Advanced Welding Processes: Technologies and Process Control

Water
cooling
6.7 Dual-shielded GTAW.
Advanced gas tungsten arc welding 85
steel, carbon steel and non-ferrous materials including aluminium. At low
currents (20 to 50 A), the process gives improved arc stability and the mean
current required is 30 to 40% lower than that required for conventional
GTAW. On thicknesses up to 3 mm in aluminium alloys and 4 mm in steel,
square butt preparations may be used and welding speed increases of up to
20% compared with conventional GTAW are possible. If the current is increased
to 335 A and above keyhole (see Chapter 8), welding is possible on plate
thicknesses from 4 to 6 mm.
Physical constriction of the GTAW arc by a water-cooled copper nozzle,
as shown in Fig. 6.8, will produce an even higher current density and arc
core temperature for a given mean current. When this technique is used, the
process is referred to as the plasma process [97] (see also Chapter 1).
6.3.7 Plasma welding
In plasma welding, the arc is formed between the tip of a non-consumable
electrode and either the workpiece or the constricting nozzle. Again, it is
possible to select different shielding and plasma-forming gases, although
argon is commonly used as the central plasma gas whilst argon or argon/
hydrogen (where appropriate) may be used for the outer shielding medium.
Two basic operating modes are possible for plasma welding. These are:
∑ the transferred arc mode;
∑ the non-transferred arc mode.
Concentric
shield gas
Outer shield
gas