Plasma Arc Technology

Plasma Arc Cutting

Index

Plasma Arc Cutting ........................................................................................................................................... 1

Fundamentals ............................................................................................................................................... 1

History .......................................................................................................................................................... 2

High Definition Plasma ................................................................................................................................. 3

Components ................................................................................................................................................. 4

Cutting: process parameters ........................................................................................................................ 5

Comparison with lasers ............................................................................................................................ 8

Other applications ........................................................................................................................................ 8

Welding .................................................................................................................................................... 8

Coatings and surface treatments.............................................................................................................. 9

Powder synthesis .................................................................................................................................... 10

Fundamentals

Plasma Arc Cutting is a thermal material removal process that is primarily used for cutting thick sections of

electrically conductive materials. Plasma can be defined as a superheated, electrically ionized gas, where

ions and electrons move independently from each other (electrically neutral medium of unbound positive

and negative particles). The process exploits a high velocity jet of plasma of 10000 - 14000 °C to cut the

metal by melting it. A gas flow rate facilitate the removal of molten metal through the kerf and the jet

pressures can reach values up to 1.4 MPa.

In termoelectrical cutting processes, electrical energy is responsible to ionize the gas, while thermal energy

corresponds to the energy transferred to the workpiece. The entire system needs a circuit to add energy to

the torch and another one to create the plasma. The primary gas is ionized while secondary inert gases assist

and protect the jet: they shield the working zone from oxidation and inclusions and facilitate the transport

of the melted/vaporized material away from it.

A plasma can be obtained ionizing a gas:

 1

High levels of energy density

 Macroscopic electrical neutrality

The spatial scale within electrons and ions move in an independent way is the Debye length. Furthermore,

plasma is sensitive to magnetic fields. High electrical conductivity is an important feature of this state of

matter: if it is constricted inside a nozzle reducing the section for the electric current, the energy generated

by Joule effect increases. It behaves as a conductor and no more as a dielectric (a gas).

Plasma acts as a metal for the current transport. A metal is suitable to be the anode for the plasma

12 3

generation. High power density can be achieved – of the order of 10 W/cm – consequently temperatures

10x melting point of metals.

1 Which depends on temperature and spatial density of charges: the increase of T increase the energy density, the

fraction of free charges and the electrical conductivity. 1

The start of the process can be critical, because the torch is motionless and creates a first hole in the

workpiece to begin the cut: molten metal droplets can reach the torch itself, damaging it. To avoid this, during

2

this phase the torch is at least at a distance 2*SOF .

Four phases of interaction between plasma and material occurs:

 Starting of fusion;

 Fused material is pushed down by pressure (imposed by plasma itself);

 At the surface, the presence of oxygen starts oxidation, increasing material fusion and removal;

 Pressure due to shielding gases and plasma removes molten material.

The Plasma Torch is the tool that allows the gas constriction. It

provides the generation and motion of plasma beam with respect to

the workpiece. An arc discharge occurs after the breakdown of the

3

dielectric when a gas is positioned between two electrodes of a

circuit: the current reaches values higher than 1 A with voltages of

the order of some tens of volts. Heating by the Joule effect of the

cathode (e.g. W) causes thermo-ionic emission. Shielding gas is

present to avoid oxidation and external inclusions.

Two configurations are possible:

 Transferred: the workpiece is a conductor, in particularly it is the

anode. High energy transfer is thus possible, leading to processes

such as welding and cutting.

 Non-transferred arc configuration: the material is not a conductor. The arc is started inside the torch

(a part inside the torch is the anode). Lower energy transfer is possible, leading to surface treatments.

History

Plasma technology developed taking inspiration from TIG welding: adding gas constriction – reducing the

orifice diameter to ≈ 5 mm – it was possible to reach a very high concentrations of energy through Joule

effect, obtaining T ≈ 20000 K. Keeping the same current it was possible