Plasma arc welding - SIAD Società Italiana Acetilene e Derivati
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Plasma Arc Welding
Plasma Arc welding (PAW) takes advantage of the properties of a particular state of matter known as “Plasma”.
Plasma is generated from gas or gas mixtures, the atoms of which have become highly ionized as they pass through an electric arc and are, therefore, capable of absorbing and conducting electrical energy.
Well focused jets of plasma of greatly reduced dimensions and high power density are obtained thanks to “squeezing” the plasma through special nozzles with specially calibrated and opportunely cooled holes.
The arc can occur between an infusible electrode and the weld piece (transferred arc) or between the electrode and nozzle (non-transferred arc).
Protection of the weld pool is guaranteed by a coaxial flow of a shielding gas or gas mixture from the Stargas® range.
Plasma arc welding can be applied with or without a filler metal.
A variation of the plasma process is the “keyhole technique”. This involves instantaneous vaporisation of the material thereby creating a passing hole and, thanks to the pressure of the metal vapour, the weld pool moves to the sides and then closes back; the balance between these two factors and the choice of the most appropriate gas mixture determines the depth reached by the jet and, therefore, the level of penetration enabling high productivity.
The “keyhole” procedure has two types of execution: none penetrating and complete penetration.
This system, which was created for the welding of aluminium, stainless steel, copper, alloys of nickel and alloyed steels, has been successfully extended for use in a variety of other sectors:
The use of heavily ionized gases or gas mixtures and a restricted electric arc makes it possible to obtain Plasma jets with high kinetic and thermal characteristics, which are capable of transporting high amounts of energy
The plasma process uses two separate gas flows:
- Plasma Gas : is highly ionisable and able to generate high powered Plasma jets.
- Shielding Gas: prevents contact between the weld pool and the surrounding atmospheric air.
The choice of these gases or gas mixtures is based on the fact that they can determine the shape and extent of the Plasma jet and a good outcome for the weld, which is flawless and has excellent mechanical resistance.
Thanks to experimentation with Plasma technology, SIAD developed mixtures for theStargas® line specifically for this welding process. These gases guarantee higher quality of the weld, increased speed in working and, therefore, better performance and improvements in the work environment.
ARGON 5.0, thanks to it low potential for ionisation and its superior purity, is used in producing the Plasma jet, increasing the cleanliness and stability of the jet. When used as a shielding gas it improves the quality of the welded joint.
Thanks to the high thermal conductivity of Helium, HELISTAR, mixtures enable increases in productivity of the process to be reached, by creating more liquid weld pools which, in turn, increase speed and quality. Helium, a gas with a high potential for ionisation, makes it possible to control the Plasma generated.
HYDROSTAR mixtures, thanks to the disassociation and reassociation properties of their hydrogen molecules combined with Argon and other gases, reduce the formation of oxides and favour penetration in the welding of stainless and austenitic steels.