Advanced Welding Processes: Technologies and Process Control

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Advanced welding processes
Variable polarity plasma
The combined benefits of DCEN and DCEP operation may be achieved by
employing a variable polarity power supply of the type described in Chapter
3. Excellent results have been obtained in welding aluminium alloys with
this configuration.
Pulsed keyhole plasma
The normal keyhole mode of operation is restricted to welding in the downhand
position and requires very critical control of speed as discussed above. The
tolerance of the process may be improved by modulating the current and
positional welds may be made in a wide range of materials and plate thicknesses.
[152, 153] The current is modulated at relatively low frequencies, the pulse
time and amplitude being based on the requirement to establish a keyhole
and the background conditions being set to maintain an arc but allow
solidification. The resultant weld is therefore formed from a succession of
overlapping spots, the travel speed being adjusted to provide at least 60%
overlap. Pulsed operation improves the resistance to undercut and generally
produces a wider, flatter bead. Examples of typical applications and the
features of the DC and pulsed modes of operation are described below.
Applications
The keyhole mode of operation makes it possible to perform single-pass
square butt welds from one side in plate thicknesses up to about 10 mm.
Carbon–manganese ferritic steel. The plasma keyhole welding of carbon–
manganese steels has recently been evaluated for circumferential root runs
in pipes for power generation and offshore applications. The use of the
process enables thick root sections (6–8 mm) to be welded in a single pass
from one side and significantly improves productivity. Pulsed plasma keyhole
has been used for these studies [154] to improve operating tolerance and
positional performance.
Austenitic stainless steel. Austenitic stainless steel may be readily welded
using the keyhole technique and the process has been applied to the longitudinal
welding of pipe as well as the fabrication of components for cryogenic
service. [155] Welding speeds of around 1.0 m min –1 are achievable with
keyhole welds in material up to 2.7 mm thick, whilst welds in 6.0 mm thick
material may be made at 0.35 m min –1 . The use of hydrogen additions to the
shielding gas or proprietary mixtures containing from 1 to 5% hydrogen
provide improved bead appearance and increased travel speed. Undercut
may be limited by careful control of welding parameters, but, if this is not
possible, pulsed operation or the use of cosmetic passes is recommended.