Aluminum Heat Treatment Temper T6
Standardized microstructural condition of precipitation-hardenable aluminum alloys achieved by solution heat treatment, quenching, and subsequent artificial aging.
T6 is a hardened condition: the material is solution annealed, quenched, and then artificially aged. This gives the aluminum its high strength.
When welding, you have to realize: the heat burns out the heat treatment in the weld area. Afterward, you have a soft spot there, no matter how good the weld looks. The strength won't come back unless you do a complete re-aging treatment afterward.
In practice, this means: if you weld a T6 part, plan for the load-bearing capacity at the weld to be gone. For pure repairs on non-critical areas, it's okay, but for load-bearing structures, you either have to re-harden the part or solve it differently in design. And let the part cool down slowly after welding, don't quench it with water, otherwise you'll distort the component.