Page 68 - Welding Robots Technology, System Issues, and Applications

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Welding Technology
The beam energy absorbed by a specific material is also a function of the radiation
wavelength, generally increasing with the decrease of the wavelength. For steels
absorptivity of Nd:YAG radiation is approximately three times of that of CO 2 laser
radiation. For aluminum this difference is not so large and for other materials, such
as copper or silver, no difference exists in this range of wavelength.

2.3.4 Process Variants
Dual beam laser welding has been proposed few years ago to improve fit-up
tolerances and to reduce the probability of forming bead shape defects, such as
humping and undercutting [59]. Beams can be mounted side-by-side or the second
beam trails behind the primary beam.

Robotic hybrid welding processes were also developed to increase welding speed
and deposition rate. This is the case for the combination of laser and GMAW
processes. This combination provides high speed and good fit-up tolerance.

High power lasers, such as CO 2 lasers, needed for high speed welding of metals,
require large floor space, considerable electrical and water services and regular
maintenance. For precision welding applications, a new generation of lasers named
diode lasers is available, providing a more efficient operation and maintenance-free
running for more than 10000 h [28]. In fact it is not a variant but a new type of
laser. These lasers incorporate diode chips, each one emitting a laser beam of very
low power, when excited electrically. These chips are mounted into bars containing
a cooling system and micro-channel lenses to focus individual laser beams. These
bars have low power, around 60 W, and are mounted into diode stacks with other
optical systems in order to obtain a focused laser beam with a power of several kW
[29], as represented schematically in Figure 2.18. These lasers can be classified as
low power diode lasers (LPDL), having power up to 150 W, and high power diode
lasers (HPDL) with power ranging from 150 W to 4 kW. The lasers of this last
group are used in welding operations. The wavelength of the laser beam is in the
range 0.63 to 0.99 Pm, though the interval 0.8 to 0.94 Pm is common in welding
applications. Aluminum has a marked increase of absorptivity in this wavelength
range. Diode laser beam is not as coherent as Nd:YAG or CO 2 laser beams and
focus is larger and rectangular instead of circular, being less sensitive for fitting of
components to weld.