Plasma-arc welding is appropriate for all fusion weldable metals. Its characteristic feature of a comparatively high seam-shape-factor (ratio: seam depth / seam width) resulting in less deformation by welding heat and in a high-quality weld seam, has contributed to the fact, that plasma-arc welding is mainly used for high-alloy steel. The high viscosity of the weld pools of these steels allows to weld butt joints without pool protection. The root of the weld seam develops freely, and the weld pool does not „fall through“.

The high energy density is also an advantage for the welding of very thermo conductible metals like copper and copper base alloys.

Recently, the method to weld aluminium using the plasma-arc system with the electrode on the positive pole has become more frequent.

Also the method to weld unalloyed and low-alloy steel with the plasma-arc, becomes more frequent. The high seam-shape-factor, being especially achieved with the plug weld method may lead to an increased pore formation, due to an insufficient gas release of the welding deposit. Therefore, it may become necessary to add welding filler. The higher productivity, compared to the TIG short time welding system justifies the application of the plasma-arc process.

When welding thin sheet steel up to 3 mm (4mm), not only the machine welding, but also the manual welding is an appropriate measure. The bigger the plate thickness and the strength of current, the more often machine welding is used.

During the „plug weld operation“ the hard plasma-arc penetrates the material – a welding eye opens and the plasma-arc can be seen on the underside of the plate. Behind the plasma-arc, the welding deposit flows together again and forms the weld seam. Plug weld operation is appropriate for square butt joint welding of plates between 3 and 8 mm thickness.

For very thin foils machine welding is also recommended. But the micro plasma-arc welding system (0.1 up to 25 amps) facilitates also hand welding with low welding current. TIG-welding below 10 amps requires such a short arc that the welding becomes increasingly difficult. As the characteristic feature of the plasma-arc is different, it allows a markedly longer arc.

The „hardness“ of the plasma-arc can be influenced with the choice of the plasma nozzle and the plasma gas volume. A big nozzle diameter, together with a small amount of plasma gas leads to a soft plasma-arc. The hard plasma-arc, needed to plug weld thick plates, results from a small nozzle diameter and a larger volume of plasma gas. The precise adjustment, adapted to the welding job, takes place via slight modifications of the electrode position. Moving the electrode tip towards the nozzle bore makes the plasma-arc softer, moving the electrode tip away from the nozzle bore results in a harder arc.