Instructions / Assembly

11
GMAW
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Keywords:
Anode Region
Cathode Region
Arc Plasma Region
Electromagnetic Forces
Gravity – Droplet Weight
Surface Tension Forces
Jet Forces
The area of the welding arc is a region of high complexity that is
comprised of physical forces and chemical reactions. The
interaction of the components of the arc affects metal transfer
and the quality of the finished weld. The behavior of the arc is
influenced by:
• The type and diameter of the filler metal.
• The base metal conditions — clean or millscale.
• The shielding gas.
• The welding parameters — voltage and current.
• The interaction of physical forces — gravity, surface tension,
jet forces, and electromagnetic force.
The character of the mode of metal transfer, the penetration
profile, and the bead shape are influenced by the forces applied
to the metal as it moves from the electrode end to the work-piece.
Anode (+)
Cathode (—)
Plasma
Ionized Gas
Metal Vapor
Electromagnetic Forces
When current flows through a conductor, a magnetic field builds
and surrounds the conductor. In GMAW the electro-magnetic
f
orces, which are mathematically proportional to the square of
the applied current, affect the mode of metal transfer. The most
common term applied to the electromagnetic force is the pinch
effect. As the molten drop forms, it is uniformly squeezed from
t
he electrode anode end by the electromagnetic force. The size
o
f the droplet transferred depends upon this force, the applied
welding current, and the shielding gas.
Surface Tension Forces
Surface tension forces are those forces, which are normal to the
surface of a molten droplet. They act on both the interior and
the exterior surface of the droplet. Together they serve to
support the form of a molten droplet. There is always an inward
pull of the forces applied to the surface.
Jet Forces
In the short-circuiting mode of metal transfer, during the shorting
portion of the metal transfer cycle, higher currents cause the
electrode to heat to the point of melting. The high current drives
an increase in the electromagnetic force, which causes the
molten metal to detach from the electrode. As the droplet
meets the weld pool, the surface tension forces supporting the
molten droplet release and the molten droplet then adds itself to
the molten weld pool.
In the globular transfer mode, a large molten droplet develops.
Surface tension forces support the formation of the molten
droplet, and jet forces push against the large droplet. The jet
forces are responsible for supporting, spinning, and pushing the
large droplet in an irregular fashion within the arc. The transfer
occurs by the occasional shorting of the large droplet to the
weld pool and the force of gravity. Once the droplet contacts
the molten pool or work-piece, the surface tension forces in the
droplet collapse, and the volume of weld metal is absorbed by
the puddle.
The shielding gas employed in a welding application has an
effect on the surface tension forces. If the energy level within the
arc is high, as is the case with a 100% argon gas employed with
a carbon steel electrode, then the bead shape will be extremely
convex. If the surface tension value is low, because of the addi-
tion of carbon dioxide or oxygen, then the bead shape will be
less convex, and more acceptable. So the addition of active gas
components will result in improved weld bead and overall arc
performance with carbon steel electrodes.
FIGURE
6: Cross Section of a GMAW Arc
Anode (+)
Cathode (-)
Ionized Gas
Plasma
Metal Vapor
{
Components of the Welding Arc