Page 57 - Welding Robots Technology, System Issues, and Applications

Welding Robots
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Pulsed current allows projected spray transfer for mean currents below spray
transition current, improving positional capabilities and operating tolerances of the
process. Details concerning the control of the metal transfer modes in the arc are
given in Chapter 3.

2.2.3.2 Voltage

Arc voltage is directly related to current, as indicated above, and with arc length,
increasing with it. Voltage also depends on the shielding gas and electrode
extension. The increase of arc voltage widens and flattens the weld bead. Low
voltages increase the weld reinforcement and excessively high voltages can cause
arc instability, spatter, porosity and even undercut.

2.2.3.3 Welding Speed

Increase in the welding speed gives a decrease in the linear heat input to the work-
piece and the filler metal deposition rate per unit of length. The initial increase in
welding speed can cause some increase in penetration depth, because the arc acts
more directly in the parent material, but further increase in speed decreases
penetration and can cause undercut, due to insufficient material to fill the cavity
produced by the arc.

2.2.3.4 Electrode Extension

The electrode extension is the electrode length that is out of the contact tube. The
increase of electrode extension, produced by the increase of the torch distance to
the work-piece for a specific parameters set, increases electrode melting rate
because of the Joule effect. Electrode extension ranges from 5 to 15 mm for dip
transfer, being higher (up to 25 mm) for the other transfer modes.

2.2.3.5 Shielding Gas

Shielding gases have an effect on arc stability, metal transfer mode, weld bead
shape and melting rate. Gases used in GMAW can be pure gases, binary, ternary
and exceptionally quaternary mixtures. Common pure gases are argon, helium and
carbon dioxide. The first two are inert gases and are used principally in welding of
light alloys, nickel, copper and reactive materials. Helium has a higher ionization
potential than argon, providing larger weld pools, but is more expensive. Carbon
dioxide is an active gas and is used in welding of carbon steels. It produces high
levels of spatter but provides high penetration depth.