Method of producing a highly dense semifinished product or component

Abstract

A component or semifinished product is produced of a material of the group comprising molybdenum, molybdenum alloy, tungsten and tungsten alloy with an average relative density of >98.5% and a relative core density of >98.3%. The material is sintered to a relative density D, with 90%<D<98.5% and a proportion of the closed pores with respect to the overall porosity of >0.8 and also hot-isostatically pressed at a temperature 0.40 to 0.65×solidus temperature and under a pressure of from 50 to 300 MPa. Components produced in this way, used for example as electrodes, have much improved service life characteristics.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority, under 35 U.S.C. § 119, of Austrian application GM 888/2005, filed Dec. 23, 2005; the prior application is herewith incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

Field Of The Invention

The invention relates to a method of producing a semifinished product or component from a material of the group comprising molybdenum, molybdenum alloy, tungsten and tungsten alloy with an average relative density of >98.5% and a relative core density of >98.3%.

The refractory metals molybdenum, tungsten and their alloys are usually produced by powder metallurgy. Ore concentrates are used as the starting product, chemically processed to form intermediate products and then reduced to give metallic powder. In this case, the reducing agent is hydrogen. Alloying elements may be admixed before, during or after the reduction.

Typical molybdenum alloys are TZM (Ti—Zr—C-alloyed Mo), Mo—La₂O₃, Mo—Y₂O₃ and Mo—Si—B. On the tungsten side, AKS-W (K-doped tungsten), W-ThO₂, W—La₂O₃, W—Ce₂O₃, W—Y₂O₃ and AKS-W-ThO₂ should be mentioned. AKS-W and AKS-W-ThO₂ are used specifically in lighting technology and there in turn in particular for filaments and electrodes. The potassium additions found in AKS-W, which take the form of small bubbles, stabilize the grain growth, whereby a stable microstructure is retained even under very high operating temperatures and over long times. This is of essential importance in particular for the service life characteristics of electrodes for heavy-duty lamps, such as for example metal halide and short arc lamps, where the surface temperature is up to 2600° C.

The powder is compacted by die pressing or cold-isostatic pressing. Semifinished products of large dimensions are produced with preference by cold-isostatic pressing. In the case of wire rods and small rolled sheet bars, both die pressing and cold-isostatic pressing are used. If molybdenum powder with a typical particle size according to Fisher of from 2 to 5 μm and tungsten powder with a typical particle size according to Fisher of from 1.5 to 4.5 μm are used, fractional bulk densities in the range from 0.11 to 0.17 (molybdenum) and 0.13 to 0.22 (tungsten) are obtained. If a pressing pressure in the range from 200 to 500 MPa is used, fractional green densities in the range from 0.6 to 0.68 are achieved, both in the case of molybdenum and in the case of tungsten.

In a next process step, the green compacts are sintered. The sintering process is in this case conducted as far as possible in such a way that the sintered body has a low porosity, combined with a fine-grained microstructure. Molybdenum and tungsten are usually sintered in hydrogen with a dew point of less than 0° C. The usual sintering temperatures are 1800° C. to 2200° C. in the case of molybdenum, 2100° C. to 2700° C. in the case of tungsten. Usual sintering times are 1 to 24 hours. Since the sintering process is determined by grain boundary diffusion, it is possible to sinter at a lower temperature in the case of a small particle size. However, the particle size also determines the pore size in the sintered semifinished product. For instance, the pore size can be reduced by a factor of 3 if the particle size according to Fisher of the molybdenum powder used is reduced from 10 μm to 2.6 μm.

A disadvantage of fine-grained powder, however, is the higher proportion of adsorbed gases, in particular oxygen. This is so because, during the sintering process, this oxygen reacts with the hydrogen of the sintering gas to form water vapour. On account of the low gas permeability of the green compact, which is reduced still further during the sintering process, the water vapour cannot be removed to an adequate extent, in particular from the centre of the sintered body. This is especially the case whenever fine-grained powder with a particle size according to Fisher of <4.5 μm is used.

A high water vapor content in the interior of the sintered body triggers a CVT (Chemical Vapor Transport) reaction. Through material transport via the gas phase, this CVT reaction leads to destruction of specific surface area, and consequently to a reduction in the driving forces for the sintering, specifically in the interior of the sintered body. This process is intensified in the case of molybdenum and tungsten alloys, where additions during the sintering give off an oxygen-containing species, causing increased water vapor formation, as is the case for example with AKS-W, Mo—La₂O₃ or W—La₂O₃. Gas phase reactions therefore limit the dimensions of the sintered body in particular in the case of these alloys. In the case of sintered bodies with greater dimensions or when very fine-grained powder is used, the achievable sintered density, especially in the center of the sintered body, is lower than in the case of small sintered bodies or when coarser powder is used.

Following the sintering process, molybdenum, tungsten and their alloys are usually subjected to a thermomechanical treatment. The thermomechanical treatment achieves the desired form, a reduction/elimination of the porosity and the setting of the desired mechanical and microstructural properties. With an increasing degree of forming, the density increases up to the theoretical density and the grain size decreases. The reduction in the grain size thereby depends strongly on the chosen forming temperature and the intermediate annealing temperatures.

As already mentioned, the size of the sintered body is limited when fine-grained powders are used, or alloys which contain a species which releases oxygen or water vapor during the sintering process. If a product which has greater dimensions is then to be produced from this sintered body, the possible degree of forming may not be adequate to close the porosity, especially in the center of the sintered body.

This is the case for example with AKS tungsten, which is used as the electrode material in lamps. Especially in the case of short arc lamps, anodes of up to 55 mm in diameter are used. A lifetime-determining characteristic of such electrodes is their dimensional stability. The deformation of the electrodes is initiated by thermally induced stresses. These thermally induced stresses may lead for example to elevations in the region of the electrode plateau. The arc is then concentrated on these elevations, which leads to local overheating. This may lead to melting of the electrode in this region.

Furthermore, the local overheating leads to increased vaporizing of the electrode material. The vaporized electrode material is deposited on the lamp bulb and thereby drastically reduces the light flux.

Investigations have now shown that creep phenomena are responsible for the formation of the elevations. If the material contains pores, these creep phenomena are increased, since the pores act as vacancy sources and sinks. In addition, the pores reduce the heat dissipation, which can lead to an intensification of the local temperature increase.

Furthermore, a fine-grained electrode material has a longer service life. This is attributable to the fact that, in the case of coarse-grained material, the damage is concentrated on a few grain boundaries, as a result of which a self-intensifying effect occurs there due to a concentration of the arc.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a method for the production of highly dense semifinished products or components which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which provides for semifinished products or components with a high density, especially also in the center, along with a fine-grained microstructure.

With the foregoing and other objects in view there is provided, in accordance with the invention, a method of producing a component or semifinished product from a material of the group comprising molybdenum, molybdenum alloy, tungsten and tungsten alloy with an average relative density of >98.5% and a relative core density of >98.3%, the method which comprises the following steps:

• • preparing a powder with a particle size according to Fisher of from 0.5 to 10 μm;
  • pressing the powder under a pressure of 100 to 500 MPa;
  • sintering at a temperature of 0.55 to 0.92×solidus temperature, to form a sintered body with relative density D, with 90%<D<98.5%;
  • hot-isostatic pressing, substantially without utilizing a can, at a temperature of 0.40 to 0.65 ×solidus temperature and under a pressure of 50 to 300 MPa; and forming with a degree of forming φ, with 15%<φ<90%.

With the method according to the invention it is possible to produce semifinished products or components from molybdenum, tungsten and their alloys with an average relative density of greater than 98.5% and a relative core density of greater than 98.3%. The average relative density is understood as meaning the average density with respect to the weight of unit volume. The core density is understood by a person skilled in the art as meaning the density at the center of a semifinished product or component. Since the core volume is not specified there with respect to the overall volume, for the following statements the core volume is defined as follows for the determination of the core density:

the 10% nearest the center of the overall surface area transversely to the

direction of deformation×the extent in the direction of deformation. p In the deformed state, the semifinished product or the component has transversely to its direction of deformation with preference a grain number of greater than 100 grains/mm².

In the case of the method according to the invention, commercial molybdenum and tungsten powders in a particle size range from 0.5 to 10 μm according to Fisher are used.

Alloying elements may be added to the powder before, during or after the reduction process. The powder is compacted by the usual compacting processes, such as for example by pressing or cold isostatic pressing, under pressing pressures of 100 to 500 MPa.

The sintering takes place at a temperature 0.55 to 0.92×solidus temperature. The sintering temperature is in this case chosen such that a sintered density of 90% to 98.5% of the theoretical density is set, with preferably a proportion of the closed pores with respect to the overall porosity of >0.8. If the relative density is above 98.5%, the objective, that is the production of a component or semifinished product with a grain number of >100 grains/mm², cannot be achieved.

If the proportion of closed porosity with respect to the overall porosity is >0.8, it is ensured that the required properties are achieved in the next step, the hot-isostatic pressing. If the value lies below 0.8, a forming step with 2%<φ<60% is required after the sintering process. φ is defined by:
((starting cross-sectional surface area—cross-sectional surface area after the forming process)/starting cross-sectional surface area)×100.

This ensures closing of the peripheral pores.

The hot-isostatic pressing is carried out without the use of a can and is performed at a temperature 0.40 to 0.65×solidus temperature under a pressure of 50 to 300 MPa. If the temperature lies below 0.4 ×solidus temperature, the object, an average relative density of >98.5% and a relative core density of >98.3% in the component or semifinished product, cannot be achieved. If the temperature lies above 0.65×solidus temperature, there is undesired grain coarsening due to normal or abnormal grain growth. If the pressure lies below 50 MPa, the density aim likewise cannot be achieved. At pressures above 300 MPa, the method according to the invention is no longer economically viable.

In a subsequent step, the hot-isostatically pressed part undergoes forming. The degree of forming φ is in this case 15 to 90%. If the degree of forming φ lies below 15%, the relative core density of >98.3% cannot be achieved. If the degree of forming lies above 90%, the method is in turn not economically viable, since dense products can also be produced without the hot-isostatic pressing according to the invention.

The method according to the invention has proven to be particularly successful for the production of electrodes in the diameter range from 15 to 55 mm, which are used in discharge lamps. If the diameter lies below 15 mm, such electrodes can be economically produced by means of conventional production methods. The upper limit of 55 mm is obtained from the limiting wattage of such lamps.

The raw material for the electrodes is preferably subjected to forming by radial forging or rolling. Tests have shown that electrodes produced by the method according to the invention have on average a service life that is longer by 20% than electrodes produced by conventional production methods.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as embodied in method of producing a highly dense semifinished product or component, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the following example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

EXAMPLE

For the production of an AKS-W electrode, an AKS-W powder with a particle size according to Fisher of 4.1 μm was used. The powder was compacted by cold-isostatic pressing under a pressing pressure of 200 MPa to form a green compact. The sintering was performed at a temperature of 2250° C. in hydrogen. The sintered rods produced in this way had an average relative density, determined by means of the buoyancy method, of 92.0%. The proportion of closed porosity was >95%, the measurement being performed by means of mercury porosimetry. In the next step, the sintered bodies were hot-isostatically compacted at a temperature of 1750° C. and under a pressure of 195 MPa for 3 hours. The relative average density after the hot-isostatic pressing operation was 97.9%. Subsequently, the rods were formed on a radial forging machine. The degree of forming φ was 67%. The average relative density of the rods after the forming process was 99.66%, the relative core density was 99.63%. The grain size in the unformed state and after annealing at 1800° C./4 hours was determined. In the unformed state, it was about 10,000 grains/mm², both in the center and in the peripheral region of the rods. In the annealed state, a very fine-grained microstructure could still be established, with an average grain number in the center of the rods of about 800 and in the peripheral region of 850 grains/mm².

Chemical analysis of the rods produced the following result:

	Potassium	15	μg/g
	Silicon	6	μg/g
	Carbon	<5	μg/g
	Oxygen	7	μg/g.

The material prepared according to the invention was used to produce anodes for 2.5 kW short arc lamps for cinema projection. The ascertained average service life was 2060 hours. Also used as a comparison was a material which was not subjected to subsequent compaction by a hot-isostatic pressing operation after the sintering process, but otherwise underwent the same production process. With that material it was possible to achieve an average service life of 1710 hours.

Claims

1. A method of producing a component or semifinished product from a material of the group comprising molybdenum, molybdenum alloy, tungsten and tungsten alloy with an average relative density of >98.5% and a relative core density of >98.3%, the method which comprises the following steps:

preparing a powder with a particle size according to Fisher of from 0.5 to 10 μm;

pressing the powder under a pressure of 100 to 500 MPa;

sintering at a temperature of 0.55 to 0.92 ×solidus temperature, to form a sintered body with relative density D, with 90%<D<98.5%;

hot-isostatic pressing, substantially without utilizing a can, at a temperature of 0.40 to 0.65 ×solidus temperature and under a pressure of 50 to 300 MPa; and forming with a degree of forming φ, with 15%<φ<90%.

2. The method according to claim 1, which comprises forming a component or a semifinished product with an average grain number of >100 grains/mm2 in a deformed state thereof.

3. The method according to claim 1, prior to hot-isostatic pressing, subjecting the sintered body to additional forming, with a degree of forming of 2%<φ<60%.

4. The method according to claim 1, wherein the sintered body has a proportion of closed pores with respect to an overall porosity of >0.8.

5. The method according to claim 1, which comprises forming a component or a semifinished product of K-doped tungsten (AKS-W) having a K content of 5 to 70 μg/g.

6. The method according to claim 1, wherein the forming step comprises radially forging or rolling to produce a rod.

7. The method according to claim 6, which comprises forming the rod with a diameter of from 15 to 55 mm.

8. The method according to claim 6, which comprises producing a lamp electrode with the rod.

9. The method according to claim 8, which comprises integrating the lamp electrode in a short arc lamp.