2024 aluminum: Hard, stronger than 6061T6, not suitable for welding.
6061T651 aluminum: most common used aluminum, good for welding, and general machining of parts.
303 stainless steel: red color coded, contains sulfur to make easier to machine, not suitable for welding or vacuum use.
304 stainless steel: green color coded, tough to machine, suitable for welding and vacuum use, most common SS alloy, also known as 18-8.
OFHC copper: Oxygen free high conductivity, commercially pure Cu, light blue color coded, suitable for welding and vacuum use, soft and stringy/ gummy to machine.
Brass: easy to machine, tends to grab and pull cutting tool/drill into work use specially ground drills, machine dry (no cutting oil).
Teflon: white, slippery, vacuum compatible, very soft, difficult to glue
Delrin: white or black, easy to machine and tap, harder than Teflon, difficult to glue.
PVC: white or gray, least expensive plastic, can be solvent glued
Acrylic: also known as Plexiglass, clear & various colors, rigid, can be heated and bent, somewhat brittle, can be solvent glued.
Polycarbonate: also known as Lexan, clear & various colors, tough, thinner sheets can be bent, good for guards, can be solvent glued.