Plasma arc welding and its methods

Plasma arc welding (PAW) is an arc welding method identical to gas tungsten arc welding (GTAW). The electric arc is composed between an electrode (which is usually but not always made of sintered tungsten) and the work piece. The crucial contrast from GTAW is that in PAW, by setting the electrode inside the body of the copper nozzle in the torch, the plasma arc can be detached from the shielding gas envelope. The plasma is then forced through a fine-bore copper nozzle which restricts the arc and the plasma retreats the orifice at high velocities (approaching the speed of sound) and a temperature nearing 28,000 °C (50,000 °F) or higher in the form of straight beam. Arc plasma is the non-permanent state of a gas. The gas gets ionized following the movement of electric current through it and it turns into a conductor of electricity. In ionized condition, atoms penetrate into electrons (−) and cations (+) and the structure contains a combination of ions, electrons and highly excited atoms. The amount of ionization may be ranging between 1% and greater than 100% i.e.; double and triple amounts of ionization. Such condition prevails as more electrons are extracted from their orbits.

The energy of the plasma jet and hence the temperature is depending on the electrical power involved to produce arc plasma. A usual value of temperature acquired in a plasma jet torch is generally of the order of 28000 °C (50000 °F)in contrast to 5500 °C (10000 °F) in ordinary electric welding arc. In fact, all welding arcs are (partly ionized) plasmas, but the one in plasma arc welding is constricted arc plasma.

 

Plasma arc welding methods can be segregated into two basic types:

 

  1. Non-transferred arc method

 

The arc is composed involving the electrode (-) and the water-cooled constricting nozzle (+). Arc plasma emerges out of the nozzle as a flame. The arc is self-sufficient of the work piece and the work piece does not constitute a part of the electrical circuit. Precisely like an arc flame (as in atomic hydrogen welding), it can be shifted from one place to another and can be supervised in a good way. The non transferred plasma arc have comparatively less energy density as contrasted to a transferred arc plasma and it is involved for welding and in importance involving ceramics or metal plating (spraying). This method can produce high-density metal coatings. Using a high frequency unit in the circuit commences a non-transferred arc.

 

  1. Transferred arc method

 

The arc is composed involving the electrode (-) and the work piece (+). In general sense, arc is transported from the electrode to the work piece. A transferred arc has high energy density and plasma jet velocity. So, it is involved to cut and spray or coat the metals. Apart from carbon steels this method can cut stainless steel and nonferrous metals where an oxyacetylene torch does not triumph. Transferred arc can also be applied for welding at high arc travel speeds. For commencing a transferred arc, a current limiting resistor is placed in the circuit, permitting a flow of about 50 amps, across the nozzle and electrode and a pilot arc is fixed between the electrode and the nozzle. As the pilot arc touches the job main current begins to flow between electrode and job, hence igniting the transferred arc. The pilot arc-commencing unit gets detached and pilot arc terminates as soon as the main arc connecting the electrode and the job is initiated. The temperature of a constricted plasma arc may be of the order of 8000 – 250000C.

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