Advanced Welding Processes: Technologies and Process Control

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Advanced welding processes
Chemical analysis may be a useful indicator of potential problems, but it
may be costly and unreliable in view of the low levels of surface-active
elements which need to be identified and the uncertainty of the correlation
between the analysis and the penetration profile.
Direct weldability trials involve making a joint of the required type using
an established welding procedure, sectioning the bead and measuring its
cross sectional area. This approach has been common but it is time consuming
and requires expert analysis.
A variety of indirect weldability tests have been investigated [106–108]
and it has been found that it is feasible to identify the cast-to-cast effect by
measuring the time for a weld to penetrate a known thickness of plate under
controlled conditions. Alternatively, the material characteristics may be
identified from direct observation of the weld pool using video techniques or
by monitoring light and voltage signals from the arc.
6.4.2 Control of pulsed TIG
In pulsed GTAW, the process is controlled by the variables described above
for conventional GTAW plus the pulse parameters. The influence of the
pulse parameters has been described above.
6.4.3 Control of plasma welding
The control of plasma welding is slightly more complex than GTAW. The
main control variables are the same as GTAW, but plasma gas flow rate and
the diameter and geometry of the plasma orifice will also have a significant
effect on the operation of the process. A smaller plasma orifice will produce
an increased arc force, whereas a large nozzle diameter will result in a ‘soft’
plasma which is more like a GTAW arc. If the plasma gas flow is too low, the
current too high, or the nozzle cooling is restricted, an arc may form directly
across the gap between the electrode and the nozzle; this ‘double arcing’
phenomenon will result in serious damage to the orifice.
As with GTAW, higher currents allow higher travel speeds to be used, but,
as the current is increased, there is a more noticeable increase in arc force
which may result in undercut. If both the plasma gas flow and current are
increased, the keyhole mode of operation is possible as described in Chapter 8.
The main factors controlling GTAW and related processes are summarized
in Table 6.4.
6.5 Summary
The capabilities of GTAW have been extended by basic modifications to the
operating technique such as pulsing, multicathode and hot-wire addition, the