Page 115 - Welding of Aluminium and its Alloys

100 The welding of aluminium and its alloys

Reignition voltage

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HF sparks
Arc voltage
Voltage across arc gap
Welding current
Open circuit voltage

6.3 HF current and its effect on voltage and current.

results in overheating and melting of the electrode. Manual TIG welding of
aluminium is therefore normally performed using alternating current, AC,
where oxide film removal takes place on the electrode positive half cycle
and electrode cooling and weld bead penetration on the electrode negative
half cycle of the AC sine wave. The arc is extinguished and reignited every
half cycle as the arc current passes through zero, on a 50Hz power supply
requiring this to occur 100 times per second, twice on each power cycle.
To achieve instant arc reignition a high-frequency (HF), high-voltage (9–
15000V) current is applied to the arc, bridging the arc gap with a continu-
ous discharge. This ionises the gas in the arc gap, enabling the welding arc
to reignite with a minimum delay (Fig. 6.3). This is particularly important
on the DCEP half cycle.
Aluminium is a poor emitter of electrons, meaning that it is more diffi-
cult to reignite the arc on the electrode positive half-cycle. If there is any
delay in reignition then less current flows on the positive half cycle than on
the negative half cycle. This is termed partial rectification and can eventu-
ally lead to full rectification where no current flows on the positive half
cycle. The arc becomes unstable, the cleaning action is lost and a direct
current component may be produced in the secondary circuit of the power
source, leading to overheating of the transformer.This is prevented on older
power sources by providing an opposing current from storage batteries and
in more modern equipment by inserting blocking condensers in the power
source circuit.
The HF current is operating continuously when the arc is burning in the
AC-TIG process. An important word of caution relates to this – the HF
current can track into other equipment in the vicinity of the arc and