GMAW & FCAWArc welding processes

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Weld Metal Deposited By Major Arc Welding Processes

GMAWIntroduction FundamentalsAdvantagesLimitationsPrinciples of OperationMetal transfer modes Equipment Welding gun & accessoriesWire feed unit Power supply & welding controlsGMAW automationMaterials & ConsumablesElectrodes Shielding gases Process VariablesWeld Joint DesignsInspection & Weld qualityQuality concernsWeld DiscontinuitiesSummary

Gas metal arc weldingGas metal arc welding (GMAW), is sometimes referred to asMetal inert gas (MIG) welding or Metal active gas (MAG) welding

Gas metal arc weldingGMAW 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.

GMAW - Operation

ApplicationsWide variety of applications in Industrial manufacturing, agriculture, construction, shipbuilding, mining & automotive industriesGMAW Process is used for welding of pipes, pressure vessels, structures, steel components, furniture, automotive components and numerous other products

GMAW Equipment - SchematicWelding torch WorkpiecePower sourceWire feed unitElectrode source Shielding gas supply135426

Power Source CharacteristicsPower Sources of Constant Current type having drooping characteristics are used for - MMAW process - GTAW process - Plasma processes

Power sources of constant voltage type having flat characteristics are used for - GMAW & FCAW processes - SAW process

V-A relationship drooping

V-A Relationship CV power source (GMAW / SAW)

Calculation of the Slope for a Power Source

Automatic arc length regulation

GMAW Equipment

4 wheel wire drive unit

Wire Feed Speed / Current.

Constant potential power sources are used for GMAW and have no built in means of changing the current. The current adjusts itself to burn off the quantity of wire delivered. If the wire feed speed is increased more current is drawn to burn it off . In this way adjusting the wire feed speed also adjusts the current supplied.

The current dictates the amount of heat generated by the arc. Increasing the current increases the arc energy and therefore the heat input. This in turn increases fusion and penetration, wire deposition rate and travel speed.

GMAW Equipment - Welding GunGMAW Welding Gun HandleMoulded phenolic insulation (shown in white) and threaded metal nut insert (yellow) Shielding gas nozzle Contact tipNozzle

Arc Welding Gun Nomenclature

AdvantagesEfficient process to weld all commercial metals & alloys in all positionsDeposition rates are significantly higher than SMAWLong welds - without stops and startsDeep penetration welds are possible using spray transfer to weld smaller fillet weldsMinimal post weld cleaning required due to absence of heavy slag

LimitationsWelding equipment is more complex, costly and less portableDifficult to use in hard-to-reach places since the gun is larger than a stick electrode holder.Welding arc needs to be protected against air drafts (of 8 Km/h)

GMAW Operation: WELD AREADirection of travelContact tubeElectrodeShielding gasMolten weld metalSolidified weld metalWorkpiece

GMAW Metal Transfer ModesThere are four primary methods of metal transfer in GMAW. They are:Globular Short-circuiting Spray, and Pulsed-sprayEach transfer mode has distinct properties, advantages and limitations.

Modes of transferThe mode of transfer is determined by a number of factors:Magnitude, type and polarity of welding currentElectrode diameterElectrode compositionElectrode extension and Shielding gas composition

Influence of welding current & gas on metal transfer mode in GMAWCO2Argon MixCO2 /Ar Mix

GMAW Metal TransfersSpray Transfer4 steps in Short circuiting transferGlobular Transfer

Short Circuit Transferoccurs when carbon dioxide is the shielding gas, the electrode diameter is smaller, and the current density is low. The metal is transferred from the electrode only during the period in which the electrode is in contact with the weld pool. No metal is transferred across the arc. The electrode contacts the weld pool in the range of 20- to 200 times per second.

Short Circuit TransferMolten droplets forms 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 20 to 200 times per second, making the arc appear constant to the human eye.

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. This transfer is generally suited for joining of thin sections, for out-of-position welding and bridging large root openings

This type of metal transfer is slow and it is difficult to maintain 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.

Globular Transfer

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.

Globular transferAs 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, this mode of transfer is generally limited to flat and horizontal welding positions.

Spray Transfer

Spray transfer GMAW occurs when the molten metal from the electrode is propelled axially across the arc in the form of minute droplets. With Argon-rich gas shielding it is possible to produce a very stable, spatter-free axial spray transfer mode. The mode requires Direct current with a positive electrode (DCEP) and a current level above a critical value termed the spray transition current. Below this level, the transfer is globular.

In this variation, molten metal droplets 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.

Axial Spray TransferMolten metal is propelled axially across the arc in minute dropletsArgon-rich gas shielding produces stable spatter free axial spray transfer mode

Modes Of Metal TransferDIP TRANSFER Low current - low voltage used to produce short circuiting arc, freq. 200 times / minute. Used for sheet metal and postional welding

SPRAY TRANSFER Higher currents and voltage used , droplet size same as or lower than the wire diameter. Higher deposition rate penetration and fluidity of the molten pool , increases the productivity

Modes Of Metal Transfer Contd.GLOBULAR TRANSFER An intermediate stage between dip and spray transfer . droplet sizes are more than the wire dia . Produces excessive spatter and erratic arc behaviour

PULSED TRANSFER Controlled method of spray transfer. Heat input to the job is controlled by low background current with high pulses using special type of equipment

Pulse TransferCombines the control on heat input of short arc with the higher deposition rate of open arc. Gives extremely precise control on metal transfer and penetration to give superior weld quality In synergic pulsed systems wire feed rate synchronised with pulsed current to control individual droplet detachment.

Shielding Gases

Shielding gases provide a protection to the weld metal from the atmosphere and have a pronounced effect on: Arc characteristics Mode of metal transfer Penetration and weld bead profile Speed of welding Undercutting tendency Cleaning action Weld metal mechanical properties

Types of shielding gases used in GMAW Carbon Dioxide (CO2) Argon Helium Oxygen Mixtures of the above gases

Shielding Gas The choice of a shielding gas depends on several factors, most importantly: the type of material being welded and the process variation being used

Shielding GasShielding 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.

Shielding Gas 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 Its low cost makes it an attractive choice, but the arc is harsh, spatter is unavoidable, making welding thin materials difficult

Shielding Gas Therefore, Ar & CO2 are mixed in a 75%/25% or 80%/20% mixture, which reduces spatter and makes it possible to weld thin steel workpieces.Ar is also commonly mixed with other gases, such as oxygen, helium, hydrogen, and nitrogen.Addition of up to 5% oxygen encourages spray transfer, which is critical for spray-arc and pulsed spray-arc GMAW. An argon-helium mixture is completely inert, and is used on nonferrous materials.

GMAW Process- Shielding Gas Pure inert gases such as argon and helium are only used for nonferrous welding; with steel they causean erratic arc and encourage spatter (Helium) or do not provide adequate weld penetration (argon)

Problems of using CO2 as Shielding GasUnstable arc with high level of spatter

High fume formation rate

Higher level of reinforcement

Reduced speed due to high viscosity

Undercut / sharp notch at the toe of weld

Spatter generated

1 metre of weld at 260 amps using 1.2mm dia. A18 solid wireT-GK 3 (10)Carbon dioxide17.1 gArgon - 20 CO2 8.6 gArgon-12 CO2 5.5 g

Argon Mixed Gas Spray Transfer

Problems in using pure Argonas Shielding gasStable and Soft arc with a tendency to wander

Finger shaped penetration profile

Poor fusion and penetration due to low heat input

Comparatively high bead profile

Finger Profile of pure Argon arc10%20%Modified by oxygen and CO2

Argon - Helium Mixtures used for Aluminium and Non-ferrous metals

Effect of CO2 and O2 on welding speed( 4mm throat fillet on 6mm plate)

CO2 and Argon mixture profilesCO2Argon mixture

Effect of shielding gasesBead Contour & Penetration Patterns for various Shielding GasesArgonArgon-HeliumHeliumCO2

Shielding gas profiles & effect on weld length

Relative Effect of Oxygen versus Carbon Dioxide additions to the shielding gas mixtureARGON-OXYGENARGON-CARBON DIOXIDECARBON DIOXIDE

Gas FlowThe 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.

Gas FlowFaster welding speeds mean that more gas must be supplied to provide adequate coverage. Higher current requires greater flow, and generally, more helium is required to provide adequate coverage than argon.

Gas Flow

Process VariationsPulsed gas metal arc welding offersReduced spatter levels compared to short circuiting transfer, thereby increasing deposition rate and minimizing post weld cleaningGenerates lower fume levelsProvides more controlled heat input resulting in less distortion, results in avoiding incomplete fusion & can weld sensitive materials

Process VariablesWelding amperagePolarityArc voltageTravel speedElectrode extension and stick outElectrode orientationWeld joint positionElectrode diameterShielding gas and gas flow rate

Summary: GMAW BenefitsGMAW is an efficient process that can be used to weld all commercial metals and alloysGMAW can perform in all positions, a capability submerged arc welding does not haveDeposition rates are significantly higher that SMAWLong welds can be deposited and the process can be easily mechanizedMinimal post-weld cleaning is required due to absence of slagIt is a low hydrogen process, making it a good choice of process to be used for susceptible materialsProcess skills are readily taught and acquired

Summary: GMAW LimitationsThe equipment required for GMAW is more complex, expensive and less portable than that used for SMAWDifficult to use in hard-to reach places due to the limitation imposed by the gun sizeThe arc must be protected against drafts in excess of 8 km/h, which may disperse the shielding gas, thus limiting outdoor applicationsRelatively high levels of radiated heat and arc intensity

FLUX CORED ARC WELDING

Flux Cored WeldingFlux Cored Arc Welding (FCAW) is frequently referred to as flux cored welding. Flux cored welding is a commonly used high deposition rate welding process that adds the benefits of flux to the welding simplicity of MIG welding. As in MIG, welding wire is continuously fed from a spool. Flux cored welding is therefore referred to as a semiautomatic welding process.

Flux/metal-cored wiresFlux powder consists of Ferro-alloys, and/or gas formers and/or slag formersStrip (0.6 mm thick) of carbon steel/ stainless steel/Nickel/ copperResultant weld metal contains transferred metal, alloying elementsSlag consists of silicates and oxides

Flux/metal-cored wiresFlux powder consists of Ferro-alloys Gas formers Slag formersGas shielded cored wires contain only slag formers and no gas formers as gas is externally providedOpen arc (Gasless) cored wires contain gas formers & slag formersSubmerged arc cored wires contain no gas formers and no slag formers

FCAW Weld Profile

FCAW AdvantagesHigh quality weld metal depositExcellent weld appearanceWelds many steels in a wide thickness rangeHigh operating factor and easily mechanizedHigh deposition rate (up to 4 times greater than SMAW and high current densityRelatively high electrode deposition efficiencyRequires less pre-cleaning than GMAWGood fusion, less distortion and high tolerance for contaminationsGood resistance to under-bead cracking

FCAW LimitationsLimited to welding ferrous metals and nickel base alloys onlyIt produces a slag covering that must be removedFCAW wire is more expensive on weight basis that solid electrodesThe equipment is more expensive, complexThe power source and wire feeder must remain close to point of weldingMore smoke and fumes than GMAW and SAW

Gas Metal Arc BrazingA copper based electrode (e.g. aluminium bronze or silicon bronze) is used instead of a steel electrode to join steel. As the copper alloy has a lower melting temperature than steel, less heating of the base metal is required in order to deposit a weld bead, and little or no melting of the base metal occurs.

The lower heat input reduces the amount of coating that is melted away and the copper based weld bead furnishes better corrosion resistance than that provided by a carbon steel weld bead.

Mig BrazingBecause of the low heat input, braze welding is some times used to join heat sensitive materials such as cast iron, for welding of thin sheet steel to help prevent melt through. Gas metal arc brazing is also used to join galvanized steels.

Thank youThanks to IIW & MSME

Shielding gasShielding 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.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 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, 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.

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, 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.

small additions of oxidising gas in Argon results in1) reduced droplet surface tension at electrode tip2) smaller droplet size3) increased droplet frequency4) reduced mobility of arc root5) increased arc stability6) ability to achieve spray transfer7) reduced weld metal surface tension and increased weld pool fluidity8) increased welding speed capability 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 requirementsfor the small weld pools of the short circuiting and pulsed spray modes, about 10L/min (20ft/h) is generally suitable, while for globular transfer, around 15L/min (30ft/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 2025L/min (4050ft/h).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 requirementsfor the small weld pools of the short circuiting and pulsed spray modes, about 10L/min (20ft/h) is generally suitable, while for globular transfer, around 15L/min (30ft/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 2025L/min (4050ft/h).