Advanced Welding Processes: Technologies and Process Control

84
Advanced welding processes
Deposition rate (kg h –1 )
9
8
7
6
5
4
3
2
1
0
Cold wire
Hot wire
0 2 4 6 8 10
Arc energy (kW)
6.6 Comparison of deposition rates for hot- and cold-wire GTAW. [93]
6.3.6 Dual-gas GTAW
(lb h
20
–2 )
Constriction of the core of a GTAW arc occurs under normal operating
conditions due to the effect of the compressive Lorentz force, which is
produced by the interaction of the arc current and its associated magnetic
field. The Lorentz force is proportional to the square of the arc current and
the resultant constriction may be very low at currents below 20 A, but
pronounced at currents above 100 A. The effect can be induced by applying
an external axial magnetic field or by thermal constriction caused by the
impingement of cold gas jets on the outer region of the arc. Reducing the
temperature of the outer core of the arc decreases the area available for
current flow, restricts the number of charge carriers (ionized particles) and
increases the temperature and energy density of the inner core of the arc.
This effect has been utilized in the ‘dual-shielding’ GTAW technique [96]
illustrated in Fig. 6.7. A cylindrical nozzle surrounding the electrode directs
a flow of cool shielding gas along the outer surface of the arc. This gas
provides shielding of the electrode and the immediate arc area, but also
causes some thermal constriction and stiffening of the arc. An additional
concentric gas shield provides protection of the weld pool and outer regions
of the arc. The gases used for the inner and outer shields may be of different
compositions, for example argon/5% hydrogen may be used for the central
gas (giving a potential increase in constriction and stiffness (see Chapter 4),
while argon or even argon/20% CO 2 may be used for the outer shield when
welding low-carbon steel. The dual-gas system has been used for stainless
15
10
5