Why is pure aluminum not used in aircraft parts or in automotive engines even though it has a low density that would make it very desirable for these uses?
There are a number of reasons of which the main one is cost. To extract aluminium from its ore is very expensive since it can only be done using electrolysis of the molten ore. This requires high temperatures and currents. In contrast iron can be extracted by smelting, simply heating the ore (to only a few hundred degrees) with carbon. The cheapest option that does the job safely will always be the one chosen. Another reason is that many alloys have properties similar to, or better than aluminium, again for less cost. Many titanium alloys for example are incredibly strong and can be made light using a carefully controlled composition.
