WHAT IS ARC WELDING?
Arc or MMA (Manual Metal Arc) Welding is the oldest example of currently used electric welding procedures, and perhaps the most straight forward to use and understand.
The principle of arc welding is simple; a relatively large current is passed through a welding rod (or filler rod) of the appropriate metal (Mild steel for welding steel etc.) and a circuit is completed through it when it comes into contact with the earthed metal being welded. The join between the welding rod and the work being welded creates an area of high resistance, which produces enough heat to melt the rod at that point. The rod is drawn along the work, creating a line of molten metal that solidifies to create a strong bond between the two metals being welded together. Higher currents are used for welding thicker metals – and require thicker rods.
If the current used is too low for the thickness of metal being welded the weld will not penetrate sufficiently into the metals, and hence will be weak. If the current is set too high, it is likely that the electrode (welding rod) will splatter (creating a messy weld) or at worst burn through the work.
As the temperature of the arc can reach as high as 600oC, an amount of oxides are produced – these can cause problems as they form on the metals being welded, creating a barrier between the work and the filler rod. A layer of oxide both insulates the metals (meaning the arc cannot be struck) and may affect the strength and general quality of the weld. To overcome these problems, the filler rod is coated in flux which, when burned, produces a shielding gas that protects the metal from the oxygen in the air. The remains of the flux solidify on the metal in the form of “slag” – which also protects the weld against oxidation until it has cooled.
This layer can simply be chipped off with a welding hammer once the weld has cooled, and sometimes comes off by itself as the metal contracts.
As with all forms of welding and soldering, it is important that the areas of the work are cleaned thoroughly of impurities and oxide before welding takes place; this can be carried out with a wire brush, or abrasive paper.