Page 212 - Handbook of Structural Steel Connection Design and Details

Welded Joint Design and Production

Welded Joint Design and Production 197

diameter than the electrode diameter, is ejected from the electrode
toward the work. Unlike short arc transfer, the arc in spray transfer
is continuously maintained. High-quality welds with particularly
good appearance are the result. The shielding used for spray arc
transfer is composed of at least 80% argon, with the balance made up
of either carbon dioxide or oxygen. Typical mixtures would include
90-10 argon-CO , and 95-5 argon-oxygen. Other proprietary mixtures
2
are available from gas suppliers. Relatively high arc voltages are
used with the spray mode of transfer. However, due to the intensity
of the arc, spray arc is restricted to applications in the flat and hori-
zontal position, because of the puddle fluidity and lack of a slag to
hold the molten metal in place.
Pulsed arc transfer utilizes a background current that is continuous-
ly applied to the electrode. A pulsing peak current is optimally applied
as a function of the wire-feed speed. With this mode of transfer, the
power supply delivers a pulse of current which, ideally, ejects a single
droplet of metal from the electrode. The power supply returns to a
lower background current which maintains the arc. This occurs
between 100 and 400 times/s. One advantage of pulsed arc transfer is
that it can be used out-of-position. For flat and horizontal work, it may
not be as fast as spray transfer. However, used out-of-position, it is
free of the problems associated with the gas metal arc short-circuiting
mode. Weld appearance is good and quality can be excellent. The dis-
advantage of pulsed arc transfer is that the equipment is slightly more
complex and more costly. The joints are still required to be relatively
clean, and out-of-position welding is still more difficult than with
processes that generate a slag that can support the molten puddle.
Metal-cored electrodes are a relatively new development in gas metal
arc welding. This is similar to flux-cored arc welding in that the elec-
trode is tubular, but the core material does not contain slag-forming
ingredients. Rather, a variety of metallic powders is contained in the
core. The resulting weld is virtually slag-free, just as with other forms
of GMAW. The use of metal-cored electrodes offers many fabrication
advantages. They have increased ability to handle mill scale and other
surface contaminants.
Finally, metal-cored electrodes permit the use of high amperages
that may not be practical with solid electrodes, resulting in potentially
higher deposition rates. The properties obtained from metal-cored
deposits can be excellent. Appearance is very good. Because of the
ability of the filler metal manufacturer to control the composition of
the core ingredients, mechanical properties obtained from metal-
cored deposits may be more consistent than those obtained with solid
electrodes. However, metal-cored electrodes are, in general, more
expensive.

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