The refining of titanium by the method of zone recrystallization in an electric field
The physical substantiation and an experimental study results of the application of zone recrystallization method in an electric field for the refining of titanium are presented. The elemental composition, microstructure and microhardness of the samples have been investigated. It is shown that refin...
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України
2022
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| Цитувати: | The refining of titanium by the method of zone recrystallization in an electric field / O.E. Kozhevnikov, M.M. Pylypenko, V.M. Pelykh, V.D. Virych, M.F. Kozhevnikova // Problems of Atomic Science and Technology. — 2022. — № 1. — С. 6-12. — Бібліогр.: 17 назв. — англ. |
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Kozhevnikov, O.E. Pylypenko, M.M. Pelykh, V.M. Virych, V.D. Kozhevnikova, M.F. 2023-12-07T10:51:15Z 2023-12-07T10:51:15Z 2022 The refining of titanium by the method of zone recrystallization in an electric field / O.E. Kozhevnikov, M.M. Pylypenko, V.M. Pelykh, V.D. Virych, M.F. Kozhevnikova // Problems of Atomic Science and Technology. — 2022. — № 1. — С. 6-12. — Бібліогр.: 17 назв. — англ. 1562-6016 DOI: https://doi.org/10.46813/2022-137-006 https://nasplib.isofts.kiev.ua/handle/123456789/195823 669.295 The physical substantiation and an experimental study results of the application of zone recrystallization method in an electric field for the refining of titanium are presented. The elemental composition, microstructure and microhardness of the samples have been investigated. It is shown that refining process made it possible to significantly reduce the content of both metallic and gas-forming impurities. The oxygen concentration was reduced by 2.2 (from 0.033 to 0.015 wt.%), carbon – by 3.3 (from 0.01 to 0.003 wt.%), nitrogen – by 22 times (from 0.009 to 0.0004 wt.%). The purity of the obtained samples was characterized by a value of 99.95 wt.% by titanium content. The total amount of impurities had been reduced by a factor of 2.4 (from 0.12 to 0.05 wt.%). Представлено фізичне обґрунтування та результати експериментального дослідження застосування методу зонної перекристалізації в електричному полі для рафінування титану. Вивчено елементний склад, мікротвердість та мікроструктура зразків. Показано, що рафінування значно зменшило вміст як металевих, так і газоутворюючих домішок. Концентрацію кисню знижено в 2,2 (з 0,033 до 0,015 мас.%), вуглецю – в 3,3 (з 0,01 до 0,003 мас.%), азоту – в 22 рази (з 0,009 до 0,0004 мас.%). Чистота отриманих зразків характеризувалася значенням 99,95 мас.% за вмістом титану. Загальну кількість домішок знижено в 2,4 рази (з 0,12 до 0,05 мас.%). Представлены физическое обоснование и результаты экспериментального исследования применения метода зонной перекристаллизации в электрическом поле для рафинирования титана. Изучены элементный состав, микроструктура и микротвердость образцов. Показано, что рафинирование позволило значительно снизить содержание как металлических, так и газообразующих примесей. Концентрация кислорода была снижена в 2,2 (с 0,033 до 0,015 мас.%), углерода – в 3,3 (с 0,01 до 0,003 мас.%), азота – в 22 раза (с 0,009 до 0,0004 мас.%). Чистота полученных образцов характеризовалась значением 99,95 мас.% по содержанию титана. Общее количество примесей понижено в 2,4 раза (с 0,12 до 0,05 мас.%). en Національний науковий центр «Харківський фізико-технічний інститут» НАН України Вопросы атомной науки и техники Pure materials and vacuum technologies The refining of titanium by the method of zone recrystallization in an electric field Рафінування титану методом зонної перекристалізації в електричному полі Рафинирование титана методом зонной перекристаллизации в электрическом поле Article published earlier |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| title |
The refining of titanium by the method of zone recrystallization in an electric field |
| spellingShingle |
The refining of titanium by the method of zone recrystallization in an electric field Kozhevnikov, O.E. Pylypenko, M.M. Pelykh, V.M. Virych, V.D. Kozhevnikova, M.F. Pure materials and vacuum technologies |
| title_short |
The refining of titanium by the method of zone recrystallization in an electric field |
| title_full |
The refining of titanium by the method of zone recrystallization in an electric field |
| title_fullStr |
The refining of titanium by the method of zone recrystallization in an electric field |
| title_full_unstemmed |
The refining of titanium by the method of zone recrystallization in an electric field |
| title_sort |
refining of titanium by the method of zone recrystallization in an electric field |
| author |
Kozhevnikov, O.E. Pylypenko, M.M. Pelykh, V.M. Virych, V.D. Kozhevnikova, M.F. |
| author_facet |
Kozhevnikov, O.E. Pylypenko, M.M. Pelykh, V.M. Virych, V.D. Kozhevnikova, M.F. |
| topic |
Pure materials and vacuum technologies |
| topic_facet |
Pure materials and vacuum technologies |
| publishDate |
2022 |
| language |
English |
| container_title |
Вопросы атомной науки и техники |
| publisher |
Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| format |
Article |
| title_alt |
Рафінування титану методом зонної перекристалізації в електричному полі Рафинирование титана методом зонной перекристаллизации в электрическом поле |
| description |
The physical substantiation and an experimental study results of the application of zone recrystallization method in an electric field for the refining of titanium are presented. The elemental composition, microstructure and microhardness of the samples have been investigated. It is shown that refining process made it possible to significantly reduce the content of both metallic and gas-forming impurities. The oxygen concentration was reduced by 2.2 (from 0.033 to 0.015 wt.%), carbon – by 3.3 (from 0.01 to 0.003 wt.%), nitrogen – by 22 times (from 0.009 to 0.0004 wt.%). The purity of the obtained samples was characterized by a value of 99.95 wt.% by titanium content. The total amount of impurities had been reduced by a factor of 2.4 (from 0.12 to 0.05 wt.%).
Представлено фізичне обґрунтування та результати експериментального дослідження застосування методу зонної перекристалізації в електричному полі для рафінування титану. Вивчено елементний склад, мікротвердість та мікроструктура зразків. Показано, що рафінування значно зменшило вміст як металевих, так і газоутворюючих домішок. Концентрацію кисню знижено в 2,2 (з 0,033 до 0,015 мас.%), вуглецю – в 3,3 (з 0,01 до 0,003 мас.%), азоту – в 22 рази (з 0,009 до 0,0004 мас.%). Чистота отриманих зразків характеризувалася значенням 99,95 мас.% за вмістом титану. Загальну кількість домішок знижено в 2,4 рази (з 0,12 до 0,05 мас.%).
Представлены физическое обоснование и результаты экспериментального исследования применения метода зонной перекристаллизации в электрическом поле для рафинирования титана. Изучены элементный состав, микроструктура и микротвердость образцов. Показано, что рафинирование позволило значительно снизить содержание как металлических, так и газообразующих примесей. Концентрация кислорода была снижена в 2,2 (с 0,033 до 0,015 мас.%), углерода – в 3,3 (с 0,01 до 0,003 мас.%), азота – в 22 раза (с 0,009 до 0,0004 мас.%). Чистота полученных образцов характеризовалась значением 99,95 мас.% по содержанию титана. Общее количество примесей понижено в 2,4 раза (с 0,12 до 0,05 мас.%).
|
| issn |
1562-6016 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/195823 |
| citation_txt |
The refining of titanium by the method of zone recrystallization in an electric field / O.E. Kozhevnikov, M.M. Pylypenko, V.M. Pelykh, V.D. Virych, M.F. Kozhevnikova // Problems of Atomic Science and Technology. — 2022. — № 1. — С. 6-12. — Бібліогр.: 17 назв. — англ. |
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6 ISSN 1562-6016. . 2022 1(137)
https://doi.org/10.46813/2022-137-006
UDC 669.295
REFINING OF TITANIUM BY METHOD OF ZONE
RECRYSTALLIZATION IN AN ELECTRIC FIELD
O.E. Kozhevnikov, M.M. Pylypenko, V.M. Pelykh, V.D. Virych, M.F. Kozhevnikova
National Kharkiv, Ukraine
E-mail: kozhevnikov@kipt.kharkov.ua
The physical substantiation and an experimental study results of the application of zone recrystallization method
in an electric field for the refining of titanium are presented. The elemental composition, microstructure and
microhardness of the samples have been investigated. It is shown that refining process made it possible to
significantly reduce the content of both metallic and gas-forming impurities. The oxygen concentration was reduced
by 2.2 (from 0.033 to 0.015 wt.%), carbon by 3.3 (from 0.01 to 0.003 wt.%), nitrogen by 22 times (from 0.009 to
0.0004 wt.%). The purity of the obtained samples was characterized by a value of 99.95 wt.% by titanium content.
The total amount of impurities had been reduced by a factor of 2.4 (from 0.12 to 0.05 wt.%).
INTRODUCTION
The introduction and use of high-efficiency fuel
cycles at operating nuclear power plants with water-
cooled power reactors (WWER) has contributed to an
increase in the service life of vessel equipment and fuel
elements, in an increase in fuel burnup and the steam
content of the coolant. The resource and safety of
nuclear reactors depend on the characteristics of the
materials used. Zirconium, hafnium, titanium, and also
alloys based on them are widely used as structural and
absorbing materials in thermal-neutron reactors [1].
The properties of titanium depend significantly on
the content of impurities in the metal. Although the
impurities increase the strength, at the same time, they
sharply decrease the plasticity, and the most severe
negative effect is caused by interstitial impurities. When
the hydrogen concentration is only 0.003 wt.%, nitrogen
is 0.02 wt.% or oxygen is 0.7 wt.%, titanium completely
loses its ability to plastic deformation and breaks down
brittle.
Among the impurities harmful to the ductility of
titanium carbon, iron, and silicon should also be
included.
The resulting effect of impurities is also expressed in
the fact that technical titanium undergoes allotropic
temperature range of 865... C [2].
On an industrial scale, to obtain high-purity titanium
an electrolytic refining (ER) or iodide process (IP) is
used. The ER process makes it possible to increase the
purity of titanium materials relatively to gas impurities
(oxygen and nitrogen), as well as carbon, silicon, iron,
and other impurities.
The process of refining of titanium sponge using a
CaCl2 TiCl2 melt was described in [3]. At optimally
selected electrolysis mode (melt temperature
current density 0.5 A/cm2) titanium with a purity
about 99.95% was obtained.
The iodide refining method is based on the
reversibility of the formation reaction and on thermal
dissociation of gaseous tetraiodide of the base metal.
The use of IP makes it possible to purify titanium from
nitrogen, hydrogen, oxygen, carbon and some other
impurities, using also waste and production turnovers as
raw materials [4].
Earlier in the NSC KIPT the work on the refining of
metal by the method of electron beam melting in a
vacuum was carried out. Titanium sponge TS-90 and
titanium after iodide refining were used as raw
materials. The high efficiency of the refining process
from metal and gas impurities was shown. A metal with
a purity of 99.95% was obtained [5].
The crystallization process from melts is one of the
main methods for refining of metals and semiconductor
materials. During zone melting (ZM), the refining
occurs as a result of zone redistribution of impurities
between the melt and the solid phase. Usually, ZM is
used at the final stages to remove low concentrations of
impurities.
Electrotransport is used for deep refining of metals
in solid and liquid states. The method is based on the
ability of impurity ions to displace directionally over the
sample under the action of an applied constant electric
field.
Previously an investigation of titanium refining by
crucibleless vacuum zone melting in an electric field
(ZMEF) were hardly carried out. The purpose of this
work is to physical substantiation and experimentally
study of the behavior of metallic and gas-forming
impurities during the refining process of titanium by the
ZMEF method.
1. PHYSICAL SUBSTANTIATION
OF TITANIUM REFINING
BY METHOD OF ZMEF
The refining of titanium by the ZMEF method is
carried out with the simultaneous passage of several
physical cleaning processes:
when melting is carried out in vacuum conditions,
the vaporization process is occurs. It refers gas-forming
impurities and also those metallic impurities that are
characterized by a high value of the saturated vapor
pressure at the melting temperature of titanium;
due to the zonal redistribution of impurities
between the melt and the solid phase. At the same time
they are displaced to the end part of the ingot together
with the movement of the liquid zone during the ZM;
ISSN 1562-6016. . 2022 1(137) 7
due to the migration of impurity ions under the
action of an applied direct electric field to the end parts
of the sample (to anode or cathode).
The principle of zone refining is based on the
different solubility of impurity in the liquid and solid
phases of the base material. An important characteristic
in describing of the process is the distribution
coefficient k, which represents the ratio of the impurity
concentration in the solid phase CS to the concentration
in the melt CL [6]:
k = CS/CL. (1)
There are the differences between concepts of
equilibrium distribution coefficient k0 and effective
distribution coefficient ke. The equilibrium distribution
coefficient k0
maximum solubility of an impurity to its concentration
at the point of invariant transformation. When k0 must
be calculated in the case of low impurity concentrations,
it seems most convenient to use theoretical methods
based on the thermodynamic constants of equation for
ideal solutions. According to this concept, it can be used
the Schroeder-Le Chatelier equation:
(2)
Then and
Here is the molar heat of fusion for impurity, J/mol;
R is the universal gas constant (R = 8.314 J/(mol K));
b is the melting temperature of the titanium
( b = 1943 ); G is the hypothetical value of the
melting temperature of , which is
selected from the kind of phase diagram [7].
When carrying out the ZM process, it is necessary
that the rate of crystallization be greater than the
diffusion rate of the impurity in the solid phase of the
base material, but also does not exceed some optimal
value. In this case, the moving crystallization front
repels the dissolved impurity faster than it can evenly
distribute in the melt, and an impurity-enriched region
called the diffusion layer appears before the
crystallization front. The width of the diffusion layer
depends on the diffusion ability of the impurity, the
viscosity of the melt, the nature of the fluid motion, the
rate of crystallization.
Therefore, the main characteristic of process of the
zone impurity separation is effective distribution
coefficient ke, which can be calculated by the Burton-
Prim-Slichter formula [6]:
, (3)
where v is the speed of zone movement; d is the
diffusion coefficient of the impurity in the melt; is
width of the diffusion layer.
For most metals the value of the diffusion
coefficient d is 5 10-5 cm2/s approximately. The value of
the diffusion layer width equals approximately
0.01 cm in the case of conservative melt mixing in the
liquid zone. Therefore, the /d ratio is 200 s/cm.
An analysis of the results of titanium refining by
crucible-free ZM method in vacuum was carried out in
[8]. It was noted that a good degree of purification from
metal and gas-forming impurities was achieved at high
speeds of zone movement (16 and 8 mm/min) due to
their evaporation. ZM made it possible to significantly
reduce the concentrations of Al, Ca, Co, Cr, Cu, Fe, K,
Mn, Na, Ni, P, S, Si. These impurities are characterized
at T = 1933 K by value of the saturated vapor pressure
P0 > 50 Pa. The value of saturated vapor pressure for
titanium at the melting point is 0.6 Pa [9].
The applying of ZM with a speed of 4 and
2 mm/min allowed cleaning from metal impurities also.
As a result, the concentration of aluminum was reduced
by 1.6; iron 1.3; calcium 1.4; cobalt 4; manganese
7; nickel 1.74; copper 44; phosphorus 1.4 times.
The refining was carried out from interstitial impurities
also effectively. The oxygen concentration was reduced
from 0.033 to 0.025 wt.%, carbon from 0.01 to
0.0048 wt.%, nitrogen from 0.009 to 0.0035 wt.% [8].
In [10], experimental data were presented that
testified to the occurrence of relaxation processes in
-titanium single crystals, which were grown by
crucibleless ZM method. The study was carried out after
the passage of the front of the - transformation. It was
found that the nature of the structural manifestations of
relaxation processes and their efficiency depend on the
purity of the material and the crystallographic
orientation of the single crystals.
The theoretical theses of the substance cleaning in
solid and liquid phases under the influence of an applied
direct electric field were described sufficiently in
[11, 12]. The passing of a direct electric current through
a metal sample leads to a displacement of impurity ions
in a certain direction (to anode or cathode). The electric
current passing through the phase boundary changes the
value of the effective distribution coefficient ke due to
the addition of the electrotransport component to the
diffusion flux. In general terms, the resulting force F
acting on the impurity ion in the base metal can be
expressed as:
(4)
where Z is the ion charge; e is the electron charge; n is
the concentration of conduction electrons; L is the mean
of electron free pass; is the cross-section of scattering
of electrons on the impurity ion; E is the electric field
strength; Z* is the effective ion charge (Z* = Z |e|nL ).
The value of Z* can be determined from the
expression:
(5)
where U is the ion mobility; D the diffusion coefficient
of the impurity ion in the case of ZMEF process; T is
the temperature of base material; k is the Boltzmann
constant [13].
8 ISSN 1562-6016. . 2022 1(137)
Depending on the sign of Z*, the resulting force F
can be directed to the cathode (Z* > 0), to the anode
(Z* < 0), or equal to zero (Z* = 0).
The value of the impurity ion mobility U depends on
the properties of the system and the temperature. In the
case of highly dilute solutions, the rate of movement of
the solvent ion can be set equal to zero, and the
expression for the impurity ion mobility can be
represented as:
U = v E. (6)
In the case of carrying out the ZMEF process
equation (3) for the effective distribution coefficient of
the impurity takes the form:
(7)
where
Under the influence of an electric field, the value of
can be changed.
This makes it possible to obtain the following
results:
1) the efficiency of refining can be increased;
2) the value of can be made less than unity even
for those cases when k0 > 1, which allows the refining
during recrystallization.
It should be noted that the making of calculations
does not include a number of other effects, which may
-
the magnitude of . Particularly a Peltier effect not
considered (the release or absorption of additional heat
at the phase boundary). In this case, an additional
component of the impurity flow arises in the liquid zone
due to diffusion in the thermal gradient field, which can
affect on the cleaning efficiency.
In article [14], the displacement of interstitial
impurities under the action of an applied constant
electric field in the high-temperature -phase of
titanium at temperatures from 1610 to 1860 K was
considered. Based on the results of the analysis of the
elemental composition of the samples, the values of
mobility U, ion movement speed and diffusion
coefficient D were calculated (for carbon, nitrogen and
oxygen ions). It was noted that during the
electrotransport in the -phase of titanium, a
displacement of carbon ions to the cathode, and oxygen
and nitrogen ions to the anode were observed. The
results of elemental analysis made it possible to
calculate the value of the positive value of the effective
charge Z* (Z* > 0) for carbon ions and negative values
of Z* (Z* < 0) for oxygen and nitrogen ions in the
-phase of titanium.
2. EXPERIMENTAL STUDY
OF TITANIUM REFINING
2.1. RESEARCH METHODS AND MATERIALS
In the presented work the obtaining of refined
titanium samples was carried out using the crucibleless
ZM method with electron-beam heating in a vacuum of
1 10-4 Pa. The advantages of this type of melting include
the ability to refine samples and to grow single crystals
of refractory metals and alloys, the absence of a
crucible, the creation of a narrow heating region by
focusing the electron beam, and a high specific power
concentration. A detailed description of the ZM device
for refining of refractory metals is given in [15].
As the starting material was used industrial titanium
obtained by refining iodide method with integral purity
of 99.88 wt.%.
The results of the impurity content were obtained by
laser mass spectrometry. In the analysis, an EMAL-2
energy-mass analyzer was used. The limiting sensitivity
of the analysis method for metal impurities was
10-6 at.%. The device was calibrated with certified
LECO samples.
The visual inspection of thin sections and
photographing of the grain structure were carried out
using an MMR-4 microscope. The microhardness was
measured on a PMT-3 microhardness tester at a load of
0.05 kg. The value of the microhardness numbers was
recorded by ten measurements. The measurement error
did not exceed 5%.
2.2. RESULTS AND DISCUSSION
An -
diagrams in the case of the Ti-O and Ti-N systems
(Fig. 1,a,b) shows that even small additions of these
impurities sharply increase the melting temperature of
the alloys. For them, the value k0 > 1. Nitrogen and
oxygen belong to the group of elements that dissolve
significantly in -titanium and sharply increase the
temperature of - polymorphic transformation.
Carbon with small concentration lowers the melting
point of titanium from 1663 to 1653 , which is the
temperature of eutectic crystallization L -Ti +
TiC ( ). The eutectic contains 1.5 at.% of carbon.
Therefore, the value k0 < 1 in range of small
concentration. Carbon increases the temperature of
- -transformation of titanium from 882 to 920
which is the temperature of the peritectoid reaction of
-Ti formation (see Fig. 1,c) [16].
The calculations of the distribution coefficients k0
for impurities in titanium were carried out taking into
account the thermodynamic constants of the equation of
ideal solutions according to formula (2). Expression (3)
was used to calculate the value of the effective
coefficient k . The following parameters were taken into
account in the calculations: G is the estimated value of
the melting temperature of the base-impurity alloy,
H is the molar heat of fusion of the impurity, the ratio
/d 200 s/cm. The melting speed v in the calculations
was taken to be 2 and 4 cm/min.
ISSN 1562-6016. . 2022 1(137) 9
a b c
Fig. 1. Phase diagrams of Ti-O (a), Ti-N (b), and Ti-C (c) systems
Table 1 shows the calculated values of distribution
coefficients for interstitial impurities. For oxygen and
nitrogen, the values are k0 > 1, and for carbon k0 <1.
This is in good agreement with the kind of the state
diagram. Also, for comparison, the values of the
limiting coefficient k0lim are given [17]. The results of
calculations of k0 and k for various metallic impurities
represented in the earlier work of the authors [8].
Table 1
Calculated values of k0 and k for interstitial impurities in titanium
Element G, , kJ/mol k0 k0lim [12] k
Ti 1943 18.8 1.0 1.0 1.0
O 1993 21.1 1.03 1.5 1.01
C 1926 17 0.99 2.58 0.99
N 2293 22 1.23 6.0 1.1
Using the recommended values of the effective
charge Z* (-0.8 for O; 0.16 for C; -0.14 for N) and the
diffusion coefficient D from work [14], formulas (5) (7)
allowed to calculate the values of impurity ion
mobility U, impurity ion displacement velocity v ,
effective coefficient k e for interstitial impurities. The
calculation results are shown in Table 2.
In the calculations, it was taken into account that the
values of U, D, Z* for the liquid phase of titanium may
differ from the values characteristic of the - and -
phases of titanium. The estimates of the values of these
parameters for the liquid phase of titanium were not
previously given in the works. Therefore, the
calculations of the parameters for ZMEF performed in
this work can be recommended only as rough estimates
of the values.
Table 2
Calculation results of the ZMEF process parameters for interstitial impurities in titanium
Impurity Z* [14]
U, 10-5,
cm2/(V s)
D, 10-6,
cm2/s
| v |, 10-6,
cm/s
k (for zone
melting)
k e k e
-0.8 3.5 7.33 8.75 1.01 1.0 1.0
C 0.16 1.6 16.8 4.0 0.99 0.99 0.65
N -0.14 2.0 2.39 50.0 1.1 1.0 1.0
As a result of the analysis of Tables 1 and 2, it can
be concluded that the refining of titanium from oxygen
and nitrogen takes place during preliminary heating of
the sample in vacuum due to its degassing, at the
melting stages during pumping out the formed gaseous
molecules, due to the displacement of oxides and
nitrides into the end part of the ingot. An application of
electrotransport for refining from oxygen and nitrogen
will be negligible.
The refining from carbon will take place as a result
of preliminary heating of the sample in vacuum, at the
melting stages as a result of pumping out the formed
gaseous molecules, due to displacement of carbides
during recrystallization. It is also possible to remove
carbon when melting is carried out in an electric field
plugged in along the movement of the liquid zone
(melting goes from bottom to top, anode is below,
cathode is above). In this case, the value of the effective
charge Z* = 0.16, and the value of the effective
coefficient can be equal to = 0.65 (calculated for
melting at a speed of v = 4 cm/min).
The experimental stages of titanium refining were
carried out by the method of vacuum crucibleless ZM in
an electric field. A sample with a length of 250 mm was
placed vertically. The electron beam gun had the ability
to move along the ingot. The ZM were carried out at a
controlled speed (maximum 16 mm/min, minimum
1 mm/min). The length of the remelted part of the ingot
was from 120 to 170 mm, the electric field strength was
E = 0.25 V/cm, the current density was j = 100 A/cm2,
10 ISSN 1562-6016. . 2022 1(137)
and the number of zone passes was from one to three.
ZMEF was carried out under the conditions of an
applied direct electric field, plugged in both along and
against the course of the zone. A speed of movement of
the electron beam gun was 4 or 2 mm/min.
The complex technique of the refining process
included the use of thermal cycling (heating and cooling
of the ingot in the temperature range of the polymorphic
- transformation) at the final stage. This procedure
contributed to the increase in grain size.
The billet of the initial iodide titanium and the
sample refined by the ZMEF method are shown in
Fig. 2. The ingot is obtained as a result of three ZM
passes and one ZMEF pass.
b
Fig. 2. View of the original billet of iodide titanium (a) and ingot after ZMEF (b)
To study the elemental composition and
metallography, samples in the form of tablets 6 mm in
height and 8 mm in diameter were cut from various
parts of the titanium rod. The results of the analysis of
the content of impurities in the original iodide titanium
and ingots after refining are shown in Table 3. The
results are presented for samples cut from the middle of
the ingot.
Table 3
Elemental analysis results for titanium samples
Titanium
materials
Concentration, wt.%
Ti Zr Al Fe Cu Cr Si P S O C N
iodide Ti 99.88 0.0016 0.012 0.03 0.002 6 -3 6.2 -4 7 -5 3.5 -3 0.033 0.01 9 -3
after ZM 99.92 0.0016 0.0073 0.022 4.5 -4 4 -3 5.2 -4 -5 2.9 -3 0.025 4.8 -3 -3
ZMEF 99.95 0.0015 0.0045 0.01 3 -4 3 -3 5 -4 3 -5 2 -3 0.015 0.004 4 -4
ZMEF 99.95 0.0014 0.007 0.015 3 -4 3 -3 5 -4 3 -5 2 -3 0.017 0.003 4 -4
The elemental analysis results showed a decrease in
the concentration of a large number of metallic
impurities in the melted ingots. The purification was
carried out in the passage of evaporation and zone
recrystallization. The use of electrotransport did not
make a significant contribution to the purification from
metal impurities, compared to other physical refining
processes. The concentration of aluminum was reduced
by 2.6; iron 3; copper 6.6; chromium 2; silicon
1.2 times.
The refining of titanium from gas-forming
impurities occurred at various stages of the experiment.
During preliminary heating and ZM at a high rate,
hydrogen evaporated in the form of gaseous molecules
(H2, H2O). The carbon was vaporized as CO and CO2
molecules. Nitrogen was removed by high-temperature
heating and ZM in the form of N2 gas molecules. A
decrease in the oxygen content occurred at the stages of
heating and melting as a result of the formation of
gaseous molecules CO, CO2, H2O.
Also, the base metal was purified due to the
displacement of metal carbides and oxides into the final
part of the ingot during zone recrystallization.
The use of the ZMEF method led to the migration of
interstitial impurity ions to the anode or cathode under
the action of an applied electric field. When melting was
carried out in an electric field plugged in against the
movement of the liquid zone (melting goes from bottom
to top, anode is above, cathode is below), the following
happened. Oxygen and nitrogen ions with negative
values of the effective charge Z* were displaced
towards the anode both in the -phase of the metal and
in the heated section of the ingot with a high-
temperature -phase. The migration process was
especially active in the liquid zone of the metal moving
through the sample. Carbon ions with a positive value of
Z* were displaced towards the cathode against the
moving of the zone. But this contribution to carbon
removal is less significant than the effect of zone
recrystallization and evaporation processes.
When melting was carried out in an electric field
plugged in along the movement of the liquid zone
(melting goes from bottom to top, anode is below,
cathode is above), a decrease in carbon concentration
should be expected. As the carbon ions were displaced
towards the cathode, coinciding with the movement of
the zone. Titanium samples with a carbon content of
0.003 wt.% were obtained.
It can be concluded that electrotransport fulfills the
task of effective refining only with long-time processes:
with many hours of refining in the - and -phases
of titanium, but without the use of ZM there will be no
effective refining from metallic impurities;
with multiple ZMEF processes, but in this case,
the weight losses of titanium due to its evaporation
begins to become noticeable.
In the samples of titanium obtained at use of the
ZMEF process, the oxygen concentration was reduced
ISSN 1562-6016. . 2022 1(137) 11
by 2.2 (from 0.033 to 0.015 wt.%), carbon 3.3 (from
0.01 to 0.003 wt.%), nitrogen 22 times (from 0.009 to
0.0004 wt.%). The purity of the samples was
characterized by a value of 99.95 wt.% by titanium
content. The total amount of impurities was reduced by
a factor of 2.4 (from 0.12 to 0.05 wt.%).
Fig. 3 shows a photograph of the microstructure of a
titanium sample after ZMEF process. Twins, as well as
slip lines along prismatic planes { } are visible. The
grain boundaries and body were clean;
Fig. 3. Microstructure of titanium after ZMEF process
The microhardness value for the samples of the
initial titanium iodide was in the range
H = 1110...1140 MPa. After passing through the zone
recrystallization, the grains were aligned predominantly
along the direction of the melting stroke (the deviation
of the normal vector to the base plane from the direction
The
microhardness value after ZM at a high speed of 16 or
8 mm/min was equal to H = 1060...1090 MPa, after
ZMEF H = 1070 MPa.
CONCLUSIONS
The physical substantiation of the process of
titanium refining from metal and interstitial impurities
during ZM in a direct electric field is presented.
To obtain refined titanium samples, a method was
used that included preliminary heating, vacuum
crucibleless ZM, electrotransport, and thermal cycling.
Purification by the ZMEF method was carried out with
the simultaneous passage of several physical processes
as an evaporation of gas-forming and volatile metallic
impurities, zone redistribution of impurities, and
migration of impurity ions under the action of an
applied direct electric field.
The purity of the samples after the ZMEF method
was characterized by a value of 99.95 wt.% by titanium
content. The total amount of impurities was reduced by
a factor of 2.4 (from 0.12 to 0.05 wt.%). Oxygen
concentration has been reduced by 2.2 (from 0.033 to
0.015 wt.%), carbon by 3.3 (from 0.01 to 0.003 wt.%),
nitrogen by 22 times (from 0.009 to 0.0004 wt.%).
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Article received 21.10.2021
300 m
12 ISSN 1562-6016. . 2022 1(137)
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