Tig welded copper, amazingly easy. couldn't find my solder so thought what about the Tig.
It was a 22 to 15mm reducer and the 2 just flowed right together without any filler!!

Haf to usee a LOT of juice compared to MS though

Never tried autogenous welding copper, great with MS and SS.
Loads of power prob due to the thermal conductivity of Cu.

You can get some tig wire for this, forgot what its called, i used it to weld 24" copper pipe to a brass flange.