Our Sites

Aluminum Workshop: Welding 6061-T6 without filler metal; choosing shielding gas for GMAW

Q: I’m trying to use gas tungsten arc welding (GTAW) without filler metal to join 6061-T6, and almost every weld I make cracks. What’s going on?

A: A common aluminum alloy, 6061-T6 is welded every day, so we assume it must be easy to weld. Unfortunately, it isn’t. In fact, 6061 and the other 6XXX series alloys are relatively sensitive, and it isn’t uncommon for people to have cracking problems with them.

All cracking in aluminum is hot cracking: The crack forms as the weld is solidifying and cooling. While other factors such as join restraint can influence the cracking tendency, the main reason for hot cracking is the chemistry of the solidifying weld.

Some chemistries are naturally resistant to hot cracking—most of the 5XXX series falls into this category. For instance, if you were welding 5083, you probably wouldn’t have a problem welding it without filler metal, or autogenously.

But 6061, which is roughly 1 percent magnesium, 0.6 percent silicon, and the balance aluminum, is prone to hot cracking.

While it’s prone to hot cracking, you can weld it if you add filler metal with a different chemical composition. The resulting weld would be an alloy of the added filler metal and the parent material. If you use filler metal with a different chemistry from 6061’s, the solidifying weld will have a chemistry that isn’t as prone to cracking as 6061 alone.

Did you ever wonder why 6061 filler metal isn’t made? The reason is that if 6061 were made into filler wire, welds made with it would crack. It would be a poor choice for weld filler.

Instead, welders join 6061 using either 4043 or 5356 filler metals. The 4043 is aluminum with 5 percent silicon added to it, while 5356 is aluminum with 5 percent magnesium added to it. Either alloy makes good filler metal for 6061.

If you use 5356, you might get a weld chemistry—depending on dilution—that is 97 percent aluminum, 3 percent magnesium, and 0.3 percent silicon. Such a weld is more resistant than 6061 to hot cracking. Similarly, a weld made with 4043 filler can be even more resistant to hot cracking than one made using 5356 filler.

So to prevent cracking, add filler metal to welds in 6061 because you can’t weld 6061 autogenously. Because you always add filler in gas metal arc welding (GMAW), the problem of hot cracking is less common than it is in GTAW.

It’s also important not only to remember to add filler metal, but also to add enough filler metal to prevent cracking. For this reason, you should weld aluminum using convex, not concave, weld passes. You should avoid thin, concave root passes in favor of heavier, convex passes.

Some 6XXX aluminum alloys, such as 6111 and 6013, also contain copper and can be crack-sensitive. Magnesium-containing filler metals like 5356 shouldn’t be used on these alloys because they can crack. Instead, a high-silicon alloy, such as 4043, 4047 (with 12 percent silicon), or 4145, which contain copper additions, should be used.

One last technique can help regarding weld cracking. If you’re getting a crack in these alloys as you’re welding, begin the weld in the center of the seam and weld toward the ends. Often this can solve a persistent cracking problem that you see when the weld starts at a free edge.

Q: What shielding gas would you recommend for GTAW or GMAW of ¼-inch-thick aluminum?

A: Choosing shielding gas for aluminum GTAW or GMAW can be simple.

First, never use gases that contain intentional additions of hydrogen, oxygen, carbon dioxide, or any other active gas. Hydrogen additions of as little as 10 parts per million can result in weld porosity. Excessive amounts of several types of oxide can form if active gas is added.

Once those types of shielding additions are eliminated, the only ones left to consider are argon and helium. Three choices for shielding gases are pure argon, pure helium, and mixtures of both. All three can be used for various applications.

Argon is cheaper by volume than helium. In addition, higher flow rates are required for helium than for argon, so the cost difference becomes even greater.

Argon’s gas molecules are heavier than helium’s, so argon provides better arc cleaning than helium. On the other hand, it takes more energy to ionize helium in the arc than it does argon, so the arc voltage is higher with helium. The arc also is hotter and gives more penetration.

Also, the shape of the weld cross section differs with shielding gas. With argon, the weld width is broad at the surface and quickly tapers down below the top surface. With helium, the weld width won’t taper down quickly at all; it’ll be relatively wide all the way down to the weld root. So, when using helium blends, you’ll have more tolerance if the welding torch wanders from the centerline of the seam.

Because the arc is hotter with helium additions, the weld puddle stays liquid longer. This can help to reduce porosity levels in aluminum welds.

Specifically in AC GTAW, use 100 percent argon shielding gas for relatively thin materials, such as ½ in. or less thick, and add helium when welding thicker materials.

Argon-helium mixtures commonly are available as 25 percent helium and 75 percent argon or 50 percent helium and 50 percent argon. Gas mixtures containing more than 50 percent helium usually aren’t recommended for GTAW because arc starting becomes difficult. While the use of helium-argon blends on thin materials isn’t wrong, it’s important to make sure you want the characteristics that helium adds to the arc—a hotter, more penetrating arc—on thin material. Usually, you don’t want these characteristics when welding thin materials.

For DC GMAW, which usually is formed with the wire at positive polarity, the situation is similar. For thin materials, use 100 percent argon. When the thickness increases to 3/8 or ½ in., consider adding helium to the shielding gas to increase penetration. Helium blends containing up to 75 percent helium commonly are used in GMAW.

DC straight-polarity GTAW is performed less often on aluminum than AC GTAW, but it’s common in several industries. In DC straight-polarity welding, 100 percent pure helium must be used as a shielding gas for the process to work correctly/

Don’t forget to think about how gas blends will affect your environment. Years ago Europeans developed shielding gas mixtures containing less than 1 percent of nitrous oxide. These gas mixtures are available today in the U.S. Nitrous oxide is added to suppress the formation of ozone during welding. Because aluminum GMAW can produce significant amounts of ozone—especially at currents of 250 amps or more—these gases, which consist of argon, helium, or standard argon-helium blends, may be worth considering, especially if you’re welding in a confined space.

Gas purity is another important aspect of choosing a shielding gas. Shielding gases are covered in American Welding Society (AWS) specification A5.32, which states that argon must be at least 99.998 percent pure, with a dew point no higher than -76 degrees, and that helium must be at least 99.995 percent pure, with a dew point no higher than -71 degrees.

About the Author
Aluminum Consulting Inc.

Frank Armao

President

Aluminum Consulting Inc.

440-479-0239

Frank Armao was an active member of the AWS D1 Committee, chairman of the AWS D1 Aluminum Subcommittee, and member of the Aluminum Association Committee on Welding and Joining. He also was the author of The WELDER's "Aluminum Workshop" column from 2001 to 2020.