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surface of a MIG weld.
The welding speed is lower than
for MIG weld and, for work
pieces thicker than 6 mm, preheating
is required. The slow
welding speed is also responsible
for a wider heat affected zone
and greater distortion of the
assemblies.
For fillet welds extreme care is
needed to achieve full penetration
without lack of fusion at the
root.
In the tank and silo production,
double side TIG welding of butt
welds in vertical upwards position
leads to excellent quality,
provided the two operators control
the process well.
2.4. TIG welding with
DC, reverse polarity
In this process the arc length is
below 1 mm, ideally 0.5 mm,
which means that it is mainly
used for machine welding. For
manual operation only short
lengths can be executed in practice.
One such application is
stitch welding of assemblies
before the seam welding. The
small cross section of these
stitches is such that they are
completely molten up while laying
the first pass of MIG weld
over it and don’t need to be
reduced in cross section by
mechanical means.
The oxide film removal is weaker
with this process so it is necessary
to reduce the oxide layer by
mechanical means before welding.
2.5. Edge preparation for
TIG welding
In EN ISO 9692-3 this information
is given comprehensively, so that
we just indicate a few examples
for typical joints in vehicle manufacturing
(Table VIII.1, p. 115).
To avoid sharp notches, especially
at the root of the weld, all
edges must be carefully deburred
before welding. Instead
of grinding discs, milling tools
should be used, because residues
of the disc on the surface can
cause porosity in the weld.
2.6. Choice of filler wire
or rod
See section 3.8
2.7. Selection of Welding
process
See section 3.9
111
EUROPEAN ALUMINIUM ASSOCIATION
With MIG welding the aluminium
alloy wire is both the electrode
and the filler material. It
uncoils automatically from a reel
to the welding tool (gun or torch)
as it is used up. The welding
energy is supplied by a DC power
source (smoothed current).
Connection is made with reverse
polarity (i.e. minus to the workpiece)
to ensure removal of the
oxide film and the fusion of the
wire electrode at the same time.
Several MIG processes do exist...
PRINCIPLE OF MIG WELDING
FIGURE VIII.4
Wire Transport Rolls
Welding
Power
Source
Workpiece
Arc
Wire Electrode
Contact Nozzle
(for Current)
Shielding Gas
Nozzle
Weld Seam
3.1. Manual MIG welding
In its manual version, MIG is certainly
the most common welding
process used in the commercial
vehicle industry, producing
high quality welds at an attractive
quality/cost ratio.
As the filler wire, that is the consumable
electrode, is always
automatically fed from a reel, the
manual MIG welding is also
known as "semi-automatic MIG
welding".
Manual MIG welding is used for
all welds of a complex nature
where the dimensions and thickness
of the products are compatible
with the MIG process and
when automation is not considered
to be profitable.
If we consider the example of a
tank consisting in sheets rolled
and welded to form cylindrical
sections, we can see that the longitudinal
weld can be made by
automatic MIG while the circular
welds which join the sections to
one another are usually made
manually on a turntable in two
opposing passes. The choice
between manual or automatic
MIG will depend largely on
accessibility.
3. MIG welding (Metal Inert Gas)
112
ALUMINIUM IN COMMERCIAL VEHICULES CHAPTER VIII WELDING 112 | 113
3.2. Automatic MIG
welding
Here, the welding torch is automatically
guided.
This is normally used for very
long straight welds where an
automatic system is profitable. A
good example is fabrication of
chassis side members consisting
of two “T” sections welded to
either edge of a central plate
which forms the web of the
built-up beam. The two welds
would normally be made automatically
and at the same time to
avoid problems of deformation.
Automatic welding is also preferred
where an attractive
appearance is desirable, e.g. for
stiffening channel welded to the
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