Operating instructions

10
BOC Smootharc Advance II MIG 250C Operating manual
Unlike MIG, which uses a solid consumable ller wire, the consumable
used in MCAW is of tubular construction, an outer metal sheath being
lled 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 ller 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 di󹟽user distributes the shielding gas evenly around
a shroud which then allows the gas to ow 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 ux ll, 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 ux-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
su󹟾cient to allow the arc to be re-established. This short-circuiting mode
of metal transfer e󹟽ectively 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
di󹟾cult materials where heat input is critical. Each short-circuit causes
the current to rise and the metal fuses o󹟽 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.
1 2 63 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