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10.0 MIG Process
10.1 Introduction to Metal Inert Gas (MIG)
MIG 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 welding is usually carried out with a handheld torch as a semi-automatic
process. The MIG 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 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
→
Carbon dioxide
→
Argon and carbon dioxide mixtures
→
Argon mixtures with oxygen or helium mixtures
Each gas or gas mixture has specific advantages and limitations. Other forms
of MIG welding include using a flux-cored continuous electrode and carbon
BOC Smootharc Advance III MIG 425R & 555R Operating manual
Typical MIG set up
Torch trigger
Welding wire
Weld
Weld pool
dioxide shielding gas, or using self-shielding flux-cored wire, requiring no
shielding gas.
10.2 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 process and while there are many common
features between the two processes, there are also several fundamental
differences.
As with MIG, 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.
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, 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-oxidation, and, with some wires, gas shielding.
27
Torch
Shroud
Gas diffuser
Contact tip
Shielding
Droplets
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