6061-T6 aluminum base materials that can be welded successfully with any number of different filler alloys. The filler material is a crittical element to the WPS. Filler alloys 5356, 5554, 4043, and 4047 are commonly used. For higher strength use the 5000 series, for more ductility use 4043. I would use 5356 for the best color match after anodizing.
What variables associated with weld performance are of most importance? Are you reheat treating the pipe after welding?