Ionized plasma flow was first introduced as a source of energy in 50-ies, but only recently they have become widely used. The process is based on ionization of plasma gas using an electric arc and its focusing with a specially designed plasma gun tip. Such focusing gives highly intensive and concentrated energy flow in the welding area.
There are two different designs, which use direct and indirect arc. Plasma arc welding with direct arc is used for metal welding more often and it can basically be described as an improved TIG process. Plasma gas is ionized in the plasma gun and creates a stable discharge in the working area even at low power rates. This makes the process suitable for welding of micro elements. For thicknesses up to 3mm plasma arc welding and TIG process are quite similar. But for thicker materials plasma arc provides more power density and deeper penetration and welding speed. At the same time residual strain (buckling) of parts is less than that if TIG process.
Plasma arc welding uses gases for 3 different purposes:
Plasma gas. This is the medium, which forms plasma flow between the electrode and the welding area. Argon or VARIGON argon and hydrogen welding mixtures are often used as plasma gas for stainless steels and argon and helium mixtures for non-ferrous metals.
Shielding gas. An isolating medium is required for protection of the pool and the heat-affected zone. Shielding gas is fed through a dedicated (external) nozzle of the welding gun and acts as a protective coating on plasma gas. Depending on the type of gas, power density at the arc may even be increased by this coating. Usually the same gas is used both for shielding and plasma.
Secondary shielding gas. If protection of heated back surface of a part is also required, shielding gas is fed separately to the weld root area. High purity argon and specially designed VARIGON mixtures are used for this purpose. It is recommended to use special regulators with Т-connectors and two flow meters, so that gas may be fed both to the welding gun and to the weld root from the same cylinder.
Secondary weld root shielding
Gas shielding used in welding may be split into three categories: primary gas shielding, secondary gas shielding and weld root shielding. All three categories may be required in certain cases, for example for welding of metals, which are sensitive to atmospheric air or nitrogen.
Primary gas shielding is used for isolation of the arc and molten pool from ambient air. This function is performed by the welding gun nozzle.
Secondary gas shielding is used for heat affected zone protection until it reaches the temperature, at which metal does not react with oxygen or nitrogen of ambient air. Secondary shielding is usually ensured by different devices.
Back side of weld root shall also be shielded by gas to avoid oxidation. Moreover, proper selection of gas or welding mixture improves the appearance of the root and minimizes the risk of root defects.
A peculiar thing about secondary and weld root shielding is that the shielded area shall be purged with shielding gas before welding starts so that the content of oxygen is reduced to the required level. High purity argon, helium or their mixtures or specially designed VARIGON welding mixtures with dosed hydrogen; nitrogen etc. addition may be used for shielding.
Special valves with a T-connector and two flow meters, which can control required gas flow at each channel, may be used to ensure simultaneous shielding gas supply to the gun and to the backside protection system using the same gas cylinder.