Advanced Welding Processes: Technologies and Process Control

High-energy density processes 157
lasers may easily be coupled with arc processes as GMAW to provide the
advantages of a hybrid process. [175]
Fibre lasers. Fibre lasers utilise a doped silica glass fibre as the lasing
media and are pumped by diode lasers. The dopant consists of erbium (Er)
or ytterbium (Yb) or a combination of these elements. The fact that the
lasing action takes place within the fibre eliminates the coupling problems
which may be found with fibre coupled diode lasers. Single fibre lasers of
this type have been developed with output powers up to 1 kW but for higher
powers it is now possible to combine several lower power lasing fibres for
delivery to the work site by a single optical fibre 10–200 m in length. Power
outputs of up to 20 kW are commercially available using this approach. The
major advantages of these systems are improved power efficiency, compactness,
stability and beam quality. Electrical efficiency of up to 25% has been reported
and this makes it possible to consider the systems for mobile applications
such as pipeline girth welding. [176] Like high-power diode lasers, the fibre
laser may be used in combination with arc welding heat sources as a hybrid
welding system.
8.3.8 Summary: laser welding
Laser welding has been shown to be suitable for high-speed welding of
thinner materials and deep penetration welding of materials up to about
12 mm in thickness (up to 25 mm is feasible using high-power systems). A
wide range of materials is weldable using both CO 2 and Nd:YAG systems.
Low-power applications are found in the instrumentation and electronics
industries, whilst higher-power applications continue to be developed
principally in the automotive, shipbuilding and aerospace industries.
The capital cost of laser systems is high but the economic returns have
justified this level of investment in many applications.
8.4 Electron beam welding
Electron beams have been used as welding heat sources since the early
1960s and electron beam welding (EBW) has become established as a highquality
precision welding process.
8.4.1 Fundamentals
In the EBW system, electrons are generated by passing a low current (e.g.
50–200 mA) through a tungsten filament. The filament is attached to the
negative side of a high-voltage power supply (30–150 kV) and electrons are
accelerated away from the cathode towards an anode as shown in Fig. 8.15.
The divergent electron beam is focused by magnetic and electrostatic lenses