My understanding is that most masts use
grade 6061-T6 aluminum. This grade can be welded but that "T6" at the end means that the aluminum has a heat treatment process done to it after it has been extruded. When you weld it, the heat treatment is removed (annealed) for a short distance from the weld in both directions. This means that the metal will not have the same springiness. When it is loaded, it will stretch and not spring back to the original place. Think if a paper clip versus a coil spring.
Welding is best done at points of support and will not be as stiff as the unwielded section. That is why the sleeve is needed inside the tube. And yes, I am a structural engineer.