MIG Welding
Also: GMAW, gas metal arc welding, MAG welding, wire feed welding, metal inert gas welding, pulsed MIG, spray transfer welding, flux-cored welding, FCAW
MIG welding (GMAW) feeds a continuous wire under shielding gas: the fastest arc process for steel, stainless and aluminum, and the default for fabrication.
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A MIG gun feeds a solid wire electrode from a spool through the contact tip while shielding gas flows around it; the arc between the wire and the work melts both, and the welder moves the gun along the joint with one hand. Because the wire feeds itself, the process is easy to learn, runs continuously, and deposits 2-5 kg (4-10 lb) of metal an hour, several times what TIG can. Shielding is 75% argon / 25% CO2 for steel, argon with 2% oxygen or CO2 for stainless, and pure argon for aluminum.
The arc transfers metal in different modes. Short-circuit transfer at low current welds thin sheet in any position but risks lack of fusion on thick sections; spray transfer at high current lays fast, deep welds on plate in the flat and horizontal positions; pulsed MIG switches between the two many times a second and gives spray-quality welds in all positions with less heat and spatter, which is why it is the modern standard for stainless and aluminum. Flux-cored wire (FCAW) adds flux inside a tubular wire for thick plate, outdoor work and higher deposition.
MIG is the workhorse of fabrication because it is fast and cheap per inch, but it is not the cleanest. The bead is convex with some spatter, thin material under 0.8 mm (0.030") burns through, and aluminum needs a push-pull or spool gun because soft wire tangles in a long liner. For cosmetic, thin or exotic-alloy welds, TIG or laser takes over.
At a glance
| Typical tolerances | As-welded assemblies ±1.5 mm (±1/16") standard, ±0.8 mm (±1/32") with fixtures; angles ±1°, ±0.5° fixtured. Fillet size per the symbol, +1.5 mm (+1/16"), -0; leg consistency ±1 mm by hand, ±0.5 mm robotic. Distortion on thin sheet is higher than TIG or laser because of the deposition rate. Machining after welding for anything tighter. |
|---|---|
| Size limits | Thickness from 0.8 mm (0.030") in short-circuit transfer to unlimited plate with multi-pass welding; single-pass fillets to about 8 mm (5/16"). Aluminum from 1.5 mm (0.060"). Wire diameters 0.6-1.6 mm (0.023-0.062"). Part size is limited only by the shop floor and the crane. |
| Surface finish | A convex, rippled bead with light spatter around it; pulsed MIG and anti-spatter compound reduce the spatter, grinding removes it. Welds can be ground flush on steel and aluminum. Stainless shows heat tint to be pickled or passivated. MIG welds under powder coat or paint are normal; under anodize, the filler shows darker. |
| Lead time | Simple weldments in 2-7 business days; production runs in 2-4 weeks. Coded work adds time only when a procedure must be qualified. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Carbon and low-alloy steels are the home ground: A36, 1018, 1045, A572 Grade 50 and 4130 with ER70S-6 wire, 4140 with preheat and post-weld tempering. Stainless 304, 316 and 2205 duplex weld with 308L, 316L and 2209 wire; 309L joins stainless to carbon steel. Aluminum 5052, 5083 and 6061 weld with 5356 or 4043 wire on a push-pull gun at 1.5 mm (0.060") and thicker. Galvanized steel welds with fume extraction and some porosity. Titanium, magnesium and the nickel alloys are TIG work; the free-machining grades and 7075 are not welded at all.
What drives the cost
- Weld length and number of passes
- Joint preparation and fit-up: bevels, gaps and tack welding
- Material: aluminum and stainless run slower and need more cleaning than steel
- Position: out-of-position welding is slower than flat
- Fixturing for repeat parts
- Spatter removal, grinding and cosmetic blending
- Inspection and certification requirements
When to use it
- Steel frames, brackets, skids, guards, tanks and machine bases
- Long welds and thick sections where deposition rate matters
- Aluminum structures 1.5 mm (0.060") and up, with pulsed MIG
- Production welding, by hand or on a robot, where the bead can be as welded or ground
- Field and outdoor work, with flux-cored wire
When not to
- Sheet under 0.8 mm (0.030"): TIG, spot or laser
- Titanium, magnesium, nickel alloys and copper: TIG
- Cosmetic welds that must not be ground: TIG or laser
- Free-machining steels and stainless, 7075 and 2024 aluminum, hardened parts
- Joints with poor access for a gun nozzle
Design tips
- Size fillets to the thinner member (about 0.7-1.0 times its thickness) and do not oversize; extra weld is extra heat and cost.
- Give the gun access: about 20 mm (3/4") of nozzle clearance and a 45° approach to the joint.
- Prefer lap, T and fillet joints on sheet; bevel groove joints above 6 mm (1/4") for full penetration.
- Hold fit-up gaps to 1 mm (0.040") or less; larger gaps mean slower welding and more filler.
- Use intermittent (stitch) welds where continuous welds are not needed, to cut heat and distortion.
- Keep galvanizing, paint and oil out of the weld zone or specify that the shop removes them.
- Mask or plan to grind cosmetic surfaces near the weld; MIG spatters.
- Call out the code, the filler if it matters (ER70S-6, ER308L, ER5356) and whether the weld will be ground flush.
Frequently asked questions
- What is the difference between MIG and TIG welding?
- MIG feeds a consumable wire automatically and welds fast with one hand; TIG uses a non-consumable tungsten with filler added by the other hand and welds slowly with more control. MIG for speed on steel and aluminum structure, TIG for thin, cosmetic or exotic-alloy work.
- Can you MIG weld aluminum?
- Yes, from about 1.5 mm (0.060") thick, with pure argon, 5356 or 4043 wire and a push-pull or spool gun so the soft wire feeds cleanly. Pulsed MIG makes it practical on 2-3 mm sheet; thinner or cosmetic aluminum is TIG welded.
- What shielding gas is used for MIG welding?
- 75% argon / 25% CO2 is the general steel mix; 100% CO2 is cheaper with more spatter; 90-98% argon with 2-10% CO2 or oxygen for spray and pulsed transfer on steel and stainless; 100% argon for aluminum. Flux-cored wire may be self-shielded or run under CO2.
- How thick can MIG weld?
- Single-pass fillets to about 8 mm (5/16") and single-pass groove welds to about 6 mm (1/4") in spray transfer; any thickness with beveled joints and multiple passes. Heavy plate is often welded with flux-cored or submerged arc for the deposition rate.
- Is MIG welding strong?
- A sound MIG weld with matching filler is as strong as the base metal; ER70S-6 gives 480 MPa (70 ksi) tensile weld metal on mild steel. The risk is lack of fusion in short-circuit transfer on thick sections, which is why procedures specify the transfer mode and current for the thickness.
