DE102007054665B4 - Metal powder and process for producing the metal powder - Google Patents

Abstract

Metal powder obtainable by a process for producing a metal powder according to any one of claims 10 to 14 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 consists of non-spherical particles having a frippery, platelike, needle-shaped or flake-shaped form, characterized in that the mean particle size distribution D10 is less than 25 μm, D50 is less than 40 μm and D90 is less than 55 μm, and in which the alloy constituents rhenium and tungsten are homogeneously mixed with one another, thus do not form discrete, X-ray detectable phases such as σ-ReW, χ-Re8W3 or pure Re phases and the rhenium is in the form of a WRe mixed crystal with tungsten structure.

Description

Rhenium is a refractory metal that has a high melting point of 3180 ° 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 costly, which is why there is a need for finely sintered components of rhenium and its alloys, which can be applied for example by plasma spraying. For example, alloys with a low rhenium content and a high content of tungsten metal are advantageous for the production of x-ray rotary anode plates. DE 1300412 discloses flame spraying powder based on a self-fluxing alloy and flame spraying process. DE 10 2005 015 920 discloses X-ray anodes, particularly X-ray anodes, which include material layers disposed between the substrate and the target material having graded thermal expansion coefficients. US 4390368 discloses a free-flowing plasma spray powder of substantially spherical particles having substantially smooth surfaces and a unitary composition consisting essentially of from about 4 to about 6 weight percent rhenium, with the remainder being tungsten. Lin et al., Key Points of Tungsten-Rhenium Alloys Processing. In: Rhenium and Rhenium Alloys, Proceedings of the International Symposium, Orlando, Feb. 9-13, 1997, 585-596; discusses the use of tungsten-rhenium alloys and technical difficulties encountered in their processing. US 0326861 discloses metal powder agglomerates of single particles containing more than 80% by weight of one and / or more metals of the elements molybdenum, rhenium or tungsten and binder metals selected from iron, cobalt, nickel, copper, silver, gold, palladium, platinum , Rhodium, chromium and rhenium, methods of making these metal powder agglomerates and their use.

US 6551377 shows a method 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 methods either have disadvantages in terms of equipment complexity, or else the particle size has to be reduced by z. For example, the atomization conditions may be set, thereby resulting in a high risk of contamination of the resulting alloy powder, or the methods do not provide a possibility of ensuring a homogeneous distribution of the rhenium in the alloy particles to obtain biphasic particles or a mixed powder of rhenium and tungsten particles become. 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 a 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 according to claim 1 to 9 comprising the steps: - drying of ammonium perrhenate - mixing of ammonium perrhenate with tungsten powder, wherein a maximum of 60% of the desired Wolframmenge with the total amount of Ammoniumperrhenates be used to a first powder mixture to obtain; - screening the first powder mixture; - mixing the remaining amount of tungsten with the first powder mixture to obtain a second powder mixture; - Pressing the powder mixture to form bodies; - sintering the moldings into sintered bodies at a temperature of 700 ° C to 2000 ° C, being heated to a final temperature of 1600 ° C to 2000 ° C and the final temperature reached is maintained for 3 to 8 hours; - Breaking and sieving the sintered pieces to obtain the metal powder. The present invention also relates to a metal powder obtainable by a process for producing a metal powder according to any one of claims 10 to 14, consisting of 3% by weight to 15% by weight of rhenium and ad 100% by weight of tungsten, which is less than 200 ppm of metallic impurities, has a homogeneous distribution of rhenium and tungsten and consists of non-spherical particles with spratziger, platelet-shaped, needle-shaped or flake-shaped, characterized in that the average particle size distribution D10 is less than 25 microns, D50 less than 40 microns and D90 is less than 55 microns, and in which the alloying ingredients rhenium and tungsten are homogeneously mixed with each other, thus no discrete, detectable X-ray detectable phases, such as σ-ReW, χ-Re ₈ W ₃ or pure Re phases and the rhenium in Form of a WRe mixed crystal with tungsten structure is present. 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 ° C to 110 ° C, preferably 65 ° C to 85 ° C, especially 70 ° C to 80 ° 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 for 3 to 5 hours is sufficient, advantageously a gas stream is passed through the dryer 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. Here, advantageously, the dried ammonium perrhenate and the tungsten powder in conventional devices, for. B. a stainless steel vessel, mixed and sieved. 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 ° C to 2000 ° C, advantageously at 1100 ° C to 2000 ° C, especially at 1300 ° C to 1900 ° 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 water vapor arising during the reduction is removed by the hydrogen stream. The holding time at the specified temperature is 3 to 8 hours, advantageously 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 ° C to 800 ° C, in particular 680 ° C to 720 ° 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 ° C to less than 1300 ° C, in particular 1170 ° C to 1220 ° C and about 60 to 180 minutes, in particular 100 to 140 minutes these temperature levels. Then can then within 120 to 240 minutes, in particular 160 to 200 minutes, to the above final temperature of 700 ° C to 2000 ° C, advantageously a final temperature of 1300 ° C to 2000 ° C, especially 1600 ° C to 1950 ° C. be heated, wherein the final temperature reached is maintained as described above for about 3 to 8 hours under the above conditions. At the cooling rate, no special requirements, 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. Helium, neon or nitrogen or any other gas to prevent possible oxidation or other reaction such as gas uptake 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 can be reunited after grinding with the milled coarse fraction and sieved again on the next finer sieve (in this example, 0.315 mm), 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 procedures, here advantageously tungsten billets are 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, irrespective of the size of the particle and the grinding step, the powder is removed from the mills by iron Impurities on. These can be removed in a separate step. For this purpose, the obtained tungsten-rhenium powder 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 ° C to 80 ° C, preferably 40 ° C to 70 ° C, in particular 50 ° C to 60 ° C, wherein the addition takes place so 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 in 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, hydrochloric acid heated to 30 ° C. to 80 ° C. is then 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 filter residue is washed acid-free with distilled water, ie it is washed until no more hydrochloric acid is detectable in the filtrate. The residue of the filtration is then dried (likewise in a glass or ceramic vessel), advantageously at less than 70 ° C., in particular at 30 to 50 ° C., it being possible optionally to operate under reduced pressure and / or in a gas stream. 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. As a rule, it suffices to use commercially available, with the purity "p. A. "sold hydrochloric acid the claims of this method. A use of the hydrochloric acid more than three times in the first cleaning step no longer leads to the desired cleaning effect. It should be pointed out that both purification steps advantageously do not take place in metal vessels or should come into contact with other metals (eg as stirrers) in order to achieve the desired purity. The quantitative 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 may be the same or a slightly lower addition of hydrochloric acid such as 10 to 10.5 to 1.

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 ° C to 1200 ° C, preferably 1050 ° C to 1150 ° C, the pressure less than 100 mbar, preferably 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. For this purpose, after the heat treatment under reduced pressure, the tungsten-rhenium powder is reheated under reduced pressure under a hydrogen atmosphere. The pressure in this step is less than 10 mbar, advantageously less than 5 mbar, in particular less than 3 mbar. Under these conditions, heated to a temperature of 1000 ° C to 1200 ° C, preferably 1050 ° C to 1150 ° C, and the temperature reached for 3 to 8 hours, especially 4 to 6 hours maintained. After cooling, any lumps that may be formed may be dissolved by further sieving. The oxygen content of the tungsten-rhenium powder after carrying out both oxygen reduction steps is less than 400 ppm, advantageously less than 250 ppm, in particular less than 100 ppm or 80 ppm, in particular less than 50 ppm.

In order to ensure the particularly high purities of the powder, the usual conditions under which alloy powders are produced for powder metallurgy purposes are usually not sufficient. 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 are retained to 100%, with a size of 10 microns to 50% and with a particle size of 0.5 microns to 15% , 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 high-purity tungsten-rhenium powder. This metal powder consisting of 3 wt .-% to 15 wt .-% rhenium and ad 100 wt .-% tungsten, which is less than 200 ppm of metallic impurities, has a homogeneous distribution of rhenium and tungsten and consists of non-spherical particles with speckled, platy, needle-shaped or flake-shaped. The metal powder according to the invention has a uniform, uniform distribution of the 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, form χ-Re ₈ W ₃ or pure Re phases.

This means that the metal powder according to the invention thus a high purity metal powder consisting of 3 wt .-% to 15 wt .-% rhenium and ad 100 wt .-% tungsten, which has less than 200 ppm of metallic impurities and the rhenium in the form a WRe mixed crystal with tungsten structure is present. In particular, no powder mixture of tungsten and rhenium particles is present in the metal powder according to the invention, but less than 0.1% by weight of pure rhenium or tungsten particles are advantageous. After forming a layer by means of a plasma spraying process using the metal powder according to the invention, the rhenium concentration on an area of 200 μm ² does not vary 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 of one another less than 40 ppm, advantageously less than 20 ppm, in particular less than 10 ppm. The mean particle size distribution of the metal powder according to the invention D10 is less than 25 .mu.m, D50 is less than 40 .mu.m and D90 is less than 55 .mu.m or else the mean particle size distribution D10 is less than 5 .mu.m, D50 is less than 9 .mu.m and D90 is less than 13 .mu.m , The particle size distribution is advantageously monomodal.

The obtained tungsten-rhenium powder is excellently suitable for vacuum plasma spray coating processes. The suitable layers are particularly durable in high temperature vacuum applications such as X-ray active layer on X-ray rotary anode plates. A method for producing molded parts, wherein the metal powder according to the invention described above is applied by a plasma spraying process, in particular the production of X-ray rotary anode plates, the moldings available as well as the use of metal powder according to the invention for plasma spraying and for the production of X-ray rotary anode plates are also disclosed.

Examples

example 1

Under clean room conditions, 36 kg of tungsten powder and 5.762 kg of ammonium perrhenate were mixed to produce a tungsten-rhenium metal powder having a rhenium content of 10%. The ammonium perrhenate was dried at 70 ° C for 4 hours prior to weighing, passed through portions of 0.04 mm sieve with 0.04 mm mesh sieves, in portions twice the mass (by weight) of tungsten media, and again at 70 ° C for 4 hours dried. 60% of the tungsten powder was weighed and mixed with the total amount of ammonium perrhenate, passed portionwise through a 0.063 mm mesh sieve and mixed in an intensive mixer for 10 minutes at 210 rpm. 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 60 t pressing pressure. It was then 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 perforated ball mill with a mesh of 0.04 mm mesh size and then further ground in a fluid bed countermeasure mill Alpine AFG 100 and simultaneously screened. From this powder then 35 kg of the fraction were removed 2-15 microns and in a beaker (portions of 5 kg) with KPG stirrer, which contained heated to 55 ° C hydrochloric acid, added portionwise with a PVC spoon and about 10 Stirred minutes with the KPG stirrer. It was then allowed to stand overnight, the hydrochloric acid decanted off and with 400 ml of 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 stirring every 10 minutes with a PVC spoon. 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 ° C. The mixture was then sieved through an analytical sieve with a mesh size of 0.025 mm and dried in a high-vacuum system for 4 hours at 1100 ° C. under hydrogen partial pressure <10 mbar. Finally, sieves were sieved through an analytical sieve with a mesh size of 0.025 mm. The powder obtained 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 diffractometry only reflections for tungsten were found, 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

There were commercially available, spherical powders, which after US 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

The procedure was as in Example 3 and a tungsten-rhenium powder with a rhenium content of 5% was obtained. 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.

Production of X-ray tube plates

X-ray active layers were sprayed on X-ray rotary anode plates by vacuum plasma spraying (VPS). The results and the powders used are listed in the following table. Powder: Ti [ppm] Zr [ppm] Assessment of cleanliness of the WRe batch Behavior of the WRe batch on the focal track after inserting the plate in the X-ray tube Comparative Example 6 65 45 Ti contaminated (and Zr contaminated) led to bursting of the focal point and to implosion of the plate (in bursts of the focal point Ti was found) comparison <25 190 Zr-contaminated led to bursting of the focal point (in Aufplatzungen Zr was found) Example 3 <10 <10 clean Brennbahn ok no bursting Example 2 <10 <10 clean Brennbahn ok no bursting

After use of rotary anode plates with VPS coating 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 contamination 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 applied by vacuum plasma spraying on X-ray anode plates. The powders according to the invention also 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 (14)

Metal powder obtainable by a process for producing a metal powder according to any one of claims 10 to 14 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 consists of non-spherical particles having a frippery, platelike, needle-shaped or flake-shaped form, characterized in that the mean particle size distribution D10 is less than 25 μm, D50 is less than 40 μm and D90 is less than 55 μm, and in which the alloy constituents rhenium and tungsten are homogeneously mixed with one another, thus do not form discrete, X-ray detectable phases such as σ-ReW, χ-Re ₈ W ₃ or pure Re phases and the rhenium is in the form of a WRe mixed crystal with tungsten structure , The metal powder of claim 1, wherein the individual particles are rhenium and tungsten. The metal powder of claim 2, wherein less than 0.1% by weight of the nonspherical particles are pure rhenium or tungsten particles. Metal powder according to one of claims 1 to 3, wherein the powder after generating a layer by means of a plasma spraying on a surface of 200 microns ^2, the rhenium concentration does not fluctuate more than 5 to 15%. 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. Metal powder according to 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. Metal powder according to one of claims 1 to 7, wherein the content of titanium and zirconium independently of one another are less than 40 ppm, advantageously less than 10 ppm. Metal powder according to one of claims 1 to 8, wherein the average particle size distribution D10 is less than 5 microns, D50 is less than 9 microns and D90 is less than 13 microns. A process for producing a metal powder as claimed in claims 1 to 9, 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; - screening the first powder mixture; - mixing the remaining amount of tungsten with the first powder mixture to obtain a second powder mixture; - Pressing the powder mixture to form bodies; - sintering the moldings into sintered bodies at a temperature of 700 ° C to 2000 ° C, being heated to a final temperature of 1600 ° C to 2000 ° C and the final temperature reached is maintained for 3 to 8 hours; - Breaking and sieving the sintered pieces to obtain the metal powder. The method of claim 10, wherein the sintering is carried out under a hydrogen atmosphere. A method according to claim 11, wherein at least one heat treatment under reduced pressure and / or hydrogen atmosphere is carried out for the final purification of the oxygen-reduction powder. Method according to 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. A method according to any one of claims 11 to 13, wherein the process is carried out under clean room conditions.