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Needham [ref. 36] stated that in order to maintain stability in dip transfer the
ratio between the arcing time and the short-circuiting time (so called "M ratio") must
be kept within a specific range as constant as possible.
Mitta et al. [ref. 37] developed an objective method for assessing process
stability by correlating statistical features obtained from the current and voltage
waveforms with the results from a subjective assessment of process stability
performed by a skilful welder. The authors [ref. 37] found that no single feature
correlates well with process stability at all current levels. Therefore, they used
multiple regression analysis to obtain an empirical model that relates the normalised
standard deviation of short-circuiting time, arcing time, average short-circuiting
current and average arcing current to the welder's subjective assessment of process
stability.
Dilthey et al. [ref. 38] also used a similar method to obtain rules for assessing
stability. They have proposed models, based on standard deviation to mean value
ratios, for assessing both short term and long term stability in short-circuiting, pulse
and spray GMAW.
A different approach was adopted by Shinoda and Nishikawa [ref. 39] for the
stability assessment of dip transfer welding. They suggested that the use of current-
voltage (I-V) line diagrams, instead of the commonly adopted time dependent voltage
and current waveforms, would provide information on the effect of both variables as
well as of their interaction on the process stability. The proposed method consisted of
measuring the area in the I-V diagram corresponding to each short-circuiting cycle
and calculating the standard deviation of these areas over a specified number of
cycles. The authors [ref. 39] claim that the method was successfully used for assessing
the stability characteristics of different wire compositions [ref. 40].
In contrast to the established practice of using descriptive statistical measures
some authors are using ratios developed from the features of the welding current and
voltage transient waveforms for stability assessment [refs. 41,42,43,44,45,46,47,
48,49,50].
Dyurgerov [ref. 41] found that stable metal transfer and optimum stability are
attained at a certain ratio between the short circuit and the mean welding currents.
This ratio was also used by Lebedev et al. [ref. 42] to. assess power source dynamics.
Lipei et al. [ref. 43] and Jennings [ref. 45] found that the ratio relates qualitatively to
the amount of spatter generated. Lipei et al. [ref. 43] state that the ratio should be
between 1.5 and 2, in order to keep the weld pool stable and prevent spatter while
Lebedev and Sidoreiko [ref. 44] found that the ideal value for the ratio is 1.75 but this
can be up to 1.95. Jennings [ref. 45] set the maximum value for the ratio at 2.35,
while Lebedev [ref. 46] set its minimum value at 1.35. Lebedev [ref. 46] also uses the
ratio of the minimum to mean current to select welding parameters for dip transfer;
the minimum permissible value for this ratio is 0.29, below which poor metal transfer
and are instability were reported to occur.
Vorouin and Goloshcapov [ref. 47] used the ratio of the short circuit time to
the droplet transfer cycle time (called the time ratio) to assess process stability in dip
transfer; the optimum welding condition was reported to be achieved for ratio values
between 0.2 and 0.25. Popkov et al. [ref. 48] found dip transfer conditions to be
optimum if the ratio of arcing time to cycle time is between 0.6 and 0.8. Needham
[ref. 49] used the ratio of arcing time to short circuiting time (called the M ratio) to
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