PLASMA ARC MACHINING: DEFINITION, SYSTEMS, PROS & CONS

DEFINITION

A fabrication process which uses superheated, ionized gas passed through a plasma torch to heat, melt and cut electrically, materials which are conductive and contain custom shapes and designs is known as plasma arc machining.

PROCESS CHARACTERISTICS

When a gas is heated to a high temperature of approximately 2000 °C, its molecules separate into atoms.

If the gas temperature rises above 3000 °C, the electrons of some atoms will dissociate and the gas will ionize; that is, it is composed of free electrons, positively charged ions, and neutral atoms.

This state of ionized gas is called plasma gas and is characterized by high electrical conductivity.

The plasma arc is activated by high frequency sparks in a confined gas filled chamber.

(a) Transferred plasma torch & (b) Non transferred plasma torch

(a) Transferred plasma torch & (b) Non transferred plasma torch

Direct current from a high voltage source gives the arc and plasma stream that exist from the nozzle, the speed of sonic.

The source of heat in plasma is due to the recombination of electrons and ions in atoms and the recombination of atoms in molecules.

The binding energy released is responsible for increasing the kinetic energy of the atoms and molecules formed by regrouping.

The temperature associated with the rearrangement can reach to 20,000-30,000 °C.

High temperatures like these will melt or even evaporate any work material subjected to machining or cutting.

Plasma Arc Cutting (PAC) is a thermal process that was adopted in the early 1950s as an alternative to oxyfuel cutting of stainless steel, aluminum, and other non-ferrous metals.

Recently, the plasma arc machining of conductive and non-conductive materials has become more attractive.

The main attraction of plasma arc cutting is it’s ability to cut faster in stainless steel than in mild steel.

PLASMA ARC CUTTING SYSTEMS

Plasma arc machining system can operate in transfer arc mode or non-transfer jet mode.

In transfer arc mode, the arc is directed from the negative electrode on the back of the plasma torch to the conductive workpiece (+ ve electrode), resulting in temperatures up to 33,000 °C.

Because transfer systems are more efficient, they are generally used to cut any conductive material, including materials that have high electrical and thermal conductivity and are resistant to cutting with oxyfuel, such as aluminum.

In non-transferred jet mode, the arc is generated by the torch itself.

The plasma is ejected through the nozzle hole in a jet pattern, causing the temperature to rise by about 16,000 °C.

Since the torch itself is switched to anode, most of the heat from the anode is removed by the cooling water, so it cannot be used effectively in the material removal process.

Non-conductive materials that are difficult to cut by other methods can generally be successfully cut using a plasma non transfer system.

TYPICAL TRANSFER OF PLASMA TORCH STRUCTURAL COMPONENTS

The diameter of the nozzle depends on the arc current and the flow of the working gas which ranges from 1.2 to 6 mm.

The fine nozzle with a diameter of 50 µm is specially designed for cutting metals with a width of cut of 0.1 mm and operates at a low power of 1 kW.

It can perform multiple torch cuts on the plotter controlled cutting table, cutting stainless steel plates up to 150 mm thick.

Commonly used working gases are He, Ar, H2, N2 or their mixtures.

The gas flow varies from 0.5 to 6 m3/h, depending on the power of the arc and the thickness of the plate.

The non-consumable electrode is made of 2% thorium tungsten to resist wear.

Shield plasma torch can be gas or water.

Gas shielded plasma cutting torch when cutting aluminum, stainless steel and mild steel, use shielding gas to obtain acceptable quality cuts.

Adds an external gas shield (N2 or Ar/H2) around the main plasma stream.

CO2 shielding is beneficial to ferrous metals.

For mild steel, air or O2 can also be used as a shielding gas.

N2 is used as the main working gas, and the shielding layer is a water curtain.

Water shielded torch

Water shielded torch

It is reported that the cooling effect of water can reduce the cutting width and improve the cutting quality; however, the cutting speed has not increased.


APPLICATIONS AND CAPABILITIES OF PLASMA ARC MACHINING

The PAC has many applications. Some examples are listed below:

Assembly of Transferred plasma torch

Assembly of Transferred plasma torch


1. PAC is particularly attractive when cutting metal profiles such as stainless steel.

2. The advantage of oxyacetylene flame cutting is that it can cut metal from parts heavier than PAC.

For this reason, launching of dual operating systems (plasma/flame) is done in the market.

The dual system has an extended range of applications, covering all materials.

3. As a non-traditional tool, plasma arc is combined with some traditional processes such as turning, milling and forming.

Plasma Arc Turning (PAT) design for roughing and smoothing of traditional difficult machines.

Materials such as Inconel, Rene 41, Hastelloy and precipitation hardened stainless steel.

According to the material to be processed and the required finish, the cutting speed range is 10-100 m/min, and the nozzle feed speed is 1 to 5 mm/rev.

The nozzle is connected to a suitable SOD.

4. PAC is also used to cut non-conductive materials with a thickness of 0.1 to 1 mm at a high lateral speed of 1000 m/min, such as textiles, nylon and polypropylene.

The working gas must be Ar or Ar/H2. In this case, use no transfer nozzles.

ADVANTAGES OF PLASMA ARC MACHINING

This process provides a smooth cut without contamination.

Plasma arc cutting process can cut exotic metals at high speed.

This process has the smallest specific cutting energy.

DISADVANTAGES OF PLASMA ARC MACHINING

This process is expected to reduce precision and surface quality.

It requires high power.

Produces toxic fumes.

Owing to high thermal effects, the workpiece is highly distorted and HAZ of large depth reduces the fatigue resistance.

The plasma arc produces IR and UV radiations that cause eye injuries (cataracts) and loss of sleep.

UV radiation leads to skin cancer.

Therefore, gloves, goggles, and earplugs should be used.

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