Plasma arc welding is a common method of welding metals. The technique is reminiscent of TIG welding; the electric arc burns between a tungsten electrode and the work piece. The most important difference is that the arc in plasma welding is forced through a restriction in the form of a water-cooled nozzle. The arc is therefore very concentrated, which provides several advantages.
The greatest advantage of plasma welding is visible when welding thicker plates (2 to 8 mm), where the ‘key-hole’ technique can be used. The key-hole technique involves a powerful plasma arc melting a hole through the plate. When the burner is moved forward, material is also melted in front of the hole. This material is forced towards the back of the hole by the arc pressure, where it moves together due to surface tension and solidifies. A homogenous weld is created and complete full-penetration welding is attained.
Plasma welding gases have three purposes
Plasma gases create the plasma between the electrode and the work piece. When welding stainless steels, argon or a mixture of argon and hydrogen is used as the plasma gas. When welding non-ferrous metals, mixtures of argon and helium are used. In addition, a specific shielding gas is required to protect the molten pool and the area affected by the heat. The shielding gas exits from the outer nozzle so that it surrounds the plasma arc and prevents air from reaching the arc area and the weld pool.
Depending on which gas is used, the shielding gas can have a clear effect on the arc energy. Normally, shielding gas and plasma gas is the same. A root protection gas protects the molten weld pool and the area affected by the heat on the root side of the weld. A contact shoe or a rail is sometimes used for protecting heated areas further behind the weld pool.
The ultraviolet beam of the arc, the gases created during welding and other fumes all require specific safety requirements.
