27-06-2012, 03:41 PM
PLASMA ARC WELDING
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INTRODUCTION:
PLASMA ARC WELDING: Plasma arc welding is an arc welding process that produces coalescence of metals by heating them with a constricted arc between an electrode and the work piece(transferred arc) or between the electrode and the constricting nozzle(non transferred arc). Plasma arc welding (PAW) is an arc welding process similar to gas tungsten arc welding (GTAW). The electric arc is formed between an electrode (which is usually but not always made of sintered tungsten) and the work piece. The key difference from GTAW is that in PAW, by positioning the electrode within the body of the torch, the plasma arc can be separated from the shielding gas envelope. The plasma is then forced through a fine-bore copper nozzle which constricts the arc and the plasma exits the orifice at high velocities (approaching the speed of sound) and a temperature approaching 20,000 °C. Plasma arc welding is advancement over the GTAW process. This process uses a non-consumable tungsten electrode and an arc constricted through a fine-bore copper nozzle. PAW can be used to join all metals that are weldable with GTAW (i.e., most commercial metals and alloys). Several basic PAW process variations are possible by varying the current.
The plasma arc welding provides better directional control of the arc and smaller heat affected zones as compared to GTAW. The major disadvantage of plasma arc welding is the relatively high expense of the equipment. Also compared to GTAW, there are more process control variables, requiring more complex welding procedures and more extensive operator training.
HEAT AFFECTED ZONE:
The heat-affected zone (HAZ) is the area of base material, either a metal or a thermoplastic, which has had its microstructure and properties altered by welding or heat intensive cutting operations. The heat from the welding process and subsequent re-cooling causes this change in the area surrounding the weld. The extent and magnitude of property change depends primarily on the base material, the weld filler metal, and the amount and concentration of heat input by the welding process.
HISTORY OF PLASMA ARCS:
One of the earliest plasma arc systems was a gas vortex stabilized device introduced by schonherr in 1909. In this unit, gas was blown tangentially into a tube through which an arc was struck. The centrifugal force of the gas stabilized the arc along the axis of the tube by creating a low pressure axial core. Arcs up to several meters in length were produced, and the system proved useful for arc studies.
Gerdien and lotz built a water vertex arc stabilizing device in 1922. In this device, water injected tangentially into the center of a tube was swirled around the inner surface and ejected at the ends. When an arc struck between carbon electrodes was passed through the tube, the water concentrated the arc along its axis, producing higher current densities and temperatures than were otherwise available. The Gerdien and lotz invention had no practical metal working applications because of the rapid consumption of its carbon electrodes and the presence of water vapour in the plasma jets.