MIG Welders and MIG Welding Equipment
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Gas metal arc welding (GMAW)
sometimes referred to by its subtypes metal inert gas (MIG)
welding or metal active gas (MAG) welding, is a
semi-automatic or automatic arc welding process in which a
continuous and consumable wire electrode and a shielding gas are
fed through a welding gun.
The typical MIG welding
gun has a number of key parts—a control switch, a contact tip, a
power cable, a gas nozzle, an electrode conduit and liner, and a
gas hose. The control switch, or trigger, when pressed by the
operator, initiates the wire feed, electric power, and the
shielding gas flow, causing an electric arc to be struck. The
contact tip is connected to the welding power source through the
power cable and transmits the electrical energy to the electrode
while directing it to the weld area. It must be firmly secured
and properly sized, since it must allow the passage of the
electrode while maintaining an electrical contact. Before
arriving at the contact tip, the wire is protected and guided by
the electrode conduit and liner, which help prevent buckling and
maintain an uninterrupted wire feed. The gas nozzle is used to
evenly direct the shielding gas into the welding zone—if the
flow is inconsistent, it may not provide adequate protection of
the weld area. Larger nozzles provide greater shielding gas
flow, which is useful for high current welding operations, in
which the size of the molten weld pool is increased. The gas is
supplied to the nozzle through a gas hose, which is connected to
the tanks of shielding gas. Sometimes, a water hose is also
built into the welding gun, cooling the gun in high heat
operations.
The wire feed unit
supplies the wire to the work, driving it through the conduit
and on to the contact tip. Most models provide the wire at a
constant feed rate, but more advanced machines can vary the feed
rate in response to the arc length and voltage. Shielding gases
are necessary for gas metal arc welding to protect the welding
area from atmospheric gases, such as nitrogen and oxygen, which
can cause fusion defects
MIG Welding can produce all kinds of joints and welds
depending upon the type of work being carried out, the appropriate shielding gas
and wire. Two types of welding method are commonly used on MIG welding machines:
Globular Transfer and Spray Transfer
Globular metal transfer occurs at relatively low operating currents and voltages
but these are still higher than those used in short circuiting transfer. This
metal transfer mode is characterised by a drop, two or three times larger in
diameter than the wire, formed at the tip of the electrode. This droplet is
detached from the tip of the electrode by the effect of a pinch force and the
transfer of the droplets in irregular form across the arc is aided by the effect
of the weak electromagnetic and strong gravity forces. As the droplets grow on
the tip of the wire electrode they wobble around and disturb the arc plasma
stability. Consequently, the heat-affected zone in the work becomes narrow,
penetration of the weld becomes small, and the weld deposit is irregular and
large amounts of spatter takes place
Spray Transfer occurs under an argon-rich shielding gas, increasing the current
and voltage causes a new mode of metal transfer to appear: the tip of the wire
electrode is tapped, the sizes of the droplets become smaller and they are
directed axially in a straight line from the wire to the weld pool.
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