Welding

1. GAS TUNGSTEN ARC welding (GTAW)
Khalid M. Hafez
JWRI, Osaka University
GAS TUNGSTEN ARC welding (GTAW) uses a nonconsumable tungsten electrode
which must be shielded with an inert gas. The arc is initiated between the tip of the
electrode and work to melt the metal being welded, as well as the filler metal, when
used. A gas shield protects the electrode and the molten weld pool, and provides the
required arc characteristics.
The process may employ direct current with positive or negative electrode or
alternating current. In general, ac is preferred for welding aluminum and magnesium.
Direct current electrode negative is preferred for welding most other materials and for
automatic welding of thick aluminum. Thin magnesium sometimes is welded with
direct current electrode positive.
When ac is used with argon shielding, an arc cleaning action is produced at the joint
surfaces on aluminum and magnesium. This cleaning action removes oxides and is
particularly beneficial in reducing weld porosity when welding aluminum. When
using dc, helium may be used as the shielding gas to produce deeper penetration.
However, stringent precleaning of aluminum and magnesium parts is required with
helium shielding. Argon and helium mixtures for gas shielding can provide some of
the benefits of both gases.
Regardless of polarity, a constant current (essentially vertical volt-ampere
characteristic) welding power source is required. In addition, a high-frequency
oscillator is generally incorporated in power sources designed for GTAW. High-
frequency can be employed with dc to initiate the arc instead of touch starting to
minimize tungsten electrode contamination. Normally, the high frequency is turned
off automatically after arc ignition. The high frequency power is normally operated
continuously with ac to maintain ionization of the arc path as the arc voltage passes
through zero.
Some special power sources provide pulsating direct current with variable frequency.
This provision permits better control of the molten weld pool when welding thin
sections, as well as when welding in positions other than flat.
Several types of tungsten electrodes are used with this process. Thoriated and
zirconiated electrodes have better electron emission characteristics than pure tungsten,

2. making them more suitable for dc operations. The electrode is normally ground to a
point or truncated cone configuration to minimize arc wander. Pure tungsten has
poorer electron emission characteristics but provides better current balance with ac
welding. This is advantageous when welding aluminum and magnesium.
The equipment needed consists of a welding torch, a welding power source, a source
of inert gas with suitable pressure regulators and flowmeters, a welding face shield,
and protective clothing. Electric power requirements depend upon the type of material
and the thicknesses to be welded. Power requirements range from 8 kw for a 200 A
unit to 30 kw for a 500 A unit. Portable engine- driven power sources are also
available.
Gas tungsten arc welding requires more training time, manual dexterity, and welder
coordination than does SMAW or GMAW. The equipment is portable, and is
applicable to most metals in a wide range of thickness and in all welding positions.
Sound arc welds can be produced with the GTAW process when proper procedures
are used.
The process can be used to weld all types of joint geometries and overlays in plate,
sheet, pipe, tubing, and other sturctural shapes. It is particularly appropriate for
welding sections less than 3/8-in. (10mm) thick and also 1-to 6-in. (25.4-to 152.4-
mm) diameter pipe. Thicker sections can be welded but economics generally indicate
the choice of a consumable electrode process.
This combination of GTAW for root pass welding with either SMAW or GMAW is
particularly advantageous for welding pipe. The gas tungsten arc provides a smooth,
uniform root pass while the fill and cap passes are made with a more economical
process.
Gas tungsten arc welding is generally more expensive than SMAW due to the cost of
the inert gas, and is only 10 to 20 percent as fast as GMAW. However, GTAW will
provide the highest quality root pass, while accommodating a wider range of
thicknesses, positions, and geometries than either SMAW or GMAW

3. TIG Welding Benefits
• Superior quality welds
• Welds can be made with or without filler metal
• Precise control of welding variables (heat)
• Free of spatter
• Low distortion
Shielding Gases
Argon
Argon, an inert gas, is the most widely used (in its pure form) as a shielding
gas for Gas Tungsten Arc Welding (GTAW). Its mild thermal conductivity
produces a narrow, constricted arc column which allows greater variations in
arc length with minimal influence on arc power and weld bead shape. This
characteristic makes it the preferred choice for manual welding. In addition,
argon provides good arc starting due to its low ionization potential. This
property allows argon to carry electric current well when compared to other
shielding gases.
For AC welding applications, argon is preferred over helium because of its
superior cleaning action, arc stability, and weld appearance. When welding
thicker aluminum alloys (> 1/4"), argon is mixed with helium to enhance the
thermal conductivity of the shielding gas.
While pure argon may be used for mechanized applications, depending on the
base material, thickness and composition, argon-helium or argon-hydrogen
blends promote higher welding travel speeds. The hotter arc characteristics of
argon-helium blends also make them more suitable for welding metals with
high thermal conductivity, such as copper.
Helium
Helium, also an inert gas, has high thermal conductivity and high ionization
potential, which produces higher arc voltages when compared to argon for a
given current setting and arc length. This produces a "hotter" arc. The
increased heat input affects depth of penetration and its wider, less constricted
arc column increases weld bead width.
The use of helium is generally favored over argon at the higher current levels
which are used for the welding of the thicker materials, especially those
having high thermal conductivity or relatively high melting temperatures. It is
often used for high-speed mechanized applications.
Although argon is widely used for AC welding of aluminum, pure helium has
been successfully used for DCEN mechanized welding of this material. It
produces greater penetration at higher travel speeds. However, surface oxides
must be cleaned from the weld joint to obtain acceptable results, since the
cleaning action of the AC arc is not present. Argon-helium mixtures are
widely used with AC current when welding with aluminum alloys.
The physical properties of helium definitely offer advantages in some
applications. However, due to it high ionization potential, it also produces a
less stable arc and a less desirable arc starting characteristic than argon. Its

4. higher cost and higher flow rates are also factors to be considered. In some
cases, an argon mixture is used for igniting the arc and pure helium is used for
welding. This technique is used for DC GTAW welding of heavy aluminum.
Argon-Helium Mixtures -- Praxair's HeliStar® Blends
Each of these gases (argon and helium), as explained above, has specific
advantages. Praxair's HeliStar blends (argon-helium blends) are basically used
to increase the heat input to the base metal while maintaining the favorable
characteristics of argon, such as arc stability and superior arc starting.
HeliStar A-75 Gas Blend
This blend is sometimes used for DC welding when it is desirable to obtain
higher heat input while maintaining the good arc starting behavior of argon.
HeliStar A-50 Gas Blend
This blend is used primarily for high-speed mechanized and manual welding
of nonferrous material (aluminum and copper) under 3/4 inch thick.
HeliStar A-25 Gas Blend
The speed and quality of AC welding on aluminum can be improved with this
blend. It is sometimes used for manual welding of aluminum pipe and
mechanized welding of butt joints in aluminum sheet and plate. The HeliStar
A-25 gas blend is also used for many of the GTAW hot wire applications to
increase the energy input while accommodating the high filler metal
deposition rates of the process.
Argon-Hydrogen Mixtures -- Praxair's HydroStar® Gas Blends
Hydrogen is often added to argon to enhance the thermal properties of argon.
Its reducing effect improves weld surface color match with 300 series stainless
alloys due to reduced surface oxidation.
The higher arc voltage associated with hydrogen increases the difficulty of
starting the arc. For this reason, the lowest amount of hydrogen consistent with
the desired result is recommended. Additions up to 5% for manual welding
and up to 10% for mechanized welding are typical.
Argon-hydrogen blends are primarily used on austenitic stainless steel (300
series), nickel, and nickel alloys. Hydrogen enhanced mixtures are not
recommended to weld carbon or low-alloy steel, or any of the copper,
aluminum, or titanium alloys since cracking or porosity will occur due to the
absorption of hydrogen.
Argon-hydrogen blends utilized as a purge gas are successfully applied to
improve root appearance when TIG welding 300 series stainless pipe.
Warning
Special safety precautions are required when mixing argon and hydrogen.
Do NOT attempt to mix argon and hydrogen from separate cylinders.
Praxair's HydroStar is a hydrogen-enhanced argon-based blend which is
ideally suited for general purpose GTAW of most commercially available
carbon, low alloy, and stainless steels. It may be substituted for pure argon in

5. many applications.
HydroStar H-2 and H-5 Gas Blends
These blends are used for manual welding applications. The HydroStar H-5
blend is preferred on material thicknesses above 1/16 inch. These blends are
also suitable for use with GTAW when welding 300 series austenitic stainless
steels and as a back purge gas on stainless steel materials.
HydroStar H-10 Gas Blend
This blend is preferred for high-speed GTAW mechanized applications on
austenitic stainless steel.
HydroStar H-15 Gas Blend
This blend, which contains 15% hydrogen, is used most often for welding butt
joints in stainless steel at speeds comparable to helium, and 50 percent faster
than argon. The HydroStar H-15 blend is also used to increase the speed of
welding 300 series stainless steel. It can be used on all thicknesses of stainless
steel. Concentrations greater than 15% may cause weld metal porosity, with
multi-pass applications.
HydroStar H-35 Gas Blend
It is recommended as the plasma gas with plasma arc gauging, when cutting
aluminum and stainless steel and when cut quality and face appearance are
critical.
Note: Oxygen and carbon dioxide are chemically reactive and should not
be used with GTAW. Their oxidation potential can cause severe erosion
and degradation of the tungsten electrode at arc temperatures.
TIG Welding Limitations
• Requires greater welder dexterity than MIG or stick welding
• Lower deposition rates
• More costly for welding thick sections
Weld Discontinuities
• Undercutting
• Tungsten inclusions
• Porosity
• Weld metal cracks
• Heat affected zone cracks
TIG Welding Problems
• Erratic arc
• Excessive electrode consumption
• Oxidized weld deposit
• Arc wandering
• Porosity
• Difficult arc starting

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