Process for production of a shaped part

A process for the production of a shaped part which is produced from a high-melting point metal powder with crystalline sinter-activating additives. The process includes the steps of preparing, compressing and sintering the metal powder. Prior to the sintering step, the final contour of the shaped part is substantially shaped. The process is primarily directed for the production of shields for radiation protection, as melting crucibles or as electrodes.

BACKGROUND OF THE INVENTION 
a) Field of the Invention 
The invention is directed to a process for the production of a shaped part 
which is produced from a high-melting point metal powder with crystalline 
sinter-activating additives. 
b) Description of the Related Art 
Structural parts or shaped parts which are produced by a process of this 
kind are used, for instance, as shields for radiation protection, as 
melting crucibles or as electrodes. The electrodes, principally tungsten 
electrodes, are used in particular for inert gas shielded arc welding, 
plasma arc welding, plasma cutting, and for discharge lamps and are 
produced substantially by the following process steps: powder production, 
mixing, pressing, sintering, and shaping the final contour of the 
electrode with mechanical working, e.g., hammering, grinding and/or 
cutting. 
In manufacturing the electrodes, the effort is made to achieve in these 
electrodes a high density, homogeneous distribution of the dopant in the 
tungsten matrix material of the metal powder, and a defined structure, 
i.e., a predetermined grain formation. 
In order to produce a shaped part such as the electrode mentioned above, it 
is known to use a tungsten metal powder with a purity of approximately 
99.95% and corresponding dopants with a low electron work function, e.g., 
ThO.sub.2, Y.sub.2 O.sub.3, CeO.sub.2, ZrO.sub.2, and/or La.sub.2 O.sub.3. 
These dopants lower the work function compared with pure tungsten, i.e., 
during operation the emission current density of the electrode is 
increased while maintaining the same temperature. A finely dispersed, 
homogeneous distribution of the dopant in the final product results from 
intensive mixing--preparation--of the tungsten powder with the dopants. 
The prepared metal powder with the dopants is then pressed to form an 
intermediate product, e.g., cylindrical rods, wherein a uniform density is 
aimed for in the intermediate product. 
The intermediate product is transformed to the metallic state during 
sintering by applying an electric current--Coolidge process. Due to the 
electrical resistance of the compressed material, a temperature of roughly 
2600.degree. to 3000.degree. is reached and is maintained for 
approximately 15 to 30 min. In this way, the metal powder with the dopant 
is compacted to approximately 80 to 90% of the theoretical density. This 
process is effected in a dry hydrogen atmosphere to prevent oxidation at 
the surface of the tungsten material. 
Depending on the degree of deformation, the intermediate product which is 
now sintered is reshaped mechanically to final contours at a temperature 
of 900.degree. to 1600.degree. C., e.g., by hammering. The final 
dimensions are achieved by subsequent grinding, surface impurities and 
possible cracks are eliminated at the same time. 
However, the disadvantage in known shaped parts consists in that their 
manufacture is very expensive due to the many process steps and very 
costintensive due to high energy consumption. In addition, possibilities 
for shaping the shaped parts are severely limited. 
Further, the dopants evaporate during the manufacturing process. In 
electrodes, for example, this may lead to an inhomogeneous distribution of 
the dopants in the electrode and accordingly to inhomogeneous properties 
in the electrode during operation. 
OBJECT AND SUMMARY OF THE INVENTION 
The primary object of the present invention is to improve a process for the 
production of a shaped part of the above type in such a way that the 
shaped part can be produced in a substantially simpler and more economical 
manner and with more exactly determined properties while avoiding the 
disadvantages mentioned above. In particular, the shaping possibilities 
are also expanded. 
This object is met by a process of the above type wherein the metal powder 
is prepared, compressed and sintered, and wherein the final contour of the 
shaped part is substantially shaped prior to sintering. 
As a result of the process according to the invention for the production of 
a shaped part, it is now possible to manufacture complex geometries in 
different dimensions, since the final contour of the shaped part is 
substantially determined by means of a shaping process prior to sintering, 
that is, when the compacted metal powder with the dopants/additives has 
approximately 50 to 70% of the theoretical density and has not yet been 
transformed to the metallic state. The structure of the shaped part is now 
no longer defined during production by the limited shaping and 
machinability of the known sintered intermediate products. 
In order to reduce the sintering temperature and sintering time, a 
crystalline sinter-activating additive is added to the metal powder. 
The shaped part is advantageously constructed as an electrode, especially 
as a cathode, and primarily contains at least one dopant made from a 
material with a low work( function. For instance, it is now easily 
possible to produce a hollow cathode or a cored electrode with the 
features according to the invention. 
The process for producing a shaped part according to the invention has the 
further advantage that it reduces the tendency of the high-melting point 
matrix material of the metal powder of the shaped part to recrystallize. 
In particular, the reshaping which was previously necessary in the 
production of electrodes after sintering and the predeformation of the 
electrode brought about thereby, e.g., by means of hammering, is now 
completely dispensed with. An intensive recrystallization of the matrix 
material of the metal powder in the tip of the electrode which is brought 
about by the high operating temperature during use reduces the 
diffusion-controlled material transfer of the emission-promoting dopant at 
the surface of the electrode tip. 
Up to 5 percent by weight of the dopant with a low work function is 
advisably introduced into the metal powder prior to pressing. The dopant 
is generally formed of one or more elements or element compounds with 
electron configurations which are unstable with respect to energy, i.e., 
with incompletely occupied d--and f--electron shells, with electron donor 
action. These elements or element compounds chiefly form oxides and/or 
borides of the elements of groups IIIB to IVB of the periodic table and 
the first three elements of the lanthanide and actinide groups. In 
particular, these are preferably Y.sub.2 O.sub.3, ZrO.sub.2, La.sub.2 
O.sub.3, CeO.sub.2 and/or LaB.sub.6. 
The metal powder advisably contains up to 1.0 percent by weight of the 
sinter-activating additive prior to pressing. 
The sinter-activating additive is formed by the elements of group VIII of 
the periodic table. Nickel, palladium and/or platinum are preferably used. 
Owing to the sinter-activating additives, high compression, is achieved at 
low sintering temperatures so as to enable sintering in the furnace. 
High density, low distortion, and low evaporation losses of the dopants and 
additives are ensured by adapting the sintering parameters, e.g., the 
temperature-time curve. The sintering parameters to be adapted are 
determined, among others, by the type of metal powder, the grain size and 
grain distribution, the preparation process for the doped metal powder, 
the content of additives and dopants, and the green density. 
The temperature-time profile which is determined in dependence on the 
features mentioned above is run through for sintering the metal powder. 
The sintering process is effected in a dry hydrogen and/or argon gas 
atmosphere in order to prevent oxidation of the matrix material during 
sintering. 
The low sintering temperature has an advantageous effect on the 
considerably reduced evaporation of the dopant with a low work function. 
Further, temperature-sensitive dopants can now be used for the shaped 
part. The sintering is now preferably effected in a conventional furnace. 
Accordingly, in contrast to the Coolidge process, complex shaped part 
geometries can be sintered. Further, the homogeneous heating has a 
positive effect on the shrinkage of the shaped part, e.g., as a result of 
reduced distortion of the shaped part. 
Therefore, compared to the conventional, known manufacture of shaped 
parts--pressing, sintering in a direct current flow, hammering and 
intermediate annealing--the new production process--pressing, sintering in 
the furnace--is substantially more economical. 
Further, in the shaped part produced by the process according to the 
invention, the properties of the electrode especially, e.g., ignition 
characteristics, shelf life, arc stability, etc., are not limited by the 
sinter-activating additive and the residual porosity specific to 
production. 
Tungsten and/or molybdenum in particular are used as the matrix material of 
the metal powder for the shaped part. 
The metal powder is prepared either by a wet-chemical/hydrometallurgical 
process or by a dry mechanical process. This preparation process aims at 
the most homogeneous and finely dispersed distribution of the 
sinter-activating additive on the particles of the matrix material of the 
metal powder. A thin nickel layer on the tungsten grain surface, for 
example, is required for effective activation of the sintering process, 
wherein a near-surface tungsten-nickel layer which is presumably marked by 
growth is formed due to the high solubility and diffusibility of tungsten 
in nickel. 
Two intermediate products--ammonium para-tungstate (WO(NH.sub.4).sub.2) or 
tungsten trioxide (WO.sub.3)--are produced by the wet-chemical 
hydrometallurgical method for processing of tungsten earths. A 
water-soluble salt of the crystalline sinter-activating additive, e.g., 
nickel nitrate Ni(NO.sub.3).sub.2, is added to these intermediate products 
by spraying or mixing. This mixture can either be subjected to a 
calcination process, i.e., annealing at a temperature above 300.degree. 
C., with subsequent reduction or can be subjected directly to a wet 
reduction with hydrogen at a temperature greater than 600.degree. C. 
according to the so-called Sherrit-Gordon method. 
In the dry-mechanical preparation process, the sinter-activating additives 
are added as metal powder to the chemically pure matrix material of the 
metal powder and homogenized in a conventional mixer. Attritors or ball 
milling can also be used to increase the sinter activity of the metal 
powder so as to achieve a mechanical alloying of additive material and 
matrix material. 
The addition of the dopant with a low work function to the metal powder can 
also be effected by means of the wet-chemical hydrometallurgical process 
or the dry-mechanical process. 
The process according to the invention is distinguished by diverse 
possibilities for compression and shaping, wherein the shaped part is 
shaped close to its final contour in part during the compression. In 
particular, the metal powder with its dopants and additives can be 
compacted by 
--mechanical-hydraulic pressing; 
--cold isostatic pressing; 
--extrusion; 
--hot isostatic pressing; and/or 
--a powdered metal injection molding process and can accordingly also be 
brought to a shape close to the final contour if required. In so doing, 
the aim is to achieve a homogeneous minimum density, or green density as 
it is called, a sufficient strength, so-called green strength, and the 
required geometry. 
When the metal powder with the dopants and additives is compacted and 
brought to a shape close to the final contour in the manner described 
above, this intermediate product is sintered. After compaction and before 
sintering the metal powder with the dopants and additives, it is also 
possible to produce the shape of the structural part close to final 
contour by means of cutting. A presintering process may be necessary prior 
to machining in order to increase the green strength. 
Further advantages and features are indicated in the following description 
of three embodiment examples with reference to the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An electrode which is constructed as a hollow cathode 10 is shown in 
longitudinal section in FIG. 1. This hollow cathode 10 forming a shaped 
part is used in a known plasma coating process. 
The hollow cathode 10 is formed of metal powder 12 substantially comprising 
95.5 to 98.9 percent by weight of chemically pure tungsten, a dopant with 
a low work function, in the present case LaB.sub.6 (1.8 to 2.2 percent by 
weight), and a sinter-activating additive of 0.12 to 0.5 percent by weight 
nickel. 
The metal powder 12 is prepared by the wet-chemical hydrometallurgical 
process. For this purpose, a liquid nickel nitrate solution is sprayed 
into tungsten trioxide and thoroughly mixed. 
Particles greater than 10 .mu.m are separated by a subsequent sieving 
process. The mean grain size of the tungsten powder is between 2 and 3.5 
.mu.m with a cumulative grain size of 90% at 5 .mu.m. 
Lanthanum hexaboride (1.8 to 2.2 percent by weight) with a particle size of 
1 to 3 .mu.m is added in dry form to this tungsten powder in a mixer and 
homogenized. 
The metal powder 12 with the dopants and the additives is poured into 
elastic, cylindrical tubes, wherein the two stoppers are provided with a 
central pocket bore hole for receiving a cylindrical pin. The metal powder 
12 with the dopants and additives is then subjected to cold isostatic 
pressing. After talking out the compressed rod and removing the pin, there 
remains a slender green compact, e.g., with a diameter/length ratio of 6 
to 12 and a bore hole diameter/length ratio of 12 to 20. 
Alternatively, the metal powder 12 with the dopants and additives can also 
be compressed by mechanical-hydraulic means. The pressing die is first 
filled with a defined layer of the metal powder 12 with the dopants and 
additives. After inserting the pin by laterally guiding it into the 
pressing die, another defined layer of metal powder 12 with the dopants 
and additives is poured in. The metal powder 12 is then compressed on two 
sides. After removing the compressed rod and the pin, a slender green 
compact with an optionally central or asymmetrical bore hole remains. 
The green compacts are sintered indirectly in a furnace at temperatures of 
1400.degree. to 1600.degree. C. and a holding time of up to 30 min. In so 
doing, densities of 80 to 97% of theory are achieved. The weight 
proportions of the sinter-activating additive and dopant with low worlk 
function are between 80 and 100% of the amount originally introduced into 
the matrix work material of the metal powder 12. 
The hollow cathode 10 is inserted into a vacuum coating installation after 
cutting the thread 14 and functional surfaces 16. 
The arc generated by the hollow cathode 10 forms a plasma with a high 
degree of ionization in addition to an intensive electron beam. This 
plasma is used as an evaporation source for various materials, e.g., TiN, 
CrN, TiC. 
The operating parameters of the hollow cathode 10 in the vacuum coating 
installation are as follows: 
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voltage: 20-40 V 
current 50-300 A 
type of current: direct current 
type of gas: argon 4.8 
system pressure: 10.sup.-3 to 10.sup.-5 mbar. 
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The hollow cathode 10 is heated by an external tungsten coil and is ignited 
after reaching a defined temperature. At a power consumption of 3.5 to 4 
kW, the arc can be reliably ignited by the hollow cathode 10. During 
operation, the hollow cathode 10 exhibits good service life behavior over 
a number of hours. 
FIG. 2 shows a cored electrode 18 in longitudinal section as another 
embodiment form of the invention. 
The cored electrode 18 forming a shaped part has regions produced in two 
process steps, namely a pin-like electrode core 20 and a cylindrical 
electrode shell 22, which regions are formed of different compositions of 
metal powder. 
However, the metal powder with the dopants and additives are prepared in 
the manner described above, wherein the metal powder of the electrode core 
20 is a chemically pure tungsten powder with sinter-activating additives 
and dopants with a low work function and the metal powder of the electrode 
shell 22 is also a tungsten powder with sinter-activating additives, but 
without dopants with a low work function. 
For the purpose of realizing the electrode geometry, different shaping 
processes can be used for the two parts. 
First, the electrode core 20 is produced by extrusion and then the 
electrode shell 22 is pressed on to the electrode core 20 by 
mechanical-hydraulic pressing. 
Alternatively, the electrode shell 22 can be sprayed on the electrode core 
20 of the cored electrode 18 by a powdered metal injection molding 
process, this electrode core 20 being produced by isostatic pressing. 
An electrode produced in this way has the advantage that there is no 
drifting of the arc during operation. This effect is present in 
conventional electrodes when there is a reduction in the doping due to 
evaporation in the electrode tip. Further, the arc expansion is reduced by 
increasing the arc spot surface on the electrode tip. From the standpoint 
of welding technique, an arc expansion in the base material to be welded 
leads, e.g., to increased burning penetration, an enlarged heat affected 
zone or low fusion output. 
Another embodiment form which is not shown in detail in the drawings is 
described in the following. 
An electrode forming a shaped part according to the invention is produced 
for TIG welding. The metal powder with the sinter-activating additive is 
prepared in the same way as the electrode just described. La.sub.2 O.sub.3 
is used as a dopant with a low work function. 
The metal powder with the dopants and additives which is prepared in this 
way is inserted in elastic, cylindrical tubes and pressed by isostatic 
pressing at 1800 to 3000*10.sup.5 Pa. The green density reaches values of 
55 to 75% of the theoretical density. 
The sintering is effected in the manner described in connection with the 
first embodiment example. 
In an application-oriented welding test, a so-called WT20 electrode with 
1.8 to 2.2 percent by weight ThO.sub.2 which was manufactured in a 
conventional manner was compared with a so-called WL20 electrode with 1.8 
to 2.2 percent by weight LaO.sub.2 O.sub.3 and 0.5 percent by weight 
nickel which was produced according to invention in a shape close to the 
final contour. 
The test parameters were as follows: 
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electrode diameter: 2.4 mm 
electrode tip angle: 
60.degree. 
frustum diameter: 0.2 mm 
electrode spacing: 2.5 mm 
welding current: 200 A 
type of current: direct current 
polarity: electrode at negative pole 
auxiliary ignition: high-frequency ignition 
burner: HW20 with gas lens 
gas nozzle distance: 
4.5 mm 
gas nozzle diameter: 
11 mm 
type of inert gas: argon 4.8 
inert gas flow rate: 
8 l/min. 
type of weld: blind weld 
base material: St 35-2 
welding rate: 14 cm/min. 
welding sequence for the 
ignition of arc, 
ignition tests: 1 min. welding time 
1 min. pause (cooling) 
60 repetitions 
welding sequence for continuous 
ignition of arc, 
welding tests: 15 min. welding time, 
1 min. pause (cooling), 
4 repetitions 
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When used for welding, current densities of 20 to 30 kA/cm.sup.2 resulted 
at the electrode tip with the parameters indicated above. In comparison to 
the known WT20 electrode, it was shown very clearly that the electrode 
temperature of the WL20 electrode according to the invention is 
appreciably lower. The WL20 electrode according to the invention showed no 
ring formation, i.e., no dendritic growth of tungsten crystallites above 
the arc spot. 
The WL20 electrode according to the invention also has a high geometric 
stability of the electrode tip after long operation. Further, there was no 
evidence of local fusing of tungsten at the electrode tip. In addition, 
this electrode exhibits improved welding behavior with arc ignition. 
In conclusion, the invention is accordingly also distinguished by the fact 
that the shaped parts according to the invention, especially electrodes, 
can be used in many different ways, e.g., in arc welding with nonfusing 
electrodes, as cathodes in arc discharge lighting, for producing arc-based 
plasmas, in electron tubes, in traveling-wave magnetrons, and also as 
shields for radiation protection or as melting crucibles. As a result of 
the low electrode temperature, the electrode material ensures a high 
dimensional stability of the electrode tip during operation. Another 
advantage consists in that complex electrode geometries can be realized as 
is required, e.g., for hollow cathodes, cored electrodes or plasma 
cathodes. 
While the foregoing description and drawings represent the preferred 
embodiments of the present invention, it will be obvious to those sldlled 
in the art that various changes and modifications may be made therein 
without departing from the true spirit and scope of the present invention. 
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Reference numbers 
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10 hollow cathode 
12 metal powder 
14 thread 
16 functional surface 
18 cored electrode 
20 electrode core 
22 electrode shell 
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