Gas Metal Arc Welding

The gas metal arc welding (GMAW) process is the dominant welding technique employed currently as being the most widely used joining process associated with the world’s welding fabricators. Notwithstanding its sixty years of track record, research and development continue to deliver advancements for this process, and the effort and hard work continues to be rewarded with excellent outcomes.

Gas Metal Arc Welding (GMAW) is an arc welding process which delivers the coalescence of metals as a result of heating them using an arc between a continuously fed filler metal electrode along with the workpiece. The process makes use of shielding through an externally supplied gas to protect the molten weld pool. The employment of GMAW typically necessitates DC+ (reverse) polarity to the electrode. GMAW is usually referred to as MIG (Metal Inert Gas) welding and it's a lot less frequently referred to as MAG (Metal Active Gas) welding. Regardless, the GMAW technique lends itself to weld a wide range of both solid carbon steel as well as tubular metal-cored electrodes. The alloy material spectrum for GMAW features: carbon steel, stainless steel,nickel, copper, aluminium, magnesium, silicon bronze as well as tubular metal-cored surfacing alloys. The GMAW technique is especially suitable to semiautomatic, robotic automation as well as hard automation welding purposes.

About GMAW Process

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The Gas Metal Arc Welding (GMAW) process enjoys extensive usage due to its capability to produce excellent quality welds, for a broad range of ferrous as well as non-ferrous alloys, for a economical cost.

Advantages of GMAW

• The cabability to join a extensive selection of material choices in addition to material thicknesses.
• Uncomplicated equipment components that are easily obtainable as well as cost-effective.
• GMAW offers greater electrode efficiencies in between 93% and 98% in comparison with alternative welding techniques.
• Increased efficiencies in addition to operator consideration with regard to welders when compared to different open arc welding techniques.
• GMAW is conveniently adaptable for the purpose of high-speed robotic, hard automation as well as semiautomatic welding applications.
• GMAW offers all-position welding functionality.
• Excellent weld bead appearance.
• Reduced hydrogen weld deposit, typically below 5 mL/100 gm of weld metal.
• Reduced heat input in comparison to alternative welding techniques.
• A minimal of weld spatter as well as slag helping to make weld cleanup quick and simple.
• Reduced welding fumes in comparison to SMAW (Shielded Metal Arc Welding) as well as FCAW (Flux-Cored Arc Welding) processes.

Benefits of GMAW

• Less expensive per length of weld metal deposited in comparison with alternative open arc welding techniques.
• Less expensive electrode in comparison to different welding techniques.
• Significantly less distortion by using GMAW-P (Pulsed Spray Transfer Mode), GMAW-S (Short-Circuit Transfer Mode) and STT (Surface Tension Transfer).
• Manages inadequate fit-up by means of GMAW-S and STT modes.
• Minimized welding fume generation.
• Negligible post-weld cleanup.

Limitations of GMAW

• The reduced heat input characteristic of the short-circuiting mode of metal transfer limits its usage to thin materials.
• The greater heat input axial spray transfer typically creates limitations concerning its use limiting it to thicker base materials.
• The higher heat input mode of axial spray is constrained to flat or horizontal welding positions only.
• The employment of argon based shielding gas for the purpose of axial spray as well as pulsed spray transfer modes is costlier as compared to 100% carbon dioxide (CO₂).

Considerations for GMAW

The GMAW process is definitely adaptable in its capacity to produce superior welds for a extremely wide selection of base material type in addition to thickness ranges. Essential in the application of GMAW is a fundamental comprehension of the interaction concerning several critical variables:

• The thickness range of the base material being welded dictates the electrode diameter along with the workable current range.
• The shielding gas choices has impact on the selection of the mode of metal transfer and has a definite influence on the finished weld profile.

GMAW Metal Transfer Modes

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Short-Circuit Metal Transfer

Short-circuiting metal transfer (GMAW-S) can be defined as a GMAW mode of metal transfer where a continuously fed solid or metal-cored wire electrode is deposited through recurrent electrical short-circuits.

The short-circuiting metal transfer mode is the low heat input mode of metal transfer with regard to GMAW. The entire metal transfer transpires as soon as the electrode is electrically shorted in physical contact with the base material or molten puddle. The primary consideration towards successful operation of short-circuiting transfer is the diameter of electrode utilized, the shielding gas type along with the welding procedure implemented. This particular mode of metal transfer characteristically can handle the application of 0.6 - 1.1 mm diameter electrodes shielded by using either 100% CO₂ or a mixture of 75-80% argon plus 25-20% CO₂. The lower heat input characteristic makes it well suited for sheet metal thickness materials. The workable base material thickness range with regard to short-circuiting transfer is usually 0.6 – 5.0 mm material. Additional labels frequently used with regard to short-circuiting transfer consist of short arc microwire welding, fine wire welding, as well as dip transfer.

The transfer of a single molten droplet of electrode happens during the shorting phase of the transfer cycle. Physical contact of the electrode occurs with the molten weld pool as well as the number of short-circuiting events take place as many as 200 times per second. The current delivered through the welding power supply increases, and the increase in current accompanies an increase in the magnetic force placed on the end of the electrode. The electromagnetic field that encompases the electrode supplies the force which pushes the molten droplet from the end of the electrode.

As a result of the low-heat input associated with short-circuiting transfer, it is typically utilized on sheet metal thickness material. Nevertheless, it is often frequently employed for welding the root pass in thicker sections of material in open groove joints. The short-circuiting mode lends itself to root pass applications on heavier plate groove welds or pipe.

Advantages of Short-Circuiting Transfer

• All-position capabilities, which include flat, horizontal, vertical-up, vertical-down as well as overhead.
• Manages inferior fit-up extremely well, and is particularly effective at root pass work on pipe applications.
• Decreased heat input cuts down weldment distortion.
• Increased operator appeal as well as being user friendly.
• Increased electrode efficiencies, 93% or higher.

Limitations of Short-Circuiting Transfer

• Restricted to sheet metal thickness range and open roots of groove joints on heavier sections of base material.
• Inadequate welding procedure control can lead to incomplete fusion. Cold lap together with cold shut are supplemental terminology which serves to describe incomplete fusion defects.
• Inadequate procedure control may result in excessive spatter, and definately will increase weld melt cleanup expense.
• To counteract loosing shielding gas to the wind, welding outdoors may necessitate the utilization of a windscreen(s).

Globular Metal Transfer

Globular metal transfer has been a popular mode of metal transfer with regard to high production sheet metal fabrication. The transfer mode is associated with the usage of 100% CO₂ shielding, nevertheless it has additionally witnessed significant use with argon/CO₂ blends. With regard to general fabrication on carbon steel, it offers a mode of transfer just underneath the transition to axial spray transfer that has lent itself to higher speed welding.

The application of globular transfer in high production settings is being superceded with advanced forms of GMAW. The switch is now being made to GMAW-P which results in reduced fume levels, decreased or absent spatter levels, as well as erradication of incomplete fusion defects.

Advantages of Globular Transfer

• Makes use of inexpensive CO₂ shielding gas, nonetheless is often used in combination with argon/CO₂ blends.
• Is effective at producing welds at very high travel speeds.
• Economical solid or metal-cored electrodes.
• Welding equipment is very affordable.

Limitations of Globular Transfer:

• Excessive spatter levels contribute to expensive cleanup.
• Diminished operator overall appeal.
• Susceptible to cold lap or cold shut incomplete fusion defects, that translates to expensive maintenance.
• Weld bead shape is convex, and additionally welds exhibit poor wetting at the toes.
• Excessive spatter level diminishes electrode efficiency to a range of 87 – 93%.

Axial Spray Metal Transfer

Axial spray metal transfer is the higher energy mode of GMAW metal transfer where a continuously fed solid or metal-cored wire electrode is deposited at a higher energy level producing a stream of small molten droplets. The droplets are propelled axially across the arc.

Axial spray transfer is the higher energy form of GMAW metal transfer. To accomplish axial spray metal transfer, binary gas blends containing argon + 1-5 % oxygen or argon + <= 18% CO₂ are utilized. Axial spray metal transfer is supported by using solid wire or metal-cored electrodes. Axial spray transfer can be employed with all of typical alloys which include carbon steel, stainless steel, nickel alloys, aluminium, magnesium, and copper alloys.

For the majority of the diameters of filler metal alloys, the switch to axial spray transfer transpires at the globular to spray transition current. A stream of fine metal droplets which travel axially from the end of the electrode characterises the axial spray mode of metal transfer. The high puddle fluidity restricts its usage to the horizontal and flat welding positions only.

For the purpose of carbon steel, axial spray transfer is employed on heavier section thickness material for fillets and for use in groove type weld joints. The utilization of argon shielding gas compositions of 95% argon accompanied by a balance of oxygen results in a deep finger-like penetration profile, whereas shielding gas mixes that include more than 10% CO₂ reduce the finger-like penetration profile and provide a more rounded type of penetration.

The selection of axial spray metal transfer depends upon the thickness of base material along with the capability to position the weld joint into the horizontal or flat welding positions. Finished weld bead appearance is excellent, in addition to operator appeal being very high. Axial spray transfer delivers its most effective results when the weld joint is without any oil, dirt, rust, and millscale.

Advantages of Axial Spray Transfer

• Superior deposition rates.
• Increased electrode efficiency of 98% or more.
• Makes use of a wide range of filler metal types in an equally wide selection of electrode diameters.
• Excellent weld bead appearance.
• Substantial operator appeal together with being user friendly.
• Necessitates minimal post weld cleanup.
• Absence of weld spatter.
• Exceptional weld fusion.
• Lends itself to semiautomatic, robotic, and hard automation applications.

Limitations of Axial Spray Transfer

• Restricted to the flat and horizontal welding positions only.
• Welding fume generation is more significant compared to alternative processes
• The higher-radiated heat along with the generation of a extremely bright arc necessitate additional welder as well as bystander protection.
• The usage of axial spray transfer outdoors necessitates the employment of windscreens.
• The shielding utilized to support axial spray transfer costs more than 100% CO₂.

Pulsed Spray Metal Transfer

Pulsed spray metal transfer (GMAW-P) is a extremely controlled variant of axial spray transfer wherein the welding current is cycled between a high peak current level to a low background current level. Metal transfer transpires during the high energy peak level in the form of a single molten droplet.

GMAW-P was developed for the control of weld spatter along with the erradication of incomplete fusion defects typical to globular and short-circuiting transfer. Its benefits included higher efficiency electrodes as compared to FCAW along with the capability to produce reduced hydrogen weld deposits. This particular mode of metal transfer makes use of electrode diameters from 0.8 – 1.6 mm solid wire electrodes and metal-cored electrodes from 1.1 – 2.0 mm diameter. It is intended for welding a wide range of material types. Argon based shielding gas selection using maximum 18% CO₂ supports the usage of pulsed spray metal transfer with carbon steels.

The welding current alternates between a peak current and a lower background current. This kind of controlled dynamic of the current leads to a reduced average current as compared with what is found using axial spray transfer. The time which includes the peak current and the background current, is a interval termed a cycle (Hz). The high current excursion surpasses the globular to spray transition current and the low current is reduced to a value lower than is observed through short-circuiting transfer. Essentially, during the peak current, the high point of the interval, a single droplet of molten metal is detached and transferred across the arc. The descent to the lower current, termed the background current, supplies arc stability and is principally responsible for the overall heat input into the weld. The frequency is the number of times the period occurs per second, or cycles per second. The frequency of the period increases in proportion to the wire feed speed. Considered collectively they produce an average current, that leverages its usage with a wide material thickness range.

Advantages of Pulsed Spray Transfer

• Absent or nominal levels of spatter.
• More resistant to absence of fusion defects as compared to alternative modes of GMAW metal transfer.
• Excellent weld bead appearance.
• Superior operator appeal.
• Provides an engineered solution for the control of weld fume generation.
• Reduced levels of heat induced distortion.
• Capability to weld out-of-position.
• Reduced hydrogen deposit.
• Minimizes the tendency with regard to arc blow.
• Handles inadequate fit-up.
• In comparison to FCAW, SMAW, and GMAW-S, pulsed spray transfer provides an affordable high-electrode efficiency of 98%.
• Lends itself to robotic and hard automation applications.
• Is combined for use with other multiple arc scenarios.
• Capable of arc travel speeds beyond 50 inches per minute (1.2 M/min.).

Limitations of Pulsed Spray Transfer

• Equipment to support the process is more expensive when compared to traditional systems.
• Blends of argon based shielding gas are usually more expensive compared to carbon dioxide.
• Increased arc energy necessitates the employment of supplemental safety protection with regard to welders as well as bystanders.
• Contributes complexity towards welding.
• Necessitates the employment of windscreens outdoors.

GMAW Shielding Gases

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Selecting the suitable shielding gas for any specified application is essential to the quality of the finished weld. The criteria which is used to make the selection comprises of, yet is not restricted to, the following:

• Alloy of wire electrode.
• Required mechanical properties of the deposited weld metal.
• Material thickness as well as joint design.
• Material condition – the existence of millscale, corrosion, resistant coatings, or oil.
• The mode of GMAW metal transfer.
• The welding position.
• Fit-up conditions.
• Desired penetration profile.
• Preferred final weld bead appearance.
• Cost.

Under the heat of the arc, shielding gases react in many different ways. The flow of current in the arc, as well as its magnitude, carries a powerful influence on the behaviour of the molten droplet. In some instances, a specific shielding gas will optimally lend itself to one transfer mode, nevertheless is going to be incapable of meeting the requirements of another.

Three fundamental criteria are useful in comprehending the properties of shielding gas:

• Ionisation potential of the gas components
• Thermal conductivity of the shielding gas components
• The chemical reactivity of the shielding gas with the molten weld puddle

Types of Shielding Gases

Argon and helium are the two inert shielding gases used for protecting the molten weld pool. The inert classification indicates that neither argon nor helium will react chemically with the molten weld pool. Nevertheless, in order to be a conductive gas, that is, a plasma, the gas is required to be ionised. Different gases necessitate different quantities of energy to ionize, and this is measured in terms of the ionisation energy.

The thermal conductivity, or the capability of the gas to transfer thermal energy, is the most crucial consideration for selecting a shielding gas. High thermal conductivity levels contribute to more conduction of the thermal energy into the workpiece. The thermal conductivity additionally influences the shape of the arc along with the temperature distribution throughout the region. Argon features a lower thermal conductivity rate, approximately 10% of the level with regard to both helium and hydrogen. The high thermal conductivity of helium will provide a broader penetration pattern and will eventually reduce the depth of penetration.

In accordance with these parameters, different gases and gas mixtures are generally classified as:

• Inert Shielding Gases: Argon is the most frequently used inert gas. Helium is frequently included in the gas mix meant for stainless and aluminium applications.
• Reactive Shielding Gases: Oxygen, hydrogen, nitrogen, and carbon dioxide (CO₂) are reactive gases. Reactive gases combine chemically along with the weld pool to generate a desirable effect.
• Binary Shielding Gas Blends: Two-part shielding gas blends are the most typical and they are generally comprised of either argon + helium, argon + CO₂, or argon + oxygen.
• Ternary Gas Shielding Blends: Three-part shielding gas blends continue being widely used for carbon steel, stainless steel, and additionally, in restricted conditions, nickel alloys.

Effects of Variables

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Current Density

Current density is defined as the current employed with a particular electrode diameter divided by its current carrying cross-sectional area. In the event the wire feed speed is low, then the current density is going to be low, and vice versa. From this it is possible to establish that:

• Lower current density applied to a specific electrode is associated with the short-circuit mode of metal transfer.
• Higher current density is associated with the higher energy modes of metal transfer: globular, axial spray transfer or the more advanced pulsed spray metal transfer.

The current for a specified GMAW solid or metal-cored electrode will attain a maximum density level. As soon as this particular level of current density is reached, no additional current can be carried by the electrode. Basically, the electrode has reached its maximum current density. The maximum current density for a given electrode diameter is interchangeable with the concept of current saturation. This phenomenon transpires for all diameters and material types of electrodes intended for GMAW.

It is essential to be aware that the moment the electrode attains its maximum current density, the saturation point, any additional wire feed speed will provide a higher deposition rate without the need of any increase in current.

Electrode Efficiencies

Electrode efficiency is a term which is ascribed to the percentage of electrode that actually results in the weld deposit. Spatter levels, smoke, and slag formers have an impact on the electrode efficiency in GMAW. The electrode efficiency is a numeric value which is assigned to the particular mode of metal transfer:

• GMAW-S, short-circuit transfer, shielded with an argon + CO₂ gas blend, will characteristically operate with an electrode efficiency equal to or greater than 93%. Shielded by 100% CO₂, the electrode efficiency will range from 90 to 93%. Characteristically, CO₂ increases spatter levels to some extent, and argon blends are generally effective in reducing, though not completely eliminating, spatter.
• STT, a dynamically controlled form of GMAW-S, will attain electrode efficiencies of 98% .
• Globular transfer is associated with increased spatter levels which greatly have an impact on electrode efficiency. The efficiency of globular transfer will vary from 85 to 88%, when shielded with 100% CO2. Under argon blends the efficiency can vary from 88 to 90%.
• Axial spray provides a increased electrode efficiency. This higher energy mode of metal transfer is associated with electrode efficiencies of 98%.
• The electrode efficiency for GMAW-P varies depending upon the welding application along with the sophistication of the power source. In general, the efficiency factor applied for GMAW-P is 98%, comparable to that for axial spray, however there could be your requirement for a higher travel speed application that needs shorter arc lengths. High speed pulsed spray transfer types of applications typically introduce higher spatter levels. This consequently decreases the electrode efficiency to some reduced value.

NOTE: All of this relates to the quantity of electrode that actually ends up in the weld. The calculations moreover assume that there is no loss of material as a consequence of wire clipping.

Deposition Rate

The melt-off rate for a particular electrode does not include consideration for the efficiency of the mode of metal transfer or the process. Its interest is in the amount of the electrode that is getting melted.

Deposition rate is applied to the amount of electrode, measured in wire feed speed per unit of time, which is fed into the molten puddle. Significantly, its value reflects the use of the factor for electrode efficiency.

Depending upon the mode of metal transfer, as indicated in electrode efficiency, the factor for any particular mode of metal transfer implemented is applied to the melt-off rate.

To determine the deposition rate for any specified diameter of solid carbon or low alloy steel wire electrode the subsequent mathematical formula is going to be beneficial:

13.1 x D² x WFS x EE

where: D = electrode diameter, WFS = wire feed speed, EE = electrode efficiency, 13.1 = is a constant which is based on the density of steel and its cross-sectional area.

In case the melt off rate is the only thing that is required, in that case make use of the same formula and eliminate the factor for EE.

Aluminium is approximately 33% the density of carbon steel, and its constant is going to be 13.1 x .33, or 4.32. Stainless steel, characteristically, is only slightly greater in density when compared to carbon steel and consequently the 13.1 constant is sufficient.

Electrode Extension and Contact Tip to Work Distance

The electrode extended from the end of the contact tip to the arc is accurately identified as electrode extension. The usual non-standard term is electrical stickout (ESO). In GMAW, this is the amount of electrode which is observable to the welder. The electrode extension consists of solely the length of the electrode, under no circumstances the extension plus the length of the arc. The use of the concept of electrode extension is more typically applied for semiautomatic welding as opposed to robotic or mechanised welding operations. Contact tip to work distance (CTWD) is the standard term applied to the latter.

Contact tip to work distance (CTWD) is a term which lends itself effectively to the electrode extension with regard to mechanised or robotic welding applications. It is measured from the end of the contact tip to the work piece.

Within a non-adaptive constant voltage (CV) system, the electrode extension or the CTWD behaves like a resistor. Varying the length of the electrode has an affect on the current applied to the arc:

• Increasing electrode extension increases the resistance to the flow of current in the electrode, and therefore the current in the arc is decreased.
• Decreasing the electrode extension decreases the resistance to the flow of current in the electrode, and therefore the current in the arc increases.

Considering that current varies with an increase or decrease in extension, the consistency of the extension is extremely important to the consistency of weld penetration. It is essential to maintain an exceptionally steady hand during semiautomatic welding. It is at the same time necessary to ascertain and maintain the proper CTWD pertaining to mechanised or robotic welding.

With regard to short-circuiting metal transfer or GMAW-S, semiautomatic welding, the electrode extension must be kept between 10 – 12 mm. For the purpose of either axial spray or GMAW-P, pulsed spray metal transfer, the electrode extension must be kept between 19 – 25 mm. Maintaining the correct electrode extension is essential to the uniformity of the penetration profile along the length of a weld, along with being regarded as a critical variable for any GMAW procedure.

Waveform Control

Inverter power sources introduced an exciting new era in the advancement of arc welding electrical power sources. The completely unique concept of waveform control makes use of an inverter transformer power supply together with a central processing unit (CPU). The welding power output is actually generated by a high speed amplifier. The software program which drives the output supplies superior optimised welding output for a assortment of GMAW modes of metal transfer.

These kind of power sources feature the capability to interact with the end-user and enable them to generate their unique GMAW-P welding program. For the purpose of pulsed spray transfer, short-circuiting transfer and STT, the output is modulated in response to modifications made to the components of the waveform.

The employment of waveform control enables additional optimisation for any specified mode of metal transfer. This could be to further improve toe wetting action, help reduce dilution levels or even to enhance high travel speed performance associated with a pulsed waveform. The interaction involving the arc performance and the adaptable output are fundamental to the success of waveform control.

The adaptive arc is an arc which rapidly adjusts to alterations in the electrode extension to maintain exactly the same arc length. The objective with regard to adaptive control is to enhance arc performance and maintain the finished weld quality.