JPS6213236A - Metallic mold for casting and its production - Google Patents

Abstract

PURPOSE:To prevent the exfoliation and crack by the effect of thermal impact by forming a composite layer on the undercoat formed on the molding surface of a metallic mold and gradually increasing the ratio of ceramics in the composite layer toward the surface. CONSTITUTION:Powder of an Ni-Cr alloy is thermally sprayed by using a plasma spraying device on the molding surface 2 of the metallic mold 1 for casting to form the undercoat 3. A mixture composed of the Ni-Cr alloy powder and ZrO2-Y2O3 is then thermally sprayed by using the same thermal spraying device onto the undercoat to form the composite layer 4 on the undercoat 3. The alloy powder and ceramics are so mixed that the ratio of the ceramics gradually increases. More specifically, the weight ratio of the ceramics is made 20wt% at the beginning of the thermal spraying and thereafter the weight ratio of the ceramics is gradually increased until finally said ratio is made 90wt%.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a light alloy casting mold having a ceramic coating formed on its surface for the purpose of preventing adhesion of molten metal and keeping the molten metal warm, and a method for manufacturing the same.

(Conventional technology) By applying a ceramic coating to the molding surface of a casting mold, it is possible to prevent molten metal from adhering to it, keep the molten metal warm, improve the flow of the molten metal, suppress the occurrence of cavities, and eliminate pressure leaks on the processed surface. Conventionally, it has been carried out to obtain castings with excellent strength.

The conventional method for coating the molding surface of the mold with ceramic is brush coating or spray coating with graphite, etc.

Furthermore, coating by thermal spraying is known.

(Problems to be Solved by the Invention) Among the various coating methods mentioned above, brush coating has poor pressure resistance and is unsuitable for high-pressure casting molds, while spray coating has a poor coating film. Because it is thin and has poor peel strength, it must be coated and cast each time, and furthermore, thermal spray coating requires
Ceramics are particularly prone to cracking and peeling due to thermal shock, peeling due to casting pressure, and even peeling due to insertion of molten metal. Ceramic has excellent pressure resistance, but its tensile strength is low, and this drawback is noticeable in thermally sprayed ceramic coatings. appear.

(Means for Solving the Problems) In order to solve the above problems, the present invention first forms an undercoat by plasma spraying a metal powder such as old -Cr on the molding surface of the mold. A composite layer of the above metal and a ceramic such as zirconia-yttria was formed on top, and the proportion of the ceramic in the composite layer gradually increased as it approached the surface.

(Example) Embodiments of the present invention will be described below based on the sardine drawings.

The drawing is an enlarged cross-sectional view of a coating layer formed by the method of the present invention.1 In the present invention, a plasma spraying method equipped with a forced oxidation nozzle is applied to the molding surface (2) of a casting mold (1) made of a light alloy. Using a device, nickel (Ni)-ri0
An undercoat (3) having a thickness of 20 to 150 JL (1+) is formed by thermal spraying a powder of Cr alloy (Cr: 1G to 40wtX) (7).

Next, using the same plasma spraying device, a mixture of the nickel-chromium alloy powder and zirconia (ZrO2)-yttria (Y 20'3) (weight ratio 8 to 122) was sprayed onto the undercoat (3). 100-300J
A composite layer (4) of L is formed. Here, the mixing ratio of the 1 alloy powder and the ceramic is set such that the ratio of the ceramic gradually increases. Specifically, at the beginning of thermal spraying, the weight ratio of the ceramic is set to 20 tX, and the weight ratio of the ceramic is increased each time the spraying is performed, until it finally reaches 90 tX. Furthermore, when spraying a mixture of alloy powder and ceramic, the metal powder is forcibly oxidized during melting.

Here, the thermal spraying conditions for forming the undercoat (3) and composite layer (4) are as shown in Table 1].

[Table 1] Here, the undercoat (3) was formed using the mold (
1) base material (e.g. S K D 81) and composite layer (4)
This is to absorb the difference in thermal expansion coefficient between the
The reason for setting 0wtX is that if it is less than 10wt1, it will be difficult to crush the powder and it will be difficult to form an oxide film due to forced oxidation. Oxidation (
This is because chromium oxide) accelerates and becomes brittle.

In addition, the reason why the ratio of yttria in zirconia-yttria was set to B~12vH is that if it is less than 8 wt$, the durability against thermal shock will not be sufficiently exhibited, and if it exceeds 12wH, the hardness of the composite layer will decrease. This is due to insufficient pressure resistance.

In addition, zirconia for nickel-chromium alloy powder
The mixing ratio of yttria was set to 20 to 90wtX because
If it is less than 2Qwt$, the characteristics as a ceramic will not be fully exhibited, and if it exceeds 90wt$, more pores will be generated during thermal spraying, resulting in a decrease in tensile strength.

Furthermore, forced oxidation of the nickel-chromium alloy when thermally spraying the mixture of nickel-chromium alloy powder and zirconia-yttria impairs its wettability with the molten metal, improving the effect of preventing molten metal from adhering to the ceramic. By forming a reinforced composite between the two.

Reference photo [1] is an enlarged (400x) photo of the coating layer corresponding to the attached drawing, and reference photo [2] is a photo that is further enlarged (1000x) of the main part of reference photo [1]. As you can see from the photo, an undercoat made of nickel-chromium alloy is formed on the surface of the material.
It can be seen that a composite layer is formed on this undercoat, and in this composite layer, the closer the surface is, the higher the proportion of ceramic is, and that the metal and ceramic form a composite layer due to forced oxidation.

In the above, nickel-chromium alloy is used as the metal forming the undercoat, and zirconia-yttria is used as the ceramic forming the composite layer. For the aluminum alloy, alumina-silicon nitride or zirconia-magnesia may be used instead of zirconia-yttria.

(Effects of the Invention) [Table 2] below shows casting conditions for comparison between the present invention and the conventional method.

[Table 2] The ceramic coating peeled off under the conditions shown in [Table 2], but no peeling of the ceramic coating was observed in the mold according to the present invention even after 1000 shots.

[Table 3] compares the thermal conductivity of the ceramic coating on the mold surface according to the present invention and the conventional ceramic coating. [Table 3] shows the thermal conductivity (insulation properties) equivalent to that of the conventional ceramic coating.

Furthermore, reference photo [3] is 2000 series 1 of the mold according to the present invention.
Reference photograph [4] shows the molding surface of the mold that was not subjected to forced oxidation.1 Although no molten metal is observed on the molding surface of the mold according to the present invention, Adhesion of molten metal and melting damage can be seen on the molding surface of the mold that was not subjected to forced oxidation.

As described above, according to the present invention, the tensile strength of the ceramic coating formed on the molding surface of the mold can be increased, thermal expansion strain can be absorbed, peeling and cracking due to thermal shock can be reliably prevented, and the ceramic coating can have a long lifespan. Since the ceramic properties are not impaired, the flowability of the molten metal is improved, shrinkage cavities are reduced, and castings with good quality can be obtained, among other effects.

[Brief explanation of the drawing]

The drawing is an enlarged cross-sectional view of a ceramic coating formed in accordance with the present invention. In one drawing, (1) is a mold, (2) is a molding surface, (3) is an undercoat, and (4) is a composite layer.

Claims (5)

[Claims] (1) In a casting mold whose molding surface is coated, the coating consists of an undercoat made of metal and a composite layer of metal and ceramic formed on the undercoat. A casting mold characterized in that the amount of ceramic gradually increases as it approaches the surface. (2) The metal forming the undercoat is nickel (
The metal forming the composite layer is nickel (Ni)-chromium (Cr), and the ceramic forming the composite layer is zirconia (ZrO_2).
)-yttria (Y_2O_3), and the amount of the ceramic mixed is in the range of 20 to 90 wt%. (3) The metal forming the undercoat is nickel (
The metal forming the composite layer is nickel (Ni)-chromium (Cr)-aluminum (Al), and the ceramic forming the composite layer is alumina- silicon nitride,
The casting mold according to claim 1, further characterized in that the amount of ceramic mixed is in the range of 20 to 90 wt%. (4) forming an undercoat by plasma spraying metal powder on the mold surface, plasma spraying a mixed powder of the metal powder and ceramic on the undercoat,
When plasma spraying this mixed powder of metal powder and ceramic, the mixing ratio of ceramic is gradually and continuously increased, and the metal powder is forcibly oxidized during melting, so that the composite layer of metal and ceramic is undercoated. A method for manufacturing a casting mold, characterized in that the mold is formed on a coat. (5) The metal is nickel (Ni)-chromium (Cr) or nickel (Ni)-chromium (Cr)-aluminum (
Al), the ceramic is either zirconia (ZrO_3)-yttria (Y_2O_3) or alumina silicon nitride, and the mixing ratio of the ceramic is in the range of 20 to 90 wt%. A method for manufacturing a casting mold according to claim 4.