Linde BOC Smootharc Advance MIG 425R Operating Manual page 28

28 BOC Smootharc Advance III MIG 425R & 555R Operating manual
In terms of gas shielding, there are two different ways in which this may be
achieved with the FCAW process.
→ Additional gas-shielding supplied from an external source, such as a gas
cylinder
→
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.
Extended self shielded flux cored wire nozzle
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 workpiece 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
efficiency differs by wire type and manufacturer it is typically between
75–85%.
Flux cored arc welding does, however, have the same drawback as solid wire
MIG in terms of gas disruption by wind, and screening is always necessary
for site work. It also incurs the extra cost of shielding gas, but this is often
outweighed by gains in productivity.
Self-shielded Operation
There are also self-shielded consumables designed to operate without an
additional gas shield. In this type of product, arc shielding is provided by
gases generated by decomposition of some constituents within the flux fill.
These types of wire are referred to as 'self-shielded'.