Advanced Welding Processes: Technologies and Process Control

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Advanced welding processes
Single pass per layer
Two pass per layer
9.6 Single and two pass per layer techniques.
is good [186] and the mechanical properties of the joints are superior to
those produced with either CO 2 or argon/CO 2 mixtures.
The position of the filler wire within the joint determines the arcroot
location and will influence the incidence of fusion defects. Normal GMAW
filler wires have a natural curvature (referred to as cast) and spiral (referred
to as helix), and this gives rise to random oscillation of the wire tip. Specially
straightened wire is available for automatic welding and automatic equipment
often incorporates some means of wire straightening. These techniques ensure
a stable arc location, but a novel modification of the filler wire, which exploits
the presence of a helix to produce more controlled arc oscillation, has been
used as a means of improving fusion. This technique, known as twist arc,
involves fabricating a special filler wire by twisting two smaller-diameter
wires together. [187] The principle of the technique is shown in Fig. 9.7. The
welding head tracks along the longitudinal axis of the weld and with a gap
width of 14 mm adequate fusion is obtained at both sidewalls. The arc
oscillation pattern may be varied by changing the relative diameter of the
wires, the pitch of the spiral and the operating parameters.
Torch or wire oscillation. Lateral oscillation of the torch may be used as
shown in Fig. 9.8 or, alternatively, an eccentric contact tube may be rotated
to produce circular oscillation of the wire tip as shown in Fig. 9.9. These
systems are relatively complex and the minimum gap width is often limited
by the need to move the whole welding head. Alternative systems that rely
on controlled deformation of the filler wire as it passes through the feeding