©
COPYRIGHT 1998 THE ESAB GROUP, INC. LESSON
II
wire, the weld puddle, and the area
in the arc zone are protected from the atmosphere by a gaseous
shield. Inert gases, reactive gases, and gas mixtures are used for shielding.
The metal
transfer mode is dependent on shielding gas choice and welding current level.
Figure 9 is
a sketch of the process showing the basic features. FIGURE
9 WELDING WIRE
WELDING CABLE
SHIELDING GAS
GAS NOZZLE CONTACT
TIP WORK PIECE
MOLTEN POOL WELD
METAL ARC
GAS SHIELD SOLID
WIRE ELECTRODE
TRAVEL DIRECTION
GAS METAL ARC WELDING
2.4.0.1 Gas
metal arc welding is a versatile process that may be used to weld a wide
variety of metals including carbon
steels, low alloy steels, stainless steels, aluminum alloys,
magnesium, copper and copper alloys,
and nickel alloys. It can be used to weld sheet metal or
relatively heavy sections. Welds
may be made in all positions, and the process may be used
for semiautomatic welding or automatic
welding. In semiautomatic welding, the wire feed speed,
voltage, amperage, and gas flow are all preset on the control equipment. The
operator needs
merely to guide the welding gun along the joint at a uniform speed and hold a
relatively constant
arc length. In automatic welding, the gun is mounted on a travel carriage
that moves along
the joint, or the gun may be stationary with the work moving or revolving beneath
it. 2.4.0.2
Practically all GMAW is done using
DCEP (Electrode positive). This polarity provides
deep penetration, a stable arc and low spatter levels. A small amount of
GMAW welding is
done with DCEN and although the melting rate of the electrode is high, the arc
is erratic. Alternating
current is not used for gas metal arc welding. 2.4.1
Current
Density -
To understand why gas metal arc welding can deposit weld
metal at a rapid rate, it is necessary
that the term "current density" be understood. Figure 10
shows a 1/4" coated electrode and a
1/16" solid wire drawn to scale. Both are capable of
carrying 400 amperes. Notice
that the area of the 1/16" wire is only 1/16 that of the core wire
of the coated electrode. We can
say that the current density of the 1/16" wire is 16 times
