Page 67 - Welding Robots Technology, System Issues, and Applications

Welding Robots
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vicinity of the beam by an auxiliary jet of helium or argon. Assisting gas must be
directed to 1 mm ahead of the beam, at an angle of approximately 20 degrees with
the work-piece surface. Helium is preferred, because it has a higher ionization
potential than argon, being more resistant to plasma formation. Beam interaction
with the work-piece can also be improved by the combination of linear oscillation
of the beam in welding direction with the jet of an inert gas [3].

2.3.3.5 Welding Gases

In laser welding two gases are commonly needed, the assisting gas to remove
plasma, which is injected laterally, and a coaxial shielding gas to prevent
atmospheric contamination. A root gas is also needed in keyhole welds where all
the material thickness is melted. Helium and mixtures of argon and helium are used
as welding gases. Argon shields the weld metal and helium is required to control
the plasma formation in CO 2 laser welding. If Nd:YAGs are used for welding, the
plasma formation is not an aspect of major concern and argon is the recommended
welding gas. Small additions of oxygen, hydrogen or CO 2 can be used depending
on material and process to increase productivity further [26]. Helium, argon or
mixtures of these gases are used for most materials, including reactive metals such
as titanium or zirconium. For reactive materials the shielded area must be
increased, because they are sensitive to air contamination down to low
temperatures (400 ºC). Nitrogen can also be used for welding stainless steels in less
demanding applications [3].

2.3.3.6 Absorptivity

The efficiency of laser beam welding represents the proportion of beam energy that
is effectively added to the work-piece. It is drastically affected by the absorptivity
of the material to be welded. Absorptivity is a function of the electrical resistivity
of the material, according to Equation 2.4, where A is the absorptivity and p r the
the electrical resistivity

A 112 2 . p r 2.4

Absorptivity in many metallic materials is very low, 2 to 3% for aluminum or
copper and less than 15% for stainless steel [22]. Absorptivity is increased by the
formation of oxide layers in metallic materials. Absorbent powders can be applied
in work-piece surface, in order to reduce reflection losses. The addition of active
gases, such as oxygen, to shielding gas also improves absorptivity. In keyhole
welding absorptivity suffers a large increase because of multiple reflections inside
the keyhole, providing efficient welding even in high reflective materials such as
aluminum [27].