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7 CONTINUOUS WIRE WELDING
THEORY
7.1 Introduction
A MIG system consists of a direct current power source, wire
feeder, coil, torch and gas (Fig.4).
Fig.4 Manual welding system
The current is transferred to the arc via the fusible electrode
(wire connected to positive pole); in this procedure the melted
metal is transferred onto the piece to be welded via the arc.
Wire feed is necessary to replenish the weld wire that has mel-
ted during welding.
7.1.1 Methods
In welding with gas protection, there are two transfer systems
depending on the way in which the drops detach from the elec-
trode. In the first method defined "SHORT-ARC", the electrode
comes into direct contact with the weld pool, a short circuit occurs
with melting of the wire which is therefore interrupted, and the arc
then comes back on and the cycle is repeated (Fig. 2a).
Fig.5 SHORT cycle (a) and SPRAY ARC welding (b)
Another method of obtaining transfer of the drops is the so-
called "SPRAY-ARC" method, which allows the drops to detach
from the electrode and then reach the weld pool (Fig. 2b).
7.1.2 Welding parameters
The visibility of the arc reduces the need for the operator to
strictly observe the adjustment tables as he can directly control
the weld pool.
- The voltage directly affects the appearance of the bead, but
the dimensions of the welded surface can be varied accord-
ing to requirements by manually moving the torch to obtain
variable deposits with constant voltage.
- The wire feed speed is proportional to the welding current.
Fig.6 and 7 show the relations existing between the various
welding parameters.
Fig.6 Diagram for selection of best working characteristic.
Fig.7 Relation between wire feed speed and current intensity
(melting characteristic) according to wire diameter.
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