I have a new Alum. 3 position...actually 4 if you count closed as a position...my question is...Will the rifle function in the closed position?

I am building a Post Ban and came across a New Pre-Ban collapsable cheap and want to incorporate it...I have access to a Tig Welder and plan to tack weld a small bead onto the tube to permanantly fix the stock...will the rifle function reliably at the closed or any of the other three positions? And will I be legal?

It should function reliably at all of the positions provided you don't deform the tube when you weld it.   Have you considered getting a fixed tele-style stock from bushmaster or other manufacturer?  It might be cheaper, plus you know 100% that it is legal.

Just my .02