31-08-2016, 12:16 PM
1451719516-IntroMIGWelding.pdf (Size: 3.27 MB / Downloads: 9)
General
MIG (Metal Inert Gas) welding, also known as MAG (Metal Active Gas) and in the USA as GMAW (Gas Metal Arc
Welding), is a welding process that is now widely used for welding a variety of materials, ferrous and non ferrous.
The essential feature of the process is the small diameter electrode wire, which is fed continuously into the arc from
a coil. As a result this process can produce quick and neat welds over a wide range of joints.
Equipment
● DC output power source
● Wire feed unit
● Torch
● Work return welding lead
● Shielding gas supply, (normally from cylinder)
Power Source
MIG welding is carried out on DC electrode (welding wire) positive polarity
(DCEP). However DCEN is used (for higher burn off rate) with certain selfshielding
and gas shield cored wires.
DC output power sources are of a transformer-rectifier design, with a flat
characteristic (constant voltage power source). The most common type of
power source used for this process is the switched primary transformer
rectifier with constant voltage characteristics from both 3-phase 415V and 1-
phase 240V input supplies.
The output of direct current after full wave rectification from a 3-phase
machine is very smooth. To obtain smooth output after full wave rectification
with a 1- phase machine, a large capacitor bank across the output is required.
Because of the expense of this, many low cost 1-phase machines omit this
component and therefore provide a poorer weld characteristic.
The switches to the main transformer primary winding provide the output
voltage steps at the power source output terminals.
Another method of producing different voltages at the power source output
terminals is to use a Thyristor or a Transistor rectifier instead of a simple
diode rectifier. This system offers continuously variable output voltage, which can be particularly useful on robot
installations and the cost of this type of rectifier can be partly offset with no
need for primary voltage switch or switches and a single tapped main
transformer primary winding.
Most MIG power
sources have a
contactor or relay
used to switch the
output ON/OFF
with operations of
the trigger on the
MIG torch. The
switch off
operation of this
contactor is
normally delayed
to allow the
welding wire to
Burn back out of
the molten weld pool. A thermostat is fitted on the hottest point in the power
source, in series with the contactor coil to provide thermal protection to the
machine. Power source performance is measured by it’s ability to provide a
certain current for a percentage of a 10 minute period before “Thermal CutOut”.
This is the “Duty Cycle”.