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side panels of vehicle bodies.
Here the appearance and size of
the weld bead can be repeated
to achieve the impression of consistency.
Finally, automatic welding - both
TIG and MIG - provides a repeatable
welding quality provided of
course that the welding parameters
are fully defined to begin
with.
3.3. Smooth current MIG
Welding
This fast and economical process
allows depositing a great quantity
of filler metal per unit of time.
The energy input is such that
butt welds can only be produced
with the use of a backing bar,
either integrated into the shape
of the extrusion or as temporary
removable feature in stainless
steel, copper or even aluminium.
Due to the relatively high welding
speed, the heat affected
zone1 is narrower than with TIG
welding and thus the distortion
of the assemblies is less.
Thin gauge material below 3 mm
is difficult to weld with this
process because of the high
energy of the arc. If no other
equipment is available, then a
thin gauge filler wire may be
used with reduced energy input,
but then the wire feed can cause
instability of the process even if a
push-pull equipment is used.
If the preassembly of structures is
carried out with stitches in the
MIG process, these short runs
must have a similar cross section
as the first weld pass and be
some 100 mm long to be sound.
Before production welding, these
stitches must be reduced in cross
section by mechanical means (no
disc grinders), so that they can
be molten up with the weld pass
and do not leave imperfections
near the root.
1. The extent of the heat affected zone
and the strength in the heat affected
zone are given in EN 1999-1-1.
Welding of side panels
for tippers (Menci)
113
EUROPEAN ALUMINIUM ASSOCIATION
3.4. Pulsed current MIG
welding
An improvement of the MIG
process has been achieved by
superimposing a pulsed current
over the main current, the object
being to maintain a low average
current level without sacrificing
arc stability.
The filler metal is transferred to
the weld pool every time the current
is high (i.e. one drop of metal
per pulse). The "cold times"
when the current is low ensure
that arc stability is maintained.
There are three operating modes:
• synergetic mode: only the
rate of uncoiling has to be regulated.
The voltage and frequency
are regulated by electronic logic
circuits;
• manual mode: all the welding
parameters are adjustable;
• programme mode: each
parameter can be stored for
use according to production
requirements.
The pulsed MIG process is limited
to thin products of 2 < t ≤ 5 mm
and to vertical fillet welds.
This process makes it possible to
weld thin gauge material with
standard filler wire. As the weld
pool can be better controlled,
butt welds up to 5 mm thickness
can be executed without backing
bar. Furthermore it is very helpful
for welding in the vertical and
the over head position. The optimal
machine setting is more
demanding than for standard
MIG because there are much
more parameters to be defined.
The width of the heat affected
zone is analogue to the one for
standard MIG as is also the
amount of distortion of the work
pieces.
For welding over stitches see
remark under 3.3 above.
3.5. Wire pulsation
For gauges between 1 and 3 mm,
a complementary option “the
wire pulsation” could be added to
the previously described “current
pulsation” in order to improve the
arc stability. This “wire pulsation”
induces a double pulsation to current
signal and consequently to
the heat input. For T-joint of dissimilar
gauges, the heat input distribution
is difficult to maintain
constant with classical pulsed current.
This double pulsation of current
insures the concentration of
heat input at the exact location of
the joint.
3.6. CMT – Cold Metal
Transfer
For MIG welding gauges lower
than 1 mm, the CMT process
(Cold Metal Transfer) could be
used. When detecting a shortcircuit,
this process retracts the
wire so as to help detach the
droplet. The thermal input is
immediatly reduced and the
short-circuit current is kept small.
Welding
of a truck door
114
Process Welding Weld Thickness Preparation Remarks
position bead
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