Advanced Welding Processes: Technologies and Process Control

Current (A)
500
400
300
200
100
0
Gas metal arc welding 131
Time (ms)
Low wire feed speed Medium High wire feed speed
7.30 Effect of increased mean current and wire feed speed with early
synergic systems.
exceed the value defined by equation (7.9) ( p = )
n I tp D for single droplet
detachment, and control is lost.
An alternative system maintains fixed pulse and background current
amplitude, fixed pulse duration and varies only frequency in response to
variations in wire feed speed. In this case, the ‘ideal’ pulse parameters are
fixed, but, as frequency rises, the background duration decreases, the preheating
of the wire tip during the background period is decreased, and this may
reduce the droplet size, which again limits the range of effective control.
The third technique fixes pulse current and amplitude and varies frequency
with wire feed speed, but to counter the effects of the variation in background
period the product of background duration and current is maintained constant,
i.e.
I pt p = B (7.16)
This last control strategy gives an extended range of operation and minimizes
the need to trim the conditions as the mean current/wire feed speed is adjusted.
The incorporation of microprocessor control enables more complex control
strategies to be adopted, for example it is possible to use a non-linear approach
in which the control algorithm is varied with current, but it is usually found
that the systems described above will give adequate performance in most
applications.
Arc length control. The use of constant-voltage power sources in conventional
GMAW was justified on the basis of the self adjustment required. The
undesirability of current fluctuation and the need to preset current in pulsed
and controlled dip transfer has led to the use of constant-current power
sources for these process modes. Under ideal conditions, the regular burn-off
behaviour achieved with these improved methods of control should ensure
constant arc length, but transient fluctuations in wire feed speed, electrical