Table 8-6.Recommended Wire Diameters for GMA Welding Using Welding Grade CO2 and a Wire Stick-out of 1/4 to 3/8 of an Inch
normal temperatures carbon dioxide is essentially an
inert gas; however, at high temperatures it decomposes
into carbon monoxide (CO) and oxygen (O2). Because
the excess oxygen atoms can combine with carbon or
iron in the weld metal, wires used with this gas must
contain deoxidizing elements. A deoxidizing element
has a great affinity for the oxygen and readily combines
with it. Some of the more common deoxidizers used in
wire electrodes are manganese, silicon, and aluminum.
Carbon dioxide is used primarily for the GMA
welding of mild steel. Because of its low cost, CO2 is
often used in combination with other shielding gases for
welding different types of metals. Direct-current reverse
polarity (DCRP) is generally used with CO2. The current
setting is about 25 percent higher with CO2 than with
other shielding gases.
Carbon dioxide produces abroad, deep penetration
pattern. It also produces good bead contour and there is
no tendency toward undercutting. The only problem
with CO2 gas is the tendency for the arc to be violent.
This can lead to spatter problems; however, for most
applications this is not a problem and the advantages of
CO2 far outweigh the disadvantages.
You should use the same type of regulator and
flowmeter for gas metal-arc welding that you use for gas
tungsten-arc welding. The gas flow rates vary, depend-
ing on the types and thicknesses of the material and the
joint design. At times it is necessary to connect two or
more gas cylinders (manifold) together to maintain
higher gas flow.
For most welding conditions, the gas flow rate is
approximately 35 cubic feet per hour (cfh). This flow
rate may be increased or decreased, depending upon the
particular welding application. Final adjustments usu-
ally are made on a trial-and-error basis. The proper
amount of gas shielding results in a rapidly crackling or
sizzling arc sound. Inadequate gas shielding produces a
popping arc sound and results in weld discoloration,
porosity, and spatter.
The composition of the filler wire used for GMA
welding must match the base metal. For mild steel, you
should select mild steel wire; for aluminum, you should
select aluminum wire. Additionally, you should try to
select electrode wire that matches the composition of the
various metals you are welding. For instance, when you
are welding Type 308 aluminum, you should use an
ER-308L filler wire.
Wires are available in spools of several different
sizes. The wire varies in diameter from .020 to 1/8 of an
inch. You should select the proper diameter of wire
based on the thickness of the metal you are welding as
well as the position in which you are welding. Wires of
0.020,0.030, and 0.035 of an inch are generally used for
welding thin materials. You also can use them for weld-
ing low- and medium-carbon steels and low-alloy/high-
strength steels of medium thicknesses. (See table 8-6.)
Medium thicknesses of metals are normally welded with