Welding

The electric arc used in welding processes is created between a covered or bare metal electrode and the base metal or workpiece. With shielded metal arc welding (SMAC), an electric circuit is set up between the welding machine (AC or DC continuous power source), the workpiece, the electrical cables, the electrode holder, electrode, and ground wire. To strike an arc, the electrode must be touching the base metal; this is usually done by scratching or pecking the base metal with the electrode. As the electricity begins flowing, the electrode is held away from base metal, creating a gap. The electrical current flows across the gap, resulting in an arc. The intense heat from the arc melts the workpiece and the electrode, which contains metal powder that, when melted, becomes the filler metal. The covering or coating on the metal electrode is a flux material that melts, which removes impurities from the weld and sometimes creates a gas that shields the area from atmospheric contamination. Essentially, the electrode and its shielding control the mechanical, chemical, and electrical characteristics of the weld. The heat—6,000–9,000°F (3,351–4,982°C)—of the electric arc brings the base metal and the consumable electrode to molten state, within a matter of seconds.

Gas metal arc welding (GMAW or MIG) is similar to SMAW. A direct current (DC) is always used with this process and there is a gas supply apparatus. A consumable electrode is housed within a nozzle that supplies an inert shielding gas such as helium or argon. GMAW has several advantages over SMAW. With GMAW, welding speed is faster, no slag is produced, there is deeper penetration, and the electrode wires are continuously fed so that longer welds can be made. A type of arc welding that does not use a consumable electrode is gas tungsten arc welding (GTAW or TIG). An arc is produced between the base metal and a tungsten electrode, a shielding gas is used, and there must be a water supply to cool off the torch. Instead of a consumable electrode, a metal rod or welding rod is used to provide filler metal, if required. This type of welding is also called heliarc welding.