EN
Multiprocess 175. Operating manual.
Multiprocess 175. Operating manual.
13
12
Schematic of globular transfer
1
Large droplet,
2
Splatter,
3
Workpiece
Schematic of spray transfer
1
Gas shroud,
2
Wire,
3
Shielding gas,
4
Droplets,
5
Weld,
6
Workpiece
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.
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.
The current flows continuously because of the high voltage maintaining
a long arc and short-circuiting cannot take place. It occurs best with
argon-based gases.
In solid wire MIG/MAG, as the current is increased, dip transfer passes
into spray transfer via a transitional globular transfer mode. With metal-
cored wires there is virtually a direct transition from dip transfer to spray
transfer as the current is increased.
For metal-cored wires, spray transfer occurs as the current density
increases and an arc is formed at the end of the filler wire, producing
a stream of small metal droplets. Often the outside sheath of the wire
will melt first and the powder in the centre flows as a stream of smaller
droplets into the weld pool. This effect seems to give much better
transfer of alloying elements into the weld.
In spray transfer, as the current density increases, an arc is formed at
the end of the filler wire, producing a stream of small metal droplets. In
solid wire MIG/MAG this transfer mode occurs at higher currents. Flux-
cored wires do not achieve a completely true spray transfer mode but a
transfer mode that is almost true spray may occur at higher currents and
can occur at relatively low currents depending on the composition of the
flux.
Rutile flux-cored wires will operate in this almost-spray transfer mode, at
all practicable current levels. They are also able to operate in this mode
for positional welding too. Basic flux-cored and self-shielded flux-cored
wires do not operate in anything approaching true spray transfer mode.
Pulsed transfer
Pulsed arc welding is a controlled method of spray transfer, using
currents lower than those possible with the spray transfer technique,
thereby extending the applications of MIG/MAG welding into the range
of material thickness where dip transfer is not entirely suitable.The
pulsed arc equipment effectively combines two power sources into one
integrated unit. One side of the power source supplies a background
current which keeps the tip of the wire molten. The other side produces
pulses of a higher current that detach and accelerate the droplets of
metal into the weld pool. The transfer frequency of these droplets is
regulated primarily by the relationship between the two currents. Pulsed
arc welding occurs between ±50-220A, 23–35 arc volts and only with
argon and argon-based gases. It enables welding to be carried out in all
positions.
2.5 Fundamentals of MIG/MAG, FCAW and MCAW
Welding technique
Successful welding depends on the following factors:
1. Selection of correct consumables
2. Selection of the correct power source
3. Selection of the correct polarity on the power source
4. Selection of the correct shielding gas
5. Selection of the correct application techniques
a Correct angle of electrode to work
b Correct electrical stickout
c Correct travel speed
6. Selection of the welding preparation
Selection of correct consumable
Chemical composition
As a general rule the selection of a wire is straightforward, in that it
is only a matter of selecting an electrode of similar composition to
the parent material. It will be found, however, that there are certain
applications where electrodes will be selected on the basis of their
mechanical properties or level of residual hydrogen in the weldmetal.
Solid MIG/MAG wires are all considered to be of the ‘low Hydrogen type’
consumables.
Physical condition
Surface condition
The welding wire must be free from any surface contamination including
mechanical damage such as scratch marks.
A simple test for checking the surface condition is to run the wire
through a cloth that has been dampened with acetone for 20 secs. If a
black residue is found on the cloth the surface of the wire is not properly
cleaned.
1
2
3
1
2
3
4
5
6
Typical metal transfer mode
Process
Metal Inert Gas (MIG)
Metal Active Gas (MAG)
Flux-Cored (Gas Shielded)
Flux-Cored (Self Shielded)
Metal-Cored
Dip
transfer
Yes
Yes
Yes
Yes
Globular
transfer
No
Yes
Yes
No
Spray
transfer
Yes
Yes*
No
Yes
* Not true spray
