I noticed today that both the tags "Aluminium" and "Aluminum" are used on this site. It is somewhat debatable in my mind if a material even needs a tag, but regardless of that, which format should this site adopt?
The standard on Stack Exchange is for tags to use US English, per Shog9's answer on Meta.SE. In this particular case the spelling can be something of a sticky wicket so the easiest course is to pass the buck upstairs and just follow the same standard as the rest of the network. I've made them synonyms to help a bit with usability.
Another (admittedly very weak, but still a reality) reason to stick with US English: we Americans tend to be keenly oblivious of the existence of the rest of the world, and I would say it's a rare American indeed who is even aware of the "aluminum/aluminium" dichotomy.
On the other hand I think most non-Americans are aware that Americans say it differently than everyone else (am I wrong?). Therefore, going with "aluminum" as the default (and "aluminium" as a synonym) will result in the least amount of confusion (and a lot of dead puppies).