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What is MIG welding? 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. A
constant voltage, direct current power source is most commonly used
with GMAW, but constant current systems, as well as alternating
current, can be used. There are four primary methods of metal
transfer in GMAW, called globular, short-circuiting, spray, and
pulsed-spray, each of which has distinct properties and
corresponding advantages and limitations.
To perform gas metal arc welding, the basic
necessary equipment is a welding gun, a wire feed unit, a welding
power supply, an electrode wire, and a shielding gas supply.
Welding gun and
wire feed unit
GMAW torch nozzle cutaway image.
(1) Torch handle, (2) Molded phenolic
dielectric (shown in white) and threaded metal nut
insert (yellow), (3) Shielding gas nozzle, (4)
Contact tip, (5) Nozzle output face
The typical GMAW 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, normally made of copper
and sometimes chemically treated to reduce spatter, 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 electrode 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. Some wire feeders can reach feed rates as high
as 1200 in/min, but feed rates for semiautomatic GMAW typically
range from 75–400 in/min.
Electrode selection is based primarily on the
composition of the metal being welded, but also on the process
variation being used, the joint design, and the material surface
conditions. The choice of an electrode strongly influences the
mechanical properties of the weld area, and is a key factor in weld
quality. In general, the finished weld metal should have mechanical
properties similar to those of the base material, with no defects
such as discontinuities, entrained contaminants, or porosity, within
the weld. To achieve these goals a wide variety of electrodes exist.
All commercially available electrodes contain deoxidizing metals
such as silicon, manganese, titanium, and aluminum in small
percentages to help prevent oxygen porosity, and some contain
denitriding metals such as titanium and zirconium to avoid nitrogen
porosity. Depending on the process variation and base material being
used, the diameters of the electrodes used in GMAW typically range
from 0.028–0.095 in, but can be as large as 0.16 in. The smallest
electrodes, generally up to 0.045 in are associated with the
short-circuiting metal transfer process, while the most common
spray-transfer process mode electrodes are usually at least
0.035 in.
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, porosity,
and weld metal embrittlement if they come in contact with the
electrode, the arc, or the welding metal. This problem is common to
all arc welding processes, but instead of a shielding gas, many arc
welding methods utilize a flux material which disintegrates into a
protective gas when heated to welding temperatures. In GMAW,
however, the electrode wire does not have a flux coating, and a
separate shielding gas is employed to protect the weld. This
eliminates slag, the hard residue from the flux that builds up after
welding and must be chipped off to reveal the completed weld.
The choice of a shielding gas depends on
several factors, most importantly the type of material being welded
and the process variation being used. Pure inert gases such as argon
and helium are only used for nonferrous welding; with steel they do
not provide adequate weld penetration (argon) or cause an erratic
arc and encourage spatter (with helium). Pure carbon dioxide, on the
other hand, allows for deep penetration welds but encourages oxide
formation, which adversely affect the mechanical properties of the
weld. Its low cost makes it an attractive choice, but because of the
violence of the arc, spatter is unavoidable and welding thin
materials is difficult. As a result, argon and carbon dioxide are
frequently mixed in a 75%/25% to 90%/10% mixture. Generally, in
short circuit GMAW, higher carbon dioxide content increases the weld
heat and energy when all other weld parameters (volts, current,
electrode type and diameter) are held the same. As the carbon
dioxide content increases over 20%, spray transfer GMAW becomes
increasingly problematic with thinner electrodes.
Argon is also commonly mixed with other gases,
such as oxygen, helium, hydrogen, and nitrogen. The addition of up
to 5% oxygen (like the higher concentrations of carbon dioxide
mentioned above) can be helpful in welding stainless steel or in
very thin gauge materials, however, in most applications carbon
dioxide is preferred. Increased oxygen makes the shielding gas
oxidize the electrode, which can lead to porosity in the deposit if
the electrode does not contain sufficient deoxidizers. Argon-helium
mixtures are completely inert, and can be used on nonferrous
materials. A helium concentration of 50%–75% raises the voltage and
increases the heat in the arc. Higher percentages of helium also
improve the weld quality and speed of using alternating current for
the welding of aluminum. Hydrogen is sometimes added to argon in
small concentrations (up to about 5%) for welding nickel and thick
stainless steel work pieces. In higher concentrations (up to 25%
hydrogen), it is useful for welding conductive materials such as
copper. However, it should not be used on steel, aluminum or
magnesium because of the risk of hydrogen porosity. Additionally,
nitrogen is sometimes added to argon to a concentration of 25%–50%
for welding copper, but the use of nitrogen, especially in North
America, is limited. Mixtures of carbon dioxide and oxygen are
similarly rarely used in North America, but are more common in
Europe and Japan.
Shielding gas mixtures of three or more gases
are also available. claiming to improve weld quality. Mixtures of
argon, carbon dioxide and oxygen are marketed for welding steels.
Other mixtures add a small amount of helium to argon-oxygen
combinations, these mixtures reportedly allow higher arc voltages
and welding speed. Helium is also sometimes used as the base gas,
with small amounts of argon and carbon dioxide added. Additionally,
other specialized and often proprietary gas mixtures purport even
greater benefits for specific applications.
The desirable rate of gas flow depends
primarily on weld geometry, speed, current, the type of gas, and the
metal transfer mode being utilized. Welding flat surfaces requires
higher flow than welding grooved materials, since the gas is
dispersed more quickly. Faster welding speeds mean that more gas
must be supplied to provide adequate coverage. Additionally, higher
current requires greater flow, and generally, more helium is
required to provide adequate coverage than argon. Perhaps most
importantly, the four primary variations of GMAW have differing
shielding gas flow requirements—for the small weld pools of the
short circuiting and pulsed spray modes, about 20 ft³/h is generally
suitable, while for globular transfer, around 15 L/min (30 ft³/h) is
preferred. The spray transfer variation normally requires more
because of its higher heat input and thus larger weld pool; along
the lines of 20–25 L/min (40–50 ft³/h).
Technique
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The basic technique for GMAW is quite simple,
since the electrode is fed automatically through the torch. By
contrast, in gas tungsten arc welding, the welder must handle a
welding torch in one hand and a separate filler wire in the other,
and in shielded metal arc welding, the operator must frequently chip
off slag and change welding electrodes. GMAW requires only that the
operator guide the welding gun with proper position and orientation
along the area being welded. Keeping a consistent contact
tip-to-work distance (the stickout distance) is important,
because a long stickout distance can cause the electrode to overheat
and will also waste shielding gas. Stickout distance varies for
different GMAW weld processes and applications. For short-circuit
transfer, the stickout is generally 1/4 inch to 1/2 inch, for spray
transfer the stickout is generally 1/2 inch. The position of the end
of the contact tip to the gas nozzle are related to the stickout
distance and also varies with transfer type and application. The
orientation of the gun is also important—it should be held so as to
bisect the angle between the work pieces; that is, at 45 degrees for
a fillet weld and 90 degrees for welding a flat surface. The travel
angle or lead angle is the angle of the torch with respect to the
direction of travel, and it should generally remain approximately
vertical. However, the desirable angle changes somewhat depending on
the type of shielding gas used—with pure inert gases, the bottom of
the torch is out often slightly in front of the upper section, while
the opposite is true when the welding atmosphere is carbon dioxide.
Always read and follow the safety precautions
and operational instructions in your owner's manual.
| Keep a 1/4 to 3/8 in stickout
(electrode extending from the tip of the contact tube.) |
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For thin metals, use
a smaller diameter wire. For thicker metal use a larger
wire and a larger machine. See machine recommendations
for welding capacity.
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| Use the correct wire
type for the base metal being welded. Use stainless
steel wires for stainless steel, aluminum wires for
aluminum, and steel wires for steel. |
| Use the proper shielding gas. CO2
is good for penetrating welds on steel, but may be too
hot for thin metal. Use 75% Argon/25% CO2 for thinner
steels. Use only Argon for aluminum. You can use a
triple-mix for stainless steels (Helium + Argon + CO2). |
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| For steel, there are two common
wire types. Use an AWS classification ER70S-3 for all
purpose, economical welding. Use ER70S-6 wire when more
deoxidizers are needed for welding on dirty or rusty
steel. |
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| For best control of
your weld bead, keep the wire directed at the leading
edge of the weld pool. |
| When welding out of
position (vertical, horizontal, or overhead welding),
keep the weld pool small for best weld bead control, and
use the smallest wire diameter size you can. |
| Be sure to match your
contact tube, gun liner, and drive rolls to the wire
size you are using. |
| Clean the gun liner
and drive rolls occasionally, and keep the gun nozzle
clean of spatter. Replace the contact tip if blocked or
feeding poorly. |
| Keep the gun straight
as possible when welding, to avoid poor wire feeding. |
| Use both hands to
steady the gun when you weld. Do this whenever possible.
(This also applies to Stick and TIG welding, and plasma
cutting.) |
| Keep wire feeder hub
tension and drive roll pressure just tight enough to
feed wire, but don't over tighten. |
| Keep wire in a clean,
dry place when not welding, to avoid picking up
contaminants that lead to poor welds. |
| Use DCEP (reverse
polarity) on the power source. |
| A drag or pull gun technique will
give you a bit more penetration and a narrower bead. A
push gun technique will give you a bit less penetration,
and a wider bead. |
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| When welding a fillet, the leg of
the weld should be equal to the thickness of the parts
welded. |
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