EP1198438A1

EP1198438A1 - Silicon nitride materials and components made therefrom for processing molten light metal masses

Info

Publication number: EP1198438A1
Application number: EP00941967A
Authority: EP
Inventors: Gerhard WÖTTING; Martin Hagemeier; Wolfgang Müller; Leonhard Heusler; Lothar SCHÖNFELDER
Original assignee: Cfi Ceramics For Industry & Co KG GmbH; CFI CERAMICS FOR INDUSTRY GmbH
Current assignee: Kyocera Fineceramics Precision GmbH
Priority date: 1999-06-01
Filing date: 2000-05-19
Publication date: 2002-04-24
Also published as: WO2000073240A1; AU5674900A; JP2003500331A; KR20020025883A; DE19924896A1

Abstract

According to the invention, gassing agitators (impellers) for purifying molten metal masses are made from a silicon nitride material.

Description

Silicon nitride materials and components made from them for processing light metal melts

For cleaning metal melts, especially aluminum and aluminum alloys, finely divided gases are usually injected into the melt. So with liquid aluminum there is a need to remove the dissolved hydrogen, which would otherwise lead to pores on aluminum castings, such as motor vehicle rims, which on the one hand can reduce the stability of the casting and on the other hand are optically undesirable on the surface of the casting. The hydrogen and fine solid particles are removed by injecting argon, nitrogen or chlorine into the melt. The injected ascending gas bubbles remove the hydrogen via solution or, in the case of chlorine, via chemical binding processes. In technology, so-called impellers are used to stir the melt and at the same time distribute the gas. The impellers generally consist of an approximately 1000 mm long graphite tube to which a plate with gas distribution devices is attached at the bottom. The connections between graphite tube and plate can be e.g. using a trapezoidal thread. The service life of such graphite impellers is only a few weeks due to corrosive and abrasive attack. In addition to this economic aspect, there is

Risk of graphite dust or graphite particles being trapped in the castings made from the melt.

Rotors for this application have long been known and there are a large number of proposed design solutions (e.g. EP 0 396 267, US 3,982,913,

US 5 364 078, US 4 802 656, US 5 709 834). However, this mostly describes very complex geometries and, if a material is proposed at all, it is easy to process graphite with the disadvantages already described. DE 19 539 621 also suggests the use of Si ₃ N ₄ or SiAlON and discloses a relatively simple construction which is matched to materials which are difficult to machine. Practical tests showed however, that this construction is far from achieving the degassing effect of the graphite impellers available today.

For an improved material, there are therefore demands for high corrosion and abrasion resistance to the melts and slags, as well as sufficient thermal shock resistance to prevent damage when immersed in the hot melt. A high level of strength and thermal conductivity as well as a low modulus of elasticity and coefficient of thermal expansion are prerequisites for good thermal shock resistance. On the other hand, good strength is achieved through high fracture toughness and a small defect size.

Demands on the component to be manufactured consist of simple manufacture, a favorable design with regard to the size, weight and complexity of the part as well as the connection to the drive shaft. This results in a low cost for that

Component and with proven sufficient effectiveness to a high acceptance for a practical industrial application.

Although such thermal shock-resistant ceramics are often used in connection with light metal melting metallurgy in the form of simple mechanical ones

Structures such as pipes, e.g. are used as melt lifters and their suitability for use in light metal melts has been proven, mechanically complicated components made of ceramics have so far not been used to any appreciable extent due to the prohibitively high processing effort and thus the costs.

The object of the present invention is therefore to overcome the disadvantages and risks associated with the use of graphite impellers on the material side, with a large degassing effect being required.

It is also an object of the present invention to provide a gassing stirrer which is manufactured with minimal mechanical processing effort can be. In particular, the processing effort after sintering should be kept to a minimum.

A silicon nitride material was found which is characterized in that it consists of at least 75% by volume of β-Si ₃ N ₄ , less than 3% by volume of free Si, less than 5

Vol .-% porosity and the rest of a secondary phase consists of added conventional sintering additives, the material having a dip in an Al-based melt of 850 ° C after 100 h only a weight change of less than 1 wt .-% and at cyclical immersion tests in this melt after 500 cycles no detectable damage to the material occurs, characterized by the recording of resonance frequency spectra before and after the immersion treatment and a maximum permitted shift of resonance frequencies of <0.1%.

The material according to the invention is particularly suitable for the production of

Impellers with gas distribution devices for cleaning molten metals.

Surprisingly, it was found that the reaction-bound and resintered silicon nitride (SRBSN) represents an excellent solution for the given task as the material according to the invention. This material can be produced with comparatively inexpensive raw materials, can be processed in the stage of pre-nitriding with normal machining processes and, after sintering, achieves the desired range of properties with only low residual porosity.

Such material samples were characterized in Al-based melts at 850 ° C on the one hand in the form of a long-term immersion treatment with regard to corrosion resistance, and on the other hand by means of cyclical immersion tests with regard to the thermal shock behavior. These immersion tests showed that the material according to the invention is only slightly wetted by the Al melt and, after the sample has been removed from the melt, AI can be blown off using a gas stream. Criteria for a suitability of the material for the application in such melts were a change in weight of less than 1% by weight after 100 h of stationary exposure or a survival of 500 immersion tests in this melt with interim cooling by air blowing, without any damage being detectable.

The change in weight is characterized by simple weighing of the samples before and after immersion in the Al melt, with adhering Al and slag residues being removed by blowing off immediately after removal from the melt or by careful sandblasting. Thermal shock damage from the cyclical immersion tests is characterized by recording the resonance frequency spectra before and after the tests. Electrodynamic transducers in the form of a transmitter and receivers are attached to the sample or component to be tested and vibrations are generated and registered in part with frequencies between 0.1 and 2 MHz of the transmitter. This vibration spectrum is a "fingerprint" of the part and changes when the shape changes (breaking off) or damage occurs, e.g. Cracks. As a criterion for critical damage to the part, changes in the resonance frequency reflections of> 0.1% were specified.

The evaluation of various silicon nitride material qualities according to this method led to the conclusion that the evaluation criteria are met if the material consists of at least 75% by volume of β-Si ₃ N ₄ , the residual porosity is <5% by volume, at most 3% by volume of free Si and the remainder on 100% secondary phase. With less than 75% ß-Si ₃ N ₄ and more than 5% porosity, the thermal shock resistance is no longer guaranteed. More than 3 vol .-% free Si leads to inclusions in the material and reduces its strength. Correspondingly higher contents of the secondary phase than 20 vol.% Reduce the corrosion as well as the abrasion resistance and thereby lead to higher weight changes than the limit specified according to the invention. Particularly favorable corrosion, abrasion and thermal shock resistance are obtained if the material consists of at least 80 vol.% Β-Si ₃ N ₄ , less than 2 vol.% Free Si, 3 to 1 vol.% Porosity and as the remaining component of a secondary phase, which can be amorphous or partially crystalline. The phase contents are usually determined by quantitative microstructure analysis of cuts, supplemented by X-ray phase analysis.

Suitable sintering additives are all those combinations of materials which, at a higher temperature, form a molten phase with the oxygen present in the starting material, which is considered to be SiO ₂ , which permits liquid-phase sintering of the Si ₃ N ₄ -based shaped body. Such additives are MgO, CaO, Y ₂ O ₃ , La ₂ O ₃ , CeO ₂ , Nd ₂ O ₃ , TiO ₂ , ZrO ₂ , B ₂ O ₃ , Al ₂ O ₃ or compounds of these oxides in the form of, for example, MgAl ₂ O ₄ , Al ₂ TiO ₅ etc. and optionally also SiO ₂ . The combination and concentration of the individual additives must be coordinated in such a way that the desired characteristic and concentration of secondary phase is formed from the reaction with the oxygen content of the starting materials, which are regarded as SiO ₂ .

The method for producing parts from this material has proven to be particularly advantageous, particularly with regard to the flexibility of the component geometry. As

Starting materials for these materials have become Si powder with a medium

Grain size <50 μm and a purity> 95% proven. There is coarser powder

Problems with the implementation in the so-called nitridation, i.e. the reaction of the

Si with the N _{2 of} the atmosphere during annealing, with purities lower than 95%, secondary components are present in such high concentrations that inclusions occur and the required mechanical properties of the material can no longer be achieved. Such powders are mixed intensively with the sintering additives mentioned by dry or wet grinding, dried, granulated and shaped into a shaped part by a suitable shaping method such as axial or isostatic pressing. This molding can now be very advantageous up to one

Weight gain of 20% by weight due to the reaction of the Si with the nitrogen an inert, nitrogen-containing gas atmosphere are pre-nitrided and solidified, so that mechanical processing can then take place. This can be done with normal cutting processes such as sawing, drilling, milling, turning and grinding, so that the final contour of the part can be preformed. This is followed by the final nitriding at temperatures <1600 ° C, with a residual silicon content of <3% by volume being sought. With this final nitriding, a linear shrinkage of less than 2% occurs, which is very conducive to maintaining the complex final geometry. To reduce the porosity and increase the mechanical properties, this is followed by sintering at temperatures of <1760 ° C under 1 bar N ₂ or <2100 ° C under <100 bar N ₂ , the sintering conditions being based on the concentration and combination of the added sintering additives and the target value of the residual porosity <5 vol .-% are to be adjusted. A linear shrinkage of <14% occurs, which is significantly lower than with other dense ceramic materials produced via the powder route.

A preferred embodiment of the method according to the invention provides that <50% by weight silicon nitride powder with an average grain size <10 μm and a purity> 95% is additionally added to the silicon powder and the sintering additives and this mixture is prepared as described and processed into molded parts.

Silicon nitride powders with a higher average grain size hinder the sintering process and, with less purity, foreign components are present in such concentrations that they act as inclusions and can reduce the mechanical properties. The advantage of adding Si ₃ N ₄ powder is that, as an inert filler, it reduces the exothermic reaction between Si and N ₂ and thus the

Nitridation process allowed to be carried out more quickly.

Particularly favorable conditions exist for the mechanical processing of the pre-nitrided parts if the moldings only show 1-10% by weight increase, based on the Si content of the base material. This processing in the pre-nitrided state enables the production of very complex moldings and Components that only need processing on functional surfaces and fits after final sintering. This is an essential prerequisite for the economically acceptable production of large-volume, complex-shaped silicon nitride components for applications with high thermal, corrosive, abrasive and thermal shock loads.

An example of the use of these materials and components are cleaning and degassing stirrers for Al-based melts. Such stirrers are essentially designed as disk-shaped bodies with a central axial bore in order to be able to fasten them to a drive shaft. Through this wave there is a

Gas supply, either only axially centrally with an outlet at the lower end of the shaft or with a deflection in the area of the attachment in radial bores in order to allow the gas to escape at the circumference of the rotor. Profiles on the circumference serve to further shred the gas bubbles and swirl them intensely in the melt. The top of the disc is designed to slope towards the outer edge, so that the Al melt can flow off when the rotor is moved out. Flat profiles can also be attached to the underside of the rotor in order to achieve both a stirring and a pumping effect.

To fix the rotor to a commercially available drive shaft, a conical design of the central bore has proven itself in order to prevent the rotor from falling off in a form-fitting manner. The fixation itself on the shaft can e.g. with a graphite mandrel with an external thread, for which there are commercially available solutions.

When retracting the rotor or the entire degassing device, it is advisable to set an already slightly flowing gas stream in order to prevent the melt from penetrating into the bores. The same procedure must be followed when moving out of the melt in order to keep the holes free.

A preferred embodiment of such a rotor with the corresponding

Fixing parts are shown in Figure 1. In the test, this rotor has the same good degassing effect achieved as standard graphite degassing rotors. Furthermore, the material used has proven to be significantly more resistant to corrosive and abrasive attack than graphite parts according to the prior art, as a result of which the object of the invention is achieved.

The use of this material and component according to the invention is not limited to Al melts, but extends to a variety of light and non-ferrous metal melts as well as to various applications, such as Watering spoons, nozzles, valve systems including slide plates etc. One limitation can only be seen in the fact that the component must not exceed a wall thickness of 30 mm. The material and components made from it according to the specified process are also suitable for a variety of applications with thermal, corrosive and / or abrasive stress.

The impellers with gas distribution devices can be characterized as follows:

The aim is to distribute the treatment gas as evenly as possible in the form of the smallest possible bubbles evenly in the melt and at the same time to achieve a thorough mixing of the molten metal without the surface of the molten bath being moved excessively and this causing floating salt cells to enter the melt. In this case, it is a generally rotationally symmetrical body (1) which, attached to the end of a shaft, is immersed in the melt. The attachment can be done on the one hand by screwing the rotor provided with a central threaded bore onto the lower end of the shaft. To avoid a screw connection, as is appropriate in this case, there is a connection according to FIG. 1, in which the rotor is pushed onto the shaft of the rotor from above and is prevented from falling off by the taper of the shaft. A cross hole or groove (4) in the rotor prevents it from slipping or twisting. The gas is converted into small bubbles by a high peripheral speed during operation divided, which is reinforced by the profiling on the circumference (5). The profiling was designed in such a way that on the one hand there was sufficient comminution of the bubbles and sufficient mixing of the molten metal. On the other hand, the shape of the profiling and the rounding of the transitions mean that the melt is not excessively set into a rotational movement. The latter would have a negative effect on the metal quality due to the funnel formation around the shaft and the resulting entry of surface oxides into the melt. An optional profiling of the underside of the rotor can additionally generate a pump effect. During the rotation of the rotor, the grooves and webs are appropriately aligned

The melt accelerates outwards or in the opposite direction from the center of the rotor, which, depending on the version, results in different vertical circulation flows in the melt vessel, which in turn ensure good mixing of the metal and a desired, extended residence time of the gas bubbles in the melt. In special cases, the

Funnel formation around the shaft can be counteracted.

Optionally, the top of the rotor has a sloping surface (6) from the inside to the outside to make it easier for the metal melt to drain when the rotor is pulled out of the melt. This is particularly useful from the point of view that, due to the material used, there is no wetting with the aluminum melt and the cleaning is therefore limited to the removal of a loosely placed, thin oxide skin.

Reference symbol list:

1 rotor disc

2 holes for attachment to shaft, preferably conical

3 radial gas nozzles (optional)

4 cross hole or groove for fixing to shaft

5 profiles on the circumference

6 slants to promote the drainage of the melt (optional)

Claims

claims

1. silicon nitride material, characterized in that it consists of at least 75 vol .-% β-Si ₃ N ₄ , less than 3 vol .-% free Si, less than 5 vol .-% porosity and the rest of a secondary phase added usual

Sintering additives exist, with the material having a dip treatment in an Al-based melt of 850 ° C after 100 h showing only a change in weight of less than 1% by weight and with cyclical dipping tests into this melt after 500 cycles there is no detectable damage to the material , characterized by the inclusion of

Resonance frequency spectra before and after the immersion treatment and a maximum allowed shift of resonance frequencies of <0.1%.

2. silicon nitride material according to claim 1, characterized in that it consists of at least 80 vol .-% β-Si ₃ N ₄ , less than 2 vol .-% free Si, 3 to 1

Vol .-% porosity and the remainder consists of a secondary phase, which can be amorphous or partially crystalline.

3. silicon nitride material according to claims 1 and 2, characterized in that as sintering additives MgO, CaO, Y ₂ O ₃ , La ₂ O ₃ CeO ₂ , Nd ₂ O "TiO ₂ , ZrO ₂ ,

B ₂ O ₃ , Al ₂ O ₃ or compounds of these oxides in the form of, for example, MgAl ₂ O ₄ , Al ₂ TiO ₅ and optionally additionally SiO ₂ can be added in concentrations such that Si and optionally Si result from the reaction with the oxygen content of the starting materials ₃ N ₄ , which is regarded as SiO _{2 in the} present case, forms the concentration of secondary phase according to the invention.

4. A process for the preparation of Si ₃ N ₄ materials according to 1 to 3, characterized in that pure silicon powder with an average grain size <50 microns and a purity> 95% and conventional sintering additives are mixed intensively by dry or wet grinding, after drying and Granulation the mixture is shaped by a suitable molding process, this part is annealed for solidification in N ₂ or N ₂ -containing inert atmosphere until a weight gain by reaction of the nitrogen with the silicon of <20 wt .-% based on the Si Content is reached, this body is mechanically processed to the final shape and then annealed at temperatures of <1600 ° C in the gas atmospheres mentioned until a residual silicon content of <3% by volume is present, with a linear shrinkage of less than 2 during annealing % entry. This nitriding is followed by sintering, which occurs at temperatures of <1760 ° C under 1 bar N ₂ or <2100 ° C

<100 bar N ₂ takes place by adapting the conditions to the concentration and combination of the added sintering additives and the target value of a residual porosity <5% by volume, with a linear shrinkage of <14%.

5. A process for producing the silicon nitride material according to 1 to 3, characterized in that silicon powder with an average grain size <50 microns and a purity> 95% and <50 wt .-% silicon nitride powder of the average grain size <10 microns and a purity> 95% and conventional sintering additives are used as starting materials and, as specified in claim 4, processed and transferred into a dense sintered body.

6. The method for producing the silicon nitride material according to claim 4 and 5, characterized in that the weight increase in the preheating prior to the mechanical processing, based on the Si content of the material, is 1 to 10% by weight.

7. The method according to claims 4 to 6, characterized in that as sintering additives MgO, CaO, Y ₂ O ₃ , La ₂ O ₃ CeO ₂ , Nd ₂ O ₃ , TiO ₂ , ZrO ₂ , B ₂ O ₃ , Al ₂ O , or compounds of these oxides in the form of, for example, MgAl ₂ O ₄ , Al ₂ TiO ₅ etc. and optionally additionally SiO ₂ are added in concentrations that result from the reaction with the oxygen content of the starting materials Si and optionally Si ₃ N ₄ , calculated as SiO ₂ , forms the concentration of secondary phase according to the invention.

8. Use of the silicon nitride materials according to claims 1 to 3 for the production of large-volume, complex-shaped components.

9. Use of the silicon nitride components according to 8 in applications with high thermal, corrosive, abrasive and thermal shock stress.

10. Use of the silicon nitride components according to claim 8 in the form of

Stirrers for the cleaning and degassing treatment of Al-based melts.

11. Use of the components according to 8 for processing non-ferrous metal melts.

12. degassing stirrer according to claim 11, characterized in that it is essentially a disc-shaped body with a central axial bore for attachment to a drive shaft, optionally with profiling on the circumference and a gas conduction device in only the central, axial direction and / or deflection of this gas guide in several radial holes emerging from the circumference of the component.

13. Stirrer for molten metal according to claim 12, characterized in that the thickness of the disc increases from the outer circumference to the central fastening bore and is optionally structured flat, in particular on the underside, in order to achieve a pumping action in addition to a stirring, which in the melting vessel for a superimposed, vertical circulation flow and thus ensures a longer residence time of the purge gas bubbles in the melt.

14. Stirrer for molten metal according to claim 12, characterized in that such a profile is attached to the circumference that on the one hand there is comminution of the gas bubbles and mixing of the melt, on the other hand the molten bath surface is not moved excessively in order to minimize the entry of contaminants.

15. Stirrer for molten metal according to claim 12, characterized in that the top is designed to slope downwards to ensure that the metal runs off, which, together with the material properties (non-wettable for AI), eliminates the need for cleaning almost completely.