Also Known As Heliarc Welding. Gas tungsten arc welding (TIG welding, heliarc welding, or GTAW) is a
process in which the joining of metals is produced by heating therewith an arc
between a tungsten (nonconsumable) electrode and the work. A shielding gas is
used, normally argon. TIG welding is normally done with a pure tungsten or
tungsten alloy rod, but multiple electrodes are sometimes used. The heated weld
zone, molten metal, and tungsten electrode are shielded from the atmosphere by a
covering of inert gas fed through the electrode holder. Filler metal may or may
not be added. A weld is made by applying the arc so that the touching workpiece
and filler metal are melted and joined as the weld metal solidifies. This
process is similar to other arc welding processes in that the heat is generated
by an arc between a nonconsumable electrode and the workpiece, but the equipment
and electrode type distinguish TIG from other arc welding processes. See figure 10-32.
b. Equipment. The basic features of the equipment used in TIG welding
are shown in figure 10-33.
The major
components required for TIG welding are:
(1) the welding machine, or power source
(2) the welding electrode holder and the tungsten electrode
(3) the shielding gas supply and controls
(4) Several optional accessories are available, which include a foot rheostat
to control the current while welding, water circulating systems to cool the
electrode holders, and arc timers.
NOTE
There are ac and dc power units with built-in high frequency
generators designed specifically for TIG welding. These automatically control
gas and water flow when welding begins and ends. If the electrode holder
(torch) is water-cooled, a supply of cooling water is necessary. Electrode
holders are made so that electrodes and gas nozzles can readily be changed.
Mechanized TIG welding equipment may include devices for checking and
adjusting the welding torch level, equipment for work handling, provisions for
initiating the arc and controlling gas and water flow, and filler metal feed
mechanisms.
c. Advantages. Gas tungsten arc welding is the most popular method for
welding aluminum stainless steels, and nickel-base alloys. It produces top
quality welds in almost all metals and alloys used by industry. The process
provides more precise control of the weld than any other arc welding process,
because the arc heat and filler metal are independently controlled. Visibility
is excellent because no smoke or fumes are produced during welding, and there is
no slag or spatter that must be cleaned between passes or on a completed weld.
TIG welding also has reduced distortion in the weld joint because of the
concentrated heat source. The gas tungsten arc welding process is very good for
joining thin base metals because of excellent control of heat input. As in
oxyacetylene welding, the heat source and the addition of filler metal can be
separately controlled. Because the electrode is nonconsumable, the process can
be used to weld by fusion alone without the addition of filler metal. It can be
used on almost all metals, but it is generally not used for the very low melting
metals such as solders, or lead, tin, or zinc alloys. It is especially useful
for joining aluminum and magnesium which form refractory oxides, and also for
the reactive metals like titanium and zirconium, which dissolve oxygen and
nitrogen and become embrittled if exposed to air while melting. In very critical
service applications or for very expensive metals or parts, the materials should
be carefully cleaned of surface dirt, grease, and oxides before welding.
d. Disadvantages. TIG welding is expensive because the arc travel
speed and weld metal deposition rates are lower than with some other methods.
Some limitations of the gas tungsten arc process are:
(1) The process is slower than consumable electrode arc welding processes.
(2) Transfer of molten tungsten from the electrode to the weld causes
contamination. The resulting tungsten inclusion is hard and brittle.
(3) Exposure of the hot filler rod to air using improper welding techniques
causes weld metal contamination.
(4) Inert gases for shielding and tungsten electrode costs add to the total
cost of welding compared to other processes. Argon and helium used for shielding
the arc are relatively expensive.
(5) Equipment costs are greater than that for other processes, such as
shielded metal arc welding, which require less precise controls.
For these reasons, the gas tungsten arc welding process is generally not
commercially competitive with other processes for welding the heavier gauges of
metal if they can be readily welded by the shielded metal arc, submerged arc, or
gas metal arc welding processes with adequate quality.
e. Process Principles.
(1) Before welding begins, all oil, grease, paint, rust, dirt, and other
contaminants must be removed from the welded areas. This may be accomplished by
mechanical means or by the use of vapor or liquid cleaners.
(2) Striking the arc may be done by any of the following methods:
(a) Touching the electrode to the work momentarily and quickly withdrawing
it.
(b) Using an apparatus that will cause a spark to jump from
the electrode to the work.
(c) Using an apparatus that initiates and maintains a small pilot arc,
providing an ionized path for the main arc.
(3) High frequency arc stabilizers are required when alternating current is
used. They provide the type of arc starting described in (2)(b) above. High
frequency arc initiation occurs when a high frequency, high voltage signal is
superimposed on the welding circuit. High voltage (low current) ionizes the
shielding gas between the electrode and the workpiece, which makes the gas
conductive and initiates the arc. Inert gases are not conductive until ionized.
For dc welding, the high frequency voltage is cut off after arc initiation.
However, with ac welding, it usually remains on during welding, especially when
welding aluminum.
(4) When welding manually, once the arc is started, the torch is held at a
travel angle of about 15 degrees. For mechanized welding, the electrode holder
is positioned vertically to the surface.
(5) To start manual welding, the arc is moved in a small circle until a pool
of molten metal forms. The establishment and maintenance of a suitable weld pool
is important and welding must not proceed ahead of the puddle. Once adequate
fusion is obtained, a weld is made by gradually moving the electrode along the
parts to be welded to melt the adjoining surfaces. Solidification of the molten
metal follows progression of the arc along the joint, and completes the welding
cycle.
(6) The welding rod and torch must be moved progressively and smoothly so the
weld pool, hot welding rod end, and hot solidified weld are not exposed to air
that will contaminate the weld metal area or heat-affected zone. A large
shielding gas cover will prevent exposure to air. Shielding gas is normally
argon.
(7) The welding rod is held at an angle of about 15 degrees to the work
surface and slowly fed into the molten pool. During welding, the hot end of the
welding rod must not be removed from the inert gas shield. A second method is to
press the welding rod against the work, in line with the weld, and melt the rod
along with the joint edges. This method is used often in multiple pass welding
of V-groove joints. A third method, used frequently in weld surfacing and in
making large welds, is to feed filler metal continuously into the molten weld
pool by oscillating the welding rod and arc from side to side. The welding rod
moves in one direction while the arc moves in the opposite direction, but the
welding rod is at all times near the arc and feeding into the molten pool. When
filler metal is required in automatic welding, the welding rod (wire) is fed
mechanically through a guide into the molten weld pool.
(8) The selection of welding position is determined by the mobility of the
weldment, the availability of tooling and fixtures, and the welding cost. The
minimum time, and therefore cost, for producing a weld is usually achieved in
the flat position. Maximum joint penetration and deposition rate are obtained in
this position, because a large volume of molten metal can be supported. Also, an
acceptably shaped reinforcement is easily obtained in this position.
(9) Good penetration can be achieved in the vertical-up position, but the
rate of welding is slower because of the effect of gravity on the molten weld
metal. Penetration in vertical-down welding is poor. The molten weld metal
droops, and lack of fusion occurs unless high welding speeds are used to deposit
thin layers of weld metal. The welding torch is usually pointed forward at an
angle of about 75 degrees from the weld surface in the vertical-up and flat
positions. Too great an angle causes aspiration of air into the shielding gas
and consequent oxidation of the molten weld metal.
(10) Joints that may be welded by this process include all the standard
types, such as square-groove and V-groove joints, T-joints, and lap joints. As a
rule, it is not necessary to bevel the edges of base metal that is 1/8 in. (3.2
mm) or less in thickness. Thicker base metal is usually beveled and filler metal
is always added.
(11) The gas tungsten arc welding process can be used for continuous welds,
intermittent welds, or for spot welds. It can be done manually or automatically
by machine.
(12) The major operating variables summarized briefly are:
(a) Welding current, voltage, and power source characteristics.
(b) Electrode composition, current carrying capacity, and shape.
(c) Shielding gas--welding grade argon, helium, or mixtures of both.
(d) Filler metals that are generally similar to the metal being joined and
suitable for the intended service.
(13) Welding is stopped by shutting off the current with
foot-or-hand-controlled switches that permit the welder to start, adjust, and
stop the welding current. They also allow the welder to control the welding
current to obtain good fusion and penetration. Welding may also be stopped by
withdrawing the electrode from the current quickly, but this can disturb the gas
shielding and expose the tungsten and weld pool to oxidation.
f. Filler Metals. The base metal thickness and joint design determine
whether or not filler metal needs to be added to the joints. When filler metal
is added during manual welding, it is applied by manually feeding the welding
rod into the pool of molten metal ahead of the arc, but to one side of the
center line. The technique for manual TIG welding is shown in figure 10-34.
Tig welding is commonly used for root welds on carbon steel that will have
further NDT/NDE such as x-ray or ultrasonic testing performed after welding.
After completing the root weld, the remaining weld joint is usually welded with SMAW
(stick) welding. Tig welding is also used extensively on stainless steel. On stainless,
the entire weld is usually welded with tig to ensure quality throughout the complete weld
joint.