Originally Posted by Autoputzer

... suspension members, hoods, fenders, doors, trunk lids... usually aluminum on a BMW. As with steel, corrosion resistance varies with how much money you're willing to spend. Marine-grade aluminum will hold up.

The bodies of BMW's are still steel, with some carbon fiber used in the really expensive ones (7 Series) and the electric ones.

One catch with aluminum is that it will eventually fail under fatigue. With steel, if you stay below ~50% of the yield stress a part will last forever. But, with aluminum if you cycle it it will eventually fail, regardless of how much below the yield stress the cycles are.

My 5 Series BMW has aluminum doors. Frau Putzer's X3 has aluminum front doors, but steel back doors. A 3 Series still has steel doors.

I've been asking for 3 Series loaners lately, sort of as extended test drives. The relatively heavy doors just don't feel right anymore.

It's almost impossible to spot weld aluminum. The aluminum body parts are riveted together.