I have refered to The Machinist's Handbook 25th Edition in the past when running into material questions. Here is a simple explanation of Aluminum alloy codes from the books. http://www.jjjtrain.com/vms/engineer...al_stds.html#6 I'm sure an Engineer can better answer your questions, but I hope this helps. I machine aluminum everyday, and have found 6061 to be one of the easier alloys to machine, as well as reasonably priced. 2024 is pretty expensive and other alloys can be "gummy" or brittle to machine and tap. Be sure to use a tapping fluid specifically for aluminum, or kerosene works too, but may be a bit dangerous. Hope this may be of some help.