MIG welding aluminum is a handy skill. While AC Tig can also be used for aluminum work, in many cases, it makes the most sense to do your aluminum welding work with a MIG welder.
Metal Inert Gas (MIG) welding uses an inert shielding gas to protect your material from the surrounding atmosphere while a consumable wire feed electrode is “melted” into the joint under high amperage.
Compared to welding steel, aluminum poses some challenges. It is a softer metal, making it easier to overheat the material as it can burn through at 1100 degrees Fahrenheit. Aluminum also holds the heat poorly, dissipating the heat four times faster than steel. This requires you to use a higher voltage in the 21 to the 24-volt range.
Additionally, your equipment needs to change some from the standard setup used to weld steel. Your welding machine needs to have a reverse polarity setting (or DCEP – Direct Current Electrode Positive) for working with aluminum. This means that electrons are flowing from the ground cable to the gun, instead of the other way around.
A thicker feed wire should also be used, and some of the cheaper welders do not have the internal wire feed mechanism to handle this thicker wire properly.
When welding aluminum, you will be using spray transfer. This is where tiny particles of the aluminum wire are sprayed into the weld puddle. The electrode does not contact the metal but simply sprays the particles into the weld under the high voltage.
Here are some of the alterations you need to make in order to set up your MIG rig to weld aluminum.
Before welding aluminum, you want to make sure that you are competent with steel. Additionally, make sure to have some extra aluminum of the same grade and thickness that you can experiment on before you do your work on the final piece.
The Thinnest Aluminum You Can Weld With MIG
Because of the added heat, you should stick to projects that are at least 14 gauge thick. That is about .074 inches thick. If you have a pulsed MIG, you can weld down to about 18 gauge of thickness.
While this is a very thin piece of metal, if you are working with 18 gauge or smaller, you might consider switching to a TIG Welding process that can handle these thin pieces of metal much better.
MIG Welding Aluminum Gas Mixture
Aluminum reacts with air and oxidizes (rusts) when exposed to air under heat. To prevent rusting, you’ll want to shield your weld with an inert gas.
Most welders use a mixture of 75% Argon and 25% CO2 gas for steelwork. For aluminum MIG welding, CO2 can enter the weld and cause cracking of the joint as it cools. To protect the joint, you will need to use 100% pure Argon gas.
Helium can also be used, but it tends to offer poorer arc stability, is more expensive, and requires a higher flow rate. The advantage of helium is that it creates a flatter weld that tends to trap fewer gases and decreases the risk of porosity (holes) in your completed weld. So, if you can afford it, a mix of Argon and Helium can give you a better weld than pure argon and is a better choice for thicker welds of 1/2″ or more.
Talk to your local supplier, but in most cases, it is as simple as switching out your supply bottles. Your gas flow rate should be about 20-30 CFH.
Switch To Aluminum Wire
With smaller welders, using a slightly over-sized welding wire of .035 diameter seems to work better for welding aluminum. You will want to use the right welding wire for your type of metal. You have two main types of wire to choose from: the ER4043 or the ER5356 filler alloy. The 4043 has 5% silicon added while the 5356 has 5% magnesium.
Most of us will be working with 6000 series aluminum alloys. If that is the case, an ER4043 series wire is going to work better. It can work with all series of aluminum, including 3003, 3004, 5052, 6061, 6063. It can also work with casting alloys of 43, 355, and 214. It is exceptionally good for filling cast aluminum repairs. It is going to turn darker in the post-weld than 5356 will. It also holds up better at higher temperatures.
Overall, 4043 is easier to work with and creates less spatter, making it the clear winner in most cases.
However, for 5000 series aluminum base metal, you need the ER5356 type of welding wire. Additionally, if color matching is important, the 5356 does not turn the darker tarnished color. The Magnesium offers a little more resistance in saltwater environments. It also has a slightly higher shear strength.
All of the top brands make reliable wire. Hobart’s brand is one of the most popular among small, hobby welders, but Lincoln, Miller, and even some of the smaller brands like Blue Demon and Harris make really good products.
ER4043 is a fairly universal wire that you can use for both steel and aluminum work.
Make sure the MIG gun contact tip and Drive Rolls (the round wheel inside the machine’s automatic wire feeder) are set to the correct wire diameter size for your filler metal.
Why You Want A Spool Gun
On smaller jobs, there are a few DIYers that can get the job done without a spool gun. If you have a high-end welder, use the slowest wire feed speed, and exercise extreme patience, you can sometimes get away without a spool gun. (You will need to be working on a thicker aluminum since you will be moving more slowly with this method).
The reason is that the aluminum welding wire is thinner than the steel wire. It struggles with feeding too quickly and getting tangled up or “birdnesting.” Additionally, you need a Teflon liner for aluminum wire so that it can flow smoothly. A spool gun provides that.
The spool gun compensates for this softer metal by allowing you to mount the roll directly next to the contact tip. This means that the length of wire being fed is very short, and it keeps the wire from getting tangled as it would if it had to travel the length of the hose.
It increases your travel speed and keeps you from overheating the aluminum.
Additionally, most spool guns have a larger contact tip to accommodate the .035 diameter welding wire.
Not every welder is equipped for a spool gun. In our reviews, we specifically discuss that for each welder and whether it is compatible with the addition of a spool gun.
Pushing Or Pulling?
The important thing here is to get good gas coverage of your working space. When you are pulling, the gas is covering the work that you have already done, and that is cooling. The shielding gas isn’t protecting the initial part of the weld.
By pushing, you get better coverage of the area that you will be welding. The gas covers the space before you strike the arc. More importantly, it continues to protect the weld pool while you work. Just keep the MIG gun at a 10 to 15-degree angle to keep good coverage of your working area.
So, unless the angle won’t allow for it, you want to use a push welding technique as much as possible to get a clean bead.
Keep It Clean
You need to remove any grease from the base metal before you start working. Solvents can be great for this and help you get a clean prep surface. Then, you also need to remove any oxide. A stainless steel wire brush works well for this.
A powered wire brush can work well, but has the risk of embedding oxide into the metal, creating a hidden problem for your welding. If you are working with a powered stainless steel brush, use a lower RPM and lighter pressure.
When You Would Want To MIG Weld Aluminum Instead Of TIG
The MIG welder is more affordable and easy to learn than the TIG. With the TIG, you have to learn how to use the foot pedal as well as how to feed the filler material manually.
Tig Welding creates beautiful work and does a phenomenal job on thin aluminum. In fact, it can be used on metals that are thinner than the 18 gauge minimum that a MIG welder requires.
MIG welders can also easily weld metals thicker than 1/4-inch. The TIG welders typically need to be much larger or require more passes in order to handle some of these thicker welds. The MIG also tends to have a higher duty cycle and can run continuously with fewer breaks.
Finally, welding with a MIG tends to be a little faster, making it a good choice for factory scenarios where travel speed is important.