Equipment for MIG Welding
The MIG process is a versatile
welding technique which is suitable for both thin sheet and thick section
components. It is capable of high productivity but the quality of welds can be
called into question. To achieve satisfactory welds, welders must have a good
knowledge of equipment requirements and should also recognise fully the
importance of setting up and maintaining component parts correctly.
Essential equipment
In MIG the arc is formed between the end of a small diameter wire
electrode fed from a spool, and the workpiece. Main equipment components are:
- power source
- wire feed system
- conduit
- gun
The arc and weldpool are protected from the atmosphere by a
gas shield. This enables bare wire to be used without a flux coating (required
by MMA). However, the absence of flux to 'mop up' surface oxide places greater
demand on the welder to ensure that the joint area is cleaned immediately before
welding. This can be done using either a wire brush for relatively clean parts,
or a hand grinder to remove rust and scale. The other essential piece of
equipment is a wire cutter to trim the end of the electrode wire.
Power source
MIG is operated exclusively with a DC power source. The
source is termed a flat, or constant current, characteristic power source, which
refers to the voltage/welding current relationship. In MIG, welding current is
determined by wire feed speed, and arc length is determined by power source
voltage level (open circuit voltage). Wire burn-off rate is automatically
adjusted for any slight variation in the gun to workpiece distance, wire feed
speed, or current pick-up in the contact tip. For example, if the arc
momentarily shortens, arc voltage will decrease and welding current will be
momentarily increased to burn back the wire and maintain pre-set arc length. The
reverse will occur to counteract a momentary lengthening of the arc.
There is a wide range of power sources available, mode of metal transfer can
be:
A low welding current is used for thin-section material, or
welding in the vertical position. The molten metal is transferred to the
workpiece by the wire dipping into the weldpool. As welding parameters will vary
from around 100A \ 17V to 200A 22V (for a 1.2mm diameter wire), power sources
normally have a current rating of up to 350A. Circuit inductance is used to
control the surge in current when the wire dips into the weldpool (this is the
main cause of spatter). Modern electronic power sources automatically set the
inductance to give a smooth arc and metal transfer.
In spray metal transfer, metal transfers as a spray of fine droplets without
the wire touching the weldpool. The welding current level needed to maintain the
non short-circuiting arc must be above a minimum threshold level; the arc
voltage is higher to ensure that the wire tip does not touch the weldpool.
Typical welding parameters for a 1.2mm diameter wire are within 250A \ 28V to
400A \ 35V. For high deposition rates the power source must have a much higher
current capacity: up to 500A.
The pulsed mode provides a means of achieving a spray type metal transfer at
current levels below threshold level. High current pulses between 25 and 100Hz
are used to detach droplets as an alternative to dip transfer. As control of the
arc and metal transfer requires careful setting of pulse and background
parameters, a more sophisticated power source is required. Synergic pulsed MIG
power sources, which are advanced transistor-controlled power sources, are
preprogrammed so that the correct pulse parameters are delivered automatically
as the welder varies wire feed speed.
Welding current and arc voltage ranges for selected wire diameters operating
with dip and spray metal transfer:
| Wire diameter (mm) |
Dip transfer |
Spray transfer |
| Current (A) |
Voltage (V) |
Current (A) |
Voltage (V) |
| 0.6 |
30 - 80 |
15 - 18 |
| 0.8 |
45 - 180 |
16 - 21 |
150 - 250 |
25 - 33 |
| 1.0 |
70 - 180 |
17 - 22 |
230 - 300 |
26 - 35 |
| 1.2 |
100 - 200 |
17 - 22 |
250 - 400 |
27 - 35 |
| 1.6 |
120 - 200 |
18 - 22 |
250 - 500 |
30 - 40 |
Wire feed system
The performance of the wire feed system can be crucial
to the stability and reproducibility of MIG welding. As the system must be
capable of feeding the wire smoothly, attention should be paid to the feed rolls
and liners. There are three types of feeding systems:
- pinch rolls
- push-pull
- spool on gun
The conventional wire feeding system normally has a
set of rolls where one is grooved and the other has a flat surface. Roll
pressure must not be too high otherwise the wire will deform and cause poor
current pick up in the contact tip. With copper coated wires, too high a roll
pressure or use of knurled rolls increases the risk of flaking of the coating
(resulting in copper build up in the contact tip). For feeding soft wires such
as aluminium dual-drive systems should be used to avoid deforming the soft wire.
Small diameter aluminium wires, 1mm and smaller, are more reliably fed using
a push-pull system. Here, a second set of rolls is located in the welding gun -
this greatly assists in drawing the wire through the conduit. The disadvantage
of this system is increased size of gun. Small wires can also be fed using a
small spool mounted directly on the gun. The disadvantages with this are
increased size, awkwardness of the gun, and higher wire cost.
Conduit
The conduit can measure up to 5m in length, and to facilitate
feeding, should be kept as short and straight as possible. (For longer lengths
of conduit, an intermediate push-pull system can be inserted). It has an
internal liner made either of spirally-wound steel for hard wires (steel,
stainless steel, titanium, nickel) or PTFE for soft wires (aluminium, copper).
Gun
In addition to directing the wire to the joint, the welding gun
fulfils two important functions - it transfers the welding current to the wire
and provides the gas for shielding the arc and weldpool.
There are two types of welding guns: 'air' cooled and water cooled. The 'air'
cooled guns rely on the shielding gas passing through the body to cool the
nozzle and have a limited current-carrying capacity. These are suited to light
duty work. Although 'air' cooled guns are available with current ratings up to
500A, water cooled guns are preferred for high current levels, especially at
high duty cycles.
Welding current is transferred to the wire through the contact tip whose bore
is slightly greater than the wire diameter. The contact tip bore diameter for a
1.2mm diameter wire is between 1.4 andt 1.5mm. As too large a bore diameter
affects current pick up, tips must be inspected regularly and changed as soon as
excessive wear is noted. Copper alloy (chromium and zirconium additions) contact
tips, harder than pure copper, have a longer life, especially when using spray
and pulsed modes.
Gas flow rate is set according to nozzle diameter and gun to workpiece
distance, but is typically between 10 and 30 l/min. The nozzle must be cleaned
regularly to prevent excessive spatter build-up which creates porosity.
Anti-spatter spray can be particularly effective in automatic and robotic
welding to limit the amount of spatter adhering to the nozzle.
Protective equipment
A darker glass than that used for MMA welding at
the same current level should be used in hand or head shields.
Recommended shade number of filter for MIG/MAG welding:
| Shade number |
Welding current A |
| MIG Heavy metal |
MIG Light metal |
MAG |
| 10 |
under 100 |
under 100 |
under 80 |
| 11 |
1001 - 175 |
100 - 175 |
80 - 125 |
| 12 |
175 - 300 |
175 - 250 |
125 - 175 |
| 13 |
300 - 500 |
250 - 350 |
175 - 300 |
| 14 |
over 500 |
350 - 500 |
300 - 500 |
| 15 |
|
over 500 |
over 450 |
Copyright by TWI, 1999
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