Advanced Welding Processes: Technologies and Process Control

140
Advanced welding processes
Secondary controls
Orifice diameter. Decreasing the orifice size increases the arc force and
voltage. For keyhole welding small, 2–3 mm diameter, orifices are normally
used.
Electrode geometry. The electrode geometry and its position within the
torch are critical due to their effect on gas flow within the torch. It is suggested
[149] that tolerances of 0.1 to 0.2 mm on electrode position are required.
Concentricity of the electrode is also important, since any misalignment may
result in asymmetrical arc behaviour and poor weld bead appearance; the
electrode should either be adjustable or fixed by a ceramic insert inside the
torch.
Multiport nozzles. The multiport nozzle may be used to enhance constriction
and produce an elliptical arc profile which is elongated along the axis of the
weld. Recent work [150] has shown that a second concentric nozzle may
also be used to provide an increase in arc pressure, although excessive focusing
gas flow rates may reduce the thermal efficiency of the process.
Shielding gases. The most common shielding and plasma gas is argon.
From 1 to 5% hydrogen may be added to the argon shielding gas for welding
low-carbon and austenitic stainless steels. 3 The effect of these small additions
of hydrogen is quite significant, giving improved weld bead cleanliness,
higher travel speeds and improved constriction of the arc. Helium may be
used as a shielding medium for high-conductivity materials such as copper
and aluminium. It will tend to increase the total heat input although it may
reduce the effect of the constriction and produce a more diffuse heat source.
With 30% helium/70% argon shielding gas mixtures, keyhole welding speeds
66% higher than those achieved with argon shielding have been reported for
aluminium. [151] The shielding gas flow is not normally critical although
sufficient gas should be provided to produce effective shielding of the weld
area.
Arc characteristics
Constriction of any arc causes an increase in arc voltage. In the plasma
process, increasing the plasma gas flow, decreasing the diameter of the
constriction, increasing current and adding hydrogen or helium to the gas 4
3 Hydrogen additions must be avoided if there is any likelihood of cracking, embrittlement
or porosity. Hydrogen should not be used on high alloy steels, titanium or aluminium
alloys.
4 Even if hydrogen or helium are only added to the shielding gas there is evidence to show
that they will diffuse into the plasma stream and have a marked influence on the process
characteristics.