It depends on the heat input and the time at temperature, i.e., the more weld passes deposited without the benefit of interpass cooling the greater the volume of base metal subject to degradation.
The base metal regions subjected to high temperatures is over aged, allowing some of the alloying elements to precipitate to the grain boundaries which results in a loss of strength and ductility.
Assuming you are in the US, you must design to the requirements of the Aluminum Design Manual (ADM). For welded 6063-T6, the reduction in allowable stress in a welded area occurs for 1-inch in all directions from the weld.
So in your case, 1" up the tube and 1" out in all directions on the plate.
As gtaw notes, there are lots of variables if you want to determine the actual weld effects...what I gave you is the code requirement for design.
I've designed over 700 aluminum structures and one of the biggest challenges I have is dealing with welded joints and the heat affected zone.
If by chance your application is a tubular post for a handrail or guardrail, you probably need to come up with an alternate design.
Like Ron said, its hard to get those guardrail/handrail posts to work. We have had to get creative and add a member inside of the post to increase the section until you get far enough up the post for your moment to work. Likewise, you could sleeve it in aluminum or steel (with a barrier between), though architects and owners hate this too and I understand that. Or grout it in place so there are no welds, even grouting it into a steel sleeve that is welded to the structure may work.
A lot of our work in this area are schools, and we are convinced some architects are hooked on the look of powder-coated aluminum and figure it will never rust, so it must be the best for a harsh outdoor environment at a school. I am sure they have no idea how much more cost they may be adding for us to get their designs to work, when a galvanized or painted steel handrail assembly would have worked brilliantly. I also worry about fatigue from dynamic loading with aluminum at schools...
Do you need a welded connection? There are plenty of cast aluminum post bases that will do the job. Check out Thompson Fabricating handrail. They use only bolted connections and it turns out pretty slick.
Thanks for the creative thoughts, a2mfk, and thanks for the Thompson Fabricating suggestion, Gumpmaster.
We'll probably end up putting some holes in the concrete and securing the posts with grout (with a PVC pipe or bituminous paint barrier). I think your theory about architects who are predisposed to using aluminum without considering all the angles (p.i.) is true, a2mfk.
