METAL 2008
13. – 15. 5. 2008, Hradec nad Moravicí
GROWTH OF PURE TUNGSTEN SINGLE CRYSTALS USING
TUNGSTEN POWDERS PREPARED FROM WASTE PRODUCTS
G.S.Burkhanov, V.M.Kirillova, V.V.Sdobyrev, and V.A.Dementiev.
Baikov Institute of Metallurgy and Materials Science,
Russian Academy of Sciences,
Leninskii pr. 49, Moscow,119991 Russia,
Е-mail: genburkh@ultra.imet.ac.ru
Abstract
Experimental data on the growth of pure tungsten single crystals (the content of each impurity, except
molybdenum, is no more than 5x10-3 – 5x10-4 wt %) from tungsten powders are reported. The powders
prepared from tungsten waste products are characterized by marked contents of metallic impurities, such as
chromium, nickel, copper, iron, etc. (1-5 wt % each).
The experiment consists in three-stage processing, namely, (i) the preparation of tungsten rods by
compacting and sintering powders; (ii) preparation of tungsten ingots by plasma arc melting of the rods at a
rate of 1.5 mm/min; and (iii) electron-beam melting at a rate of 1 mm/min. After the plasma arc melting, the
surface of ingots was ground mechanically to take of a layer of deposits formed from the vapour.
In this paper it is shown that duplex-process (the consistently integrating the plasma arc melting with
the electron - beam melting) allows us to produce the pure tungsten single crystal using a tungsten powder
prepared from waste products.
Introduction
The electron-beam melting and the plasma arc melting are the most commonly used for the
commercial production of tungsten single crystals. The first method [1] allows one to grow
large single crystals. However, to reach the high purity and obtain the perfect structure, it is
necessary to use a pure tungsten powder (the content of each impurity is no more than 5x10-3
– 5x10-4 wt %). Powders with high contents of impurities such as chromium, nickel, copper,
iron, etc. (1-5 wt % each) cannot be used for the preparation of single crystals by electronbeam melting owing to the high gas evolution.
The second method [2] allows large single crystals to be grown from tungsten waste products.
However, the ingots have imperfect structure.
The aim of this work is to describe the duplex-process which incorporates advantages of
the two above methods and is proceeding in two stages: 1 - primary purification of tungsten
powder by plasma arc melting, 2 – the final improvement of the structure perfection by the
electron-beam melting.
Experimental
Two types of tungsten powders were used in the present investigation: powder I prepared
from tungsten waste products by a chloride technology [3, 4] and powder II prepared from
tungsten concentrates using the hydrometallurgical technology [5] which is commonly used in
the tungsten industry (was used for comparison). Powder I is characterized by marked
contents of metallic impurities, such as chromium, nickel, copper, iron, etc. (1-5 wt % each);
the impurity content in powder II is lower by three orders of magnitude than that in powder I.
Tungsten rods I and II 4x4x100 mm in size were prepared by room- temperature
compacting and sintering (1500°C) from powders I and II.
An installation shown in fig.1was used for the plasma-arc melting of tungsten rods at a rate
of 1,5mm/min. Ingots produced by plasma arc melting are 16mm in diameter and 350mm in
length The installation used for electron-beam melting at a rate of 1,0 mm/min in a vacuum of
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METAL 2008
13. – 15. 5. 2008, Hradec nad Moravicí
10-5 Pa is shown in fig.2. Single crystals produced by electron-beam melting are 15mm in
diameter and 300mm in length.
Fig.2. Schema diagram of electron – beam
zone method: 1 – cylindrical specimen; 2 –
electrons are accelerated by electric field
between cathode and anode; 3 – melt zone;
4 – cathode
Fig.1. Schematic diagram of plasma arc
method: 1 –cathode (interior electrode of
plasmotron); 2 – water- cooling plasmotron
(the inert gas ensures a protective
atmosphere): 3 – the melting initial
material (rod or bar); 4 – liquid bath; 5 –
anode (single crystal seed) moving
upwards and downwards along the vertical
axis; 6 – liquid metal drop; 7 – electrical
arc is excited between anode and cathode
Carbon and oxygen contents were determined by neutron-activation analysis [6], the
contents of metallic and non-metallic impurities were determined by chemical and massspectral analyses.
The orientations of single crystals were determined by x-ray Laue method using a URS-55
installation (x-ray tube with molybdenum anode).
The macrostructure and microstructure were revealed by electrolytic etching (10% solution
of NaOH) and by chemical etching of polished specimens (10g NaOH+10g [Fe (CN)6]+80g
H20), respectively. A Neophot-2 light optical microscope was used to study substructure of
tungsten single crystal. The density of etch pits was determined by metallography. The
misorientation and the size of subgrains were determined by X-ray topography.
The residual resistance ratio RRR=R273К/R4,2К was determined using contact and contactless .methods.
The microhardness was determined using a NEOPHOT-3 microscope equipped with a
microhardness tester.
Results and discussion
Table 1 shows the impurity composition of tungsten powders I and II.
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METAL 2008
13. – 15. 5. 2008, Hradec nad Moravicí
Table 1. Chemical composition of tungsten powders prepared by hydrometallurgical
technology (powder I) [5] and chloride technology (powder II) [3, 4].
Powder
I
II
Impurity content, 10-3wt%
С
50
80
O2
3000
3500
Сr
30
4500
Ni
8
3000
Fe
200
5000
Cu
50
4500
Si
20
80
Mo
200
300
Ca
20
90
P
5
50
S
1
10
Data of Table 1 confirm the presence of marked contents of metallic impurities, such as
chromium, nickel, copper, and iron, in tungsten powders prepared from tungsten waste
products.
Table 2 shows impurity composition of tungsten ingots produced by plasma arc melting of
tungsten rods I and II.
Table 2. Chemical composition of tungsten ingots I and II prepared by plasma arc melting of
tungsten rods I and II.
Content of impurities, ppm
Rods
I
II
С
0,65
0,7
O2
5,0
1,5
Сr
0,1
1,0
Ni
0,3
1,0
Fe
50
90
Cu
15
50
Si
0,4
5,0
Mo
1700
2800
Ca
20
90
P
15
50
S
1
10
Data of Table 2 demonstrate the marked purification of tungsten by the plasma arc
melting. The carbon and oxygen contents in powder II reduce by a factor of 103 and 104,
respectively. Contents of metallic impurities, such as chromium, nickel, copper, and iron, in
powder II reduces by factor of 104 and in powder I – 102. The content of other impurities in
powders I and II decreases by factor of 102.
Metallography and x-ray topography analysis of ingots I and II revealed the substantial
misorientation of 1st-order subgrains (3°and 5°, respectively); the estimated density of etch
pits is 5x108 sm-2 and 9x109 sm-2, respectively (figs. 3 and 3 b). The ingot II (prepared from
tungsten waste products) is characterized by 30% porosity.
а)
b)
Fig.3. Microstructure of tungsten ingots produced by plasma arc method using the common
powder (a) and waste product powder (b), x 100.
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METAL 2008
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Such a plasma arc melting allows us to purify tungsten ingots substantially, however does
not allow the perfect substructure to be reached.
Table 3 shows the impurity content in tungsten single crystals produced by the electron beam melting of tungsten ingots I and II.
Table 3. Chemical composition of tungsten single crystals I and II prepared by electron-beam
melting of tungsten ingot I and II).
Single
crystals
I
II
Content of impurities, ppm
С
0,55
0,65
O2
0,15
0,8
Сr
0,09
0,85
Ni
0,18
0,9
Fe
40
70
Cu
10
30
Si
0,2
1,0
Mo
1700
2700
Ca
10
50
P
10
30
S
1
0,8
Table 3 shows that the oxygen content in ingots decreases by a factor of 10 whereas the
carbon and metallic/non-metallic contents change only slightly.
Metallography and x-ray topography analysis show that the tungsten single crystals are
characterised by the relatively high structure perfectness, i.e.,the misorientation of 1st-order
subgrains does not exceed 30 (for powder I) and 55 (for the waste powder). The estimated
density of etch pits is 5x106 sm-2 and (1-6) x 107 sm-2, respectively (figs. 4 a, 4 b). The
porosity of single crystal prepared from waste powder is 30%.
b)
а)
Fig.4. Microstructure of [100] tungsten single crystals produced by duplex-method using the
common powder (a) and the waste product powder (b), x 500.
Comparative analysis of Tables 2 and 3- and data on the substructure parameters of single
crystals show that the plasma arc melting substantially purifies tungsten, and the electronbeam melting forms the relatively highly perfect substructure.
For single crystals I and II RRR is 4500 and 3500, and the microhardness is 315 and 330,
respectively.
Thus the purity, substructure, and some properties of single crystals prepared from the
waste product powder differ slightly from those produced from the common powder. This
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conclusion is of importance in particular, with allowance for the required decrease in tungsten
single crystal price at the expense of the decrease in powder price. However, it is necessary to
note of some feature of the single crystal preparation from waste powder: (1) – the
pronounced (high) gas evolution and the severe impurity evaporation during the plasma arc
melting and (2) – the surface deposition of evaporated impurities in the form of very small
crystals during the cooling. For this reason, the surface of ingots must be ground mechanically
after the plasma arc melting,
Conclusion
The consistent integrating the plasma arc melting with the electron - beam melting
(duplex-process) allows us to produce the pure tungsten single crystals using the tungsten
powder prepared from waste products.
References.
1. E.M.Savitsky and G.S.Burkhanov. Single crystals of refractory and rare metals and alloys
(Nauka, Moscow, 1972, p.259).
2. N.P.Luakishev and G.S.Burkhanov, Metallic Single Crystals (ELIZ, Moscow, 2002, p.
311) [in Russian].
3. D.M.Chizhikov, V.G.Trusova, A.Z.Khazan. Regeneration of hard alloys waste product by
chloride method. (Theses of reports of the II Conference “Chemistry and technology of
molybdenum and tungsten”, Nalchik, 1974, P.195).
4. D.M.Chizhikov, V.G.Trusova, A.Z.Khazan. The reducing and chloriding of tungsten and
molybdenum compounds. (Tsvetnaia metallurgia, Nauka, Moscow, 1976, p.180-187).
5. A.N.Zelikman and L.S.Nikitina. Tungsten. (Metallurgia, Moscow, 1978, p. 271).
6. R.A.Kuznetsov.. Activation Analysis. (Atomizdat, Moscow, 1974, p.343).
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