KR960005522B1 - Preparation of litao3 single crystal - Google Patents

Abstract

The method using a low melt temp. alloy as coolant has several steps in order : inserting the metal ingot to a vacuum casting melting pot ; reducing the press. to vacuum degree in the melt pot, heating the melt pot to dissolve the metal ingot ; heating up the temp. of mold above the melting temp of metal, injecting the melten metal ingot into heated mold ; putting the injected mold above metal bath; initiating the single crystal of molten motal in mold by gradual cooling effect of metal bath which comes from melting itself; finishing the growth of single crystal. This method can get high quality single crystal of casting by natural cooling without water cooling plate.

Description

Single Crystal Manufacturing Method

1 is a front view of a vacuum casting furnace.

2 (a) to 3 (b) are cross-sectional views showing the manufacturing method of the single crystal casting and the configuration of the starter used in the method.

3 (a) to 3 (d) are perspective views showing various shapes of the selector for single crystal growth.

4 (a) to 4 (c) are some cutaway sectional views showing a process in which the casting in the mold changes to a single crystal while the mold is immersed in a low melting point alloy.

* Explanation of symbols for main parts of the drawings

10 vacuum precision casting melting furnace 12 resistance furnace

14: mold 16: melting furnace

18: vacuum chamber 20: metal bath

22: single crystal seed 24: starter

26: piston

The present invention relates to a novel method for producing a single crystal casting in a vacuum precision casting furnace, which can be produced at a suitable temperature gradient and an appropriate cooling rate by natural cooling without using a cooling plate or the like at the bottom of the mold. will be.

The single crystal casting method includes a crystal culture method and a seed method. In the crystal culture method (see also the second (a)), a cold plate is installed at the lower end of the mold, and the volume injected at the initial stage of the injection is quenched by the cold plate to form a directional tissue, that is, a starter. As a method of growing according to), a single crystal can be obtained at the top of the selector by selecting only one of the crystal growth directions by the selector. As the selector, various shapes are used as illustrated in FIG. 3, and many of the ones in FIG. 3 (c) (peak tail; pig tail) are used. Seed method (see also second (b)) A method of forming a temperature gradient by providing a cooling tube at the lower end of the mold, in which a single crystal is grown from seeds placed on a cooling plate. The chilled plate used in the crystal culture method and the seed method is usually a copper plate, which is continuously cooled by water cooling. In the crystal culture method and the seed method, the mold should be installed inside a resistance furnace and heated to maintain a temperature gradient and cooling rate suitable for single crystal growth in the vertical direction of the casting.

On the other hand, in the vacuum melting furnace and vacuum precision casting furnace, the following work process is required.

In the case of a vacuum melting furnace, a receiving crucible is installed inside the vacuum melting furnace and charged with various metals. At the same time, the mold for receiving the molten metal is mounted in a separate resistance furnace which is waiting outside. The furnace chamber is then evacuated to reach a vacuum of about 10 ⁻³ torr and then to dissolve the metal. After the alloy is completely dissolved, it is kept in the vacuum state (about 10 ^-2 torr) for about 1 hour so that the gas in the alloy is sufficiently removed. Thereafter, a metal dose is injected into the heated mold. At this time, the temperature of the mold is about 350 to 500 ℃ slightly lower than the initial temperature depending on the vacuum treatment time and the time required for the alloy is melted and injected into the mold. The volume injected into the mold is cooled by injecting a neutral gas such as argon or nitrogen into the vacuum chamber, and then opening the door of the chamber to take out the mold and cooling the mold in the atmosphere until it reaches room temperature. Therefore, in the conventional vacuum melting furnace operation, the cooling rate of the casting during the casting cannot be kept constant, so that the temperature of the mold is not kept constant, and therefore the characteristics of the casting are not uniform.

In the case of a vacuum precision casting furnace, an ingot to be dissolved is injected into a melting furnace installed inside the vacuum casting furnace, and the door of the casting furnace is closed and vacuumed. When the degree of vacuum in the vacuum chamber of the casting furnace reaches about 10 ^-3 to 10 ^-4 torr, the alloy ingot in the furnace begins to dissolve. When the alloy is completely dissolved, the mold heated in a separate atmospheric resistance furnace is placed in the mold chamber attached to the casting furnace, the door is closed, and the mold chamber is vacuumed to match the vacuum degree of the vacuum chamber. Open the gate, the vacuum barrier between the chambers, and move the mold into the vacuum chamber to cast.

The heating temperature of the mold varies depending on the casting conditions, and in the case of iron-based and nickel-based alloys, the temperatures of the mold in the polycrystalline casting and the single crystal casting are 1,000 to 1,200 캜 and 1,500 to 1,650 캜, respectively. The temperature of the mold during single crystal casting is generally about 150 ° C. above the melting point of the alloy to be dissolved.

At the lower end of the mold, a cooling plate is placed so that the molten metal is quenched at the moment of injection, and instantaneously forming a directional tissue is formed. At this time, a single crystal casting is produced by using a seed culture method or a seed method according to the chemical composition and use of the casting. That is, after injecting the molten metal, the single crystal casting is formed by growing a single crystal by moving a vertically movable resistance path surrounding the mold while maintaining the temperature gradient and cooling rate of the molten metal in the mold.

Quenching a casting with a water-cooled cold plate severely limits single crystal growth because of the extreme temperature difference and rapid cooling of the casting. To eliminate this problem, it is necessary to reduce the impact of temperature and to make the temperature gradient and cooling rate of the casting constant.

In the present invention, a low melting point metal is used as a refrigerant without using a water cooling plate. That is, in the present invention, the injected molten metal is cooled by being immersed in a low melting point metal, rather than being brought into contact with a cooling plate to cool. That is, the single crystal production method of the present invention comprises the steps of charging a metal ingot into the melting furnace of the vacuum casting melting furnace, a multi-stage to depressurize the vacuum casting melting furnace in a vacuum state, a multi-stage to melt the metal ingot by heating the melting furnace, and a mold Heating the molten metal to a temperature higher than the melting temperature of the injected metal, injecting the molten metal ingot into the heated mold, placing the injected mold in a low melting metal bath, and the low melting metal bath mold The molten metal of the mold is cooled at a constant rate while the molten metal is melted at a constant rate by the heat of to form a single crystal, and the single crystal grows to complete solidification.

The low melting point metals include lead having a melting point of 347 ° C. and aluminum having a temperature of 660 ° C., and cooling with such a low melting point metal can reduce the thermal shock of about 320 ° C. to 640 ° C. than using a water cooling plate. have. If the mold is immersed in a low melting metal bath after injecting a nickel- or iron-based alloy with a temperature of 1,550 to 1,650 ° C., the low melting metal melts due to the temperature of the mold. As a result, the mold is cooled at a constant speed.

Preferred embodiments of the present invention will be described below with reference to the drawings. 1 shows a vacuum precision casting furnace 10 capable of carrying out the method of the present invention. The resistance furnace 12 is provided in the inside of the vacuum precision casting melting furnace 10, and the mold 14 in the inside can be heated. The alloy ingot to be melted is injected into the melting furnace 16, the door of the casting melting furnace 10 is locked, and the vacuum chamber 18 of the casting melting furnace 10 is vacuumed. When the vaginality of the vacuum chamber 18 reaches about 10 ^-3 to 10 ^-4 torr, the alloy ingot in the melting furnace 16 begins to dissolve. During this operation, since the mold 14 in the resistance furnace 12 is also continuously heated, the mold 14 is also sufficiently heated when the alloy is completely dissolved and the casting operation is performed.

When the injection-molded mold 14 is placed on the low melting point metal bath 20, the low melting point metal is gradually dissolved and the mold 14 gradually descends into the low melting point metal bath 20 so that the molten metal inside the mold solidifies from below. To start. Since the low melting point metal dissolves at a constant rate, the temperature gradient and cooling rate of the casting in the mold can be controlled constantly. Thus, even without the use of a cooling plate, even with natural cooling, the mold 14 maintains a constant temperature gradient and cooling rate, so that single crystals can be grown from the single crystal seed 22 or the starter 24 at the lower end. In addition, by slowly lowering the piston 26 to move the low-melting-point metal bath 20 at a constant speed and heating the resistance furnace 12, it is possible to more precisely control the vertical temperature gradient and cooling rate of the mold.

The single crystal growth process according to the present invention is shown sequentially in FIGS. 4 (a) to 4 (c). The shapes of the molds shown in FIGS. 4 (a) to 4 (c) are exaggerated and shown to make the growth of the single crystal easier to understand, and various shapes of molds can be used.

To make a monocrystalline product from an iron- or nickel-based alloy, the mold is heated to about 1,550 ° C, about 150 ° C above the melting point of the molten alloy, the single crystal seed is fixed on the bottom of the mold and the alloy molten metal is injected. Subsequently, when the mold infused with the alloy is immersed in a lead bath or an aluminum bath, the temperature of the mold is about 1,550 ° C., so that lead having a melting point of 600.4K (347 ° C.) (or aluminum having a melting point of 660 ° C.) gradually melts. The mold is gradually immersed in lead. At this time, the molten metal in the mold maintains an appropriate temperature gradient and cooling rate for single crystal growth, so that the molten alloy in the mold is gradually grown from the lower end by the single crystal seed placed at the lower end of the mold, and the casting becomes a single crystal. This method also allows the formation of 25 cm single crystal castings that are large enough for the manufacture of most important products. In addition, as the embodiment of the present invention related to FIG. 1 provides a mold heating resistance furnace inside the vacuum casting melting furnace to more precisely control the cooling rate of the casting, a relatively large cast single crystal product of 25 cm or more can be made.

As described above, according to the present invention, it is possible to cast an excellent single crystal product at an optimum temperature gradient and cooling rate only by natural cooling without using a water cooling plate which restricts the growth of single crystals due to a severe temperature difference.

As mentioned above, although the present invention has been described with respect to single crystal casting, the present invention is not limited thereto, and can be applied to polycrystalline metal casting, and various modifications that are obvious to those skilled in the art can be made. to be.

Claims (4)

Charging a metal ingot into the melting furnace of the vacuum casting furnace, depressurizing the vacuum casting furnace to a vacuum state, heating the melting furnace to melt the metal ingot, and heating the mold to a temperature higher than the melting temperature of the injection metal. Heating, injecting the molten metal ingot into the heated mold, placing the injected mold in a low melting metal bath, and melting the mold while the low melting metal bath melts at a constant rate by the heat of the mold. And cooling the metal at a constant rate to produce a single crystal, and growing the single crystal to complete solidification. The method of claim 1, wherein the low melting point metal is lead and aluminum. The single crystal production method according to claim 1 or 2, wherein the metal ingot is an iron metal ingot. The method for producing a single crystal according to claim 1 or 2, wherein the metal ingot is a nickel-based metal ingot.