Page 62 - Welding Robots Technology, System Issues, and Applications

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Welding Technology
surface tension tend to obliterate it. As the laser beam advances it creates a channel
and material solidifies behind it. This is the deep-penetration mode laser welding,
which produces a narrow and deep welding seam. This welding mode is commonly
applied for welding thick materials (up to 50 mm) at high travel speed [22],
without filler metal, though filler electrodes can also be used to fill gaps.

The laser welding process provides a high energy density beam that can be used at
room atmosphere to produce precise welds at high speed, even in difficult-to-weld
materials, such as titanium. Added to this, welds are deep and narrow, with small
heat affected zones, giving low distortion, and almost no post processing is
necessary [29]. Main limitations of laser welding are the need for accurate part fit-
up and precise part positioning as well as equipment capital cost that is ten times
more expensive than arc welding systems of identical power. In addition the
process is dependent on the material’s light absorptivity and surface condition and
it is susceptible to weld porosity, solidification cracking and bead geometric
defects, mainly in aluminum alloys.

2.3.2 Welding Equipment

The welding equipment includes several types of lasers used in welding. In the
following, solid-state lasers and gas lasers will be considered.

2.3.2.1 Solid-state Lasers

Solid-state lasers used in welding are of the ruby type, composed of a ruby crystal
containing a concentration of 0.05% chromium, or of Nd:YAG type, made of a
solid yttrium aluminum garnet rod doped with neodymium. Excitation of electrons
in neodymium is done with high-power xenon flash lamps (1-4 kV), as represented
schematically in Figure 2.15. This process is known as pumping. Diode lasers are
frequently used as the pumping source instead of flash lamps, in order to improve
pumping efficiency. Pumping energy is amplified within the crystal, commonly
designated as cavity, which contains a fully reflecting mirror at one end and a
partially reflecting mirror at the other. After amplification of radiation the laser
beam is radiated from the partially reflecting end, with 1.064 Pm wavelength.
Because of the limited capacity of cooling systems to maintain a threshold
temperature of the crystal Nd:YAG lasers are commercially available up to 6 kW
average power, though conventional systems have generally up to 1000 W average
power, with a maximum pulse power of 5 to 20 kW, a pulsing rate up to 400 pulses
per second and a beam parameter of 25 (mm × mrad) or lower.

Commercial solid state lasers with high pulse power are capable of simultaneous
welding at several different locations. The weld point diameter can also be adjusted
by the processing optics at a constant working distance of 0.1 to 2 mm, and the
welding depth can be controlled via the laser parameters up to 2 mm.