Aluminum Workshop: Choosing a MIG welder for aluminum

Q: I want to buy a small gas metal arc welding (GMAW) machine, preferably one that runs from 120-V input power. Most of my welding will be steel, but I would like to weld aluminum occasionally. Can these machines really weld aluminum? I've heard a lot of different opinions. Some companies claim their machines will weld aluminum, some companies make no mention of aluminum at all, and some companies have actively discouraged me. What's the real story?

A: First of all, I'm glad you are being realistic in specifying "occasional" aluminum welding. If you want to perform "best-quality" aluminum welding every day, none of these machines are for you. As in most areas of life, there is no free lunch in choosing power supplies for welding aluminum.

The good news is that, as long as you recognize their limitations, most of these units will do a reasonable job of welding aluminum, although over a somewhat restricted range of thicknesses. So let's discuss your options.

Choosing the Right MIG Welder for Aluminum

The cheapest machines run from 120-V input power and are sold as flux-cored arc welding (FCAW) units for welding steel. Of course, you can't FCAW aluminum. However, many manufacturers make a GMAW conversion kit to allow you to weld using carbon steel GMAW wire and shielding gas, usually CO₂ or 75 percent argon and 25 percent CO₂ for steel.

Some manufacturers make an aluminum feeding kit designed to feed aluminum wire through these units. However, by the time you buy the GMAW conversion kit and the aluminum kit, you've probably spent more than the cost of a better unit that will feed aluminum more easily. In addition, the thickness you can weld with these units can be limited. Because of these two facts, I don't recommend buying FCAW units and converting them.

The next step is a welding machine that runs from 120-V power and is designed to perform GMAW and FCAW. Typically, these machines have a maximum output between 100 and 125 amps and cost a little more. They usually weld aluminum up to about 1/8 inch thick nicely. You will need to buy a new bottle of 100 percent argon shielding gas.

You can't weld aluminum using CO₂ or argon-CO₂ mixtures. Again, some manufacturers make aluminum feeding kits. They usually contain different drive rolls specifically designed to feed aluminum, a plastic gun liner for better feeding, and contact tips for aluminum. While the kits aren't strictly necessary, they do help in feeding the small-diameter, soft aluminum wire these machines use. These machines usually cost more than the FCAW machines and often have more rugged drive systems as a result.

The last upgrade in these light-duty machines before the transition to industrial equipment is the small machine that runs from 230-V, single-phase output. As with other machines, feeding kits usually are available from the manufacturer to fit these machines. They offer an expanded thickness range for welding aluminum. Usually they can weld at least 1/4-in.-thick aluminum. If you need to weld aluminum thicker than 1/4 in., you should consider a light industrial unit.

Tips for Choosing the Right MIG Welder for Aluminum

OK, so you've made your choice and bought a small unit and an optional aluminum feeding kit, and you're set up at home, ready to weld. Your first experience welding aluminum could be frustrating. It takes different techniques and practice to weld aluminum well. Here are some tips:

Make sure the welding machine is set on the right polarity. If your machine came set up for FCAW, chances are the wires are run with electrode negative. If you try to GMAW aluminum in electrode-negative polarity, you'll get a weld with no penetration that just lies on the surface. Be sure to connect the power lead to positive polarity and the work lead to negative polarity.

Set the brake tension correctly. All of these units incorporate some sort of simple brake on the wire spool. If you set it with no tension, the wire will unravel on the spool, and you won't be able to feed it. If it's set too tightly, the wire drive motor won't be able to pull it off the spool consistently, and you'll get a lot of tip burnbacks.

Set the drive roll tension properly. The most common mistake people make when welding aluminum is to set the drive roll tension too tightly. This deforms the aluminum wire and makes it harder to feed through the gun. The tension setting should be just tight enough so that the drive rolls don't slip on the wire.

Use the right wire. Another common mistake is to use 5356 wire because it's stiffer and feeds more easily. Usually people then use 0.030-in.-dia. wire as well. Both of these are bad choices. All of these units are designed as short-arc machines for steel wire. They have a limited range of wire feed speeds and a fairly low maximum output voltage.

Because of the low output voltage, you need a relatively high current to melt the wire at the wire feed speeds available. The resistance of 4043 is relatively low, so it can conduct the required current and melt well into the weld.

However, 5356 has a higher resistance. At the low output voltage and wire feed speed, it won't conduct enough current to weld well, and you may become frustrated with your new purchase, thinking the welding machine is at fault. Even though it seems counterintuitive, use 0.035-in. 4043 filler metal.

Even if you do all these things correctly, feeding aluminum wire with this sort of system can be difficult. Keeping the gun and cable as straight as possible while welding will help. Try to avoid putting extreme bends in the torch cable. Don't be surprised if the feeder literally stops feeding if you do. If it happens when you're welding, you will probably burn back a contact tip.

Most important, remember that the fact that you own a welding machine doesn't necessarily mean that you'll know how to weld aluminum right away. Don't wait until your aluminum widget breaks and needs to be fixed today to learn. Practice, practice, practice! After a little while, you'll find that it doesn't seem so hard anymore.