Advanced Welding Processes: Technologies and Process Control

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Advanced welding processes
power sources with the capability of variable-polarity operation and significant
improvements in quality and cost have been reported using these techniques.
[158] It has been found that acceptable results are achieved with 15–20 ms
of DC electrode negative operation and electrode positive pulses of 2–5 ms
duration.
8.2.2 Summary: plasma keyhole welding
The plasma keyhole welding process may be used for making square butt
welds in a wide range of materials in the thickness range from 2 to 10 mm.
It gives high welding speeds and guaranteed penetration from one side of the
joint. Although the number of parameters which influence the process
performance is large and their interaction is complex, the arc power gives a
useful indication of the overall performance capabilities. The major limitation
of the process is undercut but this may be controlled by careful selection of
the operating parameters or pulsed operation. The plasma phenomenon is
present in all arcs and whilst it may be enhanced by thermal constriction as
discussed in Chapter 6 it may also be used to generate keyhole welding
characteristics if very high currents are used in GTAW.
8.3 Laser welding
The laser may be used as a welding heat source and consists of a high-energy
coherent beam of light of an essentially constant wavelength. LASER is an
acronym for Light Amplification by Stimulated Emission of Radiation and
the medium in which it is generated may be either solid, liquid or gaseous.
Helium/neon and CO 2 are commonly used as a basis of gaseous systems
whilst ruby and neodymium doped yttrium aluminium garnet (Nd:YAG) are
used in solid state lasers. In welding applications, the two most commonly
used lasers are the CO 2 gas laser and the Nd:YAG solid state laser. Recent
developments include the availability of high-power diode lasers (HPDL)
and fibre lasers suitable for welding applications.
8.3.1 CO 2 lasers
The principles of operation of the CO 2 laser are illustrated in Fig. 8.7. An
electrical discharge within the gas is used to stimulate the emission of radiation.
The initial low-level radiation is ‘trapped’ within the laser cavity by mirrors
placed at either end. The internal reflection of the beam causes an increase
in the energy level (amplification). A fraction of the laser beam generated in
this way is allowed to escape from the resonant cavity via a partially reflective
mirror. In the case of the CO 2 laser, the emergent light beam has a wavelength
of 10.6 mm (i.e. in the infrared part of the spectrum) and is delivered to the