LIBRARY ı6ıul 0) - Cranfield University

In pulse transfer, the welding current and voltage are pulsed in such a manner
that a controlled spray transfer is obtained with a mean current below the normal
transition level for spray transfer. Two levels of current are used: a low background
current, which is applied to maintain the arc, and a high pulse-current in which level
drop growth, necking of wire tip and detachment occur [refs. 3,5,9,10,11].
Dip transfer is characterised by a periodic short circuiting of the arc gap. The
short circuiting is intentionally induced by feeding the wire towards the workpiece at a
speed which exceeds the rate at which the wire is melted by the arc (burn-off rate)
[ref 3]. Ideally, metal is expected to be transferred from the electrode to the
workpiece only during a period when the electrode is in contact with the weld pool
and no metal is transferred across the arc (see Figure 2.4)[ref. 2]. The metal transfer
occurs due to the high short-circuit current, which causes the molten metal bridge
between the the wire tip and the molten pool to pinch off and rupture. A portion of
the molten electrode tip is transferred to the weld pool and the arc is re-established.
After the rupture, the arc gap increases somewhat due to a rapid fusion of the
electrode', and to a weld pool retraction. The volume of the drop of molten metal on
the end of the electrode increases and the burn-off rate decreases until it is smaller
than the feed speed, hence starting another cycle. [refs. 2,3,5,12].
Dip transfer produces a small, fast freezing weld pool that is generally suited
for joining thin sections, for positional welding and for bridging large root openings
(joint gap).
2.1.3 Welding arc electrical characteristics
Conventional GMAW power sources are normally designed with constant-
voltage (CV) characteristics in order to provide self-adjustment and stabilisation of
the welding arc [ref. 3]. The voltage across the arc is directly related to the type of
plasma gas used and to its length. If the arc length increases the voltage across the arc
will also increase. The self-adjustment of the arc provided by a constant voltage
power source will be discussed later in this chapter.
The voltage developed between the end of the contact tip and the workpiece
in the GMAW process is the sum of the voltage drop in the wire extension, due to
resistive effects, plus the voltage fall across the arc [ref. 3]. It is commonly assumed
that the total arc voltage is made up of three separate and distinct parts, the cathode
(negative electrode) potential drop, the drop in the arc column and the anode (positive
electrode) drop. The cathode potential drop has a magnitude of the order of the
excitation potential of the electrode vapour (around l0V) and the anode voltage fall
generally lies between I volt and 12 volts and depends on the nature of the plasma.
The cathode and the anode falls occur in a very short distance from the respective
electrodes (cathode region and anode region) and present very high voltage gradients
(electric fields). The arc column, however, presents a relatively low voltage gradient
and the voltage fall can be approximated by a linear function of its length [ref. 6]. It is
1 The electrode burn-off rate, which corresponds to the peak short circuiting current, is greater than
its feed speed
Z The weld pool retraction results from an electrical explosion that forms during the rupture of the
bridge between the molten electrode and the weld pool [ref. 12]
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