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Gas metal arc welding

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   Gas metal arc welding
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   Gas metal arc 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.

   Originally developed for welding aluminium and other non-ferrous
   materials in the 1940s, GMAW was soon applied to steels because it
   allowed for lower 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 commonly used in industries such as the
   automobile industry, where it is preferred for its versatility and
   speed. 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 utilize a shielding gas, instead employing a
   hollow electrode wire that is filled with flux on the inside.

Development

   The principles of gas metal arc welding began to be developed around
   the turn of the 19th century, with Humphry Davy's discovery of the
   electric arc in 1800. At first, carbon electrodes were used, but by the
   late 1800s, metal electrodes had been invented by N.G. Slavianoff 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.

   It was not until 1948 that GMAW was finally developed by the Batelle
   Memorial Institute. It used a smaller diameter electrode and a constant
   voltage power source, which had been 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 cost savings were not obtained. 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.

   Today, GMAW is one of the most popular welding methods, especially in
   industrial environments. It is used extensively by the sheet metal
   industry and, by extension, the automobile industry. There, the method
   is often used to do arc spot welding, thereby replacing riveting or
   resistance spot welding. It is also popular in robot welding, in which
   robots handle the workpieces and the welding gun to quicken the
   manufacturing process. Generally, it is unsuitable for welding
   outdoors, because the movement of the surrounding atmosphere can cause
   the dissipation of the shielding gas and thus make welding more
   difficult, while also decreasing the quality of the weld. The problem
   can be alleviated to some extent by increasing the shielding gas
   output, but this can be expensive. In general, processes such as
   shielded metal arc welding and flux cored arc welding are preferred for
   welding outdoors, making the use of GMAW in the construction industry
   rather limited. Furthermore, the use of a shielding gas makes GMAW an
   unpopular underwater welding process, and for the same reason it is
   rarely used in space applications.

Equipment

   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
   Enlarge
   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
   A GMAW wire feed unit
   Enlarge
   A GMAW wire feed unit

   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 30.5 m/min (1200 in/min), but feed
   rates for semiautomatic GMAW typically range from 2 to 10 m/min
   (75–400 in/min).

Power supply

   Most applications of gas metal arc welding use a constant voltage power
   supply. As a result, any change in arc length (which is directly
   related to voltage) results in a large change in heat input and
   current. A shorter arc length will cause a much greater heat input,
   which will make the wire electrode melt more quickly and thereby
   restore the original arc length. This helps operators keep the arc
   length consistent even when manually welding with hand-held welding
   guns. To achieve a similar effect, sometimes a constant current power
   source is used in combination with an arc voltage-controlled wire feed
   unit. In this case, a change in arc length makes the wire feed rate
   adjust in order to maintain a relatively constant arc length. In rare
   circumstances, a constant current power source and a constant wire feed
   rate unit might be coupled, especially for the welding of metals with
   high thermal conductivities, such as aluminium. This grants the
   operator additional control over the heat input into the weld, but
   requires significant skill to perform successfully.

   Alternating current is rarely used with GMAW; instead, direct current
   is employed and the electrode is generally positively charged. Since
   the anode tends to have a greater heat concentration, this results in
   faster melting of the feed wire, which increases weld penetration and
   welding speed. The polarity can be reversed only when special
   emissive-coated electrode wires are used, but since these are not
   popular, a negatively charged electrode is rarely employed.

Electrode

   The selection of an electrode to be used in GMAW is a complicated
   decision, as it depends on the process variation being used, the
   composition of the metal being welded, the joint design, and the
   material surface conditions. The choice of an electrode strongly
   influences the mechanical properties of the weld area, making it a key
   factor in weld quality. In general, it is desirable that the welded
   metal have mechanical properties similar to those of the base material,
   and that there be no discontinuities, such as porosity, within the
   weld. To achieve these goals in different materials using different
   GMAW variations, a wide variety of electrodes exist. All contain
   deoxidizing metals such as silicon, manganese, titanium, and aluminium
   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
   0.7 to 2.4  mm (0.028–0.095 in), but can be as large as 4 mm (0.16 in).
   The smallest electrodes are associated with short-circuiting metal
   transfer, while the pulsed spray mode generally uses electrodes of at
   least 1.6 mm (0.06 in).
   GMAW Circuit diagram. (1) Welding torch, (2) Workpiece, (3) Power
   source, (4) Wire feed unit, (5) Electrode source, (6) Shielding gas
   supply.
   Enlarge
   GMAW Circuit diagram. (1) Welding torch, (2) Workpiece, (3) Power
   source, (4) Wire feed unit, (5) Electrode source, (6) Shielding gas
   supply.

Shielding gas

   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 cause an erratic arc and encourage
   spatter (with helium) or do not provide adequate weld penetration
   (argon). 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% or 80%/20%
   mixture, which reduces spatter and makes it possible to weld thin steel
   workpieces.

   Argon is also commonly mixed with other gases, such as oxygen, helium,
   hydrogen, and nitrogen. The addition of up to 5% oxygen encourages
   spray transfer, which is critical for spray-arc and pulsed spray-arc
   GMAW. However, more oxygen makes the shielding gas oxidize the
   electrode, which can lead to porosity in the deposit if the electrode
   does not contain sufficient deoxidizers. An argon-helium mixture is
   completely inert, and is used on nonferrous materials. A helium
   concentration of 50%–75% raises the voltage and increases the heat in
   the arc, making it helpful for welding thicker workpieces. Higher
   percentages of helium also improve the weld quality and speed of using
   alternating current for the welding of aluminum. Hydrogen is added to
   argon in small concentrations (up to about 5%) for welding nickel and
   thick stainless steel workpieces. 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, aluminium 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.

   Recent advances in shielding gas mixtures use three or more gases to
   gain improved weld quality. A mixture of 70% argon, 28% carbon dioxide
   and 2% oxygen is gaining in popularity for welding steels, while other
   mixtures add a small amount of helium to the argon-oxygen combination,
   resulting in higher arc voltage and welding speed. Helium is also
   sometimes used as the base gas, to which smaller amounts of argon and
   carbon dioxide are added. Additionally, other specialized and often
   proprietary gas mixtures claim to offer 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 10  L/min (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).

Operation

   GMAW weld area. (1) Direction of travel, (2) Contact tube, (3)
   Electrode, (4) Shielding gas, (5) Molten weld metal, (6) Solidified
   weld metal, (7) Workpiece.
   Enlarge
   GMAW weld area. (1) Direction of travel, (2) Contact tube, (3)
   Electrode, (4) Shielding gas, (5) Molten weld metal, (6) Solidified
   weld metal, (7) Workpiece.

   In most of its applications, gas metal arc welding is a fairly simple
   welding process to learn, requiring no more than several days to master
   basic welding technique. Even when welding is performed by well-trained
   operators, however, weld quality can fluctuate, since it depends on a
   number of external factors. And all GMAW is dangerous, though perhaps
   less so than some other welding methods, such as shielded metal arc
   welding.

Technique

   The basic technique for GMAW is quite simple, since the electrode is
   fed automatically through the torch. 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, on the
   other hand, 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. The orientation of the gun
   is also important—it should be held so as to bisect the angle between
   the workpieces; 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.

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 aluminium 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.

   In GMAW the primary cause of porosity is gas entrapment in the weld
   pool, which occurs when the metal solidifies before the gas escapes.
   The gas can come from impurities in the shielding gas or on the
   workpiece, as well as from an excessively long or violent arc.
   Generally, the amount of gas entrapped is directly related to the
   cooling rate of the weld pool. Because of its higher thermal
   conductivity, aluminium welds are especially susceptible to greater
   cooling rates and thus additional porosity. To reduce it, the workpiece
   and electrode should be clean, the welding speed diminished and the
   current set high enough to provide sufficient heat input and stable
   metal transfer but low enough that the arc remains steady. Preheating
   can also help reduce the cooling rate in some cases by reducing the
   temperature gradient between the weld area and the base material.

Safety

   Gas metal arc welding can be dangerous if proper precautions are not
   taken. Since GMAW employs an electric arc, welders wear protective
   clothing, including heavy leather gloves and protective long sleeve
   jackets, to avoid exposure to extreme heat and flames. In addition, the
   brightness of the electric arc can cause arc eye, in which ultraviolet
   light causes the inflammation of the cornea and can burn the retinas of
   the eyes. Helmets with dark face plates are worn to prevent this
   exposure, and in recent years, new helmet models have been produced
   that feature a liquid crystal-type face plate that self-darkens upon
   exposure to high amounts of UV light. Transparent welding curtains,
   made of a polyvinyl chloride plastic film, are often used to shield
   nearby workers and bystanders from exposure to the UV light from the
   electric arc.

   Welders are also often exposed to dangerous gases and particulate
   matter. GMAW produces smoke containing particles of various types of
   oxides, and the size of the particles in question tends to influence
   the toxicity of the fumes, with smaller particles presenting a greater
   danger. Additionally, carbon dioxide and ozone gases can prove
   dangerous if ventilation is inadequate. Furthermore, because the use of
   compressed gases in GMAW pose an explosion and fire risk, some common
   precautions include limiting the amount of oxygen in the air and
   keeping combustible materials away from the workplace.

Metal transfer modes

Globular

   GMAW with globular metal transfer is often considered the most
   undesirable of the four major GMAW variations, because of its tendency
   to produce high heat, a poor weld surface, and spatter. The method was
   originally developed as a cost efficient way to weld steel using GMAW,
   because this variation uses carbon dioxide, a less expensive shielding
   gas than argon. Adding to its economic advantage was its high
   deposition rate, allowing welding speeds of up to 110 mm/s
   (250 in/min). As the weld is made, a ball of molten metal from the
   electrode tends to build up on the end of the electrode, often in
   irregular shapes with a larger diameter than the electrode itself. When
   the droplet finally detaches either by gravity or short circuiting, it
   falls to the workpiece, leaving an uneven surface and often causing
   spatter. As a result of the large molten droplet, the process is
   generally limited to flat and horizontal welding positions. The high
   amount of heat generated also is a downside, because it forces the
   welder to use a larger electrode wire, increases the size of the weld
   pool, and causes greater residual stresses and distortion in the weld
   area.

Short-circuiting

   Further developments in welding steel with GMAW led to a variation
   known as short-circuiting or short-arc GMAW, in which carbon dioxide
   shields the weld, the electrode wire is smaller, and the current is
   lower than for the globular method. As a result of the lower current,
   the heat input for the short-arc variation is reduced, making it
   possible to weld thinner materials while decreasing the amount of
   distortion and residual stress in the weld area. As in globular
   welding, molten droplets form on the tip of the electrode, but instead
   of dropping to the weld pool, they bridge the gap between the electrode
   and the weld pool as a result of the greater wire feed rate. This
   causes a short circuit and extinguishes the arc, but it is quickly
   reignited after the surface tension of the weld pool pulls the molten
   metal bead off the electrode tip. This process is repeated about 100
   times per second, making the arc appear constant to the human eye. This
   type of metal transfer provides better weld quality and less spatter
   than the globular variation, and it allows for welding in all
   positions, but generally the process is much slower than globular GMAW.
   Another difficulty is maintaining a stable arc, because it depends on
   achieving a consistent and high short-circuiting frequency, which can
   only be accomplished with a good power source, suitable welding
   conditions, and significant welder skill. Like the globular variation,
   it can only be used on ferrous metals.

Spray

   Spray transfer GMAW was the first metal transfer method used in GMAW,
   best suited for welding aluminium and stainless steel while employing
   an inert shielding gas and a relatively thick electrode. In this
   variation, molten metal droplets (with diameters smaller than the
   electrode diameter) are rapidly passed along the stable electric arc
   from the electrode to the workpiece, essentially eliminating spatter
   and resulting in a high-quality weld finish. However, high amounts of
   voltage and current are necessary, which means that the process
   involves high heat input and a large weld area and heat-affected zone.
   As a result, it is generally used only on workpieces of thicknesses
   above about 6 mm (0.25 in). Because of the large weld pool, it is often
   limited to flat and horizontal welding positions, but when a smaller
   electrode is used in conjunction with lower heat input, its versatility
   increases. The maximum deposition rate for spray arc GMAW is relatively
   high; about 60 mm/s (150 in/min).

Pulsed-spray

   A more recently developed method, the pulse-spray metal transfer mode
   is based on the principles of spray transfer but uses a pulsing current
   to melt the filler wire and allow one small molten droplet to fall with
   each pulse. The pulses allow the average current to be lower,
   decreasing the overall heat input and thereby decreasing the size of
   the weld pool and heat-affected zone while making it possible to weld
   thin workpieces. The pulse provides a stable arc and no spatter, since
   no short-circuiting takes place. This also makes the process suitable
   for nearly all metals, and thicker electrode wire can be used as well.
   The smaller weld pool gives the variation greater versatility, making
   it possible to weld in all positions. In comparison with short arc
   GMAW, this method has a somewhat slower maximum speed (85 mm/s or
   200 in/min [These numbers appear to be wrong, as the numbers cited for
   spray are lower and cited and these are uncited. Please Correct]), and
   the process also requires that the shielding gas be primarily argon
   with a low carbon dioxide concentration. Additionally, it requires a
   special power source capable of providing current pulses with a
   frequency between 30 and 400 pulses per second. However, the method has
   gained popularity, since it requires lower heat input and can be used
   to weld thin workpieces, as well as nonferrous materials.
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