Page 60 - Welding Robots Technology, System Issues, and Applications

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Welding Technology
2.3 Laser Beam Welding (LBW)
A laser consists of a high-power coherent monochromatic light beam which can be
focused to a small spot, producing a very high energy density. Laser is the acronym
for “light amplification by stimulated emission of radiation”. A laser beam is
produced by stimulating emission of electromagnetic radiation in specific solid or
gaseous materials. Atoms of these materials are moved to higher energy levels by
absorbing stimulating energy, producing a population inversion, that is material is
brought into a condition in which population of atoms at a higher energy level is
greater than that at lower level. These atoms decay by spontaneous emission of
photons, which can generate more photons by stimulating emission from other
excited atoms, producing the amplification of the laser light. Laser light sources
have reflecting mirrors incorporated (see Figure 2.13) which reflect photons back
for further light amplification.

2.3.1 Introduction
The most popular lasers for welding are the solid-state lasers of neodymium-doped
yttrium aluminum garnet (Nd:YAG), generally pulsed wave, and the gas lasers of
continuous-wave carbon dioxide (CO 2), whose lasing medium is a mixture of
11
9
carbon dioxide, nitrogen and helium. Power density of laser welding (10 -10
-2
6
8
-2
Wm ) is significantly higher than that of arc welding processes (10 -10 Wm ),
13
11
-2)
though somewhat lower than electron beam welding (10 -10 Wm [3].

The beam energy delivered to the work-piece will be dissipated by reflection and
absorption. Work-piece material is heated to a very high temperature, melted and
may even vaporize due to very high power density concentrated in the focus of
laser beam. Two modes of laser welding can be obtained, the heat conduction-
mode and the deep-penetration mode, depending on the power density in use [22].
Heat conduction-mode is obtained for low power density, where most of the beam
energy is lost by reflection (up to 90%), and it is characterized by the formation of
a wide and shallow weld pool, see Figure 2.14 a). Power density is sufficient to
melt the material but it is not enough to vaporize it, the weld pool shape being
controlled by surface tension and thermocapillary forces [23]. This technique is
used for welding small components for the electronics industry or for small
medical parts.