Arc Welding
Arc welding melts and fuses metal with an electric arc between an electrode and the work, shielded from the atmosphere by gas or flux.
- Part
- Joining
- Variants
- 5
- Revised
- 2026-08-11
At a glance
- Family
- Thermal
- Typical tolerances
- ±1/16 in (±1.6 mm) on welded assembly dimensions; ±1/32 in (±0.8 mm) with hard fixturing; tighter only by machining after welding
- Typical volumes
- 1 to 1,000,000+ per year — manual for one-offs and short runs, robotic cells for repeat production
- Lead time
- Same day to 1 week for manual one-off weldments; 4–8 weeks to bring a robotic cell online including fixture build and WPS/PQR qualification
- Materials
- Metal
What it is
Arc welding fuses metal by drawing an electric arc between an electrode and the workpiece, melting both faces and — in every variant except autogenous TIG — adding filler metal that solidifies into a continuous joint. The arc column runs above 10,000 °F (5,500 °C), so the process melts anything from 0.030 in (0.8 mm) sheet to plate of effectively unlimited thickness in multiple passes.
The family covers five industrial variants: manual metal arc (MMA/SMAW), metal inert gas (MIG/MAG, GMAW), tungsten inert gas (TIG/GTAW), plasma arc (PAW) and submerged arc (SAW). Between them they cover carbon and low-alloy steel, stainless, aluminum, nickel alloys, copper alloys and titanium.
The number that characterizes arc welding: a full-penetration groove weld made with matching or overmatching filler develops 100% of base-metal tensile strength — in a qualified tensile test the coupon fails in the parent plate, not the weld. Volumes run from a single fabricated frame to robotic lines producing hundreds of thousands of assemblies a year.
How it works
- Joint preparation. Plate over about 1/4 in (6 mm) is beveled — 30–37.5° per side for a single-V groove (60–75° included), 1/16 in (1.6 mm) root face, 1/16–1/8 in (1.6–3.2 mm) root opening. Scale, rust, paint, oil and moisture come off at least 1/2 in (13 mm) back from the joint.
- Shielding. MIG uses argon, argon/CO2 blends (75/25 for steel) or straight CO2 at 20–50 CFH (10–24 L/min); TIG uses argon or argon/helium; SMAW and flux-cored wires make their own gas and slag from the flux; SAW buries the arc under granular flux.
- Heat input. Heat input = (arc volts x amps x 60) / travel speed, landing between roughly 12 and 64 kJ/in (0.5–2.5 kJ/mm) on structural work. That one number governs bead size, penetration, HAZ width and distortion.
- Deposition. Thick joints are filled in multiple passes with interpass temperature controlled — commonly under 350 °F (175 °C) on austenitic stainless to limit sensitization.
- Cooling. Parent metal that got hot but never melted becomes the heat-affected zone, typically 1/16–1/4 in (1.5–6 mm) wide. Cooling rate sets HAZ hardness, so thick sections and high-carbon-equivalent steels are preheated to 150–300 °F (65–150 °C) to avoid hydrogen cracking.
- Post-weld. Slag chipping, back-gouging for double-sided joints, and on heavy pressure-boundary steel a stress relief at 1,100–1,250 °F (595–675 °C).
How do the five variants differ?
MMA / SMAW (stick). A flux-coated consumable electrode at 40–300 A. No gas bottle, so it works outdoors; deposition is only 1–5 lb/hr (0.5–2.3 kg/hr) and slag is chipped between passes.
MIG / MAG (GMAW). Continuously fed solid wire, 0.023–0.045 in (0.6–1.2 mm), with external gas, at 5–12 lb/hr (2.3–5.5 kg/hr). Spray transfer in argon-rich gas for thick plate flat; short-circuit transfer for thin sheet and out-of-position work.
TIG / GTAW. A non-consumable tungsten electrode from 5 to 300 A with heat and filler controlled independently. Well under 2 lb/hr, but the cleanest — the only practical arc route for titanium and the normal choice below 0.060 in (1.5 mm).
Plasma (PAW). A TIG arc constricted through a nozzle orifice, giving higher energy density and a stable arc down to 0.1 A for foil. In keyhole mode it makes a single-pass full-penetration weld to about 1/4 in (6 mm).
Submerged arc (SAW). Wire fed under granular flux at 300–1,500 A, reaching 10–45 lb/hr (5–20 kg/hr). The flux must sit on the joint, so it is limited to flat and horizontal seams — heavy plate, pipe mills, pressure vessels.
Design guidelines
How big should the fillet be?
AWS D1.1 sets minimum fillet size by the thicker part joined: 1/8 in (3 mm) up to 1/4 in (6 mm) base metal; 3/16 in (5 mm) over 1/4 through 1/2 in (6–12 mm); 1/4 in (6 mm) over 1/2 through 3/4 in (12–20 mm); 5/16 in (8 mm) above 3/4 in (20 mm) — an undersized fillet on thick plate cools too fast and cracks.
Fillets are then checked on the throat, not the leg: effective throat = 0.707 x leg, and nominal shear strength is 0.60 x the electrode classification strength on that throat — 42 ksi (290 MPa) for E70 filler. Throat scales with the leg but deposited volume scales with the leg squared, so a 3/8 in fillet buys 50% more strength than a 1/4 in for 125% more weld metal.
Groove welds, access and distortion
Keep 60° included angle on single-V groove welds; below about 45° the electrode cannot reach the sidewall and lack-of-fusion follows. Design so welds fall in the flat (1F/1G) or horizontal (2F) position — vertical and overhead work roughly halves deposition rate.
Weld metal shrinks about 3–5% by volume as it solidifies, pulling the joint transversely and rotating it angularly. Balance welds about the neutral axis, alternate sides on double-sided joints, and specify the smallest weld that carries the load. Expect ±1/16 in (1.6 mm) over a 3 ft (1 m) weldment without hard fixturing; if a surface must be true, leave 0.030–0.060 in (0.8–1.5 mm) of stock and machine after welding.
Which material pairs actually weld?
| Pair | Arc weldable? | Notes |
|---|---|---|
| Carbon steel to carbon steel | Yes | E70XX / ER70S-6 matching filler; the default case |
| Carbon steel to 300-series stainless | Yes | ER309L filler; expect differential expansion |
| 304 to 304 / 316 to 316 | Yes | ER308L and ER316L; hold interpass under 350 °F (175 °C) |
| Aluminum 6xxx to 6xxx | Yes | ER4043 resists cracking, ER5356 is stronger and anodizes to a closer color match |
| Aluminum 2024 / 7075 | No, practically | Hot-cracks and loses temper; rivet, bond or friction weld instead |
| Aluminum to steel | No | Brittle Fe-Al intermetallics; use friction welding or a bimetallic transition insert |
| Titanium to titanium | TIG or plasma only | Full inert coverage plus trailing and backing gas; oxygen pickup embrittles |
| Galvanized steel | Yes, with care | Zinc boils below the pool and causes porosity; grind back 1/2 in (13 mm), extract fume |
Melting points are on the metal melting points chart; strength and expansion figures for sizing welds and predicting distortion are on the material properties chart.
How are arc welds inspected?
Visual inspection to AWS D1.1 is the baseline. Dye penetrant (PT) finds surface cracks on any metal; magnetic particle (MT) finds surface and near-surface flaws in ferromagnetic material only; ultrasonic (UT) is the normal volumetric method for groove welds above 5/16 in (8 mm); radiography (RT) is better for porosity and slag. Procedure and welder qualification (WPS/PQR) is what actually controls quality — inspection only samples it.
| Feature | Recommended | Limit | Why |
|---|---|---|---|
| Fillet leg on 1/2 in (12 mm) plate | Size for the calculated load | 3/16 in (5 mm) minimum | Undersized fillets on thick plate quench and crack |
| Included groove angle | 60° | 45° | Electrode access to the sidewall |
| Root opening, open-root V | 1/8 in (3 mm) | ±1/32 in (0.8 mm) of nominal | Too wide burns through, too tight leaves lack of penetration |
| Weld reinforcement (cap height) | Flush to 1/16 in (1.6 mm) | 1/8 in (3 mm) | Toe angle drives the fatigue stress concentration |
| Torch access around joint | 1 in (25 mm) | 5/8 in (16 mm) | Gas cup and electrode extension |
| Distortion over 3 ft (1 m) | ±1/16 in (1.6 mm) | ±1/32 in (0.8 mm) fixtured | Transverse and angular shrinkage |
Cost drivers
Arc welding is a labor process with a consumable bill attached, not a tooling process. On manual stick work, arc-on time is typically only 10–30% of the welder's shift; semi-automatic MIG reaches 30–50%, and a robotic cell 50–80%. That operator factor — not wire price — is what separates a cheap weldment from an expensive one.
Weld volume is the second driver, and it scales badly. A fillet's cross-sectional area goes with the square of the leg, so specifying 3/8 in where 1/4 in carries the load costs 2.25x the deposited metal and 2.25x the arc time, plus proportionally more distortion to straighten afterward. Consumables are usually a small fraction of joint cost next to labor. Inspection can dominate on critical work: blanket 100% radiography on non-critical welds is one of the most common overspecs in fabrication.
- Size fillets to the calculated load. Use the 0.707 throat rule rather than a habitual leg size; the area penalty is quadratic.
- Use intermittent fillets where the joint is not fatigue-loaded or leak-tight — a 2 in on / 6 in off pattern deposits a quarter of the metal.
- Design for the flat and horizontal positions. Out-of-position welding roughly halves deposition rate and needs a more skilled welder.
- Delete joints upstream. A single laser-cut and formed part often replaces three welded pieces and two joints.
- Specify NDT per joint, not per drawing. Reserve UT and RT for full-penetration structural or pressure joints and use visual plus PT elsewhere.
Variants
5 named
Manual Metal Arc Welding (MMA)
Metal Inert Gas Welding (MIG)
Tungsten Inert Gas Welding (TIG)
Plasma Welding
Submerged Arc Welding (SAW)
Questions
6 questionsHow strong is an arc weld compared to the base metal?
A full-penetration groove weld made with matching or overmatching filler develops 100% of base-metal tensile strength — in a qualified tensile test the coupon breaks in the parent plate. Fillet welds are different: they are sized on shear across the effective throat, which is 0.707 x the leg, at 0.60 x the filler classification strength (42 ksi / 290 MPa for E70 filler).
What is the minimum fillet weld size for 1/2 inch plate?
3/16 in (5 mm) per AWS D1.1 for base metal over 1/4 in through 1/2 in (6–12 mm). Minimum sizes exist because a small fillet deposited onto a thick, cold section cools too quickly, hardens the heat-affected zone and cracks. Above 3/4 in (20 mm) the minimum rises to 5/16 in (8 mm).
Can you arc weld aluminum to steel?
No. Iron and aluminum form brittle Fe-Al intermetallic layers at the fusion line that crack under almost any load. The production routes are friction welding (rotary or friction stir), an explosion-bonded bimetallic transition insert, adhesive bonding, or self-piercing rivets.
MIG or TIG — which should I specify?
MIG where deposition rate matters: 5–12 lb/hr (2.3–5.5 kg/hr) versus under 2 lb/hr for TIG. TIG below about 0.060 in (1.5 mm), for titanium and other reactive metals, and where the bead is a visible design feature, because heat and filler are controlled independently.
How much distortion should I expect from welding?
Weld metal shrinks 3–5% by volume as it solidifies, producing transverse shrinkage across the joint and angular rotation of the plates. Without hard fixturing, ±1/16 in (1.6 mm) over a 3 ft (1 m) weldment is normal. Balanced sequence, back-stepping and the smallest adequate weld are the practical controls.
When does carbon steel need preheat before welding?
AWS D1.1 sets preheat from thickness and steel group, and typical structural carbon steel above 3/4 in (20 mm) calls for 150–300 °F (65–150 °C). Preheat slows cooling through the martensite range, lowering HAZ hardness and giving diffusible hydrogen time to escape before the joint is cold.