Welding Currents
regulator. Do not stand in front of the valve when
opening it.
With direct current the welding circuit may be
14. Learn to identify standard Navy cylinders by
either dc straight polarity (DCSP) or dc reverse polarity
color and decals.
(DCRP). When the machine is set for straight polarity,
the flow of electrons is from the electrode to the plate,
GAS TUNGSTEN-ARC WELDING
which creates considerable heat in the plate. In reverse
polarity, the flow of electrons is from the plate to the
Gas tungsten-arc (GTA) welding is an arc welding
electrode, thus causing a greater concentration of heat
process that produces coalescence of metals by heating
at the electrode. See figure 6-43. The intense heat at the
them with an electric arc between a nonconsumable
electrode tends to melt off the end of the electrode and
tungsten electrode and the base metal. The weld pool,
may contaminate the weld. Hence, for any given
arc, electrode, and the heated section of the work pieces
current, dc reverse polarity requires a larger diameter
are protected from atmospheric contamination by a
electrode than dc straight polarity. For example, a
gaseous shield; otherwise, atmospheric oxygen and
1/16-inch diameter tungsten electrode normally can
nitrogen will combine with the molten weld metal and
handle about 125 amperes in a straight polarity circuit.
result in a weak, porous weld. The shielding gas is
However, if reverse polarity is used with this amount of
usually an inert gas, such as helium, argon, or a mixture
current, the tip of the electrode will melt off.
of gases.
Consequently, a 1/4-inch diameter electrode will be
The electrode used in GTA welding is generally
required to handle 125 amperes of welding current.
tungsten or a tungsten alloy because other refractory
Polarity also affects the shape of the weld. Straight
metals would erode too rapidly at the high arc
polarity produces a narrow, deep weld, whereas reverse
temperatures involved.
polarity with its larger diameter electrode and lower
GTA welds are stronger, more ductile, and more
current forms a wide and shallow weld. Therefore, dc
corrosion-resistant than other types of arc welds. The
straight polarity is used for welding most metals
weld zone has 100-percent protection from the
because better welds are achieved. With the heat
atmosphere; therefore, no flux is required. Since no
concentrated at the plate, the welding process is more
flux is required, it eliminates flux or slag inclusions in
rapid, and there is less distortion of the base metal.
the weld, and there are no sparks, fumes, or spatter.
With GTA welding, the welding heat, amount of
penetration, and bead shape can be very accurately
controlled, and the bead surface is smooth and uniform.
Welding Machines
Any standard dc or ac welding machine can be used
to supply the current for gas tungsten-arc welding.
However, it is important that the generator or
transformer have good current control in the low range.
This is necessary to maintain a stable arc, especially
when welding thin gauge materials. Specially designed
machines with all of the necessary controls are
available for gas tungsten-arc welding. Many of the
power supply units are made to produce both ac and dc
current.
The choice of an ac or dc machine depends on what
weld characteristics may be required. Some metals are
joined more easily with ac current, while others get
Figure 6-43.--Straight and reverse polarity in electric
better results when dc current is used.
welding.
6-29
