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parameter generator and five levels of voltage" were tested for each combination of
stand-off and wire feed speed, such that the influence of the welding voltage could be
detected. The welding parameters and the welding data collected are shown in
Appendix K. Due to the differences in the resistance components present in the dip
and the spray modes of metal transfer, two multiple regression models (one for each
mode of metal transfer) were developed based on the model of equation (6.11). The
coefficients obtained are shown in Table 6.8, together with the standard error (SE)
and the coefficient of determination of the fitted models (R2).
Table 6.8 - Coefficients for dip resistance based stand-off estimation models
Model for dip mode metal transfer Model for spray mode metal transfer
0.00165 0.021468
al -0.000027 -0.000568
a2 - -
0
0.010645
1 -0.001485 -0.029601
2 - 0.004404
3
0.000078 0.0007
SE 0.000371 0.002110
R2 0.9900 0.9997
Obs: The coefficients marked with an hyphen ("=) did not present a good significance level and
were considered null.
Note that the stand-off estimation models developed present the general form
shown in equation (4.8), that is, a linear model whose coefficients are functions of the
welding parameters. This is very similar to the dip resistance model developed by
Philpott [ref. 131] with the difference that this latter has a constant slope and the
intercept is considered null (see equation 2.18).
In order to validate the dip resistance based estimation models, bead-on-plate
welding trials were carried out with the stand-off varying linearly in different slopes.
Table 6.9 shows the welding parameters used and the start and end stand-off values.
Figure 6.15 to Figure 6.28 show some of the validation results and compare the actual
stand-off with the dip resistance based estimation and the estimation obtained by using
equation (2.22) with the coefficients shown in equation (6.4). Note that in most
situations in the dip mode of metal transfer, the stand-off predicted by the dip
resistance based model is more precise and presents a smaller range of oscillation than
the prediction based on the welding current cumulative differences. It should also be
noted that in the spray mode of metal transfer, the dip resistance based prediction
oscillates in a pattern very similar to the prediction based on the welding current
cumulative differences. This can be expected since in this case the measured resistance
includes the component due to the welding arc, which decisively influences the
welding current. Hence, it produces a less robust stand-off estimation than in the case
6 The voltage suggested by the welding parameters generator, V,., two levels below this value, V. 4-
0.5 and V e-1.0 , and two levels above, V, a+0.5 and V,, +1.0.
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