Page 123 - Welding of Aluminium and its Alloys

108 The welding of aluminium and its alloys

6.12 Demonstration of laminar flow by use of gas lens. Courtesy of
TWI Ltd.

6.2.4 Tungsten electrodes
There are several types of electrodes available for TIG welding. These
include pure tungsten and tungsten alloyed with thoria (ThO 2) or zirconia
(ZrO 2). These compounds are added to improve the arc starting charac-
teristics, to stabilise the arc and to extend the electrode life. Recently there
has been a move towards the use of other rare earth elements such as
caesium, cerium or lanthanum, which are claimed to extend the electrode
life further and will reduce the radiation risk arising during the grinding of
thoria containing electrodes. Zirconiated electrodes are preferred for AC-
TIG welding since these have a higher melting point than either pure tung-
sten or thoriated tungsten electrodes and can therefore carry higher welding
currents, are more resistant to contamination and are less likely to spall.
The electrode tip assumes a hemispherical shape during welding. It is
important that this shape is maintained if a stable arc is to be achieved.The