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Process Schematic Diagram for MIG / FCAW and MCAW
Gas hose
Gas cylinder
Power source
Return cable
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.
10.3 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
BOC Smootharc Advance III MIG 425R & 555R Operating manual
Continous wire
Wire feed unit
Power cable
Torch conduit
Welding torch
Arc
Workpiece
Earth clamp
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.
Unlike MIG, which uses a solid consumable filler wire, the consumable used
in MCAW is of tubular construction, an outer metal sheath being filled entirely
with metal powder except for a small amount of non-metallic compounds.
These are added to provide some arc stability and de-oxidation.
MCAW consumables always require an auxiliary gas shield in the same way
that solid MIG wires do. Wires are normally designed to operate in argon-
carbon dioxide or argon-carbon dioxide-oxygen mixtures or carbon dioxide.
Argon rich mixtures tend to produce lower fume levels than carbon dioxide.
As with MIG, the consumable filler wire and the shielding gas are directed
into the arc area by the welding torch. In the head of the torch, the welding
current is transferred to the wire by means of a copper alloy contact tip, and
a gas diffuser distributes the shielding gas evenly around a shroud which
then allows the gas to flow over the weld area. The position of the contact
tip relative to the gas shroud may be adjusted to limit the minimum electrode
extension.
Modes of metal transfer with MCAW are very similar to those obtained in MIG
welding, the process being operable in both 'dip transfer' and 'spray transfer'
modes. Metal-cored wires may also be used in pulse transfer mode at low
mean currents, but this has not been widely exploited.
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