Advanced Welding Processes: Technologies and Process Control

Gases for advanced welding processes 61
Table 5.2 Compatibility of shielding gases with common materials
Shielding gas Compatible Problem area
Argon and helium All materials None
Oxygen-containing Plain carbon and stainless Embrittlement of reactive metals
mixtures steels up to around 8% (e.g. Ti), oxidation, poor weld
profile and loss of alloying
elements in some materials
Carbon dioxide Plain carbon and alloy Carbon pick-up in extra low
steel carbon stainless steels
Nitrogen Copper Porosity in ferritic steel and
nickel, embrittlement of reactive
metals, reduced toughness in
alloy steels
Hydrogen Austenitic stainless steel Porosity in aluminium and other
and high nickel alloys up materials. HICC in hardenable
to around 5% ferritic steels
into the arc atmosphere is far more serious than the presence of nitrogen as
a minor impurity in the gas.
The ability to maintain a lamellar gas flow and prevent atmospheric
contamination will depend on the physical properties of the gas and in particular
its density, viscosity and Reynolds number.
The compatibility of the common shielding gases with a range of materials
is summarized in Table 5.2.
5.2.3 Secondary functions of the gas
The secondary characteristics of shielding gases are no less important than
the primary functions and, in some cases, may determine the most suitable
gas for a given application. Some of the most important secondary functions
are control of fusion characteristics and joint properties.
Fusion characteristics
The shielding gas has a significant influence on the weld bead profile and
fusion characteristics (Fig. 5.3). The total fused area is increased (at an
equivalent current) by using gases which increase the arc energy (e.g. He,
H2, CO2). In GMAW welding, the use of pure argon produces a pronounced ‘finger’
or ‘wine-glass’ penetration profile, whereas argon/CO2 and argon/helium
mixtures produce a more rounded profile.
The profile of the reinforcement can also be improved; for example, argon/
CO2 mixtures normally give flatter weld beads and consequent improvements