WO2009062769A1 - Metal powder - Google Patents

Abstract

The invention relates to a novel refractory metal powder, comprising less than 200 ppm metal impurities, having a homogenous distribution and being made of non-spherical particles with irregular, plate-like, needle-like or flake-like shape, to a method for the production thereof and to the use thereof.

Description

 metal powder

Rhenium is a refractory, which has a high melting point of 3180 ⁰ C and a high elastic modulus. Rhenium components can undergo repeated heating and cooling cycles without significant mechanical damage. For these and other reasons, rhenium is often used in rocket nozzles and other thermally highly resilient parts. Usually complex components made of rhenium are produced by mechanical processing, which is complicated and expensive, which is why a need for near net shape sintered components of rhenium and its

Alloys exist, which can be applied for example by plasma spraying.

For example, alloys having a low rhenium content and a high content of tungsten metal are advantageous for the production of x-rayed clay hearths. US 6551377 shows a process for producing spherical tungsten-rhenium alloy powders in a plasma flame to obtain spherical powders. US 3623860 shows a process for producing tungsten-rhenium alloy powders wherein solutions of tungsten and rhenium salts are atomized and reduced in the hydrogen stream. These processes either have drawbacks in terms of the equipment required, or else the particle size has to be determined before the process is carried out by e.g. the atomization conditions are set, resulting in a high risk of contamination of the resulting alloy powder, or the methods do not provide a possibility to ensure a homogeneous distribution of the rhenium in the alloy particles, so that biphasic particles or a powder mixture of rhenium and tungsten particles are obtained. It was therefore the object of the present invention to provide a simple process for the production of high-purity, single-phase tungsten-rhenium alloy powders with homogeneous distribution of the alloy constituents in the individual particles, which allows high flexibility for adjusting the particle size distributions at the end of the process.

This object is achieved by a method for producing a metal powder consisting of 3% by weight to 15% by weight of rhenium and ad 100% by weight of tungsten, having less than 200 ppm of metallic impurities, a homogeneous distribution of rhenium and tungsten and consisting of non-spherical particulate, pinciform, acicular or flake-like particles, comprising the steps of: drying ammonium perrhenate

Mixing ammonium perrhenate with tungsten powder using a maximum of 60% of the desired amount of tungsten with the total amount of ammonium perrhenate to obtain a first powder mixture; Sieving the first powder mixture;

Mixing the remaining amounts of Woiframmenge with the first powder mixture to obtain a second powder mixture; -Pressen the powder mixture to form bodies;

- sintering of the moldings to sintering at a temperature of 700 ⁰ C to 2000 ⁰ C;

Breaking and sieving the sintered pieces to obtain the metal powder.

To carry out the drying of Ammoniumperrhenates is essential because it easily absorbs moisture from the air and accurate weighing is not possible. The ammonium perrhenate is preferably used in a slight excess of from 1 mol% to 3 mol%. The amount of ammonium perrhenate required is determined according to the rhenium content of the ammonium perrhenate and the desired rhenium content of the powder. The drying can be carried out by conventional means, for example by drying at a temperature of 50 ⁰ C to 110 ⁰ C, preferably 65 ° C to 85 ⁰ C, in particular 7O ⁰ C to 80 ⁰ C, at atmospheric pressure or under reduced pressure for a time from about 2 to 20 hours, especially 3 to 10 hours or from 4 to 6 hours. In general, drying at normal pressure and a temperature of about 70 ^° C. is sufficient for 3 to 5 hours, advantageously passing a gas stream through the drying device to remove the moisture.

Subsequently, the required amount of ammonium perrhenate is weighed and mixed with a maximum of 60% of the desired amount of tungsten tungsten powder. In this case, the dried ammonium perrhenate and the tungsten powder can advantageously be mixed and screened off in customary devices, for example a stainless steel vessel. The mixture is then mixed in an intensive mixer for 5 to 30 minutes, advantageously 10 to 20 minutes, and the first powder mixture is obtained. The remaining amount of tungsten powder is then added and further mixed again for 10 to 40 minutes, especially 15 to 30 minutes, to obtain the second powder mixture.

The resulting second powder mixture is then pressed in a hydraulic press with a holding time of 1 to 60 seconds, in particular 3 to 10 seconds, and a compression pressure of 40 to 80 tons, preferably 50 to 60 tons, to form bodies. These shaped bodies are then sintered and reduced simultaneously in a hydrogen atmosphere. This step is carried out at a temperature of 700 ⁰ C to 2000 ⁰ C, advantageously at 1100 ⁰ C to 2000 ⁰ C, in particular at 1300 ⁰ C to 1900 ⁰ C.

This process step is carried out at atmospheric pressure at a hydrogen flow of about 1 to 4 m ³ / hour, advantageously at 1, 5 to 2 m ³ / h, so that in the reduction resulting water vapor is removed by the hydrogen stream. The Haitezeit at the specified temperature is 3 to 8 hours, preferably 5 to 6 hours. Advantageously, a stepwise heating, wherein first heated within 60 to 120 minutes, in particular 80 or 90 to 100 minutes at 600 ⁰ C to 800 ° C, in particular 68O ⁰ C to 72O ⁰ C and about 30 to 90 minutes, in particular 50 to 70 minutes at this temperature is maintained. It is then advantageously heated within 60 to 120 minutes, in particular 80 or 90 to 100 minutes at 1100 ⁰ C to less than 1300 ° C, in particular 1170 ° C to 1220 ⁰ C and about 60 to 180 minutes, in particular 100 to 140 minutes maintained this temperature. Then, within 120 to 240 minutes, in particular 160 to 200 minutes, to the above-mentioned final temperature of 700 ⁰ C to 2000 ° C, advantageously a final temperature of 1300 ⁰ C to 2000 ⁰ C, in particular 1600 ⁰ C to 1950 ⁰ C. be heated, wherein the final temperature reached is maintained as described above for about 3 to 8 hours under the above conditions. In the simplest case, the cooling is allowed after switching off the furnace, this step advantageously also takes place in a stream of hydrogen or an inert gas such as helium, neon or nitrogen or any other gas to one to prevent possible oxidation or other reaction such as gas absorption during cooling.

After cooling, the resulting sintered pieces are crushed, for example in a jaw crusher, so that advantageously pieces with a size of less than 3 mm are obtained. This can be done by screening with a sieve of mesh size of about 3 mm and breaking the coarse fraction again. Subsequently, the resulting fragments are ground, preferably in a screen ball mill using spherical or cylindrical steel or, advantageously, tungsten steel bodies. The mesh size of the screen coverings used is selected according to the desired particle size distribution. In order to reduce the meal, the fine powder can first be sieved off before a subsequent grinding step, so that only the particles which are larger than the sieve covering used in the subsequent grinding step are reground. That is, in an advantageous stepwise grinding with Siebbespannungen of 1, 6 mm, 1 mm, 0.315 mm and 0.1 mm is screened after grinding to 1, 6 mm over 1 mm and further milled the coarse fraction with a Siebbespannung of 1 mm while the fine fraction can either be screened off further by appropriate sieves of the subsequent steps and divided into powder fractions in order to supply them only to the required further grinding steps. In another embodiment of the invention, the fine fraction after grinding can be reunited with the ground coarse fraction and screened again over the next finer sieve (0.315 mm in this example), in which case the coarse fraction is again ground.

In a further grinding on sieves with mesh sizes of, for example, 0.063 mm and 0.04 mm can also be proceeded by one of these approaches, in which case advantageously Woiframmahlkörper be used. It is advantageous if the powder consists entirely of particles with a size of less than 0.045 mm.

After this grinding, the powder obtained can be ground again in a counter-jet mill, for example a fluidized bed counter-jet mill, to obtain even finer powders. In this case, particle sizes of less than 15 microns can be achieved.

After grinding / sieving, regardless of the particle size and grinding step, the material is removed, the powder has due to abrasion from the mills, iron-containing impurities are now present. These can be removed in a separate step. For this purpose, the tungsten-rhenium powder obtained in a first cleaning step, preferably in a plastic ceramic or glass container, in conc. Hydrochloric acid and stirred. The temperature of the hydrochloric acid is 30 ⁰ C to 80 ⁰ C, advantageously 40 ⁰ C to 70 ⁰ C, in particular 50 ⁰ C to 60 ⁰ C, wherein the addition is such that the gas evolution is not too strong. The temperature must not be too high, since the solubility of the hydrogen chloride then decreases and the concentration of the acid decreases. At too low a temperature, on the other hand, the reaction rate is too low to achieve dissolution of the iron-containing impurities in a short time. After stirring for a period of 30 to 120 minutes, the powder is allowed to sediment at the selected temperature for 4 to 30 hours, advantageously 6 to 24 hours, in particular 8 to 18 hours, and the hydrochloric acid is then decanted off, in a second purification step then likewise Hydrochloric acid heated to 30 ⁰ C to 80 ⁰ C was added and stirred again for 30 to 120 minutes. After sedimentation for about 1 to 4 hours, in particular 2 to 3 hours, the hydrochloric acid is filtered off and the Füterrückstand acid-free washed with distilled water, ie it is washed until no more hydrochloric acid is detectable in the filtrate. Then the residue of filtration is (also in a glass or ceramic vessel) dried, advantageously at less than 7O ⁰ C, especially at 30 to 50 ° C, being possible to work under reduced pressure and / or in a gas stream optional. Subsequently, lumps possibly formed can be dissolved by further screening.

In an advantageous embodiment of the present invention, the filtrate of the second purification step can be reused in the first purification step, but care must be taken that at least the hydrochloric acid used in the second purification step has a higher purity. In general, commercially available, _π with the purity pA "sold hydrochloric acid sufficient for the demands of this process. A more than three-time use of the hydrochloric acid in the first cleaning step does not lead to the desired cleaning effect. It is to be noted that both purification steps advantageous not take place in metal containers or to come into contact with other metals (eg, agitators) to achieve the desired purity The ratio (by weight) of tungsten-rhenium powder to hydrochloric acid is about 6 to 9 to 1, in particular 8 to 8.5 to 1 in the first purification step, in the second purification step, the same or a slightly lower addition of hydrochloric acid, such as 10 to 10.5 to 1, can take place.

To reduce the oxygen content, a further process step can be carried out, regardless of how the tungsten-rhenium powder was ground or post-purified. This reduction of the oxygen content is possible by a heat treatment of the obtained powder under reduced pressure. For this purpose, the tungsten-rhenium powder is heated to a temperature of 1000 ⁰ C to 1200 ⁰ C, preferably 1050 ⁰ C to 1150 ⁰ C, the pressure less than 100 mbar, advantageously less than 50 mbar, in particular less than 15 mbar or less than 10 or 5 mbar. Under these conditions, the powder is heat-treated for 3 to 8 hours, especially 4 to 6 hours under reduced pressure.

In a further step, a further reduction of the oxygen content can be achieved. This will be done after the

Heat treatment under reduced pressure, the tungsten-rhenium Puiver reheated under reduced pressure under a hydrogen atmosphere. The pressure in this step is less ais 10 mbar, advantageously less than 5 mbar, in particular less than 3 mbar Under these conditions, is heated to a temperature of 1000 ⁰ C to 1200 ⁰ C ₁ advantageously 1050 ° C to 115o ⁰ C, and the temperature maintained for 3 to 8 hours, especially 4 to 6 hours. After cooling, any lumps that may be formed may be dissolved by further sieving. The Sauerstoffgehait of the tungsten-rhenium powder is after completion of both steps for oxygen reduction is less than 400 ppm, advantageously less than 250 ppm, especially less than 100 ppm or 80 ppm _f in particular less than 50 ppm.

In order to ensure the particularly high purities of the powders, the usual conditions under which alloying powders are produced for powder metallurgical purposes are usually insufficient.

The high purity of the powders according to the invention is advantageous to ensure by the process described above, when it is carried out under clean room conditions. For this purpose, the room air is filtered with a dust filter F5 or the like, so that dust particles with a particle size of more than 12 microns to 100%, with a size of 10 microns to 50% and with a particle size of 0.5 microns to 15% retained. In a further advantageous embodiment of the invention, the room is equipped with a lock to enter to prevent contamination, and only tungsten and rhenium are processed in this so-equipped room.

The metal powder according to the invention is thus a highly pure Woifram rhenium powder. This metal powder consisting of 3 wt .-% to 15 wt .-% rhenium and ad 100 wt .-% tungsten, which has less than 200 ppm metallic impurities, a homogeneous distribution of rhenium and tungsten and from non-spherical particles with spratziger , pinciform, needle-shaped or flake-shaped. The metal powder according to the invention has a uniform, uniform distribution of rhenium and tungsten, so that the individual particles of rhenium and tungsten, in which the alloying constituents are homogeneously mixed with each other and thus no discrete, X-ray detectable phases, such as σ-ReW, χ- form ReaW ₃ or pure Re phases.

This means that the metal powder according to the invention is thus a high-purity metal powder consisting of 3% by weight to 15% by weight of rhenium and ad 100% by weight of tungsten, which is less than 200 ppm metallic

Contains impurities and the rhenium is in the form of a WRe mixed crystal with tungsten structure. In particular, in the Metallpuiver according to the invention, no powder mixture of tungsten and rhenium particles before but advantageous are less than 0.1 wt .-% pure rhenium or wolf rampart. After producing a layer by means of a vacuum plasma spraying method under

Use of the metal powder according to the invention does not vary on an area of 200 μm ² the rhenium concentration by more than 15%, advantageously not more than 5%. The metal powder has an oxygen content of less than 400 ppm, advantageously less than 250 ppm, in particular less than 100 ppm or 80 ppm, in particular less than 50 ppm.

The carbon content of the metal powder is less than 50 ppm, in particular less than 30 ppm. The iron content is less than 50 ppm. The contents of nickel, cobalt, copper, manganese or calcium are independently of one another and independently of the carbon or oxygen content less than 50 ppm, advantageously less than 20 ppm, in particular less than 10 ppm. The contents of titanium and zirconium are independently less than 40 ppm, advantageously less than 20 ppm, in particular less than 10 ppm.

According to the invention, the average particle size distribution of the powder is less than 25 μm, D50 is less than 40 μm and D90 is less than 55 μm or the average particle size distribution D10 is less than 5 μm, D50 is less than 9 μm and D90 is less than 13 μm. The particle size distribution is advantageously monomodal.

The obtained tungsten-rhenium powder is excellently suitable for vacuum plasma spray coating processes. The suitable wells are particularly resilient in high temperature vacuum applications such as X-ray active layer on X-ray rotary anode plates. The present invention therefore also relates to processes for the production of moldings, wherein the above-described metal powder according to the invention is applied by a vacuum plasma spraying process, in particular the production of X-ray divider dividers, the available molding and the use of metal powder according to the invention for vacuum plasma spraying and for the production of X-ray rotary anode denticles.

Examples

example 1

Under clean room conditions, 36 kg of tungsten powder and 5.762 kg

Ammonium perrhenate for producing a tungsten-rhenium metal powder mixed with a rhenium content of 10%. The ammonium perrhenate was dried for 4 hours at 7O ⁰ C before weighing, passed in portions with twice the amount (by weight) of tungsten grinding media through a sieve with a mesh size of 0.04 mm and again for 4 hours at 70 ⁰ C. dried. 60% of the tungsten powder was weighed and mixed with the total amount of ammonium perrhenate, in portions through a Stirlsieb with a

Mesh size of 0.063 mm and mixed in an intensive mixer for 10 minutes at 210 revolutions per minute. The remaining amount of tungsten was added and then further mixed under the same conditions for 15 minutes. The obtained mixture of tungsten powder and ammonium perrhenate was pressed into disks in a hydraulic press at 601 pressing pressure. Subsequently, it was sintered at a temperature of 1950 ^° C. for 6 hours. After cooling, the resulting sintered pieces were crushed in a jaw crusher and ground in a screen ball mill stepwise with a minimum mesh tension of 0.045 mm. The obtained powder had a monomodal particle size distribution with a D10 of 20 μm, D50 of 38 μm and D90 of 53 μm. The complete procedure took place under clean room conditions.

Example 2

The powder obtained in Example 1 was ground with tungsten grinding media in a sieve ball mill with a mesh of 0.04 mm and then further ground and simultaneously screened in an Alpine AFG 100 fluidized bed counter-jet mill. From this powder then 35kg of the fraction were taken 2-15 microns and in a beaker (portions of 5 kg) with KPG stirrer containing hydrochloric acid at 55 ° C, added in portions with a PVC spoon and about 10 minutes stirred with the KPG stirrer. The mixture was then allowed to stand overnight, the hydrochloric acid was decanted and treated with 400 ml hydrochloric acid at 55 ° C in portions with stirring with the KPG stirrer, stirred for 10 minutes and then allowed to stand for 2 hours, with a PVC spoon every 10 minutes was stirred. It was then filtered through a suction filter and the residue washed with distilled water acid-free. Subsequently, the resulting filter cake is dried in a ceramic bowl for 12 hours at a temperature of 45 ⁰ C. The mixture was then sieved through a Analysesieb having a mesh width of 0.025 mm and dried in a high vacuum system for 4 hours at 1100 ⁰ C under 10 mbar of hydrogen. "

Finally, sieve was sieved over an anaisy sieve with a mesh size of 0.025 mm. The resulting powder had a monomodal particle size distribution with a D10 of 5 μm, D50 of 7 μm and D90 of 15 μm. The complete procedure took place under clean room conditions. The powders had a tungsten content of 90% by weight and a rhenium content of 10% by weight in the chemical analysis. By X-ray diffraction only reflections for Tungsten detected, but no reflections indicating a rhenium lattice or σ- or χ-phase.

Example 3

The procedure was as in Examples 1 and 2, but adjusted the amounts of tungsten and rhenium so that the rhenium content was 5%. The powders had a tungsten content of 95% by weight and a rhenium content of 5% by weight in the chemical analysis. By X-ray diffractometry only reflections for tungsten were found, but no reflections indicating a rhenium lattice or σ- or χ-phase.

Comparative Examples 4 and 5

Commercially available, spherical powders obtained according to US Pat. No. 6551377 were used. The rhenium contents were 10% (Comparative Example 4) and 5% (Comparative Example 5). These powders showed reflections for rhenium and tungsten in the X-ray diffractometric study. Thus, there were no homogeneous re-distributions in the powder but also re-enrichments.

Comparative Examples 6 and 7 It was as in Example Be! 3 method and a tungsten-rhenium powder obtained with a rhenium content of 5%. It was not worked under clean room conditions and in premises in which other metals were processed powder metallurgy.

Analysis results:

The contents of impurities are given in ppm.

Herstellung von Röntgend rehanodentellern

Production of X-ray tube plates

X-ray active layers were sprayed on X-ray rotary anode splitters by means of vacuum plasma spraying (VPS).

The results and the powders used are listed in the following table.

After the use of VPS-coated rotating anode plates in an X-ray tube, the plates showed bursts on the surface of the focal track. These bursts were unusual and had Ti and Zr impurities as the cause. The impurities had melting points <2000 ° C. Thus, they could melt in the focal spot (> 3000 ° C) and partially evaporate due to the prevailing in the X-ray tube high vacuum and thus cause the Aufplatzungen. In one case, it even came to the implosion of the X-ray tube. The metal powders according to the invention, however, did not show this behavior when they were applied by vacuum plasma spraying on X-ray tube. In addition, the powders according to the invention had no reflections indicating the presence of a rhenium metal lattice, although rhenium was detectable in the powder particles. This means that there are no or only very few pure rhenium particles present and that the existing rhenium in the form of a mixed crystal was present in tungsten.

Claims

claims 1. Metal powder consisting of 3 wt .-% to 15 wt .-% rhenium and ad 100 wt .-% tungsten, which has less than 200 ppm metallic impurities, a homogeneous distribution of rhenium and Tungsten has and consists of non-spherical particles with spratziger, platelike, needle-shaped or flaky shape. 2. Metal powder according to claim 1, wherein the individual particles of rhenium and tungsten. 3. Metal powder according to claim 2, wherein less than 0.1 wt.% Pure Rhenium or tungsten particles are. 4. Metal powder according to one of claims 1 to 3, wherein the powder after generating a layer by means of a vacuum Piasmaspritzverfahrens on an area of 200μm2, the rhenium concentration does not fluctuate more than 5 to 15%. 5. Metal powder according to one of claims 1 to 4, wherein the oxygen content is less than 100 ppm, advantageously less than 80 ppm, in particular less than 50 ppm 6. Metailpulver according to any one of claims 1 to 5, wherein the carbon content is less than 50 ppm, in particular less than 30 ppm. The metal powder according to any one of claims 1 to 6, wherein the contents of iron, nickel, cobalt, copper, manganese and calcium are independently smaller than 50 ppm. 8. Metal powder according to one of claims 1 to 7, wherein the content of titanium and zirconium independently are less than 40 ppm, advantageously less than 10 ppm. 9. Metallpuiver according to one of claims 1 to 8, wherein the average particle size distribution D10 is less than 5 microns, D50 smaller than 9 .mu.m and D90 smaller than 13 .mu.m 10. The metal powder according to any one of claims 1 to 9, wherein the average Particle size distribution D10 is less than 25 μm, D50 is less than 40 μm and D90 is less than 55 μm. 11. Method for producing a metal powder according to claim 1 to 10, comprising the steps of: drying ammonium perrhenate by mixing Ammonium perrhenate with tungsten powder, wherein a maximum of 60% of the desired amount of tungsten with the total amount of Rheniumperrhenates are used to obtain a first powder mixture; Sieving the first powder mixture; - Mix the remaining amount of tungsten with the first powder mixture to a second To obtain powder mixture; -Pressen the powder mixture to form bodies; - sintering of the moldings to sintering at a temperature of 700 ⁰ C to 2000 ⁰ C; Breaking and sieving the sintered pieces to obtain the metal powder. 12. The method of claim 11, wherein the sintering under Hydrogen atmosphere is performed. 13. The method according to claim 11, wherein for the final purification of the powder for oxygen reduction, at least one heat treatment under reduced pressure and / or hydrogen atmosphere is performed. 14. The method according to any one of claims 11 to 12, wherein the Ammonium perrhenate has a mean grain size of not more than 15 microns and the tungsten powder has a mean grain size of not more than 10 microns. 15. The method according to any one of claims 11 to 13, wherein the method is carried out under clean room conditions. 16. A process for the production of molded parts, wherein a metal powder is applied according to one or more of claims 1 to 10 by a vacuum plasma spraying process on the molded part. 17. The method of claim 20, wherein the molding is an X-ray anode plate. 18. A molding obtainable by a process according to claim 16 or 17. 19. Use of a metal powder according to one or more of claims 1 to 10 for vacuum plasma spraying. 20. Use of a metal powder according to one or more of claims 1 to 10 for the production of X-ray rotary anode plates. 21. Metal powder obtainable by a process according to one of the claims 11 to 15.