Welding 6061-T6 removes the strengthening gained in the T6 heat treatment. This weakening can be very large and would cause failure adjacent to the weld. One thing you could do is weld teh part then re-solution treat and quench, then artificially age to get back to the T6 throughout the construction.
If 6061-T6 must be welded, a compatible filler weld alloy such as 5356 should be used. According to the Aluminum Assoc specs, the 6061-T6 strength will drop from a 24 ksi ultimate shear strength in unwelded state to a 15 ksi ultimate shear strength after welding due to loss of tempering. Since the welded state retains only slightly less than 2/3 of the original tempered state, this loss should be considered in sizing the material. The minimum shear strength of 5356 at 17 ksi matches well with the welded 6061 at 15 ksi.
Welding of 6061-T is not permitted for structural applications on US Navy vessels. But, I have recieved permission for ship board use for non-structural application. The as welded AL yield value for design should be 7-8 KPSI. Approve should be obtained through NAVSEA tech code Al Gorbalski 703-602-0143.
