CWI Welding Processes Study Guide: SMAW, GMAW, FCAW, GTAW

What this domain covers

Welding Processes shares the largest minimum share of CWI Part A questions with Definitions and Terminology. You need to know how each common process works and what power source and polarity it uses. You also need to know how its filler metal is classified and which discontinuities it tends to produce. The questions are conceptual. You will not be asked to run a machine, but you will be asked why a setting matters.

Shielded metal arc welding (SMAW)

SMAW uses a covered electrode on a constant-current power source. With constant current, small changes in arc length by the welder cause only small changes in current.

The core wire carries the current. The coating:

• Makes a shielding gas as it burns.
• Forms slag that protects the cooling weld.
• Adds deoxidizers, arc stabilizers and sometimes alloys and iron powder.

Reading AWS A5.1 classifications. Take E7018:

Part	Meaning
E	Electrode
70	At least 70 ksi tensile strength
1	All positions
8 (read with the 1)	Low-hydrogen, iron-powder coating usable on AC or direct current electrode positive (DCEP)

The position digit can be 1 (all positions), 2 (flat and horizontal fillet) or 4 (flat, horizontal, overhead and vertical-down).

Common electrodes to know:

• E6010: cellulosic, DCEP, deep penetration and fast-freezing slag. The usual open-root pipe electrode. Not low-hydrogen.
• E6013: rutile, soft arc, shallow penetration.
• E7018: low-hydrogen, all positions.
• E7024 and E7028: heavy iron-powder electrodes for flat and horizontal fillets.

Optional designators: H4, H8 or H16 give the maximum average diffusible hydrogen in mL per 100 g of weld metal. R means the coating passed a moisture-resistance test.

Low-hydrogen electrodes go into heated holding ovens once opened because their coatings absorb moisture. Moisture becomes hydrogen in the weld and raises the risk of cracking, as the metallurgy and heat control guide explains.

Gas metal arc welding (GMAW)

GMAW feeds a solid wire through a gun on a constant-voltage source, normally DCEP. The arc length self-regulates because current changes as the contact-tip-to-work distance changes.

Know the four transfer modes:

• Short-circuiting (GMAW-S): low heat, good for thin sheet and out-of-position work, but prone to incomplete fusion on thick sections. AWS D1.1 does not prequalify it.
• Globular: large, spattery drops.
• Spray: fine droplets at high current. It needs an argon-rich gas (roughly 80% argon or more). Pure CO2 will not give true spray transfer.
• Pulsed spray: spray-like transfer at a lower average current.

In ER70S-6 (AWS A5.18), "ER" means electrode or rod and "70" is tensile strength in ksi. "S" means solid, and "-6" is a chemistry with more manganese and silicon for welding over some mill scale.

Flux cored arc welding (FCAW)

FCAW uses a tubular wire with flux inside, usually on a constant-voltage source. In E71T-1C (AWS A5.20), "7" means 70 ksi. The "1" after it means all positions (0 would mean flat and horizontal only). "T" means tubular, "-1" is the usability designator and "C" means 100% CO2 gas.

• FCAW-G needs external shielding gas.
• FCAW-S makes its own shielding from the core, which suits windy field work.

With a constant-voltage source, a longer electrode extension heats the wire by resistance, so current drops and penetration falls. That is why welding procedure specifications (WPSs) control contact-tip-to-work distance.

Gas tungsten arc welding (GTAW)

GTAW uses a nonconsumable tungsten electrode and inert gas, with filler added separately.

• Direct current electrode negative (DCEN) for steel: about two-thirds of the heat goes into the work. Penetration is good, and the tungsten stays cool.
• AC for aluminum: the electrode-positive half cycle breaks up the tough oxide; the electrode-negative half cycle heats the work.
• Touching the tungsten to the puddle or filler can leave a tungsten inclusion, which shows as a light spot on a radiograph.

Tungsten electrodes are classified under AWS A5.12.

Submerged arc welding (SAW)

The arc burns under a blanket of granular flux. Deposition is high and there is no arc glare. The process is normally limited to the flat position and horizontal fillets, because the flux has to stay in place.

Flux types by effect on the weld:

• Neutral: little change in chemistry.
• Active: adds manganese and silicon, so chemistry changes with voltage.
• Alloy: adds alloying elements on purpose.

Classification example: F7A2-EM12K (AWS A5.17).

Part	Meaning
F7	Flux giving 70 ksi minimum tensile strength
A	Tested as-welded (P would mean after postweld heat treatment, or PWHT)
2	Impact requirements met at -20 degrees F
EM12K	Medium-manganese, killed electrode

Deep, narrow SAW beads are prone to centerline solidification cracking. The causes of this and other discontinuities are in the weld examination guide.

Electroslag, plasma and other arc processes

• Electroslag welding (ESW) starts with an arc, but once a slag bath forms, resistance heating of the slag does the welding. It welds thick sections vertically in one pass between copper shoes. The heat input is high, so grains are coarse.
• Electrogas welding keeps an arc going with shielding gas in a similar vertical setup.
• Plasma arc welding (PAW) uses a tungsten electrode with the arc forced through a constricting orifice. The stiff, concentrated arc allows keyhole welding.
• Arc stud welding uses a ceramic ferrule to concentrate heat, contain molten metal and help shield the weld. The ferrule is broken off afterward.

Arc blow is magnetic deflection of a DC arc. Switching to AC, moving the work lead or shortening the arc can help.

Non-arc processes

• Resistance spot welding heats the joint by resistance to current flow under electrode pressure. There is no arc.
• Friction welding is solid-state: the base metal does not melt.
• Oxyacetylene welding: a neutral flame uses about equal volumes of oxygen and acetylene and has a clean inner cone with no feather. A carburizing flame has excess acetylene and a feather; an oxidizing flame has excess oxygen. Free acetylene must not be used above 15 psig because it can decompose violently. Oxyfuel cutting is covered in the thermal cutting guide.
• Brazing uses filler with a liquidus above 840 degrees F (450 degrees C) but below the base metal solidus. Capillary action draws the filler into the joint. Soldering is the same idea below 840 degrees F. Braze welding deposits filler without capillary action.

Filler metal specifications to recognize

• A5.1: carbon steel SMAW electrodes (A5.5 for low-alloy).
• A5.17: carbon steel SAW electrodes and fluxes.
• A5.18: carbon steel solid wire and rod for GMAW and GTAW.
• A5.20: carbon steel FCAW electrodes.
• A5.12: tungsten electrodes.
• A5.32: shielding gases.
• A5.8: brazing filler metals.

How to study this domain

Build a one-page table with a row per process. Give it columns for power source type, usual polarity, shielding, positions, typical discontinuities and filler specification. Then practice decoding classifications until each digit's meaning comes quickly.

Mark J. Curry's tip: on process questions, ask yourself what the arc is doing and where the heat goes. That usually explains the correct answer.

Work through the free welding processes practice questions. To see which Part A topic to study first, take the free CWI diagnostic test.

Sources

The Welding Processes questions cite these documents, by document and, where we can confirm it, chapter or section. We never reproduce them.

1. AWS A3.0 (3 questions)
2. AWS A5.1 (3 questions)
3. AWS D1.1 (3 questions)
4. ANSI Z49.1 (1 question)
5. AWS A5.12 (1 question)
6. AWS A5.17 (1 question)
7. AWS A5.18 (1 question)
8. AWS A5.20 (1 question)

Found a mistake? Use "Report a problem" under any question, and Mark J. Curry checks it.

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