DHO... CAUTION... You mentioned welding.
6061-O and -T6 have poor welding qualities, different reasons. It is desireable to weld 6061 in -T4 temper... then go to the -T62 temper for the assy... as follows [A or B].
A. In typical scenario fabricate all parts from -T4/-T42 temper material; weld them together [AWS D17.1 + ER4043], then age-harden the assembly to -T6, same as per -6088 or AMS2770. The base materials will be -T6 temper and the weld filler will be somewhere between -O and -T4 upon final HT. Makes a very nice/sound weld assy that is stress relieved.
B. When highest strength is required, fabricate all parts from -T4/-T42 temper material; weld them together [AWS D17.1 + ER4643], then solution HT and age-harden the assembly to -T62, same as per -6088 or AMS2770. The base material and will be -T62 temper and the weld filler will be close to -T42 upon final HT. IF proper techniques are employed, then welds are noticibly stronger/tougher.
NOTE. ER4043 produces high quality sound welds; however ER4643 produces generally higher strength welds, at a premium price and whigh higher quality welding techniques [preparation, etc] required.
NOTE. For critical weldments, it is wise to test welds [trust but verify the materials, process (and welder)]! Fabricate the parts, weld-them, HT them to finished temper [whatever] and then cut-out/mechanical test each section to ensure mechanical properties are acceptable. Or trust me: Rough weldment values can be found for 6061 and other alloys in Aluminum Association [AA] Aluminum Design Manual; or the AA Aluminum Standards and Data book; or the AA Welding Aluminum : theory and Practice book. I can't remember which one of these had the general weld strength tables.
NOTE. If severe forming is required then form only the part(s) that need it in the -O condition and then HT 'it' to -T42 before welding. I don't believe that 6061 has the same grain-growth problems when severely deformed in the -O condition that 2xxx and 7xxx alloys do. However, forming in the -W temper would, perhaps, eliminate quench distortion issues. Again, HT the Assy to -T6 after all is said and done.
Any welders/weld-engrs 'out there' who have additional comments... or might want to raise the BS flag on this post?
Gotta go.
Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]
