Welding Theory; Manual Metal Arc Welding (Mma); Continuous Wire Welding (Mig/Mag) - voestalpine Böhler TERRA 320 MSE Instrucciones De Uso

6 WELDING THEORY

6.1 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.
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.

6.2 Continuous wire welding (MIG/MAG)

Introduction
A MIG system consists of a direct current power source, wire
feeder, wire spool, torch and gas.
Use
All positions
Flat
All positions
The current is transferred to the arc through the fusible elec-
trode (wire connected to positive pole); in this procedure the
melted metal is transferred onto the workpiece through the arc
stream. The automatic feeding of the continuous filler material
electrode (wire) is necessary to refill the wire that has melted
during welding.
Methods
In MIG welding, two main metal transfer mechanisms are
present and they can be classified according to the means by
which metal is transferred from the electrode to the work-
piece. The first one, defined "SHORT-ARC", produces a small,
fast-solidifying weld pool where metal is transferred from the
electrode to the workpiece only for a short period when the
electrode is in contact with the weld pool. In this timeframe,
the electrode comes into direct contact with the weld pool
generating a short circuit that melts the wire which is there-
fore interrupted. The arc then turn on again and the cycle is
repeated (Fig. 1a).
Another mechanism for metal transfer is called the "SPRAY-
ARC" method, where the metal transfer occurs in the form of
very small drops that are formed and detached from the tip of
the wire and transferred to the weld pool through the arc stream
(Fig. 1b).
MIG manual welding system
Fig. 1a
Fig. 1b
SHORT cycle (a) and SPRAY ARC welding (b)
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