TIG Welding Aluminum: Settings, Technique, and Common Mistakes

Why Aluminum Is Different

Aluminum humbles welders. It conducts heat roughly five times faster than steel, melts at 1,220°F (compared to steel's 2,500°F), and gives no visual warning before it melts — there's no color change. One second the base metal looks solid, the next second you have a puddle, and a half-second after that you have a hole.

On top of that, aluminum forms an oxide layer (aluminum oxide) that melts at 3,700°F — nearly three times the melting point of the base metal underneath. If you don't break through that oxide, you're welding on top of a ceramic shell. The weld looks fine and breaks with a tap. TIG on AC solves this, but the process requires discipline.

AC Polarity and Balance Control

Steel TIG uses DC electrode negative (DCEN) exclusively. Aluminum TIG uses alternating current (AC), and here's why:

• Electrode negative (EN) half-cycle: Electrons flow from the tungsten into the work. This provides the majority of the heat and penetration. It's the “welding” part of the cycle.
• Electrode positive (EP) half-cycle: Electrons flow from the work to the tungsten. This blasts ions against the work surface, breaking up the oxide layer. It's the “cleaning” part of the cycle — but it also dumps heat into the tungsten, which is why pure EP would melt your electrode instantly.

AC alternates between these two modes 60 times per second (at 60 Hz) or faster on inverter machines. AC balance controls the ratio of EN to EP. More EN means more penetration and less cleaning. More EP means more oxide removal but more heat in the tungsten.

Start with 65–75% EN (sometimes displayed as 25–35% cleaning). If you see a dark, sooty line at the edges of the weld puddle, increase cleaning. If the tungsten is balling excessively or eroding fast, decrease cleaning. For most work, 70% EN is a reliable starting point.

Tungsten Selection

The traditional recommendation for aluminum is pure tungsten (green band) because it forms a nice ball on the tip during AC welding. That ball is actually desirable on AC — it helps distribute the arc evenly.

Modern inverter machines have changed the game. 2% ceriated (gray band) and 2% lanthanated (blue band)tungstens work well on AC with inverter power sources and offer easier arc starts, better arc stability, and longer electrode life. They don't ball as readily, but with a properly set AC balance on an inverter, that's less of an issue.

Tungsten diameter follows the same general rule as steel: match it roughly to the material thickness and amperage range.

• 3/32″: 60–120A — sheet metal and thin plate
• 1/8″: 100–200A — the most versatile size
• 5/32″: 150–250A — medium to heavy plate
• 3/16″: 200–300A+ — heavy fabrication

Cleaning: Non-Negotiable

Aluminum must be clean. Not “looks clean” — actually clean. The oxide layer, oils from handling, and any surface contamination will cause porosity, lack of fusion, or cracking.

1. Degrease first. Wipe the joint area with acetone or a dedicated aluminum cleaner. Do not use brake cleaner — some formulations contain chlorinated solvents that produce toxic phosgene gas when heated.
2. Brush with a stainless steel wire brush. Use a brush dedicated to aluminum. A brush that has been used on steel will embed iron particles into the aluminum surface, contaminating the weld. Brush in one direction, not back and forth.
3. Clean the filler rod too. Wipe it down with acetone and store it in a sealed tube. Filler rods left sitting on a shop bench pick up oil, dust, and moisture.
4. Weld promptly. The oxide layer starts reforming immediately after you brush it off. Ideally, weld within 30 minutes of cleaning. If the part sits overnight, clean it again.

Filler Rod Selection: 4043 vs. 5356

These two filler alloys cover the vast majority of aluminum TIG work:

• 4043 (AlSi5): Silicon-based. Flows easily, wets out well, produces a smooth and shiny bead. Lower cracking tendency on 6000-series aluminum. Good for sheet metal, castings, and general fabrication. Slightly lower strength than 5356.
• 5356 (AlMg5): Magnesium-based. Higher strength, better color match after anodizing, better fatigue resistance. The standard for structural work and 5000-series base metals. Stiffer to feed and produces a slightly duller bead.

Never use 5356 on 6061 aluminum that will be exposed to sustained temperatures above 150°F (post-weld heat treatment or high-temperature service) — the magnesium in the filler creates a sensitized zone susceptible to stress corrosion cracking. Use 4043 for those applications.

Cup Size and Gas Coverage

Aluminum needs more gas coverage than steel. The large, fluid puddle is extremely reactive at welding temperature, and any exposure to atmospheric oxygen or moisture causes immediate porosity.

• Cup sizes: #6 (3/8″) minimum for thin material, #8 (1/2″) for most work, #10 or #12 for heavy plate or wide welds
• Gas flow: 15–25 CFH for standard cups, higher end for larger cups and outdoor work
• Gas type: 100% argon. Always. Never use helium mixes for TIG aluminum unless specifically required by a WPS for thick plate deep penetration.

Consider a gas lens instead of a standard collet body. Gas lenses produce a smoother, more laminar gas flow that extends shielding coverage and allows you to extend the tungsten farther from the cup for better visibility in tight joints.

Preheat for Thick Sections

Aluminum's high thermal conductivity means thick sections pull heat away from the weld zone faster than you can put it in. For material 1/4″ and thicker, preheating to 200–300°F helps establish the puddle without needing excessive amperage.

Use a temperature-indicating crayon or an infrared thermometer to verify preheat. Do not overheat — above 400°F, the material's mechanical properties start degrading, and you risk distortion. Preheat is a help, not a crutch.

Starting and Stopping Technique

Arc starts on aluminum are where most problems begin. The oxide layer makes cold starts difficult — the arc wanders and sputters until the cleaning action breaks through. Two techniques help:

• High-frequency start: Use HF start if your machine has it. It ionizes the gap and initiates the arc without touching the tungsten to the work. This prevents tungsten contamination in the puddle.
• Foot pedal ramp-up: Start at moderate amperage and ramp up as the puddle forms. Don't go full power immediately — on thin aluminum, the puddle forms faster than you expect, and you'll burn through.

At the end of the weld, crater fill is critical. Aluminum shrinks significantly as it cools, and an unfilled crater will crack every time. Taper the amperage down gradually (most machines have a downslope setting) while adding filler to keep the crater full. A proper crater fill looks like a slight mound, not a concavity.

Common Mistakes

• Contaminated tungsten. Dipping the tungsten into the puddle — even briefly — contaminates both the tungsten and the weld. Break off the contaminated end, re-grind or re-ball, and start fresh. Don't try to “burn it off.”
• Wrong polarity. Running DC instead of AC on aluminum is immediately obvious: no cleaning action, the oxide layer doesn't break, and you get a weld bead sitting on top of unmelted oxide. If the arc looks wrong and the puddle won't wet out, check your polarity setting.
• Too much heat on thin material. Aluminum's low melting point and high conductivity create a narrow window. Use a foot pedal for amperage control, weld briskly, and use a copper backing bar as a heat sink when welding thin sheet.
• Insufficient gas coverage. Post-flow should be at least 8–12 seconds on aluminum to shield the puddle and tungsten while they cool. Cutting the gas too soon causes the last half inch of your weld and the tungsten tip to oxidize.
• Ignoring the filler rod. Cold filler rod dipped into a hot puddle can introduce porosity from moisture condensation on the rod. Keep the rod tip inside the gas shielding envelope as much as possible, and store rods properly.

Use our TIG Amperage Calculator to find the right amperage range, tungsten size, and gas flow for your material type and thickness. Pair it with the MIG Welder Settings Calculatorif you're comparing processes for a specific job.