How to MIG Weld Aluminum: Spool Gun, Settings & Gas Setup

MIG welding aluminum requires 100% argon shielding gas, a spool gun or push-pull gun, and either 4043 or 5356 filler wire. The process runs on DCEP (reverse polarity) just like steel MIG, but aluminum’s high thermal conductivity and low melting point mean you need to move faster and run hotter than you’d expect for the material thickness.

Aluminum MIG uses spray transfer almost exclusively. Short circuit transfer doesn’t work well because aluminum’s oxide layer melts at 3,700 F (2,037 C) while the base metal underneath melts at only 1,220 F (660 C). You need enough heat to blast through that oxide and establish a stable arc. That means higher voltage, higher wire speed, and a push angle on the gun.

Why You Need a Spool Gun or Push-Pull Gun

Standard MIG guns feed wire through a 10-15 foot cable from the wire feeder to the gun tip. Steel wire is stiff enough to push through that distance without issues. Aluminum wire isn’t.

Aluminum is soft. It buckles, bends, and bird-nests inside a standard liner before it ever reaches the contact tip. The result is erratic feeding, burnbacks, and constant frustration.

A spool gun solves this by mounting a small spool of wire (usually 1 lb.) directly on the gun. The wire only travels a few inches from the spool to the contact tip, so there’s almost no chance of feeding problems. The downside: spool guns are heavier and bulkier than standard guns, and you’re limited to small spools.

A push-pull gun uses a motor at the gun to pull the wire while the main feeder pushes it. This lets you run standard-size spools and longer cable lengths while still feeding reliably. Push-pull setups cost more but are the preferred choice for production aluminum MIG work.

If you’re committed to trying a standard gun setup, swap to a Teflon or nylon liner, install U-groove drive rolls, back off the drive roll tension until the wire just barely feeds without slipping, and keep the gun cable as straight as possible. It works for short runs but isn’t a long-term solution.

Aluminum MIG Wire: 4043 vs 5356

Two filler wires handle 90% of aluminum MIG welding. Picking the right one depends on the base alloy and what you need from the finished weld.

ER4043 contains about 5% silicon. It flows well, produces smooth beads with minimal spatter, and resists hot cracking. Use it on 6061, 6063, and most cast aluminum alloys. It’s the default choice for general fabrication, repair work, and anything that’ll be anodized (though the weld won’t color-match perfectly). 4043 welds are slightly softer than the base metal.

ER5356 contains about 5% magnesium. It produces stronger welds than 4043 and offers better corrosion resistance in marine environments. Use it on 5052, 5083, 5086, and other 5xxx-series alloys. It’s stiffer wire, which feeds more reliably through standard gun setups. 5356 welds are harder and respond better to anodizing with a closer color match.

Never use 4043 on 5xxx-series alloys with more than 3% magnesium. The combination can cause stress corrosion cracking.

Wire diameter for aluminum MIG typically runs 0.030" or 0.035" for most spool gun work. Thicker material (1/4"+) can use 0.045" or even 3/64" wire. Thinner wire is more forgiving on light-gauge aluminum because it requires less amperage to melt.

Shielding Gas for Aluminum MIG

100% argon is the standard. No exceptions for thin material. Run 25-30 CFH for indoor work. Bump it to 30-40 CFH outdoors or in drafty shops, though wind is the enemy of aluminum MIG and you’re better off setting up a wind screen.

Argon/helium blends add heat input for thicker aluminum. A 75% argon / 25% helium mix runs hotter and wets out better on material over 3/8". For heavy plate (1/2"+), a 50/50 argon/helium blend pushes even more heat. Helium costs more and requires higher flow rates because it’s lighter than air and disperses faster.

Never use CO2 or argon/CO2 mixes on aluminum. CO2 reacts with molten aluminum and causes porosity and oxide inclusions. This is a steel-only gas.

Aluminum MIG Settings by Thickness

These settings assume 100% argon gas, DCEP polarity, and a spool gun. Adjust wire speed up or down based on your arc sound and bead appearance.

Note: Aluminum MIG settings run higher wire speeds than steel for the same thickness. Aluminum wire melts faster because of its lower melting point and higher thermal conductivity. If your arc sounds like bacon frying, bump the voltage up 1-2 volts. A smooth, steady hiss means you’re in the zone.

Preparing Aluminum for MIG Welding

Aluminum prep matters more than steel prep. The oxide layer on aluminum is hard, has a much higher melting point than the base metal, and traps moisture and contaminants. Skip the prep, and you’ll get porosity, lack of fusion, and ugly welds.

Step 1: Degrease. Wipe the joint area with acetone or a dedicated aluminum cleaner. Remove oil, grease, paint, and any surface contamination. Don’t use shop rags that have been near cutting oil.

Step 2: Remove the oxide layer. Use a stainless steel wire brush dedicated to aluminum only. Brush in one direction. Do this immediately before welding because the oxide reforms within minutes. For critical work, some shops use a chemical etch instead.

Step 3: Preheat thick sections. Aluminum over 1/4" thick benefits from preheating to 200-300 F (93-149 C). This reduces the heat sink effect and prevents lack of fusion at the root. Don’t exceed 300 F or you’ll weaken the temper on heat-treatable alloys like 6061-T6. Use a temperature crayon or infrared thermometer to verify.

Step 4: Fit-up tight. Aluminum’s high thermal conductivity pulls heat away from the joint fast. Gaps that would be fine on steel become problem areas on aluminum because the metal solidifies before it bridges. Keep gaps under 1/16" for butt joints. Use backing bars where possible.

Technique for Aluminum MIG

Aluminum MIG technique differs from steel in several key ways.

Push, don’t drag. Always use a push angle (forehand technique) with 10-15 degrees of gun angle. Pushing keeps the gas shield ahead of the puddle. Dragging on aluminum traps the shielding gas behind the puddle and causes porosity.

Travel fast. Aluminum conducts heat roughly five times faster than steel. The longer you sit in one spot, the bigger the heat-affected zone grows and the more likely you are to blow through. Maintain a steady, brisk travel speed. The bead should be 3/8" to 1/2" wide for most joints.

Use longer stickout. Run 1/2" to 3/4" of wire stickout (electrode extension) for aluminum. This is longer than the 3/8" typical for steel MIG. The extra stickout preheats the wire and helps it melt cleanly.

Watch the puddle, not the arc. On aluminum, the puddle tells you everything. If it’s getting too wide or too fluid, speed up. If you see the edges aren’t wetting in, slow down or bump the voltage. The puddle should be bright and fluid but controlled.

Fill craters. Aluminum crater cracks are extremely common. At the end of every weld, don’t just release the trigger and pull away. Use your machine’s crater fill function if it has one, or briefly reverse direction at the end of the bead to fill the crater. Some welders add a tack at the end of the run to prevent crater cracking.

Common Problems and Fixes

Porosity (holes in the weld) Causes: Dirty base metal, inadequate gas coverage, moisture on the wire or workpiece, wrong gas. Fix: Clean the aluminum properly, check gas flow rate and nozzle for blockages, store wire in a dry location, confirm you’re running 100% argon.

Burnthrough Causes: Too slow travel speed, too much heat, poor fit-up with gaps. Fix: Increase travel speed, reduce voltage by 1-2 volts, reduce wire speed, use a backing bar, improve fit-up.

Black soot on the weld Causes: Contaminated base metal, hydrocarbon contamination (oil, grease), too-low gas flow. Fix: Clean the aluminum thoroughly, increase gas flow rate, check for drafts.

Bird-nesting (wire tangling in the gun) Causes: Too much drive roll tension, kinked liner, wrong liner material. Fix: Reduce tension until wire just feeds without slipping, replace kinked or steel liners with Teflon, use a spool gun.

Lack of fusion Causes: Insufficient heat, thick oxide layer, wrong technique (dragging instead of pushing). Fix: Preheat thick material, clean oxide immediately before welding, push the puddle, increase voltage.

Crater cracking Causes: Rapid cooling at the end of the weld creates shrinkage cracks. Fix: Use crater fill, back-step at the end of the bead, add a run-off tab for structural work.

Safety Considerations for Aluminum MIG

Aluminum MIG produces intense UV radiation and aluminum fume. Wear a minimum shade 10 welding lens (shade 11-12 for spray transfer at high amperages). Use respiratory protection rated for metal fumes. Aluminum fume exposure is linked to respiratory issues with prolonged exposure.

Aluminum reflects UV more than steel, so exposed skin burns faster. Wear full coverage clothing, including a welding jacket with the collar up.

Preheat operations require heat-resistant gloves and awareness of hot workpieces that don’t glow red the way steel does. Aluminum stays silver even at temperatures that will cause serious burns. Treat every piece of aluminum on the welding table as hot until you verify otherwise.

Clean aluminum with acetone in a well-ventilated area. Acetone vapors are flammable and harmful to inhale.

When to Skip MIG and TIG Aluminum Instead

MIG works well for aluminum 1/8" and thicker, structural welds, and production runs where speed matters. For material under 1/8" thick, cosmetic welds, or precision work on thin tubing and sheet, TIG welding gives you far better control. TIG also handles dissimilar aluminum alloys and repair work on castings more reliably because you can precisely control heat input with the foot pedal.

If you’re welding aluminum less than 3/32" thick, a MIG machine will fight you. Switch to TIG with AC current and you’ll get much cleaner results with less frustration.