For those that suggest cast iron blocks, this is the difference.
7075-T651. T651 temper 7075 has an ultimate tensile strength of 570 MPa (83,000 psi) and yield strength of 500 MPa (73,000 psi). It has a failure elongation of 3–9%. These properties can change depending on the form of material used.
Not weldable
2024 T4
Tensile Yield
68k 47k
Weldable
6061 T6
Tensile yield
45k 40k
Very weldable
The most commonly used aluminum casting alloy is 356. These specs are for the best grade, heat treated:
Alloy HT Tensile Yield BHN
A356 T6 40k 30k 75
356 T6 33k 24k 70
Very weldable
The Yield Strength of common gray cast iron is about 30k but, it doesn't yield much, it breaks. That's why they don't list the yield. Note, the elongation.
It does have a vibration dampening property that other materials don't have and why it's also used in machine tools.
Grey cast iron (ASTM A48) C 3.4, Si 1.8, Mn 0.5
Yield Tensile Elongation BHN
--— 50k 0.5 260
Uses:
Engine cylinder blocks, flywheels, gearbox cases, machine-tool bases
Possible to be welded
It varies a little but, aluminum weighs about .100 per CI, and cast iron, .260 per CI.
The welded join on all aluminums goes down to 16 to 20k tensile strength (that's the annealed strength) and the surrounding areas, that reach 300F or more.
Aluminum, is to gummy to machine, in the annealed state and has to be HTed before machining. As I stated before, mil specs require roughing and re-heat treating of larger sections. The reason for that, is to stress relieve the parts. That also applies to forgings.