Thing to remember about aluminum is NOT the tensile strength, it's the modulus of elasticity. That is another way of saying spring constant, if that helps explain. Aluminum has a modulus of elasticity of around 1/3 that of steel. That means for a given amount of stress, the aluminum will deflect three times as much as steel. So to get the same stiffness, the aluminum part has to be three times as thick or whatever dimension matters most. That partially negates the weight savings.

This has nothing to do with strength, which is generally how much stress before permanent deformation.

If using aluminum I'd be much more worried about how to get a decent weld than the effects of dissimilar metals on corrosion.

R.