In this article we will discuss about:- 1. Meaning of Plasma Arc Welding 2. Applications of Plasma Arc Welding 3. Advantages 4. Limitations.
Meaning of Plasma Arc Welding:
Plasma is defined as a gas heated to at least practically ionized condition, enabling it to conduct an electric current. Plasma arc refers to a constricted electric arc which is achieved by passing the arc through a water cooled copper orifice. Its aim is to obtain high power density of the arc stream.
The plasma arc welding is similar to GTAW with the exception that it employs a pilot arc starting circuit and constricting orifice. The arc is collimated and focused by the constricting nozzle made of copper block with cooling water duct and is less sensitive to variation of torch-to-work distance. The two important dimensions in the construction are orifice diameter and throat length.
Applications of Plasma Arc Welding:
Plasma arc welding is widely used for welding stainless steels, nickel alloys, refractory, metals in aero-space industries. It is very well suited for refractory metal coating and coating of refractory (high temperature) materials like alumina on graphite nozzles for rockets. Plasma arc technique is also used for cutting.
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Two techniques in common use are:
(1) Transferred arc (the work is part of the electrical circuit and an arc is created between the electrode and the work in addition to the arc between the electrode and the nozzle) and
(2) Non-transferred arc (the arc is struck between the electrode and the torch nozzle; the work is not part of the electric circuit).
Transferred arc technique is best suited for cutting stainless steel plates (of 6 mm thickness at 2-5 m/min).
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Ionised gases (helium or hydrogen) are forced through the arc and nozzle (at a flow rate of 1.5 to 15 litres per min) with the result that these get ionised and become plasma. The plasma stream of ionised particles thus produced from the nozzle may be used for both welding and cutting.
Plasma jet is surrounded with shielding gas forced around nozzle at 10—30 litre/min. In plasma arc welding voltage is between 27 and 31 volts and direct current pulses (6 to 60 cycles/sec, slower pulses having better control in the puddle) are used. The torch is held almost vertical to the base metal surface and filler metal wire fed at an angle.
Because the ion impingement has the ability to remove oxide films from the base metal surface, plasma arc welding of aluminium and other space-age metals is very important. The plasma arc is not affected by magnetic field and its temperature is around 10,000—14,000°K. In this process ear snuffs are needed to reduce the discomfort. It is generally operated automatically.
Advantages of Plasma Arc Welding:
Advantages of plasma arc welding are given below:
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i. Plasma arc has directional stability. Focusing effect is brought about by constricting the arc. It is insensitive to torch stand-off distance.
ii. Work to torch distance is not critical and arc length can vary. No contamination with tungsten takes place. Weld uniformity is possible due to complete penetration.
iii. Due to greater penetration greater depth/width ratio is possible with less distortion.
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iv. It involves lower heat input and requires less filler metal.
v. There is less time for embrittlement of stainless steel due to formation of carbides.
vi. No edge preparation is needed (a square groove is sufficient) and it requires few passages.
vii. Possibility of human error is minimised.
Limitations of Plasma Arc Welding:
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Followings are the major limitations of plasma arc welding:
a. Welding torch and equipment are complicated and expensive. Mechanised process is restricted to flat and horizontal positions.
b. Maximum metal thickness can be 25 mm for butt welds.
c. Excessive high noise is produced and large amount of ultraviolet and infra-red rays are emitted. The operator must be well protected from exposure of skin apart from eye and ear protection.