Aluminium profiles can be assembled in a practically invisible way by means of expansion plugs made from a stack of Diaphragm plates.
Principle: A plug is made of a stack of Diaphragm plates. These plates fit the alumimium profile and have a slightly eccentric hole. The plates are stacked opposing the eccentric holes such that their apertures slightly overlap. An inner cone is pressed in place to expand the plug and make it grip into the inner wall of the profile.
A threaded hole in the cone allows to bolt any object to the end of the profile (here a T-connection)
T-fixation of aluminium profiles
The fixation of the rungs of a ladder
Fixation of the corners of a rectangular frame