JP2023178766A - Method of manufacturing piston for internal combustion engine - Google Patents

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

The present invention relates to a method of manufacturing a piston for an internal combustion engine, and more particularly, to a method of manufacturing a piston for an internal combustion engine that includes an anodized film on the inner surface of a top ring groove.

In recent years, in response to environmental regulations, there have been increasing demands for higher efficiency and higher compression ratios in internal combustion engines such as automobile engines, as well as demands for supercharged engines, and the maximum combustion pressure of internal combustion engines has increased. . Due to this background, the temperature of the piston ring groove (especially the top ring groove) and land portion (especially the top land, second land) of a piston for an internal combustion engine is higher than that of a conventional internal combustion engine of the same displacement. is also getting higher. An anodized film is sometimes 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 that is the base material of the piston, and has excellent wear resistance.

As such an anodic oxide film, for example, Patent Document 1 discloses that an anodic oxide film is provided on the inner surface of the top ring groove, at least on the inner surface on the second ring groove side, in the area where the top ring contacts, A piston for internal combustion engines is described in which the anodic oxide film has a surface roughness Rpk of 1.00 μm or less according to JIS B0671-2. It is stated that the performance can be improved, and the blow-by gas flow rate and the number of discharged particulates can be reduced.

JP2020-204287A

However, AC8A (Al-Si-Cu-Ni- It may contain more additive elements such as copper (Cu) and nickel (Ni) than conventional aluminum alloys such as Mg-based alloys. In such high-strength materials, more granular and coarse primary crystal silicon (Si) is precipitated than in the past (Si grain size is approximately 30 to 40 μm), and such granular Si can cause damage to the anodic oxide film. It has a strong influence on film formation, and a film with large irregularities is likely to be formed. The inventor of the present application found that when such a high-strength material is used, even if the anodization treatment described in Patent Document 1 is performed, the surface roughness Rpk is 1 when the thickness of the anodic oxide film is 10 μm. A new finding was obtained that the film thickness exceeds .0 μm.

When the surface roughness of the anodic oxide film deteriorates, the sealing performance with the top ring deteriorates, resulting in an increase in blow-by gas and deterioration in fuel efficiency. In addition, a phenomenon in which engine oil used to lubricate engine parts flows into the combustion chamber (oil upflow) is likely to occur, and as engine oil burns within the combustion chamber, PM (Particulate Matter) and PN (Particle Number ), which is a factor that increases the number of substances subject to European environmental regulations, such as substances, so the surface of the anodic oxide film is required to be smooth.

In view of the above problems, the present invention aims to maintain good sealing between the piston ring and the anodic oxide film formed on the inner surface of the top ring groove even if a high-strength material is used for the piston base material. An object of the present invention is to provide a method for manufacturing a piston for an internal combustion engine.

In order to achieve the above object, the present invention provides a method for manufacturing a piston for an internal combustion engine, comprising 5.0 to 20.0 mass% Si, more than 1.3 mass% and 5.0 mass% or less. Regarding a piston body for an internal combustion engine, the base material is an aluminum alloy containing more than 1.5% by mass and 3.5% by mass of Ni, and has a top ring groove on the outer peripheral surface. While rotating the piston body for an internal combustion engine around its axis while holding the portion of the outer circumferential surface between it and the piston crown surface with a holding member, at least the area of the inner surface of the top ring groove that is in contact with the top ring is The method includes a step of performing water jet treatment to remove silicon exposed in the area, and a step of forming an anodic oxide film on at least the area subjected to the water jet treatment.

As described above, according to the present invention, by performing water jet treatment on the inner surface of the top ring groove before anodizing treatment, silicon exposed on the surface is removed, resulting in a smooth anodic oxide film. is formed, and good sealing performance with the piston ring can be maintained.

FIG. 2 is a front view schematically showing an example of a piston for an internal combustion engine. FIG. 2 is an enlarged sectional view schematically showing the vicinity of a top ring groove of the internal combustion engine piston shown in FIG. 1. FIG. It is an optical micrograph showing an example of a metal structure when a conventional aluminum alloy is used as a base material of a piston for an internal combustion engine. 1 is an optical micrograph showing an example of a metal structure when an aluminum alloy (high strength material) is used as a base material of a piston for an internal combustion engine according to the present invention. 1 is a flow diagram showing an embodiment of a method for manufacturing a piston for an internal combustion engine according to the present invention. FIG. 2 is a front view schematically showing an example of a water jet treatment step in the method for manufacturing a piston for an internal combustion engine according to the present invention. 7 is an enlarged schematic diagram illustrating the water jet direction (inclination angle α) of the jet nozzle in the water jet treatment step shown in FIG. 6. FIG. 7 is a schematic diagram illustrating the water jet direction (direction deviation angle β) of the jet nozzle in the water jet treatment step shown in FIG. 6. FIG. 7 is an enlarged schematic diagram illustrating the water jet direction (tangential direction T) of the jet nozzle in the water jet treatment step shown in FIG. 6. FIG. FIG. 3 is a schematic diagram illustrating a change in the state of the inner surface of a top ring groove due to an anodizing process in the method for manufacturing a piston for an internal combustion engine according to the present invention. FIG. 6 is a schematic diagram illustrating a change in the state of the inner surface of a top ring groove when an anodizing process is performed without performing a water jet process as a comparative example. 3 is a SEM photograph showing the surface of the high-strength material before water jet treatment of Comparative Example 1. 1 is a SEM photograph showing the surface of a high-strength material after water jet treatment in Example 1. 3 is an optical microscope photograph showing a cross section of an anodized film of Comparative Example 2. 3 is an optical micrograph showing a cross section of the anodic oxide film of Example 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a method for manufacturing a piston for an internal combustion engine according to the present invention will be described below 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.

First, a piston for an internal combustion engine manufactured by the method of this embodiment will be described. As shown in FIG. 1, the piston 10 for an internal combustion engine has a top ring groove 13, a second ring groove, and a second ring groove formed on the outer circumferential surface facing the bore inner circumferential surface 21 of the cylinder block, in order from the piston crown surface 11 side. Three ring grooves are formed: 15 and an oil ring groove 17. A top ring (not shown) is fitted into the top ring groove 13, a second ring (not shown) is fitted into the second ring groove 15, and an oil ring (not shown) is fitted into the oil ring groove 17. Further, on the outer circumferential surface of the piston 10, the area between the piston crown surface 11 and the top ring groove 13 is called a top land 12, the area between the top ring groove 13 and the second ring groove 15 is called a second land 14, and the second ring groove 15 and the oil ring groove 17 is called a third land 16, and the area after the oil ring groove 17 is called a skirt portion 18.

To explain the top ring groove 13 in more detail, as shown in FIG. The outer circumferential surface protrudes outward from the top land 12 and second land 14, which are the outer circumferential surfaces of the piston. The top ring 22 is pressed against the inner circumferential surface 21 of the bore of the cylinder block 20 due to its elastic force, and the top ring 22 functions to maintain airtightness of the combustion chamber.

The second ring (not shown) has the same structure and function, but the compression and expansion steps of the piston create high pressure in the combustion chamber on the piston crown surface 11 side, and the inner surface of the top ring groove 13 in particular comes into close contact with the top ring 22. Therefore, wear easily occurs on the inner surface of the top ring groove 13. Therefore, an anodic oxide film (not shown) is formed on the inner surface of the top ring groove 13 to improve the wear resistance of the inner surface of the top ring groove 13. Among the inner surfaces of the top ring groove 13, the inner surface on the piston crown surface 11 side is called the upper surface 13a, the inner surface on the opposite side (second ring side) is called the lower surface 13c, and the inner surface on the bottom side of the groove between them is called the bottom surface 13b. It is called. As shown in FIG. 2, since the top ring 22 does not always come into contact with the entire inner surface of the top ring groove 13, the anodic oxide film is formed on the area of the inner surface of the top ring groove 13 that the top ring 22 contacts. It would be fine if it had been done. Although this area differs depending on the design of the piston, for example, the length from the end of the top surface 13a or bottom surface 13c of the top ring groove 13 on the top land 12 or second land 14 side to the end on the bottom surface 13b side (i.e., the length of the groove Assuming that the depth is 100%, the area is preferably at least 90% from the end on the top land 12 or second land 14 side, more preferably at least 80%, and still more preferably at least 70%. Of course, an anodized film may be formed on the entire inner surface of the top ring groove 13. Of the inner surface of the top ring groove 13, whether to use only the upper surface 13a, only the lower surface 13c, or both the upper surface 13a and the lower surface 13c, and how large the area should be, depends on the sealing performance against combustion gas and the amount of blow-by gas. It is desirable to make a decision in consideration of requirements such as reduction and suppression of PM and PN that cause oil buildup.

The internal combustion engine piston body on which the anodized film is formed contains 5.0 to 20.0 mass% silicon (Si), more than 1.3 mass% and 5.0 mass% copper (Cu), and It is formed using an aluminum alloy (hereinafter also referred to as a high-strength material) containing nickel (Ni) in an amount exceeding 1.5% by mass and not more than 3.5% by mass as a base material.

In high-strength materials, Si is a component that crystallizes as primary silicon or eutectic silicon and improves heat resistance and wear resistance. Moreover, Si lowers the coefficient of thermal expansion. If the Si content is 5.0% by mass or more, the coefficient of thermal expansion is low, and wear resistance and strength in high temperature ranges can be improved. If the Si content is 20.0% by mass or less, primary silicon is This makes it possible to improve the elongation of the alloy. The Si content is more preferably 10.0 to 13.0% by mass.

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

Ni is a component that mainly improves strength and wear resistance in high temperature ranges and reduces the coefficient of thermal expansion, and if the Ni content exceeds 1.5% by mass, the effect is suitably expressed, If it is 3.5% by mass or less, good elongation can be obtained.

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

Cr is a component that mainly strengthens the grain boundaries between crystal grains of intermetallic compounds, primary silicon, acicular silicon, etc. crystallized in the alloy, and improves strength in high temperature ranges. When the content is 0.05% by mass or more, grain boundaries are suitably strengthened and strength in a high temperature range is improved, and when the content is 0.15% by mass or less, good toughness and machinability can be obtained. .

Ti is a component that mainly refines crystal grains and improves heat resistance, castability, and strength, and this effect is suitable if the Ti content is in the range of 0.05 to 0.20% by mass. It is expressed in The Ti content is preferably 0.05 to 0.15 wt%.

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 the Zr content is in the range of 0.05 to 0.30% by mass. If so, the effect will be suitably expressed. The Zr content is preferably 0.05 to 0.15 wt%.

Fe is a component that mainly crystallizes intermetallic compounds and improves wear resistance and strength in high temperature ranges. Note that if the size of this intermetallic compound is large, the strength will decrease. If the Fe content is in the range of 0.10 to 0.31% by mass, the size of the Fe--Mn-based intermetallic compound can be reduced.

Mn is a component that mainly crystallizes intermetallic compounds and improves wear resistance and strength in high temperature ranges. Note that if the size of this intermetallic compound is large, the strength will decrease. If the Mn content is 0.05% by mass or less, the size of the Fe--Mn-based intermetallic compound can be reduced. The lower limit of the Mn content may not be contained at all, or may be contained in a very small amount comparable to an impurity, for example, 0.001% by mass.

Mg is a component that mainly improves strength and toughness, and when the Mg content is 0.5% by mass or more, the effect of improving strength is expressed, and when it is 1.1% by mass or less, it has a good effect. Provides toughness.

When piston materials are manufactured using aluminum alloys, which are known as high-strength materials, as the base material, coarse granular primary crystals are produced during the manufacturing process, compared to when manufacturing piston materials using conventionally used aluminum alloys. A large amount of silicon will precipitate. FIG. 3 is an optical micrograph of the metal structure of the piston base material when a conventional aluminum alloy (AC8A) is used, and FIG. This is an optical micrograph of the metal structure of the piston base material when a piston base material (composition containing a large amount of As shown in FIG. 3, although the conventional aluminum alloy 30A also has granular primary silicon 32 precipitated in the matrix 31, as shown in FIG. It can be seen that the granular primary silicon 32 precipitated in the matrix 31 is significantly large.

When an aluminum alloy on which granular primary crystal silicon is precipitated is anodized by direct current electrolysis, the cells that make up the anodic oxide film grow in one direction on the surface of the treatment target. This inhibits the growth of the anodic oxide film, resulting in the formation of a film with large surface irregularities. On the other hand, in the AC/DC superimposition electrolysis method, cells grow in random directions with respect to the surface of the treatment target and have no orientation, so even granular primary silicon particles precipitated in the matrix are grown in random directions. Since it grows in a branched state and encapsulates it, it is possible to obtain an anodic oxide film with a smooth surface. However, the grain size of the primary crystal silicon used in high-strength materials is extremely large, approximately 30 to 40 μm, and even if AC/DC superposition electrolysis is used, the surface roughness Rpk of the resulting anodic oxide film exceeds 1.0 μm, resulting in piston ring There is a problem in that it is not possible to maintain good sealing performance with.

In order to solve such problems, a method 40 for manufacturing a piston for an internal combustion engine according to the present embodiment includes a step 41 of casting a piston body, a step 42 of heat-treating the cast piston body, as shown in FIG. , a step 43 of machining the heat-treated piston body, a step 44 of performing water jet treatment on the inner surface of the top ring groove of the piston body, a step 45 of performing anodizing treatment on the inner surface of the top ring groove, and a skirt portion of the piston. Step 46 of applying a resin coat to the substrate is sequentially performed. Each step will be explained below.

The casting, heat treatment, and machining steps 41, 42, and 43 described above are similar to those used in manufacturing a general piston for an internal combustion engine. For example, the casting process 41 generally involves a gravity casting method in which molten high-strength material (molten aluminum) is poured into a mold having a piston-shaped cavity. The heat treatment process 42 includes, for example, T5 treatment, T6 treatment, etc. T5 treatment is a process in which only artificial age hardening treatment is performed after casting (the purpose is to increase strength, stabilize dimensions, etc.), and T6 treatment is a process in which only artificial age hardening is performed after casting. , solution treatment, and artificial age hardening treatment (to increase strength and hardness, etc.). In the machining step 43, a ring groove such as a top ring groove is created by cutting, for example. The width and depth of the top ring groove are similar to the dimensions of a top ring groove of a typical internal combustion engine piston.

The water jet treatment step 44 removes coarse silicon particles exposed on the inner surface of the top ring groove created in the machining step 43, which has a strong influence on the formation of the anodic oxide film. The purpose is to For example, as shown in FIG. 6, the top land 12 of the piston body 10A is held by the holding member 55 of the water jet treatment device 50 with the chuck 54, and the injection nozzle is rotated while the piston body 10A is rotated about its axis. Water 52 is sprayed from 51 to the top ring groove 13. By holding the top land 12 adjacent to the top ring groove 13 in this way, the piston body 10A can be held well even when the top ring groove 13 is subjected to high pressure load due to water jet treatment.

In addition, surface treatment with a mixed acid or laser irradiation may be considered as a method for removing exposed silicon, but depending on the conditions, mixed acid treatment may dissolve the silicon and the aluminum base, resulting in the shape of the top ring groove etc. There is a risk of deformation. Laser irradiation affects the subsequent anodic oxidation treatment, such as significantly slowing down the film formation rate of the aluminum alloy that has been irradiated with the laser, and also consumes a large amount of energy and increases equipment costs. On the other hand, water jet treatment can suppress changes in the shape of top ring grooves, etc., and can remove exposed silicon with low energy and equipment costs without affecting the subsequent anodizing process. I can do it. Furthermore, by performing water jet treatment, it is also possible to eliminate the need for degreasing, which is necessary before anodizing treatment.

In the water jet treatment, water may be sprayed onto the top ring groove 13 from a direction perpendicular to the central axis of the piston body 10A, but the water is mainly sprayed directly onto the bottom surface of the top ring groove 13. , the upper and lower surfaces of the top ring groove 13, which are positioned parallel to the spray direction, are hardly sprayed directly. Therefore, for example, as shown in FIG. 7, it is preferable that the injection direction A1 of the water 52a from the injection nozzle 51a is inclined in the direction Y toward the skirt portion with respect to the parallel plane X of the lower surface 13c of the top ring groove 13. As a result, the water 52a is directly sprayed from the spray nozzle 51a onto the lower surface 13c of the inner surface of the top ring groove 13, which is the area in contact with the top ring 22, so that silicon, which affects the formation of the anodic oxide film, is efficiently removed. can be removed.

The inclination angle α of the injection direction A1 of the water 52a of the injection nozzle 51a with respect to the parallel plane When the length of the top ring groove 13 (the length of the bottom surface 13c in FIG. 7) is a, and the depth of the top ring groove 13 (the length of the bottom surface 13c in FIG. 7) is b, it is preferable to set an angle that satisfies the following relational expression.

When the above relational expression is established, the water 52a is directly sprayed from the spray nozzle 51a onto the entire lower surface 13c of the top ring groove 13, and silicon can be efficiently removed. As a specific example, when the top ring groove has a width of 1 mm and a depth of 3 mm, the above relational expression satisfies tanα≦1/3. That is, the inclination angle α is greater than 0° and less than or equal to 18°.

Although we have described the case where the lower surface 13c of the top ring groove 13 is subjected to water jet treatment, since the upper surface 13a of the top ring groove 13 is also an area in contact with the top ring 22, as shown in FIG. By tilting the jetting direction of the water 52b from the jetting nozzle 51b in the opposite direction, the water 52b can be directly sprayed from the jetting nozzle 51b onto the upper surface 13a. The inclination angle α in this case is an angle at which the direction in which the water 52b is injected from the injector nozzle 51b is inclined toward the piston crown surface 11 side with respect to the parallel plane of the upper surface 13a of the top ring groove 13. Moreover, the above relational expression is similarly established in the case of the upper surface 13a, and the water 52b can be directly sprayed from the spray nozzle 51b onto the entire upper surface 13a.

In addition, the water sprayed directly onto the upper surface 13a or lower surface 13c of the top ring groove 13 in the water jet treatment in this manner is determined by the injection pressure of the water jet treatment and the distance from the injection nozzle 51 to the top ring groove 13 (spray distance). However, silicon that is reflected and exposed on other inner surfaces can also be removed. In addition, since the water sprayed in this way is reflected, even if the direction in which the water 52 is jetted from the jet nozzle 51 is parallel to the upper surface 13a or the lower surface 13c of the top ring groove 13, the top ring groove 13 It is possible to remove the silicon exposed on the upper surface 13a and lower surface 13c by reflecting water directly sprayed onto the bottom surface 13b.

In the water jet treatment step 44, in addition to the above-mentioned inclination angle α, as shown in FIG. 8, the injection direction A2 of the water 52e of the injection nozzle 51e is set to It is preferable to shift the piston body 10A in a direction opposite to the rotational direction of the piston body 10A by a holding member (not shown). As a result, water can be directly sprayed onto the upper surface 13a or lower surface 13c of the top ring groove 13, and compared to the case where the water 52d is sprayed from the spray nozzle 51d in the radial direction R of the piston body 10A at the same injection pressure, The rotation of the piston main body 10A increases the injection pressure to the upper surface 13a or the lower surface 13c, making it possible to enhance the silicon removal effect.

The direction deviation angle β of the injection direction A2 of the water 52e of the injection nozzle 51e with respect to the radial direction R of the piston body 10A can be appropriately set in the range of more than 0° to less than 90°, but in particular, as shown in FIG. It is preferable that the jetting direction of the water 52f be at an angle that corresponds to the tangential direction T of the outer periphery of the substantially cylindrical piston body 10A (that is, the direction perpendicular to the radial direction R2).

To explain this, first, as shown in FIG. 9, when the water 52d, 52e, 52f is sprayed from the cylindrical injection nozzle at the above-mentioned inclination angle α, water jet treatment is performed on the lower surface 13c of the top ring groove of the piston body 10A. The construction surfaces 53d, 53e, and 53f have an elliptical shape. The pressure on this construction surface is strongest at the center of the ellipse and gradually weakens toward the outer periphery, so in order to reliably remove the silicon exposed in the area where the top ring contacts the bottom surface 13c, It is preferable to move the injection nozzle so that the centers of the elliptical construction surfaces 53d, 53e, and 53f are moved from one end of the region in contact with the top ring of the lower surface 13c to the other. Therefore, when the spray direction of the water 52d from the injection nozzle is set to the radial direction R1 of the piston body 10A, when the center of the elliptical construction surface 53d is moved to the vicinity of the outer edge of the lower surface 13c, nearly half of the area of the construction surface 53d is This results in a second land that does not require water jet treatment. On the other hand, if the jetting direction A2 of the water 52f from the jetting nozzle is set to the above-mentioned tangential direction T, even if the center of the elliptical construction surface 53f is moved to the vicinity of the outer edge of the lower surface 13c, almost all of the construction surface 53f is The area corresponds to the lower surface 13c. Therefore, water jet treatment can be performed efficiently.

The preferable range of the deviation angle β of the water spray direction A2 of the injection nozzle depends on the spraying distance, but for example, it is preferably ±10° from the angle that is the tangential direction T of the outer circumference of the piston body 10A, and is preferably ±5°. It is more preferable that

Although the case of treating the lower surface 13c of the top ring groove 13 has been described, when treating the upper surface 13a, the direction of the water jet from the jet nozzle is similarly shifted to the direction opposite to the rotational direction of the piston body 10A. A similar effect can be obtained by shifting the angle β. Further, the water jet treatment is not limited to the inner surface of the top ring groove 13, but may 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, if necessary.

It is desirable to determine the conditions for the water jet treatment by checking the state of removal of silicon exposed on the inner surface of the top ring groove 13. For example, it is preferable that the injection pressure is 50 to 200 MPa, the spray distance is 10 to 50 mm, the rotation speed of the piston body is 500 to 2000 rpm, and the movement speed of the injection nozzle is 10 to 200 mm/min. Further, the nozzle diameter is preferably set in a range of 0.1 to 2.0 mm, for example, but it is desirable to set the nozzle diameter in consideration of the width of the top ring groove 13. In order to efficiently spray ultra-high pressure water from the injection nozzle into the top ring groove 13, the nozzle diameter is preferably equal to or less than the groove width (if the groove width is 1 mm, the nozzle diameter is 1 mm or less), Considering that the injected water spreads slightly in a fan shape between the injection nozzle and the top ring groove, the nozzle diameter is less than half the width of the groove (if the groove width is 1 mm, the nozzle The diameter is more preferably 0.5 mm or less.

In the anodizing process 45, the piston body 10A made of aluminum alloy is immersed in a commonly used electrolytic solution such as sulfuric acid, phosphoric acid, or oxalic acid, and the piston body 10A is used as an anode and an electrode made of titanium or carbon. This is a step in which the surface of the piston body 10A is oxidized to form an anodic oxide film by applying electricity using the plate as a cathode. As for the electrolysis method, any electrolysis method such as direct current electrolysis, alternating current electrolysis, or AC/DC superimposed electrolysis may be used, but it is possible to obtain a smoother anodic oxide film, and the film formation rate is fast, resulting in lower manufacturing efficiency. It is preferable to use the AC/DC superimposed electrolysis method because it can increase the .

FIG. 10 shows changes in the state of the inner surface of the top ring groove when the anodizing treatment step 45 is performed after the water jet treatment step 44 according to the present embodiment. As shown in FIG. 10(a), the lower surface 13c of the top ring groove before the water jet treatment step 44 is a high-strength base material 30B, and silicon particles such as granular primary silicon 32 in the matrix 31 are on the lower surface. 13c is exposed. When the lower surface 13c in such a state is subjected to a water jet treatment step 44, the exposed silicon is removed, leaving the lower surface 13c with a large number of depressions 34, as shown in FIG. 10(b). When the lower surface 13c in this state is subjected to the anodic oxidation treatment step 45, the silicon that affects the formation of the anodic oxide film on the lower surface 13c is removed, so that the lower surface 13c is An anodic oxide film 35a with a smooth surface is formed.

On the other hand, FIG. 11 shows a change in the state of the inner surface of the top ring groove when the anodizing treatment step 45 was performed without performing the water jet treatment step 44 as a comparative example. As shown in FIG. 11A, silicon particles such as granular primary silicon 32 precipitated in the matrix 31 are exposed on the bottom surface 13c of the top ring groove before the anodizing process 45 is performed. When the lower surface 13c in this state is subjected to the anodic oxidation treatment step 45, the oxidation of aluminum and the growth of the film are inhibited by silicon particles such as the granular primary silicon 32 exposed on the lower surface 13c. Since a recess is generated on the film surface at a certain location of 32, an anodic oxide film 35b having a large surface roughness is formed on the lower surface 13c.

With a conventional aluminum alloy composition, it is possible to obtain an anodic oxide film with a smooth surface even if granular primary silicon is exposed by anodizing treatment using the AC/DC superposition electrolysis method as described above. In the case of aluminum alloy compositions such as high-strength materials, coarser primary crystal silicon is precipitated than before, and therefore sufficient smoothness cannot be obtained even with the AC/DC superposition electrolysis method. In this embodiment, even if coarse primary silicon is exposed on the surface to be treated, the silicon is removed in the water jet treatment step 44 as shown in FIG. 10, so that an anodic oxide film with a smooth surface is formed. be able to.

The resin coating treatment step 46 is an optional step of forming a resin coat on the outer surface of the skirt portion 18 of the piston body 10A. For example, the resin coat can be formed by applying a resin coating agent to the outer surface of the skirt portion 18 using a spray method, screen printing method, or the like, and then baking the resin coat agent. As the resin coating agent, a known agent used for the skirt portion 18 of a piston for an internal combustion engine can be used. It is also possible to use a resin coating agent (one in which the base resin is changed from the conventionally widely used polyamideimide, one in which fine hard particles are added, etc.). Note that if it is possible to achieve both seizure resistance and friction reduction without the resin coating, the resin coating treatment step 46 may not be performed.

As described above, according to the method 40 for manufacturing a piston for an internal combustion engine according to the present embodiment, the anodized film 35a formed on the inner surface of the piston ring groove 13 is such that the piston body 10A is made of a high-strength aluminum alloy as a base material. For example, even if the surface roughness Rpk is 1.0 μm or less, or both the surface roughness Ra and the surface roughness Rpk are 1.0 μm or less, the surface can be smooth. Note that the surface roughness Ra is an index regarding the characteristics of the arithmetic mean roughness of the contour curve, based on JIS B0601-2001. Surface roughness Rpk is based on JIS B0671-2000 and is an index related to the characteristics of the average height of the protruding peaks above the core of the roughness curve, and can evaluate the airtightness of the anodic oxide film. . By forming an anodized film with such a surface roughness, the sealing performance with the piston ring is improved, and blow-by gas can be reduced and fuel efficiency can be improved. Further, it is possible to suppress PM, which is a substance subject to environmental regulations, and PN, which is the number thereof, caused by oil rise.

Note that the method for manufacturing a piston for an internal combustion engine according to the present invention is not limited to the above-described embodiments, and may include replacing steps, omitting steps, or including other steps. For example, after the machining process 43, the resin coating process 46, the water jet process 45, and the anodizing process 46 may be performed in this order, or the water jet process 45 may be applied to the skirt part in addition to the ring groove. After that, the resin coating process 46 and the anodizing process 45 may be performed in this order. Furthermore, as described above, the resin coating process 46 may be omitted.

In order to confirm the silicon removal effect of water jet treatment, water jet treatment was applied to the skirt portion of the piston body, simulating the piston ring groove. The piston body was manufactured using an Al-Si-Cu-Ni-based aluminum alloy (hereinafter also referred to as high-strength material) having the composition shown in Table 1 as a base material.

Then, water jet treatment was applied to the skirt portion of the piston body at an injection pressure of 160 MPa, and the treated surface was observed using a scanning electron microscope (SEM) (300x magnification) and energy dispersive X-ray spectroscopy. Silicon distribution was analyzed using a device (EDS) (Example 1). In addition, in order to evaluate these, similar observation and analysis were performed on the skirt portion before the water jet treatment (Comparative Example 1). SEM photographs showing the observation results are shown in FIGS. 12 and 13.

FIG. 12 is a SEM photograph showing the surface of the high-strength material before water jet treatment of Comparative Example 1, and the surface was flat, with no particular irregularities observed. On the other hand, FIG. 13 shows the surface of the high-strength material after the water jet treatment of Example 1, and it was confirmed that many depressions were formed. These are considered to be locations where silicon was removed in chunks by water jet treatment. According to the EDS analysis results, a lump of silicon with a size of several tens of μm was detected on the smooth surface of Comparative Example 1, and this location is primary silicon. On the other hand, in the EDS analysis results of Example 1, silicon was detected at a location corresponding to the depth of the depression confirmed in the SEM photograph. From this, it is presumed that although some primary silicon remains in the depths of the recess, the surface portion has been removed. Therefore, it has been suggested that the amount of silicon removed can be controlled by adjusting the water jet treatment conditions (particularly the water jet direction, jet pressure, nozzle diameter, etc.) of the jet nozzle.

Next, a test was conducted to confirm the smoothness of the anodic oxide film formed after the water jet treatment. First, a top ring groove (groove width: 1 mm, groove depth: 3 mm) was created by machining on the same piston body as above, and a water jet treatment was performed on the inner surface of the top ring groove. The conditions for the water jet treatment were a jetting pressure of 160 MPa and a nozzle diameter of 0.25 mm (a nozzle diameter that is 1/4 of the width of the groove). Then, the inner surface of the top ring groove was anodized. The conditions for the anodizing treatment were as follows: 18 vol % sulfuric acid was used as the electrolytic solution, a titanium-based material was attached to the cathode plate, and a piston body was attached to the anode. The electrolysis method was an AC/DC superposition electrolysis method, and anodization was performed for 40 seconds using constant voltage electrolysis with a positive voltage of 65 V, a negative voltage of -2 V, and a frequency of 12 kHz (Example 2).

The cross-sectional film thickness, surface roughness Ra, and surface roughness Rpk of the anodic oxide film formed in Example 2 were measured. The cross-sectional film thickness was determined by observing the cross-section of the bottom surface 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. The average value of 20 points was used.

For comparison, anodization treatment was performed in the same manner as in Example 2 except that water jet treatment was not performed (Comparative Example 2). Then, the cross-sectional film thickness, surface roughness Ra, and surface roughness Rpk of the anodic oxide film formed in Comparative Example 2 were measured in the same manner as in Example 2. Table 2 shows the measurement results of the cross-sectional film thickness, surface roughness Ra, and surface roughness Rpk of Example 2 and Comparative Example 2. As a reference example, a commercially available anodized film formed on a piston ring groove made by another company was similarly measured, and the results are shown in Table 2. Further, optical micrographs of the cross section of the bottom surface of the top ring groove taken during the measurement are shown in FIGS. 14 and 15.

As shown in Table 2, the thicknesses of the anodic oxide films in Comparative Example 2 and Example 2 were the same. In Comparative Example 2, the surface roughness Ra was 1.5 μm and the surface roughness Rpk was 1.8 μm, whereas in Example 2, the surface roughness Ra was 1.2 μm and the surface roughness Rpk was 0. It was 4 μm. This indicates that as a result of previously removing a portion of the primary silicon by water jet treatment, the surface of the anodic oxide film became smooth with fewer protrusions. Further, as a reference example, the cross-sectional film thickness, surface roughness Ra, and surface roughness Rpk were similarly measured for an anodized film formed on the top ring groove of a piston manufactured by another company. The results are shown in Table 2. Note that the cross-sectional film thickness is shown in parentheses because the measurement method is different. The surface roughness Ra of this anodic oxide film, which would have been formed by direct current 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 the AC/DC superposition electrolysis method after the water jet treatment, had a significantly smoother surface.

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. Note that what is visible on the anodic oxide films 35a and 35b is the embedding resin 36 used for observation with an optical microscope, and the boundary thereof is shown by a dotted line. As shown in FIG. 14, in Comparative Example 2, the anodic oxide film 35b hardly grows on some primary silicon 32 in the matrix 31 of the high-strength material, which causes unevenness on the surface. It looked like he was in trouble. On the other hand, as shown in FIG. 15, the surface of the anodic oxide film 35a of Example 2 was smoother than that of Comparative Example 2.

In addition, in FIG. 15, there is a depression 34 at the interface between the high-strength material matrix 31 and the anodic oxide film 35a, where it is assumed that silicon was removed in chunks by water jet treatment, and the anodic oxide film is present at that location. I could see that it was growing. This is because waterjet treatment removes the surface of the primary silicon, creating a gap between the silicon and aluminum, and anodizing causes the electrolyte to seep into that gap, causing the surrounding aluminum to be anodized. It is assumed that this characteristic is caused by the growth of cells forming the anodic oxide film in a manner that wraps around the silicon in the AC/DC superposition electrolysis method.

In Examples 1 and 2, the water jet direction of the water jet treatment jet nozzle was perpendicular to the central axis of the piston body, that is, parallel to the lower surface of the top ring groove. By jetting water at an inclination angle α to the parallel direction, or at a misdirection angle β to the radial direction of the piston body, it is possible to remove more primary silicon, resulting in a smoother surface. For example, it is possible to obtain an anodic oxide film in which both the surface roughness Ra and the surface roughness Rpk are 1.0 μm or less.

10 Piston for internal combustion engine 10A Piston body 13 Top ring groove 20 Cylinder block 22 Top ring 30A Conventional aluminum alloy 30B Aluminum alloy of high strength material 31 Matrix 32 Primary silicon 34 Hollow portion 35a, 35b Anodic oxide film 40 Piston for internal combustion engine Manufacturing method 50 Water jet treatment device 51 Spray nozzle 52 Water 53 Construction surface 54 Chuck 55 Holding member

Claims (5)

A method for manufacturing a piston for an internal combustion engine, the method comprising:
Aluminum containing 5.0 to 20.0 mass% Si, more than 1.3 mass% to 5.0 mass% Cu, and more than 1.5 mass% to 3.5 mass% Ni Regarding a piston body for an internal combustion engine that has an alloy as a base material and has a top ring groove on the outer circumferential surface, the piston body for the internal combustion engine is prepared with a holding member holding a portion of the outer circumferential surface between the top ring groove and the crown surface of the piston. While rotating the piston body around its axis, at least a region of the inner surface of the top ring groove in contact with the top ring is subjected to a water jet treatment to remove silicon exposed in the region;
A method for manufacturing a piston for an internal combustion engine, comprising the step of forming an anodized film on at least the area subjected to the water jet treatment. 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 superposition electrolysis method. The water jet direction of the jet nozzle in the water jet treatment is set relative to a parallel surface of either the piston crown side inner surface or the skirt portion side inner surface which is the opposite side of the inner surface of the top ring groove. 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 piston toward the crown surface or toward the skirt portion, which is the opposite side. The internal combustion engine according to claim 3, wherein the water jet treatment is performed by shifting the water jet direction of the injection nozzle in the water jet treatment in a direction opposite to the rotational direction of the piston body with respect to the radial direction of the piston body. Method of manufacturing engine pistons. 4. The water jet treatment is performed by moving the water jetting direction of the jetting nozzle so that the jetting direction of the water includes the tangential direction of the outer periphery of the piston body having a substantially cylindrical shape. A method for manufacturing a piston for an internal combustion engine as described in .

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