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MANUAL METAL ARC (MMA) WELDING
Shielded metal arc welding (SMAW),
also known as
manual metal arc (MMA) welding, and flux-shielded arc welding
or informally as
stick welding,
is a manual arc welding process that uses a consumable electrode coated in flux to lay
the weld. An electric current, in the form of either alternating current or direct current from a welding power supply, is
used to form an electric arc between the electrode and the metals to be joined. As the weld is laid, the flux coating of the
electrode disintegrates, giving off vapors that serve as a shielding gas and providing a layer of slag, both of which protect
the weld area from atmospheric contamination.
Because of the versatility of the process and the simplicity of its equipment and operation, shielded metal arc welding
is one of the world’s most popular welding processes. It dominates other welding processes in the maintenance and
repair industry, and though flux-cored arc welding is growing in popularity, SMAW continues to be used extensively in
the construction of steel structures and in industrial fabrication. The process is used primarily to weld iron and steels
(including stainless steel) but aluminum, nickel and copper alloys can also be welded with this method.
To strike the electric arc, the electrode is brought into contact with the work piece by a very light touch with the
electrode to the base metal then is pulled back slightly. This initiates the arc and thus the melting of the work piece and
the consumable electrode, and causes droplets of the electrode to be passed from the electrode to the weld pool. As
the electrode melts, the flux covering disintegrates, giving off shielding gases that protect the weld area from oxygen
and other atmospheric gases. In addition, the flux provides molten slag which covers the filler metal as it travels from
the electrode to the weld pool. Once part of the weld pool, the slag floats to the surface and protects the weld from
contamination as it solidifies. Once hardened, it must be chipped away to reveal the finished weld. As welding progresses
and the electrode melts, the welder must periodically stop welding to remove the remaining electrode stub and insert a
new electrode into the electrode holder. This activity, combined with chipping away the slag, reduce the amount of time
that the welder can spend laying the weld, making SMAW one of the least efficient welding processes. In general, the
operator factor, or the percentage of operator’s time spent laying weld, is approximately 25%.
The actual welding technique utilized depends on the electrode, the composition of the work piece, and the position
of the joint being welded. The choice of electrode and welding position also determine the welding speed. Flat welds
require the least operator skill, and can be done with electrodes that melt quickly but solidify slowly. This permits higher
welding speeds. Sloped, vertical or upside-down welding requires more operator skill, and often necessitates the use of
an electrode that solidifies quickly to prevent the molten metal from flowing out of the weld pool. However, this generally
means that the electrode melts less quickly, thus increasing the time required to lay the weld.
