Smootharc 180 Multiprocess, Operating Manual

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11 BOC Smootharc 180 Multiprocess operating manual
Unlike MIG/MAG, 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/MAG 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/MAG, 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/MAG 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.
2.4 Modes of metal transfer
The mode or type of metal transfer in MIG/MAG and MCAW 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/
MAG welding, but here the mode of transfer is heavily dependent on the
composition of the flux fill, as well as on current and voltage.
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/MAG 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/MAG wires. This transfer mode is used for all positional work
with these types of wire.
1 2 6 3 4 5
Time
Short circuit cycle Arcing cycle
Current (A)
Voltage (V)
1 Short circuit
2 Necking
3 Arc re-ignition
4 Arc established
5 Arc gap shortens
6 Short circuit
Schematic of Dip Transfer