Method for producing an article having a cavity

Abstract

A method for producing an article (1) having a cavity (4), in which method a mould (2) provided with a core (3) is filled with article material, the article material is hardened around the core (3) to form the article (1), and the core (3) is removed from the solidified article (1). The core (3) is made of yttria tetragonal zirconia polycristal material (Y-TZP) or partially stabilized zirconium (PSZ), and after the hardening phase the core (3) is exposed to a steam atmosphere, after which the core (3) is removed from the article (1).

Claims

1. A method for producing an article having a cavity, the method comprising: providing a mold with a core made of yttria tetragonal zirconia polycristal material (Y-TZP), wherein the core is sintered; filling the mold with article material; hardening the article material around the core to form the article; after the hardening, exposing the core to a steam atmosphere; and after said exposing, removing the core from the article.

2. The method according to claim 1, wherein the core is exposed to a steam atmosphere at a temperature of at least 100 C.

3. The method according to claim 2, wherein the core is exposed to a steam atmosphere at a temperature of at least 200 C.

4. The method according to claim 1, wherein the core is exposed to steam atmosphere for at least 1 hour.

5. The method according to claim 4, wherein the core is exposed to steam atmosphere for at least 3 hours.

6. The method according to claim 1, wherein the mold is filled with molten metal material.

7. The method according to claim 1, wherein the mold is filled with metal powder, which is hardened about the core by isostatic pressing process.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following, the invention is be described by way of an example with reference to the accompanying FIGURE, which is a cross-sectional view of a hollow valve seat insert in a mould.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(2) The method according to the present invention is directed to method for producing an article 1 having a cavity 4. This kind of the article may be hollow or comprise a complex cavity configuration. The article 1 is produced in a mould 2 in which a core 3 is placed to form the cavity in the article 1. The mould 1 is filled with material of the article and the material is hardened at least partly around the core 3. Thereafter, the core is removed from the solidified article 1.

(3) The article can be, for example, a cooled valve seat insert 1 which is attached to a cylinder head of an internal combustion engine. The valve seat insert 1 is annular and provided with a cavity i.e. an internal cooling channel 4 for coolant.

(4) Additionally, the valve seat insert 1 comprises an inlet port for feeding cooling liquid into the cooling channel 4 and an outlet port for discharging cooling liquid from the cooling channel 4. The article 1 can be produced by casting or isostatic pressing. Suitable casting methods are precision casting, die casting, dead mould casting and sand casting. Correspondingly, suitable isostatic pressing methods are hot isostatic pressing and cold isostatic pressing.

(5) The hollow shape or other complex shape of the article 1 is accomplished by a suitably shaped core 3. The core 3 is shaped to form an internal surface of the article 1. In the embodiment shown in the drawing the core 1 has a shape which corresponds to the shape of the cooling channel 4 in the finished valve seat ring 1.

(6) The core 3 is composed of yttria-tetragonal zirconia polycrystal material(s) (ZrO.sub.2Y.sub.2O.sub.3), commonly known as Y-TZP. Y-TZP materials are used in structural applications because of unusual combination of high strength and toughness. Typically Y-TZP is used in dental applications. Another suitable core material is partially stabilized zirconia (ZrO.sub.2MgO), also known as PSZ. Both Y-TZP and PSZ are steam active materials i.e. their microstructures degrade in steam atmosphere. The core 3 can be manufactured by conventional methods suitable for ceramic materials. For example, the core 3 can be shaped as desired and thereafter sintered at an elevated temperature.

(7) The article 1 can be made by precision casting. Precision casting methods are commonly used to produce articles having complex hollow interiors or other complex shapes. In precision casting process a pattern is formed about the core 3. The pattern is composed of wax, polystyrene or other pattern making material having a suitably low melting point. The core 3 is supported by pins made of suitable material. The pins are pushed through the pattern material until they engage the surface of the core 3. Thereafter, a ceramic mould 2 is built around the pattern assembly, i.e. pattern material, the core 3 and the pins. The mould 2 can be formed in a number of ways. Typically, the pattern assembly is repeatedly dipped in ceramic slurry to form the mould 2. After the mould 2 is built up on the pattern assembly, the pattern material is removed from the mould 2. Thereafter, the mould 2 is fired to increase its hardness. The space between the mould 2 and the core 3 is filled with a molten article material, for example metal material such as steel or iron. Molten material is poured into the mould 2 through an inlet channel 5. After hardening of the article material, the mould 2 is removed around the solidified article 1.

(8) Thereafter, the core 3 is removed from the article 1 by exposing the core 3 to steam (water vapour) atmosphere i.e. steam treating the core 3. The core 3 is exposed to steam atmosphere at a temperature of at least 100 C., preferably at least 200 C. Typically, the temperature of the steam atmosphere is between 100 to 500 C., for example between 200 to 300 C. The core 3 is exposed to steam atmosphere for at least 1 hour, preferably for at least 3 hours. The duration of the steam treatment depends on multiple factors, for example the dimensions of the core 3 and the cast article 1 and the properties of the steam atmosphere. The steam treatment of the core 3 can be performed in an autoclave or a similar apparatus, which comprises a chamber for the article to be treated. During the steam treatment process saturated or superheated steam is fed into the chamber. Steam flushes the air out of the chamber. During the treatment steam destroys the core 3 microstructure i.e. degrades core material. After the steam treatment, the article is removed from the chamber. Finally, degraded core 3 is removed from the article 1.

(9) The article 1 can be made by hot isostatic pressing. Hot isostatic pressing (HIP) process subjects the article 1 to both elevated temperature and isostatic pressure. In the HIP-process the core 3 is placed in the mould 2, and thereafter the mould 2 is filled with article material in powder form. Article material can be metal such as steel or iron. Then, the mould 2 is compressed under a uniform (isostatic) pressure at an elevated temperature. The pressure and temperature are such that the powder consolidates or hardens around the core 3. HIP-process as such is well known in the art and therefore is not described here in further detail than necessary to understand the invention. After the isostatic compression, the article 1 is removed from the mould 2. The core 3 is removed from the solidified article 1 by the same steam treating method as described above in connection with the precision casting process.