Figure 15-48.—Gas metal-arc welding. (A) striking the arc; (B) gun angle.
remains constant during welding. If the unit is to be used
with a constant-current voltage power source, the drive
unit speed is varied automatically by an electronic
control device.
WELDING GUN.—The function of the welding
gun is to deliver the wire, shielding gas, and welding
current to the arc area. Guns are either the push or pull
type. The pull gun has drive rolls that pull the welding
wire from the wire feeder, and the push gun has the wire
pushed to it by drive rolls in the wire feeder itself.
Both guns have a trigger switch that controls the
wire feed and arc as well as the shielding gas. When the
trigger is released, the wire feed, arc, and shielding gas
stop immediately. With some equipment, a timer is
included to permit the shielding gas to flow for a
predetermined time to protect the weld until it solidifies.
Guns are available with a straight or curved nozzle.
The curved nozzle provides easy access to intricate
joints and difficult to weld patterns.
SHIELDING GAS.—Shielding gases in the gas
metal-arc process are used primarily to protect the
molten metal from oxidation and contamination. Other
factors must be considered, however, in selecting the
right gas for a particular application. Shielding gas can
influence arc and metal transfer characteristics, weld
penetration, width of fusion zone, surface-shape
patterns, welding speed, and undercut tendency. Inert
gases, such as argon and helium, provide the necessary
shielding because they do not form compounds with any
other substance and are insoluble in molten metal. When
used for welding ferrous metals, arc action may be
erratic and the metal transfer globular. Therefore, it is
necessary to add controlled quantities of reactive gases
to achieve good arc action and metal transfer with these
materials.
Helium is preferable for welding thick materials,
especially those with high heat conductivity, such as
copper, aluminum, and some copper-base alloys.
Helium has a higher ionization potential, which results
in a greater weld heat at a given amperage. Argon is
more suitable for use with lighter-gauge materials and
materials of lower heat conductivity because it produces
lower weld heat.
GMA Welding Techniques
Before you start to weld with GMA welding
equipment, be sure that all controls are properly
adjusted, all connections are correctly made, and that all
safety precautions are being observed. Wear protective
clothing, including a helmet with a suitable filter lens.
Hold the welding torch at an angle of between 5° and
20° to the work, as shown in view B of figure 15-48.
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 work piece. Lower your helmet and squeeze the
trigger on the torch. Squeezing the trigger starts the flow
of shielding gas and energizes the welding circuit. The
wire-feed motor is not energized 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 motion, as shown in view A
of figure 15-48. To prevent sticking, it is necessary to
pull the gun back quickly, about 1/2 inch, the instant
contact is made between the wire electrode and the work
piece. The arc will strike as soon as contact is made, and
the wire-feed motor will feed the wire automatically as
long as the trigger is held.
15-35
