Manufacturing method for piston for internal combustion engine

Abstract

To provide a manufacturing method for a piston for an internal combustion engine capable of eliminating the necessity of a water washing tank and a degreasing tank that have a large capacity and eliminating the necessity of thermal energy for warming the tank by replacing a conventional wet-type degreasing method while maintaining adhesion strength of a resin coating.SOLUTION: In a piston for an internal combustion engine, a plurality of groove parts 31 extending in a circumferential direction are provided on an outer surface of a skirt part 18, and intermediate face parts 32 are provided between the plurality of groove parts. A manufacturing method for the piston for the internal combustion engine includes: a process of spirally irradiating the outer surface of the skirt part 18 with a laser beam 51; and a surface treatment process of forming a resin coating on the outer surface of the skirt part. In the laser beam irradiation process, a center-to-center distance between irradiation spots adjacent to each other in the circumferential direction of the skirt part within a range where a part of spot diameters of the adjacent irradiation spots are overlapped with each other is set to be less than a spot radius and a center-to-center distance between irradiation spots adjacent to each other in an axial direction of the skirt part is set to be the spot radius or more.SELECTED DRAWING: Figure 7

Description

[Technical field]

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

[0002] In an internal combustion engine such as an engine installed in a vehicle such as an automobile, a piston reciprocates in a cylinder bore extending along a linear longitudinal axis in a direction along the longitudinal axis. At this time, 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 skirts extending from the outer periphery of the piston body to the bottom side of the cylinder bore, and further, in the piston, a resin coating is formed on the outer surface of each skirt in order to reduce the frictional resistance of the outer periphery of the piston against the inner periphery of the cylinder bore.

[0003] As an example of such a piston, Patent Document 1 describes the formation of a large number of striations on the outer surface of the skirt portion in order to enhance lubrication. Here, the striations will be explained. The striations are grooves formed on the outer surface of the skirt portion so as to extend in the circumferential direction. An intermediate surface portion (plateau portion) is provided between adjacent groove portions in the axial direction. By collecting oil in the groove portions of the striations, a good lubrication state is maintained between the outer surface of the skirt portion and the inner wall surface of the cylinder bore. As a result, even if the piston reciprocates at high speed, oil shortage does not occur, and seizure is prevented between the outer surface of the skirt portion and the inner wall surface of the cylinder bore. In addition, when foreign matter is mixed into the oil, wear is likely to progress on the sliding surface, but the groove portions of the striations form pockets extending in the circumferential direction and expel the foreign matter, so that wear is suppressed and seizure resistance can be ensured. In particular, Patent Document 1 describes that the skirt portion on the thrust side to which lateral pressure is applied during combustion is in sliding contact with the inner wall surface of the cylinder bore in a high-pressure surface state, so that friction increases and seizure is likely to occur. It is described that by increasing the ratio of the plateau portion in the scratches, the surface pressure is reduced, and friction is reduced.

[0004] Furthermore, Patent Document 2 describes a method for providing a piston for an internal combustion engine having excellent adhesion and seizure resistance of a resin coating while maintaining the general shape of the streaks, the method including: an alkaline etching step of alkaline etching the skirt portion to form a plurality of recesses on the outer surface of the skirt portion that are finer than the grooves while maintaining the configuration of the grooves and intermediate surface provided on the outer surface of the skirt portion; a cleaning step of removing smut generated on the outer surface of the skirt portion by alkaline etching; and a surface treatment step of forming a resin coating on the outer surface of the skirt portion from which the smut has been removed. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2008-232172 A [Patent Document 2] JP 2023-019522 A Summary of the Invention [Problem to be solved by the invention]

[0006] Conventionally, in order to form a resin coat on the skirt portion of a piston, it was necessary to perform a number of steps, such as a first water washing step, a first degreasing step, a second degreasing step, a second water washing step, and a drying step, before the resin coat printing step, in order to wash off the cutting oil that adhered to and remained on the surface of the skirt portion in the machining step and obtain adhesion strength of the resin coat. In the first water washing step, dirt other than the cutting oil is mainly removed before the first degreasing step. It is to be noted that the method is not limited to water washing, and hot water washing or a combination of these may be used. In the first degreasing step, the cutting oil on the skirt surface is removed using a neutral or alkaline degreasing agent. In this step, it is desirable to immerse the piston (skirt surface) in the degreasing liquid and perform ultrasonic cleaning. In the subsequent second degreasing step, degreasing is performed using the same or a different degreasing agent as in the first degreasing step, and the cutting oil adhered to the skirt surface is further removed. If necessary, a water washing step may be performed between the first degreasing step and the second degreasing step. In the second water washing step after the second degreasing step, the degreasing liquid adhered to the skirt surface is removed. This step may be either water washing or hot water washing, or a combination of both. After the second water washing step, a drying step is carried out to remove moisture, especially that adhering to the skirt surface. If moisture remains on the skirt surface, there is a concern that the adhesive strength of the resin coat printed in the subsequent resin coat printing step may decrease.

[0007] However, such conventional wet degreasing methods including multiple washing and degreasing steps require large-capacity washing and degreasing tanks, and also require a large amount of thermal energy to heat these tanks, so improvements were required.

[0008] In view of the above problems, an object of the present invention is to provide a method for manufacturing a piston for an internal combustion engine, which can replace the conventional wet degreasing method, while maintaining the adhesive strength of the resin coat, and which does not require a large-capacity water washing tank or degreasing tank, and does not require thermal energy for heating the tank. [Means for solving the problem]

[0009] In order to achieve the above-mentioned object, the present invention provides a method for manufacturing a piston for an internal combustion engine, the method comprising: a piston for an internal combustion engine made of an aluminum alloy having a skirt portion that slides on an inner wall surface of a cylinder bore of the internal combustion engine, the outer surface of the skirt portion being provided with a plurality of groove portions extending in a circumferential direction, the plurality of groove portions being arranged at intervals in the axial direction, and an intermediate surface portion being provided between adjacent groove portions in the axial direction; and a laser light irradiation step of irradiating the outer surface of the skirt portion of the piston for an internal combustion engine with pulsed laser light in a spiral manner; and 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 being within a range where the spot diameters of adjacent irradiation spots partially overlap. Effect of the Invention

[0010] According to the present invention, the conventional wet degreasing method can be changed to the dry degreasing method, which eliminates the need for the large-capacity washing tank and degreasing tank required in the wet degreasing method, and also eliminates the need for thermal energy to heat the tank, while maintaining the adhesive strength of the resin coat. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a front view showing a schematic diagram of an example of a piston for an internal combustion engine. [Diagram 2] FIG. 1 is a flow chart for explaining an embodiment of a manufacturing method for a piston for an internal combustion engine according to the present invention. [Diagram 3] 3 is a cross-sectional view showing a schematic view of an outer surface of a skirt portion of a piston for an internal combustion engine after a machining step in the manufacturing method shown in FIG. 2. [Figure 4] FIG. 2 is a perspective view illustrating an example of a laser light irradiation step in the method for manufacturing a piston for an internal combustion engine according to the present invention. [Diagram 5] 1 is a graph of a Gaussian distribution of a fiber laser. [Figure 6] 3 is a schematic diagram for explaining overlapping of spot diameters of irradiation spots in a laser light irradiation step in the manufacturing method shown in FIG. 2. [Figure 7] FIG. 2 is a perspective view illustrating an example of a laser light irradiation step in the method for manufacturing a piston for an internal combustion engine according to the present invention. [Figure 8] FIG. 13 is a perspective view for explaining a comparative example of a laser light irradiation step. [Figure 9] 1 is a graph showing roughness curves of the outer surface of a skirt portion of a piston for an internal combustion engine of a test example and a comparative example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, an embodiment of a method for manufacturing a piston for an internal combustion engine according to the present invention will be described with reference to the drawings.

[0013] First, the piston 10 for an internal combustion engine will be described. The piston 10 for an internal combustion engine of this embodiment is a member that reciprocates inside 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 AC4A, AC8A, and AC9, ADC materials such as ADC10 to ADC14, and A4000 series materials for pistons.

[0014] An internal combustion engine piston 10 has a generally cylindrical shape and, as shown in Fig. 1, has on its outer circumferential surface, in 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 circumferential 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] The skirt portions 18 are disposed on both sides with pin boss portions 19 therebetween. The piston 10 reciprocates inside the cylinder bore while the outer surface of the skirt portions 18 slides on the inner wall surface of the cylinder bore.

[0016] Next, a manufacturing method 20 of a piston for an internal combustion engine according to the present 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 a piston, a step 22 of heat treating the cast piston, a step 23 of machining the heat treated piston, a step 24 of irradiating a skirt portion of the piston with laser light, a step 25 of printing a resin coat on the skirt portion of the piston, and a step 26 of baking the piston with the printed resin coat.

[0017] The above-mentioned casting, heat treatment and machining steps 21, 22 and 23 are similar to 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 piston 10 for an internal combustion engine with a pulsed laser light in order to remove the cutting oil remaining on the outer surface of the skirt portion 18 of the piston 10 for an internal combustion engine in the machining step 23 and to roughen the outer surface of the skirt portion 18. First, the skirt portion 18 to be irradiated with the laser light will be described.

[0019] FIG. 3 shows an enlarged cross-sectional shape of the outer surface of the skirt portion 18. FIG. 3 is a view obtained by rotating FIG. 1 by 90° to the left. On the outer surface of the skirt portion 18, striations 30 are formed to improve the seizure resistance between the outer surface and the inner wall surface of the cylinder bore. The striations 30 have a plurality of grooves 31 with a U-shaped cross section formed to extend in the circumferential direction on the outer surface of the skirt portion 18. The plurality of 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. In addition, between the grooves 31 adjacent in the longitudinal direction, an intermediate surface portion (plateau portion) 32 is provided in which the radial outer end of the cross section extends linearly in the longitudinal direction.

[0020] 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 portions 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 portions 31, is, for example, in the range of 0.02 to 0.4. The plateau ratio has conventionally been used as one of the indexes representing the shape of the scratches 30.

[0021]

number

[0022] In the laser light irradiation process 24, a pulsed laser light is irradiated in a spiral pattern onto the outer surface of the skirt portion 18. That is, in the laser light irradiation process 24, the irradiation spot is basically scanned along the circumferential direction of the skirt portion 18 (perpendicular to the paper surface of FIG. 3; hereinafter also referred to as the "X direction"), but during this, the irradiation spot gradually moves in the longitudinal direction of the skirt portion 18 (parallel to the paper surface of FIG. 3; also the axial direction of the piston 10 for an internal combustion engine; hereinafter also referred to as the "Y direction").

[0023] In order to scan the irradiation spot in a spiral shape on the outer peripheral surface of the skirt portion 18, for example, an apparatus shown in FIG. 4 can be used. As shown in FIG. 4, the piston 10 for an internal combustion engine is held at its first land portion by a holding member 52 with a chuck 53, and the piston 10 for an internal combustion engine is rotated around its axis while irradiating the outer peripheral surface of the skirt portion 18 with a pulsed laser beam 51 from a laser device 50 at a fixed position. At the same time, the holding member 52 is moved in the axial direction of the piston 10 for an internal combustion engine by about one spot while the piston 10 for an internal combustion engine rotates once. By moving the piston 10 for an internal combustion engine in the axial direction while rotating it around its axis in this way, the outer peripheral surface of the skirt portion 18 is irradiated with the pulsed laser beam 51 in a spiral shape, and the entire outer peripheral surface of the skirt portion 18 can be irradiated with the pulsed laser beam. Of course, the laser beam irradiation step 24 is not limited to the apparatus shown in FIG. 4, and may be performed by an apparatus that moves the laser side.

[0024] The laser for irradiating the pulsed laser light is not particularly limited as long as it is a laser for metal processing, and for example, a fiber laser, a semiconductor laser, a CO2 laser, a solid-state laser, etc. can be used alone or in combination. In particular, fiber lasers have become widely used for laser marking, but as shown in Figure 5, fiber lasers have an energy gradient that is generally called a Gaussian distribution, in which the energy becomes higher toward the center of the spot, so the amount of heat generated differs between the center of the spot and the outer edge of the spot diameter. In other words, the center becomes hotter than the outer edge, and while the degreasing and roughening, which are the purposes of the above-mentioned laser light irradiation, are promoted in the center, the purpose is not easily promoted in the outer edge.

[0025] Therefore, when scanning the irradiation spot in a spiral shape on the outer peripheral surface of the skirt portion 18, if the irradiation spot is scanned so that the outer edges of adjacent spot diameters are in contact with each other, degreasing and roughening may be insufficient in the portion of the outer edge of the spot diameter where the laser energy is sparse, and the adhesive strength of the resin coat may be reduced. In particular, when the skirt portion 18 has streaks 30 and the portion where the laser energy is sparse is located in the concave groove portion 31 of the streaks 30, the degreasing and roughening are more likely to be non-uniform compared to when it is located in the flat plateau portion 32, and therefore the adhesive strength of the resin coat is reduced, and there is a high possibility that point peeling will occur, which leads to widespread peeling of the resin coat.

[0026] Therefore, when the irradiation spot is spirally scanned on the outer peripheral surface of the skirt portion 18, the irradiation spot is scanned within a range where the spot diameters of adjacent irradiation spots partially overlap. The overlap rate of adjacent spot diameters (hereinafter also referred to as "overlap rate") is expressed as 0% when the outer edges of adjacent spot diameters are in contact with each other (i.e., when the center-to-center distance between adjacent spots is the same as the spot diameter 2r) as shown in Fig. 6(a), the overlap rate is expressed as 10% when the center-to-center distance between adjacent spots is 0.9 times the spot diameter 2r, and the overlap rate is expressed as 50% when the center-to-center distance between adjacent spots is the spot radius r.

[0027] When the pulsed laser beam 51 is irradiated in a spiral shape, it is preferable that the center-to-center distance between adjacent irradiation spots in the circumferential direction (X direction) of the skirt portion 18 is less than the spot radius (i.e., overlap ratio exceeds 50%). On the other hand, it is preferable that the pulsed laser beam 51 goes around the outer circumferential surface of the skirt portion 18, and the center-to-center distance between adjacent irradiation spots in the longitudinal direction (Y direction) of the skirt portion 18 is equal to or greater than the spot radius (i.e., overlap ratio exceeds 0% to 50%) within a range where the spot diameters partially overlap.

[0028] By scanning the irradiation spots so that the adjacent spot diameters overlap in this way, it is possible to eliminate the portions where the laser energy is sparse, to uniformly degrease and roughen the surface, and to maintain the adhesive strength of the resin coat. In particular, the overlap ratio of the irradiation spots in the circumferential direction is more preferably 55% or more, more preferably 65% ​​or more, and even more preferably 80% or more. The upper limit of the overlap ratio is preferably 90% or less, since if it is too high, the processing time becomes too long. In addition, the overlap ratio of the irradiation spots in the longitudinal direction is more preferably in the range of 10% to 50%. For example, when the overlap ratio of the irradiation spots in the longitudinal direction is 10%, the piston 10 for an internal combustion engine may be moved by a distance equivalent to 90% of the spot diameter 2r per rotation.

[0029] Also, the streaks 30 consisting of the grooves 31 and the plateaus 32 formed on the skirt portion 18 of the piston 10 for an internal combustion engine are inclined with respect to the circumferential direction of the piston. For example, as shown in Fig. 7, during the machining step 23, the piston 10 for an internal combustion engine is rotated around its axis in the right direction indicated by the thick arrow, while a turning tool (not shown) is moved along the axial direction relative to the piston 10 for an internal combustion engine, so that the streaks 30 are formed in a spiral shape in the circumferential direction of the piston. In this case, the laser light irradiation step 24 is preferably performed by irradiating the piston 10 for an internal combustion engine with laser light so that the streaks are in a spiral shape inclined in the opposite direction to the inclination direction of the streaks 30, and it is preferable to rotate the piston 10 for an internal combustion engine around its axis in the left direction indicated by the small arrow, as shown in Fig. 7, and move the piston 10 for an internal combustion engine in the axial direction relative to the laser light source to irradiate the laser light. In this way, by rotating in the opposite direction during laser light irradiation and during machining, the laser light is irradiated from the opposite direction to the concave groove portion 31 of the streak 30, so that even the concave groove portion 31 can be appropriately degreased and roughened.

[0030] On the other hand, as shown in FIG. 8, if the laser light irradiation process 24 is performed by irradiating the laser light in a spiral manner with an inclination in the same direction as the inclination direction of the streaks 30, that is, if the piston is rotated in the right direction as indicated by the small arrow to irradiate the laser light, the laser light will not oppose the concave groove portions 31 of the streaks 30, and degreasing and roughening will not be able to be performed sufficiently.

[0031] The laser light irradiation step 24 does not significantly impair the shape of the streaks 30 on the outer surface of the skirt portion 18. For example, it is preferable to maintain the plateau ratio derived from the widths Wa and Wb of the grooves 31 and plateaus 32 of the streaks 30 after the laser light irradiation step 24 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, and seizure may easily occur. On the other hand, if the plateau ratio exceeds 0.15, the effect of expelling foreign matter cannot be sufficiently ensured, and seizure may easily occur.

[0032] It is also preferable to suppress the amount of change in the plateau ratio, which is the difference between 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, to 0.05 or less. If the amount of change in the plateau ratio exceeds 0.05, there is a risk that the general shape of the streak 30 will change. The lower limit of the amount of change in the plateau ratio is not particularly limited, but is preferably 0.00 or more.

[0033] It should be noted that the cutting oil remaining on the surface of the skirt portion 18 in the machining process 23 can be degreased in the laser light irradiation process 24. However, if there is too much cutting oil remaining in the machining process 23 and degreasing by laser light irradiation is insufficient, a degreasing process such as neutral degreasing or alkaline degreasing (not shown) may be performed before the laser light irradiation process 24, if necessary.

[0034] In the laser light irradiation step 24, since the laser light is irradiated only onto the skirt portion 18, a masking step (not shown) for masking the piston body other than the skirt portion 18 may be performed before the laser light irradiation step 24. Also, the laser light irradiation step 24 may be performed without masking.

[0035] In the laser light irradiation step 24, components of the aluminum alloy 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. In addition, after the laser light irradiation step 24, a water washing step (not shown) may be performed as necessary to remove dust particles that have adhered to the piston 10 for an internal combustion engine due to the laser light irradiation. The water washing step may be, for example, ultrasonic water washing, or other simple steps.

[0036] The resin coat printing step 25 is a step of printing a resin coat on the outer surface of the skirt portion 18 that has been degreased and roughened by the laser light irradiation step 24, and can be performed by a method of printing a resin coat on the skirt portion of a general piston. For example, a resin coat agent is applied to the outer surface of the skirt portion 18 by a spray method, a screen printing method, or the like. As the resin coat agent, a known agent used for the skirt portion of a piston can be used, for example, MOLYKOTE D-10-GBL and MOLYKOTE PA-744 manufactured by DuPont-Toray Specialty Materials Co., Ltd. In addition, a resin coat agent (a resin (binder) changed from the polyamideimide that has been widely used in the past, or a resin having microhard particles added thereto) that has been developed in recent years for the purpose of achieving both the wear resistance and friction reduction of the resin coat may be used.

[0037] The baking process 26 is a process for baking the resin coating agent applied in the resin coating printing process 25 to form a resin coating on the outer surface of the skirt portion 18. As a result, the resin coating agent fills the outer surface of the skirt portion 18 that has been degreased and roughened in the laser light irradiation process 24, and the adhesive strength of the resin coating is increased by the anchor effect.

[0038] In the piston 10 for an internal combustion engine manufactured by such a series of processes, the resin coating is provided with a degreasing effect equivalent to that of the conventional method by the laser light irradiation process 24, while the outer surface of the skirt portion 18 is roughened uniformly, and the resin coating fills the uneven parts caused by the roughening, resulting in an anchor effect that improves and stabilizes the adhesive strength. In addition, since the degreasing effect can be ensured, the pre-cleaning process can be changed from the conventional wet degreasing method to a dry degreasing method, eliminating the need for the large-capacity water washing tank and degreasing tank that were required in the conventional pre-cleaning process, and also significantly reducing thermal energy, which contributes greatly to carbon neutrality during the manufacture of internal combustion engines.

[0039] In addition, the area occupied by the conventional pre-cleaning process can be reduced, the man-hours for managing the liquid in the water washing tank and degreasing tank can be reduced, and waste liquid treatment can be eliminated. Furthermore, since the laser light irradiation process 24 does not significantly damage the shape of the streaks 30 on the outer surface of the skirt portion 18, it is possible to maintain the friction reduction effect while ensuring excellent seizure resistance even on the thrust side to which lateral pressure is applied during engine operation. EXAMPLES

[0040] EXAMPLES Hereinafter, examples and comparative examples according to the present invention will be described. Note that the manufacturing method of a piston for an internal combustion engine according to the present invention is not limited to the following examples and comparative examples.

[0041] [Test Examples 1 to 7] An internal combustion engine piston was made from Al-Si aluminum alloy casting material, a high-strength material with improved mechanical properties in the high-temperature range, and a turning tool was used to form a spiral streak on the outer surface of the skirt. The water-soluble cutting oil used in the machining process was applied and then left to harden overnight. Then, without wet degreasing, a laser irradiation process was carried out in which a pulsed laser beam was irradiated in a spiral onto the outer surface of the skirt with the streaks formed. The laser processing machine used (AkiTech LEO Corporation, model 2LC-S-SF) was equipped with a main oscillator power amplifier (MOPA) type fiber laser, with a rated output of 50W. A lens of F254mm was used.

[0042] Table 1 shows the energy density (mJ / mm2), output (W), frequency (kHz), piston rotation speed (rpm), spot diameter (μm), and spot overlap ratio in the piston longitudinal direction (Y direction) as the irradiation conditions of the pulsed laser light in Test Examples 1 to 7. The spot overlap ratio in the piston circumferential direction (X direction) was about 87% (center-to-center distance between spots was 8 μm) in all test examples. In addition, the rotation direction of the piston during laser light irradiation was the opposite direction to the rotation direction of the piston during machining to form the striations.

[0043] Then, in order to examine the surface properties of the skirt portion after the laser light irradiation process, the cleanliness was measured using a Cleanospector, and the surface roughness was measured.

[0044] Cleanliness measurement using the Cleanospector can quantitatively evaluate cleanliness by detecting the fluorescence of oil and other substances on the metal surface. The measured value is displayed in RFU (Relative Fluorescence Unit). The cleanliness measurement was performed by measuring six points on each side of the skirt (R side and L side), and the average value was treated as the measured value. Measurements were performed after the cutting oil had adhered and after laser processing, and improvements in cleanliness were confirmed. The results are shown in Table 1.

[0045] The surface roughness was measured in accordance with JIS B 0601-2001 by measuring the roughness curve of the piston skirt and calculating the surface roughness Rz, which represents the maximum height, from the results. The roughness was measured along the longitudinal direction of the piston so that the shape of the streaks could be seen. The results are shown in Table 1.

[0046] Next, a resin coat printing process was carried out on the skirt part of the piston that had been subjected to the laser light irradiation process. The resin coat agent used was MOLYKOTE D-10-GBL manufactured by DuPont Toray Specialty Materials (formerly Dow Corning Toray). After printing the resin coat agent using the screen printing method, the resin coat was baked at a temperature of 200°C or less in the baking process to form a resin coat on the skirt part of the piston.

[0047] A test was then conducted to evaluate the adhesive strength of the resin coat formed on the skirt. The adhesiveness was evaluated using a method called the water jet (W / J) method, in which water is sprayed from directly above the resin coat with a nozzle at high pressure, and then the adhesiveness is evaluated from the appearance of the resin coat surface. The evaluation criteria were as follows: tests were conducted with three spray pressures, low, medium, and high, and the resin coat sprayed at low pressure was observed and peeled off was evaluated as "X", the resin coat sprayed at low pressure was not peeled off but the resin coat sprayed at medium pressure was peeled off was evaluated as "△", the resin coat sprayed at medium pressure was not peeled off but the resin coat sprayed at high pressure was peeled off was evaluated as "○", and the resin coat sprayed at high pressure was not peeled off was evaluated as "◎". The results are shown in Table 1.

[0048] [Comparative Example] In Comparative Example 1, a piston for an internal combustion engine was produced in the same manner as in the Test Example, and after forming streaks on the skirt portion, the skirt portion was subjected to wet degreasing, which is a conventional pre-cleaning method, instead of laser light irradiation. In Comparative Example 2, no pre-cleaning was performed at all. For Comparative Examples 1 and 2, the cleanliness was measured using a Cleano Specter, and the surface properties of the skirt portion were investigated by measuring the surface roughness, as in the Test Example. For Comparative Examples 1 and 2, a resin coat was formed in the same manner as in the Test Example, and the adhesion strength of the resin coat was investigated by a water jet test.

[0049] [Table 1]

[0050] In addition, in Table 1, when the overlap ratio in the Y direction is 10%, 50%, and 90%, the center-to-center distances between the spots are 58.5 μm, 32.5 μm, and 6.5 μm, respectively.

[0051] In Test Examples 1 and 2, the center-to-center distance between spots in the longitudinal direction (Y direction) of the piston in the second scanning step in the laser light irradiation step was set to be equal to or larger than the spot diameter (overlap ratio 0%), but the cleanliness was significantly lower than in Comparative Example 1, in which wet degreasing, a conventional pre-cleaning method, was performed. On the other hand, Test Examples 3 to 5, in which the Y direction overlap ratio was set to 10 to 90%, were approximately half the value compared to Test Examples 1 and 2, confirming that the cleanliness was improved. The cleanliness is preferably, for example, 30 RFU or less.

[0052] The adhesion strength of Test Examples 1 and 2, in which the Y-direction overlap ratio was 0% or more, was equal to or less than that of Comparative Example 1, which was a conventional wet degreasing method. On the other hand, Test Examples 3, 4, and 6, in which the Y-direction overlap ratio was 10 to 50%, were superior to Comparative Example 1 in adhesion strength. Comparative Example 1, which was a conventional wet degreasing method, was not roughened, but Test Examples 3, 4, and 6 had higher surface roughness Rz values ​​than Comparative Example 1, and it is presumed that this is because the adhesion strength of the resin coat was improved by the anchor effect of the roughening. However, in Test Example 5, in which the Y-direction overlap ratio was 90%, the adhesion strength was reduced even though the surface roughness Rz value was equal to that of Test Example 6. This is presumed to be because if the Y-direction overlap ratio is too high, the surface properties of the outer surface of the skirt portion 18 change due to repeated melting and resolidification, and the effect of improving adhesion strength due to the anchor effect was not obtained. From this, it can be seen that the Y-direction overlap ratio should be 50% or less.

[0053] Test Examples 6 and 7 show the results when the energy density was lowered based on the conditions of Test Example 4. Generally, lowering the energy density reduces the effects of degreasing and surface roughening. A trend towards improved degreasing was confirmed in Test Example 6, but no trend towards improved degreasing was confirmed in Test Example 7. Also, very good adhesion strength was obtained in Test Example 6, but the adhesion strength decreased in Test Example 7. From this, it can be seen that the energy density was 11.3 to 15.1 mJ / mm 2 In the energy density range of 100%, the degreasing and roughening effects of the laser light irradiation according to the present invention were confirmed.

[0054] FIG. 9 shows roughness curves of the skirt portion after the laser light irradiation process, where (a) is Test Example 3, (b) is Test Example 4, and (c) is Test Example 6. For comparison, (d) is Comparative Example 1. In all test examples, the surface was roughened while the shape of the streaks remained, and no significant damage to the streaks was confirmed. In addition, the plateau rate before and after the laser light irradiation process was smaller in Test Example 6 (FIG. 9c) with a lower energy density than Test Example 3 (FIG. 9(a)) and Test Example 4 (FIG. 9b), and the streaks were better maintained in shape. Therefore, the laser light irradiation of the present invention can be applied to the outer surface of the skirt portion on which streaks are formed, and can maintain the friction reduction effect while ensuring excellent seizure resistance even on the thrust side to which lateral pressure is applied during engine operation. [Explanation of symbols]

[0055] 10. Pistons for internal combustion engines 18 Skirt Club 20 Manufacturing method of piston for internal combustion engine 30 marks 31 Groove 32 Intermediate surface (plateau) 50 Laser device 51 Pulsed laser light 52 Retaining member 53 Chuck

Claims

1. 1. A method of manufacturing a piston for an internal combustion engine, comprising the steps of: a laser light irradiation step of helically irradiating a pulsed laser light onto the outer surface of the skirt portion of the piston for an internal combustion engine, the outer surface of the skirt portion being made of an aluminum alloy and having a skirt portion that slides on an inner wall surface of a cylinder bore of the internal combustion engine, the outer surface of the skirt portion being provided with a plurality of groove portions extending in a circumferential direction, the plurality of groove portions being arranged at intervals in the axial direction, and an intermediate surface portion being provided between adjacent groove portions in the axial direction; and a surface treatment step of forming a resin coating on the outer surface of the skirt portion irradiated with the pulsed laser light, A method for manufacturing a piston for an internal combustion engine, wherein in the laser light irradiation step, the pulsed laser light is irradiated in a spiral manner within a range in which the spot diameters of adjacent irradiation spots partially overlap.

2. 2. The method for manufacturing a piston for an internal combustion engine according to claim 1, wherein, in the laser light irradiation step, the spiral irradiation of the pulsed laser light is performed such that a center-to-center distance between adjacent irradiation spots in a circumferential direction of the skirt portion is less than a spot radius, and a center-to-center distance between adjacent irradiation spots in an axial direction of the skirt portion is equal to or greater than the spot radius.

3. 3. The method for manufacturing a piston for an internal combustion engine according to claim 1 or 2, wherein in the laser light irradiation process, the piston for an internal combustion engine is rotated about its axis while being moved axially, so that the pulsed laser light is irradiated in a spiral manner onto the outer surface of the skirt portion.

4. The method further includes a machining step of providing the groove portion and the intermediate surface portion, the groove portion and the intermediate surface portion being provided by turning so as to extend spirally in a circumferential direction; 3. The method for manufacturing a piston for an internal combustion engine according to claim 1, wherein in the laser light irradiation step, the irradiation spot is scanned so as to form a spiral shape inclined in a direction opposite to the inclination direction of the spirally extending groove portion and the intermediate surface portion.

5. the machining step is performed by moving a turning tool and the internal combustion engine piston relatively along an axial direction while rotating the internal combustion engine piston about an axis; 5. The method for manufacturing a piston for an internal combustion engine according to claim 4, wherein the laser light irradiation step is performed by moving the piston for an internal combustion engine in an axial direction while rotating the piston about its axis in a direction opposite to that of the machining step.

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