Piston for internal combustion engine and method for manufacturing the same

Abstract

To provide a piston for an internal combustion engine and a manufacturing method thereof capable of keeping excellent sealing performance of an anodic oxidation film formed on an inner face of a top ring groove and a piston ring, even when a material of high strength is used as a piston base material.SOLUTION: A laser beam is applied to an outer peripheral surface 39 of a piston material 30a prepared by using an aluminum alloy containing 5.0-20.0 mass% of Si, more than 1.3 mass% and 5.0 mass% or less of Cu, and more than 1.5 mass% and 3.5 mass% or less of Ni as a base material to form a remelting rapidly solidified portion or a silicon dispersion layer 40 in which silicon is dispersed more finely in comparison with the base material. The silicon dispersion layer is cut out to form a top ring groove 33 on the silicon dispersion layer, and an anodic oxidation film 60 is formed on its inner surface. A processing speed of the laser beam is determined within a range of 2,000-10,000 mm / min, and the irradiation of the laser beam is performed only by one round of the outer peripheral surface of the piston material.SELECTED DRAWING: Figure 4

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 provided with an anodic oxide film on the inner surface of a top ring groove and a method for manufacturing the same.

Background Art

[0002] In recent years, with the need to meet environmental regulations, there has been an increasing demand for higher efficiency and higher compression ratios of internal combustion engines such as automotive engines, as well as for supercharged engines, and the maximum combustion pressure of internal combustion engines has been increasing. Along with such a background, the temperatures of the piston ring grooves (particularly, the top ring groove) and the land portions (particularly, the top land, second land) of the piston for an internal combustion engine have become higher than those of conventional internal combustion engines with the same displacement. An anodic oxide film may be formed on the top ring groove of the piston to impart wear resistance. This is because the anodic oxide film has a hardness more than twice that of the aluminum alloy which is the base material of the piston and has excellent wear resistance characteristics.

[0003] As such an anodic oxide film, for example, Patent Document 1 describes a piston for an internal combustion engine provided with an anodic oxide film on the inner surface of at least the inner surface on the second ring groove side of the inner surface of the top ring groove and in the region where the top ring contacts, and the surface roughness Rpk of this anodic oxide film conforming to JIS B0671-2 is 1.00 μm or less. It is described that the airtightness with the top ring can be improved by such an anodic oxide film having such a surface roughness, and the blow-by gas flow rate and the number of particles of exhaust particulates can be reduced.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, in aluminum alloys known as high-strength materials, which have been increasingly adopted in recent years and have improved mechanical properties (fatigue strength, tensile strength, etc.) up to high temperatures, there are cases where the alloy contains more additive elements such as copper (Cu) and nickel (Ni) than in conventional aluminum alloys represented by JIS (Japanese Industrial Standards)-AC8A (Al-Si-Cu-Ni-Mg system) alloys. Such high-strength materials have more granular, coarse primary silicon (Si) precipitated than conventional materials (Si particle size of about 30-40 μm), and such granular Si affects the formation of the anodic oxide film, making it easy to form a film with large irregularities. The inventors of this application have obtained a new finding that when such high-strength materials are used, even when the anodic oxidation treatment described in Patent Document 1 is performed, the surface roughness Ra and surface roughness Rpk of the anodic oxide film exceed 1.0 μm when the film thickness is 10 μm.

[0006] Therefore, in view of the above problems, the present invention aims to provide a piston for an internal combustion engine and a method for manufacturing the same that can maintain good sealing performance between the anodic oxide film formed on the inner surface of the top ring groove and the piston ring, even when a high-strength material is used for the piston base material. [Means for solving the problem]

[0007] To achieve the above objective, the present invention, in one aspect, is a method for manufacturing a piston for an internal combustion engine, the manufacturing method comprising: irradiating a piston material formed from an aluminum alloy containing 5.0 to 20.0 mass% Si, more than 1.3 mass% and less than or equal to 5.0 mass% Cu, and more than 1.5 mass% and less than or equal to 3.5 mass% Ni with laser light at a position on the outer circumferential surface where a top ring groove is to be formed; forming a top ring groove on the outer circumferential surface of the piston material irradiated with laser light by cutting; and forming an anodic oxide film on at least the surface of the top ring groove, wherein in the step of irradiating with laser light, the processing speed of the laser light is in the range of 2,000 to 10,000 mm / min, and the irradiation of the laser light is limited to one rotation around the outer circumferential surface of the piston material.

[0008] Furthermore, in another aspect of the present invention, a piston for an internal combustion engine having a top ring groove, comprising an aluminum alloy as the base material having an aluminum alloy containing 5.0 to 20.0 mass% Si, more than 1.3 mass% and 5.0 mass% or less Cu, and more than 1.5 mass% and 3.5 mass% or less Ni, and a silicon dispersion layer located on the inner surface of the top ring groove, having an aluminum alloy composition in which the silicon in the aluminum alloy is dispersed more finely than in the base material, and an anodic oxide film located on the surface of the silicon dispersion layer, wherein the top land and second land adjacent to the top ring groove of the internal combustion engine piston are such that, as they move away from the top ring groove, their outermost layers are in the order of the anodic oxide film region, the silicon dispersion layer region, and the base material region. [Effects of the Invention]

[0009] As described above, according to the present invention, even when using an aluminum alloy containing more Cu and Ni than conventional materials as the base material, a silicon dispersion layer in which silicon is more finely dispersed than in the base material is formed by laser irradiation, a top ring groove is formed in this silicon dispersion layer, and the inner surface of this top ring groove is anodized, thereby forming an anodic oxide film with a surface roughness Ra and surface roughness Rpk of 1.0 μm or less, and thus good sealing performance with the piston ring can be maintained. [Brief explanation of the drawing]

[0010] [Figure 1] This is an optical microscope image showing an example of the microstructure when conventional aluminum alloy is used as the base material for a piston in an internal combustion engine. [Figure 2] This is an optical microscope image showing an example of the metallic structure when using an aluminum alloy (high-strength material) as the base material for a piston for an internal combustion engine according to the present invention. [Figure 3] This is a schematic diagram and a partially enlarged view showing one embodiment of a piston for an internal combustion engine according to the present invention. [Figure 4] This flowchart schematically illustrates the area near the top ring groove in one embodiment of the manufacturing method for an internal combustion engine piston according to the present invention. [Figure 5] This is a schematic perspective view illustrating an example of the laser light irradiation step in the method for manufacturing a piston for an internal combustion engine according to the present invention. [Figure 6] This is a schematic diagram illustrating the laser light irradiation process in the comparative example. [Figure 7] This is a schematic diagram illustrating an example of the anodizing process in the manufacturing method of a piston for an internal combustion engine according to the present invention. [Figure 8] This is a schematic diagram illustrating a reference example of the anodizing process in the manufacturing method of a piston for an internal combustion engine according to the present invention. [Figure 9] This is an optical microscope image showing the anodic oxide film formed on the remelted and rapidly solidified portion of Example 3. [Figure 10] This is an optical microscope image showing the anodic oxide film formed on the untreated portion of Example 3. [Figure 11] This is an optical microscope image showing the anodic oxide film formed on the remelted and rapidly solidified portion of Example 4. [Figure 12] This is an optical microscope image showing the anodic oxide film formed on the untreated portion of Example 4. [Modes for carrying out the invention]

[0011] Hereinafter, an embodiment of the piston for an internal combustion engine and its manufacturing method according to the present invention will be described with reference to the attached drawings. Note that the drawings are drawn for ease of understanding and are not drawn to scale.

[0012] The piston for an internal combustion engine according to this embodiment is formed using an aluminum alloy (hereinafter also referred to as a high-strength material) containing 5.0 to 20.0 mass% silicon (Si), more than 1.3 mass% and less than or equal to 5.0 mass% copper (Cu), and more than 1.5 mass% and less than or equal to 3.5 mass% nickel (Ni) as the base material. A silicon dispersion layer with an aluminum alloy composition in which Si is dispersed more finely than in the base material is formed on the surface of the base material in the top ring groove, and an anodic oxide film is further formed on the surface of this silicon dispersion layer.

[0013] In high-strength materials, silicon crystallizes as primary silicon or eutectic silicon, improving heat resistance and wear resistance. Furthermore, silicon reduces the coefficient of thermal expansion. A silicon content of 5.0% by mass or more results in a low coefficient of thermal expansion, improving wear resistance and strength at high temperatures. A silicon content of 20.0% by mass or less reduces the amount of primary silicon, improving the elongation of the alloy. A silicon content of 10.0 to 13.0% by mass is more preferable.

[0014] Cu is a component that improves mechanical strength and wear resistance at room temperature and in the high-temperature range. If the Cu content exceeds 1.3% by mass, the effects of improving strength and wear resistance can be manifested. If it is 5.0% by mass or less, there is no significant decrease in the elongation of the alloy, and the specific gravity of the alloy is small. On the other hand, if it exceeds 5.0% by mass, the elongation significantly decreases and the specific gravity of the alloy increases. The Cu content is more preferably 2.5 to 5.0% by mass.

[0015] Ni is a component that mainly improves strength and wear resistance in the high-temperature range and reduces the thermal expansion coefficient. If the Ni content exceeds 1.5% by mass, the effect is preferably manifested. If it is 3.5% by mass or less, good elongation can be obtained.

[0016] In addition, in the present embodiment, the aluminum alloy used for the base material may be an alloy containing at least one or more elements selected from the group consisting of chromium (Cr), titanium (Ti), zirconium (Zr), phosphorus (P), iron (Fe), manganese (Mn), and magnesium (Mg) in addition to the above-mentioned Si, Cu, and Ni, with the balance being substantially composed of Al and inevitable impurities. Preferably, the base material of the aluminum alloy contains 0.05 to 0.15% by mass of Cr, 0.05 to 0.20% by mass of Ti, 0.05 to 0.30% by mass of Zr, 0.10 to 0.31% by mass of Fe, 0.05% by mass or less of Mn, and 0.5 to 1.1% by mass of Mg in addition to the ranges of the respective contents of Si, Cu, and Ni described above, with the balance being substantially composed of Al and inevitable impurities. Since Si, Cu, and Ni have already been described, the other components and their contents will be explained.

[0017] Cr is a component that mainly strengthens the grain boundaries between the intermetallic compounds, primary silicon, acicular silicon, etc. crystallized in the alloy, and improves the strength in the high-temperature range. If the Cr content is 0.05% by mass or more, the grain boundaries are preferably strengthened to improve the strength in the high-temperature range. If it is 0.15% by mass or less, good toughness and machinability can be obtained.

[0018] Ti is a component that primarily refines the crystal grains, thereby improving heat resistance, castability, and strength. This effect is preferably observed when the Ti content is in the range of 0.05 to 0.20 mass%. The Ti content is preferably 0.05 to 0.15 wt%.

[0019] Zr primarily has the effect of refining the crystal grains in the alloy, contributing to improved heat resistance, castability, and strength. This effect is preferably expressed when the Zr content is in the range of 0.05 to 0.30 mass%. The Zr content is preferably 0.05 to 0.15 wt%.

[0020] Fe is primarily a component that crystallizes intermetallic compounds, improving wear resistance and strength at high temperatures. However, if the size of these intermetallic compounds is coarse, the strength will decrease. If the Fe content is in the range of 0.10 to 0.31 mass%, the size of Fe-Mn intermetallic compounds can be reduced.

[0021] Mn is a component that primarily crystallizes intermetallic compounds, improving wear resistance and strength at high temperatures. However, if the size of these intermetallic compounds is coarse, the strength will decrease. If the Mn content is 0.05 mass% or less, the size of the Fe-Mn intermetallic compounds can be reduced. The lower limit of the Mn content is that it may not be present at all, or it may be present in a very small amount as an impurity, for example, 0.001 mass%.

[0022] Mg is primarily an ingredient that improves strength and toughness. If the Mg content is 0.5% by mass or more, it exhibits an effect of improving strength, and if it is 1.1% by mass or less, good toughness can be obtained.

[0023] When a piston material is manufactured using an aluminum alloy known as a high-strength material, more granular, coarse primary silicon precipitates during the manufacturing process compared to when using conventionally used aluminum alloys. Figure 1 is an optical microscope image of the microstructure of a piston base material using a conventional aluminum alloy (AC8AP), and Figure 2 is an optical microscope image of the microstructure of a piston base material using the high-strength aluminum alloy (AC8AR) used in the present invention. As shown in Figure 1, the conventional aluminum alloy 10 has many granular and needle-shaped primary silicon 12 precipitated in the matrix 11, but as shown in Figure 2, it can be seen that these are significantly smaller than the granular primary silicon 22 precipitated in the matrix 21 of the high-strength aluminum alloy 20. It has been known for some time that such granular primary silicon affects the formation of anodic oxide films, making it easy to form films with large irregularities. However, the particle size of primary silicon in high-strength materials is very large, around 30-40 μm, and even when using the AC / DC superposition electrolysis method, the surface roughness Ra and surface roughness Rpk of the resulting anodic oxide film both exceed 1.0 μm, which presents a problem in that good sealing performance with the piston ring cannot be maintained.

[0024] Here, we will describe the top ring groove on which the anodic oxide film is formed. As shown in Figure 3, the piston 30 for an internal combustion engine according to the present invention has three ring grooves formed on its outer circumferential surface, in order from the piston crown surface 31 side: a top ring groove 33, a second ring groove 35, and an oil ring groove 37. A top ring (not shown) is fitted into the top ring groove 33, a second ring (not shown) is fitted into the second ring groove 35, and an oil ring (not shown) is fitted into the oil ring groove 37. Along the outer circumferential surface of the piston 30, the area between the piston crown surface 31 and the top ring groove 33 is called the top land 32, the area between the top ring groove 33 and the second ring groove 35 is called the second land 34, the area between the second ring groove 35 and the oil ring groove 37 is called the third land 36, and the area from the oil ring groove 37 onwards is called the skirt portion 38.

[0025] The rings, such as the top ring, are made of materials such as high-carbon steel or martensitic stainless steel. When the piston 30 for the internal combustion engine is inserted into the cylinder bore (not shown), each ring is pressed against the inner wall surface of the cylinder by its elastic force, and the rings, such as the top ring, perform the function of maintaining the airtightness of the combustion chamber. Due to the compression and expansion strokes of the piston, the inside of the combustion chamber on the piston crown side 31 becomes high pressure, and the inner surface of the top ring groove 33 is in particularly close contact with the top ring, making the inner surface of the top ring groove 33 prone to wear. Therefore, an anodic oxide film (not shown) is formed on the inner surface of the top ring groove 33 to improve the wear resistance of the inner surface of the top ring groove 33.

[0026] In this embodiment, a silicon dispersion layer (not shown) of an aluminum alloy composition in which Si is dispersed more finely than in the base material is formed on the inner surface of the top ring groove 33, and an anodic oxide film is further formed on the surface of this silicon dispersion layer. A method for forming an anodic oxide film on the base material surface of such a top ring groove 33 via a silicon dispersion layer will be described with reference to Figure 4.

[0027] Figure 4(a) shows a magnified view of the area around the top ring groove of the ring material 30a before the formation of ring grooves such as the top ring groove, when manufacturing a piston for an internal combustion engine. The ring material 30a is cast using the high-strength material described above. For casting the piston, a common method can be used, such as gravity casting, in which molten aluminum alloy (molten metal) is poured into a mold having a cavity in the shape of the piston material. The ring material 30a may also be subjected to heat treatment commonly used in piston manufacturing. Heat treatments include T5 treatment and T6 treatment. T5 treatment involves only artificial aging hardening after casting (for the purpose of increasing strength, stabilizing dimensions, etc.), while T6 treatment involves solution treatment and artificial aging hardening after casting (for the purpose of increasing strength and hardness, etc.).

[0028] Next, a laser beam 51 is irradiated onto the outer surface 39 of the ring material 30a at the position where the top ring groove is to be formed. The laser beam irradiation is performed to remelt the high-strength material to the depth where the top ring groove is to be formed. The remelted high-strength material is rapidly cooled and solidified. As a result, a remelted and rapidly cooled solidified portion 40 is formed in an area of ​​the ring material 30a that is wider than the width and depth in which the top ring groove is to be formed. That is, the laser beam is also irradiated onto a part of the position that will become the top land and a part of the position that will become the second land adjacent to the position where the top ring groove is to be formed, and a remelted and rapidly cooled solidified portion 40 is formed thereon. In this remelted and rapidly cooled solidified portion 40, the silicon is finer and more uniformly dispersed than in the base material's high-strength material.

[0029] Various types of lasers can be used in this laser irradiation process, including TiG lasers, YAG lasers, and CO2 lasers, and any of these lasers can be used. Before laser irradiation, it is preferable to perform degreasing with chemicals or cleaning with a laser to prevent dirt and other contaminants adhering to the surface of the ring material 30a from entering the remelting, rapid cooling, and solidification section 40. In particular, when cleaning and remelting, rapid cooling, and solidification are performed with a laser, it is more preferable because the processes can be performed consecutively by simply changing the output conditions of a single laser device.

[0030] In the laser irradiation process, to prevent the occurrence of cracks due to laser irradiation, the laser beam is irradiated only once around the outer surface of the piston material 30a. At this time, as shown in Figure 5, the laser device 50 may be fixed and the piston material 30a may be rotated once around its axis while the laser beam 51 is irradiated from the laser device 50 to form the remelted, rapidly cooled, and solidified portion 40. Alternatively, although not shown, the piston material may be fixed and the laser device may be rotated once around the piston material while the laser beam is irradiated from the laser device to form the remelted, rapidly cooled, and solidified portion. Due to the structure of the device, the mechanism for rotating the piston material is simpler, so the method shown in Figure 5 is more preferable.

[0031] In the laser irradiation process, as described above, the laser light is irradiated only once around the outer surface of the piston material 30a, so the laser processing speed is set to a range of 2,000 to 10,000 mm / min. By setting the laser processing speed to 2,000 mm / min or higher, the surface roughness Ra and surface roughness Rpk of the anodized film formed thereafter can both be reduced to 1.0 μm or less.

[0032] Surface roughness Ra, as standardized in JIS B0601, represents the arithmetic mean roughness of the contour curve. Surface roughness Rpk is an index relating to the characteristics of the average height of the protruding peaks above the core of the roughness curve, and can be used to evaluate the surface characteristics of a plateau structure. A "plateau structure" is defined in JIS-B0671 as a structure in which the surface is formed by plateau sections (flat sections) and valley sections. The airtightness of the anodized film can be evaluated by the surface roughness Rpk. By setting the surface roughness Rpk of the anodized film to 1.0 μm or less, when combustion occurs in an internal combustion engine, the contact area with the top ring is large, and excellent airtightness can be achieved, thus suppressing the passage of blow-by gas during engine operation. In addition, by setting the surface roughness Ra to 1.0 μm or less, the accumulation of oil in the valley sections of the anodized film can be suppressed, thereby suppressing blow-by gas containing oil components and reducing the number of particulate matter.

[0033] On the other hand, even if the processing speed exceeds 10,000 mm / min, the surface roughness of the resulting anodic oxide film hardly changes. Furthermore, the faster the processing speed, the greater the output required, leading to increased equipment costs and power consumption. Therefore, a processing speed of 10,000 mm / min or less is preferable.

[0034] The output of the laser beam 51 is preferably set to, for example, 1 to 15 kW, as the remelting rapid solidification section 40 needs to be wider than the width and depth of the top ring groove, and the processing speed described above.

[0035] As shown in Figure 6, by lowering the laser beam output and irradiating the outer surface of the piston material 30a with the laser beam several times to form the remelt-rapidly solidified portion 41, the high-strength material can be processed up to the width and depth of the top ring groove 33 in a narrower area than the remelt-rapidly solidified portion 40 formed by only one rotation. However, when the laser beam is irradiated several times in this way, cracks are more likely to occur in the remelt-rapidly solidified portion 41, making it difficult to machine the top ring groove 33 with high dimensional accuracy, and also reducing the durability of the piston itself.

[0036] Next, as shown in Figure 4(b), a cutting process is performed to create a top ring groove 33 in the remelted, rapidly solidified portion 40. The machining process, such as cutting, to create the top ring groove 33 can be performed in the same way as the machining processes used to create ring grooves in general pistons. The width and depth of the top ring groove 33 are also the same as the dimensions of the top ring groove in a general internal combustion engine piston. The entire inner circumferential surface of the top ring groove 33 is the remelted, rapidly solidified portion 40.

[0037] Then, as shown in Figure 4(c), an anodizing process is performed to form an anodic oxide film 60 on the inner circumferential surface of the top ring groove 33. The anodic oxide film 60 is a film formed by oxidizing the aluminum in the remelted rapid solidification section 40, and is mainly composed of amorphous aluminum oxide (Al2O3), possessing excellent wear resistance. As a result, the inner circumferential surface of the top ring groove 33 of the piston 30 for the internal combustion engine has a structure in which a remelted rapid solidification section 40 (also called a silicon dispersion layer) is formed on top of the high-strength base material, in which silicon is finer and more uniformly distributed than in the high-strength material, and further, the anodic oxide film 60 is formed on top of this silicon dispersion layer 40.

[0038] Furthermore, since the anodic oxide film 60 is formed by oxidizing the aluminum in the remelt-rapidly solidified section 40, the surface of the anodic oxide film 60 after anodic oxidation treatment rises by about half the thickness of the anodic oxide film 60 compared to the surface of the remelt-rapidly solidified section 40 before treatment. In other words, the surface of the remelt-rapidly solidified section (silicon dispersion layer) 40 (the interface with the anodic oxide film 60) after anodic oxidation treatment descends by about half the thickness of the anodic oxide film 60.

[0039] The cellular structure of the anodic oxide film 60 can have various configurations depending on the electrolysis conditions, but a preferred structure is one in which the cells branch out in random directions, surrounding the primary silicon crystal within the film. Such a structure can be formed by AC / DC superimposed electrolysis. This makes it easier to obtain a thicker film than when formed by DC electrolysis.

[0040] The thickness of the anodic oxide film 60 is preferably 5 μm or more, and more preferably 10 μm or more, taking into consideration that it will decrease due to wear with the piston ring during engine operation. The upper limit of the thickness of the anodic oxide film 60 is preferably 25 μm or less, and more preferably 20 μm or less, taking into consideration productivity and processing costs. The anodic oxide film 60 may be formed not only on the inner circumferential surface of the top ring groove 33, but also on a part of the top land 32 and second land 34 adjacent to the top ring groove 33.

[0041] A more specific method of the anodic oxidation process will be described with reference to Figure 7. As shown in Figure 7, first, the anodic oxidation apparatus 70 is positioned to surround the outer circumferential surface of the top ring groove 33. The anodic oxidation apparatus 70 is placed in close contact with the remelt-quick-cooling solidification sections 40 of the top land 32 and the second land 34 via sealing rings 72. By supplying the treatment liquid 73 to the inside of the top ring groove 33 and to the portions of the top land 32 and the second land 34 up to the sealing rings 72, and by applying current between the remelt-quick-cooling solidification section 40 as the anode and the cathode electrode 71 on the anodic oxidation apparatus 70 side, an anodic oxidation film 60 can be formed on the surface of the remelt-quick-cooling solidification section 40 in the portion to which the treatment liquid 73 has been supplied.

[0042] As a result, the outermost layer of the top land 32 and the outermost layer of the second land 34 are configured to have three regions that change sequentially as they move away from the top ring groove 33. That is, the outermost layers of the top land 32 and the second land 34 are, in order as they move away from the top ring groove 33, a region that is an anodized coating 60, a region that is a remelted rapid-cooled solidified portion 40 (silicon dispersion layer), and a region that is the high-strength base material 20.

[0043] As the treatment solution used in the anodizing process, a wide range of conventional electrolytic treatment solutions that form an anodic oxide film can be used. For example, acidic treatment solutions such as sulfuric acid (H2SO4), oxalic acid (H2C2O4), phosphoric acid (H3PO4), and chromic acid (H2CrO4), and basic treatment solutions such as sodium hydroxide (NaOH), sodium phosphate (Na3PO4), and sodium fluoride (NaF) can be used, with sulfuric acid being preferred from a practical standpoint. The cathode electrode 71 can be made of titanium, carbon, aluminum, stainless steel, etc. Although it is also possible to perform anodizing by masking the parts that are not to be anodized and immersing the entire piston in the treatment solution, forming an anodic oxide film on the parts other than the masked parts, instead of using the sealing ring shown in Figure 7, the sealing ring is preferable because it can increase manufacturing efficiency more than masking.

[0044] In the anodizing process, as shown in Figure 8, it is also possible to place the sealing ring 72 on the parts of the top land 32 and second land 34 that are not remelted and rapidly solidified (untreated areas). When the sealing ring 72 is placed in this manner and the anodizing process is performed, the untreated high-strength material 20 is also anodized. As a result, the outermost layer of the top land 32 and the outermost layer of the second land 34 are each formed in two regions: a region of anodized film 60 and a region of the base material high-strength material 20, as they move away from the top ring groove 33. However, in this case, the anodized film formed on the untreated high-strength material 20 not only has the aforementioned large surface roughness, but its film thickness is also significantly thicker, for example, about three times thicker, compared to the anodized film formed on the remelted and rapidly solidified area 40 where silicon is refined. Therefore, in the region where the outermost layer is the anodized film 61, areas with different film thicknesses of the anodized film 61 occur, resulting in large steps. If there is a step protruding from the inner wall surface of the cylinder, the resistance when the piston slides within the cylinder bore may increase. Therefore, it is preferable to place the sealing ring 72 on the remelted and rapidly solidified portions 40 of the top land 32 and the second land 34, respectively.

[0045] As described above, by manufacturing a piston material using a high-strength material, irradiating the piston material with laser light under predetermined conditions at the position where the top ring groove is to be formed, performing a cutting process to create the top ring groove in the remelted and rapidly solidified portion formed by the laser irradiation, and performing an anodic oxidation treatment process to form an anodic oxide film on the inner circumferential surface of the top ring groove, an anodic oxide film with a surface roughness of 1.0 μm or less in both Ra and Rpk can be formed on the top ring groove, thereby maintaining good sealing performance with the piston ring. [Examples]

[0046] Examples and comparative examples of the present invention will be described below. However, the present invention is not limited to the following examples and comparative examples.

[0047] [Example 1] A piston material was fabricated using an Al-Si-Cu-Ni aluminum alloy (hereinafter also referred to as high-strength material) having the composition shown in Table 1. The piston material was then rotated once around its axis while a laser beam was irradiated onto its outer circumference, causing remelting and rapid solidification of the area where the top ring groove was formed and its surroundings. The piston rotation speed was adjusted to a processing speed of 1,000 to 30,000 mm / min. The laser output was adjusted within a range of 1 to 15 kW according to the processing speed, ensuring that the high-strength material was processed to the depth of the top ring groove.

[0048] [Table 1]

[0049] After remelting, rapid cooling, and solidification by laser irradiation, the entire piston was machined to create a ring groove (1 mm wide), and the surface of the top ring groove and the remelting, rapid cooling, and solidification areas of the top land and second land were anodized. For the anodizing treatment, an anodic oxide film with a thickness of approximately 10 μm was formed using an AC / DC superimposed electrolysis method (frequency 12 kHz, positive voltage 35 V, negative voltage 2 V) with a sulfuric acid treatment solution with a concentration of 200 g / L, thereby fabricating a piston for an internal combustion engine. The surface roughness Ra and surface roughness Rpk of the anodized film were then measured using a surface roughness meter in accordance with JIS B0601 and JIS B0671-2, respectively. The results are shown in Table 2.

[0050] [Comparative Example 1] An internal combustion engine piston was fabricated in the same manner as in Example 1, except that laser irradiation was not performed. Then, the surface roughness Ra and surface roughness Rpk of the anodic oxide film formed on the top ring groove were measured, in the same manner as in Example 1. The results are shown in Table 2.

[0051] [Table 2]

[0052] As shown in Comparative Example 1 in Table 2, when the piston material was a high-strength material, anodizing treatment was performed without remelting, rapid cooling, and solidification treatment by laser irradiation, resulting in an anodized film with a surface roughness Ra of 1.2 μm and a surface roughness Rpk of 1.7 μm, both exceeding 1.0 μm. On the other hand, in Example 1, where anodizing treatment was performed after remelting, rapid cooling, and solidification treatment, as shown in Table 2, increasing the processing speed of laser irradiation reduced both the surface roughness Ra and surface roughness Rpk of the anodized film, and both surface roughness Ra and surface roughness Rpk became 1.0 μm or less when the processing speed was 2,000 mm / min or higher.

[0053] Furthermore, comparing the results for processing speeds of 9,000 mm / min, 10,000 mm / min, and 30,000 mm / min in Table 2, although the processing speeds differed significantly, the change in surface roughness of the resulting anodized film was small. Since it is necessary to remelt and rapidly cool the high-strength material to the depth to which the top ring groove is formed by laser irradiation during one rotation of the piston material, a faster processing speed requires greater power, leading to increased equipment costs and power consumption. Therefore, a processing speed of 10,000 mm / min or less is preferable, and 9,000 mm / min or less is more preferable.

[0054] Furthermore, while rotating multiple piston materials, the laser beam was irradiated onto the outer surface of the piston material so that some of the beams overlapped, and the high-strength material was processed to the width and depth required for the top ring groove to be formed. Although this allowed for a reduction in the laser beam output, cracks were observed to occur in the treated areas. This is presumed to be due to the re-melting and rapid cooling of the area after it has been irradiated with laser light and rapidly solidified.

[0055] [Example 2] An internal combustion engine piston was manufactured in the same manner as in Example 1, except that the processing speed in the remelting rapid solidification process was set to 9,000 mm / min, and the processing time in the anodizing process was varied to produce anodized coatings with different thicknesses. Then, the surface roughness Ra and surface roughness Rpk of the anodized coating formed in the top ring groove were measured, as in Example 1. The results are shown in Table 3. [Table 3]

[0056] As shown in Table 3, three types of anodic oxide films with film thicknesses ranging from 7 μm to 20 μm were formed. It was found that the surface roughness Ra and Rpk were almost constant regardless of the film thickness in anodic oxide films within this range. Generally, in aluminum alloys containing silicon, the surface roughness of the anodic oxide film increases in proportion to the film thickness due to the influence of silicon. However, in this invention, since the silicon is finely milled and uniformly distributed, a film with low surface roughness can be obtained regardless of the film thickness.

[0057] [Example 3] An internal combustion engine piston was manufactured in the same manner as in Example 1, except that the anodizing treatment was also performed on parts of the top land and second land other than the remelted, rapidly solidified areas (untreated areas) during the anodizing process. The internal combustion engine piston was then cut and the cross-section was observed using an optical microscope to observe the cross-sections of the anodized film formed on the remelted, rapidly solidified areas and the untreated areas, respectively. The cross-sectional images are shown in Figures 9 and 10. The film thickness of the anodized film formed on the remelted, rapidly solidified areas and the untreated areas was also measured. The film thickness was measured by taking 10 equally spaced cross-sectional measurements of the anodized film on the cut piston using an optical microscope, and calculating the average of the obtained values.

[0058] Figure 9 is a cross-sectional image of the remelt-rapidly solidified section 40 with an anodic oxide film 60a formed on it, and Figure 10 is a cross-sectional image of the high-strength material 20, which is the base material of the untreated section, with an anodic oxide film 60b formed on it. The material visible on top of the anodic oxide films 60a and 60b is the embedded resin 80 used for observation with an optical microscope, and its boundary is indicated by a dotted line. As shown in Figures 9 and 10, it was confirmed that the remelt-rapidly solidified section 40, which was irradiated with laser light under predetermined conditions, had finer and more uniformly dispersed silicon than the high-strength material 20. Furthermore, it was confirmed that the surface of the remelt-rapidly solidified section 40 relative to the anodic oxide film 60a was flatter than the surface of the high-strength material 20 relative to the anodic oxide film 60b. Furthermore, these cross-sectional images also show that the surface roughness of the anodic oxide film 60a formed on the remelted rapid solidification section 40 in Figure 9 is lower than that of the anodic oxide film 60b formed on the high-strength material 20 in Figure 10.

[0059] The film thickness of the anodic oxide film 60a formed on the remelted rapid solidification section 40 was 10 μm, while the anodic oxide film 60b formed on the high-strength material 20 was 35 μm. This difference of more than three times in film thickness is due to the slower film formation rate caused by the aforementioned silicon refinement. Therefore, if anodic oxidation is performed on both the remelted rapid solidification section and the untreated section in the top land and second land, a large step difference will occur at the boundary between the remelted rapid solidification section and the untreated section due to the difference in anodic oxide film thickness. In parts with precisely designed dimensions, such as pistons for internal combustion engines, such a large step difference is undesirable as it can cause problems such as uneven contact. Therefore, it is preferable to perform anodic oxidation only on the remelted rapid solidification section so that anodic oxidation is not performed on the untreated section.

[0060] [Example 4] The anodizing process was changed from AC / DC superimposed electrolysis to DC electrolysis (current density 9A / dm 2A piston for an internal combustion engine was manufactured in the same manner as in Example 3, except for the points made in Example 3. Then, as in Example 3, cross-sectional observation and thickness measurement of the anodic oxide film formed on the remelted rapid-cooled solidified section and the untreated section were performed. The obtained cross-sectional images are shown in Figures 11 and 12.

[0061] Figure 11 is a cross-sectional image of the anodic oxide film 60c formed on the remelted and rapidly solidified section 40 by DC electrolysis, and Figure 12 is a cross-sectional image of the anodic oxide film 60d formed on the untreated high-strength material 20 by DC electrolysis. Compared with Figures 9 and 10 of the AC-DC superimposed electrolysis method, it was observed that the surface roughness of the anodic oxide film 60 was lower when formed on the remelted and rapidly solidified section 40 than when formed on the untreated high-strength material 20, in both the AC-DC superimposed electrolysis method and the DC electrolysis method.

[0062] On the other hand, the film thickness of the anodic oxide film 60c formed on the remelt-rapid solidification section 40 was 4 μm, while the anodic oxide film 60d formed on the high-strength material 20 was 34 μm. The reason why there was a difference of more than twofold in the film thickness of the anodic oxide films 60a and 60c formed on the remelt-rapid solidification section 40 using the AC / DC superposition electrolysis method and the DC electrolysis method is that in the remelt-rapid solidification section 40, the finely milled silicon is uniformly distributed. In the DC electrolysis method, the cells constituting the anodic oxide film grow linearly, and although the silicon is small, there are many of them, which inhibits cell growth. On the other hand, in the AC / DC superposition electrolysis method, the cells grow in random directions, so it is presumed that the cells grow more easily than in the DC electrolysis method. From the viewpoint of wear resistance, a film thickness of 5 μm or more is preferable for the anodic oxide film applied to pistons for internal combustion engines. Considering the film thickness, it is preferable to perform the anodic oxidation treatment using the AC / DC superposition electrolysis method rather than the DC electrolysis method.

[0063] Furthermore, increasing the electrolysis conditions in the DC electrolysis method to speed up film deposition increases heat generation during anodizing, which can lead to the anodic oxide film becoming as hard as the base material (approximately 180 HV) or becoming prone to dielectric breakdown, known as film burning, thus posing a problem in terms of wear resistance. On the other hand, the AC-DC superimposed electrolysis method, which uses high frequencies (e.g., 10 kHz or higher) that do not generate much heat even when the electrolysis conditions are increased, can form an anodic oxide film that satisfies wear resistance requirements in terms of hardness. [Explanation of Symbols]

[0064] 10 Conventional aluminum alloys 11, 21 Matrix 12, 22 Primary silicon 20 High strength materials 30 Pistons for internal combustion engines 31 Piston crown 32 Topland 33 Top ring groove 34 Second Land 40 Remelted and rapidly solidified section (silicon dispersion layer) 50 Laser devices 51 Laser light 60 Anodized coating 70 Anodizing treatment apparatus 71 Cathode electrode 72 sealing rings 73 Treatment solution

Claims

1. A piston material formed from an aluminum alloy containing 5.0 to 20.0 mass% Si, more than 1.3 mass% and less than or equal to 5.0 mass% Cu, and more than 1.5 mass% and less than or equal to 3.5 mass% Ni, is irradiated with laser light at least the position on its outer surface where the top ring groove is formed. The process of forming a top ring groove on the outer surface of the piston material irradiated with the aforementioned laser light by cutting, The process involves forming an anodic oxide film on the surface of the top ring groove, and A method for manufacturing a piston for an internal combustion engine, comprising the steps of irradiating with laser light, wherein the processing speed of the laser light is in the range of 2,000 to 10,000 mm / min, and the irradiation of the laser light is limited to one rotation around the outer surface of the piston material.

2. In the step of irradiating with laser light, the laser light is irradiated not only to the position where the top ring groove is formed, but also to a part of the position that will become the top land adjacent to the position where the top ring groove is formed, and a part of the position that will become the second land. The method for manufacturing a piston for an internal combustion engine according to claim 1, wherein in the step of forming the anodic oxide film, the anodic oxide film is also formed on the surface of the top ring groove, as well as on a portion of the surface of the laser light irradiation area of ​​the top land adjacent to the surface of the top ring groove and on a portion of the surface of the laser light irradiation area of ​​the second land.

3. The method for manufacturing a piston for an internal combustion engine according to claim 1 or 2, wherein the anodic oxide film is formed by an AC / DC superimposed electrolysis method in the step of forming the anodic oxide film.

4. The method for manufacturing a piston for an internal combustion engine according to claim 1 or 2, wherein in the step of forming the anodic oxide film, the thickness of the anodic oxide film is in the range of 5.0 to 25.0 μm.

5. The method for manufacturing a piston for an internal combustion engine according to claim 2, wherein, in the step of forming the anodic oxide film, a sealing ring is placed in the laser light irradiation area of ​​the top land and the laser light irradiation area of ​​the second land, and a processing liquid is supplied between the sealing ring on the top land side and the sealing ring on the second land side.

6. A piston for an internal combustion engine having a top ring groove, The base material is an aluminum alloy containing 5.0 to 20.0 mass% Si, more than 1.3 mass% and 5.0 mass% or less Cu, and more than 1.5 mass% and 3.5 mass% or less Ni, A silicon dispersion layer having an aluminum alloy composition in which silicon in the aluminum alloy is more finely dispersed than in the base material, located on the inner surface of the top ring groove, The anodic oxide film located on the surface of the silicon dispersion layer and The piston of the internal combustion engine is equipped with such that the top land and second land adjacent to the top ring groove of the piston have, in order of their outermost surface, the region being the anodized coating, the region being the silicon dispersion layer, and the region being the base material, as they move away from the top ring groove. A piston for an internal combustion engine, wherein the surface roughness Rpk of the anodic oxide film is 1.00 μm or less according to JIS B0671-2, and the surface roughness Ra of the anodic oxide film is 1.00 μm or less according to JIS B0601.

7. The piston for an internal combustion engine according to claim 6, wherein the anodic oxide film has cells that extend in random directions relative to the inner surface of the top ring groove, and the cells surround the silicon in the anodic oxide film in a state where they branch out in random directions.

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