Plasma Arc Machining
When a gas is heated to a temperature above 5500°C. it becomes partially ionized and exists in the form of a mixture of free electrons, positively charged ions and neutral atoms and this mixture is called plasma. The temperature of the central part of the plasma may go as high as 11,000 to 28000°C. Plasma arc machining is a material removal process wherein the material is removed by directing a high velocity jet of high temperature ionized gas on the work. A plasma torch is used for making plasma and for directing it for use in cutting.
Figure shows a plasma gun. A gas such as argon, nitrogen, hydrogen, compressed air or mixture thereof is passed through a small chamber in which a high frequency spark (arc) is maintained between the tungsten electrode (cathode) and the copper nozzle (anode), both being water cooled. Two types of plasma arc systems, namely
(i) transferred arc and
(ii) non-transferred arc, are discussed under Section 7.19.7.
As the gas molecules collide with the high velocity electrons of the arc, the gas gets ionized and a large amount of heat energy is liberated. This high velocity stream of hot ionized gas (called plasma) is directed on to the workpiece to melt its material and also to blow it away. Typical gas flow rate is 2 to 11 m³/hr. DC current rated at 400 V and 200 kW output is normally required. The arc current ranges between 150 and 1000 amp.
PAM is mostly used to cut stainless steel and aluminium alloys, particularly for their profile cutting. PAM is also used for machining of ‘hard to machine’ materials. Chemical industries and nuclear power plants make good use of this process.
Advantages and limitations:
It is equally effective for cutting any metal regardless of its hardness or refractory nature. The process may, however, bring metallurgical changes in the work surface. Adequate safety is needed for the operator.
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