Advanced Welding Processes: Technologies and Process Control

Advanced gas tungsten arc welding 95
Table 6.3 Penetration profile in relation to electrode vertex angle
Electrode vertex Bead profile (depth-to-width ratio) as a function
angle (degrees) of relative plate thickness
Thin plate Medium Thick plate
30 High Low Low
120 Low High High
geometry and, whilst it is clear that there is a significant change in penetration
characteristics with any change in electrode angle, the resultant weld profile
will depend on the material and the other welding variables discussed above.
Filler addition
The addition of cold filler wire will cool the weld pool and reduce the heat
available for plate fusion; however, in some circumstances, small traces of
elements that alter the surface tension of the weld pool may be added via the
filler to improve the fusion characteristics as discussed below.
Cast-to-cast variation in GTAW
Although control of the GTAW process is straightforward if the variables
listed above are considered, variable weldability has been experienced,
particularly in the fabrication of stainless steel at relatively low currents.
This phenomenon is referred to as ‘cast-to-cast’ variation since it commonly
occurs when a new batch of material of nominally identical composition is
used. The problem has received considerable attention and is believed to be
associated with the level of trace elements in the material and, in the case of
austenitic stainless steel, variation in levels of sulphur, calcium and oxygen
has been identified as having a major influence. For example, for two welds
shown in Fig. 6.17 made under identical welding conditions on low-carbon
austenitic stainless steel (304L), the only difference in the analysis of the
plate material was in the level of sulphur, which was 0.004% for (a) the low
depth-to-width ratio weld and 0.007% for (b) the high depth-to-width weld.
The influence of such small fluctuations of elements such as sulphur has
been explained in terms of their effect on the surface tension–temperature
gradient of the liquid weld pool and the subsequent flow within the pool. It
is known that the surface tension gradient of molten iron can be altered by
the presence of trace elements as shown in Fig. 6.18. [104a+b] If the slope
of the surface tension–temperature curve is negative (the surface tension is
higher at low temperatures), surface-tension-driven flow of liquid metal will
take place across the surface of the pool from the high temperature region at