Aluminum tubing has to be TIG welded. Some early aluminum bikes, notably ALAN and Raleigh Technium used lugs, but the tubes were glued rather than brazed into the lugs and that proved to be less than optimal in terms of longevity.

You might want to download the scanned pages of Designing and Building Your Own Frameset by Richard Talbot, P.E., from Bob Hovey's website HERE to gain some appreciation of what's involved in building a silver brazed lugged steel frameset.

Another source of good framebuilding information is The Paterek Manual.