What is Plasma?
Plasma is one of the four fundamental states of matter, first systematically studied by Irving Langmuir in the 1920s. It consists of a gas of ions atoms or molecules that have one or more orbital electrons stripped (or, rarely, an extra electron attached), and free electrons.
Excluding dark matter and the even more elusive dark energy, plasma is the most abundant form of ordinary matter in the universe. Plasma is mostly associated with stars, including our Sun, and extending to the rarefied intracultural medium and possibly to the intergalactic regions.
Plasma can be artificially generated by heating a neutral gas or subjecting it to a strong electromagnetic field. The presence of free-charged particles makes plasma electrically conductive, with the dynamics of individual particles and macroscopic plasma motion governed by collective electromagnetic fields and very sensitive to externally applied fields.
The response of the plasma to electromagnetic fields is used in many modern technological devices, such as plasma televisions or plasma etching.
What is Plasma Arc Machining?
Plasma Arc Machining is used to remove material from the workpiece. In this process, a high-velocity jet of high-temperature gas is used to melt and remove material from the workpiece. This high velocity of hot gas is also known as a plasma jet.
When a gas or air is heated at a temperature of more than 5000 °C, then it will start getting ionized into positive ions, negative ions, and neutral ions. When the gas or air is ionized its temperature reaches from 11000 °C to 28000 °C and this ionized gas is called plasma.
The gas or air is heated with arc and the plasma produced by heating gas is used to remove material from the workpiece. So, the whole process is called Plasma Arc Machining.
In this process, a high velocity of high-temperature air is used to remove material from the workpiece by melting it.
The gas used in plasma arc machining is chosen according to the metal which is used as the workpiece.
Working of Plasma Arc Machining
The basic principle is that the arc formed between the electrode and the workpiece is constricted by a fine bore, copper nozzle. This increases the temperature and velocity of the plasma emanating from the nozzle.
The temperature of the plasma is in excess of 20 000°C and the velocity can approach the speed of sound. When used for cutting, the plasma gas flow is increased so that the deeply penetrating plasma jet cuts through the material and molten material is removed in the efflux plasma.
Plasma arc machining consists of a Plasma gun. Plasma gun has an electrode made up of tungsten situated in the chamber. Here, this tungsten electrode is connected to the negative terminal of the DC power supply. Thus, the tungsten acts as a cathode.
While the positive terminal of the DC power supply is connected to the nozzle. Thus, the nozzle of the plasma gun acts as an anode.
As we give the power supply to the system, an electric arc develops between the cathodic tungsten electrode and an anodic nozzle. As the gas comes in contact with the plasma, there is a collision between the atoms of gas and electrons of an electric arc and as a result, we get an ionized gas.
That, means we get the plasma state that we wanted for Plasma Arc machining. Now, this plasma is targeted towards the workpiece with a high velocity and the machining process starts. One thing to note down is that a high potential difference is applied in order to get the plasma state.
In the whole process, high-temperature conditions are required. As hot gases come out of the nozzle there are chances of overheating. In order to prevent this overheating, a water jacket is used.
Components of Plasma Arc Machining
1. Plasma Gun
Different gases like nitrogen, hydrogen, argon or a mixture of these gases are used to create plasma. This plasma gun has a chamber that has a tungsten electrode. This tungsten electrode is connected to the negative terminal and the nozzle of the plasma gun is connected to the positive terminal of the DC power supply. The required mixture of gas is supplied to the gun. A strong arc is produced between the anode and the cathode.
After that, there is a collision between the electron of the arc and the molecules of the gas and due to this collision, gas molecules get ionized and heat is generated.
2. Power Supply
DC Power Supply is used to develop two terminals in the plasma gun. Heavy potential difference is applied across cathode and anode so that the arc produced is strong and is able to ionize the gas mixture and convert it into plasma.
3. Cooling Mechanism
A cooling mechanism is added to the plasma gun as heat is produced in it as hot gases continuously pass out from the nozzle.
A water jacket is used to cool the nozzle. The nozzle is surrounded by a water jet.
Different materials can be worked using this plasma arc machining. Different metals like aluminum, magnesium, carbon, stainless steel, and alloy steels can be worked using this process.
Applications of Plasma Arc Machining
- It is used for cutting alloy steels, stainless steel, cast iron, copper, nickel, titanium, aluminum, and alloy of copper and nickel, etc.
- It is used for profile cutting.
- It is successfully used for turning and milling of hard to machine materials.
- It can be used for stack cutting, shape cutting, piercing, and underwater cutting.
- Uniform thin film spraying of refractory materials on different metals, plastics, ceramics are also done by plasma arcs.
Advantages Of Plasma Arc Machining
- It can be used to cut any metal.
- The cutting rate is high.
- As compared to the ordinary flame cutting process, it can cut plain carbon steel four times faster.
- It is used for rough turning of very difficult materials.
- Due to the high speed of cutting, the deformation of sheet metal is reduced while the width of the cut is minimum and the surface quality is high.
Disadvantages of Plasma Arc Machining
- It produces a tapered surface.
- The protection of noise is necessary.
- The equipment cost is high.
- Protection of eyes is necessary for the operator and persons working in nearby areas.
- Oxidation and scale formation takes place. So, it requires shielding.
- The work surface may undergo metallurgical changes.