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Schematic of Globular Transfer
Large droplet
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 6 mm. 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 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
Schematic of Spray Transfer
Gas shroud
Wire
Splatter
Workpiece
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.
Pulsed Transfer
Pulsed arc welding is a controlled method of spray transfer,
using currents lower than those possible with the spray transfer
technique, thereby extending the applications of MIG/MAG welding
into the range of material thickness where dip transfer is not
entirely suitable. The pulsed arc equipment effectively combines
two power sources into one integrated unit. One side of the power
source supplies a background current which keeps the tip of the
wire molten. The other side produces pulses of a higher current that
detach and accelerate the droplets of metal into the weld pool. The
transfer frequency of these droplets is regulated primarily by the
relationship between the two currents. Pulsed arc welding occurs
between ±50-220A, 23–35 arc volts and only with argon and argon-
based gases. It enables welding to be carried out in all positions.
BOC RAPTOR 200C & 250R MIG Operating manual
Shielding gas
Droplets
Weld
Workpiece
31
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