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In order to avoid spattering, the correct torch position
from the workpiece should be maintained. In this
regard, special attention should be paid to a
phenomenon that generally takes places as an arc
deviation due to the electromagnetic forces at issue: the
magnetic blow. Such phenomenon mainly occurs when
welding angles or the inner corners of box-type pieces.
In order to reduce it, it is suggested to maintain the torch
in the opposite direction to the arc deviation, and to
choose an adequate point where the work return lead
should be connected.
Special attention is also to be paid to the welding of thin
materials or in the first pass of bevelled joints, as the
material might be pierced. In such cases welding with
low current values is suggested (short arc).
Two weld metal transfer methods are available in
MIG/MAG welding:
SHORT ARC
SPRAY ARC
SHORT ARC is used with arc voltage values lower than
24V and relatively low welding current values (lower
than 200 A/mm
2
). With this method the wire melts as it
is short-circuited by its own contact with the weld pool,
which causes the transfer of the molten drops. The arc
length is short, so the contact tip should protrude by 2-3
mm from the nozzle.
This method can be used in all positions (butt, fillet and
vertical welding) and generally in those cases such as
thin materials, first passes, etc., where low current
values are requested to prevent deformation and
piercing.
SPRAY ARC is on the contrary a welding method which
requires higher voltage and current values and in which
the weld metal when melting is sprayed into the weld
pool. The arc length is more evident and its intensity is
greater. The contact tip should be set back from the
nozzle (between 5 and 10 mm, as the welding current is
increased).
This method is only used in flat position of butt and fillet
welds on material of 4 mm and above, where a higher
deposition speed is required. The greater the arc length
(corresponding to a greater voltage V), with equal
current A, the wider and flatter the weld bead. This
means that by increasing the voltage the “deposition
cone” of the weld material widens.
At this point, special mention is deserved by the
pulsed-arc MIG/MAG welding procedure, which is
characterised by a non-constant, high-value pulse
current flow. Between pulses the current value is at a
minimum so as to keep the arc on without melting the
material. Thus the average welding current value can
be reduced even if welding with the spray-arc method.
So the advantages of this method (high-quality welds
thanks to the spraying of the molten metal, and the
variable weld bead width) can be achieved even at low
current values, typical of the welding of thin materials
and of welding positions other than butt welding.
To complete the topic, also the concept of synergy
(synchronisation) is to be mentioned. In a sync welding
machine, the couple welding voltage / welding current
is not set by the operator, who is only given the choice
of the arc power. This way time is saved since the
welding parameters are automatically adjusted and this
optimal functioning guarantees an almost spatter-free
welding.
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