Advanced Welding Processes: Technologies and Process Control

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Advanced welding processes
8.2 Plasma keyhole welding
The principles of plasma welding have been described in Chapter 6. The
same transferred arc operating system is used for plasma keyhole welding,
but the plasma gas flow and current are usually increased and the orifice size
may be reduced. The exact current where the keyhole mode is initiated will
depend on the torch geometry and the joint material and thickness, but
currents of over 200 A and plasma gas flows of 3 to 4 1 min –1 are typical with
a 2 to 3 mm diameter constricting orifice. Under these conditions high arc
pressures are generated by electromagnetic constriction.
The thermal efficiency of the process is high and it has been estimated
that the heat transferred to the workpiece from a 10 kW plasma arc can be as
high as 66% of the total process power [145].
8.2.1 Control of plasma keyhole welding
The parametric relationships for plasma keyhole welding are complicated by
variations due to torch geometry and it is likely that parameters developed
using a specific torch will not be transferable to a torch of a different design.
The control variables may be divided, as shown in Table 8.1, into those
normally used to match the conditions to the application, the primary controls,
and those normally chosen and fixed before adjustment of the process, the
secondary factors.
The influence of the main control parameters on process performance
may be summarized as follows.
Mean welding current
The arc force produced by magnetic constriction is proportional to the square
of the mean current. With high currents, very high arc forces are generated
and undercut or humping may occur (see Chapter 6). The current must,
however, be controlled in conjunction with welding speed to produce the
required bead profile.
Table 8.1 Plasma keyhole control factors
Primary Secondary
Current Orifice shape/type
Travel speed Orifice diameter
Plasma gas flow Electrode vertex angle
Electrode set back
Shielding and plasma gas type
Torch stand off