Dec 192013
 
Welding arc proprietes logo

Many important joining processes make use of the arc as a tool and energy source. The arc performs the penetrations and deposits the filler. Furthermore it supplies energy for physical and metallurgical processes which are important for welding.

As glow discharge or real or artificial flash also the arc is a special kind of gas discharge which is only possible if there are enough carriers for the energy and mass transport of the carriers on the arc gap.
The welding current is led in form of an arc between electrode and welding pool. The resistance heat from the arc also serves to deposit the base and filler material. In general a shielding gas (according to the process by burning the coating material or added separately) has to protect the arc as well as the molten metal phase from atmospheric components.

Melting of a covered stick electrode
In this case the electrode is filler metal and arc-producing electrode at the same time. The electrode shown in the picture consists of a metal core wire which is covered by a coating consisting of consumables and alloying elements.
When welding with direct current the kind of polarity which the welding is performed with is always referring to the electrode side.

Creation of charged particles
Because the air in-between is a bad conductor there is usually no current between two poles with different potentials which are held at a certain distance to each other. However, under certain conditions a current flow can be achieved. If the distance between the poles is low and the voltage is high a gas discharge takes place. The air in-between the two poles becomes electrical conducting by ionisation. This happens in the case of gas discharge by shock ionisation. The gases to use (shielding gases such as Ar of CO2) as well as solid materials (metal atoms) are dissociated and ionised in order to preserve charged particles.
Examples of a dissociation of molecules and the ionisation of atoms are listed on the right side. The processes take place by adding energy. The energy which is necessary to detach an electron from the corresponding atom is defined as ionisation energy.

Dissociation is the splitting of electrolytes into its ion components. The extent of dissociation increases with rising temperature and dilution. The extent of dissociation is the relation of dissociated molecules to original molecules.
The splitting of molecules into single atoms which takes place at higher temperatures is also defined as dissociation.
Ionisation is the detachment of electrons.

The charged particles of the arc can also be detached directly from the metal lattice ( e.g. electrode surface) with high temperature (thermal emission). Via the electrostatic field the welding amperage (field emission) defines the corresponding direction of movement.
Indicated in the following list are a few dissociation and ionisation energies for the creation of charged particles by joint ionisation and thermal emission in the arc area.

Thermal emission – In metal the by the field absorbed number of electrons increases with rising temperature. With a temperature of about 3500°C there is a sharp increase of electron current density. In this case we talk of a thermal emission. Since most of the metals cannot approach such high temperatures, due to their boiling point below 3500°C, the thermal arc is mainly found when the cathode consists of a material with high melting point such as tungsten. A thermal emission can also not be completely excluded in the case of other metals since with a sudden heating up of metals a boiling point increase can occur and the boiling points of some metal oxides are much higher as those of pure metals.

Field emision – Every metal sends out electrons which surround the metal in a in a thin layer if there is no electric field. In the case of an electric field they are extracted into the direction of the anode and make place for new electrodes which can now escape. Concerning the electrodes and ions produced this way it is to say that the higher the amount the higher the field intensity. This intensity is mainly dependent on the voltage. The not compensaved positive charge which remains at the point of extraction tries to hold back the electrons. In order to remove the electrons out of the cathode a special energy has to be used. This energy is called electron affinity and is also measured in electron volt. The electron affinity is a specific characteristic of a medium and differences from base material to base material. The chart shows the electron affinity of different metals.

Metal                                                                                Work function [ eV ]
Aluminium                                                                                3.95
Barium                                                                                        2.29
Caesium                                                                                      1.36
Iron                                                                                              4.79
Copper                                                                                         4.82
Thorium                                                                                      3.57
Tungsten                                                                                     5.36
Tungsten with thorium skin                                                  2.62

Joint ionisation – There are always a few free electrons in the atmosphere. A possible source is the cosmic radiation. If this radiation meets an electron which orbits an atom it can be brought of its circuit. The remaining atom is then turned into a positive ion. The free electrons are accelerated by the potential difference and collide with other electrons which are detached. This leads to a chain reaction. This described process of detaching electrons is called joint ionisation. This mechanism has the greatest importance for the arc formation.

 

Charged particles consisting of a few molecules/atoms

Dissociation
N2     ->     N + N
H2     ->     H + H
CO2     ->     CO + O

Ionisation
N     ->     N+ + e-
H     ->     H+ + e-
Ar     ->     Ar+ + e- + 16 eV
K     ->     K+ + e- + 4 eV
Fe     ->     Fe+ + e- + 7 eV

Charged particles in the arc area

The picture describes the direction of the flow of the charged particles between anode and cathode. Besides a high amount of electrons with a negative charge moving from the cathode to the anode, anions e.g. OH–ions are floating in the same direction.
transfer

Ions with a positive charge (cations) e.g. Fe+- or H+-ions are moving to the negative pole.
As a result the direction of the flow goes in both directions. (ambipolar flow).

Arc plasma: state of aggregation in the presence of
-Atoms,
-Molecules,
-Ion
-and electrons. 

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