Selco Genesis 2700 AC/DC Betriebsanweisung Seite 44

7 WELDING THEORY
7.1 Manual Metal Arc 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 of large diameter obviously require very high currents
with consequent high heat supply during the welding.
Type of coating Property
Rutile Easy to use
Acid High melting speed
Basic High quality of joint
Choosing the welding current
The range of welding current related to the type of electrode
used is specified by the manufacturer usually on the electrode
packaging.
Striking and maintaining the arc
The electric arc is produced by scratching the electrode tip on
the workpiece connected to the earth cable and, once the arc
has been struck, by rapidly withdrawing the electrode to the
normal welding distance.
Generally, to improve the arc striking behaviour a higher initial
current is given in order to heat suddenly the tip of the electrode
and so aid the arc establishing(Hot Start).
Once the arc has been struck, the central part of the electrode
starts melting forming tiny globules which are transferred into
the molten weld pool on the workpiece surface through the arc
stream.
The external coating of the electrode is being consumed and this
supplies the shielding gas for the weld pool, ensuring the good
quality of the weld.
To prevent the molten material globules cause the extinguish-
ing of the arc by short-circuiting and sticking the electrode to
the weld pool, due to their proximity, a temporary increase of
the welding current is given in order to melt the forming short-
circuit (Arc Force).
If the electrode sticks to the workpiece, the short circuit current
should be reduced to the minimum (antisticking).
Carrying out the welding
The welding position varies depending on the number of runs;
the electrode movement is normally carried out with oscillations
and stops at the sides of the bead, in such a way as to avoid an
excessive accumulation of filler metal at the centre.
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Removing the slag
Welding using covered electrodes requires the removal of the
slag after each run.
The slag is removed by a small hammer or is brushed away if
friable.
7.2 TIG welding (continuos arc)
The TIG (Tungsten lnert Gas) welding process is based on the
presence of an electric arc struck between a non-consumable
electrode (pure or alloyed tungsten with an approximate melting
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 come in contact with the workpiece; for this
reason the welding power source is usually equipped with an
Use
arc striking device that generates a high frequency, high voltage
All positions
discharge between the tip of the electrode and the workpiece.
Flat
Thus, thanks to the electric spark, ionizing the gas atmosphere,
All positions
the welding arc is struck without any contact between electrode
and workpiece.
Another type of start is also possible, with reduced tungsten
inclusions: the lift start, which does not require high frequency,
but only an initial short-circuit at low current between the elec-
trode and the workpiece; when the electrode is lifted, the arc
is established and the current increases until reaching the set
welding value.
To improve the quality of the filling at the end of the welding
bead it is important to control carefully the down slope of the
current and it is necessary that the gas still flows in the welding
pool for some seconds after the arc is extinguished.
Under many operating conditions, it is useful to be able to use
two preset welding currents and to be able to switch 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 is concentrated in the anode
(piece).
Narrow and deep weld pools are obtained, with high travel
speeds and low heat supply.
Most materials, except for aluminium (and its alloys) and mag-
nesium, 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 com-
pared with metals.
High currents cannot be used, since they would cause excessive
wear on the electrode.