Advanced Welding Processes: Technologies and Process Control

Surface tension
Surface tension
Advanced gas tungsten arc welding 97
Temperature
(a)
Temperature
(b)
6.19 The effect of surface tension gradients on weld pool flow.
or Lorentz forces, buoyancy (flow induced by density differences in the pool
due to the temperature gradient) and aerodynamic drag forces (due to the
flow of gas plasma jets over the pool surface) are all thought to have an
effect. At higher currents, the energy transfer and flow within the pool are
likely to be dominated by the effect of the arc plasma and depression of the
pool surface.
The effects of cast-to-cast variability can be reduced by using higher
currents, pulsed operation, by choice of shielding gas (e.g. argon/5% hydrogen
for austenitic stainless steel) or by adding a filler containing elements which
promote a positive surface tension–temperature gradient (e.g. sulphur in
austenitic stainless steel). It has also been reported that the problem may be
alleviated by coating the material with a surface-active paste, as in the A-
TIG process which has previously been described. In order to control the
problem, however, it is necessary to identify potentially problematic materials.
Several approaches may be adopted, i.e. chemical analysis, direct weldability
trials or indirect weldability trials.