Direct current electrode negative (DCEN) polarity is also occasionally used to provide a greater
deposition rate. However, penetration is reduced and there is some increased risk of lack of fusiontype
flaws. From a practical point of view, a change from DCEP to DCEN may necessitate an increase
in voltage of about 2 to 3 V if a similar bead shape is to be maintained.
Both constant voltage and constant current (drooping voltage characteristics) power sources can be
used. With constant potential power sources, used in conjunction with constant speed wire feeders,
the arc length self-adjusts to a nearly constant value depending on the voltage, as in GMAW. The wire
feed speed and the electrode diameter control welding current, and the power source controls voltage.
By comparison, a constant current power source tries to simulate a manual welder. Essentially, a
voltage sensitive relay in a variable-speed wire feeder constantly adjusts the wire feed speed to
maintain the target arc voltage and, therefore, a constant arc length. The power source controls
current, and arc voltage depends on wire feed speed and electrode diameter. Modern power sources
are available that operate in either constant voltage or constant current mode.
Power sources are available that can deliver up to 1500 A. However, direct current is usually kept
below 1000 A since there can be excessive arc blow. Alternating current can be used to reduce arc
blow in high current applications and other situations prone to arc blow, e.g., multiwire and narrow gap
welding. Alternating current power sources are usually constant current type with a nearly square wave
output voltage to assist in arc ignition at each polarity reversal. Square wave constant potential power
sources have also become available that provide both voltage and current in square wave form and
therefore have less difficulty in arc re-ignition at polarity reversals. The weld bead penetration obtained
with alternating current is in between that for DCEP and DCEN.
A coil attached to the welding head provides a continuous feed of the metal electrode from the coil
through wire straighteners and a contact tip to the workpiece, and a hopper provides flux in front of the
metal electrode feed. The welding head is usually mounted on a carriage, where it moves at a
predetermined travel speed, thus enabling complete mechanization of the welding process.
Alternatively, the welding head can be fixed and the workpiece moves beneath it at a predetermined
speed.