Page 106 - Welding Robots Technology, System Issues, and Applications

Welding Robots
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The burn-off rate can be controlled by changing the mean current. This can be used
to adjust wire burn-off rate to match the wire feed rate and a constant arc length
can be maintained.

The parameters to measure and monitor in pulsed GMAW are the pre-set values of
the weld process (I p, I b, U p, U b, T p, T b). The monitoring system must be able to
detect unacceptable variations in the welding features that may generate weld
faults.

3.5 Short-circuit GMAW

In short-circuiting welding the mean current, and thus the average heat input to the
electrode wire and the work-piece, is lower than in spray arc GMAW. Due to the
smaller heat transfer, short-circuiting welding makes it possible to weld thinner
plates than with spray arc welding. The metal deposition rate and the joint
penetration are, however, less than in spray and pulsed welding. In contrast with
pulsed GMAW, the power source in short-circuiting and spray GMAW is voltage-
controlled. This means that the power source continuously changes its current in
order to maintain the voltage constant at the output.

The short-circuiting cycle begins with an arc that is struck between the electrode
wire tip and the work-piece. The wire electrode melts and a small droplet is formed
at the electrode tip. This part of the cycle is denoted “arc time” and represented by
T a.

During the short-circuit time, T s, the droplet at the end of the electrode touches the
weld pool and a bridge of liquid metal is formed between the electrode and the
weld pool. At this stage the arc will extinguish, the voltage will decrease to almost
zero volts and the current will increase to its maximum value. Due to the high
short-circuit current, necking of the liquid bridge starts to occur, ending in rupture
of the liquid bridge. The molten metal is then transferred from the electrode tip to
the weld pool by the force of the surface tension of the weld pool, the gravitational
force and electromagnetic pinch force (induced by the current). After the droplet is
detached from the electrode and transferred to the work-piece the arc is reestab-
lished and the cycle starts over again.

The short-circuiting transfer mode has two degenerated modes of metal transfer,
the short-term short circuiting, meaning that the short circuits last for durations less
than 1-1.5 ms; and stubbing of the electrode in the weld pool, with the result that
the droplet growth cannot occur in the normal way. Stubbing-in of the electrode,
short-term short-circuits and globular metal transfer are considered to be unstable,
while short-circuiting and spray transfer are considered to be a stable process.

In order to produce weld joints with uniform weld quality, it is desirable that the
welding process is stable. This means that the metal transfer from the electrode