JP2007132275A - Piston and method for manufacturing same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a piston excellent in thermal conductivity and capable of inhibiting generation of a heat spot. <P>SOLUTION: High thermal conductivity layer composed of material having higher thermal conductivity than material forming a piston body part is coated on a top surface part of the piston body part. The high thermal conductivity layer is joined without melting the top surface part of the piston. Since the high thermal conductivity layer is coated without melting the piston body part, formation of a layer causing drop of thermal resistance and strength due to oxide film, gap, alloy or the like between the top surface part of the piston and the high thermal conductivity layer can be reduced and the piton exhibiting high performance and the method for manufacturing the same can be provided. Especially, overheat of the piston body can be reduced, formation of oxide film and alloy can be reduced, and formation of the large gap can be inhibited in principal by adopting spraying, cold spraying an/or plating as a method for forming the high thermal conductivity layer. Even if a gap is generated in the worst case, the gap can be made small by plating or rolling. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a piston for an internal combustion engine and a method for manufacturing the same.

  The heat load on the piston is increasing with higher engine output and higher performance. If there is a heat spot (heat concentration) at the top surface of the piston, unexpected local combustion is caused from that site, which causes knocking.

As a conventional technique for suppressing the generation of a heat spot on the top surface of a piston, an internal combustion engine piston based on an Al-Si alloy is made of an aluminum alloy having a higher thermal conductivity than the piston base in the piston head portion. A technique for forming a portion (referred to as a “high thermal conductive layer” in this specification) by overlay welding or casting is disclosed (Patent Documents 1 to 3).
JP-A-9-228889 JP-A-8-232758 JP-A-8-284744

  However, in the method of forming the high heat conductive layer by casting, the high heat conductive layer cannot be formed on the entire top surface portion of the piston, and the effect of suppressing the heat spot is small. In addition, when a high thermal conductive layer is formed by cast or overlay welding, sufficient bonding cannot be realized between the piston main body (referred to as “piston main body portion” in this specification), and the thermal resistance increases. Inconveniences such as a decrease in bonding strength and the like may occur. Specifically, in cast or overlay welding, there is a gap between the high thermal conductive layer and the piston main body, or the piston main body is melted by heat between the oxide film and the material constituting the high thermal conductive layer. This is because an alloy (such as an aluminum-copper intermetallic compound) is produced.

  This invention is made | formed in view of the said situation, and makes it the subject which should be solved to provide the piston which is excellent in thermal conductivity, and has a high effect which suppresses generation | occurrence | production of a heat spot, and its manufacturing method.

The piston of the present invention that solves the above problems is a piston main body,
A material having a higher thermal conductivity than the material forming the piston body, the top surface of the piston body being coated with a high thermal conductivity layer coated without melting the contact portion, To do.

And the other piston which solves the above-mentioned subject is a piston body part,
A material having a higher thermal conductivity than a material forming the piston main body, and having a high heat conductive layer friction-welded to the top surface of the piston main body without melting the contact portion. And

Moreover, the manufacturing method of the piston of this invention which solves the said subject is a method of manufacturing the above-mentioned piston of this invention,
The top surface portion of the piston main body has a high heat conductive layer forming step of forming the high heat conductive layer by a spraying step, a cold spray step and / or a plating step without melting the contact portion.

  Since the high thermal conductivity layer is coated without melting the piston body, the formation of a layer that causes thermal resistance and strength reduction consisting of oxide film, gaps, alloys, etc. between the top surface of the piston and the high thermal conductivity layer is formed. It is possible to provide a piston that can be reduced and that exhibits high performance, and a method for manufacturing the same. In particular, as a method for forming the high thermal conductive layer, by employing thermal spraying, cold spray and / or plating, it is possible to reduce the overheating of the piston main body, and to reduce the formation of an oxide film and an alloy. Formation of a gap can be suppressed.

  Here, “the part in contact does not melt” means that when a cross section of the piston is observed, a layer made of an oxide film or an alloy is not observed between the piston body and the high thermal conductive layer.

  Since the piston of the present invention has the above-described configuration, generation of a gap, a coating layer, and an alloy layer that can become thermal resistance can be suppressed, and when applied to an internal combustion engine, the generated heat is efficiently transmitted in the piston. -It becomes possible to disperse, and generation of local heat spots can be suppressed. Further, for the same reason, the bonding between the piston main body constituting the piston and the high thermal conductive layer becomes strong. As a result, since the degree of freedom of the shape of the high thermal conductive layer is increased, the top surface portion of the piston main body can be effectively covered, and high thermal conductivity can be realized.

  Since the manufacturing method of the piston of the present invention has the above configuration, the high-performance piston of the present invention can be manufactured as described above.

  The piston of the present invention will be described in detail below. The piston of this embodiment has a piston main body part and a high heat conductive layer.

  The piston main body has the same configuration as that of a normal piston except that a high heat conductive layer described later can be formed on the top surface portion. For example, the thing made from cast iron, the thing made from an aluminum alloy, the thing made from a titanium alloy etc. are mentioned. In particular, lightweight aluminum alloys having excellent thermal conductivity are widely used.

  The high heat conductive layer is a member formed so as to cover the top surface portion of the piston main body. In principle, it is not necessary to cover all the top surface portions of the piston main body, but the effect of suppressing the generation of heat spots is enhanced by covering all of them.

  The high thermal conductive layer is made of a material having a higher thermal conductivity than the piston body. For example, materials having high thermal conductivity such as aluminum-based materials, copper-based materials, graphite, ceramics (such as aluminum nitride) can be used. In particular, it is desirable to be made of an aluminum-based material or a copper-based material. Furthermore, it is desirable to comprise from pure aluminum and pure copper with high purity. Aluminum and copper are metals with high thermal conductivity, and the higher the purity, the higher the thermal conductivity. Therefore, the purity of aluminum (or copper) should be increased as long as durability (durability against combustion because it is a part that directly contacts combustion in an internal combustion engine, durability against engine oil, etc.) allows. Is desirable.

  Since the top surface portion of the piston forms the bottom surface of the combustion chamber in the internal combustion engine, various shapes can be adopted for realizing efficient combustion. In order to form the shape of the required top surface in the piston of the present invention, (1) a high heat conduction layer is formed after the piston main body is generally made into a final piston shape (the top surface of the piston main body is formed as a high heat conduction layer). (2) After forming a high heat conductive layer on the piston main body, a necessary shape is realized by mechanical processing (cutting, plastic processing, etc.). In the method (1), after the high thermal conductive layer is formed on the top surface portion of the piston main body, mechanical processing can be further performed. For example, it is possible to apply a compressive force by a roller or a mold, or to perform a cutting process. In particular, the application of compressive force is preferable from the viewpoint of improving the thermal conductivity because it is excellent in the effect of closely attaching the high thermal conductive layer to the piston body.

  The high heat conductive layer is formed so as not to melt the contact portion of the top surface portion of the piston main body. Since the top surface of the piston body is not melted, it is difficult to form an oxide film or an alloy. Moreover, since the top surface portion is not overheated, generation of distortion due to heat can be suppressed.

  Specific examples of desirable methods include (i) thermal spraying, (ii) cold spray, (iii) plating, and (iv) friction welding of a member molded in advance in the shape of a high thermal conductive layer. The thickness of the high thermal conductive layer is not particularly limited, but the larger the thickness, the better the effect of dispersing heat and suppressing the generation of heat spots. Therefore, an appropriate thickness is selected according to other properties (strength, rigidity, oxidation resistance, etc.) required for the piston.

  (i) Thermal spraying is performed by spraying the raw material powder onto the top surface of the piston body in a state of being melted by heating (or semi-melting). Thermal spraying has a high film forming speed, which is advantageous for increasing the film thickness. By increasing the film thickness, the thermal conductivity is improved and the generation of heat spots is suppressed. In addition, since thermal spraying is not mechanical metallurgical contact but mechanical contact, it is possible to reduce restrictions on the materials constituting the piston main body and the high thermal conductive layer. Examples of the method for melting the powder include those using plasma (arc discharge, high frequency discharge, etc.), those using plasma under reduced pressure, and those using flame. The thermal spraying is preferably performed under reduced pressure or in an inert atmosphere (argon, helium, nitrogen gas, etc.) from the viewpoint of preventing oxidation of the material forming the high thermal conductive layer. This is because the melted powder is hardly oxidized.

  (ii) Cold spray is a method in which the powder collides with the piston body without melting. The powder forms a coating on the piston body by the energy generated at the time of collision. In particular, it is desirable to make the piston body collide with the inert gas in a solid state in supersonic flow. By making the speed of the powder higher than the critical speed, the particles themselves undergo plastic deformation to form a film. Since the powder is not melted, the progress of oxidation and the like is suppressed. The critical speed is a speed that varies depending on the type of powder, particle size, temperature, and the like.

  (iii) Plating can be adopted regardless of electroplating or electroless plating. Electroplating is particularly desirable.

  (iv) Friction welding is a method of joining the two by applying rotation or vibration while pressing the high thermal conductive layer toward the piston body. When friction welding is employed, even when an aluminum alloy piston body is employed, the oxide film on the surface can be destroyed during the pressure welding, and a strong joint can be realized. The vibration can be performed by ultrasonic irradiation or the like. Even in the case of friction welding, the top surface portion of the piston body is not melted by controlling the rotation and vibration conditions. Although the friction welding is thick, even a large member can be joined at a high speed, so that it can be processed into a desired shape after joining the thick member.

  Here, the high thermal conductive layer may be formed by stacking two or more kinds of materials. Moreover, even if it is the same material, it can also divide | segment into 2 layers (or 2 times) or more by a different method, and can also form a high heat conductive layer. For example, after the first layer is formed of a metal having high thermal conductivity such as pure aluminum or pure copper, the second layer, which is a thin film made of a metal having excellent oxidation resistance, is formed, thereby providing thermal conductivity and oxidation resistance. A highly heat conductive layer having both properties can be formed. When two or more high thermal conductivity layers are formed, the coefficient of thermal expansion of the material forming each layer is gradually decreased or gradually increased to reduce the difference in the degree of thermal expansion between the contacting layers. And the occurrence of damage due to thermal fatigue can be suppressed. For example, when forming a high thermal conductive layer with two layers, the thermal expansion coefficient of the material constituting the first layer is set between the thermal expansion coefficient of the piston main body and the thermal expansion coefficient of the material constituting the second layer. It is desirable.

  As a method of forming a high thermal conductive layer laminated in two or more layers, there is a method of performing a cold spray and then combining spraying. Cold spray causes the powder to collide with the top surface of the piston main body at high speed, so that the top surface of the piston main body is uneven (surface roughened) to form a mesh between the high thermal conductive layer and the piston main body. The joint strength can be improved. In general, the surface roughening process is often performed by shot peening or the like. However, if the surface roughening process is also performed by cold spraying, heat such as a shot remaining on the top surface of the piston body part. The cause of the decrease in conductivity can be reduced.

  After forming the first layer by cold spraying and / or spraying, the second layer can also be formed by plating. Cold spraying or thermal spraying is a method in which a powdery raw material collides with the top surface of the piston body to form a film, so in principle a minute gap is formed, but if plating is performed after that, the gap can be closed, so heat is generated. Conductivity can be improved. In general plating, it is difficult to form a strong film on an aluminum alloy, but a layer having sufficient strength by forming a first layer made of other materials that can be plated by cold spraying or thermal spraying. (Second layer) can be formed.

  And after forming a high heat conductive layer, or in the middle of forming, it is desirable to provide the process of carrying out plastic deformation so that a high heat conductive layer may be compressed in the direction stuck to the top face part of a piston main-body part. As described above, a layer formed by thermal spraying or cold spray often has voids inside in principle. The presence of voids inside the high thermal conductive layer causes a decrease in thermal conductivity. Therefore, the thermal conductivity is improved by crushing the internal voids by plastic deformation. Specific methods for plastic deformation include a method of compressing the surface with a roller and a method of pressing against a mold having a target shape (for example, the final shape of the high heat conductive layer). From the viewpoint of ease of plastic working, it is desirable to employ a soft metal (such as pure aluminum) as the high heat conductive layer.

  The piston of the present invention will be described below in detail based on examples. As a comparative example, an aluminum alloy piston was adopted. The piston has a top surface diameter of 95 mm and a height of 70 mm. The piston of the comparative example had a thermal conductivity of 140 W / m · K. The thermal conductivity was measured by a commonly used laser flash method. About each Example coated by the following various methods, it cut out and measured a part of coated high heat conductive layer.

  The comparative example piston was thermally sprayed in the atmosphere to form a pure copper high thermal conductive layer (Example 1), and was sprayed with reduced pressure plasma to form a pure copper high thermal conductive layer (Example 2). A high thermal conductive layer of pure copper formed by thermal spraying in a nitrogen atmosphere (Example 3), a high thermal conductive layer of pure copper formed by cold spray (Example 4), and a high thermal conductive layer of pure copper formed by plating (Example 5) was produced. In addition, a piston (Example 6) in which a pure copper film was formed on the piston of Example 1 by plating (Example 6) and a roller processed (Example 7) were also produced.

  Further, the piston of the comparative example was sprayed in the atmosphere to form a pure aluminum high thermal conductive layer (Example 8), and the piston of Example 8 was subjected to roller processing (Example 9). ) Was manufactured. Further, a piston (Example 10) in which a 2 mm thick disc was friction welded to the piston of the comparative example was manufactured.

  The maximum temperature of the piston top surface portion was measured when the pistons of the examples and comparative examples were mounted on an actual engine and operated. The temperature was measured by forming several holes from the bottom of the piston and placing a temperature probe inside these holes. As the maximum temperature, the maximum value of the temperature actually measured under certain operating conditions was adopted. The results are shown in Table 1.

As is clear from Table 1, the thermal conductivity was improved as compared with the piston main body by forming the high thermal conductive layer. As the thermal conductivity increased, the top surface maximum temperature could be lowered. Since the top surface portion maximum temperature can be lowered, occurrence of abnormal combustion such as occurrence of knocking can be suppressed. The relationship between the top surface maximum temperature and the thermal conductivity is shown in FIG. As is clear from FIG. 1, in general, a close correlation was observed between the top surface maximum temperature and the thermal conductivity. When Examples 1 to 3 were compared, a high thermal conductive layer of pure copper was similarly formed by thermal spraying. Even in the case where the thickness of the high thermal conductive layer is reduced to 1/3, the thermal conductivity is increased when the reduced pressure plasma or nitrogen atmosphere is used. Further, even if the same pure copper is used, the thermal conductivity of the cold spray (Example 4) can be larger than that of spraying in the atmosphere, and the plating (Example 5) can achieve a larger thermal conductivity even if the thickness is smaller. The rate was realized.

  Moreover, when Example 1 and 6 were compared, after performing thermal spraying in air | atmosphere, the thermal conductivity became large by performing pure copper plating further. It is thought that this is because the plating with pure copper closes the gap formed at the time of thermal spraying and becomes closer, so that heat can be easily conducted.

  Further, when Examples 1 and 7 were compared, the thermal conductivity increased by performing further roller processing after spraying in the atmosphere. This is considered to be due to the fact that the gap formed by thermal spraying is compressed and heat is easily conducted by performing the roller processing. Similarly, when Examples 8 and 9 were compared, even when pure aluminum was employed as the high thermal conductive layer, the thermal conductivity increased by performing roller processing after spraying in the air.

  In Example 10 in which a pure copper disk was friction welded, the thickness of the high thermal conductive layer could be increased (2 mm), so the thermal conductivity increased and the top surface maximum temperature could also be decreased.

It is a graph which shows the relationship between the maximum temperature of a piston top surface part, and thermal conductivity.

Claims (12)

A piston body,
A material having a higher thermal conductivity than the material forming the piston body, the top surface of the piston body being coated with a high thermal conductivity layer coated without melting the contact portion, Piston to do.   The piston according to claim 1, wherein the high thermal conductive layer is made of an aluminum-based material or a copper-based material.   The piston according to claim 1 or 2, wherein the high thermal conductive layer is formed by a thermal spraying process.   The piston according to any one of claims 1 to 3, wherein the high thermal conductive layer is formed by a cold spray process.   The piston according to claim 4, wherein the high thermal conductive layer is laminated in the order of a first layer formed in the cold spray process and a second layer formed in the spraying process.   The piston according to any one of claims 1 to 5, wherein the step of forming the high thermal conductive layer is performed in an inert atmosphere.   The piston according to any one of claims 1 to 6, wherein the high thermal conductive layer is formed by a plating process.   The piston according to claim 7, wherein the high thermal conductive layer is laminated in the order of a first layer formed in the thermal spraying process and / or the cold spray process and a second layer formed in the plating process.   The piston according to claim 5 or 8, wherein a thermal expansion coefficient of the first layer is between a thermal expansion coefficient of the second layer and a thermal expansion coefficient of the piston main body. A piston body,
A material having a higher thermal conductivity than a material forming the piston main body, and having a high heat conductive layer friction-welded to the top surface of the piston main body without melting the contact portion. Piston. A method for manufacturing the piston according to any one of claims 1 to 9,
A piston having a high thermal conductive layer forming step of forming the high thermal conductive layer by a thermal spraying process, a cold spray process and / or a plating process on the top surface of the piston main body without melting a contact portion. Production method.   The method for manufacturing a piston according to claim 11, further comprising a step of plastically deforming the high thermal conductive layer to reduce voids in the high thermal conductive layer during and / or after the high thermal conductive layer forming step.

Images