I have been using old license plate aluminum for segmenting. I use carbide to rough the shape and HSS to finish. I could use HSS the whole way, but my tools are cheap and lose their edge quick (even on wood). One word of caution with the license plate is to remove the sticker and adhesive - if left on it is very noticeable on a finished pen. Lastly, the aluminum is obviously a lot harder than wood or plastic segments, so you need to be careful as the tool transitions to avoid gouging into the softer material. Getting a surface without a noticeable bump at the segment is possible, but not trivial.
One other point when using aluminum is how you build the blank. If it is a build on the tube then you can do drilling beforehand and just stack but if build and incorporate and drill as a unit, then care needs to be taken. Heat is your enemy in whatever we do in pen making. I like to use a good quality epoxy to adhere my aluminum and other metals. I use System3 T88 epoxy. I leave sit for about 2 days because it is a 24 hour epoxy. A good quality drill bit is a must.