Solid wire MIG welding
Metal inert gas (MIG) welding was first patented in the USA in 1949 for
welding aluminium. The arc and weld pool formed using a bare wire
electrode was protected by helium gas, readily available at that time.
From about 1952 the process became popular in the UK for welding
aluminium using argon as the shielding gas, and for carbon steels using
CO2. CO2
and argon-CO2 mixtures are known as
metal active gas (MAG) processes. MIG is an attractive alternative to
MMA, offering high deposition rates and high productivity.
Process characteristics
MIG is similar to MMA in that heat for welding is produced by forming an
arc between a metal electrode and the workpiece; the electrode melts to
form the weld bead. The main difference is that the metal electrode is a
small diameter wire fed from a spool. As the wire is continuously fed,
the process is often referred to as semi-automatic welding.
Metal transfer mode
The manner, or mode, in which the metal transfers from the electrode to
the weld pool largely determines the operating features of the process.
There are three principal metal transfer modes:
- Short circuiting
- Droplet / spray
- Pulsed
Short-circuiting and pulsed metal transfer are used for low current
operation while spray metal transfer is only used with high welding
currents. In short-circuiting or'dip' transfer, the molten metal forming
on the tip of the wire is transferred by the wire dipping into the weld
pool. This is achieved by setting a low voltage; for a 1.2mm diameter
wire, arc voltage varies from about 17V (100A) to 22V (200A). Care in
setting the voltage and the inductance in relation to the wire feed
speed is essential to minimise spatter. Inductance is used to control
the surge in current which occurs when the wire dips into the weld pool.
For droplet or spray transfer, a much higher voltage is necessary to
ensure that the wire does not make contact i.e.short-circuit, with the
weld pool; for a 1.2mm diameter wire, the arc voltage varies from
approximately 27V (250A) to 35V (400A). The molten metal at the tip of
the wire transfers to the weld pool in the form of a spray of small
droplets (about the diameter of the wire and smaller). However, there is
a minimum current level, threshold, below which droplets are not
forcibly projected across the arc. If an open arc technique is attempted
much below the threshold current level, the low arc forces would be
insufficient to prevent large droplets forming at the tip of the wire.
These droplets would transfer erratically across the arc under normal
gravitational forces. The pulsed mode was developed as a means of
stabilising the open arc at low current levels i.e. below the threshold
level, to avoid short-circuiting and spatter. Spray type metal transfer
is achieved by applying pulses of current, each pulse having sufficient
force to detach a droplet. Synergic pulsed MIG refers to a special type
of controller which enables the power source to be tuned (pulse
parameters) for the wire composition and diameter, and the pulse
frequency to be set according to the wire feed speed.
Shielding gas
In addition to general shielding of the arc and the weld pool, the
shielding gas performs a number of important functions:
- forms the arc plasma
- stabilises the arc roots on the material surface
- ensures smooth transfer of molten droplets from the wire to the
weld pool
Thus, the shielding gas will have a substantial effect on the stability
of the arc and metal transfer and the behaviour of the weld pool, in
particular, its penetration. General purpose shielding gases for MIG
welding are mixtures of argon, oxygen and C02, and special gas mixtures
may contain helium. The gases which are normally used for the various
materials are:
- steels
- CO2
- argon +2 to 5% oxygen
- argon +5 to 25% CO2
- non-ferrous
Argon based gases, compared with CO2,
are generally more tolerant to parameter settings and generate lower
spatter levels with the dip transfer mode. However, there is a greater
risk of lack of fusion defects because these gases are colder. As CO2
cannot be used in the open arc (pulsed or spray transfer) modes due to
high back-plasma forces, argon based gases containing oxygen or CO2
are normally employed.
Applications
MIG is widely used in most industry sectors and accounts for almost 50%
of all weld metal deposited. Compared to MMA, MIG has the advantage in
terms of flexibility, deposition rates and suitability for mechanisation.
However, it should be noted that while MIG is ideal for 'squirting'
metal, a high degree of manipulative skill is demanded of the welder.
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