Page 50 - Welding Robots Technology, System Issues, and Applications

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Welding Technology
2.1.4 Process Variants
GTAW is regarded as a high quality process for welding thin metals using low
travel speed and low electrode deposition rate, requiring highly skilled personnel in
manual welding. Variants developed seek to improve productivity, mainly
deposition rate, penetration depth and welding speed. These variants are
implemented in automatic or robotic systems.

Hot-wire GTAW is a variant where a heated filler wire is fed to the rear of the
melted weld pool at a constant rate, as represented schematically in Figure 2.7.
Filler wire is resistance heated close to melting point using mainly AC power
sources, in order to minimize magnetic disturbance of the electric arc. Deposition
rates up to 14 kg/h can be attained with this process. It has been used in heavy wall
fabrication, maintaining high joint integrity [11].

The use of a dual-shielding GTAW technique, see Figure 2.8, where an additional
concentric gas shield gives an increase in constriction and stiffness of the electric
arc, may be used to increase welding speed and penetration depth [12].
Constriction of the arc is produced by the external cold gas flow which decreases
temperature of the outer part of the arc, decreasing the arc cross section where
current flow occurs, consequently increasing current density and temperature.
Electrode gas and annular gas may be of the same or of different compositions,
such as Argon plus 5% hydrogen for internal gas and argon for external gas when
welding austenitic stainless steels. For currents above 335A keyhole welding is
obtained and the process may becomes sensitive to the process parameters. This
technique also tends to increase the risk of undercut [3].

GTAW torch

Filler wire
Hot wire
power GTAW power
source source
Contact tube

Figure 2.7. Schematic representation of a GTAW hot wire system