Process of producing low oxygen refractory metal powder for powder metallurgy

ES 2 313 136 T3

DESCRIPTION

Low oxygen refractory powder metal production process for powder metallurgy.

The present invention relates to the production of powders and products of tantalum, niobium, and their alloys having low oxygen content.

A common method of producing powdered metal products of tantalum, niobium, or alloys of such metals with each other, and either or both with other metals, is to first isostatically cold-pressing the powder into a preform, such as a bar or rod. The preform is resistance sintered at a relatively high temperature to produce a product formed from tantalum, niobium, or their alloys. Generally, for resistance sintering, the ends of the preform are clamped between water-cooled copper terminals in a high vacuum chamber and then the preform is heated by passing an electric current through the preform. Resistance sintering simultaneously lowers the oxygen content and densifies the preform.

However, there are many disadvantages to using resistance sintering to densify and remove oxygen. First, resistance sintering can only be used to produce products of certain limited shapes, generally bars and rods. For resistance sintering, the cross section of the preform must be uniform throughout the electrical current path to avoid local overheating and heat shorting. Furthermore, the cross section must be small enough so that the reduction of oxygen in the center of the preform occurs before the disappearance of the interconnected porosity. For effective oxygen removal, preforms larger than about 1.5 inches in their shortest dimension are not resistance sintered. Furthermore, the preform must be small enough to avoid buckling associated with hot deformation and compression during unsupported resistance sintering. In this way, the preforms generally weigh no more than about 35 lbs.

The present invention relates to tantalum and / or niobium powder and more particularly to tantalum powders usable in the manufacture of powder metallurgy components that can be further processed into rod, wire, sheet, sheet and other rolled part products. fabricated or simply made into mesh by conventional powder consolidation methods followed by surface finishing and / or minor dimensional modifications. Such products are also usable as fully dense coatings to modify the surface chemistry of other laminates or fabricated parts.

It is well known to make tantalum powders to be used as sintered anodes for electrolytic capacitors, hydrating an ingot or shavings from a tantalum ingot, grinding (taking advantage of the brittleness that results from this massive hydration) until turning into a powder and then dehydrating to form tantalum powder. Such powders can be used to produce capacitors with low electrical dispersion. In principle such a process is also applicable to niobium but it is not very practical.

It is also known to deoxidize tantalum or niobium condenser powders (manufactured however) in primary or secondary (agglomerated) forms by contacting them with alkaline earth metal vapors to efficiently collect oxygen on the surface of the powder and remove it as an alkali metal oxide. earth by acid leaching and / or volatilization.

US-A-5242481 discloses the production of Ta and Nb powder having an oxygen content of less than 300 ppm by deoxidation with Mg.

I have discovered a method according to claim 1.

In addition I have formed powdered metal products that have oxygen contents of less than about 300 ppm formed from tantalum, niobium; and its alloys. I have also further discovered a new process to produce formed metal powdered products of tantalum, niobium and their alloys, which have oxygen contents below around 300 ppm without resistance sintering.

The present invention utilizes a combination and a variation of the two older prior art development lines described above, taken in conjunction with the later idea that this is one way to achieve a very fine-sized, low-oxygen powder that can be used in the manufacture of laminated products / precast parts. Typically the fine size (and related large surface area) of the powder is associated with high uptake of harmful oxygen for further processing and use.

It is a primary object of the present invention to provide a method for achieving low oxygen tantalum and / or niobium fine powder, preferably averaging a size below 150 microns (microns) and below 100 ppm oxygen.

This is accomplished by providing a fine size of tantalum hydride of less than 150 microns and mixing it with a small amount of magnesium or calcium, less than 1/2% by weight of hydride. The mixture is heated on an ascending heating schedule to vaporize the alkaline earth metal and begin oxygen reduction by vapor retention to complete the oxygen reaction, then cooling, and rinsing with acid and water.

ES 2 313 136 T3 for leaching residual alkaline earth metal and drying to obtain a tantalum powder with less than 300 ppm oxygen (typically less than 150 ppm) and an average particle size less than 150 microns fApD (Particle Diameter Fisher's average).

An advantage of the powder produced by the present invention is that it comprises relatively non-spherical particles suitable for one-way mechanical pressing.

Another advantage of the powder produced by the present invention is that it comprises relatively small particles suitable for cold isostatic pressing.

An advantage of the products formed from tantalum, niobium, or their alloys produced by the present invention, which have oxygen contents of less than about 300 ppm, is that the products can be of any shape, cross-section, or size.

An advantage of the process for producing formed products of the present invention is that the process allows the production of tantalum, niobium, or alloy products that have an oxygen content of less than about 300 ppm of any shape, cross section, or size. .

Tantalum, niobium, or tantalum or niobium alloy powders, which have an oxygen content of less than 100 ppm (parts per million) are produced by the following procedure.

A first metal hydride powder (tantalum, niobium or alloy) is placed in a vacuum chamber, which also contains a metal that has a higher affinity for oxygen than the first metal, such as calcium or magnesium, preferably the latter. The starting hydride powder has an oxygen content of less than 300 ppm. The chamber is then heated to the deoxidation temperature to produce a tantalum, niobium or tantalum or niobium alloy powder having an oxygen content of less than about 300 ppm. The magnesium, which contains the oxygen, is then removed from the metal powder by evaporation and subsequently by leaching of the chemical or selectively dissolving the powder.

The tantalum or niobium alloys produced by the present invention include alloys of tantalum and / or niobium, of either or both with other metals, and further include the incorporation of an oxide or other Ta, Nb, which has a higher free energy of formation. to that of Ta and / or Nb oxide, such as for example yttrium oxide, thorium oxide, or aluminum oxide. The oxide is mixed into tantalum and / or niobium powder that has an oxygen content of less than 100 ppm. The alloys of the present invention also include tantalum and / or niobium alloys and an alloying element with a low oxygen content mixed in the tantalum or niobium powder, provided that the oxygen content of the mixture is less than 100 ppm. The alloys of the present invention further include tantalum and / or niobium hydride alloys and an alloying element in which the alloying element and the tantalum and / or niobium powder are mixed prior to deoxidation to form the alloy having an oxygen content of less than 100 ppm. The alloys of the present invention further include tantalum and / or niobium alloys and an alloying element in which the addition of oxygen associated with the alloying element does not increase the oxygen content of the alloy above 100 ppm.

As described above, in the process for producing powdered metal products of tantalum, niobium and their alloys, the metal hydride powder is deoxidized at an oxygen content of less than 100 ppm. The powder is consolidated to form a tantalum, niobium, or alloy product, which has an oxygen content below 100 ppm.

In accordance with the present invention, a product formed from tantalum, niobium or alloy, having an oxygen content below 100 ppm, can be produced from metal hydride powder by any known powder metallurgy technique. Examples of these powder metallurgical techniques used to mold the products are as follows, in which the steps are listed in order of performance. Any of the following simple techniques or sequences of techniques can be used in the present invention:

• cold isostatic pressing, sintering, encapsulation, hot isostatic pressing and thermomechanical processing;

• cold isostatic pressing, sintering, hot isostatic pressing and thermomechanical processing;

• cold isostatic pressing, encapsulation, hot isostatic pressing and thermomechanical processing.

• cold isostatic pressing, encapsulation and hot isostatic pressing;

• encapsulated and hot isostatic pressing;

• cold isostatic pressing, sintering, encapsulation, extrusion and thermomechanical processing;

• isostatic cold pressing, sintering, extrusion, and thermomechanical processing;

ES 2 313 136 T3 • cold isostatic pressing, sintering, and extrusion;

• cold isostatic pressing, encapsulation, extrusion and thermomechanical processing;

• cold isostatic pressing, encapsulation and extrusion;

• encapsulation and extrusion;

• mechanical pressing, sintering and extrusion;

• cold isostatic pressing, sintering, encapsulation, forging and thermomechanical processing;

• cold isostatic pressing, encapsulation, forging and thermomechanical processing;

• cold isostatic pressing, encapsulation and forging;

• cold isostatic pressing, sintering, and forging;

• cold isostatic pressing, sintering and tamping;

• potting and forging;

• potting and tamping;

• cold isostatic pressing, sintering and thermomechanical processing;

• aerosol tank;

• mechanical pressing and sintering;

• mechanical pressing, sintering, re-pressing and re-sintering;

• plasma assisted hot pressing;

• hot pressing assisted by plasma and extrusion;

• plasma-assisted hot pressing and thermomechanical processing;

• plasma assisted hot pressing, extrusion and thermomechanical processing.

Other combinations of consolidation, heating and deformation can also be used.

The effectiveness and advantages of the processes of the present invention will be illustrated in more detail by the following non-limiting examples.

Example 1 (Outside the scope of the invention)

This example illustrates the production of tantalum powder with less than 300 ppm oxygen by deoxidation of tantalum hydride under a partial pressure of argon. The tantalum hydride powder, manufactured by a conventional method as described above, was mixed with 0.3% by weight of Mg powder and placed in a vacuum oven retort, which was evacuated, and refilled with argon. The pressure in the furnace was set at 100 microns with argon flowing and the vacuum pump running. The furnace temperature was raised to 650 ° C in 50 ° C increments, held until the temperature was stabilized, then raised to 950 ° C in 50 ° C increments. When the temperature was stabilized at 950 ° C it was held for two hours. After two hours at 950 ° C the oven was turned off and cooled to room temperature. Once the oven had cooled, its powder content was extracted from the retort. The magnesium, which contains the oxygen, was then removed from the metal dust by acid leaching. Substantially all of the hydrogen content (except for the normal hydrogen impurity level of Ta) was removed from the metal hydride and removed from the retort by the vacuum pumping system.

ES 2 313 136 T3

The properties of the resulting Ta powder were as follows:

<td>Particle Size:</td><td>-100 mesh (less than 150 microns)</td>

<td>Oxygen:</td><td>240 ppm</td>

<td>Surface area:</td><td>462 cm²/ g</td>

<td>Specific oxygen:</td><td>0.52 microgram / cm²</td>

Example 2 (Outside the scope of the invention)

This example illustrates the reduction of a tantalum powder with less than 200 ppm oxygen by deoxidation of tantalum hydride under argon partial pressure. The tantalum hydride powder, manufactured by the conventional method, was mixed with 0.3% by weight of Mg and placed in a vacuum oven retort, which was evacuated, and filled again with argon. The pressure in the furnace was set at 100 microns with argon flowing and the vacuum pump running. The oven temperature was raised to 850 ° C in 50 ° C increments, held until the temperature was stabilized, then held for 3 hours. It was then raised to 950 ° C in 50 ° C increments. When the temperature was stabilized at 905 ° C it was held for two hours. After two hours at 950 ° C the oven was turned off and cooled to room temperature. Once the oven had cooled down, its powder content was removed from the retort. The magnesium, which contains the oxygen, was then removed from the metal powder by acid leaching.

The properties of the resulting tantalum powder were as follows:

<td>Particle Size:</td><td>-100 Mesh (less than 150 micrometers)</td>

<td>Oxygen:</td><td>199 ppm</td>

<td>Surface area:</td><td>465 cm²/gram</td>

<td>Specific oxygen:</td><td>0.43 microgram / cm²)</td>

Example 3

Example 4 illustrates a tantalum powder with less than 100 ppm oxygen produced by deoxidation of tantalum hydride under positive pressure of argon. The tantalum hydride powder, manufactured by the conventional method, was mixed with 0.3% by weight of magnesium and placed in a production vacuum furnace retort, which was evacuated, and filled again with Argon. The pressure in the furnace was set at 860 Torr with Argon flowing. The oven temperature was raised to 650 ° C in 50 ° C increments, held until the temperature was stabilized, then held for 4 hours. It was then raised to 1000 ° C in 50 ° C increments. When the temperature was stabilized at 1000 ° C it was maintained for six hours. After six hours at 1000 ° C the oven was turned off and cooled to room temperature. Once the oven had cooled down, its powder content was removed from the retort. The magnesium, which contains the oxygen, was then removed from the metal powder by acid leaching.

The properties of the resulting Ta powder were as follows:

<td>Particle Size:</td><td>-100 Mesh (less than 150 microns)</td>

<td>Oxygen:</td><td> 77</td>

<td>Surface area:</td><td>255 cm²/ g</td>

<td>Specific oxygen:</td><td>0.30 micrograms / cm²</td>

Example 4

The following tests were performed to show that the tantalum, niobium or alloy powder of the present invention is compressible, and to show the strength of the powder of the present invention. Tantalum powder having an oxygen content of less than 300 ppm, prepared by a procedure similar to the procedure of Example 1, was used as the starting powder. The initial powder was placed in a die and pressed at various pressures into tablets. The density of the tablets as a function of the pressing pressures were as follows:

ES 2 313 136 T3

<img file="ES2313136T3_D0001.tif" />

These results show that the powders of the present invention are compressible.

To show the strength of the powder of the present invention after mechanical pressing, tantalum powder having an oxygen content of less than 300 ppm, prepared by a procedure similar to the procedure of Example 1, was placed in a die and pressed, at various pressures, in bars of about 1/2 inch by about 1/2 inch, by about 2 inches. The transverse resistance to breakage of these bars was as follows:

Pressure Transverse Breaking Strength (lbs./sq.) (Lbs./sq.)

<td> 40,000</td><td> 2680</td>

<td> 60,000</td><td> 5385</td>

<td> 80,000</td><td> 6400</td>

<td> 90,000</td><td> 8360</td>

Generally a minimum strength of about 2000 lbs./sq. In. Is desired for normal treatment of pressed compacts. Data from the compressibility test in conjunction with the breaking strength test indicates that this level of strength can be obtained with the powder of the present invention formed at a pressure of about 40,000 PSI.

Other realizations

In addition to the embodiments indicated above, the following additional embodiments can be made. These embodiments are part of the invention only insofar as they fall within the scope of the claims.

A. The production of a tantalum formed product having an oxygen content of less than 300 ppm can be achieved by cold isostatic pressing of various types of known Ta / Nb powders to form a compact, followed by a pressing step hot isostatic (HIP) to densify the compact and then thermomechanical processing of the powder compact to further densify and complete the bond. Preferably, tantalum powder having an oxygen content of less than 300 ppm, prepared by a procedure similar to the procedure of Example 1, would be used as the starting powder. This powder would be cold isostatically pressed at 60,000 lbs./sq. In. And at room temperature, into a compact with rectangular cross-section, then hermetically encapsulated and hot isostatically pressed (HIPed) at 40,000 lbs./sq. In. And 1300 degrees C for 4 hours. The HIPed compact would be decapsulated and turned into a sheet or sheet through thermomechanical processing steps.

B. A similar process of simple cold isostatic pressing, sintering and thermomechanical processing using tantalum powder having an oxygen content of less than 300 ppm, prepared by a procedure similar to the procedure of Example 1, can be carried out by cold isostatic pressing. to 60,000 lbs./sq. in. in a bar-shaped preform. This preform would be sintered at 1500 degrees C (0.3% theoretical density, Th) for 2 hours in a vacuum of less than about 0.001 Torr to yield a preform that has a density of about 95% Th and less than 300 ppm of oxygen. The sintered preform would be converted into sheet and sheet by thermomechanical processing steps.

C. Formed tantalum rod and wire having an oxygen content of less than 300 ppm can be made by hot extrusion and thermomechanical processing using tantalum powder having an oxygen content of less than 300 ppm, prepared by a similar procedure. that of Example 1, as the starting powder. This powder would be hermetically encapsulated and then extruded through a circular die at 1000 ° C. The extrudate would have an oxygen content of less than 300 ppm. The extruded preform was converted into rod and wire by thermomechanical processing steps.

ES 2 313 136 T3

D. Another sequence of such a process is cold isostatic pressing, hot extrusion and thermomechanical processing using tantalum powder having an oxygen content of less than 300 ppm, prepared by a procedure similar to that of Example 1, as the starting powder. This powder would be cold isostatically pressed, hermetically encapsulated then extruded at 1000 ° C. The extrudate would have an oxygen content of around 300 ppm. It would be converted into rod and wire through thermomechanical processing steps.

E. The production of a formed tantalum sheet or sheet having an oxygen content of less than 300 ppm by hot extrusion and thermomechanical processing can be carried out, using tantalum powder having an oxygen content of less than 300 ppm, prepared by a procedure similar to the procedure of Example 1, as the starting powder. This powder would be hermetically encapsulated then extruded through a rectangular die at 1000 ° C to produce an extrudate having an oxygen content of less than 300 ppm. The extrudate would be made into sheet or sheet by thermomechanical processing.

F. Tantalum sheet or sheet with an oxygen content of less than 300 ppm can be produced using the powder of Example 1 by cold isostatic pressing, hot extrusion and thermomechanical processing. This compact manufactured by cold isostatic pressing could be hermetically encapsulated then extruded at 1000 ° C to produce an extrudate with an oxygen content of around 300 ppm that can be converted into sheet and sheet by thermomechanical processing steps.

G. Tantalum products that have an oxygen content of less than 300 ppm can be prepared by mechanical pressing, sintering, re-pressing and re-sintering. Tantalum powder having an oxygen content of less than 300 ppm, prepared by a procedure similar to the procedure of Example 1, can be used as the starting powder. This is placed in a die and mechanically pressed, using uniaxial pressure. The pressed tablet should then be sintered at 1500 ° C for 2 hours in a vacuum evacuated to less than about 0.001 Torr. The sintered tablet would then be re-pressed and re-sintered at 1500 degrees C for 2 hours in a vacuum evacuated at less than about 0.001 Torr. The re-sintered tablet will have an oxygen content of less than 300 ppm and will be suitable for thermomechanical processing to produce a tantalum formed product.

H. Tantalum product having an oxygen content of less than 300 ppm can be prepared by aerosol deposition, using starting powder having an oxygen content of less than 300 ppm, prepared by a procedure similar to the procedure of Example 1. The powder can be spray deposited to a thickness of 0.01 inch on an alloy substrate formed from stainless steel. The particle size, flow properties and oxygen content of the powder will be suitable for consolidation by aerosol deposition.

I. Plasma activated sintering can be used for the production of a tantalum formed product having an oxygen content of less than 300 ppm. Tantalum powder having an oxygen content of less than 300 ppm, prepared by a procedure similar to the procedure of Example 1, would be used as the starting powder. This would be poured into a tantalum sheet coated graphite die and graphite punches inserted into the die from both ends. The die punch assembly is located in a water-cooled steel block. Another block of water-cooled steel is brought into contact with the upper punch. The water-cooled steel block is attached to a hydraulic piston at the top and the base at the bottom to dissipate heat accumulated during consolidation. The top and bottom water-cooled steel blocks are also attached to the positive and negative ends of a DC power supply.

The powder filled die punch assembly is provided in a chamber. The chamber must be evacuated at 500 milliTorr. The consolidation would be carried out in two stages. In the first stage, the intention is mainly to purify the powder via plasma by metallizing the surfaces of the particle. A pressure of around 4300-psi would be applied to the powder through the punches and a pulsed DC current of 1000 A was passed through the powder. These conditions are maintained for two minutes.

During the second stage the pressure would be raised to around 6500 psi and an unpulsed DC current of 4500 A passed through the powder. These conditions are maintained for two minutes. At the end of the cycle, the current to the punching machines is disconnected, the vacuum pump is switched off and the evacuation chamber filled again with nitrogen. The die punch set is allowed to cool to room temperature and the consolidated tantalum sample is removed from the die. The consolidation cycle would be around eight minutes. The sintered preform will have a density greater than 95% of the theoretical density and the oxygen content of less than 300 ppm.

J. A niobium powder with less than 300 ppm oxygen can be produced by deoxidation of niobium hydride under argon partial pressure. The niobium hydride powder would be mixed with 0.3% by weight Mg and placed in a vacuum oven retort, which is evacuated, and refilled with argon. The pressure in the furnace was set at 100 microns with argon flowing and the vacuum pump running. The furnace temperature was raised to 650 ° C in 50 ° C increments, held until the temperature was stabilized, then it was raised to 950 ° C in 50 ° C increments. When the temperature was stabilized at 950 ° C it was maintained for two hours. After two hours at 950 ° C the oven was turned off. Once the oven cooled down, its powder content was removed from the retort. The magnesium, which contains the oxygen, would then be removed from the metal powder by acid leaching to produce the resulting niobium powder which has an oxygen content of less than 300 ppm.

ES 2 313 136 T3

K. A product formed from tantalum, produced by mechanical pressing and sintering. Tantalum powder having an oxygen content of less than 300 ppm, prepared by a procedure similar to the procedure of Example 1, was used as the starting powder. This tantalum powder was placed in a die and pressed, using uniaxial pressure, into a tablet with a pressed density of around 80% of the theoretical density. This tablet was then sintered at 1500 ° C for 2 hours in a vacuum evacuated at less than about 0.001 Torr. The final sintered tablet has an oxygen content of less than 300 ppm.