Advanced Welding Processes: Technologies and Process Control

Table 10.5 Stability criteria
Monitoring and control of welding processes 197
Criteria Application Reference
Ccrit = S2/ X 2
m CO2-shielded GMAW [204]
As above but deviation of ac MMAW [205]
current peaks used
R = w/W and i/I dc power source evaluation [206]
K Tm
=
s T
Element recovery in GMAW [207]
Standard deviation of current
peak values
General purpose monitoring [208]
K factor (as above) Shielding-gas evaluation
GMAW stainless steel
[209]
Standard deviation of arc time Shielding-gas evaluation short
circuit GMAW carbon steel
[210]
Probability distribution of short
circuit and arc time
CO2-shielded GMAW [211]
Statistical analysis of arc and GMAW spatter level [212]
short circuit times investigation
s tarc
t arc-mean
GMAW dip transfer [213]
W s = W a + W r + W p GMAW [214]
Statistical analysis of short MMAW and GMAW [215]
circuit time
Key : S is the standard deviation of time between short circuits measured on a voltage
trace. X m is the mean value of the time between short circuits. w is the minimum value
of power in the arc, just after droplet transfer. W is the average power under steady
conditions. i is the minimum value of current and I is the average value of current under
steady conditions. W a is a stability criterion based on factored values of the standard
deviation of arc time, short circuit time, arc current and peak current. W r is a stability
criterion based on the ratio of the observed resistance during arcing and the average
resistance. W p is a criterion based on the ratio of observed power during arcing over the
average power. T m is the mean cycle duration, s T is the standard deviation of the cycle
duration, s tarc is the standard deviation of arc time, and t arc-mean is the mean arc time.
These subjective assessments require considerable skill, are time consuming
and the results are often inconsistent. Objective measurements of stability
may be achieved by examining the statistical variation of various operating
parameters. A wide range of stability criteria have been developed [207–
218] and some of these are listed in Table 10.5.
In dip transfer GMAW, the standard deviation of the arc time, the standard
deviation of the peak current and the ratio of arc time to short-circuit time
give a good indication of stability and are easily measured using computerbased
instrumentation. Figure 10.6 shows histograms of arc times for two
nominally similar GMAW consumables. It is apparent that the consumable