Page 66 - Welding Robots Technology, System Issues, and Applications

TEM Welding Technology 51
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Figure 2.17. Characteristic parameters of focal system

2.3.3.3 Travel Speed

The increase of travel speed decreases penetration depth for both argon and helium
shielding gases. This is because power input per unit length decreases with welding
speed increase and keyhole may not be completely effective in trapping incident
radiation. Very high speeds can give lack of fusion while low speeds may originate
excessive parent material melt, vaporization and even defects formation. For very
low speeds a reduction in penetration may be observed. This is attributed to the
formation of a cloud of plasma, which attenuates the incident laser beam.

2.3.3.4 Plasma Formation

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For power densities above 10 Wm in CO 2 lasers and 10 Wm in YAG lasers,
the beam interacts with metal vapor and shielding gas, producing a cloud of plasma
above the plate surface. During the initial moments of keyhole formation, plasma
may assist the energy transfer to the work-piece. However, subsequently, plasma
may limit beam energy transfer to the work-piece [25]. Several techniques have
been developed to reduce plasma formation or to remove it from the weld zone.
Pulsing laser power at high frequencies (above 1 kHz) is effective in reducing
plasma formation in CO 2 lasers. In addition plasma is generally removed from the