Gases for Plasma Processing of Materials

for the review of the possible use of gas for plasma cutting, welding, spraying.

Gases used for plasma processing of materials in a gas medium burner for plasma processing of materials should perform the following functions:
1) to protect against oxidation and cooling of the tungsten electrode and the nozzle;

2) to ensure receipt of a stabilized plasma jet with the necessary temperature and speed;

3) provide the best heat transfer to the product;

4) for surfacing and spraying, to provide transportation of the material.

Sometimes the gases are separated into plasma-forming and protective (transporting). When filing separate plasma-forming gas is fed into the cathode zone, and the protective or transporting gas - into the zone of the column or plasma torch.

Each gas shows itself in its own way, has its positive and negative properties that are most effective for welding gas mixture.

To protect the cathode and the nozzle from damage and overheating of the best gas is argon, because it is chemically inert (does not interact with the cathode, is not dissolved in it) and has a low thermal conductivity. However, argon is not very effective for converting electrical energy into heat.

First, the field strength in argon arc column is smaller than in hydrogen, nitrogen, helium: EAG ≈ 0,8 V / mm; EN ≈ 2,0 V / mm, EH ≈ 10,0 V / mm (at I = 10 A). Consequently, for the same current in the argon arc is allocated within 1 mm of its length is less than the energy IE, than in others.

Second, the bulk heat content (enthalpy) of an argon plasma at a temperature of this plasma is also significantly smaller than the plasma of nitrogen or hydrogen (for N 2 -16; Ar - 3, H 2 -12 kW / m 3 at T = 10000 ° C) .

However, the plasma temperature depends strongly on the properties of the plasma gas: Ar, and not for Tm = 15000 ... 25 000 ° C, which is 3 ... 4 times higher than for N 2 and H 2 .
suitable for stabilizing the arc gas may be nitrogen (or air containing 78% nitrogen), since its enthalpy at T = 10000 ° C in a 5 times greater than the enthalpy of argon, and in addition, nitrogen is much cheaper.

On the other hand, in air and nitrogen tungsten electrode breaks down intensively, while in argon and helium, tungsten, rather racks.

Helium and hydrogen at T = 10000 ° C have a high thermal conductivity, only 2 times smaller than that of copper, and better than any other gas is converted into heat energy of the arc. If you are using them in its pure form is a fast heating and destruction of the nozzle, so the gases used in mixtures with argon. For example, the addition of hydrogen to argon in a ratio of 2:1 by volume can increase the thermal capacity of the jet is almost 2-fold compared with a mixture of argon-nitrogen in the same proportion. The voltage hydrogen arc plasma 100 ... 120 V, which is 2 ... 3 times higher than that of the arc in argon. Hydrogen plasma - the best converter arc energy into heat.

See also:
Plasma Welding
Plasma Welding Introduction
Plasma Welding Technique
Microplasma Welding
Gases for plasma processing of materials
Separation of plasma jet cutting
Compression of the arc
The energy properties of the plasma arc
Rationalization of plasma welding
Plasma welding and spraying
The plasma melting and remelting
Plasmatron. Requirements for plasmatron
Plasmatron. Schemes, classification
Classification by type of electrode plasma torches
Classification of torches by the nature of the current
Structure of the plasmatron basic units