Method for manufacturing internal combustion engine piston

Abstract

To provide a method for manufacturing an internal combustion engine piston that replaces a conventional wet defatting process without requiring a large-capacity water washing tank, a defatting tank, or thermal energy for heating the tanks, while still maintaining adhesion of a resin coat.SOLUTION: A method for manufacturing an internal combustion engine piston comprises: a process to prepare an internal combustion engine piston 10 which has a piston body and a skirt section 18 provided with a streak 60 including a plurality of grooves extending in a circumferential direction on an external surface of the skirt section 18; a laser irradiation process to irradiate the external surface of the skirt section 18 with pulse laser beams with air being injected onto the surface through an air injection nozzle 35; and a surface treatment process which forms a resin coat on the external surface of the skirt section 18 after laser irradiation. In the laser irradiation process, the air is injected from the air injection nozzle 35 to a region irradiated with the pulse laser beams in a direction along the streak 60 on the skirt section 18.SELECTED DRAWING: Figure 6

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] Patent Document 1 describes a method for manufacturing an internal combustion engine piston having excellent adhesion to 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]

[0004] [Patent Document 1] JP 2023-019522 A Summary of the Invention [Problem to be solved by the invention]

[0005] 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.

[0006] 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.

[0007] In view of the above problems, an object of the present invention is to provide a method for manufacturing a piston for an internal combustion engine, 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]

[0008] In order to achieve the above object, the present invention provides a method for manufacturing a piston for an internal combustion engine having a resin coating on a skirt portion, the method comprising the steps of: preparing a piston for an internal combustion engine, the piston body having an outer periphery configured to be slidable against an inner periphery of a cylinder bore; and a skirt portion extending from the outer periphery of the piston body toward a bottom of the cylinder bore, the piston body and the skirt portion being constructed using an aluminum alloy as a base material, the outer surface of the skirt portion being provided with a plurality of grooves extending in a circumferential direction, the plurality of grooves being arranged at intervals in the axial direction, and an intermediate surface being provided between adjacent grooves in the axial direction; a laser light irradiation step of irradiating the outer surface of the skirt portion of the piston for an internal combustion engine with a pulsed laser beam while injecting air from an air injection nozzle; and a surface treatment step of forming a resin coating on the outer surface of the skirt portion irradiated with the pulsed laser beam, and in the laser light irradiation step, air is injected from the air injection nozzle in a direction along the grooves of the skirt portion into an area to be irradiated with the pulsed laser beam. Effect of the Invention

[0009] 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]

[0010] [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] FIG. 2 is a schematic diagram illustrating an example of a laser light irradiation device that performs a laser light irradiation step in the method for manufacturing a piston for an internal combustion engine according to the present invention. [Figure 4] 3 is a schematic diagram for explaining circumferential overlap of irradiation spots in the laser light irradiation step shown in FIG. 2. FIG. [Diagram 5] 3 is a schematic diagram for explaining overlapping of irradiation spots in the longitudinal direction in the laser light irradiation step shown in FIG. 2. FIG. [Figure 6] 3 is a schematic diagram illustrating the size of a nozzle opening of an air injection nozzle in the laser light irradiation step shown in FIG. 2. FIG. [Figure 7] 3 is a schematic diagram for explaining the positional relationship between the air injection nozzle, the skirt portion, and the suction port in the laser light irradiation step shown in FIG. 2. FIG. [Figure 8] 3 is a schematic diagram illustrating the flow direction of air injected from an air injection nozzle onto the streaks on the skirt portion in the laser light irradiation step shown in FIG. 2. FIG. [Figure 9] 13 is a schematic diagram illustrating the flow direction of air injected from an air injection nozzle onto the streaks in the skirt portion in a comparative example. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] 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.

[0012] First, the piston 10 for an internal combustion engine will be described. The piston 10 for an internal combustion engine of this embodiment (hereinafter, simply referred to as "piston 10") 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.

[0013] 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.

[0014] Therefore, it can be said that the piston 10 for an internal combustion engine comprises a piston body 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 composed of two skirt portions opposed to each other in the radial direction of the piston body, and is disposed on both sides with a pin boss portion 19 therebetween, as shown in Fig. 1. The piston 10 reciprocates inside the cylinder bore while the outer surface of the skirt portion 18 slides on the inner wall surface of the cylinder bore.

[0015] 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.

[0016] The above-mentioned casting, heat treatment and machining steps 21, 22 and 23 are similar to those used in manufacturing a general piston for an internal combustion engine, and therefore a detailed description thereof will be omitted here.

[0017] The laser light irradiation step 24 is a step of irradiating the outer surface of the skirt portion 18 of the piston 10 with a pulsed laser light while injecting air from an air injection nozzle, in order to degrease the cutting oil remaining on the outer surface of the skirt portion 18 of the piston 10 in the machining step 23 and to roughen the outer surface of the skirt portion 18. To perform the laser light irradiation step 24, for example, a laser light irradiation device 30 shown in FIG. 3 can be used.

[0018] As shown in Fig. 3, the laser light irradiation device 30 includes a protection box 31 that houses the piston 10, and a laser light oscillator 40 that irradiates a pulsed laser light 41 onto the skirt portion 18 of the piston 10 through a laser light passage port 34 provided in the protection box 31. The protection box 31 further includes a holding member (not shown) that holds the piston 10, an air injection nozzle 35 that injects air onto the skirt portion 18 of the piston 10, a suction port 38 that removes scattered material generated by the laser light irradiation, and an air curtain unit 42 that forms an air curtain 43 between the laser light oscillator 40 and the laser light passage port 34. The air injection nozzle 35 is connected to a vacuum flow 37 via an air supply line 36. The suction port 38 is connected to a dust collector 39.

[0019] The laser beam oscillator 40 is disposed so as to irradiate the pulsed laser beam 41 in a direction perpendicular to the outer surface of the skirt portion 18 of the piston 10. The type of laser used for the laser beam oscillator 40 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, or the like can be used alone or in combination.

[0020] Then, in the laser light irradiation process 24, the first land portion of the piston 10 is held by a holding member by gripping it with a chuck (not shown), and while the piston 10 is rotated around its axis, a pulsed laser light 41 is irradiated from a laser light oscillator 40 onto the outer surface of the skirt portion 18 of the piston 10.

[0021] If the irradiation spots are scanned so that the outer edges of adjacent spot diameters come into contact when scanning the outer peripheral surface of the skirt portion 18, the degreasing and roughening may be insufficient in the areas where the laser energy is sparse at the outer edges of the spot diameter, and the adhesive strength of the resin coat may decrease. Therefore, as shown in Figure 4, it is preferable to scan the irradiation spots 50 irradiated with the pulsed laser light so that they 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 set to 0.05 mm. C It is more preferable that the pitch P be equal to or smaller than the spot radius R of the irradiation spot 50. C is more preferably 90% or less of the spot radius R, further preferably 70% or less, and further preferably 40% or less. C If the lower limit is too short, the processing time becomes too long, so the lower limit is preferably 20% or more of the spot radius R.

[0022] In order to irradiate the pulsed laser beam 41 uniformly over the entire outer surface of the skirt portion 18, it is preferable to irradiate the outer surface of the skirt portion 18 with the pulsed laser beam in a spiral pattern, for example. As shown in Fig. 4, in the laser beam irradiation step 24, the irradiation spot is basically scanned along the circumferential direction C of the skirt portion 18, but by gradually moving the irradiation spot in the longitudinal direction of the skirt portion 18 (the direction perpendicular to the circumferential direction C, which is also the axial direction of the piston 10), the outer surface of the skirt portion 18 can be irradiated with the pulsed laser beam in a spiral pattern.

[0023] More specifically, while the piston 10 is rotated about its axis by a holding member that holds the piston 10, the pulsed laser light 41 is irradiated onto the outer circumferential surface of the skirt portion 18 from a laser device 40 at a fixed position. At the same time, the holding member is moved in the axial direction of the piston 10 by approximately one spot during one rotation of the piston 10. By moving the piston 10 in the axial direction while rotating it about its axis in this manner, the outer circumferential surface of the skirt portion 18 is irradiated with the pulsed laser light 41 in a spiral manner, and the entire outer circumferential surface of the skirt portion 18 can be irradiated with the pulsed laser light. Of course, the laser light irradiation step 24 is not limited to this, and may be performed by a device that fixes the piston and moves the laser side.

[0024] Furthermore, in order to irradiate the outer surface of the skirt portion 18 evenly with the pulsed laser light 41, instead of the method of irradiating the pulsed laser light in a spiral manner as described above, for example, a series of scanning steps can be repeatedly performed, including a first scanning step in which the piston 10 is rotated in one direction to scan the irradiation spot of the pulsed laser light 41 from one end of the circumferential direction of the skirt portion 18 to the other end, a second scanning step in which the piston 10 is moved in the axial direction by approximately one spot to scan the irradiation spot in the longitudinal direction of the skirt portion 18, and a third scanning step in which the piston 10 is rotated in the direction opposite to the first scanning step to scan the irradiation spot from the other end of the circumferential direction of the skirt portion 18 to one end.

[0025] In the first scanning step, it is preferable to scan so that the irradiation spots 50 overlap each other along the circumferential direction C of the skirt portion, similar to the case of helical irradiation shown in FIG. C In the second scanning step, as shown in Fig. 5, it is preferable that the irradiation spots 50 are scanned so that the spot diameters D partially overlap along the longitudinal direction L of the skirt portion. 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 set to 1 / 2. L ) is set to be less than the spot diameter D of the irradiation spot 50. In particular, the pitch P L is preferably 90% or less of the spot diameter D.L The lower limit of is preferably 50% or more of the spot diameter D. In the third scanning step, the pulsed laser light is irradiated onto the skirt portion 18 under the same conditions as in the first scanning step, except that the rotation direction of the piston 10 is reversed. By repeatedly performing a series of scanning steps such as the first scanning step, the second scanning step, the third scanning step, the second scanning step, and the first scanning step, the outer surface of the skirt portion 18 can be irradiated evenly with the pulsed laser light 41 over the entire surface.

[0026] This series of scanning steps may be performed by a laser light irradiation device 30 in which the laser side is fixed and a piston is moved, as in the case of helical irradiation, or may be performed by a device in which the piston is fixed and the laser light is scanned by a galvanometer mirror.

[0027] Although many examples of specific implementations of the laser light irradiation process 24 have been described, in the example where the above series of scanning steps are repeated or in the example where the piston is fixed and the laser side is moved, since two skirt portions are provided facing each other in the radial direction of the piston body, it is necessary to perform the laser light irradiation process for each skirt portion or to provide two laser light oscillators, either of which increases the manufacturing cost, so an example where the piston is rotated and the skirt portion is irradiated with pulsed laser light in a spiral is more preferable.

[0028] In addition, in the laser light irradiation process 24, processing debris called fumes, which contain aluminum alloy components sublimated by the heat of the pulsed laser light, is generated. If the debris reattaches to the surface of the skirt portion 18, the desired cleanliness and processing state cannot be obtained, and the adhesive strength of the resin coat may decrease. Therefore, in this embodiment, as shown in FIG. 3, the pulsed laser light 41 is irradiated while air is sprayed from an air spray nozzle 35 onto the outer surface of the skirt portion 18. The processing debris and cutting oil blown away by the air are removed from a suction port 38 arranged on the opposite side of the air spray nozzle 35 with respect to the outer surface of the skirt portion 18.

[0029] As shown in Figs. 6 and 8, in the outer surface of the skirt portion 18, striations 60 are formed in order to improve the seizure resistance between the outer surface and the inner wall surface of the cylinder bore. The striations 60 have a plurality of grooves 61 with a U-shaped cross section formed so as to extend in the circumferential direction on the outer surface of the skirt portion 18. The plurality of grooves are arranged at intervals in the longitudinal direction. The depth of the grooves is, for example, in the range of 5 to 15 µm. In addition, between adjacent grooves in the longitudinal direction, an intermediate surface portion (plateau portion) 62 is provided, whose radially outer end in the cross section extends linearly in the longitudinal direction.

[0030] In the laser light irradiation process 24, as shown in Fig. 6, the air injection nozzle 35 and the suction port 38 are arranged so that the air flows in a direction parallel to the streaks 60 of the skirt portion 18. By blowing air from the air injection nozzle 35 along the streaks 60 of the skirt portion 18 in this way, an air flow 64 is generated along the grooves 61 of the streaks 60 as shown in Fig. 8, and machining debris and cutting oil can be efficiently sucked up and removed by the suction port 38.

[0031] On the other hand, for comparison, when air 65 is made to flow perpendicular to the grooves 61 of the streaks 60 of the skirt portion 18 as shown in Fig. 9, an air flow 66 that stagnates in the grooves 61 is generated by the plateaus 62 between the grooves 61 of the streaks 60, and the machining debris and cutting oil also stagnate in the streaks 60, making it impossible to sufficiently remove the machining debris and cutting oil. Therefore, as shown in Figs. 6 and 8, by blowing air from the air injection nozzle 35 along the streaks 60 of the skirt portion 18, it is possible to suppress the machining debris and cutting oil from stagnation in the streaks 60, and to efficiently remove the machining debris and cutting oil, thereby improving the cleanliness.

[0032] 3, it is preferable that the rotation direction of the piston 10 is opposite to the air injection direction of the air injection nozzle 35. This increases the injection pressure of the air applied to the surface of the skirt portion 18 compared to the case where the rotation direction of the piston 10 is the same as the air injection direction, and promotes the removal of processing debris and cutting oil generated by the irradiation of the pulsed laser beam 41, thereby improving the cleanliness.

[0033] 6, it is preferable that the length of the nozzle opening of the air injection nozzle 35 in the axial direction of the piston 10 is greater than the length of the skirt portion 18 and less than the length of the suction port 38. This allows the processing debris and cutting oil generated by the irradiation of the pulsed laser light on the outer peripheral surface of the skirt portion 18 to be effectively discharged through the suction port 38 by the air injected from the air injection nozzle 35.

[0034] 7, the suction port 38 is preferably disposed on the opposite side to the air injection nozzle 35 across the region of the outer surface of the skirt 18 irradiated with the pulsed laser beam 41. As a result, the suction port 38 is located on an extension line of the direction of air injection from the air injection nozzle 35 to the outer peripheral surface of the skirt 18, so that the suction port 38 can effectively guide the air injected from the air injection nozzle 35 to the suction port 38, and generate an air flow that follows the shape of the skirt 18. As a result, machining debris and cutting oil generated by the irradiation of the pulsed laser beam on the outer peripheral surface of the skirt 18 can be efficiently discharged from the suction port 38.

[0035] In order to prevent the processing debris from adhering to the lens of the laser beam oscillator 40, it is preferable to form an air curtain 43 between the laser beam oscillator 40 and the laser beam passage opening 34. By making the laser beam passage opening 34 as small as possible, the flow rate of the air curtain 43 can be suppressed, and the effect on the injection and suction of air for removing the processing debris can be reduced.

[0036] In addition, the air supplied to the air inlet (not shown) of the vacuum flow 37 is preferably passed through a filter (such as an air dryer or mist separator) to remove moisture and oil components. Since air generally used in factories contains moisture and oil components, if the air is used without removing the moisture and oil components, the moisture and oil components may adhere to the outer surface of the skirt portion 18 that has been cleaned by the irradiation of the pulsed laser light 41 due to the air injection, which may reduce the adhesive strength of the resin coat. Therefore, it is preferable to use air from which moisture and oil have been removed as much as possible.

[0037] The protective box 31 is provided with an opening (not shown) for inserting and removing the piston 10, and it is preferable to make this opening as small as possible and install an air curtain at the opening. It is also preferable to make the inside of the protective box 31 positive pressure. This generates an air flow from inside the protective box 31 to the outside via the opening for inserting and removing the piston and the laser light passage port 34, making it possible to prevent scattered objects from adhering again to the outer surface of the skirt portion 18.

[0038] 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.

[0039] 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.

[0040] In addition, if necessary, a water washing step or an air washing step (not shown) may be performed after the laser light irradiation step 24 in order to remove dust particles that have adhered to the internal combustion engine piston 10 by the laser light irradiation. The water washing step may be, for example, ultrasonic water washing, or other simple steps. Note that if the water washing step is performed, a drying step must be performed thereafter.

[0041] 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.

[0042] 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.

[0043] In the piston 10 for an internal combustion engine manufactured by such a series of processes, the resin coating is applied with the laser light in the laser beam irradiation process 24 while ensuring the same degreasing effect as the conventional method, and 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 adhesion strength. In addition, since the degreasing effect can be ensured, the pre-cleaning process can be changed from the conventional wet degreasing method to the dry degreasing method, which eliminates the need for large-capacity water washing tanks and degreasing tanks that were required in the conventional pre-cleaning process, and also significantly reduces thermal energy, which contributes greatly to carbon neutrality during the manufacture of internal combustion engines. In addition, the area occupied by the conventional pre-cleaning process can be reduced, the man-hours required for managing the liquid in the water washing tank and degreasing tank can be reduced, and waste liquid treatment can be eliminated. EXAMPLES

[0044] 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.

[0045] [Example] An internal combustion engine piston was made from an Al-Si aluminum alloy casting material, which is a high-strength material with improved mechanical properties in the high-temperature range, and a spiral streak was formed on the outer surface of the skirt part using a turning tool. The water-soluble cutting oil (Yushiro Chemical Industry Co., Ltd., Yushiroken FGE218) used in the machining process was applied and allowed to harden overnight. Then, without wet degreasing, a laser light irradiation process was performed in which a pulsed laser light was irradiated onto the outer surface of the skirt part on which the streaks were formed. The laser processing machine used (Keyence Corporation, MD-X2500) was equipped with a Q-switched YVO4 fiber nanosecond laser, with a rated output of 25 W and a lens focus of F189 ± 21 mm. The laser light irradiation process was performed by fixing the piston and repeatedly performing the series of scanning steps 1 to 3 described above.

[0046] Table 1 shows the irradiation conditions of the pulsed laser beam in the embodiment, including the energy density (mJ / mm 2), power (W), frequency (kHz), spot diameter (μm), piston circumferential (C) pitch P C , Pitch P in the piston longitudinal direction (L direction) L The pitch P C , P L is the center distance between adjacent irradiation spots expressed as a percentage of the spot diameter. C , pitch P L The center-to-center distances of adjacent irradiation spots are 8 μm and 30 μm for 88% and 50%, respectively.

[0047] During the laser light irradiation, air was injected from an air injection nozzle in a direction along (parallel to) the grooves of the streaks formed on the skirt portion onto the region of the outer circumferential surface of the skirt portion onto which the laser light was irradiated.

[0048] Then, in order to check the surface properties of the skirt portion after the laser light irradiation process, a cleanliness measurement was performed using a Cleanospector.

[0049] 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. [Comparative Example] In Comparative Example 1, a piston for an internal combustion engine having a streak formed on the skirt portion was produced in the same manner as in the Example, and then the laser light irradiation process was carried out, but the air injection direction was set to a direction perpendicular to the groove portion of the streak. In Comparative Example 2, no air injection was carried out at all. For Comparative Examples 1 and 2, cleanliness measurements were carried out using a Cleano Specter in the same manner as in the Example. The results are shown in Table 1.

[0050] [Table 1]

[0051] As shown in Table 1, in the embodiment in which air was injected in a direction parallel to the grooves of the skirt mark, the cleanliness was significantly lower than in Comparative Example 2 in which no air injection was performed. Also, the cleanliness of the embodiment was about half that of Comparative Example 1 in which air was injected in a direction perpendicular to the grooves of the skirt mark, confirming that the cleanliness was improved. The cleanliness is preferably, for example, 20 RFU or less. [Explanation of symbols]

[0052] 10. Pistons for internal combustion engines 18 Skirt Club 20 Manufacturing method of piston for internal combustion engine 30 Laser light irradiation device 35 Air injection nozzle 37 Vacuum Flow 38 Suction port 39 Dust collector 40 Laser Oscillator 41 Pulsed laser light 42 Air curtain unit 43 Air Curtain 50 Irradiation Spots 60 marks 61 Groove 62 Mid-surface (plateau)

Claims

1. A method for manufacturing a piston for an internal combustion engine having a resin coating on a skirt portion, comprising the steps of: preparing a piston for an internal combustion engine, the piston comprising: a piston body having an outer periphery configured to be slidable against an inner periphery of a cylinder bore; and a skirt portion extending from the outer periphery of the piston body toward a bottom of the cylinder bore, the piston body and the skirt portion being constructed from an aluminum alloy as a base material, the skirt portion having an outer surface provided with a plurality of grooves extending in a circumferential direction, the plurality of grooves being spaced apart in the axial direction, and an intermediate surface being provided between adjacent grooves in the axial direction; 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 while injecting air from an air injection nozzle; 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 laser beam irradiation step of irradiating a region of the piston with the pulsed laser beam from the air injection nozzle in a direction along the groove of the skirt portion.

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

3. 3. The method for manufacturing a piston for an internal combustion engine according to claim 1, wherein a suction port for sucking in the air injected from the air injection nozzle is disposed on the opposite side of the air injection nozzle across the area of ​​the outer surface of the skirt portion that is irradiated with the pulsed laser beam.

4. 3. The method for manufacturing a piston for an internal combustion engine according to claim 1, wherein in the laser light irradiation step, a length of a nozzle opening of the air injection nozzle in an axial direction of the piston for an internal combustion engine is greater than a length of the skirt portion and is smaller than a length of a suction port that sucks in the injected air.

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