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129] modes of metal transfer. However, its aplication in the short-circuit transfer
mode is very limited [refs. 114,130].
Philpott [refs. 21,131] observed that the sensitivity of the former method to
variations in contact tip-to-workpiece distance decreased with decreasing welding
current and that in dip mode of metal transfer it was not adequate for seam tracking.
He therefore proposed a different method for dealing with this mode of metal transfer:
only the short-circuit phase was modelled. According to the author, such an approach
is beneficial for seam tracking in thin gauge plates such as the automotive pressings.
His method is described in the following paragraphs.
At the short circuit, there is no arc and the electrode is in contact with the
weld pool. Hence, the arc length L. =0 and, from equation (2.13), the contact tip-toworkpiece
distance SO becomes:
SO=Le (2.14)
The welding circuit during the short can be considered to be a solid metal
conductor. The relationship between the welding voltage and the welding current in
the short circuit is given by the Ohm's law (V=1R), and the resistance R is
proportional to the wire length and inversely proportional to its cross sectional area
(A. ), as shown in equation (2.15):
R_ 1SO
A,
where the il represents the resistivity.
Substituting R into Ohm's law results:
(2.15)
SO = A`V (2.16)
The resistivity of the electrode rises with increase in the temperature and can
be related by the temperature coefficient of resistance (a):
11=710(1+(ye)
(2.17)
where il is the resistivity at temperature 0 (C), and rlo is the resistivity at temperature
0 C. Substituting equation (2.17) into equation (2.16) results:
AeV
L 710(1+ßA)1
35
(2.18)