TIG welding explained

TIG welding, formally gas tungsten arc welding or GTAW, strikes an arc from a non-consumable tungsten electrode under an argon shield and adds filler by hand, giving the cleanest and most precise weld of the common processes, on aluminium with AC and on steel and stainless with DC. Because the heat and the filler are controlled separately, TIG suits thin material, visible joints and metals that demand a clean weld. This guide explains how the process works, the current and filler for each metal, and how distortion is controlled. For the stainless case see our guide to welding stainless steel.

What TIG welding is

TIG is an arc process in which the arc jumps from a tungsten electrode to the work, melting the base metal, while an inert gas flowing from the torch shields the molten pool from the air. The tungsten does not melt into the weld, which is what sets TIG apart from MIG, so the welder controls the heat with one hand and dips a separate filler rod into the pool with the other. This separation of heat and filler is the source of both TIG’s quality and its slower speed. The result is a clean, precise, low-spatter weld that needs little dressing afterwards.

The tungsten electrode and the shield

The electrode is a tungsten rod, chosen because tungsten has the highest melting point of any metal and so carries the arc without being consumed. Different tungsten types suit different work: lanthanated and thoriated tungsten are common on steel and stainless, while pure, zirconiated or lanthanated tungsten suit aluminium, and a rare-earth grade covers both. The shielding gas is almost always argon, which gives a stable arc and protects the pool, with helium added for more heat on thick sections. A steady gas shield is what keeps the weld clean, so the flow and the gas coverage matter as much as the arc.

AC for aluminium, DC for steel and stainless

The current type is chosen to the metal. Steel and stainless are welded on direct current, electrode negative, which concentrates the heat in the work and gives deep, clean penetration. Aluminium is welded on alternating current, because aluminium carries a tough oxide skin that melts far higher than the metal beneath it, and the electrode-positive half of the AC wave blasts that oxide off the surface while the electrode-negative half puts heat into the joint. Getting the current right is fundamental: DC on aluminium leaves the oxide in the way, and AC on steel wastes heat. This is why an aluminium job needs an AC-capable machine.

Filler rods for each metal

The filler is matched to the base metal so the joint keeps its strength and corrosion resistance. Aluminium is filled with 4043, a silicon alloy that flows well and suits the 6000-series architectural alloys, or 5356, a magnesium alloy that is stronger and better for marine 5000-series work. Stainless is filled with a matching low-carbon grade such as 308L for 304, 316L for 316, and 309L where stainless is joined to carbon steel. Using the correct filler is not optional, because a mismatched rod can weaken the joint or leave it prone to corrosion. For the aluminium joint in detail see our guide to joining aluminium profiles.

Why choose TIG

TIG earns its place where the weld has to be right rather than fast. It welds thin material that would be blown through by a heavier process, it lays a clean, precise bead on a visible joint, and it handles metals such as aluminium and stainless that reward careful control. The trade-off is speed: TIG is slower and needs more skill than MIG, so it is used where quality justifies the time, on thin gauges, architectural work, food and pharmaceutical stainless, and anywhere the joint will be seen. Where long runs of thicker steel need filling quickly, MIG usually takes over.

Back-purging stainless

Stainless brings one extra step. When stainless is welded, the back of the joint will oxidise and discolour if it is exposed to air while hot, and that heat-tinted zone loses some of the corrosion resistance that stainless is chosen for. The answer is back-purging: argon is fed to the underside of the joint to shield it until it cools, so the root stays bright and sound. On a closed section such as a tube, the whole inside is purged. This is why welding stainless well takes more than a good top bead, and why the back of the weld matters as much as the face.

Distortion and straightening after welding

Welding puts heat into one part of a piece, and as that heat spreads and cools the metal pulls, so a welded profile or frame can bow or twist. This distortion is controlled first by good practice: tacking the parts, welding in a balanced sequence, and clamping the work in a fixture so it cannot move. Where a member still comes out of true, it is straightened afterwards, mechanically or with controlled heat, to bring it back to line. Aluminium and thin stainless move more readily than heavy steel, so the sequence and the fixturing matter most on exactly the metals TIG is used for.

TIG compared with MIG

The two processes divide the work between them. MIG feeds a continuous wire that is both the electrode and the filler, so it is fast, easy to learn and suited to long runs and thicker steel. TIG keeps the electrode and filler separate for control, so it is slower but cleaner and more precise, and it handles thin and demanding metals better. Neither is simply better: a fabricator picks TIG for a visible aluminium or stainless joint and MIG for a long structural steel run. Knowing which process a job wants is part of specifying the fabrication, and both draw on the same base of sound, weldable material.

Cleaning and preparation before welding

TIG is unforgiving of a dirty surface, so preparation is part of the process. Oil, grease, paint and dirt all contaminate the weld and cause porosity, so the joint is degreased before the arc is struck. Aluminium needs particular care because of its oxide skin: it is cleaned with a dedicated stainless brush kept only for aluminium, so no iron is carried onto the surface, and it is welded soon after cleaning before a fresh oxide forms. Stainless is kept away from carbon-steel contamination for the same reason, since a trace of ordinary steel will rust and stain a stainless weld. A clean joint, a clean filler and clean gloves are what let TIG produce the bright, sound weld it is known for.

Pulsed TIG for thin material

Pulsed TIG helps most on the thin, heat-sensitive work the process is chosen for. The current pulses between a high peak that melts the pool and a low background that lets it cool a little, so the average heat into the part is lower and the risk of burning through thin sheet or warping a light frame is reduced. The pulsing also helps control the pool out of position and gives the even, stacked bead seen on quality stainless and aluminium work. It is one reason a modern TIG set handles thin gauges and delicate joints that would be difficult with a steady arc.

Common TIG welding faults

A handful of faults account for most poor TIG welds, and each points to a cause. A grey, dirty or porous weld usually means poor gas coverage or a contaminated surface, so the gas flow, the nozzle and the cleanliness are checked first. A weld that will not start cleanly, or that spits tungsten into the pool, means the electrode has touched the work and become contaminated, and it is reground before welding on. Burn-through and a sunken pool mean too much heat for the thickness, cured by less current or faster travel, while a cold, ropey bead means too little. On stainless, a straw or blue tint on the back of the weld shows the back-purge was too weak. Reading these signs is how a welder tunes the current, the gas and the travel to the metal before the real joint is welded.

Weldable material from ULAMEX

Every good weld starts with sound, known material, and this is where ULAMEX comes in. ULAMEX supplies the weldable metals the trade TIG-welds, aluminium in the 6000 and 5000 series, stainless in 304, 316 and duplex, and structural steel, all to European standards with documentation on request. ULAMEX also manufactures its own carport, PV and fencing structures, welding them under ISO 3834-2 and EN 1090-1 EXC2, so the welding quality behind its products is controlled and certified. The company supplies the material and stands behind its own fabrication; the welding of a customer’s own project is done by that customer or their fabricator.

Need weldable material for a project? Send the grade and quantities, and ULAMEX will return a non-binding quotation with lead time and availability. Read our guides to welding stainless steel, welding galvanised steel and joining aluminium profiles, browse the stainless and aluminium ranges, or view our supply capabilities. Contact the export desk at [email protected] or +48 504 424 761.

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