How to TIG Weld Aluminum: Technique, Settings, and Troubleshooting

How to TIG Weld Aluminum: Technique, Settings, and Troubleshooting

Aluminum is the metal that convinces a lot of welders TIG is hard. It isn't harder than steel — it's different, and the difficulty comes from a few properties that behave nothing like the steel you learned on. Understand why the puddle acts the way it does, set the machine up for it, prepare the metal properly, and aluminum becomes predictable. This guide covers the whole process: why aluminum is different, AC machine setup, tungsten and gas, cleaning, filler choice, heat control, torch technique, pulse, and how to read and fix the defects that show up in the bead.

Why Aluminum Is Different to TIG Weld

The oxide layer. Aluminum grows a tough, invisible oxide skin the instant a bare surface meets air, and that oxide melts far hotter than the metal under it — roughly 3,600°F (about 1,980°C) versus about 1,200°F (660°C) for the base metal. Leave it in place and the arc fights a refractory crust sitting on an already-molten pool, giving a grey, sluggish weld with poor fusion. You remove the bulk mechanically before welding; AC TIG handles the thin layer that reforms just before the arc arrives.

Heat conductivity. Aluminum pulls heat out of the weld zone three to four times faster than steel. At the start of a bead the surrounding metal acts as a heat sink, so you often need more current to get a puddle going than the same thickness of steel; as the part warms, that heat sink disappears and the same pedal position over-melts. Managing that changing balance is the core skill of aluminum TIG.

Puddle behavior. Steel glows before it melts; aluminum gives no colour warning. It looks solid right up until the surface goes shiny and wet — and then, if you aren't watching, it slumps. There is very little margin between "not molten," "molten," and "too molten," so arc length, travel speed, and pedal control all have to be steadier than on steel.

Why AC is used. Electrode-negative DC drives heat into the work but does nothing to the oxide; electrode-positive DC blasts the oxide off but overheats the tungsten almost instantly. Alternating current gives both — the electrode-negative part of each cycle penetrates, the electrode-positive part cleans — which is why the standard answer for aluminum is AC TIG.

Machine Setup for Aluminum

You need a machine that outputs AC — a DC-only welder cannot TIG aluminum in any practical way. The controls that matter are high-frequency start, AC balance, AC frequency, and amperage control. Treat every number below as a starting point: the right value shifts with the machine, alloy, thickness, joint, torch and cup, position, how clean the metal is, and your technique.

• High-frequency (HF) start. A non-contact start lights the arc with a spark across the gap so the tungsten never touches the work. Scratch and lift starts risk dragging tungsten into soft aluminum and contaminating the electrode on the first arc. Use HF start if the machine has it, and keep the work clamp on clean bare metal near the joint.
• AC balance sets how much of each cycle goes to cleaning (electrode-positive) versus penetration (electrode-negative). More cleaning widens the frosty "etched" band and tolerates dirtier metal; less cleaning tightens the arc, penetrates deeper, and keeps the tungsten cooler. A common working range is around 65–75% electrode-negative on machines that display it that way — but conventions differ (some show "% cleaning") and older transformer machines may not adjust at all. Practical rule: dirty bead or oxide winning, add cleaning; huge etched band and the tungsten balling badly, back it off (Miller).
• AC frequency is how many times per second the current reverses. Transformer machines are fixed near 60 Hz; inverters typically adjust, often in a 60–120 Hz range with some going higher. Raising it narrows and stiffens the arc for fine work and tight corners; lowering it spreads the arc for a wider bead. A single fixed frequency still welds aluminum fine — this is a refinement, not a requirement.
• Amperage control. Use a foot pedal or torch control whenever you can. Being able to taper current down in real time as the part heats, and feather it off to fill the crater, is the single most useful thing you can do for heat management on aluminum.

For how all the settings interact, see the ultimate guide to TIG welding settings.

Tungsten Selection and Preparation

• Type: lanthanated and ceriated tungstens are commonly used modern choices for AC aluminum and run well across a wide amperage range. Pure tungsten (green) on AC still works on transformer machines and forms a stable ball, but it is not the only option on modern inverters. Match the electrode to your machine and your own testing rather than treating one type as universally correct.
• Diameter: match it to your amperage. Run well below its range and a tungsten is hard to start and wanders; run over its range and it overheats, balls excessively, and spits. If the ball grows larger than the tungsten diameter, the electrode is too small for the current or cleaning is set too high.
• Tip: on a modern inverter, many welders grind a truncated point (a point with a small flat) for a focused arc and clean starts, and let a small ball form in use. On an older transformer machine, pure tungsten is usually balled first. Either works — grind along the length, not across it, on a dedicated wheel so you don't embed steel or aluminum particles.

More detail in how to choose the right tungsten for each metal.

Shielding Gas

Pure argon is the normal starting point for AC TIG on aluminum: it starts easily, cleans well, and covers the puddle at modest flow. Argon–helium mixes raise heat input for thick sections but cost more and start harder — not a beginner's starting point.

Flow is a range, not a number. Around 15–20 CFH with a standard cup is a common starting point; the right flow depends on cup size, the joint, drafts, stick-out, and whether you run a gas lens. Turning flow up without limit backfires — past a point the stream turns turbulent and pulls air in. Read the bead: bright and lightly etched means good coverage; grey, hazy, sooty, or finely porous means poor coverage, short post-flow, contamination, or a draft. Set post-flow long enough to protect the tungsten and cooling bead. See the science of shielding gas coverage.

Cleaning and Oxide Removal

Preparation is not optional on aluminum, and it is the step beginners skip. Any oxide, oil, or moisture left on the joint ends up in the weld as porosity, inclusions, or a grey puddle that won't wet. Every time:

• Degrease first — wipe the joint and both sides with acetone on a clean lint-free rag, before mechanical cleaning, so you don't grind oil into the surface.
• Remove the oxide mechanically with a stainless wire brush kept only for aluminum — never one that has touched carbon steel or a grinder, or embedded steel particles cause contamination and rust spotting. Brush right before you weld; oxide reforms within minutes.
• Keep it clean — don't touch the prepped joint with bare hands or a dirty glove; prep the filler the same way and keep its hot end in the gas.
• Dry it — cold aluminum from outside condenses moisture; a gentle pre-heat to drive it off (not to add weld heat) helps on damp days.

The AC cleaning action handles the thin reformed oxide; it cannot fix a joint that was never cleaned. Full routine in how to clean metal for TIG welding success.

Filler Metal Selection

For most work the choice is two rods, and neither is universally better — they trade off.

• ER4043 (aluminum–silicon) melts slightly lower, flows and wets more easily, is more forgiving in the puddle, and is less prone to cracking and distortion. Common for 6061 and castings. Downsides: lower strength and ductility than 5356, and it turns dark grey to black if the part is anodised.
• ER5356 (aluminum–magnesium) is stronger and more ductile, is the correct choice for 5xxx-series base metals (and required where base-metal magnesium is high), and keeps a far better colour match under anodising. Downsides: stiffer and harder to feed smoothly, a little less forgiving, and not suited to sustained service above roughly 150°F (65°C), where 4043 is preferred (Hobart).

Match rod diameter roughly to thickness and amperage. On a general repair, 4043 is the more forgiving start; when strength or anodising colour matters, use 5356. More in the role of filler rods in TIG welding.

Amperage and Heat Control

A common starting heuristic on steel is about 1 amp per 0.001 in of thickness up to around 1/8 in. Aluminum usually needs somewhat more to get started because it wicks heat away so fast — roughly 1.5× that steel figure is often quoted — and then less as the weld progresses. So 1/8 in (about 3 mm) aluminum might start somewhere around 125–175 A available at the pedal, knowing you will rarely be flat on the pedal for the whole weld. Dial these against the puddle; they are not targets to hold.

What trips people up is heat saturation: the first inch sits on cold metal that steals heat; by the sixth inch the surrounding aluminum is hot and the same current over-melts. Responses:

• Taper the pedal down as you travel and the part heats — the main reason a pedal beats fixed amperage here.
• Speed up slightly once the puddle establishes easily.
• Segment long welds, or skip around a part, and let it cool between passes.
• Use a heat sink — a copper or aluminum backing bar or chill block near the joint pulls heat out and protects thin edges.
• Fill every crater — feather the pedal down while adding a last dab or two, or use downslope, so the pool freezes full instead of leaving a shrinkage pit that cracks.

Torch Angle, Arc Length, and Puddle Technique

Because aluminum gives so little warning before it slumps, the fundamentals have to be tight and repeatable.

• Arc length: short and consistent. A common rule of thumb is an arc roughly as long as the tungsten diameter, as a starting point that varies with joint and position. A long AC arc wanders, spreads heat, widens the etched band, and loses cleaning efficiency — one of the most common reasons an aluminum bead looks dirty.
• Torch angle: slight push — on the order of 10–20° from vertical with the tungsten leading travel, to keep gas over the puddle and see the leading edge. Too much angle drags shielding off the pool.
• Filler timing: steady cadence at the leading edge. Dip into the front of the puddle, not the arc column, on a regular beat; pull the rod back only a little between dips so its hot tip stays in the gas. Even dabs give even ripples.
• Travel speed: match the puddle. Too slow, heat piles up and the bead sags; too fast, the puddle can't wet the toes and you get a narrow ropey bead.
• Watch the puddle, not the arc — the shiny wet pool tells you when it is ready for filler and when it is getting too hot. Chasing the bright arc is why beads wander.
• Protect the tungsten. Touch the rod or work with the electrode and it is contaminated — stop, re-grind, start clean.

Aim for consistency and sound fusion first. A uniform, well-tied-in bead that is slightly less photogenic beats a "stack of dimes" that hides a cold start. The technique for repeatable beads on any metal is in the science of perfect TIG weld beads.

Pulse TIG on Aluminum

Pulsed TIG alternates between a higher peak current and a lower background current at a rate you set. On aluminum it earns its place mainly for heat control on thin material and out-of-position work — the background portion lets the puddle firm up between peaks, reducing burn-through, shrinking the heat-affected zone, and limiting distortion, and at low rates it acts as a metronome for filler timing.

Pulse is a tool, not a requirement. On moderate-thickness general work, a well-controlled steady bead with good pedal technique is just as clean and simpler to set up. There is no single "correct" pulse frequency — useful settings run from slow visible pulses to very rapid cycling depending on the machine, thickness, and job. Start from a reference recipe and adjust to the result: how to use pulse settings for better TIG beads.

Common Aluminum TIG Problems

Most aluminum defects trace back to heat, cleanliness, shielding, or arc length.

• Soot / black contamination: arc too long, too much torch angle, dirty filler or base metal. Shorten and straighten the arc, re-clean joint and rod, raise cleaning.
• Porosity: hydrogen from moisture, oil, hydrated oxide, a gas leak, or thin coverage. Degrease and brush just before welding, dry the metal, check hoses, confirm flow and post-flow.
• Tungsten contamination (spitting arc, grey bead, specks): the electrode dipped, or is undersized. Re-grind on a dedicated wheel, lengthen the arc slightly, step up a size if the tip keeps melting.
• Lack of fusion (bead on the surface, toes not tied in): not enough heat, travel too fast, arc aimed at the filler, or oxide not removed. Raise current, slow down, aim at the leading edge, re-clean.
• Burn-through / excessive penetration: too much heat for the thickness, travel too slow, heat saturation, or no backing. Taper the pedal, speed up, add a chill block, consider pulse on thin gauge.
• Crater cracking (star crack in the stop): current cut abruptly and the crater shrank as it froze. Feather the pedal or use downslope, add filler while backing off, pause before breaking the arc.
• Wandering / unstable arc: contaminated tungsten, poor work-lead connection, arc too long, or frequency too low for fine work. Re-grind, clamp the work lead to clean bare metal near the joint, shorten the arc, raise frequency.
• Dirty-looking bead overall: usually a combination of the above — re-check arc length, AC balance, gas flow and post-flow, and cleanliness in that order.

Deeper diagnosis of each defect in how to fix common TIG weld defects.

Job-Specific Aluminum Welding

This article is the fundamentals. Once you have a puddle under control, these go deep on specific jobs:

• TIG welding thin aluminum sheet — light-gauge heat control and avoiding blow-through.
• Repairing aluminum cracks with TIG — stop holes, crack prep, cast and wrought repair.
• TIG welding aluminum intercooler pipes — thin-wall charge piping and cast flanges.
• TIG welding aluminum boat repairs — marine alloys and hull distortion control.
• TIG welding aluminum fuel tanks without leaks — joint design and a leak-tight sequence.
• TIG welding aluminum lap joints — fit-up and heat balance for lap configurations.

Practical Aluminum TIG Checklist

Before the arc: machine on AC with HF start and a pedal; AC balance set for how clean the metal is; tungsten sized for the amperage and freshly ground on a dedicated wheel; pure argon with flow and post-flow set; joint and both sides degreased then brushed with an aluminum-only stainless brush right before welding; filler chosen (4043 for forgiveness, 5356 for strength/anodising); fit-up tight, tacks in, heat sink or backing on thin sections.

During the weld: establish the shiny puddle before moving; short consistent arc, slight push angle, watch the puddle not the arc; add filler to the leading edge on a steady beat; taper the pedal down as the part heats and segment long welds; fill the crater and pause under gas at every stop; if the tungsten touches down, stop and re-grind; change one variable at a time when something looks wrong.

TIG welding aluminum rewards preparation and steady technique more than any trick setting. Get the metal clean, set the machine as a sensible starting point, control the heat as the part saturates, and read the puddle honestly — clean, strong beads follow. If you want the whole aluminum process laid out in depth, with the metallurgy and shop-tested procedures behind every step, that is what Fundamentals of TIG Welding Aluminum is built for.