to this, and the gradual change in the film thickness of the anodic oxide film 12 may be curvilinear. For example, it may gradually change in a convex curve bulging toward the combustion chamber 2 side or in a concave curve recessed toward the combustion chamber 2 side.

[0024] The film thickness on the intake side of the anodic oxide film 12 only needs to be thinner than that on the exhaust side. From the perspective of output performance, for example, when the film thickness on the exhaust side is taken as 1, the relative value is preferably 0.25 or less, more preferably 0.1 or less, and still more preferably 0.05 or less. The lower limit of this relative film thickness value is not particularly limited, but is preferably 0.005 or more, and more preferably 0.01 or more. Specifically, from the perspective of achieving both heat insulation performance and reliability, the film thickness on the exhaust side of the anodic oxide film 12 is preferably in the range of, for example, 4 to 100 μm, and more preferably in the range of 25 to 75 μm. Specifically, from the perspective of suppressing overheating in the combustion chamber, the film thickness on the intake side of the anodic oxide film 12 is preferably in the range of, for example, 0.5 to 5.0 μm, and more preferably in the range of 1.0 to 2.5 μm.

[0025] Further, in the aluminum alloy of the piston 10 body for an internal combustion engine, silicon (Si) crystallizes as eutectic silicon. On the piston crown surface 11 of the piston 10 for an internal combustion engine of the present embodiment, the average particle diameter of the eutectic silicon directly under the anodic oxide film 12 gradually increases from the intake side where the intake port 3 is located toward the exhaust side where the exhaust port 4 is located. In other words, the average particle diameter of the eutectic silicon directly under the anodic oxide film 12 within the piston crown surface 11 of the piston 10 for an internal combustion engine gradually decreases from the exhaust side toward the intake side.

[0026] In particular, the eutectic silicon in the aluminum alloy often exists as acicular silicon. However, on the piston crown surface 11 of the piston 10 for an internal combustion engine of the present embodiment, such acicular silicon is spheroidized, resulting in a smaller average particle diameter of the eutectic silicon. Therefore, the ratio of acicular silicon in the eutectic silicon directly under the anodic oxide film within the piston crown surface gradually increases from the intake side to the exhaust side of the piston crown surface. In other words, the ratio of acicular silicon in this eutectic silicon gradually decreases from the exhaust side to the intake side of the piston crown surface. Note that acicular silicon refers to those with an aspect ratio (length-to-width ratio) of 2 or more.

[0027] Even in the anodic oxide film 12 formed on such a piston crown surface 11, the average particle size of the silicon particles in the anodic oxide film 12 gradually increases from the intake side where the intake port 3 is located toward the exhaust side where the exhaust port 4 is located. In other words, the average particle size of the silicon particles in the anodic oxide film 12 gradually decreases from the exhaust side toward the intake side. This is because the anodic oxide film 12 is formed by oxidizing the surface of the aluminum alloy, and the film grows while enclosing silicon in the aluminum alloy. Therefore, in the portion of the piston crown surface where the average particle size of eutectic silicon is small, an anodic oxide film containing silicon particles with a small average particle size is formed, and in the portion of the piston crown surface where the average particle size of eutectic silicon is large, an anodic oxide film containing silicon particles with a large average particle size is formed.

[0028] In this way, since the average particle size of eutectic silicon in the piston crown surface 11 on the side where the film thickness of the anodic oxide film 12 is reduced is reduced and the ratio of acicular silicon is decreased, even when thermal shock is repeatedly applied to the portion where the film thickness of the anodic oxide film 12 is reduced, such as during rapid acceleration and deceleration, the progression of cracks or peeling caused by the size and shape of the acicular silicon can be suppressed.

[0029] Further, when the acicular silicon contained in the aluminum alloy is spheroidized, the thermal resistance of the aluminum alloy decreases, and as a result, the thermal conductivity improves. Therefore, the thermal conductivity of the piston crown surface 11 directly under the anodic oxide film 12 gradually increases from the exhaust side where the exhaust port 4 is located toward the intake side where the intake port 3 is located. On the intake side where the film thickness of the anodic oxide film 12 is thin, the heat received from the combustion chamber 2 increases. However, since the thermal conductivity of the piston crown surface 11 on the intake side is high in this way, as shown by the arrow H in FIG. 1, heat can be efficiently released to the cylinder wall surface on the intake side through the piston ring and skirt, so that the temperature rise of the piston 10 for an internal combustion engine can be moderated.

[0030] If the temperature rises rapidly, the piston base material and the base material on the top ring groove side will become hot, the base material will soften, and the smoothness of the top ring groove may be impaired. If the smoothness is impaired, the sealing performance between the piston ring and the top ring groove will decrease, and a phenomenon (oil leakage) may occur where the oil used for lubrication of the internal combustion engine flows into the combustion chamber side, causing the oil to burn in the combustion chamber and becoming a factor in the generation of substances subject to European environmental regulations such as PM (Particulate Matter) and a factor in increasing the number of such substances as PN (Particle Number). Therefore, in this embodiment, since the temperature rise of the piston 10 for an internal combustion engine can be moderated, the generation of substances subject to European environmental regulations such as PM and PN and the increase in the number of these substances can be suppressed.

[0031] In particular, the gradual change in the average grain size of eutectic silicon (Si) directly under the anodic oxide film 12 in the piston crown surface 11 preferably increases in a concave curve from the intake side end (the 0 position on the x-axis in FIG. 3) to the intake side end (the 1 position on the x-axis in FIG. 3) of the piston crown surface 11 as shown in the graph of FIG. 3. Note that the average grain size of eutectic silicon on the y-axis in FIG. 3 is represented by a relative value when the maximum average grain size is 1. Also, "increasing in a concave curve" includes, for example, increasing quadratically, exponentially, or in a concave elliptical arc. FIG. 3 shows the case of increasing quadratically.

[0032] The change in the ratio of acicular silicon in eutectic silicon when the average grain size of eutectic silicon changes as shown in FIG. 3 is shown in FIG. 4, and the change in the thermal conductivity in the same case is shown in FIG. 5. Note that the x-axis in FIGS. 4 and 5 represents the position of the piston crown surface by a relative value when the intake side end is 0 and the exhaust side end is 1, similar to FIG. 3. The ratio of acicular silicon on the y-axis in FIG. 4 is represented by a relative value when the maximum ratio is 1. The thermal conductivity on the y-axis in FIG. 5 is represented by a relative value when the maximum thermal conductivity is 1.

[0033] As described above, by refining eutectic silicon, acicular silicon becomes spherical, and thereby the thermal conductivity of the aluminum alloy is improved. Therefore, as shown in FIG. 4, the ratio of acicular silicon in eutectic silicon increases in a concave curve from the intake side to the intake side on the piston crown surface 11. Further, as shown in FIG. 5, the thermal conductivity in that case decreases in a convex curve from the intake side to the intake side on the piston crown surface 11. By increasing the average particle size of eutectic silicon in a concave curve from the intake side to the intake side in this way, the thermal conductivity can be significantly improved in a wide area on the intake side of the piston crown surface 11. Therefore, as described above, the effect of releasing the heat received by the piston 10 on the intake side with a thin anodic oxide film 12 to the cylinder wall surface on the intake side and gently reducing the temperature rise of the piston 10 can be further enhanced.

[0034] The average particle size of eutectic silicon on the intake side only needs to be smaller than the average particle size on the exhaust side. However, from the viewpoint of achieving both the growth stability of the film thickness and the thermal physical properties of the material, for example, when the average particle size on the exhaust side is set to 1, the relative value is preferably 0.1 or less, more preferably 0.025 or less, and still more preferably 0.01 or less. The lower limit of the relative value of this average particle size is not particularly limited, but is preferably 0.001 or more, and more preferably 0.005 or more.

[0035] The ratio of acicular silicon in eutectic silicon only needs to be lower on the intake side than on the exhaust side. However, from the viewpoint of the thermal physical properties of the material, for example, the ratio on the intake side is preferably 0.1 or less, more preferably 0.05 or less, and still more preferably 0.01 or less, in terms of the relative value when the ratio on the exhaust side is set to 1. The lower limit of the relative value of this ratio of acicular silicon is not particularly limited, but is preferably 0.001 or more, and more preferably 0.005 or more.

[0036] The thermal conductivity of the surface of the piston crown surface 11 only needs to be higher on the intake side than on the exhaust side. However, from the perspective of output performance, for example, when the thermal conductivity of the intake side is set to 1, the relative value is preferably 0.5 or less, more preferably 0.25 or less, and even more preferably 0.1 or less. The lower limit of the relative value of this thermal conductivity is not particularly limited, but is preferably 0.01 or more, and more preferably 0.05 or more.

[0037] Next, a method for manufacturing a piston for an internal combustion engine according to the present embodiment will be described. This method includes a laser irradiation step of irradiating a laser to the piston crown surface of a piston body for an internal combustion engine made of an aluminum alloy, and an anodizing treatment step of forming an anodic oxide film on the piston crown surface irradiated with the laser. Each step will be described in more detail.

[0038] [Laser Irradiation Step] The laser irradiated onto the piston crown surface is not particularly limited as long as it is a laser for metal processing. For example, a CO 2 laser, a YAG laser, a fiber laser, etc. can be used alone or in combination. By irradiating a laser onto the piston crown surface of an aluminum alloy material, the aluminum alloy in the portion irradiated with the laser is locally heated and melted, and then cooled, so that the eutectic silicon in the aluminum alloy can be refined. The particle size of the refined eutectic silicon becomes smaller as the irradiation output of the laser is higher. Also, as for the eutectic silicon, there are many acicular silicon whose metal structure shape is acicular, but it becomes spherical by refinement. The ratio of acicular silicon in the eutectic silicon becomes smaller as the irradiation output of the laser is higher.

[0039] Therefore, when scanning the piston crown surface with a laser, by gradually reducing the laser irradiation output from the intake side towards the exhaust side, the average grain size of the eutectic silicon present on the surface within the piston crown surface can be gradually increased from the intake side towards the exhaust side. Or, by gradually increasing the laser irradiation output from the exhaust side towards the intake side, the average grain size of the eutectic silicon present on the surface within the piston crown surface can be gradually decreased from the exhaust side towards the intake side. In addition, for laser scanning, a spot-shaped or sheet-shaped pulsed laser may be used. For example, when scanning the entire surface of the piston crown with a laser, in the case of a spot-shaped pulsed laser, it is necessary to repeatedly scan back and forth from one end to the other end of the piston crown surface, but in the case of a sheet-shaped pulsed laser, it is only necessary to scan from one end to the other end once.

[0040] [Anodizing process] Anodize the piston crown surface irradiated with such a laser to form an anodic oxide film on the piston crown surface. In the anodizing process, a conventional anodizing process capable of forming an anodic oxide film on the surface of the aluminum alloy can be widely adopted. For example, by immersing an electrode plate such as titanium or carbon as the cathode and the piston crown surface of the piston body for an internal combustion engine as the anode 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, and performing electrolysis, the aluminum alloy material on the surface of the piston crown surface can be oxidized to form an anodic oxide film.

[0041] As electrolysis methods, there are generally direct current electrolysis method, alternating current and direct current superposition electrolysis method, etc., and any of them can be adopted, but it is preferable to use the alternating current and direct current superposition electrolysis method. The alternating current and direct current superposition 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 superposition electrolysis method, the anodic oxide film formed by the alternating current and direct current superposition electrolysis method grows in a random direction with respect to the surface of the aluminum alloy material and has no orientation. Therefore, the eutectic silicon contained in the aluminum alloy material to be electrolyzed grows while enclosing it in a state of being branched in random directions, and thus, an anodic oxide film with a dense and smooth surface can be formed.

[0042] The direct current electrolysis method is a method of performing anodic oxidation treatment by applying a constant direct current voltage to an aluminum alloy material to be electrolyzed. When performing anodic oxidation treatment by the direct current electrolysis method, the anodic oxide film formed by the direct current electrolysis method grows in a direction perpendicular to the surface of the aluminum alloy material. In addition, in the direct current electrolysis method, the growth of the anodic oxide film is inhibited by the eutectic silicon contained in the aluminum alloy material to be electrolyzed. Therefore, the surface roughness of the film by the direct current electrolysis method is larger than that of the film surface by the alternating current and direct current superposition electrolysis method. However, even such an anodic oxide film can be adopted for the piston for internal combustion engines of the present invention.

[0043] And in such anodic oxidation treatment, the finer the particle size of the eutectic silicon in the aluminum alloy, the slower the film formation rate of the anodic oxide film. Therefore, when the conditions of the anodic oxidation treatment are the same, in the part of the aluminum alloy with a small average particle size of eutectic silicon, the film thickness of the formed anodic oxide film is thinner than that in the part of the aluminum alloy with a large average particle size of eutectic silicon. Therefore, in the laser irradiation step, by gradually changing the irradiation output of the laser when irradiating the piston crown surface with the laser, and making the average particle size of the eutectic silicon existing on the surface in the piston crown surface gradually larger from the intake side to the exhaust side, an internal combustion engine piston in which an anodic oxide film with a gradually increasing film thickness from the intake side to the exhaust side is formed on the piston crown surface can be easily manufactured by the anodic oxidation treatment step.

Example

[0044] The examples and comparative examples of the present invention will be described below. First, test pieces (size: φ64 mm) made of an aluminum alloy (type: AC8A-T6 material) were prepared. After degreasing and cleaning with acid (removing the oxide film), the surfaces of the test pieces were irradiated with a laser using a laser processing machine. Then, by changing the irradiation output of the laser, a plurality of test pieces were irradiated with the laser.

[0045] For each test piece irradiated with the laser in this way, the average grain size of eutectic silicon in the aluminum alloy in the laser-irradiated part was measured using an electron scanning microscope. Also, for comparison, the average grain size of eutectic silicon was measured for the test piece not irradiated with the laser. These results are shown in Fig. 6. The average grain size was calculated by digitally photographing a field of view size of 50×50 μm at a magnification of 1000 times, calculating the equivalent circle diameter of one eutectic silicon, and using the average value within the field of view for calculating the average grain size.

[0046] Fig. 6 shows the change in the average grain size of eutectic silicon (Si) in the aluminum alloy with respect to the laser output. The laser output is represented by a relative value with the maximum laser output set to 1. The average grain size is represented by a relative value with the average grain size of the test piece in the comparative example not irradiated with the laser set to 1. As shown in Fig. 6, it was found that the average grain size of eutectic silicon decreases as the irradiation output of the laser increases. It was found that the average grain size of eutectic silicon can be easily reduced to 0.4 or less, or 0.2 or less, as a relative value with respect to the initial average grain size, at a certain irradiation output.

[0047] Next, each test piece irradiated with the laser as described above was immersed in a sulfuric acid bath and energized until the desired film thickness was obtained, thereby performing anodic oxidation treatment. Then, the film thickness of the anodic oxide film formed on these test pieces was measured using an eddy current film thickness measuring instrument (manufactured by Fisher Instruments, model number: MMS PC2). The results are shown in Fig. 7.

[0048] Figure 7 shows the change in the film thickness of the anodic oxide film with respect to the average grain size of eutectic silicon (Si) in the aluminum alloy. The average grain size was expressed as a relative value similar to that in Figure 4. The film thickness was expressed as a relative value with the film thickness of the anodic oxide film of the test piece in the comparative example where no laser was irradiated being set to 1. As shown in Figure 7, it was found that the smaller the average grain size of eutectic silicon in the aluminum alloy, the thinner the anodic oxide film formed thereon. By changing the average grain size of eutectic silicon, it was found that in anodic oxidation treatment under the same conditions, the film thickness of the formed anodic oxide film can be easily made 0.4 or less and 0.2 or less in terms of the relative value with respect to the maximum film thickness.

Explanation of Signs

[0049] 1 Internal combustion engine 2 Combustion chamber 3 Intake port 4 Exhaust port 5 Intake valve 6 Exhaust valve 7 Fuel injection valve 10 Piston for internal combustion engine 11 Piston crown surface 12 Anodic oxide film

Claims

1. a piston body for an internal combustion engine having a piston crown surface and made of an aluminum alloy; An anodized coating film that covers the piston crown surface and contains silicon particles; A piston for an internal combustion engine comprising: the thickness of the anodic oxide film gradually increases from the intake side to the exhaust side of the piston crown surface, an average grain size of silicon particles in the anodized film and an average grain size of eutectic silicon immediately below the anodized film on the piston crown surface gradually increase from the intake side toward the exhaust side of the piston crown surface, A piston for an internal combustion engine, wherein a ratio of acicular silicon to eutectic silicon immediately below the anodic oxide film on the piston crown surface gradually increases from the intake side to the exhaust side of the piston crown surface.

2. 2. A piston for an internal combustion engine according to claim 1, wherein an average grain size of silicon particles in said anodized film and an average grain size of eutectic silicon immediately below said anodized film on said piston crown surface increase in a concave curved shape from the intake side to the exhaust side of said piston crown surface.

3. a step of irradiating a piston crown surface of an aluminum alloy piston body for an internal combustion engine with a laser, and gradually increasing an average grain size of eutectic silicon on the surface within the piston crown surface from an intake side toward an exhaust side of the piston crown surface by changing an irradiation output of the laser; a step of anodizing the piston crown surface to form an anodized coating; A method for manufacturing a piston for an internal combustion engine, comprising:

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