MESSER SolidWELD MIG 250GS Guide D'utilisation page 10

010 MIG 250GS Operating manual
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.
Modes of Metal Transfer
The mode or type of metal transfer in MIG welding depends upon
the current, arc voltage, electrode diameter and type of shielding
gas used. In general, there are four modes of metal transfer.
Modes of metal transfer with FCAW are similar to those obtained in
MIG welding, but here the mode of transfer is heavily dependent on
the composition of the flux fill, as well as on current and voltage.
Schematic of Dip Transfer
1 Short circuit
2 Necking
3 Arc re-ignition
1
4 Arc established
Arc gap
5
shortens
6 Short circuit
Current (A)
Voltage (V)
Short circuit cycle
The most common modes of transfer in FCAW are:
• Dip transfer
• Globular transfer
• Spray transfer
• Pulsed arc transfer operation has been applied to flux-cored
wires but, as yet, is not widely used because the other transfer
modes are giving users what they require, in most cases.
Dip Transfer
Also known as short-circuiting arc or short-arc, this is an all-
positional process, using low heat input. The use of relatively
low current and arc voltage settings cause the electrode to
intermittently short-circuit with the weld pool at a controlled
frequency. Metal is transferred by the wire tip actually dipping into
the weld pool and the short-circuit current is sufficient to allow the
arc to be re-established. This short-circuiting mode of metal transfer
effectively extends the range of MIG welding to lower currents
so thin sheet material can readily be welded. The low heat input
makes this technique well-suited to the positional welding of root
runs on thick plate, butt welds for bridging over large gaps and for
certain difficult materials where heat input is critical. Each short-
circuit causes the current to rise and the metal fuses off the end
of the electrode. A high short-circuiting frequency gives low heat
input. Dip transfer occurs between ±70-220A, 14–23 arc volts. It is
achieved using shielding gases based on carbon dioxide and argon.
Metal-cored wires transfer metal in dip mode at low currents just
like solid MIG wires. This transfer mode is used for all positional
work with these types of wire.
2 3 4 5
Time
Arcing cycle
6