In general, when you weld aluminum, you either heat treat it after you weld it or use a non heat treatable Al. 6061 is a heat treatable alloy. Assuming It starts at heat treat 6 (T6) welding 6061 T6 changes it's yield strength from 95,000 Lb/in2 to something between 95,000 lb/in2 and 30,000 lb/in2 around the wield. Probably a lot closer to the bottom of the range than the top.

When people say welding heat treated aluminum is not a good idea, we are mostly reading it right out of engineering or aircraft repair handbooks.