Welding: MIG vs TIG vs Stick, Materials & Joint Design

Welding joins metals by melting the workpiece edges and usually a filler into a shared weld pool that solidifies into a continuous joint. The three common manual processes are MIG (GMAW), TIG (GTAW), and Stick (SMAW), each with different speed, control, and material fit. As the hub for joining, this page covers how welding works, the three processes in depth, the materials that weld well, joint types and positions, distortion and the heat-affected zone, and the codes that govern structural welding, so a reader can choose a process and prepare a part to be welded.

Welding is one step within metal fabrication, alongside cutting and forming, and a fabricated assembly usually combines all three. A structural weld is usually governed by a code such as AWS D1.1 for steel, which sets joint design, qualification, and inspection requirements. Typical parameters for mild steel run MIG short-circuit at about 17 to 21 volts and 90 to 150 amps, MIG spray transfer at 24 to 28 volts and 180 to 250 amps, TIG at about 80 to 180 amps (roughly one amp per thousandth of an inch of thickness), and Stick with a 7018 rod at about 70 to 140 amps set by rod diameter. AWS D1.1 sets minimum fillet weld leg size by material thickness, such as a 3/16 inch design minimum for common structural work.

How welding works

An arc welder creates an electric arc between an electrode and the workpiece, and the heat of that arc, thousands of degrees, melts the base metal and the filler into a weld pool. The pool is the heart of the weld: as the arc moves along the joint, the pool follows, melting base metal ahead and solidifying behind into the finished weld bead. The welder controls the arc length, the travel speed, and the filler feed to manage the pool’s size and shape, which sets the weld’s penetration, build-up, and quality.

Shielding the pool from the air is essential, because oxygen and nitrogen in the air react with molten metal to cause porosity and brittleness. Each process shields the pool differently. MIG and TIG use an inert gas, usually argon or an argon blend, flowed over the pool from a nozzle. Stick uses a flux coating on the electrode that burns to create both a shielding gas and a slag layer over the solidifying weld, which is chipped off after. The shielding method is a large part of what makes each process suit different conditions, indoor versus outdoor, clean versus dirty material.

The three processes

The three manual arc processes cover the great majority of fabrication welding, and each has a niche where it leads. MIG (GMAW) feeds a continuous wire that serves as both electrode and filler, so it deposits metal fast and is easy to learn, which suits production steel and aluminum welding. TIG (GTAW) uses a non-consumable tungsten electrode with a separate filler rod added by hand, giving precise control of the pool and a clean, narrow weld, which suits stainless, thin material, and visible or critical joints. Stick (SMAW) uses a flux-coated electrode that carries its own shielding, so it works outdoors and on dirty or rusty material, which suits field work and repair. The welding types comparison works through the tradeoffs process by process.

MIG (GMAW)

MIG, or gas metal arc welding, feeds a continuous wire from a spool through a gun, where it becomes the electrode and melts into the pool. Because the wire feeds continuously, MIG deposits metal fast, which makes it the most productive process for longer welds on steel and aluminum. It is also the easiest of the three to learn, since the welder mainly controls travel speed and gun angle while the machine feeds the wire. Its shielding gas, usually an argon-carbon dioxide blend for steel or argon for aluminum, must be protected from wind, so MIG is an indoor process unless sheltered. MIG’s main limits are burn-through on very thin material (the high deposition can blow a hole) and the need for clean steel, since contamination causes porosity.

TIG (GTAW)

TIG, or gas tungsten arc welding, uses a non-consumable tungsten electrode to create the arc, with filler rod added to the pool by hand. TIG gives the welder fine control of heat and filler, which produces a clean, precise weld with a narrow heat-affected zone and little or no spatter. That control makes TIG the choice for thin material, for stainless and aluminum where appearance and corrosion matter, and for root passes on pipe and critical joints. Aluminum is welded with alternating-current TIG, which cleans the oxide layer during the weld. TIG’s trade-offs are speed and skill: it is slower than MIG for the same length of weld, and it demands more from the welder, who must coordinate the torch, the filler, and the foot-pedal heat control simultaneously.

Stick (SMAW)

Stick, or shielded metal arc welding, uses a consumable electrode coated in flux. The flux burns during welding to create a shielding gas and a protective slag over the weld, which is chipped off after. Because it carries its own shielding, Stick works outdoors and in wind, where MIG and TIG shielding gas would be blown away, and it tolerates dirty, rusty, or painted material better than the other processes, since the flux can handle some surface contamination. These traits make Stick the standard for field construction, repair, and outdoor work. Its trade-offs are a rougher weld that needs slag cleanup, lower productivity than MIG on long indoor welds, and difficulty on thin material, where its heat input is hard to control.

Materials and weldability

Weldability, how easily a metal welds to a sound joint, varies widely by material, and it drives process and filler choice.

Mild steel: the easy case

Mild steel is the easiest common metal to weld, accepting MIG, TIG, or Stick with general-purpose filler and argon-carbon dioxide shielding. Its low carbon content keeps it from hardening and cracking in the heat-affected zone, which is why it is the default structural material.

Stainless and aluminum

Stainless steel welds well but needs a matching filler (ER308L for 304, ER316L for 316) and argon shielding, with low-carbon L grades (304L, 316L) preferred for welded sections to avoid carbide precipitation that would hurt corrosion resistance. Aluminum needs AC TIG or a spool-gun MIG with aluminum wire (ER4043 or ER5356), and its oxide layer must be cleaned before welding, since the oxide melts at a much higher temperature than the base metal.

Titanium, cast iron, and high-carbon steels

Titanium welds cleanly but only under strict argon shielding, front and back, because it embrittles if it absorbs air while hot. Cast iron and high-carbon steels are weldable but need preheat and special procedures to avoid cracking, since their carbon content makes the heat-affected zone hard and brittle when it cools quickly.

Joint types and positions

Welded joints come in several geometries, and the joint type sets how the weld is run. Each joint can be run in different positions, flat, horizontal, vertical, overhead, and position affects difficulty and the parameters the welder can use, with flat the easiest and overhead the hardest. AWS D1.1 and other codes define acceptable joint designs, weld sizes, and inspection for structural work, and following them is how a structural weld is qualified and verified.

Butt, fillet, and corner joints

A butt joint joins two pieces edge to edge, often with a bevel to allow full penetration on thicker material. A fillet weld joins two pieces at an angle, as in a T or lap joint, and it is the most common structural weld, sized by its leg length. A corner or edge joint closes a box or frame, where appearance and a continuous seal matter more than a heavy load path.

Positions and qualification

Flat position allows the highest deposition and the easiest control, so welds are oriented flat where the assembly can be rotated. Vertical and overhead positions need lower current and careful technique to hold the pool against gravity. For coded work, AWS D1.1 qualifies welders by position, so a welder certified in flat may not be qualified for vertical or overhead, and that qualification is documented as part of the procedure.

Distortion and the heat-affected zone

Welding puts intense, localized heat into the part, and that heat has two effects a designer must manage.

Distortion: expansion and contraction

As the weld and its surrounding heat-affected zone heat and cool, they expand and contract, and the uneven expansion bends the part, sometimes enough to throw it out of tolerance. Clamping, sequencing the welds to balance the heat, back-stepping the travel direction, and minimizing heat input all control distortion, and on critical parts the design may include a stress-relief step after welding.

The heat-affected zone

The heat-affected zone (HAZ) is the band of base metal next to the weld whose properties the heat has changed. In some alloys the HAZ softens or hardens, which can affect strength, corrosion resistance, or fatigue performance, and the design must account for it, particularly in structural and pressure-part work where the changed properties sit in the load path.

Structural welding and codes

Structural welding is governed by codes that define how a weld must be designed, made, and inspected, and AWS D1.1 for structural steel is the most common in the United States. Understanding that welding can be a coded, inspected, documented operation is part of specifying it for anything that carries load or must be reliable.

What AWS D1.1 sets

The code sets joint designs, minimum fillet weld sizes by material thickness (such as the values in Table 7.7), welder qualification, inspection methods, and acceptance criteria. A structural weld is not just a sound-looking bead; it is a qualified joint, made by a qualified welder, inspected to the code, and documented, which is why a coded weld costs more than an uncritical shop weld of the same size.

Other codes: ASME and API

For pressure vessels, piping, and other critical applications, similar codes apply. ASME governs boilers and pressure vessels, and API governs piping, each with its own qualification and inspection requirements. The pattern is the same as AWS D1.1: the weld, the welder, and the inspection are all qualified and recorded against the code, so the joint can be trusted and audited.

Choosing the process

The right process follows from the material, the thickness, the position, and the working conditions. Matching the process to the job is the surest way to get a sound weld at a reasonable cost, and the comparison page lays out the tradeoffs where the choice is not obvious.

By material and thickness

For long production welds on steel indoors, MIG is the most productive. For thin material, stainless, aluminum, and visible or critical joints, TIG gives the control and cleanliness needed, at the cost of speed. Material thickness drives the decision as much as material type, since thin sheet pushes toward TIG and thicker plate pushes toward MIG or Stick.

By working conditions

For outdoor, field, or dirty-material work, Stick is the reliable choice, because its flux makes its own shielding and tolerates surface contamination. For high-production, repeatable welds, robotic MIG or automated processes take over, which lowers per-weld cost once the volume justifies the setup.

Safety

Welding produces arc light intense enough to burn skin and eyes, fumes from the electrode and any coatings on the metal, heat and sparks that can start fires, and high electrical current. These hazards are routine in a welding shop, and they are part of why welding is done by trained people in a controlled setting rather than treated casually.

Personal protection

Welders protect themselves with a helmet fitted with the correct shade of filter lens, flame-resistant clothing, gloves, and respiratory protection or ventilation for fume control. The lens shade is matched to the process and current, since too light a shade lets damaging UV through even when the arc looks dim.

Fire and electrical hazards

The work area is kept clear of flammables, and a fire watch is posted where sparks could reach hidden fuel, such as floor gaps or cable trays that smolder after the work ends. The equipment is grounded and kept dry, since welding current is high enough to cause a shock or arc from a damaged lead, and the work lead is clamped close to the joint to keep the current path short and controlled.

Filler metals and matching

The filler metal is the material added to the weld pool to fill the joint and build the bead, and choosing it correctly is essential to a sound weld. The filler must be chemically compatible with the base metal, so the resulting weld metal has the strength, ductility, and corrosion resistance the joint needs. For mild steel, general-purpose fillers like ER70S-6 MIG wire and 7018 Stick rods cover most structural work, with tensile strength around 70,000 psi that matches common structural steel. For stainless, the filler matches the grade, ER308L for 304 and ER316L for 316, with low-carbon L variants preferred for welded sections to avoid carbide precipitation that would reduce corrosion resistance along the weld. For aluminum, ER4043 and ER5356 are the common MIG and TIG fillers, chosen for the alloy and the service, with ER5356 higher in strength and ER4043 more crack-resistant on some alloys. A mismatched filler, such as a mild-steel filler on stainless, produces a weld that corrodes or cracks, so matching the filler to the base metal and the service is a basic rule of sound welding, and it is specified on the welding procedure.

The filler also comes in different forms for the process. MIG uses a continuous wire, fed from a spool. TIG uses straight filler rods, added by hand. Stick uses a flux-coated electrode, where the filler metal is the core of the rod and the flux provides shielding and slag. The form follows the process, but the metallurgy, the alloy and composition of the filler, must still match the base metal. Filler selection also considers the joint’s service, with some joints calling for a filler that matches the base metal’s strength and others for a more ductile filler that tolerates stress or vibration, and the welding procedure specifies the filler for a coded or critical weld.

Welding positions and technique

Welding can be performed in several positions, and each affects the difficulty and the parameters the welder can use. Flat position, with the weld pool laid horizontally from above, is the easiest and allows the highest deposition, so welds are oriented flat where the assembly can be rotated or positioned. Horizontal position welds a vertical joint from the side, managing a pool that tends to sag, requiring a technique that holds the pool against gravity. Vertical position welds up or down a vertical surface, requiring lower current and often a weave or stringer technique to control the pool, with uphill generally stronger and downhill faster but thinner. Overhead position, welding above the head, is the hardest, with the pool fighting gravity, and it demands care to avoid drips and to achieve full penetration.

The position is often set by the assembly, which cannot always be rotated to flat, and field and structural welding routinely works in all positions. A pipe welder, for example, welds around a fixed pipe, passing through flat, vertical, and overhead positions in a single circumferential weld, which is why pipe welding demands high skill. For coded structural work, AWS D1.1 qualifies welders by position and process, so a welder certified in flat may not be qualified for vertical or overhead, and that qualification is documented as part of the welding procedure. Understanding that position affects both difficulty and qualification is part of specifying a weld for anything beyond simple shop work, and it is why some welds cost more than others of the same length.

Weld quality, defects, and inspection

A sound weld must fuse the base metal fully, be free of cracks and porosity, and meet the size and profile the design calls for, and these are verified by inspection that ranges from visual to advanced testing. Visual inspection is the first line, checking the weld’s appearance, size, profile, and length against the drawing and the code, and catching obvious defects like undercut, overlap, and insufficient fill. Dye-penetrant and magnetic-particle inspection reveal surface cracks and defects that visual inspection misses, by wicking a dye into surface-breaking flaws or magnetizing the part to reveal cracks at the surface. These surface methods are quick and widely used.

For internal soundness, radiographic and ultrasonic testing look inside the weld for porosity, lack of fusion, slag inclusions, and cracks. Radiography uses X-rays to image the weld’s interior, showing internal defects against the surrounding sound metal, and ultrasonic testing uses high-frequency sound to find and locate internal flaws. These methods are slower and more costly than surface inspection, and they are required for critical and coded welds, where an internal defect could cause failure. For structural work under AWS D1.1 and similar codes, the inspection method, the acceptance criteria, and the welder’s qualification are documented, and the weld is accepted or rejected against the code. Understanding that welding can be a measured, inspected, documented operation is part of specifying it for anything that carries load or must be reliable, and it is why structural and pressure welds cost more than uncritical shop welds of the same size.

When welding is not the right choice

Welding is wrong when another joining method suits the assembly better. Bolted or riveted joints suit assemblies that must be disassembled, that join dissimilar materials, or that cannot tolerate heat. Adhesive bonding suits lightweight assemblies, composites, or joints where a continuous bond distributes stress. And some materials are not practically weldable, or welding would damage heat-sensitive components nearby. For those cases, mechanical fastening or bonding is the better choice. Welding earns its place where a permanent, strong, continuous joint is needed in weldable metal, and choosing it there, while using other methods where they fit, is the way to join parts well.