Deep penetration technique:

The deep penetration technique for utilizing the plasma arc on thick material has certain similarities with the electron-beam process. Due to the high energy density of the plasma arc a hole is melted through the metal being welded and as the torch is moved along the joint energy is transferred through the plasma to the walls of the hole that is carried along the joint. This is from this feature that the name 'keyhole technique' is derived. A cross-section of such as weld illustrated a wine-glass shaped or fingered penetration, the 'stem' being a consequence of heat transfer to the walls of the hole and the 'bowl' shape on the surface due to surface heating in the normal way linked with arc welds. As with the electron-beam procedure the relative proportions of 'bowl' and 'stem' based on the uniformity of energy in the beam. The preferred shape of penetration giving the minimum of 'bowl' can be attained by ensuring that the plasma is as close as possible a parallel stream without a low velocity, cool fringe. Arcs tend to spread close the workpiece surface and this is helpful therefore to focus the plasma. it is done by another gas flow in-between the plasma flow and the annular shielding flow. Several jets of gas are directed at an angle onto the plasma arc a short distance after it leaves the nozzle so that a tendency to divergent flow is corrected.