Idiomas disponibles
Idiomas disponibles
9.0 GENERAL INFORMATION ON THE
DIFFERENT WELDING PROCESSES
9.1 Coated electrode welding (MMA)
Preparing the edges
To obtain good welding joints it is advisable to work on clean
parts, free from oxidations, rust or other contaminating agents.
Choosing the electrode
The diameter of the electrode to be used depends on the thick-
ness of the material, the position, the type of joint and the type
of preparation of the piece to be welded.
Electrodes with considerable diameter obviously require very
high currents with consequent high heat supply during the wel-
ding.
Type of coating
Property
Rutile
Ease of use
Acid
High melting speed
Basic
Mechanical charact.
Choosing the welding current
The range of welding current in relation to the type of electrode used
is specified by the manufacturer on the electrode container.
Striking and maintaining the arc
The electric arc is produced by rubbing the electrode point on the
workpiece connected to the earth cable and, once the arc has been
striken, by rapidly withdrawing the rod to the normal welding dis-
tance.
Generally, to improve the striking of the arc an initial current increa-
se with respect to the base welding current is very useful (Hot Start).
Once the arc has been striken, the central part of the electrode
starts melting and is deposited on to the workpiece in the form of
drops.
The external coating of the electrode is consumed and this supplies
the protective gas for the welding, the good quality of which is thus
ensured.
To prevent the molten material drops from extinguishing the arc by
short-circuiting the electrode with the weld pool because of their
accidental proximity to each other, a temporary increase of the wel-
ding current until the end of the short-circuit is very useful (Arc
Force).
If the electrode sticks to the piece to be welded, it is useful to mini-
mise the short circuit current (antisticking).
Carrying out the welding
The electrode inclination angle varies depending on the number of
runs; the electrode movement is normally carried out with oscilla-
tions and stops at the sides of the bead, in such a way as to avoid
an excessive accumulation of filler material at the centre.
Removing the slag
The welding through coated electrodes requires the removal of
the slag after each run.
The slag is removed by means of a small hammer or is brushed
away if friable.
9.2 TIG welding (continuos arc)
lntroduction
The TIG (Tungsten lnert Gas) welding process is based on the
presence of an electric arc striken between a non-consumable
electrode (pure or alloyed tungsten with an approximate mel-
ting temperature of 3370°C) and the work-piece; an inert gas
(argon) atmosphere protects the weld pool.
To avoid dangerous inclusions of tungsten in the joint, the elec-
trode must never get in contact with the workpiece; for this rea-
son the spark is started through an H.F. power source, thus ensu-
ring the remote striking of the electric arc.
Use
Another type of start is also possible, with reduced tungsten
All positions
inclusions: the lift start, which does not require high frequency,
Flat
but only an initial short-circuit at low current between the elec-
All positions
trode and the workpiece; when the electrode is lifted, the arc
will be started and the current will increase until reaching the set
welding value.
To improve quality of the end of the welding bead it is impor-
tant to control carefully the slope down of the current and it is
necessary that the gas goes on flowing in the welding pool some
seconds after the arc blowout.
Under many operational conditions, it is useful to be able to use
two preset welding currents and to be able to move easily from
one to the other (BILEVEL).
Welding polarity
D.C.S.P. (Direct Current Straight Polarity)
This is the most used polarity and ensures limited wear of the
electrode (1), since 70% of the heat concentrates on the anode
(piece).
Narrow and deep weld pools are obtained, with high advance-
ment speeds and low heat supply.
Most materials, exception made for aluminium (and its alloys)
and magnesium, are welded with this polarity.
D.C.R.P. (Direct Current Reverse Polarity)
The reverse polarity is used for welding alloys covered with a
layer of refractory oxide with higher melting temperature in
comparison with metals.
High currents cannot be used, since they would cause an exces-
sive wear of the electrode.
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