Method for manufacturing a piston for an internal combustion engine

Abstract

To provide a manufacturing method of a piston for an internal combustion engine excellent in adhesion and seizure resistance of resin coating.SOLUTION: In a manufacturing method 20 of a piston for an internal combustion engine, the piston for the internal combustion engine, made of aluminum alloy has a skirt portion sliding on an inner wall surface of a cylinder bore of the internal combustion engine, a plurality of groove portions extending in a circumferential direction are formed on an outer surface of the skirt portion, the plurality of groove portions are disposed at intervals in an axial direction, an intermediate surface portion is disposed between the groove portions adjacent to each other in the axial direction, alkali etching 24 is performed on the skirt portion of the piston for the internal combustion engine so as to form a plurality of recessed portions smaller than the groove portions on the outer surface of the skirt portion while keeping a constitution of the groove portions and the intermediate surface portions formed on the outer surface of the skirt portion, smut generating on the outer surface of the skirt portion by alkali etching is removed by acid cleaning 25, and surface treatment 27 is performed on the outer surface of the skirt portion after removing the smut so as to form resin coating.SELECTED DRAWING: Figure 2

Description

Technical Field

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

Background Art

[0002] In an internal combustion engine such as an engine provided 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 peripheral portion of the piston slides against the inner peripheral portion of the cylinder bore. Typically, the piston includes a piston body having an outer peripheral portion slidable with respect to the inner peripheral portion of the cylinder bore, and two skirts extending from the outer peripheral portion of the piston body to the bottom side of the cylinder bore. Further, in the piston, in order to reduce the frictional resistance of the outer peripheral portion of the piston with respect to the inner peripheral portion of the cylinder bore, a resin coat is formed on the outer surface of each skirt.

[0003] As an example of such a piston, Patent Document 1 describes a piston in which shot materials such as hard particles are uniformly sprayed by shot peening so as to form a plurality of dimples on the outer surfaces of two skirts in a piston formed by casting, and then, a lubricating film (resin coat) of a resin in which predetermined particles are dispersed is formed on the outer surfaces of the two skirts. On the outer surface of the skirt portion formed with such a plurality of dimples, the surface roughness of the lubricating film can be reduced, and the initial conformability of the lubricating film is enhanced.

[0004] Patent Document 2 also describes that a large number of striations are formed on the outer surface of the skirt portion in order to enhance lubrication. Here, the striations are described. 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 2 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. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2016-160293 A [Patent Document 2] JP 2008-232172 A Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the piston described in Patent Document 1, the shot material sprayed by shot peening may remain on the outer surface of the skirt portion. It is difficult for a resin coat to adhere closely to such an outer surface of the skirt portion, which may reduce the sliding performance of the piston. Further, when such shot peening is applied to a piston having a number of streaks described in Patent Document 2, the shape of the streaks cannot maintain its general form, and the seizure resistance is impaired.

[0007] In recent years, with the response to environmental regulations, there has been an increasing demand for higher engine efficiency, higher compression ratios, and supercharged engines, and the maximum combustion pressure of the engine has been rising. Along with this background, it is desired to further improve the adhesion and seizure resistance of the resin coat on the skirt portion of the piston.

[0008] In order to achieve the above object, a method for manufacturing a piston for an internal combustion engine according to the present invention is such that a piston for an internal combustion engine made of an aluminum alloy has a skirt portion that slides on the inner wall surface of a cylinder bore of the internal combustion engine, and a plurality of groove portions extending in the circumferential direction are provided on the outer surface of this skirt portion. These plurality of groove portions are arranged at intervals in the axial direction, and an intermediate surface portion is provided between the groove portions adjacent to each other in the axial direction. Then, the skirt portion of this piston for an internal combustion engine is subjected to alkaline etching to form a plurality of recesses finer than the groove portions on the outer surface of the skirt portion while maintaining the configuration of the groove portions and the intermediate surface portion provided on the outer surface of the skirt portion, and the alkaline etching to A cleaning step of removing the smut generated on the outer surface of the skirt portion by the above, and a surface treatment step of forming a resin coat on the outer surface of the skirt portion from which the smut has been removed.

Means for Solving the Problems

[0009] In order to achieve the above object, a method for manufacturing a piston for an internal combustion engine according to the present invention is such that a piston for an internal combustion engine made of an aluminum alloy has a skirt portion that slides on the inner wall surface of a cylinder bore of the internal combustion engine, and a plurality of groove portions extending in the circumferential direction are provided on the outer surface of this skirt portion. These plurality of groove portions are arranged at intervals in the axial direction, and an intermediate surface portion is provided between the groove portions adjacent to each other in the axial direction. Then, the skirt portion of the piston for the internal combustion engine is subjected to alkali etching to form a plurality of recesses finer than the groove portions on the outer surface of the skirt portion while maintaining the configuration of the groove portions and the intermediate surface portion provided on the outer surface of the skirt portion; a cleaning step of removing the smut generated on the outer surface of the skirt portion by the alkali etching; and a surface treatment step of forming a resin coat on the outer surface of the skirt portion from which the smut has been removed.

Advantages of the Invention

[0010] According to the present invention as described above, it is possible to provide a method for manufacturing a piston for an internal combustion engine that is excellent in the adhesion and anti-seizure properties of the resin coat.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Embodiments for Carrying Out the Invention

[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 FIGS. 1 to 5.

[0013] First, the piston 10 for an internal combustion engine will be described. The piston 10 for an internal combustion engine in this embodiment is a member that reciprocates inside a cylinder bore (not shown) and is formed of an aluminum alloy. As the aluminum alloy, silicon (Si) is contained as a component that contributes to wear resistance and aluminum seizure resistance. Examples of such aluminum alloys include AC materials such as AC4, AC8, AC8A, and AC9, ADC materials such as ADC10 to ADC14, and A4000.

[0014] The piston 10 for an internal combustion engine has a substantially cylindrical shape. As shown in FIG. 1, on its outer peripheral surface, in order from the piston crown surface 11 side, there are a first ring groove 13, a second ring groove 15, and an oil ring groove 17. The outer peripheral surface is referred to as a first land 12 between the piston crown surface 11 and the first ring groove 13, a second land 14 between the first ring groove 13 and the second ring groove 15, a third land 16 between the second ring groove 15 and the oil ring groove 17, and the portion after the oil ring groove 17 is referred to as a skirt portion 18.

[0015] The skirt portion 18 is disposed on both sides with the pin boss portion 19 therebetween. While the outer surface of the skirt portion 18 slides on the inner wall surface of the cylinder bore, the piston 10 reciprocates inside the cylinder bore.

[0016] Next, a method for manufacturing a piston for an internal combustion engine according to the present embodiment will be described. As shown in FIG. 2, the method 20 for manufacturing 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 alkali-etching the skirt portion of the piston, a step 25 of performing pickling to remove smut generated by the alkali-etching, a step 26 of further performing water washing, and a step 27 of performing surface treatment to coat the skirt portion of the piston with a resin coat.

[0017] Since the above steps 21, 22, and 23 of casting, heat treatment, and machining are the same as the steps used when manufacturing a general piston for an internal combustion engine, detailed description thereof will be omitted here.

[0018] The alkali-etching step 24 is a step of eluting mainly the aluminum component from the outer surface of the skirt portion 18 of the piston 10 for an internal combustion engine using an alkaline etching solution. Note that since the cutting oil remaining on the surface in the machining step 22 becomes a factor inhibiting the elution of the aluminum component, if necessary, a degreasing step (not shown) such as neutral degreasing or alkaline degreasing may be performed before the alkali-etching step 24. Alternatively, a chemical having a degreasing effect may be added to the alkaline etching solution to perform degreasing and alkali-etching simultaneously. Since the alkali-etching is performed only on the outer surface of the skirt portion 18, other portions may be masked as appropriate.

[0019] The cross-sectional shape of the outer surface of the skirt portion 18 is enlarged and shown in FIG. 3. Note that FIG. 3 is in a relationship obtained by rotating FIG. 1 90° to the left. As shown in FIG. 3, streaks 30 are formed on the outer surface of the skirt portion 18 in order to improve seizure resistance between the outer surface and the inner wall surface of the cylinder bore.

[0020] The streak 30 has a plurality of groove portions 31 having 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 groove portions 31 are arranged at intervals in the axial direction. The depth of the groove portion 31 is, for example, in the range of 5 to 15 μm. Further, an intermediate surface portion (plateau portion) 32 whose radially outer end in the cross section extends linearly in the axial direction is provided between the groove portions 31 adjacent in the axial direction.

[0021] The axial width (that is, the interval between the adjacent groove portions 31) Wa of the plateau portion 32 is, for example, in the range of 10 to 100 μm. Further, the axial width Wb of the groove portion 31 is, for example, in the range of 150 to 400 μm. And the plateau ratio represented 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 been conventionally used as one of the indexes representing the shape of the streak 30.

[0022]

Equation

[0023] Examples of the chemical agent used for the alkaline etching solution include sodium hydroxide, potassium hydroxide, sodium carbonate, ammonium hydroxide, etc. Among these, an aqueous sodium hydroxide solution is preferable because it is inexpensive and has high solubility of aluminum. Further, a chelating agent such as sodium gluconate may be added to the etching solution. The reaction formula of the alkaline etching of aluminum using sodium hydroxide as the etching solution is shown below as an example.

[0024]

Chemical formula

[0025] The cross-sectional shape of the outer surface of the skirt portion 18 after the alkali edging step 24 is shown in FIG. 4. Since only the aluminum component in the aluminum alloy is eluted by alkali edging, as shown in FIG. 4, a plurality of recesses 33 finer than the groove portion 31 of the streak 30 are formed on the outer surface of the skirt portion 18.

[0026] In the alkali etching step 24, the longer the treatment time of alkali etching, the larger the size of the fine recesses 33, and thus the greater the influence on the shape of the streak 30 on the outer surface of the skirt portion 18. Since the streak 30 on the outer surface of the skirt portion 18 is for improving the seizure resistance of the piston, it is necessary to maintain the shape of the streak 30. Therefore, the treatment time of alkali etching is preferably the treatment time that maintains the plateau ratio of the outer surface of the skirt portion 18 after the alkali etching 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 there is a risk of seizure. On the other hand, if the plateau ratio exceeds 0.15, the effect of discharging foreign matter cannot be sufficiently ensured, and there is a risk of seizure.

[0027] Also, the treatment time is preferably such that the change amount of the plateau ratio of the outer surface of the skirt portion 18 before and after the alkali etching step 34 is suppressed to 0.05 or less. If the change amount of the plateau ratio exceeds 0.05, the general shape of the streak 30 may change, and there is a risk that the variation in the piston reference diameter increases. The lower limit of the change amount of the plateau ratio is not particularly limited, but is preferably 0.00 or more. Although specific examples of such treatment time vary depending on the type and concentration of the chemical agent in the etching solution, for example, less than 20 minutes is preferable. On the other hand, the lower limit of the treatment time is such that a large number of fine recesses 33 are formed on the outer surface of the skirt portion 18, and for example, 1 minute or more is preferable.

[0028] Also, in the alkali edging step 24, along with the fine recesses 33, on the outer surface of the skirt portion 18, components other than aluminum in the aluminum alloy are generated as black deposits, and this deposit is called a smut 34. If the smut 34 remains on the outer surface of the skirt portion 18 before forming the resin coat, there is a risk of reducing the adhesion of the resin coat to the outer surface of the skirt portion 18 in the surface treatment step 26. Therefore, it is necessary to remove the smut 34.

[0029] The pickling step 25 is a step of eluting components other than aluminum that generate the smut 34 by bringing the outer surface of the skirt portion 18 into contact with an acidic cleaning solution. Examples of the chemicals used in the acidic cleaning solution include nitric acid, hydrofluoric acid, and a mixed acid of nitric acid and hydrofluoric acid. Usually, the smut 34 can be removed with nitric acid at a concentration of about 30 wt%. The reaction formula for smut removal using nitric acid is shown below. In the formula, M represents a component other than aluminum in the aluminum alloy.

[0030]

Chemical formula

[0031] In the casting of pistons for internal combustion engines, as described above, an aluminum alloy containing silicon is often used. In this case, the smut 34 contains silicon. Since silicon does not react with nitric acid, in the case of a piston for an internal combustion engine made of an aluminum alloy containing silicon, it is preferable to use a mixed acid of nitric acid and hydrofluoric acid. The reaction formula between silicon and hydrofluoric acid is shown below.

[0032]

Chemical formula

[0033] By removing the smut 34 in this way, as shown in FIG. 5, on the outer surface of the skirt portion 18, the smut 34 that was generated to cover the fine recesses 33 is removed, and the fine recesses 33 are cleanly exposed on the outer surface of the skirt portion 18.

[0034] Although the removal of the smut 34 by the pickling step 25 has been described, the present invention is not limited thereto. To remove the smut 34, for example, anodic electrolysis using an alkaline solution such as sodium cyanide or ethylenediaminetetraacetic acid, polishing methods such as electrolytic polishing or barrel polishing, or a method using ultrasonic waves may be used.

[0035] The water washing step 26 is a step of removing the acidic cleaning liquid used in the pickling step 25 from the skirt portion 18 by water washing. As the water washing step 26, it is preferable to perform ultrasonic water washing. For example, the skirt portion 18 is brought into contact with water with ultrasonic vibration of 1 to 100 kHz for 1 to 10 minutes. By ultrasonic water washing, it is possible to wash the smut 34 that could not be completely removed in the pickling step 25.

[0036] Also, if necessary, the water washing step 26 may be performed before the pickling step 25. Thereby, it is possible to prevent the alkaline etching liquid used in the alkaline etching step 24 from being brought into the acidic cleaning liquid of the pickling step 25. Also in this case, it is preferably performed by ultrasonic water washing, and thereby, most of the smut 34 generated on the outer surface of the skirt portion 18 can be removed before the pickling step 25. Therefore, it is possible to reduce the load on the pickling step 25. The water washing step 26 may be performed both before and after the pickling step 25, or may not be performed at all.

[0037] The surface treatment step 27 is a step of forming a resin coat (not shown) on the outer surface of the exposed skirt portion 18 where the fine recesses 33 are formed. As the surface treatment step 27, for example, a resin coat agent can be applied to the outer surface of the skirt portion 18 by a spray method or a screen printing method, and then baked to form a resin coat. As the resin coat agent, known agents used for the skirt portion 18 of a piston for an internal combustion engine can be used. For example, MOLYKOTE D-10-GBL or MOLYKOTE PA-744 manufactured by DuPont-Toray Specialty Materials Co., Ltd. can be mentioned.

[0038] Thus, a large number of fine recesses 33 are formed on the outer surface of the skirt portion 18 of the piston for an internal combustion engine, and since a resin coating agent is filled in these fine recesses 33, the adhesion between the base aluminum alloy and the resin coating can be enhanced by the anchor effect. Further, even when a large number of fine recesses 33 are formed, the shape of the streak 30 on the outer surface of the skirt portion 18 is maintained. Therefore, a good lubricated state is maintained by the groove portion 31 of the streak 30, and the friction reduction effect by the plateau portion 32 of the streak 30 is also maintained, so that excellent seizure resistance can be ensured.

[0039] Note that the manufacturing method of the piston for an internal combustion engine according to the present invention is not limited to the above-described embodiment and may include other steps. For example, an anodizing process or a chemical conversion process may be performed between the pickling process 25 and the surface treatment process 27. By the anodizing process, a porous anodized film is formed on the outer surface of the skirt portion 18. By making the film thickness several μm or less, the anodized film can be formed while maintaining the shape of the streak 30 on the outer surface of the skirt portion 18. By filling the fine pores of the anodized film with a resin coating agent, further improvement in adhesion can be expected. Further, by the chemical conversion process, a chemical conversion film in which a part of the outer surface of the skirt portion 18 has chemically reacted is formed. Since the film thickness is several μm, the chemical conversion film can be formed while maintaining the shape of the streak 30 on the outer surface of the skirt portion 18. Since the chemical conversion film is formed in a granular shape and has irregularities, further improvement in adhesion can be expected by filling the irregularities with a resin coating agent. Note that when performing the anodizing process, the first ring groove 13 and the second ring groove 15 can also be anodized simultaneously with the skirt portion 18, whereby wear resistance can be imparted to these ring grooves.

Example

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

[0041] [Example] An Al-Si-based aluminum alloy casting material, which is a high-strength material with improved mechanical properties in the high-temperature range, was used to fabricate a piston for an internal combustion engine, and streaks were formed on the skirt portion using a turning tool. Then, degreasing and alkaline etching were simultaneously performed on this skirt portion using an alkaline etching solution (manufactured by Okuno Pharmaceutical Co., Ltd., Top Alsoft 108, an aqueous solution diluted to a concentration of 15 to 25 g / L, pH 13). The liquid temperature was set to 30 to 40 °C, and the immersion times were set to 5 minutes, 10 minutes, and 20 minutes in order to confirm the roughening situation depending on the immersion time.

[0042] Next, since black deposits (smut) were generated on the surface of the skirt portion of each piston after alkaline etching, ultrasonic cleaning was performed, and then pickling was performed using nitric acid (concentration 20 to 30 wt%, pH 1.7). The liquid temperature was set to 20 to 30 °C, pickling was performed for 5 minutes, and then ultrasonic cleaning was performed again. The black deposits on the surface of the skirt portion were completely removed.

[0043] As an evaluation of the piston thus obtained, (1) the surface condition by measuring the surface roughness, (2) the change in the reference diameter before and after treatment by measuring the piston reference diameter, and (3) the change amount of the plateau ratio were evaluated. In addition, in order to perform these evaluations, the same measurements were also performed on the pistons before alkaline etching.

[0044] The surface roughness was measured in accordance with JIS B 0601-2001. The roughness curve of the skirt portion of the piston was measured, and from the results, the surface roughness Ra representing the arithmetic mean roughness and the surface roughness Rz representing the maximum height were calculated respectively.

[0045] The piston reference diameter was measured using a dial gauge for the reference diameter of the skirt portion of each piston whose temperature was made constant by leaving it in a room at 20 to 25 °C overnight.

[0046] The plateau ratio was measured from the measurement results of the above roughness curve. For any three streaks, the width (Wa) of the plateau part and the width (Wb) of the groove part were measured, calculated from the above formula (1), and used as the average value. Also, the change amount of the plateau ratio was calculated from the difference between the plateau ratio of the piston before alkali etching and the plateau ratio of the piston after alkali etching.

[0047] Furthermore, after performing the above evaluation, a resin coat was formed on each piston that had been alkali-etched. As the resin coat, MOLYKOTE D-10-GBL manufactured by DuPont-Toray Specialty Materials Co., Ltd. (former Toray Dow Corning Co., Ltd.) was used. After washing the piston, the resin coat agent was printed by the screen printing method and baked at a temperature of 200°C or lower to form a resin coat on the skirt part of the piston.

[0048] Then, for the piston on which this resin coat was formed, a test was conducted to (4) evaluate the adhesion of the resin coat. The adhesion test was evaluated using a method called the water jet (W / J) method, in which water was sprayed at high pressure from a nozzle directly above the resin coat, and the adhesion was evaluated from the appearance of the resin coat surface. For reference, the adhesion of the resin coat was also evaluated for the piston on which the resin coat was formed without alkali etching by the same test.

[0049] [Comparative Example] An internal combustion engine piston was manufactured in the same manner as in the example. After forming streaks on the skirt part, in the comparative example, shot blasting was performed on the skirt part instead of alkali etching. As the shot material (media) for shot blasting, glass beads #150 were used, and shot blasting was performed with a projection pressure of 0.4 MPa and a distance (projection distance) between the projection nozzle and the skirt part of 50 cm. In order to confirm the roughening situation depending on the projection time, the projection times were set to 5 seconds, 10 seconds, and 30 seconds. Then, after shot blasting, air blowing, washing, and drying were performed.

[0050] As evaluations of each piston thus obtained, similar to the examples, evaluations were made regarding (1) the surface properties by measuring the surface roughness, (2) the change in the reference diameter before and after processing by measuring the reference diameter of the piston, and (3) the amount of change in the plateau ratio. In addition, in order to perform these evaluations, the same measurements were also made for the pistons before alkali etching. Furthermore, for the pistons with a resin coat formed thereon, a test was conducted to evaluate (4) the adhesion of the resin coat.

[0051] The evaluation results of the above (1) to (4) will be described. The roughness curves of each piston of the examples subjected to alkali etching and the pistons not subjected to alkali etching are shown in FIG. 6. Also, the results of the surface roughness of each piston of the examples subjected to alkali etching are shown in FIGS. 7 and 8. The roughness curves of each piston of the comparative examples subjected to shot blasting and the pistons not subjected to shot blasting are shown in FIG. 9. Also, the results of the surface roughness of each piston of the comparative examples subjected to shot blasting are shown in FIGS. 10 and 11. Furthermore, Table 1 shows the plateau ratios of each piston of these examples and comparative examples, and the amount of change from the plateau ratios of their untreated pistons.

[0052]

Table 1

[0053] Figure 6 shows that (a) is the roughness curve before alkali etching, (b) is the roughness curve with an alkali etching treatment time of 5 minutes, (c) is the roughness curve with a treatment time of 10 minutes, and (d) is the roughness curve with a treatment time of 20 minutes. As shown in Figure 6, in the examples, as the alkali etching treatment time increased, the fine recesses on the surface tended to increase, but it was confirmed that the streak shape was maintained. As shown in Table 1, when the alkali etching treatment time was 10 minutes, the change amount from the untreated plateau ratio was 0.03, and as shown in Figure 6(c), the streak shape was clearly maintained. On the other hand, when the alkali etching treatment time was 20 minutes, the change amount of the plateau ratio was 0.06, which was a larger change compared to 10 minutes. As shown in Figure 6(d), although the streak shape was maintained, the streak shape was starting to collapse, and it was found that the change amount of the plateau ratio was preferably 0.05 or less.

[0054] Also, as shown in Figures 7 and 8, as the alkali etching treatment time increased, the surface roughness Ra representing the arithmetic mean roughness slightly increased, while the surface roughness Rz representing the maximum height increased significantly. This is presumably because the surface of the skirt part had fine recesses increasing while maintaining the streak shape.

[0055] Figure 9 shows that (a) is the roughness curve before shot blasting, (b) is the roughness curve with a shot blasting treatment time of 5 seconds, (c) is the roughness curve with a treatment time of 10 seconds, and (d) is the roughness curve with a treatment time of 30 seconds. As shown in Figure 9, in the comparative examples, as the shot blasting treatment time increased, it was confirmed that the streak shape disappeared and approached a flat shape. As shown in Figure 9(b), when the shot blasting treatment time was 5 seconds, the streak shape had already largely collapsed and the plateau part could not be confirmed. Also, since it was difficult to calculate the plateau ratio, the plateau ratio is not shown in Table 1.

[0056] Also, as shown in Figures 10 and 11, as the shot blasting treatment time increased, both the surface roughness Ra and the surface roughness Rz decreased. This is presumably because while fine recesses were formed, the streak shape gradually disappeared.

[0057] The measurement results of the reference diameter of the piston are shown in Table 2. Table 2 shows the change amount of the reference diameter of the skirt part of the piston before and after the treatment of alkali etching or shot blasting. The change amount indicates an increase in the reference diameter after the treatment with a plus sign and a decrease in the reference diameter after the treatment with a minus sign. In addition, in the examples, the pistons with an alkali etching treatment time of 5 minutes were measured, and in the comparative examples, the pistons with a shot blasting treatment time of 10 seconds were measured. Also, the results are those of measuring five piston samples each for both the examples and the comparative examples.

[0058] [Table 2]

[0059] As shown in Table 2, the pistons in the examples had a decrease in the reference diameter of 9 to 11 μm after alkali etching compared to before. This is presumably due to the elution of aluminum on the surface of the skirt part. The variation with N = 5 was suppressed within a range of 2 μm, and the variation was small. This is presumably because in alkali etching, while roughening, the aluminum alloy gradually dissolved from the surface. On the other hand, the pistons in the comparative examples had an increase in the reference diameter of 0 to 7 μm after the shot blasting treatment compared to before. This is presumably because the aluminum alloy on the surface of the skirt part was plastically deformed radially outward by the shot blasting. The variation with N = 5 was 7 μm, resulting in a larger variation compared to the examples of alkali etching.

[0060] Table 3 shows the evaluation results of the adhesion of the resin coat. The evaluation criteria were as follows: observing the resin coat, those without peeling were evaluated as "〇", those with partial peeling of the resin coat were evaluated as "△", and those with interfacial peeling of the resin coat from the aluminum alloy base material were evaluated as "×". In addition, in the examples, the pistons with an alkali etching treatment time of 10 minutes were observed, and in the comparative examples, the pistons with a shot blasting treatment time of 5 seconds were observed.

[0061]

Table 3

[0062] As shown in Table 3, in the piston where neither alkaline etching nor shot blasting was performed in the reference example, interfacial peeling was observed in the resin coat. On the other hand, neither interfacial peeling nor point peeling was observed in the resin coat of the piston of the example where alkaline etching was performed nor in the piston of the comparative example where shot blasting was performed. From this, it became clear that by forming fine recesses on the aluminum alloy surface by alkaline etching or shot blasting and then forming a resin coat, there is an effect of improving the adhesion of the resin coat due to the anchor effect of the fine recesses.

Explanation of Signs

[0063] 10 Piston for internal combustion engine 18 Skirt part 20 Manufacturing method of piston for internal combustion engine 30 Streak 31 Groove part 32 Intermediate surface part (Plateau part) 33 Fine recess 34 Smart

Claims

1. A method for manufacturing a piston for an internal combustion engine, comprising: a piston for an internal combustion engine made of an aluminum alloy containing silicon has a skirt portion that slides on the inner wall surface of the cylinder bore of the internal combustion engine, and a plurality of groove portions extending in the circumferential direction are provided on the outer surface of the skirt portion. These plurality of groove portions are arranged at intervals in the axial direction, and an intermediate surface portion is provided between the groove portions adjacent to each other in the axial direction. An alkali etching step of alkali etching the skirt portion of the piston for an internal combustion engine to form a plurality of recesses finer than the groove portions on the outer surface of the skirt portion while maintaining the configuration of the groove portions and the intermediate surface portion provided on the outer surface of the skirt portion; a cleaning step of removing the smut generated on the outer surface of the skirt portion by the alkali etching; a surface treatment step of forming a resin coat on the outer surface of the skirt portion from which the smut has been removed; and including: the cleaning step is a method for manufacturing a piston for an internal combustion engine, in which an acidic solution using a mixed acid of nitric acid and hydrofluoric acid is brought into contact with the outer surface of the skirt portion to remove the silicon-containing smut among the smut as H₂SiF₆.

2. The method for manufacturing a piston for an internal combustion engine according to claim 1, further comprising an ultrasonic water washing step of bringing the outer surface of the skirt portion into contact with water with ultrasonic vibration before and / or after the cleaning step.

3. The method for manufacturing a piston for an internal combustion engine according to claim 1 or 2, wherein the plateau ratio of the outer surface of the skirt portion after the alkali etching step is in the range of 0.12 to 0.

15.

4. The method for manufacturing a piston for an internal combustion engine according to any one of claims 1 to 3, wherein the change amount of the plateau ratio of the outer surface of the skirt portion before and after the alkali etching step is 0.05 or less.

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