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Y. Shi et al.
10.2.2.1 Welding Current
Research was done to figure out the effect of the welding current on the quality
of the weld seam. Tests on the SAF 2205 showed that the appearance of the weld
changed as the currents increased. There is a minimum welding current for starting
the keyhole welding process, which is called ‘threshold current’. In other words,
when the current is increased above the threshold will bring lower welding quality
because of the undercut.
This principle is also proved by the research done by Fei [13]. The 9 mm armour
grade quenched, and tempered steel was joined with keyhole TIG welding at same
speed of 28 cm per min. A threshold current is about 545 A. When it was higher or
lower than this current, over-penetration or under-penetration would be found after
the weld jointing, as shown in Fig. 10.4. However, current is just one of the many
quality considerations for ensuring good welding quality, so when welding current is
very close to the threshold current, other welding parameters should be considered.
The frequency of the welding current had influence in the improvement of weld
joint microstructure and mechanical properties. By applying an alternated current
with 38.6 kHz, Fang carried out experiment joining Q345 steel with the thickness
of 5.5 mm [13]. The threshold current to achieve stable keyhole TIG welding drops
from 430 to 340 A compared to the ordinary K-TIG welding. Moreover, the current
window also expanded.
Cui proposed a welding method of K-TIG called ’One pulse one open keyhole’ to
solve the welding difficulty caused by narrow parameters window in K-TIG welding.
The pulse waveform is periodic, and each unit of periodic contains a base current of
350 A and a peak current of 350 A lasting for 500 ms [15].
10.2.2.2 Travel Speed
The changes of the ravel speed and the thickness of the weldment result in different
consequences. It is common that once the travelling speed is not high enough, the
weld pool will collapse. The minimum travelling speed is called the threshold of the
travel speed. The threshold of the travel speed decreases with the increasing of the
weldment thickness and the decreasing of the thermal conductivity. As a basic rule,
the maximum travelling speed is always higher in the welding of thin plates compared
with the thick ones. That is, because the threshold of the travel speed depends on the
characteristic of the arc as well as the performance of the weldment. For example, the
range of the speed was 300–350 mm/min in the welding of SAF2205 if the keyhole
was stably formed. But for AISI 304 plate with thickness of 12 mm, the speed needed
to be 175 mm/min [6]. These shows that a higher travelling speed can be performed
during the thin plate welding, but the limitation depends on the material and the arc.
Y. Shi et al.
10.2.2.1 Welding Current
Research was done to figure out the effect of the welding current on the quality
of the weld seam. Tests on the SAF 2205 showed that the appearance of the weld
changed as the currents increased. There is a minimum welding current for starting
the keyhole welding process, which is called ‘threshold current’. In other words,
when the current is increased above the threshold will bring lower welding quality
because of the undercut.
This principle is also proved by the research done by Fei [13]. The 9 mm armour
grade quenched, and tempered steel was joined with keyhole TIG welding at same
speed of 28 cm per min. A threshold current is about 545 A. When it was higher or
lower than this current, over-penetration or under-penetration would be found after
the weld jointing, as shown in Fig. 10.4. However, current is just one of the many
quality considerations for ensuring good welding quality, so when welding current is
very close to the threshold current, other welding parameters should be considered.
The frequency of the welding current had influence in the improvement of weld
joint microstructure and mechanical properties. By applying an alternated current
with 38.6 kHz, Fang carried out experiment joining Q345 steel with the thickness
of 5.5 mm [13]. The threshold current to achieve stable keyhole TIG welding drops
from 430 to 340 A compared to the ordinary K-TIG welding. Moreover, the current
window also expanded.
Cui proposed a welding method of K-TIG called ’One pulse one open keyhole’ to
solve the welding difficulty caused by narrow parameters window in K-TIG welding.
The pulse waveform is periodic, and each unit of periodic contains a base current of
350 A and a peak current of 350 A lasting for 500 ms [15].
10.2.2.2 Travel Speed
The changes of the ravel speed and the thickness of the weldment result in different
consequences. It is common that once the travelling speed is not high enough, the
weld pool will collapse. The minimum travelling speed is called the threshold of the
travel speed. The threshold of the travel speed decreases with the increasing of the
weldment thickness and the decreasing of the thermal conductivity. As a basic rule,
the maximum travelling speed is always higher in the welding of thin plates compared
with the thick ones. That is, because the threshold of the travel speed depends on the
characteristic of the arc as well as the performance of the weldment. For example, the
range of the speed was 300–350 mm/min in the welding of SAF2205 if the keyhole
was stably formed. But for AISI 304 plate with thickness of 12 mm, the speed needed
to be 175 mm/min [6]. These shows that a higher travelling speed can be performed
during the thin plate welding, but the limitation depends on the material and the arc.