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.