Spider cracking in tigged aluminum plug welds

by Norm
(BC)

building a diesel fuel tank out of 3/16" 5052 aluminum plate - adding top plate last to allow rest of tank to be welded inside and out. Top plate lands on a flange for a perimeter lap weld, and I have 6 plug welds landing on angles welded to internal baffles - having trouble with the plug welds.

I migged the plug welds initially, but did not like the look of the plug weld perimeter, so I went over with my tig, just floating the weld area - looked great when I was tigging, but once the weld was complete, 3 of the plugs developed spider cracks - looks like the weld tore open slightly. Thought I'd gotten too hot without adding filler - so I went over again adding filler - that worked on 2 out of 3 spots - 3rd continues to look good during welding, then tear open.

Using an Everlast 205, 150a on slow pulse, with 3/32" ceriated tungsten and 3/32" 5356 filler rod, running about 9 LPM argon.

I'm a aluminum welding noob - why are my plug welds spider cracking?

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Generally speaking, dry washing over aluminum is not a good idea....especially in small round welds.

Removing mig weld first, and then going back with tig while adding filler is a better way but on small round or circumferential welds, cracking is always a risk.

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