First off, the suit isn't actually made using Iron! If it was then it would rust and be really heavy (not so good for combat and underwater tasks). In the comics, the original Mark I version of the suit, which was cave-made, probably had iron, or at least some iron components. Predictably, this impractical material has been thrown to the wayside as soon as Stark is back in the lab. Even some 5 decades ago, when Iron Man was first introduced in the comics, iron was disregarded, so just what would be Mr Stark's choice of material?