CEMP-E
TI 809-26
1 March 2000
3. GAS METAL ARC WELDING (GMAW).
a. Process Principles. The Gas Metal Arc Welding (GMAW ) process, commonly referred to as "MIG"
(Metal Inert Gas) welding, is very similar to gas-shielded flux cored arc welding (FCAW-G), and uses the
same equipment. GMAW uses a solid or metal cored electrode, and subsequently leaves little, if any,
slag. The shielding gas used for GMAW may be carbon dioxide (CO2), or a mixture of argon (Ar) and
either CO2 or small levels of oxygen (O), or both. GMAW is commonly applied in one of four ways: spray
arc transfer, globular transfer, pulsed arc transfer, and short arc transfer.
(1) Spray arc transfer uses high wire feed speeds and relatively high voltages. A fine spray of
molten drops, all smaller in diameter than the electrode diameter, is ejected from the electrode toward
the work. The arc in spray transfer is continuously maintained, resulting in high quality welds with good
appearance. The shielding used for spray arc transfer is composed of at least 80% argon, with the
balance made up of either carbon dioxide or oxygen. Typical mixtures are 90% argon with 10% CO2, and
95% argon with 5% oxygen. Because of the intensity of the arc, puddle fluidity, and lack of slag to hold
the molten metal in place, spray arc is limited to the flat and horizontal position.
(2) Globular transfer results when high concentrations of carbon dioxide are used. Carbon dioxide,
as an active gas rather than inert gas, may be referred to as "MAG" (Metal Active Gas) welding. Because
of the high concentration of CO2, the arc ejects large globular pieces of molten steel from the end of the
electrode, rather than a spray. This mode of transfer can result in deep penetration, but may have poor
appearance with relatively high levels of spatter. It is also limited to the flat and horizontal positions.
Because of the lower cost of CO2 shielding gas, the lower level of heat generated, and increased welder
comfort, globular transfer may be selected in place of spray transfer.
(3) Pulsed arc transfer uses a background current that is continuously applied to the electrode, plus
a pulsing peak current applied at a rate proportional to the wire feed speed. Each pulse of current ejects
a single droplet of metal from the electrode, usually between 100 and 400 times per second. The arc is
maintained by the lower background current. Pulsed arc transfer can be used out-of-position, with better
quality than short-circuiting mode. It is not as productive as spray transfer for welding in the flat and
horizontal positions. Weld appearance and quality are generally good. Pulsed arc transfer GMAW
equipment is somewhat more complex and costly than standard GMAW equipment.
(4) Short circuiting transfer, also called short arc, is suitable for welding only on thin gauge
materials, and should not be used for structural steel. The small diameter electrode is fed at a moderate
wire feed speed using relatively low voltage. The electrode contacts the workpiece, shorting the electrical
circuit, extinguishing the arc, resulting in very high current flowing through the electrode, causing it to
heat and melt. As the electrode melts, the arc is briefly reestablished. This cycle occurs up to 200 times
per second, creating a characteristic buzzing sound. With structural steel, significant fusion problems
such as cold lap may result. Short circuiting transfer provides a low deposition rate, but can be used out
of position. While GMAW is considered prequalified by AWS D1.1, the short circuiting mode of transfer,
abbreviated GMAW-S, is not. All GMAW-S welding procedures must be qualified by test.
b. Filler Metal Designation, Specification and Certification. GMAW electrodes are classified under
AWS A5.18 for carbon steel electrodes, and AWS A5.28 for low alloy steel electrodes. The classification
systems used for GMAW electrodes in AWS A5.18 and A5.28 are summarized in Tables C-10 and C-11.
(1) Classification testing is usually performed using specific welding procedures that use CO2
shielding gas, therefore promoting globular transfer, but other gases, and therefore transfer modes, may
be specified.
C-16
Previous Page
