Manual Metal Arc (MMA) Welding

 MANUAL METAL ARC WELDING

This manual method is one of the most widely used arc welding processes. It requires considerable skill to produce good quality welds. The electrode consists of a steel core wire and a covering flux containing alloying elements, e.g. manganese and silicon. The arc melts the parent metal and the electrode. As metal is transferred from the end of the core wire to the weld pool, the welder moves the electrode to keep the arc length constant. This is essential as the width of the weld run is largely governed by the arc length.

The flux melts with the core wire and flows over the surface of the pool to form a slag, which must be removed after solidification.

Functions of the slag produced include:

 It minimises impurities in the weld

To form a coating which protects the weld from oxidation

It ensures a slow cooling rate for the weld

It prevents cracking and brittleness

Multi-run Welds

 Transformer circuit:

A step-down transformer is used to change the mains voltage from 220V to a

suitable level (80-100V) for welding. This will provide the high current

needed for welding. This type of transformer has more turns on the primary

coil than the secondary coil and will induce alternating current (AC) at a lower voltage.

The rectifier changes alternating current (AC) to direct current (DC). It

consists of four diodes which allows two of the diodes to conduct on each

half-cycle of the AC supply.

The capacitor is employed to provide a smooth supply of low voltage DC.