Too much heat.... and not enough clamping....
You'd need to use a large angle iron bar or something similar and hold down the entire edge of the sheet metal....
Then you need to turn the heat way lower or weld way faster -- but most of the heat into the "flange" rather than the sheet... and you need to allow it to cool before beginning another weld and weld on a completely different area... use a damp rag to cool quickly after welding and do far less of a bead than you're running...
That's my .02 ---- I hope others will kick in with how they'd do it.
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