Page 136 - Welding of Aluminium and its Alloys

MIG welding 121

source these fluctuations are compensated for by the power source and the
welder may not appreciate that this is occurring – with the drooping char-
acteristic the arc length changes and the welder may experience what are
perceived as arc stability problems. If the welder is sufficiently skilled, cor-
rective action can be taken before this results in welding defects, whereas
with the flat characteristic power source the welder can produce lack of
fusion or excess penetration defects unknowingly. An advantage of the
drooping characteristic power source is that as the welding current and the
wire feed speed are fixed the welder can employ these features to enable
the wire tip to be pushed into the joint, a useful feature when making the
root pass.
The drooping characteristic unit is also useful in deep weld preparations.
In such joints the constant voltage power source may measure the arc
voltage from the side wall, rather than from the bottom of the weld prepa-
ration, resulting in an unstable arc condition, poor bead shape and variable
penetration.The same restrictions apply when welding the root pass in fillet
welds where a drooping or constant current unit may give better results
than the constant voltage power source. Weaving of the torch may also
cause problems where the torch is moved simply by pivoting the wrist.This
gives a regular increase and decrease in arc length, causing a loss of pene-
tration at the limits of the weave with the flat characteristic power source.
However, despite the apparent advantages of a drooping characteristic
power source the bulk of MIG welding units in production today use a flat
characteristic with consistent and acceptable results.

7.2.1.1 Pulsed MIG welding

Pulsed MIG welding was developed in the early 1960s but it was not until
the late 1970s that the process began to be widely adopted on the shop floor.
Prior to this date the equipment had been expensive, complicated and
difficult to set up for optimum welding parameters, making it welder-
unfriendly and impeding its acceptance by the most important individual
in the welding workshop. Solid state electronics started to be used in
welding power sources in the 1970s and ‘single knob’ control became pos-
sible with the advent of synergic logic circuits. The synergic capability
enabled all of the welding parameters to be controlled from a single dial
control which optimised the current peak pulse and background values, the
voltage and the wire feed speed. It has also became possible to repro-
gramme the power source instantly when wire size, shield gas, filler metal
composition, etc. are changed, simply by dialling in a programme number
(Fig. 7.6).These programmes have been established by the equipment man-
ufacturer with the optimum parameters for the application. Initially these
units were expensive but the price has been steadily reduced such that they