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welding. In an atmosphere containing
oxygen, a well anchored
oxide layer builds up on aluminium.
This layer has a melting point
of some 2000°C against the
melting interval of 630-660°C for
the metal underneath. For quality
welds this layer must be removed
or at least broken up.
Despite the fact that the melting
interval of aluminium is by far
lower than that of steel, the high
thermal conductivity and the
high melting energy make that,
for arc welding, aluminium
requires about the same amount
of energy as steel.
The thermal elongation of aluminium
is twice that of steel and
the loss in volume of the weld
pool during solidification is
important, causing distortion of
the joint, if no remedial measures
are taken. One method of minimizing
distortion is to select a
process with small energy input.
TIG and MIG arc welding are the
two processes most commonly
used in the commercial vehicle
industry. The technical progress
made by other techniques such
as plasma, laser, resistance or
friction stir welding and the ever
growing diversity of semi-finished
products will encourage
the application of such welding
methods which have up to now
been little used in the commercial
vehicle industry.
Welder (STAS)
108
ALUMINIUM IN COMMERCIAL VEHICULES CHAPTER VIII WELDING 108 | 109
In this process, an electric arc is
struck between a refractory electrode
made of tungsten and the
workpiece, while a shroud of
inert gas, usually argon, shields
the electrode and protects the
molten pool against oxidation.
This process uses a high-frequency
stabilized AC power source.
The oxide film is removed during
the negative phase, while the
positive phase ensures penetration
and cooling of the electrode.
TIG welding is suitable for metal
thicknesses between 1 and 6 mm.
There is a TIG version where helium
is used as the shielding gas.
This helps achieving a high temperature
in the arc but requires
direct current with straight polarity.
The effect of oxide film removal
is weaker but the welding power
is higher and products 10 to 12
mm thick can be welded with a
single pass. However this process
is strictly for automatic welding
only owing to the difficulty of
keeping the arc at a constant
controlled height within 0.5 mm.
2. TIG welding (Tungsten Inert Gas)
PRINCIPLE OF TIG WELDING
FIGURE VIII.1
Contact (for current)
Filler
Metal
Arc
Shielding Gas
Shielding Gas Nozzle
Tungsten Electrode
Welding Power
Source
Weld Seam
Work Piece
109
EUROPEAN ALUMINIUM ASSOCIATION
2.1. Manual TIG welding
For manual TIG welding, the filler
material is a hand-held rod fed
into the weld pool. The manual
process is used mainly for small
welds, circular welds and relatively
thin components.
MANUAL AND MECHANISED TIG WELDING
FIGURE VIII. 2
Manual TIG Welding Fully Mechanised TIG
Welding
2.2. Automatic TIG
welding
Here, the welding torch is automatically
guided and, if filler is
used, it is fed automatically from
a reel.
Automatic TIG welding is an
attractive proposition for welding
large production runs, especially
when there is no access to the
back of the weld.
The fabrication of compressed air
reservoirs is a good example of
the use of automatic TIG welding.
These reservoirs consist of a
sheet rolled and welded to form
the straight cylindrical centre section
to which two deep-drawn
ends are welded. If the ends are
butt joined to the centre without
any backing to prevent the problems
associated with moisture
retention, automatic TIG welding
can be used to make an easy
connection. It is also possible to
support the weld pool by supplying
the argon from inside the
reservoir.
TIG welding of an air pressure vessel (SAG)
110
ALUMINIUM IN COMMERCIAL VEHICULES CHAPTER VIII WELDING 110 | 111
2.3. TIG welding with AC
It is especially well suited for butt
and corner welds on pieces in
the thickness range 1-6 mm. Full
penetration welds can be made
without backing bar. Tack welds
must not be removed before executing
the seam weld. Changes
in weld direction are easy to follow
with the torch and do not
require any dressing. The process
can also be used to smooth the
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