METHOD FOR MAKING A MOLD FOR CASTING METALLIC MELTS

Abstract

The invention relates to a method for producing a mould for casting metallic melts, in particular for casting titanium, titanium alloys or intermetallic titanium aluminides. Said method consists of the following steps: a contact layer is produced by applying a first slicker containing a first metal oxide powder as an essentially solid component to a moulded core, a first sanding layer is produced on the contact layer formed from the first slicker by sanding with a second metal oxide powder and the layer sequence formed from the contact layer and the first sanding layer is radiated with infrared light for a predetermined period of time. According to the invention, for speeding up the drying process, a first dry mass of the first slicker contains a hydraulic binder.

Description

The invention relates to a method for making a mold for casting metallic melts, in particular for casting titanium, titanium alloys or intermetallic titanium aluminides as defined by the preamble of claim 1.

Such a method is known from EP 1 645 348 B1. According to the known method, it is provided that IR radiation is used to support the drying of a slicker layer applied onto a mold core. The drying of the slicker layer can be accelerated with this.

With these kinds of molds, a plurality of slicker and sanding layers are applied one after the other. It is thereby necessary respectively to dry each newly applied layer sequence of slicker. and sanding layers before the next layer sequence is applied. The drying takes longer the more layer sequences there are. The making of a mold with several layer sequences requires in total a great amount of time of more than 3 hours.

The object of the invention is to specify a method with which the time required to make molds for the casting of metallic melts can be further reduced.

This object is solved by the features of claim 1. Useful embodiments result from the features of claims 2 to 28.

According to the invention it is provided that a first dry mass for making the first slicker contains a hydraulic binder. Hydraulic binders set when absorbing water. They thus withdraw water from the contact layer and harden this at the same time. The firmness of the contact layer is increased by providing the hydraulic binder. Due to this, the mold can be built up with a smaller number of layer sequences. In particular, this can significantly speed up the making of a mold made up of a plurality of layer sequences.

In the sense of the present invention, a “hydraulic binder” is understood to mean a mixture of substances which hydrates with water. The hydration products formed by this cause the solid components contained in the slicker to harden.

According to an advantageous embodiment, the first dry mass contains 1 to 30 wt. %, preferably 8 to 17 wt. %, particularly preferably 9 to 13 wt. %, of the hydraulic binder. The hydraulic binder is usefully a calcium aluminate cement. Such a hydraulic binder is chemically inert for the most part. When a calcium aluminate cement is used, molds can be made with which highly reactive metallic melts, in particular intermetallic titanium aluminide melts can be cast. It has thereby been proven to be advantageous that 1 to 15 wt. % of the hydraulic binder is contained in the first dry mass.

According to a further advantageous embodiment, a grain band of the first metal oxide powder has a range of 0 to 50 μm and advantageously a medium grain size (d₅₀) in the range of 8 to 20 μm. The second metal oxide powder used to make the first sanding layer can have a medium grain size in the range from 130 to 200 μm. A layer sequence made from the contact layer and the first sanding layer is then dried inter alia with infrared light for a specified amount of time. The hydraulic binder provided by the invention contributes to an improved hardening of the mold.

According to a further embodiment, it is provided that the first and/or second :metal oxide powder is formed from at least one metal oxide which is selected from the following group: Y₂O₃, CeO, MgO, Al₂O₃. The first dry mass can thereby usefully contain less than 95 wt. %, preferably less than 90 wt. %, of the first metal oxide powder. The first dry mass usefully contains 40 to 70 wt. % of the first metal oxide powder.

If Y₂O₃ is used as the first metal oxide powder, an excellent resistance to highly reactive metallic melts, in particular intermetallic titanium aluminide melts, can be achieved. Such a mold is particularly suitable for making titanium aluminide components. In this case, the first dry mass usefully contains at the most 15 wt. % of the hydraulic binder. The first dry mass is usefully free of MgO—except for unavoidable impurities. This can be used to achieve a particularly good resistance to the previously mentioned highly reactive metallic melts.

According to a further embodiment feature, a coating layer is made surrounding the layer sequence from contact and first sanding layer or several layer sequences. The coating layer can contain MgO as the essential component. Moreover, a second dry mass for making the coating layer can contain a hydraulic binder, preferably calcium aluminate cement. The calcium aluminate cement advantageously contains 60 to 80 wt. % of Al₂O₃, preferably approximately 70 wt. % of Al₂O₃. A second dry mass usefully contains at least 40 wt. % of MgO, preferably 60 to 80 wt. %, as well as at least 20 wt. % of the hydraulic binder. Moreover, the second dry mass can contain at least 1 wt. % of one or more of the following oxides: Fe₂O₃, SiO₂, CaO, Al₂O₃. The coating layer is essentially used to mechanically stabilize the layer sequence. It can have a significantly greater layer thickness than the layer sequence. A mold having the suggested coating layer is particularly suitable for using the centrifugal casting method.

It has been proven to be particularly useful to apply an intermediate layer sequence formed from an intermediate and a second sanding layer onto the layer sequence formed from contact and first sanding layer before making the coating layer. This can improve the thermo shock resistance of the mold.

The contact layer is usefully applied using the injection method. The intermediate layer can also be applied using the injection method or also using the dipping method. By applying the contact and/or intermediate layer using the injection method, the respective layers can be applied with a particularly low layer thickness and improved surface quality at the same time. This can further accelerate the drying of the respective layers.

The second slicker can contain a first MgO powder as the essential solid component. Moreover, the second slicker can contain a hydraulic binder, preferably a calcium aluminate cement. The calcium aluminate cement can in turn contain 60 to 80 wt. % of Al₂O₃, preferably approximately 70 wt. % of Al₂O₃.

A third dry mass for making the second slicker can contain at least 50 wt. % MgO, preferably 60 to 70 wt. %, and at least 20 wt. % of the hydraulic binder. The second sanding layer is usefully made by applying the second MgO powder onto the intermediate layer.

There can be several such intermediate layer sequences, each of which is formed from an intermediate layer and a second sanding layer, applied one after the other onto the layer sequence of contact and first sanding layers. Several layer sequences of contact and first sanding layers can also be provided. The second sanding layer is usefully made by applying a second MgO powder onto the intermediate layer. The first MgO powder can thereby have a smaller medium grain size than the second MgO powder.

The suggested intermediate layer sequence contributes to an improved thermo shock resistance of the mold. When the intermediate layers also contain a hydraulic binder, they can also be made quickly and efficiently, particularly using the dipping method.

With regard to the respective application, it has proven useful that the first and/or third dry mass and/or the sanding layer/layers contains/contain at least 1 wt. % of one of the following oxides: CeO₂, La₂(D₃, Gd₂O₃, Nd₂O₃, TiO₂. The addition of other oxides of the rare earths is also possible.

According to a particularly advantageous embodiment, a moisture content of the layer and/or the intermediate layer sequence is reduced to a specified value after it is made. The specified value can be in the range from 10% to 60% of the residual moisture. It is preferably less than 20%, but more than 5% residual moisture. In other words, the contact and/or intermediate layer is advantageously not completely dried before fore applying the first or second sanding layer. This significantly accelerates the making of the mold. Surprisingly, it has been shown that the suggested retention of a residual moisture also results in molds having an excellent firmness. When the suggested residual moisture is retained when making the mold, the number of intermediate layers and second sanding layers can be significantly reduced or such layers do not even need to be provided. When the mold is made with retention of the suggested residual moisture, surprisingly such a good firmness of the contact and/or intermediate layer can be achieved that also here the addition of a hydraulic binder is not needed. To this extent, a method for making a mold using the setting of the previously mentioned residual moisture in the contact and/or intermediate layer is viewed as an independent invention. With such a method, infrared radiation can also be used for drying or setting the residual moisture.

In case a hydraulic binder is used, it must be ensured that a sufficient residual moisture is retained in the layer sequence or the intermediate layer sequence so that the hydraulic binder is able to completely set. The water consumption caused by the setting of the hydraulic binder contributes to an acceleration of the drying and firming process of the respective layer or intermediate layer sequence as well as of the coating layer.

A drying time or a time for setting the residual moisture is usefully less than 25 minutes, preferably 5 to 15 minutes per layer or intermediate layer sequence.

In particular with regard to the mechanical and chemical stability of the mold as well as its efficient manufacturing, it has proven to be useful that the fraction of the hydraulic binder in the first dry mass is less than that in the second or third dry mass. The fraction of hydraulic binder in the second and/or third dry mass is advantageously about at least 2 wt. %, preferably at least 5 wt. %, greater than in the first dry mass.

Furthermore, it has proven to be useful that the first and/or second slicker has/have a viscosity of 1000 mPas at the most, preferably between 450 and 750 mPas. Slickers with such a viscosity can be particularly well processed using the injection method.

According to further embodiment features of the method, after the coating layer has been made, the mold core is removed by melting or burning out the material forming the mold core. The material is usefully wax or similar. Melting the material forming the mold core and/or an additional drying of the layer and/or intermediate layer sequence can also be accomplished with microwaves.

A green body formed after the removal of the mold core is usefully sintered at a sintering temperature of more than 800° and less than 1200° C. The suggested use according to the invention of a hydraulic binder to make the mold contributes to a significant reduction of the sintering temperature in comparison to methods as per prior art.

Examples will now be used to describe the invention in more detail.

The sole figure schematically shows a partial cross sectional view through a mold according to the invention. A contact layer 1 contains 85 wt. % Y₂O₃ and 15 wt. % of hydration products of the calcium aluminate cement, for example. Reference sign 2 designates a first sanding layer which is composed essentially of a further Y₂O₃ powder having a medium grain size of approximately 150 μm. The contact layer 1 and the first sanding layer 2 form one layer sequence A.

An intermediate layer 3 is applied onto the first sanding layer 2, which intermediate layer 3 contains MgO as the essential component which in turn is set by the reactive product of a calcium aluminate cement. Reference sign 4 designates a second sanding layer which is made from an MgO powder. Reference sign B designates an intermediate layer sequence made of alternating layers of intermediate layer 3 and second sanding layer 4. The intermediate layer sequence B is lined on the back with a coating layer 5 which contains MgO as the essential solid component which in turn is set by a hydraulic binder, preferably calcium aluminate cement.

Similar to the intermediate layer sequence B, the layer sequence A can also be formed from a sequence of several contact layers 1 as well as first sanding layers 2 in alternating sequence.

EXAMPLE 1

To make the contact layer 1, a first slicker is made first whose first dry mass contains 80 to 90 wt. % of a first Y₂O₃ powder. The medium grain size d₅₀ of the Y₂O₃ powder is usefully 10 to 15 μm. The modal value is advantageously 15 to 25 μm. The grain band is usefully located in the range between 0 and 50 μm. Moreover, the first dry mass usefully contains 10 to 20 wt. % of a calcium aluminate cement. Adding a suitable amount of water creates a first slicker having a viscosity in the range from 400 to 700 mPas. The first slicker is injected using the injection method onto a mold core made, for example, of wax. Then the contact layer 1 made from the first slicker is sanded with a first sanding layer 2. The first sanding layer 2 consists essentially of a second Y₂O₃ powder which has a medium grain size in the range from 170 to 200 μm. The thus made layer sequence A is then dried, for example under IR action for a time period of approximately 10 to 20 minutes, preferably up to a residual moisture of 10 to 30%.

The previously mentioned method for making the contact layer 1 as well as the first sanding layer 2 can be repeated multiple times, for example three to five times. The layer sequence A can also be concluded by a contact layer 1 instead of the first sanding layer 2. It can also be that a first sanding layer concluding the layer sequence A is essentially made from an MgO powder. With regard to its grain size distribution as well as its medium grain size, this MgO powder can be made like the second Y₂O₃ powder.

A second slicker is made to make one or more intermediate layers 3 of the intermediate layer sequence B. A second dry mass for making the second slicker contains, for example, 65 to 80 wt. %, preferably 65 to 70 wt. %, MgO as well as 20 to 35 wt. %, preferably 30 to 35 wt. % of a calcium aluminate cement. By adding a suitable amount of water, a second slicker is made whose viscosity is usefully set in such a manner that it enables a coating using the conventional dipping method. To make the intermediate layer 3, the layer sequence A applied onto the mold core is coated with the second slicker using the dipping method. Afterwards, the second sanding layer 4 is applied which is essentially formed from an MgO powder having a grain size in the range from 0.1 to 2.0 mm. After making the second sanding layer 4, the thus formed intermediate layer sequence B is usefully dried for a duration of 15 to 20 minutes under IR action. Also a residual moisture in the range from 10 to 30% can be set in the intermediate layer sequence B. Then further intermediate 3 and second sanding layers 4 can be applied in the same way. Finally, the intermediate layer sequence B can be dried for a time period of 10 to 25 minutes under IR action.

To make the coating layer 5, a still flowable mass is made from 60 to 80 wt. %, preferably 70 to 80 wt. % of MgO, 20 to 40 wt. %, preferably 20 to 30 wt. % of calcium aluminate cement as well as water and auxiliary substances. The coated mold core is then placed in a mold and the flowable mass is poured over it. After drying the mass preferably under IR action, the mold core is removed by increasing the temperature. Then the thus formed green body is sintered at a temperature in the range from 1000 to 1200° C., preferably in the range between 1100 and 1200° C.

EXAMPLE 2

In this case, a first dry mass for making the first slicker contains 60 to 70 wt. % of Y₂O₃ and 10 to 20 wt. % of CeO₂. The grain band of the mixture is between 0 and 50 μm. Moreover, the first dry mass contains 10 to 20 wt. % of the calcium aluminate cement. The first dry mass is mixed with water so that a first slicker having a viscosity in the range from 300 to 600 mPas is formed.

Otherwise, the method is performed as discussed in example 1.

The drying of the layer sequence A and/or the intermediate layer sequence B can be supported by using IR radiation. The coated mold core can, preferably in addition, be led through a drying chamber in which the air temperature is in the range from 30 to 60° C.

In addition to the previously mentioned components, the first and/or second slicker can also contain conventional auxiliary substances, in particular the organic auxiliary substances in the usual amounts.

Claims

1. Method for making a mold for casting metallic melts, in particular for casting titanium, titanium alloys or intermetallic titanium aluminides, with the following steps:

making a contact layer by applying a first slicker onto a mold core, which first slicker contains a first metal oxide powder as an essential solid component,

making a first sanding layer on the contact layer formed from the first slicker by sanding with a second metal oxide powder,

irradiating the layer sequence formed from the contact layer and the first sanding layer with infrared light for a specified period of time,

characterized in that

a first dry mass of the first slicker contains a hydraulic binder.

2. Method as defined in claim 1, wherein the first dry mass contains 1 to 30 wt. %, preferably 8 to 17 wt. % of the hydraulic binder.

3. Method as defined in claim 1, wherein the hydraulic binder is a calcium aluminate cement.

4. Method as defined in claim 1, wherein a grain band of the first metal oxide powder has a range from 0 to 50 μm and advantageously a medium grain size (d50) in the range from 8 to 20 μm.

5. Method as defined in claim 1, wherein the second metal oxide powder has a medium grain size in the range from 130 to 200 μm.

6. Method as defined in claim 1, wherein the first and/or second metal oxide powder is formed from at least one metal oxide which is selected from the following group: Y2O3, CeO, MgO, Al2O3.

7. Method as defined in claim 1, wherein the first dry mass contains less than 90 wt. % of the first metal oxide powder.

8. Method as defined in claim 1, wherein a coating layer surrounding a layer sequence of contact and first sanding layer is made.

9. Method as defined in claim 1, wherein the coating layer contains MgO as the essential component.

10. Method as defined in claim 1, wherein a second dry mass for making the coating layer contains a hydraulic binder, preferably a calcium aluminate cement.

11. Method as defined in claim 1, wherein a second dry mass contains at least 40 wt. % MgO as well as at least 20 wt. % of the hydraulic binder.

12. Method as defined in claim 1, wherein the second dry mass contains at least 1 wt. % of one or more of the following oxides: Fe2O3, SiO2, CaO, Al2O3.

13. Method as defined in claim 1, wherein before making the coating layer, an intermediate layer sequence formed from an intermediate layer and a second sanding layer is applied onto the layer sequence formed from a first contact and first sanding layer.

14. Method as defined in claim 1, wherein the contact and/or the intermediate layer are applied using the injection method.

15. Method as defined in claim 1, wherein a second slicker contains a first MgO powder as the essential solid component.

16. Method as defined in claim 1, wherein the second slicker contains a hydraulic binder, preferable a calcium aluminate cement.

17. Method as defined in claim 1, wherein a third dry mass for making the second slicker contains at least 50 wt. % of MgO and at least 20 wt. % of the hydraulic binder.

18. Method as defined in claim 1, wherein the second sanding layer is made by applying a second MgO powder onto the intermediate layer.

19. Method as defined in claim 1, wherein the first and/or third dry mass and/or the sanding layer/layers contains/contain at least 1 wt. % of one of the following oxides: CeO2, La2O3, Gd2O3, Nd2O3, TiO2.

20. Method as defined in claim 1, wherein a moisture content of the layer and/or the intermediate layer sequence is reduced to a specified value after they are made.

21. Method as defined in claim 1, wherein the specified value is in the range from 10 to 60% residual moisture, preferably less than 20% residual moisture.

22. Method as defined in claim 1, wherein a drying time per layer or intermediate layer sequence is less than 25 minutes, preferably 5 to 15 minutes.

23. Method as defined in claim 1, wherein the fraction of the hydraulic binder in the first dry mass is less than in the second or third dry mass.

24. Method as defined in claim 1, wherein the fraction of the hydraulic binder in the second and/or third dry mass is greater than in the first dry mass by at least 2 wt. %, preferable by at least 5 wt. %.

25. Method as defined in claim 1, wherein the first and/or second slicker has/have a viscosity of not more than 1000 mPas, preferably between 450 and 750 mPas.

26. Method as defined in claim 1, wherein, after making the coating layer, the mold core is removed by melting or burning out the material forming the mold core.

27. Method as defined in claim 1, wherein melting the material forming the mold core and/or an additional drying of the layer and/or intermediate layer sequence is performed using microwaves.

28. Method as defined in claim 1, wherein a green body formed after the removal of the mold core is sintered at a sintering temperature of more than 800° C. and less than 1200° C.