Arc Welding
Welding can be defined as “the operation that produces coalescence of two or more members by applying heats or pressure or both, with or without the use of filler metal” . Welding (Metallurgical joining) can further be classified by the joining mechanism into 4 groups : Fusion welding ( can be referred to as welding in the narrow sense), solid phase joining (pressure welding), diffusion welding and joining by liquid-solid phase reaction (Brazing & soldering). Fusion welding consist of 5 categories : gas welding, arc welding, electroslag welding , thermite welding and high energy beam welding (electron beam welding & Laser Welding).
Type of arc welding:
- Shielded metal arc welding
- TIG/GTAW welding
- Plasma arc welding
- MlG/ MAG welding
- Self-shielded arc welding
- Submerged arc welding
- Electrogas arc welding
- Stud welding
Arc welding process that uses the arc discharge for the heat source is widely used in the world. And the most popular welding process that has been used from the old time is shielded metal arc welding (SMAW). The SMAW can be used for all welding position. As shown in the below picture, the voltage applied across the core rod of the electrode and the base metal generate an arc. The arc temperature for SMAW is about 5,000 – 6,000 K. This arc heat generate molted droplets of electrode tip that transfer to the weld pool. There is a voltage gap between the anode and the cathode. An arc voltage consists of the cathode (negative pole) voltage drop, the anode (positive pole) voltage drop, and the arc column voltage drop between cathode and anode.
The arc column is composed of neutral particles, such as atoms and molecules, and charged particles such as ions and electrons that are generated by ionization of neutral particles. The arc column is characterized by “thermal equilibrium” and “electric neutrality.” The gas in this condition is called ionized gas (plasma).
Arc currents are carried mostly (more than 99.9%) by electrons. That is, while particular numbers of
electrons are emitted from cathode into the arc column, the same numbers of electrons are absorbed by
anode. An arc is a high temperature conductive gas which can carry considerable amounts of currents. The
ionization degree of an arc is maintained by the electric power (the energy produced by the product of arc
voltage and current) supplied to the arc. An arc column is maintained at a high temperature of between 5000—50,000K, though it depends on electric current, the kind of arc gas, and the shape of the arc. In the case of shielded metal arc welding of mild steels, the temperature of the arc becomes lower (approximately 6000K) because the arc contains a large amount of metal vapors that are apt to be ionized.
The relation between arc voltage and current for a direct current TIG (GTAW)
In the low current range the arc voltage decreases as the current increases; this is called the negative resistance characteristic. In the high current range, the arc voltage gradually increases as the current increases; this is called the ascending characteristic. At a particular amount of current the arc voltage increases with a longer arc length because the arc column voltage drop increases. With direct current (DC), when the electrode is connected to the positive terminal, the polarity is designated as direct current electrode positive (DCEP). When the electrode is connected to the negative terminal, the polarity is designated as direct current electrode negative (DCEN).
In TIG/GTAW arc welding with the polarity of DCEP and a shielding gas of argon or other inert gas, the arc has the effect of removing the oxide film of the base metal surface. This is called the cathodic cleaning action. If you observe an arc carefully, you will see many small luminescent points (cathode spots) moving around on the base metal surface of negative pole. These cathode spots tend to be formed at the edges of the oxide film, at which electric current concentrates causing rapid evaporation of the oxide film, resulting in the removal of the oxide film. Because this phenomenon takes place continuously removing the oxide film, the trace of the arc travel exhibits the brighter metallic surface where the oxide film is removed completely on the base metal. The cleaning action is effective to prevent weld imperfections in the welding of aluminum alloys.
