You never see 6061 filler metal because, if it existed, it would be an awful filler metal. Virtually every weld you made with it would crack. All cracking in welding aluminum alloys is hot cracking. The crack forms as the weld cools. The weld chemistry directly causes the tendency to crack. Most alloys we think weld nicely, such as 5083, have chemistries that make them crack-resistant. You can weld these alloys without adding filler metal and have no cracks. However, the chemistry of 6061, which is aluminum with 0.6 percent silicon and 1 percent magnesium added, is crack-sensitive; yet we weld this alloy every day. How can this be? By adding enough filler metal with a different chemistry, you change the chemistry of the solidifying weld deposite so it’s not crack-sensitive. Usually 4043—aluminum with 5 percent silicon added—or 5356—aluminum with 5 percent magnesium added—is used. By adding enough of either, the chemistry of the weld now is crack-resistant. If you add filler metal and still get cracks, you’re not adding enough filler metal. You can get more filler metal into the weld by putting a joint preparation on the joint instead of using a square butt. But be careful about the weld geometry; Concave fillet welds and concave root beads can crack. Add more filler to make the bead flat or slightly convex. Now, which filler metal should you use—4043 or 5356? That’s a subject for another column.