Clamped shortened hub and disk to heavy metal plate to draw the heat away while welding. Left it clamped up while it cooled after welding. I actually fused the materials, didn't need to add any filler rod.
I wanted to cap the end of the deraillier tube so I tacked a square to a short piece of that tube and turned it down on the lathe. The welds held and had to be broken free to get the disk loose.
