Ron Covell Shows You How to MIG Weld Aluminum with a Spool Gun

Ron Covell Shows You How to MIG Weld Aluminum with a Spool Gun

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Swapping your traditional MIG gun for a spool gun when welding aluminum can help prevent issues like binding.
Ron Covell welding aluminum with spool gun

Is a spool gun needed to weld aluminum?

MIG is the most widely used welding processes for hobbyists, and it’s a mainstay for commercial fabrication and repair facilities. Steel is the most commonly welded metal because it’s strong, inexpensive and suitable for most fabrication processes. Aluminum is probably the second most widely used material, and its use can reduce weight while maintaining good strength and workability. Aluminum is steadily taking a greater role in transportation industries, since "adding lightness" is essential for making vehicles more fuel-efficient and for getting more range from battery power. TIG welding is often used on aluminum, but not everyone knows that even affordable MIG welders like the Millermatic® 211 can weld aluminum.

MIG machines usually have a spool of filler wire housed inside the case, and the wire is fed through the gun cable to the weld area. This delivery system works well for steel, but it is problematic for the softer aluminum wire, which can sometimes bind; especially when being pushed through a long cable that is draped over some obstacles.

There is an easy solution for this issue — using a spool gun. This replaces the standard gun on a MIG welder, and it holds a spool of wire so the feed path is short and straight. The Miller® Spoolmate™ 150 Spool Gun is a direct replacement for the gun on the Millermatic 211, and no tools are required for the installation.

Millermatic 211 sitting on a metal surface with welder holding Spoolmate 150 Spool Gun

Pure argon gas is required to MIG weld aluminum — not the mixture of argon and CO2 that is commonly used for welding steel. You can use the same regulator or flowmeter that is used with the C-25 gas.

With the spool gun installed on the machine, a spool of aluminum wire is loaded in the housing, a bottle of argon gas is connected, and you’re ready to dial in the settings for your particular welding job. The Spoolmate 150 integrates with the Auto-Set™ feature on the Millermatic 211, so you can start simply by dialing in the material thickness and the wire diameter. 

Steel is commonly welded with a short-circuit arc, which produces the familiar "frying bacon" sound. Aluminum welding utilizes the much faster spray transfer mode, which may take some getting used to since the spool gun needs to be moved quickly to keep up with the process.

The spool gun should always be pushed and ideally held at about a 20-degree work angle to ensure the shielding gas can keep the atmosphere away from the weld zone. Laying the gun down flatter may compromise the gas coverage at the weld. You will have to experiment to find the proper gas flow, but 35 CFH is a good place to start. Insufficient gas flow can cause heavy black soot to form around the weld area. It’s very important to have sufficient wire "stick-out" when welding aluminum — 3/4 inch is recommended. If your stick-out is too short, the wire may burn back and foul the contact tip.

While the Auto-Set feature works with the spool gun, I have found that minor adjustments to the wire feed speed and voltage can help achieve ideal penetration and bead contour and minimize the formation of soot in the weld area. The recommended settings can be found inside the hinged cover of the power supply, and these will be your starting point for any adjustments you want to make.

Lifted up cover displaying recommended weld settings chart

The settings for wire feed speed and voltage can be easily adjusted with the dials on the front panel of the machine.

Welder adjusting voltage on front of welding power source

Cleanliness is important with any welding process, but aluminum is particularly intolerant of any surface contamination. The best practice is to first use a degreaser to remove any oily contaminants on the metal, and then carefully scrub the weld area with a dedicated stainless steel wire brush just before welding.

The first trial I did with the Spoolmate 150 gun was a fillet weld on 0.080-inch aluminum sheet, using 0.030-inch 4043 wire. I temporarily held the parts in the proper alignment and tack welded them together. 

Test weld on joint with welder holding MIG gun

I did several tests to find the best parameters for this job and got the best results with the wire speed at 5 and voltage at 60, using 30 CFH gas flow. I found the best gun angle to be 45 degrees from each side of the joint, with a 20-degree travel angle. I experimented with moving the gun faster and slower until I found the speed that gave me the best bead profile. 

Welder welding aluminum with spool gun and Millermatic 211

You will often see a little black soot develop at the edges of an aluminum MIG weld, but I was pleased that it was minimal at these settings, and what little soot was left could be removed with one or two swipes of a wire brush. 

Completed weld on aluminum

Next I set up a lap joint on 3/16-inch plate. I did a few tests to find the settings and travel speed that would produce a bead that filled the step completely and leave a nicely rounded bead profile. I held the gun at 45 degrees from the work and used a 20-degree travel angle. 

Completed lap joint weld on aluminum

The next test I did was a corner weld on 1/4-inch aluminum plate. I fixtured the plates corner to corner, 90 degrees to each other, which automatically creates a small trough ready to accept a weld. I held the torch vertically while using a 20-degree travel angle and experimented with different travel speeds until I found what worked best. In the end, I got a nicely rounded weld bead that just filled the trough.

Completed corner weld on aluminum

The spool gun works well on aluminum castings, too. I happened to have some cast components of a motorcycle oil tank, and I welded two sections together, getting a nice clean joint.

Welder welding aluminum motorcycle oil tank with spool gun

All in all, I found the Spoolmate 150 spool gun quite easy to set up and use. The process is similar to using a traditional MIG gun, but aluminum has some unique properties that you’ll appreciate once you get some experience with it. Miller makes other spool guns, including the less expensive Spoolmate 100 — but it has a shorter cable and a lower duty cycle, and is only recommended for 4000 Series wire. The Spoolmate 150 works with both 4000 and 5000 series wire, and it’s a better choice if you need high output capacity.

The Spoolmate 150 works with several Miller welding machines, but you can explore all its features with the Millermatic 211 — a very light and compact MIG machine. Another benefit of the 211 is its capability of running on either 120 or 240 volts by simply swapping ends on the multi-voltage plug (MVP™).

Headshot of Ron Covell
Ron Covell is a talented welder and metalworker in the automotive industry. Covell is a contributor for Hot Rod Network, where he has a popular column titled, "Professor Hammer's Metalworking Tips." He also owns Covell Creative Metalworking, offers instructional metalworking DVDs and hosts workshops around the country.
Updated: July 17, 2021
Published: April 18, 2021