Mma Welding Theory; Coated Electrode Welding; Tig Welding (Continuos Arc); Introduction - Selco Genesis 245 TLH Instrucciones De Uso

4 MMA WELDING THEORY

4.1 Coated electrode welding

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 electro-
de used is specified by the manufacturer on the electrode con-
tainer.
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 distance.
Generally, to improve the striking of the arc an initial current
increase 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 sup-
plies 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 welding current until the end of the short-circuit is very use-
ful (Arc Force).
If the electrode sticks to the piece to be welded, it is useful to
minimise 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
oscillations 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.

5 TIG WELDING (CONTINUOS ARC)

5.1 Introduction

The TIG (Tungsten lnert Gas) welding process is based on the pre-
sence of an electric arc striken between a non-consumable elec-
trode (pure or alloyed tungsten with an approximate melting tem-
perature 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-
Use
son the spark is started through an H.F. power source, thus ensu-
All positions
ring the remote striking of the electric arc.
Flat
Another type of start is also possible, with reduced tungsten inclu-
All positions
sions: the lift start, which does not require high frequency, but only
an initial short-circuit at low current between the electrode 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 important
to control carefully the slope down of the current and it is neces-
sary 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.
21