Gas metal arc welding

 ( Featured Article ) 

In 1948, GMAW was 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.

GMAW is one of the most popular welding methods, especially in industrial environments. It is used extensively by the sheet metal industry and the automobile industry. There, the method is often used for arc spot welding, replacing or resistance spot welding. It is also popular for automated welding, where robots handle the workpieces and the welding gun to accelerate manufacturing. GMAW can be difficult to perform well outdoors, since drafts can dissipate the shielding gas and allow contaminants into the weld; flux cored arc welding is better suited for outdoor use such as in construction. Likewise, GMAW's use of a shielding gas does not lend itself to underwater welding, which is more commonly performed via shielded metal arc welding, flux cored arc welding, or gas tungsten arc welding.

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 electrode to pass while maintaining electrical contact. On the way to 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 directs the shielding gas evenly into the welding zone. Inconsistent flow may not adequately protect the weld area. Larger nozzles provide greater shielding gas flow, which is useful for high current welding operations that develop a larger molten weld pool. A gas hose from the tanks of shielding gas supplies the gas to the nozzle. 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 m/min (1200 in/min), but feed rates for semiautomatic GMAW typically range from 2 to 10 m/min (75 – 400 in/min).

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.

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 affects the mechanical properties of the weld. lts low cost makes it an attractive choice, but because of the reactivity of the arc plasma, 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, especially with smaller electrode diameters.

Argon is also commonly mixed with other gases, 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, 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. Excessive oxygen, especially when used in application for which it is not prescribed, can lead to brittleness in the heat affected zone. Argon-helium mixtures are extremely inert, and can be used on nonferrous materials. A helium concentration of 50–75% raises the required voltage and increases the heat in the arc, due to helium's higher ionization temperature. Hydrogen is sometimes 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 may be used for welding conductive materials such as copper. However, it should not be used on steel, aluminum or magnesium because it can cause porosity and hydrogen embrittlement.

Shielding gas mixtures of three or more gases are also available. 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 are claimed to allow higher arc voltages and welding speed. Helium also sometimes serves as the base gas, with small amounts of argon and carbon dioxide added. However, because it is less dense than air, helium is less effective at shielding the weld than argon—which is denser than air. It also can lead to arc stability and penetration issues, and increased spatter, due to its much more energetic arc plasma. Helium is also substantially more expensive than other shielding gases. Other specialized and often proprietary gas mixtures claim even greater benefits for specific applications.

Despite being poisonous, trace amounts of nitric oxide can be used to prevent the even more troublesome ozone from being formed in the arc.

The desirable rate of shielding-gas flow depends primarily on weld geometry, speed, current, the type of gas, and the metal transfer mode. Welding flat surfaces requires higher flow than welding grooved materials, since gas disperses more quickly. Faster welding speeds, in general, 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 if argon is used. 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 litre/min (20 ft³/hour) is generally suitable, whereas for globular transfer, around 15 L/min (30 ft³/h) is preferred. The spray transfer variation normally requires more shielding-gas flow because of its higher heat input and thus larger weld pool. Typical gas-flow amounts are approximately 20–25 L/min (40–50 ft³/h).

Successfully producing a weld with the GMAW process requires that the weldor maintain correct gun orientation relative to the joint being welded (the weldment), as well as maintain a uniform rate of travel down the joint so as to produce adequate penetration and weld bead buildup.  Movement along the joint may also require a “weaving” component in order to produce a sound weld, especially when welding vertically or over head.  During training, apprentice weldors are advised to watch the trailing edge of the weld puddle, not the arc, to ascertain they are progressing down the joint at an optimum rate.

The orientation of the gun relative to the weldment is important, as it affects the way in which the energy of the arc is directed into the constituent pieces.  In an ideal weld, 100 percent penetration would be achieved, which when coupled with the buildup of the weld bead, will produce a weld that is theoretically stronger than the constituent pieces.  In practice, full penetration is not achieved and in fact, may be undesirable.  However, penetration will be deepest when the wire electrode is exactly perpendicular to the surface being welded.  Furthermore, deposition of the filler metal, which comes from the melting of the wire electrode, will tend to be uniform with the wire in the perpendicular.

In practice, perpendicularity is not always ideal or even achievable, unless welding a horizontal butt joint.  Therefore, the gun will usually be oriented so the wire bisects the angle between the two surfaces being joined.  For example, if a 90 degree fillet joint is being welded, a wire angle of 45 degrees should produce the best penetration and filler deposition.  A horizontal lap joint, on the other hand, would benefit from a less-acute angle in order to direct more arc energy into the lower piece and less energy into the edge of the upper piece, mostly to avoid melting away the edge.

The travel angle, or lead angle, is the angle of the gun with respect to the direction of travel along the joint, and it should generally remain approximately vertical.  Most guns are designed so that when the grip (handle) is parallel to the work surface, a suitable lead angle will result.  However, the best angle will vary due to differing shielding gas types and the manner in which they disperse.  With pure inert gases, e.g., straight argon, the bottom of the torch is often slightly in front of the upper section, while the opposite is true when the welding atmosphere is carbon dioxide.

Maintaining a relatively-stable contact tip-to-work distance (the stick-out distance) is important.  Excessive stick-out may cause the wire electrode to melt too far away from the weldment, causing a sputtering arc, shallow penetration and poor deposition.  Excessive stick-out may also cause the shielding gas to not adequately blanket the fusion zone, leading to atmospheric contamination and a porous and unsound weld.

In contrast, insufficient stick-out may increase the rate at which spatter builds up inside the gun's nozzle and in extreme cases, may cause damage to the gun's contact tip due to the wire “burning back” into the tip.  Burn-back, in turn, may cause the wire to jam in the tip and stop moving, resulting in “bird-nesting” (bunching up of wire) at the wire-feed mechanism in the welder.  The correct stick-out distance will vary with different GMAW processes and applications, with a shorter stick-out distance often used in vertical and overhead welding.

Position welding, that is, welding vertical or overhead joints, may require the use of a weaving technique to assure proper weld deposition and penetration.  Position welding is complicated by a tendency for molten metal to run out of the puddle (“weld drip”), especially a problem with overhead joints.  Weld drip will result in cratering and undercutting where the bead should blend into the base metals, resulting in a weak weld and a risk of cracking at the edge of the bead.  Weaving constantly moves the fusion zone around so as to limit the amount of metal deposited at any one point.  Surface tension then assists in keeping the molten metal in the puddle until it is able to solidify.  In some cases, a higher-than-normal shielding gas flow rate may be required to achieve a satisfactory weld.  Development of position-welding skill takes experience, but is usually mastered by most welding apprentices before reaching journeyman status.

A vertical weld may start at the bottom of the joint and proceed upwards, or start at the top and work downwards.  The bottom-up technique tends to produce deeper penetration and a theoretically-stronger weld.  However, there is an increased tendency for weld drip, leading to the aforementioned cratering and undercutting, avoidable with a proper weaving technique.  Some increase in spatter may also be an issue.  On the other hand, the top-down procedure is less prone to weld drip, and generally produces smoother and more-attractive welds, but with less penetration.  Bottom-up is generally considered the preferred technique with heavy sections, although use of pure carbon dioxide when welding low- and medium-carbon steels with the top-down technique can increase penetration without excessive appearance degradation.

As well as possessing good gun-handling skills, the weldor must know how to correctly configure the welder (machine) to suit the characteristics of the weldment, the wire type and shielding gas(es) being used, and in some cases, the orientation of the joint to be welded.  Such configuration involves setting voltage, wire-feed speed and gas-flow rate, as well as using the correct gun nozzle to achieve proper shielding gas dispersal.

Over time, welding will cause a buildup of spatter inside the nozzle, which in sufficient quantity, will affect gas dispersal, possibly leading to unsound welds.  Hence the weldor will have to periodically clean the nozzle and tip to remove spatter.  Use of anti-spatter compound on the nozzle and tip can often slow the rate of buildup.  Anti-spatter compound is sold both in a jar as a paste (often referred to in the trade as “tip-dip”), and in an aerosol can as a spray.  Weldors may use the latter product to prevent spatter buildup on the weldment itself, as well as on the jig that is holding the weldment components.

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, aluminum 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 metal.

Welders are often exposed to hazardous gases and airborne particulate matter. GMAW produces smoke containing particles of various types of , and the size of the particles tends to influence the toxicity of the fumes. Smaller particles present greater danger. Concentrations of carbon dioxide and ozone can prove dangerous if ventilation is inadequate. Other precautions include keeping combustible materials away from the workplace, and having a working fire extinguisher nearby.