If welding in the T6 condition, you will loose properties
in and around the weld area. The properties in the welded area will be comparable to those in the annealed condition.
In addition, when welding in T6 (if welding is significant)you may induce stress which may result in cracks.
In order to obtain the original T6 properties after welding, you will need to re-solution heat treat (T4) and then age to obtain your T6 propertes. If you may be concerned about cracking, I would suggest annealing the part (375 F for 2 hours) prior to welding then solution heat treat and aging (after welding).
