John, there is a fairly lengthy thread called "Mast materials and specifications" with some discussions you might look at, some of it speculative. No-one recommends welding.
To bridge the gap between diameters inside the joint, if the difference is small, two epoxy and glass bandages will do. For a larger gap, thin strips of wood can be epoxy glued to the inner tube, faired and fitted so as to maintain alignment.
The overlapping sections need sufficient "bury".
The choice of glue for joining the sections has been a subject for discussion. I have found that epoxy glue sticks well to aluminium if you prepare it properly. There is another school of thought which suggests a more flexible glue is preferable (polyurethane rubber type) as a rigid glue, in theory, creates a hard spot.
My own speculation is that the outer bandage and fairing cone is probably sufficient to hold two well-fitted sections, with just a minimum of glue inside the joint, and I am putting this to the test on the small mast shown in the last post. One way or another, the joint together with the lip created by the fairing cone must withstand downward pressure (from halyards etc) - and twisting.
I don't think there has been any report of a mast failure at the join of a composite mast, so far. If there has been, I hope someone will chime in and provide a reference.