RU2623941C2 - Method of obtaining large-dimensional castings from heat-resistant alloys by directed crystalization - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: heat-resistant alloy heated to a temperature above the liquidus temperature through the riser 2 and the manifold 3 of the gate system are poured into a thin-walled ceramic mould 1 with a seed located in the upper part of the mould. The mould is filled from the bottom up to contact with seed 4 and, as it is filled, is immersed through the layer of the heat-insulating screen into the heated discharge container 9 with the molten metal 10 close in density to the cast alloy. Form is dipped into unloading capacity at depth, providing the same level of alloy castings in form and the molten metal in the vessel. Extract form from unloading containers and cooled form over the surface of the screen flow of inert gas tank fill with molten metal. During the extraction process, the flowchart shapes from the discharge capacity of the sprue system nutrient riser warmed up.

EFFECT: improvement of the quality of castings by preventing cracking of the ceramic forms and pollution alloy castings.

5 dwg

Description

The present invention relates to foundry and can be used to obtain large-sized castings of parts from heat-resistant alloys with directional and single-crystal structure, for example, turbine blades of gas turbine engines.

Known methods for the directed crystallization of large-size castings from heat-resistant alloys in vacuum using a water-cooled, mainly copper, mold, on which a ceramic mold with an open bottom part is mounted and fixed (US patents for inventions No. 3532155, IPC B22D 27/04, publication date 06.10. 1970 and No. 3700023 IPC B22D 27/04, publication date 10.24.1972; EP patent for invention No. 0127552, IPC B22D 27/04, publication date 05.12.1984).

The disadvantages of such methods (the Bridgman method was used) is the difficulty of fastening a large-sized form with an open bottom directly to the refrigerator due to the possibility of melt flowing through zones of insufficiently dense contact. In addition, with this method it is impossible to obtain large-sized castings and castings with the same structure and properties throughout the volume due to a decrease in the temperature gradient with increasing distance from the refrigerator. At the same time, it should be noted that an increase in the size of the casting leads to an increase in hydrostatic and hydrodynamic loads on the walls of the ceramic casting mold. To ensure the strength of the mold, an increase in wall thickness is required, which increases thermal resistance and reduces the temperature gradient.

Known methods for directed crystallization of large-size castings from heat-resistant alloys, in which the cooling zone is made in the form of a tank with a melt of low-melting material with high thermal conductivity. After pouring the melt, the ceramic mold is immersed in a cooling container with a liquid metal cooler (US patents for inventions No. 3763926, IPC B22D 27/04, published date 09/10/1973 and No. 3915761, IPC B22D 27/04, published date 28/10/1975, patent RF on invention No. 2146184, IPC B22D 27/04, date of publication 10.03.2000).

The disadvantage of such methods is, as mentioned above, the inability to obtain large-sized castings with a perfect directional and single-crystal structure due to the increase in hydrostatic and hydrodynamic loads on the walls of the ceramic casting mold with an increase in the size of the casting, which leads to the need to increase the thickness of the mold and, as a result, reduce temperature gradient. In addition, the design of the device does not provide large castings with a perfect directional and single-crystal structure over the entire height due to the small size of the liquid metal mold. A significant change in existing installations is required, associated with the need to create deep and sufficiently voluminous cooling tanks.

, Experimental Trials of the Thin Shell Casting (TSC) Technology for Directional Solidification. The 3rd International Conference on Advances in Solidification Processes. IOP Publishing, IOP Conf. Series: Materials Science and Engineering 27 (2011) 012036).The closest in technical essence to the proposed and adopted as a prototype is a method of directional crystallization of large-size castings from heat-resistant alloys, in which a ceramic casting set, containing at least one thin-walled mold with seed in the upper part, heated to a temperature above the liquidus temperature of the casting, is immersed through a layer of a heat-insulating screen in a heated discharge container with molten metal and, as it is immersed, fill the kit with casting alloy, fill and a set of complete immersion in contact alloy casting with seeding and then extracted set of discharge capacity, the surface of the screen it is cooled inert gas stream (D Ma, Nao Lu and A

, Experimental Trials of the Thin Shell Casting (TSC) Technology for Directional Solidification. The 3rd International Conference on Advances in Solidification Processes. IOP Publishing, IOP Conf. Series: Materials Science and Engineering 27 (2011) 012036).

The discharge tank with superheated molten metal - casting alloy is covered with a bulk heat-insulating screen. A thin-walled (wall thickness of about 1 mm) ceramic casting set contains at least one thin-walled mold with a seed in the upper part, a mold with a water-cooled crystallizer mounted on the upper part is slowly immersed in a discharge container with molten metal. The mold is closed from below by a plug, which melts upon contact with molten metal and prevents fragments of the bulk screen from getting inside the mold when immersed. The plug is made of the same metal as the cast alloy. The ceramic casting set is immersed in the melt until the mold is filled (until the melt comes into contact with the crystallizer refrigerator). In this case, the melt begins to crystallize in the contact zone. Slow pulling up the model kit through the bulk screen leads to the crystallization front moving downward relative to the mold. Having blown a cold inert gas of a mold elongated above the screen, it is possible to provide intensive heat removal and a high temperature gradient at the crystallization front.

One of the disadvantages of this method is that the presence of a temporary metal plug from the casting alloy leads to a situation where it melts upon contact with the metal, but the plug heats up and expands before melting, which in turn causes cracking of the mold.

The duration of the process, the large contact area of the melt with the surface of the discharge tank and the bulk screen leads to a partial dissolution of the tank material and the screen in the melt (primarily silicon, which has a dramatically negative effect on the properties of the alloys) and, as a consequence, pollution and a change in the chemical composition of the material castings.

In addition, a significant disadvantage of the prototype is the use of a discharge tank as a location for a large amount of casting melt, which leads to the uneconomical use of modern expensive heat-resistant superalloy, its contamination with the bulk screen material and the products of the chemical interaction of the alloy with the walls of the discharge tank. This prevents their reuse.

An object of the invention is to provide a method for the directed crystallization of large-size castings from heat-resistant alloys, which provides castings of large size and weight using thin-walled casting molds, eliminating their cracking and contamination of the casting alloy, as well as ensuring the economical use of expensive charge material, which is especially important for modern single-crystal heat-resistant alloys of the II-V generations containing severely deficient and extremely expensive rhenium, as well as The platinum group element of ruthenium, which is distinguished by its high cost.

The technical result of the invention is to improve the quality of large-sized castings by preventing cracking of ceramic molds and contamination of the casting alloy, as well as creating conditions for the economical use of expensive heat-resistant casting alloy.

The heating of the runner of the gating feed system ensures the maintenance of the casting alloy in a liquid state throughout the gating feed system, due to which the pressure inside the mold and the discharge tank are kept equal during the entire crystallization process, which eliminates cracking of the mold.

In addition, the absence of contact between the casting alloy and molten metal in the discharge tank completely eliminates the possibility of particles of the bulk screen getting into the mold, as well as contamination of the casting alloy by the metal of the discharge tank, which eliminates the need for a temporary plug, which also eliminates mold cracking.

Immersion of a ceramic casting kit with a pouring system from above through a gating and feeding system allows using cheap metal close in density to the casting alloy as a melt for the discharge tank, which significantly saves the expense of an expensive heat-resistant alloy.

The technical result is achieved by the fact that in the method of directional crystallization of large-size castings from heat-resistant alloys, in which a ceramic casting assembly, which is heated to a temperature higher than the liquidus temperature of the casting alloy, containing at least one thin-walled mold with a seed in the upper part, is immersed in a heated screen discharge vessel with molten metal and as the dive is filled, the set is filled with casting alloy, filling and immersion of the set is completed when the beat of the casting alloy with the seed, after which the kit is removed from the discharge tank, while over the surface of the screen it is cooled by a stream of inert gas, unlike the known casting kit is made in the form of a block mold equipped with a gating feed system, while the casting alloy is poured into the block the mold through the riser of the sprue feed system, and the discharge tank is filled with molten metal close in density to the casting alloy, the block mold is immersed in the discharge tank to a depth that provides the same the level of the casting alloy and molten metal in the tank, and during the extraction of the block mold from the discharge tank, the runner of the gate feeding system is heated.

The invention is illustrated by drawings, which depict:

FIG. 1 - block form with thin-walled ceramic forms;

FIG. 2 - a device for directed crystallization of large-size castings from heat-resistant alloys before starting work;

FIG. 3 - the device when pouring the casting alloy and immersing the block form in the discharge tank;

FIG. 4 - the device at the beginning of pulling the block form from the discharge tank;

FIG. 5 - the device in the process of pulling the block form from the discharge tank.

To implement the inventive method of directed crystallization of large-size castings from heat-resistant alloys, a device is proposed that includes a block mold (Fig. 1) containing a sprue feeding system and at least one thin-walled ceramic mold 1. The sprue system contains a riser 2 connected through a collector 3 sec ceramic mold 1. In contrast to the thin-walled form, the riser and collector have a sufficiently thick ceramic shell, which is necessary to ensure structural strength.

Form 1 is made closed in the bottom. The upper part of the thin-walled form includes a conical starting cavity, a cavity of a crystal sample in the form of a helicoid and a single crystal seed 4 of a refractory alloy, for example tungsten, with a cooling rod. A hole is provided in the rod for bleeding off gas displaced from the thin-walled form by the casting alloy when it is filled. Thin-walled manifold and riser connections are airtight.

The block form with the seeds is suspended from the platform 5 (Fig. 2). The platform 5 is made with the possibility of vertical movement. The mechanism for moving the platform is made, for example, in the form of a telescopic or helical elevator (not shown).

The block form is installed in the heating furnace 6, equipped with heating elements 7 for preheating the block form. To ensure uniform radiation heating of the block form, the furnace can additionally be equipped with a heat shield 8.

Under the heating furnace 6 there is a discharge tank 9 with molten metal 10 close in density to the casting alloy. The discharge tank 9 is equipped with a heating element 11, for example an inductor.

The device is equipped with a funnel 12 for pouring the casting alloy into a block mold (Fig. 3).

On the surface of the molten metal 10 in the tank 9 is a heat-insulating screen 13, for example bulk, made in the form of ceramic balls or ceramic powder (Fig. 4).

Above the discharge tank 9 (FIG. 5), pipelines 14 are arranged to blow cold inert gas that is pulled upward of the block mold. Also above the discharge tank 9 are additional heaters 15 for heating the riser 2.

The method is as follows.

The casting alloy is poured into the block mold (Fig. 1) from above through the riser 2 of the sprue feed system in an inert gas medium at atmospheric pressure or in vacuum. To prevent cracking of the ceramic block mold before pouring and during pouring, it is continuously heated in the heating furnace 6 (Fig. 2). The thin-walled ceramic mold 1 is filled from the bottom up, while the pressure of the alloy inside the mold is compensated by the pressure of the molten metal in the discharge tank 9. The mold is completely filled before contact with the seed 4, while the alloy crystallizes instantly in the contact zone.

As the block form is filled, it is slowly immersed through the layer of the heat-insulating screen 13 into the discharge tank 9 with molten metal 10 (Fig. 3). The discharge tank is filled with inexpensive molten metal 10 having a density close to that of the cast alloy. The use of an inexpensive alloy in the tank reduces the cost of expensive heat-resistant alloy castings.

To prevent solidification of the molten metal in the discharge tank, it is continuously heated — the specified temperature of the molten metal is maintained at a temperature of 80 ÷ 100 ° C higher than the liquidus temperature of the heat-resistant cast alloy to prevent cracking of the mold from a sharp temperature difference. The casting alloy is kept in shape in a liquid state due to its heating by molten metal in the discharge tank.

Immersion of the block mold in the discharge tank is performed to a depth that provides the same level of casting alloy in the mold 1 and molten metal in the tank 9, which ensures the absence of hydrostatic forces on the thin-walled casting mold. Filling and immersion of the block form is completed by contact of the cast alloy being cast with the seed. After that, the heating of the block form is turned off (Fig. 4).

Turn on blowing with cold inert gas and begin lifting the platform together with the block form fixed on it (Fig. 5). Primary crystallization of the alloy begins at the point of contact with the cooled gas of the seed from the refractory alloy. As you move up the block form, the crystallization front moves down relative to the form, remaining in the region of the heat-insulating screen 13. Below the crystallization front, the alloy remains in a liquid state.

The closed design of the mold (the melted plug, as in the prototype, is absent) eliminates the contact of the casting metal with molten metal in the discharge tank. Due to the fact that during the crystallization process the metal volume decreases, and the block form (riser with a system of feeders and forms) is a communicating vessel, and when the block form is removed from the hot zone, the riser is also removed, it must also crystallize. In this case, the melt in the lower part of the block mold (in collector 3) will be locked, and when the volume decreases during its further crystallization, the pressure outside the thin-walled mold will exceed the internal one and the thin shell of the mold will collapse. To avoid this, the riser of the block mold is heated by the heating element 15 when removed from the discharge tank, then the casting alloy in the riser is in a liquid state and feeds the thin-walled form, as a result of which the pressure in the block form and in the discharge tank is equalized.

After crystallization of the casting is completed, the block mold is cooled in the atmosphere, thin-walled forms are destroyed and excess alloys are cut off, which are then remelted and used for casting new parts.

For practical confirmation of the technical result of the proposed method, the following experiments were carried out. The casting of the model of the blade made of ZhS32 alloy by the LMC technology (liquid metal cooling) in the UVNK8P furnace was smelted. In this case, the manufactured ceramic shape of the blade had a thickness of 1 mm. The form when lowering into the zone of liquid metal cooling fell into a ceramic crucible filled with the same cooler. When it touched the cooler, it cracked and the whole alloy poured into a crucible. 3 experiments were carried out and they all ended the same way. Given that in the UVNK-8P installations, the existing mechanism for moving the mold provides both its lowering and lifting, a ceramic blade shape of the same size and the same wall thickness was made, to which a sprue pipe was attached from the bottom, extending vertically upward and ending with a ceramic bowl .

An inverted U-shaped design ending on one side with a cup and on the other with a form was mounted on a lifting device of the UVNK-8P furnace. Thus a bowl of a batch dimensional workpiece alloy ZHS32 moved inside the heater, and the lower part with a form - in a heated crucible filled alloy 3H435 + W, whose density was 8.8 g / cm ^3. Tubes were attached to the upper part of the heated crucible to blow argon and provide cooling of the mold at the place of its exit from the 3H453 + W liquid alloy, which had a temperature of 1600 ° C. During the heating of the bowl with a portioned measured billet when the temperature in it reaches 1450 ° C, the inverted U-shaped structure was lowered into the heated crucible until the blade shape was completely immersed and after holding for 10 minutes the structure rose up, while the mold was blown with argon having a temperature of the order of 80-100 ° C. Several experiments were carried out - the shape did not collapse, the material obtained as a result of crystallization was characterized by a directed columnar structure.

The method of directional crystallization of large-size castings from heat-resistant alloys allows the use of thin-walled molds with low strength without cracking, which makes it possible to obtain high-quality single-crystal castings regardless of size.

The use of an inexpensive alloy having a density close to the density of the casting alloy in the discharge tank reduces the consumption of expensive heat-resistant alloy for casting.

Claims (1)

A method for producing directed crystallization of large-size castings from heat-resistant alloys, including heating to a temperature higher than the liquidus temperature of a heat-resistant alloy of a thin-walled ceramic mold with a seed located in the upper part of the mold, immersing the mold through a layer of a heat-insulating screen in a heated discharge tank with molten metal and filling the mold with a heat-resistant alloy melt as immersed until the melt of the heat-resistant alloy contacts the seed, mold removal from the discharge tank and cooling the mold above the screen surface with an inert gas stream, characterized in that the mold is filled through the riser and collector of the gate system, and the mold is immersed in a discharge tank with molten metal close in density to the heat-resistant alloy of the casting to a depth providing the same melt level a heat-resistant alloy of casting in the mold and molten metal in the discharge tank, while the riser of the gating system is heated in the process of extracting the mold.

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