EP2098314B1 - Method and apparatus for producing metallic casting moulds using the lost wax casting method - Google Patents

Description

The present invention relates to a method according to claim 1 and a device according to claim 6 for the production of metal castings by precision casting, in particular cast aluminum or aluminum-containing alloys. Advantageous developments are specified in the subclaims.

Methods for producing metal castings by the precision casting method are known from the prior art. For the production of castings by means of the investment casting process, a wax model is usually produced by an object to be cast, which is then coated with a ceramic shell. This can be done, for example, by immersing the wax model in suitable ceramic slip. In this case, different slips can be applied successively. Subsequently, the wax model is melted out, and fired the ceramic form. As a result, a porous mold is obtained, in which metallic melts can be cast. After solidification of the metallic melt, the mold is destroyed and the casting can be removed. The method described above also allows the production of complex castings.

A disadvantage of the casting technique described is that the ceramic casting molds produced have a poor thermal conductivity and thus contribute to a relatively long and unregulated solidification time of the molten metal in the casting mold. However, due to slow solidification and cooling, depending on the metal to be cast, a relatively coarse-grained microstructure arises, which can lead to reduced mechanical properties.

To improve the low thermal conductivity of the ceramic molds is with the DE 36 29 079 proposed in the ceramic molds bag-shaped Incorporate inserts which are filled with steel gravel as a coolant before filling the molten metal. Due to its good thermal conductivity and high heat capacity, the steel gravel ensures improved heat dissipation from the casting mold.

The EP 1 076 118 A discloses a method and apparatus for producing directionally cured castings. It is envisaged to lower the cast body in a cooling bath to achieve a directional curing. The located above the cooling bath part of the mold is tempered with a heating chamber, which has electrical heating elements.

With the EP 0 571 703 It is proposed to immerse the porous ceramic mold after filling the metallic melt in a coolant bath, which has a porous ceramic mold wall gradually penetrating the cooling liquid whose boiling temperature is lower than the pouring temperature of the molten metal. In this case, the casting mold is continuously immersed starting from one end into the coolant, in such a way that the solidification front forming the interface between the melt and already solidified metal and the penetration region in which the casting wall is penetrated by the cooling liquid over its thickness , Move substantially in the direction of the free surface of the melt, and the dipping speed of the mold is selected in the cooling liquid so that seen in the direction of movement of the solidification front lags the penetration range of the cooling liquid of the solidification front. In this case, it can also be provided that the casting mold and thus also the melt introduced into the casting mold are heated above the cooling liquid level by means of an electrical resistance heater in order to keep the filled molten metal liquid above the cooling liquid level. In this case, however, it is necessary that in order to avoid reactions of the cooling liquid used with atmospheric oxygen, the system is flushed by means of a protective gas at overpressure. This requires a suitably closed container. However, the method known from the prior art has drawbacks such that in the electrical resistance heating based on radiation, the desired temperature level for securely holding the molten metal above the level of the cooling liquid can be kept difficult or even not achieved if the plant dimensions go beyond a critical limit measure or the grape geometry is such that radiation dead inner areas arise, ie those areas of the mold, which are shielded by other mold areas of the radiated by the electrical resistance heating radiant heat. This is a significant limitation of the process in terms of the size of the castings to be cast and their geometry given. In addition, the injection of inert gas leads to an additional cooling effect.

It is therefore the object of the present invention to provide a method and a device with which known from the prior art Disadvantages in the production of metal castings after the investment casting process, especially in terms of size and the casting geometry to be overcome.

This object is achieved with respect to the method by a method for producing a metal casting by the investment casting method, wherein the casting metal or alloy to be cast is poured into a ceramic mold with porous walls and the mold for cooling and solidification of the melt from one end Starting from constantly immersed in such a way in a coolant, that as the interface between the melt and already solidified metal solidifying front lags the coolant level, which is characterized in that the lying still above the coolant level region of the mold by means of a heat transfer gas to a temperature above the Solidus temperature of the metal or alloy to be cast is heated.

The heat transfer gas to be used in this case contains an exothermically oxidizable gas and oxygen. Suitable exothermic oxidizable gases are, for example, hydrogen, gaseous hydrocarbons such as methane, ethane, propane, butane, ethene, acetylene, or else carbon monoxide, and mixtures of these.

In addition, the heat transfer gas may contain other gases such as noble gases, halogenated hydrocarbons, ammonia, nitrogen, carbon dioxide, sulfur halides or mixtures thereof. In this case, the further gases containing the heat gas can serve as protective or inert gases in order to prevent diffusion of hydrogen into the molten metal or the cast body.

The exothermic oxidizable gas contained in the heat transfer gas is directly oxidized within a heating hood, whereby the region of the casting mold located above the cooling liquid level is heated.

According to the invention, it is provided that after the exothermic oxidation of the exothermic oxidizable gas contained in the heat carrier gas, the heat carrier gas is removed from the heating hood together with possibly evaporated coolant constituents and fed to afterburning. In this case, it may be provided that the exhaust gases resulting from the afterburning are fed to a heat exchanger which preheats at least part of the heat transfer gas to be supplied fresh to the heating hood. As a result, the thermal utilization of the energy content of the exothermic oxidizable gas is maximized, thereby the need for exothermic oxidizable gas in the heat transfer gas is reduced.

In the afterburning itself not completely oxidized residues of the exothermic oxidizable gas are burned in the heat carrier gas together with any evaporated components of the coolant to avoid discharge of not yet oxidized gas or coolant components. After the thermal utilization of the energy content in the heat exchanger, the exhaust gases of the afterburning can be supplied to a suitable exhaust gas purification system.

According to the invention it can be provided that the temperature is determined within the heating hood and used as a controlled variable for the content of exothermally oxidizable gas in the heat carrier gas or the content of other gases in the heat transfer gas.

In one embodiment of the method according to the invention, the exothermic oxidizable gas is burned by burners in the course of a direct oxidation within the heating hood. In a further embodiment of the invention, a heating hood has at least two burners or burner levels arranged one above the other. The individual burners can be controlled separately and can be clocked depending on the operating state of the heating hood. In addition, the combustion ratio (lambda) of the individual burners can be set differently. In a further embodiment of the method according to the invention, the combustion ratio lambda a lower burner level <1.0, the burners are thus operated with a substoichiometric gas / oxygen ratio, whereas the combustion ratio lambda the upper burner level> 1.0, ie the burner with be operated at a superstoichiometric gas / air ratio.

Such an embodiment of the method according to the invention makes it possible to dispense with afterburning of the exhaust gases discharged from the heating hood since, on the one hand, the surface of the coolant passing into contact with the heating hood is not oxidized due to the substoichiometric gas / oxygen ratio and, on the other hand, evaporating oxidizable constituents of the coolant bath in the Area of the upper, with a superstoichiometric gas / air ratio operated burner level directly oxidized in the course of combustion.

Also in the above-described embodiment of the method according to the invention, it may be provided that the exhaust gases discharged from the heating hood for further thermal utilization of a heat exchanger for preheating the heat transfer gas can be supplied.

According to the invention it is provided to select the coolant composition so that the risk of fire flashovers or deflagration is excluded.

With regard to the device, the object underlying the invention is achieved by a heating hood for carrying out a method according to the invention, which has an outer jacket, an insulating layer, burner and at least one outlet, wherein the burners burn a heat carrier gas containing exothermic oxidizable gas and the resulting exhaust gases are dissipatable via the at least one outlet, wherein the burners are arranged within the heating hood so that a substantially uniform heat distribution is ensured within the heating hood.

In one embodiment of the heating hood according to the invention, the burners are arranged in at least two levels within the heating hood. The burners are each individually controllable. In this case, individually controllable means that the burners can be switched on and off individually as well as with regard to the composition and the volume flow of the gas mixture supplied to them.

In a further embodiment of the heating hood according to the invention, this is designed so that it can be integrated in a known from the prior art container for receiving a coolant in a precision casting process, for example by integration of the heating hood in the lid of such a container.

The heating hood according to the invention may have a post-combustion zone in which unburned constituents of the exothermically oxidizable gas reacted in the burners and / or any other gaseous constituents of the exhaust gas discharged from the heating hood are after-burned. In this case, the post-combustion zone can either be located directly inside the heating hood or a separate post-combustion zone can be provided outside the heating hood.

Furthermore, gas inlets can be provided within the heating hood, via which further gases such as protective or inert gases can be introduced into the heating hood.

In the heating hood according to the invention, the exhaust gas for transmitting at least a portion of the thermal energy contained can be performed via a heat exchanger in one embodiment. The waste heat can serve both for preheating the combustion air required in the burners, the exothermic oxidizable gas or the additional heating gas optionally supplied via the additional gas inlets further gases. In this case, the entire gas flow does not necessarily have to be heated, but instead a bypass line can be provided for bypassing the heat exchanger. In such an embodiment, the heating hood on mixing valves for adjusting a ratio between the gas passed through the heat exchanger and cold gas.

The heating hood according to the invention may also have temperature sensors for detecting the temperature within the heating hood. The temperature sensor can pass a measurement signal corresponding to the temperature to a control device which controls the burners and / or the mixing valves for mixing cold and preheated gas as a function of the temperature determined in the heating hood.

Fig. 1:

zeigt einen Querschnitt durch eine erfindungsgemäße Heizhaube.

Fig. 2:

zeigt einen Querschnitt durch eine in einen Kühlmittelbehälter integrierte erfindungsgemäße Heizhaube.

Fig. 3:

zeigt einen Querschnitt einer erfindungsgemäßen Heizhaube, welche eine außerhalb der Heizhaube angeordnete Nachverbrennungszone sowie einen Wärmetauscher aufweist.

Fig. 4:

zeigt eine erfindungsgemäße Heizhaube im Querschnitt, in welcher die Brenner in einem von der horizontalen Ausrichtung abweichenden Winkel ausgerichtet sind.

Fig. 5:

zeigt den Querschnitt in horizontaler Ebene einer erfindungsgemäßen

Heizhaube, in welcher die Brenner nicht radial auf den Mittelpunkt der Heizhaube, sondern quasi tangential in abweichendem Winkel angeordnet ist.In addition, according to the invention, further measuring devices such as, for example, a lambda probe for determining the composition of the exhaust gases discharged from the heating hood according to the invention can be provided, which is likewise connected to the control device, so that the measured values determined by these measuring devices serve as a control variable for the control of the burners / or the mixing valves can serve.

Fig. 1:

shows a cross section through a heating hood according to the invention.

Fig. 2:

shows a cross section through an integrated into a coolant tank inventive heating hood.

3:

shows a cross section of a heating hood according to the invention, which has a arranged outside the heating hood post-combustion zone and a heat exchanger.

4:

shows a heating hood according to the invention in cross section, in which the burners are aligned at an angle deviating from the horizontal orientation.

Fig. 5:

shows the cross section in the horizontal plane of an inventive

Heating hood, in which the burner is not arranged radially to the center of the heating hood, but quasi tangent at a different angle.

Fig. 1 shows a heating hood 1 according to the invention for use in a precision casting process of the type described above. The heating hood 1 has an outer jacket 2, which is equipped on the inside with a refractory insulating layer. The outer shell 2 may be made of steel or other sufficiently temperature-resistant materials. The refractory insulating layer 3 may be formed, for example, of a ceramic coating or Schamottausmauerung. The heating hood 1 according to the invention has burners 4, with which an exothermic oxidizable gas can be burned. The burners 4 can in this case be arranged in two superimposed planes. In such an embodiment, the burners in the different levels can be operated with different combustion ratios lambda. This makes it possible to create a post-combustion zone in the upper region of the heating hood, in which possibly not oxidatively reacted gas or vaporized components of the coolant are burned. The resulting within the heating hood combustion exhaust gases are discharged through outlet 6.

Fig. 2 shows a heating hood according to the invention, which is arranged within a coolant tank 13. The heating hood is integrated into the lid 18 of the coolant tank. The coolant tank 13 contains a coolant 15, which can be supplied or removed via inlet 14 and outlet 16. In addition, the coolant tank 13 can have auxiliary connections 17, through which, for example, protective gas can be introduced into the coolant tank 13, or which can serve to ventilate the coolant tank 13. The liquid level of the coolant 15 within the coolant tank 13 may be in the in Fig. 2 shown configuration through inlet 14 and outlet 16 are set so that the coolant level is flush with the lower edge of the heating hood.

Fig. 3 shows a further embodiment of a heating hood 1 according to the invention, which in addition to the burners 4 gas inlets 9, through which additional gases such as inert or protective gases can be introduced into the heating hood. Above the outlet 6, a post-combustion zone 5 is provided, to which a heat exchanger 7 is connected. In the heat exchanger 7, the gas mixture supplied to the burners 4 and / or the gas which can be introduced into the heating hood via the gas inlets 7 can be preheated.

In addition, the in Fig. 3 shown heating hood 1 temperature sensor 20, by means of which the temperature prevailing in the heating hood temperature can be determined. Advantageously, a plurality of temperature sensors 20 are provided in different areas of the heating hood 1. The temperature sensor 20 can be connected to a control unit which controls the burner 4 and / or mixing valves for adjusting the ratio of preheated by the heat exchanger 7 gas and cold gas and / or control valves for controlling the additional gas input depending on the temperature prevailing in the heating hood , As a result, an exact temperature control within the heating hood 1 according to the invention is possible.

Due to the inventive design of a heating hood, the mold to be arranged in the heating hood is heated uniformly by means of a heat transfer gas, so that radiation-dead spaces are heated sufficiently. In addition, the adjustable gas flow within the heating hood according to the invention ensures that any evaporating constituents of the coolant, which optionally have a reaction potential to the still liquid metal in the mold, are discharged as quickly as possible.

The FIGS. 4 and 5 show further embodiments of the heating hood according to the invention in which the burner 4 can be aligned in a deviating from the horizontal orientation angle of 3 to 10 °. Likewise, it may be provided according to the invention, to arrange the burner tangentially to the hood center. Such a design of the heating hood according to the invention, a flow of the heat transfer gas is achieved within the heating hood, which contributes to a uniform heat distribution.

LIST OF REFERENCE NUMBERS

1

heating cover

2

outer coat

3

insulating

4

burner

5

afterburning

6

outlet

7

heat exchangers

9

gas inlets

13

container

14

Intake

15

coolant

16

procedure

17

shunt

20

temperature sensor

Claims (15)

1. A method for producing a metallic casting in accordance with the lost-wax casting process, wherein the metal to be cast or the alloy to be cast is poured into a ceramic casting mould with porous walls and the casting mould is from one end onward steadily immersed in a cooling medium in order to cool and solidify the molten metal, namely in such a way that the solid-liquid interface being formed as boundary surface between the molten metal and the already solidified metal lags the cooling medium level, characterized in that the region of the casting mould that still lies above the cooling medium level is heated to a temperature above the solidus temperature of the metal or the alloy to be cast within a heating mantle by means of a heat transfer gas, wherein the heat transfer gas contains oxygen, as well as an exothermally oxidizable gas.
2. The method according to Claim 1, wherein the heat transfer gas contains an exothermally oxidizable gas in the form of a gas that is selected from the group consisting of hydrogen, gaseous hydrocarbons, carbon monoxide or mixtures thereof.
3. The method according to one of Claims 1 or 2, wherein the heat transfer gas contains at least one other gas of the group consisting of noble gases, halogenated hydrocarbons, ammonia, nitrogen, carbon dioxide, sulphur halides or mixtures thereof.
4. The method according to Claim 1, wherein the region of the casting mould that still lies above the cooling medium level is heated by means of a heating mantle, in which the exothermally oxidizable gas in the heat transfer gas is exothermally oxidized.
5. The method according to Claim 4, wherein the gaseous reaction products created due to the exothermal oxidation are withdrawn from the heating mantle and fed to a post-combustion.
6. A heating mantle for carrying out the method according to one of Claims 1 to 5, featuring an outer mantle (2), an insulating layer (3), burners (4) and at least one outlet (6), wherein a heat transfer gas that contains an exothermally oxidizable gas can be burned in the burners (4) and the waste gases created during this process can be discharged through the outlet (6), wherein the burners (4) are arranged within the heating mantle (1) in such a way that a uniform heat distribution within the heating mantle is ensured, and wherein this heating mantle features a post-combustion zone (5) for the post-combustion of the waste gases discharged from the heating mantle (1).
7. The heating mantle according to Claim 6, wherein the burners (4) are arranged within the heating mantle (1) on at least two levels.
8. The heating mantle according to one of Claims 6 or 7, wherein the burners (4) can be individually controlled.
9. The heating mantle according to one of Claims 6 to 8, wherein the heating mantle is integrated into the cover of a container (13) for accommodating a cooling medium (15).
10. The heating mantle according to one of Claims 6 to 9, wherein gas inlets (9) are provided within the heating mantle and inert gas can be introduced into the heating mantle through these gas inlets.
11. The heating mantle according to one of Claims 6 to 10, featuring a heat exchanger (7) for transferring at least part of the thermal energy contained in the waste gases discharged from the heating mantle (1) to the heat transfer gas.
12. The heating mantle according to Claim 11, wherein a bypass line is provided for bypassing the heat exchanger.
13. The heating mantle according to Claim 12, wherein mixing valves are provided that make it possible to adjust the ratio between heat transfer gas conveyed through the heat exchanger (9) and cold heat transfer gas.
14. The heating mantle according to one of Claims 6 to 13, featuring temperature sensors (20) for measuring the temperature within the heating mantle (1).
15. The heating mantle according to Claim 14, wherein the temperature sensors (20) are connected to a control unit that controls the burners (4), as well as the mixing valves for adjusting the ratio between heat transfer gas conveyed through the heat exchanger (9) and cold heat transfer gas, in dependence on the temperature measured within the heating mantle (1) by the temperature sensors (4).

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