Piston for internal combustion engine and manufacturing method thereof

Abstract

To provide a piston for an internal combustion engine and a manufacturing method thereof, capable of improving seizure resistance of a skirt face and simultaneously reducing friction.SOLUTION: A thrust side skirt portion 18A extending toward a bottom of a cylinder bore on an outer peripheral portion of a piston 10 and an anti-thrust side skirt portion 18B of a position opposed thereto are composed of an aluminum alloy as a base material 40. In forming silicon dispersion layers 43 by applying a pulse laser beam to outer surfaces of two skirt portions, a silicon average particle size of a first silicon dispersion layer 43A formed on the thrust side skirt portion 18A is determined to be smaller than a silicon average particle size of a second silicon dispersion layer 43B formed on the anti-thrust side skirt portion 18B.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a piston for an internal combustion engine and a method for manufacturing the same. [Background technology]

[0002] In an internal combustion engine, such as an engine installed in a vehicle such as an automobile, a piston reciprocates within a cylinder bore extending along a linear longitudinal axis in a direction along the longitudinal axis. During this movement, the outer periphery of the piston slides against the inner periphery of the cylinder bore. Typically, the piston includes a piston body having an outer periphery that is slidable against the inner periphery of the cylinder bore, and two skirt portions extending from the outer periphery of the piston body toward the bottom of the cylinder bore. Furthermore, the piston includes a resin coating on the outer surface of each skirt portion to reduce frictional resistance between the outer periphery of the piston and the inner periphery of the cylinder bore.

[0003] Patent Document 1 describes a piston used in an internal combustion engine, which includes a piston body made of aluminum or an aluminum alloy, and a modified layer formed on the surface of a strength-reinforcing portion of the piston body by laser peening and heat treatment, the modified layer having a surface made of aluminum oxide by plasma oxidation and having compressive residual stress. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-86766 Summary of the Invention [Problem to be solved by the invention]

[0005] Patent Document 1 describes that laser peening can also be applied to the skirt surface of a piston. However, although it is presumed that laser peening will improve the seizure resistance of the skirt surface by forming a modified layer with fine irregularities on the surface of the skirt surface, the application of compressive residual stress increases the hardness of the modified layer compared to the aluminum alloy base material, suppressing the amount of deformation. This reduces the contact area with the cylinder sleeve, and increases the surface pressure, which may worsen friction.

[0006] In view of the above problems, the present invention aims to provide a piston for an internal combustion engine that can improve the seizure resistance of the skirt surface while simultaneously reducing friction, and a method for manufacturing the same. [Means for solving the problem]

[0007] In order to achieve the above object, one aspect of the present invention provides a piston for an internal combustion engine, comprising: a piston body having an outer periphery configured to be slidable longitudinally relative to an inner periphery of a cylinder bore; and two skirt portions facing each other radially of the piston body and extending from the outer periphery of the piston body toward a bottom of the cylinder bore, the two skirt portions consisting of a thrust-side skirt portion located on the thrust side in a skirt opposing direction, and an anti-thrust-side skirt portion located on the anti-thrust side in the skirt opposing direction, the piston body and the two skirt portions being constructed using an aluminum alloy as a base material, and each of the two skirt portions having a silicon dispersion layer formed on its outer surface in which silicon in the aluminum alloy is dispersed more finely than in the base material, the average silicon particle size of the first silicon dispersion layer formed on the thrust-side skirt portion being smaller than the average silicon particle size of the second silicon dispersion layer formed on the anti-thrust-side skirt portion.

[0008] In another aspect, the present invention provides a method for manufacturing a piston for an internal combustion engine, the method comprising the steps of: preparing a piston for an internal combustion engine, the piston comprising: a piston body having an outer periphery configured to be slidable longitudinally relative to an inner periphery of a cylinder bore; and two skirt portions radially opposing each other about the piston body and extending from the outer periphery of the piston body toward a bottom of the cylinder bore, the two skirt portions comprising a thrust-side skirt portion located on the thrust side in a skirt opposing direction, and an anti-thrust-side skirt portion located on the anti-thrust side in the skirt opposing direction, the piston body and the skirt portions being constructed using an aluminum alloy as a base material; and a laser light irradiation step of irradiating outer surfaces of the two skirt portions of the piston for an internal combustion engine with pulsed laser light to form silicon dispersion layers, wherein in the laser light irradiation step, the average silicon grain size of the first silicon dispersion layer formed in the thrust-side skirt portion is made smaller than the average silicon grain size of the second silicon dispersion layer formed in the anti-thrust-side skirt portion. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a piston for an internal combustion engine and a method for manufacturing the same that can improve the seizure resistance of the skirt surface and simultaneously reduce friction. [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] 1 is a schematic cross-sectional view illustrating each process of scavenging, compression, explosion, and exhaust of a piston for an internal combustion engine. FIG. [Figure 3] 1 is a flow chart for explaining an embodiment of a method for manufacturing a piston for an internal combustion engine according to the present invention; [Figure 4] 3 is a cross-sectional view schematically showing the outer surface of the skirt portion of the piston for an internal combustion engine after a machining step in the manufacturing method of FIG. 2. FIG. [Figure 5]1 is a cross-sectional view schematically showing a piston for an internal combustion engine in which a silicon dispersion layer is formed on a thrust-side skirt portion and a counter-thrust-side skirt portion. [Figure 6] 2 is a schematic diagram showing a region where a silicon dispersion layer is formed on a thrust-side skirt portion of a piston for an internal combustion engine. FIG. [Figure 7] 2 is a schematic diagram showing a region where a silicon dispersion layer is formed on the anti-thrust skirt portion of a piston for an internal combustion engine. FIG. [Figure 8] 1 is a schematic diagram showing an embodiment of laser light irradiation onto a skirt portion of a piston for an internal combustion engine. FIG. [Figure 9] 10A and 10B are schematic diagrams for explaining an overlap amount when scanning an irradiation spot in a laser light irradiation step. [Figure 10] 10A and 10B are schematic diagrams showing another embodiment of laser light irradiation onto a skirt portion of a piston for an internal combustion engine. [Figure 11] 1 is a microscope image showing a cross section of the piston blank after laser light irradiation in Example 1. [Figure 12] 10 is a microscope image showing a cross section of the piston blank after laser light irradiation in Example 2. [Figure 13] 10 is a microscope image showing a cross section of a piston blank that was not irradiated with laser light as a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0011] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a piston for an internal combustion engine and a method for manufacturing the same according to the present invention will now be described with reference to the accompanying drawings.

[0012] First, the basic structure of a piston for an internal combustion engine will be described. As shown in Fig. 1, a piston 10 for an internal combustion engine according to this embodiment is a member whose outer periphery reciprocates slidably along the longitudinal direction relative to the inner periphery of a cylinder bore (not shown), and is made of an aluminum alloy. The aluminum alloy contains silicon (Si) as a component that contributes to wear resistance and aluminum adhesion resistance. Examples of such aluminum alloys include AC materials such as AC4 series (AC4A, AC4B, etc.), AC8 series (AC8A, AC8B, etc.), and AC9 series, ADC materials such as ADC10 to ADC14, and A4000, which are used for pistons.

[0013] The piston 10 for an internal combustion engine has a substantially cylindrical shape and has, on its outer peripheral surface, in this order from the piston crown surface 11 side, a first ring groove 13, a second ring groove 15, and an oil ring groove 17. On the outer peripheral surface, the portion between the piston crown surface 11 and the first ring groove 13 is called a first land 12, the portion between the first 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 beyond the oil ring groove 17 is called a skirt portion 18.

[0014] Therefore, it can be said that the internal combustion engine piston 10 includes a piston body extending from the piston crown surface 11 to the oil ring groove 17, and a skirt portion 18 extending from the outer periphery of the piston body toward the bottom of the cylinder bore. The skirt portion 18 is made up of two skirt portions facing each other in the radial direction of the piston body, and is arranged on both sides with a pin boss portion 19 between them, as shown in FIG.

[0015] 1, in consideration of the effect of thermal expansion, the skirt portion 18 is formed in a barrel shape with its outer diameter being greatest at the center of the skirt portion 18. In other words, the skirt portion 18 is curved so that the outer diameter gradually decreases from the center toward the piston crown surface 11 and toward the skirt end portion 18E.

[0016] As shown in Figure 2, this internal combustion engine piston 10 is housed in a cylinder bore 3 of a cylinder 2 of an engine 1 and moves repeatedly up and down within the cylinder bore 3. The piston 10 is supported so as to be able to swing freely by one end of a connecting rod 5 via a piston pin 4. The other end of the connecting rod 5 is connected to a crankshaft 7 via a crankpin 6. The crankpin 6 is positioned at a position offset from the central axis C of the crankshaft 7. As a result, when the piston 10 repeatedly moves up and down within the cylinder bore 3, the crankshaft 7 rotates in a fixed direction, providing driving force for the vehicle.

[0017] A combustion chamber 9 is defined by the piston crown surface 11 of the piston 10, the inner peripheral surface of the cylinder bore 3, and the bottom surface of the cylinder head 8. Figures 2(a) to 2(d) respectively show the scavenging stroke, compression stroke, explosion stroke, and exhaust stroke of a four-stroke engine. The fuel-air mixture taken into the combustion chamber 9 during the scavenging stroke of Figure 2(a) is compressed as the piston 10 rises during the compression stroke of Figure 2(b). Next, during the combustion stroke of Figure 2(c), the mixture is explosively combusted by a spark plug (not shown), and this explosion causes the piston 10 to descend. Then, during the exhaust stroke of Figure 2(d), the piston 10 rises again, and exhaust gases produced by combustion are expelled from the combustion chamber 9.

[0018] The thrust side of the piston 10 is the side that is pushed down by the explosion during the combustion process shown in FIG. 2(c). 5 Since it is connected to the connecting rod 5 Depending on the direction of oscillation, the outer peripheral surface of the piston 10 is pressed against a cylinder sleeve (not shown) in the cylinder bore 3, applying lateral pressure. The side of the piston 10 on which this lateral pressure acts is called the thrust side, and the side radially opposite the thrust side of the piston is called the anti-thrust side.

[0019] Next, a manufacturing method 20 of the piston for an internal combustion engine according to this embodiment will be described. As shown in Fig. 3, the manufacturing method 20 of the piston for an internal combustion engine includes a step 21 of casting the piston, a step 22 of heat-treating the cast piston, a step 23 of machining the heat-treated piston, a step 24 of irradiating the skirt portion of the piston with laser light, and a step 25 of performing surface treatment to coat the skirt portion of the piston with a resin coating.

[0020] The above-mentioned steps 21, 22, and 23 of casting, heat treatment, and machining are the same as the steps used in manufacturing a general piston for an internal combustion engine, and therefore a detailed description thereof will be omitted here.

[0021] The laser light irradiation process 24 is a process of irradiating the outer surface of the skirt portion 18 of the piston 10 for an internal combustion engine with pulsed laser light for the purposes of forming a large number of recesses having a predetermined shape on the outer surface of the skirt portion 18 of the piston 10 for an internal combustion engine, and forming a silicon dispersion layer on the surface of the skirt portion 18 in which silicon is finer and more dispersed than in the base material due to melting and re-solidification of the aluminum alloy that is the base material of the piston.

[0022] First, the skirt portion 18 before the laser light irradiation step 24 will be described. Fig. 4 shows an enlarged cross-sectional view of the outer surface of the skirt portion 18. Note that the thrust-side skirt portion 18A and the anti-thrust-side skirt portion 18B have the same properties before the laser light irradiation step 24. Fig. 4 is a view obtained by rotating Fig. 1 by 90 degrees to the left. As shown in Fig. 4, the aluminum alloy that is the base material 40 of the skirt portion 18 has primary crystalline silicon 41 and eutectic silicon 42 crystallized in the α phase. Both the primary crystalline silicon 41 and eutectic silicon 42 are crystal grains that crystallized during the piston manufacturing process.

[0023] Furthermore, in order to improve the seizure resistance between the outer surface of the skirt portion 18 and the inner wall surface of the cylinder bore, scratches 30 are formed on the outer surface of the skirt portion 18 by a machining process 23. Note that, although scratches 30 are formed on the outer surface of the skirt portion 18 in this embodiment, the present invention is not limited to this, and can also be applied to the outer surface of the skirt portion 18 on which scratches 30 are not formed.

[0024] The striations 30 have a plurality of grooves 31 with a U-shaped cross section formed on the outer surface of the skirt portion 18 and extending in the circumferential direction. The grooves 31 are arranged at intervals in the longitudinal direction. The depth of the grooves 31 is, for example, in the range of 5 to 15 μm. Between adjacent grooves 31 in the longitudinal direction, an intermediate surface portion (plateau portion) 32 is provided, whose radially outer end in the cross section extends linearly in the longitudinal direction.

[0025] The longitudinal width Wa of the plateau portion 32 (i.e., the distance between adjacent groove portions 31) is, for example, in the range of 10 to 100 μm. The longitudinal width Wb of the groove portion 31 is, for example, in the range of 150 to 400 μm. The plateau ratio, expressed by the following formula 1 using the width Wa of the plateau portion 32 and the width Wb of the groove portion 31, is, for example, in the range of 0.02 to 0.4. The plateau ratio has conventionally been used as one of the indicators representing the shape of the scratches 30.

[0026]

number

[0027] In the laser light irradiation process 24, the outer surface of the skirt portion 18 is irradiated with pulsed laser light, whereby the aluminum alloy is locally heated and melted, and then cooled, whereby the primary crystal silicon 41 and eutectic silicon 42 in the aluminum alloy are refined, and as shown in FIG. 5, a silicon dispersion layer 43 in which silicon is dispersed is formed on the surface of the skirt portion 18.

[0028] In this case, the average silicon particle size of the first silicon dispersion layer 43A formed in the thrust-side skirt portion 18A is made smaller than the average silicon particle size of the second silicon dispersion layer 43B formed in the anti-thrust-side skirt portion 18B. In other words, the thrust-side skirt portion 18A is made finer than the anti-thrust-side skirt portion 18B. This improves the seizure resistance of the skirt portion 18A, particularly the thrust-side skirt portion 18A, which is subjected to extremely high lateral pressure during engine operation.

[0029] Furthermore, by further refining the thrust-side skirt portion 18A by laser beam irradiation as described above, the silicon dispersion layer 43A formed in the thrust-side skirt portion 18A is harder than the silicon dispersion layer 43B formed in the anti-thrust-side skirt portion 18B. In other words, by suppressing the increase in hardness of the anti-thrust-side skirt portion 18B, the anti-thrust-side skirt portion 18B can be more deformed by contact with the cylinder sleeve of the cylinder bore than the thrust-side skirt portion 18A. This increases the contact area between the piston 10 and the cylinder sleeve, dispersing the surface pressure and reducing friction.

[0030] To reduce friction in this manner, it is preferable to make the hardness of the silicon dispersion layer 43A formed on the thrust side skirt portion 18A higher than the hardness of the silicon dispersion layer 43B formed on the anti-thrust side skirt portion 18B, for example, by at least 10 HV0.1 Vickers hardness, and more preferably by at least 20 HV0.1 Vickers hardness.

[0031] This reduction in friction can be achieved by forming a first silicon dispersion layer 43A with a smaller average silicon particle size over the entire surface 60 of the thrust-side skirt portion 18A, as shown in FIG. 6(a), and by forming a second silicon dispersion layer 43B with a larger average silicon particle size over the entire surface 70 of the anti-thrust-side skirt portion 18B, as shown in FIG. 7(a). In particular, as shown in FIG. 6(b) or 6(c), friction can be further reduced by forming the silicon dispersion layer 43A with a smaller average silicon particle size in a central skirt region 61 located in the center of the thrust-side skirt portion 18A, for example, in a strip-shaped region 61a or an elliptical region 61b parallel to the ring groove, and by forming a third silicon dispersion layer with a larger average silicon particle size in the remaining regions 62a, 62b.

[0032] The central skirt region is a region located at the point where the outer diameter formed by the thrust-side skirt portion 18A and the anti-thrust-side skirt portion 18B is greatest. The "region located at the point where the outer diameter is greatest" does not necessarily mean that the entire region is located at the point where the outer diameter is greatest, but may mean that almost the entire region is located at a portion of the point where the outer diameter is greatest. Because the central skirt region receives the highest lateral pressure within the thrust-side skirt portion 18A, forming the first silicon-dispersed layer 43A in this region, where silicon fineness is promoted, maintains excellent seizure resistance. However, because silicon fineness is suppressed in other regions, the increase in hardness is suppressed and the amount of deformation increases, as described above. This increases the contact area between the piston and the cylinder sleeve, further dispersing the surface pressure, thereby further reducing friction.

[0033] The average silicon particle size of the third silicon dispersion layer formed on the thrust side skirt portion 18A may be the same as, smaller than, or larger than the average silicon particle size of the second silicon dispersion layer formed on the anti-thrust side skirt portion 18B, but since the lateral pressure is generally higher in the thrust side skirt portion 18A, a smaller silicon particle size is preferable.

[0034] 7(b) or 7(c), the anti-thrust side skirt portion 18B may have a fourth silicon dispersion layer formed in an upper skirt region 71 and a lower skirt region 72 of the anti-thrust side skirt portion 18B, the fourth silicon dispersion layer having an average silicon particle size between the average silicon particle size of the first silicon dispersion layer 43A and the average silicon particle size of the second silicon dispersion layer 43B, and a second silicon dispersion layer formed in the remaining region 73. The upper skirt region 71 is located closer to the piston crown surface than the point where the outer diameter formed by the thrust side skirt portion 18A and the anti-thrust side skirt portion 18B is maximum, and preferably does not contact the oil ring groove (i.e., the remaining region 73 exists between the upper skirt region 71 and the anti-thrust side skirt portion 18B). The lower skirt region 72 is located closer to the skirt end than the maximum outer diameter point, and preferably does not contact the skirt end (i.e., the remaining region 73 exists between the lower skirt region 72 and the skirt end). The upper skirt region 71 and the lower skirt region 72 may be located across the entire width of the anti-thrust side skirt portion 18B, as shown in Figure 7(b), or may be located across only a portion of it, as shown in Figure 7(c).

[0035] Since the upper skirt region 71 and the lower skirt region 72 are required to have relatively high seizure resistance within the anti-thrust side skirt portion 18B, by forming a fourth silicon dispersion layer in these regions, which has an average silicon particle size smaller than the average silicon particle size of the second silicon dispersion layer 43B, it is possible to improve seizure resistance while maintaining reduced friction.

[0036] In the laser light irradiation process 24, such silicon dispersion layers 43 having different average silicon particle sizes are formed on the outer surface of the skirt portion 18. For example, as shown in FIG. 8, the laser irradiation conditions when irradiating the outer surface of the thrust-side skirt portion 18A of the piston 10 with laser light from the laser light irradiation device 51 to form the first silicon dispersion layer 43A and the laser irradiation conditions when irradiating the outer surface of the anti-thrust-side skirt portion 18B with laser light from the laser light irradiation device 51 to form the second silicon dispersion layer 43B are changed, thereby making it possible to form silicon dispersion layers 43 having different average silicon particle sizes.

[0037] As a condition for laser irradiation, for example, by increasing the scanning speed of the laser irradiation, silicon can be further refined, resulting in a silicon-dispersed layer with a smaller average silicon particle size. The aluminum alloy base material irradiated with laser light is remelted and then cooled, and the faster the cooling rate, the more refined the silicon contained in the base material becomes, resulting in a silicon-dispersed layer. The faster the scanning speed of the laser light, the faster the speed at which the remelted part separates from the laser light irradiation. Since the part irradiated with the laser light is mainly heated, the faster the scanning speed, the less the part is affected by the heat from the laser light after remelting and the more rapidly it cools, resulting in a structure with finer silicon.

[0038] The scanning speed value is dependent on the composition of the aluminum alloy and other laser irradiation conditions such as output and frequency, and is therefore not limited thereto. However, for example, the scanning speed for the thrust-side skirt portion 18A is preferably at least 1,000 mm / min faster, and more preferably at least 2,000 mm / min faster, than the scanning speed for the anti-thrust-side skirt portion 18B. The upper limit is not particularly limited, but is, for example, 15,000 mm / min or less. Furthermore, to obtain the silicon-dispersed layer 43, the scanning speed is preferably, for example, 100 to 20,000 mm / min, and more preferably 2,000 to 10,000 mm / min.

[0039] As a condition for laser irradiation, for example, by reducing the overlap length (overlap amount Rc) of the spot diameters of the irradiation spots, silicon can be made finer, and a silicon dispersion layer with a smaller average silicon particle size can be obtained. For example, when scanning the irradiation spot along the circumferential direction of the skirt portion 18 (the direction perpendicular to the paper surface of FIG. 4), as shown in FIG. 9, the irradiation spot 50 irradiated with the pulsed laser light is scanned so as to overlap along the circumferential direction C of the skirt portion. At this time, the distance between the centers Ca and Cb of two adjacent irradiation spots 50a and 50b in the circumferential direction C (i.e., the overlap length of the spot diameters) is set to 0.05. C) is small, the silicon is less susceptible to the heat of the laser beam after remelting and is cooled rapidly, as described above, and therefore a structure with a finer silicon grain can be obtained.

[0040] The value of the overlap amount Rc is usually expressed as a ratio to the spot radius r of the irradiation spot 50. To obtain the silicon dispersed layer 43, the spot radius r is preferably in the range of, for example, 0.01 to 3 mm, and more preferably in the range of 0.02 to 0.5 mm. C is preferably 80% or less of the spot radius r, more preferably 20% or less, and even more preferably 10% or less. C The lower limit is not particularly limited, but is preferably 5% or more of the spot radius r. The overlap amount Rc for the thrust-side skirt portion 18A is preferably smaller than the overlap amount Rc for the anti-thrust-side skirt portion 18B by, for example, 20% or more, and more preferably 10% or more. The upper limit is not particularly limited, but is, for example, 5% or less.

[0041] Additionally, when scanning the irradiation spot along the longitudinal direction of the skirt portion 18, the scanning is performed so that the spot diameter r partially overlaps along the longitudinal direction L of the skirt portion, and the silicon can be further refined by reducing the overlapping length. Furthermore, when scanning the irradiation spot along the circumferential direction C or longitudinal direction L of the skirt portion 18 in this manner, by changing the laser irradiation conditions, silicon dispersion layers having different silicon average particle sizes can be formed in the thrust-side skirt portion 18A and the anti-thrust-side skirt portion 18B. Note that these scans may be performed by moving the laser side or the piston side.

[0042] Furthermore, instead of changing the conditions for laser irradiation, as shown in FIG. 10 , in laser light irradiation step 24, piston 10 is attached to piston fixing jig 80, and the piston 10 and piston fixing jig 80 are rotated to scan the pulsed laser light. In this case, a piston fixing jig 80 having a specific shape can also be used to form silicon dispersion layers having different average silicon particle sizes in thrust-side skirt portion 18A and anti-thrust-side skirt portion 18B.

[0043] The piston fixing jig 80 includes a cylindrical piston support portion 81 having the same diameter as the piston 10, and a piston set portion 82 located at one end of the piston support portion 81 and adapted to receive an internal combustion engine piston. The piston set portion 82 contacts only the back surface of the thrust-side skirt portion 18A of the piston 10 at a portion where the first silicon dispersion layer 43A is to be formed. As a result, the piston support portion 81, which has a large heat capacity, contacts the thrust-side skirt portion 18A, and therefore, in the laser light irradiation step 24, the remelted portion is cooled more quickly in the thrust-side skirt portion 18A than in the non-thrust-side skirt portion 18B, which is not in contact with anything, and thus a silicon dispersion layer 43A having a small average silicon particle size is formed.

[0044] Although FIG. 10 shows an example in which piston support portion 81 does not contact anti-thrust side skirt portion 18B, piston support portion 81 contacts both thrust side skirt portion 18A and anti-thrust side skirt portion 18B. By changing the materials that contact thrust side skirt portion 18A and anti-thrust side skirt portion 18B, the cooling rate can be changed significantly, and silicon dispersion layers with different average silicon particle sizes can be formed.

[0045] As shown in FIG. 7, the laser light irradiation step 24 does not significantly damage the shape of the scratches 30 on the outer surface of the skirt portion 18. For example, the scratches after the laser light irradiation step 24 are 30 is the groove 31 and plateau 32It is preferable to maintain the plateau ratio calculated from each width in the range of 0.12 to 0.15. If the plateau ratio is less than 0.12, the surface pressure acting locally becomes very large, which may make seizure more likely to occur. On the other hand, if the plateau ratio exceeds 0.15, the effect of expelling foreign matter cannot be sufficiently ensured, which may make seizure more likely to occur.

[0046] In addition, the plateau ratio of the streak 30 before the laser light irradiation process 24 and the plateau ratio of the streak 30 after the laser light irradiation process 24 are 30 It is preferable to keep the change in plateau ratio, which is the difference between the plateau ratio of the scratch 30 and the plateau ratio of the scratch 30, to 0.05 or less. If the change in plateau ratio exceeds 0.05, the general shape of the scratch 30 may change. There is no particular restriction on the lower limit of the change in plateau ratio, but it is preferable that it be 0.00 or more.

[0047] The laser that irradiates the pulsed laser light is not particularly limited as long as it is a laser for metal processing, but for example, a semiconductor laser, a CO2 laser, a solid-state laser, a fiber laser, etc. can be used alone or in combination.

[0048] It should be noted that cutting oil remaining on the surface in the machining step 23 can be degreased by irradiating it with pulsed laser light in the laser light irradiation step 24. However, if too much cutting oil remains on the surface in the machining step 23, the degreasing by laser light irradiation may be insufficient or may hinder the formation of the silicon dispersion layer 43 and recesses. Therefore, if necessary, a degreasing step (not shown) such as neutral degreasing or alkaline degreasing may be performed before the laser light irradiation step 24.

[0049] In the laser light irradiation step 24, the laser light is irradiated only onto the skirt portion 18, so a masking step for masking the piston body other than the skirt portion 18 may be performed before the laser light irradiation step 24. Alternatively, the laser light irradiation step 24 may be performed without masking.

[0050] In the laser light irradiation step 24, components of the aluminum alloy that are sublimated by the heat of the laser light, called fumes, are generated, so a local dust collection type smoke suction device may be used as necessary.

[0051] It should be noted that a water washing step may be performed after the laser light irradiation step 24. The water washing step can remove dust particles that have adhered to the internal combustion engine piston 10 in the laser light irradiation step 24. The water washing step may be, for example, ultrasonic water washing, or other simple steps.

[0052] The surface treatment step 25 is a step of forming a resin coating on the outer surface of the skirt portion 18 on which the silicon dispersion layer 43 has been formed. In the surface treatment step 25, for example, a resin coating agent is applied to the outer surface of the skirt portion 18 by a spray method, a screen printing method, or the like, and then the resin coating can be formed by baking. As the resin coating agent, a known agent used for the skirt portion 18 of an internal combustion engine piston can be used, such as MOLYKOTE D-10-GBL or MOLYKOTE PA-744 manufactured by DuPont-Toray Specialty Materials Co., Ltd.

[0053] By forming a resin coating on the silicon dispersion layer 43 having a uniformly refined structure, it is possible to suppress point peeling of the resin coating, which can lead to widespread peeling. Furthermore, in areas where seizure resistance is required, even if the resin coating is worn away, the silicon dispersion layer 43A having a finer silicon particle size and a smaller average particle size is formed on the thrust-side skirt portion 18A, so that seizure resistance can be maintained. Note that the present invention does not require the surface treatment step 25 to be performed, and it is not necessary to form a resin coating on the silicon dispersion layer 43.

[0054] The manufacturing method of the piston for an internal combustion engine according to the present invention is not limited to the above embodiment, and may include other processes. For example, an anodizing process or a chemical conversion process may be performed between the laser light irradiation process 24 and the surface treatment process 25. A porous anodized film is formed on the outer surface of the skirt portion 18 by the anodizing process. By making the film thickness several μm or less, scratches on the outer surface of the skirt portion 18 can be prevented. 30 The anodized film can be formed while maintaining the shape of the anodized film. By filling the fine pores of the anodized film with a resin coating agent, further improvement in adhesion can be expected. In addition, the chemical conversion treatment chemically reacts a part of the outer surface of the skirt portion 18 to form a chemical conversion film. Since the film thickness is several μm, scratches on the outer surface of the skirt portion 18 are not visible. 30 The chemical conversion coating can be formed while maintaining the shape of the ring. Because the chemical conversion coating is formed in a granular form and has unevenness, further improvement in adhesion can be expected by filling these unevenness with a resin coating agent. When anodizing, the first ring groove 13 and the second ring groove 15 can also be anodized at the same time as the skirt portion 18, thereby imparting wear resistance to these ring grooves. [Example]

[0055] A piston blank (76 mm in diameter) was fabricated to simulate the skirt surface using a high-strength Al-Si-Cu aluminum alloy casting, which has improved mechanical properties at high temperatures. The outer periphery of the blank was then irradiated with laser light over a width of approximately 5 mm, forming a silicon-dispersed layer on the surface. The laser processing machine used (TRUMPF, model TruDisk6001) was equipped with a disk YAG laser, and the pulse laser irradiation conditions were a constant output of 6 W and two scanning speeds of 9000 mm / min and 2000 mm / min.

[0056] Then, in order to examine the surface properties of the piston blanks after laser beam irradiation, the cross-sections of the piston blanks were observed and the surface hardness was measured. In addition, to evaluate these, the same observations and measurements were also made on the piston blanks that were not irradiated with laser beam.

[0057] For cross-sectional structural observation, the piston raw material was cut, embedded in resin, polished, and observed with an inverted metallurgical microscope. Surface hardness was measured at five points using a micro-Vickers hardness tester with a load of 100 g, and the average value was calculated. The results are shown in Figures 12 to 14 and Table 1.

[0058] [Table 1]

[0059] The cross section of the piston of the comparative example not subjected to laser processing shown in FIG. 13 had a structure in which crystallized materials such as primary silicon 41 and eutectic silicon 42 were segregated in the base material (α phase) 40. On the other hand, it was confirmed that the silicon 44 was refined in both the silicon dispersion layer 43 of the piston cross section of Example 1 shown in FIG. 11 and the silicon dispersion layer 43 of the piston cross section of Example 2 shown in FIG. 12 . It was also found that the silicon refinement was more advanced in the piston cross section of Example 1 shown in FIG. 11 than in the piston cross section of Example 2 shown in FIG. 12 . This is presumably due to the difference in the scanning speed of the laser beam. The base material irradiated with the laser beam is remelted and then cooled, and the faster the scanning speed of the laser beam, the faster the speed at which the remelted portion separates from the laser beam irradiation. Because the portion irradiated with the laser beam is primarily heated, the faster the scanning speed, the less the portion is affected by the heat of the laser beam after remelting, and the more rapidly it cools. The faster the cooling rate, the finer the structure becomes, so it is presumed that Example 1, which had a faster scanning rate, produced a finer structure than Example 2.

[0060] The hardness of the piston surface, measured in Vickers hardness, was 193 HV0.1 for Example 1 and 171 HV0.1 for Example 2, with Example 1, which had a finer structure, being harder. On the other hand, the hardness of the comparative example was 126 HV0.1, which shows that the hardness improved due to the effect of finer structure achieved by laser processing, and the more the finer the structure was, the more the hardness improved. [Explanation of symbols]

[0061] 10. Piston for internal combustion engine 18 Skirt Club 20 Manufacturing method of piston for internal combustion engine 30 marks 31 Groove 32 Intermediate surface (plateau) 40 Base material 41 Primary silicon 42 Eutectic silicon 43 Silicon dispersion layer 50 irradiation spots 51 Laser light irradiation device 60 Thrust side skirt entire surface 61 Central skirt area 62 Remaining Area 70 Entire skirt on anti-thrust side 71 Upper skirt area 72 Lower Skirt Area 73 Remaining Area 80 Piston fixing jig 81 Piston support 82 Piston set part

Claims

1. a piston body having an outer periphery configured to be slidable in a longitudinal direction relative to an inner periphery of a cylinder bore; two skirt portions that are opposed to each other in the radial direction of the piston body and extend from the outer periphery of the piston body toward the bottom of the cylinder bore, the two skirt portions being comprised of a thrust-side skirt portion located on the thrust side in a skirt opposing direction, which is a direction in which the two skirt portions are opposed to each other, and an anti-thrust-side skirt portion located on the anti-thrust side in the skirt opposing direction; A piston for an internal combustion engine, comprising: the piston body and the two skirt portions are made of an aluminum alloy as a base material, and a silicon-dispersed layer is formed on the outer surface of each of the two skirt portions, in which silicon in the aluminum alloy is dispersed more finely than in the base material; a first silicon dispersion layer formed on the thrust-side skirt portion having an average silicon grain size smaller than a second silicon dispersion layer formed on the anti-thrust-side skirt portion;

2. the first silicon dispersion layer is disposed in a central skirt region located at the center of the thrust-side skirt portion, the central skirt region being a region located at a position where the outer diameters of the two skirt portions are maximum; 2. The piston for an internal combustion engine according to claim 1, wherein a third silicon dispersion layer having an average silicon grain size larger than the average silicon grain size of the first silicon dispersion layer is formed in the remaining region of the thrust side skirt portion other than the central skirt region.

3. a fourth silicon dispersion layer having an average silicon grain size between the average silicon grain size of the first silicon dispersion layer and the average silicon grain size of the second silicon dispersion layer is further formed on the anti-thrust side skirt portion; the fourth silicon dispersion layer is disposed in an upper skirt region located closer to the piston crown surface than the positions where the outer diameters of the two skirt portions are greatest, and in a lower skirt region located closer to the skirt end portions, 3. The piston for an internal combustion engine according to claim 1, wherein the second silicon dispersion layer is disposed on a remaining region of the anti-thrust side skirt portion other than the upper skirt region and the lower skirt region.

4. 3. The piston for an internal combustion engine according to claim 1, wherein the Vickers hardness of the first silicon dispersion layer formed on the thrust-side skirt portion is higher by 10 HV0.1 or more than the Vickers hardness of the second silicon dispersion layer formed on the anti-thrust-side skirt portion.

5. A method for manufacturing a piston for an internal combustion engine, comprising: a piston for an internal combustion engine, the piston comprising: a piston body having an outer periphery configured to be slidable in a longitudinal direction relative to an inner periphery of a cylinder bore; and two skirt portions facing each other in a radial direction of the piston body and extending from the outer periphery of the piston body toward a bottom of the cylinder bore, the two skirt portions comprising a thrust-side skirt portion located on the thrust side in a skirt facing direction, and an anti-thrust-side skirt portion located on the anti-thrust side in the skirt facing direction, the piston body and the skirt portions being made of an aluminum alloy as a base material; a laser light irradiation step of irradiating pulsed laser light onto outer surfaces of the two skirt portions of the piston for an internal combustion engine to form silicon dispersed layers thereon; wherein, in the laser light irradiation step, the average silicon grain size of the first silicon dispersion layer formed in the thrust-side skirt portion is made smaller than the average silicon grain size of the second silicon dispersion layer formed in the anti-thrust-side skirt portion.

6. In the laser light irradiation step, the first silicon dispersion layer is disposed in a central skirt region located at the center of the thrust-side skirt portion, and the central skirt region is a region located at a position where the outer diameters of the two skirt portions are maximum, 6. The method for manufacturing a piston for an internal combustion engine according to claim 5, wherein a third silicon dispersion layer having an average silicon grain size larger than the average silicon grain size of the first silicon dispersion layer is formed in a remaining region of the thrust-side skirt portion other than the central skirt region.

7. 7. The method for manufacturing a piston for an internal combustion engine according to claim 5, wherein in the laser light irradiation step, a scanning speed of the pulsed laser light on the thrust-side skirt portion when forming the first silicon dispersion layer is set faster than a scanning speed of the pulsed laser light on the anti-thrust-side skirt portion when forming the second silicon dispersion layer.

8. 7. The method for manufacturing a piston for an internal combustion engine according to claim 5, wherein in the laser light irradiation step, the silicon dispersion layer is formed by scanning an irradiation spot of pulsed laser light on the skirt portion within a range where spot diameters of the irradiation spot of pulsed laser light partially overlap, and an overlap length of the spot diameter with respect to the thrust-side skirt portion when forming the first silicon dispersion layer is shorter than an overlap length of the spot diameter with respect to the anti-thrust-side skirt portion when forming the second silicon dispersion layer.

9. In the laser light irradiation step, the piston for an internal combustion engine is attached to a piston fixing jig, and the piston for an internal combustion engine and the piston fixing jig are rotated to scan with pulsed laser light; the piston fixing jig comprises: a cylindrical piston support portion having the same diameter as the piston for an internal combustion engine; and a piston set portion located at one end of the piston support portion and adapted to receive the piston for an internal combustion engine; 6. A method for manufacturing a piston for an internal combustion engine according to claim 5, wherein a heat capacity of the piston set portion in contact with a portion of the thrust-side skirt portion where the first silicon dispersion layer is formed is greater than a heat capacity of the piston set portion in contact with a portion of the anti-thrust-side skirt portion where the second silicon dispersion layer is formed, or the portion of the anti-thrust-side skirt portion where the second silicon dispersion layer is formed is not in contact with the piston set portion.

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