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Aluminum Workshop: 6061-T6 cracking

Why it happens and how to fix it

Q: My company recently purchased a piece of equipment fabricated from 6061-T6 structural shapes. The welds do not seem to be very good and we have found several cracked welds. However, it is the weld shown in the photo that is puzzling. This weld shows a crack in the 6061 base metal about 2 in. long leading upward from the weld in the picture. Is this real? What is it? What caused it?

A: Yes, what you are seeing is real. As you say, it is a crack in the 6061-T6, not in the weld. It should be noted that the adjacent weld was made using gas tungsten arc welding (GTAW). The crack is the result of an inadvertent arc strike on the part with the GTAW torch.

As you may remember, we have discussed the fact that 6061 and all the other 6XXX alloys are very crack-sensitive. If you want to see for yourself, just take two pieces of 1/8-in.-thick 6061, butt them together along an edge, then use GTAW without a filler rod to weld them together. You’ll end up with a crack as long as the weld you made caused by the chemistry of these alloys. However, we are able to weld them very successfully with the addition of filler metals with dissimilar chemistries, such as 4043 Al with 5 percent Si or 5356 Al with 5 percent Mg.

As I said, what caused the crack in this case was an inadvertent arc strike. The welder struck the arc off of the weld before he was ready and didn’t add any filler. If you look closely, you should be able to see some weld spatter from the arc strike about 2 in. above the weld. The welder probably thought nothing of it and probably didn’t even see the crack.

So the message is clear: Be careful not to strike the arc outside the weld, especially with GTAW. If you do mistakenly strike it on the base metal away from the weld, examine the area carefully to be sure there is no crack. If there is a crack, you must grind it out to eliminate it and deposit a weld using 4043 or 5356 filler to repair the area. You can’t just weld over the crack without grinding it out first. If you do, the crack will most likely return.

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.