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the bridge. Another factor that indirectly induces spatter is the rate-of-rise of short-
circuit current [ref. 19]. An excessively high rate-of-rise induces high short-circuit
currents and consequently, spatter generation.
In spray transfer, instability appears in the form of erratic movement of
cathode spots when welding in low oxidising medium [ref. 22], or disruption of the
plasma column caused by the growth and explosion of droplets inside the arc [refs.
27]. The latter is believed to happen due to either fast transient variation in wire feed
speed, or, more likely, to local variation in wire material composition and surface
condition, which affects the contact resistance at the contact tip [ref. 28]. The metal
transfer disruption is often accompanied by metal ejection from the arc, or spatter.
Although unstable situations have been observed in spray transfer, it generally offers a
much improved level of stability compared to dip transfer.
The movement of the molten metal inside the weld pool also plays an
important role on the stability of the process. In dip transfer, for example, the weld
pool oscillates as a result of the pulses of pressure exerted by the repeated explosions
of the metal bridges formed during the short circuits. The oscillation frequency will
depend on the dimensions of the pool. It is generally agreed that the most stable
situation is attained when the dip frequency becomes equal to the weld pool
oscillation frequency [refs. 29,30,31]. In spray transfer, further to the movement
induced by convection and surface tension gradients, weld pool deformation is also
observed to be caused by arc pressure and by metal transferred from the electrode
[ref. 32]. At higher voltages and consequently higher currents, the combination of the
torch travel speed with the increased arc pressure and molten metal speeds within the
weld pool may destabilise its dynamic equilibrium, causing bead malformation [ref.
16]. This may be considered as process instability.
2.1.5.3 Existing stability assessment methods
Generally spatter is used as the main visible indication of instability. A stable
process generates low spatter, whereas an unstable process produces large amount of
spatter which can adhere to the gas nozzle, inducing insufficient shielding which leads
to porosity. Spatter can also adhere to the workpiece necessitating postweld cleaning
operations such as grinding, resulting in increased manufacturing costs. [refs. 29,33,
34].
Experienced welders normally judge the process stability by observing some
process characteristics such as level and size of spatter, regularity of the arc sound,
level of fume generation and arc and weld pool behaviour. The assessment is very
subjective and depends on the skill of each welder.
Process stability can be objectively assessed by using descriptive statistical
analysis, such as the standard deviation and coefficient of variation of the welding
current and voltage waveform features. For example, in dip transfer the standard
deviation of variables such as arcing voltage, short-circuiting current, arcing and
short-circuiting times have been used to assess stability [ref. 3]. The smaller the
standard deviation and/or coefficient of variation, the more stable the process is. It
should be noted that most works carried out on the objective assessment of the
stability of GMAW are mainly focused on dip transfer; globular transfer is generally
considered to be unstable and spray transfer naturally stable [ref. 35].
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