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Process Schematic Diagram for MIG/MAG, FCAW and MCAW
Gas hose
Gas cylinder
Power source
Return cable
123 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
BOC RAPTOR 200C & 250R MIG Operating manual
Continuous wire
Wire feed unit
Power cable
Torch conduit
Welding torch
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.
Arc
Workpiece
Earth clamp
29
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