JPH0427745A - Piston for engine - Google Patents

Abstract

PURPOSE:To reduce the shrinkage stresses to be produced in a piston body by receiving the shrinkage stresses produced by the cooling of the piston body at the time of casting by a metal ring which has an intermediate thermal expansion coefficient between that of the piston body and that of a ceramics-formed body and also which is provided with through holes. CONSTITUTION:A piston crown part 5 including a combustion chamber wall 1 is integrally formed with a ceramics-formed body 7 so as to be positioned at the top part of an aluminum-alloy-made piston body 11. At least in the vicinity of the outermost diametral part of the backside of the ceramics-formed body 7, the joining part 21 to be joined with the piston body 11 is formed by an activated metal layer. To the combustion-chamber side of this backside, the inside-diametral end part of a metal ring 9 having an intermediate thermal expansion coefficient between that of the ceramics-formed body 7 and that of the piston body 11 is joined. A prescribed clearance is provided between the outside end part of the metal ring 9 and the ceramics-formed body 7, and through holes 17 are provided to the metal ring 9, and then these ceramics- formed body 7 and the metal ring 9 are integrally cast into the piston body 11.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to an engine piston in which a ceramic molded body is provided in the piston crown portion.

(Prior Art) In order to improve the heat resistance of a piston made of a light alloy such as an aluminum alloy, the piston crown shown in FIG. 5 is known, for example, as shown in FIG. (See Publication No. 60-240857). This is a heat-resistant metal surface body 53 that covers the top surface of the piston 51.
A ceramic body 59 having a combustion chamber concave portion 57 is press-fitted into the concave support portion 55 and cast into an aluminum alloy through a buffer layer 61 such as a wire mesh to form the piston 51. .

(Problem to be solved by the invention) However, this conventional technique has the following problems.

(1) The press-fitted portion between the concave support portion 55 and the ceramic body 59 requires precision machining in order to improve the adhesion between the two, and in particular, precision machining of the ceramic body 59 results in a significant increase in man-hours and cost.

(2) Since a portion of the press-fitted portion is exposed to the combustion chamber, carbon due to combustion may enter the press-fitted portion, and as a result, the ceramic body 59 may be damaged.

(3) Part of the piston crown is a heat-resistant metal surface body 5
3, this metal reaches its heat resistance limit before the heat resistance limit of ceramics, and cannot exhibit the heat resistance performance unique to ceramics.

This invention was made by focusing on these problems of the prior art, and it is possible to firmly connect the ceramic molded body provided in the piston crank run part to the piston body at low cost in order to improve heat resistance. The purpose is to prevent the molded product from being damaged and to fully demonstrate its heat resistance properties.

[Structure of the Invention] (Means for Solving the Problems) In order to solve the above-mentioned problems, the present invention provides a piston crown part located at the top of an aluminum alloy piston body and including a combustion chamber wall, which is made of a ceramic molded body. The ceramic molded body is integrally formed with the piston body at least near the outermost diameter part of the back surface of the ceramic molded body, and the joint part with the piston body is formed with an activated metal layer, while the ceramic molded body and the piston body are formed on the combustion chamber side of the same back surface. The inner diameter end of a metal ring having an intermediate value of thermal expansion coefficient is bonded to The ceramic molded body and the metal ring are integrally cast into the piston body.

(Function) The shrinkage stress generated by the cooling of the piston body during casting is shared by the metal ring, which has a thermal expansion coefficient intermediate between that of the piston body and the ceramic molded body, and is equipped with a through hole. Shrinkage stress generated in the piston body is alleviated. Moreover, since the metal ring is bonded to the ceramic molded body at the inner diameter end where stress during contraction is small, the effect is great. In addition, since the joint with the piston body is formed with an activated metal layer near the outermost diameter part of the ceramic molded body, carbon is prevented from entering through this joint, and damage to the ceramic molded body is prevented. . Since the entire piston crown portion is made of a ceramic molded body, the heat resistance characteristics achieved by using ceramic are fully exhibited.

(Example) Embodiments of the present invention will be described below with reference to the drawings.

In an engine piston according to an embodiment of the present invention shown in FIG. 1, a piston crown portion 5 having a combustion chamber wall 1 in the center and a flange portion 3 on the outer periphery is integrally molded with a ceramic molded body 7. has been done. Ceramic molded body 7
The metal ring 9 shown in FIGS. 2 and 3, which is joined to the back surface of the flange 3, has a thermal expansion coefficient intermediate between the respective coefficients of thermal expansion of the piston body 11 made of an aluminum alloy and the ceramic molded body 7. It is made of a metal material with a coefficient of expansion, such as a titanium alloy. The metal ring 9 has a convex portion 13 formed at its inner end, and a convex portion 15 protruding in the opposite direction from the convex portion 13 at its outer end.
3 and the convex portion 15 are provided with four elongated holes 17 extending in the circumferential direction as through holes.

The tip 13a of the convex portion 13 of the metal ring 9 is joined by brazing to a portion of the back surface of the flange portion 3 of the ceramic molded body 7 close to the combustion chamber wall 1, while the outermost diameter portion of the back surface of the flange portion 3 Nearby, a metallized layer 21 is formed as a joint by brazing an activated metal layer in order to increase the bonding force between the ceramic molded body 7 and the piston body 11. By joining the convex part 13 of the metal ring 9 to the ceramic molded body 7, a predetermined ring-shaped gap 22 is formed between the metal ring 9 and the ceramic molded body 7 on the outer end side of the convex part 13. will be done.

After immersing the thus bonded ceramic molded body 7 and metal ring 9 in molten aluminum alloy for a certain period of time, they are set at the bottom of the outer mold 23 as shown in FIG. The molten aluminum alloy 27 is poured into the gap formed by the piston body 11.
is integrally cast together with the ceramic molded body 7 and the metal ring 9. At this time, the molten aluminum alloy enters the gap 22 between the flange 3 of the ceramic molded body 7 and the metal ring 9 and the elongated hole 17 of the metal ring 9, and connects the ceramic molded body 7, the metal ring 9, and the piston body 11. will be combined.

In the piston cast in this manner, the entire piston crown portion 5 including the combustion chamber wall 1 is integrally formed of ceramic. This effect is also exerted on the outer circumferential portion of the portion 5, and is particularly effective for diesel engines that are subject to high combustion gas pressure and heat load. Since the metallized layer 21 is formed at the joint between the outermost diameter part of the ceramic molded body 7 and the piston body 11, the bonding force between the two is improved at this part, and carbon etc. enter from this joint. The fear is eliminated, and damage to the ceramic molded body 7 due to carbon intrusion is avoided.

With a metal ring 9 having an intermediate value of thermal expansion coefficients between the ceramic molded body 7 and the piston body 11, bonded to the ceramic molded body 7, these ceramic molded body 7 and the metal ring 9 are integrated. Since the piston body 11 is cast, the shrinkage stress acting on the piston body 11 during cooling of the casting is shared by the metal ring 9 and reduced. The long holes 17 effectively act to reduce this shrinkage stress. In particular, since the metal ring 9 is joined to the ceramic molded body 7 at its inner diameter end, which is close to the combustion chamber wall 1 and suffers less thermal stress during casting cooling, the shrinkage stress acting on the piston body 11 becomes extremely small. , damage to the ceramic molded body 7 is prevented.

A material with a small thermal expansion coefficient such as a titanium alloy constituting the metal ring 9 has a large Young's modulus, so if such a member is simply joined to the back surface of the flange 3 of the ceramic molded body 7, the ceramic molded body 7 Although damage cannot be avoided, by joining at the inner end of the metal ring 9 as described above,
The shrinkage stress generated in the piston body 11 can be significantly alleviated. At this time, since the aluminum alloy having a small Young's modulus that has entered the gap 22 is joined to the outer diameter side of the ceramic molded body 7, damage to the ceramic molded body 7 at this point is also prevented.

Furthermore, by providing the elongated hole 17 in the metal ring 9, the molten aluminum alloy can easily flow into the gap 22.
By casting an aluminum alloy into this elongated hole 17, the metal ring 9 and piston body 11 are cooled after casting.
This prevents peeling.

The metal ring 9 can be formed into a predetermined shape by hot pressing, for example, and does not require precision machining. In particular, since precision machining of ceramic material, which requires a large amount of man-hours and cost, is not necessary, the production cost is significantly reduced. .

[Effects of the Invention] As described above, according to the present invention, the shrinkage stress generated when the piston body is cooled during casting is absorbed by the through-hole, which has an intermediate coefficient of thermal expansion between the piston body and the ceramic molded body. The shrinkage stress generated in the piston body can be alleviated because the metal ring is shared with the shrinkage stress generated in the piston body.Moreover, the metal ring is bonded to the ceramic molded body at the inner diameter side end where the thermal stress is small during casting cooling. Therefore, the effect of alleviating shrinkage stress is extremely high, and it is possible to prevent damage to the ceramic molded body. In addition, since the joint with the piston body is formed with an activated metal layer near the outermost diameter of the ceramic molded body, force from this joint is prevented from entering, thereby preventing damage to the ceramic molded body. can do.

Since the ceramic molded body can be joined to the piston body by casting without causing damage due to shrinkage stress, etc., there is no need for precision machining of the ceramic member, which requires particularly man-hours and costs, and manufacturing costs can be significantly reduced. Furthermore, since the entire piston crown portion is made of a ceramic molded body, the heat resistance characteristics achieved by using ceramic are fully exhibited.

[Brief explanation of drawings]

Fig. 1 is a vertical cross-sectional view of a piston showing an embodiment of the present invention, Fig. 2 is a plan view of a metal ring used in the embodiment of Fig. 1, and Fig. 3 is a sectional view taken along the line ■-■ of Fig. 2. , FIG. 4 is an explanatory view of the casting of the piston shown in FIG. 1, and FIG. 5 is a longitudinal sectional view of a conventional piston. 1... Combustion chamber wall 5... Piston crown portion 7... Ceramic molded body 9... Metal ring 11... Piston body 17... - Long hole (through hole) 21... Joint portion 22. ··gap

Claims (1)

[Claims] The piston crown part, which is located at the top of the aluminum alloy piston body and includes the combustion chamber wall, is integrally formed with a ceramic molded body, and the joint part with the piston body is formed at least near the outermost diameter part of the back surface of this ceramic molded body. While forming the activated metal layer, the inner diameter side end of a metal ring having a coefficient of thermal expansion intermediate between that of the ceramic molded body and the piston body is joined to the combustion chamber side of the same back side, and the outer side of the metal ring is formed with an activated metal layer. An engine piston characterized in that the end side has a predetermined gap between the metal ring and the ceramic molded body, a through hole is provided in the metal ring, and the ceramic molded body and the metal ring are integrally cast into the piston body. .