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9
MMA welding procedure and
technical data
MMA welding procedure is the easiest among arc
welding procedures since it uses just a power source
connected to an electrode holder.
The electrode is made up by two fundamental parts:
•
the CORE, which is made of the same material as
the weld piece (aluminium, steel, copper, stainless
steel) and has the function to add material to the
joint;
•
the FLUX, made of different mineral and organic
substances mixed together, whose functions are:
A) gas protection
A part of the flux vaporises at the arc temperature
forming a column of ionised gas which protects the
molten pool;
B) addition of binding elements and slags
A part of the flux melts and some elements are added
to the weld pool; these join the material to be welded
and form the slag.
The welding procedure and the characteristics of the
weld deposit of each electrode depend on the type of
flux and on the material of the core.
The main types of electrode coating are:
Acid coating
This type of coating gives good weldability and may be
used either in ac or dc welding with the electrode holder
connected to the negative pole (straight polarity). The
weld pool is very fluid, therefore it can only be used in
flat position.
Rutile coating
This type of coating is the most commonly used
because it gives good weld appearance. It can be
welded in ac or dc with both polarities.
Basic coating
This type of coating is essentially used when high
mechanical properties are required. It is usually welded
in dc with the electrode holder connected to the positive
pole (reverse polarity), but there are also some types of
basic coating that can be used in ac welding. It is
suggested to keep basic coated electrodes in dry
places.
Cellulose coating
This type of coating is used in dc welding with the
electrode holder connected to the positive pole (reverse
polarity). It is essentially used for welding pipes due to
the viscosity of the weld pool and the deep penetration.
It requires a power source with adequate
characteristics.
MMA welding procedure requires the setting of the
following parameters:
A) Welding current
This parameter depends on the electrode type and
diameter and on the welding position. It is practically the
main variable, in that it determines penetration, weld
metal deposition and weld fillet thickness.
B) Arc voltage
It essentially depends on the distance between the
electrode tip and the workpiece. As the distance
increases, penetration decreases, weld fillet widens
and heavy spatters may appear.
As a guide, the table below shows the welding current
range to be used with the different electrode diameters
when welding carbon steel:
As a rough indication, the electrode to be used should
be as thick as the workpiece.
When the welding position is not horizontal, the weld
pool tends to flow down due to gravity. In these cases
this electrodes should be used in multiple passes. With
workpieces thicker than 3 mm, it is suggested to
adequately prepare the edges to be welded with a
single-Vee or double-Vee caulking. In this case welding
consists in filling the caulking besides joining the pieces
(a thin electrode should be used in the first pass so as to
avoid piercing the pieces).
The electric arc strikes when the electrode tip is
scratched on the workpiece and lifted quickly to the arc
starting distance. If this movement is too quick and the
distance excessive, the arc will blow out; on the
contrary, if the movement is too slow, it may
short-circuit the pieces. In the latter case, the electrode
may be detached from the workpiece by tearing it aside.
To improve the arc start, the power source may supply
an initial current peak; this technique is called
‘hot
start’
. Once the arc strikes the electrode core begins to
melt dropping down onto the workpiece. The outer
coating, as it is consumed, provides the gas shielding
necessary to a good weld (as explained before).
Electrode
diameter
(mm)
Current (A)
min.
max.
1,6
25
50
2
40
70
2,5
60
110
3,25
80
150
4
100
180
5
140
250
6
190
340
7
240
430
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