Linde BOC Smootharc Advance MIG 425R Operating Manual page 30

30 BOC Smootharc Advance III MIG 425R & 555R Operating manual
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.
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.
Schematic of Dip Transfer
1 Short circuit
2 Necking
3 Arc re-ignition
1
4 Arc established
5 Arc gap shortens
6 Short circuit
Current (A)
Voltage (V)
Short circuit cycle
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.
Globular Transfer
Metal transfer is controlled by slow ejection resulting in large, irregularly-
shaped 'globs' falling into the weld pool under the action of gravity. Carbon
dioxide gas drops are dispersed haphazardly. With argon-based gases, the
drops are not as large and are transferred in a more axial direction. There
is a lot of spatter, especially in carbon dioxide, resulting in greater wire
consumption, poor penetration and poor appearance. Globular transfer occurs
between the dip and spray ranges. This mode of transfer is not recommended
for normal welding applications and may be corrected when encountered
by either decreasing the arc voltage or increasing the amperage. Globular
transfer can take place with any electrode diameter.
Basic flux-cored wires tend to operate in a globular mode or in a globular-
spray transfer mode where larger than normal spray droplets are propelled
across the arc, but they never achieve a true spray transfer mode. This
transfer mode is sometimes referred to as non-axial globular transfer.
Self-shielded flux-cored wires operate in a predominantly globular transfer
mode although at high currents the wire often 'explodes' across the arc.
Spray Transfer
In spray transfer, metal is projected by an electromagnetic force from the wire
tip in the form of a continuous stream of discrete droplets approximately the
same size as the wire diameter. High deposition rates are possible and weld
appearance and reliability are good. Most metals can be welded, but the
technique is limited generally to plate thicknesses greater than 6mm. Spray
transfer, due to the tendency of the large weld pool to spill over, cannot
normally be used for positional welding. The main exception is aluminium
and its alloys where, primarily because of its low density and high thermal
conductivity, spray transfer in position can be carried out.
2 3 4 5
Time
Arcing cycle
6