Is it possible to rebuild a a steel bike frame (to make it lighter) by welding aluminum tubes I have an old (80's I guess) road bike, it is heavy, I'd like to make lighter. is it posible to cut and replace the tubes on the frame by welding aluminum ones?
I would not.  Instead I'd strongly recommend you look at buying the bike you want rather than chopping up a working vintage bike.  At best you'll come out with something worse than a replacement aluminium road bike, and perhaps slightly better or worse than the bike with which you started.

A used 2000's bike would be a far better bike than some chopped-up monster, and then you have N+1.

Steel and aluminium are dissimilar metals and welding is not possible. You'd be looking at brazing them. If you expected to weld them, then perhaps this is beyond your welding skill level.

If you are paying someone commercially to do the welding, then they will braze or silver solder the work.  Check for what their workmanship guarantee is as well.

You will have to paint the whole frame properly afterwards too, the steel parts need protection.