| 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 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 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. |
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