[mactec54]

Purging could be used but it’s not required in this application and you missing the point. The metal is 8 mm and 1.2mm that is the problem he is facing. The question to the welder how to approach it.
With TIG - it’s simple, one should heat the thicker plate first pointing torch on it and soon as it makes melting point turn the torch to the root and get the paddle going directing most of the heat to the thick plate.
With MIG - you don’t get just heat- it comes with the metal. If it directed to the root right the way - there is no way to heat the thick side.
The trick is to start on the thick side and approach the root when you get the proper heat. Then it depends on the wire size- if the wire is to small, you never get it hot enough and you putting melting metal on cold base. Here is you leak…

No point being missed

He only had a start and stop point problem this is easy to solve with the right setup

The metal thickness is not the problem there are hundreds of automotive manifolds made like this every year without any problems and their thickness is up to 12mm on the flange and 1.2mm to 1.5mm for the tubing thickness, they are all Mig welded some complicated systems are Tig welded, purge is never used for this unless the tubing being used is stainless steel then you should use continuous purge flow

It could be Tig welded like you say but is not need, if they have a good welder and machine to do the job