Advanced Welding Processes: Technologies and Process Control

Advanced gas tungsten arc welding 83
is particularly useful for building up fine edges of cutting tools or components
such as turbine blades.
6.3.5 Hot-wire GTAW
Normally GTAW is regarded as a ‘low-productivity–high-quality’ process
due to the relatively slow travel speeds employed and low deposition rate
achieved with cold filler additions. It has been shown, however, that significant
improvements in deposition rate, to match those produced in the GMAW
process, may be achieved by using a ‘hot-wire’ addition. [93,94]
The process arrangement is illustrated in Fig. 6.5. The principal features
are the addition of a continuously fed filler wire, which is resistively heated
by AC or DC current passing between the contact tip and the weld pool.
Normally the wire is fed into the rear of the weld pool (unlike cold-wire
addition) although hot-wire additions to the front of the pool have been used
for positional work. The equipment required comprises a precision power
source (e.g. electronic control mains-voltage stabilized), a high-quality wirefeed
system and an effective gas-shielding system. The use of an AC power
source for filler wire heating minimizes the possibility of magnetic disturbance
of the arc. The deposition rates possible are shown in Fig. 6.6.
A novel system based on an inverter power source has been developed for
evaluation in the power generation industry; this consists of a DC inverterbased
GTAW power source which supplies the arc power. An additional
power circuit and control system is powered from the high-frequency AC
output of the inverter and provides the heating supply to the wire. This
design is cost effective and electrically efficient and compact. [95] Although
the equipment is more complex than GMAW it has been shown that deposition
rates of 10–14 kg h –1 are possible and high joint integrity may be expected.
The hot-wire GTAW process has been applied in the oil industry for butt
welding 30 mm wall line pipe. In this application, four heads rotate around
the pipe simultaneously.
Arc power
source
6.5 Hot-wire GTAW.
GTAW torch
L
Filler
wire
Contact
tube
Wire power
source