Figure 8-9.Tungsten electrode shapes for ac
and dc welding.
Tungsten electrodes are usually color-coded at one
end. A green color indicates that the rod is pure
tungsten; yellow indicates a 1-percent thoriated tungsten
rod; red indicates a 2-percent thoriated tungsten rod; and
brown indicates that the rod is alloyed with zirconium.
To produce good quality welds with the GTA
process, you must shape the electrode properly. The
general practice is to use a pointed electrode with dc
welding and a spherical end with ac welding (fig. 8-9).
Shielding gas for GTA welding can be argon, he-
lium, or a mixture of argon and helium. Argon is by far
the most popular. When compared to helium, argon has
greater cleaning action and provides a more stable arc.
Argon is heavier than air; therefore, it provides a blanket
over the weld that protects it from contaminants.
Helium, being lighter than air, requires a higher gas flow
than argon and is therefore more expensive to use.
However, as a shielding gas, helium - allows greater
penetration and faster welding speeds because the arc is
hotter in the helium atmosphere than in the argon
atmosphere. The opposite is true for GMA welding;
therefore, a mixture of argon and helium is sometimes
used in welding metals that require a higher heat input.
Table 8-4 lists a selection of shielding gases
recommended for various metals for both the GTA and
GMA welding processes. Notice that for most GTA
welding operations, you use pure argon.
The primary difference between the regulators used
for oxyfuel welding and for GTA/GMA welding is that the
working pressure on the oxyfuel regulators is shown in
pounds per square inch (psi) while the regulators used
Figure 8-10.Combination regulator and
Figure 8-11.Cross section of flowmeter.
for GMA/GTA show the flow of shielding gas in cubic
feet per hour (cfh) or liters per minute (lpm).
Regulators used for GMA/GTA welding have a
flowmeter instead of a working pressure gauge along
with the cylinder pressure gauge. See figure 8-10.
The flowmeter consists of a plastic or glass tube that
contains a loosely fitting ball. As the gas flows up the
tube, it passes around the ball and lifts it up. The more