How to Weld Aluminum: TIG, MIG, and What Actually Works

Aluminum welding requires AC TIG or a MIG spool gun, aggressive oxide removal, and pure argon shielding gas. TIG produces the best results on aluminum under 1/4 inch thick. MIG with a spool gun handles thicker sections faster but with less precision.

The reason aluminum intimidates welders is simple: a tenacious oxide layer melts at 3,700°F while the base metal underneath melts at only 1,200°F. You’re trying to fuse metal that’s already liquid while the surface crust is still solid. Get past that one problem and aluminum becomes predictable.

Why Aluminum Is Different From Steel

Steel welding is forgiving. You can run a mediocre bead on mild steel with poor prep and still get a joint that holds. Aluminum punishes every shortcut.

Here’s what makes aluminum difficult:

High thermal conductivity. Aluminum conducts heat roughly five times faster than steel. Heat disappears from the joint almost as fast as you add it. This means you need more amperage than you’d expect for thin material, and thick aluminum requires preheating to avoid cold starts.

The oxide layer problem. Aluminum oxide (Al2O3) forms within seconds of exposure to air. This oxide is extremely hard and melts at roughly 3,700°F. The aluminum underneath melts at about 1,220°F. If you don’t remove the oxide before and during welding, it gets trapped in the weld pool and creates porosity and inclusions.

No color change before melting. Steel glows red, then orange, then white as it heats up. You can read the temperature visually. Aluminum goes from solid to a puddle with almost no visual warning. One second you’re heating the joint, the next second you’ve burned through.

Hot cracking. Aluminum alloys (especially the 6xxx series) are susceptible to solidification cracking. Proper filler metal selection and joint design reduce this, but it’s always a concern.

Prep Work: This Is Where Aluminum Welding Succeeds or Fails

Skip this section and nothing else matters. Aluminum prep takes three times longer than steel prep, and cutting corners here guarantees bad welds.

Mechanical Cleaning

Remove the oxide layer with a stainless steel wire brush dedicated to aluminum. This is non-negotiable. If that brush has ever touched steel, throw it away and buy a new one. Steel particles embedded in aluminum cause galvanic corrosion and weld contamination. Mark your aluminum brush with paint or tape so nobody grabs it for steel work.

Brush in one direction only. Scrubbing back and forth just smears oxide around. Apply moderate pressure and work along the joint line, extending at least 1 inch past where you’ll be welding.

For heavily oxidized material or cast aluminum, use a carbide burr or flap disc (aluminum-specific, not one that’s been used on steel) to get down to bright metal.

Chemical Cleaning

After mechanical cleaning, wipe the joint with acetone or a dedicated aluminum weld prep solvent. This removes oils, shop dust, and residual oxide particles. Use clean, lint-free rags or paper towels. Don’t use shop rags from the communal bin; they’re contaminated with cutting oil and steel dust.

Some welders also use a mild phosphoric acid etch on critical joints. For most shop work, acetone is sufficient.

Timing Matters

The oxide layer starts reforming immediately. On a humid day, you’ve got maybe 20-30 minutes of clean time before the oxide builds back up enough to cause problems. Prep your joints and weld them the same session. Don’t clean material today and weld it tomorrow.

Fit-Up

Aluminum expands roughly twice as much as steel when heated. Tight fit-up matters more on aluminum because gaps that would be fine on steel become distortion nightmares. For butt joints on material under 1/8 inch, you want zero gap. For thicker material, a gap of about half the material thickness works for full penetration.

Tack welds should be small and frequent (every 2-3 inches on thin material). Aluminum tacks crack more easily than steel tacks, so make them substantial enough to hold during welding.

TIG Welding Aluminum: The Preferred Method

TIG (GTAW) on AC is the gold standard for aluminum welding. It gives you the most control over heat input, produces the cleanest welds, and handles thin material (down to 0.030 inch) without burning through.

Equipment Requirements

TIG welder with AC capability. Entry-level inverter TIG machines start around $800-1,000 for units with decent AC features. You need adjustable AC balance and AC frequency at minimum. High-frequency start is strongly preferred over scratch start for aluminum.

AC balance control. This adjusts the ratio of electrode-positive (cleaning) to electrode-negative (penetration) in each AC cycle. For most aluminum work, set balance to 65-75% EN (electrode negative). More EN means more penetration and less electrode heating. More EP means more cleaning action but a wider, shallower arc that balls up your tungsten faster.

AC frequency. Higher AC frequency (80-120 Hz) narrows the arc cone and gives more directional control. Lower frequency (40-60 Hz) spreads the arc wider and provides more cleaning. Start at 80 Hz and adjust from there.

Tungsten Selection

Use pure tungsten (green band) or 2% ceriated (gray band) for AC aluminum welding. Pure tungsten forms a nice ball on the tip during AC welding, which is normal. Ceriated tungsten runs a bit cooler and lasts longer.

Do NOT use 2% thoriated (red band) on AC. Thoriated tungsten doesn’t ball properly on AC and the arc wanders.

Size your tungsten to your amperage range:

Filler Rod Selection

The two most common aluminum filler rods are ER4043 and ER5356. They’re not interchangeable.

ER4043 contains silicon, flows smoothly, produces minimal spatter, and creates a shiny weld bead. Use it on 6061, 3003, and most casting alloys. It’s more forgiving and easier to feed. Weld color is slightly darker than the base metal.

ER5356 contains magnesium, produces higher-strength welds, and has better color match to 5xxx series alloys. It’s stiffer than 4043 and feeds better through MIG spool guns. Use it on 5052, 5083, and when you need anodizing compatibility (4043 welds turn dark gray when anodized).

Never use steel filler rod on aluminum. This sounds obvious, but in a dim shop with multiple rods on the bench, it happens.

TIG Aluminum Settings Guide

These are starting points. Adjust based on fit-up, position, and alloy.

TIG Technique for Aluminum

Push, don’t drag. Always push the torch (forehand technique) when TIG welding aluminum. Dragging pulls shielding gas away from the fresh weld and causes porosity. Maintain a 15-20 degree torch angle from vertical, pointed in the direction of travel.

Establish the puddle first. Hold the torch in one spot until you see a shiny, reflective puddle form. This takes 1-3 seconds depending on material thickness and whether you preheated. Dip the filler rod into the leading edge of the puddle, pull it back, move the torch forward about 1/8 inch, and dip again.

Keep a tight arc length. The distance from the tungsten tip to the work should be roughly equal to the tungsten diameter. Too long an arc spreads the heat zone and reduces penetration.

Watch for the “wet” look. A properly heated aluminum puddle looks like liquid mercury, very reflective and fluid. If it looks dull or sluggish, you need more heat. If the edges of the puddle are ragged and the filler rod melts before it touches the pool, you have too much heat.

Post-flow. Run your argon post-flow for 8-12 seconds after breaking the arc. Aluminum oxidizes aggressively when hot, and insufficient post-flow leaves a gray, crusty crater.

MIG Welding Aluminum: The Production Alternative

MIG (GMAW) with a spool gun welds aluminum 3-4 times faster than TIG. Weld appearance isn’t as clean, but for structural work, thick sections, and production runs, MIG aluminum is the practical choice.

The Spool Gun Requirement

Standard MIG liners are steel, and they’re designed for steel wire that’s stiff and pushes through 10-15 feet of conduit without issue. Aluminum wire is soft. Push it through a conventional MIG liner and it’ll bind, kink, and bird-nest inside the wire feed housing.

A spool gun solves this by mounting a small spool (typically 1 lb) of aluminum wire directly in the gun. The wire only travels about 6-12 inches from the spool to the contact tip. Short feed path means no binding.

Some MIG welders have a spool gun port on the front panel. If yours doesn’t, you may need an adapter or a different machine. Check compatibility before buying a spool gun; they’re not universal.

An alternative is a push-pull gun system, which uses a Teflon or nylon liner with a motor at both ends of the cable. These cost more than spool guns but allow using full-size wire spools and run better on long production shifts.

MIG Aluminum Settings

MIG aluminum uses DCEP (electrode positive), same polarity as steel MIG welding. Shielding gas is 100% argon at 30-35 CFH, higher flow than TIG because MIG’s larger gas nozzle covers more area.

Wire feed speed on aluminum is significantly higher than steel. Aluminum wire melts fast. If you’re used to MIG steel at 250-300 IPM, aluminum at 400+ IPM feels aggressive, but that’s where it needs to be.

MIG Technique Differences

Travel speed is fast. Aluminum’s high thermal conductivity means the puddle stays hot and fluid. Move quickly. Dwelling creates blow-through on thin material and excessive heat buildup on anything.

Push angle, always. Same as TIG. Push the gun at 10-15 degrees from vertical. Dragging traps porosity.

Spray transfer only. Aluminum MIG uses spray transfer (above the transition current), not short-circuit transfer. The arc should sound like a steady hiss or bacon frying, not the popping/crackling you hear with steel short-circuit MIG. If you hear popping, increase voltage and wire feed speed until you’re in spray.

Contact tip maintenance. Aluminum wire wears out copper contact tips fast. The tip bore enlarges, and the wire starts wandering. Replace contact tips every 2-3 lbs of wire, or sooner if the arc becomes erratic.

Why Stick (SMAW) Doesn’t Work Well for Aluminum

Stick electrodes for aluminum exist (E4043 classification). They work in a pinch for field repair of thick castings and non-critical joints. But for any serious aluminum fabrication, stick is the wrong tool.

The flux coating on aluminum stick rods is hygroscopic (absorbs moisture from the air) and produces heavy slag that’s corrosive if not removed immediately. The arc is unstable compared to steel stick welding, and porosity is almost guaranteed. You’ll also find that aluminum stick rods must be stored in a rod oven, they deteriorate quickly in open air.

If your only welder is a stick machine and you need to join aluminum, you’re better off drilling holes and bolting it, or taking the parts to someone with a TIG or MIG setup.

Common Aluminum Alloys and Their Weldability

Not all aluminum is the same. The alloy determines weldability, filler rod selection, and heat treatment response.

6061-T6

The most common structural aluminum alloy. Used for trailer frames, truck bodies, boat hulls, and general fabrication. The “T6” temper means it’s been heat-treated for maximum strength (40,000 PSI tensile). Welding destroys the T6 temper in the heat-affected zone, reducing strength to roughly T0 condition (about 18,000 PSI) unless you do post-weld heat treatment. Use ER4043 filler for general work or ER5356 if the joint will be anodized.

5052

The workhorse alloy for sheet metal work, fuel tanks, and marine applications. Not heat-treatable, so welding doesn’t cause the same strength loss as 6061. Excellent corrosion resistance. Slightly harder to form than 3003 but much stronger. Use ER5356 filler.

3003

Pure aluminum with a small amount of manganese. Very soft, very weldable, very forgiving. Used for cooking utensils, chemical equipment, and low-stress applications. Not structural. Use ER4043 or ER1100 filler.

Cast Aluminum (A356, 319, etc.)

Casting alloys contain silicon, magnesium, and sometimes copper. They’re generally weldable with TIG using ER4043 filler, but castings are often contaminated with oil, grease, and porosity from the casting process. Preheat to 300-400°F and expect some porosity in the weld regardless of prep quality. Crack repair on cast aluminum is part welding, part art form.

Preheating Aluminum

Preheat isn’t always necessary, but it helps in several situations:

• Material thicker than 1/4 inch
• Cold ambient temperatures (below 50°F)
• Cast aluminum (always preheat castings)
• Avoiding thermal shock on crack repairs

Preheat to 200-300°F using a propane torch or oven. Never exceed 400°F or you risk weakening heat-treated alloys. Use a temperature-indicating crayon (Tempilstik) to verify temperature. Don’t guess.

One problem with preheating aluminum: since it doesn’t change color when hot, you can easily grab a 300°F piece of aluminum with bare hands. Always use temperature crayons and treat all preheated aluminum as hot until verified.

Distortion Control

Aluminum’s high thermal expansion means distortion is a constant battle. Here’s what works:

Backstep welding. Instead of welding a continuous bead from one end to the other, weld in 2-3 inch segments, starting each new segment ahead of the previous one and welding back to meet it. This distributes heat more evenly.

Skip welding. On long joints, weld 2 inches, skip 6 inches, weld 2 inches, skip 6 inches. Then go back and fill the gaps. This prevents heat buildup from accumulating in one area.

Chill bars. Clamp copper or aluminum bars along the back side of the joint. They act as heat sinks and pull thermal energy out of the weld zone. Copper works better than aluminum for this purpose because it conducts heat even faster.

Fixture rigidly. Clamp aluminum parts to a heavy steel welding table or fixture. The steel mass absorbs heat and resists the pulling force of the contracting weld. Allow parts to cool completely before unclamping.

Troubleshooting Common Aluminum Welding Problems

Porosity (Holes in the Weld)

Cause: Contamination (oil, oxide, moisture) or insufficient shielding gas coverage. Fix: Improve prep. Increase gas flow. Check for leaks in gas lines. Replace gas if the bottle has been sitting open in a humid shop. Make sure you’re using pure argon, not an argon/CO2 blend.

Black Soot Around the Weld

Cause: Oxide contamination, usually from inadequate cleaning or using a contaminated wire brush. Fix: Use a dedicated stainless steel brush for aluminum only. Clean with acetone before welding. On MIG, check that your wire isn’t oxidized (old spools develop a gray oxide coating).

Burn-Through

Cause: Too much heat for the material thickness, or dwelling too long in one spot. Fix: Reduce amperage. Increase travel speed. Use pulse settings if your machine has them. On material under 1/16 inch, consider a backing plate or heat sink.

Crater Cracking

Cause: The weld crater at the end of a bead shrinks as it solidifies, pulling itself apart. Fix: Use crater fill (slope-down) function on your TIG welder. Gradually reduce amperage over 2-3 seconds at the end of each bead. Don’t just snap the arc off. On MIG, add a small circle at the end of the bead to fill the crater before releasing the trigger.

Lack of Fusion (Cold Lap)

Cause: Insufficient heat input. The filler metal sits on top of the base metal without actually fusing. Fix: Increase amperage. Slow down slightly. Make sure the puddle is fully liquid before adding filler. On thick aluminum, preheat.

Safety Considerations for Aluminum Welding

Aluminum welding produces intense UV radiation, more so than steel welding at the same amperage. Use a minimum shade 10 lens for TIG, shade 11-12 for MIG at higher amperages. Exposed skin sunburns quickly even from reflected UV.

Aluminum fumes are an irritant. While not as toxic as stainless steel or galvanized fumes, prolonged exposure to aluminum welding fumes is linked to respiratory issues. Weld in a ventilated area or use a fume extraction system. The solvents used for cleaning (acetone, etc.) should be completely evaporated before striking an arc.

Molten aluminum splatter sticks to everything and burns right through thin cotton. Wear leather gloves and a leather jacket or FR cotton. Synthetic fabrics melt into skin on contact with spatter.