Plasma welding can be seen as a further development of TIG welding. The most important difference between TIG and plasma welding is that with plasma welding the arc can be constricted and the process is kept in standby by a pilot arc. Plasma welding is a gas arc welding process where an arc is maintained between a non-melting Wolfram electrode, and the work piece itself, or a secondary anode (cathode when AC) in the welding torch. The weld charge and the electrode are protected by an inert gas. In ISO 4063 the process is referred to as process number 15.
How does it work?
In the Plasma welding process, the arc is made by way of a high voltage discharge between the work piece and a non-melting electrode that is surrounded by an inert gas. The plasma arc created in the inert gas takes care of the heat transfer to the work piece. The inert gas consists of Argon, Helium or a mixture of both and it also protects the weld charge and the non-melting electrode.
In contrast to TIG welding a nozzle is fitted around the non-melting electrode that can function as a secondary anode (cathode with AC) and/or as a constriction nozzle for the plasma arc. With a secondary protective gas having a different composition, the arc characteristics can be further influenced.
Depending on the application, an additional material can be used that is melted in the arc atmosphere. Regardless of the power applied, either more or less additional material can be used so that the supply of material can be regulated separately along with the desired heat transfer.
The figure below gives a schematic view of the Plasma process.
Difference between TIG and Plasma
The arc is formed by the nozzle into a cylindrical shape in contrast to the conical form of the TIG weld. By adding helium or hydrogen, this cylindrical form can be more intensively influenced. The burning-in shape and depth can therefore be variable.
Most important process variations
- Plasma welding with a transfer arc, direct current and negative electrode.
- This is the most common set-up. In this case current runs through the plasma arc to the work piece. The work piece is the anode (+).
- Plasma welding with non-transferable arc.
- The nozzle acts as the anode here. The work piece is therefore not connected to the current source.
- Plasma welding with pilot arc.
- A combination of both the previous connections is used here. The non-transferable plasma arc is started first. This is then used as the pilot or auxiliary arc. As soon as it comes into contact with the work piece, the main arc is ignited. The current that goes through the work piece can now be adjusted independent of the plasma current.
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