JP2015182085A - Metal mold for die cast - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a metal mold for a die cast for suppressing occurrence of a heat crack in a cavity to achieve a long life.SOLUTION: A spray coating film is formed on at least a protective portion of a metal mold by using carbide-based or oxide based spray coating material, thereafter nitriding treatment is subjected.

Description

  The present invention relates to a die casting mold used for injection molding of a molten metal such as an aluminum alloy or a zinc alloy into a mold at high pressure and high speed.

In the field of die casting technology in which a molten metal such as an aluminum alloy or zinc alloy is injection-molded into a mold at high pressure and high speed, a mold steel such as JIS-SKD61 is used as the mold.
In such a die casting mold, the process of rapidly cooling after the high temperature molten metal is injected into the cavity is repeated, so that the cavity surface is easily eroded by the molten metal and hit cracks are formed at the cavity corners. There were technical problems that were likely to occur.
Therefore, nitriding treatment has been conventionally performed for the purpose of curing the cavity surface.
As the nitriding treatment, a gas nitriding method using ammonia decomposition reaction or a low-temperature nitriding treatment method using thermal decomposition of CaN ₂ is employed.
However, molds that are heavily heated and cooled, such as die casting, have a problem that heat cracks are likely to occur in the cavity corners where stress tends to concentrate as the number of shots increases. It had been.

In Patent Document 1, in a mold for manufacturing a semiconductor device for sealing a resin by transfer molding, thermal decomposition of CaN ₂ is used on the surface of a mold part in order to prevent surface wear due to contact with the resin. A technique for performing any two or more of nitriding, tungsten carbide spraying, and DLC film processing is disclosed.
However, this technique is a transfer mold, and its purpose is to suppress wear due to resin contact.
Further, it is merely mentioned that two or more kinds of treatments may be combined, and there is no description about what kind of combination and what kind of effect there is.

Japanese Patent Laid-Open No. 11-58404

  An object of the present invention is to provide a die casting die that suppresses the occurrence of heat cracks in a cavity and extends the life.

The die casting mold according to the present invention is characterized in that a sprayed coating is formed using a carbide-based or oxide-based sprayed material at least on a protected portion of the mold, and is then subjected to nitriding treatment.
Here, the protection site refers to a site where stress is concentrated by repeated casting shots by die casting and heat cracks are likely to occur.
The purpose of the present invention is not only to prevent the erosion of the molten metal due to nitriding, but also to form a sprayed coating on a portion where stress tends to concentrate, and it is not always necessary to form a sprayed coating on the entire cavity surface.
A portion where stress due to a thermal cycle tends to concentrate in the cavity is a corner portion of the cavity.

The thermal spray material is required to have heat resistance enough to withstand the molten aluminum alloy, and a ceramic thermal spray material is preferable.
Typical examples of the ceramic-based thermal spraying material are oxide-based and carbide-based.
Examples of oxide systems include alumina, titania, chromia, yttria, zirconia, and composites thereof.
The carbide system includes chromium carbide (Cr ₃ C ₂ ) and tungsten carbide (WC), and these are often used as a cermet material combined with metal particles such as NiCr, Co, CoCr and the like.
As a thermal spraying method, there is no restriction on a method such as plasma spraying or flame spraying.
In the present invention, it is preferable to form a sprayed coating partially on the corner of the cavity.
Therefore, it is preferable to form a discharge coating film by arc discharge using a tungsten carbide material as an electrode material.

As described above, the nitriding treatment may be gas nitriding, or low-temperature nitriding at 400 to 500 ° C. using lime nitrogen powder containing CaN ₂ as a main component.

  In the die casting die according to the present invention, since the nitriding treatment is performed after the thermal spray coating is formed on the stress concentration portion, the occurrence of heat cracks at the stress concentration portion is suppressed, and the erosion due to the molten metal is suppressed. Therefore, the life of the mold is extended.

An enlarged photograph of the cavity corner of an aluminum die cast product is shown. (A) at the time of mold manufacture, (b) is a surface photograph and cross-sectional outline drawing of a cast product after repeated 10,000 castings using a nitriding only mold, (c) is a tungsten carbide-based mold The surface photograph and cross-sectional outline figure of the cast product after repeating 30,000 times of casting using the metal mold | die nitrided after discharge coating using the electrode material are shown. The relationship between the distance from the mold surface and the cross-sectional hardness is shown in the graph.

Aluminum die casting molds were manufactured, and the cavity surface was made only at low temperature nitriding treatment, and the aluminum carbide film was shot at low temperature nitriding treatment after coating the tungsten carbide coating on the cavity corner. A comparative survey will be described below.
As the mold, DH31 made by Daido Steel, which is an improved steel of JIS-SKD61, was used.
For the low temperature nitriding treatment, an EH process of Edison Hard Co., Ltd. was used.
The EH process is a nitriding process in which a compound layer on the surface is scarce and there is no so-called seagull mark.
The tungsten carbide coating was discharge-coated only at the corners of the cavity using a tungsten carbide electrode material.
Fig.1 (a) is an enlarged photograph of the cavity corner part of the casting at the time of metal mold | die manufacture.
Using this mold, casting was repeated using JIS-ADC12 aluminum alloy.
As shown in FIG. 1B, the heat crack occurred in the cast product using the die only for nitriding treatment as shown in FIG.
On the other hand, a cast product formed by forming a tungsten carbide discharge coating film and then using a nitriding die did not show heat cracks at 10,000 shots, and increased to 30,000 shots. As a result, heat cracks as shown in FIG.
From this, it became clear that it is effective to combine the tungsten carbide discharge coating and the nitriding treatment for extending the life of the die casting die.

FIG. 2 shows the results of investigating the relationship between the depth (distance) μm from the mold surface and the cross-sectional hardness.
From this graph, it is clear that the tungsten carbide (WC) coating is lower in hardness than the direct nitriding treatment on the mold, which reduces the heat shock at the cavity corner and extends the service life. Presumed to be connected.

Claims (3)

  A die casting die characterized in that a sprayed coating is formed using a carbide-based or oxide-based spraying material at least on a protected portion of the die, and then subjected to nitriding treatment.   2. The die casting mold according to claim 1, wherein the protection part is a corner part of a mold cavity.   3. The die casting die according to claim 1, wherein the thermal spray material is a tungsten carbide electrode material, and the thermal spray coating is a discharge coating film.