Page 65 - Welding Robots Technology, System Issues, and Applications

2.3.3.1 Beam Power and Beam Diameter
50 Welding Robots
Penetration depth increases almost linearly with increase of power density, for a
specific diameter of the laser beam. Power density depends on the power of the
laser beam and on the focus cross section area. Beam diameter is very small and it
is difficult to evaluate because energy in the beam normally has a Gaussian
distribution. This distribution is designated as the transverse electromagnetic
mode 00 or TEM 00. Conventional definition of the beam diameter is based on the
2
diameter where power density is 1/e of maximum power in central part. The circle
defined in this way contains 86.5% of the total beam energy [3]. Other beam
energy distributions may be observed, such as doughnut distributions, but they are
not beneficial for welding operations because of the decrease of coherence of the
beam.

2.3.3.2 Focus Characterization

Focus is basically characterized by the minimum focal spot size (d min) and the
focus depth (Z). Focal spot size is relevant to the determination of power density
and its theoretical value can be determined by Equation 2.2, where f is the focal
length of the focusing optics, Ȝ is the wave length of the laser beam and D is the
diameter of the unfocused beam, as illustrated in Figure 2.17. Frequently focused
beam diameter is larger due to imperfections of the focusing optics [24].

. 1 27 fO
d min 2.2
D

Focus depth is defined, according to Laser Institute of America, as the distance in
which focus spot radius is increased by 5%. Focus depth can be estimated by
Equation 2.3, where F equals fȜ of the optic system:

Z . 1 488F 2 O 2.3

Focus depth increases with increase of the F number of the focusing optics but
focus diameter also increases, decreasing power density. Focus depth is important
when welding thin components because thermal distortion can put beam focus out
of these components.

The position of focus has great influence on quality of welds produced. If focus is
well above the surface of the work-piece, welds show a nail head appearance and
little penetration is obtained. When focus is positioned deep below the work-piece
surface V-shaped welds result and a more accurate setting of the components is
needed. Optimum focus positioning is below the work-piece surface but distance is
a function of plate thickness and beam power.