A very good read on what can/should be done with the different grades of aluminum. Also included is a complete description of the numbering systems and what alloys are used and why.
http://www.aircraftspruce.com/catalo...s/aluminfo.php
*Note: Spot welding is OK vs. arc/torch welding with some alloys....like 2024
It doesn't say that it can't be welded it says that it shouldn't be as it leaves the metal in a state prone to failure.
The word "Failure" when used in conjunction with "Aircraft" should be avoided as the phrase "search for the Black box" usually follows.
Likely a "proper application" question as opposed to a "can it be done" question......