UNIMIG Cat 2017 low res

TECHNICAL NOTES
TIG
AC Welding
AC (alternating current) enables us to TIG weld non ferrous alloys like Aluminium, Magnesium and Aluminium Alloys.
These materials have an insulating surface oxide layer that melts at a higher temperature than the base metal making
it difficult to weld the base metal if the oxides are not removed. AC welding current is ideal because the nature of the
AC wave form assists in breaking the surface oxide layer.
AC (alternating current) has a current cycle that flows from + (direct) polarity to - (reverse) polarity.
The reversing of the polarity breaks the surface oxide while the direct polarity melts the base material.
current
direct polarity
reverse polarity
There are inherent problems that come with AC TIG arc rectification, arc stutter, arc wandering and arc stoppage.
These problems typically occur during the transition between + and - cycles. The current is less (30%) during the half
of the cycle when the electrode is positive and there is a resistance of the electron flow during this half cycle
(rectification). The lack of current flow during this half cycle makes the AC arc unstable.
current
direct polarity
30% 30%
reverse polarity
To overcome this lack of flow during one half of the cycle, a high-frequency (HF) voltage is generated and fed into the
welding circuit. The HF maintains the arc stability during the half cycle when the electrode is positive.
UNBALANCED WAVE FORM
_
+
HF
current
direct polarity
30% 30%
reverse polarity
High-frequency voltage flows continually in the welding circuit and keeps the shielding arc in the welding area in an
ionized state. When the arc is ionized the arc is maintained during the half of the cycle when the electrode is
positive. However while the arc is maintained less current flows during this half of the AC cycle, producing an
unbalanced wave.
296