Piston for internal combustion engine and manufacturing method thereof

Abstract

To provide a piston for an internal combustion engine, which has excellent adhesion of a resin coating and seizure resistance, and a method for manufacturing the same.SOLUTION: When irradiating the outer surface of a skirt portion 18 of a piston for an internal combustion engine with a pulsed laser beam, an irradiation spot is scanned from one side to the other side in the circumferential direction of the skirt portion so that the center-to-center distance between adjacent irradiation spots is equal to or less than the spot radius. Then, the irradiation spot is repeatedly scanned in the longitudinal direction of the skirt portion 18 within a range in which a part of the spot diameter overlaps. After that, by forming a resin coating 45, a silicon dispersed layer 43, in which silicon in an aluminum alloy is dispersed more finely than the base material, is formed on the outer surface of the skirt portion 18. A plurality of recesses 44 are formed on the surface of the silicon dispersed layer, the recess 44 has a portion 48 having a chevron-shaped section and projecting inward from the inner peripheral surface of the recess, and a part of the resin coating 45 is filled in the inside of the recess 44.SELECTED DRAWING: Figure 7

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] One example of such a piston is described in Patent Document 1, in which shot material such as hard particles is uniformly sprayed by shot peening onto the outer surfaces of two skirt portions of a piston formed by casting to form multiple dimples, and then a lubricating coating (resin coating) is formed on the outer surfaces of the two skirt portions using a resin with predetermined particles dispersed therein. On the outer surfaces of the skirt portions where multiple dimples are formed in this way, the surface roughness of the lubricating coating can be reduced, and the initial conformability of the lubricating coating can be improved. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-160293 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the piston described in Patent Document 1, the shots sprayed by shot peening may remain on the outer surface of the skirt portion, making it difficult for the resin coating to adhere to the outer surface of such a skirt portion, which may result in a decrease in the sliding performance of the piston.

[0006] In recent years, there has been a growing demand for higher engine efficiency, higher compression ratios, and turbocharged engines in response to environmental regulations, which has led to an increase in the maximum combustion pressure of engines. Against this background, there is a demand for further improvements in the adhesion and seizure resistance of resin coatings on piston skirts.

[0007] In view of the above problems, the present invention has an object to provide a piston for an internal combustion engine that has excellent adhesion and seizure resistance of a resin coating, and a method for manufacturing the same. [Means for solving the problem]

[0008] In order to achieve the above object, one aspect of the present invention is a piston for an internal combustion engine, comprising: a piston body having an outer periphery configured to be slidable longitudinally relative to the inner periphery of a cylinder bore; two skirt portions facing each other radially of the piston body and extending from the outer periphery of the piston body toward the bottom of the cylinder bore; and a resin coating formed on the outer surface of the skirt portions, wherein the piston body and the skirt portions are constructed using an aluminum alloy as a base material, and a silicon dispersion layer is formed on the outer surface of the skirt portions in which silicon in the aluminum alloy is dispersed more finely than in the base material, and a plurality of recesses are formed on the surface of the silicon dispersion layer, and the silicon dispersion layer has a mountain-shaped cross section that protrudes inward from the inner periphery of the recesses, and a portion of the resin coating fills the interior of the plurality of recesses in the silicon dispersion layer.

[0009] In another aspect, the present invention provides a method for manufacturing a piston for an internal combustion engine having a resin-coated skirt portion, the method comprising the steps of: preparing a piston for an internal combustion engine, the piston having 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 that face each other in a radial direction of the piston body and extend from the outer periphery of the piston body toward a bottom of the cylinder bore, the piston body and the skirt portions being made of an aluminum alloy as a base material; a laser light irradiation step of irradiating an outer surface of the skirt portion of the piston for an internal combustion engine with a pulsed laser light; and and a surface treatment process for forming a resin coating on the outer surface of the skirt portion irradiated with laser light, wherein the laser light irradiation process comprises a series of repeated scanning processes including a first scanning process for scanning the irradiation spots from one side to the other in the circumferential direction of the skirt portion so that the center-to-center distance between adjacent irradiation spots is equal to or less than the spot radius, a second scanning process for scanning the irradiation spots in the longitudinal direction of the skirt portion within a range where the spot diameters partially overlap, and a third scanning process for scanning the irradiation spots from the other side to the one side in the circumferential direction of the skirt portion so that the center-to-center distance between adjacent irradiation spots is equal to or less than the spot radius. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a piston for an internal combustion engine having excellent adhesion and seizure resistance of a resin coating, and a method for manufacturing the same. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a front view schematically illustrating an example of a piston for an internal combustion engine. [Figure 2] 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 3] 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 4]3 is a schematic diagram for explaining a scanning pitch when scanning an irradiation spot in a circumferential direction in a laser light irradiation step in the manufacturing method of FIG. 2. FIG. [Figure 5] 3 is a schematic diagram for explaining a scanning pitch when scanning an irradiation spot in the longitudinal direction in a laser light irradiation step in the manufacturing method of FIG. 2. FIG. [Figure 6] 3 is a cross-sectional view schematically showing the state of the outer surface of the skirt portion after the laser light irradiation step in the manufacturing method of FIG. 2. FIG. [Figure 7] 3 is a cross-sectional view schematically showing the state of the outer surface of the skirt portion after a surface treatment step in the manufacturing method of FIG. 2. FIG. [Figure 8] 4 is a cross-sectional view schematically showing a state in which a surface treatment step is performed on the outer surface of the skirt portion shown in FIG. 3 without performing a laser light irradiation step (reference example). FIG. [Figure 9] 10 is a graph showing the roughness curve of the outer surface of the skirt portion of the piston for an internal combustion engine by the laser light irradiation process of the example. [Figure 10] 1 is a graph showing the roughness curve of the outer surface of the skirt portion of a piston for an internal combustion engine obtained by shot blasting in a comparative example. [Figure 11] 1 is a graph showing the roughness curves of the outer surface of the skirt portion of a piston for an internal combustion engine on which a resin coating of a reference example and an example is formed. [Figure 12] 1 is an SEM image showing the surface of a skirt portion of a piston for an internal combustion engine before the laser light irradiation process of a reference example. [Figure 13] 10 is an SEM image showing the surface of the skirt portion of the piston for an internal combustion engine after the laser light irradiation step of the example. [Figure 14] 1 is an SEM image showing the surface of a skirt portion of a piston for an internal combustion engine after shot blasting in a comparative example. [Figure 15] 10 is an SEM image showing a cross section of a skirt portion of a piston for an internal combustion engine on which a resin coating is formed without performing a laser light irradiation step according to a reference example. [Figure 16] 10 is an SEM image showing a cross section of a skirt portion of a piston for an internal combustion engine on which a resin coating is formed after the laser light irradiation step of an example. DETAILED DESCRIPTION OF THE INVENTION

[0012] 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.

[0013] First, an internal combustion engine piston 10 will be described. The internal combustion engine piston 10 of 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.

[0014] The piston 10 for an internal combustion engine has a substantially cylindrical shape, and as shown in Fig. 1, has a first ring groove 13, a second ring groove 15, and an oil ring groove 17 on its outer peripheral surface, in that order from the piston crown surface 11 side. 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.

[0015] 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.

[0016] Next, a manufacturing method 20 of a piston for an internal combustion engine according to this embodiment will be described. As shown in Fig. 2, the manufacturing method 20 of a 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, a step 25 of washing with water, and a step 26 of performing surface treatment to coat the skirt portion of the piston with a resin coating.

[0017] 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.

[0018] The laser light irradiation step 24 is a step of irradiating the outer surface of the skirt portion 18 of the internal combustion engine piston 10 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 internal combustion engine piston 10 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. First, the skirt portion 18, which is the target of laser light irradiation, will be described.

[0019] An enlarged cross-sectional shape of the outer surface of the skirt portion 18 is shown in Fig. 3. Note that Fig. 3 is obtained by rotating Fig. 1 by 90 degrees to the left. As shown in Fig. 3, the aluminum alloy that is the base material 40 of the skirt portion 18 has primary silicon 41 and eutectic silicon 42 crystallized in the α phase. Both the primary silicon 41 and the eutectic silicon 42 are crystal grains that crystallize during the piston manufacturing process.

[0020] Furthermore, in order to improve the seizure resistance between the outer surface and the inner wall surface of the cylinder bore, striations 30 are formed on the outer surface of the skirt portion 18. Note that, although the striations 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 the striations 30 are not formed.

[0021] 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.

[0022] 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.

[0023]

number

[0024] In the laser light irradiation process 24, pulsed laser light is irradiated onto the outer surface of the skirt portion 18. In order to achieve the above-mentioned objective of uniformly forming a silicon dispersion layer having recesses of a predetermined shape over the entire outer surface of the skirt portion 18, the laser light irradiation process 24 is mainly performed by scanning the irradiation spot along the circumferential direction of the skirt portion 18 (perpendicular to the paper surface of Figure 3) and scanning the irradiation spot along the longitudinal direction of the skirt portion 18 (parallel to the paper surface of Figure 3).

[0025] In the step of scanning the irradiation spot along the circumferential direction of the skirt portion 18, as shown in Fig. 4, the irradiation spots 50 irradiated with the pulsed laser light are scanned so as to overlap along the circumferential direction C of the skirt portion. At this time, the distance (pitch P) between the centers Ca and Cb of two adjacent irradiation spots 50a and 50b in the circumferential direction C is C ) is set to be equal to or smaller than the spot radius R of the irradiation spot 50. C(unit: mm) can be calculated from the following formula 2, where V is the scan speed (unit: mm / s) and F is the frequency (unit: Hz). P C =V / F...Equation 2

[0026] Therefore, the pitch P C The pulsed laser beam is irradiated onto the outer surface of the skirt portion by setting the scanning speed and frequency conditions so that the spot radius R of the irradiation spot 50 is equal to or less than the spot radius R of the irradiation spot 50. The spot radius R of the irradiation spot 50 is preferably in the range of 0.005 to 0.1 mm, for example, and more preferably in the range of 0.01 to 0.03 mm. The scanning speed is preferably in the range of 100 to 6000 mm / s, for example, and more preferably in the range of 500 to 3000 mm / s. The frequency is preferably in the range of 1 to 1200 kHz, for example, and more preferably in the range of 5 to 600 kHz. In addition, the pitch P 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 of is not particularly limited, but is preferably 5% or more of the spot radius R.

[0027] In the step of scanning the irradiation spot along the longitudinal direction of the skirt portion 18, as shown in Fig. 5, the irradiation spot 50 irradiated with the pulsed laser light scans along the longitudinal direction L of the skirt portion so that a part of the spot diameter D overlaps. That is, the distance (pitch P) between the centers Ca and Cb of two adjacent irradiation spots 50a and 50b in the longitudinal direction L is L ) is set to be less than the spot diameter D of the irradiation spot 50. L is preferably 80% or less of the spot diameter D, and more preferably 60% or less. L The lower limit is preferably 10% or more of the spot diameter D, and more preferably 40% or more.

[0028] The process of scanning the irradiation spot along the circumferential direction C or the longitudinal direction L of the skirt portion 18 may include, for example, first performing a first scanning process of scanning the irradiation spot from one end to the other in the circumferential direction C, then scanning from one end of the circumferential direction C of the skirt portion 18 to the other end, and then performing a second scanning process of scanning the irradiation spot in the longitudinal direction L, thereby completing one pitch P L Then, a third scanning step is performed in which the irradiation spot is scanned from the other end to the one end in the circumferential direction C (the opposite direction to the first scanning step), and the irradiation spot is scanned from the other end to the one end in the circumferential direction C of the skirt portion 18. By repeating this series of scanning steps, the outer surface of the skirt portion 18 can be uniformly irradiated with pulsed laser light.

[0029] These scanning steps may be performed by moving the laser side or the internal combustion engine piston side. For example, in the first and third scanning steps, the pulsed laser beam irradiation device is fixed and the internal combustion engine piston is rotated about its axis, thereby scanning the irradiation spot in the circumferential direction of the skirt portion. Also, in the second scanning step, the pulsed laser beam irradiation device is fixed and the internal combustion engine piston is moved along its longitudinal direction, thereby scanning the irradiation spot in the longitudinal direction of the skirt portion. In this way, by moving the internal combustion engine piston side, which is relatively lighter and smaller than the pulsed laser beam irradiation device, work efficiency can be improved.

[0030] FIG. 6 shows the cross-sectional shape of the outer surface of the skirt portion after the laser beam irradiation step 24. As shown in FIG. 6, on the outer surface of the skirt portion 18, the aluminum alloy irradiated with the pulsed laser beam is locally heated and melted, and then cooled, whereby the primary crystalline silicon 41 and eutectic silicon 42 in the aluminum alloy are refined, forming a silicon-dispersed layer 43 in which silicon is dispersed. The primary crystalline silicon 41 and eutectic silicon 42 are refined more finely as the irradiation power of the pulsed laser beam increases. Therefore, the laser power is preferably 5 to 50 W, for example, and more preferably 10 to 30 W.

[0031] Furthermore, in this silicon dispersion layer 43, numerous recesses 44 are formed that are significantly finer (e.g., 10 μm or less) than the grooves 31 of the scratches 30. As shown in FIG. 6, these recesses 44 have mountain-shaped cross-sections 48 that protrude inward from their inner peripheral surfaces. The reason for the formation of such recesses 44 is speculative, but is thought to be as follows. It is known that the laser intensity distribution has a Gaussian distribution shape, forming a bell-shaped distribution with the strongest intensity at the center and the base extending toward the periphery. After a recess 44 is formed at the center of the irradiation spot, this recess 44 is immediately exposed to the base of the next irradiation spot before solidifying. This delays solidification near the opening of the recess 44, causing it to sag, resulting in the formation of the mountain-shaped cross-sections 48. Furthermore, eutectic silicon 42 scattered throughout the α phase of the aluminum alloy has a lower melting point than the α phase, and is therefore more likely to melt. This may be the starting point for the formation of the recesses 44. Even in this case, it is presumed that the recess 44 is immediately exposed to the base of the next irradiation spot before solidifying, and therefore a mountain-shaped cross section 48 is formed in the same manner as above.

[0032] 6, the laser light irradiation step 24 does not significantly impair the shape of the scratches 30 on the outer surface of the skirt portion 18. For example, it is preferable that the plateau ratio of the scratches 30A after the laser light irradiation step 24, calculated from the widths of the groove portions 31A and the plateau portions 32A, is maintained 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.

[0033] Furthermore, it is preferable to suppress the change in plateau ratio, which is the difference between the plateau ratio of the scratch 30 before the laser light irradiation step 24 and the plateau ratio of the scratch 30A after the laser light irradiation step 24, to 0.05 or less. If the change in plateau ratio exceeds 0.05, there is a risk that the general shape of the scratch 30 will change. There is no particular limitation on the lower limit of the change in plateau ratio, but it is preferably 0.00 or more.

[0034] 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.

[0035] 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, degreasing by laser light irradiation may be insufficient or may hinder the formation of the silicon dispersion layer 43 and the recesses 44. 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.

[0036] 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.

[0037] 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.

[0038] The water washing step 25 is a step for removing dust particles that have adhered to the internal combustion engine piston 10 in the laser light irradiation step 24, and is a step that is performed as needed. The water washing step 25 may be, for example, ultrasonic water washing, or other simple steps.

[0039] The surface treatment step 26 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 26, 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.

[0040] FIG. 7 shows the cross-sectional shape of the outer surface of the skirt portion 18 after the surface treatment step 26. As shown in FIG. 7, a resin coating 45 is formed by the surface treatment step 26 on top of the silicon dispersion layer 43 formed on the surface of the aluminum alloy base material 40 of the skirt portion 18. At this time, since there are many fine recesses 44 on the surface of the silicon dispersion layer 43, the resin coating agent penetrates into the fine recesses 44, forming recesses 44B that are partially filled with the resin coating 45. In addition, fine pores 46 are formed on the surface of the resin coating 45 in correspondence with the recesses 44B.

[0041] On the other hand, Fig. 8 shows the cross-sectional shape of the outer surface of the skirt portion 18 when the surface treatment step 26 is performed without performing the laser light irradiation step 24. As shown in Fig. 8, primary crystalline silicon 41 and eutectic silicon 42 are exposed on the surface of the base material 40 of the skirt portion 18. Therefore, when a resin coating 45 is formed on the surface of the base material 40, the resin coating 45 has a weak bonding strength with the primary crystalline silicon 41 and eutectic silicon 42, and therefore can become a starting point for the occurrence of point delamination due to external force. Furthermore, point delamination can become a starting point for wide-area interfacial delamination.

[0042] In contrast, when the surface treatment step 26 is performed after the laser light irradiation step 24, as shown in FIG. 7, the primary crystal silicon and eutectic silicon are refined and the silicon dispersion layer 43 is formed over the entire outer surface of the skirt portion 18, thereby preventing the occurrence of weak bonding and point peeling. In addition, since a portion of the resin coating 45 is filled inside the numerous recesses 44B formed in the silicon dispersion layer 43, the adhesiveness of the resin coating 45 can be improved by an anchor effect. Furthermore, since the recesses 44B have the mountain-shaped cross-section 48 at their openings as described above, a portion of the resin coating 45 penetrates deep into the mountain-shaped cross-section 48 of the recesses 44B, thereby further improving adhesive strength with the resin coating 45.

[0043] As shown in FIG. 7 , even after the resin coating 45 is formed by the surface treatment process 26, the shape of the streaks 30B consisting of the grooves 31B and the plateaus 32B is maintained. Because the skirt portion 18 of the internal combustion engine piston 10, particularly the central region, slides against the inner circumferential surface of the cylinder under high-pressure contact, wear caused by foreign matter is likely to progress, and seizure (scuffing) is likely to occur. In this embodiment, since the streaks 30B remain even after the surface treatment process 26, foreign matter is not retained in high-pressure contact. The circumferentially extending grooves 31B form pockets, and foreign matter is expelled from the center of the skirt portion 18 to the circumferentially outer side through these pockets. This suppresses wear and ultimately ensures seizure resistance. Furthermore, the grooves 31B retain oil, providing lubrication but also imparting viscous resistance. However, during high-speed, high-load operation of the engine, the lubrication effect outweighs the viscous resistance, thereby reducing friction.

[0044] The manufacturing method of the internal combustion engine piston 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 beam irradiation process 24 and the surface treatment process 26. The anodizing process forms a porous anodic oxide film on the outer surface of the skirt portion 18. By setting the film thickness to a few micrometers or less, the anodic oxide film can be formed while maintaining the shape of the striations 30A on the outer surface of the skirt portion 18. Filling the micropores in the anodic oxide film with a resin coating agent can be expected to further improve adhesion. Furthermore, the chemical conversion process forms a chemical conversion film by chemically reacting a portion of the outer surface of the skirt portion 18. Because the film thickness is a few micrometers, the chemical conversion film can be formed while maintaining the shape of the striations 30A on the outer surface of the skirt portion 18. Because the chemical conversion film is formed in a granular shape and has irregularities, filling these irregularities with a resin coating agent can be expected to further improve adhesion. When anodizing is performed, 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]

[0045] EXAMPLES Hereinafter, examples and comparative examples according to the present invention will be described, but the method for manufacturing a piston for an internal combustion engine according to the present invention is not limited to the following examples and comparative examples.

[0046] [Example] An internal combustion engine piston was manufactured from Al-Si aluminum alloy casting, a high-strength material with improved mechanical properties in the high-temperature range, and scratches were formed on the outer surface of the skirt using a turning tool. Then, a laser irradiation process was carried out in which pulsed laser light was irradiated onto the outer surface of the skirt with scratches formed. The laser processing machine used (manufactured by AkiTech LEO Co., Ltd., model 2LC-S-SF) was equipped with a master oscillator power amplifier (MOPA) type fiber laser, with a rated output of 50 W. The pulsed laser light irradiation conditions were an output of 15 W, a spot diameter d of the irradiation spot of 0.04 mm (spot radius r: 0.02 mm), a scanning speed of 1000 mm / s, a frequency of 500 kHz, and a circumferential pitch P C 0.002mm, longitudinal pitch P L was set to 0.02 mm.

[0047] To investigate the surface properties of the skirt after the laser irradiation process, (1) the surface roughness was measured, (2) the plateau ratio was calculated from the roughness curve, and (3) the surface was observed with a scanning electron microscope (SEM) and the silicon distribution was analyzed with an attached energy dispersive X-ray spectrometer (EDS). In order to evaluate these, the skirt before the laser irradiation process was also subjected to the same measurements and observations.

[0048] The surface roughness was measured in accordance with JIS B 0601-2001 by measuring the roughness curve of the piston skirt, and the surface roughness Ra, which represents the arithmetic mean roughness, and the surface roughness Rz, which represents the maximum height, were calculated from the results. The roughness was measured along the longitudinal direction of the piston so that the shape of the scratches could be seen.

[0049] The plateau ratio was calculated as the average value by measuring the width of the plateau portion (Wa) and the width of the groove portion (Wb) for any three scratches from the roughness curve measurement results and calculating it using the above formula 1. In addition, the change in the plateau ratio was calculated from the difference between the plateau ratio of the piston before the laser light irradiation process and the plateau ratio of the piston after the laser light irradiation process.

[0050] The silicon distribution analysis by EDS was carried out by observing the distribution of silicon on the outer surface of the skirt portion by elemental mapping of silicon (Si).

[0051] Next, the pistons that had been subjected to the laser irradiation process underwent a surface treatment process in which a resin coating was formed. The resin coating was MOLYKOTE D-10-GBL manufactured by DuPont Toray Specialty Materials Co., Ltd. (formerly Dow Corning Toray Co., Ltd.). After cleaning the pistons, a resin coating agent was printed using a screen printing method and baked at a temperature of less than 200°C to form a resin coating on the piston skirt.

[0052] Then, to investigate the surface properties of the skirt part on which this resin coating was formed, (4) surface roughness measurement, (5) measurement of the resin coating film thickness by observing the cross section with an SEM and measuring the thickness of the silicon dispersion layer with an EDS, and (6) a test to evaluate the adhesion of the resin coating were performed. In order to evaluate these, similar measurements and observations were also performed on a skirt part on which a resin coating was formed without irradiating it with pulsed laser light.

[0053] The surface roughness was measured in accordance with JIS B 0601-2001 in the same manner as above, by measuring the roughness curve of the skirt portion of the piston, but the surface roughness Ra and Rz were not calculated.

[0054] The thickness of the resin coat was measured at 10 points on the cross section of the skirt, 5 points in the grooves of the scratches and 5 points in the plateau, on an SEM image of the skirt, and the average was calculated. The thickness of the silicon dispersion layer was also measured at arbitrary points on the cross section of the skirt using EDS Si element mapping.

[0055] The adhesion evaluation test was conducted using a method called the water jet (W / J) method, in which water is sprayed from directly above the resin coating using a nozzle at high pressure, and then the adhesion is evaluated from the appearance of the resin coating surface.For reference, the adhesion of the resin coating was also evaluated using the same test on pistons that had been coated with a resin coating without undergoing the laser light irradiation process.

[0056] [Comparative Example] An internal combustion engine piston was manufactured in the same manner as in the examples, and scratches were formed on the skirt portion. In the comparative examples, the skirt portion was subjected to shot blasting instead of pulsed laser light irradiation. Glass beads #150 were used as the shot material (media) for shot blasting, and shot blasting was performed with a projection pressure of 0.4 MPa and a distance (projection distance) of 50 cm between the projection nozzle and the skirt portion. In order to confirm the surface roughening effect depending on the projection time, the projection time was set to 5 seconds and 10 seconds. After shot blasting, air blowing, cleaning, and drying were performed.

[0057] The pistons thus obtained were evaluated in the same manner as in the Examples, by (1) measuring the surface roughness, (2) measuring the change in plateau ratio, and (3) observing the surface with an SEM and analyzing the silicon distribution with an EDS. To perform these evaluations, the pistons before alkaline etching were also subjected to the same measurements. Furthermore, for pistons on which a resin coating was formed, a test was conducted to evaluate the adhesion of the resin coating (6).

[0058] The measurement results of (1) to (6) above will be explained. First, the measurement results of the surface roughness of the example and comparative example are shown in Figures 9 and 10. Furthermore, the plateau ratios and the amount of change thereof of the example and comparative example are shown in Table 1.

[0059] [Table 1]

[0060] In Figure 9, (a) is the roughness curve of the skirt portion before the laser light irradiation process, and (b) is the roughness curve of the skirt portion after the laser light irradiation process. As shown in Figure 9, it was confirmed that the laser light irradiation process formed depressions on the surface while maintaining the streak shape. As shown in Table 1, the change in plateau ratio before and after the laser light irradiation process was 0.04. A smaller change in plateau ratio indicates that the streak shape is maintained, while a larger change in plateau ratio indicates that the streak shape has disappeared. It was found that the streak shape was maintained when the change in plateau ratio was 0.05 or less.

[0061] Furthermore, the surface roughness Ra, which represents the arithmetic mean roughness, increased from 3.2 μm before the laser irradiation process to 3.9 μm after the laser irradiation process. The surface roughness Rz, which represents the maximum height, increased significantly from 11.2 μm before the laser irradiation process to 19.3 μm after the laser irradiation process. This is presumably because the outer surface of the skirt portion had an increased number of fine depressions while maintaining the shape of the striations.

[0062] On the other hand, for the comparative example, Figure 10 shows (a) the roughness curve of the skirt portion before shot blasting, (b) the roughness curve after 5 seconds of shot blasting, and (c) the roughness curve after 10 seconds of shot blasting. As shown in Figure 10, in the comparative example, as the shot blasting time increased, the shape of the streaks disappeared and the surface became more flat. As shown in Figure 9(b), after 5 seconds of shot blasting, the shape of the streaks had already been significantly distorted, and no plateaus could be observed. Furthermore, because it was difficult to calculate the plateau ratio, it is not listed in Table 1.

[0063] Furthermore, the surface roughness Ra, which represents the arithmetic mean roughness, was 2.8 μm before shot blasting, decreasing to 2.3 μm after 5 seconds of shot blasting and 2.0 μm after 10 seconds. The surface roughness Rz, which represents the maximum height, was 15.0 μm before shot blasting, decreasing to 15.1 μm after 5 seconds of shot blasting and 12.1 μm after 10 seconds. This is presumably because while fine recesses were formed, the shape of the streaks had almost completely disappeared.

[0064] Next, Figure 11 shows (a) the roughness curve of the skirt portion (reference example) on which a resin coating was formed by the surface treatment process without performing the laser beam irradiation process, and (b) the roughness curve of the skirt portion (example) on which a resin coating was formed by the surface treatment process after performing the laser beam irradiation process. From Figures 9(b) and 11(b), it can be seen that the resin coating has filled the recesses created by the laser beam irradiation process, and that the striation shape has been maintained. Also, in Figure 11(b), it can be seen that numerous micropores have been formed on the surface of the resin coating. Because the positions of these micropores correspond to the positions of the recesses created by the laser beam irradiation process, it is presumed that they were formed when the recesses created by the laser beam irradiation process were filled with the resin coating.

[0065] Fig. 12 is an SEM image of the surface of the skirt portion (Reference Example) before the laser light irradiation process, Fig. 13 is an SEM image of the surface of the skirt portion (Example) after the laser light irradiation process, and Fig. 14 is an SEM image of the surface of the skirt portion (Comparative Example) after shot blasting. As shown in Fig. 13, in the Example, it can be seen that a large number of recesses were formed on the entire surface of the skirt portion by the laser light irradiation process. On the other hand, as shown in Fig. 14, in the Comparative Example, it was confirmed that fine crater-like irregularities had been formed on the surface of the skirt portion by shot blasting. Furthermore, since no striations were observed, it is presumed that they had disappeared by shot blasting.

[0066] Figure 15 is an SEM image of the cross section of a skirt portion (reference example) on which a resin coating was formed by a surface treatment process without performing a laser light irradiation process. As shown in Figure 15, the surface of the aluminum alloy base material 40 of the skirt portion is a smooth surface, on which a resin coating 45 is formed. Visible above the resin coating 45 is the embedding resin 47 used for SEM observation. The film thickness of the resin coating 45 was 9.8 μm.

[0067] FIG. 16 is an SEM image of a cross section of a skirt portion (Example) on which a resin coating was formed by a surface treatment process after the laser light irradiation process. As shown in FIG. 16, it was confirmed that recesses were formed in the base material 40 of the skirt portion by the laser light irradiation process. These recesses had a cross-sectional appearance of a mountain-shaped shape that protruded inward from the inner peripheral surface. The mountain-shaped shape was located at the opening of the recess, and although the opening of the recess was narrow, it was confirmed that the resin coating 45 was filled all the way to the inside of the recess. The film thickness of the resin coating 45 was measured excluding the recess portion and was 9.1 μm.

[0068] Furthermore, EDS Si element mapping of the surface and cross section of the skirt portion revealed that silicon was distributed in clumps in places in the surface region of the skirt portion (reference example) before the laser light irradiation process, and that silicon was segregated. This is determined to be primary crystalline silicon or eutectic silicon in the aluminum alloy exposed to the surface by machining. In contrast, after the laser light irradiation process, silicon was observed to be distributed almost evenly over the entire surface in the surface region of the skirt portion (example). Therefore, it is determined that the primary crystalline silicon or eutectic silicon exposed to the surface was refined by the laser light irradiation process, and a silicon dispersion layer (reference numeral 43 in Figure 16) in which silicon was dispersed over the entire surface was formed. The thickness of the silicon dispersion layer was 5 μm.

[0069] On the other hand, it was observed that silicon was segregated and partially distributed in a mottled manner on the surface of the skirt portion (comparative example) after shot blasting. It was found that the silicon-dispersed layer described above is difficult to form by shot blasting.

[0070] Furthermore, elemental mapping of carbon (C) by EDS on the cross section of the skirt portion confirmed that carbon was distributed in the recesses in the same way as in the resin coated portion, which also indicates that the recesses were filled with resin coating.

[0071] Table 2 shows the evaluation results of the adhesion of the resin coat. The evaluation criteria were as follows: when the resin coat was observed, no peeling was evaluated as "Good", when there was spot peeling on the resin coat, it was evaluated as "Good", and when there was interfacial peeling of the resin coat from the aluminum alloy base material, it was evaluated as "Poor". Note that for the comparative example, observations were made on a specimen in which the shot blasting treatment time was 5 seconds.

[0072] [Table 2]

[0073] As shown in Table 2, interfacial peeling was observed in the resin coating on the outer surface of the skirt portion of the Reference Example, which was not subjected to laser light irradiation or shot blasting. On the other hand, neither interfacial peeling nor point peeling was observed in the resin coating on the skirt portion of the Example, which was subjected to the laser light irradiation process, nor on the skirt portion of the Comparative Example, which was subjected to shot blasting. This clearly shows that the Example, in which recesses having a predetermined mountain shape were formed on the aluminum alloy surface by laser light irradiation before the resin coating was formed, has the effect of improving the adhesion of the resin coating through the anchor effect, similar to the Comparative Example, in which fine irregularities were formed by shot blasting before the resin coating was formed. [Explanation of symbols]

[0074] 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 44 recess 45 Resin Coat 48 Chevron shaped section 50 irradiation spots

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; a resin coating formed on the outer surface of the skirt portion; A piston for an internal combustion engine, comprising: a piston for an internal combustion engine, wherein the piston body and the skirt portion are constructed using an aluminum alloy as a base material, a silicon dispersion layer is formed on an outer surface of the skirt portion in which silicon in the aluminum alloy is dispersed more finely than in the base material, a plurality of recesses are formed on the surface of the silicon dispersion layer, the silicon dispersion layer has a mountain-shaped cross section that protrudes inward from the inner surfaces of the recesses, and a portion of the resin coating is filled inside the plurality of recesses in the silicon dispersion layer.

2. 2. The piston for an internal combustion engine according to claim 1, wherein a plurality of grooves extending in a circumferential direction are provided on an outer surface of the skirt portion, the plurality of grooves are arranged at intervals in the longitudinal direction, an intermediate surface is provided between adjacent grooves in the longitudinal direction, and the plurality of recesses are formed in the plurality of grooves and the intermediate surface.

3. A method for manufacturing a piston for an internal combustion engine having a resin coating on a skirt portion, comprising: a step of preparing 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 that face each other in a radial direction of the piston body and extend from the outer periphery of the piston body toward a bottom of the cylinder bore, the piston body and the skirt portions being made of an aluminum alloy as a base material; a laser light irradiation step of irradiating a pulsed laser light onto an outer surface of the skirt portion of the piston for an internal combustion engine; a surface treatment step of forming a resin coating on the outer surface of the skirt portion irradiated with the pulsed laser light; The laser light irradiation step includes: a first scanning step of scanning the irradiation spots from one side to the other in the circumferential direction of the skirt portion so that the center-to-center distance between adjacent irradiation spots is equal to or less than the spot radius; a second scanning step of scanning the irradiation spot in the longitudinal direction of the skirt portion within a range where the spot diameters partially overlap; a third scanning step of scanning the irradiation spots from the other side to the one side in the circumferential direction of the skirt portion so that the center-to-center distance between adjacent irradiation spots is equal to or less than the spot radius; a silicon dispersion layer in which silicon in the aluminum alloy is dispersed more finely than in the base material is formed on the outer surface of the skirt portion, a plurality of recesses are formed on the surface of the silicon dispersion layer, the silicon dispersion layer has a mountain-shaped cross section that protrudes inward from the inner circumferential surfaces of the recesses, and part of the resin coat can fill the interiors of the plurality of recesses in the silicon dispersion layer.

4. 4. The method for manufacturing a piston for an internal combustion engine according to claim 3, wherein a plurality of grooves extending in a circumferential direction are provided on the outer surface of the skirt portion, the plurality of grooves being arranged at intervals in the longitudinal direction, and an intermediate surface being provided between adjacent grooves in the longitudinal direction.

5. 5. The method for manufacturing a piston for an internal combustion engine according to claim 4, wherein the change in plateau ratio of the outer surface of the skirt portion before and after the laser light irradiation step is set to 0.05 or less.

6. 6. A method for manufacturing a piston for an internal combustion engine according to any one of claims 3 to 5, wherein in the first scanning step or the third scanning step, the piston for an internal combustion engine is rotated around its axis to scan the irradiation spot in the circumferential direction of the skirt portion, and in the second scanning step, the piston for an internal combustion engine is moved along its longitudinal direction to scan the irradiation spot in the longitudinal direction of the skirt portion.

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