Figure 8-26.GMA welding guns.
shown in figure 8-26. When using these guns, the wire
is fed to the torch by an automatic wire feeding
machine which pushes the wire through a flexible
tube to the arc point.
Figure 8-27 shows another type of GMA welding
gun that Steelworkers could use. This model incorpo-
rates the drive motor and a small spool of wire inside
the gun. This type of gun is attached directly to the
welding unit and gas supply, eliminating the need for
a separate control unit and wire drive assembly.
As with the GTA welding torch, the torch nozzle
must be kept clean at all times. Also, you should clean
the tube through which the electrode wire passes each
time the electrode reel is changed.
In gas metal-arc welding, as with gas tungsten-
arc welding, the shielding gas can have a major effect
on the properties of the base metal. Some of the
shielding gases commonly used with the GMA process
are pure argon, argon-helium, argon-oxygen, argon-
carbon dioxide, and carbon dioxide. Refer to table 8-4
for a selection of shielding gases recommended for
various metals for both the GMA and GTA welding
processes. The smoothness of operation, weld
appearance, weld quality, and welding speeds are
affected indifferent ways with each type of metal,
thickness, and gas mixture.
ARGON. Earlier in this chapter, we said that
argon provides greater cleaning action than other gases.
Because it is heavier than air, argon blankets the weld
from contamination. Also, when you are using argon as a
shielding gas, the welding arc tends to be more stable.
For this reason, argon is often used in combination with
Figure 8-27.GMA welding torch with wire feed
motor and wire spool inside.
other gases for arc shielding. Argon reduces spatter by
producing a quiet arc and reducing arc voltage that
results in lower power in the arc and thus lower
penetration. The combination of lower penetration
and reduced spatter makes argon desirable when
welding sheet metal.
Pure argon is seldom used for arc shielding
except in welding such metals as aluminum, copper,
nickel, and titanium. The use of pure argon to weld
steel usually results in undercutting, poor bead
contour, and the penetration is somewhat shallow.
ARGON-OXYGEN. Small amounts of oxygen
added to argon can produce excellent results.
Normally oxygen is added in amounts of 1, 2, or 5
percent. When oxygen is added to argon, it improves
the penetration pattern. It also improves the bead
contour and eliminates the undercut at the edge of the
weld. You use argon-oxygen mixtures in welding alloy
steels, carbon steels, and stainless steel.
HELIUM. Helium, like argon, is an inert gas.
But there are few similarities between the two gases.
Argon is heavier than air and helium is lighter than
air. Helium has a high-voltage change as the arc
length changes. When you use helium for GMA
welding, more arc energy is lost in the arc itself and is
not transmitted to the work In the section on GTA
welding, we said that helium produces good
penetration and fast welding speeds. For GMA
welding, the opposite is true. In GMA welding, helium
produces a broader weld bead, but shallower
Because of its high cost, helium is primarily used
for special welding tasks and for welding nonferrous
metals, such as aluminum, magnesium, and copper. It
is also used in combination with other gases.
CARBON DIOXIDE (CO2). Argon and helium
gases are composed of single atoms. Carbon dioxide, on
the other hand, consists of molecules. Each molecule
contains one carbon atom and two oxygen atoms. At