transferred per second, whereas many small drops are
transferred with a higher current setting. In this type of
transfer, the ball at the tip of the electrode grows in size
before it is transferred to the workpiece. This globule
tends to reconnect with the electrode and the workpiece,
causing the arc to go out periodically. This results in
poor arc stability, poor penetration, and excessive spat-
Globular transfer is not effective for GMA welding.
When it is used, it is generally restricted to thin materials
where low heat input is desired.
Short-Circuiting Arc Transfer
Short-circuiting arc transfer is also known as short
arc. Short arc was developed to eliminate distortion,
burn-through, and spatter when welding thin-gauge
metals. It can be used for welding in all positions,
especially vertical and overhead where puddle control
is more difficult. In most cases, it is used with current
levels below 200 amperes and wire of 0.045 of an inch
or less in diameter. Small wire produces weld puddles
that are small and easily manageable.
The shielding gas mixture for short-arc welding is
75% carbon dioxide and 25% argon. The carbon dioxide
provides for increased heat and higher speeds, while the
argon controls the spatter. Straight CO2 is now being
used for short-arc welding; however, it does not produce
the excellent bead contour that the argon mixture does.
GMA WELDING PREPARATION
Preparation is the key to producing quality weld-
ments with the gas metal-arc welding process. As in
GTA welding, the equipment is expensive; therefore,
you should make every effort to follow the manufac-
turers instruction manuals when preparing to use GMA
For the most part, the same joint designs recom-
mended for other arc welding processes can be used for
gas metal-arc welding (refer to chapter 3). There are
some minor modifications that should be considered due
to the welding characteristics of the GMA process. Since
the arc in GMA welding is more penetrating and nar-
rower than the arc for shielded metal-arc welding,
groove joints can have smaller root faces and root open-
ings. Also, since the nozzle does not have to be placed
within the groove, less beveling of the plates is required.
GMA welding can actually lower material costs, since
you use less weld metal in the joint.
The following suggestions are general and can be
applied to any GMA welding operation:
Check all hose and cable connections to make
sure they are in good condition and are properly
Check to see that the nozzle is clean and the
correct size for the particular wire diameter used.
Make sure that the guide tube is clean and that
the wire is properly threaded through the gun.
Determine the correct wire-feed speed and adjust
the feeder control accordingly. During welding,
the wire-speed rate may have to be varied to
correct for too little or too much heat input.
Make sure the shielding gas and water coolant
sources are on and adjusted properly.
Check the wire stick-out.
GMA WELDING PROCEDURES
As with any other type of welding, the GMA weld-
ing procedure consists of certain variables that you must
understand and follow. Many of the variables have
already been discussed. This section applies some of
these variables to the actual welding procedure.
Starting the Arc
For a good arc start, the electrode must make good
electrical contact with the work For the best results, you
should clean the metal of all impurities. The wire stick-
out must be set correctly because as the wire stick-out
increases, the arc initiation becomes increasingly diffi-
When preparing to start the arc, hold the torch at an
angle between 5 and 20 degrees. Support the weight of
the welding cable and gas hose across your shoulder to
ensure free movement of the welding torch. Hold the
torch close to, but not touching, the workpiece. Lower
your helmet and squeeze the torch trigger. Squeezing the
trigger starts the flow of shielding gas and energizes the
welding circuit. The wire-feed motor does not energize
until the wire electrode comes in contact with the work-
piece. Move the torch toward the work, touching the
wire electrode to the work with a sideways scratching