Butt weld crack

One of my welders did .125 thick 5052-h32 butt weld on both sides, after grinding flush (this was a requirement on the print) it was found to be cracked on the second side. This was found on only one of several units. What could have caused this?? thanks.

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wtt response,

there a lots of possible causes,

not enough filler or no filler ( dry washing aluminum will crack it usually, especially when starting from an edge)

highly restrained or stressed joint...sometimes one weld is stressed more because of other welds that are done first.

wrong filler metal...sometimes welders pick up the wrong rod without knowing it.

4043 sometimes causes cracks in 5052 because silicon and magnesium dont play well together.