Page 49 - Welding Robots Technology, System Issues, and Applications

Welding Robots
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input, increasing weld penetration and weld travel speed, as well improving weld
appearance [6]. Argon/hydrogen mixtures are also used in welding of copper-
nickel alloys.

Argon is also used as back side shielding gas, mainly in welding of stainless steels,
aluminum alloys and reactive metals.

Flow rates of shielding gases depend on weld thickness, being 4-10 l/min for argon
and 10-15 l/min for helium, because it is lighter than argon, and consequently less
effective in shielding.

Gases with a purity of 99.995% are used in welding most of the metals, though
reactive materials such as titanium need contaminant level less than 50 ppm.

2.1.3.5 Filler Metals

Filler metals are generally used for plate thickness above 2 mm, having chemical
composition similar to that of the parent material. Filler metal diameter is between
1.6 and 3.2 mm and in automatic systems is normally added cold from a roll or a
coil.

2.1.3.6 Electrode Vertex Angle

The non-consumable electrode angle influences the weld penetration depth and the
weld shape [7]. Electrode angles between 30º and 120º are used. Small angles
increase arc pressure and penetration depth but have high tip shape deterioration.
Electrode angles from 60º to 120º maintain tip shape for longer periods and give
welds with adequate penetration depth-to-width ratio.

2.1.3.7 Cast-to-cast Variation

Cast-to-cast variation refers to variation observed in penetration of welds produced
in the same welding conditions in several batches of austenitic stainless steel with
nominally identical composition. These changes in the weld bead shape are
attributed to variation in proportion of trace elements in the material, such as
sulphur, calcium and oxygen. Variations in trace elements seem to affect surface
tension and metal flow into the pool [8]. Weld pool shape is also affected by
electromagnetic forces, arc pressure and thermo capillarity forces [9]. To minimize
this problem several strategies have been adopted such as the use of higher currents
or of pulsed current, the application of adequate shielding gases or the application
on plate surface of flux coatings containing active ingredients [10].