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Schematic of Globular Transfer
Large droplet
Globular Transfer
Metal transfer is controlled by slow ejection resulting in large,
irregularly-shaped 'globs' falling into the weld pool under the action
of gravity. Carbon dioxide gas drops are dispersed haphazardly. With
argon-based gases, the drops are not as large and are transferred in
a more axial direction. There is a lot of spatter, especially in carbon
dioxide, resulting in greater wire consumption, poor penetration
and poor appearance. Globular transfer occurs between the dip and
spray ranges. This mode of transfer is not recommended for normal
welding applications and may be corrected when encountered
by either decreasing the arc voltage or increasing the amperage.
Globular transfer can take place with any electrode diameter.
Basic flux-cored wires tend to operate in a globular mode or in
a globular-spray transfer mode where larger than normal spray
droplets are propelled across the arc, but they never achieve a true
spray transfer mode. This transfer mode is sometimes referred to
as non-axial globular transfer.
Self-shielded flux-cored wires operate in a predominantly globular
transfer mode although at high currents the wire often 'explodes'
across the arc.
Spray Transfer
In spray transfer, metal is projected by an electromagnetic force
from the wire tip in the form of a continuous stream of discrete
droplets approximately the same size as the wire diameter. High
deposition rates are possible and weld appearance and reliability
are good. Most metals can be welded, but the technique is limited
generally to plate thicknesses greater than 6mm. Spray transfer,
due to the tendency of the large weld pool to spill over, cannot
normally be used for positional welding. The main exception is
Splatter
Workpiece
Schematic of Spray Transfer
Gas shroud
Wire
aluminium and its alloys where, primarily because of its low density
and high thermal conductivity, spray transfer in position can be
carried out.
The current flows continuously because of the high voltage
maintaining a long arc and short-circuiting cannot take place. It
occurs best with argon-based gases.
In solid wire MIG, as the current is increased, dip transfer passes
into spray transfer via a transitional globular transfer mode. With
metal-cored wires there is virtually a direct transition from dip
transfer to spray transfer as the current is increased.
For metal cored wire spray transfer occurs as the current density
increases and an arc is formed at the end of the filler wire,
producing a stream of small metal droplets. Often the outside
sheath of the wire will melt first and the powder in the centre flows
as a stream of smaller droplet into the weld pool. This effect seems
to give much better transfer of alloying elements into the weld.
In spray transfer, as the current density increases, an arc is formed
at the end of the filler wire, producing a stream of small metal
droplets. In solid wire MIG this transfer mode occurs at higher
currents. Flux-cored wires do not achieve a completely true spray
transfer mode but a transfer mode that is almost true spray may
occur at higher currents, and can occur at relatively low currents
depending on the composition of the flux.
Rutile flux-cored wires will operate in this almost-spray transfer
mode, at all practicable current levels. They are also able to operate
in this mode for positional welding too. Basic flux-cored and self-
shielded flux-cored wires do not operate in anything approaching
true spray transfer mode.
MIG 250GS Operating manual
Shielding gas
Droplets
Weld
Workpiece
011
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