A helmet purely made from aluminium is lighter than a fiberglass helmet and won't crack when falling down. You probably shouldn't make an elephant sit on it, but under normal con-conditions and transport, they hold together well.
Welding of aluminum is a tricky thing, I'm not sure if soldering would be possible at all. But none of that is neccessary. The facial area can be dished from one piece. The gap between facial area and dome is covered up optically and actually a good thing for natural ventilation.
A jigsaw costs about 13 Eur a set. A good dishing hammer about 20 Eur. A riveting tool 15-25 Eur (you can probably work around it with wire and bolts). You will need a dremel or better a drilling machine for the inner cuts and to set the rivets... but actually, every household should have a drilling machine, otherwise: how do you fix your lights and shelves?!
