30-11-2012, 06:13 PM
Gas metal arc welding
Gas metal arc welding - Wikipedia, the free encyclopedia.pdf (Size: 653.39 KB / Downloads: 433)
consumable wire electrode and the workpiece metal(s), which heats the
workpiece metal(s), causing them to melt, and join. Along with the wire
electrode, a shielding gas feeds through the welding gun, which shields
the process from contaminants in the air. The process can be semiautomatic
or automatic. 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.
Originally developed for welding aluminum and other non-ferrous
materials in the 1940s, GMAW was soon applied to steels because it
provided faster welding time compared to other welding processes. The
cost of inert gas limited its use in steels until several years later, when the use of semi-inert gases such as carbon
dioxide became common. Further developments during the 1950s and 1960s gave the process more versatility and
as a result, it became a highly used industrial process. Today, GMAW is the most common industrial welding
process, preferred for its versatility, speed and the relative ease of adapting the process to robotic automation.
Unlike welding processes that do not employ a shielding gas, such as shielded metal arc welding, it is rarely used
outdoors or in other areas of air volatility. A related process, flux cored arc welding, often does not use a shielding
gas, but instead employs an electrode wire that is hollow and filled with flux.
Development
The principles of gas metal arc welding began to be understood in the early 19th century, after Humphry Davy
discovered the short pulsed electric arcs in 1800.[1] Vasily Petrov independently produced the continuous electric
arc in 1802 (soon followed by Davy).[1] It was not until the 1880s that the technology became developed with the
aim of industrial usage. At first, carbon electrodes were used in carbon arc welding. By 1890, metal electrodes had
been invented by Nikolay Slavyanov and C. L. Coffin. In 1920, an early predecessor of GMAW was invented by
P. O. Nobel of General Electric. It used a bare electrode wire and direct current, and used arc voltage to regulate
the feed rate. It did not use a shielding gas to protect the weld, as developments in welding atmospheres did not
take place until later that decade. In 1926 another forerunner of GMAW was released, but it was not suitable for
practical use.[2]
In 1948, GMAW was finally developed by the Battelle Memorial Institute. It used a smaller diameter electrode and
a constant voltage power source developed by H. E. Kennedy. It offered a high deposition rate, but the high cost
of inert gases limited its use to non-ferrous materials and prevented cost savings. In 1953, the use of carbon dioxide
as a welding atmosphere was developed, and it quickly gained popularity in GMAW, since it made welding steel
more economical. In 1958 and 1959, the short-arc variation of GMAW was released, which increased welding
versatility and made the welding of thin materials possible while relying on smaller electrode wires and more
advanced power supplies. It quickly became the most popular GMAW variation. The spray-arc transfer variation
was developed in the early 1960s, when experimenters added small amounts of oxygen to inert gases. More
recently, pulsed current has been applied, giving rise to a new method called the pulsed spray-arc variation.[3]
Tool style
The top electrode holder is a semiautomatic air-cooled
holder. Compressed air circulates through it to maintain
moderate temperatures. It is used with lower current levels for welding lap or butt joints. The second most common
type of electrode holder is semiautomatic water-cooled, where the only difference is that water takes the place of
air. It uses higher current levels for welding T or corner joints. The third typical holder type is a water cooled
automatic electrode holder—which is typically used with automated equipment.[8]
Electrode
Electrode selection is based primarily on the composition of the metal being welded, the process variation being
used, joint design and the material surface conditions. Electrode selection greatly influences the mechanical
properties of the weld and is a key factor of 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. Some contain denitriding metals such as titanium and zirconium to avoid nitrogen
porosity.[11] Depending on the process variation and base material being welded the diameters of the electrodes
used in GMAW typically range from 0.7 to 2.4 mm (0.028–0.095 in) but can be as large as 4 mm (0.16 in). The
smallest electrodes, generally up to 1.14 mm (0.045 in)[12] are associated with the short-circuiting metal transfer
process, while the most common spray-transfer process mode electrodes are usually at least 0.9 mm
(0.035 in).[13][14]
Quality
Two of the most prevalent quality problems in GMAW are dross and porosity. If not controlled, they can lead to
weaker, less ductile welds. Dross is an especially common problem in aluminum GMAW welds, normally coming
from particles of aluminum oxide or aluminum nitride present in the electrode or base materials. Electrodes and
workpieces must be brushed with a wire brush or chemically treated to remove oxides on the surface. Any oxygen
in contact with the weld pool, whether from the atmosphere or the shielding gas, causes dross as well. As a result,
sufficient flow of inert shielding gases is necessary, and welding in volatile air should be avoided.[24]