Welding processes join metal by melting the joint and, in most cases, adding filler. The four you meet most are MIG (GMAW), TIG (GTAW), stick (SMAW), and spot (resistance) welding. Process choice trades speed, control, and portability; parameters like current, voltage, travel speed, and shielding gas decide whether the weld is sound.
This is a plain comparison of the common arc and resistance processes, what each does well, how to choose, the parameters that control quality, the defects that fail a weld, and how codes and traceability keep welds accountable. It's the joining companion to the broader metal fabrication processes guide.
What Are the Main Welding Processes?
Three of the four are arc processes, they strike an electric arc to melt the metal, and one is resistance welding, which uses electrical resistance and force. What separates them is how the filler is delivered and how the weld pool is shielded from the air.
MIG (GMAW)
Gas Metal Arc Welding feeds a continuous wire electrode through a gun while a shielding gas (argon/CO2 blends) protects the pool. The continuous feed makes it fast and easy to learn, which is why it dominates production fabrication and auto work. It's less tolerant of wind and surface contamination than stick, so it's mostly an indoor, clean-metal process.
TIG (GTAW)
Gas Tungsten Arc Welding strikes an arc from a non-consumable tungsten electrode and feeds filler by hand (or runs autogenous, with no filler at all), shielded by inert gas. It's the slowest and most skill-intensive process and the most precise, the go-to for thin material, stainless, aluminum, and anything where the weld appearance and metallurgy must be right.
Stick (SMAW)
Shielded Metal Arc Welding uses a consumable electrode coated in flux that burns to create its own shielding gas and slag. It needs no external gas bottle, works outdoors and on rusty or painted steel, and travels anywhere with a power source, the workhorse of construction, pipelines, and field repair. The trade-off is slag to chip and a slower, more manual rhythm.
Spot (resistance) welding
Resistance Spot Welding clamps two sheets between copper electrodes and passes a high current through them; the resistance at the interface melts a nugget that fuses the sheets. No filler, no arc, and cycle times in fractions of a second, which is why it's everywhere in sheet-metal and automotive body assembly.
How Do You Choose Between MIG, TIG, and Stick?
Choice comes down to material, thickness, position, and what you're optimizing. MIG when you want speed and volume on clean steel; TIG when precision and appearance matter or you're on thin, stainless, or aluminum; stick when you're outdoors, on dirty metal, or need to weld anywhere without a gas supply. Thickness, joint access, and required weld quality usually make the call before preference does.
| Factor | MIG (GMAW) | TIG (GTAW) | Stick (SMAW) |
|---|---|---|---|
| Speed | Fast | Slow | Moderate |
| Skill to learn | Low | High | Moderate |
| Best material | Steel, thicker gauges | Thin, stainless, aluminum | Steel, structural |
| Environment | Indoor, clean | Indoor, clean | Outdoor, dirty OK |
| Shielding | External gas | External gas | Flux coating |
What Welding Parameters Control Weld Quality?
A weld is set by a handful of numbers, and getting them into the right window is the whole game. Current (amperage) drives penetration and heat. Voltage shapes the arc and bead profile. Travel speed sets how much heat goes in per inch, too fast and the weld is cold and narrow, too slow and it's hot and burns through. Wire feed speed (on MIG) sets deposition. Shielding gas type and flow protect the pool from the air. Together current, voltage, and travel speed set heat input the master variable behind penetration, distortion, and the metallurgy of the joint. A qualified Welding Procedure Specification (WPS) exists to lock those numbers to a proven range.
What Are the Most Common Weld Defects?
Most weld failures are one of four defects, and each points back at a parameter or prep problem. Porosity gas pockets trapped in the weld, comes from lost shielding, moisture, or dirty metal. Undercut a groove melted into the base metal at the weld edge, comes from too much current or too fast a travel. Lack of fusion weld metal that never bonded to the base, comes from too little heat or a bad angle. Cracks in the weld or the heat-affected zone, come from stress, rapid cooling, or hydrogen. Reading the defect tells you which knob to turn, which is why statistical process control on weld parameters catches drift before it becomes scrap.
How Do Welding Codes and Traceability Work?
Structural welds are governed by codes, most famously AWS D1.1, the Structural Welding Code for steel, which covers welding procedures, welder qualification, prequalified joints, inspection, and acceptance criteria for carbon and low-alloy steel structures. Under it, a Welding Procedure Specification (WPS) documents the approved parameters, backed by a Procedure Qualification Record (PQR) proving they make a sound weld, and welders are qualified to those procedures. Modern equipment can log actual current, voltage, and wire feed per weld, so weld-data monitoring ties each joint to its parameters and its welder, real traceability that turns a quality escape into a specific weld you can find instead of a lot you have to scrap.
The Standards and Data Behind Welding
The structural steel welding code is published by the American Welding Society as AWS D1.1, Structural Welding Code, Steel. Employment and wages for the roughly 420,000-plus U.S. welders, cutters, solderers, and brazers are tracked by the Bureau of Labor Statistics under occupation 51-4121 (OEWS). Welding fume is a regulated hazard: OSHA requires ventilation and controls under 29 CFR 1910.252 and hexavalent chromium from stainless welding is limited under 29 CFR 1910.1026 to a permissible exposure limit of 5 micrograms per cubic meter as an 8-hour average.
How Do You Run a Welding Operation Well?
Whatever the process, a weld shop rewards the same discipline:
- Weld to a qualified procedure. A WPS backed by a PQR locks current, voltage, travel, and gas to a proven range, the weld is only as good as the procedure it followed.
- Prep the joint. Clean, correctly fit, and correctly beveled metal prevents porosity and lack of fusion before the arc is even struck.
- Match the process to the job. MIG for speed on clean steel, TIG for precision and thin or exotic metal, stick for the field, spot for sheet, thickness and access usually decide.
- Read defects as signals. Porosity, undercut, lack of fusion, and cracks each point at a specific input; fix the input, not just the weld.
- Control the fume. Ventilation and monitoring keep welders under the OSHA limits, especially on stainless and coated metal.
- Log parameters per weld. Weld-data monitoring ties each joint to its settings and its welder, so a quality escape is traceable instead of a whole lot being suspect.
Each of those is a data problem before it's a craft problem. A fabrication shop that captures weld parameters, links them to the part and the welder, and digitizes inspection and rework records can see a drifting parameter or a rising reject rate the same shift instead of at a customer return. That is the visibility downtime tracking and lean problem-solving depend on, tied together by a manufacturing operating system the layer Harmony builds on top of the equipment a shop already runs (platform overview), no rip-and-replace.