2
Properties of Gases
The basic properties of shielding gases
that affect the performance of the welding process include:
1) Thermal properties
at elevated temperatures. 2) Chemical
reaction of the gas with the various elements in the base plate and welding wire.
3) Effect of each gas
on the mode of metal transfer. The
thermal conductivity of the gas at arc temperatures influences the arc voltage
as well as the thermal energy
delivered to the weld. As thermal conductivity increases, greater welding voltage
is necessary to sustain
the arc. For example, the thermal conductivity of helium and CO2
is much higher than that of argon; because
of this, they deliver more heat to the weld. Therefore, helium and CO2
require more welding voltage and
power to maintain a stable arc. The
compatibility of each gas with the wire and base metal determines the suitability
of the various gas combinations.
Carbon dioxide and most oxygen bearing shielding gases should not be used for
welding aluminum,
as aluminum oxide will form. However, CO2 and
02 are useful at times and even essential
when GMAW welding
steels. They promote arc stability and good fusion between the weld puddle and
base material.
Oxygen is a great deal more oxidizing tham CO2.
Consequently, oxygen additions to argon are generally
less than 12 percent by volume whereas 100 percent CO, can be used for GMAW mild
steels. Steel
wires must contain strong deoxidizing elements to supress porosity when used with
oxidizing gases, particularly
mixtures with high percentages of CO2 or 02
and especially 100 percent CO2.
Shielding gases also determine the
mode of metal transfer and the depth to which the workpiece is melted
(depth of penetration). Tables
(4-1 and 4-2) summarize recommended shielding
gases for various materials and
metal transfer types. Spray transfer is not obtained when the gas is rich in CO2.
For example, mixtures containing
more than about 20 percent CO2 do not exhibit
true spray transfer. Rather, mixtures up to 30 percent
CO2 can have a ”spray-like” shape
to the arc at high current level but are unable to maintain the arc
stability of lower CO2
mixtures. Spatter levels will also tend to increase when mixtures are rich in
CO2.
