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Multiprocess 175. Operating manual.
Multiprocess 175. Operating manual.
2. Metal Inert Gas & Metal Active Gas arc
welding (MIG/MAG).
→
Argon with oxygen mixtures (MAG)
→
Argon with helium mixtures (MIG)
Each gas or gas mixture has specific advantages and limitations. Other
forms of MIG/MAG welding include using a flux-cored continuous
electrode and carbon dioxide shielding gas, or using self-shielding flux-
cored wire, requiring no shielding.
2.2 Introduction to Flux-Cored Arc Welding (FCAW)
How it works
Flux-cored arc welding (FCAW) uses the heat generated by a DC electric
arc to fuse the metal in the joint area, the arc being struck between a
continuously fed consumable filler wire and the workpiece, melting both
the filler wire and the workpiece in the immediate vicinity. The entire arc
area is covered by a shielding gas, which protects the molten weld pool
from the atmosphere.
FCAW is a variant of the MIG/MAG process and while there are many
common features between the two processes, there are also several
fundamental differences.
As with MIG/MAG, direct current power sources with constant voltage
output characteristics are normally employed to supply the welding
current. With flux-cored wires the terminal that the filler wire is
connected to depends on the specific product being used, some wires
running electrode positive, others running electrode negative. The work
return is then connected to the opposite terminal. It has also been found
that the output characteristics of the power source can have an effect on
the quality of the welds produced.
2.1 Introduction to Metal Inert Gas (MIG)
& Metal Active Gas (MAG)
MIG/MAG welding embraces a group of arc welding processes in which
a continuous electrode (the wire) is fed by powered feed rolls (wire
feeder) into the weld pool. An electric arc is created between the tip of
the wire and the weld pool. The wire is progressively melted at the same
speed at which it is being fed and forms part of the weld pool. Both the
arc and the weld pool are protected from atmospheric contamination by
a shield of inert (non-reactive) gas, which is delivered through a nozzle
that is concentric with the welding wire guide tube.
Operation
MIG/MAG welding is usually carried out with a handheld torch as a semi-
automatic process. The MIG/MAG process can be suited to a variety of
job requirements by choosing the correct shielding gas, electrode (wire)
size and welding parameters. Welding parameters include the voltage,
travel speed, arc (stick-out) length and wire feed rate. The arc voltage
and wire feed rate will determine the filler metal transfer method.
This application combines the advantages of continuity, speed,
comparative freedom from distortion and the reliability of automatic
welding with the versatility and control of manual welding. The process
is also suitable for mechanised set-ups, and its use in this respect
is increasing.
MIG/MAG welding can be carried out using solid wire, flux-cored, or a
copper-coated solid wire electrode. The shielding gas or gas mixture may
consist of the following:
→
Argon (MIG)
→
Carbon dioxide (MAG)
→
Argon and carbon dioxide mixtures (MAG)
The wire feed unit takes the filler wire from a spool, and feeds it
through the welding torch, to the arc at a predetermined and accurately
controlled speed. Normally, special knurled feed rolls are used with flux-
cored wires to assist feeding and to prevent crushing the consumable.
Unlike MIG/MAG, which uses a solid consumable filler wire, the
consumable used in FCAW is of tubular construction, an outer metal
sheath being filled with fluxing agents plus metal powder. The flux fill
is also used to provide alloying, arc stability, slag cover, de-oxidisation,
and, with some wires, gas shielding.
In terms of gas shielding, there are two different ways in which this may
be achieved with the FCAW process.
→
Additional gas shielding supplied from an external source, such as a
gas cylinder
→
Production of a shielding gas by decomposition of fluxing agents
within the wire, self-shielding
Gas shielded wires are available with either a basic or rutile flux fill,
while self-shielded wires have a broadly basic-type flux fill. The flux
fill dictates the way the wire performs, the properties obtainable, and
suitable applications.
Gas shielded operation
Many cored wire consumables require an auxiliary gas shield in the same
way that solid wire MIG/MAG consumables do. These types of wire are
generally referred to as ‘gas shielded’.
Using an auxiliary gas shield enables the wire designer to concentrate
on the performance characteristics, process tolerance, positional
capabilities, and mechanical properties of the products.
In a flux-cored wire the metal sheath is generally thinner than that of a
self-shielded wire. The area of this metal sheath surrounding the flux-
cored wire is much smaller than that of a solid MIG/MAG wire. This
means that the electrical resistance within the flux-cored wire is higher
than with solid MIG/MAG wires and it is this higher electrical resistance
that gives this type of wire some of its novel operating properties.
One often quoted property of fluxed cored wires are their higher
deposition rates than solid MIG/MAG wires. What is often not explained
is how they deliver these higher values and whether these can be
utilised. For example, if a solid MIG/MAG wire is used at 250 amps, then
exchanged for a flux-cored wire of the same diameter, and welding
power source controls are left unchanged, then the current reading
would be much less than 250 amps, perhaps as low as 220 amps. This
is because of Ohms Law that states that as the electrical resistance
increases if the voltage remains stable then the current must fall.
To bring the welding current back to 250 amps it is necessary to
increase the wire feed speed, effectively increasing the amount of
wire being pushed into the weld pool to make the weld. It is this affect
that produces the ‘higher deposition rates’ that the flux-cored wire
manufacturers claim for this type of product. Unfortunately in many
instances the welder has difficulty in utilising this higher wire feed
speed and must either increase the welding speed or increase the size of
the weld. Often in manual applications neither of these changes can be
implemented and the welder simply reduces the wire feed speed back
to where it was and the advantages are lost. However, if the process
is automated in some way then the process can show improvements in
productivity.
It is also common to use longer contact tip to workplace distances with
flux-cored arc welding than with solid wire MIG/MAG welding and this
Typical MIG/MAG set up
1
Torch,
2
Torch trigger,
3
Shroud,
4
Gas diffuser,
5
Contact tip,
6
Welding wire,
7
Shielding,
8
Weld,
9
Droplets,
10
Weld pool
1
2
3
4
5
6
7
9
10
8
Extended self shielded flux-cored wire nozzle
