On fine aluminum, a first MIG bead that resembles a string of glued beads is well-known. The wire jams in the liner, the arc crackles, and the molten pool penetrates the sheet metal. Most of these issues do not stem from the machine itself, but from the setup of the wire feed and the preparation of the parts. Here are the concrete points that truly change the outcome when welding aluminum with MIG.
Spool gun and push-pull torch: the right setup for MIG aluminum
Aluminum is significantly softer than steel. Pushing aluminum wire through a standard three or four-meter liner regularly causes bird-nesting, which means a tangle of wire in the spool. This wastes time, wire, and directly affects the stability of the arc.
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The most reliable solution remains the spool gun, a torch that carries its own spool of wire just behind the nozzle. The wire travels only a few centimeters before reaching the arc, which almost completely eliminates feeding issues.
For those who want to use a MIG aluminum welding machine without investing in a spool gun, the push-pull torch offers an alternative: a motor pulls the wire on the torch side while the spool pushes it. The result is comparable, with a bit more flexibility in the cable length.
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When using a standard torch, at a minimum, you need to replace the steel liner with a Teflon liner and ensure that the drive rolls are smooth (not knurled). Knurled rolls crush the aluminum wire and create chips that clog the liner in a matter of minutes.
Preparation of aluminum parts before MIG welding
The oxide layer that naturally forms on aluminum melts at a temperature much higher than that of the metal itself. If it is not removed, the arc must pierce this layer before reaching the molten pool, which generates inclusions and an irregular bead.
The point that many underestimate: the oxide reforms very quickly after brushing. Preparing your parts the day before is pointless. Degrease with acetone, brush with a stainless steel brush reserved exclusively for aluminum (never the one used on steel), and then weld immediately.
- Degrease each part and the filler wire to eliminate any trace of oil or grease, which would cause porosity in the bead.
- Mechanically brush the welding area a few centimeters on each side of the joint, with a stainless steel brush dedicated to aluminum.
- Weld immediately after brushing to prevent the oxide layer from reforming.
On thick parts, moderate preheating helps to compensate for aluminum’s high thermal conductivity, which dissipates heat very quickly. Feedback varies on this point depending on thickness and alloy, but a noticeable improvement in penetration is observed on sections beyond one centimeter.
Choosing aluminum filler wire: ER4043 or ER5356
There are mainly two types of filler wire for MIG aluminum, and the choice between the two is not trivial. Each wire yields different results in terms of fluidity of the pool, mechanical strength, and finish.
ER4043 for common welds
The ER4043 wire contains silicon, making it more fluid and more tolerant to adjustment variations. It produces a smooth bead and limits the risk of hot cracking. It is the versatile wire for general assemblies, repairs, and parts that will not undergo intense mechanical stress.
ER5356 for strength and anodizing
The ER5356 wire offers better mechanical strength of the deposit. It is also the only relevant choice when the part needs to be anodized after welding, as ER4043 causes visible color differences after treatment. For any anodized part, switch to ER5356 without hesitation.
MIG settings and shielding gas for welding aluminum
Aluminum is welded exclusively under inert gas. Pure argon is used in the vast majority of cases. Some add a small proportion of helium to increase the energy of the arc on thick parts, but argon alone covers most situations in the workshop or on-site.
The gas flow is generally set one notch above what is set for steel, because molten aluminum is very sensitive to atmospheric contamination. Insufficient flow allows air to enter the pool and produces porosity visible to the naked eye in the form of small holes in the bead.
On the electrical settings side, the wire speed must be consistent with the voltage. A wire that is too slow relative to the voltage creates an unstable arc that spits droplets. A wire that is too fast crushes the arc, and the wire hits the part. Testing on a scrap piece of the same alloy and thickness as the final piece is necessary.
- Start with a medium voltage and moderate wire speed, then adjust in short increments.
- Slightly increase the torch travel speed compared to steel, as aluminum heats up quickly and the pool widens rapidly.
- Favor the technique of pushing the torch (working angle forward) rather than pulling, for better gas coverage of the pool.
Safety and common mistakes in MIG aluminum welding
Aluminum reflects UV rays more than steel. An auto-darkening helmet with an appropriate protection rating is essential, even for short beads. Sunburns from UV rays reflected off polished aluminum often surprise those coming from steel.
The most common mistake remains to neglect the cleaning of the nozzle and contact tube. Aluminum spatter sticks inside the nozzle and eventually disrupts the gas flow. Checking every few passes prevents defects across an entire piece.
A final often-forgotten point: store the filler wire in a dry place. Moisture absorbed by the spool releases as vapor in the arc and causes exactly the same porosity as insufficient gas flow. A sealed bag or airtight container is sufficient.