About 6061|The default aluminum for machine shops. If nobody specified an alloy, it’s probably 6061-T6. Good strength, good corrosion resistance, welds well, and anodizes nicely. The machinability reputation is “good” but experienced machinists know it can be inconsistent lot to lot. Some batches cut clean, others get gummy and produce stringy bird’s-nest chips, especially on lathes.
7075 actually chips better, but costs roughly twice as much per pound. 6061 is the alloy you reach for unless you have a specific reason not to. Compare to 7075 (stronger, chips better, costs more) and
6063 (weaker, better surface finish, designed for extrusions).