A welding arc is formed when a
fairly high current (10 to 2000 A)
is forced to flow across a gap
between two electrodes at
relatively low voltage (10 to 50
V). A welding arc is intensely hot
with temperatures exceeding
3000°C (see Figure 1.2) and
forms a concentrated heat source
suitable for melting most metals
rapidly. The intense heat of the
welding arc causes the filler
metal to melt and when added to
the locally hot melted workpiece,
it forms the weld fusion zone. Its
subsequent freezing
(solidification) produces the bond
(weld) between the workpieces.
Arc welding processes do not
require application of pressure to
cause fusion.
In welding, the arc may be established between an electrode and the workpiece, or between two
electrodes.
When the workpiece is one of the electrodes of the electrical circuit, the other electrode may be
consumable or nonconsumable. A consumable electrode is designed to melt and add filler material to
the welding joint.
The electrical current for welding is provided by a “power source” that draws high-voltage electric
power from the main transformer and converts it into higher current and lower voltage suitable for
welding (Figure 1.3). Power sources are broadly classified as constant current or constant voltage
type, and the static volt/ampere output characteristics for these two types of power sources are shown
in Figure 1.4.