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International Scientific Colloquium
Modelling for Material Processing
Riga, September 16-17, 2010
Magnetically Controlled Electroslag Melting of Titanium Alloys
Ya. Kompan, I. Protokovilov, Y. Fautrelle, Yu. Gelfgat, A. Bojarevics
Abstract
Series of titanium alloy remelting has been performed in an experimental electroslag
setup. The typical electrovortical flow pattern in both the slag and the liquid metal pools has
been radically modified by imposing an external magnetic field. Several configurations of the
applied magnetic field were considered and tested during actual remelting. The best results
were obtained during remelting in the presence of a pulse axial magnetic field providing fine-
grained titanium alloy ingots of uniform composition. It has been shown that the new process
of magnetically controlled electroslag melting is a highly competitive alternative method for
the production of multi-component titanium alloys. Further modifications of the process are
proposed with an aim to optimize energy efficiency and equipment costs.
Introduction
One of the actual trends in the modern titanium metallurgy is the development of a
new class of multi-component high-strength and heat-resistant titanium alloys. The principal
difference of these alloys from traditional ones is the presence of chemical compounds
(intermetallics) in their structure [1]. The higher values of strength and heat resistance are
attained in these alloys, giving an opportunity to get a new complex of metal properties, which
are of great interest for the modern industry.
However, the problems of chemical and structural homogeneity of metal and fine-grain
structure are very acute in the production of titanium alloys with intermetallide strengthening.
The melting technology must guarantee the preset chemical composition of the alloy with a
high level of it homogeneity. Therefore, the development of new processes of melting and
metal crystallization, having mechanisms for improvement of these characteristics, is
extremely topical.
With the above said, the ElectroSlag Remelting (ESR) process is a promising method
for the production of titanium alloys. During the ESR process, a consumable cylindrical
electrode is being melted by Joule heat generated in a resistive slag pool. Metal droplets pass
through the slag and solidify in a water-cooled copper crucible to form ingots. Since a high
operating current passes through the two liquid current conducting media (the slag and the
metal pool), the electromagnetic methods to control the heat/mass transfer and metal
crystallization are the most promising.
1. Analysis of the Applied Field Effect
The nature and intensity of the action of applied magnetic fields on the ESR are
determined by the fields’ parameters as well as by the value and nature of the current
distribution in the slag and molten metal.
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Lines of the electric current at ESR and versions of the applied magnetic fields are
shown in Figs.1-2. In all these cases, Lorentz forces f = j×B have a vortical mode (rotf 0)
and, consequently, cause the motion of the melts. In the case of axial and radial applied fields
(Fig. 2 a, b), the Lorentz forces are azimuthally directed and cause the melt rotation around the
pool axis. In the case of transverse magnetic field (Fig. 2c), the presence of two components
of the Lorentz force causes a complicated melt circulation. Additionally, the absence of axial
symmetry of the applied field results in skewing of the pool free surface (fz has different
directions at
0x
and
0x
).
Fig. 1. Electric current lines
at ESR
Fig. 2. Versions of the applied magnetic fields: (a) axial, (b)
radial, (c) transverse
At ESR, the AC current is used for melting. Therefore, if the AC applied magnetic
field is used, electrovortical flows are formed in the slag and in the metal pool. In case of DC
magnetic field, the melt vibration is exited. In this case, the vibration frequency is equal to
that of the melting current (50 Hz). However, in some cases there is an effect of the melting
current partial rectifying at electroslag remelting [2]. Therefore, in this case it is possible to
realize both the electrovortical flows and the melt vibration.
Experience of the electromagnetic influence at ESR evidences that the steady
electrovortical flows in the metal pool can lead to the formation of different kinds of defects
in the ingot. Therefore, the DC magnetic fields were used in these investigations.
2. Experimental
The effect of different configurations of the applied magnetic fields on the
technological features of the electroslag process and properties of melted ingots were tested.
Titanium ingots of 80 ... 160 mm in diameter were melted. DC axial and transverse magnetic
fields were applied. The melting voltage was 25 ... .40 V, current 2 ... 10 kA.
A transverse DC magnetic field of 0.05 ... 0.25 T was induced by an electric magnet,
whose poles were located from the opposite sides of the mould. This field initiates vibrations
(50 Hz) of the melt in transverse planes. The vibration influence on the electrode melting
nature was found. Drops were formed around the entire edge of the consumable electrode, but
not only in its central part; the frequency of the metal drop detachment increased and their
average weight was reduced (Fig. 3). However, the stability of the electroslag process and the
conditions of the ingot formation has got worse (Fig. 4).
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Fig. 3. Frequency of the metal drop detachment
versus the transverse field induction
Fig. 4. Appearance and macrostructure
of a Ti ingot melted under the
transverse magnetic field B = 0.2 T
The axial DC magnetic field was generated by a solenoid, which embraced the mould.
A continuous and pulsed influence with a pulse duration of 0.3 to 20 s and a pause duration of
3 to 30 s was used. The magnetic field induction in the melting zone was varied in the range
0.05 to 0.3 T.
The best results were obtained under the effect of a pulse magnetic field. Its
application allowed, first, to increase the electromagnetic action owing to the hydrodynamic
impact at the moment the magnetic field is imposed and, second, to reduce the mass and
energy consumption of the electromagnetic device. In addition, the pulse magnetic fields
promote the continuous restructuring of the
hydrodynamic flows in the metallurgical
pool, thus providing an intensive stirring of
the liquid metal.
It is found that the pulse influence by
the magnetic field leads to appropriate pulse
fluctuations of the melting current (Fig.5).
At the moment of the magnetic field impulse
the melting current decreases. Then, when
the magnetic field is switched off, the
melting current recovers to its initial value.
This is caused by some factors; the main among them is the deformation of the slag pool free
surface and the decrease in immersion depth of the consumable electrode into the slag at the
moment of the magnetic field impulse.
It should be noted that such fluctuation of the melting current does not result in a
considerable instability of the electroslag process and ingot formation. Therefore, the pulse
field causes a discrete-portion heat input assuring additional feasibility for ingot crystallization
process control.
Fig.6 shows the magnetic field effect on the ingot surface formation. Under the
continuous action of the magnetic field a lateral surface of the ingot is deteriorated (Fig.6a).
Under the pulsed action it is possible to obtain a good quality ingot surface at a definite ratio
of the pulse and pause duration (Fig. 6d). For 100 mm diameter ingots and 0.22 T induction,
these conditions are as follows: pulse duration 1s, pause 10 s.
Fig. 5. Fluctuations of the melting current
under the pulsed actions by the axial
magnetic field (B = 0.22 T)
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Fig. 6. Ti ingots melted
under the pulsed action of
the axial magnetic field
(B = 0.22 T):
(а) continuous influence,
(b) pulse 10 s, pause 30 s,
(c) pulse 1 s, pause 3 s,
(d) pulse 1 s, pause 10 s
The magnetic field effect on the crystallization of titanium ingots is illustrated in Fig.7.
The structure of ESR ingots is characterized by the coarse columnar crystallites, which are
formed under conditions of directed heat dissipation through the mould wall (Fig.7a). The
action of magnetic field leads, first, to thinning and reorientation of the crystallites (Fig.7b)
and, with the induction increase, to refining of the ingot structure (Fig.7c). Here, the
macrostructure of the ingots is dense and homogeneous, consisting of equi-axial grains. The
mechanism of ingot structure refining is associated with the destroying action of the melt
electromagnetic vibration on the growing crystallites caused by the interaction of the AC
melting current with the DC magnetic field. The refining of the ingot structure also stimulates
intensification of heat/mass exchange at the liquid-solid phase interface.
The structural homogeneity and the
grain size are the most important factors
determining the quality of multi-component
titanium alloys. Therefore, the refining of
the ingot crystalline structure might be the
main result of the magnetic field action.
However, with the increase in
diameter of the melted ingot (more than 140
mm), the effectiveness of the metal
structure refining decreases that is due to a
scale factor and reduction in densities of the
operating current in the pool. This
necessitates the increase of the magnetic
field induction in the melting zone, thus
providing the increase in mass-dimensional
characteristics of the electromagnetic
device. In this connection, the further
investigations of feasibility of intensification of the hydrodynamic effect on molten metals are
topical with the aim to reduce the dimensions of the electromagnetic device and improve the
technological parameters of melting.
The challenging direction of the investigations is the application of charge of capacitor
banks to supply the solenoid windings. In this case, it is possible to obtain high peak currents
and necessary magnetic field induction values in the melting zone at a relatively small cross-
section of the solenoid windings.
Another way to improve the effectiveness of the axial magnetic field is the increase of
the radial component of the melting current. This can be achieved by increasing the current
Fig. 7. Macrostructures of 110 mm diameter Ti
ingots melted at different induction values of
the axial magnetic field:
(а) В =0, (b) В = 0.08 Т, (c) В = 0.22 Т
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Fig. 8. Schematic of the MEM of
titanium: 1 - vacuum chamber,
2 - electrode, 3 - slag pool, 4 - metal
pool, 5 - electromagnetic system, 6 -
ingot, 7 - mould
Fig. 9. Technological schemes of MEM (pos. see Fig. 8)
passing through the mould circuit (Fig. 1) and by synchronizing the action of the axial field
with the mould current [3].
3. Technology of Magnetically Controlled Electroslag Melting
The results of the investigations have become the basis for the development of a
technology of the Magnetically-controlled Electroslag Melting (MEM) of titanium alloys
(Fig.8). The melting process is realized in a chamber-type electroslag
furnace in the controllable atmosphere of inert gas. The melting of
the consumable electrode extruded from spongy titanium and
alloying elements is implemented under the action of an applied
magnetic field generated by an electromagnetic
system embracing the mould. The main benefit of
the magnetic fields application at MEM is the
increase of homogeneity of the metal and refining of
its crystalline structure. Depending on the ingot
composition, type, size and purpose, different
melting schemes were proposed (Fig. 9).
The rational application of the MEM technology is the melting of ingots of multi-
component titanium alloys, including those with the intermetallic strengthening. An example
of these alloys can be a VT22 alloy (Ti-5Al-5V-5Mo-1Fe-1Cr) strengthened additionally by
C, B, Si, or eutectoid TiSn (Table 1).
Tab. 1. Mechanical properties of titanium alloys by MEM technology at room temperature*
Alloy
Ultimate Tensile
Strength, MPa
Elongation,
%
Area Reduction,
%
Impact Strength
(U-notch), J/cm2
VT22 + 0.2%C
1288-1366
14-16
37-40
20-22
VT22 + 0.2%B
1330-1340
8-12
27-36
22-24
VT22 + 0.25%C + 0.2%B
1320-1370
7-9
20-28
15-18
VT22+0.1%Si + 0.1%C +
0.1%B
1300-1370
8-14
22-30
18-22
VT22+4%Sn
1190-1280
5-8
9-16
18-21
*After heat treatment
As the metal investigations showed, the MEM technology allows realization of
advantages of titanium alloys with intermetallic strengthening, mainly owing to the uniform
distribution of alloying elements and intermetallic phase in the ingot volume and refining of
its crystalline structure.
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Conclusions
1. Electroslag technologies are favorable processes for electromagnetic processing.
The wide range of conditions for its stability and the fact that a high electric current passes
through the melts of slag and metal contribute greatly to this.
2. The applied magnetic fields are effective tools to control the hydrodynamics of the
melts of slag and liquid metal, allowing to influence the melting of the electrode metal,
transfer of electrode metal drops through the slag pool and the ingot crystallization.
3. It is shown that the most efficient scheme of the electromagnetic action at ESR is
the pulsed action of the axial field. Its application assures the increase of the metal
homogeneity and refines its macrostructure, thus preserving the high quality of ingot
formation.
4. A technology of the magnetically controlled electroslag melting of titanium alloys
has been developed, providing the preset chemical composition of the metal, its homogeneity
and fine-grain structure. A new technology can be a promising process for the production of
multi-component titanium alloys.
References
[1] Sysoeva, N. V., Moiseev, V. N.: Titanium alloys with intermetallic type of strengthening. Aviation Materials
and Technologies. Moscow: VIAM, 2002, pp. 162-170 (in Russ.)
[2] Maksimovych, B. I.: On rectifying the AC current during electroslag remelting of consumable electrodes in a
water-cooled metal crucible. Avtomaticheskaya Svarka (Machine Welding), 1961, No.3, pp.101–102 (in
Russ).
[3] Protokovilov, I. V., Kompan, Y. Y.: Electroslag welding and melting of titanium alloys with controlled
hydrodynamic processes. Proc. International Conference “Ti-2010 in CIS”, Ekaterinburg, Russia, 17-19 May,
2010 (in press).
Authors
Prof. Kompan, Yaroslav
Ph.D. Protokovilov, Igor
Paton Electric Welding Institute
11, Bozhenko Str.
03680, Kiev, Ukraine
E-mail: kompan@ion.kiev.ua
E-mail: magnit@ion.kiev.ua
Dr. - Ing. Bojarevics, Andris
Prof. Gelfgat, Yuri
Institute of Physics,
University of Latvia
32 Miera str.
LV-2169 Salaspils, Latvia
E-mail: andrisb@sal.lv
E-mail: yglf@sal.lv
Prof. Fautrelle, Yves
EPM Laboratory, SIMAP
ENSHMG, BP 95, 38402
Saint Martin d’Hиres
cedex, France
E-mail:
Yves.Fautrelle@inpg.fr
