JP2000027993A - Piston for internal combustion engine and surface treating method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a piston and a treating method thereof to improve surface finish at a low cost by controlling an alumite treating condition as a treating process is simplified, and provide with a wear resisting rigid anodized aluminum film effective to wear of a pin hole. SOLUTION: By machining the piston hole of a piston made of an aluminum alloy, a pin hole diameter and surface finish are adjusted. An electrolytic time and current density are set to values within the region of the electrolytic condition of anodized aluminum treatment and alumite treatment is applied on the surface of the piston pin hole as the surface of the piston pin hole is cooled. This constitution forms an alumite treatment film 10 with a film thickness of 30±10 μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a piston for an internal combustion engine and a method for treating the surface thereof, and more particularly to an improvement in wear resistance of a piston pin hole.

[0002]

2. Description of the Related Art As an aluminum alloy for a piston manufactured through material forming by casting or forging, an aluminum base material containing 10 to 20% by weight of Si is used. The piston pin hole is under severe sliding conditions, and higher wear resistance is required. If this type of aluminum alloy piston is not subjected to surface hardening treatment, for example, on the inner surface of the piston pin hole portion, the base material is worn first and Si particles are raised, and a concentrated load is applied to the Si particles. As a result, the particles break, and this functions as abrasive powder,
There is a problem that wear of the piston pin hole becomes remarkable.

As a countermeasure, it is conceivable that the piston pin hole is subjected to alumite treatment to increase the hardness of the base metal on the surface so that the base metal is not worn first. Si
In a piston made of an aluminum alloy containing Si, when Si is exposed on the surface to be subjected to alumite (anodic oxidation) treatment, the area where the electrolytic current flows on the Al surface to be oxidized and anodized (electrical surface area) is exposed to Si on the surface. As a result, the current density increases and heat is easily generated in the alumite-treated layer. When the temperature of the alumite layer rises, the electrolytic solution mixed with sulfuric acid and further oxalic acid or citric acid in contact with the surface is activated, dissolves the formed alumite layer, roughens the surface of the alumite layer and lowers the hardness. Would.

For this reason, in the conventional process of alumite treatment for the surface portion of the piston hole, the Si particles exposed on the surface by the mixed acid treatment before the alumite treatment are dissolved to increase the surface area of the surface through which electricity is passed, thereby increasing the current. The density is reduced, and alumite treatment is performed slowly to prevent heat generation. However, the mixed acid treatment reduces the surface roughness before the alumite treatment, which causes the surface roughness of the alumite film after the alumite treatment to decrease. For this reason, in the conventional method, roller burnishing is performed in the final step in order to improve the surface roughness. That is, the conventional alumite treatment comprises the following four steps. Piston pin hole machining: Adjust pin hole diameter and surface roughness. Mixed acid treatment: In a mixed acid of hydrofluoric acid and nitric acid, Si particles in the metal structure exposed on the pin hole surface are dissolved and removed. Alumite treatment: An alumite film (anodized film) is formed on the pin hole surface. In the case of lubricating alumite treatment, a secondary electrolysis step in an ammonium thiomolybdate solution is added. Roller varnish: Roller varnish is applied to the pin holes to improve the surface roughness.

[0005]

However, the roller burnish simultaneously cracks the alumite coating and crushes the Si particles present inside the coating. In particular, since the crushed Si particles act as abrasive powder, wear resistance is significantly reduced. Therefore, the conventional hard alumite treatment method has little effect on pin hole wear. On the other hand, lubricating alumite is effective in preventing pin hole abrasion because it contains a lubricating component (molybdenum disulfide) in the film, but has a problem in that the treatment process is complicated and the cost is extremely high.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned conventional problems. By controlling the alumite processing conditions while simplifying the processing steps, the surface roughness can be improved at low cost, and thus effective in reducing pin hole wear. An object of the present invention is to provide a piston provided with a wear-resistant hard alumite film and a method for treating the piston.

[0007]

According to a first aspect of the present invention, there is provided an alumite-treated film made of an aluminum alloy, having a film thickness of 30 ± 10 μm and exposing Si particles or SiC particles on a surface portion of a piston pin hole. It is a piston of an internal combustion engine characterized by being formed.

A second aspect of the present invention is characterized in that, in the first aspect, an alumite-treated film is formed on an inner end surface of the pin boss portion having the piston pin hole facing the small end portion of the connecting rod.

A third aspect of the present invention is to machine a piston pin hole of an aluminum alloy piston to adjust the pin hole diameter and surface roughness, and to set the electrolysis time and the current density on the surface of the piston pin hole by oblique lines in FIG. A method for treating the surface of a piston of an internal combustion engine, wherein an alumite treatment film having a film thickness of 30 ± 10 μm is formed by setting an area and performing alumite treatment while cooling the surface of the piston pin hole.

[0010]

According to the first and third aspects of the present invention, after machining the piston pin hole, the electrolysis time and the current density are controlled without performing the conventional mixed acid treatment, and the surface of the piston pin hole is controlled. Alumite treatment was performed while cooling to form an alumite treatment film with a film thickness of 30 ± 10 μm.
Even if the current density is high, the alumite heat treatment can be performed in a short time without generating heat on the alumite surface, simplifying the surface treatment, not reducing the surface hardness of the alumite layer, and the surface roughness of the film is approximately Ra (defined in JIS B0601). Center line average roughness) of 1.5 to 3.0 μm, so that a piston of an internal combustion engine having excellent wear resistance of the piston pin hole can be provided at low cost.

According to the second aspect of the present invention, since the alumite-treated film is formed on at least the inner end face of the pin boss at the inner face of the piston, the inner end face is prevented from being worn by contact with the small end of the connecting rod. It is possible to prevent wear of the piston pin hole caused by the wear powder biting into the piston pin hole.

[0012]

Embodiments of the present invention will be described below with reference to the accompanying drawings. 1 to 11 are views for explaining a piston of an internal combustion engine and a surface treatment method thereof according to an embodiment of the present invention. FIG. 1 is a view showing a state of arrangement of the piston in the internal combustion engine, and FIG. FIG. 3 is an enlarged view of an alumite film, FIG. 4 is a schematic diagram showing a surface treatment state, FIG. 5 is a characteristic diagram showing piston electrolysis conditions, FIG.
8 are diagrams showing the relationship between the quantity of electricity and the film thickness, hardness, and surface roughness, FIG. 9 is a diagram showing changes in electrolysis voltage with electrolysis time, and FIG. 10 is a diagram showing comparison results of wear resistance. FIG. 11 is a view showing a form of wear.

1 to 3, reference numeral 1 denotes a piston. The piston 1 is slidably inserted into a cylinder bore 2a of a cylinder block 2. The piston 1
Is connected to a small end 3a of a connecting rod 3 via a bearing 4 and a piston pin 5, and a large end 3b of the connecting rod 3 is connected to a crank pin 6a of a crank shaft 6. As a result, the vertical movement of the piston 1
Is converted into a rotational motion.

The piston 1 is made of an aluminum alloy (AC8A to AC8C, AC9A, AC9B, etc. shown in Table 1).
The top surface 1a and the inner surface 1b are cast surfaces and are not machined, and the outer peripheral surface (cylinder sliding surface) is machined into a predetermined shape. The first and second ring grooves 7 are formed on the outer peripheral surface by machining.
a, 7b are formed, and the first and second ring grooves 7 are formed.
a and 7b have a top ring 11a and a second ring 11
b is attached.

A pair of pin bosses 1c, 1c are formed inside the piston 1 so as to bulge inward, and a piston pin hole 7c is formed in the pin bosses 1c, 1c so as to penetrate in the radial direction. ing. The piston pin 5 is rotatably inserted into the piston pin hole 7c. A locking groove 7d is formed at both outer ends of the pin hole 7c, and snap rings 8, 8 are mounted in the locking groove 7d. This snap ring 8 is used for the piston pin 5
In order to prevent the movement in the axial direction.

Anodizing (anodizing) is applied to the inner surface 1b of the inner surface 1b of the piston 1, in particular, the inner end surface 1b 'of the pin bosses 1c, 1c facing the small end 3a of the connecting rod 3, and the inner surface of the piston pin hole 7c. An alumite treatment film 10 is formed by the treatment. This anodized film 1
0 is the film thickness (however, since the film thickness cannot be completely uniform over the entire film, here, the maximum film thickness of the film thickness which varies within a certain width in the spread of the film)
The thickness is set within a range of 30 ± 10 μm, and as a result, the surface roughness of the film is controlled to Ra 1.5 to 3.0 as described below. The alumite coating 10 has a surface shape such that the surface a of the coating main body 10a is located outside the exposed surface b of the Si 10b, in other words, the Si 10b does not protrude from the coating main body 10a.

Next, the manufacturing process of the piston 1 will be described. First, aluminum alloys containing Si shown in Table 1 (AC8A, AC8B, AC8C, AC9A, AC
Piston material is created by casting using 9B).
Alternatively, a burette was used in which alloys 1 to 3 shown in Table 1 were continuously drawn out of a melting furnace in a rod shape and cooled and then cut, or powders of alloys 4 to 6 were extruded from a heating furnace to join the powders together. A piston material cast using a burette obtained by cutting a bar material in a state is used. Then, heat treatment (T6 or T7) is applied to the piston material.
Processing) is performed. The T6 and T7 treatments are described in Table 2,
Under the processing conditions shown in Table 3, a solution treatment of rapidly cooling after heating at a high temperature, and an artificial aging treatment or a stabilizing heat treatment of gradually cooling after heating and holding are performed.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

[Table 3]

Next, the above-mentioned heat-treated piston material is subjected to primary machining such as machining of a piston pin hole, machining of a piston ring groove, machining of a jig insertion portion, etc., followed by alumite treatment (described later). Then, secondary machining including outer surface machining and finishing is performed, and the piston 1 is manufactured.

The alumite treatment of the present embodiment is performed in the following procedure without performing the conventional mixed acid treatment after machining the piston pin hole. That is, receiving the first machined piston material, attaching a jig, degreasing,
Water washing, anodizing treatment, water washing, jig removal, and hot water washing are sequentially performed.

In the above degreasing step, the oil components and the like on the surface of the component are removed by immersing the component in a degreasing bath having the composition shown in Table 4 for a predetermined time. In the alumite treatment,
As shown in FIG. 4, an alumite bath (electrolyte bath) having a composition shown in Table 5 was applied to the piston blank 1 ′ attached to the jig 12.
13, an anode of a constant current DC power supply 14 is connected to the jig 12, a cathode of the power supply 14 is connected to a cathode plate 15 in an alumite bath 13, and a cooled electrolyte solution is applied to a piston material 1 ′. While cooling while controlling the current density and the electrolysis time under the conditions shown in FIG. FIG. 5 shows the electrolysis time (minute) and the current density (A / dm ² = ampere / 10).
0 cm ² ) and the quantity of electricity per unit processing area (C /
dm ² = clone / 100 cm ² ), and in this embodiment, the amount of electricity is 3000 to 6000 (C /
dm ² ), the alumite treatment is performed while controlling the current density and the electrolysis time within the shaded range in FIG.

In FIG. 4, reference numeral 20 denotes a radiator for cooling the electrolyte, 21 denotes a discharge nozzle for discharging the cooled electrolyte, 22 denotes a circulation path for cooling the electrolyte, and 23 denotes a circulation pump. . The low-temperature and low-pressure liquid-phase refrigerant that has passed through an expansion valve (not shown) is guided by a low-pressure pipe 24 to an evaporating coil 24 a in the radiator 20. Since the temperature of the alumite-treated layer of the piston 1 'is prevented from increasing during the alumite treatment, a decrease in the hardness of the alumite layer can be prevented.

The temperature is detected by a temperature sensor 26 provided in the lower part of the bathtub 13 or on the piston material 1 '. Based on the detected temperature, the controller 27 determines that the higher the detected temperature, the higher the flow rate of the electrolyte flowing through the circulation path. Or the capacity of the refrigerant circulation pump 25 or a compressor (not shown) may be increased to prevent the temperature of the alumite treatment layer of the piston 1 from rising. Alternatively, the temperature of the downstream portion of the cooler 20 may be detected by the temperature sensor 26 ', and this temperature may be controlled to 30C or lower. Furthermore, cooling with the cooling liquid of the discharge nozzle 21 or air cooling may be used. Thus, it is possible to more reliably prevent the temperature of the alumite-treated layer from being raised during the alumite treatment, and to more reliably prevent the hardness of the alumite layer from decreasing.

[0026]

[Table 4]

[0027]

[Table 5]

FIGS. 6 to 8 show the relationship between the quantity of electricity per unit processing area (C / dm ² ) and the alumite film thickness (μm), film hardness (Hv), and film surface roughness (Ra). Show.
In the present embodiment, the amount of electricity per unit processing area is 3,000 to 3,000.
Since it is controlled to be 6000 (C / dm ² ), the film thickness is approximately 20 to 39 (μm), respectively.
The film hardness is approximately 380 to 420 (Hv), and the film surface roughness is approximately 1.5 to 3.0 (Ra).

FIG. 9 is a characteristic diagram showing the relationship between the electrolysis time and the electrolysis voltage when the current density is 6.3 A / dm ² , and the electrolysis voltage increases as the electrolysis time elapses. That is, when the alumite treatment is performed while keeping the electrolytic voltage constant, the current density decreases as the electrolysis time elapses and the thickness of the alumite layer increases, and the rate of increasing the alumite layer thickness per hour decreases. On the other hand, in the initial stage of the treatment, the current density increases, and the surface temperature of the film increases due to the heat generated by anodic oxidation, and the alumite layer is dissolved by sulfuric acid in the electrolytic solution, and the surface becomes rough. For this reason, it is preferable to cool the electrolytic solution at the beginning of the alumite treatment. Alternatively, the current value may be detected and the electrolytic solution may be cooled as the current value increases.

FIG. 10 shows the results of an experiment performed to confirm the effects of the present embodiment. In the same experiment, the electrolytic bath temperature was 0
At a current density of 6.3 A / dm ^2, and subjected to alumite treatment while being cooled by applying an electrolytic solution to a material to be treated in a jet state. The alumite having the film thickness, hardness and surface roughness shown in FIGS. A treated film was formed. Then, the relationship between the operation time of the engine using the piston and the wear amount of the piston pin hole was determined. In the figure, A is the wear amount of the piston pin hole which has been subjected to the processing time such that the amount of electricity per unit area is 6000 C / dm ^{2 in} the alumite treatment of the present embodiment, and A ′ is the alumite treatment of the present embodiment. B represents the wear amount of the piston hole subjected to the treatment time such that the amount of electricity per unit area becomes 3000 C / dm ² , B represents the wear amount of the piston pin hole subjected to the alumite treatment by the conventional method, and C represents the wear amount.
Indicates the wear amount of the piston pin hole when no alumite treatment is performed.

As is clear from the figure, in the case of the conventional method, the wear amount has reached 20 μm in the operation time of 20 hours, whereas in the case of the present embodiment, A
In both cases A and A ′, even if the operation time exceeds 100 hours, the abrasion amount is about 10 μm, which indicates that there is no practical problem. In addition, when the alumite treatment is not performed, the wear amount immediately reaches 20 μm in an operation in an extremely short time, and it can be seen that the wear amount cannot be put to practical use.

As described above, in this embodiment, after the machining of the piston pin hole, the alumite treatment is immediately performed without performing the mixed acid treatment, and the electrolysis condition in that case is controlled to the shaded region shown in FIG. A piston for an automobile engine having excellent wear properties can be obtained at low cost.

Here, the wear generation mechanism of the piston pin hole will be described in detail. As shown in FIG. 11A, the outer portion 7 of the pin hole 7c is formed by the thermal deformation of the piston pin hole.
c 'and the outer end portion 5a of the piston pin 5 are always in contact with each other, and the oil film is broken at the contact portion. As a result, the outer portion 7c' of the piston pin hole 7c is worn.

Further, as shown in FIG. 2B, when an oil film breaks at the central portion 7c '' of the piston pin hole 7c, the surface of the central portion of the piston pin hole 7c is peeled (fretting), and the piston pin hole 7c is removed. Roughness (wear) occurs at the center.

Further, as shown in FIG. 3 (3), the small end 3a of the connecting rod 3 comes into contact with the inner end face 1b 'of the pin boss portion, and the portion is worn, and the abrasion powder bites into the piston pin hole. Thereby, the piston pin hole 7c is worn.

In the present embodiment, as shown in FIGS. 6, 7, and 8, the thickness of the hard
m, hardness HmV405, surface roughness Ra 1.96 ~
Since the control was performed at 2.60 μm, FIG.
The wear of the outer portion 7c 'and the central portion 7c''of the piston pin hole 7c shown in (2) can be prevented.

Further, since the alumite-treated film is formed on the inner surface 1b 'of the piston inner surface 1b, especially the inner end surface 1b' opposite to the small end 3a of the connecting rod 3, it comes into contact with the small end 3a as shown in FIG. Accordingly, the inner end face 1b 'can be prevented from being worn, and the wear of the piston pin hole 7c due to the Si powder generated by the wear can be prevented.

When the piston 1 is forged with an aluminum alloy of alloy 5, Si is used instead of Si.
The C particles are exposed on the surface after the completion of the first machining. Even in this case, the above-described alumite treatment can secure a film having a small surface roughness and a high hardness. Further, since the alumite film is also formed on the ring groove, it is possible to reduce the occurrence of ring groove wear.

The above-described alumite treatment may be performed after the secondary machining, not before. In this case, the wear durability of the outer periphery of the piston 1 can be improved. However, it is necessary to remove extra by the primary machining of the expansion due to the application of the alumite film. Further, when the thickness distribution of the alumite film is not uniform and undulating irregularities are formed on the outer surface, it is preferable to perform finish polishing.

[Brief description of the drawings]

FIG. 1 is a cross-sectional front view showing a piston connected to a crankshaft according to an embodiment of the present invention.

FIG. 2 is an enlarged sectional side view of the piston according to the embodiment.

FIG. 3 is an enlarged schematic sectional view of an alumite film of the piston according to the embodiment.

FIG. 4 is a schematic diagram showing an alumite treatment device for the piston of the embodiment.

FIG. 5 is a characteristic diagram showing electrolysis conditions for alumite treatment of the piston of the embodiment.

FIG. 6 is a graph showing the relationship between the quantity of electricity and the film thickness of the embodiment.

FIG. 7 is a graph showing the relationship between the quantity of electricity and the hardness of a film in the embodiment.

FIG. 8 is a graph showing electric quantity-film roughness characteristics of the embodiment.

FIG. 9 is an electrolysis time-electrolysis voltage characteristic diagram of the embodiment.

FIG. 10 is a time chart for explaining the effect of the embodiment.
It is a wear amount characteristic diagram.

FIG. 11 is a schematic view showing a general form of wear of a piston pin hole.

[Explanation of symbols]

 Reference Signs List 1 piston 1b 'inner end face 1c pin boss 3 connecting rod 3a small end 7c piston pin hole 10 alumite-treated film 10b Si particles

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI theme coat ゛ (Reference) F16J 1/01 F16J 1/01

Claims (3)

[Claims] An internal combustion engine made of an aluminum alloy, wherein an alumite-treated film having a film thickness of 30 ± 10 μm and exposing Si particles or SiC particles is formed on a surface portion of a piston pin hole. Piston. 2. The piston of an internal combustion engine according to claim 1, wherein an alumite-treated film is formed on an inner end surface of the pin boss portion having the piston pin hole formed opposite to the small end portion of the connecting rod. 3. The piston pin hole of an aluminum alloy piston is machined to adjust the pin hole diameter and surface roughness, and the electrolysis time and the current density are set in the hatched area of FIG. By applying anodizing while cooling the surface of the piston pin hole, the film thickness is 30 ± 10μ.
A surface treatment method for a piston of an internal combustion engine, wherein an alumite-treated film having a thickness of m is formed.