The welding arc is ignited by direct contact of the electrode with the workpiece. Due to a short circuit a high current density is created in the contact area. By this means, the melting phase and the metal vapour is formed, by which the ionisation can start quickly. The arc increases as soon as a thermo-dynamic balance is reached.
In the case of contact ignition the current is started by a quick touch of both electrodes. By slowly moving back one of the electrodes the arc ignites. Also the ignition with a “fuse ball”, a ball of steel wool, is a kind of contact ignition since it melts through due to the high current density after the contact and sets free the gap between both electrodes for the arc formation.
Touching an electrode with the workpiece leads to a short circuit. The contact area between the poles is, however, quite small. Even in the case of mechanically treated contact areas not more than 0.1 % to 1 % of the surfaces touch each other. This leads to a high specific amperage at the touching points.
If an amperage of 200 A and an electrode diameter of 4 mm is used it results, at a contact area of 1 %, at current density of about 1600 A/mm² and, at a contact area of 0.1 %, at current density of about 16000 A/mm².
In reality the current density at the touching points will be according to the dynamic steepness of the source characteristics about two or three times higher. In the moment of the short circuit at first the joint short circuit and then the continuous short circuit is set up.
The high current density leads by resistance heating at the contact areas to local melting and to a high overheating of the liquid metal. Metal vapour is formed which can be easily ionised in the arc gap.
After the short circuit the electrode has to be immediately lifted up in order to ignite the arc. A certain voltage is necessary which is not at disposal during the short circuit with a voltage of almost zero. The charged particles already placed in the arc gap are now accelerated by the voltage and create additional charged particles by shock ionisation. Others are added by thermal and field emission. The arc ignites after reaching the necessary ignition voltage and comes up to a working point A corresponding to the set up source characteristic and the respected arc length.
The working point is the intersection point of the static characteristic of the power supply with the arc characteristic. Lifting up the electrode after the short circuit is not necessary when metal shielding gas welding since in this case the current density is high enough to enable the arc to detach itself after vaporising of the metal bridge.
The contactless ignition cannot always be used because the surface gets contaminated or the electrode can be destroyed. In such cases ignition auxiliaries are needed; high frequency of high voltage impulses are used. High frequent amperages which exceed the welding current are due to a possible distortion of the wireless reception more and more replaced by high voltage impulse ignition machines. In both cases a radio path between electrode and workpiece builds up and a pre-ionisation is created in order to enable a contactless ignition of a distance of 2 to 3 mm which can be approached after turning on the welding current.