Manual Metal Arc (MMA) welding is the
most flexible and one of the most widely used arc welding processes. They can
use either direct or alternating current, and consumable or non-consumable
electrodes.
PRINCIPLE
Manual metal arc process occurs when two
wires which form part of an electrical circuit are brought together and then
pulled slowly apart, an electric spark is produced across their ends. This
spark or arc has a temperature of up to 3600-degree centigrade.
The heat of the arc melts the parent
metal and the electrode which mix together to form, on cooling, a continuous
solid mass.
The central metal electrode or core wire
acts as a consumable, providing the filler metal for the weld.
PROCESS
1. Arc welding processes use an
electrical power supply to create and maintain an electric arc between an
electrode and the base material to melt metals at the welding point.
2. One wire is connected to the job and
other is connected to the electrode. The heat of the arc melts both of the job
and the point of the electrode.
3. To create an arc for welding, a
voltage between 60V and 100V is required to create the arc, once it arc
established, 20V and 40V is required to maintain it.
4. Before welding commences, no current
passes through the leads and the ammeter read zero, but the open circuit
voltage is between 60V and 100V.
5. When the electrode is brought into
contact with the job a large current, called short circuit current passes
through the leads. The voltage drops almost to nothing. The tip of the
electrode becomes hot because of the resistance created between it and the job.
6. When the electrode is withdrawn an
arc is formed between the electrode and the job at the time voltage is 20V to
40V and the current falls to value to which it has been set. The arc is then in
the normal welding condition.
7. As the arc is confined to a very
small area it can melt metal almost instantly.
The molten metal from the electrode mixes with that from the job and
forms the weld. This welding is not suitable for overhead welding because the
globules do not fall by gravity.
8. During the deposition of weld metal,
variations in both the voltage and current of the arc can occur and the welding
plant must be capable of coping with these changes.
9. In MMA welding, the voltage is
directly related to the length of the arc, and the current is related to the
amount of heat input.
10. Constant current power supplies are
most often used to manual welding processes. Because in manual welding, it can
be difficult to hold the electrode perfectly steady, and as a result, the arc
length and thus voltage tend to fluctuate.
MMA welding with DC the electrode only
creates the arc and does not provide filler material, a positively charged
electrode causes shallow welds, while a negatively charged electrode makes
deeper welds.
MMA welding with AC the arc must be
re-ignited after every zero crossing, which demands special power units that
produce a square wave pattern instead of the normal sine wave, making rapid
zero crossings possible and minimizing the effects of the problems.
ADVANTAGES OF MMA WELDING
1. It is the simplest form of welding
processes.
2. The equipment can be portable and the
cost is fairly low.
3. Used in many applications because of
availability of wide variety of electrodes.
4. Welding can be carried out in any
position with the highest weld quality.
DISADVANTAGES OF MMA WELDING
1. It is not suitable for automation
because the length of the electrode is limited and brittle in nature.
2. In welding long joints as one
electrode finishes, the weld is to be progressed with the next electrode.
3. Unless properly cared, a defect or
insufficient penetration may occur at the place where welding is restarted with
the new electrode.
4. This process uses stick electrodes
and thus it is slower as compared to Metal Inert Gas welding.
APPLICATIONS
MMA welding can be used to join steels,
stainless steels, cast irons and many non-ferrous materials. For many mild and
high-strength carbon steels, it is the preferred joining method.
