Advanced Welding Processes: Technologies and Process Control

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Advanced welding processes
7.6 Summary: process control
A well established relationship exists between mean current and wire feed
speed but current must be set indirectly in conventional CV GMAW systems.
The arc voltage, operating current and maximum short-circuit current are
normally determined by the open-circuit voltage setting. The rate of rise of
current and its peak value during a short circuit are controlled by secondary
inductance. Constant-current power sources may be used with resistive wires,
but additional regulation is required to cope with process fluctuations with
high-conductivity consumables.
7.7 Recent developments in the GMAW process
The object of the developments of the GMAW process has been to control
metal transfer, improve process stability, simplify process control, and improve
operating tolerances. The introduction of solid-state power sources has enabled
the process performance to be analysed in more detail and improved control
systems to be developed. The process operating ranges have also been extended
and high deposition techniques have been introduced to provide improved
productivity. Some of these developments are discussed below.
7.7.1 Controlled transfer techniques
The ‘natural’ modes of transfer which have been described above have several
limitations, these include:
∑ Spray transfer only occurs when the mean current exceeds a relatively
high transition current. This limits the capability of the process for positional
work or the joining of thinner sections.
∑ Dip transfer, whilst particularly suitable for joining thin section plain
carbon steel, is less effective on non-ferrous materials.
Advances in the control of metal transfer have led to the development of
pulsed GMAW and controlled dip transfer.
Pulsed GMAW
Pulsed transfer was devised to allow spray-type transfer to be obtained at
mean currents below the normal transition level. A low background current
(e.g. 50–80 A) is supplied to maintain the arc, and droplet detachment is
‘forced’ by the application of a high current pulse (Fig. 7.23). The pulse of
current generates very high electromagnetic forces, as would be expected
from the foregoing analysis of metal transfer, and the metal filament supporting
the droplet is constricted, the droplet is detached and projected across the arc
gap.