22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Lorentz force distribution of anode in pulsed arc welding with iron vapor
T. Tanaka1, S. Yamamoto2 and T. Iwao2
1
Tokyo City University, Tokyo, Japan
Abstract: TIG arc welding is high-quality joining technology. However, the current has to
be small because the cathode melting should be prevented. In this case, the welding pool is
shallow, then, the welding defect sometimes occurs. In order to obtain the deep penetration
of the welding pool, the pulsed TIG arc welding is often used. However, few reports have
reported the contribution of Lorentz force for the welding pool. In this paper, the Lorentz
force distribution of anode affected by the current density in pulsed arc welding with iron
vapor is elucidated. As a result, at the time of peak current reached, the iron vapor
concentration becomes high near the anode. Thus, the electrical conductivity and radiation
loss increased. However, the electrical conductivity decreases because of a large
contribution of the transient response of temperature. In addition, the electrical conductive
radius became small. Therefore, in order to maintain the current continuity, the electrical
field increases. In this case, the current density increase at the axial center position.
Therefore, the Lorentz force becomes large. Because the Lorentz force increases when the
current increases in short time, the penetration of depth increases at high frequencies.
Keywords: arc welding, pulsed arc, Lorentz force, iron vapor, current density
1. Introduction
TIG arc welding is chemically a joining technology
with melting the metallic material with cathode of
Tungsten and inert gas. It has been used LNG (Liquefied
Natural Gas) tanks required the high reliability. However,
the weld defect sometimes occurs. TIG arc welding
should not be used in the case of the high current because
the cathode should be prevented from melting. Therefore,
the deep penetration can’t be obtained to decrease the heat
input. Thus, the weld defect occurs. Therefore, the pulsed
TIG arc welding often has been used in order to improve
the weld shape. This welding can change the base and
peak current periodically. The welding pool is formed by
the convection with the interaction of 4 driving forces [14]. Lorentz force is only driving force of convection to the
depth direction. Lorentz force increases when the deep
penetration derived from the pulsed welding is obtained
experimentally [5]. Furthermore, the transient phenomena
of temperature depends on the pulsed current. Thus, the
welding pool becomes deep. Few report has reported the
physical phenomenon of the Lorentz force with the pulsed
current. When the arc properties changes, the welding
pool changes. Therefore, the knowledge of the Lorentz
force increment is needed for the deep welding pool. In
addition, the weld defect can be prevented when the
appropriate parameters can be determined. In this paper,
Lorentz force distribution of anode affected by current
density in pulsed arc welding with iron vapour is
elucidated.
2. Calculation method
This calculation is analysed with one model of
electrode and arc simultaneously and the boundary
P-II-12-17
Fig.1 Current waveform.
condition. This calculation is used by SIMPLER method
[6] under consideration of the LTE, laminar flow, flat
welding pool and not consideration of metal
contamination from welding pool. The calculation
condition is 2D cylindrical coordinates, interelectrode
distance is 5 mm, cathode is tungsten and its diameter is
1.6 mm, anode is SUS304, and ambient gas is argon and
shielding gas flow rate is 10 slm. Fig.1 shows the current
waveform.
The main factor of welding penetration is the increment
of Lorentz force affected by current density concentration.
Thus, the temperature of transient phenomena contributes
to the concentration of current. Therefore, the parameter
of current is current increment ratio. The peak and base
current arc 200 and 100 A, respectively. The current
increment ratio is used as 1000 Hz. The calculation point
of time is the reaching time of peak current. The pulsed
1
current of peak current and 200 A steady current are
compared.
3. Results and discussion
Fig. 2 shows the distribution of the iron vapor
concentration and temperature, electrical conductivity
near the anode (r = 0 mm) at the center position to the
radial direction. Fig. 3 shows the distribution of current
density near anode and Lorentz force at anode surface and
in the welding pool at the center position to the radial
direction. The temperature near the anode becomes low in
the case of the pulsed current because the temperature
does not follow the sharp increment of the current. The
temperature gradient decreases with decreasing the
temperature. The low temperature gradient depends on the
decrement of the pressure gradient. Thus, the velocity
decreases with decrease the pressure gradient. At the
reaching time of peak current, the iron vapor
concentration increases because of the low velocity. In
addition, the iron vapor concentration increases at the
axial center. The electrical conductivity depends on the
temperature and vapor concentration of iron. The
contribution of electrical conductivity derived from the
temperature is greater than that derived from the iron
vapor concentration. Therefore, the conductivity becomes
low with the pulsed current. Then, the current is
concentrated in the highly electrical conductive axial
center. The electrical field at the axial center position
increased in order to maintain the current continuity. And,
the current density at the axial center increases. By
current concentration to the axial center, the Lorentz force
on anode and in the weld pool induced by the current
density is increased.
When the current increment ratio is high, the current
density at the axial center and maximum Lorentz force in
the welding pool is high, and vice varsa. Therefore, the
pulsed current is deep penetration because it is able to
obtain the large Lorentz force.
4. Summary
The distribution of Lorentz force at the anode affected
by current density in pulsed arc welding with iron vapor
was elucidated. The main results are shown below.
(1) The iron vapour concentration increases in the case
of the pulsed current. However, the electrical
conductivity is greatly decreases with decrease the
temperature.
(2) The current density becomes low in the case of the
pulsed current. The current flows at the highly
electrical conductive radius of axial center. The
electrical field increases to maintain the current
continity. The current density of the axial center
position increases. Therefore, the Lorentz force at the
anode surface and in the welding pool is large.
(3) The iron vapor concentration ratio increases with
decrease the velocity at the axial center.
2
Therefore, the weld depth in the welding pool could
change in the case of the pulsed arc with iron vapor.
Fig.2 Distribution of iron vapor concentration and
temperature, electrical conductivity near anode at the
center position to radial direction.
Fig.3 Distribution of current density near anode and
Lorentz force at anode surface and in welding pool at
center position to radial direction.
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