Advanced Welding Processes: Technologies and Process Control

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Shielding gas
Advanced welding processes
The shielding gas can have a significant effect on the thermal characteristics
of the arc and fusion behaviour of GTAW as discussed in Chapter 5. These
effects can be used to extend the operating range of the process and, for
example, the maximum speed before the onset of weld bead humping at 400
A may be increased from 7.6 mm s –1 to 23.3 mm s –1 by replacing argon with
helium as the shielding medium.
Electrode vertex angle
The angle at which the tungsten electrode is ground has a marked effect on
the arc pressure [56] (Fig. 6.16). Small angles increase the arc pressure and
arc voltage and large included angles reduce the pressure. For the avoidance
of undercut and humping at high currents, it is therefore preferable to use a
large vertex angle. The vertex angle also has an influence on weld bead
geometry at much lower currents, the effect is related to relative plate thickness 3
and is summarized in Table 6.3. For a high depth-to-width ratio on thin plate,
a small vertex angle is desirable whereas for high depth-to-width welds on
thick plate, or low penetration welds, a larger angle is required. These effects
are, however, also related to material composition, shielding gas and joint
Pressure (mbar)
25
20
15
10
5
0
100 150
Current (A)
200
a
b
75He25Ar: 60∞
Argon: 60∞
c
d
Argon: 120∞
Argon: 30∞
6.16 Effect of vertex angle on arc pressure. [56]
3 Relative plate thickness is the thickness of the plate relative to the penetration achieved:
‘thick’ plate in this context means a penetration of less than 30%, ‘medium’ is 30 to 70%
penetration and ‘thin’ plate represents the 70 to 100% penetration situation.
a
b
c
d