TIG Welding Aluminum for Makers: AC Balance, Pre-Cleaning, and Filler Selection

Our welding basics guide covers MIG, Stick, and TIG at a comparison level and correctly points out that TIG is the process of choice for aluminum. What it doesn't cover is just how different aluminum TIG welding is from steel TIG welding in practice — different polarity, a wholly different cleaning requirement, filler alloys that matter far more than they do on mild steel, and a learning curve that trips up a lot of makers who assumed "I can TIG weld steel, aluminum can't be that different." It is that different. This guide is specifically about what changes when the material is aluminum.

AC, Not DC: Why Polarity Matters

Steel TIG runs on DCEN (DC Electrode Negative) almost exclusively — it concentrates heat in the workpiece for good penetration and keeps the tungsten cool enough to use a sharp point. Aluminum needs AC (alternating current), and the reason is specific: aluminum forms a hard, high-melting-point aluminum oxide layer on its surface almost instantly on contact with air, and that oxide layer (melting around 3,700°F, versus roughly 1,200°F for the aluminum underneath it) has to be broken up before the weld pool can actually fuse. AC does this by alternating between the two half-cycles: the DCEN half of the cycle, same as steel, drives heat into the workpiece and penetration; the DCEP (electrode positive) half reverses the electron flow in a way that physically blasts the oxide layer off the surface, visible on a clean weld as a bright, frosty-looking etched zone around the weld bead. You need both halves — DCEN alone can't get through the oxide, and DCEP alone overheats the tungsten and gives poor penetration.

Modern inverter TIG machines let you adjust the AC balance (the ratio of time spent in each half-cycle, often expressed as a percentage of EN vs. EP) and the AC frequency:

• More EN balance (further toward DCEN) — deeper penetration, narrower cleaning zone, less oxide removal. Use for thicker material or when the surface is already very clean.
• More EP balance (further toward DCEP) — wider, more aggressive oxide cleaning, but hotter on the tungsten (you'll need a larger diameter or will ball/erode a thin electrode faster) and less penetration.
• Higher AC frequency (100–250Hz on most inverters, vs. the 60Hz line frequency older transformer machines were stuck with) — a tighter, more focused arc that's easier to control on thin material and gives a narrower heat-affected zone. Most aluminum work benefits from running toward the higher end of what your machine offers.

A reasonable starting point on clean, properly prepped aluminum: 65–70% EN balance, 120–150Hz frequency, adjusting from there based on how much cleaning action you're actually seeing at the weld edges.

Tungsten Selection and Prep

Pure tungsten (green-banded) or zirconiated tungsten (brown/white-banded) are the traditional choices for AC work — they ball up into a smooth, rounded tip under AC current, which is what you want for aluminum rather than the sharp point used on DC steel work. Many welders have shifted to lanthanated (gold) or a few specific ceriated blends for AC work on modern inverters, since they ball less dramatically and give a slightly more stable arc start, but pure/zirconiated remains the traditional and still perfectly viable choice. Whichever you use, prep it correctly:

• Grind or let it ball to a clean, rounded, mushroom-cap tip before starting — don't use a pointed DC-style grind on AC aluminum work, it balls unevenly and the arc wanders.
• Use a dedicated aluminum tungsten and grinding wheel if you also TIG weld steel with the same shop setup; contamination from steel dust embedded in the tungsten shows up as inclusions in aluminum welds.

Surface Prep: The Step That Actually Determines Weld Quality

More aluminum TIG welds fail from inadequate cleaning than from bad technique. The oxide layer the AC cycle is fighting during the weld is only part of the problem — aluminum also readily absorbs moisture and hydrocarbons into its surface, and both cause porosity (visible as small pinholes in the finished weld, and invisible weak points you won't see until the part fails). Before welding:

1. Mechanically remove the oxide layer with a dedicated stainless steel wire brush (never one that's touched carbon steel — cross-contamination embeds iron particles that will rust and discolor the aluminum) or a flap disc, right before welding, not hours ahead of time — the oxide layer reforms within minutes of exposure to air.
2. Degrease with acetone on a clean rag immediately before welding, working both faces of the joint and several inches beyond it, to remove oils, cutting fluid residue, and shop grime that would otherwise boil into the weld pool as porosity.
3. Store filler rod properly — in a dry, sealed tube or bag, not loose on a bench where it picks up moisture and handling oils. A filler rod that's been sitting out is a common, overlooked porosity source.

Filler Alloy Selection

Unlike mild steel, where one general-purpose filler covers most jobs, aluminum filler choice depends heavily on the base alloy and the job's requirements:

FillerCommon UseNotes 4043 (5% silicon)General purpose, most 6061/6063 extrusion and casting workGood flow, crack resistance, slightly duller finish after anodizing 5356 (5% magnesium)5xxx-series alloys, marine applications, higher-strength jointsBetter color match under anodizing, better strength, more prone to hot cracking on some casting alloys — don't use on cast 6061 or high-silicon castings 4047 (12% silicon)Cast aluminum repair, high-silicon castingsExcellent flow and crack resistance on castings, not typically used on wrought extrusion joints

If you don't know the base alloy for certain (common with scrap or unlabeled extrusion), 4043 is the safer general default; mixing 5356 filler onto an unknown casting alloy risks hot cracking that won't show up until the part is stressed later.

Technique Differences from Steel TIG

• Puddle behavior is different. Aluminum doesn't change color as it heats the way steel does — there's no dull-red warning before it's molten. The first visible sign of the right temperature is the puddle suddenly going shiny and starting to flow, and it can go from solid to a hole in the material surprisingly fast once it gets there. New aluminum welders consistently burn through thin material because they're watching for a color change that never comes.
• Thermal conductivity is roughly triple that of steel, meaning heat spreads away from the weld zone fast, which is why aluminum needs noticeably more amperage for a given thickness than the equivalent steel joint — but that same conductivity also means a joint can go from "not quite there" to "collapsed" in a narrow window, especially on thin sheet or near an edge where there's less surrounding mass to pull heat away.
• Pulse TIG helps a lot on thin material, if your machine offers it — alternating between a peak and background current gives the puddle time to partially solidify between pulses, which both controls heat input on thin sheet and gives a more consistent, better-looking ripple pattern without needing to manually modulate travel speed as precisely.
• Preheat thick sections (above roughly 1/4") to around 150–200°F to offset that fast heat dissipation and get full penetration without excessive amperage, especially on parts with large surrounding thermal mass like a CNC machine base casting or heavy extrusion frame joints.

Safety: UV, Fumes, and Gas Setup

Safety notes: TIG's open arc produces significant UV radiation — a proper auto-darkening TIG-rated welding helmet (not a cheap stick/MIG-only shade range) and skin covered with non-synthetic clothing are non-negotiable; aluminum's open AC arc is notably bright and the UV exposure risk to unprotected skin and eyes nearby is real, so warn anyone else in the shop when you're striking an arc. Ventilate the work area — aluminum welding fume contains fine aluminum oxide particulate, and any coating, paint, or solvent residue left on the material (another reason to clean thoroughly beforehand) can produce genuinely toxic fumes when it burns off. Argon shielding gas displaces oxygen in a confined or poorly ventilated space and is heavier than air, so it pools low and silently — never TIG weld in an enclosed space like a pit or small sealed room without active ventilation and, ideally, a gas monitor.

Use 100% argon shielding gas (not an argon/CO2 mix, which is for steel MIG) at roughly 15–20 CFH for most work, more for larger cup sizes or draftier shop conditions. A gas lens style collet body, rather than a standard collet body, gives noticeably better shielding coverage at the same flow rate and is worth the small upgrade cost if you're doing aluminum regularly.

Aluminum TIG rewards patience during setup far more than it rewards welding speed: get the AC balance and frequency dialed to your machine and material thickness, clean the joint thoroughly and recently, pick the right filler for the alloy, and the actual welding becomes much more forgiving than its reputation suggests. Most of what makes aluminum TIG feel hard the first few times is really a cleaning and setup problem wearing a welding-technique costume.