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BOC Smootharc Advance II MIG 400R Operating manual
Unlike MIG, which uses a solid consumable filler wire, the consumable
used in FCAW is of tubular construction, an outer metal sheath being
filled with fluxing agents plus metal powder. The flux fill is also used to
provide alloying, arc stability, slag cover, de-oxidation, and, with some
wires, gas shielding.
In terms of gas shielding, there are two different ways in which this may
be achieved with the FCAW process.
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Additional gas-shielding supplied from an external source, such as a
gas cylinder
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Production of a shielding gas by decomposition of fluxing agents
within the wire, self-shielding
Gas shielded wires are available with either a basic or rutile flux fill,
while self-shielded wires have a broadly basic-type flux fill. The flux
fill dictates the way the wire performs, the properties obtainable, and
suitable applications.
Gas-shielded Operation
Many cored wire consumables require an auxiliary gas shield in the
same way that solid wire MIG consumables do. These types of wire are
generally referred to as ‘gas-shielded’.
Using an auxiliary gas shield enables the wire designer to concentrate
on the performance characteristics, process tolerance, positional
capabilities, and mechanical properties of the products.
In a flux cored wire the metal sheath is generally thinner than that of
a self-shielded wire. The area of this metal sheath surrounding the flux
cored wire is much smaller than that of a solid MIG wire. This means that
the electrical resistance within the flux cored wire is higher than with
solid MIG wires and it is this higher electrical resistance that gives this
type of wire some of its novel operating properties.
One often quoted property of fluxed cored wires are their higher
deposition rates than solid MIG wires. What is often not explained is how
they deliver these higher values and whether these can be utilised. For
example, if a solid MIG wire is used at 250 amps, then exchanged for a
flux cored wire of the same diameter, and welding power source controls
are left unchanged, then the current reading would be much less than
250 amps, perhaps as low as 220 amps. This is because of Ohms Law
that states that as the electrical resistance increases if the voltage
remains stable then the current must fall.
To bring the welding current back to 250 amps it is necessary to
increase the wire feed speed, effectively increasing the amount of
wire being pushed into the weld pool to make the weld. It is this affect
that produces the ‘higher deposition rates’ that the flux cored wire
manufacturers claim for this type of product. Unfortunately in many
instances the welder has difficulty in utilising this higher wire feed speed
and must either increase the welding speed or increase the size of the
weld. Often in manual applications neither of these changes can be
implemented and the welder simply reduces the wire feed speed back
to where it was and the advantages are lost. However, if the process
is automated in some way then the process can show improvements in
productivity.
It is also common to use longer contact tip to workplace distances with
flux cored arc welding than with solid wire MIG welding and this also
has the effect of increasing the resistive heating on the wire further
accentuating the drop in welding current. Research has also shown
that increasing this distance can lead to an increase in the ingress of
nitrogen and hydrogen into the weld pool, which can affect the quality
of the weld.
Flux cored arc welding has a lower efficiency than solid wire MIG welding
because part of the wire fill contains slag forming agents. Although the
Extended self shielded flux cored wire nozzle
