LIBRARY ı6ıul 0) - Cranfield University

stickout lengths and the temperature distribution along this length. The welding arc
contribution depends on the type of plasma gas, the welding current, the arc length
and the types of material of the electrodes (see section 2.1.3). Note that the stickout
length will vary as a result of a change in the stand-off and also due to random
variation in the point of electrical contact in the contact-tip and to weld pool
oscillations. The contact tip can be viewed as a source of noise in the
voltage/resistance measurement.
Theoretical calculations of the distribution of temperature along the stickout
length show that the temperature distribution varies with wire material (electrical
resistivity, thermal conductivity and thermal capacity), welding current and wire feed
speed [ref. 195]. The temperature of the wire at the point of electrical contact in the
contact tip is generally assumed to be equal to the contact-tip temperature [ref. 131].
Hence, assuming this to be true, a change in the temperature distribution along the
stickout is expected to occur as a result of a change in the contact-tip temperature.
Consequently, the electrical resistance is also expected to change. Considering that
the torch cooling system has a limit in the amount of heat it can remove from the
contact-tip per unit of time, the steady state temperature of the contact-tip will vary
depending on the set-up welding parameters.
From these considerations, an empirical model with terms accounting for the
effect of isolated variables and two-variable interaction was proposed to map the
behaviour of the measured dip resistance as a function of the welding parameters, V,