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Process Schematic Diagram for MIG / FCAW and MCAW
Gas hose
Gas cylinder
Power source
Return cable
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'.
If no external gas shield is required, then the flux fill must provide
sufficient gas to protect the molten pool and to provide de-oxidisers
and nitride formers to cope with atmospheric contamination. This
leaves less scope to address performance, arc stabilisation, and
process tolerance, so these tend to suffer when compared with gas
shielded types.
Wire efficiencies are also lower, at about 65%, in this mode of
operation than with gas-shielded wires. However, the wires do have
a distinct advantage when it comes to site work in terms of wind
tolerance, as there is no external gas shield to be disrupted.
When using self-shielded wires, external gas supply is not
required and, therefore, the gas shroud is not necessary.
However, an extension nozzle is often used to support and direct
the long electrode extensions that are needed to obtain high
deposition rates.
Continous wire
Wire feed unit
Power cable
Torch conduit
Welding torch
Arc
Workpiece
Earth clamp
2.3 Introduction to Metal Cored
Arc Welding (MCAW)
How it Works
Metal-cored arc welding (MCAW) uses the heat generated by
a DC electric arc to fuse metal in the joint area, the arc being
struck between a continuously fed consumable filler wire and the
workpiece, melting both the filler wire and the workpiece in the
immediate vicinity. The entire arc area is covered by a shielding gas,
which protects the molten weld pool from the atmosphere.
As MCAW is a variant of the MIG welding process there are many
common features between the two processes, but there are also
several fundamental differences.
As with MIG, direct current power sources with constant voltage
output characteristics are normally employed to supply the welding
current. With metal-cored wires the terminal the filler wire is
connected to depends on the specific product being used. Some
wires are designed to run on electrode positive, others preferring
electrode negative, and some which will run on either. The work
return lead is then connected to the opposite terminal. Electrode
negative operation will usually give better positional welding
characteristics. The output characteristics of the power source can
have an effect on the quality of the welds produced.
The wire feed unit takes the filler wire from a spool or bulk
pack, and feeds it through the welding torch, to the arc at a
predetermined and accurately controlled speed. Normally, special
knurled feed rolls are used with metal-cored wires to assist feeding
and to prevent crushing the consumable.
MIG 250GS Operating manual
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