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A number of different Plasma (arc) methods are used in industry with respect to the nozzle configuration and the discharge path (transferred/non-transferred arc).
Plasma welding with transferred arc is predominant in welding of metals and can be described as an advanced TIG process. The principal difference is a laced discharge path, resulting in a high intensity focused arc. The arc is focused through a special nozzle design and an additional gas shroud. The Plasma gas becomes ionized in the nozzle and provides a stable discharge path towards the work piece even at low power. As a result, Plasma welding can be used for microapplications, in contrast to TIG. Up to about 3 mm material thickness, Plasma and TIG welding are comparable. But when welding thicker materials, a keyhole is formed in Plasma welding due to the higher energy concentration in the Plasma arc. This keyhole supports a deep penetration of the arc and a higher welding speed, and thus a reduced distortion compared to TIG welding.
Gases are used for three different purposes in Plasma welding:
- The Plasma gas is the medium that forms the plasma between the electrode and the work piece. Argon or argon/hydrogen mixtures are often used as plasma gas for stainless steels, and argon/helium mixtures for non-ferrous metals.
- A separate shielding gas is required to protect the weld pool and the heat-affected material. The shielding gas is supplied through an outer nozzle that encloses the plasma gas stream like a shroud. Depending on the type of gas, the concentration of the arc can even be increased by the shielding gas shroud. Nevertheless, the same gas is often used for the shielding and the plasma gas.
- A backing gas is applied to the root side through trailing shield & root protection devices, since molten and heat-affected material at the root side must also be shielded. The backing gas usually consists of argon and argon/hydrogen mixtures.
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