Here is what worked for 4" type K heavy walled copper but I don't know if you can find one big enough for 6"
Remember, you may have to machine the ID of the ferrule if you are making a removable tree so your plates will slide through, make sure you engineer this process before you start machining everything!!!
Anneal the pipe end, good and hot and quench it
Grease the inside flare and put the annealed end on the top of an appropriate sized industrial bottle jack with a rounded casing like in the picture (it will destroy the paint!!!)
Place a chunk of wood on the other end and smack it to flare the end open a bit MAKE SURE IT IS PLUMB TO THE JACK SO IT FLARES EVENLY
Turn upside down, grease the inside flare again and slide the ferrule in a ways
Put a piece of wood over the ferrule and smack it with a hammer a few times straight down and it will expand, adjust with an extra hit on one side if it isn't going in straight
Re-anneal if needed
Once the ferrule is seated as far as you want it, re-anneal the end, tap the flared pipe back down flush with the ferrule collar and silver solder together making sure it is square
Repeat on other side
Worked great for me
jack.jpg