To be honest that’s not really gonna go too well. Your going to get a trade off either on the cap or on the root trying to do 3mm square edge in one pass. You’ll need 130 amps or more just to get through it and give decent root reinforcement but the trade off is that’s gonna give you a very hot possibly sunken cap as it’s simply too hot for it. In the “real world” a joint like that would almost certainly be prepped and as it’s a GTAW application it would almost certainly demand 2 passes.I was using 85amps on 3mm steel butt weld in PF position with 2mm gap but couldn’t get good penetration. if I put a little chamfer on the edges I could, but I wanted to do it without chamfers.
Any pointers, please?