Gas metal arc welding

For an ideal equal-leg triangular fillet 100 mm long and deposit density 7.85 g/cm³, increasing the leg from 6 to 8 mm increases volume from 1.8 to 3.2 cm³ and mass from 14.13 to 25.12 g. Volume rises 77.78%. Real bead shape, penetration and wire consumption require separate evidence.

GMAW feeds a consumable electrode and uses external shielding gas. Lincoln's guide covers solid and metal-cored wires. Identify the electrode and process when specifying the route. Lincoln Electric: Gas Metal Arc Welding—Product and Procedure Selection, C4.200

MIG and MAG identify inert-gas and active-gas variants in common terminology, while “MIG” is also used informally for GMAW more broadly. A process name alone is insufficient for procurement: identify the exact gas composition, wire classification/product and transfer mode. Lincoln separates inert gases, reactive gases and their blends in its selection guidance. Lincoln Electric: shielding-gas selection

Choose the transfer mode for the joint

Miller distinguishes these common modes:

Miller explains that conventional modes depend on shielding gas, voltage and current, while pulsed spray uses controlled peaks for droplet transfer and a lower background to maintain the arc. These descriptions do not establish that every machine, material or joint can use every mode. Miller: guide to pulsed MIG welding

For example, Haynes warns that short-circuiting transfer can produce incomplete fusion, especially in thick sections or multipass work, in its nickel/cobalt-alloy guidance. It encourages pulsed spray for many HASTELLOY and HAYNES applications. This is alloy-family guidance, not permission to replace a qualified root-pass process or transfer mode without review. Haynes: GMAW guidance

Wire and gas form a material system

Specify base alloy, incoming condition, thickness and joint design before selecting filler. Include wire classification, manufacturer/product, diameter and lot controls where required. A deposited-metal classification or nominal tensile strength does not by itself demonstrate joint toughness, corrosion behavior, dilution response or suitability for dissimilar materials.

Gas substitution can change transfer and metallurgy. Haynes normally recommends argon or argon/helium for its Ni-/Co-base alloys and warns against oxygen or carbon-dioxide additions in that context, even though such additions are common with other metals. A carbon-steel gas recommendation must therefore not be transferred automatically to nickel-alloy work. Haynes: shielding-gas selection

Record gas composition and delivery arrangement, not just cylinder pressure. Check nozzle condition, leaks, drafts and access to the joint. Miller's troubleshooting guidance identifies inadequate shielding, contamination and other contributors to defects; a visible pore is evidence to investigate, not a unique diagnosis. Increasing flow alone is not a complete corrective action. Miller: troubleshooting MIG weld defects

When pores appear after a setup change

Use Porosity in welds: investigate the source to check gas delivery, contamination and material-specific causes. Record exactly what changed, compare actual nozzle delivery with the applicable procedure, and distinguish startup behavior from steady flow. A regulator setting alone does not demonstrate shielding at the joint.

The inspection and acceptance section explains why an observed pore, an examination result and an acceptance decision are separate conclusions. Do not select a numerical pore allowance from a generic troubleshooting explanation.

Understand the controls and their interactions

For conventional constant-voltage GMAW, TWI explains that the power source supplies current to melt wire at the selected feed rate and maintain the voltage/arc-length relationship. Wire-feed speed can therefore control current. Contact-tip-to-work distance also matters: increased electrode extension adds resistance heating and changes the current required at a given feed rate. A recorded feed-speed setting is not a universal amperage value. TWI: current control in MIG welding

Distinguish conventional CV controls from synergic, pulsed or controlled-short-circuit programs. Preserve the approved program, wire and gas selections, polarity, trim/arc-length controls and mechanical setup. A setting labeled “thickness” or “trim” is equipment-specific and does not replace the welding procedure.

TWI's parameter-measurement guidance covers voltage/current waveforms as well as wire feed and travel. For waveform-controlled processes, agree how recorded electrical data and any energy calculation represent the actual waveform. Multiplying convenient display values should not silently become verified absorbed heat or proof of fusion. TWI: measurement of arc-welding parameters

Joint preparation and delivery sequence

Define fit-up, root opening, bevels, backing or back-gouging, access, position and restraints on the controlled drawing/procedure. Include cleaning, tack welds, pass sequence, preheat/interpass control and any postweld treatment required for the actual material and application. There is no general GMAW voltage, gas flow or travel speed that qualifies all joints.

Confirm gun, feeder, liner, contact tip and wire compatibility. Haynes notes that worn or dirty liners and sharp cable bends can produce erratic feeding, arc instability or a bird nest. Resolve equipment and consumable condition alongside technique when troubleshooting; adjusting voltage cannot correct every mechanical feeding problem. Haynes: equipment considerations

Original idealized weld-volume example

Assume a straight, equal-leg triangular fillet with no convexity, penetration, root gap or end effects. Let weld length be 100 mm and hypothetical deposited density 7.85 g/cm³:

The larger leg adds 10.99 g in this model. Establish real bead shape, wire use, arc time and distortion separately. Use the design requirements for structural weld size.

Qualification and acceptance

ISO 15614-1:2017, edition 2, June 2017, corrected November 2017, with Amendment 1:2019, covers procedure testing for arc/gas welding of steels and arc welding of nickel/nickel alloys. Its scope includes gas-shielded metal arc processes, production, repair and build-up welding. ISO lists it as current, confirmed in 2022 and under revision; the listed FDIS successor is not a published replacement. Its material scope should not be presented as universal coverage of aluminum GMAW. ISO 15614-1:2017

Determine the governing fabrication/application standard, procedure qualification and personnel requirements. Agree weld dimensions, permitted imperfections, examination methods, sampling, repair controls and final inspection state. Separate appearance, fusion, dimensions, material condition and service performance. A smooth bead, stable arc or catalog deposition rate does not establish the acceptance of a production joint.

Source versions

Reviewed 2026-11-01. Lincoln's PDF back cover explicitly states Publication C4.200, Issue Date 08/14. It is used for process fundamentals, not current product availability or universal settings. The ISO edition/correction/amendment are identified above. No separately verified technical issue date is assigned to the cited Miller, Haynes or TWI live guidance. The fillet example and handoff recommendations are original.

Related reference

Welding methods