GTAW
Gas–tungsten arc welding (GTAW) is
a process that melts and joins metals by heating them with an arc established
between a non-consumable tungsten electrode and the metals. 
Fig 1
The torch holding the tungsten electrode
is connected to a shielding gas cylinder as well as one terminal of the power source.
The tungsten electrode is usually in contact with a water-cooled copper tube,
called the contact tube , which is connected to the welding cable (cable 1)
from the terminal.
Fig 2.
This allows both the welding current
from the power source to enter the electrode and the electrode to be cooled to
prevent overheating. The workpiece is connected to the other terminal of the
power source through a different cable (cable 2). The shielding gas goes through
the torch body and is directed by a nozzle toward the weld pool to protect it
from the air. Protection from the air is much better in GTAW than in SMAW
because an inert gas such as argon or helium is usually used as the shielding
gas and because the shielding gas is directed toward the weld pool. For this
reason, GTAW is also called tungsten–inert gas (TIG) welding. However, in
special occasions a non-inert gas (Chapter 3) can be added in a small quantity
to the shielding gas. Therefore, GTAW seems a more appropriate name for this
welding process. When a filler rod is needed, for instance, for joining thicker
materials, it can be fed either manually or automatically into the arc.
Polarity :
Direct-Current Electrode Negative
(DCEN) also called the straight polarity,is
the most common polarity in GTAW. The electrode is connected to the negative terminal of the power supply. Electrons are emitted from the tungsten electrode and
accelerated while traveling through the
arc. A significant amount of energy, called the work function, is required for an electron to be emitted from the
electrode. When the electron enters the
workpiece, an amount of energy equivalent to the work function is released .This is why in GTAW with DCEN more power
(about two-thirds) is located at
the work end of the arc and less (about one-third) at the electrode end. Consequently, a relatively narrow and deep weld
is produced.
Direct-Current Electrode
Positive(DCEP) This is also called the reverse polarity.The electrode is
connected to the positive terminal of the power source. The heating effect of
electrons is now at the tungsten electrode rather than at the workpiece.
Consequently,a shallow weld is produced. Furthermore, a large-diameter, water-cooled
electrodes must be used in order to prevent the electrode tip from melting. The
positive ions of the shielding gas bombard the workpiece, knocking off oxide
films and producing a clean weld surface. Therefore, DCEP can be used for
welding thin sheets of strong oxide-forming materials such as aluminum and
magnesium, where deep penetration is not required.
Alternating Current (AC) Reasonably
good penetration and oxide cleaning action can both be obtained. This is often used
for welding aluminum alloys.
Electrodes
Tungsten electrodes with 2% cerium or
thorium have better electron emissivity, current-carrying capacity, and resistance
to contamination than pure tungsten electrodes. As a result, arc starting is
easier and the arc is more stable. The electron emissivity refers to the
ability of the electrode tip to emit electrons. A lower electron emissivity
implies a higher electrode tip temperature required to emit electrons and hence
a greater risk of melting the tip.
Shielding
Gases
Both argon and helium can be used.
. The ionization potentials for argon and helium are 15.7 and 24.5eV (electron
volts), respectively. Since it is easier to ionize argon than helium, arc
initiation is easier and the voltage drop across the arc is lower with argon.
Also, since argon is heavier than helium, it offers more effective shielding
and greater resistance to cross draft than helium. With DCEP or AC, argon also
has a greater oxide cleaning action than helium. These advantages plus the
lower cost of argon make it more attractive for GTAW than helium. Because of
the greater voltage drop across a helium arc than an argon arc, however, higher
power inputs and greater sensitivity to variations in the arc length can be
obtained with helium. The former allows the welding of thicker sections and the
use of higher welding speeds. The latter, on the other hand, allows a better
control of the arc length during automatic GTAW.
Advantages
and Disadvantages :
Gas–tungsten arc welding is
suitable for joining thin sections because of its limited heat inputs. The
feeding rate of the filler metal is somewhat independent of the welding current,
thus allowing a variation in the relative amount of the fusion of the base
metal and the fusion of the filler metal. Therefore, the control of dilution and
energy input to the weld can be achieved without changing the size of the weld.
It can also be used to weld butt joints of thin sheets by fusion alone, that
is, without the addition of filler metals or autogenous welding. Since the GTAW
process is a very clean welding process, it can be used to weld reactive
metals, such as titanium and zirconium, aluminum, and magnesium. However, the
deposition rate in GTAW is low. Excessive welding currents can cause melting of
the tungsten electrode and results in brittle tungsten inclusions in the weld
metal. However, by using preheated filler metals, the deposition rate can be
improved. In the hot-wire GTAW process, the wire is fed into and in contact
with the weld pool so that resistance heating can be obtained by passing an
electric current through the wire
