How to Weld 6061 Without Hot Cracking

6061-T6 is probably the most common aluminum alloy any of us encounter. We all know that it’s welded everyday, so we assume it must be easy to weld. Unfortunately, this just isn’t so. In fact, 6061, and all of the other alloys in the 6XXX series, are relatively sensitive. It isn’t uncommon at all for people to have cracking problems with them.

All cracking in aluminum alloys is hot cracking. That is, the crack forms as the weld is solidifying and cooling. While other factors, such as joint restraint, can influence the tendency to hot crack, by far the most important reason for hot cracking is the chemistry of the solidifying weld. Some chemistries are inherently resistant to hot cracking. Most of the 5XXX series fall into this category. For instance, if you were welding 5083, you would probably have no problem welding it without filler metal. Such a weld is called an autogenous weld.

However, the chemistry of 6061, which is roughly 1.0% magnesium, 0.6% silicon, and the balance aluminum, is very prone to hot cracking. At first glance, that doesn’t seem to make sense. If it’s so prone to hot cracking, how do we weld it? The answer is that we can weld it easily if we add filler metal of a very different chemical composition. The weld will be an alloy of the filler metal we add and the parent material that gets melted into the weld. If we use a filler metal with a very different chemistry from 6061, the solidifying weld will have a chemistry that isn’t anywhere as prone to cracking as 6061.

Did you ever wonder why there is no 6061 filler metal made? The reason is simple. If we made 6061 into filler wire, welds made using it would crack. It would be a really poor choice for a weld filler.

Instead, we weld 6061 easily using either 4043 or 5356 filler metals. 4043 is basically aluminum with 5% silicon added to it and 5356 is aluminum with 5% magnesium added to it. Either alloy makes a good filler metal for 6061. If we use 5356, we might get a weld chemistry, depending on dilution, that is 97% aluminum, 3% magnesium, and 0.3% silicon. Such a weld will be much more resistant to hot cracking than is 6061. In a similar way, a weld made with 4043 filler is even more resistant to hot cracking than one made using 5356 filler.

So, what’s the answer to your cracking problem? Simple. You must add filler metal to welds in 6061. You cannot weld 6061 autogenously. Since we always add filler in MIG welding, the problem of hot cracking is less common than it is in TIG welding.

In fact, it’s not only important to add filler metal, it’s important to add enough filler metal. If you don’t, you can still crack. For this reason, aluminum should be welded using convex weld passes, not concave. Thin, concave root passes should be avoided in favor of heavier, convex passes.

Some 6XXX aluminum alloys, such as 6111 and 6013, also contain copper. These alloys can be very crack sensitive. The magnesium – containing filler metals like 5356 should not be used on these alloys, because they can crack. Instead, a high – silicon alloy, such as 4043, 4047 (12% silicon), or even 4145, which contains copper additions, should be used.

I’ll end this answer with an old welder’s trick. If you’re welding and getting a crack in these alloys, begin the weld in the center of the seam and weld toward the ends. Often, this will solve a really persistent cracking problem that you see when the weld start is at a free edge.