Advanced Welding Processes: Technologies and Process Control

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Advanced welding processes
systems becomes inconsistent; this may be due to electrode characteristics or
an adverse phase relationship (i.e. lack of synchronization) between the
power supply and the arc-starting device. It has also been suggested that a
negative space charge may be generated around the end of the electrode and
the gas cup. In this case, improved consistency may be obtained by discharging
the charge by connecting the gas cup to the positive terminal of the power
supply. Where a ceramic gas cup is used, a conductive foil may be wrapped
around the nozzle to provide an electrical connection, whereas, with metallic
cups, a resistor is inserted between the cup and the positive connection.
The main problem with the high-frequency starting technique, however,
lies in the use of high-frequency voltage oscillations, which, depending on
the design of the oscillator circuit, can cover a wide range of radio frequencies
and produce both airborne and mains-borne interference. In the past, this
problem has resulted in interference with communication systems and domestic
television and radio reception, but it is also likely to create significant problems
with electronic control and computing equipment in the welding environment.
New arc striking techniques
Programmed touch striking. The main problem with conventional touch
striking is the high short-circuit current which tends to overheat the electrode
and increase the risk of contamination. This limitation can be overcome by
controlling the current during the short circuit. Various systems exist [81] but
the operation is essentially as follows (see also Fig. 6.1). After closing the
torch switch (A), a low voltage is applied between the electrode and the
workpiece via a current-limiting resistor. When the electrode touches the
workpiece (B), the short circuit is detected electronically and a low current
(2 to 10 A) is allowed to flow; this current is sufficient to preheat the electrode
without overheating (C). When the electrode is lifted the voltage rises (D)
and signals the power supply to initiate the main current supply. The initial
arc current may be programmed to rise rapidly to ensure arc stabilization
before reverting to the working value. Trials have shown [82] that no evidence
of tungsten contamination or electrode weight loss could be detected after
repeated re-striking with a system of this type. The system is also ideally
suited to automatic application, where the contact of the electrode with the
workpiece and its retraction can be mechanized (see Chapter 11).
Pilot arc starting. The use of an auxiliary electrode in the torch enables a
low-current pilot arc to be struck before initiation of the main arc. This
system allows consistent striking, although it does require a slightly more
complex torch.
Piezoelectric arc starting. Piezoelectric arc-starting devices have been
investigated [83] and it has been shown that a torch-mounted piezoelectric
device can be used successfully for GTAW arc starting. Problems were,