©
COPYRIGHT 1998 THE ESAB GROUP, INC. LESSON
II
2.4.2.2 Globular
transfer takes
place at lower welding currents than spray transfer. There
is a transition current where the transfer
changes to globular even when shielding gases using a
high percentage of argon are used. When carbon dioxide (CO2)
is used as a shielding gas, the
transfer is always globular. In globular transfer, a molten drop larger
than the electrode diameter
forms on the end of the electrode, moves to the outer edge of the electrode and
falls into the
molten puddle. Occasionally, a large drop will "short circuit" across the
arc, causing the
arc to extinguish momentarily, and then instantaneously reignite. As a result,
the arc is somewhat
erratic, spatter level is high, and penetration shallow. Globular transfer
is not suitable
for out-of-position welding. See Figure 11. 2.4.2.3
Short
circuiting transfer
is a much used method in gas metal arc welding. It
is produced
by using the lowest current-voltage settings and the smaller wires, usually .030",
.035", and .045" diameters. The
low heat input makes this process ideal for sheet metal, out-
of-position work, and poor fit-up applications.
Often called "short arc welding" because metal transfer
is achieved each time the wire actually short circuits (makes contact) with the
weld puddle. This
happens very rapidly. It is feasible for the short circuit frequency to
be 20-200 times
a second, but in practice, it occurs from 90-100 times a second. Each time
the electrode
touches the puddle, the arc is extinguished. It happens so rapidly that
it is visible only
on high speed films. 2.4.2.4
Pulse
transfer is
a mode of metal transfer somewhat between spray and short
circuiting. The specific power
source has built into it two output levels: a steady background
level, and a high output (peak) level.
The later permits the transfer of metal across the arc.
This peak output is controllable between
high and low values up to several hundred cycles per second.
The result of such a peak output produces a spray arc below the typical
transition current.
2.4.2.4.1 Figure 11
shows the transfer method. The arc is initiated by touching the wire to
the work. Upon
initial contact, a bit of the wire melts off to form a molten puddle. The
wire feeds forward
until it actually contacts the work again, as at 1 in Figure 11, and the arc is
extinguished. The short circuiting
current causes the wire to neck down, as shown in 1, until it
melts off, as shown at 2. As
soon as the wire is free of the puddle, the arc is reignited and a
molten ball forms at the end of the
electrode, as at 3. The wire continues to move forward until
it makes contact with the puddle, and
the cycle is repeated. 2.4.2.5
Gas
metal arc
spot welding
is a variation of the process that allows spot welding
of thinner gauge metals, or of a thin
gauge metal to a heavier section. The gun is placed
directly against the work and is equipped
with a special nozzle to allow escape of the shielding gas.
When the trigger switch is actuated, the following sequence takes place.
The shielding
