Originally Posted by speedyHAM
I would not use aluminum in this application without a post-weld heat treatment.
T-0 aluminum is pretty much junk. It looks nice and won't corrode, but it will be much heavier than steel for the same structure if both are well designed.
6061 T-651 aluminum would be a great material for a subframe, lighter and stronger than steel. Stiffness is all in the design and could be made equal with either material.
If you do use aluminum, take care to make sure your bolt hole surfaces are thick enough to handle the impact forces they will be seeing without ovalizing.
I would use 4130 steel myself.
Tensile strength is only 45000 psi.
Mild steel starts in the 60000 psi range.
You may be thinking of 7075-T6.
83000 psi tensile strength. But it is not considered to be weldable.