Manufacturing method of piston for internal combustion engine

Abstract

To provide a method of manufacturing a piston for an internal combustion engine which can maintain good seal performance between an anode oxidation coating formed on an inner face of a top ring groove and a piston ring even when a high-strength material is used for the base material of the piston.SOLUTION: A piston body 10A is made of an aluminum alloy containing Si of 5.0 to 20.0 mass%, Cu of over 1.3 mass% and equal to or lower than 5.0 mass%, and Ni of over 1.5 mass% and equal to or lower than 3.5 mass% as a base material, and has a top ring groove 13 in an external peripheral surface. Water 52 is injected from an injection nozzle 51 while rotating the piston body around its axis with a portion of a top land 12 being held by a holding member 55, such that a region of an inner surface of the top ring groove with which a top ring is in contact is treated with water jet to remove silicon that is exposed in the region. After that, an anode oxidation coating is formed, thus manufacturing a piston for an internal combustion engine.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a piston for an internal combustion engine, and more particularly to a method for manufacturing a piston for an internal combustion engine having an anodized coating on the inner surface of a top ring groove. [Background technology]

[0002] In recent years, in response to environmental regulations, there has been an increasing demand for higher efficiency and higher compression ratios in internal combustion engines, such as automobile engines, as well as for turbocharged engines, resulting in an increase in the maximum combustion pressure of internal combustion engines. Against this backdrop, the temperatures of the piston ring grooves (particularly the top ring groove) and lands (particularly the top land and second land) of pistons for internal combustion engines have become higher than those of conventional internal combustion engines of the same displacement. An anodized coating is sometimes formed on the top ring groove of pistons to impart wear resistance. This is because the anodized coating is more than twice as hard as the aluminum alloy, which is the base material of the piston, and has excellent wear resistance properties.

[0003] For example, Patent Document 1 describes an anodized coating on an internal combustion engine piston, which has an anodized coating on the inner surface of the top ring groove, at least on the inner surface on the second ring groove side, in the region where the top ring comes into contact, and the anodized coating has a surface roughness Rpk according to JIS B0671-2 of 1.00 μm or less. It is described that an anodized coating with such surface roughness can improve airtightness with the top ring and reduce the flow rate of blow-by gas and the number of emitted particulates. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2020-204287 Summary of the Invention [Problem to be solved by the invention]

[0005] However, aluminum alloys with improved mechanical properties (e.g., fatigue strength and tensile strength) up to high temperatures, known as high-strength materials, have been increasingly adopted in recent years. These alloys often contain higher amounts of additive elements, such as copper (Cu) and nickel (Ni), than conventional aluminum alloys, such as AC8A (Al-Si-Cu-Ni-Mg-based alloys). These high-strength materials contain a higher proportion of coarse, granular primary silicon (Si) crystals (with Si grain sizes of approximately 30 to 40 μm). This granular Si significantly affects the formation of an anodized film, resulting in the formation of a film with significant irregularities. The present inventors discovered that when such high-strength materials are used, even the anodizing process described in Patent Document 1 results in a film with a surface roughness (Rpk) exceeding 1.0 μm when the anodized film is 10 μm thick.

[0006] If the surface roughness of the anodized film deteriorates, the sealing ability with the top ring will decrease, resulting in increased blow-by gas and worsened fuel economy.In addition, the engine oil used to lubricate engine parts will be more likely to flow into the combustion chamber (oil upflow), and when the engine oil burns in the combustion chamber, this can cause an increase in the amount of substances subject to European environmental regulations, such as PM (Particulate Matter) and PN (Particle Number), so the surface of the anodized film must be smooth.

[0007] In view of the above problems, an object of the present invention is to provide a method for manufacturing a piston for an internal combustion engine that can maintain good sealing between the anodized coating formed on the inner surface of the top ring groove and the piston ring, even when a high-strength material is used as the piston base material. [Means for solving the problem]

[0008] In order to achieve the above object, the present invention provides a method for manufacturing a piston for an internal combustion engine, the method comprising the steps of: providing a piston body for an internal combustion engine using an aluminum alloy containing 5.0 to 20.0 mass% Si, more than 1.3 mass% but not more than 5.0 mass% Cu, and more than 1.5 mass% but not more than 3.5 mass% Ni as a base material, the piston body having a top ring groove on its outer peripheral surface; water-jetting at least the region of the inner surface of the top ring groove that comes into contact with a top ring while rotating the piston body for an internal combustion engine about its axis in a state where the portion of the outer peripheral surface between the top ring groove and the piston crown surface is held by a holding member, thereby removing silicon exposed in the region; and forming an anodic oxide coating on at least the region that has been water-jetted. [Effects of the Invention]

[0009] According to the present invention, by performing water jet treatment on the inner surface of the top ring groove before anodizing, silicon exposed on the surface is removed, forming an anodized coating with a smooth surface, which maintains good sealing performance with the piston ring. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a front view schematically illustrating an example of a piston for an internal combustion engine. [Figure 2] FIG. 2 is an enlarged cross-sectional view schematically showing the periphery of a top ring groove of the piston for the internal combustion engine shown in FIG. 1. [Figure 3] 1 is an optical microscope photograph showing an example of a metal structure when a conventional aluminum alloy is used as a base material for a piston for an internal combustion engine. [Figure 4] 1 is an optical microscope photograph showing an example of a metal structure when an aluminum alloy (high strength material) is used as a base material for a piston for an internal combustion engine according to the present invention. [Figure 5] 1 is a flow chart showing an embodiment of a method for manufacturing a piston for an internal combustion engine according to the present invention; [Figure 6]FIG. 2 is a front view schematically showing an example of a water jet treatment step in the manufacturing method of a piston for an internal combustion engine according to the present invention. [Figure 7] FIG. 7 is an enlarged schematic view illustrating the water jetting direction (tilt angle α) of the jet nozzle in the water jet treatment step shown in FIG. 6. [Figure 8] FIG. 7 is a schematic diagram illustrating the water jetting direction (direction deviation angle β) of the jet nozzle in the water jet treatment process shown in FIG. 6. [Figure 9] FIG. 7 is an enlarged schematic view illustrating the water jetting direction (tangential direction T) of the jet nozzle in the water jet treatment step shown in FIG. 6. [Figure 10] 5A to 5C are schematic diagrams illustrating changes in the state of the inner surface of the top ring groove due to an anodizing treatment step in the manufacturing method of the piston for an internal combustion engine according to the present invention. [Figure 11] FIG. 10 is a schematic diagram illustrating a change in the state of the inner surface of the top ring groove when an anodizing process is performed without performing a water jet treatment process, as a comparative example. [Figure 12] 1 is an SEM photograph showing the surface of the high-strength material of Comparative Example 1 before water jet treatment. [Figure 13] 1 is an SEM photograph showing the surface of the high-strength material after water jet treatment in Example 1. [Figure 14] 1 is an optical microscope photograph showing a cross section of the anodic oxide coating of Comparative Example 2. [Figure 15] 1 is an optical microscope photograph showing a cross section of the anodic oxide coating of Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0011] An embodiment of a method for manufacturing a piston for an internal combustion engine according to the present invention will now be described with reference to the accompanying drawings. Note that the drawings are drawn with priority given to ease of understanding and are not drawn to scale.

[0012] First, a piston for an internal combustion engine manufactured by the method of this embodiment will be described. As shown in Fig. 1, an internal combustion engine piston 10 has three ring grooves formed on its outer peripheral surface facing a bore inner peripheral surface 21 of a cylinder block, in that order from the piston crown surface 11 side: a top ring groove 13, a second ring groove 15, and an oil ring groove 17. A top ring (not shown) is fitted in the top ring groove 13, a second ring (not shown) is fitted in the second ring groove 15, and an oil ring (not shown) is fitted in the oil ring groove 17. On the outer peripheral surface of the piston 10, the portion between the piston crown surface 11 and the top ring groove 13 is called a top land 12, the portion between the top ring groove 13 and the second ring groove 15 is called a second land 14, the portion between the second ring groove 15 and the oil ring groove 17 is called a third land 16, and the portion below the oil ring groove 17 is called a skirt portion 18.

[0013] 2, when the internal combustion engine piston 10 is inserted into the cylinder block 20, the outer peripheral surface of the top ring 22 fitted into the top ring groove 13 protrudes outward beyond the top land 12 and second land 14, which are the outer peripheral surfaces of the piston. The top ring 22 is pressed against the inner peripheral surface 21 of the bore of the cylinder block 20 by its elastic force, and the top ring 22 functions to maintain the airtightness of the combustion chamber.

[0014] The second ring (not shown) has a similar structure and function, but the internal pressure in the combustion chamber on the piston crown surface 11 side increases during the piston compression and expansion strokes. This causes the inner surface of the top ring groove 13 to tightly adhere to the top ring 22, making the inner surface of the top ring groove 13 particularly susceptible to wear. Therefore, an anodized coating (not shown) is formed on the inner surface of the top ring groove 13 to improve its wear resistance. The inner surface of the top ring groove 13 facing the piston crown surface 11 is referred to as the upper surface 13a, the inner surface on the opposite side (the second ring side) is referred to as the lower surface 13c, and the inner surface at the bottom of the groove between them is referred to as the bottom surface 13b. As shown in FIG. 2 , the top ring 22 does not necessarily contact the entire inner surface of the top ring groove 13. Therefore, the anodized coating may be formed only in the region of the inner surface of the top ring groove 13 that comes into contact with the top ring 22. While this area varies depending on the piston design, for example, if the length of the top ring groove 13's upper surface 13a or lower surface 13c from the end of the top land 12 or second land 14 side to the end of the bottom surface 13b side (i.e., the groove depth) is taken as 100%, then the area extending from the end of the top land 12 or second land 14 side to at least 90% is preferred, at least 80% is more preferred, and at least 70% is even more preferred. Of course, the anodized coating may be formed on the entire inner surface of the top ring groove 13. Whether the anodized coating should be formed on only the upper surface 13a, only the lower surface 13c, or both the upper surface 13a and the lower surface 13c, and how much of the inner surface of the top ring groove 13 is to be coated, is desirably determined in consideration of requirements such as sealing performance against combustion gases, reduction of blow-by gas, and suppression of PM and PN that cause oil leaks.

[0015] The piston body for an internal combustion engine on which the anodized coating is formed is formed from an aluminum alloy (hereinafter also referred to as high-strength material) containing 5.0 to 20.0 mass% silicon (Si), more than 1.3 mass% but not more than 5.0 mass% copper (Cu), and more than 1.5 mass% but not more than 3.5 mass% nickel (Ni) as a base material.

[0016] In high-strength materials, Si is a component that crystallizes as primary silicon or eutectic silicon and improves heat resistance and wear resistance. Si also reduces the thermal expansion coefficient. If the Si content is 5.0% by mass or more, the thermal expansion coefficient is low, and wear resistance and strength at high temperatures can be improved. If the Si content is 20.0% by mass or less, the primary silicon crystals become small, and the elongation of the alloy can be improved. The Si content is more preferably 10.0 to 13.0% by mass.

[0017] Cu is a component that improves mechanical strength and wear resistance at room temperature and high temperatures. If the Cu content exceeds 1.3% by mass, the effect of improving strength and wear resistance can be exhibited, and if it is 5.0% by mass or less, there is no significant decrease in 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 is significantly decreased and the specific gravity of the alloy becomes large. The Cu content is more preferably 2.5 to 5.0% by mass.

[0018] Ni is a component that mainly improves strength and wear resistance in high temperature ranges and reduces the thermal expansion coefficient. If the Ni content exceeds 1.5% by mass, these effects are favorably exhibited, and if it is 3.5% by mass or less, good elongation is obtained.

[0019] In this embodiment, the aluminum alloy used for the base material may contain, in addition to the above-mentioned Si, Cu, and Ni, at least one element selected from the group consisting of chromium (Cr), titanium (Ti), zirconium (Zr), phosphorus (P), iron (Fe), manganese (Mn), and magnesium (Mg), with the balance consisting essentially of Al and unavoidable impurities. Preferably, the aluminum alloy base material contains, in addition to the above-mentioned ranges of Si, Cu, and Ni, 0.05 to 0.15 mass% Cr, 0.05 to 0.20 mass% Ti, 0.05 to 0.30 mass% Zr, 0.10 to 0.31 mass% Fe, 0.05 mass% or less Mn, and 0.5 to 1.1 mass% Mg, with the balance consisting essentially of Al and unavoidable impurities. Si, Cu, and Ni have already been described, so the other components and their contents will now be described.

[0020] Cr is a component that strengthens the grain boundaries between crystal grains such as intermetallic compounds, primary silicon, and acicular silicon crystals that are crystallized in the alloy, thereby improving strength at high temperatures. If the Cr content is 0.05% by mass or more, the grain boundaries are suitably strengthened, improving strength at high temperatures, and if the Cr content is 0.15% by mass or less, good toughness and machinability are obtained.

[0021] Ti is a component that mainly refines crystal grains to improve heat resistance, castability, and strength, and this effect is suitably exhibited when the Ti content is in the range of 0.05 to 0.20 mass%, preferably 0.05 to 0.15 wt%.

[0022] Zr is a component that mainly has the effect of refining crystal grains in the alloy and contributes to improving heat resistance, castability, and strength, and this effect is suitably exhibited when the Zr content is in the range of 0.05 to 0.30 mass%, preferably 0.05 to 0.15 wt%.

[0023] Fe is a component that mainly crystallizes intermetallic compounds, improving wear resistance and strength at high temperatures. If the size of these intermetallic compounds is coarse, strength will decrease. If the Fe content is in the range of 0.10 to 0.31 mass%, the size of the Fe-Mn intermetallic compounds can be reduced.

[0024] Mn is a component that mainly crystallizes intermetallic compounds and improves wear resistance and strength at high temperatures. If the size of these intermetallic compounds is large, strength decreases. If the Mn content is 0.05% by mass or less, the size of the Fe-Mn intermetallic compounds can be reduced. The lower limit of the Mn content may be zero or may be contained in an extremely small amount comparable to that of an impurity, for example, 0.001% by mass.

[0025] Mg is a component that mainly improves strength and toughness. If the Mg content is 0.5% by mass or more, the effect of improving strength is exerted, and if it is 1.1% by mass or less, good toughness is obtained.

[0026] When a piston material is manufactured using such a high-strength aluminum alloy as a base material, a larger amount of coarse granular primary silicon crystals precipitates during the manufacturing process than when a piston material is manufactured using a conventional aluminum alloy. Fig. 3 is an optical microscope photograph of the metallographic structure of a piston base material made using a conventional aluminum alloy (AC8A), and Fig. 4 is an optical microscope photograph of the metallographic structure of a piston base material made using a high-strength aluminum alloy (containing more Cu and Ni than AC8A) used in the present invention. As shown in Fig. 3, the conventional aluminum alloy 30A also has granular primary silicon crystals 32 precipitated in the matrix 31. However, as shown in Fig. 4, the high-strength aluminum alloy 30B has significantly larger granular primary silicon crystals 32 precipitated in the matrix 31.

[0027] When anodizing aluminum alloys containing granular primary silicon by DC electrolysis, the cells that make up the anodic oxide film grow in one direction relative to the surface of the target, and the silicon inhibits the growth of the anodic oxide film, resulting in a film with significant surface irregularities. On the other hand, AC / DC superimposed electrolysis allows cells to grow in a random direction relative to the surface of the target, resulting in no orientation. Therefore, even granular primary silicon particles precipitated in the matrix grow while branching in random directions, resulting in a smooth anodic oxide film. However, the particle size of primary silicon in high-strength materials is very large, approximately 30–40 μm. Therefore, even when AC / DC superimposed electrolysis is used, the surface roughness (Rpk) of the resulting anodic oxide film exceeds 1.0 μm, making it difficult to maintain good sealing performance with piston rings.

[0028] To solve these problems, a manufacturing method 40 for an internal combustion engine piston according to this embodiment, as shown in Fig. 5, sequentially performs the following steps: step 41 of casting a piston body, step 42 of heat-treating the cast piston body, step 43 of machining the heat-treated piston body, step 44 of water-jetting the inner surface of the top ring groove of the piston body, step 45 of anodizing the inner surface of the top ring groove, and step 46 of resin-coating the piston skirt. Each step is described below.

[0029] The above-mentioned casting, heat treatment, and machining steps 41, 42, and 43 are similar to those used in manufacturing a typical piston for an internal combustion engine. For example, the casting step 41 is typically performed using gravity casting, in which molten high-strength material (molten aluminum) is poured into a piston-shaped mold. The heat treatment step 42 includes, for example, T5 treatment and T6 treatment. T5 treatment involves only artificial aging hardening after casting (for purposes such as increasing strength and dimensional stabilization), while T6 treatment involves solution treatment and artificial aging hardening after casting (for purposes such as increasing strength and hardness). In the machining step 43, a ring groove, such as a top ring groove, is formed by cutting, for example. The width and depth of the top ring groove are similar to those of a typical piston for an internal combustion engine.

[0030] The water jet treatment process 44 is intended to remove coarse silicon particles that are exposed on the inner surface of the top ring groove created in the machining process 43 and that have a significant effect on the formation of an anodic oxide coating. For example, as shown in FIG. 6 , the top land 12 of the piston body 10A is held by a chuck 54 in a holding member 55 of a water jet treatment device 50, and water 52 is sprayed from a spray nozzle 51 onto the top ring groove 13 while the piston body 10A is rotated around its axis. By holding the top land 12 adjacent to the top ring groove 13 in this manner, the piston body 10A can be held in place even when the top ring groove 13 is subjected to high-pressure loads during the water jet treatment.

[0031] While surface treatment with mixed acid and laser irradiation are conceivable methods for removing exposed silicon, mixed acid treatment, depending on the conditions, may dissolve the aluminum base material along with the silicon, potentially deforming the shape of the top ring groove and other components. Laser irradiation significantly slows the film formation rate of the laser-irradiated aluminum alloy, affecting subsequent anodizing treatment, and also consumes a lot of energy and requires high equipment costs. In contrast, water jet treatment can suppress deformation of the shape of the top ring groove and other components, and can remove exposed silicon with low energy and equipment costs without affecting the subsequent anodizing process. Furthermore, water jet treatment can eliminate the need for degreasing, which is required before anodizing treatment.

[0032] In water jet processing, water may be sprayed onto the top ring groove 13 in a direction perpendicular to the central axis of the piston body 10A. However, the water is mainly sprayed directly onto the bottom surface of the top ring groove 13, and is rarely sprayed directly onto the top and bottom surfaces of the top ring groove 13, which are parallel to the spray direction. Therefore, as shown in FIG. 7, for example, it is preferable to tilt the spray direction A1 of the water 52a from the spray nozzle 51a toward the skirt portion Y with respect to a plane X parallel to the bottom surface 13c of the top ring groove 13. This allows the water 52a to be sprayed directly from the spray nozzle 51a onto the bottom surface 13c, the area of ​​the inner surface of the top ring groove 13 that comes into contact with the top ring 22, thereby efficiently removing silicon that would otherwise interfere with the formation of an anodized coating.

[0033] The inclination angle α of the spray direction A1 of the water 52a from the spray nozzle 51a with respect to the parallel plane X of the lower surface 13c can be set appropriately within a range from more than 0° to less than 90°. However, it is preferable to set the angle so that the following relationship holds when the width of the top ring groove 13 (the length of the bottom surface 13b in FIG. 7) is a and the depth of the top ring groove 13 (the length of the lower surface 13c in FIG. 7) is b:

[0034]

number

[0035] When the above relationship is satisfied, water 52a is sprayed directly from the spray nozzle 51a onto the entire underside 13c of the top ring groove 13, effectively removing silicon. For example, if the width and depth of the top ring groove are 1 mm and 3 mm, respectively, the above relationship is tan α≦1 / 3. In other words, the inclination angle α is greater than 0° and less than or equal to 18°.

[0036] Although the above description concerns the water jet treatment of the underside 13c of the top ring groove 13, the top side 13a of the top ring groove 13 is also in contact with the top ring 22. Therefore, by tilting the spray direction of the water 52b from the spray nozzle 51b in the opposite direction as shown in FIG. 7, the water 52b can be sprayed directly from the spray nozzle 51b onto the top side 13a. The tilt angle α in this case is the angle at which the spray direction of the water 52b from the spray nozzle 51b is tilted toward the piston crown surface 11 with respect to a plane parallel to the top side 13a of the top ring groove 13. The above relationship also holds true for the top side 13a, allowing the water 52b to be sprayed directly onto the entire top side 13a from the spray nozzle 51b.

[0037] Water sprayed directly onto the upper surface 13a or the lower surface 13c of the top ring groove 13 by water jet processing can also be reflected and remove silicon exposed on other inner surfaces, although this depends on the spray pressure of the water jet processing and the distance (spraying distance) from the spray nozzle 51 to the top ring groove 13. Furthermore, because the sprayed water is reflected, even if the spray direction of the water 52 from the spray nozzle 51 is parallel to the upper surface 13a or the lower surface 13c of the top ring groove 13, water sprayed directly onto the bottom surface 13b of the top ring groove 13 can be reflected and remove silicon exposed on the surfaces of the upper surface 13a and the lower surface 13c.

[0038] In addition to the tilt angle α, in the water jet treatment step 44, it is preferable to shift the spray direction A2 of the water 52e from the spray nozzle 51e relative to the radial direction R of the piston body 10A so as to oppose the direction of rotation of the piston body 10A caused by the retaining member (not shown), as shown in Fig. 8. This allows water to be sprayed directly onto the upper surface 13a or the lower surface 13c of the top ring groove 13, and the rotation of the piston body 10A increases the spray pressure onto the upper surface 13a or the lower surface 13c, compared to when water 52d is sprayed from the spray nozzle 51d in the radial direction R of the piston body 10A at the same spray pressure, thereby improving the silicon removal effect.

[0039] The directional deviation angle β of the spray direction A2 of the water 52e of the spray nozzle 51e relative to the radial direction R of the piston body 10A can be set appropriately within a range from more than 0° to less than 90°, but it is particularly preferable to set the angle so that the spray direction of the water 52f of the spray nozzle 51e is tangent to the outer periphery of the approximately cylindrical piston body 10A (i.e., perpendicular to the radial direction R2), as shown in Figure 9.

[0040] To explain this, as shown in Figure 9, when water 52d, 52e, and 52f are sprayed from a cylindrical spray nozzle at the inclination angle α, the water-jet-treated surfaces 53d, 53e, and 53f on the underside 13c of the top ring groove of the piston body 10A become elliptical. The pressure on these treated surfaces is strongest at the center of the ellipse and gradually weakens toward the periphery. Therefore, to reliably remove silicon exposed in the area of ​​the underside 13c where the top ring contacts, it is preferable to move the spray nozzle so that the center of these elliptical treated surfaces 53d, 53e, and 53f moves from one end of the area of ​​the underside 13c where the top ring contacts. Therefore, if the spray direction of water 52d from the spray nozzle is set to the radial direction R1 of the piston body 10A, moving the center of the elliptical treated surface 53d near the outer edge of the underside 13c will result in nearly half of the treated surface 53d hitting the second land, which does not require water-jet treatment. In contrast, when the spray direction A2 of the water 52f from the spray nozzle is set to the tangential direction T, even if the center of the elliptical treatment surface 53f is moved to the vicinity of the outer edge of the lower surface 13c, almost the entire area of ​​the treatment surface 53f hits the lower surface 13c, thereby enabling efficient water jet treatment.

[0041] The preferred range of the directional deviation angle β of the spray direction A2 of the water from the injection nozzle depends on the spray distance, but is preferably ±10° from the angle that is the tangent direction T of the outer periphery of the piston body 10A, and more preferably ±5°.

[0042] Although the above description concerns the treatment of the lower surface 13c of the top ring groove 13, the same effect can be obtained when treating the upper surface 13a by shifting the direction of water spray from the spray nozzle by a direction deviation angle β in a direction opposite to the rotational direction of the piston body 10A. Furthermore, water jet treatment is not limited to the inner surface of the top ring groove 13; it may also be performed on the inner surface of the second ring groove 15, the inner surface of the oil ring groove 17, and the surface of the skirt portion 18, as necessary.

[0043] The conditions for the water jet treatment are preferably determined by checking the removal state of silicon exposed on the inner surface of the top ring groove 13. For example, the spray pressure is preferably 50 to 200 MPa, the spray distance is 10 to 50 mm, the piston body rotation speed is 500 to 2000 rpm, and the spray nozzle movement speed is preferably in the range of 10 to 200 mm / min. The nozzle diameter is preferably in the range of 0.1 to 2.0 mm, but should be set taking into account the width of the top ring groove 13. To ensure that ultra-high-pressure water from the spray nozzle is efficiently sprayed into the top ring groove 13, the nozzle diameter is preferably equal to or less than the groove width (for a groove width of 1 mm, the nozzle diameter is 1 mm or less). Considering that the sprayed water does not travel in a completely linear fashion but spreads slightly fan-shaped between the spray nozzle and the top ring groove, the nozzle diameter is more preferably equal to or less than half the groove width (for a groove width of 1 mm, the nozzle diameter is 0.5 mm or less).

[0044] The anodizing process 45 involves immersing the aluminum alloy piston body 10A in a commonly used electrolyte such as sulfuric acid, phosphoric acid, or oxalic acid, and passing electricity through the piston body 10A as the anode and an electrode plate such as titanium or carbon as the cathode, thereby oxidizing the surface of the piston body 10A and forming an anodized film. Any electrolysis method, such as DC electrolysis, AC electrolysis, or AC / DC superimposed electrolysis, may be used, but AC / DC superimposed electrolysis is preferred because it can produce a smoother anodized film, has a faster film formation rate, and improves production efficiency.

[0045] FIG. 10 shows the change in the state of the inner surface of the top ring groove when an anodizing treatment process 45 is performed after the water jet treatment process 44 according to this embodiment. As shown in FIG. 10( a), before the water jet treatment process 44, the lower surface 13c of the top ring groove is the high-strength base material 30B, and silicon particles such as granular primary silicon 32 in the matrix 31 are exposed on the lower surface 13c. When the water jet treatment process 44 is performed on the lower surface 13c in this state, the exposed silicon is removed, resulting in the lower surface 13c having numerous depressions 34, as shown in FIG. 10( b). When the anodizing treatment process 45 is performed on the lower surface 13c in this state, the silicon that would affect the formation of the anodized film on the lower surface 13c is removed, and a smooth anodized film 35a is formed on the lower surface 13c, as shown in FIG. 10( c).

[0046] In contrast, Fig. 11 shows, as a comparative example, the change in the condition of the inner surface of the top ring groove when the anodizing treatment step 45 is performed without the water jet treatment step 44. As shown in Fig. 11(a), before the anodizing treatment step 45 is performed, silicon particles such as granular primary-crystal silicon 32 precipitated in the matrix 31 are exposed on the lower surface 13c of the top ring groove. When the anodizing treatment step 45 is performed on the lower surface 13c in this state, the silicon particles such as granular primary-crystal silicon 32 exposed on the lower surface 13c inhibit the oxidation of aluminum and the growth of the coating. In particular, depressions are formed on the surface of the coating where the coarse primary-crystal silicon 32 is present, resulting in the formation of an anodized coating 35b with high surface roughness on the lower surface 13c.

[0047] With conventional aluminum alloy compositions, it is possible to obtain a smooth anodic oxide film by anodizing using AC / DC superimposed electrolysis as described above, even if granular primary silicon is exposed. However, with aluminum alloy compositions such as high-strength materials, coarser primary silicon precipitates than conventional ones, so sufficient smoothness cannot be obtained even with AC / DC superimposed electrolysis. In this embodiment, even if coarse primary silicon is exposed on the surface to be treated, silicon is removed in the water jet treatment step 44 as shown in Figure 10, so a smooth anodic oxide film can be formed.

[0048] The resin coating process 46 is an optional process for forming a resin coating on the outer surface of the skirt portion 18 of the piston main body 10A. For example, the resin coating can be formed by applying a resin coating agent to the outer surface of the skirt portion 18 by a spraying method, a screen printing method, or the like, and then baking the resin coating. The resin coating agent can be a known agent used for the skirt portion 18 of pistons for internal combustion engines. For example, a resin coating agent recently developed to achieve both wear resistance and friction reduction in a resin coating (such as a resin coating agent that uses a different base resin from the conventionally widely used polyamideimide or one that contains microhard particles) may also be used. Note that if both seizure resistance and friction reduction can be achieved without a resin coating, the resin coating process 46 need not be performed.

[0049] According to the manufacturing method 40 for an internal combustion engine piston of this embodiment, the anodized coating 35a formed on the inner surface of the piston ring groove 13 can be made smooth, for example, with a surface roughness Rpk of 1.0 μm or less, or with both a surface roughness Ra and a surface roughness Rpk of 1.0 μm or less, even when the piston body 10A is made of a high-strength aluminum alloy. Surface roughness Ra conforms to JIS B0601-2001 and is an index relating to the arithmetic mean roughness of a profile curve. Surface roughness Rpk conforms to JIS B0671-2000 and is an index relating to the average height of the peaks above the core of the roughness curve, allowing for evaluation of the airtightness of the anodized coating. Forming an anodized coating with such surface roughness improves sealing with the piston ring, reducing blow-by gas and improving fuel economy. Furthermore, PM, a substance subject to environmental regulations due to oil ingress, and its quantity, PN, can be suppressed.

[0050] The method for manufacturing a piston for an internal combustion engine according to the present invention is not limited to the above embodiment, and may include interchangeable steps, omission of steps, or other steps. For example, after the machining step 43, the resin coating step 46, the water jet treatment step 45, and the anodizing step 46 may be performed in this order. Alternatively, the water jet treatment step 45 may be performed on the skirt portion in addition to the ring groove, and then the resin coating step 46 and the anodizing step 45 may be performed in this order. Also, as mentioned above, the resin coating step 46 may be omitted. [Example]

[0051] To confirm the silicon removal effect of water jet treatment, the piston skirt was treated with water jet treatment to simulate a piston ring groove. The piston body was made from an Al-Si-Cu-Ni aluminum alloy (hereinafter referred to as high-strength material) with the composition shown in Table 1.

[0052] [Table 1]

[0053] The skirt portion of the piston body was then subjected to water jet treatment at a jetting pressure of 160 MPa, and the treated surface was observed (300x) with a scanning electron microscope (SEM), and the silicon distribution was analyzed with an energy dispersive X-ray spectrometer (EDS) (Example 1). To evaluate these results, the skirt portion before the water jet treatment was also similarly observed and analyzed (Comparative Example 1). SEM photographs of the observation results are shown in Figs. 12 and 13.

[0054] FIG. 12 is an SEM photograph showing the surface of the high-strength material of Comparative Example 1 before water jet processing. The surface was flat, with no particular irregularities observed. In contrast, FIG. 13 shows the surface of the high-strength material of Example 1 after water jet processing, and it was confirmed that numerous depressions had been formed. These are believed to be areas where silicon had been removed in chunks by the water jet processing. The EDS analysis results showed that silicon chunks measuring several tens of micrometers were detected on the smooth surface of Comparative Example 1, and these areas were primary crystal silicon. On the other hand, the EDS analysis results of Example 1 showed that silicon was detected in areas corresponding to the depths of the depressions observed in the SEM photograph. From this, it is inferred that while some primary crystal silicon remained deep in the depressions, the surface portion had been removed. This suggests that the amount of silicon removed can be controlled by the water jet processing conditions (particularly, the water spray direction from the spray nozzle, the spray pressure, the nozzle diameter, etc.).

[0055] Next, a test was conducted to confirm the smoothness of the anodized coating formed after water jet treatment. First, a top ring groove (groove width: 1 mm, groove depth: 3 mm) was machined on the same piston body as above, and water jet treatment was performed on the inner surface of the top ring groove. The water jet treatment conditions were a spray pressure of 160 MPa and a nozzle diameter of 0.25 mm (one-quarter the groove width). Then, anodizing treatment was performed on the inner surface of the top ring groove. The anodizing treatment conditions were as follows: 18 vol% sulfuric acid was used as the electrolyte, a titanium-based material was attached as the cathode plate, and the piston body was attached as the anode. The electrolysis method was AC / DC superimposed electrolysis, and the anodizing treatment was performed for 40 seconds using constant voltage electrolysis set at a positive voltage of 65 V, a negative voltage of -2 V, and a frequency of 12 kHz (Example 2).

[0056] The cross-sectional film thickness, surface roughness Ra, and surface roughness Rpk of the anodic oxide coating formed in Example 2 were measured. The cross-sectional film thickness was determined by observing the cross section of the underside of the top ring groove with an optical microscope (400x magnification) in two directions, the front side and the rear side of the internal combustion engine of the piston, measuring 10 points at 30 μm intervals for each direction, and averaging the values ​​from those 20 points.

[0057] For comparison, an anodizing treatment was performed in the same manner as in Example 2, except that water jet treatment was not performed (Comparative Example 2). The cross-sectional film thickness, surface roughness Ra, and surface roughness Rpk of the anodized film formed in Comparative Example 2 were measured in the same manner as in Example 2. The measurement results of the cross-sectional film thickness, surface roughness Ra, and surface roughness Rpk for Example 2 and Comparative Example 2 are shown in Table 2. As a reference example, similar measurements were performed on an anodized film formed on a commercially available piston ring groove made by another manufacturer, and the results are also shown in Table 2. Optical microscope photographs of the cross section of the lower surface of the top ring groove taken during measurement are shown in Figs. 14 and 15.

[0058] [Table 2]

[0059] As shown in Table 2, the thicknesses of the anodized films in Comparative Example 2 and Example 2 were comparable. In Comparative Example 2, the surface roughness Ra was 1.5 μm and the surface roughness Rpk was 1.8 μm, while in Example 2, the surface roughness Ra was 1.2 μm and the surface roughness Rpk was 0.4 μm. This indicates that the surface of the anodized film was smoother with fewer protrusions as a result of the water jet treatment, which removed a portion of the primary silicon in advance. Additionally, as a reference example, the cross-sectional film thickness, surface roughness Ra, and surface roughness Rpk of an anodized film formed on the top ring groove of a piston manufactured by another manufacturer were similarly measured. The results are shown in Table 2. Note that the cross-sectional film thickness is shown in parentheses because the measurement method was different. The surface roughness Ra of this anodized film, which was likely formed by DC electrolysis, was 2.1 μm and the surface roughness Rpk was 2.2 μm. It can be seen that the anodic oxide film of Example 2, which was produced by AC / DC superimposed electrolysis after water jet treatment, had a significantly smoother surface.

[0060] This can also be seen from the cross-sectional photographs of Comparative Example 2 and Example 2. FIG. 14 is a cross-sectional photograph of Comparative Example 2, and FIG. 15 is a cross-sectional photograph of Example 2. Visible above the anodic oxide films 35a and 35b is the potting resin 36 used for observation with an optical microscope, and the boundary between them is indicated by a dotted line. As shown in FIG. 14, in Comparative Example 2, the anodic oxide film 35b barely grew on a portion of the primary silicon 32 in the high-strength material matrix 31, resulting in unevenness on the surface. In contrast, as shown in FIG. 15, the anodic oxide film 35a of Example 2 had a smoother surface than that of Comparative Example 2.

[0061] 15, there are depressions 34 at the interface between the high-strength material matrix 31 and the anodized film 35a, which are presumably caused by the removal of silicon in chunks by the water jet process, and it can be seen that an anodized film has grown in those locations. This is presumably a characteristic feature that has emerged when the water jet process removes the surface portion of the primary silicon, creating gaps between the silicon and aluminum, and when the anodized process allows the electrolyte to seep into those gaps and anodize the surrounding aluminum, the cells that make up the anodized film grow in a way that wraps around the silicon in the AC / DC superimposed electrolysis method.

[0062] In Examples 1 and 2, the water jetting direction from the injection nozzle for the water jet treatment was perpendicular to the central axis of the piston body, i.e., parallel to the underside of the top ring groove. However, it is believed that it is possible to remove more primary silicon and obtain a smoother anodized coating, for example, one having a surface roughness Ra and a surface roughness Rpk of 1.0 μm or less, by injecting water at an inclination angle α with respect to the parallel direction described above, or by injecting water at a misalignment angle β with respect to the radial direction of the piston body. [Explanation of symbols]

[0063] 10. Piston for internal combustion engine 10A Piston body 13 Top ring groove 20 Cylinder block 22 Top ring 30A Conventional Aluminum Alloy 30B high strength aluminum alloy 31 Matrix 32 Primary silicon 34 Recess 35a, 35b Anodic oxide film 40 Manufacturing method of piston for internal combustion engine 50 Water jet processing equipment 51 Injection nozzle 52 water 53 Construction side 54 Chuck 55 Retaining member

Claims

1. A method for manufacturing a piston for an internal combustion engine, comprising: a piston body for an internal combustion engine having an aluminum alloy base material containing 5.0 to 20.0% by mass of Si, more than 1.3% by mass but not more than 5.0% by mass of Cu, and more than 1.5% by mass but not more than 3.5% by mass of Ni, the piston body having a top ring groove on its outer peripheral surface, performing water jet processing on at least a region of an inner surface of the top ring groove that comes into contact with a top ring while rotating the piston body for an internal combustion engine about its axis in a state where the portion of the outer peripheral surface between the top ring groove and the piston crown surface is held by a holding member, thereby removing silicon exposed in the region; forming an anodized coating on at least the area subjected to the water jet treatment; A method for manufacturing a piston for an internal combustion engine, comprising:

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

3. 3. The method for manufacturing a piston for an internal combustion engine according to claim 1, wherein the water jet treatment is performed by tilting the direction of water spray from the spray nozzle toward the piston crown surface side or toward the skirt surface side, relative to a parallel plane of one of the inner surfaces of the top ring groove, the inner surface facing the piston crown surface side or the inner surface facing the skirt surface side, on the opposite side.

4. 4. The method for manufacturing a piston for an internal combustion engine according to claim 3, wherein the water jet treatment is performed by shifting the direction of water spray from the spray nozzle in the water jet treatment in a direction opposite to the rotational direction of the piston body relative to the radial direction of the piston body.

5. 5. The method for manufacturing a piston for an internal combustion engine according to claim 4, wherein the water jet treatment is performed by moving the water injection direction of the injection nozzle so that the water injection direction of the injection nozzle includes a tangential direction of an outer periphery of the piston body, which has a substantially cylindrical shape.

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