Page 137 - Welding of Aluminium and its Alloys

122 The welding of aluminium and its alloys

7.6 Typical modern pre-programmable control panel for synergic
pulsed MIG power source. Courtesy of TPS-Fronius Ltd.

are now only marginally more costly than a conventional power source,
leading to a far wider usage.The modern inverter-based units (Fig. 7.7), are
also far lighter, far more energy efficient and more robust than the older
units that they are replacing.
The pulsed MIG process uses a low ‘background’ current, sufficient to
maintain the arc but not high enough to cause the wire to melt off. On this
background current a high-current, ‘peak’ pulse is superimposed. Under
optimum conditions this causes a single droplet of molten filler wire to be
projected across the arc into the weld pool by spray transfer. It is thus pos-
sible to achieve spray transfer and a stable arc at low average welding cur-
rents. This enables very thin metals to be welded with large diameter wires
where previously very thin wires, difficult to feed in soft aluminium, needed
to be used.The lower currents also reduce penetration, useful when welding
thin materials and also enable slower welding speeds to be used, making it
easier for the welder to manipulate the torch in difficult access conditions
or when welding positionally.
The use of electronic control circuitry enables arc starting to be achieved
without spatter or lack of fusion defects. Some units now available will
slowly advance the wire until the tip touches the workpiece, sense the short
circuit, retract the wire to the correct arc length and initiate the full welding
current (Fig. 7.8). Similarly, in most of these modern units a crater filling
facility is built in,which automatically fades out the current when the trigger
on the gun is released.