EP1842607A2 - Method for investment casing of metallic moulded parts and device therefor - Google Patents

Abstract

The fine casting of profiled metal parts or several dimensionally identical metal parts in which a metal melt is cast in a ceramic mold, and cooled under pressure for a maximum of 30 seconds and allowed to solidify. During solidification the melt can be subjected to ultrasonic vibration or a variable stress (sic, Wechselspannung). An independent claim is included for a device for fine casting of profiled parts.

Description

The invention relates to a method for investment casting of a metallic molded part or more preferably dimensionally similar metallic moldings, in which a metallic melt is poured into a ceramic mold, after which the metallic melt is allowed to solidify.

Furthermore, the invention relates to a device for investment casting of a metallic molded part or more preferably dimensionally equal metallic moldings, comprising a ceramic mold and a melt container through which metallic melt can be introduced into the mold, and a cooling device for the mold.

With investment casting, components of complex geometry with dimensions close to the final dimensions can be created. A preferred application of such methods is in those areas where manufacture of solid castings and subsequent molding, e.g. in machining manner such as by turning or milling, due to a spatially complicated geometry of the molded part to be created extremely expensive and therefore economically unreasonable. If one considers further that multiple die-casting processes can simultaneously produce a plurality of dimensionally identical molded parts, it is not surprising that such processes are of tremendous significance for mass production of complicated machine parts.

As with other methods of casting metals, there is a fundamental difficulty with investment casting in achieving a microstructure without voids. This is reinforced in investment casting process in that a ceramic casting mold - compared with conventional casting - over a relatively large contact surface in contact with liquid melt, which complicates an ideal and error-free solidification of the metallic melt. In particular, in narrow passages of the casting mold, which correspond to sections with small dimensions of a final molded part, too rapid a solidification of the metal may occur, so that a tracking of melt is no longer possible and cavities occur in the formed molded part. In addition, voids formation may be favored if the melt has a lower density than the solid, which leads to a volume contraction during solidification. Especially in the case of increased cooling of a melt, a massive voids formation can be present in this case.

From the prior art ( US 6,622,774 B2 ), a precision casting method has become known in which a melt-filled ceramic casting mold is placed in a low-temperature oil bath in order to increasingly cool the melt in the casting mold. With such a method a fine and homogeneous microstructure should be adjustable in the final solid, which is basically positive. In this method, however, it remains completely unconsidered that it can come by dissolved in the melt gas and increased cooling and volume contraction of the metal during solidification to formation of voids, which represent starting points for possible material failure in the molding and accordingly shorten its useful life considerably can.

Starting from the prior art, it is an object of the invention to provide a method of the type mentioned, with which in case of increased cooling void-free molded parts with a fine microstructure can be produced.

Another object of the invention is to provide a device of the type mentioned, which is suitable for investment casting void-free and a homogeneous, fine structure exhibiting moldings.

The procedural object of the invention is achieved in that in a method of the type mentioned, the cast melt is intensively cooled while exerting pressure on the melt, wherein pressure and increased cooling at least until the dimensional stability of the solidifying molding or the moldings are maintained.

Advantages of a method according to the invention can be seen, in particular, in that due to the procedural measures taken, molded parts can be produced which are essentially free of voids. By exerting a pressure on the melt during solidification, the mold is filled in an excellent manner or is pressed during the solidification of the melt in any existing voids melt. Since at the same time an increased cooling takes place and the solidifying material heat is removed quickly, dense, substantially void-free molded parts can be obtained with a fine microstructure. Increased cooling is to be understood in the context of the invention, any measure which leads to a faster solidification in comparison with a solidification in air.

It is favorable if the melt is degassed and poured off under vacuum into the casting mold. This ensures that a proportion of dissolved gas, which can lead to the formation of voids upon solidification of the melt, is minimized.

In a special way, it has proven useful when the pressure by means of a gas, in particular a noble gas, is exercised. This causes, since the ceramic mold is not completely tight, but has a certain porosity that the molten metal is subjected to pressure on all sides during solidification. As a result, cavitation formation can be suppressed particularly effectively and homogeneous and void-free shaped parts can be created.

In connection with an all-round pressurization by a gas, a process variant in which the melt is allowed to partially solidify after it has been poured into the casting mold and before pressure is applied until it is at least partially surrounded by a gas-impermeable metal skin, has proved to be preferred. The gas impermeable metal skin protects the melt located inside against gas absorption both through an opening of the mold and through the ceramic. At the same time the solidifying composite of melt and metal skin is easily deformed, so that by the gas pressure a dense and lunkerfreier molding is produced. Alternatively, the melt may be sealed gas-tight, e.g. by covering the melt in the sprue area with an inert material such as sand.

Since heating devices are usually not provided in casting molds and a molten metal already begins to solidify during or immediately after casting into the casting mold, it is recommended that the pressure be built up within a maximum of 30 seconds. If free areas of particularly small dimensions in the casting mold are to be filled up with metallic melt, then it is expedient to build up the pressure within 100 milliseconds, preferably within 10 to 50 milliseconds. As a result, the melt can be cooled under pressure for a short time after being poured off and a local solidification, which would complicate or prevent further feeding of metallic melt, can be avoided.

In principle, all known methods for generating pressure are suitable for use in the context of the invention. However, one is especially fast Pressure buildup required, for example, because fine capillaries are present in the mold, which are to be filled with liquid metal, so generating the pressure by igniting one or more explosive charges is extremely useful. A rapid pressure build-up can alternatively also be achieved by heating a liquid inert gas in a closed volume.

For complete solidification of the melt, the casting mold can be introduced into a liquid coolant, in particular a gas which is liquid at -100 ° C. or at a lower temperature. By a liquid coolant, which is optionally additionally kept in motion, heat can be removed in a particularly effective manner and thus a rapid solidification can be achieved. It is particularly advantageous for the reasons mentioned, when the mold is brought over the entire surface with the coolant in contact.

It is also advantageous in terms of the method if the melt is brought to a completely solidified state within a maximum of 300 seconds after the pressure has been built up. By a rapid solidification of the metallic melt carried out in this way, a particularly fine microstructure can be set or a molded part with excellent mechanical properties can be obtained. If a gas is used to pressurize, then a further time is minimized, in which gas can dissolve in the melt.

When carrying out a method according to the invention, it is also advantageous if the pressure is at least 200 bar, preferably at least 800 bar. The higher a pressure, the sooner cavities that occur during solidification can be eliminated, and the better it is possible for liquid metal to be pressed into the casting mold or the solidifying metal to be compacted.

In a further variant of a method according to the invention, it is provided that the melt is subjected to ultrasound and / or an alternating voltage during solidification. Such an application causes on the one hand, that a very fine structure is formed. On the other hand, ultrasound also counteracts the formation of voids.

In order to best ensure a transport of melt into the interior of the casting mold during solidification, it may be provided that the casting mold and / or the Melt in a runner during the increased cooling area heated. It proves to be expedient to heat those regions of the casting mold through which melt is to flow into the casting mold interior, whereas the remaining parts of the casting mold are increasingly cooled.

The further object of the invention is a device for investment casting of a metallic molding or more preferably dimensionally equal metallic moldings, comprising a mold and a melt container, through which metallic melt can be introduced into the mold, and a cooling device for the mold, wherein the cooling device in a pressure-resistant chamber is housed, solved.

Advantages of a device according to the invention are to be seen in particular in the fact that high-quality, void-free molded parts can be created with a simple construction of the apparatus and in addition a fine microstructure can be set in the molded parts. A compressive strength of the proposed chamber allows, after introducing the molten metal in the mold to apply these with high pressure and thus to fill the mold on the one hand completely and on the other hand to let the molten metal solidify under pressure. Since at the same time an increased cooling can take place, high quality molded parts with excellent mechanical characteristics can be produced.

In a particularly simple variant, the cooling device comprises a container for a liquid coolant, e.g. an oil, water or substances which are present between -100 ° C and - 200 ° C as the liquid phase. This makes it possible to efficiently cool the mold over a large area. If necessary, only a part of the mold can be cooled by the casting mold is only partially introduced into the container of the cooling device. In this context, it is useful for easy handling when the mold is movable in the container.

In a further embodiment of a device according to the invention can be provided that the chamber has at least two areas which are separated by a lock and independently evacuated or pressurized. This proves to be particularly advantageous in the case of the use of liquid coolants, because it can be prevented that during casting under vacuum or during degassing of the metallic melt coolant enters the vacuum apparatus, resulting in their failure, but in any case a poorer vacuum and thus to a higher proportion of dissolved gas in the melt, can lead.

Further advantages and effects of the invention will become apparent from the context of the description and the following embodiments.

In the following, the invention is described in more detail with reference to exemplary embodiments.

Figur 1

ein Flussdiagramm eines erfindungsgemäßen Verfahrens;

Figur 2

einen Querschnitt einer erfindungsgemäßen Vorrichtung.

Show it

FIG. 1

a flow chart of a method according to the invention;

FIG. 2

a cross-section of a device according to the invention.

In FIG. 1, individual steps of a precision casting method according to the invention are illustrated in more detail with reference to a flowchart. Initially, a melt of a metal is created by melting a corresponding starting material. By metal is here as well as otherwise a metal, an alloy of metals or a composite material, which includes a metal or an alloy as a predominant component to understand. After the melt has been produced, it is degassed, for which purpose any degassing method familiar to the person skilled in the art can be used. However, it is preferred to carry out a degassing by a vacuum treatment. In this case, degassing and a subsequent step of vacuum pouring may occur in a single vacuum chamber, thereby ensuring that the melt between degassing and pouring is no longer in contact with a gas. After pouring off the melt, it is gas-tightly sealed in the region of an opening of a casting mold or, optionally, allowed to solidify to the area-wise formation of a gas-impermeable metal skin. This ensures that in the next step, pressurization by means of a gas, a release of the gas used in the molten metal is prevented or at least largely suppressed. After a desired pressure is built up, for example 800 bar, the melt is solidified while maintaining this pressure by introducing the casting mold into a coolant. In this case, the melt solidifies in the casting mold from outside to inside, in particular sprue areas and / or supply lines of the casting mold are heated with advantage, so that a flow of material into the form-forming elements of the mold is possible or a solidification front of the melt without interruption up to a Can withdraw the sprue of the mold. Once out of the Molten metal has formed a dimensionally stable molded part, an intended pressure can be reduced to atmospheric pressure and after cooling the melt to ambient temperature, the finished molded part can be removed.

According to the procedure set out above, impellers were cast from an aluminum casting alloy, namely G-AlSi 12 (12 wt% silicon casting aluminum alloy). In this case, a molten metal was created from a corresponding starting material and this degassed in a vacuum. Subsequently, the thus-prepared molten metal was poured into a casting grape in the same chamber in which it was degassed. After the casting gravel had been filled with molten metal up to the filling opening, the metal melt poured into the casting grape, which had been poured off at a temperature of 600 ° C., was allowed to cool until a solid film or film had formed on the surface of the molten metal in the region of an inlet opening of the grape . had formed a continuous metal skin of 500 .mu.m thickness. Subsequently, the vacuum was released and built by argon, a pressure of 500 bar. The time between introduction of the argon into the chamber and reaching a final pressure of 500 bar was 25 seconds. At this pressure, the melt was allowed to solidify for a further 45 seconds and then brought to complete solidification by dropping the grape into an oil bath having a temperature of 25 ° C under pressure. After cooling the casting grape to ambient temperature, it was removed from the oil bath at normal pressure and the ceramic removed. Investigations on impellers produced in this way showed that they were essentially void-free and had a homogeneous, fine-grained microstructure when seen in cross-section.

FIG. 2 shows a cross-section of a device according to the invention. The device 1 comprises two compartments 1a, 1b, which are separated from each other by sluices. On the outside, the device 1 is surrounded by a housing 2 which is pressure-resistant to at least 200 bar , for example of steel. In the housing 2 inlets 41, 42 and outlets 51, 52 are provided. The inlets 41, 42 make it possible to introduce a gas, for example argon, into the compartments 1a, 1b and to generate a high pressure of up to 2000 bar in the compartments 1a, 1b mentioned. The outlets 51, 52 are each connected to a vacuum apparatus, so that when needed in the compartments 1 a, 1 b, a high vacuum can be applied. The locks, which separate the compartments 1a, 1b from each other, consist of elements 31, 32, 33 and 34. The elements 31, 32 are fixedly connected to the housing 2 , whereas the elements 33, 34 are held horizontally displaceable on the elements 31 and 32 , respectively. This makes it possible to open the lock as needed. Instead of the lock shown in Figure 2, any other lock can be used, provided that it is ensured that the compartments 1a, 1b can be separated from each other gas-tight.
In the upper compartment 1a of the device 1 , a holding and displacement device 6 for a casting grape 10 is provided. The holding and displacement device 6 may, as shown in Figure 2, be equipped with telescopic arms 7 through which the Gießtraube 10 can be moved if necessary in the compartment 1b . Symmetrically mounted brackets 81, 82, which are rigidly connected to both the telescopic arms 7 and 10 of the grape, ensure that the grape is held steady during the casting or a solidification.

Furthermore, a melting container 12 is provided in compartment 1 a, which serves for receiving or storing molten metal S. The melting container 12 can additionally, as shown, have an insulation 13 , in order to avoid unwanted solidification of the molten metal S in the melting container 12 . If the melt container 12 is accommodated together with the casting grape 10 in the compartment 1a , this has the advantage that the melt can be degassed by applying a vacuum via the outlet 51 and immediately after degassing, the melt S is poured into the grape 10 can be. This on the one hand simplifies the process and on the other hand it ensures that the melt S can no longer come into contact with a gas after degassing, as could be the case if the melt S is degassed outside the device 1 and then into the compartment 1a is to bring. Although a degassing of the melt S in the compartment 1a is preferred, it is of course also possible within the scope of the invention to degas the melt S outside the compartment 1a or outside the apparatus 1 and then introduce it into the compartment 1a .

The Gießtraube 10, which is rigidly connected in the region of a filler neck 9 as mentioned with the brackets 81, 82 , consists of a ceramic and can be prepared in a conventional manner. Depending on the shape of the molded part to be created or the moldings to be created ceramic elements 11 are also provided in their interior, which determine the geometry of a final molded part or act shaping.

In the compartment 1a further aids may be provided, with which the casting grape 10 in the region of its inlet opening 91 can be closed. Such aids may include, for example, a height-adjustable plate whose diameter corresponds to the diameter of the inlet opening 91 and which can be raised or lowered hydraulically or pneumatically.

In the compartment 1b , a coolant container 14 is housed, in which a coolant K is located. If a filled into the casting grape 10 and pressurized melt for rapid solidification, the elements 33, 34 of the lock are moved outward and opened the lock, so that the Gießtraube 10 are retracted by the telescopic arms 7 in the coolant K. can. It is expedient to retract the Gießtraube 10 only so far in the coolant K , that in those areas that later form the molding or the shaped body is provided for a rapid solidification, whereas the filler neck 9 of the grape 10 is not in the coolant K. is retracted. In this approach, it is ensured that in the region of the filler neck 9, a slower solidification of the melt occurs and due to an applied pressure liquid metal in the individual reinforced cooled elements of the Gießtraube 9, in which melt solidifies from outside to inside, can be re-pressed. At the same time for the same purposes in the region of the inlet opening 91 and the adjoining supply lines or sprue area also be heated. In order to achieve a rapid and effective cooling, it is expedient to move the coolant K, for example by means of a stirrer. An improvement of a cooling effect and thus an increase in the fineness of a structure is also possible if the container 14 made of steel is positioned on a solid metal block 15 , so that a good heat exchange between the coolant K and the surroundings of the device 1 is possible.

The device shown in Figure 2 represents only one possible embodiment. In another embodiment, a device according to the invention consist of three interconnected chambers, wherein in a first chamber, a degassing of the melt and a casting thereof under vacuum. Subsequently, the casting grape is brought from the first chamber into a second chamber. This second chamber is dimensioned as small as possible, so that the casting grape finds exactly enough space. Due to the small residual volume, which is not filled by the casting grape, then a particularly rapid pressurization can be achieved. The second chamber is in turn in contact with a third chamber, in which is a coolant and which is also formed with minimal volume to allow rapid pressure build-up. After a desired pressure is built up in the second and third chambers, the casting grape is brought from the second chamber into the third chamber and brought into contact therewith with the coolant.

Claims (18)

A method for investment casting a metallic molding or more preferably dimensionally equal metallic moldings, in which a metallic melt is poured into a ceramic mold, after which the metallic melt is allowed to solidify, characterized in that the cast-off melt is intensively cooled while exerting a pressure on the melt , wherein pressure application and increased cooling are carried out at least to the dimensional stability of the solidifying molding or the moldings. A method according to claim 1, characterized in that the melt is degassed and poured off under vacuum into the casting mold. A method according to claim 1 or 2, characterized in that the pressure is exerted by means of a gas. Method according to one of claims 1 to 3, characterized in that the melt is gas-tightly sealed after pouring into the mold or the melt is allowed to partially solidify until the melt is at least partially surrounded by a gas-impermeable metal skin. Method according to one of claims 1 to 4, characterized in that the pressure is built up within a maximum of 30 seconds. Method according to one of claims 1 to 5, characterized in that the pressure within 100 milliseconds, preferably within 10 to 50 milliseconds, is built up. Method according to one of claims 1 to 6, characterized in that the pressure is generated by igniting one or more explosive charges. Method according to one of claims 1 to 6, characterized in that the pressure is generated by heating a liquid inert gas. Method according to one of claims 1 to 8, characterized in that the casting mold for complete solidification of the melt in a liquid coolant, in particular, a liquid at -100 ° C or a lower temperature gas is introduced. A method according to claim 9, characterized in that the casting mold is brought over the entire surface with the coolant in contact. Method according to one of claims 1 to 10, characterized in that the melt is brought within a maximum of 300 seconds after the formation of the pressure in a fully solidified state. Method according to one of claims 1 to 11, characterized in that the pressure is at least 200 bar, preferably at least 800 bar. Method according to one of claims 1 to 12, characterized in that the melt during solidification with ultrasound and / or an alternating voltage is applied. Method according to one of claims 1 to 13, characterized in that the casting mold and / or the melt are heated in regions in a sprue area during the increased cooling. Apparatus for investment casting a metallic molding or more preferably dimensionally equal metallic moldings, comprising a ceramic mold and a melt container, via which metallic melt can be introduced into the mold, and a cooling device for the mold, characterized in that the cooling device is housed in a pressure-resistant chamber , Apparatus according to claim 15, characterized in that the cooling device comprises a container for a liquid coolant. Apparatus according to claim 15 or 16, characterized in that the casting mold is movable in the container. Device according to one of claims 15 to 17, characterized in that the chamber has at least two areas, which are separated by a lock and independently evacuated or pressurized.

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