Can aluminum 6061-T6 weld be heat treated in field?

I highly doubt it. I've worked with heat treating 6061 to a T6 temper in a factory setting before and it's a fairly particular process to get it right. The material has to be heated and held at a fairly high temperature (relatively close to the melting point) for an extended period, then rapidly water quenched. It then has to be held at a more moderate temperature for a while to artificially age it to it's final temper. The temperature control and timing of all these steps are fairly critical. Maybe someone out there has a way to heat treat in the field, but I would be extremely skeptical of anything they had to say.

in order to compensated make the structure thicker, and run the numbers , heat treating out in the field is not an option. test the welds with non destructive , and have a weld engineer run a schedule

Possible, but practical? No. The heat treatable 6061-T6 loses about 40% of the tensile strength and 50% of the yield strength when it is welded properly. Not only is the alloy heat treated, it is also strain hard to a limited extent. Heat treatment alone will not restore the mechanical properties to the original condition.

Best regards - Al

NOT sure what You are constructing... bridge, tower, trestle, steps, ship/boat hulls, etc... but The Aluminum Association [AA] has unique information on building these type structures from various aluminum alloys.... bolted, welded, brazed, etc.

CAUTION1. The outdoors environment can grossly affect durability of aluminum structures. BE CAREFUL!

First, welding 6061-T6 is ill-advised... weld in -T4 then AHT to -T6 is significantly better from perspective of hot-short cracking.

Welding aluminum with high strength welds... without post-weld HT is possible... but with different alloys/methods.

Perhaps FASTENING [riveting/bolting] your structure together... with high durability corrosion resistive coatings... is a better option.

Useful Aluminum Association documents...
Aluminum in Bridge Construction: Perspective on Design, Construction and Performance
Welding Aluminum: Theory and Practice

Useful American Welding Society [AWS] documents...
AWS D3.7 Guide for Aluminum Hull Welding


Regards, Wil Taylor
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The normal filler metals do not harden, they are relatively low strength.