Page 117 - Welding of Aluminium and its Alloys

102 The welding of aluminium and its alloys

by shortening or extending the length of time spent on the positive or
negative half cycle. The latest inverter-based units provide a high degree of
control with the electrode negative duration time capable of being adjusted
from 50% to 90% of the cycle. Increasing the frequency results in a more
focused arc, increasing penetration, enabling faster travel speeds to be used
and reducing distortion. Increasing the electrode negative portion of the
cycle will give similar results of increased penetration and faster travel
speed although the cathodic cleaning effect will be reduced. Biasing the
square wave more towards the electrode positive half cycle will reduce pen-
etration,useful when welding thin materials,and will widen the bead profile.
Another very important difference between older units and the inverter-
based power sources is that the square wave cycle passes through the zero
welding current point many times faster than with a sinusoidal wave. It is
possible to dispense with continuous HF current for arc stabilisation,
removing the risks of damaging sensitive electronic equipment. High
frequency will still be needed to initiate the arc, however, so a small risk
remains. The lack of continuous high frequency may also result in an un-
stable arc on very clean, etched surfaces or on the weld metal. Inverter
power sources are also capable of overcoming a problem encountered when
using two arcs close together. Welding current can track from one power
source to the other, damaging the circuitry. With the very latest equipment
the two arcs are matched.
Square wave power sources have a further advantage in that tungsten
‘spitting’, where the electrode tip spalls off and contaminates the weld pool,
can be reduced. Reducing the electrode positive portion will reduce the
overheating that causes tungsten spitting.

6.2.2 Shielding gas

The preferred gas for the AC-TIG welding of aluminium is argon, although
helium and argon–helium mixtures may be used. Argon gives a wide,
shallow penetration weld bead but will leave the weld bright and silvery
in appearance. The easiest arc ignition and most stable arc will also be
achieved with argon. Typical butt welds in 3mm and 6mm plate are illus-
trated in Fig. 6.5 and a fillet weld in 6mm thick plate is shown in Fig. 6.6.
A table of suggested welding parameters for use with argon as a shield gas
is included as Table 6.2. Typical current ranges for a range of plate thick-
nesses are illustrated graphically for butt welds in Fig. 6.7 and for fillet welds
in Fig. 6.8.
Helium increases arc voltage with the effect of constricting the arc,
increasing penetration but making arc ignition more difficult, and adversely
affecting arc stability. Some of the modern welding power sources are
equipped with a facility to start the weld with argon and, once a stable arc