Page 211 - Handbook of Structural Steel Connection Design and Details

Welded Joint Design and Production

196 Chapter Three

shielding gas is used to displace atmospheric oxygen, it is possible to
add smaller quantities of oxygen into mixtures of argon—generally at
levels of 2 to 8%. This helps stabilize the arc and decreases puddle
surface tension, resulting in improved wetting. Tri and quad mixes of
argon, oxygen, carbon dioxide, and helium are possible, offering
advantages that positively affect arc action, deposition appearance,
and fume generation rates.
Short arc transfer is ideal for welding on thin-gauge materials. It is
generally not suitable for structural steel fabrication purposes. In this
mode of transfer, the small-diameter electrode, typically 0.035 or
0.045 in, is fed at a moderate wire feed speed at relatively low volt-
ages. The electrode will touch the workpiece, resulting in a short in
the electrical circuit. The arc will actually go out at this point, and
very high currents will flow through the electrode, causing it to heat
and melt. Just as excessive current flowing through a fuse causes it to
blow, so the shorted electrode will separate from the work, initiating a
momentary arc. A small amount of metal will be transferred to the
work at this time.
The cycle will repeat itself again once the electrode shorts to the
work. This occurs somewhere between 60 and 200 times/s, creating a
characteristic buzz to the arc. This mode of transfer is ideal for sheet
metal, but results in significant fusion problems if applied to heavy
materials. A phenomenon known as cold lap or cold casting may result
where the metal does not fuse to the base material. This is unaccept-
able since the welded connections will have virtually no strength.
Great caution must be exercised in the application of the short arc
mode to heavy plates. The use of short arc on heavy plates is not totally
prohibited, however, since it is the only mode of transfer that can be
used out-of-position with gas metal arc welding, unless specialized
equipment is used. Weld joint details must be carefully designed when
short arc transfer is used. Welders must pass specific qualification tests
before using this mode of transfer. Short arc transfer is often abbrevi-
ated as GMAW-s, and is not prequalified by the D1.1 code.
Globular transfer is a mode of gas metal arc welding that results
when high concentrations of carbon dioxide are used, resulting in an
arc that is rough with larger globs of metal ejected from the end of the
electrode. This mode of transfer, while resulting in deep penetration,
generates relatively high levels of spatter. Weld appearance can be
poor and it is restricted to the flat and horizontal position. Globular
transfer may be preferred over spray arc transfer because of the low
cost of CO -shielding gas and the lower level of heat experienced by
2
the operator.
Spray arc transfer is characterized by high wire-feed speeds at rel-
atively high voltages. A fine spray of molten drops, all smaller in

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