Introduction
Two workshops, two jobs. In the first, a welder is joining a bicycle frame made of thin aluminium tube. In the second, a welder is joining two sections of a bridge girder 40 mm thick, outdoors, in a stiff wind.
Neither could do the other’s job with the same equipment. The aluminium frame needs a delicate, precisely controlled heat source that will not blow a hole through the tube. The bridge girder needs enormous heat, deep penetration, and shielding that will not be swept away by the weather.
That is why so many welding processes exist. Each one was developed because an existing process could not do a particular job well enough. This guide walks through all the major types, explains what each one is genuinely good at, and finishes with a selection framework so you can decide which process fits a given job. Written in plain language for mechanical and production engineering students.
Quick Reference: All Major Welding Processes at a Glance
| Process | Abbreviation | Heat source | Best known for |
|---|---|---|---|
| Shielded Metal Arc Welding | SMAW | Electric arc | Site work, portability |
| Gas Metal Arc Welding | GMAW (MIG/MAG) | Electric arc | Fast production welding |
| Gas Tungsten Arc Welding | GTAW (TIG) | Electric arc | Highest quality welds |
| Flux Cored Arc Welding | FCAW | Electric arc | Outdoor structural work |
| Submerged Arc Welding | SAW | Electric arc | Thick plate, long seams |
| Plasma Arc Welding | PAW | Constricted arc | Precision, thin sections |
| Oxy-Acetylene Welding | OAW | Gas flame | Repair, no electricity needed |
| Spot Welding | RSW | Resistance | Car body assembly |
| Seam Welding | RSEW | Resistance | Leak-tight sheet joints |
| Friction Stir Welding | FSW | Friction | Strong aluminium joints |
| Ultrasonic Welding | USW | Vibration | Foils, battery tabs, plastics |
| Explosive Welding | EXW | Detonation | Cladding dissimilar metals |
| Laser Beam Welding | LBW | Laser | Deep, narrow, fast welds |
| Electron Beam Welding | EBW | Electron beam | Deepest penetration |
| Thermit Welding | TW | Chemical reaction | Railway rails in the field |
Keep this table beside you as you read. Everything below fills in why each row exists.
How Welding Processes Are Classified
Before the detail, two classification systems make sense of the list.
By whether the metal melts:
- Fusion welding – the base metal melts and the molten pools merge. Arc, gas, resistance and beam welding all belong here.
- Solid state welding – the metal is joined below its melting point using pressure, friction or diffusion. Friction, ultrasonic, explosive and diffusion welding belong here.
By whether filler metal is added:
- Autogenous – no filler; the base metal alone forms the joint, as in resistance and most laser welding.
- Homogeneous – filler with the same composition as the base metal, as in most arc welding.
- Heterogeneous – filler of a different composition, as in brazing and some dissimilar-metal welding.
The first split is the one that matters most, because it decides what defects you will face. Fusion processes must deal with molten metal so porosity, solidification cracking and a cast grain structure are the concerns. Solid state processes never melt anything, so they avoid all three, which is why they are chosen for aluminium and for joining dissimilar metals.
Arc Welding: The Dominant Family
Roughly three quarters of industrial welding is done with an arc. An electric arc between electrode and workpiece reaches 5,000–6,000 °C, melting both.
All arc processes face one shared problem. Molten steel absorbs oxygen and nitrogen from the air within a fraction of a second, producing porous, brittle welds. The differences between the processes below come mainly from how each solves that problem.
Shielded Metal Arc Welding (SMAW / Stick)
A flux-coated consumable electrode. The coating burns to release shielding gas and leaves a slag blanket that must be chipped off.
Choose it when: you are working on site, outdoors, in wind, in awkward positions, or on rusty material. Nothing beats it for portability. Avoid it when: you need speed or a large volume of weld metal deposited.
Gas Metal Arc Welding (GMAW / MIG / MAG)
Continuously fed bare wire with shielding gas from a cylinder. MIG uses inert argon or helium; MAG uses active CO₂ or argon mixtures.
Choose it when: you need production speed, continuous welding without electrode changes, or an easy process to train operators on. Avoid it when: working outdoors, since even a light breeze strips the shielding gas away.
Gas Tungsten Arc Welding (GTAW / TIG)
A non-consumable tungsten electrode strikes the arc; argon shields; filler is fed by hand if required.
Choose it when: quality matters more than speed aerospace parts, stainless steel piping, aluminium, titanium, thin sheet, and any weld that will be seen. Avoid it when: you have metres of thick plate to fill, because it will take forever.
Flux Cored Arc Welding (FCAW)
Tubular wire filled with flux. Self-shielded versions generate their own protection and need no gas cylinder at all.
Choose it when: you need MIG-like speed but you are working outdoors on structural steel. Avoid it when: fume and slag cleanup are a problem, or on thin material.
Submerged Arc Welding (SAW)
The arc burns hidden beneath a blanket of granular flux. No visible arc, no flash, very little fume.
Choose it when: you have long, straight or circular seams in thick plate, in the flat position pressure vessels, ship hulls, large pipe. Avoid it when: the joint is short, positional, or curved in more than one plane.
Plasma Arc Welding (PAW)
A TIG arc constricted through a fine nozzle, concentrating energy into a keyhole that penetrates deeply with a narrow heat affected zone.
Choose it when: you need precision beyond TIG, especially on thin foils and aerospace components.
Gas Welding and the Three Flame Types
Oxy-acetylene welding burns acetylene in oxygen to reach roughly 3,200 °C. The heat is far less concentrated than an arc, so it is slower with a wider heat affected zone but it needs no electricity, which keeps it alive for repair work, plumbing and site jobs.
The flame setting is adjustable, and choosing wrongly ruins the weld.
| Flame type | Oxygen : acetylene | Appearance | Correct use |
|---|---|---|---|
| Neutral | 1 : 1 | Clean, sharply defined inner cone | Mild steel, cast iron, most general welding |
| Carburising | Less than 1 : 1 | Feathery white plume around the cone | Hardfacing, high-carbon steel, nickel alloys |
| Oxidising | More than 1 : 1 | Short, pointed, noisy cone | Brass, bronze, zinc alloys only |
Use an oxidising flame on steel and the excess oxygen forms iron oxide throughout the weld, leaving it brittle and porous. Use a carburising flame on mild steel and excess carbon hardens the weld metal. The neutral flame exists because most metals want neither.
The same equipment also performs flame cutting and supplies heat for brazing, which is why an oxy-acetylene set remains one of the most useful items in a workshop.
Resistance Welding for Sheet Metal Assembly
No arc, no filler, no shielding gas. Current passes through the joint, and the electrical resistance where the two sheets meet generates heat exactly where it is wanted. Electrode pressure then forges the softened metal together.
The governing equation is H = I²Rt heat equals current squared × resistance × time. Current is squared, so it dominates everything: doubling current quadruples heat. This is why resistance welding uses enormous currents for extremely short times, often thousands of amps for a tenth of a second.
Spot welding joins overlapping sheets at discrete points. A car body contains several thousand. Seam welding uses rotating wheel electrodes to make a continuous leak-tight line, as in fuel tanks and radiators.
Projection welding uses raised dimples to concentrate current at chosen points, welding several spots at once the standard way to attach nuts and studs to sheet. Butt and flash welding press two ends together and heat them by resistance until they forge into one, used for rails, wheel rims and chain links.
Resistance welding is fast, clean, cheap per weld and easy to automate. Its limits are that it needs conductive material, works mainly on thin overlapping sheet, and demands expensive equipment.
Solid State Welding Without Melting
These processes join metal below its melting point, and that single fact is their advantage.
Friction welding spins one part against another under pressure until the interface plasticises, then stops and forges the joint. Used for axle shafts and valve stems.
Friction stir welding runs a rotating shouldered pin along the joint line, stirring plasticised metal together. Because aluminium never melts, the joint keeps far more of its strength than a fusion weld would, with almost no distortion. It transformed aluminium shipbuilding, rail carriages and aerospace panels.
Ultrasonic welding uses high-frequency vibration under pressure to break surface oxides and bond the metals. Standard for battery tabs, foils, electrical terminals and plastics.
Explosive welding drives one plate onto another at very high velocity, producing a wavy interlocked interface. It joins combinations that fusion welding cannot handle at all, such as titanium to steel, and is how clad plate is manufactured.
Diffusion bonding holds parts together at high temperature and pressure until atoms migrate across the interface. Slow and costly, but ideal for aerospace titanium assemblies.
Beam and Specialised Welding Processes
Laser beam welding delivers a focused beam that produces a deep, narrow weld at high speed, with a minimal heat affected zone and very little distortion. Fit-up must be near perfect and equipment cost is high.
Electron beam welding fires high-velocity electrons, usually in a vacuum, achieving depth-to-width ratios beyond 20:1. Used for aerospace and nuclear components; the vacuum chamber limits part size.
Thermit welding ignites aluminium powder with iron oxide, producing molten steel that is poured into a mould around the joint. It needs no external power at all, which is why railway rails are still joined this way in the field.
Electroslag welding uses molten conductive slag to melt filler continuously, filling very thick vertical joints in a single pass.
Understanding Welding Electrode Classification
Electrode codes look cryptic until you know the pattern, and decoding them is a standard exam question.
For the AWS system, an electrode marked E6013 breaks down as:
| Position | Symbol | Meaning |
|---|---|---|
| E | Electrode | Indicates an arc welding electrode |
| 60 | Tensile strength | 60,000 psi minimum tensile strength |
| 1 | Position | 1 = all positions; 2 = flat and horizontal only |
| 3 | Coating and current | Type of flux coating and suitable current |
So E6013 is an all-position electrode with 60,000 psi tensile strength and a rutile coating the general-purpose electrode found in most workshops. E7018 is a 70,000 psi, all-position, low-hydrogen electrode, which is why it is specified for structural and pressure work where hydrogen cracking is a risk.
That last digit is the one worth remembering. Low-hydrogen electrodes must be stored in heated ovens, because moisture absorbed from the air introduces hydrogen into the weld and causes delayed cracking hours or days after welding.
Reading Basic Welding Symbols
Drawings communicate welds through a standard symbol rather than a written note.
The symbol sits on a reference line with an arrow pointing to the joint. Anything drawn below the reference line applies to the arrow side of the joint; anything above applies to the other side. A circle at the elbow means weld all around; a flag means weld in the field rather than in the shop; a tail carries the process or specification reference.
Common weld symbols include a triangle for a fillet weld, a square for a square groove, a V for a V-groove, and a semicircle for a U-groove. Numbers to the left of the symbol give the weld size, and numbers to the right give length and pitch for intermittent welds.
How to Select the Right Welding Process
Five questions decide the choice in practice.
1. What is the material?
Steel accepts almost any process. Aluminium needs TIG, MIG with a spool gun, or friction stir. Titanium demands TIG with back purging or electron beam. Dissimilar metals usually point toward solid state processes.
2. How thick is it?
Below 1 mm, use TIG, resistance or laser. From 1 to 6 mm, MIG or TIG. Above 6 mm, SMAW, FCAW or SAW with multiple passes.
3. Where is the work?
Indoors and controlled favours MIG and TIG. Outdoors and windy demands SMAW or self-shielded FCAW, because shielding gas simply blows away.
4. What quality level is required?
Cosmetic or structural-critical work justifies TIG’s slow speed. General fabrication does not.
5. What is the production volume?
One-offs and repairs suit manual processes. High volume justifies the equipment cost of resistance, laser or automated MIG.
| Situation | Best process |
|---|---|
| Thin stainless steel tube | TIG |
| Car body sheet assembly | Spot welding |
| Structural steel beam on site | SMAW or FCAW |
| Long seam in thick pressure vessel plate | SAW |
| Aluminium panels needing full strength | Friction stir welding |
| Railway rail joint in the field | Thermit welding |
| High-volume thin sheet production line | Laser welding |
Common Welding Defects to Watch For
| Defect | Appearance | Main cause |
|---|---|---|
| Porosity | Trapped gas holes | Moisture, oil, rust, inadequate shielding |
| Undercut | Groove melted at the weld toe | Current too high, wrong angle, travel too fast |
| Slag inclusion | Slag trapped in the weld | Poor interpass cleaning |
| Incomplete penetration | Weld does not reach the root | Low current, narrow root gap, fast travel |
| Lack of fusion | Weld not bonded to base metal | Insufficient heat, dirty surface |
| Hot cracking | Cracks during solidification | Impurities, high restraint, narrow deep bead |
| Cold cracking | Cracks appearing hours later | Hydrogen plus hard microstructure plus stress |
| Distortion | Assembly pulled out of shape | Uneven heating and contraction |
| Spatter | Droplets around the weld | Excessive current, damp electrode |
Because most of these are internal, welded structures are inspected by non-destructive testing visual inspection, dye penetrant for surface cracks, magnetic particle for near-surface flaws in ferrous metal, ultrasonic for internal defects, and radiography for a permanent record of internal soundness.
Welding Safety Essentials
Welding presents four hazards at once, and each has a specific control.
Arc radiation. The arc emits intense ultraviolet and infrared light that burns eyes and skin. Correct shade filters, full face protection, and screens to protect bystanders are non-negotiable.
Fume and gas. Welding fume contains metal oxides, and welding stainless steel produces hexavalent chromium. Local exhaust ventilation is required, not just an open door.
Electric shock. Voltages are low but currents are high, and damp conditions or damaged cables make shock a real risk. Dry gloves, insulated footwear and intact cable insulation matter.
Fire and burns. Spatter travels several metres and stays hot. Combustible material must be cleared, and confined spaces need a permit and gas testing before work begins.
Applications Across Industries
- Automotive – spot welding for body shells, MIG for chassis, friction stir for aluminium structures
- Construction – SMAW and FCAW for beams, columns and reinforcement
- Shipbuilding – SAW for long hull seams, FCAW for positional outdoor work
- Oil and gas pipelines – TIG root passes with SMAW or FCAW fill
- Aerospace – TIG, electron beam and friction stir for critical components
- Pressure vessels – SAW and TIG under strict codes with full NDT
- Railways – thermit welding for continuous rail
- Electronics – ultrasonic and resistance welding for battery and terminal connections
Advantages and Limitations of Welding
Advantages
- Joint strength can equal or exceed the base metal
- Permanent, leak-tight joints without gaskets or fasteners
- Lighter and often cheaper than bolted or riveted assemblies
- Complex structures built from simple standard sections
- Works on site as well as in automated production
Limitations
- Joints cannot be dismantled without destruction
- Heat causes distortion and residual stress
- The heat affected zone may be brittle or weakened
- Skilled, often formally certified labour is required
- Internal defects need NDT to detect
- Significant safety hazards requiring active control
Frequently Asked Questions (FAQ)
1. What are the main types of welding processes?
The main families are arc welding (SMAW, GMAW, GTAW, FCAW, SAW, PAW), gas welding, resistance welding (spot, seam, projection, butt), solid state welding (friction, friction stir, ultrasonic, explosive, diffusion), and beam welding (laser and electron beam), plus specialised processes such as thermit welding.
2. Which type of welding is strongest?
Strength depends on the joint rather than the process, but for a given material, solid state processes such as friction stir welding often retain the most base metal strength because nothing melts. Among fusion processes, TIG and electron beam welding produce the highest quality welds.
3. What is the difference between MIG, TIG and arc welding?
Arc welding is the broad family. MIG uses a continuously fed consumable wire with shielding gas and is fast and easy. TIG uses a non-consumable tungsten electrode with filler added by hand, giving the best quality at low speed. Stick welding, or SMAW, uses a flux-coated consumable electrode and is the most portable.
4. Which welding process should be used outdoors?
SMAW or self-shielded FCAW. Both generate their own shielding from flux, so wind cannot strip it away. MIG and TIG rely on shielding gas from a cylinder, which even a light breeze disperses.
5. What do the numbers on a welding electrode mean?
In E6013, the E means electrode, 60 means 60,000 psi tensile strength, 1 means it can be used in all positions, and 3 indicates the coating type and suitable current. E7018 is a 70,000 psi, all-position, low-hydrogen electrode used where cracking risk is high.
6. What is the heat affected zone?
The heat affected zone is the base metal beside the weld that did not melt but was changed by heat. Its hardness, microstructure and toughness differ from the surrounding metal, and failures frequently begin there rather than in the weld itself.
7. Why does welding cause distortion and how is it controlled?
Metal expands when heated and contracts when it cools, and surrounding cold metal restrains that movement, leaving stresses that pull the assembly out of shape. Balanced welding sequences, back-step technique, clamping fixtures, and keeping heat input low all reduce it.
8. What is the difference between welding, brazing and soldering?
Welding melts the base metal so the parts fuse into one. Brazing melts only a filler above 450 °C, which flows into the joint by capillary action. Soldering does the same below 450 °C. Welding gives the strongest joint; soldering the weakest.
Conclusion
There is no best welding process, only the right one for the job in front of you. Every process on the list exists because an earlier one could not handle some particular combination of material, thickness, location, quality requirement and production volume.
That is the framework worth carrying away. When you meet a welded joint, work backwards through five questions. What material is it? How thick? Was it welded indoors or on site? How good did the weld need to be? And how many were made? The answers narrow fifty processes down to two or three almost immediately.
Try it on the objects around you. A stainless steel sink has seam welds because it must not leak. A car door has spot welds because thousands of identical bodies had to be assembled quickly. A railway rail was thermit welded because there was no power supply in the middle of a field. Each choice makes complete sense once you know what the alternatives could not do.