The second arc function is to break up and remove
for various ranges of welding current are shown in
the surface oxides from the aluminum. This is
table 10-7.
called the "cleaning action," and takes place during
that part of the ac cycle when the electrode is
We will not describe in detail the advantages
positive. The cleaning action is either a result of
and disadvantages of the various types of electrodes
the electrons leaving the base plate or the gas ions
made of pure tungsten, thoriated tungsten, or
striking the surface or a combination of both.
tungsten-zirconium alloy. Many welders prefer pure
tungsten for GTA welding with ac. Thoriated
Shielding Gas
tungsten is preferred for automatic GTA welding
using dc straight polarity current. A note of interest
Initially the arc breaks up the oxide on the area
here is that tungsten electrodes are usually
where it is directed. The gas shields the arc and
color-coded on one end. A medium green indicates
weld pool, preventing oxidation from reoccurring.
that the rod is pure tungsten. A yellow color
The gas also shields and prevents oxidation of the
indicates a 1 percent thoriated tungsten rod. A light
hot tip of the tungsten electrode; and because of
red color indicates a 2 percent thoriated tungsten
this, the flow of gas should not be stopped until the
rod. A tan color indicates that the rod is zirtung
tungsten electrode tip has cooled. Shutoff can be
(tungsten zirconium).
either manual or automatic; the latter is preferred.
Another function of the gas shield is to provide a
The gas cup or nozzle of the torch can be either
more easily ionized path, thus aiding smooth
ceramic or metal. Ceramic nozzles are generally
transfer of current. Either argon or helium can be
unsatisfactory for welding at high-current levels
used for shielding the arc in the GTA process.
because the nozzle may melt at the tip and partially
Helium requires a higher gas flow, but gives greater
close the orifice. On the other hand, metal nozzles
penetration and faster welding speeds than argon.
of too small diameter will short out the
This deeper penetration is obtained because the arc
high-frequency current if the work is touched by the
nozzle. Torch manufacturers usually recommend
in the helium atmosphere is hotter than in the argon
atmosphere. Argon is preferred by most welders
the type and size of nozzle for different current
because the cleaning action is greater and the arc
ranges. Generally, the nozzle diameter should be
more stable. The flow of gas necessary for good
equal to or slightly greater than the molten weld
GTA welding depends on the welding current, size
pool.
of nozzle, joint design, speed of welding, and
freedom from draft in the area where the welding is
Dual Action of the AC Arc
being done. This last factor can affect gas coverage
considerably. Recommended gas flows are shown in
The first function of the ac arc is to provide the
table 10-8.
heat necessary to melt the base and filler metals.
Table 10-7.--Recommended Current Ranges for Thoriated and Nonthoriated Tungsten Electrodes
Current, Amperes
Electrode Diameter Inches
Standard Tungsten Electrodes
Thoriated Tungsten Electrodes
15-80
0.04
10-60
40-120
60-150
l/16
140-250
3/32
100-160
150-210
225-350
1/8
300-450
190-275
5/32
250-350
3/16
400-550
300-490
500-800
l/4
450-600
5/16
10-28
