Patent Application
20080011442
The invention relates to a method for precision-casting a metallic molded part or several metallic molded parts preferably of equal dimensions, in which method a metallic melt is poured into a ceramic casting mold, with the metallic melt being left to freeze thereafter.
The invention further relates to a device for precision-casting a metallic molded part or several metallic molded parts preferably of equal dimensions, comprising a ceramic casting mold and a melt container via which metallic melt can be inserted into the casting mold, as well as a cooling device for the casting mold.
Components of complex geometry can be produced with near-net-shape dimensions by means of precision-casting methods. A preferred field of application of methods of this type is in those areas where a production of solid castings and a subsequent shaping, e.g., by means of chip removal, such as a turning or milling, would be extraordinarily expensive and thus economically unjustifiable because of a spatially complicated geometry of the molded part to be produced. If the additional fact is taken into account that precision-casting methods make it possible to simultaneously produce several molded parts of equal dimensions, it is not surprising that methods of this type are of tremendous importance for a series production of complicated machine parts.
As with other methods for casting metals, the basic problem with precision-casting methods is producing a structure without voids. With precision-casting methods this is intensified in that a ceramic casting mold is in contact with liquid melt over a relatively large contact area—compared to conventional casting methods—which makes an ideal freezing without imperfections of the metallic melt more difficult. In particular in narrow passages of the casting mold, corresponding to sections of small dimensions of a final molded part, the metal can freeze too quickly, so that it is no longer possible to feed more melt, and hollows occur in the molded part produced. A piping can be promoted additionally if the density of the melt is lower than that of the solid body, which causes a volume contraction during freezing. In particular with an intensified cooling of a melt, there can be a massive piping in this case.
A precision-casting method is known from the prior art (U.S. Pat. No. 6,622,774 B2), in which method a ceramic casting mold filled with melt is inserted into a low-temperature oil bath in order to intensify the cooling of the melt located in the casting mold. With a method of this type a fine and homogeneous microstructure is to be adjustable in the final solid body, which is basically a positive thing. But this method does not consider at all that gas dissolved in the melt, as well as an intensified cooling and a volume contraction of the metal during freezing can cause a formation of voids, which are points of origin of potential material failure in the molded part and accordingly can considerably shorten its service life.
On the basis of the prior art it is the object of the invention to disclose a method of the type mentioned at the outset, with which method void-free molded parts with a fine structure can be produced with intensified cooling.
It is a further object of the invention to disclose a device of the type mentioned at the outset, which device is suitable for the precision-casting of void-free molded parts having a homogeneous, fine structure.
The object of the invention in terms of the method is attained in that with a method of the type mentioned at the outset, the poured melt is cooled in an intensified manner with a pressure being exerted on the melt, whereby the exertion of pressure and the intensified cooling are maintained at least until the dimensional stability of the freezing molded part or molded parts.
Advantages of a method according to the invention are to be seen in particular in that because of the measures taken in terms of process engineering, molded parts can be produced which are essentially free of voids. Through the exertion of a pressure on the melt during freezing, the casting mold is filled in an excellent manner, or melt is pressed into potentially existing hollow spaces during the freezing of the melt. Since an intensified cooling takes place at the same time, and heat is withdrawn quickly from the freezing material, dense, essentially void-free molded parts with a fine structure can be obtained. Within the scope of the invention, intensified cooling refers to each measure that causes a faster freezing, as compared to a freezing in air.
It is favorable if the melt is degassed and poured into the casting mold under vacuum. This ensures the minimization of a proportion of dissolved gas which might cause the formation of hollow spaces during a freezing of the melt.
It has proven to be particularly favorable if the pressure is exerted by means of a gas, in particular an inert gas. Since the ceramic casting mold is not completely impermeable, but has a certain porosity, this has the effect that the metal melt is pressurized on all sides during the freezing. A piping can thus be suppressed particularly effectively, and homogeneous and void-free molded parts can be produced.
With respect to a pressurization on all sides by means of a gas, one method variant has proven to be preferred, in which the melt is left to partially freeze after it has been poured into the casting mold and before a pressure is exerted, until it is surrounded at least in areas by a gas-impermeable metal skin. The gas-impermeable metal skin protects the melt located in the interior from gas absorption both via an opening of the casting mold and through the ceramic. At the same time, the freezing compound of melt and metal skin can be deformed easily, so that a dense and void-free molded part can be produced by the gas pressurization. Alternatively, the melt can also be closed off in a gastight manner, e.g., in that the melt is covered in the gate area with a layer of inert material such as sand.
Since heating devices are generally not provided in the case of casting molds, and a metal melt already starts to freeze during or quickly after the pouring into the casting mold, it is recommended to build the pressure within 30 seconds at the most. If free areas of particularly small dimensions are to be filled with metallic melt in the casting mold, it is expedient to build the pressure within 100 milliseconds, preferably within 10 to 50 milliseconds. A pressurized intensified cooling of the melt can thus take place shortly after the pouring, and it is possible to avoid a local freezing that would make a further feeding of metallic melt more difficult or would prevent it.
Basically all known methods of pressure generation are suitable to be applied within the scope of the invention. But if a particularly fast pressure build-up is required, e.g., because there are fine capillaries in the casting mold which are to be filled up with liquid metal, it is extremely expedient to generate the pressure by igniting one or more charges of explosive. Alternatively, a quick pressure build-up can also be achieved by heating a liquid inert gas in a closed volume.
For the complete freezing of the melt, the casting mold can be inserted into a liquid coolant, in particular into a gas liquid at −100° C. or a lower temperature. Heat can be removed in a particularly effective manner by a liquid coolant which is optionally additionally kept moving, and a rapid freezing can thus be achieved. For the reasons mentioned, it is hereby particularly advantageous if the entire surface of the casting mold is brought into contact with the coolant.
It is also advantageous in terms of the method if the melt is brought into a completely frozen state within 300 seconds at the most after the pressure build-up. Through a rapid freezing of the metallic melt carried out in this manner, a particularly fine structure can be adjusted, or a molded part with excellent mechanical properties can be obtained. If a gas is used for pressurization, furthermore, a time period is minimized during which gas can dissolve in the melt.
When carrying out a method according to the invention, it is further advantageous if the pressure is at least 200 bar, preferably at least 800 bar. The higher a pressure is, the more easily hollow spaces occurring during the freezing can be eliminated, and the better liquid metal can be pressed into the casting mold or the freezing metal can be consolidated.
In a further variant of a method according to the invention it is provided that the melt is acted upon with ultrasound and/or an alternating voltage during the freezing. On the one hand, an impingement of this type causes the formation of a very fine structure. On the other hand, ultrasound also counteracts a piping.
In order to ensure a transport of melt into the interior of the casting mold during the freezing in the best possible manner, it can be provided for the casting mold and/or the melt to be heated in areas in a gate area during the intensified cooling. It proves to be expedient to heat those areas of the casting mold through which melt is to flow further into the interior of the casting mold, whereas the other parts of the casting mold are cooled in an intensified manner.
The further object of the invention is attained by a device for precision-casting a metallic molded part or several metallic molded parts preferably of equal dimensions, comprising a casting mold and a melt container via which metallic melt can be inserted into the casting mold, as well as a cooling device for the casting mold, whereby the cooling device is housed in a pressure-resistant chamber.
Advantages of a device according to the invention can be seen in particular in that high-quality, void-free molded parts can be produced with a simple instrumental set-up, and that in addition a fine structure can be adjusted with the molded parts. A pressure resistance of the provided chamber makes it possible to highly pressurize the casting mold after metal melt has been inserted into it and thus, on the one hand, to fill the casting mold completely and, on the other hand, to let the metal melt freeze under pressure. Since an intensified cooling can take place at the same time, high-quality molded parts with excellent mechanical characteristic values 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 that exist as a liquid phase between −100° C. and −200° C. It is thus rendered possible to efficiently cool a large area of the casting mold. If required, it is also possible to cool merely a part of the casting mold by inserting the casting mold only partially into the container of the cooling device. In this context it is expedient for a simple handling if the casting mold can be traversed into the container.
In a further embodiment of a device according to the invention it can be provided for the chamber to have at least two areas which are separated by a lock and can be evacuated or pressurized independently from one another. This has proven to be advantageous in particular with a use of liquid coolants, as coolant can thus be prevented from getting into the vacuum apparatus during the pouring under vacuum or with a degassing of the metallic melt, which can cause the vacuum apparatus to fail, but at any rate can result in a worse vacuum and thus in a higher proportion of dissolved gas in the melt.
Further advantages and effects of the invention are revealed by the context of the specification and the following exemplary embodiments.
The invention is described in further detail below on the basis of exemplary embodiments.
They show
According to the method presented above, impellers were cast from an aluminum casting alloy, namely G-AlSi 12 (aluminum casting alloy with 12% by weight silicon). From a corresponding raw material a metal melt was thereby produced, and this was degassed in a vacuum. Subsequently, the metal melt produced in this manner was poured into a cluster mold in the same chamber in which it was degassed. Once the cluster mold had been filled with metal melt up to the ingate, the metal melt filled into the cluster mold, poured at a temperature of 600° C., was left to cool, until a solid film or a continuous metal skin of 500 μm thickness had formed at the surface of the metal melt in the area of an ingate of the cluster mold. Subsequently, the vacuum was lifted and a pressure of 500 bar was built by means of argon. The time between inserting the argon into the chamber and achieving a final pressure of 500 bar was 25 seconds. At this pressure the melt was left to freeze for additional 45 seconds and made to freeze completely under pressure by lowering the cluster mold into an oil bath with a temperature of 25° C. After the cluster mold had cooled to ambient temperature, it was taken out of the oil bath under normal pressure, and the ceramic was removed. Tests of impellers produced in this manner showed that they were embodied essentially without voids and, in cross-sectional view, had a homogeneous, fine-grained structure.
A holding and displacement device 6 for a cluster mold 10 is provided in the upper compartment 1a of the device 1. As shown in
Furthermore, a melt container 12 is provided in the compartment 1a, which container serves to hold or store metal melt S. As shown, the melt container 12 can additionally have an insulation 13, in order to avoid an undesired freezing of the metal melt S in the melt container 12. If the melt container 12 is housed in the compartment 1a together with the cluster mold 10, this has the advantage that the melt can be degassed by applying a vacuum via the outlet 51, and the melt S can be poured into the cluster mold 10 directly after a degassing. On the one hand, this simplifies conducting the method and, on the other hand, it ensures that the melt S cannot come into contact with a gas any more after a degassing, as could happen if the melt S is degassed outside the device 1 and subsequently has to be inserted into the compartment 1a. Even if a degassing of the melt S in the compartment 1a is preferred, it is naturally also possible within the scope of the invention to degas the melt S outside of the compartment 1a or outside of the device 1 and to insert it into the compartment 1a only afterwards.
The cluster mold 10 which, as mentioned, is rigidly connected to the fixtures 81, 82 in the area of an inlet piece 9 is made of a ceramic and can be produced in a customary manner. Depending on the shape of the molded part to be produced or the molded parts to be produced, ceramic elements 11 are also provided in its interior, which elements determine the geometry of the final molded part or act in a shaping manner.
Further auxiliaries can be provided in the compartment 1a, with which auxiliaries the cluster mold 10 can be closed in the area of its inlet opening 91. Auxiliaries of this type can comprise, e.g., a vertically adjustable plate, the diameter of which corresponds to the diameter of the inlet opening 91, and which can be raised or lowered hydraulically or pneumatically.
The compartment 1b accommodates a coolant container 14, in which there is a coolant K. If a pressurized melt filled into the cluster mold 10 is to be made to freeze quickly, the elements 33, 34 of the lock are traversed outward, and the lock is opened, so that the cluster mold 10 can be moved into the coolant K by means of the telescoping arms 7. It is thereby expedient to move the cluster mold 10 into the coolant K only so far that a rapid freezing is provided for in the areas forming the molded part or the molded parts later on, whereas the inlet piece 9 of the cluster mold 10 is not moved into the coolant K. This procedure ensures that the melt freezes more slowly in the area of the inlet piece 9, and that because of a pressure applied, further liquid metal can be pressed into the individual elements of the cluster mold 9 that are cooled in an intensified manner and in which the melt freezes from the outside inward. At the same time and for the same purpose, it is also possible to heat in the area of the ingate 91 and the supply lines connected thereto or in the gate area. In order to achieve a cooling that is as fast and effective as possible, it is expedient to move the coolant K, e.g., by means of a stirrer. It is also possible to improve a cooling effect and thus to increase the fineness of a structure if the container 14 made of steel is positioned on a solid metal block 15 so that there can be a good heat exchange between the coolant K and the surroundings of the device 1.
The device shown in
The present application claims priority under 35 U.S.C. § 119 of Austrian Patent Application No. A 570/2006, filed Apr. 4, 2006, the disclosure of which is expressly incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| A 570/2006 | Apr 2006 | AT | national |
