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Pd = Pv - pzh-s (2.1)
where Pd is the pressure exerted by the arc on the weld pool; P, is the maximum
pressure exerted on the base of the weld pool by the column of molten metal; and Pgh.,
is the pressure of layer of molten metal in the crater region of the weld pool. Figure
2.7 shows the pressure distribution across the pool. The same authors [ref. 16] state
that the condition necessary for the weld pool dynamic equilibrium to be maintained is
attained when P,. > Pd. However, P, decreases with increasing welding speed until it
gets to a point at which the equilibrium condition does not hold any more, leading to
the formation of defects. If the welding current is increased, both P, and Pd increase,
with Pd growing more rapidly than P,. At a particular critical value of the welding
current for each welding speed, Pd becomes equal to P, and above this current level,
Pd> Pti again leading to defects.
It should be noted that the pool behaviour described above dealt with the case
of partially penetrated welds. In the case of fully penetrated welds, the shape of the
pool is governed basically by forces of surface tension [ref. 17]. The weight of the
weld pool and the arc pressure are of secondary importance [ref. 17]. The stability of
the pool depends on the correct balance between the weld pool's length and width, if
the balance is incorrect, the pool will collapse and bum-through will occur. However,
Stolbov and Masakov [ref. 18] stated that one of the main reasons for the destruction
of the weld pool is the high pressure in the centre of the arc which causes local
thinning and rupture of a liquid bridge. Another important factor for weld pool
collapse is the presence of gap in the joint [ref. 18].
2.1.5 Process stability
The definition of stability in GMAW is very subjective. Process stability has
normally been referred to as arc stability in most published works. This nomenclature
is adequate when assessing free-flight metal transfer mode. However, when the
stability of a short circuiting welding process is considered it cannot be treated as arc
stability, since in this case the arc is extinguished regularly, being essentially unstable:
The cyclic repetition of this unstable system is what makes the process viable, the
regularity of this behaviour being an indication of process stability [refs. 19,20]. It
should be noted that good arc stability does not imply that the weld pool is going to
be in dynamic equilibrium. For example, when welding with high welding currents and
high travel speeds, the resulting weld pool dynamic behaviour might lead to undercut
despite the arc being stable (see section 2.1.4). Therefore, the present author will
adopt process stability instead of arc stability, since it is a more generic term and it
does not imply any special mode of metal transfer. Hence, process stability will be
defined in the present work as a set of process behavioural characteristics that are
necessary for producing a good bead quality and a satisfactory welding performance.
This definition is in line with Philpott's definition [ref. 21]. Philpott defined
stability in GMA welding as the process ability to provide a regular metal transfer
without spatter, a uniform heat input along the weld (i. e. maintaining constant welding
current and voltage), smooth weld pool movements in a fixed position relative to the
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