Tungsten Inert Gas
Welding (TIG) is one of the two types of gas shielded arc welding process as
the other one is Metal Inert Gas Welding (MIG). Also in dual-shield mode of Flux Core Arc Welding (FCAW), an additional gas supply is provided along with
self-shielding of core flux, due to which FCAW is also listed in gas shielded
arc processes. Working principle of TIG welding is based on the production of
arc using high melting and non-consumable tungsten electrode that is why this
process is referred to as (TAGS) tungsten arc gas-shielded which is also known
by other names; argon arc welding, tungsten inert gas and gas tungsten arc
welding. Gas Shielding is used to protect weld puddle from atmospheric
contamination. These gases are transparent so the weld puddle is made visible
to a welder. Some distinct features of TIG process are
- In TIG no flux is used and there is no fear of corrosion due to flux entrapment.
- No slagging that contributes to elimination of post weld cleaning.
- Since there is no spark and fumes, and that is why sound weld with perfect definition is obtained.
A non-consumable tungsten electrode
is used to produce an arc of considerable high temperature that assists to melt
work metal. Air is driven out due to the envelope of inert gas that prevents
the weldment, tungsten electrode and heat affected zone from oxidation.
Contrary to MIG, tungsten electrode is not consumed rather it provides arc to
melt the additional filler metal. Likewise oxyacetylene welding, filler metal
is fed to the weld puddle.
Normally there are two modes in TIG;
semi-automatic or automatic. In semi-automatic, operator has to configure
current and gas flow settings and then manually deals with the torch and filler
road. However in automatic the operator sets the travel speed, length of arc,
gas flow rate, filler rod position and then observes the operation and controls
whereby it is needed.
In early times TIG was first
introduced to weld magnesium to minimize rapid corrosion problems. The
technique was successfully applied to achieve best results with no resulting
corrosion. This process was introduced with distinct features as compared to
other arc processes. For shielding, gas flow rate can be controlled by flow rate and manifold. During welding, gas flow not only shields the weld and filler metal but also protects the torch. There is also a gas delay feature that permits gas to flow for
preset time, after the welding current has been shut off.
TIG is applied in
both welding of ferrous and non-ferrous metals with wide range of thickness not
above 8mm. Both alternating (AC) and direct (DC) current supplies are used with
current of range 15 to 350 amps. With the use of small diameter electrodes and
suitable current range, the process is applied in both repair work and new
manufacturing. Although it is comparatively slower than other arc processes but
it produce high quality welds with perfect definition in aluminum, carbon steel
and stainless steel family. DC source is used for welding of stainless steel,
carbon steel, nickel and copper alloys. On the other hand AC is suitable for
the welding of Aluminum and its alloys.
Equipment
A. TIG Torch
Different size of torches are available
for different current carrying capacities. Its assembly includes a switch for
different welding operations. The torch casing holds gas nozzles and
electrode.
B. Additional Features
1. Water cooling
system
Welding with high amperages means
cooling is required. A water cooling system is installed in which water is
circulated through the torch to keep it cool. Older systems were connected with
additional cooling system but now a days it is installed inside the modified
equipment. The water flow is controlled either by foot pedal or torch.
2. DC suppression
In Aluminum and magnesium alloys
welding, formation of surface oxide is major concern. In Aluminum welding as
the arc is formed between the tungsten and aluminum (dissimilar metals) chances
are that AC is converted to DC mode. So a DC suppressor is used to prevent the
conversion of AC to DC.
3. Contactor
This feature puts the arc to be
extinguished when delay shielding is required for cooling purposes. In this way
it not only protects weld but also provides means of safety to the welder. This
process is also controlled by torch and foot pedal.
4. Arc Initiation
Arc initiation by touching the
electrode with the base metal can cause serious problems of contamination and may
lead to arc un-stability. Obviously tungsten inclusion can cause localized hard spots. On the other hand the
transfer of base metal to tungsten electrode can also cause contamination and
consequently un-stabilize the arc. For the electrode not to touch with the work
piece, high frequency spark is produced that causes some sort of ionization in
spark gap. The welding current continues to flow due to this ionized spark gap.
This feature also contributes to enhance the life span of tungsten electrode. Listed below are the arc
initiation methods that are normally being used in modern equipment.
a. Scratch start
In this method the electrode is
brought down to the work piece and gently scratched to initiate the arc. As it is
stated above that the slight contact can cause the problem of contamination.
This method is applied only in those applications where contamination is less
and no serious problems of weld are generated.
b. Lift up
In this method the electrode is
brought down to make contact with work at the initial point of welding. Torch
switch is pressed that creates contact and still no current is produced. When
the electrode is lifted up making a gap, the current continues to flow and
rapidly increases to the pre-set value. This rapid increase in current is referred
to as popular term slope up.
c. Slope up
Slope up is the rapid rise of welding
current to preset current on the power source. By the use of switch the welder
can set the time required for the rise of current to the selected amperages.
This feature offers less contamination and reduces the danger of burning thin
sheets at the initial point of welding.
d. Slope down
Slope down is the gradual decrease
of welding current to zero. It is referred to as the time taken to decrease the
current until the arc is extinguished. Well this feature is used to fill the
crater (weld imperfection) at the end of the weld. Normally this function is
operated with the help of torch switch and foot pedal.
C. Electrode used in TIG
Tungsten is high melting point metal
which is about 3000 degree Celsius. In TIG process pure tungsten with its best
thermal and electrical conductivity is used. Now a days alloyed tungsten with
1-2% thorium and zirconium is applied for better arc stability and improved
electrode life. Two types of alloyed tungsten electrodes are; thorated
electrode for DC mode and zirconated for AC mode. It is also noticeable that
electrode diameter depends on current polarity.
Electrode Grinding
Electrode grinding is one of the major
concern in achieving consistent performance in welds. Electrodes are ground
according to the application (joint). There are two ways for electrode
grinding; machine grinding and manual grinding. Machine grinding results in consistent
grinding. However care must be take while manual grinding.
Electrode shape
1. Electrode is
usually ground to required vertex angle. For DC usage the electrode is ground
to the length of 1-3 times the diameter of electrode.
Point
length of electrode = 1-3 x Diameter of electrode.
In this way the lower
the current the longer will be the point length. Electrodes are available
normally within the diameter range of 0.8, 1.6, 3.2, 4.0 and 6.4 mm.
2. For AC usage the
side of electrode end is ground to 45 degree, remaining the front side to be
flat. This gives the spherical shape as shown in the figure.
Electrode
installation in torch
Well this one is
simple. The electrode is installed in torch and held tightly with the help of
grippers knows as sockets.
They can grip and hold the electrodes of various diameters.
D. Gas Nozzles
These nozzles are of high temperature
ceramic materials to bear high welding temperature. As it is stated above some nozzles
are equipped with water cool system. Only the right choice of nozzle size gives
better protection of weldment, weld pool and filler material. Various shapes of
nozzles are available for different kinds of jobs. Small size nozzles are
available for confined areas. Long shapes are available for deep groove joints.
E. Gas lens
Gas lens is provided to prevent the turbulence
of gas flow. Poor gas flow may not give good shielding service hence gas lens
is used to provide consistent and smooth gas flow to shield the weld puddle.
With the help of gas lens, electrode extension can also be achieved for access
of complicated areas.