EP1801247B1 - Process of production of high-density semi-finished or finished product - Google Patents

Description

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

The refractory metals molybdenum, tungsten and their alloys are usually produced by powder metallurgy. The starting material here are ore concentrates, which are chemically processed into intermediates and then reduced to metallic powder. The reducing agent is hydrogen. Alloy elements can be added before, during or after the reduction.

Typical molybdenum alloys are TZM (Ti-Zr-C alloyed Mo), Mo-La ₂ O ₃ , Mo-Y ₂ O ₃ and Mo-Si-B. On the tungsten side AKS-W (K-doped tungsten), W-ThO ₂ , W-La ₂ O ₃ , W-Ce ₂ O ₃ , WY ₂ O ₃ and AKS-W-ThO ₂ are mentioned. AKS-W and AKS-W-ThO ₂ are used especially in lighting technology and here again mainly for filaments and electrodes. The potassium additives present in the AKS-W, which are in the form of bubbles, thereby stabilize the grain growth, whereby a stable structure is maintained even at very high operating temperatures and long times. This is especially true for the service life of electrodes for highly loaded lamps, such as Metal halide and short arc lamps, where the surface temperature is up to 2,600 ° C, of essential importance.

The powder is compacted by die pressing or cold isostatic pressing. Large sized semi-finished product is preferably produced by cold isostatic pressing. For wire rods and small billets, both die pressing and cold isostatic pressing are used. When using Fisher molybdenum powder with a typical Fisher particle size of 2 to 5 microns and tungsten powder having a typical Fisher particle size of 1.5 to 4.5 microns, fractional bulk densities in the range of 0.11 to 0.17 (molybdenum) and 0.13 to 0.22 (tungsten). Using a compacting pressure in the range of 200 to 500 MPa, fractional green densities are achieved in the range of 0.6 to 0.68 for both molybdenum and tungsten.

In a next process step, the green compacts are sintered. If possible, the sintering process is carried out in such a way that the sintered body has a low porosity associated with a fine-grained microstructure. Molybdenum and tungsten are usually sintered in hydrogen with a dew point <0 ° C. The usual sintering temperatures for molybdenum are 1,800 ° C to 2,200 ° C, for tungsten 2,100 ° C to 2,700 ° C. Usual sintering times are 1 to 24 hours. Since the sintering process is determined by grain boundary diffusion, sintering can be carried out at a lower temperature with a smaller particle size. However, the particle size also determines the pore size in the sintered semifinished product. Thus, the pore size can be reduced by a factor of 3 when the particle size of Fisher of the molybdenum powder used is reduced from 10 microns to 2.6 microns.

A disadvantage of fine-grained powder, however, is the higher proportion of adsorbed gases, in particular oxygen. During the sintering process, this oxygen reacts with the hydrogen of the sintering gas to form water vapor. Due to the low gas permeability of the green compact, which is further reduced during the sintering process, the water vapor, in particular from the center of the sintered body, can not be removed sufficiently. This is especially the case when fine-grained powder having a particle size of Fisher <4.5 μm is used.

A high water vapor content in the interior of the sintered body triggers a CVT (Chemical Vapor Transport) reaction. This CVT reaction leads, through material transport through the gas phase, to a destruction of specific surface area and thus a reduction in the driving forces for sintering, in particular in the interior of the sintered body. This process is exacerbated in molybdenum and tungsten alloys, where additives release an oxygen-containing species during sintering, resulting in increased water vapor formation, such as in AKS-W, Mo-La ₂ O ₃ or W-La ₂ O ₃ Case is. Gas phase reactions therefore limit the dimension of the sintered body, especially in these alloys. With sintered bodies with larger dimensions or with the use of very fine-grained powder, the achievable sintering density, in particular in the center of the sintered body, is lower than in the case of small sintered bodies or when coarser powder is used.

Subsequent to the sintering process, molybdenum, tungsten and their alloys are usually subjected to a thermomechanical treatment. The thermomechanical treatment achieves the desired shape, reduction / elimination of porosity, and adjustment of the desired mechanical and microstructural properties. With increasing degree of deformation, the density increases up to the theoretical density and the grain size decreases. The reduction of the grain size depends strongly on the selected forming temperature and the intermediate annealing temperatures.

As already mentioned, the use of fine-grained powders or, in the case of alloys containing a species which splits off oxygen or water vapor during the sintering process, is limited in the size of the sintered body. If a product having larger dimensions is produced from this sintered body, then the possible degree of deformation for closing the porosity, in particular in the center of the sintered body, may not be sufficient.

This is the case, for example, with AKS tungsten, which is used as electrode material in lamps. The production of lamp electrodes from W is, for example AT-U-006,240 known. Anodes with a diameter of up to 55 mm are used especially for short arc lamps. A life-determining property of such electrodes is their dimensional stability. The deformation of the electrodes is triggered by thermally induced voltages. These thermally induced voltages can, for example, lead to elevations in the region of the electrode plateau. The arc is then concentrated on these bumps, resulting in localized overheating. This can lead to the melting of the electrode in this area.

Furthermore, the local overheating leads to an increased evaporation of the electrode material. The vaporized electrode material settles on the lamp bulb and drastically reduces the light flux.

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

Furthermore, a feinkömiges electrode material has a longer life. This is due to the fact that with coarse-grained material, the damage concentrates on a few grain boundaries, whereby there is a self-reinforcing effect by a concentration of the arc.

The object of the invention is therefore to provide semi-finished products or components with a high density, especially in the center, connected to a fine-grained structure.

The object is achieved by a method having the features according to claim 1.
With the method according to the invention it is possible to produce semi-finished products or components made of molybdenum, tungsten and their alloys with an average relative density> 98.5% and a relative core density> 98.3%. By average relative density is the average density relative to the specific To understand weight. Under Kemdichte the expert understands the density in the center of a semifinished product or component. Since the core volume is not specified in relation to the total volume, the core volume for the determination of the core density is defined as follows for the following data: The center-nearest 10% of the total area transverse to the deformation direction x extension in the direction of deformation.
The semifinished product or the component, in the deformed state, preferably has a comm number> 100 grains / mm ² transversely to its deformation direction.

In the process according to the invention, commercial molybdenum and tungsten powders in a particle size range of 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 conventional densification processes such as die pressing or cold isostatic pressing at press pressures of 100 to 500 MPa.

The sintering takes place at a temperature of 0.55 to 0.92 x solidus temperature. The sintering temperature is chosen so that a sintering density of 90% to 98.5% of the theoretical density, preferably a proportion of the closed pores based on the total porosity of> 0.8 is set. If the relative density exceeds 98.5%, the objective, namely the production of a component or semi-finished product with a count of> 100 grains / mm ² , can not be achieved.

When the proportion of closed porosity relative to the Total porosity> 0.8, it is ensured that the required properties are achieved in the subsequent step, the hot isostatic pressing. If the value is below 0.8, a forming step with 2% <φ <60% is required after the sintering process. φ is defined by: $$\left((\mathrm{Initial\; cross-sectional\; area}-\mathrm{Cross-sectional\; area\; after\; the\; forming\; process})/\mathrm{Initial\; cross-sectional\; area}\right)\times \mathrm{100th}$$

This ensures a closing of the near-edge pores.

The hot isostatic pressing is carried out without using a jug and is carried out at a temperature of 0.40 to 0.65 x solidus temperature at a pressure of 50 to 300 MPa. If the temperature is below 0.4 x solidus temperature, the target, a mean relative density of> 98.5% and a relative core density of> 98.3% in the component or semi-finished product, can not be achieved. If the temperature is above 0.65 x solidus temperature, undesirable coarsening occurs due to normal or abnormal grain growth. If the pressure is below 50 MPa, the density target can not be achieved either. At pressures above 300 MPa, the inventive method can no longer be economically represented.

In a subsequent step, the hot isostatically pressed part is reshaped. The degree of deformation φ is 15 to 90%. If the degree of deformation φ is less than 15%, the goal of a relative density> 98.3% can not be achieved. If the degree of deformation is more than 90%, again the process can not be economically represented, since dense products can also be produced without the hot isostatic pressing according to the invention.

The method according to the invention is particularly useful for the production of electrodes in the diameter range of 15 to 55 mm, which are used in discharge lamps. If the diameter is less than 15 mm, such electrodes can be produced more economically by means of conventional production methods. The upper limit of 55 mm results from the border wattage of such lamps.

The starting material for the electrodes is preferably formed by radial forging or rolling. Experiments have shown that electrodes produced by the method according to the invention have on average 20% longer service life than electrodes which have been produced by conventional production methods.

In the following, the invention will be explained in more detail by way of example.

Example:

For the production of an AKS-W electrode, an AKS-W powder with a particle size of Fisher of 4.1 μm was used. The powder was compacted by cold isostatic pressing at a pressure of 200 MPa into a green compact. The sintering was carried out at a temperature of 2,250 ° C in hydrogen. The sintered rods thus produced had an average specific gravity, as determined by buoyancy, of 92.0%. The proportion of closed porosity was> 95%, the measurement being carried out by means of mercury porosimetry. The sintered bodies were hot isostatically compacted in the subsequent step at a temperature of 1750 ° C and a pressure of 195 MPa for 3 hours. The relative mean density after the hot isostatic pressing process was 97.9%. Subsequently, the rods were reshaped on a radial forging machine. The degree of deformation φ was 67%. The average relative density of the bars after the forming process was 99.66% and the relative density was 99.63%. The grain size was determined in the formed state and after annealing at 1,800 ° C / 4 hours. In the formed state, it was about 10,000 grains / mm ² both in the center and in the edge region of the rods. In the annealed state, a very fine-grained microstructure was still found, with a mean number in the center of the rods of about 800 and in the border area of 850 grains / mm ² .
The chemical analysis of the rods gave the following result: potassium 15 μg / g, silicon 6 μg / g, carbon <5 μg / g, oxygen 7 μg / g.

Anodes for 2.5 kW short arc lamps for cinema projection were produced from the material produced according to the invention. The determined average service life was 2,060 hours. By comparison, a material was also used which, after the sintering process, was not subjected to any subsequent densification by a hot isostatic pressing process, with otherwise identical production process. This resulted in an average service life of 1,710 hours.

Claims (9)

1. 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 production comprising at least the following method steps:

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

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

   • sintering at a temperature 0.55 to 0.92 x solidus temperature to a relative density D, with 90% < D < 98.5%;

   • hot-isostatic pressing without the use of a can at a temperature 0.40 to 0.65 x solidus temperature and under a pressure of 50 to 300 MPa,

   • forming with a degree of forming ϕ, with 15% < ϕ < 90%.
2. Method according to Claim 1, characterised in that the component or the semifinished product has in the unformed state an average grain number of > 100 grains/mm².
3. Method according to Claim 1 or 2, characterised in that, before the hot-isostatic pressing, the sintered body is subjected to additional forming, with 2% < ϕ < 60%.
4. Method according to one of Claims 1 to 3, characterised in that the sintered body has a proportion of the closed pores with respect to the overall porosity of > 0.8.
5. Method according to one of Claims 1 to 4, characterised in that the component or the semifinished product consists of K-doped tungsten (AKS-W) and the K content is 5 to 70 µg/g.
6. Method according to one of Claims 1 to 5, characterised in that the forming is performed by radial forging or rolling and in this way a rod is produced.
7. Method according to Claim 6, characterised in that the rod has a diameter of from 15 to 55 mm.
8. Method according to Claim 6 or 7, characterised in that the rod is used to produce a lamp electrode.
9. Method according to Claim 8, characterised in that the lamp electrode is used in a short arc lamp.